The perovskite solar cells (PSCs) are promising candidates to reach the highest efficiency among the single junction solar cells due to the adapted bandgap around 1.55 eV, strong absorption and carrier diffusion length. To increase the PSCs efficiency, real origin of the I-V hysteresis became a big issue and has been discussed widely. In this study, we have modeled the hysteresis in the I-V curves of PSC by two dimensional modeling using the Silvaco Atlas software. The simulations has been done using a planar structure made of glass, fluorine doped tin oxide (FTO), compact titanium oxide (c-TiO2), methyl ammonium lead tri-iodide (MAPbI3), 2,2′,7,7′-Tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spiro-OMeTAD-bifluorene (spiro-OMeTAD) and gold. We have studied the effect of trap assisted charge accumulation in c-TiO2 and recombination at the c-TiO2 / MAPbI3 interface. On the one hand this study shows that those two processes can reproduce the hysteresis behavior as function of the scan rate and voltage preset in perovskite solar cells. On the other hand this study suggests that to reduce hysteresis and improve the efficiency of PSCs one should decrease charge trapping and recombination at the c-TiO2 / MAPbI3 interface.

References :

[1] L. Cojocaru, S. Uchida, P. V. V. Jayaweera, S. Kaneko, J. Nakazaki, T. Kubo, H. Segawa, Chem. Lett. 2015, 44, 1750–1752.

[2] H. J. Snaith, A. Abate, J. M. Ball, G. E. Eperon, T. Leijtens, N. K. Noel, S. D. Stranks, J. T.-W. Wang, K. Wojciechowski, W. Zhang, J. Phys. Chem. Lett. 2014, 5, 1511–1515.

[3] W. Tress, N. Marinova, T. Moehl, S. M. Zakeeruddin, M. K. Nazeeruddin, M. Grätzel, Energy Environ. Sci. 2015, 8, 995–1004.

[4] F. P. Heiman, G. Warfield, IEEE Trans. Electron Devices 1965, 12, 167–178.

   Perovskite solar cells have attracted much attention as a promising printable photovoltaic with high power conversion efficiency. The performance of perovskite solar cells highly depends on the quality of the perovskite layer, and thus the development of the fabrication technology to obtain a dense and flat perovskite layer is one of crucial issues in this field. We have recently optimized our solution method with solvent engineering using a mix solvent of DMF and DMSO, which leads to a high power conversion efficiency ~20% even in the cells composed of a pure CH₃NH₃PbI₃ perovskite as a photo-absorber. In this method, the use of DMF was found to be effective to obtain a thicker perovskite layer, which leads to high J_SC, due to high solubility of perovskite components (PbI₂ and CH₃NH₃I) in DMF, whereas the higher volatility of DMF relative to DMSO needs a rather narrow process window in its spin-coating. For the application to the larger scale fabrication of the devices, the development of fabrication method for perovskite layer with a wider process window, which leads to higher reproducibility, is strongly required.  

   During the course of our investigation of solvent effects on the perovskite film, we found that the color of CH₃NH₃PbI₃ perovskite film immediately changes from black to colorless upon exposure to DMF vapor. The SEM images exhibited that the morphology of the surface drastically changes from cubic to needle-like shape. To confirm the colorless crystalline component, we isolated colorless crystals from a mixture of PbI₂ and CH₃NH₃I in DMF. The single crystal X-ray analysis confirmed that these colorless crystals correspond to a complex of CH₃NH₃PbI₃·DMF, in which a solvent molecule of DMF is intercalated between the perovskite components. This complex shows higher solubility in DMSO (2.0 M at room temperature) compared to that of a mixture of PbI₂ and CH₃NH₃I (1.2 M), suggesting its utility as a highly purified precursor for perovskite layer even in the solution method using a DMSO solution, which enables the fabrication of perovskite layer with a wider process window due to a low volatility of DMSO. After optimization of the fabrication protocol, we achieved a J_SC value of 23.5 mA/cm², a V_OC of 1.14 V, and a FF of 0.73, leading to high PCE of 19.6% with excellent reproducibility. In this presentation, we will report the details of our fabrication method using this complex as a precursor as well as its properties.

Organic-inorganic lead halide perovskites have a high content of toxic lead which may eventually hamper their commercialization [1]. The hybrid two-dimensional drew attention because these systems can be regarded as natural multiple-quantum-wells in which the semiconducting inorganic layers act as potential "wells" and the organic layers act as potential "barriers"[2,3]. 2D perovskites (CnH2n+1NH3)2BX4; B = Mn, Cu; X = Cl, Br, I) can be alternative classes of lead free perovskite for optoelectronic application. We prepare different series of Cu and Mn perovskites by mixing of different CnH2n+1 organic ammonium cation (alkyl and aromatic) and X halide ion and we studied the optical properties in such systems. Properties were analyzed using x-ray diffraction und UV-VIS spectroscopy. Bruker D8 Advance with Bragg-Brentano geometry was used for x-ray analysis and UV3600 Shimadzu for optical characterization. The effect of different solvents on the morphology and crystallinity of 2D perovskite films were investigate using AFM.

[1] Daniele Cortecchia, Herlina Arianita Dewi et al., Inorganic Chemistry, 2016, 55(3) pp. 1044-1052.

[2] Kataoka, T.; Kondo, T.; Ito, R.; Sasaki, S.; Uchida, K.; Miura, N.Magneto-Optical Study on Excitonic Spectra in (C6H13NH3)2PbI4, Phys. Rev. B: Condens. Matter Mater. Phys.1993, 47, 2010−2018.

[3] Muljarov, E. A.; Tikhodeev, S. G.; Gippius, N. A.; Ishihara, T. Phys. Rev. B: Condens. Matter Mater. Phys. 1995, 51, 14370−14378.

FA_1−xMa_xPb(I_1−yBr_y)₃ has been shown as one of the most promising perovskite compositions with a high performance of 20.8% PCE.[1] This composition is designed for an optimal band gap and power conversion performance and not for their stability performance; but for realization of commercial photovoltaic technologies both must be considered. From our previous work, the devices of MAPbI₃ without encapsulation in different environments were examined; humidity and oxygen with light were found as the two main degradation factors for perovskite.[2] Herein, the stabilities of mixed cation and mixed halide perovskite in humidity or oxygen plus light have been examined.  The humidity tolerance of (MAPb(I_1-xBr_x)₃, x =0…1) and (FA_1−xMA_xPbI₃, x=0…1) films were examined at varying humidity levels to monitor the formation of the hydrated perovskite phase. The films were placed in an inert environmental chamber with nitrogen and controlled humidity levels. The higher bromine ratio can enhance the humidity stability owing to the thermodynamic crystal stability of MAPbBr. Interestingly, both FAPbI₃ and MAPbI₃ showed inferior stability to humidity compared to mixed cation films.Oxygen and light stability of mixed halide material series was investigated in dry air with 1 sun. For thin film studies, the longer stability was observed for MAPbBr₃, but all of the mixed halide films with the inclusion of iodide started to degrade at same time as MAPbI₃.  These results suggest iodide containing perovskite materials have an inherent instability to environmental stresses that cannot be overcome with ion tuning; however, CH₃NH₃PbBr3 perovskite shows promising stability for large bandgap photovoltaic operations.

Reference

[1] D. Bi, W. Tress, M. I. Dar, P. Gao, J. Luo, C. Renevier, K. Schenk, A. Abate, F. Giordano, J.-P. Correa Baena, J.-D. Decoppet, S. M. Zakeeruddin, M. K. Nazeeruddin, M. Gra tzel and A. Hagfeldt, Sci. Adv., 2016, 2, e1501170–e1501170.

[2] D. Bryant, N. Aristidou, S. Pont, I. Sanchez-Molina, T. Chotchunangatchaval, S. Wheeler, J. R. Durrant and S. A. Haque, Energy Environ. Sci., 2016, 9, DOI: 10.1039/c6ee00409a. 

Organic-inorganic perovskite (PVK/ CH3NH3PbI3) type solar cells have attracted much attention due to its rapid improvement in the photo conversion efficiency. Recently, these kind of solar cells have achieved over 22% of photo conversion efficiency. However, most commonly employed organic hole transport materials (HTM) such as Spiro-OMeTAD increase the production cost and hinders the large scale industrial production of these type of solar cells. With the advent of further research, finding low cost alternatives to replace these expensive HTM is a major field of research. NiO was investigated as a low cost HTM, having a higher potential to replace expensive and popular spiro-OMeTAD. Herein we report, inverted PVK solar cell with the configuration of FTO glass/NiO/Perovskite/PC61BM/BCP/Ag. NiO was deposited on FTO glass by spray pyrolysis using nickel acetyl acetonate as a precursor solution. Perovskite layer was formed by one step deposition process by spin coating 1.8M solution of CH3NH3PbI3-DMF complex in DMSO. During the spin coating, toluene dripping was conducted inside a N2 filled glove box to get smooth PVK layer. The device possesses an appreciated power conversion efficiency (PCE) of 16.1% under simulated solar irradiation with fill factor (FF) of 0.76, short –circuit current density (Jsc) of 19.60 mA·cm-2, and open circuit voltage (Voc) of 1.08 V, which had been changed with the processing temperature at spray-pyrolysis deposition. We measured the NiO characteristics with the different processing temperatures (UV-Vis, EDX, XRD, XPS, XASF, and work function) for the further consideration. 

Recently, bismuth-based lead-free double perovskites like Cs₂AgBiBr₆ have been considered as alternatives to the emerging lead-based perovskites for solar cell applications. Organic–inorganic lead halide perovskite absorbers have excellent photovoltaic properties, such as suitable bandgap, high optical absorption, and long carrier lifetimes [1]. The underlying issues are the presence of toxic lead and their instability under ambient atmosphere (e.g., O₂ and H₂O). Trivalent cations, such as Bi³⁺, along with monovalent cations, such as Ag⁺, have concurrently been introduced on the B-sites of halide perovskites, leading to B cation double perovskites with the general chemical formula of A₂B’B’’X₆. Recently, bismuth-based lead-free double perovskites like Cs₂AgBiBr₆ have been considered as alternatives to the emerging lead-based perovskites for solar cell applications. The compound crystallizes in a cubic structure, where the rock-salt arrangement of B cations is energetically favored, primarily because of the large charge difference between monovalent and trivalent B cations [3]. These Pb-free double perovskites have been reported to have long carrier recombination lifetime, [3] good stability against air and moisture, [3] and low carrier effective masses [4].                    

In our work, we report on the physical vapor deposition of a halide double perovskite, Cs₂AgBiBr₆. We compare single, dual, and triple source vapor deposition processes and explore the optoelectronic properties of the resulting films.  

References

[1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 2009, 131,6050–6051.

[2] A. H. Slavney, T. Hu, A. M. Lindenberg, H. I. Karunadasa, J. Am. Chem. Soc. 2016, 138, 2138–2141.

[3] M. Anderson, K. Greenwood, G. Taylor, K. Poeppelmeier, Prog. Solid State Chem. 1993, 22, 197–233.

[4] G. Volonakis, M. R. Filip, A. A. Haghighirad, N. Sakai, B. Wenger, H. J. Snaith, F. Giustino, J. Phys. Chem. Lett. 2016, 7, 1254–1259.

 Organic Solar Cells (OSCs) based on organic semiconductors are currently attracting a great attention due to advantage of solution process, ease of fabrication and potential of flexible device. Despite these advantages, photovoltaic performance of OPVs is still low because organic semiconductors have limited absorption property, low absorption coefficient and low electron mobility. To record the high performance for OSCs, studies have been conducted on synthesizing developed structure of organic semiconductors with novel photovoltaic property and modifying device structure to improving charge transporting property.

 In inverted-OSCs (i-OSCs), PEDOT:PSS, hole extraction layer (HEL) with high conductivity and transparency, was introduced on photoactive layer via solution process. However, the corrosive nature of PSS (polystyrene sulfonate) is harmful for photoactive layer with respect to stability of device. It can be good alternative that introducing modified HEL on photoactive layer like neutral polyelectrolytes, metal oxides and hybrid materials via solution process to increasing photovoltaic performance and stability. Among them, conjugated polyelectrolytes and metal oxide precursor hybrid layer is attractive HEL for protecting photoactive layer and improving charge transporting property.

 In this study, we fabricated i-OSCs with solution processed organic-inorganic hybrid HEL with conjugated polyelectrolytes and metal oxides precursor to enhanced performance and stability in ambient air. As a results, hybrid HEL introduced device exhibited enhanced stability in ambient air than PEDOT:PSS based device.

The potential of 4 terminal perovskite//CIGS tandem solar cell was evaluated by optical splitting method. In the 4 terminal approach, there is flexibility in cell design since the current matching required in 2 terminal device is not needed. Additionally, adjusting of the polarity of the top cell back electrode and bottom cell top electrode is not required. On the other hand, in the splitting method, the top and bottom cells can be evaluated on their original and already developed single junction structure, without any adaption such as application of transparent back electrode for the top cell used in stacked structures. The optical splitting method is realized by the use of an optical filter installed in an angle of 45 degrees between the light source and bottom cell. The optical filter used in this work was designed to reflect the light with wavelength shorter than 620 nm, and to transmit the longer wavelength light. The perovskite cell is set perpendicularly to the bottom cell to receive the light reflected by the optical filter.

The efficiencies of the perovskite and CIGS single solar cells used in this work under 1 sun condition were 13.3% (J_SC 22.0 mA/cm², V_OC 1.03 V, FF 58.4%) and 18.8% (J_SC 38.7 mA/cm², V_OC 0.651 V, FF 74.4%), respectively. The cell area and bandgap estimated from the absorption edge of the perovskite solar cell were 9 mm² and 1.5 eV, respectively. Similarly, the area and estimated bandgap of the CIGS solar cell were 15 mm² and 1.1 eV, respectively.

The efficiencies of the perovskite and CIGS solar cells after splitting were 9.0% (J_SC 12.2 mA/cm², V_OC 1.03 V, FF 71.5%) and 12.1% (J_SC 25.7 mA/cm², V_OC 0.637 V, FF 73.9%), respectively, resulting in a total efficiency of 21.1%. Interestingly, FF of the perovskite top cell improved after splitting, while V_OC remains the same. This could be related to the lower injection condition under splitting. On the other hand, V_OC of the CIGS bottom cell after splitting decreased according to the decrease in J_SC, which in turn is close to the value estimated from EQE. Considering the bandgap of the solar cells used in this work, an optical filter with a cutting wavelength of 770 nm should result in a higher total efficiency. Further efficiency improvement is expected by using an optical filter with lower absorption loss and by the application of a top solar cell with a bandgap of 1.7–1.8 eV.

Donor-acceptor (D-A) copolymer has been proved to be excellent candidates for efficient polymer solar cells (PSCs). An easy and powerful strategy of D-A copolymer design to enhance performance of PSCs would advance their industrialization. Here we demonstrate effective the molecular design way using the simple and easy molecular mechanic function of Molecular Mechanic2 (MM2) & Merck Molecular Force Field94 (MMFF94) calculation and non-covalent conformational locking effects in D-A copolymer backbone. It is shown to play an important role in D-A copolymer with highly ordered structure through intra- and/or intermolecular interactions with hydrogen bonding. We report a newly designed D-A copolymer as a donor using our strategy, poly(benzodithiophene-diphenylphenazine), P(BDP-DTPz) which exhibits good processability, highly molecular ordering and excellent charge carrier mobility balance. A maximum power conversion efficiency (PCE) of 6.2 % is achieved with P(BDP-DTPz):PC₇₁BM blend under 1.5G solar irradiation. This work will provide a new perspective and guidline for molecular design of D-A copolymers in PSCs.

Three alcohol-soluble conjugated polyelectrolytes (CPEs) based on thiophene derivatives were designed and synthesized. The CPEs were successfully incorporated into polymer solar cells (PSCs) as pH-neutral hole-transporting layers (HTLs) via solution process at room temperature. The dipole moment and work function (WF) of CPE-coated indium tin oxide (ITO) were controlled by the dipole moment of the polymer, as determined by ultraviolet photoelectron spectroscopy. By coating ITO with CPEs having different π-linkers with stronger dipole moments, the secondary cut-off of the CPE-covered ITO electrodes in the range 16.70–16.60 eV successively shifted to lower binding energies due to an increase in the electrostatic repulsion in the molecules. The dipole moment of the polymer was found to be determined by the stereochemical properties of the molecular structure. The strength and direction of the dipole moment of the monomers and CPEs were calculated using density functional theory. The results confirmed that the strength and direction of the dipole moment changed according to the number of heteroatoms and configuration in the polymer backbone. P1 with a single thiophene unit had a deeper WF than that of polymers with atactic thiophene monomers. Bulk heterojunction (BHJ) solar cells fabricated with P1 as the HTL showed the strongest generated ion-induced dipoles(μ_ID), and the short-circuit current (J_SC) and hole mobility of the devices increased. The device fabricated with P1 exhibited the best power conversion efficiency (PCE) up to 7.3%, with a J_SC of 15.6 mA·cm^-2, open circuit voltage (V_OC) of 0.697 V, and fill factor (FF) of 67.4%. The initial PCE of this device was maintained, accompanied by a 34% improvement in the stability compared with that of the PEDOT:PSS-based device. These results indicate the importance of judicious selection of a π-conjugated monomer with optimal electrical and structural properties to obtain effective CPEs for PSCs. Moreover, drawbacks of the instability of PEDOT:PSS may potentially be overcome by P1 based on its pH-neutrality and maximized dipole moment.

Perovskite solar cells composed of organic-metal-halide materials have made remarkably progress in just a few years with power conversion efficiencies from 3.8%¹ in 2009 to a certified 22.1%² in 2016. Following from the traditional structure of dye-sensitized solar cells, n-type semiconductor anatase TiO₂ mesoporous layer was commonly used for delivering high power conversion efficiency for perovskite solar cells. However, disadvantages of the TiO₂ mesoporous structure is that it requires a high-temperature (>450ºC) sintering process to improve the interconnection among the nanoparticles³ and burn out the organic binders in paste. Furthermore, the position of conduction band edge between anatase TiO₂ to organo-lead-halide perovskite is ~0.1 eV⁴ which is not satisfyingly enough for electron transfer. Inorganicmetal oxides such as SnO₂ is generally used as n-type semiconductor for electronic devaises such as display, which have a deeper conduction band edge than anatase TiO₂ and higher mobility of up to 240 cm² /(V · s). Thus, it could perform more efficient electron transfer from perovskite to electron transport layers, and elimination of hysteresis on the J–V curve due to the enhanced electron extraction by reduction of the energy barrier at the perovskite/electron transportation layer interface.⁵ In this studies, we report perovskite solar cells structured with SnO₂ compact layer made by spray pyrolysis and nano particles mesoporous layer as electron selective layers and its less hysteresis J–V curve. A superior SnO₂ compact layer was prepared from tin tetrachloride (SnCl₄) solution on the several annealing temperature, and then following with spin-coating SnO₂ nanoparticles solution onto compact layer to fabricate the mesoporous SnO₂ layer in ambient air condition; furthermore, it was annealed under 180ºC for 30 min. By combining with a triple cation perovskite layer of formamidinium, methylammonium and cesium and optimized SnO₂ layers, the resulting best performance of our devices were reached 1.16 V as open circuit voltge, 21.8 mA cm^-2 as short circuit current, 72% as fill factor, and 18.1% as power conversion efficiency.

Reference

1. A. Kojima, K. Teshima, Y. Shirai, et al., J. Am. Chem. Soc., 2009, 131, 6050–6051.

2. NREL: National Center For Photovoltaics Home Page. http:// www.nrel.gov/ncpv, Accessed July 30, 2016.

3. M. M. Lee, J. Teuscher, T. Miyasaka, et al., Science, 2012, 338, 643–647.

4. J. P. C. Baena, L. Steier, W. Tress, et al., Energy Environ. Sci., 2015, 8, 2928–2934.

5. Q. Jiang, L. Zhang, H. Wang, et al., Nature Energy, 2016, 2, 16177.

This paper reports a synergistic strategy to enhance the power conversion efficiency (PCE) of flexible planar perovskite solar cells (PSCs) by controlling the thickness of the ZnO electron transport layer (ETL), PbI2 phase, and size/morphology of the perovskite (MAPbI3) absorber layer. To optimize the size/morphology of MAPbI3 via a modifed two-step spin coating process, various volumes of CH3NH3I precursor solutions with a constant concentration were continuously coated, which greatly affected the grain growth condition of the MAPbI3. In addition, the remnant PbI2 phase in the MAPbI3, which acted as a recombination barrier, was simultaneously optimized for higher efficencies. This strategic method to synergistically combine the major factors affecting the final PCE resulted in the best efficiency of 12.3%, which is the highest efficiency among ZnO-ETL-based flexible planar PSCs to date. The flexible PSCs reported in this study will impact the development of highly effcient flexible wearable energy sources with imperoved bending durable performance.

Metal halide perovskite thin films can be crystallized via a broad range of solution based routes, however, the quality of the final films is strongly dependent upon small changes in solution composition or processing parameters. It would therefore be useful to better understand the interplay between starting solution and final perovskite films quality.With the intention to achieve both large grains of high crystalline quality, and uniform films with no pinholes, it appears sensible to revisit the formulation of the excess-organic mixed halide deposition route, specifically understanding the interplay of varying the metal-halide source upon nucleation and growth and final film quality. In this work, we present an investigation the thin-film growth from several starting perovskite solution and its influencing factors. We have adopted the different precursor compositions by “fractional substitution” of PbCl₂ with PbI₂ in the mix-halide precursor composition (3CH₃NH₃I: 1-xPbCl₂: xPbI₂) to unravel the deep crystallization mechanism. Here, we have discovered that by substituting only a 2 mol% of PbCl₂ with the PbI₂ in the mix-halide precursor composition (3H₃NH₃I:PbCl₂) we transform the nucleation and growth process in thin film fabrication. The presence of the PbI₂ results in uniformly distributed quadrilateral shaped crystallites of tens of microns in size, just after the casting of the precursor film, as opposed to “precursor crystals” which usually proceed crystallization 1 with this mixed-halide recipe.  These crystals grow and impinge to form pinhole free films with highly-crystalline domains. By incorporating these films into regular planar heterojunction perovskite solar cells fabricated in humidity controlled air (humidity: ~17%), we achieved 19.1% current-density voltage (JV) measurer power conversion efficiency and 17.2% stabilized power output (SPO). Importantly,  we reached over 20% JV power conversion efficiency with the CH(NH₂)₂-based composition in humidity controlled air using with the understanding of crystallization process into the organic-inorganic halide perovskite materials.

 FA_1-xCs_xPbI₃ with a mixed cation of Formamidinium (FA) and Cs system resulted in phase stabilization and showed better stability for moisture and performance compared to other systems such as CH₃NH₃PbI₃, FAPbI₃.Two-step sequential processcan fabricate well covered and uniform perovskite layer compared to the one-step solution process without anti-solvent washing. However, ionic radius of the FA⁺(253 pm) is larger than of CH₃NH₃⁺(217 pm), and perovskite formation rate of PbI₂ film and FAI solution is slower than of PbI₂ and CH₃NH₃I. This finding suggests that insufficient conversion of PbI₂ or uncontrolled surface roughness with long reaction times are likely to occur in the case of a planar structure.However, the simple planar structure is more desirable for low cost and high throughput manufacturing processes.

 We have investigated the fabrication process of organic-inorganic perovskite FA_1-xCsxPbI₃ thin films on compact TiO₂ layers by the reaction of porous PbI₂·CsI_x layers and high concentration FAI solution. The fabricated FA_1-xCs_xPbI₃thin films were characterized by XRD, UV-Vis absorption, PL,FESEM, and AFM. When x was increased, the δ phase and PbI₂ disappeared with a significant improvement in crystallinity, and the absorption band edge shifted to shorter wavelengths with x=0 to 0.2 and when x=0.3, CsPbI₃ was formed.The fabricated perovskite thin films have dense and pore-free surfaces with minimal surface roughness and the fabricated FA_1-xCs_xPbI₃ thin films were applied to the planar structure solar cells(Glass/FTO/TiO₂/Perovskite/spiro-OMeTAD/Ag). when x = 0.2 among x = 0 to 0.3, a PCE was highest at 14.7% with J_SC of 21.4 mA cm^-2, V_OC of 1.02 V and FF of 67.5 % in reverse scan.

Metal-free indoline dye (D149) is a promising organic molecular sensitizer for dye-sensitized solar cell (DSSC) due to its large-molar extinction coefficient and efficient electron injection properties on metal oxide semiconductor surface. However, charge recombination due to defect presence interface and/or self-aggregation among dye molecules is significantly suppress photovoltaic performance. Methylammonium lead bromide pervoskite (MAPbBr₃) has been widely employed as light absorber owing to appropiated band gap (1.9-2.3 eV) and high Voc. Herein, we investigate co-sensitization between D149 and MAPbBr₃. The perovkite solution in DMSO was deposited on D149 embedded mesoporous titania (as thickness ca. 400 nm) by spin-coating and employing spiro-OMeTAD as hole-transporting material (HTM). Bimolecular substrate shows intense broad absorption from 400 to 650 nm in which absorption of individual take place. After doping with a few content of MAPbBr₃ , the synergistic effect can be obviously observed by showing high yield of IPCE spectrum covering from 300-600 nm and improving photo-conversion efficiency (increasing as 20% compared with pure dye) as well. Bimolecular system also demonstrates improving J_sc, V_oc and FF. However, according to our knowledge, synergistic effect between bimolecules is supported by dopant infiltration and morphology of devices as well. Small concentration (approximately 0.2 M) of MAPbBr₃ fully penetrates into TiO₂ mesoporous structure without large voids or capping layer.

Perovskite solar cells using methyl ammonium lead triiodide (CH₃NH₃PbI₃) perovskite as a photoabsorber have attracted much attention as promising next-generation highly efficient solar cells. The performance of perovskite solar cells is known to highly depend on the quality of the perovskite layer. Among several methods for fabrication of perovskite layer, we focused on a one-step solution method, in which perovskite film is formed by spin-coating a mixed solution of CH₃NH₃I and PbI₂ in DMSO or DMF with toluene treatment, followed by annealing. In this process, the transparent film is formed after spin-coating, which immediately colorizes to black corresponding to perovskite upon annealing at ~100 ℃. We focused on transparent intermediate and succeeded in isolation of these crystals. X-ray structural analysis revealed that the intermediate crystals prepared from DMSO or DMF solution correspond to MA₂Pb₃I₈∙2DMSO or MA₂Pb₃I₈∙2DMF as composition formula, respectively, both of which contain intercalated solvent molecules. Thermogravimetric analysis on these crystals indicates that the intercalated solvent molecules come out from the intermediate at ~78 ℃ to become black solids. Based on these characteristics of the intermediates, we optimized our device fabrication method to achieve high power conversion efficiency (~20.3%) with high reproducibility, even in the cells using a pure CH₃NH₃PbI₃ perovskite photoabsorber. In this presentation, the details of the properties of the intermediates as well as our fabrication process will be introduced. 

 Organic-inorganic lead halide perovskite compounds have attracted much attention as a material for solar cells due to their strong light absorption coefficient, high carrier mobility, low binding energy of excitons, and tunable band gap. Additionally, the perovskite compounds can be synthesized using inexpensive sources, suggesting that high performance and inexpensive solar cells are achievable. However, perovskite solar cells, which are usually fabricated by a solution process, have poor stability against moisture because the perovskite compounds are ionic crystals, and sensitive to moisture incorporation from the solvent. As a consequence solution processes face big challenges to become a suitable process for the fabrication of perovskite solar cells on large areas in mass production scale. There have been a few reports on the fabrication of perovskite solar cells using dry processes such as vapor phase deposition or pulsed laser deposition. In this work, we propose fabrication of perovskite solar cells by sputtering, which is more suitable for large scale fabrication. We present results on the fabrication of perovskite films by sputtering and their characterization.

  PbI₂ target was fabricated from PbI₂ powder using a pressing machine under 10 MPa. Then, PbI₂ films were fabricated on glass substrates by RF sputtering using the PbI₂ target. To obtain perovskite films, sputtered PbI₂ films were annealed at 100^oC in CH₃NH₃I gas filled petri dish. XRD measurement and SEM observation were carried out to characterize the fabricated perovskite films.

 From the XRD measurement, clear diffraction patterns of CH₃NH₃PbI₃ perovskite were confirmed in the films. SEM observations show that the substrates are entirely covered with perovskite grains. We applied these films for perovskite solar cells and successfully obtained photovoltaics performance from sputter processed perovskite films.

Upon the formation of the surface complexes of organic molecules (TCNQ, 2-anthroic acid) with TiO₂ nanoparticles, new visible light absorption bands appear due to interfacial charge-transfer (ICT) transitions from these organic molecule's adsorbates to the conduction band of TiO₂ directly. Therefore, ICT transitions are expected to be a novel charge-separation mechanism for efficient photovoltaic conversion featuring one-step charge separation with energy loss ^[1]. In this study, the complexes of each organic molecules TCNQ and 2-anthroic acid with Nb doped TiO₂ electrode were fabricated such as dye-sensitized solar cells (DSSCs) and investigated the improvement of charge-transfer process of them. The Nb doped TiO₂ nanoparticles were prepared by hydrothermal method at 180 ºC for 24 h ^[2]. According to dope Nb into TiO₂ lattice, its level of the conduction band could be closer to these organic molecule's adsorbates than its of Un doped TiO₂ ^[3]. That is, the injection of ICT transitions could be shortly. This presentation will mainly discuss the surface of the Nb doped TiO₂ nanoparticles chemically absorbed with these organic molecules by using incident photon-to-current conversion efficiencies (IPCE) and I-V measurement. Furthermore, the present description will also focus on the effect of carrier recombination due to ICT transitions with the Nb doped TiO₂

Reference;

[1] J.Fujisawa phys.Chem.Chem.Phys.,2015,17,12228-12237

[2] T.Nikolay,L.Larina,O.Shevaleevskiy,B.T.Ahn Energy Environ. Sci.,2011,4,1480-1486

[3] J.Liu, Y.Duan, X.Zhou, Y.Lin Appl.Surf.Sci.,2013,277,231-236

Photovoltaic cells based on hybrid perovskite absorbers have rapidly emerged as very promising for commercial applications since they have recently exceeded 20% solar power conversion efficiencies using low-cost solution-based processes.¹ Mixed cation and halide hybrid perovskites possess superior optoelectronic properties, such as large absorption coefficient and long charge carrier diffusion length.^2,3 Optimization of device structure and processing have led to a rapid increase in the device performance over the last 6 years.^{4, 5} Despite the remarkable achievements on pushing the efficiencies to new heights, huge gaps in knowledge of the absorption and emission semiconducting properties which may involve electron transitions from one energy state to another or other materials, a fundamental material property for energy conversion of photovoltaic devices, still have not been filled. Charge carrier dynamics are also a critical issue in not only perovskite materials but also for the fundamental understanding of solar cell devices in order to optimize device performance and approach the Shockley–Queisser efficiency limit of about 31% for 1.55 eV bandgap perovskite.^{6, 7}

In this study, we investigated the excitation density dependence of the photoluminescence (PL) properties of methylammonium lead iodide (MAPI₃)  perovskite films and the PL quenching efficiency (PLQE) of perovskite in junction with PEDOT:PSS and PC₆₁BM, hole and electron acceptors respectively. Ground state optical spectroscopy in combination with femtosecond Transient Absorption Spectroscopy (fs-TAS) and time-resolved PL (tr-PL) allow us to unravel the excited state dynamics in these perovskite films and their interfaces with charge extracting layers. Our results demonstrate that the charge carrier density-dependant bimolecular recombination kinetics of MAPI₃ compete with charge extraction at light fluxes approaching 1 sun. Ultimately, the excitation densities should be carefully considered when conducting PLQE experiments in order to mimic operating conditions and extract appropriate values of carrier sxtraction.

1.     NREL efficiency chart.

2.     J. H. Im, C. R. Lee, J. W. Lee, S. W. Park and N. G. Park, Nanoscale, 2011, 3, 4088-4093.

3.     G. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Gratzel, S. Mhaisalkar and T. C. Sum, Science, 2013, 342, 344-347.

4.     Y. Rong, Z. Tang, Y. Zhao, X. Zhong, S. Venkatesan, H. Graham, M. Patton, Y. Jing, A. M. Guloy and Y. Yao, Nanoscale, 2015, 7, 10595-10599.

5.     Y. Y. Zhou, M. J. Yang, W. W. Wu, A. L. Vasiliev, K. Zhu and N. P. Padture, Journal of Materials Chemistry A, 2015, 3, 8178-8184.

6.     J. S. Manser and P. V. Kamat, Nature Photonics, 2014, 8, 737-743.

7.     W. E. I. Sha, X. Ren, L. Chen and W. C. H. Choy, Applied Physics Letters, 2015, 106, 221104.

Open circuit voltage (V_oc) of organic solar cells is determined by the difference between the Fermi level of the p-type and the n-type semiconductor. This Fermi level difference is varied with the relative energy levels between p and n type materials and the charge densities, which is related with charge recombination rate. These relative energy and recombination rate are affected by the structure of solar cells. Thus, thermal annealing, film thickness, and semiconductor composition ratio of the active layer film can affect the V_oc. Especially, the semiconductor composition ratio has large influence. Thus, the aim of this study is, to clarify the origin of the effect of composition ratio on V_oc. We used poly(3-hexylthiophene) (P3HT) as a donor and phenyl-C61-butylic acid methyl ester (PCBM) as an acceptor materials. We prepared solar cells with the composition ratio of P3HT: PCBM = 3:1, 2:1, 1:3. Charge recombination and, charge density were measured by a modified charge extraction method using a fast switch and reverse bias under extraction. As a result, the V_oc of 1:3 cells was the highest under 1 sun conditions. When V_oc was compared among the cells with different ratios at the same charge density, the cells with 1:3 had the highest V_oc. This suggests the difference of the energy levels between P3HT and PCBM was the highest among the cells.

Perovskite solar cells, in which methyl ammonium lead iodide (CH₃NH₃Pbl₃) is used as the photoactive layer, can be fabricated by simple and inexpensive solution processes.  A lot of attention has been drawn by these solar cells in the recent years because of their high power conversion of efficiencies (PCE), surpassing 20%. Such high efficiency of the perovskite solar cells can be ascribed to favorable semiconductor properties of CH₃NH₃Pbl₃ that are materialized through uniform and dense films.  Although its application in solar cell has gained major attraction, the material also finds other interesting applications such as photo detectors, light emitting diodes, X-ray detectors, memory resistors, etc. In this study, we examined methods for fabricating patterned perovskite layer with a thickness of 100 μm or higher.First, we synthesized single crystal of CH₃NH₃PbI₃ by an inverse temperature recrystallization method. Then, the crystals were grinded to powder, which was dissolved in the γ-butyrolactone (GBL). The solution was then dripped and dried on the FTO glass and the process was repeated to obtain thick perovskite films.The 100 um thick perovskite layer thus prepared showed uniquephotoconductive properties due to the bulk large crystal of perovskite.

In the dye-sensitized solar cell, when light is irradiated on the dyes, electrons are excited and injected into the semiconductor electrode and the electrons diffuse to transparent conducting substrate.During the transport in the semiconductor, the electrons may recombine with oxidized dye or oxidized species in electrolyte solution. Regarding the recombination between the electrons in the semiconductor and the oxidized dyes, many papers claim that if the dyes lie on the semiconductor electrode, the recombination is likely to occur due to short distance between the dyes and the electrode. However, for the case of Zn phthalocyanine dye, we reported that the recombination was less likely to occur when the adsorption angle was small. However only the study was performed systematically on the effect of adsorption angle.The purpose of this study is to generalize the relationship between the adsorption angle of the dye and the reduction rate. In this study, the adsorption angle was changed by adding adsorption moiety to ortho or para position of phenyl ring in metal free organic dyes. Transient absorption measurement was used to measure the reduction rate of oxidized dyes. We found similar reduction rates regardless of the adsorption angle at room temperature. We found also temperature dependent reduction kinetics from the standing dyes but not from the lying dye. 

Colloidal quantum dots (CQDs) give unique optical properties such as the size dependent absorption, which makes the quantum dots promising candidate materials toward high efficiency solar cells.  In particular, PbS CQD-based solar cells of depleted heterojunction structures showed high power conversion efficiency (PCE) over 11.3 % in 2016.  However, the short exciton and / or carrier diffusion lengths of PbS QD films limit the PbS QD active layer thickness, thereby limiting light harvesting efficiency.  Thus we focused on solar cells composed of PbS CQD/ZnO nanowire structures to achieve efficient carrier transport and light absorption in the near infrared region.  After some morphology control of the ZnO nanowire arrays, we achieved external quantum efficiency (EQE) values of approximately 60% at the first exciton peak (1020 nm) and over 80% in the visible region (J. Phys. Chem. Lett., 2013, 4, 2455).  To clarify the exact role of the ZnO NW in the PbS QD/ZnO NW solar cells, we studied photocurrent properties of the solar cells by focusing on the cell structure, and found out that, ZnO NWs act as a very efficient transporter and collector for the photogenerated electrons in the PbS QD region.  Therefore, once good electron pathways was established by incorporating ZnO NWs, holes in the PbS QD region can diffuse a distance over 1000 nm (J. Phys. Chem. C., 2015, 119, 27265).  Ensuring long-term durability of the solar cells is essential for practical applications.  Long-term stability tests under quasi-sunlight illumination were performed on the PbS QD/ZnO-NW solar cells without encapsulation.  The power conversion efficiency after 3000 hour illumination maintained approximately 90% of the initial value (Phys. Status Solidi RRL, 2014, 8, 961).

Electron transfer kinetics is described by three factors according to Marcus theory. On the other hand, not all of experimentally observed electron transfer kinetics can be explained only with Marcus theory. In this work, we examined the effect of dye structure on the regeneration kinetics of oxidized dye by reduced species in the electrolyte in dye-sensitized solar cells (DSSCs). The regeneration kinetics was studied for DSSCs using Co complex redox couples and organic dyes which have donor-π-conjugated linker-acceptor (D-π-A) structure. Between two dyes which have diphenylamine added carbazole moiety as the donor part and have one (MKY1) or three (MK118) hexylthiophene, MKY1 showed faster regeneration rate than MK118 when the free energy difference (ΔG) between the levels of the dye's HOMO and redox couple in the electrolyte was comparable in the DSSCs. For the case of two dyes which have three hexylthiophenes in the π-conjugated linker part and have diphenylamine added carbazole (MK118) or carbazole (MK1) moiety, the regeneration kinetics was comparable between MK118 and MK1 under the configuration of solar cells giving similar ΔG. These results indicates that the diphenylamine of MK118 didn't affect the regeneration kinetics while the diphenylamine of MKY1 facilitated the regeneration kinetics. Under the conditions giving similar ΔG, smaller-number of hexylthiophene resulted in faster regeneration kinetics.

In this work, we discussed about the interface between perovskite and electron transport layer (ETL) to improve the performance of the perovskite solar cells (PSCs) with reducing the energy loss at the material interfaces in the PSCs.              The TiCl₄ treatment for the compact TiO₂ layer to increase the performance of PSC has been reported. The effect of the TiCl₄ treatment was considered as the cause of the wettability increases of the precursor solution of the perovskite on TiO₂ compact layer and the efficient charge transport from perovskite layer to TiO₂ layer. However, TiCl₄ treatment may change the conduction band edge (CBE) potentials. The adjustment of the CBE potentials will be important issue to obtain the low barrier potentials for electron transfer from perovskite to TiO₂ compact layers. To study the effect of the TiCl₄ treatment on the CBE potentials, we prepared the electrochemical cells with the iodide and tri-iodide redox electrolyte and treated substrate with TiCl₄ aqueous solution and the relation between open circuit voltages (Voc) and the electron density in TiO₂ layer of the cells were measured with the charge extraction methods. The Voc as the quasi CBE potential was negatively shifted with the TiCl₄ treatment but positively shifted with heating after TiCl₄ treatment. We compared the PSCs performance as the different heating temperature at 150 °C, 300 °C, and 450 °C of the TiCl₄ treated compact TiO₂ layers. The Voc was decreased and the short circuit current was increased with increasing the temperature. The highest PCE was obtained with the heating at 300 °C.

Despite remarkable rise in efficiencies of hybrid organic-inorganic perovskite solar cells (PSCs), instability issues from intrinsic defects in perovskite layers have been main obstacle to the next generation power sources. Since this instability imposes to use expensive noble metal electrodes such as gold and platinum, which leads a dilemma between low-cost processability and long-term stability of the PSCs, low cost PSCs with long term stability would have major impact on the needs of our society. In this work, we clearly reveal that a critical self-degradation process of the PSCs arises from intrinsic ionic defects of the perovskite layer and successfully solve the problem with a new regime for long-term stable PSCs, which is versatile and universal method of using a “solution-processable chemical inhibition layer (CIL)” by neutralizing the ionic defects and inhibiting the corrosion of common metal electrodes. As a consequence, our long-term stable PSCs maintains almost ~ 80% of their initial PCEs even after the 9,000-storage hours under nitrogen condition, and ~ 80% after 200 hours even in ambient condition without any encapsulation. More importantly, we verified the universality of CIL whose amine function groups and metal oxide network medium act as “a corrosion inhibitor and a charge transport layer”, respectively. Our CIL, a solution-processable amine-mediated titanium suboxide (A-TiOx), successfully stabilizes iodide ions and protects the common metal electrodes of Al and Ag as an “active” passivation layer in the PSCs.

In optical photovoltaics, properties of donor-acceptor interface play an important role in its functionality. Regarding this point, cychlopentadithiophene-benzothiadiazole copolymer (PCPDTBT) has interesting experimental characteristics: when it has 2-ethylhexyl (EH) side chain, “face-on” configuration is dominant, while “edge-on” configurations are prevailing for n-dodecyl (C12) and n-hexadecanyl (C16) side chains. Saeki and co-workers in Osaka Univ. measured charge separation and recombination efficiencies on PCBM/PCPDTBT(EH, C12, C16) interfaces using time-resolved microwave conductivity method, and found that charge separation and recombination are faster in PCPDTBT-EH, compared to those in C12 and C16 cases. In order to understand this experimental result, we performed molecular dynamics simulation on PCBM/PCPDTBT interface as well as quantum chemistry calculation to obtain charge separation and recombination rates between PCBM and PCPDTBT monomers in various relative configurations. Our calculations suggest that donor-acceptor distances at “face-on” configuration are generally smaller than those at “edge-on” configuration, and this difference results in difference of charge separation and recombination rates.

 Perovskite solar cells (PSCs) have attracted much attention as cost-effective next generation photovoltaics. In order to further improve the device performance, it should be crucial to develop an excellent hole-transporting material (HTM) that can effectively collect holes generated in perovskite layer. We have reported a novel HTM, HND-Azulene, using four oxygen-bridged triarylamines around an azulene core.¹ This HTM having n-octyl groups was found to be superior in comparison with the currently used standard material, Spiro-OMeTAD, in PSCs. For further improvement of the device performance, we newly synthesized the derivatives of this system with a variety of alkoxy groups.

 The hole mobilities of these derivatives were measured by the space-charge-limitation of current (SCLC) method. Our PSCs are composed of the following structure: FTO / compact-TiO₂ / mesoporous-TiO₂ / perovskite CH₃NH₃PbI₃ / HTM / Au. The perovskite layers were prepared according to one-step solution method with solvent engineering.

 The SCLC measurements on these HND-Azulene derivatives revealed that their hole mobility increases as the length of alkoxy chains shortened (-OR = octyloxy < hexyloxy < butoxy < methoxy). In order to investigate the effects of alkoxy chain length on the performance in PCSs, the devices using methoxy-azulene and octyl-azulene as HTMs were fabricated. The methoxy-azulene (17.0%) showed higher PCE value in comparison with octyl-azulene (15.1%).

 In this presentation, the effects of the chain length of alkoxy groups on the performance of PSCs as well as on the solid state properties will be discussed in detail.

[1] H. Nishimura, N. Ishida, A. Shimazaki, A. Wakamiya, A. Saeki, L. T. Scott, and Y. Murata, J. Am. Chem. Soc. 2015, 137, 15656.

Organic−inorganic perovskite is a promising class of materials for photovoltaic applications and light emitting diodes. However, so far commercialization is still impeded by several drawbacks. Atomic-scale effects have been suggested to be possible causes and thus an experimental view at the atomic level is desirable. Here, we present a low-temperature scanning tunneling microscopy (STM) study of single crystal perovskite as well as ultra-thin films of methylammonium lead bromide CH₃NH₃PbBr₃ on Au(111). Topographic images of the in situ cleaved perovskite surface reveal the real-space atomic structure. Careful comparison between the experimental STM images and density functional theory (DFT) calculations suggest that the surfaces are the reconstructed (010) CH₃NH₃Br terminated surfaces. We were also able to visualize the defects in real space at the atomic level. [1] 

Reference: [1] R. Ohmann, L. K. Ono, H.-S. Kim, H. Lin, M. V. Lee, Y. Li*, N.-G. Park*, and Y. B. Qi*, J. Am. Chem. Soc. 2015, 137, 16049

     Inverse temperature crystallization (ITC) of formamidinium lead iodide perovskite (HC(NH₂)₂PbI₃) using γ-butyrolactone (GBL) as solvents have been of interest because of its simplicity to obtain single and bulk crystals. However, the detailed crystal growth mechanism is still unclear. Therefore, we investigated the relationship between crystal growth temperature and side-chain length of lactones.

     Lactones with alkyl chains, such as methylbutyrolactone and γ-hexanolactone, have a similar chemical structure to GBL, and are suitable for ITC because of their high boiling point of > 200 ºC and high solubility of HC(NH₂)₂I and PbI₂ which are the precursors of HC(NH₂)₂PbI₃. The HC(NH₂)₂PbI₃ crystals in lactones with longer alkyl chain can be formed in shorter period of less than 30 min, higher HC(NH2)₂PbI₃ yield of more than 50 % and lower temperature of less than 60 ºC, compared with those in GBL (2 hours, 15 % and 110 ºC, respectively).

     Raman and UV-vis spectroscopy of a solution of the precursors in lactones reveal a weak solvent-solute-complex strength and a formation of iodoplumbate complexes of PbI₃^- and PbI₄^2-, while there is a strong interaction to the precursors and no signals of iodoplumbate complexes in dimethyl sulfoxide or N,N-dimethylformamide. These results indicated that the weak complex strength in the solvent with alkyl substitution lead to small activation energy and rapid formation of the perovskite structure. These lactones would be promising solvents for low-temperature fabrication of perovskite solar cells.

Metal organic halide perovskites are promising materials for solar cells with a maximum certified efficiency of 22.1%. However, there are only a handful of reports on larger area modules, where efficiencies drop with increased use of active area. Chemical vapor deposition (CVD) is a technology used in many industrial applications demonstrating potential for scale up. We used a CVD process to fabricate solar cells and larger multi cell modules, and investigated scaling issues. In addition, we fabricated modules using an established MAPbI3 solution process and compared them to CVD grown modules. The solution processed cells performed better than CVD cells when comparing PCEs determined from J-V measurements, but the steady state power of solution processed solar cells decreased quickly with increased area. In contrast, FAPbI3 CVD grown solar modules maintained much of their PCEs transitioning from J-V measurements to the steady state operating conditions, suggesting that the FAI based CVD process may outperform MAI based solution processed modules when scaled up to practical sizes. [1] 

[1]       Matthew R. Leyden, Yan Jiang, Yabing Qi*, "Chemical vapor deposition grown formamidinium perovskite solar modules with high steady state power and thermal stability" J. Mater. Chem. A 4, 13125 - 13132 (2016). 

Hydogen production by water splitting is the key technology to solve energy, resources, and environmental issues. The representative artificial photosynthetic reation is water splitting. A photoelectrochemical system is one of the candidates to achieve an efficient water splitting of an artificial photosynthesis. Although various potential photoelectrodes with visible light response have been developed, most of them require an external electric assistance to split water. Recently, fablication of a combined system with such photoelectrodes and solar cells has been extensively studied to overcome that issue. For example, a perovskite solar cell with low cost and high efficiency has attracted much attention. In the present study, we demonstrate solar water splitting upon fabricating a system with a Pt/SiC photocathode, a CoOx/BiVO₄:Mo photoanode, and a pervskite solare cell. The Pt/SiC and CoO_x/BiVO₄:Mo photoelectrodes gave cathodic and anodic photocurrents under simulated sun light irradiation, respectively. The current-potential curve of the Pt/SiC photocathode intersected that of the CoO_x/BiVO₄ photoanode at around 0.8 V_RHE. Therefore, solar water splitting was tested using the combined photoelectrochemical cell with and without a perovskite solar cell. The cell without a perovskite solar cell gave a certain photocurrent and about 1 V of the photovoltage. The photocurrent was further boosted by using a perovskite solar cell. Thus, we successfully fabricated the artificial photosynthetic system for solar water splitting using photoelectrodes and a perovskite solar cell.

Perovskite solar cells (PSCs) have captivated researchers due to high efficiencies and easy fabrication. Hole transport layer (HTL) is key to attaining higher efficiencies in PSCs. Currently the most widely used HTL is spiro-MeOTAD. The addition of LiTFSI and t-BP dopants to spiro-MeOTAD HTL is necessary to achieve desirable hole transport properties. Interestingly, it is common knowledge that several hours of ambient air exposure are needed to reach highest solar cell performance. Using photoemission spectroscopy and mercury drop electrode I–V measurements on hole-only devices, we investigated the influences of H₂O vapor (RH 90%, RT 25 ºC), dry O₂, and ambient air (RH 50%, RT 25 ºC) on charge transport properties of spiro-MeOTAD films doped with LiTFSI and t-BP. Our results reveal that H₂O vapor exposure resulted in a substantial and irreversible enhancement in conductivity. On the other hand, O₂ exposure resulted in a reversible enhancement under applied bias in the devices. XPS results reveal that freshly prepared spiro-MeOTAD shows a very low concentration of LiTFSI dopants at the top surface and air exposure causes some characteristic elements (e.g., F, S, and Li) belonging to LiTFSI to migrate to the top surface. Furthermore, XPS reveals that humidity is the main force driving the redistribution of the LiTFSI dopants. The migration of LiTFSI dopants across the bulk film seems to be facilitated by two types of pinholes with different sizes, which is confirmed by AFM, SEM, and optical microscopy. In summary, our results reveal the mechanisms that drive the conductivity enhancement of LiTFSI-doped spiro-MeOTAD HTLs, eventually leading to PCEs’ enhancement in perovskite solar cells. [1, 2]

[1]  Hawash, Z.; Ono, L. K.; Qi, Y. B., Moisture and Oxygen Enhance Conductivity of LiTFSI-Doped Spiro-MeOTAD Hole Transport Layer in Perovskite Solar Cells. Advanced Materials Interfaces 2016, 3 (13).

[2]  Hawash, Z.; Ono, L. K.; Raga, S. R.; Lee, M. V.; Qi, Y. B., Air-Exposure Induced Dopant Redistribution and Energy Level Shifts in Spin-Coated Spiro-MeOTAD Films. Chem. Mater. 2015, 27 (2), 562-569.

Colloidal quantum dots (CQDs) are promising absorbers and carrier transporter for photovoltaics. This is because of high degree of freedom for controlling absorption band positions, and because the CQDs are compatible with solution-based technology. The power conversion efficiency of depleted heterojunction solar cells made up of ZnO electron transporter and CQDs has been steadily increasing, and reached over 11%. However, short carrier diffusion length of CQD films limits their film thicknesses, namely, light harvesting efficiency. Thus we focused on solar cells composed of PbS CQD/ZnO nanowire structures to achieve efficient carrier transport and light absorption. We then developed air-stable and high efficiency solar cells yielding 60% EQE at the first exciton peak (1020 nm) (H. Wang et al., J. Phys. Chem. Lett.(2013)). Here we construct PbS QD/ZnO NW solar cells with different PbS QDs giving the exciton absorption peaks between 700 nm and 1500 nm. An EQE of 26% is obtained in the solar cell made up of the CQD giving a 1500-nm exciton peak and an EQE onset of 1700 nm (0.73 eV~ Eg). Although Voc values of the solar cell decrease as the exciton peak shifts to longer wavelengths, the Voc of the solar cell giving a 1500-nm exciton peak (0.73 eV ~ Eg) remains approximately 0.4 V. It is interesting to compare the Voc with those of inorganic solar cells such as Si solar cells. The Voc of inorganic solar cells is known to empirically scale as Eg/q-0.4 (V), where Eg is the optical band gap (eV), and q the elementary charge. On the other hand, the colloidal PbS QD/ZnO NW solar cells yield the Voc higher by about 20% than 0.33 V estimated from the equation (0.73 (eV)/q-0.4 (V)). A series of experiments carried out on our solar cells indicates that the colloidal PbS QD/ZnO NW structure is useful for developing the bottom cells of multi-junction solar cells.

We studied the effects of post-synthesis thermal treatments on material properties of CH3NH3PbI3 perovskite films and their photovoltaic performance focusing on fundamental material properties. Perovskite thin films with different annealing duration showed different grain morphology and film quality confirmed by scanning electron microscope (SEM) and XRD. Kelvin probe force microscopy revealed the existence of a positive potential barrier at grain boundaries, which is known to be beneficial by suppressing carrier recombination. The height of the barrier increased with the annealing duration, which directly correlated with the device performance. The origin of the potential barrier appeared to be chemical inhomogeneity as revealed by Nano-Auger electron spectroscopy. Qualitatively similar temperature-dependence of current-density vs bias characteristics were observed regardless of the annealing duration, where efficiency collapsed at low temperatures due to a diverging series resistance. The freeze-out of free carries and/or a Schottky-type barrier(s) at the interface(s) is likely responsible for the diverging series resistance. 

The organic-inorganic hybrid perovskite has been receiving considerable attention in photovoltaic and general optoelectronic applications due to its superior characteristics including high absorption coefficient, direct band gap, long carrier lifetime, and high hole and electron mobility. Methyl ammonium lead iodide is the most common structure ever explored until now, however, all MAPbI₃–based solar cells have been based on polycrystalline MAPbI₃ thin film. As there are plenty of grains, grain boundaries, voids, and surface defects within the film, it suffers from noticeable carrier recombination and instability. It’s reported that single-crystalline MAPbI₃ possesses superior photovoltaic properties and stability due to less grains, grain boundaries, voids, and surface defects. Here we present a rapid crystal growth process to synthesize MAPbI₃ single crystals by means of inverse temperature crystallization. Unlike general crystallization process, MAPbI₃ precursor exists retrograde solubility in γ-Butyrolactone(GBL). We will explain this anomalistic phenomenon by the crystallization mechanism of MAPbI₃ in GBL.

Perovskite solar cells using CH₃NH₃PbI₃ or CH(NH₂)₂PbI₃ as photovoltaic materials have been studied extensively because of the high efficiency and low production cost. In order to obtain environmentally friendly Pb-free solar cells, Sn(II) perovskite devices are expected. Perovskite Sn(II) halides, such as CH₃NH₃SnI₃, CsSnI₃ and CH₃NH₃SnBr₃, however, are difficult to get as pure states, due to their instability in air. Since small amount of Sn(IV) impurities are incorporated into the perovskite lattice, the semiconducting property of the perovskite will be degenerated. Recently, semiconducting CsSnI₃ perovskite has been reported using SnF₂ as a reducing agent. [1] In this report, the following reaction was monitored by XRD and ¹¹⁹Sn NMR spectroscopy in order to evaluate reducing effect of SnF₂ :

                          (CH₃NH₃)₂SnBr₆ + 2SnF₂ → 2CH₃NH₃SnBr₃ + SnF₄        

Just after stoichiometric mixing of the starting materials, the color changed from pale yellow of (CH₃NH₃)₂SnBr₆ to orange, suggesting an instantaneous formation of the perovskite at the surface.  After the heating the mixture at 430 K, the formation of perovskite CH₃NH₃SnBr₃ was confirmed from the ¹¹⁹Sn NMR and XRD. [2] Furthermore, the effect of SnF₂ addition on the perovskite was discussed. The effect of SnF₂ addition could not be observed on the ¹¹⁹Sn NMR spectra and XRD patterns. However, the ¹¹⁹Sn NMR spin-lattice relaxation times became longer, since Sn(IV) impurities were removed from the perovskite.   

[1] M. H. Kumar, S. Dharani, W. L. Leong, P. P. Boix, R. R. Prabhakar, T. Baikie, C. Shi, H. Ding, R. Ramesh, M. Asta, M. Graetzel, S.G. Mhaisalkar, N. Mathews, Adv. Mater.,2014, 26, 7122-7127.

[2] K. Yamada, K. Nakada, Y. Takeuchi, K. Nawa and Y. Yamane, Bull. Chem. Soc. Jap., 84 (9), 926-932 (2011).

  Formamidinium lead iodide perovskite (FAPbI₃; FA⁺ = HC(NH₂)₂⁺) single crystal with a bandgap of ~1.4 eV has attracted much attention as a new class of absorption layers of solar cell. We investigated various electrical and optical properties of FAPbI₃ single crystal, which was made by inverse temperature crystallization method.

  Dark current – voltage measurement was carried out, and hole mobility and hole trap density were estimated by the space-charge-limited current technique. Temperature dependence of resistance with and without light irradiation indicated the existence of two activation energies.

  Absorption coefficient of a slice of single crystal was calculated from the transmittance and reflectance data, and optical bandgap of the inside of a crystal was estimated. On the other hand, the transmittance measurement of the powder milled from a single crystal showed a different optical bandgap. It is coincided with the results which a photo-luminescence spectrum of a single crystal showed two peaks. The origins of bandgap and activation energy will be discussed.

The perovskite solar cell (PSC) with a compact layer (CL) and a mesoporous scaffold is considered most promising in terms of efficiency, reproducibility and hysteresis control. Typically, CL and scaffold are made by two separated steps, which increases the complexity of the fabrication procedure. In this study, we developed a unique method to deposit a TiO₂ CL and TiO₂ scaffold on FTO glass for PSCs by applying a two-stepelectrodeposition in one single bath. The bottom-up growth characteristic of electrodeposition offers excellent controllability of the thickness and morphology of TiO₂ films by simply altering the depositing profile. As a result, we found that a satisfactory TiO₂ CL and scaffold can be obtained using a profile of -0.6V initially and -0.5V sequentially. From cyclic voltammetry, it was showed that depositing time at 250 seconds in -0.6V and then 120 seconds in -0.5V resulted in an optimal CL and scaffold in sequence. At present, the highest power conversion efficiency of PSC using this method achieved 14.6%.

We developed a novel method to fabricate CH₃NH₃PbI₃ perovskite by simply exposing lead iodide films to methylamine (MA) and hydroiodic acid (HI) gases. The process can be easily up-scaled, and can be carried out in a few minutes, at temperatures below 80°C, at 50% relative humidity. This method simplifies the process by avoiding the step of synthetizing the methylammonium iodide precursor. Solar cell efficiencies up to 15% were achieved and good stability was observed for devices stored for 130 days.¹

In addition, the perovskite films resulting from the intermediate steps of this fabrication process contain a large number of impurities in the form of lead oxide/hydroxides or PbI₂. Defects can form in the vicinity of such impurities, affecting the photovoltaic performance. We analyzed the impedance spectroscopy response of several devices fabricated with the films containing different amounts of impurities. The aim was to correlate the different resistances such as charge recombination with the amount of impurities in the film and also high quality perovskite and lead iodide only. We found minor variations in photovoltage (V_oc), independently of the content of impurities. It was also found that the recombination feature does not correlate with the low frequency arc of the Nyquist, instead, it is more likely correlated with the high-frequency arc, when the scan is measured at voltages near V_oc.²

¹ Sonia R. Raga, Luis K. Ono, Yabing Qi*, J. Mater. Chem. A, 2016, 4, 2494

² Sonia R. Raga, Yabing Qi*, J. Phys. Chem. C, 2016, 120, 28519

Lead halide based organic-inorganic perovskite semiconductors (CH3NH3PbX3, X = Br, I, Cl) have gained great attention as potential light harvesting material in thin-film photovoltaics due to its promising light-absorption characteristics, charge-transport dynamics and simple solution processibility. Recently, using lead based perovskite, worldwide research efforts have led to an incredible enhancement in power conversion efficiencies up to 22.1%. However, these mentioned devices suffer from the relatively poor stability in air and moisture, and more importantly the toxicity of lead is an obstacle for further commercialization. Lead-free perovskite materials such as Sn or Ge based solar cells have been shown to exhibit moderate performances. However, Sn and Ge based devices suffer from serious problems such as toxicity when exposed to environment and in particular stability against moisture.This presentation will discuss the possibility to replace lead by a non-toxic bismuth based light absorbing perovskite material, its physiochemical, optoelectronic properties and suitability for the futuristic solar cells in planar/meso heterojunction architecture. Furthermore, the present talk will also focus on the effect of tuning the morphology of MIB perovskite, by bringing in N-methyl-2-pyrrolidone (NMP) as a morphology controller under different operating conditions, on device parameters and thereby its effect on device performance.

Despite an expeditious rise in its power coversion efficiency organolead halide perovskite based solar cells¹ still stand way behind commercialization because of two major challenges; poor stability^2,3 and high toxicity of Pb⁴. Therefore, urgent and intense efforts are necessary to address these issues and thereby make the new technology a commercial success. Although few studies^5,6 pertaining to stability have shown impressive results of long term stability a large number of studies has directly or indirectly disclosed and siscussed the facts of both material and performance instability in the perovskite solar cells. In the recent past, a great amount of effort has been also made to replace the toxic Pb by less toxic Sn and Bi but none of such perovsite compounds has been found to be so promising in terms of efficiency. Hence, unfortunately, the “lead” leads. Moreover, Sn is also not necessarily less toxic than Pb. In such a case, a wise and otherwise approach would be to find ways that can prevent Pb from getting into environment instead of replacing the metal. In the present work, we show the attempts made by us to reuse/recycle the perovskite films from fresh and degraded devices. Planar perovskite (CH₃NH₃PbI₃) solar cells using spiro-OMeTAD as hole transport layer were aged in dry (<10% relative humidity) and ambient (about 30% humidity) conditions and were also heat-treated at different temperatures (60, 80, 100 and 120^oC) for different durations to deliberately degrade the cell performance. The performance degradation was thorughly investigated by I-V, EIS, XRD and SEM characterizations. Then, the perovskite films from these devices were recovered, examined and reused either after treatment or without any treatement. The results showed that the performance of the cells could be recovered partially or completely depending on the history of treatement of the devices. In addition, it was found that degradation in cell performance was largely caused by the chemical/electrical changes occured in the spiro-OMeTAD and the interface between perovskite and spiro-OMeTAD while the perovskite film remained almost unaffected by storing in dry conditions and heat-treatment up to 100^oC.    

References

(1) Lee, et.al, Science, 2012, 338, 643-647, (2) Han, et. al, J. Mater. Chem. A, 2015, 3, 8139, (3) Smecca, et. al, Phys.Chem.Chem.Phys.,2016, 18, 13413, (4) Babayigit, et.al, Scientific Reports, 2012, 6, Article number: 18721, (5) Mei, et. al, Science, 2014, 345, 6194, 295-298, (6) Burschka, et. al,  Nature, 499, 316–319

Electron extraction layer (EEL) as one of the most important component in organometal–halide perovskites solar cells (PSCs) has attracted considerable research efforts on both materials and its interface with the light-harvesting perovskite. In this study, Ti-Fe-O ternary oxides were incorporated into the planar-structured PSCs based on CH₃NH₃PbI₃ as the EELs compared with conventional TiO_x compact layer and pure FeO_x compact layer. It shows that for the Ti-Fe-O oxides, with the increase of the Fe content from 0%mol to 100%mol, the conductivities and energy levels of the thin films present relative changes. With the incorporation of Fe, the conduction bands get lower compared with pure TiO_x which is proved by X-ray photoelectron spectroscopy (XPS) and UV-Vis spectroscopy. Additionally, the conductivity is significantly improved according to the conductive atomic force microscopy (CAFM) and linear sweep voltammetry (LSV) measurement. As a result, the oxides with higher Fe content give better electron extractions according to the photoluminescence (PL) spectroscopies measured in both steady and transient state. On the contrary, more narrow energy gap due to higher Fe ratio would result in short-wavelength- light loss and lower open circuit voltage (V_OC). Consequently, Ti_0.5Fe_0.5O_x with the optimal Ti-Fe ratio of 1:1 gives the highest power conversion efficiency (PCE) up to 14.7% higher than pure TiO_x and FeO_x with reasonable short circuit current (J_SC), V_OC and highly reduced hysteresis.

In printable and low-cost next-generation solar cells, perovskite solar cell has gathered large attention these years for its high efficiency (> ~22%) in addition to ease of fabrication with low-cost materials.  To achieve the sufficient device reproducibility, uniformity, and reliability in the long-time use, many scientific and practical issues still remain.  Furthermore, higher efficiency is still required with large active area for the commercial applications. In this study, we focus on the scientific understanding of 1) perovskite/scaffold interface and 2) annealing process potentially affecting to the crystallization of CH₃NH₃PbI₃ for perovskite solar cell.  Firstly, we present the facet chemistry of TiO₂ scaffold with utilizing specific facet-dominant TiO₂ nanoparticles for constructing meso-porous scaffold for perovskite solar cells.  Furthermore, the solar cell properties with specific facet dominant anatase TiO₂ scaffold is discussed in detail with regarding the electron transport properties measured by a series of photoelectrochemical impedance spectroscopy including the characterization of CH₃NH₃PbI₃ perovskite crystals.  Secondary, we present the annealing effects on characterization of CH₃NH₃PbI₃ perovskite crystals. We especially propose a microwave heating applied to the annealing process of to create the CH₃NH₃PbI₃ perovskite crystalline films through the conversion of the intermediate state of as-coated precursor films into perovskite poly-crystalline films.  Observed rapid conversion process into perovskite crystals with higher crystallinity indicates that MW is potentially applicable for the rapid process of perovskite solar cell creation.  Furthermore, even the degradation of perovskite to be PbI₂ by evaporation of methylammonium iodide at relatively lower temperature to the conventional hotplate heating imply a promising process to achieve novel high throughput annealing process at low-temperature.  The direct and effective heating effect by MW is discussed from the point of the crystallinity of CH₃NH₃PbI₃.

A quasi two dimensional (2D) perovskite containing a little amount of hydrophobic alkylamine group as a part of A-site cation of a lead halide perovskite structure is promising because it will provide long term durability against humidity to the organic-inorganic hybrid perovskite solar cell which has been known as a next new generation low-cost and high-performance electric power source. However, the photovoltaic properties of 2D perovskite itself has not been investigated well because of low possibility to obtain a good photon harvesting efficiency and power conversion efficiency (PCE) due to its narrow light absorption region although it was known that the 2D perovskite has a unique charge separation ability and charge transport characters due to layered structure constructed by self-organization. In this presentation, we investigated orientation controlling method for a 2D perovskite crystal ((BA)₂PbI₄) consisting of a lead iodide and a butylamine hydroiodide as a hydrophobic cation and examined its effect on photovoltaic properties of solar cells. It was found that the crystalline orientation of 2D perovskite can be successfully controlled by optimizing preparation conditions of spin-coating for 2D perovskite layer and the oriented nanostructure of scaffold layer which assists a crystalline growth of 2D perovskite. As the result, crystalline orientation-controlled (BA)₂PbI₄ solar cell prepared on TiO₂ oriented nanowire arrays showed higher PCE than that of non-controlled 2D perovskite solar cell prepared on planer TiO₂ film scaffold.

 Improving the hole-transporting layers (HTL) of Perovskite solar cells often affect the cell performance such as conversion efficiency (η) and stability. Arylamine polymers are potential hole-transporting materials (HTM) to replace the typical HTM spiro-OMeTAD. Arylamine derivatives exhibit an appropriate HOMO level (ca. 5.4 -5.5 eV) and high charge transport ability as the HTM. A series of poly(vinyl triphenylamine)s (PVA) as the HTM were prepared through facile two steps (Wittig reaction and radical polymerization) with high yield.These polymers showed a rapid and reversible redox and an adequate HOMO level. The polymers with molecular weight >10,000 dissolved in organic solvents (THF, chlorobenzene, chloroform and so on) and yielded high quality and tough layers with a thickness of 10 nm or less by simple coatings. The perovskite solar cells fabricated with the polymers provided almost hysteresis-less J-Vcurves with the conversion efficiency of ca. 16 %. By improving the coating procedure and tuning the cell fabrication, the conversion efficiency of ca. 17% was achieved for the 1 cm² cell.

This work was partially supported by the research project of NEDO, Japan

Hole-transporting materials are one of the keys for improving the conversion efficiency, durability, and preparation cost of perovskite solar cells. Arylamine polymers are amorphous hole-transporting materials with high hole mobility and durability. Arylamine polymers were prepared with high yield via very simple one-step addition condensation of triphenylamine and phenylenediamine derivatives (substituted with methyl or methoxy groups) with paraldehyde under moderate conditions (80°C, 4h). Molecular weights of the polymers were in 6.9×10³ ~ 1.6×10⁴. The polymers were highly soluble in organic solvents (THF, chloroform, chlorobenzene and so on) via alkyl groups of the main chain and showed high glass transition temperatures (Tg>200°C). The polymers formed thin layers by spin-coating which were amorphous, homogeneous and tough. The polymers substituted with electron donating groups gave appropriate HOMO levels (-5.25−5.40 eV) for a mixed-cation perovskite solar cell. The cells yielded the conversion efficiency of 14 % and high durability or long life-time.

This work was partially supported by the research project of NEDO, Japan.

Perovskite solar cells using CH₃NH₃SnI₃ or CsSnI₃ as photovoltaic materials have been studied extensively. The conversion efficiency, however, are still 6% which is much lower than Pb devices. [1] This is due to the instability of Sn²⁺ state in air, since Sn⁴⁺ is more stable oxidation state. Therefore, small amount of Sn(IV) impurities are incorporated into the perovskite lattice and damage semiconducting property of the perovskite. However, chemical properties of Sn²⁺ are quite similar to those of Pb²⁺, Sn²⁺ perovskite analogue is expected to be a good candidate from the environmental problems.  Sn²⁺ and Pb²⁺ ions form similar valence electron configuration (5s² or 6s²) and hence, MX₂ (M=Sn, Pb, X=Cl, Br) form pyramidal MX₃^- anions in acidic solutions. Pyramidal anions are also recognized in crystals. In a suitable choice of cation, perovskite structure appears associated with strong color, such as orange, red or black. In these perovskite lattice an infinite linear chains -X-M-X-M- are formed three dimensionally. This infinite linear chain is formed mainly by p-orbitals of M and X. This bond is similar to that found in linear I₃^- anion and called “Three-center four-electron (3c-4e) bond”.  This concept is more generally accepted as a “Hypervalent bond”. [2] Characteristic band gap property and small effective mass of electron or hole are both originate from the hypervalent nature of these perovskite halides.  In this report, essential physical properties (structure, phase transition and band gap) for series of solid-solutions, CsSnCl3-xBrx and CsSnBr_3-xI_x, are summarized. Furthermore, reducing power of SnF₂ on CsSnBr₃ powder was observed by ¹¹⁹Sn NMR spectroscopy.

[1] W. Liao, D. Zhao, Y. Yu, C.R. Grice, C. Wang, A.J. Cimaroli, P. Schulz, W. Meng, K. Zhu, R.G Xiong, and Y. Yan, Adv. Mater. 2016, 28, 9333-9340.

[2] Characteristic Properties of Hypervalent Compounds: Static and Dynamic Structures of Sb(III), Sn(II), and Ge(II) Halides, K. Yamada and T. Okuda, in “Chemistry of Hypervalent Compounds,” ed. by K. Akiba, Wiley-VCH, New York (1999), Chap. 3, pp 49-80.

Lead iodide perovskite materials, e.g. CH₃NH₃PbI₃ (MAPbI₃), have a greater advantage for the photovoltaic applications because of their extremely long photocarrier diffusion lengths (μm scale), which can be regarded as a result of shallow defect levels in MAPbI₃. [1] In a similar way, the energy levels of surface states are of great account as well. Therefore, we calculated the structural stability and electronic states on the simple surfaces of tetragonal MAPbI₃ by use of first-principles calculations. We found that there are two major phases on all of the four surface facets. They can be classified as vacant- and flat- type terminations, and both terminations can coexist especially on the probable (110) and (001) surfaces. These surfaces can contribute to long carrier lifetimes in MAPbI₃ because they have no midgap surface states. [2,3]

Despite the outstanding properties of the perovskite, the dependence of current−voltage (J−V) curves on the voltage scan direction and speed makes it difficult to evaluate power conversion efficiencies. A recent experiment showed that the rate-dependent J−V curves are related to built-up potentials induced by ion displacement in perovskite photo-absorbing layers. [4] In this respect, we calculated activation barriers of ion (or vacancy) migrations in tetragonal MAPbI₃ and trigonal (NH₂)₂CHPbI₃ (FAPbI₃) by first-principles calculations. The migrations of I⁻ anions in both perovskites show low barriers of 0.3 to 0.45 eV, which values indicate these perovskites are potential ion conductors. Furthermore, MA⁺ and FA⁺ cations have rather low barriers c.a. 0.6 eV, namely molecular cations can also migrate. Based on the dilute diffusion theory, we can expect that small vacancy concentrations suppress these ion conductions. [5]

[1] W. -J. Yin, T. Shi, and Y. Yan, Appl. Phys. Lett. 104, 063903 (2014).

[2] J. Haruyama, K. Sodeyama, L. Han, and Y. Tateyama, J. Phys. Chem. Lett., 5, 2903 (2014).

[3] J. Haruyama, K. Sodeyama, L. Han, and Y. Tateyama, Acc. Chem. Res., 49, 554 (2016).

[4] W. Tress, N. Marinova, T. Moehl, S. M. Zakeeruddin, M. K. Nazeeruddin, and M. Grätzel, Energy Environ. Sci. 8, 995 (2015).

[5] J. Haruyama, K. Sodeyama, L. Han, and Y. Tateyama, J. Am. Chem. Soc., 137, 10048 (2015).

Ultrasonic spray coating is a promising pathway to scaling-up of perovskite solar cell production because it can be applied on scales from < 1 cm² laboratory samples to continuous roll-to-roll production.  The capability to optimize a scalable process in under laboratory conditions is particularly important because of dramatically lower cost of process development with table-top machines rather than large-scale industrial equipment.  Several groups have demonstrated fabrication of perovskite solar cells by ultrasonic spray coating, however the publications so far included only machine-specific parameters required to achieve optimal performance. Due to its inherent scalability, design of ultrasonic spray systems varies considerably, and settings on one machine are not necessarily meaningful on another. Even environmental factors and coated substrate area may considerably affect processing conditions and film quality for the same type of spray coating machine.

To identify machine-independent parameters that determine film quality, we demonstrate that spray-coated wet-film thickness and evaporation rate strongly correlate with dry film morphology and large-scale uniformity. To realize machine-independent process characterization, we demonstrate a system for in-situ measurement of wet film thickness and evaporation rate immediately after spray coating. We further demonstrate that with limited data, modeling can be used to compute wet film evaporation rate under conditions where in-situ measurement is not practical.  Monitoring wet film evaporation rate and thickness parameters directly would allow a process to be replicated between laboratories or manufacturing sites without the need for extensive re-optimization.  Furthermore, we demonstrate that the PbI2 films that were fabricated by the optimized process are suitable for high-performance perovskite solar cell fabrication under ambient conditions. Solar cells and modules are demonstrated with device active area between 1 cm² and 2 cm² and PCE up to 13% with negligible hysteresis. 

Charge carrier trapping at defects on surfaces or grain boundaries is detrimental for the performance of perovskite solar cells (PSCs). In practice, it is one of the main limiting factors for carrier lifetime. Also, it causes hysteresis in the current-voltage curves, which has been considered as a serious issue for operation of PSCs.

In this work, types of surface defects responsible for carrier trapping are clarified by comprehensive first-principles investigation into surface defects of tetragonal CH₃NH₃PbI₃ (MAPbI₃).

Considering defect formation energetics, it was revealed that different types of surface defects are responsible for carrier trapping, depending on the conditions: MAI-rich, intermediate, and PbI₂-rich. Under MAI-rich condition, excessive I atoms on surfaces act as carrier traps. Under PbI₂-rich condition, excessive Pb atoms or I atom vacancies do, however, they have somewhat higher formation energy. Under intermediate condition, any carrier-trapping surface defects have high formation energy, i.e. cannot be easily formed.

From the above, PbI₂-rich condition is preferred to MAI-rich one, consistent with the superiority of the two-step device fabrication method to the one-step one in terms of the photovoltaic performance. Also, it can be proposed that intermediate condition has possibility to be the best choice.

Perovskite solar cells (PSCs) have attracted considerable attention of late, because of their low cost, high performance, and facile fabrication. A key component of PSCs is the hole-transport material (HTM), which plays a crucial role in achieving high performance by hindering charge recombination. The most frequently used HTM is wide-band-gap 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamino)-9,9'-spirobifluorene (spiro-OMeTAD), which to date has proven to be the most effective HTM. However, a major drawback of this compound is its low conductivity, which directly affects the photovoltaic parameters. An important strategy to overcome this limitation and improve device performance is p-type doping to spiro-OMeTAD for inducing more charge carriers.We report herein the synthesis of new cobalt complexes tris[2-(1H-pyrazol-1-yl)-4-trifluoromethylpyridine]cobalt(III) tris[bis(trifluoromethylsulfonyl)imide] (Co1), tris[4-(1,1-difluoroethyl)-2-(1H-pyrazol-1-yl)pyridine]cobalt(III) tris[bis(trifluoromethylsulfonyl)imide] (Co2), bis[6-(1H-pyrazol-1-yl)-2,2'-bipyridine]cobalt(III) tris[bis(trifluoromethylsulfonyl)imide] (Co3) and bis[4,4'-dimethyl-6-(1H-pyrazol-1-yl)-2,2'-bipyridine]cobalt(III) tris[bis(trifluoromethylsulfonyl)imide] (Co4), which we use as a dopant to create a HTM in perovskite solar cells. The addition of Co1–Co4 to spiro-OMeTAD in chlorobenzene changes the color of the material, which indicates that charge transfer occurs between spiro-OMeTAD and Co1–Co4. These complexes have suitable redox potentials for oxidation of spiro-OMeTAD and the conductivity is enhanced in films with Co1–Co4. Devices made from spiro-OMeTAD doped by Co1, Co2, and Co4 perform better than those made from Co3 because of the presence of hydrophobic alkyl groups on the ligands. An overall power conversion efficiency of 14.8% is obtained by using Co2 as a dopant, which exceeds that of tris[2-(1H-pyrazol-1-yl)-4-tert-butylpyridine]cobalt(III) tris[bis(trifluoromethylsulfonyl)imide] (FK209) under the same fabrication conditions.These results indicate that the Co(III) complex Co2 is a good candidate for a p-dopant of the HTM. 

This research was funded by the New Energy and Industrial Technology Development Organization (NEDO).

Functionalized Porphyrins as New Hole-Transporting Materials for High Performance Perovskite-Based Solar CellsHsien-Hsin Chou,a Yu-Hsien Chiang,b Peter Chen,*,b and Chen-Yu Yeh*,aaDepartment of Chemistry, National Chung Hsing University, Taichung 402, Taiwanb Department of Photonics, National Cheng Kung University, Tainan 701, Taiwan  AbstractPerovskite-based solar cells (PSCs) are the most fascinating solar-to-electricity converting technologies in the new century. The urge of searching for alternative hole-transporting materials (HTMs) for high-performance perovskite-based solar cells is growing. Recently we have developed new category of porphyrin-based hole-transporting materials which are structurally simple, easy to prepare and well performed. New zinc porphyrins Y2 and Y2A2 have been utilized in perovskite solar cells (PSCs) specifically as hole-transporting materials (HTMs) rather than photosensitizers. The combination of MAPbI3 as photosensitizer and porphyrins as HTMs is potential alternative to well-known MAPbI3/Spiro-OMeTAD hybrids owing to high performance and versatility toward molecular engineering of porphyrin families. A high efficiency of 16.60% is achieved by n-butyl tethered Y2 HTM (VOC = 0.99 V; JSC = 22.82 mA cm-2) which is comparable to that of Spiro-OMeTAD of 18.03% (VOC = 1.06 V; JSC = 22.79 mA cm-2). Both materials possess similar HOMO level and same order of magnitude of hole-mobility at 10-4 cm2 V-1 s-1. The slightly poorer performance of 10.55% (VOC = 1.01 V; JSC = 17.80 mA cm-2) is obtained for n-dodecyl tethered Y2A2 HTM. This is believed to stem from more surface pinholes while deposited on perovskite leading to an order of magnitude slower mobility. To our knowledge, it is the highest-performance solar cell device (including dye-sensitized solar cells and organic photovoltaics) ever reported that is incorporated with porphyrin materials.   

In the present paper, we present the role of the isopropyl alcohol (IPA） solvent in the synthesis of organic-lead-halide perovskite　CH(NH2)2PbIxBr3-x [FAPbIxBr3-x] thin films　by a two-step reaction of an amorphous (a-)PbIxBr2-x layer and FAIxBr1-x solution diluted in IPA　using spectroscopic ellipsometry (SE) as well as X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning microscopy (SEM) observation. An optical dispersion model was developed to extract the complex refractive index N (=n+ik), optical transition, and film thickness of FAPbIxBr3-x perovskites by SE analysis as a function of immersion time in a solution of FAIxBr1-x diluted in IPA. SE combined with X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning microscopy (SEM) revealed that IPA promoted film crystallization of a-PbIxBr2-x accompanied by the formation of surface roughness, grain boundaries, and voids, followed by enhanced diffusion of FAIxBr1-x into the grain boundaries/voids in the mesoporous crystallized PbIxBr2-x network.IPA played a significant role to promote crystallization of the a-PbIxBr2-x film through the removal of coordinated CPH from the a-PbIxBr2-x network, accompanied by the formation of grain boundaries, voids, and surface roughness.The diffusion of FAIxBr1-x into the voids and/or grain boundaries resulted in the simultaneous formation of large amounts of free volume and an increase in the film thickness, resulting in a decrease in the refractive index. The red-shift of absorption edge from 3.4 to 1.73 eV is attributed to a phase transition from 0D to 3D [PbX6]4- octahedral clusters. These processes contribute synergistically　to the growth of the perovskite structure.

In printable and low-cost next-generation solar cells, perovskite solar cells, the interface engineering is considered as the next key technology for improving the cell efficiency and long-term stability of device.  As TiO₂ is the most commonly used material for the oxide scaffold layer, our interest focuses on controlling TiO₂ surface to engineer the crystallization and electron transfer properties at the interface of scaffold/perovskite.  In this presentation, we especially focus on the facet chemistry of the oxide scaffold, and we report a few recent studies of perovskite solar cells with utilizing the facet chemistry of oxide scaffold.  In our studies, the single crystal of the facet oriented TiO₂ and specific facet-dominant TiO₂ nanoparticles are utilized for constructing a model system and device configuration, respectively.  In the model system, the affinity of component of perovskite , Pb and Cl, revealed the difference among each facet of TiO₂, and therefore suggests that the facet control of scaffold is an important factor for the film formation and ultimately the device properties.  Furthermore, the solar cell properties with specific facet dominant anatase TiO₂ scaffold  is discussed in detail with regarding the electron transport properties measured by a series of photoelectrochemical measurements.  

Bismuth based perovskite with the structure (CH3NH3)3Bi2I9 (MBI) are rapidly emerging as eco-friendly and stable semiconducting material as a substitute for the lead halide perovskites. Relatively higher bandgap of MBI (about 2.1 eV) has been found to be a bottle-neck in realizing the high photovoltaic performance similar to that of lead halide based perovskites. We would like to demonstrate the bandgap engineering of novel bismuth based perovskites obtained by in-situ sulfur doping of MBI via the thermal decomposition of Bi(xt)3 (xt = Ethyl Xanthate) precursor. Color of the obtained films were clearly changed from orange to black when annealed from 80áµ’C to 120áµ’C. Formation of sulfur doped MA3Bi2I9 was confirmed by XRD and the presence of sulfur was confirmed through XPS. In this work, obtained sulfur doped bismuth perovskites exhibited a bandgap of 1.45 eV which is even lower than that of most commonly used lead halide perovskites. Hall-Effect measurements showed that the carrier concentration and mobility are much higher as compared to that of undoped MA3Bi2I9.

Mesoscopicsolar cells (MSCs) based on three-dimensional interpenetrating networkjunctions have attracted much attention due to the advantages of low materialcosts and simple fabrication processes.[1] Dye-sensitizedsolar cells (DSSCs), the first-generation MSCs, have made various breakthroughson working mechanisms, functional materials and device architectures in thepast years, which motivated the research on this photovoltaic (PV) technology from laboratory investigations to practical applications.[2, 3] Recently, the second-generation MSCs, mesoscopic perovskite solar cells (MPSCs) based on hybrid organic–inorganic halide perovskite semiconductors have boosted thedevelopment of this PV technology. Benefiting from the optimization of depositionapproaches, design of new material systems, and diversity of device concepts,the efficiency of MPSCs has increased from 2.19% in 2007 to a certified 22.1%in 2016. Inspired by the latest amazing advances, more and more researchersattempted to develop large-area mesoscopic solar modules to promote the commercializationof this technology.[4] Due to the advantages of both material costs andproduction costs, MPSCs have been regarded as one of the most promisingcandidates of the next-generation PV technology. It is believed that the dream of generating electricity with a cost-effective PV strategy will be realized inthe near future.[5]

However, large area perovskite solar modules based on the conventional perovskite structures suffer from stability concern and a complicated process. Herein we demonstrate a high efficient, stable and large-area perovskite solar module that can be fabricated under ambient conditions. The module employs a triple layer architecture of mesoporous TiO2, ZrO2 and carbon, which allows us to use screen-printing technique for large-scale production and reduce the cost by ignoring either the expensive hole-transport layer or the noble-metal counter electrode. Upon optimization of each component of the device and the fabrication method, a highest efficiency of more than 10% for 100 cm2 module has been obtained under AM1.5G at 1 Sun illumination condition. No degradation of peformance was observed under extensive stability tests, including an indoor light soaking test , a local outdoor test and a shelf-life dark test. This provides a promising path towards realizing efficient and stable perovskite photovoltaics.

1.    M. Grätzel, Acc. Chem. Res. 42, 1788 (2009).

2.    H. Han, Udo Bach et al., Appl. Phys. Lett.94, 103102 (2009).

3.    Y. Rong, H. Han et al., Sol. Energy Mater. Sol. Cells105, 148 (2012).

4.    S. Si, Y. Rong, H. Han et al.,Front. Optoelectron., In press.

5.    Y. Rong, L. Liu, H. Han et al.,Adv. Energy Mater. 1501066 (2015).

Exploration and development of ‘green’ materials for photovoltaic devices have attracted intense attention since green plants exemplify solar energy conversion. Inspired by the sophisticated system of nature plants, here, we have demonstrated an eco-friendly and effective light sensitizer to be applied in solar cells, which can be prepared in one step via hydrothermal treatment of celery leaves extractive. Heat treatment will degrade the natural dyes and introduce carbon quantum dots (CQDs) into the extractive. The effects of hydrothermal temperatures on the CQD formation and the nature dye survival have been investigated. Experimental results indicate after heat-treatment at 120°C for 2.5 hours, the formed CQDs have particles size ranged from 4~6 nm together with some dyes surviving. The nanocrystalline TiO₂ solar cells using this hydrothermally treated extractive as light sensitizer can achieve the best power conversion efficiency of 0.482% under AM 1.5G one full sun illumination, which is 1.7 times higher than the solar cell using the original extractive as the sensitizer and 1.5 times higher than the one based on pure CQD sensitizer. The doped CQDs can improve the energy efficiency by facilitating the electron/energy transfer in the dye-semiconductor complex and thus may provide a useful approach for the development of highly efficient photoelectric devices.

Perovskite solar cells (PSCs) are one of most promising next generation solar cells due to their high conversion efficiency, low materials cost, and ease for production. After the conversion efficiency has achieved over 20%, interests of researchers gradually spread across lead-free materials, flexible device, and high durability of performance. Among them, technology to minimize the production cost is a major issue for industrialization. We have focused on a low temperature fabrication of PSC, which allows use of plastic substrates for development of flexible and low cost devices. For low temperature fabrication process, organic carrier transporting materials are generally useful. However, inorganic materials are superior in chemical stability if materials of thin compact film can be prepared by low temperature method. As a candidate of such materials, amorphous metal oxide semiconductors are promising because of their high stability and low material's cost.   We employed amorphous Nb-doped TiO₂ [TiO₂(Nb)] as a compact layer (CL) material and combined it with binder-free Brookite TiO₂ nano-particles to prepare mesoscopic TiO₂ substrate. Based on the substrate, we fabricated formamidinium (FA) and methylammonium (MA) mixed perovskite [(FAPbI₃)_0.85(MAPbBr₃)_0.15] solar cells. PSCs fabricated below 100°C and 150 °C successfully achieved conversion efficiencies of up to 15% and over 17%, respectively. Additionally, the amorphous TiO₂(Nb) based device showed less hysteretic behavior in J–V curve compared to high temperature (550 °C) sintered crystalline non-doped TiO₂ CL based device.

Mass production and reliability are considered as key factors for commercial dye sensitized cells. To meet requirements of mass production, the efficiency of each fabricated cell must be controlled in an allowable difference and fabricating time should be short as possible. A step by step and semi-automatic producing process, including screen printing of electrode material, dye adsorption, lamination, electrolyte filling, and encapsulation, was established to realize the change of efficiency in practice. The other parameters, such as different operator and batch number of material also are considered. The results show that the change of efficiency can be reduced under ten percent with well-controlled parameters. The operator of screen printing process affects efficiency change significantly and the batch number of material is a minor issue. In addition, the cell lifetime was also estimated by accelerated aging test and Arrhenius analysis, showing that cell lifetime can fulfil the requirement of indoor application.

We theoretically study hysteresis in the current-voltage characteristics caused by the transient response of the electric field from the accumulated charges. We derived an equivalent circuit from the mechanism of charge carriers accumulation in the charge transport layer of a solar cell. By solving the equivalent circuit model, we show some of the features of hysteresis in perovskite solar cells.

Recently, to reduce or even to eliminate this hysteresis, the TiO2 electron transport layer was replaced with another layer.  The results presented here are consistent with the reduced hysteresis that was obtained by introducing these structures, because the charges would accumulate at the interface between the charge generation layer and the electron transport layer.

While our results are qualitative, they do clearly reproduce the basic hysteresis features, such as a higher current when scanned from the open circuit (OC) side as compared with that obtained when scanned from the short circuit (SC) side, and the large hysteresis at the OC side when compared with that on the SC side for the planar structured cells.

K. Seki, Appl. Phys. Lett. 109, 033905 (2016).

Hybrid lead halide perovksite, MAPbX₃(MA=CH₃NH₃+, X=Br- or I-) emerged with remarkable efficiency increasing and examination on its physical properties are of particular concern. One of the issues is hysteresis phenomenon in current-voltage characteristics. Current-voltage curves tend to show a strong dependence on the sweep rate and direction. Several mechanisms have been proposed including difference in trapping efficiency, ion migration, imbalance between electron and hole fluxes into an electron transfer layer and a hole transfer layer, and ferroelectricity or spontaneous polarization of the perovskites. We report our observations on the non-free charge behaviors in the thin films by measuring piezoelectric responses using piezoelectric force microscopy. In addition, we verify the details of the crystal structure in terms of configuration of the methylammonium ligands. We propose a device model to enhance charge transfer through increasing open circuit voltage in the perovskite solar cells when we have a preferred polarization in the materials.

Lead halide based perovskite semiconductors have recently gained wide interest in solid state photovoltaic devices with impressive power conversion efficiencies (now above 20%).[1] While offering a relatively simple and low cost processability, the poor control of their growth processes in thin films represents a major roadblock. Inks based on colloidal suspensions of nanoparticles (NCs) could lead to a better control of the material quality and device reliability, but current NC synthesis methods use bulky, high-boiling point ligands and solvents, limiting their use in applications.[2,3] We report a fast, room-temperature synthesis of inks based on CsPbBr₃ perovskite NCs using short, low boiling-point ligands and solvents.[4] Requiring no post-synthesis treatments, the inks are directly used to fabricate thin films of high optoelectronic quality, exhibiting photoluminescence quantum yields higher than 30% and an amplified spontaneous emission threshold as low as 1.5 µJ/cm^₂. The robustness of such properties is demonstrated by the fabrication of the first nanocrystal-based solar cells, with a density of short circuit current higher than 6 mA/cm^₂ and open circuit voltages as high as 1.5 V, close to the maximum theoretical values achievable. 

References:

[1] N. J. Jeon, et al., Nat Mater (2014) 13, 897-903

[2] L. Protesescu, et al., Nano Letters (2015), 15, 3692-3696

[3] Q. A. Akkerman, et al., J. Am. Chem. Soc. (2015), 137, 10276-10281

[4] Q. A. Akkerman, et al., submitted

Three novel triphenylamine-based hole transport materials (HTMs), BTPA-4, BTPA-5, and BTPA-6, were synthesized through a facile two-step procedure with low cost starting materials. Different from the spirobifluorene core in commonly-used spiro-OMeTAD, spiro(fluorene-9,9-xanthene) skeleton was employed in these three materials as the central bridge. More than 70° water contact angle rendered these three HTMs to effectively protect the corresponding solar cell devices from humidity. Moreover, thermogravimetric analysis of these three materials shows that they decompose above 300 °C. We systematically studied the effect of the chemical structure of the HTMs on the perovskite solar cell performance by varying the conjugated system and the number of terminated triphenylamine charge transporting components. Although central bridge in BTPA-6 was somewhat more twisted out-of-plane compared with BTPA-4 and BTPA-5 which generated the hypsochromic shift of BTPA-6, BTPA-6 exhibited better solar cell performance (12.10%) than BTPA-4 and BTPA-5, which is also comparable to the cell fabricated with expensive spiro-OMeTAD(13.25%). The present results highlight BTPA-6 as highly competitive HTMs for future large scale application in perovskite solar cells.

Organic-inorganic perovskite solar cells containing various device structures (mesoporous, planar and inverted structured), which origin from conventional dye sensitized solar cells (DSSCs), have surged great waves in photovoltaic field owing to their peculiar advantages. However, comparing with the relatively mature development of charge carrier recombination research in DSSCs, the related studies are still on the initial stage in perovskite solar cells. The most accepted charge carrier recombination mechanism of trap-limited charge carrier recombination in DSSCs is not suitable for perovskite solar cells, especially the mesoporous structured perovskite solar cells, which has the most similar device structure with DSSCs. In our work, conventional DSSCs and mesoporous structured perovskite solar cells were prepared and their charge carrier recombination properties were measured and investigated by transient photovoltage decay method. In DSSCs the charge carrier recombination process is predominated by the trap states in mesoporous TiO₂ phase, and thus, leading to a mono-exponential recombination decay process. However, in mesoporous structured perovskite solar cells, we found that the perovskite phase also plays a crucial role in charge carrier recombination process, i.e., at low Fermi level, photo-generated electrons predominately populate in perovskite phase, while at high Fermi level, most electrons occupy traps in mesoporous TiO₂. As a result, the dynamics of charge carrier recombination is respectively dominated by perovskite phase and mesoporous TiO₂ in these two cases.¹ Further investigation suggests that the demarcation voltage between perovskite-dominated and TiO₂-dominated recombination process is influenced by the trap state distribution & density of perovskite.² Recently, a physical model based on multiple trap theory is proposed by taking into account both the contributions of perovskite phase and mesoporous TiO₂ phase, which is suitable to describe the biphasic charge carrier recombination process in mesoporous structured perovskite solar cells.³ These findings provide a novel horizon for the understanding of the charge carrier recombination process in perovskite solar cells.  

1. H. Y. Wang, Y. Wang, M. Yu, J. Han, Z. X. Guo, X. C. Ai, J. P. Zhang and Y. Qin, Phys. Chem. Chem. Phys., 2016, 18, 12128-12134.

2. H. Y. Wang, M. Y. Hao, J. Han, M. Yu, Z. X. Guo, P. Zhang, Y. Qin, X. C. Ai, J. P. Zhang, Under review.

3. H. Y. Wang, Y. Wang, M. Y. Hao, J. Han, M. Yu, Z. X. Guo, P. Zhang, Y. Qin, X. C. Ai, J. P. Zhang, In preparation.

In this research, strontium titanate (SrTiO₃) doped cobalt (Co) were used as counter electrodes in dye-sensitized solar cells. SrTi_1-XCo_XO₃ nanoparticles were prepared by hydrothermal method at 220 ^oC for 24 h. The surface morphology of SrTi_1-XCo_XO₃ nanoparticles were observed by the scanning electron microscopy, and the particle sizes are about 50 - 150 nm. The SrTi_1-XCo_XO₃ nanoparticles were mixed with PEDOT-PSS polymer blinder, and the films were coated on fluoride-doped tin oxide substrate (FTO glass) by tape casting technique. Cyclic voltammetry (CV) measurements indicated that SrTi_1-XCo_XO₃ can be functioned as a catalysts in iodide/triiodide redox reaction. Electrochemical impedance spectroscopy (EIS) result shows charge transfer resistance of counter electrodes reduce after dope strontium titanate with cobalt.  The solar energy conversion efficiency increase with increasing Co dope, the highest efficiency of 8.31% was obtained in cell using SrTi_1-XCo_XO₃ with x=0.075. The solar cell performance of strontium titanate doped cobalt with x=0.075 is slightly higher than the Pt DSSC (~8.27%).

    Organolead triiodide perovskite (OTP)-based solar cells, as a promising low-cost photovoltaic (PV) devices, have attracted great research interest since they were reported by miyasaka et al. firstly in 2009. With the application of OTP in all-solid-state solar cells, variety of fabrication technology and interface engineering were developed to improve the power conversion efficiency (PCE). To date, the certificated PCE has come to 22.1%, which is on par with the best conventional thin-film PV devices.      

    The perovskite solar cells with high efficiencies, low cost, high stability, nontoxicity and reproducibility are our final goal to pursue. The charge transport layer plays a key role in protecting the perovskite photoactive layer from exposure to moisture and air, thus achieving highly stable perovskite-based photovoltaic cells.  Therefore, to select the optimal charge transport material has become crucial, not only for energy level matching and charge transport, but also to achieve high stability. Moreover, the interface deterioration by chemical reaction between the perovskite layer and the metal electrode (e.g., Al, Ag) in the ambient environment have been reported to dominate the initial degradation of perovskite solar cells, which occurs far ahead of the degradation of perovskite grains by moisture or heat. The selection of metal electrode is also very important for the stability of device.    

    Here, we report the high stabilty and low-temperature processed perovskite solar cells with graphene oxide (GO) as HTM and carbon as cathode, with the structure of  ITO/GO/MAPbI3/PCBM /Carbon. This construction shows great potential for the advancement of practical application of perovskite solar cells.

  Perovskite solar cells (PSCs) are promising low-cost photovoltaic technologies with high solar-to-electric power conversion efficiency (PCE). Inverted-structured devices typically employing a perovskite–fullerene (such as, [6,6]–phenyl–C61–butyric acid methyl ester (PCBM)) planar heterojunction structure owe many advantages like simple device fabrication, high stability and small hysteresis.[1] However, in spite of numerous studies, the PCEs of inverted-PSCs are still lower than those of normal-structured PSCs, especially on the reporting of independently certified results.[2] Further device structure designs and materials development are essential for improving the performance of inverted-PSCs. The inverted-PSCs are featured with the photoelectron collection layer behind the light absorbing material of perovskite. As the lead halide hybrid perovskites have very high light absorption coefficients, a large proportion of photoelectrons are generated near the front electrode from which the light is incident. These photoelectrons have to travel a relatively long distance through the perovskite layer to be collected in the inverted planar heterojunction PSCs (PHJ). Consequently, the electron transporting characters of the semiconducting perovskite layer as well as the structure of electron extraction heterojunction play important roles on affecting the photovoltaic performance of inverted-PSCs.

  Here, we report efficient inverted-structured PSCs with a perovskite-fullerene graded heterojunction (GHJ), which is featured with a gradient distribution of electron accepting material in the light absorption perovskite layer.[3] This structure is found capable of enhancing the PCE of inverted structured PSCs as it improves the photoelectron collection and reduces recombination loss, especially for the formamidinium (FA) cation based perovskites that have superior spectra response and thermal stability. The conformal fullerene coating on perovskite during the GHJ deposition facilitates a full coverage of fullerene with reduced layer thickness, thus minimize the resistive loss in larger size devices. This GHJ structure enable the fabrication of centimeter-scale PSCs showing high efficiency with small hysteresis and good stability. A remarkable PCE of 18.21% with FF as high as 0.784 was certified by a public test center (Calibration, Standards and Measurement Team at the Research Center for Photovoltaics, AIST, Japan) based on a cell with an aperture area of 1.022 cm². To our best knowledge, this is the best results of inverted-PSCs to date.

Reference:

[1] Chen, Wei, et al., Science 350.6263 (2015): 944-948.

[2] Bi, Dongqin, et al., Science advances 2.1 (2016): e1501170.

[3] Wu, Yongzhen, et al., Nature Energy 1 (2016): 16148.

Organic­-inorganic hybrid perovskite solar cells have attracted a lot of interest recently due to their high efficiency (over 22 %) solution-based processing. While much research has been done into the general structure of the CH₃NH₃PbI₃ (MAPbI₃) perovskite material using X­-ray, neutron diffraction, and spectroscopy analyses, transmission electron microscopy (TEM) investigations have not been done extensively. This lack is possibly due to the highly damaging effect the electron beam has on the soft perovskite material, leading to many inconsistent findings being reported. In this work, low dose conditions have been specifically developed which allow for detailed study of the micro- and crystal structure of MAPbI₃ using TEM. It was found that fragile crystallographic twinning mirrored across the {112} planes is intrinsic to MAPbI₃ at room temperature, but disappears after a few minutes of very low electron dose rate (approximately 10² e/(nm² s)) exposure. The twinning was found to disappear once the MAPbI₃ was heated above its tetragonal to cubic phase transition temperature (57 °C), but reappeared when allowed to cool back down to room temperature. This indicates that the twinning is a result of strain alleviation in the crystal following a change in the unit cell volume during phase transition. Local high symmetry twin planes in other materials have been shown to act as charge transport highways, significantly facilitating movement of charges throughout the crystal. Since most reported MAPbI₃ films undergo an annealing step above the phase transition temperature, the twins can be considered to be ubiquitous in MAPbI₃-based solar cells and can be part of the explanation of the excellent charge transport properties of this material.

Perovskite solar cells (PSCs) based on lead halide perovskite (e.g., CH₃NH₃PbI₃) have recently attracted worldwide attention in the photovoltaic technology field because of their low costs and high efficiency. The power conversion efficiencies (PCEs) of these devices have increased from approximately 4.0 % to 22.0 %. However, the large-scale application of highly efficient PSCs has been restricted by the poor long-term stability, which is induced by several complicated degradation processes, such as photoreaction at TiO₂ surface, thermal-instability of perovskite, iodide diffusion at bias, water incursions or corrosion induced by additives in organic hole-transporting materials (HTMs).

Herein, we fabricated mesoscopic PSCs with various pyridine-based additives in 2,2′,7,7′-tetrakis(N,N-di-pmethoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) and compared their photovoltaic performance with that of a 4-tertbutyl pyridine (TBP)-based device. We observed that Lewis-base pyridine additives could strongly enhance the photovoltaic performance of the PSCs. We also confirmed that most of the investigated pyridine derivatives corroded perovskite due to the formation of new complexes with PbI₂. This corrosion phenomenon led to chemical decomposition of the perovskite and deteriorated the interface between the perovskite and the HTMs. Most importantly, we found a new additive of pyridine with a long alkyl chain substituted at the o-position chemically stable with no corrosion to perovskite, thereby substantially improving PSC stability.

Low-cost and high energy conversion efficiency are the crucial factors for large scale application of solar cells. In recent years, a promising high cost-performance photovoltaic technology, organometal halide perovskite solar cells (PSCs), has attracted great attention. However, most of the reported high efficiencies were obtained on a small working area of about 0.1 cm² with the material utilization ratio of only 1% during film deposition, which actually hinders the advancement in future application of PSCs. Here we present the soft-cover deposition (SCD) method where surface wettability, solution viscosity and thermal crystallization are the processing key factors for the deposition of uniform perovskite films with high material utilization ratios. Scaling-up, pinhole-free, large crystal grains and rough-border-free perovskite films were obtained over a large area of 51 cm², which were processed continuously in ambient air with a significant enhancement in the material utilization ratio up to ~80%. Unit solar cells fabricated via SCD perovskite films obtained PCEs up to 17.6% in a working area of 1 cm² with very small hysteresis and high reproducibility. At present, we also applied this unique technology to the deposition of inorganic and organic semiconductor solution processed thin-films which serve as electron and hole transport layers in perovskite solar cell and high performance has been achieved.

We show that modifications of the optical constants occur in planar CH3NH3PbI3 layers well before any structural change is observed by X-ray diffraction. We demonstrate that environment and temperature are key parameters driving changes in the dielectric function and therefore the absorption coefficient. The measurements were performed in controlled atmosphere of non-reactive gas (dry Ar or N2) and reactive gas (O2 or humid air) as a function of the working temperature, in the range 40-80°C, indeed across the tetragonal-cubic transition. With respect to truly inert Ar used as benchmark to sign the thermal behavior of the material, annealing in N2 ambient not only assures the reversibility of the optical behavior during thermal cycles, thus preventing/counteracting degradation effects, but also slightly improves the optical response during time. While it is clearly elucidated that the optical constants of the material can be recovered during thermal cycles in N2 and Ar ambient, the presence of humidity in the air, instead, causes the absorption coefficient to monotonically and inexorably degrade especially at the crossover of the phase transition (above 60°C). An intermediate case is represented by the saturated O2 ambient. The appropriate use of N2 to stabilize the layer indeed results as an effective strategy to preserve the optical response of the material under thermal operation conditions

Organo-metal halide perovskite based solar cells (PSCs) have attracted great research attention as a low-cost photovoltaic technology. The state-of-art research has achieved very high power conversion efficiency (PCE) of 22%. However, most of the high PCEs were achieved on cells with tiny working area of ~0.1 cm2 and very short lifetime. As well known, the measurement errors will be enlarged when the active cell area gets smaller. Moreover, the poor stability and large hysteresis in current-voltage scanning may further open the debate on the reliable characterization of the device performance. Therefore, it is crucial to advance efficient and stable PSCs with larger working area, which will accelerate the research of perovskite solar cells for future application.

In this presentation, I will demonstrate our works to achieve high cost-performance PSCs with working area of over 1 cm2. [1-2] First, we developed a new method to enable a better charge separation in planar heterojunction PSCs. This work can enhance the long-term stability of the device which performed a good stability of 1,000 hours under simulated solar light (AM 1.5G). Second, we developed the soft-cover deposition method to enable high cost-performance perovskite solar cell. Third, an annealing-free deposition method will also so be introduced for large-area high quality perovskite films.

Reference:

[1] Ye, F.; Chen, H.; Xie, F. X.; Tang, W. T.; Yin, M. S.; He, J. J.; Bi, E. B.; Wang, Y. B.; Yang, X. D.*; Han, L. Y.*, Energy Environ. Sci. 2016, 9 (7), 2295-2301.

[2] Yin, M. S.; Xie, F. X.; Chen, H.*; Yang, X. D.; Ye, F.; Bi, E. B.; Wu, Y. Z.; Cai, M. T.; Han, L. Y.*, J. Mater. Chem. A 2016, 4 (22), 8548-8553

A low-cost and simple process tandem structure including a top cell with high bandgap and a bottom cell with lower bandgap would be a promising candidate for high efficiency solar cells.  In this work, perovskite solar cells (as a top cell) and p-type c-Si solar cells (as a bottom cell) were fabricated and perovskite/crystalline silicon tandem structure was introduced. In order to form a tandem structure, perovskite solar cell was stacked to the crystalline silicon solar cell with an inter junction layer.  J-V curves were measured for the sample with an area of 0.09 cm².  The best fabricated tandem solar cell reaches the efficiency of 15.4% with J_SC of 16.3 mA・cm^-2, V_OC of 1.42 V and FF of 66.6%.  Considering the current match of series-connected top and bottom cells and improving the antireflection performance on the bottom cell, the perovskite-silicon tandem cells can be improved.  The detail about tandem structure will be shown in this conference. 

Cs₂SnI₆ has been successfully incorporated as an efficient hole transport material (HTM)[1] for dye sensitized solar cells (DSSCs), indicating its p-type semiconductor behavior. However, some of the reports claimed its n-type behavior only [2]. It has also been reported that the formation of pure, dense and thin films of desired thickness of Cs₂SnI₆ by solution processable method is difficult task to perform due to its poor solubility in any solvent [2] . Hence, in this work first time we fabricated pure thin films of Cs₂SnI₆ by improvising solution process method.  Despite the use of Cs₂SnI₆ as a hole transporter, the quantitative information on the charge transfer at the interface of different n-type and p-type semiconductor is still lacking and investigation for detailed information is needed. This work investigates the Cs₂SnI₆ film for utilizing as an absorber layer where facile electron and hole mobility is important.  In our work, we realized the interfacial charge transfer for Cs₂SnI₆ with n-type PCBM layer and p-type P3HT layer by photoluminescence (PL) as well as transient absorption spectroscopy ( TA) data, supporting for both intrinsic electron and hole mobility. However, no change of absorption decay signal in case of PCBM when measured in microsecond scale unravel the slow electron mobility compared to hole mobility.The absorption coefficient for Cs₂SnI₆ was estimated to be 8 * 10³ cm^-1 at 700 nm, indicating that the penetration depth for 700 nm light is 1.25 μm or at least 1.25 μm thin film need to absorb light effectively, which is comparable to conventional Pb based solar cells. Excitation carrier life time was monitored by time resolved PL decay which revealed fast decay of 0.717 ns followed by a slow decay lasting for 900 ns within the material. Mobile charge carrier life time as measured by time resolved microwave photoconductivity (TRMP) suggested for values of more than 300 ns. This was further supported by nanosecond transient absorption spectroscopy (ns TA) indicating slow charge decay lasting up to 20μs.

References.

1. B. Lee and M. G. Kanatzidis et.al, Jour. Am. Chem. Soc., 2014, 136, 15379-15385.

2. B. Saparov and D. B. Mitzi, Chem. Mater., 2016, 28, 2315-2322.

Building solar cells on a flexible plastic film is an important issue for roll to roll mass production and wearable devices. For this purpose, low temperature fabrication process of perovskite solar cells has been intensively studied.Although fullerene derivatives and ZnO, which can be prepared at low temperature, are often used as electron collectors of flexible solar cells, cheap and stable metal oxides are more desirable. Here, we have developed brookite TiO₂ mesoporous layer prepared at oC as an electron collector. Although conventional anatase TiO₂ requires an organic binder in the mesoscopic precursor paste, which is to be removed by sintering at ~500 ^oC, to ensure inter-particle necking and the adhesiveness of TiO₂ nanoparticles on a conductive glass, the brookite TiO₂ layer can be formed at oC low temperature from a binder-free slurry based on dehydration reaction of surface hydroxyl groups between nanoparticles. The mesoporous brookite layer with high hydrophilicity works as an effective scaffold for crystal growth of CH₃NH₃PbI_3-xCl_x perovskite film with high coverage and large crystal size compared to anatase TiO2 and leads to hysteresis-less and higher photovoltaic performance. In combination with amorphous metal oxide hole-blocking layer, the brookite electron collector enables 18.9% power conversion efficiency for perovskite solar cells prepared at low temperature on ITO glass substrates. Furthermore, with amorphous tin oxide blocking layer, the flexible brookite-based perovskite solar cells formed on plastic films give efficiency up to 14% with hysteresis-less property. The flexible cells exhibited high mechanical stability against repeated bending with radius of 1.5 cm.