Halide perovskites have arisen as a potential candidate to implement in wide range of photovoltaic and optoelectronic devices such as solar cells, LEDs, amplifier, lasers and also transistors. Among other virtues, halide perovskite preserve its excellent optoelectronic properties deposited on flexible substrate property that do not share the Silicon Solar cells. Here we present a new approach in order to deposit CH3NH3PbI3 on abundant, biodegradable, flexible and transparent substrates, including nanopaper and nanocellulose. Plastic is not renewable and cannot be decomposed through biodegradation in the same way as organic materials. Also (organic) optoelectronic devices require precise control over the optical properties of substrate. Fundamentally different from plastic substrates, the optical properties of transparent cellulosic substrates are tunable due to the availability of different building blocks derived from cellulosic fibers with hierarchical structure. With this approach, we have successfully deposited Lead Iodide Perovskite on nanocellulose substrate and characterized with XRD, SEM and PL measurements. Also the effect of diethyl ether as the anti-solvent during the deposition process on the crystal morphology of perovskite layers were examined. Furthermore, the role of insulating polymer, PMMA, on the stability of perovskite layers on nanocellulose substrate was tested.
Organo-lead halide perovskite (e.g. CH3NH3PbX3, with X=I,Br,Cl) have been widely used to prepare highly efficient solar cell devices with efficiencies already surpassing 20%. Most of the highest efficiency devices use perovskite absorbers with mixed halide composition (I,Cl or I,Br) that are prepared by wet-chemical methods such as spin-coating. While the wet-chemical preparation methods allow fast and cheap material synthesis, vapor deposition has also been proven to be a successful alternative method that may be better suited for large scale production. Furthermore, issues concerning dissolubility and the influence of the substrate/atmospheric pressure are avoided or at least reduced, making vacuum-based co-evaporation well suited for a fast screening of the formation limits of the different perovskite phases if the phase formation under different molecular flux ratios can be monitored. In our group with have built an evaporation chamber equipped with a dedicated X-ray diffractometer, which allows us to study the formation and evolution of the different phases during thin film growth. In this contribution, we show our advances in investigating the MAPb(I,Cl)3 and MAPb(I,Br)3 compositional ranges and compare them to the pure MAPbI3 perovskite. In contrast to the case of MAPb(I,Cl)3 and to theoretical predictions, we were not able to identify an explicit miscibility gap for the MAPb(I,Br)3 solid solution in our experiments. However, we do observe the formation of PbI2 and PbBr2 secondary phases alongside the perovskite when co-evaporating CH3NH3I (MAI) and PbBr with varying flux ratios. Furthermore, the phase evolution under thermal stressing and the critical temperature for the decomposition of the perovskite thin films is investigated and compared for the different halide precursors.
Solution based processes show great potential for synthesis of mixed-halide perovskite thin-films, as they do not need cost intensive vacuum technology and no high-temperature annealing steps. However little is known about the exact mechanisms during film formation. We present a detailed investigation of the chlorine derived synthesis of MAPbI3 perovskite. By simultaneously applying synchrotron based in-situ quick scanning X-ray absorption spectroscopy (QEXAFS), X-ray diffraction (XRD) as well as X-ray fluorescence (XRF) in combination with in-situ optical reflection and photoluminescence spectroscopy a conclusive picture of the film formation can be drawn. The reaction is conducted in a custom designed in-situ reactor with control of temperature and atmosphere. Using this set of complementary real-time time -resolved characterization techniques simultaneously, we can correlate the evolution of the chemical reaction from XAS, the formation of crystalline phases from XRD and the chemical composition from XRF with the evolution of the optoelectronic properties of the film on one single timeline. Furthermore, the combination of compositional and structural properties with the evolution of optoelectronic properties allows a meaningful interpretation of the optoelectronic data. Hence, we are able to correlate the delayed formation of crystalline MAPbI3 with the decreasing chlorine content of the sample. Correlating changes of the optoelectronic properties with simultaneously measured structural and compositional properties allows an interpretation of lab-based in-situ optical spectroscopy. Our results give insight into the detailed formation process and can help to provide a mechanistic understanding of the reactions and intermediates involved in the formation of MAPbI3 from Cl-containing precursors. This knowledge could be utilized to optimize synthesis strategies and tunability of the optoelectronic properties of organic-metal-halide perovskite semiconductors.
Organic inorganic (hybrid) lead halide perovskites are interesting materials for photovoltaic applications, low-threshold lasers and light-emitting diodes (LEDs). Their application in LEDs requires a precise control over their morphology, since this determines their optical and electronic properties. In particular, materials with high photoluminescent quantum yield (PLQY) are desirable for the preparation of efficient LEDs. One of the most suitable strategies for controlling the morphology and enhancing the photoluminescence of perovskites is the preparation of nanostructured materials. The recent breakthroughs in the PLQY of narrow-band emitting perovskite nanoparticles (NPs) (full width at half-maximum, FWHM ∼20 nm) make these materials promising candidates for light-emitting applications.
In this work, methylammonium lead bromide nanoparticles are synthetized with a new ligand (11-aminoundecanoic acid hydrobromide) by a non-template method. Upon dispersion in toluene they show a remarkable photoluminescence quantum yield of 80%. In addition, the bifunctional ligand allows to anchor the nanoparticles on a variety of conducting and semiconducting surfaces, showing bright photoluminescence films with a quantum yield exceeding 50%. This opens a path for the simple and inexpensive preparation of multilayer light-emitting devices.
Organic inorganic (hybrid) perovskite solar cells are considered one of the most promising technologies for future photovoltaics, due to the fast rise of the achievable power conversion efficiency (PCE). Besides the thorough understanding of the effect of morphology and crystallinity on the optical and electronic behavior of this class of semiconductors, the recent breakthrough in terms of device efficiency have been achieved through a rational chemical design of the perovskite absorbers. Apart from the substitution of the monovalent cation A and the halide X in the general perovskite formula AMX3, the impact of the partial exchange of the divalent metal M on the optoelectronic properties of the semiconductor has been investigated on a much more limited basis. In this work we describe the partial replacement of Pb2+ by Sr2+ in MAPbI3 and its effect on the morphological, optical and electronic behavior of the material. We found the addition of Sr2+ in MAPbI3 to strongly enhance the charge carrier collection efficiency of the cells leading to increased current densities and very high fill factors (FF), going from 78% for the pure perovskite, up to 85% in the presence of 2% Sr2+. To gain insight on the marked improvements in the FF and current collection, the charge carrier dynamics of the MAPbI3:Sr2+ films were investigated using time-resolved microwave conductivity (TRMC) measurements. These measurements indicate that, at low charge carrier concentrations (~1014 cm-3), the charge carrier lifetime of Sr2+ containing perovskites is in excess of 40 μs. Such carrier lifetimes are among the longest observed for this family of perovskites, and even longer than those reported for perovskite single crystals. We also examine the surface electronic properties of the perovskite absorber layer, by means of photoemission spectroscopy to further detail the origin of the high charge carrier lifetimes as well as the enhancement in fill factor. We observe a small change in the perovskite work function as well as a significant Sr2+ enrichment at the thin-film surface, which might explain, in part, the altered photovoltaic performance. The observations of perovskite solar cells with fill factors as high as 85%, obtained merely by the addition of very small amounts of a different cation, is remarkable and can be easily extended to other hybrid lead halide perovskites, potentially boosting further the already high power efficiencies.
Lead halide perovskite semiconductors have recently gained wide interest following their successful embodiment in solid-state photovoltaic devices with impressive power-conversion efficiencies, while offering a relatively simple and low-cost processability. Although the primary optoelectronic properties of these materials have already met the requirement for high efficiency optoelectronic technologies, industrial scale-up requires more robust processing methods, as well as solvents that are less toxic than the ones that have been commonly used so successfully on the lab-scale.
Here we report a fast, room-temperature synthesis of inks based on CsPbBr3 perovskite nanocrystals using short, low-boiling-point ligands and environmentally friendly solvents. Requiring no lengthy post-synthesis treatments, the inks are directly used to fabricate films of high optoelectronic quality, exhibiting photoluminescence quantum yields higher than 30% and an amplified spontaneous emission threshold as low as 1.5 uJ.cm-2. Finally, we demonstrate the fabrication of perovskite nanocrystal-based solar cells, with open-circuit voltages as high as 1.5V.
Further ongoing development of this work consists in the substitution of bromide anions with iodine through different post-synthesis anion-exchange reaction strategies to yield a lower-bandgap material which will be beneficial for higher power conversion efficiency solar cells, as well as enable the fabrication of the first all-perovskite nanocrystal-based tandem solar cell.
 Akkerman et al., Nature Energy, 2016, 2, 16194
Hybrid (organic-inorganic) perovskites have become widely studied semiconducting materials, mostly due to their impressive photovoltaic performances. These are consequence of their unique properties, such as high absorption coefficient, high carrier mobility and long range carrier diffusion. On the other hand, the optical and electronic properties of hybrid perovskites vary largely with the material composition and morphology. Recently, hybrid perovskites such as methylammonium lead bromide (MAPbBr3) have emerged as potential candidate for future electroluminescent devices. Generally, in LEDs, a high photoluminescence quantum yield (PLQY) of the active layer is desirable, since it is thought to determine the final efficiency of the device. Unfortunately, when the pure MAPbBr3 is prepared by simple solution methods (single step spin-coating of the precursor solution), its PLQY is rather limited, at least if measured at low excitation intensity. In analogy to inorganic semiconductors, the most promising strategy to promote radiative recombination is the spatial confinement of the charge carriers. In this work we present strategies to improve the PLQY of hybrid perovskites by limiting their growth using a template material (i.e. polymer, small molecule or a porous inorganic scaffold) which is compatible with the solution processing of highly luminescent thin-films. The structural and optical properties, together with the application of such materials in LEDs, will be discussed.
Cs-based inorganic perovskites represent a very promising alternative to hybrid organic perovskites, given their better long-term stability and the impressive radiative yield. Therefore such materials are extremely attracting not only for tandem photovoltaic cells but also for the realization of efficient LEDs and lasers, being possible to cover a spectral range from NUV to NIR depending on the halide composition. In this contribution we present time-integrated (TI) and picosecond time-resolved (TR) photoluminescence (PL) results to assess the exciton recombination dynamics in high quality CsPbBr3 thin films, varying several experimental parameters ( temperature, excitation intensity, excitation photon energy). An increase of the PL decay time is reported increasing the temperature, suggesting the thermal activation of a transfer process from non-radiative states. By the comparison of reflectivity and PL measurements we correlate the emission bands to the resonances in the reflectivity spectra, proving the high sample quality. Moreover, by local thermal heating with laser pulses we show that we can change the relative weight of bulk and nanocrystal emission, opening the route to a controllable modification of the film nanotexture.
The electron transport layer (ETL) plays a fundamental role in perovskite solar cells. Recently, graphene-based ETLs have been proved to be good candidate for scalable fabrication processes and to achieve higher carrier injection with respect to most commonly used ETLs. In this work we present an experimental study addressing the effects of different graphene-based ETLs in sensitized MAPI solar cells. By means of time-integrated and picosecond time-resolved photoluminescence techniques, the carrier recombination dynamics in MAPI films embedded in different ETLs is investigated. Using graphene doped mesoporous TiO2 (G+mTiO2) with the addition of a lithium-neutralized graphene oxide (GO-Li) interlayer as ETL, we find that the carrier collection efficiency is increased by about a factor two with respect to standard mTiO2. Taking advantage of the absorption coefficient dispersion, we probe the samples, and in particular the MAPI layer morphology, along the thickness. We find that the MAPI embedded in the ETL composed by G+mTiO2 plus GO-Li brings to a very good crystalline quality of the MAPI layer with a trap density about one order of magnitude lower than that found with the other ETLs. In addition, this ETL influences also the phase transition of MAPI from tetragonal to orthorhombic, blocking MAPI at the tetragonal phase, regardless of the temperature. Graphene-based ETLs can therefore open the way to significant improvement of perovskite solar cells.
Perovskite solar cells (PSCs) have attracted the interest of the scientific community in the last few years due to their high performance produced by interesting physical effects and low cost solutions. In spite of the spectacular advances in cell efficiency, many aspects of this system are poorly understood. One interesting phenomenon is the hysteresis response observed in current-voltage curves upon illumination. Recently, the origin of these hysteresis cycles has been associated to capacitive effects and the movement of ions , pointed that the transport in perovskite is a mixed ionic-electronic conduction. The movement of ions can be assisted by anion and cation vacancies. First-principle studies in MAIPb prevskites have shown that the most diffusive species is the iodide anion . In the literature, the movement of ions is modeled in different ways. In some cases, anions and cations are considered to move at the same time . Other works consider only the movement of iodide cation vacancies . The nature of semiconductor is also modeled differently. Some authors assume the perovskite to be intrinsic , . Others question the intrinsic nature of the perovskite semiconductor. All these questions can be solved with the comparison between experiments and the simulation of the PSCs.
In this work, we have systematically analyzed the effect of the distribution of mobile ions and their vacancies on p-type, n-type, or intrinsic PSCs under illumination by means of the drift-diffusion transport equations. They include ion charges at the bulk and at the perovskite/electrode interfaces. We study the profiles of the ion, electron and hole densities, at different applied voltages. In particular, the analysis is focused on the short-circuit and open-circuit regimes of the solar cell. We distinguish between the movement of single and both type of ions. The distribution of mobile ions modifies the band bending in the bulk and close to the perovskite-metal interfaces. Different distributions of mobile ions appear in the cases under study. In general they act as if the doping of the semiconductor is changed. However, not all of them produce an accumulation of free charge carriers at the interfaces which can be the origin of the hysteresis phenomena observed in experiments .
. B. Chen et al. J. Phys. Chem. Lett. 2015, 6, 4693-47002.
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. R. Gottesman et al. Chem, 1, 776-789 (2016)
Organic-inorganic metal halide perovskite solar cells (PSCs) have reached power conversion efficiences over 20%. Two archetypal PSC architectures are reported in the literature: mesoporous and planar PSCs. In the former one, a mesoporous TiO2 scaffold is incorporated into the cell. Because sizes of the mesopores are typically small compared to wavelengths of visible light, the scaffold barely scatters light.
In this work, we propose to periodically structure a porous TiO2 scaffold incorporating pores with diameters comparabe to wavelengths of visible light thanks to the use of colloidal crystal templating fabrication method. The resulting TiO2 scaffold filled with perovskite will constitute an opal-like photonic crystal incorporated in the solar cell, which will strongly interact with light.
Through Finite Difference Time Domain (FDTD) simulations, we demonstrate that the photonic crystal induces resonances that can be exploited to modulate light harvesting in the macroporous TiO2 layer. Sunlight absorption by the PSC will be presented and discussed with dependency of pore sizes and number of opal layers.
The water solubility and toxicity of components in lead containing perovskite thin-film solar cells raise concerns over their implementation as feasable commercial products. Substitution of the lead (Pb) in the perovskite with another post-transition metal such as tin (Sn) could result in a wider acceptance. The best known efficiency using CH3NH3SnI3 was published in 2014 by Noel, N.K., et al., who achieved a record PCE of 6,4% using the spin-coating technique . We intend to build fully vacuum processed perovskite solar cells. In our labs we could produce and analyse lead (Pb) based perovskite solar cells using Glass/FTO/c-TiO2/m-TiO2/CH3NH3PbI3/spiro-MeOTAD/Au by means of spin coating with a highest recorderd efficiency of 14,97%. Working on the transition to lead-free perovskite solar cells, a Vacuum Flash Evaporation prototype has been built and tested, producing stoichiometric films of cubic methylammonium tin iodide perovskite (CH3NH3SnI3), which has been analysed by means of XPS and XRD. As upcoming research, tuning the band gap of the material using mixtures of bromide (Br-) and iodide (I-) in the perovskite and building working devices could be proven useful for constructing tandem solar cell devices.
 Noel, N.K., et al., Lead-free organic–inorganic tin halide perovskites for photovoltaic applications. Energy & Environmental Science, 2014. 7(9): p. 3061-3068
Perovskite compounds, used either in mesoscopic or planar solar cells, have allowed preparing highly efficient solid-state devices (>20%).
In this study, we propose to design photoanodes with photonic structure in order to modulate light interaction. The periodic structure of porosity could add specific optical properties likely to increase light harvesting and reduce reflexion losses. Besides, current efficiencies reported for mesoscopic perovskite solar cells using an inorganic porous scaffold are slightly lower than those reported for planar perovskite cells mainly due to issues in perovskite infiltration. The control of the porous network architecture in terms of pore organization, size and connectivity could overcome this limitation.
TiO2/perovskite and perovskite-only photoanodes with an inverse opal porous structure are prepared from templating techniques, using polystyrene beads as structuring agent. The photoanode microstructure is further characterized by scanning electron microscopy (SEM) and X-Ray diffraction (XRD). In parallel, light interaction is modeled in order to find the best compromise in terms of photonic architecture (pore size, organization, thickness…).
The big potential for low-cost production in the perovskite technology is due the possibility of deposited the major components of device with a low-temperature coating process using inexpensive, abundant materials. The possibility of use solution processed methods makes perovskites a promising candidate for inexpensive printing or roll to roll industrial mass solar cell production on light weight, large area and flexible substrates. Precursor solvent, replacement of antisolvent treatment, annealing process and additives are the main parameters that significantly present influence on organo-metal halide perovskite crystallization. A suitable combination of green, low cost and effective precursor solvents, anti-solvents, additives, direct or multi-step annealing processing are needed in order to improve the crystallization and morphology of up-scalable perovskite films. In this work we focus on Hansen Solubility Parameters (HSP) approach as a tool for the design of green ink formulations containing organo-metal halide perovskites. Solvent engineering based on HSPs has been used for achieving an increase of perovskite concentration, a better control of the evaporation rate, and to promote a better crystallization of solution processed perovskite films. A list of potential green solvents for CH3NH3PbI3 perovskites as an alternative to hazardous DMF, DMSO, NMP and DMAC, among others, has been obtained. Processing parameters (such as boiling point, relative evaporation rate, etc.) together with environmental and toxicological aspects have also been considered. Furthermore, thin films of the most promising green solvents were fabricated and characterized. The results for solubility tests and thin film deposition are shown and compared to the use of common toxic solvents. Solvent engineering by HSP considering solubility/insolubility, environmentally friendly solvents, and influence of processing and annealing parameters will help to improve the macromolecular order of perovskite structures replacing toxic solvents that are not industrially up-scalable. The development of this innovative solvent engineering approach will contribute to improve reliability on the wet processing by using more environmentally friendly inks for perovskite absorbers.
Formamidinium lead iodide (FAPbI3) has a broader absorption spectrum and improved thermal stability than the more famous methylammonium lead iodide system, thus providing great promise for its integration into photovoltaic (PV) devices. Of the two main FAPbI3 polymorphs, by far the most interesting for PV applications is the high temperature metastable α-phase (trigonal perovskite) modification – over its low temperature δ–phase (hexagonal non-perovskite) counterpart – because of its superior optical and electronic properties1; from its long carrier lifetime (484 ns), to the roughly one order of magnitude increase in conductivity (1.1x10-7 (Ωcm)-1), when compared to methylammonium PbI3 (MA PbI3). FAPbI3 is well known to undergo a δ- to α-phase transition above 150 ºC,2 however the real-world effects of transforming this material in oxygen-rich environments are yet to be explored. In this report, phase-sensitive Raman scattering and x-ray diffraction (XRD) methods are employed to investigate the structural properties of δ- and α-phase FAPbI3 transformed via thermal annealing under atmospheric conditions. The kinds of information the two techniques yield are notably different, with Raman scattering probing surface structure, rather than the overall average, like in XRD. Raman is able to fingerprint both δ- and α-phases readily,1 however it also reveals the formation of non-trivial PbO polymorph surface deposits after thermal treatment. Parallels can be drawn here to the related MAPbI3 system, which is prone to oxygen intercalation.3 In fact, PbO compound Raman bands where actually inadvertently contained in the Raman data reported by Han et al.1, for the low temperature delta-phase, even before undergoing thermal annealing. We conclude that such a susceptibility to lead oxide surface incorporation must be considered when designing and fabricating stable optical devices based on the thermally transformed metastable α -FAPbI3 materials.
 Q. Han et al., “Single Crystal Formamidinium Lead Iodide (FAPbI3): Insight into the Structural, Optical, and Electrical Properties”, Adv. Mater. 28, 2253 (2016).
 N. J. Jeon et al., “Compositional engineering of perovskite materials for high-performance solar cells”, Nature 517, 476 (2015).
 W. Kong et al., “Oxygen Intercalation Induced by Photocatalysis on the Surface of Hybrid Lead Halide Perovskites”, J. Phys. Chem. C 120, 7606 (2016).â€‹
Hybrid organic-inorganic halide perovskite (PS) derivatives, with general formula ABX3, have attracted enormous attention due to their outstanding properties that make them suitable for the development of high performing optoelectronic devices. In the last years, photovoltaic solar cells based on bulk PS layers with efficiency values over 22% have been produced. Besides its success in the photovoltaics field, a new generation of photodetectors, lasers, and in light-emitting diodes (LEDs) that use bulk PS as the active semiconductor layer are being currently studied. However, the preparation of PS based nanoparticles that show extremely high fluorescence quantum yields (Φf ≈ 90%) has been considered a significant breakthrough in the field. The PS based quantum dots (PS-QD) show tunable electro-optical properties depending on the particle size and composition, and exceptionally narrow emission spectra. Therefore, PS-QDs are considered excellent candidates for the development of high efficiency, low-cost, wide gamut and high purity color displays. In this work we show the synthesis and characterization of PS-QDs and their application for the development of high performance LEDs through solution-processing methods.
In the last few years, hybrid halide perovskites have awakened interest due to their great properties for the development of photovoltaic devices. On the other hand, these materials present uncommon properties compared with other photovoltaic materials as a fast ionic diffusion coefficient. In order to benefit from this property in the growth of the perovskite and to control the morphological and optoelectronic properties, we report a two-step deposition method involving not just organic cation migration but anion migration. In this work, methylammonium lead bromide (MAPbBr3) film is formed after dipping separately three different lead salts (PbI2, PbBr2, PbCl2) in a MABr solution. This method results in films with different morphological and optoelectronic properties depending on the initial lead halide salt and also produces different performances of the photovoltaic devices. On one hand, in the devices prepared using PbI2 and PbCl2 salts the charge recombination is reduced and so they have a higher open circuit potential, especially with PbI2. On the other hand, the photocurrent increases using PbBr2. The final films and devices are characterized by measuring photoluminescence, light absorption and J-V curves. In addition, they are analyzed by X-Ray Diffraction, Impedance Spectroscopy, Scanning Electron Microscopy and Energy Dispersive X-ray spectroscopy.
We have provided a novel method to passivate perovskite layer by introducing additives through the anti-solvent deposition step. Different additives, namely, a twisted hexaazatrinaphthylene (HATNA) and a bisthiadiazolefused tetraazapentacenequinone (DCL97), were chosen to verify the method. Optoelectronic devices based on perovskite passivated such as solar cells, LEDs and waveguide light amplifiers have been investigated. We have observed a significant improvement of the performance of all three devices. Photoconversion efficiency of solar cells has been increased 25% respect the reference cell, with no additive, while current/potential hysteresis has been decreased. We have also detected an increase of EQE for LEDs in a factor 3 and the reduction of the light amplification threshold energy in a factor of 2 to 4 when organic additives are introduced. Systematic characterization points the main role of additives is to passivate the grain boundaries and/or perovskite interface with organic additives, thus producing a reduction of non-radiative recombination that leads to a notable increase of the main figures of merit of perovskite optoelectronic devices.
Perovskite Solar Cells (PSCs) have emerged as a very promising alternative in comparison to silicon based solar cells due to the combination of high light absorption coefficients and outstanding solar energy conversion efficiencies of more than 20% with simple and cheap methods of fabrication like spin coating or other fluid-to-solid transformation processes [1,2].
The dynamic response of photo voltage (IMVS) and photocurrent (IMCS) on light, modulated with changing frequencies, is generally addressed as Intensity Modulated Photo Spectroscopy (IMPS) [3,4,5]. It is a linear, small signal method close to Electrochemical Impedance Spectroscopy (EIS). IMPS is a technique popular for instance in the fields of Dye Sensitized Solar Cells (DSSC) when evaluating the competition between photo charge carrier lifetime and diffusion speed. IMPS intentionally uses bias light superimposed with a small modulation in order to maintain linearity [3,4,5]. On the contrary, intensity transients leave linearity. This is due to the large perturbation induced by switching light from the on-state to the off-state, or vice versa. A couple of characteristic properties are assumed to be in steady state in the case of IMPS, but they are changing dramatically under light transients. The latter technique is offering the possibility of getting additional insights into ultrafast processes. It is therefore advantageous, when linear dynamic measurements under frequency variation, like IMPS and IMVS, can be put in relation to measurements of transient behaviour in the time domain. In the present work IMPS data of an Organic Solar Cell (OSC) sample were recorded at specific bias intensities. The spectra were interpreted by means of AC modelling and fitting. For comparison, the same samples were characterized by means of fast intensity transients, again. Therefore, the intensity was switched off within 80 ns, with and without keeping the background intensity at a certain bias. The results were compared with the IMPS results and the contribution of the photoconductivity could be identified.
1 N.-G. Park, J. Phys. Chem. Lett., 2013, 4, 2423–2429.
2 G. Niu, X. Guo and L. Wang, J. Mater. Chem. A, 2015, 3, 8970–8980.
3 E. A. Ponomarev and L. M. Peter, J. Electroanal. Chem., 1995, 396, 219–226.
4 Y. Zhao, A. M. Nardes and K. Zhu, Faraday Discuss., 2014, 176, 301–312.
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Metal halide perovskites such as methylammonium lead iodide (CH3NH3PbI3) are generating great excitement due to their outstanding optoelectronic properties, which lend them to application in high efficiency solar cells and light-emission devices. However, there is currently debate over what drives the second order electron-hole recombination in these materials. Here, we propose that the band gap in CH3NH3PbI3 has a direct-indirect character. Time-resolved photo-conductance measurements show that generation of free mobile charges is higher for excitation energies just above the bandgap (1.7 eV) than further above the bandgap (>1.8 eV). Furthermore, we find that second-order band-to-band recombination of photo-excited electrons and holes is retarded when the temperature is decreased. A thermal activation energy of 47 meV is found, which is on the same order of magnitude as the difference between the direct and indirect bandgaps predicted from literature. These results provide a new framework to understand the optoelectronic properties of metal halide perovskites and analyze spectroscopic data.
 Hutter, E. M. et al. Direct–indirect character of the bandgap in methylammonium lead iodide perovskite. Nat. Mater. 16, 115–120 (2017).
 Motta, C. et al. Revealing the role of organic cations in hybrid halide perovskite CH3NH3PbI3. Nat. Commun. 6, 7026 (2015).
In organic-inorganic hybrid perovskite solar cells, optimization of hole transport materials (HTMs) is important for enhancing solar power conversion efficiency and improving stability. At OIST, a team of researchers in the Energy Materials and Surface Sciences Unit has been making concerted efforts to study 2,2’,7,7’-tetrakis[N,N-di-(4-methoxyphenyl)amino]-9,9’-spirobifluorene (spiro-MeOTAD), which is the most widely used HTM in perovskite solar cells [1-7]. In this talk, we will present our latest understanding of fundamental interactions between Li-bis(trifluoromethanesulfonyl)-imide (LiTFSI), 4-tert-butylpyridine (t-BP) and spiro-MeOTAD. We will also show how gas exposure (e.g., exposure to O2, H2O, N2) influences electronic structures and conductivity of such HTM films. In addition, we will propose further strategies to improve perovskite solar cell performance and stability [4,6].
 E.J. Juarez-Perez, M.R. Leyden, S. Wang, L.K. Ono, Z. Hawash, Y.B. Qi*, Chem. Mater. 28 (2016) 5702.
 Z. Hawash, L.K. Ono, Y.B. Qi*, Adv. Mater. Interfaces 3 (2016) 1600117.
 Z. Hawash, L.K. Ono, S.R. Raga, M.V. Lee, Y.B. Qi*, Chem. Mater. 27 (2015) 562.
 L.K. Ono+, S.R. Raga+, M. Remeika, A.J. Winchester, A. Gabe, Y.B. Qi*, J. Mater. Chem. A 3 (2015) 15451 (+These authors contributed equally)
 Y. Kato, L.K. Ono, M.V. Lee, S. Wang, S.R. Raga, Y.B. Qi*, Adv. Mater. Interfaces 2 (2015) 1500195.
 M.C. Jung, S.R. Raga, L.K. Ono, Y.B. Qi*, Sci. Rep. 5 (2015) 9863.
 L.K. Ono, P. Schulz, J.J. Endres, G.O. Nikiforov, Y. Kato, A. Kahn, Y.B. Qi*, J. Phys. Chem. Lett. 5 (2014) 1374
A new facile preparation approach for synthesis of perovskite nanowires with different compositions is proposed using a highly reproducible method of tuning the morphology by simple varying of chemical reactants. MAPbI3 and FAPbI3 nanowires are obtained by dipping of PbI2 films into solutions of MAI and FAI dissolved in mixtures of isopropanol and dimethylformamide (DMF) and allows to control chemically the morphology of the perovskite crystals. Nanowires with different anionic composition MAPbBr3, MAPbCl3 or FAPbBr3 are obtained via an ionâ€‘exchange in isopropanol solutions of MABr, MACl and FABr respectively.
We found that the nanowires formation sequence includes intermediate phases such as MAI-PbI2-DMF and FAI-PbI2-DMF acting as structure directing agents. The 1D shape of the adduct is preserved during the conversion to perovskite. Thus, the adducts play the role of key precursors controlling the final product morphology. Systematic investigations of the observed phase transformations and morphology features on multiple length scales revealed the effectiveness of the suggested synthetic route utilizing an original pseudomorph formation mechanism of the 1D structures to produce partly oriented films and textured layers of the nanowires via a few experimental steps. Understanding of the formation mechanism of nanowires through the adduct phases contribute to the Lewis acidâ€‘base adduct approach for obtaining high performance materials for perovskite solar cells.
This work was supported by Ministry of Education and Science of Russian Federation, Project Number: 14.613.21.0053.
For thin-film solar cells, interfaces play an important role in understanding mechanisms of charge separation and loss. Photoemission spectroscopy, including XPS and UPS, is used to reveal the nature of interfaces. Significant measure of interfaces are band bending, offsets in the conduction band for electrons and offsets for the holes in valence band or chemical reactions. Photoemission spectroscopy conveys the intricate electronic structure of the device, further contributing to device performance improvements. For these studies, it is necessary to grow the cell in ultra-thin layers and measure photoemission after each step. Therefore, it is mandatory to prepare the different layers inside of a UHV system to avoid any influence of the atmosphere. The preparation chambers are therefore directly connected to the measurement device, which allows in situ photoemission characterization. The growth of ultra-thin layers allows tracking of the electronic structure throughout the whole device. Our study focused on the two absorber materials CH3NH3PbI3 and CH3NH3SnI3 with different selective contacts. The interfaces to some of the most common hole transport materials, such as spiro-MeOTAD, CuSCN, and CuI, as contacts to TiO2 and Au, were analyzed. A better understanding of the interface helps to advance the functionality of photodiodes. Further optimization of the cell requires more effective interfaces to minimize voltage losses and reduce recombination at the interfaces.
Organic-inorganic hybrid perovskite absorbers for solar cell applications attracted enormous attention of the scientific community during the last years. Nowadays, perovskite solar cells have reached efficiencies above 20% . Still, to find fabrication processes applicable to upscaling from laboratory to commercial solar cell sizes remains a task to work on. The closed space sublimation (CSS) proved to be such an upscalable process for other thin film technologies like CdTe solar cells. In our lab we combine a conventional evaporation of PbCl2, PbI2 or SnI2 with the transformation of those base layers to CH3NH3PbI3 (MAPI) and CH3NH3SnI3 (MASI) perovskites via a CSS process. Both processes are carried out under ultra high vacuum (UHV) conditions. In addition, a XPS/UPS system is directly connected to our fabrication chambers, which allows in situ PES experiments on MAPI and MASI layers, without breaking the UHV conditions. We present the comparison between an open transformation process of lead salt layers to MAPI with a CSS process, that should allow shorter transformation times and higher substrate temperatures. In addition, the fabrication conditions required for the production of MAPI and MASI perovskite layers with the CSS process will be shown. The analysis of the layers is mainly done with XPS, XRD and SEM. Emphasis will be laid on the electronic and chemical surface properties of the CSS perovskites. Furthermore, the potential the CSS process offers for tuning the morphology of the MAPI and MASI layers will be discussed.
 NREL solar cell efficiency chart (http://www.nrel.gov/ncpv/)
The perovskite solar cells introduced in 2009 and attract the attention of many research groups in less than one decade. One of the important issues regarding these cells is light harvesting. In this work, we have used light-trapping structures (LTS) to improve the ability of the cell to confine more light in the active layer and thus, more absorbed light. Some of LTSs are based on photonic or plasmonic nano-structures. Depend on the configurations, these nano-structures can improve the light absorption in the cell. Our proposed structure is based on plasmonic nano-particles (NPs) to increase the light harvesting in the absorber layer. The plasmonic NPs create localized surface plasmon as a result of light interaction to free electrons and increase the intensity of the electromagnetic field. We have used plamonic nano-rings (NRs) that have three parameters to controlling, outer radius, inner radius and thickness. By accurate tuning of these parameters, plasmonic NR lead to high electromagnetic field intensity and thus high light absorption. For this reason, we have defined two aspect ratios, one for the proportionality of the outer radius to thickness and another for the proportionality of outer radius to the inner one. We have considered two cases, close-packed and non-close-packed NP array. According to the numerical simulation and by considering the best position of NRs in the active layer, the absorption increases in both close-packed and non-close-packed NP arrays and more lights have been confined around the NPs compared to the reference cell which is without any NPs. But, this increased absorption is not necessarily lead to greater efficiency because of the absorption loss due to the parasitic absorption.
Organometal Halide Perovskite (OMHP) based Solar Cells (SC) have recently experienced an intensive research due to their low cost and high efficiency.1,2 In particular, the OMHP presents very attractive characteristics such as superior charge transport properties with a long-range ambipolar behaviour, low exciton binding energy or magnetic field effects.3,4 One important motivation of researchers is the fundamental understanding of OMHP since many aspects around fundamental physics governing device operation are still under debate and needs to be unravelled to understand and optimize its behaviour.3
The microstructure is a crucial aspect which governs the optoelectronic properties of OMHP materials.5,6 Many examples can be found in literature of OMHP with different microstructures such as layered structures or micro and nanocrystals.5,6 But only few examples can be found in literature about their one dimensional(1D) structuration, which is foreseen to have a crucial impact on many aspects of SC performance such as efficiency, absorption of light or antireflection capabilities.7,8 The demonstration that 1D nanostructures represents a feasible technology for future photovoltaics requires suitable synthesis methods to fabricate the required multilayer architecture in one-dimension and a direct comparison with its two dimensional analogue for their fundamental understanding.
Nowadays, most of OMHP devices are fabricated via wet methods and thus the interfaces are exposed to liquids, water or environmental gases. For example, perovskites are usually deposited under nitrogen atmosphere to avoid hydrolysis9 that leads to an important deterioration of the SC performance and therefore should be avoided. The improvement of crucial aspects of the cells such as efficiency or durability requires a clean and dry approach to avoid the exposition of the relevant interfaces to the atmosphere.
In this work we report a full vacuum methodology to develop one-dimensional OMHP nanostructures. A full microstructural, optical and luminescence characterization is presented which shows the optoelectronic modifications induced by the microstructure. This work demonstrates the feasibility of the approach and paves the way for the fabrication of highly efficient one-dimensional OMPH solar cells.
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The high efficiency conversion (up-to 21.6 % ) of alkylammonium lead halide perovskite solar cells has attracted the attention of the scientific community in the recent years. Despite the spectacular advances in cell efficiency, many aspects of the methylammonium lead iodide (MAPbI3) are poorly understood. In particular, the origin of its colossal dielectric constant, which is amplified under illumination, remains still unsettled. Additionally, it is well known that moisture affects the MAPbI3 in many ways (morphology, crystallization, etc.) and that under moisture conditions at room temperature it suffers a phase transformation into MAPbI3.H2O, leading to an inclusion of water molecules inside its structure.
In this context, the goal of this work is try to obtain experimental evidence about the origin of large dielectric constant of the MAPbI3 and the role of moisture conditions.
In this work, we have measured the complex dielectric permittivity and carry out an impedance spectroscopy analysis on a pellet samples and thin films as a function of frequency (1 Hz - 1 MHz) and temperature (100 K - 350 K). Also, we have studied the influence of the moisture conditions on the dielectric and conductivity response of the MAPbI3 compound. Very interestingly, the experimental findings point out that the presence of moisture induces several changes at the values of the dielectric constant and conductivity on the MAPbI3 material at room temperature.
(1) Grätzel, M. et al. Science 2016, 354, 206-209.
(2) Bisquert, J. et al. J. Phys. Chem. Lett. 2014, 5, 2390.
(3) Barnes, P.R.F. et al. Chem. Mater. 2015, 27, 3397-3407.
CH3NH3Pb(Br,I)3 single crystals are interesting for understanding fundamental properties and potential applications for large-area crystalline substrates. Most studies of all, examination on high photo-conversion efficiency thin-films are proceeded to comprehend the conducting characteristics. Grain growth behavior in perovksite is not fully controlled in terms of compositions, textures, and even believed as a trapping source of ionic migration. Surface potential of perovskites is known to depend strongly on the grain boundaries [1,2]. Singe crystals with no grain boundaries are, therefore, a material form for studying intrinsic properties. We investigated perovskite thin-films grown on TiO2 mesoscopic or planar bottom electrodes. We measured temperature dependent photoluminescence and Raman scattering spectroscopy in order to investigate bandgaps and vibrational characteristics. The distribution of surface electric potential and current transport was studied by Kelvin probe force microscopy and conductive atomic force microscopy respectively. Surface potentials of iodide and bromide films exhibit distribution around 4.4 eV and 4.6 eV, respectively. Current level of the mono-grain is small but some spots exhibit large current values at high external voltage bias. Consolidated information of the band gap and workfunction will provide a schematic picture on the electronic structure of the perovskite materials.
 G. Y. Kim, S. H. Oh, B. P. Nguyen, W. Jo, B. J. Kim, D. G. Lee, H. S. Jung, “Efficient Carrier Separation and Intriguing Switching of Bound Charges in Inorganic−Organic Lead Halide Solar Cells”, Journal of Physical Chemistry Letters, 6, 2355 (2015). Daehan Kim, Gee Yeong Kim, Changhyun Ko, Seong Ryul Pae, Yun Seog Lee, Oki Gunawan, D. Frank Ogletree, W. Jo, and Byungha Shin, “The effect of post-synthesis annealing duration on grain properties and photovoltaic performance of organic-inorganic hybrid perovskites”, Journal of Physical Chemistry-C, 120, 21330 (2016).
Significant progress has been made in power conversion efficiency (PCE) and device stabilization of organoplumbate-trihalide (MAPbX3 perovskite solar cells. From the humble 4% reported in 2009 to a certified 22% efficiency in 2015; these developments are spectacular. Interest in the transport properties of the MAPbX3’s was triggered as soon as the first high efficiency devices appeared. A remarkable feature in MAPbX3 devices is the low diffusion coefficients and mobility: 0.05–0.2 cm2s-1 and 1–30 cm2V-1s-1, respectively, in polycrystalline material. Mobilities in single crystalline material are marginally higher than 100 cm2V-1s-1. This is in spite of the low effective masses that are comparable to those in prototypical Si and GaAs solar cells. Structural defects and not impurities are thought to be responsible for the low mobility figures. Furthermore, defects are also implicated in the hysteresis typical of these materials and in the intrinsic doping. Bandgap engineering has been demonstrated by introducing Sn and Ge to partially substitute Pb in this materials system. Again, the effect of defects introduced in this manner on the transport properties is still unknown. Knowledge about defects is critical for further improvements in materials properties. We have studied highly resistive single crystal light p-type methylammonium lead-bromide perovskite using thermal admittance spectroscopy (TAS) and current-mode deep levels transient spectroscopy (I-DLTS). Multy-frquency capacitance-temperature (C-T) scans reveal a phase transition around 150 K and the presence of a frequency dispersion around 275 K; the same dispersion does not appear in the C-T spectra when the sample is cooled and measured under forward-bias. The C-T spectra also reveal anomalous capacitance profiles; capacitance decrease with increasing temperature, suggesting deep-lying defects. Light on/off C-T measurements supports this characterization. Using Laplace-DLTS at various biases at 2000 s-1 rate window, we detected an electron/hole traps at Ev+282 eV and Ev+0.174 eV with apparent cross section ∼10-18cm-2. There are two other defects which are still under investigation so that we can reveal their signatures.The defect-complex becomes almost undetectable by I-DLTS post vacuum-annealing at 380 K.
Hybrid halide perovskite compounds have recently revolutionized the field of photovoltaics with a vertiginous enhancement in light-to-electricity power conversion efficiency currently reaching outstanding 22.1% values. In parallel to photovoltaic uses, organometal perovskites have been proposed as their use as active material for lithium-ion battery (LIB) anodes. It is presented here promising results and progress into the understanding of the storage electrochemical mechanism of lead halide perovskite materials. Methylammonium lead bromide (MAPbBr3) exhibit rather stable specific capacity ≈200 mA h g -1 with an excellent rate capability. These preliminary results are comparable to current commercial anodes capacities. Moreover, perovskite electrodes allow checking ion diffusion dynamics of extrinsic defects (Li + ) in a fully controllable way. Our strategy avoids the inherent uncertainty found in analyzing native defect migration in a multicomponent environment, which confirms the fast ionic conductor character of perovskite materials from a direct measurement that does not rely on simulation tools. Ion diffusion within the host matrix dominates the charging rates that are accessible by means impedance spectroscopy.
The opto-electronic properties of hybrid perovskites can be easily tailored by changing their components. Specifically, mixing the common short organic cation (methyl ammonium (MA)) with a larger one (e.g. butyl ammonium (BA)) creates multilayered 2D perovskites: (BA)2(MA)n-1PbnI3n+1. These materials, also known as Ruddlesden-Popper phases,have proven to make highly efficient, solution-processed and stable LEDs (EQE = 8.8%) and photovoltaic solar cells (PCE = 12.5%). We have studied 2D (BA)2(MA)n-1PbnI3n+1 Ruddlesden-Popper hybrid perovskites using two distinct TRMC techniques with different excitation sources: high-energy electron pulse and laser photo-excitation. Our combined experimental results show a clear increase of the mobility, probability of exciton dissociation and lifetime of charges with the thickness of the [(MA)n-1PbnI3n+1]2- slabs. The increase in mobility is consistent with DFT calculations that show a decrease of the effective mass of holes. The larger exciton dissociation yield and longer lifetime of charges are explained by a decrease of the Coulombic interactions and exciton binding energy. We estimated the binding energies of these materials combining the temperature trend of the charge mobility (PR-TRMC) with the photo-conductivity TRMC. The obtained temperature trend of the yield of exciton dissociation was analyzed in the framework of the Saha equation to show that the exciton binding energies range between ~80 meV and ~370 meV depending on the thickness of the [(MA)n-1PbnI3n+1]2- slabs. This finding was confirmed by temperature dependent photo-luminescence measurements that show the presence of bound excitons at low temperature. These results demonstrate that the opto-electronic properties of these 2D materials are highly tunable for specific applications.
Solar cells based on APbX3 halide perovskites with (A = CH3NH3, HC(NH2)2, Cs and X = Br, Cl, I) have shown record efficiencies of up to 21.1%.1 However, there are concerns related to these absorbers, such as the toxicity of Pb and solar cell instability from UV induced damage to the organic component. One route to reduce the toxicity of the absorber material is replacing Pb by Sn. Moreover, completely substituting the organic cation(s) with (inorganic) Cs can improve device longevity.2 Incorporating these modifications leads to a class of inorganic Pb-free perovskites – CsSnX3 – as promising alternative absorbers. Devices based on CsSnX3 have thus far exhibited only relatively low efficiencies, possibly due to the oxidization from Sn+2 to Sn+4 producing deep defects in the absorber.3 Adding SnF2 during preparation inhibits this oxidation,4 however, the exact mechanism behind the SnF2 treatment, in particular its impact on the electronic structure, is not yet fully understood. Another approach to preventing Sn oxidation is changing the deposition environment/technique. Sn is more likely to oxidize when CsSnX3 is prepared wet-chemically (e.g., via spin-coating) than if it is prepared by thermal evaporation under vacuum conditions. For the latter, depositing the absorber in ultra-high vacuum (UHV) allows preparation of films without Sn oxidation.
To identify the roles of the SnF2 treatment, halide composition, and deposition route, thin films of (spin-coated) CsSnBr3/compact-TiO2/FTO/glass and (UHV deposited) CsSnCl3/Mo/glass were characterized by hard x-ray photoelectron spectroscopy (HAXPES). We find improved substrate coverage when SnF2 is added to the precursor solution or when the absorber is deposited via thermal evaporation in UHV. Furthermore, we can identify different chemical species depending on the preparation process. Our measurements also reveal an impact of the SnF2 treatments on the perovskite electronic structure, i.e., it enhances the density of states close to the valence band maximum (VBM). Also, we find a VBM shift of approximately 1 eV away from the Fermi level if CsSnCl3 is compared to CsSnBr3.
In our contribution, we will discuss in detail the chemical and electronic properties of inorganic Pb-free perovskites. These findings can lead to insights required to identify pathways for efficiency improvements of solar devices based on solution- and thermal-deposition-processed CsSnX3 perovskite absorber layers.
1Saliba et al., Energy Environ. Sci.9 (2016)1989.
2Kulbak et al., J. Phys. Chem. Lett. 7 (2015) 167.
3Chung et al., J. Am. Chem. Soc. 134 (2012) 8579.
4Kumar et al., Adv. Mater. 26 (2014) 7122.
The past seven years have seen a huge increase in the efficiency of perovskite based solar cells going from 3.8% to 22.1%. The 2.2 eV bandgap of methylammonium lead bromide (CH3NH3PbBr3) makes this material a possible candidate to serve as a top cell in a tandem solar cell, combined with a crystalline Si bottom cell. To realise this, it is necessary to understand the light-induced charge carrier dynamics in this perovskite, specifically the charge carrier lifetime and mobility. CH3NH3PbBr3 has three phases with transitions from cubic to tetragonal at T = 236.9K and to orthorhombic at T = 144.5K. We chose single crystals to study the intrinsic properties of CH3NH3PbBr3, since there are limited number of grain boundaries and the crystals are phase pure. These crystals were studied by temperature dependent complementary time-resolved microwave conductivity (TRMC) and photoluminescence (PL) measurements. In the cubic phase, TRMC results show on excitation at 560 nm a mono exponential decay with 250 ns lifetime which is probably due to Shockley Read Hall recombination. On excitation at 500 nm, which leads to generation of charge carriers close to the surface, a much faster multi-exponential decay is observed. This faster decay might originate from intraband-gap surface states that lead to fast recombination. Furthermore, the yield of charges decreases on higher excitation densities. This reduction in yield, which is in favour of exciton formation in agreement with the Saha equation. The intensity independent PL lifetime measurements with lifetimes in the order of several nano seconds are in accordance with above view. Interestingly, in the orthorhombic phase apart from excitonic emission at 560 nm, a broad second emission at 620 nm (DE = 0.2 eV) was observed. The TRPL of this 620 nm peak was characterised by a slow rise of several tens of ns and a decay extending into the microsecond regime. This slow PL decay corresponds with the TRMC trace, implying that the PL originates from the radiative decay of mobile charges. We attribute the slow rise to carrier diffusion followed by localisation at domain boundaries. Rashba splitting of the valence band due to symmetry breaking at the boundaries can explain the low energy emissive states. These boundaries might be related to the differences in crystal orientation in the orthorhombic phase. On heating the crystal back to the tetragonal phase leads again to disappearance of this low energy PL.
The steady-state current-voltage relation of a solar cell (j-V) provides its characteristic power conversion efficiency in long-term operation under sunlight. However, measured curves on the same sample may vary depending on the magnitude, rate, and direction of the scanning potential, resulting in hysteretic effects that make the extracted efficiencies unreliable. In spite of the many hypothesis reported in the literature, this issue is still unclear and open to further research.
We hereby present a theoretical model that explains dynamical hysteresis in perovskite solar cells. It takes into account not only the specifications of the measurement but also the past history of the sample, expressed as an initial surface polarization voltage and charge, which yield a displacement current. The total current is then defined based on this current, a recombination current at the surface, and the photocurrent. The suggested model faithfully reproduces experimental data upon MAPI solar cells, and could be used as well to predict hysteretic effects in other perovskite-based solar cells.
In perovskite solar cells (PSCs) the current density–voltage (J–V) curves have been found to present a hysteresis-like distortion when the measurement is done by sweeping the applied bias at different scan conditions. The so-called hysteresis effect has raised many concerns about the feasibility and long-term stability of this kind of photovoltaic technology. However, there is a absence of distinction among different hysteretic phenomena which is needed to unravel its underlying physical and chemical mechanisms. In our work we have distinguished between capacitive and noncapacitive currents giving rise to specific hysteretic responses in the J–V curves of PSCs. It has been found that capacitive current producing hysteresis dominates in regular structures with TiO2 as bottom electron selective layer. This is mainly caused by the charge, both ionic and electronic, accumulation ability of the TiO2/perovskite interface but has no influence on the steady-state operation. Noncapacitive hysteresis has been observed at slow enough scan rates in all kind of architectures. Inverted structures, including organic compounds as bottom hole selective layers and fullerene materials as top contact, show larger noncapacitive distortions that we associate with the inherent reactivity of contact materials and the absorber perovskite.
The overwhelming interest in perovskite solar cells asks for an in-depth understanding of the underlying operation mechanisms. Device simulation is an important tool to shed light on the working principle of perovskite solar cells and thus facilitates the further optimization of the cells. Several attempts to model perovskite solar cells with a one dimensional drift-diffusion approach have been made in the recent past [1,2,3] by including ion migration in the model. In this contribution, we compare the different drift-diffusion modelling approaches [1,2,3] and investigate the influence of the model ingredients such as boundary conditions, mobile or static ion density and mobilities on the transient current response. We therefore solve the set of partial differential equations containing Poisson’s equation and the continuity equations for electrons, holes and mobile ions fully coupled with Newton’s algorithm in the time as well as frequency domain.To illustrate the beneficial use of this 1D drift-diffusion model, we analyse the transient simulation results such as the current response to a voltage step or to a light pulse over several orders of magnitude in time. Moreover, we study impedance spectroscopy data and the resulting capacitance and conductance. We discuss the evolution of charge carrier and ion distribution profiles over time. The transient response is altered significantly upon application of a pre-bias. In forward biasing conditions, the transient electroluminescence signal can be evaluated. The results are compared with transient measurements of planar perovskite solar cells and the pre-biasing conditions are considered.
 G. Richardson et al., Energy Environ. Sci.,2016, 9, 1476-1485.
 P. Calado et al., ArXiv160600818. (2016).
 Neukom, HOPV2016.
We report a general and effective procedure for the synthesis of NCs and bulk crystals of methylammonium (MA) lead halide perovskites employing N-methylformamide (NMF) as source of MA ions. This procedure takes advantage of the in-situ formation of MA via transamidation with alkylamine or acid-catalyzed hydrolysis of NMF, hence, reducing the amount of work, chemicals toxicity and cost required to realize high quality perovskites. Strongly fluorescent MAPbX3 NCs with photoluminescence quantum yields reaching 74% for MAPbBr3 and 60% for MAPbI3 could be synthesized using NMF and oleylamine by a simple reprecipitation process, without the need of degassing any solvents. Moreover, only through this NMF-hydrolysis strategy, bulk crystals can be grown without heating and without any anti-solvent. In principle, with the method described here, MA ions can be released “on-demand” during a process, for example by increasing the acidity of the medium or by raising the local temperature, which might enable a better control on the fabrication of this important class of materials. These possibilities are currently being studied in our groups.
Hybrid lead halide perovskite solar cells (PSSCÊ¹s) have emerged as one of the most promising and hottest topics in the photovoltaics field in the last few years, mainly due to the impressive evolution of the power conversion efficiency (PCE) achieved in a relatively short period of research.1 Despites these outstanding PCE values, the full understanding of the photo-physical/chemical mechanisms that participate during the charge generation and extraction, the exact origin of the J-V hysteresis, the slow dynamic processes observed, as well as the giant dielectric constant of the PSSCÊ¹s at the low frequency region of the impedance spectra, constitute crucial questions to be elucidated. We have recently demonstrated that illuminating a PSSC induces irreversible modifications of the hole transport material broadly employed, i.e. photo-induced oxidation of Spiro-OMeTAD, which contribute significantly to the enhancement of the J-V hysteresis via charge accumulation.2 In the present work, we analyse the light-induced effects that suffer the PS films upon illumination and how these effects influence the performance of the devices including the high overall efficiency values and their relatively limited long-term stability.
1http://www.nrel.gov/ncpv/images/efficiency_chart.jpg2 R. S.Sanchez,
2E. Mas-Marza, Solar Energy Materials & Solar Cells 158(2016)189–194.
We will present a simple process to convert a metallic film of Pb(0), Sn(0) or a mixture of those to an ABX3 halide perovskite by introducing to AX [e.g., methylammonium iodide (MAI), MABr, CsI, etc.] salts dissolved in simple alcoholic solvents.(1) The novel approach shows a reproducible and high-quality films of various (including mixed) halide perovskites. This together with the significantly reduced toxicity suggest a promising rough towards scale-up. A much lower toxicity of this fabrication method is achieved by avoiding the use of polar aprotic solvents, such as dimethylformamide or dimethylsulfoxide, which are commonly used and become very toxic when containing Pb cations.We will describe of our findings, including examples of the direct transformation from Pb and Sn to, for example, MAPbI3, MAPbBr3, MAPb(Br,I)3, FAPbI3, MASnI3 and the pseudo-perovskite Cs2SnI6. Apart from I-V characterizations of full devices, morphological and electro-optical characterizations will be presented.
(1) Y. Rakita, N. Kedem, D. Cahen, G. Hodes, Pat.Appl. # IL 245536 – ‘process for the preparation of halide perovskites and perovskite-related materilas’
Hybrid organic-inorganic perovskites with the general formula ABX3 have attracted great attention as light-harvesting materials for the next generation of photovoltaics. Since their first use as photoactive material in 2009, power conversion efficiencies (PCE) have already increased to a certified value of 22.1%, thereby competing with established thin-film PV technologies. Improvements in PCE have been mostly achieved by advances in the deposition process of CH3NH3PbI3 or mixed halide CH3NH3PbI3-xClx absorber layers. Optimized film uniformities and coverages as well as larger crystalline domains seem to be the most important characteristics leading to high performance in the planar heterojunction. Compared to one-pot synthesis of perovskite films from a mixed precursor solution, the sequential deposition of PbI2 and CH3NH3I, first developed in 2014 by Liu et al. for a planar heterojunction device architecture, enables better control over film coverage and crystallite formation. The PbI2 precursor acts as a template for the growth of the final perovskite film, resulting in higher surface coverages and better reproducibility of morphology and JV-performance. Especially for a planar heterojunction configuration, a compact and pin-hole free perovskite layer is necessary to avoid any shunting between the n- and p-type sides as well as low light absorption in the solar cell. We present a way to easily control the morphology of pure iodine CH3NH3PbI3 films by varying the dipping conditions of the PbI2 precursor layer into an alcoholic CH3NH3I-solution. We show that final film coverage, smoothness and crystallite sizes can be finely tuned via concentration, temperature, atmosphere and water content of a CH3NH3I/IPA solution, as well as by the thickness of the PbI2 precursor layer and the dielectric constant of the alcoholic solvent itself. Scanning electron microscopy images will show that it is possible to obtain very compact perovskite layers with large grain sizes in the micrometer range and narrow grain boundaries by simply controlling the immersion-conditions. JV-curves reveal that JSC, VOC and fill factors are significantly improved with increasing surface coverage. As these film properties are the main necessity for high performance perovskite solar cells in a planar heterojunction configuration, our results indicate a potentially alternative and easy route compared to e.g. “solvent engineering techniques”.
Lead (Pb)-based ‘halide perovskites’ (HaPs), have shown exceptionally high power conversion efficiencies in recent years. Structural tolerance in these systems allows incorporation of multiple cations and anions within the perovskite lattice, leading to >21% solar-cell efficiencies, which made it ‘the next big thing’ in photovoltaics. However, the toxicity of lead (Pb), which is used in the most studied cells, may affect its large-scale uses. Hence we explore completely Pb-free HaPs such as CsSnBr3, for solar-cell applications. Addition of SnF2 was observed to impart beneficial effects on several parameters of CsSnBr3-based cells, which is our major focus.Cell efficiency was observed to increase dramatically (~200 times) when SnF2 was added to the perovskite deposition solution compared to what is achieved without SnF2, possibly due to a combined effect of (1) filling of Sn2+ vacancies, (2) minimizing oxidation of Sn2+ or, (3) removal of trap states. To understand the effect of SnF2 concentration on PV parameters, various concentrations of SnF2 were added during CsSnBr3 preparation and 20 mol% was found be optimal to achieve improved device parameters such as short circuit current (JSC), open circuit potential (VOC) and fill factor (FF). In order to understand the role of SnF2 on the energetics (work function-WF and top of the valence band-EVBM) of the CsSnBr3 (with and without SnF2 addition), Ultraviolet Photoelectron Spectroscopy (UPS) was monitored on titania (TiO2) and gold (Au) substrates. SnF2 addition was found to aid in the alignment of the bands with some selective contacts in a solar cell, which explains the improvement in the critical cell parameters. Furthermore, continuous X-irradiation was found to be detrimental for pristine CsSnBr3, and resulted in metallic tin (Sn0) formation. However, addition of SnF2 protected the halide perovskite from beam damage, possibly because of a SnF2-induced change in the Sn2+ electrochemical potential that makes the reaction to form Sn0, energetically less probable.
Improvement of the performance of perovskite solar cells and optoelectronic devices may benefit from a better understanding of the intrinsic photophysics of materials. Thin polycrystalline films present a variability of properties connected to their grain structure. In order to investigate the intrinsic properties of CH3NH3PbI3, high quality single crystals are ideally suited. Low temperature photoluminescence spectroscopy of CH3NH3PbI3 single crystals reveals the existence of a sharp free excitonic peak, with a full width at half maximum of only a few meV at 10K.This emission line is completely absent of the thin films spectra which are dominated by trap state emission. We highlight a strong thermal broadening of the free exciton (FE) emission connected to exciton Ì¶ LO-phonons coupling down to low temperatures. Additionally, the FE emission presents a fast, nonexponential decay with subnanosecond characteristic time, connected to the efficient trapping of excitons. In comparison, trap state emission spectra and recombination dynamics are very similar in thin films and single crystals, showing that trap states are likely formed at the surface/interface of material. The results highlight the importance of single crystals as a reference material for the study of hybrid organic-inorganic perovskites.
 G. Grancini, A.R. Srimath Kandada, J.M. Frost, A.J. Barker, M. De Bastiani et al., Nature Photonics (2015), 7 (10), 695-702.
 H. Diab, G. Trippé-Allard, F. Lédée, K. Jemli, C. Vilar et al., J. Phys. Chem. Lett. 10.1021/acs.jpclett.6b02261
This work has been performed within the GOTSOLAR project, which has received funding from the European Union’s Horizon 2020 research and innovation Programme under the grant agreement No 687008. The information and views set out in this abstract are those of the authors and do not necessarily reflect the official opinion of the European Union. Neither the European Union institutions and bodies nor any person acting on their behalf may be held responsible for the use which may be made of the information contained herein.
Organic-inorganic (hybrid) perovskites are interesting semiconductor materials for optoelectronics devices. Beside their application in photovoltaics, perovskites are emerging as promising components of light-emitting diodes (LEDs). In electroluminescent devices, a high photoluminescence quantum yield (PLQY) is desirable in order to efficiently convert the injected charges into photons. Unfortunately, the PLQY of 3-dimensional (3D) perovskites such as methylammonium lead bromide (MAPbBr3) strongly depends on excitation intensity, and reaches high values only at high excitation fluence where radiative bimolecular recombination dominates. On the other hand, trap-assisted non-radiative recombination dominates at low excitation, resulting in a low PLQY (< 5%). As a consequence, LEDs need high current densities in order to produce significant light emission, resulting in a low power conversion efficiency. A strategy to enhance the luminescent properties of perovskite films and hence the efficiency of LEDs is the use of so-called quasi-2D structures. Quasi-2D perovskites consist of mixed cation compounds with stoichiometry (RA)2(MA)n-1PbnX3n+1 where n represents a number of 3D perovskite layers between the more bulky organic linkers RA. The addition of the organic linkers impedes the growth of extended 3D crystals, thereby increasing the radiative recombination rate and hence the PLQY. We present a quasi-2D perovskite where the organic linker is butylammonium bromide (BABr), enabling the assembly in very uniform thin films with reduced particle size, low roughness and very high PLQY (>80 %). Thin-films of these materials have been applied in multilayer LEDs, and the use of energetically optimized charge transport layers together with the passivation of the perovskite surface resulted in current efficiency exceeding 3 cd/A.
The photoconversion efficiency of perovskite solar cells (PSCs) has been enhanced by the deposition of inorganic nanoparticles (NPs) at the interface between the compact TiO2 electron selective contact and the mesoporous TiO2 film. The NPs used have been core/shell Au@SiO2, where a thin SiO2 coating protects the Au core from the direct chemical interaction with CH3NH3PbI3 halide perovskite used as light harvesting material. Samples prepared with the Au@SiO2 NPs exhibits higher external quantum efficiency in all the complete wavelength range at which perovskite presents light absorption and not just at the wavelengths at which Au@SiO2 NPs presents their absorption peak. This fact rules out a direct plasmonic process as the responsible on cell performance enhancement. A detail characterization by photoluminescence, impedance spectroscopy and open circuit voltage decay unveil a modification of the interfacial properties with an augmentation of the interfacial electrostatic potential that increases both photovoltage and photocurrent. This work highlights the dramatic role of interfaces in PSC performance. The use of reduced quantities of highly stable inorganic compounds to modify the PSC interface instead of the extensively used organic compounds opens the door to a new surface engineering based on inorganic compounds
Reproducible Hybrid Halide Perovskites Solar Cells with High Efficiencies Fabricated in Ambient Conditions by Solvent and Additives Engineering
Clara Aranda1*, C. Cristóbal2, Antonio Guerrero1, Juan Bisquert1
1 Institute of Advanced Materials (INAM), Universitat Jaume I, 12006 Castelló, Spain
2 Universidad Nacional de Ingeniería, Lima, Perú
Photovoltaic devices based on lead halide perovskites solar cells, have erupted as strong candidates to compete with already commercially available technologies. Significant efforts have been devoted towards the achievement of high efficiencies leading to 20.9 %, using methylammonium lead triiodide perovskites. Solvent engineering has proved as a key to obtain increased crystallinity of the perovskite layer.1 In this work the coordination ability of different solvents such as DMF, DMSO or GBL is systematically studied.2 It is shown that the coordination ability of the solvent plays a very important role. By controlling the coordination chemistry we show that highly efficient perovskites solar cells can be fabricated (18.6 %) in ambient conditions. This work demonstrates that efficient perovskite materials may be produced even in the absence of an inert atmosphere.
1. Namyoung Ahn, Dae-Yong Son, In-Hyuk Jang, Seong Min Kang, Mansoo Choi and Nam-Gyu Park, Journal of the American Chemical Society, 2015, 137, 8696−8699
2. Sara Rahimnejad, Alexander Kovalenko, Sergio Martí Forés, Clara Aranda and Antonio Guerrero, ChemPhysChem, 2016, DOI: 10.1002/cphc.201600575
Organic-inorganic lead halide perovskites (mainly CH3NH3PbI3) are being extensively studied because their excellent photovoltaic properties, such as suitable bandgap, high optical absorption and long carrier lifetime. To improve their photovoltaic performance, it is important to understand the impact of point defects on their electronic structure. In this work, we report the electronic structure of new CH3NH3PbI3 perovskite derivatives, in which deep defects were obtained by replacing Pb+2 atoms. To deal with the bandgap underestimation problem of common DFT methods, quasiparticle calculations have been applied via the G0W0 approximation. Bandgap value based on GW has considerably improved theoretical results compared to experimental one for the native perovskite. The investigation of the electronic structure of new CH3NH3PbI3 perovskites suggests that the presence of point defects play an important role in the coupling of two low energy photons to achieve a higher energy electron excitation (like in the Z-scheme of photosynthesis), which would maximize the photovoltaic performance.
Acknowledgements. This work was partially supported by the Comunidad de Madrid project MADRID-PV (S2013/MAE/2780) and by the Ministerio de Economía y Competitividad through the project BOOSTER (ENE2013-46624-C4-2-R). The authors acknowledge the computer resources and technical assistance provided by the Centro de Supercomputación and Visualización de Madrid (CeSViMa).
Solar cells are one of the most important routes to harvest renewable energy. The so-called third generation solar cell based on the solution-processed method has attracted significant interests. Polymer solar cells have reached ~10% power conversion efficiency for single-junction bulk heterojunction (BHJ) architectures. However, the efficiency is still too low to reach the point for a large-scale deployment. Recently, organolead halide perovskite-based solar cells demonstrated their high PCE as that of Si-based solar cells. It creates remarkable interests in the world. Nevertheless, poor stability and containing toxic Pb in the structure will be the key barriers for future applications.Various attempts have been under investigation to solve these problems. In this report, we will present a new route for fabrication of solar radiation absorbers by blending a photoactive conjugated polymer with an organolead halide perovskite to create a composite photoactive layer for solar light harvesting. The photoactive polymer did not only contribute to generation of charges, but also enhance stability of solar cells by providing a barrier protection to halide perovskites. Given that versatile of the conjugated semi-conductive polymers and halide perovskites in terms of their properties in light absorbing wavelength, bandgap, and stability, it enables to create various combinations as novel light harvesting materials with wide light absorbance, high conversion efficiency, and excellent stability for future solution-processed solar cells. The morphology and crystallinity of the composite perovskite films were investigated by using AFM, SEM and XRD respectfully. In addition, UV-Vis absorption and photoluminescence PL were used to check the optical properties. The power conversion efficiency of 14.4 % was achieved for the the best devices with JsC of 22.8 mA/cm2, VoC of 0.9 and FF of 70 %
Photovoltaic devices based on lead halide perovskites have drawn the attention of the scientific community due to the impressive power conversion efficiency evolution now reaching 21.6 %.1 So far one of the main limitations that have not allowed this technology to reach commercial applications is the low stability observed under operation conditions indicating that further work in this direction is needed. In this work we identify ion migration towards the interfaces followed by chemical reactivity with the contacts as one of the main degradation mechanisms. First, ion migration is monitored by means of electrical techniques such as cyclic voltammetry or Capacitive measurements.2,3 It is shown that the electrical properties of the ETL/perovskite interface is severely affected by ions accumulation and this is responsible for hysteresis behavior. In addition, chemical reactivity with the external contacts such as the reaction of iodide ions with the oxidized form of spiro-OMeTAD leads to a reduction of conductivity that can be regarded as an important degradation mechanism. In order to avoid chemical reactivity with the contacts it is shown that the introduction of a thin layer of Cr deposited between the external contact and the ETL can enhance the stability of perovskite devices.4
We thank financial support by MINECO of Spain under project (MAT2013-47192-C3-1-R) and Generalitad Valenciana for financial support on the DISOLAR2 Project (PROMETEOII/2014/020). Additionally, A. G. would like to thank the Spanish Ministerio de Economía y Competitividad for a Ramón y Cajal Fellowship (RYC-2014-16809).
1.Saliba, M. et al. Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance. Science 2016. DOI:10.1126/science.aah5557
2.Carrillo, J.; Guerrero, A. et al. Ionic reactivity at contacts and aging of methylammonium lead triiodide perovskite solar cell Advanced Energy Materials 2016, 6 (9), 1502246.
3.Almora, O.; Guerrero, A.; et al. Ionic charging by local imbalance at interfaces in hybrid lead halide perovskites. Applied Physics Letters 2016, 108 (4), 043903.
4.Guerrero, A.; et al. Degradation of Planar Lead Halide Perovskite Solar Cells. ACS nano 2016, 10, 218–224.
2D derivatives of hybrid halide perovskites are currently receiving increasing attention not only for their applications in photo-voltaics but also in other fields for their characteristic high photoluminescence and color tunability. The hydrophobic long chain cations act as an asset for their higher stability. The low dimensionality of the inorganic layer in 2D perovskites leads to quantum confinement effects and thus high exciton binding energy. Also, the low dielectric constant of organic cations leads to lower screening and thus lesser separation of charges. In this study we have tried to design new 2D perovskites by substituting the long organic cations with highly electron deficient cations targeting charge separation. We have performed DFT and TD-DFT calculations with lead-iodide inorganic layers coupled with electron-deficient naphthalene diimide dibutylammonium (NDI-dbu), and perylene diimide dibutylammonium (PDI-dbu)) cations. Subsequently the cations were substituted with electron donating and withdrawing groups. Through this study we demonstrate the effects of electron deficient linker groups in between the inorganic layers on the electronic structure of the 2D perovskites. Our study shows that the electron deficient organic cations have low-lying electronic levels and thus lead to small band gap. The pi-conjugated core of the organic molecules dominates the energy states of the conduction band whereas the covalently linked inorganic layer dominates the valence band. This has implications on the charge mobility of electrons and holes. The substitution of the electron donating groups at the bay area leads to an increase in the band gap of the material. With higher conjugation in the organic cation, the effective masses in the perovskite layer tend to decrease. We have also looked into the absorption properties of these materials and found increased absorption for higher conjugated organic cations.
Physical modeling of hysteretic behavior in I-V curves of perovskite solar cells (PSCs) is necessary for further understanding of power generation mechanism and improvements of power conversion efficiencies (PCEs). In some cases, reduction of hysteresis in planar structure PSC (p-PSCs) has been achieved by using PCBM layer mostly for inverted structure. In such cases, the opposite trend of the I-V hysteresis, where the forward scan shows higher efficiency than that of reverse scan has been observed. In this paper, an equivalent circuit model with inductance was newly proposed. This model consists of Schottky diode (SBD) involving parasite inductance focusing PCBM/Al(Ca) interface and well represents the opposite I-V hysteresis of the p-PSC with inverted structure. Furthermore, based on the equivalent circuit we have constructed physical real device composed of 2 Si photo diodes and a diode, an inductor and resistor. Photovoltaic response for modeled perovskite solar cells based on equivalent circuit with 2 Si photo diodes, a diode, an inductor and resistor. The I-V curves for this device well supported our equivalent circuit having 2 Si photodiode and SBD diode, and conclude that the inverted trend of hysteresis is coming from the inductance originated from the interface PCBM/Al(Ca).