Enormous efforts have been made in the last 8 years to up-grade the performance of lead halide hybrid perovskite solar cells (PSCs).1 Although efficiency level has reached 25%, PSCs face serious challenges of practical stability and durability required for industrialization. Compositional engineering of lead halide perovskites by mixing different cations and anions, using modulator molecules and mixing 2D and 3D structures have improved the stability of perovskites against heat and moisture. However, organic cations in halide perovskites (methylammonium, etc.) and use of diffusible ionic dopants in hole transport materials (HTMs) are responsible for low stability of perovskites at high temperatures (>120oC). In this respect, use of all-inorganic perovskite materials and dopant-free HTMs is highly desired.2 We have conducted some work in this direction which includes stabilization of CsPbI3 black phase3 and use of dopant-free HTMs. We could show that PSCs with all-inorganic perovskites and dopant-free HTMs are capable of efficiency up to 15%.2 Further, Open-circuit voltage was found to be enhanced over 1.4V. In preparation of all-inorganic perovskites, a big challenge should be directed to development of lead-free perovskite materials for environmental safety in practical applications.2

Among extensive applications of PSCs for outdoor and indoor power generation, use of PSCs in space environments is also promising because thin perovskite photovoltaic films demonstrate high stability and tolerance against exposure to severe space environment having high energy particle irradiations (proton and electron beams).4 Thin absorbers (<500 nm) avoid accumulation of particles and due to intrinsic defect tolerant nature of perovskites, radiation-induced collision damage is highly suppressed. Our current efforts in making PSCs based on both lead and lead-free perovskites, and future perspectives of perovskite photovoltaics will be introduced.

REFERENCES

1. A. K. Jena, A.，Kulkarni, T. Miyasaka, Chem. Rev. 2019, 119, 3036–3103.

2. T. Miyasaka, A. Kulkarni, Gyu Min Kim, Senol Öz, and A. K. Jena, Adv. Energy. Materials 2019, 1902500, 1-20.

3. A. K. Jena, A. Kulkarni, Y. Sanehira, M. Ikegami, and T. Miyasaka, Chem. Mater. 2018, 30,

6668-6674.

Tin oxide (SnO2) is a promising material for the electron transport layer in planar perovskite solar cells (P-PSCs) due to its suitable energy level and high electron mobility. SnO2-based P-PSCs show the highest power conversion efficiency among planar structure devices, but the PCE remains still low compared to the mesoporous TiO2-based PSCs and there is a lack of thermal stability study. In this study, we develop a simple interface engineering to improve optoelectronic properties and the thermal stability of the P-PSCs. The modified SnO2 shows high conductivity, effective charge extraction ability and high recombination resistance. Empirically, efficiencies of 21.43% and 20.5% were reached for the device with doped Spiro-OMeTAD and with dopant-free asy-PBTBDT, respectively, in the present study. The devices with modified SnO2 show excellent stability under mild (humidity of 25%) and harsh conditions (humidity of 85%; temperature of 85°C). Thus, our newly developed method guarantees highly efficient and thermal stable P-PSCs.

solar cells, with efficiencies exceeding 22%, are presently the forerunner amongst the next generation photovoltaic technologies. However, in the past year, further efficiency improvements have decreased significantly. For any new efficiency breakthroughs, fresh perspectives and approaches must be developed. Recent discoveries of slow hot carrier cooling phenomenon in halide perovskites [1-3] revealed that such perovskites are highly promising hot-carrier absorber materials capable of unlocking disruptive high-efficiency hot-carrier photovoltaics to overcome the Shockley-Queisser limit. In this talk, I will trace the developments on the slow hot carrier cooling properties of halide perovskites beginning with the discovery of slow hot hole cooling in 2013. [1] Our group further uncovered that hot electrons cooling is slow as well and is relatively balanced with the hot holes. [4] I will also focus on our group’s efforts: (i) to further retard the hot carrier cooling using perovskite nanoparticles; (ii) to achieve efficient extraction of the hot carriers; [3] and (iii) to understand origins and mechanisms of slow hot-carrier cooling in halide perovskites [5]. Our latest findings in multiple exciton generation and hot carrier trapping dynamics will also be discussed.

References

G. C. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Graetzel, S. Mhaisalkar and T. C. Sum*, “Long-Range Balanced Electron and Hole Transport Lengths in Organic-Inorganic CH3NH3PbI3”, Science, 342 (6156) 344-347 (2013),

Y. Yang, David P. Ostrowski, R. M. France, K. Zhu, J. van de Lagemaat, J. M. Luther & Matthew C. Beard, “Observation of a hot-phonon bottleneck in lead-iodide perovskites”, Nature Photonics 10, 53–59 (2016)

M. J. Li, S. Bhaumik, T. W. Goh, M. S. Kumar, N. Yantara, M. Graetzel, S. G. Mhaisalkar, N. Mathews, and T. C. Sum*, “Slow Cooling and Highly Efficient Extraction of Hot Carriers in Colloidal Perovskite Nanocrystals”, Nature Communications 8 14350 (2017)

T. C. Sum*, N. Mathews, G. Xing, S. S. Lim, W. K. Chong, D. Giovanni, H. A. Dewi, “Spectral Features and Charge Dynamics of Lead Halide Perovskites: Origins and Interpretations (Invited Article)”, Accounts of Chemical Research 49 (2) 294-302 (2016),

J. Fu, Q. Xu, G. Han, B. Wu, C. H. A. Huan, M. L. Leek & T. C. Sum*, “Hot carrier cooling mechanisms in halide perovskites”, Nature Commununications 8:1300 (2017)

Solution-processed thin films of lead halide perovskites show exceptional properties, making them interesting candidates for a range of optoelectronic applications such as solar cells and light-emitting diodes. The thickness and crystallite-size in these films is typically in the sub-micron range, resulting in an abundance of grain-boundaries. Here we will describe techniques for growing thin single-crystalline lead perovskite crystallites with edge lengths of several tens of microns and thicknesses of around 1 micron, starting from single-source 1D lead halide perovskite precursors. In a first study we used platelet-shaped MAPbBr₃ single-crystals to fabricate active electro-optical modulators (AEOM) exhibiting > 98% light transmission intensity modulation. In a second study we successfully grew mixed halide MAPbBr_(3-x)I_x single crystals to study photoinduced halide demixing in crystalline materials, using a range of different spectroscopic techniques such as narrowband fluorescence imaging and time-resolved spectroscopy. In the final section of this talk we will report on the recent progress in the development of back-contact perovskite solar cells, including the use of mask-free lithography techniques and novel honey-comb-like back-contact architectures.

Tetrabenzoporphyrin (BP) is a p-type organic semiconductor characterized by the large, rigid p-framework, excellent stability, and good photoabsorption capability. However, the extremely low solubility of BP hampers solution processing not only for thin-film preparation, but also for chemical modification and purification. Ono and coworkers judiciously circumvented the solubility issue via the use of a thermally convertible precursor, CP[1]. CP quantitatively transforms to BP upon heating by releasing ethylene, an easily removable gaseous compound, as a solo by-product. Accordingly, a high-purity thin film of BP can be prepared by depositing CP as a solution and then heating it to effect the retro-Diels–Alder reaction in situ. However, the control of the solid-state arrangement of BP frameworks, especially in solution-processed thin films, has not been intensively explored, and charge-carrier mobilities observed in BP-based materials have stayed relatively low as compared to those in the best organic molecular semiconductors. This work concentrates on engineering the solid-state packing of BP derivatives toward achieving efficient charge-carrier transport in its solution-processed thin films[2].

In the last decades to reproduce an AM1.5G spectrum, matching the A class, scientists were practically obliged to use expensive solar simulators mounting complex optics and using xenon arc bulbs as light sources. Such light source has multiple disadvantages and to this day spare bulbs are still quite expensive and with a limited life time (1000 h-1500 h) that make such technology not worth for long life testing, large area spot or frequent switching on and off.

In the last 5 years LED technology improved drastically and in the face of a constant improvement in quality, reliability and an increase in radiant power, a remarkable reduction in prices was observed, especially concerning critical bandwidth as the ones between 350 nm and 400 nm and the between 700 nm and 1100 nm. These allowed a very fast development of high efficient, long life, stable but expensive Solar Simulators based on LEDs technology.

Greatcell Solar Hyperion is, without doubts, a breakthrough on LED Solar simulators technology as it assures a high reliability, high accuracy and long life (>15000 h) at a very competitive price. Greatcell Solar Hyperion matches triple A+ class, respecting the most stringent international standards, over 16 cm X 16 cm spot (making it compatible i.e. with standard C-Si solar cells) and a triple A class over 22 cm X 22 cm illumination area, making it suitable to test multiple small and large areas devices.

Thanks to the innovative concept of Greatcell Solar Hyperion, it was possible to contain electrical consumption keeping the latter below a remarkable value of 600 W/h. Furthermore, through a dedicated driving software, it is possible to customise Hyperion spectrum modifying individually each one of the 20 LED families mounted. Greatcell Solar Hyperion LED solar simulator is was conceived as a modular structure, this opens the possibility to expand the illumination spot to host much larger substrates respect standard lab ones.

Greatcell Solar Hyperion can surely be considered a unique sun light emulator, capable to reproduce solar spectrum in a very faithful way as never done before with traditional xenon arc lamps sun simulators.

Light Fidelity (Li-Fi) have been show the next-generation communication system with the features of high security, high speed and low interference. It is of great urgency to exploration of corresponding devices such as light-emitting diodes (LEDs), image sensors, opto-couplers, photodiodes, microcontrollers and non-volatile flash photomemory. Among the plethora of derivatives in optical wireless communications, non-volatile flash photomemory is of particular interest since it is the essential building block of computation technology for nowadays big database storage device with ultrafast programming rate. Herein, versatile photoactive material with exotic photo-physic properties have been utilized as floating gate to develop the ultrafast responsive non-volatile flash photomemory. Time-resolved photoluminescence and cryogenic photoluminescence have been revealed the fact that the charge transfer rate between photoactive layer and active channel. Through the delicate material selection and morphology manipulation, the devices possess wavelength/power/illumination time-distinguishable multilevel behavior within very short photo-programming time down to~ 1 ms. The ultrafast programming time of non-volatile photomemory trigger the realization of Li-Fi in the near future.

Colloidal quantum dots (CQDs) are attractive semiconducting materials for broad optoelectronic devices, serving an ease of device fabrication compatible with low-temperature processing and low-cost deposition methods. Fabrication of CQD thin-film transistors (TFTs), is so far finalized with conventional thermal annealing, which requires prolonged processing time; yet it leaves insulating organic residues behind, suppressing charge carrier mobility (~10^-2 cm²V^-1s^-1). Here we demonstrate a millisecond heat treatment of lead sulfide (PbS) CQD films using photonic curing, which results in high carrier mobility in TFT devices. In comparison with conventional thermal annealing, the photonic curing is found to effectively suppress organic residues, yet it preserves quantum confinement in the films. With increasing flashing power, the electron mobility on SiO₂-gated devices increases up to 0.2 cm²V^-1s^-1, attributed to suppression and decomposition of organic residues, as revealed from ellipsometry and x-ray photoelectron spectroscopy (XPS) measurements. At high flashing power, the electron mobility decreases which is associated with the formation of traps and hole doping in the films. According to XPS analysis, we find that some of Pb atoms are ablated from the films, responsible for hole doping and dangling bond formation at high flashing power. Finally, we operate the PbS CQD TFTs with top gate configuration using polymethyl methacrylate (PMMA) and high capacitance polymer dielectrics, with their good interface and superior trap filling, enabling us to obtain high electron mobility of 0.48 and >1 cm²V^-1s^-1, respectively. Our works show the great importance of photonic curing for the fabrication of high performance CQD TFTs, standing out as effective replacement of conventional thermal annealing, which is potentially applicable for other optoelectronic applications such as CQD photodetectors and solar cells.

The field of photovoltaics has seen significant developments in recent years. Among the available technologies, dye sensitised solar cells (DSSCs) have attracted considerable attention[1]. Here, the majority of the research efforts are centred on the dye sensitizer, since it influences many of the key electron transfer processes that impact photovoltaic performance. Designing new dyes requires various aspects (wide absorption in the visible region, high molar extinction coefficients, photochemical stability, stable oxide surface anchoring etc.) to be taken into account. The traditional routes adopted thus far involve the determination of relevant properties of a large number of potential candidates, via high-throughput experiments or computations[2]. This approach, however, is laborious and time-consuming.

With a view to streamline the design process, we have combined data-driven approaches with Darwin-inspired evolutionary algorithms[3,4,5]. While the former makes use of statistics and machine learning to provide on-demand property estimates, the latter optimises constituent fragments of the dye molecule in a synthetically tractable manner, leading to the design of dyes with given target properties[6]. In this presentation, we demonstrate how data-driven molecular engineering can be gainfully employed to accelerate materials discovery. We will also present other applications of the method for designing dye-based optical filters for low cost fluorescence detection.

Near-infrared (NIR) light emitting diodes (LEDs) are useful in a wide range of applications including night-vision devices, optical communication, biomedical imaging and medical treatments. The NIR emitters like organic compounds and colloidal quantum dots (QDs) have been widely investigated. However, their less charge transport ability and luminescence efficiency limit the improvement of external quantum efficiency (EQE) in NIR LEDs, which is still far from practical application. As compared with QDs or organic semiconductor based LEDs, the film-based structure with high carrier mobility, such as lead halide perovskites, enhances radiative recombination by minimizing charge trapping losses, resulting in higher EQE value in LEDs. Lead halide perovskites also exhibit significant potential for applications in LEDs because of their high color purity and a narrow full-width at half-maximum (FWHM) over the entire visible light spectrum, as well as their low-cost solution processing without high temperature treatments, however, pure lead based perovskite LEDs only emit below 800 nm. In this report, we present a new approach for developing highly efficient NIR LEDs based on an energy transfer system composed of all inorganic perovskite (CsPbCl₃) film as an energy donor and ytterbium ions (Yb³⁺) as an acceptor. The NIR emission of Yb ion at around 1000 nm is found to be sensitized by CsPbCl₃ in a thin film structure with a photoluminescence quantum yield over 60%. The Yb³⁺:CsPbCl₃ based LEDs also exhibit a bright electroluminescence around 1000 nm with the highest EQE (~6.0%) ever reported for NIR LEDs capable of emission beyond 900 nm, which was achieved by high carrier transporting ability and effective sensitized emission property in the solid-film structure.  

Recent reports on dye-sensitized solar cells (DSSCs) under ambient light conditions ^[1][2] have reopened the path towards widespread indoor photovoltaics. DSSCs possess promising features such as solution-based fabrication, low-toxicity, colour-tunability and light-weight. However, challenges remain including the appropriate choice of the electrolyte or solid hole conductor. A 2015 report by Freitag et al.^[3] proposes the use of a copper phenanthroline electrolyte towards the development of efficient liquid- and solid-state DSSCs. Their solid-state DSSCs are obtained by slow evaporation of the electrolyte in liquid-state cells.

Originating from the above work, we have studied the co-sensitization effect of two organic dyes in a copper bipyridyl electrolyte system.^[4] A high-performing costly dye (XY1)^[5] was co-adsorbed with a moderately-performing cheaper dye (5T)^[6] to achieve similar (9.5% at 1 sun) or superior (10.2% at 0.1 sun) performance to the XY1 dye alone. This translates to a 1.4-fold increase in cost performance of the sensitizer. Photophysical measurements suggested that the co-sensitized devices (XY1+5T) have better dye coverage and exhibit longer electron lifetimes especially at lower light intensities. Further testing of XY1+5T under indoor fluorescent lighting revealed a power conversion efficiency (PCE) of 29% at 1000 lux, which is among the highest reported under similar conditions. We believe our findings revalue the co-sensitization method as a cost-reduction strategy for DSSCs. Proposals towards more practical DSSCs (e.g. solid-state) will also be discussed during the talk.

With the emergence of low-power electronic devices such as wireless sensor nodes, the Internet of Things (IoT) is rapidly developing and spreading. While the IoT has significant potential to benefit our lives, commerce, and industry, the use of large number of wireless or portable devices would need to be powered by distributed energy sources at the lowest possible cost, especially in regions that are limited to the range of a few milliwatts. However, conventional batteries, such as coin cells, require periodic replacement and maintenance, thus spurring the demand for emerging energy-harvesting systems that utilize ambient energy to semipermanently operate indoor electronic devices.

 Thus, various methodologies have been proposed for harvesting energy from local environments, including light, heat, movement/vibration, and electromagnetic waves. Of these technologies, photovoltaic (PV) energy conversion has shown great potential because of its higher energy density and output voltages. Thus, recent developments of PV devices not only target conventional solar cells under 1-sun illumination (i.e., 100 mW cm⁻²) but also dim-light indoor applications (with a factor 100–1000 lower incident power). As sunlight is not available always and at all locations, artificial indoor lighting sources, such as white light-emitting diodes (LEDs) and fluorescence lamps, can steadily supply small amounts of energy for powering sensors and electronic components inside buildings. Unlike the AM 1.5G solar spectrum, typical indoor illumination spectra are limited to the visible wavelength region, as they are optimized for the human eye. Here, eco-friendly organic photovoltaics (OPVs)^9,10 serve to be a promising indoor energy-harvesting technology, which offer various inherent advantages such as lightweight, mechanical flexibility, solution processability, and cost-effective large-area manufacturing capability. Additionally, the characteristics of OPV materials such as a high absorption coefficient and a tunable absorption range are well suited for indoor applications when compared to robust inorganic silicon devices. Recently, some research groups, including us, have investigated whether OPVs can demonstrate indoor performance superior to that of silicon solar cells, and the results revealed that OPVs indeed show higher power conversion efficiencies (PCEs) under white LED lighting conditions. It is necessary to further optimize OPV materials and devices, and thereby improve their PCEs for future practical applications. Nevertheless, thus far, only a few OPV materials including representative semiconducting polymers and low-bandgap small molecules have been tested for their potential in terms of organic energy-harvesting devices; thus, a systematic study on indoor PV characteristics and mechanism is still lacking.

In this presentation, we propose some small-molecule based light absorbers and evaluate their OPV performances using a binary bulk-heterojunction (BHJ) system with fullerene acceptor under LED illumination with different illuminances (200–10000 lx). Furthermore, we demonstrated flexible energy-harvesting modules of ~10 cm² under dimly lit conditions of 200 lx. Our research paves the way for practical indoor applications of solution-processed OPVs.

Recent advances in atomically thin two dimensional (2D) transition metal dichalcogenides (TMDs) have shown remarkable impacts in optoelectronic technologies which span from photodetectors, field-effect transistors, sensors, and solar cells. Several methods have been reported to synthesize atomically thin TMDs which include mechanical cleavage, chemical vapour deposition and liquid phase exfoliation (LPE). Amongst them, LPE is the most promising one since it serves as a comparably low-cost, fast and simple method, which results in high yield suspension of TMD nanosheets. LPE also enables film deposition by a solution-phase process for instance by printing or coating methods. Here we demonstrate the synthesis of MX₂ nanosheets (WS₂ and MoS₂) using LPE in aqueous ammonia (NH₃(aq)) and their application as hole transporting layer (HTL) in organic solar cells. By employing this technique, we are able to produce stable dispersion containing monolayer and multilayer  MX₂ nanosheets with lateral sizes ranging between 10s-100s nanometers, for both MoS₂ and WS₂. The low boiling point of the NH₃ and water makes them more favourable than the commonly used high boiling-point solvent such as N-Methyl-2-pyrrolidone (NMP) or N-Cyclohexyl-2-pyrrolidone (CHP). Hence no high-temperature post-deposition annealing is required. The successful exfoliation of TMDs is shown via atomic force microscopy (AFM), transmission electron microscopy (TEM) and Raman spectroscopy. The semiconducting nature of the MX₂ nanosheets is revealed by X-ray photoelectron (XPS), UV-Vis and photoluminescence (PL) spectroscopies. Finally, we integrated the synthesized nanosheets in non-fullerene organic solar cells (NF-OSCs) as an HTL replacing the hygroscopic and acidic poly(3,4-ethylene dioxythiophene): poly(styrenesulfonate) (PEDOT: PSS). Ultraviolet photoelectron spectroscopy (UPS) shows that the work function of ITO increases upon deposition of TMDs. Consequently, the thin layer of MX₂ can improve contact with the active layer and hole extraction. The photovoltaic measurements show that integration of the MX₂-based HTLs in NF-OSCs can enhance the performance of the devices. PCE of the NF-OSC with WS2- and MoS2-based HTLs can be increased to 11.8% and 11.4%, respectively. These results are comparable to conventional PCE of PEDOT:PSS-based NF-OSCs. This study carries great importance of 2D MX₂ for the performance enhancement of organic solar cells. Furthermore, the simplicity of the synthesis of MX₂ by LPE in NH₃(aq) and their deposition make the approach feasible to implement in broader applications.

Fabricating solar cells with tandem structure is an effective way for increasing the power conversion efficiency (PCE) beyond that of single-junction organic photovoltaics (OPV). To obtain high PCE of the tandem OPV, carefully engineered front-cell and back-cell are required. However, wide-bandgap materials for front cells that have both high short-circuit current density (J_SC) and open-circuit voltage (V_OC) are scarce. In this contribution, we developed and studied two new acceptor molecules namely IDTA and IDTTA with optical bandgaps (E_g^opt) of 1.90 and 1.75 eV, respectively. When blended with wide bandgap polymer PBDB-T, single-junction cells with PCE up to 7.4% for IDTA and 10.8% for IDTTA (with high V_OC = 0.98 V, J_SC = 15.8 mAcm^-2, and FF = 70%) are demonstrated. The latter PCE value is the highest reported to date for wide-bandgap (E_g^opt ≥ 1.7 eV) OPVs. Our detailed transport and recombination studies show that the improved charge transport in IDTTA-based cells leads to higher fill-factor and improved charge generation than IDTA-based devices. Moreover, IDTTA-based OPVs show improved shelf lifetime and thermal stability. Nanoplasmonic spectroscopy analysis reveals that the PBDB-T:IDTTA layer exhibits significantly higher glass transition temperature, which could explain its superior thermal stability at 80°C. Finally, with the aid of optical-electrical device simulation, we were able to combine PBDB-T:IDTTA as the front-cell with PTB7-Th:IEICO-4F as the back-cell in tandem OPVs and demonstrate cells with PCE = 15%, open circuit voltage of 1.66 V and short circuit current of 13.6 mA/cm²; in good agreement with our theoretical predictions. These results highlight IDTTA as a promising acceptor for high performance tandem OPVs.

The stability of final setups is one of the prominent challenge for photovoltaic applications. There is thus a vital need for a better understanding of the degradation mechanisms and thereby the possible mitigation strategies. Flexible organic photovoltaic (OPV) modules are commonly encapsulated by two gas-barrier films to prevent moisture and oxygen degradations. Although this encapsulation process can significantly be diversified, its effect on both initial and in-use performances are scarcely described in the literature. In addition, several on-site studies showed that the mechanical degradation could be more critical on optoelectronic effects than the photo-chemical counterpart. It seems that the stability of the overall complex device architecture may be altered according to several very dissimilar mechanisms: 1- within the active components, 2- at the layers’ interfaces and 3- from the external envelop. Presented work focuses on the last two scales using different ageing conditions (in inert atmosphere, and in severe 85°C/85%RH conditions). For this purpose, two variants in the encapsulation process are compared: the roll-to-roll lamination of a pressure sensitive adhesive and the vacuum lamination of a hot-melt thermoplastic.

The adhesion between the different layers within the device is a key factor for the development of flexible OPV devices reliable after all the processing steps, and during in-use. We here propose a way to individually quantify the adhesion strength of each interface in samples. The 180° peeling test mechanical characterization was adapted for and then applied to the flexible devices. In addition, non-destructive imaging characterization techniques were developed: the laser-beam induced-current mapping, and the luminescence emission imaging under optical and electrical excitation. These latter techniques largely confirmed the hypothesis of a mechanical degradation during the roll-to-roll lamination process.

All these investigations revealed two weak interfaces and several methods have been tested to try and enhance them. We show that improving these interfaces directly increases both the overall performance of the device, and its resilience to roll-to roll encapsulation. Using the imaging techniques previously developed, we will finally study the stability of the encapsulated OPV devices during accelerating aging tests in inert atmosphere at 85°C and 85°C/85%RH. An occurring aging mechanism was proposed, which allows one to explain the localization of the degradation but also the failure type, either optical or electrical.

Nickel oxide (NiO_x) has stood out as an excellent hole transporting material (HTM) for perovskite solar cell (PSCs). Its high optical transmittance, and matching valence band energy level, along with the high stability in ambient atmosphere, gives NiO_x an edge over the other popularly used HTM’s for PSCs. Despite these advantages, the power conversion efficiency (PCE) values for NiO_x HTM based devices lag significantly behind the high performing cells based on organic HTMs such as spiro-OMeTAD, and PTAA. Amongst the various factors, affecting the device performance, one of the major influence comes from the nature of substrate, which plays an influential role in governing the interfacial properties of the device as well as the bulk properties of the perovskite films. 

In the current study, we attempt to understand the possible reasons for low efficiency of sputtered NiO_x based PSCs, by investigating how the NiO_x under layer is directly affecting the growth and crystallisation of compositionally engineered perovskite film and study its correlation with the perovskite optoelectronic properties and device performance. In addition to varying perovskite composition, we also employed different deposition conditions to probe the relation between the crystal growth and charge carrier dynamics at the perovskite/NiO_x interface. This work further emphasizes on the importance of specific deposition conditions and compositionally designed perovskite that are suitable for enhancing the efficiency of sputtered NiO_x based perovskite solar cells. A deep insights into direct correlation of different deposition conditions, and growth mechanism of perovskite with device long term stability against ambient atmosphere (~50-60% humidity) and continuous light illumination will also be covered.

  Organometal halide perovskite materials have emerged as the strong candidates for the next generation photovoltaic materials. The power conversion efficiency (PCE) of perovskite solar cells (PSCs) has reached 25.2%, which is the highest efficiency in thin-film photovoltaics.[1] To further boost the PCE of PSCs, we previously proposed that the replacing the widely-used sandwich structure with a new back-contact structure would lead to PSCs with higher efficiency, lower cost, and better compatibility for upscaling.[2] However, since the distance between two electrodes is longer in back-contact structure than that in the traditional sandwich structure, the performance of back-contact PSCs is significantly affected by the grain boundaries in the charge transporting path.

  Here, we report that the performance of back-contact PSCs can be boosted by forming high-quality perovskite films with large crystal size on the back-contact substrates. The results of photoluminescence microscopy explicitly demonstrate that the charge collecting efficiency is largely improved when the crystal size is close to or larger than the distance between two adjacent electrodes. I will discuss the film-formation method and the performance of the devices in the presentation.

Introduction

There are two methods fabricating the perovskite solar cells. In general, solution process that adds anti-solvent to spin coating solution MAPbI3 is easy to use. The process of solution shows low cost, but is difficult to control. The whole device manufacturing leads to low error tolerance. Another method of perovskite is thermal evaporation, which makes mass production possible and has high error tolerance, but it requires more time to fabricate and needs more cost.

Fabrication

Combining the advantages of thermal evaporation and solution process is the new method which has been created by our laboratory, called sandwich evaporation technique (SET).

PEDOT:PSS was spin-coated on ITO conductive glass. Then MAI/DMF solution was prepared and spin-coated on ITO substrates coated with PEDOT:PSS. Then PbI₂ was evaporated and MAI powder was evaporated in low pressure chamber to complete the active layer. Finally, PC₆₀BM was spin-coated as the electron transport layer. Then BCP was evaporated as the surface modification layer and Ag electrode respectively.

Results and discussions

Based on the above steps, we found that crystallization of perovskite improved when kept in low vacancy lower than 0.13 Pa. In order to further improve the quality, the process was applied to SET process to slow down the reaction rate.

As shown in the SEM (Scanning Electron Microscope), we compared the cross-section of devices that just finished and have been controlled at 0.13Pa for several days. The cross-section of the perovskite that has just been fabricated in Fig. (a) is not flat. The Electron-hole pairs will recombine in the crevice, which seriously affects the performance of device. The crystalline grain shown in fig. (b) after several days of preserving under low vacancy has relatively better flatness. The unreacted perovskite in the evaporation process can continue to finish completely, and the sandwich structure in MAI-PBI₂-MAI further promoted the full progress of the reaction. Finally, the highly quality perovskite solar cells were formed.

The PL (Photoluminescence Spectroscopy) of perovskite which fabricated with SET also first increased for several days and reached a maximum intensity when kept at low vacancy for several days.

CH3NH3Iaq.⇋CH3NH2aq.+HIaq.

4HIaq.+O2⇋I2s+H2Oaq.

2HIaq.⇋H2g+I2s

The intensity of X-ray we measured was increased from 1^st day to 7^th day and we got the maximum X-ray pattern and 7^th day.

The absorption of our samples had the same phenomenon. And the increasing absorption, explained that the improvement of the short current Jsc. Hence we got the highest Jsc , 23.66 mA/cm² at 7^th day.

Conclusion

The MAPbI₃ formed with MAI-PbI₂-MAI was successfully applied to the SET process. MAPbI₃ film first Further optimized the crystalline by controlling at low-pressure. The continuous and uniform perovskite layer have good performance for the 0.91V of V_oc, 23.66 mA/cm² of J_sc, 76.4% of fill factor, and 16.51% of efficiency. We believe that the SET process shows the great potential and has universal use in any of ABX₃ perovskite solar cells’ fabrication.

Perovskite solar cells (PSCs) are one of the most promising future solar cell candidates because they show high power conversion efficiencies (PCEs) approaching those of silicon- and compound-semiconductor based solar cells. Moreover, PSCs can be produced by a low-temperature processing and coating method, so they can be fabricated on flexible substrates such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) films. This capability would make roll-to-roll processes possible, meaning that PSCs can potentially realize low-cost, flexible, lightweight, and large-area perovskite photovoltaic (PPV) modules that can be installed on roofs with low load-bearing capacity, curved surfaces, walls, and windows. However, perovskite films generally exhibit low reproducibility, and it is difficult to fabricate large, pinhole-free homogeneous perovskite films. It is therefore crucial to establish fabrication processes that realize both large and highly efficient perovskite PPV modules.

In order to fabricate uniform layers with low thickness fluctuation over a large area, we have been developing the “meniscus coating method” [1, 2]. The perovskite photoactive layer and buffer layers of the film-based PPV modules were fabricated by this coating method. It is difficult to achieve low electrical resistance between adjacent cells in organic film-based solar modules fabricated using mechanical scribing, because organic film substrates are so soft that the mechanical scribing blades can easily damage them. To address this issue, we thoroughly investigated low-pressure mechanical scribing conditions, and successfully removed the layers on an ITO electrode without damaging either the ITO electrode or the PEN film substrate. Additionally, we replaced the amorphous ITO with crystallized ITO to reduce series resistance by changing the indium-to-tin ratio.

Using the above-described meniscus coating method and experimental results, we achieved a PCE of 11.7% in a film-based PPV module with a designated area of 703 cm², as measured by the National Institute of Advanced Industrial Science and Technology (AIST). This result is described in the “solar cell efficiency tables (version 54)” [3].

Carbon based Perovskite Solar cells (C-PSCs) have emerged in recent years as the most promising candidates for commercialisation in perovskite photovoltaics. Constructed by screen-printing mesoporous Titania, Zirconia and Carbon and subsequent perovskite infiltration, these devices are highly stable, low cost and make use of easily scaled manufacturing techniques. However, while the lack of an expensive hole transport material decreases device cost and enhances stability, the limited conductivity of carbon electrodes also inhibits performance and represents a significant barrier to commercial application.

This work presents a method for electrode conductivity enhancement through the application of Aluminium and Copper grids to prefabricated C-PSCs. Adhered to cells using a low temperature carbon ink, metallic grids were found to dramatically reduce electrode series resistance, leading to a large improvement in Fill Factor and efficiency enhancement of up to 23.3 % in 1 cm² devices. Average power conversion efficiencies (PCEs) of 13.15 ± 0.12 % and 12.83 ± 0.06 % were obtained for Copper and Aluminium respectively, up from ~11 % pre-application. Performance is also significantly augmented in the case of larger-scale 11.7 cm² modules, where PCEs went from 7.73 % to 9.97 % and 7.70 % to 11.05 % for Aluminium and Copper grids respectively. 

Grid structures are easily applied as the mesh structure allows ink permeation and hence top down deposition. The resultant bilateral coverage protects the grid and imparts mechanical stability without damaging the underlying layer. This technique offers a fast, cheap and low temperature route to high-performance, large-scale stable C-PSCs and could therefore have serious potential for application to the high-volume manufacture of large-scale perovskite devices.

Perovskite solar cells based on carbon materials as the back electrode (C-PSCs) have attracted significant attention due to their low cost and excellent stability. In general, the device structure of based C-PSCs has two types, one is hole transport layer (HTL)-free and another one is with hole transport layer. For these two cases, the carbon paste electrodes (CPEs) both need directly collect the photo-generated holes or/and transport the holes. Therefore, the interfacial engineering between the perovskite layer and carbon electrode layer plays a crucial role in charge collection and affects the performance of C-PSCs. 

    Our group have been carried out the interfacial engineering between carbon layer and perovskite layer using several methods to improve the performance of the C-PSCs. In this paper, we will report our recent results, including development a simple process for fabricating an effective sandwich structured perovskite solar cell based on the carbon material and doing the interfacial engineering between the perovskite layer and carbon electrode layer for perovskite solar cells.

Perovskite solar cells (PSCs) using organic-inorganic halides as a light-absorbing layer exhibit high power conversion efficiencies (PCEs; 25.2%) in small-area solar cells [1]. The film quality dramatically affects the performance of PSCs and depends on the fine structure of the perovskite layers, including the crystallinity, surface coverage, and roughness. Varied fabrication methods have been employed including a one-step and, a sequential two-step process which can be either solution or vacuum processed for perovskite thin-film preparation [2]. Currently, mainly one-step spin-coating with an antisolvent is used to precipitate perovskite thin films. This method can fabricate relatively uniform and compact perovskite layers that lead to significantly increased PCEs of the PSCs [3,4], but this method requires technical skill, and a large volume of antisolvent such as toluene and chlorobenzene have an enormous environmental impact. In this conference, we report the fabrication of methylammonium-free FA⁺ and Cs⁺ mixed cation perovskite thin films with micrometer-sized grains using a simple one-step spin-coating method with the use of two additives and without an antisolvent [5,6].

 Furthermore, this organic-inorganic perovskite thin films were treated with a fluorinated ammonium iodide solution, and the thin fluorinated two-dimensional layer was formed on the surface, resulting in a significant improvement in photoluminescence intensity. When applied to PSCs, untreated PSCs exhibited PCEs of 18.6% and 17.0% during the reverse and forward scanning, respectively. The PSCs treated with the optimal concentration of fluorinated ammonium iodide showed improved open-circuit voltage, fill factor, and PCEs of 20.6% and 18.5% during reverse and forward scanning, respectively.

 Also, we report the fabrication of perovskite thin films with a fluorinated polymer (FP) additive via a one-step spin coating method for improving the perovskite-layer passivation quality and the crystalline-phase grain boundary. Here, the FP self-organizes and forms a very thin overlayer atop the perovskite layer surface during film formation upon using an FP-added precursor solvent, which suppresses the surface roughness (measured as the RMS value) of the perovskite layer. Further, the PCE increases from 17.2% for FP-free PSCs (control) to 19.1% for FP-added PSCs operating in the reverse scan (RS) mode due to the enhanced interfacial passivation capability upon addition of the surface-segregated FP.

Recently, solution-processable organolead halide perovskite materials have been attracting great interest as a cost-effective and high performance light-harvesting material of solar cells. Since perovskite crystals are formed by coating and drying process of precursor solutions, control of crystal growth has been important issue for high power conversion efficiency (PCE). Recently, it is reported that ratio of PbI₂ and methylammonium iodide (MAI) in precursor solutions has great influence on perovskite crystal formation. Excess amount of PbI₂ contained in a precursor solution promotes crystal growth of perovskite and yields the layer of high crystallinity, which gives high PCE of solar cells.[1] On the other hand, excess amount of MAI in precursor solutions yields perovskites with less trap density.[2] To obtain perovskites with high crystallinity and low trap density, in this study, we performed post-treatment with MAI solution to highly crystalline perovskite crystals formed from a PbI₂-rich precursor solution. As a result, high PCE up to 20.7% was achieved.

   Perovskite layers was fabricated from a precursor solution containing 10 mol% excess PbI₂. To the perovskite layers (PbI₂-rich perovskite), MAI solution was spin-coated, and the substrates was annealed at 100 ^oC. Diffraction peak of PbI₂ was observed in X-ray diffraction pattern of perovskite without the MAI treatment. When the perovskite was treated with 10 mM MAI solution, the PbI₂ peak disappeared. This indicates that in the perovskite treated with >10 mM MAI solutions, PbI₂ was removed and excess MAI was incorporated.

  Photovoltaic performance of perovskites with and without the MAI treatment was compared. The perovskites without MAI treatment exhibits PCE at 19.2%. Even when perovskite was treated with 10 mM MAI solution and PbI₂ was removed, the PCE was same. However, by >10 mM MAI solution treatment, PCE increased up to 20.7%. This indicates that excess amount of MAI in perovskite layer is important for high PCE. Dark current analysis shows that trap density was drastically decreased in >10 mM MAI-treated perovskite. We conclude that PCE was improved due to decrease in perovskite trap density based on tuning MAI content by post-treatment.

  Metal-halide perovskites are exciting materials for solar cell applications due to their high photogeneration efficiencies and ease of fabrication.  The stability of perovskite solar cells, however, is poor – particularly under full sunlight and harsh outdoor conditions.  Ambient light energy harvesting in indoor environments, where stability concerns are less acute, is therefore an attractive near term application for emerging perovskite technologies.  Not all solar cells operate efficiently in low light, however, so in this study we set out to examine the properties and performance of our highly efficient mixed-composition Cs_0.05FA_0.80MA_0.15PbI_2.75Br_0.25 perovskite devices¹ as a function of light intensity.

  A typical cell was found to retain about 25% of the maximum efficiency down to the lowest measured light intensities of 0.03 mW/cm² (1/3000^th of full sunlight, or 30 Lux).  The maximum efficiency is observed at 10 mW/cm² (1/10^th of full sunlight, 10 000 Lux).  In addition to current-voltage analysis, impedance spectroscopy was employed to investigate the underlying mechanisms governing the change in cell performance.  The parallel resistance, r­­_p, resolved by the impedance measurements, is compiled as a function of applied bias voltage and incident light intensity.   While usefully high at ambient light levels, the parallel resistance falls dramatically in stronger light, particularly at moderate voltages, resulting in significant loss of fill factor.  The FF loss is shown to be the primary cause of the reduction in cell efficiency above 10 mW/cm².  These results can be comprehensively explained by introducing a field-dependent photocurrent into the standard equivalent circuit, and is accurately modelled using the constant-field approximation developed previously for amorphous Si photodiodes.^2,3

Abstract

Organic-inorganic lead halide perovskite solar cells (PSCs) are one of the most promising technologies for solar energy harvesting. The record power conversion efficiency (PCE) of PSCs has now reached 25.2%. Excess/unreacted lead iodide (PbI₂) crystals are often seen in perovskite films, and are believed to increase PCEs of PSCs because of passivating the trap states. Nevertheless, how PbI₂ crystals affect long-term stability of PSCs under continuous light irradiation has been rarely studied. Therefore, the clarification of basic degradation mechanisms related to unreacted PbI₂ is important for improving the stability of PSCs. Here, we show that unreacted PbI₂ crystals are one of the main reasons for degradation of PSCs. Under continuous illumination of light, unreacted PbI₂ undergone the photo-decomposition by forming metallic Pb. Indeed, the degradation of PSCs was accelerated by metallic Pb. By carefully reducing the amount of the unreacted PbI₂, the stability of PSCs is greatly improved; almost no degradation of PCEs was observed even after 500 hours of continuous illumination with maximum power point tracking.

The efficiency of Pb-perovskite solar cells with more than 1 cm2 is 20.9 % which became close to those of inorganic multi-crystalline solar cells such as MC-Si, CIGS, and CdTe. In small cells with less than 1cm2, the efficiency of 25.2% has just been reported for the perovskite solar cells.

Conventional perovskite solar cells consisting of Pb have band gap of 1.5-1.6 eV and can harvest the light in the visible region up to 850nm.  According to Shockley-Queisser limit, light harvesting layer with 1.2-1.4 eV band gap gives the highest efficiency. Mixed metal PbSn perovskite materials have a narrow band gap of around 1.2 eV.  Therefore, mixed metal SnPb perovskite solar cell is expected to give higher efficiency than Pb-perovskite solar cells.  In addition, the narrow band gap solar cell is useful for bottom cells for all perovskite tandem solar cells. When SnPb mixed metal perovskite solar cells was firstly reported by us, the efficiency was around 4%. However, the efficiency has been enhanced gradually and recently efficiency higher than 20% has been reported by several groups including our Lab.  How the efficiency was enhanced will be discussed in the presentation^1-4.

  The conventional perovskite layer consists of Pb ions. The use of Pb ions is limited by the law such as RoHS directive in Europe.  From this view point, there is a strong request for Pb-free perovskite solar cells solar cells. Bismuth halide compounds such as Cs3Bi2I9, MA3Bi2I9 Ag3BiI6, AgBi2I7, Cs2AgBiBr6, antimony halide compounds such as Rb3Sb2I9, titanium halide compounds such as Cs2TiBr6, and copper halide compounds such as MA2CuI4 have been reported to replace lead. However, the solar cell efficiency based on these materials were less than 5% and was not satisfactory.  Among all lead-free perovskite materials, Sn-based perovskite is one of the most promising candidates as the light harvesting layer for Pb-free PSCs, because they have perovskite structure similar to Pb perovskite.  In addition, the band gap is narrow (1.4eV). The efficiency has been enhanced to 10% by several research groups including us, however, the efficiency is not still satisfactory, when compared with Pb-perovskite. The cause of the low efficiency and how the efficiency will be enhanced is discussed^5-7.

  Perovskite solar cells with wide gap of 1.7-1.8 eV are needed for making tandem cells consisting of SnPb perovskite solar cells as the bottom layer. CsPbI2Br is one of the candidates for the top perovskite layers. However, the efficiency was not satisfactory because of large Voc losses.  We will report CsPbI2Br solar cells with 1.34V (Voc) and 14% efficiency.  We discuss how Voc loss is decreased to enhance the efficiency^8,9.

Perovskite solar cells have achieved a high efficiency over 25% in spite of solution based processes. The production cost of perovskite PV modules is expected to drastically decrease by the solution based processes with a low temperature annealing under atmospheric. In order to produce large area modules with a high efficiency and low cost, it is necessary to prepare highly uniform perovskite films with a high printing speed. We have selected an ink jet printing method from several large area coating methods because ink droplets controlled accurately from a lot of nozzles can form highly uniform thickness of films with a high-speed printing. We have investigated the uniformity of the cell performance in-plane. Precursors of the perovskite films were prepared by coating the solution containing CsI, MAI, FAI and PbI2 using the ink jet printing on 17 substrates of a structure of glass/TCO/c-TiO2/mp-TiO2 with a 2.5 x 2.5 cm2 size located randomly on a 30 x 30 cm2 area. The ink jet printing method can coat the films only on each substrate by drawing patterns. Toluene as an anti-solvent was quickly dropped onto each precursor during the substrate spun. The films were then dried to form the perovskite layers. PTAA films as a HTM prepared by spin coating on the perovskite films. Au films as a back electrode were deposited by vacuum evaporation. 66 cells with an aperture area of 0.1cm2 on 17 substrates showed an average efficiency of 19.4% with a distribution of plus minus 1.7%. High efficiencies and high uniformity in plane of 30 x 30 cm2 area have achieved by the ink jet printing. These results show that the ink jet printing is suitable to produce the large area perovskite modules with a high efficiency. We have also fabricated 30 x 30 cm2 sized modules by the ink jet printing. The performance of the modules will be described in detail at the conference.

Organic lead halide perovskite based solar cells (HPSCs) have achieved certified power conversion efficiency (PCE) over 25.0%. Despite the high efficiency achieved, there are still many concerns about the large-scale applicability of these solar cells because of their Pb²⁺ content. The simpler approach to address this issue is to find a benign or less toxic metal to replace the lead atom in the perovskite structure, obtaining a perovskite displaying similar excellent optical and electrical properties as the Pb-based compounds.

Tin based perovskite holds the promise to give rise to similar or even higher PCE compared to their Pb counterpart. However, for relatively long time the tin-based HPSCs held a PCE lower than 7% though intensive research efforts were devoted to their investigation. The facile formation of tin vacancies and easy oxidation of Sn²⁺ have been identified as the main reason determining the low PCE.

In my presentation I will report as a small amount of 2D tin perovskite templates the growth of highly crystalline and oriented 3D FASnI₃ grains (2D/3D mixtures), suppressing effectively the appearance of tin vacancies and Sn²⁺ oxidation. [[1]] As a consequence of the reduced background charge carrier density and trap assisted charge recombination, the device showed a 50% improvement in the PCE (up to 9%) compared to that using pure 3D tin perovskite. We further succeeded reducing the defects in the 2D/3D tin perovskite films by adding ethylammonium iodide (EAI) into the corresponding perovskite precursor solution even starting from precursors of limited purity [2]. These films display larger crystalline grains and a more compact and uniform film morphology when compared to their counterparts without EA cation. These features lead to a much lower trap density, background charge carrier density and charge recombination loss in the corresponding devices. Results, which allow hopping for future highly efficient Sn-based hybrid perovskite solar cells.

My talk in this section can be divided into two parts. First, we investigated the doping effect of bulky organic cations with ethylenediammonium diiodide (EDAI2) as a co-additive to enhance the performance and stability of the FASnI3 perovskite solar cells.[1] The additive EDAI2 plays a key role to cause slow passivation of the surface and relaxation of crystal strain such that the device performance increases gradually with increasing duration of storage. In the presence of EDAI2 additive (1 %) the FASnI3 device attained the best initial efficiency 7.4 % and the device performance continuously increased as a function of duration of storage; the maximum PCE, 8.9 %, was obtained for a device stored in a glove box for over 1400 h with only slight degradation for storage beyond 2000 h. I will report hybrid tin-based perovskite solar cells that incorporate a non-polar organic cation, guanidinium (GA+), in varied proportions into FASnI3 crystal structure in the presence of 1 % EDAI2.[2] The device performance was optimized at precursor ratio GAI:FAI = 20:80 to attain PCE 8.5 % when prepared freshly; the efficiencies continuously increased to attain a record PCE 9.6 % after storage for 2000 h, which is a world record at the time the paper was published.[3] For photocatalysis, we report a new series of bismuth-based perovskite nanocrystals (PeNCs) as effective photocatalysts for CO2 reduction to produce methane and carbon monoxide with great performance by gas-solid reaction. Detailed studies have been performed by using electron paramagnetic resonance (EPR) and Diffuse Reflection Infrared Fourier Transfer (DRIFT) spectral techniques to propose the plausible mechanism for CO and CH4 formation via CO2 photoreduction by Bi-based PeNC photocatalysts.

Reference:

[1] E. Jokar, C.-H. Chien, A. Fathi, M. Rameez, Y.-H. Chang and E. W.-G. Diau,* Energy Environ. Sci., 2018, 11, 2353-2362

[2] E. Jokar, C.-H. Chien, C.-M. Tsai, A. Fathi and E. W.-G. Diau,* Adv. Mater., 2019, 19, 1804835

[3] E. W.-G. Diau,* E. Jokar and M. Rameez, ACS Energy Lett., 2019, 4, 1930-1937

Increasing the efficiency of solar cells further is now the most direct avenue to make solar electricity even cheaper and ease the energy transition. This involves designing solar cells that achieve efficiencies beyond the practical limit of 27% of crystalline silicon (c-Si) solar cells, the technology dominating the photovoltaics market. In addition, this solar cell of tomorrow should be processed at low costs, employ earth-abundant elements and exhibit a long life time in the field. One emerging cell design may meet these criteria: multi-junction solar cells combining metal halide perovskites and c-Si. The tuneable bandgap, soft processing conditions and high single-junction performance of perovskite cells indicate that a c-Si solar cell could be upgraded with one or more perovskite top cell(s) to reach efficiencies >30% through a few extra process steps and hence at low additional process costs.

Fulfilling this potential is first a processing challenge. For maximum performance and compatibility with existing c-Si process flows, the perovskite solar cell should be deposited directly on the textured front side of the c-Si solar cell, a texture that improves light management in the c-Si. But this pyramidal texture is not compatible with standard perovskite solution-based deposition protocols as these lead to non-conformal coatings and electrical shunt paths. A hybrid evaporation/solution perovskite processing route was developed at EPFL PV-Lab to enable the conformal deposition of perovskite top cell(s) on the pyramidal texture of c-Si cells to achieve high photocurrents (Panels a of TOC). This contribution will discuss how a careful analysis of the device structure on the nanometre scale enabled the identification of performance-loss mechanisms and helped guiding processing efforts. High-efficiency (>25%) perovskite/c-Si tandems were demonstrated on different n- and p-type c-Si technologies [1,2], notably by identifying i) optimal bottom cell contact structures [3], ii) crystallographic and chemical features enabling the recombination junction to quench shunts [4], iii) hole-selective contact instabilities depending on recombination junction and perovskite crystallisation conditions (Panel b). This contribution will then elaborate on the next performance-loss mechanisms that should be addressed to achieve an efficiency of >30%.

The second challenge, and likely the most difficult one to tackle before any commercialisation of the technology can be envisaged, is related to the instability of perovskite solar cells. Degradation pathways triggered by reverse voltages, which may appear when a module becomes partially shaded, or during long-term operation at maximum power point at various temperatures will be discussed [5]. These emphasise the dynamic nature of the perovskite nanostructure (ionic migration within the absorber and also into the contacts (Panel c), volatilization of species, crystallographic phase change/decomposition, shunt formation due to metal migration) depending on the external stimuli and its influence on the solar cell performance. Overall, these findings demonstrate that opaque solar cell architectures commonly employed by the community are particularly unstable in reverse bias, prompting the need for urgent research efforts.

Metal halide perovskite solar cells have attracted increasing attention as one of the most promising photovoltaic devices.  Currently, the power conversion efficiency (PCE) of lead-based perovskite solar cells has exceeded 25%.  Although it is approaching to the theoretical limit, there is still room for further improvement in PCE.  In most cases, charge recombinations are still one of the main loss processes in perovskite solar cells.  In this talk, I will focus on voltage losses due to various charge recombinations in perovskite solar cells.  First, I will talk about the origin of the initial improvement in open-circuit voltage (V_OC) and fill factor (FF) of lead-based perovskite solar cells after storage in the ambient atmosphere.  We estimated trap density in perovskite solar cells with different storage durations by analyzing the intensity dependence of V_OC with direct and SRH recombination model as reported previously [1].  As a result, we found that trap density is decreased with increasing storage time and hence V_OC is also improved.  In order to address the origin of the decrease in trap density, we measured external quantum efficiency (EQE) spectra over the wide wavelength above and below the bandgap energy E_g.  Consequently, EQE signals were observed at wavelengths even below E_g for the device before the storage, suggesting that there are some defects in sub-bandgap states.  After the storage, the EQE signals below E_g were effectively reduced.  A similar reduction in the EQE at sub-bandgap was observed for the device with surface passivation treatment.  We therefore conclude that the initial improvement in V_OC is due to the decreasing defects in sub-bandgap states, which are probably located at surface or boundary of perovskite grains.  We further discuss the origin of voltage losses in tin-based perovskite solar cells [2].

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1. Introduction

In the IoT(Internet of Things) society, vast many types of sensors are embedded into the wireless communication module in order to collect various ambient data for energy managements like the BEMS and HEMS. These systems need a lot of self-powered sensor nodes with energy harvester instead of wired power source or primary batteries.

The organic solar cells have high photovoltaic performance in the indoor low light condition. So, this study demonstrates the energy saving verification using by wireless sensor nodes with organic solar cell under the various indoor condition.

2. Experimental

Organic solar cells are fabricated in our laboratories. Figure 1.(a) shows the organic solar cell panel of single cell type. The electron-donor material (TR-SCP197) of organic photovoltaic layer is also synthesized in our laboratories. The environmental sensor nodes (temperature, humidity, and brightness) are assembled by NISSHA Co., Ltd. And the occupancy sensor nodes are assembled by OPTEX Co., Ltd. The conventional amorphous silicon solar cells (a-Si) are also prepared and tested at same time in order to compare the photovoltaic performance and driving performance of wireless sensor nodes under the low light condition. The wireless sensor nodes are installed in an office and a conference room. For example, the brightness and occupancy data are wirelessly transmitted to the gateway in order to maintain the optimal brightness of room lightings by specific area and turn off the room lighting automatically if no one is there. Figure 1.(b) shows the energy management system using wireless sensor nodes in this system

3. Result

The photovoltaic performance of organic solar cells are 1.3 times electric power generation compared to a-Si under the 50 to 200 Lux LEDs. Though a 1-year verification test, the environmental sensor nodes and occupancy sensor nodes work with the very dim light condition at 40 and 70 Lux each. And the electrical power consumption of room lightings are saved about 40% after installing this system.

The performance of polymer: acceptor blends for use as a light-harvesting layer in organic photovoltaic (OPV) cells depends strongly on the structural features of the active layer, including the extent of intermixing, vertical phase segregation, and generally, its phase morphology. Recent studies show that in most popular bulk heterojunction (BHJ) systems the phases are not pure and a significant volume is occupied by mixed domains that may be rich in either donor or acceptor. [1] Therefore, one crucial aspect of understanding optimized OPV performance is to find characterization techniques to gain a detailed picture of the mixed phases in BHJ systems. This is especially important with the introduction of multicomponent systems such as ternaries where multiple acceptors (A), or donors (D), with different mixing behavior with the other (donor or acceptor) component can be used. [2]

In this study, we present the observations of the phase behavior of non-fullerene acceptors (NFAs), namely non-planar O-IDFBR, planar O-IDTBR, and its branched side-chain analog EH-IDTBR, in semi-crystalline P3HT using thermal and optical characterization techniques and evaluate the optical methods we have used as probes of microstructure. We use a combination of non-destructive optical probes to analyze the microstructure of blend films of P3HT: NFAs as a function of composition. Spectroscopic ellipsometry (SE) helps analyze the optical properties of multicomponent systems in terms of the composition, and Raman spectroscopy allows us to understand the state of order of the conjugated backbone, and chemical structure.  Moreover, e demonstrate how UV-vis and PL can be used to capture the degree of mixing of the thin films. We interpret the optical properties of binary blends of different composition in terms of the phase behavior of the blends and compare our findings with a picture of the phase behavior obtained using differential scanning calorimetry (DSC). The detailed picture of the microstructure allows us to correlate the impact of NFA composition and crystallinity on the microstructure with photocurrent generation by photovoltaic devices.

These optical studies demonstrate that the less planar NFA (O-IDFBR) mixes finely into semicrystalline P3HT at weight contents up to 40 wt%, beyond which it disrupts the order of the P3HT. However, the more planar NFAs (O-IDTBR and EH-IDTBR), could be accommodated up to 70 wt% in the blend without disrupting the polymer. We observe the maximum of the power conversion efficiency (PCE) of P3HT: O-IDTBR peaks at the eutectic composition where crystals of both D/A form and based on the optical probes the binary is phase-separated. Surprisingly, we observe the maximum of the PCE of P3HT: O-IDFBR to lie around 30-50 wt% O-IDFBR, which is far below the apparent eutectic composition, as shown in the figure attached. This is assigned to the loss of P3HT transport before reaching the eutectic, based on Raman analysis.

The results show that device performance is dictated by short circuit current density (J_sc). In order to focus on understanding the interplay between the blend film morphology and the J_sc of the corresponding devices we estimate composition-dependent internal quantum efficiency (IQE) and compare with our results for phase behavior. We find that the O-IDTBR contribution to IQE is notably higher than that of O-IDFBR, which can be assigned to the higher crystallinity of O-IDTBR compared to O-IDFBR as well as due to P3HT crystallinity is rather disrupted when mixed with O-IDFBR than O-IDTBR. It appears that the optimized performance is strongly dependent on the degree of polymer and NFA crystallization.

The minimum energy offset required to ensure efficient charge separation at organic donor-acceptor heterojunctions is a matter of current debate.^1-2 Here, we investigate the charge generation in a series of donor – acceptor bulk heterojunction blends including the small molecule donor DR3, two high-efficiency donor polymers (PBDB-T-2F, PCE10), and eight different non-fullerene acceptors (NFA), spanning a power conversion efficiency range from <1% to above 15%.

In principle, both electron affinity (EA) and ionization energy (IE) offsets should equally control the charge generation. However, we demonstrate that despite large EA offsets, it is the IE offset, which controls both the exciton quenching and charge separation efficiency, as ultrafast energy transfer from the donor to the acceptor competes with photoinduced electron transfer. This implies, the IE offset cannot be reduced to zero to maximize the Voc as it leads to inefficient exciton quenching and reduced charge separation efficiencies and in turn to lower device photocurrents and performance.

The microstructure of donor: acceptor blend photoactive layers in organic solar cells directly influences the properties of interfacial charge-transfer (CT) states, and thereby influences the minimum value of the energy loss that can be achieved in the photovoltaic device. However, relatively few studies are able to relate blend phase behaviour directly to device energy losses. In this work, we study the correlation between the phase behaviour of two poly-3-hexylthiophene (P3HT): acceptor blends made using different non-fullerene acceptors, namely IDTBR and IDFBR, and the energy losses of the blend devices. Whilst the two acceptors are chemically very similar, small differences in molecular structure result in large differences in the degree of crystallisation and mixing in the blend films. In particular, P3HT:IDTBR blends shows a higher degree of crystallinity and more phase separated domains, whereas P3HT:IDFBR blends show lower crystallinity and more mixing of polymer and acceptor. The different blends exhibit different trends in terms of energy loss as a function of blend composition, indicating disparate mechanisms on energy losses. To understand this, I will quantify the radiative and nonradiative component of energy losses and apply a model of interfacial charge recombination [1] and models of charge transport in order to relate the measured energy losses to the transport and recombination dynamics in the different blends and finally to the blend microstructure.  I will bring the observations together to discuss the relationship between phase behaviour and recombination losses using the P3HT:acceptor systems as a model.

In order to achieve semi-transparency in perovskite and organic solar cells, the electrode materials must be as transparent as possible. In this work, MoO_x/ITO/Ag/ITO (MoO_x/IAI) and MoO_x/IZO thin films with respective high-average-transmittance of 79.90% and 85% between 400 nm and 900 nm were introduced as the top transparent electrode to explore its influences on optoelectronic properties of the fabricated perovskite and organic solar cells. MoO_x has been demonstrated previously as protection from sputtering damage using a conventional ITO or IZO top electrode, however it is shown here to provide protection from a sputtered IAI and IZO film that provides superior transparency and conductivity and is deposited using more favourable low temperature processing conditions. MoO_x and Ag were thermally evaporated and ITO and IZO was radio-frequency magnetron sputtered at room temperature. The resulting semi-transparent solar cells showed power conversion efficiency of 12.85% for IAI based PSCs, 16.03% for IZO based PSCs and 4.58% for IZO based OPV, respectively.  The semi-transparent perovskite devices showed a much-reduced degradation rate as compared to the reference device with only an Ag/Au top electrode. Notably, we show that an unencapsulated semi-transparent perovskite cell using MoOx/IZO maintains its full performance under continuous illumination for over 770 hours in nitrogen environment. With such a combination of performance and transparency, this work shows great promise in application of perovskite solar cells into window glazing products for building integrated photovoltaic applications (BIPV), powering internet of things (IoT) and combining into tandem solar cells with industrially mature photovoltaic technologies such as silicon and copper indium gallium di-selenide (CIGS).

Porphyrins materials such as tetraarylporphyrins, subphthalocyanines, phthalocyanines, and oxasmaragdyrins have shown compatibility as a hole transporting material (HTM) in perovskite solar cell (PSC) architecture. The devices with porphyrinic macrocycles as HTM show comparable power conversion efficiency (PCE) to those with Spiro-OMeTAD as the HTM.^1-6 Nevertheless, the influence of the light-harvesting properties of porphyrinic macrocycles to the efficiency of hole migration has not been investigated. We discuss herein the overall consequence of light-harvesting of these compounds to the PCE performance using the derivatives of SM09, the best HTM among examined oxasmaragdyrins in our previous report, as model compounds.⁷ The alkyl chains with different chain lengths are introduced in the core position of SM09 to replace the fluorine atoms and attenuate the intermolecular distance which might alter the rate of energy transfer or self-quench. A series of core modification of SM09, encoded by SM09-B(OMe)2, SM09-B(OEt)2, SM09-B(OBut)2, SM09-B(OHex)2 and SM09-B(OOct)₂ will be examined and compared.

The presence of methoxy groups in SM09-B(OMe)2 slightly improved the PCE performance of PSC in comparison with parent SM09, consistent with our previous report about the effects of the methoxy group as the HTM to PSC performance. Additionally, we have found that diminished PCE observed upon increasing the alkyl length from two-carbon SM09-B(OEt)2 to four-carbon SM09-B(OBut)₂ likely due to the furtherer molecular distance which affected the hole mobility. Interestingly, we found a unique phenomenon in the J-V curves of the devices with SM09-B(OHex) and SM09-B(OOct) as HTMs, i.e. an unsteady Jsc with current continuously decrease at low voltage region. This phenomenon leads to unreasonable Fill Factor (FF) and an unusual low of short-circuit current density (J_SC). After a series of examinations, we believe that this phenomenon is related to the self-quenched of SM09 derivatives. The observation that this current drop at low voltage region can be precluded by increasing the scan rate suggests that it is not a rapid exchange process. We used a filter light to measure the PCE performance and found that the dropping current was not observed while the device exposed with short-wavelength light (<600 nm) but still appear when it exposed by long-wavelength light (>600 nm). The presence of longer alkyl chains generates a larger intermolecular distance between the molecule which has not only decreased the hole mobility but also suppress self-quenched of boryl oxasmaragdyrins.  Thereafter, the excited HTM will inject photoelectrons to the LUMO of perovskite cell, which then either retards the excitation of perovskite layer or increases the rate of charge recombination from electrons at LUMO of perovskite and holes on the HOMO of HTM. Additional photophysical lifetime measurements will be conducted to provide conclusive evidence of this phenomenon. 

Searching of new HTMs as replacement of spiro-OMETAD has been a hot topic in PSC studies. Our finding for the first time reveals the effects upon using photoactive materials as HTM. Under the circumstance that there is no significant alternation on Voc and Jsc upon changing the scan rates during the measurements, the solution to obtain a more accurate PCE can be achieved through increasing scan rates. However, the competition on photon absorptions between HTM and perovskite layer appears to exert negative effects to the overall performance of PSC and should be taken into consideration upon the design of HTMs.

Perovskite solar cells (PSCs) offer a magnificent opportunity to harness solar energy in an efficient and low cost way. The ambition for commercialization has been greatly encouraged by the surge in device performance from 3.8% in 2009 to the state-of-the-art 25.2%¹. To obtain high power conversion efficiency altering the interfacial properties is essentially important. New charge selective contacts have been investigated to provide possible solutions to overcome this hurdle. Being in a molecular scale, self-assembled monolayers (SAMs) are promising affordable candidates for interface modification. SAMs are essentially organic assemblies formed by the adsorption of molecular constituents from solution onto the surface of solids. In the literature, several studies have shown that SAMs have many positive effects for PSCs, including the improvement of the energy level alignment, positively affecting the morphology, and passivating trap states². They are offering the benefits of uniformly formed layers with minimized thickness that will be suitable for large-scale production³.

In the present talk, we will discuss semiconductor self-assembled monolayer to use as hole transporting layer instead of PEDOT:PSS for PiN type perovskite solar cells. We will present preparation procedure and photovoltaic characterization of devices, which have improved power conversion efficiency.

The Organometal halide perovskite solar cells (PSCs) have been dramatically researched and developed in the power conversion efficiency (PCE) certified over 24% [1] which is comparable to other solid state solar cells as crystalline silicon, CIGS, CdTe, etc. The modification of the components of perovskite is one of the key word for the refinement of quality and morphology of crystal and layer with increasing the abundance of photoactive phase such as tetragonal, cubic, trigonal, and decreasing the surface roughness and void in the layer, especially, with A-site cation in perovskite structure ABX₃ [2]. In this talk, we report the potassium cation doping effect to perovskite absorber about properties of the expanded crystal lattice constant, red-shifted light absorption, up shifted conduction and valance band position, disappeared grain boundary in film, diminished hysteresis in photovoltaic I-V curve with over 20% PCE [3,4], furthermore, we have reached new composition as MA-Free system with over 21% PCE. The scaling up the photo active area with small less of FF without drop of Voc from single cells was maintained by development of device fabrication process point of view. Recently, the PCE was reached near 21% by 1 cm² and 2.76 cm² with less hysteresis based on the better flatness and less impurity of perovskite film by optimizing the process.

References

[1] Martin A. Green, et al.  Prog Photovolt Res Appl. 27, 565–575 (2019).

[2] Saliba, M., Grätzel, M., et al.  Science, 354, 206–209 (2016).

[3] Z. Tang, T. Bessho, H. Segawa, et al.  Sci. Rep., 7, 12183 (2017).

[4] Z. Tang, S. Uchida, T. Bessho, H. Segawa et al.  Nano Energy, 45, 184–192, (2018).

Solution-processed tandem solar cells, that stack two or more single-junction subcells with different band gaps to harvest photons in the full solar spectrum more efficiently, have attracted increasing attention recently. Organic photovoltaics and perovskite solar cells are promising candidates for the top and/or middle subcells of tandem solar cells because the solar cells are able to capture visible and near-infrared photon energy. While PbS and PbSe colloidal quantum dots (CQDs) have been gaining much attention for short-wave infrared solar cells owing to their wide-range bandgap tunability and solution process compatibility. Thus, we have focused on PbS QD/ZnO nanowire (NW) structures with the aim of achieving efficient carrier transport and light absorption in the infrared region simultaneously ^1-2. We then investigated the performance of PbS QD/ZnO NW solar cells using PbS CQDs with the first exciton absorption peak locating in the infrared region (940 nm-1840 nm) ³. We recently constructed high-efficiency infrared PbS QD/ZnO NW solar cells with a record high EQE of 47% (at 1560 nm). The solar cell installed with an 870 nm sharp-cut filter yielded a PCE of 2.02 % (Jsc=14.8 mAcm^-2; Voc=0.285 V; FF=47.9 %) under a filtered one-sun illumination. Based on these results, we will discuss the potential of PbS QD / ZnO NW solar cells toward the bottom subcell of multi-junction solar cells. 

The emerging technology of perovskite solar cells (PSCs) has a revolutionary influence on the research community due to the unprecedent growth rate of its power conversion efficiency (PCE). Despite PSC has a history that shows a seemly promising future, most studies have been focusing on PCE and lifetime optimization instead of verifying the performance governing factor of PSCs. Recently, time-of-flight secondary-ion mass spectrometry (ToF-SIMS) has been utilized for getting insights into the perovskite material due to its extremely high detection limit and its ability of detecting molecular component signals. These features have made ToF-SIMS a powerful tool in uncovering the fundamental property of the perovskite film, which is crucial for turning PSCs into a transformative technology. Unfortunately, very few have been aware that the probing beam must be carefully chosen during ToF-SIMS analysis otherwise misleading artifacts will appear [1],[2].

During this presentation, the origin of depth profile artifacts induced by the most commonly used sputter beams, O₂⁺ and Ar-gas cluster ion beam (Ar-GCIB), will be discussed. These artifacts, although often ignored, could be widely observed in the perovskite depth profiles. We demonstrated an approach to eliminate depth profile artifacts by replacing O₂⁺/Ar-GCIB with either C₆₀⁺ or Ar⁺ sputtering. Based on this finding, we were able to reveal the true component distribution of PSCs and identified the performance governing factor. We verified that different manufacturing procedures could result in different component distributions of the perovskite film, which would greatly influence the PCE and J-V response of the device. As for stability issues, we confirmed that the major compositional difference between a fresh and an aged PCS was resulted from iodide diffusion. These results provided evidences that could support and facilitate the development of a more efficient and stable PSC.

Gold electrode mitigation impact: elucidation of both degradation and safeguard mechanisms in a mixed-ion perovskite solar device

Lara Perrin^a, Manon Spalla^a,b, Emilie Planès^a, Muriel Matheron^b, Solenn Berson^b, Lionel Flandin^a

^aUniv. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, Grenoble INP, LEPMI, 38000 Grenoble, France

^bUniv. Grenoble Alpes, CEA, LITEN, INES, 73375 Le Bourget du Lac, France

In the photovoltaic field, perovskite devices have already proved their potential to overcome the performance limits of current technologies and achieve low cost and high versatility. Several challenging developments are nowadays in progress on, for instance, low temperature processes for flexible devices compatibility, semi-transparent devices for tandem applications … Nevertheless, this technology is known to be sensitive to environmental factors as temperature, oxygen and humidity.

The degradation behaviour of a selected pertinent mixed-ion perovskite device has been here draft when submitted to incremental stress conditions (see adjacent figure). The presented device architecture “ITO/SnO₂/mixed-ion perovskite/PTAA/Au” is potentially compatible for both flexible and tandem applications. In order to find possible mitigation strategies, a careful study was conducted allowing to elucidate occurring degradation mechanisms thanks to an optimized characterization set composed of either imaging techniques (light beam induced current and photoluminescence), physico-chemical analyses (X-ray diffraction, UV-visible absorption and photoluminescence spectroscopy) and photovoltaic parameters measurements (power conversion and external quantum efficiencies).

As exposed on the presented figure (top view photographs of solar devices), it is possible to detect a strong difference in the perovskite layer already with an unaided eye, according to applied constraints (temperature only, additional air, followed by additional moisture). First of all, the talk will detail the degradation mechanisms highlighted using several complementary physico-chemistry characterizations. In particular, let us examine pictures presented on the right side (devices with gold electrode): the close yellow color resulting from conditions 1 and 3 were in fact proved to originate from a totally different downgrading product. Deeper imaging techniques were also employed and proved to be relevant to investigate the homogeneity of both the perovskite layer and its interfaces integrity.

Secondly, as evidenced on pictures on the left side (devices after top gold electrode delamination), it is obvious that gold plays a protective role against both air and moisture: degradation deceleration through a reduced permeation rate resulting in different perovskite layer chemical compositions and mechanical strengths. As regards condition 1 aging, at first sight it could seem obvious and rational that the electrode has no potential for protection against a sole temperature stress. However, advanced examinations brought to light a strong difference in the perovskite layer chemical evolution when scouting either under or apart the top electrode and here again the perovskite occurring degradation mechanism was found to be significantly lowered in the presence of gold electrode. This time gold has been proved to collaborate through an interaction with the hole transporting layer constituents. To give an idea of this significant contribution, devices were both aged with and without top electrode: completed devices loose 72% of photovoltaic efficiency after 500 h of aging at 85°C without gold, against 28% when aged with gold.

For years, considerable research effort worldwide has been invested in the development of new thin-film photovoltaic (PV) technologies that may offer lower cost production, new applications or both [1]. At present dye-sensitized solar cells (DSC) have been on the market, and in the case of perovskite solar cells (PSC), a number of companies and research centres are devoted to technology transfer from laboratory to market, working on device stability and reliability, up-scaling and compatibility of the cell manufacturing with industrial process, such as roll-to-roll deposition [2]. However, there are considerable difficulties associated with reliable measurement of power conversion efficiency for some kinds of DSC and PSC that are a serious concern for technology transfer from laboratory to market. Although the measurement procedures described in the IEC 60904 series and IEC 60891 are highly effective for “well-behaved” devices, such as most wafer-based silicon solar cells, these standards lack sufficient direction to address the complex challenges presented by these devices [3]. As a consequence, these devices need some additional measurement challenges that are at present not dealt with in these standards. In this paper we present what additional measures may be needed for accurate determination of the power conversion efficiency, and how these measures might be done for the devices exhibiting complex current response to applied voltage.

Multi-junction solar cells based on solution-processed metal halide perovskites offer a route to increased power conversion efficiency (PCE); however, the limited options for infrared (IR)-absorbing back cells have constrained progress. Colloidal quantum dot (CQD)-based solar cells, which are solution-processed and have bandgaps tunable to wavelengths beyond 1000 nm, are attractive candidates for this role. Here we report a solution-processed four-terminal (4T) tandem solar cell comprised of a perovskite front cell and a CQD back cell. The 4T tandem provides a PCE exceeding 20%, the highest PCE reported to date for a perovskite-CQD tandem solar cell. The front semi-transparent perovskite solar cell employs a dielectric-metal-dielectric (DMD) electrode constructed from a metal film (silver/gold) sandwiched between dielectric (MoO3) layers. The highest-performing front semi-transparent perovskite solar cells exhibit a PCE of ~18%. By tuning the wavelength-dependent transmittance of the DMD layer based on the zero-reflection condition of optical admittance, we build semi-transparent perovskite solar cells with a 25% increase in IR transmittance compared to baseline devices. The back cell is fabricated based on an IR CQD absorber layer complementary to the IR transmittance of the semi-transparent perovskite front cell. Solution-processed hybrid tandem photovoltaics (PV) combining these technologies offer to contribute to higher-efficiency solar cells for next-generation flexible photovoltaic (PV) devices.

In this paper, we developed a simulation tool to analyze the hysteresis effect of JV curve in Perovskite solar cells. Some studies have shown that the ion migration is the main reason leading to the hysteresis effect of JV curve [1-3]. However, the typical commercial simulation software can only demonstrate the fixed ion distribution [4],which can only  model the effect of ion accumulation at two sides. However, it's hard to explain as the dynamic changes of ion migration under different bias scan rates. Hence, we developed a 2D simulation program that can handle the time dependent ion migration movement. This program also model  the light absorption with texture surface as well as the carrier transport in the devices. In this program, the optical field was solved by FD-TD method and electrical properties were solved by Poisson & drift-diffusion solver, the detail can be referred to Ref. [5]. The time-dependent drift-diffusion model especially for ion migration was added into this program and couple with the Poisson and drift-diffusion solver for carriers. Hence, we can consider the electrical field variation with ion migration in our modeling. Partial simulation results were shown in this abstract due to limit space.. The structure was formed by Au (80nm) / PCBM (20nm) / MAPbI3 (300nm) / PEDOT:PSS (20nm) / ITO (150nm) [6]. Fig. (a) is the ion accumulation with different pre-bias for 10 seconds. In the Fig. (b), the solid line is simulation result of conduction band without ion migration effect and the dash line is the conduction band that considered the ion accumulation. These figures show that the ion accumulation was mainly decided by pre-bias condition (the electrical field at this bias). To understand the effect of ion distribution in electrical properties, the different voltage-bias scan rate is was performed to understand the degree of ion migration. Fig. (c) shows the hysteresis effect under different scan rate. The trend of simulation results was close to the experiment data [1]. And some studies have shown that the ion migration would lead to the defect generation in Perovskite [1]. Fig. (d) shows the result with consideration of defect generation after ion migration, we can observe that the open-circuit voltage would be strongly affected by different defect state density and ion accumulation. By consideration the defects, the trend of simulation result is more close to the experiment result [1].

A projection of worldwide CO₂ emissions of the future PV industry is initially presented. We investigate the development of gross global CO₂-emissions from the PV industry towards a sustainable energy future. This is the basis for our motivation to propose a fully printed integrated carbon-based PV module concept as alternative pathway to strongly reduce the carbon footprint to the ultimate lower limit of the glass substrate fabrication. For this glass-glass sealed perovskite solar cell (PSC) concept in which the perovskite is introduced “in-situ” as a last fabrication step, we show a certified-stabilized efficiency of 9.3 %. We calculated for the in-situ PSC an emission of 3.67 g CO­₂-eq/kWh, which is just 4.8 % of the value for mono-Si PV modules.

With the goal to increase the efficiency towards an ambitious - yet achievable - 20%, we propose three approaches aimed at increasing the light-harvesting efficiency and at obtaining a uni-directional charge transport. These approaches have been experimented on monolithic carbon-based PSCs (C-PSCs):

1. A perovskite molten-salt approach obtained through liquefaction and recrystallization of CH₃NH₃PbI₃ perovskite with methylamine - MA⁰(CH₃NH₂) gas. The fabricated C-PSC showed optimized perovskite self-assembling and improved crystallization with efficient charge transport and extraction, resulting in a high V_OC of 1 V, which is the highest V_OC reported for a monolithic HTL-free MAPbI₃ device. This device has been certified under steady-state with a value of 12.6%. Furthermore, we unravel how methylamine interacts with perovskite during its liquid-to-solid transition by using a combination of Raman spectroscopy, single-crystal XRD and a real-time photoluminescence (PL) monitoring during the crystallization.

2. Monolithic C-PSCs commonly use thick >1 µm printed electrically insulating space layer to prevent charge recombination at the mp‑TiO₂/carbon interface. We show a reproducible large-area procedure to replace this thick space layer with an ultra-thin dense 40 nm sputtered Al₂O₃, which is able to prevent ohmic shunts and to efficiently reduce the charge recombination at the mp-TiO₂/carbon interface. Herewith, transport limitations related so far to the hole diffusion path length inside the thick mesoporous space layer have been omitted by concept. A stable V_OC of 1 V using MAPbI₃ perovskite has been achieved with stabilized device performance of 12.1%.

3. The third approach show how embedding cross-linked poly(methyl-methacrylate) (PMMA) nanoparticles of tunable size into the mesoporous TiO₂ scaffold via sol-gel process can directly influence the pore size of the final film and, therefore, enhance the light-harvesting efficiency. SEM and 3D nano-tomography (from FIB-SEM) demonstrate the pore size engineering of the mp-TiO₂ layer. The impregnated C-PSC filled with double-cation mixed halide FA_0.83Cs_0.17PbI_2.64Br_0.36 perovskite photoabsorber, reaches V_OC close to 1 V with a power conversion efficiency (PCE) of 12.7%.

Lead-free tin perovskite solar cells (PSCs) show the most promise to replace the more toxic lead-based perovskite solar cells. However, the efficiency is significantly less than that of lead-based PSCs as a result of low open-circuit voltage (V_OC). This is due to the tendency of Sn²⁺ to oxidize into Sn⁴⁺ in the presence of air together with the formation of defects and traps caused by the fast crystallization of tin perovskite materials. Here, post-treatment of the tin perovskite layer with edamine Lewis base to suppress the recombination reaction in tin halide PSCs results in efficiencies higher than 10%, which is the highest reported efficiency to date for pure tin halide PSCs. Larger crystal sizes is obtained with EDA post-treatment and the V_OC improved by as much as 0.1 V at an optimum EDA concentration. The X-ray photoelectron spectroscopy data suggest that the recombination reaction mainly originates from the nonstoichiometric Sn:I ratio in addition to the large Sn⁴⁺:Sn²⁺ ratio. The amine group in edamine bonded the undercoordinated tin, passivating the dangling bonds and defects, resulting in suppressed charge carrier recombination. This work provides an evident that the surface recombination also needs to be addressed especially in the case of tin perovskite solar cells in order to achieve better device performance.

The capacitance inside the perovskite solar cell device is crucial factor related with the hysteresis in I-V curve[1]. Nevertheless the understanding of capacitance is still not yet well understood because the conventional impedance analysts is a function of evaluation frequency and can not separate the charge / discharge current (capacitance) specially under the illumination condition.

Here in this study, we newly measured the internal capacitance of perovskite solar cell by using stepped light-induced transient measurements of photocurrent and voltage (SLIM–PCV) together with conventional inductance-capacitance-resistance (LCR) meter as a reference. By the SLIM-PCV measurement, we have seen a general trend of capacitance against the light intensity, that can be easily observed in the silicon solar cells. The perovskite solar cells measured by using the LCR-meter under 100 mW·cm-2 irradiation condition provides the huge capacitance values as a function of frequencies that is already reported in several literatures. The EBIC analysis is interesting and back supported the charge accumulation behavior at CH3NH3PbI3/TiO2 interface. We will discuss about this technique to correlate with the device durability by monitoring the capacitance.

Figure 1: Capacitance measurement for perovskite solar cell using LCR meter under the dark and 100 mW·cm-2 conditions

Tin perovskite solar cells (TPSCs) as the most promising candidate for lead-free PSCs have attracted much attention all over the world. However, tin perovskite films deposited by the solution-based strategy usually undergo a much faster crystallization rate than that of the lead analogs,[1] which leads to abundant pinholes and random crystal orientation that induce serious charge recombination and poor device performance.[2] Besides, tin perovskite materials show poor environmental stability, they easily undergo phase transition or oxidization process when exposed to the air. Additive engineering is a promising way to improve both the film quality and stability, but there is a lack of design principle for the additive molecules to further improve the efficiency and long-term stability of TPSCs.

In this manuscript, we introduced the additive molecules designed with a π-conjugated unit and Lewis base functional groups to govern the crystallization kinetics of FASnI₃ perovskite.[3] It's found that the π-conjugated units with strong electron-donating ability significantly increase the electron density of Lewis base groups, resulting in more stable intermediate phase formed with the Lewis acid Sn²⁺ components, leading to a compact and uniform perovskite film with much longer carrier recombination lifetime. Moreover, the hydrophobic nature of π-conjugated system also retards the permeation of moisture into perovskite crystal and therefore prevents the degradation of FASnI₃ film in air.

As a result, we achieved a stabilizing power conversion efficiency of 10.1% for the TPSCs, and a certified steady-state efficiency of 9.2% was also obtained from the accredited test center, National Institute of Advanced Industrial Science and Technology (AIST, Japan). In addition, the TPSCs treated with CDTA maintained over 90% of its initial PCE after 1000-hours light soaking in air.

The success of perovskite photovoltaics is underpinned by their impressive optoelectronic properties, notably their combination of high charge-carrier mobilities with low rates of charge-carrier recombination^[1]. Sub-bandgap trap states in hybrid lead halide perovskites can act as nonradiative recombination centres, leading to shorter charge-carrier lifetimes and limiting the open-circuit voltage (V_oc) in perovskite solar cells^[2]. It is therefore essential to understand the nature and energy scale of these trap states for the development and optimization of technology based on these materials.

In this study^[3] we investigated the influence of sub-bandgap trap states on charge-carrier recombination through an analysis of the low-temperature photoluminescence (PL)  of FAPbI_3, a perovskite material used in some of the most efficient and stable perovskite solar cells ^[4]. We observed a power-law time dependence in the emission intensity and an additional low-energy emission peak that exhibits an anomalous relative Stokes shift. Using a rate-equation model and a Monte Carlo simulation, we revealed that both phenomena arise from an exponential trap-density tail with characteristic energy scale of ≈3 meV. Since charge-carrier recombination from sites deep within the tail causes emission with energy downshifted by up to several tens of meV, such phenomena may in part be responsible for V_oc losses commonly observed in these materials. We propose that the origin of the band-tail states in FAPbI3 may lie in the rotational freedom of the polar organic cation. These results underline the suitability of viewing hybrid perovskites as classic semiconductors, whose electronic bandstructure picture is moderated by a modest degree of energetic disorder.

The incorporation of cesium (Cs) and rubidium (Rb) ions into multiple-cation mixed lead halide perovskites increases their photovoltaic performance. In this study, the reasons for the performance increase are investigated by a set of steady-state and transient spectroscopy techniques. Analyzing the band edge absorption using Elliott’s model shows that the Cs/Rb-ion incorporation increases the band gap, while the excitonic binding energies remain low, in the range of a few milli-electronvolts. Low Urbach energies determined by photothermal deflection spectroscopy suggest optimized microstructures upon Cs/Rb incorporation. The charge carrier recombination dynamics indicate that Cs/Rb-incorporation reduces not only the first-order (trap-assisted) recombination, but also the second-order recombination in these perovskite films. Upon excitation, carrier density-induced broadening of the photo-bleaching following the Burstein-Moss model is observed and effective carrier masses are determined to be in the range of a few tenths of the electron rest mass, explaining the excellent charge carrier mobilities of these perovskite films. Sub-picosecond hot carrier cooling is observed, indicating a strong charge-phonon coupling. Our results unfold the impact of cesium/rubidium incorporation on the photophysics of multiple-anion lead halide perovskites and provide guidelines for future material engineering and device design.

I will present different approaches we have adopted to improving the efficiency, and fundamental stability of the perovskite absorber materials and devices. I will give further insight into what factors influence stability, and how to mitigate degradation.

In order to compete with, and deliver a superior technology to existing silicon PV, I will highlight how moving from a single absorber layer, to a multi-junction cell leads to much higher efficiencies, and I will show experimental realization of progress along such a road map for both all-perovskite and perovskite-on-silicon tandem cells.

Finally, I highlight progress towards the industrialization and manufacturing scale up of perovskite-on-silicon tandem solar cells at Oxford PV. 

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