Ordered Perovskite Structure with Functional Units for High Performance and Stable Solar Cells.

Ion migration is one of the most critical challenges that affects the stability of metal-halide perovskite solar cells (PSCs). However, the current arsenal of available strategies for solving this issue is limited. Here, novel perovskite active layers following the concept of ordered structures with functional units (OSFU) to intrinsically suppress ion migration, in which a three-dimensional (3D) perovskite layer is deposited by vapor deposition for light absorption and a 2D layer is deposited by solution process for ion inhibition, are constructed. As a promising result, the activation energy of ion migration increases from 0.36 eV for the conventional perovskite to 0.54 eV for the OSFU perovskite. These devices exhibit substantially enhanced operational stability in comparison with the conventional ones, retaining >85% of their initial efficiencies after 1200 h under ISOS-L-1. Moreover, the OSFU devices show negligible fatigue behavior with a robust performance under light/dark cycling aging test (ISOS-LC-1 protocol), which demonstrates the promising application of functional motif theory in this field.

LiTFSI-Free Hole Transport Materials for Robust Perovskite Solar Cells and Modules with High Efficiencies.

Lithium bis(trifluoromethane) sulfonamide (LiTFSI) and oxygen-doped organic semiconductors have been frequently used to achieve record power conversion efficiencies of perovskite solar cells (PSCs). However, this conventional doping process is time-consuming and leads to poor device stability due to the incorporation of Li ions. Herein, aiming to accelerate the doping process and remove the Li ions, we report an alternative p-doping process by mixing a new small-molecule organic semiconductor, N2,N2,N7,N7-tetrakis (4-methoxyphenyl)-9-(4-(octyloxy) phenyl)-9H carbazole-2,7-diamine (labeled OH44) and its preoxidized form OH44+(TFSI-). With this method, a champion efficiency of 21.8% has been achieved for small-area PSCs, which is superior to the state-of-the-art EH44 and comparable with LiTFSI and oxygen-doped spiro-OMeTAD. Moreover, the stability of OH44-based PSCs is improved compared with those of EH44, maintaining more than 85% of its initial efficiency after aging in an ambient condition without encapsulation for 1000 h. In addition, we achieved efficiencies of 14.7 and 12.6% for the solar modules measured with a metal mask of 12.0 and 48.0 cm2, respectively, which demonstrated the scalability of this method.

Grain Boundary Elimination via Recrystallization-Assisted Vapor Deposition for Efficient and Stable Perovskite Solar Cells and Modules.

Vapor deposition is a promising technology for the mass production of perovskite solar cells. However, the efficiencies of solar cells and modules based on vapor-deposited perovskites are significantly lower than those fabricated using the solution method. Emerging evidence suggests that large defects are generated during vapor deposition owing to a specific top-down crystallization mechanism. Herein, a hybrid vapor deposition method combined with solvent-assisted recrystallization for fabricating high-quality large-area perovskite films with low defect densities is presented. It is demonstrated that an intermediate phase can be formed at the grain boundaries, which induces the secondary growth of small grains into large ones. Consequently, perovskite films with substantially reduced grain boundaries and defect densities are fabricated. Results of temperature-dependent charge-carrier dynamics show that the proposed method successfully suppresses all recombination reactions. Champion efficiencies of 21.9% for small-area (0.16 cm2 ) cells and 19.9% for large-area (10.0 cm2 ) solar modules under AM 1.5 G irradiation are achieved. Moreover, the modules exhibit high operational stability, i.e., they retain >92% of their initial efficiencies after 200 h of continuous operation.

Mitigating the Internal Ion Migration of Organic-Inorganic Hybrid Perovskite by a Graphene Oxide Interlayer.

Organic-inorganic hybrid perovskite solar cells (PSCs) have attracted great research attention due to their outstanding optoelectronic properties. The low-temperature synthesizing process of organic-inorganic hybrid perovskites can provide a significant advantage of reducing the manufacturing cost of solar cells. However, at the same time, this also brings challenges to PSCs in the form of long-term stability. Because of the low vacancy formation energy, organic-inorganic hybrid perovskites suffer from serious ion migration issue. Also, this ion migration will lead to a series of stability problems, which can hardly be addressed by encapsulation. Currently, modifying the surface of perovskite by an ion-blocking layer is a common strategy for achieving highly stable PSCs. These strategies could effectively address the stability issues caused by the interfacial ion diffusion between perovskite and the charge transport layer. However, the ion migration inside the perovskite layer could be still a knotty problem, which is difficult to be solved through surface modification. Herein, we propose a novel strategy to mitigate the internal ion migration by inserting two-dimensional graphene oxide (GO) into a perovskite layer. Close-space sublimation and ultrasonic spray coating were employed to prepare perovskite and GO layers, respectively. We found that the ion migration in the as-prepared perovskite/GO/perovskite can be successfully mitigated by the GO interlayer. As a result, the champion PSC with a GO interlayer maintained 85% of its initial power conversion efficiency (PCE) after 96 h of continuous illumination. By contrast, the efficiency of the PSC without a GO interlayer declined rapidly and maintained only 50% of the initial value. We believe that this novel interlayer strategy could provide a new idea and approach to preparing highly stable PSCs.

All-vacuum deposited perovskite solar cells with glycine modified NiO x hole-transport layers.

Organic-inorganic hybrid perovskite solar cells (PSCs) have attracted enormous research attention due to their high efficiency and low cost. However, most of the PSCs with high efficiencies still need expensive organic materials as their hole-transport layer (HTL). Obviously, the highly expensive materials go against the low-cost concept of advanced PSCs. In this regard, inorganic NiO x was considered as an idea HTL due to its good transmittance in the visible region and outstanding chemical stability. But for most of the PSCs with a NiO x HTL, the hole-extraction efficiency was limited by the unmatched valence band and too many surface defects of the NiO x layer, especially for the vacuum-deposited NiO x and perovskite. Herein, we developed a facile strategy to overcome this issue by using self-assembled glycine molecules to treat the NiO x surface. With glycine on the surface, the NiO x exhibited a deeper valence band maximum and a faster charge-extraction at the NiO x /perovskite interface. What's more, the vacuum-deposited perovskite showed a better crystallinity on the NiO x + glycine substrate. As a result, the PSCs with a glycine interfacial layer achieved a champion PCE of 17.96% with negligible hysteresis. This facile approach is expected to be further developed for fabricating high-efficiency PSCs on textured silicon solar cells.

Printing strategies for scaling-up perovskite solar cells.

Photovoltaic technology offers a sustainable solution to the problem of soaring global energy demands. Recently, metal halide perovskite solar cells (PSCs) have attracted worldwide interest because of their high power conversion efficiency of 25.5% and great potential in becoming a disruptive technology in the photovoltaic industry. The transition from research to commercialization requires advancements of scalable deposition methods for both perovskite and charge transporting thin films. Herein, we share our view regarding the current challenges to fabrication of PSCs by printing techniques. We focus particularly on ink technologies, and summarize the strategies for printing uniform, pinhole-free perovskite films with good crystallinity. Moreover, the stability of perovskite solar modules is discussed and analyzed. We believe this review will be advantageous in the area of printable electronic devices.

Ink Engineering for Blade Coating FA-Dominated Perovskites in Ambient Air for Efficient Solar Cells and Modules.

To accelerate the commercial application of organic-inorganic hybrid perovskite solar cells (PSCs), it is necessary to develop simple and low-cost methods to prepare pinhole-free large-area perovskite films with high quality. A one-step blade coating method is regarded as a scalable technique. It is demonstrated that with the addition of N,N'-dimethylpropyleneurea (DMPU) in an FA-dominated perovskite precursor, a large-area high-quality perovskite film can be obtained by blade coating, achieving improved photovoltaic performance, thermal stability, and storage stability. It is found that the strong interaction between DMPU and Pb2+ ions is beneficial to delay the nucleation crystallization process, increase the size of crystal grains, and improve the crystallinity of the perovskite film. Planar n-i-p solar cells introducing DMPU exhibit power conversion efficiencies of 20.20% for 0.16 cm2 devices and 17.71% for 5 × 5 cm2 modules with an aperture area of 10 cm2. In addition, the devices without encapsulation placed at 50 °C for 500 h and with a relative humidity of 20 ± 5% for 1000 h still maintain efficiencies above 80 and 90%, respectively, showing outstanding stability.

Bandgap adjustment assisted preparation of >18% Cs y FA1-y PbI x Br3-x -based perovskite solar cells using a hybrid spraying process.

The preparation of Cs y FA1-y PbI x Br3-x -based perovskite by ultrasonic spraying has valuable application in the preparation of tandem solar cells on textured substrates due to its excellent stability and the advantages of large-area uniform preparation from the spraying technology. However, the bandgap of perovskite prepared by spraying method is difficult to adjust, and perovskites with a wide bandgap have the issue of phase instability. Here, we improved the crystallinity of the perovskite by simply controlling the post-annealing temperature. The results show that perovskite film prepared by hybrid spray method has the best crystallinity and device performance at a post-annealing temperature of 170 °C. On this basis, the bandgap of perovskite was changed from 1.53 eV to 1.76 eV by controlling the ratio of the organic halogen precursor solution. When the bandgap is 1.57 eV, a perovskite solar cell with an efficiency of 18.31% is obtained.

Multifunctional Polymer-Regulated SnO2 Nanocrystals Enhance Interface Contact for Efficient and Stable Planar Perovskite Solar Cells.

Perovskite solar cells (PSCs) have rapidly developed and achieved power conversion efficiencies of over 20% with diverse technical routes. Particularly, planar-structured PSCs can be fabricated with low-temperature (≤150 °C) solution-based processes, which is energy efficient and compatible with flexible substrates. Here, the efficiency and stability of planar PSCs are enhanced by improving the interface contact between the SnO2 electron-transport layer (ETL) and the perovskite layer. A biological polymer (heparin potassium, HP) is introduced to regulate the arrangement of SnO2 nanocrystals, and induce vertically aligned crystal growth of perovskites on top. Correspondingly, SnO2 -HP-based devices can demonstrate an average efficiency of 23.03% on rigid substrates with enhanced open-circuit voltage (VOC ) of 1.162 V and high reproducibility. Attributed to the strengthened interface binding, the devices obtain high operational stability, retaining 97% of their initial performance (power conversion efficiency, PCE > 22%) after 1000 h operation at their maximum power point under 1 sun illumination. Besides, the HP-modified SnO2 ETL exhibits promising potential for application in flexible and large-area devices.

Interface modification effect on the performance of CsxFA1-xPbIyBr3-y perovskite solar cells fabricated by evaporation/spray-coating method.

In this study, high quality CsxFA1-xPbIyBr3-y perovskite thin films were successfully fabricated by an evaporation/spray-coating hybrid deposition method. In this method, CsI and PbI2 were first deposited via thermal evaporation, and then FAI/FABr mixed solution was sprayed on the CsI/PbI2 substrate to form the CsxFA1-xPbIyBr3-y film. As confirmed by x-ray diffraction, scanning electron microscopy, and atomic force microscopy, a perovskite film with full surface coverage and small surface roughness was obtained. Then, the effect of interface modification materials on the performance of perovskite solar cells (PSCs) was investigated: the devices with the [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) interlayer incorporated via vacuum evaporation deposition between SnO2 and perovskite showed remarkably higher performance than those with the C60 interlayer, which was attributed to enhanced charge extraction and reduced recombination at the SnO2/PCBM/perovskite interface. As a result, a high power conversion efficiency (PCE) of 18.21% was obtained for the 0.16 cm2 device. To the best of our knowledge, it is the highest efficiency of CsxFA1-xPbIyBr3-y based PSCs fabricated by the spray technique. Furthermore, we fabricated mini-modules with the size of 5 × 5 cm2 and achieved a PCE of 14.7%.

A pressure-assisted annealing method for high quality CsPbBr3 film deposited by sequential thermal evaporation.

All-inorganic CsPbBr3 perovskite solar cells have triggered incredible interest owing to their superior stability, especially under high temperature conditions. Different from the organic-inorganic hybrid perovskites, inorganic CsPbBr3 perovskite always need a high annealing temperature for the formation of a cubic phase. Generally, the higher temperature (over 300 °C) and longer annealing time will promote the growth of CsPbBr3, resulting in larger grain sizes and lower trap density in the crystals. However, CsPbBr3 perovskite can also be damaged by excessive annealing temperature (∼350 °C) and time, since PbBr2 only has a melting temperature close to 357 °C. To address this issue, herein, we developed a novel pressure-assisted annealing method to prevent the sublimation of PbBr2 at high temperature. The CsPbBr3 films were firstly deposited by sequential thermal evaporation, and then annealed at 335 °C in an alloy pressure vessel. By controlling the pressure of the vessel, we obtained CsPbBr3 films with various morphologies. At normal atmospheric pressure, the as-prepared CsPbBr3 film exhibited small grain sizes and was full of pinholes. With the increase of annealing pressure, the grain sizes of the film showed a significant increasing trend, and the pinholes gradually vanished. When the pressure value came to 10 MPa, compact and uniform CsPbBr3 films with large grain sizes were obtained. Based on these films, CsPbBr3 perovskite solar cells with FTO/compact-TiO2/CsPbBr3/carbon architecture achieved a champion power conversion efficiency of 7.22%.

Fabrication of Efficient and Stable Perovskite Solar Cells in High-Humidity Environment through Trace-Doping of Large-Sized Cations.

Solution-processed organic-inorganic lead halide perovskites have shown photovoltaic performance above 23 %, attracting great attention. However, the champion devices require fabrication in a controlled inert/dry atmosphere. The development of highly efficient and stable perovskite solar cells under high-humidity atmosphere conditions for future commercialization is still challenging, especially for CH3 NH3 PbI3 (MAPbI3 ), which is vulnerable to moisture. In this study, a large-sized tert-butylammonium [C(CH3 )3 NH3 + ] organic cation was incorporated into the MAPbI3 crystalline structure, which could form a more stable 3 D crystalline structure and alleviate the decomposition caused by the humidity. It delivered a power conversion efficiency of 19.3 % upon preparation under a humid environment condition of 50 % relative humidity as well as improved humidity and thermal stability. Our work provides a facile strategy for improving perovskite performance and stability by introducing a new chemical additive for the future application of perovskite solar cells.

Enhancing the thermal stability of the carbon-based perovskite solar cells by using a Cs x FA1-x PbBr x I3-x light absorber.

Despite the impressive photovoltaic performance with a power conversion efficiency beyond 23%, perovskite solar cells (PSCs) suffer from poor long-term stability, failing by far the market requirements. Although many efforts have been made towards improving the stability of PSCs, the thermal stability of PSCs with CH3NH3PbI3 as a perovskite and organic hole-transport material (HTM) remains a challenge. In this study, we employed the thermally stable (NH2)2CHPbI3 (FAPbI3) as the light absorber for the carbon-based and HTM-free PSCs, which can be fabricated by screen printing. By introducing a certain amount of CsBr (10%) into PbI2, we obtained a phase-stable Cs x FA1-x PbBr x I3-x perovskite by a "two-step" method and improved the device power conversion efficiency from 10.81% to 14.14%. Moreover, the as-prepared PSCs with mixed-cation perovskite showed an excellent long-term stability under constant heat (85 °C) and thermal cycling (-30 °C to 85 °C) conditions. These thermally stable and fully-printable PSCs would be of great significance for the development of low-cost photovoltaics.

Room-temperature synthesized SnO2 electron transport layers for efficient perovskite solar cells.

Tin oxide (SnO2) is widely used as electron transport layer (ETL) material in perovskite solar cells (PSCs). Numerous synthesis methods for SnO2 have been reported, but they all require a proper thermal treatment for the SnO2 ETLs. Herein we present a simple method to synthesize SnO2 nanoparticles (NPs) at room temperature. By using butyl acetate as a precipitator and a proper UV-Ozone treatment to remove Cl residuals, excellent SnO2 ETLs were obtained without any thermal annealing. The highest power conversion efficiency (PCE) of the prepared PSCs was 19.22% for reverse scan (RS) and 18.79% for forward scan (FS). Furthermore, flexible PSCs were fabricated with high PCEs of 15.27%/14.74% (RS/FS). The low energy consuming SnO2 ETLs therefore show great promise for the flexible PSCs' commercialization.

Universal passivation strategy to slot-die printed SnO2 for hysteresis-free efficient flexible perovskite solar module.

Perovskite solar cells (PSCs) have reached an impressive efficiency over 23%. One of its promising characteristics is the low-cost solution printability, especially for flexible solar cells. However, printing large area uniform electron transport layers on rough and soft plastic substrates without hysteresis is still a great challenge. Herein, we demonstrate slot-die printed high quality tin oxide films for high efficiency flexible PSCs. The inherent hysteresis induced by the tin oxide layer is suppressed using a universal potassium interfacial passivation strategy regardless of fabricating methods. Results show that the potassium cations, not the anions, facilitate the growth of perovskite grains, passivate the interface, and contribute to the enhanced efficiency and stability. The small size flexible PSCs achieve a high efficiency of 17.18% and large size (5 × 6 cm2) flexible modules obtain an efficiency over 15%. This passivation strategy has shown great promise for pursuing high performance large area flexible PSCs.

Enhanced Crystallinity of Low-Temperature Solution-Processed SnO2 for Highly Reproducible Planar Perovskite Solar Cells.

Low-temperature solution-processed SnO2 as a promising electron-transport material for planar perovskite solar cells (PSCs) has attracted particular attention because of its outstanding properties such as high optical transparency or high electron mobility. However, low-temperature sol-gel processes used in the synthesis are inevitably affected by the humidity of the atmosphere, which results in a wide distribution in the performance of the prepared PSCs owing to the inability to control crystallinity and defects. Herein, a highly crystalline SnO2 film is synthesized using a simple water bath post-treatment, which can remove the surface residuals of SnCl4 on the SnO2 films, which is beneficial for the interface charge transport from the perovskite to the SnO2 electron-transport layer. An improved performance of the PSCs can be easily obtained applying this treatment, giving rise to a high power conversion efficiency (PCE) of 19.17 %, much higher than that of the pristine SnO2 -based device (17.59 %). Most importantly, the reproducibility of the devices has been greatly improved, independent of the environmental humidity. Therefore, the enhanced crystallinity of SnO2 has shown promise for future commercial PSC applications: 5 cm×5 cm PSC modules have achieved a PCE of 16.16 %.

Structural and Chemical Changes to CH3 NH3 PbI3 Induced by Electron and Gallium Ion Beams.

Organic-inorganic hybrid perovskites, such as CH3 NH3 PbI3, have shown highly promising photovoltaic performance. Electron microscopy (EM) is a powerful tool for studying the crystallography, morphology, interfaces, lattice defects, composition, and charge carrier collection and recombination properties at the nanoscale. Here, the sensitivity of CH3 NH3 PbI3 to electron beam irradiation is examined. CH3 NH3 PbI3 undergoes continuous structural and compositional changes with increasing electron dose, with the total dose, rather than dose rate, being the key operative parameter. Importantly, the first structural change is subtle and easily missed and occurs after an electron dose significantly smaller than that typically applied in conventional EM techniques. The electron dose conditions under which these structural changes occur are identified. With appropriate dose-minimization techniques, electron diffraction patterns can be obtained from pristine material consistent with the tetragonal CH3 NH3 PbI3 phases determined by X-ray diffraction. Radiation damage incurred at liquid nitrogen temperatures and using Ga+ irradiation in a focused ion beam instrument are also examined. Finally, some simple guidelines for how to minimize electron-beam-induced artifacts when using EM to study hybrid perovskite materials are provided.

Efficient and Stable Inverted Planar Perovskite Solar Cells Using a Triphenylamine Hole-Transporting Material.

Inverted perovskite solar cells (PSCs) with a p-i-n structure have attracted great attention. Normally, inorganic p-type metal oxides or polymers are used as the hole-transport material (HTM), a vital component in the inverted PSCs. However, this type of HTM often requires high processing temperatures and/or high costs. On the other hand, a commonly used organic HTM, poly(3,4-ethylenedioxythiophene polystyrene sulfonate (PEDOT:PSS), is sensitive to humidity and thus affects the stability of the PSCs. Herein, we employ a small molecule, 4,4',4''-tris(N-3-methylphenyl-N-phenylamino) triphenylamine (m-MTDATA) to replace PEDOT:PSS as a new HTM for inverted PSCs. Compared to a PEDOT:PSS-based device, m-MTDATA-based PSCs exhibit enhanced performance. The highest power conversion efficiency (PCE) was notably improved from 13.44 % (PEDOT:PSS) to 18.12 % (m-MTDATA), suggesting that m-MTDATA could be an efficient HTM to achieve high performance inverted PSCs. Furthermore, the m-MTDATA-based device demonstrated improved stability (retaining 90 % PCE) under ambient conditions over 1000 h compared with the PEDOT:PSS-based devices (retaining 40 % PCE).

Low-Temperature Presynthesized Crystalline Tin Oxide for Efficient Flexible Perovskite Solar Cells and Modules.

Organic-inorganic metal halide perovskite solar cells (PSCs) have been emerging as one of the most promising next generation photovoltaic technologies with a breakthrough power conversion efficiency (PCE) over 22%. However, aiming for commercialization, it still encounters challenges for the large-scale module fabrication, especially for flexible devices which have attracted intensive attention recently. Low-temperature processed high-performance electron-transporting layers (ETLs) are still difficult. Herein, we present a facile low-temperature synthesis of crystalline SnO2 nanocrystals (NCs) as efficient ETLs for flexible PSCs including modules. Through thermal and UV-ozone treatments of the SnO2 ETLs, the electron transporting resistance of the ETLs and the charge recombination at the interface of ETL/perovskite were decreased. Thus, the hysteresis-free highly efficient rigid and flexible PSCs were obtained with PCEs of 19.20 and 16.47%, respectively. Finally, a 5 × 5 cm2 flexible PSC module with a PCE of 12.31% (12.22% for forward scan and 12.40% for reverse scan) was fabricated with the optimized perovskite/ETL interface. Thus, employing presynthesized SnO2 NCs to fabricate ETLs has showed promising for future manufacturing.