Investigation of Oxygen Passivation for High-Performance All-Inorganic Perovskite Solar Cells.

Defect passivation using oxygen has been identified as an efficient and convenient approach to suppress nonradiative recombination and improve the photovoltaic performance of hybrid organic-inorganic halide perovskites (HHPs). However, oxygen can seriously undermine the chemical stability of HHPs due to the reaction of superoxide with protonated organic cations such as CH3NH3+ and [(NH2)2CH]+, thus hindering the deep understanding of how oxygen affects their defect properties. Here we substitute free-proton inorganic Cs+ for organic moiety to avoid the negative effect of oxygen and then systematically investigate the oxygen passivation mechanism in all-inorganic halide perovskites (IHPs) from theory to experiment. We find that, in contrast to conventional oxygen molecule passivation just through physisorption on the surface of perovskites, the oxygen atom can provide a better passivation effect due to its stronger interaction with perovskites. The key point to achieve O-passivated perovskites rather than O2 is the dry-air processing condition, which can dissociate the O2 into O during the annealing process. O-passivated IHP solar cells exhibit enhanced power conversion efficiency (PCE) and better air stability than O2-passivated cells. These results not only provide deep insights into the passivation effect of oxygen on perovskites but also demonstrate the great potential of IHPs for high photovoltaic performance with simplified ambient processing.

Single-Crystalline Nanosheets of Hybrid Perovskite Fabricated by a Vapor-Solution Sequential Deposition Route.

Organic-inorganic hybrid perovskite single crystals have attracted much attention due to their superior optoelectronic properties. Herein, we report a facile vapor-solution sequential route to prepare single-crystalline nanosheets of hybrid lead triiodide perovskite. It is found that this two-step deposition is able to fabricate sizeable high-quality single-crystalline nanosheets with no need of delicate control of crystallization conditions such as concentration or temperature for normal single crystal growth. The resulting perovskite nanosheets show good reproducibility and single crystallinity with bright and uniform photoluminescence. Our study provides a promising strategy for scalable fabrication of perovskite single crystals with great potential in optoelectronic applications.

A Two-Dimensional Hole-Transporting Material for High-Performance Perovskite Solar Cells with 20 % Average Efficiency.

A readily available small molecular hole-transporting material (HTM), OMe-TATPyr, was synthesized and tested in perovskite solar cells (PSCs). OMe-TATPyr is a two-dimensional π-conjugated molecule with a pyrene core and four phenyl-thiophene bridged triarylamine groups. It can be readily synthesized in gram scale with a low lab cost of around US$ 50 g-1 . The incorporation of the phenyl-thiophene units in OMe-TATPyr are beneficial for not only carrier transportation through improved charge delocalization and intermolecular stacking, but also potential trap passivation via Pb-S interaction as supported by depth-profiling XPS, photoluminescence, and electrochemical impedance analysis. As a result, an impressive best power conversion efficiency (PCE) of up to 20.6 % and an average PCE of 20.0 % with good stability has been achieved for mixed-cation PSCs with OMe-TATPyr with an area of 0.09 cm2 . A device with an area of 1.08 cm2 based on OMe-TATPyr demonstrates a PCE of 17.3 %.

General Space-Confined On-Substrate Fabrication of Thickness-Adjustable Hybrid Perovskite Single-Crystalline Thin Films.

Organic-inorganic hybrid perovskite single-crystalline thin films (SCTFs) are promising for enhancing photoelectric device performance due to high carrier mobility, long diffusion length, and carrier lifetime. However, bulk perovskite single crystals available today are not suitable for practical device application due to the unfavorable thickness. Herein, we report a facile space-confined solution-processed strategy to on-substrate grow various hybrid perovskite SCTFs in a size of submillimeter with adjustable thicknesses from nano- to micrometers. These SCTFs exhibit photoelectric properties comparable to bulk single crystals with low defect density and good air stability. The clear thickness-dependent colors allow fast visual selection of SCTFs with a suitable thickness for specific device application. The present substrate-independent growth of perovskite SCTFs opens up opportunities for on-chip fabrication of diverse high-performance devices.

Microscopic investigations on the surface-state dependent moisture stability of a hybrid perovskite.

Hybrid organic-inorganic perovskite (HOIP) materials have caught significant attention in photovoltaics and photoelectronics for their outstanding photovoltaic properties. However, their instability to various environment, such as illumination, temperature, moisture and oxygen, hinders their way to commercialization. To figure out the interaction mechanism between H2O and CH3NH3PbI3 (MAPbI3), extensive theoretical studies have been carried out; however, the experimental results are insufficient and inconsistent. Here, we systematically investigate and compare the influence of H2O on MAPbI3 perovskite films with or without DMF) post-annealing in dark or light condition. The interaction between H2O and the surface of pristine MAPbI3 leads to the fusion of grain boundaries thus grain growth into micron level in short-time moisture exposure. While the penetration of H2O into MAPbI3 results in swelled crystalline whisker, cracking into smaller grains in long-time exposure upon the release of H2O. However, no degradation occurs in dark condition. As the DMF post-annealing treatment changes the surface states of MAPbI3, the interactions between the external H2O and internal MAPbI3 significantly varies from the pristine MAPbI3. Three different surface states with different topographies have influence on the interaction process and mechanism with H2O, leading to different decomposition rates, the striped surface that is the most rough among the three and experiencing the minimum change in surface potential with exposure to 80% humidity decomposes into PbI2 fastest. However, the addition of light will once again affect the aforementioned process. It is found that even ambient light could severely speed up the moisture-induced decomposition of MAPbI3, while the N,N-dimethylformamide (DMF) post-annealing treatment significantly improves the stability of MAPbI3 films upon exposure to humidity and illumination, benefiting from the MAI-deficient thus H2O resistant surface.

Tuning the Fermi-level of TiO2 mesoporous layer by lanthanum doping towards efficient perovskite solar cells.

Tuning the band alignment is proved to be an effective way to facilitate carrier transportation and thus enhance the power conversion efficiency (PCE) of solar cells. Doping the compact layer with metal ions or modifying the interfaces among functional layers in perovskite solar cells (PSCs) can appreciably improve the PCE of PSCs. Inspired by the rare earth elemental doping of TiO2, which has witnessed the success in photocatalysis and dye-sensitized solar cells, we firstly demonstrated here that La3+ doping in the mesoporous TiO2 layer of a mesostructured PSC can tune its Fermi level and thus significantly enhance the device PCE. Systematic analysis reveals that doping La3+ into TiO2 raises the Fermi level of TiO2 through scavenging oxygen and inducing vacancies, which subsequently increases the open circuit voltage and the fill factor while reducing the series resistance of the PSC using La3+-doped TiO2 as a mesoporous layer. As a result, a PCE of 15.42% is achieved, which is appreciably higher than the PCE of a device with undoped TiO2 (12.11%).