CH3NH3PbI3‐xClx is a commonly used chemical formula to represent the methylammonium lead halide perovskite fabricated from mixed chlorine‐ and iodine‐containing salt precursors. Despite the rapid progress in improving its photovoltaic efficiency, fundamental questions remain regarding the atomic ratio of Cl in the perovskite as well as the reaction mechanism that leads to its formation and crystallization. In this work we investigated these questions through a combination of chemical, morphological, structural and thermal characterizations. The elemental analyses reveal unambiguously the negligible amount of Cl atoms in the CH3NH3PbI3‐xClx perovskite. By studying the thermal characteristics of methylammonium halides as well as the annealing process in a polymer/perovskite/FTO glass structure, we show that the formation of the CH3NH3PbI3‐xClx perovskite is likely driven by release of gaseous CH3NH3Cl (or other organic chlorides) through an intermediate organometal mixed halide phase. Furthermore, the comparative study on CH3NH3I/PbCl2 and CH3NH3I/PbI2 precursor combinations with different molar ratios suggest that the initial introduction of a CH3NH3+ rich environment is critical to slow down the perovskite formation process and thus improve the growth of the crystal domains during annealing; accordingly, the function of Cl− is to facilitate the release of excess CH3NH3+ at a relatively low annealing temperatures.

Benzylamine is introduced as a surface passivation molecule that improves the moisture-resistance of the perovskites while simultaneously enhancing their electronic properties. Solar cells based on benzylamine-modified formamidinium lead iodide perovskite films exhibit a champion efficiency of 19.2% and an open-circuit voltage of 1.12 V. The modified FAPbI films exhibit no degradation after >2800 h air exposure.

Metal halide perovskites have achieved great success in photovoltaic applications during the last few years. The solar to electrical power conversion efficiency (PCE) of perovskite solar cells has been rapidly improved from 3.9% to certified 22.7% due to the extensive efforts on film deposition methods, composition and device engineering. Further investigation on eliminating the defect states in perovskite absorbers is necessary to push forward the PCE of perovskite solar cells approaching the Shockley-Queisser limit. In this review, we summarize the defect properties in perovskite films and present methodologies to control the defects density, including the growth of large size crystals, photo-curing method, grain boundary and surface passivation, and modification of the substrates. We also discuss the defects-related stability and hysteresis issues and highlight the current challenges and opportunities in defects control of perovskite films.

Developing environmentally friendly perovskites has become important in solving the toxicity issue of lead-based perovskite solar cells. Here, the first double perovskite (Cs AgBiBr ) solar cells using the planar structure are demonstrated. The prepared Cs AgBiBr films are composed of high-crystal-quality grains with diameters equal to the film thickness, thus minimizing the grain boundary length and the carrier recombination. These high-quality double perovskite films show long electron-hole diffusion lengths greater than 100 nm, enabling the fabrication of planar structure double perovskite solar cells. The resulting solar cells based on planar TiO exhibit an average power conversion efficiency over 1%. This work represents an important step forward toward the realization of environmentally friendly solar cells and also has important implications for the applications of double perovskites in other optoelectronic devices.

Recently, there have been extensive research efforts on developing high performance organolead halide based perovskite solar cells. While most studies focused on optimizing the deposition processes of the perovskite films, the selection of the precursors has been rather limited to the lead halide/methylammonium (or formamidium) halide combination. In this work, we developed a new precursor, HPbI3, to replace lead halide. The new precursor enables formation of highly uniform formamidium lead iodide (FAPbI3) films through a one‐step spin‐coating process. Furthermore, the FAPbI3 perovskite films exhibit a highly crystalline phase with strong (110) preferred orientation and excellent thermal stability. The planar heterojunction solar cells based on these perovskite films exhibit an average efficiency of 15.4% and champion efficiency of 17.5% under AM 1.5 G illumination. By comparing the morphology and formation process of the perovskite films fabricated from the formamidium iodide (FAI)/HPbI3, FAI/PbI2, and FAI/PbI2 with HI additive precursor combinations, it is shown that the superior property of the HPbI3 based perovskite films may originate from 1) a slow crystallization process involving exchange of H+ and FA+ ions in the PbI6 octahedral framework and 2) elimination of water in the precursor solution state.