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The traditional way to stabilize α‐phase formamidinium lead triiodide (FAPbI3) perovskite often involves considerable additions of methylammonium (MA) and bromide into the perovskite lattice, leading to an enlarged bandgap and reduced thermal stability. This work shows a seed‐assisted growth strategy to induce a bottom‐up crystallization of MA‐free perovskite, by introducing a small amount of α‐CsPbBr3/DMSO (5%) as seeds into the pristine FAPbI3 system. During the initial crystalization period, the typical hexagonal α‐FAPbI3 crystals (containing α‐CsPbBr3 seeds) are directly formed even at ambient temperature, as observed by laser scanning confocal microscopy. It indicates that these seeds can promote the formation and stabilization of α‐FAPbI3 below the thermodynamic phase‐transition temperature. After annealing not beyond 100 °C, CsPbBr3 seeds homogeneously diffused into the entire perovskite layer via an ions exchange process. This work demonstrates an efficiency of 22% with hysteresis‐free inverted perovskite solar cells (PSCs), one of the highest performances for MA‐free inverted PSCs. Despite absented passivation processes, open‐circuit voltage is improved by 100 millivolts compared to the control devices with the same stoichiometry, and long‐term operational stability retained 92% under continuous full sun illumination. Going MA‐free and low‐temperature processes are a new insight for compatibility with tandems or flexible PSCs. [ABSTRACT FROM AUTHOR]

The commonly used electron transport material (6,6)-phenyl-C61 butyric acid methyl ester (PCBM) for perovskite solar cells (PSC) with inverted planar structures suffers from properties such as poor film-forming. In this manuscript, we demonstrate a simple method to improve the film-forming properties of PCBM by doping PCBM with poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) as the electron transport layer (ETL), which effectively enhances the performance of CH3NH3PbI3 based solar cells. With 5 wt % F8BT in PCBM, the short circuit current (JSC) and fill factor (FF) of PSC both significantly increased from 17.21 ± 0.15 mA·cm−2 and 71.1 ± 0.07% to 19.28 ± 0.22 mA·cm−2 and 74.7 ± 0.21%, respectively, which led to a power conversion efficiency (PCE) improvement from 12.6 ± 0.24% to 15 ± 0.26%. The morphology investigation suggested that doping with F8BT facilitated the formation of a smooth and uniform ETL, which was favorable for the separation of electron-hole pairs, and therefore, an improved performance of PSC. [ABSTRACT FROM AUTHOR]

Lead halide complexes are efficient precursors to obtain high quality perovskite films and high performance solar cells. However, how the lead halide complex affects the crystallization kinetics and thus the film quality of perovskite has not been systematically studied. Herein, two PbI 2 (X) complexes with different coordination strength of ligand X in complexation with PbI 2 (PbI 2 (DMF) and PbI 2 (DMSO)) are synthesized. The correlation between the morphology, evolution of the perovskite formation, defects state, carrier lifetime and the subsequent photovoltaic performance are investigated in details via a two-step coating method with various PbI 2 (X) complexes. We find that PbI 2 (DMSO) derived perovskite film shows larger grains and vertically oriented grain boundaries as well as enhanced photoluminescence intensity and longer carrier lifetime. As a result, highly efficient inverted planar perovskite solar cells with a power conversion efficiency up to 17.0% are achieved from PbI 2 (DMSO) complex under 1 sun illumination. This research could open up a new pathway to further improve the performance of perovskite solar cells through the control of perovskite crystallization kinetics by choosing proper PbI 2 (X) complex precursors. [ABSTRACT FROM AUTHOR]