Insights into the interplay of different recombination mechanisms and their origins (bulk, surface) are provided comparing fresh, light-soaked and aged devices.

Methylammonium (MA)- and formamidinium (FA)-based organic-inorganic lead halide perovskites provide outstanding performance as photovoltaic materials, due to their versatility of fabrication and their power conversion efficiencies reaching over 22%. The proposition of guanidinium (GUA)-doped perovskite materials generated considerable interest due to their potential to increase carrier lifetimes and open-circuit voltages as compared to pure MAPbI. However, simple size considerations based on the Goldschmidt tolerance factor suggest that guanidinium is too big to completely replace methylammonium as an A cation in the APbI perovskite lattice, and its effect was thus ascribed to passivation of surface trap states at grain boundaries. As guanidinium was not thought to incorporate into the MAPbI lattice, interest waned since it appeared unlikely that it could be used to modify the intrinsic perovskite properties. Here, using solid-state NMR, we provide for the first time atomic-level evidence that GUA is directly incorporated into the MAPbI and FAPbI lattices, forming pure GUA MAPbI or GUA FAPbI phases, and that it reorients on the picosecond time scale within the perovskite lattice, which explains its superior charge carrier stabilization capacity. Our findings establish a fundamental link between charge carrier lifetimes observed in photovoltaic perovskites and the A cation structure in ABX-type metal halide perovskites.

The presence of surface and grain boundary defects in organic-inorganic halide perovskite films can be detrimental to both the performance and operational stability of perovskite solar cells (PSCs). Here, we study the effect of chloride additives on the bulk and surface defects of the mixed cation and halide PSCs. We find that using an anti-solvent technique, the perovskite film is divided into two layers, i.e., a bottom layer with large grains and a thin capping layer with small grains. The addition of formamidinium chloride (FACl) into the precursor solution removes the small-grained perovskite capping layer and suppresses the This article is protected by copyright. All rights reserved. 2 formation of bulk and surface defects, providing a perovskite film with enhanced crystallinity and large grain size of over 1 μm. Time-resolved photoluminescence measurements show longer lifetimes for perovskite films modified by FACl and subsequently passivated by 1adamantylamine hydrochloride (ADAHCl) as compared to the reference sample. Impedance spectroscopy measurements show that these treatments reduce the charge carrier recombination in the PSCs, leading to a champion device with power conversion efficiency of 21.2% and a V oc of 1152 mV under AM1.5 solar spectrum, and with negligible hysteresis during current-voltage sweeps. The Cl treated PSC also shows improved operational stability with only 12% PCE loss after 700 h under continuous illumination.

Interfacial studies and band alignment engineering on the electron transport layer (ETL) play a key role for fabrication of high‐performance perovskite solar cells (PSCs). Here, an amorphous layer of SnO2 (a‐SnO2) between the TiO2 ETL and the perovskite absorber is inserted and the charge transport properties of the device are studied. The double‐layer structure of TiO2 compact layer (c‐TiO2) and a‐SnO2 ETL leads to modification of interface energetics, resulting in improved charge collection and decreased carrier recombination in PSCs. The optimized device based on a‐SnO2/c‐TiO2 ETL shows a maximum power conversion efficiency (PCE) of 21.4% as compared to 19.33% for c‐TiO2 based device. Moreover, the modified device demonstrates a maximum open‐circuit voltage (Voc) of 1.223 V with 387 mV loss in potential, which is among the highest reported value for PSCs with negligible hysteresis. The stability results show that the device on c‐TiO2/a‐SnO2 retains about 91% of its initial PCE value after 500 h light illumination, which is higher than pure c‐TiO2 (67%) based devices. Interestingly, using a‐SnO2/c‐TiO2 ETL the PCE loss was only 10% of initial value under continuous UV light illumination after 30 h, which is higher than that of c‐TiO2 based device (28% PCE loss).