Homologous Bromides Treatment for Improving the Open-circuit Voltage of Perovskite Solar Cells.

Solution-processed organic-inorganic mixed halide perovskite solar cells have achieved rapid improvements in power conversion efficiency (PCE). However, it is imperative to minimise voltage deficit (Woc = Eg /q-Voc ) for the state-of-the-art cells for their PCE to approach the theoretical limit. Herein, we report the strategy of depositing homologous bromide salts on the perovskite surface to achieve a surface and bulk passivation for the fabrication of solar cells with high open-circuit voltage. A Woc of 0.39 V (Voc = 1.24 V) in 1.63 eV bandgap (determined by EQE) perovskite device (PCE = 22.1%) and a Woc of 0.38 V (Voc = 1.18 V) in 1.56 eV bandgap perovskite device (PCE = 23.7%) are demonstrated on 0.159 cm2 . Meanwhile, we also achieved an efficiency of 21.8% for a large area device of 1 × 1 cm2 . Distinct to the conclusions given by previous works that the homologous bromides such as FABr only react with PbI2 to form a large bandgap perovskite layer on top of the original perovskite, our work finds that the bromide also penetrates the perovskite film and passivates the perovskite in the bulk. This is confirmed by the small bandgap enlargement observed by absorbance and photoluminescence (PL), bromide element ratio increasing in bulk by time-of-flight secondary ion mass spectrometry (TOF-SIMS) and depth-resolved X-ray photoelectron spectroscopy (XPS). Furthermore, a clear suppression of non-radiative recombination in the perovskite device was confirmed by a variety of characterisations. The best non-encapsulated device with homologous bromide passivation maintained 97% of its initial efficiency after 2500 hours of ambient storage and 59% after 520 hours of thermal stability test at 85°C. Our work provides a simple and universal way to reduce the Woc of single-junction perovskite solar cells and it would also shed light on developing other high performance optoelectronic devices, including perovskite-based tandems and light-emitting diodes (LEDs). This article is protected by copyright. All rights reserved.