Impact of Oxygen on the Electronic Structure of Triple-Cation Halide Perovskites

Alloyed triple A-cation
perovskites containing a mixture of Cs,
methylammonium (MA), and formamidinium (FA) cations are attracting
intense attention because of their high photovoltaic performance and
relative stability. However, there is limited fundamental understanding
of their vacancy defect behaviour and influence of molecular oxygen
on their electronic and stability properties. In this combined computational–experimental
study, we investigate the (FA,MA,Cs)­Pb­(I,Br)3 model system
with its simulated atomistic structure presented for the first time
and supported by X-ray diffraction data. We examine how iodide vacancies
and O2 molecules influence the local geometry and electronic
structure. Our calculations, supported by Kelvin Probe contact potential
difference and photoluminescence measurements, show that introduction
of O2 leads to a p-doped triple-cation perovskite and passivates
iodide vacancies, resulting in enhanced luminescence efficiency. These
results have important implications for the performance and stability
of mixed-cation perovskites in optoelectronic devices.