Shining a light on the photochemistry of methylammonium lead iodide perovskite for solar cell application

Since their first use in solar cells in 2009, lead halide perovskites have been employed in a wide variety of solar cell architectures leading to an impressive 25.5 % power conversion efficiency in 2020. However, the complex properties of perovskite materials as well as their instability in ambient atmosphere is impeding their development. In particular, trap states, through their formation and passivation, are believed to be responsible for the instability and subsequent degradation of the perovskite, as well as decreasing the device performances. In this work, 3 aspects of the methylammonium lead iodide (MAPI) perovskite are investigated. First, the steady-state photoluminescence and degradation of MAPI were investigated using fluorescence spectroscopy, microscopy, and X-ray diffractometry. Photobrightening, due to the passivation of trap states, and photodarkening, due to the degradation of the perovskite by moisture and oxygen, are found to be competing processes, making the onset of degradation difficult to accurately determine. Second, two models used to simulate charge carrier recombination and time-resolved photoluminescence (TRPL) in perovskite materials, and extract information related to trap states are investigated. The importance of the excitation fluence and repetition period used is highlighted. The models are then used to fit TRPL decays of a MAPI thin-film and a bimolecular rate constant of ∼ 50 × 10−20 cm3/ns is obtained. Finally, the optical, morphological, photoluminescence and crystallographic properties, and stability of MAPI and 5-aminovaleric acid iodide-modified MAPI (AVA-MAPI) infiltrated in mesoporous carbon perovskite solar cells are investigated. Superoxide formation was measured using the dihydroethidium fluorescence probe and AVA-MAPI is shown to generate less superoxide than MAPI when infiltrated in mesoporous layers (ZrO2, TiO2/ZrO2 and TiO2/ZrO2/C). This is partially explained by the presence of a uniform capping layer in AVA-MAPI, hindering oxygen diffusion in the film hence reducing superoxide formation