Atomistic modelling – impact and opportunities in thin-film photovoltaic solar cell technologies

Impact of iodine antisite (IPb) defects on the electronic properties of the (110) CH3NH3PbI3 surface

The power conversion efficiency of perovskite solar cells can be significantly improved if recombination losses and hysteresis effects, often caused by the presence of structural and chemical defects present at grain boundaries and interfaces, can be minimized during the processing of photoactive layers. As a crucial first step to address this issue, we performed density functional theory calculations to evaluate the electronic structure of the energetically favored (110) perovskite surface in the presence of the widely reported I_Pb antisite defects. Our calculations indicate that the nature of trap states formed is different for the perovskite surface with exposed methylammonium (MAI) and lead iodide (PbI₂) terminating groups. While, in MAI terminated surfaces, I_Pb antisite defects lead to shallow states close to the valence band, both deep and shallow states are created in the bandgap region in the PbI₂ terminated surface. Furthermore, we determined contribution from individual atoms to the trap states and inferred that the trap states originate from the clusters of iodine atoms that are formed near the defect site. The exact nature of the defect state is strongly correlated with the atomic structure of these clusters and can be potentially tuned by controlling the processing conditions of the perovskite film.