A theoretical study of the molecular passivation of p-/n-type defects on tin- and germanium-mixed perovskite surfaces using Lewis base/acid.

Perovskite solar cells have been attracting considerable attention because of their high power conversion efficiency (PCE). However, their efficiency is compromised by the defect sites on the perovskite surfaces, where charge carriers (excitons) are trapped and recombined. In this study, based on the density functional theory method, we theoretically explore a molecular passivation process for coating a perovskite surface to reduce the defect concentration. This study focuses on Pb-free perovskite materials, such as MASnI, MAGeI, and MASnGeI (MA = CHNH), employing ethylenediamine and iodopentafluorobenzene as excellent passivation molecules. Our theoretical calculations show that the adsorption of these passivation molecules on the Pb-free perovskite surfaces could remove defect energy levels from the bandgap. Particularly, we discuss the effectiveness of a Lewis base to reduce deep defect levels for a Sn-Ge-mixed perovskite. Furthermore, we present a molecule-defect-level interaction model to understand the passivation mechanism, focusing on the chemical effects of Lewis bases/acids.