Impact of PbI₂ Passivation and Grain Size Engineering in CH₃NH₃PbI₃ Solar Absorbers as Revealed by Carrier-Resolved Photo-Hall Technique

Abstract

With power conversion efficiencies now exceeding 25%, hybrid perovskite solar cells require deeper understanding of defects and processing to further approach the Shockley-Queisser limit. One approach for processing enhancement and defect reduction involves additive engineering—, e.g., addition of MASCN (MA = methylammonium) and excess PbI2 have been shown to modify film grain structure and improve performance. However, the underlying impact of these additives on transport and recombination properties remains to be fully elucidated. In this study, a newly developed carrier-resolved photo-Hall (CRPH) characterization technique is used that gives access to both majority and minority carrier properties within the same sample and over a wide range of illumination conditions. CRPH measurements on n-type MAPbI3 films reveal an order of magnitude increase in carrier recombination lifetime and electron density for 5% excess PbI2 added to the precursor solution, with little change noted in electron and hole mobility values. Grain size variation (120–2100 nm) and MASCN addition induce no significant change in carrier-related parameters considered, highlighting the benign nature of the grain boundaries and that excess PbI2 must predominantly passivate bulk defects rather than defects situated at grain boundaries. This study offers a unique picture of additive impact on MAPbI3 optoelectronic properties as elucidated by the new CRPH approach.