Role of Residual PbI₂ in Two-step Vapor Processed Perovskite Solar Cells with Methylamine Treatment

Methylamine (MA) treatment of vapor-processed perovskite films is used to improve performance of perovskite solar cells (PSC) by a significant increase in open-circuit voltage. In sequential processing of PSCs, the presence of residual PbI<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> in the film impacts device performance. This study aims to understand the role of residual PbI<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> after MA treatment and its effect on PSCs. We characterize properties of MA-treated PbI<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> films and MA-treated perovskite films with different amounts of PbI<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> from two-step vapor processing. Our findings suggest that MA interacts with PbI<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> and changes its optical properties. However, the role of excess PbI<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> in MA-treated and as-deposited PSCs does not change, with Voc improvement after MA treatment depending on the amount of PbI<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> in the as-deposited film. An optimized MA-treatment gives approximately a 10% increase in V<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</inf> compared to untreated PSC devices.