Mixed-Organic-Cation (FA)x(MA)1–xPbI3 Planar Perovskite Solar Cells with 16.48% Efficiency via a Low-Pressure Vapor-Assisted Solution Process

Abstract

Compared to that of methylammonium lead iodide perovskite (MAPbI), formamidinium lead iodide perovskite (FAPbI) has a smaller energy band gap and greater potential efficiency. To prevent the transformation of α-FAPbI to δ-FAPbI, preparation of (FA)(MA)PbI was regarded as an effective route. Usually, the planar (FA)(MA)PbI perovskite solar cells are fabricated by a solution process. Herein, we report a low-pressure vapor-assisted solution process (LP-VASP) for the growth of (FA)(MA)PbI perovskite solar cells that features improved electron transportation, uniform morphology, high power conversion efficiency (PCE), and better crystal stability. In LP-VASP, the (FA)(MA)PbI films were formed by the reaction between the PbI film with FAI and MAI vapor in a very simple vacuum oven. LP-VASP is an inexpensive way to batch-process solar cells, avoiding the repeated deposition solution process for PbI films, and the device had a low cost. We demonstrate that, with an increase in the MAI content, the (101) peak position of FAPbI shifts toward the (110) peak position of MAPbI, the (FA)(MA)PbI perovskites are stable, and no decomposition or phase transition is observed after 14 days. The photovoltaic performance was effectively improved by the introduction of MA with the highest efficiency being 16.48% under conditions of 40 wt % MAI. The carrier lifetime of (FA)(MA)PbI perovskite films is approximately three times longer than that of pure FAPbI. Using this process, solar cells with a large area of 1.00 cm were fabricated with the PCE of 8.0%.