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

Compared to that of methylammonium lead iodide perovskite (MAPbI3), formamidinium lead iodide perovskite (FAPbI3) has a smaller energy band gap and greater potential efficiency. To prevent the transformation of α-FAPbI3 to δ-FAPbI3, preparation of (FA)x(MA)1-xPbI3 was regarded as an effective route. Usually, the planar (FA)x(MA)1-xPbI3 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)x(MA)1-xPbI3 perovskite solar cells that features improved electron transportation, uniform morphology, high power conversion efficiency (PCE), and better crystal stability. In LP-VASP, the (FA)x(MA)1-xPbI3 films were formed by the reaction between the PbI2 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 PbI2 films, and the device had a low cost. We demonstrate that, with an increase in the MAI content, the (101) peak position of FAPbI3 shifts toward the (110) peak position of MAPbI3, the (FA)x(MA)1-xPbI3 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)x(MA)1-xPbI3 perovskite films is approximately three times longer than that of pure FAPbI3. Using this process, solar cells with a large area of 1.00 cm2 were fabricated with the PCE of 8.0%.