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%.

Formamidinium‐Based Lead Halide Perovskites: Structure, Properties, and Fabrication Methodologies

“…[47] This strategy has been explored both in single crystals [47] and thin films. [7,10] Ambient stabilization of the desirable α-phase has been demonstrated for both partial substitutions with MA [24,34,42,163] and Cs. [57,125] It was also reported that the stable cubic phase is obtained in FA x MA 1−x PbI 3 for 0.2 ≤ x ≤ 0.9, while the tetragonal phase is obtained for low values of x (high MA content).…”

“…[134] It should also be noted that a significant improvement (three times longer carrier lifetime) was reported for FA x MA 1−x PbI 3 compared with FAPbI 3 . [163] Furthermore, the carrier lifetime in mixed perovskite FA x MA 1−x PbI 3−3x Br 3x single crystals was found to be more than 20 times longer compared with that in FAPbI 3 single crystals. [41] Thus, mixed-composition perovskites can provide a potential advantage, not only in terms of phase stabilization but also in terms of improved charge transport.…”

“…Thus, a common approach to improve the phase stability of FAPbI 3 is to introduce other cations such as MA + or Cs + . Phase stability under ambient conditions has been demonstrated for FA x MA 1−x PbI 3 perovskites, [163] as well as [90] Reproduced with permission. [90] Copyright 2016, ACS Publications.…”

“…In this method, the solution-processed PbI 2 film is exposed to FAI/MAI vapor, and FA x MA 1−x PbI 3 films are formed. [163] A similar approach, i.e., the use Small Methods 2018, 2, 1700387 [43] Reproduced with permission. [43] Copyright 2016, AAAS.…”

Room‐Temperature Partial Conversion of α‐FAPbI3Perovskite Phase via PbI2Solvation Enables High‐Performance Solar Cells

“…It begins with deposition of a metal halide, such as crystalline PbI 2 , which is subsequently exposed to the cation salt and thermally converted into the perovskite phase. This process has been studied and optimized extensively for MAPbI 3 and promises similar high‐quality FAPbI 3 films, but has yielded much lower PCE, given the higher energy of formation of FAPbI 3 compared to MAPbI 3 , as well as the formation of nonperovskite intermediates . Incorporating Cs + in the two‐step approach would be useful to promote conversion like in the one‐step process, but it is quite challenging because of the low solubility of CsI in isopropanol (IPA) solvent used for deposition of methylammonium halide salts .…”