The Stabilization of Formamidinium Lead Tri‐Iodide Perovskite through a Methylammonium‐Based Additive for High‐Efficiency Solar Cells

Abstract

Nowadays, complex chemistry and precursor solution compositions are developed to stabilize hybrid perovskite films and boost the efficiency of perovskite solar cells (PSCs). In this context, determining the actual composition of these layers, especially in organic cations, and understanding the chemistry behind is challenging. Herein, the introduction of methylammonium (MA+) in formamidinium lead iodide (FAPbI3) 3D perovskite is considered to stabilize the α‐phase, whose quantity must be minimized to reduce the material hydrophilicity and its possible destabilization by degassing. The key effects of methylammonium chloride (MACl) additive on the growth of FA1–xMAxPbI3 perovskite layers are studied. Liquid nuclear magnetic resonance (NMR) is used to analyze the photovoltaic layers. NMR peaks and their origin are identified. The MA and FA content in films actually used in PSCs is reliably measured and prepared over a large additive molar concentration ratio. x is quantified at 0.06 ± 0.01 for pure films, which corresponds to the best entropic compound stabilization. It results in PSCs with a stabilized power conversion efficiency as high as 22.06%. These PSCs are shown to be highly stable under solar irradiation and high moisture.

A Coadditive Strategy for Blocking Ionic Mobility in Methylammonium‐Free Perovskite Solar Cells and High‐Stability Achievement

“…It confirms our observation made for FA 1− x MA x PbI 3 PVK layers prepared in the presence of MACl additive that this element is mostly eliminated upon the annealing step above 150 °C. [ 18,19 ] K element was detected in the samples prepared with KCl additive and appeared quite homogeneously distributed.…”

“…They also promoted the formation of large grains and produced a monolithic structure which is the best one for a fast charge carriers’ transfer to the interfaces. [ 18 ] On XRD patterns in Figure 2e, the presence of PbI 2 was suppressed by KCl, whereas this phase was dramatically reduced in the KCl/NH 4 Cl layer which was mainly made of α‐phase Cs 0.1 FA 0.9 PbI 3 . Moreover, XRD peaks were more intense for the mixture due to a better crystallinity.…”

“…They present long charges carriers diffusion lengths that favor their collection at the contacts, low exciton binding energy in the case of the 3D compounds that favors the charge separation and a direct optical transition. [ 1–19 ] Moreover, their optical bandgap can be tuned by playing on the material composition and can approach the Schockley–Queisser limit for the maximum power conversion efficiency (PCE). [ 9 ] As a result of the recent research efforts from the international scientific community, they now reach a certified record PCE of 25.5%.…”

“…It leads to the formation of pure α‐phase PVK films with large grains due to the presence of chloride ion. [ 18,19 ] However, the full elimination of MA + must be targeted for a better stabilization. [ 20–33 ] The most popular approach for MA‐free PSCs consists in replacing MA + by Cs + and/or by Rb + cations, two alkali metals.…”

Chlorides, other Halides, and Pseudo‐Halides as Additives for the Fabrication of Efficient and Stable Perovskite Solar Cells

“…Besides, long‐term stability is also a challenge for the commercialization and broad diffusion of PSCs [30,31] . Thus, many efforts have focused on fabricating highly oriented crystal films, increasing grain size, reducing traps, increasing heat, humidity, illumination stability, and so on [32–36] . Among all the improvement methods, the employment of halide and pseudo‐halide additives for the preparation of various layers and treatments of interfaces is of major interest.…”