Stabilization and Crystallization Control of α-FAPbI₃—Towards Efficient and Stable Perovskite Solar Cells

Formamidinium lead triiodide (FAPbI₃) is gaining interest with an optimal band gap of ~ 1.5 eV. Although α-FAPbI₃ is the most thermodynamically stable photoactive phase, it can kinetically stabilize δ-FAPbI₃ in ambient. Many attempts were studied to stabilize the black α-phase, for example, pseudo-halide anion engineering, ionic liquid engineering, and incorporation of additives. Incorporating with methylammonium chloride (MACl) as an additive not only induces the stabilized α-FAPbI₃, but also affects the crystallization kinetics of α-FAPbI₃. Along with the role of methylammonium chloride (MACl) as a ‘stabilizer’ in the formation of αFAPbI₃, we pointed out the additional role as a ‘controller’ in the crystallization kinetics. Herein, we examined the crystallization process of MACl, as higher concentration of MACl induces slower crystallization kinetics. Which results in larger grain size and [100] preferred orientation in a-FAPbI₃ films as the MACl concentration increased from 10 to 20, 30, and 40%. Microscopic observations, such as electron back-scattered diffraction (EBSD), confocal photoluminescence (PL), time-resolved photoluminescence (TRPL) mapping, and conductive atomic force microscopy (C-AFM), were conducted to examine the optoelectronic properties of [100] preferentially oriented grains. Reduction in the non-radiative recombination, a longer lifetime of charge carriers and less photocurrent deviation between each grain induce higher short-circuit current density (<i>J_sc</i>) and fill factor (FF) in a-FAPbI₃based perovskite solar cells (PSCs) with MACl40%. Resulting the PSCs with MACl40% to attain the highest power conversion efficiency (PCE) of 23.5%. Furthermore, we confirmed these observations by enhancing the grain size and [100] preferred orientation of the MACl10% sample through a two-step heat treatment. As the lower heat treatment induced suppression of nucleation and enhancement of the crystal growth in a-FAPbI₃films with MACl10%. The two-step processing resulted in an improvement in the PCE from 18.4% to 21.8%. It indicates the impact of adding MACl to control the crystallization kinetics resulting in a preferred orientation and a large grain size.<br/>It highlights the importance of crystallization kinetics resulting in desirable microstructures for device<br/>engineering.