Controlling Crystallization of Perovskite Layers and Understanding the Effects of By-products

Abstract

Perovskite solar cells (PSCs) have arisen as a game changer in photovoltaic technology due to their outstanding optoelectronic properties and low-cost processability. However, realizing both high efficiency and long-term stability remains a major challenge, primarily due to uncontrolled crystallization and unstable phase transitions of perovskite layers. My doctoral research has focused on controlling the crystallization kinetics and phase stability to achieve highly crystalline and stable perovskite thin films. Through various strategies based on organic cations and halides, the growth of perovskite layers were effectively regulated and systematically investigated. In Chapter 2, the incorporation of alkylammonium chlorides (RACl) into the formamidinium lead iodide (FAPbI3) precursor enabled precise control over crystallization kinetics, preferred orientation, and film morphology. In particular, propylammonium chloride (PACl) effectively facilitated the phase transition and yielded smooth, defect-minimized films, leading to a record power conversion efficiency of 25.73% in 2022, along with excellent operational stability. This work was published in Nature (Nature, 2023, 616, 724). In Chapter 3, a quasi-2D scaffolding approach was applied to Cs-rich wide-bandgap perovskites, improving film crystallinity and phase stability. The quasi-2D acted as an intermediate scaffold, resulting in high crystallinity, uniform surface morphology, while reducing defect density. This study was published in Small (Small, 2025, 21, 2500197). In Chapter 4, the spontaneous formation of formamidinium-based by-products, such as methylformamidinium iodide (MFAI) and propylformamidinium iodide (PFAI) were identified as key factors influencing the phase transition and stability of FAPbI3. The characteristics of the by-products are governed by the size of the cations, which in turn affects their decomposition kinetics and molecular configuration. Compared with MFAI, PFAI promotes a more favorable phase transition and enhances the phase stability of FAPbI3. Consequently, the adoption of PACl over MACl leads to improved long-term stability of perovskite solar cells, owing to the decreased residual MA+ and MFA+, and increased retention of PFAI. This research is still ongoing state, and the manuscripts are being prepared. Overall, my doctoral research work provides fundamental insights into the crystallization and phase transition mechanisms of perovskites, offering effective design principles for achieving highly efficient and stable PSCs.