The Influence of Ammonium Acids Additives on Performance and Stability of Metal Halide Perovskite Solar Cells and Their Crystallization Pathways

Abstract

Additive engineering has been demonstrated as a simple and highly effective approach to mitigate surface defects in metal halide perovskite films and improve power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs), addressing one of the most significant barriers to the commercialization of perovskite-based technologies. However, the interactions between additives and perovskite precursors and films are not fully understood on the molecular level. In this dissertation, we focus on one of the most commonly used amino/ammonium additives, 5-amino/ammonium acid (5-AVA/5-AVAX), to investigate their interactions with perovskite and elucidate how these interactions influence the perovskite crystallization process and film properties. In Chapter 1, I systematically summarize the recent research progress on additive engineering in perovskite solar cell applications and discuss the future research trends. Chapter 2 provides detailed experimental methods for fabrication and characterization of the 5-AVA/5-AVAX modified perovskite thin film and devices. In Chapter 3, I investigate the influence of amino/ammonium acid additives on the prototypical methylammonium lead triiodide (MAPbI3) perovskite through experimental measurements and density functional theory (DFT) calculations and highlight the critical role of halide compositions on film properties and device performance. Specifically, the ammonium-based salts—5-AVAI and 5-AVACl—have stronger interactions with perovskite compared to 5-AVA, leading to better control of perovskite crystallization and formation of larger grains in the final film. Moreover, 5-AVACl significantly reduces the nonradiative recombination, resulting in the best device PCE and stability among these three additives. In Chapter 4, I extend my investigation to explore the role of 5-AVAI and 5-AVACl additives in the formamidinium lead triiodide (FAPbI3) perovskite, which possesses a more ideal bandgap than MAPbI3. The inclusion of 5-AVAI and 5-AVACl is shown to stabilize the photoactive α-FAPbI3 phase and suppress the non-photoactive δ-FAPbI3 formation without distorting the lattice or affecting the bandgap. Notably, Cl− is found to be incorporated into the perovskite lattice during the early stage of crystallization, strengthening its interaction with perovskite and further enhancing the stability of α-FAPbI3. As a result, the devices treated with 5-AVACl achieve a better PCE compared to those with 5-AVAI. In Chapter 5, I study the effect of 5-AVA/5-AVAX additives on the nucleation and crystal growth of MAPbI3 during spin coating and subsequent annealing process. By employing in situ photoluminescence (PL) measurement, we monitor the PL evolution throughout the fabrication process and demonstrate the retarded effect of 5-AVA/5-AVAX additives on perovskite nucleation and crystal growth. Combined with ex situ GIWAXS for structural characterization of the final films, we find that the halide composition in 5-AVA/5-AVAX influences the phase distribution throughout the film. Specifically, 5-AVA and 5-AVACl favor to form two-dimensional (2D) perovskite, with 2D concentrated predominantly at the bottom interface. Conversely, 5-AVAI mainly stays at the perovskite top surface without forming 2D structures.