Optimizing Solvent Chemistry for High-Quality Halide Perovskite Films

Abstract

Over the past decade, solution-processed organic–inorganic hybrid perovskite solar cells (PSCs) have emerged as a viable alternative to traditional crystalline silicon photovoltaics, with power conversion efficiency (PCE) increasing notably from 3.8% to over 26%. This remarkable advancement is attributed to the unique band structures and exceptional defect tolerance of the hybrid perovskites. The bandgaps in perovskites derive from their antibonding orbitals at both the valence band maximum and conduction band minimum. Consequently, bond breaking creates states away from the bandgap, resulting in either shallow defects or states within the valence band. Despite defect densities up to 10⁶ times higher than single-crystal silicon, polycrystalline perovskite films (<1 μm thick) can still achieve comparable device performance due to their high defect tolerance. Superior photovoltaic performance in perovskite films depends on an efficient wet-chemical process, offering a notable advantage over silicon-based photovoltaic technology. Evidently, solvent characteristics and their potential interaction with perovskites significantly impact crystal growth from precursor inks, subsequent polycrystalline film quality, and the ultimate performance of devices. Understanding solvent properties in relation to film formation processes is essential for informing solvent selection in the emerging perovskite photovoltaics and its future commercialization. In this Account, we present a thorough analysis of solution-processed perovskite films, encompassing the crystallization process and phase transition of perovskite-related solvated complexes, and structure passivation of perovskite phase. We systematically categorize the prevalent solvents utilized in film preparation and outline a solvent roadmap for producing high-quality perovskite films from a chemical perspective, considering their interaction with the perovskite structure. We also address often-overlooked factors in solvent selection in current research. First, middle-polarity dispersion solvents fundamentally govern nucleation and growth kinetics of perovskite solvated films in the solution phase, thereby significantly shaping film morphology. However, control over the solvation interaction between dispersion solvent and perovskite structure for morphology regulation remains insufficient. Second, high-polarity binding solvents interact with the perovskite structure via solvent-involved intermediates, optimizing crystallization kinetics in the solution phase (sol–gel state) and controlling phase-transition kinetics of the intermediate phase. This interaction influences the crystal and structural properties of the resultant perovskite phase though managing the intermediate phase remains challenging. Third, low-polarity modification solvents, combined with functional passivation molecules, are employed to modulate interface energetics of perovskite films by enabling both chemical defect passivation and physical field-effect passivation. However, achieving optimal interface energetics by forming heterojunctions or homogeneous interfaces through solvent selection is still difficult. By integrating fundamental solvent mechanisms and design criteria, comprehensive strategies can be formulated to achieve high PCE and stability in photovoltaics. Finally, we discuss key challenges and future perspectives in commercializing solution-processed perovskite photovoltaics, with the goal of inspiring innovative material designs and solvent engineering approaches.