Elucidating Crystallization Pathways in Hybrid Organic–Inorganic Perovskites

Abstract

Hybrid organic–inorganic perovskites (HOIPs), such as methylammonium lead iodide, have attracted much attention for inexpensive, high-performance solar cells. Controlling HOIP thin-film quality and morphology, which is essential to achieve consistent solar-cell efficiencies, requires a fundamental understanding of the link between solution chemistry and crystallization pathways. In this work, we use a multi-technique approach to probe the early stages of HOIP crystal nucleation and growth. Solution-phase nuclear magnetic resonance (NMR) spectroscopy provides information on solution speciation and intermolecular interactions, such as H-bonding between solvent molecules and organic cations. 1H, 207Pb, and 81Br NMR support the formation of solution clusters in weakly coordinating solvents such as dimethylformamide. In situ fluid-cell atomic force microscopy (AFM) allows for the measurement of temperature-dependent crystal growth kinetics and the determination of equilibrium temperatures and kinetic coefficients for growth. Our in situ AFM data reveals the role of formic acid as an additive for HOIP crystal growth. By combining density functional theory calculations and in situ X-ray diffraction, we elucidate the effect of solvent and solution speciation on the crystallization pathway. Our results indicate that the solution coordination chemistry determines the crystallization pathway. Collectively, our findings could help design HOIP ink formulations for a wide variety of solution-based syntheses, providing control over crystallization rate, crystal size, morphology, composition, and defect distribution. These synthesis optimizations will advance our ability to make reproducible, well-defined HOIP films and, thus, high-efficiency devices.