Controlling Crystallization of Methylammonium Lead Iodide
Authors:   , Jain Crystal Nayak , Baxter Jason B. , Fafarman Aaron T. 
Published: August 2022

Perovskites are a fast-emerging material for photovoltaic applications and offer many benefits in terms of manufacturability, material availability, and tunable band gaps. Methyl ammonium lead iodide (MAPbI3) is an organic-inorganic halide perovskite of interest for solar power since it is solution processable in many solvents, in particular N, N Dimethyl formamide (DMF) and gamma-Butyrolactone (GBL). Controlling the crystallization from these solvents to produce films with large grain sizes and smooth surfaces are important for photovoltaic applications. Crystallization of perovskites from DMF is already well studied. Experiments with a commercial roll-to-roll slot-die coater showed that drying at room temperature in ambient conditions causes macroscopic defects on the edges of the film, forming long lengths of needle-like crystals rather than grains. Less ambient drying time during coating improved the microstructures of the films. Larger wet film thickness showed some larger grain sizes but also needlelike crystals as film thickness increased. Increasing the concentration of the ink with the same wet film thickness (which kept amount of solvent to be evaporated approximately the same) showed worse morphology but improved coverage due to the increase in perovskite deposited. Keeping the estimated dry film thickness constant (which decreased drying time with increased concentration) and increasing concentration of ink lead to more needlelike crystals in the film. Overall, these experiments showed that kinetic effects from drying did not affect the overall morphology of the film but did affect the size of the crystals. Faster drying lead to higher nucleation rates with smaller grain sizes, while slow drying lead to the opposite effect. Regardless of kinetics, it is apparent that the needlelike crystals, which may derive from a PbI2:DMF Lewis adduct, are an effect of thermodynamics, and, in this study, could only be overcome by raising the temperature at which the film dries. In GBL, MAPbI3 is retrograde soluble at temperatures above 60°C which can be leveraged to encourage bottom-up crystallization as well. We sought to understand the effect of temperature on the morphology of crystals formed from GBL. To study this, droplets of highly concentrated ink were dropped onto glass substrates and observed through in-situ microscopy on a heated stage. In GBL, the black phase crystallized directly out of solution above 60°C, even with evaporation. The yellow phase, which may be the solvent-solute complex of MAPbI3 in GBL, is what first appeared out of solution at temperatures below 60°C and held its morphology once converted to the black phase by further evaporation. This agreed with conclusions that were made in the previous studies, where morphology is dependent on thermodynamics. The possibility of using intense illumination to overcome kinetic barriers of inverse temperature crystallization was explored through use of a fluorescence microscope with a partially sealed cell to allow for slow evaporation. It was hypothesized that some specific light induced crystallization mechanism exists, but other wavelengths were not evaluated. The initial results are reported here with blue light illumination and show the movement of yellow phase crystals in solution towards already forming black phase crystals.