Effect of Co-Solvents on the Crystallization and Phase Distribution of quasi-2D Perovskites

Quasi-2D perovskites are promising candidates for stable and highly efficient solar cells. This class of perovskites consists of both small cations that fit in the A-site, such as methylammonium (MA), and larger spacers, such as butylammonium (BA). The resulting structure is a layered perovskite with perovskite layers (which number is referred to as n) sandwiched between the bulky spacers. On top of the superior environmental stability compared to their traditional 3D counterpart, q2D PVKs possess tunable optoelectronic properties that can be exploited for a variety of applications. Changing the number of inorganic layers between the spacers, for example, allows for bandgap tuning. However, centering the phase distribution on a specific n-value is challenging. Together with a variety of processing techniques, solvent engineering has previously been used in the literature to tune both phase purity and device efficiency. Nevertheless, no reports have investigated the effect of co-solvents on the crystallization mechanism and the reasons behind the changes in phase distribution.

In our work, we investigated the kinetics of film formation of BA₂MA₃Pb₄I₁₃ processed from DMF/co-solvent mixtures (DMSO and NMP) via in-situ absorption measurements. By combining these results with optical and crystallographic characterization, we found that the crystallization starts at the liquid-air interface as a 3D perovskite, and it proceeds towards the bottom by forming a variety of perovskite structures with different values of n. We correlated these changes in phase distribution at the bottom of the film with the solvent-precursors interaction in the perovskite precursor solution. The interaction of co-solvents with the precursors influences the availability of MA and BA to react, thus strongly impacting the crystallization of different perovskite phases and the efficiency of resulting solar cell devices.