Equilibrating bonding energy between solvent and solute for optimized crystallization enables efficient perovskite solar cells†

Low-cost solution processing has enabled perovskite solar cells to rapidly improve their efficiency. However, the uncontrolled morphology of the photoactive layer hinders their further enhancement. Specifically, the solvent–solute interactions in the precursor solution lead to a special stage during film formation, namely the solvate phase, playing a vital role in determining the quality of perovskites via subsequent phase transitions. We herein investigate the impact of host/guest solvent systems on the crystallisation of photovoltaic perovskites and their corresponding effect on device figures of merit. In particular, we found that the barrier of the dominating phase transformation process could be manipulated by balancing the interaction of solvent–PbI₂ and solvent–FAI. Additionally, we reveal that the strong binding energy between the solvent and PbI₂ restrains solvent evaporation at the bottom of the perovskite crystals, leading to the presence of voids during annealing. Enhanced solvent–FAI intermolecular interactions, combined with the improved miscibility between the solvent component and antisolvent could facilitate the extraction of solvents from the saturated perovskite precursor solution, avoiding the formation of voids. By systematically manipulating the above processes, we obtained an optimized morphology with enhanced crystal quality with device efficiency increasing from 22.06% to 23.24%. We believe the simple methodology presented here provides new insights into understanding the solution-solid transitions of photovoltaic perovskites.