Unveiling the Effect of Polymerization Values on the Crystallization Behavior of Perovskite Films for Efficient and Stable Solar Cells

Obtaining high-quality perovskite films is crucial for reducing non-radiative recombination and improving device performance. As reported, the crystallization process of perovskite films can be effectively tailored by introducing multifunctional additives. In this work, polyvinyl pyrrolidone (PVP) with varying degrees of polymerization (DP) values (K15, K30, and K90) are systematically screened and utilized as an additive to regulate defect density and control the crystallization process of perovskite films. This regulation is achieved through coordination interactions between perovskites and their multi-carbonyl groups of PVP. Among the selected polymers, the K30 carbonyl polymer exhibits a suitable viscosity and bind strength, which facilitated a significant delay in the intermediate phase transition and accelerated nucleation. As a result, high-quality perovskite films with enhanced crystallinity, prolonged PL lifetime, and no residual PbI₂ are achieved. The K30-treated MA-free device demonstrated a PCE of 24.16% with a remarkable V_OC of 1.2 V. Moreover, this strategy can be extended to RbCsFAMA quadruple-cation perovskite films, delivering an impressive PCE of 25.35% with a minimal V_OC deficit of 0.35 V. This work provides an effective approach to modulate the crystal growth of perovskite films and highlights the significance of the DP values for polymeric additives.