Polyvinyl Pyrrolidone Induced “Confinement Effect” on PbI2 and the Improvement on Crystallization and Thermal Stability of Perovskite

Polyvinyl pyrrolidone is blended in PbI₂ with varied concentration, so as to study the coarsening dynamics of perovskite during the two-step growth method. It is observed that polyvinyl pyrrolidone hinders the crystallization of PbI₂ and helps to form a more amorphous PbI₂ matrix, which then improves perovskite crystallization. As the blending concentration increases from 0 to 2 mM, average crystallite/grain size of perovskite increases from 40.29 nm/0.79 µm to 84.35 nm/1.02 µm while surface fluctuation decreases slightly from 25.64 to 23.96 nm. The observations are caused by the "confinement effect" brought by polyvinyl pyrrolidone on PbI₂ . Elevating blending concentration of polyvinyl pyrrolidone results in smaller PbI₂ crystallites and more amorphous PbI₂ matrix, thus reducing the diffusion/reaction barrier between PbI₂ and organic salt and favoring perovskite crystallization. As blending concentration increases from 0 to 2 mM, the device efficiency rises from 19.76 (± 0.60) % to 20.50 (± 0.89) %, with the optimized value up to 22.05%, which is further improved to 24.48% after n-Octylammonium iodide (OAI)-basing surface modification. The study enlarges the scope of "confinement effect" brought by polymer molecules, which is beneficial for efficient and stable perovskite solar cell fabrication.