Polyvinyl Pyrrolidone Induced "Confinement Effect" on PbI and the Improvement on Crystallization and Thermal Stability of Perovskite.

Polyvinyl pyrrolidone is blended in PbI 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 PbI and helps to form a more amorphous PbI 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 PbI . Elevating blending concentration of polyvinyl pyrrolidone results in smaller PbI crystallites and more amorphous PbI matrix, thus reducing the diffusion/reaction barrier between PbI 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.