Surface-Orientation Elimination of Vapor-Deposited PbI2 Flakes for Efficient Perovskite Synthesis on Curved Solar Cells.

Vapor deposition of perovskite solar cells (PSCs) has attracted considerable interest for its dry processing characteristics. However, a two-step sequential vapor deposition method suffers from ineffective conversion of PbI2 to perovskite with reasons still unclear. In this report, we carefully investigated the crystallization orientation of PbI2 films deposited by physical vapor deposition via synchrotron grazing-incidence wide-angle X-ray scattering (GIWAXS) and observed an asymmetric scattering pattern with respect to the qz-axis. The observed oriented morphology and texture hinder the diffusion of MAI molecules in the PbI2 films synthesized by vapor deposition, resulting in over 15% PbI2 remaining at the buried interface after reaction with MAI vapor. As a result, the MAPbI3 synthesized in this way was also highly oriented, especially in the surface layers. Surface fumigation (SF) step was introduced to decrease the orientational anisotropy of PbI2, which successfully breaks the diffusion barriers of MAI molecules by forming a complex layer on the PbI2 surface with polar solvent vapors, like dimethyl sulfoxide or 1,3-dimethyl-2-imidazolidinone. We infer that the SF treatment changes the vapor-solid reaction mechanism from reaction-crystallization to dissolution-recrystallization, which largely promotes the conversion of PbI2 to perovskite. Defects were reduced in perovskite synthesized in this way, and a p-i-n device with 19.56% efficiency was fabricated, which is among the highest efficiencies reported for sequential-vapor-deposited PSCs. Notably, this method enables the fabrication of conformal perovskite layers on uneven substrates. An exemplary PSC showing efficiency of 8.93% was fabricated on a precurved substrate. We believe that the method is applicable to the fabrication of tandem or curved PSCs that are compatible with wearable or building/autocar-integrated photovoltaics in the future.