Residual PbI2 Beneficial in the Bulk or at the Interface? An Investigation Study in Sputtered NiOx Hole-Transport-Layer-Based Perovskite Solar Cells

Investigation of the perovskite (MAPbI(3); MA(+) = CH3NH3+) crystallization, location of residual lead iodide (PbI2), and its influence on the optoelectronic and photovoltaic properties is important to resolve the low-efficiency issue in radio frequency (RF) sputtered nickel oxide (sp-NiOx) based perovskite solar cells (PSCs). Despite obtaining the perovskite layer from a solution of the stoichiometric ratio of precursor materials using a one-step method (OSM) for spin-coating, residual PbI2 grains are observed (from the top surface scanning electron microscopy (SEM) image) in the bulk of perovskite (deposited on sp-NiOx). To investigate the dependence of residual PbI2 on the preparation methods, we prepared the perovskite (on sp-NiOx) by a two-step method (TSM) in which methylammonium iodide (MAI) solution is spin-coated on top of the deposited PbI2 layer. The cross-sectional SEM image of the TSM-based perovskite device, confirms the presence of the residual PbI2 layer (20-50 nm) at the interface of perovskite/NiOx. Although a higher X-ray diffraction (XRD) peak intensity of residual PbI2 was observed, the higher crystallinity of the perovskite film prepared by TSM was confirmed based on the evaluation of the photoluminescence (PL) peaks and the PL lifetime. In comparison with the devices prepared by OSM, the higher photovoltaic performance of PSCs is observed for devices fabricated using TSM. An investigation on the relation of the device performance with the location of residual PbI2 is presented in this work and can be insightful to further improve the performance of sp-NiOx-based PSCs.