Roughness and structural modification of PbI2 thin films by isopropanol treatment to improve methylammonium lead halide formation and solar cell efficiency

Perovskite thin films can be prepared by two-step spin coating technique, in which lead iodide (PbI2) is first deposited followed by methylammonium halide. The addition of isopropanol (IPA) in the first deposition leads to more porous and less crystalline PbI2 thin films. To understand the effect of IPA treatment, we study the surface morphology parameters of PbI2 thin films by using statistical fractal analysis on their Atomic Force Microscopy images at different image scales. We found that the surface roughness of IPA-treated PbI2 layer (r-PbI2) is almost twice than that of the PbI2 layer without such treatment (c-PbI2). Moreover, the surface of r-PbI2 thin films shows a self-similar characteristic, which means that the roughness pattern may repeat at different scales. This is confirmed by perfilometry measurement, showing a uniform roughness pattern of r-PbI2 samples in a millimeter scale. Furthermore, larger grain size and crystallinity of perovskite thin films are obtained with r-PbI2, independent of the partial substitution of iodide by chloride or bromide. The perovskite solar cells based on r-PbI2 exhibit average power conversion efficiency (PCE) of 10.27% over 127 cell samples, whereas those fabricated with c-PbI2 films show a lower average PCE of 7.20% over 107 samples.