Strategy for the Complete Conversion of Thermally Grown PbI 2 Layers in Inverted Perovskite Solar Cells

We suggest a combination of morphological control and the introduction of an additive as a strategy to completely convert thermally grown a PbI2 layer into the perovskite structure for inverted organic–inorganic hybrid perovskite solar cells (PSCs). For 2-step PSCs based on thermal evaporation, the complete conversion of PbI2 into the perovskite structure is a key parameter for device performance. To achieve complete conversion, we regulated the morphology of the PbI2 layer by controlling the growth rate. Compared to rapidly grown PbI2 layers, a PbI2 layer grown at a slow rate contained pinholes that enhanced the diffusion of methyl ammonium iodide (MAI) from the PbI2 surface. Despite the assistance of the pinholes, the conversion of the PbI2 was incomplete due to the limited interaction between the MAI and PbI2. Complete conversion of the pinhole-rich PbI2 occurred by introducing an additive into the MAI solution. The conversion of PbI2 was measured by grazing-incidence wide-angle X-ray scattering using synchrotron X-rays. By controlling the incidence angle of the X-rays, we quantitatively compared the amount of PbI2 as a function of the X-ray penetration depth. Using a completely converted 50 nm-thick PbI2 layer, we obtained a power conversion efficiency of 9.44%.