As mentioned previously in earlier chapters, mesoscopic perovskite solar cell (PSC) was initially considered as a sensitized solar cell. The hole-transporting materials (HTMs) that had been commonly used in solid-state dyesensitized solar cells, such as spiro-OMeTAD and P3HT, were first used in the PSCs. 1,2–6,7 As more research revealed later that perovskite material has also good charge transport properties, thin film perovskite planar devices were realized and further optimized to have device performance beyond 15%. 8–10 As an example, in Figure 7.1a, the perovskite planar thin film developed through vapor-assisted process by Chen et al., has very smooth thin formation and large grains (Figure 7.1b), both of which are beneficial for the device performance. With the high quality, planar perovskite film fabricated, Chen et al. obtained the high power-conversion efficiency (PCE) of 12.1%, with the J–V curve shown in Figure 7.1c, the highest efficiency for planar architecture PSCs at that time.