Organic-inorganic metal halide perovskite solar cells (PVSCs) have experienced rapid development in recent years, with its power conversion efficiency (PCE) improving from 3.8% in 2009 to an impressive 25.7% in 2021. [1] Thus, the metal halide perovskite is currently considered as a promising emerging photovoltaic (PV) material due to its excellent optoelectronic properties such as high optical absorption coefficient, long charge carrier lifetime, and low nonradiative recombination rates. [2] It is believed that the PVSCs can enter the specific PV market segments, such as building-integrated PV, in the coming years after upscaling the device fabrication and achieving decent device operational stability.The pioneer work of PVSCs was conducted by Miyasaka et al. using CH 3 NH 3 PbBr 3 and CH 3 NH 3 PbI 3 as photoactive materials in the liquid-based dye-sensitized solar cells (DSSCs). [3] The perovskite-based DSSCs achieved an efficiency of 3.8% but exhibited very poor stability because the metal halide perovskites are chemically reactive to the liquid electrolyte. Subsequently, the PCE of perovskite-based DSSCs was further improved to 6.5% using CH 3 NH 3 PbI 3 quantum dots (QDs) as the light absorber in 2011. Although the authors demonstrated enhanced efficiency by this way, the device performance degraded by 80% upon continuous irradiation for only 10 min, because the redox electrolyte dissolved the perovskite QDs due to its strong polar property. [4] To resolve this issue, Park et al. proposed a transition from DSSC to a solid-state PVSC by introducing a solid-state organic hole conductor, namely, 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9-9'spirobifluorene (spiro-OMeTAD). The solid-state hole conductor provides efficient hole extraction from the perovskite layer to the electrode, achieving a record PCE of 9.7%. [5] In addition, the stability of such solid-state PVSCs has improved to over 500 h under AM 1.5 G light illumination. This research has triggered rapid progress in the research activities on PVSCs and the related publications have increased exponentially since 2012. The yearly progress of PVSC is summarized in Figure 1 and the current certified record PCE of PVSCs is 25.7%, achieved by Ulsan National Institute of Science and Technology (UNIST) in 2021. [1]