Organic-inorganic hybrid perovskite solar cells (PSCs) have achieved tremendous development in recent years and their certified PCEs have already reached above 25%. [1][2][3][4][5] Despite the success in obtaining dramatically increased photovoltaic performance, there are certain concerns on the long-term thermal and operational stability, including either the organic-inorganic perovskite absorber layer and/or the organic CTLs. [6][7][8][9] The volatilization of organic components such as methylammonium and formamidinium in these absorber materials is a major obstacle to their stability. [10][11][12][13] To resolve this issue, a facile and effective method is to replace the organic cations completely with inorganic counterparts, such as cesium (Cs þ ).Cesium lead halides (CsPbX 3 , X ¼ I, Br, Cl, or mixed halides) have demonstrated excellent stability toward moisture, light soaking, and high temperatures of up to 300-400 C. [14][15][16][17] Among all the Cs-based PSCs exploited thus far, CsPbBr 3 is the most phase-stable inorganic perovskite but its wide bandgap makes it difficult to realize high-efficiency PSCs. [18][19][20] In contrast, CsPbI 3 possesses an appropriate bandgap of %1.73 eV, but it is structurally unstable under ambient conditions at room temperature, which can spontaneously transform into the undesired yellow phase (δ-phase). [21][22][23] Recently, cesium lead mixed-halide perovskite, CsPbI 2 Br, which balances the trade-off between the reasonable broad bandgap of 1.92 eV and phase stability, has been successfully applied in photovoltaic and semitransparent, even tandem devices. [24][25][26][27] However, in these reported high-efficiency inorganic PSCs, organic compounds, such as 2,2 0 ,7,7 0 -tetrakis-(N,N-di-pmethoxyphenylamine)-9,9 0 -spiro-bifluorene (spiro-MeOTAD) and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA), are generally used as hole transport materials (HTMs) in regular n-i-p structure devices and [6,6]-phenyl C 61 butyric acid methyl ester (PCBM), C 60 , and their derivatives as electron transport materials (ETMs) in regular p-i-n-structured devices. [24][25][26] The relatively high cost and instability of these HTMs and ETMs are undesirable for the large-scale commercialization of PSCs. [28][29][30][31] Thus, the development of affordable and stable inorganic CTLs is essential for the continued evolution of high-performance perovskite photovoltaics. Among the inorganic