Perovskite solar cells (PSCs) have received great attention due to their ever-increasing power conversion efficiency (PCE), low-cost materials, and easy solution preparation. The certified efficiency of PSCs reached 25.5% [1] based on a lab scale, exceeding the performance of copper indium gallium selenide (CIGS) and CdTe thin-film solar cells and approaching the highest reported value of the mainstream silicon solar cell. [2] The highest-efficiency devices generally use the regular n-i-p structure composed of a perovskite absorber between a metal oxide electron transport layer (MOETL) and an organic hole transport layer (HTL). [1,3] Miyasaka et al. first adopted CH 3 NH 3 PbI 3 and CH 3 NH 3 PbBr 3 as light absorbers in liquid electrolyte-based dyesensitized solar cells using a thick TiO 2 layer (%10 μm) as an electron collector. The first PSCs achieved a PCE of 3.8%. [4] After that, tremendous efforts have been made for the PSCs based on TiO 2 . The PSCs included an n-type TiO 2 ETL, a perovskite absorber layer, a p-type 2,2 0 ,7,7 0tetrakis-(N,N-di-4-methoxyphenylamino)-9,9 0 -spirobifluorene (Spiro-OMeTAD) HTL, which has been the most typical and successful structure. Grätzel et al. achieved the first certified efficiency of 14.14% based on the aforementioned n-i-p structure with compact TiO 2 (c-TiO 2 )/mesoporous TiO 2 (m-TiO 2 ). [5] In the following years, the performance of the PSCs based on TiO 2 achieved tremendous progress. To remove the organic material in TiO 2 paste and achieve high-quality m-TiO 2 layer, a hightemperature sintering process (>500 C) was generally needed. [6] To avoid high-temperature processing, low-temperature processed SnO 2 was developed. Tian and co-workers adopted SnO 2 thin films attained by spin coating SnO 2 nanoparticles on indium-doped tin oxide (ITO) followed by low-temperature annealing at 200 C, and a 13% PCE was achieved. [7] Huge progress of SnO 2 -based PSCs was reported by Fang and co-workers. They adopted thermal decomposition of SnCl 2 •2H 2 O precursor at 180 C in ambient air to form SnO 2 film and achieved 17.21% efficiency. [8] Later on, Hagfeldt et al. used low-temperature chemical bath deposition to grow SnO 2 ETL and obtained almost hysteresis-free PCEs of 20.8%. [9] In 2019, the PCE of the planar SnO 2 PSCs reached a certified efficiency of 23.32%, which first surpassed that of m-TiO 2 PSCs. [10] Therefore, SnO 2 has increasingly attracted great attention as an ETL for PSCs, and it is considered as the most promising alternative to TiO 2 . Up to date, all the world records of the certified PCE were achieved based on TiO 2 or SnO 2 n-i-p conventional structures since the PSCs appeared in 2009 (Figure 1). [1,3,5,6,[10][11][12][13][14][15] Recently, a PCE of 25.8% (certified 25.5%) has been achieved based on Cl-bonded SnO 2 . [1] Thus, the important key to achieve state-of-the-art PSCs is the application of TiO 2 or SnO 2 , due to its excellent electrical properties.