Organic-inorganic hybrid perovskite materials have gained remarkable attention in the photovoltaic community due to their superior optical and electrical properties, including high carrier mobility, large light absorption coefficient, tunable bandgap, low trap density, etc. [1][2][3][4] Nowadays, the certified record power conversion efficiency (PCE) of perovskite solar cells (PSCs) has been boosted from 3.8% to 25.5%, [5,6] benefitting from great endeavors focusing on interface engineering, composition modulation, solvent engineering, crystallinity regulation, etc. [7][8][9] As one of extensively used architectures, the typical planar-type PSC comprises transparent conductive substrate, compact electron transport layer (ETL), perovskite absorber layer, hole transport layer (HTL), and metal electrode. [10][11][12] Thereinto, ETL plays a critical role in extracting and transport electrons from perovskite materials to conducting substrate. Hence, it is crucial to design high-quality ETLs with outstanding electron extraction capability for high-performance PSCs. [13,14] It is known that TiO 2 serves as a conventional widely used ETL that delivers PSCs with efficiencies >20%. [15,16] However, the preparation of TiO 2 ETLs usually suffers from high-temperature sintering processes (>450 C); moreover, the low electron mobility and high-density oxygen vacancy trap states of TiO 2 also restrict the further development of TiO 2 -based PSCs. [17,18] Under such circumstances, researchers are dedicated to explore more promising low-temperature processed ETLs for PSCs, such as SnO 2 , CdS, ZnS, SnS 2 , etc. [19][20][21][22] Of these ETLs, CdS shows unique advantages, including: 1) CdS thin films
