less than one decade, the power conversion efficiency (PCE) of perovskite solar cells (PSCs) have rapidly been achieved to 22.7%, i.e., a level that is nearly on par with traditional silicon solar cells. [2] In addition, efficient, flexible, [3] color-tunable, and semitransparent PSCs [4] have also been demonstrated, showing the merit for application in portable devices and building-integrated photovoltaics (BIPV). Therefore, PSCs are considered to be the most promising candidate for next-generation high efficiency solar cell technology, and they hold great potential in photovoltaic market in the near future.Perovskite has a general ABX 3 structure. For newly emerged organic-inorganic halide perovskites for photovoltaic application, A represents methylammonium (namely, CH 3 NH 3 + , abbreviated as MA + ), formamidinium (namely, CH(NH 2 ) 2 + , abbreviated as FA + ), and Cs + (including its mixture with MA + and/or FA + ), B represents metal ions, including Pb 2+ and/or Sn 2+ , and X represents halide anions, such as I − , Br − , and Cl − (including the mixture of them). [1f,5] The basic structures of PSCs are shown in Figure 1a. Generally, there are two kinds of structures, namely, mesoporous structure and planar structure. A mesoporous-structured PSC consists of transport conductive oxide (TCO)/compact layer TiO 2 (cl-TiO 2 )/mesoporous layer (TiO 2 or Al 2 O 3 )/perovskite/ hole transport layer (HTL)/metal anode. The planar-structured PSC has an architecture: cathode/electron transport layer (ETL)/ perovskite/HTL/anode for conventional structure or anode/ HTL/perovskite/ETL/cathode for inverted structure. Both of the mesoporous and the planar PSCs are reported to yield high PCE. [6] As can be seen from Figure 1a, a PSC consists of several layers, including electrodes (cathode and anode), absorber layer, and carrier transport layers. The absorber layer is basically metal halide perovskite (such as MAPbI 3 and FAPbI 3 ), [7] mixed metal halide perovskite (such as FA 1−x MA x Pb(I 1−y Br y ) 3 ), [8] or inorganic perovskite (CsPbI 3 ). [9] For the ETL or HTL, it can be either inorganic (such as TiO 2 , SnO 2 , ZnO, CuI, NiO, etc.) [10] or organic material (such as phenyl-C61-butyric acid methyl ester (PCBM), fullerene (C 60 ), 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD), poly(3,4ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS), copper(I) thiocyanate (CuSCN), polytriarylamine (PTAA), poly(3-hexylthiophene) (P3HT), etc.). [11] Figure 1b simply shows a schematic representation of the interfaces and the energy levels in a PSC with planar structure (regardless of The rapid progress of organic-inorganic metal halide perovskite solar cells (PSCs) has attracted broad interest in photovoltaic community. A typical PSC consists of anode/cathode, a perovskite layer as absorber, and carrier transport layer(s) (electron/hole transport layer(s)), which are stacked together, resulting in multi-interfaces between these layers. Charge extraction and transport in these solar...