power conversion effi ciency (PCE) more than 20% has been achieved using a solution process. [6][7][8][9][10] Among the different processes for fabrication of perovskite solar cells, including one-step solution process, [ 11,12 ] sequential deposition, [ 13,14 ] vacuum deposition, [ 15 ] and chemical vapor deposition, [ 16 ] a two-step solution process has benefi ts such as good control of the crystal structure for perovskite fi lm. [ 17 ] In order to fabricate perovskite solar cells, different types of electron transfer layers (ETLs) and hole transfer layers (HTLs) have been employed. Among them, a conducting polymer such as poly(3,4-ethylenedioxythiophene):polystyr enesulfonate (PEDOT:PSS) is commonly used as the HTL in perovskite solar cells. However, the high acidity of this polymer deteriorates the ITO layer, and its high hygroscopicity can also accelerate degradation of the device due to water absorption. [ 18 ] Recently, many research groups have focused on inorganic materials that can work as an HTL, such as CuSCN and NiO. [19][20][21] Regarding the ETL materials, more stable metal oxides, such as titanium dioxide (TiO 2 ), zinc oxide (ZnO), and tin oxide (SnO 2 ) have been developed for perovskite solar cells. [ 22 ] Among these metal oxides, TiO 2 -nanoparticles (NPs) have been commonly used as the ETL to provide an effi cient charge collection. [ 23 ] In order to prepare a TiO 2 mesoporous layer, a sintering process with high temperature (≈500 °C) is required. [ 18 ] This temperature is too high for fabrication of perovskite on plastic substrate. In addition, few groups already have reported ETL-free perovskite solar cell with descent PCE on plastic substrates; however, their PCE is not comparable with those devices fabricated on rigid substrates.ZnO NPs are, therefore, a promising candidate to replace TiO 2 NPs as the ETL layer in perovskite solar cells. However, only a few works have reported perovskite solar cells based on ZnO NPs. Intriguingly, the devices on ZnO NPs show a maximum PCE of close to 15.7%, [ 24 ] which is lower than those on TiO 2 NPs. To fabricate the device on ZnO NPs, a twostep solution process is usually employed. Depending on the annealing temperature (70 °C-100 °C) of the perovskite fi lm, different works have reported a device performance from 8.9% to 15.7%. [24][25][26][27] In addition, a few works have demonstrated the instability of perovskite materials on top of ZnO NPs due to Fabrication of organohalide perovskite materials on the top of ZnO nanoparticles (NPs) has some benefi cial advantages such as room temperature processing; however, the perovskite is not stable on ZnO NPs layer during the annealing process. In fact, there are only a few reports about the fabrication of perovskite solar cells on ZnO NPs layer. Herein, the decomposition mechanism of CH 3 NH 3 PbI 3 perovskite materials on ZnO is reported, and it is found that the perovskite fi lm on the top of the ZnO layer is converted into PbI 2 during the annealing process due to the existence of hydroxide groups on...
