Solution processed zinc oxide (ZnO) nanoparticles (NPs) with excellent electron transport properties and a low-temperature process is a viable candidate to replace titanium dioxide (TiO2) as electron transport layer to develop high-efficiency perovskite solar cells on flexible substrates. However, the number of reported high-performance perovskite solar cells using ZnO-NPs is still limited. Here we report a detailed investigation on the chemistry and crystal growth of CH3NH3PbI3 perovskite on ZnO-NP thin films. We find that the perovskite films would severely decompose into PbI2 upon thermal annealing on the bare ZnO-NP surface. X-ray photoelectron spectroscopy (XPS) results show that the hydroxide groups on the ZnO-NP surface accelerate the decomposition of the perovskite films. To reduce the decomposition, we introduce a buffer layer in between the ZnO-NPs and perovskite layers. We find that a commonly used buffer layer with small molecule [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) can slow down but cannot completely avoid the decomposition. On the other hand, a polymeric buffer layer using poly(ethylenimine) (PEI) can effectively separate the ZnO-NPs and perovskite, which allows larger crystal formation with thermal annealing. The power conversion efficiencies of perovskite photovoltaic cells are significantly increased from 6.4% to 10.2% by replacing PC61BM with PEI as the buffer layer.

We show the effects of chlorine incorporation in the crystallization process of perovskite film based on a lead acetate precursor. We demonstrate a fabrication process for fast grain growth with highly preferred {110} orientation upon only 5 min of annealing at 100 °C. By studying the correlation between precursor composition and morphology, the growth dynamic of perovskite film in the current system is discussed. In particular, we found that both lead acetate precursor and Cl incorporation are beneficial to perovskite growth. While lead acetate allows fast crystallization process, Cl improves perovskite crystallinity. Planar perovskite solar cells with optimized parameters deliver a best power conversion efficiency of 15.0% and average efficiency of 14.0% with remarkable reproducibility and good stability.

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Despite the advanced progress in power conversion efficiency (PCE), developing air‐processed high‐efficiency perovskite solar cells (PVSCs) for future commercialization is still challenging. Here, we report that besides the general wisdom of the effect of moisture, oxygen in air has a severe impact on the quality of the solution‐deposited perovskite films. Interestingly, as opposed to moisture that induces fast crystallization of PbI2 upon deposition, oxygen exacerbates the wettability of the PbI2 solution on the substrates. We find that simply preheating the substrate and PbI2 solution a fully covered and uniform PbI2 film deposited in air can be obtained. This is possibly due to the increased vapor pressure of the solvent at higher temperatures to reduce the ingress of oxygen and moisture during the PbI2 deposition. Using this simple method, an air‐processed PVSC made under a humid atmosphere of 70% RH has a record PCE of 18.11%. Our work not only reveals the origin of the detrimental effects on perovskite film formation in ambient air, but also provides a simple practical strategy to develop air‐processed high‐efficiency PVSCs for future commercialization.