Bilayer Low-Temperature-Processed Metal Oxides as Electron Transporting Materials for Perovskite Solar Cells with Over 18% Efficiency

Solar cells which can generate electricity from absorbing sunlight is one of the most promising clean energy devices nowadays. During the last decade, a new type solar cell named metal halide perovskite solar cells (PSC) has undergo a tremendous development and now becoming a superstar in photovoltaic field. However, most of the high-efficiency PSC is made based on a high-temperature-processed (over 500 °C) TiO₂ electron transporting materials (ETM), which hinders the further commercialization of this technology. Therefore, developing a low-temperature-processed ETM to replace the current TiO₂ is urgently required in the PSC research field. In this work, the low-temperature-processed ZnO layer has been used as ETM first. It is found that the perovskite film spin-coated on the ZnO layer is not totally converted to perovskite with a little amount of residue PbI₂, which is bad for PSCs. To solve this problem, we employed a ZnO/SnO₂ bilayer as ETM to enhance the contact between ETM and perovskite film. It is clearly observed that the formation of perovskite (CH₃NH₃PbI₃) on ZnO/SnO₂ bilayer ETM is feasible and no PbI₂ remining in the perovskite film after thermal annealing, as evidenced by X-ray diffraction (XRD) results. We further conduct Atomic Force Microscope (AFM) measurements of the films. The AFM images of CH₃NH₃PbI₃ films deposited on ZnO and ZnO/SnO₂ show that perovskite film is smoother on ZnO/SnO₂ bilayer ETM. Furthermore, it is found that the PSC based on ZnO/SnO₂ bilayer ETM shows a higher power conversion efficiency (PCE) of 18.0% compared to the 14.9% obtained in the control device based on pristine ZnO ETM. The enhancement of device performance with ZnO/SnO₂ bilayer device is due to the improvement of device photovoltaic parameters. It is found that the fill factor of the target device with ZnO/SnO₂ bilayer ETM is 80.7%, significantly improved from 69.2% of the control device based on pristine ZnO layer. This can be ascribed to the efficient charge extraction ability of ZnO/SnO₂ bilayer ETM as demonstrated by the photoluminescence (PL) intensity of CH₃NH₃PbI₃ spin coated on ZnO/SnO₂ bilayer. The PL signal of perovskite on ZnO/SnO₂ bilayer has an obvious blue shift, indicating that the perovskite on ZnO/SnO₂ bilayer has less defects compared to perovskite in ZnO layer. Finally, by using the ZnO/SnO₂ bilayer ETM, we successfully fabricated a large-area PSC with an active area of 1.2 cm², exhibiting a PCE of 9.3%. Our results show a great potential of the ZnO/SnO₂ bilayer ETM for both PSC and other optoelectronic devices.