Study On The Preparation And Property Of ZnO-based Perovskite Solar Cells Under Ambient Conditions

Perovskite solar cells are considered as one of the most promising next-generation photovoltaic devices due to their high efficiency,low-cost production and compatibility with the fabrication of flexible devices.The latest certificated efficiency has reached 23.7%,which is comparable to the commercial silicon solar cells.However,most high-efficiency perovskite solar cells are based on high-temperature treated nanostructured TiO₂ as the electron transport layer.High temperature annealing process not only increases the manufacturing cost,but is also unsuitable for the preparation of flexible perovskite solar cells.In addition to similar band position and physical properties,ZnO has a higher electron mobility comparing with TiO₂.Moreover,high crystallinity ZnO film can be obtained by various of low-temperature techniques.Serving as electron transport layer,ZnO is one of the ideal alternatives for TiO₂.Nonetheless,it is usually difficult to form thermally stable perovskite films on ZnO layers,so the efficiency and stability of ZnO-based perovskite solar cells remain relatively low.Additionally,most of device fabrication and measurement processes are completed in the glovebox to ensure efficiency,which seriously hinders the commercialization of perovskite solar cells.Therefore,the preparation of high-efficiency and air-stable ZnO-based perovskite solar cells under ambient condition is still a grand challenge.In view of this,the room-temperature aging and optimizing fabrication process of ZnO layer as well as the modifications of Au nanorods?Au NRs?and cetyltrimethylammonium bromide?CTABr?were proposed to improve the efficiency and stability of the ZnO-based perovskite solar cell under ambient conditions.Perovskite materials deposited on the ZnO layer are easy to decompose during annealing process.The room-temperature aging strategy was used to treat the ZnO layer,aiming to improve the thermal stability of the CH₃NH₃PbI₃?MAPbI₃?film deposited on it.The results showed that the aging step did effectively remove the residual hydroxyl and acetate ligands on the ZnO surface that could accelerate the decomposition of perovskite materials.As expected,the thermally stable ZnO/MAPbI₃ bilayer was achieved.Meanwhile,the environmental stability of the ZnO-based perovskite solar cells fabricated under air conditions were also significantly improved.Although the room-temperature aging had effectively improved the stability of the ZnO-based perovskite solar cells,the efficiencies of devices were still relatively low.Therefore,the preparation processes of ZnO and MAPbI₃ layer were further optimized on the basis of room-temperature aging to improve the device performance.The ultrasonic treatment for the ZnO nanoparticles dispersion effectively improved the dispersibility in the solvent,beneficial for forming a denser ZnO film.Instead of immersing the PbI₂ film into the CH3NH3I solution,both of grain size of the MAPbI₃crystal and compactness of the MAPbI₃ film were significantly improved by spin-coating method.As a consequence,the efficiency of the ZnO-based perovskite solar cell prepared under air conditions was increased from 10.76%to 14.25%.Meanwhile,the average efficiency of the unencapsulated devices could still keep at 86.25%of their initial value after storing at air atmosphere with 40%relative humidity for 45days.The device performance was further optimized by spin-coating the Au NRs N,N-dimethylformamide?DMF?dispersion at the interface of ZnO and MAPbI₃ films.The results presented that the plasmon resonance effect of Au NRs could effectively increase the light absorption of MAPbI₃.Also,the analysis of X-ray absorption near edge structure showed that the injection of generated hot electrons from the Au NRs to the ZnO conduction band could also significantly increase the electron transport rate of ZnO,and balance the transport of interface charges.Furthermore,the DMF solvent using for dispersing Au NRs also could effectively reduce the growth rate of perovskite crystals,increase the diffusion length of the precursor solution,and prolong growth period of perovskite grains.This facilitated the formation of larger-size grains and denser MAPbI₃ film with less grain boundaries.The Au NRs modified ZnO-based perovskite solar cell showed a significantly improved hysteresis effect and an efficiency of up to 16.51%.Importantly,the environment and illumination stabilities of devices were further improved.Although the efficiency and stability of devices have been improved to some extent based on the above research work,there is still a big gap from the actual application level.To further improve the stability of device,the CTABr molecule with hydrophobic long alkyl chain and Br^-ion was selected as a protective layer of the perovskite film.The results showed that the CTABr molecules anchored on the top of MAPbI₃ film as a hydrophobic insulating layer could effectively inhibit the invasion of water.Meanwhile,the density functional theory calculations showed that there was a lock-in effect between CTA⁺and I^-ion.Coupled with the lattice shrinkage induced by the Br^-ion doping,ion migration in the MAPbI₃ film could be suppressed effectively.The CTABr modified MAPbI₃ film exhibited excellent self-healing ability.The efficiency of the as-prepared devices could almost keep constant after storing for 2150 h under a harsh environment with 60-65%relative humidity.