Intrinsic charge carrier dynamics and device stability of perovskite/ZnO mesostructured solar cells in moisture

Abstract

Understanding the intrinsic behaviour of perovskite solar cells is as important as breaking a new PCE record. In particular, the essential mechanisms (e.g. the effect of moisture, oxygen or temperature) behind their chemical instability remain controversial. In this work, we unconventionally and specially fabricate a robust CH₃NH₃PbI₃ perovskite solar cell device in ∼70% relative humidity (RH) with an un-optimized PCE of 8.76%, because our main aim is to understand the essential mechanisms, not the efficiency, which drastically relies on the device assembling technique. Such a device exhibits excellent chemical and performance stability, with more than 72% of the initial PCE still maintained after 250 days storage under 40 ± 3% RH conditions without any encapsulation. In addition, the mechanism underlying the chemical and device stability is fully understood from the charge carrier dynamics and variation of the perovskite crystal. These high RH conditions may facilitate a slight and rapid decomposition/separation of CH₃NH₃PbI₃ to form a PbI₂ passivation layer as a moisture defender, which on the contrary plays a crucial role in stabilizing the PCE of CH₃NH₃PbI₃ mesoporous solar cells. This PbI₂ insulation layer made the CH₃NH₃PbI₃/ZnO p–n junction a CH₃NH₃PbI₃/PbI₂/ZnO p–i–n junction, and maintained good junction quality. Another key factor contributing to the high stability lies in the formation of regular nano-sized ZnO arrays. Consequent transient photovoltaic (TPV) tests revealed that not only fresh CH₃NH₃PbI₃, but also pure PbI₂ thin films and CH₃NH₃PbI₃ thin films stored for a long time, all possess a long minority carrier life time. In particular, the stored CH₃NH₃PbI₃/ZnO thin films still maintain a relatively fast photogenerated carrier transfer stability compared with the fresh CH₃NH₃PbI₃/ZnO thin film. We demonstrated the relatively positive role of a moderate amount of PbI₂ in perovskite solar cells. This robust device fabrication and TPV measurements give a new perspective for understanding the heterojunction quality and PCE variation in perovskite solar cell fabrication.