Impact of PC71BM layer on the performance of perovskite solar cells prepared at high moisture conditions using a low temperature annealed ZnO thin film as the electron transport layer

ZnO is a promising electron transport material with high electron mobility compared to TiO2 and SnO2. However, its high basicity and the presence of hydroxyl groups at the ZnO surface induces thermochemical decomposition of hybrid perovskites though proton transfer reactions. In perovskite solar cells (PSCs), these deprotonation reactions produce chemical products at the interface between ZnO and perovskite, which obstacle charge carrier extraction process and lead to low efficiency of the solar cells. In this work, PC71BM thin films of three different thickness, 19, 11 and 6 nm, were deposited on top of ZnO layers, prepared by sol–gel spin coating and annealed at 150 °C. It is found that low temperature prepared ZnO films contain deep trap states, and the effective optical band gap of ZnO/PC71BM double layers is slightly reduced with the thickness of the fullerene derivative. The presence of an interfacial PC71BM layer on top of ZnO enhances the stability of the upcoming perovskite coatings and promotes the passivation of trap states at the ZnO surface. Interestingly, the best PC71BM-passivated PSC, fabricated under relative humidity (RH) of 60–65%, achieves a maximum power conversion efficiency (PCE) of 13.3%, whereas those PSCs with only ZnO as the electron transport layer show an average PCE of 5.5%. However, the stability under continuous illumination of PC71BM based PSCs is significantly lower than expected, probably due to the PC71BM degradation under high RH conditions.