ZnO has been widely used as an electron transport layer (ETL) in organic photovoltaics (OPVs) to facilitate photo-generated electrons extraction due to its appropriate band alignment with various active absorbing layers. Typically, ZnO ETLs are prepared by solution-based techniques such as spin coating. However, with spin coating, it is difficult to control the oxygen stoichiometry, which has great impacts on the defects formation and hence materials properties of ZnO. For example, carrier mobility and optical transparency would affect charge transport and light utilization of the active layer. Moreover, surface defects at the interface between ZnO and the active layer could have great impacts on the stability of the devices.

In this work, we deposited ZnO thin films by radio frequency magnetron sputtering and studied the effects of deposition conditions on the materials properties of the ZnO. Furthermore, the sputtered ZnO were integrated in OPVs with a non-fullerene acceptor-based absorber, PM6:Y7. Our results show that sputtered ZnO on indium tin oxide (ITO) coated glass are polycrystalline and have high optical transparency of >85% in the visible region, which is comparable to a bare ITO glass substrate. Substrate temperature, oxygen flow rate ratio of the sputtering gas ([O₂]/[Ar+O₂]) and thickness of the ZnO all have significant effects on the OPV device performance. The roles of oxygen stoichiometry on OPV device performance were investigated by photoemission spectroscopy and J-V measurements. Our results indicate that a higher [O₂]/[Ar+O₂] effectively reduces the concentration of oxygen-related defects, resulting in improved device performance. Utilizing a two-step sputtering process to suppress interfacial defects, solar cells using optimized sputtered ZnO deliver a higher power conversion efficiency than using spin-coated ZnO. Lifetime measurements under controlled environments were carried out to evaluate the stability of the devices. The difference in stability is explained by the ZnO/PM6:Y7 interfacial quality, and can be related to the difference in defect concentration at the ZnO surface.

In summary, we have investigated the effects of different sputtering conditions of ZnO ETLs on PM6:Y7 device performance. Devices using optimized sputtered ZnO ETLs outperform those using spin-coated ZnO ETLs. Moreover, our results suggest that the tuning of ZnO surface defects through the manipulation of oxygen stoichiometry plays a crucial role on the interfacial quality between ZnO ETL and the active layer, which in turn can be used to optimize the interface stability and device lifetime. Our findings highlight the potential of sputtered ZnO as a promising ETL in highly efficient non-fullerene acceptor based organic photovoltaics.