Effect of semiconducting nature of ZnO interfacial layer on inverted organic solar cell performance

Abstract

The light-soaking effect is one of the major drawbacks for inverted organic solar cells (OSCs) if metal oxides are used as the electron transport layer (ETL). The oxide ETL primarily originates the above effect from the energy barrier, deep level defects, and excess carriers tunneling. Here, electron-beam evaporated high-quality pristine and post-treated e-ZnO thin films are utilized to fabricate inverted OSC as the ETL between the transparent cathode and active bulk-heterojunction PBDB-T-2Cl:PC61BM layer to study the influence on device performance. Various experimental techniques, including AFM, XRD, XPS, and UPS, are utilized to identify the surface and semiconducting properties of differently treated interfacial e-ZnO films precisely. XPS results reveal the variation of oxygen vacancies and adsorbed oxygen species on the surface of e-ZnO layers. The semiconducting nature of various e-ZnO thin films for the use of ETL are also probed with the help of UPS results, which accurately locate the valence band maximum and Fermi level position. After correlating the property of e-ZnO systematically with the respective OSC device performance, it is found that the deeper valence band top and higher n-type nature of e-ZnO is desirable to depict the light soaking free highest solar cell efficiency and large open-circuit voltage of about 0.97 V in a single junction. The presence of lesser chemisorbed oxygen species over the e-ZnO surface might be an added advantage to demonstrate the light soaking free operation in inverted OSC devices.