Spray-Deposited Aluminum-Doped Zinc Oxide as an Efficient Electron Transport Layer for Inverted Organic Solar Cells

Spray-deposited thin films of zinc oxide (ZnO) and aluminum-doped zinc oxide (Al-ZnO) are characterized in detail to get insight into the role of a dopant in the matrix. ZnO and Al-ZnO are implemented as electron transport layers (ETLs) in inverted organic solar cells (IOSCs) with PTB7-Th as a donor and IEICO-4F as a nonfullerene acceptor, forming the bulk heterojunction (BHJ) photoactive layer. Organic solar cells (OSCs) based on the ZnO ETL exhibit a short-circuit current density (J_SC) of 24.46 mA/cm² and an open-circuit voltage (V_OC) of 0.68 V, yielding a power conversion efficiency (PCE) of 9.3%. A solar cell based on the Al-ZnO ETL yields a higher J_SC of 25.16 mA/cm² and a V_OC of 0.71 V, resulting in a PCE of 10.5%, which indicates that Al doping improves the device performance. Time-delayed collection field (TDCF) measurements yielded field-independent charge generation for both devices. Furthermore, steady-state photoluminescence (PL), time-resolved PL, and transient absorption measurements confirm reduction in the number of defect states in Al-ZnO thin films compared to ZnO thin films and efficient charge transfer, yielding an overall improved IOSC device performance.