Polymer solar cells based on spontaneously-spreading film with double electron-transporting layers

Although metal oxides such as ZnO are frequently used in inverted polymer solar cells (PSCs) to serve as an electron-transporting layer, how to effectively improve its interface with the photoactive layer is still an open question. In this study, we exploited small-molecule (SM) compounds usually used as electron acceptors in PSCs to act as an interlayer on ZnO and constructed an SM/ZnO double electron-transporting layer to improve the device's photovoltaic performance. To avoid the solvent erosion to SM interlayer during the deposition of upper-lawyer film, spontaneously-spreading-film technique was employed to prepare the photoactive layer. We used two SM acceptors (ITIC and IDT-BOC6) with different energy levels to serve as the interlayer. Results show that devices based on ITIC interlayer, which has similar LUMO level with materials in its adjacent layers, possess a better electron-transporting ability than IDT-BOC6 based ones which lack of such an energy-level structure. Moreover, SM interlayer is beneficial to flatten the interface and improves the contact between the electron-transporting layer and the photoactive layer. Consequently, we demonstrate a new way to improve the collection of electrons in PSCs by using commonly available electron acceptors, which provides a large choosability in interfacial materials.