Imidazole-Functionalized Fullerene as a Vertically Phase-Separated Cathode Interfacial Layer of Inverted Ternary Polymer Solar Cells

By using a facile one-pot nucleophilic addition reaction, we synthesized a novel imidazole (IMZ)-functionalized fullerene (C60-IMZ), and applied it as a third component of inverted ternary polymer solar cells (PSCs), leading to dramatic efficiency enhancement. According to FT-IR, XPS spectroscopic characterizations, and elemental analysis, the chemical structure of C60-IMZ was determined with the average IMZ addition number estimated to be six. The lowest unoccupied molecular orbital (LUMO) level of C60-IMZ measured by cyclic voltammetry was −3.63 eV, which is up-shifted relative to that of 6,6-phenyl C61-butyric acid methyl ester (PC61BM). Upon doping C60-IMZ as a third component into an active layer blend of poly(3-hexylthiophene) (P3HT) and PC61BM, the power conversion efficiency (PCE) of the inverted ternary PSCs was 3.4% under the optimized doping ratio of 10 wt %, dramatically higher than that of the control device ITO/P3HT:PC61BM/MoO3/Ag based on the binary P3HT:PC61BM blend (1.3%). The incorporation of C60-IMZ results in enhancement of the absorption of P3HT:PC61BM blend film, increase of the electron mobility of the device, and rougher film surface of the P3HT:PC61BM active layer beneficial for interfacial contact with the Ag anode. Furthermore, C60-IMZ doped in P3HT:PC61BM blend may migrate to the surface of ITO cathode via vertical phase separation as revealed by XPS depth analysis, consequently forming a cathode interfacial layer (CIL), which can lower the work function (WF) of ITO cathode. Thus, the interfacial contact between the active layer and ITO cathode is improved, facilitating electron transport from the active layer to ITO cathode. The effectiveness of C60-IMZ as a vertically phase-separated CIL on efficiency enhancement of inverted ternary PSCs is further verified by doping it into another active layer system comprised of a low-bandgap conjugated polymer, poly(thieno[3,4-b]-thiophene/benzodithiophene) (PTB7), blended with [6,6]-phenyl C71-butyric acid methyl ester (PC71BM). Under the optimized C60-IMZ doping ratio of 10 wt %, the PCE of the PTB7:PC71BM-based inverted ternary PSC device reaches 5.3%, which is about 2 times higher than that of the control binary device (2.6%).