Amphiphilic Diblock Fullerene Derivatives as Cathode Interfacial Layers for Organic Solar Cells

A new amphiphilic diblock fullerene derivative [6,6]-phenyl-C₆₁-butyricacid-4-(9,9,9',9'-tetrakis(3-bromopropyl)-9H,9'H-[2,2'-bifluoren]-7-yl)phenol-(N,N,N-trimethylpropan-1-aminium) bromide (C₆₀-4TPB) was synthesized and applied in organic solar cells. Solvent annealing by toluene could obviously induce the self-assembly of the C₆₀-4TPB layer, which can be tested by the measurements of the water contact angle. After the treatment with toluene, a vertical-like arrangement in the ultrathin layer of the C₆₀-4TPB molecule will be formed between electron-collecting zinc oxide (ZnO) layers and the active layer (blend system of PTB7:PC₇₁BM), leading to the improvement of the interfacial compatibility between the active layer and the ZnO layer. On the top surface of the C₆₀-4TPB layer, the C₆₀ molecules can be expected to induce the enrichment of PC₇₁BM and block the hole, resulting in further increase in the open-circuit voltage (V_OC) and fill factor (FF). After spin-coating the C₆₀-4TPB solutions onto the ZnO layer with a concentration of 0.5 mg/mL in dimethyl sulfoxide, obviously improved performances were obtained with a power conversion efficiency of 8.07%, which can be attributed to the optimized interface morphology between hydrophilic ZnO and hydrophobic PTB7:PC₇₁BM.