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

A new amphiphilic diblock fullerene derivative [6,6]-phenyl-C 61 -butyricacid-4-(9,9,9′,9′-tetrakis(3-bromopropyl)-9 H,9′ H -[2,2′-bifluoren]-7-yl)phenol-( N, N, N -trimethylpropan-1-aminium) bromide (C 60 -4TPB) was synthesized and applied in organic solar cells. Solvent annealing by toluene could obviously induce the self-assembly of the C 60 -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 C 60 -4TPB molecule will be formed between electron-collecting zinc oxide (ZnO) layers and the active layer (blend system of PTB7:PC 71 BM), leading to the improvement of the interfacial compatibility between the active layer and the ZnO layer. On the top surface of the C 60 -4TPB layer, the C 60 molecules can be expected to induce the enrichment of PC 71 BM and block the hole, resulting in further increase in the open-circuit voltage ( V OC ) and fill factor (FF). After spin-coating the C 60 -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:PC 71 BM.