Variations in the open-circuit voltage (Voc ) of ternary organic solar cells are systematically investigated. The initial study of these devices consists of two electron-donating oligomers, S2 (two units) and S7 (seven units), and the electron-accepting [6,6]-phenyl C71 butyric acid methyl ester (PC71 BM) and reveals that the Voc is continuously tunable due to the changing energy of the charge transfer state (Ect ) of the active layers. Further investigation suggests that Voc is also continuously tunable upon change in Ect in a ternary blend system that consists of S2 and its corresponding polymer (P11):PC71 BM. It is interesting to note that higher power conversion efficiencies can be obtained for both S2:S7:PC71 BM and S2:P11:PC71 BM ternary systems compared with their binary systems, which can be ascribed to an improved Voc due to the higher Ect and an improved fill factor due to the improved film morphology upon the incorporation of S2. These findings provide a new guideline for the future design of conjugated polymers for achieving higher performance of ternary organic solar cells.
Keywords: energy of charge transfer state; oligomer; open-circuit voltage; power conversion efficiency; ternary organic solar cells.
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