Unique insight into phase separation in polymer solar cells from their electric characteristics

A series of polymer solar cells (PSCs) were fabricated with indene-C60 bisadduct (ICBA) or [6,6]-phenyl-C61-butyric acid methyl ester (PC₆₁BM) as an electron acceptor and with PBDT-TS1 as an electron donor. The donor/acceptor (D/A) phase separation was adjusted with different solution processing methods, consisting of cool (room temperature, 20 °C) solution, hot (70 °C) solution and the solutions with solvent additive 1,8-diiodideoctane (DIO). The champion PCE of PSCs with ICBA or PC₆₁BM as an electron acceptor is 4.32% or 5.97% for the active layers prepared from hot solution with DIO additive or cool solution with DIO additive, respectively. The improved PCEs should be attributed to the optimized D/A phase separation in the active layers by adjusting the redistribution of PC₆₁BM or the ICBA among the PBDT-TS1 networks. The degree of phase separation of the active layers with different acceptors was evaluated according to the current density-voltage (J-V) curves of hole-only and electron-only devices. The distribution of PC₆₁BM or ICBA molecules in the normal direction can be simply judged from the symmetry degree of J-V curves of electron-only devices measured under the forward and reverse bias.