Photon energy loss in ternary polymer solar cells based on nonfullerene acceptor as a third component

Abstract

Understanding photon energy loss caused by the charge recombination in ternary blend polymer solar cells based on nonfullerene acceptors (NFAs) is crucial for achieving further improvements in their device performance. In such a ternary system, however, the two types of donor/acceptor interface coexist, making it more difficult to analyze the photon energy loss. Here, we have focused on the origin of the voltage loss behind a high open-circuit voltage (V_OC) in ternary blend devices based on one donor polymer (poly(2,5-bis(3-(2-butyloctyl)thiophen-2-yl)-thiazolo[5,4-d]thiazole) [PTzBT]) and two acceptors, including a fullerene derivative ([6,6]-phenyl-C₆₁-butyric acid methyl ester [PCBM]) and an NFA ((2,2′-((2Z,2′Z)-(((4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-sindaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(4-((2-ethylhexyl)oxy)thiophene-5,2-diyl))bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) [IEICO-4F]), which exhibit V_OC similar to that of fullerene-based PTzBT/PCBM binary devices. From the temperature-dependent V_OC, we found that the effective interfacial bandgap is the same between them: the PTzBT/PCBM/IEICO-4F ternary blend device is the same as the PTzBT/PCBM fullerene-based binary device rather than the PTzBT/IEICO-4F nonfullerene-based binary device. This means that the recombination center of the ternary blend device is still the interface of PTzBT/PCBM regardless of the incorporation of a small amount of NFA. On the basis of detailed balance theory, we found that the radiative and nonradiative recombination voltage losses for PTzBT/PCBM/IEICO-4F ternary devices significantly reduced compared to those of fullerene-based PTzBT/PCBM binary counterparts. This is ascribed to the disappearance of charge transfer absorption due to overlap with the absorption of NFA and the reduction of energetic disorder due to the incorporation of NFA. Through this study, the role of NFAs in voltage loss is once again emphasized, and a ternary system capable of achieving high V_OC resulting from significantly reduced voltage loss in ternary blend solar cells is proposed. Therefore, we believe that this research proposes the guidelines that can further enhance the power conversion efficiency of polymer solar cells.