The first application of isoindigo-based polymers in non-fullerene organic solar cells

Although isoindigo (IID)-based polymers can realize high charge mobility, these materials are currently confined to fullerene-based organic solar cells (OSCs). Herein, we designed a pair of alternative D-pi-A type copolymers,PE71andPE72, to expand the application in non-fullerene OSCs, where benzo[1,2-b:4,5-b ']thiophene (BDT), thieno[3,2-b]thiophene (TT) and IID units were used as D, A and pi-bridge, respectively. The aim of optimizing the length of alkyl chains on TT bridge is to ensure polymer solubility, crystallinity as well as miscibility with acceptor molecules. We find thatPE71andPE72exhibit similar optical and electronic properties, butPE71with shorter hexyl chain tends to aggregate into fiber-like structure. In the end, Y6 is selected as the electron acceptor because of suitable energy levels and complementary absorption spectrum. Finally,PE71:Y6 device realizes a power conversion efficiency (PCE) of 12.03%, which is obviously higher than that ofPE72:Y6 device (9.74%) and is also the highest value for IID-based photovoltaic polymers. The charge transport, molecular aggregation, film morphology and energy loss analysis were systematically investigated. The improved photovoltaic performance ofPE71:Y6 mainly originates from the better interpenetrating network structure toward facilitating exciton seperation and free charge carrier transportation. Our results indicate that IID-based D-pi-A polymers can also be utilized in non-fullerene OSCs and the alkyl chains on the thieno [3,2-b]thiophene pi-bridge have a vital effect on the photovoltaic performance.