Changing the π-bridge from thiophene to thieno[3,2-b]thiophene for D-π-A type polymer enables high performance fullerene-free organic solar cells

For photovoltaic polymers with a D–π–A backbone, there are a great deal of D and A units, but the choice of π bridge is relatively limited and thiophene (T) is still the most effective one. Here, we utilize two D–π–A polymers, J52-FS with thiophene as the π bridge and PE2 with thieno[3,2-b]thiophene (TT) as the π bridge, to combine with a low band-gap non-fullenere Y6, respectively. The photovoltaic cells based on PE2:Y6 can realize a PCE of 13.50%, which is obviously higher than the 10.58% PCE of the J52-FS:Y6 analogue. Our work demonstrates that using TT as the π-bridge is a simple and promising approach to construct efficient photovoltaic polymers.