Incorporating a weak acceptor unit into PTB7-Th to tune the open circuit voltage for non-fullerene polymer solar cells

PTB7-Th is one of the most effective polymer donors which have drawn huge attention in organic solar cells. However, the highest occupied molecular orbital (HOMO) level of PTB7-Th is relatively high, which limit open circuit voltage (V_oc) of the cell devices. In this work, to develop the new members of PTB7-Th family materials with higher V_ocs, two new random terpolymers PTB7-Th(0.9) and PTB7-Th(0.8) have been synthesized, in which contains 10% or 20% benzo[1,2-b:4,5-b′]dithiophene-2,6-dicarboxylate (VBDTC, a weak acceptor unit) moiety in PTB7-Th. It was found that the incorporating of VBDTC unit has great influence on optical and electronic properties of the related polymers. Comparison with PTB7-Th, the PTB7-Th(0.9) and PTB7-Th(0.8) exhibit blue shift in the UV−visible spectra, which forming a more complementary absorption with the low band gap non-fullerene acceptors, like ITIC and Y6. Increasing the percentage of VBDTC moiety, it can sufficiently deepen the low-lying HOMO level of the related polymers, which would expect to an enhancement in V_oc. Non-fullerene solar cells blended with ITIC, PTB7-Th(0.9) exhibited a V_oc of 0.839 V, with power conversion efficiency (PCE) of 4.86%. Interestingly, PTB7-Th(0.8) showed superior photovoltaic performance with a V_oc of 0.862 V, and PCE of 5.24%. And when blended with Y6, the PCE of PTB7-Th(0.8) based device was further improved to 6.13%, due to increasing the short-circuit current (J_sc). The comprehensive study reveals that it is an effective strategy to tune the absorption spectra and deepens the HOMO energy level of the PTB7-Th related polymers by incorporation of a weak acceptor unit, thus, achieving higher V_oc as well as photovoltaic performance.