D–A1–D–A2 Copolymers with Extended Donor Segments for Efficient Polymer Solar Cells

Abstract

Typically a donor–acceptor (D–A) design strategy is used for engineering the bandgap of polymers for solar cells. However, in this work, a series of alternating D–A1–D–A2 copolymers PnTQTI(F) were synthesized and characterized with oligothiophenes (nT, n = 1, 2, 3) as the donor and two electron-deficient moieties, quinoxaline and isoindigo, as the acceptors in the repeating unit. We have studied the influence of the donor segments with different numbers of thiophene units and the effect of the addition of fluorine to the quinoxaline unit of the D–A1–D–A2 polymers. The photophysical, electrochemical, and photovoltaic properties of the polymers were examined via a range of techniques and related to theoretical simulations. On increasing the length of the donor thiophene units, broader absorption spectra were observed in addition to a sequential increase in HOMO levels, while the LUMO levels displayed very small variations. The addition of fluorine to the quinoxaline unit not only decreased the HOMO levels of the resulting polymers but also enhanced the absorption coefficients. A superior photovoltaic performance was observed for the P3TQTI-F-based device with a power conversion efficiency (PCE) of 7.0%, which is the highest efficiency for alternating D–A1–D–A2 polymers reported to date. The structure–property correlations of the PnTQTI(F) polymers demonstrate that varying of the length of the donor segments is a valuable method for designing high-performance D–A1–D–A2 copolymers and highlight the promising nature of D–A1–D–A2 copolymers for efficient bulk-heterojunction solar cells.