New narrow-band-gap thiazoloquinoxaline-containing polymers and their use in solar cells with bulk heterojunction

Two new regioregular polymers P1 and P2 with structure of type D–A1–D–A2 have been prepared. The polymers exhibit strong light absorption in the range 300–1100 nm and have band gaps of 1.09 and 1.11 eV, respectively. The HOMO and LUMO energies for P1 and P2 are–5.08/–3.81 and–5.16/–3.85 eV, respectively. Polymer solar cells (PSC) based on P1: PC₇₁BM (1: 2, v/v) and P2: PC₇₁BM (1: 1, v/v) have open-circuit voltage V _oc, short circuit current J _sc, and efficiency of 0.79 and 0.84 V, 8.32 and 9.54 mA/cm², 3.5 and 4.7%, respectively. The PSC based on P2 exhibits higher characteristics due to the presence of fluorine atoms in the structure: their strong electron-withdrawing properties decrease the HOMO level of polymer P2 as compared with that of P1, which increases the V _oc value. Moreover, the formation of additional S???F contacts leads to the growth of ordering and crystallinity of polymer P2 as compared with P1, which favors an increase in the values of J _sc and filling factor.