Design, synthesis and photovoltaic properties of a new D-π-A polymer with extended π-bridge units

New low band gap copolymers PBDTT-DTTBT and PBDTT-DTBT using BDT-T and BT as donor and acceptor, with different π-bridge units thieno[3,2-b]thiophene and thiophene, respectively, were synthesized. The absorption spectra, electronic energy levels, and photovoltaic properties of the polymers were characterized. PBDTT-DTTBT and PBDTT-DTBT films show broad absorption in the visible range with an absorption edge at over 751 nm, and possess relatively low-lying HOMO levels at -5.11 eV and 5.15 eV. A high hole mobility of 1.97 × 10^-3 cm² V^-1 s^-1 was recorded for PBDTT-DTTBT, that is two orders of magnitude higher than PBDTT-DTBT (1.58 × 10^-5 cm² V^-1 s^-1), which is ascribed to the application of thieno[3,2-b]thiophene as π-bridge units. The PCE of the PSC device based on PBDTT-DTTBT/PC₇₀BM (1-1, w/w) reached 6.03% with a J_sc of 12.46 mA cm^-2, a V _oc of 0.78 V and a FF of 0.62, while PBDTT-DTBT/PC₇₀BM (1-1, w/w) only has a PCE of 2.34% with a V_oc of 0.82 V, a J _sc of 5.78 mA cm^-2 and a FF of 0.495 under the illumination of AM 1.5 G, 100 mW cm^-2. These results indicate that PBDTT-DTTBT is a promising photovoltaic material. Furthermore, it can be concluded that extending the conjugation of the backbone units, as demonstrated in this work, has little influence on the molecular energy levels, but can be seen as a feasible strategy to improve the photovoltaic properties of D-π-A conjugated polymers by enhancing intermolecular interactions.