Synthesis and photovoltaic properties of narrow band gap copolymers of dithieno[3,2-b:2′,3′-d]thiophene and diketopyrrolopyrrole

Two new sets of donor-acceptor and donor-π-acceptor type copolymers with a narrow band-gap, PDTTDPP and PTDTTTDPP, based on dithieno[3,2-b:2′, 3′-d]thiophene (DTT) and dithieno[3,2-b:2′,3′-d]thiophene bridged with thiophene (TDTTT) as a short π-conjugated spacer with diketopyrrolopyrrole (DPP), were successfully synthesized by Stille polymerization. Incorporation of linear long chain alkyl (decanyl) groups in the DTT core and branched alkyl (ethylhexyl) groups in the DPP unit improved the solubility and processability of the resulting co-polymers. UV-vis absorption spectroscopy and cyclic voltammetry results demonstrated that introduction of thiophene as a shorter conjugated spacer between the donor and acceptor units in the PTDTTTDPP copolymer facilitated tuning of the absorption capability between the donor and acceptor resulting in a red-shifted absorption as compared to the PDTTDPP copolymer. Thermogravimetric analysis of the copolymers showed high thermal decomposition temperatures of 375°C for PDTTDPP and 370 °C for PTDTTTDPP. The hole mobility of the copolymers, measured using an organic thin film transistor device, was 1.0 × 10^-6 cm² for PDTTDPP and 4.5 × 10^-3 cm² for PTDTTTDPP. Photovoltaic properties of PDTTDPP and PTDTTTDPP with PC₆₁BM were evaluated using different device fabrication conditions and photovoltaic devices based on the polymer:PC₆₀BM (1:1 ratio (w/w)) bulk hetero-junction demonstrated a maximum PCE of 1.39% and 0.29% for PTDTTTDPP:PC₆₁BM and PDTTDPP:PC₆₁BM, respectively, in chlorobenzene.