2,1,3-benzothiadiazole-5,6-dicarboxylicimide based semicrystalline polymers for photovoltaic cells

Two semicrystalline low band gap polymers based on highly electron-deficient 2,1,3-benzothiadiazole-5,6-dicarboxylicimide (BTI) were synthesized by considering the chain planarity via intrachain noncovalent coulombic interactions. The thiophene-BTI and thienothiophene-BTI based PPDTBTI and PPDTTBTI have a low band gap (∼1.5 eV) via strong intramolecular charge transfer interaction, showing a broad light absorption covering 300∼850 nm. Semicrystalline film morphology was observed for both polymers in the grazing incidence wide angle X-ray scattering measurements. Interestingly, PPDTBTI showed a pronounced edge on packing structure but PPDTTBTI showed predominantly a face on orientation in both pristine and blend films. Different packing patterns influenced significantly the charge carrier transport, recombination and resulting photovoltaic characteristics. The best power conversion efficiency was measured to be 5.47% for PPDTBTI and 6.78% for PPDTTBTI, by blending with the fullerene derivative, PC₇₁BM. Compared to the PPDTBTI blend, PPDTTBTI: PC₇₁BM suffered from the lower open-circuit voltage but showed the substantially higher hole mobility and short-circuit current density with smaller charge recombination, showing very good agreements with molecular structures and morphological characteristics.