6-(2-Thienyl)-4H-thieno[3,2-b]indole based conjugated polymers with low bandgaps for organic solar cells

Two copolymers of donor-acceptor (D-A) type based on 6-(2-thienyl)-4H-thieno[3,2-b]indole (TTI), electron-pushing unit, were synthesized and explored in organic solar cells. 4H-thieno[3,2-b]indole (TI), which has indole fused with thiophene, was coupled with another thiophene to form TTI which was connected with electron-pulling units 2,1,3-benzothiadiazole (BT) and 2,2-dimethyl-2H-benzimidazole (MBI) for efficient ICT effect to generate low bandgaps. PTTIBT, with TTI and BT units, and PTTIMBI, with TTI and MBI units, are expected to provide broader absorption spectra and smaller bandgaps than polymers based on carbazole. PTTIBT and PTTIMBI, with good solubility in organic solvents, show optical bandgaps of 1.69 and 1.57 eV in films, respectively. The PTTIMBI shows higher PCE value as compared to that of PTTIBT with the device fabricated using a PTTIMBI:PC71BM(1:1.5w/w) blend.