Improved photovoltaic performance of two-dimensional low band-gap conjugated polymers with thieno[3,2-b]thiophene and diketopyrrolopyrrole units by altering pendent position of conjugated side chain

• Two-dimensional (2-D) polymers with thienothiophene-alt-diketopyrrolopyrrole frame were synthesized.
• Photovoltaic performance was improved for the 2-D polymers by altering pendent position.
• Polymer P2 pending alkyl-thiophene groups into thienothiophene core exhibited higher PCE.
• The P2-based device presented a high hole-mobility of 0.74 × 10−4 cm2 V−1 s−1.

Two-dimensional low band-gap copolymers of P1 and P2 with a framework of (2,5-di(thiophen-2-yl)thieno[3,2-b]thiophene)-alt-(2,5-bis(2-ethylhexyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione) (DTTT-alt-DTPP) were synthesized, in which two 2,3-dioctylthiophene-5-yl groups were pended into the 3-positions of two thiophenyl rings close to the thieno[3,2-b]thiophene (TT) unit for P1 and the 3, 6-positions of the TT unit for P2, respectively. The influence of pending positions on optophysical and photovoltaic properties were studied. Similar broad absorption spectra from 350 to 950 nm were observed with a low band-gap of 1.25 eV. However, significantly different photovoltaic properties were obtained in the bulk heterojunction polymer solar cells with a device structure of ITO/PEDOT:PSS/active layer/LiF/Al. The P1-based devices showed a power conversion efficiency (PCE) of 0.79%, but the P2-based devices displayed a notably increasing PCE up to 2.12% under an AM 1.5G irradiation with an intensity of 100 mW cm−2. Our results indicate that altering pendent position of alkyl thiophene is effective way to improve the photovoltaic properties for the two-dimensional copolymers.