Novel Donor-Acceptor Copolymers Based on Dithienosilole and Ketone Modified Thieno[3,4-b] thiophene for Photovoltaic Application

Two novel donor-acceptor (D-A) copolymers containing dithienosilole (DTS) donor unit and ketone modified thieno[3,4-b]thiophene (TT) acceptor unit, namely, poly{4,4-bis(2-ethylhexyl)dithieno[3,2-b:2,3-d]silole-5,5-diyl-alt-1-(thieno[3,4-b]thiophen-2-yl)-2-ethylhexan-1-one} (PDTSTT) and poly{4,4-bis(2-ethylhexyl)dithieno[3,2-b:2,3-d]silole-5,5-diyl-alt-1-(4,6-bis(4-ethylhexylthien-2-yl)thieno[3,4-b]thiophen-2-yl)-2-ethylhexan-1-one} (PDTSDTTT), were synthesized for the application in polymer solar cells (PSCs), and the effects of thiophene bridge on the structural geometry and photovoiltaic performance have been investigated. The polymer PDTSTT and PDTSDTTT have the electrochemical bandgaps of 1.54 and 2.02 eV, together with low-lying HOMO energy levels of -5.47 and -5.37 eV, respectively. Molecular geometry simulation result shows that compared with PDTSTT, the insertion of thiophene bridge in PDTSDTTT can well relieve the steric hindrance between the TT and DTS unit, as confirmed by the reduced dihedral angle between DTS and DTTT unit. Under the illumination of AM 1.5G, 100 mW/cm(2), the PSC based on PDTSDTTT/PC61BM (1:1, w:w) demonstrates a power conversion efficiency of 1.0%, which is significantly improved in comparison with the device based on PDTSTT/PC61BM (1:2, w:w) under the same experimental condition. This is because the enhanced planarity and increased effective conjugation of the main chain in PDTSDTTT promote the more favorable morphology for charge transportation, although PDTSTT possesses the broader absorption band than PDTSDTTT.