Straight chain D-A copolymers based on thienothiophene and benzothiadiazole for efficient polymer field effect transistors and photovoltaic cells

Three types of linear and planar-structured donor (D)-acceptor (A) type alternating copolymers were synthesized by incorporating intrachain noncovalent Coulombic interactions, based on thieno[3,2-b]thiophene and benzothiadiazole (BT) moieties. The chain linearity and fine adjustment of interchain organization by the incorporation of different numbers of electronegative fluorine atoms onto BT, significantly affected the frontier energy levels, film morphology, and the resulting charge transport properties. The semi-crystalline morphology and charge carrier transport properties were studied by grazing incidence wide-angle X-ray scattering and polymer field-effect transistor (PFET) characteristic measurements. A hole mobility as high as 0.1 cm² V^-1 s^-1 in PFET was obtained for poly[2,5-bis(decyltetradecyloxy)benzene-alt-4,7-bis(thieno[3,2-b]thiophene)-5,6-difluoro-2,1,3-benzothiadiazole] (PPDTT2FBT), suggesting a strong self-organization due to the linear chain configuration with conformation lock. The difluorinated PPDTT2FBT also showed the highest power conversion efficiency (PCE, 6.4%) by blending with PC₇₁BM, but a poorer photovoltaic performance was obtained compared to the wavy-structured counterpart, poly[2,5-bis(2-hexyldecyloxy)phenylene-alt-5,6-difluoro-4,7-di(thiophen-2-yl)-2,1,3-benzothiadiazole] (PPDT2FBT), reported previously. The mainly edge-on orientation of PPDTT2FBT (with π-π stacking in both xy and z directions) is attributed to the moderate PCE in the blends. Fine modulation of chain linearity may suggest an effective way to control the desirable interchain ordering and bulk film morphology for specific application in polymer solar cells or field effect transistors.