Highly Conjugated Side-Chain-Substituted Benzo[1,2-b:4,5-b′]dithiophene-Based Conjugated Polymers for Use in Polymer Solar Cells

A series of novel benzo[1,2-b:4,5-b']dithiophene (BDT)-based conjugated polymers were synthesized using a Stille cross-coupling reaction. These polymers contained dithienyl thieno[3,4-c]pyrrole-4,6-dione (DTTPD) as an acceptor. Alkylthienylenevinylehe thiophene side groups were introduced into. the BDT units, and the solubility, absorption spectra, energy levels, charge transport, blend film morphology, and photovoltaic properties of the resulting polymer (poly[4,8-bis{2,2'-(5-ethylhexyl)-thienylenevinylenethiophene}benzo [1,2-b;3,4-b] dithiophene2,6-diyl-alt-1,3-di(thien-5'-yl)-5-octyldodecyl[3,4-c]pyrrole- 4,6-dione-2,2'-diyl], PBDTTVT-DTTPD) were investigated. In addition, an alkylthienyl-substituted BDT-based polymer (poly[4,8-bis(5-(2-ethylhexypthiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-1,3-di(thien-5'-yl)-5-octyldodecyl[3,4-c]pyrrole-4,6-dione-2,2'-diy;], PBDTT-DTTPD) was synthesized to compare the optoelectronic and photovoltaic properties of the polymers. The weight-averaged molecular weights (M,,,) of PBDTT-DTTPD and PBDTTVT-DTTPD were found to be 41 000 and 58 100 Da, with polydispersity indices of 2.0 and 2.2, respectively. Photophysical studies revealed low bandgaps of 1.88 eV for PBDTT-DTTPD and 1.87 eV for PBDTTVT-DTTPD. The PBDTTVT-DTTPD thin film was able harvest a broad solar spectrum covering the range from 300 to 700 nm. Solution-processed field-effect transistors fabricated from these polymers displayed p-type organic thin film transistor characteristics. The field-effect mobilities of PBDTT-DTTPD and PBDTTVT-DTTPD were measured to be 3.2 x 10(-3) and 1.8 x 10(-3) cm(2) V-1 s(-1), respectively. A polymer solar cell device prepared using PBDTTVT-DTTPD as the active layer exhibited a power conversion efficiency (PCE) of 6.04% with a high open circuit voltage of 0.87 V under AM 1.5 G (100 mW/cm(2)) conditions.