High-mobility low-bandgap conjugated copolymers based on indacenodithiophene and thiadiazolo[3,4-c]pyridine units for thin film transistor and photovoltaic applications

Two new semiconducting polymers based on indacenodithiophene and thiadiazolo[3,4-c]pyridine units were synthesized via Stille coupling polymerization. The polymers, PIDTPyT and PIDTDTPyT, exhibited main absorption bands in the range of 550-800 nm while their absorption maxima were located at around 700 nm in films. With two additional thiophene spacers, PIDTDTPyT showed a broader absorption band but a 20 nm blue-shifted maximum peak compared to that of PIDTPyT. Both of the polymers possess low bandgaps (∼1.6 eV) and deep energy levels for both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Organic field-effect transistors (OFETs) device measurements indicate that PIDTPyT and PIDTDTPyT have high hole carrier mobilities of 0.066 and 0.045 cm² V^-1 s ^-1, respectively, with the on/off ratio on the order of 10 ⁶. Bulk heterojunction photovoltaic devices consisting of the copolymers and PC₇₁BM gave power conversion efficiencies (PCE) as high as 3.91% with broadband photo-response in the range of 300-800 nm. The relationships between the photovoltaic performance and film morphology, energy levels, hole mobilities are discussed. © 2011 The Royal Society of Chemistry.