Synthesis and characterization of thieno[3,2-b]thiophene-isoindigo-based copolymers as electron donor and hole transport materials for bulk-heterojunction polymer solar cells

A novel class of thieno[3,2-b]thiophene (TT) and isoindigo based copolymers were synthesized and evaluated as electron donor and hole transport materials in bulk-heterojunction polymer solar cells (BHJ PSCs). These π-conjugated donor-acceptor polymers were derived from fused TT and isoindigo structures bridged by thiophene units. The band-gaps and the highest occupied molecular orbital (HOMO) levels of the polymers were tuned using different conjugating lengths of thiophene units on the main chains, providing band-gaps from 1.55 to 1.91 eV and HOMO levels from -5.34 to -5.71 eV, respectively. The corresponding lowest unoccupied molecular orbital (LUMO) levels were appropriately adjusted with the isoindigo units. Conventional BHJ PSCs (ITO/PEDOT:PSS/active layer/interlayer/Al) with an active layer composed of the polymer and PC ₇₁BM were fabricated for evaluation. Power conversion efficiency from a low of 1.25% to a high of 4.69% were achieved with the best performing device provided by the D-π-A polymer with a relatively board absorption spectrum, high absorption coefficient, and more uniform blend morphology. These results demonstrate the potential of this class of thieno[3,2-b]thiophene-isoindigo- based polymers as efficient electron donor and hole transport polymers for BHJ PSCs.