Two-dimensional photovoltaic copolymers with spatial D-A-D structures: synthesis, characterization and hetero-atom effect

Abstract

A series of two-dimensional (2D) conjugated copolymers with spatial D-A-D structures (PTNBTB, PTCBTB, and PTSBTB) consisting of hetero-atom-bridged dithiophene and phenylvinyl-substituted benzothiadiazole blocks in the main chain have been designed, synthesized, and characterized. The structure-property relationships of the resulting copolymers were systematically investigated. The effects of the bridging atoms (N, C, and Si) on their thermal, optical, electrochemical and charge-transporting properties were also studied. PTNBTB exhibits a smaller band gap with red-shifted absorption, whereas PTSBTB possesses deeper HOMO level and higher hole mobility than PTCBTB or PTSBTB. Bulk heterojunction (BHJ) solar cells were fabricated and characterized with the conventional configuration of ITO/PEDOT:PSS/copolymer:PC71BM (1:1)/Ca/Al. As expected, PTSBTB devices showed the highest PCE, up to 4.01%, which was due to the lower HOMO level, higher carrier mobility, and stronger optical response as well as the finer nanoscale phase separation of the pristine polymer and/or the corresponding blending active layer with PC71BM. The primary results offer useful insights in designing 2D copolymers with spatial D-A-D backbone and different hetero-atom bridged donor units to finely tune the absorptions, electronic energy levels, carrier mobilities and the photovoltaic properties.