Thieno[3,2-b]thiophene-Bridged Conjugated Polymers Based on Dithieno[3,2-b:2′,3′-d]silole and Thieno[3,4-c]pyrrole-4,6-dione for Polymer Solar Cells: Influence of Side Chains on Optoelectronic Properties

Two new D-pi-A copolymers combining dithieno[3,2-b:2 ',3 '-d]silole as donor unit and thieno[3,4-c]pyrrole-4,6-dione as acceptor unit are designed and synthesized. The influence of different side chains on the optical properties, the energy levels of the highest occupied molecular orbitals and lowest unoccupied molecular orbitals, electronic structures, morphologies of blend films, and the photovoltaic properties of the conjugated polymers are investigated. As a result, the polymers reveal different light absorption and energy levels, which contributes to the short-circuit current and the open-circuit voltage, respectively. The dihedral angles between the donor unit and the pi bridge are greatly increased due to the steric effect caused by introducing the alkyl chain to the pi bridge. Eventually, a maximum power conversion efficiency of 2.65% is obtained.