Polycyclic Aromatics with Flanking Thiophenes: Tuning Energy Level and Band Gap of Conjugated Polymers for Bulk Heterojunction Photovoltaics

In the pursuit of donor−acceptor low band gap polymers for photovoltaic applications, finding an optimal donor monomer which maximizes the photovoltaic efficiency is a complex synthetic optimization problem. We synthesized three different bithiophenes flanking a center aromatic ring (pyrrole, benzene, or pyridine) as the donor monomers (DTPr, DTBn, and DTPn) with decreasing electron-donating ability. An array of six electrochemically and optically unique polymers were prepared by copolymerizing these three monomers with 2,1,3-benzothiadiazole and with thiophene. The optical, electronic, and photovoltaic properties of these polymers were investigated. Among this series, we found that the HOMO energy level of the polymer is dominated by the most electron-rich ring in the polymer backbone. The optical band gap, conversely, involves the entire polymeric system. Among the three donor monomers investigated, we identified that DTBn-based polymers exhibited the most potential in photovoltaic applications due to their moderately low band gaps and low HOMO energy levels.