Benzodithiophene-thiophene-based photovoltaic polymers with different side-chains

A series of benzodithiophene-thiophene-based alternating copolymers were synthesized with different side-chains, and their photovoltaic characteristics were examined. The choice of solubilizing side-chains influences significantly the chain conformation, frontier orbital energy levels, intermolecular organization, and the resulting optical, morphological, and photovoltaic properties. The incorporation of an e-withdrawing carbonyl group in the side-chain decreased the highest occupied molecular orbital (HOMO, ca. -5.4 eV) level and improved the chain planarity through intrachain hydrogen bonding. The shortest π-π stacking distance (3.72 Å) was also measured for the alkylcarbonyl-substituted BDTCOT:PC₇₁BM blended film by two dimensional grazing incidence X-ray scattering. With compared to other polymers, the BDTCOT:PC₇₁BM device showed a substantially improved open-circuit voltage and short-circuit current density, leading to a 4.66% power conversion efficiency. The side-chains need to be designed to be multifunctional to induce a deep HOMO level and chain planarity (for interchain ordering) as well as good solution processability.