Quantum Chemical Calculations of Side-Group Stacking and Electronic Properties in Thiophene-Quinoxaline Polymers

Organic bulk heterojunction (BHJ) solar cells offer a viable source of solar energy. Structural organization is crucial in BHJ cells but hard to achieve and assess due to limitations in experimental methodology. Quantum chemical methods have here been used to gain further insight into the geometric and optical properties of a promising light-harvesting polymer, poly[2,3-bis(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1). Calculations show that favorable positions of the two alkoxyphenyl side groups on each TQ1 monomer allow nonbonded side-group stacking interactions with the neighboring units in both directions. This yields a unique, helical geometry with enhanced intramolecular ordering that promotes extensive electronic conjugation. Adequate description of this effect requires computational methods that include dispersion corrections. A strategy based on such side-group interactions is proposed for designing new polymers.