Temperature-Dependent Optical Properties of Flexible Donor - Acceptor Polymers.

Optical properties of five donor-acceptor polymers of interest for light-harvesting in organic photovoltaic devices have been studied experimentally and computationally. Experimentally recorded absorption spectra in solution of the five polymers are shown to be significantly temperature-dependent. The polymers were subjected to a first-principles computational treatment using density functional theory optimizations and excitation calculations. For two of the polymers, APFO-3 and PTI-1, a methodology that accounts for a thermally induced distribution of conformations based on Boltzmann statistics is applied to produce size- and temperature-converged optical results. This provides a deeper understanding of the temperature dependence of optical properties and improves the computational predictions of absorption wavelength and intensity at experimentally accessible temperatures, as compared to results from traditional quantum chemical calculations based on optimized polymers. Together, the combined experimental and computational temperature studies elucidate and quantify the significant influence of structural flexibility on the optical absorption properties of typical donor-acceptor polymers.