Effects of Nuclear Vibrations on the Energetics of Polythiophene: Quantized Energy Molecular Dynamics

Abstract

The effects of nuclear dynamics on the energetics of polythiophene relevant for the performance of organic solar cells are studied for the first time. Nuclear motions change the expectation values of frontier orbital energies and the band gap by ~0.1 eV versus values at the equilibrium geometry, which is expected to have a significant effect on light absorption, charge separation, and donor regeneration. A new molecular dynamics algorithm that accounts for the quantum nature of vibrations is introduced. It reproduces effects of temperature and deuteration that are lost in the standard molecular dynamics. Inclusion of quantized vibrations leads to a broadening of the band gap and of energy levels by ~20 % at 300 K, while having little effect on their expectation values (which change by up to 0.03 eV). Increase in temperature from 300 to 400 K and deuteration cause an additional broadening of the spectrum by ~26 and 21 % respectively.