PBC-DFT : An Efficient Method to Calculate Energy Band Gaps of Conducting Polymers used in Solar Cells

Abstract

In recent years, conducting polymers have gained attention for their promising application in solar cells due to their potential low cost, lightweight, and flexibility. Desirable polymers have a small band gap and a low HOMO energy level. Methods of finding this band gap exist using density functional theory (DFT) by calculating the energy gaps of increasing oligomer lengths (n), and plotting the HOMO-LUMO gap (in eV) as a function of the reciprocal polymer length (1/n). This method, however, proves time consuming and computationally costly. An alternative, less time-consuming method using periodic boundary conditions (PBC) exists. In our research, we studied existing donor-acceptor polymers from the literature and used PBC to calculate their band gaps for comparison with experimental data. To perform these calculations we used DFT at the B3LYP/3-21G(d) level of theory on optimized dimers. The PBC method yields results consistent with experimental values and can be useful in determining theoretical band gaps prior to synthesis which can aid in saving valuable lab time.