Organic photovoltaics: A charge transfer perspective in the study of donor-acceptor systems

The present research involves the study of donor-acceptor (D/A) dyad complexes from a charge transfer energy perspective. The aim is to provide insight and predictive understanding into the charge transfer processes of the molecular-level components in donor-acceptor based organic solar cells using computational methods to describe photochemical processes at the quantum mechanical level within the Density Functional Theory (DFT) approximation. Predictive understanding is anchored in reproducing experimental results, wherein the present work a perturbative excited-state DFT method is described in detail and shown to give Charge Transfer (CT) energies in excellent agreement with benchmark experimental data. With an accurate excited state method for calculating CT excitation energies at hand, the present research applies the method to the study of D/A pairs employed in photovoltaic devices. An examination is made of the effect on the CT energetics of varying the donor and acceptor component in the dyad and the changes in the frontier orbital energy levels and CT energies with respect to a varying D/A distance and D/A relative orientation. The results of the perturbative excited state DFT calculations provide direct insight into photovoltaic device efficiency since the CT energy determines the achievable open circuit voltage of a donor-acceptor based organic solar cell device.