Paper presented to the 3rd Southern African Solar Energy Conference, South Africa, 11-13 May, 2015.
Charge transfer states at the donor–acceptor interface in organic solar cells determine the device performance. Therefore a basic description, understanding and estimation of the energetics of the charge transfer states at donor-acceptor interfaces are crucial for the improvement of the efficiency especially of organic solar cells. In the presentation, we report about the results of theoretical studies on donor–acceptor combinations used in molecular solar cells. As an example, we discuss especially zinc phthalocyanine – C60 blends. Several computational schemes based on constrained density functional theory (c-DFT), Hartree-Fock based configuration interaction of singles (CIS) and time-dependent density functional theory (TD-DFT) using the hybrid functional B3LYP were used to assess the energy of the lowest charge transfer (CT) state in such systems. The results of the calculations are discussed in comparison with available spectroscopic data, and in comparison with solid state calculations. For example, the results from c-DFT and CIS calculations reproduce the correct Coulomb asymptotics between cationic donor and anionic acceptor configurations, whereas TD-DFT gives qualitatively wrong excitation energies.