DFT and TD-DFT calculation of new thienopyrazine-based small molecules for organic solar cells

Background: Novel six organic donor-p-acceptor molecules (D-pi-A) used for Bulk Heterojunction organic solar cells (BHJ), based on thienopyrazine were studied by density functional theory (DFT) and time-dependent DFT (TD-DFT) approaches, to shed light on how the pi-conjugation order influence the performance of the solar cells. The electron acceptor group was 2-cyanoacrylic for all compounds, whereas the electron donor unit was varied and the influence was investigated. Methods: The TD-DFT method, combined with a hybrid exchange-correlation functional using the Coulomb-attenuating method (CAM-B3LYP) in conjunction with a polarizable continuum model of salvation (PCM) together with a 6-31G(d,p) basis set, was used to predict the excitation energies, the absorption and the emission spectra of all molecules. Results: The trend of the calculated HOMO-LUMO gaps nicely compares with the spectral data. In addition, the estimated values of the open-circuit photovoltage (V-oc) for these compounds were presented in two cases/PC60BM and/PC71BM. Conclusion: The study of structural, electronics and optical properties for these compounds could help to design more efficient functional photovoltaic organic materials.