Density functional theory design and characterization of D-A-A type electron donors with narrow band gap for small-molecule organic solar cells

For seeking the high-efficiency narrow-band-gap donor materials to enhance short-circuit current density (J(sc)) for organic solar cells, a series of D-A-A type small molecules derived from the synthesized donor DTDCTB and with different heterocyclic bridges were designed and characterized by using density functional theory (OFF) and time-dependent DFT calculations. According to the electron-withdrawing strength of the heterocyclic bridges and molecular energy matching relationship, we designed and ultimately chose four ideal donors (3, 5, 16 and 19) which have the suitable energy levels to match with PCBM and narrower band gaps than DTDCTB. Next, the properties affecting open-circuit voltage (V-oc), J(sc) and fill factor (FF) were investigated by calculating the geometrical structures, frontier molecular orbital energy levels, absorption spectra, light harvesting efficiencies, chare-transfer indexes, exciton binding energies and hole mobilities of the ideal donors, as well as DTDCTB for comparison. The results show that 3 has lower HOMO level, higher absorption efficiency, more favorable excion dissociation and hole transport than others, facilitating the improvement of the V-oc, J(sc) and FF. Finally, 3 would be the most promising one in this series of donors and further boost the device efficiency. (C) 2013 Elsevier B.V. All rights reserved.