Modulation of Electron-Donating Ability in D–A–A Small Molecules for Application in Organic Solar Cells

Comparative studies of a class of dithienopyr-role (DTP)-based D-A-A type small-molecule donor materials used in heterojunction solar cell have been carried out by means of quantum chemical calculations. The chemical modification of DTP group will be expected to modulate the electron-donating ability of D-A-A small molecules. To gain a better understanding of the effects of structural modification of DTP on the photophysical properties and the photovoltaic performance, the geometrical and electronic structures, absorption spectra, energy loss, intramolecular charge transfer, exciton dissociation and charge recombination rates at the interface, and hole-transport properties are analyzed in detail to establish structur-property relationships for the investigated D-A-A molecules. This work provides an intuitive explanation why the modification of DTP could modulate the frontier orbital levels, light-absorbing capacities, intra- and intermolecular charge-transfer properties, and interfacial exciton dissociation and recombination. The calculated results also demonstrate that the fine tuning of the electron-donating ability of small-molecule donor material could be an effective approach to improving the cell performance. This study could give theoretical guidelines for molecular design and pave the way to synthesize efficient small-molecule donor materials.