In order to further improve the short-circuit current (J(SC)) and the overall performance of the already synthesized donor molecule (1s) used in organic heterojunction solar cells, six new A-A-D-A-A type small molecules composed of different donor (D, electron rich) and same acceptor (A, electron poor) moieties were designed and characterized by using density functional theory (DFT), time-dependent density functional theory (TD-DFT) and Marcus theory. Through calculating frontier molecular orbital energy levels and spectra properties, we found that HOMO level goes up while E-g goes down along with the cycle extension of the center part flanked by bithiophene. Next, character of excited state, ionization potentials (IPs) and charge transport properties were also investigated to provide an in-depth insight into the charge transfer/transport characteristics and the J(SC) of organic solar cells. The results reveal that compounds 3c, 3n and 3 degrees exhibit favorable J(SC) and comparable performance to original molecule (1s) and show promising potential in organic solar cells. (C) 2016 Elsevier B.V. All rights reserved.