Design of a novel series of small molecule donors for application in organic solar cells

In an effort to study high-efficient electron donor materials for application in organic solar cells (OSC), density functional theory (DFT) calculations were performed to predict a variety of small molecules (SMs) with terminal acceptor-n bridge-central donor-n bridge-terminal acceptor (A-pi-D-pi-A) framework. An interesting A-pi-D-pi-Atype SM containing phenothiazine as D unit has been recently synthesized and applied to in the OSC. Motivated by this study, here, we have systematically predicted 10 SM donor materials based on the phenothiazine as the central D unit as well as several A and pi units that have been used into materials representing excellent SC as well as charge transport characteristics. Then, the structural, electronic, and optical properties of the predicted SM donors have been investigated. The fundamental band gap energies, orbital spatial distributions, and intrinsic dipole moments were calculated using DFT while the optical band gap energies were obtained using time-dependent density functional theory (TD-DFT). Several SC properties of the predicted SMs were then computed and compared with the available experimental results of OSCs. Our suggested SMs represented the enhancement in the open circuit voltage (V-OC) and charge transport properties which may lead to OSCs with improved power conversion efficiency (PCE) compared to the previously synthesized SMs. Moreover, small reorganization energies, large transfer integrals, and high intra-molecular coupling obtained from dense pi-stacking give rise to increased electron mobility in the predicted SMs. However, this strategy can be helpful for further improving the performance of SMs in OSCs.