A class of small molecule donors configured in a donor-acceptor-acceptor’ (d-a-a’) structure have been studied for vacuum-deposited OPVs. They consist of an electron-donating (d) functional unit connected to two consecutive electron-accepting (a, a’) groups. The rigid and rod-like molecular backbones with strong push-pull interactions between the ‘d’ and ‘a’ units result in a large ground state dipole moment along the backbone axis. This leads to antiparallel π-π stacking that favors intermolecular charge transfer. In this work we synthesized two vacuum-deposited small molecules that are modified from previously reported donors with similar structures. All molecules studied have the same molecular backbone with different side chains attaching to an asymmetric heterotetracene donor block. Single crystal analysis and thin film grazing incidence x-ray diffraction are performed. The donor with a shorter branched side chain yields the highest single crystal packing density, corresponding to the largest absorption coefficient and short circuit current (JSC) among the three molecules studied. The preferred face-on stacking arrangement that facilitates charge transport in the vertical direction also leads to a higher fill factor (FF). A power conversion efficiency of 9.3% is achieved with JSC = 16.5 mA/cm2, VOC = 0.94 V and FF = 0.60, which is one of the highest performance single junction OPVs grown by vacuum thermal evaporation. By relating the side chain shape with the crystal packing habit and the device performance, we provide a means of molecular structure modification leading to significant performance improvements.
 X. Che, C.-L. Chung, X. Liu, S.-H. Chou, Y.-H. Liu, K.-T. Wong, S. R. Forrest, Advanced Materials 2016, 28, 8248.