Abstract: Polythiophene (PTs) is a promising electron donor in organic solar cells (OSCs) due to its simple structure and excellent synthetic scalability. However, the high HOMO (highest occupied molecular orbital) energy level of PTs and the unfavorable active layer morphology limit the performance improvement of PTs-based OSC device. Herein, in order to solve the disadvantage of high HOMO energy level of PTs, a highly planar polymer donor poly(thiazole-ethylene) derivatives(PTzV-T) was designed and synthesized by introducing an ester-functional thiazole-vinyl structure as electron-withdrawing unit into the reported poly(thiophene vinylene) derivative (PTVT-T). The aggregation behavior and energy structure were characterized by ultraviolet-visible (UV-Vis) absorption spectroscopy, electrochemical cyclic voltammetry and two-dimensional grazing incidence wide-angle X-ray scattering. Results show that the polymer PTzV-T has a low HOMO energy level (−5.88 eV) and solid planarity, and shows different preaggregation behaviors and temperature dependence in chloroform, chlorobenzene and o-xylene solvents. The films prepared with these three solvents also show different crystal packing modes and crystallinity.