Effect of Polymer Chain Regularity on the Photovoltaic Performance of Organic Solar Cells

In this work, regio-regular and random polymers (reg-PTF and ran-PTF) comprising benzodithiophene and 3-fluorothiophene units are developed to investigate the influence of polymer chain regularity on the photovoltaic performance. Interestingly, reg-PTF exhibits higher HOMO energy level (∼0.2 eV) than that of ran-PTF due to the more ordered molecular structure and higher crystallization. As the result, the HOMO energy level offset between donor and acceptor can be well controlled by selecting different acceptors. Using BTP-ec9-4F with a much deep HOMO energy level as acceptor, reg-PTF-based devices display an inferior PCE of 7.66% compared with ran-PTF based ones (11.78%) due to the too large HOMO energy levels offset. When blending with Y18-1F, the PCE of reg-PTF-based devices is increased to 13.23% due to the appropriate energy level. In comparison, the ran-PTF-based devices nearly do not work because of the negative HOMO energy levels offset. This work indicates that both the polymer chain regularity and the energy level offset between the donor and acceptor material are important to the performance of organic solar cells.