Influence of polymer side chains on the photovoltaic performance of non-fullerene organic solar cells

Abstract

Novel polymers comprising a 3-fluoro-5-alkylthiophenyl benzodithiophene donor unit and a 5-fluoro-6-alkoxy (or alkylthio)-2,1,3-benzothiadiazole (BT) acceptor unit were synthesized. Both POF and PSF possess low HOMO and LUMO energy levels due to the incorporation of fluorine atoms. Additionally, alkoxy and alkylthio substitution on the BT unit also had a great influence on the molecular packing and the energy level of the resulting polymers. The introduction of the alkylthio side chains on the BT unit of PSF led to a significant downshift of the HOMO energy level in comparison to that of POF with an alkoxy substituent due to the weaker electron-donating properties of the sulfur atom than that of oxygen. However, the steric hindrance caused by the large sulfur atoms resulted in reduced planarity of the backbone of PSF, which might influence the charge transport and the morphology of the blend film. As a result, POF based NF-PSCs exhibited a PCE of 7.28%, with a V_oc of 0.86 V, a J_sc of 14.9 mA cm⁻², and an FF of 0.47, while a low PCE of 1.55% with a V_oc of 0.95 V, a J_sc of 5.6 mA cm⁻², and an FF of 0.29 was obtained for PSF based non-fullerene polymer solar cells (NF-PSCs). Therefore, the side chain engineering of the donor polymer is crucial for maximizing both J_sc and V_oc values to achieve high performance polymer solar cells.