Non-Electron Withdrawing Unit Copolymerization to Enhance Photo-Stability of Thiophene-Based Organic Solar Cells

The impact of non-electron withdrawing unit copolymerization on photostability of fullerene-based organic solar cells is investigated using two donor polymers namely: benzodithiophene-4,8-dione (BDD) unit copolymerized with α-quaterthiophene (4 T) unit (PBDD4T) and poly-3-hexyl-thiophene (P3HT). The optical and electrochemical properties of the polymers reveal a deeper highest occupied molecular orbital (HOMO) and broader absorption in PBDD4T in comparison to P3HT owing to the BDD copolymerization. The copolymer achieves a higher power conversion efficiency (PCE) compared to the P3HT-based device due to its higher V_OC and J_sc that resulted from its deeper HOMO level and broader absorption. The photostability study reveals that PBDD4T-based devices lost 14% of its initial PCE relative to the 48% reduction in PCE for P3HT-based devices after 7 h of irradiation. Further investigation into the reasons behind the difference in the photostability suggests that photooxidation and recombination induced by irradiation in PBDD4T-based devcie are suppressed by BDD-copolymerization, maintaining better stability. In contrast, P3HT:PC₇₁BM-based solar absorber shows bimolecular recombination due to photoaging processes, which have negatively impacted the device stability. The reduction in stability of both devices is evident by lower photogenerated current attributed to reduced charge mobility and increased surface roughness.