Synthesis and photovoltaic evaluation of fluorinated PTB7-Th derivatives in binary and ternary organic solar cells

Three donor-acceptor conjugated polymers derived from PTB7-Th were synthesized via Stille cross-coupling polymerization and systematically evaluated in binary and ternary organic photovoltaic (OPV) devices. The polymers, PTB7-Th, PTB7-TF (fluorinated donor unit), and PTB7-BF₂ (BF₂-chelated donor unit), were characterized by GPC, UV-Vis spectroscopy, cyclic voltammetry, and profilometry to establish structure-property-performance relationships. Fluorination in PTB7-TF effectively deepened the HOMO level and promoted improved molecular organization, resulting in smoother and thinner films that facilitated charge transport and reduced recombination losses. As a consequence, PTB7-TF-based devices exhibited the best performance, reaching a power conversion efficiency (PCE) of 8.46% in a ternary blend with Y6:PC₇₀BM. In contrast, although PTB7-BF₂ presented relatively favorable energy levels and the highest open-circuit voltage, its performance was severely limited by reduced short-circuit current and fill factor. This behavior is attributed to increased backbone rigidity, suboptimal nanoscale morphology, and inefficient charge transport, highlighting that energy level tuning alone is insufficient to ensure high device efficiency. The incorporation of PC₇₀BM in ternary systems improved film homogeneity and charge transport pathways, but only when structural compatibility between donor and acceptor was preserved. Overall, the results demonstrate that OPV performance is governed by the interplay between electronic structure and morphological organization, providing mechanistic insights and design guidelines for PTB7-Th-derived polymers in non-fullerene organic solar cells.