Ultrafast charge dynamics of diketopyrrolopyrrole-based terpolymers for optoelectronic applications: impact of acceptor concentrations and thermal annealing

Abstract

Diketopyrrolopyrrole (DPP)-based terpolymers have emerged as promising donor materials for organic solar cells (OSCs), offering tunable optoelectronic properties, broadband absorption, and enhanced morphological stability through random terpolymerization strategies. This study investigates the ultrafast charge carrier dynamics of three DPP-based terpolymers, (P1–P3), incorporating different ratios of fluorobenzotriazole (FTAZ) and thienothiophene-capped DPP (TTDPP) acceptors. Using steady-state absorption and femtosecond transient absorption spectroscopy, we examine how acceptor concentrations and thermal annealing influence photophysical processes in pristine terpolymers and their bulk heterojunction blends with PC71BM. Increasing the TTDPP content enhances backbone planarity, π−π stacking, and charge delocalization, thereby reducing recombination losses and promoting efficient charge separation. Compared to P2 and P3, the P1 terpolymer, with the highest TTDPP ratio, exhibits the most favorable intramolecular charge transfer kinetics and balanced carrier mobilities, consistent with its superior photovoltaic performance. As previously reported, the corresponding terpolymer-based devices exhibited power conversion efficiencies of 5.58%, 3.10%, and 3.57% for P1, P2, and P3, respectively. Thermal annealing, however, reduces long-lived charge populations in all blends due to fullerene aggregation, which diminishes phase separation. These findings highlight how rational tuning of acceptor ratios in DPP-based terpolymers provides a pathway toward optimizing both efficiency and stability in OSCs.