Low-Cost Polymer Donors Composed of Non-Halogenated Benzodithiophene and Cyanoacrylate-Substituted (Oligo)thiophene for Efficient Organic Solar Cells

Abstract

The fluorinated thienyl side chain and ring-fused electron-accepting (A) unit inevitably increase the synthesis complexity and product costs of state-of-the-art polymer donors (PDs), such as PM6 and D18, which may hinder the commercial application of organic solar cells (OSCs). Herein, we developed nonfused ring A units based on cyanoacrylate-substituted (oligo)thiophenes (OThs) through two or four steps, which were then copolymerized with a non-halogenated benzodithiophene (BDT)-based electron-donating (D) unit to produce a series of D–A-type PDs: PY-TQ, PY-2TQ, and PY-3TQ. The resulting polymers possess shorter synthetic routes and reduced costs compared with commonly used high-performance PDs. It was found that both cyanoacrylate substitution and the number of thiophene units have significant effects on the molecular structures and the optoelectronic and photovoltaic properties of these PDs. All polymers show excellent solubility, high molecular weight, and weak crystallinity due to their random chemical structure. Theoretical calculations reveal that PY-3TQ has a more coplanar backbone than the other two polymers because the inserted thiophene π-bridge can effectively reduce steric hindrance. Moreover, these PDs exhibit medium optical bandgaps (1.79–1.89 eV) and low-lying HOMO levels (approximately −5.40 eV), which are beneficial for their coordination with low-bandgap nonfullerene acceptors to construct efficient OSCs. The PY-2TQ:Y6-based OSC achieves a higher power conversion efficiency (PCE = 11.90%) compared to PY-TQ (5.83%) and PY-3TQ (8.20%), owing to more balanced carrier mobility, higher charge dissociation probability, weaker charge recombination, and superior active layer morphology. Additionally, it is proven that PY-2TQ has a certain generality to pair with other nonfullerene acceptors. More importantly, using PY-2TQ as the additional donor, the PM6:PY-2TQ:Y6 ternary OSCs achieve a decent PCE of up to 17.54%. This study highlights the potential of cyanoacrylate-substituted OThs as simple and efficient A units for designing high-performance and cost-effective PDs for OSC applications .