Synergetic Effect of Side-Chain Engineering of Polymer Donors and Conformation Tuning of Small-Molecule Acceptors on Molecular Properties, Morphology, and Photovoltaic Performance

It is demonstrated that the side-chain engineering of polymer donors and molecular conformation of small-molecule acceptors (SMAs) plays a crucial role in the morphology and photovoltaic performance of polymer solar cells (PSCs). However, the synergetic effect of these two aspects has been rarely reported. Herein, two wide-band gap donor–acceptor (D–A) copolymers (PST-TTC and PSPT-TTC) featuring the same main-chain backbone but different conjugated side chains were developed and combined with three low-band gap symmetric SMAs (ITIC, DTCFO-ICCl, and Y6) with various molecular conformations (S-shape, C-shape, and U-shape) to investigate the synergetic effect of side-chain engineering of polymer donors and conformation manipulation of SMAs on molecular properties, device physics, film morphology, and photovoltaic performance. The results indicate that PST-TTC with an alkylthiothiophene side chain exhibits a broader absorption spectrum, a smaller optical band gap (1.95 vs 1.99 eV), and a deeper-lying highest occupied molecular orbital (HOMO) level (−5.39 vs −5.36 eV) as compared to PSPT-TTC with an alkylthiophenylthiophene side chain. The PSCs were constructed according to the different donor/acceptor combinations based on the two as-synthesized polymers and three selected SMAs. After optimization, PST-TTC paired with S-shaped ITIC provides a power conversion efficiency (PCE) of 10.10% and the PCE can be further improved up to 10.60% when C-shaped DTCFO-ICCl was used to replace ITIC as the acceptor. However, the device performance becomes worse, accompanied with a sharply reduced PCE of 7.36% after the substitution of ITIC for U-shaped Y6. On the contrary, when the paired donor was changed to PSPT-TTC, the PSPT-TTC:Y6-based PSC achieves a remarkably increased PCE of up to 11.46%. These delicate observations suggest that the polymer donor with different conjugated side chains should collaborate with symmetric SMAs possessing various molecular conformations to realize superior morphology and device performance.