Conjugated polymers (CPs) are a novel class of highly crosslinked materials with extended π- conjugated systems, predominantly composed of low-cost, earth-abundant elements such as C, N, S, O, and H. Their tuneable optoelectronic and photophysical properties make CPs ideal for catalysis in water splitting and related energy conversion applications. The donor-acceptor (D- A) approach is a powerful strategy for tailoring these properties. By carefully selecting donor and acceptor units, D-A conjugated polymers with low band gaps can efficiently harvest a broader spectrum of solar energy, enhance charge transfer, and improve separation efficiency, making them well-suited for applications such as organic photovoltaics (OPVs), photodetectors, and photoelectrochemical (PEC) water splitting.
In this study, we utilized oxidative chemical vapor deposition (oCVD)—a solution-free, versatile, and scalable thin-film fabrication technique—to synthesize homopolymers from 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBTD) and benzo[1,2-b:3,4-b′:5,6-b″]trithiophene (BTT) by varying the oxidant-to-monomer ratios. DTBTD inherently possesses a donor-acceptor structure, where the benzothiadiazole unit serves as the acceptor and the thiophene groups act as the donor. Additionally, we synthesized copolymers of BTT-DTBTD with different monomer-to-monomer ratios to explore their compositional versatility. The objective of copolymerization was to strengthen the donor component by incorporating benzo[1,2-b:3,4-b′:5,6-b″]trithiophene (BTT) and to explore its impact on the overall electronic properties of the copolymer.
The successful polymerization of DTBTD and BTT via oCVD was confirmed through HRMS, which identified oligomers with up to 12 repeating units in the oCVD polymer films of pBTT, pDTBTD, pBTT-DTBTD—absent in sublimed monomer thin films. SEM revealed distinct morphological differences between the monomer and polymer films, showcasing the transformative effect of polymerization on thin-film structure. UV-Vis-NIR spectroscopy provided insights into the electronic structure of the oCVD films, showing a significant red shift in absorption spectra compared to their monomer counterparts. This shift signifies extended π-conjugation within the polymerized films, a critical feature for efficient light harvesting. XPS data further confirmed the chemical composition and bonding environments within the films, solidifying evidence of successful polymerization.
Energy band diagrams from UV-Vis and XPS data demonstrated the tunability of electronic properties via oCVD. The band gap of pDTBTD decreased from 2.34 eV in the sublimed monomer to 1.34 eV after polymerization, while pBTT showed a reduction from 3.91 eV to 2.76 eV. Copolymerization further adjusted the band gaps of pBTT-DTBTD to a range of 1.71–1.77 eV, depending on the monomer-to-monomer ratios. Photoelectrochemical analysis of the oCVD films under simulated sunlight demonstrated their capability for efficient water reduction. The photocurrent densities measured at 0.331 V vs. RHE were 4.23, 11.9, and 20.83± 2 μA·cm⁻² for pBTT, pDTBTD, and pBTT-DTBTD copolymers, respectively. These values highlight the superior performance of the copolymer, driven by combination of donor and acceptor units.
These results emphasize the potential of oCVD for the synthesis, engineering and integration of donor-acceptor homopolymer and copolymer thin films as scalable, efficient, and environmentally friendly materials for renewable energy applications. This work represents a critical advancement in sustainable energy solutions, combining innovative materials with a green fabrication process to enable high-performance photoelectrocatalysis.