Pyrazine-based sensitizers for dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) have emerged as a major technology in solar energy conversion. Ruthenium dyes are commonly used in DSSCs due to their high stability and power conversion efficiency (PCE), but the complex and costly synthesis of ruthenium complexes hinders their commercialization. Metal-free sensitizers attract significant attention in DSSC technologies. They are eco-friendlier with more facile synthesis and offer diverse structural designs for tuning electronic, optical and morphological properties. Metal-free dyes have been reported with PCEs surpassing Ru-based N3 and N719 benchmark sensitizers in DSSCs. Pyrazine-based sensitizers demonstrate favorable photophysical and electrochemical properties due to their unique structural features and versatile synthetic approaches enabling functionalization at different positions. Several pyrazine sensitizers have been reported with strong absorption extending to the near-infrared region, high molar extinction coefficients, and balanced hole and electron transport. The donor-π-acceptor (D-π-A) design with pyrazine as the π-bridge is conventional to favor intramolecular charge transfer in DSSCs. Other pyrazine architectures, e.g., D-A-π-A′, demonstrated high PCEs, reaching up to 12.5%. This review highlights the advances in pyrazine-based sensitizers focusing on the pyrazine core as a principal electron acceptor, π-auxiliary acceptor, and even as a unit for functionalization as an electron-donating moiety. The reported sensitizers for DSSCs since 2008 are summarized, including metal-free dyes and pyrazines conjugated to Ru and porphyrin dyes. The dyes are classified into quinoxaline, thienopyrazine, pyridopyrazine, and pyrrolopyrazine cores in different sections. The DSSC parameters are summarized, discussing the electronic structure-property and structure-function relationships and offering insights into future architectures that accelerate their commercialization.