Photophysical Properties of Donor-Acceptor-π Bridge-Acceptor Sensitizers with a Naphthobisthiadiazole Auxiliary Acceptor: Toward Longer-Wavelength Access in Dye-Sensitized Solar Cells

Accessing longer-wavelength photons is a critical research direction in dye-sensitized solar cells (DSSCs). Currently, dye-sensitized solar cells are exceptional at generating high photovoltages with shorter wavelength photons, but few examples exist of DSSCs using photons in the shortwave infrared region (>1000 nm). New dye building blocks are critical toward accessing these longer-wavelength photons. With this in mind, five new triphenylamine-based metal-free dyes derived from a donor-auxiliary acceptor-pi bridge-acceptor (D-A'-pi-A) structure are characterized theoretically for application in dye-sensitized solar cells (DSSCs). The driving force of electron injection, the spontaneity of dye regeneration, charge-transfer length, and partial density of states of the isolated and TiO2-bound dyes, which are all critical to DSSC performance, are systematically investigated via first-principles calculations. We find that replacing the high-performing benzothiadiazole (BTD) auxiliary acceptor building block with a longer conjugation length and stronger electron-withdrawing building block, naphtho [1,2-c:5,6-c'] bis( [1,2,5]thiadiazole) (NTz), is beneficial toward the kinetics of charge injection along with reducing the optical energy gap of the designed dyes. The obtained results imply that using the NTz unit extends the absorption spectrum toward longer wavelengths and improves charge separation due to the planarity and conjugation length extension that arises from the NTz fragment. The shift of the conduction band of TiO2 (Delta E-CB value) is higher for the designed NTz-based dyes than for the BTD-based dye on TiO2, suggesting that the open-circuit voltage (V-OC) of a dye-sensitized solar cell device will also be higher. Concerning the photophysical properties of the dyes, the NTz-based dyes are promising as they possess a longer excited state and longer radiative lifetime than a BTD-based dye. We synthesize a model NTz-based dye and a BTD analogue for comparison with computational results. The optical properties of the NTz-based dye are computed to validate our computational approach, which agrees with the experimental observations. Our combined computational and experimental approach sheds light on the physical principle of molecular photogenerated charge transfer and provides valuable guidance for the further molecular synthesis of long-wavelength absorbing materials.