Donor-Free Dyes with an Additional Acceptor Unit for Dye-Sensitized Solar Cells

This study examines the design of organic 'donor-free cyanoacrylic end-functionalized oligothiophene dyes involving four thiophene (4T) rings with benzothiadiazole as an additional electron acceptor unit (A) at different positions (1, 2, 3, 4 and 5) of 4T. The electronic and optical properties are then compared theoretically with the reference 4T dye for dye-sensitized solar cells (DSSCs). Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were used to gain insight into the factors responsible for photovoltaic performance. The absorption spectrum of the dyes differed in form because of the different energy levels of molecular orbital (MO) of each dye and the intramolecular energy transfer. The additional auxiliary acceptors (A) in 4T dyes improve the visible-light light harvesting, with a large red shift observed in the light response, and they also give an additional peak in the blue wavelength, which leads to a rise in light-harvesting efficiency. This work suggests that optimizing the, position of electron acceptors in dye sensitizers based on A-sr-A chains would improve photovoltaic properties for dye-sensitized solar cells (DSSCs).