Impact of neutral and anion anchoring groups on the photovoltaic performance of triphenylamine sensitizers for dye-sensitized solar cells

Anchor groups play a vital role in dye-sensitized solar cells (DSSCs), acting as a bridge for electron injection from the sensitizers to the metal oxide semiconductor. Carboxylic acids (COOH) are widely used anchors for most sensitizers because of their strong adhesion to the semiconductor surface. Electron injection occurs through this adhesion, which is the main process that initiates the electrical circuit in a DSSC. Owing to the proton (H+) of the COOH anchoring group, the conduction band of the semiconductor is shifted positively after sensitizer adsorption, leading to open circuit photovoltage (V-OC) loss. In this study, the triphenylamine-based sensitizer with a carboxylate group (COO-) was synthesized as an anchor. Although the power conversion efficiency of the anionic triphenylamine sensitized solar cells was lower than that of the neutral-base cell due to the decreased amount of dye on the photo-electrode surface, it exhibited an enhanced VOC compared to that of the neutral form. Density functional theory (DFT) and time dependent DFT studies were also carried out to theoretically characterize the two dyes and explore the difference between the carboxylic and carboxylate anchor groups.