Correlating Dye Adsorption Behavior with the Open-Circuit Voltage of Triphenylamine-Based Dye-Sensitized Solar Cells

To study the relationship between dye adsorption behavior and open-circuit voltage (V-OC) of dye-sensitized solar cells (DSCs), four triphenylamine-based organic sensitizers with closely related molecular structures but with two different adsorption orientations were engineered and compared. The origin of V-OC was investigated in terms of band-edge movement of the TiO2 conduction band (CB) and interfacial charge recombination, with the latter found to be the governing factor. The two dyes with cyanoacetic acid as an anchoring group (TC dyes) adopt a standing adsorption mode and exert a larger surface dipole potential on TiO2 than their counterparts bearing rhodanine-3-acetic acid (TR dyes), which lie along the surface. TR dyes exhibit a greater extent of charge recombination than TC dyes because of the low surface-blocking efficiency of the dye layer and the intimacy between the I-3(-)-bound dyes and the TiO2. The differences in both CB movement and charge recombination between TR and TC dyes amplify with the expansion of the pi-conjugated system. The present result shows that molecules able to stand on the TiO2 surface upon adsorption would be structures of interest in the design of organic sensitizers for DSCs.