Establishing nanoscopic structure-function relationships in the working electrodes of dye-sensitized solar cells
Dye-sensitized solar cells are a strong contender for next-generation photovoltaic technology with niche applications as solar-powered windows. The performance of a DSSC is particularly susceptible to the dye sensitizer, which is adsorbed onto the surface of a wide-band-gap semiconductor such as TiO2, to form the working electrode. The nature by which such surfaces are sensitized stands to influence the resulting dye···TiO2 interfacial structure and thence the operational performance of the DSSC working electrode. To this end, atomic force microscopy is first used to image the nanoscopic formation of dye···TiO2 interfacial structures. Four well-known DSSC dyes (N3, N749, MK-2, SQ-2) adsorbed onto amorphous TiO2 substrates were chosen for this study. The results show that all of these dyes present some form of aggregation upon the sensitization on TiO2, whose spatial distributions reveal distinct nanoaggregate particle characteristics. This nanoparticle structural assembly persists even when these dye···TiO2 interfaces are fabricated using hundredfold diluted dye sensitization concentrations. Time-tracking scanning of the N749 dye reveals the forming progress of the nanoaggregates. Simulations and experiments were subsequently combined to conduct systematic studies on the optical and structural properties of two commercial dyes, RK-1 and SQ-2. The results reveal that the deprotonation of the cyanoacrylic acid group in RK-1 occurs upon dye adsorption onto TiO2, which induces a hypsochromic shift of its absorption band. DFT modeling also indicates the involvement of cyano groups in anchoring to TiO2 which makes the dye···TiO2 binding structure more stable. Fourier transform infra-red spectroscopy data prove that COO/CN (A2) and bridging bidentate (BB) binding modes co-exist in the RK-1···TiO2 interfacial structure. As for SQ-2, its simulation explains the distinct optical properties of this squaraine dye. Additionally, desorption experiments reveal a low dye loading of SQ-2 on TiO2. Based on the aforementioned study, DSSCs co-sensitized with RK-1 and SQ-2 were studied using spectral and electrical characterization, with supporting DFT simulations. Results show that the dye absorption band can be effectively extended with this co-sensitization strategy to realize panchromatic DSSCs. The co-sensitized DSSCs can gain higher short-circuit current density and power conversion efficiency with specific molar ratios of the co-sensitizing solution, although open-circuit voltage remains lower than that of the cell based on pure RK-1 dye sensitizers. The analysis also suggests that the decrease in the open-circuit voltage may result from the forming of aggregation when SQ-2 is co-sensitized with RK-1. The thesis ends with suggesting future work corresponding to the present study.
DSSC, working electrode, structure, function, imaging, modelling
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