Spectroscopic and photoelectrochemical studies of metal-free dyes for applications in dye-sensitized solar cells
Mulhern, Kacie Ryan
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In this dissertation, we present a series of novel chalcogenorhodamine dyes bearing phosphonic acids and carboxylic acids for sensitizers of nanocrystalline TiO 2 in dye-sensitized solar cells (DSCs). We studied the effect of surface-attachment functionality and aggregation on the persistence, electron transfer reactivity, and overall photoelectrochemical performance of the dyes on TiO 2 for DSCs. The dyes were constructed around a 3,6-bis(dimethylamino)chalcogenoxanthylium core and varied in the 9-substituent: 5-carboxythien-2-yl in dyes 1-E (E = O, Se), 2-carboxythien-3-yl in dyes 2-E (E = Se), 5-phosphonothien-2-yl in dyes 3-E (E = O, Se), 4-carboxyphenyl in dyes 4-E (E = O, S), and 4-phosphonophenyl in dyes 5-E (E = O, Se). Monolayers of 1-E3-E4-Eand 5-E on nanocrystalline TiO 2 films consisted of both H-aggregated and non-aggregated dyes, whereas 2-E underwent little or no aggregation upon adsorption. With the exception of 2-Esurface coverages of dyes and the extent of H-aggregation varied minimally with surface-attachment functionality, structure of the 9-aryl group, and identity of the chalcogen heteroatom. Carboxylic acid-functionalized dyes 1-E and 4-E desorbed rapidly and completely from TiO 2 into acidified CH 3 CN, but phosphonic acid-functionalized dyes 3-E and 5-E persisted on TiO 2 for days. We used transient absorption spectroscopy to characterize excited-state electron injection from a 1-Se2-Seand 3-Se to TiO 2 . Injection of electrons from photoexcited dyes into TiO 2 yielded the dication radical ( 1-Se +2-Se +and 3-Se + ) and an associated transient absorption at wavelengths shorter than 540 nm, the amplitude of which was proportional to the quantum yield of electron injection (Q inj ). Our data reveal the Q inj for H-aggregated 1-Se was approximately 2-fold greater than Q inj for non-aggregated 1-Se and approximately 3-fold greater than Q inj for non-aggregated 2-Se . Additionally, the Q inj from H-aggregated 3-Se was (2.0 ± 1.3)-fold greater than from monomeric 3-Se . Therefore, H-aggregation increased the efficiencies of both light-harvesting and electron injection. Comparison of the analogous carboxylic acid-functionalized dye ( 1-Se ) and phosphonic acid-functionalized dye ( 3-Se ) revealed that Q inj via the carboxylate linkage was (2.3 ± 1.1)-fold greater than via the phosphonate linkage. Thus, electron-injection reactivity is sensitive to both the aggregation state and the surface-anchoring mode of these chalcogenorhodamine dyes. Short-circuit photocurrent action spectra of DSCs corresponded closely to absorptance spectra of dye-functionalized films; thus, H-aggregation did not decrease the electron-injection yield or charge-collection efficiency. Maximum monochromatic incident photon-to-current efficiencies (IPCEs) of DSCs ranged from 53% to 95% and were slightly higher for carboxylic acid-functionalized dyes 1-E and 4-E . The photoelectrochemical performance (under monochromatic or white-light illumination) of 1-E and 4-E decayed significantly within 20-80 min of assembly of DSCs, due primarily to desorption of the dyes. In contrast, the performance of phosphonic acid-functionalized dyes ( 3-E and 5-E ) remained stable or improved slightly on similar time scales. Power-conversion efficiencies of DSCs under white-light illumination were low (<1%), suggesting that dye regeneration was inefficient at high light intensities. Preliminary transient photovoltage results support this proposition. Our findings suggest that controlled aggregation of organic dyes may represent an attractive strategy for improving the global energy-conversion efficiencies of organic dye-sensitized solar cells and photocatalysts. In addition, replacing carboxylic acids with phosphonic acids increased the inertness of chalcogenorhodamine-TiO 2 interfaces without greatly impacting aggregation of dyes or the interfacial electron-transfer reactivity. The decrease of Q inj for phosphonic acid-bearing dyes is offset by its enhanced stability and persistence on TiO 2rendering the phosphonic acid-functionalized and H-aggregating dyes particularly attractive sensitizers.