Two-electron photo-oxidation of betanin on titanium dioxide and potential for improved dye-sensitized solar energy conversion

Abstract

The plant pigment betanin is investigated as a dye-sensitizer on TiO 2 with regard to its potential to undergo twoelectron oxidation following one-photon excitation. Electrochemical, spectroelectrochemical and transient absorption measurements provide evidence for two-electron proton-coupled photo-oxidation leading to a quinone methide intermediate which rearranges to 2-decarboxy-2,3-dehydrobetanin. Time-resolved spectroscopy measurements of betanin on nanocrystalline TiO 2 and ZrO 2 films were performed on femtosecond and nanosecond time-scales and provide evidence for transient species with absorption bands in the blue and the red. The results shed light on previous reports of high quantum efficiencies for electron injection and point the way to improved solar conversion efficiency of organic dyesensitized solar cells.

Dynamics of Interfacial Electron Transfer from Betanin to Nanocrystalline TiO2: The Pursuit of Two-Electron Injection

“…The latter is consistent with the wavelength range of the steady state fluorescence spectrum of Bt. 27 The results of global fits to a biexponential relaxation model plus a constant background (nondecaying on the ps time window of our experiment) were reported in ref 27 and are shown graphically in the Supporting Information ( Figure S2). Table 1 summarizes the results from a multiexponential global analysis of the transient absorption dynamics at selected wavelengths (450, 525, and 675 nm).…”

“…The SEC measurements of Bt on TiO 2 suggest that oxidation of Bt in its ground electronic state takes place in the vicinity of 1.9 V Ag/AgCl (2.1 V NHE). This is considerably more positive than the redox potential of Bt at a gold electrode in aqueous solution at pH 7, about 0.44 V Ag/ AgCl, 27 perhaps as a result of the poor dark conductivity of the TiO 2 electrode. There is ample evidence that electron injection from Bt* to the CB of TiO 2 is thermodynamically favorable, resulting in good external quantum yields and energy conversion efficiency.…”

“…This is consistent with partial irreversible oxidation of Bt on TiO 2 but not on ZrO 2 , in agreement with the comparison of steady state absorption spectra of the films before and after laser exposure. 27 Second, the relative amplitudes of the fast and slow decay components differ greatly, especially when looking at the positive transient in the vicinity of 450 nm. For Bt on ZrO 2 , the fast component of the blue TA has an amplitude similar to that of the slow component.…”

“…Femtosecond and nanosecond transient absorption spectra were recorded as described in ref 27. Briefly, femtosecond pump−probe spectra were recorded at an excitation wavelength of 580 nm generated by a non-collinear optical parametric amplifier from the output of a Ti:sapphire laser at 778 nm, resulting in sub-50 fs pulses after compression.…”

Charge separation and carrier dynamics in donor-acceptor heterojunction photovoltaic systems

“…Significantly slower charge injection into nanocrystalline titanium dioxide was witnessed for the plant-pigment betanine 39,40 and the D-π-A organic dye-sensitizer Y123, 41 where the interfacial charge transfer process was observed to take place with time constants of 6–8 ps and 2 ps, respectively. The conclusion provided by X-ray absorption experiments that electrons can be injected directly into trap states in the proximity of the dye-sensitizer molecules they originate from, rather than escaping quickly as hot carriers has important consequences.…”