Abstract
The ultrafast photoinjection and subsequent relaxation steps of the indoline dye D149 were investigated in detail for a mesoporous electrodeposited ZnO thin film and compared with experiments on sintered TiO(2) and ZrO(2) thin films, all in contact with air, using pump-supercontinuum probe (PSCP) transient absorption spectroscopy in the range 370-770 nm. D149 efficiently injects electrons into the ZnO surface with time constants from ≤70 fs (time-resolution-limited) up to 250 fs, without the presence of slower components. Subsequent spectral dynamics with a time constant of 20 ps and no accompanying change in the oscillator strength are assigned to a transient Stark shift of the electronic absorption spectrum of D149 molecules in the electronic ground state due to the local electric field exerted by the D149˙(+) radical cations and conduction band electrons in ZnO. This interpretation is consistent with the shape of the relaxed PSCP spectrum at long times, which resembles the first derivative of the inverted steady-state absorption spectrum of D149. In addition, steady-state difference absorption spectra of D149˙(+) in solution from spectroelectrochemistry display a bleach band with distinctly different position, because no first-order Stark effect is present in that case. Interference features in the PSCP spectra probably arise from a change of the refractive index of ZnO caused by the injected electrons. The 20 ps component in the PSCP spectra is likely a manifestation of the transition from an initially formed bound D149˙(+)-electron complex to isolated D149˙(+) and mobile electrons in the ZnO conduction band (which changes the external electric field experienced by D149) and possibly also reorientational motion of D149 molecules in response to the electric field. We identify additional spectral dynamics on a similar timescale, arising from vibrational relaxation of D149˙(+) by interactions with ZnO. TiO(2) exhibits similar dynamics to ZnO. In the case of ZrO(2), electron injection accesses trap states, which exhibit a substantial probability for charge recombination. No Stark shift is observed in this case. In addition, the spectroelectrochemical experiments for D149˙(+) in dichloromethane and acetonitrile, which cover the spectral range up to 2000 nm, provide for the first time access to its complete D(0)→ D(1) absorption band, with the peak located at 1250 and 1055 nm, respectively. Good agreement is obtained with results from DFT/TDDFT calculations of the D149˙(+) spectrum employing the MPW1K functional.
Highlights
The ultrafast photoinjection and subsequent relaxation steps of the indoline dye D149 were investigated in detail for a mesoporous electrodeposited ZnO thin film and compared with experiments on sintered TiO2 and ZrO2 thin films, all in contact with air, using pump–supercontinuum probe (PSCP) transient absorption spectroscopy in the range 370–770 nm
The spectra have a blue-shifted maximum (518 nm in the case of ZnO and ZrO2, 524 nm for TiO2) and exhibit a significant broadening which is probably due to heterogeneous binding environments and a certain fraction of D149 aggregates
We have shown that the indoline dye D149 very efficiently injects electrons on an ultrafast timescale (r250 fs) into a highly porous electrodeposited ZnO thin film, initially forming an eÀ–D149+ complex
Summary
Production.[1,2,3] The top efficiency of such devices has currently reached 12.3%, employing a mesoporous TiO2 electrode co-sensitized with Zn-porphyrin and cyclopentadithiophene dyes and a cobalt(II/III) based redox electrolyte.[4,5] Another convenient material for the mesoporous electrode in a DSSC is ZnO, which has a band gap of 3.2 eV and a conduction band edge position of À4.3 eV, both similar to TiO2.6–8 Direct low-temperature fabrication of highly porous crystalline ZnO films from aqueous solution has been achieved by co-electrodeposition with water soluble dyes, e.g. eosin Y, as structure-directing additives and subsequent removal of the dye by treatment with aqueous KOH.[9,10,11] Subsequent readsorption of another dye with a broad absorption band and a high absorption coefficient results in the formation of a. We characterize the differential absorption spectrum of D149+ by spectroelectrochemistry up to 2000 nm
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