J. Phys. Chem. C 2007, 111, 8979-8987
8979
Effect of Insulating Oxide Overlayers on Electron Injection Dynamics in Dye-Sensitized Nanocrystalline Thin Films† Jianchang Guo, Chunxing She, and Tianquan Lian* Department of Chemistry, Emory UniVersity, Atlanta, Georgia 30322 ReceiVed: December 27, 2006; In Final Form: February 3, 2007
Insulating metal oxide coatings has been investigated as a potential approach for improving the efficiency of dye-sensitized solar cells. Thin insulating overlayers were thought to improve the cell efficiency by retardation of the recombination kinetics of injected electrons in semiconductor thin films with the oxidized sensitizer molecules and the electrolytes. However, how they affect the electron injection dynamics is still unclear. In this work, we examined the effects of Al2O3 overlayers on the electron injection dynamics in Ru and Re bipyridyl complexes sensitized nanocrystalline TiO2 and SnO2 films. Electron injection dynamics was measured as a function of the number of overlayers by ultrafast infrared transient absorption spectroscopy. The uniformity of the coating was also probed using interfacial charge-transfer complexes. The injection rate was found to decrease when the number of Al2O3 overlayers increases, and the mechanisms by which the overlayers affect the injection dynamics were discussed.
kET(r) ) k0 e-βrr
Introduction Controlling interfacial electron transfer (ET) rates between molecular adsorbates and semiconductor nanoparticles has been a subject intense recent interest because it is a potential approach for optimizing the efficiency of dye-sensitized solar cells.1,2 In the most-efficient dye-sensitized solar cells, using Ru(dcbpy)2(NCS)2 [dcbpy ) (4,4′-dicarboxy-2,2′-bipyridine)](RuN3) sensitized TiO2 nanocrystalline thin films, a solar to electric power conversion efficiency as high as 10% has been achieved and incident-photon-to-current conversion efficiency (IPCE) near unity at peak wavelength has been reported.1,3,4 The high conversion efficiency was attributed to ultrafast electron injection from the RuN3 excited states to TiO25-22 and a much slower charge recombination from TiO2 to the oxidized dye molecules and redox electrolytes.23-31 The ultrafast injection process, consisting of a primary 10 Å) generally varies exponentially with the length of the bridge, r33,41 †
Part of the special issue “Kenneth B. Eisenthal Festschrift”. * Corresponding author. E-mail:
[email protected].
(1)
where k0 is the extrapolated rate constant at r ) 0 and βr is the exponential decay constant. The βr values were found to be 0.81.2/Å for methylene spacers and smaller (0.2 to 0.6 Å-1) for unsaturated spacers such as polyene, phenylene phenylenevinylene, and phenylene-ethynylene.38-40 The observed exponential dependence is in good agreement with theoretical predictions and reflects the exponential variation of the coupling strength with the distance for electron tunneling processes.38-40 These studies showed that the dependence at shorter bridge lengths (
