J. Phys. Chem. C 2008, 112, 5203-5212
5203
Comparison of Electron Injection Dynamics from Rhodamine B to In2O3, SnO2, and ZnO Nanocrystalline Thin Films Jier Huang, David Stockwell, Abdelaziz Boulesbaa, Jianchang Guo, and Tianquan Lian* Department of Chemistry, Emory UniVersity, 1515 Dickey DriVe, Atlanta, Georgia 30322 ReceiVed: September 17, 2007; In Final Form: NoVember 14, 2007
Interfacial electron transfer (ET) dynamics from excited rhodamine B (RhB) to three semiconductor nanocrystalline thin films (In2O3, SnO2, and ZnO) was investigated to examine their dependence on semiconductors. The injected electrons in the semiconductors were directly measured by their transient absorption in the mid-IR region (∼5 µm), and the evolution of the adsorbate in its ground, excited, and cation states was monitored by its transient absorption in the visible region. The formation of the IR absorption of injected electrons correlates well with the decay of the RhB excited state, allowing an unambiguous determination of electron injection rates from the adsorbate excited state to the semiconductor nanoparticles. The recombination processes were monitored by following the decay of injected electrons and RhB cations and the recovery of the RhB molecules in the ground state. The effects of dye aggregation and excited-state quenching on the injection dynamics were also examined and were shown to be negligible under low dye coverage and excitation power density. Injection times to In2O3 and SnO2 are similar (1.2 ps) and are ∼6 times faster than to ZnO (7.0 ps). Possible reasons for the semiconductor dependent injection rates are discussed.
Introduction Interfacial electron transfer (ET) dynamics between dye sensitizers and semiconductor nanoparticles has been the subject of intense interest because of its importance to many applications of nanoparticles, including dye sensitized solar cells (DSSCs).1-3 The highest efficiency in DSSCs has been reported for Ru(dcbpy)2(NCS)2 [dcbpy is (4,4′-dicarboxy-2,2′-bipyridine)] (RuN3) sensitized TiO2 nanocrystalline thin films, which exhibit solar to electric power conversion efficiencies as high as 10% and an incident photon-to-current conversion efficiency (IPCE) near unity at peak absorption wavelengths.4,5 Cells based on other combinations of sensitizers and semiconductors, such as ZnO,6-8 Nb2O5,6,7,9-13 SnO2,6,14,15 WO3,7 Ta2O5,7 and In2O36,7 showed a lower conversion efficiency. The reasons for their low efficiencies remain unclear. The high IPCE in cells based on RuN3 sensitized TiO2 films was attributed to ultrafast electron injection from the RuN3 excited states to TiO216-33 and a much slower charge recombination from TiO2 to the oxidized dye molecules and redox electrolytes.34-42 A comparison of electron injection and recombination kinetics in different sensitizer/ semiconductor combinations may lead to an understanding of the dependence of ET dynamics on the properties of sensitizers and semiconductors and suggest possible ways to improve cell efficiencies. The injection dynamics from RuN3 dyes to several different semiconductor films, such as TiO2, SnO2, ZnO, and In2O3, has been reported.7,19,21,23,26-28,32-34,36,43-47 Electron injection to TiO2 was shown to be biphasic, with an ultrafast component (
