LETTERS Electron Injection and Recombination in Ru(dcbpy) 2 (NCS) 2 Sensitized Nanostructured ZnO Christophe Bauer, Gerrit Boschloo, Emad Mukhtar, and Anders Hagfeldt* Department of Physical Chemistry, Uppsala UniVersity, Box 532, S-751 21 Uppsala, Sweden ReceiVed: NoVember 9, 2000; In Final Form: March 19, 2001 The dynamics of electron-transfer processes between bis(tetrabutylammonium) cis-bis(thiocyanato)bis(2,2′- bypiridine-4,4′-dicarboxylato)ruthenium(II) (called N719) and nanostructured ZnO films have been investigated by femtosecond and nanosecond spectroscopy. The incident photon to current conversion efficiency (IPCE) for these dye-sensitized electrodes was 36% in the maximum of 530 nm, corresponding to a quantum efficiency of 80%. The highest IPCE values were obtained when the electrodes were prepared under conditions where formation of dye aggregates in the pores of the nanostructured films is avoided. For such films, the electron injection time was in the subpicosecond regime (<300 fs), which is comparable to the N719-TiO 2 system. The back electron-transfer kinetics between conduction band electrons and oxidized dye molecules were biexponential with time constants of 300 ns and 2.6 μs. Variation of the light intensity did not affect the time constants, but only their relative weights. The kinetics of back electron transfer in the N719-ZnO and N719-TiO 2 systems were found to be identical. Introduction Solar cells based on the dye-sensitization of nanostructured metal oxides are promising for low-cost solar energy conversion and are the subject of intense research. 1 Most research is focused on the system with cis-bis(dithiocyanato)bis(2,2′-bipyridine-4,4′- dicarboxylato)ruthenium(II) (or Ru(dcbpy) 2 (NCS) 2 ) sensitizer, nanostructured TiO 2 (anatase) films and I - /I 3 - redox electrolyte. This combination is particularly successful for a number of reasons: (1) The electron injection from the dye excited state into the conduction band of TiO 2 is ultrafast (femtosecond regime). 2,3 (2) The recombination of injected electrons with oxidized dye molecules is much slower (nanosecond to micro- second regime). 4 (3) The electron transfer of injected electrons with triiodide is very slow (millisecond to second regime). 5 (4) The regeneration of the oxidized dye with the iodide is fast (nanosecond regime). 4 To gain more fundamental insight in the nanostructured solar cell and to approach a general understanding of the factors governing efficient electron transfer at semiconductor surfaces, we replaced TiO 2 with ZnO, as this material has a similar band gap and conduction band position but has different electronic properties. Initial studies of Ru(dcbpy) 2 (NCS) 2 -sensitized nano- structured ZnO were not very promising in terms of solar cell efficiency, 6 but recently we achieved considerable improve- ments. 7,8 In this Letter, the electron injection and recombination kinetics of efficient Ru(dcbpy) 2 (NCS) 2 -sensitized ZnO elec- trodes are discussed. Recently, Asbury et al. reported relatively slow and multiexponential electron injection in Ru(dcbpy) 2 - (NCS) 2 -sensitized ZnO. 9 We will demonstrate that in efficient Ru(dcbpy) 2 (NCS) 2 -sensitized ZnO photoelectrodes electron injection is ultrafast (<300 fs) and that the recombination rate of injected electrons with oxidized dye molecules is comparable in the N719-ZnO and N719-TiO 2 systems. Experimental Section Preparation of Transparent Nanostructured ZnO Films. Colloidal ZnO was prepared using a modification of a previously published method 10 and will be discussed in detail in a * Corresponding author. E-mail: anders.hagfeldt@fki.uu.se. © Copyright 2001 by the American Chemical Society VOLUME 105, NUMBER 24, JUNE 21, 2001 10.1021/jp004121x CCC: $20.00 © 2001 American Chemical Society Published on Web 05/26/2001