Article pubs.acs.org/JPCC
Ultrafast Relaxation as a Possible Limiting Factor of Electron Injection Efficiency in Black Dye Sensitized Nanocrystalline TiO2 Films Ryuzi Katoh,*,†,§ Akihiro Furube,† Nobuhiro Fuke,‡ Atsushi Fukui,‡ and Naoki Koide‡ †
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ‡ SHARP Corporation, 282-1 Hajikami, Katsuragi, Nara 639-2198, Japan ABSTRACT: The photoinduced electron injection process in black dye (BD, trithiocyanato (4,4′,4″-tricarboxy-2,2′:6′,2″terpyridine)ruthenium(II); Ru(tcterpy)(NCS)3) adsorbed on nanocrystalline TiO2 films was studied by means of transient absorption and luminescence spectroscopy with subnanosecond time resolution in air and in 0.5 M solution of 4-tertbutylpyridine in acetonitrile. The resulting intensities and temporal profiles differed substantially between these conditions, whereas the electron injection efficiencies were almost the same. Generation efficiency of triplet excited state can be also evaluated, and thus the branching ratio of photoexcited BD/TiO2 can be obtained. These results clearly indicate that ultrafast relaxation from the primary excited state to the ground state occurs efficiently, probably owing to ultrafast recombination at the BD/TiO2 interface, and non-electron-injecting dyes exist on the surface. Accordingly, we conclude that the electron injection efficiency in BD/TiO2 is not simply limited by competition between the rate of deactivation in excited state and the rate of electron injection.
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INTRODUCTION Photoinduced electron transfer (ET) reactions are one of the most fundamental chemical reactions and have been studied extensively.1,2 As shown in Scheme 1, upon photoexcitation, an
Photoinduced ET reactions are known to be key reactions for solar energy conversion in a variety of photovoltaic devices, including dye-sensitized solar cells (DSSCs)5−7 and organic photovoltaic cells (OPVs).8 To realize high-performance photovoltaic devices, the efficiency of charge separation must be high, and therefore much research has been dedicated to achieving high efficiencies. DSSCs are one type of promising solar cell device, and many studies of their primary processes have been carried out with the goal of improving DSSC performance.5−7 Kinetics of electron injection in a DSSC sensitized with a conventional Ru−complex dye are illustrated in Scheme 2. Upon photoexcitation of the Ru−complex dyes, a singlet excited state (S*) is populated, and then electron injection into the TiO2 conduction band (CB) occurs in competition with ultrafast intersystem crossing (ISC) to an excited triplet state (T*). From T*, electron injection can also occur in competition with deactivation process (luminescence and internal conversion, IC) to the ground state. In this scheme, the electron injection efficiency is simply limited by the competition between the rate of electron injection and the rate of deactivation from the excited state. In Scheme 2, the electron injection efficiency from S* (Φinjsinglet) and T* (Φinjtriplet) is expressed using the rate constants of electron injection and deactivation processes.
Scheme 1. General Reaction Pathway of Photoinduced Electron Transfer
excited state of a donor molecule (D*) is populated and gives an electron to an acceptor molecule (A) with a given efficiency (ΦET). An intermediate ion-pair consisting of a donor cation (D+) and an acceptor anion (A−) is formed as a result of ET. This intermediate ion-pair can separate into free ions, but it may alternatively recombine to the ground state (geminate ionpair recombination) in a competitive process. Thus, the efficiency of free ion formation ΦFree is determined both by ΦET and by the efficiency of recombination (ΦRec) in the intermediate. Notably, the role of the intermediate is very important to discuss ΦFree. In this context, much effort has been put forth to clarify the nature of the intermediate state.3,4 © 2012 American Chemical Society
Received: March 22, 2012 Revised: September 29, 2012 Published: October 4, 2012 22301
dx.doi.org/10.1021/jp302768q | J. Phys. Chem. C 2012, 116, 22301−22306
The Journal of Physical Chemistry C
Article
Scheme 2. Kinetic Competition for Electron Injection Processes in DSSCs
Scheme 3. Possible Kinetic Channels in Photoexcitation in DSSCs
In contrast with a conventional ET process (Scheme 1), the electron injection process in DSSCs does not involve formation of an intermediate ion-pair (Scheme 2); namely, direct injection from an excited dye to CB of TiO2 has been assumed. Actually, electron injection in DSSCs occurs quickly (
