NCPV preprints for the 2. world conference on photovoltaic solar energy conversion Page: 26 of 144
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PHOTOCHEMICAL SOLAR CELLS BASED ON DYE-SENSITIZATION OF NANOCRYSTALLINE T102
S. KDeb, R. Ellingson, S.Ferrere,A. J.Frank,B.A. Gregg,A. J.NozikN.Park, and G. Schlichthorl
National Renewable Energy Laboratory (NREL),1617 Cole Boulevard, Golden, CO 80401-3393 USA
ABSTRACT: A photoelectrochemical solar cell that is based on the dye-sensitization of thin nanocrystalline films of TiO2 (anatase)
nanoparticles in contact with a non-aqueous liquid electrolyte is described. The cell, fabricated atNREL, shows a conversion efficiency of
-9.2% atAM1.5, which approaches the best reported value of 10%-11% by Gratzel atEPFL in Lausanne, Switzerland. The femtosecond
(fs) pump-probe spectroscopy has been used to time resolve the injection of electrons into the conduction band ofnanocystalline TiO2 films
under ambient conditions following photoexcitation ofthe adsorbed Ru(11)-complex dye. The measurement indicates an instrument-limited
-50 fs upper limit on the electron injection time. We also report the sensitization of nanocrystalline TiO2 by a novel iron-based dye,
CIS-[Fe r(2,2'-bipyridine4,4'-dicarboxylic acid)z(CN)2], a chromophore with an extremely short-lived, nonemissive excited state. The dye
also exhibits aunique "band selective" sensitization through one ofits two absorption bands. The operational principle ofthe device has been
studied through the measurement of electric field distribution within the device structure and studies on the pH dependence of dye-redox
potential. The incorporation of W03-based electrochromic layer into this device has led to anovelphotoelectrochromic device structure for
"smart window" application.
Keywords: Dye-sensitized photoelectrochemical solar cel-1: Photoelectrochromic window-2
The idea of dye-sensitization of inorganic materials goes back
several decades, and a vast body of literature exists on the
dye-sensitization of AgBr for photographic applications. The
first use of dye-sensitized TiO2 for solar energy conversion was
reported in a U.S. patent issued in 1978, in which a
photoelectrochemical cell based on dye-sensitization of TiO2
particles, particularly in anatase form, was reported . The
dye used was N-methylphenazinium ion which extended the
spectral response of iO2to the 500-nm region. However,the
conversion efficiency of such a device was relatively low and
the stability of dye was an issue. A breakthrough occurred in
recent years when M. Gritzel and B. O'egan reported a
photochemical solar cell using nanoparticle TiO2 sensitized by
a more efficient and stable Ru(11)-complex dye . The
standard dye used in present cells  is
Ru(11X4,4'-dcarboxy-2,2-bipyridine)2(NCS)2 (absorption peak
at 550 rnm); this system shows conversion efficiencies of
7%-10% under standard solar conditions. Arecentnew "black"
dye (4,9,14-tricarboxy 2,2T-6,6'-terpyridyl
ruthenium(11)trithiocyanate) has recently been reported by
Grtzel that produces an efficiency of nearly 11% .
The photoelectrode of a dye-sensitizedPV cell consists of a
10-20-pm film of nanocrystalline TiO2 particles (10-30 rim in
diameter) that contain a monolayer of adsorbed dye molecules;
the dye-coated particles are supported on a transparent
conducting glass substrate (e.g., tin oxide) (see Fig. 1). The
pores of the nanocrystalline TiO2 film are filled with a liquid
electrolyte containing the iodide/triiodide redox couple in anon-
aqueous electrolyte, such as acetonitrile. A transparent counter
electrode is placed over the nanocrystalline TiO2, and the edges
of the cell are sealed. Upon photoexcitation of the cell, the
excited dye molecules inject electrons efficiently into the TiO2
conduction band, effecting charge separation. The injected
electrons traverse the nanocrystalline film with little loss and are
collected at the conducting glass substrate. After passing
through the external circuit and delivering power to a load, the
electrons re-enter the cell at the counter electrode and reduce
triiodide to iodide, which then diffuses into the pores of the
TiO2 film to reduce the photo-oxidized dye back to its original
P T02 e
D, D la/
D by D
- Liquid electrolyte
- To particles
counter elecrode (TCO)
Dye Sensitized Nanocrystalline TiO2
Photochemical Solar Cell
Figure 1: Band diagram and structure ofphotochemical solar
In this paper we report the fabrication of an -92% -efficient
cell using an essentially similar technology. Dynamics of
electron injection into nanocrystalline TiO2 films sensitized with
Ru-based dye has been studied by transient infrared absorption
spectroscopy. We also report the sensitization of TiO2 by an
iron bipyridyl complex, a chromophore with an extremely
short-lived, nonemissive excited state.
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NCPV preprints for the 2. world conference on photovoltaic solar energy conversion, article, September 1, 1998; Golden, Colorado. (digital.library.unt.edu/ark:/67531/metadc707815/m1/26/: accessed November 15, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.