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2. THENREL PHOTOELECTROCBEMICAL SOLAR
The Photoelectrochemical Solar Cell project at NREL is an
integrated program of basic and applied research that is jointly
planned and funded bytheU.S. Department ofEnergy's Office
of Energy Research and Office of Energy Efficiency and
Renewable Energy. In this section we discuss our research on
the development of dye-sensitized photovoltaic cells and
elucidate the operating principle of these devices.
2.1 NREL Cell Preparation and Efficiency
We have fabricated a dye-sensitized Ti02 solar cell (with no
antireflection coating) with a conversion efficiency of 9.2%.
This value approaches the best achieved, to date, by the group
of Gratzel atEPFL (10%-11%) and is substantially higher than
efficiencies reported by other laboratories. The -9.2% cell
displayed a short-circuit photocurrent Jx of 17.899 mA/cm2, an
open-circuit photovoltage Von of 756 mV, and a fill factor of
The preparation of TiOzfilm was adapted from the literature
[5-6]. A conducting glass plate (Asahi Glass;
F-doped SnO2overlayer, 80 transmittancee inthe visible, 14%
haze, 7./sq) was used as the substrate for depositing TiO2
films. To control the thickness of the TiO2 film and to mask
electrical contact strips, 0.5-cm width of the conducting glass
plate was covered along the length of each edge with adhesive
tape. A mixture of TiO2 colloids and TiO2 powder were
deposited on the surface of the conducting glass. After
removing the adhesive tapes, the assemblage was heated in air
for 30 min at 450*C and then allowed to cool. The TiO2-
covered plate was cut into a 1.25 x 1.2 cm electrode. The film
was then immersed in an aqueous 0.3 M TiCl4 solution for 2
days in a closed chamber, washed with distilled water, and
annealed again at 450 *C for 30 min. The thickness of the
resulting film was about 12 pm.
The TiO2 electrodes were coated with [RuLL'(NCS)2] (L =
2,2'-bypyridy-4,4'-dicarboxylic acid, IV = 2,2'-bipyridyl-4,4-
ditetrabutylammoniumcarboxylate). The dye-coated
nanocrystalline TiO2 electrode was soaked in 4-tert-
butylpyridine for 15 min and then dried under aN2 stream. Pt
counter electrodes with a mirror finish were prepared by
electron beam, depositing a 60 nm layer ofPton top ofa40 nm
layer of Ti on a glass plate. The Pt electrode and the dye-coated
TiO2 electrode were sealed together with 25-pm thick strip of
Surlyn (Dupont), sandwiched along the length of each edge.
Sealing was accomplished by pressing the two electrodes
together at a pressure of 900 psi and a temperature of 120 *C.
A small quantity of a redox electrolyte, consisting of an alkyl
methylimidazolium iodide and 2in acetonitrile, was introduced
into the porous structure of the dye-covered TiO2 film by
capillary action. The dye-coated TiO2 film was illuminated
through the transparent conducting glass support
2.2 ImprovingthePhotovoltage Through Surface
A major factor limiting the conversion efficiency of present
dye-sensitized TiO2 solar cells is the low photovoltage ,
which is substantially below the theoretical maximum [8-10].
Charge recombination at the nanociystallite/redox electrolyte
interface is expected to play a significant role in limiting the
photovoltage. There are two likely recombination pathways
occurring at the interface. The injected conduction-band
electrons may recombine with oxidized dye molecules or react
with redox species in the electrolyte. Owing to the rapid rate of
reduction of the oxidized dye molecules by r ions, which are
present at high concentration, the contribution of this latter
energy-loss channel to the recombination current can usually be
ignored . The net recombination process, controlling the
photovoltage, is represented by the reaction :
2e + 13 -- 3r
Some suppression of back electron transfer in BO2, as
manifested by a higher open-circuit photovoltage V, has been
reported [12,13] as a result of chemically treating the surface
In this paper, we report on the effect of various surface
modifying reagents on V~ and the underlying mechanism
[14,15] for their action. An unexpected resultisthe discovery
that the reaction rate for recombination is second -order in
triiodide ion concentration. The mass-transport theory is also
applied to determine whetherthe nanoporous TiO2film impedes
the diffusion oftriiodide ions in the cell.
The fill factor is also not significantly changed by surface
treatment. The major effect of surface treatmentisto increase
V. and consequently the cell efficiency. The improved V.
with respect to the untreated surface (V,. = 0.57 V) ranges
from 0.64 V for vinylpyridine-treated electrodes to 0.73 V for
polyvinylpyridine-treated electrodes, corresponding to respective
increases of 70 and 160 mV. The largest improvement was for
an NH3-treated electrode, which yielded a Ve of 0.81 V,
corresponding to an increase of 240 mV, and a conversion
efficiency of 7.8 %.
To determine whetherthe nanoporous Tizfilm impedes the
diffusion of 3 ions in the liquid phase, the dependence of V.
on the radiant power at low I3 concentration was studied, and
mass transport theory was applied to the experimental data to
obtain the diffusion coefficient of I3. The calculated curve
coincides closely with the experimental data for an optimized
diffusion coefficient of 7.55 10- cm2s for 13 ions in
CHaCN/NMO (5050 wt%)/TiO2. After correcting forthe TiC2
porosity (0.3) , the diffusion coefficient of I3 ions in the
solution phase was determined to be 2.5 10.5 cm2/s, which is in
good agreement with values obtained for I3 ions in CH3CN
[(8.5-30)10' cm2/s] and NMO (2.8 104 cm2s) [17-19]. The
similarity of our measured value of the diffusion coefficient
with those reported in the literature implies that, in the
concentration range investigated, most of the 13 remains in
solution and is not adsorbed to the TiO2 surface. In other
words, the porous structure of the TiO2 films does not
significantly retard the diffusion of Ii ionsinthe solution phase.
23.1 Time-Resolved Infrared Spectroscopic Studies of
Electron Injection in Dye-Sensitized TiC2
We have used femtosecond pump-probe spectroscopy 
to time resolve the injection of electrons into nanocrystalline
TiO2 film electrodes under ambient conditions following
photoexcitation of the adsorbed dye [Ru(4,4=-dicarboxy-2,2=-
bipyridine)2(NCS)2] (N3). Pumping at one of the metal-to-
ligand charge transfer absorption peaks and probing the
absorption by injected electrons in the TiO2 at 1.52 pm and in
the range of4.1 to 7.0 pm,we have directly observed the arrival
of electrons injected into the TiO2 film. Our measurements
indicate an instrument-limited 50-fs upper limit on the electron
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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/m1/27/: accessed December 15, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.