Materials Science Forum
ISSN: 1662-9752, Vol. 771, pp 71-89
doi:10.4028/www.scientific.net/MSF.771.71
© 2014 Trans Tech Publications, Switzerland
Online: 2013-10-25
ZnO Nanoforest based New Generation Dye Sensitized Solar Cells
Pragnesh N Dave1,a, Puneet R Malpani2,b
1
Department of Chemistry, Krantiguru Shyamji Krishna Verma Kachchh University,
Mundra Road, Bhuj (Gujarat), India
a
[email protected], b [email protected]
Keywords: Dye Sensitized Solar Cells, Zinc Oxide, Photoanode, Metal Oxide.
Abstract
ZnO is gaining importance in the electronics industry because of its availability of large sized single
crystals, strong luminescence demonstrated in optically pumped lasers and the possibility of gaining
control over its electrical conductivity. Dye Sensitized Solar Cells (DSSC’s) is a
photoelectrochemical system that incorporates a porous structured wide-bandgap oxide
semiconductor (TiO2 or ZnO) film as the photosensitized anode that offers increased surface area
for dye molecule adsorption. ZnO Nanoforest is comprised of high density, branched ZnO nanowire
photoanodes. The overall light-conversion efficiency of the branched ZnO nanowire DSSC’s is
almost 5 times higher than the efficiency of DSSC’s constructed by upstanding ZnO nanowires. The
efficiency increase is due to increased surface area for higher dye loading and light harvesting, and
also due to reduced charge recombination phenomena by providing direct conduction pathways
along the crystalline ZnO nanoforest.
Introduction
Photoelectrochemical solar cells (PSCs), consisting of a photoelectrode, a redox electrolyte,
and a counter electrode. Several semiconductor materials such as n- and p-Si, n- and p-GaAs,
n- and p-InP, and n-CdS, have been used as photoelectrodes. These materials, when used with a
suitable redox electrolyte, can produce solar light-to-current conversion efficiency of
approximately 10%. However, under irradiation, photocorrosion of the electrode in the electrolyte
solution frequently occurs, resulting in poor stability of the cell, so efforts have been made
worldwide to develop more stable PSCs.
Oxide semiconductor materials have good stability under irradiation in solution. However,
stable oxide semiconductors cannot absorb visible light because of relatively wider band gaps.
Sensitization of wide band gap oxide semiconductor materials, such as TiO2 , ZnO, and SnO2 ,
with photosensitizers, such as organic dyes, brought the concept of Dye Sensitized Solar Cells
(DSSC) in the world. In the sensitization process, photosensitizers adsorbed onto the
semiconductor surface absorb visible light and excited electrons are injected into the conduction
band of the semiconductor electrodes [1].
Recent drastic improvements in the performance of DSSC have been made by Grätzel and
coworkers at the Swiss Federal Institute of Technology (EPFL). They achieved a solar energy
efficiency, η, of 7 to 10% under AM1.5 irradiation using a DSSC consisting of a nanocrystalline
TiO2 thin-film electrode having a nanoporous structure with large surface area, a novel Ru
bipyridyl complex, and an iodine redox electrolyte [2, 3]. They also developed a Ru terpyridine
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Potential Development in Dye-Sensitized Solar Cells for Renewable Energy
complex that absorbs in the near-IR region up to 900 nm as a photosensitizer for a
nanocrystalline TiO2 photoelectrode: the resulting cell obtained η = 10.4% under AM1.5 with a
short circuit photocurrent density, JSC , of 20.5 mA cm−2 , an open-circuit voltage, VOC , of
0.72 V, and a fill factor (ff ) of 0.70 [4, 5]. Below mentioned is an image of the Grätzel cell
showing its working and mechanism.
Fig. 1. (1) Sunlight entering the cell strikes the dye molecules on the surface of TiO2. Absorbed
energy creates an excited state of the dye from which an electron in injected into the TiO 2 particles.
2) The released electrons move by diffusion to the anode on top and transfer to an external circuit.
3) Meanwhile the dye molecule strips an electron from Iodide in electrolyte below the TiO2,
replacing the one it has lost. 4) The Iodide recovers its missing electron by mechanically diffusing
to the bottom of the cell, where the counter electrode reintroduces the electron after flowing through
the external circuit [6].
Fig. 2. Electron-hole interaction after the excitation of dye molecule with sunlight. Here,
photoexcited electrons are injected from the dye to the conduction band (denoted as ‘‘c.b.’’) of the
nanocrystallite (1), the dye is regenerated by electron transfer from a redox couple in the electrolyte
(3), and a recombination may take place between the injected electrons and the dye cation (2) or
redox couple (4). The latter (4) is normally believed to be the predominant loss mechanism.
Electron trapping in the nanocrystallites (5) is also a mechanism that causes energy loss.
Materials Science Forum Vol. 771
73
The possible mechanism of electron hole interaction during DSSC operation is illustrated in Figure
2. After the successful excitation of dye molecules with the sun light, the ground state electrons at
the highest occupied molecular orbital will be photoexcited to the unoccupied molecular orbital.
The photoexcited electron will pass through the nanomaterials to reach electrodes for completion.
Selection of ZnO as Photosensitized anode for DSSC
A direct wide band gap metal oxide material is zinc oxide (ZnO), a wurtzite type semiconductor
with an energy gap of 3.37 eV at room temperature. Due to its large bandgap, ZnO is an excellent
semiconductor material like other wide bandgap materials GaN and SiC. The crystal structure,
energy band gap, electron mobility and electron diffusion coefficient of both ZnO and TiO 2
nanomaterial’s were summarized in Table 1 for comparison. The band gap energy of ZnO
nanomaterial’s is almost same as that of TiO2 but the electron mobility and electron diffusion
coefficient of ZnO showed much higher values than the TiO2 which facilitates its importance in the
DSSC application. ZnO has higher electronic mobility as compared to TiO2 providing faster means
of charge transfer and reduced recombination losses which is a very important criteria for DSSC.
[10]
Table 1. Comparison of physical and electrical properties of ZnO and TiO2 [10]
Crystal structure
Energy band gap [eV]
Electron mobility [cm2 / Vs]
Refractive index
Electron effective mass[me]
Relative dielectric constant
Electron diffusion coefficient
[cm2 / s1]
ZnO
rocksalt, zinc blende,
and wurtzite
3.2-3.3
205–300 (bulk ZnO), 1000
(single nanowire)
2.0
0.26
8.5
5.2 (bulk ZnO), 1.7x104
(nano-particulate film)
TiO2
rutile, anatase, and brookite
3.0-3.2
0.1-4
2.5
9
170
0.5 (bulk TiO2) 108-104
(nano- particulate film)
The major objective behind selecting a photoanodic nanomaterial’s film for DSSC is that it offers
large internal surface area whereby to adsorb sufficient dye molecules for the effect capture of
incident photons from the solar power. Thus high electron mobility, high thermal conductivity, wide
and direct band gap in the blue and ultraviolet region, large free-exciton binding energy due to
which emission processes persists even at room temperature or above, make ZnO suitable to be
used as Photosensitized anode for DSSC. [7]
ZnO Generation Dye Sensitized Solar Cells
Dye-sensitized solar cell (DSSC) using inorganic semiconductor and organic dyes/metallorganiccomplex dye is considered as an alternative to conventional silicon solar cell because they are
flexible, inexpensive and easier to manufacture. One important limiting factor in the DSSC cell
performance is electron transport. During its traversal to the photoelectrode, an electron is estimated
to cross 103 to 106 nanoparticles [3]. The disorder structure of the nanoparticles film leads to
enhanced scattering of free electrons, thus reducing electron mobility and causing electron
recombination especially at the grain boundaries between the nanoparticles [8]. The replacing of the
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Potential Development in Dye-Sensitized Solar Cells for Renewable Energy
nanoparticle film with an array of nanorod offers the potential for improved electron transport
leading to higher photo-efficiencies. The pathways provided by the nanorods ensure the rapid
collection of carriers generated throughout the device as the nanorods provide a direct path from the
point of photogeneration to the conducting substrate. This greatly reduces the electron
recombination losses of the photogenerated charge-carriers due to the fewer grain boundaries in
charge transportation process. Moreover, electron transport in the crystalline rod is expected to be
several orders of magnitude faster than percolation through a random polycrystalline network [9].
1. Fabrication
Synthesis of ZnO Nanoparticles
Zn(Ac)2.2H2O is dissolved in boiling ethanol at atmospheric pressure and the solution was directly
cooled to 0°C. A white powder precipitated close to room temperature. X-ray diffraction showed
that this was anhydrous zinc acetate, Zn(Ac)2. LiOH.H2O was dissolved in ethanol at room
temperature in an ultrasonic bath and cooled to 0 °C. Hydroxide containing solution was added
dropwise to the Zn(Ac)2 suspension under vigorous stirring at 0 °C. The reaction mixture became
transparent when LiOH had been added. The ZnO sol was stored at ≤ 4 °C to prevent rapid particle
growth. Precipitation of ZnO aggregate can be achieved by addition of water, washing of the
precipitate with cold ethanol and again dispersing in ethanol . Precipitates can also be improved by
addition of other organic solvent. Precipitated ZnO nanoparticle will becomes seed for the growth
of ZnO nanorods, ZnO nanowires, ZnO Nanoforest.[11]
ZnO Nanorods as Photoanodic Material for DSSC
Conducting glass substrate is thoroughly cleaned by acetone/ethanol using sonication process. ZnO
nanoparticles dispersed in ethanol/methanol is spread uniformly on conducting glass substrate
through spin coating, this particles serves as seed on the substrates to be grown as nanorods.
Nanorods are grown by immersing seeded substrates in aqueous solution containing Zinc Nitrate
hydrate and hexamethylenetetramine at 90 °C for few hours. During growth stirring is done in order
to maintain homogeneous solution concentration. Growth Starts with polycrystalline film and ends
with densely packed nanorods. Keeping the design of solar cell in mind polycrystalline film is
necessary to prevent short circuit between hole conductor/collector and conducting glass substrate.
The length and diameters of the ZnO nanorods depends on the reaction parameters such as reactant
concentration, growth time and temperature. Nanorods are then rinsed with deionized water and
dehumidified to remove any residual organics in order to optimize cell performance. [12][13]
Ruthenium Dye solution is prepared by dissolving the dye in ethanol. To bring up a dye monolayer
on ZnO Nanorods they are immersed in it and heated at about 80 °C for couple of hours. Dye
loaded Nanorods are then purged with ethanol in order to remove non-adsorbed dye molecules. For
preparing CuSCN solution it should be dissolved in Di-n-propyl sulfide and stored overnight.
Supersaturated solution is allowed to settle. In order to load CuSCN , dye loaded ZnO Nanorods is
kept on a hotplate at 80 °C and the solution of CuSCN is repeatedly dropped at short intervals in
order to obtain a desired thickness of uniform coated film of CuSCN over the dye loaded ZnO
Nanorods to prevent short circuiting. On CuSCN, Ti/Au contacts with diameter of 0.5 mm were
deposited using e-beam evaporation through a shadow mask. Figure 3 shows a schematic of ZnO
Nanorods DSSC.
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Fig. 3. Schematic of ZnO Nanorods/Dye/CuSCN solar cell.
ZnO Nanowire as Photoanodic Material for DSSC
Arrays of ZnO nanowires were synthesized in an aqueous solution using a seeded-growth process.
This method employed fluorine-doped tin oxide (FTO) substrates that were thoroughly cleaned by
acetone/ethanol sonication. A thin film of ZnO quantum dots (dot diameter = 3–4 nm, film
thickness = 10–15 nm) was deposited on the substrates via dip coating in a concentrated ethanol
solution. Nanowires were grown by immersing the seeded substrates in aqueous solutions
containing 25 mM zinc nitrate hydrate, 25 mM hexamethylenetetramine, and 5–7 mM
polyethylenimine (PEI) at 92°C for 2.5 h. After this period, the substrates were repeatedly
introduced to fresh solution baths in order to obtain continued growth until the desired film
thickness was reached. The arrays were then rinsed with deionized water and baked in air at 400 °C
for 30 minutes to remove any residual organics and to optimize cell performance. The use of PEI, a
cationic polyelectrolyte, is particularly important in this fabrication, as it serves to enhance the
anisotropic growth of nanowires. As a result, nanowires synthesized by this method possessed
aspect ratios in excess of 125 and densities up to 35 billion wires per square centimeter. The longest
arrays reached 20–25 µm with a nanowire diameter that varied from 130 to 200 nm. These arrays
featured a surface area up to one-fifth as large as a nanoparticle film. [14]
Nanowire arrays were first sensitized in a solution (0.5 mmol / l) of (Bu4N)2Ru(dcbpyH)2(NCS)2
(N719 dye) in dry ethanol for one hour and then sandwiched together and bonded with thermally
platinized FTO counter electrodes separated by 40 µm thick hot-melt spacers. The internal space of
the cell was filled with a liquid electrolyte (0.5M LiI, 50mM I2, 0.5M 4-tertbutylpyridine in 3methoxypropionitrile) by capillary action.
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Potential Development in Dye-Sensitized Solar Cells for Renewable Energy
Fig. 4. Schematic of ZnO array nanowire DSSC [13] .
ZnO Nanoflower as Photoanodic Material for DSSC
Fluorine doped SnO2 (FTO) glass substrate was firstly immersed in a 100 ml 5 mM zinc chlorine
aqueous solution with a small amount of ammonia in a bottle with autoclavable screw cap, and then
the solution was kept at 95 ° C for 15–40 h. for deposition of the nanoflower film, a clear FTO
substrate without any surface modification was used. It took about 16 for nanoflower to deposit
uniformly on the substrate.
After ZnO-nanostructure fabrication, the ZnO nanostructure substrates were treated by oxygen
plasma for 5 min and baked in oven at 100 ° C. Then the substrates were immersed in a ethanolic
solution containing 0.2 mM cis-bis(isothiocyanato)bis(2,2’- bipyridyl-4,4’-dicarboxylato)ruthenium (II) bis-tetrabutylammonium for 1 h for dye loading. The counter electrode used was a
400 Å Pt fabricated by e-beam evaporation on a commercial indium tin oxide glass. The two
electrodes separated by 50µm thermal-plastic Suryln spacers were bonding together, and the
electrolyte composed of 0.1 M I2, 0.1 M LiI, 0.5M ter-butylpyridine and 0.6 M 1-hexyl-3methylimidazolium iodide in methoxy-acetonnitrile was introduced between the electrodes by
capillary action. The active cell area was typically ~0.4 cm2.
Figure 5 shows the current-voltage (I-V) characteristics for DSSCs constructed using nanorod and
nanoflower films measured in dark and under a simulated illumination with a light intensity of 100
mW/cm2 (AM1.5G). The short circuit current (Jsc), open-circuit voltage (Voc), FF and PCE
derived from the I-V curves under AM1.5G illumination for both nanorod and nanoflower based
DSSCs are also presented in the inset table in figure 5. From figure 5, it can be seen that Jsc, Voc
and FF for the cell constructed using ZnO nanoflower film (Jsc = 5.5 mA/cm 2 , Voc= 0.65V and FF
= 0.53) represent clear improvement over the cell constructed using nanorod array (Jsc = 4.5
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mA/cm2 , Voc= 0.63V and FF = 0.36). Due to the much improved Jsc and FF, the nanoflower-based
cell reached a total power conversion efficiency of 1.9%, outperforming that of the nanorod-based
cell (1.0%) by 90%.
Fig 5. Current-voltage (I-V) characteristics for solar cells constructed using ZnO nanorods and ZnO
nanoflowers under dark and under illumination (light) with 100 mW/cm2 AM1.5G simulator.
DSSC’s performance (Jsc, Voc, FF and PCE) taken from the I-V characteristics under illumination
for both nanorod and nanoflower based cells.
Figure 6(a) and 6(b) show schematically two nanostructure arrays (upstanding nanorod and
nanoflower, respectively) shined under sunlight. Theoretically, upstanding nanorod array may be
not favorable for light harvesting (or light-dye interactions) because some photons could possibly
fall on the gap between adjacent ZnO nanorods, not being able to be absorbed by the dye interfacing
ZnO. Even for the light shining vertically on the nanorod, the absorption is not complete because
the light might only passes through one thin layer of dye-ZnO interface at the end of the rod.
Therefore, the light loss may be significant for upstanding ZnO arrays. In the nanoflower
morphology, the random branches of the nanoflowers benefit both a larger surface area and an
increased light-dye interactions, meanwhile, not sacrificing the good electron transportation. using
nanoflower photoanode helps us to increase the dye loading and light harvesting, while retain good
electron conductivity as in the upstanding nanorod photoanode. With nanoflower ZnO photoanode,
a DSSC with power conversion efficiency of 1.9% was attained, which is 90% better compared to
the control DSSC using upstanding ZnO-nanorod array as photoanode. [14,15]
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Potential Development in Dye-Sensitized Solar Cells for Renewable Energy
Fig 6. Schematic of upstanding Nanorod (a) and flowerlike (b) arrays shined under light.
ZnO Nanoforest for photoanodic material for DSSC
“Nanoforest” of high density, long branched “treelike” multigeneration hierarchical ZnO NW
photoanodes can significantly increase the power conversion efficiency. The efficiency increase is
due to substantially enhanced surface area enabling higher dye loading and light harvesting and also
is due to reduced charge recombination by direct conduction along the crystalline ZnO nanotree
multigeneration branches. This approach mimics branched plant structures with the objective to
capture more sunlight.
Nanoforest of hierarchical ZnO NWs is grown by a hydrothermal growth approach. Depending on
the growth conditions, two types of growth modes are observed, lengthwise growth (LG) and
branched growth (BG). LG can yield ZnO NWs of increased length by extending the growth at the
tip of the backbone ZnO NW. On the other hand, BG produces highly branched ZnO NWs by
multiple generation hierarchical growth. As shown in figure 7a,b, first generation (backbone) ZnO
NWs are grown from ZnO quantum dot seeds deposited on a substrate immersed in an aqueous
precursor solution. ZnO quantum dots (3-4nm)in ethanol are drop casted on a F-SnO2 conductive
glass (FTO) substrate to form uniform seeds for ZnO NW growth. NWs were grown by immersing
the seeded substrate into aqueous solutions containing 25 mM zinc nitrate hydrate, 25 mM
hexamethylenetetramine (C6H12N4,HMTA) and 5–7 mM polyethylenimine (PEI) at 65-95 °C for 37 hours. [13] After the reaction was complete, the grown ZnO NWs were thoroughly rinsed with
Milli-Q water and dried in air to remove residual polymer. Longer ZnO NW can be produced by
repeating the hydrothermal growth process in a fresh aqueous precursor solution per the LG mode
sketched in Figure 7c. Dramatic change in the ZnO NW structure could occur by heating the first
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79
generation ZnO NW at 350 °C(10min),adding seed NPs and subsequently applying hydrothermal
growth. Instead of LG, highly BG of ZnO NW on the sidewalls of the first generation ZnO NW
could be observed (Figure 7d). . It should be noted that BG differs from LG only in that it involves
the presence of a heating step and a seeding step before the regular hydrothermal growth. The
combination of multiple LG and BG steps can be applied for more complex hierarchical ZnO NW
structuring.
Fig 7. Two routes for hierarchical ZnO NW hydrothermal growth. Length growth (LG) (a-b-c),
branched growth (BG) (a-b,d), and hybrid (a- e). Notice polymer removal and seed NPs for
branched growth.
While a single hydrothermal reaction LG process can produce 2-8 µm long, vertically aligned ZnO
NWs (130-200 nm diameter), multiple LG growth steps can grow 40-50 µm long ZnO NWs of high
aspect ratio (>100). Figure 8a displays ZnO NW after 1,2,3 times LG steps. The length extension
becomes smaller as the step order increases. Vertically aligned long ZnO NWs grown by multiple
LG steps can be used as the backbone of the hierarchical branched ZnO NW forest. High quality
hierarchical branched ZnO NW forest can be grown only after both (1) removal of polymer
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Potential Development in Dye-Sensitized Solar Cells for Renewable Energy
(HMTA, PEI) by heating the ZnO NW and (2) coating with seed NPs on the backbone of ZnO NW
surface. Figure 8 b,c, depicts the seed effect and figure 8d,e, shows the polymer removal effect.
HMTA and PEI hinder only lateral growth but allow axial growth of the ZnO NWs in the solution,
thus yielding high aspect ratio NWs. [13] The polymer can be removed by heating the ZnO NWs at
350 °C for 10 min. Branched growth (BG) that had been previously suppressed by HMTA and PEI
during regular LG could be induced once the polymer was removed from the backbone NW, as
shown in Figure 8b. In addition to random and sparsely branched NW growth on the side walls, the
diameter of first generation backbone ZnO NW also increased (~1 µm) due to lateral growth after
the removal of the HMTA and PEI polymer layer. After the polymer removal, seed NP coating on
the first generation backbone NWs could induce growth of densely packed higher order generation
BG while suppressing the diameter increase of the first generation backbone NW (Figure 8c).
ZnO seed NP coating on the backbone ZnO NW without polymer removal could grow just sparsely
branched ZnO NW (Figure 8d). In contrast, high quality hierarchical branched ZnO NW forest
could be achieved by coating the backbone ZnO NW with ZnO seed NP after polymer removal
(Figure 2e). ZnO NW growth from the seed NP on HMTA and PEI polymer may be less favorable
than on the ZnO NW surface without polymer. This signifies that both polymer removal and seed
layer addition are important for realizing high density hierarchical branched ZnO NW forest
growth.
Fig 8. SEM pictures of ZnO NWs. (a) Length growth (1,2,3 times growth). Seed effect: first
generation branched growth (b) without seeds and (c) with seeds after polymer removal. Polymer
removal effect: first generation branched growth (d) without polymer removal and (e) with polymer
removal after seed NP deposition. Polymers on ZnO NW are removed after 350 °C heating for 10
min. [16] ZnO NWs were rinsed with DI water and baked in air at 350 °C for 30 min to remove any
residual organics and optimize solar cell performance.
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81
Nanowire arrays were first sensitized in a solution (0.5mM cis-bis(isothiocyanato)bis(2,2’bipyridyl-4,4’-dicarboxylato)-ruthenium(II)bis-tetrabutylammonium (N719 dye) in dry ethanol for
five hours and then sandwiched together and bonded with thermally platinized FTO counter
electrodes separated by 40 µm thick hot-melt foil spacers. The internal space of the cell was filled
with a liquid electrolyte (0.1 M LiI, 50mM I2, 0.5M 1,2-dimethyl-3-propylimidazolium iodide,
0.03M I2 and 0.5M tert-butylpyridine in acetonitrile) by capillary action. The current density versus
voltage (J-V) characteristics of the cells were measured under AM1.5G 100 mW/cm2 illumination
from a solar simulator immediately after cell assembly.
Fig 9. (a) ZnO Nanoforest DSSC, (b) J-V curve of Dye-Sensitized solar cells with “Nanoforest”
ZnO NW.
Table 2. Summarized characteristic of Nanoforest DSSC in figure 8b
Symbol
Backbone
NW length
µm
LG1
7
LG2
13
LG3
BG1
Branching
times
Efficiency
(%)
Jsc
(mA/cm2)
Voc
(V)
FF
0.45
1.52
0.636
0.480
0.71
2.37
0.640
0.486
18
0.85
2.87
0.645
0.484
7
2.22
7.43
0.681
0.522
2.51
8.44
0.683
0.531
2.63
8.78
0.680
0.530
0
Configuration
1
BG2
13
BG3
2
Figure 8b shows the J-V characteristics for solar cells with both BG and LG ZnO nanostructures
and the DSSC characteristics are summarized in Table 2. J-V curves for upstanding ZnO NW with
various lengths (LG1, 7 µm; LG2, 13 µm; LG3, 16 µm)are presented in Figure 8b. The short circuit
current density (Jsc) and the overall light conversion efficiency increased as the length of the LG
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Potential Development in Dye-Sensitized Solar Cells for Renewable Energy
ZnO NW increased (LG1, 0.45%; LG2, 0.7%; LG3,0.85%) due to effective surface area increase.
One-time branched growth on LG1 (7 µm) and LG2 (13 µm) yielded BG1 (from LG1) and BG2
(from LG2) and two-time branched growth produced BG3 (from LG2). By implementing additional
NW generations, the short circuit current density (Jsc ) and overall light conversion efficiency could
be significantly increased for high density hierarchical branched ZnO NW nanoforest. The
respective enhancement was in the range of by 350-500% for the same backbone NW length (LG1,
0.45% to BG2, 2.22%; LG2, 0.71% to BG2, 2.51%). The efficiency increase can be explained by
considering a combination of several effects. First, the enhanced photon absorption associated with
the augmented surface area results in increased dye loading and correspondingly to large Jsc
increase. LG can grow only upstanding NWs by adding length to the first generation backbone NW.
However, BG can grow multibranched NWs from just a single first generation backbone NW, thus
surface area can be increased dramatically. The measured NW number density for upstanding LG
NW (109/cm2) could be increased by 1-2 orders of magnitude (1010-11/cm2 ) by branched NW
growth. Second, a dense network of crystalline ZnO NWs can increase the electron diffusion length
and electron collection because the NW morphology provides more direct conduction paths for
electron transport from the point of injection to the collection electrode. Third, randomly branched
NWs promote enhanced light-harvesting (light-dye interaction) without sacrificing effective
electron transport. Furthermore, branched NWs can increase light- harvesting efficiency by
scattering enhancement and trapping [16].
Dynamics Charge Transport mechanism and Recombination principles affecting Design
features for a ZnO Nanoforest based Dye sensitized solar cells
The power conversion efficiency η of a solar cell is given by η = ( FF x |Jsc| x Voc)/Pin, where FF
is the fill factor, |Jsc| absolute value of the current density at short circuit, Voc photovoltage at open
circuit, and Pin is incident light power density. Our ultimate aim is to increase the overall power
conversion efficiency and increase the lifetime of the DSSC.
Improving range of spectral absorbance by modifying the dye. The semiconductor morphology
must
also provide both high surface area to maximize dye absorption and efficient electron transport that
delivers the electron to the collection electrode without recombination. High surface area required
for dye adsorption. [14]
The nanowire-based cells absorb only 9% of the incident light at the dye’s absorbance maximum
and have dye loadings of 9x10-9 mol/cm2 , about an order of magnitude less than the nanoparticle
films. Growing longer wires with smaller diameter (i.e. increasing aspect ratio) and increased
nucleation density will increase surface area and thus light harvesting, which will substantially
improve the current densities and efficiencies of cells made from nanowires. The low fill factor
most likely results from recombination between photoexcited electrons in the nanowires and triiodide ions in the electrolyte.[16]
If one can avoid simultaneously accelerating reverse electron transfer from the photoelectrode to the
dye or regenerator, speeding up electron transport provides opportunities for enhancing the
performance of DSSCs in several ways. The useful thickness of a photoanode is determined by its
effective electron diffusion length,
Materials Science Forum Vol. 771
Ln = (Dnτn)1/2
83
(1)
Where Dn is the effective diffusion coefficient for the electron, within the photoelectrode and τn is
the survival time of the electron with respect to recombination with the oxidized dye or regenerator.
DSSCs employing solid-state hole-conductors in particular suffer from an Ln less than the thickness
required to absorb the majority of incident light. The result is low Light Harvesting Efficiency
(LHE) or poor charge collection efficiency (ηc), both of which limit the Incident Photon-to Current
Efficiency (IPCE) according to
IPCE = LHE * φinj * ηc
(2)
Where φinj is the efficiency of electron injection from the excited dye into the semiconductor
framework. Faster transport, therefore, can increase Ln and thus increase LHE and photocurrent.
In the most efficient liquid electrolyte DSSCs, Ln is already greater (at most wavelengths) than the
thickness required to collect most of the incident photons. Here, the result of faster transport (larger
Dn) would be to make the cells tolerant to faster recombination dynamics (shorter τn; recall that the
electron collection efficiency is a measure of the competition between transport and recombination).
In principle, faster redox shuttles could then be employed. As dye regeneration by inherently faster
shuttles need not be accelerated by large overpotentials, this change could directly address the
problem of low photovoltage that has plagued DSSCs since their inception.
DSSCs harness high-surface-area sintered-nanoparticlenetworks loaded with organic dyes to
achieve good light harvesting efficiency and excellent exciton dissociation. While DSSCs generally
have excellent absorbed photon to current efficiencies, the long and tortuous path through sintered
particle networks in state-of the-art DSSCs results In high electron concentrations under standard
solar illumination, increasing the rate of charge recombination. Relatively slow diffusion of
electrons through the network requires that efficient cells use a slow redox couple (I-/ I3-) to avoid
recombination with electrons in the semiconductor. Thus, a significant overpotential of the redox
couple is required to accelerate the regeneration of the ground state of the dye, ultimately resulting
in low photovoltages. Average electron transport times, τd and effective diffusion coefficient are
inversely related:
τd = constant * L2/ Dn
(3)
where L is the thickness of the photoelectrode, and the magnitude of the constant (typically between
0.25 and ~0.4) depends on factors such as the uniformity (or otherwise) of carrier generation
through the thickness of the electrode.
Intensity modulated photocurrent and photovoltage spectroscopy probes the rates of recombination
and charge transfer in operating DSSCs. Figure 10 shows real and imaginary components of the
photomodulated current for a nanoparticle electrode and a nanorod array electrode as a function of
modulation frequency, f = /2. The average transport time may be estimated from the minimum
angular frequency in the imaginary plot:
τd = -1d,min
(4)
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Potential Development in Dye-Sensitized Solar Cells for Renewable Energy
The second trace shows the results for a nanorod array electrode. The striking feature is a shift of
two orders of magnitude towards higher characteristic lag frequency, indicating the transport
dynamics are two orders of magnitude faster for electrons within the nanorod array electrode.
Similar analysis of the photomodulated voltage under open circuit conditions yields average charge
lifetimes, τn.
Fig. 10 Real (black) and imaginary (grey) components of the photomodulated current for a 4.5 µm
thick nanorod electrode as a function of modulation frequency. Open circles show the current
corrected for RC attenuation (eqn (4)), where R and C take typical literature values, 15Ω and 30µF
cm-2.
Fig 11. Electron lifetime (triangles) and average transit time (circles) over a range of illumination
intensities. Arrows highlight the difference between τn and τd for nanoparticle (filled symbol) and
nanorod (open symbol) devices.[18]
The charge dynamics as function of light intensity are summarized in Figure. 11. The measured
ratio of recombination time to transport time for the nanorod electrode varies from 280 to 1850. If
the results are extrapolated to the number of photons incident in the absorbing range of the dye
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under AM 1.5 illumination, the ratio becomes ~64 or about 18 times that seen with the nanoparticle
electrode geometry. Thus, nanorod array cells should be capable of sustaining efficient charge
collection over much greater thicknesses than nanoparticle-based cells.
In summary, photomodulation experiments with dye-sensitized ZnO solar cells show that electron
transport is tens to hundreds of times faster in nanorod array electrodes than in nanocrystalline
particulate electrodes. Recombination, on the other hand, is slightly slower. Taken together, these
findings support the contention that nanorod geometries are likely to provide very substantial
dynamical advantages in operating dye-sensitized solar cells.
Limitation of ZnO based DSSC
It has been demonstrated that ZnO has approximately the same band gap and band position as TiO2.
Furthermore, ZnO possesses a high electron mobility, low combination rate, and good
crystallization into an abundance of nanostructures. Many efforts have been made on ZnO-based
DSCs with either nanocrystalline films or films consisting of various nanostructures that are highly
efficient in electron transport and/or photon capture. However, so far, the results obtained for dyesensitized ZnO solar cells have still shown relatively low overall conversion efficiencies when
compared with TiO2-based systems. The limited performance in ZnO-based DSCs may be
explained by the instability of ZnO in acidic dye (i.e., protons from the dyes cause the dissolution of
Zn atoms at ZnO surface, resulting in the formation of excessive Zn2+/dye agglomerates) and the
slow electron-injection kinetics from dye to ZnO.
Instability of ZnO in Acidic Dyes
Commercially available dyes such as N3, N719, or ‘‘black’’ dye derived from ruthenium–
polypyridine complexes have been widely used as a sensitizer for TiO2-based DSCs. The molecules
of these dyes have carboxyl groups that connect with TiO2. However, direct use of these dyes with
ZnO is difficult because the surface structure of the ZnO crystals may be destroyed when they are
soaked in an acidic dye solution containing a Ru complex for an extended period of time. By
preparing N3-adsorbed ZnO-nanoparticle films and comparing the transient absorption and
fluorescence spectra of those films immersed in dye solution for different durations of time,
Horiuchi et al. studied the formation of the Zn2+/dye-complex layer on ZnO nanoparticle
surface.[19]They found that such a complex layer could always be observed if the immersion time
was longer than 3 h. Below an immersion time of 10 min, no complex was formed. Westermark et
al. [20] and Chou et al. [21] verified that a short sensitization time may be favorable to avoid the
formation of Zn2+/dye-complex. However, the formation of the Zn2+/dye-complex seems to be very
sensitive to the experimental conditions, resulting in a considerable difference in the optimization of
the sensitization time. The Zn2+/dye-complex can agglomerate to form a thick covering layer
instead of a monolayer, and is therefore inactive for electron injection. This means that the
sensitization time of ZnO in acidic dyes is limited by its stability, resulting in insufficient dye
adsorption and electron injection and thus poor performance of DSCs.
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Potential Development in Dye-Sensitized Solar Cells for Renewable Energy
Low Electron-Injection Efficiency
Electron-injection efficiency describes the probability of photogenerated electrons transferring from
the dye molecules to semiconductor. In term of DSSC performance, the electron injection
efficiency, φinj , and the incident photon-to-current conversion efficiency (IPCE) are related by eqn
(2). The electron-injection efficiency is, above all, determined by the electronic coupling between
the dye and semiconductor, and it is also judged by the relative energy levels of the dye and
semiconductor, the residential lifetime of photogenerated electrons in the dye molecules, and the
density of electron-accepting states in the semiconductor. In particular, for ZnO sensitized with Rucomplex acidic dyes, it has been demonstrated that the injection process may also be influenced by
the formation of Zn2+/dye-complex agglomerates, resulting in a low electron-injection efficiency.
For either ZnO or TiO2 , the injection of electrons from Ru-based dyes to a semiconductor shows
similar kinetics that include a fast component of less than 100 fs and slower components on a
picosecond time scale. [22–27] Such biphasic kinetics are caused by competition processes between
the ultrafast electron injection and molecular relaxation. However, for ZnO with Ru-based dyes, the
electron injection is dominated by slow components, whereas for TiO2 [28] it is dominated by fast
components, leading to a difference of more than 100 times in the injection rate constant. Based on
a two-state injection model [24] and using ultrafast infrared transient absorption spectroscopy, a
quantitative study has been given to describe the electron-injection dynamics from Ru–polypyridyl
complexes to nanocrystalline ZnO or TiO2 film, revealing that the injection time scales from
unthermalized and relaxed excited states to ZnO are estimated to be 1.5 and 150 ps, respectively,
both of which are one order of magnitude slower than those of TiO2.[22] The different injection
dynamics most likely originate from the conduction-band structures of the semiconductors. That is,
the ZnO conduction bands are largely derived from the empty s and p orbitals of Zn2+, while the
TiO2 conduction band is comprised primarily of empty 3d orbitals from Ti4+. The difference in band
structure results in a different density of states and, possibly, different electronic coupling strengths
with the adsorbate. Enright et al. estimated that the density of conduction band states near the band
edge is as much as two orders of magnitude higher in TiO2 by reason of the larger effective mass of
the conduction-band electron in TiO2 (5-10me) than that in ZnO (≈0.3me). [29] The slower electroninjection dynamics for ZnO with Ru-based dyes is also proposed to be a result of the electron
injection proceeding stepwise via intermediate states, as described by [30]
N3 + ZnO → N3* + ZnO →
intermdediate →
N3+ + e-CB
(5)
Where N3* and N3+ represent the excited and oxidized states, respectively, and e-CB indicates a
conducting electron in ZnO. The origin of the intermediate states has been ascribed to the
interaction between the photoexcited dye and localized surface states on the ZnO surface. The
electron transfer is considered to occur slowly due to the existence of intermediate states and
therefore results in a low injection efficiency.
Conclusion
ZnO is believed to be a superior alternative material to replace the existing TiO 2 photoanodic
materials used in DSSC and has been intensively explored in the past decade due to its wide band
gap and similar energy levels to TiO2 . More important, its much higher carrier mobility is favorable
Materials Science Forum Vol. 771
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for the collection of photoinduced electrons and thus reduces the recombination of electrons with
tri-iodide. Although the formation of Zn2+/dye complex is inevitable due to the dissolution of
surface Zn atoms by the protons released from the dye molecules in an ethanolic solution, selection
of other alternative dye molecules will definitely help to boost the conversion efficiency to much
higher level. Therefore, the recent development on the synthesis of metal-free dye molecules will
lead the DSSC device fabrication to the new height as for the cost effectiveness and simple
technique are concern.
References
[1]
K. Hara, H. Arakawa, Dye-Sensitized Solar Cells, in: A. Luque, S. Hegedus (Eds.),
Handbook of Photovoltaic Science and Engineering, John Wiley & Sons ltd., New York,
(2003) 663-700.
[2]
Md. K. Nazeeruddin, A.Kay, I.Rodicio, R. Humphry-Baker, E. Müller, P. Liska, N.
Vlachopoulos, M. Grätzel, Conversion of Light to Electricity by cis-X2Bis (2,2’ bipyridl4,4’ dicarboxylate) Ruthenium (II) Charge-Transfer Sensitzers (X= Cl- , Br- , I- , CN- and
SCN-) on Nanocrystalline TiO2 Electrodes, J. Am. Chem. Soc. 115 (1993) 6382-6390.
[3]
B. O’Regan, M. Grätzel, A low-cost, high-efficiency solar cell based on dye-sensitized
colloidal TiO2 films, Nature 353 (1991) 737-740.
[4]
Md. K. Nazeeruddin, P. Pèchy, M. Grätzel, Efficient panchromatic sensitization of
nanocrystalline TiO2 films by a black dye based on atrithiocyanato–ruthenium complex,
Chem. Commun. (1997) 1705-1706.
[5]
Md. K. Nazeeruddin, P. Pèchy, T. Renouard, Shaikh M. Zakeeruddin, R. Humphry-Baker,
P. Comte, P. Liska, Le Cevey, E. Costa, V. Shklover, L. Spiccia, G.B. Deacon,
C.A.Bignozzi, M. Grätzel, Engineering of Efficient Panchromatic Sensitizers for
Nanocrystalline TiO2-Based Solar Cells, J. Am. Chem. Soc. 123 (2001) 1613-1624.
[6] l
earning from nature: dye sensitized solar cells: By Kathy Li Dessau 2nd November 2010,
Information on: http://www.solarnovus.com
[7]
A. Janotti, C.G. Van de Walle, Fundamentals of Zinc Oxide as a Semi-Conductor, Rep.
prog. Phys. 72 (2009) 1-29
[8]
K.D. Benkstein, N. Kopidakis, J.V. Lagemaat and A.J. Frank, Influence of the percolation
network geometry on electron transport in dye-sensitized titanium dioxide solar cells, J.
Phys. Chem. B, 107 (31) (2003) 7759–7767.
[9]
M. Grätzel, Perspectives for dye-sensitized nanocrystalline solar cells, Prog. Photovolt.
Res. Appl., 8 (1) (2000) 27-38.
[10]
A. P. Uthirakumar, Fabrication of ZnO Based Dye Sensitized Solar Cells, in:
L.A.Kosyachenko (Eds.), Solar Cells-Dye-Sensitized Devices, InTech, Europe ( 2011) 435456.
88
Potential Development in Dye-Sensitized Solar Cells for Renewable Energy
[11]
E. A. Meulenkamp, Synthesis and growth of ZnO nanoparticles, J. Phys. Chem. B., 102
(1998) 5566-5572.
[12]
B. Postels, A. Kasprzak, T. Buergel, A. Bakin, E. Schlenker, H.H. Wehmann, A. Waag,
Dye-sSensitized solar cells on the basis of ZnO nanorods, J. Kor. Phys. Soc., 53 (2008) 115118.
[13]
M. Law, L.E. Greene, J.C. Johnson, R. Saykally, P. Yang, Nanowire dye-sensitized solar
cells, Nat. Mat., 4 (2005) 455-459.
[14]
Q. Zhang, C.S. Dandeneau, X. Zhou, G. Cao, ZnO Nanostructures for dye-sensitized solar
cells, Adv. Mater., 21 (2009) 4087-4108.
[15]
C. Y. Jiang, X. W. Sun, G.Q. Lo, D. L. Kwong, Improved Dye-Sensitized Solar Cells with a
ZnO-nanoflower Photoanode, Appl. Phys. Lett., 90 (2007) 263501.
[16]
S.H. Ko, D. Lee, H.W. Kang, K.H. Nam, J.Y. Yeo, S.J. Hong, C.P. Grigoropoulos, H.J.
Sung, Nanoforest of Hydrothermally Grown Hierarchical ZnO Nanowires for a High
Efficiency Dye-Sensitized Solar Cell, Nano Lett., 11 (2011) 666-671.
[17]
J. B. Baxter, E. S. Aydil, Nanowire-based dye-sensitized solar cells, Appl. Phys. Lett., 86
(2005) 053114.
[18]
A. B. F. Martinson, J. E. McGarrah, Md. O. K. Parpia, J. T. Hupp, Dynamics of charge
transport and recombination in ZnO nanorod array Dye-Sensitized Solar Cells, Phys. Chem.
Chem. Phys., 8 (2006) 4655-4659.
[19]
H. Horiuchi, R. Katoh, K. Hara, M. Yanagida, S. Murata, H. Arakawa, M. Tachiya, Electron
Injection Efficiency from Excited N3 into Nanocrystalline ZnO Films: Effect of (N3-Zn2+)
Aggregate Formation, J. Phys. Chem. B, 107 (2003) 2570-2574.
[20]
K. Westermark, H. Rensmo, H. Siegbahn, K. Keis, A. Hagfeldt, L. Ojamae, P. Persson, PES
Studies of Ru(dcbpyH2)2(NCS)2 Adsorption on Nanostructured ZnO for Solar Cell
Applications, J. Phys. Chem. B, 106 (2002) 10102-10107.
[21]
T. P. Chou, Q. F. Zhang, G. Z. Cao, Effect of Dye Loading Conditions on the Energy
Conversion Efficiency of ZnO and TiO2 Dye-Sensitized Solar Cells, J. Phys. Chem. C, 111
(2007) 18804-18811 .
[22]
N. A. Anderson, X. Ai, T. Q. Lian, Electron Injection Dynamics from Ru Polypyridyl
complexes to ZnO Nanocrystalline Thin Films, J. Phys. Chem. B, 107 (2003) 14414-14421.
[23]
J. B. Asbury, E. Hao, Y. Q. Wang, H. N. Ghosh, T. Q. Lian, Ultrafast Electron Transfer
Dynamics from Molecular Adsorbates to Semiconductor Nanocrystalline Thin Films, J.
Phys. Chem. B, 105 (2001) 4545-4557.
[24]
J. B. Asbury, N. A. Anderson, E. C. Hao, X. Ai, T. Q. Lian, Parameters Affecting Electron
Injection Dynamics from Ruthenium Dyes to Titanium Dioxide Nanocrystalline Thin Film,
J. Phys. Chem. B, 107 (2003) 7376-7386.
Materials Science Forum Vol. 771
89
[25]
D. Kuciauskas, J. E. Monat, R. Villahermosa, H. B. Gray, N. S. Lewis, J. K. McCusker,
Transient Absorption Spectroscopy of Ruthenium and Osmium Polypyridyl complexed
adsorbed onto Nanocrystalline TiO2 photoelectrodes, J. Phys. Chem. B, 106 (2002) 93479358.
[26]
G. Benko, J. Kallioinen, J. E. I. Korppi-Tommola, A. P. Yartsev, V. Sundstrom,
Photoinduced Ultrafast Dye-to-Semiconductor Electron Injection from Nonthermalized and
Thermalized Donor States, J. Am. Chem. Soc., 124 (2002) 489-493.
[27]
J. B. Asbury, Y. Q. Wang, T. Q. Lian, Multiple-Exponential Electron Injection in
Ru(dcbpy)2(SCN)2 Sensitized ZnO Nanocrystalline thin films, J. Phys. Chem. B, 103 (1999)
6643-6647.
[28]
N. A. Anderson, T. Lian, Ultrafast Electron Injection from Metal Polypyridyl Complexes to
Metal-Oxide Nanocrystalline thin films, Coord. Chem. Rev., 248 (2004) 1231-1246.
[29]
B. Enright, D. Fitzmaurice, Spectroscopic Determination of Electron and Hole Effective
Masses in a Nanocrystalline Semiconductor Film, J. Phys. Chem., 100 (1996) 1027-1035.
[30]
A. Furube, R. Katoh, K. Hara, S. Murata, H. Arakawa, M. Tachiya, Ultrafast Stepwise
Electron Injection from Photoexcited Ru-Complex into Nanocrystalline ZnO Film via
Intermediates at the surface, J. Phys. Chem. B, 107 (2003) 4162-4166.
Potential Development in Dye-Sensitized Solar Cells for Renewable Energy
10.4028/www.scientific.net/MSF.771
ZnO Nanoforest Based New Generation Dye Sensitized Solar Cells
10.4028/www.scientific.net/MSF.771.71
DOI References
[7] A. Janotti, C.G. Van de Walle, Fundamentals of Zinc Oxide as a Semi-Conductor, Rep. prog. Phys. 72
(2009) 1-29.
10.1088/0034-4885/72/12/126501
[3] B. O'Regan, M. Grätzel, A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2
films, Nature 353 (1991) 737-740.
10.1038/353737a0