Ga-doped ZnO transparent electrodes with TiO2 blocking layer/nanoparticles for dye-sensitized solar cells.

5 MB·English

by  Kim, Ji-Hong

Kimetal.NanoscaleResearchLetters2012,7:11
http://www.nanoscalereslett.com/content/7/1/11
NANO EXPRESS Open Access
Ga-doped ZnO transparent electrodes with TiO
2
blocking layer/nanoparticles for dye-sensitized
solar cells
Ji-Hong Kim, Kyung-Ju Lee, Ji-Hyung Roh, Sang-Woo Song, Jae-Ho Park, In-Hyung Yer and Byung-Moo Moon*
Abstract
Ga-doped ZnO [GZO] thin films were employed for the transparent electrodes in dye-sensitized solar cells [DSSCs].
The electrical property of the deposited GZO films was as good as that of commercially used fluorine-doped tin
oxide [FTO]. In order to protect the GZO and enhance the photovoltaic properties, a TiO blocking layer was
2
deposited on the GZO surface. Then, TiO nanoparticles were coated on the blocking layer, and dye was attached
2
for the fabrication of DSSCs. The fabricated DSSCs with the GZO/TiO glasses showed an enhanced conversion
2
efficiency of 4.02% compared to the devices with the normal GZO glasses (3.36%). Furthermore, they showed
better characteristics even than those using the FTO glasses, which can be attributed to the reduced charge
recombination and series resistance.
Introduction ofthe ZnO-based materialsmay bedestroyedwhenthey
Dye-sensitized solar cells [DSSCs] have been recognized are immersed in the acidic dye solution containing a Ru
as an alternative to the conventional p-n junction solar complexforalongtime.Besides,theelectricalconductiv-
cells because of their simple fabrication process, low ityofGZOfilmscandeteriorateafterthermalannealingat
production cost, and transparency. A typical DSSC con- high temperature which is required to form the TiO
2
sists of a transparent conductive oxide [TCO] electrode, semiconductornanoparticlelayer[6].
a dye-sensitized oxide semiconductor nanoparticle layer, In this paper, we suggest the use of GZO transparent
a liquid redox electrolyte, and a Pt counter electrode electrodes with a TiO blocking layer for DSSCs. The
2
[1,2]. Generally, fluorine-doped tin oxide [FTO] is com- TiO blocking layer can protect the GZO electrodes
2
monly used for DSSCs as TCO due to its good thermal from the acidic dye solution and the oxidation at high
stability. However, FTO films have some drawbacks temperature. The use of a thin TiO blocking layer can
2
including high cost, insufficient conductivity, and low also reduce the recombination of electrons at the elec-
optical transmittance. Therefore, new TCO materials are trode/electrolyte interface.
required to replace FTO glasses [3].
ZnO-based materials have emerged as a promising Experimental details
material for transparent electrodes in solar cell applica- GZO thin films were deposited on glass substrates by
tions.SinceundopedZnOshowshighresistivityowingto using a pulsed laser deposition [PLD] system for trans-
low carrier concentration, group-III elements are doped parent electrodes. A ZnO ceramic target containing 2
into ZnO. Among them, Ga-doped ZnO [GZO] has sev- wt.% Ga O was ablated using a Q-switched Nd:YAG
2 3
eraladvantagessuchashigherresistancetooxidationand laser with a wavelength of 355 nm (Surelite III; Conti-
less lattice deformation compared to the other materials nuum, Santa Clara, CA, USA). During the deposition,
[4,5].Nevertheless,littleefforthasbeenspentonattempts oxygen partial pressure and substrate temperature were
touseGZOasTCOforDSSCssincethesurfacestructure kept at the optimal conditions, which were 20 mTorr
and 400°C, respectively. The electrical properties of the
deposited GZO thin films were investigated by the van
*Correspondence:[email protected] der Pauw Hall measurement system. After the deposi-
DepartmentofElectricalEngineering,KoreaUniversity,5-1Anam-dong,
tion of the GZO films, a thin TiO layer was also
Seongbuk-gu,Seoul,136-713,SouthKorea 2
©2012Kimetal;licenseeSpringer.ThisisanOpenAccessarticledistributedunderthetermsoftheCreativeCommonsAttribution
License(http://creativecommons.org/licenses/by/2.0),whichpermitsunrestricteduse,distribution,andreproductioninanymedium,
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Kimetal.NanoscaleResearchLetters2012,7:11 Page2of4
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deposited on the GZO surface for the blocking layer by (0002) appears at 2θ = 33.64°, which means that the c-
a PLD method. A metal Ti target was ablated in oxygen axis oriented GZO films were grown on glass substrates.
atmosphere at a substrate temperature of 400°C. The GZO thin films are well known to have a hexagonal
structural and optical properties were examined by X- wurtzite structure with a preferred growth orientation
ray diffraction [XRD] and UV-Vis spectroscopy. along the c-axis due to their lowest surface free energy.
Using the fabricated GZO glasses, DSSCs were manu- The GZO films on glass showed a very low resistivity of
factured. Anatase TiO slurry (colloid) was prepared 5.95 × 10-4 Ω cm (sheet resistance of 6.53 Ω/sq), which
2
with a conventional method [7] and coated for a TiO is comparable to that of the commercially used FTO
2
nanoparticle layer. The coating was carried out on the glasses. The carrier concentration and Hall mobility of
TiO blocking layer with an active area of 0.36 cm2 by the GZO films were 6.82 × 1020/cm3 and 15.37 cm2/Vs,
2
doctor blading. For comparison, the GZO without the respectively. The high carrier concentration of the GZO
blocking layer and commercially used FTO glasses were films can be attributed to the substitution of Ga3+ for
also employed to fabricate the DSSCs. After being sin- Zn2+ caused by the supply of sufficient thermal energy
tered at 510°C, they were immersed into the solution of [8]. Furthermore, the deposition at the optimal substrate
N719 ruthenium-based dye. Pt-coated counter electro- temperature leads to high mobility with the improve-
des were prepared by screen printing on the GZO- ment in crystallinity. From these results, it is concluded
deposited glasses and heated at 400°C. The surface that the deposited GZO films can be used for transpar-
morphologies of the TiO nanoparticles on the GZO ent electrodes. However, as mentioned above, a blocking
2
and GZO/TiO glasses were observed by scanning elec- layer should be needed in DSSC applications because of
2
tron microscopy [SEM]. Then, the DSSC cells were susceptibility to acid and oxidation at high temperature.
assembled by sealing the TiO electrode and Pt elec- Figure 2 shows the optical transmittance spectra for
2
trode together using a hot melt film (Bynel; DuPont, the GZO glasses with the TiO blocking layer. The aver-
2
Wilmington, DE, USA), and an electrolyte was injected age transmittance is approximately 80% in the visible
into the space between the electrodes through a pre- region, which implies that the fabricated GZO glasses
drilled hole in the counter electrode. Finally, the hole are highly transparent enough to be applied to the
was sealed with Bynel and a cover glass. The photovol- DSSCs even after the deposition of the TiO blocking
2
taic performance of the DSSCs was evaluated using a layer.
solar simulator at one sun (AM1.5, 100 mWcm-2) con- Figures 3a and 3b show the SEM images of the TiO
2
dition. In order to investigate the electron transport nanoparticles coated on the GZO glasses with the TiO
2
properties of the DSSCs, electrochemical impedance blocking layer and normal GZO glasses, respectively. It
spectroscopy analysis was performed. can be seen that the porous TiO nanoparticles adhere
2
uniformly on both glasses. There is no large difference
Results and discussion between them, but the morphology of the TiO nano-
2
Figure 1 shows the XRD θ-2θ spectra of the GZO thin particles on the TiO blocking layer is more ordered
2
films on glass substrates. The diffraction peak of GZO and spherical.
Figure1XRDθ-2θpatternoftheGZOthinfilmsdepositedon Figure 2 Optical transmittance spectra for the GZO glasses
glasssubstrates. withtheTiO blockinglayer.
2
Kimetal.NanoscaleResearchLetters2012,7:11 Page3of4
http://www.nanoscalereslett.com/content/7/1/11
Table 1Photovoltaic parametersofthe fabricated DSSCs
Sample V (V) J (mA/cm2) FF h(%)
oc sc
GZO/TiO 0.652 11.05 0.558 4.021
2
GZO 0.673 10.64 0.47 3.363
FTO 0.656 11.04 0.531 3.844
summarized in Table 1. It should be noted that the per-
formance of the DSSCs with the normal GZO glasses is
relatively poor (3.36%) compared to that with the FTO
though the GZO and FTO have similar electrical resis-
tivity. However, when the TiO blocking layer is
2
employed, the fabricated DSSCs show an improvement
in the short-circuit current [I ] and fill factor [FF], and
sc
as a result, a conversion efficiency of 4.02% is obtained,
which is 19.6% higher than that of the DSSCs with the
normal GZO glasses. These results prove that the TiO
2
blocking layer plays a role in protecting the GZO films
as was expected. Furthermore, the DSSCs with the
GZO/TiO glasses show slightly better characteristics
2
than those with the commercially used FTO glasses,
which is ascribed to the fact that the TiO blocking
2
layer can prohibit the recombination of injected elec-
trons in the GZO with the electrolyte effectively. In par-
ticular, the improvement in FF is clearly found, which is
due to the improved electrical contact between the
GZO and TiO nanoparticles [9].
2
Figure 5 presents the Nyquist plots of the electroche-
mical impedance spectra of the fabricated DSSCs with
and without the TiO blocking layer. It is known that
Figure 3 SEM images of the TiO nanoparticles. The TiO 2
nanoparticleswerecoatedonthe(a)G2ZOglasseswiththeTiO 2 the semicircles in the frequency regions of 103 to 105, 1
2
blockinglayerand(b)normalGZOglasses. to 103, and 0.1 to 1 Hz are associated with the charge
transport at the TiO /TCO or Pt/electrolyte interface,
2
TiO /dye/electrolyte interface, and Nernstian diffusion
Figure 4 shows the photocurrent density-voltage [J-V] 2
in the electrolyte, respectively [10]. The first circle of
characteristics of the fabricated DSSCs using the GZO/
the DSSCs with the GZO/TiO glasses is smaller than
TiO , GZO, and FTO glasses measured under one sun 2
2 that with the GZO glasses, which indicates that the
condition. The estimated photovoltaic parameters are
Figure4J-VcharacteristicsofthefabricatedDSSCsusingthe Figure5Electrochemicalimpedancespectraofthefabricated
GZO/TiO2,GZO,andFTOglasses. DSSCswithandwithouttheTiO2blockinglayer.
Kimetal.NanoscaleResearchLetters2012,7:11 Page4of4
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charge transport at the TiO nanoparticles/GZO is 6. ZhangQF,DandeneauCS,ZhouXY,CaoGZ:ZnOnanostructuresfordye-
2
easier with the TiO blocking layer. The larger shunt sensitizedsolarcells.AdvMater2009,21:4087-4108.
2 7. WuS,HanH,TaiQ,ZhangJ,ChenBL,XuS,ZhouC,YangY,HuH,ZhaoX-
resistance of the DSSCs with the GZO/TiO glasses cor-
2 Z:Improvementindye-sensitizedsolarcellswithaZnO-coatedTiO2
responding to the second circle is also seen. Therefore, electrodebyrfmagnetronsputtering.ApplPhysLett2008,92:122106-3.
the improvement in the efficiency with the TiO block- 8. ParkS-M,IkegamiT,EbiharaK:Effectsofsubstratetemperatureonthe
2 propertiesofGa-dopedZnObypulsedlaserdeposition.ThinSolidFilms
ing layer can be explained by the small series resistance
2006,513:90-94.
and large shunt resistance [11]. 9. XiaJ,MasakiN,JiangK,YanagidaS:DepositionofathinfilmofTiOxfrom
atitaniummetaltargetasnovelblockinglayersatconductingglass/
TiO interfacesinionicliquidmesoscopicTiO dye-sensitizedsolarcells.
Conclusions JPh2ysChemB2006,110:25222-25228. 2
GZO thin films were deposited on glass substrates for 10. LeeS,NohJH,HanHS,YimDK,KimDH,LeeJ-K,KimJY,JungHS,HongKS:
DSSC applications. The GZO films showed a low resis- Nb-dopedTiO2:anewcompactlayermaterialforTiO2dye-sensitized
solarcells.JPhysChemC2009,113:6878-6882.
tivity of 5.95 × 10-4 Ω cm. A TiO blocking layer was
2 11. XiaJB,MasakiN,JiangKJ,YanagidaS:Fabricationandcharacterizationof
inserted between the GZO and TiO nanoparticles to thinNb2O5blockinglayersforionicliquid-baseddye-sensitizedsolar
2
protect the GZO and enhance the photovoltaic proper- cells.JPhotochemPhotobiolAChem2007,188:120-127.
ties. The fabricated DSSCs with the GZO/TiO2 glasses doi:10.1186/1556-276X-7-11
showed a conversion efficiency of 4.02%, which was Citethisarticleas:Kimetal.:Ga-dopedZnOtransparentelectrodes
withTiO blockinglayer/nanoparticlesfordye-sensitizedsolarcells.
19.6% higher compared to 3.36% of the DSSCs using the Nanoscale2ResearchLetters20127:11.
GZO glasses. Furthermore, they showed slightly better
characteristics than those using the commercially used
FTO glasses. These results can be ascribed to the
reduced charge recombination and series resistance
caused by the influence of the TiO blocking layer.
2
Therefore, it can be concluded that the GZO films have
the ability to be employed to the high-efficiency DSSCs
as transparent electrodes.
Acknowledgements
ThisworkwassupportedbyKoreaUniversityGrantandtheNew&
RenewableEnergyoftheKoreaInstituteofEnergyTechnologyEvaluation
andPlanning(KETEP)grantfundedbytheKoreagovernmentMinistryof
KnowledgeEconomy(No.20103030010040-11-1000).
Authors’contributions
JHKcarriedouttheexperimentsandpreparedthemanuscriptinitially.KJL,
JHR,andSWSparticipatedinperformingtheexperimentsand
characterizations.JHPandIHYparticipatedinanalyzingtheexperimental
data.BMMconceivedofthestudyandparticipatedinitsdesignand
coordination.Allauthorsreadandapprovedthefinalmanuscript.
Competinginterests
Theauthorsdeclarethattheyhavenocompetinginterests.
Received:10September2011 Accepted:5January2012
Published:5January2012
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