Synthesis of Amorphous Er+ -Yb 3
+Co-doped TiOi and Its Application as a Scattering Layer for Dye-sensitized Solar C
이ls
Chi-Hwan Han,* Hak-Soo Lee, Kyung-won Lee, Sang-Do Han,and Ishwar Singh*
PhotovoltaicResearch Center, Korea InstituteofEnergyResearch, Yuseong, Daejeon 305-343,Korea
* E-mail:hanchi @kier.re.kr
tDepartment of Chemistry,MaharshiDayanandUniversity, Rohtak-124001, India ReceivedAugust 14, 2008, AcceptedOctober8, 2008
T1O2doped with Er3+ and Yb3+ was used for fabricating a scattering layer and a nano-crystalline TiO2electrode layer to be used in dye-sensitized solar cells. The material was prepared using a new sol-gel combustion hybrid method with acetylene black as fuel. The Er3+- Yb3+ co-doped titanium oxide powder synthesized at 700oC had embossed structure morphology with a size between 27 to 54 nm that agglomerated to produce micron size particles, as ob
served by the scanning electron micrographs. The XRD patterns showed that the Er3+- Yb자 co-doped titanium oxide had an amorphous structure, while using the same method without doping Er3+ or Yb3+, TiO2was obtained in the crystallite form with thea dominance of rutile phase. Fabricating a bilayer structure consisting of nano-crystalline TiO2 and the synthesized Er3+- Yb3+ co-doped titanium oxide showed better scattering property, with an overall in
crease of 15.6% in efficiency of the solar cell with respect to a single nano-crystalline TiO2 layer.
Key Words: Dye-sensitizedsolar cell, Scatteringlayer, Er3+-Yb3+ co-dopedtitanium oxide, Amorphous structure
Introduction
Since the breakthrough paper by O'Reganand Gratzel, dye-sensitizedsolarcellshave made goodprogress as anal ternative to theconventionalsilicon-based solarcells because oftheir low-cost productionandhighperformance.1 These cells efficientlyusethedye'scapacity to inject photo-excited electrons into the conduction band of wide bandgap nano
crystalline TiO2. However, there are severallimitationsthat restrict theefficiency of thedye-sensitized solarcells,such as (i) particle size of TiO2, (ii) inefficient light absorption by dyes in the nearinfra-red region, (iii) sub-optimumphoto voltage output, (iv) resistance losses, (v) light intensity de pendantrecombination,(vi) and non-ideal diode darkcurrents among others. Efforts have led to maximum power con
version under full sunlighttoabout 10% for a cell with an ac
tive area of0.25 cm2.2
Themost importantpart of thecellis TiO2 (nano-crystal line) film,which issizedependent and utilizesmorephotons due to alarger quantity of adsorbed dye.3 It has also been sug gested that a mixture of submicron-sized particles with nano-crystallineTiO2 or a bilayer structureconsisting of light scattering layerand nano-crystalline semi-transparent TiO2
layercan improvephotocurrentdensityinstead of usingonly nano-crystallineTiO2 film.4,5The idea ofusing thesubmicron particles of TiO2 g with nano-crystalline film comes fromthe factthatlight scatters strongly when colliding with thelarge particles,which increasesthepathlength of theincidentlight inthenano-crystalline TiO2 film.Hereagain, thescattering of lightis dependentonthe size of theTiO2 particles of thesup porting layer6 as wellason its refractiveindex.5 Some work
ers, therefore, had made effortstowards modifyingtheTiO2
electrodes.7-13 Kusama etal.'*1 have, however, stressedthe incorporation of additives in theelectrolytic solutions for in
creasingthe efficiency of thecells, which is another promis ing area of research.
Sofarasmodification of TiO2 electrode is concerned, effi
ciency of thedye-sensitizedsolar cellscould beenhanced us
ing WO3,7 electrophoreticdeposition and compression of tita nia particles,8 incorporating carbon powder,9,10 zirconia,11 ZnO,12 AkO3-coated SnOz/TiO? composite13 etc. Recently, up-conversion, in termsofabsoption of two or more lower en ergyphotons followed by emission of higherenergyphoton, of Er3+-Yb3+ co-dopedmaterial hasattracted much attention because of potential applicationinoptical devices.16 However,
no work has been done for making a TiO2 scattering layer modifying bydoping with rare earthelements. Wefound an improvementin efficiency over 15%ifthe TiO2 electrodeis fabricated with a bilayer consisting of nano-crystallineTiO2
and asynthesizedTiO2 materialdoped with Er3+ and Yb3+.
Experiment시
Synthesis of E产 and Yb3+ co-doped T1O2 powder Tita- nium(IV)isopropoxide,erbium(III)nitratepentahydrate,and ytterbium(III) nitrate pentahydrate were purchased from Aldrichand were used as the starting material. Acetylene black was purchasedfrom the Chevron Phillips Chemical Company. Fig. 1 showsthe flow scheme of the combustion processused for thesynthesisof Er3+ and Yb3+ co-dopedTiO2
powder.
Tothemixture of 2.0mLacetic acid and 16.0mL of etha nol, 3.4 mL oftitanium isopropoxide and 0.2 g of acetylene black were added to themixture of 2.0 mLacetic acid and 16.0 mL of ethanol. The solwas stirred for 30minwhile 1.0 mL waterwas added dropwise. Then 0.566 g of erbium(III)nitrate pentahydrate and 0.573 g of ytterbium(III) nitrate pentahy
drate wereadded and the solwas stirred for one morehour.
220 Bull. KoreanChem.Soc. 2009, Vol. 30, No. 1 Chi-Hwan Han et al.
Titanium isopropoxide
Ethanol
+ Er(NO3)3 + YB(NO3)3 + Acetic acid
t Acetylene black Mixed paste
卜一
D. I. WaterSol-gel process
卜--
Drying & heat treatmentTiO2 powder
Figure 1. Process for synthesis of Er3+-Yb3+ co-doped TiO2 amor
phous material.
This precursorwasdried at130oCin an oven for 12 h andthen heatedat700oC ina furnace. For comparison purposes, TiO2
powder was also synthesizedby the same method without addingtheerbium(III) and ytterbium(III)nitrates.
Thesynthesizedpowders wereexaminedby powder X-ray diffraction(XRD;Rigaku, Ultima Plus diffractometer D/Max 2000). The morphology and size of theparticleswere inves
tigated by using a field emission scanning electronmicro
scope (FE-SEM; Hitachi, S-4300). Thermal analyses were carried out using a simultaneous thermal analyzer (STA;
Scinco, STA S-1500)withaheatingrate of 5oC/min.
Preparationof T1O2 photoelectrode andRu(II)dyecoating.
The TiO2 paste (Ti-nanooxide D, Solaronix) was deposited onto conducting glass with a fluorine-doped stannic oxide layer (FTO, TEC 8/2.3 mm, 8 Q/D, Pilkington) using a screen-printingmethod. Theresultinglayer was calcined for 2 h at 470oC in a muffle furnace. This process was repeated three times until a thickness of 15
卩
m wasobtained. The area of the prepared porousTiO2 electrode was 25 mm2 (5mmx 5 mm). Dye absorption was carried out by dipping the TiO2electrode in a 4x10
-
4 Mt-butanol/acetonitrile (Merck, 1:1) solution of thestandard ruthenium dye:N719(Solaronix) for 48 h at25oC. The photoelectrode was then washed,dried,and immediately used to measure theperformance of a solar cell.Scattering layerscoating. Scatteringlayerswereprepared with the synthesizedEr3+ and Yb3+ co-dopedTiO2 powder, and for comparisonpurposes, withtheTiO2 powder only.Er3+ and Yb3+co-dopedTiO2 powder and TiO2 powder were dis
persedin ethanol usinganultrasonichorn. After sonification, the colloidal suspensions were concentrated using a rotary evaporator and transformed into a screen-printablepaste by the addition ofethylcellulose and terpineol. The prepared scattering layer pastes were deposited onto nano-crystalline TiO2 layerusing a screen-printing method. UV-Vis trans
mission spectra of the dye-adsorbedTiO2 films were meas ured with UV-Vis spectrophotometer (Lambda 2, Perkin Elmer).Since the transmittance of TiO2 film may bedifferent depending on whether the TiO2 film(scatteringlayer) is in contactwith electrolyteor air, the transmission was measured witha completely fabricated dye-sensitized solarcell,notjust TiO2 films on FTOglasses.
Photovoltaic characterization.Transparent counterelectro
des were preparedby placing a few dropsof 10 mM hydrogen hexachloroplatinate(IV)hydrate (99.9%, Aldrich) and 2-prop- anol solutionon FTOglass(TEC 8/2.3 mm, Pilkington) and calciningit at450oC for 2h.Theliquidelectrolytewascom posed of 0.70M 1,2-dimethyl-3 -propyl-imidazoliumiodide (Sanko),0.10 M Lil (Aldrich), 40 mMiodine (Aldrich), and 0.125 M 4-tert-butylpyridine (Aldrich) in acetonitrile. The photoelectrochemical properties of the prepared dye-sensi
tized solarcell were measuredby using a computer-controlled digital source meter (Potentiostat/Galvanostat Model273A, EG & G) anda solar simulator (AM 1.5, 100 mW/cm2, Driel) as alight source.
Results and Discussion
Synthesis and characterization of Er+-Yb3+ co-doped T1O2 nanopowders. When thedried gel a was heated ina high form crucible, the powder onthe inner side of thecrucibledid not burn well, and the acetylene black remained unburnt.
Hence,the dried gel was well dispersed on a boat-type cru
cible and then heated in amufflefurnace.
TheDSC/TGAplots of theproductsobtained bythecom bustion of thesampleareshown in Fig. 2. Thedriedgel con
tainingEr3+ and Yb3+ showed a broadendothermic peakat 100oCand threeexothermic peaks at 276oC, 369oC and 536oC as depicted in Fig. 2a. Theendothermicpeak at 100oC canbe attributedto the evaporation of the remaining solvent. The first exothermic peaksat 276oC maybe attributedto the de composition of theorganicmaterials.The peaksat 369oC and 536oC couldbe attributedto the formation oftitanium oxide
8 6
0
5
0
(%)
으
危JL
흐 으 - 그
75 5 0
5
0
5
0 7 a
&
9
&
(次 )。
%
」关
으豈
0 100 200 300 400 500 600 700 800 Temperature (oC)
Figure 2. TG and DTA plots of the dried gels recorded between 30 and 900oC at a heating rate of 5oC/min. (a) Er3+-Yb3+ co-doped TiO2 and (b) TiO2
트
WQ Q
그드
WQ Q 0 0 0 2
4
6
o
o
o 2
w 6 6
4
2
0
6 4 2
o x
Bull. Soc.2009, 30, No.
(a)
Figure 3. SEM images of (a) Er3+-Yb3+ co-doped TiO2 and (b) TiO2, heat treated at 700oC.
ac o. q」 v
>) wl UH U_
20 3 0 40 50 60 70 80
2©
Figure 4. XRD results of (a) Er3+-Yb3+ co-doped TiO2 and (b) TiO2, heat treated at 700oC.
and thedecomposition of acetyleneblack, respectively. The driedgel without Er3+ and Yb3+also showedalsoa broaden dothermicpeak at 100oC and twoexothermic peaksat 380oC, and 568oC as depicted in Fig. 2b.
Fromthe thermal analysis, synthesistemperature wasde termined to be 700oC, the temperature at which acetylene black completelyburns out. The colorthematerialwhen dop ed with Er3+ and Yb3+ was light pink when heated at 700oC, while theTiO2 had pure white if no materialwasdoped.
Itwas observedthatthematerial obtainedafter doping with Er3+ and Yb3+ at 700oC was in the amorphous powder (Fig.
3a). The samematerial,if prepared without dopingbyEr3+- Yb3+, changed to crystallites whenheated at700oC (Fig. 3b).
Possibly,thedoping of Er3+and Yb3+ ions in theTiO2 crystal lites broke the rutile and/oranatase structure before finally converting to the amorphousmaterial.This was confirmed by XRDspectra (Fig. 4), asno crystallitephase wasshown by the doped material (Fig. 4a), while the material with the same method of application without dopingshowed clear crystal
lites having mixed anatase and rutile phases with the domi nance of rutile structure (Fig. 4b). Amorphous structure of Er3+-Yb3+ co-doped titanium oxide may stemfrom the size difference of theions between Er3+-Yb3+ and Ti4+. When it is six coordinated,the ion size of Ti4+is 0.605 A, while those of Er3+ and Yb3+ are0.89 and 0.868respectively.17As observed by thescanningelectron micrographs, the particles of thedop edmaterial showed size between 27 to54 nmwith embossed structuredmorphology that agglomerated to produce micron sized particles.
Based ontheXRD and SEMstudies,it wasconcluded that the acetyleneblackburntslowlyup to 670oC preventingand
increase in size of theparticlesbyreleasingCO2 gas. For the undopedTiO2 material, a mixed structureofrutile and anatase phase with 45〜222nmcrystallitesize was obtained at 700oC.
However, theEr3+-Yb3+ co-doped titanium oxideobtainedat 700oChad amorphous particles without anycrystalline phase.
Fig. 5 showstheradiationspectrum of the TiO2 synthesized by the presentmethoddoping withand without Er3+- Yb3+ tak en ina light intensity of 100mW/cm2 and spectralirradiance of AM 1.5. The weak absorption spectrumshows a pale pink color of the materialwhendoping was done. Thematerials did notshowanyemission in thegreen region underthestandard conditions of sunlight.Thismeans that thevisible-to-infrared
5
4
3
2
1
0 0
0
0
0
0
m-su 옫
-
400 500 600 700 800
Wavelength(nm)
Figune 5. Radiation spectrum of (a) Er3+-Yb3+ co-doped TiO?, (b) T1O2 with a light intensity of 100 mW/cm2 and spectral irradiance of AM 1.5.
222 Bull. KoreanChem.Soc. 2009, Vol. 30, No. 1 Chi-Hwan Han et al.
Figure 6. Transmittance of nano-crystalline T1O2 photoelectrode
3+ 3+
with (a) scattering layer of Er -Yb co-doped TiO2, (b) scatter
ing layer of TiO2, and (c) without scattering layer.
up-conversion luminescence is absentand not utilizing the higher wavelengths for enhancingefficiency.Fig. 6 (a) shows the transmittance of scatteringlayer produced by Er3+-Yb3+
co-dopedTiO2,(b) is a scattering layerofundoped TiO2,and (c) does not have a scattering layer i.e. having a layer of nano-crystalline TiO2 without any supporting layer ofTiO2
particles.Itwasobserved thatthetransmittanceincreased as wavelength increased. The transmittance is higher when no scattering layer is fabricated, while incomparison to the TiO2
scatteringlayer,theEr3+-Yb3+ co-dopedTiO2 layerhad mini
mum transmittanceevenat higher wavelengths. This shows thatthe present morphology of the scattering layer is well suited.
Effect of light scatteringEi3+-Yb3+ co-doped T1O2 layer The double layer consisting of transparent nano-crystalline TiO2 and microcrystalline light scatteringEr3+-Yb3+ co-dop- ed-TiO2 was used for the cell’s photocurrent enhancement.
Fig. 7 depictsthephotovoltaic curves showingthecompari son with TiO2 electrodes fabricated withor without scattering
15
10
5
Z( E 으
vtu) m
wu op
-u 은
능
20
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Voltage(V)
Figure 7. Photocurrent-voltage characteristics of solar cells of (a) scattering layer of Er3+-Yb3+ co-doped TiO2, (b) scattering layer of TiO2, and (c) without scattering layer. The measurements were carried out at room temperature with a light intensity of 100 mW/cm2, spectral irradiance of AM 1.5, and an active cell area of 0.25 cm2.
Table 1. Photovoltaic performance of dye-sensitized solar cells based on the N719 dye with or without scattering layers
Voc (V)
J
sc(mA) FF Efficiency (%) With scattering layer of
Er3+- Yb3+ co-doped TiO2 0.773 18.9 0.615 8.98 With scattering layer of TiO2 0.773 18.4 0.614 8.72 Without scattering layer 0.743 17.1 0.613 7.80
layers. Thetotalcell area waskeptat 25 mm2 With nolight scattering layer in thecellthe parameters measured wereJsc = 17.1 mA, Voc = 0.743 V FF = 0.613 and n = 7.80%.Upon fabri
cation of the light scattering TiO2 layerwithout doping any material,theparameters showed enhancement with Jsc = 18.4 mA, Voc = 0.773 V FF = 0.615, and n= 8.72%, while the scat
tering layer when made byEr3+-Yb3+ co-doped TiO2 hadJsc = 18.9mA, Voc = 0.773 V, FF=0.615, and n = 8.98%. The re
sults relevant efficiencyparametersare shown inTable 1.
The light scattering layer actsasa photontrapping system and isalso active in photovoltaicgeneration. Rutile and ana tase forms of theTiO2 havetetragonalcrystalstructures pos
sessinghighrefractiveindices (rutile: 2.903; anatase: 2.49);
therefore, rutileform hasa better scattering property.5 Therefore, peoplehavemadeeffortsbased on the idea that a supporting layerof modifiedcrystallineTiO2 oflarger particle sizemight scatter theincident light bycolliding witha largeparticle,re sulting in the increase of cell’s efficiencyby increasing the path length of the incident light to be madeavailable tothe nano-crystallineTiO2 electrode.7-13Inthe presentcase, amor phousEr3+-Yb3+ co-dopedTiO2 film has a furtheradvantage of having no grain boundary,thereforeproducingbetteropti calscattering properties16and sincerefractive index is closely related to the film density, the packing density of theamor phousTiO2 would behigherthan the crystalline forms.This is evident from theresultswith an overall increase of 15.6% in efficiency of thecellswhen an amorphousEr3+-Yb3+ co-dop- ed-TiO2 material was used while anincrease of 11.7% ineffi
ciencywasrecorded when undoped TiO2 was used as a scat
tering layer. This alsoconfirms that ifmodifications aremade inthe TiO2 electrode utilizing a bilayer systemconsistingof both nano-crystallineTiO2 and amorphous Er3+-Yb3+ co-dop- ed-TiO2 material, the efficiency of the solarcells could be increased.
Conclusion
A newsol-gel combustion hybridmethodwas utilized that offered aneffective route for synthesis to prepare Er3+-Yb3+
co-doped-TiO2. The method produced amorphous powder when doped with Er3+ and Yb3+ and a crystalline rutile and anatase mixed phase, when no dopingwasperformed. Since theEr3+-Yb3+ co-doped-TiO2 material possessed amorphous nature and showed betterlight scattering as well as lower transmittance properties,it was thus found suitable as a light scattering layer. The light scatteringlayerincreasedthe effi
ciency of thedye-sensitizedsolar celltoby 15.6%.
References
1. O’Regan, B.; Gratzel, M. Nature 1991, 353, 737.
2. Kroon, J. M.; Bakker, N. J.; Smit, H. J. P; Liska, P; Thampi, K.
R. ; Wang, P; Zakeeruddin, S. M.; Gratzel, M.; Hinsch, A.; Hore, S. ; Wurfel, U.; Sastrawan, R.; Durrant, J. R.; Palomares, E.;
Pettersson, H.; Gruszecki, T.; Walter, J.; Skupien, K.; Tulloch, G.
E. Prog. Photovolt.: Res. Appl. 2007, 15, 1.
3. Han, C.-H.; Lee, H.-S.; Han, S.-D. Bull. Korean Chem. Soc.
2008, 29, 1495.
4. Ito, S.; Zakeerudin, S. M.; Baker, R. H.; Liska, P; Charvet, P;
Comte, P.; Nazeeruddin, M. K.; Pechy, P.; Takata, M.; Miura, H.;
Uchida, S.; Gratzel, M. Adv. Mater. 2006, 18, 1202.
5. Hore, S.; Vetter, C.; Kern, R.; Smit, H.; Hinsch, A. Sol. Energy Mater. Sol. Cell 2006, 90, 1176.
6. Vargas, W. E. J. Appl. Phys. 2000, 88, 4079.
7. Cheng, P.; Deng, C.; Dai, X.; Li, B.; Liu, D.; Xu, J. J. Photochem.
Photobiol. A: Chem. 2008, 195, 144.
8. Grinis, L.; Dor, S.; Ofir, A.; Zaban, A. J. Photochem. Photobiol.
A: Chem. 2008, 198, 52.
Bull. Chem. 2009, 30, No.
9. Kang, S. H.; Kim, J.-Y.; Sung, Y.-E. Electrochim. Acta 2007, 52, 5242.
10. Kang, S. H.; Kim, J.-Y.; Kim, Y.-K.; Sung, Y.-E. J. Photochem.
Photobiol. A: Chem. 2007, 186,234.
11. Menzies, D.; Dai, Q.; Cheng, Y.-B.; Simon, G. P.; Spiccia, L.
Mater. Lett. 2005, 59, 1893.
12. Kim, S.-S.; Yum, J.-H.; Sung, Y.-E. J. Photochem. Photobiol. A:
Chem. 2005, 171, 269.
13. Liu, Z.; Pan, K.; Liu, M.; Wang, M.; Lu, Q.; Li, J.; Bai, Y; Li, T.
Electrochim. Acta 2005, 50, 2583.
14. Kusama, H.; Sugihara, H. J. Photochem. Photobiol. A: Chem.
2007, 187, 233.
15. Kusama, H.; Sugihara, H. J. Photochem. Photobiol. A: Chem.
2006, 181, 268.
16. Shang, Q.; Yu, H.; Kong, X.; Wang, H.; Wang, X.; Sun, Y.;
Zhang, Y.; Zeng, Q. J. Lumin. 2008, 128, 1211.
17. Shannon, R. D. Ecta Crysta. 1976, A32, 751.
18. Zhang, M.; Lin, G.; Dong, C.; Wen, L. Surf. Coating Tech. 2007, 201, 7252.
