preparation and characterization of electrospun ti02 for

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A i e C h e m i s Vo l. 12, No. I, May 2 ( 53-56

Preparation and Characterization of Electrospun Ti02 forDye-Sensitized Solar Cell

Dcp[/rlment of Chemical and B c h e m i c a l Engincering, Chosun Unil'crsity.l a l / 501-759. Aarea

AbstractApplication of electrspun Ti02 nanofibers was reported for an electrode fordye-sensitized solar cells (DSSC). This paper repo rt s on structural characterizaion andmorphology of titanium dioxide (Ti02) nanofibers prepared by electrospinning using asolution that contained poly (vinyl acetate) (PVAc) and titanium (IV) isopropoxide (Tip)and acetic acid as a catalyst for sol- gel reaction in dimethyl formamide (DMF). Ti02nanofibers with diameters of 80-100 nm were successfully obtained from calcination ofthe as-electrospun Ti02/PVAc composite nanofibers at above 400 "C in air for 5h r. Theas-electrospun and calcined Ti02IPVAc composite nanofibers were characterized bySEM.XRD,FT- IR ,TGA ,BET. The results indicated a significant effect of calcinationtemperature on the crys tal line phase in the form of either an atase or mixedanatase-rutile and the morphology of the nanofibers.Keywords Electrospinning, Titanium dioxide , Dye-sensitized solar cell1. INTRODUCTIONDye sensi tized solar cells (DSSCs) have been receiving much attention because theyhave many advantages such as low cost, less toxic manufacturing , easy scale-up , lightweight , and use of flexible panels compared to the convent ional p-n junction devices[1 ,2J . It has been pointed out that further works on the development ofnano-structured materials as well as the analysis of the electron transport dynamicsshould be conducted to enhance the low energy conversion efficiency of DSSC. Therehave been many studies on the synthesis and characterization of titania nanocrystallitesas well as the development of dyes for DSSC. However , systematic studies on theinfluence of adsorption propert ie s between dye molecules and titania films on the powerconversion efficiency of DSSC are very limited.The aim of this work is to investigate the influence of Ru (II) dye adsorption

properties on the conversion efficiency of DSSC. For this purpose . experimental andtheoret ical s tudies on the adsorpt ion equi libr ium and kinetic studies as functions ofsolution pH and temperatures were conducted to cont rol the adso rpt ion amount and alsounderstand the adsorpt ion mechanism of N719 dye on nano-structured titania particles.Adsorption kinetic data were ana lyzed by employing the pseudo-second-order model[3J. In addit ion , the solar cell performances including the overall conversion efficiency( T/ ), fill factor ( F : , open-circuit voltage (VaJ and short-circuit current ( . ) of the

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Ti02/dye sensitized nanocrystalline TiOz solar cells were investigated in accordancewith the adsorption quantity of N719 dye. An equivalent circuit analysis using theone-diode model was used to evaluate the influences of adsorption quantity on theenergy conversion efficiency of DSSC [4 ,5] . /\. nonlinear least-square optimizationmethod was used to determine five model parameters.2. EXPERIMENTAL DETAILSTi02 fibers were electrospun directly onto a Sn02:F-coated glass substrate (FTO. 1em x 1 em. TEC-8 , Pilkingotn) from mixture containing 3 g poly (vinyl acetate)(PVAc ,MW = 500.000 g/moD (Aldrich ), 6 g titanium (IV) isopropoxide (TiP) (Aldrich)and 2 .4 g of acetic acid as a catalyst for sol-gel reaction in dimethyl formamide CDMF)(37.5 mL). In a typical electrospinning , the precursor solution was loaded into a syringeconnected to a high-voltage power supply (ConverTech Model SHV120). An electricfield of 15 kV was applied between a metal orifice and the FTO substrate at a distanceof 1a em. The spinning rate was controlled by a syringe pump (KD Scienti fic Model220) at 60Llmin. Electrospun Ti0 2 web was treated with tetrahydrofuran (THF) vaporin a c losed chamber for 1 h prior to calcination. The calcination was carried outstepwise at each temperature (duration in min) as: 100 .C (5) ; 150 .C (5) ; 325 .C(5): 450 .C (30 min) in air. To grow Ti02 rutile crystal epitaxially , Ti02 web plate wasimmersed into a 0.1M titanium tetrachloride (TiCI4) (Aldrich) aqueous solution in aclosed chamber for 24 h. The 0.1M TiCI 4 aqueous solution was prepared in thefollowing manner. F ir stly, 2M concentrated TiCl 4 solutionwas prepared by addingdirectly titanium tetrachloride into a flask containing ice and then , the solution wasfurther diluted to 0.1 M. The Ti02 web electrode was immersed overnight in anethanolic solution containing 3 x l O.4M of ruthenium dye. RuL2 (NCS) 2 (L =2,2-bipyridyl-4 ,4 -dicarboxylic acid) (N3 , Solaronix). The electrode was rinsed anddried after its removal from the dye solution. The liquid electrolyte we used consistedof 0.6M1-hexyl-2 ,3-dimethyl-imidazolium iodide (C6DMlm) , 0.05M iodine (Iz). O.lMlithium iodide (L iI ) and 0.5M 4-tertbutylpyridine dissolved in 3-methoxyacetonitrile.Pt-spuUered Sn02:F glasswas used as thecounter electrode. Adye-sensitized solar cellcontaining a polymer gel elect rolyte was characterized: this electrolyte consisted ofpoly (vinylidenefluorideco- hexafluoropropylene) (PVDF- HFP) (Kynar 280 I , 0.13g) ,C6DM1m


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p a r a t i o n Characterization of Electrospun T i for Dye-Sensitized Solar Ce ll

3. RESULTS AND DISCUSSION

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?\;anoporous electrospun Ti02 electrodes improve the penetration of polymer gelelectrolyte effectively. Electrospun Ti0 2 v e for DSSC in this work shows thewel l-organized porous e lect rode s truc tu re a s shown in Fig. 1a after calcination at 450'C in air remov ing PVAc from the as-spun fibers. Several studies have previouslyreported that treatment of nanocrystalline Ti0 2 with TiCI 4 solution. Results in asignificant improvement in device performance. The nanocrystalline Ti0 2 electrode withT i C I ~ treatment increased the necking between the nanoparticles of the film , thus.facilitating the diffusion of photoinjected electron between particles and lowering theprobabili ty of recombination. In this study. electrospun T i O ~ web was treated with O. IMTiCl! aqueous solution at different temperatures. h Ti02 crystal grown from T i C isknown to the rutile s truc ture . The base Ti02 web is anatase and the epi taxial ly grownrutile is confirmed from the mixture of anatase and rutile peaks a ft er tr eatmen t. Theepitaxial growth rate of rut il e crystal on the anatase T i O ~ nanofiber surface depends ontemperatures. After treatment at 20 'e for 24 h. T i O ~ fi be rs were covered with a sm ClIlamount of rutile crystal as shown in Fig. lb . At 40 'r, the diameter of Ti0 2 fiberincreased twice compa red to the untreated as shown in Fig. 1c and also themacroscopic porous morphology was retained. However , the treatment a t. highertemperature. such as GO "e increases the additional rutile on the nanofibers and t.heporosity decreases to prevent the penetration of gel electrolyte. (Fig. 1)The lPCE can be expressed theoretically as the product of the light absorptionefficiency of the dye. the quantum yield of electron i nj ec tion and the eff ic iency ofcollecting the in jected electrons at the conducting glass sub str at e. The IPCE wasmeasured for a DSSC of electrospun T i ( ) ~ electrode with liquid electrolyte before andafter TiCL1 treatmen t. Both spectra show very similar spectral characteristics except thepeak at 350 nm , which is contributed by the light absorption in Ti02. The broad peakcorrelates to the sensitization of dye on Ti02. The overall IPCE after TiCI, treatment ish ighe r than the untreated. In par t. icular, the contribution from Ti02 increases after thetreatment due to the increase in packing density.Fig 2 presents the photocurrent density- voltage curves for cells base

Table 1. Photovoltaic performance of dye-sensitized solar cel lsNo. VO" [V] Isc (mNcm2) FF rtl%]

Ti02 nanoparticle 0.65 13.95 0.66 5.98Ti02 nanofiber 0.67 16.33 0.59 6.52

Ti02 nanofiber-TiCl4 0.60 18.97 0.66 7 .45

A p l C l l S t r y Vol. 12, No.1, 2


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4. CONCLUSION

- Ti02 nanofiber- T i 0 2 n a n i b e r - T i C l - Ti02nanopar tide

3027

6300.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Potential,VFig.2 Photocurrent density-voltage

characteristics of DSSCs with electrospunTi02 electrode

Fig.1 SEM IMAGE on Ti02 Fibers

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In summary , the energy conversion efficiency obtained from the DSSC with PVDF- HFPgel electrolyte was over 90% of that obtained from liquid electrolyte. In order toimprove the short-circuit photocurrent , we treated the electrospun T i O ~ electrode withT i C I ~ aqueous solution. The rutile crystalline l ayer was gro\'m epitaxial ly on Ti02 f ibers./ \n addit ional T i O ~ l ayer inc reased the volume fraction of active materials in porouselectrode resul ting in an increase of sensitizer adsorption. The photocurr en t of theDSSC with the electrospun TiO;! electrode increased more than 20% after TiCl 4treatmen t.References and Notes1. T. Hoshikawa. T. Ikebe , M. Yamada. R. Kikuch i, K. Eguchi. J. Photochem. Photobiol.

A 184. 78 (2006 ).2. J. Xia. F. Li , C. Huang. J. Zhai. L. Jiang , Solar Energy Materials & Solar Cells 90 ,

944 (2006).3. M.H. Kalavathy . T.S. Kar th ikeyan. L.R. Miranda. J. Colloid Interface Sci. 292. 354

(2005).4. J. A. Anta. F. Casanueva. and G. Oskam. J. Phys. Chem. B 110. 5372 (2006) .5. Z. Ouennoughi and M. Murayama. Solid-State Electronics 43 , 1985 (1 999).

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preparation and characterization of electrospun ti02 for

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