Synthesis of ZnO composited TiO2 nanoparticle and its applicationin dye sensitized solar cells: A novel approach in enhancingopen-circuit voltage

Animesh Layek, Somnath Middya, Arka Dey, Mrinmay Das, Joydeep Datta,Partha Pratim Ray nQ1

Department of Physics, Jadavpur University, Kolkata 700032, India

a r t i c l e i n f o

Article history:Received 25 February 2014Accepted 3 April 2014

Keywords:NanocompositeOptical material and propertiesSolar energy materials

a b s t r a c t

In this study, hydrothermal synthesis has been introduced to incorporate ZnO nanoparticle within TiO2

matrix by using PVP (polyvinylpyrrolidone) as a surfactant. Inclusion of ZnO within TiO2–ZnOnanocomposite increases optical absorption and reduces optical energy band gap. Within this TiO2–ZnO composite, recombination of photogenerated charge carrier reduces and carrier diffusion length (LD)increases approximately by 10% which confirms its superiority over TiO2 in the application of dyesensitized solar cell (DSSC). The open circuit voltage (Voc) of the DSSC with configuration ITO/ZnO–TiO2/Dye/I�3/ITO has been improved by 97 mV compared to ITO/TiO2/Dye/I�3/ITO device. Instead of TiO2,introduction of novel ZnO composited TiO2 also reduces the series resistance (Rs) and improves thefill-factor (FF) of the solar cell. The Short circuit current density (Jsc) of this nanocomposite based devicehas also been increased slightly. The increase in mobility-lifetime (μτ) product of photogenerated chargecarriers of the DSSC fabricated with novel ZnO composited TiO2 confirms its superiority in deviceapplication.

& 2014 Published by Elsevier B.V.

1. Introduction

DSSCs are extremely promising because they are made of low-cost materials and do not need sophisticated apparatus to manu-facture, while still providing reasonable energy-conversion effi-ciency. As clean photoelectric conversion, and potential alternativeto the traditional photovoltaic devices, nanocrystalline TiO2 basedDSSCs were investigated in the recent past [1,2]. The recombina-tion of photo-injected electrons in the conduction band of TiO2

with the oxidized dye is one of the inevitable problems [3]. Therelatively slow electron transport rate resulting from multipletrapping/detrapping events occurring within grain boundaries,will lead to high interface recombination and limit the deviceefficiency. In DSSC, the individual particle size is so small that theformation of a space charge region is impossible. This indicatesthat the recombination rate of the photo-injected electrons is veryhigh due to the absence of an energy barrier at the electrode/electrolyte interface [4]. Literature survey reveals many systematicefforts to reduce the recombination at the interface including

formation of bilayer, composite, and passivation by electropoly-merization [5].

Herein we represent a novel approach for making photosensingTiO2–ZnO nanocomposite with higher absorption compared toTiO2. Such kind of enhancement in absorption ability with reduc-tion in optical band gap energy of surfactant guided synthesizedTiO2–ZnO composite has so far been remain unexplored. Toimprove the performance of DSSC, it was a challenge to us toincrease the rate of carrier generation with effective mobility (meff)and to reduce the recombination by improving the absorptionability and modifying the band gap of the active materials. At lastthe paramount composite material was synthesized and appliedsuccessively in fabrication of DSSC. Herein DSSCs were fabricatedwith configuration ITO/TiO2/Dye/I3�/ITO and ITO/TiO2–ZnO/Dye/I3�/ITO and characterized for comparison. The TiO2–ZnO nano-composite based DSSC show better performance with improved mτproduct and LD of the photogenerated charge carriers compared toTiO2 based device. The overall improvement of the TiO2–ZnObased device has been observed with the improvement in Voc,Jsc, FF and Rs. The time of flight (τ) and diffusion length (LD)of photogenerated charge carrier of the DSSCs was estimated bySpace-Charge-Limited-Current (SCLC) and Einstein–Smoluchowskiequation.

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Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/matlet

Materials Letters

http://dx.doi.org/10.1016/j.matlet.2014.04.0330167-577X/& 2014 Published by Elsevier B.V.

n Corresponding author. Tel.: þ91 94 75 23 7259; fax: þ91 33 2413 8917.E-mail address: partha@phys.jdvu.ac.in (P. Pratim Ray).

Please cite this article as: Layek A, et al. Synthesis of ZnO composited TiO2 nanoparticle and its application in dye sensitized solar cells:A novel approach in enhancing open-circuit voltage. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.04.033i

Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎

2. Synthesis

In this typical synthesis, 5 g of Titanium tetrachloride (TiCl4)was added with 100 ml of water and the solution was divided intotwo aliquot volumes. 0.5 g of Zinc-acetate was dissolved in 50 mlof water in another beaker. First part of the stock solutionwas mixed with Zinc-acetate solution and was stirred vigorously.Pre-prepared 0.1 N of NaOH was added drop wise with these twosolutions by maintaining the pH (� 6–7) and were stirred for next2 h. Few drops of PVP (polyvinylpyrrolidone) were added ascapping reagent in both of them. The white precipitate (ppt) wasobserved in both the cases, which were transferred into linearTeflon autoclave to heat at 160 1C for two days. After cooling downto room temperature white ppt was washed with DI water andethanol repeatedly and sequentially by centrifuge technique andwas dried under vacuum at 100 1C to make free from solvent.All the AR-Grade reagents were procured from Merck.

3. Material characterization

The morphology of the composite material has been studiedby powder X-ray diffraction (PXRD) (Bruker make D8 X-RayDiffractometer) and Transmission Electron Microscopy (TEM)(JEOL make JEM-1400 TEM). To trace out the functional groups,Fourier Transmission Infra-red (FTIR) spectra have been recordedwith the help of FTIR-8400 S Spectrophotometer of Shimadzu. UV–vis absorption has been recorded with 2401PC spectrophotometerof Shimadzu.

4. Device fabrication and characterization

Herein two different DSSCs were fabricated (illustrated inSupplementary) with configuration ITO/TiO2/Dye/I3�/ITO andITO/TiO2–ZnO/Dye/I3�/ITO. In this typical part Brilliant Greenwas taken as the active dye.

The DSSCs were characterized with the help of Keithley 2400sourcemeter, interfaced with PC. The photovoltaic performance ofdevices was measured under illumination of intensity 80 mW cm�2.

5. Results and Discussions

Fig. 1(A) represents the powder X-ray diffraction (PXRD)spectra of (a) TiO2 and (b) TiO2–ZnO nanocomposite respectively.JCPDS card no. 84-1286, 84-1750 and 36-1451 approved the

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100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132Fig. 1. (A) PXRD patterns of (a) TiO2 and (b) TiO2–ZnO, (B and C) TEM images of TiO2 and TiO2–ZnO, (inset B and C) SEAD images of TiO2 and TiO2–ZnO.

Fig. 2. (a and b) Tauc's plot for TiO2 and TiO2–ZnO, inset (a and b) represent UV–visabsorption spectra of TiO2 and TiO2–ZnO.

Fig. 3. Current Density–Voltage (J–V) characteristics of (a) ITO/TiO2/Dye/I3�/ITOand (b) ITO/TiO2–ZnO/Dye/I3�/ITO based devices.

A. Layek et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎2

Please cite this article as: Layek A, et al. Synthesis of ZnO composited TiO2 nanoparticle and its application in dye sensitized solar cells:A novel approach in enhancing open-circuit voltage. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.04.033i

synthesis of TiO2 and TiO2–ZnO materials with phase identity.Using the Scherrer's broadening equation (illustrated in Supple-mentary), L¼λk/Bcosθ, the average particle size (L) was measuredas 26 nm and 39 nm, respectively. Fig. 1(B) and (C) show the TEMimages of TiO2 and TiO2–ZnO respectively, which exhibit that thesize of the particles are in nanoscale. The crystalline of theparticles are confirmed by SAED patterns.

Inset of Fig. 2 represents the UV–vis absorption spectra of TiO2

and TiO2 -ZnO which illustrates that the absorption edge of TiO2

has been significantly shifted due to incorporation of ZnO. The rateof chemical growth of the nanomaterial highly depends on thechain length of the surfactant and the chemical vapor pressure [6].This results the accumulation of more charge carriers in valenceband which eventually reduces the energy band gap. The opticalenergy band gap was estimated as 3.2 eV and 3.7 eV for TiO2–ZnOand TiO2 (Fig. 2) from Tauc's equation (see Supplementary). Due toits lower band gap and higher absorption, TiO2–ZnO compositecan serve as an important material in DSSCs.

Fig. 3 represents the current density vs. voltage (J–V) charac-teristics of DSSCs with configuration (a) ITO/TiO2/Dye/I3�/ITOand (b) ITO/TiO2–ZnO/Dye/I3�/ITO. DSSC based on TiO2–ZnOnanocomposite exhibits high Voc and slight increment in Jsc withimproved efficiency and FF compared to the TiO2 based one(Table 1). With all these parameters along with lower Rs provesthe overall improvement in the performance of TiO2–ZnO basedsolar cell.

We have observed a large increase in Voc (from 509 mV to606 mV) of TiO2–ZnO based DSSC, which might have occurreddepending upon the elevation of Fermi energy level according todoping composition of the synthesized material. The Voc isattributed to the charge recombination with either oxidized dyeor tri-iodide electrolyte [7]. To get further insight of the devices wehave qualitatively analyzed charge transport properties by usingSpace-Charge-Limited-Current (SCLC) theory and Einstein-Smoluchowski equation. (See the Supplementary) From this ana-lysis we see that LD and μτ product (Table 1) of the TiO2–ZnObased device has been increased although the material behavesitself as a resistive material [8]. This confirms the improvement ofthe charge transport properties within the device with decreasedrate of recombination. FF and Rs of the TiO2–ZnO based devicehave been improved probably due to the improvement of thejunction properties within the device.

The above demonstration approved the superiority of the novelTiO2–ZnO composite within DSSC for the fabrication of efficientdevice with better quality. The performance of the device mayfurther be improved by optimizing the thickness of the active layer.

6. Conclusion

In this letter, we have reported a novel technique to synthesizehierarchical TiO2–ZnO nanocomposite with PVP surfactant andinvestigated its successive application to enhance the overallperformance of DSSCs. There is a significant improvement in Voc

of the TiO2–ZnO nanocomposite based device. Here we have triedto tune the desired material property for device application bysuitable surfactant assisted controlled growth. The quantitativeand qualitative analysis reveals the superiority of the materialwith improved charge transport properties for device application.

Acknowledgment

The authors appreciate the University Grand Commission(UGC), Govt. of India for the financial support under project 39-508/2010(SR).

Appendix A. Supplementary material

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.matlet.2014.04.033.

References

[1] Liu J, Wang Y, Sun D. Renew Energy 2012;38:214–8.[2] Abdullah MH, Rusop M. Appl Surf Sci 2013;284:278–84.[3] Chang S, Wang H, Hua Y, Li Q, Xiao X, Wong WK, Wong WY, Zhu X, Chen T.

J Mater Chem A 2013;1:11553–8.[4] Zaban A, Chen SG, Chappel S, Gregg BA. Chem Commun 2000;134:2231–2.[5] Gregg BA, Pichot F, Ferrere S, Fields CL. J Phys Chem B 2001;105:1422–9.[6] Layek A, Middya S, Ray PP. J Mater Sci Mater Electron 2013;24:3749–55.[7] Roh SJ, Mane RS, Min SK, Lee WJ, Lokhande CD, Han SH. Appl Phys Lett

2006;89:253512–3.[8] Marc G, Augugliaro V, Martin C, Palmisano L, Rives V, Schiavello M, Tilley RJD,

Venezia AM. J Phys Chem B 2001;105:1026–32.

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Table 1Characteristic parameters of the DSSCs.

Device configuration JSC (mA cm�2) VOC (V) FF Efficiency (%) Rs (Ω) LD (nm) lτ (cm2 V�1)

ITO/TiO2/Dye/I3- /ITO 1.164 0.509 0.49 0.288 416 56 6.89�10�10

ITO/TiO2:ZnO/Dye/I3- /ITO 1.189 0.606 0.51 0.365 397 61.5 8.36�10�10

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Please cite this article as: Layek A, et al. Synthesis of ZnO composited TiO2 nanoparticle and its application in dye sensitized solar cells:A novel approach in enhancing open-circuit voltage. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.04.033i