Thin Solid Films 568 (2014) 74–80

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Photocurrent enhancement by surface plasmon resonance of goldnanoparticles in spray deposited large area dye sensitized solar cells

Nikhil Chander a,⁎, Puneet Singh a, A.F. Khan b, Viresh Dutta a, Vamsi K. Komarala a

a Photovoltaic Laboratory, Centre for Energy Studies, Indian Institute of Technology Delhi, New Delhi 110016, Indiab Department of Electronics and Information Technology, Ministry of Communications & Information Technology, Government of India, New Delhi 110003, India

⁎ Corresponding author. Tel.: +91 11 2659 1255; fax: +E-mail address: vamsi@ces.iitd.ac.in (V.K. Komarala).

http://dx.doi.org/10.1016/j.tsf.2014.08.0020040-6090/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 6 February 2014Received in revised form 26 July 2014Accepted 6 August 2014Available online 13 August 2014

Keywords:Dye-sensitized solar cellsSurface plasmon resonanceSpray depositionGold nanoparticlesThin films

A facile method for fabricating large area TiO2 and TiO2–Au nanocomposite films for dye sensitized solar cells(DSSCs) is presented using a spray technique. Pre-synthesized gold nanoparticles (AuNPs)were sprayed togeth-er with the TiO2 NPs and composite films with brilliant coloration due to surface plasmon resonances of Au NPswere prepared. Composite films containing ~15 nm sized Au NPs exhibited enhanced absorption in the visibleregion of the electromagnetic spectrum. DSSCs with a large area of ~4.5 cm2 were fabricated and a photocurrentenhancement of ~10% was obtained for plasmonic DSSC containing 0.3 wt.% of ~15 nm Au NPs. Incident photonto current conversion efficiency data conclusively showed enhanced currents in the visible region of the poly-chromatic spectrum arising due to plasmon enhanced near-field effects of Au NPs around the absorbing dyemolecules.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Metal nanoparticles (NPs) embedded in dielectric matrices havevarious technological applications like solar cells [1–3], photocatalysis[4,5] and sensors [6,7]. For plasmonic dye-sensitized solar cell (DSSC)applications large area films of TiO2–metal nanocomposites (NCs) arerequired. The fabrication of such thin film NCs can be performed bytechniques like magnetron sputtering, liquid phase deposition andchemical vapor deposition [8]. These techniques are either very expen-sive or unsuitable for depositing large area films on different substrates.In this paper, an easy to use and industrially viable spray depositiontechnique has been utilized to fabricate TiO2–Au NC films on glass sub-strates for plasmonic DSSCs.

The spray technique is inexpensive and can be used to fabricate largearea films on various substrates at different deposition temperatures.The spraymethod has been used previously to deposit Au NP films, syn-thesis of CdS nanospheres, ZnO thin films and even fluorine-doped tinoxide (FTO) layers on glass substrates [9–12]. An efficiency of 3.2%was reported for a spray deposited small area DSSC [13]. However,there is only one study on the use of the spray technique for fabricatinglarge area TiO2–Au NC films, in which the researchers reported the fab-rication of compositefilms for glass coatings bymixingmetal NPswith atitanium precursor and the tuning of surface plasmon resonance (SPR)was obtained by annealing the films at different temperatures [14].Here we use commercially available TiO2 NPs instead of a precursor

91 11 2659 1251.

and chemically synthesized Au NPs to form large area films, which canbe used for DSSCs and any other suitable application. We show theapplication of these large area films for fabricating DSSCs with an activearea of ~4.5 cm2. Plasmonic effects of AuNPs have also been studied andenhanced efficiencies from plasmonic DSSCs are demonstrated.

2. Experimental methods

2.1. Fabrication of large area TiO2 and TiO2–Au films for optical studies

TiO2 films were deposited on glass substrates by modification of apreviously reported spray deposition technique [15]. About 1 g of TiO2

NPs were dispersed in 100 ml of 1-butanol and sonicated for 30 min.This dispersion was sprayed using a specially designed nozzle, onglass substrates kept at ~200 °C on a hot plate. Nitrogen was used asthe carrier gas; the solution flow rate and gas pressure were 2 ml/minand 10 kPa, respectively. The distance between the spray nozzle andsubstrates was kept constant at 20 cm. The solution was sprayed incycles of 20 s followed by an interval of 20 s to ensure that the substratetemperature was maintained at ~200 °C. The whole spray apparatuswas kept inside a specially designed enclosed chamber so as to mini-mize human contact with the chemical fumes resulting from the sprayprocess (see Fig. 1a). Gold NPs were prepared by the citrate reductionmethod [16] with chloroauric acid (HAuCl4·3H2O) as the gold precur-sor. Thus synthesized Au NPs were mixed with TiO2 NPs (Avg.size ~ 20 nm, Sigma Aldrich) in Au–TiO2 weight ratio of 0.3%. A similarprocess as mentioned above was used to fabricate TiO2–Au NC films.The films thus prepared had a thickness of 4 ± 0.2 μm. The prepared

Fig. 1. (a) Schematic of spray deposition apparatus and (b) the digital photograph of fabricated TiO2 and TiO2–Au composite films. (For interpretation of the references to color in thisfigure, the reader is referred to the web version of this article.)

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films were roughly square in shape with dimensions 2.5 cm × 2.5 cm.The square shaped films were fabricated so as to completely cover theentry and exit ports of integrating sphere assembly of a spectrophotom-eter to measure the total reflectance and total transmittance, whichhave diameters of approximately 1 in. All the films were annealed at~500 °C in air to remove any organic material and to improve adhesionof films on the substrates.

2.2. Fabrication of large area TiO2 and TiO2–Au NC films for DSSCpreparation

The films for fabricating DSSCs were thicker and prepared on FTOglass substrates by slight modification of the above-mentioned process.FTO glasses were cleaned using soap water, de-ionized water, acetoneand propanol in an ultrasonic bath. A compact layer of TiO2 was formedon FTO substrates by the spray pyrolysis process. For this purpose, asolution of titanium diisopropoxide bis(acetylacetonate) (SigmaAldrich) diluted in ethanol was sprayed on FTO substrates kept on ahot plate at 450 °C. In this fashion, a compact TiO2 layer of 70–80 nmwas prepared. Following this step the temperature of the substrateswas reduced to 200 °C and the dispersion of TiO2 NPs (or mix of TiO2–

Au NPs) in 1-butanol was sprayed as mentioned previously. The filmswere annealed at 500 °C following the spray deposition. The substrateswere allowed to cool down to room temperature. In this manner, filmswith an area of ~7.5 cm2 were deposited. The thickness of the

mesoporous titania layers thus prepared was 12 ± 0.5 μm. Afterwards,the prepared TiO2 and TiO2–Au NC films were refluxed in a 1 Mpropanol solution of titanium(IV) isopropoxide (TIP) (Sigma Aldrich)for 15 min and subsequently annealed at 500 °C. The thickness of thefilms was controlled by adjusting the amount of TiO2 (or TiO2–Au)NPs dispersion sprayed on the substrates.

In general, large area films (area N 1 cm2) could not be prepared bythe doctor blade method because of non-uniformity of film thicknessesand inhomogeneous nature of the prepared films. Some large area films(~7.5 cm2) were fabricated with the doctor blade method and it wasfound that the thickness varied significantly at different points on thefilms and the films were not as smooth and uniform as those preparedby the spray method. Due to these reasons, large area doctor bladedfilms were not used for device fabrication.

2.3. Fabrication of small area films using the spray and doctor blademethods

Apart from the large area films prepared by the spray deposition,some small area films were also prepared using the spray and conven-tional doctor blade methods for comparing the two deposition tech-niques. Small area spray films were deposited in the same manner asdescribed earlier, but a metal mask was used to define the depositionarea of ~0.2 cm2. The doctor blade method was also used for fabricatingTiO2 and TiO2–Au NC films with an area of ~0.25 cm2 on FTO glasses

Fig. 2. (a) XRD pattern of spray deposited TiO2 film on fluorine doped tin oxide substrate.(b) FESEM image of the spray deposited TiO2 film. (c) TEM image of TiO2 nanoparticles.

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using a paste consisting of TiO2 NPs (or amix of TiO2–AuNPs), terpineoland ethyl cellulose. The small area films prepared by the spray anddoctor blade methods were annealed at 500 °C. The resulting filmshad a thickness of 12 ± 0.5 μm. An ~3 μm scattering layer was also de-posited on top of themesoporous layer by the doctor blademethod. Thefilms were annealed again at 500 °C. Following this step, the films wererefluxed in a titanium isopropoxide solution and heated up to 500 °C.

2.4. Assembly of DSSCs

Above prepared films were dipped in 0.3 mM ethanol solutions ofN719 (di-tetrabutylammonium cis-bis(isothiocyanato) bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II)) dye (Solaronix) for 20 h.Large area films were dipped in a 75 ml dye solution while small areafilms were dipped in a 10 ml dye solution. The counter electrodeswere prepared by a sputter deposition of ~100 nm platinum thin filmson indium-doped tin oxide coated glass substrates. The electrolyteconsisted of 0.6 M 1-propyl-2,3-dimethyl imidazolium iodide (DMPII),0.1 M LiI, 0.05 M I2 and 0.5 M tert-butylpyridine in acetonitrile. DSSCswere assembled in a sandwich configuration using the photoelectrodeand the counter electrode. Large area DSSCs were left un-sealed andtested using a 3 cm × 1.5 cm aperture to define an active area of~4.5 cm2. Small area DSSCs were sealed using Surlyn and tested withan aperture of area ~0.16 cm2.

2.5. Instruments

The extinction, total transmittance and total reflectance spectra ofdye, Au NPs and TiO2–Au NC filmswere recorded using a Perkin Elmer'sLambda 1050 UV–Vis–NIR spectrophotometer. For total reflectance andtransmittancemeasurements, a 150mm integrating sphere attachmentwas used. Spray deposited TiO2 film was examined under a Zeiss fieldemission scanning electronmicroscope (FESEM) at an operating voltageof 3 kV to study itsmorphology. X-ray diffraction (XRD) patterns of TiO2

coated FTO substrates were recorded in bulk mode using a Philips XRDsystem (Philips X'PERT PRO) with Cu K-α incident beam (λ = 1.54 Å).Transmission electron microscopy (TEM) studies were carried outusing a Tecnai G2 F20 electron microscope at an acceleration voltageof 200 kV by scrapping off parts of the TiO2 film and dispersing themin ethanol. Thicknesses of all the films were measured using a Dektak-XT (Bruker) surface profiler. Current density versus voltage (J–V)measurements were performed using an Oriel Sol3A (Newport, USA)Class AAA solar simulator and a Keithley 2440 source-meter. AM1.5Gillumination corresponding to an incident light power of 100 mW/cm2

was employed for J–V measurements. A calibrated silicon solar cellwas used as reference. Incident photons to current conversion efficiency(IPCE) spectra were recorded using a ReRa SpeQuest (Netherlands)quantum efficiency measurement system. Incident monochromaticlight was chopped at 20 Hz and a bias light of 0.1 sun (10 mW/cm2)was employed for IPCE measurements. All measurements were carriedout at room temperature.

3. Results and discussions

3.1. Structural and optical studies

A schematic of the apparatus used for depositing TiO2 and TiO2–Aucomposite films by the spray method is shown in Fig. 1a. Nanocompos-ite film containing Au NPs appears pink-violet in color compared to thepure white color of the bare TiO2 film (see Fig. 1b). The adhesion of thespray deposited and TIP post-treated film on FTO substrate is quitegood. The deposited films could not be removed from the substratesby wiping with fingers or tissue paper. However, the films depositedon normal glass slides for optical studies, which were not subjected toTIP post-treatment, have relatively poor adhesion. The films could beremoved from glass slides by rubbing with a tissue paper; so they

were handled only from the edges with care. The use of TiO2 blockinglayer and TIP post-treatment greatly improves the adhesion of filmson FTO substrates and they are mechanically as stable as the films

Fig. 3. Extinction spectrum of gold nanoparticles in water and (inset) transmissionelectron micrograph of the nanoparticles.

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deposited by the doctor blademethod. The spraymethod can be used tofabricate films on a large area substrate with control over depositionrate and thickness. As mentioned in the Experimental methods section,the spraydepositionwasperformed for a short duration of 20 s followedby an interval of 20 s. This ensures that the substrate temperature ismaintained at the desired level (200 °C) and also that the films areuniform in nature and no pin-holes or cracking of films occur. Solutionspray rate and gas flow pressure have to be also optimized to ensurepinhole-free uniform films. We optimized these parameters and wereable to fabricate TiO2 and TiO2–Au films uniformly over an area of~7 cm2. These films were used for fabricating control and plasmonicDSSCs. The photovoltaic behavior of the DSSCs is discussed later.

Fig. 2a shows the X-ray diffraction pattern of a spray deposited TiO2

film on FTO substrate in 2θ range of 20–50°. The peaks in the XRDpattern correspond to the anatase phase of TiO2 (JCPDS no. 01-0562)with (101) as the dominant peak. The well defined peaks in the XRDpattern prove that the deposited film consists of highly crystalline ana-tase TiO2 NPs. The average particle size of TiO2 NPs is ~20 nm as can beseen fromTEMmicrograph (Fig. 2c). The FESEM image of a spray depos-ited TiO2film can be seen in Fig. 2b. The extinction spectrumof AuNPs isshown in Fig. 3; the inset of Fig. 3 shows the TEMmicrograph of Au NPshaving the sizes around 15 nm. The SPR peak position of extinctionspectrum is in good agreement with the published literature for theAu NP sizes of around 15 nm [17,18].

Fig. 4a andb shows the total reflectance and transmittance spectra ofbare TiO2 film and TiO2–AuNCfilm containing 0.3wt.% Au NPs recordedusing an integrating sphere, and also the calculated absorptance spectra.

Fig. 4. Absorptance, total reflectance and total transmittance spectra of (a) TiO2 and (b) TiO2–A

Absorptance was calculated according to the relation A+ R+ T= 100,where A, R and T stand for absorptance, reflectance and transmittance,respectively. The reflectance and transmittance were measured atthree different points on the deposited films and no significant differ-ences were observed. This shows that the spray films are depositeduniformly and also that the Au NPs are distributed evenly in the meso-porous TiO2 matrix. From Fig. 4 it can be inferred that the band edge ofTiO2 is about 400 nm; the incorporation of Au NPs (0.3 wt.%) does notseem to affect the band edge of TiO2. In the film with Au NPs the reflec-tance spectrum shows a sharp decline in the 400–650 nm wavelengthregion. For the Au NPs of mean size ~15 nm, the dip or minima corre-sponding to SPR is at ~561 nm. This value of SPR is red shifted comparedto the peak exhibited by Au NPs (~519 nm) in an aqueous medium(Fig. 3) because the particles are now present in TiO2 matrix, whichhas a higher value of refractive index than water. It can be seen fromthe figure that absorption in the visible region is enhanced because ofthe incorporation of Au NPs in TiO2 film. According to Mie theory,absorption and scattering are size dependent properties and absorptionis dominant for small metal NPs [19]. Previously published theoreticalstudies also show that small sized metal NPs are best suited for applica-tions where absorption enhancement is desirable [18,20,21]. Efficiencyenhancements have been reported in DSSCs and organic solar cells byusing small sized metal NPs [3,22,23]. We must mention here that~15 nm sized Au NPs and 0.3 wt.% Au–TiO2 weight ratio are not theoptimized values for obtaining maximum efficiency enhancement.This size and concentration of Au NPs have been chosen to demonstrateproof of concept large area spray deposited plasmonic DSSCs. Weare currently working on optimizing the size and concentration ofAu NPs to be used in DSSCs for obtaining maximum photocurrentenhancement.

Fig. 5a shows the extinction spectrum of N719 dye (in ethanol) usedfor sensitizing the TiO2 and TiO2–Au films. The absorbance of dye sensi-tized films is shown in Fig. 5b. Plasmonic NC film has higher absorbancein the 400–600 nmwavelength region compared to dye-sensitized pureTiO2 film. This enhanced absorption corresponds to the increasedabsorptance of TiO2–Au film (Fig. 4b) due to SPRs of Au NPs incorporat-ed in TiO2 matrix. From 500–600 nm the absorptance values are in therange of ~30% (peak value is 40%) indicating that plasmon enhancedabsorption is significant in this wavelength region (see Fig. 4b). But,since the gold NPs are used along with the dye molecules we have totake into account the combined contributions (both from dye andgold NPs) to absorption which is shown in Fig. 5b. The absorbanceincreases in a broadband region from 450 to 600 nm. Since the dyemol-ecules have greater contribution in light absorption as they are presentin very large numbers compared to the gold NPs, the spectral features ofabsorbance of TiO2–Au dye sensitized film correspond mainly with theTiO2 dye sensitized film. That is why it is difficult to see the enhanced

u films. Digital photographs of the films used for optical studies are also shown in insets.

Fig. 5. (a) Extinction spectrum of N719 dye used for sensitizing the titania films. (b) Absorbance spectra of spray deposited and dye sensitized TiO2 and TiO2–Au films.

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absorption in Fig. 5b corresponding to only the peak position of absorp-tance, i.e. ~550 nm, as observed in Fig. 4b. Rather, we see a broadbandenhancement from 450 to 600 nm. The same thickness and same areafilms were used for these measurements and any extra or enhancedabsorption observed in the spectra is indeed because of SPR of goldNPs. The absorptance peak of TiO2–Au is resonant with the absorbanceof dye sensitized films. This is desirable from a view point of plasmonicenhancement of photocurrent. If the SPR of Au NPs is off-resonant withthe dye absorption then light harvesting may not be improved signifi-cantly because the Au NPs would offer enhanced absorption in a wave-length region where the dye molecules' excitation does not occur or isvery feeble.

3.2. Performance of DSSCs

ThreeDSSCs of each typewere prepared and their photovoltaic char-acteristics were studied in order to ensure reproducibility of fabricateddevices and collect reliable data. Table 1 displays the average values ofparameters along with their standard deviations. The current–voltagecurves of large area spray deposited DSSCs are shown in Fig. 6. Thedeposited films have dimensions of 1.7 cm × 4.5 cm approximately. Amask with dimensions 1.5 cm × 3 cm corresponding to an area of~4.5 cm2 was utilized while making the measurements. This was donein order to ensure that all measured large area DSSCs have exactly thesame active area so that a fair comparison can bemade. The area of uni-form illumination provided by the solar simulator is roughly 5 cm× 5 cm; therefore, the length of active area (3 cm) is kept less than5 cm so that uniform illumination corresponding to AM1.5G class AAAis received by the entire active area. Large area control DSSC (CellA) provides a photocurrent in excess of 40 mA with a current densityof 9.12 mA/cm2. Large area plasmonic DSSC (Cell B) consisting of~0.3 wt.% of ~15 nmAu NPs shows ~10% enhancement in photocurrentwith a current density reaching up to 10.05 mA/cm2. Small incrementsin open circuit voltage and fill factor are also observed (see Table 1).An overall power conversion efficiency (PCE) enhancement of ~18% isobtained for the large area plasmonic DSSC compared to the controldevice. The counter electrode for large area DSSCs consisted of sputter

Table 1Photovoltaic parameters of the DSSCs fabricated in the present study.

Name of device Type of device

Cell A Spray deposited control DSSC large area (~4.5 cm2)Cell B Spray deposited plasmonic DSSC large area (~4.5 cm2)Cell C Spray deposited control DSSC small area (~0.16 cm2)Cell D Spray deposited plasmonic DSSC small area (~0.16 cm2)Cell E Control DSSC by doctor blade (~0.16 cm2)Cell F Plasmonic DSSC by doctor blade (~0.16 cm2)

coated platinum thin film on ITO glass substrates. The large area sputtercoated platinum electrodes have been shown to provide enhanced fillfactors for large area DSSCs compared to the conventional counter elec-trodes platinized using chemical methods [24]. This enabled us toachieve fill factors of ~60% for large area DSSCs (see Table 1).

A post-treatment involving the use of TIP has been used for all DSSCsfabricated in the present study. This TIP treatment has been reported toimprove DSSC performance and prevent metal NPs from corrosion byiodide electrolyte [23,25]. The small improvement observed in opencircuit voltage of Cell B can be attributed to Fermi level shifting of TiO2

because of the incorporation of Au NPs [26]. But the dominant factorfor enhanced efficiency is photocurrent enhancement due to SPRs ofAu NPs. The IPCE spectra of Cells A & B are shown in the inset of Fig. 6.The IPCE spectrum of plasmonic DSSC clearly shows enhancement inthe 400–600 nm region. This correlates well with the increased absorp-tion of TiO2–Au film (Figs. 4b & 5b). Since the enhanced absorption isbroadband in nature, as discussed earlier in Section 3.1, we see a broad-band response in IPCE curves. It means that the enhanced photocurrentis due to near-field effects of the incorporated Au NPs. Along with theincident light's far-fields, Au NPs' near-fields also contribute in dyemol-ecules' excitations, and thus improve the performance of DSSC. Plas-monic effect has been used for improving efficiencies of DSSCs whichemploy synthetic ruthenium based dyes, but it can also be used for im-proving performance of DSSCs based on non-toxic, low-cost naturalsensitizers or other types of metal-free dyes [27,28].

For large area plasmonic DSSC we have achieved an efficiency of~4.5% compared to the baseline efficiency of ~3.8%. These values arehigher than some previously reported efficiency values obtained viathe spray deposition method [13]. Still higher efficiencies for largearea DSSCs could have been obtained by the use of a scattering layer.Large sized TiO2 NPs (200–400 nm) are generally used as a scatteringlayer in high efficiency DSSCs [29]. However, the small area deviceswere coated with a scattering layer using a commercial scattering TiO2

paste (WER-2-O, Dyesol).Current density versus voltage (J–V) curves of small area control and

plasmonic DSSCs fabricated by the spray and doctor blade methods areshown in Fig. 7. The PCE of small area control DSSCs fabricated using

Voc (mV) Jsc (mA/cm2) Fill factor (%) Efficiency (%)

700 ± 4 9.12 ± 0.1 59 ± 1.6 3.77 ± 0.05710 ± 6.5 10.05 ± 0.12 62.5 ± 1 4.46 ± 0.07630 ± 4 14.1 ± 0.16 68 ± 1 6.04 ± 0.11642 ± 6 16.22 ± 0.18 68.5 ± 1 7.13 ± 0.08652 ± 1.6 14.71 ± 0.07 68 ± 1 6.52 ± 0.03658 ± 5 17.2 ± 0.24 68 ± 1 7.7 ± 0.14

Fig. 6. Current–voltage curves of large area spray deposited control and plasmonic DSSCs.Inset of figure shows IPCE spectra of control and plasmonic DSSCs.

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spray (Cell C) and doctor blade (Cell E) is comparable. These devicesexhibit similar values of Jsc, FF and PCE (N6%). This implies that thespray depositionmethod for fabricatingDSSCs is comparable to the doc-tor blade method in terms of device performance. Plasmonic DSSCs

Fig. 7. Current density–voltage curves of small area (a) spray deposited and (b) doctorblade DSSCs. Insets of figures are IPCE spectra of control and plasmonic DSSCs.

exhibited ~15% (Cell D) and ~17% (Cell F) enhancement in photocurrentwhen compared to the control devices of the spray and doctor blademethods, respectively. Overall PCE of well over 7% is obtained for plas-monic DSSCs. The IPCE spectra of control and plasmonic devices areshown in insets of Fig. 7a–b. Enhanced photocurrents, especially in the400–600 nm wavelength region, can be observed from the graphswhich correspond to the enhanced absorption of TiO2–Au film (seeFig. 5b). This indicates that photocurrents are indeed increasing becauseof surface plasmon enhanced near-field effects of the Au NPs incorpo-rated in TiO2 film.

4. Conclusions

Large area TiO2 and TiO2–Au composite films have been fabricatedby the spray deposition method. Composite film shows enhancedabsorption in the visible wavelength region of the electromagneticspectrum due to SPRs of the incorporated Au NPs. The spray depositedfilms have been used for fabricating DSSCs with a large area of~4.5 cm2 and an efficiency of 4.46% has been reached by plasmonicDSSC containing 0.3 wt.% of ~15 nmAuNPs. Spray deposited and doctorblade DSSCs of small area have been compared and found to providenearly the same level of device performance. The spray depositionmethod provides a fast, easy and inexpensive approach to make largearea DSSCs. The spray technique presented here can be effectivelyused for fabricating the geometry in which metal or any other types ofNPs are embedded in the TiO2/ZnO active layer. It is especially suitablefor fabricating large area films on various kinds of substrates for DSSCapplication.

Acknowledgments

The authors would like to acknowledge the partial support from theDept. of Sci. & Tech. (DST), Govt. of India under the Solar EnergyEnabling Research grant number RP02468. One of the authors (P.S.)would like to acknowledge the financial support from DST ESCORT(RP02499) research project. Authors thank Sanjay K. Sardana andEshwar Thouti (Centre for Energy Studies, IIT Delhi) for providing sput-ter deposited platinum counter electrodes on ITO glass substrates.

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