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Enhanced DSSC performance with high surface area thin anataseTiO2 nanoleaves

Vivek Dhas a, Subas Muduli a, Shruti Agarkar a, Abhimanyu Rana a, Beatrice Hannoyer b,Rahul Banerjee a, Satishchandra Ogale a,⇑

a National Chemical Laboratory, Council of Scientific and Industrial Research, Pune 411 008, Indiab Universite de Rouen, GPM UMR 6634 CNRS – BP 12, 76801 Etienne du Rouvray Cedex, France

Available online 24 March 2011

Communicated by: Associate Editor Hari Upadhyaya

Abstract

Thin anatase TiO2 nanoleaves (NLs) with high surface area (�93 m2/g) are synthesized by hydrothermal route and dye-sensitizedsolar cells (DSSC) made using such NLs are compared with those made with hydro-thermally synthesized TiO2 nanoparticles and theDegussa P25 powder. The NLs-based DSSCs show increment of 16% and 24% in the total conversion efficiency over the cells made withNPs and P25 respectively. Interestingly, the highest increment in the conversion efficiency (�35%) is achieved for cells made using a 50:50(wt/wt) mixture of NLs and NPs. In this case the dye loading is found to be >50% higher than the case of NP films. Moreover the DCresistance of diffusion of I�3 in electrolyte (as revealed by the electrochemical impedance spectroscopy) is substantially lower than the NPfilm case, other cell parameters being nominally comparable.� 2011 Elsevier Ltd. All rights reserved.

Keywords: Dye-sensitized solar cells; Hydrothermal; Electrochemical impedance spectroscopy

1. Introduction

Dye-sensitized solar cells (DSSC) based on nano-struc-tured TiO2 electrodes have been attracting intense scientificand industrial attention for quite some time because oftheir unique hybrid architecture supporting interestingoptoelectronic phenomena, potential for low cost scale upof the corresponding synthesis procedure and applicabilityfor diffuse light harvesting. (O’Regan and Gratzel, 1991;Gratzel, 2001, 2005) There are a number of factors in thecomplex DSSC architecture which can limit their perfor-

mance and need to be optimized simultaneously for thebest performance. In particular, higher dye loading of theinternal surfaces of the photo-anode and enhanced mobil-ity of the electrons through 1D metal oxide nano-structuresare the most desired goals. Photoanodes comprising ofanisotropic nanostructures (e.g. nanorods, nanotubes,nanowires etc.) are being examined with interest in thiscontext, though they may not necessarily optimize boththe aspects mentioned above simultaneously (Jiu et al.,2006; Zhu et al., 2007; Feng et al., 2008). Nanophase ana-tase TiO2 is the most investigated material in the DSSCcontext, but in most studies this material is used in spher-ical morphology or as faceted nanoparticles (NPs). It isto be noted that unlike ZnO, the crystal habit of TiO2 doesnot yield natural growth of uniform anisotropic structures.(Dhas et al., 2008; Nakade et al., 2002). Law et al. foundthat the electron diffusion coefficient of ZnO nanowires isseveral hundred times larger than the coefficient for ZnO

0038-092X/$ - see front matter � 2011 Elsevier Ltd. All rights reserved.

doi:10.1016/j.solener.2011.02.029

⇑ Corresponding author. Address: National Chemical Laboratory,Council of Scientific and Industrial Research, Pune 411008, India andThe Indian Institute of Science Education and Research, Pune, India. Tel.:+91 20 2590 2260; fax: +91 20 2590 2636.

E-mail address: sb.ogale@ncl.res.in (S. Ogale).

www.elsevier.com/locate/solener

Available online at www.sciencedirect.com

Solar Energy 85 (2011) 1213–1219

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or TiO2 nanoparticle film. (Law et al., 2005) Ohsaki et al.reported that the electron lifetime of a TiO2 nanotube filmis 3 times higher than that in a nanoparticle film resulting inan improvement of diffusion length (Ohsaki et al., 2005).Mor et al. have also shown that the nanotube films havesuperior electron lifetime than the nanoparticle films(Mor et al., 2006). These authors have further shown thatelongated nanostructures can lead to useful light harvestingeffects. Adachi et al. showed that the current density ofDSSC made of a network of nanowires is more than twicethat of NPs DSSC (Adachi et al., 2004). However there areonly a few reports on the use of a mixed composite ofNanorods (NRs)/Nanoparticles (NPs) structured to bal-ance the advantages of each other. Tan et al. showed thatincorporation of 20 wt.% of nanowires can increase the effi-ciency of DSSC from 6.7% to 8.6% .Here they have shownthat in case of composite films current density hasincreased and also due to addition of nanowires crack-freethick films can be made. Also, the photoelectrochemicalproperties of NPs/NRs composite-based electrodes getmodified since the decrease of NPs contents leads to smal-ler surface area for dye adsorption, while the incorporationof NRs can enhance the electron transport rate and lightscattering effect (Tan and Wu, 2006). However, Wu et al.recently claimed rather contradictory results that the effi-

ciency of the NRs–NPs composite DSSC as well as NRsDSSC is lower than that of the NPs DSSC (Wu et al.,2008). Their Electrochemical Impedance Spectroscopy(EIS) studies showed that electron diffusion decreases inNRs DSSC, when compared to the NPs DSSC. It has beenshown that vertical alignment of NRs within the anodes iscrucial for achieving a high electron transport rate.

In this work, we have hydro-thermally synthesized ana-tase TiO2 nanoleaves (NLs) following the work of Yanget al. (2008) and have made DSSCs using such NLs. Ouridea was to examine the possible advantages of the highsurface area and electrical connectivity afforded by thenano-leave architecture in the DSSC context. Our conjec-ture was that the higher surface area would enhance dyeloading and the good extended connectivity may offer briskcharge transport. However, enhanced surface area couldalso enhance scattering and the charge recombination dueto surface defect states, hence, it is essential to examineexperimentally whether and how the improvement in cellparameters, if any, can be realized. We have found thatwhile the NLs themselves yield a better performance thanthe NPs, the DSSCs made with a mixture of NLs andNPs yield a far superior performance than the cells madeseparately with NLs and NPs. We present, analyze and dis-cuss these data in the paper.

(c)

20 30 40 50 60 70 80

(a)

(004

)

(116

)

(204

)

(220

)(2

15)

(105

)(2

11)

(200

)

(101

)

2θ

NLs NPs

400 600 800

(b)642.94

520401.7

Wavenumbers(cm-1)

NLs NPs

(d)

Fig. 1. (a) X-ray diffraction (XRD) patterns and (b) Raman spectra for TiO2 nanoparticles (NPs) and nanoleaves (NLs). HRTEM images for NLs andNPs are shown in (c) and (d) respectively. Inset of (c) shows AFM data for NLs deposited on silicon substrate.

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2. Experimental methods

2.1. Materials and synthesis protocol

The chemical agents used in this work were procuredform Aldrich Co. The N719 dye and the Fluorine dopedSnO2 (FTO) electrodes (sheet resistance 15 X/square) wereprocured from Solaronix Co.

1 mL (3.5 mM) of titanium (IV) isopropoxide and 3 mL(29 mM) of triethyl amine (NEt3) were mixed in 76 mL ofwater by ultrasound to obtain a translucent solution. Thesolution was sealed in a Teflon-lined stainless steel vesseland heated at 140 �C for 48 h. After centrifugation thewhite milky product was washed using 10 M HNO3 anddeionized water until the pH value was neutralized. Thenthe product (nanoleaves) was collected and dried at 60 �Cin vacuum to get a white powder.

The TiO2 nanoparticles were also prepared by usinghydrothermal method. Titanium isopropoxide (2 mL) was

hydrolyzed by adding sufficient amount of deionized waterfollowed by sonication for 5 min. The solution was thentransferred to Teflon lined autoclave vessel along with3 mL of H2SO4 (1 M). This autoclave vessel was kept at175 �C for 24 h. The resulting product was washed thor-oughly with deionized water and dried at 50 �C in a dustproof environment to produce white powder of TiO2

nanoparticles.

2.2. Characterization

Various techniques such as X-ray diffraction (XRD, Phi-lips X’Pert PRO), Raman (Horiba Jobin Yvon LabRAMHR System), High-Resolution Transmission ElectronMicroscope (HR-TEM, FEI Tecnai 300), Atomic forceMicroscopy (AFM, RHK Technology) and Electrochemi-cal Impedance Spectroscopy (EIS, Autolab PGSTAT30(Eco-Chemie)) were used to characterize the samples. Theimpedance measurements were made at a roomtemperature.

2.3. Fabrication of dye-sensitized solar cell

To make and study the DSSC, doctor blading methodwas employed. After making the films they were annealed

Reflectance at 450nm

NPs : 75% ,

Nlves: 64% and

NPs + Nlves: 78%

(c)

400 600 800

40

60

80

(a)

%R

Wavelength (nm)

NPs NLs NPs + NLs

0 2 4 6 8 100

200

400

600

800

1000

(b)

NLs NPs P25 NLs + NPs

Cur

rent

(μA

)

Applied Voltage (V)

400 500 600 7000.00

0.05

0.10

0.15

0.20

0.25

(d)

Abs

orba

nce

Wavelength (nm)

NPs P25 NLs NLs + NPs

Fig. 2. (a) Diffused reflectance spectra of nanoparticles (NPs), nanoleaves (NLs) and 1:1 Nps + NLs, (b) current perpendicular to plane (CPP) transportdata for different DSSC architectures of interest, (c) schematic diagram of current perpendicular to plane (CPP) measurement, (d) optical absorption ofsolutions containing dye detached from doctor bladed films of different cases of interest (film area of 2 cm2 dye extracted in 10 mL of 1 mM KOH).

Table 1BET surface area measurements of TiO2 nanoparticles (NPs) andnanoleaves (NLs).

Name Surface area (m2/g) Pore volume (cc/g) Pore radius (A)

NPs 20.87 0.177 22.586NLs 92.95 0.147 15.104

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at 450 �C for 30 min. For sensitization, the films wereimpregnated with 0.5 mM N719 dye in ethanol for 24 hat room temperature. The samples were then rinsed withethanol to remove excess dye on the surface and were air-dried at room temperature. This was followed by redoxelectrolyte addition and top contact of Pt coated FTO asdiscussed elsewhere (Muduli et al., 2009). The electrolyteused was 1 M 1-hexyl-2, 3-dimethyl-imidazolium iodide,0.05 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine inacetonitrile. I–V characteristics were measured using a

solar simulator (Newport) at 100 mW/cm2 (1 sun AM1.5). Standard Silicon solar cell (SER NO. 189/PVM351)from Newport, USA was used as reference cell.

3. Results and discussion

Fig. 1a compares the XRD patterns for TiO2 NPs andNLs. The peaks in Fig. 1a at 25.28 (101), 37.80 (004),48.18 (200) and 54.09 (105) clearly represent pure anataseTiO2 phase (PCPDFWIN # 211 272). Fig. 1b shows theRaman spectra for the TiO2 NPs and NLs. These spectrashow clear signatures at about 402 (B1g) cm�1, 520(A1g + B1g) cm�1 and 643 (Eg) cm�1, which are in accor-dance with the bands observed for anatase TiO2, (Muduliet al., 2009). Fig. 1c and d shows TEM images of theNLs and NPs, respectively. The nanoparticles are facetedand fairly mono-dispersed with a size of �10 nm. The mor-phology of NLs is planar without branches. The length ofNLs is about 120 ± 40 nm, while the width is about 30 nm.The inset of Fig. 1c shows AFM data for NLs. Althoughthe leaves appear to be bowed up a bit upon depositionon silicon substrate, the line scan shows that their thicknessis <8 nm.

In order to investigate the effect of the NLs on the per-formance of DSSC we used these as material for workingelectrode. Fig. 2a shows the diffused reflectance spectra(DRS) of electrodes made of TiO2 NPs, NLs and the mix-ture of NPs – NLs. In the case of NLs it is observed thatthe percentage reflectance is low (64%) compared to NPs(75%). This is due to the loss of radiation through thinNLs compared to agglomerated NPs which scatter lightto a large extent, giving it an opaque nature compared tosemi-transparent in NLs case. In order to optimize thereflectance – transmittance ratio we mixed these NPs andNLs in 1:1 weight ratio. The percentage reflectance thusimproved to (78%). We also performed current-perpendic-ular-to-plane (CPP) conductivity measurements on ourfilms before dye loading (i.e. after annealing at 450 �C)using aluminum top electrode. The voltage was swept from0 V to 10 V with a step of 0.5 V and a delay of 0.5 s. Theresults of these measurements are summarized in Fig. 2b.

The inset shows a schematic of the measurement proce-dure. It can be noted that the current is higher for casesinvolving NLs as compared to the NPs and P25. Also,the current is highest for the NL films as compared tothe NP and NL (50:50) mixture case, establishing the cor-responding higher electrical connectivity. However as dis-cussed next the dye loading was highest for the case ofmixture as compared to the case of NLs alone.

To obtain dye loading data the dye was detached fromvarious cell configurations (all with 2 cm2 area) in 10 mLof 1 mM KOH. The calculated dye loading from the opticaldata is shown in Fig. 2c. For the films made of P25, NPs,NLs and the (50:50) mixture of NLs and NPs the dye load-ing is found to be 5 � 10�8, 4.9 � 10�8, 6.28 � 10�8 mol/cm2 and 7.8 � 10�8 mol/cm2 respectively. The significantenhancement in dye loading for cases involving NLs can

0.0 0.2 0.4 0.6 0.80.000

0.001

0.002

0.003

(a)

Cur

rent

(A)

Voltage (V)

P25 NPs NLs

0.0 0.2 0.4 0.6 0.80.000

0.001

0.002

0.003

0.004

(b)

Cur

rent

(A)

Voltage (V)

NPs + NLs NLs

Fig. 3. (a) Comparison of Solar cell characteristics for DSSCs made withTiO2 nanoleaves and those made with P25 and TiO2 nanoparticles (filmthickness is 12 lm) and (b) comparison of Solar cell characteristics forDSSCs made with TiO2 nanoparticles and the mixture of nanoparticlesand nanoleaves (film thickness is 10 lm).

Table 2DSSC: anatase TiO2 nanoparticles (NPs), nanoleaves (NLs) only andNLs + NPs.

Name Voc(V)

Isc(mA)

Jsc(mA/cm2)

Fillfactor(FF)

Efficiency(g) %

Degussa P25 (12 lm) 0.69 2.5 10 65 4.5 ± 0.2NPs (12 lm) 0.7 2.6 10.5 67 4.8 ± 0.2NLs (12 lm) 0.77 2.7 10.7 68 5.6 ± 0.2NLs (10 lm) 0.77 2.1 8.3 62.8 4.0 ± 0.2NLs + NPs (1:1 wt.%

ratio) 10 lm0.74 3.4 13.7 65 6.5 ± 0.2

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be attributed to the very high surface area of 93 m2/g forNLs as compared to 21 m2/g for NPs as shown in Table 1;the areas being measured by the standard BET technique.The theoretical calculations for area/weight (m2/g) esti-mated for NPs with 10 nm diameter is about 142 m2/g; how-ever the observed value is only 21 m2/g. This clearlyindicates that in the powder itself the synthesis process leadsto a degree of agglomeration. On the other hand, in the caseof NLs the estimated area/wt is about 80–90 m2/g, which isclose to the observed value of 93 m2/g. The significantincrease in dye loading for the mixture case as compared

to NLs alone can be attributed to porosity aspect of the spe-cific architecture discussed later.

Fig. 3a and b and Table 2 compare the solar cell charac-teristics for DSSCs prepared with electrodes made of TiO2

NPs, TiO2 NLs and Degussa P25 powder and the mixtureof NPs and NLs (all cell with 0.25 cm2 area). It can be seenthat the optimized mean efficiencies obtained by our proce-dure for P25 and NPs are about 4.5% and 4.8%, respec-tively. On the other hand the efficiency is considerablyhigher for the case of DSSC made with NLs alone(5.6%). While this is already a significant enhancement, a

(A)

(B) Fig. 4. (A) FE-SEM images of (a1) NPs, (a2) NLs and (a3) NPs + NLs. (B) Schematic diagram showing the comparison between DSSC architecturesinvolving TiO2 nanoparticles (NPs), nanoleaves (NLs) and the mixture of NLs + NPs.

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further dramatic increase in efficiency (6.5%) is seen for thecase of a mixture of NPs and NLs (50:50).

FE-SEM images in Fig. 4A and its cartoon representa-tion in Fig. 4B helps to bring out the differences in the nat-ure of TiO2 photoanodes for the cases comprising of NPs,NLs and the mixture of the two. The lower current in thecase of NP film (a1) can be understood based on the corre-sponding high interface density. The higher current in theNL case (a2) as compared to NP only can be attributedto the expected flattened and dense architecture. Sucharchitecture is however not conducive to better dye loadingand indeed shows lower dye loading as compared to thecase of mixture. The architecture of the mixture (a3) orients

some NLs towards the vertical direction for conductanceand would also give enhanced paths for dye incorporation.Thus, it shows higher current and higher dye loading ascompared to the purely NLs case leading to considerablyhigher conversion efficiency.

In order to study the electron transport and chargerecombination in these TiO2 films Electrochemical Imped-ance Spectroscopy (EIS) measurements were performed.The frequency range chosen for the measurement was from10�2 Hz to 106 Hz with ac amplitude of 10 mV. The imped-ance spectrum of DSSC (Adachi et al., 2006) (Nyquist plot)mainly consists of 3 semicircles attributed to the imped-ances for the Pt| Electrolyte (Z1), TiO2|Dye |electrolyte(Z2) and the diffusion of I�3 (Z3) in the electrolyte, respec-tively, as depicted in Fig. 5a. The data for our three casesof interest (NPs, NLs and NPs + NLs) are shown inFig. 5c and indeed show the expected three semicircle fea-ture. The resistance elements R0, Rk, and RD are describedas the real part of Z1, Z2 and Z3, respectively shown inFig. 5b. It is found that the resistance element R0 in thehigh frequency range over 1 MHz is influenced by the sheetresistance of TCO and the contact resistance between theTCO and TiO2. It can be seen from 5c that for the caseof the mixture the value of R0 is intermediate between thatfor NPs and NLs, as may be expected. The parameters ofsignificance to the cell performance can be extracted fromthe impedance data from (Nyquist plot) (Adachi et al.,2006) as follows:

(a) keff is estimated from the peak frequency xmax of thecentral arc, which describes the rate of recombinationof electron in the film.

(b) seff is the inverse of keff which gives the informationabout the lifetime of electron throughout the workingelectrode.

(c) Rk is the diameter of the central arc which gives thecharge transfer resistance including recombinationof electrons with I�3 at the TiO2/electrolyte interface.

(d) Deff is the effective diffusion coefficient of electrons iscalculated as,

Deff ¼ ðRk=RwÞL2keff ;

where L is the thickness of working electrode and fortrue circle Rk� Rw.

(e) RD is the DC resistance of diffusion of I�3 in electro-lyte calculated from the diameter of the third arc(low frequency in the mHz range).

20 30 40 50 60 700

10

20

30

ωmax = 3.85 Hzωmax = 14.17 Hz

ωmax = 4.64 Hz

R0

Z'' (

Ohm

)

Z' (Ohm)

NLs NPs + NLs NPs

(a)

(c)

(b)

30 40 50 600

10

20

30

Z3

Z2

Z1 ωmax = keffR0

RDRkZ'' (

Ohm

)

Z' (Ohm)

Typical Nyquist Plot

Fig. 5. Electrochemical impedance spectroscopy (EIS) measurements: (a)schematic diagram showing the three interfaces, (b) typical Nyquist Plotwith specified parameters and (c) Nyquist plot of nanoparticles (NPs),nanoleaves (NLs) and 1:1 NPs + NLs.

Table 3Properties determined by electrochemical impedance spectroscopymeasurements.

Name keff

(s�1)seff

(s)Rk

(X)Deff

(cm2 s�1) � 10�4Con(X cm s�1)

RD

(X)

NPs 3.85 0.26 24.11 1.33 0.11 6.00NLs 14.17 0.07 20.0 4.08 0.34 3.34NPs + NLs

(1:1)4.64 0.21 23.42 1.56 0.13 3.88

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(f) Con describes the charge transport resistance and itsrecombination rate along the entire thickness ofworking electrode and is calculated as

Con ¼ RkLkeff :

Fig. 5c and Table 3 compare the three cases of TiO2 mate-rials used as working electrode for the same film thickness.Notable differences are seen between the NPs and NLs casesas reflected in the RD, seff, Deff and Con values. Although theparameter Deff, which represents the electron diffusion coef-ficient through the TiO2 nanostructure, is significantlyhigher in the case of NLs as compared to the NP case, thisadvantage is offset by the significantly higher recombinationrate (much smaller lifetime) in the case of nanoleaves, mostpossibly due to surface defect states due to enhanced surfacearea. This is also reflected by the higher Con value. The NL-based architecture further enables reduction of RD, the DCresistance of diffusion of I�3 in electrolyte. The combinedeffect of all these is the higher efficiency observed for nano-leaves (5.6%) as compared to the nanoparticle case (4.8%).In case of the highest efficiency cell i.e. 1:1 mixture of NPsand NLs optimum values of the impedance parameters areseen to have been realized. Interestingly, the recombinationrate is only somewhat higher than the nanoparticle case, butthe parameter RD (the DC resistance of diffusion of I�3 inelectrolyte) is significantly lower. This leads to the muchhigher efficiency, since other parameters are nominally com-parable with the nanoparticle case. It seems that processingof the nanoleaves film along with a significant percentage ofnanoparticles cleans a large fraction of the surface defects onone hand and yields a microstructure that facilitates dyeloading as well as I�3 diffusion in the electrolyte.

4. Conclusion

Thin anatase TiO2 nanoleaves with high surface area areused in dye-sensitized solar cells (DSSC) architectures andthe corresponding results are compared to the DSSCs madewith nanoparticles and Degussa P25. It is observed thatDSSCs made with NLs show a higher conversion efficiency(�5.6%) than those made with NPs (�4.8%) and P25(�4.5%), but the highest efficiency (�6.5%) is realized forDSSCs made using a (50:50) mixture of NLs and NPs, withthe film thickness optimized in each case for the best perfor-mance (without any separate light harvesting strategy). Inthe nanoleaves only case although the carrier diffusion isbrisk the carrier recombination is also enhanced (signifi-cantly low carrier lifetime) due to higher surface area (possi-bly due to surface defects). Yet, much lower resistance to I�3diffusion allows higher efficiency as compared to the NP onlycase. In the case of the NP and NL mixture the dye loading isfound to be >50% higher than the case of NP films. More-over the DC resistance of diffusion of I�3 in the electrolyteis substantially lower, other cell parameters being nominallycomparable. This leads to the highest efficiency.

Acknowledgments

The authors from NCL, Pune gratefully acknowledgefunding for this work by the Department of InformationTechnology (Dr. K. Kumar and Dr. S. Chatterjee), Govt.of India. SBO and BH also acknowledge funding supportfrom CEFIPRA (Grant No. 3808-2).

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