Electrochimica Acta 152 (2015) 360–367

Novel Method to Improve Performance of Dye-sensitized Solar CellsBased on Quasi-solid Gel-Polymer Electrolytes

E.N. Jayaweera a,b, C.S.K. Ranasinghe a,b, G.R.A. Kumara b,*, W.M.N.M.B. Wanninayake a,c,K.G.C. Senarathne a,b, K. Tennakone d, R.M.G. Rajapakse a,b, O.A. Ileperuma a,b

a Postgraduate Institute of Science, University of Peradeniya, Peradeniya, Sri LankabDepartment of Chemistry, Faculty of Science, University of Peradeniya, Sri LankacDepartment of Physics, Faculty of Science, University of Peradeniya, Sri LankadDepartment of Physics, Georgia State University, Atlanta, USA

A R T I C L E I N F O

Article history:Received 25 August 2014Received in revised form 24 November 2014Accepted 25 November 2014Available online 26 November 2014

Keywords:Dye-sensitized solar cellsQuasi-solid gel-polymer electrolytesNanocrystalline TiO2

Conversion efficiencyIPCE

A B S T R A C T

This manuscript is concerned with the successful attempts we have made to circumvent the problemsassociated with I�/I3� redox couple-containing, ethylene carbonate (EC) and propylene carbonate (PC)-plasticized, polyacrylonitrile (PAN)-based gel polymer electrolyte used in dye-sensitized solar cells(DSCs). We identify the poor pore filling by a quasi-solid to be the major obstacle impeding theperformance of such DSCs. In the systematic study reported here, we have prepared four types of DSCs,(a) with only the redox couple containing plasticized gel-polymer electrolyte sandwiched between twoelectrodes, (b) same electrolyte but hot-pressed for the gel to better penetrate into the pores of the dyed,interconnected, nanocrystalline TiO2matrix, (c) pores filled with the usual liquid electrolyte (acetonitrilecontaining I�/I3� redox couple) but reducing the problems of volatile liquids by sealing the porescontaining the liquid electrolyte by pressed PAN gel electrolyte and (d) DSC with the usual liquidelectrolyte. The efficiencies of the DSCs from (a) to (d) are 4.1%, 5.2%, 8.4% and 9.8%, respectively. Theenhanced efficiencies in this order are clearly due to significant enhancements in the short-circuitphotocurrent densities of the cells. Our novel invention of (c) cells overcome the problems associatedwith DSCs based on quasi-solid state gel polymer electrolytes as well as those based on usual less viscousliquid electrolytes. The efficiencies of such former cells (c) are very close to those of the latter cells (d).This simple method can be universally adopted for all quasi-solid-state electrolyte-based DSCs in order toimprove their performance and durability.

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Electrochimica Acta

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1. Introduction

The Dye-sensitized Solar Cell (DSC) utilizing a mesoporous, TiO2

films was first reported by O’Regan and Grätzel in 1991. The devicehad an impressive power conversion efficiency, h of 7.1%–7.9% inAM 1.5 illumination and 12% in the diffuse daylight with more than80% IPCE (Hereinafter efficiencies reported are for 1.5 AMillumination unless otherwise stated) [1]. This solar cell iscomposed of a dye-coated, mesoporous TiO2 film attached ontoa conducting surface of fluoride-doped tin oxide (FTO) coated glassplate, a lightly platinized FTO counter electrode and a redoxelectrolyte. The working principal of the DSC is well known. Theelectrolyte containing a redox couple (e.g., I�/I3�) present in theDSC is responsible for several key roles. These include the transportof the redox species and establishing the cell potential. The

* Corresponding author.

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transport of the reduced species of the redox couple (I�) towardsthe oxidized dye molecules enabling the injection of an electronfrom I� to the oxidized dye molecule for the regeneration ofordinary dye molecules, and the transport of the oxidized speciesof the redox couple (I3�) towards the counter electrode to enablethe regeneration of the reduced species in the electrolyte, are alsoessential steps in the DSCs based on liquid electrolytes. Theconventional liquid electrolyte is composed of LiI, and I2 dissolvedin a polar organic liquid such as acetonitrile. The cations present inthe liquid electrolytes also play an important role in setting up ofthe Fermi level of the active electrode [2–4]. Intercalation ofcations such as Li+ in TiO2 particles is known to enhance theelectron transfer from the excited dye molecules to the conductionband (CB) of the semiconductor nanoparticles [2]. Although theless viscous organic liquids provide a fast ionic transport, theirvolatility at high temperatures, precipitation of salts at lowtemperature and the ability for leakage, hamper the practicalapplication. These problems have been well addressed and several

E.N. Jayaweera et al. / Electrochimica Acta 152 (2015) 360–367 361

solutions have been proposed though the proposed solutionsthemselves may have some other disadvantages. The use of non-volatile, ionic liquids in place of simple organic liquids is one suchattempt. In this sense, iodide salts of imidazolium cations havebeen used and the DSCs fabricated gave reasonably highconversion efficiencies [5–9]. These ionic liquids also offer theadvantages such as high thermal stability, wide electrochemicalwindow and high ionic conductivity although they too suffer fromthe disadvantages associated with liquids such as the ability forleakage. Yamanaka et al. have reported conversion efficiency of2.30% by introducing ionic liquid crystals of 1-dodecyl-3-imida-zolium iodide in place of the imidazolium iodide liquid [10]. Zhaoet al. used 1-methyl-3-hydroxymethyl imidazolium iodide as thecharge transport layer in ionic liquid crystal-based DSC andreported an efficiency of 3.10% [11].

Another approach to circumvent the problems of liquidelectrolytes has been to replace the liquid electrolyte by a quasi-solid polymer electrolyte, and a vast literature is available for use ofsuch electrolytes in DSCs. Wang et al. reported a quasi-solid-stateDSC with a polymer gel electrolyte giving an efficiency exceeding6% [12]. Plasticized solid polymer electrolytes offer a pseudo liquidmedium to provide a reasonably higher ionic transport whileretaining the cohesive properties of solids. The use of plasticizerssuch as ethylene carbonate (EC) and propylene carbonate (PC) withPEO in gel-polymer electrolytes used in DSCs has resulted inconversion efficiencies of 3.6% [13]. Using poly(acrylonitrile) (PAN)polymer with EC, PC and acetonitrile plasticizers and NaI/I2 redoxcouple gave 4.4% conversion efficiency at 30 mW cm�2 illumina-tion [14]. The replacement of smaller cations in iodide salts such asLi+ by bulky cations, such as tetra n-butylammonium or 1,2-dimethyl-3-propyl imidazolium, has enhanced the performancesof DSCs based on solid polymer electrolytes and efficiencies up to7.23% has been reported [14]. In this case, the relatively immobilebulky cations enhance the mobility of iodide and triiodide ionswithin the quasi-solid thus enhancing the solar cell performance.The best efficiencies reported, so far, for PAN-based DSCs is in therange from 6.0%–7.0%. A power conversion efficiency of 7.72% hasbeen reported for the DSCs based on novel needle-like polymer gelelectrolyte containing latent, chemically-cross-linked gel electro-lyte precursors by Wang et al. [15]. The use of ultra-thin poly(vinlydine fluoride-co-hexaflouopropylene) polymer membraneelectrolyte has resulted significantly higher efficiency of 8.35% [16].There is a very recent paper on the use of yet another very specialpolymer matrix called 3D polymer-network membrane (3D-PNM)in such solar cells by Park et al. [17], who have reported 9.1%efficiency for this quasi-solid-state solar cell.

One of the major drawbacks of solid electrolytes is the poorpore-filling, due to the difficulty for the penetration of the solidmaterial into the pores of the dye-coated, nanocrystallinesemiconductor particles. This leads to problems of dye regenera-tion and, consequently, enhancing the recombination, thuslowering the efficiency. Hot-pressing of the solid polymer filmcontaining the redox couple enhances the pore filling though not asmuch as that could be obtained by a liquid electrolyte. In this work,we have, therefore, studied three types of DSCs and fourth with theusual liquid electrolyte-based DSC. The first DSCs are based on poly(acrylonitrile) (PAN) polymer electrolyte containing I�/I3� in thepolymer matrix. In the second type of DSCs, the procedure of hot-pressing was employed where the PAN electrolyte containingI�/I3� redox couple is sandwiched between the two electrodes andheated to about 80 �C and pressed so as to fill the pores by the PANelectrolyte. Third types of cells were constructed by first filling thepores of the nanocrystalline TiO2 matrix with acetonitrile-basedliquid electrolyte containing I�/I3� and then covering the uppersurface with PAN electrolyte containing I�/I3� redox couplebetween the two electrodes. In this manuscript, we compare the

solar cell performance of the above four types of DSCs underidentical conditions. Our simple modification as in the third type ofDSC yielded the highest efficiency of 8.4% for such common andlow-cost polymer like PAN-based DSCs. This method can berecommended for any quasi-solid-state DSCs for their bettermentand we are currently investigating the applicability of this methodin other polymer-based DSCs also.

2. Experimental

2.1. Preparation of transparent colloidal TiO2 solution

Titanium tetraisopropoxide (Sigma Aldrich, USA, 97%)(20.0 cm3) and acetic acid (Wako Chemicals, Japan, 99.7%)(2.5 cm3) were mixed with ethanol (Hayman, England, 99.9%)(25.0 cm3) and steam was passed through the solution for 2 min.Rapid hydrolysis of titanium tetraisopropoxide and the expulsionof ethanol by steaming then produces a transparent solid massconsisting of TiO2 nanoparticles. This solid mass was ground with50.0 cm3 of water in a motor and subsequently autoclaved at 150 �Cfor 3 hours.

2.2. Deposition of the TiO2 film and dye coating

TiO2 suspension was prepared by mixing titanium colloidalsolution (20.0 cm3), acetic acid (5.5 cm3) and Triton X-100 (Sigma,USA, 99.5%) (5 drops) and it was further diluted by adding ethanol(20.0 cm3). The cleaned FTO glass plates were kept on a hot plateheated to 150 �C. Then, the above TiO2 suspension was spread onthe FTO glass plate by spraying using a purpose-built spray gun.These plates were sintered at 450 �C for 30 min, in air, and allowedto gradually cool down to about 80 �C. The warm plates were thenplaced in and kept soaked overnight in a 0.3 �10�3M solution ofthe di-tetrabutylammonium-cis-bis(isothiocyanato) bis(2,2’-bipyridyl-4,4’-dicarboxylate) ruthenium(II) (N719) dye in a 1:1 sol-vent mixture of acetonitrile and tertiary-butyl alcohol.

2.3. Preparation of PAN gel polymer electrolyte

A mixture of ethylene carbonate (Aldrich, USA, 98%) (0.525 g),polyacrylonitrile (Aldrich, USA) (0.225 g), propylene carbonate(Winlab, USA, 99.5%) (0.750 g) and Pr4N+I� (Alfa Aesar, England,98%) (0.132 g) were mixed at 80 �C until the mixture turns into aclear, homogeneous viscous gel. Then, iodine (Vickers laboratories,England, 99.5%) (0.012 g) was added and mixed well. Next the hotgel-electrolyte was pressed in between two clean glass plates andleft in a vacuum desiccator overnight to remove any adsorbedmoisture.

2.4. Preparation of Cr/Pt counter electrode

Cleaned FTO substrate was fixed inside the vacuum chamber ofthe sputtering machine. Two source guns were used to hold thechromium (Cr) and platinum (Pt) targets. Then under an ultra highvacuum, chromium sputtering was carried out for 30 min. todeposit a uniform, mirror-like Cr film with a thickness of 600 nm.Then, using the Pt target, sputtering was carried out for 3 min. toobtain a 20 nm thick Pt layer, on the Cr layer.

2.5. Fabrication of Dye-sensitized solar cells (DSCs) with differentelectrolytes

The dye-coated TiO2 electrode each was sandwiched withlightly-platinized, mirror-type chromium-coated FTO glass andfour DSCs were fabricated using different electrolytes. In the firstcell (a), the electrolyte used was a solid thin film of poly

0

5

10

15

20

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

(a)

(b)

(c)

(d)

Cell Potential/V

Phot

ocur

rent

Den

sity/

mA

cm-2

Fig. 2. I-V characteristics of the solar cells (a) FTO/TiO2/N719 dye/PAN polymerelectrolyte/Cr-coated, lightly platinized FTO (b) FTO/TiO2/N719 dye/hot-pressedPAN polymer electrolyte/Cr-coated, lightly platinized FTO and (c) FTO/TiO2/N719 dye/liquid electrolyte/PAN polymer electrolyte/Cr-coated, lightly platinizedFTO (d) FTO/TiO2/N719 dye/liquid electrolyte/Cr-coated, lightly platinized FTOunder AM 1.5 irradiation.

362 E.N. Jayaweera et al. / Electrochimica Acta 152 (2015) 360–367

(acrylonitrile) (PAN) containing Pr4N+I� and I2 and, therefore, thecell (a) is FTO/TiO2/N719 dye/PAN polymer electrolyte/lightlyplatinized mirror-type chromium-coated FTO glass. In the cell (b)the PAN film was subjected to hot-pressing at 80 �C to give the cellFTO/TiO2/N719 dye/hot-pressed PAN polymer electrolyte/lightlyplatinized, mirror-type chromium-coated FTO glass. In the cell (c),the pores of the TiO2 film was filled with the liquid electrolytecomposed of 0.1 M LiI, 0.05 M I2, 0.6 M dimethylpropylimidazoliumiodide and tertiarybutylpiridine in acetonitrile, the liquid phase onthe TiO2 surface was wiped off and a PAN gel electrolyte film wasplaced in the space between the two electrodes and pressed (Nothot-pressed but pressed at RT). The resulted cell (c) is abbreviatedas FTO/TiO2/N719 dye/liquid electrolyte/PAN polymer electrolyte/lightly platinized mirror-type chromium-coated FTO glass. Thefourth cell (d) is the usual FTO/TiO2/N719 dye/liquid electrolyte/lightly platinized mirror-type chromium-coated FTO glass. Aschematic representation of the DSCs (b) and (c) are shown in Fig.1.

2.6. Characterization

The corresponding IV characteristics for the DSCs, with anactive cell area of 0.25 cm2, were obtained under AM 1.5 illumina-tion using a solar simulator (PECcell-L01) coupled to a digitalsource meter (Keithley, 2400 model). The incident photon-to-current conversion efficiencies (IPCE) for the DSCs comprising thegel-polymer and liquid electrolytes were measured using a PVE300 (Bentham) and a TMc 300 (Bentham) monochromator-basedsolar cell evaluation system. Fig. 2 and Fig. 3 depict thecorresponding JV and IPCE spectra.

Impedance Spectroscopy (IS) studies were carried out in orderto investigate the electrochemical processes involved in DSCs.Fig. 4 depicts the impedance spectra obtained for the DSCsfabricated with (a) gel-polymer electrolyte, (b) hot-pressed gel-polymer electrolyte, (c) both gel-polymer electrolyte and liquidelectrolyte, and (d) liquid electrolyte, in the frequency range of100 kHz- 0.01 Hz. A 10 mV sinusoidal signal was used to obtainimpedance spectra in the dark under forward bias conditions.

Further, the Linear Sweep Voltammetric (LSV) measurementswere carried out, at a scan rate of 5 mV s�1, to determine theapparent diffusion coefficients (Dapp) of I3� ions in differentelectrolyte systems. Slow scan rates were employed in order toensure that the steady-state conditions were achieved. An FTOplate and a Cr-coated, lightly-platinized FTO plate were used as the

Glass

Dye -coated TiO 2

FTO

PAN gelelec trolyte

Liqu idelec trolyte

Glass

Dye -coa ted TiO 2

FTO

PAN gelelec trolyte

Unfill ed pore s

(a)

(b)

Fig. 1. A schematic representation of the DSCs (a) pore filled with liquid electrolyteand covered with the gel-polymer electrolyte and (b) with hot-press gel-polymerelectrolyte.

working and counter electrodes respectively. The IS and LSVstudies were conducted using a potentiostat (Autolab) equippedwith a frequency response analyzer.

3. Results and discussion

3.1. Photovoltaic characteristics and IPCE measurements of DSCs

Fig. 2 shows the JV characteristics of the solar cells fabricatedfrom (a) TiO2/N719 dye/PAN polymer electrolyte (b) TiO2/N719 dye/hot-pressed PAN polymer electrolyte (c) TiO2/N719 dye/liquid electrolyte/PAN polymer electrolyte and (d)TiO2/N719 dye/liquid electrolyte under AM 1.5 irradiation. Thelight-to-electricity conversion efficiencies of these solar cells are4.1%, 5.2%, 8.4% and 9.8% respectively. The lowest efficiency isobtained for the solar cell made with plasticized PAN electrolytecontaining Pr4N+I� and I2 (cell (a)). This is due to two reasons: poorfilling of pores of the interconnected nanoporous TiO2 particlematrix by the gel-polymer electrolyte and the sluggish transport ofredox species in the gel electrolyte compared to that in a lessviscous, liquid electrolyte. When the gel-polymer electrolyte issubjected to hot-pressing, as in cell (b), the gel electrolyte canpenetrate into the pores to some extent, giving a better pore fillingthan that without hot-pressing. This would help the efficientregeneration of the dye due to enhanced capability of electron

0

10

20

30

40

50

60

70

300 400 500 600 700 800 900

IPC

E (

%)

Wavelength/ nm

Fig. 3. The IPCE spectrum of FTO/TiO2/N719 dye/liquid electrolyte/PAN polymerelectrolyte/Cr-coated lightly platinized FTO solar cell.

Ω

Fig. 4. Impedance spectra of TiO2-based DSCs with (a) gel-polymer electrolyte, (b) hot-pressed gel-polymer electrolyte, (c) gel-polymer electrolyte and liquid electrolyte, and(d) liquid electrolyte under a bias potential of -0.68 V. The special feature corresponding to the Warburg diffusion is shown in the insets.

Table 1Photovoltaic parameters and IPCE values of TiO2-based DSCs employing differentelectrolyte systems

Electrolyte JSC/mA cm�2 VOC/V FF h(%) IPCE(%)at 532 nm

Gel-polymer 10.8 0.660 0.582 4.14 54Hot-pressed gel-polymer 13.0 0.655 0.614 5.23 57Gel-polymer with liquid 17.6 0.736 0.652 8.44 62Liquid 20.1 0.694 0.702 9.81 64

E.N. Jayaweera et al. / Electrochimica Acta 152 (2015) 360–367 363

injection from iodide ions to the oxidized dye molecules.Therefore, the efficiency of the cell made with hot-pressed PANelectrolyte is better by about 27% than that without hot-pressingthe PAN electrolyte. When the pores of the interconnected TiO2

matrix is filled with the liquid electrolyte and the PAN gelelectrolyte is placed above the liquid electrolyte-filled TiO2 matrix,as in cell (c), the efficiency is even higher, little more than twicethat of the cell (a) (from 4.1% to 8.4%). This is due to theimprovement of pore-filling achieved using the liquid electrolytethus providing better contact of I� ions to the oxidized dyemolecules. Further to this, the liquid-wetted back surface of thepolymer gel also provides a favorable medium for faster iontransport through the gel. When the liquid electrolyte alone is usedthe efficiency rises to 9.8%, which is a satisfactory efficiency forsuch a cell without reflecting particles to enhance repeated lightabsorption and dense layer of TiO2 on FTO to block therecombination at the FTO/TiO2/electrolyte triple-junction. Al-though the solar cell based on liquid electrolyte alone gives thehighest conversion efficiency, this cell is associated with typicalproblems such as the evaporation and leakage of the liquid, whichare well known impediments in such cells. The new concept usedhere is based on the fact that a better pore-filling has been obtainedby filling the pores with the liquid electrolyte but its evaporationand the leakage problems have been reduced by sealing porescontaining the liquid electrolyte with the pressed PAN polymer gelelectrolyte. This cell gives an efficiency of 8.4% and the decrementof the efficiency from the cell made with liquid electrolyte alone isonly 1.4%. This increase in the efficiency of the cells from (a) to (d) isclearly due to a similar increase in the photocurrent densities from10.8 mA cm�2 for the PAN cell, 13.0 mA cm�2 for the hot-pressedPAN cell, 17.6 mA cm�2 for the cell in which pores are filled with theliquid electrolyte but the working electrode is sealed using the PANgel polymer electrolyte, to 20.1 mA cm�2 for the cell fabricated

with liquid electrolyte alone. In the cell (a), the electrolyte onlytouches the upper surfaces of the TiO2 particle matrix and henceonly the geometric surface area is covered with the polymerelectrolyte. The dye molecules adsorbed on to the pore surfaces ofthe TiO2 matrix are not in contact with I� ions. Therefore, the onlymechanism for the reduction of oxidized dye molecules generatedin these regions is by the recombination with the electrons injectedinto the TiO2 particles. When the PAN gel polymer electrolyte ishot-pressed, the gel electrolyte penetrates into the pores to someextent, leading to better current density and, consequently, higherefficiency than that without hot-pressing. In the case of (c), theoxidized dye molecules are in much better contact with I� ionspresent in the liquid electrolyte. The higher current density andconversion efficiency of the cell (c) compared to those of cells (a)and (b) can, therefore, be explained by the efficient regeneration ofoxidized dye molecules with I� ions present in the liquidelectrolyte. The performance of the cell (d) is the best, since theliquid electrolytes provides not only better pore-filling but also afaster transport of ions, towards the desired sites when they aredepleted, than that is achieved by a quasi-solid state, polymer gelelectrolyte. Therefore, the cell made with pores filled by the liquidelectrolyte, but the evaporation and leakage of the liquid is

(b)

TiO2

CB

VB

FL

I-/I3-

D*/D+

D/D*

-1.0

-1.5

-0.5

+0.5

+1.0

0.0

+1.5

+2.0

VOC

+2.5

+3.0TiO2

CB

VB

FLI-/I3

-

D*/D+

D/D*

-1.0

-1.5

-0.5

+0.5

+1.0

0.0

+1.5

+2.0

(a)

+2.5

+3.0

E vs SHE/V E vs SHE/V

Fig. 5. Schematic energy level diagrams at (a) zero applied potential and (b) at an applied bias potential equal to VOC.

364 E.N. Jayaweera et al. / Electrochimica Acta 152 (2015) 360–367

prevented by sandwiching a PAN electrolyte between the twoelectrodes, has a reasonably high efficiency and a longer life-time.

These results are further confirmed by the IPCE values obtainedfor the DSCs. The corresponding photovoltaic parameters and theIPCE values for the DSCs employing different electrolytes aretabulated in Table 1. The IPCE obtained for the cells (a), (b), (c) and(d) are 54%, 57%, 62% and 64%, respectively, at 532 nm. The highestphotocurrent response is attained for the liquid electrolyte-basedDSC. The DSC comprising both gel polymer electrolyte and liquidelectrolyte gives an IPCE value of 62% at 532 nm, and above 40% inthe entire visible region as can be observed from Fig. 3. Thecorresponding current densities and IPCE values are in a goodagreement thus confirming the improved pore-filling obtained asthe liquid electrolyte is utilized together with the gel-polymerelectrolyte.

Fig. 6. Simplified transmission line equivalent circuit diagrams for a DSC forward biasepotential equal to VOC, in the dark, where, Rs is the total series resistance of the DSC, Zd(elecfilm, rr and Rr are the recombination resistance at the TiO2/electrolyte interface, Cm is thspecies in the electrolyte, Rpt is the charge transfer resistance at the counter electrode

3.2. Impedance spectroscopic analysis of DSCs

In order to acquire in-depth information on any material ordevice, from IS data, it is required to model and represent thecorresponding system using an equivalent circuit. As theorized byseveral research groups, the diffusion-recombination modelaccounts for the different chemical and electronic processesoccurring in a DSC, ascribing to the characteristic IS behaviorobserved [18,19]. Especially, comprehensive work by Bisquertet al. has provided an ideal theoretical framework for analyzing ISdata. In general, the equivalent circuit for a DSC comprises a largenumber of circuital elements connected in series and parallel.Thus, to reduce this complicated equivalent circuit to a simpletwo-channel transmission line model, the perturbation to thesystem is introduced via an applied forward bias potential, in the

d (a) at very low applied potentials (b) at intermediate potentials (c) at an applied) is the diffusion impedance of the electrons travelling through the mesoporous TiO2

e chemical capacitance, Zd is the diffusion impedance corresponding to the redox, and Cpt is the double layer capacitance at the Pt/electrolyte interface.

Table 2The calculated electron life time (te) values for DSCs comprising differentelectrolyte systems

Electrolyte Rr/V Cm� 10�3/F te/s

Gel polymer 102 2.17 0.221Hot-pressed gel-polymer 77.1 2.13 0.164Gel-polymer with liquid 55.2 2.84 0.156Liquid 52.6 2.16 0.114

E.N. Jayaweera et al. / Electrochimica Acta 152 (2015) 360–367 365

dark. Then, certain processes, which take place in an illuminatedDSC, i.e. the electron injection by the exited dye molecules to theconduction band (CB) of TiO2, regeneration of the dye by I� ionsand the recombination of photogenerated electrons with theoxidized dye, are eliminated simplifying the transmission lineequivalent circuit.

Hence, only the following reactions are considered to take placein a DSC, in the dark, under forward bias conditions. The appliedbias potential referred in here is negative with respect to thecounter electrode. As now the counter electrode is positive withrespect to the working electrode, it behaves as the anode, whereasthe dyed-mesoporous TiO2 film behaves as the cathode. Therefore,at the cathode, the externally supplied electrons to the FTO plateare injected to the conduction band of the TiO2, and are diffusedthrough the mesoporous TiO2 film.

e�FTO þ TiO2 ! e�CB (a)

Then, at the uncovered sites of TiO2, at the TiO2/electrolyteinterface, electrons recombine with the I3� ions to give I� ions.

I3� þ 2e�CB ! 3I� (b)

At the anode, I3� ions are reducedto give I� ions.

3I� ! I3� þ 2e� (c)

Therefore, these processes, which take place in the dark, undera bias potential, can be represented by IS data, as they areresponsible for the distinct features observed in the correspondingIS spectra, and can also be represented by the circuit elements ofthe equivalent circuit.

In the dark, the applied bias potential, being the key factor,determines the characteristic features of the impedance spectrum.The schematic energy level diagrams are shown in Fig. 5,illustrating the shift of the Fermi level upon the applied biaspotential. The corresponding energies are given with respect to thestandard hydrogen electrode (SHE).

At very low applied potentials, which are well below the bottomof the CB of TiO2, it fails to inject electrons to the CB of TiO2 thusensuing higher resistance of the TiO2 layer. Hence, the contributionof the mesoporous, TiO2 nanoparticulate network to the ISspectrum becomes negligible, and the charge recombination andthe capacitance of the back contact dominates the IS spectrum. Atmoderate bias potentials, the equivalent circuit becomes compli-cated as all the electrochemical processes which take place in theDSC leave their signature in the IS spectrum. Hence, all thecomponents of the equivalent circuit given in Fig. 6 (b) are neededto be taken in to account when determining the correspondingparameters from IS data. At higher bias potentials, which are closerto the open circuit potential (VOC) of the DSC, the complicatedtransmission line model (Fig. 6 (c)) simplifies, owing to the factthat resistance of the TiO2 becomes considerably small. In thisspecial situation, the observed three semicircles which are athigher, intermediate and lower frequency regimes correspond to(i) the charge-transfer process at the counter electrode, (ii)recombination resistance at the TiO2/electrolyte interface andthe chemical capacitance of the mesoporous TiO2 film, and (iii) thediffusion process of the redox species in the electrolyte,respectively. Further, if carefully examined, a straight line whichis inclined at an angle of 45�, can be identified at the higherfrequency region of the intermediate-frequency semicircle whichaccounts for the charge transport resistance associated with theinterconnected TiO2 particulate network [18–21]. In this work, thecorresponding IS spectra are obtained in the dark, under a forward

bias potential of -0.68 V, which is equivalent to VOC, so that thetransmission line equivalent circuit simplifies to that given in Fig 6(c).

Since the following equation accounts for the electron transportresistance in the mesoporous TiO2 thin film and the recombinationof electrons at the TiO2/electrolyte interface, it can be used todetermine the corresponding physical parameters as theorized byBisquert et al. [21].

ZðvÞ ¼ RtRr

1 þ iv�vrec

!1=2

coth Rt�Rr

� �1=21 þ iv

.vrec

� �1=2" #

(1)

Where, Rt is the charge transport resistance in the mesoporousTiO2 films, Rr is the recombination resistance at the TiO2/electrolyte interface, vrec is the characteristic frequency for therecombination and v is the angular frequency.

Further, the correlation between the two competing factors Rtand Rr can be expressed as,

vd

vrec¼ Rr

Rt¼ Ln

L

� �2

(2)

Where, vrec is the characteristic frequency for the recombination,vd is the characteristic frequency for the electron diffusion in theTiO2 film, Ln is the electron diffusion length and L is the active filmthickness.

For a DSC to yield reasonable power conversion efficiency, Rrshould be greater than Rt, ensuring efficient electron collection atthe FTO surface. This suggests that the electron life time in the TiO2

film should be higher than the electron transport time, through themesoporous TiO2 film. The corresponding electron life time, te, canbe calculated using Eq. (3).

te ¼ 1vrec

¼ RrCm (3)

Where, vrec is the characteristic frequency for the recombination,Cm is the chemical capacitance and Rr is the recombinationresistance.

Also, the charge transport time td through the mesoporous TiO2

film is given by Eq. (4).

td ¼ 1vd

¼ RtCm (4)

Where, vd is the characteristic frequency for the electron diffusionin the TiO2 film, Cm is the chemical capacitance and Rt is the chargetransport resistance in the mesoporous TiO2 films.

The corresponding Ln te and td values for the DSCs comprisingdifferent electrolyte systems, calculated using Eqs. (2)–(4), aretabulated in Table 2 and Table 3.

The semicircle appeared in the low frequency regime in theNyquist plot corresponds to the Nernst diffusion impedance, ZN, ofthe redox species in the electrolyte. By applying Fick’s law and theappropriate boundary conditions, the mathematical expression forthe Nernst diffusion impedance, ZN, can be simplified to,

ZN ¼ Zoffiffiffiffiffiffiiv

p tanhffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiivtdðelÞ

q(5)

Table 3The calculated charge transport time (td) and diffusion length values for DSCscomprising different electrolyte systems

Electrolyte Rt/V Cm� 10�3/F td/s Ln/mm

Gel polymer 9.87 2.17 0.021 13.6Hot-pressed gel-polymer 11.1 2.13 0.024 11.9Gel-polymer with liquid 9.5 2.84 0.027 10.2Liquid 13.5 2.16 0.029 8.37

Table 4Calculated diffusion coefficients of I3� in different electrolytes

Electrolyte Dapp� 10�6 /cm2 s�1

Gel polymer 0.98Gel-polymer with liquid 2.03Liquid 19.6

366 E.N. Jayaweera et al. / Electrochimica Acta 152 (2015) 360–367

Where, v is the angular frequency, Zo is the Warburg parameterand td(el) is the characteristic diffusion time constant. Further, Zoand td are given by the following equations.

Zo ¼ RT

n2F2CoAffiffiffiffiD

p (6)

td elð Þ ¼ d2.

D(7)

Where R is the gas constant, T is the absolute temperature, F is theFaraday constant, Co is the bulk concentration of I3� ions, A is theelectrode area, D is the diffusion coefficient of the redox species, nis the number of electrons transferred per reaction event in eachreaction (n = 2) and d is the thickness of the Nernst diffusion layer(d = 0.5del;del is the distance between the two electrodes). Since theconcentration of I� ions in the electrolyte is higher than that of theI3� ions, the diffusion impedance is determined by the diffusion ofI3� ions. The diffusion coefficient of I3� ions, in different electro-lytes, can be calculated using Eqs. (7) and (8).

tdðelÞ ¼1

vdðelÞ(8)

Where, vd(el) is the characteristic frequency for diffusion of theredox species in the electrolyte.

3.3. Linear Sweep Voltammetric (LSV) studies

Diffusion of the ions, I� and I3�, contributes to the currentthrough the electrolyte. If the diffusion coefficients of the redoxspecies are of the same order, the limiting current corresponds tothe diffusion of ions with low concentration, through theelectrolyte. As the concentration of I� ions is considerably higherthan that of I3� ions, the limiting current is determined by the

-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0

-1.0x10-4

-5.0x10-5

0.0

(a) Gel-p olymer elect rolyte(c) Gel-p olymer an d li quid electrol ytes(d) Liquid electrol yte

Potential/V

Cur

rent

/A

Fig. 7. LSV measurements for TiO2-based DSCs with (a) gel-polymer electrolyte, (c)gel-polymer electrolyte and liquid electrolyte, and (d) liquid electrolyte.

diffusion of I3� ions. The following equation was used to calculatethe apparent diffusion coefficients (Dapp) of I3� ions in gel-polymerelectrolyte, (b) gel-polymer electrolyte and liquid electrolyte, and(d) liquid electrolyte [22,23].

Jlim ¼ 2nFCI�3 DI�3

l(9)

Where, F is the Faraday constant, Jlim is the limiting current densityfor the reduction of triiodide ions in the electrolyte, DI3

� is thediffusion coefficient of the triiodide ions,n(n = 2) is the number ofmoles of electrons involved per mole of I3� ions reduced, CI3

� is themolar concentration of the I3� ions considering that all the I2 addedto the electrolyte is converted to I3� and l is the distance betweenthe two electrodes.

The LSV spectra obtained for the different electrolyte systemsare shown in Fig 7. To determine the diffusion coefficient of I3�

ions, when the liquid and the gel-polymer electrolytes are usedtogether, the average concentration of I3� ions was calculated byconsidering the concentration of I3� ions in each electrolyte. Thediffusion coefficient of I3� ions, in different electrolytes, calculatedusing LSV data is listed in Table 4.

As evident from IS studies, the recombination resistance ishigher in the gel-polymer electrolyte-based DSCs than those in theDSCs employing hot-pressed gel-polymer electrolyte, both liquidand gel-polymer electrolyte, and liquid electrolyte. But at the sametime the LSV data reveals that diffusion coefficient of I3� is low ingel-polymer electrolyte compared to other electrolytes. Thus,despite the larger recombination resistance possesses by the gel-polymer electrolyte, low diffusion coefficient of I3� hinders theperformance of the gel-polymer electrolyte-based DSC, resulting inlower power conversion efficiencies. Hence, it can be deduced thatthe advantage of having a large recombination resistance isdiminished by lower diffusion coefficient of I3� in the electrolyte.The recombination resistance of DSCs decreases as, cell (a) > cell (b)> cell (c) > cell (d) whereas, the diffusion coefficients of I3� in theelectrolytes decreases in the order of liquid electrolyte, liquid andgel-polymer electrolyte together, and gel-polymer electrolyte.Consequently, the highest power conversion efficiency is obtainedfor the liquid electrolyte-based DSC while the second highest, thirdhighest and the lowest power conversion efficiencies are obtainedfor both liquid and gel-polymer electrolyte-based DSC, hot-pressedgel-polymer electrolyte based DSC, and gel-polymer electrolyte-based DSC respectively.

4. Conclusion

In this work, a novel method has been proposed to harness theadvantages of low-viscous, liquid electrolytes and quasi-solidstate, polymer gel electrolytes, by utilizing both together tofabricate a DSC which is largely stable and leak proof. In order tocompare the properties and performance of such DSCs with theDSCs employing liquid or gel-polymer electrolyte, four differenttypes of DSCs have been fabricated. The light-to-electricityconversion efficiencies of these DSCs employing gel-polymerelectrolyte, hot-pressed gel-polymer electrolyte, both liquid andgel-polymer electrolyte, and liquid electrolyte are 4.1%, 5.2%, 8.4%and 9.8%, respectively, under AM 1.5 illumination. The novelinvention, which is more suitable for long-term practical

E.N. Jayaweera et al. / Electrochimica Acta 152 (2015) 360–367 367

applications, is superior to the first two by 105% and 62%,respectively, but inferior to usual liquid electrolyte-based DSCsonly by 16.6%. These novel cells give an IPCE of 62% at 532 nm, andabove 40% in the entire visible region where as the other DSCscomprising gel-polymer electrolyte, hot-pressed gel-polymerelectrolyte, and liquid electrolyte gives IPCE of 54%, 57%, and64% at 532 nm respectively. Further, the calculated recombinationresistances of DSCs and diffusion coefficients of I3� ions inrespective electrolyte systems substantiate the power conversionefficiencies obtained for the DSCs employing different electrolytes.

Acknowledgments

This work was supported by the National Science Foundation ofSri Lanka (NSF/Fellow/2011/02).

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