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nnCorresponding aGwangju 500-712, S

E-mail addresses1These authors co

RAPID COMMUNICATION

Statistical TiO2/dye-mass dependenceand dye-regeneration efficiencyon dye-sensitized solar cells

Kyung-Jun Hwanga,1, Yonkil Jeongb,1, Chulmin Choia,Young Jin Kimc, Gunwoo Kimc, Yong-Kook Choid, Sungho Jina,c,n,Dong-Won Parkb,nn

aDepartment of Mechanical and Aerospace Engineering, University of California, San Diego (UCSD),La Jolla, CA 92093, USAbResearch Institute for Solar and Sustainable Energies, Gwangju Institute of Science and Technology(GIST), Gwangju 500-712, South KoreacMaterials Sciences and Engineering, University of California, San Diego (UCSD), La Jolla,CA 92093, United StatesdDepartment of Chemistry, Chonnam National University, Gwangju 500-757, South Korea

Received 28 May 2015; received in revised form 16 July 2015; accepted 17 July 2015Available online 28 July 2015

KEYWORDSDye-sensitized solarcells;TiO2;Dye-mass;Charge-transferkinetics;Dye-regenerationefficiency

0.1016/j.nanoen.2lsevier Ltd. All rig

thor at: Departm

uthor at: Researcouth Korea.: jin@ucsd.edu (S.ntributed equally

AbstractDye-sensitized solar cells (DSCs) were statistically investigated on TiO2 thickness and dye-massdependence. The DSC devices exhibited a trend of performance saturation from an approxi-mately 15 μm TiO2 thickness with 10�7 mol/cm2 dye-mass, which was estimated to be amaximum engineering condition required for effective charge-transfer kinetics in this investi-gation. The prominent factor for power conversion efficiency was increase of short-circuitcurrent density by decrease of charge-transfer resistance with a heavy load of dye-mass. Thedye-regeneration efficiency evaluated from a minimum electron density and a transferredcounterpart ionic carrier density was approximately ranged between 1.5 and 3.0%.& 2015 Elsevier Ltd. All rights reserved.

015.07.016hts reserved.

ent of Mechanical and Aerospace Engineering, University of California, San Diego (UCSD), La Jolla,

h Institute for Solar and Sustainable Energies, Gwangju Institute of Science and Technology (GIST),

Jin), widipark@gist.ac.kr (D.-W. Park).to this work.

K.-J. Hwang et al.384

Introduction rinsed with absolute ethanol. Two pin-holes for injecting the

Dye-sensitized solar cells (DSCs) based on nanocrystallineTiO2 have attracted attention owing to their low-costmanufacturability, simple fabrication, available building-integrated photovoltaics, and robust thermal stability [1–7]. Typically the DSCs have the device configuration ofglass/transparent conducting oxide (TCO)/nanoporous TiO2

film/dye molecules adsorbed onto the TiO2 surface/redoxcouple electrolyte I� =I�3

� �counter electrode such as pla-

tinum (Pt) or carbon deposited on TCO glass substrates.Here, because the TiO2 film thickness, the TiO2 particleproperties, and the adsorbed dye-mass are particularlycritical to the DSC performance of relevance to electrondiffusion and transport, effectiveness and optimization ofthe TiO2 film thickness have been extensively studiedthrough an understanding on charge-transfer kinetics andcharge photodynamics, which is related with charge recom-bination and dye regeneration lifetimes [8–11]; the dye isregenerated by the process that an electron excited fromthe dye molecules is transferred into the TiO2 conductionband and followed by the oxidized dye, forming a N719 dyecation radical (N719 dye+�) is subsequently reduced byelectron donation from the electrolyte containing I�. Theoxidized dye may also recombine with electrons injected inthe TiO2 surface, which causes the charge recombinationprocess [12–14]. An in-depth study of charge-transferkinetics and charge photodynamics in interface betweensolid and liquid is still an ongoing and interesting issue forphotoelectrochemistry. A statistical evaluation of deviceperformance according to the TiO2 film thickness and dye-mass is also necessary in a point of view of reviewing and itis to promptly meet the scientific needs for a minimumrequirement of device engineering.

In this work, we statistically investigated and reviewedthe DSCs performance according to the TiO2 film thicknessand the dye-mass loading, including the dye-regenerationefficiency by a study of charge-transfer kinetics usingelectrochemical impedance spectroscopy (EIS).

Figure 1 The statistical plots of the DSC performancesaccording to the TiO2 film thickness and the dye-mass.

Experiments

A fluorine-doped tin oxide (FTO; TEC 8, Pilkington) conduct-ing glass substrate was first cleaned in acetone using anultrasonic bath for 15 min, rinsed with 95% ethanol (Aldrich)and water, and finally dried in an oven at 80 1C for 1 h. Amask, with an open area of 0.5 cm� 0.5 cm was placed onthe FTO glass for photoelectrode deposition. The homemadeTiO2 paste (with an average TiO2 particle size of 15 nm)[15,16] was placed in the mask opening using the screenprinting technique. The FTO substrate with screen-printedTiO2 paste was stored in a clean box saturated with absoluteethanol (Aldrich) for approximately 10 min to reduce thesurface irregularity of the coated paste. The thickness of theTiO2 film was controlled by repeated cycles of screen printingfollowed by drying at 120 1C for 5 min (see Figures S1 and S2).The fabrication of the TiO2 thin film was completed bysintering at 500 1C for 30 min. The TiO2 film was then soakedfor 24 h at room temperature in a cis-RuII(dcbpy)2(NCS)2 dyesolution (0.3 mM N719 dye, Solaronix). After preparing thephotoelectrode, excess dye molecules on the TiO2 film were

electrolyte were drilled into the rinsed FTO glass using adrilling machine (DREMEL 4200, Robert Bosch Tool). The Ptcounter electrode was prepared by depositing a Pt solution(0.05 M H2PtCl6, Aldrich) onto the FTO glass via spin coating(ACE-1020, Dong Ah Trade) at 3000 rpm for 60 s and then,annealing at 400 1C for 30 min in air. The prepared photo-electrode and the counter electrode were sealed with a hot-melt sealing sheet (SX 1170-100, Solaronix) with a thicknessof 100 μm. The DSC device with an active area of 0.25 cm2

was completed by injecting a redox electrolyte consisting of0.6 M 1-methyl-3-propylimidazolium iodide (PM II, Solaronix),0.03 M I2 (Aldrich), 0.1 M guanidinium thiocyanate (GuSCN,Aldrich), and 0.5 M 4-tertbutylpyridine (TBP, Aldrich) in amixture of anhydrous acetonitrile (Aldrich) and 99.5% valer-onitrile (Aldrich) (v/v=85:15). The TiO2 film thickness wasmeasured using a stylus profiler (XP-100, Ambios Technology).The morphologies and thickness of TiO2 film were identifiedusing field-emission scanning electron microscopy (FE-SEM;S-4700, Hitachi). The adsorption capacity of the TiO2 film forN719 dye was estimated by complete dye desorption fromthe TiO2 electrodes using 0.1 M NaOH solution/anhydrousethanol (50/50 vol%, Aldrich) and analysis of desorptionconcentration using UV/vis spectrometry (UV-210, Shi-madzu). The current density–voltage (J–V) curves weremeasured under the standard 100 mW/cm2 irradiation(1-sun) using a source-measure unit under white-light irra-diation from a 450 W xenon lamp (Thermo Oriel Instrument).Before measurement, the light intensity was calibrated byusing a Si reference diode equipped with an AM 1.5G filter(KG-5). The external quantum efficiency (EQE) was measuredusing an incident photon to current efficiency system in thewavelength range from 300 to 900 nm (QEX7, PV Measure-ments). The electrochemical impedance spectroscopy (EIS;Versastat4, Princeton Applied Research) measurements wereperformed using the a.c. impedance in the frequency rangeof 1 to 106 Hz with amplitudes of75 mV.

Results and discussions

Figure 1 shows the statistical DSC device performancesaccording to the TiO2 film thickness and the loaded dye-mass. The devices exhibit a trend of performance saturationfrom the approximately 15 μm TiO2 thickness with 10�7

385TiO2dye-mass dependence and dye-regeneration efficiency

mol/cm2 dye-mass, which has the performance with a short-circuit current density (Jsc) of 17.3 mA/cm2, an open-circuitvoltage (Voc) of 0.732 V, a fill factor (FF) of 66.7%, and apower conversion efficiency (η) of 8.45%. According to theTiO2 film thickness and the dye-mass, the Voc and the FF arelinearly changed, whereas the Jsc and η are logarithmicallychanged and saturated almost at 15 μm. The η is sensitive tothe Jsc change, which is dominated by the dye-mass. Thedye-mass and the device performance according to the TiO2

film thickness were selectively investigated using sixsamples. In this study, the TiO2 films were denoted DSC-Xseries such as DSC-1 (2.5 μm), DSC-2 (5 μm), DSC-3 (7.5 μm),DSC-4 (10.0 μm), DSC-5 (12.5 μm), and DSC-6 (15.0 μm),respectively.

Figure 2(a) shows the optical transmission spectra of thedye adsorbed TiO2 films as a function of thickness. Over theTiO2 film thickness of 12.5 μm, the transmittance of dyeadsorbed TiO2 films approaches to zero in the visible regionbelow 600 nm. This is attributed to an increase of availableTiO2 surface area for the adsorbed dye-mass (D), whereas theadsorbed dye concentration (Cdye) is almost constant to0.6570.3 mol/μm3 as shown in Figure 2(b). The Cdye has ahomogeneous distribution in the overall TiO2 film volume.The behavior of D determines the overall behavior of Jsc. Thevalue of D was determined using the following relation [17]:

D¼ C0�Cð Þ � VA

ð1Þ

Figure 2 The UV/vis transmittance spectra of N719 dyeadsorbed TiO2 films (a), and the behaviors of the calculated Dand Cdye (b) according to the TiO2 film thickness.

where D (mol/cm2) is the equilibrium amount of N719 dyeadsorbed onto the TiO2 film, C0 and C (mol/L) are the initialand equilibrium concentrations of the dyes, V (L) is thevolume of the solution, and A (cm2) is the cut end sectionalarea of the TiO2 film used. The value of Cdye was defined bythe following relation [18]:

Cdye mol=μm3� �¼D �10�8 mol=cm2� �

Hfilm μmð Þ ð2Þ

where Hfilm is the TiO2 film thickness.Figure 3 shows the J–V characteristic curves (a) and the

external quantum efficiency (EQE) curves (b) of the DSCdevices according to the TiO2 film thickness and the loadeddye-mass. The trend of the Jsc change is consistent with thatof the change of the TiO2 film thickness and the loaded dye-mass. In Figure 3(b), at 520 nm wavelength, the maximumEQE peaks of the sample devices are almost saturated abovethe TiO2 film thickness of 12.5 μm, whereas it is observablethat the EQE curves consistently increase in the longwavelength region due to a uniform charge generationthroughout the TiO2 film [19,20].

Figure 4 shows the EIS characteristic curves (a) and thecalculated the charge-transfer parameters (b) of the DSCdevices according to the TiO2 film thickness and the

Figure 3 The J–V (a) and the EQE characteristic curves (b) ofthe DSC devices according to the TiO2 film thickness and thedye-mass.

Figure 4 The EIS characteristic curves (a) and the behaviors ofcharge-transfer kinetics (b) according to the TiO2 film thicknessand the dye-mass. The inset shows the behaviors of eachresistive component as a function of TiO2 thickness.

Figure 5 The suggested dye-regeneration mechanism.

K.-J. Hwang et al.386

dye-mass. The inset shows the behaviors of each resistivecomponent as a function of TiO2 thickness. The EIS plotsexhibit the typical behaviors of the charge-transfer resis-tance (RCE) at the Pt/electrolyte interface in the highfrequency region, the charge-transfer resistance (RPE) atthe TiO2/dye/electrolyte interfaces in the middle frequencyregion, and the Nernstian diffusion (RD) within the electro-lyte in the low frequency region. The resistance componentRS can be related to the series resistance of the cell, whichinvolves the sheet resistance of the FTO, resistivity of theelectrolyte and electrical contacts of the cell [21,22]. Asshown in the inset figure, all DSCs display similar values ofthe RS (close to 9.670.4) but RPE decreases rapidly with theincrease of film thickness while the other resistance com-ponents maintained with a small change. The thicker TiO2

film obviously reduces the charge-transfer resistance at theTiO2/dye/electrolyte interfaces in the middle frequencyregion. In other words, the thicker TiO2 film reduces backelectron transfer (recombination) probability to I�3 inelectrolytes. The charge-transfer properties according tothe TiO2 film thickness and the dye-mass can be quantita-tively evaluated using an analysis of charge-transfer kineticsas shown in Figure 4(b). The charge-transfer lifetime (τct)

can be roughly calculated using the relation of τct=1/fp,where the peak frequency (fp) is the characteristic fre-quency in the middle frequency region [23,24]. A number ofN719 dye+� regeneration (G) is estimated using a one-sidedabrupt junction approach of current with J�qneμeE, whereq is elementary charge, ne is minimum electron densityequivalent to the G, μe is electron mobility of the TiO2 filmwith approximately 1 cm2/V s [25], and E is electric fieldcreated in a space charge region (SCR) of the TiO2 film. TheE is roughly estimated by the information; the SCR width inthe TiO2 film is known to be approximately 50 nm [26,27],where an electrical double layer between TiO2 surface andelectrolyte is not considered because of a negligible spaceless than nanometer level. The potential difference isreplaced by the experimentally obtained Voc of averagely0.75 V in the SCR. The deduced E becomes 1.5� 105 V/cm.Under the short-circuit condition (Jsc), the G is obtained asshown in Figure 4(b). The dye-regeneration depends on anumber of the transferred counterpart ionic I� carrier (T),which can be estimated by electrical equilibrium condition.The interface between semiconductor and electrolyteshould satisfy the relation of qneμeE=qnctμctE, where theqnctμctE is current density in terms of the counterpart ionicI� movement in the electrolyte. This condition induces thesimple relation of neμe=nctμct. The charge-transfer mobility(μct) can be calculated by the following approaches: theionic diffusion coefficient (Dct) is roughly obtained using therelation of Dct=d2/τct, where the d is a mechanical distance(d) between TiO2 surface and Pt counterpart electrode withapproximately 40 μm, and followed by the Einstein relationof μct=qDct/kT, where the kT/q is thermal voltage[10,23,24]. The obtained nct is equivalent to the T. Thedye-regeneration efficiency (G/T) is evaluated to beapproximately 1.5–3.0%. Fundamentally, this low efficiencyis attributed to be a large difference of the mobility ofcharge carriers. This is discussed in the next section.

Figure 5 displays the suggested dye-regeneration mechan-ism. Photon (solar light) is equivalent to the electromagneticwave (EMW) and its alternative electric and magnetic fieldsinduce the negative charge buildup to the opposite directionof photon incidence through the absorption. Here excitedelectrons which donate the conduction current are equivalentto the Jsc and they recombine only with the ionic carriersadjacent to the Pt counter electrode, whereas at the sametime, many electrons will repeat excitation and confinementin the dye due to an ambiguity of the ionic carrier position; ifmany ionic carriers are adjacent to the Pt counterpart

387TiO2dye-mass dependence and dye-regeneration efficiency

electrode, many electrons will be excited and leave from thedye (dye+�) and the many recombination will be processed aswell as the Jsc will be increased. The current is continuousphysical quantity and it should be the same in the interfacebetween semiconductor and electrolyte. Eventually, the cur-rent is limited by the low mobility ionic carriers, which can beone of the reasons of no more current gain despite of dye-massincrease. By the electrical equilibrium condition, the dye+� inthe interface is small in number compared with the number ofthe negatively charged ionic I� =I�3

� �carriers and the dye-

regeneration rate is proportional to the number of dye+�.

Conclusions

We statistically investigated and reviewed the deviceperformance on the TiO2 film thickness and the dye-massloading, and studied charge-transfer kinetics using anelectrochemical impedance spectroscopy (EIS). In our inves-tigation, the 15 μm TiO2 film thickness with an approxi-mately 10�7 mol/cm2 dye-mass exhibited the optimizeddevice performance. The overall trend of device efficiencywas dominantly controlled by the behavior of currentincrease with the dye-mass. The EIS was directly evidencedfor the reduction of charge-transfer resistance as well asused for the analysis of the charge-transfer kinetics. Thedye-regeneration efficiency was evaluated by the calcula-tion of a number of N719 dye+� regeneration (G) and anumber of transferred counterpart ionic I� carrier (T). Theefficiency (G/T) was evaluated to be approximately 1.5–3.0%, which low efficiency was considered to be caused by alarge difference of the mobility of charge carriers.

Acknowledgments

This research was supported by Basic Science Research Programthrough the National Research Foundation of Korea (NRF)funded by the Ministry of Education (2013R1A6A3A03021144).

Appendix A. Supporting information

Supplementary data associated with this article can befound in the online version at http://dx.doi.org/10.1016/j.nanoen.2015.07.016.

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Dr. Kyung-Jun Hwang received his Ph.D.degree in chemical engineering from Cho-sun University in 2011. After postdoctoralwork at the Korea Electronics TechnologyInstitute, he is working as postdoctoralscholar in Department of Mechanical andAerospace Engineering, UC San Diego. Hisresearch field is optical materials that areresponsive to environmental light excita-tions, and synthesis of nanoscale semicon-

ductors for solar energy harvesting and energy related applications.His current research addresses low cost issue of perovskite solarcells as well as dye-sensitized solar cells by eliminating the use ofexpensive and high electrical resistance fluorinated tin oxide glass.For this, he has been fabricating the micro- or nano-patternedmetal mesh electrode for highly efficient, large-scaled solar cell.

Dr. Yonkil Jeong received his Ph.D. inSchool of Materials Science from JapanAdvanced Institute of Science and Technol-ogy, Japan, in 2008. From 2008 to 2009, hejoined a development group of microscopein JAIST as a post-doctoral researcher. In2009, he moved to Research Institute forSolar and Sustainable Energies, GwangjuInstitute of Science and Technology, Koreaas a research scientist. His research field is

compound semiconductor photovoltaic devices. In particular, he iscarrying out developments of III-V compound semiconductor solar

K.-J. Hwang et al.388

cells for concentrated photovoltaics and Cu(In,Ga)Se2 / Cu2ZnSn(S,Se)4 thin film solar cells. His recent interest is an energy balancebetween electronic and ionic devices.

Dr. Chulmin Choi received his Ph.D. degreein Material Science and Engineering fromUniversity of California at San Diego (UCSD)in 2010. He is currently a postdoctoralfellow in Department of Mechanical andAerospace Engineering (MAE) at the UCSD.His main research interests focus on nanos-tructure design and fabrication for newenergy materials related to nano-patternedmetal mesh electrode for highly efficient

large-scaled solar cell, selective infrared reflectance materials forsmart windows on energy saving building and super-hydrophobiccoating solution for self-cleaning substrate and smart cloth.

Young Jin Kim is a Ph.D. student in Materi-als Science and Engineering program atUniversity of California San Diego (UCSD).Prior to enrolling the Ph.D. program, hereceived his B.S. in Mechanical Engineeringfrom UCSD. His current research interestsare focused on synthesis, characterizationof thermo chromic materials for energysaving smart buildings.

Gunwoo Kim is currently a graduate stu-dent in Materials Science and Engineering atUniversity of California, San Diego (UCSD).His research is focused on synthesis andcharacterization of polymer, inorganicnanoparticles, and nano sized structuresfor energy saving devices. He studied Bio-molecule and Physical Chemistry at TokyoInstitute of Technology (2003-2007) forBachelors degree, and Biophysics at Univer-

sity of Tokyo (2007-2009) for Masters degree. Prior to enter UCSD,he was a research engineer at Samsung Electro-Mechanics in Suwon,South Korea (2009-2013).

Prof. Yong-Kook Choi is a Professor in theDepartment of Chemistry at ChonnamNational University (CNU), Korea. Hereceived his Ph. D. degree from ChonbukNational University in Chemistry in 1985. Hejoined CNU in 1992 and his current researchinterest includes electrode materials andinterface reaction for energy device. He haspublished over 100 articles until now.

Prof. Sungho Jin received his Ph.D. inMaterials Science & Engineering from UCBerkeley in 1974. After joining Bell Labora-tories at Murray Hill, New Jersey, he carriedout forefront materials and devicesresearch for more than 26 years. He thenjoined UC San Diego (UCSD) in 2002 as aProfessor and Iwama Endowed Chair, alsoserving as the Director of UCSD-wide Mate-rials Science & Engineering Program, and is

now a professor emeritus at UCSD. His research interests andactivities include R&D of nano materials, magnetic materials,electronic materials and devices, energy materials, and bio materi-als. He is a member of the US National Academy of Engineering(elected in 1999), Fellow of American Physical Society (2003),Fellow of American Society for Metals (1994), TMS Fellow (2000),Inaugural MRS Fellow (2008). He received various awards includingJohn Bardeen Award (2007), Nano 50 Award (2005), Ho-Am Engi-neering Prize (2000), Albert-Sauveur Achievement Award (2009),and CRS Jorge Heller JCR Award (2012).

Dr. Dong-Won Park received his Ph.D.degree in chemistry from Chonnam NationalUniversity in 2007. After postdoctoral workat the Korea Atomic Energy Research Insti-tute, he worked as senior researcher atResearch Institute for Solar and SustainableEnergies, Gwangju Institute of Science andTechnology (2009-2013). Currently, he isworking as a senior scientist at Institute ofEngineering Thermodynamics, German

Aerospace Center in Germany. His current research interests arefocused on synthesis, characterization, and modeling of newelectrode architectures/materials for energy conversion andstorage.