Dye Sensitized Solar Cells for Economically Viable PhotovoltaicSystemsHyun Suk Jung and Jung-Kun Lee*

Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, UnitedStates

School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon, Korea

ABSTRACT: TiO2 nanoparticle-based dye sensitized solar cells(DSSCs) have attracted a significant level of scientific and technologicalinterest for their potential as economically viable photovoltaic devices.While DSSCs have multiple benefits such as material abundance, a shortenergy payback period, constant power output, and compatibility withflexible applications, there are still several challenges that hold back largescale commercialization. Critical factors determining the future of DSSCsinvolve energy conversion efficiency, long-term stability, and productioncost. Continuous advancement of their long-term stability suggests thatstate-of-the-art DSSCs will operate for over 20 years without a significantdecrease in performance. Nevertheless, key questions remain in regardsto energy conversion efficiency improvements and material costreduction. In this Perspective, the present state of the field and theongoing efforts to address the requirements of DSSCs are summarized with views on the future of DSSCs.

A dye-sensitized solar cell (DSSC) is a photoelectrochem-ical cell whose device physics is different from traditional

p−n junction type solar cells.1 In DSSCs, a thin layer of dyemolecules that is coated on the surface of the mesoporous oxidesemiconductor films functions as a photosensitizer. The dyemolecules absorb incoming photons and generate electron−hole pairs. These electrons are then quickly injected into theconduction band of the semiconductor and a redox couple inthe electrolyte later regenerates the dye molecules.2

DSSCs offer significant economic and environmentaladvantages over conventional photovoltaic devices becausethey can be manufactured relatively inexpensively, and in anenergy-efficient and environment-friendly manner. In addition,due to their unique electrochemistry and physics, DSSCsexhibit several features, which are not available in other types ofthe solar cells. Compared with Si solar cells, the maximumpower of DSSCs is less dependent on ambient temperatureand, furthermore, the drop of the energy conversion efficiency

between 20 and 50 °C is negligible.3 These features enableDSSCs to produce more power in outdoor applications thanother solar cells, although their efficiencies are similar at roomtemperature. The performance of DSSCs increases at a lowerlight intensity (<100mW/cm2). At one-third normal sunlight(∼ 50mW/cm2), which is a typical lower-light, real-worldcondition, the DSSC module shows an efficiency of 7.48%.4 Inaddition, the change in the light incident angle has a smallereffect on the efficiency of DSSCs. Consequently, they providemore stable power output over the course of a day, comparedto Si solar cells. Moreover, their transparency and color can becontrolled for the aesthetic integration into a variety of surfaces.This is beneficial when producing building-integrated photo-voltaic (BIPV) products, in particular for replacement ofnormal window glass. Therefore, DSSCs have been extensivelyexplored as an alternative to conventional Si solar cells.The theoretical efficiency of the DSSC is as high as 20.2%,

which is comparable to that of commercial silicon-based solarcells.5 For the last two decades, significant progress has beenmade, and the best energy conversion efficiency of the DSSCsat the laboratory scale has already surpassed 11%. AlthoughDSSCs still have problems to circumvent before being widelyproliferated, the efficiency of ∼10% in Sony’s submodule andthe stable operation for over 25 000 h at 55 °C, AM 1.5condition in Dyesol’s prototype products are regarded aspioneering footsteps toward commercialization.6,7 In this

Received: January 17, 2013Accepted: April 25, 2013

DSSCs offer significant economicand environmental advantagesover conventional photovoltaicdevices because they can be

manufactured relatively inexpen-sively, and in an energy-efficientand environment-friendly man-

ner.

Perspective

pubs.acs.org/JPCL

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Perspective, we introduce current issues of DSSCs and recentadvances toward their economic viability. We also brieflysummarize the development history of DSSCs and theiroperating principles.Recent Progress on Conventional Liquid Electrolyte Based Dye

Sensitized Solar Cells. The high efficiency breakthrough ofDSSCs was made when mesoporous TiO2 nanoparticle filmswere used to absorb dye molecules and collect photogeneratedelectrons. A benefit of this nanostructure is to capture a largeamount of incident photons, because its high surface areadramatically promotes dye adsorption. An important require-ment of the nanostructured film is to maintain the energy of thephotogenerated electrons in the transport process. The chargecollection efficiency of state-of-the-art DSSCs is higher than90% in the state-of-the-art nanoparticle photoelectrode.However, carrier trapping at the defects and the misalignedposition of the band edges reduce the energy of thephotogenerated carriers and cause a decrease in the open-circuit voltage of the device. To suppress the energy loss of thephotogenerated electrons, the charge transport and recombi-nation processes have been studied extensively. This has led toa new design of nanostructured films and/or new wide bandgapsemiconductors for the photoelectrodes. For example, one-dimensional (1-D) nanomaterials such as nanorods, nanowires,and nanotubes have been used as photoelectrode materials,instead of TiO2 spherical nanoparticles.8,9 The electrondiffusion coefficients (De) for TiO2 nanotube photoelectrodesare around 10−6−10−4cm2/s, which is slightly larger than thosefor TiO2 nanoparticle photoelectrodes (10−7−10−4cm2/s).10,11

Recently, anatase nanowire-based photoelectrodes exhibitsignificantly increased De (10

−4−10−3cm2/s).12 The nanowiresalso improved the electron diffusion length (Ln), which showscollective information about charge transport and recombina-tion. Ln is expressed as Ln = (Deτe)

1/2, where τe is electronlifetime. While Ln for TiO2 particle-based DSSCs is 10−20 μm,Ln of the TiO2 nanotubular DSSCs ranges from 30 to 40μm.11,13 Moreover, the Ln for anatase TiO2 nanowire-basedDSSCs is as high as 20−100 μm, which indicates that the 1-Dmaterials possess better charge transport and recombinationproperties than nanoparticle materials.12 Although they haveshown superior charge transport behavior and lower carriertrapping, DSSCs based on 1-D materials have problems relatedto small short circuit current. Since the roughness factor of the1-D arrays is lower than that of the nanoparticle films, the

amount of dye molecules adsorbed on the surface is smaller forthe 1-D nanoarrays than the nanoparticle films. To increase theroughness factor, nanowires with branches, nanowire/nano-particle mixtures, and very long nanowires were examined.14,15

Another way to control the carrier recombination is tochange the electrical properties of the wide bandgap semi-conductors by coating the surface of the nanoparticles ordoping the impurity. Several metal oxides such as SrO, Al2O3,and MgO have successfully suppressed the charge recombina-tion and/or bend the band edges at the nanoparticle-electrolyteinterface.16 Recently, an Nb-doped TiO2 film has been appliedto DSSCs.17 A small amount of Nb doping also prevents thecarrier recombination and improves the open circuit voltage(Voc) and the short circuit current (Jsc). The effect of surfacecoating and impurity doping are pronounced in DSSCs withlower efficiency, but their effects on improvement are morelimited as the efficiency of the DSSCs gets close to 10%.An additional function of the oxide particles in DSSCs is to

enhance light harvesting efficiency by scattering the incominglight. A typical light scattering layer is composed of relativelylarge TiO2 particles with a size of several hundred nanometers.Due to the light trapping effect, the incident photons are able toefficiently stimulate the photosensitizers. The efficiency ofDSSCs containing light scattering layers has been increasedfrom 7.6% to 9.2%, compared with DSSCs composed of justTiO2 active layers.18 Most of the highly efficient cells (over10%) have employed light scattering layers, which demonstratethat these layers are absolutely necessary to achieve a highefficiency. The dye adsorption in the scattering layer is notsignificant, because of the large particle size. Hence, bifunc-tional light scattering materials have been tested to improve thedye loading properties. These new materials are hollow sphereswith a size of several hundred nanometers, which are composedof TiO2 nanocrystals with a size of ∼20 nm. Park et al. reportedthat highly porous TiO2 hollow spheres showed a higherphotocurrent (15.8 mA/cm2) than commercial light scatteringparticles (14.6 mA/cm2).19 Hahn et al. exploited a quasi-inverseopal layer based on highly crystalline TiO2 nanoparticles forboth light scattering and dye adsorption, which is shown inFigure 1.20 The ordering of hollow spheres is not periodic,which is favorable for light scattering. Also this layer iscomposed of TiO2 nanocrystals, leading to the formation of ananoporous structure whose dye adsorption is approximately 2-fold higher than commercial scattering particles. Recently, Liao

Figure 1. Field-emission scanning electron microscopy (FESEM) images of (a) a quasi inverse opal based photoelectrode and (b) a magnified quasiinverse opal layer. (c) IPCE spectra of the DSSCs with and without the inverse opal structure (CCIC stands for a commercial light scattering layermade by Catalysts & Chemicals Industries Co. (Japan) and 970- and 420- QIO stand for quasi inverse opal layers with 970 nm- and 420 nm- sizedhollows). Reproduced with permission from ref 20. Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

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et al. synthesized hierarchical TiO2 spheres consisting ofanatase nanorods and nanoparticles, which showed trifunction-ality including high dye loading, efficient light scattering ability,and higher charge-collection efficiency. By using this newmaterial an energy conversion efficiency of 10.3% wasachieved.21

The Shockley−Queisser (S-Q) limit shows that themaximum efficiency of the single junction Si solar cells isclose to about 31%. In DSSCs, the gap between the highestoccupied molecular orbit (HOMO) and the lowest unoccupiedmolecular orbit (LUMO) is larger than the band gap of Si andthe theoretical limit is about 20% which depends on the lightabsorption spectra of the day. The theoretical efficiency of thesolar cells can be increased if the single junction structure issubstituted with the multiple junction structure. To push the S-Q limit upward, a tandem structure that uses two different bandgap materials has been proposed. When the band gaps arearound 1.5 to 1.7 eV for the top cell and 0.8 to 0.9 eV for thebottom cell in a series-connected double-junction device, theefficiency limit can be extended over 45%.22 To fabricatetandem structure DSSCs, p-type DSSCs connected to theconventional n-type DSSCs should be developed. A primarybarrier toward the p-type DSSCs is the lack of p-type oxidesemiconductors, which is equivalent to TiO2 in the conven-tional DSSCs. A nanostructured NiO film is a good candidatefor the photoelectrode material of p-type DSSCs, since theunique defect structure of NiO produces a large amount ofholes. However, p-type DSSCs in the literature have notexhibited an energy conversion efficiency at the same level asconventional DSSCs, which has been ascribed to the lowdiffusion coefficient of holes in p-type semiconductors.23 Thecarrier diffusion coefficient of NiO-based DSSCs is 3 orders ofmagnitude lower than the typical values of TiO2-based DSSCs.This indicates that new p-type semiconductors with a highcarrier diffusion coefficient will cause a remarkable impact onthe efficiency of tandem DSSCs.

Dye molecules adsorbed on the surface of metal oxidenanoparticles capture incident photons, generate electron−holepairs that are excitons, and then inject photocarriers into theoxide semiconductor. Important parameters of dyes areHOMO/LUMO energy levels, light extinction coefficient,oxide surface anchoring, electron−hole separation, andmanufacturing cost. Recent research on new dye moleculeshave focused on (1) controlling the spectral response bychanging HOMO/LUMO levels and (2) reducing the materialcost by removing Ru from the dye molecules and increasing theextinction coefficient.Metal complex-based dye molecules, especially Ru-based

complexes, have performed the best for DSSCs. N3 dye (cis-RuL2(NCS)2 (L:2,2′-bipyridyl-4,4′-dicarboxylic acid)) and itsderivative, N719 dye, have shown outstanding photon-electronconversion behavior, due to electron transition from Ru to thep* orbital of diimine, which is directly attached to TiO2. TheN3 dye, however, does not adequately harvest the light whosewavelength is greater than 700 nm. Therefore, there have beenextensive studies to lower the LUMO level. One of the most

successful methods is to replace a bipyridine group by aterpyridine group and a cyanide group. In this dye, known as“black dye” (N749), the light absorption edge is extended toaround 900 nm. Consequently, the energy conversion efficiencyincreases to 11.1%.24

One weakness of N-series dyes is that the extinctioncoefficient of the dye molecules is small, which means that athick photoelectrode is required. To increase the extinctioncoefficient, the conjugation length of the ligand is controlled. Agood example is to attach multiple ligands to the core Ru as anexploitation of heteroleptic ruthenium complexes with highermolar extinction coefficients compared to the N719 dye. Byextending the π conjugated system of ancillary ligands, Gao etal. synthesized the C101 dye molecule (NaRu(4,4′-bis(5-hexylthiophen-2-yl)-2,2′-bipyridine)(4-carboxylicacid-4′-car-boxylate-2,2′-bipyridine)(NCS)2 with a high extinction coef-ficient, resulting in an efficiency of 11%.25 These resultsdemonstrated that the molecular design of the ligands iscapable of improving light harvesting efficiency.In synthesizing new organic dyes, efforts have been made to

improve the spectral response of the dye and reduce materialcost. Organic dyes are generally composed of donor-conjugatedspacer-acceptor parts, and the extinction coefficient and thelight absorption spectra can be widely tuned by changing donorand acceptor parts. Therefore, the molar extinction coefficientof the organic dyes can be as large as ∼1.9 × 105 M−1 cm−1 andthe onset light absorption wavelength can be extended to nearIR regions.26,27 New dye molecules have increased lightharvesting efficiency (LHE) continuously by controlling theextinction coefficient and light absorption range. However, aconventional structure of DSSCs still has other factors limitingthe enhancement of LHE. Inherent parasitic reflection andabsorption at TCO glass decrease the effective number ofincident photons absorbed by DSSCs. The transmittance forFTO glass coated by TiO2 blocking layer is reported as 70% at550 nm.28 Recently, the optical transmittance problem of thephotoelectrode has been addressed by adjusting opticalinterference between FTO and TiO2 layers.

28

To date, a universal design rule for optimum organic dyes hasnot been found, and the progress of the related field relies onempirical approaches. Although some organic dyes performedbetter than Ru-based dyes in p-type DSSCs, most of the organicdyes exhibited lower efficiency in n-type DSSCs. The bestenergy conversion efficiency of p-type and n-type organic dyesensitized solar cells is 0.5% and 10.3%, respectively.29,30

Because of the flexible light absorption spectrum and the highextinction coefficient, cosensitization by multiple Ru-based andorganic dyes has also been explored to increase the lightharvesting efficiency at the near IR region. Recently, Fan et al.reported a 10.2% efficiency for a DSSC employing cosensitiza-tion of a Ru-based dye (JK-142) and an organic dye (JK-62).31

The combination of an organic dye that possesses a relativelyhigh extinction coefficient and a Ru-based dye that absorbs arelatively broad range of wavelengths boosts the energyconversion efficiency. Also, this cosensitization techniqueprovides a new way to position different dyes on a singleTiO2 film by controlling molecule size. The large size JK142dye can penetrate only the upper region of a nanoporous TiO2film, and the small JK-62 dyes can be adsorbed onto the entirepart of the TiO2 films. Compared with the tandem DSSCs, thisapproach is commercially viable.A theoretical maximum of the photovoltage of DSSCs is

determined by the difference between the quasi Fermi energy

New p-type semiconductors witha high carrier diffusion coefficientwill cause a remarkable impact onthe efficiency of tandem DSSCs.

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of electrons in TiO2 and the redox potential energy of theelectrolyte. However, the photovoltage of the DSSCs isgenerally smaller than this theoretical limit, and one of thereasons is a backward reaction between the dye and the redoxelectrolyte. If holes in the oxidized dye do not move to theelectrolyte quickly, the photoexcited electrons of the dye canreact with the oxidized ionic species of the electrolyte and thephotovoltage decreases. Therefore, efforts on adjusting thecomposition of electrolyte have been made (1) to control theredox potential for increasing the theoretical maximum of Vocand (2) to retard the backward electron transfer from the dyeto the electrolyte. A difficulty in adjusting the redox potential ofthe electrolyte is due to the fact that a driving force for theregeneration of oxidized dye molecules is an energy differencebetween the redox potential level of the electrolyte and theHOMO level of the oxidized dye molecule. A change in theredox potential to increase Voc, in turn, decreases the drivingforce for the dye regeneration and promotes the backwardelectron transfer, thereby reducing Isc and Voc. This is one ofreasons why many groups perform research on new redoxcouples that can replace the I−/I3

− redox couple.Liquid electrolytes based on the I−/I3

− redox couple havebeen mostly used in DSSCs because of the suitable redoxpotential for regeneration of oxidized dyes and relatively fastdiffusivity. However, the large driving force of the dyeregeneration in the I−/I3

− redox (>0.5 V) adversely affectsVoc, and the photovoltage of DSSC using the I−/I3

− redoxcouple is restrained to ∼0.8 V.32,33 Since redox couples play akey role in determining Voc, researchers have focused on findingalternative redox couples. Various halides, organic compounds,and transition metal systems have been tested as redox couples.However, most new redox couples have not outperformed I−/I3−. Recently, Co(2+/3+) tris(bipyridine)-based redox electrolyte

matching with donor−π−bridge−acceptor zinc porphyrin dyeas a sensitizer (YD2-o-C8) significantly improved energyconversion efficiency up to 12.3% (Figure 2).34 This highefficiency is due to the retarded back electron transfer ofphotoelectrons to Co complexes, thereby increasing Voc and JSCto 0.94 V and 17.7 mA/cm2. The presence of the four octyloxygroups in the YD2-o-C8 dye suppresses the access ofCo3+tris(bipyridyl) to the TiO2 surface and retards the carrierrecombination rate. This result indicates that study on the dyesneeds to be performed in conjunction with electrolyte andsemiconductor film studies to match their electrochemical

properties and maximize the energy conversion efficiency ofDSSCs.

Emerging Strategies in DSSCs. Levelized cost of energy(LCOE) is the fairest methodology to compare thesustainability of different energy supply technologies. In thecase of a photovoltaic system, the LCOE is determined byseveral variables such as solar insolation, energy conversionefficiency, and system degradation rate.35 The detailed equationfor LCOE has been reported in previous studies on energyeconomics.35−37 In addition to high energy conversionefficiency, a lower system degradation rate that is closelyrelated to long-term stability of the devices is important inreducing the LCOE and making the photovoltaic system moreeconomically viable.From the stand point of long-term stability and reliable

operation, a problematic component of DSSCs is the liquidelectrolyte. If the sealing of the device is not perfect, the liquidelectrolyte gradually evaporates away and impurities such aswater and oxygen molecules permeate into the cell. Therefore,the assembly of DSSCs containing liquid electrolyte is requiredto minimize solvent leakage/vaporization. To improve thestability of DSSCs, different kinds of electrolytes have beenextensively studied to supersede the liquid-type electrolyte. Asolid-state hole conductor is an ideal form of the electrolyte forcommercialization of DSSCs, since this addresses the problemof conventional liquid electrolytes such as leakage andevaporation. Therefore, the compatibility of p-type inorganicsemiconductors and organic hole conductors with DSSCs hasbeen investigated in detail. At present, Spiro-OMeTAD(2,2(,7,7(-tetrakis-(N,N-dipmethoxyphenylamine)9,9′-spirobi-fluorene) is the most commonly used organic hole conductorfor solid state dye sensitized solar cells (SDSSCs), due to itssmall molecular size and high solubility in organic media. In

Figure 2. (a) The molecular structures of the YD2-o-C8 porphyrin dyes and (b) APCE as a function of wavelength for the YD2-o-C8 porphyrinadsorbed at the surface of a 6-m-thick nanocrystalline TiO2 film in contact with Co(II/III)tris(bipyridyl)-based electrolyte. Reproduced withpermission from ref 34. Copyright 2011 American Association for the Advancement of Science.

Study on the dyes needs to beperformed in conjunction withelectrolyte and semiconductorfilm studies to match their elec-trochemical properties and max-imize the energy conversion

efficiency of DSSCs.

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addition, the redox potential of Spiro-OMeTAD is morepositive than that of I−/I3

− couple, which is beneficial toincreasing the open circuit voltage (Voc) of SDSSCs. WhenSpiro-OMeTAD is combined with a novel high extinctionorganic dye using 4,4′-didodecyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophene as a spacer between donor and acceptor groups,the efficiency of SDSSCs is as high as 6.08%.38

Compared with the liquid electrolyte, the solid holeconductors have lower charge carrier mobility. In Spiro-OMeTAD, the hole mobility is only 10−4 cm2/(V s), whichis much smaller than the charge carrier diffusivity in the liquidelectrolyte. The low carrier mobility increases the probability ofcarrier recombination during the transport process and reducesthe photocurrent density. Although certain p-type organicsemiconductors such as PEDOT/PSS have better electricalconductivity (10−3 to 500 S/cm), the size of PEDOT/PSS in asecondary or tertiary structure is too large to pass through themesopores of TiO2 nanoparticle films. This partial filling of thepores is caused by the small pore size of the mesoporous films.The unfilled portion of the mesoporous photoelectrode and thelow carrier mobility of the solid electrolyte are the source forelectron−hole recombination and parasitic current in SDSSCswhere the charge transport is controlled via a trap-limiteddiffusion.Although the large molecules clearly have a problem filling

the mesopores, the pore filling capability of the small moleculessuch as Spiro-OMeTAD is controversial. Melas-Kyriazi et al.reported that it is difficult to fully fill the pores of the thickphotoelectrode with Spiro-OMeTAD, although it is welldissolved in the organic media.39 Their depth profiling analysisshows that the filling fraction of the 2.5 um thick mesoporousfilm is 60−65%. As the film becomes thicker than 3 um, thepore filling fraction and energy conversion efficiency ofSDSSCs decreases. However, Docampo et al. claimed thatthe pore filling fraction is 80% in 2-um-thick mesoporous filmsand 60% in the 5-um-thick mesoporous film.40 Theirconclusion is that the fast carrier recombination at the interfaceof the hole conductor limits the energy conversion efficiency ofSDSSCs.New p-type conductors are extensively studied to solve

problems of existing solid electrolytes. First, the molecularstructure of imidazole-based ionic liquids is modified to

increase melting temperature and hole mobility. For instance,when the alkyl chain in imidazolium salts is replaced by an estergroup, dimers of conductor molecules are formed and three-dimensional (3-D) ionic channels of iodides are created.41

These changes facilitate charge transfer along the polyiodidechain and increase conductivity of the solid electrolyte. Thesecond research direction is to develop a new kind of polymerelectrolyte. One example is an organic ionic plastic crystal thatis in the intermediate stage between solid crystals and liquids.Different ionic plastic crystals such as N-methyl-N-ethyl-pyrrolidinium dicyanamide [C2mpyr] [N(CN)2] and 1-ethyl-1-methyl pyrrolidinium iodide (P12I) have been tested as theelectrolyte of SDSSCs.42 Their efficiency is higher than 5%,which is similar to the efficiency of SDSSCs using Spiro-OMeTAD. In addition, these ionic plastic crystals allow for thestable operation of SDSSCs even at 80 °C. The third researchdirection is to gelate the liquid electrolyte by adding lowmolecular weight polymers or oxide nanoparticles. Recently,cyclohexanecarboxylicacid-[4-(3-octadecylureido)phenyl]amidewas used as the gelator of the 3-methoxypropionitrile (MPN)-based liquid electrolyte.43 This electrolyte is not in a completesolid state, but in a quasi-solid state. The efficiency of quasi-solid dye sensitized solar cells (QSDSSCs) with the newmolecular weight gelator is 9.1%, which is close to that of theliquid electrolyte DSSCs.As an alternative to polymer-based solid electrolytes,

inorganic p-type semiconductors including CuSCN, CuI,NiO, CuAlO2, and CsSnI3 have been also investigated.CuSCN is a typical inorganic solid electrolyte that has beenexplored for DSSCs. It is relatively stable and providesreasonable energy conversion efficiency (∼2%).44 However,the low electric conductivity of CuSCN (∼10−2 S/m) preventsfurther improvement of SDSSCs, which motivates subsequentresearch on the doping effect. Cl− in the thiocynate site andtriethylamine coordinated Cu(I) in the cuprous site are foundto increase the electric conductivity to ∼1 S/m by increasingthe carrier concentration. However, the best efficiency ofSDSSCs using CuSCN in the literature is still 3.4% at AM 1.5.45

CuI-based SDSSCs exhibit better energy conversion efficiency(3−6%) than those that are CuSCN-based.46 However, CuI isquickly photodegenerated to Cu2O or CuO at the CuI/TiO2interface. Very recently, Chung et al. reported a new type of p-

Figure 3. (a) Temperature dependence of electrical conductivity (s, filled squares) and Seebeck coefficient (S, filled circles) of CsSnI3. (b) Energylevels of the components of the CsSnI3 solid-state solar cell. The valence band maximum (VBM; orange) and the conduction band minimum (CBM;blue) are represented in eV, along with the energy difference between the edges. The ground (HOMO; orange) and excited states (LUMO; blue) ofN719 dye is also shown. Reproduced with permission from ref 47. Copyright 2012 Nature Publishing Group.

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type inorganic semiconductors, CsSnI3, which successfullyaddresses major problems of the solid electrolyte for highlyefficient SDSSCs.47 As shown in Figure 3, the band edges ofCsSnI3 allow for hole transfer from the photoexcited dye toCsSnI3. Hole mobility of CsSnI3 is mh = 585 cm2/(V s), whichis 106 times larger than that of Sprio-OMeTAD. When F isdoped into CsSnI3, the hole concentration increases, and theenergy conversion efficiency of SDSSCs reaches 10.2% withouta mask on the device. This recent article indicates thatinorganic SDSSCs can be comparable to state-of-the-art liquidelectrolyte DSSCs.When the size of the metal medium is smaller than the mean

free path of the electrons, the collective oscillation of theelectrons is bound to the surface of the nanostructured metals,and the plasma frequency is quantized. This is known as surfaceplasmons. When both the energy and momentum of theincident light match those of the surface plasmons, theextinction coefficient of the plasmonic nanostructures can besignificantly increased. The unique optical properties of thesurface plasmons have been used to improve the energyconversion efficiency in DSSCs. Standridge, et al. used atomiclayer deposition (ALD) to conformally coat arrays of silvernanoparticles with a very thin layer of TiO2 and studied thecorrelation between photocurrent density and TiO2 layerthickness.48 Their results, shown in Figure 4, demonstrated thatdye molecules in close proximity to silver nanoparticlesproduce more electrons than those on bare TiO2. However,these thin film type plasmonic structures are not suitable forDSSCs using mesoporous TiO2 films, since the near-field of thesurface plasmons decay dramatically a few hundred nanometersaway from the metal component. To solve this problem, theplasmonic particles were directly added to mesoporous TiO2films by mixing noble metal (i.e., Au and Ag) nanoparticles andTiO2 nanoparticles.Recent studies on photoelectrodes with embedded metal

nanoparticles are classified into three groups. One researchdirection is dedicated to preventing the erosion of metalnanoparticles and the trapping of photogenerated carriers,which can occur by adding bare nanoparticles into DSSCs. Forthis purpose, Au or Ag nanoparticles were coated with thedielectric layer such as SiO2 and TiO2 or were placed insideTiO2 hollow spheres.49,50 The experimental results show thatthe surface plasmons can promote the light absorption ofDSSCs and increase their energy conversion efficiency by 30−100%. It is noted that the plasmon-assisted carrier generationbecomes dominant over the plasmons’ oscillation dampingwhen photogenerated electrons are transferred from the dye tothe TiO2 within 10 fs. The second research direction ofplasmonic DSSCs is to tune the plasmonic frequency of thenanostructured metals. Since the resonance frequency is mainly

determined by the refractive index of the metal and thesurrounding dielectrics, it is difficult to control the plasmonicfrequency of metal nanoparticles. Therefore, several groupshave changed the shape of the nanostructure metals and shiftedthe surface plasmon frequency within the absorption spectrumof DSSCs. Ding et al. explored the effect of the Ag nanoshell,and Chang et al. investigated the role of Au nanorods.51,52 Bothgroups proved that a change in the shape of nanostructuredmetals enhances the photon-electron conversion of DSSCs,which is more pronounced at the modified plasmonic frequencyof the nanoshell or nanorods. The third research direction isbased on the surface plasmon polariton that is confined nearthe metal-dielectric interface. When the two-dimensional (2-D)plasmonic layer of nanoscale Ag domes is used as the back-reflector of SDSSCs, the short circuit current of the deviceswith high extinction dyes is increased by more than 10%.53

Although the nanostructured metals have been found toimprove the performance of DSSCs, Choi et al. raise a questionon the mechanism of that enhancement.54 They have arguedthat stored electrons in the metal nanoparticles can change theFermi energy level of the semiconductor−metal nanocompo-sites. Consequently, the open circuit voltage and the energyconversion efficiency can be increased. This implies that thenanostructured metals may play two different roles inenhancing the performance of DSSCs.In inorganic semiconductor nanoparticles, the energy level in

the conduction band and the valence band becomes discretedue to the quantum confinement. This has led to extensiveresearch on nanoparticle-based photovoltaics that can poten-tially solve the problems of silicon-based solar cells. The initialinorganic nanosensitizer solar cells used inorganic nanoparticlescoated on mesoporous TiO2 films.

55 The network of TiO2 orZnO nanoparticles works as the electron acceptor, whichcollects electrons from the inorganic sensitizers. The energyconversion efficiency of the early solar cells was lower than 1%,due to the fast carrier recombination at the inorganicnanoparticle sensitizers.To improve the carrier injection process from the inorganic

nanomaterials, the photoelectrode structure and the sensitizingmaterials have been modified. For example, 1-D nanowires andnanotubes were sensitized by inorganic nanoparticles. Leschkieset al. showed that the extremely thin PbSe layer on ZnOnanowires absorbs most of the incoming solar light, and theelectrons go through the ZnO nanowires.56 The power outputof their device was significantly improved, but the efficiency isstill about 2% when illuminated with 100 mW/cm2 light. Thiswas attributed to recombination at the CdSe/ZnO interface.Therefore, several following studies on inorganic nano-sensitizers have been aimed at minimizing the electron−holerecombination near the sensitizing component. A TiO2/CdS/

Figure 4. (a) Configuration of solar cells containing silver nanoparticles (NPs) and dye. (b) IPCE of the solar cell with dye only, with silver NPsonly, and with dye and silver NPs.48 Reproduced with permission from ref 48. Copyright 2009 American Chemical Society.

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CdSe multilayer structure was found to form a stepwise band-edge level and facilitate the cascade-like extraction of electronsand holes, leading to an energy conversion efficiency close to4%.57 In Sb2S3-sensitized TiO2 solar cells, the screening effectof the nanostructured film and the strong interfacialinteractions of Sb2S3 with the p-type solid electrolyte havebeen found to suppress the carrier recombination and increasethe fill factor.58 When the surface area of the TiO2 film wasmaximized and the functional group of the solid electrolyte waschelated to Sb2O3, Jsc and the energy conversion efficiency ofthe solar cell becomes larger than 15 mA/cm2 and 6% at AM1.5.A remaining issue of solar cells with inorganic nanosensitizers

is how to increase the open circuit voltage. For this purpose,Lee et al. implemented mesoporous Al2O3 film to replace theelectronically disordered, low-mobility n-type TiO2.

59 In their“meso-superstructured solar cell” shown in Figure 5, the layeredperovskites of organometal halides (CH3NH3PbI3) and spiro-OMeTAD are sequentially coated on the mesoporous Al2O3film as the light absorber and the p-type conductor. Since theinsulating Al2O3 provides only a scaffold function to the solarcells, the devices operate as two-component hybrid solar cellsinstead of sensitized solar cells. The modified structure leads tolow fundamental carrier loss, large open circuit voltage (∼1.1eV), and very high energy conversion efficiency (∼10.9%). Kimet al. have also shown that the perovskite-type nanomaterial,(CH3NH3)PbI3, is a promising light absorber for highperformance solar cells.60 In their recent report on SDSSCs,a combined use of (CH3NH3)PbI3 and SpiroOMeTAD led to ahigh short circuit current (>17 mA/cm2), large open circuitvoltage (0.888 V), and remarkable energy conversion efficiency(9.7%) at AM 1.5 condition. These results indicate thatperovskite-structured inorganic sensitizers may offer a break-through to DSSCs.

Perspectives on DSSC Commercialization. Critical factorsdetermining the commercialization of DSSC are energyconversion efficiency, raw material abundance, energy payback

period, long-term stability, and production cost. DSSCs haveadvantages in terms of raw material abundance and energypayback period. TiO2 is a very abundant material, and theexpected consumption of Ru for the DSSC dye is only a few %of annual usage. Also, the energy payback period of DSSCs isabout 1 year, which is shorter than that of Si solar cells (>3years). On-going studies on the electrolyte, the sensitizingmaterial, and the hermetic sealing technique, indicate that themarginal degradation of the device performance is expectedafter 20 years of operation.However, the energy conversion efficiency of DSSCs may be

only slowly improved. The energy conversion efficiency of thecommercially available module is not expected to be muchhigher than 10%. The lower efficiency of the solar cells meansthat the balance of system (BOS) is increased. Compared withthe 10% efficiency module, the 15% efficiency module and the25% efficiency module reduce the installation costs by 20% and35%, respectively.61 This leads to the conclusion that themodule price of DSSCs should be close to $0.6/Wp to competewith current Si PV modules (∼$0.8/Wp) and eventually reach$0.35/Wp to achieve wide grid parity. In the U.S. Departmentof Energy roadmap, the target price and efficiency of the solarcell modules for the grid parity is $0.5/Wp and 25% by 2020.62

Since the current material cost of solar cells is slightly less thanhalf of the module price, the material cost of economicallyviable DSSCs with 10% module efficiency must range from$15/m2 to $30/m2. In a recent cost analysis of DSSCs,however, the material cost production is projected to be lessthan $50/m2, based on 100 000 m2 production of 10%efficiency module per year by 2020. This indicates thatelectricity generation by DSSCs will be more costly than bySi solar cells in 2020. Therefore, efforts to reduce the amountand cost of materials required to fabricate DSSCs areimperative for the widespread use of DSSCs.For this purpose, intensive research is ongoing for all

components of DSSCs. Dye and TiO2 nanoparticles take upmore than 50% of the production cost, which is attributed tocomplex synthesis methods.3,63 Ti is an abundant and cheapmaterial, and even the price of Ru is 5−10% of the dye cost onthe assumption that the annual production of DSSCs is 7 MWp.Time-consuming synthesis of high purity dye molecules andhydrothermal growth of highly crystalline TiO2 nanoparticlesare responsible for the increase in production cost. While

Figure 5. (a) Ultraviolet to visible (UV−vis) absorbance spectra of the photoactive layer in the solar cell (mesoporous oxide; perovskite absorber;spiro-OMeTAD) sealed between two sheets of glass in nitrogen and exposed to simulated AM1.5 sunlight (Inset: extracted optical density at 500 nmas a function of time. (b) Current density−voltage characteristics at AM1.5 condition for Al2O3-based cells, one cell exhibiting high efficiency (redsolid trace with crosses) and one exhibiting VOC > 1.1 V (red dashed line with crosses); for a perovskite-sensitized TiO2 solar cell (black trace withcircles); and for a planarjunction diode with structure FTO/compact TiO2/ CH3NH3PbI2Cl/spiro-OMeTAD/Ag (purple trace with squares).Reproduced with permission from ref 59. Copyright 2012 American Association for the Advancement of Science.

Perovskite-structured inorganicsensitizers may offer a break-

through to DSSCs.

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several companies are developing alternative synthesis routes toeasy ligand exchange, fast dye purification, and simplecrystallization of uniform TiO2 nanoparticles, breakthroughshave not been found yet. It is expected that the cost reductionof dye and TiO2 will be connected to the production volumeand the extinction coefficient of the dyes. An 80% reduction ofthe dye cost is projected when the production volume of thedye is increased 100-fold. In addition, a small increase in theextinction coefficient of the dye will easily decrease thethickness of the dye-coated TiO2 film by 50%. The next mostexpensive component is glass panels. Since the cost of thetransparent conducting oxide-coated glass is about $10/m2 andthat of the Pt-coated glass is about $6/m2, the removal of evenone glass piece will bring the material cost of DSSCs close tothe generally targeted value. In addition, the replacement of theglass is required to utilize DSSCs in mobile applications.Therefore, intensive research is being conducted to change thetraditional structure of DSSCs. Conductor materials connectingindividual cells are also challenging, since the cost of Ag used asthe conductor material is about 15% of the DSSCs. A moresignificant problem is that the price of Ag is continuouslyincreasing. This is a universal issue in the solar cell industry.Consequently, several companies are working to reduce theamount of Ag in metal conductors or substitute Ag with cheapmetals such as Cu and Ni. Although the complete replacementof Ag is ideal in terms of the cost, cheap metal conductors sufferfrom high electric resistivity and easy degradation. To preventdegradation, the cheap metal lines need to be fired in the inertambience and encapsulated quickly after the firing process. Thecost of the electrolyte is not problematic when only the solvent-based electrolyte is used. In contrast, the ionic liquid or thesolid hole conductor is still costly, and further work is required.

If research into decreasing the amount of the materials perDSSCs module is not successful, there is a chance that DSSCswill be used in niche applications such as flexible solar cells orcompete with amorphous Si solar cells for the low-end market.At present, a promising strategy for the DSSC industry is tomodify existing products and add extra value rather than tobuild a photovoltaic power plant. A very recent market researchstudy forecasts that the global market for DSSCs will grow veryslowly to $290 million by 2023 and the indoor and mobileelectronic sections will command half of that market.64 DSSCsin the initial commercialization stage are aimed at chargers,solar bags, and wireless solar keyboards. Later, BIPVs andautomobile PVs are expected to be important applications ofDSSCs. In these areas, the high temperature stability of thedevices becomes critical, since DSSCs need to be installed inglass windows, building walls, and steel roofs. The temperatureof DSSCs in contact with and 5 cm away from the outside ofbuilding walls was increased to 80 and 60 °C, respectively.Hence, DSSCs for BIPVs and automobile PVs are required topass an accelerated aging test under harsh environments(temperature of 85 °C and humidity of 85%) to prove devicereliability. It is noteworthy that expensive ionic liquid-basedelectrolytes must be used for DSSCs operated at high

temperature, due to the volatility of the cheap solvent-basedelectrolytes.In the following section, recent studies to reduce the cost of

DSSCs by changing the conventional cell structure or replacingexpensive constituents are summarized. During the last twodecades, most of the DSSCs have been fabricated in the form ofa sandwich that employs 2-D flat transparent conducting oxides(TCO) such as Sn-doped indium oxide (ITO) and F-dope tinoxide. However, this structure has limitations on fabricatingflexible DSSCs and reducing material costs due to theemployment of TCO. Although flexible DSSCs have beenrealized by using polyethylene terephthalate (PET) orpolyethylene naphthalate (PEN) films coated with TCO, theTCO films on organic substrates can be easily cracked afterbending or stretching, subsequently deteriorating their electricalconductivity. Recently, newly structured DSSCs including (1)Ti foil-, (2) metal wire- and (3) metal mesh-type DSSCs havebeen exploited to overcome these aforementioned problems.DSSCs built on Ti foil electrodes have garnered a

considerable amount of attention, because of their lowerprice and easy fabrication process. A major strength of Ti foilover FTO is that Ti foil is compatible to the low-costfabrication method such as a roll-to-roll mass production. Fromthe raw material cost, Ti foil and TCO glass are comparable. Itis expected that the price of 0.2 mm thick Ti foil is as expensiveas TCO glass (∼$10/square meter).65,66 In Ti-foil DSSCs,TiO2 films were deposited onto Ti-foil instead of FTO glasses.Binder burn-out and thermal annealing processes can beapplicable in this type of the photoelectrode. Back illumination,i.e., light going through a backside Pt counter electrode, isrequired to operate this type of DSSC because the Ti foil is nottransparent. Ito et al. reported 7.2% efficiency flexible DSSCsbased on Ti foil photoelectrodes.66

Metal wire-type DSSCs use metal wires coated with TiO2 orZnO active layers and counter electrode wires.67,68 However,the wire-type DSSCs have shown a fairly low energy conversionefficiency, because of the very small surface area for the dyeadsorption. This problem can be avoided by growing nanowireor nanotubes arrays on the metal wires.Fan et al. reported wire-shaped flexible dye-sensitized solar

cells (WSF-DSSCs) with a simple helical twisting structureformed by two fiber-like electrodes.67 They observed that theshort circuit current of the devices was in direct proportion tothe wire length. The 5 cm long WSF-DSSC performedrelatively poorly, i.e., the Voc, Isc, and fill factor were 610 mV,0.06 mA, and 0.38, respectively. Due to the easy synthesismethod, ZnO nanowire arrays were grown on the metal wiresto increase surface area. However, when the wire-type DSSCswere fabricated on the ZnO nanowire-coated metal wires, theenergy conversion efficiency of the device did not exhibit a highperformance. This was attributed to the fact that the inherentsurface instability of ZnO prevents the monolayer coating ofthe dye molecules. Recently, a remarkably improved efficiencyof 7% was achieved by employing a Ti fiber photoelectrodebased on TiO2 nanotube arrays, which significantly boostedenergy conversion efficiency of the wire-type DSSCs as shownin Figure 6a.68 Techniques used for high efficiency wire-typeDSSCs can be applied to other substrates such as wovencurtains, tents, and bags. This indicates that DSSCs can be builton nontraditional substrates that are widely used in day-to-daylife.An understanding of metal wire-based DSSCs allows for the

emergence of new DSSC structures built on the metal mesh

Efforts to reduce the amount andcost of materials required to

fabricate DSSCs are imperative forthe widespread use of DSSCs.

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that is the network structure of the metal wires. Flexiblestainless steel meshes coated with TiO2 nanoparticles werestudied initially. The surface modification of steel meshes bycoating ZnO nanowire and Zn2SnO4 nanowire arrays has alsobeen explored.69 Replacement of the metal mesh by Ti meshhas significantly improved the performance of the mesh-typeDSSCs, since Ti mesh is more compatible with TiO2nanotubes. Rustomji et al. reported an energy conversionefficiency of 5% by employing the TiO2 nanotube arrays in theTi mesh (Figure 6b).70 Wang et al. built 3-D DSSCs on double-deck cylindrical Ti mesh substrates.71 The photoanode is the Timesh that has been anodized to form a TiO2 nanotube layer,and the counter electrode is the Ti mesh that is platinizedthrough electrodeposition. The advantage of the 3-D DSSCs isthat the efficiency of the solar cells does not depend on theincident solar beam angle, due to its axial symmetrical structureas seen in Figure 6c. The optimized energy conversionefficiency was 5.5% under standard AM 1.5 conditions.In general, the mesh-type DSSCs have employed Pt

nanolayer or TCO-coated substrates, which are unfavorablefor achieving low cost and high flexibility. Recently, a glasspaper that contains the electrolyte was utilized as a supportingmaterial for a new type of flexible DSSCs.72 TCO-free andhighly bendable DSSCs were constructed on a pair of the metalmesh and the glass paper, which is depicted schematically inFigure 6d. This flexible device was inspired by the traditionalKorean door structure and showed an energy conversion

efficiency of 2%, which was maintained even under bending ofthe device until the radius of curvature reached 2 cm.Pt-coated conducting glass has been used as a counter

electrode in DSSCs due to its high electronic conductivity andcatalytic activity. A Pt-loaded counter electrode fabricated bythermal deposition of Pt chloride showed fairly low chargetransfer resistances of less than 1 Ω cm2. The only detriment tousing Pt is that it is a noble metal. Therefore, cheap carbon andorganic materials have been studied as alternative materials.Counter electrodes based on carbon nanopowders have beentested. Their charge transfer resistance can be decreased to 0.74Ω cm2, which is comparable to that of the Pt electrode.73

Recently, new carbon materials such as carbon nanotubes andgraphene have been introduced. Han et al. fabricatedpoly(styrene-4-sodiumsulfonate) (PSSNa)-grafted multiwallcarbon nanotube (MWCNT-g-PSSNa) films using an electro-static spray (e-spray) technique.74 With this approach, anenergy conversion efficiency of more than 7% was achievedalong with a charge transfer resistance of 1.52 Ω cm2 at athickness of 0.31 μm. Graphene supported by Pt nanoparticleshas been also exploited as the counter electrode material ofDSSCs.75 Its charge transfer resistance was 2.36 Ω cm2, whichis not much different than that of other counter electrodes. Inaddition, poly(3,4-ethylenedioxythiophene) (PEDOT) dopedwith toluenesulfonate anions also showed an excellent chargetransfer resistance of 0.95 Ω cm2.76 Overall energy conversionefficiencies for DSSCs employing these cheap counter electro-

Figure 6. (a) Actual optical photo of flexible fiber-shaped cell. Reproduced with permission from ref 68. Copyright 2012 Royal Society of Chemistry.(b) Low-magnification SEM images of TiO2 nanotube mesh anode; the inset figure is the surface of nanotube array. Reproduced from ref 70.Copyright 2010 american Chemical Society. (c) Schematic diagram of a 3D DSSC and the photovoltaic parameters of the 3D DSSC illuminated indifferent angles of the incident sunlight. The inset shows the schematic diagram of the 3D DSSC under illumination in different angles. Reproducedwith permisison from ref 71. Copyright 2010 American Institute of Physics. (d) Design of TCO-free highly bendable DSSC and the cross-sectionalview shown in the circle. Reproduced with permisison from ref 72. Copyright 2012 Royal Society of Chemistry.

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des is slightly lower than that of Pt-counter-electrode-basedDSSCs. Recently, a DSSC employing a nanostructured PEDOTcounter electrode demonstrated an efficiency of 8.3%, which isthe highest efficiency so far achieved by PEDOT counterelectrodes.77

Other materials that have been examined to replace Pt aresulfide and carbide compounds such as CoS, Mo2C, and WC.CoS coated on ITO/PEN exhibited good charge transferresistance of 1.8 Ω cm2 with good catalytic properties.Therefore, the CoS nanorod counter electrode increased theefficiency of the DSSC to 7.7%.78 Mo2C and WC with orderedmesoporous carbon also demonstrated a large exchange currentdensity on the electrode surfaces, which confirms that thesealternative electrodes catalyze the reduction of triiodide toiodide in the electrolyte.79

While the efficiency of DSSCs using alternative counterelectrodes approximates that of conventional DSSCs, theirlong-term stability and temperature-dependence are not wellcharacterized at this time. This information is essential toevaluating the true potential of the alternative counter electrodematerials.This Perspective reviews the recent progress with DSSCs and

prospects for their commercialization. Due to their uniquephysical characteristics, the economical and environmentallyfriendly fabrication process, the short energy payback period,and the material abundance, DSSCs have the potential tocompete with other thin film solar cell technology. Given thisrecent progress, it is expected that DSSC modules will soon becommercially available for the niche market where aestheticaspects or the mechanical flexibility of the device is important.Although the electrical, optical, and chemical properties ofDSSCs have been improved significantly, the widespread use ofDSSCs still requires further optimization of device perform-ance. Moreover, efforts to reduce the cost of the DSSCsmodules should continue. To make DSSCs economically viable,their module price needs to be lower than at least $0.6/Wp.This challenge requires researchers to improve the energyconversion efficiency of the module by broadening the lightabsorption spectrum and decreasing the energy loss of carriersthrough a transport process. Another way to lower the cost is toreduce the consumption of raw materials by redesigning thestructure of conventional DSSCs, which is also the subject ofongoing research carried out in both academia and industryaround the world.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: jul37@pitt.edu.

NotesThe authors declare no competing financial interest.Biographies

Jung-Kun Lee is an associate professor in the Department ofMechanical Engineering and Materials Science at the University ofPittsburgh. He received his Ph.D. degree from the Department ofMaterials Science and Engineering at Seoul National University, Koreain 2000. Then, he won the highly competitive Director’s PostdoctoralFellowship of Los Alamos National Laboratory (LANL) in 2001.Later, he was promoted to a technical staff member at LANL. Afterover 5 years of service at LANL, he joined the University of Pittsburghin 2007. His major research topics include sophisticated processingand characterization of nanostructured materials and electronicmaterials for photovoltaic and information technology. The scientific

quality of his research is validated by more than 130 publications inrefereed journals. He also holds 10 patents on the dielectric and opticalapplications of functional materials.

Hyun Suk Jung is an associate professor in the School of AdvancedMaterials Science & Engineering at Sungkyunkwan University(SKKU). He received his BS, MS, and Ph.D. degrees in materialsscience & engineering from Seoul National University (SNU), in 1997,1999, and 2004, respectively. He joined Los Alamos NationalLaboratory (LANL) as a director’s postdoctoral fellow in 2005. Hebegan working for Kookmin University (KMU) in 2006, and joinedSungkyunkwan University in 2011. He has published over 90 peer-reviewed papers regarding the synthesis of inorganic nanomaterialsand dye-sensitized solar cells. He presently researches flexible solarcells and inorganic sensitized solar cells. http://home.skku.edu/∼hjung/

■ ACKNOWLEDGMENTS

Dr. Lee acknowledges the financial support from the NationalScience Foundation (Grant No. DMR-0847319 and CBET-1235979). A portion of Dr. Jung’s research was supported bythe National Research Foundation (2012M3A6A7054864 and2012M3A7B4049986).

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