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Large photocurrent generation of an ITO electrode modifiedwith a red copper(II) complex

Lin-Ying Li a, Meng-Yun Xu a, Xi Chen a, Qian-Jin Zhang b, Ke-Zhi Wang a,

a College of Chemistry, Beijing Normal University, Beijing 100875, Chinab

College of Life Sciences, Beijing Normal University, Beijing 100875, China

Received 17 November 2010; received in revised form 19 March 2011; accepted 18 April 2011Available online 14 May 2011

Communicated by: Associate Editor Sam-Shajin Sun

Abstract

A film of copper complex [CuL](NO3)2 (L = 2,3,8,9-tetraphenyl-1,4,7,10-tetraazacyclododeca-1,3,7,9-tetraene) formed on indiumtinoxide (ITO) coated glass by the solvent evaporation of the acetonitrile solution of the complex onto the ITO substrate, was characterizedby ultraviolet-visible absorption spectroscopy, cyclic voltammetry, scanning electron microscopy, and photoelectrochemistry. The pho-toelectrochemical cell by using [CuL](NO3)2 modified ITO, a saturated calomel electrode, and platinum wire as working, reference andcounter electrodes respectively in 1 M Na2SO4, was found to exhibit a large prompt and reproducible cathodic photocurrent density of71 lA/cm2 under white light irradiation of 70 mW/cm2 at an applied potential of 0.4 V, and an incident photon to current efficiency(IPCE) of 1.1% at k = 660 nm. This Cu(II) complex photosensitizer has advantages of simple synthesis, low-cost, environmentally benignand good photoelectrochemical performance. 2011 Elsevier Ltd. All rights reserved.

Keywords: Copper; Shiff base; Photoelectrochemical property; Photocurrent

1. Introduction

Fossil fuels as a finite resource are overwhelminglybelieved to lead to climate-altering accumulation of CO2in the atmosphere. In this context, low-cost and efficientconversion of solar energy have emerged as a crucial goal,as solar energy is the only renewable energy source with theproven capacity to meet the worlds increasing energyneeds (Robertson, 2008). Since high-efficiency conversionof solar radiation to electrical power has been achievedby Gratzel and coworkers, who proposed a very efficientsolar cell with TiO2 and ruthenium complexes, dye-sensi-tized solar cells (DSSCs) have been under active investiga-tion as alternatives to silicon-based photovoltaic devices

for solar energy utilization (Anandan et al., 2005; Leeet al., 2009). Over the last four decades, Ru(II) polypyridylcomplexes have attracted tremendous interest as photosen-sitizers for conversions of solar energy into chemical orelectrical energy (Anandan et al., 2005; Lee et al., 2009).However, there are some potential drawbacks of noblemetal-based devices, including high costs and environmen-tal concerns. It is therefore worthwhile to develop alterna-tives that overcome these drawbacks.

In this regard, one could be eager for other families ofmetal complexes that are comparable to, or even surpassRu(II)polypyridyl complexes. Since copper(I) complexeswith 2,9-disubstituted-1,10-phenanthroline ligands wererecognized to possess similar photophysical properties toarchetypal [Ru(bpy)3]

2+ (bpy = 2,2-bipyridine) salts (Vanteet al., 1983), these copper(I) complexes of bpy and 1,10-phe-nanthroline derivatives have attracted attention as

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

doi:10.1016/j.solener.2011.04.018

Corresponding author. Tel./fax: +86 10 58805476.E-mail address: kzwang@bnu.edu.cn (K.-Z. Wang).

www.elsevier.com/locate/solener

Available online at www.sciencedirect.com

Solar Energy 85 (2011) 17801786

http://dx.doi.org/10.1016/j.solener.2011.04.018mailto:kzwang@bnu.edu.cnhttp://dx.doi.org/10.1016/j.solener.2011.04.018http://dx.doi.org/10.1016/j.solener.2011.04.018mailto:kzwang@bnu.edu.cnhttp://dx.doi.org/10.1016/j.solener.2011.04.018

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sensitizers for solar cells (Alonso-Vante et al., 1994; Sakakiet al., 2002; Bessho et al., 2008), and related researchadvances have been given in two elegant recent reviews(Robertson, 2008; Ardo and Meyer, 2009). In addition,Cu(II) complexes have also played important roles as bothsensitizers and electron-transfer mediators in photoelectro-

chemical solar cells. The copper(II) complexes of [(-)-spar-teine-N,N0](maleonitriledithiolato-S,S0)copper ([Cu(SP)(mmt)])0/, bis(2,9-dimethy-1,10-phenanthroline)copper([Cu(dmp)2]

2+/+), and bis(1,10-phenanthroline)copper ([Cu(phen)2]

2+/+), and Ru dye cis-dithiocyanato-N,N0-bis(4-car-boxylato-4-tetrabutylammonoiumcarboxylate-2,20-bipyri-dine)ruthenium(II) (N719) were reported as electron-transfermediators and sensitizer in dye-sensitized solar cells, achiev-ing light power conversion efficiencies of 0.1%, 1.4%, and1.3%, respectively (Hattori et al., 2005). Sing and coworkersreported copper(II) phthalocyanine-based solid-stateorganic hetero-junction solar cell of indiumtin oxide (ITO)/poly(3,4-ethylenedioxythiophene):polystyrene sulfonic acid/

copper phthalocyanine/3,4,9,10-perylenetetracarboxylicbisbenzimidazole/aluminum, obtaining an open-circuitphotovoltage of 1.15 V and a short-circuit photocurrentdensity of 0.125 mA/cm2 (Singh et al., 2006). Valli et al.reported copper(II) tetrakis-(isopropoxy-carbonyl)phtha-locyanine modified TiO2 nanocrystal hybrid heterojunctiondevice with a low photocurrent density of $15 nA/cm2

(Ingrosso et al., 2009). Zakeeruddin and Gratzel et al.reported a Cu(II) complex 2,7,12,17-tetrapropionic acidof 3,8,13,18-tetramethyl-21H,23Hporphyrin sensitizedTiO2 nanocrystalline solar cell with a power conversionefficiency of 3.8% (Alibabaei et al., 2010). By coordination

of copper(II)-2,20

:60

,200

-terpyridine (CuII

tpy) with oxidizedcarbon nanohorns (CNHsCOOH), Tagmatarchis andIto et al. prepared CNHs-COO-CuIItpy metallo-nanocom-plexes, and confirmed intercomponent photoinducedcharge separation with the aid of an electron mediator ofhexyl-viologen dication (Rotas et al., 2008). Zhong et al.reported polythiophene carrying 8-hydroxyquinoline com-plex with Cu(II) as a dye sensitizer in dye-sensitized solarcell, giving an open-circuit photovoltage of 0.63 V, ashort-circuit photocurrent density of 1.872 mA/cm2, anda power conversion efficiency of 0.78% (Xiao et al.,2010). Although the photoelectric conversion efficienciesso far reported for Cu(I) and Cu(II) complexes remainedlower than those for Ru(II) dyes, the metal copper hasadvantages of the natural resource abundance of copperore, and environmentally friendly over other metals. Tothe best of our knowledge, copper complexes reported inliterature as photoelectrochemical solar cell sensitizers orelectron-transfer mediators have been very rare comparedto Ru(II) sensitizers (Robertson, 2008; Ardo and Meyer,2009). Here we report a red 12-membered tetra-imine mac-rocycle Cu(II) complex as a good photoelectrochemicalsensitizer for ITO electrode with a large cathodic photo-current density of 71 lA/cm2 under white light irradiationof 70 mW/cm2, and an incident photon to current

efficiency (IPCE) of 1.1% at k = 660 nm.

2. Experimental

2.1. Materials

[CuL](NO3)2 (L = 2,3,8,9-tetraphenyl-1,4,7,10-tetraaza-cyclododeca-1,3,7,9-tetraene) was prepared according to

the literature (Bhoon and Singh, 1981), and its molecularstructure is shown in Fig. 1. The other reagents were com-mercially available, and were used without furtherpurification.

2.2. Substrates

Indiumtin oxide (ITO) coated glass substrates werewashed by ultrasonication in detergent and deionized waterfor 10 min each, and were then put into a mixture consist-ing of 25% NH3H2O, 30% H2O2, and deionized water (v/v/v, 1:1:5) at 70 C for 20 min, followed by rinsing with

copious deionized water and then vacuum dried.

2.3. Preparation of the film electrode [CuL](NO3)2/ITO

The ITO sheets precleaned as described above, werecovered with the acetontirile solutions of the copper com-plex with varied concentrations from 4 to 20 mM bydropping pipettes, and were then vacuum dried at 55 Cfor 30 h. The thus obtained film electrodes were soakedin 1 M Na2SO4 aqueous solutions and were subjected towhite light irradiation (70 mW/cm2) for 5 min prior toUVvisible (UVvis) spectroscopy, scanning electronmicroscopy, cyclic voltammetry and photoelectrochemicalmeasurements.

2.4. Instrumentation

Ultraviolet-vis spectra of [CuL](NO3)2 films on ITOwere measured on a Cintra 10 e UVVis spectrophotome-ter with a blank ITO as the reference. All of the electro-chemical and photoelectrochemical experiments wereperformed on a CHI-601 voltammetric analyzer in athree-electrode conventional cell at the ambient laboratorytemperature (22 2 C) in 1 M Na2SO4 aqueous solution,equipped with an ITO glass modified with the copper com-plex, a platinum wire and a saturated calomel electrode(SCE) as working, counter and reference electrodes, respec-tively. Throughout the experiments, an IR light filter was

Fig. 1. Molecular structure of [CuL](NO3)2.

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fitted with a 500 W xenon lamp system, in order to protectelectrodes from heating. The white light intensities weremeasured with a Model ST-900M Photometer (Photoelec-tric Instrument Factory of Beijing Normal University).To acquire the photocurrent action spectrum, a monochro-matic light was obtained from a 500 W xenon lamp

(Changtuo Photoelectronic Technology Ltd., Beijing, PRChina) fitted with a certain additional bandpass filter withthe spectral width of 5 nm. The monochromatic lightintensities were measured with a light gauge radiometer,which is corrected by standard silicon cells (FZ-A, Photo-electric Instrument Factory of Beijing Normal University).Morphology and composition analysis of a bare ITO glassand [CuL](NO3)2 films on ITO glass were performed on aHitachi X-650 scanning electron microscope with anEDAX-9100 energy dispersive spectrum device, using anaccelerating voltage of 10 kV.

3. Results and discussion

3.1. Scanning electron microscopy

The surface morphology and composition analysis ofthe films were characterized by scanning electron micro-scope (SEM), and the SEM images are shown in Fig. S1,supporting information. Clearly, the bare ITO film washomogeneous and flat without continuous island-like clus-ters or aggregates (see Fig. S1a), while the surface morphol-ogy (see Fig. S1b) of [CuL](NO3)2 on ITO showed closelypacked and randomly stacked clusters with diameters rang-ing from several hundreds nanometers to several microme-

ters, making the film rough and jaggy in surface. Theelemental compositions of both the bare ITO and theCu(II) complex-coated ITO were also determined usingthe SEM coupled with the energy-dispersive X-ray(EDX) analyzer system, and the composition analysisresults as shown in Table S1 in the supporting information,indicating that the composition elements of N and Cu werefound in the spectrum of [CuL](NO3)2 film on ITO, andwere absent in that of the bare ITO.

3.2. UVvis absorption spectroscopy

UVvis absorption spectra of [CuL](NO3)2 in acetoni-trile and in the film on ITO are compared in Fig. 2. Thecopper complex in acetonitrile showed an absorption peakcentered at 600 nm which is assignable to the dd transitionfor a distorted tetragonal copper(II) complex (Karabocekand Karabocek, 1997; Williams et al., 2002). The coppercomplex film on ITO exhibited a broad dd absorptionband centered at $700 nm, which is red-shifted by 85 nmwith respect to the acetonitrile solution, due most probablyto aggregate formation of the copper complex in the filmdue to the intermolecular pp* interactions. Note that theabsorption for the copper complex film extends into near-infrared region which is desirable for solar light harvesting,

and for photoelectric conversion accordingly.

3.3. Cyclic voltammetry

Cyclic voltammograms (see Fig. 3) of [CuL](NO3)2 onITO showed a quasi-reversible redox couple with a half-wave potential E1/2 {E1/2 = (Epa + Epc)/2} being found at+0.088 V vs. SCE, which corresponds to the metal-basedCuII/CuIII redox reaction (Ma et al., 2004; Fernandez-Get al., 2002; Yildirim et al., 2002; Raman et al., 2004; Palet al., 2005; Farias and Bastos, 2009). A linear increase incathodic peak currents with scan rates was obtained upto a scan rate of 0.4 V/s, indicating that the redox reactionwas a surface-confined rather than a diffusion controlledprocess (Bryce et al., 2003; Cheng and Dong, 2000; Chenget al., 2001). From an onset oxidation potential Eonset(Ox)

at +0.064 V vs. SCE (+0.305 V vs. normal hydrogen elec-trode, NHE), the value of the highest occupied molecularorbital (HOMO) energy level of the copper complex,EHOMO, was calculated according to Eq. (1) to be4.80 eV with respect to the vacuum level based on a valueof 4.74 eV for SCE with respect to the zero vacuum level(Wang et al., 2002).

Fig. 2. UVVis absorption spectra of [CuL](NO3)2 in acetonitrile (solidline) and the film on ITO glass substrate (dashed line).

Fig. 3. Cyclic voltammograms of a [CuL](NO3)2/ITO film in 0.1 Maqueous Na2SO4 at different scan rates. Insert shows a plot of the peak

currents vs. scan rates.

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EHOMO ef4:74 EonsetOxg 1

The value of the lowest unoccupied molecular orbital(LUMO) energy level of the copper complex, ELUMO,was calculated according to Eq. (2) to be 3.23 eV by tak-ing an onset absorption wavelength konset at 786 nm (seeFig. 2) for the Cu(II) complex on ITO

DE ELUMO EHOMO 1240=konset 2

3.4. Photoelectrochemistry

The effects of spin-coating solution concentrations onphotoeletrochemical responses were studied. The photo-currents were found to enhance steadily with increasingconcentrations of [CuL](NO3)2 from 1 to 4 mM, andremain almost constant over concentration range[Cu] = 430 mM (see Fig. S2). A concentration of 4 mMfor the spin-coating acetonitrile solution was thus chosen

for the preparation of the films for the following photo-electrochemical measurements.

Shown in Fig. 4 are the photocurrent responses for[CuL](NO3)2/ITO film in aerated and Ar-degassed 1 MNa2SO4 aqueous solution at applied potentials of 0.4,0.3 and 0.1 V vs. SCE under a white light irradiation(730 nm > k > 325 nm) of 70 mW/cm2. The prompt catho-dic photocurrent responses were observed by switching theirradiation on, and the photocurrent traces were reproduc-ible under several on/off cycles. At the applied potential of0.4 V vs. SCE, the film electrode in the aerated NaClsolution exhibited a stable cathodic photocurrent densityof 71 lA/cm2; and as expected for the cathodic photocur-rent generation, the cathodic photocurrent density of thefilm electrode in the degassed solution was reduced toabout 20 lA/cm2. In contrast, a bare ITO electrode showeda negligible response, demonstrating clearly that the coppercomplex was responsible for the photocurrent generation.It is noteworthy that the photocurrent density of 71 lA/cm2 above-mentioned for [CuL](NO3)2 film is the highest

among those listed in Table 1 for some representative thinfilms previously reported (Ingrosso et al., 2009; Lang et al.,1998; Taniguchi et al., 1999; Yamada et al., 2003; Torreset al., 2000; Zou et al., 2010; Aoki et al., 1999; Wu et al.,1999; Han et al., 2007; Ju et al., 2010; Masayuki et al.,2002), and even more than 80-fold greater than those we

previously reported for the electrostatically self-assembledfilms of [Ru2(bpy)4(bpbp)2]4+/WO3 and [Ru2(b-

py)4(eip)2]4+/PB under similar experimental conditions

(Zou et al., 2010; Ju et al., 2010). An incident photon tocurrent efficiency (IPCE) value at a wavelength k of660 nm for [CuL](NO3)2/ITO film was calculated to be1.1%, according to Eq. (3) (Nazeeruddin et al., 1993):

IPCE% 1240IlA=cm2

knmPInc:W=m23

where I is the produced photocurrent, and PInc. is the inci-dent light power at the wavelength k. Interestingly, the

[CuL](NO3)2 film is much thicker than the self-assembledand LangmuirBlodgett films, and resulting in thatstrongly absorbed [CuL](NO3)2 film at k = 660 nm exhib-ited IPCE value of only 1.1%, which still compares favor-ably with those for the films listed in Table 1. It shouldbe pointed out that this efficiency value of 1.1% is much lessthan those reported for some Ru(II) complex-sensitizedTiO2 nanocrystalline solar cells, e.g. the two solar cellswe recently reported (Fan et al., 2009, 2010). The depen-dence of photocurrents on the applied bias voltages wasalso studied and the results are shown in Fig. 5. The photo-currents decreased with increasing bias voltages, demon-

strating that the photocurrents were generated byelectron moving from ITO electrode to the Cu(II) complex,and being cathodic. This cathodic photocurrent polaritywas also evidenced by additions of electron acceptormethyl viologen (MV2+) and electron donor ascorbic acidinto the electrolyte solution. It was found that photocur-rents increased with increasing viologen concentrationsover applied potential range from 0.2 V to 0 V, but al-most disappeared in the presence of the ascorbic acid.The photocurrent action spectrum and UV/Visible absorp-tion spectrum of [CuL](NO3)2/ITO film are overlaid inFig. 6. The both spectra are similar to each other, suggest-ing that the photocurrents originated from photoexcitationof [CuL](NO3)2 in the film.To understand the photocurrentgeneration mechanism, one should look closely at the elec-tron-transfer mechanism that took place within the film.Fig. 7 shows the energy level arrangements for the speciesinvolved in the photocurrent generation processes, in whichthe reduction potentials of methyl viologen and O2 as wellas Fermi level of ITO are taken to be 0.4, 0.26 and0.3 V vs. NHE, respectively (Ju et al., 2010; Imahoriet al., 2000). Clearly, the photocurrent was generated byfollowing photoinduced electron transfer cascade: (1) pho-toexcitation of ground state copper complex resulted in ex-cited complex [CuL]2+*, (2) and from which the electron

was transferred to MV2+, and (3) thus oxidized copper

Fig. 4. Photocurrent responses for [CuL](NO3)2/ITO film in aerated (solidline) and N2-degassed (dotted line) 1 M Na2SO4 aqueous solution atapplied potentials of0.4, 0.3 and 0.1 V vs. SCE under a white light

irradiation of 68 mW/cm

2.

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complex [CuL]3+ accepted electron from the Fermi level ofITO. It is conceivable that the high efficiency photoelectricconversion could result from matching between the

HOMO energy level of [CuIIL]/[CuIIIL] and the Fermi level

of ITO, and between the LUMO level of [CuIIL]/[CuIIIL]and redox potential of electron donor O2 or MV

2+.

4. Conclusions

A copper(II) complex-modified ITO electrode was fabri-cated by the solvent-evaporation method. The modifiedelectrode was demonstrated to exhibit good photoelectro-chemical properties with a photocurrent density and anincident photon to current efficiency (IPCE) values of ashigh as 71 lA/cm2 and 1.1%, respectively. The impressivephotoelectric properties exhibited by the copper complexcompare favorably with some representative sensitizerspreviously reported under similar experimental conditions,and but significant lower than Ru(II) complex-sensitizedTiO2 nanocrystalline solar cells. In view of its simple syn-thesis, low-cost and good photoelectrochemical properties,

[CuL](NO3)2 complex is an attractive candidate for further

Table 1The comparisons of photocurrent density (J) under white light irradiation and the incident photon to current efficiency (IPCE) of thin films prepared bysolvent evaporation (SE), self assembly, electrodeposition, and LangmuirBlodgett (LB) techniques.

Filmsa Film-forming technique J (lA/cm2) IPCE(%) Reference

[CuL](NO3)2 SE 71 1.1 This workTIPCuPc/TiO2 NC SA 0.015 Ingrosso et al. (2009)odqi LB 0.350.46 0.5 Lang et al. (1998)

[Ru(bpy)2(bpy-Poly-tPA)]2+ LB 0.16

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molecular modification, e.g. by enhancing its molar extinc-tion coefficients and grafting an anchoring group, for appli-cations in photoelectric devices.

Acknowledgements

The authors thank the National Natural Science Foun-dation (20771016, 20971016, and 90922004), Beijing Natu-ral Science Foundation (2072011), the FundamentalResearch Funds for the Central Universities, and Analyti-cal and Measurements Fund of Beijing Normal Universityfor financial supports.

Appendix A. Supplementary data

Scanning electron micrographs of bare ITO and[CuL](NO3)2/ITO film at top view under 70,000 magnifica-tion, and the effects of concentrations of the spin-coatingacetonitrile solutions of the copper complex on the photo-

currents obtained at a bias voltage of 0.1 V vs. SCE for[CuL](NO3)2/ITO films were available from supplementarydata. Supplementary data associated with this article canbe found, in the online version, at doi:10.1016/j.solener.2011.04.018.

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