One-Step Synthesis of CdS Sensitized TiO 2 Photoanodes for Quantum Dot-Sensitized Solar Cells by Microwave Assisted Chemical Bath Deposition Method

Published: May 02, 2011

r 2011 American Chemical Society 1472 dx.doi.org/10.1021/am200520q |ACS Appl. Mater. Interfaces 2011, 3, 1472–1478

RESEARCH ARTICLE

www.acsami.org

One-Step Synthesis of CdS Sensitized TiO2 Photoanodes forQuantum Dot-Sensitized Solar Cells by Microwave AssistedChemical Bath Deposition MethodGuang Zhu, Likun Pan,* Tao Xu, and Zhuo Sun

Engineering Research Center for Nanophotonics & Advanced Instrument, Ministry of Education, Department of Physics,East China Normal University, Shanghai 200062, China

bS Supporting Information

’ INTRODUCTION

Sensitized solar cells have attracted considerable attention andrepresent a key class of cell architecture that has emerged as apromising candidate for the development of next generationsolar cells because of their acceptable power conversion effi-ciency and low production cost.1�15 As sensitizers for sensitizedsolar cells, inorganic semiconductor quantum dots (QDs), suchas PbS, CdS, CdSe, and InAs, have been suggested along withorganometallic or organic dyes because QDs have advantages ofhigh extinction coefficient, spectral tunability by particle size, andgood stability, which are known to increase the overall powerconversion efficiency of solar cells.16�19 In quantum-dot-sensi-tized solar cells (QDSSCs), the performance of QDs and theirinterconnectivity with TiO2 substrate is the key for deviceperformance. Many studies have been devoted to exploredifferent fabrication techniques to attach QDs onto TiO2. Sofar, the QDs have been successfully sensitized on the surface ofTiO2 by self-assembled monolayer via linker assistance or directadsorption,16,20,21 chemical bath deposition (CBD),19,22,23 elec-trochemical deposition,24,25 and photodeposition26 techniques.Recently, Gao et al.27 and Lee et al.28 studied a close spacesublimation technique in which CdS powder was heated at500 �C in a furnace with Ar or Ar/H2 gas flow to form CdSnanoparticles onto the surface of TiO2 for QDSSCs. Lee et al.

29

reported that QDSSCs based on CdS coated TiO2 electrodesusing spray pyrolysis deposition (SPD) method showed a powerconversion efficiency of 1.84% in I�/I3

� electrolyte and 0.87% inpolysulfide electrolyte.

As an inexpensive, quick, clean, versatile technique, microwaveirradiation induces interaction of the dipole moment of polarmolecules or molecular ionic aggregates with alternating electro-nic and magnetic fields, causing molecular-level heating whichleads to homogeneous and quick thermal reactions.30,31 How-ever, microwave technique is seldom studied by now to coat CdSQDs onto TiO2 electrode for QDSSCs, although such a methodhas been used successfully to fabricate CdS QDs,32�34

nanowires,35 and nanotubes.36

In this work, we fabricate sensitized-type solar cells based onTiO2 photoanode and CdS QDs as sensitizers, in which CdSQDs are prepared using microwave assisted chemical bathdeposition (MACBD) method, and investigate their photovol-taic performance. Compared with those methods, MACBDtechnique can synthesize rapidly CdS QDs and control preciselytheir sizes with a narrow distribution, improve the wettability ofTiO2 surface and form a good contact between CdS QDs andTiO2 layer due to rapidly elevated temperature duringmicrowaveirradiation. Furthermore, this technique can offer an easy controlover all experimental parameters without the requirement ofrepetitive immersing operation, organic linker or high tempera-ture heating. The as-synthesized cell shows a high short-circuitcurrent density of 7.20 mAcm-2 and conversion efficiency of1.18% under one sun illumination as compared with the cellsfabricated using CBD and SPD methods.

Received: January 11, 2011Accepted: May 2, 2011

ABSTRACT: Sensitized-type solar cells based on TiO2 photo-anodes and CdS quantum dots (QDs) as sensitizers have beenstudied. CdS QDs are grown on TiO2 films, utilizing one-stepmicrowave assisted chemical bath deposition (MACBD) method.This method allows a facile and rapid deposition and integrationbetween CdS QDs and TiO2 films. The photovoltaic perfor-mances of the cells fabricated using CdS precursor solutions withdifferent concentrations are investigated. The results show that thecell based on MACBD deposited TiO2/CdS electrode achieves amaximum short circuit current density of 7.20 mAcm-2 and powerconversion efficiency of 1.18 % at one sun (AM 1.5G, 100 mW cm-2), which is comparable to the ones prepared using conventionaltechniques.

KEYWORDS: solar cells, quantum dots, electrodes, microwave-assisted chemical bath deposition, titanium dioxide

1473 dx.doi.org/10.1021/am200520q |ACS Appl. Mater. Interfaces 2011, 3, 1472–1478

ACS Applied Materials & Interfaces RESEARCH ARTICLE

’EXPERIMENTAL SECTION

TiO2 electrode was prepared by screen printing of TiO2 paste37 on

the F-doped SnO2 (FTO) (resistivity: 14 Ω/0, Nippon Sheet Glass,Japan). The electrode configuration was a transparent layer of nano-crystalline (NC)-TiO2 (P25, Degussa) with a mean size of 25 nm and ascattering layer microcrystalline (MC)-TiO2 with a mean size of 200 nm(DalianHeptaChroma SolarTechCo., Ltd.). The as-prepared electrodeswere sintered at 500 �C for 30 min.Subsequently, CdS sensitizer was deposited on the mesoporous TiO2

electrodes by MACBD technique using a precursor aqueous solution ofCd(NO3)2 and CH4N2S (Sinopharm Chemical Reagents Co. Ltd.). Inthe precursor solution, the concentrations of Cd(NO3)2 and CH4N2Sare same. The TiO2 electrodes were immersed into a vessel withprecursor aqueous solution, and then the vessel was put into anautomated focused microwave system (Explorer-48, CEM Co.) andtreated at 150 �Cwithmicrowave irradiation power of 100W for 30min.In this work, TiO2/CdS films fabricated using 0.01, 0.015, 0.025, 0.05,and 0.1 M Cd(NO3)2 and CH4N2S solutions are named as electrodes 1,2, 3, 4, and 5, respectively.In this work, in order to compare the performances of QDSSCs

fabricated using different methods, CdS QDs are also deposited on theTiO2 film by traditional CBD1,19 and SPD38 methods. In CBD process,TiO2 film was dipped into an ethanol solution containing 0.5 M

Cd(NO3)2 for 5 min, rinsed with ethanol, and then dipped for another5 min into a 0.5 M Na2S methanol solution and rinsed again withmethanol. The two-step dipping procedure was considered to be onecycle. This sequential coating was repeated for several cycles. In the SPDmethod, CdS sensitizer was deposited on the preheated TiO2 films to430 �C for 15 min using a precursor aqueous solution of 0.1 M cadmiumchloride and 0.1 M thiourea.

The morphology and structure of TiO2 and CdS QDs incorporatedTiO2 (TiO2/CdS) electrodes were characterized by using aM21XVHF2Z(Mac Science Co. Ltd) X-ray diffractometer with Cu KR radiation (V =35 kV, I = 20 mA), a Renishaw Raman spectrometer (ViaþReflex) systemwith a 514.5 nm excitation source, aHitachi S-4800 field-emission scanningelectron microscopy (FESEM), and a JEOL-2010 high-resolution trans-mission electron microscope (HRTEM), respectively. The UV-vis absorp-tion spectra of electrodes were detected using a UV-vis spectrophotometer(Hitachi U-3900).

The CdS QDSSCs were sealed in a sandwich structure with a 25 μmspacer (Surlyn) by using thin Au-sputtered FTO glass as counterelectrode. Water/methanol (3:7 by volume) solution was used as aco-solvent of the polysulfide electrolyte.39 Electrolyte solution consistsof 0.5 M Na2S, 2 M S, and 0.2 M KCl. The active area of the cell is0.2 cm2. J�V measurement was performed with a Keithley model 2440Source Meter and a Newport solar simulator system (equipped with a1 kW xenon arc lamp, Oriel) at one sun (AM 1.5 G, 100 mW cm2).Incident photon to current conversion efficiency (IPCE) was measuredas a function of wavelength from 300 to 800 nm using an Oriel 300Wxenon arc lamp and a lock-in amplifier M 70104 (Oriel) undermonochromator illumination. Electrochemical impedance spectroscopy(EIS)measurements40�42 were carried out in dark conditions at forwardbias: 0�0.6 V, applying a 10 mV AC sinusoidal signal over the constantapplied bias with the frequency ranging between 100 kHz and 0.1 Hz(Autolab, PGSTAT 302N and FRA2 module).

’RESULTS AND DISCUSSION

Images a and b in Figure 1 show the FESEM images (topview) of NC-TiO2 underlayer and MC-TiO2 overlayer films,respectively. The TiO2 electrode is constructed by a randomagglomeration of TiO2 particles. The porous structure of TiO2

favors an easy penetration of electrolyte, as well as Cd and Sprecursors, during deposition. The cross-section FESEM imagein Figure 1c displays clearly the two layer TiO2 electrodeconsisting of a 10 μm thick compact P25 NC-TiO2 transparentlayer and a 3 μm thick loose MC-TiO2 scattering layer. Suchdouble-layer structure can favor the contact between the sub-strate and electrode and enhance the light scattering ability,

Figure 1. Surfacemorphologies of (a) NC-TiO2 underlayer, (b)MC-TiO2

overlayer, (d) electrode 1, (e) electrode 2, (f) electrode 3, (g) electrode 4,and (h) electrode 5 measured by FESEM. Insets are correspondingmagnified FESEM images. (c) Cross-section FESEM image of as-preparedTiO2 electrode.

Figure 2. EDS spectrum of electrode 4.



which can improve the performance of the cells.43 Figure 1d�hshows the FESEM images (top view) of the electrodes 1�5,

respectively. It can be observed from Figure 1d and e that smallamount of CdS has been distributed on the surface of TiO2 filmas compared with bare TiO2 film (Figure 1b) because of a lowconcentration of precursor solution. A more compact surfacewith porous structure is observed from Figure 1f and g, indicat-ing that with the increase in the precursor concentration, theamount of CdS deposited on the TiO2 film increases gradually.However, excessive deposition of CdS blocks most pores in theelectrode, as seen from Figure 1h, which is not beneficial tothe access of electrolyte deep into the surface porous structure.The composition of electrode 4 was identified by energydispersive X-ray spectroscopy (EDS) linked to FESEM, asshown in Figure 2. Quantitative analysis of the EDS spectrumgives a Cd:S atomic ratio of about 1, indicating that high-gradeCdS particles are formed. Electrodes 1, 2, 3, and 5 have similarEDS results.

Figure 3a shows a high-magnification HRTEM image of CdSQDs prepared by using 0.05 M precursor aqueous solution ofCd(NO3)2 and CH4N2S with microwave irradiation power of100 W at 150 �C for 30 min. It is clearly found that as-preparedCdS QDs have a fine crystallite and a narrow size distribution in∼5 nm. Figure 3b shows a low-magnification HRTEM image ofelectrode 4. It is observed that the aggregation is composed ofsmall NC-TiO2 and bigger MC-TiO2 particles. CdS is difficult tobe observed in low-magnification HRTEM image. The corre-sponding selected area electron diffraction (SAED) pattern in anupper right inset in Figure 3b indicates that TiO2/CdS film is apolycrystalline structure. Figure 3c shows a high-magnificationHRTEM image of electrode 4. The larger crystallites are identi-fied to be MC-TiO2 (left) and NC-TiO2 (right). The latticespacing measured for the crystalline plane is 0.352 nm, corre-sponding to the (101) plane of anatase TiO2 (JCPDS 21�1272).Around the TiO2 crystallite edge, a fine crystallite is observed.The crystallite with a size of ∼5 nm connecting to the TiO2 haslattice fringes of 0.335 nm which is ascribed to (111) plane ofCdS (JCPDS 80-0019). The HRTEM image and the elementalanalysis of EDS linked to HRTEM (see the Supporting Informa-tion, Figure S1,) confirm the existence of CdS in the electrode.The structure of the obtained CdS QDs byMACBDmethod wasfurther characterized by X-ray diffraction (XRD) measurement.Figure 4 shows the XRD patterns of pure TiO2 and TiO2/CdSfilm (electrode 4). Compared to pure TiO2, TiO2/CdS filmexhibits new peaks corresponding to (111), (220), and (311)planes of CdS, indicating the presence of the cubic phase CdS(JCPDS 80-0019).

Figure 3. (a) High-magnification HRTEM image of CdS prepared byMACBD; (b) low-magnification and (c) high-magnification HRTEMimages of electrode 4, inset is the corresponding SAED pattern.

Figure 4. XRD patterns of pure TiO2 electrode and TiO2/CdSelectrode 4.



Figure 5 shows the Raman spectra of pure TiO2, TiO2/CdS(MACBD, electrode 4), TiO2/CdS(CBD), and TiO2/CdS-(SPD) films, respectively. The spectrum of TiO2 is dominated by395, 516, and 637 cm�1 bands, characteristic of TiO2 anatasephase. Compared with pure TiO2 film, a longitudinal-optical(LO) mode at around 300 cm�1, together with its overtones at600 and 900 cm-1 for CdS is observed in the spectra of all TiO2/CdS films, showing that a combination of two semiconductingcharacteristic bands. It should be noticed that the LO peak ofTiO2/CdS(CBD) and TiO2/CdS(SPD) exhibit a small red-shiftas compared with that of TiO2/CdS(MACBD), which reflects acertain degree of phonon confinement and indicates that theparticle size of CdS QDs via MACBD deposition is somewhatlarger.44 Furthermore, the LO bandwidths of both TiO2/CdS(MACBD) and TiO2/CdS(SPD) are almost same butnarrower than that of TiO2/CdS(CBD), indicating that as com-pared with CBD method, a higher degree crystallinity and fewerstructural defects can be attained via MABCD method, whichdoesn’t require the high-temperature treatment as used in the SPDmethod.

Figure 6 displays the UV�vis absorption spectra of pure TiO2

film and TiO2/CdS electrodes 1-5. Compared with the absorp-tion spectra of pure TiO2 film, there is an obvious absorption

peak near 500 nm for TiO2/CdS films, which is ascribed to thecontribution from CdS QDs. The band gap of CdS QDscorresponding to the absorption edge is about 2.38 eV. Theabsorbance gradually increases with the increase in the concen-tration of the CdS precursor solution, indicating that more CdSQDs have been deposited onto the TiO2 film. This result isconsistent to the FESEM observation.

The J�V curves of CdS QDSSCs with different electrodes1�5 are shown in Figure 7a. The open circuit potential (Voc),

Figure 5. Raman spectra of pure TiO2, TiO2/CdS(MACBD), TiO2/CdS(CBD), and TiO2/CdS(SPD) electrodes, respectively.

Figure 6. UV�vis absorption spectra of pure TiO2 and TiO2/CdSelectrodes 1�5.

Figure 7. J�V curves of CdS QDSSCs with electrodes 1�5 (a) underone sun illumination (AM 1.5 G, 100 mW cm�2) and (b) in darkconditions; (c) IPCE curves of CdS QDSSCs with electrodes 1�5.



short circuit current density (Jsc), fill factor (FF) and conversionefficiency (η) of all cells are listed in Table 1. The Jsc and Voc ofthe cell with electrode 1 are 4.49 mA cm�2 and 0.40 V,respectively, resulting in a very low value of conversion efficiency(0.63%).Whenmore CdSQDs are deposited onto the TiO2 film,Jsc, Voc and η increase obviously and reach maximum values of7.20 mA cm�2, 0.46 V and 1.18% at a precursor concentration of0.05M, and then decrease with a further increase of CdS amount.At the beginning of the deposition, the MACBD process issupposed to increase the coverage ratio of CdS on the TiO2

surface by replenishing the uncovered area and the thickness ofCdS layer increases with the increase of the concentration of theCdS precursor solution. Such increment of CdS loading leads tomore excited electrons under the illumination of light, which isadvantageous to the photocurrent. An optimized thickness ofCdS layer is obtained at some concentration of precursorsolution, which results in a highest Jsc and η by providing a goodinterfacial structure between TiO2 and CdS films and reducingthe recombination of the injected electrons from TiO2 to theelectrolyte because of a well-covered CdS layer on the TiO2

surface. However, as the thickness of CdS layer further increases,it will be more difficult to transport an electron from the CdSlayer into the TiO2 film because of the increase in the averagedistance and electron injection time from CdS to TiO2.

44 In themeantime, the CdS/electrolyte contacting area will decrease withthe increase of the CdS amount because more pores are probablyblocked by the additional loading of CdS, leading to unfavorableelectron transportation at TiO2/CdS/electrolyte interface.45,46

This inference was confirmed by the J�V curves of the cells withdifferent electrodes 1�5 in dark conditions, as shown inFigure 7b. The applied voltage required to drive the electronsacross the photoelectrodes is highest for the cell with electrode 4,which is due to a well-covered CdS layer on the TiO2 surface.This result indicates that the cell with electrode 4 has a superiorinterfacial structure to inhibit the interfacial recombination of theinjected electrons from TiO2 to the electrolyte,

47,48 which is alsoresponsible for its higher conversion efficiency. The incidentphoton to current conversion efficiency (IPCE) curves of CdSQDSSCs with electrodes 1�5, as shown in Figure 7c, exhibit asimilar trend as J�V curves. A maximum IPCE value of 65 % at470 nm is obtained for the cell with electrode 4.

The charge transfer and recombination behavior in the CdSQDSSCs were further studied by analyzing the EIS spectra atvarious applied voltage in dark conditions. Figure 8a shows thetypical Nyquist plots of the cells with electrodes 1�5 obtained atan applied voltage of 0.4 V. The EIS spectra are characterized bythe presence of two semicircles in a Nyquist plot.40,41 The highfrequency semicircle is related to the charge transfer resistance(Rct) at the interfaces of the electrolyte/counter electrode andthe low-frequency one is due to the contribution from thechemical capacitance of nanostructured TiO2 (Cμ) and the

charge recombination resistance (Rrec) between TiO2 and thepolysulfide electrolyte.40,41 The corresponding equivalent circuitis shown in the inset of Figure 8a.49,50 Figure 8b shows the Rrecof the cells with electrodes 1-5 at various applied potentials(0-0.6 V) obtained from EIS fitting. It can be observed that Rrec

decreases with the increase of applied potential and the cell withelectrode 4 shows a highest recombination resistance (i.e., lowestrecombination) compared to other cells, which explains thehighest Jsc measured for the cell with electrode 4. It should bepointed out that lower Rrec of the cell with electrode 5 ascompared with the cell with electrode 4 should be ascribed tothe loss of effective electrochemical active area of the electrodebecause excessive CdS hinders the access of electrolyte deep intothe surface porous structure.21 The EIS result is consistent withthe dark current measurement in Figure 7b.

Table 1. Photovoltaic Parameters of CdS QDSSCs withElectrodes 1�5

electrode Jsc (mA cm�2) Voc (V) FF (%) η (%)

1 4.49 0.40 35.5 0.63

2 6.56 0.44 33.8 0.98

3 6.93 0.44 33.8 1.04

4 7.20 0.46 35.1 1.18

5 4.98 0.44 35.8 0.79

Figure 8. (a) Nyquist plots of the cells with electrodes 1�5 at anapplied voltage of 0.4 V. Inset displays the corresponding equivalentcircuit and (b) recombination resistance (Rrec), as a function of appliedvoltage.

Table 2. Comparison of Photovoltaic Performances of CdSQDSSCs Fabricated Using CBD, SPD, and MACBDMethods

method Jsc (mA cm�2) Voc (V) FF (%) η (%)

CBD 5.45 0.49 39 1.06

SPD 4.00 0.50 37 0.76

MACBD 7.20 0.46 35 1.18



The photovoltaic performances of QDSSCs based on TiO2/CdS(MACBD), TiO2/CdS(CBD), and TiO2/CdS(SPD) elec-trodes under one sun illumination (AM 1.5 G, 100 mW cm�2)are compared (see the Supporting Information, Figure S2) andthe results are summarized in Table 2. It can be observed thathigher Jsc and η values are achieved via MACBD method ascompared with CBD and SPD methods. The higher Jsc and η forthe cell using MACBD method should be ascribed to thefollowing reasons: (i) Good contact between CdS QDs andTiO2 layer is formed due to rapidly elevated temperature duringmicrowave irradiation. CdS deposition on the surface of TiO2 viaMACBD method is a three-step process (see the SupportingInformation, Figure S3). In the first step, positively chargedCd2þ ions form a complex with TiO2 film. In the second step,thermal decomposition of CH4N2S in the presence of microwaveirradiation releases S2- which reacts with Cd2þ to produce CdSnuclei. The CdS nuclei grow, crystallize and stabilize on the TiO2

under microwave irradiation,51 leading to a good contact be-tween CdS and TiO2. On the other hand, microwave irradiationhas been used to prepare the hydrophilic nanoparticles52 or toimprove the surface wettability of polymer by increasing surfacefree energy.53,54 The improvement of surface wettability of TiO2

film by microwave treatment has been observed by contact anglemeasurement of water droplets on the surface of TiO2 electrodeunder microwave treatment (see the Supporting Information,Figure S4), which also can favor CdS deposition on TiO2 film

55

and form a good contact between them. Such a superior interfacebetween TiO2 and CdS via MACBD method can inhibit theinterfacial recombination of the injected electrons from TiO2

to the electrolyte, which is responsible for its higher Jsc and η.19

(ii) Microwave irradiation can heat up the aqueous solutionhomogeneously and fast because of the penetration characteristicof microwaves and high utilization factor of microwaveenergy.56,57 Therefore, the nucleation and growth of CdS QDscan be finished in an extremely short period of time, which isextraordinarily beneficial for reducing the concentration of sur-face defects of QDs.58,59 The carrier recombination at surfacedefects of QDs is correspondingly suppressed and thus the cellperformance is increased.60,61

’CONCLUSIONS

In summary, we have demonstrated a simple, rapid, andeffective MACBD method to deposit CdS on TiO2 film asphotoanode for QDSSCs. This method can synthesize CdSQDs rapidly and form good contact between CdS and TiO2

film. Amaximum 7.20mA cm�2 short-circuit current density and1.18% conversion efficiency under one sun illumination has beenachieved by using MACBD method, which is comparable tothose using conventional CBD and SPD techniques. The presentsynthetic strategy should be a promising fabrication techniquefor QDSSCs.

’ASSOCIATED CONTENT

bS Supporting Information. The elemental analysis by EDSlinked to HRTEM (Figure S1); J�V and IPCE curves ofQDSSCs based on different electrodes (Figure S2); schematicdiagram of the process of CdS deposition on TiO2 film undermicrowave irradiation (Figure S3); photographic images of waterdroplets on the surface of TiO2 electrode under microwave

treatment (Figure S4) (PDF). This material is available free ofcharge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*Tel: 86 21 62234132. Fax: 86 21 62234321. E-mail: [email protected].

’ACKNOWLEDGMENT

This workwas supported by Special Project forNanotechnologyof Shanghai (1052 nm02700).

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