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The CdS/CdSe/ZnS Photoanode Cosensitized Solar Cells Basedon Pt, CuS, Cu2S, and PbS Counter Electrodes

The CdS/CdSe/ZnS Photoanode Cosensitized Solar Cells Basedon Pt, CuS, Cu2S, and PbS Counter... Hindawi Publishing Corporation Advances in OptoElectronics Volume 2014, Article ID 397681, 9 pages http://dx.doi.org/10.1155/2014/397681 Research Article The CdS/CdSe/ZnS Photoanode Cosensitized Solar Cells Basedon Pt, CuS, Cu S, and PbS Counter Electrodes 1 2 3 Tung Ha Thanh, Dat Huynh Thanh, and Vinh Quang Lam Faculty of Physics, Dong aTh p University, Dong aTh p Province, Cao Lanh City 870000, Vietnam Vietnam National University, Ho Chi Minh City 700000, Vietnam University of Science, Vietnam National University, Ho Chi Minh City 700000, Vietnam Correspondence should be addressed to Tung Ha aTh nh; tunghtvlcrdt@gmail.com Received 14 October 2013; Revised 25 December 2013; Accepted 25 December 2013; Published 27 February 2014 Academic Editor: Armin Gerhard Aberle Copyright © 2014 Tung Ha Thanh et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Highly ordered mesoporous TiO modified by CdS, CdSe, and ZnS quantum dots (QDs) was fabricated by successive ionic layer adsorption and reaction (SILAR) method. eTh quantity of material deposition seems to be aec ff ted not only by the employed deposition method but also and mainly by the nature of the underlying layer. The CdS, CdSe, and ZnS QDs modification expands the photoresponse range of mesoporous TiO from ultraviolet region to visible range, as confirmed by UV-Vis spectrum. Optimized anode electrodes led to solar cells producing high current densities. Pt, CuS, PbS, and Cu Shavebeenusedaselectrocatalysts on counter electrodes. The maximum solar conversion efficien cy reached in this work was 1.52% and was obtained by using Pt electrocatalyst. CuS, PbS, and Cu S gave high currents and this was in line with the low charge transfer resistances recorded in their case. 1. Introduction a good choice. However, their deposition on plain FTO electrodes does not always produce materials with sufficiently As an alternative to dye molecules, semiconductor quantum high specicfi surface or with structural stability. dots (QDs) like CdS, CdSe [1], PbS [2], InAs [3], InP [4], and In this paper, we studied the effects of comodification others [5] as well as extremely thin inorganic absorber layers by CdS, CdSe, and ZnS QDs on the photovoltaic response [6, 7] have been used. QDs are very attractive because of their of mesoporous TiO basedQDSSC.ThemesoporousTiO 2 2 size-dependent optical band gap, the possibility to design were treated by SILAR of CdS, CdSe, and ZnS QDs and hierarchical multilayer absorber structures, and the potential were used as photoanodes in QDSSC. We demonstrated that to use them for multiexciton generation from a single photon. the comodied fi mesoporous TiO possesses superior pho- One potential method for improving the performance of tovoltaic response compared to the single QD sensitized quantum dots solar cells (QDSSCs) is by constructing desired devices. Pt, CuS, PbS, and Cu Shavebeenusedaselectrocat- energy band structures using multiple QDs. Niitsoo and co- alysts on counter electrodes. eTh na fi l TiO /CdS/CdSe/ZnS workers have rfi stly demonstrated that a desired cascade photoanode leads to high efficiency QDSSCs. structure can be formed by sequential deposition of CdS and CdSe layers onto the TiO nanoparticle films [ 8]. Recently, Lee et al. have also reported a self-assembled TiO /CdS/CdSe 2 2. Experiment structure that exhibited a signicfi ant enhancement in the pho- tocurrent response [9, 10]. In addition, nanostructured CuS, 2.1. Materials. Cd(CH COO)⋅2H O(99%),Cu(NO ) , 3 2 2 3 2 PbS, Cu S, andPthavebeenusedaselectrocatalystsonthe Na S, Zn(NO ) ,Sepowder,Spowder,Na SO ,and Brass 2 2 3 2 2 3 counter electrode. Alternative catalysts have been proposed foil were obtained from Merck. TiO paste was obtained by several researchers [9–12]. Metal sulfides are considered from Dyesol, Australia. 2 Advances in OptoElectronics FTO/TiO /QDs FTO/TiO 2 FTO 2 (a) (b) (c) 300 C, vacuum 500 C SILAR Printed silk (d) (e) (f ) CdS CdSe ZnS Figure 1: eTh diagram shows the process to prepare the TiO /CdS/CdSe/ZnS photoanode. 2.2. To Prepare TiO Films. The TiO thin films were fab- of every layer of CdS, CdSe, and ZnS are 40 nm, 43.3 nm, 2 2 ricated by silk-screen printing with commercial TiO paste. 40 nm, respectively. eTh ir sizesrangedfrom10to20nm. Twolayersoffilm with thickness of 8𝜇 m were measured by microscope. eTh n, 2.4. Construction of the Counter Electrodes. PbS films were ∘ ∘ the TiO film was heated at 400 Cfor 5min and500 Cfor 2 deposited on u fl orine doped tin oxide (FTO) conductive glass 30 min. Afterwards, the film was dipped in 40 mmol TiCl electrode by cyclic voltammetry (CV) from the solution of ∘ ∘ solution for 30 min at 70 C and heated at 500 Cfor 30min. Pb(NO ) 1.5 mM and Na S O 1.5 mM. CV experiments 3 2 2 2 3 The specific surface area of the mesoporous TiO was 2 were carried out at various potential scan rates in a potential investigated by using the N adsorption and desorption range 0.0 to –1.0 V versus Ag/AgCl/KCl electrode, pH from isotherms before and aer ft the calcination. eTh surface area is 2.4 to 2.7, and ambient temperature. Pt films were fabricated 2 −1 120.6 m g (measured by BET devices). This result indicates by silk-screen printing with commercial Pt paste. en, Th the Pt that the synthesized material has wider mesoporous struc- films were heated at 450 C for 30 min. CuS was also deposited ture. on FTO electrodes by a SILAR procedure, by modifying the method presented in [13]. Precursor solutions contained 0.5 mol/L Cu(NO ) in methanol and 1 mol/dm Na S⋅9H O 3 2 2 2 2.3. To Prepare TiO /CdS/CdSe/ZnS Films. The highly in a 1 : 1 water : methanol mixture. An FTO electrode was ordered TiO was sequentially sensitized with CdS, CdSe, immersed for5mininthe metalsaltsolution, copiously andZnS QDsbySILAR method.First,the TiO film was washed with triple-distilled water, dried in an air stream, dipped in 0.5 mol/L Cd(CH COO) -ethanol solution for 3 2 immersed for 5 min in the Na S⋅9H Osolution, andfinally 2 2 5 min, rinsed with ethanol, dipped for 5 min in 0.5 mol/L washed and dried again. This sequence again corresponds to Na S-methanol solution, and then rinsed with methanol. one SILAR cycle. 10 SILAR cycles were performed. Finally, The two-step dipping procedure corresponded to one the electrode with deposited CuS film was rfi st dried and SILARcycle andthe incorporated amount of CdSQDs ∘ then it wasput for5mininanovenat100 C. The counter was increased by repeating the assembly cycles for a total electrode was a Cu S film fabricated on brass foil. Brass foil of three cycles. For the subsequent SILAR process of CdSe wasimmersedinto37% HClat70 C for 5 min, rinsed with QDs, aqueous Se solution was prepared by mixing Se water, and dried in air. Aeft r that, the etched brass foil was powder and Na SO in 50mL pure wateraeft radding 2 3 dipped into 1 mol/L S and 1 mol/L Na Saqueous solution, 1mol/L NaOH at 70 Cfor 7h.TheTiO /CdS samples were resulting in a black Cu S layer forming on the foil [14]. dipped into 0.5 mol/L Cd(CH COO) -ethanol solution 3 2 for 5 min at room temperature, rinsed with ethanol, 2.5. Fabrication of QDSSCs. The polysulfide electrolyte used dipped in aqueous Se solution for 5 min at 50 C, and in this work was prepared freshly by dissolving 0.5 M Na S, rinsed with pure water. eTh two-step dipping procedure 0.2 M S, and 0.2 M KCl in Milli-Q ultrapure water/methanol corresponds to one SILAR cycle. Repeating the SILAR (7 : 3 by volume). The CdS/CdSe/ZnS cosensitized TiO cycle increases the amount of CdSe QDs (a total of four photoanode and Pt counter electrode were assembled into a cycles). The SILAR method was also used to deposit the ZnS sandwich cell by heatingwithaSurlyn.Theelectrolyte was passivation layer. eTh TiO /CdS/CdSe samples were coated filled from a hole made on the counter electrode, which was with ZnS by alternately dipping the samples in 0.1 mol/L later sealed by thermal adhesive film and a cover glass. The Zn(NO ) and 0.1 mol/L Na S-solutions for 5 min/dip, 3 2 2 active area of QDSSC was 0.38 cm . rinsing with pure water between dips (a total of two cycles). Finally, it was heated in a vacuum environment with different temperatures to avoid oxidation (see Figure 1). The 2.6. Characterizations and Measurements. The morphology thickness of TiO /CdS/CdSe/ZnS photoanodes were meas- of the prepared samples was observed using field-emission ured by Microscopic. The results of the average thickness scanning electron microscopy (FE-SEM, S4800). eTh crystal Advances in OptoElectronics 3 mau3-TiO -CdS-CdSe-ZnS Cd Zn Na Ti Ti 600 Ti Cd Si Se 300 Zn Cd Ti Se Zn Zn (keV) (b) (a) (c) Figure 2: (a) FE-SEM images of the TiO /CdS/CdSe/ZnS photoanode, (b) energy dispersiveanalysisofX-ray spectra(EDAX)ofthe TiO /CdS/CdSe/ZnS photoanode, (c) cross-sectional view of the TiO /CdS/CdSe/ZnS photoanode. 2 2 structure was analyzed with an X-ray diffractometer (Philips, (111), (220), and (331) of cubic CdS (JCPDS Card number 41- PANalytical X’pert, CuK𝛼 radiation). eTh absorption prop- 1049) and CdSe (JCPDS Card number 75-5681), respectively. ∘ ∘ erties of the nanotube array samples were investigated using Two peaks can be observed at 48 and 54.6 ,which can a diffuse reflectance UV-Vis spectrometer (JASCO V-670). be indexed to (220) and (331) of cubic ZnS, respectively. Photocurrent voltage measurements were performed on a It demonstrates that the QDs have been crystallized onto Keithley 2400 sourceMeter using a simulated AM 1.5 sunlight the TiO film. Figure 3(b) is the Raman spectrum of the TiO /QDs photoelectrodes. It shows that an anatase structure with an output power of 100 mW/cm produced by a solar of the TiO films has vfi e oscillation modes corresponding simulator (Solarena, Sweden). −1 wave numbersat143,201,395,515,and 636cm . In addition, −1 twopeaks canbeobservedat201,395,and 515cm ,which 3. Results and Discussion can be indexed to the cubic structure of CdS and CdSe. The results of the Raman are likely similar to the results of the ShowninFigures 2(a) and 2(b) are the FESEM images XRD. The optical performance of the QDs coated TiO film of TiO /CdS/CdSe/ZnS photoanode film. Figure 2(a) shows 2 is characterized by absorbance. Figure 3(c) shows the UV-Vis highly uniform porous morphology with the average inner absorption spectra of the sensitized electrodes measured aeft r diameter of nanostructure around 60 nm. For photovoltaic each cycle of SILAR. As expected, the absorbance increased applications, the structure of QDs adsorbed TiO should 2 with the deposition cycles of CdS, CdSe, and ZnS. However, meet at less two criteria. First, the QDs should be uniformly only absorption spectra with SILAR cycles of the electrode deposited onto the TiO surface without aggregation, so that 2 TiO /CdS(3)/CdSe(3)/ZnS(2) show the best photovoltaic the area of TiO /QDs can be maximized. Second, a moderate 2 performance as discussed in the following section. In short- amount the QDs should be deposited so that TiO is not 2 wavelength region (380–550 nm), the increase of absorbance blocked. is due to the fact that more CdS was loaded on TiO film and Figure 2(c) is a cross-sectional image showing that thecoabsorptionofCdS,CdSe, andZnS.Inlong-wavelength the QDs are well deposited onto the TiO with an 2 region (550–629 nm), the deposition of higher amounts of average thickness of about 12𝜇 m by the microscope. CdSe and ZnS on TiO /CdS electrode results in the increase Figure 2(b) is the energy dispersive X ray spectroscopy of the of absorbance. Moreover, the increasing successive deposi- TiO /CdS/CdSe/ZnS film. It shows that the Ti and O peaks 2 tion cycles also trigger a red shift of absorption spectrum are from the TiO film; and Cd, Se, Zn, and S peaks, clearly 2 whichisdue to aslightlossofquantum connfi ement eeff ct visibleinthe EDSspectrum, arefromthe QDs. eTh Si is from [15]. The evaluated sizes of CdS and CdSe are consistent with theFTO andCis from thesolvent organic. aTh tshows that the sizes measured from the FE-SEM images. eTh eect ff of theQDs arewelldeposited onto theTiO . 2 deposition cycles of CdS, CdSe, and ZnS can be clearly seen The structure of the TiO /QDs photoelectrodes for pho- on the energy band gap values of CdS/CdSe/ZnS cosensitized tovoltaic applications, shown in Figure 3(a),isstudied by the TiO films. eTh estimated band gaps vary from 1.97 eV to XRDpatterns.ItrevealsthattheTiO has an anatase structure 2.7 eV, which are higher than the values reported for CdS and with a strong (101) peak located at 25.4 ,which indicatesthat CdSe in bulk (2.25 eV and 1.7 eV resp.[16]), indicating that the the TiO films are well crystallized and grow along the (101) sizes of CdS, CdSe, and ZnS on TiO films are still within the 2 2 direction (JCPDS Card number 21-1272). Three peaks can be scale of QDs. eTh diameter of QDs was calculated from 2 nm ∘ ∘, ∘ observed at 26.4 ,44 and 51.6 ,which canbeindexed to to 6 nm by (1). A higher absorption is thus obtained because Counts 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 4 Advances in OptoElectronics −1 TiO (101) 1000 2 E 40000 143 cm CdS(111) CdSe(111) TiO (004) ZnS(220) 400 E CdSe(220) −1 ZnS(331) 636 cm CdS(220) 10000 A 1g B , 2LO CdSe(331) E , LO 1g g −1 −1 −1 515 cm CdS(331) 201 cm 395 cm −1 251 cm 30 40 50 60 100 200 300 400 500 600 700 −1 2𝜃 (deg) Raman shift (cm ) CdSe TiO /CdS/CdSe/ZnS CdS TiO /PbS/CdS/CdSe/ZnS at 150 C TiO ZnS TiO -anatase CdSe 2 2 CdS (a) (b) 460 nm 583nm 629 nm 400 500 600 700 800 Wavelength (nm) TiO /CdS(3)/CdSe(1)/ZnS(2) TiO /CdS(1)/CdSe(3)/ZnS(2) TiO /CdS(3)/CdSe(2)/ZnS(2) TiO /CdS(2)/CdSe(3)/ZnS(2) TiO /CdS(3)/CdSe(4)/ZnS(2) TiO /CdS(3)/CdSe(3)/ZnS(2) TiO /CdS(3)/CdSe(5)/ZnS(2) TiO /CdS(4)/CdSe(3)/ZnS(2) TiO /CdS(5)/CdSe(3)/ZnS(2) TiO /CdS(3)/CdSe(3)/ZnS(1) 2 2 (c) Figure 3: XRD (a), Raman (b), and UV-Vis (c) of the TiO /CdS/CdSe/ZnS photoanodes. theabsorptionspectrumofZnS complementsthose of the eTh XRDpatternswereusedtocharacterizethe crystalstruc- CdSe andCdS QDs. Furthermore, ZnSactsasapassivation ture of the obtained products. As shown in Figure 4(a),itcan layer to protect the CdS and CdSe QDs from photocorrosion be seen that the XRD pattern of the PbS counter electrode is [17]. Consider the following equation by Yu et al. [18]group: in conformity with cubic (𝑎=𝑏=𝑐=5.93 A). eTh observed peaks could be assigned to diffraction from the (111), (200), −9 4 −6 3 −3 2 (220), (311), and (222) faces and there is no characteristic peak 𝐷=1.6122⋅10 𝜆 −2.6575⋅10 𝜆 +1.6242⋅10 𝜆 for other impurities. This indicates that pure crystalline PbS −0.4277𝜆+41.57. wasformedvia thecyclicvoltammetry process. Figure 4(b) illustrates the XRD pattern of the synthesized Cu Saeft r (1) Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) Advances in OptoElectronics 5 Commander sample ID (coupled 2𝜃/𝜃 ) 20 30 40 50 20 30 40 50 60 2𝜃 2𝜃 (deg) PbS 0.25.brml (displacement) Cu S PDF 00-001-0880, PbS, Gatena PDF 00-046-1088, SnO , Cassiterite, syn (a) (b) 2000 FTO (100) 111 Cu O (101) 103 CuS 108 CuS 110 CuS 20 30 40 50 60 20 30 40 50 60 2𝜃 (deg) 2𝜃 (deg) CuS Pt (d) (c) Figure 4: XRD of the PbS, Cu S, Pt, and CuS counter electrodes. 1 h by chemical bath deposition (CBD) method. eTh peaks of 1.7 eV, the absorption of bulk CdSe is also limited below of corresponding crystal planes were indexed in the gur fi e, approximately 760 nm. eTh conduction band of CdSe is matching to the hexagonal phase chalcocite𝛽 -Cu S (JCPDS slightly lower than that of TiO , so the electrons would flow 2 2 ˚ ˚ card number 46-1195,𝑎 = 3.96 A,𝑐 = 6.78 A). Figure 4(c) from CdSe to TiO [20]. In addition,wehavecoatedtwo illustrates that the XRD pattern of the Pt films were fabricated layers of ZnS QDs, which could be attributed to several by silk-screen printing with commercial Pt paste. eTh peaks reasons. First, as the absorption edge of ZnS is at about of corresponding crystal planes were indexed in the gur fi e, 345 nm, a higher absorption can be obtained due to the matching to the hexagonal phase. As shown in Figure 4(d),it complement of the absorption spectrum of the ZnS with that canbeseenthattheXRDpatternoftheCuScounterelectrode of the CdSe and CdS QDs. Second, ZnS acts as a passivation is in conformity with the hexagonal phase. It is in agreement layer to protect the CdS and CdSe QDs from photocorrosion. with the reported data for CuS (JCPDS Card. number 79- u Th s, the photoexcited electrons can efficiently transfer into 2321). the conduction band of TiO . Third, the outer ZnS layer A relative energy level of different components is shown can also be considered to be a potential barrier between the in Figure 5(a). According to the data reported in the literature interface of QDs materials and the electrolyte. ZnS has a very [16, 19], thebandgap of TiO (3.2 eV) limits its absorption wide band gap of 3.6 eV; it is much wider than that on the rangebelow thewavelengthofabout 400nm. CdSe has CdS and CdSe QDs. As a result, the leakage of electrons a higher conduction band (CB) edge than TiO ,which is from theZnS,CdSe, andCdS QDsintothe electrolytecan favorable for electron injection. However, with a band gap be inhibited. As a result, an ideal model for the cosensitized Counts Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) 6 Advances in OptoElectronics ZnS ZnS −3.0 −1.5 −3.0 −1.5 CdSe −3.5 −1.0 CdS −3.5 −1.0 CdS TiO TiO CdSe −4.0 −0.5 −4.0 −0.5 CB CB −4.5 0.0 −4.5 0.0 −5.0 0.5 −5.0 0.5 −5.5 1.0 −5.5 1.0 −6.0 1.5 −6.0 1.5 −6.5 2.0 −6.5 2.0 −7.0 2.5 −7.0 2.5 VB VB −7.5 3.0 −7.5 3.0 −8.0 3.5 −8.0 3.5 (a) (b) Figure 5: (a) Relative energy levels of TiO , CdS, CdSe, and ZnS in bulk phase, (b) the proposed energy band structure of the TiO /CdS/CdSe/ZnS nanostructure interface. All the energy levels are referenced to NHE scale. CB and VB are conduction band and valence band [16, 19]. TiO electrode is shown in Figure 5(b).Aeft rCdSeand ZnS and is helpful to collect excited electrons from ZnS, CdSe, and QDs are sequentially deposited onto a TiO /CdS film, A CdS to TiO film. 2 2 cascade type energy band structure is constructed for the FF is determined from measurement of the IV curve and cosensitized photoanode. eTh best electron transport path is is defined as FF =𝑉 ⋅𝐼 /𝑉 ⋅𝐼 . FF depending max Max OC sc from the conduction band of ZnS and n fi ally reaching the on𝑉 values, the junction quality (related with the series OC conduction band of TiO . Meanwhile, this stepwise structure 𝑅 ), and the type of recombination in a solar cell. From 2 𝑠 is also favorable for the hole transport. Table 1,𝑉 values change according to the film thickness OC We prepared the photoanodes with many different layers from 0.29 to 0.76, corresponding to the change in FF from of QDs. Firstly, we have prepared CdS or CdSe films. How- 0.26 to 0.41. Therefore, the FF is the low value because ever, the results were of very low performance. Therefore, we 𝑉 is low. On the other hand,𝑉 values depend on the OC OC decided covering with ZnS layer for the following reasons. recombination process; they are particularly large;𝑉 gives OC Firstly, extendpeakadsorptionspectruminthe visiblelight low open-circuit voltages. In addition, FF is eeff cted by 𝑅 . region. Secondly, the ZnS layers which acted as the agent The equations of 𝑅 can be calculated by o Th ngpron and passivated the surface of QDs. Moreover, they protected coworkers [21] as follows: the light corrosion. u Th s, the conversion excited electrons 𝐼 +𝐼 −𝐼 𝑉 −𝑉 1 ph 𝑜 1 through the conduction band of TiO better. Thirdly, ZnS 1 2 𝑅 = − ln[ ]. (2) layers separatedthe surfaces of theCdS andCdSewith 𝐼 −𝐼 𝜆(𝐼 −𝐼 ) 𝐼 +𝐼 −𝐼 2 1 2 1 ph 𝑜 2 electrolyte. The ZnS has a very wide band gap of about 3.6 eV, much larger than other CdS and CdSe QDs. As a Two operating points are (𝐼 ,𝑉 )and (𝐼 ,𝑉 )onasingle𝐼 -𝑉 1 1 2 2 result, electrons move from CdS, CdSe, and ZnS to the curve.𝜆=𝑞/𝑛𝐾𝑇 ;𝐼 ,𝐼 are the photocurrent and the diode ph 𝑜 electrolyte can be inhibited. Figure 6(a) shows that the power reverse saturation current.𝑅 values are calculated from 55 to 2− conversion efficiencies of QDSSCs are increasing with the 158 mΩcm . This result indicate that fill factor will decrease SILARcycle number of CdS, CdSe,and ZnSat3,3,and 2, when VOC increase. respectively. It is noted that lower power conversion efficiency Four main types of counter electrodes have been studied. was obtained for those cells with either less than 3 CdS Their synthesis is detailed in experiment and method. eTh and CdSe SILAR cycles or more than 3 CdS and CdSe FESEM images of the corresponding electrocatalytic films SILAR cycles (Figure 6(b)). The TiO /CdS(3)/CdSe(3)/ZnS are shown in Figure 7 (inset). In the rfi st case, PbS films device shows an open-circuit voltage (𝑉 )of0.76V,a were deposited on u fl orine doped tin oxide (FTO) conductive OC short-circuit current density (𝐽 )of4.79mA/cm ,fill factor glass electrode by cyclic voltammetry (CV) from the solution sc (FF) of 0.41, and an energy conversion ecffi iency of 1.52%. of Pb(NO ) 1.5 mM and Na S O 1.5 mM. CV experiments 3 2 2 2 3 When the deposition cycles of CdS and CdSe increase, slight were carried out at various potential scan rates in a potential changes in𝑉 and FF values were obtained. Remarkably, range 0.0 to −1.0 V versus Ag/AgCl/KCl electrode, pH OC the 𝐽 decreases, which results in a substantial reduction from 2.40 to 2.70, and ambient temperature. CuS was also sc of efficiency from 1.52% to 0.45% ( Table 1). These results deposited on FTO electrodes by a SILAR procedure, by indicate that although better light absorption performance modifying the method presented in [13]. The electrode with wasobtainedwhenmoreCdSewas loaded on TiO /CdS, the deposited CuS film was rfi st dried and then it was put for excessive CdSe on TiO /CdS films may lead to an increase of 5 min in an oven at 100 C. The counter electrode was a Cu S 2 2 recombination in photoanodes. On the contrary, the increase film fabricated on brass foil. Brass foil was immersed into of ZnS leads to the increasing generation of photoelectron 37% HCl at 70 C for 5 min and then rinsed with water and Vacuum (eV) NHE (eV) 3.2 eV 2.25 eV 1.7 eV 3.6 eV Vacuum (eV) NHE (eV) 3.2 eV 2.39 eV 1.8 eV 3.6 eV Advances in OptoElectronics 7 1.6 5 1.2 0.8 0.4 24 68 10 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Sample Voltage (V) TiO /CdS(1)/CdSe(3)/ZnS(2) TiO /CdS(3)/CdSe(1)/ZnS(2) Experiment point 2 2 TiO /CdS(2)/CdSe(3)/ZnS(2) TiO /CdS(3)/CdSe(2)/ZnS(2) 2 2 TiO /CdS(3)/CdSe(3)/ZnS(2) TiO /CdS(3)/CdSe(4)/ZnS(2) 2 2 TiO /CdS(3)/CdSe(5)/ZnS(2) TiO /CdS(4)/CdSe(3)/ZnS(2) 2 2 TiO /CdS(3)/CdSe(3)/ZnS(1) TiO /CdS(5)/CdSe(3)/ZnS(2) (b) (a) Figure 6: (a) eTh J-V curves of the QDSSCs with different photoanodes under one sun illumination and (b) diagram shows the values efficiency of solar cells. Table 1: Photovoltaic performance parameters of QDSSCs based on different photoanodes. Solar cells 𝐽 (mA/cm ) 𝑉 (V) Fill factor FF Efficiency 𝜂 (%) sc OC TiO /CdS(1)/CdSe(3)/ZnS(2) 2.18 0.29 0.35 0.22 TiO /CdS(2)/CdSe(3)/ZnS(2) 4.28 0.54 0.37 0.86 TiO /CdS(3)/CdSe(3)/ZnS(2) 4.79 0.76 0.41 1.52 TiO /CdS(4)/CdSe(3)/ZnS(2) 5.73 0.39 0.31 0.68 TiO /CdS(5)/CdSe(3)/ZnS(2) 3.05 0.45 0.32 0.45 TiO /CdS(3)/CdSe(1)/ZnS(2) 6.05 0.356 0.256 0.55 TiO /CdS(3)/CdSe(2)/ZnS(2) 4.21 0.55 0.38 0.88 TiO /CdS(3)/CdSe(4)/ZnS(2) 3.30 0.48 0.31 0.50 TiO /CdS(3)/CdSe(5)/ZnS(2) 2.08 0.33 0.27 0.18 TiO /CdS(3)/CdSe(3)/ZnS(1) 7.03 0.39 0.26 0.73 Table 2: Photovoltaic parameters of solar cell modified by various cathodes. Solar cells 𝐽 (mA/cm ) 𝑉 (V) Fill factor FF Efficiency 𝜂 (%) sc OC PbS cathode 6.14 0.43 0.24 0.63 CuS cathode 5.72 0.38 0.31 0.68 Cu S cathode 4.2 0.55 0.376 0.87 Pt cathode 4.79 0.76 0.41 1.52 dried in air. Aeft r that, the etched brass foil was dipped into printing with commercial Pt paste. Then, the Pt films were 1mol/L S and 1mol/L Na S aqueous solution, resulting in heated at 450 Cfor 30min. In thehighmagnicfi ationimage ablack Cu S layer forming on the foil [14]. Figures 4(a), of Figure 4(d), one can distinguish the big blocks of FTO 4(b),and 4(c) show the image of PbS, CuS, Cu Sfilmsthat covered with Pt nanoparticles [22]; Figure 4 and Table 2 present a rough nanostructure, which are suitable for counter show that the maximum efficiency reached in the present electrodes. eTh similar Pt films were fabricated by silk-screen work, that is, 1.52%, was obtained with Pt on the counter Current density (mA/cm ) Efficiency (%) 8 Advances in OptoElectronics 0.0 0.1 0.2 0.3 0.4 0.1 0.2 0.3 0.4 Bias voltage V (V) Bias voltage V (V) OC OC PbS cathode CuS cathode (a) (b) 0.0 0.1 0.2 0.3 0.4 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Bias voltage V (V) Bias voltage V (V) OC OC Cu S cathode Pt counter electrode (c) (d) Figure 7: J-V curves of solar cells modified by various cathodes. electrode. The Pt electrocatalysts, that is, Cu S, CuS, and electrode. PbS, CuS, Cu S, andPtwereusedaselectrocata- 2 2 PbS, gave higher current densities than Pt but lower 𝑉 lysts on counter electrodes in combination with a polysulfide OC than Pt. On the contrary, open-circuit voltage values were electrolyte. The maximum solar conversion efficiency of practically not aeff cted by the electrocatalyst. eTh major 1.52% was obtained with a Pt counter electrode. eTh most problem encountered in the present work was with the value important ndin fi g of this work is the importance of the rfi st of the fill factor (FF). It remained below 0.42 and this limited nanostructure layer deposited on the mesoporous TiO film, the overall efficiency, even though, the current densities which aeff cted the quantity and the quality of the subsequent presently recorded were high. The search for a higher FF is QDs layers and the ensuing cell efficiency. High current an open question and has occupied many other researchers. densities were obtained with all cells having optimized anode It is believed that higher FFs will be obtained with even better electrodes. Among them, the highest currents were obtained electrocatalysts and more functional counter electrodes. with Pt electrocatalysts. 4. Conclusions Conflict of Interests QDSSCs have been constructed and optimized by combining eTh authors declare that there is no conflict of interests TiO with CdS, CdSe, and ZnS nanostructure on the anode regarding the publication of this paper. 2 2 Photocurrent density J (mA/cm ) Photocurrent density J (mA/cm ) SC SC Photocurrent density J (mA/cm ) Photocurrent density J (mA/cm ) SC SC Advances in OptoElectronics 9 Acknowledgments [15] H.M.Pathanand C. D. Lokhande,“Deposition of metal chalcogenide thin films by successive ionic layer adsorption and This work was supported by Vietnam National University reaction (SILAR) method,” Bulletin of Materials Science,vol.27, with the name of the project being B 2012-18-05TD, the pp. 85–111, 2004. University of Science of Ho Chi Minh City, and Dong Thap [16] M. Gratzel ¨ , “Photoelectrochemical cells,” Nature,vol.414,no. University. 6861, pp. 338–344, 2001. [17] Z. Tachan, M. Shalom, I. Hod, S. Ruhle ¨ , S. Tirosh, and A. Zaban, “PbS as a highly catalytic counter electrode for polysulfide- References based quantum dot solar cells,” JournalofPhysicalChemistry C, vol. 115, no. 13, pp. 6162–6166, 2011. [1] M. Shalom, S. Ruhle, I. Hod et al., “Energy level alignment in CdS quantum dot sensitized solar cells using molecular dipoles,” [18] W. W. Yu, L. Qu, W. Guo, and X. Peng, “Experimental deter- Journalofthe American Chemical Society,vol.131,no. 29,pp. mination of the extinction coeicient of CdTe, CdSe and CdS 9876–9877, 2009. nanocrystals,” Chemistry of Materials,vol.15, no.14, pp.2854– 2860, 2003. [2] P.Yu, K. Zhu, A. G. Norman,S.Ferrere,A.J.Frank,and A. J. Nozik, “Nanocrystalline TiO solar cells sensitized with InAs [19] C. G. Van de Walle and J. Neugebauer, “Universal alignment quantum dots,” JournalofPhysicalChemistry B,vol.110,no. 50, of hydrogen levels in semiconductors, insulators and solutions,” pp. 25451–25454, 2006. Nature,vol.423,no. 6940,pp. 626–628, 2003. [3] A.Zaban,O.I.Mici ´ c, ´ B. A. Gregg, and A. J. Nozik, “Photosensi- [20] Y. L. Lee and Y. S. Lo, “Highly efficient quantum-dot-sensitized tization of nanoporous TiO electrodes with InP quantum dots,” 2 solar cell based on Co-sensitization of CdS/CdSe,” Advanced Langmuir, vol. 14, no. 12, pp. 3153–3156, 1998. Functional Materials,vol.19, no.4,pp. 604–609,2009. [4] S. Ruhle ¨ , M. Shalom, and A. Zaban, “Quantum-dot-sensitized [21] J. Thongpron, K. Kirtikara, and C. Jivacate, “A method for the solar cells,” ChemPhysChem, vol. 11, no. 11, pp. 2290–2304, 2010. determination of dynamic resistance of photovoltaic modules [5] C. Herzog, A. Belaidi, A. Ogacho, and T. Dittrich, “Inorganic under illumination,” in Proceedings of the Technical Digest of solid state solar cell with ultra-thin nanocomposite absorber the 14th International Photovoltaic Science and Engineering basedonnanoporousTiO and In S ,” Energy and Environmen- 2 2 3 Conference (PVSEC14 ’04),Bangkok,Thailand,January 2004. tal Science,vol.2,no. 9, pp.962–964,2009. [22] N. Balis, T. Makris, V. Dracopoulos, T. Stergiopoulos, and P. [6] S.-J. Moon, Y. Itzhaik, J.-H. Yum, S. M. Zakeeruddin, G. Hodes, Lianos, “Quasi-solid-state dye-sensitized solar cells made with and M. 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Liau, “CdS/CdSe co-sensitized TiO photoelectrode for efficient hydrogen generation in a photoelectrochemical cell,” Chemistry of Materials,vol.22, no. 3, pp. 922–927, 2010. [11] A. L. Efros, M. Rosen, M. Kuno, M. Nirmal, D. J. Norris, and M. Bawendi, “Band-edge exciton in quantum dots of semicon- ductors with a degenerate valence band: dark and bright exciton states,” Physical Review B,vol.54, no.7,pp. 4843–4856, 1996. [12] L. E. Brus, “Electron-electron and electron-hole interactions in small semiconductor crystallites: the size dependence of the lowest excited electronic state,” The Journal of Chemical Physics , vol. 80, p. 4403, 1984. [13] T.-L. Li, Y.-L. Lee, and H. Teng, “High-performance quantum dot-sensitized solar cells based on sensitization with CuInS quantum dots/CdS heterostructure,” Energy and Environmental Science,vol.5,no. 1, pp.5315–5324,2012. [14] J. Tian, R. Gao, Q. 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The CdS/CdSe/ZnS Photoanode Cosensitized Solar Cells Basedon Pt, CuS, Cu2S, and PbS Counter Electrodes

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Hindawi Publishing Corporation
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Copyright © 2014 Tung Ha Thanh et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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10.1155/2014/397681
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Hindawi Publishing Corporation Advances in OptoElectronics Volume 2014, Article ID 397681, 9 pages http://dx.doi.org/10.1155/2014/397681 Research Article The CdS/CdSe/ZnS Photoanode Cosensitized Solar Cells Basedon Pt, CuS, Cu S, and PbS Counter Electrodes 1 2 3 Tung Ha Thanh, Dat Huynh Thanh, and Vinh Quang Lam Faculty of Physics, Dong aTh p University, Dong aTh p Province, Cao Lanh City 870000, Vietnam Vietnam National University, Ho Chi Minh City 700000, Vietnam University of Science, Vietnam National University, Ho Chi Minh City 700000, Vietnam Correspondence should be addressed to Tung Ha aTh nh; tunghtvlcrdt@gmail.com Received 14 October 2013; Revised 25 December 2013; Accepted 25 December 2013; Published 27 February 2014 Academic Editor: Armin Gerhard Aberle Copyright © 2014 Tung Ha Thanh et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Highly ordered mesoporous TiO modified by CdS, CdSe, and ZnS quantum dots (QDs) was fabricated by successive ionic layer adsorption and reaction (SILAR) method. eTh quantity of material deposition seems to be aec ff ted not only by the employed deposition method but also and mainly by the nature of the underlying layer. The CdS, CdSe, and ZnS QDs modification expands the photoresponse range of mesoporous TiO from ultraviolet region to visible range, as confirmed by UV-Vis spectrum. Optimized anode electrodes led to solar cells producing high current densities. Pt, CuS, PbS, and Cu Shavebeenusedaselectrocatalysts on counter electrodes. The maximum solar conversion efficien cy reached in this work was 1.52% and was obtained by using Pt electrocatalyst. CuS, PbS, and Cu S gave high currents and this was in line with the low charge transfer resistances recorded in their case. 1. Introduction a good choice. However, their deposition on plain FTO electrodes does not always produce materials with sufficiently As an alternative to dye molecules, semiconductor quantum high specicfi surface or with structural stability. dots (QDs) like CdS, CdSe [1], PbS [2], InAs [3], InP [4], and In this paper, we studied the effects of comodification others [5] as well as extremely thin inorganic absorber layers by CdS, CdSe, and ZnS QDs on the photovoltaic response [6, 7] have been used. QDs are very attractive because of their of mesoporous TiO basedQDSSC.ThemesoporousTiO 2 2 size-dependent optical band gap, the possibility to design were treated by SILAR of CdS, CdSe, and ZnS QDs and hierarchical multilayer absorber structures, and the potential were used as photoanodes in QDSSC. We demonstrated that to use them for multiexciton generation from a single photon. the comodied fi mesoporous TiO possesses superior pho- One potential method for improving the performance of tovoltaic response compared to the single QD sensitized quantum dots solar cells (QDSSCs) is by constructing desired devices. Pt, CuS, PbS, and Cu Shavebeenusedaselectrocat- energy band structures using multiple QDs. Niitsoo and co- alysts on counter electrodes. eTh na fi l TiO /CdS/CdSe/ZnS workers have rfi stly demonstrated that a desired cascade photoanode leads to high efficiency QDSSCs. structure can be formed by sequential deposition of CdS and CdSe layers onto the TiO nanoparticle films [ 8]. Recently, Lee et al. have also reported a self-assembled TiO /CdS/CdSe 2 2. Experiment structure that exhibited a signicfi ant enhancement in the pho- tocurrent response [9, 10]. In addition, nanostructured CuS, 2.1. Materials. Cd(CH COO)⋅2H O(99%),Cu(NO ) , 3 2 2 3 2 PbS, Cu S, andPthavebeenusedaselectrocatalystsonthe Na S, Zn(NO ) ,Sepowder,Spowder,Na SO ,and Brass 2 2 3 2 2 3 counter electrode. Alternative catalysts have been proposed foil were obtained from Merck. TiO paste was obtained by several researchers [9–12]. Metal sulfides are considered from Dyesol, Australia. 2 Advances in OptoElectronics FTO/TiO /QDs FTO/TiO 2 FTO 2 (a) (b) (c) 300 C, vacuum 500 C SILAR Printed silk (d) (e) (f ) CdS CdSe ZnS Figure 1: eTh diagram shows the process to prepare the TiO /CdS/CdSe/ZnS photoanode. 2.2. To Prepare TiO Films. The TiO thin films were fab- of every layer of CdS, CdSe, and ZnS are 40 nm, 43.3 nm, 2 2 ricated by silk-screen printing with commercial TiO paste. 40 nm, respectively. eTh ir sizesrangedfrom10to20nm. Twolayersoffilm with thickness of 8𝜇 m were measured by microscope. eTh n, 2.4. Construction of the Counter Electrodes. PbS films were ∘ ∘ the TiO film was heated at 400 Cfor 5min and500 Cfor 2 deposited on u fl orine doped tin oxide (FTO) conductive glass 30 min. Afterwards, the film was dipped in 40 mmol TiCl electrode by cyclic voltammetry (CV) from the solution of ∘ ∘ solution for 30 min at 70 C and heated at 500 Cfor 30min. Pb(NO ) 1.5 mM and Na S O 1.5 mM. CV experiments 3 2 2 2 3 The specific surface area of the mesoporous TiO was 2 were carried out at various potential scan rates in a potential investigated by using the N adsorption and desorption range 0.0 to –1.0 V versus Ag/AgCl/KCl electrode, pH from isotherms before and aer ft the calcination. eTh surface area is 2.4 to 2.7, and ambient temperature. Pt films were fabricated 2 −1 120.6 m g (measured by BET devices). This result indicates by silk-screen printing with commercial Pt paste. en, Th the Pt that the synthesized material has wider mesoporous struc- films were heated at 450 C for 30 min. CuS was also deposited ture. on FTO electrodes by a SILAR procedure, by modifying the method presented in [13]. Precursor solutions contained 0.5 mol/L Cu(NO ) in methanol and 1 mol/dm Na S⋅9H O 3 2 2 2 2.3. To Prepare TiO /CdS/CdSe/ZnS Films. The highly in a 1 : 1 water : methanol mixture. An FTO electrode was ordered TiO was sequentially sensitized with CdS, CdSe, immersed for5mininthe metalsaltsolution, copiously andZnS QDsbySILAR method.First,the TiO film was washed with triple-distilled water, dried in an air stream, dipped in 0.5 mol/L Cd(CH COO) -ethanol solution for 3 2 immersed for 5 min in the Na S⋅9H Osolution, andfinally 2 2 5 min, rinsed with ethanol, dipped for 5 min in 0.5 mol/L washed and dried again. This sequence again corresponds to Na S-methanol solution, and then rinsed with methanol. one SILAR cycle. 10 SILAR cycles were performed. Finally, The two-step dipping procedure corresponded to one the electrode with deposited CuS film was rfi st dried and SILARcycle andthe incorporated amount of CdSQDs ∘ then it wasput for5mininanovenat100 C. The counter was increased by repeating the assembly cycles for a total electrode was a Cu S film fabricated on brass foil. Brass foil of three cycles. For the subsequent SILAR process of CdSe wasimmersedinto37% HClat70 C for 5 min, rinsed with QDs, aqueous Se solution was prepared by mixing Se water, and dried in air. Aeft r that, the etched brass foil was powder and Na SO in 50mL pure wateraeft radding 2 3 dipped into 1 mol/L S and 1 mol/L Na Saqueous solution, 1mol/L NaOH at 70 Cfor 7h.TheTiO /CdS samples were resulting in a black Cu S layer forming on the foil [14]. dipped into 0.5 mol/L Cd(CH COO) -ethanol solution 3 2 for 5 min at room temperature, rinsed with ethanol, 2.5. Fabrication of QDSSCs. The polysulfide electrolyte used dipped in aqueous Se solution for 5 min at 50 C, and in this work was prepared freshly by dissolving 0.5 M Na S, rinsed with pure water. eTh two-step dipping procedure 0.2 M S, and 0.2 M KCl in Milli-Q ultrapure water/methanol corresponds to one SILAR cycle. Repeating the SILAR (7 : 3 by volume). The CdS/CdSe/ZnS cosensitized TiO cycle increases the amount of CdSe QDs (a total of four photoanode and Pt counter electrode were assembled into a cycles). The SILAR method was also used to deposit the ZnS sandwich cell by heatingwithaSurlyn.Theelectrolyte was passivation layer. eTh TiO /CdS/CdSe samples were coated filled from a hole made on the counter electrode, which was with ZnS by alternately dipping the samples in 0.1 mol/L later sealed by thermal adhesive film and a cover glass. The Zn(NO ) and 0.1 mol/L Na S-solutions for 5 min/dip, 3 2 2 active area of QDSSC was 0.38 cm . rinsing with pure water between dips (a total of two cycles). Finally, it was heated in a vacuum environment with different temperatures to avoid oxidation (see Figure 1). The 2.6. Characterizations and Measurements. The morphology thickness of TiO /CdS/CdSe/ZnS photoanodes were meas- of the prepared samples was observed using field-emission ured by Microscopic. The results of the average thickness scanning electron microscopy (FE-SEM, S4800). eTh crystal Advances in OptoElectronics 3 mau3-TiO -CdS-CdSe-ZnS Cd Zn Na Ti Ti 600 Ti Cd Si Se 300 Zn Cd Ti Se Zn Zn (keV) (b) (a) (c) Figure 2: (a) FE-SEM images of the TiO /CdS/CdSe/ZnS photoanode, (b) energy dispersiveanalysisofX-ray spectra(EDAX)ofthe TiO /CdS/CdSe/ZnS photoanode, (c) cross-sectional view of the TiO /CdS/CdSe/ZnS photoanode. 2 2 structure was analyzed with an X-ray diffractometer (Philips, (111), (220), and (331) of cubic CdS (JCPDS Card number 41- PANalytical X’pert, CuK𝛼 radiation). eTh absorption prop- 1049) and CdSe (JCPDS Card number 75-5681), respectively. ∘ ∘ erties of the nanotube array samples were investigated using Two peaks can be observed at 48 and 54.6 ,which can a diffuse reflectance UV-Vis spectrometer (JASCO V-670). be indexed to (220) and (331) of cubic ZnS, respectively. Photocurrent voltage measurements were performed on a It demonstrates that the QDs have been crystallized onto Keithley 2400 sourceMeter using a simulated AM 1.5 sunlight the TiO film. Figure 3(b) is the Raman spectrum of the TiO /QDs photoelectrodes. It shows that an anatase structure with an output power of 100 mW/cm produced by a solar of the TiO films has vfi e oscillation modes corresponding simulator (Solarena, Sweden). −1 wave numbersat143,201,395,515,and 636cm . In addition, −1 twopeaks canbeobservedat201,395,and 515cm ,which 3. Results and Discussion can be indexed to the cubic structure of CdS and CdSe. The results of the Raman are likely similar to the results of the ShowninFigures 2(a) and 2(b) are the FESEM images XRD. The optical performance of the QDs coated TiO film of TiO /CdS/CdSe/ZnS photoanode film. Figure 2(a) shows 2 is characterized by absorbance. Figure 3(c) shows the UV-Vis highly uniform porous morphology with the average inner absorption spectra of the sensitized electrodes measured aeft r diameter of nanostructure around 60 nm. For photovoltaic each cycle of SILAR. As expected, the absorbance increased applications, the structure of QDs adsorbed TiO should 2 with the deposition cycles of CdS, CdSe, and ZnS. However, meet at less two criteria. First, the QDs should be uniformly only absorption spectra with SILAR cycles of the electrode deposited onto the TiO surface without aggregation, so that 2 TiO /CdS(3)/CdSe(3)/ZnS(2) show the best photovoltaic the area of TiO /QDs can be maximized. Second, a moderate 2 performance as discussed in the following section. In short- amount the QDs should be deposited so that TiO is not 2 wavelength region (380–550 nm), the increase of absorbance blocked. is due to the fact that more CdS was loaded on TiO film and Figure 2(c) is a cross-sectional image showing that thecoabsorptionofCdS,CdSe, andZnS.Inlong-wavelength the QDs are well deposited onto the TiO with an 2 region (550–629 nm), the deposition of higher amounts of average thickness of about 12𝜇 m by the microscope. CdSe and ZnS on TiO /CdS electrode results in the increase Figure 2(b) is the energy dispersive X ray spectroscopy of the of absorbance. Moreover, the increasing successive deposi- TiO /CdS/CdSe/ZnS film. It shows that the Ti and O peaks 2 tion cycles also trigger a red shift of absorption spectrum are from the TiO film; and Cd, Se, Zn, and S peaks, clearly 2 whichisdue to aslightlossofquantum connfi ement eeff ct visibleinthe EDSspectrum, arefromthe QDs. eTh Si is from [15]. The evaluated sizes of CdS and CdSe are consistent with theFTO andCis from thesolvent organic. aTh tshows that the sizes measured from the FE-SEM images. eTh eect ff of theQDs arewelldeposited onto theTiO . 2 deposition cycles of CdS, CdSe, and ZnS can be clearly seen The structure of the TiO /QDs photoelectrodes for pho- on the energy band gap values of CdS/CdSe/ZnS cosensitized tovoltaic applications, shown in Figure 3(a),isstudied by the TiO films. eTh estimated band gaps vary from 1.97 eV to XRDpatterns.ItrevealsthattheTiO has an anatase structure 2.7 eV, which are higher than the values reported for CdS and with a strong (101) peak located at 25.4 ,which indicatesthat CdSe in bulk (2.25 eV and 1.7 eV resp.[16]), indicating that the the TiO films are well crystallized and grow along the (101) sizes of CdS, CdSe, and ZnS on TiO films are still within the 2 2 direction (JCPDS Card number 21-1272). Three peaks can be scale of QDs. eTh diameter of QDs was calculated from 2 nm ∘ ∘, ∘ observed at 26.4 ,44 and 51.6 ,which canbeindexed to to 6 nm by (1). A higher absorption is thus obtained because Counts 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 4 Advances in OptoElectronics −1 TiO (101) 1000 2 E 40000 143 cm CdS(111) CdSe(111) TiO (004) ZnS(220) 400 E CdSe(220) −1 ZnS(331) 636 cm CdS(220) 10000 A 1g B , 2LO CdSe(331) E , LO 1g g −1 −1 −1 515 cm CdS(331) 201 cm 395 cm −1 251 cm 30 40 50 60 100 200 300 400 500 600 700 −1 2𝜃 (deg) Raman shift (cm ) CdSe TiO /CdS/CdSe/ZnS CdS TiO /PbS/CdS/CdSe/ZnS at 150 C TiO ZnS TiO -anatase CdSe 2 2 CdS (a) (b) 460 nm 583nm 629 nm 400 500 600 700 800 Wavelength (nm) TiO /CdS(3)/CdSe(1)/ZnS(2) TiO /CdS(1)/CdSe(3)/ZnS(2) TiO /CdS(3)/CdSe(2)/ZnS(2) TiO /CdS(2)/CdSe(3)/ZnS(2) TiO /CdS(3)/CdSe(4)/ZnS(2) TiO /CdS(3)/CdSe(3)/ZnS(2) TiO /CdS(3)/CdSe(5)/ZnS(2) TiO /CdS(4)/CdSe(3)/ZnS(2) TiO /CdS(5)/CdSe(3)/ZnS(2) TiO /CdS(3)/CdSe(3)/ZnS(1) 2 2 (c) Figure 3: XRD (a), Raman (b), and UV-Vis (c) of the TiO /CdS/CdSe/ZnS photoanodes. theabsorptionspectrumofZnS complementsthose of the eTh XRDpatternswereusedtocharacterizethe crystalstruc- CdSe andCdS QDs. Furthermore, ZnSactsasapassivation ture of the obtained products. As shown in Figure 4(a),itcan layer to protect the CdS and CdSe QDs from photocorrosion be seen that the XRD pattern of the PbS counter electrode is [17]. Consider the following equation by Yu et al. [18]group: in conformity with cubic (𝑎=𝑏=𝑐=5.93 A). eTh observed peaks could be assigned to diffraction from the (111), (200), −9 4 −6 3 −3 2 (220), (311), and (222) faces and there is no characteristic peak 𝐷=1.6122⋅10 𝜆 −2.6575⋅10 𝜆 +1.6242⋅10 𝜆 for other impurities. This indicates that pure crystalline PbS −0.4277𝜆+41.57. wasformedvia thecyclicvoltammetry process. Figure 4(b) illustrates the XRD pattern of the synthesized Cu Saeft r (1) Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) Advances in OptoElectronics 5 Commander sample ID (coupled 2𝜃/𝜃 ) 20 30 40 50 20 30 40 50 60 2𝜃 2𝜃 (deg) PbS 0.25.brml (displacement) Cu S PDF 00-001-0880, PbS, Gatena PDF 00-046-1088, SnO , Cassiterite, syn (a) (b) 2000 FTO (100) 111 Cu O (101) 103 CuS 108 CuS 110 CuS 20 30 40 50 60 20 30 40 50 60 2𝜃 (deg) 2𝜃 (deg) CuS Pt (d) (c) Figure 4: XRD of the PbS, Cu S, Pt, and CuS counter electrodes. 1 h by chemical bath deposition (CBD) method. eTh peaks of 1.7 eV, the absorption of bulk CdSe is also limited below of corresponding crystal planes were indexed in the gur fi e, approximately 760 nm. eTh conduction band of CdSe is matching to the hexagonal phase chalcocite𝛽 -Cu S (JCPDS slightly lower than that of TiO , so the electrons would flow 2 2 ˚ ˚ card number 46-1195,𝑎 = 3.96 A,𝑐 = 6.78 A). Figure 4(c) from CdSe to TiO [20]. In addition,wehavecoatedtwo illustrates that the XRD pattern of the Pt films were fabricated layers of ZnS QDs, which could be attributed to several by silk-screen printing with commercial Pt paste. eTh peaks reasons. First, as the absorption edge of ZnS is at about of corresponding crystal planes were indexed in the gur fi e, 345 nm, a higher absorption can be obtained due to the matching to the hexagonal phase. As shown in Figure 4(d),it complement of the absorption spectrum of the ZnS with that canbeseenthattheXRDpatternoftheCuScounterelectrode of the CdSe and CdS QDs. Second, ZnS acts as a passivation is in conformity with the hexagonal phase. It is in agreement layer to protect the CdS and CdSe QDs from photocorrosion. with the reported data for CuS (JCPDS Card. number 79- u Th s, the photoexcited electrons can efficiently transfer into 2321). the conduction band of TiO . Third, the outer ZnS layer A relative energy level of different components is shown can also be considered to be a potential barrier between the in Figure 5(a). According to the data reported in the literature interface of QDs materials and the electrolyte. ZnS has a very [16, 19], thebandgap of TiO (3.2 eV) limits its absorption wide band gap of 3.6 eV; it is much wider than that on the rangebelow thewavelengthofabout 400nm. CdSe has CdS and CdSe QDs. As a result, the leakage of electrons a higher conduction band (CB) edge than TiO ,which is from theZnS,CdSe, andCdS QDsintothe electrolytecan favorable for electron injection. However, with a band gap be inhibited. As a result, an ideal model for the cosensitized Counts Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) 6 Advances in OptoElectronics ZnS ZnS −3.0 −1.5 −3.0 −1.5 CdSe −3.5 −1.0 CdS −3.5 −1.0 CdS TiO TiO CdSe −4.0 −0.5 −4.0 −0.5 CB CB −4.5 0.0 −4.5 0.0 −5.0 0.5 −5.0 0.5 −5.5 1.0 −5.5 1.0 −6.0 1.5 −6.0 1.5 −6.5 2.0 −6.5 2.0 −7.0 2.5 −7.0 2.5 VB VB −7.5 3.0 −7.5 3.0 −8.0 3.5 −8.0 3.5 (a) (b) Figure 5: (a) Relative energy levels of TiO , CdS, CdSe, and ZnS in bulk phase, (b) the proposed energy band structure of the TiO /CdS/CdSe/ZnS nanostructure interface. All the energy levels are referenced to NHE scale. CB and VB are conduction band and valence band [16, 19]. TiO electrode is shown in Figure 5(b).Aeft rCdSeand ZnS and is helpful to collect excited electrons from ZnS, CdSe, and QDs are sequentially deposited onto a TiO /CdS film, A CdS to TiO film. 2 2 cascade type energy band structure is constructed for the FF is determined from measurement of the IV curve and cosensitized photoanode. eTh best electron transport path is is defined as FF =𝑉 ⋅𝐼 /𝑉 ⋅𝐼 . FF depending max Max OC sc from the conduction band of ZnS and n fi ally reaching the on𝑉 values, the junction quality (related with the series OC conduction band of TiO . Meanwhile, this stepwise structure 𝑅 ), and the type of recombination in a solar cell. From 2 𝑠 is also favorable for the hole transport. Table 1,𝑉 values change according to the film thickness OC We prepared the photoanodes with many different layers from 0.29 to 0.76, corresponding to the change in FF from of QDs. Firstly, we have prepared CdS or CdSe films. How- 0.26 to 0.41. Therefore, the FF is the low value because ever, the results were of very low performance. Therefore, we 𝑉 is low. On the other hand,𝑉 values depend on the OC OC decided covering with ZnS layer for the following reasons. recombination process; they are particularly large;𝑉 gives OC Firstly, extendpeakadsorptionspectruminthe visiblelight low open-circuit voltages. In addition, FF is eeff cted by 𝑅 . region. Secondly, the ZnS layers which acted as the agent The equations of 𝑅 can be calculated by o Th ngpron and passivated the surface of QDs. Moreover, they protected coworkers [21] as follows: the light corrosion. u Th s, the conversion excited electrons 𝐼 +𝐼 −𝐼 𝑉 −𝑉 1 ph 𝑜 1 through the conduction band of TiO better. Thirdly, ZnS 1 2 𝑅 = − ln[ ]. (2) layers separatedthe surfaces of theCdS andCdSewith 𝐼 −𝐼 𝜆(𝐼 −𝐼 ) 𝐼 +𝐼 −𝐼 2 1 2 1 ph 𝑜 2 electrolyte. The ZnS has a very wide band gap of about 3.6 eV, much larger than other CdS and CdSe QDs. As a Two operating points are (𝐼 ,𝑉 )and (𝐼 ,𝑉 )onasingle𝐼 -𝑉 1 1 2 2 result, electrons move from CdS, CdSe, and ZnS to the curve.𝜆=𝑞/𝑛𝐾𝑇 ;𝐼 ,𝐼 are the photocurrent and the diode ph 𝑜 electrolyte can be inhibited. Figure 6(a) shows that the power reverse saturation current.𝑅 values are calculated from 55 to 2− conversion efficiencies of QDSSCs are increasing with the 158 mΩcm . This result indicate that fill factor will decrease SILARcycle number of CdS, CdSe,and ZnSat3,3,and 2, when VOC increase. respectively. It is noted that lower power conversion efficiency Four main types of counter electrodes have been studied. was obtained for those cells with either less than 3 CdS Their synthesis is detailed in experiment and method. eTh and CdSe SILAR cycles or more than 3 CdS and CdSe FESEM images of the corresponding electrocatalytic films SILAR cycles (Figure 6(b)). The TiO /CdS(3)/CdSe(3)/ZnS are shown in Figure 7 (inset). In the rfi st case, PbS films device shows an open-circuit voltage (𝑉 )of0.76V,a were deposited on u fl orine doped tin oxide (FTO) conductive OC short-circuit current density (𝐽 )of4.79mA/cm ,fill factor glass electrode by cyclic voltammetry (CV) from the solution sc (FF) of 0.41, and an energy conversion ecffi iency of 1.52%. of Pb(NO ) 1.5 mM and Na S O 1.5 mM. CV experiments 3 2 2 2 3 When the deposition cycles of CdS and CdSe increase, slight were carried out at various potential scan rates in a potential changes in𝑉 and FF values were obtained. Remarkably, range 0.0 to −1.0 V versus Ag/AgCl/KCl electrode, pH OC the 𝐽 decreases, which results in a substantial reduction from 2.40 to 2.70, and ambient temperature. CuS was also sc of efficiency from 1.52% to 0.45% ( Table 1). These results deposited on FTO electrodes by a SILAR procedure, by indicate that although better light absorption performance modifying the method presented in [13]. The electrode with wasobtainedwhenmoreCdSewas loaded on TiO /CdS, the deposited CuS film was rfi st dried and then it was put for excessive CdSe on TiO /CdS films may lead to an increase of 5 min in an oven at 100 C. The counter electrode was a Cu S 2 2 recombination in photoanodes. On the contrary, the increase film fabricated on brass foil. Brass foil was immersed into of ZnS leads to the increasing generation of photoelectron 37% HCl at 70 C for 5 min and then rinsed with water and Vacuum (eV) NHE (eV) 3.2 eV 2.25 eV 1.7 eV 3.6 eV Vacuum (eV) NHE (eV) 3.2 eV 2.39 eV 1.8 eV 3.6 eV Advances in OptoElectronics 7 1.6 5 1.2 0.8 0.4 24 68 10 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Sample Voltage (V) TiO /CdS(1)/CdSe(3)/ZnS(2) TiO /CdS(3)/CdSe(1)/ZnS(2) Experiment point 2 2 TiO /CdS(2)/CdSe(3)/ZnS(2) TiO /CdS(3)/CdSe(2)/ZnS(2) 2 2 TiO /CdS(3)/CdSe(3)/ZnS(2) TiO /CdS(3)/CdSe(4)/ZnS(2) 2 2 TiO /CdS(3)/CdSe(5)/ZnS(2) TiO /CdS(4)/CdSe(3)/ZnS(2) 2 2 TiO /CdS(3)/CdSe(3)/ZnS(1) TiO /CdS(5)/CdSe(3)/ZnS(2) (b) (a) Figure 6: (a) eTh J-V curves of the QDSSCs with different photoanodes under one sun illumination and (b) diagram shows the values efficiency of solar cells. Table 1: Photovoltaic performance parameters of QDSSCs based on different photoanodes. Solar cells 𝐽 (mA/cm ) 𝑉 (V) Fill factor FF Efficiency 𝜂 (%) sc OC TiO /CdS(1)/CdSe(3)/ZnS(2) 2.18 0.29 0.35 0.22 TiO /CdS(2)/CdSe(3)/ZnS(2) 4.28 0.54 0.37 0.86 TiO /CdS(3)/CdSe(3)/ZnS(2) 4.79 0.76 0.41 1.52 TiO /CdS(4)/CdSe(3)/ZnS(2) 5.73 0.39 0.31 0.68 TiO /CdS(5)/CdSe(3)/ZnS(2) 3.05 0.45 0.32 0.45 TiO /CdS(3)/CdSe(1)/ZnS(2) 6.05 0.356 0.256 0.55 TiO /CdS(3)/CdSe(2)/ZnS(2) 4.21 0.55 0.38 0.88 TiO /CdS(3)/CdSe(4)/ZnS(2) 3.30 0.48 0.31 0.50 TiO /CdS(3)/CdSe(5)/ZnS(2) 2.08 0.33 0.27 0.18 TiO /CdS(3)/CdSe(3)/ZnS(1) 7.03 0.39 0.26 0.73 Table 2: Photovoltaic parameters of solar cell modified by various cathodes. Solar cells 𝐽 (mA/cm ) 𝑉 (V) Fill factor FF Efficiency 𝜂 (%) sc OC PbS cathode 6.14 0.43 0.24 0.63 CuS cathode 5.72 0.38 0.31 0.68 Cu S cathode 4.2 0.55 0.376 0.87 Pt cathode 4.79 0.76 0.41 1.52 dried in air. Aeft r that, the etched brass foil was dipped into printing with commercial Pt paste. Then, the Pt films were 1mol/L S and 1mol/L Na S aqueous solution, resulting in heated at 450 Cfor 30min. In thehighmagnicfi ationimage ablack Cu S layer forming on the foil [14]. Figures 4(a), of Figure 4(d), one can distinguish the big blocks of FTO 4(b),and 4(c) show the image of PbS, CuS, Cu Sfilmsthat covered with Pt nanoparticles [22]; Figure 4 and Table 2 present a rough nanostructure, which are suitable for counter show that the maximum efficiency reached in the present electrodes. eTh similar Pt films were fabricated by silk-screen work, that is, 1.52%, was obtained with Pt on the counter Current density (mA/cm ) Efficiency (%) 8 Advances in OptoElectronics 0.0 0.1 0.2 0.3 0.4 0.1 0.2 0.3 0.4 Bias voltage V (V) Bias voltage V (V) OC OC PbS cathode CuS cathode (a) (b) 0.0 0.1 0.2 0.3 0.4 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Bias voltage V (V) Bias voltage V (V) OC OC Cu S cathode Pt counter electrode (c) (d) Figure 7: J-V curves of solar cells modified by various cathodes. electrode. The Pt electrocatalysts, that is, Cu S, CuS, and electrode. PbS, CuS, Cu S, andPtwereusedaselectrocata- 2 2 PbS, gave higher current densities than Pt but lower 𝑉 lysts on counter electrodes in combination with a polysulfide OC than Pt. On the contrary, open-circuit voltage values were electrolyte. The maximum solar conversion efficiency of practically not aeff cted by the electrocatalyst. eTh major 1.52% was obtained with a Pt counter electrode. eTh most problem encountered in the present work was with the value important ndin fi g of this work is the importance of the rfi st of the fill factor (FF). It remained below 0.42 and this limited nanostructure layer deposited on the mesoporous TiO film, the overall efficiency, even though, the current densities which aeff cted the quantity and the quality of the subsequent presently recorded were high. The search for a higher FF is QDs layers and the ensuing cell efficiency. High current an open question and has occupied many other researchers. densities were obtained with all cells having optimized anode It is believed that higher FFs will be obtained with even better electrodes. Among them, the highest currents were obtained electrocatalysts and more functional counter electrodes. with Pt electrocatalysts. 4. Conclusions Conflict of Interests QDSSCs have been constructed and optimized by combining eTh authors declare that there is no conflict of interests TiO with CdS, CdSe, and ZnS nanostructure on the anode regarding the publication of this paper. 2 2 Photocurrent density J (mA/cm ) Photocurrent density J (mA/cm ) SC SC Photocurrent density J (mA/cm ) Photocurrent density J (mA/cm ) SC SC Advances in OptoElectronics 9 Acknowledgments [15] H.M.Pathanand C. D. 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