Co-Sensitized Tio₂ Photoelectrodes by Multiple Semiconductors (Pbs/Pb_0.05cd_0.95s/Cds) to Enhance the Performance of A Solar Cell

Introduction

Increasing demand for energy is forcing us to seek into an alternative new resources. Among many available candidates, solar energy is ideal to meet the target because of its abundant, clean, and inexhaustible characteristics. Recently, quantum dot-sensitized solar cells (QDSSCs) have received much attention for alternative of dye-sensitized solar cells (DSSCs) owing to their band gap tune ability, higher absorption coefficient of quantum dots compared to dye molecules, and the ability of multiple-carrier generation^1-4. However, the efficiency of QDSSCs is lower compared with other type DSSCs,  which  restricts  their  potential  application. Thus it is still a crucial issue to enhance its efficiency. A  typical  QDSSC,  just  like  DSSC,  consists  of  a  QDs-sensitized  TiO₂ film photoanode, a redox electrolyte, and a platinized counter electrode^5,6. As an important component, the photoanode plays a role  in  the  separation  of  photogenerated  charge  carriers  and  the transportation  of  electrons.  TiO₂ films have been demonstrated to be promising candidate as photoanodes due to their appropriate energy band position and both thermal and chemical stability in solution. However, the large band gap of TiO₂ (3.2 eV) limits its absorption only to the ultraviolet region, which takes only 3 – 5% of the whole solar spectrum reaches earth surfaces.

To  improve  the  performance  of  QDSSCs,  various  nanostructures  have  been  employed  to  fabricate  the  photoanodes,  such  as mesoporous  films^7–9,  hierarchical  sphere¹⁰,  nanowires  (NWs), nanorods  (NRs)  and  nanotubes  (NTs)  arrays.  Recently,  many new  strategies,  especially  the  surface  and  interface  treatment,  were also  employed  to  modify  the  nanostructures  of  the  photoanodes, aiming  to  reduce  the  recombination  and  improve  the  transport of  the  photo-excited  carriers  at  the  interface¹⁰. To extend the activity of a photoelectrode into the visible light region, various approaches were employed including doping TiO₂ with other impurities, using dye-sensitized solar cells (DSSCs), and very recently using the composite semiconductors such as QDs-sensitized solar cells (QDSCs)^11-13.

Many kinds of small band gap semiconductors that can absorb light in the visible region, e.g., CdS¹³, PbSe¹⁴, CdSe¹⁵, InP¹⁶, and CdTe¹⁷, have been used as sensitizers. Among them, CdS is semiconductor with direct band gap (E_gap) of 2.25 eV, which means CdS can trigger wider light absorption range compared to TiO₂. However, to accelerate charge separation in bulk/single crystal material, the level of the conduction band (CB) edge which provides the electron injection driving force from QDs to TiO₂ should be taken into account. It is known that CdS has a high CB edges with respect to that of TiO₂, and a higher electron injection efficiency of CdS was found¹⁸.  PbS is also fascinating sensitizer for QDSSC.  The bulk band gap of PbS (0.41 eV) can be easily manipulated by changing the particle size. The poor stability of PbS quantum dots is a major drawback for QDSSC application. Thus in order to solve the problem, CdS shells are coated on the PbS sensitizer to prevent the corrosion by the polysulfide electrolyte and suppress the recombination at the semiconductor/electrolyte interface^19-21. Different from the dye molecules, quantum dot sensitizers may have defects or dislocation. Therefore, enhancing the quality of the quantum dots is an important issue for optimizing QDSSCs²².

In this study, we prepared TiO₂ photoanode sensitized by multiple semiconductors, which include PbS, Pb_0.05Cd_0.95S, CdS and assembled it into a typical QDSSC. The effect of adding a layer of quantum dot / sensitizer to the value of the photocurrent and efficiency of the solar cells, then was investigated and will be discussed herein.

Materials and Methods

Materials

All chemicals and solvents used in this study were of reagent grade. Titanium (IV) isopropoxide, polyethylene glycol (Mr = 1000), triethanolamine, H₂PtCl₆, sulphur were purchased from Sigma Aldrich. While methanol, ethanol, 2-propanol, Cd(CH₃COO)₂.2H₂O, Na₂S.9H₂O, Pb(NO₃)₂, KCl were purchased from Merck. Distilled water produced from a BioPure purification system was used throughout the experiments and homemade FTO (75X25X1,4 mm³, sheet resistance = 21Ω/sq) were made by spray pyrolysis method²³.

Instrumentation

XRD patterns of the prepared film were conducted on X-ray Diffractometer XRD 7000 Shimadzu Cu Ka radiation (l= 0.15418 nm) that operated at 40 kV and 30 mA. FESEM images were taken using FESEM (FEI-Inspect F50). The percentage of elemental composition in the composite film was obtained from energy dispersive spectroscopy (EDS Apollo X). The oxidation state of titanium, cadmium, lead, sulfur, and platinum were examined using Thermo VG Scientific-Sigma Probe X-ray photoelectron spectroscopy  (XPS)  with  mono-chromated  Al  Kα radiation and operating pressure in the sampling chamber was below 5 × 10⁻⁹ Torr. Photocurrent was measured by electrochemical workstation with a three-electrode configuration, which was connected to a computer-controlled potentiostat (e-DAQ/e-recorder 401) to record the generated photocurrent. TiO₂ photoanode was placed as working electrode, while a Pt wire and Ag/AgCl was placed as counter and reference electrode, respectively. The supporting electrolyte was Na₂S 0.1M. As source of visible irradiation is 60W wolfram lamp with the intensity 2.20 mW/cm². Solar cells performance evaluation was measured by Edaq potensiostat connected with e-recorder 401.

Procedure

Preparation of FTO/TiO₂ film²⁴

Titanium tetraisopropoxide (TTIP), polyethylene glycol (PEG, Mr = 1000), triethanolamine, ethanol, FTO (fluorine tin oxide) and distilled water were used in this experiment. TiO₂ sol was prepared by mixing of 7.5 mL TTIP, 2.4 mL triethanolamine, and 36 mL of ethanol. The mixture was stirred with a magnetic stirrer at room temperature for 1.5 hours, then was added by adequate ethanol-water (4.5:0.5) and 2 grams of PEG, and stirred with a magnetic stirrer for 1.5 hours. The resulting sol then was used to coat the FTO glass by a dip coating method, and then annealed at 500ºC for 1 hour. This step was repeated 5 times to obtain TiO₂ film on the FTO surface (FTO/TiO₂).

Preparation of  FTO/TiO₂/CdS Film

CdS quantum dot (QD) was prepared by a SILAR (successive ionic layer adsorption and reaction) method. Individual solution containing Cd(CH₃COO)₂ and Na₂S were used as a Cd and S precursor to obtain CdS quantum dot. The CdS was deposited directly by immersing the FTO/TiO₂ glass into an aqueous solution of Cd(CH₃COO)₂ 0.3M for 2 minutes, then rinsed with distilled water. Then was immersed in aqueous solution of  Na₂S 0.1M for 2 minutes, then rinsed with distilled water. This step was repeated 10 times until the optimum CdS layer was formed. The CdS layer on FTO/TiO₂ surface then was dried at room temperature for few minute to obtain FTO/TiO₂/CdS film.

Preparation of  FTO/TiO₂/PbS/CdS Film

PbS was prepared by a SILAR (successive ionic layer adsorption and reaction) method. The FTO/TiO₂ was soaked in the solution of Pb(NO₃)₂ 0,02M in methanol for 2 minutes and then rinsed with ethanol. Then was soaked in a solution of Na₂S 0,01M in methanol: water (1:1) for 5 minutes, then rinsed with ethanol. This step was performed up to three cycles in order to obtain desired thickness. Then proceed with the CdS layer in the same manner as above. The FTO/TiO₂/PbS film was immersed into an aqueous solution of Cd(CH₃COO)₂ 0.3M for 2 minutes, then rinsed with distilled water. Then was  immersed in aqueous solution of 0.1M Na₂S for 2 minutes, then rinsed with distilled water. This step was repeated 10 times until the optimum CdS layer was formed. The CdS layer on FTO/TiO₂/PbS surface then was dried at room temperature for few minute to obtain FTO/TiO₂/PbS/CdS film.

Preparation of  FTO/TiO₂/PbS/ Pb_0.05Cd_0.95S /CdS Film

The FTO/TiO₂ was soaked in the solution of Pb(NO₃)₂ 0,02M in methanol for 2 minutes and then rinsed with ethanol. Then was further soaked in a solution of Na₂S 0,01M in methanol : water (1:1) for 5 minutes, then rinsed with ethanol. These steps were done in three repeating cycles in order to obtain desired thickness. Then proceed with the Pb_0.05Cd_0.95S layer using SILAR method. First, a solution of Pb/Cd 0,02M in methanol (with a molar ratio of Pb/Cd = 5/95) was prepared by mixing a solution of Pb(NO₃)₂ and Cd(CH₃COO)₂ ²⁴. QD deposition was performed with SILAR method, firstly it was soaked in a solution of Pb/Cd 0,02M for 2 minutes, then rinsed with ethanol. Then it was soaked in a solution of Na₂S 0,01M for 5 minutes, then rinsed with ethanol. This step was performed up to three cycles in order to obtain the desired thickness. Then it was proceed with the CdS layer in the same manner as above to obtain FTO/TiO₂/PbS/Pb_0.05Cd_0.95S/CdS film

Characterization

Characterization of the film structure was done by using X-ray diffractometer (Shimadzu XRD 7000) with CuKα radiation (λ = 1.5418 Å), at 30 kV, 10 mA. Film thickness and morphology of the film surface was observed with FESEM (FEI-Inspect F50). The composition of the constituent elements measured by Energy Dispersive Spectroscopy (Apollo X). The oxidation states of constituent elements were examined by X-ray photoelectron spectroscopy. Photocurrent response was measured using an electrochemical workstation (e-DAQ/e-recorder 401) using 60W wolfram lamp as visible light source. Ag/AgCl electrode was used as a reference electrode, while platinum (Pt) was used as a counter electrode and Na₂S 0,1M as an electrolyte. Photocurrent measurements using multipulse amperometry with a potential = 0 volts and scan time = 100 second.

Performance Evaluation

Performance evaluation was done by using Edaq potensiostat connected with e-recorder 401. TiO₂ photoanode was placed as a solar cell working electrode and FTO/Pt was placed as counter electrode.  Redox couples Na₂S 0.2M, S 1M, KCl 0.02M solution was used as an electrolyte and 150W halogen lamp (3.50 mW/cm² was measured by lux meter) was used as irradiation sources.  Isc (short circuit current), and Voc (open circuit voltage) was measured by linear sweep voltammetry (LSV) with  scan rate of 40 mV/s from 500 mV to 0 mV under visible illumination.

After solar cells have been fabricated, it should be evaluated for a number of parameters which provide its performance information. These include Isc (short circuit current, mA/cm²), Voc (open circuit voltage, mV), FF (fill-factor) and h (power conversion efficiency). Based on I–V curve, the fill factor (FF) is defined as :

where Im (mA/cm²) and Vm (mV) are the photocurrent and photovoltage for maximum power output (Pm, mW/cm²), The overall energy conversion efficiency (h) is defined as:

Results and Discussion

Preparation and characterization of the nanostructured TiO₂ photoanode

Composition and crystal structure of the nanostructured TiO₂ photoanode

Based on patterns of X-ray diffraction in figure 1.a, it appears that FTO has a value of 2θ = 26.4º, 33.7º, 37.7º, 51.6º, 54.4º, 61.6º, 64.8º and 65.7º with the indexes Miller (110), (101), (200), (211), (220), (310), (112), and (301). This shows that according to JCPDS 41-1445,  our FTO has cassiterite structure . After FTO coated with TiO₂, it appears the new peaks at 25.3º and 47.9º and the other peak with increased intensity in 37.8º (figure 1.b). This corresponds to JCPDS 21-1272, thus it can be concluded that our TiO₂ has crystallite structure of anatase.

The anatase crystallite size of TiO₂ was estimated using the Scherrer equation.

where the constant k is a shape factor usually ∼0.9, λ is the wave length of X-ray (0.15418 nm), β is the FWHM in radians and θ is the Bragg’s angle.

The  estimation  shows that  the  mean  crystallyte  size  of  the  anatase  TiO₂ individual  particles  calculated  from  Eq.  (5)  is  22–50  nm. While the dominant peak of rutile TiO₂ at 27.4º, 36.1º and 54.3º (JCPDS 21-1276) are not visible at all.

Figure 1 : X-ray diffraction patterns of TiO2 nanostructured photoanode Click here to View figure

 

From XPS spectra, useful information was obtained to assign the complete chemical composition of the sample surface. The survey spectrum in figure 2.a gives XPS profile of the as-synthesized TiO₂ nanoparticles.

Figure 2 : XPS spectrum of FTO/TiO2 thin film Click here to View figure

 

The constituents peaks were identified to be titanium (Ti), oxygen (O) and carbon (C). The X-ray photoelectron spectroscopy (XPS) spectra were analyzed  for  the  TiO₂ film  in  order  to  study  the  surface  composition  of the  TiO₂  thin  film. The Ti2p_3/2 and Ti2p_1/2 spin-orbital splitting photoelectrons for TiO₂ (figure 2.b) are located at binding energies of 458.7 and 464.5 eV, respectively, which is consistent with the values of Ti⁴⁺ in the TiO₂ lattices, as observed earlier by Hung, et al.²⁵. According to the literature, the Ti2p_3/2 binding energy of TiO₂ is in the range of 458.7–459.2 eV, while the binding energy of titanium in the oxidation state 3+ is in the range of 456.2–457.4 eV²⁶. Therefore, titanium exists on the surface in the form of TiO₂ with oxidation state 4+. The photoelectron spectra of O1s can be deconvoluted into two peaks (figure 2.c). The O1s main peak at 529.9 eV is assigned to the metallic oxides, which is consistent with the binding energy of O^2- in the TiO₂ lattices. A shoulder to the main O1s peak can be observed at high binding energy, 531.8 eV which can be attributed to the hydroxyl groups or chemisorbed water molecules adsorbed on TiO₂ surface. In addition, it also shows the presence of C peak (figure 2.d) at 285.1, 286.8 and 288.6 eV. The deconvolution of the C1s peaks indicates the presence of two peaks at 286.3 and 288.8 eV other than the main peak of sp2-carbon at 284.6 eV. The peaks at 286.3 and 288.8 eV can be attributed to hydroxyl carbon (C–OH) and carboxyl carbon (O=C–O), respectively²⁷. This indicates that there are more carbon-containing  residues on the films. This residues were suspected from uncompleted combustion of PEG molecules during calcination.

After CdS deposited on the surface of TiO₂, it appears a different X-ray diffraction patterns. In the figure 1.c, it appears that the peak 26.4º and 51.6º increased in intensity and appears a small peak at 47.4º. This shows that the CdS has successfully synthesized by SILAR method and according to JCPDS 10-0454, this CdS has  hawleyite crystallite structure. The small intensity of CdS peaks was caused by a small quantities of CdS deposited on the TiO₂ surface. It was only about 1: 6 (Cd: Ti), as shown in the EDS spectrum in figure 3.b. According to Balaz, et al.²⁸, CdS has two structures, these are hexagonal α-CdS called greenockite (JCPDS 41-1049) and cubic β-CdS called hawleyite (JCPDS 10-0454). Our CdS is in accordance with the hawleyite structure. Besides coated by CdS, TiO₂ is also coated by PbS. Characterization of FTO/TiO₂/PbS/Pb_0.05Cd_0.95S/CdS layer with X-ray diffraction does not show any clear PbS peaks. Only a small peak seen in 53.4º (figure 1.d), this due to the amount of PbS deposited on the TiO₂ surface is very small, only about 1:12 (Pb: Ti) as shown in the EDS spectrum in figure 3.c. Beside that reason, PbS and CdS layer is only dried at room temperature without any calcination, so likely PbS and CdS still have an amorphous structure and the resulted peak tends to be broaden. PbS has only a galena single crystal structure, so it can be ascertained that our PbS has galena structure (JCPDS 05-0592).

Figure 3 : EDS spectrum and elemental composition of TiO2 nanostructured photoanode Click here to View figure

The spectrum of the overall survey scan of FTO/TiO₂/PbS/Pb_0.05Cd_0.95S/CdS thin layer (figure 4.a) indicates the presence of Ti, O, Pb, Cd, and S in the sample. The Cd3d_5/2 and Cd3d_3/2 binding energies are equal to 405.0 eV and 411.8 eV (figure 4.b) and it corresponds to Cd²⁺.

Figure 4 : XPS spectrum of FTO/TiO2/PbS/Pb0.05Cd0.95S/CdS thin films Click here to View figure

 

For cadmium sulfide, the Cd3d_5/2 binding energy is in the range of 404.4 – 405.7 eV²⁹. Figure 4.d shows XPS spectra of S2p levels of CdS nanoparticles. S2p peaks observed at 161.3 eV (for S2p_3/2) and 162.7 eV (for S2p_1/2) are attributed to metal sulfide. Observed values of the S2p_3/2 binding energy for sulfur in the sulfides is in the range of 161.1–161.7 eV²⁹; sulfates and sulfites are characterized by the higher S2p binding energy in the range of 168–170 eV³⁰.  The resolution of the XPS spectra  of  the  Pb  resulted  in  peaks  at  different  binding  energies.  The Pb4f_7/2 and Pb4f_5/2 spin-orbital splitting photoelectrons for PbS (figure 4.c) are located at binding energies of 138.2 eV and 142.9 eV, respectively, which is consistent with the values of Pb²⁺ in the PbS lattices³¹. In addition, it also shows the presence of C peak (figure 4.e) as well as O peak (figure 4.f) at 285.0 and 530.0 eV, respectively, as impurities. The O1s main peak at 530.0 eV is assigned to the metallic oxides, which is consistent with the binding energy of O^2- in the TiO₂ lattices. A shoulder to the main O1s peak can be observed at higher binding energy, 532.0 eV which can be attributed to the hydroxyl groups or chemisorbed water molecules adsorbed on TiO₂ surface. The deconvolution of the C1s peaks indicates the presence of two peaks at 286.3 and 288.8 eV other than the main peak of sp2-carbon at 284.6 eV. The peaks at 286.3 and 288.8 eV can be attributed to hydroxyl carbon (C–OH) and carboxyl carbon (O=C–O), respectively. This peaks were suspected from uncompleted evaporation of methanol / ethanol solvent used in Pb²⁺ and S^2- solution.

Surface morphology of the nanostructured TiO₂ photoanode

 From figure 5.a shows that the TiO₂ deposited on the surface of the FTO, has a porous structure with a diameter of less than 1 µm. Pores are from PEG template that lost during calcination. Pores are useful to hold sensitizer in larger quantities. CdS and PbS will fill the pores of TiO₂ so that these pores will be full after the sensitizer enter into it, as shown in figure 5.b and 5.c. But in wide pores, sensitizer only able to coat the inner walls of the pores so that the pores are still open with a smaller diameter (figure 5.c). PbS and CdS largely occupy space in the pores and only a few of this sensitizer which occupies on the surface of TiO₂. This reason is supported by a cross-sectional images in figure 6, that  the thickness of TiO₂/PbS/Pb_0.05Cd_0.95S/CdS layer is only slightly thicker than the TiO₂ layer.

Figure 5 : SEM images of TiO2 nanostructured photoanode Click here to View figure

Figure 6 : Cross-sectional SEM images of TiO2 nanostructured photoanode a) FTO/TiO2; b) FTO/TiO2/PbS/Pb0.05Cd0.95S/CdS Click here to View figure

 

Photoelectrochemical response of the nanostructured TiO₂ photoanode

Light absorption, particularly in the visible region, can be increased by using a narrow bandgap quantum dots. Two or more different band gap quantum dots can be used at a same time so that the light is not absorbed by the first quantum dot, can still be absorbed by the second quantum dot. This phenomenon is called as co-sensitization, the sensitization was carried out by two or more sensitizers. With the co-sensitization makes the larger absorption of the incident light. The more light is absorbed, the more likely the resulting photocurrent. In this study two sensitizers, CdS and PbS were used.  CdS has a bandgap 2.25 eV coupled with PbS which has a band gap of 0.41 eV. CdS has absorption in the visible region is at a wavelength of 550 nm while PbS has absorption in the infra red region (+ 3000nm). With the merger of two quantum dot, it is expected to increase the absorption of visible light so that it will increase its photocurrent. From the observation of the photocurrent in figure 7, it is clear that the two sensitizer PbS and CdS able to improve the higher photocurrent .

Figure 7 : The photocurrent of TiO2 nanostructured photoanode A)FTO/TiO2, B)FTO/TiO2 /CdS, C)FTO/TiO2/PbS/CdS, D)FTO/TiO2/PbS/Pb0.05Cd0.95S/CdS (illuminated with 60W wolfram lamp, 2.20 mW/cm2; electrolyte = Na2S 0.1M, bias potensial = 0 Volt) Click here to View figure

 

Mixture of PbS and CdS, is Pb_0.05Cd_0.95S which has a bandgap between them, was able to increase the absorption in the visible region and simultaneously improve its photocurrent. FTO/TiO₂/CdS layer generates a photocurrent of about 0.190 mA/cm² while FTO/TiO₂/PbS/CdS about 0.302 mA/cm², so that the PbS able to increase the photocurrent of 59%. The existence of Pb_0.05Cd_(0.95S layer between PbS and CdS layer is useful to  accelerate the transfer of electrons from CdS layer to PbS.  FTO/TiO₂/PbS/ Pb_0.05Cd_0.95S /CdS layer able to generate photocurrent up to 0.363 mA/cm². Therefore, combination of PbS and  Pb_0.05Cd_0.95S layer able to increase the photocurrent up to 91%.

The mechanism of electron flow is highly dependent on the position of the conduction band of each semiconductor. CdS semiconductor which has a conduction band edge of -4.0 eV is estimated as the beginning of a flow of electrons. CdS with the band gap 2,25eV will be excited when exposed by the light with a wavelength smaller than 550 nm. From the CdS conduction band, the electrons will flow into the conduction band PbS of -4.1 eV. In addition to receive an electrons from CdS, PbS semiconductor can also be a source of electrons. PbS with a band gap of 0.41 eV will be excited when exposed by the light with a wavelength of less than 3000 nm. Electrons from the PbS valence band will move toward the PbS conduction band. Furthermore, from the PbS conduction band, electrons proceed to the TiO₂ conduction band of -4.2 eV. From the TiO₂ conduction band, the electrons move toward the SnO₂ conduction band of -4.8 eV, to get to the external circuit. Meanwhile, to prevent electron-hole recombination is required hole transporting material is S^2- from the redox electrolyte. For more details of the electrons flow, can be seen in figure 8 below.

Figure 8 : Schematic diagram illustrating the electron flow of PbS-CdS co-sensitized solar cells Click here to View figure

 

Preparation of Pt counter electrode

Pt counter electrode is made by depositing Pt on the surface of the FTO. From X-ray diffraction pattern in figure 9, it appears that the metals Pt have been successfully deposited on the surface of the FTO.

Figure 9 : X-ray diffraction patterns of Pt counter electrode Click here to View figure

 

It appears from the new peak appearing at 39,4º and 66.4º which is the peak of platinum according to JCPDS 88-2343. The amount of Pt  deposited on the surface of the FTO is very small that is around 1:10 (Pt: Sn) as shown in the EDS spectra in figure 10, so that the resulted peak in X-ray diffraction pattern has also  a small intensity. Although the amount of Pt is very small but platinum crystals have a uniform shape, as shown in figure 11.

Figure 10 : EDS spectrum and elemental composition of Pt counter electrode Click here to View figure

Figure 11: SEM images of Pt couter electrode Click here to View figure

 

Figure 12.a is the typical survey spectrum of FTO/Pt thin film, showing the presence of Pt, Sn, O and C.  The corresponding Pt4f spectra are shown in figure 12.b. It is seen that the Pt4f_7/2 binding energy is 70.7 eV that is typical of metallic platinum. According to Kalinkin, et al.³², for the chloride complexes, the Pt4f spectrum is shifted to higher binding energies by a value proportional to the oxidation state of platinum. For example, for K₂PtCl₄ (the oxidation state of platinum is Pt²⁺) and K₂PtCl₆ (Pt⁴⁺), the binding energy (Pt4f_7/2) values are 73.1 eV and 75.2 eV, respectively. As a consequence, the shift of the Pt4f_7/2 photoemission line with respect to that of the metal is 2.0 eV for Pt²⁺ and 4.1 eV for Pt⁴⁺, i.e. about 1 eV per unit oxidation number. Binding energies of Sn3d_3/2 and Sn3d_5/2 electrons in the surface layer of the FTO materials were found to be 494.9 eV and 486.7 eV respectively, which corresponded to Sn⁴⁺ in the tin dioxide

Figure 12 : XPS spectrum of FTO/Pt thin film Click here to View figure

 

Performance evaluation of the solar cells

Solar cells was constructed by assembling the working electrode and the counter electrode by using a redox electrolyte Na₂S/S. Solar cell performance evaluation was done by using the potentiostat and 150W halogen lamp as a light visible source. Intensity of halogen lamp was measured by a lux meter and was obtained the intensity at a distance of 10 cm of 3.50 mW/cm². From 3 different configurations are FTO/TiO₂/CdS, FTO/TiO₂/PbS/CdS, FTO/TiO₂/PbS/ Pb_0.05Cd_0.95S/CdS obtained I-V curves as indicated in figure 13.

Figure 13 : Left : I-V curve for performance evaluation of solar cells; Right : Color of TiO2 nanostructured photoanodes as working electrode B)FTO/TiO2/CdS, C)FTO/TiO2/PbS/CdS, D)FTO/TiO2/PbS/Pb0.05Cd0.95S/CdS Click here to View figure

 

Based on the I-V curve in figure 13, it can be determined the value of the fill factor, Isc, Voc, and efficiency, using the formula described above, in order to obtain the following results, as indicated in table 1.

Table 1: The value of the fill factor, I_sc, V_oc, and efficiency of solar cells

Sensitizer	Imax	Vmax	Isc	Voc	FF	Pmax	Pin	h
	mA/ cm2	mV	mA/ cm2	mV		mW/ cm2	mW/ cm2	(%)
CdS	0.1172	162	0.2298	327	0.253	0.0190	3.501	0.54
PbS/CdS	0.1462	257	0.2446	469	0.328	0.0376	3.501	1.07
PbS/Pb0.05 Cd0.95S/CdS	0.2285	217	0.4286	452	0.256	0.0496	3.501	1.42

From the table 1, it is obtained the efficiency for CdS single sensitizer = 0:54% and using (PbS+CdS+Pb_0.05Cd_0.95S) = 1.42%. Thus an increase in efficiency of around 160% on solar cells using multiple semiconductor compared with solar cells using a single semiconductor of CdS.

Conclusion

Based on the results obtained, it can be concluded that TiO₂ nanostructured photoanode have been successfully prepared by utilizing two semiconductor namely CdS and PbS. Quantum dot of CdS, PbS and Pb_0.05Cd_0.95S have been successfully prepared using SILAR method and obtained photocurrent of 0.190 mA/cm² for configuration FTO/TiO₂/CdS, 0.302 mA/cm² for FTO/TiO₂/PbS/CdS and 0.363 mA/cm² for FTO/TiO₂/PbS/Pb_0.05Cd_0.95S/CdS. Sensitizers which use multiple semiconductors namely PbS, CdS and Pb_0.05Cd_0.95S able to increase the photocurrent of 91% greater than that using a single semiconductor of CdS. Based on the incident light of 3.5 mW/cm²,the all three types constructed solar cells show efficiency of 0.54%, 1.07% and 1.42%, respectively. It can be concluded that the efficiency has been increase of around 160% when multiple semiconductors PbS, CdS, Pb_0.05Cd_0.95S were used.

Acknowledgements

One of the authors (Supriyono) wish to thank to Industrial Education and Training Center, Ministry of Industry of Indonesia for providing the research funding. Partial support from Research Cluster Scheme Universitas Indonesia, and opportunity provided by KAIST (Korean Advanced Institute of Science and Technology) for SEM and XPS measurements are also acknowledged.

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