Capitulo 2. Descripción de las Experiencias de Educación para La Paz
2.3 Proyecto Giho. La historia oral como recurso
6.1 Introduction
Cadmium sulphide (CdS) is a member of group II-VI binary compound semiconductors which has found wide applications in the world of electronic devices. It is a direct bandgap semiconductor having energy of 2.42 eV for single crystalline material at room temperature [1,2]. It is n-type in electrical conduction and is widely used in the fabrication of electronic devices due to its unique optoelectronic features. Some of the applicable areas where CdS semiconductors have found optimum usefulness are: gas sensors [3–5], thin film field effect transistors [6], photoresistors [7], photosensors [8–
10], light emitting diodes [11,12], Schottky diodes [13] and solar cells [14–16]. CdS thin films have been found to be a suitable window material to some low-bandgap absorber semiconductor layers. Typical examples of semiconductors using CdS as hetero-junction partner for solar cell applications are CdS/CIS [17], CdS/CIGS [18], CdS/Cu2S [19] and CdS/CdTe [20–22]. In this work, CdS has been used as an hetero-partner to CdTe thin films to develop glass/FTO/n-CdS/n-CdTe/Au solar cell device structures.
Numerous deposition techniques have been used to-date for the deposition of CdS thin films. Some of these techniques include: chemical bath deposition (CBD) [23], spray pyrolysis [24], vacuum deposition [25], close spaced sublimation (CSS) [26], screen printing [3], sputtering [27], metal-organic chemical vapour deposition (MOCVD) [28]
and electrodeposition [29–31]. Polycrystalline CdS thin films with good quality can be obtained using the aforementioned growth techniques. However, the initial cost of setting up instrument for techiques like CSS and MOCVD is very high. In growth technique like CBD, generation of large toxic waste containing Cd is a great disadvantage and disposing these wastes often introduce additional expenses into the overall production cost. In a production line, it is therefore advantageous to use a continuous deposition process like electrodeposition to develop CdS and its other solar cell hetero-partner. This will ensure a reduction in the production cost. In this research, the growth of CdS thin films was achieved using electrodeposition technique with a two-electrode set up. The main aim of this work is to establish the right cathodic
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deposition potential for growing CdS thin films for applications in optoelectronic devices most especially solar cells.
6.2 Preparation of CdS electrolytic bath
Two chemicals namely CdCl2.xH2O (99.999% purity) and (NH4)2S2O3 (98% purity) were used in preparing the electrolyte for the electrodeposition of CdS thin films. The two chemicals were purchased from Sigma Aldrich, United Kingdom. The precursors CdCl2.xH2O serve as the cation source while (NH4)2S2O3 serve as the anion source. The electrolytic bath contains 0.3 M CdCl2.xH2O and 0.03 M (NH4)2S2O3 in 400 ml of de-ionised water. After mixing the two chemicals together, the initial pH was measured to be 1.44±0.02. The pH of the bath was adjusted to 2.50±0.02 using diluted NH4OH and HCl. The solution was allowed to stir continuously for ~6 hours to ensure full dissolution of the chemicals in the de-ionised water. The electroplating of CdS thin films were carried out at ~80oC using a heater with an embedded magnetic unit which controls the magnetric stirrer during deposition.
6.3 Voltage optimisation and growth of CdS thin films
This section discusses some analytical techniques used for estimating the range of cathodic potentials that would be suitable for the growth of nearly stoichiometric CdS layers and for material characterisation.
6.3.1 Cyclic voltammogram
The suitable voltage range to grow nearly stoichiometric CdS thin films was obtained using a cyclic voltammogram. A range of cathodic potentials from 0 to 1600 mV was applied through the electrodes inside the electrolyte at a sweep rate of 180 mVmin-1. The pH of the solution and deposition temperature were maintained at 2.50±0.02 and
~80oC respectively. The I-V curve of the electrolyte containing aqueous solutions of 0.3 M CdCl2 and 0.03 M (NH4)2S2O3 in both forward and reverse directions is shown in Figure 6.1. The two main atoms making up the CdS thin films are cadmium (Cd) and sulphur (S) from CdCl2 and (NH4)2S2O3 precursors respectively. Out of these two atoms, sulphur has the tendency to deposit first before cadmium because S has a more positive redox potential
Eo than Cd. The Eoof sulphur and cadmium with respect to the standard hydrogen electrode (SHE) are +0.449 and -0.403 V respectively [32].160
Figure 6.1. Cyclic voltammogram of electrolyte containing 0.30 M CdCl2 and 0.03 M (NH4)2S2O3 in 400 ml of de-ionised water (pH = 2.50±0.02, T = 80oC). Inset shows the transition voltage at which sulphur starts to deposit.
The diagram shown at the inset of Figure 6.1 illustrates the potential at which S begins to deposit while the point labelled A in Figure 6.1 descibes the potential at which Cd deposition starts to take place. The deposition of S starts to take place at ~85 mV while that of Cd begins at ~550 mV. This result further illustrates the initial explanation given about S depositing first before Cd due to its Eovalue.
A steady rise was observed in the forward current from point A to point B as the cathodic voltage increases from ~550 to 1118 mV. This steady increase shows that more S and Cd are being deposited to form a mixture of sulphur and CdS thin films. The slight reduction observed at point B before the sudden rise again is due to the deposition of elemental Cd and co-deposition of CdS on the working electrode. The rise in deposition current density after ~1118 mV shows rapid discharge of Cd and reaction between Cd and S to form CdS. The point C indicated on the reverse cycle of the I-V curve shows that the transition point of current flow from the positive to the negative takes place at ~1138 mV. Point C indicates the dissolution of elemental Cd and removal of Cd from CdS layer deposited on the cathode. At this point, the deposition current density is zero because the dissolution rate of materials is equal to its deposition rate.
The dissolution of S from the surface of the working electrode occurs at the broad peak
-0.5 -0.3 -0.1 0.1 0.3 0.5 0.7 0.9
0 200 400 600 800 1000 1200 1400 1600
Current Density (mAcm-2)
Cathodic Potential (mV) B
A
C
D
Q
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labelled D. At low cathodic growth potential (≤1118 mV), a S-rich CdS layer is expected to be formed. As the deposition potential increases, the amount of Cd in the CdS layers gradually increases thus allowing near stoichiometric CdS layers to be deposited. Thus, a voltage range (~1150 mV to 1250 mV) labelled Q in Figure 6.1 has been identified as being suitable to grow near stoichiometric CdS layers according to this experimental result. The overall chemical reaction for the deposition of CdS thin films on the cathode is stated in Equation (6.1) [30].
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2 3 2
2 S O 2e CdS SO
Cd (6.1)
6.3.2 Visual appearance
The visual appearance of 11 samples of electroplated CdS layers grown between cathodic voltages of 1150 to 1250 mV is shown in Figure 6.2. As earlier discussed, the range of voltages used in this work was determined from the result of the I-V curve of the glass/FTO inside the CdS electrolyte. All the CdS layers shown in Figure 6.2 were deposited at a temperature of ~80oC for 30 minutes duration and the pH of the bath was maintained at 2.50±0.02 at the start of deposition.
Figure 6.2. Typical images showing the visual appearance of as-deposited CdS thin films grown at different cathodic potentials.
As observed from Figure 6.2, all the ED-CdS layers exhibited uniform yellowish colour in appearance. Visual appearance is one way in which qualitative information can be obtained about electroplated semiconductors as previously explained in Chapter 4 when
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the colour of CdSe layers changes with pH. None of the CdS layers appear dark in colour despite the variation in deposition potential. As reported by Diso et al. [33], CdS layers deposited at higher cathodic potential (for example, at 1500 mV) appears dark in colour due to Cd-richness. Attempts made to grow at very high cathodic potential of 1500 mV in this work resulted in formation of Cd dendrytes on the top surface of the CdS thin films as a result of high deposition current density. However, as illustrated in Figure 6.2 (b), the CdS layers grown at this Vg did not become dark as suggested by Diso et al. [33]. This may be due to a lot of factors such as the concentration of Cd salts in the bath and the pH of the electrolyte. The colour of CdS samples grown at low cathodic potential of 900 mV differ from the rest of the samples explored between 1150 to 1250 mV. The colour appears light yellowish; this is due to deposition of more sulphur at this low cathodic potential [34]. As earlier discussed, sulphur has the tendency to deposit easily and faster than Cd because its redox potential is more positive than that of Cd. The energy bandgap obtained for as-deposited CdS layers grown at 900 mV is ~2.28 eV. This value is lower than the Eg of bulk CdS and is closer to the Eg of CdO which has been reported to be in the range 2.20 to 2.30 eV [5,35] . CdO thin films have also been classified as a potential window layer as a result of their Eg value [36].
6.3.3 X-ray diffraction
The XRD spectra of as-deposited CdS layers grown between cathodic potentials of 1170 to 1230 mV is illustrated in Figure 6.3 (a). This voltage range was studied so as to determine the optimum deposition potential to grow nearly stoichiometric CdS layers.
The CdS layers used in this investigation were grown for ~30 minutes. As shown in Figure θ.γ (a), two main peaks were observed at the position β =βη.0ηo and 26.66o along the (100) and (002) planes respectively. The presence of these peaks makes the as-deposited (AD) CdS layers to be polycrystalline. The crystal structures of the AD-CdS layers were found to be hexagonal by comparing the observed XRD measured data with the reported data from JCPDS file with reference codes 01-080-0006 and 01-077-2306. Hexagonal crystal phase has been reported to be the stable phase for CdS thin films [37]. Several reports in the literature also show that the cubic phase of CdS thin films convert to hexagonal after annealing [30,38,39].
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Figure 6.3.(a) XRD spectra of as-deposited CdS layers grown at the cathodic potentials ranging from 1170 mV to 1230 mV, and (b) the (100)H and (002)H peak intensity versus cathodic potentials ranging from 1170 mV to 1230 mV for as-deposited CdS layers.
The peak at β in the range (βθ.θ0-26.69)o represents the preferred orientation peak since it has the highest intensity. However, this peak coincides with the underlying FTO peak at β = βθ.θηo. For this reason, the most intense diffraction peak along (002) plane was not used for the CdS analysis and crystallites size estimation. The second peak observed from the XRD spectra of AD-CdS thin films at β =βη.0ηo along (100) plane was utilised to calculate the crystallite sizes and to determine the optimum potential to electrodeposit nearly stoichiometric CdS thin films. Figure 6.3 (b) shows the plot of peak intensity versus cathodic potentials for diffraction peaks along (002) and (100) plane.
The results presented in Figure 6.3 (b) indicate a similar trend in the peak intensities for both planes as the cathodic potential is increased from 1170 mV to 1230 mV. However, the intensities of peaks along (002) plane are generally higher than those of (100) plane.
As observed from Figure 6.3 (b), the peak intensity gradually increases from 1170 mV to 1200 mV along both (002) and (100) planes. As the cathodic deposition potential increases beyond 1200 mV to 1230 mV, a drop was observed in the peak intensity. It is however interesting to see that despite the overlapping which occurs between CdS peak along (00β) plane and underlying FTO peak at β = βθ.θηo, both planes show similar trends at cathodic deposition potentials between 1170 to 1230 mV.