Deposition and Characterisation of Zinc Telluride as a Back Surface Field Layer in Photovoltaic Applications N Srimathy, A Ruban Kumar

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N Srimathy, A Ruban Kumar. Deposition and Characterisation of Zinc Telluride as a Back Surface Field Layer in Photovoltaic Applications. Mechanics, Materials Science & Engineering MMSE Journal. Open Access, 2017, 9, ￿10.2412/mmse.32.15.18￿. ￿hal-01504786￿

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Distributed under a Creative Commons Attribution| 4.0 International License Mechanics, Materials Science & Engineering, April 2017 – ISSN 2412-5954

Deposition and Characterisation of Zinc Telluride as a Back Surface Field Layer in Photovoltaic Applications

Srimathy N.1, A. Ruban Kumar1,a

1 – School of Advanced Sciences, VIT University, Vellore. 632014, India a – [email protected]

DOI 10.2412/mmse.32.15.18 provided by Seo4U.link

Keywords: thermal evaporation, XRD, AFM, Raman spectroscopy, cubic structure.

ABSTRACT. Zinc Telluride films developed by Thermal evaporation technique has wide application in photovoltaic and optoelectronic applications. ZnTe films at 423K and 473K were deposited onto glass substrates and annealed at 573K. Structural studies were carried out by XRD technique and Morphological study was done by AFM which in turn shows the high intensity peak at annealed condition. Optical properties was studied by UV-VIS spectrometer to find the energy distribution and thereby, bandgap is calculated, which ranges from 1.89eV to 2.42eV. Raman analysis was done to find the phonon distribution and molecular longitudinal modes.

Introduction. Zinc Telluride is an interesting II–VI p-type semiconductor material for its application for photovoltaic and optoelectronic applications. It is highly transparent towards visible region with a band gap ranging from 2.4 to 2.6 eV. Because of these high stability propertiesThe inability of the other types of solar cells can be defined due to other losses in efficiency say, surface recombination, disqualified solar spectrum and other factors. These factors can be overcome by using efficient direct band gap semiconductors with optimized bandgap [2]. Commercially, it is proven that Thermal evaporation is the user-friendly and easy handling technology for Zinc Telluride deposition for various reasons. Since the distance between the substrate and the source can be adjustable, the thickness can be monitored and stabilized for various applications. Experimental Description. Zinc Telluride is red- brownish polycrystalline powder with the purity of 99.999%, loaded with the tungsten boat for evaporation. The chamber is maintained with the base pressure of approx. 10-5 torr. Once the source attains the melting temperature of the Zinc Telluride, Evaporation starts and the film get deposited into the glass substrate. Here the substrate temperature is maintained at 423K. The evaporation was done for 15 min, and the thickness of the film was measured using the quartz crystal monitor attached to the thermal evaporation chamber. Unique thickness can be maintained by fixed deposition charge and time. The deposition process depends on the various factors such as the substrate temperature, nature of the source material, melting point of the source material and the base vacuum pressure. The thickness was maintained at 300 nm for both the temperatures. Now the samples were annealed at 573K in an inert atmosphere , here, Argon atmosphere for 5hr under vacuum. The entire procedure is repeated, only changing the substrate temperature to 473K. The deposition procedure and the thickness is maintained the same with the only difference of the substrate temperature, followed by same annealing procedure. The samples are then investigated with various characterization techniques before and after annealing for comparison. XRD (X-Ray Diffraction) technique (XRD, Brucker D8 Advance, Cu Kα radiation, λ= 1.5406 Å), was used to analyse the structural properties of ZnTe. The surface properties was studied by Atomic force microscopy (AFM). Optical properties like Transmission and Reflection was measured using UV-VIS spectrometer which in turn, the absorption coefficient, α, can be calculated [4].

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where d is the thickness of the film, R and T represent the reflection and transmission coefficients from the reflection and transmission spectrum respectively. Results and discussion Structural Properties. XRD is an important analytical tool to find the structural properties of any material. Bruker X-ray diffractometer is used for this study, which is operated with 40kV and 10 mA, with the azimuthal distribution of copper with a wavelength of λ = 1.540 Å .The sample holder was rotated inside with a speed of 1 deg/min placed vertically. A graph was plotted with the intensities versus azimuthal angle. Typical XRD patterns obtained shows that the ZnTe films show the (Cubic) Zinc Blende structure which is an essential characteristic for the back surface layer to find its application in Photovoltaic. The interplanar spacing with corresponding diffraction intensities were calculated by Bragg’s Equation [5]

where dhkl represents the interplanar spacing, hkl, the corresponding miller indices. The results obtained are compared with the standard, JCPDS 04-0850 file data.The orientation of the XRD peaks are found to be in (111) planes, with the additional peaks in (222) and (220) planes. After Annealing, the peaks were found to be more prominent compared to the initial peaks. It is clearly seen from the Figure 1.

Fig. 1. XRD patterns for ZnTe at 423K and 473K, before and after annealing.

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The patterns clearly show that the intensity of peaks increases after annealing. This shows that the temperature annealing cleanses the surface contamination and hence providing a promising BSF layer [6]. Thus, a peak at 2θ at 450, was observed to show that the Cubic Telluride crystals with Zinc Blende structure. This peak shows that the telluride crystals are found huge in the film which is due to the annealing treatment. Because of the diffusing character of Telluride crystals, the Zinc acts as a host to accept the crystals to form a uniform thin film layer. Usually Zinc Telluride films at low temperatures have high dislocation density and formation of strain in films, which is not observed in Films deposited at higher temperatures. The annealing at 573K, has capability of decreasing the trend of strain and thus increases the crystallites formation. Also, the size of the crystallites varies as the temperature increases. ZnTe films have the capability of aligning themselves to the nature of heat treatment and processing. Physical Properties.The surface morphology of the ZnTe films was determined by Atomic Force Microscopy (AFM). The method used to analyse the surface properties of ZnTe was by Non-contact method, wherein the points of contact of probe will not contact the surface of the film. Here, ZnTe films developed at 423K and 473K with their corresponding annealed films were investigated. Usually, ZnTe films at 300 to 400 nm thickness, are highly minced, with pointed grain size. The effects of temperature dependence of the films were evaluated with their annealing behavior[7]. Film deposited at 423K exhibits an average roughness of about 4.23nm, and at 473K, it is 5.76nm. After annealing, it is observed that the large grains of the film protrude due to the temperature absorption. Hence, this shows the high intense peaks of the film at higher temperatures. The average roughness was found to be 4.01nm at 423K and 4.98nm at 473K respectively. Figure.2, 3, 4 & 5 shows the AFM images of ZnTe films deposited at 423K and 473K before and after annealing respectively. It is clearly shown that there were large crystallites of the film in annealed films after heat treatment in Argon atmosphere [8]. This is due to the fact the process of reduction occurs in the films when annealed at 573K. It is clear from the earlier publications that the film has the capability absorbing higher temperatures which in turn does not affect the actual properties of the film. Hence such prominent behavior of ZnTe films have wide application towards the Photovoltaic properties. It is also clear from the AFM images that as the substrate temperature increases the density of the film increases and the intensity of the peaks become highly fixed to exhibit their structural behavior suitable for the solar applications.

Fig. 2. AFM image of ZnTe deposited at 423K before annealing.

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Fig. 3. AFM image of ZnTe deposited at 423K after annealing.

Fig .4. AFM image of ZnTe deposited at 473K before annealing.

Fig. 5. AFM image of ZnTe deposited at 473K after annealing.

Optical Properties. Semiconductor with the 1.85eV to 2.5eV can be used in photovoltaic application, which is an essential property for the film, as BSF layer. Transmission and absorption spectra was obtained from the Shimadzu, UV-VIS spectrometer. It is clear that the films before annealing treatment exhibits were highly transparent compared to the films after annealing [9]. It is clear from the Figure.6 and Figure.7 that the transmission spectra of the film deposited at 423K and 473K, before and after annealing, which is highly discrete with sequential format.

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Fig. 6. Transmission Spectra of ZnTe film deposited at 423K, and 473K before annealing.

Fig. 7. Transmission Spectra of ZnTe film deposited at 423K, and 473K after annealing.

The decrease in transmission spectra of films after annealing was due to the fact that there was a change in stoichiometry and structural improvement in the films due to the high absorption of temperature. It is clearly shown from the results of transmission spectra of the film. The swanpoel method [10] was used to calculate the absortion coefficients of the film from the reflection measured [11]. Also, the Bandgap is also measured from the following formula:

α⋅ hυ = Aa (hυ− Eg )1/ 2 where hυ represents the energy of the incident photon, Eg represents the energy band gap. It is found to be in the range of 1.89eV to 2.42eV. It is an important property for a film to exhibit its performance, as a BSF layer [12]. Raman spectroscopy. Raman is proved to be one of the useful tool for the spectroscopic analysis of the thin films. As ZnTe films are one of the promising films for the photovoltaic applications, it is essential for the film to exhibit Raman properties [13]. Due to various factors such as molecular vibration, material stress and its structural disorder, there can be variation of Raman spectra of the film deposited at different temperatures [14]. It is clear from the images that the annealing affect of the films exhibit good Raman scattering at both elastic and inelastic modes of spectra. Any mismatch in the lattice structures as well as molecular distribution can be overcome by annealing at higher temperatures. There is also a possibility of heating effect of high power laser density which in turn can damage the film under investigation [15].

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Fig. 8. Raman Spectra of ZnTe at 423K and 473K before and after annealing respectively.

It is clear from the images that the ZnTe films exhibit the Raman Spectra at 205cm-1, 410cm-1 and 620cm-1 respectively. These longitudinal mode of phonon is identical for the photovoltaic application for its application as a BSF field. The Raman spectra obtained from both the films are due to the Telluride bands that exists in the ZnTe films which is due to the 205cm-1 longitudinal phonon mode [16]. Thus the results of Raman spectroscopy helps in better understanding of the phonon level distribution of molecules in the film. It is clearly depicted that the Films annealed at higher temperatures have good quality of performance as a BSF layer inside the photovoltaic thin films [17]. Still, this Raman spectra does not profile the complete analysis of films, further study at different intensities of study is required to map the behavior of such thin films. Summary. The study shows that the annealed films at higher temperatures exhibit a good BSF layer, compared to the film properties before annealing. XRD studies clearly shows that Cubic Zinc Blende structure and which is an essential structure for a molecule as a BSF layer. AFM results also depicted a prominent high intensity peaks of annealed film than a non- annealed film [18]. Absorption spectra and the bandgap thus calculated ranges from 1.89eV to 2.42eV, which is the most optimistic character for a BSF layer in thin film solar cells. Raman spectroscopy yielded very unique information about the phonon properties and the surface molecular composition of the ZnTe before and after annealing. This investigation shows that the annealed ZnTe exhibits better BSF layer properties compared to the non-annealed ZnTe films. Still further characterisation studies can provide better information, which will be investigated further. Acknowledgement. Authors would like to thank Mr. Sanjith for his kind support and Mrs. Sunita for the facilities provided for this study. I like to gratefully acknowledge their presence in this study. References [1] Nazar Abbas Shah, Waqar Mahmood, Thin solid Films, “Physical properties of sublimated zinc telluride thin films for solar cell applications”. DOI: 10.1016/j.tsf.2013.03.088 [2] T. L. Chu, Shirley S. Chu, F. Firszt, and Chuck Herrington, Journal of Applied Physics, “Deposition and properties of zinc telluride and cadmium zinc telluride films”. DOI: http://dx.doi.org/10.1063/1.336514 [3] Michael Neumann-Spallart, Christian Kiinigstein , Thin Solid Films, “Electrodeposition of zinc telluride”. DOI: http://dx.doi.org/10.1016/0040-6090(95)06641-1 [4] M. Rusu, I. Salaoru, M. E. Popa, G. I. Rusu, Intern.J. Mod. Phys. B18, 1287 (2004). DOI:10.1.1.518.6368...

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Cite the paper

Srimathy N., A. Ruban Kumar (2017). Deposition and Characterisation of Zinc Telluride as a Back Surface Field Layer in Photovoltaic Applications. Mechanics, Materials Science & Engineering, Vol 9. doi:10.2412/mmse.32.15.18

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