Thermally evaporated CdTe thin films for solar cell applications: Optimization of physical properties
Graphical abstract
Introduction
Cadmium telluride (CdTe) is one of the most promising II–VI group compound semiconductor because of its potential use for wide varieties of applications such as solar energy conversion devices and substrate for γ-ray, x-ray and IR detectors [1]. CdTe has a band gap of 1.5 eV for efficient photo conversion and a high optical absorption coefficient ( > 104 cm−1) at band edge [2]. Properties of CdTe thin films are strongly dependent upon the fabrication technique, annealing treatment, film thickness, doping, chemical etching and substrate temperature etc. [3].
CdTe thin films have a polycrystalline nature and can exist in both cubic and hexagonal forms [4], [5]. Polycrystalline CdTe films can be prepared by several techniques [6], [7], [8], [9], [10]. Among them, the thermal evaporation technique is popular because of its high deposition rate, low material consumption and low cost of operation [3].
Polycrystalline semiconductor thin films typically exhibit poor optoelectronic properties due to the existence of grain boundaries and other lattice defects. Thus, it is extremely important to minimize those through passivation by post-deposition treatments [11]. Among various post-deposition treatments, wet chemical etching of CdTe is generally used to create Te-rich regions on the CdTe surface, for forming a low resistance back contact to polycrystalline CdTe thin films. Usually, etching treatments are made on CdTe by using either a dilute solution of bromine in methanol (Br2-MeOH) or a mixture of nitric and phosphoric acids in water (NP etchant). These etchings allow CdTe surface smoothening, cleaning it from oxides and other contaminations, creating a Te-rich surface layer on CdTe [12].
Therefore, in this work CdTe thin films were fabricated by thermal evaporation at different substrate temperatures. Influence of substrate temperature on structural properties have been characterized using grazing incidence x-ray diffraction (GIXRD) technique and optical properties has been characterized by UV–vis spectrophotometry. The influence of etching on electrical properties has also been investigated for films fabricated under different substrate temperatures using van der Pauw method.
Section snippets
Experimental details
CdTe thin films were fabricated by thermal evaporation using Edwards Vacuum Coating Unit on cleaned soda lime glass substrate with the help of an alumina crucible and commercially available CdTe powder (Sigma-Aldrich, < 250 μm, deposition grade, ≥ 99.99%). Glass substrates were kept fixed at 12 cm above the crucible, inside the chamber after cleaning. Steps of the cleaning procedure can be found elsewhere [13], [14]. The chamber was emptied at a pressure better than 2 × 10−5 Torr by combination of
Structural analysis
Fig. 1 shows the diffraction patterns obtained for films fabricated at different substrate temperatures. According to the figure, each diffraction pattern has 10 reflections which corresponding to (111), (220), (311), (400), (331), (422), (511), (440), (531) and (620), suggesting polycrystalline nature with cubic zinc-blende structure according to ICDD-PDF file 15-0770. However, no phase changes corresponding to CdTe were observed with changing substrate temperature. Furthermore, additional
Conclusion
Structural analysis revealed that the fabricated CdTe films are polycrystalline in nature with cubic zinc-blende structure. Larger crystallite size lowest strain and lowest dislocations densities were found for films deposited at a substrate temperature of 250 °C, suggesting better crystallinity of the films. The transmittance spectra of the films fabricated at 250 °C show a better interference pattern, indicating smoothness of the surface and uniformity of thickness than the films deposited at
Acknowledgements
Financial assistance from National Science Foundation of Sri Lanka (NSF, RG/12/BS/03), Prof. S. Sivananthanthan of University of Illinois at Chicago (USA) and the Postgraduate Institute of Science of University of Peradeniya are gratefully acknowledged. Prof. M.A.K.L. Dissanayake of National Institute of Fundamental Studies of Sri Lanka, Prof. K. Premaratne of Department of Physics at University of Peradeniya, Mr. P.G.S.A. Bandara of University of Mississippi, Oxford, MS, (USA), Mr. R.G.S.A.
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