Impact of thermal annealing on physical properties of vacuum evaporated polycrystalline CdTe thin films for solar cell applications
Graphical abstract
The physical properties of polycrystalline CdTe films are investigated with thermal annealing treatment and results indicate that annealed films may be used as absorber layer in CdTe solar cells.
Introduction
The rapid increment in energy demand of the present world has paid an attention toward the development of low-cost and highly efficient solar energy sources. At present, the development of thin film solar cells is an active area of research due to low-cost, high efficiency and excellent stability. The study of II–VI binary semiconductor compounds has been intensified in order to find new suitable materials for solar cells in last few years [1], [2], [3]. Among these, the cadmium telluride (CdTe) has been recognized as one of the most promising candidates owing to its ideal direct band gap 1.45 eV and high absorption coefficient (>105 cm−1) in the visible range of solar spectrum. The conventional CdTe/CdS heterojunction is commonly used as p–n junction to the solar cell applications. In this junction, p-type CdTe is used as absorber layer together with n-type CdS as window layer [4], [5], [6], [7]. An extensive research has been done on CdTe thin films in last decade mainly due to its enormous potential applications in the field of thin film solar cells and optoelectronic devices like photo detectors, light-emitting diodes (LEDs), field effect transistors, radiation detectors, X-ray detectors, optical filters, nonlinear integrated optical devices, lasers etc. [8], [9], [10], [11], [12]. These applications have increased the importance of this material and motivated to investigate low-cost and high efficiency CdTe thin film devices.
CdTe thin films can be fabricated by a number of physical and chemical techniques such as pulsed laser deposition, magnetron sputtering, electro deposition, spray pyrolysis, close-space sublimation, metal organic chemical vapor deposition, thermal vacuum evaporation etc. [13], [14], [15], [16], [17], [18], [19]. Thermal vacuum evaporation is one of the most commonly used techniques to fabricate thin films because of its some advantages like being most productive, very high deposition rate and low material consumption as well as low-cost of operation. The film thickness-uniformity study on surface morphology, electrical and optical properties of sputtered CdTe thin films was reported by Choi et al. [20] for large-area semiconductor heterostructured solar cells. They observed that the photovoltaic properties of thin films were affected by the film thickness. Lee et al. [21] reported the electrical and optical properties of CdTe thin films employing vacuum evaporation and found that the dark resistivity was reduced with growth temperature and the photovoltaic properties were improved. The structural reproducibility of CdTe thin films deposited on different SnO2-coated glass substrates by close space sublimation method was reported by Potlog et al. [22]. These films were found polycrystalline nature with cubic phase and the optical band gap varied in the range 1.485–1.495 eV. Kosyak et al. [23] reported the effect of condensation temperature on structural and photoluminescence properties of close-spaced sublimated polycrystalline CdTe thin films. Recently, studies on effect of film thickness on physical properties of vacuum evaporated CdTe thin films were reported by Chander and Dhaka [19], [24]. They observed that the grain size and optical energy gap were decreased with film thickness while the micro strain was increased. The physical and chemical properties of the thin films are strongly dependent upon the fabrication techniques, film thickness, annealing, substrate, doping and substrate temperature. The annealing may be performed in vacuum, air and gaseous medium like N2, H2, Ar etc.
The literature survey invites an attention towards the study on impact of post-deposition thermal annealing on the physical properties of CdTe thin films. Thus in the present work, an attempt has been made to investigate the impact of low temperature annealing on the structural, optical, electrical and surface topographical properties of CdTe thin films which may be used as absorber layer. The films of thickness of 500 nm were deposited on glass and ITO coated glass substrates using thermal vacuum evaporation technique. The physical properties have been investigated using the XRD, UV‐Vis spectrophotometer, source meter, AFM, SEM coupled with EDS.
Section snippets
Deposition of CdTe thin films
CdTe powder of purity 99.999% and ITO coated glass substrates were procured from Sigma Aldrich. The thermal vacuum evaporation technique was employed to fabricate the CdTe thin films of 500 nm which were grown on ITO and 7059 corning glass substrates of dimension of (10 mm×10 mm×1 mm) at room temperature and working pressure 10−6 mbar. The glass substrates were used to find structural, optical, surface topographical and compositional analysis while ITO coated glass substrates for electrical
Structural analysis
The X-ray diffraction patterns of as-fabricated and annealed CdTe thin films are shown in Fig. 1.
The diffraction peak in the XRD pattern is observed at position 2θ=23.86° corresponding to orientation (111) for as-fabricated CdTe thin film and the orientation coincide well with the JCPDS data files 65-0880 and 15-0770 [28]. No diffraction peak is observed corresponding to cadmium, tellurium or other compounds. The (111) orientation is close-packed and this type of textured growth has been
Conclusion
This work reports the effect of post-deposition thermal annealing on the physical properties of polycrystalline CdTe thin films grown by thermal vacuum evaporation followed by post-deposition thermal annealing in air atmosphere at low temperature range 150–350 °C. The structure of films was found to be zinc-blende cubic with preferred orientation (111) and polycrystalline in nature. The crystal structure remains unchanged with low annealing temperature and increase in the intensity of the
Acknowledgments
The authors are thankful to the University Grants Commission, New Delhi, India for providing financial support under Major Research Project vide F.No.42-828/2013 (SR) and to the Centre for Non-Conventional Energy Resources, University of Rajasthan, Jaipur for providing deposition facility. MSD is thankful to the UGC for Raman Postdoctoral Fellowship vide F.No. 5-1/2013(IC) in USA.
References (36)
- et al.
Thin Solid Films
(2014) - et al.
Mater. Lett.
(2012) - et al.
Thin Solid Films
(2013) - et al.
Sol. Energy Mater. Sol. Cells
(2004) - et al.
Sol. Energy
(1980) - et al.
Sol. Energy
(2014) - et al.
Mater. Sci. Semicond. Process.
(2015) - et al.
Thin Solid Films
(1999) - et al.
Sol. Energy Mater. Sol. Cells
(2001) - et al.
Vacuum
(2013)
Mater. Sci. Eng. B
Sol. Energy Mater. Sol. Cells
J. Cryst. Growth
Physica E
Solid State Sci.
Mater. Sci. Semicond. Process.
Sol. Energy
Surf. Coat. Technol.
Cited by (65)
Influence of In-dopant on the optoelectronic properties of thermal evaporated CuAlTe<inf>2</inf> films
2023, Solid State Communications