Impact of thermal annealing on physical properties of vacuum evaporated polycrystalline CdTe thin films for solar cell applications

https://doi.org/10.1016/j.physe.2016.01.012Get rights and content

Highlights

  • The CdTe films have zinc-blende structure with preferred orientation (111).

  • Optical band gap is found to decrease from 1.64 eV to 1.48 eV with thermal annealing.

  • The as-grown films are uniform, homogeneous and free from crystal defects.

  • The surface roughness is increased with annealing temperature.

Abstract

A study on impact of post-deposition thermal annealing on the physical properties of CdTe thin films is undertaken in this paper. The thin films of thickness 500 nm were grown on ITO and glass substrates employing thermal vacuum evaporation followed by post-deposition thermal annealing in air atmosphere within low temperature range 150–350 °C. These films were subjected to the XRD, UV‐Vis NIR spectrophotometer, source meter, SEM coupled with EDS and AFM for structural, optical, electrical and surface topographical analysis respectively. The diffraction patterns reveal that the films are having zinc-blende cubic structure with preferred orientation along (111) and polycrystalline in nature. The crystallographic parameters are calculated and discussed in detail. The optical band gap is found in the range 1.48–1.64 eV and observed to decrease with thermal annealing. The current–voltage characteristics show that the CdTe films exhibit linear ohmic behavior. The SEM studies show that the as-grown films are homogeneous, uniform and free from defects. The AFM studies reveal that the surface roughness of films is observed to increase with annealing. The experimental results reveal that the thermal annealing has significant impact on the physical properties of CdTe thin films and may be used as absorber layer to the CdTe/CdS thin films solar cells.

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.

fx1
  1. Download : Download full-size image

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)

  • O.K. Echendu et al.

    Thin Solid Films

    (2014)
  • I. Ban et al.

    Mater. Lett.

    (2012)
  • A. Salavei et al.

    Thin Solid Films

    (2013)
  • S. Lalitha et al.

    Sol. Energy Mater. Sol. Cells

    (2004)
  • H.J. Goldsmid et al.

    Sol. Energy

    (1980)
  • S.D. Gunjal et al.

    Sol. Energy

    (2014)
  • S. Chander et al.

    Mater. Sci. Semicond. Process.

    (2015)
  • L.R. Cruz et al.

    Thin Solid Films

    (1999)
  • A. Romeo et al.

    Sol. Energy Mater. Sol. Cells

    (2001)
  • L.R. Cruz et al.

    Vacuum

    (2013)
  • Y.O. Choi et al.

    Mater. Sci. Eng. B

    (2010)
  • J.H. Lee et al.

    Sol. Energy Mater. Sol. Cells

    (2003)
  • V. Kosyak et al.

    J. Cryst. Growth

    (2010)
  • A. Purohit et al.

    Physica E

    (2015)
  • S.S. Shinde et al.

    Solid State Sci.

    (2008)
  • S.P. Nehra et al.

    Mater. Sci. Semicond. Process.

    (2015)
  • V.M. Nikale et al.

    Sol. Energy

    (2011)
  • S.S. Lin et al.

    Surf. Coat. Technol.

    (2004)
  • Cited by (65)

    View all citing articles on Scopus
    View full text