Effect of oxidation on structural, optical and electrical properties of CdS thin films grown by sputtering

doi:10.1016/j.ijleo.2015.07.078

Optik

Volume 126, Issue 21, November 2015, Pages 3177-3180

https://doi.org/10.1016/j.ijleo.2015.07.078 Get rights and content

Abstract

CdS thin films of 150 nm thick were grown by sputtering technique on top of commercially available FTO coated glass substrates. The films are oxidized by subsequently annealed in oxygen ambient at 350 °C. XRD, EDX, FTIR analysis, UV–vis spectrometry and Hall Effect measurement were used to characterize the films. XRD studies confirm the polycrystalline nature of the deposited films with phase transition from hexagonal CdS to orthorhombic CdS:CdO mixed structure. It has been observed from the UV–vis and EDX characterizations that the band gap increases with the increase of oxygen concentration incorporated to the annealed CdS thin films as well as with the increase of the oxidation time. Band gaps of the films were found to be in the range of 2.52 – 2.89 eV. Cd single bond S and CdO stretching vibration was observed from the FTIR spectra. The film resistivity and mobility are observed to change inversely with the inclusion of O₂ in the CdS thin films.

Introduction

In recent years, extensive research has been carried out on the area of preparation and characterization of CdS thin films due to its application in different type of most promising photovoltaic solar cells such as CdTe [1], CZTS [2] and CIGS [3]. CdS is the group II – VI compound semiconductor with a direct optical band gap of around 2.45 eV, and CdS/CdTe, CdS/CZTS or CdS/CIGS contact is the energy converting interface in the among solar cells [4], [5]. The light is transmitted through the n-type CdS layer to the p-type absorber layer and the photogenerated electron-hole pairs are divided in the depletion region of the CdS/CdTe, CdS/CZTS or CdS/CIGS heterojunction. Moreover, CdS thin film in a solar cell structure does not just act as a window layer, but contribute to carrier generation due to its high photosensitivity [6], [7]. Therefore, the optical, structural and electrical properties of the CdS layers can influence the characteristics of CdS/CdTe, CdS/CZTS or CdS/CIGS heterojunction interface and consequently, the performance of the whole cells [8].

In this work, CdS thin films are oxidized by thermal annealing in an attempt to increase its band gap for efficient, stable and low-cost solar cells. The structural, optical and electrical properties of annealed CdS thin films have been analyzed by XRD, EDX, FTIR analysis, UV–vis spectrometry and Hall Effect measurement.

Section snippets

Experimental

Commercially available FTO coated soda lime glass substrates were used to deposit CdS thin films. Substrates were degreased by ethanol–acetone–ethanol and then dried by dry N₂. CdS layers of 150 nm were deposited by sputtering at a substrate temperature 300 °C and 30 W of RF power with 18 mTorr Ar pressure. The prepared films were annealed to oxidize in a vacuum furnace at 350 °C for 15, 30 and 45 min. The chamber pressure maintained in the range 300 – 320 Torr by continuous N₂:O₂ (99:1) gas flow as

Results and discussions

Fig. 1 shows the X-ray diffraction patterns for films oxidized at 350 °C for 30 min (sample 3) and 45 min (sample 4) in oxygen ambient. Examining the Fig. 1, it is observed that for the oxidized films, the XRD reflections peaks are corresponding to the CdS hexagonal phase (JCPDS 01-077-2306), where, the additional reflection peaks corresponding to the CdO:CdSO₄, (JCPDS 00-026-0382) phases are also observed. The film annealed for 30 min is found in (0 0 2) hexagonal phase oriented but, once the

Conclusion

The CdS thin films are oxidized by thermal annealing in oxygen ambient and the resultant films properties are extensively studied in this work. XRD patterns indicate that the resulting films show mixed structure with CdS and CdO phases, which may be described as CdS – CdSO4 – CdO. However, it is confirmed that O₂ is substituted into the Cd single bond S lattice, from FTIR studies. It has been found that the oxidization of CdS by thermal annealing in oxygen ambient increases its band gap by incorporating

Acknowledgment

This work is supported by NPST Program through King Saud University (KSU) with research grant code 10-ENE-1039-02. Authors would also like to acknowledge the Solar Energy Research Institute (SERI) of The National University of Malaysia (UKM) for other supports.

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Effect of oxidation on structural, optical and electrical properties of CdS thin films grown by sputtering

Abstract

Introduction

Section snippets

Experimental

Results and discussions

Conclusion

Acknowledgment

Sol. Energy Mater. Sol. Cells

Sol. Energy

Sol. Energy Mater. Sol. Cells

Thin Solid Films

J. Solid State Chem.

Sol. Energy Mater. Sol. Cells

Energy Procedia

Appl. Surf. Sci.

Vacuum

Sol. Energy Mater. Sol. Cells