Optoelectronic properties of R-F magnetron sputtered Cadmium Tin Oxide (Cd2SnO4) thin films for CdS/CdTe thin film solar cell applications
Graphical abstract
Introduction
The current thrust in research on transparent conducting oxide (TCO) thin films has been foused by increase in mass-produced optoelectronic gadgets using highly electrical conducting and optical transparent thin nano structures [1]. TCOs are mostly used as passive transparent electrodes in optoelectronic devices, like flat panel displays, organic light emitting diodes (OLED) and chalcogenide/chalcopyrite thin film solar cells [2], [3], to name a few. In addition, TCOs are pertinent to transparent optoelectronics because of their inimitable features of transparency in the visible region of electromagnetic spectrum and tunable electrical conductivity, from almost insulating to degenerate semiconducting behavior. Indium tin oxide (ITO) is the utmost used TCO material for the optoelectronic applications because of its high electrical conductivity, low optical absorption and the ability to be prepared at low substrate temperatures [4]. However, the dearth and the cost factors of indium, along with the need for higher performing TCOs, with exceptional functionality and possessing extremely high mobility for selective applications have stimulated further research to search for new TCO materials [2], [5], [6].
Recently, multicomponent oxides have been attracting much attention as new TCO materials, due to the ability to tune their electrical and optical properties by varying the cationic proportions. At present, multicomponent transparent metal oxides such as CdIn2O4 [7], Zn2SnO4 [8], ZnGa2O4 [9] and Cd2SnO4 [10] have shown excellent properties and serve as a favorable alternative to ITO for solar cell applications. Amongst these TCOs, Cd2SnO4 has outstanding electrical and optical properties that are potentially superior to the conventional TCOs, such as ZnO, SnO2 and In2O3:Sn [11]. Especially, the Cd2SnO4 films are the center of interest for the applications in CdS/CdTe solar cells replacing the SnO2 layer. Wu et al. [12] reported 14% efficiency for the Cd2SnO4-based CdS/CdTe polycrystalline solar cells. The electrical resistivity (ρ) of a novel TCO material should be around 10−5 Ω cm, with typical absorption coefficient less than 104 cm−1 in the near UV and visible range, and with an optical band gap around 3 eV. Cadmium stannate films are much attracted for its low optical absorption (α) in the visible region and high thermal stability with high electron conductivity (σ) and mobility [3], [10]. Compared to other TCOs, Cd2SnO4 also has larger value of σ/α ratio (figure of merit for rating TCOs). A higher value of σ/α indicates better performance of the TCOs [3]. In addition, the Cd2SnO4 films are chemically inert, easily etchable using HCl or HF [10] and have smoother surface compared to ITO thin films. These assets make the Cd2SnO4 as potential TCO material that warrants further distinctive applications, besides the toxicity of cadmium.
The properties of cadmium tin oxide thin films were first reported by Nozik [13], who prepared the CdSnO3 films by radio-frequency (R-F) sputtering. Since then, sputtering is considered to be the foremost suited method to prepare the Cd:Sn:O films. The properties of Cd2SnO4 thin films prepared by R-F magnetron sputtering, along with post heat treatment in Ar or Ar/CdS atmosphere at various elevated temperatures, have been already reported elsewhere [7], [8], [14], [15]. However, most of the superstrate device structures and/or devices such as Cu(In,Ga)Se2 (CIGS) based solar cells that use flexible substrates such as polymer substrates which cannot sustain at very high temperature (>450 °C) [16]. Devices with superstrate structure require the as-prepared TCO films in which high temperature thermal annealing of the TCOs should be avoided to maintain the device structures and quality by avoiding the diffusion of layers at very high temperature annealing/thermal processing of the TCOs. Hence, the detailed study on the as-prepared crystalline Cd2SnO4 thin films needs to be clarified. Also, the properties of Cd2SnO4 thin films as a function of varying substrate temperature have been discussed only in few reports [17], [18], [19].
This present work aims to elaborate on the conditions for preparing device quality, “as-prepared,” crystalline Cd2SnO4 thin films that have high transmission to light and low resistivity, along with detailed structural and surface properties. The effect of substrate temperature on these properties and the scattering mechanism are systematically explored with supportive experimental evidences, illustrations and interpretations, especially, with X-ray photoelectron spectroscopy.
Section snippets
Experimental procedures
The sputtering target, from which the Cd2SnO4 thin films were sputtered, was prepared using 99.9% pure CdO and SnO2 powders (Merck India, Bengaluru, India), added in stoichiometric proportions with acetone and pelletized into disk-shape of 2 in. diameter using a uniaxial pressure of 3 tons/cm2. The target was then pre-heated at 150 °C for 6 h to remove the acetone, followed by sintering at 1100 °C for 4 h in air atmosphere. Prior to deposition, the prepared target was sputter-cleaned in the Ar
Structural characterizations
The thickness of the deposited Cd2SnO4 thin films was found to be in the range of 0.4–1 μm. The variation of film thickness with the substrate temperature is shown in Fig. 1. The film thickness decreased with the increase in substrate temperature. This can be attributed to either well known vapor pressure phenomena or adatoms diffusion process. As the substrate temperature was increased from RT to 300 °C, the particle ejected from the target, gaining kinetic energy on heating the substrate. The
Conclusion
Cd2SnO4 films have been deposited on to the glass substrates by varying the substrate temperature from room temperature to 300 °C, successfully, without any post deposition heat treatment. The as-prepared films were characterized and based on the results, an optimized condition for the preparation of Cd2SnO4 thin films was evolved. XRD results show that a structural phase transition was observed and the intensity of the reflections showed a dependence on substrate temperature. Surface
Acknowledgements
One of the authors K. Jeyadheepan would like to acknowledge SASTRA University for the financial support to carryout the research under TRR scheme. One of the authors, K.J. thanks Dr. Moses Ezhil Raj, Scott Christian College, Nagercoil and Dr. Vijayakumar Rajendran, SASTRA for their help in the preparation of this manuscript & Prof. M. Sridharan, SASTRA for his support.
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