Optical, electrical and structural investigations on Cd1−xZnxSe sintered films for photovoltaic applications

doi:10.1016/S0927-0248(02)00291-X

Solar Energy Materials and Solar Cells

Volume 76, Issue 3, 31 March 2003, Pages 399-415

https://doi.org/10.1016/S0927-0248(02)00291-X Get rights and content

Abstract

II–VI polycrystalline semiconducting materials have come under increased scrutiny because of their wide use in the cost reduction of devices for photovoltaic applications. Cd_1−xZn_xSe is an important semiconducting alloy because of the tunability of its physical parameters such as band gap and lattice parameters by controlling its stoichiometry. Many more material characteristics of it would be altered and excellently controlled by controlling system composition x.

Polycrystalline thin films of Cd_1−xZn_xSe with variable composition (0⩽x⩽1) have been deposited onto ultra-clean glass substrates by sintering process. The optical, structural and electrical transport properties of Cd_1−xZn_xSe thin films have been examined. The optical band gap and optical constants of these films were determined by using double beam spectrophotometer. The DC conductivity and activation energy of the films were measured in vacuum by two-probe technique. The Schottky junction of Cd_1−xZn_xSe with indium was made and the barrier height and ideality factor were determined using current–voltage characteristics. The nature of sample, crystal structure and lattice parameters were determined from X-ray diffraction patterns. The films were polycrystalline in nature having cubic zinc-blende structure over the whole range studied.

Sintering is very simple and viable compared to other cost intensive methods. The results of the present investigation will be useful in characterizing the material, Cd_1−xZn_xSe, for its applications in photovoltaics.

Introduction

Pseudobinaries of II–VI, IV–VI and III–V group compounds are attracting a great deal of attention because of their potential abilities in a wide spectrum of optoelectronic devices [1], [2], [3], [4], [5], [6]. High absorption coefficients, high efficiency of radiative recombination and nearly matching band gaps with the visible region of the solar spectrum are the root causes of the popularity of II–VI semiconductors. Ternary materials provide a possibility of tailoring their properties as per requirements and hence project themselves as important semiconducting material for future advancements in the field of device fabrication. Cd_1−xZn_xSe is a promising ternary material because of the tunability of its physical parameters such as band gap and lattice parameter by controlling its stoichiometry. The band gap of this material can be tuned from 1.70 to 2.70 eV with 0⩽x⩽1. It is an efficient absorber in the visible region of solar spectrum [7], [8]. The applications of Cd_1−xZn_xSe in thin film devices like laser screen materials in projection color TV's [9], [10], nuclear radiation detectors [4], light emitting diodes, laser diodes, electroluminescent, photoluminescent, photovoltaics [1], [2], [3], [4], [11], [12], etc. have shown its prominence and ability. The band structures, optical properties and crystal structures of both CdSe and ZnSe are very similar and therefore the system Cd_1−xZn_xSe would not only result in the feasibility of a graded energy gap of a broad spectral sensitivity but many more material characteristics can be altered and excellently controlled by the system composition x. Both CdSe and ZnSe are known to exist in either cubic zinc blende or hexagonal wurtzite crystal forms depending on the composition and the conditions of preparations. Compared to CdSe, Cd_1−xZn_xSe is more stable and would replace CdS as a window material in solar cells. Further, the incorporation of ZnSe into CdSe has shown pronounced effect in enhancing the electrochemical power conversion efficiency [13], [14], [15].

The research on renewable energies includes the photovoltaic conversion of solar energy and important investigations of novel materials and structures. Photovoltaics are one of the most fascinating ways for direct solar energy conversion. Thin film solar cells give hope to meet the cost goals, which are necessary to provide the needs for energy production by photovoltaics.

It is generally recognized that any large-scale application must rely on cheap polycrystalline materials. The use of thin film polycrystalline semiconductors has attracted much interest in an expanding variety of applications in various electronic and optoelectronic devices. The technological interest in polycrystalline-based devices is mainly caused by their very low production costs. Different workers [9], [16], [17], [18], [19] prepared Cd_1−xZn_xSe films by different techniques and studied their structural, optical and photo-electro-chemical properties.

Our intention is to employ this material for the fabrication of photovoltaic devices and therefore as a part of our continuing program we concentrated on our bifocal interest: (i) to develop a suitable method and optimize the different deposition conditions so as to obtain good quality Cd_1−xZn_xSe thin films and (ii) to investigate and analyze the structural, optical and electrical transport properties of these films. Sintering technique is one of the best techniques allowing the preparation of polycrystalline semiconductor films with ease, low costs and large area applications. It is extremely simple and viable compared to other cost intensive methods [20], [21], [22]. In this paper, optical, structural and electrical characterization of polycrystalline Cd_1−xZn_xSe sintered films has been carried out.

Section snippets

Synthesis of Cd_1−xZn_xSe films

The films of Cd_1−xZn_xSe (x=0, 0.2, 0.4, 0.6, 0.8, 1) were prepared by screen printing followed by sintering method. To make different compositions of Cd_1−xZn_xSe, we used CdSe and ZnSe of high purity (99.999%). Cd_1−xZn_xSe alloy was prepared by taking the stoichiometric ratio of CdSe and ZnSe compounds. The different compounds have been taken as follows:

Weight of CdSe=191.36 (1−x) g.
Weight of ZnSe=144.33 x g.
Weight of ZnCl₂·H₂O=10% weight of (CdSe+ZnSe) g.

As these weights were being very large, we

Optical studies

The optical reflectance measurements of Cd_1−xZn_xSe (x=0, 0.2, 0.4, 0.6, 0.8, 1) sintered films were recorded at room temperature from 400 to 800 nm wavelength range using a Hitachi U3400 UV–VIS–NIR double beam spectrophotometer. The optical band gaps of these films were determined with the help of reflection spectra. Almost all the II–VI compounds are direct band gap semiconductors. According to Tauc relation [24], the absorption coefficient for direct band gap material is given by $αhν=A(hν−E_{g})^{1/2}$

Conclusion

The optical, structural and electrical transport properties of CdSe–ZnSe pseudobinary sintered films were investigated. (1) The absorption coefficients of the films of Cd_1−xZn_xSe system is of the order of 10⁴ cm⁻¹ and their band gaps can be engineered from 1.69–2.58 eV by varying x from 0–1, which are suitable for efficient absorption in the visible region of the solar spectrum. (2) All the films of Cd_1−xZn_xSe system are found to be polycrystalline in nature and have cubic zinc-blende structure

Acknowledgements

Thanks are due to Department of Science and Technology (DST), Council for Scientific and Industrial Research (CSIR), New Delhi (India) and CONACYT (G38618-U), Mexico for providing financial assistance in the form of major research projects.

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Optical, electrical and structural investigations on Cd1−xZnxSe sintered films for photovoltaic applications

Abstract

Introduction

Section snippets

Synthesis of Cd1−xZnxSe films

Optical studies

Conclusion

Acknowledgements

Thin Solid Films

Solid State Commun.

J. Cryst. Growth

Mater. Chem. Phys.

Thin Solid Films

Sol. Energy Mater. Sol. Cells

Sol. Energy Mater.

Thin Solid Films

J. Phys. Chem. Solids

J. Phys. Chem. Solids

J. Opt. Mater.

J. Opt. Mater.

J. Cryst. Growth

Indian J. Pure Appl. Phys.

J. Electrochem. Soc.

Appl. Phys. Lett.

Optical, electrical and structural investigations on Cd_1−xZn_xSe sintered films for photovoltaic applications

Synthesis of Cd_1−xZn_xSe films