Formation of the band gap energy on CdS thin films growth by two different techniques
Introduction
The new technologies for massive production of solar cells, are based on the use of materials with thin film geometry due to the low cost solar energy conversion, low materials consumption and the facility to obtain very small integrated modules [1], [2], [3], [4]. In this way, the cost of the produced power/year can be approximately 10 times less than the cost for crystalline silicon technology. For these reasons, thin films are actually the most promising technology for solar cells production. However, a combined solution deposition and thermal evaporation technology have been found the most promising processes for solar cells mass-production. Considering the different materials for photovoltaic applications, the heterostructure glass/conductor-oxide/CdS/CdTe/metal is one of the most low cost efficient converters of solar radiation in electricity (15.8%) [5], [6], and much effort has been made on their research and development. Actually, CdTe is one of the most important semiconductors because of the direct band gap energy (1.45 eV), and the high conversion efficiency of terrestrial solar light in electricity. This semiconductor is normally prepared by sublimation, by heating CdTe powder of high purity at 700°C and deposited on a substrate at 500°C. However, CdTe does not grow stoichiometrically [7], having slight Cd vacancies deficiency, consequently growing as p-doped. Thus, a natural n-doped material is required as a partner to achieve a good junction with high efficiency in solar conversion. An optimal partner for CdTe is cadmium sulfide, CdS. Normally, CdTe is grown on CdS thin films previously prepared by chemical deposition method. The wider band gap of CdS allows the sunlight to enter the CdTe material more readily, acting as a window effect.
As a rule, CdS thin films grow as n-type semiconductor due to the donor centers formed during deposition. Vacancies of sulfur cause deviations from stoichiometry. However, by means of thermal diffusion of impurities, such as Cu and In, after preparation, it is possible to obtain p-type CdS [8], [9].
CdS is normally prepared by a low cost and a low temperature technique: the chemical bath deposition (CBD) [10], [11], but is also prepared at high temperature by evaporation-sublimation technique [12], or by spray pyrolysis [13], for the production of large-areas, being the last one an intermediate method between gas phase and solution techniques.
For solar cells manufacturing, typical film thickness suggested is approximately 4 μm for CdTe, and 0.1 μm for CdS. The formed CdTe/CdS interface presents a large mismatch between compounds (10%) [14] and usually, a further heat treatment needs to be made to obtain a good homojunction. The formed junction and the interface characteristics are the key for good development of the solar cell; however, an excessive mix of the materials in the interface can be the reason for the homojunction disappearing and the solar cell performance diminishing [4]. Similar results can be obtained if mixing in the interface is not good, because generated defects can reduce the solar cell efficiency. Therefore, it is necessary to achieve an intermediate mixed conditions such that thin layers of CdS deposited on CdTe only remove the mismatch defects, and maintains the characteristics of the heterojunction.
The band gap energy of the thin films is one of the most important parameters of transparent films for optical window applications. CdS polycrystalline films possess a direct band gap of 2.42–2.45 eV at room temperature (RT). For solar energy applications, CdS films require to have: high optical transparency, low electrical resistivity and high structural orientation at RT, being the first requirement related with this work. Optical transparency depends on the band gap energy value and the thickness measured on the films, and shows a strong dependence with the film preparation procedure.
In this work we studied the formation of the band gap of CdS thin films during the first stages of growth, when they are prepared by CBD and close spaced sublimation (CSS) techniques, and the development of the morphological and the structural properties achieved. The CBD technique was applied with two modalities: magnetic agitation on the bath solution and by ultrasonic vibration on the whole deposition system. The role of the thickness on the CdS films and its relation with the measured band gap energy for solar applications are discussed.
Section snippets
Experimental procedure
CdS films were deposited on Corning glass 7059 substrates (10×15 mm) cleaned with carbon tetrachloride, acetone, and isopropyl alcohol and rinsed with distilled water in each step. Films were deposited by two techniques: chemical bath deposition (CBD) and close spaced sublimation (CSS).
Details of the CBD technique are widely described in the literature [15], [16]. Briefly, the chemical bath is formed by an aqueous solution of cadmium chloride (CdCl2), potassium hydroxide (KOH), ammonium nitrate
Band gap energy formation
Optical properties were measured on CdS films, prepared with different deposition times (i.e. different thickness) by means of the optical transmittance property. Energy vs. squared absorption coefficient (α2) curves were plotted in order to calculate the Eg value using Eq. (1). Fig. 1 shows the results obtained for the CdS films prepared by CBD with magnetic agitation and different deposition times. The absorption process is directly related to the curves behavior. Curves correspond to films
Conclusions
CdS thin films were deposited on glass substrate by chemical bath deposition and close spaced sublimation techniques. For films grown using the CBD technique, two modalities for bath agitation was used: magnetic agitation and ultrasonic vibration. For these techniques we used small and crescent deposition times, in order to produce very thin films for the band gap energy formation study, and to obtain the corresponding morphological and structural properties. Morphology of the surface obtained
Acknowledgements
This work was supported by CONACYT (México) trough project 28778-E. Authors thank M.S. Daniel Aguilar for the X-ray measurements, Wilian Cauich for thickness determination by Auger, and Mr Roberto Sánchez for CSS films preparation.
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