AFM Studies on the Morphological Evolution of Chemically Deposited In2S3 Thin Films as a Function of Deposition Time
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AFM Studies on the Morphological Evolution of Chemically Deposited In2S3 Thin Films as a Function of Deposition Time Merida Sotelo-Lerma1, Omar Armando Castelo-Gonzalez1, Rafael Ramirez-Bon2, and Francisco Javier Espinoza-Beltran2 1 Departamento de Investigacion en Polimeros y Materiales, Universidad de Sonora, Apdo. Postal 130, Hermosillo, Sonora, 83190, Mexico 2 Centro de Investigación y Estudios Avanzados del IPN. Unidad Querétaro, Apdo. Postal 1-798, Querétaro, Querétaro, 76001, Mexico ABSTRACT Indium sulphide (In2S3) is a very promising semiconductor material for window layers in solar cell devices. It is currently being investigated for high efficiency solar cells based on Cu(In,Ga)Se2-In2S3 heterostructures. The chemical bath deposition (CBD) technique is one of the most convenient methods to obtain In2S3 films because of its simplicity, low cost and some other advantages. Amorphous and polycrystalline In2S3 films on glass substrates have been obtained by the CBD technique. Like in many others CBD processes, the deposition mechanism and kinetic growth of In2S3 films on glass substrates is not very well understood [1-6]. In this work we have chemically deposited In2S3 films for different times from 6 up to 39 hours, in order to study by atomic force microscopy (AFM) the formation of the films on glass substrates. The AFM measurements were performed in a liquid medium in which the cantilever tip and the sample are completely immersed in the liquid. A specially designed AFM cell is composed of a tip attached to a circular transparent window, the liquid level is between the upper and lower surface of the window, and a circular meniscus is established around the window, preventing the tip could be affected or destroyed by the surface tension of the liquid. By using this liquid AFM technique, we can at real-time observe the thin film forming process, and thereby clearly reveal the growing mechanism. It is an ideal and more practical tool for in situ investigation of samples which are normally found in liquid environments. INTRODUCTION Nowadays solar cells based on Cu(In,Ga)Se2 (CIGS)-CdS heterostructures with efficiency as high as 17% on a laboratory scale are being the focus of intensive research. The solar cells with highest efficiencies have been obtained with CdS windows layers deposited by means of the chemical bath deposition (CBD) technique. However, the chemical deposition of the CdS layer in large-scale production of solar cells represents an environmental problem because the great amount of toxic Cd-containing waste. Indium sulfide (In2S3) is one of the most promising semiconductor materials to substitute CdS in CIGS-CdS solar cells, because the high efficiencies, up to 15.7 %, attained by CIGS-In2S3 solar cells [1, 7]. The energy band gap values reported for In2S3 films are between 2.0 and 2.75 eV, depending on the composition and deposition parameters [8, 9]. There are reports on direct gap and indirect gap transitions. The crystalline structure of In2S3 has three crystalline phases α, β a
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