Characterization of indium sulfide thin films containing copper
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1165-M08-21
Characterization of indium sulfide thin films containing copper N. Barreau and M. Tessier Institut des Matériaux Jean Rouxel (IMN)-UMR 6502, Université de Nantes, CNRS, 2 rue de la Houssinière, BP 32229, 44322 Nantes cedex 3, France ABSTRACT The crystalline, optical and electrical properties of In2S3 containing copper thin films are investigated. Increasing the amount of copper within the In2S3 crystalline matrix yields reduced bandgap value and hindered conductivity. The films investigated being synthesized at low temperature (200 °C), it is likely they have similar properties as the materials formed at the CuIn1-xGaxSe2/In2S3 interface. INTRODUCTION Indium sesquisulfide In2S3 is among the candidates to substitute the chemical bath deposited (CBD)CdS buffer layer in CuIn1-xGaxSe2 (CIGSe) thin films solar cell [1]. High efficiencies have been achieved with devices buffered with In2S3 grown by numerous techniques such as evaporation [2], sputtering [3], atomic layer deposition [4], spray pyrolysis [5] or spray ion layer gas reaction [6]. The interfaces formed between the CIGSe and these In2S3 buffers all share the specificity that copper as well as sodium diffuse from the absorber into the buffer. Consequently, the absorber surface is copper depleted and the buffer layer is not pure In2S3 but In2S3 containing copper and sodium. The properties of these compounds have been studied during the last decade from powder samples [7]. These works led to the conclusion that copper as well as sodium occupy the vacant tetrahedral sites of the spinel-like In2S3 crystalline structure; moreover, in order to keep the electrical neutrality, the insertion of three copper/sodium atoms is accompanied with the substitution of one indium. The crystalline structure of these compounds can thus be described as: [In16]Oh[In5.33-x/3Ax 2.67-2x/3]TdS32 (1) with x ≤ 4, represents vacant sites and A the copper/sodium atoms [7]. Although the properties of the limit composition, i.e. x = 4, are now rather well known, there remains a lack of knowledge concerning the compounds corresponding to x < 4. The present work aims at studying the properties of In2S3 containing copper thin films, referred to as In2S3:Cu. The influence of the amount of copper introduced within the In2S3 matrix on the structural, optical and electrical properties of the films is investigated. EXPERIMENT Thin films synthesis The process used in order to synthesize the In2S3:Cu thin films consists of two steps; firstly the growth of stoichiometric In2S3 films through the co-evaporation of elemental indium and sulfur, secondly the co-evaporation of copper and sulfur (Cu+S) onto these In2S3 films.
Sulfur is also provided with copper because according to formula (1), the In2S3:Cu phases contain higher chalcogen to metal ratio compared to In2S3. The amount of copper provided during this latter process sequence is the parameter controlling the copper content of the final films (value of x in (1)). One should note that between the In2S3 deposition and the Cu+S evaporation,
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