Improved Routes towards Solution Deposition of Indium Sulfide Thin Films for Photovoltaic Applications
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Improved Routes towards Solution Deposition of Indium Sulfide Thin Films for Photovoltaic Applications: Kuveshni Govender, David Smyth-Boyle and Paul O’Brien Department of Chemistry and the Materials Science Centre, University of Manchester, Oxford Road, Manchester, M13 9PL, UK ABSTRACT Conditions necessary for the reproducible deposition of pristine In2S3 thin films on TOglass substrates by low temperature solution deposition have been identified. Baths containing carboxylic acids yield adherent, specular and crystalline films, within a defined pH range, dependent on the particular acid employed. Films have been characterised by XRD, SEM, XPS and electronic spectroscopy. As–deposited films were found to crystallise as tetragonal β-In2S3, no evidence for incorporation of hydroxy-indium species was found by XPS measurements.
INTRODUCTION The III-VI semiconductor indium sulfide (In2S3) has been the focus of much attention during the past two decades, primarily due to its optoelectronic properties. The compound has inspired applications in the preparation of green and red phosphors [1] and more recently in semiconductor-sensitized solar cells [2]. Indium sulfide is an n-type semiconductor that exists in three forms, viz. a defect cubic structure α- In2S3 under ambient conditions, which transforms into a defect spinel β-In2S3 at 693 K and into a layered structure γ-In2S3 at 1013 K [3-5]. The bandgap of In2S3 varies between 2.0 and 2.45 eV depending upon the composition [6]. An important research goal within the photovoltaic industry is the replacement of toxic heavy metals, such as cadmium, with more benign materials. The replacement of n-type CdS layers in polycrystalline heterojunction thin film solar cells employing CIS, CIGS or CdTe absorber layers, is of particular interest. Thin films of In2S3 have been demonstrated to be a suitable substitute for such layers in CIGS based cells. Films are most commonly deposited by the chemical bath method (CBD). Efficiencies of 15.7 % have been obtained for such devices, comparable to 16 % obtained for the standard CBD CdS buffer layer [7]. It is known that the composition, structure and morphology of CBD films are highly sensitive to deposition conditions (e.g. bath chemistry and nature of substrate surface). Such sensitivity grants the investigator the potential for tailoring the material properties of films by judicious selection of CBD conditions. Various groups have reported CBD-Inx(OH,S)y [7-9] but to date, no convincing studies have been presented for the CBD of pristine and crystalline In2S3. The most satisfactory films have been deposited from baths containing acetic acid. In preliminary studies, we have identified the conditions necessary for reproducible CBD-In2S3 on TO-glass from baths containing carboxylic acids. Our approach may offer a general route towards low temperature deposition of crystalline thin films of β-In2S3 with no hydroxide incorporation.
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EXPERIMENTAL DETAILS CBD of In2S3 thin films Thin films of In2S3 were deposited on commercial TO glas
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