An investigation of the dip-Ion Layer Gas Reaction process to produce ZnO films with increased deposition rates Tristan Koehler, Sophie Gledhill ⁎, Alexander Grimm, Nicholas Allsop, Christian Camus, Andreas Hänsel, Wolfgang Bohne, Jörg Röhrich, Martha Lux-Steiner, Christian Herbert Fischer Helmholtz Zentrum Berlin, Glienicker Straße 100, D-14109 Berlin, Germany abstract article info Article history: Received 27 June 2007 Received in revised form 4 November 2008 Accepted 3 December 2008 Available online 11 December 2008 Keywords: ZnO Non-vacuum deposition Buffer layer Chalcopyrite solar cell Intrinsic ZnO thin film layers have previously been deposited using the dip-ILGAR ‘Ion Layer Gas Reaction’ method. This deposition method and material has been effectively employed to deposit the buffer layers in chalcopyrite solar cells [M. Bär, H.-J. Muffler, Ch.-H. Fischer, S. Zweigart, F. Karg, M.C. Lux-Steiner Prog. Photovolt: Res. Appl.10 (2002) 173]. The original parameters for the ZnO dip ILGAR deposition process were optimised for film quality. These parameters, however, were not suitable for an up-scaled technology transfer to tape deposition as the dip speed and growth rate meant impractically long deposition times. The results presented here are from an investigation, using the laboratory scale ILGAR apparatus with 2.5×5 cm 2 substrates, into parameters e.g. solvent, salt and apparatus parameters, which could allow an increased deposition rate and dip speed yet retains film quality. Simple dip mechanics and the Landau–Levich equation, which describes film thickness as a function of dip withdrawal speed, gravitational acceleration and the properties of the solution, are considered. The recently optimised deposition parameters given here will allow a dip speed of more than 7 m/min and deposition rate of 7.5 nm/cycle. © 2008 Elsevier B.V. All rights reserved. 1. Introduction ZnO is a material of great general commercial and scientific interest to a wide range of manufactured devices including solar cells, flat panel displays and gas sensors. In specific ZnO is used in chalcopyrite cells for the n-type window layer and buffer layer. In our laboratory a non-vacuum deposition technique ‘Ion Layer Gas Reaction’‘ILGAR’ was developed and patented [1,2–5]. ILGAR is a cyclic and sequential process. In the first step a solid film of metal precursor compound (‘ion layer’) is deposited by dipping the substrate into a metal salt solution. On removal from the solution the solid precursor is chemically converted to the desired chalcogenide by exposure at moderately elevated temperature to the corresponding hydrogen chalcogenide gas (‘gas reaction’). In order to form ZnO a zinc salt is utilised and the reactant gas is a basic H 2 O/NH 3 vapour. Previously high quality solar cells have been produced using a ZnO buffer layer or the so called ZnO ‘window extension layer’. Efficiencies of 15% have been reported [5] on Cu(In,Ga)(S,Se) 2 absorber structures. The ILGAR technique is scalable and particularly applicable to a tape coating process which would then process flexible solar cells. Flexible chalcopyrite solar cells have been produced in the Helmholtz Zentrum Berlin by conventional methods with efficiencies up to 16.7% [6]. Within the laboratory a 16 m long looped ILGAR tape coater for large scale production is under construction to deposit the buffer layer. The original ILGAR process used dip speeds of 0.138 m/min and deposited layers at a rate of 1 nm/dip [7]. For the looped tape process the dip rate and speed had to be accelerated to allow the tape to be coated with sufficient thickness in a practical time frame. To prepare for the technology transfer the investigation is done utilising the laboratory dip coater for substrates up to 2.5×5 cm 2 . The deposition rate, defined in the present work as the average thickness of the layer grown per ILGAR cycle, is dependent on a number of factors which include how well the solution wets the substrate, how much of the solution coats the substrate, how fast the solvent evaporates and how the film growth proceeds after drying of solvent and heating in oven. These factors are governed by a number of controllable parameters of the ILGAR dip coater system, such as the dip speed and are also dependent on the inherent properties of the solution and substrate such as the solution viscosity, density, boiling point and surface tension at both the solution–substrate and air– solution interfaces. Presented here is a systematic investigation into which parameters gave the optimum deposition rate whilst maintaining a high quality ZnO film in terms of homogeneity, density and crystal growth. Thin Solid Films 517 (2009) 3332–3339 ⁎ Corresponding author. E-mail address: sophie.gledhill@helmholtz-berlin.de (S. Gledhill). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.12.005 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf