Towards Lower Deposition Temperatures of Spray Deposited ZnO Films
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1012-Y02-10
Towards Lower Deposition Temperatures of Spray Deposited ZnO Films Sophie E Gledhill, Nicholas Allsop, Pablo Thier, Christian Camus, Martha Lux-Steiner, and Christian Herbert Fischer Solar Energy, Hahn Meitner Institute, Glieniker Strasse, 100, Berlin, D-14109, Germany
ABSTRACT A non-vacuum spray deposition method has been used to deposit ZnO thin films. This method is particularly relevant as the deposition technique is readily up-scalable to roll to roll coating processes. At a deposition temperature of 250°C the undoped spray deposited ZnO is conducting enough to replace the sputtered intrinsic ZnO layer used in Cu(In, Ga)(S, Se)2 solar cells. As the deposition temperature increases the resistivity of the spray deposited ZnO decreases, although for Cu(In, Ga)(S, Se)2 solar cells subsequent processing temperature must remain under 250°C other applications for transparent conducting oxides are not so temperature restricted. At substrate temperatures above 400°C we are able to dope the ZnO with Aluminium and attain resistivites of 5x10-3Ωcm and free charge carrier concentrations of 10-20cm-3. ZnO film growth and quality are sensitive to the precursor solution. For a non-vacuum process the properties of the films are excellent. 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. Specifically ZnO is used in chalcopyrite cells for the n-type window layer. All high efficiency thin film solar cells based on Cu(In, Ga)(S, Se)2 absorbers have been achieved using sputtered ZnO. The standard two layer (RF) sputtering process from ceramic targets consisting of a high ohmic pure ZnO layer followed by aluminium doped ZnO is often used [1,2]. The pure ZnO layer will be referred to as the intrinsic ZnO (i-ZnO) layer, although this layer grows as n-type with no intentional doping. The RF sputtering process is not practical for large area deposition of ZnO. The high cost of the RF equipment thus motivates research for alternative ZnO deposition methods. In this work we use a simple non-vacuum spray deposition method to deposit ZnO. The method is more akin to an aerosol assisted chemical vapor deposition (CVD) [3] than spray pyrolysis. Aerosol assisted CVD relies on CVD to form the film. The main distinction from CVD is the method by which the precursors are delivered to the substrate surface. In aerosol assisted CVD the droplets are formed by a spray route in which volatile precursors are dissolved in solvents which are to be nebulized. The solvent evaporates and reactants vaporise, at least partially, before reaching the substrate. The technique is capable of being carried out on a band or tape coater. In our laboratory a 10cm wide tape coater has been built for processing similar films on flexible substrates. The films presented here have been prepared on apparatus on a laboratory scale.
At deposition temperatures low enough for the absorber / buffer interface to be stable at (250°C) l
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