The Formation of CuIn(S,Se) 2 Thin Film Solar Cell Absorbers from Electroplated Precursors
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1012-Y03-29
The Formation of CuIn(S,Se) 2 Thin Film Solar Cell Absorbers from Electroplated Precursors Stefan Jost1, Frank Hergert1, Rainer Hock1, Torsten Vofl2, Jˆrg Schulze2, Andreas Kirbs2, Michael Purwins3, Volker Probst4, and Jˆrg Palm4 1 Chair for Crystallography & Structural Physics, University of Erlangen-N¸rnberg, Staudtstr. 3, Erlangen, D-91058, Germany 2 Atotech Deutschland GmbH, Erasmusstr. 20, Berlin, D-10553, Germany 3 Crystal Growth Laboratory, University of Erlangen-N¸rnberg, Martensstr. 7, Erlangen, D91058, Germany 4 AVANCIS GmbH & Co. KG, Otto-Hahn-Ring 6, M¸nchen, D-81739, Germany
ABSTRACT In this article structural properties as well as morphological aspects of CuIn(S,Se)2 thin film solar cell absorbers, produced by annealing of electroplated precursors, are discussed. Realtime X-ray diffraction (XRD) experiments while precursor annealing have shown, that a reduced amount of electrodeposited selenium is the key parameter to realize a chalcopyrite formation mechanism similar to the one known for sputtered stacked elemental layer (SEL) precursors. Absorber layers processed from precursors produced by simultaneous electrodeposition of copper, indium and selenium show a preferable absorber morphology with an average grain size on the micrometer scale when the electrochemically deposited amount of selenium is reduced to [Se] / [In] = 0.1. The amount of selenium, missing for the formation of a stoichiometric chalcopyrite, was deposited in a second process step prior to precursor annealing. Solar cells produced from these absorbers show light conversion efficiencies up to 10%. INTRODUCTION The production of Cu(In,Ga)(S,Se)2 thin film solar cell absorbers from electroplated precursors offers a great potential of cost reduction due to the substitution of production steps involving high vacuum technology and excellent resource utilization. Solar cell efficiencies of 11.5% have been demonstrated with a CuIn(S,Se)2 thin film absorber, produced from precursors fabricated by electrochemical deposition of copper, indium, selenium and sulfur [1]. The phase composition of electroplated precursors can be adjusted by an adaptation of different deposition parameters [2]. Real-time XRD experiments during the precursor annealing process have shown, that the initial precursor phase composition exerts a strong influence on the CuInSe2 formation mechanism as well as on the resulting absorber morphology [2]. Three different types of precursors ñ classified according to the observed differences concerning the semiconductor formation while annealing ñ were identified (table I). Precursor type I comprises the nanocrystalline as-deposited phases Cu2-xSe and InSe, which react to the chalcopyrite CuInSe2 starting at a sample temperature of 470 K. Precursor type II shows an inhibited CuInSe2 formation from the initial precursor phases γ-In2Se3 and Cu2-xSe starting already at 400 K.
Absorbers produced from type I and II precursors show an undesirable absorber morphology (for details see [2]). The chalcopyrite crystallization
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