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1210-Q06-08

Inherent Buffer-layer Formation on Chalcopyrite Absorbers C. Pettenkofer, A. Hofmann, C. Lehmann, A. Dombrowa Helmholtz- Zentrum Berlin, Institute E-I4, Albert-Einstein Str 15, 12489 Berlin, Germany ABSTRACT We report on epitaxial growth of ZnO on polycrystalline and (112) orientated CuInS2 and CuInSe2 thin films. Step-by-step growth and investigation by photoelectron spectroscopy (PES) and low energy electron diffraction (LEED) provided information on the growth mode and the electronic structure of the ZnO-CuInS2-interface. During the initial growth no ZnO is deposited. Instead a monolayer of ZnS is formed by depletion the CuInS2 surface of excess sulfur. Thereafter, the ZnO growth starts on the ZnS buffer layer. The band alignment derived from PES shows that the ZnS buffer layer is thin enough to provide a beneficial band alignment for photovoltaic applications. CuInSe2 (112) samples showed a similar behaviour, but at the chosen deposition temperature of 450°C only ZnSe growth is detected. At lower temperatures ZnO growth on top of ZnSe is observed. XPEEM experiments show an inhomogeneous interface. INTRODUCTION CuInS2 (CIS) and CuInSe2 (CISE) as compound semiconductors of the chalcopyrite family are promising materials for thin film solar cells. For an optimized band alignment and defect reduction between absorber and TCO window a buffer layer is necessary. Technologically, CdS prepared by wet chemical processing is used as buffer up to now despite a variety of drawbacks of this material. CdS diminishes the transmission within the blue spectral range and thereby the efficiency of the solar cell. Other concerns about Cd beside the material itself are long processing times (15 min) and interruption of the vacuum processes between absorber preparation and TCO deposition and initiated investigations for optimization of the buffer layer. In previous work we investigated the intimate CISE/CdS interface /1/. Here we report on preparation and investigation of buffer layers and TCO using modern surface sensitive electron spectroscopies and a dedicated UHV based preparation of the films to gain more insight into the fundamental parameters

determining the energetic, electronic and chemical conditions at the interface. Starting from these data a method is proposed which will introduce a complete in line vacuum process for the solar cell fabrication with no need of intermediate wet chemical processing. CuInS2 and CuInSe2 films are prepared by MBE in (112) orientation (main growth direction of chalcopyrite thin films). ZnO is deposited in a MOMBE process using Diethylzinc (DEZ) and water as precursors as described in /2/. EXPERIMENT CuInS2 (CIS) /1,4,3/ and CuInSe2 (CISE) epitaxial films are prepared in (112) orientation and characterized by LEED, XPS and UPS. Sulfide films are deposited using TBDS as a sulfur source /5/, CuInSe2 is deposited from elemental sources at 500°C on Si (111) or GaAs(111)A substrates. For both films clean and bright LEED pattern are observed and band structure investigations by ARUPS p