Growth of Co-evaporated Cu(In,Ga)Se 2 - The Influence of Rate Profiles on Film Morphology
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Growth of Co-evaporated Cu(In,Ga)Se2 – The Influence of Rate Profiles on Film Morphology M. Bodegård, J. Kessler, O. Lundberg, J. Schöldström and L. Stolt Ångström Solar Center, Uppsala University P.O. Box 534 751 21 Uppsala, Sweden ABSTRACT The influence of the evaporation rate profiles on the microstructure of co-evaporated Cu(In,Ga)Se2, (CIGS), is discussed. The influence of Cu excess in the beginning of the CIGS growth has been investigated. In addition, the Ga rate has been varied in order to create bandgap grading in the CIGS film. By studying CIS and CGS films separately and as CGS/CIS stacks results on interdiffusion of In and Ga interdiffusion have been obtained. The resulting thin films are investigated mainly using Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD). Solar cell devices were prepared and IV measurements performed on samples with varying CIGS deposition parameters. INTRODUCTION The Boeing company was the first to superseed 10% efficiency for a thin film CuInSe2 solar cell [1]. The so-called Boeing recipe uses co-evaporation of the elements and is made in two steps. The first 2/3 of the evaporation is carried out under Cu-rich conditions. In the final 1/3, the Cu evaporation rate is reduced in order to obtain an overall Cu-poor stoichiometry. Variations of this approach have been used in this paper, expressed in different evaporation rate profiles. Bandgap profiles can be obtained by varying the Ga/(Ga+In) ratio during growth. Increased Ga concentration in CIGS material leads to an increase in the bandgap. This increase will mainly be in the conduction band energy level[2]. Beneficial effects on the solar cell device behaviour using high Ga contents near the back contact have previously been reported [3, 4]. We have observed that the main effect of the higher Ga concentration in the back of the solar cell is that it increases the open circuit voltage. Apart from having effects on the electric properties of the solar cells there is also an effect on the growth if a high Ga concentration is used. Higher Ga content typically leads to smaller grains. EXPERIMENTAL We presently have two deposition systems for the CIGS layer, the first being a Balzer BAK 550 vacuum system with mass-spectrometer feed-back control of the metal evaporation rates. The metals are evaporated from open boats and selenium is evaporated from a quartz crucible heated with a heat coil. The second system is a modified Balzer UMS500P with Luxel Radak II effusion sources for the metals. The same Se source design is used in both systems. The composition of the films in the UMS system is monitored by detection of the emittance change which occurs when the CIGS layers change from a Cu-rich to a Cu-poor composition. The H2.2.1
control is obtained by tracing the output power to the substrate heater, keeping the substrate temperature constant. As the emittance decreases, the power needed to heat the substrates also decreases. This change is rather abrupt and can easily be detected. When the transition is complete the outp
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