Optical and Physical Characterization of Cu 2-x Se Thin Films for Real Time Spectroscopic Ellipsometry on Cu(In,Ga)Se 2
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1210-Q06-02
Optical and Physical Characterization of Cu2-xSe Thin Films for Real Time Spectroscopic Ellipsometry on Cu(In,Ga)Se2-based Photovoltaic Devices. James D. Walker, Himal Khatri, Scott Little, Vikash Ranjan, Robert Collins, and Sylvain Marsillac Wright Center for Photovoltaics Innovation and Commercialization, University of Toledo, Toledo OH 43606, USA ABSTRACT In situ, real time spectroscopic ellipsometry (RTSE) has been used to study the growth processes and optical properties of Cu2-xSe - an important binary compound in the fabrication of high efficiency copper indium gallium diselenide (CIGS) photovoltaic devices. It was found that the high surface roughness of the Cu2-xSe layers necessitated a “graded” optical model in order to extract meaningful dielectric functions at both 550 ºC and room temperature. The optical model was verified at room temperature against SEM micrographs and reflectance measurements carried out ex situ. The growth temperature dielectric functions presented in this study are expected to allow for a greater level of control and understanding of the so-called 2- and 3-stage processes for CIGS fabrication in which a Cu2-xSe phase, present at the CIGS grain boundaries, acts as a fluxing agent for the growth of photovoltaic quality CIGS. Real time optical feedback via RTSE combined with the growth temperature dielectric functions presented here could play an important role in improving material fabrication on both the laboratory and industrial scales. INTRODUCTION The current state-of-the-art CIGS laboratory solar cells employ a 3-stage co-evaporation growth process obtaining conversion efficiencies as high as 20% [1]. In this 3-stage process, the CIGS layer is made non-stoichiometrically Cu-poor, proceeding to Cu-rich, and finally returning to a slightly Cu-poor phase [2,3]. This process has proven itself useful in obtaining precise control over the atomic Cu to group III ratio - critical for photovoltaic quality CIGS. In the Curich phase, stoichiometric CIGS exists simultaneously with a Cu2-xSe phase which phenomenologically enhances CIGS grain quality and size. The return to slightly Cu-poor conditions and growth termination is monitored indirectly by observing the power delivered to the substrate heater assembly. As the Cu2-xSe phase is consumed, the film goes through a substantial change in emissivity which alters the power requirements needed to hold the substrate at a fixed temperature. This change in power delivered to the substrate heater indicates that the Cu2-xSe phase has been depleted. Note that holding the power constant and observing the temperature change is also possible. While this is a robust and simply implemented method for controlling the CIGS stoichiometry, we are interested in probing these phase changes via RTSE to hopefully gain a greater understanding of the dynamics involved and possibly direct the process towards increased device efficiencies. Towards this end, the current study first looks at understanding the Cu2-xSe phase itself by depositing in conditi
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