Detailed study of metastable effects in the Cu(InGa)Se 2 alloys: Test of defect creation models
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Detailed study of metastable effects in the Cu(InGa)Se2 alloys: Test of defect creation models JinWoo Lee1, Jennifer T. Heath2, J. David Cohen1 and William N. Shafarman3 1 Department of Physics, University of Oregon, Eugene, OR 97403 U.S.A. 2 Department of Physics, Linfield College, McMinnville, OR 97128 U.S.A. 3 Institute of Energy Conversion, University of Delaware, Newark, Delaware 19716 U.S.A.
ABSTRACT We have investigated metastable changes to Cu(InGa)Se2 solar cells in response to infrared optical exposure and forward injection current. Using the drive level capacitance profiling and admittance spectroscopy techniques, we are able to distinguish changes in the free hole carrier concentration from those of a deeper bulk trap. For both types of treatment, changes in these concentrations clearly follow a 1:1 ratio, in agreement with recent predictions of a microscopic model proposed by Lany and Zunger. The creation kinetics follow a strongly sub-linear power law at 250K, and we demonstrate that we can account for this using rate equations in which the metastable effect is initiated by electron capture into an empty defect state. The dependence on time and light intensity is (time) 0.22± 0.03 and (intensity) 0.53± 0.06, except at high light intensities where saturation is observed.
INTRODUCTION It has been well established that metastable changes in Cu(InGa)Se2 (CIGS) solar cells occur in response to optical exposure or carrier injection [1-4]. A couple of mechanisms have been proposed to explain the physical mechanism behind the light-induced metastability in this material [1,5]. In particular, the recent microscopic model proposed by Lany and Zunger predicts a 1:1 relationship between the increases in free hole carrier concentration and increases in deep bulk acceptors [5]. In their model the vacancy complex of (VSe-VCu) produces p-type persistent photoconductivity through its conversion from a positive donor level near the conduction band edge to a state consisting of a negatively charged vacancy complex plus two holes in the valence band. After the capture of one of the released holes, this would then result in the 1:1 ratio between the final free hole carrier density, p, and the metastable deep acceptor (hole trap) density, NT. Our recent experiments have established more firmly the quantitative link between metastable changes in the deep defect densities and the free carrier densities. Indeed, we successfully confirm the 1:1 relationship between the metastable changes in the free hole carrier densities and deep acceptor densities following light exposure over a wide range of intensities, as well as by forward-bias current injection in the dark. In addition, we have examined the creation kinetics of this effect and find that it obeys strongly sub-linear time dependence at all intensities. Finally, a rate-equation analysis is presented that is generally consistent with the microscopic model proposed by Lany and Zunger.
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INSTRUMENTATION AND SAMPLE PREPARATION Our high frequency admittance s
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