Metastable Defect Distributions in CIGS Solar Cells and Their Impact on Device Efficiency
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1012-Y04-01
Metastable Defect Distributions in CIGS Solar Cells and Their Impact on Device Efficiency Malgorzata Igalson Faculty of Physics, Warsaw University of Technology, Koszykowa 75, Warszawa, 00 660, Poland ABSTRACT Metastabilities in the electrical characteristics of Cu(In,Ga)Se2-based devices are commonly observed phenomena originating from persistent changes of shallow and deep level distributions within the absorber. We examine characteristic changes induced by voltage bias and light together with their relaxation behavior and interpret them as the consequences of ìa negative-Uî type of centers predicted by theoretical calculations of Lany and Zunger. It is shown how the properties of these centers justify a model of p+ layer explaining specific features of light and dark current-voltage characteristics. Discussion showing the impact of various charge distributions on carrier transport is presented. The arguments are provided, that centers responsible for metastable effects are also to blame for majority of photovoltaic losses exhibited in various devices.
INTRODUCTION Metastabilities in the electrical characteristics of Cu(In,Ga)Se2-based (CIGS) devices have been investigated for more than 10 years already and are still a subject of a debate. Illumination and voltage bias induce changes in doping profiles observed by capacitance-voltage [1-3] and drive level capacitance profiling DLCP [4], and also changes in defect spectrum seen by DLTS [1] and admittance [5]. Since current voltage (IV) characteristics and hence photovoltaic parameters of the solar cells are also affected [6-8], this effect needs in-depth explanation - and then introduction of technological procedures to minimize or eliminate, if possible, its detrimental influence. So far, two explanations of this phenomenon have been proposed: defect relaxation [1] and copper ions diffusion [9]. Recently more evidence both theoretical [10, 11] and experimental [2] have been found supporting the first explanation. Here we will review some of the older data and also show new experiments, which can be interpreted in the framework of the Lany-Zunger (L-Z) model [10,11] ] explaining persistent changes of doping distribution by ënegative-Uí behavior of VSe-VCu double vacancy. The theoretical calculations show that this defect can be either a shallow donor or shallow acceptor, depending on the charge state. Non-equilibrium carriers supplied either by light or voltage bias act to change the occupation of defects, and in consequence the distribution of these two configurational states in the junction. We will show how a model of a ìdefectedî or ìp+î layer is a bridge linking the Fermi-level dependent defect distributions (predicted by the L-Z model) with their impact on current transport and photocarrier loss. The conclusion derived from this model will be used to explain induced changes of the photovoltaic parameters in a given device - and also to interpret the differences between performance of devices from various laboratories with various efficiencies. F
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