Kinetics of Charge Trapping and Emission in CIGS Solar Cells
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1012-Y12-16
Kinetics of Charge Trapping and Emission in CIGS Solar Cells Aleksander Urbaniak1, Malgorzata Igalson1, and Susanne Siebentritt2 1 Faculty of Physics, Warsaw University of Technology, Koszykowa 75, Warszawa, PL 00662, Poland 2 Hahn-Meitner-Institut, Glienicker Strafle 100, Berlin, D-14109, Germany ABSTRACT In this work we investigate bias ñ induced metastability in CIGS solar cells. Long - term capacitance transients have been measured for two baseline CIGS devices with different efficiencies and a CGS cell in order to analyze carrier trapping processes. Based on the results we discuss hole emission process which leads to metastable increase of net acceptor density and also hole capture related to its relaxation. Time constants and activation energies for hole emission and capture have been obtained. Apart of carrier trapping processes we have also distinguished an interface-related change of capacitance. Our results indicate processes involving thermal lattice relaxation. We explain them in light of properties of (VSe+VCu) divacancy, defect with negative correlation energy, which can exist in both donor and acceptor configuration. INTRODUCTION Bias ñ induced metastabilities in Cu(In,Ga)Se2 based devices are typical and well certified features for those structures. Even in highly efficient devices such characteristic phenomena are easily observed. Applying voltage to the sample at room temperature causes persistent changes in its electrical characteristics. Effect which is observed after applying voltage pulse is a long lasting change in capacitance which can persist even for days. Bias stressing induces also significant alteration of space charge distribution and net acceptor concentration profiles, shift in admittance spectra and also can strongly deteriorate current ñ voltage characteristics [1,2]. These metastable changes are temperature dependent. We are able to induce those effects around room temperature and above it. If we cool down the sample to temperatures below 200 K with applied voltage, the metastable effects are freezed and we can return to the previous state of the cell only by heating up the structure. TSCAP (Thermally Stimulated Capacitance) measurements show that metastable increase of the capacitance obtained after reverse bias treatment relaxes at temperatures above 250 K. Metastable effects occurring in CIGS solar cells are described in detailed way in [1,2]. As current ñ voltage characteristics change after bias treatment understanding of the origin of observed phenomena is of importance for explaining factors having impact on the cell performance. In this contribution we employ transient capacitance techniques to investigate carrier capture and emission processes which lead to creation and relaxation of reverse bias ñ induced metastabilities in Cu(In,Ga)Se2 based devices. Long ñ term capacitance kinetics are investigated to examine changes of occupation of defects. We interpret the results within the framework of a model of (VSe + VCu) center [3], which we believe is responsible
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