Anomalous Reverse Breakdown of CIGS Devices: Theory and Simulation
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Anomalous Reverse Breakdown of CIGS Devices: Theory and Simulation Marco Nardone and Saroj Dahal Bowling Green State University, E Wooster Street, Bowling Green, OH, 43403, U.S.A ABSTRACT Copper indium gallium selenium (CIGS) photovoltaic (PV) devices exhibit unique reverse breakdown characteristics in terms of the dependence on temperature, light intensity, photon energy, and buffer layer material. In this work, the theoretical basis of potential reverse breakdown mechanisms are described and compared to available data. Quantitative analysis performed with semiconductor device simulation indicates that none of the conventional reverse breakdown mechanisms can account for the observations. Further work to better understand the reverse current-voltage characteristics of CIGS PV devices will provide insight to improve performance and reliability. INTRODUCTION The reverse bias characteristics of copper indium gallium selenium (CIGS) based photovoltaic devices are important for understanding their behavior under conditions of nonuniform voltage and illumination. Partial shading, which can cause significant reverse voltage in a module, has been identified as a key reliability issue [1]. Several studies have shown that partial shading/reverse bias can cause shunts to form [2–5]. Theoretical work suggests that current crowding [4] and weak diode spots [6] can result in permanent damage to a CIGS module when subjected to partial shade. The reverse breakdown physics of CIGS devices is unclear. In this work we provide a review of the pertinent experimental data and a theoretical analysis of reverse breakdown mechanisms, along with supporting semiconductor device simulations. Various aspects of reverse breakdown in CIGS devices have been attributed to mechanisms such as avalanche (impact ionization) at low temperatures (T < 200 K) and tunneling near room temperature [7–9]. Quantitative evaluations of those mechanisms were not provided. A tunneling-assisted Poole-Frenkel (PF) conduction model was suggested to account for the light-enhanced reverse breakdown and temperature dependence [10]. However, the analytical expression for that model did not include features of tunneling. Also, the numerical values and physical appropriateness of the fitting parameters were not discussed. As described below, the PF effect can act as an enhancement to reverse breakdown which is otherwise governed by thermal excitation and/or tunneling. OVERVIEW OF EXPERIMENTAL DATA Reverse bias current-voltage (IV) measurements of CIGS devices have exhibited anomalous characteristics. Such properties are typically described by a breakdown voltage, 𝑉 . In the literature, 𝑉 is loosely quantified as the reverse voltage at which the current begins to increase exponentially, except for in [7] where it is given a more concrete definition based on an intersection point between two slopes of a reverse bias IV curve. The following are some of the
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