Point Contact Admittance Spectroscopy of Thin Film Solar Cells
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Point Contact Admittance Spectroscopy of Thin Film Solar Cells Anthony Vasko, Kristopher Wieland and Victor Karpov University of Toledo, 2801 W. Bancroft, Toledo, OH 43606, U.S.A. ABSTRACT We present the new characterization technique of multi-dimensional admittance measurements. In standard admittance measurements, a semiconductor device is probed in the transverse dimension, between flat plate contacts. We extend such measurements to distributed, possibly non-uniform solar cells where one of the two contacts has very small (point-like) dimensions. As a result, both the real and displacement currents spread into lateral directions while flowing between the electrodes. Correspondingly, the probing electric field may result in contact voltages that are laterally not equipotential. The spatial voltage distribution will depend on the probing DC bias and AC frequency. The resulting measurement will give information about the system’s lump parameters, such as open circuit voltage, sheet and shunt resistances, as well as the presence and location of shunts. Understanding of the measurement is developed through intuitive and analytic models. Numerical models, utilizing finite element circuits, are used to verify the analytic results, and also may be directly compared to or used to fit experimental data. While our focus is on introducing the physical theory, early experimental results demonstrating spatial scaling are shown. INTRODUCTION Large area solar cells may have non-uniformities or inhomogeneities that can impair device performance or influence degradation mechanisms. Techniques for studying such nonuniformities include laser beam induced current (LBIC) [1], electron beam induced current (EBIC) [2], lock-in thermography [3], and local surface photovoltage [4]. Numerical modeling has also been used to simulate devices with non-uniformities[5]; in this work, we extend such modeling to simulate admittance measurements on devices with non-uniformities, with the potential for using these measurements on real devices to reverse engineer their parameters. C
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Figure 1. The experimental connections for an admittance measurement (A) and equivalent circuit (B). Contacting for (C) 1-dimensional and (D) 2-dimensional measurements. In C and D, the lower layer is a low resistance contact, the middle layer is the active semiconductor, and the upper layer is relatively resistive. In C, a low resistance 1-dimensional contact causes the voltage variation in the resistive contact to be in one dimension (perpendicular to the contact) only. Admittance measurements have long been used as a characterization method for thin-film solar cells, giving information such as doping profiles and density of states[6]. The typical
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admittance measurement is done on a semi-conductor structure sandwiched between two parallel, flat, equipotential electrodes (such as transparent conducting oxide and metal). The resulting complex current I response to an applied voltage V is interpreted in terms of a leaky capacitor, as shown in figure 1, gi
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