Computer Simulation of Edge Effects in a Small-Area Mesa N-P Junction Diode
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1123-P02-05
Computer Simulation of Edge Effects in a Small-Area Mesa N-P Junction Diode Jesse Appel1, Bhushan Sopori1 and N.M. Ravindra2 1 National Renewable Energy Laboratory, Golden, CO 80401, USA. 2 New Jersey Institute of Technology, Newark, NJ 07102, USA.
ABSTRACT The influence of edges on the performance of small-area solar cells is determined using a modified commercial, finite-element software package. The n+/p mesa device is modeled as having a sub-oxide layer on the edges, which acquires positive charges that result in development of an electric field within the device. Our computer simulations include generation/ recombination at the diode edges as well as the influence of light on the recombination characteristics of the edges. We present a description of our model, dark and illuminated characteristics of devices with various surface charge concentrations, and the dynamics of carrier generation/recombination. The influence of edge geometry on diode performance is determined.
INTRODUCTION Use of mesa diode arrays, fabricated by chemical etching, is a valuable technique for detailed characterization of photovoltaic properties of single-crystalline and multicrystalline silicon wafers. The chemical etching process creates a sub-oxide, which is rich in hydrogen and passivates the edges. The diode array consists of 100-mil-diameter devices that are etched 3 to 4 microns deep and have a blanket back Al contact common to all devices. Each device has a front Al contact, 10 mils in diameter, for ease in automatic probing. The diodes are electrically isolated from one another and can be probed to measure the current density versus voltage curves under dark and illuminated conditions [1,2]. We find that the degree of passivation depends strongly on the process conditions. To understand the details of the edge passivation, we modeled an n-p junction diode using a commercial, finite-element software package. These simulations have led to a determination of the self-consistent solution to the continuity equations for electrons and holes using the steady-state drift-diffusion model for carrier dynamics coupled with electric potential determined from Poisson’s equation [3]. The purpose of these simulations is to determine the influence of edge conditions on the overall performance of mesa diodes under dark and illuminated conditions. In particular, we examine the effect of edge shape on the I-V characteristics of the diode. The underlying mechanisms of bulk and surface recombination have been well established for crystalline silicon semiconductor devices [4-7]. We have applied them to our mesa device using the COMSOL software. Our simulations show that the space-charge region becomes extended along the vertical edge of the mesa diode due to the fixed positive surface charge. At the intersection of the vertical edge and step, a strong electric field is produced because it has a small convex radius of curvature [8]. Depending on the sharpness of this intersection, the entire device can become significantly shunted. Simu
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