Effect of Shunts on Thin-Film CdTe Module Performance.
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1165-M05-22
Effect of Shunts on Thin-Film CdTe Module Performance. Galymzhan T. Koishiyev, James R. Sites Physics Department, Colorado State University, Fort Collins, CO, United States ABSTRACT A 2-D numerical circuit model is used to analyze the impact of shunts on basic performance parameters of a CdTe thin-film module. A numerical estimate of module-efficiency loss in the worst-case scenario due to shunts of different severity and fractional module area is presented. It is shown that absolute module-efficiency loss ∆η (%) varies in systematic fashion with these shunt parameters. Estimates of ∆η based on simple area-weighted efficiency are typically low by 3-4 times. Furthermore, the distribution pattern of shunts over the module plays a significant role in the module loss. A reliable parameter P to characterize the distribution of shunts is introduced, and its effect on module-efficiency loss, as well as individual-parameter (FF, VOC and JSC) losses, is shown. Furthermore, higher transparent-conductive-oxide (TCO) sheet resistance is shown to increase shunt isolation and consequently mitigate the efficiency decrease. INTRODUCTION Thin-film modules have now been on the market for more than two decades, and the minimization of loss mechanisms remains a primary goal of research and development laboratories. One such loss mechanism, which under certain conditions can be a critical one, is the loss due to localized shunts in the module. The general design and geometry of most thinfilm modules is essentially the same. A typical thin-film module consists of a number of elongated cells connected in series and separated from each other by scribe-lines. Various imperfections in scribing techniques can result in localized shunt paths. In many cases, the shunts are not random but occur preferentially at cell cusps and grain-boundary corners along cell edges due to physical chipping from the scribing procedure [1]. Imperfection in the scribing procedure, however, is not the only cause of shunt paths in thin-film modules. Generally during deposition, thin-film growth produces a granular structure with the main grain axis perpendicular to the film plane. Penetration of the junction depletion layer throughout a module by such grain boundaries can also lead to shunting conductance [2]. Still, another mechanism that can cause localized shunt is a flaw during the deposition process of the layers, when the window layer is not properly deposited and is locally too thin or is not continuous. The primary goals of the current work are to understand collective impact of several shunts on module performance, what the appropriate shunt parameters are, and how shunt parameters correlate with each other in their effect on the module. To address these questions, a 2-D circuit model of a CdTe thin-film module, which assumes typical rectangular geometry and distributed TCO sheet resistance ρS (Ω/sqr), was used. A reliable parameterization of shunt severity, fractional area, and distribution pattern was introduced. Absolute module-efficiency lo
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