Lateral Nonuniformity And Mesoscale Effects in Giant Area Electronics
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Lateral Nonuniformity And Mesoscale Effects in Giant Area Electronics V. G. Karpov, Diana Shvydka, and Yann Roussillon Department of Physics and Astronomy, University of Toledo, Toledo, OH 43606, U.S.A. ABSTRACT The recently developed physics of thin-film photovoltaics is suggested to be representative of other giant area electronics. We show that (i) giant-area devices are intrinsically nonuniform in the lateral directions, (ii) the nonuniformity spans length scales from millimeters to meters depending on external drivers such as light intensity and bias, and (iii) it significantly impacts the device performance. We derive a fundamental length scale that discriminates between the cases of small and large-area devices, and beyond which a new physics emerges. In addition, we present a practical method of mitigating the nonuniformity effects. INTRODUCTION Opto-electronic applications of giant area including flexible substrate photovoltaics (PV), light-emitting devices (LED), and liquid-crystal displays (LCD) can be thought of as arrays of non-linear elements integrated through conducting electrodes. Because of the large area requirements, these elements are typically non-crystalline, and are made of either amorphous or polycrystalline materials. Due to their inherent disorder, they are not quite identical and are characterized by certain distributions of parameters; hence, random arrays of nonlinear elements. In this work we discuss the effects of randomness on the integral parameters of giant area electronics. In many cases, the array elements are diodes, such as constituting thin-film transistors in LCD drivers or photo-diodes responsible for PV and LED effects. In the simplest case these elements are connected in parallel. Such are, for example, solar cells and light emitting diodes (Fig. 1). metal p-n junction TCO l
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(a) (b) Figure 1. Sketch of a PV system with elements connected in parallel: structure (a) and equivalent circuit (b). A parallel connection has obvious restrictions related to the electrode resistivity. Indeed, it is typical that at least one electrode in a device is made transparent in order to let the light in (for PV applications) or out (for LED). The transparent conductive oxides (TCO) used for such interconnects are quite resistive. If the area is too large (see below), this translates into substantial resistive loss that ruins the device performance. This
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problem is circumvented by using low resistive interconnects between individual small area elements constituting a giant area device. This concept and its corresponding equivalent circuit are illustrated in Fig. 2 for the case of large area terrestrial PV modules, where individual cells are quasi-linear in shape and are connected through low resistive metal scribes. Because each cell is quite long (~ 1m), the device parameter variations along its length are substantial. Therefore, it is better represented by a set of random elements in parallel, as shown in Fig. 2b. Because of relatively high conduction in TCO and interco
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