Physics of CdTe Photovoltaics: from Front to Back
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Physics of CdTe Photovoltaics: from Front to Back V. G. Karpov, Diana Shvydka, and Yann Roussillon Department of Physics and Astronomy, University of Toledo, Toledo, OH 43606, USA ABSTRACT We discuss physical mechanisms underlying the performance and stability of CdTe based thin-film PV. The processes in (i) photovoltaic junction, (ii) back contact, (iii) nonuniformities, (iv) grain boundaries, and (v) light-induced degradation are addressed including their interactions. The physics of thin film PV turns out to be quite different from that of crystalline PV. High surface-volume ratio and lack of crystallinity result in strong interfacial effects, lateral nonuniformities, and shunting-like and adhesion instabilities in thin film structures. This paper is aimed at presenting a ‘big picture’; also, it suggests practical ways of improving thin-film PV. 1. INTRODUCTION Device physics has been a key factor in established semiconductor technologies, such as crystalline Si, Ge, and various AIIIBV. However, after more than two decades of extensive research, some features unique to thin-film PV have become apparent that are not readily understood in the existing framework. Such are, for example, variations between the characteristics of nominally identical devices, extremely high sensitivity to minute surface treatments, unusual degradation kinetics, and super-additive effects of independent factors. In general, the film small thickness and non-crystalline structure seem to be the key factors underlying the observed peculiarities of thin-film PV. This manifesto is aimed at presenting our view of physical mechanisms by which these factors determine thin film PV properties. The paper is organized as follows. In Sec. 2 we describe the generic structure and basic parameters of CdTe based solar cells. Sec. 3 gives a phenomenological view of nonuniformities applicable to subsequent topics. In Sec. 4 we consider the physics of the device main junction. Sec. 5 deals with the back contact, another junction that can be highly non-ohmic. Sec. 6 explains interaction between the main junction and back contact. Sec. 7, 8 briefly discuss the role of grain boundaries and recombination. Sec. 9 addresses the physics of degradation. Conclusions are given in Sec. 10. 2. CELL STRUCTURE AND BASIC PARAMETERS This section contains a compressed introductory compilation of the basic facts from several sources, of which we mention the reviews in [1]. The superstrate structure sketch in Fig. 1a can represent practical devices in a wide range of their constituting layer thickness: 0.1 – 0.3 µm for the conductive transparent oxide (TCO) and buffer layers, 0.08 – 0.3 µm for CdS, 1.5 – 7 µm for CdTe, all deposited on 1 – 3 mm thick glass [1]. CdCl2 treatment and Cu introduced through the back contact are necessary elements of many successful recipes. The corresponding band diagram (Fig. 1b) shows a one-dimensional view including three conceivable models for the CdS/CdTe conduction band offset. The component forbidden gaps are approximately 2.4 eV f
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