Role of Interface Quality and Film Doping Density in Amorphous Si / Crystalline Si Heterojunctions for Photovoltaic Appl
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Role of Interface Quality and Film Doping Density in Amorphous Si / Crystalline Si Heterojunctions for Photovoltaic Applications M. Farrokh Baroughi and S. Sivoththaman Department of Electrical & Computer Engineering University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada ABSTRACT We have studied some key aspects of the (n+)a-Si/(p)c-Si heterojunction structures photovoltaic applications. The importance of the hetero-interface quality has been studied by both theoretical simulations and experimental characterization. The surface treatment prior to the film deposition is very critical. Even though H-plasma passivation can help passivate bulk defects in low quality Si, extreme care should be taken to avoid plasma induced surface damage which will result in a defective hetero-interface. Furthermore, (n+) a-Si films with different doping levels have also been characterized and their influence on the device performance has been studied. INTRODUCTION The hydrogenated amorphous Si/crystalline Si (a-Si:H/c-Si) heterojunction has high potential for cost-effective photovoltaic (PV) applications. The low temperature(T) nature of the process provides an alternative for diffused pn junction solar cells on crystalline Si. The high potential of this technology was recently presented by a (p+)a-Si/(i)a-Si/(n)c-Si/(i)a-Si/(n+)a-Si structure called HIT cell (heterojunction with intrinsic thin layer), resulting in a high efficiency of 20.7% [1]. Different aspects of the cell structure has been reported by several research groups [2-6], employing n+ (or p+) a-Si emitter and a p-type (or n-type) crystalline wafer with or without a very thin i-a-Si:H as crystalline wafer surface passivation layer. The main advantage of this process is its low-T nature, allowing the use of low-cost (and hence somewhat defective) crystalline Si, which usually degrade after several high-T excursions. The low-T process also allows to implementing hydrogen defect passivation schemes prior to device fabrication. The a-Si film properties and the interfacial quality critically affect the heterojunction properties. In this paper we study, both by theoretical simulations and by experiments, some important aspects of the hetero-interface and the effects of doping in the (n+)a-Si films. Presence of interfacial defects & intrinsic a-Si layer (i-layer) and electrical properties of (n+)a-Si film are analyzed. AMPS (Device Simulator for Microelectronic and Photonic Structures, Penn. State. University) program has been used for simulations. Test structures are fabricated and analyzed by spectral response, I-V, C-V, and I-V-T measurements. ASPECTS OF THE QUALITY OF THE HETERO-INTERFACE The quality of the hetero-interface is very critical for PV devices because it is responsible for the device high open circuit voltage. An important part of the interface quality is related to the treatment of the wafer surface prior to the a-Si film deposition. Improper wafer cleaning processes and plasma induced damage can degrade the interface quality. An intrinsic layer of a
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