Imaging electron transport across grain boundaries in an integrated electron and atomic force microscopy platform: Appli
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Imaging electron transport across grain boundaries in an integrated electron and atomic force microscopy platform: Application to polycrystalline silicon solar cells M. J. Romero1, F. Liu1, O. Kunz2, J. Wong2, C.-S. Jiang1, M. M. Al-Jassim1, and A. G. Aberle2 1
National Renewable Energy Laboratory (NREL), 1617 Cole Boulevard Golden, CO 80401-3393 2 ARC Photovoltaics Center of Excellence, The University of New South Wales (UNSW) Sydney NSW 2052, Australia ABSTRACT We have investigated the local electron transport in polycrystalline silicon (pc-Si) thin-films by atomic force microscopy (AFM)-based measurements of the electron-beam-induced current (EBIC). EVA solar cells are produced at UNSW by EVAporation of a-Si and subsequent solidphase crystallization (SPC)–a potentially cost-effective approach to the production of pc-Si photovoltaics. A fundamental understanding of the electron transport in these pc-Si thin films is of prime importance to address the factors limiting the efficiency of EVA solar cells. EBIC measurements performed in combination with an AFM integrated inside an electron microscope can resolve the electron transport across individual grain boundaries. AFM-EBIC reveals that most grain boundaries present a high energy barrier to the transport of electrons for both p-type and n-type EVA thin-films. Furthermore, for p-type EVA pc-Si, in contrast with n-type, charged grain boundaries are seen. Recombination at grain boundaries seems to be the dominant factor limiting the efficiency of these pc-Si solar cells. INTRODUCTION Silicon is the leading semiconductor in terrestrial solar energy applications, and is expected to dominate the photovoltaic industry for at least another decade. Although both expansion of the silicon production and advances in solar-grade silicon (such as innovations in upgraded metallurgical silicon) will drive down the price of the feedstock, the added costs from fabricating wafers will continue at current levels. A wafer-replacement proposal such that provided by polycrystalline silicon (pc-Si) thin films grown on inexpensive substrates is therefore of great interest for the large-scale deployment of silicon-based photovoltaics at low cost. If solar-grade silicon is obtained and appropriate light-trapping strategies are implemented, pc-Si thin films (5– 40 µm in thickness) on foreign substrates (not to be confused with silicon thin wafers) can realistically reach 15% solar conversion efficiency. There are many different strategies for the fabrication of pc-Si thin films at temperatures compatible with borosilicate glass substrates (< 650 ºC) [1,2]. One approach towards improved solar cells makes use of an ultrathin seed layer as 'template' for the silicon epitaxy, with the purpose of producing high-quality epitaxial pc-Si. AIC (Aluminum-Induced Crystallization) is the most widely used seeding method [3]. As for the epitaxy, HWCVD (Hot-Wire Chemical Vapor Deposition)[4,5], IAD (Ion-Assisted Deposition) [6], and SPC (Solid-Phase Crystallization) [7,8] have all been explor
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