Solid-phase crystallization of evaporated silicon thin films on glass for photovoltaics: A combined SEM and TEM study Fude Liu,* M.J. Romero, K.M. Jones, and M.M. Al-Jassim National Center for Photovoltaics, National Renewable Energy Laboratory, Golden, CO 80401, USA O. Kunz, J. Wong, and A.G. Aberle $ ARC Photovoltaics Centre of Excellence, The University of New South Wales, Sydney NSW 2052, Australia $ Now with: Solar Energy Research Institute of Singapore, National University of Singapore, Singapore 117576 * E-mail address: fude.liu@nrel.gov; Tel.: 001 303 384 7677; fax: 001 303 384 6490. ABSTRACT The material-quality limiting factors of evaporated solid-phase crystallized (SPC) poly-Si thin films fabricated on planar glass for photovoltaic applications are investigated by a study combining scanning electron microscopy and transmission electron microscopy. The grains in the investigated thin films are found to be randomly oriented, with an average grain size of ~2.1 μm. In general, the grains are found to have a high defect density, although some grains are more defective than others. We also observe a high level of impurity incorporation, in particular, oxygen, into the film. The optical activity of the Si films is dominated by deep band tail states. We conclude that the high intragrain defect densities and the high impurity levels are two major limiting factors for obtaining high-quality evaporated SPC poly-Si thin films for photovoltaics. Keywords: Photovoltaics; Thin-film solar cells; Polycrystalline silicon; Glass; Solid-phase crystallization; Intragrain defects; Impurities; Evaporation 1. INTRODUCTION Crystalline silicon thin-film solar cells can potentially get around the cost barrier of solar energy by using much less material than conventional bulk modules. In addition, unlike non-silicon thin-film photovoltaic (PV) technologies, they do not suffer from limited supply of basic materials or toxicity of the components [1]. Efficient solar cells can still be achieved even with a very thin Si layer (~2 μm), if proper light trapping is applied [2]. Compared to hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) films, polycrystalline silicon (poly-Si) (grain size: 1 μm to 1 mm) thin films have better transport properties and are not subject to light-induced degradation effects [3,4]. Poly-Si thin-film solar cells are an emerging technology that aims to combine the advantages of crystalline Si and a thin- film approach [3]. The films are usually formed on inexpensive substrates (e.g., glass, metal-alloy tapes, and ceramics) at a temperature ranging from 400 to 1200 ºC. Although the development of poly-Si thin films is less mature than a-Si:H and μc-Si:H, rapid progress has been made and some promising results have been shown in the last several years. In particular, solid-phase crystallization (SPC) of a-Si has been the most successful and widely studied poly-Si film formation technique without using a seed layer [5−7]. EVA solar cells on glass—where EVA stands for “solid-phase crystallization of EVAporated Si”—have been developed at the University of New South Wales (UNSW) as one of the promising approaches for lowering the cost of photovoltaic (PV) solar electricity [8]. The cell structure is usually glass/SiN/p + /n/n + (or glass/SiN/n + /p/p + )The cells are made as follows: The a-Si is e-beam evaporated and in situ doped at low substrate temperature in a non-ultra-high vacuum (non-UHV) environment, and then crystallized ex situ by atmospheric-pressure SPC in a tube furnace (N 2 Thin Film Solar Technology, edited by Alan E. Delahoy, Louay A. Eldada, Proc. of SPIE Vol. 7409, 740906 · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.823622 Proc. of SPIE Vol. 7409 740906-1