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SPIN-DEPENDENT PROCESSES IN THIN-FILM SILICON SOLAR CELLS K. LIPS, R. MÜLLER, P. KANSCHAT, F. FINGER1, W. FUHS, Hahn-Meitner-Institut, Abt. Silizium Photovoltaik, Kekuléstr. 5, D-12489 Berlin, Germany Institut für Schicht und Ionentechnik, Forschungszentrum Jülich, D-52425 Jülich, Germany

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ABSTRACT We report on electrically detected magnetic resonance (EDMR) studies in three different types of thin-film silicon solar cells: (i) c-Si absorbers with epitaxially grown silicon thin-film emitters (ii) c-Si absorbers with hydrogenated amorphous silicon (a-Si:H) emitters, and (iii) microcrystalline silicon (µc-Si:H) pin diodes. Although cells of type (i) and (ii) are of a similar structure their EDMR spectra are completely different. We identify surface recombination via Pb0 centers in cells with c-Si emitters but hopping transport through conduction bandtail states in the 30 nm thin a-Si:H emitter layer. No signals related to interface recombination are detected in either cell. The EDMR signals of cell (iii) are identified as hopping among tail states of the conduction band with a subsequent nonradiative tunneling transition to neutral dangling bonds. At elevated temperature recombination is suggested to be dominated by direct capture. The EDMR signal of the dark current can be described with a simple diode model only involving diffusion and recombination currents. INTRODUCTION Electron spin resonance (ESR) is a unique tool for the identification of the microscopic nature of localized states in semiconductors. Such states are known to largely influence the electronic properties of devices like solar cells. Unfortunately, common device characterization methods like I-V or capacitance measurements only yield macroscopic information and their interpretation has to rely on assumptions of the fundamental transitions which involve such localized states. Moreover, spatial resolution of conventional characterization techniques is limited if very thin films are involved, i.e. distinctions between surface and bulk recombination in thin-film emitters is in many cases not possible. The above limitations can be partly overcome if one combines current measurements with ESR to a technique referred to as electrically detected magnetic resonance (EDMR). EDMR allows to identify the microscopic nature of transport and recombination paths. The sensitivity of EDMR is not limited by the absolute number of paramagnetic centers like in the case of ESR. In this report we will discuss in detail spin-dependent processes observed in a variety of different types of thin-film silicon solar cells prepared by PECVD and ECR-CVD. The cells under study are (i) thin-film emitter c-Si solar cells with a-Si:H and homoepitaxially grown emitters and (ii) microcrystalline silicon (µc-Si:H) pin diodes. BASICS OF SPIN-DEPENDENT TRANSPORT AND EXPERIMENTAL SETUP The first observation that a resonant microwave field induces changes in transport was reported by Honig and Maxwell for c-Si [1]. In 1970 Lepine and Prejan demonstrated that also A18.2.1

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