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A5.7.1

Interpretation of Transient Photocurrents in Coplanar and Sandwich PIN Microcrystalline Silicon Structures Steve Reynolds, Vladimir Smirnov, Charlie Main, Friedhelm Finger1 and Reinhard Carius1, School of Computing and Advanced Technologies, University of Abertay Dundee, Bell Street, Dundee U.K. 1 Forschungszentrum Jülich, Institute for Photovoltaics, D-52425 Jülich, Germany.

ABSTRACT We report on the use of coplanar transient photoconductivity and post-transit time-of-flight spectroscopy techniques in the study of carrier transport in microcrystalline silicon films prepared over a range of crystallinities. Coplanar samples are susceptible to post-deposition oxidation and reversible adsorption of atmospheric gases, which may alter the apparent density of states. Coplanar measurements suggest lower deep defect densities in more highly crystalline films, but this is due at least in part to an increased occupancy of these states. A comparison of results obtained using both techniques suggests anisotropic transport, with reduced band tailing (greater structural order) along the direction of film growth, a larger defect concentration around the column boundaries, and a higher defect density within the amorphous tissue than in optimised single-component amorphous silicon films.

INTRODUCTION The promise of continued improvement in stable photovoltaic conversion efficiencies provides a major stimulus for research on the optoelectronic properties of microcrystalline silicon (µc-Si:H) films [1]. However, although carrier transport in this material has been studied intensively for a number of years, there is still no comprehensive electronic model. Films are frequently mixed-phase in character, containing columns or clusters of crystalline grains, disordered regions, voids, and hydrogen [2]. Measurements suggest the presence of band tails and defects, as in hydrogenated amorphous silicon (a-Si:H), but additional complexities are introduced by grain and column boundaries, band offsets and structural anisotropy. It has been proposed that dangling bonds are located in the disordered phase around crystalline columns, while boundaries between the smaller grains within columns or larger grains cause deviations in bond lengths and angles and thus give rise to tail states [3]. It is also known that films, particularly those with porous structure, may be affected strongly by exposure to various gases, including air [4,5]. Here, we consider the prospects for application of transient photocurrent spectroscopies in the study of transport, and specifically the density of localised states (DOS), in microcrystalline silicon. Two procedures have been used in this work – transient photoconductivity (TPC), on films equipped with coplanar ohmic contacts, and post-transit time-of-flight spectroscopy (PTTOF), on p-i-n photovoltaic structures. The decay in photocurrent is measured over several orders of magnitude of time following the application of a short laser flash. If trap-limited band transport predominates, the decays may be analys