Optimization of p-type Nanocrystalline Silicon Thin Films for Solar Cells and Photodiodes
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1153-A06-02
Optimization of p-type Nanocrystalline Silicon Thin Films for Solar Cells and Photodiodes Y. Vygranenko1, E. Fathi2, A. Sazonov2, M. Vieira1, G. Heiler3, T. Tredwell3, A. Nathan4 1 Electronics Telecommunications and Computer Engineering, ISEL, Lisbon, 1950-062, Portugal 2 Electrical and Computer Engineering, University of Waterloo, Waterloo, N2L 3G1, Canada 3 Carestream Health Inc., Rochester, NY, 14652-3487, USA 4 London Centre for Nanotechnology, UCL, London, WC1H 0AH, United Kingdom
ABSTRACT We report on structural, electronic, and optical properties of boron-doped, hydrogenated nanocrystalline silicon (nc-Si:H) thin films deposited by plasma-enhanced chemical vapor deposition (PECVD) at a substrate temperature of 150°C. Film properties were studied as a function of trimethylboron-to-silane ratio and film thickness. The film thickness was varied in the range from 14 to 100 nm. The conductivity of 60 nm thick films reached a peak value of 0.07 S/cm at a doping ratio of 1%. As a result of amorphization of the film structure, which was indicated by Raman spectra measurements, any further increase in doping reduced conductivity. We also observed an abrupt increase in conductivity with increasing film thickness ascribed to a percolation cluster composed of silicon nanocrystallites. The absorption loss of 25% at a wavelength of 400 nm was measured for the films with optimized conductivity deposited on glass and glass/ZnO:Al substrates. A low-leakage, blue-enhanced p-i-n photodiode with an nc-Si p-layer was also fabricated and characterized.
INTRODUCTION Boron-doped hydrogenated nanocrystalline silicon (nc-Si:H) is an attractive material for large-area electronics and photovoltaic applications. It has advantages over hydrogenated amorphous silicon (a-Si:H) and silicon carbide with respect to higher conductivity and lower optical absorption in the visible range [1]. Thin p-type nc-Si:H films are used as the window layer in p-i-n solar cells with an a-Si:H or nc-Si:H intrinsic layer [2, 3]. To achieve a low absorption loss in the p-layer, the deposition conditions have to be optimized specifically for the thin (
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