Micro-Raman Studies of Mixed-phase Hydrogenated Silicon Solar Cells
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Micro-Raman Studies of Mixed-phase Hydrogenated Silicon Solar Cells Jessica M. Owens1, Daxing Han1, Baojie Yan2, Jeffrey Yang2, Kenneth Lord2, and Subhendu Guha2 1 Department of Physics & Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, U.S.A. 2 United Solar Systems Corp., 1100 W. Maple Road, Troy, MI 48084, U.S.A. ABSTRACT The open-circuit voltage (Voc) of mixed-phase hydrogenated silicon solar cells has been found to increase after light soaking. In this study, we use micro-Raman to investigate the heterogeneous structure of solar cells in the amorphous-to-nanocrystalline transition region. For a cell with Voc = 0.981 V, Raman spectra show a typical broad Gaussian lineshape around 480 cm-1, a signature of typical amorphous material. A cell with Voc = 0.674 V displays a sharp Lorentzian peak around 516 cm-1, indicative of nanocrystallinity. A cell with Voc = 0.767 V was systematically scanned for 20 different positions in 500 µm increments. Most spectra show a typical Gaussian lineshape around 480 cm-1, several spectra reveal a hint of a nanocrystalline shoulder around 512 cm-1, and one spectrum exhibits a distinct nanocrystalline peak. We conclude that the nanocrystallite distribution in the mixed-phase material is very non-uniform even within a mm dot. This result provides direct evidence supporting a recently proposed twodiode equivalent-circuit model to explain the light-induced effect. INTRODUCTION We recently observed a significant light-induced enhancement in the open-circuit voltage (Voc) of mixed-phase silicon solar cells in the amorphous-to-nanocrystalline transition region [1, 2]. Since the average grain size is in the order of a few nanometers in the mixed-phase material, we use the term nanocrystalline instead of microcrystalline silicon. The magnitude of the variation in Voc (∆Voc) depends on the intrinsic (i) layer thickness, the deposition temperature, the initial Voc, and the light-soaking intensity. Under intense light soaking, a ∆Voc as large as 150 mV has been observed. Subsequent annealing of the solar cells substantially restored the original Voc values. In-situ photoluminescence (PL) spectral studies found that the PL intensity and peak-energy position associated with the amorphous component of the mixed-phase material increases upon light soaking. We proposed that a reduction of nanocrystalline volume fraction (Xc) or size is responsible for the observed Voc enhancement [1, 2]. However, recent studies by conventional Raman and X-ray diffraction spectroscopy found no observable change before and after light soaking, prompting us to carry out further investigations. Raman spectroscopy is a sensitive tool that provides valuable structural information about hydrogenated amorphous silicon (a-Si:H) materials. In a-Si:H, all phonon modes of the transverse acoustic (TA), longitudinal acoustic (LA), longitudinal optical (LO), and transverse optical (TO) modes are Raman active. Thus, Raman spectra from a-Si:H at room temperature yield a reasonable spectral compari
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