Electronic Properties Of Nanocrystalline Silicon Deposited With Different Crystallite Fractions And Growth Rates
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Electronic Properties Of Nanocrystalline Silicon Deposited With Different Crystallite Fractions And Growth Rates P. G. Hugger1, J. David Cohen1, Baojie Yan2, Guozhen Yue2, Xixiang Xu2, Jeffrey Yang2, and Subhendu Guha2 1 Department of Physics, University of Oregon, Eugene, OR, 97405 2 United Solar Ovonic, LLC, Troy, MI, 48084 ABSTRACT Junction capacitance measurements were used to characterize the properties of nanocrystalline silicon (nc-Si:H) solar cells. These methods included drive-level capacitance profiling (DLCP) to obtain spatially-resolved defect densities, as well as transient photocapacitance (TPC) and transient photocurrent (TPI) spectra to reveal optically responsive states in the band-gap, and to estimate minority carrier behavior before and after prolonged light exposure (lightsoaking). Crystalline volume fractions were estimated using Raman spectroscopy. Previously we had identified at least two types of distinct behaviors in such nc-Si:H materials that correlated with the crystalline volume fraction. Here, in one case, we report results indicating that both types of behavior can occur in a single sample, possibly indicating that the structural properties of that sample have evolved during growth. INTRODUCTION Hydrogenated nanocrystalline silicon (nc-Si:H) has shown promise as a thin film photovoltaic material for it’s long wavelength optical response, high growth rates, and it’s high resilience to optical degradation. The mixed-phase nature of nc-Si:H accounts for these benefits but also contributes to the unique and complex electrical behavior of this material. In this study, junction capacitance measurements were used in conjunction with Raman spectroscopy to probe both the bulk electrical properties and the structural characteristics of n-i-p nc-Si:H solar cell devices. The samples consisted of several series of solar cells deposited on stainless steel substrates (SS/n+/i nc-Si:H/p+/ITO). Intrinsic layers were deposited using both RF and MVHF glow-discharge methods and were grown under conditions producing samples with different average crystalline volume fractions, thicknesses, and nanocrystallite distributions. We typically examined samples both in their annealed and light-soaked state. The latter was produced by exposing the samples to tungsten-halogen light through a 610nm long-pass filter at an intensity of 200mW/cm2 for 100 hours. EXPERIMENTAL METHODS Densities of states in the bandgap were investigated using drive-level capacitance profiling (DLCP) [1]. Similarly to C-V profiling methods, the DLCP measurement takes advantage of the fact that the depletion region width, i.e. the junction capacitance, is intimately related to the density of states in the bandgap. It can be shown [2] that under AC biasing conditions (C = C0+C1dV+C2dV2+…) the DLCP density can be written as an integral over the band-gap density of states:
3
N DLCP
E
F C0 =− = n + ∫ g ( E , x)dE 2qεA 2 C1 EC − Ee
(1)
Here Ee represents a thermal emission cutoff energy: Ee = kBT ln(ν/2πf), where T is the measu
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