Temperature Dependence of Dark Current-Voltage Characteristics of Hydrogenated Amorphous and Nanocrystalline Silicon Bas
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0910-A26-02
Temperature Dependence of Dark Current-Voltage Characteristics of Hydrogenated Amorphous and Nanocrystalline Silicon Based Solar Cells Baojie Yan, Jeffrey Yang, and Subhendu Guha United Solar Ovonic Corporation, 1100 West Maple Road, Troy, Michigan, 48084 ABSTRACT Systematic measurements of dark current density versus voltage (J-V) characteristics were carried out on a-Si:H, a-SiGe:H, and nc-Si:H solar cells at different temperatures from 23°C to 150°C. In a first order approximation, the dark J-V characteristics follow the standard diode characteristic formula for most high quality solar cells. The temperature dependence of the reverse saturation current can be used for deducing the intrinsic layer bandgap for the three types of solar cells. From a detailed analysis using the derivative of the measured dark J-V characteristics, we obtain different features in the plot of ideality factor versus bias voltage for the three types of solar cells. We also deduce the distribution of density of states using a recently proposed procedure. In general, a peak near the middle of the bandgap appears for the three types of solar cells; below the middle of the bandgap, there is an exponential-like broad distribution of gap states. However, the calculated gap state distribution shows a temperature dependence, which may be caused by the simplification of the calculation procedure. INTRODUCTION Measurement of dark current density versus voltage (J-V) characteristics is a useful characterization technique for hydrogenated amorphous silicon (a-Si:H), silicon germanium alloy (a-SiGe:H), and nanocrystalline silicon (nc-Si:H) solar cells because the measurement is conducted directly on solar cells, thus avoiding possible inconsistency between material properties obtained from specially designed samples and the intrinsic layer in solar cells. The simple classical formulation of dark J-V characteristics was deduced from crystalline semiconductor diodes. In general, the J-V curves follow the standard diode characteristic equation of [1] J (V ) = J 0 {exp(qV / nkT ) − 1} ,
(1)
where J0 is the reverse saturation current density, q unit charge, n diode ideality factor, k the Boltzmann constant, and T the measurement temperature. The parameters of J0 and n have clear correlations with material properties in crystalline semiconductors. J0 relates to the band gap Eg through the relationship of J 0 = J 00 (exp( E g / 2kT ) ) ,
(2)
where J00 is weakly dependent on T (~T2/3) and such dependence is ignored in most cases. The n value is determined by the transport mechanisms such that n=1 when the transport is dominated by diffusion and n=2 by recombination. In amorphous semiconductor diodes, the J-V characteristics may still follow the standard crystalline diode equation, but the ideality factor n is between 1 and 2 [2-6]. The value of n depends on the material properties and even changes with bias voltage [7-9]. The interpretations of the obtained parameters are still controversial. Although significant work was done during the
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