Junction Capacitance Study of a-SiGe:H Solar Cells Grown at Varying RF and VHF Deposition Rates
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Junction Capacitance Study of a-SiGe:H Solar Cells Grown at Varying RF and VHF Deposition Rates Peter G. Hugger1, Jinwoo Lee1, J. David Cohen1, Guozhen Yue2, Xixiang Xu2, Baojie Yan2, Jeff Yang2 and Subhendu Guha2 1 Department of Physics, University of Oregon, Eugene, OR 97403, U.S.A. 2 United Solar Ovonic LLC, 1100 West Maple Road, Troy, MI, 48084, U.S.A.
ABSTRACT Significant advances have been made in increasing the deposition rate of hydrogenated silicon germanium alloys (a-SiGe:H) using a modified VHF glow discharge deposition method while also maintaining good electronic properties important for its application in photovoltaic devices. We examine the electrical and optical properties of these alloys deposited either by RF (13.56MHz) or the modified VHF methods over deposition rates varying from 1 to 10 Å/s. The electronic properties of a series of 1.4 eV optical gap a-SiGe:H i-layers, in many cases in solar cell device configurations, were characterized. Drive-level capacitance profiling was used to determine the deep defect densities, and transient photocapacitance measurements allowed us to determine the Urbach energies. Results were obtained for both the annealed and light-soaked degraded states and these results were correlated to the cell performance parameters. In general the a-SiGe:H layers deposited using the modified VHF excitation exhibited improved electronic properties at higher growth rates than the RF deposited samples. INTRODUCTION Hydrogenated amorphous silicon germanium alloys (a-SiGe:H) are technologically important for their incorporation as the low and mid-gap optical components in amorphous silicon (a-Si:H) based multijunction solar cells. Typically, RF (13.56MHz) glow discharge has been the predominant method of depositing these materials, albeit with the drawback that deposition rates have been limited to ≤ 3 Å/s. Alloys grown using RF methods at deposition rates higher than this have generally exhibited a significantly higher degree of light-induced degradation and lower stable efficiencies [1,2]. Depositing a-Si:H itself at rates higher than 3 Å/s while maintaining good electronic properties has been achieved at United Solar through its development of a “modified VHF” glow discharge deposition technique (hereafter also referred to as “VHF”), This technique utilizes significantly higher frequencies than for the RF glow discharge process and incorporates some further proprietary alterations to the standard VHF deposition method. Recently this method has been applied to deposition of a-SiGe:H films with some success of achieving better electronic properties at higher deposition rates [3]. In order to better understand the increased stability of a-SiGe:H films deposited using the modified VHF method, we have studied a series of low-gap (ETauc ≈ 1.4 eV) a-SiGe:H devices grown using both RF and VHF techniques. Here we report the results of junction capacitance measurements such as drive-level capacitance profiling (DLCP) and transient photocapacitance and photocurrent (TPC and T
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