Devices Fabrication with Narrow-Bandgap a-SiGe:H Alloys Deposited by HWCVD
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Devices Fabrication with Narrow-Bandgap a-SiGe:H Alloys Deposited by HWCVD Yueqin Xu, Baojie Yan,* Brent P. Nelson, Eugene Iwaniczko, Robert C. Reedy, A.H. Mahan, and Howard Branz National Renewable Energy Laboratory 1617 Cole Blvd., Golden, CO 80401 *United Solar Ovonic Corporation 1100 W. Maple Road, Troy, MI 48084 ABSTRACT We incorporate narrow-gap amorphous silicon germanium (a-SiGe:H) alloys grown by hot-wire chemical vapor deposition (HWCVD) into single-junction n-i-p solar cells, and improve both fill factor (FF) and open-circuit voltage (Voc) by bandgap grading. The Tauc bandgap (ET) of the a-SiGe:H is as low as about 1.25 eV. Previously [1], we obtained a short-circuit current density (Jsc) up to 20 mA/cm2 in an n-i-p device incorporating an ungraded 120-nm i-layer of 1.25-eV a-SiGe:H. However, without buffer layers or bandgap profiling, the fill factor was only 38%, likely due to an abrupt bandgap transition and poor hole collection. To overcome these problems, we have used composition bandgap profiling throughout the i-layer and improved both Voc and FF significantly without any Jsc loss. The solar cell efficiency is improved from 3.55% to 5.85% and Voc rises from 0.475 to 0.550 eV. This improved single-junction a-SiGe:H solar cell has a quantum efficiency of about 48% at λ=800 nm and about 15% at λ=900 nm. We present details of the bandgap profiling and its effect on device performance. INTRODUCTION It is now recognized that multijunction, multigap amorphous-silicon-based solar cells can have higher efficiency with better stability than single-junction solar cells. In 1988, Guha et al. [2, 3] reported a multijunction solar cell with an efficiency of 13.7%, which incorporated high quality a-SiGe:H (ET =1.50 eV) in the bottom cell. By further increasing the Ge content in the bottom cell up to ~40%, for a bandgap of about 1.40 eV, they were able to enhance the longwavelength response. By incorporating optimized bandgap profiling, they achieved an improved efficiency of 14.6% [4]. This is highest value reported up to date for an a-Si-based solar cell [5]. These results showed that one key to achieving a high-efficiency multijunction solar cell is the development of high quality, low band-gap material to capture low-energy photons in the longwavelength (λ> 800 nm) region of the solar spectrum. Last year, we reported that the narrow gap (1.25 eV < Et < 1.30 eV) a-SiGe:H alloys grown by HWCVD could be improved by adjusting the filament temperature, filament diameter, and substrate temperature [6]. By lowering the tungsten (W) filament temperature from more than 2000°C to less than 1850°C, using a thinner W filament (0.38-mm diameter), and lowering the substrate temperature from 350°C to 250°C, we obtained high quality a-SiGe:H alloys at a deposition rate of 0.3 to 0.4 nm/sec. These 1.25 to 1.30 eV bandgap materials have a germanium content 60 to 65 at. %, are compact (low microvoid concentration) and have low compositional heterogeneity as detected by small angle x-ray scattering. FTIR measurements als
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