Measurement of the Electric Potential on Amorphous Silicon and Amorphous Silicon Germanium Alloy Thin-Film Solar Cells b
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A9.42.1
Measurement of the Electric Potential on Amorphous Silicon and Amorphous Silicon Germanium Alloy Thin-Film Solar Cells by Scanning Kelvin Probe Microscopy C.-S. Jiang, H. R. Moutinho, Q. Wang, M. M. Al-Jassim National Renewable Energy Laboratory, Golden, CO 80401 B. Yan, J. Yang, and S. Guha United Solar Ovonic Corporation, Troy, MI 48084 ABSTRACT We report on direct measurements of surface potentials on cross sections of a-Si:H and aSiGe:H n-i-p solar cells using scanning Kelvin probe microscopy. External bias voltage (Vb) induced changes in the electric field distributions in the i layer were further deduced by taking the derivative of the Vb-induced potential changes. This procedure avoids the effect of surface charges or surface Fermi-level pinning on the potential measurement. We found that the electric field does not distribute uniformly through the i layer of a-Si:H cells, but it is stronger in the regions near the n and p layers than in the middle of the i layer. The non-uniformity is reduced by incorporating buffer layers at the n/i and i/p interfaces in the a-Si:H solar cells. For a-SiGe:H solar cells, the electric field at the p side of the i layer is much stronger than at the n side and the middle. The non-uniformity becomes more severe when a profiled Ge content is incorporated with a high Ge content on the p side. We speculate that the increase in defect density with increasing of Ge content causes charge accumulation at the i/p interface. INTRODUCTION Photovoltaic performance of hydrogenated amorphous silicon (a-Si:H) and amorphous silicon germanium alloy (a-SiGe:H) solar cells has been greatly improved in recent years by optimizing the device structure and fabrication processes [1]. In many cases, the improvements are identified through the conventional characterizations of apparent device properties of opencircuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and conversion efficiency. However, the device physics of a-Si:H and a-SiGe:H remains unclear due to the complexity of material properties and device structure. For example, the spatial distribution of the electric potential across the n-i-p structures is a very important parameter for device design but is unknown in most situations. It is well known that the electric potential plays a major role in photovoltaic devices because it is responsible for the collection of photoexcited carriers. In contrast to intensive theoretical modelings on the potential distribution [2,3], however, experimental characterizations have been limited to indirect measurements such as current density versus voltage (J-V), electroabsorption, and capacitance versus voltage (C-V) measurements. None of these techniques measure the spatial distribution of the potential. In the last decade, nano-electrical property measurements based on atomic force microscopy (AFM) technique, such as scanning Kelvin probe microscopy (SKPM) and scanning capacitance microscopy, have been developed and applied to the characterization of semiconductor devices.
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