Study of Solar Cells by SEM Dark Voltage Contrast
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STUDY OF SOLAR CELLS BY SEM DARK VOLTAGE CONTRAST S.MIL'SHTEIN*, S.IATROU*,D.KHARAS**, R.O.BELL, AND D.SANDSTROM. *EE DEPT UMASS LOWELL, MA. **CHELMSFORD HIGH SCHOOL, MA. MOBIL SOLAR CO., BILLERICA, MA. ABSTRACT
For the first time the Scanning Electron Microscopy (SEM) Dark Voltage Contrast (DVC) technique was used to examine the field distribution in a solar cell made of polycrystalline silicon. The samples were cut off solar cells made of Edge-defined Film-fed Grown (EFG) Silicon and lapped at 2 and 5 degree angles. The DVC measurements were performed at forward, reverse, and zero bias conditions using a Hitachi S-570 SEM and a Kevex-8000 microanalyzer. The I-V curves were recorded to test the electric performance of the samples. We examined two groups of solar cells. One group of cells demonstrated, after regular processing, reasonable electrical performance and abrupt DVC profile. The other group was intentionally doped with titanium, which presumably was agglomerated in the p-n junction area thus degrading completely the electrical performance of the cells and weakening the electric field by a factor of 4-6. INTRODUCTION
The performance of semiconductor devices depends greatly on the profile of the electric field in a p-n junction, a major element of most diodes, transistors, etc. The field distribution (Ep-n ), i.e. graded or abrupt, depends on the distribution of impurities, and could be impacted by the presence of electrically active defects in the bulk, or surface part of a depleted area. Electrically active nonuniformities might cause leakage current in a p-n junction, intensify recombination processes in the depleted region, accelerate electrical breakdown process at small bias voltages, or lower the efficiency of operation. Widely used characterization methods such as I-V and C-V measurements [1-2] identify the inadequate performance of a p-n junction. However they do not provide information about performance failures on a microscale. Microscopic techniques such as Scanning Electron Microscopy (SEM) mode EBIC (Electron Beam Induced Conductivity) provide information [3] about electrically active defects around a p-n junction but do not show direct profiling of Ep-n. New SEM Mat. Res. Soc. Symp. Proc. Vol. 283. ©1993 Materials Research Society
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techniques using tunneling microscopes [4-6] provide high resolution images of the p-n junction, but do not provide quantitative information on Ep-n. The recently developed [7] SEM-Dark Voltage Contrast (DVC) methods provide the plot of Ep-n versus depth of a p-n junction. Deviation from linearity of the plot indicates the presence of electrically active nonuniformities, which can be simultaneously observed on the SEM screen. Recently [7] we established that the efficiency of a solar cell is proportional to the abruptness of Ep-n. In this paper, we report that electrical nonuniformities in the depleted region affect the efficiency of the device. EXPERIMENT
The Edge-defined Film-fed Grown (EFG) solar cells were lapped at 20-50 as shown in Fig. 1.
ELECTRON
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