Improvement in Diffusion Length Estimation using the Photocurrent-Capacitance Method in CuInSe 2 -based Cells
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Improvement in Diffusion Length Estimation using the Photocurrent-Capacitance Method in CuInSe2-based Cells Clifford H. Champness Electrical and Computer Engineering Department, McGill University, 3480 University Street, Montreal, Quebec, Canada, H3A 2A7. ABSTRACT The experimental technique in the photocurrent-capacitance method of estimating minority diffusion lengths, as applied to CuInSe2-based photovoltaic cells, has been improved, principally by replacing the earlier chopped light and monochromator technique by one employing a steady strong stable light source and a long pass optical filter. The filter requires an absorption edge at an appropriate wavelength to provide the penetrating light at sufficient intensity, which has been found WREH PIRU&X,Q6H2FHOOVDQG PIRU&X ,Q*D 6H2 cells . In addition, the parallel capacitance is determined with reverse bias under the same illumination as that during the photocurrent measurements and at a suitable frequency, such as 10 kHz. With these changes, the technique has been found to be more tolerant to cell imperfections and reproducible estimates of diffusion length have been obtained on CuInSe2 and Cu(In,Ga)Se2 cells.
INTRODUCTION For some time, a simple nondestructive method of estimating the minority diffusion length in a photovoltaic cell has been the photocurrent-capacitance method, which has been used with a variety of measuring techniques [1-5]. The method has been applied to CuInSe2 cells in the author s laboratory using a chopped light technique, which has worked well in freshly fabricated devices [6,7]. However, this technique was found to be less successful when applied to cells with detectable internal junction current shunting and series resistance, such as often occurs in many real devices, especially those that have aged with shelf life. Accordingly, the measuring technique was changed to make the method more tolerant to device imperfections and the present paper describes the changes made for studying CIS and CIGS cells. Briefly, the basic principle of the method is as follows. If a perfect n+p junction (i.e. one with zero internal shunt current and zero series resistance) is alternately illuminated with long wavelength SHQHWUDWLQJ OLJKWIRUZKLFK
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against 1/Cp , extrapolating back from the linear region to the 1/Cp abscissa, intersecting it at a negative value of -1/Ci , the diffusion length is g
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