A new method of determination of minority carrier diffusion length in the base region of silicon solar cells

摘要

A new method of determination of the minority carrier diffusion length (L) in the base region of an n/sup +/-p-p/sup +/ silicon solar cell using the spectral response of the cell in a middle wavelength (/spl lambda/) range, e.g., 0.75>/spl lambda/>0.90 /spl mu/m is presented. In this method Q/sub int/ or if required Q/sub int//f where Q/sub int/ is the internal quantum efficiency of the cell and f=exp(/spl minus/(x/sub b//spl alpha//sub /spl lambda//))(L/sup 2//spl alpha//sub /spl lambda///sup 2//L/sup 2//spl alpha//sub /spl lambda///sup 2//spl minus/1), x/sub b/ being the distance of base region from the front surface, is plotted against the reciprocal absorption coefficient (/spl alpha//sub /spl lambda///sup /spl minus/1/ of silicon. The Q/sub int/ versus /spl alpha//sub /spl lambda///sup /spl minus/1/ or else Q/sub int//f versus /spl alpha//sub /spl lambda///sup /spl minus/1/, plot gives an intercept L/sub MW/ on the /spl alpha//sub /spl lambda///sup /spl minus/1/-axis and a unit intercept on the other axis. The intercept length L/sub MW/ is related to L through d/L and S/sub B/, where d is the thickness of the base region and S/sub B/ is the back surface recombination velocity of minority carriers. For d/L<2.5, L=L/sub MW/ and is independent of S/sub B/. However, for d/L>2.5, the true value of L which may be somewhat different from L/sub MW/ can be determined if S/sub B/ is known. While most existing long wavelength spectral response (LWSR) methods require d/L to be large (d/L<2.5) is such that tanh(d/L)/spl ap/1, this method has no such restriction on d/L. It is highly suitable for cells for which L is large but x/sub b/ is small. We have applied the MWSR and LWSR methods to a few n/sup +/-p-p/sup +/ silicon solar cells and have found that the former is much superior to the latter if d/L>2.5.