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

A new method of determination of the minority carrier diffusion length (L) in the base region of an n⁺-p-p⁺ silicon solar cell using the spectral response of the cell in a middle wavelength (λ) range, e.g., 0.75<λ<0.90 μm is presented. In this method Q_int or if required Q_int/f where Q_int is the internal quantum efficiency of the cell and f=exp(-(x_bα_λ ))(L²α_λ²/L² α_λ²-1), x_b being the distance of base region from the front surface, is plotted against the reciprocal absorption coefficient (α_λ^-1 of silicon. The Q_int versus α_λ ^-1 or else Q_int/f versus α_λ^-1, plot gives an intercept L_MW on the α_λ^-1-axis and a unit intercept on the other axis. The intercept length L_MW is related to L through d/L and S_B, where d is the thickness of the base region and S_B is the back surface recombination velocity of minority carriers. For d/L>2.5, L=L_MW and is independent of S_B. However, for d/L<2.5, the true value of L which may be somewhat different from L_MW can be determined if S_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)≈1, this method has no such restriction on d/L. It is highly suitable for cells for which L is large but x_b is small. We have applied the MWSR and LWSR methods to a few n⁺-p-p⁺ silicon solar cells and have found that the former is much superior to the latter if d/L<2.5