Standard analysis

C. 2.1 Front illumination

The standard technique for the evaluation of internal quantum efficiency spectra [36] assumes a cell that is thick, compared to the optical absorption length La = l/as, and has a spatially homogeneous minority carrier diffusion length. Here, as denotes the absorp­tion coefficient of crystalline Si. The profile of the minority carrier generation rate g(Z) decays exponentially with the distance from the cell surface. When the optical absorp­tion in the emitter of the solar cell is neglected, the inverse internal quantum efficiency

IQE’= + LJLq (C.8)

depends linearly on the optical absorption length. Hence an effective diffusion length Lq may be determined as the slope of the inverse internal quantum efficiency IQE~ when plotted versus the optical absorption length La.

Experimental example

Figure C.5 shows an example of the standard IQE evaluation technique. We measure the internal quantum efficiency of a monocrystalline Si solar cell with a base doping of 5xl018 cm-3. The P-doped emitter is 0.5 pm thick. The cell thickness is Wf= 525 pm. The effective diffusion length is only LQ = 6.4 pm, due to the high doping. The minority carriers do not reach the back surface of the cell, and consequently the effective diffu­sion length equals the diffusion length L in the base. At absorption lengths La that are larger than the film thickness Wfi the optical reflection at the back surface enhances the quantum efficiency IQE, and the measured IQE~X data deviate from the linear relation expressed by Eq. (C.8).

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