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).
