Applicability of electrical short circuit current decay for solar cell characterization: limits and comparison to other methods

Abstract

The electrical short-circuit current decay (ESCCD) method for determination of the bulk diffusion length L and the effective rear surface recombination velocity S is investigated with regard to application in solar cell development. To solve the major experimental problem, the fairly damped oscillations caused by parasitic capacitances and inductances, we introduced a calculation technique that fits the exponential decay plus three damped oscillations to the experimental curve. The measured effective diffusion lengths are compared to the results of an open-circuit voltage decay (OCVD) measurement and an evaluation of the spectral response of the solar cell, showing that only the ESCCD results are exclusively related to the base of the cell. The validity of the method is confirmed by the good agreement of the measured diffusion lengths and surface recombination velocities with those from two comparative measurements — determination of the bulk diffusion length performed on the unprocessed wafers using photoluminescence profile measurement and determination of L and S by evaluating the asymptotic decay time constants of the short-circuit current and the open-circuit voltage decay after optical excitation. Applying the method to electron-irradiated cells, decreasing diffusion lengths and increasing rear surface recombination velocities with increasing fluences could be observed. A discussion of the errors related to the ESCCD method shows that the accuracy in L and S depends critically on the knowledge of the device parameters.