Method to Determine the Recombination Characteristics of Minority Carriers in Graded-Band-Gap Solar Cells

Graded-band-gap solar cells have been shown to be a promising way of improving the power conversion efficiency of solar cells. However, due to the mixture of drift and diffusional motion of charge carriers induced by the band-gap grading in the solar cells, existing assessment techniques fail to properly characterize the electrical quality of the graded-band-gap solar cells. I develop a simple method to simultaneously characterize recombination characteristics including the minority carrier diffusion length L-d and the back-surface recombination velocity S-b through depth-resolved minority carrier collection length by diffusion in the graded-band-gap solar cells. The suggested method relies on wavelength-dependent lateral photocurrent in a simple custom-designed device structure, which enables to determine the recombination characteristics of the minority carrier, regardless of the band-gap grading. As a proof of concept, the suggested method is applied to a graded-band-gap Cu(In, Ga)Se-2 solar cell without even knowing the depth profile of the band-gap grading. The determined L-d and S-b values match well with previous reports of these values for Cu(In, Ga)Se-2 solar cells. This result suggests that this approach can be thus readily extended to other types of solar cells because this approach does not require the depth profile of the band-gap grading.