Structure optimization for a high efficiency CIGS solar cell

This paper uses numerical simulation to study the effects of Ga concentration profile on the performance of CuIn_1-xGa_xSe₂ (CIGS) solar cell, including the effects of acceptor type Cu antisite defects whose concentration depends on Ga composition. These defects are the dominant deep traps in the CIGS material system. The concentration and spatial distribution of these traps affect the solar cell performance. The trap density model used in this work follows experimental reports in the literature. The trap concentration is 4.3×10¹⁵ cm^-3 for CIS (x=0) and decreases to 1.2×10¹⁴ cm^-3 when the Ga mole fraction, x, reaches 0.24. The trap concentration increases exponentially above x=0.30. Applying this model to solar cells with uniform composition absorber layer predicts that the power conversion efficiency reaches a maximum value of 14.6%, at x=0.24 and decreases with increasing Ga content above x=0.30, in good agreement with experimental results. When this model is used to simulate a solar cell where the Ga composition in the absorber layer is graded, the electric field produced by compositional grading improves the efficiency because of the reduced recombination rate. However compositions where x is higher than 0.45 lead to a drop in performance due to the high trap density and shorter lifetime. Both grading from the CdS/CIGS interface (forward grading), and back grading where the Ga concentration increases from the junction into the CIGS film were studied. In forward grading, the maximum efficiency is achieved when the Ga concentration is graded such that x decreases from 0.35 at the surface to 0.24 at 0.4 μm into the CIGS film. In back grading, the maximum efficiency is achieved when x increases from 0.45 at the surface to 0.5 at 0.4 μm into the CIGS film.