Quantitative Assessment of Optical Gain and Loss in Submicron-Textured CuIn1-xGaxSe2 Solar Cells Fabricated by Three-Stage Coevaporation

The external quantum efficiency (EQE) spectra of high-efficiency CuInGaSe2 (CIGS)-based solar cells fabricated by a standard three-stage process are simulated by incorporating the effects of (i) the V-shaped Ga-compositional profile within a 1.8-mu m-thick CIGS layer and (ii) the light scattering caused by the submicron-textured structure. For the EQE calculation of the CIGS solar cells, we develop a simulation scheme in which the total light absorption in the solar cell is assessed using the experimental reflectance spectrum, whereas the absorptance of each solar-cell layer is deduced by assuming a flat optical model. The optical effect of the double-graded Ga-compositional profile in the CIGS layers is calculated explicitly based on a complete CIGS optical database established recently. A highly accurate EQE simulation for CIGS solar cells is made possible by the developed simple calculation method. In particular, our technique allows the determination of the partial EQE contributions for different thicknesses and wavelengths in the CIGS-based solar cells. The EQE analysis reveals that the carrier collection efficiency in the CIGS light-absorber layers is almost unity, but the light absorption in the 1-mu m-thick CIGS bottom region is negligible, confirming that the bottom layer with higher Ga content plays a dominant role as a back-surface field with the conduction-band grading. We find that the major optical loss occurs in the ZnO:Al, CdS, and Mo component layers with a corresponding current loss of approximately 3 mA/cm(2) in each layer. Furthermore, an EQE simulation method for arbitrary CIGS solar-cell structures is developed by imposing the antireflection condition in the calculation of the reflectance spectra. By applying this technique, the effects of the Ga-compositional profiles and thicknesses of various solar-cell component layers on the EQE spectrum are determined.