Modeling of defect-tolerant thin multi-junction solar cells for space application

Abstract

Using drift-diffusion model and considering experimental III-V material parameters, AM0 efficiencies of multijunction solar cells have been calculated and the effects of dislocations and radiation damage have been analyzed. It is shown that ultrathin multi-junction devices perform better in presence of dislocations or/and radiation harsh environment compared to conventional thick counterpart. Under high radiation doses the optimization of the device design for Ga 0.51 In 0.49 P/GaAs multijunction devices leads to a significant improvement of end of life (EOL) efficiencies (i.e. under a dose of 10 16 cm -2 1MeV electron fluence the device efficiency can be improved 2.5 fold by fine tuning the design . In addition these calculations indicate that for radiation doses in excess of an equivalent 1x10 15 cm -2 1 MeV electron fluence (~10 years in Geostationary orbit) the optimized design makes these design somewhat insensitive to the presence of large of other minority carrier traps such as those introduced by crystalline defects (i.e. dislocations). The results suggest that by optimizing the device design, one can obtain nearly the same EOL efficiencies for highly dislocated metamorphic/heteroepitaxial solar cells than those of an ideal defect free device. These findings could be of importance to the manufacture of current metamorphic space cells as to some extent they indicate possibilities for circumventing the need for sophisticated defect filtering techniques. 1-INTRODUCTIONSince the launch of Vanguard, the first man-made solar powered satellite, the quest for solar cells with higher efficiencies and improved radiation tolerance has been of paramount importance to space power system designers. Within this context solar cells based on III-V materials, mere laboratory curiosities of early nineties, have established themselves as the prime candidates for space orbital operations. . III-V lattice-matched multijunction devices based on GaInP-GaAs-Ge have shown efficiencies of in excess of 30% AM0 [1]. In order to achieve higher efficiencies in multijunction solar cells, flexibility in choice of band gaps is required which is not constraint by lattice matching condition. This flexibility in choice of band gap is used in metamorphic solar cells. Metamorphic solar cells have demonstrated efficiencies which are higher than conventional latticed matched multi junction solar cells. But the major drawback in using metamorphic devices is the presence of crystalline defects which are in the range of 5x10 6 to 10 8 cm -2 [2,3] arising due to lattice mismatch between sub cells. These defects cause reduction in minority carrier lifetime and result in efficiency degradation, hence thick buffer layer are used in order reduce these defects. Space radiation also causes reduction in diffusion length and lifetimes. Hence solar cells in space slowly degrade due to presence of this radiation. It is been shown that optimization of single junction GaAs solar cell would give better end of life performance (EOL) than conventional solar cells ...