Modeling of Bragg reflectors to increase performance of high efficiency III-V multijunction solar cells

Abstract

High efficiency III-V solar cells are important for solar concentrator systems and space applications. To increase the efficiency of these cells, creative methods have been employed such as adding quantum wells in stress-balanced superlatticies to get closer to the ideal bandgaps for three junction cells. However, due to the strain of the quantum wells leading to limited possible growth thickness this addition of quantum wells is insufficient to completely absorb light in the regime they are designed to absorb. This work details modeling work done on Bragg reflectors in between the second and third subcells of a three junction device, behind the quantum wells in an attempt to increase efficiency in the range of the quantum wells. Modeling to increase the power to the second and third subcells using genetic algorithms was found to increase the overall power of these two subcells. This entails increasing the absorbance and therefore power of the second cell while keeping transmission high in the range of the final subcell. This is a challenge because in traditional Bragg reflectors the peak reflectance is proportional to the sidelobe reflectance that decreases power to the final subcell. Other optimization techniques such as optimizing for reflection profiles and current were also attempted but were found to be less useful due to imprecise or lesser abilities to account for the impact on the bottom subcell which turned out to be very significant. As such, it was found that the Bragg reflectors designed were only helpful when the open circuit voltage (Vop) of the final subcell was low or the absorbance of the quantum wells was high, when loss to the bottom subcell was either less significant.