Light management for improved photon absorption in thin-film radiation-tolerant multijunction space photovoltaics

Maintaining photon absorption in optically thin III-V multijunction space photovoltaics can be accomplished by integrating back surface reflectors (BSR) to increase the photon path length inside the thin solar cell and enhance the photogenerated current. This research investigates the integration of textured BSRs with thin-film 1-eV InGaAs solar cells to improve radiation tolerance and maintain device performance of thin-film inverted metamorphic (IMM) solar cells. The developed textures include surface treatments using reactive-ion etching (RIE) and in situ processing during the epitaxial growth of the solar cells. The textured layers achieve higher surface roughness than the pre-textured surface, indicating angular photon scattering. The InGaAs solar cells with textured BSRs show an increase in the short-circuit current density compared to the flat BSR with no degradation in the open-circuit voltage. The planar and RIE BSR solar cells result in lifetime enhancement factors of 2.4 and 3.6, respectively, indicating an increased photon path length due to the textured reflector. Improving the photon path length in the ultrathin InGaAs solar cells can be accomplished by using a low-index total internal reflection layer between the textured semiconductor and mirror. The results and discussion provided in this work support the integration of 600 nm-thick InGaAs subcells into a standard IMM design to achieve highly efficient and radiation-hardened space photovoltaic devices.