Growth of GaSb with low threading dislocation density directly on GaAs may be possible with the strategic strain relaxation of interfacial misfit arrays. This creates an opportunity for a multijunction solar cell with access to a wide range of well-developed direct bandgap materials. Multijunction cells with a single layer of GaSb/GaAs interfacial misfit arrays could achieve higher efficiency than state-of-the-art inverted metamorphic multi-junction cells while forgoing the need for costly compositionally graded buffer layers. To develop this technology, GaSb single junction cells were grown via molecular beam epitaxy on both GaSb and GaAs substrates to compare homoepitaxial and heteroepitaxial GaSb device results. The GaSb-on-GaSb cell had an AM1.5g efficiency of 5.5% and a 44-sun AM1.5d efficiency of 8.9%. The GaSb-on-GaAs cell was 1.0% efficient under AM1.5g and 4.5% at 44 suns. The lower performance of the heteroepitaxial cell was due to low minority carrier Shockley-Read-Hall lifetimes and bulk shunting caused by defects related to the mismatched growth. A physics-based device simulator was used to create an inverted triple-junction GaInP/GaAs/GaSb model. The model predicted that, with current GaSb-on-GaAs material quality, the not-current-matched, proof-of-concept cell would provide 0.5% absolute efficiency gain over a tandem GaInP/GaAs cell at 1 sun and 2.5% gain at 44 suns, indicating that the effectiveness of the GaSb junction was a function of concentration.
This study presents a new design for a single-junction InAlAs solar cell, which reduces parasitic absorption losses from the low band-gap contact layer while maintaining a functional window layer by integrating a selective etch stop. The etch stop is then removed prior to depositing an anti-reflective coating. The final cell had a 17.9% efficiency under 1-sun AM1.5 with an anti-reflective coating. Minority carrier diffusion lengths were extracted from external quantum efficiency data using physics-based device simulation software yielding 170 nm in the n-type emitter and 4.6 μm in the p-type base, which is more than four times the diffusion length previously reported for a p-type InAlAs base. This report represents significant progress towards a high-performance InAlAs top cell for a triple-junction design lattice-matched to InP.
Ultrathin III-V solar cells with proper light management have become more attractive than their optically thick counterparts as they are less expensive and lightweight, can maintain photon absorption, and have high radiation tolerance for space-related applications. Comprehensive optical modeling efforts have provided pathways to improve device efficiency in ultrathin GaAs solar cells with light trapping structures. Usually, the absorption mechanism known as free-carrier absorption (FCA) is ignored in these models due to the ultrathin layers and the direct bandgap of GaAs. This manuscript reports the significance of considering FCA as a parasitic loss caused by the optical enhancement in highly doped non-active layers between the ultrathin solar cell and backside light trapping structures. We model FCA based on Drude theory in a p-type AlGaAs layer behind ultrathin GaAs solar cells with a planar mirror and cylindrical gratings. Our results show that, depending on the AlGaAs thickness and doping concentration, free carriers will absorb transmitted photons and reduce the backside reflectance, degrading the current and voltage output from ideal conditions. One example shows that for a 300 nm-thick GaAs solar cell, the Ag mirror's peak reflectance decreases nearly 12% when the AlGaAs back layer is 800 nm-thick at a doping concentration of 4x1019 cm−3. Notably, the cylindrical grating designs with 38.5%, 46.5%, and 64.9% AlGaAs coverage resulted in an absolute efficiency reduction of 0.6%, 1.8%, and 2.9% at a doping concentration of 4x1019 cm−3, respectively. This novel study demonstrates that FCA in non-active layers must be properly addressed in the device design to progress the efficiency of ultrathin III-V solar cells with light trapping structures.
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