GaSb on GaAs interfacial misfit solar cells

Abstract

Growth of GaSb with low threading dislocation density directly on GaAs may be possible with the strategic strain relaxation of interfacial mist arrays. This creates an opportunity for a multi-junction solar cell with access to a wide range of well-developed direct bandgap materials. Multi-junction cells with a single layer of GaSb/GaAs interfacial mist arrays could achieve higher eciency than state-of-the-art inverted metamorphic multi-junction cells while forgoing the need for costly compositionally graded buer 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 eciency of 5.5% and a 44-sun AM1.5d eciency of 8.9%. The GaSb-on-GaAs cell was 1.0% ecient 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 eciency gain over a tandem GaInP/GaAs cell at 1 sun and 2.5% gain at 44 suns, indicating that the eectiveness of the GaSb junction was a function of concentration.The state-of-the-art single-substrate multi-junction solar cell is the inverted metamorphic (IMM) cell, where lattice mismatched subcells are grown monolithically via compositionally graded buer layers. 13 Growth of a typical triple-junction (3-J) IMM starts with a GaAs (lattice