Investigation and Mitigation of Shunts for Higher Efficiency Epitaxial GaSb/GaSb and GaSb/GaAs Solar Cells

Multi-junction cells using GaAs/GaSb interface misfit arrays for metamorphic growth can achieve comparable efficiency to conventional inverted metamorphic multijunction cells without the need for costly graded buffer layers. In this work, GaSb single junction cells were grown via molecular beam epitaxy on both GaSb and GaAs substrates to compare homoepitaxial and heteroepitaxial GaSb material quality. The homoepitaxial cell achieved 4.1% efficiency under 1 sun AM1.5g while the IMF cell achieved 0.70%. The IMF cell had higher non-radiative dark current likely caused by defects related to the mismatched growth. Lock-in thermography was used to explore shunt causing defects and sidewall shunting was passivated by dielectric deposition on device sidewalls. Homoepitaxial cells with sidewalls passivated by $\mathbf{Al}_{2}\mathbf{O}_{3}$ had $\sim 5\mathbf{x}$ higher average shunt resistance compared to unpassivated devices. Reverse bias thermograms indicated that MBE-related defects filled with front grid metal were the cause of device-crippling shunts which lowered device yield and limited growth of large-area devices.