Current Crowding in a High-Efficiency Black Phosphorus Light-Emitting Diode Using a Reflective Back Contact

We demonstrate a high-performance mid-infrared (MIR) light-emitting diode (LED) based on a black phosphorus (b-P)/n-MoS₂ heterojunction. A gold back contact combined with a rhenium-doped n-type MoS₂ layer is used to enhance light extraction. The device shows a MIR peak external quantum efficiency (EQE) of (1.6 ± 0.2)% at room temperature and a record (7.0 ± 0.5)% EQE at 77 K, with a maximum radiant power density of (108 ± 8) W/cm². Finite-element simulations highlight the importance of phonon-assisted band-to-band tunneling under reverse bias and the influence of carrier velocity saturation under forward bias. The simulations also reveal that the high ideality factors extracted from the current-voltage characteristic are due to current crowding at the heterojunction and a consequence of the device geometry. These findings establish a new high-performance b-P LED architecture and provide crucial insights into the physics of MIR sources based on 2D materials.