Efficient Short-Wave Infrared Light-Emitting Diodes based on Heavy-Metal-Free Quantum Dots

Short-wave infrared (SWIR) light emission is important for a diverse range of modern applications, such as eye-safe depth sensing, light detection and ranging (LiDAR), facial recognition, eye tracking, optical communication, and health-monitoring technologies. However, there are a very limited number of known semiconductors that can emit efficiently in the SWIR spectral range. Presently, SWIR light-emitting diodes (LEDs) based on colloidal quantum dots (CQD) are dominated by lead chalcogenide systems (PbS and PbSe CQDs), despite the presence of heavy metal and modest efficiencies (~8%). Here, we report highly efficient SWIR LEDs based on heavy-metal-free In(Zn)As-In(Zn)P-GaP-ZnS core-shell CQDs. The shell layers were judiciously chosen to possess decreasing lattice constant which allowed the lattice strain caused by mismatch to be gradually relaxed across the core–shell structure and was crucial in passivating surface traps in the In(Zn)As core which resulted in a constant increase in the PLQE from 2% to 10%, 30%, and 73% upon the growth of the In(Zn)P, GaP, and ZnS shells, respectively.<br/><br/>Using solution processing methods, an indium–tin oxide (ITO)/zinc oxide (ZnO)/ poly(9-vinylcarbazole) (PVK)/polyethylenimine ethoxylated (PEIE)/CQDs/poly(N,N′-bis-4-butylphenyl-N,N′-bisphenyl)benzidine (poly-TPD)/molybdenum trioxide (MoO₃)/silver (Ag) electroluminescent device was also fabricated using the synthesized CQDs. The implementation of an otherwise hole-transporting PVK layer on the electron-injecting side of the device stack led to a surprising enhancement in device performance, giving high external quantum efficiencies (EQE) of 13.3% at long emission wavelengths of 1006 nm. Single-carrier device and optical investigations reveal the origins of enhancement to be the electronic decoupling of the CQD layer with the electron-injecting ZnO layer, which mitigated luminescence quenching and improved charge balance. This work marks one of the highest efficiencies reported for heavy-metal-free solution-processed LEDs in the SWIR spectral region (&gt;1000 nm), and could find significant applications in emerging consumer electronic technologies.