Spatial Composition Tuning of Colloidal NiMgO Hole Transport Materials for All-Inorganic Quantum Dot Light-Emitting Diodes

Rapid progress in the device performance of colloidal quantum dot (QD) based light-emitting diodes (QD-LEDs) proposed the new technical opportunity for next-generation displays. Beyond the premise of QD-LEDs in displays, scientists in this field are now looking forward to realizing brighter and robust lighting sources based on QDs, previously achieved solely by inorganic LEDs and lasing technologies. Unfortunately, the state-of-the-art QD-LEDs employing the hybrid device architectures, organic hole transport layers (HTLs) and inorganic electron transport layers (ETLs), seem to be incapable of the direction toward high power devices due to limited hole conductivity and thermal instability of organic HTLs. This intrinsic demerit of organic HTLs naturally calls for the development of inorganic HTLs featuring enhanced hole conductivity and stability. Among various candidates, nickel oxide (NiO_x, <i>x</i> &lt; 1) has gained attention due to its wide band gap and p-type conductivity granted by Ni vacancy (V_Ni). However, prior attempts to implant NiO<i>_x</i> in QD-LEDs have exhibited suboptimal performance due to the large hole injection barrier and difficulty to match electrical property of ETL counterparts. <br/>To address the disparity of valence band maximum (VBM) and conductivity of NiO<i>_x</i> to typical ETLs, herein, we propose the colloidal NiMgO nanocrystals (NCs) with spatially controlled Mg content. The NiMgO NPs was synthesized by co-condensation of Ni(OH)₂ and Mg(OH)₂ at elevated temperature. X-ray photoelectron spectroscopy (XPS) and photoemission yield spectroscopy (PYS) confirmed that the incorporation of Mg to the Ni–O lattice increased the amount of V_Ni <i>within and on</i> the NCs, and lowered VBM position. We verified that the internal Mg content was limited to ~2% and residual Mg(OH)₂ coordinated to V_Ni sites on NCs. Interestingly, V_Ni on NCs’ surface had a marginal effect on the hole concentration but significantly suppressed hole mobility by an order of magnitude. Maneuvering the spatial Mg content, we successfully optimized NiMgO NCs to minimize the energy offset between the VBM of NiMgO and the 1S_h state of QDs, and to achieve suitable hole mobility similar to that of ZnMgO ETLs. This optimization was also beneficial to reduce photo-induced hole transfer from QDs to NiMgO that was identified as a serious efficiency-limiting process at NiO<i>_x</i>–QD junctions. All-inorganic QD-LEDs employing our NiMgO HTLs exhibited a record peak efficiency of 16.3% and T₅₀ of 382,239 hr (at 100 nit), surpassing previous benchmarks. Our achievement highlights the potential of inorganic HTLs in paving the way for high power QD-LEDs in the future.