Over 30% efficient and highly stable Co-SnO₂-based QLEDs through ultrathin MgCl₂ interface engineering

SnO₂-based quantum dot light-emitting diodes (QLEDs) offer superior environmental stability over ZnO-based devices, yet their electroluminescent performance is often limited by interfacial exciton quenching and unbalanced carrier injection. Here, we employed solution-processed Co-doped SnO₂ (CSO) nanoparticles as the electron transport layer to reduce oxygen vacancies and optimize energy-level alignment. As a result, the red QLED with 5 wt.% CSO achieved an external quantum efficiency (EQE) of 21.5% at a luminance of 187 cd/m². However, at high luminance the carrier-balance improvement cannot be maintained and the CSO-device still shows significant efficiency roll-off. To further optimize the interface, we introduced an ultrathin MgCl₂ interlayer between the quantum dot layer and the CSO layer. The MgCl₂ layer acts as a functional modifier, passivating surface defects, tuning energy levels, and improving carrier balance. Consequently, the device achieved a maximum EQE exceeding 30% and a long T₉₅ operational lifetime (at 1000 cd/m²) of 3367.7 h. This synergistic strategy provides a practical pathway toward high-performance and stable QLEDs.