Tailored SnO₂ electron transport layer delivers over 27% external quantum efficiency in quantum dot light-emitting diodes compatible with multiple hole transport layers

Achieving balanced charge transport is crucial for high-performance quantum-dot light-emitting diodes (QLEDs), yet it remains a significant challenge. This issue is notably evident when using high-mobility metal oxides, such as ZnO and SnO₂ nanoparticles, as electron transport layers (ETLs), due to their excessive electron mobility which leads to a severe mismatch with most organic hole transport layers (HTLs). Consequently, the balanced charge injection and high efficiency in conventional QLEDs have been largely confined to high-mobility HTLs like poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl) diphenylamine) (TFB). In this study, we present a universal strategy to precisely tune the electron mobility of SnO₂ nanocrystals through controlled Zn²⁺ doping concentration. This approach enables synergistic matching with a range of commonly used HTLs, achieving a near-ideal charge balance across all systems. As a result, we fabricated high-performance QLEDs with universal HTL compatibility, where all optimized devices exhibited a maximum external quantum efficiency (EQE) exceeding 27%. Importantly, the positive aging effect commonly observed in ZnMgO-based devices is completely eliminated in all our SnO₂-based QLEDs. This work provides a general and universal ETL materials for fabricating highly efficient and stable QLEDs compatible with diverse hole transport materials without the need to consider their hole mobility.