Stable ZnS Electron Transport Layer for High-Performance Inverted Cadmium-Free Quantum Dot Light-Emitting Diodes

We report high-efficiency and long-lifetime inverted green cadmium-free (InP-based) quantum dot light-emitting diodes (QLEDs) using a stable ZnO/ZnS cascaded electron transport layer (ETL). We have successfully developed a strategy to spin-coat stable ZnS ETLs with a relatively higher conduction band minimum (CBM) and lower electron mobility than that of ZnO, which leads to balanced carrier injection and an improved device lifetime. Analysis shows that by using the ZnO/ZnS cascaded ETL, electron injection is reduced, resulting in an improved charge balance in the QD layer and suppressed exciton quenching, which preserves the emission properties of QDs. Optimized devices with ZnO/ZnS cascaded ETLs show a maximum external quantum efficiency of 10.8% and a maximum current efficiency of 37.5 cd/A; these efficiency values are an almost 2.2-fold improvement compared to those of reference devices without ZnS. The QLED devices also showed a remarkably long lifetime (LT70) of 265 h at an initial luminance of 1000 cd/m2. The predicted half-lifetime (LT50) at 100 cd/m2 is 60,255 h, which, to our knowledge, is currently the longest lifetime yet reported for InP-based green QLEDs.

Automated summary

This study reports the development of high-efficiency and long-lifetime inverted green cadmium-free quantum dot light-emitting diodes (QLEDs) using a stable ZnO/ZnS cascaded electron transport layer (ETL). The use of ZnO/ZnS cascaded ETL allows for reduced electron injection, improving charge balance and suppressing exciton quenching in the QD layer. The optimized devices achieved significantly improved efficiency and long lifetime compared to reference devices.