Balacing Charge Extends QD Efficiencies

Chinese researchers have taken quantum dot (QD) LEDs above 20% external quantum efficiency (EQE) – thought to be the technology’s theoretical maximum (Display Monitor Vol 20 No 20) – for the first time.

By adding an insulator layer to the design structure, a team at Zhejiang University achieved an EQE of 20.5%, with an operational lifetime of 100,000 hours. Previous work by MIT (Display Monitor Vol 18 No 30) raised EQE to 18%, but device lifetime remained a challenge.

QD LEDs suffer from low lifetimes due to an imbalance between electron injection and hole injection (the charge levels). The electrons (charge) injected into the device were not all used. The team added an insulating layer to slow the electrons down.

Specially-designed QDs, with a high quantum yield, good solution processibility and proper band structure, were required to test the theory. The final structure was formed of six layers, sandwiches between an ITO electrode and a silver electrode. Conducting-polymer layers were used, beneath a layer of cadmium selenide-cadmium sulfide (CdSe-CdS) core-shell QDs. These layers assisted with hole transport. A zinc oxide (ZnO) nanoparticle layer provided the electron transport. The insulating layer was formed of poly(methylmethacrylate) (PMMA), placed between the QDs and ZnO layer.

Aside from the silver electrode (added through vacuum deposition), all LED components were solution-processed. Inkjet printing – or other plastic-compatible methods – could also be used to apply the silver layer.

Lifetime could be further improved using industrial equipment, rather than the university materials. In addition, work needs to be done with other LED colours (the LEDs used were red).