Enhancing Quantum Dot LED Efficiency by Tuning Electron Mobility in the ZnO Electron Transport Layer

Quantum‐dot (QD) light‐emitting diodes (QLEDs) are an important new class of optoelectronic device. Despite the ubiquity of ZnO as the electron‐transport material in QLEDs, little is known about how its properties influence QLED performance. Here, it is demonstrated that the defect density and electron mobility of the ZnO nanoparticle electron‐transport layer strongly affect QLED device efficiency and can be used to balance electron and hole injection into the QD layer. Films of ZnO nanoparticles exhibiting electron mobilities tuneable over an order of magnitude are made by annealing out defect states in suspensions of ZnO nanoparticles prior to deposition. By incorporating these ZnO films into a typical QLED device, it is demonstrated that a clear maximum in QLED external quantum efficiency can be found at ZnO mobilities around 2–4 × 10 −4 cm 2 V −1 s −1 , yielding over 10 000 cd m −2 at low operating voltage. The work demonstrates a simple method for enhancing QLED performance without modification of the device architecture and provides valuable insights into the physics of QLED operation.