Balancing charge injection in quantum dot light-emitting diodes to achieve high efficienciy of over 21%

Quantum dot light-emitting diodes (QLEDs) have attracted considerable attention in displays owing to their high color purity, wide gamut, narrow emission band, and solution-processed characteristics. However, a major problem of the unbalanced carrier (electrons and holes) injection in QLEDs deteriorates their performance. Here, we balanced the charge injection in QLEDs by optimizing the carrier transport layers. Different organic hole transport layers (HTLs) with a suitable thickness were employed to match the electron transport layer (ETL) of ZnO. Mg 2+ was doped into the ZnO (MZO) ETL to decrease the electron mobility and match the hole mobility of the HTL. Consequently, the QLEDs exhibited an excellent external quantum efficiency (EQE) of 21.10% at a luminance of 4661 cd m −2 . In the luminance range of 100–30,000 cd m −2 , EQE roll-off was considerably low, and more than 80% of the initial EQE value could be maintained, indicating less Auger recombination because of the balanced carrier injection. This work reveals that compared with energy level matching, the charge transfer capability of the transport layers is more instrumental in the charge balance regulation of QLED devices.