Inverted quantum dot light-emitting diodes with defect-passivated ZnO as an electron transport layer

Abstract

Colloidal quantum dot (QD) light-emitting diodes (QLEDs) are of high interest for future display devices owing to the potential for realizing a wide color gamut. The performances of the QLEDs are approaching those of organic light-emitting diodes (OLEDs) in terms of lifetime as well as efficiency. Recently, low-temperature sol–gel processed ZnO has been utilized as an electron transport layer (ETL) of inverted QLEDs owing to the simple processability and the ease of property modification. However, the devices with the sol–gel ZnO suffer from high leakage current and resultant low efficiency especially at low driving voltages due to intrinsic defect states of ZnO. Here we report inverted QLEDs showing enhanced performance by introducing ethylenediaminetetraacetic acid (EDTA) for defect-passivation in ZnO. We found that EDTA effectively fills up the oxygen vacancies in ZnO by chelating function with little electrical conductivity degradation. As a result, the QLEDs with EDTA–ZnO as the ETL showed an order of magnitude lower current density at low voltage regions (<3 V) and approximately 25% higher external quantum efficiency than the device with pristine ZnO. We investigated the effects of EDTA on the properties of the ZnO films and the QLEDs using them with various analyses. We anticipate that the concepts and results shown here will help with further development of low-temperature processible, high performance inverted QLEDs.