Increasing the brightness and efficiency of quantum dot light-emitting diodes by optimizing the PMMA electron-blocking layer

Quantum dots (QDs) are promising materials for advanced light-emitting diodes (LEDs). Their high thermo- and photostabilities compared to the currently used organic materials allow achieving a greater device brightness due to a higher current density. However, the imbalance of the carrier injection/transport rates is one of the weakest points of QD-based LEDs (QDLEDs), because excess charges accumulated in the emitting layer quench light emission due to various nonradiative processes. The imbalance of charge carrier transport rates in QDLEDs is related to the high potential barrier for hole injection into the QD layer, accompanied by a greater mobility of negative charges in the electron transport layer. To solve this problem, an electron-blocking layer (EBL, made, e.g., of PMMA) can be introduced, which makes it possible to regulate the flow of electrons into the emitting layer. Here, we have theoretically and experimentally investigated the dependence of the luminosity and current efficiency of a multilayer QDLED with the ITO/PEDOT:PSS/poly-TPD/PVK/QDs/PMMA/ZnO/Al structure on the thickness of its EBL. For this purpose, a series of devices was fabricated with the PMMA layer thickness ranging from 0.13 to 3.1 nm. By tuning this thickness, we have fabricated a device with a brightness exceeding that of the control device without EBL by a factor of four, current efficiency increased by almost an order of magnitude, and turn-on voltage lowered by about 1 V. Therefore, it can be concluded that tuning the EBL of a QDLED is a promising strategy to improve charge carrier balance and thereby achieve efficient light emission.