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 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 control the flow of electrons into the emitting layer. Here, we have theoretically and experimentally investigated the dependence of the luminosity and current efficiency of an ITO/PEDOT:PSS/poly- TPD/PVK/QDs/PMMA/ZnO/Al multilayer QDLED 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 obtained a device with a brightness exceeding that of the control device without an EBL by a factor of four, current efficiency increased by almost an order of magnitude, and turn-on voltage lowered by about 1 V. Furthermore, we have developed a theoretical model of QDLEDs under study, which is consistent with their measured current-voltage characteristics. Using our model, we show that the brightness of the device can be significantly increased by an increase in the thickness of the polymer hole-transport layer (HTL) compared with the QD layer. Therefore, it can be concluded that fine-tuning the thicknesses of both hole- and electron transport layers of a QDLED is a promising strategy to improve charge carrier balance and thereby achieve efficient light emission.