Origin of enhanced efficiency and stability in diblock copolymer-grafted Cd-free quantum dot-based light-emitting diodes

The efficiency and operational lifetime of quantum dot (QD) based light-emitting diodes (QLEDs) are essentially affected by the electron–hole charge balance. Although various methods have been reported to improve the charge balance, these methods cause issues at the same time, such as increasing a driving voltage and complicating a device structure. In this work, we introduce hybrid InP/ZnSeS/ZnS QDs, in which the oleic acid ligands are substituted with semiconducting diblock copolymer units possessing hole-transporting carbazole groups, to facilitate hole injection and to reduce electron leakage. As a result, the efficiency and the operational lifetime were improved by 1.4-fold and 4-fold, respectively, with lower driving voltages. Origins of the results were systematically investigated, and we found that the diblock copolymer units surrounding the QDs made the inter-dot distance farther apart from one another, leading to reduced Forster resonance energy transfer between QDs which can cause reduced photoluminescence quantum yields. Also, the hole transporting ability of carbazole groups facilitated hole injection into QDs and blocked excess electron leakage at the same time. The impedance spectroscopy on the devices with the pristine and hybrid QDs before and after bias stress revealed that hole accumulation at the hole transport layer/QD interface significantly affected the operational lifetime of the QLEDs. This work offers a design for the QD emission layer to achieve efficient and stable QLEDs with systematic understanding of the origins.