Effect of Charge Transfer to Quantum Dots on Electroluminescence

Recently, quantum dot light-emitting diodes (QLEDs) have extensively utilized electron transport layers (ETLs) based on the metal oxides such as ZnO, which stands out for their high performance in QLEDs. Despite the successful implementation of metal oxide-based ETLs, there is still a puzzle surrounding how they effectively drive QLEDs. Previous studies have primarily attributed this advancement to the modification of energy levels within the ETLs. However, despite the energetic considerations, several reports on metal oxide-based QLEDs have shown driving voltages lower than the optical bandgap of quantum dots (QDs). Additionally, the efficiencies of QLEDs often surpass the theoretical limit dictated by the emissive probability of neutral QDs. This intriguing finding suggests the existence of a concealed mechanism behind the performance improvement of QLEDs.<br/>In this study, we demonstrate that the charge transfer between ZnO-based ETLs and QDs not only affects the energy alignment of QDs in QLEDs but also influences their optical properties. The study for ultraviolet photoelectron spectroscopy represents the overall energy landscape of QLEDs being rearranged centered on the surface states of QDs. The consequence of charge transfer from ZnO-based ETL to the QD surface increases the work function of QDs, providing electrostatic potential gain for hole injection. The spectrally-resolved photoluminescence and electroluminescence results show that carriers injected into individual QDs regulate the injection rate of opposite carriers through Coulombic interaction. By carefully assessing the carrier occupation in QDs, we are able to quantify the emissive probability of QDs on metal oxides. The transfer of electrons from the metal oxides to QDs electrically passivates the surface trap states on QDs, thereby increasing the emissive probability of QDs. The electrobrightening of QDs by metal oxide ETLs pushes the efficiency envelopment of QDs by surface defects.<br/>Our series of analyses indicate an oversimplification in considering the QD ensemble as a single emitter and the ligand-passivated surface as a mere insulator within QLEDs. From a practical perspective, we emphasize the importance of considering individual QDs as discrete emitters and the optoelectronic changes resulting from charge exchange. We believe this consideration would pave the way for the design of efficient QLEDs.