Hole-Injection-Barrier Effect on the Degradation of Blue Quantum-Dot Light-Emitting Diodes

Abstract

Inefficient hole injection presents a major challenge in achieving stable and commercially viable solution-processed blue electroluminescent devices. Here, we conduct an in-depth study on quantum-dot light-emitting diodes (QLEDs) to understand how the energy levels of common electrodes and hole-transporting layers (HTL) affect device degradation. Our experimental findings reveal a design rule that may seem nonintuitive: combining an electrode and HTL with matched energy levels is most effective in preventing voltage rise and irreversible luminance decay, even though it causes a significant energy offset between the HTL and emissive quantum dots. Using an iterative electrostatic model, we discover that the positive outcomes, including a T 95 lifetime of 109 h (luminance = 1000 nits, CIE-y = 0.087), are due to the enhanced p-type doping in the HTL rather than the assumed reduction in barrier heights. Furthermore, our modified hole injection dynamics theory, which considers distributed density-of-states, shows that the increased HTL/quantum-dot energy offset is not a primary concern because the effective barrier height is significantly lower than conventionally assumed. Following this design rule, we expect device stability to be enhanced considerably.

Improved Charge Injection Balance in Quantum Dot Light-Emitting Diodes through Interface Texture

“…That corresponds to the abrupt photoluminescence drop within the first 0.1 second (sampling time interval) of device operation, shown in Figure c. Moreover, the state-of-the-art red CQDs can maintain the photoluminescence intensity during device operation and fully recover the loss once the device is turned off, which is consistent with our earlier reports. , Therefore, the percentage of photoluminescence loss can be used as a measure of CQD charging due to imbalanced charge injection. Figure d reveals that interface texture lowers the CQD charging by 11% due to the improved charge balance.…”

“…Despite the significant advancement over other solution-processed LEDs, the best-reported QLEDs still suffer from inefficient hole injection, the mechanism of which has recently been discussed in detail using electrostatic approaches and hopping theories. − Meanwhile, enhancing electron injection is much less challenging, especially for low-bandgap QLEDs. This imbalanced charge injection leads to unfavored device phenomena, including low luminance efficiency under low operating voltages and nontypical luminance decay curve shapes, which contrast with organic LEDs. , In-depth spectroscopic studies of low-bandgap QLEDs reveal that many CQDs tend to get intermittently negatively charged until neutralized by an incoming hole .…”

“…This imbalanced charge injection leads to unfavored device phenomena, including low luminance efficiency under low operating voltages and nontypical luminance decay curve shapes, which contrast with organic LEDs. , In-depth spectroscopic studies of low-bandgap QLEDs reveal that many CQDs tend to get intermittently negatively charged until neutralized by an incoming hole . Photoluminescence studies suggest that most CQDs remain charged for hours after device operation …”

“…Efforts have been made to improve hole injection through energy-level matching. Still, these attempts have not been very successful due to the significant gap between the work function of common anodes and the ionization energy of CQDs. , Surface-ligand engineering of CQDs is effective in creating interfacial dipoles and modifying energy levels, but the trade-off between energy barrier, electrical conductivity, and processability significantly limits their applications. , In addition to the difficulty in matching energy levels, the Fermi level pinning between the anode and the hole transporting layer (HTL) has been a hindrance . One possible solution is to enhance the carrier concentration by p-doping the organic HTL.…”

Charge-generation structures and their applications in light-emitting devices

“…These advantages include overcoming the restrictions of energylevel matching between electrodes and charge transport layers, and facilitating charge balance in devices. For LEDs with a single emission unit employing charge transport layer, charge injection is a notable issue, with the barrier between electrodes and charge transport layers being more crucial than that between charge transport layers and EMLs [55]. The introduction of CGLs means that carriers are no longer injected into the device via the electrodes but are generated internally within the device.…”