Abstract
This work investigates the root causes of the limited stability of electroluminescent quantum dot light-emitting devices (QDLEDs). Studies using electrical measurements, continuous UV irradiation, and both steady-state and transient photoluminescence (PL) spectroscopy reveal that exciton-induced degradation of the hole transporting material (HTM) in QDLEDs plays a role in limiting their electroluminescence (EL) stability. The results indicate that there is a correlation between device EL stability and the susceptibility of the HTM to exciton-induced degradation. The presence of quenchers in the HTM layer can lead to a decrease in the luminescence quantum yield of QDs, suggesting that energy transfer between the QD and HTM films may play a role in this behavior. The results uncover a new degradation mechanism where excitons within the HTM limit the EL stability of QDLEDs.
Highlights
We have studied the effect of exciton damage of the hole transporting material (HTM) on quantum dot light-emitting devices (QDLEDs) stability
Energy transfer from quantum dots (QDs) to quenching molecules in the HTM may play a role in this effect, representing a pathway through which degradation of the HTM can negatively impact QD quantum yield over time
The findings show that aside from its influence on device efficiency, energy transfer from the HTM to QDs is an important factor to be considered for more stable QDLEDs
Summary
The unique properties of colloidal quantum dots (QDs) such as high photoluminescence (PL) quantum yield approaching 100%, narrow and tunable electroluminescence (EL) spectra, and solution-compatibility for low cost processing make them advantageous for use as an electroluminescent material in light-emitting devices.[1,2,3,4,5,6,7] Significant progress in developing improved QD materials and device structures, including the use of inorganic (typically ZnO) materials for the electron transport layer and organic materials for the hole transport (HTM) layer, has drastically improved the performance of quantum dot light-emitting devices (QDLEDs) over the last two decades.[8,9,10,11] external quantum efficiency (EQE) and EL lifetime (LT50) as high as 20.5% and 300 000 hours (for an initial luminance of 100 cd m−2) were reported for CdSe-core QDLEDs, respectively.[12,13] the lifetime of most QDLEDs remains inadequate for commercial applications. These excitons arise from the drift of electrons past the QD emissive layer and their recombination with holes in the HTM layer.[21] Knowing that organic materials are susceptible to damage by excitons, a phenomenon that has been found to have a detrimental effect on the stability of organic layers in light-emitting devices,[22,23,24] the possibility of exciton-induced degradation of the HTM playing a role in the limited electroluminescence stability of QDLEDs must be considered This notion is compelling once the remarkable stability of the other layers is taken into consideration. ZnO and other metal oxides used as transport materials are known to generally have high chemical and electrical robustness.[10,11,13,26,27]
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