Differences in Spontaneous Electron Transfer in Red, Green, and Blue Quantum Dot Light-Emitting Devices and Its Benefits for Electroluminescence Efficiency

In this study, transient and steady-state photoluminescence (PL) measurements reveal that the spontaneous electron transfer that is known to occur from the electron transport layer (ETL) into the quantum dot (QD)-emissive layer (EML), which occurs under zero bias, decreases as the band gap of the QDs-EML becomes wider and therefore decreases as one goes from red (R)-QDs to green (G)-QDs and then to blue (B)-QDs. PL measurements, simulation results, and tests on both full devices as well as single carrier devices demonstrate that there is a correlation between the magnitude of the spontaneous electron transfer and the sequence of carrier injection into the QDs-EML. Specifically, in R-QLEDs, where the spontaneous electron transfer is strongest, the sequence of charge injection into the QDs-EML is "electron first, hole next," whereas in the case of B-QLEDs, where it is weakest, the sequence is "hole first, electron next". Also, the observed disparities in the spontaneous electron transfer coincide with significant differences in the external quantum efficiencies (EQEs), with EQEs of 8.91 and 0.77% in the R and B-QLEDs, respectively. This suggests that charging the QDs-EML with electrons first, as opposed to holes first, appears to be more favorable for efficiency enhancement, a behavior that is attributed to the lower Auger recombination rate of negative trions versus that of positive trions. These results highlight the importance of spontaneous electron transfer, resulting in charging the QDs-EML with electrons rather than holes as a prerequisite for efficient QLEDs.