Origin of the\nEfficiency Roll-off in Quantum Dot Light-Emitting\nDiodes: An Electrically Excited Transient Absorption Spectroscopy\nStudy

In situ characterizations of charge injection dynamics,\nequilibrated\nconcentration, and electric field distributions shed light on the\ncritical mechanisms of quantum dot light-emitting diodes (QD-LEDs).\nIn this work, we developed electrically excited transient absorption\nspectroscopy, which can provide the above key information, to investigate\nthe efficiency roll-off of QD-LEDs. We found that the average electron\npopulations per QD are low when QD-LEDs exhibit efficiency roll-off,\nexcluding Auger recombination as the main cause. We also revealed\nthat the weak electrical field inside the QD layer under forward biases\nhas a negligible impact on the efficiency. Interestingly, we found\nthat as the voltage increases the electron concentration in the QD\nlayer saturates at very low levels. When combined with the concomitant\nefficiency roll-off, we propose electron leakage is the main loss\nat elevated driving voltages. We further demonstrate that increasing\nthe electron confinement potential with the ZnS shell enables us to\nefficiently mitigate the efficiency roll-off.