Charge carrier analysis via impedance spectroscopy and the achievement of high performance in CdSe/ZnS:di-[4-(N,N-di-p-tolyl-amino)-phenyl]cyclohexane hybrid quantum dot light-emitting diodes

Abstract

Despite the importance of charge carrier injection balance for achieving high performance with quantum dot light-emitting diodes (QLEDs), there have been comparatively few relevant studies. Thus, we extensively analyzed charge carrier behaviors of QLEDs using an impedance spectroscopy (IS) method and a QLED-equivalent circuit. This yielded both the capacitive and resistive values of each of the emission layer (EML) and the charge transport layer (CTL), revealing the relationships between these values and device performance. Using this analysis method, we examined the effects of the combination of hybrid quantum dot (QD) EMLs and CTLs on QLED performance. The CdSe/ZnS QD and di-[4-(N,N-di-p-tolyl-amino)-phenyl]cyclohexane blended hybrid QD EML, in combination with the di-[4-(N,N-di-p-tolyl-amino)-phenyl]cyclohexane hole transport layer, produced electron–hole balance in EMLs and resulted in a remarkable improvement in device performance. The maximum current efficiency of a green QLED using this combination was 43.7 cd/A, which was approximately two-fold greater than the maximum current efficiency of a QLED with a conventional CdSe/ZnS QD EML and much better than previously reported values for green QLEDs with CdSe/ZnS-based EMLs. In summary, we have greatly improved QLED performance via charge balance optimization. Herein, we present the methods for this improvement, along with an analysis of carrier behavior. • A CdSe/ZnS and TAPC hybrid layer improved light emission efficiency 1.9 times. • Hybrid emission layer and TAPC hole transport layer combination was the most efficient. • Energy band levels of the hybrid light emission layer could be controlled. • Capacitance and resistance of each layer were extracted by Impedance spectroscopy. • The device operation mechanism was well explained by the impedance spectroscopy.