Correlation between the structural morphology and device characteristics of quantum dot based emission layer blended with small molecular hole transport material

Quantum dot-based light-emitting diodes (QD-LED) have advantages over organic-based light-emitting diodes (OLED) with narrow bandwidth and easy tuning of the wavelength range. A blending of the organic hole transport material (HTM) into the quantum dot (QD) emission layer (EML) can improve the efficiency of the QD-LED at an optimized concentration. However, the relationship between structural morphology and efficiency has not been fully understood. In this work, it is found that the efficiency of green QD-LED is improved when we mixed 10 wt% of 4,4′,4′′-tris(N-carbazolyl)triphenylamine (TCTA), a small molecular HTM, to QD EML. We find that the hole transportability is improved as the mixing ratio of TCTA increases up to ∼10 wt%. Above 10 wt% of TCTA, the electrons flowing from the cathode are relatively blocked to prevent exciton formation. There is no significant ordering change for the QD mixed with 10 wt% of TCTA compared to the QD-only film using the grazing incidence small X-ray scattering technique. With increasing TCTA ratio, TCTA induces QDs aligned in 2D hexagonal close-packing. It turns out that the excess TCTA enhances the ordering of QDs but deteriorates the efficiency of the devices due to a carrier leakage of imperfect charge injection into QDs.