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

초록

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 QDLED 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 similar 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.

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