E-mail: h2aziz uwaterloo. They also provide new insights into the importance of managing charge distribution in the charge transport layers for realizing high stability QDLEDs and new approaches to achieve that. One can also see that increasing the PEI concentration from 0. The higher V th and lower current density suggest that the PEI makes electron injection and transport more difficult, an effect that can be attributed to its low conductivity.
An examination of the EQE vs. The passivation of ZnO surface defects by the PEI may also be contributing to this efficiency enhancement. The EL spectra Fig. There is a distinct difference between the trends of the control device and the PEI containing devices in the driving voltage trends of Fig.
In general, an increase in driving voltage during electrical driving can be attributed to the formation of space charges within the device layers that create internal electric fields of opposite direction to the field produced by the external bias which require an increase of the external bias to offset their effect and maintain the same current flow.
One also notes the different curvatures i. That this increase in driving voltage does not seem to negatively affect device efficiency as inferred from the stable EL suggests that it may be arising from space charges that are formed far away from the QD EML. In order to investigate the root causes of the stability enhancement we first investigate the surface topography of the ETLs.
The images are shown in Fig. The surface topographies of the ETLs are very similar and roughness measurements indicate that all films have very smooth surfaces. This is consistent with previous studies showing that coating PEI on ZnO brings about some surface planarization effect. This is consistent with the TRPL results that show that introducing even a small amount of PEI affects the TRPL decay rates which indicates that the PEI is effective in passivating a significant fraction of ZnO surface defects and points to strong interactions between the two materials and hence good dispersion.
