Reducing the Efficiency Roll Off and Applied Potential-Induced Color Shifts in CdSe@ZnS/ZnS-Based Light-Emitting Diodes

Green light-emitting CdSe@ZnS/ZnS (QD) nano-particles were synthesized; the photophysical and morphological properties of their films, which were prepared by spin coating from six different concentrations, corresponding to absorbance values of 0.6, 1.1, 1.6, 2.1, 2.8, and 4.0, were determined. Increasing the absorbance value from 0.6 to 4.0 did not change the photophysical properties of QD films to a large extent, whereas it resulted in an increment in QD film thickness from 20 to 110 nm. The films were utilized as an emissive layer in QD light-emitting diodes with poly(9-vinylcarbazole) (PVK), PVK:2-(4-biphenyllyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD), and PVK:1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl]-phenylene (OXD-7) hole-transport layers (HTLs). The presence of PBD or OXD-7 in PVK reduced the efficiency values but played a positive role in the color purity and efficiency roll off. The maximum color temperature and electroluminescence wavelength shifts obtained with applied potential were 109, 50, and 50 K and 11, 5, and 5 nm for pure-PVK, PVK:PBD, and PVK:OXD-7-based devices, respectively. Hole mobility, capacitance (at 10(3) Hz), and charge-transfer efficiency values were 9.0 x 10(-7), 6.8 x 10(-7), and 4.2 x 10(-7) cm(2) V s(-1), 1.7, 1, and 1 nF, and 6.90%, 15.50%, and 16.10% for pure-PVK, PVK:PBD, and PVK:OXD-7-based devices, respectively. Enhanced color purity and lowered efficiency roll off obtained with PVK:PBD and PVK:OXD-7 HTLs were attributed to decreased capacitance, increased charge-transfer efficiency, and reduced Joule heating.