Colloidal quantum dots (QDs) have been attracting intense attention as promising candidates for new generation photoluminescent materials due to their fascinating optical properties, such as wide-band absorption, narrow-band emission, and high photoluminescence efficiency. These unique properties originate from semiconducting nature of the QD cores. The core is usually protected by an inorganic shell made of wider-gap semiconductors, resulting in photoluminescence quantum yield (PLQY) as high as 80%. CdSe-CdS and CdSe-ZnS core-shell QDs are such typical examples.
Electroluminescence (EL) devices containing QDs in their emitting materials, which are known as QLEDs, have been demonstrated as a new type of display device providing better monochromaticity than conventional organic LED (OLEDs). However, the electroluminescence efficiencies of QLEDs have been inferior to that of OLEDs due probably to mismatch of the structure that has been developed based on OLEDs. Typically, the deep energy level of a valence band-edge (VB-edge) of the QDs hinders hole injection into QDs from HOMO of organic hole transport materials. Although developing new type of hole transport material is essential, one of the “indirect” strategies to solve this problem would be use of ZnO nanoparticles as an electron transport layer (ETL), which make use of deeper energy level of a VB-edge of ZnO nanoparticles. The resulting structure allows to accumulate holes in QD layer and facilitates hole injection into the QDs. However, the device structure of QLEDs still needs to be improved for more enhancing electroluminescence efficiencies.
This study was carried out to design QLEDs structure which allows efficient charge injection into QDs by changing materials and thickness of the hole injection layer (HIL), and electron transport layer (ETL). The device performance was evaluated by voltage– current and voltage–luminance plots, and external quantum efficiency (EQE). CdSe-CdS-ZnS core-shell-shell nanoparticles used here were synthesized by the previously reported methods. Figure 1(a) shows the structure of the devices which consisted of layers of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene): polystyrene (PEDOT:PSS, 6 nm)/poly(N-vinylcarbazole) (PVK, 37 nm)/CdSe-CdS-ZnS core-shell-shell QDs (14 nm)/ZnO nanoparticles (x nm)/LiF (1 nm)/Al (100 nm), where the thickness of the ZnO layer was varied between 10 nm and 70 nm. Except for LiF and Al layers, which was deposited using the vacuum thermal evaporation, each layer was sequentially formed on ITO-patterned glasses by spin-coating. Figure 1(b) shows the normalized EL spectrum of the QLEDs having an emission peak at 628 nm with a narrow full-width at half-maximum of 35 nm. The inset shows the picture of device (3×3 mm) demonstrating the formation of uniform emission. Figure 1(c) shows the voltage–luminance plots for CdSe-based QLEDs. The device of x = 12 nm only attained a maximum luminance of 298 cd/m2 and a maximum EQE of 0.063% although enough current was flowing (~100 mA). This result indicates the occurrence of charge imbalance, probably because the holes were leaked out to reach the cathode (Al) due to the insufficient ZnO nanoparticle layer thickness. Then, the thickness of ZnO nanoparticle layer was gradually increased to x=31, 48, 68 nm, raising steady the values of maximum luminance and EQE. These results suggested that the thickness of each layer might have effects on hole-electron balance.
The hole-only and electron-only devices were fabricated using the same CdSe-CdS-ZnS core-shell-shell QDs and voltage–current curves were measured. The electron-only device passed larger current than the hole-only device, indicating the lower resistance for electron injection and transport than hole injection and transport. These facts corresponded with the previous results that increment of ZnO layer improved the device performance.
Finally, the device was modified based on that with x = 68 nm. The use of PEDOT:PSS (HIL) having lower resistance were fabricated to further improve the charge balance. As a result, the device achieved a maximum luminance of 6500 cd/m2 and a maximum EQE of 1.3%.
Figure 1
