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The work mechanism and sub-bandgap-voltage electroluminescence in inverted quantum dot light-emitting diodes.

Ji W, Jing P, Zhang L, Li D, Zeng Q, Qu S, Zhao J - Sci Rep (2014)

Bottom Line: Further, the EL from QD-LEDs at sub-bandgap drive voltages is achieved, which is in contrast to the general device in which the turn-on voltage is generally equal to or greater than its bandgap voltage (the bandgap energy divided by the electron charge).The high energy holes induced by Auger-assisted processes can be injected into the QDs at sub-bandgap applied voltages.These results are of important significance to deeply understand the EL mechanism in QD-LEDs and to further improve device performance.

View Article: PubMed Central - PubMed

Affiliation: State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, 3888 Dongnanhu Road, Changchun 130033, China.

ABSTRACT
Through introducing a probe layer of bis(4,6-difluorophenylpyridinato-N,C2)picolinatoiridium (FIrpic) between QD emission layer and 4, 4-N, N- dicarbazole-biphenyl (CBP) hole transport layer, we successfully demonstrate that the electroluminescence (EL) mechanism of the inverted quantum dot light-emitting diodes (QD-LEDs) with a ZnO nanoparticle electron injection/transport layer should be direct charge-injection from charge transport layers into the QDs. Further, the EL from QD-LEDs at sub-bandgap drive voltages is achieved, which is in contrast to the general device in which the turn-on voltage is generally equal to or greater than its bandgap voltage (the bandgap energy divided by the electron charge). This sub-bandgap EL is attributed to the Auger-assisted energy up-conversion hole-injection process at the QDs/organic interface. The high energy holes induced by Auger-assisted processes can be injected into the QDs at sub-bandgap applied voltages. These results are of important significance to deeply understand the EL mechanism in QD-LEDs and to further improve device performance.

No MeSH data available.


(a) The structure schematic diagram of the QD-LEDs in our work. (b) The flat energy levels of the materials used in our study. Inset shows the cross-section SEM image of Device A. (c) The absorption and PL spectra of QDs used in our work. Inset represents the representative UV-illuminated photo of QDs dispersion in toluene. (d) The TEM image of QDs. Inset is the SEM image of sample with a structure ITO/ZnO/QDs.
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f2: (a) The structure schematic diagram of the QD-LEDs in our work. (b) The flat energy levels of the materials used in our study. Inset shows the cross-section SEM image of Device A. (c) The absorption and PL spectra of QDs used in our work. Inset represents the representative UV-illuminated photo of QDs dispersion in toluene. (d) The TEM image of QDs. Inset is the SEM image of sample with a structure ITO/ZnO/QDs.

Mentions: Figure 2a schematically shows the structure of the device with a FIrpic probe layer in the QD-LEDs. The QD emission layer is sandwiched between the ZnO nanoparticle electron transport layer (ETL) and CBP HTL for the control device. The FIrpic layer with different thicknesses is introduced at the QDs/CBP interface to probe the position of exciton formation by assessing the effect of FIrpic layer on device performances. As can be seen from Figure 2b, the exciton recombination zone should be located at QD/CBP or QD/FIrpic interface according to the energy alignment. The inset shows the cross-section SEM image of the device without a FIrpic layer. Figure 2c shows the absorption and PL spectra of the CdSe/CdS/ZnS QDs (in toluene) used in our devices, as well as the EL spectrum of QD-LEDs. The PL emission peak is located at 602 nm and the full-width at half-maximum (FWHM) is 42 nm. We can see that the EL spectrum is almost consistent with the PL one. The FWHM of EL spectrum is also 42 nm with a little redshift of 3 nm compared with the PL one. This well complete PL-EL spectral overlap demonstrates the efficient recombination of holes and electrons on the QDs in the QD-LED. The quantum yield of CdSe/CdS/ZnS QDs is ~60% in solid powder form measured by an integrating sphere. The TEM image of the CdSe/CdS/ZnS QDs is shown in Figure 2d. High crystallinity of individual QDs is obtained and the average diameter of the QDs is ~8.0 nm. The inset shows the top-view SEM image of ITO/ZnO/QDs and we can see the QDs on ZnO film are close-packed.


The work mechanism and sub-bandgap-voltage electroluminescence in inverted quantum dot light-emitting diodes.

Ji W, Jing P, Zhang L, Li D, Zeng Q, Qu S, Zhao J - Sci Rep (2014)

(a) The structure schematic diagram of the QD-LEDs in our work. (b) The flat energy levels of the materials used in our study. Inset shows the cross-section SEM image of Device A. (c) The absorption and PL spectra of QDs used in our work. Inset represents the representative UV-illuminated photo of QDs dispersion in toluene. (d) The TEM image of QDs. Inset is the SEM image of sample with a structure ITO/ZnO/QDs.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4225534&req=5

f2: (a) The structure schematic diagram of the QD-LEDs in our work. (b) The flat energy levels of the materials used in our study. Inset shows the cross-section SEM image of Device A. (c) The absorption and PL spectra of QDs used in our work. Inset represents the representative UV-illuminated photo of QDs dispersion in toluene. (d) The TEM image of QDs. Inset is the SEM image of sample with a structure ITO/ZnO/QDs.
Mentions: Figure 2a schematically shows the structure of the device with a FIrpic probe layer in the QD-LEDs. The QD emission layer is sandwiched between the ZnO nanoparticle electron transport layer (ETL) and CBP HTL for the control device. The FIrpic layer with different thicknesses is introduced at the QDs/CBP interface to probe the position of exciton formation by assessing the effect of FIrpic layer on device performances. As can be seen from Figure 2b, the exciton recombination zone should be located at QD/CBP or QD/FIrpic interface according to the energy alignment. The inset shows the cross-section SEM image of the device without a FIrpic layer. Figure 2c shows the absorption and PL spectra of the CdSe/CdS/ZnS QDs (in toluene) used in our devices, as well as the EL spectrum of QD-LEDs. The PL emission peak is located at 602 nm and the full-width at half-maximum (FWHM) is 42 nm. We can see that the EL spectrum is almost consistent with the PL one. The FWHM of EL spectrum is also 42 nm with a little redshift of 3 nm compared with the PL one. This well complete PL-EL spectral overlap demonstrates the efficient recombination of holes and electrons on the QDs in the QD-LED. The quantum yield of CdSe/CdS/ZnS QDs is ~60% in solid powder form measured by an integrating sphere. The TEM image of the CdSe/CdS/ZnS QDs is shown in Figure 2d. High crystallinity of individual QDs is obtained and the average diameter of the QDs is ~8.0 nm. The inset shows the top-view SEM image of ITO/ZnO/QDs and we can see the QDs on ZnO film are close-packed.

Bottom Line: Further, the EL from QD-LEDs at sub-bandgap drive voltages is achieved, which is in contrast to the general device in which the turn-on voltage is generally equal to or greater than its bandgap voltage (the bandgap energy divided by the electron charge).The high energy holes induced by Auger-assisted processes can be injected into the QDs at sub-bandgap applied voltages.These results are of important significance to deeply understand the EL mechanism in QD-LEDs and to further improve device performance.

View Article: PubMed Central - PubMed

Affiliation: State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, 3888 Dongnanhu Road, Changchun 130033, China.

ABSTRACT
Through introducing a probe layer of bis(4,6-difluorophenylpyridinato-N,C2)picolinatoiridium (FIrpic) between QD emission layer and 4, 4-N, N- dicarbazole-biphenyl (CBP) hole transport layer, we successfully demonstrate that the electroluminescence (EL) mechanism of the inverted quantum dot light-emitting diodes (QD-LEDs) with a ZnO nanoparticle electron injection/transport layer should be direct charge-injection from charge transport layers into the QDs. Further, the EL from QD-LEDs at sub-bandgap drive voltages is achieved, which is in contrast to the general device in which the turn-on voltage is generally equal to or greater than its bandgap voltage (the bandgap energy divided by the electron charge). This sub-bandgap EL is attributed to the Auger-assisted energy up-conversion hole-injection process at the QDs/organic interface. The high energy holes induced by Auger-assisted processes can be injected into the QDs at sub-bandgap applied voltages. These results are of important significance to deeply understand the EL mechanism in QD-LEDs and to further improve device performance.

No MeSH data available.