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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 current efficiency-voltage characteristics of QD-LEDs; (b) The efficiency-current density curves of FIrpic based OLEDs.
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f3: (a) The current efficiency-voltage characteristics of QD-LEDs; (b) The efficiency-current density curves of FIrpic based OLEDs.

Mentions: It is well-known that the excitons on QDs can be formed by either DCI into QDs from charge transport layers of ZnO and CBP or ET process from CBP to QDs. In order to determine the EL mechanism in inverted QD-LEDs, one and only method is to distinguish the DCI and ET processes by rationally designing device structure, such as changing the charge transporting layer surrounding the QD layer to adjust the DCI and ET processes. In the control device, replacing the CBP with other common used HTL materials will simultaneously affect the DCI and ET processes from HTL to QDs, by which we cannot precisely evaluate the influence of HTL on the DCI and ET processes. As a result, we cannot determine the EL mechanism in QD-LEDs. To overcome this issue, we insert a phosphorescent small molecule material of FIrpic between QDs and CBP layer due to the unique properties of FIrpic, including concentration quenching and moderate charge injection from FIrpic to QDs. Simultaneously, the ET process from Firpic to QDs is also inefficient as reported by Cheng25. Due to these properties, FIrpic offers a platform to investigate the EL mechanism in inverted QD-LEDs. More specifically, if the EL in the FIrpic-containing QD-LEDs originates from ET process, the device efficiency will be dramatically decreased due to the severe exciton quenching and inefficient ET processes in neat FIrpic layer. In contrary, if the EL mechanism in QD-LEDs is DCI-type, the introduction of FIrpic layer into the devices will have little effect on the efficiency due to the similar HOMO energy levels of CBP and FIrpic as seen in Figure 2b. Firstly, in order to demonstrate the exciton quenching (i.e., the concentration quenching) in neat FIrpic film, the FIrpic based OLEDs with a structure of ITO/PEDOT:PSS/CBP/Emission layer/TPBi/LiF/Al were fabricated. The detailed fabrication procedures are shown in the Methods. The emission layers are composed of CBP:FIrpic (12 wt%) and neat FIrpic for the two OLEDs, respectively. We can see from Figure 3a that the peak efficiency is decreased by 70%, from 15.0 cd/A for CBP:FIrpic based OLED to 4.6 cd/A for neat FIrpic layer based device. These results are in excellent agreement with the discussed in the Introduciton section.


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 current efficiency-voltage characteristics of QD-LEDs; (b) The efficiency-current density curves of FIrpic based OLEDs.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f3: (a) The current efficiency-voltage characteristics of QD-LEDs; (b) The efficiency-current density curves of FIrpic based OLEDs.
Mentions: It is well-known that the excitons on QDs can be formed by either DCI into QDs from charge transport layers of ZnO and CBP or ET process from CBP to QDs. In order to determine the EL mechanism in inverted QD-LEDs, one and only method is to distinguish the DCI and ET processes by rationally designing device structure, such as changing the charge transporting layer surrounding the QD layer to adjust the DCI and ET processes. In the control device, replacing the CBP with other common used HTL materials will simultaneously affect the DCI and ET processes from HTL to QDs, by which we cannot precisely evaluate the influence of HTL on the DCI and ET processes. As a result, we cannot determine the EL mechanism in QD-LEDs. To overcome this issue, we insert a phosphorescent small molecule material of FIrpic between QDs and CBP layer due to the unique properties of FIrpic, including concentration quenching and moderate charge injection from FIrpic to QDs. Simultaneously, the ET process from Firpic to QDs is also inefficient as reported by Cheng25. Due to these properties, FIrpic offers a platform to investigate the EL mechanism in inverted QD-LEDs. More specifically, if the EL in the FIrpic-containing QD-LEDs originates from ET process, the device efficiency will be dramatically decreased due to the severe exciton quenching and inefficient ET processes in neat FIrpic layer. In contrary, if the EL mechanism in QD-LEDs is DCI-type, the introduction of FIrpic layer into the devices will have little effect on the efficiency due to the similar HOMO energy levels of CBP and FIrpic as seen in Figure 2b. Firstly, in order to demonstrate the exciton quenching (i.e., the concentration quenching) in neat FIrpic film, the FIrpic based OLEDs with a structure of ITO/PEDOT:PSS/CBP/Emission layer/TPBi/LiF/Al were fabricated. The detailed fabrication procedures are shown in the Methods. The emission layers are composed of CBP:FIrpic (12 wt%) and neat FIrpic for the two OLEDs, respectively. We can see from Figure 3a that the peak efficiency is decreased by 70%, from 15.0 cd/A for CBP:FIrpic based OLED to 4.6 cd/A for neat FIrpic layer based device. These results are in excellent agreement with the discussed in the Introduciton section.

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.