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Study of sequential dexter energy transfer in high efficient phosphorescent white organic light-emitting diodes with single emissive layer.

Kim JW, You SI, Kim NH, Yoon JA, Cheah KW, Zhu FR, Kim WY - Sci Rep (2014)

Bottom Line: The dopant concentrations of FIrpic, Ir(ppy)3 and Ir(piq)3 were adjusted and optimized to facilitate the preferred energy transfer processes attaining both the best luminous efficiency and CIE color coordinates.The presence of a deep trapping center for charge carriers in the emissive layer was confirmed by the observed red shift in electroluminescent spectra.White PHOLEDs, with phosphorescent dopant concentrations of FIrpic-8.0%:Ir(ppy)3-0.5%:Ir(piq)3-0.5% in the mCP host of the single emissive layer, had a maximum luminescence of 37,810 cd/m(2) at 11 V and a luminous efficiency of 48.10 cd/A at 5 V with CIE color coordinates of (0.35, 0.41).

View Article: PubMed Central - PubMed

Affiliation: Department of Green Energy &Semiconductor Engineering, Hoseo University, Asan, Korea.

ABSTRACT
In this study, we report our effort to realize high performance single emissive layer three color white phosphorescent organic light emitting diodes (PHOLEDs) through sequential Dexter energy transfer of blue, green and red dopants. The PHOLEDs had a structure of; ITO(1500 Å)/NPB(700 Å)/mCP:Firpic-x%:Ir(ppy)3-0.5%:Ir(piq)3-y%(300 Å)/TPBi(300 Å)/Liq(20 Å)/Al(1200 Å). The dopant concentrations of FIrpic, Ir(ppy)3 and Ir(piq)3 were adjusted and optimized to facilitate the preferred energy transfer processes attaining both the best luminous efficiency and CIE color coordinates. The presence of a deep trapping center for charge carriers in the emissive layer was confirmed by the observed red shift in electroluminescent spectra. White PHOLEDs, with phosphorescent dopant concentrations of FIrpic-8.0%:Ir(ppy)3-0.5%:Ir(piq)3-0.5% in the mCP host of the single emissive layer, had a maximum luminescence of 37,810 cd/m(2) at 11 V and a luminous efficiency of 48.10 cd/A at 5 V with CIE color coordinates of (0.35, 0.41).

No MeSH data available.


(a) Current density-voltage (J-V) and luminescence-voltage (L-V) measured for devices A1–A4, and (b) plots of luminous efficiency as a function of luminescence.
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f2: (a) Current density-voltage (J-V) and luminescence-voltage (L-V) measured for devices A1–A4, and (b) plots of luminous efficiency as a function of luminescence.

Mentions: It has been reported that quenching processes due to triplet-triplet exciton annihilation (TTA) and triplet-polaron annihilation are responsible for the efficiency roll-off18. In this work, the dopant concentration of FIrpic used in the PHOLEDs was adjusted (to achieve high efficiency), by optimizing the EL spectra, taking into account the self-quenching and/or TTA of the dopant molecules. We fabricated devices A1–A4 by varying the doping concentration of FIrpic from 0% to 12% as described in Table 1. Fig. 2 (a) shows the current density–voltage (J–V) and luminance–voltage (L–V) measurements for devices A1–A4, while Fig. 2 (b) shows luminous efficiency as a function of luminance (~10000 cd/m2). As shown in Fig. 2 (a), the initial current density of devices A2–A4 increased with the FIrpic concentration. Device A4 has higher current density than that measured for devices A2 and A3 until 50 mA/cm2. After that, the current density curve of the devices A2, A3 and A4 are similar to each other. Charge carriers moved through the guest molecules at the initial lower voltages and then moved through both the host and guest molecules in the EML. This is due to the fact that the FIrpic dopant has a smaller energy gap, lower HOMO and LUMO levels than that of the ETL, the HTL and the mCP host as shown in Fig. 1. However, driving voltages of devices A2, A3 and A4 is higher than that of device A1, which has an undoped EML. Device A1 has a threshold voltage of 3 V, while that for devices A2 and A3 is 4 V, and it is 4.5 V for device A4. This phenomenon arises from the band gap difference in host (broader) and dopants (narrower) in the EML. Hence, the dopant molecules are function as deep trapping centers for charge carriers in the EML causing an increase in driving voltage19. Therefore, self-quenching and TTA by directly recombining excitons in the dopant molecules are inevitable in a host-dopant system with higher doping concentration.


Study of sequential dexter energy transfer in high efficient phosphorescent white organic light-emitting diodes with single emissive layer.

Kim JW, You SI, Kim NH, Yoon JA, Cheah KW, Zhu FR, Kim WY - Sci Rep (2014)

(a) Current density-voltage (J-V) and luminescence-voltage (L-V) measured for devices A1–A4, and (b) plots of luminous efficiency as a function of luminescence.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4228348&req=5

f2: (a) Current density-voltage (J-V) and luminescence-voltage (L-V) measured for devices A1–A4, and (b) plots of luminous efficiency as a function of luminescence.
Mentions: It has been reported that quenching processes due to triplet-triplet exciton annihilation (TTA) and triplet-polaron annihilation are responsible for the efficiency roll-off18. In this work, the dopant concentration of FIrpic used in the PHOLEDs was adjusted (to achieve high efficiency), by optimizing the EL spectra, taking into account the self-quenching and/or TTA of the dopant molecules. We fabricated devices A1–A4 by varying the doping concentration of FIrpic from 0% to 12% as described in Table 1. Fig. 2 (a) shows the current density–voltage (J–V) and luminance–voltage (L–V) measurements for devices A1–A4, while Fig. 2 (b) shows luminous efficiency as a function of luminance (~10000 cd/m2). As shown in Fig. 2 (a), the initial current density of devices A2–A4 increased with the FIrpic concentration. Device A4 has higher current density than that measured for devices A2 and A3 until 50 mA/cm2. After that, the current density curve of the devices A2, A3 and A4 are similar to each other. Charge carriers moved through the guest molecules at the initial lower voltages and then moved through both the host and guest molecules in the EML. This is due to the fact that the FIrpic dopant has a smaller energy gap, lower HOMO and LUMO levels than that of the ETL, the HTL and the mCP host as shown in Fig. 1. However, driving voltages of devices A2, A3 and A4 is higher than that of device A1, which has an undoped EML. Device A1 has a threshold voltage of 3 V, while that for devices A2 and A3 is 4 V, and it is 4.5 V for device A4. This phenomenon arises from the band gap difference in host (broader) and dopants (narrower) in the EML. Hence, the dopant molecules are function as deep trapping centers for charge carriers in the EML causing an increase in driving voltage19. Therefore, self-quenching and TTA by directly recombining excitons in the dopant molecules are inevitable in a host-dopant system with higher doping concentration.

Bottom Line: The dopant concentrations of FIrpic, Ir(ppy)3 and Ir(piq)3 were adjusted and optimized to facilitate the preferred energy transfer processes attaining both the best luminous efficiency and CIE color coordinates.The presence of a deep trapping center for charge carriers in the emissive layer was confirmed by the observed red shift in electroluminescent spectra.White PHOLEDs, with phosphorescent dopant concentrations of FIrpic-8.0%:Ir(ppy)3-0.5%:Ir(piq)3-0.5% in the mCP host of the single emissive layer, had a maximum luminescence of 37,810 cd/m(2) at 11 V and a luminous efficiency of 48.10 cd/A at 5 V with CIE color coordinates of (0.35, 0.41).

View Article: PubMed Central - PubMed

Affiliation: Department of Green Energy &Semiconductor Engineering, Hoseo University, Asan, Korea.

ABSTRACT
In this study, we report our effort to realize high performance single emissive layer three color white phosphorescent organic light emitting diodes (PHOLEDs) through sequential Dexter energy transfer of blue, green and red dopants. The PHOLEDs had a structure of; ITO(1500 Å)/NPB(700 Å)/mCP:Firpic-x%:Ir(ppy)3-0.5%:Ir(piq)3-y%(300 Å)/TPBi(300 Å)/Liq(20 Å)/Al(1200 Å). The dopant concentrations of FIrpic, Ir(ppy)3 and Ir(piq)3 were adjusted and optimized to facilitate the preferred energy transfer processes attaining both the best luminous efficiency and CIE color coordinates. The presence of a deep trapping center for charge carriers in the emissive layer was confirmed by the observed red shift in electroluminescent spectra. White PHOLEDs, with phosphorescent dopant concentrations of FIrpic-8.0%:Ir(ppy)3-0.5%:Ir(piq)3-0.5% in the mCP host of the single emissive layer, had a maximum luminescence of 37,810 cd/m(2) at 11 V and a luminous efficiency of 48.10 cd/A at 5 V with CIE color coordinates of (0.35, 0.41).

No MeSH data available.