High efficiency phosphorescent organic light-emitting diodes using carbazole-type triplet exciton blocking layer

Abstract

Device performances of green phosphorescent organic light-emitting diodes using ͑4, 5-benzene ͑mCP͒ as an exciton blocking layer were investigated. CBP and mCP were introduced between hole transport layer and emitting layer to block triplet exciton quenching and efficient hole transport to emitting layer. The efficiency of green devices could be improved by more than three times by using mCP exciton blocking layer. There have been many studies about light emission and high efficiency in PHOLEDs. [3][4][5][6][7] Many different device architectures were tried to improve the light-emitting efficiency of PHOLEDs. A hole blocking layer or exciton blocking layer ͑EBL͒ was introduced in PHOLEDs to block hole injection from light-emitting layer ͑EML͒ to electron transport layer and it was effective to get high efficiency in PHOLEDs. [3][4][5] An electron blocking layer was also used in blue PHOLEDs to block electron injection from EML to hole transport layer ͑HTL͒. 5 Fac-tris͑1-phenylpyrazolato-N , C2Ј͒ iridium ͓Ir͑ppz͒ 3 ͔ with its lowest unoccupied molecular orbital ͑LUMO͒ level of 1.7 eV was efficient as an electron blocking material. Other than these, a double EML structure was used and high quantum efficiency of 19.3% was reported. 2,6 A graded doping structure was studied by our group and it also gave a long lifetime as well as high power efficiency. 7 In this work, we studied the use of ͑4,4Ј-N , NЈ-dicarbazole͒biphenyl ͑CBP͒ and N , NЈ-dicarbazolyl-3,5-benzene ͑mCP͒ as an EBL to get high efficiency in green PHOLEDs. A detailed mechanism for exciton blocking of CBP and mCP was clarified and device performances were investigated.The standard device structure used in this experiment was indium tin oxidetris͑2-phenylpyridine͒ iridium ͓Ir͑ppy͒ 3 ͔ ͑30 nm, 5% doping͒/biphenoxy-bi͑8-hydroxy-3-methylquinoline͒ aluminum ͑5 nm͒/tris͑8-hydroxyquinoline͒ aluminum ͑20 nm͒ / LiF ͑1 nm͒ /Al ͑200 nm͒. Three devices with different HTL structures were fabricated to investigate the effect of CBP and mCP EBLs on device performances. Device I had NPB ͑30 nm͒ as a HTL and device II had both NPB ͑20 nm͒ and CBP ͑10 nm͒ as a double layer HTL, while device III had NPB ͑20 nm͒ and mCP ͑10 nm͒ as a double layer HTL. PH1 was supplied from Merck Co. and it has a spirobifluorene-type backbone structure with high electron transport properties because of spirobifluorene units. The triplet band gap of PH1 was 2.4 eV and the highest occupied molecular orbital ͑HOMO͒ and LUMO were 5.9 and 2.8 eV, respectively. The current density-voltageluminance characteristics of the devices were measured with Keithley 2400 source measurement unit and PR 650 spectrophotometer.It is important to confine excitons in EML to increase the recombination efficiency of holes and electrons in OLEDs by blocking charge carriers and exciton diffusion out of EML. 3 Hole and exciton blocking in PHOLED was effective in electron transport layer side and electron and exciton blocking in HTL side is expected to give high recombination efficiency through charge and exciton confin...

The Efficient Green Emitting Iridium(III) Complexes and Phosphorescent Organic Light Emitting Diode Characteristics

“…The maximum external quantum efficiency (EQE) of 25.6% and current efficiency of 84.4 Cd/A were observed for Ir(msippy) 3 based device with CIE color coordinates of (0.31, 0.64). PHOLEDs based on Ir(ppy) 3 shows the green EL emission peak at 512 nm with shoulder peak around 539 nm originating from the triplet excited states of the Ir(ppy) 3 dopant in the EML of the device, which is consistent with earlier report (Zhang et al 2005;Cheng et al 2003;Kim et al 2007). The small additional emissions from CBP host and TcTa layers were observed for the devices based on Ir(ppy) 3 complex, which indicates that the energy and/or charge transfer from host to dopant is incomplete.…”

Recombination zone study of phosphorescent organic light-emitting diodes with triplet mixed host emitting structure

“…In addition to charge balance, recombination zone of holes and electrons is critical to lightemitting efficiency of OLEDs [2]. The recombination zone affects charge injection, exciton quenching effect and optical properties of devices [2][3][4][5]. Therefore, it is very important to study recombination zone of OLEDs and precise control of recombination zone can give better device performances.…”

Stable efficiency roll-off in phosphorescent organic light-emitting diodes

“…High QE in TCTA:TPBI mixed host devices is mostly due to holes and electron balance in light emitting layer and additional improvement of QE by TCTA originated from blocking of triplet exciton quenching and further charge balance in EML. 10 To compare efficiency roll-off in mixed host devices, QE of all devices was normalized and normalized QE is also shown in Fig. 1.…”