Efficient triplet harvest for orange-red and white OLEDs based exciplex host with different donor/acceptor ratios

Highlights

• Orange-red/white OLEDs based exciplex host with different donor/acceptor ratios and doping concentrations are fabricated.

• EQE of 18.3% in orange-red, 17.8% in warm white and 16.6% in cool white OLEDs are realized, respectively.

• Triplet exciton harvest on exciplex host through RISC plays the key role for high device efficiency.

Abstract

Highly efficient orange-red and white organic light-emitting diodes (OLEDs) based exciplex host with different donor/acceptor ratios and doping concentrations are fabricated for efficient triplet excitons harvest. As a result, the maximum efficiency is achieved at 5:5 ratios exciplex host in orange-red OLEDs with current efficiency, power efficiency and external quantum efficiency (EQE) of 37.0 cd/A, 37.3 lm/W and 18.3%, respectively. Furthermore, the white OLEDs under two different doping concentrations give warm white (0.48, 0.41) and cool white (0.28, 0.37) emission with maximum current efficiencies, power efficiencies and EQEs of 38.3/37.6 cd/A, 41.3/43.7 lm/W and 17.8/16.6%, respectively. We found that the triplet exciton harvest on exciplex host through reverse intersystem crossing process and efficient energy transfer are responsible for the high device efficiency.

Introduction

Exciplex organic light-emitting diodes (OLEDs) received more and more attention since the thermally activated delayed fluorescent (TADF) mechanism was discovered by Adachi group in 2012 [[1], [2], [3]]. TADF is the singlet exciton radiative transition that from triplet exciton reverse intersystem crossing (RISC) process due to the small singlet-triplet state energy level splitting (ΔE_ST). Exciplex that formed from intermolecular charge transfer conducts spatially separated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), which present natural small ΔE_ST for high triplet exciton harvest and utilization. Therefore, a series of high efficiency exciplex OLEDs with blue, green and yellow emission were explored by suitable donor/acceptor materials selection [[4], [5], [6], [7], [8]]. In generally, the donor materials are hole transport materials and acceptor materials are electron transport materials, respectively. And the mixed ratio of donor/acceptor materials is 1:1 in most of exciplex OLEDs.

Furthermore, mixed exciplex could be applied as host to sensitize dopant for high efficiency OLEDs due to the excellent charge transport bipolarity, efficient triplet harvest and energy transfer property [9,10]. The mixed hole and electron transport materials make the exciplex layer exhibit high hole and electron transport ability, which could improve charge recombination efficiency, extend exciton formation zone and reduce exciton concentration. High efficiency exciplex presents efficient TADF behavior, which could improve triplet harvest for high exciton utilization. The highly efficient triplet exciton RISC process could also enhance the long range Förster energy transfer between singlet state energy level of host and dopant. These outstanding characteristics guarantee the efficient application of exciplex in host role and the dopants of traditional fluorescent, phosphorescent and TADF emitter all could be employed in exciplex host to achieve highly efficient dopant emission. Kim et al. reported a series of high efficiency blue, green, orange and white OLEDs by utilizing phosphorescent emitter as dopant and exciplex as host since 2013 [[11], [12], [13], [14], [15]]. Traditional fluorescent emitter of 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetra-methyljulolidyl -9-enyl)-4H-pyran (DCJTB) could be also doped into exciplex host to break 10% external quantum efficiency (EQE) [16]. While TADF emitter of 9-[4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl]-N,N,N′,N′‐tetraphenyl‐9H‐carbazole-3,6‐diamine (DACT-II) acts as dopant to apply in exciplex host could reach to 34.2% EQE through two RISC processes of TADF emitter and TADF exciplex host [17]. Thus, the high efficiency OLEDs could be realized in exciplex host by efficient triplet exciton harvest. However, most of donor/acceptor ratio in exciplex host reported to now is 5:5, the different mixed ratio donor/acceptor in exciplex host is very rare. Besides, the effect of small ratio donor or acceptor in exciplex host is also need be explored to extend more application to develop OLEDs based exciplex further.

In this work, we fabricated four different donor/acceptor ratios exciplex host to sensitize orange-red phosphorescent dopant of Iridium (III) bis(2-phenylquinoline) acetylacetonate (Ir (pq)₂acac). As a result, the 5:5 ratio achieved the best electroluminescence (EL) performance with maximum current efficiency, power efficiency and EQE of 37.0 cd/A, 37.3 lm/W and 18.3%, respectively. While other OLEDs with mixed ratios of 7:3, 8:2 and 9:1 also obtained a high efficiency level with maximum EQEs of 17.3%, 17.4% and 16.5%, respectively. Further, highly efficient warm and cool white OLEDs with the same exciplex host but smaller acceptor ratio of 1% were realized. The maximum current efficiency, power efficiency and EQE of 38.3 cd/A, 41.3 lm/W and 17.8% in warm white OLEDs and 37.6 cd/A, 43.7 lm/W and 16.6% in cool white OLEDs were realized. Our results demonstrated that the acceptor ratio had little effect on exciplex host to achieve highly efficient OLEDs and the white OLEDs could be also realized with small acceptor ratio and low doping concentration.