Phosphorescence organic light-emitting diodes (PHOLED) can facilitate low-power consumption full-color display and solid-state lighting applications because they possess an internal quantum efficiency of 100% in theory 1–4. Compared to red and green PHOLED, high-efficiency blue PHOLED is more difficult to realize, since it requires a host material with large triplet energy (TE) to confine the triplet excitons on phosphorescence emitters 5. Moreover, the neighboring hole- and electron-transporting materials should have higher TE than that of the blue phosphor 6 to prevent the triplet exciton quenching. However, these kinds of materials are rare and hard to design 7, 8. Based on a conventional blue phosphorescent dopant and their host materials, a common approach of improving the performance is to optimize the emitting layer (EML), such as introduction of a mixed-host architecture 9, 10, double hosts 11, and stepwise EML 12–16, and so on.
Compared to the conventional organic light-emitting diode (OLED), stepwise-doping EML allows for better control of charge transport and recombination, which can be classified as two kinds of EML. One type of stepwise-doping EML consists of graded bipolar transport hosts and constant concentration of dopant. Chwang et al. 12 reported a stepwise graded bipolar transport emissive layer with 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (NPB) and tris-(8-hydroxyquinoline)aluminum (Alq3) as hosts and 1% 2,3,6,7-tetrahydro-1,1,7,7,-tetramethyl-1H,5H,11H-10-(2-benzothiazolyl)quinolizino-[9,9a,1gh]coumarin (C545T) as the dopant. The power efficiency was 1.6 times larger than that of a uniformly mixed device. Erickson and Holmes 13 reported a single-layer green PHOLED based on a graded-composition EML by mixing both hole- and electron-transporting host materials (HTMs and ETMs, respectively). The graded EML consists of nearly 100% HTM on the anode side and nearly 100% ETM on the cathode side. A peak external quantum efficiency of 19.3 ± 0.4% was realized at a luminance of 600 cd m−2. The other type of stepwise-doping EML is comprised of a dopant with graded concentration in a host material. The emitter has either hole-transporting or electron-transporting property, which is usually opposite to the host material. Lei et al. 14 reported the first stepwise doping blue PHOLED by doping bis[(4,6-difluorophenyl)-pyridinato-N,C2′](picolinate)Ir(III) (FIrpic) in N,N′-dicarbazolyl-1,4-dimethene-benzene (DCB) with varied concentrations of 5, 10, 20, and 45% in each step. The device showed a peak luminous efficiency of 15.4 cd A−1, which is almost 80% higher than that of the conventional FIrpic-based PHOLED. However, the shoulder at 500 nm of the electroluminescence (EL) spectrum was enhanced and became the main emission peak in the graded doped device. The bathochrome issue of the EL spectrum is caused by a shift of the recombination zone far away from the reflective cathode. Recently, Lee 15 has investigated the charge-trapping effects in PHOLED by changing the doping concentration of emitter in EML. In the device based on the (4,4′-N,N′-dicarbazole)biphenyl (CBP):FIrpic host–guest system, the performance was not influenced by three different stepwise-doping profiles (2–6–10%, 10–6–2%, 10–6–10% with direction from HTL to ETL), indicating that there is little hole and electron trapping effect in the device. The result is contradictory to the reports published previously 14, 16. In order to get a better understanding and further improve the performance of blue phosphorescent OLED, we have performed a detailed investigation on the effects of stepwise-doping profile.
In this work, an irregular stepwise-doping EML architecture is presented that permits the realization of both high luminous efficiency and power efficiency. The layer composition is formed by inserting a high-doping EML between two relatively low-doping EMLs. This irregular stepwise-doping profile allows for high hole–electron recombination probability and suppressed triplet exciton quenching in EML.