The light emitting efficiency and the stability of the phosphorescent devices, whose emission characteristics are strongly
dominated not only by the energy transfer but also by the charge carrier trapping influenced by the heterostructured
emissive layers and charge injection layer, are studied in terms of the charge injection behavior, carrier transporting
mobility, and balancing of devices. The enhancement of the light emitting properties (higher efficiency and lower driving
voltage) by use of heterostructures at emitting layer, either multilayers or mixing of hole- and electron-transporting
materials (such as 4,4",4"-tris(N-carbazolyl)-triphenylamine); TCTA and bis(10-hydroxybenzo[h]quinolinate) beryllium;
Bebq2) was characterized. By a design of emitting layer structure for recombination, 30~50lm/W efficiency of devices
with Ir(ppy)3 and conventional transporting layer was possible.
The charge conduction properties of the organic phosphorescent emission layer doped with iridium-based green and
red phosphorescent emitters, fac-tris(2-phenylpyridine) iridium(III) (Ir(ppy)3) and bis(2-(2'-benzo
[4,5-a]thienyl)pyridinato-N,C3')iridium(acetyl-acetonate) (btp2Ir(acac)), were studied and compared to those of the
reference host of 4,'-N,N'-dicarbazole-biphenyl (CBP). In the CBP host layer, both dopants act as hole traps but
they affect the electron transport differently. Compared with the pristine CBP film, the electron mobility is similar for
Ir(ppy)3-doped CBP but it is more than two orders of magnitude lower for btp2Ir(acac)-doped CBP. Because of such
difference in the electrical conduction properties between the Ir(ppy)3- and btp2Ir(acac)-doped CBP, the main
recombination zone position and the electron-hole balance changes. Based on these findings, we optimized white
organic light emitting diodes (OLEDs) with multi-emitting layer (EML) structures in which CBP layers doped with
Ir(ppy)3 and btp2Ir(acac) and fluorescent dopant of
4,4'-bis[2-{4-(N,N-diphenylamino)phenyl}vinyl]biphenyl
(DPAVBi) were used as green (G), red (R), and blue (B) EMLs, respectively. The white OLEDs with the R/G/B EML
sequence show improved electron and hole balance, resulting higher efficiency, better color stability and longer
lifetime compared to the G/R/B EML sequence. A high luminous current efficiency of 13.5 cd/A at 100 cd/m2 was
achieved with the R/G/B EML sequence.
In this paper, the light emitting efficiency, spectrum, and the lifetime of the phosphorescent devices, whose
emission characteristics are strongly dominated not only by the energy transfer but also by the charge carrier trapping
induced by the emissive dopant, are explained by differences in the energy levels of the host, dopant, and nearby
transport layers. On the basis of our finding on device performance and photocurrent measurement data by time-of-flight
(TOF), we suggest a detailed emission mechanism, along with a physical interpretation and practical design scheme for
improving the efficiency and lifetime of devices. Moreover, the effects of the modification of charge (both hole and
electrons) transporting layers which results in a drop of operating voltage and improved efficiency on the performance of
electrophosphorescent device are investigated experimentally. Using the Ir-based phosphorescent emitter, vacuumevaporated,
solution-processed, and composition of their hybrid structures are designed for better charge carrier balance
and efficient exciton blocking behavior.
In this paper, we demonstrated that controlled charge trapping, both at the emission layer and charge transport layer with
energy level alignment, is essential for charge-balanced and effective electrophosphorescent organic light-emitting
device (OLED). Conditions for enhanced of efficiency and lifetime of OLED were obtained with graded doping profile
at the light-emission layers (varied host-dopant concentration) and different hole (exciton) blocking materials.
Conceptual device physics presented in this study can be applied at an initial design of charge-confined, balanced
structure of highly efficient electrophosphorescent devices.
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