Investigation of blue phosphorescent organic light-emitting diode host and dopant stability
Graphical abstract
Introduction
To a large extent, the emergence of OLEDs as an important technology for display and lighting applications can be attributed to the development of highly efficient phosphorescent emitters. The best known class of phosphorescent emitters includes the cyclometallated complexes with iridium (III) as the central atom [1], [2], [3], [4]. With specifically designed chelating ligands for color tuning, RGB (red, green and blue) emitters with nearly 100% (internal) quantum efficiency have been demonstrated. The operating lifetime, however, remains an issue. Although excellent lifetime (>100,000 h) has been achieved for the red and green emitters, the lifetime for the blue emitters is relatively short (typically less than 10,000 h) [5], and decreases with increasing blue shift in wavelength. Among the blue emitters, FIrpic, bis(4,6-difluorophenyl-pyridinato-N,C2) picolinate iridium (III) [6], [7], is perhaps the most widely used. Incorporated as a dopant in a host matrix, it is reportedly capable of achieving 100% internal quantum efficiency with peak emission around 470 nm and light-blue CIE x, y coordinates of x = 0.16 and y = 0.40 [8], [9], [10], [11], [12]. However, FIrpic is unstable with an operating lifetime reported in the range of 0.2–60 h, depending on the test conditions and device architecture [13], [14], [15]. A recent study [14] showed that FIrpic undergoes irreversible chemical dissociation in an operating device, producing fragments that can quench excitons and reduce the OLED efficiency. Several possible pathways for FIrpic degradation have been proposed, suggesting mainly the involvement of FIrpic excited states formed as a result of electron–hole recombination in the dopant/host matrix. In this work, we have investigated the effect of charge transport in a FIrpic doped mCBP matrix on the stability of the OLED device. We conclude that hole transport on FIrpic and charge recombination on mCBP both lead to rapid device degradation and derive insight into materials selection criteria.
Section snippets
Experimental
OLED devices were fabricated on patterned indium-tin-oxide (ITO, 110 nm and 15 ohm/sq) coated glass substrates. The substrates were batch wise cleaned by, sequentially, soaked and mechanically scrubbed in detergent solution, washed in acetone and isopropanol using an ultrasonic bath, rinsed, dried and then further treated with O2 plasma. All films were prepared by thermal deposition (<10−6 Torr) without breaking vacuum until the OLED device was completed with the top aluminum electrode. After
Results and discussion
The layer structures of a series of OLEDs designed to probe the effect of HTL formulation on OLED performance are listed in Table 1. The highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) for each of the materials used in the devices are shown in Fig. 1. All devices have in common an HTL/ETL device structure with Alq as the ETL and emitter. While not shown, all observed electroluminescence originates from Alq (see supplementary material). The thicknesses of the HTL and
Acknowledgements
This material is based upon work supported by (a) the National Science Foundation (NSF) Graduate Research Fellowship Program under Grant No. 0935947 and (b) the Department of Energy (DOE) under Award No. DE-EE0003296. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF or DOE.
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