OLED devices

The possibility of controlling ihc morphological and structural order in the solid is therefore a fundamental requirement for the control and reproducibility of the emission properties of a luminescent material within an organic light emitting diode (OLED) device. 

The characteristics of ITO described in the previous paragraph make it a useful material for use as the anode in an OLED. At the same time, they arc the cause of many difficulties which have been observed in the reproducibility and stability of OLED devices. We shall return to this topic in more detail later, but suffice it... 

OLED devices are fabricated on a glass, plastic, metal, or ceramic substrate as a multilayer-stacked structure represented in Figure 3.1. 

FIGURE 3.1 Schematic of a multilayer OLED device structure. 

Closely related to the anode modifications described above, the use of a HIL material to improve charge injection into the OLED device has spawned a number of materials, which have been shown to provide benefits, particularly in terms of lower operating voltages and extended lifetimes of devices. 

The performance of OLED devices employing CuPc as a HIL is unstable due to thermally induced HTM crystallization on the CuPc surface [27]. One approach to improve the hole injection and enhance the device stability is to overcoat the CuPc or else to directly deposit... 

Due to the relatively high mobility of holes compared with the mobility of electrons in organic materials, holes are often the major charge carriers in OLED devices. To better balance holes and electrons, one approach is to use low WF metals, such as Ca or Ba, protected by a stable metal, such as Al or Ag, overcoated to increase the electron injection efficiency. The problem with such an approach is that the long-term stability of the device is poor due to its tendency to create detrimental quenching sites at areas near the EML-cathode interface. Another approach is to lower the electron injection barrier by introducing a cathode interfacial material (CIM) layer between the cathode material and the organic layer. The optimized thickness of the CIM layer is usually about 0.3-1.0 nm. The function of the CIM is to lower... 

Fujikawa et al. studied a series of triphenylamine (TPA, 14) oligomers from the dimer TPD up to the related pentamer and used them as HTMs [71]. Their results indicated that the thermal stability of the OLEDs was dramatically improved using a HTM TPTE (15) (Scheme 3.9), a tetramer of TPA. The resulting OLED devices show uniform light emission in continuous operation up to 140°C without breakdown [72],... 

The material that gets most of the glory in OLED devices is naturally that which generates the light output. In many cases, however, the so-called EML is actually a mixture of two or more... 

The phenomenon of organic EL was first demonstrated using a small-molecule fluorescent emitter in a vapor-deposited OLED device. The Kodak group first used metal oxinoid materials such as the octahedral complex aluminum tris-8-hydroxyquinoline (Alq3) (discussed above as an ETM) as the fluorescent green emitter in their pioneering work on OLED architectures [167],... 

The merit of these chromene dopants is their relatively long emission wavelength peaks compared to DCM or DCJTB materials due to the more conjugated chromene moiety, and this contributes to the more saturated red emission. In fact the EL spectra of OLED devices of ITO/TPD/Alq3 chromene-dopants/Alq3/Mg Ag exhibited satisfactory red emission color, especially for Chromene-1 and Chromene-2 dopants. However, these chromene-based red emitters showed lower fluorescent quantum yield (18%, 15%, and 54% for Chro-... 

Recently, Chen s group reported a deep blue OLED based on an asymmetric mono(styryl) amine derivative DB1 (192) as shown in Scheme 3.59. PL spectra of this deep blue dopant in toluene solution showed a peak emission of 438 nm, which is about 20 nm hypsochromic shift compared with DSA-amine symmetric dopant, due to the shorter chromophoric conjugated length of the mono(styryl) amine. OLED device based on this blue dopant achieved a very high efficiency of 5.4 cd/A, with CIE coordinates of (0.14, 0.13) [234]. 

Spiro-FPAl/TPBI/Bphen Cs/Al. A very low operating voltage of 3.4 V at luminance of 1000 cd/m2 was obtained, which is the lowest value reported for either small-molecule or polymer blue electroluminescent devices. Pure blue color with CIE coordinates (0.14, 0.14) have been measured with very high current (4.5 cd/A) and quantum efficiencies (3.0% at 100 cd/m2 at 3.15 V) [245]. In another paper, Spiro-FPA2 (126) was used as a host material with an OLED device structure of ITO/CuPc/NPD/spiro-FPA2 l%TBP/Alq3/LiF that produces a high luminescent efficiency of 4.9 cd/A [246]. 

Stable white emission with CIE coordinates of (0.3519, 0.3785) was obtained in such a rare-earth-based OLED device. The authors mentioned that the QE of the device was not good, possibly due to the inefficient energy transfer process between the ligand and the rare-earth metal. A suitable choice of the ligand may improve this type of device performance. 

Other work by Tsuboyama et al. reported a very highly efficient red PHOLED with power efficiency of 8.0 lm/W at 100 cd/m2 using Ir(piq)3 as a dopant [362], Most exciting, however, is the relatively recent demonstration of exceptional lifetimes for these materials in OLED devices where work from UDC has claimed a 14 cd/A red CIE (0.65, 0.35) with a lifetime of 25,000 h at 500 nit. Such performance promises much for phosphorescent red emitters in commercial devices and even higher efficiencies have been realized in systems that compromise the chromaticity toward the deep red with CIE (0.67, 0.33) and lifetimes >100,000 h at 500 cd/m2 [363],... 

T.K. Hatwar, J.R. Vargas, and V.V. Jarikov, Stabilized white-light-emitting OLED devices employing a stabilizing substituted perylene material, U.S. Patent 2,005,089,714, pp. 21 (2005). 

Figure 7.5 shows a schematic example of the electroluminescent process in a typical two-layer OLED device architecture. When a voltage is applied to the device, five key processes must take place for light emission to occur from the device. 

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