Emitter Material

The apphcation of a high electric field across a thin conjugated polymer film has shown the materials to be electroluminescent (216—218). Until recentiy the development of electroluminescent displays has been confined to the use of inorganic semiconductors and a limited number of small molecule dyes as the emitter materials. Expansion to the broad array of conjugated polymers available gives advantages in control of emission frequency (color) and facihty in device fabrication as a result of the ease of processibiUty of soluble polymers (see Chromogenic materials,electrochromic). 

Star-shaped molecules with oligophenyl arms derived from 9,9-spirobifluorene as the central unit have been synthesized by Tour et al. [58] and Salbeck et al. [59] and have been suggested as potential emitter materials for blue LEDs. 

Suppression of the aggregate emission is possible in two quite different ways. At first, the aggregate emission could be almost completely shut out by simply diluting the LPPP 12 with a matrix polymer. LEDs with 1% LPPP 12 in poly(9-vinylcarbazole) PVK as emitter material are characterized by a pure blue light emission with a quantum efficiency of ca. 0.15% in single-layer configuration (lTO/1% LPPP 12 in PVK/Ca) 135],... 

Thinking Critically The form of your second graph is completely determined by two values the work function <, which is a property of the emitter material, and Planck s constant h, which is a fundamental constant of nature. The graph would look precisely the same if the lamp had been twice as bright. Explain why this observation leads to the conclusion that light has a particle-like aspect. 

There are two distinct advantages of the self-powered neutron detector (a) very little instrumentation is required—only a millivoltmeter or an electrometer, and (b) the emitter material has a much greater lifetime than boron or U235 lining (used in wide range fission chambers). 

One disadvantage of the self-powered neutron detector is that the emitter material decays with a characteristic half-life. In the case of rhodium and vanadium, which are two of the most useful materials, the half-lives are 1 minute and 3.8 minutes, respectively. This means that the detector cannot respond immediately to a change in neutron flux, but takes as long as 3.8 minutes to reach 63% of steady-state value. This disadvantage is overcome by using cobalt or cadmium emitters which emit their electrons within 10 14 seconds after neutron capture. Self-powered neutron detectors which use cobalt or cadmium are called prompt self-powered neutron detectors. 

A bewildering array of materials has been used as emitters in SMOLEDs since this early work on Alq3. In the following sections, we will present a brief review of host-guest emitter materials and give a perspective description of all the current state-of-the-art small molecule materials for emission at the three primary colors needed for full-color display applications. 

Several groups have studied naphthalene substituted anthracene derivatives as hosts or emitter materials in blue OLEDs (121, 202-205) (Scheme 3.63). The Kodak group used ADN as a host and TBP as a dopant in ITO/CuPc/NPD/ADN TBP/Alq3/Mg Ag [241]. They achieved a narrow vibronic emission centered at 465 nm with CIE (0.154, 0.232) and a luminescent efficiency as high as 3.5 cd/A. In comparison, the undoped device shows a broad and featureless bluish-green emission centered at 460 nm with CIE (0.197, 0.257) and an EL efficiency below 2.0 cd/A. The operational lifetimes of the doped device and the undoped device were 4000 and 2000 h at an initial luminance of 636 cd/m2 and 384 cd/m2, respectively. 

Other derivatives have been reported such as the spiro-linked fluorene-anthracenes (126, 206), which preserve the optical and electrochemical properties of anthracene while reducing the tendency for crystallization and enhancing the solubility and Ts (Scheme 3.64). Highly efficient deep blue OLEDs have been demonstrated by using Spiro-FPAl (206) as an emitter material in a p-i-n type OLED structure ITO/MeO-TPD 2%F4-TCNQ/Spiro-TAD(44)/... 

Bis(dimesitylboryl)-2,2 -bithiophene (BMB-2T, 242) forms a stable amorphous glass and emits pure blue color with a high fluorescence QE of 86% in THF solution [270]. However, an OLED with ITO/m-MTDATA/TPD/BMB-2T/Mg Ag emits with a broad emission due to an exciplex with TPD. The exciplex can be prevented by insertion of a thin layer of 1,3,5-tris(biphenyl-4-yl)benzene (TBB) between TPD and BMB-2T, leading to a pure blue emission. It seems that the boron complex or boron-containing compounds easily form an exciplex with common HTMs. Other similar blue emitter materials also demonstrate such behavior. 

FIGURE 7.8 Molecular structures of several organic emitter materials. Their positions on the diagram are arranged as a function of their emission color with blue emitters on the left, green in the center, and red on the right hand side. 

The reactivity of a surface depends on many factors. These include the adsorption energies of chemical species and their dissociation behavior, their diffusion on the surface, the adatom-adatom interactions, the active sites where a chemical reaction can occur, and the desorption behavior of a new chemical species formed on the surface. The site specificity depends on at least three factors the atomic configuration of the surface, the electronic structures of the surface, and the localized surface field. In atom-probe experiments, the desorption sites can be revealed by a timegated image of an imaging atom-probe as well as by an aiming study with a probe-hole atom-probe, the electronic structure effect of a chemical reaction can be investigated by the emitter material specificity, and the surface field can be modified by the applied field. 

R. W. Whatmore and R. Watton, Pyroelectric Materials and Devices, Published in Infrared Detectors and Emitters Materials and Devices, P. Capper and C. T. Elliott ed., Chapman and Hall, London, 99, 2000. 

For example, rhenium(I) complexes have been used as emitter materials in electroluminescent devices 17-20) and biological probes (11), as dye for dye-sensitized solar cells (21), as chromophores for photochemical electron or energy transfer studies (14,17,22), and as a redox photosensitizer (23). 

In addition, the success of multilayer studies has caused the reexamination of single-layer devices. Although the combinations and contents of well-organized carrier transport and emitter materials resulted in the enhancement of EL efficiency, a higher efficiency than the multilayer structures has never obtained.59,60 Thus, we anticipate that organic/organic interfaces will especially provide efficient carrier recombination sites. 

Here, we describe the design concept of hole-transport materials (HTMs), electron-transport materials (ETMs), emitter materials (EMs), and dopant materials. We also mention the material design for obtaining highly efficient and durable OLED devices. 

Finally, we should mention that the above-mentioned emitter materials are occasionally useful in a DH structure. Moreover, it is possible to form a single-layer EM if we could achieve balanced hole and electron injection and transport by equipping adequate carrier injection electrodes. In fact, BAS96 and BczVBi100 were known to show a fairly high EL efficiency in a single-layer structure. 

FIGURE 2.9. Molecular structures adequate for an EML. Emitter materials are classified into two categories ETM-EM and HTM-EM. 

Approaches for Realizing Stable Blue Emitter Materials. 288... 

From the above results it can be concluded that keto defect sites are preferably formed during polymer synthesis when non- or monoalkylated fluorene species are present in the reaction mixture. This points to the necessity of avoiding even small amounts of these components in order to provide polyfluorenes and polyfluorene-based materials without such centers of degradation and to realize high molecular weight polymers. This prerequisite for polyfluorenes with increased stability can also be transferred to other blue emitter materials as shown, e.g., by Romaner et al. [43] for ladder-type polyparaphenylenes. In this study, again, full alkylation was identified as an important parameter for highly stable materials as it was derived from com-... 

---