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standards? The answer lies in doping phosphorescent dyes into the organic layer. The abundant organic material is known as the “host.” The other much less profuse phosphorescent dye is the “guest”. These two parts are made to work together to harvest both singlet and triplet excitons. An internal quantum efficiency of 100% is theoretically possible. The process of luminescence in an OLED begins when voltage is applied. The cathode injects electrons into the organic host/guest mixture as the anode injects holes; excitons are created as these charge carriers meet. Since the host compound is more abundant, the majority of the excitons are formed there. Exciton decay is an extremely fast process in the fluorescent host. It is so fast (10—15 seconds) that the guest materials absorb nearly all of the host’s emitted photons. The newly formed excitons in the guest emit light via phosphorescence. The guest material is specifically designed to promote phosphorescence. Normally, the decay of triplets is a spin-forbidden process. To solve this problem, the guest is made of an organic heavy metal-ligand complex. Large atomic weight metals such as Osmium, Iridium, or Platinum are used in these important molecules. They possess the ability to give the excited triplet electrons singlet-like character. Spin-orbit coupling from the heavy metal ligand complex produces the effect. This intersystem crossing makes triplet decay an allowed process. The guest material can, thus, phosphoresce and produce very efficient emission. Why is a host even needed if the guest material actually produces the useful emission of the device? Answer: The π-π interaction between ligands in the guest molecules causes them to form aggregates as they become more concentrated. This quenches the light and decreases device performance. The purpose of a host is to dilute the guest enough to eliminate this problem. The objective of this research in particular, is to develop host materials that fluoresce in the UV-blue region and have higher triplet energy levels than those of the guest. Such a difference in bandgap will facilitate an exothermic energy transfer from the host to the guest. The larger bandgaps of the host will also ensure higher triplet energy levels to prevent a back energy transfer. The efficiency of light emission will decrease if the triplet energy level of the host is lower than that of the guest. As of now, creating such host molecules is a very challenging task in research. Most of the host materials available now have relatively low triplet energy levels, which are not suitable for the blue phosphors. Figure 5. Chemical structure of host materials A series of materials (Figure 5) have been synthesized as hosts for the phosphorescent metal complex. DiCzPF was synthesized by the Ullman coupling reaction between 2,7-dibromo- 9,9-diphenyl fluorene and carbazole, while DiCzBPF was synthesized by the Suzuki coupling between 9,9-diphenyl fluorene diboronic ester and 3,5-dicarbazole-1-bromo benzene. Two phenyl groups at the 9 th position of the fluorene give the molecules a certain rigidity that keeps them from forming aggregates. Methods to determine whether a compound would be a potentially good host material are to take UV-Visible and Photoluminescent (PL) Spectroscopy. UV-Vis indicates the wavelengths of light a compound absorbs. A PL spectrum shows the wavelengths that are emitted. The onset of the UV spectrum can be used to determine a material’s bandgap. The UV-Vis and PL spectra comparing the emissions and absorptions of the two compounds are shown below (Figure 6). Although DiCzBPF (tetraphenyl) has a much longer conjugation length comparing with DiCzPF (biphenyl), the onsets of their UV spectra are very similar, indicating that these two molecules have similar bandgaps (~3.3eV). Both molecules have emission in the UV-blue region. 52 CMDITR Review of Undergraduate Research Vol. 1 No. 1 Summer 2004
Figure 6. UV and PL spectra of the host materials. The emission of DiCzPF only slightly blueshifted (5 nm) relative to DiCzBPF. UV and PL measurements demonstrate that increasing the conjugation length in DiCzBPF does not significantly affect their energy levels. This may be because the bulky dicarbazole benzene units in DiCzBPF can significantly twist its conjugation out of the plane. The preliminary results from UV and PL study show that these two molecules are potentially good candidates to be used as the host for the blue phosphorescent OLEDs. Acknowledgements National Science Foundation (NSF) Center on Materials and Devices for Information Technology Research (CMDITR) University of Washington (UW) References 1. Köhler, A., J. Wilson and R. Friend. 2002. Fluorescence and phosphorescence in organic materials. Advanced Materials. 10:701-707. 2. Optoelectronics Industry Development Association (OIDA). 2002, Organic light emitting diodes (OLEDs) for general illumination update 2002. [Online]. August. 20. http://lighting.sandia.gov/lightingdocs/OID A_SSL_OLED_Roadmap_Full.pdf 3. Parthasarathy, G., J. Liu and A. Duggal. 2003. Organic light emitting devices from displays to lighting. The Electrochemical Society Interface. Summer. 42-47 CMDITR Review of Undergraduate Research Vol. 1 No. 1 Summer 2004 53
Page 1 and 2: CMDITR Review of Undergraduate Rese
Page 3 and 4: Welcome to the premier issue of the
Page 5 and 6: Extended Abstracts Table of Content
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Page 11 and 12: Acknowledgements Brian Ratliff, Gra
Page 13 and 14: Figure 2. Synthesis of the dendrime
Page 15 and 16: Toward the Synthesis of a Magnetic
Page 17 and 18: Results and Discussion Synthesis of
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Page 21 and 22: References 1. (a) MacDiarmid, A.G.
Page 23 and 24: suggests that any effects of these
Page 25 and 26: Electrospinning of Piezoelectric Ma
Page 27 and 28: varies and is close to the critical
Page 29 and 30: Synthesis of Azaindoly-phenanthroli
Page 31 and 32: Experimental Physical Measurements.
Page 33 and 34: [60] fullerene enable close packing
Page 35 and 36: Crystallization. We attempted to cr
Page 37 and 38: number of solvents including aceton
Page 39 and 40: References 1. Dalton, L.R. 2003. No
Page 41 and 42: though the beads were moving becaus
Page 43 and 44: Feasibility of Hydroxy-Chalcone Lin
Page 45 and 46: absorbance is proportional to conce
Page 47 and 48: Results Linear Dichroism Metripol c
Page 49 and 50: Pararosaniline Xylene Cyanole FF Cr
Page 51: the source of the light emitted by
Page 55 and 56: Method The hybrid chromophore will
Page 57 and 58: Results The ligand was synthesized
Page 59 and 60: Following the cooling down, the pro
Page 61 and 62: Two-Photon Induced Free Radical Pol
Page 63: Future Work When the results of the

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