New heights for organic LEDs
Indium lines up for fame ELECTRONIC MATERIALS
Carbon-based semiconducting polymers exploit the virtues of both semiconductors and plastics. However, the report of a new synthesis [Hill et al., Science (2006) 311, 1904] brings in another periodic group to be explored for molecular semiconductor applications. Carbon (and, to a lesser degree, Group 14) is well known for the ease with which it forms extended homonuclear chains, or catenates, contributing to the delocalization of electrons and the semiconducting properties of the molecule. Other elements do not catenate so easily, restricting the number and diversity of polymers that are possible. But by reacting indium iodide with N-[4-(3,5-dimethylphenylamino)pent-3-en2-ylidene]-3,5-dimethylbenzenamine in the presence of a potassium base and THF in a ‘one-pot’ synthesis, Michael S. Hill and colleagues at Imperial College London and the University of Sussex in the UK have produced a catenated chain of In atoms, the first Group 13 chain to be synthesized. It is energetically difficult to get Group 13 atoms to catenate, because of their increased atomic weight. In fact, based on their previous work on In and Sn compounds, the researchers were expecting a cyclic oligomer. Instead, X-ray crystallography shows that the molecule is composed of a partially helical open chain of six In centers capped at the ends with iodine. Each In atom is bound to a single large β-diketiminate ligand, but unsupported by any bridging ligands. Density functional theory calculations based on the X-ray structure support absorption spectroscopy evidence for electron delocalization. Hill believes that the creation of this hexaindium oligomer broadens the
Central chain of In centers (violet) capped by iodine (red) and surrounded by β-diketaminate ligands of nitrogen (blue) and carbon (black). [Courtesy of Mike Hill (Murray Roberts-Visual Elements).]
horizons for future syntheses involving main group metals, which he refers to as “using the periodic table as a paint palette”. The ease and reproducibility of the synthesis bodes well for creating longer oligomers and polymers. In common with organic polymers, the synthesis does not require high temperatures, and because the procedure is entirely solution based, would enable spray-coating in the future and make it easy to produce thin films. While there is no immediate application for the In oligomer, it represents a huge step toward integrating inorganics with organic semiconductors. With the rise of organic polymers, many inorganic synthetic routes have faded but, as Hill notes, many of these methods are as relevant as ever for pursuing further research into inorganic oligomers and polymers. D. Jason Palmer
Polymers surprisingly similar to metals POLYMERS Researchers from the University of California at Santa Barbara, Pusan National University, and Ajou University in Korea have synthesized a polymer that shows truly metallic transport behavior typical of a conventional metal [Lee et al., Nature (2006) 441, 65]. The polymer, polyaniline (PANI) in the emeraldine base (EB) form, was prepared using a process that suppresses undesirable side reactions and produces high quality samples with a low density of structural defects. The PANI-EB samples are then doped with camphor sulfonic acid (CSA). The PANICSA samples show two of the signatures typical of metallic behavior. Firstly, this polymer, unlike other conducting polymers, shows a monotonic decrease in resistivity with temperature. At room temperature, the electrical conductivity of PANI-CSA is in excess of ~103 S/cm, but increases by a factor of ~2.5 as the temperature is lowered to 5 K. Secondly,
JUNE 2006 | VOLUME 9 | NUMBER 6
reflectance measurements of PANI-CSA in the infrared adhere to the Drude model typical of a metal, without having to take account of disorder-induced localization theory. The researchers attribute the metallic behavior of PANI-CSA to the improved molecular structure of the material compared with other polymers, which have both molecular-scale disorder and structural inhomogeneities at mesoscopic scales. X-ray diffraction data indicate that the polymer has improved interchain stacking and a more planar chain conformation. The researchers suggest that improving the quality of known conducting polymers could enhance their behavior in applications. However, in a commentary on the work, Richard Friend of the University of Cambridge [Friend, Nature (2006) 441, 37] cautions that the results also indicate the limits of conductivity for such polymers.
Organic light-emitting devices (OLEDs) could revolutionize lighting and significantly improve efficiency compared with incandescent lighting. Two new research findings have important implications for the field. Researchers from Princeton University and the University of Southern California, Los Angeles have designed a new device architecture for allphosphor-doped devices that offers the potential for 100% internal quantum efficiency [Sun et al., Nature (2006) 440, 908]. This type of device currently uses phosphorescent molecules to harness triplet excitons, which make up three-quarters of the bound electron-hole pairs and would otherwise recombine nonradiatively. The researchers replace the phosphorescent dopant with a blue fluorescent molecule in conjunction with green and red phosphor dopants. The singlet energy is therefore harnessed by the blue fluorescent dopant, while the remaining triplet energy is harvested by green and red phosphorescent dopants. By removing the energy loss associated with having a blue phosphorescent dopant, power efficiency can be improved by ~20%. The alternative to small-molecule devices is to use polymers, but their performance has typically lagged behind. Now researchers from the Universities of Köln and Potsdam in Germany have demonstrated red, blue, and green multilayer polymer electrophosphorescent devices, which can be solution processed, that exhibit efficiencies comparable to smallmolecule devices [Yang et al., Adv. Mater. (2006) 18, 948]. "This is the first example of a solutionbased OLED with an internal quantum efficiency of close to 100% (external quantum efficiency of 18.8%)," says Dieter Neher from Potsdam.