Brighter Organics On Display

Most of today’s cell phones, flat-screen TVs, and portable media players rely on liquid-crystal display technology. An alternative based on organic light-emitting diodes (OLEDs) holds the promise of displays that are thinner, lighter, brighter, and flexible—including ones that can be rolled up, worn like a bracelet, and embedded in fabrics and clothing.

But the high cost and low power efficiency of OLED devices have limited their development. A new family of organic compounds may provide a way around those problems.

Researchers in Japan have designed and synthesized low-cost compounds based on carbazolyl dicyano­benzene (CDCB) and show they efficiently emit light in response to an electric current (Nature, DOI: 10.1038/nature11687). The family members, which differ in the number of carbazolyl units and the presence of other organic substituents, emit a wide spectrum of colors ranging from sky blue to orange.

The earliest OLED displays, introduced roughly 25 years ago, were based on all-organic fluorescent materials that inherently convert just a small fraction of electrical energy input to light. OLEDs featuring phosphorescent metal-organic compounds proved to be more efficient emitters, and they are now the standard in this area. Yet they are costly because they include rare metals such as iridium and depend on exotic metal catalysts for their synthesis.

By tailoring the structure of compounds that include electron-donating carbazole groups and electron-accepting dicyanobenzene units, Kyushu University chemists Chihaya Adachi and Hiroki Uoyama and coworkers have now designed metal-free compounds with a tiny energy gap between the molecules’ excited singlet and triplet electronic states. The low-cost, all-organic compounds were made in one step from commercially available starting materials. Because of their electronic structures, the compounds exhibit electroluminescent efficiencies comparable to today’s best phosphorescent OLEDs.

The group has “convincingly demonstrated the molecules’ potential” for use in highly efficient OLEDs, says Brian D’Andrade of Exponent, an engineering and scientific consulting firm. In a commentary about the work, D’Andrade adds that challenges regarding OLED emission colors and operational lifetimes need to be addressed before the compounds can make their way into production.