Number 437 (Story #2), July 2, 1999 by Phillip F. Schewe and Ben Stein|
ELECTROPHOSPHORESCENCE GETS THE GREEN LIGHT In organic light emitting devices (OLEDs) electrical energy injected onto a host molecule is often transferred to luminescent "guest" molecules which then light up. Using this approach, OLEDs have been fabricated to emit colors ranging from violet to the near infrared and have been incorporated into displays already on the market. So far OLED researchers have concentrated on maximizing fluorescent emission of light. Fluorescent OLEDs use a process whereby the energy transfer occurs between a singlet-state (total spin of zero) host molecule and a singlet-state guest molecule. Phosphorescent OLEDs, by contrast, transfer energy from a triplet-state (total spin value of one) host to a triplet-state guest, which subsequently emits the energy as light. Phosphorescence is inherently a slower and less efficient process, but triplet states constitute the majority of electrically excited states, so putting them to work can increase the overall luminescence. This is exactly what scientists at Princeton (Stephen Forrest, firstname.lastname@example.org, 609-258-4532) and the University of Southern California have now done. Using both singlet and triplet states for producing green light, they have achieved quantum efficiencies (photons out divided by electrons in) of up to 8% and power efficiencies (optical power out divided by electrical power in) of up to 30 lumens/Watt. These high efficiencies are unprecedented and may have a great impact on display technology. (Baldo et al., Applied Physics Letters, 5 July 1999).