EFFICIENT BLUE POLYMER LIGHT-EMITTING-DIODES FROM A SERIES OF SOLUBLEPOLY(PARAPHENYLENE)S

Three derivatives of poly(paraphenylene) (PPP) have been synthesized, all with excellent solubility in common organic solvents. Efficient bl ue polymer light-emitting diodes (LEDs) are demonstrated using these P PPs as the semiconducting and luminescent polymers. Double-layer polym er LEDs (consisting of a hole transport layer in addition to the elect roluminescent layer) emit blue light with external quantum efficiencie s between 1% and 3% photons per electron, when using indium tin oxide as the anode and calcium as the cathode. Using internal field emission (Fowler-Nordheim tunneling) of single carrier devices for both electr ons and holes, the energies of the top of the pi band and the bottom o f the pi band have been determined as, respectively, 5.7 and 2.3 eV b elow the vacuum. The operating voltages of these LEDs have been lowere d by using a porous polyaniline anode, or by blending PPP with a hole transport material. LEDs using air stable cathodes, silver, indium, al uminum, and copper, were also demonstrated. Device quantum efficiencie s, between 0.3% and O.8%, were obtained. (C) 1996 American Institute o f Physics.