Efficient blue polymer light-emitting diodes from a series of soluble poly(paraphenylene)s

Three derivatives of poly(paraphenylene) (PPP) have been synthesized, all with excellent solubility in common organic solvents. Efficient blue polymer light-emitting diodes (LEDs) are demonstrated using these PPPs as the semiconducting and luminescent polymers. Double-layer polymer LEDs (consisting of a hole transport layer in addition to the electroluminescent layer) emit blue light with external quantum efficiencies 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 electrons and holes, the energies of the top of the pi band and the bottom of the pi* band have been determined as, respectively, 5.7 and 2.3 eV below the vacuum. The operating voltages of these LEDs have been lowered by using a porous polyaniline anode, or by blending PPP with a hole transport material. LEDs using air stable cathodes, silver, indium, aluminum, and copper, were also demonstrated. Device quantum efficiencies, between 0.3% and O.8%, were obtained. (C) 1996 American Institute of Physics.