EFFECTS OF POLYMERIC ELECTRON TRANSPORTERS AND THE STRUCTURE OF POLY(P-PHENYLENEVINYLENE) ON THE PERFORMANCE OF LIGHT-EMITTING-DIODES

A series of new electroactive monomers containing 2,5-diphenyl-1,3-oxazole, 2,5-diphenyl-1,3,4-oxadiazole, and 3,4,5-triphenyl-1,2,4-triazole heterocycles have been synthesized in good yield. These monomers were incorporated as either pendant groups or directly into the backbone of 10 high molecular weight polymers [poly(arylmethacrylate), poly(arylmethacrylamide), poly(aryl formal), and poly(aryl ether)]. The polymers appear to be amorphous and exhibit glass transition temperatures in the range 115-208 degrees C, and most have excellent thermal stability in air (decomposition > 400 degrees C). Thin, clear, pinhole free-films are readily deposited on a variety of substrates (e.g., silicon, quartz) by spin coating. These materials were used as the electron transport (ET) layer in light-emitting diodes (LEDs) having an ET layer deposited on PPV with aluminum and indium tin oxide electrodes (i.e., Al/ET layer/PPV/ITO). The ET materials contain as much as 97 mol % of the electroactive moiety, while conventional electron transporters (e.g., PBD dissolved in PMMA) contain 46 mol %. LEDs containing these ET polymers were much more stable than devices without an ET. Many were also more stable than those having a conventional electron transporter. Relative to LEDs without ETs, the internal quantum efficiencies using ETs were higher in some cases and lower in others. In addition to varying the ET layer, two different types of PPV (crystalline and amorphous) were also used to construct four different types of devices. In terms of diode efficiency, the most important factor is the PPV conjugation length and not the type of ET used. The internal quantum efficiencies ranged from 0.2 to 0.0004%. Finally, the current/voltage curves of some of the LEDs were fitted to four different models in order to determine which best describes the device physics.