Dependence of the performance of light-emitting diodes on the molecular weight of the electroluminescent polymer PFO-MEH-PPV

Controlled synthesis of the electroluminescent polymer PFO-MEH-PPV (poly[(9,9-dioctyl-2,7-divinylenefluorenylene)-alt-co-(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene)]) provided samples of varying molecular weight (Mw) in the range 20-360 kDa, as determined by gel-permeation chromatography and light scattering. The samples were used as the active layers in organic light-emitting diodes (OLEDs), and the performanceofthe devices was examinedasafunctionofMw. Turn-on voltages fellinthe range 1.92-2.78 V, luminances varied from 231 to 5826 cd/m (2), and luminous efficacies ranged from 0.06 to 0.90 lm/W. The emitted colour was found to vary from green to yellow as Mw increases. Optimal performance was attained by using PFO-MEH-PPV with Mw = 100 kDa. To help reveal how Mw determines the performance of OLEDs, relative quantum yields of photoluminescence in solutions and films were measured, and films were characterized by atomic force microscopy and transmission electron microscopy. Key words: organic light-emitting diodes (OLEDs), electroluminescent polymers, molecular weight, photoluminescence quantum yield. La synthese controlee du polymere electroluminescent PFO-MEH-PPV (poly[(9,9-dioctyl-2,7-divinylenefluorenylene)-alt-co-(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene)]) a permis de generer des echantillons de divers poids moleculaires (Mw) dans l'intervalle de 20 a 360 kDa, que nous avons determines par chromatographie sur gel permeable et diffusion de la lumiere. Nous avons utilise ces echantillons comme couches actives dans des diodes electroluminescentes organiques (DELO) dont nous avons examine la performance en fonction du Mw. Les valeurs de tension d'activation se trouvaient dans l'intervalle de 1,92 a 2,78 V, les valeurs de luminance variaient de 231 a 5826 cd/[m.sup.2] et les valeurs d'efficacite lumineuse, de 0,06 a 0,90 lm/W. Nous avons observe que la couleur emise passe duvertau jaunea mesure queleMwaugmente. La performance optimale a ete atteinte avecle PFO-MEH-PPV de Mw = 100 kDa. Afin de mieux comprendre la maniere dont le Mw determine la performance des DELO, nous avons mesure les rendements quantiques relatifs de photoluminescence en solutions et dans les films, et nous avons caracterise ces derniers par microscopie a force atomique et microscopie electronique en transmission. [Traduit par la Redaction] Mots-cles : diodes electroluminescentes organiques (DELO), polymeres electroluminescents, poids moleculaire, rendement quantique de photoluminescence.
Introduction The world needs renewable sources of energy to reduce dependency on fossil fuels, and efforts must also be made to moderate the increasing global demand for energy. At present, [...]