Polydispersity Effects on Conjugated Polymer Light-Emitting Diodes

This paper reports the effects of polydispersity in conjugation length on polymer light-emitting diode (PLED) device performance. For this study a di-alkoxy substituted pentamer of p -phenylenevinylene (5PV) was blended with a small amount of a lower band gap poly( p -phenylenevinylene) (PPV) or a small amount of a di-alkoxy substituted p- phenylenevinylene type nonamer (9PV). We also fractionated two different di-alkoxy substituted oxadiazole-PPVs and blended the high molecular weight, lower band gap fraction, into a matrix of the lower molecular weight, higher band gap, polymer. It was found that incorporation of a small amount of the low band gap material significantly degraded PLED device performance. To further test the significance of polydispersity we synthesized a low molecular weight narrow polydispersity PPV. Devices made with this PPV were about 2 orders of magnitude better in LED external quantum efficiency when compared to a high molecular weight, higher polydispersity PPV of identical structure. These findings are significant in that they point to the importance of controlling the polydispersity in conjugation length for obtaining efficient PLEDs. They also indicate the importance of controlling polydispersity when comparing one polymer structure to another.