Hole and electron transport in poly(9,9-dioctylfluorene) and poly (9,9-dioctylfluorene- co -benzothiadiazole)

Abstract

Time-of-flight photocurrent (TOF) measurements on the blue emitting electroluminescent polymer poly(9,9-dioctylfluorene) (PFO) show that the room temperature hole mobility can vary from 10^-2 to 5x10^-5 cm²/Vs depending on how the polymer films are prepared. It also undergoes irreversible increases when the samples are annealed. These results can be related to PFOs complex phase behavior and show the importance of understanding and controlling the polymer film structure for device applications. We also present new TOF measurements on the green emitting electroluminescent polymer poly(9,9-dioctylfluorene-<i>co</i>-benzothiadiazole) (BT). Previous TOF measurements have shown that BT exhibits dispersive electron transport and that holes are very heavily trapped, no hole transport signal being measurable using this technique. The new TOF measurements on a recently synthesized batch of BT show less dispersive electron transport with a mobility of 10^-3 cm²/Vs as well as non-dispersive hole transport with a mobility of 2 x 10^-3 cm²/Vs. This new batch therefore exhibits the highly desirable property of both good electron and hole transport in the same electroluminescent polymer. It is proposed that this is due to a change in the molecular weight and/or polydispersity of the material and indicates the importance of further development of relatively well known materials. TOF measurements of the variation of the hole and electron mobility with temperature are examined within the framework of the Gaussian disorder model.