Dipolar alignment and consequent enhanced charge transport in poly (9, 9′ di octyl fluorene)-2, 7-ylene ethylnylene

Abstract

Current density-voltage characteristics of poly (9, 9′ di octyl fluorene)-2,7-ylene ethylnylene thin films (∼120 nm) have been studied in hole only device configuration at different temperatures (290-100 K) in unpolarized and polarized samples. The hole mobility has been found to be enhanced as a result of dipolar alignment by exposure to a dc electric field via cooling at all elevated temperatures. At higher field, current density has been found to be governed by trapped charge limited currents (TCLC) with hole mobility strongly dependent on electric field and their respective charge transport parameters have been obtained for both samples. The density of trap states has been found to be decreased on polarization from 1.1 × 10 18 to 7.6 × 1017 cm-3 and trap energy has correspondingly decreased from 43 to 35 meV. The TCLC model with Poole-Frenkel-type field-dependent mobility has been fitted into the data and found to be in excellent agreement. Temperature dependence of zero field mobility (0) and disorder parameter (σ) also has been estimated. We conclude that the relatively higher hole mobilities may be due to the orientational ordering of polar molecules and displacement of excess charges. © 2011 American Institute of Physics.