Thermally activated field assisted carrier generation and transport in N, N′ -di-[(1-naphthalenyl)- N, N′ -diphenyl]-(1, 1′ biphenyl)- 4, 4′ -diamine doped with 2,3,5,6-tetrafluoro- 7, 7′,8, 8′ -tetracyanoquinodimethane

Abstract

Current density-voltage (J-V) characteristics of N, N′ -di-[(1-naphthalenyl)- N, N′ -diphenyl]-(1. 1′ biphenyl)- 4, 4′ -diamine (α-NPD) doped with 2,3,5,6-tetrafluoro- 7, 7′,8, 8′ -tetracyanoquinodimethane have been studied as a function of doping concentration (0-0.8 wt %) and temperature (105-300 K). The current density was found to increase with increase in doping concentration. In the doped samples as field increases above 3.3× 104 V/cm the current abruptly starts increasing at a higher rate, which is ascribed as due to increased free charge carrier generation in the bulk. The enhanced free charge carrier generation is due to field assisted thermal dissociation of donor-acceptor pairs (Poole-Frenkel process) as well as charge injection at the interface. The released carriers increase the charge carrier density which brings the Fermi level near the highest occupied molecular orbital level of the α-NPD and reduces the space charge region near the interface favoring the tunneling of charge carrier across the interface, which is enough to support Ohmic conduction. The carrier generation has been found to be a thermally activated process. At higher fields (i.e., above 1.52× 105 V/cm) the nonlinear J-V characteristics have been explained as due to field dependent mobility of holes. © 2008 American Institute of Physics.