Hole transport in the organic small molecule material α-NPD: evidence for the presence of correlated disorder

Abstract

In this paper the hole mobility in the amorphous small molecule material N,NЈ-bis͑1-naphthyl͒-N,NЈ-diphenyl-1 , 1Ј-biphenyl-4 , 4Ј-diamine ͑␣-NPD͒, which is frequently used in organic light-emitting diodes, is studied. From an analysis of the temperature and layer thickness dependence of the steady-state current density in sandwich-type ␣-NPD-based hole-only devices, it is found that a conventional mobility model assuming a Poole-Frenkel type field dependence and neglecting the carrier density dependence is not appropriate. Consistent descriptions with equal quality are obtained within the framework of two forms of the Gaussian disorder model ͑GDM and CDM͒, within which the presence of energetic disorder is described by a Gaussian density of states and within which spatial correlations between the site energies are absent or are included, respectively. Both models contain a carrier density dependence of the mobility. Based on a comparison of the site densities as obtained from both models with the molecular density, we argue that the analysis provides evidence for the presence of correlated disorder.

Ab initiocharge-carrier mobility model for amorphous molecular semiconductors

“…In this model, the value C = 0.25 is obtained. The simulated values of σ were 0.130 eV (α-NPD) and 0.227 eV (Alq 3 ), with the former value close to the EGDM value found in the modeling of α-NPD devices [12], but incompatible with the ECDM value. The GEMM prediction of the T dependence of the mobilities was found to agree rather well with experiment, even at low T .…”

“…The c dependence of both models was shown to be essentially the same [11]. Good agreement between measured T -dependent current density-voltage (J -V ) characteristics of hole-only devices of α-NPD [N,N -Di(1-naphthyl)-N,N -diphenyl-(1, 1 -biphenyl)-4,4 -diamine] and those calculated with both the EGDM and ECDM could be obtained, but with different values of σ and N t : σ = 0.14 eV, N t = 0.20 × 10 27 m −3 for the EGDM and σ = 0.10 eV, N t = 3.7 × 10 27 m −3 for the ECDM [12]. Because for the ECDM N t was closer to the actual density of molecules than for the EGDM, it was suggested that site energies in α-NPD are spatially correlated.…”

Monte Carlo study of charge transport in organic sandwich-type single-carrier devices: Effects of Coulomb interactions

“…On the other hand, a similar comparison of EGDM and ECDM modeling of current-voltage characteristics of hole-only devices of the molecule N,N -bis(1-naphthyl)-N,N -diphenyl-1,1 -biphenyl-4,4 -diamine (α-NPD) and of electron-only devices of the molecule bis(2-methyl-8-quinolinolato)(4-phenylphenolato) aluminum (BAlq) shows that the intersite distance as obtained from a fit with the ECDM modeling is more realistic than the one obtained with the EGDM modeling. 12,13 This work suggests that for the investigated polymers and small-molecule semiconductors the EGDM and the ECDM, respectively, are thus the best models to describe charge transport.…”

Electric field dependence of charge carrier hopping transport within the random energy landscape in an organic field effect transistor

“…22,23 This model has been applied to describe the charge carrier mobility measured in an organic sandwich-type diode device. [23][24][25] Note that the electric field in such diode devices is normally much larger than the lateral electric field in an OFET due to very different interelectrode distances.…”