Simulation of OLEDs with a polar electron transport layer

Abstract

Organic light-emitting diodes (OLEDs) rely on the use of functional materials with suitable energy levels and mobilities for selective charge carrier injection and transport of one species only at the respective electrode. Until recently, however, the dipolar nature of many organic semiconductors has been largely ignored in this context. In particular, electron transports layers (ETLs) often exhibit spontaneous orientation polarization leading to interfacial charges that modify the electrical potential landscape inside a hetero-layer device. Here we demonstrate that the effect of polar ETLs can be simulated using the well-established Poisson and drift-diffusion formalism, if these interfacial charges are taken into account. Impedance spectroscopy is used in order to validate our approach and to characterize the polarity of the material. Finally, simulations allow to quantify the impact of polar ETLs on device performance.

The use of charge extraction by linearly increasing voltage in polar organic light-emitting diodes

“…6(c), it would thus be possible to determine the electron mobility in this material. Simulated equilibrium charge and field profiles for this case can also be found in the recent publication by Altazin et al 38 It would then also be feasible that by dipolar doping of Al(7-Prq) 3 into an HTL the hole mobility of the HTL host could be determined (Fig. 6(d)).…”

“…3(c), confirming that there is a driving force for holes to be injected already below V bi . 38 For comparison, Figs. 3(d)-3(f) show the corresponding situation in a standard MIS device.…”

“…39 Drift-diffusion simulations to calculate the CELIV currents and charge density profiles have been generated using the commercial tool Setfos 4.4 by Fluxim AG, 40 which was already successfully used to characterize OSCs 23,30,41 and OLEDs. 38,42 Hereby, the macroscopic polarization was taken into account as demonstrated in our previous publication. 38 …”

“…38,42 Hereby, the macroscopic polarization was taken into account as demonstrated in our previous publication. 38 …”

“…The most commonly used electron transport materials for OLEDs, like Alq 3 , TPBi, BCP, and BPhen, 36,37 exhibit a permanent macroscopic polarization due to spontaneous orientation of molecular dipoles upon layer deposition, forming an effective sheet charge density on both sides of the layer. 38 It has been shown that this polarization effect leads to a reduction of the hole injection voltage V t , dependent on the thickness of the ETL and on the interface charge density, according to the following equation: 33,34 …”