Kinetic Monte Carlo Study of the Sensitivity of OLED Efficiency and Lifetime to Materials Parameters

Abstract

wileyonlinelibrary.comand excitonic processes and determining the values of the relevant material-specifi c parameters, ii) supporting the layer stack design process by providing an effi cient route for developing understanding of the benefi ts and disadvantages of the use of novel materials and layer stack concepts. The third (vertical) axis in the fi gure indicates examples of the performance characteristics which may be studied using the simulations. We envisage that simulations can be used to clarify the sensitivity of the OLED device performance to materials and layer stack parameters, and can provide insights in the functioning of OLEDs beyond the capability of experiments. For example by a much more precise control of the device structure, by providing insight in all processes at an ultimate (molecular-scale) spatial resolution and at the shortest relevant time scales, and by providing the opportunity to explore the performance under conditions which are experimentally not realizable. A long-term goal of our research is therefore to develop and apply a simulation method which can address any combination of the three application types, and which provides a platform within which novel insights as obtained in each of the fi elds indicated in the fi gure can be readily adopted.So far, commonly used OLED simulations methods are based on one-dimensional drift-diffusion approaches (see Coehoorn and Bobbert and references therein) [ 2 ] in which the charge transport and excitonic processes are included in a semi-empirical manner. However, it is well-known that as a result of the energetic disorder, resulting from the structural disorder, the current density in OLEDs is fi lamentary, rather than uniform. [ 3 ] We have shown from molecular-scale three-dimensional kinetic Monte Carlo (3D-KMC) modelling that, as a consequence, the emission takes place on certain preferred molecular sites, rather than being laterally uniform. [ 4 ] It is thus necessary to develop a full 3D-KMC model for accurately simulating the effect of the loss processes, such as triplet-polaron quenching and triplet-triplet annihilation in phosphorescent OLEDs. These losses lead to an effi ciency decrease with increasing current density and luminance (roll-off of the internal quantum effi ciency, η IQE ). We have recently extended the 3D-KMC method to include the charge transport and all excitonic processes in an integral manner. [ 5 ] The simulations were found to provide a good description of the roll-off of archetypical green and red emitting OLEDs studied intensively in the literature, [ 6 ] and were used to The performance of organic light-emitting diodes (OLEDs) is determined by a complex interplay of the optoelectronic processes in the active layer stack. In order to enable simulation-assisted layer stack development, a threedimensional kinetic Monte Carlo OLED simulation method which includes the charge transport and all excitonic processes is developed. In this paper, the results are presented of simulations including degradation processes...

Förster-type triplet-polaron quenching in disordered organic semiconductors

“…The excitons are allowed to decay with default radiative and nonradiative lifetimes τ rad,0 = 1.84 μs and τ nr,0 = 5.52 μs, respectively, so that the effective lifetime is τ 0 = 1.38 μs. These are realistic values for the phosphorescent orange-red emitter bis(2-methyldibenzo[f,h]quinoxaline)(acetylacetonate)-iridium(III) [Ir(MDQ) 2 (acac)] [20]. However, the effect of varying τ while keeping the ratio τ nr /τ rad fixed was also studied.…”

Generalized Kinetic Monte Carlo Framework for Organic Electronics

“…In contrast to the Förster rate, the Dexter type energy transfer rises due to the exchange interaction and may occur between next neighbors [92]. Following [22], the Dexter type hopping is modeled as a Miller-Abrahams hopping defined by:…”

“…A separation rate a s exs = 1/τ s exs is enabled if the singlet is located at an interface. Dissociation of a triplet exciton is interpreted as an electron hopping from its current position towards an unoccupied neighboring site [21][22][23]. The separation rate a t exs is calculated with the Miller-Abrahams rate using an energy difference of ∆E = ∆E LUMO + E CT,B − E t B .…”

“…Another limitation of existing models is the simplified treatment of excitonic processes, which are of high importance regarding organic devices. Most kMC models either account for singlet [10,[17][18][19] or triplet exciton dynamics [20][21][22][23] for the use in OSCs or OLEDs, respectively. However, the interplay between singlets and triplets can be used to modify exciton lifetimes and diffusion lengths, which is a promising way to tailor efficiencies in organic devices and therefore requires a better understanding [24][25][26].…”

Simulating charge transport in organic semiconductors and devices: a review

“…They also show the more experimentally-accessible luminance vs. lifetime relationship, showing that lifetime reduces with increasing current (Figure 9a) and disorder ( Figure 9b). Coehoorn et al [442] have also used a KMC model to examine degradation in OLEDs, and found that exciton-polaron quenching was the major cause of both internal quantum efficiency (IQE) roll-off and degradation. This in turn allowed development of an analytical expression that relates the lifetime to parameters available in steady-state measurements.…”