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(a) Sketch of the OLED stack that is made with three different emissive layer (EML) matrix materials. (b) Eigenvectors for the emission zone obtained by SVD for a 120 nm thick ETL, shapes for other ETL thicknesses are qualitatively similar. (c) Four largest eigenvalues plotted in log scale versus ETL thickness. Curve styles in (b) and (c) depict corresponding eigenvectors and eigenvalues.
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The position of light-emitting molecules can be identified using interferometric approaches. Standard schemes utilize constructive interference to obtain a sectioned area of interest with high detection efficiency. The examination of organic light-emitting diodes (OLED) removes the common constraint of low light levels and enables a more generalize...
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Citations
... The air mode is one of the optical modes in OLEDs, which is coupled out into the air. There have been plenty of research results on the optical modeling and experimental measurement of the air mode, which discuss the device optimization [10][11][12], the analysis of the emission zone profile [13][14][15][16][17], as well as the emitter orientation in OLEDs [18][19][20][21]. In particular, the internal profiles of the emission zone were analyzed in OLEDs with respect to the dipole orientation and light polarization in the combination of in situ far-field measurement and optical reverse simulation [13][14][15][16][17][18][19][20][21]. ...
... There have been plenty of research results on the optical modeling and experimental measurement of the air mode, which discuss the device optimization [10][11][12], the analysis of the emission zone profile [13][14][15][16][17], as well as the emitter orientation in OLEDs [18][19][20][21]. In particular, the internal profiles of the emission zone were analyzed in OLEDs with respect to the dipole orientation and light polarization in the combination of in situ far-field measurement and optical reverse simulation [13][14][15][16][17][18][19][20][21]. In these investigations, the output optical power of the air mode was calculated on the basis of the electromagnetic methods, such as the Fabry-Perot formulation, source-term method, or point dipole model, which proved to be mathematically equivalent for the air mode [9]. ...
We propose a theoretical formulation to calculate the internal profile of the air mode in the organic light-emitting diode (OLED) on the combination of the transfer matrix method and source-term method. The spatial distributions of the air mode are calculated in a top-emitting OLED with respect to the light polarization, extraction angle, dipole orientation, and dipole position. Air modes are also calculated on the basis of the previously used external source model, where the input optical wave is injected from the air into the OLED multilayer. Comparison of the calculated air modes between two models checks the validity of the external source model. In addition, we propose an improved formula to determine the optimal emitter positions that maximize the two-beam interference of the micro-cavity effect. In the improved formula, a non-ideal reflection phase shift at a reflective metal anode is treated as the skin depth of the air mode. Finally, the effect of the dipole orientation on the air mode is investigated. Compared with the air mode emitted by the horizontally oriented dipole, the air mode generated by the vertically oriented dipole has relatively small intensity and shows the opposite dependence of the emitter position variation. The calculation results of the internal profile of the air mode within the emission layer are matched with the profile of the emission zone obtained by output radiant flux on the basis of the currently used point dipole model.
... Additional deposition runs have been performed to access tooling factors and layer thicknesses, where the ETL's thickness is the most important parameter for accurate optical reverse simulations. The design thickness of the ETL needs to be optimized for ideal emission zone estimation of the TE polarized pattern [16] as well as for optimum visibility of perpendicular emitters in TM polarization [15]. Both considerations are plotted in Figure 2 and yield an optimum ETL thickness for emission zone analysis of 134 nm (Figure 2a), while the best visibility of perpendicular emitters is predicted in the 140…145 nm range (Figure 2b). ...
... The simulation relies on the dyadic Green's function approach [17] implemented for OLED modelling in the tool RadiatingSlabs as reported previously [4,7,9,15,16,18]. This numerical model includes arbitrary orientation distributions, Purcell effect consideration as well as multiple incoherent reflexes in the substrates of large area sources. ...
... In a first step, the TE-polarized emission patterns are analyzed to extract the emission zone of the device [16], thereby deriving the intrinsic spectrum as well [21]. Subsequently, the ensemble orientation is derived from the TM polarized emission pattern while assuming the emission zone and intrinsic spectrum as obtained from the first step. ...
Spontaneously aligning emitter molecules can improve the external efficiency of OLED devices and have been reported for different types of emitters doped into suitable hosts. Similar to the morphological alignment of the active molecules, a birefringence of the host affects the apparent emitter orientation. In order to investigate this effect experimentally, Ir(ppy)3 is co-evaporated into the emitting layer of OLED using hosts with different birefringence properties. Analyzing
the electroluminescence emission patterns quantitatively reveals the apparent orientation distribution of the emitters. The results emphasize that host birefringence can be used to suppress perpendicular emitter contributions while amplifying the impact of the horizontal dipoles, thus leading to an enhanced forward emission.
... Additional deposition runs have been performed to access tooling factors and layer thicknesses, where the ETL's thickness is the most important parameter for accurate optical reverse simulations. The design thickness of the ETL needs to be optimized for ideal emission zone estimation of the TE polarized pattern [16] as well as for optimum visibility of perpendicular emitters in TM polarization [15]. Both considerations are plotted in Figure 2 and yield an optimum ETL thickness for emission zone analysis of 134 nm (Figure 2a), while the best visibility of perpendicular emitters is predicted in the 140…145 nm range (Figure 2b). ...
... The simulation relies on the dyadic Green's function approach [17] implemented for OLED modelling in the tool RadiatingSlabs as reported previously [4,7,9,15,16,18]. This numerical model includes arbitrary orientation distributions, Purcell effect consideration as well as multiple incoherent reflexes in the substrates of large area sources. ...
... In a first step, the TE-polarized emission patterns are analyzed to extract the emission zone of the device [16], thereby deriving the intrinsic spectrum as well [21]. Subsequently, the ensemble orientation is derived from the TM polarized emission pattern while assuming the emission zone and intrinsic spectrum as obtained from the first step. ...
Aligned emitters increase OLED outcoupling but their orientation averaging has not yet been studied. This averaging is measured after the introduction of plasmonic losses in the emitter’s near field causing orientation dependent lifetime changes.
... On the other hand, the spatial distribution of light generation in OLEDs, often referred to as the emission zone (EMZ), is subject to continuous investigation for many reasons [19][20][21][22]. Simulation studies of the EMZ are important, as they provide design-based engineering of OLED stacks, to estimate the internal electro-optical conversion efficiency from far-field measurements [23][24][25]. The importance of adapted stack architectures to observe the origin of emission accurately has been pointed out recently [26][27][28][29][30][31]. ...
The outcoupling efficiency of a hybrid OLED device with a hole-injection layer formed by either an organic semiconducting material, namely PEDOT:PSS, or an under-stoichiometric molybdenum oxide is examined. Because the device is considered for the three primary colour emissions, the active layer is composed of a wide band-gap blue emitting host polymer doped with either phosphorescent or fluorescent guest emitters. Critical parameters, such as the emission zone profile for the doped active layer with either the organic hole-injection layer or the inorganic metal oxide, are studied. Contour charts have been derived providing the areas of maximum outcoupling efficiency with the corresponding thickness pairs of the active layer and the hole-injection layer. (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
... 12,[15][16][17][18][19][20][21] These methods heavily utilize fitting procedures, which may yield highly resolved evaluation, however, usually require extensive data sets, and naturally rely on advanced numerical techniques, which tend to obscure the underlying physical phenomena. 19,22,23 In recent work, we have presented a different approach to this problem, developing analytical closed-form formulae to extract the emission zone location from measured emission pattern extrema, assuming the excitons are concentrated in a very narrow region. 24 We have shown therein that the angles in which maximum or minimum emission occurs are related to the emitter location via a generalized Bragg's condition, which stems from the interference between the radiating source and its image, induced by the reflecting cathode. ...
... The method is based on three analytical steps, employed on the measured transverse electric (TE) polarized emission pattern, 27 which is less sensitive to the dipole orientation. 18,20,22 First, we apply a simple division operation to the measured emission pattern to isolate the image-source (IS) interference term. Second, we apply a simplified form of our previous theory to determine the mean value of the exciton spatial distribution from Bragg's condition. ...
... Before we dive into the rigorous formulation, it is worthwhile to emphasize the two main merits of our approximate method, in view of the availability of highly accurate numerical tools. 12,[15][16][17][18][19][20][21][22] First, for some OLED engineering tasks, the complexity involved in employing the numerical methods is not very cost effective. For initial design stages and routine verification processes, for instance, it seems that a more intuitive, computationally efficient, approach, as the one presented in this paper, would be a better choice. ...
We present an analytical method for evaluating the first and second moments of the effective exciton spatial distribution in organic light-emitting diodes (OLED) from measured emission patterns. Specifically, the suggested algorithm estimates the emission zone mean position and width, respectively, from two distinct features of the pattern produced by interference between the emission sources and their images (induced by the reflective cathode): the angles in which interference extrema are observed, and the prominence of interference fringes. The relations between these parameters are derived rigorously for a general OLED structure, indicating that extrema angles are related to the mean position of the radiating excitons via Bragg's condition, and the spatial broadening is related to the attenuation of the image-source interference prominence due to an averaging effect. The method is applied successfully both on simulated emission patterns and on experimental data, exhibiting a very good agreement with the results obtained by numerical techniques. We investigate the method performance in detail, showing that it is capable of producing accurate estimations for a wide range of source-cathode separation distances, provided that the measured spectral interval is large enough; guidelines for achieving reliable evaluations are deduced from these results as well. As opposed to numerical fitting tools employed to perform similar tasks to date, our approximate method explicitly utilizes physical intuition and requires far less computational effort (no fitting is involved). Hence, applications that do not require highly resolved estimations, e.g., preliminary design and production-line verification, can benefit substantially from the analytical algorithm, when applicable. This introduces a novel set of efficient tools for OLED engineering, highly important in the view of the crucial role the exciton distribution plays in determining the device performance.
... This task has been solved by investigating the forward emission of polymer OLEDs, 3,4 by intensity comparison of different devices, 5 or by utilizing full angular and wavelength resolved spectra. 6,7 The impact of fitting algorithms has been discussed 8,9 and simplified methods have been suggested. 10 The importance of adapted stack architectures to observe the origin of emission accurately has been pointed out recently. ...
... We restrict the discussion in this paper to systems comprising of thin emitting layers prepared by vacuum co-evaporation that emit light through the substrate glass into the air. Different experimental approaches utilize emission patterns that have been measured either in air 3,6 or inside the substrate glass by attaching an index matched half ball lens. [7][8][9] These approaches differ with respect to the angular range of analysis. ...
... 7 The wavelength discretization is chosen to be 5 nm to match relevant experimental values. 6,7 The two common methods for measuring the electroluminescence emission pattern are sketched in Figure 1(a). In the simplest case the emission pattern of the OLED stack is measured in air. ...
Highly efficient state of the art organic light-emitting diodes (OLED) comprise thin emitting layers with thicknesses in the order of 10 nm. The spatial distribution of the photon generation rate, i.e. the profile of the emission zone, inside these layers is of interest for both device efficiency analysis and characterization of charge recombination processes. It can be accessed experimentally by reverse simulation of far-field emission pattern measurements. Such a far-field pattern is the sum of individual emission patterns associated with the corresponding positions inside the active layer. Based on rigorous electromagnetic theory the relation between far-field pattern and emission zone is modeled as a linear problem. This enables a mathematical analysis to be applied to the cases of single and double emitting layers in the OLED stack as well as to pattern measurements in air or inside the substrate. From the results, guidelines for optimum emitter – cathode separation and for selecting the best experimental approach are obtained. Limits for the maximum spatial resolution can be derived.
Recently, research in small molecule organic semiconductors have achieved great progress owing to their well-defined structure, easy synthesis and promising optoelectronic properties. In this work, we report a theoretical study based on two novel cyclopentadithiophene-naphthalene derivatives based small molecules for their potential applications in organic electronic devices such as organic light-emitting diodes (OLEDs) and organic solar cells (OSCs). The effects of electron-withdrawing groups such as fluorine (- F) and cyano (- CN) on the electronic, optical and charge transfer properties were explored using the Density Functional Theory (DFT). This computational investigation can provide a deep understanding of the structure-property relationships of the studied compounds. Our calculations clearly show that these compounds exhibit rigid co-plane skeletal structures. These relevant structures will be successfully employed to design a new photo-luminescent class of materials with low electronic band gap, deep HOMO energy levels, large absorption in the visible range and green-yellow emission. The designed molecules have shown their ability to serve as luminescent materials for OLEDs based on the electric (I–V) simulations and as donor materials for OSCs based on the Scharber diagram with a PCE that reaches 10 %.
Purposely tailored thin film stacks sustaining surface waves have been utilized to create a unique link between emission angle and wavelength of fluorescent dye molecules. The knowledge of the thin film stack’s properties allows us to derive the intrinsically emitted luminescence spectrum as well as to gain information about the orientation of fluorophores from angularly resolved experiments. This corresponds to replacing all the equipment necessary for polarized spectroscopy with a single smart thin film stack, potentially enabling single shot analyses in the future. The experimental results agree well with those from other established techniques, when analyzing the Rubrene derivative in a 2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine (T2T) host used for the fabrication of optimized organic light-emitting diodes. The findings illustrate how resonant layered stacks can be applied to integrated spectroscopic analyses.
We propose the Poynting vector analysis of the air mode in a top-emitting organic light-emitting diode (OLED) by combining the transfer matrix method and dipole source term. The spatial profiles of the time-averaged optical power flow of the air mode are calculated inside and outside the multilayer structure of the OLED with respect to the thickness of the semi-transparent top cathode and capping layer (CPL). We elucidate how the micro-cavity effect controlled by the thickness variation of the semi-transparent top cathode or CPL affects the internal optical power and absorption loss inside the OLED multilayer and the external optical power coupled into the air. When the calculated absorption loss and external power obtained by the proposed Poynting vector and currently-used point dipole models are compared, two calculation results are identical, which demonstrates the validity of the two models.
