Tuning the Microcavity of Organic Light Emitting Diodes by Solution

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Tuning the Microcavity of Organic Light Emitting Diodes by Solution Processable Polymer:Nanoparticle Composite Layers Jan Benedikt Preinfalk, Fabian R. Schackmar, Thomas Lampe, Amos Egel, Tobias Schmidt, Wolfgang Brutting, Guillaume Gomard, and Uli Lemmer ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10717 • Publication Date (Web): 08 Jan 2016 Downloaded from http://pubs.acs.org on January 14, 2016

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ACS Applied Materials & Interfaces

Tuning the Microcavity of Organic Light Emitting Diodes by Solution Processable Polymer:Nanoparticle Composite Layers Jan B. Preinfalk, *,† Fabian R. Schackmar,† Thomas Lampe,§ Amos Egel, † Tobias D. Schmidt,§ Wolfgang Brütting,§ Guillaume Gomard*,†,‡ and Uli Lemmer*,†,‡ †

Light Technology Institute, Karlsruhe Institute of Technology (KIT), Engesserstr. 13, 76131

Karlsruhe, Germany, ‡Institute of Microstructure Technology, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany and §

Institute of Physics, Universitätsstr. 1, 86135 Augsburg, Germany.

Keywords organic light emitting diodes, light outcoupling, microcavity, nanoparticle composites, silica nanoparticles, conductive polymer, solution processing.

Abstract: In this study, we present a simple method to tune and take advantage of microcavity effects for an increased fraction of outcoupled light in solution-processed organic light emitting diodes. This is achieved by incorporating non-scattering polymer:nanoparticle composite layers. These tunable layers allow to optimize the device architecture even for high film thicknesses on a single substrate by gradually altering the film thickness using a horizontal dipping technique. Moreover, it is shown that the optoelectronic device parameters are in good agreement with transfer matrix simulations of the corresponding layer stack, which offers the possibility to

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numerically design devices based on such composite layers. Lastly, it could be shown that the introduction of nanoparticles leads to an improved charge injection which combined with an optimized microcavity resulted in a maximum luminous efficacy increase of 85 % compared to a nanoparticle-free reference device.

Introduction In the recent years, organic light emitting diodes (OLEDs) have attracted a lot of interest owing to their low power consumption, their thin film architecture, the possibility to fabricate them on flexible substrates and the large variety of emitting materials whose emission spectra can cover the whole visible range.1–3 High device efficiencies have already been reported4–6 and the technology successfully made its way into the mobile phone display mass market. Although intensive research has brought up highly efficient materials,6–10 the device efficiencies are still too low for lighting applications to be a serious competitor with inorganic LEDs. The device efficiency is significantly limited by low light outcoupling arising from different loss channels such as internal reflections, parasitic absorption and coupling to substrate, surface plasmon polariton and waveguide modes.11,12 Those losses can be circumvented by implementing light extraction schemes based on periodical structures (e.g., gratings or planar photonic crystals), 13–15 disordered structures 16,17 or alternatively, by the use of nanoparticle based scattering layers. 18–23 In our recent work,19 silica nanoparticles with a diameter of 30 nm were incorporated in the hole injection layer consisting of poly(3,4-thylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) to form a scattering layer. As the refractive index contrast between the nanoparticles and the polymer is quite low, the scattering effect reported can be attributed to strong morphological changes such as the creation of air voids acting as volume scattering centers or the increase of surface roughness.


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The external quantum efficiency can also be increased by designing suitable microcavities which change the modal distribution favoring radiation modes and also alter the local density of optical states leading to an increase in the internal quantum efficiency due to an enhanced spontaneous emission rate.24–29 This can be achieved by introducing mirror-like structures (e.g. Bragg mirrors) at the anode and/or cathode side.30–33 In the present study, we introduce a versatile way to increase the fraction of outcoupled light by tuning the microcavity with the use of a solution processed non-scattering and low-absorbing polymer:nanoparticle composite layer. This is realized by incorporating silica nanoparticles with a small diameter of 7 nm and a low refractive index contrast with PEDOT:PSS to minimize the haze and ensure good layer homogeneity. The composite layers allow the optimization of the device architecture from an optical point of view by tailoring the hole injection layer thicknesses in terms of cavity improvement while simultaneously enhancing the electrical layer properties, i.e. charge injection and conductivity even at large film thicknesses up to 400 nm without significantly introducing parasitic absorption or high Ohmic losses. By varying the thickness and the effective refractive index of the composite layer, the modal distribution of the waveguide modes (including surface plasmon polariton modes in the following) can be gradually changed and optimized for a wavelength range around the emission peak. This is achieved by doctorblading a wedge-shaped composite layer on top of an ITO-covered glass substrate in a thickness range between around 50 nm and 400 nm while keeping the SiO2 content constant. Those steps are described in more detail in the experimental section. The influence of the composite layer on the OLED performance is then investigated in the results and discussion section. The contribution of optical effects to the efficiency is shown by simulations and the electrical effects


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are studied by performing conductive atomic force microscopy (c-AFM), Kelvin probe force microscopy (KPFM) and photo-electron spectroscopy in air (PESA). Experimental Section In order to prepare the composite layers, PEDOT:PSS (Heraeus Clevios P VP.AI 4083 Special Grade, solid content 2.85 wt.% in H2O) was mixed with the nanoparticles dispersion (W.R. Grace Ludox SM colloidal silica, 30 wt.% in H2O) in different ratios. In the framework of this study, two different concentrations were examined: first, a ratio of PEDOT:PSS to silica dispersion of 10:1 (56 wt.% / 33 vol.%. of SiO2) and second, a ratio of 5:1 (72 wt.% / 50 vol.%. of SiO2). The weight and volume concentrations were calculated by using the initial concentrations in the dispersions and the specific densities of the solid materials. Higher concentrations were tested, however the resulting layers showed strong film inhomogeneities and high surface roughness and were therefore unsuitable for further experiments. The resulting dispersion was treated in an ultrasonic bath for 5 minutes to prevent agglomeration and to maintain a homogenous blend. To investigate the influence of the composite layer thickness on the microcavity and on the device efficiency, we used the OLED layout according to Figure 1, which depicts an OLED array of 19x2 pixels equidistantly distributed along a rectangular substrate. Before depositing the composite layers, a glass substrate covered with 130 nm of ITO (Rsq=12 Ω/sq) was structured by wet etching, leaving an ITO bar of 7.5 mm x 1.7 mm in the middle of the substrate. After an additional cleaning step using an ultrasonic treatment for 10 minutes in acetone and 10 minutes in isopropanol, the substrate was treated with oxygen plasma for further cleaning and to ensure good wetting of the composite material.


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After structuring and cleaning, the composite layer was doctor-bladed on top. To achieve a thickness gradient along the substrate, we used a modified doctor-blading system (Zehntner ZAA 2300) which allows for a dynamic control of the blade speed.34,35 The thickness gradient then depends on the speed, the viscosity (which in turn is influenced by the nanoparticle content), the blade gap and the applied fluid volume. As the layers were deposited with thicknesses in the range of around 50 nm to 400 nm, the blading and material parameters had to be adjusted according to the corresponding material composition. A cylindrical coating bar with a radius of 6.5 mm and a gap of 200 µm was used on a 65 °C pre-heated substrate. The precise blading parameters are reported in the supporting information, the resulting layer thicknesses for the corresponding nanoparticle concentrations are shown in Figure 2. After the deposition of the composite layer, the sample was heated up to 150 °C for 20 minutes in a vacuum oven to remove the residual water. The resulting layer showed a homogenous distribution of nanoparticles and a smooth surface with a roughness of 5 nm (root mean square, measured with a white light interferometer over a surface of 254 µm x 190 µm), as seen in the scanning electron microscope (SEM) cross section in Figure 3a and the atomic force microscope (AFM) image in Figure 3b. As the nanoparticles exhibit an average diameter of 7 nm and as no significant particle agglomeration occurred while avoiding air voids in the layer, a negligible haze of less than 1 % was measured (reported in the supporting information). As the emitting material, the polymer emitter PDY-132 (“Super Yellow”, Merck KGaA) with a peak emission wavelength at 550 nm was dissolved in toluene at a concentration of 4 mg/ml. The material was deposited in the same way as the composite material, but with a constant film thickness. To compensate the thickness gradient caused by the decrease in material volume along


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the substrate, the blade speed was accelerated from 35 mm/s at the beginning to 50 mm/s at the end on the substrate leading to a constant film thickness of 60 nm. The variation of the film thickness is 5 nm and did not affect the overall performance significantly. A coating bar with a gap of 400 µm and a radius of 14 mm was chosen and the substrate was heated at 65 °C. For the cathode deposition, a 1 nm lithium fluoride and 200 nm aluminum electrode was thermally evaporated through a shadow mask. The deposition of the composite layers was performed under normal atmosphere, whereas all the subsequent steps, including the electrooptical characterization, were carried out under nitrogen atmosphere to avoid further contamination with oxygen and moisture.

Device characterization: The electrical characteristics of the OLEDs were recorded by a source measurement unit (Keithley 2400), the luminous flux was measured using an integrating sphere coupled to a spectrometer (Instrument Systems CAS140), current efficiencies were calculated assuming Lambertian emission. The AFM measurements were carried out using a Bruker Dimension Icon with PFTUNA tips for conductive AFM (bias 1 V, ITO as electrical ground, peak force 5 nN) and SCM-PIT tips for KPFM. The Riken Keiki AC-2E was used for photo-electron spectroscopy, a linear fit to the cubic root of the photoelectric yield was used to extract the work function, the average was determined from 3 measurements per configuration. The SEM is a Zeiss Supra VP55. The layer thicknesses were determined by a profilometer (DektakXT, Bruker), the layer profile was determined with a white light interferometer (PLu Neox, Sensofar).


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Results and Discussion The obtained luminous efficacies and current efficiencies for the OLEDs using pure PEDOT:PSS and the two nanoparticle concentrations examined at a luminance of 1000 cd/m² are plotted for different composite layer thicknesses in Figure 4. A clear oscillation in the efficiencies can be observed for all measurements, as expected for weak microcavities formed between a reflecting layer and a semi-transparent substrate. When the composite layer thickness is varied, the emission into the far field is affected by constructive (or destructive) interferences, such that the decay rate into these radiative channels is enhanced (diminished) with respect to the overall decay rate. In the present case, this leads to two maxima, one for a small thickness of around 100 nm and another one for a larger thickness around 325 nm, separated by a minimum located at a thickness of about 210 nm. A slight color shift due to cavity effects is observed as evidenced in the angular spectra in S4 in the supporting information. Compared to a reference device with a 20 nm thick and pure PEDOT:PSS layer which showed a luminous efficacy of 8.3 lm/W (10.5 cd/A), the efficiency could be increased by up to 60 % (27 % in cd/A) for a nanoparticle concentration of 56 wt.% and up to 85 % (40 % in cd/A) at a concentration of 72 wt.%. However, in the case of pure PEDOT:PSS layers with thicknesses in this range, the device efficiency is significantly decreased, although the oscillatory behavior caused by the modified microcavity is still clearly visible. Due to the comparatively low viscosity of the material, thicknesses of up to only 300 nm were possible. To further separate optical and electrical influences of the composite layers on the efficiency, optical simulations were performed by a numerical tool based on a transfer matrix formalism.36,37 The refractive indices of the composite layer were determined by spectroscopic ellipsometry at variable angles (WVASE, J.A. Woollam Co. Inc.), modeled with a Lorentz and Tauc-Lorentz


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oscillator in the ordinary plane, and two Gaussian oscillators in the extraordinary plane to include the anisotropy of the materials.38,39 The PEDOT:PSS/silica-nanoparticle blend was modeled by applying a Bruggemann effective medium approximation (EMA) with the corresponding volume concentrations of silica. The coupling of the emitting dipole to the different modes was then calculated for varying composite layer thicknesses in the range of 20 nm to 400 nm. The thicknesses used were 130 nm for the ITO layer, 20 nm to 400 nm for the composite layer, 60 nm for the emitting layer using a combination of 10 % isotropic and 90 % parallel oriented dipoles, and 200 nm for the aluminum layer. For the composite layer, the anisotropic behavior was modeled according to Penninck et al.40 The dipoles were positioned in the middle of the emitting layer. The calculated distribution of the different modes as a function of the composite layer thickness is displayed in Figure 5a for a nanoparticle concentration of 56 wt.%. As expected and as observed in the experiments, an oscillation in the outcoupled fraction of light can be observed with a maximum at a composite layer thickness of 90 nm and a corresponding coupling to radiation modes of 23.7 % of the total generated light. The increase in outcoupled light comes along with a dip in the amount of waveguiding. Figure 5a shows another maximum occurring at 300 nm and a minimum at 224 nm. These findings are in good qualitative agreement with the experimental results which are included for comparison in Figure 5b. Although the simulation result predicts a much lower increase in outcoupling efficiency, the experimentally observed efficiency enhancement is even more pronounced. The structures discussed here complement other approaches towards light management in OLEDs. In general, we can discuss two classes of internal mode extraction structures. The first one being tailored nanostructures such as Bragg gratings, the second being the modification of


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the mode distribution favoring outcoupled modes by tuning the layer thicknesses without adding additional structures. In the former case, enhancement factors of ~2 were reported resulting from a complex interplay between the different optical channels.11,13,41 These internal structures are not easily wet-processable and may induce electrical defects. In the second approach, layer thickness modifications in evaporated small molecule devices have been applied to improve the optical performance of the device by optimizing the device cavity.37,42,43 However, parasitic absorption and a decreased internal quantum efficiency may occur. Here we show that the use of non-absorbing nanoparticles allows for the realization of even thicker cavities in solution based devices. Although the oscillatory behavior in efficiency can be explained by the modified microcavity, the strong increase in the luminous efficiency for the nanoparticle composites is mainly due to a modified electrical behavior of the injection material. This is also evident from the comparison of the JV-characteristics of different nanoparticle concentrations displayed in Figure 6a to Figure 6c. With the introduction of nanoparticles, the current density increases significantly by a factor of 3 to 5, with the highest current densities observed for a nanoparticle concentration of 56 wt.%. Furthermore, as seen in Figure 6d, the highest nanoparticle concentration seems to lower the onset voltage, especially compared to a 100 nm thick layer of pure PEDOT:PSS. The nanoparticles most likely modify the morphology of the native PEDOT:PSS layer leading to an improved and more balanced injection. It might also be possible that the conductivity of PEDOT:PSS is modified by the presence of the nanoparticles. As discussed in Refs.,44–47 the conductivity of PEDOT:PSS is anisotropic (higher in-plane conductivity) and relies on the nanomorphology of this material which is probably altered in the presence of the silica nanoparticles.


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To further analyze the effects of the nanoparticles on the electrical properties of the composite layers, conductive AFM measurements in peak force tapping mode were carried out to map the vertical conductive paths. The results are shown in Figure 7. Here, the pure PEDOT:PSS layer (Figure 7a) shows less conductive paths and a lower absolute current as the composite layers (Figures 7b–e). The measurements show the largest number of conductive paths and also the highest current density at a silica concentration of 56 wt.% (Figure 7d). This can explain the high current densities and low ohmic losses in the OLEDs even at large film thicknesses. Although the nanoparticles are insulating and an increase in ohmic losses and device failure might be expected, the good electrical properties probably result from the adhesion of the nanoparticles to the anode by electrostatic forces during the deposition of the layer and the voids being filled with PEDOT:PSS due to capillary forces, as similarly described in Ref.48 This could lead to a more complex network of conductive paths inside the layer, but also to an enlarged interface between the composite layer and the emitting layer. As the anisotropic conductivity in PEDOT:PSS layers arises from the separation of PEDOT- and PSS-rich areas during deposition and drying,44–47 the nanoparticles might distort this self-assembly, resulting in different conductive pathways which are beneficial for vertical hole transport and therefore lead to a more isotropic electrical conductivity. Furthermore, a change in the injection can explain the strong increase in current density and luminous efficacy of the OLEDs. Hence, the work functions of the composite layers were measured by Kelvin probe force microscopy (KPFM) and the reference work function of the pure PEDOT:PSS layer was measured in absolute values by photo-electron spectroscopy in air at a UV intensity of 500 nW. For KPFM a potential of 1000 mV was used. The results are shown in Figure 8 and indicate a strong increase of the work function from 5.13 eV (pure PEDOT:PSS) to


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5.38 eV (56 wt.%), which could explain an improved injection. As it was demonstrated in the case of charged SiO2 nanoparticles in Ref.,49 bringing a negatively charged region in the vicinity of the anode induces a dipole moment resulting in an increased local electric field across the interface, therefore promoting charge injection. A similar mechanism could take place in our composite layers. Conclusions In summary, we have investigated the potential of composite hole injection layers consisting of PEDOT:PSS and embedded silica nanoparticle for light extraction in OLEDs. Those nonscattering and low-absorbing layers are solution-processed and the method proposed here is therefore easily up-scalable. The layer thicknesses were adjusted from few tens to few hundreds of nanometers to optimize microcavity effects and to increase the coupling to radiation modes. We found that those composite layers not only allowed an optical optimization of the OLED architecture, but could simultaneously improve the electrical properties, i.e. charge injection and transport. Consequently, the luminous efficacy of the devices was improved by factor of 85 % with respect to an OLED based on a nanoparticle-free hole injection layer. Further studies are required to understand the vertical and in-plane morphological changes of the PEDOT:PSS film following the introduction of the silica nanoparticles, and their relationship to electrical properties.

Associated Content Supporting Information. Doctor blading parameters, absorption, haze and refractive indices of the composite layers, surface morphology measured by white light interferometry, angle-


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resolved OLED emission spectra. This material is available free of charge via the Internet at http://pubs.acs.org. Author information Corresponding Author *E-Mail: [email protected] *E-Mail: [email protected] *E-Mail: [email protected] Funding sources Bundesministerium für Bildung und Forschung (BMBF) within the project OLYMP (number 13N12240). Notes The authors declare no competing financial interest. Acknowledgments The authors would like to thank the financial support by the Bundesministerium für Bildung und Forschung (BMBF) within the project OLYMP (number 13N12240). J.P. and A.E. acknowledge support by the Karlsruhe School of Optics & Photonics (KSOP). G.G. acknowledges the funding by the Alexander von Humboldt-Foundation. The authors would like to thank Thomas Eiselt from the Institut für Mikrosystemtechnik (IMTEK) and Stefan Höfle (LTI) for useful discussions and Stefan Gärtner, Tanja Pürckhauer, Adrian Mertens and Alexander Colsmann (LTI) for the possibility to perform conductive AFM and PESA measurements and the assistance with the experiments.


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Abbreviations OLED organic light emitting diode, PEDOT:PSS poly(3,4-thylenedioxythiophene) polystyrene sulfonate, ITO indium tin oxide, c-AFM conductive atomic force microscope, KPFM Kelvin probe force microscope, SEM scanning electron microscope, AFM atomic force microscope, PESA photo-electron spectroscopy in air.

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Figure 1. Fabrication process of an OLED with a wedge-shaped composite hole injection layer. (a) Structuring of the ITO-coated glass substrate for the anode contact. (b) Doctor-blading of the composite layer with a wedge-shaped thickness gradient. (c) Doctor-blading of the light emitting polymer with a constant film thickness. (d) Thermal evaporation of the cathode materials.


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Figure 2. Film thickness at equidistant positions on the substrate at nanoparticle concentrations of (a) 0 wt.%, pure PEDOT:PSS, (b) 56 wt.% and (c) 72 wt.%. The precise doctor blading parameters can be found in the supporting information.


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Figure 3. (a) SEM picture of a cross section of a composite layer showing the high package density of the nanoparticles and the smooth surface. (b) AFM image of the surface indicating a low roughness due to marginal agglomeration and very few large particle clusters.

Figure 4. (a) Measured luminous efficacies and (b) the corresponding current efficiencies at nanoparticle concentrations of 56 wt.% and 72 wt.%. The dashed lines indicate the reference value for a 20nm layer of pure PEDOT:PSS.


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Figure 5. (a) Simulated mode distribution according to different composite layer thicknesses at a concentration of 56 wt.% (33 vol.%). (b) Comparison of simulation and measurements for the two silica concentrations investigated.


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Figure 6. Measured current density-voltage characteristics of the devices for several composite layer thicknesses and for a nanoparticle concentration of (a) 0 wt.%, (b) 56 wt.% and (c) 72 % wt. (d) Onset voltages of the different configurations at a fixed layer thickness of 100 nm.


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Figure 7. Conductive-AFM measurements of different grades of the composite layers on top of ITO-covered glass substrates at a thickness of 150 nm. (a) Pure PEDOT:PSS (b) Composite layer, 30 wt.% SiO2 (c) 40 wt.% (20 vol.%) SiO2. (d) 56 wt.% (33 vol.%) SiO2. (e) 72 wt.% (50 vol.%) SiO2.

Figure 8. Work functions of pure PEDOT:PSS (0 wt.%) and of the composite layers of different nanoparticle concentrations measured by KPFM and PESA.


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ToC Graphic


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Tuning the Microcavity of Organic Light Emitting Diodes by Solution

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