This PDF file contains the front matter associated with SPIE Proceedings Volume 10687, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

In the past few decades Organic Light-Emitting Diodes (OLEDs) have matured to become a widespread display technology. Despite their commercial success in TVs, smartphones and tablet screens, several key issues still remain, especially if OLEDs are to become a viable technology for lighting. Metalorganic emitters based on Ir or Pt are widely used in OLED technology, due to their facile colour tunability, short phosphorescence lifetime and efficient intersystem crossing (ISC), which enables emission from both singlet and triplet states to be harnessed, leading to 100% internal quantum efficiency (IQE). However, these emitters are based on rare and toxic noble metals. In addition, efficient and stable blue and especially deep-blue phosphorescent emitters, which are essential for displays and lighting, are still to be demonstrated in order to replace currently used less efficient fluorescent blue OLEDs. Recently, purely organic emitters exhibiting thermally activated delayed fluorescence (TADF) mechanism were also shown to be capable of 100% IQE. Since efficient ISC in these compounds is based on the small singlet and triplet gap rather than heavy atom effect, TADF compounds represent a new class of potentially inexpensive emitters for next generation OLED displays and lighting. In this work, a series of four deep blue-to-green emitting TADF compounds have been synthesized and characterised. The compounds are based on the same scaffold as the dicarbazoyldicyanobenzene (2CzPN) reported by Adachi and co-workers, but with the replacement of the cyano groups by less electron-withdrawing oxadiazole moieties. The weaker acceptor strength of oxadiazole compared to cyano groups translates to more blue-shifted emission compared to 2CzPN. Additionally, higher spatial separation of HOMO and LUMO levels between acceptor and donor units, respectively, leads to a smaller singlet-triplet gap. This allows us to demonstrate efficient deep blue TADF OLEDs with CIE coordinates (0.16, 0.13), as compared to (0.16, 0.30) for 2CzPN devices. A thorough photophysical study of model compound 2CzPN and oxadiazole acceptor based derivatives will be presented both for solution and thin films. The blue oxadiazole compounds show up 75% quantum yield in solid state. The electrochemical and photophysical properties, as well as crystal structures are compared to the theoretical quantum chemical calculations of the studied compounds. The efficient emission properties coupled to the small singlet-triplet gap in these molecules allows us to demonstrate efficient electroluminescence (up to 10 % EQE) from the vacuum deposited OLEDs.

Thermally Activated Delayed Fluorescence (TADF) process is the new paradigm for Organic Light-Emitting Diodes (OLEDs). Despite all the efforts, a complete mechanistic understanding of TADF materials has not been fully uncovered yet. Part of the complexity arises from the apparent dichotomy between the need for small energy difference between the lowest singlet and triplet excited states (EST) which has to carry a significant charge transfer (CT) character; and for a significant spin-orbit coupling which according to El-Sayed rules requires the involved singlet and triplet excited states to have very different natures. In this contribution, we will show: (i) How the nature of these excited can be characterized and how this nature can be tuned by varying the nature of the electron donating (D) or accepting (A) units in D-A(-D) compounds. (ii) How this dichotomy can be resolved once accounting in a fully atomistic model of reference carbazole derivatives for thermal fluctuations of the molecular conformations and discrete electronic polarization effects in amorphous films. For both topics, we will demonstrate that, electronic excitations involved in the TADF process have a mixed CT-locally excited character being dynamically tuned by torsional vibrational modes and that overall, the conversion of triplet-to-singlet is a dynamic process gated by conformational fluctuations.

Organic light-emitting devices (OLEDs) are already widely used for common applications like OLED TVs or smartphone displays. Nevertheless, it is still a challenge for both science and industry to develop OLED systems for lighting applications that combine true-color white light, high efficiencies and high brightness at the same time. Since white emission in OLEDs is usually a combination of two or more different emitters with individual colors it is necessary that all included systems are efficient. It has been shown that the concept of thermally activated delayed fluorescence (TADF) allows to synthesize very efficient light-emitting molecules with various emission colors. In our work, we use the sky-blue TADF emitter 9-[2,3,4,5-tetra(carbazol-9-yl)-6-(trifluoromethyl)phenyl]carbazole (5CzCF3Ph) with an emission maximum at a wavelength of 495 nm in thin films, reaching a photoluminescence quantum yield of 70 %. In an OLED, the emitter delivered up to 18 % external quantum efficiency (EQE). This is beyond the theoretical limit of conventional fluorescent OLEDs. To achieve warm-white emission, we combine the sky-blue emission of 5CzCF3Ph with the red emission of the common phosphorescent emitter Ir(MDQ)2(acac) within one emission layer. Due to the very broad blue emission (FWHM ~ 95 nm), a dedicated deep blue emitter becomes obsolete and it is possible to tune the combined two-color spectrum in such a way, that a high color rendering index of over 80 and correlated color temperatures about 2800 K can be obtained by this strategy. EQEs of up to 17 % and luminous efficacies of 16 lm/W have been measured for the hybrid white OLEDs. This two-color concept paves the way towards future utilization of TADF emitters in lighting applications by simplifying the required sequence of organic layers inside the OLED. In our approach, the excitons are formed mostly on the TADF emitter itself. To achieve a suitable amount of red light for the white emission, it is necessary to enable efficient exciton transition pathways between 5CzCF3Ph and Ir(MDQ)2(acac). Due to the variety of potential local and charge-transfer excited states in the emitter system, there are several probable scenarios for the energy transfer. Utilizing time-correlated single photon counting (TCSPC) with a wavelength-sensitive detection, we study the exciton decay of both the TADF prompt and delayed fluorescence as well as the phosphorescent emission channel in detail. With this technique, we deliver a thorough investigation of the exciton transfer and exchange mechanisms in the emitter system of our warm-white hybrid OLEDs.

We will discuss the photophysics and particularly the role of spin states in thermally activated delayed fluorescence (TADF) in films and organic light emitting diodes (OLED). In particular, whether the TADF process is spin-dependent and, if yes, what is the exact mechanism and what are the relevant precursor states. We perform direct spin-sensitive measurements on TADF OLED devices applying multi-frequency electroluminescence- and current-detected magnetic resonance (ELDMR, EDMR). The idea behind these experiments is that the static magnetic field applied to devices modifies only the energy levels of spin-carrying states due to Zeeman splitting, thus changing the emission rates. We observe that the resonant microwave radiation, applied to OLEDs, leads to enhancement of the EL intensity. The effect was found to be very sensitive to experimental conditions, thus modifying the resonance frequency, temperature and microwave power we were able to shed light on the underlying mechanism of the reverse intersystem crossing and the spin states involved. With temperature-dependent ELDMR, the singlet-triplet splitting ∆EST can be determined, as we reported for two different donor-acceptor systems [1]. Comparing ELDMR, EDMR and photoluminescence detected magnetic resonance (PLDMR), we revealed differences in TADF processes under optical excitation and electrical injection. Finally, we compare the mechanisms of triplet-singlet conversion in poorly emissive charge-transfer states in OPV donor-acceptor blends [2] with those in highly emissive TADF systems. The information gained from magnetic resonance experiments can potentially help to design new OLED materials as well as to further improve their performance. [1] S. Väth, K. Tvingstedt, M. Auth, A. Sperlich, A. Dabuliene, J. V. Grazulevicius, P. Stakhira, V. Cherpak, V. Dyakonov, Adv. Opt. Mater. 5, 1600926 (2017). [2] S. Väth, K. Tvingstedt, A. Baumann, M. C. Heiber, A. Sperlich, J. A. Love, T.-Q. Nguyen, V. Dyakonov, Adv. Energy Mater. 7, 1602016 (2017).

Blend thin films, prepared by dip-coating, of polyfluorene (F8 or PFO), acting as an electron donor, and [6,6]-phenyl-C₆₁- butyric acid methyl ester (PC₆₀BM), acting as the electron acceptor, have been characterized by UV/VIS absorption spectroscopy, static and dynamic fluorescence, and atomic force microscopy. Four different solvents were used for the film preparation; the monohalogenated fluorobenzene and chlorobenzene and their dihalogenated counterparts odifluorobenzene and o-dichlorobenzene, respectively. Fluid mechanics calculations were used to determine the withdrawal speed for each solvent, in order to prepare wet films of comparable thicknesses. The resulting dry films were also of similar thicknesses. It was found that the choice of solvent influences the ability for F8 to form its β-phase.

Diketopyrrolopyrrole polymers (DPP’s) are an important class of donor materials for organic solar cells owing to their supreme charge carrier mobility and optical absorption which extends into the NIR (until  950-1000 nm). The former allows making efficient solar cells with rather thick active layers while the latter makes them a good candidate to be used in tandem devices. In this study, we synthesized four different DPP polymers with thiophene and thienothiophene conjugation segments in the backbone. For each of the backbones, we changed the branching point of the solubilizing alkyl chains (at 2nd or 6th carbon position). Solar cells were fabricated in the inverted configuration under ambient conditions following the device architecture: ITO/PEIE/active layer/MoOx/Ag. In general, thienothiophene based polymers performed better yielding maximum PCE’s close to 6.5 %. Interestingly, the short-circuit current varied from 7mA/cm2 to around 18mA/cm2 for the best performing system. The morphology was investigated using TEM and grazing incidence wide angle x-ray scattering (GIWAXS). While - stacking was not influenced by the conjugation segments, GIWAXS measurements reveal closer - stacking ( 3.5 Å) in polymers with farther alkyl branching (at 6th carbon position) as compared to polymers with branching at the 2nd carbon position (- stacking distance  3.6 Å). Alkyl lamellar spacing for branching at the 6th-position was  28 Å while for the 2nd- position lamellar spacing was  17 Å. Pole figures of the - stacking peak were calculated to get an idea about the distribution of crystallite orientation. For the thiophene substituted DPP’s most of the crystallites had face-on orientation while for thienothiophene substituted DPP’s, population of both face-on and edge-on crystallites were observed. By integrating the peak intensity as a function of polar angle, the relative degree of crystallinity (rdoc) was determined for the four polymer systems. TEM images revealed a fibrillar morphology for the four blended systems. The average polymer fibril width varied among the four polymer systems. For the thiophene-based DPP polymers, fibers widths were 35-50 nm (much larger than the typical exciton diffusion length  10nm). To study the effect of polymer fiber width and fiber purity on charge generation we measured fluorescence quenching in the blend films by selectively exciting polymer domains. To shed further light on phase purity of polymer fibrils, carbon/sulphur elemental maps were obtained using TEM. Overall, we try to correlate the effect of alkyl branching on the formation of mixed-phase morphology and how it affects the device performance.

Organic semiconducting materials are typically subject to energetic and positional disorder and localization of electronic states. The electronic behavior of these materials are therefore strongly influenced by thermalization of charge carriers in the localized density of states (DOS). Consequently, non-equilibrium processes play an important role in the operation of devices made of such materials. We show as an example that measurements of recombination dynamics, conducted under transient or steady-state conditions, can easily be misinterpreted when a detailed understanding of the interplay of thermalization and recombination is missing. To enable adequate measurement analysis, we solve the multiple-trapping problem for recombining charge carriers and analyze it in the transient and steady excitation paradigm for different DOS distributions. We show that recombination rates measured after pulsed excitation are inherently time-dependent, since recombination gradually slows down as carriers relax in the DOS. When measuring the recombination order after pulsed excitation, this leads to an apparent high-order recombination at short times. As times goes on, the recombination order approaches an asymptotic value. For the Gaussian and the exponential DOS distributions, this asymptotic value equals the recombination order under continuous excitation. For a more general DOS distribution, the recombination order can also depend on the carrier density, under both transient and steady-state conditions. However, we show that there are cases where thermal equilibrium is never attained in the device. We conclude that transient experiments can provide rich information about recombination in and out of equilibrium and the underlying DOS occupation provided that consistent modeling of the system is performed.

Here, we report the strategies to increase the photon harvesting in single junction organic photovoltaics by band gap engineering. Low band-gap non-fulllerene small molecule acceptors yield remarkable short-circuit current (26.6 mA/cm²) which comparable to existing high efficiency photovoltaic technologies.

Age-related retinal diseases lead to blindness due to progressive loss of image-capturing photoreceptor neural cells, which are responsible for the conversion of light entering the eye into electrical signals that are sent to the brain. Retinitis pigmentosa and age-related macular degeneration are two leading causes of severe visual losses in adult individuals, affecting over 1 million people worldwide. In these diseases, rod and cone photoreceptor cells are progressively lost while neural cells in the retinal network remain functional. Electronic retinal prostheses have great potential to restore sight by electrical stimulation of the surviving neurons. In this project, we aim at developing an artificial retinal implant based on organic photodetectors (OPDs). Neural stimulation is provided by an OPD pixel array processed on ultrathin plastic foil. Upon illumination with near-infrared (NIR) light, photo-generated electrical charge in each OPD pixel is delivered to the biological tissue via stimulating electrodes. Flexibility and softness of organic materials allow to interface intimately with neurons so that electrical signals generated by the implant are translated into bio-signals. Firstly, we investigated the photovoltaic performance of NIR-sensitive OPDs based on polymer:fullerene bulk heterojunction. We used PDPP3T:PCBM photoactive layer to detect NIR light up to 930 nm. We showed that by going from a single to a tandem OPD the open-circuit voltage can be doubled and the charge threshold for neural stimulation can be reached at lower light intensities. This results in a more efficient cellular stimulation while maintaining high implant resolution. Furthermore, we analysed the stimulating electrode behavior in a biological-like environment. We characterized the double-layer capacitance of gold, platinum and titanium nitrite (TiN) electrodes by pulsed-voltage measurements in phosphate buffered saline solution (PBS). We observed that TiN exhibits the highest charge capacitance due to the high surface roughness, which enables to maximize the charge injection into the electrolyte. Using a combination of experimental work and modeling we studied the process of charge injection into the biological tissue. We simulated the injected charge during 1 ms NIR light pulse under different illumination conditions. We compared the performance of retinal implants based on single or tandem OPD pixels coupled to 20 µm stimulating electrodes. As we expected, tandem OPD pixels always maximize charge injection into the electrolyte due to their higher photo-voltage. Moreover, the high double-layer capacitance of TiN electrodes metal electrodes results in sufficient charge injection levels for neural stimulation, which is generally achieved between 1 and 4 nC. In conclusion, we predicted that neural activity can be triggered using organic photovoltaic pixels in combination with high charge capacitance electrode materials. These findings are paving way to the development of a high-resolution retinal prostheses based on organic soft materials.

Today OLEDs are established light-sources for automotive rear-lights. OLEDs having a small number of segments can be found in various cars and are about to spread further. Beyond this state-of-the-art OLEDs being flexible as well as featuring a higher number of segments are desired by OEMs. Unfortunately conventional display approaches like passive and active matrix displays are struggling to deliver the high luminance and fill factor required for automotive applications and are therefore unable to fulfill the requirements by the automotive lighting industry. In this work we are going to present our approach on flexible and highly segmented OLED applications for Automotive to overcome this problem. An investigation of a novel device concept is demonstrated, as well as details on our substrate technology development, encapsulation and electronic/driving schemes. A strong focus has been put on the possibility to use standard lithography for substrate creation to have, if possible, no impact on existing supply chains and a maximum of reliability due to the use of well-known processes. The core concept for flexible and highly segmented OLED devices is derived from this processes. Challenges that occurred during the manufacturing of the substrates and devices are described in detail as well as solutions to circumvent these unexpected problems. Measurements of OLED key parameters are presented together with visual impressions of the devices. Driving schemes and electronics needed to control the OLED segments are introduced in short to complete the study of the overall concept of flexible and highly segmented OLEDs for automotive applications.

Organic light-emitting diodes (OLEDs) have reached a huge market as technology for small displays, e.g. in smartphones, and are entering the larger display and solid-state lighting markets as well. In parallel to these commercial successes, the OLED technology is adapted to a multitude of promising new applications, such as in optogenetics and medical therapy. However, it is still challenging to ensure good stability in applications that require high brightness (or high optical power density), in part due to the resulting resistive heating. Increased temperature can lead to a change in morphology of one or several organic layers, e.g. via crystallization of organic molecules, which then reduces electrical and optical performance and likely results in rapid device failure. Aside from an intrinsic resistive heating, heating can also be due to environmental effects during operation or fabrication and encapsulation of devices. For instance, atomic layer deposition (ALD) is a promising technique to form thin, yet highly protective encapsulation layers. However, state-of-the-art ALD processes require relatively high temperature during deposition (< 80 °C). 4,7-diphenyl-1,10-phenanthroline (BPhen) has been widely used as electron transporting layer (ETL) due its high electron mobility, particularly in an organic matrix-dopant system. However, it is well known that thin films of BPhen tend to recrystallize spontaneously. Annealing accelerates crystallization even further due to the relatively low glass transition temperature (Tg) of BPhen (62 °C). A straightforward way to enhance the device thermal stability is to make use of a high Tg material, yet materials have to be carefully adopted to provide appropriate functionality in OLEDs. In this contribution, we report the improvement of the thermal stability of OLEDs with BPhen based electron transport layers (ETLs) by cesium (Cs) doping. To verify the role of the Cs dopant in the BPhen matrix, recrystallization features of Cs-doped BPhen films with different doping concentrations were investigated using optical microscopy and atomic force microscopy. We also examined the photophysical properties of the films, i.e. photoluminescence (PL) and absorption. PL spectra exhibit monotonic red-shifts and broadening as the Cs doping concentration increases. This presumably indicates formation of metal complexes via interaction between the 1,10-phenanthroline group of BPhen molecules and the Cs ions. It was found that Cs plays a critical role, not only in inhibiting undesired recrystallization of BPhen molecules in a thin-film, but also in allowing BPhen layers to be thermally stable beyond the Tg of neat BPhen. Next, the electrical and optical properties of blue and red OLEDs that contain BPhen layers with different Cs-doping concentrations as ETL were characterized after annealing at temperatures between 60 and 100 °C. We find that higher doping concentrations lead to a marked increase in thermal device stability (quantified by current density and luminance at a fixed voltage). Making use of this observation, we successfully encapsulated BPhen based OLEDs with thin-film oxide layers using ALD. The results shown in this work may be transferable to other material systems and can thus provide a useful guideline to enhance the intrinsic thermal durability of organic devices and to render them compatible with processes involving thermal treatment.

The efficiency of phosphorescent organic light emitting diodes (OLEDs) shows a decrease with increasing luminance ("roll-off"). One of the contributions to the roll-off is triplet-triplet annihilation (TTA). TTA is the process of energy transfer from one exciton to another, after which the excited exciton decays non-radiatively to the lowest triplet state. There is ongoing debate on the mechanism of TTA [1, 2]: (i) when can TTA be described as a single step Förster-type process? (ii) when does exciton diffusion enhance the TTA rate? (iii) is exciton diffusion the result of Förster or Dexter-type interaction processes? (iv) when does the exciton confinement on the guest molecules become insufficient, so that also diffusion via the matrix can contribute to the TTA rate? In previous work [2] it was shown that it is possible to determine when exciton diffusion starts playing a role by using a refined analysis method of time-resolved photoluminescence (PL) measurements. This was done for a single system, CBP:Ir(ppy)2(acac) [3, 4]. In this study, we will go beyond this work by studying the role of exciton confinement by systematically varying the host material, for the case of the three phosphorescent guest molecules Ir(ppy)2(acac) (green), Ir(bt)2(acac) (yellow) and Ir(MDQ)2(acac) (red). In total, thirteen systems are included. We find a systematic monotonic decrease of the TTA rate coefficient with increasing confinement energy, which we attribute to diffusion via the host, as long as the confinement energy is smaller than approximately 0.3 eV. For TPD:Ir(ppy)2(acac), e.g., (confinement energy only 0.15 eV), we find an enhancement of the TTA rate of more than a factor of two as compared to the value obtained for a system with excellent confinement, such as TCTA:Ir(ppy)2(acac) (confinement energy 0.45 eV). This enhancement is found to be thermally activated. For systems with a strong exciton confinement, guest-guest exciton diffusion starts playing role for guest concentrations above 6 mol%. We have employed measurements of the temperature dependence of the TTA rate, which is found to be thermally activated, to study the mechanism of guest-guest exciton diffusion. The experimental results are analyzed more in-depth using kinetic Monte Carlo simulations, within which all processes are treated in a mechanistic manner. The simulations include the effect of triplet energy disorder. For systems with strong confinement, such as TCTA:Ir(ppy)2(acac), TTA can be described well assuming Förster-type TTA processes with a Förster radius of 3.5 nm. Exciton guest-guest diffusion can be equally well be described using Förster-type processes with a Förster radius of 2.7 nm or Dexter-type processes using a wavefunction decay length of 0.35 nm and rate to the first neighbor molecules of kD,1 ~ 10^9 s-1. These results enable us to more precisely determine the optimal degree of confinement and guest concentration, required in high efficiency OLEDs. [1] Y. Zhang, S.R. Forrest, Chemical Physics letters, 590 (2013) 106-110 [2] H. van Eersel et al., Journal of Applied Physics, 117 (2015) 1155002 [3] L. Zhang et al., Chemical Physics Letters, 652 (2016) 143-147 [4] L. Zhang et al., Chemical Physics Letters, 662 (2016) 221-227

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.

As one of the major candidates for future lighting technologies, organic light-emitting diodes (OLEDs) have reached a mature status fulfilling industrial performance requirements for display applications [1]. Their overall efficiency is currently mainly limited by the large amount of initial power dissipated into optical loss modes due to the high refractive index of the organic layers. A key method of reducing the trapping of light inside the OLED is to orient the transition dipole moments (TDM) of the light-emitting molecules parallel to the substrate interface. The most established experimental technique of determining the so-called anisotropy coefficient as a measure of the average TDM orientation of the molecules are angular resolved photoluminescence measurements [2]. Although this method has been applied by several groups with varying experimental realizations, a quantitative discussion of the effect of specific setup configurations is - to the best of our knowledge - missing so far. For instance, the accurate positioning and size of the optical excitation spot is one of the crucial requirements of this measurement. Also, the distance between the rotation center and the detector is important. In this work, we present a ray optics model which accounts for such macroscopic effects. By solving the developed equations numerically, we show that already a small displacement of the excitation spot leads to remarkable changes in the measured emission spectra. For specific setup configurations the accuracy of experimental data fits can be drastically improved using the mentioned corrections. Since some of the requirements for almost ideal experimental conditions are hard to realize, for instance due to varying substrate thicknesses, this numerical model might be a step towards better comparability of determined anisotropy coefficients. Additionally, it enables for the first time to quantify measurement deviations caused by non-idealities of the setup and with that to optimize them in a controlled fashion. [1] T. Tsujimura, OLED display fundamentals and applications, John Wiley and Sons (2017) [2] T. D. Schmidt, T. Lampe, D. Sylvinson M. R., P. I. Djurovich, M. E. Thompson and W. Brütting, Phys. Rev. Applied 8, 037001 (2017)

We present improvements in light outcoupling for the example of red, bottom-emitting, ITO free OLEDs. As an optimization tool we use experimentally verified coupled modelling approach, where we simulate a complete OLED device, including thin-film coherent stacks as well as thick microtextured incoherent layers (substrate). We calibrate the combined model on a fabricated small sample OLED. The research of lateral limitations and limited integrating sphere opening effects show that small area effects can lead to large deviations in outcoupling efficiency with respect to the large area devices commonly used in lighting applications. On the large area devices, we focus on the optimization of the thinfilm stack cavity in the OLED by tuning the thicknesses of thin layers. We show the importance of including the complete device in the optimization process, including the thin-film stack and the thick substrate with the outcoupling textures. We show that an OLED with an optimized planar cavity and applied external positive shaped dome texture can reach up to 50.5 % light extraction efficiency according to simulations.

It is known that organic light emitting diodes (OLEDs) can reach an internal quantum efficiency close to 100 %¹ . Outcoupling of the generated photons however is not that efficient resulting in an extraction efficiency of only around 20 %² . This is mainly due to total internal reflection at the OLED-substrate and substrate-air interfaces. In recent literature^1,3 , lenses are proven to be an adequate solution, but lens production techniques are complex, expensive and unsuitable for mass production. The aim of this research is therefore to investigate the development of a cost-effective lens array film by inkjet printing. These inkjet printed lenses are validated by pixelated OLEDs. Firstly, circular patterns of anisole are printed in a regular hexagon on PMMA-foil. Due to the coffee ring effect, reservoirs are formed in this foil which prevent the liquid lenses from merging. Afterwards these lenses, i.e. spherical droplets of NOA74, are deposited into these reservoirs and cured by ultraviolet light. Finally, the lenses are connected to printed pixelated OLEDs. The developed lens array film increases the OLED’s outcoupling efficiency by more than 20 % as is also expected from a theoretical study on these light extraction principles. The combination of the above-mentioned route for lens printing with the deposition of patterned OLED pixels, will not only improve the outcoupling to a large extend but will also help to develop OLEDs with a tailored emission pattern. A throughout understanding of the principles behind it will lead to optimized extraction efficiencies for large area printed OLED panels.

Organic light-emitting transistors (OLETs) combining the dual functions of electrical switching and light emission are promising devices to push large amounts of charge carriers into an isolated recombination area. Most OLETs proposed so far, however, suffer from hole-electron current imbalance when driven at high currents. This results in a light emission zone very close to one of electrodes. The proximity of the electrode is detrimental to efficient light emission as the metal significantly quenches excitons. In addition, in single gate OLETs, the emission zone can unpredictably switch from one contact to the other upon small bias changes since electron and hole currents are not controlled individually. Dual-gate architectures have been proposed to independently control the transport of both types of charge carriers towards the recombination zone. In the split-gate OLET, where both gates lie side by side in the same plane, light is exclusively emitted from the center of the channel, far from the electrodes. But the unavoidable horizonal gap between the gates creates a highly resistive region in the vicinity of the recombination zone that lowers the electrical efficiency of the device. Here, we introduce the overlapping-gate OLET, a novel dual-gate architecture in which one gate partially covers the other, leaving no horizontal gap between the gates. By accumulating charge carriers in the transport layer, each gate independently opens a unipolar gapless channel to transport and inject charge directly into the recombination zone, thereby avoiding transport through highly resistive ungated regions. For the active layer, we propose a vacuum-evaporated multi-layered structure of organic semiconductors: The transport layers are based on high mobility p- and n-type materials for efficient lateral transport. The recombination zone is made of a fluorescent host-guest emissive layer. Thanks to this architecture, the red light emitted by the overlapping-gate OLET is precisely located along the edge of the top gate that overlaps with the bottom gate. Therefore, light emission stays localized in the center of the channel, isolated from the quenching electrodes. Besides, the independent control over the supply of both types of charge carriers enables balanced transport up to high current densities. As a result, high performance red-emitting OLETs are demonstrated with an external quantum efficiency of 5.6% at a high luminance over 2000 cd m−2. Furthermore, the device shows no efficiency roll-off up to a current density of 30 mA/cm2. The conditions for balanced transport are rationalized through the development of an equivalent-circuit model. This shows that balanced transport is achieved when both transport channels are biased in the linear regime and that charge densities at the boundaries of the emissive layer are equal. The overlapping gate light emitting transistor is a promising step towards the development of bright organic light emitting devices that combine high efficiency at high current densities.

During the last two decades, small organic molecules have been widely studied for potential applications in organic solid-state lasers due to low-cost production, simple processing possibility and physical property tuning ability through chemical structure synthetic modifications. One of the most investigated and applied compound in dye lasers is 4- (dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM). It has shown remarkable properties as a dye in solid-state lasers. One of the drawbacks of this compound is high intermolecular interactions which reduce emission efficiency. Therefore it can be applied only in doped systems in low concentration (around 2 wt%). Recently we have demonstrated that incorporation of bulky triphenyl groups in the low molecular mass organic compounds enables the ability to form good optical quality transparent glassy films by solution processing. Additional such bulky groups reduce intermolecular interaction thus increase photoluminescence quantum yield in the thin film. In the presentation, we will show optical properties of new 2-cyanoacetic derivatives where two different bulky groups (9H-carbazole fragment and triphenyl group) are attached to molecule electron donating and accepting parts. Synthesized compounds have light absorption from 400nm to 600nm and photoluminescence from 600 nm up to 800 nm. Dyes with only one incorporated electron donating fragment showed 16% PLQY and ASE excitation threshold values (below 52 μJ/cm²) in neat thin films. Two electron donating fragment containing molecules have PLQY of 7% and ASE excitation threshold 223 μJ/cm².

New solution-processable materials based on well-known green iridium(III) heteroleptic complexes (ppy)₂Ir(acac) and (ppy)₂Ir(pic) were acquired by chemical modification of ppy ligand with functionable hydroxyl groups and subsequent esterification with 3,3,3-triphenylpropionic acid fragment. Photoluminescence quantum efficiencies up to 0.90 were measured for the compounds in solution. Emission characteristics in pure solid films and different guest-host systems with hole transporting materials were investigated. Green light emitting OLEDs (organic light emitting devices) was prepared and characterized.

In this work photoluminescence and amplified spontaneous emission properties of new original 2-cyanoacetic acid derivative in different concentration mixed in polyvinyl carbazole (PVK) matrix were investigated. Ethyl 2-(2-(4-(bis(2- (trityloxy)ethyl)amino)styryl)-6-tert-butyl-4H-pyran-4-ylidene)-2-cyanoacetate (KTB) is recently synthesised nonsymmetric red light emitting laser dye, that in previous experiments with neat thin films showed low amplified spontaneous emission (ASE) threshold value. Based on PVK high refractive index it has been used as a polymer to ensure the preparation of good planar waveguide. Luminescence quenching is expected in neat amorphous thin films according to previous experiments which reduces photoluminescence quantum yield and increases ASE excitation threshold energy. It could be overcome by a decrease of the intermolecular interactions between laser active molecules by doping them in polymer matrix thereby decreasing photoluminescence quenching effect in the system by increasing distance between organic molecules which in turn results in lowering ASE excitation threshold energy. The lowest threshold value of ASE was achieved at 20wt% of KTB molecule in PVK matrix. Ability to significantly decrease intermolecular interactions and excitation threshold energy of investigated compound in host-guest systems makes it promising to be used as a laser dye in preparation of organic solid state lasers.

In this study we report novel 3,3′-bicarbazole based charge transporting materials mainly designed for a use in systems containing phosphorescent iridium (III) complex emitters. A low-cost oxidative coupling reaction using FeCl₃ was employed in the synthesis of 3,3′-bicarbazole compounds. Different derivatives of 3,3′-bicarbazole with 4-ethoxyphenyland ethyl- substituents at 9,9′- positions and (2,2-diphenylhydrazono)methyl- and 4-(dimethylamino)styryl- substituents at 6,6′- positions were synthesized. Obtained (2,2-diphenylhydrazono)methyl- derivatives exhibit glass transition temperatures that are sufficient for applications in electronic devices. Thin amorphous films of good optical quality can be produced from synthesized materials using spin-coating method. The effect of (2,2-diphenylhydrazono)methylsubstituents at 6,6′- and 4-ethoxyphenyl- substituents at 9,9′- positions on the charge transport properties of the 3,3′-bicarbazole derivatives was investigated. With the introduction of both electron acceptor and donor moieties to 3,3′-bicarbazole structure material electron and hole drift mobilities reach approximately 1·10^-5 cm²/V·s. Molecule ionization (I_f) levels and electron affinity (EA_f) levels in thin films were determined using photoelectric effect experiment. Depending on the nature of substituents at 6,6′- and 9,9′- positions If levels range from -5.19 to -5.13 eV and EA_f levels are from -2.44 to -2.38 eV.

We explore whispering gallery mode (WGM) assisted random lasing under circular spot excitation from a 4- (dicyanomethylene)-2-methyl-6-(4dimethylaminostyryl)-4h-pyran (DCM) dye-doped poly(vinylalcohol) (PVA) thin film coated silica microsphere when excited above the threshold excitation intensity. The output spectrum consists of randomly positioned narrow spectral lines. Under circular spot-excitation, the allowed WGMs in the microcavity provide a stripe like path along the equator for amplification. The high-power density built within these microcavities due to WGM and the weakly scattering random lasing effect, along with stripe like excitation combined with the high fluorescence quantum yield of DCM dye, give rise to threshold pump energies as low as 33 μJ for free space pulsed excitation at 532 nm. This makes these DCM-PVA coated microspheres attractive tiny sources for sensing applications.

Organic light emitting diodes (OLED) have found their applications in the mobile and TV screens. Till now the commercially available diodes are made by expensive thermal evaporation in a vacuum. The costs of OLED fabrication could be decreased by applying low-cost wet casting methods, for example, spin-coating. In this work, we have studied a group of blue light emitting purine derivatives which could potentially be used in OLEDs. The advantage of these compounds is their ability to form amorphous thin films from solutions. All the thin films were prepared by the spincoating method from chloroform solution on ITO glass. The position of hole and electron transport energy levels is important for efficient OLED fabrication. Ionization energy was determined using photoelectron yield spectroscopy. The gap between ionization energy and electron affinity was determined using photoconductivity measurements. Electron affinity (E_a) then was calculated as a difference between ionization energy (I) and photoconductivity threshold value (E_th). Changes in the energy level values depending on the molecule structure were investigated. The position of electron acceptor group strongly affects the gap between ionization energy and electron affinity, while with the help of the attached substitute groups it is possible to alter the ionization energy. Fine “tuning” of the ionization energy values can be achieved by altering the length of the “tail” where the inactive bulky group is attached.

Remote phosphor-converted LEDs (rpc-LEDs), which rely on a phosphor layer located away from the LED chip, are a particularly attractive technology benefitting from a higher luminous efficiency and from an improved stability compared with on-chip LEDs. However, systems based on thin-film remote phosphor layers still face a low color conversion efficiency (CCE). This mostly originates from an insufficient interaction of the exciting blue light with the phosphors. To overcome this limitation, we propose to couple the thin-film converting layers to a micro-concavity array (MCA) designed to enhance the optical pathlength of the exciting light, resulting in an improved CCE. This is achieved by exploiting the excellent light scattering and retro-reflection properties of MCA. We experimentally verify that the MCA transmit 95% of the incoming blue light into the converting layer, whereby 84% of this share corresponds to scattered light. Moreover, the measured retro-reflection amounts to 21% for normally incident light. The potential of the fabricated MCA films is tested by integrating them on the illuminated side of remote light converting thin-film layers with sub-millimeter thickness. Two examples, including quantum dots (QDs)- and rare-earth phosphor- based LEDs, are investigated. Our results show that the CCE of both rpc-LEDs are improved due to the enhanced excitation of the downconverted materials and to the effective extraction of the backscattered light. Thus, the CCE values of QDs-based and phosphor-based and rpc-LEDs are increased by 8.1% and by 12.7%, respectively, compared to devices without MCA films. In the latter case, the angular color uniformity is additionally improved under the effect of light scattering.