Recently, organic light-emitting diodes (OLEDs) have gained increasing interest as light source for biophotonic applications. Especially in the context of wearable and implantable electronics, OLEDs are particularly interesting due to their mechanical flexibility, the possibility to form densely patterned microscopic pixels, to achieve fast response time, and to tune the emission properties. In addition, OLEDs allow stacking of individual pixels, which enables multi-color light emission from exactly the same area. In this contribution, we present our recent progress on tailoring OLED properties for use in optical stimulation of cells by means of optogenetics. With the development of light-sensitive proteins capable of both activating and inhibiting cellular activity using two different stimulation wavelengths, light sources are required that provide switchable bi-color light emission. We developed two-terminal, bi-color OLEDs that are capable of switching the emission color by changing the applied voltage polarity and show how these can trigger bidirectional photocurrents in ND7/23 cells and elicit electrical activation and silencing of neuronal activity in fruit fly larvae.

We report a family of thermally activated delayed fluorescence (TADF) dendrimers that exhibit both vanishingly small singlet-triplet gaps and large oscillator strengths. Host-free green solution-processed OLEDs show outstanding maximum external quantum efficiencies of up to 28.7%, current efficiencies of 98.8 cd A-1 and power efficiencies of 91.3 lm W-1.

Research focus in photon energy upconversion (UC) utilizing organic compounds that can induce triplet-triplet annihilation (TTA) is associated with promising applications in photocatalysis, bioimaging, night vision, memory devices, targeted drug delivery, and many more. TTA-UC is particularly attractive because of its efficient operation under incoherent excitation and at low power densities (~ mW/cm2) such as those provided by the sunlight. This implies novel strategies in photovoltaics to harvest unabsorbed near-infrared (NIR) photons via their conversion to visible light. The best-performing NIR-to-vis UC systems realized so far, both in solution and solid-state, rely on rubrene (Rub) emitter. However, in the solid-state emissive properties of Rub are heavily influenced by the film morphology. Here, we introduce a straightforward UC film fabrication technique based on thermal evaporation in an inert atmosphere. In this way obtained highly concentrated Rub films display one order of magnitude higher fluorescence quantum yield (ФFL) compared to the films produced by solution-processing or sophisticated thermal deposition in a vacuum. Moreover, we confirm that in these films emitter and sensitizer are well dispersed and intermixed to result in triplet energy transfer (ФTET > 60%). As a result, the UC films demonstrate record-high NIR-to-Vis UC efficiency (ФUC) of 1.2%. The attained ФUC is at least 2-fold higher compared to that of other bicomponent NIR-UC films reported in the literature so far. Moreover, we evaluate Rub potential as a TTA emitter by calculating the statistical probability ( f ) to obtain one singlet from two triplets via TTA. We determine f to be ~19.7 %. The value is found to be similar to the previously estimated one for Rub in a solution (15.5%). This indicates that f of the Rub is not significantly influenced by the environment and is a more intrinsic property.

For bright area light sources, such as OLED lighting tiles, the electric resistivity of transparent electrodes induces a non-uniform current distribution within the device. At high driving currents, the interaction between conductivity, heat, and power dissipation results in a positive electrothermal feedback loop, which drastically exacerbates inhomogenities in local device temperature and luminance. Such nonlinear behavior induces unprecedented electrothermal effects that compromise the performance of bright area light sources. In this contribution, we discuss how Joule self-heating sqeezes temperature and current into a tiny device region, while the remainder of the active area decreases in luminance (Switch-back effect). We further introduce how the dimension of the active area governs the current-voltage characteristics of the OLED and how two and even three stable operating branches develop (tristability) that can promote destructive incidences.

The organic light-emitting diode (OLED) technology has become the state-of-the-art for mobile displays, is penetrating the TV display sector, and is a light-source with further application potential in the area of automotive lighting, indoor lighting, and wearable and health-monitoring electronics. Recently, J. Rafols-Ribe et al. showed that the efficiency and lifetime of OLEDs can be increased by the layer-by-layer optimization of its fabrication [J. Rafols-Ribe et al., Sci. Adv., 4 (2018)]. This was possible by heating the substrate during deposition. In this work, we investigate how each heated OLED-layer influences the overall device-performance change and compare it to post-annealing treatment by studying the optoelectrical and photoluminescent characteristics of the OLEDs. Additionally, we employ impedance spectroscopy to calculate the giant surface potential of the thin films in an attempt to learn more about the nanoscopic changes of the layer morphologies. Here, different emitter systems, i.e., phosphorescence- or thermally activated delayed fluorescence-based ones, are tested. We find a clear correlation between the surface potential of a transport layer and efficiency of the OLED. Furthermore, we discuss that the efficiency is improved because of a reduced non-radiative decay rate. We show that the in-situ heating increases the efficiency only for OLEDs containing polar layers. These findings contribute to the understanding of the in-situ heating of OLED layers and provide further levers for OLED improvement.

Polymer-wrapping has enabled the sorting and purification of large amounts of semiconducting and monochiral single-walled carbon nanotubes (SWNTs) that can be applied in a wide range of (opto-)electronic devices with outstanding charge transport properties. Their near-infrared emission properties are tunable either by strong-light matter coupling, i.e., the formation of exciton-polaritons in Fabry-Pérot cavities (Nat. Mater. 2017, 16, 911 & ACS Photonics 2021, 8, 182) or by controlled introduction of different luminescent sp³-defects. These quantum defects with characteristically red-shifted emission and long photoluminescence (PL) lifetimes enable higher PL quantum yields and single-photon emission at room temperature. They can be created in a controlled manner in polymer-wrapped semiconducting nanotubes in organic solvents (ACS Nano 2019, 13, 9259). Their impact on charge transport and electroluminescence has been investigated in ambipolar field-effect transistors with dense SWNT networks (ACS Nano 2021, 15, 10451). Various substituents of sp³ defects are possible including stable organic radicals, which directly interact with the excitons localized at the defects (ACS Nano 2021, 15, 5147). Furthermore, the type and wavelength of the defect emission is governed by the precise binding configuration for chiral (6,5) SWNTs. A simple synthetic protocol relying on nucleophilic addition instead of radical-based reactions facilitates the introduction of even more red-shifted defects that are excellent single-photon emitters at room temperature (Nat. Commun. 2021, 12, 2119). Here, I will give an overview of our recent progress on the targeted functionalization of polymer-wrapped (6,5) SWNTs, their emission and charge transport properties with and without defects.

Organic photodetectors (OPDs) with a performance comparable to that of conventional inorganic ones have recently been demonstrated for the visible regime.[1] However, near-infrared photodetection at a high detectivity has been proven to be more challenging and, to date, the true potential of organic semiconductors in this spectral range (800‒2500 nm) remains largely unexplored. We have recently shown that the main factor limiting the specific detectivity ( ) is non-radiative recombination, which is also known to be the main contributor to open-circuit voltage losses in organic photovoltaics.[2] Based on this finding we concluded that OPDs have the potential to be a useful technology up to 2 μm, given that high external quantum efficiencies can be maintained at these low photon energies. To further elaborate on the fundamentals defining these limitations, a next-generation of ‘defect-free’ organic semiconductors is synthesized. This allows us to investigate the influence of material imperfections (end-capping, homocoupling defects, and other impurities) on the material properties and device performance. In this contribution, we give an overview of the synthetic approaches applied and the latest organic semiconductors for high performance NIR OPDs, approaching their intrinsic limits. [1] N. Li et al., Mater. Sci. Adn Eng., 146, 100643 (2021) [2] S. Gielen et al., Adv. Mater., 32, 2003818 (2020)

All polymer solar cells (all-PSCs), consisting of a binary blend of a polymer donor (PD) and a polymer acceptor (PA), possess important advantages including enhanced morphological stability, improved mechanical flexibility, and better compatibility with large-area roll-to-roll production over small molecule acceptor (SMA)-based PSCs. High efficiency and mechanical robustness are both crucial for the practical applications of all-PSCs in stretchable and wearable electronics. In this regard, we report a series of new polymer acceptors (PAs) by incorporating a flexible conjugation-break spacer (FCBS) to achieve highly efficient and mechanically robust all-PSCs. Incorporation of FCBS affords the effective modulation of the crystallinity and pre-aggregation of the PAs, and achieves the optimal blend morphology with polymer donor (PD), increasing both the photovoltaic and mechanical properties of all-PSCs. In particular, the all-PSC based on PYTS-0.3 as a PA incorporated with 30% FCBS and a PD (PBDB-T) demonstrates a high power conversion efficiency (PCE) of 14.68% and excellent mechanical stretchability with a crack onset strain (COS) of 21.64% and toughness of 3.86 MJ m–3, which is significantly superior to those of devices with the PA without the FCBS (PYTS-0.0, PCE = 13.01% and toughness = 2.70 MJ m–3). To date, this COS is the highest value reported for PSCs with PCEs of over 8% without any insulating additives. These results reveal that the introduction of FCBS into the conjugated backbone is a highly feasible strategy to simultaneously improve the PCE and stretchability of PSCs resulting in their reliable applications in stretchable electronics.

Hybrid Organic Inorganic Perovskites (HOIPs) have attracted a lot of attention since in 2009 a thin-film solar cell was produced with a PCE of almost 4%. Since then record after record in PCEs of perovskite solar cells using so-called 3D hybrid perovskites has been broken, reaching nowadays a PCE of more than 25%. The main drawback relates to the limited stability of performances by defects and/or ion mobility in the inorganic lattice. In an effort to improve stability and reduce hysteresis of the solar cells quasi 2D hybrid perovskites have been explored, quite often using besides methylammonium cations larger cations, e.g. butylammonium or more often phenylethylammonium cations. In recent years our research group is exploring the use of more complex and potentially functional larger cations composed of pyrene or carbazole subunits. We could demonstrate a substantial improvement of the thermal stability and under high relative humidity (77%) of active layers for perovskite solar cells composed of such quasi 2D hybrid perovskites. In this contribution, we discuss the basic mechanism of this stability effect. Furthermore, we present some recent results obtained using oligothiophene and a benzothieno[3,2-b]benzothiophene (BTBT) alkylammonium cation into the organic layer of a 2D layered lead iodide perovskite. Structural characterization, phase stability, and photoconductivity measurements of (n=1) and quasi 2D perovskites will be presented. Extraordinary high stability was observed for such layers under thermal stress (>240°C) and under high moisture conditions. Potential mechanisms and implications for alternative structures will be discussed.

Supramolecular structures essentially determine photonic and electronic material properties.1 To control supramolecular structures, we utilize interface assembly of amphiphilic dyes, essentially via the Langmuir-Blodgett (LB) technique, to yield well-defined layers and stacks with tailored optoelectronic properties. We demonstrate how different condensed phases can be accessed and how molecular orientations2 as well as intermolecular binding energies can be characterized directly at the interface via Langmuir isotherms and their hysteresis, respectively.3, 4 Using the latter techniques, free-standing, though being only covalently bound, bifacial dye membranes have been developed. For the example of amphiphilic fullerenes, we demonstrate a new continuously operating rolling Langmuir-Schaefer transfer of monolayers at varied packing densities, thus enabling a LUMO energy variation of the films by 70 meV as determined using square-wave-voltammetry. Such LUMO energy shifts could be transferred linearly to variation in open circuit voltage of corresponding pi-Si-hybrid solar cells.5 REFERENCES: 1. S. Das and M. Presselt, Journal of Materials Chemistry C, 2019, 7, 6194-6216. 2. M. L. Hupfer, S. Ghosh, Y. Wang, T. Opsomer, W. Dehaen and M. Presselt, Advanced Materials Interfaces, 2021, DOI: 10.1002/admi.202101490. 3. M. L. Hupfer, M. Kaufmann, F. Herrmann-Westendorf, T. Sachse, L. Roussille, K. H. Feller, D. Weiss, V. Deckert, R. Beckert, B. Dietzek and M. Presselt, ACS Appl Mater Interfaces, 2017, 9, 44181-44191. 4. M. L. Hupfer, M. Kaufmann, S. May, J. Preiss, D. Weiss, B. Dietzek, R. Beckert and M. Presselt, Phys Chem Chem Phys, 2019, 21, 13241-13247. 5. S. Das, J. Preiß, J. Plentz, U. Brückner, M. von der Lühe, O. Eckardt, A. Dathe, F. H. Schacher, E. Täuscher, U. Ritter, A. Csáki, G. Andrä, B. Dietzek and M. Presselt, Advanced Energy Materials, 2018, 8, 1801737.

Photovoltaic spatial light modulators form a new class of dynamic glazing that could be of interest to smart windows applications. The structure of the modulators includes a twisted nematic liquid crystal layer and an organic donor-acceptor bulk heterojunction. The latter is in contact with the liquid crystal and is used as a molecular alignment layer. In addition, under illumination, the bulk heterojunction generates an electric field that can be strong enough to orient the liquid crystal molecules homeotropically and change the device optical transmittance, without requiring an external power source. The transmittance of this hybrid device adjusts spontaneously to ambient light within less than a second, with a sensitivity that can be tuned by a passive resistor. While this unique combination of features is desirable for smart windows, the device maximum transmittance in the clear state is currently limiting the possible scope of application. In this contribution, we will firstly present the detailed structure, elaboration procedure and optical properties of a first generation of photovoltaic spatial light modulators that are based on commercially available polymer:fullerene blends and liquid crystals. The physical mechanism underlying the device operation will be demonstrated by crossed-polarizer intensity measurements as a function of incident light intensity and applied voltages. Furthermore, the time-dependent transmittance of a device that is exposed to a pulsed light source will be presented in order to assess its response time and reversibility. In the second part we will describe various routes that we are following to improve the device optical properties in terms of maximum transmittance and sensitivity to ambient light. In particular, a new high band-gap semiconducting molecule that has been designed to achieve a highly transparent bulk heterojunction layer and increase the photo-induced electric field will be presented and its utilization in photovoltaic spatial light modulators will be shown.

In recent works, we have used transient absorption and ultrafast charge extraction measurements to show that photocreated charges do not thermalize in the disordered density of states that is typical of organic semiconductors. [1] Here, we discuss how this impacts the performance of the device and how the slow thermalization might be used to boost the open circuit voltage Voc and the fill factor FF. The basis of this work is a calibrated kinetic Monte Carlo (kMC) model that we compare to a wide range of experiments and to drift-diffusion calculations. [2] Specifically, comparing the kMC model to both DD simulations and to an analytical model based on Rau’s reciprocity model, [3] shows that the non-equilibrium nature of the photogenerated charge carriers leads to a 0.1-0.2 eV enhancement of Voc as compared to a situation where thermalization is instantaneous. [4] In IV-curves, this is directly visible from the difference between the curves that are calculated by DD and kMC using the same parameters. Interestingly, carefully chosen composition gradients can be used to further enhance Voc and the FF. [5] REFERENCES: [1] A. Melianas and M. Kemerink, Photogenerated Charge Transport in Organic Electronic Materials: Experiments Confirmed by Simulations, Advanced Materials 31, 1806004 (2019). [2] S. Wilken, T. Upreti, A. Melianas, S. Dahlström, G. Persson, E. Olsson, R. Österbacka, and M. Kemerink, Experimentally Calibrated Kinetic Monte Carlo Model Reproduces Organic Solar Cell Current-Voltage Curve, Sol. RRL 4, 2000029 (2020). [3] T. Kirchartz, J. Nelson, and U. Rau, Reciprocity between Charge Injection and Extraction and Its Influence on the Interpretation of Electroluminescence Spectra in Organic Solar Cells, Phys. Rev. Applied 5, 054003 (2016). [4] T. Upreti, S. Wilken, H. Zhang, and M. Kemerink, Slow Relaxation of Photogenerated Charge Carriers Boosts Open-Circuit Voltage of Organic Solar Cells, J. Phys. Chem. Lett. 8 (2021). [5] O. Andersson and M. Kemerink, Enhancing Open-Circuit Voltage in Gradient Organic Solar Cells by Rectifying Thermalization Losses, Solar RRL 4, 2000400 (n.d.).

The integration of multiple devices in a single functional unit is boosting the advent of a series of compact optical sensors for rapid and on-site analysis. In this context, the huge potential of plasmonic-based sensors has been affected by the strict constraints of the detection scheme. The need for laboratory equipment, such as laser sources and expensive prism-based optics, results therefore in not-portable systems. Here, an ultra-compact plasmonic sensor is demonstrated through the smart-integration of an organic light-emitting transistor (OLET), an organic photodiode (OPD), and a nanostructured plasmonic grating (NPG).[1] The direct integration of the OPD onto the planar structure of the OLET provided an unprecedented high degree of proximity of the light-source and light detecting areas, which enabled the exploitation of the angle-dependent sensing characteristics of the NPG. The most effective 3D layout of integration, including the optimal size and relative positioning of the three elements (i.e. OLET, OPD, and NPG), was unravelled by an advanced simulation tool, which also predicted the signal variation of the sensor under different conditions. Accordingly, the effectiveness of the new plasmonic-based detection scheme was demonstrated by the dependence of the OPD photocurrent on the surrounding environment of the NPG. In particular, a variation of the OPD photocurrent of about 10-9 A was recorded when exposing the NPG from water to alcoholic solutions at different concentrations. A miniaturized plasmonic sensor with a total size of 0.1 cm3 was therefore obtained through the smart integration of nanometer-thick optoelectronic and plasmonic components. [1] M. Prosa, et. al. Adv. Funct. Mater. 2021, 2104927. https://doi.org/10.1002/adfm.202104927

Hybrid organic-inorganic perovskites (HOIPs) have received a lot of research attention over the past decade, related to the rapid increase in the power conversion efficiency of perovskite solar cells. The materials used for solar cells are mainly three-dimensional (3D) HOIPs, with a general formula of ABX3 with A being a small monovalent organic cation, B a divalent metal ion, and X a halide anion. More recently, the related material class of 2D HOIPs, with a general formula of (A*)2BX4, is receiving increased attention by combining a generally enhanced material stability compared to 3D HOIPs with a much higher degree of compositional flexibility. 2D HOIPs can accommodate bulkier organic cations (A*) with a conjugated organic core. Depending on the relative alignment between the frontier energy levels of the organic core and the inorganic framework, energy/charge transfer between the components of the hybrid is possible. We built in a carbazole derivative as the organic cation into a 2D HOIP. Through electron paramagnetic resonance experiments combined with computational calculations, we show that excitons generated in the inorganic layer undergo charge transfer at the organic−inorganic interface, resulting in a positive polaron delocalized over several carbazole moieties. In another material system, we incorporate an organic charge-transfer complex (CTC) with a pyrene derivative as the donor and TCNQ as the acceptor into the organic layer of a 2D HOIP. Based on time-resolved spectroscopy, we show that holes are transferred to the inorganic layer upon excitation of the CTC while electrons stay localized on TCNQ acceptor molecules.

In recent years, organic molecules with stable open-shell ground states have attracted growing interest due to their outstanding properties, i.e. responsive spin structures, high-spin ground states, two-photon absorption, or small band gap. Although a growing number of interesting materials has appeared, molecules often lack in thermal stability impeding their application in electronic devices. In this presentation, we will highlight routes but also dead ends in the quest for high-spin configurations in hydrocarbons. We benchmark a computational approach for the characterization of open-shell organic structures, which combines predictability with appropriate simulation resources. For polycyclic heteroaromatic hydrocarbons containing a benzoisoindole core, we explain why a supposedly open-shell material does not provide the desired characteristics [1]. On the contrary, we discuss the promising characteristics of stable polycyclic hydrocarbon diradicaloids as well as related tetraradicaloids [2,3,4]. We demonstrate that significant optimization of material properties can be achieved already by chemical functionalization, while the full potential of promising material groups like bisphenalenyl-based molecules is far from being fully exploited. References: [1] M. Richter, K. S. Schellhammer, et al., Org. Chem. Front. 4, 847 (2017). [2] J. Ma, et al., Angew. Chem. Int. Ed. 56, 3280 (2017). [3] J. Ma, et. al., Chem. Sci. 10, 4025 (2019). [4] M. R. Ajayakumar et al., Angew. Chem. Int. Ed. 60, 13853 (2021).

By using the room temperature phosphorescence of organic materials, transparent and flexible tags with resolutions of up to 700 dpi and an activation time of 120 ± 20 ms can be achieved. Any pattern can be written into the film by selectively illuminating the film with ultraviolet light through a shadow mask. The inscribed information can then be erased again with the aid of a heat source, and the overall cycle can be repeated many times (> 40 tested). The functional layer of these devices consists of organic biluminescent emitters doped into a poly(methyl methacrylate) (PMMA) host matrix covered with an oxygen barrier layer. By wet processing under ambient conditions, molecular oxygen is present in the emission layer, hence quenching the phosphorescence effectively. During this quenching process, highly reactive singlet oxygen is generated, which can chemically bond to the surrounding environment. Therefore, the molecular oxygen concentration decreases in the illuminated areas. The decreasing oxygen quenching is at some point outcompeted by the radiative rate of the emitter, enabling locally resolved phosphorescence in shape of the used mask. The oxygen permeability of the barrier layer is temperature dependent and increases with rising temperature. This enables an oxygen refilling of the functional layer by heating the device and thus increases the quenching rate to a value prior to the activation process. After a short cooling phase, the film can be rewritten. Under continuous-wave (cw) illumination, any luminescence that is independent of the oxygen concentration, be it fluorescence or phosphorescence with a high radiative rate, limits the contrast between activated and oxygenated areas. Therefore, different material systems are developed and tested, showing reduced fluorescence but maintaining the oxygen quenching ability of the phosphorescence. The emitter 4,4‘-dithianthrene-1-yl-benzophenone (BP-2TA), with a phosphorescence lifetime of 30 ms and a high phosphorescence-to-fluorescence ratio of about 20, fulfills these requirements best and thus enables high-contrast tags that can also be read under cw illumination.

Several emerging applications relying on absorption and re-emission of light by molecular species require a photo-luminescence quantum yield (PLQY) near unity. However, when the PLQY approaches unity and the optical density is low, it is surprisingly difficult to measure accurately, as the most commonly used techniques rely either on exact knowledge of material parameters or on a comparison to a measurement of a non-fluorescent material, introducing errors in the measurement, further complicated by possible non-linearities of the detection system. In this work, we propose a new setup which circumvents these issues. It involves a simultaneous relative measurement of the produced heat, the amount of re-emitted light and the average energy of the emitted photons, allowing us to determine the fraction of absorbed energy which is converted to heat. Crucial to the set-up is a tunable continuous-wave laser, allowing to perform these measurements at varying excitation energies. The photon energy dependent part of the fraction of absorbed energy converted to heat is due to an increased thermalization energy with increasing incident photon energy. The remaining heat produced by the fully thermalized excitation determines the fraction of absorbed energy not converted to photons and thus the deviation of the PLQY from unity. Using this new method, we determine the PLQY of seven different luminescent molecules in low concentration solutions, and use a rigorous error analysis to achieving PLQY measurements with an unprecedented accuracy down to ±0.3% within 95% confidence intervals. Perylene orange is found to have the highest PLQY of the molecules studied, being 99.0±0.3%.

Strong light-matter interactions in optical microcavities can form exciton-polaritons – a superposition state of photons and excitons. Above a threshold excitation density, these bosonic quasiparticles can condense into the lowest energy state as a polariton laser. This does not require population inversion and so offers potential to generate coherent light with very low threshold. Organic semiconductors are attractive materials for microcavity polaritons due to their high exciton binding energy and oscillator strength. It is well reported in inorganic semiconductor microcavities that the threshold for polariton lasing is lower than that for conventional photon lasing. However, polariton lasing thresholds in organic semiconductors are relatively higher than organic photon lasers. Here, we present a study of strong coupling and polariton lasing in highly soluble fluorene-carbazolylstyryl materials, which exhibit high absorption coefficients, PLQY and radiative rates. We observe polariton lasing under 100 fs optical pumping, with absorbed pump threshold densities as low as 2.2 µJ/cm2. This is the lowest threshold reported to date for polariton lasing in organic microcavities, and comparable to state-of-the-art organic cavity surface-emitting lasers. A step-like blueshift at polariton lasing threshold indicates the interplay between different exciton depletion channels. Our study also provides insight to the design principles for an optimal polariton lasing threshold. Through a comparison of the photophysics and polariton laser performance of these materials with those of other reported polariton lasers, we identify key materials and cavity properties required for further development of low threshold organic polariton lasers.

Recently Frenkel exciton polaritons (EPs) in organic materials and biological structures attracted considerable interest in relation to their Bose-Einstein condensation, low threshold polariton lasing, and polariton chemistry in microcavity. The concept of EPs suggests strong light-matter interaction that is supported by a considerably larger oscillator strength of organic systems compared to inorganic semiconductors. Polariton absorption of electron-vibrational systems was considered in nanofibers of organic dyes using a realistic non-Markovian model of molecular vibrations [1], and in a single-mode cavity in the presence of Brownian dissipation [2]. Theoretical description of EP luminescence in molecular systems is a challenge since in this case both the interaction with radiation field and electron-vibrational interaction are strong. Elastic cavity emission of polaritons in a single-mode microcavity in the presence of Brownian dissipation from molecular vibrations was considered in Ref.[2] for a specific case when polariton linewidth is the mean of cavity and molecular linewidth, although only at resonance when the cavity frequency is equal to that of a molecular resonance. Meanwhile, the interaction of the polariton with molecular vibrations should depend on exciton contribution to EP. In this work using a realistic non-Markovian theory for the description of the polariton-molecular vibrations interaction we calculate polariton luminescence in the polariton basis. We show that the frequency shift and broadening of polariton luminescence spectra strongly depend on the exciton contribution to the exciton polariton that is a function of frequency. Our non-Markovian theory predicts Fano resonances in EP luminescence and narrowing of its spectrum with an increase in the number of molecules in a single-mode microcavity. We also consider non-equilibrium (hot) EP luminescence that opens a way for its observation in organic-based nanodevices. [1] B.D. Fainberg, J. Phys. Chem. C 123, 7366 (2019). [2] K.S.U. Kansanen, J.J. Toppari, and T.T. Heikkila, J. Chem. Phys. 154, 044108 (2021).

Organic solids can be used as emitters in organic solid-state lasers. As one of the most popular red light-emitting organic laser dyes is DCM that has been widely used in a dye laser. It has been used in solid-state lasers. The active material was host-guest systems where DCM was doped at low concentration due to large intermolecular interactions. We previously developed original glass-forming DCM derivatives were attached bulky triphenyl groups reduced intermolecular interaction thus it was possible to observe amplified spontaneous emission (ASE) in the neat films. In this work, we are presenting new derivatives with modified electron acceptor and donor groups. The investigated compounds are pyranyliden derivatives with malononitrile and 1H-indene-1,3(2H)-dione as acceptor and bis(5,5,5-triphenylpentyl)amino- and 5,5,5-triphenylpentyl-piperazin-1-yl- as donor. Photoluminescence and ASE were investigated in thin films. All compounds emit light in red spectral regain with the photoluminescence quantum yield of up to 6%. Amplified spontaneous emission excitation threshold energy was about 100 μJ/cm2. Silver nanoparticles were used to increase the performance of the compounds. Triangular nanoparticles with the surface plasmon resonance (SPR) overlapping the absorption and emission spectra of compounds were used. Metal nanoparticles were dissolved in the organic solution with investigated compounds. The solution was ultrasonicated prior to deposition on a glass substrate by the spin-coating method. SPR enhance the absorption and emission of the investigated compounds. Accordingly, it was observed as at least two times increase in the PLQY of mixed thin films and more than two times were decreased excitation threshold energy of ASE. More detailed results and metal nanoparticle influence on emission properties of investigated compounds will be discussed.

Organic electronics offer many new applications with properties like like flexibility, easy manufacturing, biocompatibility, etc. While OLED displays are commercially successful, organic circuits have not yet reached broad applications, mainly since device performance is lacking. In this talk, I will we present new approaches based on highly ordered organic multilayer structures which have the potential to increase device performance by orders of magnitude. Specifically, triclinic rubrene crystals are created by heating of amorphous layers and can be electrically doped during the epitaxial growth process to achieve hole or electron conduction. Analysis of the space charge limited current in these films reveals record vertical mobilities of 10cm2/Vs. To demonstrate the performance of this materials system, pin-diodes with record threshold frequency of 3.7 GHz are built, paving the way for future high-performance organic electronic devices based on carefully designed heterostructures. Furthermore, these structures allow comparatively long minority carrier diffusion lengths, allowing to realize the first organic bipolar transistor.

Superfluorescence is a quantum phenomenon in which an ensemble of photoexcited dipoles are spontaneously synchronized leading to the formation of a macroscopic quantum coherence phase, forming a giant dipole and emitting a huge photon burst. Similar to other second order quantum phase transitions such as Bose-Einstein condensation and superconductivity, superfluorescence also requires the formation of a macroscopic coherence of the photoexcited quantum oscillators. The key to reach this macroscopic coherent quantum state is its phase stability and the synchronization time. Typically, due to phonon scattering, superfluorescence in solids can only occur at extremely low temperatures. In this presentation, we will present results of our recent discovery of high temperature superfluorescence in hybrid perovskites. Here, we report superfluorescence in hybrid perovskites at temperatures beyond 78 K, which is significantly higher than SF phase transition temperatures reported in other solids. Based on our analysis, we propose a model describing how the quantum phase is protected in hybrid perovskites and explain why high temperature superfluorescence can be observed in this unique material system. Specifically, using our model, we will explain how the synchronization time and the quantum phase stability are affected by the material composition and temperature. We postulate that our model can be used to explain other quantum phenomena as well.

The development of thermally activated delayed fluorescent (TADF) emitters has led to the realization of 100% in internal quantum efficiencies (ηint) of organic light-emitting diodes (OLEDs). Realizing 100% in external quantum efficiencies (ηext), however, continues to be limited by only 20% in the light outcoupling efficiency (ηout). Outcoupling enhancement (ηout) typically employs structures that redirect substrate guided and organic waveguided modes, however, they often require complex and multiple fabrication steps that may induce colour shifting with increasing viewing angle. Conversely, structures that suppress the colour shifting have no effect on the Δηout. This work demonstrates a solution in using a simple, one-step Yb-doped femtosecond laser (IMRA America, Inc.) used to pattern inverted holes at the air/glass substrate interface of the OLED which are useful for simultaneously reducing spectral shifting and redirecting substrate guided modes for Δηout [1,2]. Compared to reference devices which utilize a planar glass substrate, a maximum Δηout of about 31% and 60% was achieved for patterned devices (with intrinsically high and low transmittance, respectively) having conically shaped holes of optimized periodicity of 7 μm and a depth of 10 μm. Additionally, where the EL spectra peak shifts by ~11 nm in reference devices between 0o to 70o viewing angle, all patterned devices demonstrate only a 3-5 nm shift. The Δηout and reduction in spectral shifting is attributed to the extraction of substrate guided modes into free space as a result of smaller ray contact angle made with the conical surface and suppression of interference effects due to light scattering, respectively [1,2]. This work is expected to further the attractiveness of OLED devices to the general lighting industry given high light extraction efficiency and maintenance of colour purity. Where femtosecond laser technology has primary applications in the medical (LASIK surgery) and industrial fields (lithography), their broader applicability to the field of organic opto-electronics has now been demonstrated. [1] S. Lloyd, T. Tanigawa, H. Sakai and H. Murata, IEICE Trans. Electron. 102 (2), 180 (2019). [2] S. Lloyd, K. Higashimine, T. Tanigawa and H. Murata, Jpn. J. Appl. Phys., 60 (SB), SBBG01 (2021).

In the current work we will present the transfer of graphene pixels and arrays. The process we used to accomplish the transfer was the Laser Induced Transfer technique. We will exhibit the advantages of the certain technique, the resolution of the transferred pixels and the characterization of the samples. Also, we will demonstrate the transfer of graphene arrays on both flexible polymeric and rigid substrates. The transfer of the graphene pixels was accomplished for resolution from 40 μm to 15 μm, whereas the digital manipulation of the transfer enables the deposition of arrays with dimension up to 1 mm2. Furthermore, in this work we will present the fabrication of a flexible capacitance touch sensor. The device is composed by LIFT transfer bottom electrodes and top electrodes and between them we deposit a insulating layer with the assistance of a spin-coater. For bottom electrodes we used silver nanoparticles ink, which was LIFT transferred and laser sintered in order to be conductive and form two pads with a line to connect them. The top electrode is a transferred graphene array with dimension at 1 mm2 and the insulating layer we used is an in-house fabricated PDMS, which was deposited on the half of the bottom electrodes. The quality and quantity of graphene layers were measured and characterized with Scanning Electron Microscopy and micro-Raman spectroscopy. Electrical measurements were conducted on the same samples in order to receive the sheet resistance and the capacitance values. The capacitance measurements were conducted at the top electrode of the flexible touch sensor. Finally, we will introduce the next step in the demonstration of the touch sensor, which is the application of tensile or compressive stress on the flexible touch sensor, which will result in accumulation of different capacitance.

Organic semiconductors have received increasing interest in sensing the explosive molecules emitted from landmines and improvised explosives devices (IEDs), especially when used as fluorescent sensors which are highly sensitive, easy to fabricate, and relatively cheaper when compared to other sensing technologies. Sensitivity down to ppb can be observed as quenching of photoexcited excitons in the fluorescent films, due to electron transfer from the excitons in the film to the nitroaromatic explosives molecules. However, for many organic fluorescent sensors, the quenching is irreversible or exhibits extremely slow reversibility due to strong binding interactions between the sensors and analytes, which imposes a limitation in terms of reusability. Here we present a study of the control of a fluorescent sensor’s temperature to overcome the binding interaction, and make the quenching reversible. Specifically, we study the thermal release of 2,4-DNT from the commercial fluorescent polymer, Super Yellow. Thermal cycling of the sensors results in recovery of fluorescence making them reusable. Interestingly, the release of 2,4-DNT occurs at a specific temperature which can be explored to discriminate which type of analytes are being released from the sensors. To optimise the sensors, blends of SY with Poly carbazole (PVK) were fabricated, and the thermal release of various nitroaromatic analytes were characterised. This method can be applied to other organic fluorescent sensors in a route to make them reusable, and give a selective response based on the temperature at which analytes are released.

Materials for organic light-emitting diodes could be divided into several generations were last 3rd generation organic compounds exhibiting TADF which exhibit up to 100% singlet and triplet state conversation to light without the presence of heavy metal. Organometallic complexes consisting of Cu metal atom is a conceptually different class of low-cost TADF emitters. While this element does not exhibit fast room-temperature phosphorescence, it still can promote a rapid TADF, despite the considerable ΔEST gaps met in such compounds (< 0.05 eV). In this work, we present a novel approach towards TADF emitters where charge transfer through-space and metal-assisted take place. Cu complexes bearing carbazolide and carbene ligands were synthesized. Imidazole-based N-heterocyclic carbenes (NHCs) were used, with electron-accepting sulphonyl groups introduced at 4-position of the N-bound 2,6-diisopropylphenyl (Dipp) substituents. Photoluminescence bands, decays and quantum yields of complexes in various solutions, as well as PMMA-doped films with emitter concentration 5 wt%, were taken. Emission properties of thin films in the temperature diapason from 10 to 300 K were measured to estimate the charge transfer mechanism. For PL bands at room temperature excitation wavelength was either 300 or 375 nm. For temperature-induced CT emission band shifts in vacuum 345 nm excitation wavelength was used, delay 50 ns and width 10 μs. To get PL decays at room temperature a laser with a wavelength of 375 nm was used and decay was measured at the peak wavelength. For temperature-induced decays in vacuum used excitation was 345 nm, time range 50 µs. To get quantum yields at room temperature, emission and scattering ranges of the sample were measured separately with 375 nm excitation. Photoluminescence quantum yield from 0.2 to 0.3 was obtained in solutions but in thin films, it increased up to 0.9 due to the decreased torsional degree of freedom. Emission decays could be described by three exponential functions in all temperature intervals. One featuring a prompt and two delayed emission components. For both delayed emissions, the fastest and slowest emission rate decrease is observed throughout the cooling range, designating a thermally activated nature of the emissive process. The corresponding ΔEST values were estimated from Arrhenius plots and are in the range of 0.0062−0.0075 eV. By combining through-space CT architecture, giving compounds the characteristic low ΔEST values, with the presence of a heavy metal atom, which provides small, but still present SOC.