We report experimental and theoretical investigations with high-speed μ-OLEDs and demonstrate promising optical pulse responses as short as 400 ps using Alq3. These observations indicate that high-speed μ-OLEDs can be used for light communication in the GHz regime. The measurements are for in-house fabricated μ-OLEDs without cavity and size of 100 μm × 100 μm. With a validated model for an electrically pumped OLED, we simulate the generation of ultra-short optical pulses. The model includes Stoke-shifted reabsorption and field-enhanced Langevin recombination rate. For the Alq3 system we compare the results with the above-mentioned measurements. The good agreement between the measurement and the simulation is the basis for further study of the prospects for ultra-short dynamics and organic laser diode operation on the ps time scale.

External light-outcoupling structures in organic light-emitting diodes (OLEDs) present an efficient and cost-effective solution to improve optical performance of devices. External structures can be realized, for example, by applying a microtextured foil on the emitting side of the device. We employ advanced three-dimensional optical simulations to investigate the effects of OLED structures with different external textures in relation to different emitting dipole orientations and layer thicknesses of the OLED stack. We investigate light outcoupling of a red, ITO free, large-area, bottom-emitting OLED, by applying various sinusoidal and pyramid-like external microtextures in periodic (rectangular and hexagonal) and random arrangements. It is shown that there are minimal differences in outcoupling efficiency for the optimized textures for different texture feature shapes, under condition that sufficiently high aspect ratio (height / period) of the texture is applied. With optimized external texture with three-sided micropyramids, horizontally aligned dipole sources and optimized layer thicknesses of OLED stack more than 62 % outcoupling efficiency is predicted. Interestingly, the results show that the highest efficiencies for individual orientation of dipoles, can be achieved with the same shape and size of the texture, regardless of the preferential orientation of the emitting dipoles, which simplifies further research and development of external light extraction.

Organic semiconductor materials such as planar conjugated small molecules are of great interest to the photovoltaic community. In thin films, the exciton and charge carrier dynamics, which are crucial to photovoltaic device operation, depend in a non-trivial way on the organic molecular structure and on the molecular organization in the solid state. Recently, the exciton diffusion has been found to strongly depend on the crystalline order of the organic thin films. This work presents the study of the exciton lifetime in an innovative class of molecular semiconductors able to present different crystalline order. This family of molecules has a “dumbbell-shaped” structure based on triazatruxene units that act as a π-stacking platform. Such molecules with different side-chains have been found to self-assemble into various crystalline and liquid crystalline phases. We have studied the steady-state photoluminescence and the exciton lifetime for several triazatruxene-based derivatives with different side-chains, in solution and in thin films for different solid state phases. In solution, the fluorescence lifetime corresponds to the reference value that can be obtained without intermolecular interaction. In thin films, we measured the exciton lifetime for different molecular structures in order to correlate the exciton dynamics with the molecular stacking. The results reveal a significant increase in the exciton lifetime with the enhancement of the structural order.

Exciton diffusion plays a decisive role in determining the recent rise in charge-generation yield (CGY) and powerconversion efficiency brought about by non-fullerene acceptor (NFA) solar cells. In this presentation a technique, named pulsed-PLQY, is introduced to measure the exciton diffusion length in organic semiconductors through exciton-exciton annihilation (EEA) without the need for temporal measurements. Pulsed-PLQY is validated by comparing to established EEA techniques using both simulated and experimental results. It is found that Pulsed-PLQY conserves the validity of established EEA techniques while the implementation is faster, easier, significantly simplifies the equipment required, and is less sensitive to experimental conditions than traditional EEA techniques. Utilising pulsed-PLQY, it is found that NFA’s have increased diffusion lengths, compared to fullerene acceptors, and that this increase is driven by increases in diffusivity

Helical molecular orbitals forming in oligoynes (chains of sp hybridized carbons) are intriguing for control of electron’s spin in simple organic molecules. Spin-Orbit coupling induced by mixing of orthogonal p electron orbitals leads to enhanced singlet-to-triplet intersystem crossing. However, the role of conformers for spin-orbit coupling is still unclear in oligoyne-bridged systems. Herein, we report a theoretical study of spin-orbit coupling and intersystem crossing rates as a function of axial torsion between bifluorene end-fragments bridged by various length oligoynes. Density functional theory computations revealed that conformers can exist at room temperature due to low torsional barrier. Highest calculated intersystem-crossing rates were found for torsional angles around 30° and agreed well with experimental values. Elongation of oligoyne bridge up to three triple bond fragments leads to steep increase of spin-orbit coupling, and thus, intersystem-crossing rates. Interestingly, at intermediate torsional angles oligoyne bridge allows to maintain high oscillator strength of the lowest singlet transition together with substantial spin-orbit coupling, which is a desirable property for photosensitizer and emitter molecules.

In this study we are interested in the implementation of mixed processes for the realization of symmetrical structures shaped in thin layers for integrated photonics and based on silicon plus organic UV210. We used the so called UV210 polymer for shaping the core waveguide. The UV210 polymer made up of poly (p-hydroxystyrene) and poly (t-butyl acrylate) is a chemically amplified resin; a photo-acid generator is added to the matrix of the copolymer in order to increase the sensitivity of the resin and create a chain reaction during developments so as to develop sub-wavelength patterns. Several families of multilayers structures have been produced by specific sub-wavelength lithography plus PECVD, and then properly characterized by including stoichiometry analyses plus imaging by Raman. The advantage of achieving Si/SiO₂/UV210/Si/SiO₂ symmetry relates first of all to the equations of electromagnetism and guidance which no longer impose a cut-off thickness (or frequency) during extreme miniaturization, but also for an adequate protection of the components by an upper layer of silicon covering the surface of the chip for sensor applications and specific detection of aggressive substances / agents. All structures, including the addition of silicon directly onto the organic, exhibit excellent mechanical strength and optical stability; the last silicon/silica bilayer also acts as a thin protective shell. Various families of resonant photonic structures could be cleanly characterized on platform. Furthermore, by statistical measurements of resonance parameters, we conclude that the processes and properties of the materials obtained have good reproducibility. This opens the way to the realization of sensors dedicated to aggressive substances directly in contact with the resonant elements probing it.

The synthesis of a novel imidazolidine type N-heterocyclic carbene (NHC) Cu (I) complex with asymmetrically attached phenylsulfonyl- acceptor group is presented. The asymmetrical ligand was used for the preparation two Cu(I) carbene-metal-amide (CMA) complexes with carbazolide (complex 1) or 1,8-dimethylcarbazolide (complex 2) as the amide donors. Obtained complexes exhibit efficient thermally activated delayed fluorescence (TADF) with luminescence quantum yields (QY) reaching 0.80 in PMMA matrix. Metal promoted through-space charge transfer approach in the emitter design lowers the ΔΕ_ST gap and the small spin-orbit coupling (SOC) provided by metal atom enables high radiative rates (k_r = 2.21×10⁵ s^-1 for complex 2). The photophysical properties of the asymmetrical (1-2) and previously reported symmetrical (3-6) molecular designs are compared. Obtained results suggest a closely similar photophysical behavior for both the asymmetrical and symmetrical CMA complexes.

The impact of metal atom on the photophysical properties of luminescent organometallic carbene-metal-amide complexes exhibiting through-space charge transfer is investigated. The substitution of copper atom with gold alters the excited state energy level configuration of the emitter. While in the copper-based emitters the lowest triplet excited state (T₁) is related to a through-space charge transfer between the carbazolide donor and carbene-bound phenylsulphonyl acceptor, in the gold-bearing structural analogue T₁ level is accompanied by an additional closely situated triplet state T₂, which features a charge transfer between the carbazolide donor and imidazolidine carbene acceptor. Because of a significant spin-orbit coupling provided by Au atom T₂ state exhibits relatively fast phosphorescence rate of 8×10⁴ s^-1. Consequently, the emissive process for the gold-functionalized compound can be characterized with a co-occurring thermally activated delayed fluorescence (TADF) and phosphorescence, in contrast to the copper-based structural analogues, where only TADF is observed.

Light-emitting electrochemical cells (LEEC) are relatively easy-to-fabricate thin-film lighting devices. Therefore, LEECs have been already successfully used in high-end smart lighting applications such as light-emitting clothes and emissive textiles, rollable and stretchable wallpaper-like lamps, and biocompatible light sources for in vivo or epidermal medical devices. However, most of the LEECs utilize ionic metal complexes as emitters that consist of the rear element Iridium. Ionic small molecules (SM) as light emitters could be a better solution to the low-cost device. In the work, we have investigated purely organic ionic compound that could exhibit aggregate induced emission. The compounds consist of the carbazole group and pyridinium ion as a cation. Perchlorate was used as an anion. Emission spectra, fluorescence kinetics and photoluminescence quantum yield (PLQY) were measured in polymer doped thin films. Polymethyl methacrylate (PMMA) was used as an additive. Photoluminescence quantum yield is 73% in powder and 45% in the pure thin film. It is further decreasing on the increase of PMMA mass ratio in the system which is the proof of aggregation-induced emission effect. The sample with the emitter:PMMA at mass ratio 9:1 exhibited the same emission properties as neat films but in addition was also optically clear those this system was used in LEEC. The LEEC cells with the simple structure ITO/PEDOT:PSS/Emitter:PMMA(9:1)/Al were prepared. The maximum brightness was 27 cd/m2 with the turn-on voltage of 8.7 V and CIE coordinates x=0.21 and y=0.29.