"Microdisplays", exhibiting pixel densities well beyond 1000ppi at screen sizes well below 1.4" have become key components for near-to-eye displays in assisted-, augmented-, virtual and mixed-reality (short: extended-reality "XR") devices. Specifically, their use in wearables (e.g., Smart Glasses), or electronic viewfinders puts strong performance requirements (e.g., high resolution, ultra-low power consumption, slim formfactor) on such components, due to limited space, weight and battery capacity, e.g., in spectacles' frames. This paper is reviewing major requirements and discussing approaches and achievements.

OLED microdisplays are of high interest for augmented-reality (AR) applications. In this paper, we present highly efficient monochrome single (1-u), tandem (2-u) and triple (3-u) unit top emitting OLEDs with a peak wavelength of 560 nm. Current efficiency, emission peak and brightness of these devices are compared using passive test substrates. A transfer from the test structure to a 0.62 ''-CMOS-backplane with SXGA resolution is shown. The influence of multi-stacked OLEDs on crosstalk is discussed.

Using smaller CMOS nodes for the design of backplanes for light modulator enables the integration of additional features. This paper reports on a backplane designed in 28nm technology which embeds a complete framebuffer as well as a programmable high speed data interface which can realize up to 576Gbit/s data transfer rate to pixel array utilizing a 1440 x 1080 resolution with a 2.5 micron pixel capable for LCOS, OLED and micro-LED front planes. The programmable modulation scheme will be discussed in detail using typical scanning examples.

Emissive OLED-on-silicon microdisplays have been considered being opaque only so far. However, modern and advanced silicon CMOS process nodes are increasingly made on silicon-on-insulator (SOI) substrates. By separating the SOI handle wafer from the buried oxide (BOX) layer (that has the active silicon on top) and applying space-cautious layout design of the CMOS active devices as well as wiring layers it is possible to achieve semitransparent, high-resolution CMOS backplanes for microdisplays. Similar to regular OLED-on-silicon the emissive frontplane becomes embedded by waferlevel OLED post-processing. Yet, depending on pixel density and array layout a microdisplay transparency of <20% can be achieved now. Consequently, the semi-transparent microdisplay becomes the optical combiner itself, eliminating the exit pupil expander (EPE), which drastically improves the optical efficiency from the light source into the eye box. Additionally, new high-brightness OLED achieving <35kcd/m² in monochrome, or 10kcd/m² in color versions, and their integration onto the OLED-on-SOI platform and an ultra-low power pixel cell backplane architecture (power consumption <10mW) pave the way for matching both form factor and battery life requirements in optical see-through NTE, enabling new optical concepts for augmented-reality (AR) devices.

OLED-on-Silicon technology has reached industry maturity for OLED (organic light emitting diode) microdisplays, e.g., in near-to-eye applications. Anyway, there is a huge amount of further applications, which can be addressed with an organic frontplane on an integrated CMOS backplane, e.g., OPD (organic photo diode). This paper will report on latest results in the field of OPD-on-silicon sensing as well as OLED-on-silicon displays, sketching up the impact of a universal photonic platform comprising emitters, photodetectors and CMOS driving and read-out circuitry, based on application scenarios.

OLED microdisplays have entered several professional and consumer near-to-eye visualization devices, such as VR/AR, assisted-reality or electronic viewfinder. Head-, helmet- or eyeglass frame-mounted displays, smart glasses or visors provide user information for human-machine interaction, situational awareness, personal safety, remote support or training. Display architecture and parameters, such as screen size, pixel density, resolution, color range and auxiliary functions can be varied to achieve high-resolution extended full-HD for VR/AR or ultra low-power options for long battery life in true wearables. This report is focusing on the design and characteristics of newly developed ultra-low power and slim form factor OLED microdisplay devices, featuring <0.2” screen diagonal, QVGA resolution at pixel density <2150ppi and monochrome as well as color versions.

Controlling the alignment of emitter molecules in the active layer of organic light-emitting diodes has become a main approach to maximize the device efficiency when emitter molecules with luminescence quantum yields approaching 100% are used. In order to guarantee stable device performance, the initial molecular orientation should not change over time. In this work, we study this property for a time frame of 1.5 years and storage temperatures up to 80 degrees C which may be reached in displays exposed to direct sun light. For the studied material systems, this temperature is close to the glass transition at which drastic morphological changes occur and a randomization of the molecule arrangement is expected. We compare two different phosphorescent emitter molecules and, additionally, investigate the impact of the substrate temperature during evaporation. Concluding this long-term study, we prove experimentally that the emitter orientation remains unchanged under those device-critical storage conditions. On the contrary, the fatal potential of heat-induced reorientation is revealed by post-annealing experiments that show a strong change of the emitter orientation at about 20 K above the glass transition temperature.

This paper reports on the latest development on ultra low‐power microdisplays. Initially this architecture has been presented in a monochrome version in [4]. Due to the demand of extended applications beyond pure information displays towards situation aware notifications a new colored version has been developed with the main achievement of cutting down the pixel size by more than a factor of two to enable color sub‐pixels. Furthermore special attention was paid on very high brightness to also enable implementation in situation‐aware augmented reality optics.

Organic light‐emitting materials exhibiting thermally activated delayed fluorescence (TADF) show great promise for improving display applications. Recently, intermolecular effects between emitting molecules have been given more attention, revealing strong solid‐state solvation or aggregation induced changes of sample performance. Implications of this on device performance are not yet fully covered. In this work, a thorough investigation of a novel TADF emitter, methyl 2,3,4,5,6‐penta(carbazol‐9‐yl)benzoate (5CzCO2Me), is provided. Steady‐state emission spectra reveal a luminescence redshift with increasing emitter concentration in a small molecule host. In all investigated concentrations, the emission profile remains the same; thus, the redshift is attributed to the solid‐state solvation effect. The highest photoluminescence quantum yield (PLQY) is achieved in the 20 wt% sample, reaching 66%. The best organic light‐emitting diode (OLED) in terms of current–voltage–luminance and external quantum efficiency (EQE) parameters is the device with 60 wt% emitter concentration, reaching maximal EQE values of 7.5%. It is shown that the emitter transports holes and that charge‐carrier recombination does not take place on the bandgap of the host, but rather, a mixed host–guest concentration‐dependent recombination is seen. The hole‐transporting properties of 5CzCO2Me allow for a new dimension in tuning the device performance by controlling the emitter concentration.

The feedback loop of OLED microdisplay information and human action is used to study human-machine interaction with the aim to create new ways of remote work and learning environments.

The efficiency of organic light-emitting diodes that utilize the principle of thermally activated delayed fluorescence (TADF) depends sensitively on the host material in which the TADF emitter molecules (guests) are embedded. Potential loss processes are “ deconfinement, ” the transfer of excitons from the guest to the host, and “ dissociation, ” the formation of intermolecular charge-transfer states. We investigate how both processes can be suppressed by studying the photoluminescence efficiency, emission spectrum, and time-resolved emission intensity of eight thin-film systems in which 5 mol. % of the sky-blue TADF emitter 4-carbazolyl-methylphthalimide (abbreviated here as CzPIMe) is embedded in various host materials. Deconfinement is found to be entirely suppressed if the triplet energy of the host is 0 : 25 eV or more above that of the guest. For systems allowing for deconfinement, the dependence on the energy difference is consistent with a recent theoretical analysis [C. Hauenstein et al. , J. Appl. Phys. 128 , 075501 (2020)]. Dissociation, due to hole transfer to a host molecule, is found to be suppressed if the host ’ s highest occupied molecular orbital energy is not more than about 0 : 2 eV higher than that of the guest. Otherwise, we observe an efficiency loss, a spectral redshift, and the disappearance of distinct prompt and delayed emission regimes. A comprehensive rate-equation model is developed from which we study the sensitivity of these observations to the energy level structure, the intermolecular interaction rates, and the photophysical rates that follow from a fit to the experimental data for the CzPIMe:TCTA [tris(4-carbazoyl-9-ylphenyl)amine] system.

Materials exhibiting thermally activated delayed fluorescence (TADF) have been extensively explored in the last decade. These emitters have great potential of being used in organic light-emitting diodes because they allow for high quantum efficiencies by utilizing triplet states via reverse intersystem crossing. In small molecules, this is done by spatially separating the highest occupied molecular orbital from the lowest unoccupied molecular orbital, forming an intramolecular charge-transfer (iCT) state and leading to a small energy difference between lowest excited singlet and triplet states (ΔEST). However, in polymer emitters, this is harder to achieve, and typical strategies usually include adding known TADF units as sidechains onto a polymer backbone. In a previous work, we proposed an alternative way to achieve a TADF polymer by repeating a non-TADF unit, polymerizing it via electron-donating carbazole moieties. The extended conjugation on the backbone reduced the ΔEST and allowed for an efficient TADF polymer. In this work, we present a more in-depth study of the shift from a non-TADF monomer to TADF oligomers. The monomer shows non-TADF emission, and we find the delayed emission to be of triplet-triplet annihilation origin. An iCT state is formed already in the dimer, leading to a much more efficient TADF emission. This is confirmed by an almost two-fold increase of photoluminescence quantum yield, a decrease in the delayed luminescence lifetime, and the respective spectral lineshapes of the molecules.

Hierarchical textures are researched experimentally and by optical simulations in the role of external outcoupling solution for OLEDs. Their full outcoupling potential, limiting factors and pathways for further increase in outcoupling are addressed.

Persistent luminescence from triplet excitons in organic molecules is rare, as fast non-radiative deactivation typically dominates over radiative transitions. This work demonstrates that the substitution of a hydrogen atom in a derivative of phenanthroimidazole with an N -phenyl ring can substantially stabilize the excited state. This stabilization converts an organic material without phosphorescence emission into a molecular system exhibiting efficient and ultralong afterglow phosphorescence at room temperature. Results from systematic photophysical investigations, kinetic modeling, excited-state dynamic modeling, and single-crystal structure analysis identify that the long-lived triplets originate from a reduction of intrinsic non-radiative molecular relaxations. Further modification of the N -phenyl ring with halogen atoms affects the afterglow lifetime and quantum yield. As a proof-of-concept, an anticounterfeiting device is demonstrated with a time-dependent Morse code feature for data encryption based on these emitters. A fundamental design principle is outlined to achieve long-lived and emissive triplet states by suppressing intrinsic non-radiative relaxations in the form of molecular vibrations or rotations.

Luminescent organic materials with high photostability are essential in optoelectronics, sensor, and photocatalysis applications. However, small organic molecules are generally sensitive to UV irradiation, giving rise to chemical decompositions. In this work, we demonstrate two novel CN-substituted two-dimensional sp(2)-carbon-linked conjugated polymers (2D CCPs) containing a chromophore triphenylene unit. The Knoevenagel polymerization between 2,3,6,7,10,11-hexakis(4-formylphenyl)-triphenylene (HFPTP) and 1,4-phenylenediacetonitrile (PDAN) or 2,2'-(biphenyl-4,4'-diyl)diacetonitrile (BDAN), provides the crystalline 2D CCP-HFPTP-PDAN (2D CCP-1) and 2D CCP-HFPTP-BDAN (2D CCP-2) with dual pore structures, respectively. 2D CCP-1 and 2D CCP-2 exhibit the photoluminescence quantum yield (PLQY) up to 24.9 and 32.3%, which are the highest values among the reported 2D conjugated polymers and pi-conjugated 2D covalent organic frameworks. Furthermore, compared with the well-known emissive small molecule tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN), both 2D CCPs show superior photostability under UV irradiation for 2 h, profiting from the twisted and rigid structures of the CN-substituted vinylene linkages. The present work will trigger the further explorations of novel organic emitters embedded in 2D CCPs with high PLQY and photostability, which can be useful for optoelectronic devices.

Emitters showing thermally activated delayed fluorescence (TADF) in electroluminescent devices rely on efficient reverse intersystem crossing (rISC) arising from small thermal activation barriers between the lowest excited triplet and singlet manifolds. A small donor-acceptor compound consisting of a demethylacridine donor and a methylbenzoate acceptor group is used as a model TADF emitter. The spectroscopic signatures of this system are characterized using a combination of photoluminescence and photoluminescence excitation, and the photoluminescence decay dynamics are recorded between delays of 2 ns and 20 ms. Above T = 200 K, our data provide convincing evidence for TADF at intermediate delays in the microsecond range, whereas triplet-triplet annihilation and slow triplet decay at later times can be observed over the entire temperature range from T = 80 K to room temperature. Moreover, close to room temperature, we find a second and faster up-conversion mechanism, tentatively assigned to reverse internal conversion between different triplet configurations. An interpretation of these experimental findings requires a calculation of the deformation patterns and potential minima of several electronic configurations. This task is performed with a range-separated hybrid functional, outperforming standard density functionals or global hybrids. In particular, the systematic underestimation of the energy of charge transfer (CT) states with respect to local excitations within the constituting chromophores is replaced by more reliable transition energies for both kinds of excitations. Hence, several absorption and emission features can be assigned unambiguously, and the observed activation barriers for rISC and reverse internal conversion correspond to calculated energy differences between the potential surfaces in different electronic configurations.

Organic light-emitting diodes (OLEDs) have successfully entered the display market and continue to be attractive for many other applications. As state-of-the-art OLEDs can reach an internal quantum efficiency of almost 100%, light outcoupling remains one of the major screws left to be turned. The fact that no superior outcoupling structure has been found underlines that further investigations are needed to understand their prospect. In this paper, we use two-dimensional titanium dioxide block arrays as a model of an internal light outcoupling structure and investigate the influence of its geometrical parameters on achieving the highest external quantum efficiency (EQE) for OLEDs. The multivariable problem is evaluated with the visual assistance of scatterplots, which enables us to propose an optimal period range and the block width-to-distance ratio. The highest EQE achieved is 45.2% with internal and external structures. This work contributes to the highly desired prediction of ideal light outcoupling structures in the future.