Joining the rich photophysics of organic light-emitting materials with the exquisite sensitivity of optical resonances to geometry and refractive index enables a plethora of devices with unusual and exciting properties. Examples from my team include biointegrated microlasers for real time sensing of cellular activity and long-term cell tracking, as well as the development of photonic implants with extreme form factors and wireless power supply that support thousands of individually addressable organic LEDs and thus allow optogenetic targeting of neurons deep in the brain with unprecedented spatial control. Very recently, by driving the interaction between excited states in organic materials and resonances in thin optical cavities into the strong coupling regime, we unlocked new tuning parameters which may play a crucial role in the next generation of TVs and computer displays to achieve even more saturated colour while retaining angle-independent emission characteristics.

The outcoupling efficiency of OLEDs can be significantly enhanced by orienting the emitter molecules horizontally within the film. While some guidelines have been developed to control molecular orientation in OLEDs[1], our understanding of the factors at play remains limited by the fact that current techniques only obtain average values of what in reality is a distribution of orientations. Here, we develop a method to obtain orientation distributions of emitters in thermally evaporated films of host-guest emissive layers relevant to OLED displays[2]. We achieve this by adapting out-of-focus fluorescence imaging of individual molecules for use in thermally evaporated systems. We show how the orientation distribution of emitters depends on processing conditions and on the nanoscale environment of the emitters. Crucially, we also show that emitters can adopt different orientation distributions that would be indistinguishable in ensemble-averaging measurements.

[1] F. Tenopala-Carmona, … M. C. Gather, Adv. Mater. 2021, 33, 2100677
[2] F. Tenopala-Carmona, … M. C. Gather, Nat. Comm. 2023, 14, 6126

In this presentation, I discuss our recent study investigating the influence of strong coupling in polariton organic light-emitting diodes using time-resolved electroluminescence studies [1]. We fabricated bottom-emitting polariton OLEDs, employing the well-established polariton TDAF molecular semiconductor between aluminum electrodes. Our analysis, based on a model of coupled rate equations, considered all major mechanisms contributing to delayed electroluminescence in organic emitters. We found that in our devices emission dynamics remained unmodified in the presence of strong coupling. I will also discuss the prospects of strong coupling and photonics as an alternative route to investigate material properties that are usually inaccessible. This direction may offer new avenues for OLEDs to benefit from polaritonic research [2].

[1] A. G. Abdelmagid, H. A. Qureshi, M. A. Papachatzakis, O. Siltanen, M. Kumar, A. Ashokan, S. Salman, K. Luoma, and K. S. Daskalakis, arXiv:2309.12737, (2023).
[2] E. Palo, M. A. Papachatzakis, A. Abdelmagid, H. Qureshi, M. Kumar, M. Salomaki, and K. S. Daskalakis, The Journal of Physical Chemistry C 127, 14255, 2023.

Conjugated polymers (CPs) hold promise for modern organic electronics due to their adaptability and cost-effectiveness. However, their molecular-scale characterization is still challenging. In this talk, I will show how the high-resolution imaging capabilities of scanning tunnelling microscopy (STM) can be used to analyse CPs and surpass conventional analytical limits. With a series of examples, I will demonstrate that this method allows molecule-by-molecule characterisation, revealing self-assembly, length distribution, sequence, and chemical structure of the polymers. These can be used to investigate microscale behaviour, comparing polymerization techniques, understanding side-chain effects, and fully characterise CPs where traditional methods fall short, due to aggregation or mass limitations.

State-of-the-art alternating semiconducting polymers, synthesized via established academic protocols, often contain homocoupling defects, causing the true structure to deviate from the anticipated perfectly alternating polymer backbone. These structural defects significantly hinder the reproducibility across different polymer batches, posing a challenge to the commercial viability of the organic semiconductor field, while simultaneously imposing performance limitations in different applications by creating defected chains, limiting the attainable molecular weight and increasing the dispersity. In this study, two synthesis methods – conventional Stille polymerization and a novel defect-free route – are employed to create the p-type accumulation mode OECT (organic electrochemical transistor) benchmark material pgBTTT. The effect of homocoupling, and its absence, is investigated by comparing the bulk properties of the two polymers and evaluating their respective OECT performances.

In this contribution, we will present different spectroscopic methods used to investigate organic mixed ionic-electronic conductors (OMIECs) which constitute a promising material class for interfacing biological systems with electronics. Organic electrochemical transistors (OECTs) are a solution for this task and are often used to benchmark OMIECs performance. Different studies have showed that polymers designed with hydrophilic solubilizing chains show better OECT performance, mostly due to their higher ionic uptake and stability in aqueous environments. We explore how side chain engineering in poly(3-hexylthiophene) (P3HT) with different content of TriEthylene Glycol (TEG) impacts its electronic and ionic transport properties.

This study explores the impact of ethylene glycol side chain modification in pgBTTT polymers, a key component in organic electrochemical transistors. By varying the concentration of these side chains from 50% to 100%, we observed a significant influence on the polymers' volumetric capacitance, with an interesting deviation at 90% concentration. Additionally, we investigated the efficacy of blending techniques to enhance this capacitance. Two blending approaches were tested: pgBTTT with pBTTT (OR)2 and pgBTTT with pgBTTT-OEG-OR. These blends demonstrated superior volumetric capacitance compared to copolymers, especially at higher side chain ratios. Importantly, blends with matching side chain ratios exhibited improved kinetics in doping and dedoping processes. These findings offer valuable insights for optimizing the structure and performance of organic electrochemical devices, paving the way for more efficient and effective electrochemical transistors and mixed conductors.

The use of singlet fission to enhance device efficiency or enable new capabilities raises the need to investigate it on a slow operational timescale. Using a transistor-based measurement to examine temperature-dependent singlet fission in tetracene single crystals, we observe that it is activated to 210 K, at which point it undergoes an optoelectronic phase transition. We compare these results to those of pentacene and suggest that the phase change is due to a change in the singlet fission kinetics. We further examine the interplay between this readout and other extrinsic and intrinsic device properties, such as disorder and trap states. Our results give insight not only into the readout mechanism of this OFET-based measurement, but also strategies to manipulate and tune the response.

We have presented findings on TADF emitters consisting of a strong D-A structure, exhibiting hybridized electronic excited states that encompass a primary donor-to-acceptor long-range (LR) interaction and an ancillary short-range (SR) charge-transfer characteristic. This configuration successfully balances a small ΔEST and a large f value. Our thorough theoretical and experimental analysis has uncovered that the addition of peripheral donor units to the core D-A backbone leads to the formation of multiple triplet excited states between the S1 and T1 states, exhibiting locally excited or hybridized characteristics. The close alignment of the 1CT and 3LE states accelerates the spin-flip process, resulting in a sizable radiative rate nearly equivalent to the intersystem crossing (ISC) rate. Additionally, a decent reverse ISC rate (107 s-1) is simultaneously obtained in one emitter, leading to a short (ns) delayed lifetime and effectively suppressing efficiency roll-off.

Afterglow emission allows for imaging that is independent of autofluorescence under ambient conditions. Although higher-resolution afterglow is required for increasing the quality of the autofluorescence-free emission imaging, increasing the excitation intensity to generate brighter afterglow emission decreases the resolution of afterglow images. Therefore, methods and materials that provide afterglow imaging with higher resolution remain to be developed. In this study, we performed photoinduced triplet depletion and demonstrated improved resolution of bright afterglow emission using the depletion. Triplet depletion is related to charge separation via photoionization from a triplet state caused by the depletion and subsequent rapid singlet formation. Triplet excitons that accumulated in a solid material by excitation were depleted under irradiation using a depletion beam with a longer wavelength than the absorption wavelength of the material. A higher-resolution afterglow image with an improvement of 25% was obtained by simultaneously focusing a donut-shaped depletion beam and an excitation beam.

Luminescent solar concentrators (LSC) are an attractive emerging concept for semi-transparent, building-integrated photovoltaics at low cost and weight. Hereby, large-area, luminescent wave-guide foils collect solar radiation, being harvested at the wave-guide’s edge by a small-size solar module. To further improve the performance of LSCs, emitter materials with photoluminescence quantum yields (PLQY) near-unity are required, as this minimizes losses due to photon re-absorption events. This presentation show-cases a highly sensitive approach to experimentally determine the PLQY of emitters, combining spectroscopic and photothermal techniques. Screening the PLQY of six emitter molecules in solution, we are able to measure a maximum value of 99.4% with an unprecedented precision down to ±0.3% – which is about ten times better than established techniques. This newly developed method will therefore contribute to the development of future highly efficient LSCs, which require emitters with extremely high PLQYs, well above 99%. We further show that such emitters can perform thermally assisted photon upconversion, illustrating their potential for optical refrigeration.

We present a design for programmable luminescent tags fully made from biodegradable, ready-to-use materials (bioPLTs) allowing for waste-free information storage. Quinine embedded in polylactic acid as host material provides sufficient room temperature phosphorescence (RTP) for easy readout even under continuous-wave illumination. Exceval is used as oxygen blocking layer to locally control the oxygen-sensitive RTP emission for high-resolution writing of information. Accordingly, these bioPLTs exhibit all function-defining characteristics also found in their regular non-biodegradable analogs even including a flexible design when using polylactic acid foils as substrate.

The persistent emission that remains after excitation light irradiation is stopped enables high-contrast imaging without relying on surrounding autofluorescence. However, the luminance of common persistent emitting materials hardly increases even when the excitation light intensity increases. Therefore, persistent emission has not been utilized for high-resolution imaging. Here we introduce approaches to obtain high-resolution afterglow information using persistent room temperature phosphorescence (RTP). In order to obtain the high-resolution afterglow information, it is necessary to improve the RTP yield and suppress the saturation of RTP brightness under strong light excitation. We explain the molecular designs to enhance the RTP yield using unique dynamic quantum chemical calculations. For the suppression of the RTP brightness with excitation irradiance, Forster resonance energy transfer to the accumulated triplet excitons in strong excitation is discussed. Finally, afterglow emission from the individual nanoparticles of materials showing bright persistent RTP was demonstrated in an atmospheric and aqueous solution environment.

To enhance the singlet-to-triplet occupation ratio and boost radiative recombination in organic semiconductors, several schemes exist to inject charge carriers with a high degree of spin polarization. So far, the electrical current flowing across the ferromagnetic 3d metal/molecule interface has been shown to exhibit a maximum spin polarization P = 74% at 10K. Yet applications require a higher P and at room temperature, using a simple 3d metal. We will present magnetotransport results showing P=89% at T=40K across the Fe/C60 interface, and P=77% at 295K across the facile Co/C interface. These new records highlight the potential of this class of spin injectors for organic optoelectronics.

Spins in molecular films can be observed in the ground state in the form of free radials or paramagnetic ions. They can also be obtained in the photoexcited state, and harnessed to improve photoemission yields in OLEDs or efficiency in photovoltaics. One particularly intriguing phenomenon is singlet fission in pentacene, where a single photon absorbed for a singlet excitation can be converted into two triplets, potentially boosting efficiency in solar cells. This presentation will focus on elucidating the correlation between molecular structure and couplings of spins obtained by singlet photoexcitation. The impact of intermolecular interactions will be probed via pentacene dilution within an insulating matrix, and temperature-dependent spectroscopies including transient electron paramagnetic resonance. The impact of spin coupling on future applications will be considered.

Transparent photovoltaics, a burgeoning field at the intersection of materials science and renewable energy technology, have the potential to revolutionize the way in which we harness sunlight and integrate solar power into our daily lives. The performance of transparent photovoltaics is evaluated by the light utilization efficiency and necessitates a trade-off between optimizing efficiency and ensuring transparency. In this work, a comparative analysis is performed of two donor polymer materials that differ only in the presence of a single fluorine atom among the polymer backbone, examining their performance in transparent solar cells in relation to both of these critical aspects.

The light-emitting electrochemical cell (LEC) comprises mobile ions in the active material. It is the action of these ions that enables its attractive properties; notably, that complete LECs can be fabricated by scalable ambient-air printing and coating.

The redistribution of the ions causes electrochemical doping of the emissive semiconductor (p-type at anode, n-type at cathode), which results in the formation of a p-n junction doping structure. This in-situ formed doping structure enables the printing fabrication, but also poses challenges from conceptual and performance perspectives. For instance, the doping regions comprise high concentration of mobile polarons that can cause severe exciton-polaron quenching, and the position of the emissive p-n junction for constructive interference cannot be controlled by conventional spatial design during device fabrication.

Here, we present conceptual insights and rational design methods for alleviation of exciton-polaron quenching and for control of the position of the in-situ formed p-n junction for efficient emission. We also present developments towards an LEC, which is sustainable during both fabrication, operation and recycling.

Deep-red/near infrared (NIR) persistent emission from purely organic molecules possesses great potential for security protocols, imaging, and information exchange. Persistent electroluminescence, achieved by incorporating suitable materials in organic light-emitting diodes (OLEDs), is highly attractive as it allows to integrate the essential features into optoelectronic systems and applications. While first material systems using triplet-singlet Förster resonance energy transfer (TS-FRET) show promising characteristics, the concentrations of phosphorescent donor materials and fluorescent deep-red acceptor materials remain too low for OLED applications. In our contribution, we present a systematic photophysical analysis of several donor-acceptor combinations based on TS-FRET with the aim of increasing the donor concentration while keeping high photoluminescence quantum yields. Our results pave the way towards efficient deep-red/NIR down-conversion OLED pixels with persistent emission.

Triplet-polaron quenching (TPQ) is one of the main causes of exciton loss in OLED devices, which seriously limits the luminance efficiency. In host-guest blend systems, which a phosphorescent “guest” emitter is diluted in a “host” material, the polarons can be located on a host or a guest molecule, and can be holes or electrons, dependent on the energy level structure. However, the mechanism of TPQ and the relation of the interaction strength with the molecular structure are not well known. Here, we present an integrated approach of experimental and simulation methods for obtaining TPQ interaction strength.

We will report on state-of-the art, highly correlated wavefunction methods accounting for both static and dynamic electron correlation, aiming at the full characterization of the low-lying electronic excitations in (multi)chromophoric organic radicals.

These methods have been applied to design organic molecules displaying both efficient luminescence and near-unity generation yield of high-spin multiplicity excited states. We will also present a host:guest design for OLEDs that exploits energy transfer with demonstration of up to 9.6% external quantum efficiency (EQE) for 800 nm emission.

Doublet fluorescence from organic radicals has been suggested as a promising route to achieve high efficiency in electroluminescence (EL) with nanosecond decay lifetime, especially for deep red/near-infrared (NIR) emission. Here, a highly efficient and bright doublet emissive system is suggested by combining a thermally activated delayed fluorescence (TADF) host supporting both electron and hole transport and a tris(2,4,6-trichlorophenyl)-methyl-based radical emitter. Strong NIR steady-state photoluminescence (PL) by host photoexcitation demonstrates effective singlet-to-doublet Fӧrster resonance energy transfer. Strong temperature dependence in the delayed emission of transient PL profiles suggests additional energy transfer pathways, in particular triplet-to-doublet Dexter energy transfer. Turning to EL devices, a high maximum external quantum efficiency and radiance of 17.4% and 110,000 mW sr-1 m-2 is achieved with a peak emission wavelength of 707 nm. This new doublet EL design shows the disruptive potential of organic radicals for NIR light-emitting technologies.

Results related to all-solution processed distributed feedback lasers based on a top-layer polymeric resonator geometry will be presented. Such device architecture is very convenient to obtain tunable devices with low threshold and high laser efficiency. Besides, the versatility of the polymeric resonator (a photoresist layer with a surface relief grating, fabricated by holographic lithography and dry etching) has been recently expanded with the demonstration of its use as a stamp to imprint gratings on perovskites. The results presented here include, on the one hand, work related to devices in which the novelty relies on the active compound, which provides improvement on aspects such as the threshold, photostability or the extension of the spectral emission region to the near infrared; and on the other hand, advances on the use of the prepared devices for sensing applications.

Nowadays, there are many ideas for the realization of distributed feedback (DFB) lasers in either a mono- and bilayer system with a permanent or temporary holographic pattern. Because feedback in DFB lasers can be provided by gain and/or refractive index periodic perturbations, there may be several approaches to their realization, however, dynamic tuning is limited. In our studies we show that using simple systems based on a single organic dye-doped polymeric thin films for distributed feedback, we can get fully reversible spectral tuning of 150 nm. As active compounds we have applied novel push-pull luminescent pyrazoline, diphenylaminofluorene and thiophene derivatives, with different acceptor groups. Integration of such luminescent dyes with transparent polymeric medium allows fabricating real-time lasing tunability in the visible region and first biological window (650-950 nm). Also Excited-State Intramolecular Proton Transfer (ESIPT) compounds, have attracted our considerable attention. Such structure enabled real-time red-green-blue (RGB) switching of emission, both in solution and solid-state, providing white laser light emission.

Commonly, the active layers of organic photovoltaic devices (OPV) is deposited from solution-process techniques, using chlorinated and/or aromatic solvents, which have high toxicity for the humans and the environment. In this communication, we intend to present the recent achievements dedicated to development of environmentally-friendly strategy to fabricate (OPV) devices. We have been developing several water-based colloidal inks for eco-friendly process of OPV. By a fine control of the size and the morphology of the nanoparticles, high power conversion efficiencies have been achieved, approaching 10%. This work shows that it is possible to achieve high performances devices from water-based inks by careful control of the nanoparticle and active layer morphology. It opens the route for more environmentally-friendly processes not only for organic photovoltaics but also for organic electronics in general.

As a promising candidate to drive low-power, off-grid applications, organic indoor photovoltaics are beginning to attract research attention. In organic photovoltaic devices, charge transport layers are often used to promote the extraction of majority carriers while blocking minority carriers. They can however be a source of device degradation and introduce additional complexity to the fabrication of the device stack. Here, a simplified, yet performant indoor OPV architecture with extended absorber thickness, but without electron transport layer (ETL) is demonstrated. We show that the diminished impact of the ETL on indoor OPV results from a drastically reduced surface recombination in thick absorber devices. The proposed simplified device architecture with thick absorber has great potential in large-scale production.

While efficient charge generation despite ultra-low energy offsets (down to 0 eV offsets) between donor and acceptor in non-fullerene acceptor (NFA) based organic solar cells has been often reported, the physical meaning of this observation is discussable and unclear. In our work, in terms of a series of donor:NFA combinations, and by combining advanced experimental techniques and optical and electrical simualtions, the role of energetic offset and the pathways for carrier losses has been studied in detail.

We performed comprehensive studies of current transport across SWCNT-Si heterojunctions, considered as a promising component for advanced photodetectors. Low-doped n-type Si was used as a substrate and SWCNT films were deposited on its top by a wet method out of solutions. We collected current-voltage (I-V) characteristics of the heterojunctions in the 78-300 K temperature range under dark conditions. In the forward bias, the I-V curves exhibited two regimes, namely, the “low” and “high” voltage regimes. We applied the Cheung–Cheung method to evaluate the height of the Schottky barrier, the series resistance, and the ideality factor, for both regimes. For tested samples, the ideality factor is very well fitted with the T-1/2 dependence. The slope of this dependency for the “high” voltage regime decreases with the increase of the SWCNT concentration, what agrees with the Card–Rhoderick model that the slope in this regime should be inversely proportional to the density of states at the SWCNT/SiO2 interface, which in turn is proportional to the SWCNT concentration. The crossover voltage between the two voltage regimes decreased linearly with the temperature for all our samples.

Organic Thermoelectric materials (OTE) show potential as a material for the production of green energy, through conversion of heat to electricity. Among OTE materials, polythiophenes are excellent candidates, although their structure-property relationships are to date poorly understood.

Figuring out the structure-property relationships are particularly challenging for organic semiconducting polymers because of their often complex semicrystalline structures and the big level of uncertainty regarding the detailed structure and composition of the polymers. In this study we present a modelling study of the morphological properties of polythiophenes. We present a computational protocol, using molecular dynamics (MD) with GAFF2 forcefields, able to generate the complex semicrystalline structures of these polymers and explain how different simulation parameters/conditions impact the results.

On-site analysis of multiple analytes from different classes (such as heavy metals, proteins and small molecules), at the sensitivity required for a selected application, is a hard technological challenge. In this context, optical sensing in miniaturized systems has the largest potential. Baser on our previous findings,[1-3] we present here the design and optimization of a miniaturized optical sensor with multiple channels, capable of multimodal optical detection in each channel, and the proof-of-concept realization of sub-systems providing two complementary detection modes: plasmon enhanced fluorescence and localized surface plasmon resonance. The multichannel (enabling multiplexing) and multimodal optical sensor is designed to have a total size of one inch-square and optimized sensing performance, obtained by combining organic optoelectronic and nanoplasmonic components.

[12] M. Prosa et al., Adv. Funct. Mater. 31 (2021).
[13] M. Bolognesi et al., Adv. Mater. 2208719 (2023) 1–13.
[14] F. Floris et al., Mater. Proc. 14 (2023) 1–5.

While solid-state upconversion systems promise applications in solar cells, photoredox catalysis, and bioimaging; most require tremendous optical power to achieve maximum upconversion efficiency. In this work, we lower this required optical power by using the concentrated electric field of surface plasmons generated from an organic light emitting diode. Excitons formed within the diode launch surface plasmons into a metallic cathode. On the opposite side of this cathode, those surface plasmons excite the triplet sensitizer of an upconversion film with a skin depth of only tens of nanometers. We find that the power required to reach half of the maximum upconversion efficiency is lowered by over one order of magnitude compared to conventional laser excitation and is crossed at a diode current density of only 1 mA/cm2. These mild conditions may allow for more practical applications of solid-state upconversion systems.

The current state-of-the-art infrared (IR) systems use inorganic semiconductors for so-called photonic detection in the near (few μm) and mid-IR (10’s of μm) range and switch to thermal detection systems such as bolometers for the large wavelength regimes (up to mm range). In order to lower the cost of IR detectors, much effort is put into designing low-gap polymers or molecules for organic photo-detectors, but their detection range is currently still limited to 1500-1600 nm wavelengths, with modest detectivities in comparison to existing inorganic technologies in the short wave infrared (SWIR) range. In this work, we look at the possibility of using organic semiconductors in a bolometric device for infrared detection over an extensive wavelength range, spanning from the SWIR to the mid-IR. We find advantages of using organic materials compared to inorganic materials for the fabrication of bolometers and use theoretical modeling to guide us on which parameters we can use to optimize our devices. We find two key parameters determining the device performance: the polymers’ thermal conductivity and the overall device thickness.

We have developed a new type of transistor that can mimic the functions and behaviors of biological synapses, which are the connections between neurons in the brain. Our transistor has a special polymer layer of cross-linked poly-4-vinyl phenol (PVP), which can form electric-double-layers (EDL) with high capacitance. This layer allows us to control the synaptic weight and the learning ability of our transistor by applying different gate voltages. We also used zinc oxynitride for the channel, which have higher mobility than oxide materials. Unlike conventional memristors, our transistors can perform signal transmission and learning functions simultaneously. It can also emulate a variety of synaptic behaviors such as excitatory postsynaptic current, paired-pulse facilitation, long-term memory, and filtering capability. Our transistor is a promising device for artificial intelligence systems that require brain-like computing.