This conference presentation was prepared for the Frontiers in Luminescent Organic Semiconductor Materials and Devices conference at SPIE Photonex, 2022.

Many reported TADF emitters are claimed to enjoy intramolecular H-bonding interactions, although direct evidence for such interactions is scarce. Here we investigate an exemplar series or such materials, and using computation energy surfaces find that H-bonding is most likely inactive in this series. It remains unclear whether such interactions are truly present in other reported examples, although computational energy surfaces may help distinguish this in future.

Exothermic Förster-type exciton transfer to lower-energy emitters plays a crucial role in OLEDs. As is well-known, a small exothermicity partially overcomes the spectral Stokes shift, enhancing the Förster transfer rate. We demonstrate here another enhancement mechanism: transfer to higher-lying electronically excited states of the acceptor molecules. We evaluate the Förster transfer rate for 84 different donor–acceptor pairs of phosphorescent emitters. Due to the enhancement the Förster radius tends to increase with increasing exothermicity, from around 1 nm to almost 4 nm. The enhancement becomes particularly strong when the excited states have a large spin-singlet character.

Thermally Activated Delayed Fluorescence (TADF) process is the new paradigm for Organic Light-Emitting Diodes (OLEDs). Despite all the efforts, a complete mechanistic understanding of TADF materials has not been fully uncovered yet. Part of the complexity arises from the apparent dichotomy between the need for small energy difference between the lowest singlet and triplet excited states (dEST) which has to carry a significant charge transfer (CT) character; and for a significant spin-orbit coupling which according to El-Sayed rules requires the involved singlet and triplet excited states to have very different natures. In this contribution, we will show: (i) How this dichotomy can be resolved once accounting in a fully atomistic model of reference carbazole derivatives for thermal fluctuations of the molecular conformations and discrete electronic polarization effects in amorphous films. Using both computational and experimental techniques, we demonstrate that, electronic excitations involved in the TADF process have a mixed CT-locally excited character being dynamically tuned by torsional vibrational modes. Hence, we will demonstrate that the conversion of triplet-to-singlet and light emission in TADF materials are both electronic processes that are vibrationally-assisted. (ii) How doped triangle-shaped molecules can lead to (i) concomitant narrow emission, high quantum yield of emission and small dEST resulting in a whole new generation of TADF emitters, the multi-resonant TADF emitters and to (ii) a new family of compounds with an inverted singlet-triplet gap and potentially, a downwards energy RISC. To do so, we rely on high level quantum chemical calculations and show that an accurate description of electron correlation effects is key to correctly predict the excited states ordering as well as the optical properties of these compounds. (iii) How the interactions in the solid state can turn RISC from a SOC-driven to a hyperfine interaction (HFI)-driven mechanism. Combining time-resolved and transient electron paramagnetic resonance spectroscopies as well as (time-dependent) density functional theory calculations, we demonstrated that HFI-RISC occurs through delocalized charge transfer states in a curcuminoid derivative.

In the OLED industry, materials and devices are often optimized independently, making the process time-consuming and expensive. To address this, we are developing a toolchain for parameter-free simulation-aided OLED design from molecule all the way up to device. In this toolchain, the molecular level properties are calculated using accurate quantum chemistry simulations in a realistic morphology. These nanoscale morphologies are then scaled to device-scale morphologies that can be used as input for the device level simulations. In this talk, we will show how this approach enables optimizing the molecular and stack properties simultaneously and ultimately can reduce time-to-market and costs.

This talk will provide an overview of the three main approaches used within the group to aid the discovery of new materials (i) high-throughput virtual screening (ii) machine learning and (iii) bottom up construction of physical models. The relation between the tree approaches will be discussed including a methodology for selecting the best strategy given time and budget constraints. The examples cover a number of areas related to organic electronics including charge transport in molecular and polymeric materials, singlet fission, temperature activated delayed fluorescence, organic photovoltaics, emissive materials. We also introduce the DIADEM project, that enables identification and testing-in-the-lab of novel materials for organic electronics.

It is of utmost importance to understand the effects of exciton-polaron quenching (EPQ) in OLEDs, in order to be able to counteract its detrimental effects. In this work we provide a method for calculating Förster radii of EPQ processes in phosphorescent emitter-host films based on a combination of theoretical and experimental methods. We model absorption spectra using multi-scale ab initio methods. The emission spectra of emitters can still be obtained experimentally, allowing for accurate determination of the Förster EPQ radius. Using our method we have determined Förster radii for various emitter-host combinations prevalent in OLEDs.

Phosphorescence from purely organic molecules has been considered to be a rare feature with little practical use. In the last decade however, material research has shown that room temperature phosphorescence (RTP) is found in a very large set of different organic materials and can be brought to respectable photoluminescence efficiencies. Still, one of the key quantities for RTP, namely the excited state lifetime, remains orders of magnitude longer compared to conventional fluorophores or organometallic phosphors. This long lifetime renders RTP unsuited for OLED-display technology. In this presentation, I will discuss some recent developments towards two application concepts which function only because of the special features of RTP. First, I will introduce programmable luminescent tags that function as an electronics-free, thin-film, flexible and fully transparent information storage device. Second, RTP is used at the heart of a wavelength tracking sensor system. Here, a thin-film comprising an RTP emitter and an additional secondary emitter is effectively used as a spin-mixing layer that allows simple wavelength-discrimination.

Conjugated polymer nanoparticles are interesting nanomaterials with synthesis- and process-tunable optical properties that underpin applications in biology, catalysis, and the use of colloidal NP inks in flexible photovoltaic devices. Given the technological value of manipulation of polymer chain conformation and packing in (opto-) electronic devices, a question arises as to the extent of polymer chain ordering that may exist (and be monitored or controlled) within these fundamental building blocks. To address this challenge, we employ combined synthesis and optical spectroscopy methods to study the internal structure of polyfluorene nanoparticles in aqueous dispersions and establish useful structure-property relationships.

Triplet-triplet annihilation (TTA) gives rise to correlations in the positions of surviving excitons, which are ignored in the mean-field approximation of the master equation (ME). These correlations can in principle be accounted for exactly in Kinetic Monte Carlo (KMC) simulations, but these are computationally expensive. In this work, we present ME modeling of TTA that accounts for correlations. A comparison between our modeling and KMC simulations reveals the effect of correlations on the rates of radiative decay and TTA, and shows that our ME modeling is an accurate and computationally attractive alternative.