Recent Research Highlights

With regard to organic light-emitting diodes a way to avoid electron trapping is to lower the LUMO of the organic semiconductors below 3.5 eV, which then poses a new challenge, namely the injection of holes into semiconductors with HOMO levels deeper than 6 eV. We have developed a new strategy for creating Ohmic hole contacts on semiconductors with deep HOMO levels using an interlayer, for which the sole requirement is that it has a higher ionization energy than the organic semiconductor (Nature Materials (2018)). The ability to create Ohmic contacts enables us to further investigate the hole transport in a whole range of organic semiconductors. We have found that an energy window for trap-free transport exists between 3.5 eV and 6.0 eV, providing a design rule for organic semiconductors to obtain balanced transport (Nature Materials (2019)).

Using this design rule, we have demonstrated a yellow-emitting OLED based on a single-layer of a thermally-activated delayed fluorescence (TADF) emitter with state-of-the-art efficiency and very low operating voltage (Nature Photonics (2019)). This result demonstrates that the single-layer OLED can rival and even exceed the performance of complex multilayer devices. Another challenge to overcome is the limited stability of single-layer LEDs. We have demonstrated that the voltage drift of a polymer LED driven at constant current is caused by the formation of hole traps, which leads to additional non-radiative recombination between free electrons and trapped holes. The trap formation originates from exciton-polaron interactions and can be suppressed using trap-dilution, resulting in PLEDs with unprecedented stability (Nature Materials 17, p. 557 (2018)). Due to the existence of a trap-free window the upper limit for the band gap of organic semiconductors for which simultaneous trap-free electron and hole transport is possible amounts to ~2.5 eV, which poses a fundamental challenge for blue OLEDs with a bandgap of ~3 eV. We have now developed a bottom-up molecular str(ategy to avoid trapping of charge carriers in wide band gap (~3 eV) organic semiconductors and realized the first wide band gap material with trap-free transport (Nature Materials (2023)). In parallel, a combined electrical-optical OLED device model has been constructed that allows quantitative analysis of all loss processes. Based on these developments, a single layer blue OLED has been realized with a record high external quantum efficiency of 27.7% (Advanced Materials (2023)).

We unraveled the electron and hole transport of perovskite semiconductors, including the presence of mobile ions that screen the electric field and cause hysteresis in charge transport measurements (Nature Communications 11, 4023 (2020)).

Regarding meniscus guided coating we have experimentally demonstrated the crucial role of the meniscus shape in the fluid flow and crystallization of organic semiconductors and developed a multicomponent mass-transport model to predict how the morphology formation depends on materials- and processing parameters (Nature Materials 20, 68–75 (2021)).

In bioelectronics using complementary organic electrochemical transistor-based amplifiers multiscale real-time and high-sensitivity ion detection has been achieved (Nature Communications (2020)). An adaptable organic neuromorphic circuit was developed that enabled a robot to learn to follow a path to the exit of a maze, while being guided by visually indicated paths (Science Advances 7, eabl5068 (2021)). Furthermore, an organic artificial neuron consisting of a compact non-linear electrochemical element has been demonstrated, operating in biologically relevant environments that displays spiking dynamics sensitive to common ions of the biological aqueous milieu (Nature Electronics 5, 774-783 (2022)).