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Wireless magnetoelectrically powered organic light-emitting diodes

Julian F. Butscher, Sabina Hillebrandt, Andreas Mischok, Anna Popczyk, Jonathan H. H. Booth, and Malte C. Gather, Science Advances (2024).

Compact wireless light sources are a fundamental building block for applications ranging from wireless displays to optical implants. However, their realization remains challenging because of constraints in miniaturization and the integration of power harvesting and light-emission technologies. Here, we introduce a strategy for a compact wirelessly powered light-source that consists of a magnetoelectric transducer serving as power source and substrate and an antiparallel pair of custom-designed organic light-emitting diodes. The devices operate at low-frequency ac magnetic fields (~100 kHz), which has the added benefit of allowing operation multiple centimeters deep inside watery environments. By tuning the device resonance frequency, it is possible to separately address multiple devices, e.g., to produce light of distinct colors, to address individual display pixels, or for clustered operation. By simultaneously offering small size, individual addressing, and compatibility with challenging environments, our devices pave the way for a multitude of applications in wireless displays, deep tissue treatment, sensing, and imaging.

Enhancing the Open-Circuit Voltage of Perovskite Solar Cells by Embedding Molecular Dipoles within Their Hole-Blocking Layer

Julian F. Butscher, Sebastian Intorp, Joshua Kress, Qingzhi An, Yvonne J. Hofstetter, Nikolai Hippchen, Fabian Paulus, Uwe H. F. Bunz, Nir Tessler, and Yana Vaynzof, ACS Applied Materials & Interfaces (2019).

Engineering the energetics of perovskite photovoltaic devices through deliberate introduction of dipoles to control the built-in potential of the devices offers an opportunity to enhance their performance without the need to modify the active layer itself. In this work, we demonstrate how the incorporation of molecular dipoles into the bathocuproine (BCP) hole-blocking layer of inverted perovskite solar cells improves the device open-circuit voltage (VOC) and, consequently, their performance. We explore a series of four thiaazulenic derivatives that exhibit increasing dipole moments and demonstrate that these molecules can be introduced into the solution-processed BCP layer to effectively increase the built-in potential within the device without altering any of the other device layers. As a result, the VOC of the devices is enhanced by up to 130 mV, with larger dipoles resulting in higher VOC. To investigate the limitations of this approach, we employ numerical device simulations that demonstrate that the highest dipole derivatives used in this work eliminate all limitations on the VOC stemming from the built-in potential of the device.

Dipolar hole-blocking layers for inverted perovskite solar cells: effects of aggregation and electron transport levels

Julian F. Butscher, Qing Sun, Yufeng Wu, Fabian Stuck, Marvin Hoffmann, Andreas Dreuw, Fabian Paulus, A Stephen K. Hashmi, Nir Tessler and Yana Vaynzof, Journal of Physics: Materials (2020).

Herein, we report on the synthesis and investigation of two triazino-isoquinoline tetrafluoroborate electrolytes as hole-blocking layers in methylammonium triiodide perovskite photovoltaic devices with fullerene electron extraction layer. We find that increasing the thickness of the dipolar hole-blocking layer results in a gradual increase in the open-circuit voltage suggesting that aggregation of the molecules can enhance the dipole induced by the layer. This finding is confirmed by theoretical calculations demonstrating that while both molecules exhibit a similar dipole moment in their isolated state, this dipole is significantly enhanced when they aggregate. Ultra-violet photoemission spectroscopy measurements show that both derivatives exhibit a high ionization potential of 7 eV, in agreement with their effective hole-blocking nature demonstrated by the devices. However, each of the molecules shows a different electron affinity due to the increased conjugation of one of the derivatives. While the change in electron transport level between the two derivatives is as high as 0.3 eV, the difference in the open-circuit voltage of both types of devices is negligible, suggesting that the electron transport level plays only a minor role in determining the open-circuit voltage of the device. Numerical device simulations confirm that the increase in built-in potential, arising from the high dipole of the electrolyte layer, compensates for the non-ideal energetic alignment of the charge transport levels, resulting in high open-circuit voltages for a range of electron transport levels. Our study demonstrates that the application of small molecule electrolytes as hole-blocking layer in inverted architecture perovskite solar cells is a powerful tool to enhance the open-circuit voltage and provides useful guidelines for designing future generations of such compounds.

High-Brightness Blue Polariton Organic Light-Emitting Diodes

Julia Witt, Andreas Mischok, Francisco Tenopala Carmona, Sabina Hillebrandt, Julian F. Butscher, and Malte C. Gather, ACS Photonics (2024).

Polariton organic light-emitting diodes (POLEDs) use strong light-matter coupling as an additional degree of freedom to tailor device characteristics, thus making them ideal candidates for many applications, such as room temperature laser diodes and high-color purity displays. However, achieving efficient formation of and emission from exciton-polaritons in an electrically driven device remains challenging due to the need for strong absorption, which often induces significant nonradiative recombination. Here, we investigate a novel POLED architecture to achieve polariton formation and high-brightness light emission. We utilize the blue-fluorescent emitter material 4,4′-Bis(4-(9H-carbazol-9-yl)styryl)biphenyl (BSBCz), which exhibits strong absorption and a highly horizontal transition-dipole orientation as well as a high photoluminescence quantum efficiency, even at high doping concentrations. We achieve a peak luminance of over 20,000 cd/m2 and external quantum efficiencies of more than 2%. To the best of our knowledge, these values represent the highest reported so far for electrically driven polariton emission from an organic semiconductor emitting in the blue region of the spectrum. Our work therefore paves the way for a new generation of efficient and powerful optoelectronic devices based on POLEDs.

An Exciplex-Based Light-Emission Pathway for Solution-State Electrochemiluminescent Devices

Changki Moon Julian F. Butscher, Malte C. Gather, Advanced Materials (2023).

Electrochemiluminescence (ECL) allows the design of unique light-emitting devices that use organic semiconductors in a liquid or gel state, which allows for simpler and more sustainable device fabrication and facilitates unconventional device form-factors. Compared to solid-state organic LEDs, ECL devices (ECLDs) have attracted less attention due to their currently much lower performance. ECLD operation is typically based on an annihilation pathway that involves electron transfer between reduced and oxidized luminophore species; the intermediate radical ions produced during annihilation dramatically reduce device stability. Here, the effects of radical ions are mitigated by an exciplex formation pathway and a remarkable improvement in luminance, luminous efficacy, and operational lifetime is demonstrated. Electron donor and acceptor molecules are dissolved at high concentrations and recombined as an exciplex upon their oxidization/reduction. The exciplex then transfers its energy to a nearby dye, allowing the dye to emit light without undergoing oxidation/reduction. Furthermore, the application of a mesoporous TiO2 electrode increases the contact area and hence the number of molecules participating in ECL , thereby obtaining devices with a very high luminance of 3790 cd m−2 and a 30-fold improved operational lifetime. This study paves the way for the development of ECLDs into highly versatile light sources.

Energy Level Alignment in Ternary Organic Solar Cells

Vincent Lami, Yvonne J. Hofstetter, Julian F. Butscher, Yana Vaynzof, Advanced Electronic Materials (2020).

Ternary organic solar cells (TOSC) are currently under intensive investigation, recently reaching a record efficiency of 17.1%. The origin of the device open‐circuit voltage (VOC), already a multifaceted issue in binary OSC, is even more complex in TOSCs. Herein, two ternary systems are investigated with one donor (D) and two acceptor materials (A1, A2) including fullerene and nonfullerene acceptors. By varying the ratio between the two acceptors, VOC is found to be gradually tuned between those of the two binary systems, D:A1 and D:A2. To investigate the origin of this change, ultraviolet photoemission spectroscopy (UPS) depth profiling is employed, which is used to estimate the photovoltaic gap in the ternary systems. The results reveal an excellent agreement between the estimated photovoltaic gap and the VOC for all mixing ratios, suggesting that the energetic alignment between the blend components varies depending on the ratio D:A1:A2. Furthermore, the results indicate that the sum of radiative and nonradiative losses in these ternary systems is independent of the blend composition. Finally, the superiority of UPS over X‐ray photoemission spectroscopy (XPS) depth profiling is demonstrated in resolving compositional profiles for material combinations with very similar chemical, but dissimilar electronic structures, as common in TOSCs.

Energy Transfer to a Stable Donor Suppresses Degradation in Organic Solar Cells

Andreas Weu, Rhea Kumar, Julian F. Butscher, Vincent Lami, Fabian Paulus, Artem A. Bakulin and Yana Yaynzof, Advanced Functional Materials (2020).

Despite many advances toward improving the stability of organic photovoltaic devices, environmental degradation under ambient conditions remains a challenging obstacle for future application. Particularly conventional systems employing fullerene derivatives are prone to oxidize under illumination, limiting their applicability. Here, the environmental stability of the small molecule donor DRCN5T together with the fullerene acceptor PC70BM is reported. It is found that this system exhibits exceptional device stability, mainly due to almost constant short‐circuit current. By employing ultrafast femtosecond transient absorption spectroscopy, this remarkable stability is attributed to two separate mechanisms: 1) DRCN5T exhibits high intrinsic resistance toward external factors, showing no signs of deterioration. 2) The highly sensitive PC70BM is stabilized against degradation by the presence of DRCN5T through ultrafast, long‐range energy transfer to the donor, rapidly quenching the fullerene excited states which are otherwise precursors for chemical oxidation. It is proposed that this photoprotective mechanism be utilized to improve the device stability of other systems, including nonfullerene acceptors and ternary blends.

Azaacene Dimers: Acceptor Materials with a Twist

Lukas Ahrens, Julian F. Butscher, Victor Brosius, Frank Rominger, Jan Freudenberg, Yana Vaynzof and Uwe H. F. Bunz, Chemistry–A European Journal (2020).

The synthesis of five spiro‐linked azaacene dimers is reported and their properties are compared to that of their monomers. Dimerization quenches emission of the longer (≥(hetero)tetracenes) derivatives and furnishes amorphous thin‐films, the absorption is not affected. The larger derivatives were tested as acceptors in bulk‐heterojunction photovoltaic devices with a maximum power conversion efficiency of up to 1.6 %.

Azaarene Dimers

Sebastian Hahn, Julian F. Butscher, Qingzhi An, Angelina Jocic, Olena Tverskoy, Marcus Richter, Xinliang Feng, Frank Rominger, Yana Vaynzof and Uwe H. F. Bunz, Chemistry–A European Journal (2019).

Binaphthyl‐3,3′,4,4′‐tetraone was prepared and coupled to different bis(TIPS‐ethynyl)‐substituted (TIPS=triisopropyl silane) aromatic diamines, resulting in the formation of dimeric benzo‐fused azaacenes, centrally connected by a single bond. The two halves of the molecules are highly twisted with respect to each other and showed limited electronic interaction in the ground state because their absorption spectra remained very similar to those of the constituting monomers. The dimers displayed greatly reduced fluorescence when compared to the monomers, suggesting that there is a significant interaction of the two azarene units in the excited state. Preliminary investigations showed that the dimers are attractive for application as acceptors in organic photovoltaic because they significantly outperform their monomeric counterparts.

High-frequency EPR study on Cu4Cu- and Co4Co-metallacrown complexes

C. Koo, J. Park, J. F. Butscher, E. Rentschler, R. Klingeler, Journal of Magnetism and Magnetic Materials 477 (2019): 340-343.

High-frequency/high-field electron paramagnetic resonance studies on two homonuclear 12-MC-4 metallacrown complexes Cu4Cu and Co4Co are presented. For Cu4Cu, our data imply axial-type g-anisotropy with gx=2.03±0.01, gy=2.04±0.01, and gz=2.23±0.01, yielding g=2.10±0.02. No significant zero field splitting (ZFS) of the ground state mode is observed. In Co4Co, we find a mS=±3/2 ground state with g=2.66. The data suggest large anisotropy D of negative sign.

Dimeric Phenazinothiadiazole Acceptors in Bulk Heterojunction Solar Cells

Lukas Ahrens, Yvonne J. Hofstetter, Baris Celik, Julian F. Butscher , Frank Rominger, Jan Freudenberg, Yana Vaynzof and Uwe H. F. Bunz, Organic Materials, 3(02), 168-173 (2021).

Two covalently linked triisopropylsilyl-ethynylated phenazinothiadiazoles were prepared through condensation of a spirocyclic and a bicyclic tetraketone with a 5,6-diaminobenzothiadiazole. The spirobisindene- and the ethanoanthracene-based linkers render the electron acceptors amorphous in thin films. The optoelectronic properties of the non-conjugated dimers are indistinguishable from that of the crystalline monomer. Bulk heterojunction solar cells were prepared with power conversion efficiencies peaking at 1.6%. The choice of linker neither influenced optical and electrochemical properties nor device performance.