Azaacene Dimers: Acceptor Materials with a Twist.

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.

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.

Wireless magnetoelectrically powered organic light-emitting diodes.

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.

High-Brightness Blue Polariton Organic Light-Emitting Diodes.

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.

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.

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

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.