Optimizing organic light-emitting diode performance through molecular engineering
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Abstract
Organic light-emitting diodes (OLEDs) are a promising display technology, offering clear advantages over traditional display technologies such as lower power consumption, ease of fabrication, fast response times and superior contrast with perfect blacks. However, state-of-the-art OLEDs can still be optimized in terms of efficiency, simplicity and lifetime. Commercial OLED devices typically employ a multilayer architecture, incorporating several organic layers with differing functions (e.g. charge injection, exciton blocking) to confine excitons within the emissive layer and enable efficient light generation. One of the most crucial factors for efficient light generation is efficient charge transport within the employed organic emissive layer. This often-overlooked factor can significantly influence the resulting efficiency. Most organic semiconductors exhibit unipolar charge transport, where, due to charge trapping, either hole or electron transport predominates. This issue is particularly pronounced in blue-emitting organic semiconductors. To address this challenge, a novel molecular strategy is presented where the HOMO and LUMO are spatially separated within the molecule. By carefully tuning the chemical structure and molecular stacking, the electron-transporting LUMO is protected from extrinsic impurities, thereby enhancing the electron current by orders of magnitude and enabling trap-free hole and electron transport simultaneously. This approach provides a pathway toward developing large band gap organic semiconductors with balanced charge transport, which is crucial for efficient OLEDs. Using this strategy, highly efficient single-layer blue- and green-emitting OLEDs based on thermally activated delayed fluorescence (TADF) are demonstrated for the first time. These devices exhibit performance rivalling current multilayer OLED stacks, marking a significant advancement toward simpler, more efficient, and longer-lasting OLEDs.