Abstract
The energy gap law (E-G-law) and aggregation quenching are the main limitations to overcome in the design of near-infrared (NIR) organic emitters. Here, we achieve unprecedented results by synergistically addressing both of these limitations. First, we propose porphyrin oligomers with increasing length to attenuate the effects of the E-G -law by suppressing the non-radiative rate growth, and to increase the radiative rate via enhancement of the oscillator strength. Second, we design side chains to suppress aggregation quenching. We find that the logarithmic rate of variation in the non-radiative rate vs. E-G is suppressed by an order of magnitude with respect to previous studies, and we complement this breakthrough by demonstrating organic light-emitting diodes with an average external quantum efficiency of similar to 1.1%, which is very promising for a heavy-metal-free 850nm emitter. We also present a novel quantitative model of the internal quantum efficiency for active layers supporting triplet-to-singlet conversion. These results provide a general strategy for designing high-luminance NIR emitters. Light-emitting diodes: Near-infrared fluorescence from organic moleculesOrganic (carbon-based) light-emitting diodes (LEDs) that emit near-infrared light can be built by linking together large organic molecules called porphyrins, offering many potential industrial and medical applications. Organic near-infrared LEDs have several advantages over conventional LEDs based on inorganic semiconductors, including mechanical flexibility, biocompatibility and the absence of polluting heavy metals. Researchers in the UK and Italy led by Harry Anderson at the University of Oxford and Franco Cacialli at University College London explored the potential of linked porphyrin structures that fluoresce at near-infrared wavelengths. The optical properties of the materials are improved by engineering the molecular structure and a quantitative model is presented to explain the efficient emission. This research provides understanding of exciton dynamics and points towards innovative uses of near-infrared light in applications including light therapy, optical communications, biosensors and biometric systems.