Light emitting organic materials show potential for production of high performance full-colour displays. However, a deficiency of organic materials emitting red light has limited the realization of this potential. Now, novel fluorescent organic materials developed by a collaboration of researchers at Shandong and Peking Universities could lead to major advances in this field.

A major challenge in the development of any light emitting material is limiting ‘non-radiative’ processes that cause its excited states to be quenched without emission of light. Such processes occur when fluorescent molecules are packed too close together. These inefficient recombination pathways can be minimized by embedding fluorescent molecules in a host material —a technique known as doping. However, optimizing the concentration of molecules in a solid host high enough so that they emit strongly, but low enough to avoid quenching is not trivial, which makes it difficult to mass produce high performance light emitting devices.

Such problems have been largely solved for materials that emit green and blue wavelengths of light. But the performance of those that emit longer wavelength red light is still poor. To address this issue, Xu-Tang Tao and colleagues1 have developed a class of fluorescent molecules that operate in a qualitatively different manner to red light emitters explored to-date.

Fig. 1: The authors' fluorescent materials emitting yellow (top) and red (bottom) light under UV radiation, in a solution of ethanol and water (left) and in solid form (right).

Unlike dopant-based emitters, which only fluorescence efficiently when they are isolated from each other, the authors' molecules do so only when they cluster together to form aggregates —a mechanism known as aggregation-induced emission. When these materials were dispersed in a solution of pure ethanol, no light was observed under excitation by ultraviolet light. But when water was added, causing the molecules to aggregate, the molecules fluoresced brightly, emitting yellow or red light depending on their chemical composition.

More importantly, the molecules continued to fluoresce even when dried into a solid form (Fig. 1). This enabled the authors to make yellow and red light emitting diodes with low operating voltages and moderate efficiencies. The real advantage of these materials, arising from their simple chemical structure and ease of production, is likely to be their versatility in the mass production of commercial devices.