Perylene Bisimide Red Emitters as Organic Downconverters for High-Luminance Hybrid-LED Applications

Virtual-reality (VR) and augmented-reality (AR) applications require miniature displays with high-luminance in red, green, and blue, to create a full-colour display that remains visible in daylight. The miniaturization of inorganic LEDs, specifically in the case of AlInGaP for red LEDs, where pixels are reduced to micron size and pitch, can result in surface defects caused by processing steps. These defects have a notable impact on the external quantum efficiency of micro-LED devices, leading to significant attenuation.<br/><br/>Down-converting organic semiconductor materials can utilise the high-power density of blue GaN-based LEDs to offer high lighting efficiency across the colour spectrum. This can provide a high-luminance RGB display from an array of blue LEDs, especially beneficial for micro-LED displays. Organic materials avoid the processing difficulties of phosphors and quantum dots, however, they are seldom explored in high-luminance applications because of their poor photostability.<br/><br/>We present the synthesis of a series of red perylene bisimide derivatives, a family of organic materials which are renowned for their strong fluorescence and high photostability. They are investigated for their solid-state properties, blended in polymer matrices, and are deposited on blue GaN-based LEDs to explore their photostability at high-power density. Many important factors are considered, such as ensuring strong blue absorption, colour-accurate emission, and good processability.<br/><br/>Ensuring that the colour conversion film is thinner than the size of the pixel is important for micro-LED displays in order to avoid optical cross-talk and light leakage. To achieve this, donor-acceptor dye compositions are explored, where a series of strongly blue-absorbing donor dyes are incorporated, which can facilitate non-radiative transfer to the acceptor dyes. This enables stronger absorption of blue light without the need to increase the concentration of dyes to a level that would lead to aggregation quenching, thereby preserving the efficiency of the device.