Organic light-emitting diodes (OLEDs) are widely used in niche lighting, smartphones, televisions/displays, watches and automotive displays, smart phones, wearable electronics, and solid-state lighting applications, owing to their phenomenal performances, including fast response time, wide viewing angles, thinner display, better contrast ratio, low power consumption, lightweight and exciting optical and electrical properties. Commercial OLEDs still rely on expensive and rare metal iridium (cost – US$5400/Oz, global production 3 tonnes per annum). Therefore, the development of economical and sustainable emitters for the replacement of expensive metal-based emitters is indispensable. Recent years have witnessed that thermally activated fluorescence (TADF) materials could be an alternative solution to expensive metal-based emitters. The remarkable features of TADF materials are that they can achieve 100% IQE, while maintaining a facile color-tuning ability. To date, numerous TADF materials have been reported and exploited in fabricating sky blue, green, and red OLEDs.
Deep red (DR) and near-infrared (NIR) emissive materials (650 nm to 750 nm) have occupied special attention given their potential in applications such as night vision devices, optical communication etc. Though osmium, iridium, or platinum complexes are known to exhibit long wavelength emission, they suffer from large efficiency roll-offs at high current densities, making them largely unsuitable for such commercial applications. The developments of TADF-based DR/NIR materials are not in pace with other color TADF emitters because of their low photoluminescence quantum yields and OLED device performances. We have addressed the low photoluminescence quantum yields of emitters of DR/NIR TADF materials by adopting (i) a rigid and strong acceptor strategy, (ii) aggregation-induced emission active unit incorporation into the deep red TADF emitter. We obtained moderate PLQY values and then employed these emitters in OLEDs. In the course of our efforts, we discovered and assessed the potential of a weak electron donor for use in blue TADF emitter design.
