Organic light-emitting diodes (OLEDs) are rapidly becoming the preferred display technology for a range of consumer electronics, from smartwatches, mobile phones and laptops to large screen televisions and displays used in the automotive industry. Ultra-high-definition displays must meet ever more stringent industry color standards, the most recent of which being Rec 2020., which are defined according to the Commission International de l'Éclairage (CIE) 1931 as (0.131, 0.046), (0.17, 0.797) and (0.708, 0.292) for blue, green, and red, respectively. Presently commercial vacuum-deposited OLEDs rely on phosphorescent complexes as emitter materials for the red and green sub-pixels and organic fluorescent materials for the blue sub-pixel. Significant research has been devoted to developing sustainable organic emitters that operate at comparable efficiencies to phosphorescent complexes and so can replace noble metal-based emitters for red and green OLEDs, and for blue OLEDs can lead to much higher efficient devices. Organic compounds that exhibit thermally activated delayed fluorescence (TADF) have the most potential to act as replacement emitters in OLEDs as this class of material can harvest 100% of the electrically generated excitons and convert these to light emission. Though TADF OLEDs now show comparable efficiencies to phosphorescent OLEDs for each of the red, green and blue devices, the typically employed donor-acceptor design results in emission from a charge-transfer (CT) state that is broad and unstructured, leading to poor chromaticity in the device. A subclass of TADF emitters, multi-resonant TADF (MR-TADF), however, shows great promise to address the issue of colour purity. MR-TADF emitters show very narrowband emission (full width half maximum, FWHM <40 nm), which is due to their very rigid polycyclic aromatic structure and their short range charge transfer (SRCT) excited state. Their use in OLEDs would obviate the need for the use of color filters, leading to more energy efficient devices.