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In the past few decades Organic Light-Emitting Diodes (OLEDs) have matured to become a widespread display technology. Despite their commercial success in TVs, smartphones and tablet screens, several key issues still remain, especially if OLEDs are to become a viable technology for lighting. Metalorganic emitters based on Ir or Pt are widely used in OLED technology, due to their facile colour tunability, short phosphorescence lifetime and efficient intersystem crossing (ISC), which enables emission from both singlet and triplet states to be harnessed, leading to 100% internal quantum efficiency (IQE). However, these emitters are based on rare and toxic noble metals. In addition, efficient and stable blue and especially deep-blue phosphorescent emitters, which are essential for displays and lighting, are still to be demonstrated in order to replace currently used less efficient fluorescent blue OLEDs.
Recently, purely organic emitters exhibiting thermally activated delayed fluorescence (TADF) mechanism were also shown to be capable of 100% IQE. Since efficient ISC in these compounds is based on the small singlet and triplet gap rather than heavy atom effect, TADF compounds represent a new class of potentially inexpensive emitters for next generation OLED displays and lighting.
In this work, a series of four deep blue-to-green emitting TADF compounds have been synthesized and characterised. The compounds are based on the same scaffold as the dicarbazoyldicyanobenzene (2CzPN) reported by Adachi and co-workers, but with the replacement of the cyano groups by less electron-withdrawing oxadiazole moieties. The weaker acceptor strength of oxadiazole compared to cyano groups translates to more blue-shifted emission compared to 2CzPN. Additionally, higher spatial separation of HOMO and LUMO levels between acceptor and donor units, respectively, leads to a smaller singlet-triplet gap. This allows us to demonstrate efficient deep blue TADF OLEDs with CIE coordinates (0.16, 0.13), as compared to (0.16, 0.30) for 2CzPN devices.
A thorough photophysical study of model compound 2CzPN and oxadiazole acceptor based derivatives will be presented both for solution and thin films. The blue oxadiazole compounds show up 75% quantum yield in solid state. The electrochemical and photophysical properties, as well as crystal structures are compared to the theoretical quantum chemical calculations of the studied compounds. The efficient emission properties coupled to the small singlet-triplet gap in these molecules allows us to demonstrate efficient electroluminescence (up to 10 % EQE) from the vacuum deposited OLEDs.
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