Organic light-emitting diodes (OLEDs) represent the next generation display and lighting technology with notable cost saving, energy efficiency and engineering advantages enabling products with a flexible, foldable or transparent form factors. Efficiency of OLEDs relies on harvesting light from singlet and “dark” triplet excited states generated in 1:3 ratio upon electrical excitation. However, it is a challenge to find blue-emitting materials showing both high efficiency and long operating lifetimes.

We have been working intensely on developing a new class of novel emitters called carbene metal amides (CMAs) which allow 100% harvesting of singlet and triplet excited states. Briefly, triplet states, with spin equal to 1, are usually dark and must be converted to singlet states, with spin equal to 0, to be able to emit light. We have already demonstrated bright and efficient green and sky-blue OLEDs utilizing triplet states but our challenge here was to achieve higher-energy emission and produce a deep-blue CMA OLED.

We hypothesized that by substituting side groups of varying electron-donating or withdrawing strength we could tune the energy levels to achieve a wide range of emission colours, including blue, and reveal more of the underlying operating principles of these fascinating materials.

Illustration of the various CMA complexes under 360 nm UV-light.

In principle there is an almost unlimited number of substituents which can be made, however we found that many modifications resulted in unacceptable adverse effects such as reduced luminescence yields or poor stability. The addition of two CF₃ groups, for example, resulted in deep-blue electroluminescence with CIE coordinates [0.16, 0.05], but at the cost of reduced efficiency and extremely rapid degradation. Through that experience we learnt two important lessons: that the lowest energy triplet state should be of charge-transfer type across the whole molecule and that, in contrast to reports from other material classes, the presence of resonant or lower energy triplet states localised to just one part of the molecule was detrimental to the devices. This finding could be a key clue as to which excited states are involved in the process of converting the spin of dark triplet states into bright singlets and how the required the change of spin is achieved.

Ultimately, we found that the addition of an electron-withdrawing CF₃ group paired with a slightly electron-donating tert-butyl group does produce an efficient blue emitter and we demonstrated an external quantum efficiency of 20.9% with CIE coordinates (0.17, 0.17) acceptable for the mobile applications. We also confirmed and generalized our previous findings that the polarity of host materials can further shift the emission energy. Moreover, CMA emitters are uncommonly resistant to aggregation quenching of luminescence, allowing the fabrication of simplified host-free OLEDs.

Molecular structures of CMA complexes 1-4 (left) and CIE-coordinate diagram with OLEDs photographs (right). 

Research into CMAs is still in its early days but has already shown exceptional promise in the ability to produce high-efficiency OLEDs across a wide range of colours. The next challenges are to improve material stability, which must improve by orders of magnitude before CMAs can become commercially viable, and to further explain the mechanism of spin conversion which makes these materials so efficient. We have already seen in this work that the addition of tert-butyl side-groups appears detrimental to material stability, so molecular engineering may allow us to find a more stable structure while we further develop our understanding of the excited state spin dynamics fundamental to efficient OLED operation.

Patrick J. Conaghan, Campbell S. B. Matthews and Alexander S. Romanov

For more details, please, see our just accepted manuscript “Highly efficient blue organic light-emitting diodes based on carbene-metal-amides” Nat Commun 11, 1758 (2020). https://doi.org/10.1038/s41467....