|
Light emission in organic semiconductors is governed by the spin of excitons formed upon electrical excitation. Conventionally, 25% of excitons form as emissive singlets and 75% form non-emissive triplets.
Exceeding this limit for OLEDs requires designing new materials. Developments in molecular design have allowed utilization of triplet excitons through either direct phosphorescence (1) or secondary processes converting a triplet into a singlet via a spin flip, creating “delayed” fluorescence. (2)
Thermally Activated Delayed Fluorescence (TADF) has provided guidelines for creating donor-acceptor molecules, but the effects governing spin dynamics are still being explored. Increasingly, there is consensus that intersystem crossing,(ISC) cannot be understood from a static picture of the molecules; a more dynamic approach is necessary.
Carbene Metal Amide (CMA) emitters (3) provide an excellent example, displaying large spectral shifts due to conformational reorganisation and highly variable intersystem crossing rates. In solid films, they have produced solution processed green OLEDs with record efficiencies.
Here we show, starting from the green CMA archetypes, we can alter the molecular design to probe the effects of steric hindrance, spin-orbit coupling, and dipole strength on the emission properties.
Using fast time resolved cryogenic PL spectroscopy we demonstrate the impact of changing the metal bridge atom on ISC, and explore high molecular weight variants for flexible electronics.
We demonstrate these emitters can be tuned across the visible spectrum whilst retaining similar photophysical properties, and achieve efficient OLED devices via both solution and vacuum processing.
We discuss their structure property relationships for emission, explore a new set of high efficiency OLED dopants, and provide fundamental insight into their spin conversion mechanism. From these studies we derive the first set of design rules for this new class of organometallic TADF emitters.
1) Very high-efficiency green organic light-emitting devices based on electrophosphorescence, Baldo et al. Appl. Phys. Lett. 1999
2) Highly efficient organic light-emitting diodes from delayed fluorescence.
Uoyama et al. Nature 2012
3) High-performance light-emitting diodes based on carbene-metal-amides, Di et al. Science, 2017
|
