Hyperfluorescence

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What is hyperfluorescence?

Hyperfluorescence1 is an emission mechanism used in state-of-the-art OLEDs and is increasingly referred to as the fourth generation of devices. Unlike traditional device architectures that have a single emissive species doped into a host material, devices utilising hyperfluorescence dope two distinct emissive species into the light-emitting layer (Figure 1). One of the emissive species is a TADF ‘sensitiser’, which acts as an assistant dopant by converting the electrically generated triplet excitons into the singlet state of the TADF molecule [via reverse intersystem crossing (RISC)]. The singlet excitons are then transferred to the S1 state of the fluorescent emitter [via Förster resonance energy transfer (FRET)] and light emission then occurs from the fluorescent molecule. This approach allows us to combine the high internal quantum efficiencies of TADF emitters with the high colour purity of fluorescent emitters (Figure 2).

Figure 1. Emission mechanism in hyperfluorescence.
Figure 2. Hyperfluorescent device produced by Kyulux, showing the improved colour purity/saturation when comparing a TADF OLED and the same TADF emitter in a hyperfluorescent device.2
 

 

Challenges in Hyperfluorescence

Achieving the desired performance in hyperfluorescence remains a significant challenge, as the sensitiser-emitter distances and orientations are crucial for device characteristics.  Energy can be transferred from the TADF sensitiser to the fluorescent emitter by two main processes, to either advantageous or detrimental effect. FRET can transmit singlet excitons from the TADF sensitiser to the fluorescent emitter over long ranges (1-10 nm). This enables efficient fluorescence from the terminal emitter, although the process itself is strongly dependant on chromophore distances and orientations. While FRET rates are largest at short distances, at extremely short distances (<1 nm) DET can also enable the transfer of triplet excitons from the TADF sensitiser to the fluorescent emitter. This results in exciton quenching on the fluorescent emitter, which cannot emit via the triplet state. Therefore, to achieve optimal FRET energy transfer, the chromophores must be held at a precise distance and orientation to facilitate FRET without being close enough to enable DET.

The current uncontrolled approach is to simply blend the materials into a host and assume the materials are evenly distributed throughout the film, which is simply not the case3 (Figure 3).  Even so, in the case of a uniform distribution of components many of the dopant chromophores will be too close (facilitating DET) while others will be too far apart (inhibiting FRET). For hyperfluorescent devices to be efficient, the chromophore interactions must be precisely controlled.

Figure 3. Random distribution of chromophores in hyperfluorescent devices

Material Design

Both TADF and fluorescent emitters in hyperfluorescent devices can be strategically modulated to achieve some measure of control over the distances and orientations of the chromophores. There is currently relatively little research into this field, with material development thus far focussing on refining individual components of the blend rather than a holistic approach to understanding and developing the system as a whole.

In collaboration with Professor Alexander Colsmann (KIT) and Professor Karl Börjesson (The University of Gothenburg) we are exploring an entirely new approach to hyperfluorescence. If you would like to know more about this exciting research, reach out to Marie Skłodowska-Curie Postdoctoral Fellow, Dr Emma Puttock (emma puttock ∂does-not-exist.kit edu).

 

[1] H. Nakanotani et al., Nat. Commun., 2014, 5, 1–7
[2] A. Endo et al., SID Symp. Dig. Tech. Pap., 2020, 51, 57–60
[3] C. Tonnelé et al., Angew. Chemie - Int. Ed., 2017, 56, 8402–8406.