Thermally activated delayed fluorescence with 7% external quantum efficiency from a light-emitting electrochemical cell

Abstract

We report on light-emitting electrochemical cells, comprising a solution-processed single-layer active material and air-stabile electrodes, that exhibit efficient and bright thermally activated delayed fluorescence. Our optimized devices delivers a luminance of 120 cd m−2 at an external quantum efficiency of 7.0%. As such, it outperforms the combined luminance/efficiency state-of-the art for thermally activated delayed fluorescence light-emitting electrochemical cells by one order of magnitude. For this end, we employed a polymeric blend host for balanced electrochemical doping and electronic transport as well as uniform film formation, an optimized concentration (<1 mass%) of guest for complete host-to-guest energy transfer at minimized aggregation and efficient emission, and an appropriate concentration of an electrochemically stabile electrolyte for desired doping effects. The generic nature of our approach is manifested in the attainment of bright and efficient thermally activated delayed fluorescence emission from three different light-emitting electrochemical cells with invariant host:guest:electrolyte number ratio.

Color‐Stable, Efficient, and Bright Blue Light‐Emitting Electrochemical Cell Using Ionic Exciplex Host

“…Because hole/electron transport by the host in devices mimic oxidation/reduction of the host molecule, good oxidation and reduction stability mean that the host molecule can recover after it transports one hole or electron. The excellent oxidation and reduction stability indicates that tBuCAZ-ImMePF 6 and TRZ-ImEtPF 6 are competent for hole-transport (p-type doping) and electron-transport (n-type doping), [16,17,22] respectively, which benefits their use as host materials in LECs.…”

“…A wide variety of emitting materials, [ 2,6,7 ] such as fluorescent polymers and small molecules, [ 1,8,9 ] phosphorescent ionic transition‐metal complexes, [ 10–14 ] thermally‐activated delayed fluorescence (TADF) molecules, [ 15–17 ] quantum dots, [ 18 ] and perovskite, [ 18–20 ] have been used for LECs. With these emitting materials, red to blue and white LECs have been fabricated.…”

“…In consideration of these issues, it seems very difficult to develop color‐stable, efficient, bright and operationally stable blue devices based on conventional LECs. In contrast, host‐guest LECs show decoupled emission and charge‐transport owing to the dispersion of the emitting material into the host, [ 16,17,22,24–37 ] which hold great promise for blue LECs. By dispersing blue‐emitting materials into hole/electron‐transporting materials, host‐guest blue LECs have been fabricated.…”

Evidence and Effects of Ion Transfer at Active‐Material/Electrode Interfaces in Solution‐Fabricated Light‐Emitting Electrochemical Cells

“…This finding is in line with that a majority of the BF 4 anions indeed have made the transfer into the PEDOT:PSS electrode in the herein studied devices at steady‐state operation. However, it should also be acknowledged that the ion concentration for optimum LEC operation is expected to be altered when the emission zone (i.e., the p‐n junction) is shifted towards an electrode interface, [ 59 ] since this will also alter exciton‐electrode quenching, exciton‐polaron quenching, [ 37,46,60–62 ] and the influence of cavity effects. [ 63–66 ] Nevertheless, the consistent overall picture that emerges in this study is that a majority of the anions in the active material has migrated into the relatively thick (compared to the active material) PEDOT:PSS positive anode during LEC operation, and that this transfer has a profound influence on the device operation.…”

Push–Pull Dyes for Yellow to NIR Emitting Electrochemical Cells

“…Recently, Edman and co-workers demonstrated that small molecules could undergo the same peculiar LEC mechanism and could be used as alternative light-emitting materials for LECs. [13] Since then, many families of classical organic dyes, such as perylenes, [14] cyanines, [15] porphyrins [16,17] and thermally activated delayed fluorescence (TADF) dyes, [18,19] have been incorporated into the emissive layer. [20] Similar to CP-and iTMC-based LECs, the performance of LECs using small molecules is highly dependent on interchromophore interactions in thin films.…”