Solution‐Processable Ultrapure‐Blue Light‐Emitting Electrochemical Cells

Abstract

Light‐emitting electrochemical cells (LECs) have attracted considerable attention owing to their unique luminescence mechanisms and device structures, making them suitable for solution processing. However, these cells encounter problems related to color purity, which plays an important role in efficient and high‐resolution displays. Here, two types of solution‐processed LECs are reported that exhibit ultrapure blue emissions at 467 and 452 nm with narrow emission full‐widths at half‐maximum (FWHM) values of 21 and 30 nm, corresponding to the Commission Internationale de l'éclairage (CIE) coordinates of (0.12, and 0.15) and (0.16, 0. and 10), respectively. These values are obtained using the multi‐resonance‐induced thermally activated delayed fluorescence (MR‐TADF) materials, namely, ν‐DABNA and DABNA‐4TBC. The suppression of structural relaxation and vibronic coupling in these MR‐TADF materials results in high‐color‐purity emissions with a small FWHM. Host‐guest‐type LECs employing these molecules as guest emitters in a spin‐coated single active layer exhibit both ultrapure blue electroluminescence (EL) and solution processability, strongly supporting their use in building a sustainable society in the future.

High‐Performance Narrowband Light‐Emitting Electrochemical Cells Enabled by Intrinsically Ionic Multi‐Resonance Thermally Activated Delayed Fluorescence Emitters

“…[37] Tanaka et al doped a neutral N/B-embedded MR-TADF emitter and an ionic liquid into a neutral polymer host to fabricate LECs, which showed narrowband blue EL (FWHMs < 30 nm). [38] The EQEs (<1%) also need significant improvements. [38] Tang et al doped a neutral N/B-embedded MR-TADF emitter, DtBuCzB, [39] and an ionic liquid into a neutral blend host to fabricate LECs.…”

“…[38] The EQEs (<1%) also need significant improvements. [38] Tang et al doped a neutral N/B-embedded MR-TADF emitter, DtBuCzB, [39] and an ionic liquid into a neutral blend host to fabricate LECs. [40] The optimized device showed narrowband green-blue EL (𝜆 EL = 494 nm, FWHM = 32 nm) with peak brightness/EQE/half lifetime at 493 cd m −2 /3.8%/≈1 h under constant-current driving.…”

“…Narrowband TADF LECs have been fabricated by dispersing a neutral MR-TADF emitter and an ionic liquid into a neutral host. [37,38,40] The incompatibility between the neutral and ionic components would negatively affect the device performance. [16] It is envisaged that combining an ionic MR-TADF guest emitter and an ionic TADF host would be a promising avenue toward high-performance narrowband LECs.…”

Facile and controllable chemical doping of conducting polymers with an ionic liquid dopant

“…The doping level is sensitively controlled by using a single device up to very high doping levels. 17,19) Indeed, we have demonstrated that the typical semicrystalline polymer poly [2,5-bis(3-alkylthiophen-2-yl)thieno(3,2-b)thiophene] (PBTTT) exhibits a macroscopic insulator-to-metal transition with a clear peak of the thermoelectric power factor upon doping with the ionic liquid [N,N-diethyl-N-methyl-N-(2methoxymethyl)ammonium][bis(trifluoromethanesulfonyl) imide] ([DEME][TFSI]) as the gate insulator. 19,20,21) The TFSIanion is frequently used for electrolyte gating, [16][17][18] as well as in recently developed anion exchange doping methods, 10,11,22) to realize a stably doped state.…”