Highly Efficient Thermally Activated Delayed Fluorescence via J‐Aggregates with Strong Intermolecular Charge Transfer

Abstract

The development of high‐efficiency and low‐cost organic emissive materials and devices is intrinsically limited by the energy‐gap law and spin statistics, especially in the near‐infrared (NIR) region. A novel design strategy is reported for realizing highly efficient thermally activated delayed fluorescence (TADF) materials via J‐aggregates with strong intermolecular charge transfer (CT). Two organic donor–acceptor molecules with strong and planar acceptor are designed and synthesized, which can readily form J‐aggregates with strong intermolecular CT in solid states and exhibit wide‐tuning emissions from yellow to NIR. Experimental and theoretical investigations expose that the formation of such J‐aggregates mixes Frenkel excitons and CT excitons, which not only contributes to a fast radiative decay rate and a slow nonradiative decay rate for achieving nearly unity photoluminescence efficiency in solid films, but significantly decreases the energy gap between the lowest singlet and triplet excited states (≈0.3 eV) to induce high‐efficiency TADF even in the NIR region. These organic light‐emitting diodes exhibit external quantum efficiencies of 15.8% for red emission and 14.1% for NIR emission, which represent the best result for NIR organic light‐emitting diodes (OLEDs) based on TADF materials. These findings open a new avenue for the development of high‐efficiency organic emissive materials and devices based on molecular aggregates.

NIR Light‐Driving Barrier‐Free Group Rotation in Nanoparticles with an 88.3% Photothermal Conversion Efficiency for Photothermal Therapy

“…The nonradiative decay rate generally increases exponentially as the optical energy gap decrease. [18] Thus, the small HOMO-LOMO energy gap promotes the photothermal effect.…”

“…As a result, tfm‐BDP demonstrated a HOMO‐LUMO gap of 1.565 eV, smaller than H‐BDP (1.911 eV) and m‐BDP (1.835 eV), which was consistent with the bathochromic shift of tfm‐BDP seen experimentally. The nonradiative decay rate generally increases exponentially as the optical energy gap decrease 18. Thus, the small HOMO‐LOMO energy gap promotes the photothermal effect.…”

Reducing the Singlet−Triplet Energy Gap by End‐Group π−π Stacking Toward High‐Efficiency Organic Photovoltaics

“…[ 47 ] Note that the energy splitting for singlet and triplet excitons are dominated by Coulombic and exchange terms, respectively, [ 48 ] we envisage that the Δ E ST can be reduced without sacrificing f in dimers or aggregates. [ 49,50 ] As seen in Figure , for the V‐type and J‐type dimers formed by π−π stacking between the end‐groups, the triplet energy splitting is negligible due to the small intermolecular overlap; however, the singlet energy splitting can be very significant, and the lower‐energy state or both splitting states are dipole‐allowed. Consequently, the Δ E ST can be substantially reduced.…”

A Red Thermally Activated Delayed Fluorescence Emitter Simultaneously Having High Photoluminescence Quantum Efficiency and Preferentially Horizontal Emitting Dipole Orientation

“…[ 11–14 ] Red TADF emitters with main emission peaks of above 600 nm, however, significantly underperform their blue and green counterparts in terms of both EQEs and PEs. [ 15–19 ] Up to date, there are only few reports on red TADF emitters having over 20% EQEs and above 30 lm W −1 PE, [ 20–25 ] and simultaneously achieving about 30% EQE and 50 lm W −1 PE for red TADF OLEDs is still a formidable challenge for researchers.…”

“…Red TADF emitters generally suffer more significant nonradiative processes (dominated by the nonradiative internal conversion) between S 1 and the ground state ( S 0 ), largely due to vibrational overlap between the zero‐vibrational levels of S 1 and the higher‐vibrational levels of S 0 governed by the energy gap law. [ 31 ] Accordingly, most of red TADF emitters have less than 70% Φ PL s. [ 16–20 ] To address this issue, ultrahigh molecular rigidity is highly required in constructing excellent red TADF emitters to suppress detrimental internal conversion processes. To illustrate, very recently Liao et al reported an efficient red TADF OLED with a high EQE of 28.1% and an electroluminescence (EL) emission peak at 648 nm using a dibenzo[a,c]phenazine‐3,6‐dicarbonitrile‐based red TADF emitter having nearly 100% Φ PL .…”