Light Amplification in Molecules Exhibiting Thermally Activated Delayed Fluorescence

Abstract

from organic dyes for use in organic light-emitting devices such as organic light-emitting diodes (OLEDs). [1][2][3] Phosphorescent and TADF materials are of particular interest because they allow for the harvesting of nearly 100% of excitons as electroluminescence (EL) through utilization of both singlet excitons and triplet excitons, which are generated three times more often than singlet excitons upon the recombination of holes and electrons in organic emissive materials, [4] and OLEDs are already being applied to various commercial products such as displays and lighting sources. The next challenging target in the research field of organic light-emitting devices is the realization of a current-injection organic semiconductor laser. [5][6][7][8][9] However, although light amplification processes such as lasing and amplified spontaneous emission (ASE) have been successively demonstrated in organic semiconducting solid-state films under optical excitation, current-injection organic semiconductor lasers have still not been realized owing to the extremely high lasing threshold current density (a few kA cm −2 ) predicted for known materials and device structures under electrical pumping. Thus, reduction of the threshold current density is essential for approaching the realization of an organic semiconductor laser.Since lasing has currently only been demonstrated in fluorescent dyes, one critical key to reducing threshold energy is the realization of triplet laser materials [8] that can use not only singlet excitons but also triplet excitons as the energy source for light amplification. However, long-lived triplet excitons (typical triplet exciton lifetime is a few microseconds or more) generally cause strong triplet excited-state absorption, which induces intense optical losses for light amplification when the excited-state absorption spectrum overlaps with the emission wavelength. In addition, relatively low radiative decay rates (k r , typically 10 5 -10 8 s −1 ) from the lowest singlet excited state (S 1 ) (or lowest triplet excited state T 1 ) to ground state (S 0 ) in TADF (or phosphorescent) materials compared to those of fluorescent dyes (≈10 9 s −1 ) induce an increase of threshold energy for light amplification because k r is related to Einstein's B coefficient, B ∝ (c 3 /8πhv 3 )k r , where h is Planck's constant, v is the frequency of light, and c is the velocity of light. Thus, although we showed that energy transfer from TADF molecules to fluorescent laser dyes enabled light amplification in our previousTo reduce the threshold current density and move closer toward the realization of future current-injection organic semiconductor lasers, the harvesting of triplet excitons is anticipated because 75% excitons are directly formed as triplet excited states under electrical excitation according to spin statics. However, the observation of light amplification in pure phosphorescent or thermally activated delayed fluorescence (TADF) materials has nor yet been reported even under optical excitation. Herein, lig...

Thermally Activated Delayed Fluorescence Materials: Towards Realization of High Efficiency through Strategic Small Molecular Design

“…The MR‐TADF emitter DABNA‐2 was also found to possess the capability to induce light amplification . The co‐deposited film with 6 wt.…”

“…The MR-TADF emitter DABNA-2w as also found to possess the capability to induce light amplification. [48] The co-deposited film with 6wt. %D ABNA-2: 3,3-Di(9H-carbazol-9-yl)biphenyl (mCBP)s howed an onlinear increaseo fe mission intensity and also ar apidly decreasedF WHM.…”

Blocking Energy‐Loss Pathways for Ideal Fluorescent Organic Light‐Emitting Diodes with Thermally Activated Delayed Fluorescent Sensitizers

“…The dilemma for materials with TADF as emitters is a small Δ E ST intrinsically accompanied by small radiative decay rates ( k r , typically 10 5 –10 7 s −1 ), invoking instability and efficiency roll‐off at high current density . This tradeoff can be broken by delivering the singlet energy of TADF to conventional phosphorescent or fluorescent dyes through the more efficient Förster resonance energy transfer (FET) .…”

Low‐Threshold Organic Semiconductor Lasers with the Aid of Phosphorescent Ir(III) Complexes as Triplet Sensitizers

“…Various organic emitters have thus been intensively explored with involving the triplet excited states for light emission via spin mixing between singlet and triplet excited states . High‐performance phosphorescent and fluorescent OLEDs have been demonstrated by exploring efficient spin‐mixing in organic emitters facilitated by large spin–orbit interactions in heavy‐metal complexes, small singlet–triplet energy gaps in thermally activated delayed fluorescence materials, and intermolecular charge transfer complexes (exciplexes) . For instance, the IQE value of OLEDs could be lifted up to 100% by using phosphorescent complexes due to the large spin–orbit interactions in heavy‐metal complexes that allow harvesting nearly 100% excitons for electroluminescence (EL) via utilization of both singlet and triplet excitons.…”

“…As far as we are concerned, the most widely explored organic gain media are exclusively based on fluorescent emitters . In order to minimize the optical losses caused by the T 1 population in organic laser systems based on fluorescent emitters, a general strategy is to eliminate the triplet excitons by introducing triplet quenchers, i.e., oxygen, cyclooctatetraene, anthracene derivatives, and metal nanoparticles embedded in the gain media .…”