Magnetic Field Enhancement of Organic Light‐Emitting Diodes Based on Electron Donor–Acceptor Exciplex

Abstract

Several methods of harvesting singlet excitons via delayed fl uorescence have been introduced in OLED so far. These methods include up-conversion to singlet excitons by triplet-triplet annihilation (TTA) [ 3,4 ] or triplet fusion in materials that show a strong singlet fi ssion. [ 5 ] A different approach for enhancing the singlet emission that involves triplet excitons was introduced recently, whereby the triplet excitons may undergo reverse intersystem crossing (RISC) to singlet excitons and consequently give rise to thermally activated delayed fl uorescence (TADF). [6][7][8][9][10][11][12] This occurs in compounds with small electron exchange energy, and thus small singlet-triplet energy splitting, Δ E ST that enables triplet excitons to undergo thermally activated RISC to the singlet manifold. [ 13 ] A fi rm indication for TADFrelated emission in compounds that possess RISC is that the EL in these OLED is thermally activated, with activation energy E act ≈ Δ E ST Ͻ Ͻ 0.7 eV (which is Δ E ST in traditional organic semiconductors). During the last few years there has been a large interest in magnetic fi eld effect (MFE) in conjugated organic compounds, mainly because of the possibility to enhance the electroluminescence effi ciency, which was dubbed magneto-EL (MEL). [ 11,[14][15][16][17][18][19][20][21] In this effect, the magnetic fi eld changes the exchange rate between PP singlet (PP S ) and triplet (PP T ), which can be detected through the induced change in the EL emission intensity (MEL) or the current density (MC) in the device. This occurs if the PP S and PP T recombination rates ( R S , R T ) and/or dissociation rates ( d S , d T ) differ from each other. [ 17,22 ] So far the MEL maximum value, MEL max at room temperature (RT) has been less than ≈20% in OLEDs.In conventional OLEDs, spin mixing occurs within the PP states rather than at the exciton levels because the electron-hole orbitals strongly overlap in the latter species leading to large exchange energy, J , that consequently causes large energy gap, Δ E ST (=2 J ) between singlet and triplet states. In contrast, materials showing RISC may allow spin-mixing among the PP spin levels and in the exciton levels because Δ E ST is small. [ 23 ] In this case, possible spin-mixing mechanism may be the hyperfi ne interaction [ 24,25 ] and/or the Δ g mechanism, [ 26 ] where the difference, Δ g in the g -values of positive and negative carrier in the pair may promote intersystem crossing. The obtained full width Reverse intersystem crossing (RISC) from triplet to singlet states has been recently introduced to photophysics of organic chromophores. One type of RISC occurs in donor (D)-acceptor (A) composites that form an exciplex manifold in which the energy difference, Δ E ST between the lowest singlet (S 1 ) and triplet (T 1 ) levels of the exciplex is small (<100 meV) thus allowing RISC at room temperature. This adds a delayed component to the photoluminescence emission that is widely known as thermally activated delayed fl uorescence. Here, it is found t...

Direct Detection of Singlet-Triplet Interconversion in OLED Magnetoelectroluminescence with a Metal-Free Fluorescence-Phosphorescence Dual Emitter

“…Control over the emitter architecture will also allow us to probe the effect of the SOI on spin-mixing and spin-dependent transport. Finally, we note that these dual emitters provide a class of materials in between conventional singlet emitters and thermally activated delayed fluorescence emitters, which have been shown to exhibit extremely large magnetic-field effects of >1000% under constant-voltage conditions due to the formation of exciplexes [60,61]. It is an interesting question whether a suitable matrix can be found to prevent back transfer of both singlet and triplet excited states of the emitter while enabling the formation of exciplex species to enhance the overall magnetic-field response and thereby simplify optical detection of spin correlations further.…”

Ultralarge Magneto‐Electroluminescence in Exciplex‐Based Devices Driven by Field‐Induced Reverse Intersystem Crossing

“…Based on this concept, Basel et al have reported large magneto-photoluminescence (≈5%) and MEL (≈35%) in electron donor-acceptor (D-A) blend exciplex-based OLEDs. [ 19 ] However, it should be emphasized that these values are still far away from the requirement for any sensor and/or display applications. In the meanwhile, Ling et al have reported a much larger MEL (up to ≈110%) in another D-A combination exciplex-based OLEDs.…”

“…[ 18 ] Naturally, the fi eld-induced change in ρ is limited since not all the PP spin states are involved in the transformation between singlet and triplet under B -fi eld; and this also limits the MEL maximum value (MEL| max ). [ 18,19 ] In addition, the spin-lattice relaxation time, τ SL , may also limit ρ change with B ; this is described by the limiting factor, F = exp(− τ / τ SL ), where τ …”

Magneto‐Photoluminescence Based on Two‐Photon Excitation in Lanthanide‐Doped Up‐Conversion Crystal Particles

“…Magneto‐photoluminescence is a typical phenomenon in magnetic field effects (MFE) of excited states in excitonic light‐emitting materials . In excitonic light‐emitting materials, the excited states become spin dependent to form singlet and triplet states.…”