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. 2022 May 18;13(1):2744.
doi: 10.1038/s41467-022-29759-7.

Singlet and triplet to doublet energy transfer: improving organic light-emitting diodes with radicals

Feng Li #  1   2 , Alexander J Gillett #  2 , Qinying Gu  2 , Junshuai Ding  1 , Zhangwu Chen  1 , Timothy J H Hele  3 , William K Myers  4 , Richard H Friend  5 , Emrys W Evans  6   7
Affiliations

Singlet and triplet to doublet energy transfer: improving organic light-emitting diodes with radicals

Feng Li et al. Nat Commun. .

Abstract

Organic light-emitting diodes (OLEDs) must be engineered to circumvent the efficiency limit imposed by the 3:1 ratio of triplet to singlet exciton formation following electron-hole capture. Here we show the spin nature of luminescent radicals such as TTM-3PCz allows direct energy harvesting from both singlet and triplet excitons through energy transfer, with subsequent rapid and efficient light emission from the doublet excitons. This is demonstrated with a model Thermally-Activated Delayed Fluorescence (TADF) organic semiconductor, 4CzIPN, where reverse intersystem crossing from triplets is characteristically slow (50% emission by 1 µs). The radical:TADF combination shows much faster emission via the doublet channel (80% emission by 100 ns) than the comparable TADF-only system, and sustains higher electroluminescent efficiency with increasing current density than a radical-only device. By unlocking energy transfer channels between singlet, triplet and doublet excitons, further technology opportunities are enabled for optoelectronics using organic radicals.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1
Fig. 1. Light emission mechanisms and the radical energy transfer system.
Electroluminescence mechanisms for TADF-only, radical-only and energy transfer OLEDs. Spin-allowed radiative transitions from excited to ground states are indicated by blue arrows labelled ‘hv.’ a Scheme for TADF OLED mechanism with emission from singlet S1 exciton, and singlet–triplet intersystem crossing (ISC) and reverse intersystem crossing (rISC) processes with non-emissive triplet T1 exciton. b Scheme for radical OLED mechanism with emission from doublet D1 exciton, formed by direct electrical excitation. Higher energy and non-emissive quartet Q1 exciton state are shown. c Scheme for TADF:non-radical energy transfer OLED mechanism. Electrical excitation generates singlet D(S1) and triplet D(T1) excitons, with FRET singlet-singlet energy transfer to non-radical energy acceptor (A) to form emissive singlet excitons, A(S1). Dexter triplet–triplet energy transfer forms non-emissive triplet excitons, A(T1); non-radiative decay to the ground state is shown by a wavy arrow. ISC and rISC steps between D(T1) and D(S1) are indicated. Spin multiplicity of D and A pairs are denoted by 2 S+1 in 2S+1{D A}. d Scheme for TADF:radical energy transfer OLED mechanism. Electrical excitation generates singlet D(S1) and triplet D(T1) excitons, with singlet–doublet FRET and triplet–doublet Dexter energy transfer to radical energy acceptor (A) to form emissive doublet excitons, A(D1). e Chemical structures for 4CzIPN and TTM-3PCz used to test the mechanism in (d). f Absorption (black) and normalised PL (red) profiles for 4CzIPN (dotted lines) and TTM-3PCz (solid lines).
Fig. 2
Fig. 2. Transient photoluminescence studies of 4CzIPN and TTM-3PCz with 355 nm excitation.
a PL timeslices at 2.5 ns for 4CzIPN:TTM-3PCz:CBP (ratio = 0.25:0.03:0.72, red line); 4CzIPN:CBP (0.25:0.75, black line); TTM-3PCz:CBP (0.03:0.97, blue line), showing emission from both TADF and radical in the combined film. b PL timeslices for 4CzIPN:TTM-3PCz:CBP at various times from 2.5 to 50 ns, showing the 4CzIPN emission decaying relative to the radical emission at longer times. c Integrated PL fraction time profiles from 2.5 ns to 25 µs for 4CzIPN:TTM-3PCz:CBP in 650–840 nm range (red line); 4CzIPN:CBP in 450–800 nm range (black line); and TTM-3PCz:CBP in 575–840 nm range (blue line), showing faster luminescence for the combined TADF:radical film than the TADF-only film.
Fig. 3
Fig. 3. Transient absorption and temperature dependence studies of 4CzIPN and TTM-3PCz.
Picosecond to nanosecond (a) timeslices and (b) kinetic profiles from transient absorption studies of 4CzIPN:TTM-3PCz:CBP (ratio = 0.25:0.03:0.72). 400 nm excitation, fluence = 89.1 μJ/cm2. This shows the decay of the singlet PIA around 830 nm and the growth of the radical PIAs around 620 and 1650 nm. c Nanosecond to microsecond timeslices of the 4CzIPN:TTM-3PCz:CBP blend (0.25:0.03:0.72). 355 nm excitation, fluence = 17.0 μJ/cm2. Discontinuities in timeslice spectral profiles for (a) and (c) arise because multiple experiments are used to cover the studied wavelength probe regions. Transient absorption kinetic profiles for photoinduced absorption features of (d) TTM-3PCz (610–630 nm) and (e) 4CzIPN (800–830 nm). d TTM-3PCz excited-state kinetics are shown for 4CzIPN:TTM-3PCz:CBP (0.25:0.03:0.72, red squares); and TTM-3PCz:CBP (0.03:0.97, black circles). This shows delayed radical emission is active in 4CzIPN:TTM-3PCz:CBP (TADF:radical) from triplet–doublet energy transfer. e 4CzIPN excited-state kinetics are shown for 4CzIPN:TTM-3PCz:CBP (red squares); and 4CzIPN:CBP (0.25:0.75, black circles). This shows delayed radical emission in 4CzIPN:TTM-3PCz:CBP (TADF:radical) is more rapid than delayed emission in 4CzIPN:CBP (TADF only). Mono- and bi-exponential fits are indicated by solid lines in (d and e). f Ratio of integrated delayed PL contribution for 4CzIPN:CBP (black circles) and 4CzIPN:TTM-3PCz:CBP (red circles) at different temperatures. Three-point moving average and trends for these profiles are indicated by square and line plots, and show different temperature dependencies.
Fig. 4
Fig. 4. 4CzIPN and TTM-3PCz organic light-emitting diodes.
a Device architecture for OLEDs with varying emissive layer: 4CzIPN:TTM-3PCz:CBP, 4CzIPN:CBP; TTM-3PCz:CBP. b–d Current density–voltage (J–V), radiance–voltage, EQE–current density (from 10−3  mA/cm2) curves for OLEDs. e Normalised EL profiles for 4CzIPN:TTM-3PCz:CBP OLEDs with varying voltage, and 4CzIPN and TTM-3PCz emission contributions. f Magneto-electroluminescence (MEL) studies of TTM-3PCz (red squares) and 4CzIPN (red diamonds) emission in 4CzIPN:TTM-3PCz:CBP OLEDs; 4CzIPN emission in 4CzIPN:CBP (black triangles). OLED devices were biased at 8 V. MEL studies show different magnetic field dependencies for 4CzIPN and TTM-3PCz emission from 4CzIPN:TTM-3PCz:CBP devices, which supports Dexter triplet–doublet energy transfer and not the hyperfluorescence mechanism of 4CzIPN triplet exciton energy harvesting.
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