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).
