Intersystem crossing effects on spin mixing in exciplex-based organic light-emitting devices

Abstract

The electroluminescence (EL) efficiency in exciplex-based organic light-emitting devices (OLEDs) is seriously impacted by the spin sense of injected carriers that leads to spin correlated singlet and triplet polaron pairs, which subsequently form singlet and triplet exciplex, respectively. Magnetic field effect (MFE) measurements have been employed to investigate the intersystem crossing effects on spin mixing induced by hyperfine interaction (HFI), spin-orbit coupling (SOC) and Zeeman splitting on the fraction of singlets formed in the low efficiency exciplex-based OLEDs (electron donor-acceptor, <0.1 cd/A). Poly [ N , N ′-bis(4-butylphenyl)- N , N ′-bis(phenyl)-benzi （Poly-TPD）is selected as electron donor, and 2,5-bis(5- tert -butyl-2-benzoxazolyl) thiophene (BBOT) is used as the acceptor. BBOT: Poly-TPD film is sandwiched between PEDOT: PSS hole-transporting layer and aluminum cathode. In exciplex-based device, a relative larger electron-hole separation distance is obtained because the electrons and holes are located in different neighboring molecules. As a consequence, a small exchange energy ( Δ E S T ) can be achieved by modifying the electron-hole separation distance because the Δ E S T decreases exponentially with increasing its separation distance. We have obtained large (up to ≈ 28.6%) magneto-electroluminescence (MEL) upon the application of a relatively small field, B (<300 mT). In the meantime, its magneto-resistive (MR) exhibits a small value (MR ≈ −2.5%), suggesting that large EL changes are not simply caused by the magneto-resistive effect. Therefore, the external magnetic field influences primarily the radiative exciplex recombination rate but has a negligible effect on the total recombination rate. It is proved that MEL is independent on the reverse intersystem crossing (RISC) spin-mixing mechanism. We show that intersystem crossing (ISC) related spin-mixing process dominates the low efficiency response in the exciplex manifold. The triplet levels are very closer to the singlet level in the ISC process, leading to efficient conversion from singlets to triplets through the HFI and SOC. Therefore, many singlet exciplexes can convert into triplets with thermal energy release, resulting in lower EL efficiency of fluorescent exciplex-based device. • Magnetic field effect (MFE) measurements were used to investigate intersystem crossing effects on spin mixing. • A small exchange energy (△E ST ) can be achieved by separating the HOMO from the LUMO. • A large (up to ≈28.6%) MEL (magneto-electroluminescence) was obtained upon the application of a relatively small field. • Intersystem crossing (ISC) related spin-mixing process dominates the low efficiency response in the exciplex manifold.