Host-Defined Trap Polarity Enables Carrier Buffering and Regulates Exciton Dynamics in Deep-Blue MR-TADF OLEDs

Abstract

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters such as ν-DABNA have enabled highly efficient deep-blue organic light-emitting diodes (OLEDs). However, their device performance is strongly governed by the electronic structure of the host matrix. Here, we demonstrate that ν-DABNA displays a distinct host-dependent reversal of charge-trapping polarity in three representative host matrices—mSiTrz, mCBP, and PhCzBCz in an identical device architecture. Specifically, ν-DABNA behaves as a hole trap in mSiTrz, an electron trap in PhCzBCz, and a bipolar trap in mCBP according to its energy level. The electroluminescence (EL) spectra of ν-DABNA-based devices exhibit nearly identical peak positions around 470 nm, and the mCBP-based device delivers the highest peak EQE at low current density, a relatively serious efficiency roll-off at high current density, and a long device lifetime, indicating the bipolar carrier-buffering effect of doping ν-DABNA. Transient EL measurements reveal a clear decoupling between charge formation and emissive decay processes. The rise dynamics strongly depend on host-dependent trap polarity, leading to a significant variation in turn-on time, while the decay dynamics remain invariant across all devices, confirming that delayed fluorescence is exclusively governed by intrinsic ν-DABNA reverse intersystem crossing (RISC). The temperature-dependent single-carrier current measurement and transient EL decay once again demonstrate the mechanism of carrier trapping and thermal detrapping by ν-DABNA doping in different host systems. Based on these findings, we introduce a host-defined trap-polarity function, ΦH, which quantitatively captures electron–hole transport asymmetry governed by energy-level offsets to link the charge transport and exciton dynamics in ν-DABNA-based MR-TADF OLEDs. This work establishes a universal host-level-controlled trap-assisted emission model for doped MR-TADF OLEDs.