The origin of interlayer-induced significant enhancement of EQE in CzDBA-based OLEDs studied by magneto-electroluminescence

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Abstract
Over twelve-fold enhancement of external quantum efficiency (EQE) is observed in 9,10-bis(4-(9H-carbazol-9-yl)-2,6-dimethylphenyl)-9,10-diboraanthracene (CzDBA)-based organic light-emitting diodes (OLEDs) with an interlayer between the hole-transporting layer (HTL) and the emission layer, where the CzDBA emitter is a typically donor–acceptor–donor (D–A–D)-type thermally activated delayed fluorescence material. Analyses of the fingerprint magneto-electroluminescence traces indicate that the interlayer ensures the charge balance of the emission layer in devices, avoiding triplet-charge annihilation and contributing to the enhancement of EQE. Additionally, experimental results also show that intersystem crossing (ISC) and reverse ISC (RISC) processes coexist in the device with an interlayer. Notably, ISC boosts with increasing bias currents and working temperatures, respectively, exhibiting abnormal current and normal temperature dependences. This abnormal phenomenon is caused by the weakened RISC between charge-transfer states of CzDBA molecules at large bias currents. More interestingly, as bias currents increase, ISC in the device without an interlayer first exhibits normal current dependences and then turns into an abnormal one, which may attribute to the competitive effects of exciplex at the HTL/CzDBA interface and excited states of CzDBA molecules. Our findings not only unravel the underlying mechanisms in D–A–D-type molecules but also provide ideas for designing highly efficient devices.
Funding Information
  • The Research and Innovation Project of Graduate Students of Chongqing (CYB20088)
  • the National Science Foundation of China (11874305)

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