Efficient exciplex organic light-emitting diodes with a bipolar acceptor

• Intermolecular charge–transfer of binary-mixed systems was investigated.
• Exciplex systems of m-MTDATA:26DCzPPy and TAPC:26DCzPPy were demonstrated.
• A maximal EQE of 5.03% was achieved for the m-MTDATA:26DCzPPy system.
• Dual emission from exciplex and electroplex was found for the TAPC:26DCzPPy system.

Efficient exciplex organic light-emitting diodes (OLEDs) have been fabricated with an intermixed light-emitting layer comprising electron-rich m-MTDATA or TAPC as a donor and bipolar material 26DCzPPy as an acceptor. Intermolecular charge–transfer interaction between various donor materials with different highest occupied molecular orbital (HOMO) levels and 26DCzPPy are systematically investigated to explore the formation principle of the exciplex. Various techniques like steady state absorption and photoluminescence (PL) spectra, temperature dependent transient PL decay and time-resolved emission spectra (TRES) using time-correlated single-photon counting (TCSPC) principle were adopted to probe the electronic and optical properties of these exciplex systems. Strong exciplex emission was found from the m-MTDATA:26DCzPPy and TAPC:26DCzPPy mixed systems. The devices comprising these two exciplex systems as the emission layer exhibited a promising external quantum efficiency and a high exciton utilization efficiency due to the triplet exciton up-conversion from triplet to singlet states inspired by their small singlet–triplet exchange energy. Our current work contributes to know more about the unique intermolecular charge–transfer property between electron-rich donors and bipolar acceptors, and provides valuable exploration on developing novel high-performance exciplex OLEDs.

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