Systematic investigation via controlling the energy gap of the local and charge-transfer triplet state for enabling high efficiency thermally activated delayed fluorescence emitters

Thermally activated delayed fluorescence (TADF) emitters have evolved as a certified candidate in light generation technologies for producing efficient organic light-emitting diodes (OLEDs) on account of their 100% internal quantum efficiency (IQE) via reverse intersystem crossing (RISC) and toxic metal-free design. The fast rate of RISC (k(RISC)) is the ultimate requirement of an efficient TADF emitter, which can be achieved by minimizing the singlet-triplet energy gap (Delta E-ST). Here, four donor-acceptor type TADF emitters namely 3BPy-mDCz, 3BPy-mDTA, 3BPy-mDMAC, and 3BPy-mDPT were synthesized based on benzoyl pyridine (3BPy) as an unaltered acceptor and varying the donor strength ranging from carbazole to phenothiazine. These emitters show low Delta E-ST values forecasting their TADF nature. The Delta E-ST values decreased from 0.22 to 0.14 eV upon increasing the donor strength. The maximum external quantum efficiency (EQE) of 18.7% for 3BPy-mDCz, 22.5% for 3BPy-mDTA, 13.8% for 3BPy-mDMAC and 2.1% for 3BPy-mDPT was obtained. These drastic differences in the performances of 3BPy-mDTA and 3BPy-mDPT are due to the locally excited (LE)-L-3(T-2) intermediate state between the lowest singlet (S-1) and triplet (T-1). Among 3BPy-mDMAC and 3BPy-mDPT, the efficiency of 3BPy-mDMAC is superior due to less CT character and high photoluminescence quantum yield (PLQY). This work paves a new direction for efficient TADF molecular design by indicating the role of the intermediate triplet state ((LE)-L-3) despite possessing high Delta E-ST values.

Automated summary

Thermally activated delayed fluorescence (TADF) emitters with 100% internal quantum efficiency (IQE) via reverse intersystem crossing (RISC) and toxic metal-free design have become promising candidates for efficient organic light-emitting diodes (OLEDs). In this study, four donor-acceptor type TADF emitters were synthesized, and increasing the donor strength resulted in lower Delta E-ST values and higher efficiency. The performance discrepancies between different emitters were attributed to the presence of an intermediate triplet state. This work highlights the importance of the intermediate triplet state ((LE)-L-3) in the design of efficient TADF molecules.