Traditionally, OLEDs only achieved an internal quantum efficiency of 25% since only singlet excited states can be used to emit fluorescence. The other 75% is excited to a triplet state and recently heavy metals have been used to enhance intersystem crossing by strong spin-orbit coupling (SOC) increasing the theoretical internal quantum efficiency to 100%.
A downside to using heavy metals is the low lifetime of blue OLEDs due to weak metal-ligand coordination bonds. Alternatively, thermally activated delayed fluorescence (TADF) can be employed to convert the lowest triplet state (T1) to the lowest singlet state (S1) by reverse intersystem crossing (RISC). While being promising, the TADF mechanism can be hampered by competing processes if the reverse intersystem crossing does not happen quick enough. Beside a small S1-T1 energy gap, good vibrational coupling between the S1 and T1 states, the intersystem crossing rate is largely influenced by spin-orbit coupling (SOC).
A new deep-blue TADF emitter, TMCz-BO, was discovered by the OPERA research center at Kyushu University. TMCz-BO emits efficiently at 467nm in a film. Calculations with spin-orbit coupling time-dependent DFT (SOC-TDDFT) helped explain why TMCz-BO is such an efficient TADF emitter. The SOC matrix element (SOCME) between the T2 and S1 state is relatively large (0.124 cm-1) facilitating reverse intersystem crossing, while intersystem crossing back to the T1 state is hampered by the low SOCME of 0.001 cm-1.
This computational strategy shows promise in further development and molecular design of TADF emitters. It’s easy to optimize both ground and excited states with ADF. You can also get started with tutorials on tuning range separated hybrids and optimizing TADF emission with ADF.
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Jong Uk Kim, In Seob Park, Chin-Yiu Chan, Masaki Tanaka, Youichi Tsuchiya, Hajime Nakanotani & Chihaya Adachi, Nanosecond-time-scale delayed fluorescence molecule for deep-blue OLEDs with small efficiency rolloff, Nat Commun 11, 1765 (2020).Key conceptsADF DFTB OLEDs organic electronics Relativistic DFT