Date Published: April 16, 2018
Publisher: John Wiley and Sons Inc.
Author(s): Paloma L. dos Santos, Jonathan S. Ward, Daniel G. Congrave, Andrei S. Batsanov, Julien Eng, Jessica E. Stacey, Thomas J. Penfold, Andrew P. Monkman, Martin R. Bryce.
By inverting the common structural motif of thermally activated delayed fluorescence materials to a rigid donor core and multiple peripheral acceptors, reverse intersystem crossing (rISC) rates are demonstrated in an organic material that enables utilization of triplet excited states at faster rates than Ir‐based phosphorescent materials. A combination of the inverted structure and multiple donor–acceptor interactions yields up to 30 vibronically coupled singlet and triplet states within 0.2 eV that are involved in rISC. This gives a significant enhancement to the rISC rate, leading to delayed fluorescence decay times as low as 103.9 ns. This new material also has an emission quantum yield ≈1 and a very small singlet–triplet gap. This work shows that it is possible to achieve both high photoluminescence quantum yield and fast rISC in the same molecule. Green organic light‐emitting diode devices with external quantum efficiency >30% are demonstrated at 76 cd m−2.
Organic light‐emitting diodes (OLEDs) have become a central part of materials research, with the ever‐growing requirement for more efficient, higher quality display devices. There is significant interest in OLED materials which emit light via a thermally activated delayed fluorescence (TADF) mechanism1 that converts dark, triplet excited states to emissive singlet states by reverse intersystem crossing (rISC). This can be achieved using aromatic donor–acceptor (D–A) molecules, which typically are conjugationally separated with the D and A units orthogonal. These systems emit from a singlet charge transfer state (1CT), which is energetically very close to its 3CT state through minimized electron exchange. A further excited state such as a local excited triplet state (3LE) situated very close in energy to this 1CT is also required.2 Therefore, having a small singlet–triplet gap (ΔEST) is crucial, but is not the only requirement for efficient rISC. rISC can harvest up to 100% of triplet states into singlet states.3 Currently, the main challenges facing the TADF community are the long overall residence times of emitter molecules in triplet excited states, and the low oscillator strengths of the 1CT radiative transitions. Here, we report a new TADF molecular design, incorporating a rigid, planar, central donor unit with multiple acceptor units bound via C—N bridges. This new design gives a key step forward in TADF efficiency through multiple coupled singlet–triplet states. The resulting fast rISC rates lead to delayed fluorescence (DF) emission lifetimes shorter than the phosphorescence lifetimes of most Ir complexes currently used in OLEDs.4 Critically, a unity photoluminescence quantum yield (PLQY) is also maintained.
In conclusion, TAT‐3DBTO2 introduces a new design for TADF emitters. The multi‐acceptor single‐donor motif imparts a large number of energy states which gives a short prompt 1CT lifetime and unitary PLQY. Moreover, we find 12 singlet–triplet state (pairs) within 0.2 eV of each other, which we believe gives rise to a DF component with a very fast rISC rate, on the order of 1 × 107 s−1. This shows that it is possible to achieve both a unitary PLQY and a sub‐microsecond TADF lifetime in the same molecule. The conformational complexity of the molecule, however, gives rise to different rISC rates, as observed in the emission decays. Nevertheless, in devices, these optimal photophysical properties translate into an EQE which exceeds 30% at a useful brightness of 76 cd m−2. Thus, this new TADF molecular design opens up a new dimension for achieving truly high performance TADF OLEDs and provides a solution to overcome the main concerns of current TADF molecular designs.
Three types of samples were studied in this work: i) TAT‐3DBTO2 solutions (10−3–10−5m) in methylcyclohexane, toluene, and dichloromethane (CH2Cl2) solvents; ii) drop‐casted blend film of TAT‐3DBTO2:mCP 1:9 molar ratio; and iii) evaporated doped films of TAT‐3DBTO2:BCPO, 1:9 v/v. All the solutions were stirred for several hours to ensure complete dissolution. The films were dispersed onto quartz substrates.
The authors declare no conflict of interest.