Shorter Exciton Lifetimes via an External Heavy‐Atom Effect: Alleviating the Effects of Bimolecular Processes in Organic Light‐Emitting Diodes

Abstract

Multiexcited‐state phenomena are believed to be the root cause of two exigent challenges in organic light‐emitting diodes; namely, efficiency roll‐off and degradation. The development of novel strategies to reduce exciton densities under heavy load is therefore highly desirable. Here, it is shown that triplet exciton lifetimes of thermally activated delayed‐fluorescence‐emitter molecules can be manipulated in the solid state by exploiting intermolecular interactions. The external heavy‐atom effect of brominated host molecules leads to increased spin–orbit coupling, which in turn enhances intersystem crossing rates in the guest molecule. Wave function overlap between the host and the guest is confirmed by combined molecular dynamics and density functional theory calculations. Shorter triplet exciton lifetimes are observed, while high photoluminescence quantum yields and essentially unaltered emission spectra are maintained. A change in the intersystem crossing rate ratio due to increased dielectric constants leads to almost 50% lower triplet exciton densities in the emissive layer in the steady state and results in an improved onset of the photoluminescence quantum yield roll‐off at high excitation densities. Efficient organic light‐emitting diodes with better roll‐off behavior based on these novel hosts are fabricated, demonstrating the suitability of this concept for real‐world applications.

Acridan‐Grafted Poly(biphenyl germanium) with High Triplet Energy, Low Polarizability, and an External Heavy‐Atom Effect for Highly Efficient Sky‐Blue TADF Electroluminescence

“…Like phosphorescence OLEDs, an efficient host for the TADF guest requires a higher E T level than that of the guest to suppress the nonradiative process, reasonable charge balance to reduce efficiency roll‐off, and well‐matched HOMO/LUMO levels to those of the guest to reduce the charge injection barrier . A host with low polarizability and a heavy atom is also crucial to enhance emission color purity and spin–orbit coupling interactions in the TADF guest, respectively, and the latter can increase the RISC rate and compensate the increased intersystem crossing (ISC) rate to efficiently harvest triplet excitons for light emission …”

“…Concerning the external heavy‐atom effect on the TADF guest, we believe that the Ge atom with a high atomic number in P(DMAC‐Ge) should have a heavy‐atom effect, which can enhance spin–orbit coupling (H SO ) and thus promote the RISC rate in the TADF guest to enable the conversion of more triplet excitons into singlet excitons for the generation of more delayed fluorescence . To confirm whether the heavy atom Ge in P(DMAC‐Ge) has the opportunity to provide H SO to the guest DMAC‐TRZ molecule, we examined the accommodation of the guest in the vicinity of the host in a 3D plot (Figure a).…”

Recent Advances in Conjugated TADF Polymer Featuring in Backbone‐Donor/Pendant‐Acceptor Structure: Material and Device Perspectives

“…The strategy of TICT with partial twist geometry realizes a satisfied ΔE ST tailoring work, helping to simultaneously increase RISC rate and gain acceptable transition dipole moment for radiative decay. For the pure PAPTC film state, τ DF is as short as 0.68 μs, which is beneficial to reduce triplet density and thus contribute to restrain triplet‐induced quenching processes, e. g. TTA, TPA, in real non‐doped device under operation, subsequently well explaining the low roll‐off superiority. The EL performance of PAPCC device is distinctly low, merely achieving an EQE max.…”

Large Increase in External Quantum Efficiency by Dihedral Angle Tuning in a Sky‐Blue Thermally Activated Delayed Fluorescence Emitter

“…Device Fabrication : Detailed description of device fabrication can be found in the previous publication …”