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

• Published in: Advanced materials (Weinheim) Vol. 29; no. 40
• Main Authors: Einzinger, Markus, Zhu, Tianyu, de Silva, Piotr, Belger, Christian, Swager, Timothy M., Van Voorhis, Troy, Baldo, Marc A.
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 01.10.2017; Wiley Blackwell (John Wiley & Sons)
• Subjects: Bromination; Coupling (molecular); Emission spectra; Excitation; external heavy‐atom effect; Fluorescence; intersystem crossing; Light emitting diodes; Materials science; Molecular dynamics; Organic light emitting diodes; Photoluminescence; Spin-orbit interactions; spin–orbit coupling; thermally activated delayed fluorescence; intersystem crossing; thermally activated delayed fluorescence; external heavy-atom effect; spin-orbit coupling; organic light-emitting diodes
• DOI: 10.1002/adma.201701987
• ISSN: 0935-9648; 1521-4095

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. Brominated hosts with an external heavy‐atom effect lead to shorter triplet exciton lifetimes in thermally activated, delayed‐fluorescence guest molecules due to increased spin–orbit coupling and intersystem crossing rates. Combined with externally manipulated singlet–triplet splitting of the dopants, this results in enhanced onsets of the photoluminescence quantum yield roll‐off and fabrication of organic light‐emitting diodes with improved droop behavior.