Modulation of Förster and Dexter Interactions in Single‐Emissive‐Layer All‐Fluorescent WOLEDs for Improved Efficiency and Extended Lifetime

Abstract

White organic light-emitting diodes (WOLEDs) with thermally activated delayed fluorophor sensitized fluorescence (TSF) have aroused wide attention, considering their potential for full exciton utilization without noble-metal containing phosphors. However, performances of TSF-WOLEDs with a singleemissive-layer (SEL) still suffer from low exciton utilization and insufficient blue emission for proper white balance. Here, by modulating Förster and Dexter interactions in SEL-TSF-WOLEDs, high efficiencies, balanced white spectra, and extended lifetimes are realized simultaneously. Given the different dependencies of Förster and Dexter interactions on intermolecular distances, sterically shielded blue thermally activated delayed fluorescence (TADF) emitters and orange conventional fluorescent dopants (CFDs) with electronically inert peripheral units are adopted to enlarge distances of electronically active chromophores, not only blocking the Dexter interaction to prevent exciton loss but also finely suppressing the Förster one to guarantee balanced white emission with sufficient blue components. It thus provides the possibility to maximize device performances in a large range of CFD concentrations. A record high maximum external quantum efficiency/power efficiency of 19.6%/52.2 lm W −1 , Commission Internationale de L'Eclairage coordinate of (0.33, 0.45), and prolonged half-lifetime of over 2300 h at an initial luminance of 1000 cd m −2 are realized simultaneously for SEL-TSF-WOLEDs, paving the way toward practical applications.

Simultaneously Enhanced Reverse Intersystem Crossing and Radiative Decay in Thermally Activated Delayed Fluorophors with Multiple Through‐space Charge Transfers

“…As shown in Figure S12, significant spectral overlaps between the absorption of AZA‐BN and emission of sensitizers were observed. And the radii ( R 0 ) of FRET, defined as an intermolecular distance at which the energy transfer rate constant is equal to the total decay rate constant of the pristine donor without acceptor, were calculated to be 4.06 Å, 4.03 Å, and 3.99 Å for 1CTF, 2CTF, and 3CTF, respectively [18] . Those high R 0 s validates the efficient energy transfer from sensitizer to dopant.…”

Thermally Activated Delayed Fluorescence Sensitization for Highly Efficient Blue Fluorescent Emitters

“…Incomplete FRET from TADF to fluorescent molecules would decrease color purity owing to the broad emission spectrum of the TADF material, while any Dexter energy transfer from the TADF material (or host, if present) to the non-emissive triplet state of the fluorescent molecules will result in decreased efficiency. [25][26][27][28][29][30] Despite extensive efforts to build more efficient hyperfluorescencebased devices, in particular, through designing new blue TADF emitters, an in-depth understanding of the hyperfluorescence processes at molecular level is still lacking, posing a serious challenge to the systematic design and development of efficient optoelectronic devices.…”

A π–D and π–A Exciplex‐Forming Host for High‐Efficiency and Long‐Lifetime Single‐Emissive‐Layer Fluorescent White Organic Light‐Emitting Diodes

“…[6] To break this trade-off, simplified white devices with single-emissive-layer (SEL) constituting a blueemitting TADF sensitizer and complementary-color-emitting conventional fluorescent dopants (CFDs) have been particularly proposed to shorten exciton lifetimes while maintaining unity exciton utilization efficiency by virtue of fast Förster energy transfer (FET) process from sensitizer to CFDs, providing viable opportunity to unlock the full potential of both PEs and lifetimes of all-fluorescent white devices. [7,8] However, to turn theory into reality, this so-called TADF-sensitized fluorescence (TSF) SEL-WOLEDs still face formidable challenges, one of which is to eliminate the large energy barrier between transporting layers and emitting layers (EMLs) with wide-energygap host to further lower driving voltages of SEL architecture. Another fundamental but usually ignored issue is the inevitable large energy-gap offset (ΔE g ) between high-energy sensitizer and low-energy CFDs (Scheme S1, Supporting Information).…”

“…Those materials were chosen based on the fact that, on one hand, the high stability of 5TCzBN should benefit the long device lifetime; on the other hand, the electronically inert tert-butyl units on both emitters have been proved to not only guarantee enough blue component for balanced white emission but also eliminate exciton loss through the DET process. [7,21] The S 1 and T 1 of 5TCzBN were reported to be 2.77 and 2.60 eV, both lower than that of the exciplex-forming host and thus benefiting to prevent back energy transfer. The absorption and emission spectra of 5TCzBN and TBRb are illustrated in Figure 2a, revealing a blue emission spectrum peaked at 470 nm and an orange one at 570 nm with a shoulder at 610 nm, respectively.…”