Efficient fluorescent materials and OLEDs for NIR

(A) Molecular structure of the l-PN (THS) oligomer series. (B) Band diagram of the material used for OLED. TFB (Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4?-(N-(4-sec-butylphenyl)diphenylamine)]) And F8BT molecular structures are shown above and below the relative band diagram, respectively. (C) ITO pattern glass substrate, poly (styrene sulfonic acid) doped poly (3,4-ethylenedioxythiophene) (PEDOT: PSS) hole transport layer, TFB electron / exciton blocking layer, F8BT: 1- P6 (THS) NIR light emitting layer and Ca / Al cathode. Credits: Alessandro Minotto, Ibrahim Bulut, Alexandros G. Rapidis, Giuseppe Carnicella, Maddalena Patrini, Eugenio Lunedei, Harry L. Anderson, Franco Cacialli

Near-infrared emitters (NIRs) are critical not only for a variety of biomedical, security and defense applications, but also for (non-) visible light communication and the Internet of Things (IoT). Researchers in the United Kingdom and Italy have developed a porphyrin oligomer NIR emitter that provides high efficiency despite being completely free of heavy metals. They have demonstrated an 850 nm organic light emitting diode (OLED) with a peak external quantum efficiency of 3.8%, along with a new quantitative model of device efficiency.

The ability to manipulate near-infrared (NIR) radiation is for security (eg, biometrics) and ICT (information), as well as for biomedical departments (the translucency of human tissues is a clear advantage). It has the potential to enable a large number of technologies. And communication technology), the most obvious applications are (nearly or invisible) visible light communication (VLC) and related impacts, including the imminent Internet of Things (IoT) revolution. Compared to inorganic semiconductors, organic NIR sources offer a wide range of inexpensive manufacturing, mechanical flexibility, compatibility, and potentially biocompatibility.

However, the emission efficiency of organic emitters in NIR is hampered by the adverse effects of certain types of agglutination / packing of emitters in the solid state and the increase in non-emissivity commonly observed when the energy gap (EG) is reduced. You can. ), That is, the so-called “energy gap law” (EG law) of non-radiative transition. Hybrid organic / inorganic innovative materials such as perovskite methylammonium lead halides and quantum dots may provide alternatives to high external quantum efficiency (EQE), but their heavy metal content makes them particularly suitable for most applications. Prevents use in biocompatible or wearable applications. Toxicity issues can also affect phosphorescent materials that incorporate toxic heavy elements.

In a new treatise published in Light: Science and applicationAn international team of scientists, led by Professor Franco Cassiali of the University College London and Professor Harry Anderson of the University of Oxford, has launched a new non-toxic, heavy metal-free organic NIR emitter and OLED featuring luminescence that peaks at about 850 nm. I am reporting. External quantum efficiency (EQE) up to 3.8%.

The authors use optical spectroscopy to take advantage of the spatial extent of the excited state at the length of the oligomer to quantify radiative and non-radiative processes (quantified by emissivity and non-emissivity, kr and knr, respectively). Elucidate how to manipulate conflicts between them to your advantage. Suppression of aggregation. Surprisingly, instead of decreasing photoluminescence quantum yield (PLQY) with oligomer length (and thus reducing gaps), steady increase and final PLQY near hexamer (1-P6 (THS)). Saturation is observed.

Surprisingly, in these systems, conjugated triple-bond-based bridges between porphyrins allow effective intramolecular electron bonding between macrocycles and delocalized radiation (single term) excited states (excitators). You can understand this behavior when you consider enabling localization. Over the increasing part of the molecule. This increases the spatial range discrepancies of radiated (singlet) and non-radiated (triplet) excitons, taking into account the intrinsically localized nature of triplets. Such discrepancies are expected to suppress intersystem crossing (ISC) between singlet and triplet, and thus suppress non-emissivity (knr). In addition, exciton delocalization is also expected to favor decoupling from the vibration ladder (thus avoiding the EG law).

Notably, the increase in non-radiative velocities (forced by the increase in oligomer length) as a function of the reduction in the energy gap is characterized by logarithmic velocities in these systems that are orders of magnitude smaller than in previous studies. It can be attached. Second, the bulky trihexylsilyl side chain binds to the porphyrin, preventing quenching of aggregation due to steric hindrance that limits π-π interactions (see chemical structure in Figure 1).

Breakthroughs in basic photophysics and material design have been confirmed by incorporating the F8BT: 1-P6 (THS) blend into OLEDs, with an average EQE of 1.1% and a maximum EQE of 3.8% at peak wavelengths of 850 nm. It was proved that (Fig. 2). A new quantitative model has also been developed to analyze the results. This means the importance of the triplet-to-singlet conversion process (eg, intersystem crossing and / or thermally activated delayed fluorescence), spin statistics.

Efficient fluorescent materials and OLEDs for NIR

The EL spectra of OLEDs incorporating F8BT: 1-P6 (THS) as the active layer are 15V and 24V (ie maximum) with EBL-less and EBL-with (a), EQE vs. current density (b), and corresponding JVR curves, respectively. Radiated voltage) was collected. (Insert view). Credits: Alessandro Minotto, Ibrahim Bulut, Alexandros G. Rapidis, Giuseppe Carnicella, Maddalena Patrini, Eugenio Lunedei, Harry L. Anderson, Franco Cacialli

The EQE presented in this paper is, to the best of our knowledge, the highest ever reported in this spectral range from “heavy metal-free” fluorophore.

The authors summarize the importance of their work: “Our results show a modest increase in knr by EG than in the literature, but most importantly, design high-intensity NIR. We also offer a general strategy for. Emitters. “

“In the short term, we will further develop OLEDs in this challenging spectral range for a wide range of potential applications covering life sciences (biochemical wearable sensors, in vivo underground bioimaging, to name just two). It may be possible, security (eg biometrics), horticulture, and (in) visible light communication (iVLC), a serious competitor to ease the bandwidth demands of the imminent Internet of Things (IoT) revolution. . “

“More importantly, and from a perspective, these findings are important for many areas.”

An exciting era of efficient, heavy atom-free OLEDs

For more information:
Alessandro Minotto et al, Towards Efficient Near Infrared Fluorescent Organic Light Emitting Diodes, Light: Science and application (2021). DOI: 10.1038 / s41377-020-00456-8

Provided by Chinese Academy of Sciences

Quote: Https: // Efficient for NIR (2021, 28 January) obtained on 28 January 2021 Fluorescent material and OLED

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Efficient fluorescent materials and OLEDs for NIR

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