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SPIRO-QUinolinophenothiazines as Efficient Semi-conducTors for single-layer phosphorescent OLEDs – SPIRO-QUEST

SPIRO-QUinolinophenothiazines as Efficient Semi-conducTors for single-layer phosphorescent OLEDs- SPIROQUEST

To date, all the high efficiency Phosphorescent Organic Light-Emitting Diodes (PhOLEDs) are multi-layer devices, built on a stack of layers. Simplifying the multi-layers structure with the so-called Single-Layer PhOLEDs (SL-PhOLEDs), the simplest device only made of the electrodes and the emitting layer is a key step. However, high-efficiency SL-PhOLEDs are rarely reported, due to the lack of efficient host materials. SPIROQUEST aims to reach universal materials for high performance SL-PhOLEDs.

The objective of SPIRO-QUEST is to reach stable, efficient and universal hosts for simplified SL-PhOLEDs. The goal is to simplify the PhOLED technology

This proposal finds its origin in an important fact of the literature: All the high-efficiency PhOLEDs (with External Quantum Efficiency EQE>20%) are multi-layer devices. These multi-layer PhOLEDs are constituted of a stack of organic layers in order to improve the injection, transport and recombination of charges within the EMissive Layer (EML). There are usually in a PhOLED stack, a hole transporting layer, an electron transporting layer, a hole blocking layer and an electron blocking layer and these layers are even often doubled. Despite the technology is mastered, it suffers from a real complexity and a high-cost. Simplifying the multi-layers structure with the so-called Single-Layer PhOLEDs (SL-PhOLEDs), the simplest device only made of the electrodes and the EML, can also lead to a better stability and is therefore one key step for the future. However, high efficiency SL-PhOLEDs, can be only obtained by precise molecular designs of the host material used in the EML. <br />In SPIRO-QUEST, we want to develop such hosts for high-efficiency SL-PhOLEDs, which are very rare in litterature. To reach this goal, an universal host for red, green and blue SL-PhOLEDs should fulfil several precise criteria: <br />(i) a high ET of 2.7 eV to confine the triplet excitons within the red, green or blue phosphorescent guest, <br />(ii) HOMO/LUMO levels adapted to the electrodes Fermi levels allowing an efficient charges injection,<br />(iii) good and well balanced mobility of electron and hole (ambipolar character), <br />(iv) thermal and morphological stability to extend the lifetime of the device.<br /><br />A strategy to build ambipolar materials consists to incorporate both hole- and electron-transporting units in the molecular structure. However, a key parameter to control is the interaction between the hole- and electron-transporting unit. To fulfil all the above listed design criteria, we aim to use herein a specific Donor-Spiro-Acceptor molecular design, which is the heart of SPIRO-QUEST.

Several series of OSCs possessing electron accepting fragments are investigated. The first step is their syntheses (Task 1). For each OSC, the following properties is then investigated (Task 2):
- Physico-chemical properties: HOMO/LUMO energy levels (electrochemistry), morphology (AFM), thermal stability (ATG/DSC) and supramolecular arrangement (X-Ray),
- Photophysical properties: singlet state energy, triplet state energy, bipolar properties and lifetimes (UV-vis absorption and emission in solution and in the solid state),
- Charge transport properties: Mobility of the charge carriers (hole and electrons),
- Molecular modelling: Insights into the electronic structures of the OSCs (DFT and TD-DFT).
Finally, red, green and blue SL-PhOLEDs incorporating the OSCs (Task 3) will be fabricated and their stability will be investigated.

Thus, the scientific program is divided in 4 tasks.
Tache 0: Coordination
Tache 1: Synthesis
Tache 2: Properties study
Tache 3: Incorporation in PhOLEDs

SPIROQUEST has already led to many important results. Several series of host materials have been synthesized, their properties determined and their incorporation in SL-PhOLEDs performed.Of particular interest, the phenylacridine/phosphine oxide association has allowed to reach outstanding performances in red, green and blue SL-PhOLEDs. The materials have been synthesized via an efficient approach and are constructed on the association of an electron rich phenylacridine unit connected by a spiro carbon atom to three different electron-deficient diphenylphosphineoxide-substituted fluorenes. Electrochemical, spectroscopic, thermal and transport properties have been evaluated. The position (C2 and C7 vs. C3 and C6) and the number (1 vs. 2) of diphenylphosphineoxide units on the fluorene backbone have been particularly studied to highlight the best combination in term of device performances. Red, green and blue SL-PhOLEDs (RGB SL-PhOLEDs) have been fabricated and characterized. Of particular interest, we managed to reach a FIr6-based SL-PhOLED possessing an external quantum efficiency of 9.1% and a low threshold voltage (below 3 V). As far as we know, this is the first example of SL-PhOLED using this blue phosphorescent emitter. On the other hand, with notably a very high external quantum efficiency of 18% with FIrpic as sky blue emitter, another host material displays the highest overall performance in the series and the highest overall performance ever reported for RGB SL-PhOLEDs using a universal host. This not only shows that the association of phenylacridine and diphenylphosphineoxide units fulfils the required criteria for an universal host for high efficiency SL-PhOLEDs but also highlights that the arrangement of these fragments drives the device performance.
Other series of hosts are currently under investigations in order to improve the performance and/or stability of the electroluminescence.

We have reported a universal host displaying the highest performance ever reported in literature for RGB SL-PhOLEDs (J. Mater. Chem. C 2020, 8, 16354). The performance in blue is outstanding, 18%, and almost identical to the performance obtained with multi-layer devices. This shows that our design strategies fulfil the required criteria for an ideal host for SL-PhOLEDs. This is highly promising for the future of this project
Thanks to this outstanding result, we have been invited to write the first review on single-layer Phosphorescent OLEDs (Adv. Funct. Mat. 2021, 31, 2010547). This allows us to draw the first structure/properties/device performance relationship map in the field. In addition, in the light of the literature, we also noted that the ‘single-layer device’ appellation has been often wrongly used. have precisely reviewed, classified and analysed the different architectures of the devices called ‘single-layer’. We provide a precise and accurate appellation for all these families of electronic devices, allowing to properly compare their performance.

We have also developed new cyclic semi-conductors and study their transport properties prior to incorporation in SL-PhOLEDs.
This first work has been published in the prestigious journal, Journal of The American Chemical Society (J. Am. Chem. Soc. 2021, 143, 8804) and has shown the potential of the cyclic nature on the transport properties. The donor/acceptor structures are currently synthesized.
A fruitful collaboration with the group of Prof Cornil at Mons University has also allowed to work on the white fluorescent emission obtained with some materials (J. Mater. Chem. C 2020, 8, 14462), of great interest for the future of SPIROQUEST.
We are currently working on
(i) modifying the molecular structures of the host materials (modification of the electron rich and of the electron poor part) to improve the performance (EQE, threshold voltage etc)
(ii) other phosphorescent emitter
(iii) device engineering

This project has already led to four published articles in high level journals and two reviews.

1. Lucas, N. McIntosh, E. Jacques, C. Lebreton, B. Heinrich, B. Donnio, O. Jeannin, J. Rault-Berthelot, C. Quinton, J. Cornil, C. Poriel, J. Am. Chem. Soc. 2021, 143, 8804-8820


2. P. Tourneur, F. Lucas, C. Quinton, Y. Olivier, R. Lazzaroni, P. Viville, J. Cornil, C. Poriel, J. Mater. Chem. C 2020, 8, 14462-14468.

3. C. Poriel, J. Rault - Berthelot, Adv. Funct. Mat. 2021, 31, 2010547.

4. F. Lucas, C. Quinton, S. Fall, T. Heiser, D. Tondelier, B. Geffroy, N. Leclerc, J. Rault-Berthelot, C. Poriel, J. Mater. Chem. C 2020, 8, 16354-16367 ( COVER)

5. D. Thirion, M. Romain, J. Rault-Berthelot, C. Poriel, Mater. Adv. 2021, 2, 1271-1283.

6. C. Poriel, J. Rault - Berthelot, Technique de l'Igénieur 2021, E6507 v1.

Designing highly efficient Organic Semi-Conductors (OSCs) for Organic Electronics (OE) has led to the fantastic development of this technology. In this context of OE, Phosphorescent Organic Light-Emitting Diodes (PhOLEDs) are the 2nd generation of OLEDs (after fluorescent OLEDs) and have encountered a significant development for the last twenty years as they can in principle reach internal quantum efficiency of 100% by harvesting both singlet and triplet excitons. PhOLED technology is more mature than the recent 3rd generation of OLEDs based on Thermally Activated Delayed Fluorescence (TADF). A PhOLED uses a Host-Guest EMitting Layer (EML) which consists in a Triplet Emitter (Guest) disperses into an appropriate OSC (Host). For the last decade, the design of high triplet energy (ET) hosts (>2.7 eV), essential to be used with red, green and blue phosphors, has been an intense research field worldwide and has led to high-efficiency multi-layer PhOLEDs. This proposal finds its origin in an important fact of the literature: Almost all the high-efficiency PhOLEDs are multi-layer devices. These multi-layer PhOLEDs are constituted of a stack of organic layers in order to improve the injection, transport and recombination of charges within the EML. There are usually in a PhOLED stack, hole and electron transporting layers, hole and electron blocking layers and these layers are even often doubled. Despite the technology is mastered, it suffers from the complexity of the stack, a high-cost, and is time-consuming. In addition, interfacial phenomena can lead to parasite emissions and a low stability. Simplifying the multi-layers structure with the so-called Single-Layer PhOLEDs (SL-PhOLEDs), the simplest device only made of the electrodes and the EML is therefore one key step for the future. However, high-efficiency SL-PhOLEDs are very rarely reported in literature (especially blue emitting devices), due to the lack of host materials possessing all the required properties. In SPIRO-QUEST proposal, we aim to address this feature through the design of universal host materials for high performance red, green and blue SL-PhOLEDs. The goal of this project is to gather in a single molecule all the required properties to insure the energy transfers cascade within the PhOLED (particularly a high triplet state energy level), a high thermal/morphological stability for device lifetime, adequate HOMO/LUMO levels for charge injection and more importantly a good and well balanced mobility of electron and hole (ambipolar character). This ambipolarity is a key property for SL-PhOLED.
To fulfil all the above mentioned criteria, this project uses a Donor-Spiro-Acceptor molecular design which implies the judicious connection of a Donor unit to an Acceptor unit. One of the novelty of this proposal is the nature of the donor unit which is a new electron rich fragment: the Quinolinophenothiazine. This fragment is highly promising as shown in preliminary works led by this consortium. Thus, molecular design of host materials is the heart of this proposal, which possess very solid foundations as demonstrated by preliminary results published in the last 30 months.
As the development of new materials leading to high efficiency devices is one of the key direction of OE, SPIRO-QUEST targets striking results. More precisely, we aims to go beyond the state of the art in term of performance and stability of SL-PhOLEDs.
This multidisciplinary project is led by three complementary research groups, internationally recognized in their fields. SPIRO-QUEST involves a group specialized in chemistry of pi-conjugated systems (C. Poriel/J. Rault-Berthelot, ISCR - Rennes), a group specialized in microelectronics and especially in charge transport (E. Jacques, IETR- Rennes) and a group specialized in physics of OLEDs (D. Tondelier, Ecole Polytechnique - LPCIM).

Project coordination

Cyril PORIEL (INSTITUT DES SCIENCES CHIMIQUES DE RENNES)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

IETR INSTITUT D'ELECTRONIQUE ET DE TELECOMMUNICATION DE RENNES (IETR)
LPICM Laboratoire de physique des interfaces et des couches minces
ISCR INSTITUT DES SCIENCES CHIMIQUES DE RENNES

Help of the ANR 427,357 euros
Beginning and duration of the scientific project: February 2020 - 46 Months

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