Organic Light Emitting Diodes (OLEDs) offer a very promising alternative to conventional lighting but are far to be efficient enough to drastically change the future. A key requirement to reach high-efficiency OLEDs is that the emission of light should arise from phosphorescence and not from fluorescence for that the chemist need to design new organic host materials which electronic properties fit with blue inorganic phosphorescent derivatives.
In order to reduce domestic and public energy consumption, new efficient sources of low-energy white light are urgently required. The development of Phosphorescent OLEDs using host-guest emitting layers has led to numerous efficient green and red Phosphorescent OLEDs. However, the lack of effective blue host materials remains a severe limitation to this technique. In fact, these materials must have a high triplet energy level (ET), must be thermally and morphologically stable and must have good carrier transporting properties in order to get a rapid recombination of hole and electron in the active layer and a rapid transfer of the triplet excitons to their guest blue emitter. <br />Our objective is to design new host materials with * a restricted pi-conjugation length through meta-phenylene linkages, achieving hence a high ET but keeping intact the charge carrier properties, * insulating spiro-fluorene to achieve high thermal/morphological stability and the perfect separation of hole and electron transporting units. <br />This unique molecular design appears hence very promising to combine within a single molecule high ET, good thermal/morphological stability and good charge carrier properties, usually considered as antinomic. This design may pave the way to the development of pure hydrocarbon derivatives as host materials for highly efficient Phosphorescent OLED, key commitment for the future of low energy lighting. <br />This project concerns then the synthesis of new organic host materials, the determination of their physicochemical and photophysical properties and their use as host materials for blue inorganic dopant as active layer in blue Phosphorescent OLEDs. <br />
The theoretical approach coupled to experimental studies is of great importance in this project as DFT calculations will allow insights into the electronic structure and spectroscopic properties of the «3pi-2spiro« molecules considered in this work. DFT calculations predict the localisation of the electron density distributions of the HOMO and LUMO levels and their energy levels.
The synthesis of the new molecules is based on simple and efficient synthesis to reach numerous molecules starting with common intermediates. This is the expertise of the Rennes group which is at the origin of the «3pi-2spiro« concept and works for several years on spiro compounds for various applications e.g. organic electronics, photophysics of pi-conjugated systems.
Physicochemical analysis allows getting a structure properties-relationship and consists in optical studies (absorption and emission in solution and in solid state at ambient or at high temperature), electrochemical studies, studies of morphological stability… with the perspective of HOMO and LUMO levels, ET energy determination.
The most interesting new molecules (adapted ET and HOMO-LUMO levels) are then used as active layer in OLED, using eventually multilayer devices. The guest is chosen as function of the ET energy of the host with the objective of reaching good host for blue guest, however efficient green PhOLEDs are also of interest.
OLEDs characteristics are then studied in detail in terms of colour emission, efficiency and so on..
During the first part of the project (eighteen months), partner 1 have synthesized various 3pi-2spiro compounds derived from dispirofluorene-indenofluorene (DSF-IF) with central core derived from meta-terphenyl (two isomers) and ortho-terphenyl (one isomer) completing the series of DSF-IF isomers. Meta-terphenyl DSF-IF derivatives lead to interesting results in blue PhOLEDs. However, ortho-DSF-IF with a triplet level at 2.6 eV is more adapted to green dyes. A new direction has then been taken in the choice of new molecules for blue PhOLEDs. These new molecules are donor-acceptor compounds with 2pi-1spiro architecture, spirodiazafluorene-fluorene derivatives or spirobifluorene derivatives with various substituents on carbon 4 of one fluorenyl unit. Theoretical calculations predict for these molecules a high triplet energy level adapted to blue phosphorescent dyes and HOMO/LUMO levels adapted to the Fermi level of the electrodes used in the OLED devices.
Lifetimes of excited species as well as energy level of the triplet of all the new molecules have been measured in Cachan (partner 2). The values obtained are consistent with those obtained by theoretical calculations showing the interest of the preliminary theoretical calculations.
Complete physico-chemical analyses of the new molecules are achieved for most of the molecule and need to be completed for the others.
Concerning the OLED and PhOLED devices, partner 4 (polytechnique) has done a lot of measurements as soon as new molecules arrived from Rennes. New device architecture has been tested to favour the electron-hole recombination within the active matrix in order to get the exciton transfer from host to guest the more efficient as possible. Results obtained were largely improved and to our knowledge, the performances obtained with meta-DSF-IF derivatives are the highest never reported in literature with pure hydrogenocarbon molecules.
The choice of the new host molecules and of the new device architecture is promising for the future of the project.
The second part of the research will then surely be the most gainful part of the project. Theoretical calculations already done allow us to predict new molecules of high interest as host materials for blue dyes. The participation of two PhD students in the Rennes group allows being optimist for the synthesis of these new molecules.
Physicochemical analysis completed by ET measurements, lifetime of the excited species measurements and charges mobility of the molecules in the solid state will be done in collaboration of partners 1, 2 and 3.
Finally, the participation of a post-doc fully dedicated to this project, will allow partner 4 to test all the new molecules adapted to blue dyes in efficient blue PhOLEDs.
Properties dependence on positional (1,2-a)- vs. (2,1-b)-indenofluorene isomerism: influence of the ring bridging
Maxime Romain, Denis Tondelier, Jean-Charles Vanel, Bernard Geffroy, Olivier Jeannin, Joëlle Rault-Berthelot, Rémi Métivier, Cyril Pori
The present proposal deals with the design of new host materials based on oligophenylene derivatives for high-efficient blue Phosphorescent Organic Light Emitting Diodes (PhOLEDs).
As home-lighting now accounts for over 9% of all the energy consumed in France each year, new highly efficient sources of white light are urgently required. Organic Light Emitting Diodes (OLEDs) offer a very promising alternative but are far to be efficient enough to drastically change the future of lighting. A key requirement to reach high-efficient OLEDs is that the emission of light should arise from phosphorescence and not from fluorescence. However phosphorescent emitters, compared to fluorescent emitters, typically possess longer lifetimes, leading to undesired concentration quenching and/or triplet-triplet annihilation and, thereby, declining performances. To overcome these drawbacks, researchers commonly dope the triplet emitters (guest) into an appropriate organic host material. Literature reports a huge number of triplet emitters that have been used as dyes for phosphorescent OLEDs. Thus, red and more recently green phosphorescent dyes with triplet state energy level (ET) of ca. 2.0 eV and 2.35 eV respectively, have been doped into charge-transporting hosts leading to high-efficient red and/or green PhOLEDs. In contrast highly efficient blue PhOLEDs remain very rare in the literature, because of the lack of suitable host materials with very high ET (ET > 2.7 eV).
An 'ideal' host material for blue PhOLED should present a very high ET (ET>2.7 eV) but also good carrier transporting properties and high morphological stabilities, which appear to be antinomic. Indeed, good carrier transporting properties are usually linked to extended pi-conjugation whereas high ET is usually found in small molecular units with short pi-conjugation pathway. Similarly, the morphological stability that is necessary for forming stable and uniform films is also usually associated with an important molecular size of the material, antinomic again with a high ET.Thus, there is a trade-off between increasing the band gap of a material to increase singlet and triplet energies and decreasing the length of the pi-aromatic system, which may adversely affect the charge transport properties and the morphological stability.
Based on a new concept, never described in the literature, we propose an answer to this apparent antinomy through the use of original oligophenylene derivatives.
Oligophenylene derivatives and pure hydrocarbons in general, have been very rarely used as host materials for blue PhOLED. Indeed, it is known that the ET of oligophenylene derivatives strongly decreases when the number of phenyl rings increases, limiting hence their potential application as host materials. However, one way to tune the ET of oligophenylenes materials is to interrupt the pi conjugation along the oligophenylene core through the modification of the phenylene linkages (para / meta / ortho). Our idea in this work is thus to design oligophenylene derivatives with:
- a restricted pi conjugation length obtained through meta phenylene linkages, achieving hence a high ET but keeping intact the excellent charge carrier properties.
- insulating spiro-fluorene units to achieve (i) a high thermal / morphological stability and (ii) the perfect separation of the hole and electron transporting units.
This unique molecular design appears hence very promising to combine within a single molecule high ET, good thermal / morphological stability and good charge carrier properties, usually considered as antinomic. This design may pave the way to the development of pure hydrocarbon derivatives as host materials for highly efficient PhOLEDs, key commitment for the future of low energy lighting.
Our project deals then with the synthesis of new organic molecules, their physicochemical, photophysical and charge transport properties and their use as host materials in high-efficency blue phosphorescent OLEDs.
Madame Joëlle RAULT-BERTHELOT (CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE BRETAGNE ET PAYS- DE-LA-LOIRE) – Joelle.Rault-Berthelot@univ-rennes1.fr
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.
Cachan CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE ILE-DE-FRANCE SECTEUR EST
IMS INSTITUT POLYTECHNIQUE BORDEAUX
Polytechnique CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE ILE-DE-FRANCE SECTEUR OUEST ET NORD
Rennes CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE BRETAGNE ET PAYS- DE-LA-LOIRE
Help of the ANR 414,000 euros
Beginning and duration of the scientific project: December 2011 - 36 Months