Design and Characterization of Electroluminescent

Mallesham GODUMALA and Gaël ZUCCHI LPICM – Ecole polytechnique – Route de Saclay, 91128 Palaiseau - France

What is an OLED: An organic emitting diode (OLED) is a light-emitting device in which the emissive layer is a thin film of an organic compound that emits light when an electric current passes through it.  This organic layer is situated between two electrodes, at least one of which should be transparent. The microcavity is usually about 200 nanometers, leading to extremely thin devices. This, added to the flexibility of organic materials opens the way to the design of low-weight foldable devices.  An OLED display works without a backlight i.e., they are self-emitting. Applications in the fields of flat panel displays and lighting are anticipated, for instance. Multi functional conjugated Polymers:  Conjugated polymers are soluble electroluminescent materials which properties can be easily tuned by chemical engineering. This leads to multi-functional materials with controllable properties, in terms of solubility, charge transport, and color of emission. They present a great interest as they allow the design of devices with simple structures (reduced number of layers), reduced manufacturing cost and easy to fabricate.  Thanks to their good solubility of polymers, they can be coated from solution processes. Solution processes are more preferable over thermal evaporation because they allow the use of cheap techniques, and they are compatible with large area applications.  They usually possess low electron affinity (low ionization potential) which makes them more hole transporting materials.

Thermal evaporation: OLED architectures Solution process: Metal Cathode Metal Cathode Electron Transport Layer Advantages: Optimized multi-layer Advantages: cheap techniques OLED architecture structure. Drawbacks: difficult to optimize Electron Transporting Later Hole Blocking Layer Drawbacks: Expensive and not charge transportation. compatible with large area Solution: design of multi- Organic Emitter Emitting Layer deposition. functional materials. Hole Transporting Layer Hole Transport Layer ITO Anode Hole Injection Layer

ITO Anode

Goal of the work: - To develop conjugated polymers derived from blue-emitting polyfluorenes. - To increase the electron transport ability of the materials by a suitable chemical functionalization (Benzimidazole). Benzimidazole is an electron deficient moiety which transports electrons. - To control the emitted color by using specific emitters (Tb3+ for green emission and Eu3+ for red emission). The production of white light is the final objective by using these three primary colours. The advantages of Polyfluorenes in OLED applications:

 The first blue light emitting diode was produced Ambipolar polymeric materials: with a substituted polyfluorene. These materilas are useful as blue emitters. Efficient blue emitters (QY ~50% in thin films) (Zucchi et al. J. Mater. Chem. C, 2013, 1, 3207–3216).  Polyfluorene and its derivatives are most promising blue light emitting materials because of several advantages:

 High (PL) efficiencies. For efficient blue emission

N  Wide band-gaps (~ 2.90 eV) which is required for blue N emission.

N (x+y)  Good thermal stabilities which is essential for good ( y ) N N morphology. (x) (x+y)  n Electron transport segment Easy to tune their properties by substitution and/or copolymerization. used in these polymers 1.0  The fluorene core structure shows good carrier transport properties. 0.8 N th The (9 position) of is more 0.6 useful to modify charge injection and mobility, and to N N improve the thermal stability as well as solubility of the 0.4 H polymers. Typical PL spectrum of a 0.2 Benzimidazole Polyfluorene Intensité normalisée

0.0

400 500 600 700

λ /nm

Characterization of the materials:  Photophysical properties: To estimate the color, quantum yield and to understand the energy transfer processes. (collaboration with ENS Cachan)  Electrochemical properties: To evaluate the HOMO and LUMO energy levels of the materials. (collaboration with PMC, Ecole polytechnique)  Electrical properties: To asses charge carrier mobility.

Conclusions:  Multifunctional Materials with tunable properties like emission along with charge transporting properties (usually called ambipolar materials) are more preferable to reduce the cost of production of .  Solution deposition techniques offer an attractive alternative approach superior to thermal evaporation technology, in terms of their potential to enable low-cost and large area manufacturability.

Acknowledgements: Mallesham GODUMALA gratefully acknowledges the Labex Charm3at, Ecole polytechnique, and the CNRS for providing funding for his post doctoral research