Laboratoire de Physique des Interfaces et des Couches Minces

CNRS - École polytechnique - Institut Polytechnique de Paris

Organic Light Emitting Diodes (OLEDs)

Written by : Denis Tondelier

The structure of an OLED is a superposition of several organic semiconductor layers between two electrodes, one of which (at least) is transparent. Organic semiconductors used in an OLED can be either small molecules or polymers. However, the most complex structures (comprising several layers) preferentially use small molecules deposited by thermal evaporation. Polymers semi-conductors are preferentially deposited by wet techniques.

Structure of an OLED

The basic structure of an OLED consists of superimposing several layers of organic materials between a cathode and an anode, which is often transparent formed of indium tin oxide (ITO). Organic thin films typically include a hole transport layer (HTL), an emission layer (EML) and an electron transport layer (ETL). By applying an appropriate electrical voltage, electrons and holes are injected into the EML layer from the cathode and the anode. Electrons and holes combine in the EML layer to form excitons and then electroluminescence appears. The charge transfer materials, the emission layer and the choice of electrodes are fundamental parameters that determine the performance and efficiency of the OLED component.

There are three types of emissive materials: fluorescent materials, phosphorescent materials (PhOLEDs) and the latest generation thermally activated delayed fluorescence materials (TADF).

Currently at LPICM we are working on several projects concerning OLEDs.

A first collaboration with Professor Hissler’s group in Rennes aims to study a particular family of fluorescent materials based on the chemistry of phosphorus. These materials are studied in the form of small molecules which are evaporated as emitting material. In addition, these materials can be grafted onto ZnO nanoparticles, which is the subject studied in the ANR FluoHyb (ANR-17-CE09-0020).

White OLED based on phosphorus organic emitters

A second current collaboration is with the group of Doctor Poriel in Rennes. For several years we have obtained ANR funding (HOME-OLED ANR-11-BS07-0020 and SPIROQUEST ANR-19-CE05-0024) which aims to synthesize and study matrices for phosphorescent emitters. This leads us at present to study ambipolar matrices which allow the use of less complex PhOLED structures. In these structures, a single thin layer is sandwiched between two electrodes.

Flexible PhOLED

A final collaboration with doctors Pieters from CEA Saclay and Favereau in Rennes is interested in the use of TADF transmitters which emit circularly polarized light (ANR-19-CE07-0040).

OLED coupling TADF and CPL (J. Am. Chem. Soc. 2016, 138, 12, 3990–3993)