Hybrid organic-inorganic Light Emitting Diodes

Résumé

Hybrid organic inorganic light emitting diodes are nowadays attracting great attention due to their intrinsic air stability and solution processability, which could result in low-cost, large area, light emitting devices. Despite the fact that high luminance values have been already demonstrated in recent publications, the efficiency of HyLEDs has been limited by its peculiar hole-dominated electronic mechanism. In particular, the electron injection is promoted by the hole accumulation at the metal oxide EIL/organic interface, but at the same time this mechanism leads to limits the device efficiency. It is known from the research in OLEDs that when the recombination zone is close to an interface, exciton quenching and direct charge recombination can take place. In this thesis, the design rules for standard OLEDs technology have been applied to HyLEDs in order to overcome those limitations, and new successful strategies to improve the performances of this new class of devices have been presented. Firstly, the use of a charged polymer as electron injection layer from the metal oxide to the polymer was presented. This approach leads to more efficient HyLEDs and gives the possibility of using different light-emitting polymer, allowing the tuning of the emission colour of the device through the whole range of the visible spectra. This device structure is of particular interest because a multilayer structure composed by a metal oxide cathode, a conjugated polyelectrolyte EIL, and the active polymer, was prepared completely by solution processing, thanks to the orthogonality of the solvents used to deposit the subsequent materials. In chapter 3 it is demonstrated how the hole leakage through the metal oxides EIL is an important loss factor leading to a lower exciton density in the polymer layer.The use of insulating metal oxides with very deep valence band resulted in the lowering of the current density flowing through the device. This effect is due to the high barrier for holes at the organic/metal oxide interface when using insulating materials like HfO2 or MgO. Thus, through the employment of these metal oxides, the efficiency of the HyLEDs can be successfully raised. It is well known that high efficiency in OLEDs can be raised considerably only when making use of phosphorescent species. In chapter 4, the use of triplet emitters in high efficiency solution processed HyLEDs was presented. In that particular device layout, a novel doped metal oxide cathode was used in order to enhance the electron injection into the active organic layer and prevent exciton quenching. Very high efficacy values up to 15 cd/A have been obtained by tuning the composition of the active organic materials in the polymer layers. In chapter 5,the performances displayed by HyLEDs using ZnO nanocristals exceed those obtained by employing polycrystalline ZnO thin films and the effect is attributed to the larger bandgap of the ZnO NCs caused by quantum confinement. It was shown that the bandgap diminishes upon temperature assisted agglomeration which is why best device performances were obtained when simply drying the NCs at room temperature.The use of solution-processed ZnO NCs in the absence of any thermal treatment allowed for the preparation of the first bright flexible HyLEDs. This work clearly underlines the potential of this novel class of devices and it indicates HyLEDs as a real possible competitor to the current OLED technology.