A step by step strategy for solution-processed quantum dots light emitting diodes
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A step by step strategy for solution-processed quantum dots light emitting diodes Hélène Bourvon1, Stéphanie Le Calvez 1, David Vaufrey 1, Sylvia Meunier Della Gatta 1, 1
CEA-LETI, MINATEC Campus, DIHS/LTCV, 17 rue des Martyrs, 38054 Grenoble Cedex 9 France
ABSTRACT Solution-based printing and coating processes have the potential to dramatically reduce the production costs of Organic Light Emitting Diodes. This is particularly true for Quantum Dots Light Emitting Diode (QDLEDs), the newborn in the field of LEDs, due to quantum dots price prohibiting wastage. Here, we report our latest results on the development of solutionprocessed QDLEDs. We have implemented a layer by layer strategy, from a whole evaporated small molecule based OLED to a hybrid QDLED developed by wet deposition techniques for the first layers and by evaporation for the last ones. Intermediate steps are discussed in this paper. First, we have worked on a poly(3,4-ethylenedioxythiophene poly(styrenesulfonate) (PEDOT:PSS) layer. The PEDOT:PSS formulation for inkjet printing and spin coating were optimised: wettability on an ITO substrate, jettability of the inkjet formulation and baking conditions were studied. Additives as surfactant and ethylene glycol were added to the commercial inkjet grade solution to improve the deposition process. As a consequence to this study, anisotropic conductivity of PEDOT:PSS was observed and is reported here. In particular, ethylene glycol demonstrated a strong ability to increase the parallel conductivity by several orders of magnitude, but not the vertical one. Then, inkjet-printed and spin-coated device performances are compared to complete this first study. Hybrid devices with an efficacy of 12cd/A at 4V were obtained, with 2.17 % of EQE, and a luminance of 4000 cd/m² at 4V. Finally, we succeeded in the development of our first QDLED based on CdSe core/ CdSZnS shell quantum dots emitting at a wavelength of 600nm. Quantum dots were inkjet printed, in order to waste as little as possible this very expensive material. INTRODUCTION Small molecules-based Organic Light-Emitting Devices (OLEDs) are well mastered in research teams and evaporation processes are being transferred to the industry for applications like cell phone screens or TV set. Nevertheless, this technique is still very expensive and difficult to adapt for large area and mass production. That is why a way to reduce production costs has to be found. One way to face this issue could be the use of solutions. Polymers or solvable molecules could be deposited by spin coating or inkjet printing on a drop-on-demand mode, which could reduce wastage of active molecules by far, from 95% in evaporation machine to a few percents for inkjet process. Unfortunately, it is still difficult to master material properties in solution-based layers. Solvents must be chosen carefully to avoid interface troubles and dissolution of successive layers. Moreover, for specific applications, it is important to have a very narrow emission spectrum. Quantum dots (QDs) are particularly
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