ITO sputtering study for transparent OLED top electrode

Organic Light Emitting Diodes (OLEDs) are a cutting-edge lighting technology that allows to create thin, light weight, diffuse-emitting, glare-free, and even transparent light sources having low power consumption, wide color gamut, fast response time, and precise dimming. Transparent OLEDs (TrOLEDs) can be integrated into architectural glass windows, car windshields, divider panels, or lamps, thus enabling the creation of lighting objects with captivating and innovative designs, allowing natural light to pass through, since they are optically transparent when turned off. In order to achieve high-performance TrOLED, it is necessary to develop transparent electrodes with superior optical and electrical properties, to replace the opaque metal top electrodes that are commonly used in bottom-emitting OLEDs. Transparent conducting oxides (TCOs) are the most widely used transparent materials for bottom electrodes, and sputtering technology is the most common technique to deposit such materials. However, sputtering of high-quality TCO is not well suited for the fabrication of OLED top electrodes because it exposes the underlying organic layers to damages by the plasma emission of high-energy particles. In this study, the effects of sputter deposition of Indium Tin Oxide (ITO) on Tris(8-hydroxyquinoline) aluminum(III) (Alq₃) OLED active layer have been investigated, to establish the best compromise between process conditions and ITO films electro-optical properties to reduce the damage induced by sputtering. Moreover, the impact of introducing a thin thermally evaporated calcium (Ca) layer, before ITO sputtering, has also been examined. In this case, Ca acts both as a protective layer for the underlying Alq₃ and, at the same time, as a good electron injector for OLEDs.