OLED screens already equip many smartphones, televisions and smartwatches. Each pixel produces its own light directly, without a backlight. This technology offers deep blacks, high contrast and thin devices. Japanese researchers are now exploring a new way of organizing internal materials to make current flow better.
An OLED relies on several extremely thin organic layers. When electric charges pass through them, some molecules emit light. In current screens, these molecules are generally arranged in a disordered manner, in a so-called amorphous structure. This organization makes large surfaces easier to manufacture, but it can slow the movement of charges.
Close-up view of a current OLED screen.
Image Wikimedia
This internal resistance matters a lot. Poor electrical circulation can require higher voltage, produce more heat and limit the achievable brightness. Manufacturers also have to find a balance between efficiency, lifespan, color fidelity and production cost.
Masahiro Morimoto, Yuya Honda and Shigeki Naka, from the University of Toyama, tested a rubrene layer. This organic material transports charges particularly well when it adopts a crystalline structure, in which molecules follow a regular order. Until now, integrating such crystals into a thin, easily manufacturable OLED remained difficult.
The researchers first deposited a disordered rubrene layer under vacuum. A two-step heat treatment then transformed it into a crystalline film. The resulting domains reached about 1 mm in width, for only 50 nm in thickness. This thinness corresponds to about one thousandth that of a human hair.
The change strongly modified the electrical behavior of the component. At identical voltage, the current density could reach up to 1,000 times that of a comparable OLED containing amorphous rubrene. This result means charges passed through the layer much more easily.
The experimental diode began to emit around 1.3 V for a low luminance. A reduced voltage could, in the long run, help limit certain electrical losses. In a smartphone or television, this could lead to less energy-hungry screens, brighter at equal consumption, producing less heat and with a longer lifespan.
The crystalline structure also made the light emission narrower and more precise in the measured spectrum. This property could help produce better controlled colors. However, it will have to be combined with suitable red, green and blue materials, as well as the many other layers needed in a commercial panel.
The method uses vacuum deposition, already common in OLED manufacturing. This advantage could facilitate its future integration. Researchers must still verify the regularity of crystals over large areas, their compatibility with millions of pixels and their resistance to prolonged use cycles.