The research led by Professor Hideo Hosono at the Institute of Innovative Research, Tokyo Institute of Technology, was part of the Strategic Basic Research Programs of the Japan Science and Technology Agency (JST). The researchers focused on how the thin-film transistors (IGZO-TFT) are now being applied to OLED displays, and then succeeded in using transparent amorphous oxides to develop new materials for use in the electron injection layer and electron transport layer.
The newly-developed transparent oxide semiconductors, a-calcium aluminate electride (a-C12A7:e) and a-zinc silicate (a-ZSO), are said to increase the stability when applying IGZO-TFT to OLED displays, while lowering manufacturing costs.
Publishing their results in the online bulletin edition of the “Proceedings of the National Academy of the USA” in a paper titled “Transparent amorphous oxide semiconductors for organic electronics: Application to inverted OLEDs”, the a-C12A7:e is described as having an exceptionally low work function of 3.0 eV (equivalent to lithium metal) and was used to enhance the electron injection property from a-ZSO to an emission layer.
A-ZSO is reported to exhibit a low work function of 3.5 eV and high electron mobility of 1 cm2/(V · s) exceeding that of conventional organic materials by more two orders of magnitude. The a-zinc silicate can also form an ohmic contact with both conventional cathode and anode materials, making it a very versatile transport layer.
Using the new materials, the researchers were able to fabricate OLEDs with a performance equivalent to or greater than stacked structures even when using reverse-stacked structures (structures in which the cathode is at the bottom). This is because the new materials circumvent the organic semiconductor’s low electron mobility and their transparency allows any stacking order.
The researchers also demonstrated the manufacturability of their new semiconductors on a large-area substrate, at ambient temperatures, with the same ease as depositing transparent ITO electrodes. What’s more, the thin film is amorphous with superior smoothness and allows for simultaneous wet etching of the thin film together with the ITO electrodes on which it has been formed, simplifying processes for mass production.
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