Organic Devices on Flexible Substrates Advance

Although still mostly in the research phase, there have been some significant advancements in the field of organic — what some might call "plastic" — devices, such as thin-film transistors (TFTs). Organic TFTs can be "printed" by solution processing, which means that, not only can they potentially be manufactured at a lower cost than their inorganic cousins, but they can be printed on almost any substrate because of the low processing temperatures. Eventually, these could be in widespread use for various low-cost applications, such as active-matrix LCDs in mobile electronics, smart cards, price and inventory tags, and large-area sensor arrays.

In 2000, state-of-the-art organic TFTs, fabricated by Cambridge University, had a mobility of 0.02 cm²/Vsec. Four years later, in work to be presented at the upcoming International Electron Devices Meeting (IEDM), held Dec. 13-15 in San Francisco, organic TFTs with 1 cm²/Vsec mobilities will be presented by Penn State and University of Kentucky researchers (higher mobilities related to higher switching speeds and better display performance). The latest organic TFTs were fabricated using triethylsilylethynyl (TES) thienyl pentacene and a maximum process temperature of 90°C. The researchers found that, with simple gold contacts, they could achieve an extracted field-effect mobility of only 0.32 cm²/Vsec. But once they treated the contacts with pentrafluorobenzenethiol, they were able to reduce drain current crowding and improve the mobility to 1 cm²/Vsec.

This large-area, flexible and lightweight sheet image scanner consists of organic transistors and organic photodiodes. It is placed on a business card under ambient light for capturing an image of the card. The sheet has no optical or mechanical parts. (Source: University of Tokyo)

Perhaps the most interesting work comes from Infineon and Freiberg University. They've developed organic TFTs with a 2.5 nm thick molecular self-assembled monolayer gate dielectric, which allows the devices to operate with supply voltages as low as 1.5 V. To create gate electrodes that can be lithographically patterned, the team used a thin layer of aluminum, deposited by evaporation in vacuum. The natively oxidized aluminum surface presents a large density of functional groups for molecular self-assembly, which is carried out from solution or from the vapor phase.

Papers will also be presented at IEDM by MIT researchers who have developed a low-temperature, full-lithographic transistor process flow to create pentacene TFTs, as well as methods for controlling threshold voltage of pentacene TFTs based on exposure to an oxygen plasma or UV/ozone. In one paper, the researchers will describe how pentacene can be damaged by exposure to solvents, so a room-temperature CVD-deposited polymer film, parylene-C, is used to protect the pentacene layer during photolithography in two of the five masking steps. In another, they describe a solution to the problem of varying gate threshold voltages in TFTs caused by process-induced traps at the semiconductor-dielectric interface.

A paper from Princeton University and Universal Display Corp. (Ewing, N.J.) reports on organic LEDs fabricated on a dome-shaped organic substrate. Here, a pixilated array of organic LEDs was fabricated on transparent plastic and interconnected while flat, and in the last process step was shaped into a dome. The idea behind this was that ICs that cover large surfaces of arbitrary shape will be required in "human-size" electronics for distributed sensing (medical diagnostics), actuation (human prosthetics) and display (electronic garments) at high functional density. A spherical surface or dome is one among the class of non-developable surfaces, which cannot be made by the simple bending or rolling of a flat sheet. The researchers say making a flat circuit then shaping it to a dome requires large plastic deformation, which poses a tremendous challenge because mechanical design criteria are superimposed on the usual electro-optic criteria.

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