Flexible Substrate Brings Next Generation Electronics Closer to Reality

Field effect transistors marked a significant turning point for the modern electronics industry. For many years, manufacturers built semiconductors with silicon and gallium arsenide. Companies eventually replaced these semiconductor materials with amorphous silicon (a-silicon or a-Si), which makes up the primary material used to fabricate the thin film transistors (TFT) used in liquid crystal displays.

TFT consists of field effect transistors that are layers of thin film deposited on a glass substrate.  The lack of a crystal structure allows display manufacturers to employ the vapor-deposition process to apply thin-film transistors onto large substrates.  Manufacturers use TFTs in the fabrication of popular electronic devices, including e-readers, laptops, smartphones, PC monitors, navigation systems, and high-definition TVs.

The processing and performance limitation of field effect transistors have manufacturers and researchers investigating the use of semiconducting nanoparticles to complement or replace thin film transistor (TFT) devices.

David Kim, a doctoral candidate at the University of Pennsylvania, heads a team of researchers who discovered a method for applying nanoscale particles of cadmium selenide on flexible plastic substrates.

Using amorphous silicon as a marker, the cadmium selenide nanocrystals process permits a charge to move 22 times as fast as a-Si. In addition, the cadmium selenide nanocrystals can be deposited at room temperatures. The annealing process requires a milder temperature compared to the moderate heat (150°C – 400°C) required for amorphous silicon, according to the researchers.

The ability to apply this semiconducting material at lower temperatures will lead to the ability to use other flexible substrate options.

Researchers created three types of circuits: 1) an inverter, 2) an amplifier, and 3) a ring oscillator.  Selecting a substrate consisting of a flexible plastic sheet, scientists used a stencil-like tool call a shadow mask to separate one level of the circuit on the substrate. They also utilized the shadow mask to mark off small areas of gold conductors, which function as the electrical connections to the upper levels of the circuit.

Scientists applied a aluminum oxide layer as an insulator and introduced a 30nm layer of nanocrystals. The shadow mask was also used to build a final top layer of electrodes, which create the circuits.

Display manufacturers and institutions have invested a significant amount of resources into the research of electron transport in cadmium selenide, but according to Kim, “until recently we haven’t been able to get good performance out of them.” According to Kim, “The new aspect of our research was that we used ligands that we can translate very easily onto the flexible plastic; other ligands are so caustic that the plastic actually melts.”

Researchers express eagerness about the enormous potential flexible plastic substrates hold for the next generation electronic devices. The flexibility, straightforward manufacturing processes, and low power consumption of cadmium selenide nanocrystal circuits will also lead to innovative electronics and sensors that will foster innovations in biomedicine and security technology.

Read more about this exciting development in the journal Nature Communications.

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