Researchers at the University of Maryland, College Park have printed transparent transistors on transparent paper. The finished device is flexible, up to 84% transparent, and in theory this could be the first step towards green, paper-based electronics.

As we’ve covered before, printing computer circuits isn’t overly difficult — you just need to find the right conductive and semiconductive inks (which can be tricky), and then print them out on a suitable substrate until you have a transistor. Because these ink-based printed circuits are very thin, though, the smoothness of the substrate is very important. When you’re dealing with layers of ink that are a few nanometers thick, any blemish on the substrate is enough to disrupt the flow of electrons and break the circuit.

In the case of regular old paper, bumps and blemishes are usually measured in micrometers — far too irregular to print circuitry on. Not to be deterred, the researchers at the University of Maryland used nanopaper — paper created from wood pulp that’s been specially treated with enzymes and mechanically beaten. Nanopaper has a much more regular structure than normal paper, and is stronger (and transparent) as a result. More importantly, though, nanopaper is smooth to within just a few nanometers. “It’s as flat as plastic,” says Liangbing Hu, one of the researchers who worked on the project.

With the nanopaper in hand, the researchers then created some transistors by printing three inks: First a layer of carbon nanotubes, then a dielectric ink, then a semiconducting ink, and then another layer of nanotubes. The nanotubes not only act as electrodes but also act as a structural backbone. The final transistors are up to 84% transparent, and the device continues to work while bent.

Moving forward, it’s easy to imagine flexible, printed devices that are responsibly sourced using renewable sources. The fact that these printed circuits are highly transparent could also be useful, either for cosmetic reasons in wearable computing, or for building displays. Before such applications can be considered, though, the researchers will have to find a way of producing these transparent transistors using roll-to-roll printing, or another commercial, mass-producible process.

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Research paper: DOI: 10.1021/nn304407r – “Highly Transparent and Flexible Nanopaper Transistor”