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A2.02 - Logic on Paper Using Environmentally Friendly Ion Modulated Transistors 
December 1, 2014   2:00pm - 2:15pm

We have created low-voltage ion-modulated transistors on paper using environmentally friendly electrolytes. Using a method of spontaneous vertical phase separation during application of a semiconductor/insulator blend we can create thin semiconductor layers on rough paper substrates. With this, drop casting or spin casting, technique we get a thin semiconductor layer on top of a polymer insulator. This fabrication method has several advantages, namely; the semiconductor is separated from the paper substrate that contains dopants that would penetrate and chemically dope the semiconductor, the amount of semiconductor needed in the blend in order to get working transistors can be as low as 0.5 wt% reducing both the cost and the amount of harmful semiconductor material needed, and finally making the semiconductor thin resulting in fast-switching devices. From a device performance standpoint the last point will be the most important one. We have found that ions from the electrolyte will always penetrate the semiconductor during operation. This is also the reason for the high currents observed in our ion-modulated transistors. During operation ions penetrate and dope the semiconductor resulting in a bulk transport through the semiconductor. A very disadvantageous side-effect of this is the reduced switching speed of the device (as a result of the slow ion motion in the semiconductor during the doping process). A thin semiconductor will effectively nullify this problem resulting in fast switching times. The semiconductor poly(3-hexylthiophene) (P3HT) and the insulator poly(L-lactic acid) (PLLA) is mixed together in chlorobenzene and drop casted onto the paper substrate. Due to the favorable surface energies and solvabilities involved, the PLLA will precipitate first and form a layer of insulator on the paper substrate leaving a P3HT-layer on top. The electrolyte is a mixture of choline chloride and different organic compounds, such as urea or glycol. When mixed together these form deep eutectic mixtures that are liquid or semi-liquid at room temperature. They are environmentally friendly and can be solution processable and thus compatible with roll-to-roll fabrication techniques. The best one of these mixtures is blended with a commercial binder to create a solid electrolyte that can be spin casted on top of the P3HT semiconductor. We demonstrate the usefulness of these type of devices by creating inverters and five-stage ring-oscillators that oscillate at 1 Hz. We further show how logic can be created using these environmentally friendly and disposable paper transistors by creating working SR-latches and flip-flops.

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