Effect of Bending on the Electrical Characteristics of Flexible Organic Single Crystal-based Field-effect Transistors

The overall goal of this work is to demonstrate how the bending of a flat substrate will effect the property of a field effect transistor, that is prepared on the substrate. This method can help answering key questions in flexible electronic devices. Including, how bending a substrate will influence the performance of a device that is using organic transistors.

The main advantage of this technique, is for that wide quantitative data, in various bending direction. And evidence about the packing arrangements in a organic crystal. Prepare the TCDAP sample as described in the text protocol.

Put the sample at one end of a boat, and load the boat into a glass inner tube. Load the inner tube into a longer glass tube, and push it in about 17 centimeters from the opening. Next, load the long glass tube into a copper tube horizontally fixed on a rack.

Make sure the boat of TCDAP is located in the middle of the heating area, defined by a heating band around the copper tube. Purge the PBT system with helium gas at a flow rate of 30 cubic centimeters per minute. Then, turn on the transformer to heat up the heating band to 310 degrees Celsius.

Maintain at this temperature for two days. After cooling the system to room temperature, collect the crystals from the inner tube. Put double sided tape on a pre-cleaned and pre-cut 200 micron thick transparent polyethylene terephthalate, or PET substrate.

Examine the crystals under a stereo microscope. Select good quality, shining crystals, with the dimension of approximately 5 millimeters by 0.03 millimeters for device fabrication. Place a needle like TCDAP crystal parallel with the length of the PET substrate on the double sided tape, and fix it securely.

Under a stereo microscope, apply water based colloidal graphite through a microliter syringe needle, in a line that extends from the two ends of the crystal, acting as the source and the drain. Wait for about 30 minutes for the colloidal graphite to dry, and measure the distance between the two graphite spots under an optical microscope, to determine the exact channel link. Use carbon conductive tape to fix the PET substrate on a microscopic slide.

Place the slide near the end of the paralysis tube of the deposition chamber. Weigh zero point five grams of the precursor of the dielectric insulator, Paracyclophane, and place it near the inlet of the paralysis tube. Pump down the system to a vacuum of 10 to the minus 2 torr.

Preheat the paralysis zone near the center of the tube to a preset temperature of 700 degrees Celsius, and maintain at this temperature. Heat up the Paracylophane sample to 150 degrees Celsius. The vapors of the precursor, will pass through the paralysis zone, to give the monomers.

Which will condense near the end of the paralysis tube and polymerize to form a polymer coating. After the paralysis polymerization reaction continues for two hours, cool down the system and take out the samples form the paralysis tube. Determine the thickness of the deposited polymer dielectric layer, by measuring the step height of the polymer layer on the substrate using a profilometer according to the manufacturers instructions.

Apply the isopropanol base colloidal graphite through a microliter syringe needle, to form a line on the back of the dielectric layer, above the crystal, to serve as the gate electrode. Use the scalpel to cut a hole through the polymeric dielectric film, above the source drain electrode area to expose the electrodes underneath for connection. With the help of a stand in clamps, bring the electrode probes from the parameter analyzer into contact with the source drain gate electrodes.

Record the IV characteristics at different gate potentials according to the manufacturer’s instructions. To measure the properties in the tinsel state, wrap the back side of the flexible PET substrate around cylinders of different radii, and fix PET substrate the the cylinder on four sides with vacuum tape. Connect the probes, so the source drain gate electrodes and measure the IV characteristics at different gate potentials, as before.

To measure in the compressive state, wrap half of the front side of the PET substrate around the end of a cylinder, such that the crystal’s source drain gate electrodes are facing the cylinder, and yet are still exposed. Fix the PET substrate on the cylinder with vacuum tape. Finally, connect the probes to the source drain gate electrodes, and measure the IV characteristics at different gate potentials as before.

The top contact single crystal field effect transistor prepared on a flexible substrate is shown. In addition to the schematic diagrams of the bending experiment. The upward bending, or compressive state, is shown as well as the downward bending, or tinsel state.

An overlay of the transfer characteristics of the TCDAP Single Crystal Based Field Effect Transistor device in the downward bending state, is shown here. An overlay of the transfer characteristics of the TCDAP Single Crystal Based Field Effect Transistor device in the upward bending state is also shown. These plots display the measured mobility as a function of the bending radius for the TCDAP Single Crystal Based devices, for downward bending and for upward bending.

The field effect mobility of the Single Crystal Device varied in the opposite direction, depending on the bending direction. Which may be due to changes in the packing arrangement of the molecules in the crystal. This experiment is designed to understand the device property of organic transistors, upon bending of the device substrate.

This experiment utilized single crystals of organic conductor as channel material. To eliminate methodic complications. The choice of single crystal of the right size, and quality, is important.

The use of double sided tape is essential to maintain a constant contact between the organic crystal, and the dielectric layer, so that a reproducible result can be obtained. When bending the end of the device upward to reach the compressive state by wrapping around the cylinder, the device is wrapped at the end of the cylinder, so that half of the device is exposed to air. This allowed if the parameter and to be contact with electrodes.

The suggest that, in compressive state, the mobility increases, the more so, with higher bending curvature. Whereas in tinsel state, the mobility decreases, also more decrease, with higher bending curvature. In lieu of the correlation between electronic cupping and the mobility, the results are rationalized to be due to the changes in the distance between molecules in a crystal, upon bending.

This method allows measurements of currents on the tinsel state as well as in the compressive states. Which is more representative of the real situation. And this method also allows a variety of electric property to be measured directly on a flexible substrate.