Spray-Coated Melanin/PEDOT:PSS Films for Sustainable Organic Electrochemical Transistors

In this research, we developed sustainable organic electronic devices through a scalable deposition approach using low-cost setup and solution processable materials. The center devices involved with solution processing of bio-compatible agreeing materials like melanin and the integration into functional electronic device. To begin, add 0.3 grams of three four dihydroxy DL phenylalanine to a glass beaker.

Add 60 milliliters of deionized water to the beaker to dissolve the three four dihydroxy DL phenylalanine under magnetic stirring. Add 400 microliters of ammonium hydroxide to start the oxidation process. Transfer the prepared solution to a polytetrafluoroethylene liner inside a stainless steel reactor.

Then, close the reactor and increase the pressure to six atmospheres with industrial oxygen gas. Keep the mixture under magnetic stirring for six hours. Transfer the synthetic mixture into a dialysis membrane.

Dialyze against deionized water to extract and purify the melanin, replacing the water daily for seven days or until it becomes colorless. Dry the purified material in an oven at 90 degrees Celsius for 12 hours to obtain melanin powder. Then place the substrate below a shadow mask pattern to obtain five pairs of source and drain electrodes on each substrate separated by one millimeter in length and in width.

Afterwards, place the shadow mask with the substrates in the evaporation support. Now place the required amount of gold in the evaporation boat to ensure adequate film thickness and uniform deposition. Place the masked samples into the sample holder inside the evaporation chamber.

Next, configure the deposition controller to begin gold deposition at a slow rate of 0.2 angstroms per second until a thickness of five nanometers is reached for controlled nucleation and uniform film formation. Then set the deposition rate to one angstrom per second until the final thickness of 100 nanometers is achieved. Automatically evacuate the evaporation chamber until a vacuum of five times 10 to the power of negative six millibars reached and start the evaporation process.

Wait for the deposition process to complete. Once the evaporation chamber has cooled for one hour, refill the chamber until one atmosphere and remove the substrates. Then, cover the gold electrodes with an adhesive mask that has openings for the channel deposition.

Select a robust three dimensional printer with suitable dimensions to use as the base platform for the spray coating system. Then modify the three dimensional printer framework to mount the airbrush for mechanical adaptation. Remove the extruder and place the airbrush on a custom holder.

Use the x, y and z axis stepper motors to control the motion and height of the airbrush. Adapt the extruder to actuate the airbrush trigger, enabling software controlled spray release. Then center the airbrush in the XY plane directly above the hot plate, maintaining a vertical distance of 15 centimeters from the airbrush to the hot plate surface.

Now set the hot plate temperature to 100 degrees Celsius and adjust the compressed air pump to a pressure of one bar. Program the G code to move the airbrush 10 centimeters along the x axis at a speed parameter of F 8, 000. Set the spray opening width using the E 1.5 command and define a dwell time of P 5, 000 between forward and return strokes.

Then place the masked substrate with the source and drain electrodes already deposited at the center of the hot plate. Add 250 microliters of the melanin and P.PSS solution into the airbrush reservoir. Edit the desired number of spray cycles in the G code, typically setting it to 10 cycles and run the program.

Clean the airbrush reservoir with deionized water after completing the deposition. To measure the output curves of OECTs, sweep the drain voltage from zero volts to minus 0.6 volts. Record the drain current at each step.

To measure the transference curve, fix the drain voltage at minus 0.5 volts. Sweep the gate voltage from minus 0.5 volts to plus 0.6 volts and record the drain current. UV visible spectra confirmed successful incorporation of melanin into P.PSS films by the appearance of characteristic melanin absorption bands.

FTIR analysis showed interactions between melanin and P.PSS demonstrated by the disappearance of characteristic P.PSS absorption bands. Profilometry revealed that pristine P.PSS films had the highest thickness of 961 nanometers, which decreased to 342 nanometers upon addition of 10%melanin, then increased to 493 nanometers at 30%and 562 nanometers at 50%melanin. Atomic force microscopy showed that the surface roughness of pristine P.PSS films was 4.4 nanometers, decreased to 3.3 nanometers with 10%melanin and increased to 7.8 nanometers and 15.6 nanometers with 30%and 50%melanin respectively.

Output curves showed that pristine P.PSS devices produced high drain currents, which decreased upon addition of 10%melanin. Transfer characteristics for 10%melanin devices displayed a pronounced hysteresis loop and an on and off current ratio of approximately 150. Trans conductance measurements revealed that the maximum GM value shifted to more negative gait voltages as melanin concentration increased.

The normalized maximum trans conductance was highest for the 10%melanin blend, which had the lowest film thickness. We demonstrate that automated spray deposition enables controlled thickness increase leading to a higher volumetric capacitance and improved OECT performance compared to conventional deposition techniques. Melanin speed up P.PSS enhanced transduction transit in organic electronic comply, while controlling deposition and AB tuning of response timing, memory effects.

Now we aim to understand the effect of spray coating on OECT parameters and their implementation in neuromorphic and electronic devices.