Ambient Method for the Production of an Ionically Gated Carbon Nanotube Common Cathode in Tandem Organic Solar Cells

The overall goal of the following experiment is to show that a tandem solar cell can be created completely in an ambient environment with no high vacuum deposition processes involved. This is achieved by fabricating two organic photovoltaic sub cells on two separate indium tin oxide patterned glass substrates as a second step, carbon nano tube common and GA electrodes are laminated to the tops of the devices. The gait will allow control over the work function of the common electrode, which will act as a cathode for the two organic photovoltaic sub cells.

Next place ionic liquid on top of the carbon nano tube electrodes and press the two together, forming a common carbon nano tube electrode impregnated with ionic liquid. The results show carbon nano tube electrodes can be efficiently converted into cathodes via charging and parallel tandem solar cell performance can be observed on the device based on measurements of the current voltage characteristics of the device under illumination. The main benefit of this technique over existing processes, such as vacuum processing, is that we can do this in ambient conditions.

This saves money and it's also more energy efficient. But additionally, we can also do this a lot more easily because in a normal process, you stack the layers on top of each other, whereas we do a few layers and then laminate the whole thing together. This is a lot easier.

So while this method can give us insight into organic photovoltaic, it can also be applied to other systems as well. Examples include field effect transistors, light emitting diodes, and combinations of devices such as logic gates and organic solar cells with integrated energy storage Visually demonstrating carbon anti tube lamination is important because the nanotubes are lightweight, strong, and can be difficult to manipulate effectively. This video begins with the fabrication of the tandem solar cell.

Prior to this step, photovoltaic, active layers have been coated onto Indium tin oxide or ITO substrates. There are two substrates both made on glass with patterned ITO electrodes. Each substrate has two parallel indium tin oxide electrodes extending from one edge to at least a millimeter from the other edge.

Each substrate is also coated with 30 nanometers of pdot. PSS one substrate has a 200 nanometer thick layer composed of a one-to-one blend of P three HT and PC 61 bm. The other has a 100 nanometer thick layer composed of a two to three blend of PTB seven and PC 71 bm.

The next steps will be identical for the samples. The video will focus on only one, get a clean room wipe and dampen it with a small amount of toluene. Use it to wipe away polymer layers to expose the substrate edges and the indium tin oxide strip that will serve as the common electrode in the device.

Be careful not to expose portions of the other ITO electrode, which will be used in a later step. The next step is to laminate the carbon nano tube common electrode. One side of this filter paper holds single wall carbon nanotubes created in a floating catalyst chemical vapor deposition process.

Place the nano tube side of the filter paper on top of the active layer of the substrate so that it connects the layer to the ITO electrode that will be used as the common electrode. Gently apply pressure to the filter paper to transfer carbon nanotubes. Then lift off the filter paper to leave carbon nanotubes behind.

To improve adhesion of the carbon nano tube electrode, make use of the hydro fluoro ether methoxy, non fluoro butane with a pipette. Draw some of the HFE to place on the samples. Drop the HFE onto the top of the carbon nanotubes.

Allow the HFE to evaporate before proceeding. After the substrates have dried, identify the region that will have the gait electrode. Use a wipe dampened with toluene to wipe away any polymer and carbon nanotubes in that region.

Next, use a razor blade to remove any remaining polymer in the region. In order to prevent gait leakage, clean any polymer debris away with a nitrogen gun, employ a multimeter to check that the electrodes are isolated before continuing. At this point, prepare to laminate carbon nanotubes onto the cleaned region to create the gait electrode.

This silicon wafer holds a forest of highly aligned, multi walled carbon nanotubes made with a chemical vapor deposition process. Mount the wafer near the edge of a raised platform near the wafer and level with it. Two parallel capillary tubes Should protrude.

A substrate shown here in a holder should be able to fit between them. To start lamination of the nano tubes, press a razor blade into the edge of the forest and pull laterally away from the forest parallel to the wafer and toward the tubes. A freestanding sheet self assembles.

Once the sheet has been started, use a capillary tube to continue pulling it over the protruding tubes. Next, bring the device held with wafer tweezers oriented to expose the cleaned region to the sheet. Transfer the sheet supported between the tubes to the cleaned region of the device by passing the device through the sheet.

Use a capillary tube to pull an additional sheet, then add another layer. Repeat the process until five layers of the multi walled carbon nanotubes have been applied. After lamination, use a pipette to draw about 100 microliters of HFE place drops of the HFE on the gait electrode and allow it to dry.

After both the P three HT and PTB seven substrates have had electrodes added. The next step is to introduce the ionic liquid. In this video, deme tetra fluoro bore.

Here are the P three HT substrate on the left and the PTB seven substrate on the right. Arrange to mirror one another. Select one electrode to work with and use a piece of capillary tube to place several small drops of the fluid on top of the common electrode.

About 10 microliters total. Do the same with the gait electrode. Proceed by picking up the substrate without the ionic liquid and aligning it with the other.

Orient the substrates so that both common electrodes and both gait electrodes will align and face one another. Then put the substrate without the ionic liquid on top of the other. Add a photo mask over the active area of the P three HT with an aperture size smaller than the electrode size.

Use small clips to hold the device together and hold the photo mask in place for the next steps. The device has been transferred to a measurement glove box equipped with a solar simulator. The glove box has a nitrogen environment which helps prevent degradation due to air exposure.

During testing, position the tandem cell under the solar simulator. Orient the device so the P three HT cell and photo mask face the solar simulator output. Begin making the necessary electrical connections to the gate power supply, the source measure unit, and the multiplexer.

First, connect the gate power supply between the common electrode and the gate electrode with the common as ground. Next, connect the two ITO anodes to a multiplexer to allow selection of either the anode of the front P three HT cell, the back PTB seven cell, or both. The device output is input to the source measure unit.

Finally, connect the ground of the source measure unit to the common electrode. For the tandem configuration, the front cell configuration and the back cell configuration measure the device IV characteristics. In this experiment, the measurements are automated.

To select the sub cell being measured, toggle the lamp shutter and run the IV sweeps. The gate voltage is manually varied between zero and two volts. After selecting a gate voltage and waiting for the gate current to stabilize to the range of tens of nano amps, start the measurements by selecting one sub cell and running the IV sweeps in the light from negative one to one volt.

Do the same sweep with the solar simulator lamp off. Each sweep takes approximately 15 seconds. Each sub cell and the parallel combination of the two should be measured at each gate voltage.

Continue by incrementing the gate voltage to its next value and performing the six pairs of sweeps until the entire gate voltage range is explored. Here are measured IV curves for the tandem cell using two different gait voltages. The black curves correspond to a GA voltage of 1.5 volts and the red curves to a gate voltage of 2.25 volts.

Circles are data collected using both cells in tandem squares are associated with the use of the front. P three HT cell triangles are for the back PTB seven cells. The highly linear curves for the front cell and the tandem cells at 1.5 volts suggests the P three HT sub cell acts as a shunt in its off state.

In contrast, the PTB seven back cell demonstrates DDE characteristics at 1.5 volts and 2.25 volts. Where its performance is degraded, the data can be used to extract solar cell parameters as a function of gate voltage. Here are the open circuit voltage, the short circuit current, the filling factor, and the power conversion efficiency.

Black curves are for cells in tandem blue are for the front. P three HT cells, and red are for the back. PTB seven cells focusing on the open circuit voltage.

The red curves show the PTB seven cell turned on at 0.5 volts. The cell peaks at 1.5 volts. The blue curves show the P three HT cell beginning to turn on around one volt.

It is fully turned on above two volts. When performing this procedure, it is necessary to measure the resistance of all contacts to ensure that no shorting occurs, especially on the carbon nanotube common and gait electrodes. In conclusion to all the nice work which was so beautifully described by my younger colleagues who actually did all these beautiful experiments, I want to mention that this way of making monolithic devices of solid and liquid parts just by building parts on each piece of glass, adding very thin layer of liquid and a touching glass on the top of each other is a very interesting new design that will allow not only to build more complicated devices without any vacuum or many, many, many layers on top of each other, but it also opens the way to understand the physics behind the operation of organic layers.

And also this will allow to optimize the performance of organic photo tag in the future. After watching this video, you should be able to use carbon nanotubes to make a anode on top of an organic device. Next, you should be able to make a tandem device out of the two separately made devices.

And finally, you can use ionic liquids to make that anode into a cathode. Don't forget that working with carbon nanotubes and halogenated solvents is extremely hazardous. So precautions such as working glove boxes and personal protective equipment should always be used when doing this procedure.