Name: Ambient Method for the Production of an Ionically Gated Carbon Nanotube Common Cathode in Tandem Organic Solar Cells (Video) | JoVE Journal
Uploaded: 2014-11-05T15:00:00.000+00:00
Duration: 14 min 37 s
Description: Learn ambient fabrication of tandem organic solar cells using carbon nanotube common cathodes with ionic liquid gating. This method eliminates vacuum processing, enabling cost-effective solar energy conversion.

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5 minutes

Support for this work was provided by DOE STTR grant DE-SC0003664 on Parallel Tandem Organic Solar Cells with Carbon Nanotube Sheet Interlayers and Welch Foundation grant AT-1617. The authors thank J. Bykova for providing CNT forests and A. R. Howard, K. Meilczarek, and J. Velten for technical assistance and useful discussions.

1. Indium Tin Oxide (ITO) Patterning and Cleaning
NOTE: Use 15&#937;/&#9633; ITO glass, and purchase or cut the ITO glass into sizes suitable for spin coating and photolithography. It is most efficient to perform steps 1.1-1.7 on a piece of glass as large as possible, and then cut it into smaller devices. Also note that steps 1.1-1.7 require the ITO glass to be oriented with the ITO-side up. This can be checked easily with a multimeter&#8217;s resistance setting.
<ol>
	<li>Spin coat 1 ml of S1813 positive photoresist onto the ITO-side of the ITO glass at a rate of 3,000 rpm for 1 min. Use more resist for larger pieces of glass, make sure the entire glass is coated, and remove any bubbles before starting the spin coater.</li>
	<li>Anneal the resist coated glass, on a hot plate, at 115 &#176;C for 1 min.</li>
	<li>Load the sample and the photomask onto the contact aligner.</li>
	<li>Expose the photoresist coated ITO glass for an appropriate time. The exposure time is around 10 sec, but vary this time based on the UV lamp intensity, photoresist type, and thickness.</li>
	<li>Develop the UV-exposed substrates in MF311 developer. A spin processor&#8217;s automated process produces the best and most repeatable results, but development can be done manually as followed.
	<ol>
		<li>Submerge the UV-exposed substrates for 1 min in the developer, followed by rinsing in deionized (DI) water and drying with a nitrogen gun. Because the developer loses strength quickly, replace the developer between samples, or alternatively increase the development time when reusing developer.</li>
	</ol>
	</li>
	<li>Etch the ITO substrates in concentrated hydrochloric acid (HCl). This takes between 5-10 min depending on the concentration of the HCl. Rinse in DI water, dry, and test the resistivity of the etched portions with a multimeter. If any conductivity remains, etch for a longer time.</li>
	<li>Once the etching is complete, remove the photoresist with acetone. Note that prompt removal of the photoresist prevents residual HCl from over-etching the patterned ITO.</li>
	<li>If needed, cut the etched ITO glass substrates into device sizes.</li>
	<li>Clean the ITO substrates in a bath ultrasonicator in a sequence of solvents - DI water, acetone, toluene, methanol, and finally isopropyl alcohol.</li>
</ol>
2. OPV Sub-cell Fabrication
<ol>
	<li>Prepare P3HT:PC61BM solution. 
	NOTE: For the most consistent results, prepare the solutions in a nitrogen environment. It is possible to follow this procedure in ambient conditions.
	<ol>
		<li>Find and write down the mass of two clean, ~4 ml glass vial and their caps, and mark them with a permanent marker to distinguish them from another.</li>
		<li>In a nitrogen or argon glove box, transfer approximately 10 mg of poly(3-hexylthiophene-2,5-diyl) (P3HT) to one vial and approximately 10 mg of phenyl-C61-butyric acid methyl ester (PC61BM) to the other.</li>
		<li>Weigh the vials again to find the mass of the P3HT and PC61BM.</li>
		<li>Transfer the vials with P3HT and PC61BM into a glovebox for the rest of the solution making process.</li>
		<li>Add a magnetic stir bar into each vial and then add enough chlorobenzene to each to create 45 mg/ml solutions.</li>
		<li>Place the solutions on a magnetic stirring hot plate at 55 &#176;C for approximately 2 hr or until the solutes have completely dissolved.</li>
		<li>Mix equal volumes of the P3HT and PC61BM solutions together, and let the mixed solution stir for another hour prior to use.</li>
	</ol>
	</li>
	<li>Prepare PTB7:PC71BM solution.
	<ol>
		<li>Repeat steps 2.1.1 to 2.1.4 with poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b&#8242;]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7) and phenyl-7,1-butyric acid methyl ester (PC71BM) instead of P3HT and PC61BM.</li>
		<li>Make a mixture of 3% by volume 1,8-diiodooctane (DIO) in chlorobenzene. This mix is called DIO-CB.</li>
		<li>Add a magnetic stir bar into each vial and then add enough DIO-CB to the PTB7 vial to have a 12 mg/ml solution and enough DIO-CB to the PC71BM vial to have a 40 mg/ml solution.</li>
		<li>Let these solutions stir on a hot plate at 70 &#176;C for two days.</li>
		<li>Mix the solutions in a weight ratio of PTB7 to PC71BM of 1 to 1.5.</li>
		<li>Let the mixed solution stir for another hour at 70 &#176;C prior to use.</li>
	</ol>
	</li>
	<li>Filter poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonate) (PEDOT:PSS) through a 0.45 &#181;m pore size nylon filter. Note this procedure uses P VP AI4083.</li>
	<li>Spin Coat Active layers.
	<ol>
		<li>Place cleaned ITO substrates, ITO-side up, into a UV-Ozone cleaner for 5 min.</li>
		<li>Spin-coat 120 &#956;l of the filtered PEDOT:PSS onto UV-ozone treated, patterned ITO- glass substrates at 3,000 rpm for 1 min. This should yield a 30 nm thick layer.</li>
		<li>Anneal the PEDOT:PSS coated ITO substrates for 5 min at 180 &#176;C.</li>
		<li>Spin-coat 70 &#956;l of the mixed P3HT:PC61BM solution onto PEDOT:PSS coated ITO substrates at approximately 1,000 rpm for 1 min. Vary the rate as needed to deposit a 200 nm thick active layer.</li>
		<li>Anneal the P3HT:PC61BM coated substrates at 170 &#176;C for 5 min. The results may vary on optimal annealing temperature.</li>
		<li>Spin-coat 70 &#956;l of the mixed PTB7:PC71BM solution onto PEDOT:PSS coated ITO substrates at approximately 700 rpm for 1 min. Vary the rate as needed to deposit a 100 nm thick active layer.</li>
		<li>Load the PTB7:PC71BM coated substrates into a high vacuum (&#60; 2 x 10-6 Torr) chamber to remove the residual DIO. Typically, leave the samples in the chamber O/N.</li>
	</ol>
	</li>
</ol>
3. Fabricate the Tandem Device
<ol>
	<li>Laminate CNT electrodes.
	<ol>
		<li>Cut the PTB7 and P3HT substrates in half to make a tandem device. A specialized ITO pattern would not require this step. The ITO pattern should have at least two parallel ITO electrodes extending from one edge to one mm away from the other.</li>
		<li>First prepare the PTB7 and P3HT coated substrates by wiping away polymer and PEDOT from the edges of the glass, and expose the ITO strip which will be used as the common electrode as seen in the first panel of Figure 1.</li>
		<li>Laminate the CNT common electrode on top of the PTB7 and P3HT electrodes. Apply a SWCNT film by placing the CNT side of the filter paper on the device, pressing gently, and then peeling the filter paper away. This is shown in the second panel of Figure 1.</li>
		<li>Densify the CNT electrode onto the surface by applying methoxy-nonafluorobutane (C4F9OCH3) (HFE) and by coating the CNT with a small amount of the liquid and then letting it dry off.</li>
		<li>Wipe away the polymer and CNT on top of the ITO and glass which will have the gate electrode as shown in the third panel of Figure 1. Remove all the polymer from the glass to prevent gate leakage with a razor blade.</li>
		<li>Laminate the CNT gate electrode onto the cleaned area of the PTB7 and P3HT coated substrates. Laminate the MWCNT by pulling from the edge of the MWCNT forest with a razor blade and let the sheet stand freely between some capillary tubes. Pass the device through the freestanding sheet to laminate the CNT onto the device. The gate electrode should have 2-3 times the number of layers as laid onto the common electrode.</li>
		<li>Densify the gate electrode with HFE.</li>
	</ol>
	</li>
	<li>Place a small drop (&#8776;10 &#181;l) of ionic liquid, N,N-Diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate, DEME-BF4, on top of both CNT electrodes of one of the substrates.</li>
	<li>Carefully place the substrate without ionic liquid on top of the substrate with ionic liquid with the common and gate electrodes on top of each-other. This is shown in the last panel of Figure 1.</li>
	<li>Place a photomask with an aperture size smaller than the electrode size over the active area. Use small clips to hold the photomask in place as well as to hold the device together during testing.</li>
</ol>
4. Measure the Device
<ol>
	<li>Transfer the device into the measurement glovebox.</li>
	<li>Make the electrical connections.
	<ol>
		<li>Connect the gate power supply between the common electrode and the gate electrode with the common as ground.</li>
		<li>Connect the two ITO anodes to wires which are connected to a switch which allows selection of either anode or both anodes.</li>
		<li>Connect the output of the switch to the input of the source measure unit.</li>
		<li>Connect the ground of the source measure unit to the common electrode.</li>
	</ol>
	</li>
	<li>Measure the device&#8217;s IV characteristics by repeating the following steps for ascending VGate.
	<ol>
		<li>Set VGate to the next value, starting from VGate = 0 V to VGate = 2 V in increments of 0.25 V.</li>
		<li>Wait 5 min or until the gate current is stabilized. Ideally, the gate current should stabilize around 10s of nanoamperes.</li>
		<li>Set the switch to both sub-cells.</li>
		<li>Open the lamp shutter.</li>
		<li>Run a voltage sweep on the source measure unit from -1 volt to +1 volt at about 100 increments or more.</li>
		<li>Run a voltage sweep from +1 volt to -1 volt.</li>
		<li>Close the lamp shutter.</li>
		<li>Run the voltage sweeps again.</li>
		<li>Set the switch to the front sub-cell.</li>
		<li>Repeat steps 4.3.4 to 4.3.8.</li>
		<li>Set the switch to the back sub-cell.</li>
		<li>Repeat steps 4.3.4 to 4.3.8.</li>
	</ol>
	</li>
	<li>Calculate device parameters.
	<ol>
		<li>Find the short circuit current (JSC) of each sub-cell at each VGate by finding the current produced by the device when the voltage across the sub-cell is 0 V.</li>
		<li>Find the open circuit voltage (VOC) of each sub-cell at each VGate by finding the voltage produced by the device when the current through the sub-cell is 0 A.</li>
		<li>Find the maximum power output from the solar cell by multiplying each voltage value with each current value and selecting the maximum (most negative) value. This assumes that one measures photo-generated current as negative current.</li>
		<li>Find the power conversion efficiency (&#951;) by dividing the maximum power by the input light power.</li>
		<li>Find the filling factor (FF) by dividing the maximum power by the product of JSC and VOC.</li>
	</ol>
	</li>
</ol>

<ol><li>He, Z., Zhong, C., Su, S., Xu, M., Wu, H., Cao, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Enhanced+power-conversion+efficiency+in+polymer+solar+cells+using+an+inverted+device+structure%5BTitle%5D)+AND+%22Nature+Photonics%22%5BJournal%5D)">Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure.</a> Nature Photonics. 6, 591-595 (2012).</li>
<li>Yuan, Y., Huang, J., Li, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Intermediate+layers+in+tandem+organic+solar+cells%5BTitle%5D)+AND+%22Green%22%5BJournal%5D)">Intermediate layers in tandem organic solar cells.</a> Green. 1 (1), 65-80 (2011).</li>
<li>Kim, J. Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Efficient+tandem+polymer+solar+cells+fabricated+by+all-solution+processing%5BTitle%5D)+AND+%22Science%22%5BJournal%5D)">Efficient tandem polymer solar cells fabricated by all-solution processing.</a> Science. 317 (5835), 222-225 (2007).</li>
<li>Yu, B., Zhu, F., Wang, H., Li, G., Yan, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((All-organic+tunnel+junctions+as+connecting+units+in+tandem+organic+solar+cell%5BTitle%5D)+AND+%22Journal+of+Applied+Physics%22%5BJournal%5D)">All-organic tunnel junctions as connecting units in tandem organic solar cell.</a> Journal of Applied Physics. 104 (11), (2008).</li>
<li>Schueppel, R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Controlled+current+matching+in+small+molecule+organic+tandem+solar+cells+using+doped+spacer+layers%5BTitle%5D)+AND+%22Journal+of+Applied+Physics%22%5BJournal%5D)">Controlled current matching in small molecule organic tandem solar cells using doped spacer layers.</a> Journal of Applied Physics. 107 (4), (2010).</li>
<li>Hiramoto, M., Suezaki, M., Yokoyama, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Effect+of+thin+gold+interstitial-layer+on+the+photovoltaic+properties+of+tandem+organic+solar+cell%5BTitle%5D)+AND+%22Chemistry+Letters%22%5BJournal%5D)">Effect of thin gold interstitial-layer on the photovoltaic properties of tandem organic solar cell.</a> Chemistry Letters. 19 (3), 327-330 (1990).</li>
<li>Xue, J., Uchida, S., Rand, B. P., Forrest, S. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Asymmetric+tandem+organic+photovoltaic+cells+with+hybrid+planar-mixed+molecular+heterojunctions%5BTitle%5D)+AND+%22Applied+Physics+Letters%22%5BJournal%5D)">Asymmetric tandem organic photovoltaic cells with hybrid planar-mixed molecular heterojunctions.</a> Applied Physics Letters. 85 (23), 5757 (2004).</li>
<li>Tanaka, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Monolithic+parallel+tandem+organic+photovoltaic+cell+with+transparent+carbon+nanotube+interlayer%5BTitle%5D)+AND+%22Applied+Physics+Letters%22%5BJournal%5D)">Monolithic parallel tandem organic photovoltaic cell with transparent carbon nanotube interlayer.</a> Applied Physics Letters. 94 (11), (2009).</li>
<li>Mielczarek, K., Cook, A., Kuznetsov, A., Zakhidov, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((OPV+Tandems+with+CNTS%3A+Why+Are+Parallel+Connections+Better+Than+Series+Connections%5BTitle%5D)+AND+%22Low-Dimensional+Functional+Materials%22%5BJournal%5D)">OPV Tandems with CNTS: Why Are Parallel Connections Better Than Series Connections.</a> Low-Dimensional Functional Materials. , 179-204 (2013).</li>
<li>Kim, J. Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Efficient+tandem+polymer+solar+cells+fabricated+by+all-solution+processing%5BTitle%5D)+AND+%22Science%22%5BJournal%5D)">Efficient tandem polymer solar cells fabricated by all-solution processing.</a> Science. 317 (5835), 222-225 (2007).</li>
<li>Kuznetsov, A. A. <a target="_blank" href="http://scholar.google.com/scholar?hl=en&safe=off&q=%22Physics+of+electron+field+emission+by+self-assembled+carbon+nanotube+arrays%22">Physics of electron field emission by self-assembled carbon nanotube arrays</a>. , The University of Texas at Dallas. (2008).</li>
<li>Kuznetzov, A. A., Lee, S. B., Zhang, M., Baughman, R. H., Zakhidov, A. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Electron+field+emission+from+transparent+multiwalled+carbon+nanotube+sheets+for+inverted+field+emission+displays%5BTitle%5D)+AND+%22Carbon%22%5BJournal%5D)">Electron field emission from transparent multiwalled carbon nanotube sheets for inverted field emission displays.</a> Carbon. 48 (1), 41-46 (2010).</li>
<li>Zakhidov, A. A., Suh, D. -S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Electrochemically+Tuned+Properties+for+Electrolyte-Free+Carbon+Nanotube+Sheets%5BTitle%5D)+AND+%22Advanced+Functional+Materials%22%5BJournal%5D)">Electrochemically Tuned Properties for Electrolyte-Free Carbon Nanotube Sheets.</a> Advanced Functional Materials. 19 (14), 2266-2272 (2009).</li>
<li>Cook, A., Yuen, J. D., Zakhidov, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Ion-Reconfigurable+photovoltaic+cells%2C+hybrid+tandems+and+photodetectors+with+CNT+ionic+gate%5BTitle%5D)+AND+%22US+Patent+Application%22%5BJournal%5D)">Ion-Reconfigurable photovoltaic cells, hybrid tandems and photodetectors with CNT ionic gate.</a> US Patent Application. 61, (2012).</li>
<li>Cook, A. B., Yuen, J. D., Zakhidov, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Electrochemically+gated+organic+photovoltaic+with+tunable+carbon+nanotube+cathodes%5BTitle%5D)+AND+%22Applied+Physics+Letters%22%5BJournal%5D)">Electrochemically gated organic photovoltaic with tunable carbon nanotube cathodes.</a> Applied Physics Letters. 103 (16), (2013).</li>
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</ol>

The authors have nothing to disclose.

The results highlight a few considerations when designing parallel tandem solar cells. Notably, if one of the sub-cells is performing poorly, tandem performance in negatively affected. The results show that there are two main effects. If one sub-cell is shorted, e.g., shows ohmic behavior, the FFT will be no higher than the FF of the bad sub-cell. JTSC and VTOC will be similarly affected. This is the case when VGate is low and the P3HT sub-cell ha...

Polymer semiconductors are the leading organic photovoltaic (OPV) materials due to high absorptivity, good transport properties, flexibility, and compatibility with temperature sensitive substrates. OPV device power conversion efficiencies, &#951;, have jumped significantly in the past years, with single cell efficiencies as high as 9.1%1, making them an increasingly viable energy technology.
Despite the improvements in &#951;, the thin optimal active layer thicknesses of the devices limit light absorption and hinder reliable fabrication. Additionally, the spectral width of light absorption of each polymer is limited compared to inorganic materials. Pairing polymers of differing spectral sensitivity bypasses these difficulties, making tandem architectures2 a necessary innovation.
Series tandem devices are the most common tandem architecture. In this design, an electron transport material, an optional metallic recombination layer, and a hole transport layer connect two independent photoactive layers called sub-cells. Linking sub-cells in a series configuration increases the open circuit voltage of the combination device. Some groups have had success with degenerately doped transport layers3&#8211;5, but more groups have used particles of gold or silver to aid recombination of holes and electrons in the interlayer6,7.
In contrast, parallel tandems require a high conductivity electrode, either anode or cathode, joining the two active layers. The interlayer must be highly transparent, which limits series tandem interlayers containing metallic particles, and even more so for the parallel tandem interlayers composed of thin, continuous metal electrodes. Carbon nanotubes (CNT) sheets show higher transparency than metal layers. So the NanoTech Institute, in collaboration with Shimane University, has introduced the concept of using as the interlayer electrode in monolithic, parallel tandem devices8.
Previous efforts featured monolithic, parallel, tandem OPV devices with CNT sheets functioning as interlayer anodes8,9. These methods require special care to avoid shorting of one or both cells or damaging preceding layers when depositing later layers. The new method described in this paper eases fabrication by placing the CNT electrode on top of the polymeric active layers of two single cells, then laminating the two separate devices together as shown in Figure 1. This method is remarkable as the device, including an air-stable CNT cathode, can be fabricated entirely in ambient conditions employing only dry and solution processing.
CNT sheets are not intrinsically good cathodes, as they require n-type doping to decrease the work function in order to collect electrons from the photoactive region of a solar cell10. Electric double layer charging in an electrolyte, such as an ionic liquid, can be used to shift the work function of CNT electrodes11&#8211;14.
As described in a preceding paper15 and depicted in Figure 2, when the gate voltage (VGate) is increases, the work function of the CNT common electrode is decreased, creating electrode asymmetry. This prevents hole collection from the OPV&#8217;s donor in favor of collecting electrons from the OPV&#8217;s acceptor, and the devices turn ON, changing from inefficient photoresistor into photodiode15 behavior. It should also be noted that the energy used to charge the device and the power lost due to gate leakage currents is trivial compared to the power generated by the solar cell15. Ionic gating of CNT electrodes has a large effect on the work function due to the low density of states and the high surface area to volume ratio in CNT electrodes. Similar methods have been used to enhance a Schottky barrier at the interface of CNT with n-Si16.

<table><tbody><tr><td>Poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonate)</td><td>Heraeus</td><td>Clevios PVP AI 4083</td><td></td></tr><tr><td>poly(3-hexylthiophene-2,5-diyl)&amp;nbsp;</td><td>Rieke Metals&amp;nbsp; Inc.</td><td>P3HT:&amp;nbsp; P200</td><td></td></tr><tr><td>phenyl-C&lt;sub&gt;61&lt;/sub&gt;-butyric&amp;nbsp; acid methyl&amp;nbsp; ester</td><td>1- Material</td><td>PC61BM</td><td></td></tr><tr><td>Poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b&amp;prime;]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl})&amp;nbsp;</td><td>1- Material</td><td>PTB7</td><td></td></tr><tr><td>phenyl-C&lt;sub&gt;61&lt;/sub&gt;-butyric acid methyl&amp;nbsp; ester</td><td>Solenne</td><td>PC71BM</td><td></td></tr><tr><td>1,8-Diiodooctane</td><td>Sigma Aldrich</td><td>250295</td><td></td></tr><tr><td>Chlorobenzene</td><td>Sigma Aldrich</td><td>284513</td><td></td></tr><tr><td>Indium Tin Oxide Coated Glass 15 Ohm/SQ</td><td>Lumtec</td><td></td><td></td></tr><tr><td>S1813</td><td>UTD Cleanroom</td><td></td><td></td></tr><tr><td>MF311</td><td>UTD Cleanroom</td><td></td><td></td></tr><tr><td>HCl</td><td>UTD Cleanroom</td><td></td><td></td></tr><tr><td>Acetone</td><td>Fisher Scientific</td><td>A18-20</td><td></td></tr><tr><td>Toluene</td><td>Fisher Scientific</td><td>T323-20</td><td></td></tr><tr><td>Methanol</td><td>BDH</td><td>BDH1135-19L</td><td></td></tr><tr><td>Isopropanol</td><td>Fisher Scientific</td><td>A416-20</td><td></td></tr><tr><td>CEE Spincoater</td><td>Brewer Scientific</td><td></td><td>http://www.utdallas.edu/research/cleanroom/tools/CEESpinCoater.htm</td></tr><tr><td>Contact Printer</td><td>Quintel</td><td>Q4000-6</td><td>http://www.utdallas.edu/research/cleanroom/QuintelPrinter.htm</td></tr><tr><td>CPK Spin Processor</td><td></td><td></td><td>http://www.utdallas.edu/research/cleanroom/tools/CPKsolvent.htm</td></tr><tr><td>Spin Coater</td><td>Laurell</td><td>WS-400-6NPP/LITE</td><td></td></tr><tr><td colspan="4">[header]</td></tr><tr><td>Glove Box</td><td>M-Braun</td><td>Lab Master 130</td><td></td></tr><tr><td>Solar Simulator</td><td>Thermo Oriel/Newport</td><td></td><td></td></tr><tr><td>Keithley 2400 SMU</td><td>Keithley/Techtronix</td><td>2400</td><td></td></tr><tr><td>Keithley 7002 Multiplexer</td><td>Keithley/Techtronix</td><td>7002</td><td></td></tr><tr><td>Ultrasonic Cleaner</td><td>Kendal</td><td>HB-S-49HDT</td><td></td></tr><tr><td>Micropipette</td><td>Eppendorf</td><td>200 &amp;micro;l</td><td></td></tr></tbody></table>

ambient method for the production of an ionically gated carbon nanotube common cathode in tandem organic solar cells

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

A method of fabricating organic photovoltaic (OPV) tandems that requires no vacuum processing is presented. These devices are comprised of two solution-processed polymeric cells connected in parallel by a transparent carbon nanotubes (CNT) interlayer. This structure includes improvements in fabrication techniques for tandem OPV devices. First the need for ambient-processed cathodes is considered. The CNT anode in the tandem device is tuned via ionic gating to become a common cathode. Ionic gating employs electric double layer charging to lower the work function of the CNT electrode. Secondly, the difficulty of sequentially stacking tandem layers by solution-processing is addressed. The devices are fabricated via solution and dry-lamination in ambient conditions with parallel processing steps. The method of fabricating the individual polymeric cells, the steps needed to laminate them together with a common CNT cathode, and then provide some representative results are described. These results demonstrate ionic gating of the CNT electrode to create a common cathode and addition of current and efficiency as a result of the lamination procedure.

A tandem device formed from differing polymers, particularly polymers of significantly differing band gaps, is of practical interest as these devices can absorb the largest spectral range of light. In this device structure, the PTB7 sub-cell is the back cell and P3HT is the front sub-cell. This is intended to absorb the greatest amount of light as the P3HT sub-cell is largely transparent to the longer wavelength light absorbed by the PTB7 sub-cell. For the sake of clarity, the solar cell parameters, VOC, J<sub...

A method of fabricating, in ambient conditions, organic photovoltaic tandem devices in a parallel configuration is presented. These devices feature an air-processed, semi-transparent, carbon nanotube common cathode.

A method of fabricating organic photovoltaic (OPV) tandems that requires no vacuum processing is presented. These devices are comprised of two ...

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.

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Research

JoVE Journal

Engineering

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