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 ...

<ol>
	<li>He, Z., Zhong, C., Su, S., Xu, M., Wu, H., Cao, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Enhanced+power-conversion+efficiency+in+polymer+solar+cells+using+an+inverted+device+structure.">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/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Intermediate+layers+in+tandem+organic+solar+cells.">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/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Efficient+tandem+polymer+solar+cells+fabricated+by+all-solution+processing.">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/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=All-organic+tunnel+junctions+as+connecting+units+in+tandem+organic+solar+cell.">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/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Controlled+current+matching+in+small+molecule+organic+tandem+solar+cells+using+doped+spacer+layers.">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/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effect+of+thin+gold+interstitial-layer+on+the+photovoltaic+properties+of+tandem+organic+solar+cell.">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. 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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.

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			<td height="21" style="height:21px;width:312px;">Name of Material/ Equipment</td>
			<td style="width:191px;">Company</td>
			<td style="width:119px;">Catalog Number</td>
			<td style="width:167px;">Comments/Description</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonate)</td>
			<td style="width:191px;">Heraeus</td>
			<td colspan="2">Clevios PVP AI 4083</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">poly(3- hexylthiophene-2,5-diyl)&#160;</td>
			<td style="width:191px;">Rieke Metals&#160; Inc.</td>
			<td style="width:119px;">P3HT:&#160; P200</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">phenyl-C61 -butyric&#160; acid methyl&#160; ester</td>
			<td style="width:191px;">1- Material</td>
			<td style="width:119px;">PC61BM</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">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})&#160;</td>
			<td style="width:191px;">1- Material</td>
			<td style="width:119px;">PTB7</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">phenyl-C61 -butyric acid methyl&#160; ester</td>
			<td style="width:191px;">Solenne</td>
			<td style="width:119px;">PC71BM</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">1,8-Diiodooctane</td>
			<td style="width:191px;">Sigma Aldrich</td>
			<td align="right" style="width:119px;">250295</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Chlorobenzene</td>
			<td style="width:191px;">Sigma Aldrich</td>
			<td align="right" style="width:119px;">284513</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Indium Tin Oxide Coated Glass 15 Ohm/SQ</td>
			<td style="width:191px;">Lumtec</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">S1813</td>
			<td style="width:191px;">UTD Cleanroom</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">MF311</td>
			<td style="width:191px;">UTD Cleanroom</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">HCl</td>
			<td style="width:191px;">UTD Cleanroom</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Acetone</td>
			<td style="width:191px;">Fisher Scientific</td>
			<td style="width:119px;">A18-20</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Toluene</td>
			<td style="width:191px;">Fisher Scientific</td>
			<td style="width:119px;">T323-20</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Methanol</td>
			<td style="width:191px;">BDH</td>
			<td style="width:119px;">BDH1135-19L</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Isopropanol</td>
			<td style="width:191px;">Fisher Scientific</td>
			<td style="width:119px;">A416-20</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">CEE Spincoater</td>
			<td style="width:191px;">Brewer Scientific</td>
			<td style="width:119px;"></td>
			<td>http://www.utdallas.edu/research/cleanroom/tools/CEESpinCoater.htm</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Contact Printer</td>
			<td style="width:191px;">Quintel</td>
			<td style="width:119px;">Q4000-6</td>
			<td>http://www.utdallas.edu/research/cleanroom/QuintelPrinter.htm</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">CPK Spin Processor</td>
			<td style="width:191px;"></td>
			<td style="width:119px;"></td>
			<td>http://www.utdallas.edu/research/cleanroom/tools/CPKsolvent.htm</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:312px;">Spin Coater</td>
			<td style="width:191px;">Laurell</td>
			<td style="width:119px;">WS-400-6NPP/LITE</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Glove Box</td>
			<td style="width:191px;">M-Braun</td>
			<td style="width:119px;">Lab Master 130</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Solar Simulator</td>
			<td style="width:191px;">Thermo Oriel/Newport</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Keithley 2400 SMU</td>
			<td style="width:191px;">Keithley/Techtronix</td>
			<td align="right" style="width:119px;">2400</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Keithley 7002 Multiplexer</td>
			<td style="width:191px;">Keithley/Techtronix</td>
			<td align="right" style="width:119px;">7002</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Ultrasonic Cleaner</td>
			<td style="width:191px;">Kendal</td>
			<td style="width:119px;">HB-S-49HDT</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Micropipette</td>
			<td style="width:191px;">Eppendorf</td>
			<td style="width:119px;">200uL</td>
			<td></td>
		</tr>
	</tbody>
</table>

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...

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

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 ...

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9.3K vistas.  The University of Texas at Dallas. 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.