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>

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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. 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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 has not turned ON.
Conversely, if one sub-cell has good diode properties, but low VOC or JSC, then JTSC is nearly the sum of JFSC and JBSC. However, if there is a large difference between each sub-cell VOC, then VTOC is nearly equivalent to the lesser VOC. This performance is demonstrated when VGate is high and the PTB7 cell has turned off. The &#951;T can be somewhat higher in the case, but is still likely to be less than that of the better sub-cell alone.
The back sub-cell turning on earlier than the front sub-cell was unexpected as the sub-cells share gate and common electrodes. The work function of their cathode, and therefore the degree of electrode asymmetry between the ITO and cathode, should be identical. Furthermore, the back sub-cell shows a higher VOC than the front sub-cell and should require greater electrode asymmetry with respect to work function than the front sub-cell, and thus a larger VGate before turning on.
Given the lower HOMO level of the PTB7 polymer, it is possible that suppression of hole injection/extraction occurs more rapidly in PTB7, and thus the device turns on at a lower VGate. Other effects to consider are the fact that PTB7 is a copolymer, which is a polymer composed of alternating donor and acceptor units. This might have an impact on the interface dipoles generated between the polymer and the CNT common electrode.
The procedure described in this text is applicable to applications which adhere to the following constraints. The semiconducting active material must not be soluble or adversely affected by the ionic materials. In the case of light emitting or photovoltaic devices, the anode and semiconducting layers should not be opaque in the same spectral regions. Given these constraints, it is possible to apply these techniques to organic light emitting diodes, organic field effect transistors, and similar inorganic devices.
In conclusion, a parallel tandem device fabrication method that possesses benefits over conventional processing methodologies is described. The method requires no vacuum processing, is scalable, can be performed in ambient conditions, and each active layer is fabricated in an optimal manner, reducing the incidence of shorts and simplifying processing. Salient features are identified in the operation of parallel OPV tandems. While the overall efficiencies are somewhat low, further improvements can be made by optimizing photoactive layers and CNT electrodes. Additionally, if sub-cell turned on at the same time, an &#951;T over 3% would be observed.

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

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, JSC, FF, and &#951; will be decorated with a superscript T, F, or B when referring to the tandem, front or back cell respectively. Table 1 shows these abbreviations.
A selection of current voltage curves for the device are depicted in Figure 3, and extracted solar cell parameters in Figure 4. It is observed that the PTB7 sub-cell (back) turned ON at a much lower VGate than the P3HT sub-cell (front). Figure 4 shows that the PTB7 cell begins to turn ON at VGate = 0.5 V and peaks around VGate = 1.5 V. The front sub-cell shows signs of turning ON around VGate = 1.0 V, but does not fully turn ON below VGate = 2.0 V. VTOC and FFT mimic the behavior of the worse sub-cell, being slightly higher than the front sub-cell for VGate &#60; 2.0 V and slightly higher than the back cell for VGate &#62; 2.0 V. JTSC is far less than the sum of JFSC and JBSC for VGate &#60; 1.5 V, after which point the current addition is quite good. &#951;T is less than the greater of &#951;F or &#951;B for all VGate except at 1.5 V.
The poor addition of current and efficiency for low VGate appears to be due to the front sub-cell acting as a shunt in its OFF state. This is seen from the highly linear curves of the front cell and the tandem at VGate = 1.5 V in Figure 3. Conversely, after the PTB7 back cell has degraded, (at VGate = 2.25 V), it still maintains a diode characteristics, as shown by the curve in Figure 3, and thus does not act as a shunt. This results in addition of JSC, but VTOC, FFT and &#951;T are reduced due to the low VBOC. While the process draws robustness from its simplicity, there are variations due to the manual processing and batch-to-batch variations in materials. This can result in shifts in turn on voltage of &#177;0.25 V and peak efficiency by &#177;0.5%. This variation can be decreased with a more automated process.
<img alt="Figure 1" src="/files/ftp_upload/52380/52380fig1highres.jpg" /> 
Figure 1.&#160;Tandem device design process. The fabrication and lamination process used in building the ionically-gated tandem OPV, with arrows depicting the sequence. A diagram of the final device structure is shown on the bottom left corner. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/52380/52380fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/52380/52380fig2highres.jpg" /> 
Figure 2.&#160;Tandem device band and electrical diagram. The approximate band diagram of the tandem device is shown along-side the electrical diagram of the device. The shaded regions with arrows on the CNT electrodes show the shift in work function. The solid lines and circuit elements below show the electrical connections. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/52380/52380fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/52380/52380fig3highres.jpg" /> 
Figure 3.&#160;Selected IV curves. IV curves for a VGate (1.5 V) in which the back (PTB7) cell shows its best performance and a higher VGate (2.25 V) in which the front (P3HT) cell shows its best performance. &#8216;T&#8217; and circles denote the tandem curve, &#8216;F&#8217; and squares the front, and &#8216;B&#8217; and triangles the back.
<img alt="Figure 4" src="/files/ftp_upload/52380/52380fig4highres.jpg" /> 
Figure 4. Device parameters. Solar cell parameters extracted from IV measurements from VGate = 0.5 V to VGate = 2.25 V. &#8216;T&#8217; and black squares denote the tandem curve, &#8216;F&#8217; and blue triangles the front, and &#8216;B&#8217; and red circles the back. The dotted lines show results for descending voltages sweeps of the OPV device while the solid lines show the ascending results.
<table border="0" cellpadding="0" cellspacing="0">
	<tbody>
		<tr>
			<td dir="LTR" rowspan="6">OPV Parameter</td>
			<td colspan="4" dir="LTR">Sub-cell measured</td>
		</tr>
		<tr>
			<td dir="LTR"></td>
			<td dir="LTR">Tandem</td>
			<td dir="LTR">Front</td>
			<td dir="LTR">Back</td>
		</tr>
		<tr>
			<td dir="LTR">VOC</td>
			<td dir="LTR">VTOC</td>
			<td dir="LTR">VFOC</td>
			<td dir="LTR">VBOC</td>
		</tr>
		<tr>
			<td dir="LTR">JSC</td>
			<td dir="LTR">JTSC</td>
			<td dir="LTR">JFSC</td>
			<td dir="LTR">JBSC</td>
		</tr>
		<tr>
			<td dir="LTR">FF</td>
			<td dir="LTR">FFT</td>
			<td dir="LTR">FFF</td>
			<td dir="LTR">FFB</td>
		</tr>
		<tr>
			<td dir="LTR">&#951;</td>
			<td dir="LTR">&#951;T</td>
			<td dir="LTR">&#951;F</td>
			<td dir="LTR">&#951;B</td>
		</tr>
	</tbody>
</table>
Table 1.&#160;Parameter abbreviations. Summarized solar cell parameter abbreviations. Superscripts, T, F, and B denote tandem, front and back respectively.

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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 ...

ambient-method-for-production-an-ionically-gated-carbon-nanotube

Nanyang Technological University

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9.4K Views.  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.

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

Harvesting Solar Energy by Means of Charge-Separating Nanocrystals and Their Solids

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials offering a superior control over the spatial distribution of charge carriers across material interfaces. As this study demonstrates, a combination of donor-acceptor nanocrystal (NC) domains in a single nanoparticle can lead to the realization of efficient photocatalytic1-5 materials, while a layered assembly of donor- and acceptor-like nanocrystals films gives rise to photovoltaic materials.
Initially the paper focuses on the synthesis of composite inorganic nanocrystals, comprising linearly stacked ZnSe, CdS, and Pt domains, which jointly promote photoinduced charge separation. These structures are used in aqueous solutions for the photocatalysis of water under solar radiation, resulting in the production of H2 gas. To enhance the photoinduced separation of charges, a nanorod morphology with a linear gradient originating from an intrinsic electric field is used5. The inter-domain energetics are then optimized to drive photogenerated electrons toward the Pt catalytic site while expelling the holes to the surface of ZnSe domains for sacrificial regeneration (via methanol). Here we show that the only efficient way to produce hydrogen is to use electron-donating ligands to passivate the surface states by tuning the energy level alignment at the semiconductor-ligand interface. Stable and efficient reduction of water is allowed by these ligands due to the fact that they fill vacancies in the valence band of the semiconductor domain, preventing energetic holes from degrading it. Specifically, we show that the energy of the hole is transferred to the ligand moiety, leaving the semiconductor domain functional. This enables us to return the entire nanocrystal-ligand system to a functional state, when the ligands are degraded, by simply adding fresh ligands to the system4.
To promote a photovoltaic charge separation, we use a composite two-layer solid of PbS and TiO2 films. In this configuration, photoinduced electrons are injected into TiO2 and are subsequently picked up by an FTO electrode, while holes are channeled to a Au electrode via PbS layer6. To develop the latter we introduce a Semiconductor Matrix Encapsulated Nanocrystal Arrays (SMENA) strategy, which allows bonding PbS NCs into the surrounding matrix of CdS semiconductor. As a result, fabricated solids exhibit excellent thermal stability, attributed to the heteroepitaxial structure of nanocrystal-matrix interfaces, and show compelling light-harvesting performance in prototype solar cells7.

A general strategy for the development of charge-separating semiconductor nanocrystal composites deployable for solar energy production is presented. We show that assembly of donor-acceptor nanocrystal domains in a single nanoparticle geometry gives rise to a photocatalytic function, while bulk-heterojunctions of donor-acceptor nanocrystal films can be used for photovoltaic energy conversion.

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials ...

Integrating a Triplet-triplet Annihilation Up-conversion System to Enhance Dye-sensitized Solar Cell Response to Sub-bandgap Light

The poor response of dye-sensitized solar cells (DSCs) to red and infrared light is a significant impediment to the realization of higher photocurrents and hence higher efficiencies. Photon up-conversion by way of triplet-triplet annihilation (TTA-UC) is an attractive technique for using these otherwise wasted low energy photons to produce photocurrent, while not interfering with the photoanodic performance in a deleterious manner. Further to this, TTA-UC has a number of features, distinct from other reported photon up-conversion technologies, which renders it particularly suitable for coupling with DSC technology. In this work, a proven high performance TTA-UC system, comprising a palladium porphyrin sensitizer and rubrene emitter, is combined with a high performance DSC (utilizing the organic dye D149) in an integrated device. The device shows an enhanced response to sub-bandgap light over the absorption range of the TTA-UC sub-unit resulting in the highest figure of merit for up-conversion assisted DSC performance to date.

An integrated device, incorporating a dye-sensitized solar cell and triplet-triplet annihilation up-conversion unit was produced, affording enhanced light harvesting, from a wider section of the solar spectrum. Under modest irradiation levels a significantly enhanced response to low energy photons was demonstrated, yielding a record figure of merit for dye-sensitized solar cells.

The poor response of dye-sensitized solar cells (DSCs) to red and infrared light is a significant impediment to the realization of higher photocurrents ...

Making Record-efficiency SnS Solar Cells by Thermal Evaporation and Atomic Layer Deposition

Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent thermal evaporation, and it absorbs visible light strongly. However, for a long time the record power conversion efficiency of SnS solar cells remained below 2%. Recently we demonstrated new certified record efficiencies of 4.36% using SnS deposited by atomic layer deposition, and 3.88% using thermal evaporation. Here the fabrication procedure for these record solar cells is described, and the statistical distribution of the fabrication process is reported. The standard deviation of efficiency measured on a single substrate is typically over 0.5%. All steps including substrate selection and cleaning, Mo sputtering for the rear contact (cathode), SnS deposition, annealing, surface passivation, Zn(O,S) buffer layer selection and deposition, transparent conductor (anode) deposition, and metallization are described. On each substrate we fabricate 11 individual devices, each with active area 0.25 cm2. Further, a system for high throughput measurements of current-voltage curves under simulated solar light, and external quantum efficiency measurement with variable light bias is described. With this system we are able to measure full data sets on all 11 devices in an automated manner and in minimal time. These results illustrate the value of studying large sample sets, rather than focusing narrowly on the highest performing devices. Large data sets help us to distinguish and remedy individual loss mechanisms affecting our devices.

Tin sulfide (SnS) is a candidate material for Earth-abundant, non-toxic solar cells. Here, we demonstrate the fabrication procedure of the SnS solar cells employing atomic layer deposition, which yields 4.36% certified power conversion efficiency, and thermal evaporation which yields 3.88%.

Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent ...

Fabrication of Fully Solution Processed Inorganic Nanocrystal Photovoltaic Devices

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal inks. For the photoactive absorber layer, colloidal CdTe and CdSe nanocrystals (3-5 nm) are synthesized using an inert hot injection technique and cleaned with precipitations to remove excess starting reagents. Similarly, gold nanocrystals (3-5 nm) are synthesized under ambient conditions and dissolved in organic solvents. In addition, precursor solutions for transparent conductive indium tin oxide (ITO) films are prepared from solutions of indium and tin salts paired with a reactive oxidizer. Layer-by-layer, these solutions are deposited onto a glass substrate following annealing (200-400 &#176;C) to build the nanocrystal solar cell (glass/ITO/CdSe/CdTe/Au). Pre-annealing ligand exchange is required for CdSe and CdTe nanocrystals where films are dipped in NH4Cl:methanol to replace long-chain native ligands with small inorganic Cl- anions. NH4Cl(s) was found to act as a catalyst for the sintering reaction (as a non-toxic alternative to the conventional CdCl2(s) treatment) leading to grain growth (136&#177;39 nm) during heating. The thickness and roughness of the prepared films are characterized with SEM and optical profilometry. FTIR is used to determine the degree of ligand exchange prior to sintering, and XRD is used to verify the crystallinity and phase of each material. UV/Vis spectra show high visible light transmission through the ITO layer and a red shift in the absorbance of the cadmium chalcogenide nanocrystals after thermal annealing. Current-voltage curves of completed devices are measured under simulated one sun illumination. Small differences in deposition techniques and reagents employed during ligand exchange have been shown to have a profound influence on the device properties. Here, we examine the effects of chemical (sintering and ligand exchange agents) and physical treatments (solution concentration, spray-pressure, annealing time and annealing temperature) on photovoltaic device performance.

This protocol describes the synthesis and solution deposition of inorganic nanocrystals layer by layer to produce thin film electronics on non-conductive surfaces. Solvent-stabilized inks can produce complete photovoltaic devices on glass substrates via spin and spray coating following post-deposition ligand exchange and sintering.

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal ...

Influence of Hybrid Perovskite Fabrication Methods on Film Formation, Electronic Structure, and Solar Cell Performance

Hybrid organic/inorganic halide perovskites have lately been a topic of great interest in the field of solar cell applications, with the potential to achieve device efficiencies exceeding other thin film device technologies. Yet, large variations in device efficiency and basic physical properties are reported. This is due to unintentional variations during film processing, which have not been sufficiently investigated so far. We therefore conducted an extensive study of the morphology and electronic structure of a large number of CH3NH3PbI3 perovskite where we show how the preparation method as well as the mixing ratio of educts methylammonium iodide and lead(II) iodide impact properties like film formation, crystal structure, density of states, energy levels, and ultimately the solar cell performance.

We present an extensive study on the effects of different fabrication methods for organic/inorganic perovskite thin films by comparing crystal structures, density of states, energy levels, and ultimately the solar cell performance.

Hybrid organic/inorganic halide perovskites have lately been a topic of great interest in the field of solar cell applications, with the potential to ...

Chemistry

Morphology Control for Fully Printable Organic&#8211;Inorganic Bulk-heterojunction Solar Cells Based on a Ti-alkoxide and Semiconducting Polymer

The photoactive layer of a typical organic thin-film bulk-heterojunction (BHJ) solar cell commonly uses fullerene derivatives as the electron-accepting material. However, fullerene derivatives are air-sensitive; therefore, air-stable material is needed as an alternative. In the present study, we propose and describe the properties of Ti-alkoxide as an alternative electron-accepting material to fullerene derivatives to create highly air-stable BHJ solar cells. It is well-known that controlling the morphology in the photoactive layer, which is constructed with fullerene derivatives as the electron acceptor, is important for obtaining a high overall efficiency through the solvent method. The conventional solvent method is useful for high-solubility materials, such as fullerene derivatives. However, for Ti-alkoxides, the conventional solvent method is insufficient, because they only dissolve in specific solvents. Here, we demonstrate a new approach to morphology control that uses the molecular bulkiness of Ti-alkoxides without the conventional solvent method. That is, this method is one approach to obtain highly efficient, air-stable, organic-inorganic bulk-heterojunction solar cells.

A method for fully printable, fullerene-free, highly air-stable, bulk-heterojunction solar cells based on Ti alkoxides as the electron acceptor and the electron-donating polymer fabrication is described here. Moreover, a method for controlling the morphology of the photoactive layer through the molecular bulkiness of the Ti-alkoxide units is reported.

The photoactive layer of a typical organic thin-film bulk-heterojunction (BHJ) solar cell commonly uses fullerene derivatives as the electron-accepting ...

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical properties. The possibility of doping was investigated by adding monovalent cation halides with similar ionic radii to Pb2+, including Cu+, Na+, and Ag+. A shift in the Fermi level and a remarkable decrease of sub-bandgap optical absorption, along with a lower energetic disorder in the perovskite, was achieved. An order-of-magnitude enhancement in the bulk hole mobility and a significant reduction of transport activation energy within an additive-based perovskite device was attained. The confluence of the aforementioned improved properties in the presence of these cations led to an enhancement in the photovoltaic parameters of the perovskite solar cell. An increase of 70 mV in open circuit voltage for AgI and a 2 mA/cm2 improvement in photocurrent density for NaI- and CuBr-based solar cells were achieved compared to the pristine device. Our work paves the way for further improvements in the optoelectronic quality of CH3NH3PbI3 perovskite and subsequent devices. It highlights a new avenue for investigations on the role of dopant impurities in crystallization and controls the electronic defect density in perovskite structures.

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Here, we present a protocol to adjust the properties of solution-processed CH3NH3PbI3 through the incorporation of monovalent cation additives in order to achieve highly efficient perovskite solar cells.

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical ...

Well-aligned Vertically Oriented ZnO Nanorod Arrays and their Application in Inverted Small Molecule Solar Cells

This manuscript describes how to design and fabricate efficient inverted solar cells, which are based on a two-dimensional conjugated small molecule (SMPV1) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), by utilizing ZnO nanorods (NRs) grown on a high quality Al-doped ZnO (AZO) seed layer. The inverted SMPV1:PC71BM solar cells with ZnO NRs that grew on both a sputtered and sol-gel processed AZO seed layer are fabricated. Compared with the AZO thin film prepared by the sol-gel method, the sputtered AZO thin film exhibits better crystallization and lower surface roughness, according to X-ray diffraction (XRD) and atomic force microscope (AFM) measurements. The orientation of the ZnO NRs grown on a sputtered AZO seed layer shows better vertical alignment, which is beneficial for the deposition of the subsequent active layer, forming better surface morphologies. Generally, the surface morphology of the active layer mainly dominates the fill factor (FF) of the devices. Consequently, the well-aligned ZnO NRs can be used to improve the carrier collection of the active layer and to increase the FF of the solar cells. Moreover, as an anti-reflection structure, it can also be utilized to enhance the light harvesting of the absorption layer, with the power conversion efficiency (PCE) of solar cells reaching 6.01%, higher than the sol-gel based solar cells with an efficiency of 4.74%.

This manuscript describes how to design and fabricate efficient inverted SMPV1:PC71BM solar cells with ZnO nanorods (NRs) grown on a high quality Al-doped ZnO (AZO) seed layer. The well-aligned vertically oriented ZnO NRs exhibit high crystalline properties. The power conversion efficiency of solar cells can reach 6.01%.

This manuscript describes how to design and fabricate efficient inverted solar cells, which are based on a two-dimensional conjugated small molecule ...

Fabrication of Robust Nanoscale Contact between a Silver Nanowire Electrode and CdS Buffer Layer in Cu(In,Ga)Se2 Thin-film Solar Cells

Silver nanowire transparent electrodes have been employed as window layers for Cu(In,Ga)Se2 thin-film solar cells. Bare silver nanowire electrodes normally result in very poor cell performance. Embedding or sandwiching silver nanowires using moderately conductive transparent materials, such as indium tin oxide or zinc oxide, can improve cell performance. However, the solution-processed matrix layers can cause a significant number of interfacial defects between transparent electrodes and the CdS buffer, which can eventually result in low cell performance. This manuscript describes how to fabricate robust electrical contact between a silver nanowire electrode and the underlying CdS buffer layer in a Cu(In,Ga)Se2 solar cell, enabling high cell performance using matrix-free silver nanowire transparent electrodes. The matrix-free silver nanowire electrode fabricated by our method proves that the charge-carrier collection capability of silver nanowire electrode-based cells is as good as that of standard cells with sputtered ZnO:Al/i-ZnO as long as the silver nanowires and CdS have high-quality electrical contact. The high-quality electrical contact was achieved by depositing an additional CdS layer as thin as 10 nm onto the silver nanowire surface.

Fabrication of Robust Nanoscale Contact between a Silver Nanowire Electrode and CdS Buffer Layer in CuIn,GaSe2 Thin-film Solar Cells

In this protocol, we describe the detailed experimental procedure for the fabrication of a robust nanoscale contact between a silver nanowire network and CdS buffer layer in a CIGS thin-film solar cell.

Silver nanowire transparent electrodes have been employed as window layers for Cu(In,Ga)Se2 thin-film solar cells. Bare silver nanowire electrodes ...

In situ Grazing Incidence Small Angle X-ray Scattering on Roll-To-Roll Coating of Organic Solar Cells with Laboratory X-ray Instrumentation

We present an in-house, in situ Grazing Incidence Small Angle X-ray Scattering (GISAXS) experiment, developed to probe the drying kinetics of roll-to-roll slot-die coating of the active layer in organic photovoltaics (OPVs), during deposition. For this demonstration, the focus is on the combination of P3HT:O-IDTBR and P3HT:EH-IDTBR, which have different drying kinetics and device performance, despite their chemical structure only varying slightly by the sidechain of the small molecule acceptor. This article provides a step-by-step guide to perform an in situ GISAXS experiment and demonstrates how to analyze and interpret the results. Usually, performing this type of in situ X-ray experiments to investigate the drying kinetics of the active layer in OPVs relies on access to synchrotrons. However, by using and further developing the method described in this paper, it is possible to perform experiments with a coarse temporal and spatial resolution, on a day-to-day basis to gain fundamental insight in the morphology of drying inks.

This paper is a demonstration and a guideline to perform and analyze in-house (with a laboratory X-ray instrument) in situ GISAXS experiments of drying inks on roll-to-roll slot-die coated, non-fullerene organic photovoltaics.

We present an in-house, in situ Grazing Incidence Small Angle X-ray Scattering (GISAXS) experiment, developed to probe the drying kinetics of roll-to-roll ...

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

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