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

Podsumowanie

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.

Streszczenie

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.

Wprowadzenie

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, η, 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 η, 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 layers^3–5, but more groups have used particles of gold or silver to aid recombination of holes and electrons in the interlayer^6,7.

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 anodes^8,9. 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 electrodes^11–14.

As described in a preceding paper¹⁵ and depicted in Figure 2, when the gate voltage (V_Gate) is increases, the work function of the CNT common electrode is decreased, creating electrode asymmetry. This prevents hole collection from the OPV’s donor in favor of collecting electrons from the OPV’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.

Protokół

1. Indium Tin Oxide (ITO) Patterning and Cleaning

NOTE: Use 15Ω/□ 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’s resistance setting.

1. 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.
2. Anneal the resist coated glass, on a hot plate, at 115 °C for 1 min.
3. Load the sample and the photomask onto the contact aligner.
4. 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.
5. Develop the UV-exposed substrates in MF311 developer. A spin processor’s automated process produces the best and most repeatable results, but development can be done manually as followed.
   1. 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.
6. 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.
7. 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.
8. If needed, cut the etched ITO glass substrates into device sizes.
9. Clean the ITO substrates in a bath ultrasonicator in a sequence of solvents - DI water, acetone, toluene, methanol, and finally isopropyl alcohol.

2. OPV Sub-cell Fabrication

1. 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.
   1. 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.
   2. 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.
   3. Weigh the vials again to find the mass of the P3HT and PC₆₁BM.
   4. Transfer the vials with P3HT and PC₆₁BM into a glovebox for the rest of the solution making process.
   5. Add a magnetic stir bar into each vial and then add enough chlorobenzene to each to create 45 mg/ml solutions.
   6. Place the solutions on a magnetic stirring hot plate at 55 °C for approximately 2 hr or until the solutes have completely dissolved.
   7. Mix equal volumes of the P3HT and PC₆₁BM solutions together, and let the mixed solution stir for another hour prior to use.
2. Prepare PTB7:PC₇₁BM solution.
   1. Repeat steps 2.1.1 to 2.1.4 with poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]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.
   2. Make a mixture of 3% by volume 1,8-diiodooctane (DIO) in chlorobenzene. This mix is called DIO-CB.
   3. 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.
   4. Let these solutions stir on a hot plate at 70 °C for two days.
   5. Mix the solutions in a weight ratio of PTB7 to PC₇₁BM of 1 to 1.5.
   6. Let the mixed solution stir for another hour at 70 °C prior to use.
3. Filter poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonate) (PEDOT:PSS) through a 0.45 µm pore size nylon filter. Note this procedure uses P VP AI4083.
4. Spin Coat Active layers.
   1. Place cleaned ITO substrates, ITO-side up, into a UV-Ozone cleaner for 5 min.
   2. Spin-coat 120 μl 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.
   3. Anneal the PEDOT:PSS coated ITO substrates for 5 min at 180 °C.
   4. Spin-coat 70 μl 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.
   5. Anneal the P3HT:PC₆₁BM coated substrates at 170 °C for 5 min. The results may vary on optimal annealing temperature.
   6. Spin-coat 70 μl 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.
   7. Load the PTB7:PC₇₁BM coated substrates into a high vacuum (< 2 x 10^-6 Torr) chamber to remove the residual DIO. Typically, leave the samples in the chamber O/N.

3. Fabricate the Tandem Device

1. Laminate CNT electrodes.
   1. 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.
   2. 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.
   3. 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.
   4. 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.
   5. 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.
   6. 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.
   7. Densify the gate electrode with HFE.
2. Place a small drop (≈10 µl) 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.
3. 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.
4. 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.

4. Measure the Device

1. Transfer the device into the measurement glovebox.
2. Make the electrical connections.
   1. Connect the gate power supply between the common electrode and the gate electrode with the common as ground.
   2. Connect the two ITO anodes to wires which are connected to a switch which allows selection of either anode or both anodes.
   3. Connect the output of the switch to the input of the source measure unit.
   4. Connect the ground of the source measure unit to the common electrode.
3. Measure the device’s IV characteristics by repeating the following steps for ascending V_Gate.
   1. Set V_Gate to the next value, starting from V_Gate = 0 V to V_Gate = 2 V in increments of 0.25 V.
   2. Wait 5 min or until the gate current is stabilized. Ideally, the gate current should stabilize around 10s of nanoamperes.
   3. Set the switch to both sub-cells.
   4. Open the lamp shutter.
   5. Run a voltage sweep on the source measure unit from -1 volt to +1 volt at about 100 increments or more.
   6. Run a voltage sweep from +1 volt to -1 volt.
   7. Close the lamp shutter.
   8. Run the voltage sweeps again.
   9. Set the switch to the front sub-cell.
   10. Repeat steps 4.3.4 to 4.3.8.
   11. Set the switch to the back sub-cell.
   12. Repeat steps 4.3.4 to 4.3.8.
4. Calculate device parameters.
   1. Find the short circuit current (J_SC) of each sub-cell at each V_Gate by finding the current produced by the device when the voltage across the sub-cell is 0 V.
   2. Find the open circuit voltage (V_OC) of each sub-cell at each V_Gate by finding the voltage produced by the device when the current through the sub-cell is 0 A.
   3. 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.
   4. Find the power conversion efficiency (η) by dividing the maximum power by the input light power.
   5. Find the filling factor (FF) by dividing the maximum power by the product of J_SC and V_OC.

Wyniki

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, V_OC, J

Dyskusje

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 FF^T will be no higher than the FF of the bad sub-cell. J^T_SC and V^T_OC will be similarly affected. This is the case when V_Gate is low and the P3HT sub-cell ha...

Materiały

Name	Company	Catalog Number	Comments
Poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonate)	Heraeus	Clevios PVP AI 4083
poly(3-hexylthiophene-2,5-diyl)	Rieke Metals  Inc.	P3HT:  P200
phenyl-C61-butyric  acid methyl  ester	1- Material	PC61BM
Poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl})	1- Material	PTB7
phenyl-C61-butyric acid methyl  ester	Solenne	PC71BM
1,8-Diiodooctane	Sigma Aldrich	250295
Chlorobenzene	Sigma Aldrich	284513
Indium Tin Oxide Coated Glass 15 Ohm/SQ	Lumtec
S1813	UTD Cleanroom
MF311	UTD Cleanroom
HCl	UTD Cleanroom
Acetone	Fisher Scientific	A18-20
Toluene	Fisher Scientific	T323-20
Methanol	BDH	BDH1135-19L
Isopropanol	Fisher Scientific	A416-20
CEE Spincoater	Brewer Scientific		http://www.utdallas.edu/research/cleanroom/tools/CEESpinCoater.htm
Contact Printer	Quintel	Q4000-6	http://www.utdallas.edu/research/cleanroom/QuintelPrinter.htm
CPK Spin Processor			http://www.utdallas.edu/research/cleanroom/tools/CPKsolvent.htm
Spin Coater	Laurell	WS-400-6NPP/LITE
[header]
Glove Box	M-Braun	Lab Master 130
Solar Simulator	Thermo Oriel/Newport
Keithley 2400 SMU	Keithley/Techtronix	2400
Keithley 7002 Multiplexer	Keithley/Techtronix	7002
Ultrasonic Cleaner	Kendal	HB-S-49HDT
Micropipette	Eppendorf	200 µl