Making planarized transparent electrodes

Memmert vacuum oven VO is used in fabricating planar electrodes.

This article describes the findings in the journal Science and Technology of Advanced Materials, available at the National Institute for Materials Science and Taylor & Francis. You can read the full report here.

A planar electrode is an unshielded electrode embedded in an insulated plane so that the electrode surface is flush with the surrounding plane. From: Journal of Electroanalytical Chemistry

For detailed information about the Memmert vacuum drying oven,  contact the team or stay updated on latest product news. Detailed information is now available in the vacuum oven whitepaper. Register here to access it in our downloads section. For further information on drying ovens, its technology, uses, versatility and more, read this explainer blog.


Transparent electrodes, such as indium tin oxide (ITO), are an essential element of optoelectronic devices. The reason being that its optical transparency and conductivity in flat panel displays, solar cells and light diodes.

ITO has become the most utilized material for transparent electrodes due to its acceptable sheet resistance (15–60 Ω/) and high optical transparency (>90%).

There are a few drawbacks:

  • The lifetime cost analysis of Indium tin oxide (ITO) demonstrates that up to 87% energy necessary for the making of conventional organic photovoltaic (OPV) is credited to ITO fabrication.
  • Temperatures up to 400 °C is required for optimal conductivity
  • Brittle properties

– which restrict flexible applications and reel-to-reel processing.

This causes a need for an alternative means to transparent electrode properties.

Now comes the topic of ‘figure of merit’.

A figure of merit is a quantity used to characterize the performance of a device, system or method, relative to its alternatives. From: Practical Three-Way Calibration, 2014

This value helps to compare the transparent electrode. It is said in the study that with alternative transparent conductive electrodes, a trade-off ­occurs between optical transparency and conductivity. That is where the figure of merit is welcome:

  • It makes use of the sheet resistance of the electrode
  • It uses the optical transmission at 550nm for better performance

There are several ways to go about fabrication of ITO-free transparent electrodes:

The last one, silver nanowire (AgNW), a key element of this report, has a couple advantages toward fabrication of transparent conducting electrodes:

There is a catch though – greater diameter of nanowires can lead to surface roughness.

Aim of the report

The report lists previous attempts to explore the transparency and sheet resistance properties of interwoven silver nanowires and single-walled carbon nanotubes (AgNW:SWCNT) transparent electrodes. There is much room for the further development by way of modifying electrode processing conditions. This allows for the planarized surface of electrodes to take part in device fabrication.

This report discusses the manufacturing of high figure of merit planarized (<5 nm rms roughness) AgNW:SWCNT transparent electrode which embedded inside thin poly(3,4-ethylenedioxythiophene) polystyrene sulfonate/PEDOT:PSS via epoxy adhesion lift-off technique.


  • Silver nanowire (AgNW) were taken as a suspension (20.4 mg mL−1) in 2-propanol (IPA)
  • Aliquot of that was diluted to 0.1 mg mL−1 with IPA and kept for storage for later purposes
  • Carboxylate functionalized (P3 type) SWCNTs with purity levels above 90%


A summarized diagram follows the steps written below.

This diagram shows the fabrication of planar nanocomposite electrodes on a glass substrate.

The fabrication of planar AgNW:SWCNT:PEDOT:PSS nanocomposite electrodes on a glass substrate. (a) Substrate patterning through cellulose ester membrane, (b) transfer to silicon substrate, (c) spin-coating of PEDOT:PSS solution, (d) creation of epoxy sandwich structure, (e) curing the epoxy, (f) liquid nitrogen separation of electrode from silicon template. DI stands for deionized and PEN represents polyethylene naphthalate. Courtesy of Taylor & Francis Online and A J Stapleton et al.

The carboxylate functionalized SWCNT (50 mg) was treated in 3M HNO3 heated at reflux for 12 hours and filtered afterwards via 0.45 μm polycarbonate. A sample was then taken from here and suspended in water via probe sonication at 40% amplitude for a time period of 2 minutes. Later, it was with deionized water in order to establish a concentration of 0.25 mg mL−1.  The aforementioned interwoven networks were prepared for the study by vacuum filtration through mixed cellulose ester membranes. Pre-prepared AgNW (0.1 mg mL−1) and SWCNT (0.25 mg mL−1) solutions were added to 300 mL of deionized water. This allowed for surface loading of 125 mg m−2 of AgNWs in the final nanocomposite electrode.

Patterned electrodes were created by positioning a relatively smaller pore size mixed cellulose ester template underneath the 0.45 μm membrane during filtration process. Following filtration, the study proceeds to position the patterned electrodes on pre-cleaned silicon substrates. This silicon and the patterned electrodes are heated 80 °C under 0.16 kg cm−2 of pressure in a vacuum oven for time period of 30 minutes. This is laboratory vacuum drying oven from Memmert capable of several applications whose information is available in the blog.  How does a laboratory vacuum oven work? What is a vacuum oven used for?  Contact us or fill out a vacuum oven inquiry.

This is an image of the Memmert vacuum oven VO101. This is a laboratory heating and drying oven.

Memmert vacuum oven VO101.

Now, mixed cellulose ester filter paper can be removed by dissolution in acetone for one hour. This leaves behind the residue of patterned AgNW: SWCNT nanocomposite on the silicon substrate. The poly (3,4-ethylenedioxythiophene) polystyrene sulfonate/PEDOT:PSS solution that is in a composition of 33 v/v% 2-propanol and 10 v/v% sorbitol (200 μL) is spin-coated above the silver nanowires and single-walled carbon nanotubes (AgNW:SWCNT) transparent electrodes nanocomposite at 500 rpm for a 5 second run and upped to 3000rpm for 3 seconds. This nanocomposite is annealed at a temperature of 140 °C for 10 min.

Epoxy resin is applied on top of the poly (3,4-ethylenedioxythiophene) polystyrene sulfonate/PEDOT:PSS coated the silver nanowires and single-walled carbon nanotubes (AgNW:SWCNT) electrode. A glass substrate for transfer purposes is placed on top to make AgNW: SWCNT:PEDOT:PSS/epoxy/glass stack.

Using a Memmert heating and drying oven, at 65 °C for one hour, this stack went through a curing process for the epoxy. Stack is placed inside liquid nitrogen to cleave the silicon/PEDOT:PSS interface and cause formation of a planar conducting surface on this glass substrate. Sheet resistance is measured. Transmission of reflectivity is measured on the samples (25 mm2) via spectrophotometer with integrating sphere. Scanning electron microscopy (SEM) and topographical atomic force microscopy (AFM) images are taken. Root mean square roughness (Rrms) values are found from plane-fitted image scans of 10 μm2.

The electrical conductivity of the planar AgNW:SWCNT electrodes is mapped with the help of peak force tunnelling AFM (PF-TUNA). The cantilever and tip are coated with 20nm platinum and iridium to cause tip diameter at about 40 nm. The sample surface is connected electrically using a copper wire. This setup is pictured below.

This is an image of a PF-TUNA setup. It shows the nanocomposite surface, photodiode, laser, cantilever, Cu tape and substrate at work.

PF-TUNA setup. Courtesy of Taylor & Francis Online and A J Stapleton et al.

Two kinds of cells are fabricated by using two different organic semiconductor blends to the purpose of testing the AgNW:SWCNT:PEDOT:PSS electrodes in the devices with structures:

  • glass/AgNW:SWCNT/MoOx/poly (3-n-hexylthiophene-2, 5-diyl) (P3HT):phenyl-C61-butyric acid methyl ester (PC60BM)/Al
  • glass/AgNW: SWCNT/MoOx/poly(N-9″-heptadecanyl-2, 7-carbazole-alt-5, 5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)) (PCDTBT): (6, 6)-phenyl C70-butyric acid methyl ester (PC70BM)/Al

Pre-patterned ITO substrates are cleaned with Alconox (Alconox) in deionized water. Later, they are rinsed deionized water and ultra-sonicated for a 5 minute period. This is followed up with consecutive ultra-sonication in acetone and isopropanol – both for 10 minutes.

The substrates are dried with nitrogen gun. All the methods and steps of device fabrications and testing for all types of devices take place inside a nitrogen environment.  Thin film of MoOx is placed on the planarized AgNW:SWCNT film through thermal evaporation with a pressure value at 1×10−6 mbar.

A concoction of P3HT:PCBM (1:1 w/w) is made through the mixing of equal amounts of individual solutions of P3HT( Mw= 94 kDa, polydispersity index (PDI) = 1.9) and PC60BM in 1, 2-dichlorobenzene (DCB) (anhydrous grade). The concentration level was at 30 mg mL−1.  The blend is filtered and spin-coated. The PCDTBT:PC70BM active layer was made via mixing 6 mg mL−1 solution of PCDTBT in DCB. The active layer blend was spin-coated on the MoOx layer. P3HT:PCBM (1:1 w/w) and the PCDTBT:PC70BM (1:4 w/w) are dried at a temperature setting of 60 °C for 20 minute on a hot plate.

This led to a thick layer of Al deposition through thermal evaporation at a pressure rate of 1×10−6 mbar to fulfill the fabrication where the final devices consisted of active area of 0.2 cm2. Device characterization was done with a solar simulator and light intensity was calibrated.


The findings of the report are shown in the following:


Average %T (400-800 nm)

Average %R (400-800 nm)

 Sheet resistance (Ω/□)

 Figure of merit (Ω^-1)


86,0 +/- 1,0

3,4 +/- 0,3

6,6 +/- 0,5367


93,0 +/- 7,3

7,2 +/- 4,3

18,3 +/- 0,5


Summary of properties of ITO and AgNW:SWCNT:PEDOT:PSS electrodes. Courtesy of Taylor & Francis Online and A J Stapleton et al.

This is a graph showing the transmission (%T) and reflectivity (%R) of an ITO and planar nanocomposite electrode corrected for the substrate contribution.

Transmission (%T) and reflectivity (%R) of an ITO and planar AgNW:SWCNT:PEDOT:PSS nanocomposite electrode corrected for the substrate contribution. The sheet resistances, shown on the right, are an average of 15 measurements on 3 separate 25 mm2 samples. Courtesy of Taylor & Francis Online and A J Stapleton et al.

SEM images of (a) non-planarized AgNW and SWCNTs on a glass substrate and (d) the AgNWs:SWCNT:PEDOT:PSS electrode after the planarization. AFM of (b) non-planarized AgNW and SWCNTs on glass and (e) AgNWs and SWCNTs after the planarization process. The height profiles along the dotted lines in (b) and (e) are shown in (c) and (f), respectively. All scale bars are 2 μm. Courtesy of Taylor & Francis Online and A J Stapleton et al.

SEM images of non-planarized AgNW and SWCNTs on a glass substrate and the AgNWs:SWCNT:PEDOT:PSS electrode after the planarization are shown in this image.
These images show the height and peak force current map of planarized electrode surface with a bias voltage of 2V.

(a) Height and (b) peak force current map of planarized AgNW:SWCNT electrode surface with a bias voltage of 2 V. Courtesy of Taylor & Francis Online and A J Stapleton et al.

This image shows J–V characteristics of best OPV devices on planarized AgNW:SWCNT:PEDOT:PSS nanocomposite transparent electrodes and ITO with P3HT:PC60BM and PCDTBT:PC70BM active layers and performance parameters.


J–V characteristics of best OPV devices on planarized AgNW:SWCNT:PEDOT:PSS nanocomposite transparent electrodes and ITO with P3HT:PC60BM and PCDTBT:PC70BM active layers and performance parameters. Inset shows structure of an OPV device. Courtesy of Taylor & Francis Online and A J Stapleton et al.


The planar nanocomposite electrodes are fabricated with a superior figure of merit (367 Ω−1)—a combination of transparency and conductivity—than ITO (267 Ω−1) on glass.

  • Without PEDOT:PSS, the electrode surface is resistive.
  • AgNW:SWCNT:PEDOT:PSS electrodes helped make low temperature (non-annealed) devices –

Therefore demonstrating the potential for the AgNW:SWCNT:PEDOT:PSS electrode to function with a range of organic semiconducting polymer:fullerene bulk heterojunction blend systems.

About Memmert vacuum oven

Memmert GmbH + Co.KG, operating out of Schwabach and manufacturing from Büchenbach, produces the vacuum oven VO as part of its diverse heating and drying oven range.

It offers features such as:

  • Temperature range up to +200 °C
  • Vacuum control range: 5 to 1100 mbar
  • 3 model sizes (29 to 101 litres volume)
  • 1 model variant: TwinDISPLAY
  • Anti-splinter; VDE-tested door construction for all models
  • Pump control: Optimised rinsing of the pump membrane as well as signal output for switching the pump ON/OFF according to requirements.
  • Optional: Pump base cabinet and energy-efficient vacuum pump
  • Nearly exclusive use of high-quality, rust-resistant and easy to clean stainless steel for interior and exterior housing
  • Precise and homogenous temperature control thanks to a product-specific heating concept
  • A wide range of options for programming and documentation using interfaces, integrated data loggers and the AtmoCONTROL software
  • 3 years guarantee worldwide

Read the blog for other application articles. Visit the homepage, or sign up for the newsletter, for more information on products such as climate chambers, incubators, laboratory water baths, medical devices such as sterilisers, blanket warmers and more. For detailed information, please send a message via  email or inquiry form.