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
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– 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:
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.
A summarized diagram follows the steps written below.
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.
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.
Average %T (400-800 nm) | Average %R (400-800 nm) | Sheet resistance (Ω/□) | Figure of merit (Ω^-1) | |
|---|---|---|---|---|
AgNW:SWCNT:PEDOT:PSS | 86,0 +/- 1,0 | 3,4 +/- 0,3 | 6,6 +/- 0,5 | 367 |
ITO | 93,0 +/- 7,3 | 7,2 +/- 4,3 | 18,3 +/- 0,5 | 292 |
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.
(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.
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.
