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:

• Conducting polymer layers
• Ultrathin metal films
• Doped metal oxides (such as aluminiumdoped)
• Metal oxide–silver–metal oxide (mam) stacks
• Silver nanowire (AgNW) based electrodes

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

• Convenient nanowire production
• Easy to process scalable solutions. For more information, read reports on large scale, highly conductive and patterned transparent films of silver nanowires, fully solution-processed inverted polymer solar cells with laminated nanowire electrodes, silver nanowire-based transparent, flexible, and conductive thin film and spray deposition of highly transparent, low-resistance networks of silver nanowires over large areas
• High conductivity as well as mechanical ductility as found in the environmental and economic assessment of ito-free electrodes for organic solar cells and highly flexible silver nanowire electrodes for shape-memory polymer light-emitting diodes

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

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%

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 MoO_x 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 MoO_x 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:

Summary of properties of ITO and AgNW:SWCNT:PEDOT:PSS electrodes. 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.

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