Flexible solar cells with carbon nanotubes

The flexible electronic components used in mobile devices are generally composed of dielectric substrate materials—such as poylethylene terephthalate and polyethylene naphthalate—embedded with active or sensing layers. Organic (i.e., carbon-based) materials—such as conducting polymers, carbon black, graphite, or carbon nanotubes (CNTs)—are increasingly being used as active materials in flexible electronics. They offer a wide range of physical and chemical properties when mixed with a polymer filler and applied to the substrate, for example, by screen printing. We recently designed and manufactured a carbon-polymer composite on a Kapton^® foil substrate.^1–3 Fabricated by screen printing, these kind of sensors were designed as internal temperature sensors for biological monitoring (see Figure 1). Despite the wide use of flexible organic materials in traditional electronics and sensors, flexible photovoltaics (PVs) are relatively underexplored. Here, we describe the fabrication of flexible transparent contacts for solar cells using CNTs.

Figure 1. Polymer-based flexible temperature sensors for bio-signal monitoring in textronic applications.

Designing flexible PVs requires the elaboration of an appropriate semiconductor junction and definition of the optical properties of the active layer. Importantly, the PVs require suitable contacts between the components. These electrodes must be reliable, efficient, cheap to manufacture, and compatible with the rest of the solar cell structure. The most frequently used solution is to apply a transparent conductive oxide, such as indium tin oxide (ITO), to the top, emitting layer of the solar cell. ITO—a mixture of tin dioxide and indium oxide—is characterized by its high optical transmission (>90%) in the visual range and relatively low electrical resistivity. Unfortunately, ITO is incompatible with flexible PVs because of its weak resistance to mechanical stress: it breaks or is crushed too easily. Furthermore, thin ITO layers are predominantly manufactured by magnetron sputtering,⁴ which is expensive. Finally, indium resources are limited, and the global supply is expected to be exhausted within the next 15 years. Avoiding the use of ITO, we devised a novel method of fabricating flexible transparent contacts for solar cells using CNTs.⁵ These are becoming increasingly popular in electronic applications because they can be manufactured by a variety of techniques and are easily chemically diversified to expand their range of properties.^{6, 7}

We fabricated CNT layers with relatively high optical transmittance using inexpensive screen printing on both glass and elastic polymer substrates. Compared to a 160nm-thick ITO layer, our CNT layer (1.5μm thick) demonstrated an average difference of 10% in transmittance (see Figure 2).

Figure 2. Transmittance of 0.25% by weight carbon nanotube (CNT) layer (1.5μm thick) and indium tin oxide layer (ITO, 160nm thick), both on borosilicate glass.

Despite this favorable performance, the sheet resistance our of CNT layer was relatively high and varied from 0.8–10kΩ/square. Since this parameter must be reduced for efficient PV applications, we increased the CNT content and used different techniques for film deposition, such as pouring from the liquid phase. We determined that the resistance of CNT layers—in contrast to ITO—is completely independent of bending, which is critical for flexible solar cell construction. We verified this in a mechanical bending assay (see Figure 3). We prepared a 1.5μm-thick CNT layer, screen-printed onto a Kapton^® substrate. After 80 cycles of rapid mechanical bending, the resistance remained constant. This is in stark contrast to ITO, whose resistance increases on bending.⁶

Figure 3. Variation in resistance of CNT layers to bending compared to ITO.⁶

In summary, we have explored the use of CNTs in flexible transparent conductive electrodes in solar cells. We will next incorporate these electrodes into fully operational flexible solar cell structures and assess their electrical properties. We will also investigate different carbon-based materials for use in transparent conductive layers, such as graphene. Since graphene is extremely transparent in the visual range (up to 97%) and demonstrates high electron mobility (up to 200,000cm²V⁻¹s⁻¹), it is a promising candidate for future PV applications.