Recyclable organic solar cells

Dr. Neil Canter, Contributing Editor | TLT Tech Beat July 2013

A process for making green technology even greener uses a renewable source derived from cellulose nanocrystals.

KEY CONCEPTS
• Organic solar cells are less efficient than inorganic solar cells but exhibit advantages from the light absorption, fabrication, weight and flexibility standpoints.
• A more sustainable organic solar cell produced from naturally derived cellulose nanocrystals exhibits the highest power conversion efficiency seen f r om any solar cell prepared from renewable materials.
• This CNC-based organic solar cell can be recycled, which makes a green technology even greener.

IN EXAMINING THE CURRENT OPTIONS FOR GENERATING ELECTRICITY from renewable sources, using the sun appears to be one of the most promising. The main process used is based on the photovoltaic effect in which the solar radiation is converted into direct current electricity through the use of semiconductors.

One other approach to capture solar energy is through the use of a heat transfer fluid. The challenge is to optimize the characteristics of a solar mirror used to concentrate solar energy in order to increase the temperature of the heat transfer fluid to the maximum extent possible. A previous TLT article discussed work to develop a new silver- metalized reflective film that can be used in place of glass in a solar collector mirror (1). The benefits of using this film is a mirror that is more durable and is much lower in weight.

Bernard Kippelen, professor in the School of Electrical and Computer Engineering and director of the Center for Organic Photonics and Electronics at Georgia Tech in Atlanta, says, “Besides conventional solar cells based on using highly ordered crystalline silicon as the semiconductor, work is ongoing to develop organic solar cells that are based on the use of organic molecules and polymers. In a similar fashion to inorganic solar cells, organic solar cells are also based on the photovoltaic effect.”

An important component of the organic solar cell is the substrate that is placed under the bottom electrode to serve as a support for the organic semiconductors. Organic solar cells are not yet as efficient as silicon-based materials, although their performance has been increasing rapidly in recent years.

But Kippelen believes they exhibit key advantages over inorganic solar cells. He says, “Organic solar cells absorb light more strongly than silicon cells. Hence, organic cells are much thinner than their inorganic counterparts. A typical crystalline silicon cell is 180 microns thick, while organic solar cells are usually about 100 nanometers thick.”

Fabrication and assembly of multiple solar cells is also easier with organic solar cells. Kippelen explains, “The most efficient inorganic solar cells are high purity crystals that are fabricated into wafers. A lot of labor is involved in doing this process. In contrast, organic solar cells are produced as thin films through multiple techniques such as vacuum deposition and from solution. These processes are easier to scale up to commercial levels.”

A third attribute is that layers of an organic solar cell are lightweight and highly flexible compared with inorganic solar cells. Kippelen says, “Organic solar cells are also much less likely to shatter than inorganic solar cells.”

Currently, organic solar cells are prepared through the use of petroleum- based substrates such as polyethylene terephthalate and polyethersulfone. These substrates are not environmentally friendly and difficult to recycle. If a substrate could be found that combines the attractive features of these polymers and is derived from a renewable source, then solar cell technology could even become more attractive from a green standpoint. Research is now underway to evaluate the potential of such a substrate.

CELLULOSE NANOCRYSTALS
Kippelen and his fellow researchers have developed an organic solar cell using cellulose nanocrystals (CNCs) as the substrate. He says, “Our objective was to look for natural materials that exhibit the properties needed to function as the substrate of an organic solar cell. CNCs fit this criterion because they are derived from wood and can be prepared by a well established industry with a lot of infrastructure in place.”

CNCs exhibit the desired properties of low density combined with high tensile strength, high elastic modulus and a low coefficient of thermal expansion. They are also a transparent material, which facilitates the passage of light through the solar cell. Kippelen says, “Transparency is particularly good when CNCs are fabricated into thinner films that are less than 100 micrometers thick.”

Low surface roughness is also very important to facilitate the deposition of thin layers of organic semiconductors. Kippelen says, “Paper was looked at as a low cost, natural substrate but unfortunately displays high surface roughness. In contrast, we found the surface roughness for CNCs to be less than two nanometers, which is a big breakthrough in using this technology.”

The researchers prepared an organic solar cell by first depositing a semitransparent 20-nanometer thick silver layer as the bottom electrode onto the CNC substrate. The efficiency of the silver layer was enhanced through applying a 10-nanometer thick layer of an amine containing a branched polymer known as ethoxylated polyethylenimine (PEIE) through the use of spin coating.

The organic layer was a combination of two substances known as PBDTTT-C and PCBM (90 nanometers thick), which was also spin coated onto the PEIE. Finally, the top electrode is a combination of silver (150 nanometers thick) and molybdenum trioxide, which collects the positive holes.

Figure 2 shows an image of this organic solar cell, which exhibited a power conversion efficiency of 2.7 percent. This figure is the highest reported for any solar cell prepared from renewable materials.


Figure 2. A recyclable, organic solar cell has now been prepared using a renewable raw material, cellulose nanocrystals, as the substrate. This is a step in the process of making a green technology even greener. (Courtesy of Georgia Tech)

One other objective realized by the researchers was their ability to recycle all of the materials used in the organic solar cell at room temperature. Kippelen says, “We were able to recycle the organic solar cell by placing it in distilled water.”

The CNC film quickly disintegrates and can be isolated as a residue after redispersion in water. The organic semiconductor can be dissolved in an organic solvent. The other components in the solar cell then can be isolated on filter paper after removal of the water and organic solvent.

Future work for the researchers will focus on boosting the power conversion efficiency of this organic solar cell above 10 percent, which will be comparable to other organic solar cells. Kippelen says, “We intend to start by replacing the silver electrode, which currently blocks half of the light, with a transparent electrode. This should double the efficiency of the organic solar cell.”

Ultimately the researchers have a simple objective with this work. Kippelen says, “We are aiming to make the capture of solar energy, which is a green technology, even greener through the use of renewable materials and recycling.”

Further information can be found in a recently published article (2) or by contacting Kippelen at kippelen@ece.gatech.edu.

REFERENCES
1. Canter, N. (2009), “Improved Solar Power Reflector,” TLT, 65 (12), pp. 10-11.
2. Zhou, Y., Hernandez, C., Khan, T., Liu, J., Hsu, J., Shim, J., Dindar, A., Youngblood, J., Moon, R. and Kippelen, B., “Recyclable Organic Solar Cells on Cellulose Nanocrystal Substrates,” Scientific Reports, 3 (1536), DOI:10.1038/srep01536.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.