Carbon Nanotubes as Charge Carriers in Organic and Hybrid Solar Cells

Invented by: Somenath Mitra Cheng Li

Field(s) of Study: Chemistry, Environmental Science
Patent Category: Energy, Environmental

  • Patent #US9293720B2

  • Filed on Feb 19, 2008 Granted on Mar 22, 2016
  • Website

New Jersey Institute of Technology
Newark, NJ 07102

Fossil fuels
Today, we live in a society in which people use fossil fuels as though they are a harmless, infinite resource. However, this is certainly not the case. When burned, fossil fuels release gases into the Earth’s atmosphere. These gases cause the phenomenon known as the greenhouse effect. The greenhouse effect is what happens when heat from the sun stays in Earth’s lower atmosphere, causing the planet to gradually get warmer and warmer.

This poses a threat to all the inhabitants on earth, as species will struggle to adapt to the changing climate. Due to the greenhouse effect, the polar icecaps are already melting at alarming rates. If the polar ice caps continue to melt, the sea level will eventually rise to a point where a considerable amount of land inhabited by both humans and animals will disappear beneath the waves. Another issue that arises from fossil fuels is contamination of the land and other finite resources. Often, either when the fossil fuels are disposed of improperly, as when the waste is dumped into rivers and streams, or when accidents occur, such as oil spills, the land and water in the area becomes contaminated and unusable for several years. This is true even if the problem is addressed almost immediately. These are just some of the problems that arise from using fossil fuels. So where does that put us today?

Problems with solar energy
Generating energy from the sun’s power is one alternative for society’s use of fossil fuels. The issue is that the solar industry isn’t perfect either. However, the possible side effects from the solar industry are much less harmful to the earth than the side effects from using fossil fuels.

The first problem with solar energy is that solar energy generation is weather dependent. If there is a cloudy day where the sun’s light comes and goes as clouds block the sun’s rays, then there will not be enough energy to supply the energy needs of the populace. In addition, some days the sun might not shine at all, which will result in no energy production for the day. If there is a string of cloudy or rainy days in a row where the sun is either absent or only there for a few hours of the day, then, once all of the stored energy gets used, there is no more energy until the sun comes back out.

This problem is exacerbated by the fact that the solar industry is massively inefficient. Current solar cells and panels have an efficiency rate of only ten to twenty percent. This means that only about 10 to 20% of the energy obtained from the sun gets converted into useable energy. In comparison, fossil fuels have an average efficiency rate of 36%.[i]

A second issue that the solar industry has with energy efficiency is something called “line loss.” “Line loss” refers to the energy lost while electricity is traveling through a wire across a long distance. The longer the distance traveled, the more energy lost. This means that if the United States were to take a 10,000 square ft. area (which is enough to power the entire country if everything were directly next to the solar power plant) and dedicate the land strictly to the use of solar panels, a large portion of the energy generated would still be lost simply trying to get it across the country for everyone to use.

Finally, one of the biggest challenges facing the solar industry is cost. Solar technology is very expensive to produce, which translates to an even higher price for consumers. Because fossil fuels are much cheaper for consumers to purchase, cost is probably the biggest deterrent to the widespread implementation of solar energy. That is why there is currently a push to produce cheaper solar panels that also work more efficiently.

Two researchers at the New Jersey Institute of Technology have attempted to tackle these problems of solar energy using nanotechnology. Dr. Somenath Mitra and Dr. Cheng Li aim to change the way manufacturers produce solar cells with their 2016 patent “Carbon Nanotubes as Charge Carriers in Organic and Hybrid Solar Cells” (patent US9293720B2).

The Inventors
Dr. Mitra is a Professor within NJIT’s Chemistry and Environmental Science Department. Based on his research, Mitra observed that under microwave conditions nanotubes became very reactive. This phenomenon allows for efficient transport of electrons towards an electrode and an increase in efficiency for photovoltaics, a system that converts light into electricity, based on carbon nanotubes. As a postdoctoral candidate, Dr. Li worked with Mitra. His research interests focus on nanostructured materials and their applications in renewable energy technologies. After leaving NJIT, Li became the Vice Director of the Center for Advanced Materials at Chongqing Academy of Science and Technology in China.

CNT’s as Charge Carriers
Mitra and Li’s patent describes “an organic or organic/inorganic hybrid photovoltaic conversion device.” This photovoltaic device is made up of a photoactive polymer, fullerenes made of carbon, an electrode to collect electrical current, and carbon nanotubes that can be single or multi-walled and soluble or dispersible. Since this technology was meant to be used for generating solar power, naturally the substance that the photovoltaic is made of is activated by sunlight.

Somenath Mitra and Cheng Li, US Patent 9293720B2

The device is created using solvent vapor annealing and thermal annealing. To anneal an object, one would heat the object before letting it cool slowly. This process removes internal stresses within materials, increasing the strength and ductility of the material while decreasing its hardness. In the end, the photovoltaic would be comprised of a polymer acting as an electron donor, the carbon fullerenes acting as electron acceptors, and the carbon nanotubes serve as a means of transporting electrons through the device.

This patent is new and unique because of the methods it provides for enhancing the efficiency of photovoltaic devices. It provides enhanced power conversion efficiency through improved optical absorption and appropriate morphological rearrangement. This means that the efficiency of energy conversion can be increased through better light absorption and by a more appropriate arrangement of carbon nanotubes. The carbon nanotubes used in the photovoltaic allow for improved charge separation and provide better electron transport.

The invention is nonobvious because it generally includes a polymer as the electron donor, carbon as the electron acceptor, and the carbon nanotubes acting as electron transports. However, it is disclosed within the patent that a polymer, the carbon fullerenes and carbon nanotube composite, has been developed, adapted, and has already been used for photovoltaic cells. Another improvement included in the patent concerns electron transport within the photovoltaic. Instead of the usual quantum dots found on photovoltaics, they were replaced with quantum rods with lengths up to fifty millimeters.

The usefulness of this patent is that it provides a more cost effective solution to organic photovoltaic devices with increased power conversion efficiencies over previously demonstrated organic photovoltaic devices.

Because this is a newer technology, only being granted a patent in March 2016, it is not too surprising that no other inventors have cited “Carbon Nanotubes as charge carriers in organic and hybrid solar cells” in their own patents. However, there are many nanotechnology researchers working on similar ways to use carbon nanotubes to solve the problems of solar cells, in order to produce cleaner, more efficient energy for all.

By Luke Gregory, Kristen King, Julian Royal

[i] Peter Taylor, Olivier Lavagne d’Ortigue, Nathalie Trudeau, Michel Francoeur. Energy Efficiency Indicators for Public Electricity Production from Fossil Fuels. International Energy Agency. July 2008, pg 5.