If you’ve watched the movie The Day after Tomorrow, you must remember the climate-related natural disasters that ravaged the human civilization. Though the events depicted in the movie are unrealistic, we are still dramatically altering Earth’s environment and climate through our current energy policies and practices. How can we mitigate some of the effects of man-made global warming? One possible solution is to increase our use of solar energy, a plentiful, sustainable, and environmentally friendly form of energy.
We waste a lot of the solar energy that reaches the Earth every day. In the United States, solar energy provides less than 0.1% of our electricity, compared to more than two-third that is generated from fossil fuels — coal, oil and natural gas . Despite the accelerating growth in solar power plants, there is still a large amount of untapped solar energy all around us. If we find a way to efficiently harness this energy, we may one day be able to use cheap and clean energy from the Sun. It has been estimated that covering less than 0.2% of the land on earth with solar cells of 10% efficiency (which can turn 10% of the light energy hitting the cell into electrical energy) would provide twice the power that is needed to run the world .
Solar cells – Utilizing energy from the Sun
There are multiple ways to convert solar energy into electricity or thermal energy. Photovoltaic (PV) cells, commonly known as solar cells, produce electricity as long as they are exposed to enough light. The most common solar cells are made out of special materials called semiconductors, usually the element silicon. When light strikes a solar cell, it is partially absorbed by the semiconductor material. The energy of the absorbed light “excites” electrons and knocks them off the atoms in the material, allowing the electrons to flow freely as an electric current . In other words, the solar cell takes solar energy and transforms it into electricity.
Dye-sensitized solar cells — Making solar cells out of raspberries
Many universities and research institutions have joined the race to make cheap and efficient solar energy a reality. Besides the traditional silicon-based solar cells, researchers are developing new photovoltaic technologies, including dye-sensitized solar cells inspired by plants.
Plants naturally harness the energy of the sun to survive through a process called photosynthesis. In this process, plant cells use a pigment, or dye, called chlorophyll to capture solar energy. It uses this energy to free electrons (in a manner analogous to what happens in solar cells) to drive a process that ultimately turns carbon dioxide from the air into sugar for the plant’s consumption. Since the 1970s, researchers have tried to make solar cells by mimicking photosynthesis. They did this by covering crystals of a semiconductor, titanium dioxide (TiO2), with a layer of chlorophyll. This first generation of “dye-sensitized” solar cells (DSC) only had an efficiency of 0.01% . With relatively easy-to-obtain materials, you can even make a crude DSC out of raspberries right in your garage!
Making a solar cell from TiO2 and raspberries.
Recent progress in nanotechnology (the manipulation of matter at the molecular/atomic scale) has enabled scientists to overcome the problem of low efficiency. Rather than large titanium dioxide crystals, scientists have developed solar cells with a sponge-like surface of tiny semiconductor particles, each about 20 nm in diameter (1 nm is one-billionth of a meter), and covered by a very thin layer of chlorophyll-like pigment (Figure 1). Compared to conventional solar cells, this change from a flat surface to a porous surface significantly increases the surface area available for light absorption by one thousand times, which in turn greatly increases the efficiency of the conversion of light into electrical energy .
DSC technology has enabled the development of inexpensive solar-powered technologies. Conventional solar cells, made out of expensive crystalline silicon, have a high cost of production . In comparison, the materials used in the DSCs are much cheaper. For example, titanium dioxide is a common material in daily life: it is found in toothpaste, sunscreen and white paint.
Figure 1. Schematic of a dye-sensitized solar cell. Light excites the dye that coats the titanium dioxide (TiO2) nanoparticles, knocking loose electrons that jump into the transparent conductor, the negative pole of the cell. Electrons in the dye are replenished by the iodide/triiodide (I–/I3–) electrolyte (ion-containing liquid that conducts electricity), which are in turn replenished by a reaction sped up by the catalyst that coats the positive pole of the cell. (Image credit: Wikimedia Commons user oldboltonian)
Next generation solar cells — Enhancing power-conversion efficiency using genetically modified virus
Last month, researchers at MIT found a novel way to significantly improve solar cell efficiency by enlisting the service of tiny viruses to perform detailed assembly work at the sub-microscopic level . Graduate students from the lab of Angela Belcher, in collaboration with several other researchers, used a genetically engineered form of the bacteria-infecting virus M13 to organize carbon nanotubes in a DSC. These tiny tubes, made from single layers of carbon atoms (Figure 2), are ideal collectors of the excited electrons that get knocked off from the dye by solar energy, as their chemical structure allows electrons to be conducted away very quickly (see here). However, one of the challenges in their application to solar cells is the difficulty of managing their arrangement at the nanoscale. The method developed by the MIT team, called “virus template assembly”, establishes a close contact between the semiconductor nanoparticles and carbon nanotubes, enabling even faster and more efficient transport of electrons excited by light, greatly enhancing the efficiency of the solar cells. Thanks to the “magic hand” of viruses, we might now be able to harness the energy from the Sun more efficiently.
Figure 2. Chemical structure of a carbon nanotube. (Image credit: Wikimedia Commons user Saperaud)
A bright future lies ahead if we can take full advantage of the abundant solar energy that strikes the Earth everyday. This future may one day be reached through advancements in photovoltaic technology. If scientists continue to think outside the box, the “solar revolution” may come more quickly than we ever thought possible.
Jing Yang is a PhD student in Applied Physics at the Harvard School of Engineering and Applied Sciences
 Annual Energy Review 2007. (2008). <ftp://ftp.eia.doe.gov/multifuel/038407.pdf>
For example, in the state of Massachusetts, the annual average solar radiation is 4.0-4.5 kWh/ m2/ day [http://www.nrel.gov/gis/images/map_pv_us_annual10km_dec2008.jpg]. From the state electricity profile 2008 [http://www.eia.gov/cneaf/solar.renewables/page/state_profiles/massachusetts.html], the net generation of solar energy in 2008 is 42,505,478 MWh. Thus, covering 0.1% of the land in MA would provide 100% of the power used by the state. However, solar energy currently only accounts for 0.1% of total electricity generation capacity. Therefore, there is plenty of room for development in the solar market.
 How Solar Cells Work. http://science.howstuffworks.com/environmental/energy/solar-cell.htm
 Pagliaro, M, Palmisano, G, Ciriminna G. (2009) Working principles of dye-sensitized solar cells and future applications. Photovoltaics International 3:47-50. http://www.physics.hku.hk/~phys0628/DSSC%20Working%20Principle.pdf
 China Solar PV Report 2007. http://www.greenpeace.org/raw/content/eastasia/press/reports/china-pv-report.pdf
 MIT news: Solar power goes viral. http://web.mit.edu/newsoffice/2011/solar-virus-0425.html
Other videos of interest:
The future of technology – Photovoltaic Glass http://www.youtube.com/watch?v=leI9E96ZIOQ
Introduction to Solar Photovoltaics http://www.youtube.com/watch?v=2mCTSV2f36A
Solar Power and GreenFlow Energy http://www.youtube.com/watch?v=vtqxEgoaABI