OK, so here's the thing, I'm taking part in Padarth, which is the material science fest of IITB in the exposition on materials. My topic is 'Whacky Solar Cells'. The exposition happens to be tomorrow and I'm really nervous. So I'm going to lay down the structure of my talk here.
This post will just have the history of solar cells and the different types.
I have taken most of this post from an article I wrote along with 2 seniors on Solar cells. This article has already appeared in the Meta department's magazine/newsletter.
History of Solar Cells:
Photovolataic devices have become a buzzword now in the times of energy crisis but the history of photovoltaics dates more than 160 years back. The phenomenon of photoconductivity was first discovered in 1839 by a nineteen years old French experimental physicist, Alexandre Edmond Becquerel, who was experimenting with selenium electrodes to be used as high resistance rods for use in submarine applications. He observed that the conductivity of the rods increased with illumination.
In 1873, Willoughby Smith discovered photovoltaic effect in selenium. Later, in 1876, R.E. Day and William G. Adam found that illuminating a junction between selenium and platinum also has a photovoltaic effect. These discoveries led to the construction of the first solar cell based on selenium and gold junction in 1877, which was described in detail by Charles Fritts, an American scientist, in 1883. In 1888, Edward Weston received the first US patent for "solar cell" and later, in 1901, Nikola Tesla received a US patent for "method of utilizing, and apparatus for the utilization of, radiant energy".
From 1900-1949, scientists tried to find the theoretical explanation of Photovoltaic effect and first functional solar cells were made. In 1904, Albert Einstein published a paper explaining the effect, for which he was awarded the Nobel Prize in 1921. Einstein's theoretical explanation was experimentally proved by Robert Millikan in 1916. In 1914, the existence of a barrier layer in PV devices was reported.
The start of silicon based solar cells and devices can be attributed to the discovery of a method of preparing monocrystalline silicon by Jan Czochralski in 1918. The first monocrystalline silicon solar cell was made in 1941 by Russel Ohl.
Solar cell research got a big boast during 1950-1970 due to extensive interest in space research. In early 1950s, extensive laboratory research on solar cells took place. In 1951, the first germanium solar cells were made. In 1954, Rappaport, Loferski and Jenny of the RCA Laboratories published a report on photovoltaic effect in CdS.
In the same year, Bell Labs researchers Pearson, Chapin, and Fuller reported an efficiency of 4.5% in silicon solar cells; this was raised to 6% only a few months later. By the year 1955, the energy cost for PV devices was US$ 1,785 per W and the maximum efficiency achieved out of a device was 2%. By 1960, the efficiency of commercially available solar cells had touched 14%.
The energy crisis and oil embargoes of 1970s increased the interest in renewable sources of energy and development of photovoltaics got a big boast. Large photovoltaic corporations were set up around the world during this time, like the Solar Power Corporation in 1972, the Solarex Corporation in 1973 Solec International and Solar Technology International in 1975. In 1974, Japan formulated “Project Sunshine” to fuel PV research and development. By this time, a silicon solar cell of US$ 30 per W had been produced. This opened up the possibility of using solar cells in small day-to-day applications like pocket calculators, watches, radios, lanterns and other small battery-charging applications. The Solar Energy Research Institute (SERI), later to become the National Renewable Energy Laboratory (NREL), opened in Golden, Colorado in 1977. Total PV manufacturing production exceeded 500 kW by that year.
In 1984, the fist amorphous silicon solar cell was made and in 1986, ARCO Solar introduced the first commercial thin film photovoltaic module. By then University of New South Wales, Australia had made a solar cell with efficiency of 20%. In 1989, a concentrator solar cell was developed in which sunlight is concentrated onto the cell surface by means of lenses. This system achieved an efficiency of 37% due to the increased intensity of the collected energy. New thin film deposition techniques were perfected in 21st century, leading to the development of more efficient and cost-effective novel designs solar cells like amorphous silicon solar cells, organic solar cells and quantum dot solar cells. Recently, a research group at Fraunhofer Institute for Solar Energy Systems ISE, Germany, reported a whopping efficiency of 41% with tandem solar cells. Hopefully, that day is not far when solar cells will become the ultimate energy source with cheap solar panel installations on each and every household in the world!
Novel Types of Solar Cells:-
1. Quantum Dot Solar Cells
Quantum Dots are semiconductor nano crystals composed of periodic groups of II-VI, III-V, or IV-VI materials that exhibit size and compositionally tunable band gaps. This is because the size of such a semiconductor crystal is so small that it approaches the size of the material's Exciton Bohr Radius (An exciton bohr radius is the distance in an electron-hole pair) and the electron energy levels can no longer be treated as continuousi.e that there is a small and finite separation between energy levels. The absorptive and emissive behavior of a quantum dot depends strongly on its size.
The greater the bandgap of a solar cell semiconductor, the more energetic the photons absorbed, and the greater the output voltage. On the other hand, a lower bandgap results in the capture of more photons including those in the red end of the solar spectrum, resulting in a higher output of current but at a lower output voltage.
Thus, there is an optimum bandgap that corresponds to the highest possible solar-electric energy conversion, and this can also be achieved by using a mixture of quantum dots of different sizes for harvesting the maximum proportion of the incident light. Another advantage of quantum dots is they can easily be molded in convenient shapes by combining with organic polymers, dyes, or made into porous films on substrates of various materials like glass, plastics and metal sheets.
2. Organic solar cells
Organic solar cells emerged in the late 1970s, based on conjugated polymers – polymers with alternating double and single carbon-carbon bonds – when it was discovered that doping these materials – i.e. slightly contaminating with appropriate chemical elements - increased conductivity several orders of magnitude . Since then these materials have found applications in the making of solar cells.
Organic photovoltaic solar cells bear an important potential of development in the search for low-cost modules for the production of domestic electricity. One of the main differences between inorganic and organic solar cells is that photo-excitation in these materials does not automatically lead to the generation of free charge carriers, but to bind electron–hole pairs (exciton) with a binding energy of about 0.4 eV.
One of the biggest obstacles to organic solar cells is that it is difficult to control what happens after light is absorbed: whether the desired property is transmitting energy, storing information or emitting light. Experiments, however suggests it is possible to achieve control using quantum effects, even under relatively normal conditions. Organic solar cells — made out of plastic-like polymers — are much cheaper when compared by the conventional inorganic, silicon and metal-based solar cells typically favored by the solar industry. They are also flexible and are very light. Their one main disadvantage, of course, is their relatively low efficiency of just 5.4% as compared to the 20 - 40% efficiencies regularly attained by inorganic crystalline solar cells.
3. Dye sensitized solar cells
A dye sensitized solar cell is a relatively new class of thin film, low-cost solar cells. It is based on a semiconductor formed between a photo-sensitized anode and an electrolyte which functions like a photochemical system. They are also known as Grätzel cells after their inventor.
Dye-sensitized solar cells separate the two functions provided by silicon in a traditional cell design. Normally the silicon acts as both the source of photoelectrons, as well as providing the electric field to separate the charges and create a current. In the dye-sensitized solar cell, the bulk of the semiconductor is used solely for charge transport, the photoelectrons are provided from a separate photosensitive dye.
Charge separation occurs at the surfaces between the dye, semiconductor and electrolyte. In quantum efficiency (the chance that one photon (of a particular energy) will create one electron) terms, these cells are extremely efficient. Due to their "depth" in the nanostructure there is a very high chance that a photon will be absorbed, and the dyes are very effective at converting them to electrons. DSSc's are currently the most efficient third-generation solar technology available.
4. Tandem solar cells
Multijunction photovoltaic cells are a sub-class of solar cells developed for higher efficiency. These multijunction cells consist of multiple thin films. Each type of semiconductor will have a characteristic band gap energy which, loosely speaking, causes it to absorb light most efficiently at a certain color, or more precisely, to absorb electromagnetic radiation over a portion of the spectrum. The semiconductors are carefully chosen to absorb nearly all of the solar spectrum, thus generating electricity from as much of the solar energy as possible.
Tandem solar cells based on monolithic, series connected, gallium indium phosphide (GaInP), gallium arsenide GaAs, and germanium Ge pn junctions, are seeing demand rapidly rise as we can see reflected in the irse in cost of gallium and germanium. These solar cells are currently being utilized in the Mars rover missions. Triple-junction GaAs solar cells were also being used as the power source of the Dutch four-time World Solar challenge winners Nuna in 2005 and 2007, and also by the Dutch solar cars Solutra (2005) and Twente One (2007).A Dutch university set the record for thin film solar cell effiency using a single junction GaAs to 25.8% in August 2008 using only 4 µm thick GaAs layer which can be transferred from a wafer base to glass or plastic film.
My next post will cover my topic in more detail which is -Solar Cells from Blueberries
No comments:
Post a Comment