A novel solar cell based on Cu(In,Ga)Se2 was developed in the Laboratory for Thin Films and Photovoltaics (Empa, Switzerland). This device has an efficiency of almost 19%; furthermore, it can be produced at a lower cost than other solar cells, as it uses a flexible polymer film (polyimide) as a substrate.
How Do Solar Cells Work
Solar cells are devices that convert the light from the sun directly into electricity; they are also called photovoltaics (PVs). The conversion from light into electricity is known as the photoelectric effect; the materials capable of performing this typically are semiconductors. In fact, it is the bandgap present in the semiconductor which absorbs the light and, consequently, generates electric charges. These charges can be collected to deliver energy in the form of electricity.
PVs can be employed to produce energy from the sun, but more development is needed to produce the maximum possible energy from this renewable source at a price competitive to other sources of energy.
The efficiency of a PV system varies, depending on the method used for making the solar cell and the value of the bandgap energy of the employed semiconductor material. The maximum theoretical value is about 30% for a single-material solar cell, and can be obtained with a bandgap of about 1.0 – 1.5 eV. This value, however, can be increased by combining together several PV devices of different bandgap energy; these are called tandem structures. In this way, more light from the sun can be absorbed. Efficiencies between 32 and 42%, for instance, have been reported for PV multi-component systems.
The Costs of Solar Energy
Up to a few years back, manufacturing solar cells was rather expensive, as the manufacturing volume was low and the processes were quite complex. This made the cells unsuitable for commercial applications at large scale, as the price of the electricity produced with these devices would not be competitive with electricity produced from conventional energy sources such as fossil fuels. Progress during the last years has already reduced the cost; a further reduction is expected with the use of new technologies. The choice of the materials used for commercial PVs, therefore, has to be a compromise between cost and efficiency. Ideally, the solar cells should be made by simple and cheap processes; at the same time, the efficiency should also be as high as possible.
The Materials Employed in Creating Solar Cells
Today, three materials are commercially used in thin film PV manufacturing: amorphous silicon, (a-Si), cadmium telluride, (CdTe) and copper indium gallium diselenide (Cu(In,Ga)Se2 – CIGS). According to the latest figures, the proportions are 60, 27, and 13%, respectively.
Cadmium telluride thin films on glass are, at present, the cheapest PVs on the market; they are produced by First Solar Company at a cost of about 0.76 $ per peak watt.
Regarding CIGS, the forecasts for the future show that they will become the main thin film PVs, reaching about 40% of the total market by 2020. The reason for this change is the potentially lower manufacturing cost of CIGS devices compared to its competitors. The efficiency for these systems has also improved greatly in recent times: earlier this year, data were published on CIGS thin films deposited on rigid glass substrates, showing up to 20.3% efficiency.
PV devices made of CIGS thin films on a flexible substrate were prepared in the Laboratory for Thin Films and Photovoltaics, by Professor A.N. Tiwari and his coworkers. The material they used is polyimide (PI), a plastic polymer.
The use of a polymer as a substrate for CIGS solar cells has been considered before. However, the PV devices made in this way showed always poorer efficiency until recently: the maximum value reported was lower than 16% and well below the 20.3% mentioned above for CIGS PVs deposited on glass.
In their work, however, Tiwari and his research group achieved an efficiency of 18.7%; this is the highest reported up to now for CIGS on a polymer and for any type of solar cell grown on a flexible substrate.
More Uniform Gallium Distribution
The main novelty in this solar cell is the way gallium (Ga) is distributed in the thin films. Previously, this element showed a lower concentration in the center of the film and higher at the top and bottom surfaces. In this case, on the contrary, Ga shows a much more uniform distribution throughout the whole film. This affects the value of the bandgap energy and, consequently, the efficiency of the system.
Very Competitive Solar Material
When asked about these data, Professor Tiwari told Decoded Science:
“These results are important, because they show a new way of improving the efficiency of CIGS on PI. Theoretically, the gallium distribution could be optimized even more; this could lead to a material with even better performance.
The efficiency of this cell is now comparable with the ones prepared on rigid glass substrates. In this case, however, we have further advantages: the material is lighter and flexible. This makes it easier and cheaper to prepare, as the roll-to-roll manufacturing method can be used. Considering a large scale production, the manufacturing costs could potentially be as low as 0.6 $ per peak watt. Our idea now is also to use other low cost metals as substrates, such as aluminum, mild steel and stainless steel for achieving similar performance.”
The complete study on the development of this PV device was published in Nature Materials.
A. Chirilă et al. Highly efficient Cu(In,Ga)Se2 solar cells grown on flexible polymer films. Nature Materials, DOI: 10.1038/NMAT3122, 2011. Accessed October 12, 2011.
Gangadhar Adiboina. Thin Film Photovoltaic (PV) Cells Market Analysis to 2020. Alternative Energy eMagazine. Accessed October 12, 2011.
M.A. Green et al. Solar cell efficiency table (version 37). Progress in Photovoltaics, DOI: 10.1002/pip.1088. Accessed October 12, 2011.
P. Jackson et al. New world record efficiency for Cu(In,Ga)Se2 thin-film solar cells beyond 20%. Progress in Photovoltaics, DOI: 10.1002/pip.1078. Accessed October 12, 2011.
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