Research Themes
Inorganic Photovoltaic Devices
Figure 01: A solar farm using standard crystalline silicon solar modules.
Theme Leaders: Karin Hinzer and Rafael Kleiman
Presently, the largest sector of the photovoltaics industry by far has been captured by bulk and thin film inorganic materials. Inorganic materials have been well-studied by the semiconductor industry and are heavily used by the photovoltaic (PV) industry. These materials have absorption properties covering a broad range of the entire solar spectrum. Generally, they are rugged and long-lived with device lifetimes of up to 30 years or more. These materials can also exhibit solar radiation-resistance, particularly important for space applications like satellites or the international space station. The inorganic materials promise the greatest impact for near-term improvements in solar cell cost reduction in an effort to achieve comparable prices with polluting non-renewable power sources.
Figure 02: The International Space Station uses high efficiency inorganic solar cells made from GaAs.
Solar cells made from the inorganic element silicon make up more than 90% of the market. The most common silicon-based solar cells are made from either single-crystalline or poly-crystalline wafers approximately 2/10 of a millimeter thick. These two materials are essentially different “grades” of silicon. Single-crystalline is a higher grade and it performs better. However, it is also more expensive. Single-crystalline and poly-crystalline solar cells are both mature technologies and high-end solar modules using them can convert up to approximately 20% of the sun’s energy into useable electricity. Chances are you have seen silicon solar modules before, maybe on a residential or industrial building in your neighbourhood or even powering a roadside sign.
Figure 03: Inorganic thin-film solar cells made from materials like CdTe can be flexible, allowing a range of applications.
An alternative approach uses thin films of silicon to fabricate a solar cell rather than a thick wafer. The material used in thin-film cells is called amorphous silicon and it can be thought of as the lowest grade of silicon. A thin-film module can have an efficiency of up to 12%. However, because so much less material is used, thin-film modules tend to be cheaper overall. One important advantage of thin film technologies is that they can be fabricated on inexpensive supports that can be either rigid or flexible. This means it can be used for a variety of interesting applications. Some examples are: a roll-up portable laptop charger, flexible solar panels incorporated into a tent, solar panels on a car or even solar panels integrated into curved architecture.
Figure 04: Solar shingles can help offset building costs by replacing other building materials.
Other thin film technologies are based on alternative inorganic materials, including Cadmium Telluride and Copper Indium Gallium Selenide (CIGS). These devices have demonstrated similar efficiencies to the thin film Silicon technologies, and have the potential to become widely deployed via inexpensive manufacturing technologies.
A particularly promising application of either crystalline or amorphous inorganic solar cells is building-integrated photovoltaics (BIPV). In a BIPV application, solar cells can be seamlessly integrated into a buildings itself. In some cases this can even decrease the cost of construction because the solar cells may effectively replace other building materials, as is the case with solar shingles.
Figure 05: A game-changing technology developed in Canada by Morgan Solar allows sunlight to be concentrated onto a solar cell without overly bulky optical assemblies.
Certain inorganic materials, called “III-V semiconductors,” have the potential for the highest solar cell efficiencies. These materials can be used to create a stack of solar cells all within the same wafer. This is called a multi-junction solar cell and it can absorb much more light than a standard cell. However, these also are the most expensive solar cells. To reduce the cost they are usually employed in a solar concentrator system. This device is essentially a magnifying glass which focuses sunlight from a large area into a much smaller area where the solar cell is situated. This way a smaller solar cell can be used and it will produce an equivalent amount of power. The high cost of the cell is replaced by the lower cost of lenses or mirrors. Concentrator photovoltaic (CPV) systems bring the cost of energy very near to flat panel PV technologies but with the advantage of more efficient use of land area.
Inorganic materials are perhaps the most versatile category of solar cell material out there. Our research in this field may contribute to improvements across the entire spectrum of photovoltaic applications, from those that demand a high efficiency (and have a resultant high-cost) to those where a low-cost is important (and resultantly a low-efficiency), and pretty much everything in between.To Top Menu
Organic Photovoltaic Devices
Theme Leaders: Michel Côté and Mario Leclerc
Figure 06: Organic solar cells can be deposited onto very cheap flexible substrates.
Like inorganic solar cells, organic materials can be used to directly generate electricity from the sun. The difference is that the materials used for organic solar cells are molecules or polymers derived from the element carbon. “Carbon polymers” are the scientific way of saying “plastic.” The potential for organic solar cells becomes clear simply by observing how ubiquitous plastic materials have become in modern society. Plastic solar cell materials are cheaper, in some cases drastically cheaper, to produce than inorganic materials like silicon. During processing, thin-film organic solar cells can be deposited onto large, flexible surfaces at near-room temperature. This simplifies processing considerably.
Figure 07: A flexible organic solar cell can give you portable power anywhere you may need it (so long as it is sunny).
Figure 08: A funky bus shelter in San Francisco using organic solar cells on its curved roof to produce electricity.
Current commercial cell operating efficiencies are low (1%), but since organic cells are so inexpensive to produce, low operating efficiencies are acceptable. The major disadvantage of organic materials is performance degradation upon exposure to the sun. Researchers have already created laboratory cells with improved lifetime performance and 8-10% efficiency.
Our research may lead to many possible commercial applications, such as laptop-recharging briefcases, tents, umbrellas, awnings, bus shelters, and coloured window tinting (since the cells can be made semi-transparent). Some of these applications already exist. Imagine…what else is possible?To Top Menu
Hybrid Organic-Inorganic Photovoltaic Devices
Theme Leader: Ian Hill
Organic materials are comprised specifically of carbon-based molecules (plastic) while inorganic materials are made up of other substances, such as silicon. Organic-inorganic hybrid solar cells seek to capitalize on the strengths of both materials, specifically, the low-cost and strong light absorption of organic cells and the good electrical conductivity of inorganic cells. Hybrid cells encompass several different approaches, including:
Figure 09: Different colours of dye-sensitized solar cells are possible depending on the dye that is used, lending itself to applications where a unique look is desired.
- Dye-sensitized solar cells (DSSC), where an inorganic semiconducting material is coated with a strongly-absorbing organic dye layer
- Metal oxide/organic semiconductor cells, which are similar to fully organic PV devices, but one of the organic electrical contacts is replaced with an inorganic transparent metal-oxide semiconductor (such as TiO2, ZnO, SnO2)
- Cells in which traditional inorganic semiconductor materials are combined with organic materials to enhance efficiency via the light harvesting characteristics of the organic materials.
In dye-sensitized solar cells and metal oxide/organic solar cells, the niche applications of organic solar cells are retained. They are thin-film solar cells which can be cheap and flexible. They can thus be used for a variety of settings such as in building-integrated photovoltaic (BIPV) applications on flexible architecture, semi-transparent glass, portable chargers, and awnings. The goal of our researchers is to support the development of these technologies so that they may be a better choice for these applications than standard organic technology.To Top Menu
Nano-Structured Photovoltaic Devices
Theme Leader: Ray R. LaPierre
Figure 10: The narrow dimensions of silicon nanowire solar cells may allow researchers to beat previous theoretical limits by “pulling” electrons out of the material before they lose their excess heat energy.
Standard structures for solar cell design utilize materials in the form of bulk crystals, and any impurities in the crystalline structure have previously been considered to reduce device performance. Recently, researchers have begun to engineer nano- and micro-scale structures to enhance device efficiency through both electrical and optical effects. Incorporating nano-structures into PV devices has been proposed through several methods in order to exceed the theoretical limits of traditional solar cells. Optically, nano-structures can be used for light trapping, effectively extending the path of the incoming light to maximize absorption. Electronically, several proposed methods attempt to make better use of the absorbed light’s energy that is normally converted to heat, and thus wasted, in standard solar cells. The design and fabrication of nano-structures introduces new challenges at both the theoretical and the experimental levels.
Nano-structured solar cells are an emerging field and our researchers are still probing the fundamental physics of these devices in the lab. If this approach reaches commercialization it will most likely find its share of the market in some of the same areas that were listed in the “Inorganic Photovoltaic Devices” theme. However, this technology is likely to be higher in cost, and ideally performance, than your standard solar cell so it may be limited to more niche applications like concentrated photovoltaics or space applications.To Top Menu
Sources for Photos:
Figure 01: Eat Your Planet ~ November 1, 2010
Figure 02: CalFinder ~ Solar Blog page 2012
Figure 03: Got Powered ~ content upload 2011
Figure 04: Renewable Energy Magazine ~ August 22, 2012
Figure 05: The Unofficial Morgan Solar Blog ~ August 9, 2010
Figure 06: Research in Germany ~ energy technologies > research projects 2012
Figure 07: National Research Council Canada ~ archive March 24, 2010
Figure 08: Design News ~ December 16, 2010
Figure 09: Solaris Nanosciences ~ applications 2012
Figure 10: Science Prog ~ May 22, 2008