2011-10-19 | Editor : Dr Dino Ponnampalam 6138 pageviews

Thin Film Solar Cells & the Construction Industry

The prospect of harnessing solar energy to meet the energy demands of the 21st century offers a clean and reliable solution to one of the biggest issues facing the human population. Going forward, how we source our energy and how we use this energy are two aspects to the energy predicament that have resulted from using finite fossil fuels. However, with every problem or issue comes an opportunity and now might be the time to take advantage of this window and make fundamental changes to the way society utilizes energy, especially when it comes to buildings.

Embedding Solar Energy Technology into the Construction Industry

Solar energy has had a long and fruitful relationship with the construction industry; history is littered with designs and building structures where the architecture was such so as to maximize this natural resource. As electricity is a relatively modern concept, ancient civilizations oriented their homes to capture as much sunlight as possible to provide lighting, and also to keep their homes and buildings warm during the colder seasons.

This technology has progressed by leaps and bounds since ancient times, but the principles of utilizing solar energy into society still remain. By marrying ancient concepts with modern technology, by combining the best of the old with the best of the new, solar energy utilization could be extended to include real estate applications in various creative and innovative ways rather than just in the traditional way.

Using Thin Films of Solar Cells in Boosting Economic Benefit

The main material component of a solar cell is silicon. This inexpensive, abundant material is the material of choice when considering solar cell technology. First generation solar cells, composed of monocrystalline silicon, offer electrical energy conversion rates of up to 20 %. These first generation solar cells make up the backbone of the solar industry, offering advantages that are currently unrivalled. The major advantages are the cost of manufacture and the electrical energy conversion rate, factors that are vital to the solar energy industry, both to the manufacturing side and the consumer side.

While first generation solar cells offer numerous advantages, the economics associated with monocrystalline silicon solar cells were unfavorable for widespread adoption. This led to the development of less expensive solar cells, utilizing less silicon. This was achieved by the production of a thin film solar cell (TFSC). These TFSCs were constructed by depositing layers of thin films of semiconducting material onto a substrate, with the thickness of each layer precisely controlled. This control, coupled with less material being required due to the thin film nature, allowed for economic gains to be realized.

Overview of the Various TFSC Compositions

While first generation silicon solar cells are either monocrystalline or polycrystalline and in bulk or wafer form, TFSCs are thin layers comprised of nanocrystalline, polycrystalline, amorphous silicon, or black silicon. The different compositions are determined by the differing grain sizes. There are numerous types of TFSCs, ranging from amorphous silicon (a-Si) and cadmium telluride (CdTe) solar cells to copper indium gallium selenide (CIGS) solar cells and dye-sensitized solar cells (DSSC).

[a] Amorphous Silicon (a-Si)

With no grain size, silicon in this form is termed amorphous. Therefore, amorphous silicon is the non-crystalline form of silicon. Though offering lower electrical conductance levels, amorphous silicon is very flexible and can be manufactured at extremely thin ratios (around 1 % of crystalline silicon solar cells). As the cost of silicon is one of the biggest factors in the cost of a solar cell, reducing this expense will allow for savings to be made during the manufacturing process.

[b] Nanocrystalline Silicon (nc-Si)

This form, also known as microcrystalline silicon, is similar to amorphous silicon but differs in that nanocrystalline silicon contains grains of crystals in the mix, and has a grain size less than 0.1 micrometer (μm). There are numerous advantages to using nanocrystalline silicon over amorphous silicon. For instance, they have higher electron mobility (if cultivated in the appropriate way). In addition, they are able to absorb in a wider part of the electromagnetic spectrum. Moreover, they are easy to fabricate, and they are more stable than amorphous silicon. Possessing a similar band gap as crystalline silicon, around 1.12 electron volts (eV), nc-Si can be used in the construction of a multijunction tandem solar cell.

[c] Polycrystalline Silicon (poly-Si)

Polycrystalline silicon is characterized by having small grains of crystals with a grain size of between 0.1 μm to 50 μm. A key component to the construction of the solar panel, polycrystalline silicon offers manufacturers an alternative and inexpensive route for the production of solar cells but at lower efficiencies than what could be obtained with monocrystalline silicon solar cells.

[d] Black Silicon

Black silicon, strictly speaking, is not considered to be a natural state of silicon; black silicon was discovered by chance. At first, this material was the unwanted side reaction of the ion etching process, but was soon discovered to be something unique. The surface structure is needle-shaped, with each needle around 10 μm in height and less than 1 μm in diameter. These needles dramatically reduce the feature of light being reflected (around 20 – 30 % in conventional solar cells) to about 5 %, boosting the efficiency of the solar cell.

Alternatives to Silicon-Based TFSCs

In recent years, the thin film solar industry expanded to include non silicon-based solar cells. Instead of silicon, solar cells were constructed with semiconducting material(s) such as cadmium and copper. Due to their thin dimensions, manufacturing techniques such as roll-to-roll printing (a technique also used to produce paper and money) could be utilized to produce these thin solar cells. Two of the more famous non-silicon thin films include cadmium telluride (CdTe) and copper indium gallium selenide (CIGS), but a particular mention must go to dye-sensitized solar cells (DSSC).

[i] Cadmium Telluride (CdTe)

Using the elements cadmium (Cd) and tellurium (Te), a crystalline compound of CdTe is formed and this form is the lowest cost-form of thin film solar cells. This feature makes CdTe thin films particularly attractive to manufacturers, and is one reason why CdTe thin films accounted for 6 % of all solar cells in 2010.

In spite of having a low-cost base, CdTe thin film solar cells do have some material problems. For example, the availability of tellurium is an issue as tellurium is extremely rare. Furthermore, the toxicology of cadmium presents challenges as well. These two points are in addition to the technological problem, that of a lower efficiency rate. Compared to monocrystalline silicon solar cells - the industry benchmark - the efficiency rate of CdTe thin films range from 10 % to 17 %. However, this efficiency rate is constantly improving due to advances made in academia and industrial research centers. Both private and public companies are involved in the manufacture of CdTe thin films, with First Solar by far the market leader.

[ii] Copper Indium Gallium Selenide (CIGS)

CIGS are known as direct band gap semiconductors, and this feature is exploited when considering solar cells. A direct band gap semiconductor sees an equal number of holes and electrons in the conductance band and the valence band, allowing for an electron to directly emit a photon. CIGS, being a direct band gap semiconducting material, absorbs sunlight at a greater rate and this allows for less material to be utilized, allowing for cost savings to be made. By absorbing more sunlight, the efficiency rate will increase. For CIGS solar cells, the typical efficiency rate is about 20%.

Numerous companies are involved in the manufacture of CIGS such as Nanosolar (using their roll-to-roll printing technique) and Axuntek Solar Energy Company, and even Taiwan Semiconductor Manufacturing Company (TSMC). With the lower costs, higher efficiencies comparable to monocrystalline silicon solar cells, and fewer environmental and material issues (the main one being with indium), CIGS thin film solar cells has a promising future.

[iii] Dye-Sensitized Solar Cell (DSSC)

DSSCs are known as Grätzel cells, named after one of their discoverers, Michael Grätzel. Using inexpensive material, and avoiding complex and elaborate manufacturing steps, DSSCs are extremely encouraging for the future use of thin film solar cells and their deployment.

DSSCs are composed of an electrolyte coupled with a photosensitized anode (titanium dioxide), and are surprisingly strong as flexible thin sheets. DSSCs are able to resist most commonly occurring minor natural events, such as debris being blown onto the DSSC or even hail. In the laboratory, the electrical energy efficiency conversion rate can be high, but in normal operation, DSSCs offer an efficiency rate of 10 – 12 %.

This lower efficiency rate is not an issue when considering certain applications such as rooftop installations, but for large-scale projects other solar energy technologies might be more suitable. However, research is currently pushing the boundaries of this innovative technology to boost the efficiency rate. Currently, Dyesol and Tata Steel are focusing on being market leaders, with a recent announcement of the world’s largest DSSC manufacturing plant.

Changing the face of the Building Industry Through TFSC Technology

The thin film solar technologies described in this article have changed the face of the solar industry. First generation solar cells still form the backbone of the industry, but these second-generation (CdTe and CIGS) solar cells and third-generation solar cells (DSSCs) have opened up new and practical applications.

With first generation solar cells, a lot of space was required whether in an open field or on top of a building structure. Now, with these thin and flexible sheets of solar cells, creative applications have allowed for a transformation of the building industry.

Building-Integrated Photovoltaics (BIPV) allow for photovoltaic material to be integrated into traditional building material, instantly harnessing our built world in generating electricity from the sun. Rooftops and building façades no longer remain idle, but are now ‘solar energy hotspots’ allowing consumers and businesses the opportunity to harness the power of sunlight.

With thin film solar cells being flexible but robust, a natural fit would be with BIPV applications. Not being thick and fixed, like first generation solar cells, allows for ‘blending’, providing the building owner with an attractive option when constructing or restructuring. An extension to this, and again due to the development of thin film solar cells, concerns the windows found in a building structure. Using thin film technology, it is now possible to lace or construct glass windows to generate electricity, significantly accelerating the roll out of thin film solar cells.

The Promising Future of Thin Film Solar Cells

Thin film solar cell, both silicon-based and non silicon based, are promising solar applications that can help make the transition from polluting sources of energy to utilizing cleaner sources. By integrating thin films with the built environment, electricity generated from sunlight could be harnessed better and used in so many ways.

Due to this potential, research has been carried out to further make efficiency gains and manufacturing advances. Recent developments include using black silicon to boost the efficiency, and even the production of honeycomb nanostructures on the surface, effectively making the silicon thin film solar cell a three dimensional solar cell. This inevitably all points to a bright, and thin-film solar future.

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