The transition to a clean energy society is an unstoppable movement, one that will benefit consumers, industry, and the environment. This push towards a future powered by the use of renewable energy resources will be made up of numerous components, with solar energy one of the main constituents.
Solar energy has the potential to completely meet the current energy demands of society but two factors constrain the full utilization of solar power. The first factor is the manufacturing cost, and the second factor is the electrical energy efficiency conversion rate. An ideal situation would be for a solar cell with a very high efficiency rate with low manufacturing costs, but that ideal balance has not been reached.
Using current technology, the aim is to manufacture a solar module at an acceptable cost while still maintaining a sufficient efficiency rate to convert a worthwhile amount of electricity. Research into this field is constantly looking at ways of improving the efficiency rate and lowering the manufacturing costs, and with the introduction of thin film solar cells, this shift to an idealized balance has begun.
Traditional High-Cost Solar Cells Based on Silicon
Silicon-based solar cells have been the solar cells of choice for more than fifty years. Silicon is a relatively abundant element, able to provide a fairly acceptable electrical efficiency conversion rate of between 17 % and 22 % (about two-thirds the theoretical maximum, known as the Shockly-Queiser limit).
These silicon-based solar cells collectively made up the first generation of solar cell technology. However, while the efficiency rate was acceptable, the manufacturing costs were deemed too high. Even though silicon is used for solar cells, the element is actually not the best choice: a poor absorber of light with an inherent thickness requirement of several hundred micrometers (also called microns), silicon is only the material of choice due to its long history in the microelectronics industry, and to the relatively high Shockley-Queisser limit.
Replacing Silicon-Based Solar Cells with Thin Film Solar Cells
The solar industry has made advances, however, to move away from using silicon to produce solar cells that are lower in cost. Thin film solar cells, so-called due to their very thin dimensions, are changing the solar cell industry both from a manufacturer’s point-of-view and from a consumer point-of-view.
From a manufacturer’s viewpoint, thin films dramatically reduce the amount of material required for the production of a solar cell module. From requiring a thickness of several hundred microns, thin film solar cells reduce the material thickness required to just one micron. In addition, during the manufacturing process, thin films can be deposited over a wider area allowing for volume manufacturing. These two features, undoubtedly, reduce the material costs associated with the manufacture of thin film solar cells, aiding thin films to be cost-effective.
From a consumer viewpoint, thin films have allowed for the opportunity to integrate photovoltaic material into the building architecture. This spawned a new industry segment, termed Building-Integrated Photovoltaic (BIPV), and this has been one of the fastest growing sectors in the solar cell industry.
Types of Thin Film Solar Cells
Three main types of thin film solar cells have been commercialized: cadmium telluride (CdTe); copper indium gallium selenide (CIGS); and dye-sensitized solar cells (DSSC). Each type is certainly unique, and in recent years, these three examples of the progress and innovation found in the solar energy industry have experienced a considerable boom in their roll out. However, this article will focus on CdTe thin film solar cells, the advances being made and the practical implications of this unique non-silicon based solar cell.
Innovative Solar Cells without Silicon
The quest to replace silicon with alternative semiconducting layers is not a recent phenomenon. In terms of exchanging silicon with CdTe, research began in the 1950s. CdTe, a crystalline semiconducting compound, offers numerous and interesting properties: a band gap of 1.45 electron volts (eV); a high optical absorption feature; and a high chemical stability. These three properties, coupled with other electrical and chemical properties, make CdTe an excellent candidate to supplant silicon.
The best way to integrate CdTe into a solar cell is through the formation of a heterojunction. When CdTe, a good absorber (p-doped semiconductor layer), is encased in a solar module with a layer of cadmium sulfide (CdS), which is a good n-doped transparent window layer, an innovative solar cell is created. The CdTe/CdS heterojunction possesses a high fill factor (the ratio between the power output and the product of the open circuit voltage and the short circuit current). For a solar cell to be effective, a fill factor greater than 0.7 is required; CdTe/CdS solar cells have a fill factor of 0.77, making this combination a suitable one. Together with a transparent conductive oxide layer, and the two contact layers, a thin film solar cell is produced. Figure 1 shows an illustration of the composition of a thin film solar cell of CdTe/CdS.
Figure 1: Illustration showing the layer-by-layer composition of a CdTe/CdS thin film solar cell. Credit: Solar&Energy
The CdTe/CdS thin film solar cells are not without issues, but these issues are focal points for the research community to investigate; these investigations will lead to developments that will further enhance this novel solar cell. Such issues include: heterojunction contact point; transparency of TCO layer and CdS layer; ohmic contact between the CdTe layer and the back contact; and improvements to the glass layer. For CdTe thin films, the main issues are concerned with the fabrication of the CdTe layer, the ohmic contact, and the glass layer.
Challenges & Opportunities of CdTe Thin Films
[i] Fabrication technique
The meeting point between the CdTe layer and the CdSe layer results in the enhanced properties of the solar cell, thus making this heterojunction contact point vital to the solar cell, and also the point of focus for the research community to improve further the thin film solar cell. Therefore, the choice of fabrication method is of importance.
Techniques for fabrication of both CdS and CdTe: closed space sublimation (CSS), radio frequency sputtering (RF sputtering), and chemical bath deposition (CBD) for CdS preparation; and CdTe preparation utilizes CSS, electrodeposition (ED), and screen printing (SP). Each technique is flawed, but the manufacturers and academics are pursuing ways and techniques in which to reduce these flaws. For example, for RF sputtering, one way to circumvent the flaw is to carry out the procedure in an inert environment (like argon) containing triflouromethane (CHF3).
Currently, the best fabrication method to use where the heterojunction contact point is enhanced is CSS for the CdTe preparation and CBD for the CdS preparation. In 2003, the National Renewable Energy Laboratory (NREL) confirmed the efficiency rate for a CdTe/CdS thin film to be 16.3 %.
For a pure cadmium telluride thin film solar cell, one minus the CdS layer, the two industry giants first Solar and General Electric (GE) have produced solar cells with enhanced efficiency rates due to their choice in fabrication. First Solar, in particular, has produced a cell in the laboratory with a 17 % efficiency rating, tested by NREL. However, there is usually a time lag between achievements in the laboratory being transferred and scaled-up for commercialization, but with the proven technique and experience at First Solar, the wait should not be for too long. Apart form this record-breaking solar cell efficiency, First Solar offer thin films with an average solar cell efficiency of 11.7 %. The thin film solar cell on offer by GE, through their acquisition of PrimeStar Solar Inc., has an efficiency rate of 12.8 %.
[ii] Back contact
The CdTe layer has a high electron affinity, therefore, an ohmic contact is required to maximize hole transport; the p-type CdTe layer will result in a Fermi level between the absorption layer and the metal layer, resulting in a decrease in hole transport and thus, a reduced efficiency rate.
A conventional way to create an ohmic contact would be to first etch the surface with nitric-phosphoric acid, producing a layer rich in telluride ions. By then depositing copper (Cu), a Cu-Te alloy could be formed that will act as a suitable ohmic contact.
Using a buffer layer is one good solution to this ohmic contact point issue. For example, zinc telluride (ZnTe) could act as a buffer layer between the CdTe layer and the back contact, offering an electrical field that reduces recombination losses on the back contact.
Regardless of the technique selected, which range from the conventional to the novel, producing an ohmic contact will ensure that the efficiency rate of the thin film is not diminished.
[iii] Glass cover
One interesting aspect to reducing the manufacturing costs would be to replace the glass sheet with a suitable replacement. Empa, a Swiss-based research institute, together with DuPont™, has done just that. By using Kapton ®, a material that is 100 times thinner and 200 times lighter than glass, Empa produced a CdTe thin film solar cell with an efficiency rating of 13.8 %. By using Kapton ®, the weight of the solar cell had been reduced dramatically offering consumers a lightweight and flexible solar cell while reducing the balance of costs for the manufacturers and installers.
Environmental Concerns of Using Cd-Based Thin Film Solar Cells
A flexible, thin solar cell is a welcome development, pushing the roll out of this technology even wider. With the flexible nature, consumers are free to purchase and install these cells without adversely affecting the value of their property. In fact, quite the opposite: thin film solar cells are able to blend into the environment, and in most cases, significantly appreciates the appearance of the structure which might lead to an upward movement in terms of the value of the property.
However, some concerns are raised when cadmium and tellurium are exploited. Cadmium possesses toxic properties, and tellurium is one of the rarest elements on earth. Numerous studies have shown that CdTe thin films are safe, with the toxic elements safely encapsulated. First Solar, a leader in this field, has gone one step further by developing a pioneering recycling technique to ensure that thin film solar cells sold by the company will be recycled when damaged or when they reach the end of their natural life cycle. Such programs will reduce any anxiety consumers might have.
Future Outlook of Thin Film Solar Cells
The aim of utilizing renewable energy technology is to facilitate the transition from our current practice or using polluting fossil fuels to one that harnesses the power of nature. Solar energy, being one of the cleanest and most abundant natural energy resources, is just one of the many routes to achieve this major shift.
For the short-to-medium term, first generation silicon solar cells will still dominate due to efficiency concerns. However, the strides being made by companies such as First Solar and GE will hasten the changeover to thin films. And with the added benefit of the construction industry integrating thin film solar cells, residential and commercial buildings will slowly begin to change the appearance of the towns and cities we live in.
In time, the efficiency rates of CdTe thin films will be comparable to first generation solar cells. With comparable efficiency rates, low manufacturing costs, flexible nature and with the ability to integrate into the built environment, CdTe solar cells offer a powerful set of reasons to go solar. This point will mark the real boom in adopting cleaner sources of energy, bringing the transition to a clean energy society one day closer.
