By Mattie DeDoes
Energy viability ultimately comes down to the cost and benefit relationship. In a society that has become more complex, and with the true costs of traditional fuels rising, the future of solar energy continues to brighten. However, in order to gain a market advantage, the solar energy industry continues to work for improved cost-benefit technology.
The photovoltaic (PV) power industry has long been dominated by crystalline silicon solar cells because of their cost-effective production. When you consider a solar installation, you may be picturing panels which utilize the standard silicon solar cell. However, by using the same PV physical principles with a different configuration of materials, there is potential for a cheaper alternative.
Thin Film Cells
Thin film cells currently only occupy about 5% of the market share of solar energy production, but that percentage seems ready to increase. While these cells have long been limited by their low efficiency, recent developments have greatly improved the operational efficiency of these alternative designs. If this progression continues, thin film solar cells could soon compete with crystalline silicon—both in cost and performance.
Both thin film and crystalline silicon cells utilize the phenomenon of the photoelectric effect, in which light striking a semiconductor can produce an electrical current. The difference between the two types of solar cells is in the materials chosen and the method of construction. Crystalline silicon (abbreviated c-Si) solar cells are constructed by placing metallic contacts on the front and back surfaces of a piece of silicon roughly 200 microns thick. In contrast, thin film cells contain a thin layer of semiconductor (approximately 1 micron) deposited onto a substrate of glass, plastic or metal. The primary reason for the improved outlook for thin film technology is due to the nature of the manufacturing process for thin film cells. The process lends itself to lower-cost, higher-volume mass production more easily than the process for constructing c-Si cells.
By using less of the semiconducting material, the cost of materials for a thin film cell is drastically reduced when compared with conventional designs. However, the thickness limits the efficiency of the cell’s energy production. With less material, there is less opportunity for incoming light to interact with the semiconducting material and produce energy, and a greater probability that the rays of sunlight will pass right through.
Because of low efficiency, thin film solar panels require a much larger area than conventional c-Si panels in order to achieve the same power output. As a result, bulk silicon has dominated the household solar installation industry because of its ability to provide adequate amounts of energy from a rooftop-sized solar field. However, in recent years, laboratory-based thin film efficiency has risen to levels similar to c-Si cells, making them a viable competitor in the near future.
The Materials
Thin film solar cells have been constructed using a variety of semiconducting materials. The four most prominent of these materials are discussed below.
Cadmium Telluride (CdTe)
Cadmium Telluride cells are the most common form of thin film solar cell, accounting for more than 50% of the thin film market. According to the National Renewable Energy Laboratory (NREL), the current CdTe efficiency record for research cells in the lab is held by First Solar at 21.5%. First Solar has steadily increased this record from 17% in 2011, showing an impressive rate of development. In comparison, the world record efficiency for single-crystal Si cells stands at 25%, but has increased by less than 1% in the previous 15 years.
The production of CdTe cells presents two main issues that must be addressed, obtaining tellurium and disposing of cadmium.
With an abundance of about 1 µg/kg (similar to that of platinum), tellurium is an extremely rare element on Earth. The main source of tellurium is as a byproduct of a copper purifying process known as electrolytic refining. Unfortunately, it takes about 1,000 tons of impure copper to produce about 1 kg of tellurium via this process.
The issue with cadmium is not one of acquisition, but rather of safety. The Occupational Safety and Health Administration (OSHA) reports that exposure to cadmium in its basic form are highly toxic, targeting the body’s cardiovascular, neurological, and reproductive systems, and can cause cancer. However, cadmium is relatively harmless in its CdTe compound form as used in the solar cells. While there are concerns about the chance that CdTe will break down its elements, NREL tests determined that CdTe must be heated to 1,041 degrees Celsius before it breaks down. Typically, most rooftop fires burn around 800-900 degrees Celsius.
Copper Indium Gallium Selenide (CIGS)
In a Copper Indium Gallium Selenide (CIGS) cell, the semiconducting material is made up of a mixture of solid solutions of copper indium selenide and copper gallium selenide. Changing the ratio of the two substances also changes the bandgap energy - the lowest frequency of radiation that can be absorbed and converted to electricity by the material.
The current world record for CIGS cell efficiency was set at 21.7%, by the Center for Solar Energy and Hydrogen Research in Stuttgart, Germany in September of 2014. This efficiency record is comparable to that of CdTe, showing little difference in performance.
CIGS technology does not encounter the issues related to acquisition and disposal of materials that are present in CdTe cells construction. However, traditional production methods for CIGS cells uses a process called sputtering, which results in higher production costs associated with CIGS cells in comparison to their CdTe counterparts. Newer CIGS cells, including the record-setting German cell, used a different technique. This process, known as co-evaporation, is predicted to more easily lend itself to mass production than previous methods.
Amorphous Silicon (a-Si)
Most early thin film cells were based on amorphous silicon. Amorphous (non-crystalline) solids do not exhibit the long-range ordered pattern of atoms that are present in a crystalline solid. Instead, the atoms are arranged in a more random distribution.
The current efficiency world record for the a-Si cell is 13.4%, set by Sharp in 2013. Because of its low efficiency, it is unlikely that a-Si technology will compete with CdTe and CIGS technologies in the future.
However, there are specific applications that can utilize a-Si technology effectively. Devices that consume very little energy can be powered by a few a-Si cells. For example, any handheld solar-powered calculator uses a-Si cells as its power source.
Gallium Arsenide (GaAs)
The Rolls Royce of solar cells, gallium arsenide technology (GaAs) is the highest-performing and most expensive thin film technology on the market. The efficiency world record for GaAs thin film cells was recorded at 28.8% by Alta Devices. GaAs cells outperform even c-Si cells, producing more than two times the electricity per unit area and per unit weight. In addition to superior efficiency, an NREL test showed that the performance of GaAs cells did not diminish with increased operating temperature, while c-Si suffered.
Unfortunately for GaAs enthusiasts, standard c-Si panels remain much more affordable because of the abundance of silicon and the refined production process. Up until now, applications for GaAs solar cells have been restricted to satellites and concentrating photovoltaics, in which lenses, mirrors, and tracking mechanisms focus large amounts of light onto small, high-efficiency cells.
Pros and Cons of Thin Film Cells compared to c-Si
Pros
- Standard c-Si cells are thick and rigid, while thin film cells can be made flexible
- Much more resistant to high temperatures than c-Si cells
- Lower cost of materials
- Newer production processes could be easily scaled for mass production
Cons
- Because of how thin they are, thin film cells degrade faster under weathering conditions.
- Even though the cost of the materials is less for thin film cells, established economies of scale allow for cheap mass production of c-Si cells.
- Lower module efficiency at present.
- No clear-cut technology available.
To clarify, the cost of materials alone is less for most thin film cells than it is for c-Si cells. However, because there are so many conventional silicon cells being constructed, the overall manufacturing process becomes cheaper because these modules can be mass-produced.
Thin film technologies clearly possess attributes that could make them competitive with c-Si in the near future, but more improvement is needed. With multiple choices of materials for thin film development, there is no single technology that stands out above the rest in all categories (efficiency, cost, availability, safety). As a result, these technologies are currently undergoing competitive development that may cause one or more of them to eventually emerge as a winner.
If thin film technologies continue to evolve and become more widespread in application, the advantage that c-Si currently holds because of its mass production, economy of scale could decrease. In addition, certain applications might be engineered that maximize the advantages of thin film cells, such as high-temperature settings and curved surfaces. If thin film cells continue their rise in efficiency, these devices are sure to make an impact on the world of solar power in the near future.
Additional Resources:
- NREL chart for world record solar cell efficiencies
- Fraunhofer Institute’s Photovoltaics report from October 2014
