Ultimately, the world will run out of fossil fuels. Meanwhile, as the world fights over fossil fuels, our planet hangs in space a mere 93 million miles from a giant fusion reactor that’s 1.4 million kilometers wide. The sun gives us about 1.37 kilowatts per square meter of energy. If it were somehow possible to convert 100 percent of the sun’s energy into usable power, we would need only one hour of it to power the Earth for an entire year.
Using the sun for energy is hardly a new concept. Nor are photovoltaics (PV), which convert sunlight into electricity; the technology has been under commercial development since the 1970s, though up until recently, it remained difficult to achieve economies in manufacturing to make PV affordable. To date, solar panels large enough to fulfill significant energy needs have been neither very cheap nor very portable.
While the military has developed folding, semi-portable solar panels for installations in remote locations, few organizations or individuals have the kinds of budgets of the Department of Defense. Not to mention that most solar panels to date have been opaque, rigid and hard to use in places that could use a little supplemental energy from PV: atop a car or a building’s windows.
The future practicality of PV — lower price point and putting PV cells onto less rigid surfaces so they can be “wrapped” or molded around buildings, cars and even people, in the form of PV-cell clothing — seems to lie in making PV cells flexible. Enter the plastics industry and the development of thin-film solar cells (TFSC), also called thin-film photovoltaic (TFPV) cells. TFSCs are made by depositing one or more thin layers, i.e., films, of photovoltaic material onto a substrate. The thickness of the layers is small, ranging from a few nanometers to a few tens of micrometers. Existing thin-film processes generally use semiconductor materials such as copper, indium, gallium, cadmium and selenium to create the PV cells.
While the familiar crystalline silicon (c-Si) rigid solar panel process still commands the majority of the PV market, thin-film PV is gaining traction, largely due to lower manufacturing costs and more applications, as its flexibility allows it to be deployed in far more ways than rigid panels. Today, thin-film cells make up about 18 percent of the global PV market.
TFPVs can be made flexible and built into roofing, wrapped around buildings (called building integrated photovoltaics, or BIPV) and integrated pretty much anywhere a solar power source is required. Their advantage over c-Si panels is that they are lightweight, less likely to being lifted off a building by a heavy wind and can even be walked on, on a roof surface, for example.
The potential of thin-film PV is huge. Analyst GBI Research has projected that thin-film production will grow 24 percent from 2009 levels to reach 22,214 megawatts in 2020. GBI expects growth to be especially robust in Europe and the United States in the short to medium term, while Asian countries — China, Japan and India — will see accelerated growth over the medium term. Germany is currently the strongest player in the thin-film PV market and is likely to remain so in the future.
Thin-film solar cells are usually broken out into several categories based on the type of photovoltaic material used in their manufacture. Most TFPV-finished products fall into one of four categories: amorphous silicon (a-Si) and other thin-film silicon (TF-Si), cadmium telluride (CdTe), copper indium gallium selenide (CIS or CIGS) and dye-sensitized solar cell (DSC) or other organic solar cells.
Organic PV film uses organic electronics made of conductive organic polymers. The largest player in organic PV film has been Massachusetts-based Konarka Technologies Inc., which is currently the only company in the world marketing organic PV (OPV) materials in the form of a product called Power Plastic. The material has been used on portable shades, umbrellas, windows, tents, roofs and shelters, as well as in bags and chargers for portable consumer electronic devices.
A new solution to solve the world’s energy problems? Not really…not yet. While OPV materials are cheaper to produce in large quantities than c-Si solar cells, OPV still suffers from poor conversion rates, a measure of the efficiency of the solar cell. A 10 percent conversion rate for a solar cell has traditionally been considered good. (Sanyo Corp. set the record in 2010 for the world’s most efficient solar cell, clocking in an energy conversion efficiency of 20.7 percent.)
Konarka admits that its conversion efficiency of 3 percent is still too low, and the company hopes to reach an efficiency of 10 percent by 2015. Currently, OPV also has a short life span — only a few years — so Konarka is also striving to increase the product’s life span to 10 years by 2015. But an inability to secure funding forced the company into bankruptcy last month. Konarka management noted that the company had received potential offers to either acquire it or provide further funding. Interestingly, one offer came from the government of China.
The earliest success with thin-film solar materials came from using non-crystalline silicon PV cells and combining them with a membrane to create amorphous silicon (a-Si) PV film. This type of film has a better conversion efficiency than organic PV film at this time, and it’s extremely lightweight; the thickness of the amorphous silicon PV film is only about 1 millimeter, resulting in about one-tenth of the weight of conventional glass PV products. It does have a tradeoff, though, in being more costly to manufacture than OPV.
Amorphous silicon PV film is often applied to ethylene tetrafluoroethylene, or ETFE, a fluorine-based clear plastic that is being used to create roofs and even entire buildings in green building projects.
Copper Indium Diselenide
This thin-film PV material, made from a semiconductor material composed of copper, indium, gallium and selenium (as the name implies), makes up a still-tiny portion of the thin film market, but it has perhaps the highest expectations, as researchers have been able to achieve conversion efficiencies that approach those of crystalline silicon-based solar cells. In 2008, Researchers at the U.S. Department of Energy’s National Renewable Energy Laboratory were able to achieve 19.9 percent conversion efficiency — only a few tenths of a percent behind the highest efficiencies measured with multicrystalline silicon-based solar cells.
Earlier this month, Nanosolar Inc., in San Jose, Calif., a manufacturer of thin-film photovoltaic cells and panels based on copper indium gallium diselenide (CIGS) technology, announced it had raised $70 million in new capital, in addition to the $20 million it raised earlier this year. The company has been one to watch since achieving a lab-tested top cell conversion efficiency rate of 17.1 percent.
Thin-film PV based on the semiconductor material cadmium telluride has attracted some very large players thanks to its potential for high conversion efficiencies. (The record stands at 16.5 percent.) The dominant company in this sector is First Solar, which is owned by an investment arm of Wal-Mart. Last year, General Electric announced its intent to build the largest photovoltaic plant in the country, and the $600 million plant, which will be completed and operational in 2013, will be based on CdTe technology. One of CdTe’s major drawbacks, though, is the toxicity of cadmium, which means it’s subject to rigid regulation.
Printed PV, Thanks to Nanotechnology
Printed electronics is an exciting application area, and the technology is being used to create thin-film PVs with the help of nanotechnology-based silicon
printing processes. These PV materials can be printed on many different surfaces, and though the process is occasionally used on stainless steel, one of the more exciting applications is the printing of PV cells onto inexpensive, flexible film and even ordinary packaging materials, allowing for the integration of PV cells onto a wide variety of materials and a broad set of applications.
Obviously, not everything is rosy in the thin-film PV industry, and many companies have just not been able to continue production and remain afloat, generally due to a dearth of investment funds. Since April 2010, 13 thin-film PV companies have gone bankrupt or shuttered their facilities, reported PV Tech.
Another drawback that has dogged the industry is the fact that the prices for traditional c-Si cell solar equipment have come down in recent years and continue to drop, a process that has slowly been eroding the price advantage of thin-film. Without this strong price differential advantage, thin film will have to move forward on its advantage of light weight, flexibility and a broader application set.
Some researchers are pursuing even higher efficiency conversion rates by building PV materials capable of capturing a greater spectrum of wavelengths from the sun. Creating what’s known as “multijunction” cells, scientists use layers of different semiconductor materials such as germanium, gallium arsenide and gallium indium phosphide to create stacks of cells with different band gaps. However, the process is still very costly and has limited applications thus far.
Despite the constraints, imagine a time in the near future when you can purchase a “roll” of solar cells that you can simply attach to your roof to make significant cuts in your reliance on grid energy minus the expensive and bulky solar panels that require professional installation. Imagine wearing a shirt that can collect enough solar energy to charge your phone. Think of driving a car featuring PV cells printed literally right onto the steel, providing the car with power from the sun when it’s available and massively cutting down on the need for gas or electricity from dirty grid energy. Chances are, all of these technologies are closer than we think.