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The road and roadblocks to efficient solar cells

By Justin Smith, 14 April 2009

When talking about solar cells, the key figures that are repeatedly brought up are cost and the efficiency of any particular cell. While the cost of a cell is, on the surface at least, fairly straightforward, efficiency can be a bit harder to pin down at first.  Basically, the efficiency of a solar cell is measured by how much electricity is converted from the sunlight that hits it; the energy conversion efficiency of a solar cell is the percentage of sunlight converted by the cell into electricity. In traditional monocrystalline, multicrystalline and thin film cells, most of the light is not converted into electricity, but is either reflected, turned into heat or passes right through the cell.

When a photon of the proper wavelength encounters the silicon semiconductor in a cell, the electrons, which are negatively charged, will be separated from the rest of the atom in a region of the cell called the p-n junction. The p-n junction is where the positive and negative sections of a solar cell come together.Once separated from their photons, the electrons travel across one section of the cell, called the n-type semiconductor, while the holes will travel in the opposite direction across another section of the cell, called the p-type semiconductor. Since the two sections are in contact, on a regular basis some of the freed electrons will rejoin with holes, resulting in a loss of power for the cell, but this does not occur with many of the electrons.

The Shockley-Queisser limit

With today’s technology, there are considerable limitations to the amount of electricity that can be converted from sunlight. The theoretical limit for single p-n junction silicon solar cells has been placed at 30 percent, which was calculated at Shockley Semiconductor by William Shockley and Hans Queisser in 1961, and is known as the Shockley-Queisser limit.

The basic idea comes from the fact that for the photon to move from one part of the cell to another, crossing what is called the band gap, it requires energy, and the use of that energy automatically implies that 100 percent efficiency cannot be attained.  There are other limiting factors as well. Energy is often lost as heat, but some solar plants that do not use photovoltaics actually put the heat to use to create energy, although no viable solution has been made yet to combat the heat loss problem in solar cells. Light reflecting off cells is also a problem, but anti-reflective coatings helps mitigate the problem. Some cells even use reflective materials within the cell to force light to bounce around inside, giving the cell more opportunities to capture photons.

One of the greatest limiting factors on the effectiveness of photovoltaic cells is that in most cells, only one type of material, generally silicon, is used to absorb sunlight. The problem with this is that the sunlight that hits the Earth’s surface covers a wide range of the light spectrum, but silicon can only absorb light from a specific part of that spectrum. Silicon can absorb red, blue and yellow light, but not infrared light, radio waves or microwaves. The solution to this problem is the highly efficient multijunction solar cell.

Current world record efficiencies
For monocrystalline silicon cells with a single p-n junction, the highest efficiency ever recorded currently stands at 25 percent, which was accomplished in October of last year at the ARC Photovoltaics Centre of Excellence at the University of New South Wales (UNSW) in Sydney, Australia. The previous record of 24.7 percent was also set at UNSW years earlier.  ARC Professor Martin Green said the jump in performance leading to the milestone resulted from new knowledge about the composition of sunlight.

“Since the weights of the colors in sunlight change during the day, solar cells are measured under a standard color spectrum defined under typical operational meteorological conditions,” Green said. “Improvements in understanding atmospheric effects upon the color content of sunlight led to a revision of the standard spectrum in April. The new spectrum has a higher energy content both down the blue end of the spectrum and at the opposite red end with, dare I say it, relatively less green.”   More than half of all solar cells throughout the world are produced from multicrystalline silicon. In comparison to monocrystalline silicon, which makes up about a third of the market, the multicrystalline material is less expensive, but contains more defects such as grain boundaries or dislocations.

For this reason, the photovoltaic community has had to be content with efficiency values below 20 percent for cells made of multicrystalline silicon, whereas this limit was already exceeded more than 20 years ago for monocrystalline material. However, scientists at Fraunhofer Institut Solare Energiesysteme (ISE) in Freiburg, Germany, cleared that hurdle in 2004, setting the bar at 20.3 percent efficiency.

After two years of work, a doctoral candidate, Oliver Schultz, successfully developed a process that allows the problematic defects to be partially “deactivated”. Schultz explained, “The trick is to choose temperatures during the solar cell production process such that the electrical properties of the multicrystalline silicon are improved and a high-efficiency solar cell structure is built up at the same time.”

Researchers at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) have moved closer to creating a thin-film solar cell that can compete with the efficiency of the more common silicon-based solar cell.  The NREL’s copper indium gallium diselenide (CIGS) thin-film solar cell reached 19.9 percent efficiency in March of last year, setting a new world record for this type of cell. Multicrystalline silicon-based solar cells have shown efficiencies as high as 20.3 percent.

“This is an important milestone,” said NREL Senior Scientist Miguel Contreras. “The thin film people have always looked for matching silicon in performance, and we are reaching that goal.”  CIGS cells use extremely thin layers of semiconductor material applied to a low-cost backing such as glass, flexible metallic foils, high-temperature polymers or stainless steel sheets. Thin-film cells require less energy to make and can be fabricated by a variety of processes. Because of this, they provide a promising path for providing more affordable solar cells for residential and other uses. The CIGS cells are of interest for space applications and the portable electronics market because of their light weight. They are also suitable in special architectural uses, such as photovoltaic roof shingles, windows, siding and others.

Researchers were able to set the world record because of improvements in the quality of the material applied during the manufacturing process, boosting the power output from the cell, Contreras said.

Multijunction cells reach higher efficiencies
As noted earlier, the small range of the light spectrum that silicon can absorb limits how powerful a solar cell can be. Enter the multijunction solar cell, which is made up of several light-absorbing materials, each of which absorbs light from different sections of the spectrum. While the multijunction cell cannot yet solve the problem of sunlight generating heat within a solar cell, it actually makes use of the fact that light can pass through a cell by placing thin layers of the absorptive materials on top of one another, letting the light pass through to each one, increasing the amount of light that is absorbed.

Today, multijunction solar cells, which derive their name from the fact that they have multiple p-n junctions, are reaching conversion efficiency rates of just over 40 percent. However, it is anticipated that an efficiency rate of 50 percent is expected to be achieved in the coming years. In fact, it is within the realm of possibility that these cells could reach 60 percent efficiencies in the future.

Metamorphic multijunction solar cells are a special type of solar cells using what are called III-V semiconductor compounds. These cells are typically made out of thin gallium indium phosphide and gallium indium arsenide as layers on germanium substrate.

These materials can be combined together, however, only by applying a method called metamorphic growth. According to Fraunhofer ISE, in contrast to conventional solar cells, the semiconductors in these cells do not have the same lattice constant, in other words, the distance between the atoms in a crystalline structure are not matched up. This makes it difficult to grow the III-V semiconductor layers with a high crystal quality, since the point at which the materials come together can result in the different lattice constants creating dislocations and other crystal defects.

For these higher efficiency structures, it is important that the solar spectrum is divided into three equally large spectral regions by a choosing the appropriate light absorbing materials. In this way all of the three subcells generate the same amount of current. Fraunhofer ISE said that a serial-connected solar cell, where the electric current in the device is ultimately limited by the smallest current generated by one of the subcells, is a necessity for these types of cells.

Dispute of the current multijunction efficiency record
While the current world records for monocrystalline, multicrystalline and thin film solar cells are pretty much accepted by most institutions, the same cannot be said for multijunction cells. To date, three groups have each claimed to set the record, the University of Delaware (UD), Fraunhofer IES and the NREL, each with different efficiency rates.

A consortium led by the UD achieved multijunction solar cell efficiency of 42.9 percent from sunlight at standard terrestrial conditions. In 2007 UD worked with DuPont and other companies and universities striving for the 50 percent efficiency goal set by the Defense Advanced Research Projects Agency’s Very High Efficiency Solar Cell (VHESC) program.

The VHESC solar cell uses a lateral optical concentrating system that splits solar light into three different energy bins of high, medium and low, and directs them onto cells of various light sensitive materials to cover the solar spectrum. The system delivers variable concentrations to the different solar cell elements. The concentrator is stationary with a wide acceptance angle optical system that captures large amounts of light, which the UD researchers say eliminates the need for tracking devices.

No modules have been built using UD’s cell, therefore making it not yet practical or close to production, giving some dissenters an opportunity to say that this is not truly a record. However, with fresh funding and the cooperative efforts of the DuPont-UD consortium, the team at UD said it expects that the new high efficiency solar cells could be in production by 2010.

Fraunhofer ISE claims that the record efficiency level is really 41.1 percent for the conversion of sunlight into electricity. The Fraunhofer ISE module concentrates sunlight by a factor of 454 and focuses onto a five-square-millimeter multijunction solar cell made out of gallium indium phosphide, gallium indium arsenide on a germanium substrate. At a higher sunlight concentration of 880, an efficiency of 40.4 percent was measured.

The researchers at Fraunhofer ISE have succeeded in overcoming many of the obstacles inherent in growing the cells. They have managed to localize the defects in a region of the cell that is not electrically active. As a result, the active regions of the cell remain relatively free of defects, which is a prerequisite for achieving the highest efficiencies.

Finally, scientists at the NREL feel they set the world record in solar cell efficiency last August with a multijunction photovoltaic device that converts 40.8 percent of the light that hits it into electricity. The inverted metamorphic triple-junction solar cell was designed, fabricated and independently measured at NREL.

The 40.8 percent efficiency was measured under concentrated light of 326 suns. One sun is about the amount of light that typically hits Earth on a sunny day. The new cell is a natural candidate for the space satellite market and for terrestrial concentrated photovoltaic arrays, which use lenses or mirrors to focus sunlight onto the solar cells.

Instead of using a germanium wafer as the bottom junction of the device, the new design uses compositions of gallium indium phosphide and gallium indium arsenide to split the solar spectrum into three equal parts that are absorbed by each of the cell’s three junctions for higher potential efficiencies. This is accomplished by growing the solar cell on a gallium arsenide wafer, flipping it over, then removing the wafer, leaving a light and thin cell.