Solar Cells

A solar cell is, in principle, a simple semiconductor✶ device that converts light into electric energy. The conversion is accomplished by absorbing light and ionizing crystal atoms, thereby creating free, negatively charged electrons and positively charged ions. If these ions are created from the basic crystal atoms, then their ionized state can be exchanged readily to a neighbor from which it can be exchanged to another neighbor and so forth; that is, this ionized state is mobile; it behaves like an electron, and it is called a hole. It has properties similar to a free electron except that it has the opposite charge.

✶ Solar cells can be made from single crystals, crystalline and amorphous semiconductors. For simplicity this article begins with a description of crystalline material.

Each photon of the light that has a high enough energy to be absorbed by the crystal's atoms will set free an electron hole pair. The electron and hole are free to move through the lattice in a Brownian motion ; however, on average they will never move too far from each other. When the electron comes too close to a hole during their Brownian motion, they will recombine. On the other hand, when they experience an electric field, this will tend to separate the electrons from the holes; the electrons will drift toward the positive pole (the anode), and the positively charged holes will drift toward the cathode. Recombination will then take place in the external circuit (within the electric wires). Consequently a current will flow. Since it is generated by photons, one speaks of a photo current. And the semiconductor that performs this effect is called a photo conductor. Photo conductors are passive devices. They react to light by changing their electric conductivity. In order to activate them an external electric power source, such as a battery, needs to be supplied to draw a current that increases with increasing light intensity. There are many photo conductor devices in our surroundings; as for example, in cameras, in streetlight controls to switch the lights off at dawn and on at dusk, or for light barriers in garage door safety controls.

However, if an electric field is incorporated into the semiconductor, it will separate the electrons and holes. The part of the crystal that accumulates the electrons will be negatively charged; the part that accumulates the holes will be positively charged. The resulting potential difference, referred to as an open circuit, can be picked up by an electrometer. When electrodes are provided at both sides, a current can flow between them. The crystal, when exposed to sunlight, acts as a battery and becomes a solar cell (see Figure 1).

Such a built-in field is easily created in certain semiconductors that can dissolve a small quantity of different impurities; can donate a free electron, called a donor; and can also accept an additional electron, called an acceptor. When dissolving these impurities (called doping) separately in different parts of the crystal, the region that contains donors is called the n -type region, the region with acceptors is called the p -type region. Between these two regions lies an np junction. This region represents the built-in field, since the n -type region is negatively charged compared to the p -type region. Here electrons that are created by light can be separated from the accompanying holes, with the electrons moving into the attracting p -type region and the holes moving preferably into the n -type region.

This, in principle, describes the essentials of a solar cell. The following portions of the article deal with each part of the solar cell in more detail, present a quantitative description of its performance, indicate performance limitations (called the "efficiency" of the solar cell), and give a variety of solar cell materials with comparative performance.

The pn Junction

The pn -junction can be easily understood in the band model with the conduction band populated by free electrons and the valence band populated by free holes. Without light, these carriers are created in thermodynamic equilibrium by donors and acceptors respectively. Mathematically their concentration is given by the Fermi-function (Eq.1)

         (1)

with the determining Fermi level E _F, in the n -type region lying essentially in the middle between the donor level and the lower edge of the conduction band, E _c^. N _c is the effective density of states at the lower edge of the conduction band and is on the order of 10¹⁹ cm⁻³. A similar equation holds for the density of holes; here the Fermi level lies between the acceptor level and the upper edge of the valence band (see Figure 2a).

In thermodynamic equilibrium the Fermi level is horizontal throughout the crystal, thereby forcing both conduction and valence bands to bend, creating the pn -junction (see Figure 2b). Electrons from the p -type region will thereby "roll down" the hill and holes from the n -type region will "bubble up" the slope; hence both will be separated until their charge will force a reduction potential barrier so that the thermal motion across the junction will become equal from the left and from the right.

With light, additional carriers are created, hence equation 1 needs to be modified, replacing E _F with E _Fn or E _Fp, the two quasi-Fermi levels in the n -type or p -type region respectively. These quasi-Fermi levels are now split; the higher the light intensity the more they split. Close to the electrode both quasi-Fermi levels collapse toward the majority quasi-Fermi level, where they are connected to the metal Fermi level (Figure 3). This shift of the Fermi levels in the electrodes represents the open circuit voltage that can be approximated by the shift of the minority quasi-Fermi levels (Figure 3):

V _oc ≈ E _Fn (p ) – E _Fp(n )         (2)

Current-Voltage Characteristics

Materials with a pn -junction show a nonlinear, rectifying characteristic, since it is much easier to move electrons from the electrode through the p -type region and through the junction than from the n -type region over the junction barrier into the p -type region. The current-voltage characteristic through a pn -junction is given by the typical diode equation:

         (3)

as shown as the dark current curve in Figure 4.

When the device is exposed to light, the additional carriers cause an increased current, recognized by an essentially parallel shift of the characteristic downward by j_L, shown in Figure 4. (For a more precise discussion see Böer, 2002.)

The photovoltaic characteristic is consequently given by (see, e.g., Fahrenbruch and Bube 1983):

         (4)

j _L is the maximum current obtained in short-circuiting the device. With j ₀ << j _L ∼ j _SC one obtains for the open circuit voltage (when j = 0) from Eq. 4:

         (5)

Whenever the current-voltage characteristic extends into the fourth quadrant, electric power can be extracted. Its maximum value is given by the largest rectangle that can be inscribed in the characteristic (Figure 4). This identifies the maximum power current, j _mp and the maximum power voltage V_mp. The ratio of the products of these maximum power points to the products of the short circuit current and open circuit voltage is called the fill factor, which can be approximated for ideal solar cells by (see, e.g., Green 2001):

         (6)

with ν _oc = V _oc /(kT /e ). The actual fill factor is usually somewhat smaller and is a measure of the quality of the solar cell. It is a function of the diode doping and of solar cell materials.

The net carrier transport in a solar cell can be pictured as shown in figure 5, with light (hν ) coming from the left and generating (g₀) electron hole pairs in the p -type front layer (of thickness d ). The electrons move towards and through the junction, the holes in opposite direction. When both are shown as electron current (the arrow head is inverted for holes) one visualizes the continuity of the carrier flow and its building up with increasing depth of the front layer.

Solar Cell Materials

A wide variety of materials have the potential of yielding solar cells; several are considered attractive candidates for reasons of high conversion efficiency and ease of fabrication.

For a solar cell one can select a single semiconductor having a junction, usually referred to as homojunction, or a combination of two materials, with the junction at the interface referred to as heterojunction. The selected material needs to match the solar spectrum; i.e., it has to absorb most of the spectrum for maximizing the short circuit output, therefore it has to have a low band gap. However, this is counteracted by the desire to also have a large open circuit voltage, requiring a larger band gap and forcing a compromise. Consequently, for homojunction materials a band gap between 1 and 1.5 eV is preferred.

Typical representatives of this class of homojunction semiconductors are Si; several III-V compounds, most prominently GaAs; and from the class of II-VI compounds CdTe, since it can be doped p - and n -type, while others cannot. Several ternary compounds are also used, most prominently CuInSe₂ and similar ternaries. An example for a heterojunction cell is the CdS/CdTe combination (Meyers and Birkmire, 1995). For more details see Green (2001).

These materials can be employed as single crystals (Si and GaAs), as polycrystals (Si), other thin-film materials (CdTe and all ternaries), and as amorphous material (a-Si:H). Single crystals have the advantage of having high crystal quality and a minimum density of recombination centers; therefore they have a high carrier lifetime that is essential for the carriers to reach the junction after generation, in order to be separated and to contribute to the current.

Other factors that influence the output of solar cells are degradation of surfaces and of electrodes; the first causes a reduction of carrier lifetime, the second causes a reduction in solar cell life expectancies. An example of the first is the beneficial effect of a natural oxide layer that reduces surface recombination of Si solar cells, while special efforts need to be made for GaAs cells to passivate the surface. An example for the second effect is the CdTe solar cell, which has difficulties maintaining stable electrodes on the p -type side.

Optimization of Photon Absorption

In order to make maximum use of the impinging photons and obtain maximum solar cell output, one has to maximize surface penetration, minimize reflection, and reduce obstacles, such as electrodes.

A typical example for such optimization is the structure (shown in Figure 6) developed by Martin Green and his group in order to produce highly-efficient Si solar cells. In addition to an anti-reflecting coating, it contains an etched pyramidical surface that permits light capture (velvet effect) by multiple reflections downward. It also reduces the surface cover of electrodes that are of sufficient thickness to carry the current by depositing it vertically into narrow grooves. It also minimizes the probability of both types of carriers reaching the same electrode by providing a repulsive field through strong doping of a thin surface layer.

Solar Cell Efficiency

Solar cell efficiency is a most valuable measure of its performance. With sunlight impinging from the zenith on a sunny day, a surface perpendicular to the light receives about 1 kW/m². When converted by a solar cell of 10 percent efficiency (presently reached or exceeded by most commercially available solar panels), this means that 100 W/m² in electrical energy can be harvested. This is sufficient if surface areas are ample and the panels are relatively inexpensive. However, where surface areas are at a premium—e.g., on top of a solar car or in some satellites—it is essential to use more efficient solar cells. These are available from carefully engineered Si cells or from GaAs, reaching efficiencies close to 25 percent.

When still higher efficiencies are desired, one can resort to tandem solar cells made by adding a semiconductor of a lower band gap on the bottom, so that photons of lower energy that were not absorbed in the top cell have a second chance to be absorbed and produce additional electric power. Adding a third layer to such a tandem can be beneficial. An example is shown in Figure 7 for a monolytic cell (with matched current and only two electrodes). AlGaAs/GaAs tandem cells reach 27.8 percent efficiency. Mechanically stacked cells (with four electrodes) between GaAs and Si have reached 31 percent (Gee and Virshup, 1988). For more on a large variety of solar cell materials and their best efficiencies, see Green (2001) or Bube (1998). Comprehensive solar cell efficiency tables are provided in Green et al. (2000).

see also Solid-State Devices.

Karl W. Böer

Bibliography

Böer, K. W. (2002). Survey of Semiconductor Physics, Vol. II. New York: John Wiley.

Bube, R. H. (1998). Photovoltaic Materials. London: Imperial College Press.

Chung, B. C; Virshup, G. F.; and Schultz, J. C. (2000). Proceedings of the 21st IEEE Photovolt. Spec. Conf. Kissimee, FL, p. 179.

Fahrenbruch, A. L., and Bube, R. H. (1983). Fundamentals in Solar Cells. New York: Academic Press.

Gee, J. M.; and Virshup, G. F. (1988). Proceedings of the 20th IEEE Photovol. Spec. Conf. Las Vegas, NV, p. 754.

Green, M. A. (2001). Solar Energy, the State of the Art. London: James & James.

Green, M. A.; Emery, K.; Bucher, K.; King, K. L.; and Igari, S. (2000). "Solar Cell Efficiency Tables." Progress in Photovoltaics 8: 377.

Meyers, V., and Birkmire, R. W. (1995). Progress in Photovoltaics 3: 393.

Solar Cell

Background

Photovoltaic solar cells are thin silicon disks that convert sunlight into electricity. These disks act as energy sources for a wide variety of uses, including: calculators and other small devices; telecommunications; rooftop panels on individual houses; and for lighting, pumping, and medical refrigeration for villages in developing countries. Solar cells in the form of large arrays are used to power satellites and, in rare cases, to provide electricity for power plants.

When research into electricity began and simple batteries were being made and studied, research into solar electricity followed amazingly quickly. As early as 1839, Antoine-Cesar Becquerel exposed a chemical battery to the sun to see it produce voltage. This first conversion of sunlight to electricity was one percent efficient. That is, one percent of the incoming sunlight was converted into electricity. Willoughby Smith in 1873 discovered that selenium was sensitive to light; in 1877 Adams and Day noted that selenium, when exposed to light, produced an electrical current. Charles Fritts, in the 1880s, also used gold-coated selenium to make the first solar cell, again only one percent efficient. Nevertheless, Fritts considered his cells to be revolutionary. He envisioned free solar energy to be a means of decentralization, predicting that solar cells would replace power plants with individually powered residences.

With Albert Einstein's explanation in 1905 of the photoelectric effect—metal absorbs energy from light and will retain that energy until too much light hits it—hope soared anew that solar electricity at higher efficiencies would become feasible. Little progress was made, however, until research into diodes and transistors yielded the knowledge necessary for Bell scientists Gordon Pearson, Darryl Chapin, and Cal Fuller to produce a silicon solar cell of four percent efficiency in 1954.

Further work brought the cell's efficiency up to 15 percent. Solar cells were first used in the rural and isolated city of Americus, Georgia as a power source for a telephone relay system, where it was used successfully for many years.

A type of solar cell to fully meet domestic energy needs has not as yet been developed, but solar cells have become successful in providing energy for artificial satellites. Fuel systems and regular batteries were too heavy in a program where every ounce mattered. Solar cells provide more energy per ounce of weight than all other conventional energy sources, and they are cost-effective.

Only a few large scale photovoltaic power systems have been set up. Most efforts lean toward providing solar cell technology to remote places that have no other means of sophisticated power. About 50 megawatts are installed each year, yet solar cells provide only about. 1 percent of all electricity now being produced. Supporters of solar energy claim that the amount of solar radiation reaching the Earth's surface each year could easily provide all our energy needs several times over, yet solar cells have a long way to go before they fulfill Charles Fritts's dream of free, fully accessible solar electricity.

Raw Materials

The basic component of a solar cell is pure silicon, which is not pure in its natural state. Pure silicon is derived from such silicon dioxides as quartzite gravel (the purest silica) or crushed quartz. The resulting pure silicon is then doped (treated with) with phosphorous and boron to produce an excess of electrons and a deficiency of electrons respectively to make a semiconductor capable of conducting electricity. The silicon disks are shiny and require an anti-reflective coating, usually titanium dioxide.

The solar module consists of the silicon semiconductor surrounded by protective material in a metal frame. The protective material consists of an encapsulant of transparent silicon rubber or butyryl plastic (commonly used in automobile windshields) bonded around the cells, which are then embedded in ethylene vinyl acetate. A polyester film (such as mylar or tedlar) makes up the backing. A glass cover is found on terrestrial arrays, a lightweight plastic cover on satellite arrays. The electronic parts are standard and consist mostly of copper. The frame is either steel or aluminum. Silicon is used as the cement to put it all together.

The Manufacturing
Process

Purifying the silicon

• 1 The silicon dioxide of either quartzite gravel or crushed quartz is placed into an electric arc furnace. A carbon arc is then applied to release the oxygen. The products are carbon dioxide and molten silicon. This simple process yields silicon with one percent impurity, useful in many industries but not the solar cell industry.
• 2 The 99 percent pure silicon is purified even further using the floating zone technique. A rod of impure silicon is passed through a heated zone several times in the same direction. This procedure "drags" the impurities toward one end with each pass. At a specific point, the silicon is deemed pure, and the impure end is removed.

Making single crystal silicon

• 3 Solar cells are made from silicon boules, polycrystalline structures that have the atomic structure of a single crystal. The most commonly used process for creating the boule is called the Czochralski method. In this process, a seed crystal of silicon is dipped into melted polycrystalline silicon. As the seed crystal is withdrawn and rotated, a cylindrical ingot or "boule" of silicon is formed. The ingot withdrawn is unusually pure, because impurities tend to remain in the liquid.

Making silicon wafers

• 4 From the boule, silicon wafers are sliced one at a time using a circular saw whose inner diameter cuts into the rod, or many at once with a multiwire saw. (A diamond saw produces cuts that are as wide as the wafer—. 5 millimeter thick.) Only about one-half of the silicon is lost from the boule to the finished circular wafer—more if the wafer is then cut to be rectangular or hexagonal. Rectangular or hexagonal wafers are sometimes used in solar cells because they can be fitted together perfectly, thereby utilizing all available space on the front surface of the solar cell.
• 5 The wafers are then polished to remove saw marks. (It has recently been found that rougher cells absorb light more effectively, therefore some manufacturers have chosen not to polish the wafer.)

Doping

• 6 The traditional way of doping (adding impurities to) silicon wafers with boron and phosphorous is to introduce a small amount of boron during the Czochralski process in step #3 above. The wafers are then sealed back to back and placed in a furnace to be heated to slightly below the melting point of silicon (2,570 degrees Fahrenheit or 1,410 degrees Celsius) in the presence of phosphorous gas. The phosphorous atoms "burrow" into the silicon, which is more porous because it is close to becoming a liquid. The temperature and time given to the process is carefully controlled to ensure a uniform junction of proper depth.

  A more recent way of doping silicon with phosphorous is to use a small particle accelerator to shoot phosphorous ions into the ingot. By controlling the speed of the ions, it is possible to control their penetrating depth. This new process, however, has generally not been accepted by commercial manufacturers.

Placing electrical contacts

• 7 Electrical contacts connect each solar cell to another and to the receiver of produced current. The contacts must be very thin (at least in the front) so as not to block sunlight to the cell. Metals such as palladium/silver, nickel, or copper are vacuum-evaporated through a photoresist, silkscreened, or merely deposited on the exposed portion of cells that have been partially covered with wax. All three methods involve a system in which the part of the cell on which a contact is not desired is protected, while the rest of the cell is exposed to the metal.
• 8 After the contacts are in place, thin strips ("fingers") are placed between cells. The most commonly used strips are tin-coated copper.

The anti-reflective coating

• 9 Because pure silicon is shiny, it can reflect up to 35 percent of the sunlight. To reduce the amount of sunlight lost, an anti-reflective coating is put on the silicon wafer. The most commonly used coatings are titanium dioxide and silicon oxide, though others are used. The material used for coating is either heated until its molecules boil off and travel to the silicon and condense, or the material undergoes sputtering. In this process, a high voltage knocks molecules off the material and deposits them onto the silicon at the opposite electrode. Yet another method is to allow the silicon itself to react with oxygen- or nitrogen-containing gases to form silicon dioxide or silicon nitride. Commercial solar cell manufacturers use silicon nitride.

Encapsulating the cell

• 10 The finished solar cells are then encapsulated; that is, sealed into silicon rubber or ethylene vinyl acetate. The encapsulated solar cells are then placed into an aluminum frame that has a mylar or tedlar backsheet and a glass or plastic cover.

Quality Control

Quality control is important in solar cell manufacture because discrepancy in the many processes and factors can adversely affect the overall efficiency of the cells. The primary research goal is to find ways to improve the efficiency of each solar cell over a longer lifetime. The Low Cost Solar Array Project (initiated by the United States Department of Energy in the late 1970s) sponsored private research that aimed to lower the cost of solar cells. The silicon itself is tested for purity, crystal orientation, and resistivity. Manufacturers also test for the presence of oxygen (which affects its strength and resistance to warp) and carbon (which causes defects). Finished silicon disks are inspected for any damage, flaking, or bending that might have occurred during sawing, polishing, and etching.

During the entire silicon disk manufacturing process, the temperature, pressure, speed, and quantities of dopants are continuously monitored. Steps are also taken to ensure that impurities in the air and on working surfaces are kept to a minimum.

The completed semiconductors must then undergo electrical tests to see that the current, voltage, and resistance for each meet appropriate standards. An earlier problem with solar cells was a tendency to stop working when partially shaded. This problem has been alleviated by providing shunt diodes that reduce dangerously high voltages to the cell. Shunt resistance must then be tested using partially shaded junctions.

An important test of solar modules involves providing test cells with conditions and intensity of light that they will encounter under normal conditions and then checking to see that they perform well. The cells are also exposed to heat and cold and tested against vibration, twisting, and hail.

The final test for solar modules is field site testing, in which finished modules are placed where they will actually be used. This provides the researcher with the best data for determining the efficiency of a solar cell under ambient conditions and the solar cell's effective lifetime, the most important factors of all.

The Future

Considering the present state of relatively expensive, inefficient solar cells, the future can only improve. Some experts predict it will be a billion-dollar industry by the year 2000. This prediction is supported by evidence of more rooftop photovoltaic systems being developed in such countries as Japan, Germany, and Italy. Plans to begin the manufacture of solar cells have been established in Mexico and China. Likewise, Egypt, Botswana, and the Philippines (all three assisted by American companies) are building plants that will manufacture solar cells.

Most current research aims for reducing solar cell cost or increasing efficiency. Innovations in solar cell technology include developing and manufacturing cheaper alternatives to the expensive crystalline silicon cells. These alternatives include solar windows that mimic photosynthesis, and smaller cells made from tiny, amorphous silicon balls. Already, amorphous silicon and polycrystalline silicon are gaining popularity at the expense of single crystal silicon. Additional innovations including minimizing shade and focusing sunlight through prismatic lenses. This involves layers of different materials (notably, gallium arsenide and silicon) that absorb light at different frequencies, thereby increasing the amount of sunlight effectively used for electricity production.

A few experts foresee the adaptation of hybrid houses; that is, houses that utilize solar water heaters, passive solar heating, and solar cells for reduced energy needs. Another view concerns the space shuttle placing more and more solar arrays into orbit, a solar power satellite that beams power to Earth solar array farms, and even a space colony that will manufacture solar arrays to be used on Earth.

Where To Learn More

Books

Bullock, Charles E. and Peter H. Grambs. Solar Electricity: Making the Sun Work for You. Monegon, Ltd., 1981.

Komp, Richard J. Practical Photovoltaics. Aatec Publications, 1984.

Making and Using Electricity from the Sun. Tab Books, 1979.

Periodicals

Crawford, Mark. "DOE's Born-Again Solar Energy Plan," Science. March 23, 1990, pp. 1403-1404.

"Waiting for the Sunrise," Economist. May 19, 1990, pp. 95+.

Edelson, Edward. "Solar Cell Update," Popular Science. June, 1992, p. 95.

Murray, Charles J. "Solar Power's Bright Hope," Design News. March 11, 1991, p. 30.

—Rose Secrest

solar cell Device that converts sunlight directly to electricity. It normally consists of a p-type (positive) silicon semiconductors coated with an n-type (negative) one. Light radiation causes electrons to be released and creates a potential difference so that current can flow between electrodes connected to the two crystals. The cells are about 10% efficient. Solar cells are often used to power small electronic devices, such as pocket calculators. Several thousand cells may be used in panels to provide power of a few hundred watts. Panels have been used to power space vehicles and satellites.