By far the most common type of solar cell because of their long history, silicon solar cells are fast being overtaken by thin-fflm cells, which are much less complex and costly.

Crystalline silicon is expensive and takes a lot of energy to make. It also absorbs only part of the solar spectrum and does it weakly at that. Only those photons that have more energy than silicon’s bandgap can be absorbed, so the red and infrared parts of sunlight are wasted. That energy just heats up the solar cell, which is not good. The blue part of the solar spectrum is also partly wasted for the following reason. Each photon can release only one electron regardless of its energy as long as it exceeds the bandgap. So a very energetic photon at the blue end of the spec­trum uses only part of its energy to create electric current, and the rest of the energy again is lost as heat. To capture more colors of sunlight, cells made with other materials with different bandgaps are used in the basic cell instead of silicon. These other semiconducting materials are called III-V compounds, and they are explained in Box 3.4.

Box 3.4 Doped and III-V Semiconductors

The way semiconductors can be manipulated is best understood by looking at the part of the periodic table near silicon, as shown in Fig. 3.34. The Roman numerals at the top of each column stand for the number of elec­trons in the outer shell of the atom. The different rows have more inner shells, which are not active. The small number in each cell is the atomic number of the element. Silicon (Si) and germanium (Ge) are the most com­mon semiconductors and are in column IV, each with four active electrons. They share these with their four closest neighbors in what is called cova­lent bonds. These are indicated by the double lines in Fig. 3.35. These bonds are so strong that the atoms are held in a rigid lattice, called a crys­tal. The actual lattices are three-dimensional and not as simple as in the drawing. The crystal is an insulator until a photon makes an electron-hole pair by knocking an electron into the conduction band, as we saw in Fig. 3.32.

II III IV V VI 5 B 6 C 7 N 13 Al 14 Si 15 P 31 Ga 32 Ge 33 As N 49 In 50 Sn 51 Sb м

Fig. 3.34 The periodic table near silicon

Box 3.4 (continued)

However, there is another way to make Si or Ge conduct. We can replace one of the silicon atoms in Fig. 3.35a with an atom from column III, for instance, boron. We would then have a “hole.” That’s because boron (B) has only three active electrons and leaves a place in a covalent bond where an electron can go. Since holes can move around and carry charge as if there were positive electrons, this “doped” semiconductor can conduct. We can also dope Si with an atom from column V, such as phosphorus (P), as shown in Fig. 3.35b. Since phosphorus has five active electrons, it has an electron left over after forming covalent bonds with its neighbors. This is a free electron which can carry current. Note that the P nucleus has an extra charge of +1 when one electron is removed, so the overall balance of + and — charges is still maintained. The conductivity can be controlled by the number of dopant atoms we add. In any case, only a few parts in a million are sufficient to make a doped semiconductor be a good enough conductor to interface with metal wires. Any element in column III, boron (B), aluminum (Al), gallium (Ga), or indium (In), can be used to make a p-type semiconductor (those with holes). Any element in column V, nitrogen (N), phosphorus (P), arsenic (As), or antimony (Sb), can be used to make an n-type semiconductor. When the doping level is high, these are called p+ and n+ semiconductors.

Now we can do away with silicon! We can make compounds using only elements from columns III and V, the III-V compounds. Say we mix gallium and arsenic in equal parts in gallium arsenide (GaAs). The extra electrons in As can fill the extra holes in Ga, and we can still have a lattice held by cova­lent bonds. We can even mix three or more III-V elements. For instance, GaInP2, which has one part Ga and one part In from III and two parts P from V. There are just enough electrons to balance the holes. This freedom to mix

Box 3.4 (continued)___________________________________________

any of the III elements with any of the V elements is crucial in multijunction solar cells. First, each compound has a different bandgap, so layers can be used to capture a wide range of wavelengths in the solar spectrum. Second, there is lattice-matching. The lattice spacing is different in different com­pounds. Current cannot flow smoothly from one crystal to another unless the spacings match up. Fortunately, there is so much freedom in forming III-V compounds that multijunction cells with up to five compounds with different bandgaps have been matched. Figure 3.36 shows how the three layers of a triple-junction cell cover different parts of the solar spectrum.

At the bottom of Fig. 3.34, we have shown a II-VI compound, cadmium telluride (CdTe). Each pair of Cd and Te atoms contributes two electrons and holes. This particular II-VI material has been found to be very efficient in single-layer solar cells. It is one of the main types of semiconductors used in the rapid expansion of the thin-film photovoltaic industry.

Fig. 3.36 The parts of the solar spectrum covered by each subcell of a triple-junction solar cell (http://www. amonix. com/technology/index. html)

By adjusting the compositions of these III-V compounds, their bandgaps can be varied in such a way as to cover different parts of the solar spectrum. This is illus­trated in Fig. 3.37. The spectrum there will be explained in Fig. 3.40. The different cells are then stacked on top of one another, each contributing to the generated electric current, which passes through all of them. There are many layers in such a “multijunction” cell. The layers of a simple two-junction cell are shown in Fig. 3.38. The top cell has an active layer labeled n-GaInP2 and is sandwiched

Fig. 3.37 Top: the solar spectrum plotted against photon energy in eV. Long (infrared) wavelengths are on the left, and short (ultraviolet) wavelengths are on the right. The visible part is shown in the middle. Bottom: bandgaps of various semiconductors plotted on the same eV scale. The bandgaps of Ge, GaAs, and GaInP2 are fixed at the positions marked. In InGaN, half the atoms are N, and the other half In and Ga. The bandgap of InGaN, given by the data points, varies with the percentage of Ga in the InGa part. As illustrated for the marked point, the part of the spectrum on the blue side of its bandgap is captured, and the part on the red side is lost (adapted from http://emat-solar. lbl. gov)

between the current-collecting buffer layers labeled n-AlInP2 and p+GaAs. This is the basic cell structure shown in Fig. 3.33. The bottom cell has an active element labeled n-GaAs surrounded by buffer layers. Connecting the two cells is a two-layer tunnel diode, which ensures that all the currents flow in the same direction. Up to five-cell stacks have been successfully made,38 yielding efficiencies above 40%, compared with 12-19% for single-silicon cells. Each cell in a stack has three layers plus the connecting tunnel diode. However, not all the layers are equally thick as in the diagram, and the entire stack can be less than 0.1 mm thick! Pure crystalline silicon needs at least 0.075 mm thickness to absorb the light and at least 0.14 mm thickness to prevent cracking [7], but this does not apply to the other materials.

The semiconductor layers are the main part of a solar cell, but they are thin com­pared with the rest of the structure. A triple-junction cell is shown in Fig. 3.39. The support layer could be a stainless steel plate on the bottom or a glass sheet on the top. The top glass can also be grooved to catch light coming at different angles. At the bottom is a mirror to make the light pass through the cell a second time.

Antireflection coating
	AR and conductive gnd coating
Power collection grid
n — AllnP2		Top cell
n-GalnP2
p+GaAs
p+GaAe		Tunnel Diode
n+-GaAa
n-AIGaAs		Bottom cell
n-GaAa
p-GaAs
p+-GaAs		Substrate

Fig. 3.38 The parts of a two-cell stack using gallium-indium-phosphide (GaInP2) and gallium arsenide (GaAs) (http://www. vacengmat. com/ solar_cell_diagrams. html)

At the top is an antireflection coating such as we have on camera or eyeglass lenses. The current is collected by a grid of “wires,” formed by a thin film of conducting material. The top grid has to pass the sunlight, so it is made of a transparent conduc­tor like indium-tin oxide, which is used in computer and TV screens for the same purpose. The photovoltaic layers have to be in a specific order. At the top is material with the largest bandgap, which can capture only the blue light, whose photons have the highest energy. The lower energy photons are not absorbed, so they pass through to the next layer, labeled “green” here. This has a lower bandgap and captures less energetic photons. Last comes the “red” layer, which has the smallest bandgap and can capture the low-energy photons (the longest wavelengths) which have passed through the other layers unmolested. If the red layer were on top, it would use up all the photons that the other layers could have captured, but it would use them ineffi­ciently, since the voltage generated is the same as the bandgap voltage.

The voltage generated by each cell is only about 1.5 V, so cells are connected into chains that add up the voltage in series to form a module. Modules giving a

Back reflector
film layer

Flexible stainless
steel substate

Fig. 3.39 A typical multijunction solar cell assembly. All the layers in the active part of this ceil are less than 1 pm (1/1,000th of a millimeter) thick (http://www. solarnavigator. net/thin_film_ solar_cells. htm) voltage of, say, 12 V are then grouped into arrays, and thousands of arrays make a solar farm. Modules and arrays generally need to be held in a frame, adding to the cost, and the frames have to be supported off the ground. There is a problem with the series arrangement of the cells. If one cell fails, the output of the entire chain is lost, since the current has to go through all the cells in a chain. Similarly, if one of the layers in a cell fails, there can be no current going out of that cell. Fortunately, the failure rate of commercial units is known and is not bad. Solar cells can still produce 80% of their power after 25 years or more, at least for single-junction cells.

Solar cell efficiency is degraded by another effect: the colors to which a cell responds is fixed in the design of the photovoltaic layers, but the color of sunlight changes with time and place. At sunset, the light is redder and yellower. This means that the blue cell cannot put out as much current. Since the same current flows in series through the whole stack, the red cell’s larger current cannot all be used; its excess current turns into heat. The atmosphere alters the solar spectrum more than you might think. This is shown in Fig. 3.40. In space, the spectrum is almost exactly like that of a classical blackbody. In the visible part of the spectrum, about 30% the intensity is absorbed by the atmosphere. In the infrared region, large absorption bands are caused by gases in the atmosphere. This spectrum is further degraded by the atmosphere during the day as the sun goes lower in the sky.

Multijunction and crystalline silicon solar cells are so expensive that they are not suitable for solar farms, but they have two good applications. First and foremost, these are used where cost is not a prime concern: in space satellites. The ruggedness of

UV, visible	nfrared
Sunlight at Top of the Atmosphere
5250 C Blackbody Spectrum
Radiation at Sea Level

250 500 750 1000 1250 1500 1750 2000 2250 2500

Wavelength (nm)

Fig. 3.40 The solar spectrum in space ( yellow) and on the earth’s surface (red). The visible region is shown by the small spectrum at the bottom. Parts of the spectrum are heavily absorbed by water vapor, oxygen, and CO2 (http://en. wikipedia. org/wiki/Image:Solar_Spectrum. png) silicon and the efficiency of multijunction are needed out there. The sunlight is stronger, and cooling has to be considered because there is no air. Missions to the moon and Mars will no doubt have the most expensive solar cells made. On the earth, expensive solar cells can be used in concentrator PV systems. Since multijunction cells are so expensive, it is cheaper to make large-area Fresnel lenses to catch the light and focus it onto a small chip. The solar intensity can be increased as much as 500 times (“500 suns”). The solar cell will be very hot, but cooling on earth is not a problem. This idea has attracted commercial interest. The Palo Alto Research Center of Xerox Corp. has developed a molded glass sheet with bumps like bubble-wrap. Each bump contains two mirrors configured like a Cassegrain telescope to focus sunlight onto a small cell. The amount of PV material needed is reduced by at least 100 times. Making high-quality silicon is very energy-intensive, but some forms of it can be used for terrestrial solar cells. More on silicon is given in Box 3.5.

Box 3.5 The Story of Silicon

Oxygen and silicon are the most abundant elements on the earth’s crust, oxygen mostly in the form of water (H2O) and silicon in the form of rock (SiO2). These molecules are prevalent because they are very stable; it takes a lot of energy to break them up. The solar cell business got a head start because the semiconductor industry had already built up the infrastructure for producing pure silicon. Without a source of silicon, the expense of making a silicon solar cell would have been prohibitive.

Box 3.5 (continued) The integrated circuits that make computers, cell phones, iPods, and other electronic devices work are made of 99.9999% pure silicon. These chips are made of single-crystal silicon. First, pure silicon is produced from quartz. It is then melted in a crucible by heating to above 1,400°C (2,600°F). This requires a lot of energy: think of the molten rock flowing from the Kilauea caldera in Hawaii into the sea. A seed crystal is then dipped into the liquid and slowly drawn upwards, carrying some silicon with it. As the silicon solidifies, it takes on the crystalline structure of the seed; and a large cylindrical ingot is formed. The entire ingot, 400 mm (12 in.) in diam­eter, is a single crystal. This is then sliced into wafers about 0.2 mm thick. The “sawdust,” or kerf, takes up 20% of the silicon, and it cannot be re-used because of contamination by the cutting tool. To make computer chips, a wafer is processed to make hundreds of chips at once, each containing mil­lions of transistors. The wafer is then sliced into the individual chips, each no larger than 1 cm2 in size. The cost of the silicon wafer is minor, since the chips are worth a million dollars. For solar cells, however, the large areas required mean that the silicon is the main expense, even when off-grade material rejected by the semiconductor industry is used. Silicon shortages cause large fluctuations in price. Note that to form solar cells, the silicon has to be re-melted, using more energy. Single-crystal solar cells are the most efficient because electrons and holes flow easily along the lattice. However, silicon made of small crystals is cheaper and easier to make. The silicon can be poured into a crucible without the slow drawing-out process. Depending on the crystal size, this is called multicrystalline, polycrystalline, or microcrystalline silicon. In these materials, electron flow is interrupted by their bumping into grain boundaries. This causes a higher resistivity and hence loss of energy into heat. Most silicon solar cells are made of polycrystalline silicon. There is also amorphous silicon, which is really a thin-film material. The silicon atoms are not in a lattice at all but are randomly distributed. The produc­tion process is entirely different. A glass substrate is exposed to silane (SiH4) and oxygen (O2) in a plasma discharge, where the hydrogen latches on to the oxygen to form water, and the silicon is deposited onto the glass. The electrical conductivity of amorphous silicon is very poor, and it has to be improved by adding hydrogen in a subsequent hydrogenation process. The result is called a-Si:H. Its power output decreases about 28% at first use, so it has to be “light — soaked” for about 1,000 h before it stabilizes. It is also less efficient in the winter, when the temperatures are lower. The efficiency is only about 6%, but amorphous silicon is much cheaper than any crystalline form and can be used in large installations. Crystalline silicon, on the other hand, is suitable for space applications but not for large solar farms.