Info

Charge distribution

Figure 4.7 Space Charge Region Formation at a p-n Junction by Diffusion of

Electrons and Holes remain. They create a positive space charge region. Where holes have diffused into the n-region, negatively ionized atoms remain and create a negative space charge region.

An electric field between the n-region and p-region is thus created. It counteracts the charge carriers and hence the diffusion cannot continue indefinitely. Finally, a diffusion voltage k T in e it;

is created. The charge neutrality within the boundaries dn and dp of the space charge region in the n-type and p-type semiconductor region leads to:

The total width of the space charge region can then be calculated from:

For silicon with a dopant concentration of nD = 24016 cm 3 and :

1-1016

cm-3 at a temperature of T = 300 K, the diffusion voltage becomes to Vd = 0.73 V; and with £. = 11.8, the widths are dn = 0.13 ¡m and dp = 0.25 ¡m.

When electrons are lifted from the valence band into the conduction band and thus released from the atom in the space charge region, the electric field will pull them into the n-region. Similarly, generated holes will move into the p-region. This can be explained in the energy band model by band bending in the space charge region (Figure 4.8).

As described before, the solar cell can only convert a part of the photon energy into electrical current. For photon energies smaller than the band gap, the energy is not sufficient to promote an electron from the valence band to the conduction band. This is the case for wavelengths above:

Figure 4.8 Solar Cell Principle with Energy Band Model

Not all the energy of photons with wavelengths near the band gap is converted to electricity. The solar cell surface reflects a part of the incoming light, and some is transmitted through the solar cell. Furthermore, electrons can recombine with holes. In other words, they can fall back to the valence band before they are converted to electricity. Figure 4.9 describes these processes.

The solar cell only uses an amount of energy equal to the band gap of the higher energy of photons with lower wavelengths. Excess energy, i.e. energy above the band gap equivalent, is passed on to the crystal in the form of heat. Hence, the share of the usable energy mainly depends on the wavelength and the band gap. The external quantum collecting efficiency next(A) is the likelihood that an incident photon generates an electron-hole pair. It is closely related to the spectral response, which is a measure of the part of the energy converted into charge carriers. In photovoltaics, the spectral response S(A) given by:

is defined using the external quantum collecting efficiency next and the wavelength A. Figure 4.10 shows the spectral response S as a function of the wavelength A.

In the absence of an external field, i.e. if a solar cell is short-circuited, the photocurrent iph is generated. This current can be calculated using the solar cell area A, the spectral sensitivity S and the spectrum of sunlight E(A) (e.g. the air mass AM 1.5 spectrum of Chapter 2):

Reflection

Reflection

Figure 4.9 Processes in an Irradiated Solar Cell

The irradiance E absorbed by the semiconductor is a share of the incoming irradiance E0. It depends on the thickness d of the semiconductor and the material-dependent absorption coefficient a:

There are two types of semiconductor: direct and indirect. The absorption coefficient of indirect semiconductors such as silicon is significantly lower for higher wavelengths. For example, the direct semiconductor GaAs has an absorption coefficient for light with a wavelength of about 1 ¡m of a(GaAs) ~ 630 mm-1, whereas this value decreases to a(Si) ~ 7.2 mm-1 for silicon. For both semiconductors to absorb the same amount of light, the silicon will have to be 87.5 times thicker than a GaAs semiconductor. The wavelength dependence of the absorption coefficients must be considered for an exact calculation. Crystalline silicon solar cells should have a thickness of at least

0 500 1000 1500

Wavelength >. in nm

0 500 1000 1500

Wavelength >. in nm

Source: Wagemann and Eschrich, 1994

Figure 4.10 Spectral Response of a Solar Cell about 200 jm for high absorptions. So-called light trapping, which reflects the light in the material, enlarges the path length and reduces the required thickness.

Further physical details of solar cells as well as descriptions of other solar cell technologies such as metal-insulator-semiconductor (MIS) cells are not given here. Details can be found in the literature, for example, Goetzberger et al (1998); Green (1994); Luque and Heqedus (2003); Marti and Luque (2003).

Production of Solar Cells and Solar Modules Crystalline silicon solar cells

Various semiconductor materials are suited to solar cell production; however, silicon is the most commonly used material today. For this reason, only the process of producing solar cells from silicon is described here.

Silicon can mainly be found in quartz sand (SiO2). The following reduction process extracts silicon from the quartz sand at high temperatures of about 1800°C (3272°F):

The result of this reaction is so-called metallurgical-grade silicon (MG-Si) with a purity of about 98 per cent. Another process for extracting silicon is the aluminothermic reduction:

However, silicon gained by this process also has significant impurities. Silicon used by the computer industry is so-called electronic-grade silicon (EG-Si) for the production of semiconductor devices. Its impurity level is below 10-10 per cent. This high purity is not necessary for solar cell production, in which solargrade silicon (SOG-Si) is commonly used. Nevertheless, purification processes are needed for the production of SOG-Si.

Silicon is mixed with hydrogen chloride or chloric acid (HCl) in the silane process. An exothermic reaction produces trichlorosilane (SiHCl3) and hydrogen (H2):

Trichlorosilane is liquid at temperatures of 30°C. Multiple fraction distillations are used to remove the impurities. The chemical vapour deposition (CVD) process is used for silicon recovery. Silicon is deposited as a thin silicon rod at temperatures of 1350°C (2462°F), when the trichlorosilane is brought into contact with high-purity hydrogen:

The end product is a high-purity silicon rod with diameters of up to 30 cm (about 12 inches) and lengths up to 2 m (about 80 inches). These rods can be used for the production of polycrystalline solar cells, which consist of a number of crystals, rather than a single crystal. The crystals of polycrystalline silicon are differently oriented and separated by grain boundaries. They introduce some efficiency losses.

To increase solar cell efficiency, monocrystalline material can be produced from polycrystalline material applying the Czochralski or float zone process. Seeding a single crystal at high temperatures transforms the polycrystalline silicon to the desired monocrystalline silicon. No grain boundaries are present in the resulting material and thus losses within a solar cell are reduced.

Wire saws or inner diameter saws cut the crystalline silicon rods into 200-,um to 500-jUm slices. This process causes relatively high cutting losses of up to 50 per cent.

The silicon slices, or so-called wafers, are cleaned and doped in the following steps. Hydrofluoric acid removes any saw damage. Phosphorus and boron are used for doping silicon to create the p-n junction. Gaseous dopants are mixed with a carrier gas such as nitrogen (N2) or oxygen (O2) for gas diffusion, and this gas mixture flows over the silicon wafers. The impurity atoms diffuse into the silicon wafer depending on the gas mixture, temperature and flow velocity. Etching cleans the surface of the doped semiconductor.

Finally, cell contacts are added. A screen printing process adds the front and rear contacts. Materials for the contacts are metals or alloys of aluminium or silver. The rear contact usually covers the whole cell area. Thin contact fingers are used for the front contacts, because they obstruct and reflect sunlight. Only a minimum of the cell's surface should be covered by contacts in order to optimize light capture.

Finally, an antireflective coating is added to the solar cell. This coating reduces reflection at the metallic silicon surface. Titanium dioxide (TiO2) is mostly used for the coating and gives the solar cell its typical blue colour. Nowadays, it is also possible to produce antireflective coatings in other colours, allowing architects to better integrate solar modules with buildings. Figure 4.11 shows the structure of a crystalline solar cell.

Various other methods can be employed to increase the efficiency. For example, the solar cell's surface can be structured with miniature pyramids. The pyramids are shaped in such a way that any reflection of the light is directed onto the cell, hence producing a second incident beam. Furthermore, buried front contacts can reduce the reflection losses. A more detailed description of the production methods can be found in Goetzberger et al, 1998; Green, 1994; Lasnier and Ang, 1990.

Solar modules with crystalline cells

Single unprotected solar cells can be damaged rapidly as a result of climatic

Figure 4.11 Solar Cell Structure and Front View of a Crystalline Silicon Solar Cell

influences. To avoid this, further protection is necessary. Several solar cells with an edge length between 10 and 21 cm (4-8 inches) are combined in a solar module for cell protection. Many modules are made up of 32-40 cells; however, other module sizes with significantly more or fewer cells exist. The front cover is formed by low-iron glass, which has already been described for the flat solar thermal collector in Chapter 3. The back cover is made of glass or plastic. Between the front and rear cover, the solar cells are embedded into plastic. This material is usually ethylene vinyl acetate (EVA), which is cured at temperatures of 100°C (212°F) at reduced pressure. This process is called lamination. A frame is added to the finished modules in some cases. A junction box protects the contacts from water; bypass diodes are also mounted in this box (see section headed 'Series connection under inhomogeneous conditions', p143).

Technical data for solar modules are given in Table 4.6.

Thin film modules

Besides crystalline silicon, thin film modules hold promise for the cells of the future. They can be made of amorphous silicon and other materials such as cadmium telluride (CdTe) or copper indium diselenide (CuInSe2 or CIS). Thin film modules can be produced using a fraction of the semiconductor material necessary for crystalline modules and this promises lower production costs in the medium term. Therefore, the development potential of thin film modules is very high. A disadvantage is the use of toxic materials in some cells; therefore, the importance of production safety and material recycling will increase in the coming years. Furthermore, the supplies of certain materials, for example of tellurium, are restricted, whereas silicon reserves are almost inexhaustible. However, it is not yet clear which material will dominate future markets. This chapter will give only a short illustrative description of amorphous silicon thin film modules because they are the thin film modules with the longest market history.

The base for amorphous silicon solar modules is a substrate; in most cases

Aluminium Silicon Tin oxide rear contact UH-, i, p+) front contact

Aluminium Silicon Tin oxide rear contact UH-, i, p+) front contact

Glass substrate (front)

Figure 4.12 Structure of an Amorphous Silicon Solar Module this is glass or a metal foil. A spray process deposits a thin layer of transparent tin oxide on the glass substrate. This layer serves as the transparent front contact. A laser cuts this layer into strips in order to create an integrated series connection and vapour deposition at high temperatures adds silicon and dopants to the substrate. A 10-nm p-layer and afterwards a 10-nm buffer layer are deposited. An intrinsic 1000-nm layer of amorphous silicon and finally a 20-nm n-layer follow. Screen-printing processes add the back contacts, which are mostly made of aluminium. The samples are then either laminated or coated with a polymer to protect the solar module from climatic influences. Figure 4.12 shows the principle structure of an amorphous silicon solar module.

Silicon loses its crystalline structure during vapour depositing in amorphous solar cells. The main advantage of amorphous silicon cells is that they are thinner than crystalline cells by a factor of 100. This is only possible because amorphous silicon is a direct semiconductor with a much higher absorption coefficient than crystalline silicon. Due to its amorphous structure, the band gap is higher than crystalline silicon, at 1.7 eV. The production process saves a lot of material and even more efficient production is possible. Laboratory efficiencies of 13 per cent have been reached. However, amorphous silicon solar cells are mainly used in small applications such as pocket calculators or wristwatches largely because the efficiency achieved in production is only around 6-8 per cent, which is much lower than that of crystalline cells, which reach up to 20 per cent. Furthermore, amorphous silicon solar cells exhibit a degradation process. This reduces the efficiency in the first months of operation by 10-20 per cent until the performance finally stabilizes.

Other thin film materials already have started to be commercialized. Various materials and technologies are currently under development. Besides the use of new materials such as cadmium telluride (CdTe) or copper indium diselenide (CuInSe2), microcrystalline thin film cells are under development. Other promising developments are dye-sensitized cells on a base of titanium dioxide (TiO2). Which technology will become commercially viable depends on further technological developments.

Figure 4.13 Simple Equivalent Circuit of a Solar Cell

Electrical Description of Solar Cells Simple equivalent circuit

A photovoltaic solar cell is a large area diode. It consists of an n-type and p-type doped semiconductor with a resulting space charge layer. Typically, a non-irradiated solar cell has nearly the same behaviour as a diode. Therefore, a simple diode can describe the equivalent circuit.

The equation of the cell current I depends on the cell voltage (here V = VD) with the saturation current IS and the diode factor m:

The thermal voltage Vt at a temperature of 25°C is Vt = 25.7 mV. The magnitude of the saturation current IS is of the order of 10-10-10-5A. The

Figure 4.14 Influence of the Irradiance E on the I-V Characteristics of a Solar Cell

Figure 4.14 Influence of the Irradiance E on the I-V Characteristics of a Solar Cell

Figure 4.15 Extended Equivalent Circuit of a Solar Cell (One-diode Model)

diode factor m of an ideal diode is equal to 1; however, a diode factor between 1 and 5 allows a better description of the solar cell characteristics.

A current source connected in parallel to the diode completes the simple equivalent circuit of an irradiated solar cell. The current source generates the photocurrent Iph, which depends on the irradiance E and the coefficient c0:

Kirchhoff's first law provides the current-voltage characteristics of the simple solar cell equivalent circuit illustrated in Figures 4.13, and Figure 4.14 shows the characteristic curves at different irradiances):