PV System Characteristics

Materials and Cell Design

The basic PV building block is the PV cell. It is referred to as a cell because it produces dc electricity like a battery. A single PV cell is a thin semiconductor wafer, most commonly made of highly purified silicon. Groups of cells are joined together to form a PV module (or prefabricated panel) and modules may be connected into an array. The modules and arrays can provide electricity in virtually any quantity, starting at a few milliwatts to power a calculator and ranging up to power plant proportion.

The wafer is doped on one side with atoms that produce a surplus of electrons and on the other side with atoms that produce a deficit of electrons. This establishes a voltage difference between the two sides of the wafer. Metallic contacts are made to both sides of the wafer. When the wafer is bombarded by photons from solar radiation, electrons are knocked off the silicon atoms and drawn to one side of the wafer by the voltage difference and can flow through an external circuit attached to the metal contacts on each side of the wafer.

A PV wafer is typically made of silicon (which is in column IV of the periodic table with an atomic number of 14) and has a diamond crystal structure. It has 14 electrons arranged in 3 different shells. Its 10 core electrons are tightly bonded to the nucleus of the atom. The 4 valence electrons in the outer shell are covalently bonded to 4 neighboring atoms, forming its crystalline structure. At normal atmospheric temperatures, it is a very modest conductor. Its crystalline structure does not permit electrons to move about as freely as the electrons in a good conductor, such as copper.

By substituting, or doping, the host material with other atoms from columns III and V of the periodic table, its conductivity can be altered significantly. Doping with column V impurities (e.g., phosphorous, arsenic, or antimony) completes the covalent bond and leaves an additional loosely bound electron forming what is known as an n-type semiconductor. Since column V atoms have 5 electrons in their outer shell instead of 4, when these atoms bond with the silicon atoms, they have one extra electron that is more loosely held in place (only by a proton in the nucleus). This makes the material a far better conductor than pure silicon. When energy is applied to the altered substance, electrons are far more easily knocked loose from their orbit. As each electron is knocked loose, a hole is left behind where an electron could bond. The free electrons are called free carriers and can carry electrical current. The designation as an n-type semiconductor is for negative, due to the prevalence of free electrons.

Doping with column III impurities (e.g., boron, aluminum, gallium, or indium) leaves the covalent bond deficient of one electron or with a hole, forming what is known as a p-type semiconductor. Hence, in the n-type semiconductor, there are electrons looking for holes, and in the p-type, there are holes looking for electrons.

When these two types of conductors are joined, a p-n junction is formed, which is like a large diode area. Whenever different materials are placed in contact, an electric field exists at the interface. This field will exert a force on the electrons and cause current to flow whenever there are free electrons present. The concentration of electrons is greater on the n side than on the p side and the concentration of holes is greater on the p side than on the n side. The formation of the junction instantaneously allows the positive and negative electric charges to redistribute, establishing a built-in internal electric field. Equilibrium is established as an internal electric field builds up to the direction that opposes further flow of electrons from the n region and holes from the p region. If contacts are made with the two ends of the p-n junction and a voltage is applied (i.e., light impinging on the junction device), the state of equilibrium is disturbed and current will again begin to flow and a voltage will be established at the terminals, producing usable power. Figure 14-41 illustrates the electron distribution in the two materials and at the p-n junction.

Figure 14-42 illustrates the layers that form the basic construction of a typical crystalline silicon PV cell. As sunlight impinges on the top surface of the PC cell, some of the light is reflected off of the cell's grid structure and some is reflected by the surface of the cell. Antireflection coatings and texturing of the silicon surface can help to minimize surface reflection losses and promote the transmission of this light into the energy conversion layers below. Two additional electrical contact layers are also necessary. The electrical contact layer on the face of the cell where light enters is made of metallic material typically organized in a grid pattern. Since it is generally not transparent, it must have sufficient spacing so as not to cover the entire face of the cell and block light. The back electric contact layer is made of metal and functions as an electrical contact and covers the entire back surface of the cell.

Absorption Material Selection

The sun emits virtually all of its radiation energy in a spectrum of wavelengths that range from about 7 x 10-7 to 13 x 10-6 ft (2 x 10-7 to 4 x 10-6 m). The majority of this energy is in the visible region. As shown in Figure 14-43,

Fig. 14-41 Electron Distribution in N-Type and P-Type Materials and at the P-N Junction. Source: Jack Stone, NREL

each wavelength corresponds to a frequency and an energy — the shorter the wavelength, the higher the frequency and the greater the energy, expressed in electron volts (eV). Only a certain amount of energy is required to knock an electron loose for a given material. This is called the band gap energy.

The photons in some of the light (long wavelength infrared) that fall upon the cell do not have the threshold energy needed to free electrons from the silicon atoms and pass through the cell without interacting. Concurrently, the photons in some of the light (short wavelength ultraviolet) have more than enough energy to create the electron hole pairs. The excess energy transferred to the charge carriers is dissipated as heat. About 40% of the incident light energy is absorbed and effectively used in freeing electrons from silicon atoms so that they can wander in the crystal lattice. The energy of the electrons increases from the ground state energy to an excited energy state. In this exited state, electrons (or photons) are no longer associated with specific atoms in the absorber material, but are free to move. These energetic free electrons are then forced in the direction of the built-in electric field and collected by the electrical contact layers for use in an external circuit where they perform useful work.

Semiconductors are selected as absorber materials since they are a strong absorber of electromagnetic radiation in the visible range of wavelengths. Semiconductors in thicknesses of 0.004 in. (0.01 cm) or less can absorb all incident visible light. Band gap energy level is an important characteristic in material selection. The optimal balance between current and voltage is achieved at a band gap energy of about 1.4 eV. While the band gap of silicon, at 1.12 eV (which can effectively use wavelengths in the range of about 0.4 to 1.1 microns), is not ideal from this perspective, its natural abundance continues to make it the dominant material used.

In addition to silicon, other semiconductor materials that may be employed as absorbers include gallium arsenide, indium phosphide, copper indium diselendide, cadmium telluride, and titanium dioxide. The materials used for the junction-forming layers need only be dissimilar, and, to carry the electric current, they must be conductors. These may be the same materials used to produce diodes and transistors of solid-state electronics and microelectronics, which share the same basic technology as PV cells.

One way to increase overall efficiency is to create multiple-junction cells. The material with the highest band gap energy can be used on the surface, absorbing high-energy

Antireflection coating Transparent adhesive Cover glass

Sunlight

Sunlight

n-Type semiconductor p-Type semiconductor

Back contact

Fig. 14-42 Labeled Illustration of PV Cell Design. Source: NREL

n-Type semiconductor p-Type semiconductor

Back contact

Fig. 14-42 Labeled Illustration of PV Cell Design. Source: NREL

Negative ©

n-type

Positive space charge

Electric field

Free hole /

Fixed acceptor impurity ion e e e

Free electron Fixed donor impurity ion

Ultraviolet

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