Principles of solar cells

The principles of semiconductor physics are best illustrated using the example of silicon, a group 4 elemental semiconductor, shown in figure 1-1. The silicon crystal forms the so-called diamond lattice where each atom has four nearest neighbors at the vertices of a tetrahedron. The four-fold tetrahedral co-ordination is the result of the bonding arrangement which uses the four outer (valence) electrons of each silicon atom. Each bond contains two electrons, and you can easily see that all the valence electrons are taken up by the bonds. Most other industrially important semiconductors crystallise in this or closely related lattices, and have a similar arrangement of the bonding orbital.

This crystal structure has a profound effect on the electronic and optical properties of the semiconductor. According to the quantum theory, the energy of an electron in the crystal must fall within well-defined bands. The energies of valence orbitals which form bonds between the atoms represent just such a band of states, the valence band. The next higher band is the conduction band which is separated from the valence band by the energy gap, or bandgap.

The width of the bandgap Ec - Ev is a very important characteristic of the semiconductor and is usually denoted by Eg. The table 1-1 gives the bandgaps of the most important semiconductors for solar-cell applications.

Figure 1-2 gives the band diagram and electron-hole distribution. A pure semiconductor (which is called intrinsic) contains just the right number of electrons to fill the valence band,

and the conduction band is therefore empty. Electrons in the full valence band cannot move - just as, for example, marbles in a full box with a lid on top. For practical purposes, a pure semiconductor is therefore an insulator.

Semiconductors can only conduct electricity if carriers are introduced into the conduction band or removed from the valence band. One way of doing this is by alloying the semiconductor with an impurity. This process is called doping. As we shall see, doping makes it possible to exert a great deal of control over the electronic properties of a semiconductor, and lies in the heart of the manufacturing process of all semiconductor devices.

Suppose that some group 5 impurity atoms (for example, phosphorus) are added to the silicon melt from which the crystal is grown. Four of the five outer electrons are used to fill the valence band and the one extra electron from each impurity atom is therefore promoted to the conduction band. For this reason, these impurity atoms are called donors. The electrons in the conduction band are mobile, and the crystal becomes a conductor. Since the current is carried by negatively charged electrons, this type of semiconductor is called n type.

A similar situation occurs when silicon is doped with group 3 impurity atoms (for example, boron) which are called acceptors. Since four electrons per atoms are needed to fill the valence band completely, this doping creates electron deficiency in this band. The missing electrons - called holes - behave as positively charged particles which are mobile, and carry current. A semiconductor where the electric current is carried predominantly by holes is called p-type.

The operation of solar cells is based on the formation of a junction, shown in figure 1-3.

The important feature of all junctions is that they contain a strong electric field. To illustrate how this field comes about, let us imagine the hypothetical situation where the p-n junction is

formed by joining together two pieces of semiconductor, one p-type and the other n-type. In separation, there is electron surplus in the n-type material and hole surplus in the p-type.

When the two pieces are brought into contact, electrons from the n region near the interface diffuse into the p side, leaving behind a layer which is positively charged by the donors.

Similarly, holes diffuse in the opposite direction, leaving behind a negatively charged layer stripped of holes. The resulting junction region then contains practically no mobile charge carriers, and the fixed charges of the dopant atoms create a potential barrier acting against a further flow of electrons and holes. Note that the electric field in the junction pulls the electrons and holes in opposite directions.

The potential barrier of a junction permits the flow of electric current in only one direction - the junction acts as a rectifier, or diode. This can be seen in our example where electrons can only flow from the p region to the n region, and holes can only flow in the opposite direction. Electric current, which is the sum of the two, can therefore flow only from the p-side to the n-side of the junction. The I-V characteristic of a diode is shown in figure 1-4.

Photovoltaic energy conversion relies on the quantum nature of light whereby we perceive light as a flux of particles called photons. On a clear day, about 4.4 x 10¹⁷ photons strike a square centimeter of the Earth's surface every second.

Only some of these photons - those with energy in excess of the bandgap - can be converted into electricity by the solar cell. When such photon enters the semiconductor, it may be absorbed and promote an electron from the valence to the conduction band. Since a hole is left behind in the valence band, the absorption process generates electron-hole pairs, shown in figure 1-5.

Each semiconductor is restricted to converting only a part of the solar spectrum. The spectrum is plotted in figure 1-6 in terms of the incident photon flux as a function of photon energy. The shaded area represents the photon flux that can be converted by a silicon cell - about two-thirds of the total flux.

The nature of the absorption process also indicates how a part of the incident photon energy is lost in the event. Indeed, it is seen that practically all the generated electron-hole pairs have energy in excess of the bandgap. Immediately after their creation, the electron and hole decay to states near the edges of their respective bands. The excess energy is lost as heat and cannot be converted into useful power. This represents one of the fundamental loss mechanisms in a solar cell.

Because solar cells are semiconductor devices, they share many of the same processing and manufacturing techniques as other semiconductor devices such as computercom and memory chips. However, the stringent requirements for cleanliness and quality control of semiconductor fabrication are a little more relaxed for solar cells. Most large-scale commercial solar cell factories today make screen printed poly-crystalline silicon solar cells.

Single crystalline wafers which are used in the semiconductor industry can be made into excellent high efficiency solar cells, but they are generally considered to be too expensive for large-scale mass production.

Figure 1-7 shows the process of manufacturing solar cells. Generally, the first step is making texturization on the surface of a silicon wafer. Then dopants diffuse to the wafer.

Coating reflection layers is processed after removing the glass layer, produced by oxidation effects, on the surface of wafer. Then the silk screen printing, oven firing and efficiency testing is made.

Poly-crystalline silicon wafers are made by wire-sawing block-cast silicon ingots into very thin (180 to 350 micrometer) slices or wafers. The wafers are usually lightly p-type doped. To make a solar cell from the wafer, a surface diffusion of n-type dopants is performed on the front side of the wafer. This forms a p-n junction a few hundred nanometers below the surface.

Antireflection coatings, which increase the amount of light coupled into the solar cell, are typically applied next. Over the past decade, silicon nitride has gradually replaced titanium dioxide as the antireflection coating of choice because of its excellent surface passivation qualities (i.e., it prevents carrier recombination at the surface of the solar cell). It is typically applied in a layer several hundred nanometers thick using plasma-enhanced chemical vapor deposition (PECVD). Some solar cells have textured front surfaces that, like antireflection coatings, serve to increase the amount of light coupled into the cell. Such surfaces can usually only be formed on single-crystal silicon, though in recent years methods of forming them on multicrystalline silicon have been developed.

The wafer is then metallized, whereby a full area metal contact is made on the back surface, and a grid-like metal contact made up of fine "fingers" and larger "busbars" is screen-printed onto the front surface using a silver paste. The rear contact is also formed by screen-printing a metal paste, typically aluminium. Usually this contact covers the entire rear side of the cell, though in some cell designs it is printed in a grid pattern. The paste is then fired at several hundred degrees Celsius to form metal electrodes in ohmic contact with the silicon. After the metal contacts are made, the solar cells are interconnected in series (and/or parallel) by flat wires or metal ribbons, and assembled into modules or "solar panels". Solar panels have a sheet of tempered glass on the front, and a polymer encapsulation on the back.

Tempered glass cannot be used with amorphous silicon cells because of the high temperatures during the deposition process.

The solar cell was firstly developed by Chapin, Fuller, and Pearson in 1954 using a diffused silicon p-n junction[4]. Subsequently, the cadmium-sulfide solar cell was developed by Raynolds et al.[8] Historically, silicon was the first commercially used for the solar cell material and it is the most extensively studied semiconductor in the present day. To data, solar cells have been made in many other semiconductors, using various device configurations with high-purity silicon and optimized solar cell designs, the efficiencies of 23% can be experimentally achieved under normal sunlight.[9] However, the polycrystalline silicon used in the commercial cells and modules, the efficiencies of about 17% are now obtained[10].

A wide variety of compound semiconductor, such as GaAs, InP, CdTe, and CdS, which have a band gap that is more suitable to the sunlight spectrum and therefore is potential for high efficiency. Especially, III-V compound semiconductors are used for space solar cells, concentrator solar cells, and thermophotovoltaic generators. Recent studies of GaAs solar cells have even demonstrated the efficiencies of about 25%[11-14].

Generally speaking, optical losses in solar cells originate from the three major reasons:

surface reflection, top grid shadowing, and inadequate absorption, illuminated by either excess energy photons or weak energy photons. So these three directions are deserved to be explored.

According the manufacturing process, shown in figure 1-7, surface texturization is an important key to lower surface reflection so that my study is focused on this problem.