Solar cells today are mostly made of silicon, one of the most common elements on Earth. The crystalline silicon solar cell was one of the first types to be developed and it is still the most common type in use today. They do not pollute the atmosphere and they leave behind no harmful waste products.
Photovoltaic cells work effectively even in cloudy weather and unlike solar heaters, are more efficient at low temperatures. They do their job silently and there are no moving parts to wear out. It is no wonder that one marvels on how such a device would function. To understand how a solar cell works, it is necessary to go back to some basic atomic concepts. In the simplest model of the atom, electrons orbit a central nucleus, composed of protons and neutrons. Each electron carries one negative charge and each proton one positive charge. Neutrons carry no charge. Every atom has the same number of electrons as there are protons, so, on the whole, it is electrically neutral. The electrons have discrete kinetic energy levels, which increase with the orbital radius. When atoms bond together to form a solid, the electron energy levels merge into bands. In electrical conductors, these bands are continuous but in insulators and semiconductors there is an “energy gap”, in which no electron orbits can exist, between the inner valence band and outer conduction band [Book 1].
Valence electrons help to bind together the atoms in a solid by orbiting 2 adjacent nucleii, while conduction electrons, being less closely bound to the nucleii, are free to move in response to an applied voltage or electric field. The fewer conduction electrons there are, the higher the electrical resistivity of the material. In semiconductors, the materials from which solar sells are made, the energy gap Eg is fairly small. Because of this, electrons in the valence band can easily be made to jump to the conduction band by the injection of energy, either in the form of heat or light [Book 4]. This explains why the high resistivity of semiconductors decreases as the temperature is raised or the material illuminated. The excitation of valence electrons to the conduction band is best accomplished when the semiconductor is in the crystalline state, i.e. when the atoms are arranged in a precise geometrical formation or “lattice”. At room temperature and low illumination, pure or so-called “intrinsic” semiconductors have a high resistivity. But the resistivity can be greatly reduced by “doping”, i.e. introducing a very small amount of impurity, of the order of one in a million atoms. There are 2 kinds of dopant. Those which have more valence electrons that the semiconductor itself are called “donors” and those which have fewer are termed “acceptors” [Book 2].
In a silicon crystal, each atom has 4 valence electrons, which are shared with a neighbouring atom to form a stable tetrahedral structure. Phosphorus, which has 5 valence electrons, is a donor and causes extra electrons to appear in the conduction band. Silicon so doped is called “n-type” [Book 5]. On the other hand, boron, with a valence of 3, is an acceptor, leaving so-called “holes” in the lattice, which act like positive charges and render the silicon “p-type”[Book 5]. The drawings in Figure 1.2 are 2-dimensional representations of n- and p-type silicon crystals, in which the atomic nucleii in the lattice are indicated by circles and the bonding valence electrons are shown as lines between the atoms. Holes, like electrons, will remove under the influence of an applied voltage but, as the mechanism of their movement is valence electron substitution from atom to atom, they are less mobile than the free conduction electrons [Book 2].
In a n-on-p crystalline silicon solar cell, a shadow junction is formed by diffusing phosphorus into a boron-based base. At the junction, conduction electrons from donor atoms in the n-region diffuse into the p-region and combine with holes in acceptor atoms, producing a layer of negatively-charged impurity atoms. The opposite action also takes place, holes from acceptor atoms in the p-region crossing into the n-region, combining with electrons and producing positively-charged impurity atoms [Book 4]. The net result of these movements is the disappearance of conduction electrons and holes from the vicinity of the junction and the establishment there of a reverse electric field, which is positive on the n-side and negative on the p-side. This reverse field plays a vital part in the functioning of the device. The area in which it is set up is called the “depletion area” or “barrier layer”[Book 4]. When light falls on the front surface, photons with energy in excess of the energy gap (1.1 eV in crystalline silicon) interact with valence electrons and lift them to the conduction band. This movement leaves behind holes, so each photon is said to generate an “electron-hole pair” [Book 2]. In the crystalline silicon, electron-hole generation takes place throughout the thickness of the cell, in concentrations depending on the irradiance and the spectral composition of the light. Photon energy is inversely proportional to wavelength. The highly energetic photons in the ultra-violet and blue part of the spectrum are absorbed very near the surface, while the less energetic longer wave photons in the red and infrared are absorbed deeper in the crystal and further from the junction [Book 4]. Most are absorbed within a thickness of 100 Ã¦m. The electrons and holes diffuse through the crystal in an effort to produce an even distribution. Some recombine after a lifetime of the order of one millisecond, neutralizing their charges and giving up energy in the form of heat. Others reach the junction before their lifetime has expired. There they are separated by the reverse field, the electrons being accelerated towards the negative contact and the holes towards the positive [Book 5]. If the cell is connected to a load, electrons will be pushed from the negative contact through the load to the positive contact, where they will recombine with holes. This constitutes an electric current. In crystalline silicon cells, the current generated by radiation of a particular spectral composition is directly proportional to the irradiance [Book 2]. Some types of solar cell, however, do not exhibit this linear relationship.
The silicon solar cell has many advantages such as high reliability, photovoltaic power plants can be put up easily and quickly, photovoltaic power plants are quite modular and can respond to sudden changes in solar input which occur when clouds pass by. However there are still some major problems with them. They still cost too much for mass use and are relatively inefficient with conversion efficiencies of 20% to 30%. With time, both of these problems will be solved through mass production and new technological advances in semiconductors.