The free charge carriers in so called intrinsic semiconductors such as silicon are the result of the thermal excitation of electrons from a valence band into a conduction band. The excitation of the electron leaves behind a positively charge hole in the valance band which can also act a charge carrier. Because the free charge carriers are created by thermal excitation, the colder a semiconductor becomes the less free charge carriers are present and the electrical resistance become higher. This behavior is in contrast to metallic conductors which have free electrons in the conduction band at all temperatures and which actually have lower resistance at lower temperature because thermal lattice vibration do not interfere as much with the free motion of the conduction electrons. The density of free charge carriers in pure silicon at room temperature is low compared to metallic conductors, and the electrical resistivity is high.
The number of free charge carriers in silicon can be greatly increased by adding a small amount of another element called a dopant. Silicon has four electrons in its out shell. If the dopant atom has 5 electrons in its outer shell (e.g. phosphorous) then one of these electrons can easily be excited into the conduction band of silicon. The positively charge ionic core left behind is bound to the site the dopant atom and therefore cannot act a current carrier. Thus silicon doped with phosphorous has an excess of negative charge carriers and is therefore called an n-type semiconductor. If an atom with 3 electrons in is outer shell (e.g. boron) is added to silicon then one of the electrons in the valence band can easily become attached to the atom leaving behind a positively charge hole which can act as current carrier. The electron attached to the dopant atom is immobile and cannot act as a current carrier. Thus silicon doped with phosphorous has an excess of positive charge carriers and is therefore called a p-type semiconductor. Both of these types of doped silicon are called extrinsic semiconductors. Extrinsic semiconductors have much higher densities of free charge carrier and much lower resistivities than intrinsic semiconductors.
Pure silicon is capable of a absorbing sunlight and exciting electron/hole pairs, but no electromotive force exist which can drive the free electrons through an external circuit as an electrical current. In order to produce such a current a P-N junction is required. A P-N junction is an interface between two volumes of silicon one of which has an excess of negative charge carriers and the other of which has an excess of positive charge carriers. The most common method of producing such a junction is to take a wafer of p-type crystalline silicon (most commonly doped with boron) and exposing one surface of the wafer to phosphorous at high temperature. The phosphorous diffuses into the silicon and forms a thin layer of n-type silicon. The boundary between this layer and the bulk of the wafer is the p-n junction.
When electron/hole pairs are excited by light absorption in the vicinity of the p-n junction the electron moves to one side of the junction and the hole to the other side and they can recombine only after the electron has moved through an external circuit an back into the other side of the junction. This movement of the photo excited electrons is the source of the electrical current produced by PV cells.
Silicon PV cells can be produced starting with either p-type silicon or n-type silicon. It turns out that controlling the electrical resistivity of n-type silicon crystals is more difficult than controlling the resistivity of p-type silicon thus making n-type silicon with will controlled resistivity more expensive to produce. However PV cells utilizing n-type silicon wafers can achieve higher light to electricity conversion efficiency than PV-cells utilizing p-type wafers. P-type solar cells still dominate the market, but high efficiency n-type cells have found a niche in applications where space is at a premium so that the higher efficiency allows more energy production in a give area of rooftop.
Silevo is a player in the high efficiency n-type PV solar cell field. The typical method of producing a p-n junction in a n-type wafer to expose is one surface to boron gas at high temperature. The boron diffuses into the silicon and converts a thin layer of the wafer into a p-type semiconductor. Silevo creates an ultra thin p-type layer at one surface of the wafer not by doping but by allowing one surface of the wafer to become oxidized and then depositing thin film layers on top of the oxide. The actual physics by which by which the so-called inversion layer with a majority of p-type carriers is created is a mystery to me, but it obviously works. I believe that the high temperature boron diffusion process is fairly expensive so that Silevo’s thin film deposition process may represent a significant cost reduction.
PV cells of this type have a lower temperature coefficient of performance (% loss of efficiency per degree increase in temperature). The Silevo cells have a temperature coefficient of -0.22%/C compared to -0.45 to -0.50%/C for diffusion based solar cells. Silevo claims that this lower temperature coefficient leads to 5% to 12% increase in energy production under real world conditions. Obviously hot desert conditions will get the most benefit from the lower coefficient.
Silevo does not manufacture n-type silicon wafers itself. They are depending on general industry progress for lower costs in this aspect of the solar modules.
Silevo has garnered media attention recently because it was acquired by Solar City, a major player in the residential PV market. This attempt by Solar City to become a vertically integrated company who manufactures it own solar panels is somewhat risky, but if Silevo can really deliver a high performing low cost product the payoff could be very substantial.