Photovoltaic (PV) Cell Model Chuck Carpenter, W5USJ copyright1990 by Chuck Carpenter Photovoltaics are "sandwiches" of materials made from silicon, the second most abundant material in the world. One layer of silicon is treated with a substance to create an excess of electrons. This becomes the negative or "N" layer. The other layer is treated to create a deficiency of electrons, and becomes the positive or "P" layer. Assembled together with conductors, the arrangement becomes a light-sensitive NP Junction semiconductor. It's called a semiconductor, because, unlike a wire, the unit conducts in only one direction; from negative to positive. INSERT DMM ILLO An ideal PV is shown in the figure above. When exposed to sunlight (or other intense light source), the voltage is about 0.50 volts DC, and the potential current flow (amps) is proportional to the light energy (photons). In any PV, the voltage is nearly constant, and the current is proportional to the size of the PV and the intensity of the light. Actual PVs, those available in the real world, are not as perfect as the ideal model. The equivalent circuit of a PV, shown in the diagram below, is a battery with a series internal resistance. (Similar to any other practical battery.) INSERT CELL ILLO Because of the variations in internal resistance, current and voltage will vary between cells of equivalent size and structure, connected to the same load, and under the same light source. Manufacturing techniques would strive for the lowest possible internal resistance. And, the variations in internal resistance are accounted for in the panel assemblies you buy. Presently, there are two types of PV cells: crystalline, and amorphous. The crystalline units are the more common, generally blue-colored frosty looking ones. Amorphous means noncrystalline, and these look smooth and change color depending on the way you hold them. You see them now in solar-powered calculators and in some low-powered PV panels. Crystalline types are more efficient, but more expensive to manufacture.