Fundamentals of PV Cell Performance

The objective of the PV cell is to convert as much of the energy in the incoming stream of photons into electricity as possible. In this section, we will describe the phenomena that are occurring, as the individual photons are either converted to electrical charge or not, depending on the circumstances. We will also quantify the effect of rates of conversion and rates of loss on the overall efficiency of the cell.

In order to encourage the conversion of photons to electrons using the photoelectric effect, the two layers of silicon that constitute a silicon-based PV cell are modified so that they will be more likely to produce either (1) loose electrons, or (2) holes in the molecular structure where electrons can reattach. Recall that the silicon atom has 4 valence electrons in its outer shell. In one PV cell design, the upper or n-type layer is doped with phosphorus, with 5 valence electrons, while the lower or p-type layer is doped with boron, which has 3 valence electrons (see Fig. 10-2). This type of molecular structure is known as a

p-n junction and is common to most designs of PV cells. The imbalance of electrons across the cell creates a permanent electrical field which facilitates the travel of electrons to and from the external circuit and load to which the PV system is attached.

In Fig. 10-2 upper layer is n-type silicon doped with phosphorus, which has an excess of electrons; lower layer is p-type silicon doped with boron, which has extra holes for absorbing electrons. On the right-hand side, an incoming photon breaks loose an electron within the cell structure. The electron then travels toward the collector comb, while the electron hole travels toward the conductive backing, contributing to the current flow to the load outside the cell.

An arriving photon possessing an energy value equal to or greater than the bandgap energy Eg is able to break loose an electron from the structure of the PV cell. The value of Eg depends on the type of panel used; recall that energy is inversely related to wavelength, that is, E = hc/l, where h is Planck’s constant and c is the speed of light. Therefore, photons with a wavelength greater than the bandgap wavelength 1G do not have sufficient energy to convert to an electron. Electron holes can also move through the cell structure, in the opposite direction to that of the electrons, in the sense that as an electron moves to fill a hole, it creates a hole in the position previously occupied.

Electrons leave the structure of the PV cell and enter the exterior circuit via conductive metal collectors on the surface of the cell. Typical collectors are laid out in a “comb” pattern and are readily observed on the surface of many PV cells. The collector networks, where present, block the incoming sunlight from entering the PV cell. Therefore, design of the collectors involves a trade-off between having sufficient area to easily collect as many electrons as possible, but not so much area that the ability of the sun’s energy to enter the cell is greatly reduced.