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Photovoltaic cells

Photovoltaic cells are thin layers of semiconductors that produce a voltage if exposed to (sun)light. An array of just 21 small cells (having a surface area of 15 cm^2 each) was exposed to the light of the son (or simply diffuse daylight) three times under different conditions and the array’s characteristic curves were measured (Figure 1). The characteristic of solar cells is measured by hooking up load resistors having different resistances to the cells (or to the array) and determining voltage and electric current.

Interpretation

Photovoltaic or solar cells are similar to diodes, so their characteristic curves are similar to those of diodes (Figure 2). The difference is that light falling on them shifts the characteristic curve of the diode in the negative direction. Flipping the diagram about the horizontal (voltage) axis leads to the graphs seen in Figure 1. As a result, the cells work as generators, and except for the form of the curves, the characteristic diagram is similar to that of a battery (Figure 3). There is a voltage across a cell even when no electric current is flowing (this is the maximum possible value called the open circuit voltage as in the case of a battery). The voltage decreases if a current is allowed to flow through the cells in an electric circuit, and there is a maximum current called the short-circuit current (at U_PV = 0).

The short-circuit current depends more or less linearly upon the intensity of the light falling upon a PV cell (see Figure 1). In other words, the shift of the diode characteristic depends upon the intensity of the light. This tells us that the energy supplied by the light is responsible for the photovoltaic effect. Light absorbed by the doped semiconductor produces pairs of holes and electrons, and this produces the open circuit voltage of the cell. Since the transport for charge is that for semi-conducting materials, the model of a PV cell is different from the simple one applicable to a battery (Figure 3). Diffusion of charge with the chemicals of a battery leads to a resistive effect similar to that in a metal which is ohmic. In a diode it is clearly not ohmic.