An important type of photodetector is the photovoltaic cell, which generates a voltage that is proportional to the incident EM radiation intensity. These sensors are called photovoltaic cells because of their voltage-generating capacity, but the cells actually convert EM energy into electrical energy. Photovoltaic cells are very important in instrumentation and control applications because they are used both as light detectors and in power sources that convert solar radiation into electrical power for remote-measuring systems. Our emphasis here is on their use in analytical instruments.
The operating principle of the photovoltaic cell is illustrated in Figure above. The cell is a large exposed diode that is constructed using a pn junction between appropriately doped semiconductors. Photons hitting the cell pass through the thin p-doped upper and are absorbed by electrons in the n-doped layer. This causes conduction electrons and holes to be created
The upper terminal is positive and the lower negative. In general, the open-circuit voltage V that is developed on a photovoltaic cell varies logarithmically with the incident radiation intensity according to the following equation:
V= Vo ln(Ir)
Ir = the radiation intensity in W/m2
Vo = the calibration voltage in volts
V = the unloaded output voltage in volts
- The electricity produced by the solar cells is clean and silent. Because they do not use fuel other than sunlight, PV systems do not release any harmful contamination of air or water into the environment, deplete natural resources or endanger human or animal health.
- Photovoltaic systems are silent and visually discrete.
- Small-scale solar plants can take advantage of unused space on the roofs of existing buildings.
- Photovoltaic cells were originally developed for use in space, where repair is extremely expensive, if not impossible. Photovoltaic energy still feeds almost all satellites that circulate through the earth, since it works reliably for long periods of time with little maintenance.
- Some toxic chemicals, such as cadmium and arsenic, are used in the photovoltaic production process. These environmental impacts are minor and can be easily controlled by recycling and proper disposal.
- Solar energy is somewhat more expensive to produce than conventional energy sources due in part to the cost of manufacturing photovoltaic devices and partly to the conversion efficiencies of the equipment. As conversion efficiencies continue to increase and manufacturing costs continue to decline, PV will become increasingly cost competitive with conventional fuels.