The photoelectric effect [local] lent support to the notion that the energy of a particle of light was related to the frequency of the light. This historical experiment is the ionization of a metal by incident light inside a tube like a cathode ray tube. The dependence of the energy [local] of the electron emitted by the incident light is related to the frequency rather than the intensity of light. Electrons emitted from metals in a tube are moving freely to the other electrode using energy above the threshold or ionization energy as kinetic energy. The total energy of these electrons is determined by varying the voltage across the electrodes until current stops flowing.
The historical experiments are just one form of the photoelectric effect. Other forms have been implemented in light sensors for common devices and analytical chemistry instrumentation. The term photoelectric effect also includes the photoconduction effect and the photovoltaic effect used in these detectors. Light detectors in chemical instruments of diverse types use the photoelectric effect.
A CdS photocell is an example of photoconduction. The CdS cell is a resistor in the dark, but electrons are promoted to conduction bands by light above the threshold energy. The resistance of the CdS cell decreases as more electrons are promoted by higher intensity light. Photocells are used as inexpensive light detectors.
Photovoltaic cells are semiconductor versions of the photoelectric effect. These cells are frequently silicon based diodes designed to produce a voltage from light. Unlike the historical photoelectric tubes, these diodes have conduction bands as excited states rather than completely continuous kinetic energies. Consequently, light must be in the proper range of wavelengths to produce a voltage. Not only is there a threshold voltage, but very high energy light is absorbed poorly because the energy gap between the ground states and the conduction band are too large. Inexpensive versions of photovoltaic cells are used in solar cells to produce power with p- and n-doped semiconductors [local].
Light sensors or detectors in instruments are increasingly becoming semiconductors that are photovoltaic devices. Charged coupled device (CCD) and complimentary metal oxide semiconductor (CMOS) light sensors are fabricated with potential wells to collect electrons released by photoelectric events.
The expensive version is a charge coupled device, CCD, which detects light intensity in many different analytical instruments. The devices in scientific instruments are similar to the CCD sensors now available in digital and video cameras. Electrons are collected in wells, and then an entire row is read by moving electrons in the wells sequentially to one end of the row. At the end of the row the light is measured for each well separately. During the readout of the potential wells no light is collected. Timing and data collection are complex but managed easily by a computer.
The CMOS is a less complicated solid state device where each potential well of electrons is read directly. Newer versions of CMOS [local] are manufactured with more resolution and less noise than older versions. The direct readout simplifies the circuits required, reduces the power requirements, and lowers the cost of these CMOS light sensors.
See solar cells under applications below.
