Various selenium rectifiers. Many were stacks of individual selenium diodes in series.

Overview Selenium Rectifiers and Solar Cells

by Lewis Loflin


(Above) Various selenium rectifiers. Many were "stacks" of individual selenium diodes in series.

See What is a CIS Solar Cell? below.

The selenium rectifier consists of a metal base plate (usually iron or aluminum), a selenium layer, and a metal cap or electrode. The base plate, typically the source of electrons, acts as the cathode (negative terminal). The electrons flow from this metal base plate through a load to the selenium layer and then to the metal cap or electrode, the anode (positive terminal).

This setup leverages the rectifying characteristics of the selenium-metal junction, allowing current to flow predominantly in one direction.

What are the electrical properties of selenium? Selenium, as a semiconductor, exhibits several notable electrical properties.

Selenium is a p-type semiconductor with excess positive charge carriers (holes). This property is due to the presence of acceptor impurities, which create vacancies in the valence band.

Selenium can be doped with various elements to modify its electrical properties. For instance, doping with arsenic can turn it into an n-type semiconductor, altering its conductivity and behavior in electronic devices.

Like many semiconductors, selenium's electrical conductivity increases with temperature. This is because thermal energy can free more electrons from covalent bonds.

Selenium rectifiers, which were once commonly used before the advent of silicon diodes, leverage their ability to conduct electricity preferentially in one direction. This rectification property is due to the formation of a Schottky barrier at the metal-selenium interface.

A Schottky barrier (a Schottky diode or rectifying contact) is a potential energy barrier for electrons formed at a metal-semiconductor junction. This barrier arises from the difference in work functions between metal and semiconductors.

The work function indicates how easy or hard it is to dislodge an electron from the surface of a material. A lower work function means electrons are more easily removed, while a higher work function means they are more tightly bound to the material. This difference between the selenium semiconductor and the metal plate creates an imbalance of electrons and, thus, voltage or rectification.

Because electrons can more easily leave the metal due to its lower work function, they flow towards the selenium (which has a higher work function) through a load. This sets up the conditions for the Schottky barrier and gives rise to the junction's rectifying properties.

Selenium's resistivity can vary widely depending on its form and purity. Resistivity is a fundamental property that quantifies how strongly a material resists the flow of electric current. It is defined as the electrical resistance per unit length and unit cross-sectional area.

Resistivity is an intrinsic property of selenium. It doesn't depend on physical shape but is a property of the element itself. Metals like copper have low resistivity, while insulators like rubber have high resistivity. Like most semiconductors, Selenium is in between, and its resistivity can be manipulated through doping, temperature changes, or light exposure.

Selenium solar cell used as a photometer.
Selenium solar cell used as a photometer.
Click for a larger image.

Selenium photovoltaic cells are photoelectric devices that generate or control electric currents. They are used in various applications, including photographic exposure meters, Burglar alarms, electronic door opening and counting devices, Electronic control systems in factory assembly lines, and Industrial color perceptors.

Selenium was one of the first materials discovered to have photovoltaic properties. In the 19th century, its photoconductivity was found, leading to early experiments with selenium-based solar cells, among the first solid-state solar cells created.

Selenium was the world's first solid-state solar cell and initiated contemporary photovoltaic research. However, the efficiency of selenium photovoltaics has remained at 6.5% since 2017.

Selenium is highly photoconductive, meaning its electrical conductivity increases when exposed to light. This property is utilized in photocopiers and solar cells. When photons strike the selenium, they excite electrons, creating more free-charge carriers, thus increasing conductivity.

Vintage selenium solar cells for Russia.

Selenium's amorphous form has a band gap of approximately 1.95 eV (electron volts), influencing its electrical behavior. These properties make selenium helpful in various applications, although many have been superseded by silicon and other materials due to performance and efficiency reasons. Selenium's unique properties still find niche uses, especially in photoconductive applications.

Selenium has a relatively wide bandgap (approximately 1.95 eV), which is advantageous for specific photovoltaic applications. It can be used with materials with different bandgaps to capture a broader spectrum of sunlight. The bandgap determines the wavelength of light that a semiconductor can absorb or emit. For instance, materials with a smaller bandgap might be used for infrared detectors, while those with a larger bandgap might work with visible or UV light.

The bandgap of a solar cell's material influences how much solar energy can be absorbed. Different bandgap materials can be stacked in multi-junction cells to capture a broader spectrum of sunlight.

Research has shown that selenium solar cells can achieve significant open-circuit voltages, approaching 1 volt, indicating good potential for efficiency improvements. Despite their photovoltaic properties, pure selenium solar cells have historically suffered from low efficiency. The open-circuit voltage of a single silicon solar cell is 0.5 to 0.7 volts.

Interest in selenium for photovoltaics persists due to its potential in multi-junction cells or as part of more complex semiconductor alloys designed to optimize light absorption and conversion efficiency.

While selenium isn't the leading material in today's commercial solar market, its photovoltaic properties are well-recognized. It is relevant in research and development to advance solar cell technology, particularly in specialized applications like thin-film technologies.

Copper Indium Selenide Solar Cell
Copper Indium Selenide Solar Cell

What is a CIS Solar Cell?

CIS stands for Copper Indium Selenide (CuInSe₂). When gallium is added to the mix, it's known as CIGS (Cu(In,Ga)Se₂). The addition of gallium increases the bandgap of the material, which can lead to higher efficiency. They've shown potential for high conversion efficiencies, with some lab-scale CIGS cells achieving efficiencies over 20%.

While not as common as silicon in the solar industry, selenium is used in combination with other elements in some types of thin-film solar cells. One notable example is Copper Indium Gallium Selenide (CIGS) solar cells. CIGS cells use a compound semiconductor made of copper, indium, gallium, and selenium, which has a high absorption coefficient for solar radiation, making it efficient for thin-film applications.

While a Schottky barrier may be present due to contact with a metal back, CIS cells don't rely on Schottky barriers. The primary operation is based on the p-n junction or heterojunction formed within the device.

Manufacturing at scale while keeping costs down has been a challenge, though progress has been made. Besides traditional solar panels, CIS/CIGS technology is explored for use in building-integrated photovoltaics (BIPV), portable electronics, and even space applications due to their radiation resistance.

The scarcity of indium and gallium compared to silicon is a concern for large-scale production, scalability, and cost.

Selenium is an element related to sulfur producing pungent, toxic gas if burned often identified as selenium dioxide (SeO₂). This gas has a characteristic odor, somewhat reminiscent of horseradish, garlic, or decaying matter, and it's highly irritating to the respiratory system, eyes, and skin.

Layers deposited on CIGS solar cell.
Layers deposited on CIGS solar cell.

Addendum

In the context of a CIS (Copper Indium Selenide) solar cell, Mo refers to Molybdenum. Molybdenum is commonly used as the back contact material in CIS solar cells.

Molybdenum (Mo) acts as a back electrode (+) or contact layer. Provides good electrical conductivity and adhesion to the substrate (usually glass). Offers high temperature stability, which is crucial during the high-temperature deposition processes of the CIS absorber layer. Helps in preventing diffusion of contaminants from the substrate into the active layers of the solar cell. This layer is typically deposited via sputtering onto the substrate before the deposition of the CIS absorber layer.

Cu(In,Ga)Se2, also known as CIGS (Copper Indium Gallium Selenide), is similar to CIS but includes Gallium, which modifies some of its properties.

Molybdenum (Mo): Just like in CIS cells, Molybdenum serves as the back contact in CIGS solar cells is the Positive Terminal (+). The back contact is where holes are collected.

The Cd(Zn)S layer acts as a buffer layer between the CIGS absorber and the front TCO (Transparent Conductive Oxide) contact. The Cd(Zn)S layer has a wider bandgap (around 2.4 eV for CdS and adjustable with Zn content) compared to CIGS (1.1-1.5 eV), allowing shorter wavelength light to pass through to the CIGS layer while absorbing less light itself. This enhances the overall light absorption in the CIGS layer. When properly doped, it can provide a pathway for electrons while maintaining low resistance.

Negative Terminal (-): The front contact, typically a transparent conductive oxide like ZnO or ITO, where electrons are collected.

For CIGS solar cells, the open-circuit voltage typically ranges from about 0.6 to 0.8 volts. The incorporation of Gallium increases the bandgap, generally leading to a higher open-circuit voltage (Voc) compared to pure CIS cells.

Maximum Power Point Voltage (Vmp): This is slightly lower than Voc, usually around 0.5 to 0.7 volts under standard test conditions. The addition of Gallium in CIGS allows for better tunability of the bandgap, which can enhance both voltage and efficiency compared to CIS.

The manufacturing of CIGS photocells is energy-intensive, particularly due to the deposition processes. However, advancements in deposition techniques, material efficiency, and scale of production can significantly influence the overall energy intensity.

Energy Payback Time (EPBT): For CIGS modules, this has been reported to be around 1 to 2 years under European conditions, which indicates the amount of time it takes for the module to produce energy equivalent to that used in its production. This can be compared to silicon modules, which might have an EPBT of 1.5 to 3 years or more, depending on technology and location.


The above material credit GROK the X AI with additions by Lewis Loflin.

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I've been a part-time adjunct professor at a local community college teaching electricity and electronics.

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