MadSci Network: Chemistry

Re: What are the mechanics of solar cells?

Area: Chemistry
Posted By: Justin Remais, Student and Engineer Asst., University of California at Berkeley/Lawrence Berkeley National Laboratory
Date: Fri Jun 27 11:29:38 1997
Area of science: Chemistry
ID: 862532986.Ch
Hello Raj,
Photovoltaic (or PV) systems convert light energy into electricity. The 
term "photo" stems from the Greek "phos," which means "light." "Volt" is 
named for Alessandro Volta (1745-1827), a pioneer in the study of 
electricity. "Photo-voltaics," then, could literally mean "light- 
electricity." Most commonly known as "solar cells," PV systems are already 
an important part of our lives. The simplest systems power many of the 
small calculators and wrist watches we use every day. More complicated 
systems provide electricity for pumping water, powering communications 
equipment, and even lighting our homes and running our appliances. In a 
surprising number of cases, PV power is the cheapest form of electricity 
for performing these tasks.

The most important parts of a solar cell are the semiconductor layers, 
because this is where the electron current is created. There are a number 
of different materials suitable for making these semiconducting layers, and 
each has benefits and drawbacks. Unfortunately, there is no one ideal 
material for all types of cells and applications. 

In addition to the semiconducting materials, solar cells consist of a top 
metallic grid or other electrical contact to collect electrons from the 
semiconductor and transfer them to the external load, and a back contact 
layer to complete the electrical circuit. Then, on top of the complete cell 
is typically a glass cover or other type of transparent encapsulant to seal 
the cell and keep weather out, and an antireflective coating to keep the 
cell from reflecting the light back away from the cell.

The "photovoltaic effect" is the basic physical process through which a PV 
cell converts sunlight into electricity. Sunlight is composed of photons, 
or particles of solar energy. These photons contain various amounts of 
energy corresponding to the different wavelengths of the solar spectrum. 
When photons strike a PV cell, they may be reflected or absorbed, or they 
may pass right through. Only the absorbed photons generate electricity. 
When this happens, the energy of the photon is transferred to an electron 
in an atom of the cell (which is actually a semiconductor). With its 
newfound energy, the electron is able to escape from its normal position 
associated with that atom to become part of the current in an electrical 
circuit. By leaving this position, the electron causes a "hole" to form. 
Special electrical properties of the PV cell--a built-in electric 
field--provide the voltage needed to drive the current through an external 
load (such as a light bulb).

To induce the electric field within a PV cell, two separate semiconductors 
are sandwiched together. The "p" and "n" types of semiconductors correspond 
to "positive" and "negative" because of their abundance of holes or 
electrons (the extra electrons make an "n" type because an electron 
actually has a negative charge). 

Although both materials are electrically neutral, n-type silicon has excess 
electrons and p-type silicon has excess holes. Sandwiching these together 
creates a p/n junction at their interface, thereby creating an electric 

When the p-type and n-type semiconductors are sandwiched together, the 
excess electrons in the n-type material flow to the p-type, and the holes 
thereby vacated during this process flow to the n-type. (The concept of a 
hole moving is somewhat like looking at a bubble in a liquid. Although it's 
the liquid that is actually moving, it's easier to describe the motion of 
the bubble as it moves in the opposite direction.) Through this electron 
and hole flow, the two semiconductors act as a battery, creating an 
electric field at the surface where they meet (known as the "junction"). 
It's this field that causes the electrons to jump from the semiconductor 
out toward the surface and make them available for the electrical circuit. 
At this same time, the holes move in the opposite direction, toward the 
positive surface, where they await incoming electrons.

The most common way of making p-type or n-type silicon material is to add 
an element that has an extra electron or is lacking an electron. In 
silicon, we use a process called "doping." 

Silicon was the semiconductor material used in the earliest successful PV 
devices, and is still the most widely used PV material, and, although other 
PV materials and designs exploit the PV effect in slightly different ways, 
knowing how the effect works in crystalline silicon gives us a basic 
understanding of how it works in all devices.

In a PV cell, photons are absorbed in the p layer. It's very important to 
"tune" this layer to the properties of the incoming photons to absorb as 
many as possible and thereby free as many electrons as possible. Another 
challenge is to keep the electrons from meeting up with holes and 
"recombining" with them before they can escape the cell. To do this, we 
design the material so that the electrons are freed as close to the 
junction as possible, so that the electric field can help send them through 
the "conduction" layer (the n layer) and out into the electric circuit. By 
maximizing all these characteristics, we improve the conversion efficiency 
of the PV cell.

For more information on photovoltaics, check out the following site on the 

US Department of Energy PV Program

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