Re: Are solar panels based upon the principles of Einstein photoelectric effect

Photovoltaic cells (solar cells) work sort of like the Einstein photoelectric effect. PV (photovoltaic) cells are semiconductors, or more properly semiconductor devices made of n-type and p-type semiconductors joined together. An n=type semiconductor has electrons to carry current and a p-type has holes, or the absence of electrons to carry current. Holes in many ways act like positive charge moving in the direction of current.

Within a semiconductor there is a valence band of electrons which are not free to move and a conduction band of electrons which are free to move around like the electrons in a metal. Electrons (or holes) in the conduction band can carry current, but unless something provides energy the electrons are at an energy below the conduction band and no electric current will exist. The valence band is at a lower energy. Charge carriers (electrons or holes) have to be "promoted" (or given energy) to become conduction charge carriers. This feature of being somewhat of a conductor and somewhat of a insulator gives it the name semiconductor. Semiconductors can be classified by the lowest energy difference between the valence band and the conduction band.

So like the photoelectric effect electrons are set free by incoming photons, but this freedom is promotion to the conduction band where it will act like an electron in a metal. Silicon and germanium are the most commonly used materials because these materials are easily doped (made into n-type or p-type) and very plentiful. The threshold frequency can be adjusted based on the doping and slightly by the construction of the material.

Typically solar cells are designed to have energy gaps of 1.4 to 1.8 eV (electron volts). For pure crystalline silicon the energy gap is around 1.1 eV which corresponds to a wavelength of 1.12 micrometers. Photons with a wavelength smaller than 1.12 micrometers will be promoted to the conduction band. For sunlight this corresponds to about 77% of solar energy. However, much of the energy in excess of the 1.11 eV goes into heating the silicon crystal which degrades the performance. About 43% of the average absorbed photon energy goes into heat. Theoretically crystal silicon should work at 22% at room temperature. However, other losses like internal resistance of the cell reduce the efficiency to about 10 to 14%.

The continual promotion of electrons from valence band to conduction band has to offset by a process to replentish and repair the valence band. This process is not perfect in PV cell, so over time the material degrades. Dangling bonds (where electrons used to be) form and can lead to a failure of the device if not repaired. Repair usually means cooking the material in a dark environment -- sort of remaking it.

A typical "fresh" silicon PV cell might operate around 14% efficiency, but after a month of continuous exposure to sunlight, the efficiency might drop to 12% or less. Crystalline semiconductors are more susceptible to damage from exposure and a more expensive to make, so despite gathering more energy because of a lower energy gap, crystalline semiconductors are typically not used. Amorphous semiconductors (for PV celles) are usually preferred because they are more durable. These are made by "dripping" the molten semiconductor material unto a plate where it "freezes" into its solid form. In this way, the shape and structure of the material can be controlled very accurately.

You can find more information on PV cells on the Mad Sci Network http://www.madsci.org/MS_search.html by using the keyword photovoltaic.

References:
Thomas Sidley Cull, Jr., Ph.D. Thesis: NMR Structural Studies of PECVD Amorphous Silicon Films, Washington University (1997).
Jack J. Kraushaar and Robert A. Ristinen, Energy and Problems of a Technical Society, John Wiley & Sons, (1993).

Sincerely,

Tom "Amorphous Boy" Cull