From the thermodynamic point of view how does a photovoltaic cell works? Where can I find info about it ?

Question

Can anyone one around the world help me with this topic?

Where can I find information about it ?

I need to analyze from the thermodynamic point of view a photovoltaic cell, which property or law applies to this technology ?. Where can I find information about the convertion light efficiency to electrical work ?

thanks

Answer 1

Currently a maximum possible efficiency of 26% with a probably average of only 15% max since most solar cells are fixed arrays.  Those arrays that track the sun can get an extra 6% like those on the International Space Station, however most arrays are fixed.  Some energy is lost when the silicon wafers are not properly aligned to conduct electricity, more energy simply passes through and some energy only heats up the solar cell.

Germany is the current leader in solar cell technology and are in the process of converting the majority of their electrical grid to solar power.  With nanotechnology the silicon wafer elements can be laid down in straighter lines so reaching possible efficiencies of 30%-60%.  The main result for current inefficiency is that all the elements are not laid down exactly in line and so some power generated is lost, the second reason is that some of the silicon atoms only absorb the photons to heat up and the silicon wafers are so thick as to not let much sunlight reach the bottom layers.

Solar Cells operate by absorbing sunlight, some of that sunlight (roughly 1/3) hits the silicon wafers.  Some of these photons are not energetic enough to knock the outer electrons free and so only warm up.  Those photons able to knock electrons free do so by exciting the silicon electrons in the atom's outer shell and knocking some lose; that can then flow as current.  From a thermodynamic point of view the excess energy of the sun (in photons) is shined on the solar cell and it either warms up the solar cell, it is lost or it generates sufficient energy to cause an electron flow.

According to Wikipedia:  http://en.wikipedia.org/wiki/Photovoltai...

"At high noon on a cloudless day at the equator, the power of the sun is about 1 kW/m², on the Earth's surface, to a plane that is perpendicular to the sun's rays. As such, PV arrays can track the sun through each day to greatly enhance energy collection. However, tracking devices add cost, and require maintenance, so it is more common for PV arrays to have fixed mounts that tilt the array and face due South in the Northern Hemisphere (in the Southern Hemisphere, they should point due North). The tilt angle, from horizontal, can be varied for season, but if fixed, should be set to give optimal array output during the peak electrical demand portion of a typical year. For large systems, the energy gained by using tracking systems outweighs the added complexity (trackers can increase efficiency by 30% or more). PV arrays that approach or exceed one megawatt often use solar trackers. Accounting for clouds, and the fact that most of the world is not on the equator, and that the sun sets in the evening, the correct measure of solar power is insolation – the average number of kilowatt-hours per square meter per day. For the weather and latitudes of the United States and Europe, typical insolation ranges from 4 kWh/m²/day in northern climes to 6.5 kWh/m²/day in the sunniest regions. Typical solar panels have an average efficiency of 12%, with the best commercially available panels at 20%"

According to Wikipedia:  http://en.wikipedia.org/wiki/Photovoltai...

"Current research is targeting conversion efficiencies of 30-60% while retaining low cost materials and manufacturing techniques. There are a few approaches to achieving these high efficiencies."

Nanotechnology is the best so far that is under development.  With it very thin multiple layers can be laid down to absorb more of the incoming light and the silicon atoms can be arraigned in straight paths to be more efficient.

According to Wikipedia:  http://en.wikipedia.org/wiki/Photovoltai...

"Simple explanation

Photons in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon.

Electrons (negatively charged) are knocked loose from their atoms, allowing them to flow through the material to produce electricity. The complementary positive charges that are also created (like bubbles) are called holes and flow in the direction opposite of the electrons in a silicon solar panel.

An array of solar panels converts solar energy into a usable amount of direct current (DC) electricity.

Photogeneration of charge carriers

When a photon hits a piece of silicon, one of three things can happen:

the photon can pass straight through the silicon — this (generally) happens for lower energy photons,

the photon can reflect off the surface,

the photon can be absorbed by the silicon, if the photon energy is higher than the silicon band gap value. This generates an electron-hole pair and sometimes heat, depending on the band structure.

When a photon is absorbed, its energy is given to an electron in the crystal lattice. Usually this electron is in the valence band, and is tightly bound in covalent bonds between neighboring atoms, and hence unable to move far. The energy given to it by the photon "excites" it into the conduction band, where it is free to move around within the semiconductor. The covalent bond that the electron was previously a part of now has one fewer electron — this is known as a hole. The presence of a missing covalent bond allows the bonded electrons of neighboring atoms to move into the "hole," leaving another hole behind, and in this way a hole can move through the lattice. Thus, it can be said that photons absorbed in the semiconductor create mobile electron-hole pairs.

A photon need only have greater energy than that of the band gap in order to excite an electron from the valence band into the conduction band. However, the solar frequency spectrum approximates a black body spectrum at ~6000 K, and as such, much of the solar radiation reaching the Earth is composed of photons with energies greater than the band gap of silicon. These higher energy photons will be absorbed by the solar cell, but the difference in energy between these photons and the silicon band gap is converted into heat (via lattice vibrations — called phonons) rather than into usable electrical energy....

Thermodynamic Efficiency Limit

Solar cells operate as quantum energy conversion devices, and are therefore subject to the "Thermodynamic Efficiency Limit". Photons with an energy below the band gap of the absorber material cannot generate a hole-electron pair, and so their energy is not converted to useful output and only generates heat if absorbed. For photons with an energy above the band gap energy, only a fraction of the energy above the band gap can be converted to useful output. When a photon of greater energy is absorbed, the excess energy above the band gap is converted to kinetic energy of the carrier combination. The excess kinetic energy is converted to heat through phonon interactions as the kinetic energy of the carriers slows to equilibrium velocity.

Solar cells with multiple band gap absorber materials are able to more efficiently convert the solar spectrum. By using multiple band gaps, the solar spectrum may be broken down into smaller bins where the thermodynamic efficiency limit is higher for each bin.

Quantum efficiency

As described above, when a photon is absorbed by a solar cell it is converted to an electron-hole pair. This electron-hole pair may then travel to the surface of the solar cell and contribute to the current produced by the cell; such a carrier is said to be collected. Alternatively, the carrier may give up its energy and once again become bound to an atom within the solar cell without reaching the surface; this is called recombination, and carriers that recombine do not contribute to the production of electrical current.

Quantum efficiency refers to the percentage of photons that are converted to electric current (i.e., collected carriers) when the cell is operated under short circuit conditions. External quantum efficiency is the fraction of incident photons that are converted to electrical current, while internal quantum efficiency is the fraction of absorbed photons that are converted to electrical current. Mathematically, internal quantum efficiency is related to external quantum efficiency by the reflectance of the solar cell; given a perfect anti-reflection coating, they are the same.

Quantum efficiency should not be confused with energy conversion efficiency, as it does not convey information about the power collected from the solar cell. Furthermore, quantum efficiency is most usefully expressed as a spectral measurement (that is, as a function of photon wavelength or energy). Since some wavelengths are absorbed more effectively than others in most semiconductors, spectral measurements of quantum efficiency can yield information about which parts of a particular solar cell design are most in need of improvement"

There is a famous incident of President Jimmy Carter visiting a solar cell farm and almost touching an active plant that would have given him a huge shock.

While standard glass is mostly silicon it is in no way aligned to generate a current; the alignment process must be intentional.