Quantum mechanics allows heat absorbed in submicron thin metal films over the full solar spectrum to be converted to electrical current by the photoelectric effect.
In 1901, Nikola Tesla described the photoelectric effect in US patent “Apparatus for the Utilization of Radiant Energy.” Charging was accomplished by using a metal plate exposed to ultraviolet (UV) radiation. If applied to solar cells, a polished insulated metal plate will gain a positive charge as electrons ejected from the UV content in sunlight are continually drained to a capacitor. See http://en.wikipedia.org/wiki/Photoelectric_effect In his patent, Tesla noted that as the radiation falls on the metal plate, the capacitor will charge indefinitely. One of Tesla’s many US and Foreign patents is shown below.
Today, solar cells are generally not based on Tesla’s photoelectric effect. Instead, the photovoltaic (PV) effect is used where lower intensity visible (VIS) light moves electrons out of the valence band of semiconductors into higher-energy conduction bands, thereby producing electric current at a voltage related to the band-gap energy. But with PV’s made from single crystal or multi crystal semiconductors, the materials comprise up to 40% of the unit cost. Because of this, PV solar cells comprising very thin films of amorphous silicon or copper indium gallium selenide (CIGS) are of great interest because the thin films allow many more cells to be made with the same material, thereby significantly lowering costs. But unlike Tesla’s metal plate that absorbs almost all VIS and UV radiation, semiconductor and CIGS at thin film thicknesses lack the absorption necessary to efficiently capture the full solar spectrm. Organic PV cells now being considered in thin film technology are limited to thicknesses of about 3 microns. See Hong Kong Winter School on Solar Cells at http://physics.hkbu.edu.hk/home/Winterschool.html
Moreover, thin film PV cells are usually limited to the VIS part of the solar spectrum. Infrared (IR) light with a wavelength between 0.7 and 300 microns cannot be utilized in PV cells by moving electrons between the valence and conduction bands or by ejecting electrons from metals by Tesla’s photoelectric effect. Nevertheless, IR light comprises a large fraction of sunlight that is lost in the PV solar cells. At sea level, bright sunlight provides about 1000 watts per square meter at sea level. Of this, 527 watts is IR with 445 and 32 watts per square meter in the VIS and UV, respectively.
Submicron Thin Metal Films
Thin film PV technology based on silicon, CIGS, and organic materials is conceptually limited because solar radiation cannot be efficiently absorbed in thicknesses less than about 3 microns. However, thin metal films absorb from the UV to the IR even at submicron thicknesses. In effect, all solar radiation is absorbed in metals, but in thicknesses of a more than a few microns is converted to heat. Neither PV’s or Tesla’s photoelectric effect convert heat to electricity, and therefore another mechanism is required to allow thin metal films to efficiently function at solar cells.
QED Induced Radiation
QED induced radiation allows heat absorbed in thin metal films over the full solar spectrum to be converted to electricity. Here QED stands for quantum electrodynamics. In effect, Tesla’s photoelectric effect is extended to submicron thin film technology. How heat absorbed is converted to electrical current can be understood by quantum mechanics
Classically, heat is transferred by convection, radiation, and conduction, but in thin films is restricted by quantum mechanics to vanishing heat capacity in the thickness direction. Although the specific heat remains at macroscopic values in the in-plane directions, this is inconsequential because there is little if any in-plane temperature changes. See http://www.nanoqed.org/ at “Nanofluids and Thin Films”, 2009.
Quantum Mechanics Restrictions
The quantum mechanics restriction is described in the Einstein-Hopf relation for the harmonic oscillator that shows the average Planck energy of an atom at temperature is dispersed with wavelength. At room temperature, the thermal kT energy of the oscillator rapidly vanishes below wavelengths of about 50 microns, and therefore submicron thin films lack heat capacity because their thickness excludes all thermal wavelengths beyond about 1 micron. Here k is Boltzmann’s constant and T absolute temperature. What this means is all solar radiation irrespective of its wavelength that is absorbed in submicron thin films cannot be conserved by an increase in temperature.
Conservation of absorbed Solar Radiation
Nevertheless, the absorbed heat must be conserved. Typically, submicron thin films have EM confinement frequencies in the thickness direction beyond the UV. Here EM stands for electromagnetic. Since heat is low frequency EM energy, conservation may proceed by inducing the heat by QED to be frequency up-converted to levels beyond the UV. In effect, submicron thin films act as frequency up-conversion devices converting VIS and IR solar radiation to UV radiation that has the Planck energy that charges the film by Tesla’s photoelectric effect. In contrast, thin metal films having thicknesses greater than a few microns increase in temperature upon absorbing solar radiation and are inconsequential in solar energy conversion.
1. Thin film technology in PV solar cells is conceptually limited because silicon, CIGS, and organic materials lack the absorption of solar radiation at thicknesses less than about 3 microns.
2. Metal thin films regardless of thickness allow absorption of solar radiation from the UV through the VIS to the IR.
3. Thin metal films having thicknesses greater than a few microns increase in temperature upon the absorption of solar radiation. But in submicron thin films, quantum mechanics precludes the conservation of absorbed solar radiation by an increase in temperature.
4. Conservation of absorbed solar energy in submicron thin metal films may only proceed by QED induced frequency up-conversion to the EM confinement frequency of the thin film in the thickness direction, the latter in the UV and beyond.
5. Thin metal films induce the QED up-conversion of absorbed VIS and IR radiation to UV levels and beyond necessary to free electrons and charge the thin metal film. Without QED induced radiation, the VIS and IR lack the Planck energy to free electrons and produce electrical current.
6. The consequence of QED induced radiation is that submicron thin metal films by Tesla’s photoelectric effect offer the possibility of utilizing the full solar spectrum to produce electrical current.