THE FUTURE OF SOLAR CELL TECHNOLOGY
Editors Note: This submission by Giancarlo S. ('16) is a summary and explanation of the solar cells he has been studying called Intermediate Band Solar Cells (IBSC). Being that he is in 10th grade, he has not yet worked with a mentor, nor conducted any research of his own, so this article is mainly based on bibliographic research he has done.
I have been studying solar cells as my science research topic. More specifically, I am focusing on a new experimental type of solar cell, named “Intermediate Band Solar Cells” (IBSC), for their implementation of a so-called intermediate band within the device. In order to better understand why these cells are so revolutionary, I will compare them to traditional solar cells (TSC) available today.
The largest and most significant advantage IBSCs have over TSCs is their increased efficiency. TSCs have a maximum efficiency of around 32%, that is, 32% of the suns energy can be converted into electrical energy. In comparison IBSCs have a maximum theoretical efficiency of around 64%. To put this into perspective, to power an average household 600 square feet of solar paneling is required, by using IBSCs only 300 square feet would be needed. Additionally, due to the technology used in IBSCs, they work much better in low light conditions. This is significant because it means that on cloudy or overcast days IBSCs will still be producing current while TSCs will not. Finally IBSCs have the potential to be manufactured so that they are very thin and light. Reducing the footprint/mass of the cells would be beneficial on weight sensitive applications (like solar powered aircraft).
To understand how an IBSC works one first needs to understand how a TSC works. There are two layers to a TSC, the N-Layer and P-Layer. The N-Layer is commonly found on the top of the cell and possess atoms with an excess amount of electrons in its valence shell. (This excess amount of electrons is created through a process called doping).The P-Layer is traditionally found on the bottom of the cell and is composed of atoms that lack the traditional number of total valence electrons coustomary for that element. (Again, this lacking of electrons is created through doping) When a photon (light particle) strikes an electron in the N-Layer, the energy from the photon is transferred to the electron. This transfer of energy knocks the electron off the valence band and into the conduction band (the super highway for electrical current), this energized electron is now called an exciton. The exciton is eager to settle down, and so joins an atom in the P-Layer (thus filling the gap). This movement of electrons is what we call electrical current and is used to power our everyday devices.
Low energy photons (like those you might encounter on a cloudy day), however, do not possess enough energy to knock an electron from its valence shell. Unfortunately, these photons' energy is lost, and transferred into thermal energy. This “wasting” of photons is the main reason why the efficiency of TSCs is so low. A good analogy is a pool table where the cue ball is a photon, the playing balls are the electrons, the table is the N-Layer and the pockets are the P-Layer. If the cue ball (photon) does not strike the regular balls (electrons) with enough force (energy), the balls will not make it into the pocket (the P-Layer). The amount of energy, measured in eV (electron volts), required to “kick” an electron into the conduction band is called the band gap. Each material has a different band gap, and they vary based widely from metals, to semi conductors, to insulators.
In an IBSC scientists hope to utilize these previously wasted low energy photons by creating a location in which the electrons can exist that has a lower bandgap than the bandgap of the host material. For example, if the host material has a bandgap of 3eV and a electron only possesses 2eV, it cannot reach the conduction band, and the energy from the photon is lost. If, however, there is an intermediate band located at 1.5eV, than that same electron could travel to the intermediate band. A second photon could then come along and knock the electron the extra 1.5eV to the conduction band.
By utilizing this technology, solar cells would become more effective in low light conditions and be able to convert solar energy to electrical energy for more hours of the day (early morning to evening). Unfortunately scientists are still working on the practical fabrication of these devices and many issues have arisen, due to the physical fabrication, bandgap values, and structural failures. Hopefully this technology will continue to be improved and will become the next standard in solar cell production.
The largest and most significant advantage IBSCs have over TSCs is their increased efficiency. TSCs have a maximum efficiency of around 32%, that is, 32% of the suns energy can be converted into electrical energy. In comparison IBSCs have a maximum theoretical efficiency of around 64%. To put this into perspective, to power an average household 600 square feet of solar paneling is required, by using IBSCs only 300 square feet would be needed. Additionally, due to the technology used in IBSCs, they work much better in low light conditions. This is significant because it means that on cloudy or overcast days IBSCs will still be producing current while TSCs will not. Finally IBSCs have the potential to be manufactured so that they are very thin and light. Reducing the footprint/mass of the cells would be beneficial on weight sensitive applications (like solar powered aircraft).
To understand how an IBSC works one first needs to understand how a TSC works. There are two layers to a TSC, the N-Layer and P-Layer. The N-Layer is commonly found on the top of the cell and possess atoms with an excess amount of electrons in its valence shell. (This excess amount of electrons is created through a process called doping).The P-Layer is traditionally found on the bottom of the cell and is composed of atoms that lack the traditional number of total valence electrons coustomary for that element. (Again, this lacking of electrons is created through doping) When a photon (light particle) strikes an electron in the N-Layer, the energy from the photon is transferred to the electron. This transfer of energy knocks the electron off the valence band and into the conduction band (the super highway for electrical current), this energized electron is now called an exciton. The exciton is eager to settle down, and so joins an atom in the P-Layer (thus filling the gap). This movement of electrons is what we call electrical current and is used to power our everyday devices.
Low energy photons (like those you might encounter on a cloudy day), however, do not possess enough energy to knock an electron from its valence shell. Unfortunately, these photons' energy is lost, and transferred into thermal energy. This “wasting” of photons is the main reason why the efficiency of TSCs is so low. A good analogy is a pool table where the cue ball is a photon, the playing balls are the electrons, the table is the N-Layer and the pockets are the P-Layer. If the cue ball (photon) does not strike the regular balls (electrons) with enough force (energy), the balls will not make it into the pocket (the P-Layer). The amount of energy, measured in eV (electron volts), required to “kick” an electron into the conduction band is called the band gap. Each material has a different band gap, and they vary based widely from metals, to semi conductors, to insulators.
In an IBSC scientists hope to utilize these previously wasted low energy photons by creating a location in which the electrons can exist that has a lower bandgap than the bandgap of the host material. For example, if the host material has a bandgap of 3eV and a electron only possesses 2eV, it cannot reach the conduction band, and the energy from the photon is lost. If, however, there is an intermediate band located at 1.5eV, than that same electron could travel to the intermediate band. A second photon could then come along and knock the electron the extra 1.5eV to the conduction band.
By utilizing this technology, solar cells would become more effective in low light conditions and be able to convert solar energy to electrical energy for more hours of the day (early morning to evening). Unfortunately scientists are still working on the practical fabrication of these devices and many issues have arisen, due to the physical fabrication, bandgap values, and structural failures. Hopefully this technology will continue to be improved and will become the next standard in solar cell production.
