In order to understand whether luminescent down-shifting of incident photons caused by dye-doping has the potential to improve the efficiency of modern solar cells, it is first necessary to understand how solar cells function. Essentially, solar cells generate electricity by converting energy from the sun's light into electric current. Solar cells consist primarily of a semiconductor panel, which has relatively weakly bonded electrons in its valence shell. [1] If the cell absorbs a sufficient amount of energy, these electrons have the potential to break free of the bonds binding them to the atom. Therefore, solar cells absorb energy from photons of light, and they release electrons, which are then free to flow as current. Once the electron's energy is used to do useful work, it is restored to the semiconductor at its original energy, completing the cycle. The voltage delivered by a solar cell is determined by the band gap of the semiconductor, which corresponds to the amount of energy required to free an electron. [1] The photons of light that are absorbed by the solar cell must contain enough energy to free an electron, or else they simply produce heat. Therefore, the efficiency of a solar cell is determined by its ability to absorb photons and successfully extract enough energy from them to free electrons and create current.
Solar cells have numerous advantages that make them a promising area of research. Since solar cells extract their energy from the sun, they can be effective almost anywhere on the Earth's surface or even in space. [1] Furthermore, the sun's energy is truly renewable and there are minimal effects to the climate due to the lack of emissions or necessity for waste disposal. However, there are also several key drawbacks which have prevented solar cells from being more widely deployed. First, the energy generated by solar cells is unpredictable in the short term, making it difficult to integrate into an energy plan. Since the energy cannot be effectively stored, energy generated by solar cells during the day cannot be used to meet peak demands at night. Most importantly of all, however, modern solar cells are not efficient enough to compete with other energy sources. [1] As a result, there is significant ongoing research into ways to improve the efficiency of solar cells.
One common solar cell currently available on the market is based upon a semiconductor consisting of copper, indium, gallium, and selenium. [2] For this reason, they are commonly referred to as CIGS solar cells. CIGS solar cells also typically contain a high-resistance zinc oxide layer and a cadmium sulfide layer which serves as a buffer. [2] CIGS solar cells have been reported to operate at efficiencies as high as 18-20%. [2-4] However, this efficiency still falls well short of the theoretical maximum efficiency for a solar cell, which is approximately 33%. [5,6] In particular, CIGS solar cells are especially inefficient at absorbing short-wavelength photons in the range of 300-460 nm. [5,7] One of the primary factors that contributes to this loss of efficiency is the fact that short-wavelength photons are preferentially absorbed by the resistance and buffer layers, which prevents them from reaching the semiconductor. [5,7] Dye-doping is one of the potential strategies presented to counteract this effect and increase the efficiency of CIGS cells.
The idea of dye-doping is to add a layer of luminescent paint to the surface of the solar cell. [5,7] This paint layer absorbs the incident photons before they reach the other layers of the panel. After the paint absorbs the photons, it reemits them at a higher wavelength. Since the photons are now at a higher wavelength, they will no longer be absorbed by the other layers, meaning that they can successfully reach the semiconductor and generate electricity. This process of absorbing and reemitting photons at a higher wavelength is called luminescent down-shifting, where the term "down" refers to the fact that the frequency of the photons is decreased. [5,7]
There are numerous important properties that these doping layers must possess to improve the overall efficiency of the cell. Foremost, they must be carefully engineered to absorb photons only in the desired range of wavelengths. [7] Otherwise, these layers would absorb photons which would otherwise have successfully reached the semiconductor, reducing the efficiency of the cell. Also, the material must emit as many photons as possible to increase the number of photons that reach the semiconductor. Ideally, the layer would emit one photon for each one absorbed, so that no photons are lost during the absorption and reemission process. [7] It is unavoidable that the photons emitted by the luminescent layer will be emitted in all directions. Therefore, some photons will emitted back out the front of the cell and lost. However, this effect can be minimized by choosing a material with an appropriate refractive index. [7]
Experiments have shown that adding a luminescent dope layer to the surface of a solar cell can increase its efficiency with regard to short-wavelength absorption by 25-40%. [5] Furthermore, since adding a dope layer to a solar cell does not require any modification to the underlying solar cell, this technology can easily be deployed to solar cells currently in operation. Indeed, the inclusion of a dye layer in the production of solar cells is estimated to cost less than one cent per meter squared. [7] Therefore, dye-doping has the potential to be an extremely cost-effective means of increasing the efficiency of solar cells.
© Will Thomas. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] S. Hegedus and A. Luque, "Achievements and Challenges of Solar Electricity from Photovoltaics," in Handbook of Photovoltiac Science and Engineering, ed. by A. Luque and S. Hegedus, 2nd. ed. (Wiley, 2011), p. 1.
[2] W. N. Shafarman, S. Siebentritt, and L. Stolt, "Cu(InGa)Se2 Solar Cells," in Handbook of Photovoltaic Science and Engineering, ed. by A. Luque and S. Hegedus, 2nd. ed. (Wiley, Chichester, 2011), p. 546.
[3] P. Jackson et al., "New World Record Efficiency for Cu(In,Ga)Se2 Thin-Film Solar Cells Beyond 20%," Prog. Photovoltaics 19, 894 (2011).
[4] K. Ashe, "Photocell Economic History," Physics 240, Stanford University, Fall 2010.
[5] E. Klampaftis et al., "Increase in Short-Wavelength Response of Encapsulated CIGS Devices by Doping the Encapsulation Layer with Luminescent Material," Solar Energy Materials and Solar Cells 101, 62 (2012).
[6] A. Kalantarian, "Fundamental Photovoltaic Limits," Physics 240, Stanford University, Fall 2010.
[7] C. P. Thomas, A. B. Wedding and S. O. Martin, "Theoretical Enhancement of Solar Cell Efficiency by the Application of an Idea 'Down-Shifting' Thin Film," Solar Energy Materials and Solar Cells 98, 455 (2012).