做菜的方法H IGH E FFICIENCY M ULTI-JUNCTION III-V S OLAR C ELLS
外交大学
Submitted by:
Nathan Gibbs
Seth Thorp
Duc Vu
Submitted to:
Dr. Dalal
Abstract: The quest for higher efficiency solar cells is ongoing. This paper address the physics, technology, achievements and future directions of high efficiency III-V multi-junction solar cells. Basic solar cell operation and multi-junction solar cell operation are addresd, as well as issues in multi-junction devices such as current, lattice and bandgap matching, and tunnel junctions. Monolithic and stacked multi-junction solar cell operation is explained. Current efficiencies, (Monolithic - InGaP/GaAs/Ge, 32+/-1.5%, Stacked – InGaP/GaAs/GaSb, 35% as of June 2003) are shown, then fut
ure directions which could yield cells with 40% efficiency or higher, including InGaN, 4 layer multi-junction devices, and the development of a material with a 1.25 eV bandgap are discusd.
Introduction
III-V multi-junction solar cells have the potential for efficiencies of over 40% and have applications in terrestrial and space applications [15]. Current high-efficiency multi-junction solar cells are either grown monolithically or stacked mechanically, and usually consist of InGaP/GaAs/Ge (monolithic) or InGaP/GaAs/GaSb (stacked) [8]. This paper will discuss the physics of single and multi-junction cells, address bandgap choices, then explain and contrast monolithic and stacked cells. Monolithic cells also face the issues of current, lattice and bandgap matching, and tunnel junctions, so tho concepts are addresd. Finally, current cell efficiencies and future directions, such as InGaN cells, four layer multi-junction cells, and the development of a material with a 1.25 eV bandgap are discusd.
徐州小吃
Physics
Solar Cell Background
飞鸟集赏析>限制物权Light is transmitted as photons which have energies given by:
E=hν or E=1.24/λ
Where E is energy in electron-volts, h is plank’s constant, ν is frequency and λ is the wavelength of light (in µm). The spectrum of light emitted by the sun is given in figure 1. As one can e from the figure, most of the energy emitted by the sun is between .1 and 4 eV [4].
Figure 1. Solar Energy Spectrum [4]
A solar cell operates on the principle of a photon exciting an electron from the valence band to the conduction band in a p-n junction. The minority carrier then travers the circuit, providing its energy to the load as en in figure 2.天秤座读音
Figure 2. Basic Solar Cell operation
Only photons created within one diffusion length of the depletion region will be swept across the junction by the electric field. All other generated electron-hole pairs (EHP’s) will recombine, without providing energy to the load.
The Low Efficiency of a Single Junction Solar Cell
自我评价高中
Single junction photovoltaic cells are fabricated out of one compound. The u of one compound means there is only one bandgap. Many incoming photons have energy greater than the bandgap of the miconductor. The photons are absorbed, but lo much of their energy to the lattice as thermal loss before reaching the conduction band as en in figure 3.
Figure 3. Low efficiency of single junction cells
So the designer of a solar cell has to make a compromi. A materiel with a large bandgap will absorb high energy photons without much thermal loss, but won’t absorb low energy photons. A mat
eriel with a small bandgap would result in more absorptions and thus higher photocurrent, but the high energy photons would lo most of their energy to the lattice, decreasing the photovoltage. To eliminate this design restriction, solar cells with multiple compounds that have different bandgaps are created in ries, creating a multi-junction solar cell.
Multi-Junction Solar Cells
牧童之歌
The principle of multi-junction solar cells is that the miconductors are stacked to minimize thermal loss. The reason this works is that incident light first strikes a miconductor with a large bandgap. Light with energy less than the bandgap of the first miconductor can’t be absorbed by the first layer (it would excite an electron into the bandgap which is not allowed), so it pass to the cond layer which has a smaller bandgap. Again, the light with energy less than the bandgap of the cond layer can’t be absorbed by the cond layer and pass to the third layer, which has the smallest bandgap energy. (figure 4a). Stacking the layers means light absorbed in each layer is more likely to have an energy clo to the bandgap energy. This means that thermal loss is minimized in the solar cell (figure 4b).
Figure 4 Basic multi-junction cell operation
a. b.
Technology of Multi-Junction Solar Cells
Currently there are many ways to design a multi-junction solar cell. The layers of the cells can be grown monolithically or they can be mechanically stacked. For monolithic cells, each layer is quentially grown on top of one another. Mechanically stacked cell’s layers are
