Before discussing multi-junction solar cells, let’s review the basic reasons for the energy conversion limitations of single-junction solar cells. Consider an ideal solar cell made of a material with a semiconductor bandgap. When the photon energy is incident inside the cell, the photon will be absorbed and converted into electrical energy, but the electrical energy converted at this time has only excess energy, which will become heat dissipation. And when h v < E, the photons will not be absorbed into electricity. And only in the case of hν>E, the efficiency of converting light energy into electrical energy can reach the highest. It should be noted that even in the case of hv = E, the maximum energy conversion efficiency is still below 100%.
The range of the solar spectrum is very wide, ranging from 0 to 4 eV, so the conversion efficiency of single-junction solar cells to sunlight is naturally much lower than that of single-frequency light. A simple idea to solve this problem is to divide the solar spectrum into several bands according to its energy, and each band uses a solar cell with a suitable energy bandgap to convert light energy into electrical energy, so as to improve the efficiency of converting light energy into electrical energy. E.g. Divide the solar spectrum into blue bands hv1 , hv2 , hv3 , hv4 , hv4 , hv5 , where hv1 <hv2 <hv3 and the photons in these spectral ranges will use three different band gaps of the solar energy Eg1 = hv1 , Eg2 =hv2, Eg3 =hv3 to convert light energy. If the number of spectral bands can be divided into more, higher energy conversion efficiency can theoretically be achieved.
The scholar Henry has calculated under the conditions of A Ml. 5 and 1 sun. Energy conversion efficiency limit of multijunction solar cells. When the number of junctions of the solar cell is 1, 2, 3 and 36, the highest energy conversion efficiencies are 37%, 50%, 56% and 72%, respectively. When the energy conversion efficiency is changed from a single-junction solar cell to a 2-junction solar cell, the increase is most obvious, and the improvement of the energy conversion efficiency of more junction pairs becomes limited. This calculation is good news in some ways, because it is very difficult to actually make a solar cell with 4 or 5 junctions, and if a multi-junction solar cell is to really improve the energy conversion efficiency, each It is very important whether the energy band gap of a sub cell material is selected correctly.
Using high-efficiency single-crystal semiconductor solar cells on the surface to generate electricity, whether it is a single-crystal Si solar cell or a GaAs solar cell, the cost of the battery is too expensive. An alternative is to use a concentrating solar cell power generation system, using cheap concentrating modules to reduce the usage of single crystal semiconductor solar cells to one percent, or even one thousandth.
Table 1[Z] shows the structural parameters of concentrating GaAs solar cells, including P on-N type and N-on-P type. Compared with non-concentrating GaAs solar cells, the structure of concentrating GaAs solar cells has an extra layer under the base layer to reflect secondary carriers back to improve cell efficiency. The concentrating design can not only reduce the amount of solar cells, but also improve the conversion efficiency of solar cells, which is mainly due to the increase of the open circuit voltage Voe and the increase of the fill factor. At present, the highest conversion efficiency of GaAs solar cells is 27.6% (AM 1.5, 255 suns) under concentrated light conditions.
Table 1 – Structural Parameters of Concentrating GaAs Solar Cells
In concentrated solar cell power generation system. Since a single solar cell produces currents of several amperes or more, series resistance has a very large effect on efficiency. Too high series resistance will reduce the fill factor and severely reduce the cell efficiency. The main sources of series resistance include the resistance Re of the grid electrode line, the contact resistance Rs between the metal and the semiconductor (Fig. 1[2]), and the lateral sheet resistance of the current passing through the emitter layer. There are several ways to reduce the series resistance of the battery itself. click here to open to learn more.
Figure 1 – Tandem cell analysis of concentrating GaAs solar cells
(1) Increase the density of grid electrodes. Increasing the density of the grid electrode can simultaneously reduce the grid electrode line resistance, contact resistance and lateral sheet resistance of the entire solar cell, but increasing the grid electrode density often increases the shading area, thus reducing the efficiency of the cell.
(2) Increase the conductivity of the emitter layer. Increasing the conductivity of the emitter layer can reduce the lateral sheet resistance R day. Since N-type GaAs has better conductivity than P-type GaAs, the N on P structure is a better choice.
(3) Reduce contact resistance For concentrating GaAs solar cells, the contact resistance Re must usually be small. A thin layer of alloy, such as Au/Zn/Au or Au/Ge/Ni/Au, is added between the gate electrode and the semiconductor, and with suitable thermal fusion conditions, the contact resistance can be effectively reduced to the required level scope.
Figure 2 shows the electrode design of a concentrator solar cell. The radial grid electrodes guide the current from the central area of the solar cell to the periphery, although the grid electrode density is actually much higher than shown in the figure. In order to reduce the series resistance and reduce the shading area, the shading ratio of the grid electrode will be controlled at 4%~8% as much as possible.
Figure 3 shows the efficiency comparison of Pon-N type and N-on-P type concentrating GaAs solar cells under different concentrating conditions. The dense grid electrode design is used to reduce the shading area and achieve higher efficiency. However, when the concentrated light intensity exceeds 400 times, the efficiency of N-on-P type GaAs solar cells designed with low m-density grid electrodes begins to saturate, while the efficiency of N-on-P type GaAs solar cells designed with high-density grid electrodes begins to saturate. The efficiency of on-N type GaAs solar cells can continue to increase as the concentration of light increases by a factor of 1000.
Figure 3 – Efficiency comparison of Pon-N type and N-on-P type concentrating GaAs solar cells under different concentrating conditions
This result shows that the design of the grid-like concentrating GaAs solar electric electrode is the key point of the concentrating solar cell, according to the design of the grid-like electrode of the cell. The actual grid concentration magnification optimizes the design of the grid electrode, effectively reducing the density of the grid electrode, which is much higher than the series resistance shown in the figure, and at the same time reducing the shading area as much as possible to increase the photocurrent, in order to improve the efficiency of the battery.
GaAs is a material with a direct energy band gap, and the absorption effect of light is quite good. When the photon energy is larger than the bandgap of GaAs, the absorption coefficient of the material for light is about 104cm-1. Figure 1 shows the light absorption coefficients of semiconductor materials such as Si, GaAs and InP for different wavelengths, so the thickness of the GaAs solar cell structure is only about 4µm. The high absorption rate of light by the direct energy gap material is an advantage. But other problems arise when making solar cells.
There are many dangling bonds on the surface of semiconductor materials, forming a large number of surface defects. These surface defects can cause electron-hole pairs generated by illumination. Recombination on the surface of the solar cell makes the light energy unable to be converted into electrical energy, reducing the energy conversion efficiency of silver solar energy and earth. This is beneficial, and the effect has a greater impact on direct-energy semiconductor solar cells. Generally, we will use the surface recombination speed (surface recombination), the unit is cm / hour, this parameter to evaluate the influence of surface defects. The surface recombination speed of GaAs material is about 107 cm/s, which is very high. Figure 2 shows the effect of different surface recombination speeds on the optical spectral response of GaAs solar cells. The higher the surface recombination speed, the worse the quantum conversion efficiency of short-lived photons. The surface recombination speed must be reduced to below 104 cm/s, and the GaAs solar cell will have good conversion efficiency.
In order to improve the energy conversion efficiency of GaAs solar cells, the influence of surface defects must be reduced. Generally speaking, there are four methods to reduce the influence of surface defects.
(1) The first is to try to passivation the crystal surface and reduce the defect density on the surface. For example, the generation of SiO2 on the Si surface can greatly reduce the surface recombination rate. However, there is no suitable and stable oxide in GaAs material to achieve this effect.
(2) The second is to make the PN interface as close to the surface of the solar cell as possible. For materials with high light absorption efficiency such as GaAs, the depth of the PN junction must be controlled at 50 nm to reduce the influence of surface defects.
(3) The third is to use the front surface electric field (front surface field). This is the same as the principle of the back surface field of the Si solar cell, that is, a high/low doping structure is used to form a built-in electric field to prevent the secondary carriers generated by illumination from diffusing to the surface of the solar cell, so as to avoid the diffusion of the secondary carriers generated by the illumination to the surface of the solar cell. Reduce the impact of surface defects. For example, for a solar cell that was originally a P-type emitter IN-type base (P-type emitter IN-type base) structure, the P-type emitter can be changed to a high and low doped structure, and the F layer must be very thin, Reduce light absorption.
(4) The fourth is to grow another layer of window layer on the surface (window layer). The window layer is a layer of material with a large energy band gap, which can allow most of the incident light to pass through, and can prevent electrons and holes from diffusing to the surface of the solar cell and recombining under the influence of surface defects. In addition to selecting a material that allows most of the sunlight to pass through the window layer, the interface defect density between the window layer and the solar cell emitter must be very low, which means that the lattice constant of the window layer material must be very different from the emitter layer material. In order to reduce the defects caused by lattice mismatch.
GaAs solar cells can be said to be the most researched and best developed type of single-junction field V semiconductor solar cells. In the production of single-junction solar cells, the advantages of GaAs in terms of material properties are as follows
(1) The energy band gap of GaAs material is closest to the theoretical optimum value of single-junction solar cells.
(2) The radiation resistance of GaAs solar cells is better than that of Si solar cells.
(3) GaAs solar cells have a good window layer material: the lattice constant of GaAs is almost completely matched with that of GaAs, which makes it difficult to generate stacking fault defects (dislocation) when growing on GaAs, regardless of the composition ratio of Al. The high aluminum content is an indirect energy gap material, which can almost penetrate the window layer as long as the photon energy is lower than 3 eV.
(4) The conversion efficiency of GaAs solar cells is less sensitive to temperature. The energy band gap of Si semiconductor is relatively low, so when the temperature increases, the carrier density in the material changes greatly. In addition, and it is an indirect energy gap semiconductor, its carrier life time is greatly affected by phonon (phonon), that is, lattice vibration (lat t ice vibration). The carrier lifetime decreases rapidly. Therefore, the energy conversion efficiency of Si solar cells decreases rapidly as the temperature increases.
(5) The GaAs substrate is cheaper than InP, and at the same time, there are cheaper alternative substrates. The lattice constant and thermal expansion coefficient of GeoGe are almost the same as those of GaAs, which makes the Ge substrate a very good alternative substrate for the manufacture of GaAs solar cells. Moreover, the mechanical strength of Ge material is twice that of GaAs, so the thickness of GaAs solar cells made of Ge substrate can be reduced to about U = 90µm, which greatly reduces the weight of GaAs solar cells. The efficiency of solar cells made of Ge substrates is almost the same as that of solar cells made of GaAs substrates. The pool is just as good.
GaAs solar cells have been developed since the 1960s. In 1972, IBM used the LPE method to produce a heteroface GaAs solar cell. The structure of this solar cell is a P-type GaAs emitter (emitter), an N-type GaAs base (base), and a P-type AlGaAs window layer on top of the P-type GaAs. Since the AlGaAs window layer blocks the diffusion of electron-hole pairs to the surface of the GaAs solar cell and reduces the influence of surface defects of the material, the efficiency of the GaAs solar cell is greatly improved. The GaAs solar cell structure of this heteroface was also later developed into the basis of GaAs solar cells. While using LPE to develop GaAs solar cells, many people also use metal-organic vapor phase epitaxy to develop GaAs solar cells. Since metal-organic vapor phase epitaxy has excellent control over the thickness and doping concentration of the device structure, it is possible to produce high-efficiency Quite high GaAs solar cells (22%, AMO).
Figure 3 [Z] is a structural diagram of a typical GaAs solar cell, which includes an N-type epitaxial substrate, an N-type GaAs buffer layer, an N-type GaAs base, a P-type GaAs emitter, a P-type window layer, and an anti-reflection film. The epitaxial substrate can be GaAs or Ge. If it is a Ge substrate, two issues must be paid attention to in the epitaxial time.
(l) GaAs is a polar semiconductor material, while Ge is a non-polar semiconductor material. If the epitaxial conditions of the GaAs/Ge interface are not properly controlled during the epitaxial delay, an ant knock base domain will be formed on GaAs, resulting in stacking fault defects, reducing GaAs. Efficiency of solar cells.
(2) In addition, due to the high temperature of the epitaxial time, Ga and As atoms will diffuse into the Ge substrate, which will activate the Ge substrate to generate PN junction. In theory, the activated Ge substrate can form another solar cell and improve the efficiency of the GaAs solar cell, but in fact, due to the mismatch of the photocurrent between the GaAs cell and the Ge cell, the efficiency of the GaAs solar cell is reduced, so GaAs is fabricated on Ge. In the case of solar cells, it is generally tried to keep the Ge from being activated.
Solar cell structure The key to solving the above two problems lies in the N-type GaAs buffer layer. Appropriate buffer epitaxy conditions can avoid the generation of anti phase domains and Ge activation.
N-type GaAs base thickness is generally about 3. 5µm, and the doping degree is 1. 0 × 10[7 2. 0 × 1017 cm 3 o Some studies have shown that the doping material will affect the life of holes, such as using As an N-type doped GaAs material, Se has a higher hole life than Si-doped GaAs, so the conversion efficiency of the solar cell is also higher. The thickness of the P-type GaAs emitter is about 0.5 m. Generally, Zn is used as the P-type dopant, and the concentration is about 2.0 × 1018 cm. The window layer is a very important part. The higher the Al content, the higher the light transmittance of the window layer. The higher it is, it is also easy to be oxidized and deteriorated. Generally speaking, the Al ratio of the window layer is 80% ~ 85%, and then an anti-reflection film is coated as soon as possible in the solar cell manufacturing process to protect the window layer from oxidation. The thickness of the window layer is about 50 nm, and the doping concentration should be as high as possible to reduce the potential array and series resistance of the hetero interface.
The effect of antireflection coatings on solar cell performance is also very important. The optical index of refraction of GaAs is about 3.6, if there is no anti-reflection coating, more than 30% of sunlight will be reflected and cannot enter the solar cell. If a suitable thickness of SiN, single-layer anti-reflection film is used, the light reflectivity can be reduced to about 10%, while the use of MgF2/ZnS double-layer anti-reflection film can further reduce the reflected light to about 3%.
