Single Junction GaAs Solar Cells
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%.
