Researches On The Irradiation Effects Of Single Junction GaAs/Ge And Monocrystalline Silicon Solar Cells Under Laser

As a new energy in the 21 st century, the development of solar cells is the focus of world attention. The production and application of semiconductor solar cells have been the attention. In the field of laser processing of the solar cells, many countries have published numerous reports. But the damage to solar cells caused by laser irradiation is rarely reported. As the absorption coefficients of various semiconductor materials at different laser wavelengths are different, and the damage mechanisms to materials of laser at different pulses are different too. In this paper, two wavelengths at 532 nm and 1064 nm with pulsed laser at nanosecond, picosecond and continuous laser are also used to irradiate the Ga As/Ge solar cells and monocrystalline solar cells. Then the solar cells are analyzed by SEM,XPS etc. before and after the laser irradiation. At last, we calculate the temperature rise curves of semiconductor material such as silicon, germanium, gallium arsenide under laser irradiation. And we get some new phenomena and new results. The main contents are as follows:Firstly, this paper studied the laser damage mechanism to Ga As/Ge cells by nanosecond, picosecond and continuous laser at 532 nm. Si O2 and Ti O2 antireflection films are transparent to 532 nm wavelength. The initial damage occurred at the surface of Ga As solar cells,and Ga As decomposed, Ge substrate was heated to melt under laser irradiation. For continuous laser, thermal cumulative effect is obvious. And the damage is due to thermal melting and thermal stress caused by the tremendous temperature difference between the cell surface radial. The damage to the solar cells by nanosecond and picosecond pulsed laser are into heat damage and mechanical damage. Thermal effects make the material melt, gasify. The high-temperature molten material would produce gaseous material, resulting in its outward pressure injection. X-ray photoelectron spectroscopy and Raman spectroscopy micro-analyzer test results showed the damaged material is germanium, electrodes are turned down, leading to short-circuit, and the solar cell could not work.Secondly, this paper studied the damage mechanism to Ga As/Ge solar cells by nanosecond, picosecond and continuous laser at 1064 nm. If ignoring the exciton absorption, multi-photon absorption, free-carrier absorption, Ga As layer and anti-reflective layer on the surface of the solar cells are transparent to 1064 nm. 1064 nm is absorbed by the surface of Ge substrate. The initial damage occurred at the interface of Ga As and Ge. The shortest pulse used in the experiment is 25 ps, which is longer than the time of the heat transfered to the crystal lattice period(10 ps), so the heat is transferred to the lattice and deposited in the lattice, the material temperature rises. The peak power density of the pulsed laser used in the experiment can be achieved GW/cm2 or more, and reaches the threshold value generating the plasma. Therefore, with the laser energy density increasing, the laser damage to the solar cells melt firstly, the damage mechanical changed to the plasma expansion then. The damage mechanical of continuous laser to Ga As is melt and thermal stress damage down the cell surface radial.Thirdly, nanosecond, picosecond pulsed laser and continuous laser at 532 nm and 1064 nm are used to irradiate the monocrystalline silicon solar cells. Then we study the damage mechanism. When the laser is focused on the surface of monocrystalline silicon cells, changes of the output parameters of the solar cell are very small. When the laser is focused on the gate line, the changes of output parameters are not very obvious yet, but it has a downward trend, relative to the results of laser focused on the surface. Since the 532 nm and 1064 nm are both within the response band of the material silicon, the laser is absorbed at the surface layer, the electron in the valence band absorbs a photo and is transfered to the conduction band, and forms a hole in the valence band, then forms a pair of electron-hole. The remaining heat is transferred to the lattice, photon energy deposes, and the solar cell damages. The initial damage threshold of monocrystalline silicon cells at 1064 nm is slightly higher than the 532 nm wavelength, this because the spectral response of related materials is different.Finally, comparative study of the damage to Ga As/Ge solar cells and the damage to monocrystalline silicon cells is carried out. Researches discovered a new phenomenon that the damage to Ga As/Ge solar cells is only due to a single damage point. The results show that when the laser focused on the gate line, the performance parameters declined, the solar cell could not work properly, while the monocrystalline silicon solar cells is significantly different. By using the scanning electron microscopy(SEM), X-ray photoelectron spectroscopy(XPS), Raman analysis spectroscopy etc., the results showed that after the irradiation, the material of monocrystalline silicon cells is still much silicon and a small amount of silicon oxide, and the main component of the damaged area in Ga As/Ge solar cells is Ge, and a certain doping makes the conductivity of Ge improve greatly. This explains why silicon cells are hard to damage, and the Ga As/Ge solar cells are damaged easily, the results of experiments and materials analysis complement each other well.