Design And Fabrication Of Broad Spectrum Antireflective Film For GaAs - Based Multijunction Solar Cells

Improving the photoelectric conversion efficiency and reducing production costshas been the purpose for development of photovoltaic technology. Besides,multi-junction solar cell technology is one of the effective ways to improve thephotoelectric conversion efficiency of solar cells. Due to their performance superiority,GaAs-based multi-junction solar cells have achieved higher efficiency than any othertypes of solar cells. The absorption spectrum of GaAs-based multi-junction solar cell isfurther broaden, which is about300~1800nm, almost realizing the absorption of thefull solar spectrum. But the traditional double-layer antireflection coating can notachieve ideal reflection effect in the wide spectrum range. Therefore, in order toreduce the reflection loss of multi-junction solar cells, it is necessary to search forbetter anti-reflection structures with low reflectance in a wide range of bsorptionspectrum.In this paper, multi-layer antireflection coatings were designed and optimized inthe wavelength range of350~1800nm by the TFC thin-film design software. Andexperimental study was carried out on the basis of theoretical design. Much researchwas firstly done on the preparation process of the single-layer film. And the layerswere characterized with Ellipsometry, X-ray diffractometer (XRD) and atomic forcemicroscope (AFM). The results showed that the refractive indexes of TiO2and SiO2single films both increased with the decreasing of oxygen partial pressure and theincreasing of electron beam current and substrate temperature. Among them, theprocess parameters had greater effect on the properties of TiO2film. The surfacemorphology of ZnS thin film at300℃was uniform and compact, presenting thebetter structural and optical properties.Then the TiO2/SiO2double-layer, ZnS/Al2O3/MgF2and ZnS/MgF2/ZnS/MgF2multilayer antireflection coatings were prepared on GaAs substrate. And opticalspectrophotometer was used to evaluate the quality of the layers for photovoltaic applications. The average reflectance of TiO2/SiO2double-layer, ZnS/Al2O3/MgF2andZnS/MgF2/ZnS/MgF2in the whole wavelength range was respectively13%,10.82%and8.55%. Among them, the double-layer antireflection coating of TiO2/SiO2had agood antireflection effect in the wavelength range of400~1000nm, while theantireflection effect is poor in the wavelength of1000nm~1800nm.The three-layeranti-reflection coating of ZnS/Al2O3/MgF2had a good antireflection effect in thewavelength of800~1800nm, while the antireflection effect was not very satisfactoryin the visible range. The four-layer anti-reflection coating of ZnS/MgF2/ZnS/MgF2exibited the best antireflection effect in the whole wavelength range of350~1800nmand the minimum reflectances wereobtained at500nm and1200nm, respectivly.