| AVS 62nd International Symposium & Exhibition | |
| Thin Film | Friday Sessions |
| Session TF+MI-FrM |
| Session: | Thin Films for Light Trapping, Plasmonic, and Magnetic Applications |
| Presenter: | Bo Yuan, University of Delaware |
| Authors: | B. Yuan, University of Delaware B. Thibeault, University of California at Santa Barbara K. Dobson, University of Delaware A. Barnett, University of New South Wales, Australia R.L. Opila, University of Delaware |
| Correspondent: | Click to Email |
Because of the ability to exploit multiple absorption bands, Multi-junction Solar Cells (MJSCs) are the most efficient solar cells ever developed. As on single junction solar cells, Antireflection Coatings (ARCs) are utilized to achieve broadband absorption. Due to the fact that the total current of MJSCs is limited by the subcell that has the lowest generated current, ARCs on MJSCs must have the ability to minimize light loss at the range of limiting cell, but optimally all across the visible spectrum.
Unlike conventional Double Layer Antireflection Coatings (DLARCs) that can only reduce light reflection at certain wavelengths, moth eye structures are able to mitigate light loss over broadband wavelength due to their smooth change of refractive index. We report the fabrication of such a Subwavelength Structure (SWS) by using wet etching and dry etching of dielectric materials. Silicon wafers are used here as the substrate to test the quality of ARCs. ZnO has been chosen as one dielectric material because of its excellent transmittance and durability. It also has a close index match to the underlying GaInP window layer in this tandem cell. Wet etching using oxalic acid has been utilized here to texture the ZnO surface because it is a simple and cost-effective method. A low reflection (less than10%) over a broad range of wavelength (400-970nm) has been achieved. However, it turns out that wet etching is not very controllable and cannot fabricate the high aspect ratio periodic structure necessary for optimal absorption. A Zn(C2O4)2H2O bulk phase was found on the ZnO surface.
Thus, lithography and plasma etching have been employed because of their better process control capability and less dependence on the crystalline orientation of the material. We then switched to using Ta2O5 since it has similar optical properties to ZnO and also a broad bandgap to ensure its transparency. Dry etching of Ta2O5 gives us nanocones with aspect ratio (height over base diameter) of 1.26. At an angle of 8 degree from normal incidence, textured Ta2O5 achieved the averaged reflection as low as 6% over 320-900nm and itoutperforms DLARCs and textured ZnO over a wide range of wavelength. Future work will focus on fabricating this moth eye structure on III-V/SiGe tandem cells and simulating the reflectance spectra using Finite Difference Time Domain methods.