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Effect of Thickness on Surface Properties and Efficiency of Silicon Cell Thin Films
Ghayah M. Alsulaim, A.A. El-Amin and Y.H. Elbashar
Since silicon (Si) is an indirect bandgap material, it is generally observed that the thickness of Si required to absorb usable sunlight should be greater than (α band edge), where α band edge represents the wavelength (λ) absorption coefficient of light corresponding to the near-band. This implies that the wafer thickness for sufficient absorption of the solar spectrum is >700 μm. This is quite a large thickness for a Si wafer and is not desirable for commercial production of solar cells for two reasons: the wafer cost can be very high and its effectiveness for collection of photo-generated carriers will be small because it is difficult to have a minority-carrier diffusion length (MCDL) comparable to such a large wafer thickness. Thus, for practical reasons, wafer thickness must be less than this value. Furthermore, detailed models that take into account surface characteristics and the multireflections within the wafer show that absorption can be greatly enhanced; thus, the need for such a thick wafer is diminished. Later, we will show that by employing an appropriate structure, a very thin layer of Si can offer a high degree of absorption of the solar spectrum nearly as much as a thick wafer.
Keywords: Maximum achievable current density, thickness, efficiency