Silicon absorption coefficient

Fig. 8. Optical properties of silicon (a) transmissivity vs wavelength, (b) shortwavelength absorption coefficient where the transmissivity increases sharply...

The thickness of a photovoltaic cell is chosen on the basis of its ability to absorb sunlight, which in turn depends on the bandgap and absorption coefficient of the semiconductor. For instance, 5 nm of crystalline silicon are required to absorb the same amount of sunlight as 0.1 nm of amorphous silicon and 0.01 nm of copper-indium diselenide. Only MBE and MOCVD are capable of producing such extremely thin films.i l... 

The optical properties of a-Si H are of considerable importance, especially for solar-cell applications. Because of the absence of long-range order, the momentum k is not conserved in electronic transitions. Therefore, in contrast to crystalline silicon, a-Si H behaves as though it had a direct bandgap. Its absorption coefficient for visible light is about an order of magnitude higher than that of c-Si [74]. Consequently, the typical thickness (sub-micrometer) of an a-Si H solar cell is only a fraction of that of a c-Si cell. 

Absorption characteristics, of hydrogenated amorphous silicon films, 22 133-134 Absorption coefficients, 23 126 Absorption-desorption towers, 10 614 Absorption maxima, vinylene shifts of,... 

Silicon is the most popular material for photovoltaic (PV) power. Another material is gallium arsenide (GaAs), which is a compound semiconductor. GaAs has a crystal structure similar to that of silicon, but it consists of alternating gallium and arsenic atoms. It is well suited for PV applications since it has a high light absorption coefficient and only a thin layer of material is required, which reduces the cost. 

The reflectivity of silicon at 633 nm is 35% and the absorption coefficient is 3.8 x 10 m . Calculate the following qnantities for a 10 p,m-thick sample of silicon at this wavelength. Person 1 Calculate the fraction of incident light that is absorbed. 

Figure 7.14 Imaginary part of the dielectric constant 62 as a function of photon energy, hv, for amorphous and crystalline silicon. (After Pierce Spicer, 1972.) Since Sj is proportional to the optical absorption coefficient a, Sj is a dimensionless measure of optical absorption.

Optical absorption measurements give band-gap data for cubic silicon carbide as 2.2 eV and for the CC-form as 2.86 eV at 300 K (55). In the region of low absorption coefficients, optical transitions are indirect whereas direct transitions predominate for quantum energies above 6 eV. The electron affinity is about 4 eV. The electronic bonding in silicon carbide is considered to be predominantly covalent in nature, but with some ionic character (55). In a Raman scattering study of valley-orbit transitions in 6H-silicon carbide, three electron transitions were observed, one for each of the inequivalent nitrogen donor sites in the silicon carbide lattice (56). The donor ionization energy for the three sites had values of 0.105, 0.140, and 0.143 eV (57). 

The cell has an optical capability of 2.5 m, using 12 transversals. When applied to CO2, however, the absorption coefficient (ac) is of sufficient size to allow measurements using an 0.5 m (4 transversals) path. Due to the magnitude of ac, a large difference in intensity develops as the beam passes through the cell. Maximum detection and amplification efficiency can only be achieved when the intensity levels of the sample and reference beams are approximately the same. To help balance the intensity levels, the mirror coatings (multi-layer silicon/silicon dioxide) have been optimized to pass the sample beam (99 percent reflectance at 4.3 pm) and partially attenuate the reference wavelength (95 percent... 

Because of the low absorption coefficient of amorphous and microcrystalline silicon, it is mandatory to optimize light scattering within thin film silicon solar cells by the use of suitably textured (rough) interfaces and surfaces. This paragraph comments about the ideal surface roughness for ZnO layers deposited by CVD, and used as front or back contacts within amorphous, microcrystalline (and micromorph) solar cells. 

Gordon et al. [59] proposed a figure of merit defined by the ratio of the electrical conductivity over the optical absorption coefficient in the visible spectral range. They tested many dopants for AP-CVD ZnO films, and obtained the highest figure of merit for fluorine-doped ZnO, which they used as TCO for amorphous silicon solar cells. 

Fig. 8.3. Absorption coefficient a of a-Si H (dashed), pc-Si H (solid), and crystalline silicon (dotted) [32]. The re-axes represent the wavelength (bottom) and photon energy (top), respectively, of the incident light...

As described in Volume 2IB, Hydrogenated Amorphous Silicon Optical Properties, the absorption coefficient of undoped a-Si H is strongly influenced by the deposition conditions. For example, the optical gap usually increases as the substrate temperature decreases, and this effect has been attributed to an increase in the hydrogen content (Zanzucchi et al.,... 

Tellurium, which has low thermal diffusivity, can be used in very thin films ( 300 A), which accounts for its excellent writing sensitivity. Hydrogenated amorphous silicon is a wide band gap semiconductor, reducing the film thickness is impractical, since the film has a lower absorption coefficient in the red. It has been shown that ablation of a very thin Ge film in a trilayer structure requires three times more energy than Te (Bell and... 

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