Silicon bandgap

Figure 1 Energy diagrams showing the relative positions of the silicon bandgap and the chemical potential of platinum ions (left) and nickel ions (right) in fluoride solutions.

The silicon bandgap is approximately 1.12 eV which implies that silicon-based cells should absorb across aU spectra of visible light as well as capmre some near IR with wavelength shorter than 1105 nm. 

Bruno M, Palummo M, Marini A, del Sole R, Ossicini S (2007) From Si nano wires to porous silicon the role of excitonic effects. Phys Rev Lett 98 036807 Buttard D, Bellet D, Dolino G, Baumbach T (1998) Thin layers and multilayers of porous silicon X-ray diffraction investigation. J Appl Phys 83(11) 5814-5822 Calcott PDJ (1977) Experimental estimates of porous silicon bandgap. In Canham L (ed) Properties of porous silicon. INSPEC, London, p 202... 

Germanium was the semiconductor material used in the development of the transistor in the early 1950s. However, it exhibits high junction leakage current due to its narrow bandgap and is now largely replaced by silicon. It is a brittle metalloid element with semiconductor characteristics. The properties of germanium are summarized in Table 8.3.1 lP l... 

Diamond, however, is not the universal semiconductor panacea it is an indirect bandgap semiconductor and does not lase. In addition, present semiconductor materials, such as silicon and gallium arsenide, are solidly entrenched with a well-established technology, and competing with them will not be an easy task. CVD diamond will also compete with silicon carbide, which has also an excellent potential as a high-performance semiconductor material and is considerably easier and cheaper to produce. 

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. 

The electrical properties (dark conductivity and photoconductivity) are reported to first decrease and then increase upon increasing power [361]. The optical bandgap increases with increasing power, due to the increase of the hydrogen content [63, 82, 362, 363]. However, at very high power levels, microcrystalline silicon is formed [364], which causes the hydrogen content (and, consequently, the bandgap) to decrease. 

Interestingly, it has been argued that nanoparticulate formation might be considered as a possibility for obtaining new silicon films [379]. The nanoparticles can be crystalline, and this fact prompted a new line of research [380-383], If the particles that are suspended in the plasma are irradiated with, e.g., an Ar laser (488 nm), photoluminescence is observed when they are crystalline [384]. The broad spectrum shifts to the red, due to quantum confinement. Quantum confinement enhances the bandgap of material when the size of the material becomes smaller than the radius of the Bohr exciton [385, 386]. The broad PL spectrum shows that a size distribution of nanocrystals exists, with sizes lower than 10 nm. 

Assuming that an efficient D-A type of molecule can be synthesized, it should be possible to deposit these molecules as a monolayer onto a glass slide coated with a metal such as aluminum or a wide bandgap semiconductor such as Sn(>2. With the acceptor end of the molecule near the conductor and with contact to the other side via an electrolyte solution it should be possible to stimulate electron transfer from D to A and then into the conductor, through an external circuit and finally back to D through the electrolyte. This would form the basis of a new type of solar cell in which the layer of D-A molecules would perform the same function as the p-n junction in a silicon solar cell (50). Only the future will tell whether or not this concept will be feasible but if nature can do it, why can t we ... 

S. Hoger, J. McNamara, S. Schricker, and F. Wudl, Novel silicon-substituted, soluble poly(phe-nylene vinylene)s enlargement of the semiconductor bandgap, Chem. Mater., 6 171-173, 1994. 

Fig. 7.8 The below-bandgap transmission of free-standing mesoporous silicon compared to the bulk transmission of the corresponding n+ and p+ substrates. The strong increase in...

Because of its indirect bandgap, bulk crystalline silicon shows only a very weak PL signal at 1100 nm, as shown for RT and 77 K in Fig. 7.9. Therefore optoelectronic applications of bulk silicon are so far limited to devices that convert light to electricity, such as solar cells or photodetectors. The observation of red PL from PS layers at room temperature in 1990 [Cal] initiated vigorous research in this field, because efficient EL, the conversion of electricity into light, seemed to be within reach. Soon it was found that in addition to the red band, luminescence in the IR as well as in the blue-green region can be observed from PS. 

---