Microcrystalline silicon

Crystalline silicon has the tetrahedral diamond arrangement, but since the mean thermochemical bond strength between the silicon atoms is less than that found between carbon atoms (Si—Si, 226 kJmol , C—C, 356kJmol ), silicon does not possess the great hardness found in diamond. Amorphous silicon (silicon powder) is microcrystalline silicon. 

Schropp, R.E.l. and Zeeman, M. (1998) Amorphous and Microcrystalline Silicon Solar Cells (Kluwer Academic Publishers, Dordrecht). 

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. 

The application of this technique in conventional RF deposition equipment revealed the possibility of producing microcrystalline silicon at relatively low power densities and temperatures [501]. Depending on deposition conditions, (i.e., deposition cycle time, hydrogen exposure time, and power density), different microcrystalline fractions are found see [165,251,502]. 

FIG. 72. Schematic cross-section of (a) a single junction p-i-n o-Si H superstrata solar cell and (b) a tandem solar cell structure. (From R. E. I, Schropp and M. Zeman. "Amorphous and Microcrystalline Silicon Solar Cells—Modeling, Materials and Device Technology," Kluwer Academic Publishers, Boston, 1998, with permission.)... 

R. E. I. Schropp and M. Zeman, Amorphous and Microcrystalline Silicon Solar Cells—Modeling, Materials and Device Technology. Kluwer Academic Publishers, Boston, 1998. 

The industrial application of Plasma Induced Chemical Vapour Deposition (PICVD) of amorphous and microcrystalline silicon films has led to extensive studies of gas phase and surface processes connected with the deposition process. We are investigating the time response of the concentration of species involved in the deposition process, namely SiH4, Si2H6, and H2 by relaxation mass spectroscopy and SiH2 by laser induced fluorescence. 

Colins RW, Ferlauto AS, Ferreira CM, Chen C, Koh J, Koval RJ, Lee Y, Pearce JM, Wronski CR (2003) Evolution of microstrucutre and phase in amorphous, protocrystalline, and microcrystalline silicon studied hy realtime spectroscopic ellipsometry. Sol Energy Mat Sol Cells 78 143-180... 

Juska G, Arlauskas K, Viliunas M, Kocka J (2000) Extraction current transients new method of study of charge transport in microcrystalline silicon. Phys Rev Lett 84 4946... 

Vanecek M, Pomba A (2002) Fourier-transform photocurrent spectroscopy of microcrystalline silicon for solar cells. Appl Phys Lett 80 719... 

Ubara, H., Imura, T. and Hiraki, A. Formation of Si-H bonds on the surface of microcrystalline silicon covered with SiOx by HF treatment. Solid State Communications 50, 673 (1984). 

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. 

It is, however, certainly useful to include the light scattering property of TCO films in the figure of merit. Furthermore, one should also take into account the exact spectral range where the TCO has to operate (i.e., a differentiation is here necessary between amorphous and microcrystalline silicon solar cells). With this in mind, Fay et al. proposed a new, wavelength-dependent Figure of Merit (FoM(A)) [35] ... 

Furthermore, a series of ZnO B films with various doping levels has been deposited, and the thickness of the films has been adapted so as to keep the sheet resistance close to 10Qsq. This means that the thickness of the ZnO films was increased as their doping level was decreased. For this series, the highest FoM(A) of LP-CVD ZnO films on the whole visible spectral range has been obtained for the 6 j,m-thick undoped film. This prediction has then been experimentally confirmed the same microcrystalline silicon solar cell was deposited on this ZnO series, and the highest photogenerated current... 

Microcrystalline Silicon and Micromorph Tandem Solar Cells... 

Fig. 8.5. Quantum efficiency of an amorphous/microcrystalline silicon tandem junction solar cell. The individual quantum efficiency curves of the two component cells (a-Si H top (dotted) and pc-Si H bottom (dashed,)) are also included...

Fig. 8.17. (a) Cross-sectional TEM micrograph of an etched ZnO Al film covered with about 1 pm of a microcrystalline silicon p-i-n solar cell [119] (b) SEM image of an etched ZnO Al film showing cross-section and etched surface. The crater edge on the ZnO surface is indicated by a black line... 

In the short term, there is a need to reduce the cost of thin-film solar cells. To do this will require the development of deposition techniques of thin films such as microcrystalline silicon and other materials that are robust and provide high throughput rates without sacrificing film efficiencies. In the short run, thin-film deposition methods can potentially gain... 

Schroeder, B. 2003. Status Report Solar Cell Related Research and Development Using Amporphous and Microcrystalline Silicon Deposited by HW (Cat) CVD. Thin Solid Films, Vol. 430, pp. 1-6. 

Fig. 2.5. Diagram showing the typical deposition conditions for microcrystalline silicon and a-Si H films deposited from silane/hydrogen mixtures at different rf power.

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