Amorphous and Microcrystalline Silicon

Both a-Si H and pc-Si H can be doped by adding phosphorus or boron containing gases such as phosphine PH3 or trimethylboron B(CHs)3 during the deposition process, so that p-i-n homojunctions and thus photovoltaic devices become possible. 

---

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

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. 

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] ... 

The discharge activated deposition of amorphous and microcrystalline silicon (a-Si and pc-Si, respecti>%ly) is presently among the most important applications of plasma chemistry This field has been opened up by Spear and Le Comber who where the first to demonstrate in 1975-76 the possibility of substitutional doping of a-Si but only recently some, although still incomplete, understanding of the chemical processes during the deposition has been achieved The... 

Figure 4.84 Raman spectra of silicon samples ranging from pure crystalline to one containing primarily amorphous silicon. The spectra show the sharp band at 521 cm from crystalline silicon and the much broader band centered at approximately 480 cm- from amorphous silicon. Spectra were collected on a Thermo Scientific DXR Raman microscope using a 532 nm excitation laser. (From Deschaines, T.O. et al.. Characterization of amorphous and microcrystalline silicon using Raman spectroscopy. Technical Note 51735, Thermo Fisher Scientific, 2009 (www.thermofisher.com). Used with permission.)...

Deschaines, T.O. Hodkiewicz, J. Henson, P. Characterization of amorphous and microcrystalline silicon using Raman spectroscopy. Technical Note 51735, Thermo Fisher Scientific, Inc. Madison, WI, 2009. 

Structural characterisation of B- and P-implanted amorphous and microcrystalline silicon was carried out using Raman spectroscopy in the vSiH regjon. Si H stretching and bending modes were identified in the Raman spectrum of nanocrystalline porous silicon, together with vibrations of Oj-SiH and Si-O Si units. ... 

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... 

Williams et al. have investigated spin-casted PEDOT PSS as a p-layer on ITO combined with amorphous silicon and microcrystalline silicon in an organic-inorganic p-i-n stack. A power efficiency (tj) of 2.1% and an open circuit voltage of 0.883 V has been achieved, which dropped to 0.21% and 0.176 V when the PEDOT PSS layer was omitted. ... 

It may occasion surprise that an amorphous material has well-defined energy bands when it has no lattice planes, but as Street s book points out, the silicon atoms have the same tetrahedral local order as crystalline silicon, with a bond angle variation of (only) about 10% and a much smaller bond length disorder . Recent research indicates that if enough hydrogen is incorporated in a-silicon, it transforms from amorphous to microcrystalline, and that the best properties are achieved just as the material teeters on the edge of this transition. It quite often happens in MSE that materials are at their best when they are close to a state of instability. 

ZnO can be advantageously used as TCO layer in various kinds of thin film silicon solar cells, either as back or as front contact, or even as an intermediate reflector between the amorphous and the microcrystalline p-i-n junctions, within the micromorph tandem solar cell [58] (see also Chap. 8). Figure 6.48 illustrates the various possibilities for using a ZnO layer within a thin film silicon solar cell. In the present paragraph, we will comment about the use of CVD ZnO both as front contact (or window layer) and also as back contact (or part of the back reflector). 

Although ZnO has also been applied in so-called amorphous/crystalline heterojunction solar cells consisting of a (doped) silicon wafer and thin doped a-Si H layers to build the p-n junction, we will restrict ourselves here to solar cells and modules with amorphous and/or microcrystalline absorber layers, i.e., real thin film silicon solar cells. For detailed information on the use of ZnO in crystalline silicon wafer based devices, the reader is referred to the literature (see e.g. [23,24]). 

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...

High Si spin polarization obtained by dynamic nuclear polarization in microcrystalline silicon powder was reported. Unpaired electrons in this silicon powder are due to dangling bonds in the amorphous region of this intrinsically heterogeneous sample. Si nuclei in the amorphous region become polarized by forced electron-nuclear spin flips driven by off-res-onant microwave radiation while nuclei in the crystalline region are polarized by spin diffusion across crystalline boundaries. Hyperpolarized silicon microparticles have long T-1 relaxation times and could be used as tracers for MRI. 

Amorphous, microcrystalline, polycrystalline, and single-crystalline silicon are important building blocks in the construction of miniature silicon architectures by microlithography. Each silicon crystallinity has a slightly different characteristic Raman shift associated with the silicon first-phonon band. LCTF Raman chemical imaging can be applied to measure the distribution of silicon crystallinities within an integrated-circuit test pattern. The test pattern contains single-crystal silicon with a thin oxide layer and polycrystalline silicon epitaxially deposited on the monocrystalline silicon substrate. 

An alternative approach is to use microcrystalline Si as the lower cell instead of a-Si Ge (Figure 3.13). Two silicon layers - one amorphous and one crystalline - are deposited on a glass substrate. The amorphous layer absorbs the visible part of the solar spectrum, while the crystalline layer absorbs the near-infrared light. This lowers the amount of Si used in comparison with conventional technologies, reducing material and... 

---