Category Handbook of the Physics ofThin-Film Solar Cells

Phosphorus Doping Puzzle

In crystalline silicon (c-Si) phosphorus (P) substituts for a silicon atoms with 4 valence bonds in the crystal lattice. P has five valence electrons, so in the “fourfold coordinated” sites of the Si lattice, four electrons participate in the bonding and the fifth excess electron occupies a state just below the bottom of the conduction band and acts as a donor to render c-Si и-type.

In a-Si:H, however, most phosphorus atoms bond to only three silicon neighbors; they are in threefold coordinated sites. This configuration is chemically advanta­geous; phosphorus atoms normally form only three bonds (involving the three va­lence electrons in p atomic orbitals). The other two electrons are paired in s atomic orbitals, remain tightly bond to the P atom...

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Defects and Gap States

Between the band tails lie defect levels; in un-doped a-Si:H, these levels are due primarily to the dangling bonds (D-centers) measured by electron spin resonance. For example, infrared absorption at around 1.2 eV excites an electron from such defect and is proportional to the D-center density (Jackson and Amer 1982).

The D-center is “amphoteric:” there are three charge states (with +e, 0, and —e), leading to two levels (transitions between the 0/+ and —/0). The (+/0) level is about 0.6 eV below EC in low-defect-density, undoped a-Si:H (Antoniadis and Schiff 1992). The (+/0) level lies about 0.3 eV below the (—/0) levels; the dif­ference between the two levels is termed the “correlation energy” of the D-center (Antoniadis and Schiff 1992).

The level positions vary between do...

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Band Tails, Band Edges, and Band-Gaps

a-Si:H shows conduction and valence bands with an energy gap where the density- of-states is very low. For perfect crystals, the valence and conduction band edges Ey and EC are well defined, as is the band-gap EG = EC — Ey. In disordered semiconductors there is an exponential distribution of band tail states near the band edges. For instance, the valence band tail is given by

g(E) = gv exp(—(E — Ey))/AEy. (41.1)

The width AEy of this exponential distribution is identified as “Urbach” tail of the spectrum. For a-Si:H, a typical value is AEy = 50 x 10—3 eV. The band tail accounts for a low hole mobility; (Tiedje 1984; Gu et al. 1994)...

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Electronic Density-of-States

The optical and electronic properties of semiconductors are determined by the elec­tronic density-of-states, g(E) that can be measured by of electron photoemission (Ley 1989; Jackson et al. 1985), optical absorption (Cody et al. 1981), and electron or drift mobilities (Tiedje 1984).

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Defects and Metastability

The electronic and optical properties of the material depend on this configuration and the chemical bonding of its defects. A single defect may be a dangling bond and is referred to as the D-center. It dominates most of the properties in undoped a-Si:H (Street 1991), and can be created by removing a hydrogen atom in the di­luted phase. This is confirmed by an increase of dangling bonds when hydrogen is removed from a-Si:H by heating. However, the density of dangling bonds is usually much lower than the density of hydrogen lost from the bulk. This is attributed to an additional removal of some hydrogen from clustered-phase sites, which does not create dangling bonds.

Figure 41.3 shows the increase of the dangling bond density as a function of the illumination time up to 107 s...

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Atomic and Electronic Structure of a-Si:H

41.4.1 Atomic Structure

Silicon atoms in amorphous silicon mostly have the same basic structure that they have in crystal silicon: each silicon atom is connected by covalent bonds to four other silicon atoms arranged as a tetrahedron around it. However, amorphous Si has numerous atoms with only 3 bonds satisfied, the fourth bond unempty, it is a "dangeling” bond, consequently the lattice is deformed, there is no long-rang order.

In hydrogenated amorphous silicon (a-Si:H), an hydrogen atom is attached to the dangling bond. This hydrogen is invisible to X-rays, but is evident in proton mag­netic resonance (Reimer and Petrich 1989) and infrared spectroscopy (Zhao et al.

Fig. 41...

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Staebler-Wronski Effect

Amorphous silicon-based solar cells exhibit a significant decline in their efficiency during their first few hundred hours of illumination; however, the degradation of multiple layer solar cells and of nanocrystalline silicon cells is much lower. The

single-junction cell loses about 30 % of its initial efficiency after about 1000 hours; the triple-junction module loses about 15 % of its initial efficiency (see Fig. 41.2).

All amorphous silicon-based solar cells exhibit such degradation with light, which is called the Staebler-Wronski effect (Staebler and Wronski 1977a, 1977b). The effect anneals out nearly completely within a few minutes at temperatures of about 160 0 C, and anneals substantially in outdoor deployment at summer operat­ing temperatures of 60 0 C.

The Staebler-Wronski effec...

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A-Si:H Followed by a Micro-crystalline Si Layer

The micro-crystalline Si layer has also a smaller band gap than a-Si:H and increases the conversion efficiency substantially. These new layers are also referred to as nano­crystalline layers and are produced similarly to the a-Si:H layer in a high frequency ac-gas discharge at 13.56 MHz, however using a larger current density.

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