Category Handbook of the Physics ofThin-Film Solar Cells

A-Si/a-SiGe Tandem and a-Si/a-SiGe/a-SiGe Triple-Junction Solar Cells

Dual-junction a-Si/a-Si (same band-gap tandem) solar cells have lower material cost (no GeH4) than tandem cells using a-SiGe, and have slightly higher efficien­cies (0.5-1 % absolute) than a-Si single junction cells. Same band-gap a-Si/a-Si tandems have been in production for decades. Multiband-gap dual junction (a-Si/a – SiGe: Doughty and Gallagher 1990, or a-Si/nc-Si Yamamoto et al. 2004; Meier et al. 2007) and triple-junction (a-Si/a-SiGe/a-SiGe or a-Si/a-SiGe/nc-Si) solar cells use spectrum-splitting. They achieve higher conversion efficiencies, usually over 10 % stabilized for small area <1 cm2 cells (Doughty and Gallagher 1990; Kroll et al. 2007; Fujioka et al. 2006; Mahan et al. 2001). While all-amorphous a-Si (1.8 eV)/a-SiGe (1.6 eV)/a-SiGe (1...

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Bandgap Grading of a-SiGe i – Layers and of a-SiC

To enhance the fill factor of cells with a-SiGe i-layers, band gap grading is used to enhance the collection of holes (Guha et al. 1989; Zimmer et al. 1998). An asym­metric V-shaped band-gap profile is created by adjusting the Ge content across the i – layer. Wider-band-gap material is deposited closest to the p-layer.

Such a grading allows more light to be absorbed near the p – layer so that “slower” holes do not have to travel far to get collected. Also, the tilting of the valence band creates an electric field that assists holes generated in the middle or near the и-side of the i – layer to move toward the p-layer. With appropriate hydrogen dilution during growth and band-gap grading, a-SiGe cells can generate up to 27 mA/cm2 under AM1.5 light...

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Alloys to Vary the Band Gap

The band-gap of a-SiGe alloys can be continuously adjusted between 1.7 and 1.1 eV by varying the percentage of Ge. However, the optoelectronic quality (i. e. defects and carrier transport) of a-SiGe degrades rapidly when the a-SiGe bandgap is re­duced below about 1.4 eV. Figure 41.8 shows the J-V characteristics of a series of a-SiGe solar cells with different Ge concentrations in the i-layer (of constant thickness, and without a back reflector) (Agarwal et al. 2002).

As the band-gap is reduced, Voc is reduced and Jsc increases, since the narrower – band-gap absorbs more sunlight. But also the fill factor decreases as the band-gap decreases, probably because of a reduced hole drift-mobility or an increase in the defect density, hence lower lifetime and collection.

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Multijunction Solar Cells

41.7.1 Advantages of Multi-junction Solar Cells

A-Si:H solar cells can be fabricated in a stacked structure to form multi-junction solar cells. Figure 41.7(1) shows a tandem cell with two junctions (i. e. two pin photodiodes) in series. These multi-junction cells can have higher solar conversion efficiency than single-junction cells and are presently used in most commercial mod­ules. Mostly multi-junction a-Si:H based solar cells were tandem or triple junctions with an a-SiGe low band-gap absorber. Since the early 70’s, tandem “micromorph” cells and cells with a-Si/nc Si multiple band-gap tandem modules are widely used Multi-junction solar cells are spectrum splitting...

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Optical Design of a-Si:H and nc-Si:H Solar Cells

Back reflectors and substrate texturing are used to improve the power output of most thin film solar cells. Figure 41.6a shows the “quantum efficiencies” measured for a 250-nm-thick a-Si:H solar cell with varying texturing and back reflectors (Hegedus et al. 1996), along with absorptances calculated from the optical absorption coeffi­cient spectrum for a-Si:H. The dashed absorptance curve labeled “no light trapping”


Fig. 41.6 (a) Absorption spectra calculated for a 250 nm a-Si:H, and quantum efficiency spectra measured for a series of a-Si:H nip solar cells with a 250 nm thick i-layer and varying substrates and back reflectors (Hegedus et al. 1996). E: (untextured, none); V: (untextured, untext.); ▲ un­textured, textured), (textured, none); ■ (textured, untextured)...

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Influence of Thickness on Power Generation

The power generation is determined by the depth of the absorption for the more than 70 % of the blue components (this depth is approximately first 200 nm), and by the distance a photo-generated carrier can travel before it recombines. This distance is given by hx = ixdFt with the drift mobility, F the acting field, and т the carrier lifetime.

The /гт product is larger for electrons than for holes and depends on the structure of the a-Si:H. However, in nc-Si the distance a hole can travel is larger than in a – Si:H by at least a factor 100 (Schiff 2004). The drift mobility of a hole is on the order of 0.01 cm2/Vs and for electrons is ~2 cm2/Vs in a-Si:H, and for holes in nc-Si:H it is, at ~1 cm2/V s, i. e. still smaller than the electron drift mobility...

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Theory of the a-Si pin Cells

41.6.1 Electronic Structure of a pin cell

Due to the presence of a “built-in” electric field F(x) within the a-Si:H-based pin solar cell device, the bands are tilted, for EC according to eF(x) = dEC(x)dx; however, these built-in fields cannot be measured directly.

The built-in fields across the i – layer stems from the space charges created be­tween the n and p layers on both sides. This is caused by the connection of the Fermi energies at both sides (as connected to the respective electrodes). With elec­trons donated from the n-layer to the p-layer a built-in potential Vbi can be read. With light, electrons and holes are generated that will drift in the built-in field to­ward the respective electrodes.

Several computer programs were developed for solar cell calculations with a – Si...

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Alloys and Doping

As was discussed in Sect. 41.4.7, a-Si-based alloys can be deposited using a gas mixture of SiH4 with other gases such as GeH4, CH4, O2, or NO2, and NH3 for obtaining a-SiGex, a-SiCx, a-SiOx and a-SiNx, respectively. Among these alloys, only a-SiC, as a wide band-gap p-layer, and a-SiGe, as a low-band-gap absorber layer, have been used to produce solar cells. But a-SiGe with a band-gap below

1.3 eV is difficult to deposit with uniformity (Mackenzie et al. 1998; Paul et al. 1993). By taking advantage of the similar dissociation rate of GeH4 and disilane (Si2H6), a mixture of these gases permitted the fabrication of a-SiGe alloy with uniform films (Guha et al. 1987). As discussed in Sect. 41.4.6, a-Si can be doped и-type by mixing phosphine (PH3) or doped p-type by mixing it with diborane (B...

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