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MULTIPLE-JUNCTION SOLAR CELLS 545<br />

0<br />

−5<br />

Current density<br />

[mA/cm 2 ]<br />

−10<br />

−15<br />

E G (eV)<br />

1.84<br />

1.65<br />

1.50<br />

1.37<br />

0.0 0.2 0.4 0.6 0.8 1.0<br />

Voltage<br />

[V]<br />

Figure 12.23 Performance of a-Si and a-SiGe nip solar cells with different Ge concentrations in<br />

the i-layer; the i-layer band gaps are indicated in the legend. The fill factors for these cells are<br />

0.70, 0.62, 0.55, and 0.43 for the cells with i-layer band gaps of 1.84, 1.65, 1.50, and 1.37 eV,<br />

respectively [149]<br />

Quantum efficiency<br />

1.0<br />

0.8<br />

0.6<br />

0.4<br />

0.2<br />

Photon energy<br />

[eV]<br />

3.5 3 2.5 2 1.5<br />

Band gap (eV)<br />

(a) 1.84<br />

(b) 1.65<br />

(c) 1.50<br />

(d) 1.37<br />

(e) 1.50 (BR)<br />

(f) µc-Si<br />

0.0<br />

400 500 600 700 800 900 1000<br />

Wavelength l<br />

[nm]<br />

Figure 12.24 Quantum efficiency (QE) spectra for a series of a-Si- and a-SiGe-based pin<br />

single-junction solar cells. Shown in the figure are QE curves for single junction solar cells with<br />

(a) 1.84 eV a-Si i-layer, (b) 1.65 eV a-SiGe i-layer, (c) 1.50 eV a-SiGe i-layer, (d) 1.37 eV a-SiGe<br />

i-layer, (e) 1.50 eV a-SiGe i-layer, with the device deposited on a back-reflector (BR), (f) µc-Si<br />

i-layer. Curve (f) is included here for a later discussion in Section 12.5.4. Curves (a) through (e)<br />

are from [149] and curve (f) is from [150]

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