Environmental Engineering Reference
In-Depth Information
n
-type
p
-type
anode
electron
acceptor
electron
donor
cathode
(a)
(b)
Figure 6.10 More general single-bandgap
devices. Schematic [74] of p
and CuIn
1x
Ga
x
Se
2
CIGS
cells, and to
individual elements of tandemcells, generally of
high efficiency. Figure 6.10b applies to
organic
polymer
cells, generally of low efficiency and
low cost. The third type of cell, the
dye-
sensitized solar cell,
is not shown. Multiple
junction (tandem),
intermediate band,
and
quantum-dot-assisted cells are additional
categories (see text).
n junction (a) and
an organic bilayer structure (b) under
illumination. Illumination leads to output
voltage and current in external circuit. In (a),
light can be usefully absorbed in wider region
than the depletion layer, whose width includes
the diffusion lengths for minority carriers in the
semiconductor regions on both sides of the
actual junction. Figure 6.10a applies to Si, GaAs,
-
6.3
CIGS (CuIn
1x
Ga
x
Se
2
) Thin-Film Solar Cells
Low-cost semiconductor cells with ef
ciency approaching 20% have evolved making
use of alloys of selenides of copper, indium, and gallium (CuIn
1
x
Ga
x
Se
2
, known as
CIGS). These crystalline alloys are typically P-type semiconductors with appropri-
ate bandgaps, high absorption coef
cients, and quite large diffusion lengths. The cell
is typically in the formof a thinN-layer placed near the front of an absorber of CIGS
with thickness about 1
m. The PN junction, as in Figure 6.10a, sends electrons and
holes in opposite directions. The ef
ciency limit in these cells seems to be about 20%,
with 15%available in preliminary large-scale production. This is in the same range as
polycrystalline Si cells, but it appears that the CIGS cells are easier to fabricate on
large scale and can be much cheaper to produce. These cells are made commercially
by Avancis, Nanosolar, Honda Soltec, Solar Frontier, Wurth Solar, and Global Solar.
The historical origin of this line of CIGS cells may have been the CdS/Cu
2
S cell
formed by chemical surface treatment of CdS. This cell fell out of favor partly because
of the toxic nature of Cd.
m
6.3.1
Printing Cells onto Large-Area Flexible Substrates
The reasons for ascendancy of CIGS include ef
ciency and durability, but the main
reason is that these cells can be manufactured, without vacuum equipment, on a roll-
