knjiga solarna energija dobra

402 HIGH-EFFICIENCY III-V MULTIJUNCTION SOLAR CELLS
1.0
Internal quantum efficiency
0.8
0.6
0.4
0.2
Emitter
Depleted layer
0.0
2.0
Base
2.5
3.0
Photon energy
[eV]
3.5
4.0
Figure 9.16 Measured (crosses) and modeled (lines) quantum efficiency of a GaInP solar cell.
The contributions from the different layers of the solar cell are labeled and demonstrate how the
emitter dominates the blue response, whereas the base dominates the red response
Internal quantum efficiency
1.0
0.8
0.6
0.4
0.2
S front = 10 4 cm/s
L emitter = 0.5 µm
Thick window = 25 nm
S front = 10 7 cm/s
L emitter = 0.5 µm
No AlInP
window
S front =
10 4 cm/s
S front = 10 4 cm/s
L emitter = 0.05 µm
Internal quantum efficiency
1.0
0.8
0.6
0.4
0.2
3-µm
base
0.3-µm-thick base
S back = 10 4 cm/s, L base = 3 µm
S back = 10 7 cm/s, L base = 3 µm
S back = 10 4 cm/s, L base = 1 µm
0.0
2.0
2.5
3.0
Photon energy
[eV]
(a)
3.5
0.0
1.8
2.0
2.2 2.4 2.6
Photon energy
[eV]
(b)
2.8
3.0
Figure 9.17 Modeled QE of GaInP cell. (a) The solid line, relative to the “no AlInP window”
line, shows the effect of absorption of 25 nm of AlInP. The two lower curves show the degradation
from an increased front-surface recombination velocity (S front ) or from a decreased emitter diffusion
length (L emitter ). (b) Compares a thin and a thick GaInP cell
thick base layer is more sensitive to the diffusion length in the base, whereas a cell with
a thin base layer is relatively more sensitive to the quality of the back-surface field (see
Figure 9.17(b)).
There are numerous reasons why the V OC or FF may be degraded. Table 9.5 enumerates
many of these, and Figure 9.18(a) illustrates one example.
Extra junctions are more likely to be problems when working with Ge because III-
V elements dope Ge and Ge dopes the III-V materials. The back of the Ge wafer must be
etched before processing to avoid an extra junction at the back [100]. Accidental junctions
in Ge are often highly shunted, with nearly ohmic I-V characteristics. In this case, these