Environmental Engineering Reference
In-Depth Information
(a)
(b)
Test Number
Figure 9.8
The variation of
V
oc
and FF as a function of applied forward-
and reverse-bias electrical stresses on two 0.5 mm diameter solar cells.
When the right conditions have been established, it is possible
to shift the Fermi-level position to form a distorted energy band
diagram, as shown in Fig. 9.12. For example, the application of
reverse-bias stress to this device structure and removal could easily
trigger the Fermi-level shift to a lower position at the AlGaAs/GaAs
interface. When this happens, there are two diodes in the device
structureconnectedoppositetoeachotherandthetypicalI-Vcurve
represented by Fig. 9.7(b) will be the final result. This situation
corresponds to test number 4 of Fig. 9.8(a); note the drastic
reduction of solar cell parameters (
V
oc
≈
≈
0.53)
due to the weakened main diode. At large forward-bias voltages,
the main diode reaches flat-band conditions and the reverse-bias
properties of the smaller diode at AlGaAs/GaAs interface dominate
I-V characteristics, showing its saturation current in the first
quadrant.
440 mV and FF
