Environmental Engineering Reference
In-Depth Information
alloysystem[5,6].Theseresultshavebeenpresentedanddiscussed
in chapters 6, 7, and 8. GaAs/AlGaAs solar cells with high
V
oc
and
fill factor (FF) values have been used to explore the measurement
variationsandstabilityissuescommonlyobservedinthin-filmsolar
cells. This chapter takes examples from CdTe, GaAs/AlGaAs, and
CIGSsolarcellsinordertohighlightthesevariationsduringcurrent-
voltage (I-V)measurements.
9.2 Variations of I-V Characteristics of Metal/n-CdTe
Interfaces
Before discussing instabilities of fully fabricated solar cells, it is
worth summarising the work reported in 2005 on metal/n-CdTe
single interfaces in the presence of a set of defect levels [7]. Typical
forward and reverse I-V characteristics of metal/n-CdTe diodes
are shown by curve-1 in Fig. 9.1. These diodes exhibited good
rectification properties, and the ideality factors (
n
)variedinthe
range 1.10-1.95. The potential barrier height (
φ
b
) estimated using
thestandardmethod[8]wasintherangeof0.96
±
0.04eV.However,
the details of I-V curves varied from measurement to measurement
and also according to the immediate history of the diode. For
example, the value of
n
varied slightly based on the direction of
datacollection(whethertheI-Vcurvewasmeasuredfromreverseto
forwardbiasorviceversa)andtheconditionsofstorageofthediode
priortomeasurements(whetherthediodewasstoredinthedarkor
inroomilluminationconditions).Tostudythesevariationsindetail,
a selected diode was forward biased with a voltage value of 1.0 V
for a period of 1 hour and the I-V curve was measured immediately
after the removal of the bias voltage. Many diodes showed very
different I-V curves with reduced barrier heights, and a typical I-V
curve is shown by curve-2 in Fig. 9.1. These diodes also exhibited
values of
n
in the range 1.10-2.00, enabling the accurate estimation
of barrier heights. The estimated barrier heights were much less
than the original value, and typical barriers were in the range 0.70-
0.77 eV. After recording the I-V characteristics, the same diode was
exposedtoareversebiasof1.0Vforalongerperiod(over10hours).
During the application of a bias voltage, the diode was kept under
