Environmental Engineering Reference
In-Depth Information
Table 7.2 Typical values of
n
and
φ
b
measured under dark and
illuminated conditions
Under Dark Conditions
Under AM1.5 Illumination
φ
b
(eV)
φ
b
(eV)
Diode Number
n
n
>
>
1
1.41
1.60
1.13
1.79
>
>
2
1.50
1.56
1.15
1.70
>
>
3
1.68
1.48
1.22
1.70
>
>
4
1.84
1.43
1.14
1.76
>
>
5
2.26
1.27
1.91
1.34
7.3.2
Electrical Properties Under AM1.5 Illumination
The rectification properties of these devices improve under air-
mass 1.5 (AM1.5) illumination. The values of
n
estimated from I-V
curves under illumination vary in the range 1.13 to 1.91, indicating
aconsiderablereductionoftheR&Gprocessunderillumination(see
Table 7.2). This data indicates that most of the defects have reduced
R&Gactivitiesunderilluminatedconditions.Sincethevaluesof
n
are
close to unity, barrier heights can be estimated with an improved
accuracy. These measurements indicate that the
φ
b
values for these
devices are greater than 1.79 eV.
The linear-linear I-V curves observed under AM1.5 illumination
exhibit excellent PV properties and a typical curve is shown in
Fig.7.4.ThisdeviceshowsPVparameters;
V
oc
=
1
,
171mV,
J
sc
=
12
mAcm
−
2
, and fill factor (FF)
=
0.85 with an overall e
ciency of
∼
12%.
Table 7.3 presents a summary of PV parameters measured at
different laboratories for different devices processed from one
batch. The
V
oc
values observed are in the range 1,141-1,171 mV, as
expectedfromdeviceswithhigh
φ
b
values.Thesedeviceparameters
have been independently verified by measuring in five different
laboratories,andthePVparametersshowsimilarvalues,confirming
the accuracy of measurements. These
V
oc
values exceed the highest
reported values to date [8, 9] for a GaAs/AlGaAs single-device
system. Furthermore, these parameters hold when the area of the
device is increased to 5
×
5and10
×
10 mm
2
. This is expected
since the uniform epitaxial material layers are grown by the well-
established MOVPE technique.
