Environmental Engineering Reference
In-Depth Information
when
P
in
is taken as the solar power incident on a unit area. The
I
sc
should bereplaced by the current density (
J
sc
), giving
V
oc
·
J
sc
·
FF
P
in
η
=
(1.15)
The standard AM1.5 condition provides the value of
P
in
=
100
mWcm
−
2
as the solar constant to evaluate the e
ciency of the
device.
1.5.3
How to Maximise V
oc
The open circuit voltage achievable from a given junction depends
on the electronic properties of that interface. In order to obtain an
expression for the
V
oc
of a Schottky barrier solar cell, the diode
characteristics under illumination (Eq.1.11) can be used:
I
L
=
I
0
·
exp
eV
nkT
−
I
sc
(1.11)
By definition,
V
=
V
oc
when
I
L
=
0, or the external circuit is
kept open, or the current through the external circuit is zero. This
gives
0
=
I
0
·
exp
eV
oc
nkT
−
I
sc
or
·
exp
−
·
exp
eV
oc
nkT
φ
b
kT
e
SA
∗
T
2
=
I
sc
The rearrangement ofthis equation provides
V
oc
=
n
e
·
ln
J
sc
kT
φ
b
+
(1.16)
A
∗
T
2
This indicates that the magnitude of
V
oc
depends on the value of the
φ
b
, present at the interface. Therefore, to achieve high values of
V
oc
,
large potential barriers are desirable.
It is also evident that the
V
oc
depends on the value of
n
,the
idealityfactorofthediode.Thevalueof
n
foranidealSchottkydiode
with current-transport dominated by thermionic emission [13] is
equal to unity (
n
1.00). But with current-transport dominated
by R&G,
n
value is equal to 2.00, indicating a helpful parameter to
=
