Using the AC equivalent circuit, the
transfer impedance Zt() between the load
voltage VL() and the photocurrent Iph()
can be measure and written as
( )
( )
( )
ph
L
t
I
V
Z
(1)
2 2
( ) ( )
( ) ( ) ( ) ( )
( )
s L P P P L P P s L L
L P s L P P P L P P s L L
t
R R R C R X C R R R X
R R R R R C R X j C R R R X
Z
(2)
where Rp= Rd//Rsh. From the Eq. (2) the
amplitude and the phase of the transfer
impedance can be derived.
If we write CP to represent Cd in parallel
with CT, we can show that
2
1
2
2
2
( )
( )
( ) ( )
1
s L p
t
L P
P s L L
P R R R
Z
R R
R R R X
C
(3)
where
2
() Zt
is the squared amplitude of
Zt()
The transfer impedance equation, Eq. (1)
(2), and the capacitance equation, Eq. (3),
are a basis to determine Cd and CT through
measurements at various frequencies of light
modulation. We note that Rs, Rsh, and Rd can
be obtained from IV curves of solar cells.
3. EXPERIMENTS
We use one commercial 12.5*12.5 cm2
x- Si solar cell in the measurement. Three
sets of experiments are undertaken, namely,
(a) IV characterization under dark
condition to determine Rs and Rsh.
(b) IV characterization under
illumination to determine Rd, and
(c) Intensity Modulation Transfer
Impedance
3.1 Light Sources