مجلة جامعة الفاشر للعلوم التطبيقية العدد الاول

1. Jfuas No.1 June 2013
33
Characterizing and modeling a Photovoltaic Device
Made of Conjugated Polymer
Taha Adam Abdalla, University of AlFashir, Sudan
Email:abdalla_taha@hotmail.com
Ġ ĞØ
Ħª
Ğ ěKğ
Ù Ğ( λ=550nm)
F1mW/cm2EKĞ Ĥ
ĦK
Ù6mW/cm2
Ø Ğ25 mW/cm2ğ ª
ª ĜK
ª Ğ
ĝ Ò ī Ù
ěĞ K
Abstract
Photovoltaic properties of ITO/Poly[3-(4”-(1”’, 4”’, 7”’-
trioxaoctyl)phenyl)-2, 2’-bithiophene]/Al device has been investigated
by measuring the photocurrent resulting from illuminating through
ITO and Al electrodes. An open circuit voltage and a short circuit
current density are obtained under monochromatic light illumination
(λ=550nm) and under white light of intensity of 1mW/cm2
. Dark

2. ‫اﻟﻔﺎﴍ‬ ‫ﺟﺎﻣﻌﺔ‬–‫ﻣﺠﻠﺔ‬‫اﻟﻌﻠﻮم‬‫اﻟﺘﻄﺒﻴﻘﻴﺔ‬–‫اﻟﻌﺪد‬‫اﻷ‬‫ول‬–‫ﻳﻮﻧﻴﻮ‬2013‫م‬
34
current-voltage results have revealed a formation of a Schottky barrier
at Al/polymer contact. The short circuit current is found to be linearly
dependent upon the incident light intensity up to 6mW/cm2
, then it
followed square root like dependency between 6mW/cm2
and 25
mW/cm2 ,
because at low intensities free carries have been generated
and at higher intensities there has some recombination effects.
The cell is modeled by an equivalent circuit diagram of a diode.
The open circuit voltages and the short circuit currents under
monochromatic and white light of the simulated data are compared
with the experimental data. It shows that the modeled results have
slight deviation from the experimental one, that is due to
inhomogeneity of the polymer in the device..
Keywords/phrases: Schottky barrier, open circuit voltage, short circuit
current, modeling photovoltaic device.
Introduction
Polymer materials are known only for their insulating properties.
Researchers have used polymers as electrical wires protective,
handling and etc. However, in 1977 Heeger et al were able to discover
the electrical conductivity of polyacetylene by doping it with iodine.
The discovery has triggered the research in this area. A number of
conducting polymers such as polythiophene, polyaniline, polypyrrole,
etc. were then prepared (Stenger-Smith 1998). They were synthesized
by chemical and electrochemical methods.
Schottky barrier devices have been fabricated to study the
electrical and the photovoltaic properties of a number of devices made

3. Jfuas No.1 June 2013
35
of conjugated polymers, such as polyaniline (Show-An and Yih
1993), polypyrrole (Bantikassegn et al 1993 a), polyacetylence
(Waldrop et al 1981 6), polythiophene derivatives (Bantikissegn et al
1997 b) etc. The devices have yielded rectified currents and showed
some significant photovoltaic properties (Gardner Tan 1989).
However, their power conversion efficiencies are proved to be rather
low (Sharma et al 1995).
Polymer-metal contact plays an important role in the behavior of
the Schottky barrier device (Inganäs and Lundstorm 1984). When a p-
type polymer is sandwiched between two metals of different work
functions, a Schottky barrier-type is formed at the polymer/low work
function metal interface, while an ohmic contact is formed at the
polymer/high work function metal interface (Antoniadis et al 1994).
This structure is the source for the rectification effect of the Schottky
barrier device.
In this study a Schottky barrier junction made of poly[3-(4”-(1”’,
4”’, 7”’-trioxaoctyl)phenyl)-2, 2’-bithiophene] is investigated. The
chemical structure of the polymer and the schematic diagram of the
device are shown in fig (1). For the junction under study the Indium
Tin Oxide (ITO) electrode has the higher work function, and will be
biased positive with respect to the Al electrode, which has the lower
work function.
The study has covered dark-current voltage characteristics,
photocurrent-voltage characteristics, spectral response and the

4. ‫اﻟﻔﺎﴍ‬ ‫ﺟﺎﻣﻌﺔ‬–‫ﻣﺠﻠﺔ‬‫اﻟﻌﻠﻮم‬‫اﻟﺘﻄﺒﻴﻘﻴﺔ‬–‫اﻟﻌﺪد‬‫اﻷ‬‫ول‬–‫ﻳﻮﻧﻴﻮ‬2013‫م‬
36
correlation between the photoaction spectra and the polymer
absorption spectra.
The dark current-voltage characteristics are used to investigate
the device electrical properties such as the ideality factor, the Schottky
barrier height, the rectification ratio and the leakage current. On the
other hand the photocurrent-voltage characteristics are used to
determine the open-circuit voltage, the short-circuit current, the fill
factor and the power conversion efficiency of the device.
Current-voltage equivalent diagram is modeled and the
parameters of the device are calculated and compared with the
experimental results.
Experimental Details
Fig.1. Schematic diagram of the ITO/Poly[3-(4”-(1”’, 4”’, 7”’-
trioxaoctyl)phenyl)-2, 2’-bithiophene]/Al device
WŽůLJŵĞƌ
/dK
'ůĂƐƐ
S
S
n
Poly[3-(4-(1', 4', 7'-trioxaoctyl)phenyl)-2, 2'-bithiophene]
O
O
O

5. Jfuas No.1 June 2013
37
The device was fabricated by sandwiching a layer of poly[3-(4”-
(1”’, 4”’, 7”’-trioxaoctyl)phenyl)-2, 2’-bithiophene] between ITO and
Al electrodes using spin coating machine and Edward 360 Vacuum
evaporator.
The dark and photocurrent-voltage measurements were obtained
by using Pico-Ampere meter model pH 4140 interfaced with HP Test
Fixture (Model 16055A). For the photocurrent measurements, a 250W
quartz tungsten halogen lamp (QTH) source of white light was passed
through a double monochromator (Oriel-model 7240), before being
focused onto the entrance window of the polymer. The absorption
spectra of the polymer were measured in the visible region using
UV/VIS/NIR λ 19 spectrometer.
The intensity of the monochromatic incident light on the sample
was obtained using the spectral response of a calibrated silicon diode
(Hmamatsu, Model S1336-8BK), by placing it at the position of the
sample. The device response to the light was measured using
Luxemeter (Model Lx-101) by placing the Luxemeter sensor at the
position of the sample. Light intensities in both measurements were
obtained by controlling the output power source.
Results and Discussions:
1-J-V Dark Current Characteristics
The Current-voltage characteristics can be explained using
thermoionic emission theory of the charge carrier transport in a
Schottky barrier device, given by (Sze 1981):

6. ‫اﻟﻔﺎﴍ‬ ‫ﺟﺎﻣﻌﺔ‬–‫ﻣﺠﻠﺔ‬‫اﻟﻌﻠﻮم‬‫اﻟﺘﻄﺒﻴﻘﻴﺔ‬–‫اﻟﻌﺪد‬‫اﻷ‬‫ول‬–‫ﻳﻮﻧﻴﻮ‬2013‫م‬
38
]1)[exp(0
−=
nkT
qV
JJ (1)
where n is the ideality factor, T the absolute temperature, k Boltzmann
constant and J0 is the thermoionic reverse saturation current density
given by
)exp(2
0
kT
q
TAJ b
φ−
= ∗
(2)
where A*
is the Richardson constant taken to be 120Acm-2
K-2
and
b
φ is the barrier height in volts.
Figure2 shows the forward and the reverse current densities
characteristics. A linear relationship between logJ and the bias voltage
V is observed in more than one portion of the curve. This suggests that
not only the thermoionic emission is involved in the charge transport
mechanism but also the other forms of charge transports mechanisms
may come into play as well as the effect of the series resistance. These
could be the tunneling transport, Poole-Frankel or ohmic conduction.

7. Jfuas No.1 June 2013
39
-2 -1 0 1 2
1E -11
1E -10
1E -9
1E -8
1E -7
1E -6
-2 -1 0 1 2
-20 0
0
20 0
40 0
60 0
80 0
1 00 0
1 20 0
(a )
logJ
V (v olt)
(b)
J(nA/cm
2
)
V (v o lt)
Fig.2 Current-voltage characteristics (a) Semilog plot of current density
versus bias voltage (b) current-voltage plot showing the
rectifying behavior of the device.
We have calculated the dark current parameters from the linear
portion between 0.5 and 1.4V. This part is assumed to follow the
thermoionic emission transport. The reverse saturation current density
is estimated as equal to 4x10-10
A/cm2
. The rectification ratio at –2V
and +2V is 33, and the barrier height is 0.97eV. The rectification ratio
being 33 means that the current pass through the device at +2V is
about 33 times higher than at –2V, which strongly confirms the
rectifying behavior of the device. The value of J0 is small compared to
the device density current. It suggests that there is a blocking effect in

8. ‫اﻟﻔﺎﴍ‬ ‫ﺟﺎﻣﻌﺔ‬–‫ﻣﺠﻠﺔ‬‫اﻟﻌﻠﻮم‬‫اﻟﺘﻄﺒﻴﻘﻴﺔ‬–‫اﻟﻌﺪد‬‫اﻷ‬‫ول‬–‫ﻳﻮﻧﻴﻮ‬2013‫م‬
40
the junction. The ideality factor n obtained from the slope of the curve
is equal to 2.5. The ideality factor being more than one is attributed to
the existence of recombination effect in the bulk of the polymer. The
result also suggests the possibility of an inhomogeneous Schottky
barrier height. Such values of n are reported for many organic
semiconductors (Sze 1981).
The shunt resistance RSH and the series resistance RS of the
device estimated from the logJ-V plot are 2.4x109
Ω and 3.6x109
Ω
respectively. The RSH value confirms the rectifying behavior of the
device, however, the RS value suggests that there is a considerable
resistance facing the forward current. This caused the forward current
of the device to become low. The series resistance may be due to the
high bulk resistance of the polymer or the inhomogeneity of the film.
Figure 3 illustrates logJ-logV plot of the device. It shows that at
the lower voltages (0.4-1.2V) the current is approximately space
charge limited while at the higher voltages (1.2-2.0V) the
experimental data reveals that logJ-logV plot characteristics in the
forward direction play a transition from space-charge-limited to the
power law dependence i.e. J∝ Vm
with m approximately equal to 10.
This result supports the existence of tunneling transport conduction
mechanism.

9. Jfuas No.1 June 2013
41
1
1E-11
1E-10
1E-9
1E-8
1E-7
logJ
logV
Fig.3 LogJ-logV characteristics of the ITO Poly[3-(4”-(1”’, 4”’, 7”’-
trioxaoctyl)phenyl)-2, 2’-bithiophene] Al device.
2-Photocurrent-Voltage Characteristics
The curve factor or the fill factor of the photovoltaic device is
defined as the ratio between the maximum output power to the
product of Voc and JSC. It gives a measure of how much the cell power
is being optimized. FF is given by the following equation (Donald A.
Seanor 1982)
scoc
mm
IV
IV
FF
.
.
= (3)

10. ‫اﻟﻔﺎﴍ‬ ‫ﺟﺎﻣﻌﺔ‬–‫ﻣﺠﻠﺔ‬‫اﻟﻌﻠﻮم‬‫اﻟﺘﻄﺒﻴﻘﻴﺔ‬–‫اﻟﻌﺪد‬‫اﻷ‬‫ول‬–‫ﻳﻮﻧﻴﻮ‬2013‫م‬
42
where Vm and Im are the voltage and the current of the device at
the maximum power respectively. Voc is the open circuit voltage and
Isc is the short circuit current.
The power conversion efficiency η of the device is defined as
the ratio of the output power pout to the incident power pin and if the
fill factor is known it can be written as
Ap
FFJV
in
scoc
/
..
=η (4)
where A is the cell area.
The device photovoltaic parameters have been calculated from the analysis
of J-V plot in Fig.4 and compiled in table (1).
Table (1) Photovoltaic parameters for J-V characteristics
Parameter White-light
(1mW/cm2
)
Monochromatic-light
550nm (0.1 µ W/cm2
)
Voc (V) 4.40 6.20
JSC (nA) 3.60 3.90
FF 0.26 0.26
%η 0.04 0.6
RS(Ω ) 0.3x108
0.3x109
RSh(Ω ) 1.3X107
1.8X109

11. Jfuas No.1 June 2013
43
-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
-8
-6
-4
-2
0
2
4
6
8
10
12
E xp.
S im
J(nA/cm
2
)
V (volt)
-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
E xp.
S im .
J(µA/cm
2
)
V (volt)
+Fig.4 Photocurrent versus applied voltage characteristics (a) the white
light result (Exp.) and the simulated result (Sim.). (b) the
monochromatic light result (Exp.) and the simulated result
(Sim.)
;ĂͿ
;ďͿ

12. ‫اﻟﻔﺎﴍ‬ ‫ﺟﺎﻣﻌﺔ‬–‫ﻣﺠﻠﺔ‬‫اﻟﻌﻠﻮم‬‫اﻟﺘﻄﺒﻴﻘﻴﺔ‬–‫اﻟﻌﺪد‬‫اﻷ‬‫ول‬–‫ﻳﻮﻧﻴﻮ‬2013‫م‬
44
The observed fill factors represent the typical FF values usually
obtained for organic polymers. The device high series resistance has
influenced it. The estimated power conversion efficiency is rather
small, though it matches the efficiencies of the other organic Schottky
barrier devices. The shunt resistance of the device under illumination
is rather high which has indicated the rectifying effect of the device.
On the other hand the series resistance is high enough to make the
photocurrent density so small. This means that high series resistance
of the device limits the conversion efficiency.
It can be noticed that when white light of intensity 1mW/cm2
is
used, a higher short-circuit current and a relatively lower open-circuit
voltage are obtained. This is because more excitons might have been
created under such illumination, and gained enough energy to diffuse
to their dissociation site and thus produced higher photocurrent. The
photocurrent is assumed to be due to the separation and the drift of the
electron-hole pairs under the action of the space-charge electrical field
formed at Al/polymer interface.
The photocurrent-voltage results can be modeled by an
equivalent circuit diagram shown in Fig.5. The diagram consists of a
series resistance Rs, a shunt resistance Rsh, a photocurrent generator Iph
and the diode D. The series resistance represents the sum of electrodes
resistance Rj and the polymer bulk resistance Rb. The diode behavior
can be described by the Shockley equation. Therefore, using the
equivalent circuit diagram the total current as a function of the applied
voltage is described by the following equation (Dorin et al 2008):

13. Jfuas No.1 June 2013
45
)1(
]1)([exp0
+
−+−−
=
sh
s
ph
sh
S
R
R
I
R
U
IRU
nkT
q
I
I (5)
where q is the electronic charge, U the applied voltage I0 the saturation
current and Rj+Rb=Rs.
The current-voltage characteristic is simulated by the proposed
equivalent circuit and plotted in Fig.4. The current for a given voltage
is calculated by varying the equivalent circuit parameters until the
deviations of the simulated data from the experimental data were
minimum. The simulated parameters for the monochromatic light and
white light are shown in table (2).
Table (2) Photovoltaic parameters for simulated J-V
Parameter White-light
(1mW/cm2
)
Monochromatic-light
550nm (0.1 µ W/cm2
)
Voc (V) 5.40 3.5
JSC (nA) 3.90 3.6
FF 0.26 0.26
%η 0.05 0.8
RS(Ω ) 3.0x106
0.3x105
RSh(Ω ) 1.3X108
1.3X106
It can be noticed that the simulated data for the white light and
the monochromatic light have slightly deviated from the experimental
data especially for open-circuit voltage results. This may be due to the

14. ‫اﻟﻔﺎﴍ‬ ‫ﺟﺎﻣﻌﺔ‬–‫ﻣﺠﻠﺔ‬‫اﻟﻌﻠﻮم‬‫اﻟﺘﻄﺒﻴﻘﻴﺔ‬–‫اﻟﻌﺪد‬‫اﻷ‬‫ول‬–‫ﻳﻮﻧﻴﻮ‬2013‫م‬
46
high internal resistance of the device and the inhomogeneity of the
polymer forming the device.
Fig.5 The equivalent circuit diagram of the device.
3-Spectral Response
The overall quantum efficiency φ is the ratio between the
numbers of photocarriers to the number of incident photon and is
given by (Seanor 1982).
λ
φ
i
sc
I
J
q
hc
= (6)
Where, h is the Blank constant, c the speed of light, λ the wavelength of
the incident light, JSC the short circuit current density and Ii the incident light
intensity.
The spectral response is the photocurrent collected at different
wavelengths relative to the number of incident photons on a surface
(known as incident photon conversion efficiency IPCE%). It can be
represented in the form of a plot of any particular photovoltaic
/ƐŚ
Zď
/Ě
ZƐŚ
/ Zũ
h/ƉŚ

15. Jfuas No.1 June 2013
47
parameter (such as Vph, Voc, Jph, JSC, φη, , etc.) versus the wavelength
of the incident light.
Figure 6 shows the quantum efficiency (φ or IPCE% ) and the
absorption spectra of the polymer versus the wavelength. It can be
noticed that the maximum quantum efficiency corresponds to the high
energy where absorption is low. This can be interpreted as follows: at
high photons energy excitons are generated either near Al/polymer
where they can immediately dissociated or near ITO and diffuse
towards Al/polymer interface without being trapped. At low energies
excitons are also generated near Al and ITO electrodes but some of
them may get lost before reaching the Al/polymer interface.
We conclude that the generation, diffusion and drift of the
photocarriers are due to the electrical field formed at Al/polymer
space charge region, and only excitons that reach the Al/polymer
interface give rise to free carriers.

16. ‫اﻟﻔﺎﴍ‬ ‫ﺟﺎﻣﻌﺔ‬–‫ﻣﺠﻠﺔ‬‫اﻟﻌﻠﻮم‬‫اﻟﺘﻄﺒﻴﻘﻴﺔ‬–‫اﻟﻌﺪد‬‫اﻷ‬‫ول‬–‫ﻳﻮﻧﻴﻮ‬2013‫م‬
48
300 350 400 450 500 550 600 650 700 750
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
absorb.
Al
ITO
Absorbance
IPCE%
λ(nm)
Fig.6 IPCE% of the device illuminated through ITO and Al sides in
comparison to the
absorption spectra of Poly[3-(4”-(1”’, 4”’, 7”’-trioxaoctyl)phenyl)-2, 2’-
bithiophene].
4-Short-circuit Current Intensity Relation
Figure 7 shows the dependence of JSC on white light intensity for
Al/polymer/ITO junction. JSC varies linearly with the incident light
intensity up to 6mW/cm2
. The linear dependence of JSC at low light
intensities is an indication that the electrons and holes are generated
effectively at the active region of the device. In the region between

17. Jfuas No.1 June 2013
49
6mW/cm2
and 25mW/cm2
the JSC dependence follows the sublinear
type, with the slope of the curve of approximately 0.7. Generally
organic photovoltaic cells display a relationship of the following type
(Glens et al 1995, Hagen 1997 15,16).
m
nsc IJ ∝ (7)
where m is the slope of the curve, which gives an indication to the
amount of the recombination effect of the charge carriers during their
transport to the electrodes. Therefore, the highest intensity region
suggests an existence of electron-hole recombination. Such
dependence of JSC on the light intensity has been reported in many
Schottky barrier organic devices (Marks et all 1994, Fu-Ran and
Faukner 1987, Ghosh and Feng 1973). This result is consistent with
dark J-V characteristics at the higher forward voltages.

18. ‫اﻟﻔﺎﴍ‬ ‫ﺟﺎﻣﻌﺔ‬–‫ﻣﺠﻠﺔ‬‫اﻟﻌﻠﻮم‬‫اﻟﺘﻄﺒﻴﻘﻴﺔ‬–‫اﻟﻌﺪد‬‫اﻷ‬‫ول‬–‫ﻳﻮﻧﻴﻮ‬2013‫م‬
50
0.1 1 10
10
100
0 5 1 0 1 5 2 0 2 5
0
5 0
1 0 0
1 5 0
2 0 0
0 .0 0 .1 0 .2 0 .3 0 .4 0 .5 0 .6
4
6
8
1 0
1 2
1 4
1 6
1 8
(b )
logJSC
log (In ten sity)
JSC
(nA/cm
2
)
In te n s ity(m W /cm
2
)
(a )
JSC
(nA/cm
2
)
In ten s ity(m W /cm
2
)
Fig.7 Dependence of the short-circuit current density on the incident
light intensity. the insert figure (a) the linear dependence of the short-
circuit current density on the incident light intensity (b) the sublinear
dependence of short-circuit current on the higher light intensity.
Conclusion
Photovoltaic properties of ITO/ poly[3-(4”-(1”’,4”’,7”’-
trioxaoctyl)phenyl)-2,2’-bithiophene] /Al device have been studied.
Current-voltage characteristics have been investigated using
thermoionic emission model of charge transport. The result suggests a
formation of a Schottky barrier type at Al/polymer interface.
Photovoltaic measurements show that ITO/poly[3-(4”-
(1”’,4”’,7”’-trioxaoctyl)phenyl)-2,2’-bithiophene]/Al device responses

19. Jfuas No.1 June 2013
51
linearly to the low incident light intensities (6mW/cm2
) and semi-
linearly to the higher the light intensities (6-25mW/cm2). It has been
deduced that at the low light intensities many free carriers are
generated, but due to the high series resistance the output current is
very small. At the high light intensities there is some recombination
effect, which reduces output photocurrent.
A comparison between the experimental and the simulated data
shows that the effect of series resistance is high due to inhomogeneity
in the polymer.
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20. ‫اﻟﻔﺎﴍ‬ ‫ﺟﺎﻣﻌﺔ‬–‫ﻣﺠﻠﺔ‬‫اﻟﻌﻠﻮم‬‫اﻟﺘﻄﺒﻴﻘﻴﺔ‬–‫اﻟﻌﺪد‬‫اﻷ‬‫ول‬–‫ﻳﻮﻧﻴﻮ‬2013‫م‬
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