Eng4

Jfuas No.1 June 2013

33

Characterizing and modeling a Photovoltaic Device

Made of Conjugated PolymerTaha Adam Abdalla, University of AlFashir, Sudan

Email:[email protected]

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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

الفارش م2013يونيو–ولاألالعدد–التطبيقيةالعلوممجلة–جامعة

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


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


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

S

S

n

Poly[3-(4"-(1"', 4"', 7"'-trioxaoctyl)phenyl)-2, 2'-bithiophene]

O

O

O


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):


38

]1)[exp(0

−=

nkT

qVJJ (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

qTAJ b

φ−

=∗

(2)

where A*

is the Richardson constant taken to be 120Acm-2

K-2

and

bφ 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.


39

-2 -1 0 1 2

1E -11

1E -10

1E -9

1E -8

1E -7

1E -6

-2 -1 0 1 2

- 2 0 0

0

20 0

40 0

60 0

80 0

1 00 0

1 20 0

(a )

logJ

V (v o lt)

(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


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 2.4x109Ω and 3.6x10

9Ω

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.


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

IVFF

.

.

= (3)


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 (0.1µ 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


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-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

12Exp .

S im

J(nA/cm

2

)

V (vo lt)

-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

Exp .

S im .

J(µA/cm

2

)

V (vo lt)

+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.)


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):


45

)1(

]1)([exp0

+

−+−−

=

sh

s

ph

sh

S

R

R

IR

UIRU

nkT

qI

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 (0.1µ 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


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).

λφ

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


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.


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

Absorb

ance

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


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

nscIJ ∝ (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.


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 s ity)

JSC(nA/cm

2

)

In te n s ity (m W /cm2

)

(a )

JSC(nA/cm

2

)

In ten s ity(m W /cm2

)

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


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.References:

(1) Antoniadis H, Hsich B.R, Abkowitz M.A, jenekhe S.A. and

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143.

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Education

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