Journal of Surface Engineered Materials and Advanced Technology, 2011, 1, 42-50
doi:10.4236/jsemat.2011.12007 Published Online July 2011 (http://www.SciRP.org/journal/jsemat)
Effect of Self-Assembled Monola yers on the
Perfor mance of Organic Photovoltaic Cells
Hanène Bedis
1UMAO, Faculté des Sciences de Tunis, Campus U niversitai r e, Tunis, Tu nisia; 2ITODYS, 15 Rue Jean-Antoine de Baï f , Paris,
France.
Received April 1st, 2011; revised May 5th, 2011; accepted May 14th, 2011.
ABSTRACT
The improvement of the performance of organic photovoltaic cells (OPVCs) and the photogeneration process in these
devices may occur via multiple mechanisms depending on their structure and/or architecture. For this purpose we in-
vestigate how self-assembled monolayers of thiol molecules (C12H25SH and 3T(CH2)6SH) and benzoic acid molecules
(ABA and NBA) affect the efficiency and the photogeneration of free carriers in a sexithiophene based photovoltaic
cells. Firstly, we provide the results of absorption spectra for samples with SAM of thiol that show there effect on ori-
entation of 6T molecules on these structures and the organization degree of the thiol molecules on ITO substrate. Af-
terward, we describe from current vs. applied voltage after illumination, the enhancement of the performance of these
cells. In the second, we study the effect of SAM of benzoic acids molecules on the photovoltaic behavior. A theoretical
model is used for quantitative description of the open circuit voltage as a function of carrier’s generation rates at the
electrodes. The results of I-V characterization under illumination show that open circuit voltage as well as short circuit
current is dramatically affected by the dipolar layer. The orientation and the magnitude of dipole moment of benzoic
acid molecules are the crucial factors that affect the organic photovoltaic parameters.
Keywords: SAMs, Oligothiophene, Photovoltaïc Cell, Photogeneration, Efficiency , Performance
1. Introduction
Organic semiconductors materials are potential candi-
dates in electronics industry, more diversified and less
expensive. Indeed, the fact that is a synthetic materials
contrary to the mineral semiconductors, allows to mode-
ling the constitutive molecules to adjust a very precise
property. It is by proceeding that we were able to con-
ceive a multitude of electronic structures such as organic
light-emitting diodes (OLEDs) which cover the totality
of the vi sib le spectral field and organic photo voltaic cells
(OPVCs).
Organic photovoltaic cells are gaining considerable
interest motivated by a steady enhance of their conver-
sion efficiency during the last decade [1- 3]. Nevertheless,
their opera ting mechanisms are still s ubject of controver-
sial theories. In fact, the observed photocurrent in
OPVCs is an experimental evidence of free photocarrier
production. Nonetheless, the origin of photogenerated
carriers that are responsible of the observed photocurrent
may be attributed to mainly three mechanisms. The first
is the ionization of tight bound excitons or the Frenkel-
type excitons in the bulk which is described by Onsager
theory [4,5]. In this mechanism Frenkel excitons lead to
the formation of charge transfer excitons resulting in
geminate pairs of photogenerated carriers lying on adja-
cent molecules. The geminate pairs escape the strong
columbic attraction under the effect of built-in electric
field E at a given temperature T with the probability
P(E,T). The second mechanism occurs at the interface
with the electrodes. In this mechanism one of the gemi-
nate pair carriers passes through the electrode and the
other is left “free” to contribu te to photocurrent. T he last
mechanism is attributed to detrapping of trapped carriers.
In this mechanism photogenerated tight bound excitons
transfer enough energy to trapped charges by colliding
them. Actually, all of the three described mechanisms
may occur simultaneously but in some cases one among
them may be preponderant on the two others as for ex-
ample the case of materials with low mobility, in which
bulk photogeneration may be ignored because their short
lifetime and photogenerated carriers in the bulk that are

Effect of Self-Assembled Monolayers on the Performance of Organic Photovoltaic Cells
43
far from the electrodes disappear rapidly and minimizing
the photocurre nt.
The development of organic photovoltaic cells (OPVCs)
is still a matter of research despite their low efficiency
relatively to mineral ones which is precisely a crucial
factor for their commercialization. This is due to mainly
two reasons. The first one may be qualified as historical
and is related to the youthfulness of organic optoelec-
tronics by comparison to minerals. The second one is
physical and is due to mainly two reasons:
• The first one is the low dielectric constant of organic
semiconductors (~3) relatively to that of inorganic
ones (~10). This property makes photo-excited elec-
tron-hole geminate pairs much more bound, due to
columbic interaction, in organic materials indeed ex-
citons in organic semiconductors are Frenkel-type
with a binding energy in the order of some tenths of
eV [5].
• The second one is the low mobility of carriers in or-
ganic semiconductors which inhibits short lifetime
carriers from attaining elec trodes [6].
The relatively high binding energy prevents these
photogenerated species from dissociating which reveals
low free carrier production efficiency. The most probable
sketch to account for photocurrent observed experime n t-
tally in organic solar cells is that free carriers are pro-
duced from dissociation of those tightly bound geminate
pairs by interaction with the metal/Semiconductor inter-
face [5].
Self Assembled Monolayers (SAMs) which were
widely used in OLEDs are supposed to induce modifica -
tion of metallic electrode work function leading to sig-
nificant improve of carrier injection into the organic
semiconductor. Another advantage in using SAM layers
at electrode interfaces is to improve the adhesion of the
organic film onto the metal or the oxide electrode [7].
Then, the quality of the interface reflected by the degree
of the structural order, has been improved substantially
by using self-assembled monolayer (SAM) at the inter-
face either in the organic transistor [8], the organic di-
ode [9] or the organic light e mitting d iode [10].
We are interested in this work on photovoltaic cells
based of the α-sexithiophene (6T) molecules (Figure
1(a)). The characteristics of these molecules apart their
stability and facility of synthesis, they are essentially
motivated by their structural order, the degree of organi-
zati on and the big pur ity whi ch offers to oligomeres con-
trary to polymers. These molecules present by their sim-
plicity a better understanding of the optoelectronics phe-
nomena and allow to modeling the behavior of polythio-
phenes.
We present in this article the methods for improving
performance of sexithiophene based photovoltaic cell
Figure 1. Chemical structure of: (a) sexithiophene, (b) Do-
decanthiol (C12H25SH), (c) Thiol with head group the ter-
thiophene (3T(CH2)6SH/6T), (d) p-Nit ro -benzoic acid NBA
and (e ) p-Amino -benzoic acid ABA.
through introduce a self-assembled monolayer of thiols
molecules differed wit h func ti o na l gr o up s (C12H25SH a nd
3T(CH2)6SH) (Figure 1(b) and Figure 1 ( c) ) a nd benz oic
acid molecules (Amino-Benzoic Acid and Nitro-Benzoic
Acid) (Figure 1(d) and Figure 1(e)) on ITO in order to
control photocarrier generation at the interface ITO/or-
ganic and to provide an increase in device efficiency. In
this study, the evaporate 6T both on SAMs devices were
characterized electrically after illumination. We describe
from current vs. applied voltage, the enhancement of
devices efficiencies and we can estimate their photo-
voltaic parameters. A theoretical model is used for quan-
titative description of the open circuit voltage as a func-
tion of carrier’s generation at the electrodes, and can ex-
plain the effect of orientation and the magnitude of di-
pole moment of SAM of benzoic acid on the photoge-
neration rate of free carriers and their effect on the or-
ganic photovoltaic parameters.
2. Experimental
2.1. Preparation of ITO
The purpose of the preparation of the ITO is to eliminate

Effect of Self-Assembled Monolayers on the Performance of Organic Photovoltaic Cells
44
the impurities and to increase its surface reactivity and
the interactio ns with the grafted molecules. For the whole
of our exp e rimenta l wor k, the glas s ma ki n g and the whol e
set of our manipulating tools are carefully cleaned in
order to avoid to the maximum any contamination which
may trouble grafting of SAM.
Prior to SAMs grafting ITO substrates, provided by
SOLEMS, were carefully cleaned according to the fol-
lowing protocol: first the samples are immerged in an
ultrasound bath of ultrapure water for 30 mn, seco nd they
are rinsed in pure NaOH (30%) during 15 mn and third,
they are immerged in pure sulphuric acid (98%) during
1mn. After that the samples are rinsed again in an ultra-
sound bath of ultra pure water during 1mn. This chemical
treatment is used in order to render ITO surface more
hydrophilic and reactive with respect to grafted SAM.
2.2. Preparation of SAM and Devices Structure
SAMs are obtained by solution dipping of cleaned ITO
substrates according to the following procedure: we pre-
pared two solutions with the molecule shown i n Figure 1.
To deposit SAMs on the cleaned ITO substrates, we
used in the firs t for thiol mole c ules a pure solution of 2ml
of Dodecanethiol: C12H25SH (Figure 1(a)), and a pre-
pared solution of 3T(CH2)6SH (Figure 1(b)) in benzene
with a concentration of 10−3 mol/l. The samples of
cleaned ITO substrates were left for one week in these
solutions in order to maximize the SAM graftin g [11].
Afterward the samples were rinsed in pure ethanol
(C2H6O (96%)) in an ultrasonic bath, then dried with
argon. All the experimental steps were carried out at
room temperature. For acid molecules, we used pure so-
lution of 2 ml of 4- Aminobenzoic acid (ABA), 4-Nitro-
benzoic acid (NBA) Figure 1(d) and F ig ure 1(e). All the
experimental steps were performed at room temperature.
In order to obtain comparable results either between
SAM coated ITO and bare ITO we performed both sexi-
thiophene (6T) and aluminum (Al) coatings in the same
conditions. We deposited 80 nm of 6T on five sub-
strates: four of them were coated with SAMs (thiols and
benzoic acids) the last one is just for bare cleaned ITO. A
semi-transparent 20 nm thick aluminum cathode was
finally deposited through a shadow mask under 10−6 torr
leading to final structures ITO/SAM/6T/Al (Figure 2)
and ITO/6T/Al. Each sample consisted of two pixels
each of which had a rectangular shape of 2 mm/4 mm.
2.3. Ele ctr ical and Op t ical Characterization
Electrical measurements were performed with a Keithley
4200-SCS Semiconductor Characterization System. For
I-V measurement under illumination, the first irradiating
light source was a 150 W Tungsten lamp for the first
series of samples with SAM of thiols and the second ir-
radiating light source was a 150 W Xenon arc lamp
(model 150 W/1 XBO, Osram).
Absorption spectra have been recorded with a Carry
500 UV-VIS-NIR spectrophotometer that directly gives
the variation of optical density D.O or absorbance vs.
wave length of thin film of 6T. All measurements are
taken in the ambient air and at room temperature on the
optical seat by illumination of the cell through the
semi-tra nspa rent Aluminium electrod e .
3. Theory
In this section we calculate the open voltage for the sam-
ple geometry described above. We assume that for ap-
plied voltage s V = Voc the c harge densit y inside the sa m-
ple is low enough that it does not cause any band bend-
ing, it remains localized at the contact, the photocurrent
is equal to zero and the electric field remains uniform
and equal to (Vbi - V)/d, where d is the thickness of the
poly- mer layer.
The carrier conduction of in this case is limited of hole
and current den sities Jh of hole respectively is:
g
a
g
c
ITO
Al
6T
S
A
M
ABA
NBA
Figure 2. E nergy diagram of ITO/SAM/6T/Al structure.

Effect of Self-Assembled Monolayers on the Performance of Organic Photovoltaic Cells
45
( )( )
bi
hh
px
VV
Jep xeD
dx
µ
∂
−
=−+
∂
(1)
where e is the elementary charge, µh is the hole mobility,
D is the diffusion coefficient and p is the hole density.
Solving the Equation (1) we get the hole den sity [12]:
( )
11
qd
qx
caa c
qd qd
p ppep
px e
ee
−
−
−−
−−
= +
−−
(2)
where the subscripts “a” and “c” denote charge concen-
trations at the anode
( )
0x=
and the cathode
( )
xd=
,
respectively, and
( )
1
bi
eV V
qd kT
−
=
.
Using the Einstein relation between the mobility and
the diffusion coefficient one get the dark current:
( )
-
1
qd
ha hc
bi
Dqd
µp pe
VV
Jede
µ −

= −

(3)
The total current density under illumination is in the
following for m:
DL t
JJJ= +
(4)
where the steady state current density in the dark is:
( )
e
-
1
qd
ha hc
bi
Dqd
µpp
VV
Jede
µ −

= −

(5)
Since exctions in organic materials are tightly bound,
the basic charge generation mechanism for photoconduc-
tivity is believed to involve dissociation of the excited
state via transfer of charge to the metal electrode, leaving
the other charge free inside the organic layer [12]. Thus,
in order to calculate the current density under illumine -
tion one can neglect bulk photogeneration and assume
that only photogenerated carriers at the electrodes con-
tribute to photocurrent. Therefore, JL is obtained by sub-
stit uti ng r es pe ctively pa and pc in JD by gaI and gcI, wher e
I stand for the illuminatio n intensity and ga and gc are the
density of photogenerated holes respectively at the anode
and the cathod e. Thus JL is written as:
( )
-
1
qd
ha hc
bi
Lqd
µgIg Ie
VV
Jede
µ −

= −

(6)
Then, the total curre nt density is written in t he form:
( ) -
1
qd
aa cc
bi
th qd
pgIpgI e
VV
Je de µ

+−+

=
 −



(7)
Or at V = Voc, the total cur rent densi ty equals zero then
we ge t the relation between the built-in potential and the
open circuit voltage as follows:
ln
aa
oc bicc
p gI
kT
VV ep gI

+
= −
+

(8)
Since generally organic semiconductors are known to
be excellent photoconductors that is photocurrent is or-
der s o f mag nit ud e gre at er that d ar k cur r ent, then one ma y
neglect
a
p
and
c
p
with respect to a
gI
and c
gI
respectively leading to the expression:
ln a
oc bic
g
kT
VV eg

= −

(9)
On the other hand, the built-in potential
bi
V
in con-
jugated non-doped thin film devices is p roportional to the
workfunction difference between the cathode and the
anode.
( )
ITO Al
1
bi
Ve φφ = −
(10)
where ITO φ
and
Al
φ
are the workfunctions of the anode
(ITO) and o f the c athode (Al ). In the case of SAM coated
electrodes one should take into account the workfunction
shift due to dipole layer introduced by dipolar character
of SAM molecules. Then the Potential energy shift ow-
ing to a dipolar SAM layer at the ITO surface is given
by [13]:
0
D
e µ
φεε
Γ
∆=
(11)
where Γ is the number of molecules by unit surface (2 ×
1018 m−2 for ABA molecule and 1.3 × 1018 m−2 for NBA
molecule [14,15], µD is the dipole moment of the indi-
vidual molecule,
ε
0 is the vacuum permittivity and ε is
the dielectric co nstant of S AM molecules, (
ε = 5.3) [14].
Dipole moments of both molecules are calculated using
Gaussian program and they are also verified in the lite-
rature [16]. Thus, the built-in potential of SAM based
devices as /SAM/6T/Al may be written as follows:
( ) ( )
1
biITO Al
Ve φφ φ =− +∆
(12)
Note that the built in potential
bi
V
can be reduced or
enhanced depending on the sign of
D
µ
i.e. on the
orientation of the dipole moment.
Finall y o ne can write the expression of the open circuit
voltage a s :
( )
( )
1ln
a
ocITO Alc
g
kT
Ve eg
φφ φ

=−+∆ −

(13)
4. Results and Discussion
4.1. Devices with SA M of Thiol
4.1.1. Optical Characterization
The UV-Visible absorption spectra have been record to a
Carry 500 scan/UV-VIS-NIR spectrophotometer. Figure
3(a), shows the optical absorbance at normal incidence of

Effect of Self-Assembled Monolayers on the Performance of Organic Photovoltaic Cells
46
(a) (b)
Figure 3 . UV-visible absorption spect ra of 6T thin film deposited on SAM coated I TO and bare ITO substrates.
both samples: 6T on bare ITO and SAM coated ITO .
Although the 6T thin film layer is deposited simulta-
neously on both substrates, we notice that for the sample
with SAM coated ITO the optical density (O.D.) is re-
duced by a factor 2. This difference is not related to the
film thickness bus may be accounted of the position of
α-6T molecules. In fact, electronic spectra of oligothio-
phene thin films are dramatically affected by the orienta-
tion of crystallites in the layer. Indeed, since the mole-
cules of oligothiophene are lengthened and since the
*
ππ−
transition dipole moment
µ
of these molecule is
quasi-parallel to their long axis
L
, t hen lig ht absor ption
which is proportional to scalar product of the incident
light electric field
E
by the transition dipole moment
µ
is maximum when
E
and
µ
are parallel and null
when
E
and
µ
are perpendicular [16]. Therefore the
difference between the tree spectra may be accounted for
by a better orientation of 6T molecules in the case of
C12H25SH and 3T(CH2)6SH than in the case of bare ITO .
Indeed, 6T thin film seems to be much more organized
with the SAM of 3T(CH2)6SH then with C12H25SH and
bare ITO , which can induce a quasi-epitaxial growth on
terthiophene groups at the surface.
From the absorption spectra we can thus deduce the
values of the band gap energy (Eg) for sample with and
without S AM o f t hio l by usi n g the T a uc mod el [17]. Th is
model supposes a parabolic variation of edge of the ab-
sorption band with energy of the photons. The absorption
coefficient
α is then associated to the energy of the pho-
tons E and to the gap (Eg) by the following relation
which allows to extrapolate the bandwidth.
( )
g
n
EAE E α = −
(14)
where E = hν, effective energy of photon, A is constant
and n is a n index connec t to the na ture of electr onic tra n-
sition. We note according to Figur e 3 ( b) that electronic
transitions which appear in the absorption spectra are
controlled by the vibrationnels levels from the molecule
6T. In fact, the permit transition of first excited state,
correspond to the transition 1ag1au and contained in the
plan (LM), allows us to deduce gap energy and the other
transitions correspond of transitions towards the vibra-
tionnels levels of the molecule 6T. This enabled us to
adjust our spectrum on the first absorption band. We no-
tice from adjustments results that the values o f gap for n
= 1/2 are in the same order of magnitude and are in
agreement with that are deferred in the literature [16].
Moreover the best linear curve in the area of edge of ab-
sorption band corresponds to an average value of gap of
the order 2.2 eV, and all the found values show that the
gap of sexithiophene layer is independent of the func-
tional surface.
Consequently, grafting of aliphatic self assemblies
monolayer of thiols or with thiophene head group on
ITO, emerge on the level of the orientation and the or-
ganization of the layers. This results are confirmed with
the results of measurement of contact angle and the
surface energy [11] that prove the better orientation of
C12H25SH molecules towards the surface of ITO and the
layer is relatively dense, but they have tendency to dis-
organize with the time of adsorption. In addition
3T(CH2)6SH molecules are better organized on the sur-
face and form a dipolar layer with the interface which
would involve an electrical improvement of contact ITO .
4.1.2. Current-Voltage Characteristics
Figure 4 shows the tendency observed of ITO/6 T/Al
(REF) and devices with self assembled monolayer of

Effect of Self-Assembled Monolayers on the Performance of Organic Photovoltaic Cells
47
Figure 4. Curr ent density c haracteristics at applied voltage
illuminated with tungsten lamp for devices with SAMs of
C12H25SH and (3T(CH2)6SH/6T), li near plot.
thiols under illumination by a tungsten lamp inside the
Al. We clearly observe on the Figure 3 an effect of the
light on I-V characteristics. It is a photocurrent that is
strongly depends of the applied voltage. By observing
photovoltaic parameters in Table 1, we remark clearly
that the poten tial of open cir cuit and e fficiency increased
considerably for (ITO /SAM/6T/Al) compared to (ITO
/6T/Al). Moreover, these results explain that SAM of
thiol where the head groups are oligothiophenes present
the best performances of the realized devices in spite of
the fact that it absorbs a little luminous power that the
two other devices. This would be show a better organiza-
tion of the layer [11,18,19] and induced a stronger mo-
bility of the charges, affect the device efficiency. We
could thus assure that the effect of the SAM containing
thiols is limited to the photogeneration in volume and
that really there is no direct interface effect on the dis-
sociation of excitons. Thus, for devices with SAM of
thiols the improvement of efficiency is probably imputa-
ble at the photogenerate free carrier of factors which are
much l ess signi ficant with tungst en lamp. In the conti nu-
ation we will study from J-V characteristic under Xenon
illumination, the effect of dipolar SAM molecules of
benzoic acid in photo vo lta ic conversion.
4.2. Devices with SAM of Benzoic Acid
4.2.1. Current-Voltage Characteristics
The current-voltage characterization under illumination
has shown different characteristics depending on the
SAM nature (Figure 5, Figure 6). We notice that reverse
biased photocurrent in NBA coated ITO samples is by fa r
greater than the ABA coated ITO and the bare ITO sam-
ples, whereas its open circuit voltage is clearly lower
than both of the other samples. On the other hand ITO/
Figure 5. Current density characteristics at applied voltage
illuminated with Xenon lamp for devices with SAMs of
AB A and NB A, linear plot.
Figure 6. Current density characteristics at applied voltage
illuminated with Xenon lamp for devices with SAMs of
ABA and NBA, logarithmic plot.
ABA/6T/Al cell shows the largest short–circuit current
(Jsc) and also the largest o pen circ uit voltage ( Voc) (Table
2) summarizes the values of photocurrent densities (Jph),
open circuit voltages (Voc) and the efficiencies with bare
ITO and SAM-coated ITO samples.
It turns out that the conversion efficiency is dramati-
cally affected by the grafting of the acid molecules on
ITO. Indeed the ef ficienc y of I T O/NB A/6T /Al is reduced
nearly to the half of that of bare ITO sample, whereas in
ITO/NBA/6T/Al sample the efficiency in almost twice
that of bare ITO sample. The improvement of efficiency
in the case of ABA can be accounted for b y an enhance-
ment of interfacial charge carriers photogénération due to
the interactio n of excitons with the dipole layer altogeth-
er with a suitable orientation of the dipole moment, that
is the electric field lying in the d ipo lar SAM layer has the
same orientation than that of the intrinsic electric

Effect of Self-Assembled Monolayers on the Performance of Organic Photovoltaic Cells
48
Table 1. Open circuit volt age, current den sity and efficiency of co nversion results of d evi ces with SAM of thiol s .
Voc (V) Jcc (µA/cm2) FF(%)
η
×
10-2 (%)
ITO /6T/Al 0.55 0.85 18.5 2.4
ITO/C12H25SH/6T/Al 0.60 1.29 18.7 4.0
ITO/3T(CH2)6SH/6T/Al 0,65 1.20 17.8 3 .9
Table 2. Open circuit voltage, current density and efficiency of conversi on results of devices w ith SAM of benzoic acids.
Voc (V) Jcc (µA/ cm2) FF(%)
η
×
10-2 %
ITO /6T/Al 1.18 0.48 28.75 2.7
ITO/ ABA/6T/Al 1.24 1.08 21.73 4.9
ITO/ NBA/6T/Al 0.24 0.83 31.97 1.1
field. Also, we can express this result by orientation of
dipole moment of S AM in the direction favor o f increase
of interfacial energy between the ITO and the SAM
layer. After collection of carriers at the interfaces we will
have a significant generation of photoc urrent.
4.2.2. Orientation of the Dipole Moment on
Photogeneration Carriers
The change o f ITO work function is related to the mole-
cules adsorbed on the surface that are supporting a dipole
moment owing to the presence of partial charges on the
functional groups. These dipoles are aligned on the sur-
face and form a dipolar layer which can be seen as an
effective layer lying in a planar capacitor formed by
charges of opposite sign at the edges of SAM layer and
in which lays an intrinsic electric field (Figure 7). The
orientation of the dipole either facilitates or inhibits the
extraction of electrons or holes by the surface which en-
trai ns a shif t of the p hotoge nerate d carr iers resul ting in a
shift of potential surface [20- 23].
We note that SAMs of benzoic acids increase or de-
crease the work function of the anode with the additional
potential barrier with the interfaces depending on the
orientation of the dipole moment of grafted acid mole-
cules [24,25].
Therefore, at a first insight one can say that grafting
SAMs with permanent dipolar moment molecules en-
hances the creation of carriers at the interface which may
improve the performances of organic photovoltaic cells.
But, the orientation of dipole layer will increase the
built-in potential if the dipole layer field is in the same
direction o f the built-in electric field and vice-versa.
The experimental parameters according to Equation
(11), (12) and (13) for bare ITO and SAM-coated ITO
devices are summarized in (Table 3). We notice that
ITO/NBA/6T/Al cells exhibit the largest photocurrent
but the lowest open circuit voltage, whereas for ITO/
Figure 7. Photogeneration at interfaces.
ABA/6T/Al the Jph and Voc are slightly enhanced rela-
tively to ITO/6T/Al. The NBA sample which has higher
dipole moment than ABA is more efficient in creating
photocarriers, but on the other hand it has a quite low
open circuit voltage. The first feature can be interpreted
by the strength of the dipolar moment which induces a
quite strong field nearby the dipolar surface (Table 3);
hence the dissociation rate of interacting excitons is con-
siderably enhanced. Whereas the second feature is main-
ly due to the orientation of the dipolar moment, which
results in an effective field oriented in the opposite d irec-
tion of the built-in potential of the bulk sexithiophene
thin fil m (Figure 7(a)). Thus the orientation of the dipo-
lar moment o f the SAM is a c rucial factor in determining
the open circuit voltage of a thin film photovoltaic cell.
In fact, for ABA based samples in which the dipolar
moment of the molecules is oriented such as the built-in
potential is enhanced (Figure 7(b)), the open- circuit
voltage as well as photocurrent are slightly enhanced
compared to ITO/6T/Al samples.
In the NBA sample which has higher dipole moment
than ABA is more efficient in creating photogenerated
carriers, but on the other hand it has a quite low open
circuit voltage. For NBA based samples, a remarkable
decrease of built in potential that follow the magnitude

Effect of Self-Assembled Monolayers on the Performance of Organic Photovoltaic Cells
49
Table 3. Resul ts of ph otocur rent dens ity, Open ci rcuit vol tage, buil d in potent ial an d fre e-sta nding dipole mo ment of SAM of
the devices.
Dipole moment (D) [13]
∆ φ
(eV) JPh (µA/cm2) (@−1V) VOC (V) Vbi (eV) ga/gc
Bare ITO - - 0.81 1.18 0.60 8.40·10−11
ABA +2.64 +0.38 3.31 1.24 0.98 2.55·10−5
NBA −5.94 −0.55 45.44 0.24 0.05 5.17·10−4
and the orientation of dipole moment. We can assume
that strength of the dipolar moment induces a quite
strong field nearby the dipolar surface.
5. Conclusions
The grafting of self assembled monolayers with thiols
molecules and dipolar molecules of benzoic acid on ITO
may be a fashionable way to improve photovoltaic per-
formance of organic cells. Analyze of UV-Visible ab-
sorption spectra shows an effect of thiols SAM on the
orientation of the sexithiophene molecule on the sub-
strate (gap 2.2 eV), what can be related to the degree of
organization of the thin layer that is better with the mo-
lecules 3T(CH2)6SH. The current vs. applied voltage
characterisation show an enhancement of device effi-
ciency that confirm the effect of thiols molecules on the
photogeneration of free carriers in the bulk i.e. far from
electrodes and contribute to photocurrent, due to the
conjunction between the low mobility of free carriers in
organic materials and their short lifetime. Moreover, it
may be worth remembering that dipolar benzoic acids
derivative (ABA and NBA) increase the efficiency of
photovoltaic cells. This increase is significant especially
for oriented d ipole molecule of ABA at IT O/6T interface
(0.05%) reported with NBA. This improvement is af-
fected by interaction of tightly bound Frenkel excitons
with a surface dipole that may lead to efficient dissocia-
tion of gemi nate electron-ho le p airs tha t ha s a sig nifica nt
effect on the photoc urrent rate. However the contributio n
of photogenerated carriers to photocurrent is strongly
dependent on dipole orientation. In fact the NBA com-
pound has a large dipole moment but is oriented in the
sight of re d uct ion i n pho t oc ur r e nt co nt ra r y t o ABA. T he n
grafting strong dipole moment molecules oriented to-
wards the cathode would provide a significant improve-
ment of organic photovoltaic cells.
6. Acknowledgements
The author will like to thanks and express her gratitude
to Dr. Fayçal Kouki and Prof. Habib Bouchriha, directors
of research in UMAO (University El-Manar, Tunis) for
their helpful and critical discussions to accomplish the
st udy. I express al so my than ks to Mr. Gill Ho rowitz and
Mr. Philippe Lang, directors of research in ITODYS
(University Paris7) for their assistance and support in
experimental studies.
REFERENCES
[1] G. Yu and A. J. Heeger, “Polymer Photovoltaic Cells:
Enhanced Efficiencies via a Network of Internal Donor-
Acceptor Heterojunctions,” Science, Vol. 270, No. 5243,
1995, pp. 1789-1791. doi:10.1126/science.270.5243.1789
[2] P. Peumans and S. R. Forrest, “Very-High-Efficiency
Double -Hetero-Structure Copper Pthalocyanine/C60 Photo-
voltaic Cells,” Applied Physics Letters, Vol. 79, No. 1,
2001, pp.126-128. doi:10.1063/1.1384001
[3] G. Li and Y. Yang, “High Efficiency Solution Proc-
essable Polymer Photovoltaic Cells by Self-Organization
of Polymer Blends,” Nature Materials, Vol. 4, No. 11,
2005, pp . 864-868. doi:10.1038/nmat1500
[4] C. L. Braun, “Electric Field Assisted Dissociation of
Charge Transfer States as a Mechanism of Photocarrier
Production,” Journal of Ch emical Ph ysics, Vol. 80, No. 9,
1984, pp . 4157-4161. doi:10.1063/1.447243
[5] F. Kouki and H. Bouchriha, “Photogeneration Process in
Pristine Sexithiophene Based Photovoltaic Cells,” Or-
ganic Electronics, Vol. 11, No. 8, 2010, pp.1439-1444.
doi:10.1016/j.orgel.2010.06.005
[6] S. F. Alvaradoand and D. D. C. Bradley, “Direct Deter-
mination of the Exciton Binding Energy of Conjugated
Polymers Using a Scanning Tunneling Microscope,”
Physical Review Letters, Vol. 81, 199 8, pp . 108 2-1085.
doi:10.1103/PhysRevLett.81.1082
[7] X. Crispin, “Interface Dipole at Organic/Metal Interfaces
and Organic Solar Cells,” Solar Energy Mater and Solar
Cell, Vol. 83, No. 2-3, 2004, pp. 147 -168.
doi:10.1016/j.solmat.2004.02.022
[8] D.H. Kim and S. Young Oh, “Effects of ITO Surface
Modification Using Self-Assembly Molecules on the
Character istics of OLEDs,” Ultramicroscop y, Vol. 108,
No. 10, 2008, pp. 1233-1236.
doi:10.1016/j.ultramic.2008.04.026
[9] H. Sirringhaus and R. Friend, “Integrated Optoelectronic
Devices Based on Conjugated Polymers,” Science, Vol.
280, No. 5370, 1998, pp.1741-1744.
doi:10.1126/science.280.5370.1741
[10] S. Goncalves-Conto and L. Zuppiroli, “Interface Mo-
rphology in Organic Light-Emitting Diodes,” Advanced
Materials, Vol. 11, No. 2, 1999, pp.112-115.
doi:10.1002/(SICI)1521-4095(199902)11:2<112::AID-A

Effect of Self-Assembled Monolayers on the Performance of Organic Photovoltaic Cells
50
DMA112>3.0.CO;2-Z
[11] H. Bedis Ouerghemmi and F. Kouki, “Self-Assembled
Monolayer Effect on the Characteristics of Organic Dio-
des,” Synthetic Metals, Vol. 159, No. 7-8, 2009, pp.
551-555. doi:10.1016/j.synthmet.2008.11.031
[12] G. G. Malliaras and J. C. Scott, “Photovoltaic Measu r e-
ment of The Built-in Potential in Organic Light Emitting
Diodes and Photodiodes,” Journal of Applied Physics,
Vol. 84, No. 3, 1998, pp. 1583-1587.
doi:10.1063/1.368227
[13] S. G. Ray and H. Waldeck, “Organizatio n-Induced
Charge Redistribution in Self-Assembled Organic
Monolayers on Gold,” Journal of Physical Chemistry B,
Vol. 109, No. 29, July 2005, pp. 14064-14073.
doi:10.1021/jp050398r
[14] H. Bässler and G. Vaubel, “Exciton-Induced Photocur-
rents in Molecular Crystals,” Discussions of the Faraday
Soci ety, Vol. 51, 1971, pp. 48-53.
doi: 10.1039/DF9715100048
[15] L. Zuppiroli1 and M. Grätzel, “Self-assembled Monolay-
ers as Interfaces for Organic Opto-Electronic Devices,”
European Physical Journal B, Vol. 11, No. 3, 1999, pp.
505-512. doi:10.1007/s100510050962
[16] F. Kouki an d F. Garn ier, “E xperimental Determination of
Excitonic Levels in Alpha-Oligothiophenes,” Journal of
Chemical Physi cs , Vol. 113, No. 1, 2000, pp. 385-391.
doi:10.1063/1.481804
[17] J. Tauc and A. Van cu, “Opti cal Properties and Electronic
Structure of Amorphous Germanium,” Physica Status
Solidi, Vol. 15, No. 2, 1966, pp. 627-637.
doi:10.1002/pssb.19660150224
[18] N. Camaioni and Giovanna Barbarella, “Branched Thio-
Phene-Based Oligomers as Electron Acceptors for Or-
ganic Photovoltaics,” Journal of Materials Chemistry,
Vol. 15, No. 22, 2005, pp. 2220-2225.
doi: 10.1039/B503062E
[19] H. Hoppe and N. Serdar Sarici ftci, “P olymer So lar Cell s,”
Advances in Polymer Science, Vol. 214, 2007, pp.1-86.
doi: 10.1007/12_2007_121
[20] J. W. King and S . P. Mo ln ar, “ Molecul ar Structural Index
Control in P roperty-Directed Clustering and Correlation,”
International Journal of Quan tum Chemistry, Vol. 80, No.
6, 2000, p p. 1164 -1171.
doi:10.1002/1097-461X(2000)80:6<1164::AID-QUA3>3.
0.CO;2 -C
[21] I. H. Campbell and D. L. Smith, “Electrical Impedance
Measurements of Polymer Light Emitting Diodes”, Ap-
plied Physics Letters, Vol. 66, No. 22, 1995, p. 3030.
doi:10.1063/1.114267
[22] P. S. Davids and D. L. Smith, “Nondegenerate Contin-
uum Model for Polymer Light - Emitting Diodes,”
Journal of Applied Physics, Vol. 78, No. 6, 1995, pp.
4244-4252. doi:10.1063/1.359886
[23] A. J. Cambell and D. G. Lidzey, “Space-Charge Limited
Conduction with Traps in Poly (Phenylene Vinylene)
Light Emitting Diodes,” Journal of Applied Physics, Vol.
82, No. 12, 1997, pp. 6326-6342. doi:10.1063/1.366523
[24] M. Carrara and L. Zuppiroli, “Carboxylic Acid Anchoring
Groups for the Construction of Self-Assembled
Monolayers on Organic Device Electrodes,” S ynthetic
Metals, Vol. 121, 20 01 , pp. 1633-1634.
doi:10.1016/S0379-6779(00)00728-1
[25] H. Bedis, F. Kouki and H. Bouchriha, “Effect of Self
Assembled Monolayers on Carrier Photogeneration in
Organic Photovoltaic Cells,” International conference
ELECMOL’08 Grenoble-HMMNT-Mina tec, 2008.