Effect of Self-Assembled Monolayers on the Performance of Organic Photovoltaic Cells

doi:10.4236/jsemat.2011.12007

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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.

Email: [email protected]u.tn

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

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

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:

ITO

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

( )( )

Jep xeD

∂

−

=−+

∂

(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]:

( )

caa c

qd qd

p ppep

px e

−

−−

= +

−−

(2)

where the subscripts “a” and “c” denote charge concen-

trations at the anode

( )

0x=

and the cathode

( )

xd=

respectively, and

( )

eV V

qd kT

−

Using the Einstein relation between the mobility and

the diffusion coefficient one get the dark current:

( )

ha hc

Dqd

µp pe

Jede

−



= −



(3)

The total current density under illumination is in the

following for m:

JJJ= +

(4)

where the steady state current density in the dark is:

( )

ha hc

Dqd

µpp

Jede

−



= −



(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:

( )

ha hc

Lqd

µgIg Ie

Jede

−



= −



(6)

Then, the total curre nt density is written in t he form:

( )

aa cc

th qd

pgIpgI e

Je de



+−+



=

 −







(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:

oc bicc

p gI

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

and

with respect to a

and c

respectively leading to the expression:

ln a

oc bic

VV eg



= −



(9)

On the other hand, the built-in potential

in con-

jugated non-doped thin film devices is p roportional to the

workfunction difference between the cathode and the

anode.

( )

ITO Al

φφ

= −

(10)

where ITO

and

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]:

φεε

∆=

(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,

0 is the vacuum permittivity and ε is

the dielectric co nstant of S AM molecules, (

= 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:

( )

biITO Al

φφ φ

=− +∆

(12)

Note that the built in potential

can be reduced or

enhanced depending on the sign of

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

ocITO Alc

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

(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

of these molecule is

quasi-parallel to their long axis

, t hen lig ht absor ption

which is proportional to scalar product of the incident

light electric field

by the transition dipole moment

is maximum when

and

are parallel and null

when

and

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

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.

( )

EAE E

= −

(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

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

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(%)

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(%)

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

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]

∆

(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.

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