Effect of Self-Assembled Monolayers on the Performance of Organic Photovoltaic Cells
Copyright © 2011 SciRes. JSEMAT
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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