Electronic and Optical Properties of Bulk-Heterojunction Organic Solar CellsThe bilateral Slovenian-Romanian project nr. 5CB/30.05.2008 Electronic and Optical Properties of Bulk-Heterojunction Organic Solar
Cells Project
value: 83945 lei Period
of project: 18 months Project
Coordinator: University of Bucharest Project Director: Prof. Univ. Dr. Stefan ANTOHE Contact:
tel. +4021 4574535 fax. +4021
457 44 18 e-mail. santohe@solid.fizica.unibuc.ro Since Tang [1] first presented a thin
film organic solar cell based on a donor-acceptor heterojunction two decades
ago, tremendous effort has been invested in improving the power conversion
efficiency (h) of organic photovoltaic devices. The first organic photovoltaic cells were of Schottky type M1/Organic
layer/M2 (M1 and M2 metals with different work
functions, chosen in such a way that one contact is ohmic and the other one is
a blocking contact). In those structures the organic semiconductor was in the
class of monomeric phthalocyanines, merocyanines, porphiryns, etc., and the
photovoltaic response is due to the separation of the photogenerated charge
carriers in the built-in electric field present at the rectifying
metal/semiconductor interface. The power conversion efficiency was small, of
the order of 10-2 % [2]. Using two-layered structures in which the photoactive
region is the heterojunction between two organic layers, with complementary
absorption spectra, the power conversion efficiency was increased with about
two orders of magnitude [3]. Trying to enlarge the photoactive region,
three-layered structures were reported with an increased efficiency due to an
increased number of sites for exciton dissociation [4]. Recently structures based on polymeric
blends were produced, with far better
efficiency of about 4-5 %, and also cheeper than the organic monomeric
thin films [5]. Among the large range
of polymeric materials tested as active layers, polymer-fullerene bulk
heterojunction solar cells have shown promising perspectives, because of the
high quality of these materials in terms of mobility and thermal stability
[6-10]. Photovoltaic structures based on
poly(3-hexylthiophene) (P3HT), 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61
(PCBM) polymers and P3HT:PCBM(1:1) polymers blend, respectively, were prepared
by spin coating technique, using optical glass substrates covered with 30 nm
thick ITO. The Current-Voltage (I-V) characteristics, in the dark and under
illumination through ITO electrode of the ITO/PEDOT/P3HT/Al, ITO/PEDOT/PCBM/Al
and ITO/PEDOT/P3HT:PCBM(1:1)/Al, structures were measured. Their non-linearity
and asymmetry were explained on the base of electrode/organic semiconductor
interface behavior. The measured action spectra of the cells revealed the
features also observed in absorption spectra of the component polymers or the
blend. A fill factor (FF) higher than in the case of single layer structures
was measured experimentally for the blend structures. The power conversion
efficiency of the as prepared blend structures was higher,
too, than those of the single layer structures based on the ordinary P3HT and
PCBM layer, respectively. The fourth quadrant parameters of the blend
structures, determined in monochromatic light, changed in time with respect of
their values obtained on as prepared samples. The grounds of scientific collaboration
were established in 2008, between the group led by Professor Stefan Antohe
(Faculty of Physics, University of Bucharest) and Professor Gvido Bratinas group at the University of Nova Gorica.
It must be stressed that there are many bilateral Romanian-Slovenian projects
with significant results in terms of scientific research. The Slovenian partner (Laboratory for
Epitaxy and Nanostructures (LEN) from University of Nova Gorica) has a good
expertise in the preparation and the characterization of thin films made of
organic and inorganic materials, and has access to modern deposition
techniques, such as: liquid phase epitaxy and thermal vacuum evaporation
specialized for metals, AIIBVI compounds and organic
materials. The Romanian partner has access to high
quality equipments necessary to characterize structural, morphological,
electrical and optical properties of organic solar cells, as: Clean Room Class
1000, XRD with reflectometry properties, AFM and SEM with nanolitography
facilities, Perkin Elmer Lambda 35 spectrofotometer and solar simulators. As concerning the human resource, both
research teams have a good expertise in the field of the proposed project; on
the other hand, taking into account the complementarity of experimental
facilities, we appreciate that this bilateral cooperation is fruitful for both
parts. We consider
that this collaboration can lead to achieving the objectives of the project,
i.e.: 1. Preparation and
structural, electrical and optical characterizations of thin films based on
MEH-PPV blend or conjugated polymers with MoSIx nanowires (MNs), and analysis
of the electrical and optical behavior of ITO/Polymer ar Metal/Polymer. 2. Realization and
electrical and optical characterization of P3HTx/PCBM1-x photovoltaic cells . Research on photovoltaics is defined as a
priority in research politics in Europe and has gained considerable weight also
in 7th Framework program. The present proposal represents an effort
in strengthening European cooperation in this area. Specifically for Romanian
industry, the project will have an impact on the state-of-art in photovoltaics,
and will stimulate through the dissemination of the results among the
interested companies, and increased rate of investments in this type of
research and development. The obtained results were submitted for
publication in Materials Science and Engineering B, Electrical and Photoelectrical Properties of Organic Photovoltaic Cells
Based on poly(3-hexylthiophene) and
1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61, and were also presented at international and
national conferences as: 6th International Conference on
Nanosciences & Nanotechnologies, 13 15 July 2009, Thessaloniki, Greece;
Romanian Conference on Advanced Materials, 25 28 August 2009, Brasov, Romania
and 2009 Annual Scientific Conference, 5 June 2009, Faculty of Physics,
University of Bucharest, Bucharest, Romania. Other two papers are curently
under preparation and will be published in ISI journals. Experimentally obtained data confirmed
the initial assumptions. The blend cell shows an improved (wider) spectral
response as compared to the case of single layer cells. The absorption and
action spectra of the blend P3HT:PCBM [1:1] were enlarged, showing the so
called ,,co-sensitization effect. As
expected in the case of the blend structures, the number of Donor/Acceptor (D/A) interfaces was
significantly increased by blending the donor and acceptor materials. [1] Tang, C.W. Appl. Phys. Lett. 1986, 48, 183. [2] Antohe, S., Phys.Stat.Sol.(a)1993,136, 401. [3] Antohe, S.; Tugulea, L., Phys.Stat.Sol. (a) 1991, 128, 253. [4] Antohe, S.; Ruxandra, V.; Tugulea, L.; Gheorghe, V.; Inascu, D., J.Phys.III France 1996, 6, 1133. [5] Schmidt-Mende, L.; Fechtenkötter, A.;
Müllen, K., Moons, E.; Friend, R.H., MacKenzie, J.D. Science 2001, 293, 1119. [6] Padinger, F.; Rittberger, R. S.; Sariciftci, N. S. Adv. Funct. Mater. 2003,
13, 85; [7] Chirvase, D.; Parisi, J.; Hummelen, J.C.; Dyakonov, V. Nanotechnology 2004, 15, 1317; [8] Li, G.; Shrotriya, V.; Yao, Y.; Yang, Y. J. Appl. Phys. 2005, 98, 043704; [9] Schilinsky, P.; Asawapirom, U.; Scherf, U.; Biele, M.; Brabec, C.J. Chem. Mater. 2005, 17, 2175; [10] Kim, Y.; Choulis, S.A.; Nelson, J.; Bradley, D.D.C.; Cook, S.;
Durrant, J.R. Appl. Phys. Lett. 2005, 86, 063502. |
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