1 Quantum Solar Energy Linz (QSEL), A-4010 Linz, Austria.
2 Christian Doppler Laboratory for Plastic Solar Cells, Physical
Chemistry, Johannes Kepler University, A-4040 Linz, Austria.
3 Stratingh Institute and Materials Science Center, University of
Groningen, 9747 AG Groningen, The Netherlands
The demand for inexpensive, renewable energy sources is driving new
approaches to the production of low-cost photovoltaic devices. In the last couple of
years, a large amount of effort has been put in the development of solar cells based
on organic molecules [1] and conjugated polymers [2,3]. Especially polymer-based
solar cells would offer tremendous advantages for the fabrication, like low cost
roll-to-roll production of large area, flexible solar cells. Because of these
advantages, the development of polymer solar cells would have a major impact,
even if the efficiencies of these types of photovoltaic devices up to now are smaller
than the efficiencies achieved in inorganic solar cells.
For the generation of electrical power by light conversion it is necessary to
spatially separate the electron-hole (e-h) pair generated in the absorption process
before recombination can take place. In conjugated polymers, the stabilization of the
photoexcited e-h pair can be achieved by blending the polymer with an acceptor
molecule having an electron affinity that is larger than the electron affinity of the
polymer, but still smaller than the ionization potential of the polymer donor. Under
these conditions it is energetically favorable for the excited conjugated polymer to
transfer an electron to the acceptor molecule, while a hole remains in the polymer
valence band as the lowest energy level.
In our work this alignment of the energy levels is realized by blending of poly
[2-methoxy, 5-(3',7'-dimethyl-octyloxy)]-p-phenylene vinylene (MDMO-PPV) or
poly-3-octyl-thiophene (P3OT) with derivatives of C60. The polymers
act as electron donor when blended with the acceptor molecule C60
or the soluble methanofullerene [6,6]-Phenyl C61-butyric acid methyl
ester (PCBM) or a mixture of methanofullerene =91higher adducts=92 (multi-PCBM)
in which the fullerene cage bears two or more identical pending groups on various
positions. It has been shown that in these systems photoinduced electron transfer
from the polymer onto the fullerenes occurs within approximately 200 fs after the
excitation [4]. Since all other known competing relaxation processes in conjugated
polymers occur on time scales that are orders of magnitudes larger than 200 fs, this
ultra fast charge transfer has a quantum efficiency of approximately 1. Thus nearly
all photons absorbed by the polymer are converted to electrons on the
C60. In addition, the lifetime of the charge separated excited state is
in microsecond time scale at room temperature [5], allowing the generation of high
concentrations of non-equilibrium electrons and holes.
By sandwiching the blend of conjugated polymers and C60
between electrodes that have different work functions F m (for example ITO (indium tin oxide,
F ito = 4.7 eV) and Al (F Al = 4.3 eV)) an electrical field is
established across the polymer layer. At acceptor concentrations higher than the
percolation threshold of 17 mol % [6] this electric field separates the non-equilibrium
carriers generated by the incident light and a current is delivered to an electrical
circuit connected to the electrodes. However, in these polymer blends the mobilities
of electrons and holes are rather small due to the hopping transport, thereby limiting
the efficiency of the photovoltaic cells.
We compare the I-V characteristics of devices made from MDMO-PPV and
P3OT blended with various C60 derivatives. Large area photovoltaic
devices have been fabricated showing power conversion efficiencies up to 1.5 %
under monochromatic illumination at 488 nm. It is shown that among the various
combinations of materials both the open circuit voltage and the short circuit current
is maximal for a blend of MDMO-PPV and a highly soluble fullerene derivative
(PCBM). In addition, it is shown that by decreasing the device thickness to an
optimum value, the efficiency can be increased significantly.
The practical application of the conjugated polymer fullerene devices is limited
by their stability under ambient conditions (i. e. in the presence of water or oxygen).
Recent FTIR degradation studies on the polymer composites showed rapid
bleaching of the conjugated polymer and somewhat slower degradation of
C60 under oxygen and light [7]. Stabilization of these composites
was realized by the exclusion of oxygen, either by sealing or by handling the devices
under inert conditions. The embedding of the photoactive conjugated polymer fulle
rene blend in a conventional polymer matrix (guest host approach) is a sound and
promising method to improve the photoactive sample quality for the following
reasons [8]:
Polymeric photovoltaic devices consisting of a mixture of a MDMO-PPV, PCBM and a conventional polymer (as conventional polymers polystyrene (PS), polyvinylcarbazole (PVK) and polyvinylbenzylchloride (PVBC) were investigated) were fabricated and characterized under monochromatic light and by wavelength dependent photocurrent measurements. From these data power efficiencies were calculated and compared to the electron-photon conversion efficiencies.
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[6] | C. J. Brabec, F. Padinger, V. Dyakonov, J. C. Hummelen, R. A. J. Jansen and N. S. Sarciftci, in "Molecular Nanostructures", Proceedings of the International Winterschool on Electronic Properties of Novel Materials, Kirchberg 1998. |
[7] | H. Neugebauer, C. J. Brabec, and N. S. Sariciftci, Synth. Met. ICSM 98, in print. |
[8] | C. J. Brabec, H .Johannson, F. Padinger, H. Neugebauer, J. C. Hummelen and N. S. Sariciftci, submitted to Solar Energy Materials and Solar Cells |