11-01-2014, 03:18 PM
Realization of Large Area Flexible Fullerene - Conjugated Polymer Photocells: A Route to
Plastic Solar Cells
[attachment=59843]
Abstract
Various interesting photophysical phenomena in composites of fullerenes and non
degenerate ground state conjugated polymers with highly extended π-electrons in their main
chain can be explained by the ultrafast electron transfer from the conjugated polymer (donor) to
the fullerene (acceptor) upon illumination. This photoeffect was utilized for the production of
small area (mm2) photovoltaic devices which show energy conversion efficiencies ηe > 1% and
carrier collection efficiencies ηc > 20%. In this work we present efficiency and stability studies
on large area (6 cm by 6 cm) flexible solar cells based on a soluble alkoxy PPV (3,7 -
dimethyloctyloxy methyloxy poly(phenylenevinylene)) and a highly soluble fullerene derivative,
1-(3-methoxycarbonyl)propyl-1phenyl [5,6]C61 (PCBM). The enhanced solubility of PCBM
compared to C60 allows a high fullerene - conjugated polymer ratio and strongly supports the
formation of donor - acceptor bulk heterojunctions.
INTRODUCTION
The utilization of organic materials for photovoltaic devices has been investigated
intensely during the last couple of decades (for a summary of the early reports see for
example (1,2,3)). Because of the ultrafast photoinduced electron transfer (4) with long
lived charge separation, the conjugated polymer/C60 system offers the special
opportunity to produce thin film photovoltaic devices from solution. The photoinduced
charge separation happens with quantum efficiency near unity. The performance (5) of
such bulk heterojunction devices is remarkably enhanced compared to devices made
from the single components. Conjugated polymers seem to fulfill all properties
acquired for photovoltaic energy conversion: strong light absorption and the
possibility of charge separation in presence of a strong electron acceptor like
fullerenes.
RESULTS AND DISCUSSION
The photovoltaic devices have been produced by spin casting from solution,
yielding a typical film thickness around 100 - 200 nm. For the high work function
electrode, transparent ITO substrates, either on glass or on polyester, have been used.
The low work function electrode, Al, was evaporated onto the spin cast film.
Fig. 1 shows the intensity of the photoluminescence (dots, right axis) as a function
of the fullerene concentration in alkoxy PPV/PCBM composites. Luminescence
quenching already happens at very low fullerene concentrations (below 1 mol%
fullerenes). Percolation of fullerenes to a connected path (around 17 mol% for small
molecules) is not necessary for luminescence quenching. Even at low fullerene
concentrations the very effective electron transfer takes place. To obtain an efficient
photovoltaic response it is further necessary to collect these photogenerated charges.
The squares in Fig. 1 (left axis) show the short circuit current Isc as a function of the
fullerene concentration.
CONCLUSION
In plastic solar cells fullerenes act in a double role - as highly efficient e- acceptors
as well as e- conductors. The power efficiency of plastic solar cells (> 1.2 %) is limited
by charge transport. Plastic solar cells can be upscaled to large flexible substrates
without losing efficiency. Oxygen protection of plastic solar cells increases the shelf
life time over 150 days. Further improvements in device efficiencies are expected by
optimizing the composite composition, the network morphology and the charge
transport properties of the single components.
Plastic Solar Cells
[attachment=59843]
Abstract
Various interesting photophysical phenomena in composites of fullerenes and non
degenerate ground state conjugated polymers with highly extended π-electrons in their main
chain can be explained by the ultrafast electron transfer from the conjugated polymer (donor) to
the fullerene (acceptor) upon illumination. This photoeffect was utilized for the production of
small area (mm2) photovoltaic devices which show energy conversion efficiencies ηe > 1% and
carrier collection efficiencies ηc > 20%. In this work we present efficiency and stability studies
on large area (6 cm by 6 cm) flexible solar cells based on a soluble alkoxy PPV (3,7 -
dimethyloctyloxy methyloxy poly(phenylenevinylene)) and a highly soluble fullerene derivative,
1-(3-methoxycarbonyl)propyl-1phenyl [5,6]C61 (PCBM). The enhanced solubility of PCBM
compared to C60 allows a high fullerene - conjugated polymer ratio and strongly supports the
formation of donor - acceptor bulk heterojunctions.
INTRODUCTION
The utilization of organic materials for photovoltaic devices has been investigated
intensely during the last couple of decades (for a summary of the early reports see for
example (1,2,3)). Because of the ultrafast photoinduced electron transfer (4) with long
lived charge separation, the conjugated polymer/C60 system offers the special
opportunity to produce thin film photovoltaic devices from solution. The photoinduced
charge separation happens with quantum efficiency near unity. The performance (5) of
such bulk heterojunction devices is remarkably enhanced compared to devices made
from the single components. Conjugated polymers seem to fulfill all properties
acquired for photovoltaic energy conversion: strong light absorption and the
possibility of charge separation in presence of a strong electron acceptor like
fullerenes.
RESULTS AND DISCUSSION
The photovoltaic devices have been produced by spin casting from solution,
yielding a typical film thickness around 100 - 200 nm. For the high work function
electrode, transparent ITO substrates, either on glass or on polyester, have been used.
The low work function electrode, Al, was evaporated onto the spin cast film.
Fig. 1 shows the intensity of the photoluminescence (dots, right axis) as a function
of the fullerene concentration in alkoxy PPV/PCBM composites. Luminescence
quenching already happens at very low fullerene concentrations (below 1 mol%
fullerenes). Percolation of fullerenes to a connected path (around 17 mol% for small
molecules) is not necessary for luminescence quenching. Even at low fullerene
concentrations the very effective electron transfer takes place. To obtain an efficient
photovoltaic response it is further necessary to collect these photogenerated charges.
The squares in Fig. 1 (left axis) show the short circuit current Isc as a function of the
fullerene concentration.
CONCLUSION
In plastic solar cells fullerenes act in a double role - as highly efficient e- acceptors
as well as e- conductors. The power efficiency of plastic solar cells (> 1.2 %) is limited
by charge transport. Plastic solar cells can be upscaled to large flexible substrates
without losing efficiency. Oxygen protection of plastic solar cells increases the shelf
life time over 150 days. Further improvements in device efficiencies are expected by
optimizing the composite composition, the network morphology and the charge
transport properties of the single components.