1.
Title of the Project
Synthesis and optical studies of fully conjugated diblock copolymers for novel
photovoltaic devices.
2.
Applicant
M.T.W. Milder
3.
FOM-research group
FOM-G-02
Nonlinear Optics and Spectroscopy
Materials Science Center
4.
Institute
University of Groningen
Nijenborgh 4
9747 AG Groningen
The Netherlands
Tel: +31 50 3634440
Fax: +31 50 3634441
[email protected]
5.
Abstract
With the depletion of conventional energy sources the demand for renewable energy
increases continuously. Polymer based solar cells are a potential novel source of green
energy. The costs of the starting materials and the manufacturing process of these devices
are much lower than of conventional silicon solar cells. However, due to their low
efficiencies the price per kWh so far is 5 times higher. To make polymer based solar cells
an economically interesting alternative the efficiency of these devices must be increased.
Currently, the active layer of polymer based solar cells mainly consists of blends of ptype polymers and derivatives of C60. We propose the development and characterization
of novel conjugated polymers for these photovoltaic devices. Conjugated diblock
copolymers contain covalently linked p- and n-type units in a single polymer chain.
These polymer chains can self-assemble into photo active aggregates. The physical
properties of the aggregates suggest that higher efficiencies than in classic blends can be
achieved. Conjugated polymers have been the subject of many studies, however to our
knowledge there is only one report about the synthesis of fully conjugated diblock
copolymers. We propose to synthesize novel conjugated diblock copolymers with p- and
n-type blocks. Optical studies on the photoinduced processes by linear and nonlinear
spectroscopy will reveal the main relaxation pathways that compete with charge
generation. Elucidation of the photoinduced processes will lead to the development of
solar cells with higher conversion efficiencies.
1
6.
Introduction
Photovoltaic solar cells are promising sources of renewable energy. Most solar cells are
based on polycrystalline silicon and have a relatively high cost price determined by the
costs of the starting material and the expensive manufacturing process. Polymer
semiconductors are a good alternative for the silicon-based solar cells due to their
potentially low manufacturing costs and their flexibility. The efficiency of these new
materials is at the moment maximum 3.5%, which limits the use of polymers solar cells
in industry.
Since the discovery of the electrical conductivity of doped polyacetylene in 1977,
conjugated polymers (CPs) have been subject to extensive research.1 Conjugated
polymers are versatile compounds. The semiconducting properties of these materials can
be tuned by chemical functionalization, and they can readily be used to coat large
surfaces. Many types of conjugated polymers are used in device manufacture. Conjugated
polymers form a promising class of materials in the development of new solar cells.
The principle of a solar cell is shown in figure 1. The sunlight is transmitted by a
transparent electrode after which the photons are absorbed by the active material,
conjugated polymers in the case of plastic solar cells. The absorbed photons create
excitons that can relax back to the ground state by multiple pathways. One of the
pathways is the route in which charge carriers are created. The exciton diffuses to an
electron donor and acceptor interface (pn-junction) where it can form charge carriers
(electron and hole). The charge carriers drift to the electrodes, creating a current in the
photovoltaic device. There are many other photoinduced processes in conjugated
polymers that compete with the generation of charge carriers. To optimize the conversion
of the sunlight into electric energy the generation of charge carriers and their collection
efficiency at the electrodes should be maximized. The relaxation pathways in conjugated
polymer systems can be identified using spectroscopy. The results obtained from
spectroscopic studies can be applied to tune the charge generation towards higher
efficiencies.
2
Figure 1: Principle of a polymer based solar cell. The incoming (sun) light is transmitted by a
transparent electrode. The polymer layer between the transparent and the metal electrode
consists of a p-type (donor) and n-type (acceptor) material. Absorbed photons create an exciton
on the polymer chain that can result in the formation of charge carriers (electron and hole). The
charge carriers drift to the corresponding electrode creating a current.
7.
Scope of the project, the scientific problem
The conversion efficiency of sunlight into electric energy by solar cells is limited by a
number of different factors. Physical and chemical impurities can act as electron and hole
traps (carrier loss). Secondly, the absorption band of the CPs used in a device limits the
fraction of the solar spectrum that creates an electron-hole pair. Photons that have a lower
energy than the band gap will not contribute to charge generation (photon loss).
Incorporation of a lower bandgap material in the photoactive layer would increase the
fraction of sunlight that is converted. Finally, the exciton diffusion length in CPs also
limits the formation of charge carriers. The exciton has to encounter a pn-junction to
generate a charge pair. In conjugated polymers the maximum exciton diffusion length is
20 nm.2 To obtain efficient charge separation, the average domain size of donor and
acceptor should not exceed this value (exciton loss). This size depends on the
morphology of the photoactive polymer layer. The choice of polymer and processing
conditions to form the photoactive layer determines the morphology.
3
7.1
State of research
Several approaches to improve the efficiency of solar cells have been subject to research.
Control over the polymer morphology by influencing the parameters in the spincoating
process has been proposed as a solution to overcome the limited exciton diffusion length.
3
Control over the morphology can also be obtained using intrinsic properties of the
designed macromolecules. To obtain interpenetrating networks of donor and acceptor
materials, double cable donor and acceptor units have been developed to provide
continuous route for charge carriers between electrodes.4 A new class of materials,
conjugated block copolymers, has the potential to provide a new method to control
morphology in polymer devices.5
8.
Research plan
Conjugated diblock copolymers are materials containing two different conjugated blocks
that are covalently linked. If the block copolymers are formed with n and p type
monomers, the resulting material contains covalently linked donor and acceptor units and
provides phase separation. Supramolecular organization of diblock copolymers and its
effect on the electronic properties of these materials has not been studied extensively.
Diblock copolymers can self-assemble to different morphologies ranging from lamellae
to spherical vesicles (Fig. 2).6 Under specific conditions using solvophobic effects, the
morphology of diblock copolymers can be steered towards vesicles (Fig. 3). The vesicles
have a two-layer shell, the outer layer is formed by the first block of p-type polymer and
the inner layer is formed by the second block of n-type polymer. The aggregation of
polymers influences the physical properties.
Three novel aspects of diblock copolymer vesicles are important for their use in
photovoltaics. Firstly, additional electron acceptor molecules can be introduced into the
polymer layer. One of the most promising compounds acting as an electron acceptor is
C60. A disadvantage of this molecule is the limited solubility in organic solvents. This has
prevented efficient use of this compound in photovoltaics. The solubility of C60 has
already been increased by functionalization of the molecules with alkyl chains, however
the synthetic route for these substances is difficult.7 The use of polymer vesicles allows a
more simple control over the polymer/C60 ratio (Fig. 3).8 Secondly, the lifetime of the
charge separated state population can be increased. A longer lifetime of this population
means that more charges will diffuse to the electrodes before recombination. This results
in a higher collection efficiency of charge carriers at the electrodes. The acceptor block
of diblock copolymer acts as an additional energy barrier for charge recombination and
subsequent relaxation to the ground state. The presence of the acceptor block will
therefore increase the lifetime of the population of the charge separated state. Thirdly, the
conversion efficiency of the material can be increased by the addition of a second type of
polymer. The fraction of light that will be involved in the actual generation of charge
carriers depends on the bandgap of the polymers. The maximum solar photon flux is
around 1.5-1.8 eV, while most conjugated polymers have a bandgap over 2 eV.9 Addition
of the second type of polymer with a different bandgap, in this case the acceptor block,
will decrease the photon loss and enhance the conversion efficiency of the material.
4
Figure 2: Classical structures that can be formed by aggregation of block copolymers. L, C and S
stand for lamellar, cylindrical and spherical respectively.
Conjugated diblock copolymers are a promising class of materials and will be
studied in cooperation with the group of polymer science (University of Groningen). The
appropriate conjugated block copolymers will be designed and synthesized. The
morphology of the system will be determined with standard electron microscopy and
AFM. Subsequently the electronic properties of self-assembled vesicles will be studied
by both linear and ultrafast nonlinear optical spectroscopy. The photoinduced processes
of these regular self-assembled structures in solution possibly differs from what is known
from solid films and dilute solutions of CPs.10,11,12,13,14,15,16 Spectroscopic studies will
result in knowledge of the competing photoinduced processes. The knowledge can be
used to tune the conditions of the generation of charge carriers what can result in higher
efficiencies of photovoltaic devices. The diblock copolymers can self-assemble into rods
under specific conditions depending on the concentration of polymer and the solvent
polarity. These rods provide a continuous route between the two electrodes and control
the distance between donor and acceptor units (Fig.3). Therefore these compounds might
be ideal components to increase the efficiency of photovoltaic devices. The project can be
extended with optical studies of the diblock copolymers in the solid state. The application
of these polymers in photovoltaic devices will be studied in close collaboration with the
molecular electronics group (University of Groningen).
Figure 3: Schematic representation of a self-assembled diblock copolymer vesicle. The outer
layer of the vesicle (block 1) is formed by the electron donating polymer depicted in blue (p-type).
The inner layer (block 2) is formed by the electron accepting polymer depicted in green (n-type).
The grey spheres in the core of the vesicle represent C 60 molecules that can be added optionally.
5
Figure 4: Schematic representation of a solar cell based on diblock copolymer rods. The blue and
green cylinders represent the p- and n-type polymer respectively.
8.1.A
Synthesis of conjugated diblock copolymers (Polymer Science Group)
The scientific field of block copolymers is well-developed and the compounds are widely
used.17,18 Recently the use of these materials as a building block in self-assembly has
been shown.19 The inherent microscale phase separation between different blocks and the
ability to form aggregates proves that the molecules are interesting materials for
molecular device manufacture.
There are several routes to synthesize block copolymers. The synthesis of fully
conjugated diblock copolymers is challenging. The techniques used for copolymerization
are different from the methods that are applied in the synthesis of normal conjugated
polymers.20 Polymerization of the second block of the copolymer has to be performed
under conditions in which the first block is stable. To our knowledge there is only a
single record on the synthesis of conjugated diblock copolymers.21 Recently also a fully
conjugated triblock copolymer consisting of polyfluorene (PF) and polyaniline (PANI)
blocks has been synthesized. 2,22 The polymer vesicles shown in figure 3 are formed by
self-assembly of p-n-type diblock copolymers, therefore a synthetic route to form the
complex polymers has to be developed. The route described by Asawapirom et al. will
be adapted to obtain an appropriate diblock instead of the triblock copolymers. 23 The
starting material for the polymerization of the fist block has to be monodirectional, which
complicates the synthesis. The monomer fluorene has two identical reactive positions
where polymerization can occur. One of these groups has to be deactivated to allow the
polymerization to be monodirectional. 24,25, 26,27
6
Figure 5: Schematic representation of the electronic design of the D-A diblock copolymer
molecules. The bandgap is depicted as the energy difference between the HOMO (H) and LUMO
(L) levels. The bandgaps are not necessarily equal in all components. After photoexcitation,
electron and hole transfer can respectively take place between the LUMO and HOMO levels of
the components.
Before starting the synthesis of the fully conjugated diblock copolymer, a test system
needs to be synthesized. This test system is the block copolymer proposed by Jenekhe
and consists of one conjugated and one nonconjugated block.8 These rod-coil block
copolymers also form vesicles in which C60 can be solved. The vesicles will act as a
model system for optical experiments while the fully conjugated diblock copolymer is
developed.
8.1.B Monomer selection
There are several aspects that should be taken into account when choosing the two
monomers that will be used in the diblock copolymer. An important factor in
photovoltaic systems is the photoinduced charge transfer. The two monomers that will
form the two blocks need to satisfy the conditions shown in the energy diagram in figure
5 to allow electron and hole transport. To acquire electron transfer into the core of the
vesicles, the outer layer needs to donate electrons to the inner block. However, the
accepting properties of the inner block should not exceed those of the C 60 molecules to
allow electron transfer.
A second important factor is the property of the diblock copolymers to form
micelles or vesicles in solution. Solvophobic effects determine the morphology of the
diblock copolymer system. The donor block will form the outer shell of the vesicles (Fig.
3). The inner, acceptor block, has to be (slightly) insoluble in the solvent used to cause
aggregation into vesicles. A combination of two solvents can be used to obtain the
desired morphology.8 The choice of monomers is also based on the polymerization
reaction used to synthesize the copolymers. Either the donor or the acceptor block is
7
synthesized first and subsequently polymerization of the second block proceeds at the
reactive end of the first block. The polymerization of the second block of the copolymer
has to be performed under conditions in which the first block does not degrade.
8.1.C Characterization
The synthesized conjugated diblock copolymers will be characterized by several different
techniques. The purity and composition of the sample will be determined by NMR
spectroscopy. Information about the morphology will be acquired by electron microscopy
and AFM. Electrochemistry experiments will indicate the energy levels of the charge
separated states.
8.2
Photoinduced processes (Nonlinear Optics and Spectroscopy Group)
The key aspect in the operation of a solar cell is the generation of charge separation in
conjugated polymers which follow after photoexcitation. Figure 6 shows the possible
relaxation pathways in the diblock copolymer vesicle. The states of interest are the charge
separated states that are depicted by the red boxes. Applying this to the diblock
copolymer rods means that the different charges travel through different parts of the rod
(Fig. 3). To study the different photoinduced processes that can lead to the population of
the charge separated states, optical spectroscopy will be used. Knowledge of the different
processes can be used to direct more excitons towards dissociation into a separated
charge pair by changing the conditions concerning e.g. solvent polarity and chain length.
Figure 6: Schematic energy diagram describing the possible processes in diblock copolymers
induced by photoexcitation. The diblock copolymer vesicles including C 60 are represented by the
D-A-C60. The charge separated states are depicted in red, whereas localized singlet and triplet
states are depicted in blue (donor), green (acceptor) and yellow (C60). ET and CT stand for
energy transfer and charge transfer respectively.
8
Spectroscopic studies will start on the diblock copolymer vesicles proposed by Jenekhe, 8
while the fully conjugated diblock copolymer is developed. This copolymer will serve as
a test compound for the optical spectroscopy part of the project. The test block copolymer
consists of one conjugated and one nonconjugated polymer. The synthesis of this
compound is well known and the choice of monomers can be adapted as long as the
reactions as described in reference 8 can still be applied.
Diblock copolymer vesicles will be studied in solution both with and without C60
molecules in the core of the vesicles (Fig.3). Selective excitation can be achieved by
tuning the laser frequency to the maximum absorption wavelength of either the donor or
the acceptor block. Energy transfer between the two blocks or between the acceptor block
and C60 causes quenching of the fluorescence of the initially excited part. In parallel
sensitization of the singlet excited state of either the acceptor or C60 is observed. Charge
transfer between the two blocks or between acceptor block and C60 can be observed by
studying the system in solvents of different polarity. Nonpolar solvents will enhance
energy transfer, whereas polar solvents favor electron transfer by stabilizing the charge
separated state (ΔG<0). Solvent dependent changes in fluorescence intensity suggest a
photoinduced process in addition to energy transfer. The additional process is likely to be
charge transfer. Singlet oxygen sensitization can be used to investigate the yield of triplet
states formation. Because intersystem crossing to a triplet state competes with charge
separation, the oxygen fluorescence will give an indication about the population of the
charge separated states.
Direct observation of the population of the charge separated states is possible with
ultrafast nonlinear spectroscopy. The transient signal of charge separated pairs can reside
up to ~100 ps due to long charge recombination times. The signal can be observed with
pump-probe experiments. Photogeneration of charge separated pairs can also be followed
by studies of infrared active vibrational (IRAV) modes.13,14 The technique observes a
signal that is related to modes that become vibrationally active after perturbation of the
local symmetry e.g. by changes in bond length due to lattice stabilized charges.
The photoinduced processes depend on the size of the two blocks in the diblock
copolymer. The distance between donor and acceptor units should preferentially not
exceed the exciton diffusion length of 20 nm. The effect of chain lengths on the
generation of charge separated pairs will also be subject of study as will be the
temperature dependence of this process.
9
9.
Plan of work
The research will be carried out in the Nonlinear Optics and Spectroscopy group and the
Polymer Science group of the University of Groningen (RUG). Both research groups are
part of the top research school MSC+ that plays an active role in the development and
characterization of novel functional materials. Within the MSC+, the groups of
(Bio)organic Materials and Devices and Physics of Organic Semiconductors are involved
in research on photovoltaics.
The project will run from January 2006 to December 2010. In 2006 the PhD student in
Polymer Science will start synthesis of the test system and preliminary development of
the fully conjugated diblock copolymer. The synthesis of the fully conjugated diblock
copolymer will take three years. In 2007 the PhD student in the Nonlinear Optics and
Spectroscopy group will start with arranging the set-up for studies on conjugated
polymers and subsequent experiments on the model system, which will approximately
take one year. The remaining three years will be used for the characterization of the
morphology and optical studies of the conjugated diblock copolymers.
10.
Available infrastructure
10.1
Place of research
The place of research will be the Nonlinear Optics and Spectroscopy group and the
Polymer Science group of the University of Groningen (RUG). The infrastructure and
facilities in the building of mathematical and natural sciences are adequate for synthetic
work, characterization and spectroscopic studies.
The technical equipment for spectroscopic studies contains:
- basic Perkin-Elmer spectrophotometer and fluorimeter
- Ti:sapphire laser with pulse picker and harmonic generator (250-550 nm), 2 ps
time-resolution, supplied with a streak camera (Hamamatsu C5680) for
measurements on emission lifetimes
- Hurrican Ti:sapphire laser (1kHz) delivering sub-100 fs pulses, equipped with
two NOPAs (250-1000 nm) for high resolution time-dependent experiments
- Ti:sapphire laser-amplifier system delivering sub-100 fs pulses with energies up
to 15 μJ supplied with an OPA (2.5-3.5 μm) for oxygen fluorescence
measurements
11
Requested funding
11.1
Personnel positions
There will be two positions for PhD students. A technician will be employed to assist in
the optical experiments for 0.1 fte (5 k€).
10
11.2
Running budget
Per year 5 k€ is required per PhD student to attend international conferences/workshops.
11.3
Consumables
Material costs will be used for chemicals, laboratory equipment and small optical
elements (e.g. mirrors, beam splitters). The total costs will amount to 7 k€ per year per
PhD student.
11.4
Budget summary
Personnel (positions):
PhD students
Postdocs
Technicians
Personnel costs:
Running budget
Consumables
Total (requested)
12.
2006
41
0
0
2007
82
0
5
2008
82
0
5
2009
82
0
5
2010
41
0
5
5
7
53
10
14
111
10
14
111
10
14
111
5
7
58
TOTAL
328
0
20
348
40
56
444
Application in industry
The realization of efficient solar cells based on molecular material still is a scientific
challenge. The efficiency of conventional solar cells based on polycrystalline silicon is at
the moment 15-20%.28 The disadvantages of the use of silicon are the high material and
manufacturing costs. Current plastic solar cells are potentially cheaper to make, however
they have a limited efficiency of ~3.5%. Before these materials can be taken into
production, the efficiency should be increased dramatically. Development of a conjugated
diblock copolymer and characterization of its photoinduced processes that compete with
charge generation by linear and nonlinear spectroscopy are promising developments
towards higher conversion efficiencies in photovoltaics.
11
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13