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
MaterialsScienceCenter
4.Institute
University of Groningen
Nijenborgh 4
9747 AG Groningen
The Netherlands
Tel: +31 50 3634440
Fax: +31 50 3634441
5.Abstract
With the depletion of conventional energy sources the demand for renewable energy increases continuously. Polymer based solar cells are a potentialnovelsource 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 p-type 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 activeaggregates. 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 processeswill lead to the development of solar cells with higher conversion efficiencies.
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, creatinga 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.
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.
7.1State 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).8Secondly, 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.
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.10,11,12,13,14,15,16Spectroscopic studies will result in knowledge of the competing photoinduced processes. The knowledgecan be used to tune the conditions of the generation of charge carriers whatcan 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 C60 molecules that can be added optionally.
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.21Recently 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
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.BMonomer 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 C60 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 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.CCharacterization
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.
8.2Photoinduced 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 C60 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.
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 C60molecules 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.