Summary
Photovoltaic devices are considered one of the important scientific and technological challenges for conversion of the most important source of inexpensive and renewable energy (solar energy) to electricity. However, traditional photovoltaic silicon solar cells are still deemed too expensive to be a serious concurrent to fossil fuels or other renewable energy sources (wind, waterpower, biomass, etc). All-organic solar cells are considered now as being a very promising alternative for conversion of solar energy to electricity because they can be fabricated based on low-cost advanced materials and using the bulk heterojunction technology. However, the power efficiency of the organic photovoltaic devices is still lower compared with the traditional inorganic devices, due to a sum of factors, and huge research efforts have to be done to find the optimal polymer donor/acceptor combination for building commercial organic solar cells.
The inefficiency of the organic photovoltaic cells with a single layer or bi-layer active component configuration is due to the exciton limited lifetimes which only allow the diffusion at a short distance. Donor excitons created far away from the heterojunction interface decay to the ground state without the chance to reach the acceptor and leading to a very low power conversion efficiency. Only those excitons generated near the interface can be dissociated in free carriers. In order to separate the excitons into free charges, a donor/acceptor (D-A) system must be employed. When the exciton reaches the donor/acceptor interface, it will dissociate and the electron will transfer to the material with larger electron affinity while the hole will be accepted by the material with the lower ionization potential. Since the exciton diffusion length of organic materials is short (typically 1 - 10 nm) only those excitons generated within the exciton diffusion length of organic material from interface can diffuse to the interface and then dissociate into free charge carriers. Other excitons generated deeply within the active layer may recombine or be trapped before they reach the interface. Thus this problem can be overcome by molecular blending the donor and acceptor together to form a nanocomposite followed by casting from solution as thin photoactive layer between two electrodes, which is called bulk heterojunction device (BHJ). The BHJ configuration requires a single active layer to create an internal D-A heterojunction, in which donor and acceptor materials form an interpenetrating network with large D-A surface area. In a typical polymeric BHJ photovoltaic cell (Figure 1), the active layer is sandwiched between a transparent indium tin oxide (ITO) anode and a metal (Al, Ca, Mg) catode. The photoactive layer is a homogeneously mixture composed of a low band gap conjugated polymer donor and a soluble nanosized electron-accepting material. Formation of an interpenetrating network with an acceptor requires the donor polymer to have a certain interaction with the acceptor, preventing severe phase separation. In this case, the excitons are dissociated into electrons and holes at the donor/acceptor interface and are transported through the two respective phases and are collected at the electrodes and with the aid of the internal electric field generates the photocurrent and photovoltage. The formation of a bicontinuous network creates two channels to transport holes in the donor domain and electrons in the acceptor domain. The major interests in increasing the efficiency of the photovoltaic cells are to improve the photoinduced electron transfer in donor/acceptor heterojunction. Conjugated polymers combine the optical and electronic properties of semiconductors with processing advantages and serves as p-type materials in assembling organic solar cells with low weight, integrated flexibility, and low cost. The basic requirements for an ideal donor polymer include: good solubility and film-forming properties; strong and broad bandwidth absorption; high hole mobility; suitable HOMO/LUMO energy levels; high purity and molecular weight. It is commonly accepted that the open circuit voltage (Voc) of the bulk heterojunction device is proportional to the difference between the highest occupied molecular orbital (HOMO) energy level of the donor and the lowest unoccupied molecular (LUMO) energy level of the acceptor. The HOMO energy level of the p-material has to be at least 0.3 eV higher than that of the acceptor derivative in order to favor the electron transport and to overcome the binding energy of intrachain exciton. Conjugated poly(p-phenylenevinylene)s have attracted a great deal of attention due to their conducting and photoluminescent properties. Its derivatives remain the most popular conjugated polymers for this application and continue to generate considerable interest and much research for photovoltaic applications. Unfortunately, the unsubstituted PPV is insoluble and infusible and therefore difficult to process into the solid state. By incorporating arylamine derivatives (i.e., carbazole, indolocarbazole) as donor sequences in PPV structure the polymer band gap can be reduced and solubility improved by introduction of substituents at amine nitrogen.
In BHJ organic solar cells, conversion of incident solar energy to photocurrent (the photovoltaic mechanism) takes place by a cascade of four steps: (1) the absorption of sunlight by conjugated donor polymer and generation of excitons (electrically neutral bound electron-hole pairs), (2) diffusion of excitons to the heterojunction, (3) dissociation of the excitons into free charge carriers, and (4) the transport of these carriers to the contacts, as is illustrated in Figure1.
Figure 1: Energy diagram of an organic solar cell based on BHJ concept; The four subsequent processes to generate photocurrent are: (1) absorption of an incident photon to create an exciton, (2) diffusion of an exciton toward the donor-acceptor interface, (3) dissociation of an exciton into an electron in the acceptor and hole in the donor, and (4) collection of charges at the contacts.
Every step has to be optimized so that to have an overall high efficiency of the photovoltaic process. Many researchers have observed that small structural deviations on the backbone of conjugated polymers influence their photovoltaic properties. Therefore, the challenge here is to provide and study new conjugated polymer/acceptor combinations with well-defined structure and to improve every step in photovoltaic process. The optimization of organic solar cells is a fine balancing activity. It requires finding the optimal compromise among a combination of materials characteristics that can work in opposite directions.
The inefficiency of the organic photovoltaic cells with a single layer or bi-layer active component configuration is due to the exciton limited lifetimes which only allow the diffusion at a short distance. Donor excitons created far away from the heterojunction interface decay to the ground state without the chance to reach the acceptor and leading to a very low power conversion efficiency. Only those excitons generated near the interface can be dissociated in free carriers. In order to separate the excitons into free charges, a donor/acceptor (D-A) system must be employed. When the exciton reaches the donor/acceptor interface, it will dissociate and the electron will transfer to the material with larger electron affinity while the hole will be accepted by the material with the lower ionization potential. Since the exciton diffusion length of organic materials is short (typically 1 - 10 nm) only those excitons generated within the exciton diffusion length of organic material from interface can diffuse to the interface and then dissociate into free charge carriers. Other excitons generated deeply within the active layer may recombine or be trapped before they reach the interface. Thus this problem can be overcome by molecular blending the donor and acceptor together to form a nanocomposite followed by casting from solution as thin photoactive layer between two electrodes, which is called bulk heterojunction device (BHJ). The BHJ configuration requires a single active layer to create an internal D-A heterojunction, in which donor and acceptor materials form an interpenetrating network with large D-A surface area. In a typical polymeric BHJ photovoltaic cell (Figure 1), the active layer is sandwiched between a transparent indium tin oxide (ITO) anode and a metal (Al, Ca, Mg) catode. The photoactive layer is a homogeneously mixture composed of a low band gap conjugated polymer donor and a soluble nanosized electron-accepting material. Formation of an interpenetrating network with an acceptor requires the donor polymer to have a certain interaction with the acceptor, preventing severe phase separation. In this case, the excitons are dissociated into electrons and holes at the donor/acceptor interface and are transported through the two respective phases and are collected at the electrodes and with the aid of the internal electric field generates the photocurrent and photovoltage. The formation of a bicontinuous network creates two channels to transport holes in the donor domain and electrons in the acceptor domain. The major interests in increasing the efficiency of the photovoltaic cells are to improve the photoinduced electron transfer in donor/acceptor heterojunction. Conjugated polymers combine the optical and electronic properties of semiconductors with processing advantages and serves as p-type materials in assembling organic solar cells with low weight, integrated flexibility, and low cost. The basic requirements for an ideal donor polymer include: good solubility and film-forming properties; strong and broad bandwidth absorption; high hole mobility; suitable HOMO/LUMO energy levels; high purity and molecular weight. It is commonly accepted that the open circuit voltage (Voc) of the bulk heterojunction device is proportional to the difference between the highest occupied molecular orbital (HOMO) energy level of the donor and the lowest unoccupied molecular (LUMO) energy level of the acceptor. The HOMO energy level of the p-material has to be at least 0.3 eV higher than that of the acceptor derivative in order to favor the electron transport and to overcome the binding energy of intrachain exciton. Conjugated poly(p-phenylenevinylene)s have attracted a great deal of attention due to their conducting and photoluminescent properties. Its derivatives remain the most popular conjugated polymers for this application and continue to generate considerable interest and much research for photovoltaic applications. Unfortunately, the unsubstituted PPV is insoluble and infusible and therefore difficult to process into the solid state. By incorporating arylamine derivatives (i.e., carbazole, indolocarbazole) as donor sequences in PPV structure the polymer band gap can be reduced and solubility improved by introduction of substituents at amine nitrogen.
In BHJ organic solar cells, conversion of incident solar energy to photocurrent (the photovoltaic mechanism) takes place by a cascade of four steps: (1) the absorption of sunlight by conjugated donor polymer and generation of excitons (electrically neutral bound electron-hole pairs), (2) diffusion of excitons to the heterojunction, (3) dissociation of the excitons into free charge carriers, and (4) the transport of these carriers to the contacts, as is illustrated in Figure1.
Figure 1: Energy diagram of an organic solar cell based on BHJ concept; The four subsequent processes to generate photocurrent are: (1) absorption of an incident photon to create an exciton, (2) diffusion of an exciton toward the donor-acceptor interface, (3) dissociation of an exciton into an electron in the acceptor and hole in the donor, and (4) collection of charges at the contacts.
Every step has to be optimized so that to have an overall high efficiency of the photovoltaic process. Many researchers have observed that small structural deviations on the backbone of conjugated polymers influence their photovoltaic properties. Therefore, the challenge here is to provide and study new conjugated polymer/acceptor combinations with well-defined structure and to improve every step in photovoltaic process. The optimization of organic solar cells is a fine balancing activity. It requires finding the optimal compromise among a combination of materials characteristics that can work in opposite directions.
Main Objectives
The general objective of this project is to bring an original contribution to understanding in more detail of the photovoltaic process in organic materials by synthesis of novel conjugated donor polymers with efficient sunlight absorption on a large spectral domain and good hole transporting materials by modification and manipulation of the chemical structures of the conjugated polymers and architecture. Thus, new or modified monomers and co-monomers, new polymer synthesis methods and structures will be used. Extending the absorption and decreasing the band gap of the polymer donor to match solar spectrum will be in our attention. The design and synthesis of low band gap materials to harvest the low-energy photon part of the solar spectrum can be performed by introduction of acceptor groups in the donor polymer backbone (band gap engineering using intramolecular donor-acceptor interactions. The acceptor groups shift the LUMO level down while the HOMO energy level is less affected. Certainly, designing and engineering materials for organic cells involves careful balance and a comprehensive understanding of all of the processes described in Summary part. A key challenge related to organic photovoltaics is to control electronic structure, film morphology and device properties by modifying the chemical structure using monomers with more planar and extended π aromatic moieties. Carbazole, as an aromatic heterocycle, has been largely used for synthesis of electroactive polymers for applications as materials for xerography, light emitting diodes, photorefractive materials, etc. For photovoltaic applications, most of the papers have used 3,6-disubstituted carbazole due to the fact the monomers and intermediates are easier to be synthesized and functionalized. We intend to synthesize new monomers based on carbazole as 2,7-disubstituted rings and to use in synthesis of polyarylenes and polyarylenevinylenes. Having a planar structure, a proper electron-donating properties and good hole-transporting property, 2.7-carbazole based oligomers and polymers are suitable for photovoltaic applications. Because, among donor polymers, the fused-ring based polymers have achieved so far the best performance we intend also to use [3,2-b] indolocarbazole coupled by para-linkages for a higher effective conjugation length. The penta-fused heterocyclic unit of indolocarbazole provides a large coplanar π-conjugated system with effect on absorption spectrum.
Synthesis and study of oligomers with branched, star and dendrimer structures as photovoltaic materials are advantageous because they can be purified by common techniques (sublimation, recrystallization, flash chromatography, etc) and can be deposited as thin films by vacuum evaporation. Monodisperse star-shaped and dendrimer compounds with high purity and solution processability are very interesting photovoltaic materials and less studied as compared with conjugated polymers. Unlike linear conjugated polymers, star oligomers and dendrimers form nanocomposites with 2D isotropic transport of the carriers. Starting from tri- (4,4’,4”-triiodo, or tri formyl triphenylamine) used as cores, by divergent method, dendrimers will be synthesized and studied.
The most critical challenge in BHJ solar cells is that tremendous work is required to fine-tune the physical interaction between donor-acceptor components in order to obtain an optimal morphology with a well-defined nanostructure. In the BHJ solar cells, the acceptor compound (the second component) is dispersed at nano-scale size in the polymer donor solution to form a nanocomposite material. Since the discovery of photoinduced charge transfer, research efforts have dominated by the use of fullerenes as the electron acceptor. Fullerenes (C60, C70) are efficient n-type acceptors possesses high light harvesting ability, fast charge separation, slow charge recombination and high carrier mobility and studied extensively for fabrication of BHJ type solar cells. Their poor solubility in most common organic solvents and low processability are overcome using derivatized acceptors. Graphenes, two-dimensional and atomically thin layer of graphite, and its lower-dimensional derivatives have attracted interest exponentially in the last few years since high-quality graphene preparation was reported by Novoselov and Geim in 2004, work for they have received in 2010 the Nobel prize for Physics. Graphenes have different electronic and optical characteristics compared with CNT and C60. It can be functionalized to become a soluble acceptor and a competitive alternative as the electron-accepting material in BHJ device applications.
As a conclusion, the topic of this proposal can be framed to the new photovoltaic materials composed of a molecular blend of bicontinuous and interpenetrating conjugated donor polymer and acceptors, based on bulk heterojunction (BHJ) concept. The realization of the project needs several efforts in some directions, such as design and synthesis of donor polymers and oligomers, the choice of the acceptor and the optimization of photovoltaic properties of the nanocomposites. The first objective of the application is synthesis of new donor conjugated polymer structures based on triphenylamine, carbazole and indolocarbazole groups, with more planar and extended π aromatic moieties, strong and broad bandwidth absorption. Synthesis of monodisperse star-shaped and dendrimer compounds based on tri- functional cores is another objective of this application. Polymers and oligomers are of polyarylene, polyarylene vinylene and polyarylene ethynylene type. As acceptor of the nanocomposites, derivatized fullerene and graphene compounds will be studied. The polymers and oligomers are synthesized by Suzuki, Heck, Wittig-Emmons reactions and their chemical and electronic (UV and fluorescence spectra, electrochemical behavior, HOMO and LUMO levels, band gap, etc) structure determined by FTIR, NMR, UV, fluorescence, voltammetry methods. Morphology of films is studied by AFM, SEM, TEM, XRD methods.
Synthesis and study of oligomers with branched, star and dendrimer structures as photovoltaic materials are advantageous because they can be purified by common techniques (sublimation, recrystallization, flash chromatography, etc) and can be deposited as thin films by vacuum evaporation. Monodisperse star-shaped and dendrimer compounds with high purity and solution processability are very interesting photovoltaic materials and less studied as compared with conjugated polymers. Unlike linear conjugated polymers, star oligomers and dendrimers form nanocomposites with 2D isotropic transport of the carriers. Starting from tri- (4,4’,4”-triiodo, or tri formyl triphenylamine) used as cores, by divergent method, dendrimers will be synthesized and studied.
The most critical challenge in BHJ solar cells is that tremendous work is required to fine-tune the physical interaction between donor-acceptor components in order to obtain an optimal morphology with a well-defined nanostructure. In the BHJ solar cells, the acceptor compound (the second component) is dispersed at nano-scale size in the polymer donor solution to form a nanocomposite material. Since the discovery of photoinduced charge transfer, research efforts have dominated by the use of fullerenes as the electron acceptor. Fullerenes (C60, C70) are efficient n-type acceptors possesses high light harvesting ability, fast charge separation, slow charge recombination and high carrier mobility and studied extensively for fabrication of BHJ type solar cells. Their poor solubility in most common organic solvents and low processability are overcome using derivatized acceptors. Graphenes, two-dimensional and atomically thin layer of graphite, and its lower-dimensional derivatives have attracted interest exponentially in the last few years since high-quality graphene preparation was reported by Novoselov and Geim in 2004, work for they have received in 2010 the Nobel prize for Physics. Graphenes have different electronic and optical characteristics compared with CNT and C60. It can be functionalized to become a soluble acceptor and a competitive alternative as the electron-accepting material in BHJ device applications.
As a conclusion, the topic of this proposal can be framed to the new photovoltaic materials composed of a molecular blend of bicontinuous and interpenetrating conjugated donor polymer and acceptors, based on bulk heterojunction (BHJ) concept. The realization of the project needs several efforts in some directions, such as design and synthesis of donor polymers and oligomers, the choice of the acceptor and the optimization of photovoltaic properties of the nanocomposites. The first objective of the application is synthesis of new donor conjugated polymer structures based on triphenylamine, carbazole and indolocarbazole groups, with more planar and extended π aromatic moieties, strong and broad bandwidth absorption. Synthesis of monodisperse star-shaped and dendrimer compounds based on tri- functional cores is another objective of this application. Polymers and oligomers are of polyarylene, polyarylene vinylene and polyarylene ethynylene type. As acceptor of the nanocomposites, derivatized fullerene and graphene compounds will be studied. The polymers and oligomers are synthesized by Suzuki, Heck, Wittig-Emmons reactions and their chemical and electronic (UV and fluorescence spectra, electrochemical behavior, HOMO and LUMO levels, band gap, etc) structure determined by FTIR, NMR, UV, fluorescence, voltammetry methods. Morphology of films is studied by AFM, SEM, TEM, XRD methods.
