WO2015164831A1 - Molecular compositions, materials, and methods for efficient multiple exciton generation - Google Patents

Abstract

Embodiments of the present invention provides compounds, compositions, and methods for their preparation that provide efficient intramolecular fission, such that local order and strong nearest neighbor coupling is no longer a design constraint. Inventive materials include organic oligomers and polymers designed to exhibit strong intrachain donor-acceptor interactions and provide intramolecular singlet fission, whereby triplet populations can be generated in very high yields of, e.g., 170% or more. The inventive disclosure is directed to polymers of the general formula: [SA-SD]n with a strong electron acceptor (SA), a strong electron donor (SD), and n a positive integer equal to or greater than two; methods for their preparation and monomers used therein, blends, mixtures and formulations containing them; the use of the polymers, blends, mixtures and formulations as semiconductors in organic electronic (OE) devices, especially in organic photovoltaic (OPV) devices, and to OE and OPV devices comprising these polymers, blends, mixtures or formulations.

Description

MOLECULAR COMPOSITIONS, MATERIALS, AND METHODS

FOR EFFICIENT MULTIPLE EXCITON GENERATION

CROSS-REFERENCE OF RELATED APPLICATIONS

[0001] This international PCT Application claims the benefit of priority from U.S.

Provisional Patent Application No. 61/983996, filed April 24, 2014, entitled, "MOLECULAR COMPOSITIONS, MATERIALS, AND METHODS FOR EFFICIENT MULTIPLE EXCITON GENERATION," which is incorporated here by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] The present invention was made with government support under contract numbers DE-AC02-98CH-10886, DE-SC0001085, and DE-SC0012704 awarded by the U.S. Department of Energy. The United States government may have certain rights in this invention.

FIELD OF THE INVENTION

[0003] The present inventions relate to compounds that are designed to produce efficient singlet fission and their use in compositions and singlet fission processes. More particularly, the present inventions relate to organic compounds that are designed to produce efficient singlet fission and their use in compositions and singlet fission processes that can produce triplet excitons in high yields, and their use in various materials and devices.

BACKGROUND OF THE INVENTION

[0004] Solar cells, also known as photovoltaic cells, are electrical devices that convert light energy directly into electricity by, what is known as, the photovoltaic effect. The photovoltaic effect is the creation of voltage or electric current in a material upon exposure to light. The photovoltaic effect is related to the photoelectric effect, although they are different processes.

[0005] Generally, when sunlight or any other light is incident upon a material, electrons present in the valence band can absorb energy and become excited from the absorption of energy. When the light energy exceeds bandgap, an electron can be promoted to what is referred to as the conduction band, which is the range of electron energies enough to free an electron from binding with its atom to move freely within the atomic lattice of the material as a delocalized electron, and become free. Then, as the highly excited, non-thermal electrons diffuse, some reach a junction where they are accelerated into a different material by a built-in potential (referred to as Galvani potential). The result is that an electromotive force can be generated. Thus, some of the light energy absorbed may be converted into useful electric energy. The photovoltaic effect can also occur when two photons are absorbed simultaneously in a process called two-photon photovoltaic effect.

[0006] Carrier multiplication refers to the phenomenon wherein absorption of a single photon leads to the excitation of multiple electrons from the valence band to the conduction band of a semiconducting material. In a conventional silicon solar cell, each photon is, in theory, only able to excite one electron across the band gap, and any photon energy in excess of the bandgap is dissipated as heat. In a material capable of carrier multiplication, high- energy photons excite on average more than one electron across the band gap, and so in principle the solar cell can produce more useful work.

[0007] However, silicon based solar cells are fundamentally limited in their production of useful energy. For example, if an incoming photon does not have sufficient energy, the cell will not absorb it. On the other hand, if a photon has too much energy, the excess energy is wasted as heat. In addition, it is believed that a silicon solar cell cannot generate more than one electron from a single photon absorbed. Thus, the conversion efficiency of photovoltaic cells by these combined effects, known as the Shockley-Queisser limit. The Shockley-Queisser limit is the fundamental upper limit to efficiency in single junction solar cells. This thermodynamic constraint limits the efficiency of single PN- junction solar cells to 33.7%, where a PN-junction is a boundary between two types of semiconductor materials. Scientists have spent decades looking for solutions to the problems posed.

[0008] Organic solar cell research has increased over the years and has seen the introduction of new materials, improved materials engineering, and more sophisticated device structures that provide increased power conversion efficiencies. Solar cells constructed of organic materials are becoming increasingly efficient due to the discovery of the bulk heterojunction concept. See, e.g., Benanti et ah, Organic solar cells: An overview focusing on active layer morphology, Photosynthesis Research, vol. 87, pp. 73-81 (2006); and Kippelen et at, Organic Photovoltaics, Energy Environ. Sci., vol. 2, no. 3, pp. 251-261 (2009).

[0009] The field of organic solar cells has benefited from the development of light- emitting diodes based on similar technologies, which have entered the market recently. For a review of the field of organic solar cells, discussion of their different production technologies, and discussion of parameters to improve their performance,see Hoppe et ah, Organic solar cells: An overview, Journal of Materials Research, Vol. 19, Issue 07, pp 1924- 1945 (2004).

[0010] Among the several challenges to improve the performance of organic photovoltaics (OPVs) is the Shockley-Queisser limit (-33.7%), as defined above. Thermodynamic modeling predicts that using materials capable of multiple exciton generation (MEG) in a single-junction solar cell could theoretically circumvent the Shockley-Queisser limit and increase the upper limit of power conversion efficiency from 33.7% to 44%. The assumption is that SF results in forming two triplet excitons, each of which produces an electron-hole pair.

[0011] Recently it was reported that the organic dye pentacene could be useful in providing greater solar efficiency. Congreve et a , External Quantum Efficiency Above 100% in a Singlet-Exciton-Fission-Based Organic Photovoltaic Cell, cience, vol. 340, no. 6130, pp. 334-337 (2013). Pentacene is a polycyclic aromatic hydrocarbon consisting of five linearly-fused benzene rings, which acts as an organic semiconductor. As reported by Congreve, a photovoltaic cell based on pentacene could generate two electrons from a single photon, i.e., more electrical current from the same amount of sun light. Various approaches have been taken in efforts to design compounds that will produce more efficient singlet fission. See J. C. Johnson et ah, Toward Designed Singlet Fission: Solution Photophysics of Two Indirectly Coupled Covalent Dimers of l,3-Diphenylisobenzofuran,J. Phys. Chem. B, 1 17, 4680 (2013).

[0012] Singlet-exciton fission describes the process in which an arriving photon generates two "excitons" (excited states) that can be made to yield two electrons. Singlet exciton fission is a spin-allowed process for generating two triplet excitons from a single absorbed photon. Fission of singlet excitons into two triplet exciton pairs is spin conserving and, therefore, spin allowed. Theoretically, the efficiency of a conventional solar cell could be improved if a molecular material capable of singlet fission could be incorporated.

[0013] The production of two triplet excitons from the absorption of a single photon.

To implement this, the two triplets from the singlet fission material need to be successfully harvested. Singlet fission (SF) could dramatically increase the efficiency of organic solar cells by producing two triplet excitons from each absorbed photon. While this process is known, most descriptions have assumed the necessity of a charge-transfer intermediate. See Zimmerman et ah, Mechanism for Singlet Fission in Pentacene and Tetracene: From Single Exciton to Two Triplets, J. Am. Chem. Soc, 133 (49), pp. 19944-19952 (201 1). For an in depth discussion of singlet fission, see Smith et ah, Singlet Fission, Chem. Rev., 1 10, pp. 6891-6936 (2010).

[0014] Although several existing materials exhibit singlet fission, these materials are generally based on aggregates of conjugated and/ or aromatic molecules, including, for example, acenes, polyenes, and caratenoids. Singlet fission has also been previously demonstrated in polymers including poly-thiopehenevinylene and poly-pheneylenevinylene. Thiophene dioxide (TDO)-containing systems have been studied for other applications. However, these studies were predominately focused on the basic science of molecular TDO- containing entities, particularly on light emission. These previously studied molecular singlet fission systems may offer good singlet fission efficiency; however, they are not very adjustable, efficiencies are low, and triplet lifetimes are very short. The combined effects of which make applications of existing molecular singlet fission systems limited and applications of existing polymeric singlet fission systems impractical.

[0015] Conjugated polymers that have been suggested in the literature for use in organic photovoltaic devices ("OPV devices") do still suffer from certain drawbacks. For example, many polymers suffer from limited solubility in commonly used organic solvents, which can inhibit their suitability for device manufacturing methods based on solution processing, or show only limited power conversion efficiency in OPV bulk-hetero-junction devices, or have only limited charge carrier mobility, or are difficult to synthesize and require synthesis methods which are unsuitable for mass production.

[0016] While SF has been extensively studied in molecular crystals and aggregates, made possible by an intermolecular process, intramolecular SF is not well understood in molecular and polymeric materials. Fundamentally, a core challenge is the modular synthetic design of building blocks for molecules and polymers that can undergo SF. Coupling chemical structure design with the mechanistic understanding of the physical processes of multiple exciton generation ("MEG") could open avenues of exploration in parallel using families of materials, rather than the current serial approach targeting single compounds, which are generally based on acenes, oligoenes, and select polymeric materials. [0017] Intramolecular SF is a process that has been rarely invoked in soft materials - it has only been observed in oligoenes (carotenoids) and polyenes (polydiacetylene), as well as a thiophene-containing conjugated polymer. It is postulated that these materials are capable of producing multi-exciton states through charge derealization across these large molecules. However, such observation does not provide the necessary guidelines to build new materials. There is a need for designing and synthesizing novel materials for intramolecular singlet exciton fission in small molecules and polymers that are efficient and configurable singlet fission materials, which important for developing low cost, efficient organic (or hybrid) photovoltaic technologies. Also, there is a need for developing solution processable small molecule and polymeric singlet fission materials which allow for effortless device assembly through a variety of low-cost processing techniques, where these materials may additionally have applications in fuel cells.

[0018] Recently, it was found that the coupling between the singlet state and the ME states is weak, but the MEG process is mediated by a strongly coupled intermediate CT state.

[0019] Therefore, there is still a need for singlet- fission capable organic semiconducting ("OSC") materials that are easy to synthesize, especially by methods suitable for mass production, show good structural organization and film-forming properties, exhibit good electronic properties, especially a high charge carrier mobility, good processability, especially a high solubility in organic solvents, and high stability in air. Especially for use in OPV cells, there is a need for OSC materials having a low bandgap, which enable improved light harvesting by the photoactive layer and can lead to higher cell efficiencies, compared to the polymers discussed in the literature.

SUMMARY OF THE INVENTION

[0020] The present invention provides compounds and materials that are capable of multiple exciton generation. The present invention provides the design of new materials that embody two key design elements grounded on the mechanistic understanding of SF: a) reduce the singlet-triplet gap, such that the triplet energy is approximately half of the singlet energy; and b) a lowest lying optical excitation with significant charge-transfer (CT) character that can act to mediate the SF process.

[0021] Accordingly, the present invention provides compounds and materials, including organic molecules, such as oligomers and polymers, capable of singlet fission, specifically efficient intramolecular singlet fission, such that local order and strong nearest neighbor coupling is no longer a design constraint. Compounds and materials of the invention exhibit strong intrachain donor-acceptor interactions that generate triplet populations in very high yields, e.g., yields of 160%, 175%, or more. Thus, the compounds and materials of the invention are prepared by conjugating strong-acceptor and strong-donor building blocks to access a charge-transfer state that is strongly coupled to the multiple- exciton state. The technology of the present invention is applicable to similar families of polymers and small molecules, and provides development of new materials with tunable electronic structure.

[0022] Polymers of the invention include those of the general formulas: [SD-SAJz,

[WD-SA-D-SA-WD]«, , [SA-D-SA-WD]«, [SD-SP-SA]«, [SD-SP-SA-SP]«, and the like as described more fully here, wherein D represents an electron donor; SA represents a strong electron acceptor; SD represents a strong electron donor; WD represents a weak electron donor; SP represents a spacer; and n represents a positive integer, methods for their preparation and monomers used therein, blends, mixtures and formulations containing them, the use of the polymers, blends, mixtures and formulations as semiconductor in organic electronic (OE) devices, especially in organic photovoltaic (OPV) devices, and to OE and OPV devices comprising these polymers, blends, mixtures or formulations.

[0023] Other applications of the polymers of the invention include use in devices such as but not limited to hybrid photovoltaic devices, nanoparticle/Quantum dot devices, fission sensitizer in inorganic applications (e.g., silicon, CIGS, etc.).

[0024] The invention further relates to a formulation comprising a mixture or polymer blend as described above and below and one or more solvents, preferably selected from organic solvents.

[0025] The invention further relates to the use of a conjugated polymer, formulation, mixture or polymer blend as described above and below as charge transport, semiconducting, electrically conducting, photoconducting or light emitting material, or in an optical, electrooptical, electronic, electroluminescent or photoluminescent device, or in a component of such a device or in an assembly comprising such a device or component.

[0026] The invention further relates to a charge transport, semiconducting, electrically conducting, photoconducting or light emitting material comprising a conjugated polymer, formulation, mixture or polymer blend as described above and below.

[0027] The invention further relates to an optical, electrooptical, electronic, electroluminescent or photoluminescent device, or a component thereof, or an assembly comprising it, which comprises a conjugated polymer, formulation, mixture or polymer blend, or comprises a charge transport, semiconducting, electrically conducting, photoconducting or light emitting material, as described above and below.

[0028] The optical, electrooptical, electronic, electroluminescent and photoluminescent devices include, without limitation, organic field effect transistors (OFET), organic thin film transistors (OTFT), organic light emitting diodes (OLED), organic light emitting transistors (OLET), organic photovoltaic devices (OPV), organic solar cells, laser diodes, Schottky diodes, photoconductors, photodetectors, printable circuits, capacitors, and sensors.

[0029] Another embodiment includes compositions for use in preparing solar cell compositions and/or materials, and the photovoltaic solar cells prepared therefrom.

[0030] The components of the above devices include, without limitation, charge injection layers, charge transport layers, interlayers, planarizing layers, antistatic films, polymer electrolyte membranes (PEM), conducting substrates and conducting patterns.

[0031] The assemblies comprising such devices or components include, without limitation, integrated circuits (IC), radio frequency identification (RFID) tags or security markings or security devices containing them, flat panel displays or backlights thereof, electrophotographic devices, electrophotographic recording devices, organic memory devices, sensor devices, biosensors and biochips.

[0032] In addition the compounds, polymers, formulations, mixtures or polymer blends of the present invention can be used as electrode materials in batteries and in components or devices for detecting and discriminating DNA sequences.

BRIEF DESCRIPTION OF THE DRAWING FIGURES [0033] FIG. 1 is an illustration of the steps toward MEG.

[0034] FIG. 2 is an illustration of a molecular design for intramolecular SF using strong acceptor (SA), strong donor (SD), and donor (D) units.

[0035] FIG. 3 is an illustration of the connectivity in small molecules, using strong acceptor (SA) and strong donor (SD) units. Polymer design for intramolecular CT mediated singlet-fission.

[0036] The top graph of FIG. 4 shows the BDT-TDOl compound represented by the dark (black) lines, and the p-BDT-TDOl compound represented by the light (dashed grey) lines. The bottom graph of FIG. 4 shows the BDT-TD02 compound represented by the dark (black) lines and the p-BDT-TD02 compound represented by the light (dashed grey) lines. [0037] FIG. 5 is an illustration of the effect of thiophene oxidation on excited state lifetime.

[0038] FIG. 6A and 6B are illustrations of the effects of thiophene oxidation on the optical properties of the materials of Comparative Example 9 showing the linear absorption spectra.

[0039] FIGS. 7A, 7B, 7C, and 7D illustrate the TA and PRTT for all TDO-containing materials of Comparative Example 9.

[0040] As illustrated in FIG. 7A, the BDT-TDOl compound is represented by the dark (black) line, while the p-BDT-TDQ compound is represented by the dashed line. As illustrated in FIG. 7B, the BDT-TDO₂ compound is represented by the dark line, while the p- BDT-TDO₂ compound is represented by the dashed line (grey line). In both FIGS. 7A and 7B, both the BDT-TDOi and p-BDT-TDOi compounds (7A) and the BDT-TDQ> and p- BDT-TDO₂ (7B) start at approximately the same normalized AOD point at the top of each graph, and drop down equivalently, for each polymer in each graph, respectively, at about the zero (0) seconds mark.

[0041] As illustrated in FIG. 7C, the BDT-TDQ compound is represented by the dark

(black) line, the p-BDT-TDOi compound is represented by the dashed (grey) line, and the BDT-TDOi triplet is represented by circles. As can be seen, these BDT-TDQ and p-BDT- TDOi compounds initially descend on the Scaled AOD at the about 630-650 wavelength (nm); all lines approach the same Scaled AOD level and the p-BDT-TDOi triplet appears. As illustrated in FIG. 7D, the BDT-TDO₂ (dark solid line) and p-BDT-TDQz (dashed grey line) compounds, and the p-BDT-TDQz triplet (solid circles), appear at a wavelength of about 450 nm on the Scaled AOD reach their peak at about the 650-700 nm wavelength, 725 to 775 nm wavelength.

[0042] FIG. 8 illustrates a scheme showing the singlet deactivation process.

DETAILED DESCRIPTION OF THE INVENTION

[0043] Singlet fission had not been previously observed in thiophene dioxide (TDO)- containing systems, most likely due to the constraints imposed by conventional thiophene oxidation chemistry. The TDO-containing systems described here represent the first modular and highly-tunable singlet fission system with both molecular and polymeric applications. The unique intramolecular nature of fission TDO composites may also offer the possibility of other applications. Through chemical design, chromophores capable of intramolecular fission can be produced. TDO-containing systems provide a novel approach for material design to improve the utility of the singlet fission process in, for example, photovoltaic, applications. Incorporating TDO into a copolymer configuration allows for a highly tunable singlet fission system. Adjustment of the number of sequential TDO subunits allows for the triplet energy and conduction band (LUMO) energy to be tuned. The monomer or groups of monomers selected for the other component in the copolymer allows for tuning of the bandgap and the valence band (HOMO) energy. Either subunit may be functionalized to tune the polarity and solubility, which in turn controls the polymer morphology solid state modifications, allowing for extensive tuning of electronic, optical, and structural properties.

[0044] Throughout this disclosure, the term "polymer" generally means a molecule of high relative molecular mass, the structure of which essentially comprises the multiple repetition of units derived, actually or conceptually, from molecules of low relative molecular mass (PAC, 1996, 68, 2291). The term "oligomer" generally means a molecule of intermediate relative molecular mass, the structure of which essentially comprises a small plurality of units derived, actually or conceptually, from molecules of lower relative molecular mass (PAC, 1996, 68, 2291). In a preferred sense according to the present invention, a polymer means a compound having greater than 1 (>l),i.e., at least 2 repeating units, preferably greater than 5 (>5) repeating units, and an oligomer means a compound having units of between greater than 1 and less than 10 (>1 and <10), preferably less than 5 (<5), repeating units. The terms "repeating unit" and "monomeric unit" mean the constitutional repeating unit (CRU), which is the smallest constitutional unit the repetition of which constitutes a regular macromolecule, a regular oligomer molecule, a regular block or a regular chain (PAC, 1996, 68, 2291).

[0045] In one embodiment, the present invention provides a compound, e.g., an oligomer or polymer, of the general formula: [D-A}j, wherein D represents an electron donor; A represents an electron acceptor; andw represents a positive integer.

[0046] In a further embodiment, the present invention provides a compound, e.g., an oligomer or polymer, of the general formula: [SD-SA}j, wherein SD represents a strong electron donor; SA represents a strong electron acceptor; andw represents a positive integer.

[0047] In another embodiment, the present invention provides a compound, e.g., an oligomer or polymer, of the general formula: [A-D}j, wherein D represents an electron donor; A represents an electron acceptor; andw represents a positive integer. [0048] In another embodiment, the present invention provides a compound, e.g., an oligomer or polymer, of the general formula: [A-D-A j, wherein D represents an electron donor; A represents an electron acceptor; andw represents a positive integer.

[0049] In another embodiment, the present invention provides a compound, e.g., an oligomer or polymer, of the general formula: [D-SA}j, wherein D represents an electron donor; SA represents a strong electron acceptor; andw represents a positive integer.

[0050] In another embodiment, the present invention provides a compound, e.g., an oligomer or polymer, of the general formula: [SA-D}j, wherein D represents an electron donor; SA represents a strong electron acceptor; andw represents a positive integer.

[0051] In another embodiment, the present invention provides a compound, e.g., an oligomer or polymer, of the general formula: [SA-D-SA}?, wherein D represents an electron donor; SA represents a strong electron acceptor; andw represents a positive integer.

[0052] In another embodiment, the present invention provides a compound, e.g., an oligomer or polymer, of the general formula: [D-SA-D-SA-DJj, wherein D represents an electron donor; SA represents a strong electron acceptor; andw represents a positive integer.

[0053] In another embodiment, the present invention provides a compound, e.g., an oligomer or polymer, of the general formula: [WD-SA-D-SA-WD]?, wherein D represents an electron donor; SA represents a strong electron acceptor; WD represents a weak electron donor; and n represents a positive integer.

[0054] In one preferred embodiment, the present invention provides a compound, e.g., an oligomer or polymer, of the general formula: [D-SA-SD-SA-D]?, wherein D represents an electron donor; SA represents a strong electron acceptor; SD represents a strong electron donor; and n represents a positive integer.

[0055] In another preferred embodiment, the present invention provides a compound, e.g., an oligomer or polymer, of the general formula: [SA-SD-SA-SD]j, wherein SD represents a strong electron donor; SA represents a strong electron acceptor; and« represents a positive integer.

[0056] In one preferred embodiment, the present invention provides a compound, e.g., an oligomer or polymer, of the general formula: [SD-SP-SA-SP]?, wherein SA represents a strong electron acceptor; SD represents a strong electron donor; SP represents a spacer; and n represents a positive integer.

[0057] In a further preferred embodiment, the present invention provides a compound, e.g., an oligomer or polymer, of the general formula: [SD-SP-SA}j, wherein SA represents a strong electron acceptor; SD represents a strong electron donor; SP represents a spacer; and n represents a positive integer.

[0058] In another preferred embodiment, the BDT-TDO compounds or polymers with the general formulas described here have efficient singlet fission comprising a singlet fission efficiency of greater than about 25%, preferably about 100% or greater, about 100% to about 200%, and more preferably about 200% or greater. The efficiency necessary for device applications is about 100%. Any efficiency less than about 25% may be too inefficient for most applications.

[0059] With respect to the embodiments described throughout this disclosure, it is preferred that D is a strong electron donor (z.e., an SD); SA is a strong electron acceptor; WD is a weak electron donor; and n is an integer of from 1 to 100.

[0060] "Electron donor" means a chemical entity that donates electrons to another compound or another group of atoms of a compound. "Electron acceptor" means a chemical entity that accepts electrons transferred to it from another compound or another group of atoms of a compound (see, also, U.S. Environmental Protection Agency, 2009, Glossary of technical terms, hypertext transfer protocol://www.epa.gov/oust/cat/TUMGLOSS.HTM). "Spacer" means a chemical entity that serves as neither an electron donor nor an electron acceptor. Spacers include at least one monomer (or group of monomers) containing at least one pi bond, where non-limiting examples of spacers include acetylene, arylene, vinylene, phenylene, thiophene, furan, and pyrole. The spacers are preferably positioned between the electron donor and electron acceptor subunits identified in the general formulas described here. The electron donors SD, D, and WD, may each be a compound or atom; the electron acceptors A and SA may each be a compound or atom; and the spacers SP may each be a compound or atom. The formulas of the embodiments of the invention may represent one or more compounds or polymers, and the one or more compounds or polymers may be combined in polymeric form, or they may exist separately in a composition.

[0061] Non-limiting examples of electron donor monomer compounds include benzodithiophene ("BDT") and its derivatives, which are preferred strong electron donor compounds, and which have the following general structural formula I shown below:

R2 I

[0062] More preferably, the BDT monomer compound has the following general structural formula II s

II

[0063] In each of the BDT monomer compound general structures illustrated, each of

Ri and R2 may be the same or different and may be selected from: hydrogen, straight or branched chain alkyl of Q-2₀, alkenyl, alkynyl, oligoethylene glycols, aromatic rings (e.g., thiophene, benzene, furan, other heteroatom groups), and other functional alkanes; and each of the R3 and R4 groups may be the same or different and may be selected from: hydrogen, straight or branched chain alkyl of Q-2₀, alkenyl, alkynyl, alkoxy, organotin compounds, 2- ethylhex-l-yl, 2-Ethylundec-l-yl, and 3-Buten-l-yl.

[0064] Preferred alkyl substituents for R and R2 are Ci_io, i.e., methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, and decyl. Preferred R and R4 substituents are boronic acids, boronic esters, organotin compounds Sn(R)₃, wherein R₅ is selected from C 1₀, i.e., methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, and decyl.

[0065] A preferred monomer is the BDT derivative having the general structural formula III shown below:

wherein "R" is the same as defined for R and R2 in accordance with general structure II above and SnR₃ is an organo-tin compound wherein R₃ as shown represents three (3) "R" substituents selected from Q.io, i-e., methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, an oligoethylene glycol, such as hexaethylene glycol, pentaethylene glycol, tetraethylene glycol, water or polar soluble groups such as hydroxides, sulphates, and carbonates of the Group 2 elements - beryllium, magnesium, calcium, strontium and barium, alcohols, particularly lower alcohols of Q-C6, such as methanol, ethanol, propanol, butanol, pentanol, carboxylic acids of Q-C6, ketones of -C6, amines of -C6, amides of -C6, carboxylate ions of -C6, ammonium ions of Q-C6 Preferably each of the foregoing polar groups (e.g., the alcohols, carboxylic acids, ketones, amines, amides, carboxylate ions, and ammonium ion groups) will have a length of around four to five Cti units (including the C with the polar group). Alternatively, instead of the SnR substituents in the general formula III, a boronic acid substituent or a boronic ester substituent (group) may be employed.

[0066] Another preferred electron donor has the general structural formula IV shown below:

R R IV wherein "R" is the same as defined for R and R2 in accordance with general structure I above and SnR₃ is an organotin compound wherein R₃ as shown represents three (3) "R" substituents selected from Q.io, i-e., methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, an oligoethylene glycol, such as hexaethylene glycol, pentaethylene glycol, tetraethylene glycol, water or polar soluble groups such as hydroxides, sulphates, and carbonates of the Group 2 elements - beryllium, magnesium, calcium, strontium and barium, alcohols, particularly lower alcohols of Q-C6, such as methanol, ethanol, propanol, butanol, pentanol, carboxylic acids of Q-C6, ketones of -C6, amines of -C6, amides of -C6, carboxylate ions of -C6, ammonium ions of Q-C6 Preferably each of the foregoing polar groups (e.g., the alcohols, carboxylic acids, ketones, amines, amides, carboxylate ions, and ammonium ion groups) will have a length of around four to five Cti units (including the C with the polar group). Alternatively, instead of the SnR substituents in the general formula III, a boronic acid substituent or a boronic ester substituent (group) may be employed.

[0067] A preferred weak electron donor (WD) monomer compound is thiophene

("T") which has the general structure V shown below:

wherein R_1; R_2j R_3; and R₄ may be the same or different and may be selected from hydrogen, straight chain or branched alkyl of Q_-2o, alkenyl, alkynyl, alkoxy, halogen (F, CI, Br, I), sulfur, organotin compounds Sn(R₅)₃, wherein R₅ is selected from Q.io, i-e., methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, an oligoethylene glycol, such as hexaethylene glycol, pentaethylene glycol, tetraethylene glycol, water or polar soluble groups such as hydroxides, sulphates, and carbonates of the Group 2 elements - beryllium, magnesium, calcium, strontium and barium, alcohols, particularly lower alcohols of Q-C6, such as methanol, ethanol, propanol, butanol, pentanol, carboxylic acids of Q-Ce, ketones of Ci-Ce, amines of -C6, amides of -C6, carboxylate ions of Q-C6, ammonium ions of Q- Ce Preferably each of the foregoing polar groups ⁽g.g., the alcohols, carboxylic acids, ketones, amines, amides, carboxylate ions, and ammonium ion groups) will have a length of around four to five CH₂ units (including the C with the polar group). Alternatively, one or more boronic acid substituents or boronic ester substituents (groups) may be employed.

[0068] Additional donor compounds include each of the fourteen (14) general structural formulae below, wherein the "R" substituents in the formulae below are the same as defined in accordance with R and R2 in general structural formula II above, and X and Y, with respect to each formulae below, may be the same or different and may be the same as defined for R3 and R₄ in general structural formula II above, or may be selected from hydrogen, straight chain or branched alkyl of G-20, alkenyl, alkynyl, alkoxy, halogen (F, CI, Br, I), sulfur, organotin compounds Sn(R₅)₃, wherein R₅ is selected from C O, i.e., methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, an oligoethylene glycol, such as hexaethylene glycol, pentaethylene glycol, tetraethylene glycol, water or polar soluble groups such as hydroxides, sulphates, and carbonates of the Group 2 elements - beryllium, magnesium, calcium, strontium and barium, alcohols, particularly lower alcohols of Q-C6, such as methanol, ethanol, propanol, butanol, pentanol, carboxylic acids of Q-Ce, ketones of Ci-Ce, amines of -C6, amides of -C6, carboxylate ions of Q-C6, ammonium ions of Q- Ce Preferably each of the foregoing polar groups (g.g., the alcohols, carboxylic acids, ketones, amines, amides, carboxylate ions, and ammonium ion groups) will have a length of around four to five CH₂ units (including the C with the polar group), or X and Y may be a boronic acid substituent or a boronic ester substituent (group) may be employed:

[0069] Non-limiting examples of preferred electron acceptor monomer compounds include thiophene derivatives. A strong electron acceptor is (SA) monomer compound is thiophene oxide ("TO"), which has the general structure VI shown below:

o

£ rw VI wherein R_1; R₂, R3, and R4 may be the same or different and may be selected from hydrogen, straight chain or branched alkyl of Q-20, alkenyl, alkynyl, alkoxy, halogen (F, CI, Br, I), sulfur, organotin compounds Sn(R₅)₃, wherein R₅ is selected from Q.io, i-e., methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, an oligoethylene glycols, such as hexaethylene glycol, pentaethylene glycol, tetraethylene glycol, water or polar soluble groups such as hydroxides, sulphates, and carbonates of the Group 2 elements - beryllium, magnesium, calcium, strontium and barium, alcohols, particularly lower alcohols of Q-C6, such as methanol, ethanol, propanol, butanol, pentanol, carboxylic acids of Q-Ce, ketones of Ci-Ce, amines of -C6, amides of -C6, carboxylate ions of Q-C6, ammonium ions of Q- Ce Preferably each of the foregoing polar groups ⁽g.g., the alcohols, carboxylic acids, ketones, amines, amides, carboxylate ions, and ammonium ion groups) will have a length of around four to five CH₂ units (including the C with the polar group). Alternatively, one or more boronic acid substituents or a boronic ester substituents (group) may be employed.

[0070] A preferred strong electron acceptor (SA) monomer compound is thiophene

S,S-dioxide (also known as thiophene 1, 1, dioxide) ("TDO") which is a doubly oxidized counterpart of thiophene. TDO exhibits attractive electronic properties. TDOs have been demonstrated to stabilize the lowest unoccupied molecular orbital (LUMO) by their ability to increase the electron affinity, which is useful in order to localize the multiple excitons within these moieties. The TDO-containing oligomers and conjugated polymers exhibit narrowed highest occupied molecular orbital (HOMO)-LUMO bandgaps, in contrast to their unoxidized counterparts. TDO-containing materials may serve as a new type of electron acceptor. TDO has the general structure VII shown below: O O

R2 R3 VII wherein R_1; R2, R3, and R4 may be the same or different and may be selected from hydrogen, straight chain or branched alkyl of Q-20, alkenyl, alkynyl, alkoxy, halogen (F, CI, Br, I), sulfur, organotin compounds Sn(R₅)₃, wherein R₅ is selected from Ο_1-10, i.e., methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, an oligoethylene glycol, such as hexaethylene glycol, pentaethylene glycol, tetraethylene glycol, water or polar soluble groups such as hydroxides, sulphates, and carbonates of the Group 2 elements - beryllium, magnesium, calcium, strontium and barium, alcohols, particularly lower alcohols of Q-C6, such as methanol, ethanol, propanol, butanol, pentanol, carboxylic acids of Q-Ce, ketones of Ci-Ce, amines of -C6, amides of -C6, carboxylate ions of Q-C6, ammonium ions of Q- Ce Preferably each of the foregoing polar groups ⁽g.g., the alcohols, carboxylic acids, ketones, amines, amides, carboxylate ions, and ammonium ion groups) will have a length of around four to five CH₂ units (including the C with the polar group). Alternatively, a boronic acid substituent or a boronic ester substituent (group) may be employed.

[0071] The TDO compound may be employed as a single compound of TDO (?.g., mono-thiophene S,S-dioxide or "TDOl"), or it may be employed in repeat coupled units of more than one TDO compound (e.g., two or more TDO compounds, such as, bi-thiophene S,S-dioxide or "TD02", tri-thiophene S,S-dioxide or "TD03", and tetra-thiophene S,S- dioxide or "TD04", etc., also referred to as "poly TDO" or "pTDO", or TDQ, e.g., a TDO structure wherein n is an integer greater than 1, such as 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, etc.)

[0072] Another preferred electron acceptor is the TDO derivative which has the general structural formula VIII shown below:

wherein R is the same as defined in accordance with general structural formula VII above.

[0073] Another preferred electron acceptor is a thiophene derivative that has the general structural formula IX shown below:

wherein R is the same as defined in accordance with general structural formula VI above.

[0074] Additional, electron acceptors include compounds of each of the eighteen (18) general structural formulae below, wherein the R substituents are the same as defined in accordance with general structural formula VI above, and X and Y, with respect to each formulae below, may be the same or different and may be selected from hydrogen, straight chain or branched alkyl of C_1-2o, alkenyl, alkynyl, alkoxy, halogen (F, CI, Br, I), sulfur, organotin compounds Sn(R₅)₃, wherein R₅ is selected from C O, i.e., methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, an oligoethylene glycol, such as hexaethylene glycol, pentaethylene glycol, tetraethylene glycol, water or polar soluble groups such as hydroxides, sulphates, and carbonates of the Group 2 elements - beryllium, magnesium, calcium, strontium and barium, alcohols, particularly lower alcohols of Q-Ce, such as methanol, ethanol, propanol, butanol, pentanol, carboxylic acids of Q-Ce, ketones of -C6, amines of -C6, amides of -C6, carboxylate ions of Q-C6, ammonium ions of -C6 Preferably each of the foregoing polar groups ⁽g.g., the alcohols, carboxylic acids, ketones, amines, amides, carboxylate ions, and ammonium ion groups) will have a length of around four to five CH₂ units (including the C with the polar group), or X and Y may be a boronic acid substituent or a boronic ester substituent (group) may be employed:

-22-

-23 -

[0075] Rozen's reagent (HOF-CI¾CN) has revolutionized the oxidation chemistry of thiophenes. Rozen's reagent allows for oxidation of oligomers up to quaterthiophene.

Benzodithiophene (BDT) rings are prevalent electron donors as semiconductor building blocks for solar cells. By combining the BDT and TDO building blocks or subunits, a variety of oligomers and conjugated polymers may be synthesized as described here. Intramolecular singlet fission was observed in these synthesized molecules. Photophysical studies indicated that the TTD02- BDT-TD02-T oligomer is capable of intramolecular SF with low efficiency, while the singlet fission efficiency is dramatically improved by the extension of the TDO and BDT repeat units in poly-(BDT-TDO). Singlet fission will occur with a repeating unit of any number greater than or equal to 2. The number of repeating units for functionality may be greater than or equal to 2, preferably greater than or equal to 10.

Materials presented here may have, for example, 10-25 repeating units. Polymers may have a preferred donor to acceptor (D:A) ratio of 1 : 1 or 1 :2. However, any whole number ratio may be used, for example, 3: 1, 2: 1, 1 : 1, 1 :2, and 1 :3. Preferred BDT-TDO polymers are those with a D:A ratio of 1 : 1 as they were found to be the most efficient.

[0076] Essentially, useful polymers, compounds, and materials may be electron rich subunits coupled to a strongly electron deficient subunits. Polymers may be synthesized or obtained by the Stille condensation polymerization between the bis-stannylated BDT and dibromo-TDO. The synthesis of conjugated polymers by the Stille coupling reaction is generally known by a person skilled in the art, see, e.g., J.K. Stille, Pure Appl. Chem., 57(12): 1771- 1780 (1985); Bao, et al, Chem. Mater., 5 (1): 2-3 (Jan. 1993). Small molecules can also be synthesized from their respective building blocks. The modularity of the polymerization allows for the exploration of various electron rich moieties to couple with oligoTDOs. Controlling the donor strength and solubilizing alkyl groups (R) modifies the electronic effects and packing interactions on singlet fission.

[0077] Spacers include any monomer (or group of monomers) containing one or more pi bonds. The spacers are preferably positioned between the electron donating subunits and the electron accepting subunits. For example, a thiophene spacer may separate an electron donor and electron acceptor as shown in the following general structural formula of

PBDTTDOT:

wherein "m" is a positive integer, "n" is a positive integer, "p" is a positive integer, "q" is a positive integer, preferably greater than 2, and "R" is as defined for R and R2 in accordance with general structure II above, and the BDT-TDO polymer exhibits efficient singlet fission. R may be any chemical functional group, including but not limited to straight or branched chain C_1-20 alkyl, alkenyl, alkynyl, oligoethylene glycols, and aromatic rings. The R group affects the solubility of the polymer and modifying the R group may thereby assist in processing. This structure may be referred to, interchangeably or in some variation, as poly- BDT-T_m-TDO„-Tp, p-BDT-T_m-TDO„-Tp, PBDTT_mTDO„Tp, or poly-BDTT_mTDO„T_p.

[0078] In a preferred embodiment, a polymer of BDT and TDO in accordance with the invention has the following general structural formula X shown below:

PBDTTDOi x

wherein "n" is a positive integer, "q" is a positive integer, preferably any positive integer greater than 2, and "R" is as defined for R and R2 in accordance with general structure II above. R may be any chemical functional group, including but not limited to straight or branched chain C_1-20 alkyl, alkenyl, alkynyl, oligoethylene glycols, and aromatic rings, and the BDT-TDO polymer exhibits efficient singlet fission. The R group affects the solubility of the polymer and modifying the R group may thereby assist in processing. This structure may be referred to, interchangeably or in some variation, as poly-BDT-TDQ, p-BDT-TDO„, or PBDTTDO„.

[0079] In another preferred embodiment, a polymer of the invention has the following general structural formula XI shown below:

OR

OR SOTTDOn xi

wherein the benzodithiophene compounds (BDT; the center compound in accordance with general structure II above) and the thiophene compounds (end compounds in accordance with the general structural formula V) are the electron donors, and the electron acceptor compounds are the thiophene dioxide compounds (TDO in accordance with general structural formula VII) which may be poly-TDOs, "n" is a positive integer, "x" is as defined above for Ri and R4 in accordance with general structure V, "R" is as defined for R and R2 in accordance with general structure II above, and "q" is a positive integer.

[0080] Another preferred polymer has the following general structural formula XII shown below:

wherein the benzodithiophene compound (BDT; the left hand most compound in accordance with general structure II above) is the electron donor and, with respect to the BDT portion, "R" is as defined for Ri and R2 in accordance with general structural formula II above; and the thiophene derivative (similar to general structural formula IX above) is the electron acceptor and, with respect to the thiophene derivative portion, R is as defined in general structural formula IX above, and "n" is a positive integer.

[0081] Another preferred polymer has the following general structural formula XIII shown below:

R R xiii

wherein in the Si containing ring structure R is defined as in general structural formula IV above, and in the TDO portion R is the same as defined in the general structural formula VIII above.

[0082] Examples of electron donor atoms that may be employed in other embodiments in accordance with the present invention include the metals, which represent the left-hand side of the Periodic Table of Elements, i.e., everything from the left-hand side of the Periodic Table (Group I et seq) to the metalloids is a metal and represents electron donors. Everything to the right of the metalloids is a nonmetal and represents electron acceptors.

[0083] Although the materials, compounds, and polymers described here may be used in a variety of applications, one of the primary commercial applications for these materials is their use in photovoltaic systems. These TDO-containing materials present a new material class useful as a multi-functional layer in photovoltaic devices. Within this context, the TDO compounds could act as a charge acceptor or charge transport layer that also serves to absorb visible light, undergo fission, and inject down-converted excitons into a lower bandgap active layer. Due to the unique intramolecular nature of fission in TDO-containing systems, singlet fission also occurs in the solution phase. Therefore these materials may also be applied in dye-sensitized photovoltaic or photocatalytic devices. TDO-enabled singlet fission improves photovoltaic device efficiency. Another embodiment of the invention is directed to methods or the use of the polymer, compound, or materials described here as a multi-functional layer in a photovoltaic device. A further embodiment may be directed to a device comprising the polymer, compound, or materials described here which forms a multi-functional layer and the device has a singlet fission efficiency of greater than about 25%, preferably greater than or equal to about 100%, about 100% to about 200%, and more preferably greater than or equal to about 200%.

[0084] Incorporation of singlet fission materials into, for example, a solar cell allows for the Shockley-Queisser limitation to be circumvented. The resultant device may have two effective bandgaps without additional current matching constraints or the cost associated with building a secondary active layer. Even for devices far from the Shockley-Queisser efficiency limit, incorporation of a singlet fission layer can increase efficiency.

[0085] The polymers according to the present invention can also be used in mixtures or polymer blends, for example together with monomeric compounds or together with other polymers having charge -transport, semiconducting, electrically conducting, photoconducting and/or light emitting semiconducting properties, or for example with polymers having hole blocking or electron blocking properties for use as interlayers or charge blocking layers in OLED devices. Thus, another aspect of the invention relates to a polymer blend comprising one or more polymers according to the present invention and one or more further polymers having one or more of the above-mentioned properties. These blends can be prepared by conventional methods that are described in prior art and known to the skilled person. Typically the polymers are mixed with each other or dissolved in suitable solvents and the solutions combined.

[0086] Another aspect of the invention relates to a formulation comprising one or more polymers, mixtures or polymer blends as described above and below and one or more organic solvents. Preferred solvents are aliphatic hydrocarbons, chlorinated hydrocarbons, aromatic hydrocarbons, ketones, ethers and mixtures thereof. Additional solvents which can be used include 1,2,4-trimethylbenzene, 1,2,3,4-tetramethyl benzene, pentylbenzene, mesitylene, cumene, cymene, cyclohexylbenzene, diethylbenzene, tetralin, decalin, 2,6- lutidine, 2-fluoro-m-xylene, 3-fluoro-o-xylene, 2-chlorobenzotrifluoride, dimethylformamide, 2-chloro-6-fluorotoluene, 2-fluoroanisole, anisole, 2,3-dimethylpyrazine, 4-fluoroanisole, 3- fluoroanisole, 3-trifluoro-methylanisole, 2-methylanisole, phenetol, 4-methylanisole, 3- methylanisole, 4-fluoro-3-methylanisole, 2-fluorobenzonitrile, 4-fluoroveratrol, 2,6- dimethylanisole, 3-fluorobenzonitrile, 2,5-dimethylanisole, 2,4-dimethylanisole, benzonitrile, 3,5-dimethylanisole, Ν,Ν-dimethylaniline, ethyl benzoate, l-fluoro-3,5-dimethoxybenzene, 1 -methylnaphthalene, N-methylpyrrolidinone, 3-fluorobenzotrifluoride, benzotrifluoride, benzotrifluoride, diosane, trifluoromethoxybenzene, 4-fluorobenzotrifluoride, 3- fluoropyridine, toluene, 2-fluorotoluene, 2-fluorobenzotrifluoride, 3-fluorotoluene, 4- isopropylbiphenyl, phenyl ether, pyridine, 4-fluorotoluene, 2,5-difluorotoluene, l-chloro-2,4- difluorobenzene, 2-fluoropyridine, 3-chlorofluorobenzene, 3-chlorofluorobenzene, 1-chloro- 2,5-difluorobenzene, 4-chlorofluorobenzene, chlorobenzene, o-dichlorobenzene, 2- chlorofluorobenzene, p-xylene, m-xylene, o-xylene or mixture of 0-, m-, and p-isomers. Solvents with relatively low polarity are generally preferred. For inkjet printing solvents with high boiling temperatures and solvent mixtures are preferred. For spin coating alkylated benzenes like xylene and toluene are preferred.

[0087] Examples of especially preferred solvents include, without limitation, dichloromethane, trichloromethane, monochlorobenzene, o-dichlorobenzene, tetrahydrofuran, anisole, morpholine, toluene, o-xylene, m-xylene, p-xylene, 1,4-dioxane, acetone, methylethylketone, 1 ,2-dichloroethane, 1, 1, 1-trichloroethane, 1, 1,2,2-tetrachloroethane, ethyl acetate, n-butyl acetate, dimethylformamide, dimethylacetamide, dimethylsulfoxide, tetraline, decaline, indane, methyl benzoate, ethyl benzoate, mesitylene and/or mixtures thereof.

[0185] The concentration of the polymers in the solution is preferably 0.1 to 10% by weight, more preferably 0.5 to 5% by weight.

[0088] After the appropriate mixing and ageing, solutions are evaluated as one of the following categories: complete solution, borderline solution or insoluble. The contour line is drawn to outline the solubility parameter-hydrogen bonding limits dividing solubility and insolubility. Complete solvents falling within the solubility area can be chosen from literature values such as published in Crowley et ah, Journal of Paint Technology, 38, No 496, 296 (1966). Solvent blends may also be used and can be identified as described in Solvents, W. H. Ellis, Federation of Societies for Coatings Technology, pp. 9-10, (1986). Such a procedure may lead to a blend of "non" solvents that will dissolve both the polymers of the present invention, although it is desirable to have at least one true solvent in a blend.

[0089] The polymers according to the present invention can also be used in patterned

OSC layers in the devices as described above and below. For applications in modern microelectronics it is generally desirable to generate small structures or patterns to reduce cost (more devices/unit area), and power consumption. Patterning of thin layers comprising a polymer according to the present invention can be carried out for example by photolithography, electron beam lithography or laser patterning.

[0090] For use as thin layers in electronic or electrooptical devices the polymers, polymer blends or formulations of the present invention may be deposited by any suitable method. Liquid coating of devices is more desirable than vacuum deposition techniques. Solution deposition methods are especially preferred. The formulations of the present invention enable the use of a number of liquid coating techniques. Preferred deposition techniques include, without limitation, dip coating, spin coating, ink jet printing, letter-press printing, screen printing, doctor blade coating, roller printing, reverse-roller printing, offset lithography printing, flexographic printing, web printing, spray coating, brush coating or pad printing. Ink-jet printing is particularly preferred as it allows high resolution layers and devices to be prepared.

[0091] The polymers, blends, or formulations may also include the use of non- volatile additives including but not limited to dielectrics, electrolytes, or high boiling point solvents. These additives may be used alter the carrier generation efficiency, carrier lifetime, carrier mobility, film morphology, conductivity, dielectric properties, or optical properties.

[0092] Selected formulations of the present invention may be applied to prefabricated device substrates by ink jet printing or microdispensing. Preferably industrial piezoelectric print heads such as but not limited to those supplied by Aprion, Hitachi-Koki, InkJet Technology, On Target Technology, Picojet, Spectra, Trident, Xaar may be used to apply the organic semiconductor layer to a substrate. Additionally semi-industrial heads such as those manufactured by Brother, Epson, Konica, Seiko Instruments Toshiba TEC or single nozzle microdispensers such as those produced by Microdrop and Microfab may be used.

[0093] In order to be applied by ink jet printing or micro-dispensing, the polymers should be first dissolved in a suitable solvent. Solvents must fulfil the requirements stated above and must not have any detrimental effect on the chosen print head. Additionally, solvents should have boiling points greater than 100°C, preferably greater than 140°C, and more preferably greater than 150°C, in order to prevent operability problems caused by the solution drying out inside the print head. Apart from the solvents mentioned above, suitable solvents include substituted and non-substituted xylene derivatives, di-Q-2-alkyl formamide, substituted and non-substituted anisoles and other phenol-ether derivatives, substituted heterocycles such as substituted pyridines, pyrazines, pyrimidines, pyrrolidinones, substituted and non-substituted N,N-di-Ci_2-alkylanilines and other fluorinated or chlorinated aromatics.

[0094] A preferred solvent for depositing a polymer according to the present invention by ink jet printing comprises a benzene derivative which has a benzene ring substituted by one or more substituents wherein the total number of carbon atoms among the one or more substituents is at least three. For example, the benzene derivative may be substituted with a propyl group or three methyl groups, in either case there being at least three carbon atoms in total. Such a solvent enables an ink jet fluid to be formed comprising the solvent with the polymer, which reduces or prevents clogging of the jets and separation of the components during spraying. The solvent(s) may include those selected from the following list of examples: dodecylbenzene, l-methyl-4-tert-butylbenzene, terpineol limonene, isodurene, terpinolene, cymene, diethylbenzene. The solvent may be a solvent mixture, that is a combination of two or more solvents, each solvent preferably having a boiling point greater than 100°C, more preferably greater than 140°C. Such solvent(s) also enhance film formation in the layer deposited and reduce defects in the layer.

[0095] The ink jet fluid (that is mixture of solvent, binder and semiconducting compound) preferably has a viscosity at 20°C (degrees Celsius) of 1-100 mPas (millipascales), more preferably 1-50 mPas, and most preferably 1-30 mPas.

[0096] The polymers or formulations according to the present invention can additionally comprise one or more further components or additives selected for example from surface-active compounds, lubricating agents, wetting agents, dispersing agents, hydrophobing agents, adhesive agents, flow improvers, defoaming agents, deaerators, diluents which may be reactive or non-reactive, auxiliaries, colorants, dyes or pigments, sensitizers, stabilizers, nanoparticles or inhibitors.

[0097] The polymers according to the present invention are useful as charge transport, semiconducting, electrically conducting, photoconducting or light emitting materials in optical, electrooptical, electronic, electroluminescent or photoluminescent components or devices. In these devices, the polymers of the present invention are typically applied as thin layers or films.

[0098] Thus, the present invention also provides the use of the semiconducting polymer, polymer blend, formulation or layer in an electronic device. The formulation may be used as a high mobility semiconducting material in various devices and apparatus. The formulation may be used, for example, in the form of a semiconducting layer or film. Accordingly, in another aspect, the present invention provides a semiconducting layer for use in an electronic device, the layer comprising a polymer, polymer blend or formulation according to the invention. The layer or film may be less than about 30 microns. For various electronic device applications, the thickness may be less than about 1 micron thick. The layer may be deposited, for example on a part of an electronic device, by any of the aforementioned solution coating or printing techniques.

[0099] The invention additionally provides an electronic device comprising a polymer, polymer blend, formulation or organic semiconducting layer according to the present invention. Especially preferred devices are organic field effect transistors (OFETs), organic thin film transistors (OTFTs), integrated circuits (ICs), logic circuits, capacitors, radio frequency identification tags (RFID tags), organic light emitting diodes (OLEDs), organic light emitting transistors (OLETs), organic photovoltaic devices (OPVs), solar cells, laser diodes, photoconductors, photodetectors, electrophotographic devices, electrophotographic recording devices, organic memory devices, sensor devices, charge injection layers, Schottky diodes, planarising layers, antistatic films, conducting substrates and conducting patterns.

[00100] Another embodiment may be directed to applications that include the use of hybrid photovoltaic devices, nanoparticle/Quantum dot devices, and use as a fission sensitizer in inorganic applications (e.g., silicon, copper indium gallium (di)selenide (CIGS), etc.). The general construction of these devices with different polymers or oligomers are disclosed and known in the art. Hybrid photovoltaic devices are described in, for example, U.S. Patent No. 8,426,725; U.S. Patent No. 8, 106,289; and U.S. Publication No. US20130312801. Hybrid photovoltaic devices may utilize inorganic materials as the acceptor and electron transporter in a structure. The hybrid photovoltaic devices have a potential for not only low-cost, but also for scalable solar power conversion. Optical applications advantageously utilize quantum dots because of their high extinction coefficient. (Leatherdale, et al. (2002)7¾e Journal of Physical Chemistry B 106(31):7619). In electronic applications, they have been shown to operate like a single electron transistor and demonstrate the Coulomb blockade effect. Quantum dot nanoparticles of silicon, cadmium selenide, cadmium sulfide, or indium arsenide may be of particular use to increase efficiencies in producing a higher energy difference. Various nanoparticle/quantum dot devices are disclosed in, for example, U.S. Patent No. 7,868,302 and U.S. Publication Nos. US20130009131 ; US20120292594; and US20120211074. Fission sensitizers may be in the form of nanocrystals or another type of non-covalent aggregate, or may be in the form of a covalent polymer, oligomer, or dimer. Rapid and efficient singlet fission may preferably occur in inorganic applications using, for example but not limited to, silicon and CIGS. Fission sensitizers such as those that are described in, for example, U.S. Publication Nos. US20130240850; US20120228586; and US20100193011, have general constructions that are known in the art, and compounds, polymers, and oligomers of the various embodiments of the invention may be employed in the applications and devices disclosed here.

[0100] Especially preferred electronic device are OFETs, OLEDs and OPV devices, in particular bulk heterojunction (BHJ) OPV devices. In an OFET, for example, the active semiconductor channel between the drain and source may comprise the layer of the invention. As another example, in an OLED device, the charge (hole or electron) injection or transport layer may comprise the layer of the invention. Organic Photovoltaic Devices (OPVs)

[0101] A polymer in accordance with the present invention may be used in an OPV device that comprises or contains, more preferably consists essentially of, very preferably exclusively of, a p-type (electron donor) semiconductor and an n-type (electron acceptor) semiconductor. The p-type semiconductor is constituted by a polymer according to the present invention. The n-type semiconductor can be an inorganic material, such as, for example, zinc oxide or cadmium selenide, or an organic material, such as, for example, a fullerene or substituted, for example (6,6)-phenyl-butyric acid methyl ester derivatized Qo fullerene, also known as "PCBM" or "QoPCBM", as reported, for example, in Yu et ah, Science, Vol. 270, p. 1789 (1995), or a structurally analogous compound with, for example, a C70 fullerene group (C₇₀PCBM), or a polymer (see, for example, Coakley et ah, D. Chem. Mater, 16, 4533 (2004)). A preferred material of this type is a blend or mixture of a polymer according to the present invention with a Qo or C70 fullerene or substituted fullerene like C₆₀PCBM or C₇₀PCBM. Preferably the ratio polymer : fullerene is from 2: 1 to 1 :2 by weight, more preferably from 1.2: 1 to 1 : 1.2 by weight, most preferably 1 : 1 by weight. For the blended mixture, an optional annealing step may be necessary to optimize blend morphology and consequently OPV device performance.

[0102] A first preferred OPV device according to the invention comprises the following layers (in the sequence from bottom to top): a high work function electrode preferably comprising a metal oxide like for example ITO, serving as anode, an optional conducting polymer layer or hole transport layer, preferably comprising an organic polymer or polymer blend, for example of PEDOT:PSS (poly(3,4-ethylenedioxythiophene): poly(styrene-sulfonate), a layer, also referred to as "active layer", comprising a p-type and an n-type organic semiconductor, which can exist for example as a p-type/n-type bilayer or as distinct p-type and n-type layers, or as blend or p-type and n-type semiconductor, forming a BHJ, optionally a layer having electron transport properties, for example comprising LiF, a low work function electrode, preferably comprising a metal like for example aluminum, serving as cathode, wherein at least one of the electrodes, preferably the anode, is transparent to visible light, and wherein the p-type semiconductor is a polymer according to the present invention.

[0103] A second preferred OPV device according to the invention is an inverted OPV device and comprises the following layers (in the sequence from bottom to top): [0210] an electrode comprising for example ITO serving as cathode, optionally a layer having hole blocking properties, preferably comprising a metal oxide like TiQ or ZnO_x, an active layer comprising a p-type and an n-type organic semiconductor, situated between the electrodes, which can exist for example as a p-type/n-type bilayer or as distinct p-type and n-type layers, or as blend or p-type and n-type semiconductor, forming a BHJ, an optional conducting polymer layer or hole transport layer, preferably comprising an organic polymer or polymer blend, for example of PEDOT:PSS, a high work function electrode, preferably comprising a metal like for example gold, serving as anode, wherein at least one of the electrodes, preferably the cathode, is transparent to visible light, and wherein the p-type semiconductor is a polymer according to the present invention.

[0104] In the OPV devices of the present invent invention the p-type and n-type semiconductor materials are preferably selected from the materials, like the polymer/fullerene systems, as described above. If the bilayer is a blend an optional annealing step may be necessary to optimize device performance.

Organic Field Effect Transistors (OFETs)

[0105] The compound, formulation and layer of the present invention are also suitable for use in an OFET as the semiconducting channel. Accordingly, the invention also provides an OFET comprising a gate electrode, an insulating (or gate insulator) layer, a source electrode, a drain electrode and an organic semiconducting channel connecting the source and drain electrodes, wherein the organic semiconducting channel comprises a polymer, polymer blend, formulation or organic semiconducting layer according to the present invention. Other features of the OFET are well known to those skilled in the art.

[0106] OFETs where an OSC material is arranged as a thin film between a gate dielectric and a drain and a source electrode, are generally known, and are described for example in U.S. Patent No. 5,892,244, U.S. Patent No. 5,998,804, and U.S. Patent No. 6,723,394. Due to the advantages, like low cost production using the solubility properties of the compounds according to the invention and thus the processability of large surfaces, preferred applications of these FETs are such as integrated circuitry, TFT displays and security applications.

[0107] The gate, source and drain electrodes and the insulating and semiconducting layer in the OFET device may be arranged in any sequence, provided that the source and drain electrode are separated from the gate electrode by the insulating layer, the gate electrode and the semiconductor layer both contact the insulating layer, and the source electrode and the drain electrode both contact the semiconducting layer.

[0108] An OFET device according to the present invention preferably comprises: a source electrode, a drain electrode, a gate electrode, a semiconducting layer, one or more gate insulator layers, optionally a substrate, wherein the semiconductor layer preferably comprises a polymer, polymer blend or formulation as described above and below.

[0109] The OFET device can be a top gate device or a bottom gate device. Suitable structures and manufacturing methods of an OFET device are known to the skilled in the art and are described in the literature.

[01 10] The gate insulator layer preferably comprises a fluoropolymer, like e.g. the commercially available Cytop 809Μ™ or Cytop 107M™ (from Asahi Glass). Preferably the gate insulator layer is deposited, e.g., by spin-coating, doctor blading, wire bar coating, spray or dip coating or other known methods, from a formulation comprising an insulator material and one or more solvents with one or more fluorine atoms (fluorosolvents), preferably a perfluorosolvent. A suitable perfluorosolvent is, e.g., FC75™ (available from Acros, catalogue number 12380).

[01 11] Other suitable fluoropolymers and fluorosolvents are known in prior art, like for example the perfluoropolymers Teflon AF^M 1600 or 2400 (from DuPont) or FluoropeF^M (from Cytonix) or the perfluorosolvent FC 43™ (Acros, No. 12377). Especially preferred are organic dielectric materials having a low permittivity (or dielectric constant) from 1.0 to 5.0, very preferably from 1.8 to 4.0 ("low k materials"), as disclosed for example in US 2007/0102696 Al or U.S. Pat. No. 7,095,044.

[01 12] In security applications, OFETs and other devices with semiconducting materials according to the present invention, like transistors or diodes, can be used for RFID tags or security markings to authenticate and prevent counterfeiting of documents of value like banknotes, credit cards or ID cards, national ID documents, licenses or any product with monetary value, like stamps, tickets, shares, cheques, etc.

Organic Light Emitting Diodes (OLEDs)

[01 13] Alternatively, the materials according to the invention can be used in OLEDs, e.g. as the active display material in a flat panel display applications, or as backlight of a flat panel display like e.g. a liquid crystal display. Common OLEDs are fabricated using multilayer structures. An emission layer is generally sandwiched between one or more electron-transport and/or hole-transport layers. By applying an electric voltage, electrons and holes as charge carriers move towards the emission layer where their recombination leads to the excitation and hence luminescence of the lumophore units contained in the emission layer. The inventive compounds, materials and films may be employed in one or more of the charge transport layers and/or in the emission layer, corresponding to their electrical and/or optical properties. Furthermore their use within the emission layer is especially advantageous, if the compounds, materials and films according to the invention show electroluminescent properties themselves or comprise electroluminescent groups or compounds. The selection, characterization as well as the processing of suitable monomeric, oligomeric and polymeric compounds or materials for the use in OLEDs is generally known by a person skilled in the art, see, e.g., Meerholz, Synthetic Materials, 1 11-1 12, pp. 31-34 (2000); Alcala, J. Appl. Phys., 88, pp. 7124-7128 (2000), and the literature cited therein.

[01 14] According to another use, the materials according to this invention, especially those showing photoluminescent properties, may be employed as materials of light sources, e.g. in display devices, as described in EP 0 889 350 Al or by Wederet al, Science, 279, pp. 835-837 (1998).

[01 15] A further aspect of the invention relates to both the oxidised and reduced form of the compounds according to this invention. Either loss or gain of electrons results in formation of a highly delocalized ionic form, which is of high conductivity. This can occur on exposure to common dopants. Suitable dopants and methods of doping are known to those skilled in the art, e.g., from EP 0 528 662, U.S. Patent No. 5, 198, 153 or WO 96/21659.

[01 16] The doping process typically implies treatment of the semiconductor material with an oxidizing or reducing agent in a redox reaction to form delocalized ionic centers in the material, with the corresponding counterions derived from the applied dopants. Suitable doping methods comprise for example exposure to a doping vapor in the atmospheric pressure or at a reduced pressure, electrochemical doping in a solution containing a dopant, bringing a dopant into contact with the semiconductor material to be thermally diffused, and ion-implantation of the dopant into the semiconductor material.

[01 17] When electrons are used as carriers, suitable dopants are for example halogens

(e.g., 12, C12, Br2, IC1, IC1₃, IBr and IF), Lewis acids (e.g., PF₅, AsF₅, SbF₅, BF₃, BC1₃, SbCi₅, BBr₃ and S0₃), protonic acids, organic acids, or amino acids ^.g., HF, HC1, HN0₃, H₂S0₄, HCIO₄, FS0₃H and C1S0₃H), transition metal compounds (e.g., FeCl₃, FeOCl, Fe(C10₄)₃, Fe(4-CH₃C₆H₄S0₃)₃, TiC , ZrC , HfC , NbF₅, NbCl₅, TaCl₅, M0F5, M0CI5, WF₅, WC1₆, UF₆ and LnCl₃ (wherein Ln is a lanthanoid), anions (e.g., CI^", Br^", Γ, I₃ ^", HS0₄ ^", S0₄ ^2", N0₃ ^", CIO4^", BF₄ ^", PF₆ ^~, AsF₆ ^~, SbF₆ ^~, FeC , Fe(CN)₆ ^3~, and anions of various sulfonic acids, such as aryl-S(¾^"). When holes are used as carriers, examples of dopants are cations (e.g., H⁺, Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺), alkali metals (e.g., Li, Na, K, Rb, and Cs), alkaline- earth metals (e.g., Ca, Sr, and Ba), <¾, XeOF₄, (N0₂ ⁺) (SbF₆ ^~), (N0₂ ⁺) (SbCl₆ ^~), (N0₂ ⁺) (BF₄ ^~ ), AgC10₄, H₂IrCl₆, La(N0₃)₃.6H₂0, FS0₂OOS0₂F, Eu, acetylcholine, R4 ⁺, (R is an alkyl group), R4P⁺ (R is an alkyl group), ReAs⁺ (R is an alkyl group), and R₃S⁺ (R is an alkyl group).

[01 18] The conducting form of the compounds of the present invention can be used as an organic "metal" in applications including, but not limited to, charge injection layers and ITO planarizing layers in OLED applications, films for flat panel displays and touch screens, antistatic films, printed conductive substrates, patterns or tracts in electronic applications such as printed circuit boards and condensers.

[01 19] The compounds and formulations according to the present invention may also be suitable for use in organic plasmon-emitting diodes (OPEDs), as described for example in Koller et al, Nature Photonics 2008 (published online Sep. 28, 2008).

[0120] According to another use, the materials according to the present invention can be used alone or together with other materials in or as alignment layers in LCD or OLED devices, as described for example in US 2003/0021913. The use of charge transport compounds according to the present invention can increase the electrical conductivity of the alignment layer. When used in an LCD, this increased electrical conductivity can reduce adverse residual dc effects in the switchable LCD cell and suppress image sticking or, for example in ferroelectric LCDs, reduce the residual charge produced by the switching of the spontaneous polarization charge of the ferroelectric LCs. When used in an OLED device comprising a light emitting material provided onto the alignment layer, this increased electrical conductivity can enhance the electroluminescence of the light emitting material. The compounds or materials according to the present invention having mesogenic or liquid crystalline properties can form oriented anisotropic films as described above, which are especially useful as alignment layers to induce or enhance alignment in a liquid crystal medium provided onto said anisotropic film. The materials according to the present invention may also be combined with photoisomerizable compounds and/or chromophores for use in or as photoalignment layers, as described in US 2003/0021913.

[0121] According to another use, the materials according to the present invention, especially their water-soluble derivatives (for example with polar or ionic side groups) or ionically doped forms, can be employed as chemical sensors or materials for detecting and discriminating DNA sequences. Such uses are described for example in Chen et al., Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 12287; Wang et al, Proc. Natl. Acad. Sci. U.S.A. 99, 49 (2002); DiCesare et al., Langmuir 2002, 18, 7785; and McQuade et al, Chem. Rev., 100, 2537 (2000).

[0122] Throughout the application, where compositions are described as having, including, or comprising specific components, or where processes are described as having, including, or comprising specific process steps, it is contemplated that compositions of the present teachings also consist essentially of, or consist of, the recited components, and that the processes of the present teachings also consist essentially of, or consist of, the recited process steps. Unless the context clearly indicates otherwise, as used herein plural forms of the terms herein are to be construed as including the singular form and vice versa. Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", mean "including but not limited to", and are not intended to (and do not) exclude other components.

[0123] It will be appreciated that variations to the foregoing embodiments of the invention can be made while still falling within the scope of the invention. Each feature disclosed in this specification, unless stated otherwise, may be replaced by alternative features serving the same, equivalent or similar purpose. Thus, unless stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

DETAILED DESCRIPTION OF THE DRAWING FIGURES

[0124] As illustrated in FIG. 1, the steps toward MEG are shown starting from the excitation to the SI state, which is strongly coupled to the charge-transfer (CT) state, leading to the ME states. From studies of intermolecular SF, the current mechanistic understanding of SF suggests that the direct coupling between the singlet state and the multi-excitonic (triplet pair) states is weak, but coupling mediated by an intermediate CT state can be quite strong.

[0125] The connectivity and conceptual design of the building blocks for the small molecules and polymers is shown in FIGS. 2 and 3, respectively. The systems are composed of electron rich (donor) moieties and electron deficient units (acceptor). The strong acceptor (SA) character was introduced to lower the lowest unoccupied molecular orbital (LUMO) and the strong donor raises the highest occupied molecular orbital (HOMO), effectively reducing the band gap of the resultant molecule or polymer. Additionally, the selected SA moiety has a triplet energy low enough to satisfy the energetic requirement for SF. This is an important parameter, given that SF is favored when the energy of the singlet state is at least twice the energy of the triplet state (i.e., E[S1] > 2E[T₁]). Moreover, the most important feature of this model is that having the SA units conjugated with the D units could lead to effectively accessing the CT states that are required to mediate coupling to ME states. For example, considering a linear combination of the CT moieties in FIG. 3, the charge-transfer mediated SF model suggest that structures bearing δ⁺ and δ^" polarizabilty act in a similar fashion to the CT states in the simplified SF diagram (see, again, FIG. 1). The polymer design for intramolecular CT-mediated singlet-fission (i.e., [SA-SD-SA-SD]«) leads down to the linear combination of CT moities with δ⁺ and δ, which finally leads down to MEG.

[0126] As illustrated in FIG. 4, to characterize the dynamical properties of the polymers of the invention, broadband femtosecond transient absorption (TA) spectroscopy was employed. Two particular systems BDT-TDOl and p-BDT-TDOl were examined here. First, unoxidized analogues of the above materials an used as a control experiment. These materials exhibit dynamics typical of many semiconducting polymer systems. These systems show two generalized excited state pathways: excitonic relaxation and relaxation via formation and decay of charge transferred states. Both pathways repopulate the ground electronic state. The molecular system shows initial population of both excitonic and charge- transfer states. The excitonic state decays to form charge-transferred state in ca. 10 ps, which then decays to repopulate the ground state in greater than 3 ns. The polymer system shows similar dynamics, but the excitonic state shows a longer lifetime as a result of stabilization via excitonic relaxation. The result is a singlet excitonic lifetime of about 600 ps. The CT lifetime is greater than 3 ns. These properties qualitatively hold for all studied unoxidized materials.

[0127] As illustrated in FIG. 5, the effect of thiophene oxidation on the excited state lifetime is quite dramatic. Perhaps the most simple parameter for accessing the effects of oxidation is the excited state lifetime, the average lifetime of an excitation before it repopulates the ground state. This is well represented by the bleach kinetics. Upon thiophene oxidation, the greater than 3 ns lifetime of BDT-TD02 is shortened to less than 1 ps. Similar results, to varying degrees of severity, are seen for all measured TDO-containing BDT conjugates and copolymers. This up to 3 orders-of-magnitude change in lifetime strongly suggests that oxidative chemical modification introduces new photophysical relaxation pathways in BDT-TDO complexes. As can be seen, TDO-containing small molecules have an excited state lifetime several orders of magnitude shorter than their unoxidized counterparts. This is due to introduction of additional non-radiative decay pathways resembling those of polyene-containing systems.

[0128] As illustrated in FIG. 6, with reference to Comparative Example 9, thiophene oxidation results in significant absorption red-shifting in small molecules and polymers. TDO-containing molecules display a single broad absorption feature, while polymers have discrete singlet and charge -transfer exciton bands. In FIG. 6, the first dotted line (grey line) represents the compound BDT-T2, the second dashed line (black dashed line) represents the compound BDT-TD02, and the solid dark line represents BDT-TDO 1. In the second graph (right hand side), the dotted line that is lowest at the level of 300 nm represents p-BDT-T2, the middle dashed line at 300 nm represents p-BDT-TD02, and the top solid line (dark line) at 300 nm wavelength represents p-BDT-TDO l .

[0129] FIG. 7 illustrates, with reference to Comparative Example 9, the ground state recovery kinetics are compared for TDOl (7 A) and TD02 (7B) containing molecules (black solid line) and polymers (grey dashed line). The triplet spectra are also compared in TDOl (7C) and TD02 (7D) containing materials. The spectra of triplets generated from singlet fission (lines) are compared to triplets generated through pulsed radiolysis (circles). SF generated spectral traces are from TA datasets; global target analysis was used to extract the triplet spectrum when the triplet had significant temporal overlap with other populations. Since the triplet is localized to the TDO subunits, the triplet spectrum is equivalent for all materials containing the same number of sequential TDO monomers (aside from the contribution of the overlapping bleach band).

[0130] As illustrated in FIG. 8, and discussed further below, starting from the initially excited singlet charge transfer exciton (S2|CT), singlet fission (SF) can occur to form a pair of triplets (2T1) for each singlet. In some systems, internal conversion (IC) through a polyene-like dark state (SI) is observed. This internal conversion process competes with SF on a sub-picosecond timescale, and thereby reduces the SF yield.

EXAMPLES

[0131] The following Examples of the invention are provided only to further illustrate the invention, and are not intended to limit its scope. Chemicals were purchased from Sigma- Aldrich and used as received, unless otherwise noted. All reactions were performed in oven- dried round bottom flasks, unless otherwise noted. Compounds l¹, 3², 4³, 6⁴, 8² and 9¹ (the superscripts for the compounds 1, 3, 4, 6, 8, and 9 cite to references that follow the examples) were prepared by reported procedures. 1H and ¹³C nuclear magnetic resonance spectra were recorded at 300 K (unless otherwise noted) on Bruker DRX300 (300MHz) or Bruker DRX400 (400MHz) FT NMR spectrometers. High-resolution mass spectra were recorded on a JMS-HX110 HF mass spectrometer (ionization mode: FAB+). UV-vis absorption spectra were taken on a Shimadzu UV-1800 spectrophotometer. Gel permeation chromatography (GPC) was carried out on a Waters separation module equipped with a Waters 2414 refractive index detector and a Waters 2998 photodiode array detector, using THF as the eluent. Molecular weights (MWs) and polydispersity indices (PDIs) are reported relative to polystyrene standards.

EXAMPLE 1

PREPARATION OF BDT MONOMER DERIVATIVE (COMPOUND 2)

1 2, 78%

[0132] Compound 1 (1.34 g, 3.43 mmol) was dissolved in 60 mL of anhydrous THF and cooled in an ice bath. Butyllithium solution (3.43 mL, 7.55 mmol) was added dropwise under nitrogen. The mixture was kept at 0 °C for 30 min. and then at room temperature (RT) for 1 hour (a great deal of white solid precipitate appeared). The mixture was cooled in the dry ice-IPA bath, 1.5 lg of trimethyltin chloride was dissolved in 5 mL dry THF and added in one portion, the reactant turned clear rapidly. The resulting mixture was stirred at room temperature (R.T.) for overnight. The mixture was quenched with 50 mL of water and extracted with ether. The organic extraction was dried with anhydrous sodium sulfate. After removing the solvent, the crude yellow solid was recrystallized from ethanol (EtOH) to give Compound 2 as a white solid (1.92 g, yield: 78%). Results of analyses performed on Compound 2 were as follows: ^lH NMR (400 MHz, CDC1₃, ppm): δ 0.45 (s, 18 H), 0.94 (t, J = 6.0 Hz, 6H), 1.38-1.42 (m, 8H), 1.58-1.62 (m, 4H), 1.85-1.92 (m, 4H), 4.31 (t/ = 6.8 Hz, 4H), 7.52 (s, 2H). ¹³C NMR (100 MHz, CDC1₃): δ -8.34, 14.08, 22.69, 25.79, 30.51, 31.68, 73.59, 128.02, 133.00, 134.01, 139.28, 140.48, 141.16. HRMS (FAB) m/z calculated for C₂₈H₄₆0₂S₂Sn₂: 716.2240, Found: 716.2378.

EXAMPLE 2 PREPARATION OF T-TDO

[0133] 2,5-dibromothiophene-S,S-dioxide (Compound 3) (274 mg, 1 mmol) and

Compound 4 (419 mg, 1 mmol) were dissolved in 4 mL toluene under nitrogen. Pd(PPh)₄ (57 mg, 0.05 mmol) was added and the resulting mixture was stirred at 80C for 2 hours. After cooling down to room temperature, the reaction mixture was extracted with CHCI₂ and washed twice with water and then dried with N%S0₄. After removing the solvent, the crude product was purified by column chromatography on silica gel and eluted with 50% dichloromethane/hexanes to provide T-TDO (Compound5) as a yellow solid (138 mg, yield: 42%). Results of analyses performed on T-TDO (Compound 5) were as follows: ^lR NMR (400 MHz, CDCI₃, ppm): δ 2.56 (s, 3H), 6.54 (d, J = 5.2 Hz, 1H), 6.90 (d, J = 5.2 Hz, 1H), 6.98 (d, J = 4 Hz, 1H), 7.46 (d, J = 4 Hz, 1H), ¹³C NMR (100 MHz, CDC1₃): 5(ppm) 20.46, 1 16.79, 1 18.15, 128.51, 129.39, 129.49, 129.89, 136.88, 143.87.

EXAMPLE 3 PREPARATION OF T-TDO-BDT-TDO-T [0134] Compound 2 (160 mg, 0.224 mmol) and Compound 5 (145 mg, 0.45 mmol) were dissolved in 5 mL toluene under nitrogen. To this was added Pd(PPh)₄ (13 mg, 0.01 1 mmol) and the resulting mixture was stirred at 110^°C for 24 h. After cooling down to room temperature, the solvent was removed under reduce vacuum, and the crude product was purified by column chromatography on silica gel and eluted with dichloromethane/hexanes (ratio 3 :2) to provide T-TDO-BDT-TDO-T as a dark red solid (80 mg, yield: 41%). Results of analyses performed on Compound T-TDO-BDT-TDO-T were as follows: ^lH NMR (400 MHz, CDC1₃, ppm): δ 0.92-0.97 (m, 6H), 1.38-1.43 (m, 8H), 1.54-1.59 (m, 4H), 1.86-1.90 (m, 4H), 2.59 (s, 6H), 4.30 (t, J = 6.4 Hz, 4H), 6.70 (d, J = 5.2 Hz, 2H), 6.86 (d, J = 5.2 Hz, 2H), 7.02 (d, J = 4 Hz, 2H), 7.54 (d, J = 4 Hz, 2H), 7.94 (s, 2H). ¹³C NMR (100 MHz, CDCI₃): 5(ppm) 14.10, 22.66, 25.56, 30.53, 31.66, 74.50, 110.03, 117.20, 121.45, 121.74, 128.88, 129.57, 130.10, 130.29, 133.45, 135.58, 136.92, 143.73, 145.15.

EXAMPLE 4

PREPARATION OF T-TDQz (COMPOUND 7)

7,

[0135] 5,5'-Dibromo-4,4'-dihexyl-2,2'-bithiophene-[all]-S,S-dioxide (Compound 6)

(1.67 g, 3 mmol) and Compound 4 (1.26 g, 3 mmol) were dissolved in 8 mL toluene under nitrogen. To this was added Pd(PP¾)₄ (173 mg, 0.15 mmol) and the resulting mixture was stirred at 80^°C for 2 hours. After cooling down to room temperature, the reaction mixture was extracted with CH₂CI₂ and washed twice with water and then dried with N¾S0₄. After removing the solvent, the crude product was purified by column chromatography on silica gel and eluted with 25% EtOAc/hexanes to provide T-TDQ (Compound 7) as red solid (0.82 g, yield: 46%). Results of analyses performed on T-TDQ (Compound 7) were as follows: ^LH NMR (400 MHz, CDC1₃, ppm): δ 0.88-0.94 (m, 6H), 1.26-1.4 (m, 12H), 1.58-1.66 (m, 4H), 2.46 (t, J = 7.6 Hz, 2H), 2.59 (s, 3H), 2.67 (t, J = 8 Hz, 2H), 7.06 (d, J = 4 Hz, 2H), 7.21 (s, 1H), 7.58 (d, J= 4 Hz, 1H). ^ljC NMR (100 MHz, CDC1₃): 5(ppm) 13.97, 20.61, 22.42, 22.46, 26.34, 27.27, 28.84, 29.23, 30.04, 30.29, 31.33, 31.40, 1 16.90, 125.56, 127.99, 128.75, 129.29, 129.93, 130.15, 130.46, 132.36, 134.15, 142.42, 144.57. HRMS (FAB) m/z calculated for C₂₅H₃₃Br0₄S4: 605.7026, Found (isotopic pattern): 604.0445, 605.0475, 606.0425.

EXAMPLE 5 PREPARATION OF T-TDO₂-BDT-TDO₂-T

[0136] Pd₂(dba)₃ (55 mg, 0.06 mmol) and P(o-tolyl (36 mg, 0.12 mmol) under nitrogen was added to a stirred solution of Compound7 (727 mg, 1.2 mmol) and Compound 2 (430 mg, 0.6 mmol) in 30 mL toluene. The resulting mixture was stirred for 24 hours at 1 10 °C. After cooling down to room temperature, the reaction mixture was poured into water (60 mL) and extracted with CH₂CI₂. The organic layer was washed with water and then dried over Na₂S0₄. After removal of solvent, the crude product was purified by column chromatography on silica gel using a mixture of dichloromethane and hexanes (3 :2) as eluent to afford the target compound (T-TDQ>-BDT-TD0₂-T) as dark blue solid (312 mg, 36%). Results of analyses performed on T-TDO₂-BDT-TDO₂-T were as follows: ^lH NMR (400 MHz, CDCI₃, ppm): δ 0.88-0.94 (m, 18H), 1.26-1.77 (m, 44H), 1.91 (m, 4H), 2.61 (s, 6H), 2.70 (t, J = 8.4 Hz, 4H), 2.86 (t, J = 8 Hz, 4H), 4.35 (t, J = 6.4 Hz, 4H), 7.08 (d, J = 4 Hz, 2H), 7.22 (s, 2H), 7.26 (s, 2H), 7.61 (d, J = 4 Hz, 2H), 8.1 1 (s, 2H). ¹³C NMR (100 MHz, CDCI₃): 5(ppm) 14.00, 14.07, 20.68, 22.49, 22.64, 25.75, 27.31, 27.80, 29.28, 29.69, 30.40, 30.48, 30.64, 31.44, 31.63, 74.72, 123.22, 127.41, 128.02, 128.77, 129.03, 129.16, 129.41, 130.05, 130.36, 132.39, 132.56, 134.51, 134.59, 138.18, 144.35, 145.08. HRMS (FAB) m/z calcd for C₇₂H₉₄OioSio: 1440.16, Found: 1440.6490. EXAMPLE 6 PREPARATION OF POLY-(BDT-TDO)

[0137] A 20-mL reaction vial was charged with a stirrer bar, 2,5-dibromo-3-hexyl- thiophene-S,S-dioxide (Compound8) (215 mg, 0.6 mmol), Compound2 (431 mg, 0.6 mmol), Pd₂(dba)₃ (27 mg, 0.03 mmol), P(o-to¾ (18 mg, 0.06 mmol) and 4 mL chlorobenzene. The reaction vial was purged with nitrogen and securely sealed. The reaction mixture was stirred at 130 °C for 48 hours. After cooling down to room temperature, the reaction mixture was precipitated into a mixture of methanol (50 mL) and 37% HC1 (5 mL). The dark red powder was filtered off and washed with methanol, following by further purification with sequential Soxhlet extraction with methanol, hexanes and dichloromethane to afford the polymer as a dark red solid (259 mg, 73%). Results of analyses performed on poly-(BDT-TDO) were as follows: 1H NMR (400 MHz, CDCk, ppm): δ 0.93 (broad, 9 H), 1.30-1.54 (broad, 20 H), 1.91 (m, 4H), 2.86 (m, 2H), 4.34-4.36 (m, 4H), 7.65 (broad, 1H), 8.13 (m, 2H). GPC: Mn = 7.9 K, PDI = 1.48.

EXAMPLE 7

PREPARATION OF POLY-(BDT-TDQ>)

[0138] A 20-mL reaction vial was charged with a stirrer bar, 5,5'-Dibromo-4,4'- dihexyl-2,2'-bithiophene-[all]-S,S-dioxide (Compound 6) (278 mg, 0.5 mmol), Compound 9 (484 mg, 0.5 mmol), Pd₂(dba)₃ (23 mg, 0.025 mmol), P(o-tol (16 mg, 0.05 mmol) and 3 mL chlorobenzene. The reaction vial was purged with nitrogen and securely sealed. The reaction mixture was stirred at 130 °C for 36 hours. After cooling down to room temperature, the reaction mixture was precipitated into a mixture of methanol (50 mL) and 37% HC1 (5 mL). The dark powder was filtered off and washed with methanol, following by further purification with sequential Soxhlet extraction with methanol, hexanes and dichloromethane to afford the polymer as a dark blue solid (271 mg, yield: 68%). Results of analyses performed on poly-(BDT-TDQ₂) were as follows:¹!! NMR (400 MHz, CDC1₃, ppm): δ 0.96 (broad, 12 H), 1.26-1.60 (m, 24H), 1.77 (m, 4H), 1.89-1.93 (m, 4H), 2.88 (broad, 4H), 4.29- 4.37 (m, 4H), 7.32 (broad, 2H), 8.15 (broad, 2H). GPC: 19.6 K, PDI = 1.53.

References:

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EXAMPLE 8

TRANSIENT ABSORPTION SPECTROSCOPY POLYMER ANALYSIS

[0139] Transient absorption spectroscopy was conducted using a commercial

TkSapphire laser system (SpectraPhysics|800nm| 100fs|3.5mW|lkHz). Excitation light was generated via a commercial optical parametric amplifier (LightConversion). Super continuum probe light was generated by focusing 800nm fundamental into a sapphire disc. The probe light was split into signal and reference beams, both of which were detected on a shot-by-shot basis with fiber-coupled Silicon (visible) or InGaAs (infrared) diode arrays. The pump-probe delay was controlled with a mechanical delay stage (Newport). EXAMPLE 9 COMPARATIVE EXAMPLE

[0140] The initial building blocks that were tested comprised of benzodithiophene as the electron rich unit (D), thiophene as a weak donor (WD), and the mono- or bi-thiophene- 1, 1 -dioxide (TDOl, and TD02, respectively) that acted as a strong electron acceptor. Both the polymers p-BDT-TDOn and small molecules BDT-TDOn had the core architecture shown in FIG. 1 , where it is postulated that, upon exciton generation, formation of a charge transfer exciton between BDT (electron donor) and TDO (electron acceptor) moieties would provide strong charge -transfer-mediated coupling to the intramolecular ME state (see FIG. 1). The energy levels of the four SF materials exhibited low-lying LUMOs (see Table 1). The unoxidized versions of the materials were used as controls. The compounds are illustrated below:

Singlet fission polymers and small molecules

Control polymers and small mo

[0141] To understand the effects of thiophene oxidation on the optical properties of the above materials, the linear absorption spectra were first analyzed as shown in two spectra in FIG. 6. Substituting thiophene with TDO universally reduced the bandgap of the resultant oligomer or polymer, which yielded significant red-shifting of the absorption spectrum (see Table 1). The optical absorption spectra of the molecular systems (FIG. 6A) were broad visible absorption with a single peak and no prominent structure, which is a characteristic of many small molecule systems. The polymeric systems (FIG. 6B) showed distinct transitions for the singlet exciton (S) and charge transfer (CT) excitations, which is typical for polymers in donor-acceptor configurations. The energy of the singlet transition remained roughly constant for all polymers, though its relative strength varied with the number of TDO units. The reduction in bandgap upon oxidation ranges from 300 to 760 meV. Generally, the bandgap reduction was more dramatic in oligomers than polymers and in systems with a larger number of oxidized units. As confirmed by cyclic voltammetry (Table 1), this bandgap reduction was predominantly due to stabilization of the LUMO energy (lowering the conduction band).

[0142] To properly confirm that intramolecular singlet fission was occurring, three parameters had to be verified: 1) triplet formation via fission should occur on ultrafast timescales, 2) the spectrum of the SF-generated triplet should match that of triplets generated through other means, 3) the triplet pairs generated via SF decay faster than the native single triplet, 4) the associated dynamics are insensitive to the concentration of the molecules in solution. Taken together, these criteria unambiguously established the fact that the triplets were formed by intramolecular singlet fission rather than intersystem crossing, which was primarily reflected in the distinct formation and recombination dynamics associated with multiple exciton states populated on a single molecular unit.

[0143] To characterize the rates of triplet formation in the above TDO-containing materials of the invention, a technique with sub-picosecond resolution and sensitivity to singlet and triplet populations was used. For this purpose, broadband femtosecond transient absorption (TA) spectroscopy was employed. The results showed that the substitution of thiophene for TDO induced several new singlet deactivation processes, which significantly decreased the singlet exciton lifetimes. The process that is of greatest interest is singlet fission.

[0144] Beginning with p-BDT-TDOl, the TDO-containing material with the longest excited state lifetime and highest SF yield. TA spectroscopy of p-BDT-TDO l resolved three spectrally distinct states following optical excitation. The initially formed state was assigned as a singlet charge-transfer exciton (CTE), as is commonly observed in donor-acceptor polymer systems. This state and its associated nIR induced absorption band have an about 7ps lifetime. The decay of the CTE induced absorption feature was correlated with the rise of a second spectral feature (FIG. 7C). This population was assigned as a triplet exciton formed as the product of singlet fission. Approximately 85% of the initial excitations went on to form triplet pairs. Following formation, the triplet decayed via triplet-triplet annihilation with an about 70ps time constant. The third spectral feature was a very broad induced absorption spanning most of the visible spectral range. This feature was generated within the 100 fs instrument response, persisted for about a nanosecond and accounts for about 10% of the initially excited population. This feature was assigned as polaron resulting from auto- ionization and noted that excitation with high energy photons yielded more of this state, which offered further support for this assignment.

[0145] To confirm the above assignment of the triplet (Tl→Tn) induced absorption feature, a pulsed radiolysis triplet transfer (PRTT) experiment was performed. This experiment used an electron pulse to generate triplets that were then optically probed to yield the triplet induced absorption spectrum (FIGS. 7C and 7D, blue circles) and native triplet lifetime. The spectral agreement of the PRTT Tl spectra and proposed Tl induced absorption from TA confirmed that triplets were formed on an ultrafast timescale following optical excitation in all TDO-containing material studied here. The molecular PRTT Tl spectrum was used to confirm the presence of triplet in the analogous polymers. Since the triplet was localized to the TDO subunits, all TDOn-containing materials would have similar Tl spectra (aside from any overlapping ground state bleach contributions). Thus, the ultrafast production of triplet was confirmed in the four TDO-containing molecular and polymeric systems (FIG. 7C and FIG. 7D).

[0146] Once the presence of triplet had been verified, the triplet generation mechanism was evaluated to confirm that the triplet generation mechanism was indeed SF. In all studied TDO-containing systems, TA spectroscopy revealed triplet formation occurred in less than 10 ps, which supported SF as the triplet generation mechanism rather than ISC. Triplet formation was possible through ISC, but observation of ISC on such fast timescales was generally limited to systems containing heavy atoms that facilitated spin-orbit coupling as a means of circumventing angular momentum conservation selection rules. The combined lack of heavy atoms and very fast triplet formation supported the assertion that triplets were formed by SF rather than ISC.

[0147] The triplet recombination dynamics also supported SF as the mechanism of triplet generation. In all studied TDO-containing materials, triplets generated from optical excitation recombined within less than Ins, thereby suggesting that the triplet (pairs) were recombining geminately rather than decaying through intersystem crossing on a much longer timescale. This, combined with the triplet formation rate, allowed for the unlikely possibility of triplet generation via less than 10-ps intersystem crossing to be eliminated, further confirming the presence of singlet fission in p-BDT-TDOl .

[0148] To confirm that the observed singlet fission was intramolecular, BDT-TD02 was subjected to a solution concentration dependence. The steady state absorption spectrum, SF yield, and SF triplet lifetime were found to be independent of chromophore concentration over more than an order of magnitude in concentration variation. The absorption spectrum concentration independence suggested that significant aggregate formation was not occurring, at least to the extent that no significant chromophore-chromophore interactions were observed in the steady state optical properties. This supported that the studied solutions were fully dispersed rather than aggregated. In addition, SF yield and triplet lifetime were also observed as independent of chromophore concentration, which allowed for the possibility that diffusional intermolecular SF could be eliminated. This was further supported by the sub- picosecond (faster than diffusion-limited) SF timescale. These combined measurements verified that SF was intramolecular, and ruled out both potential intermolecular SF mechanisms.

[0149] Once the protocol for identifying intramolecular singlet fission was established, it was used on a variety of structures, including oligomers, to probe important design criteria for efficient fission materials. Notably, an oligomer consisting of two strong acceptors and one donor molecule was sufficient for introducing a measurable single fission yield. Though the overall efficiency was lower than in the p-BDT-TDO l polymer system, these studies suggested that this was not an inherent limitation of a small molecule system since the fast deactivation processes that lead to an overall smaller yield in the oligomers was also found in another polymer with multiple consecutive strong acceptor units, p-BDT- TD02.

[0150] The photophysical and electronic material properties are listed for the TDO- containing materials studied here. Optical bandgaps were determined with the linear absorption spectra (FIG. 6). The reduction and oxidation potentials were determined with cyclic voltammetry. The SF yield and lifetimes were determined by exponential fitting of TA bleach recovery kinetics, except for BDT-TDQ that required global analysis to deconvolute populations.

TABLE 1 : MATERIAL PROPERTIES

Bandgap,

Ered Eox

Material optical

(eV) (eV) (ps)

(eV)

BDT-TDOi 1.82 -5.5 -4.1 0.2 - 0.3 75 + 15

BDT-TD0₂ 1.62 -5.8 -4.0 0.3 - 0.7 13 + 8 Bandgap,

Ered Eox

Material optical

(eV) (eV) (ps)

(eV) p-BDT-TDO_! 1.75 -5.7 -3.8 1.7- 1.75 80 + 20 p-BDT-TD0₂ 1.50 -5.5 -3.7 0.5 - 0.6 23 + 3

Details of the Deactivation Process and its Relationship to Fission

[0151] Modification of the donor-acceptor interactions may not only affect the criteria for fission, but also the competing decay pathways that can lower the overall yield. The highest singlet fission in p-BDT-TDO 1 was observed, which showed a triplet quantum yield 1.75, based upon the bleach recovery component associated with the triplet-triplet recombination. Although singlet fission was also observed in three other TDO-containing materials presented above, an overall lower SF yield was measured due to the introduction of an efficient non-radiative relaxation process out of the singlet exciton state. Generally, BDT- TDOl, BDT-TD02, and p-BDT-TD02 showed repopulation of the ground state on two time scales: the majority of carriers underwent internal conversion within the first picosecond, and a smaller population decayed in several tens of picoseconds. The fast decay component was ascribed to an S2→S1→S0 sequential internal conversion process that replenished the ground state, and proposed that the initial S2→S1 internal conversion occurred within the approximately 100 fs instrument response of the TA system, and the decay observed in the TA data is the S1→S0 decay. The proceeding decay of the SI state iwass associated with an nIR induced absorption feature that was assigned as an Sl→Sn transition. The second decay component was assigned to triplet pairs formed by singlet fission and eliminated by triplet- triplet annihilation to replenish the ground state. Similarly to the above results for p-BDT- TDO 1, this pathway was spectrally distinct; it was associated with a visible induced absorption feature with a tail that extended into the nIR (FIGS. 7C and 7D lines). This spectral feature was assigned to a triplet transition (Tl→Tn), in agreement with PRTT results (FIGS. 7C and 7D circles). This state formed within the 100-fs instrument response and persisted for several tens of picoseconds in both molecular systems and p-BDT-TD02. While the initial S2→S1 internal conversion process was not directly observed, it was clear that it was in competition with the (also instrument response limited) SF process. Additionally, the lack of a rise in the triplet population after 100-fs confirmed that SF originating from the SI state was not occurring. Similar results were observed in BDT- TDOl, BDT-TD02, and p-BDT-TD02, where the SF yield was limited by competing internal conversion.

[0152] This ultrafast IC process can be qualitatively explained with consideration of the electronic structure of the TDO subunit. While thiophene acted as a typical aromatic moiety, TDO did not. The oxidation of a thiophene monomer resulted in a transition from a 6π→4π electron system. To an approximation, the TDO subunit was an electronic analogue of cis-butadiene (or cyclopentadiene). As a result, TDO-containing monomers and oligomers possessed electronic and photophysical properties similar to linear polyenes, like the carotenoids. The ultrafast deactivation in TDO systems was well-explained within this context. Polyenes are known for their fast recovery following excitation. The relaxation process generally occurred via two sequential internal conversion processes. The excitation formed a singlet excited state (S2, B lu symmetry), which quickly internally converted to a lower lying dark state (S I, Alg symmetry), and finally to the ground state (SO, Alg symmetry). These fast internal conversion processes were well studied within the context of the carotenoid family of polyenes, as well as polyene-containing polymers. The first internal conversion process generally occurred within a range of tens to hundreds of femtoseconds. The second internal conversion process repopulated the ground state within picoseconds to tens of picoseconds. Some TDO-containing materials studied here behaved in a qualitatively similar fashion.

[0153] The observation of singlet fission in this family of new materials provided insight into the mechanistic underpinnings of singlet fission in systems with strong intra- chain donor-acceptor interactions and allowed the establishment of the design criteria for new materials. For example, the absence of internal conversion in p-BDT-TDOl demonstrated that the formation of the "dark" singlet state observed in the other material (along with the competing fast internal conversion pathway) was not inherent to SF-sensitized copolymers or TDO-containing systems. The materials presented here provide empirical insight into the control of IC. Within the polymer systems, it was observed that a single TDO subunit could efficiently act to induce singlet fission, but multiple sequential TDO-units were necessary for formation of the detrimental polyene-like SI |Alg dark state. As a result, caution was used when using multiple sequential TDO subunits as they may induce alternative singlet deactivation pathways. The relevant parameters for control of the IC process in the molecular systems presented here were less clear, as molecular systems with both one and two sequential TDO subunits showed fast IC in competition with SF.

[0154] Based upon the above materials characterization and discussion, the singlet fission process in donor-acceptor copolymers was proposed to be a charge transfer mediated process, similar to what has been observed in molecular SF systems. Within this context, the SF process was divided into two sequential charge transfer events. For molecular aggregates, the first charge transfer process began with a singlet exciton and resulted in neighboring cationic and anionic molecules. This charge transfer was from an electron transfer event, or it was an optically coupled charge transfer event resultant from the lowest lying excitation having some charge -transfer character. This principle also applied in the case of SF in intramolecular D-A materials, though the notation must be refined. In these systems, the lowest-lying electronic excitation was a charge transfer excitation typically denoted with the hole carrier density localized on the 'donor' monomer(s) and the electron density predominately on the 'acceptor' monomer(s). However, the above approximation of the electron and hole wave functions showed a localized electron and a delocalized hole, so the strictly localized Frenkel-type excitonic picture of charge-transfer-mediated SF was perhaps an over simplification. Viewing the SF process through the lens of a charge transfer mediated process was an informative means of understanding the design of singlet fission polymers.

[0155] The introduction of charge transfer character was the predominate mechanism for engineering low bandgap polymers; however, it also served to initiate the SF process. Since the lowest-lying excitation had significant CT character, SF proceeded efficiently via a charge transfer mediated process rather than the much less efficient direct SF mechanism (FIG. la). Within this context, copolymers consisting of strong donor-acceptor moieties served as efficient materials for SF, provided that the energetic requirements for SF were met. The fulfillment of the energetic requirement was dependent on a negative (or slightly positive but thermally accessible) AE_SF, where AE_SF was defined as the difference in the energy of the lowest lying singlet (¾) and twice the triplet energy (E_T). [AE_SF=E_s-2ET]. E_s was defined by the bandgap of the copolymer, as defined by the difference of the EJOMO-DONOR and E_LUMO- ACCEPTOR_. E_s was tuned by varying the selected donor and acceptor moieties. Ideally this should be chosen to match the Es and 2E_T, since this would optimize the SF rate and yield while minimizing the amount of excess energy that is lost to thermalization. ¾^■ was defined by the polymer subunit that would host the triplet excitons. In the materials studied here, this is the TDO subunit. For localized triplet excitons, it was not expected that ¾^■ would vary significantly based upon the donor subunits that were selected for the polymer. However, the triplet energy was dependent on the number of sequential triplet acceptors, though care was taken as detrimental IC processes were introduced with this modification.

[0156] Thus, the present invention provides a family of singlet fission exhibiting materials based on a donor-acceptor copolymer configuration utilizing a bi-functional electron acceptor and singlet fission sensitizer, TDO. Singlet fission was confirmed in two molecular and two polymeric systems, the most efficient of which yielded an up to 1.85 charge carrier pairs per absorbed photon (1.75 triplet excitons and about 0.1 polaron pairs). These systems were generalized to provide a platform for the design of tunable singlet fission capable materials. Efficient singlet fission-capable polymers were constructed by assembling a copolymer of a strong donor and a strong acceptor, where one of the subunits had a triplet energy such that ES-2ET < 0.

[0157] The content of all patents, patent applications, published articles, abstracts, books, reference manuals and abstracts, as cited herein are hereby incorporated by reference in their entireties to more fully describe the state of the art to which the disclosure pertains.

[0158] All of the features disclosed in this specification may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. In particular, the preferred features of the invention are applicable to all aspects of the invention and may be used in any combination. Likewise, features described in non-essential combinations may be used separately (not in combination).

[0159] It will be appreciated that many of the features described above, particularly of the preferred embodiments, are inventive in their own right and not just as part of an embodiment of the present invention. Independent protection may be sought for these features in addition to or alternative to any invention presently claimed.

[0160] It should be understood that the foregoing description is only illustrative of the present disclosure. Various alternatives and modifications can be devised by those skilled in the art without departing from the disclosure. Accordingly, the present disclosure is intended to embrace all such alternatives, modifications and variations that fall within the scope of the appended claims.

Claims

CLAIMS We claim:

1. A polymer, comprising alternating electron donating subunits and electron accepting subunits having the formula [D - A}i, wherein D is the electron donating subunit, A is the electron accepting subunit, and n is an integer, wherein the polymer exhibits efficient singlet fission.

2. A polymer according to Claim 1, wherein the electron donating subunit is a strong electron donor and the electron accepting subunit is a strong electron acceptor.

3. A polymer according to Claim 1, wherein the polymer has the formula [SA-D-SA-WD$, wherein WD is a weak electron donor and SA is a strong electron acceptor.

4. A polymer according to Claim 1, wherein the polymer has the formula [WD-SA-D-SA- WD]«, wherein WD is a weak electron donor and SA is a strong electron acceptor.

5. A polymer according to Claim 1, further comprising at least one spacer between the electron donating subunits and the electron accepting subunit.

6. A polymer according to Claim 5, wherein the polymer has the formula [SD-SP-SA$, wherein SD is a strong electron donor, SP is a spacer, and SA is a strong electron acceptor.

7. A polymer according to Claim 5, wherein the polymer has the formula [SD-SP-SA-SP$, wherein SD is a strong electron donor, SP is a spacer, and SA is a strong electron acceptor.

8. A polymer according to Claim 1, wherein singlet fission efficiency is greater than about 100 %.

9. A polymer according to Claim 8, wherein singlet fission efficiency is about 100 % to about 200 %.

10. A polymer according to Claim 5, wherein the spacer is at least one monomer containing at least one or more pi bonds.

1 1. A polymer having the formula:

wherein "n" is a positive integer, "q" is a positive integer, and "R" may be the same or different and may be selected from: hydrogen, straight or branched chain alkyl of C-20, alkenyl, alkynyl, oligoethylene glycols, and aromatic rings, wherein the polymer exhibits efficient singlet fission.

12. A polymer according to Claim 1 1, wherein n is 1.

13. A polymer according to Claim 1 1, wherein q is greater than or equal to 2.

14. A polymer having the formula:

wherein "m" is a positive integer, "n" is a positive integer, "p" is a positive integer, "q" is a positive integer, and "R" may be the same or different and may be selected from: hydrogen, straight or branched chain alkyl of Q_-2o, alkenyl, alkynyl, oligoethylene glycols, and aromatic rings, wherein the polymer exhibits efficient singlet fission.

15. A polymer according to Claim 14, wherein m is 1.

16. A polymer according to Claim 14, wherein n is 1.

17. A polymer according to Claim 14, wherein p is 1.

18. A polymer according to Claim 14, wherein q is greater than or equal to 2.

19. Use of the polymer according to any one of Claims 1-18 as a multi-functional layer in a photovoltaic device.

20. A device comprising the polymer according to any one of Claims 1-18, wherein the polymer forms a multi-functional layer and the device has a singlet fission efficiency of greater than 25%.