In this presentation, we will review our latest advances in the theoretical description and understanding of the electronic and optical processes taking place in bulk-heterojunction solar cells. In particular, we will discuss how an approach combining molecular mechanics and molecular dynamics simulations, quantum-mechanical calculations, and structural determinations based on grazing-incidence X-ray diffraction and NMR studies, can provide detailed information on the electron-transfer processes taking place at the electron donor - electron acceptor interface.

The material class of dicyanovinyl end-capped oligothiophenes (DCV-nT) is highly suitable for investigating energy and charge transfer processes in donor-acceptor blends in a systematical way because of its tunability of electronic and morphological properties by varying e.g. length of backbone or side chains. Moreover, it has been shown to work very well as donor material in small molecule organic solar cells (OSC) achieving up to 7.0% power conversion efficiency (PCE) for a DCV5T derivative already in a non-optimized bulk heterojunction (BHJ) device (1). Despite this and other impressive achievements, the processes mediating the generation of free charge carriers from the initial excitation and how these processes can be influenced is still under debate. We use photoinduced absorption spectroscopy (PIA) to probe the long-living (µs-ms) excited species present after photoexcitation of DCVnT:C60 blends as they are used in our devices. With PIA, the generation and recombination behaviour of triplet excitons and cations (polarons) is investigated. Measurements were conducted using several DCV4T and DCV6T derivatives with small variations of the side chains. We herein investigate the impact of temperature on the generation efficiency of charge carriers in those blend films. Our first results indicate that the generation rate of cations (polarons) in these materials is strongly increased with increasing temperature whereas their lifetime decreases by more than one order of magnitude from 10K to room temperature. Furthermore, the generation efficiency of triplet excitons is also temperature dependent, which will be discussed in this presentation. Additionally, we will present first results on field-dependent PIA measurements on semi-transparent devices and discuss the changes in the recombination behavior of cations induced by different bias voltages. [(1) Fitzner et al., J. Am. Chem. Soc., submitted (2011)]

A 5,11-conjugation extended anthradithiophene-containing polymer (oADT-dTDPP) is synthesized through in situ desilylation and acetylenic coupling of a bistrimethylsilylethynyl-monomer. The resulting polymer exhibits a largely red-shifted absorption onset, leading to a low bandgap of less than 1.4 eV, owing to ADTâ?Ts additional conjugation orthogonal to the backbone. Compared to previous amorphous 2,8-conjugation extended ADT-cyclopentadithiophene systems, oADT-dTDPP exhibits order in thin film, forming lamellar structures out of the substrate plane. As a result, it exhibits field-effect hole mobilities, on the order of 0.1 cm2 V-1 s-1, a tenth to hundredth fold improvement as compared to previous acene-containing polymer systems.

Conjugated polymers owe their solution-processability to alkyl substituents appended to the polymer backbone. Unfortunately, these substituents are inherently insulting and do not contribute to the overall charge transport mechanism in thin film devices. In addition, the size and shape (branched or linear) of the solubilizing groups can influence the structural order of the resulting thin film. Therefore, a compromise needs to be found that maintains processability while minimizing insulating alkyl content. Recently, the incorporation of furan(s) along the backbone of diketo pyrrolo-pyrrole (DPP) polymers was shown to increase polymer solubility, allowing the use of relatively short branched side-chains while maintaining material solubility. In this report, we apply this concept to polymer field effect transistors using furan containing DPP polymers with shorter branched and linear n-alkyl side chain substituents. This was found to shorten Ï?-Ï? stacking distances between polymer backbones and improve overall structural order in the resulting films. Field effect hole mobilities beyond 1.5 cm2/Vs are reported.

Molecular orientation in organic crystalline films is mirrored in their electrical properties, with a high degree of order corresponding to a superior mobility. Short intermolecular distances and parallel planar molecular orientations allow for a good overlap between the Ï?-orbitals of neighboring molecules and are beneficial for charge transport. However, achieving long-range order is challenged by an inadequate knowledge of the factors that determine molecular orientation on various surfaces and the lack of practical design methods to control this order. We show that the microstructure in organic thin-films can be controlled by manipulating halogen-halogen interactions between the organic semiconductor and the self-assembled monolayers (SAMs) present at contact and dielectric surfaces. The organic semiconductors studied here are pentacene and anthradithiopene derivatives with various alkyl substituents and with backbones consisting of 5, 6 or 7 rings. Films are deposited from solution by spin-coating, spray-deposition and solvent-assisted crystallization, for comparison. We selectively choose the surface treatments to introduce targeted interactions during deposition of the organic films, and to isolate key effects behind microstructure formation. The aromatic SAMs contain either five halogen atoms or one located at positions 2, 3 or 4 on the benzene rings, respectively. We demonstrate that the halogen interactions can template and drive the self-assembly of the organic semiconductor molecules along specific growth fronts and the strength of this interaction governs molecular alignment within the organic film. As a result, we can precisely control the preferential molecular orientation on a surface and tune the charge carrier mobility from 10^(-3) cm2/Vs to 1cm2/Vs in the same material. The effect of SAM presence on thin-film transistor properties such as mobility, contact resistance, interfacial trap-density and threshold voltage will be discussed, and the results will be correlated with structural data obtained from X-ray diffraction studies. Thin film results will be compared with single crystal data, which will provide a well-defined structure, the ultimate molecular long-range order.

From the fundamental standpoint, organic semiconductors are fascinating as they are neither crystalline nor amorphous and their microstructure plays a central role in governing charge transport. I will show that understanding disorder is the key to determining charge transport mechanism. Using advanced synchrotron-based X-ray characterization techniques we are able to define and measure structural order at different length-scales. We are particularly interested in cumulative disorder (paracrystallinity), where the lattice parameter takes on a Gaussian distribution about its mean value. The disorder parameter g allows us to rank materials quantitatively on a continuous scale, from a perfectly crystalline material (g<1%) to an amorphous one (g>10%). Using disorder as a ranking parameter, I will discuss the differences in transport between small molecule and polymer films as well as their respective inherent limitations and bottlenecks. This work may help devising design rules for new materials with desirable transport properties for polarons and excitons.

A novel vacuum lamination approach to fabrication of high-performance single-crystal organic field-effect transistors has been developed. The non-destructive nature of this method allows a direct comparison of field-effect mobilities achieved with various gate dielectrics using the same single-crystal sample. The method also allows gating delicate systems, such as n-type crystals and SAM-coated surfaces, without perturbation. Ref.: H. T. Yi, Y. Chen, K. Czelen and V. Podzorov, Adv. Mater. (2011).

If discotic liquid crystalline (LC) molecules are alignable over a large area in millimeter-thick films, many interesting applications could emerge. We developed the universal molecular design strategy with "electric-field (E-field) responsive handle" that operates universally for enabling orientation of columnarly assembled extended Ï?-conjugated LC molecules in desired directions over a macroscopic length scale. The unidirectional orientation, once developed by the action of E-field, can be maintained after the E-field is switched off. The "E-field-responsive handle" consists of an aromatic amide bearing paraffinic tails, which works properly in combination with a variety of discotic structural motifs such as triphenylene, hexaphenylbenzene, oligothiophene, tetrathiafluvalene and phthalonitrile. The amide handles incorporated in their columnar assemblies are hydrogen-bonded and align along the direction of an applied E-field, thereby triggering large-area unidirectional columnar orientation.

Two novel acene-based semiconductors were investigated with respect to their performance as n-type materials in organic field-effect transistors. The partially fluorinated acene derivatives were synthesized with different degree of fluorine substitution, one with four- and another with two- fluorine substitutions in high yield. Both materials exhibit high thermal stability with decomposition temperatures above 500 °C. Since both materials are supposed to have an increased electron affinity compared to the non-fluorinated pendant, n-type operation in thin film transistors with gold source and drain contacts were expected. However, thin film transistors (TFTs) based on the twice substituted material show only weak ambipolar transport demonstrating an insufficient fluorination to switch from hole dominated to electron dominated transport. Correspondingly, high performance n-type TFTs have been achieved from the four times fluorinated material with an electron mobility of up to 1 cm2/Vs. This demonstrates that fluorinated acene derivatives are in general excellent n-type semiconductors for applications in complementary circuits. On basis of a complementary inverter employing the four times fluorinated acene derivative based TFTs as n-type transistor and a none-fluorinated analogue based TFTs as p-type transistor the potential of the novel acene derivatives for organic logic circuits is finally demonstrated.

Conjugated polymers and oligomers provide a unique encompassing set of structurally tunable optical, electronic transport, and redox properties that allows their present and potential use in a host of applications which span, field effect transistors, light emitting diodes, solar cells/photodetectors, and electrochromism, along with batteries and supercapacitors. Their properties are controlled by repeat unit, along with macromolecular and solid-state, structure; all dependent on the chemical identity of each system. In this presentation, we will teach how chemistry enables the creation of electron-rich, electron-poor, and donor-acceptor (DA) polymers where a specific property is pushed towards its limit. The flexible synthetic chemistry of dioxythiophene- and dioxypyrrole-based polymers has allowed us to develop highly reversible p-type dopable materials employed in charge storing supercapacitors. Two-band absorption induced by the incorporation of a Donor-Acceptor-Donor (DAD) triad in a conjugated polymer induces long wavelength light collection well into the near infrared for photovoltaic devices. Introducing a new dithienogermole acceptor into a DA polymer, in conjunction with fullerene blend morphology control and solar cell device architecture, have lead to AM1.5 power conversion efficiencies in excess of 7%. Isoindigo-based polymers provide n-type doping characteristics, and its use in DAD molecular systems yields bulk heterojunctions with high morphological reproducibility.

Supercapacitors are electrical energy storage devices combining the high power, rapid switching, and exceptional cycle life of a capacitor with the high-energy density of a battery. Power sources based on supercapacitors are emerging as a preferred option for applications requiring short power pulses, particularly when combined with conventional batteries. In order to maximize capacitance, switching speed, and power, materials for supercapacitors need to incorporate conductive and redox-active materials into high surface area structures. Conducting polymer are well-suited to address these challenges owing to the myriad of synthetic and processing methods which result a in a variety of nanostructures and electrical behaviors. In this research, we evaluate the influence of molecular structural variations on the electrochemical performance of poly(triarylamine-thiophene) electrodes. Polymer electrodes are electrochemically synthesized using monomers with varying thiophene compositions, which controls the length of the thiophene chain between triarylamine centers. We show that the energy density of the polymer electrodes is strongly correlated to the thiophene composition and arrangement. Next, the regularity of the polymer is controlled by incorporating methyl side-groups into the monomer, which influences the electrical and energy storage properties. When methyl groups are incorporated to direct the polymerization of the terminal thiophene moieties at the favorable 2,5-positions, the energy and power density of the polymer electrodes is significantly improved. The design criteria demonstrated for triarylamine-thiophene polymers is used to improve the energy storage properties of these materials and provide insight into the development of new polymer electrode systems.

Transferring the unique properties of individual single-walled carbon nanotubes (SWNTs) to macroscale composites such as fibers and sheets has been stymied by inadequate assembly methods. Here we describe a technique for developing multifunctional SWNT/polymer composite thin films that provides a fundamental engineering basis to bridge the gap between their nano and macroscale properties. Selected polymers are infiltrated into a Mayer rod coated conductive SWNT network to fabricate solar cell transparent conductive electrodes (TCEs), fuel cell membrane electrode assemblies (MEAs), and lithium ion battery electrodes. Our freestanding TCEs have an outstanding optoelectronic performance competing with the best literature reports for SWNTs and root mean square roughness of 3.8 nm, yet are also mechanically robust enough to withstand delamination, a step toward scratch resistance necessary for flexible electronics. We modulate the work function of the carbon nanotube network through doping and use a contact film transfer process to fabricate non-metal flexible organic solar cells. MEAs made from this method show up to four times as high platinum (Pt) utilization when compared to conventional assembly methods, demonstrating our approachâ?Ts ability to integrate ionic conductivity of the polymer with electrical conductivity of the SWNTs at the Pt surface. Our battery anodes, which show reversible capacity of ~850 mAh/g after 15 cycles, demonstrate the integration electrode, current collector and separator to simplify device architecture and decrease overall weight. Each of these applications demonstrates that our technique could maintain the conductivity of SWNT networks and their dispersion within a polymer matrix while simultaneously optimizing key complementary properties of the composite. Here, we lay the foundation for the assembly of nanotubes and nanostructured components (rods, wires, particles, etc.) into macroscopic functional materials cheaply and scalably through our solution-processed technique.

With increasing global energy consumption, efficient energy storage systems are urgently needed. Currently, lithium-ion batteries are prevalent in many of these applications because of their established reliability and high energy density. However, commercial lithium-ion batteries can be limited by cycle life, shelf stability, and safety concerns. For these reasons, much research has focused on alternative cathode materials. We have applied layer-by-layer (LbL) assembly techniques to construct hybrid electrodes containing both polyaniline (PANI) and V2O5 that successfully take advantage of properties of both materials. The advantage of using LbL assembly is that the two materials are highly intermixed. V2O5 has a high capacity and energy density, but a low conductivity. PANI is conductive in its emeraldine salt form, and is also electrochemically active. Together, PANI and V2O5 comprise a synergistic composite electrode with good conductivity and electrochemical performance. Assembly conditions (pH and concentration) were selected to produce films that grew regularly and uniformly. High and low molar mass PANI were both explored as components. PANI/V2O5 LbL films showed exponential growth and linear growth when fabricated using low molar mass PANI and high molar mass PANI, respectively. Using UV-Vis spectroscopy, we found that PANI dominated the electrochromic response. However, the electrochemical response possessed contributions from both PANI and V2O5. The electrochemical performance of this LbL system was dependent on film thickness, composition and the fraction of electrochemically accessible material. The composition was measured using X-ray photoelectron spectroscopy (XPS), and we found greater V2O5 content in LbL films fabricated using high molar mass PANI. Our best-performing films were made from low molar mass PANI had a power density of 387 mW/cm3, a charge storage capacity of 264 mAh/cm3, and an energy density of 783 mWh/cm3. By combining PANI and V2O5 in a working cathode, we have demonstrated the effectiveness of LbL assembly in producing a composite material in which both the organic and inorganic components function together as a whole.

In order to effectively use conducting polymers as energy storage materials, the conducting polymer structure needs to be sufficiently porous. Most conducting polymers suffer from limited penetration of counter-ions into the bulk material, which means that a porous structure is essential to maximise utilisation. In traditional molecular solvents, the more porous structures tend to result from the thiophene derivitives such as poly(3,4-ethylenedioxythiophene) (PEDOT), whereas polymers such as polypyrrole (PPy) tend to grow very densely. In the past in ionic liquid solvents the deposition of even the thiophene derivitives has proven difficult with the resultant layers exhibiting poor kinetics for both the charge and discharge reactions due to much denser growth. We show that two types of polymers (PEDOT and PPy) can be combined as one layer, using a mixed monomer solution in an ionic liquid as the electro-deposition media, to create a superior polymer layer. The co-deposited film is found to exhibit an improved morphology and higher ionic transport, while maintaining a similar specific capacitance to the homo-polymer PPy. The lessons learnt from these experiments have been applied to manufacture prototype flexible fabric-based batteries and supercapacitors utilising these two conducting polymers. In the initial stages of this flexible battery work, the polymerisation was carried out chemically, in molecular solvents. Improvements to the process to make these flexible batteries, however, are being attempted using electro-deposition in ionic liquid media.

Recently, supercapacitors have received considerable attention for high energy density storage devices. Various conductive polymers are being investigated as a supercapacitor electrode material due to their capability for fast switching between redox states with a high degree of electrochemical reversibility, nontoxic nature and the low cost [1]. In this context, poly(3,4-ethylenedioxythiophene) (PEDOT) is being actively researched for application as a supercapacitor electrode because of its high chemical and mechanical stability compared to several other conducting polymers used in the supercapacitor fabrication. Various techniques for PEDOT electrode preparation using the sulphonate doped aqueous emulsions and chemical polymerization or electro-oxidation of 3,4-ethylenedioxythiophene (EDOT) monomer have been reported. These methods offer no control over the electrode structure and morphology which is critical for realizing efficient ionic charge storage. In the present paper, we report on the formation of PEDOT films for the supercapacitor electrodes by a method of galvanic pulsed electro-polymerization of EDOT monomer in an organic medium. Structural studies show that pulse ON time can be effectively used to modify the polymer chain lengths and control the chain defects. The variation in the pulse OFF time, on the other hand, enables the polymer conjugation and orientation. In this work, role of the duty cycle of the current pulses in modification of the PEDOT film morphology is described. Furthermore, with the optimized microstructure an unusually high doping level by the ClO4- ionic dopants was realized as ascertained by the x-ray photoelectron spectroscopy (XPS) investigations. Supercapacitor cells were made in all solid-state configuration with the PEDOT electrodes formed using the ultra-short on- time current pulsed polymerization over transparent conducting glass substrates. The electrolyte is a novel ionic liquid gel polymer electrolyte prepared by immobilizing ionic liquid 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([EMIm]FAP) in (PVdF-HFP). Such supercapacitors show overall capacitance of about 78-85 mF cm-2 (equivalent to a single electrode specific capacitance of 95-100 F g-1 of PEDOT). A steep rising behavior of the impedance curve at low frequencies establishes the capacitive behavior of the cells and a near unity ratio of doping-to-dedoping charge with good reversibility by cyclic voltammetry (CV). This paper also presents investigations of various electrical parameters associated with the bulk properties of electrolytes and electrode-electrolyte interfaces using the a.c. impedance spectroscopy, galvanostatic charge-discharge and CV techniques. [1] G.A. Snook, P. Kao, A.S. Best, J. Power Sources 196 (2011) 1-12.

Polymer cathodes can be prepared by electrochemical oxidation of pyrrole to polypyrrole in solutions of lignin derivatives. The quinone group in the lignosulfonates is used for electron and proton storage and exchange during redox of the interpenetrating polypyrrole/lignosulfonate. The combination of an electroactive polymer and an electroactive biopolymer in an interpenetrating network considerably improves the charge density and capacitance per mass in the mixed materials. The use of renewable and cheap raw materials in polymer electrodes meets the need for low cost, intermittent electrical energy storage, and may be one element in a renewable energy future.

Recently Supercapacitors have emerged as an important energy storage device due to high power density, high cyclability and durability. Recently, we have synthesized G/ruthenium oxide (RuO2)â?"polyaniline (PANI), conducting nanocomposite materials using facile chemical oxidative polymerization approach, and studied extensively the behavior of supercapacitor using novel nanocomposite materials. The G/RuO2â?"PANI has been characterized by using structural techniques e.g. scanning electron microscopy (SEM). transmission electron microscopy (TEM), XRD, Raman spectroscopy, electrical conductivity, respectively. The electrochemical behavior of prepared material has also been characterized using electrochemical techniques e.g. cyclic voltammetry, impedance, and chronopotentiometry. The new G/RuO2â?"PANI tertiary nanocomposite material shows higher conductivity and specific capacitance than G-PANI and RuO2-PANI nanocomposite, throughout the various redox processes during charge/discharge cycles. The G/RuO2â?"PANI films have exhibited low resistivity, tunability and wider potential window and faster charge transfer rates to obtain high specific capacitance for supercapaictor applications. A specific capacitance of 450 to 550 F/g at a current density of 0.1 A/g has been observed for G/RuO2â?"PANI respectively.

In-situ stress measurements of thin films of conducting polymers (CP) have been made in different electrochemical environments using the cantilever beam-bending method1,2, in which the changes in curvature of the substrate can be used to calculate stress and strain. This electrochemically induced strain arises from ion movement as a part of the charge compensation mechanism in CPs, which is the basis for devices such as actuators, sensors and rechargeable polymer batteries3. We demonstrate the real-time measurement of stress evolution of a conducting polymer, polypyrrole (pPy) in different electrochemical environments through the multi-beam optical stress sensor (MOSS) technique, which has been used in thin film stress measurement of a variety of inorganic materials4. This technique employs an array of parallel laser beams and measures the relative changes in the spacing between them, which ensures a minimal sensitivity to vibrations. MOSS is used to track the curvature changes of a polymer/Au/Ti coated silicon wafer, which is used as the working electrode while the reverse uncoated side serves as the reflective surface necessary for optical measurement. This curvature change can be converted into biaxial stress using the Stoney equation. Electrochemically induced stress in pPy doped with different dopants (Indigo carmine, IC, Anthraquinone-2,6-disulfonate and Naphthalene-2,6-disulfonate disodium salts) were studied during film growth and potential cycling using MOSS. Different electrochemical environments have been explored to identify optimal cycling conditions, which can help maximize performance and lifetime of devices based on these materials. The magnitude of the induced stress within pPy[IC] at neutral pH correlated with the radius of the hydrated mobile ion in the order Li+>Na+>K+. A distinct â?~break-inâ? period was observed for the film as a global rise in stress, after which the stress profile became constant over extended periods. Based on the changes in the curvature (tension or compression) of the silicon substrate, we can explain the nature of ion movement occurring as a part of the charge compensation process in pPy. Electrochemical quartz crystal microbalance (EQCM) studies were also conducted on these polymer films to confirm the nature of the mobile ion. Lowering the pH (~0) enabled the quinone based dopants to undergo reversible oxidation and reduction within the range of potential under investigation, resulting in a unique mechanism of induced stress; observable due to the efflux and influx of protons in addition to the movement of the mobile ion. MOSS can thus be used to establish performance boundaries of these materials. Future studies will examine voltage-induced stress in other combinations of CP/dopant/electrolyte. References 1. Q.Pei et al, J.Phys.Chem. 96, 10507 (1992) 2. V.Tabard-Cossa et al, J.Phys.Chem.B. 109, 17531 (2005) 3. H.K. Song et al, Adv. Mater. 18 (2006) 1764 4. E.Chason et al. Surf.Eng. 19, 387 (2003)

In this paper, we report recent progress and future prospects of organic thin-film transistors (TFTs) using self-assembled monolayer gate dielectrics. First, flexible TFTs with thermal stability are fabricated and applied to biomedical sterilization processes. Organic TFTs comprise a dinaphto-[2,3-b:2,3-f]-thieno-[3,2-b]-thiophene channel layer and a single-molecule thick gate dielectric of an alkylphosphonic acid self-assembled monolayer. The TFT characteristics are stable, even after exposure to conditions typically employed for medical sterilization, i.e., heating to 150 {degree sign}C for 20 s in air. As the second topics, threshold voltage of organic TFTs is controlled spatially by stamping method. Tetradecylphosphonic acid and pentadecylfluoro-octadecylphosphonic acid are transferred to form ultrathin layers in different regions on a substrate by stamping. The stamped layers are shown to have equal insulating ability as the dipped method monolayer. The feasibility of the threshold voltage control by stamping is demonstrated by the fabrication of integrated circuits.

Organic semiconducting materials have been investigated because of their applicability to low-cost and large area electronic devices such as RFID tags, and electronic papers. In particular, solution-processable organic semiconducting small molecules have attracted for their property forming single crystalline active layers, which enable us not only to achieve high performance, but also to investigate their intrinsic electrical properties. However, since organic single crystals tend to grow in random direction, it is still challenging to fabricate uniform organic semiconducting layers in dimensions and orientations. Here we use a simple method, capillary force lithography with pre-patterned PDMS stamps, to fabricate highly textured organic semiconducting micro-patterns of the small molecule, dioctylbenzothienobenzothiophene (C8-BTBT). The dimensions of the micro-patterns are controlled by those of the pre-patterned PDMS stamps. The crystal orientations of C8-BTBT line pattern are characterized by phi-scans of X-ray diffraction techniques. In the phi scan of (020), two strong peaks are observed with interval of 180° and FWHM of 13°, which means that the lines grow with a specific orientation. The crytallinity of the single line is investigated by a selective area diffraction of TEM, and it also reveals that the C8-BTBT line preferentially nucleates at the walls of the PDMS stamp and grows inwards. To check the electrical properties of the C8-BTBT line as active layers, bottom-gate and top-contact field effect transistors are fabricated, and the highest mobility is 2.3cm2/Vs.

The functional Ï?-electron materials have been greatly explored for the application to organic semiconducting devices such as organic field-effect transistors (OFETs) and organic photovoltaics (OPVs). Although a wide variety of heteroatom containing aromatic Ï?-electron systems has been developed, to date, comprehensive study on the effect of chalcogen atoms in Ï?-electron system has been rarely investigated. In this work, we focused on the W-shape molecules: dinaphtho[1,2-b:2',1'-d]chalcogenophenes possessing oxygen (DNF-W), sulfur (DNT-W), and selenium (DNS-W) atom, respectively. Computational calculations of W-shape molecules suggest that electron density distribution of the highest occupied molecular orbitals (HOMOs) drastically change depending on the chalcogen atom. Particularly, DNS-W possesses the large orbital coefficient on the bulkier selenium atom in HOMO, which is preferable to achieve high carrier mobility. Inspired by the calculation results, we synthesized these materials and elucidated their packing structure and carrier-transporting property in the FETs. The single crystals of these materials were prepared by physical vapor transport (PVT) technique, so that needle-like crystals of DNF-W and platelet crystals of DNT-W and DNS-W were obtained, respectively. X-ray single crystal analyses revealed that DNT-W and DNS-W with large-radius chalcogen atoms form herringbone packing structure with 2-D isotropic transfer integrals (43â?"71 meV for DNT-W and 46â?"93 meV for DNS-W), whereas DNF-W composed of the only first-row atoms assumes 1-D columnar Ï?-Ï? stacking structure with the anisotropic transfer integrals (84 meV for stacking direction and 8â?"13 meV for transverse direction). To study the charge transporting properties, FETs were fabricated from the three materials. Especially, vacuum deposited thin films based on DNS-W exhibited excellent FET characteristics with the hole carrier mobility approaching 1.0 cm2/Vs due to the considerable transfer integral, for which the performance is superior to those for DNF-W and DNT-W based thin-film FETs. To the best of our knowledge, this is the first time to conduct the comprehensive study on the effect of the chalcogen atom on the molecular orbitals, crystal structures, and FET performances. In the presentation, single crystal FETs of these materials will be also discussed.

Control of molecular orientation is crucially important for the development of high performance organic electronic devices. Particularly, in organic thin-film transistors (OTFTs), charge carrier transport depends strongly on the degree of Ï?-orbital overlap between adjacent molecules. Thus, the best way to increase the carrier mobility is to grow organic molecules with an appropriately preferred orientation of the source to drain electrodes. This means that the optimal Ï?-orbital overlap direction should be aligned parallel to the charge transport direction. However, most organic semiconductors have strong anisotropy of charge transport properties on the ab crystal plane, and thin-film growth techniques in organic electronics lack control of the in-plane orientation of molecules. In this work, we have designed and synthesized novel organic semiconductor based on thienothiophene core, dibenzothiopheno[6,5-b:6â?T,5â?T-f]thieno[3,2-b]thiophene (DTBTT), which have an almost coplanar structure and crystallize into a herringbone arrangement, similar to pentacene. However, the most significant structure feature of the DTBTT crystal packing is the presence of the 3-D network with strongly intermolecular multiple interactions. These interactions facilitate charge carrier transport. Additionally, the intermolecular transfer integral between adjacent molecules in crystal structure of DTBTT are quite similar, which means charge carrier transport can occur to any direction. The DTBTTâ?"based OTFTs on an octadecyltrichlorosilane-modified SiO2/Si substrate exhibit excellent field-effect performance with the highest mobility of 5.9 cm2 Vâ?"1sâ?"1.

We have proposed a new strategy of molecular design for highly ordered smectic (rod-like) liquid crystals exhibiting high charge carrier mobility, synthesized several model materials based on the present molecular design, and characterized their phase transition behaviors by differential scanning calorimetry, texture observation with a polarized microscope, and x-ray diffraction study, and charge carrier transport properties by time-of-flight experiments. We adopted anthracene and benzothienobenzothiophene (BTBT) moiety as a core part and synthesized their derivatives. We clarified that they exhibited a highly ordered smectic mesophase of SmE at a certain temperature range next to the crystallization temperature. We found that the charge carrier transport in the SmE phase was non-dispersive and ambipolar basically. We determined electron and hole mobilities to be 0.2cm²/Vs for the BTBT derivative, while the anthracene derivative exhibited very high hole mobility of 0.3 cm²/Vs. We discuss the availability of the new strategy of molecular design for highly ordered smectic mesophases exhibiting a high carrier mobility, and show their FET application, which showed high thermal-stability up to 150^oC and high mobility over 5cm²/Vs.

Fullerenes are important components in organic bulk heterojunction (BHJ) solar cells as electron acceptors. There is significantly less known about the ordering processes in thin film of solution processable fullerenes than other organic semiconductors. [6,6]-phenyl-C61-butyric acid n-butyl ester (PCBNB) has shown promise as an acceptor in solution-processed small molecule BHJs with a benzoporphrin as the donor. The thermotropic phase behavior of PCBNB thin films was investigated using transmission electron microscopy and x-ray scattering after spin coating from toluene. The as cast structure is amorphous but a highly crystalline simple hexagonal structure with the c axis normal to the substrate is observed for films annealed at 160 °C and 180 °C (PCBNB160 and PCBNB180). The film is disordered again after a 200 °C anneal. The electron diffraction patterns of PCBNB180 and PCBNB160 are nearly identical but the PCBNB160 exhibits many additional superlattice reflections. High angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) images show that the PCBNB180 C60 cages form a hexagonal lattice and are aligned in columns along the c axis whereas in the PCBNB160 at least some, if not all, of the C60 cages are missing every fifth (100) row, giving rise to the superlattice reflections. The missing C60 cages may account for an observed decrease in electron mobility parallel to the film plane of highly crystalline PCBNB160 relative to that of the amorphous as cast PCBNB. The superlattice structure is independent of casting solvent (i.e., chloroform and chlorobenzene) and substrate (i.e., SiO2 and PSS:PEDOT) and can be obtained either on heating the as cast film or on cooling from 200 °C.

We report the synthesis and characterization of novel polycyclic aromatic hydrocarbons bearing three electron-withdrawing imide functionalities. These triimides are modeled after the perylene and naphthalene diimides, which have seen extensive use as n-type materials in organic photovoltaics and field-effect transistors. The confluence of a facile synthesis and strong electron-accepting capability renders the triimides attractive candidates to supplant the expensive fullerene derivatives currently used in photovoltaic cells. Preliminary bulk heterojunction solar cells of poly(3-hexylthiophene) (P3HT):triimide blends have exhibited a power conversion efficiency (PCE) of 0.43%, which exceeds the PCE (ca. 0.18-0.25%) of analogous P3HT:perylene diimide solar cells [1]. Efforts to optimize the PCE of these devices are presently underway. The transport characteristics of the triimides in solution-processed field-effect transistors are being studied and will also be discussed. [1] a) W. S. Shin, H.-H. Jeong, M.-K. Kim, S.-H. Jin, M.-R. Kim, J.-K. Lee, J. W. Lee, Y.-S. Gal, J. Mater. Chem. 2006, 16, 384; b) V. Kamm, G. Battagliarin, I. A. Howard, W. Pisula, A. Mavrinskiy, C. Li, K. Müllen, F. Laquai, Adv. Energy Mater. 2011, 1, 297; c) X. Guo, L. Bu, Y. Zhao, Z. Xie, Y. Geng, L. Wang, Thin Solid Films 2009, 517, 4654.

We exploit the elastic instabilities of polymer surfaces under compressive mechanical stress to generate wrinkles and deep folds with prescribed dimensions and at pre-specified coverage over large areas. These wrinkles and deep folds act as photonic structures; they increase light coupling into and trapping within polymer photovoltaics that are subsequently constructed atop such surface structures. Devices on surfaces comprising wrinkles and folds exhibit a 79% increase in the external quantum efficiency (EQE) in the visible compared to analogous devices constructed on flat surfaces. More significantly, we observe an exponential increase in near-infrared light absorption in these devices. In both experiments and in numerical simulations, we find the presence of wrinkles and deep folds to extend the useful range of energy conversion by > 200 nm for model bulk-heterojunctions comprising poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester, corresponding to a 600% increase in the EQE in the near-infrared where light is otherwise minimally absorbed. Further numerical simulations indicate this enhanced light coupling and trapping phenomenon to be general; improvements are also predicted for polymer photovoltaics comprising low bandgap polymers. While we demonstrate this concept with polymer photovoltaics, the controlled introduction of compressive stress provides a straightforward and economical route to large-scale patterning of photonic structures for flexible opto-electronics.

Solution-processing in organic optoelectronics offers new opportunities for the large-area, low-cost, and printed manufacturing technologies. In general, solubility of conjugated polymers in common organic solvents typically results from the attachment of flexible aliphatic chains as solubilizing groups onto conjugated moieties. The selection of solubilizing groups is an art of balance, as they strongly affect molecular packing, thin-film morphology and hence device performance. Branched or linear alkyl chains represent the majority in the family of solubilizing groups. However, physical properties imposed by such groups are not always beneficial. Surprisingly, little attention has so far been given to designing new solubilizing groups for conjugated polymers, in stark contrast to the tremendous efforts made in search for new conjugated building blocks. In this talk, we will present a novel hybrid solubilizing block and demonstrate its effectiveness as a side-chain in low bandgap polymer semiconductors for the application of organic transistors and photovoltaics. We found that hybrid solubilizing blocks have a very large effect on molecular packing such as a closer Ï?-Ï? stacking distance (3.58â"«), mixed crystallographic texture, and a lager crystalline coherence length, relative to the control polymer with a common aliphatic side-chain, which leads to a high average field-effect mobility of 2.00 cm2V-1s-1 in organic transistors. The maximum mobility was determined to be as high as 2.48 cm2V-1s-1. Furthermore, we studied how this hybrid solubilizing group affects donor/acceptor phase segregation behavior and efficiency in bulk heterojunction organic photovoltaics. We believe this strategy of using hybrid solubilizing groups can be extended and generalized to other conjugated systems.

We report the synthesis and evaluation of new pi-conjugated materials that contain borepin rings. These non-benzenoid systems are electron-deficient on account of the inclusion of tri-coordinate boron and are weakly aromatic suggesting materials that incorporate them could be highly polarizable as would be required for application in electronic devices. We have developed strategies to prepare two distinct types of electron-deficient acene-like scaffolds that are indefinitely stable under ambient conditions and under synthetic conditions necessary for typical chemical manipulations (such as Pd catalyzed cross-couplings). Using halogenated versions of these new scaffolds, the reactivity and the properties of the pi-conjugated oligomers and polymers that result from chemical modification or polymerization will be presented. Recent progress to prepare new borepin based scaffolds will also be reported. References: (1) A. Caruso Jr., M. A. Siegler and J. D. Tovar, â?oSynthesis of functionalizable boron-containing pi-electron molecules that incorporate formally aromatic fused borepin rings,â? in Angewandte Chemie International Edition, 2010 (49) 4213-4217 (DOI: 10.1002/anie.201000411) (2) A. Caruso Jr. and J. D. Tovar, â?oFunctionalized dibenzoborepins as components of small molecule and polymeric pi-conjugated electronic materials,â? in the Journal of Organic Chemistry, 2011 (76) 2227-2239. (DOI: 10.1021/jo2001726) (3) A. Caruso Jr. and J. D. Tovar, â?oConjugated â?oB-entacenes:â? polycyclic aromatics containing two borepin rings,â? in Organic Letters, 2011 (13) 3106-3109. (DOI: 10.1021/ol2010159)

Supramolecular engineering on well-defined surfaces provides access to a multitude of nanoscale architectures, including clusters of distinct symmetry and size. The underlying driving forces that lead to such self-assembled supramolecular structures generally involve both graphoepitaxy and weak directional nonconvalent interactions. Here we will show that a balance between very weak CH/Ï? bonding among the ethyne groups of the phenylacetylene molecules and molecule-surface interactions enables robust supramolecular self-assembly of well-defined â?omagicâ? molecular clusters, with significant degree of electronic delocalization. The choice of phenylacetylene was motivated by an array of possible CH/Ï? interactions that can be found in various relative orientations of the parent molecules. Of these combinations, only direct bonding among the ethyne groups was revealed, and it overcame the intermolecular repulsion that would otherwise be prevalent in 2D geometries of non-polar pi-conjugated molecules [1]. Subsequent supramolecular self-assembly of robust molecular clusters, with an almost perfectly uniform size-distribution, was enabled by the unique multicentric character of the CH/Ï? bonding. Specifically, the ethyne group acted as both electron density donor and proton acceptor, enabling each molecule to participate in multiple CH/Ï? bonding interactions and with several of its neighbors. At the same time, saturation of the coordination shell of the multicentric CH/Ï? bonds among the ethyne groups dictated both the â?omagicâ? shape of the supramolecular assembly, and its epitaxial relation relative to the underlying metal surface support. The involvement of CH/Ï? interactions as a dominant factor in the self-assembly of phenylacetylene was inferred from the extensive analysis of scanning tunneling microscopy images, and supported by density functional theory calculations. Finally, we have established from that despite the weakness of the CH/ Ï? interactions, close proximity of the ethyne groups provides for a significant electronic delocalization within the supramolecular assemblies. We therefore believe that CH/ Ï? interactions offer an attractive tunability via chemical functionalization, and thus a feasible strategy toward rational design of supramolecular structures composed of pi-conjugated hydrocarbons on metal and oxide surfaces, as well as their transition toward extended assemblies, all concomitant with distinct electronic properties. This research was conducted at the Center for Nanophase Materials Sciences (CNMS), which is sponsored at Oak Ridge National Laboratory by the Office of Basic Energy Sciences, U.S. Department of Energy. The computational work was performed using the resources of the CNMS and the National Center for Computational Sciences at Oak Ridge National Laboratory. [1] Q. Li, C. Han, S. Horton, M. Fuentes-Cabrera, B. Sumpter, W. Lu, J. Bernholc, P. Maksymovych, and M. H. Pan, ACS Nano in review (2011).

Molecular doping plays an essential role in small molecule based organic devices like solar cells and light-emitting diodes [1]. It allows the control of the free carrier concentration with the following advantages: active control of Fermi level position for device optimization, increased conductivity in doped electron or hole transport layers that opens the pathway to quasi-ohmic contacts between photoactive layers and external electrodes, and strongly improved charge carrier extraction or injection. This is the basis of many OLEDs and organic solar cells showing record efficiencies. Although many studies have shown that the presence of suitable dopant molecules can shift Fermi levels and increase the conductivity of the material system by orders of magnitude, the exact mechanisms of molecular doping are still far from being fully understood. In the present work, thin layers (30nm) of fullerene C60 are doped, using two different air-stable n-dopants with varying doping concentration. Conductivity and thermovoltage (Seebeck) measurements in vacuum are discussed and the influences of doping ratio and temperature are compared to investigate the nature of the doping process. The n-dopants investigated are the commonly used 3,6-bis(dimethylamino)acridine (acridine orange base, AOB) and a novel 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzoimidazole derivate (R-DMBI), derived from N-DMBI [2]. We find that doping of C60 by R-DMBI results in 10 times higher conductivities when compared to similar AOB doping concentrations. With increasing doping concentration the Seebeck coefficient is found to decrease, indicating a Fermi energy shift towards the transport state. For both dopants and different doping concentrations the energetic difference between the Fermi energy and the transport level is observed and compared to the thermal activation energy of the conductivity. Both energies show the same trend of a strong reduction with increasing doping concentration, confirming an increase of the free charge carrier density. Conductivity and Seebeck measurements are combined to estimate the mobility of the layers. These results are compared to additional C60 samples doped with air-sensitive dopants and show that R-DMBI is a promising and air-stable n-dopant for replacing AOB in the future. References: [1] K. Walzer et al. Chemical Reviews 2007, 107, 1233-71 [2] P. Wei et al. Journal of the American Chemical Society 2010, 132, 8852-3

Research in Organic/Polymeric solar cells is progressing significantly with power conversion efficiency (PCE) approaching 10% in small devices. The basic question is that how much we learned in designing new materials that will further help us to optimize the performances of the solar cell devices. In our recent work, we take an approach that combines organic/polymeric synthesis, physical studies and device optimization to gain deeper understanding in these seemingly complex systems. In this talk, we will present results on development of new materials with excellent performances and discovery of important parameters that affect the solar cell performances. We will discuss in details the effect of local dipole moment on the solar cell PCE, trying to uncover some guidance for further material designs. The take-home message to be delivered is that we need a synergistic approach to solar polymer designs and synthesis.

Acyclic diene metathesis (ADMET) chemistry can be employed to prepare a variety of substituted polythienylenevinylenes (PTVs) which can serve as low bandgap light absorbers in polymer bulk heterojunction solar cells. This talk will describe device results for a variety of PTV homopolymers and copolymers, including a systematic examination of cell performance as a function of molecular weight and copolymer composition. Additional discussion will focus on the role of the diode saturation current (J_o ) in determining open circuit voltage (V_oc) and the dependence of J_o on polymer architecture and solar cell design.

Progress in the field of organic photovoltaics has been rapid in recent years, due to the development of new design paradigm for donor polymers and the development of improved processing techniques to optimize film morphology. In general, donors are designed around fullerene-based acceptors, which have been proven to be impressive counterparts for the top-performing polymer donor systems. We reasoned that optimization of acceptor structures may be the necessary key for further improvements in organic photovoltaic performance. To that end, we recently undertook a survey of electron-deficient pentacene and anthradithiophene derivatives as acceptors (paired against P3HT), to determine which parameters were critical to device performance. We found that even weakly electron-withdrawing groups on these acenes turned them into efficient acceptors with high (> 1.0) open-circuit voltages, and that the device current was strongly correlated with the preferred crystalline habit of the acceptor molecules. We will present the results of very recent studies that have shown the reason for this correlation, as well has provided a design guideline and screening tool for the further enhancement of non-fullerene acceptor performance. The application of these findings to second-generation small-molecule acceptors will then be presented, along with the impact of factors such as donor-acceptor moieties, unresolved chiral alkyl groups, and new crystalline motifs on device performance.

Organic photovoltaic devices that based on bulk heterojunction (BHJ) structured active layer that consists of conjugated polymers as donor and nanometer sized fullerenes as acceptors have made great progress recently. The complicated two-component active layer morphology is due to the fact that the miscibility between the polymers and fullerenes are at minimum and is affected by solvents, temperature and other parameters during processing, resulting in phase-separated polymer rich and fullerene rich domain. The active layer that incorporates phase-separated domains in a BHJ solar cell plays a critical role affecting the device performance because these domains provide not only interfaces for charge separation for photogenerated excitons but also percolation pathways for charge carrier transport to the respective electrodes; the former requires fine dispersion of fullerenes in the polymer due to the small exciton diffusion length in polymer while the latter necessities decent fullerene domain size for forming interpenetrating networks. We report the PBTC12TPD/PC61BM filmâ?Ts morphology with simultaneous synchrotron grazing-incidence small-/wide-angle X-ray scattering (GISAXS/GIWAXS) and the cross-section area morphology with transmission electron microscopy electron energy loss spectroscopy (TEM/EELS) along with C-ratio for the films of PBTC12TPD/ThC61BM. We use processing solvents-chloroform , chlorobenzene , and 1,2-ortho dichlorobenzene for dissolving PBTC12TPD, PC61BM and ThC61BM for obtaining an optimum morphology for the active layer. In particular, we elucidate the difference in the nano-scale top-view and cross-section-view of the active layer morphology of the devices for understanding how the PCE of a device incorporating a PBTC12TPD/ThC61BM (1:1.5, w/w) film processed with DCB as the active layer was improved from 4.2% to 6.2%-a relative increase of 46%-after processed with CF. The samples for the EFTEM and EELS investigations have been prepared similarly to the processing condition used for device fabrication, and so the deduced morphology can be precisely related to the performance of polymer BHJ solar cells.

Control over the ratio of two blocks in a new class of all-conjugated diblock copolymers, poly(3-butylthiophene)-b-poly(3-hexylthiophene) (P3BHT) provides a facile approach to precisely tune the molecular organization and nanoscale morphology in polymer bulk heterojunction (BHJ) solar cells. In stark contrast to the power conversion efficiency, PCE, of 1.08% in poly(3-butylthiophene) (P3BT)/[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) and 3.54% in poly(3-hexylthiophene) (P3HT)/PC71BM solar cells, an attractive, high PCE of 4.02% was achieved in a P3BHT21/PC71BM BHJ device in which the molar ratio of P3BT:P3HT in P3BHT21 was 2:1. The ratio of P3BT and P3HT blocks was found to exert a noteworthy influence on the molecular organization of P3BHT, the film morphology of P3BHT/PC71BM blend, and the final performance of P3BHT/PC71BM photovoltaic devices. This enhanced performance reflected a synergy of finer phase separation of P3BHT21 and PC71BM and the formation of respective percolation networks of electron donor P3BHT and electron acceptor PC71BM. The P3HT block rendered the P3BHT chains favorable chemical compatibility for the diffusion of PC71BM molecules, allowing for finer phase separation between P3BHT crystalline domains and PC71BM domains at the nanoscale and maximizing the interfacial area of P3BHT21/PC71BM for improved charge generation. The P3BT block facilitated the self-assembly of P3BHT chains into sufficient interpenetrating pathways for efficient charge transport and collection. Moreover, a small crystalline domain with a size of 10.4 nm formed in the active layer that is comparable to the exciton diffusion length of most conjugated polymers (~10 nm).

Rapid progress in the production of highly pure, type-controlled semiconducting carbon nanotubes has recently made it possible to exploit these exceptional materials as photoabsorbers in photovoltaic devices. Semiconducting carbon nanotubes have highly attractive properties for photovoltaics, most notably tunable near-infrared (NIR) bandgaps (0.8-1.3 eV), strong optical absorptivity at their bandgap > 10^5 1/cm, exceptional charge transport mobility, and stability to thermal degradation and photo-oxidation. Here, we present on our recent successes in exploiting semiconducting carbon nanotubes as photoabsorbers in polymer-inspired photovoltaic devices.[1-2] In particular, we show that nanotubes form a type-II heterojunction with C60 fullerenes and C60 derivatives with energy offsets sufficient to result in electron transfer from optically excited nanotubes to C60. The internal quantum efficiency for exciton dissociation and charge transfer > 75%, for nanotubes of diameter < 1 nm and gaps > 1 eV. Thus, the nanotube / C60 materials-pair forms a â?odonor/acceptor heterojunctionâ? analogous to polymer solar cells with nanotubes taking on the role of the semiconducting â?opolymerâ?. We have fabricated both planar and blended devices based on this nanotube / C60 materials-pair. We show that the performance of planar devices is determined by the length-scale for exciton energy transfer, which is highly anisotropic in nanotube films and can be as long as 600 nm along the length of a nanotube but only 10 nm in the transverse direction, limited by inter-tube transfer. In blends, the need for long-range exciton energy transport is avoided and the limiting factor becomes charge collection/recombination and controlling the phase separation between the two components. Based on these initial investigations [1-2], we have realized preliminary photovoltaic devices with peak external QE > 20% across 1000-1365 nm and a monochromatic power conversion efficiency of 7% at 1052 nm. In combination with future efforts in controlling the morphology of nanotubes in films and blends, the materials- and device-studies presented here are expected to lead to new classes of high efficiency, stable, solution-processable, carbon-based photovoltaic devices. [1] D. J. Bindl, M.-Y. Wu, M. S. Arnold, Nano Letters (2011). [2] D. J. Bindl, A. S. Brewer, M. S. Arnold, Nano Research (2011).

While the efficiency of bulk heterojunction polymer solar cells increases into the 8-9% range, it is still clear that there is much room for improvement in efficiency, lifetime, and cost-effective synthetic and fabrication approaches. A major obstacle to higher efficiencies is the development of ideal donor-acceptor pairs that are optimal for light harvesting, charge generation and collection, and form stable cocontinuous morphologies. Here, several avenues toward developing optimal donor-acceptor pairs are discussed in the context of binary and ternary blends. As an extension beyond the simple donor-acceptor approach to narrow band gap polymers, a route toward multichromophoric polymers will be discussed. A new family of semi-random hexyl-thiophene based donor-acceptor copolymers was synthesized where the restricted, yet randomized linkage pattern of monomers retains a high degree of structural order in the polymers preserving attractive properties of rr-P3HT while also generating broadband absorption. In many cases, efficiencies exceeding that for P3HT are observed in solar cells. We are also investigating ternary blend solar cells based on two donor components and one acceptor component (or one donor and two acceptors), which have been recognized as a potential route to increase the absorption breadth of a solar cell and consequently the short-circuit current density. Recently, using a three-component system, we demonstrated for the first time that the open-circuit voltage of ternary blend solar cells is composition dependent and can be tuned across the full range defined by the corresponding limiting binary blends without negatively impacting the fill factor or the short circuit current. As a result, with judicious choice of components, the attainable product of short circuit current and open circuit voltage (and by extension the efficiency) in a single-layer ternary blend solar cell could be higher than is achievable with a standard binary blend solar cell. Efforts toward this end will be discussed.

Polymers, small molecules and dendrimers have been used in the two main families of â?~organicâ?T photovoltaic devices, namely dye sensitised and solid-state heterojunction solar cells. In the field of organic semiconductors the term â?~organicâ?T is generally used to include all organic materials as well as organometallic complexes. In both device architectures the devices are essentially excitonic in nature, that is, an exciton is formed before charge separation. Irrespective of the organic solar cell device platform the challenges for device architecture and materials design remain the same - one must absorb as much of the solar spectrum as possible and efficiently separate and transfer the generated charge. These processes require optimisation and careful design of the absorber (donor) and acceptor electronic properties and control of their nanophase behavior. The overall aim is to achieve this optimisation in materials that can be solution processed to create large area devices. In this presentation we will discuss recent progress in applying molecular engineering to optimize the properties of materials that can be used in organic solar cells. We will present the development of our latest non-fullerene acceptor materials: their synthesis, optical and electronic properties, processing, and device performance. We will discuss the relationship between the structure and photon harvesting capacity of the materials. Finally, we shall discuss a paradigm shift in materials design that utilizes the high electron affinity material as the light absorbing chromophore. We show that, upon photoexcitation, low band-gap high electron affinity materials can generate current by accepting an electron from a donor that is in the ground-state. We term this Channel II harvesting and it opens new possibilities for the design of complementary junctions and absorbers.

We have carefully analyzed several of the highest performing semiconducting polymers used in photovoltaic cells, including P3HT, PBTTT, PCDTBT and PBDTTPD, and made many interesting observations. We show how the performance of photovoltaic cells made by blending these polymers with fullerenes depends on factors such as the film thickness, testing conditions, annealing and polymer:fullerene ratio. We present x-ray diffraction data that makes it possible to assess how crystalline the films are and whether or not the fullerenes mix with the polymer at the molecular scale. We have made hole-only diodes by using two high work function electrodes and analyzed the space charge limited current to measure charge carrier mobilities. In some cases we have observed the presence of deep traps. We analyze the internal quantum efficiency of the devices as a function of electric field to determine recombination mechanisms, which vary widely amongst the different polymers. We will connect our morphological characterization with the device results to explain why several of the most studied semiconducting polymers behave very differently when used in solar cells. We have also measured how well the solar cells perform over long periods of time and found that P3HT cells last 3 years while PCDTBT cells last 7 years. We will discuss our current understanding of what causes degradation and how more durable cells could be made.

By developing new materials and advanced processing procedures the power conversion efficiency that has been obtained in single junction polymer:fullerene bulk heterojunction solar cells increased significantly and now approaches 10%. The efficiency can be further improved by using a tandem configuration. Compared to single junctions, tandem cells reduce thermalization losses by absorbing high energy photons in a wide band gap cell, and reduce transmission losses by absorbing low energy photons in a small band gap cell. For a tandem cell the of polymer materials used in the two photoactive layers and the recombination layer that serves to connect the two sub cells electrically are crucial. The photoactive layers must provide high conversion efficiencies, while the recombination interlayer should cause minimal resistive, optical, and energetic losses. Recent advances in this area will be discussed. Based on a new efficient low band gap polymer and an efficient wide band gap material, we will demonstrate solution processed tandem polymer solar cells in a normal polarity configuration with 7% power conversion efficiency. These tandems use a ZnO/PEDOT:PSS recombination layer. With an efficiency of 7.0%, the tandem polymer solar cell performs 20% better than each of the best single junction solar cells. We further show that a solution processed recombination layer consisting of PEDOT:PSS and ZnO nanoparticles can be used to make efficient tandem solar cells with inverted polarity. The inverted tandem cell that comprises reaches a power conversion efficiency of 5.8%, again 20% higher than that of the corresponding single junction inverted cells. The power conversion efficiencies of 7.0% for the normal tandem and of 5.8% for the inverted tandem are among the highest ones reported for this type of solar cells to date. Further advances in efficiency can be expected when polymer materials for more efficient single junction layers become available and when the resistive losses in the recombination contact can be further reduced.

To identify losses responsible for the low short circuit current density (Jsc) in many high open circuit voltage (Voc) organic solar cells we have applied a combination of NIR photoluminescence spectroscopy, photoinduced absorption spectroscopy, and photoinduced electron paramagnetic resonance spectroscopy. We observe the spectral fingerprints and CT state-energies for a series of blended low energy absorbing polymer:fullerene films with tailored frontier orbital energy offsets. Our results suggest rapid intersystem crossing and incomplete quenching of fullerene triplet excitons as an important loss pathway, being distinct from recombination via the polymer triplet as previously proposed for many blends. In blends relying strongly on fullerene absorption to achieve broadband response, this new finding may represent a critical loss mechanism. We propose strategies for kinetically redressing related Jsc losses at high Voc.

The photocurrent generation of organic bulk-heterojunction photo-active layers on laterally patterned cathode and anode electrodes has been studied. Photocarriers originated from exciton dissociation at donor/acceptor interface were transported to electrodes by built-in potential difference along the lateral directions. Photocurrent of lateral bulk heterojunction device was dramatically enhanced by introducing P3HT nanowires with high lateral mobility, resulting in an overall power-conversion efficiency of ~1.0% calculated by illumination area of incident light. The holes could be transported to electrodes by P3HT nanowires up to several micrometers without suffering from recombination. In the active layers of lateral bulk heterojunction, photocarriers near both electrodes and substrates could contribute the generation of photocurrent and other carriers recombine in bulk. For the application of lateral bulk-heterojunction to the power source of opto-electronic device system, multiple devices with series and parallel connection were fabricated.

The solution-processability of a conjugated polymer is traditionally controlled by changing the size and branching of alkyl side-chains appended to the polymer backbone. However, these substituents affect structural order and charge transport properties in thin-film devices. As a result, a balance must typically be established between a polymer's solubility and insulating alkyl side-chain content. Previously, the incorporation of furans along the polymer backbone was shown to greatly enhance polymer solubility, allowing for the use of relatively short branched side-chains while preserving high device efficiency. Here, we demonstrate that furans in the backbone allow for the use of linear n-alkyl side-chains, which promote nanostructural order in alternating furan-thiophene PDPP2FT polymers. Specifically, linear side-chains are shown to shorten polymer backbone Ï?-Ï? stacking distances and increase the correlation lengths of both Ï?-Ï? stacking and lamellar spacing. Solar cells fabricated from these n-alkylâ?"substituted PDPP2FT polymers and the electron acceptor PC71BM exhibit improved power conversion efficiencies reaching 6.5%.

Up-conversion of infrared photons (i.e., photons with energy less than the bandgap of the absorber) is considered an interesting option for enhanced photovoltaic efficiencies as one way of going 'beyond the Shockley-Queisser limit'. The presently known lanthanide-based nanocrystalline up-converters have an extremely weak and narrow absorption around 975 nm, severely limiting any practical use for improving the efficiency of real solar cells. We now report on a simple and viable method to eliminate the limited absorption problem. We show that efficient broadband near-IR up-conversion can thus be obtained. Furthermore, we show that the method allows for tuning the spectral response of the up-converters. This tuning is essential for tailoring the up-conversion spectral response (input and output) with respect to the active layer PV semiconductor(s) bandgap(s).

Optimisation of conjugated aromatic polymers as light absorbing electron donors for solar cells requires a clear understanding of the relationship between molecular structure and both electronic properties and thin film morphology. In this presentation, an optimization process is described for a series of semiconducting polymers based on an electron rich indacenodithiophene aromatic backbone skeleton. The effect of bridging atom, alkyl chain functionalisation and co-repeat unit on the morphology, molecular orbital energy levels, charge carrier mobility and solar cell efficiencies will be illustrated. This is an extremely versatile conjugated unit, having coplanar aromatic ring structure, where the electron density can be manipulated by the choice of bridging group between the rings. The functionality of the bridging group also plays an important role in the polymer solubility, with the out of plane aliphatic chains present in both the carbon and silicon bridge, promoting solubility. This particular polymer conformation however, typically suppresses long range organization and crystallinity, which had been shown to strongly influence charge transport. In many of the polymers discussed, it was possible to combine both high solubility with excellent charge transport properties, even in polymers where there was no observable evidence of polymer crystallinity. The optical bandgap of the polymers can be tuned by the combination of the donating power of the bridging unit and the electron withdrawing nature of co-repeat units, alternating along the polymer backbone. It is possible to shift the absorption into the near infra-red when strong donors and acceptors are utilized.

A series of solution-processable, small-molecule, donor materials based on an architecture consisting of two diketopyrrolopyrrole (DPP) cores with different aromatic Ï?-bridges between the DPP units and different end-capping groups were evaluated. In general, this architecture leads to desirable light absorption and electronic levels for donor materials. Out of the compounds investigated, we find that a material with a dithieno(3,2-b;2',3'-d)silole (SDT) core and 2-benzofuran (BFu) end capping groups lead to the most favorable properties for bulk heterojunction (BHJ) solar cells, capable of generating photocurrent up to 800 nm while producing an open circuit voltage of over 850 mV, indicating a small loss in electrical potential compared to other BHJ systems. Device properties can be greatly improved through the use of solvent additives, and initial attempts to optimize device fabrication have resulted in power conversion efficiencies upwards of 4%.

Any conjugated polymer for photovoltaic applications contains three key constituting components: the conjugated backbone, the side chains and the substituents. While the creative design and synthesis of conjugated backbone has received the greatest amount of attention, which ultimately drives the efficiency of bulk heterojuction (BHJ) solar cells to new record high, the side chains and the substituents have largely been overlooked until recently. We found alkyl side chains influenced the intermolecular interaction (among polymers and between polymers and fullerenes) and related stacking/packing in the solid state, all of which have a large impact on the performance of solar cells. While these side chains do not perturb the electronic and optical properties of related conjugated polymers (if anchored properly to minimize the steric hindrance), the substituents (such as F, O), on the other hand, can fine-tune these properties such as energy levels and band gaps. We recently showed that adding F atoms to the conjugated backbone leads to a higher open circuit voltage (Voc), a higher short circuit current (Jsc) and a better fill factor (FF) of these F-substituted polymers based solar cells than those of their non-fluorinated analogs based ones. In order to de-convolute these intriguing but intertwined influences on photovoltaic properties of polymer solar cells by these side chains and the fluorine substituents, we carried out a systematic study on a series of polymers containing identical conjugated backbone but with different side chains and whether or not with fluorine substituents. Interestingly, polymer with long bulky side chains and fluorine substituents exhibits the largest Voc and a very high Jsc as well as a high FF, resulting in the highest efficiency observed among all polymers, regardless of processing solvent choices (chlorobenzene or dichlorobenzene). The observed differences in Voc, Jsc and FF, depending upon the side chains and fluorine substituents, have been thoroughly investigated via simulation and calculation. Our study clearly indicates that a proper combination of side chains and F substituents on the conjugated backbone is a viable approach to simultaneously obtain large Voc, high Jsc and good FF of the related BHJ devices.

In this paper, we will report a novel synthetic strategy based on a photo cyclodehydrogenation reaction, which has been used to synthesis a group of 2D-fused thiophene units. Such units has been used to construct novel conjugated oligomers and polymers. The applications of these materials in organic electronics will be reported.