Program - Symposium Z: Conjugated Organic Materials for Energy Conversion, Energy Storage, and Charge Transport

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 approach’s 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)–polyaniline (PANI), conducting nanocomposite materials using facile chemical oxidative polymerization approach, and studied extensively the behavior of supercapacitor using novel nanocomposite materials. The G/RuO2–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/RuO2–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/RuO2–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/RuO2–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 ‘break-in” 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)

The polybenzidine has been synthesized by thermal method and characterized by X-ray diffraction and Infrared absorption studies. Further, polybenzidine has been doped with nitrate, sulfate and acetate ions. The electrical conductivity behavior of doped and undoped polybenzidine has been studied for various concentrations of dopants between room temperature and 2000C. All these polymers were found to behave like semiconductors. The activation energies of allt he samples within the semiconducting region are estimated and analysed with the variation in dopant concentration. The results are explained with the effective charge formation due to ion pair association.

Pending Post

Bulk heterojunction polymer solar cells (PSCs) based on blends of conjugated polymers and fullerene derivatives have gained increasing attention as a renewable and clean energy sources since they offer the prospects of lower manufacturing costs, lightweight, solution processability, and flexibility. Significant progress has been made in this field, and the power conversion efficiencies (PCEs) of PSCs have reached 7-8%, primarily due to the development of new low-bandgap polymers and better control of the nanoscale morphology of the interpenetrating electron donor/acceptor networks. Despite considerable progress in this field, PCE of PSCs must be further improved for commercialization. Recently, several classes of low-bandgap polymers by using internal charge transfer (ICT) from an electron-rich unit to an electron deficient moiety within the fundamental repeat unit have been developed to better harvest the terrestrial solar spectrum. In this poster, we report the highly processable, new benzodithiophene-based copolymers. We systematically investigated the synthesis, thermal stability, as well as the optical and electrochemical properties of these polymers. Detailed synthetic scheme, optical, electrochemical, and photovoltaic properties of the copolymers will be presented.

Bulk heterojunction solar cells based on a binary blend of a polymeric donor and a fullerene acceptor have seen rapid improvements in efficiency in recent years, from 2.5% to around 8%. However, the ultimate efficiency of such solar cells appears to be limited to about 10 – 12%. Ternary blend solar cells based on two donor components and one acceptor component (or one donor and two acceptors) have been recognized as a potential route to increase the absorption breadth of a solar cell and consequently the short-circuit current density (J_sc). Recently, using a three component system, we demonstrated for the first time that the open-circuit voltage (V_oc) of ternary blend BHJ 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 (FF) or the J_sc of the solar cells. As a result, with judicious choice of components, the attainable product of J_sc × V_oc (and by extension the efficiency, η = (J_sc × V_oc × FF) / P_in, where P_in is the intensity of the incident light) in a single layer ternary blend solar cell could be higher than is achievable with a standard binary blend solar cell. This approach will be discussed in the context of solar cells with PC₆₁BM as the acceptor and combinations of conjugated polymers as the donors.

Organic solar cells have been a subject of growing research interest as they promise to be low cost, lightweight, flexible and easy to incorporate into existing infrastructure. Published champion level efficiencies are around 8% but it is generally agreed that further increases in efficiency will be required before these organic solar cells can become competitive with their inorganic counterparts. A new family of semi-random hexyl-thiophene based donor-acceptor copolymers was synthesized. The restricted linkage pattern of monomers retains a high degree of structural order in the polymers preserving attractive properties of rr-P3HT while also taking advantage of the multichromophoric nature of random polymers, which allows broad spectral absorption of light. Stille-polymerization was used to synthesize six novel donor-acceptor polymers containing benzothiadiazole, thienopyrazine or diketopyrrolopyrrole as acceptors and hexyl-thiophene as the common donor. All semi-random polymers show considerably broadened absorption (up to 1000 nm) while retaining a semicrystalline morphology and hole mobilities matching P3HT. Diketopyrrolopyrrole based semi-random polymers show efficiencies between 3.6 and 4.9%, exceeding the efficiency of P3HT based solar cells. These results show that the semi-random approach to donor-acceptor copolymers is a very attractive route to effective polymers for solar cells which also benefits from a modular and straightforward synthesis.

Conjugated polymers are promising electrode materials for light-weight, flexible, organic energy storage. They can be produced and processed continuously and cheaply from a wide variety of starting materials. Chemical and electrochemical synthetic techniques can be employed to modify polymers at the molecular level, enabling fine-tuning of specific properties like solubility, electrochemical potentials, and mechanical flexibility. The development of new materials with enhanced stability, cyclability, and response is essential to developing viable polymeric energy storage. For this purpose, we have synthesized and characterized novel dithienopyrrole conjugated polymers (polyDTPs) and studied their electrochemical storage performance as electrodes in non-aqueous electrochemical cells. PolyDTPs have very stable oxidized states and are not widely explored as materials for energy storage. More commonly, polyDTP’s have been explored as donor-acceptor polymers for solar cells. However, one challenge is improving their solution-processability. Ultimately, it is desired to design water-processable polyDTP’s so as to minimize solvent usage and cost. Using targeted synthetic methods we have produced polyDTPs substituted with a variety of side-chains, allowing us to improve polyDTP’s water- and solvent-processability as well as its electrochemical stability. Each polyDTP is characterized using UV-Vis spectroscopy and standard electrochemical testing (cyclic voltammetry, galvanostatic cycling, impedance spectroscopy) in nonaqueous media. With the development of these new materials, new energy storage media may be realized to achieve flexible organic energy storage.

Control of molecular orientation in semiconducting polymer thin films is very important to achieve high performance in organic electronic devices such as field effect transistors (FETs) and solar cells. In case of the solar cells, face-on orientation, i.e. the planes of π-conjugated backbones are parallel to the substrate surface, could facilitate the vertical charge transport in thin films and contribute to the high photovoltaic performance. However, the semiconducting polymers in thin films generally tend to adopt edge-on orientation, i.e. the planes of π-conjugated backbones are perpendicular to the substrate surface. In this study, we have successfully induced the face-on orientation of a semiconducting polymer, poly(3-hexylthiophene) (P3HT), in thin films by a simple hot press method. Moreover, we have observed significant enhancement of hole mobility in the direction normal to the film plane after the hot press treatment. The P3HT film without press showed distinct peaks of the lamellar spacing in the out-of-plane X-ray diffraction (XRD) measurement, while a peak of π-π stacking was mainly observed in the in-plane measurement, indicating the edge-on orientation of P3HT chains. In contrast, XRD of the P3HT films after the hot press showed that the diffraction peaks of the lamellar spacing in the out-of-plane measurement diminished, while the peaks of the lamellar spacing appeared in the in-plane measurement. This result clearly indicates that the simple hot press of the polymer films changes the orientation of P3HT chain from the edge-on to the face-on orientations in the films. Next, we have investigated the effect of the molecular orientation in the films on the charge mobility. Time of flight (TOF) measurements of P3HT films with and without the hot press were performed. As a result, the charge mobility of the hot-pressed P3HT film that adopted the face-on orientation in the films was twofold higher than that of the films with the edge-on orientation. This result indicates that the difference in the molecular orientation affect the vertical charge transport in the films. It is expected that the higher charge mobility contributes to high power conversion efficiency (PCE) in polymer solar cells.

Over the past decade, organic field-effect transistors (OFETs) have attracted significant attention as a potential alternative of prevalent silicon based thin-film transistors (TFTs) for achieving low cost fabrication, easy processing, good mechanical property, flexibility, and so on. Aiming at the realistic transistor mobility, wide range of organic semiconductor materials have been designed and synthesized. In particular, heteroarene-containing fused aromatic systems have been intensively investigated as a backbone unit, due to their high charge carrier mobility and environmental stability. Among various heteroaromatics, nitrogen-atom-containing fused aromatics are one of the most promising materials to ensure easy functionalization and high solubility, which might be of great importance in view of solution-processed device applications. In this presentation, we report on a novel class of nitrogen-atom-containing fused aromatic system, indolo(3,2-b)indole (IDID), for high-performance OFET application. Among different IDID-based molecules synthesized in this work, para-phenylene type Ï€-extended organic semiconductor 4H4TIDID showed the highest p-type field-effect transistor (FET) performance with hole mobility values as high as 0.97 cm² V^-1s^-1 and on/off current ratio higher than 10⁵ in the thermally evaporated device. Moreover, even the simple solution-processed 4H4TIDID thin films showed hole mobility values up to 0.048 cm² V^-1s^-1 without any further annealing treatment.

Ï€-Conjugated polymers are promising for various optoelectronic applications, such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs) and organic photovoltaics (OPVs). In particular, Ï€-conjugated hyperbranched polymers are currently attracting keen interest due to their outstanding solubility, efficient intramolecular energy/charge transfer characteristics, and the most convenient easy one-pot synthetic procedure, when compared with the conventional linear polymer counterparts. In this work, we have newly synthesized benzothiadiazole containing conjugated ortho- and para-linking 1,2,4-hyperbranched polymer (1,2,4-hb-PBT), for OPVs applications. We could clearly observe that the 1,2,4-hb-PBT showed a distinct intramolecular energy transport behavior which is not shown in the conventional meta-linking 1,3,5-hyperbranched polymer (1,3,5-hb-PBT). We could easily fabricate bulk heterojunction OPV devices by blending 1,2,4-hb-PBT or 1,3,5-hb-PBT with [6,6]-phenyl C60 butyric acid methyl ester (PCBM), which showed power conversion efficiencies (PCE) as high as 0.63% and 0.39%, respectively. The larger PCE value of 1,2,4-hb-PBT compared to 1,3,5-hb-PBT is most likely due to the systematic increase in short-circuit current (J_sc) (from 2.89 to 3.85mA/cm²) occurring from structural differences. The significant improvements in the performance of OPVs based on ortho-, para-linking(1,2,4-) hyperbranched structures with respect to other conventional meta-linking(1,3,5-) hyperbranched structures are far greater than expected from decreased band gap. This result is attributed to enhanced inherent energy funneling property from the periphery to the backbone enabling the efficient excited energy transfer in ortho-, para-linking(1,2,4-) hyperbranched polymer which was evidenced by optical (UV/vis absorption and photoluminescence) characterizations.

There is an increasing interest in the design and synthesis of novel conjugated polymers due to their application in electronic and luminescent devices based on organic molecules. Among them, stimuli-responsive conjugated polymers incorporating electroactive units as substituents or backbone units have received considerable attention with a focus on the construction of molecular based devices such as molecular wires and molecular electrogenic sensors. In this respect, azulenes which show unique photophysical and redox properties derived from their unusual π-electron polarization structures can be expected to possess significant potential as building blocks for conjugated polymers. However, examples of polymers containing azulenes are limited due to the difficulty of introducing functional groups to the azulene nucleus. We have recently disclosed the regioselective [6+4]π-cycloaddition reaction of 3-alkyl-2,5-dibromothiophene-S,S-dioxides with fulvene leading to azulenes substituted at the seven-membered ring. In this study, we studied preparation of polyazulenes connected through the seven-membered ring by nickel-mediated Yamamoto-type poly condensation polymerization. Furthermore, we found that iridium-catalyzed direct borylation of C-H bonds of 4,7-dibromoazulenes enabled a boryl group to be introduced into the 5-membered rings of azulene backbones. Their homopolymerization via a palladium-catalyzed Suzuki-Miyaura-type polymerization afforded a novel hyperbranched polyazulenes alternatively linked through both five- and seven-membered rings. Their spectroscopic and electrochemical properties were examined and compared with those of the known 1,3-polyazulenes.

Conjugated polymers are of interest in organic photovoltaics due to their low cost, ease of processability, and flexible design. An intriguing approach to controlling active layer morphology in organic photovoltaics is to utilize conjugated polymer fibrils, consisting of pre-formed crystalline nanowires that can form an interpenetrating network with n-type materials (i.e., fullerene derivatives and quantum dots), with domain sizes on the order of the exciton diffusion length (~10 nm). Fibrils formed from poly(3-hexyl thiophene) (P3HT) are noted for their hole mobilities that are higher than comparable P3HT thin films, but they are not stable to thermal annealing or any of a variety of solvents that are useful for device fabrication. We have developed chemistries for cross-linking P3HT-based fibrils, from diblock copolymer precursors, to afford the desired robust fibrillar structures. We synthesized P3HT by Grignard metathesis (GRIM) polymerization, followed by chain-extension with either tetrahydropyranyl (THP) ether substituted thiophene, or n-propyl bromide substituted thiophene. THP deprotection gave a P3HT-block-poly(3-methanol thiophene) (P3MT) diblock copolymer, while substitution of the alkyl bromides with potassium phthalimide, and deprotection using hydrazine, gave a P3HT-block-poly(3-aminopropyloxymethyl thiophene) (P3AmT) diblock copolymer. Molecular weights of the diblock copolymers ranged from 10-16 kDa, with polydispersity indices (PDI) between 1.2-1.6, as estimated by gel permeation chromatography against polystyrene standards. The polymers were prepared having P3HT as the major block (70-85 mole percent), intended to drive the solution assembly while leaving the hydroxyl or amine functionalized block at the fibril exterior for subsequent chemical transformations and cross-linking. Solvent-induced crystallization of these functionalized polythiophenes was achieved by dissolving ~10 mg of polymer in 1 mL of good solvent (such as chloroform), then titrating in a marginal or poor solvent (such as methylene chloride). Fibril formation was confirmed by UV-vis spectroscopy (appearance of vibronic bands at 530, 560, and 610 nm) and transmission electron microscopy (TEM) (nanowires were observed to have diameters between ~15-25 nm and lengths ranging from ~0.5 to 5 microns). Triethylamine (TEA) and 1,6-diisocyanatohexane were added to fibril solutions, at different concentrations, to cross-link the assemblies covalently. Based on the concentration of fibrils and diisocyanate cross-linker, a range of cross-linked structures was obtained, from dense fibrillar sheets to discrete fibrils of micron length. These fibrils maintained their structural integrity in solvents that dissolved uncross-linked fibrils (such as chloroform, chlorobenzene, toluene) and temperatures that melt standard fibrils (>150 °C). The performance of these materials in the active layer of solar cells is currently under evaluation.

Organic photovoltaic (OPV) devices based on conjugated polymers and fullerene derivatives have attracted strong attention owing to the low-cost, easy fabrication, and mechanical flexibility. Recently, as efficient electron-acceptors, fullerene bisadducts have been developed with high lowest unoccupied molecular orbital (LUMO) energy level to increase open-circuit voltage (V_OC) of the OPV devices. Although significant progresses are achieved by the synthesis of new photovoltaic materials, device optimization and morphology control of bulk heterojunction layers play a crucial role in attaining the high-performance. In contrast to the well-known monoadducts of fullerenes as electron acceptors, fullerene bisadducts showed the reduced tendency for molecular ordering due to the regioisomers with the second addend attached at various positions on the fullerene ring. Consequently, the fullerene bisadducts suffer from inefficient charge dissociation and transport due to the inadequate phase-separation. We present the investigation of the morphology control of the active layers consisting of the polymer:fullerene bisadducts with the processing additives for enhanced photovoltaic performance of the OPV devices. The small amount of the additive added into the active solution assists crystallization of polymers and ordering of bisadduct-fullerenes, hence the facilitated phase-separation increases charge carrier mobilities and recombination lifetimes measured by space-charge limited current model and transient photovoltage technique, respectively. Moreover, we synthesized a fullerene bisadduct with diphenyl moiety as a symmetric second addend (DP-PCBM) and studied with the comparative characterization to the other well-known fullerene bisadducts. The OPV devices with DP-PCBM with the inherently reduced regioisomers showed the high V_OC of 0.84 V. We anticipate our study can contribute to the precise understanding for relationship between the morphology of the active layer and the photovoltaic property of the devices for academic interests as well as the successful commercialization of OPVs.

Nanowire photoconductors can lead to a variety of optoelectronic devices potentially useful in next generation nanoelectronics, optical switches, transceivers, image sensors, intrachip interconnects, solar cells, chemical and biological sensors. In addition to the quantum confinement size-regime, nanowire photoconductors can yield higher light sensitivity than their bulk counterparts due to the large surface-to-volume ratio and small dimensions. In particular, the photogenerated charges of organic single crystalline nanowires with rational molecular design, where a charge-generating moiety and a charge-transporting component are systematically combined, are considered to further enhance field-induced mobility due to their intrinsic defect-free and highly ordered nature. To date, however, only a few studies of photoconductive nanowires based on organic materials have been reported. Herein we report single crystalline nanowire phototransistors based on perylene tetracarboxylic diimides (PTCDIs) which provide high external quantum efficiency under light irradiation and exhibit highly sensitive and reproducible photo-responses. Additionally, we demonstrate the fabrication of multi-component nanowires using organic semiconductors and graphene and their optoelectronic properties. We also investigate photogenerated charge carrier behaviors of organic single crystalline nanowire photoconductors by analyzing charge accumulation and release rates from deep traps.

Over the past two decades, conjugated polymer-based organic electronic devices have attracted a great deal of attention due to their potential for cost-effective solution processing, mechanically flexible devices and light-weight constructions. Although the best performance of conjugated polymers has currently exceeded that of amorphous silicon, conjugated polymers have limitations in the practical applications due to their instability when exposed to air, solvent, or thermal treatment. To overcome these critical instabilities, we have developed an efficient crosslinking strategy in which the introduction of crosslinking bridges in conjugated polymers does not disturb their molecular packing, thereby maintaining the performance after the crosslinking. This on-demand crosslinking mechanism enables production of solvent-resistant organic transistors and thermally stable organic photovoltaics. In the experimental study of OTFT performance, the crosslinked copolymers showed remarkable solvent resistance through photo-crosslinking without decrease in mobility. In addition, they kept their electrical performance for a long time in ambient air. Furthermore, the bulk-heterojunction organic photovoltaics (BHJ OPVs) containing the crosslinked copolymers showed an average efficiency higher than 3.3% after 40 h annealing at an elevated temperature of 150°C, which represents one of the most thermally-stable OPV devices reported to date.

Polymer solar cells (PSCs) are the promising candidates for renewable energy sources due to their low cost, light weight, flexibility and solution-processability. We report on cost-effective and ITO-free PSCs fabricated with the copper (Cu) electrode. The Cu electrode was used as a bottom cathode in inverted PSCs, and each layer above the Cu was all-solution-processed. The work-functions of Cu electrodes were engineered with water-soluble interfacial dipole layer to enhance the device-performance. Highly conductive PEDOT:PSS was used as a transparent top anode, and a photoactive layer was based on P3HT:PCBM. The fabricated PSCs exhibited a power conversion efficiency (PCE) of more than 2.5%, demonstrating PSCs fabricated with Cu electrode have the great potential for cost-effective power generators. This study could be an important approach to the development of all-solution-processable and roll-to-roll PSCs using low-cost Cu electrode as a bottom cathode.

Among the approaches to solar energy conversion, dye-sensitized solar cells (DSCs) have been one of the best potential devices for high power conversion efficiency and low-cost manufacture. In addition, organic dyes are readily synthesized with a desired structure and relatively inexpensive to manufacture. They are associated with an π-extended structural variety that can be adjusted to optimize the absorption spectral properties. In this work, a series of metal-free organic dyes bridged by anthracene-mediated π-conjugations were designed and synthesized as new chromophores. Diphenylamine-thiophene unit in these dyes act as donor, while π(linker)-cyanoarylic acid group act as electron acceptors. Detailed investigations on the relationship between the dye structures, photophysical properties, electrochemical properties, and performances of DSCs will be presented.

Dye-sensitized solar cells (DSSCs) which emerged as a new generation of photovoltaic devices have attracted significant attention and have been studied extensively because of their high efficiency, low cost, and facile fabrication. And the donor-acceptor conjugated system is the fundamental feature for most metal-free organic dyes due to the effective photoinduced intramolecular charge transfer property. In this study, we synthesized novel X-shaped donor–acceptor (D–A) organic dyes for use in DSSCs. Two triphenylamine units in these dyes act as electron donors, while two cyanoacrylic acid groups act as electron acceptors. The dye structures were analyzed by means of DFT theoretical calculation, optical properties and thermal properties. Eventually, DSSCs were elaborated with these new dyes and their photovoltaic performances were investigated precisely.

Because of increasing energy demands, many efforts have been made to develop novel and efficient energy storage platforms. Flexible, organic energy storage is potentially an important area of energy storage because it enables flexible electronics and displays, for example. A variety of conjugated polymers such as polyaniline, polypyrrole, and polyacetylene are promising candidates for our purposes, having properties of both battery and capacitor electrodes. Among them, polyaniline has attracted much attention on account of its good capacity and electrochemical and thermal stability. However, polyaniline’s useful conductive form, emeraldine salt, is not easily processible in water. We desire to use water as a processing medium because it is environmentally friendly and low-cost. Recently, conductive, polymer acid doped polyaniline has been synthesized to improve its water-dispersability by using poly(2-acrylamido-2-methylpropane sulfonic acid) as a templating agent. Polyaniline: poly(2-acrylamido-2-methylpropane sulfonic acid) (PANI:PAAMPSA) was synthesized and characterized by UV-Vis, zeta-potential measurements, FTIR spectroscopy, and XPS. The PANI:PAAMPSA dispersion was quite stable as the emeraldine salt over a wide pH range (1-9.5) in aqueous solution for more than a week. In addition, the synthesized PANI:PAAMPSA bore an overall negative charge. The amount of PANI in PANI:PAAMPSA was determined to be approximately 25 wt% by XPS analysis. PANI:PAAMPSA’s electrochemical energy storage performance was evaluated using a three-electrode cell with nonaqueous electrolytes. PANI:PAAMPSA was found to have near-theoretical capacity and good cyclability over 1000 cycles. Voltages up to 4.5 V vs. Li/Li+ were readily achieved. From our findings, we conclude that PANI:PAAMPSA is an excellent candidate electrode for flexible, organic energy storage.

While nonpolar molecules are often disfavored for the construction of supramolecular crystals due to their extreme weak intermolecular interactions, we, for the first time, observed the large crystal formation of 2-dodecylanthracene from a hot liquid. When some of these crystals are organized around screw dislocation sites, large helix-like hexagonal structures show up. Since the backbone of the building blocks is made of semiconducting acene molecules, this study could have implications to flexible displays, organic field-effect transistors or even making functional nanomaterials via self-assembly.

Resonant soft X-ray scattering(RSOXS) is a complementary tool to existing reciprocal space methods, such as grazing incidence small-angle X-ray scattering, for studying order formation in polymer thin films. In particular, RSOXS can exploit differences in absorption between multiple phases by tuning the X-ray energy to one or more resonance peaks of organic materials containing carbon, oxygen, nitrogen or other atoms. We have examined the structural evolution in poly(3-hexylthiophene) (P3HT)/phenyl-C61-butyric acid methyl ester (PCBM) mixtures, which are often utilized as the active layer in organic solar cells, by tuning the X-rays to resonant absorption energies of carbon and oxygen. At high annealing temperatures, we find that the scattering profiles at the carbon and oxygen edges match, but at lower annealing temperatures the scattering profiles differ. The RSOXS results are consistent with elemental maps obtained through energy-filtered transmission electron microscopy, where we find that annealing conditions can dictate whether two or three phases form in mixtures of P3HT and PCBM. Thus, when two phases are present (pure P3HT and an amorphous mixture of P3HT and PCBM) the scattering contrast at the carbon and oxygen edges is consistent, but when three phases are present (pure P3HT, pure PCBM and a mixture of P3HT and PCBM) tuning the X-ray energy can render differences in contrast between multiple phases.

Organic solar cells have attracted much attention in recent years as an inexpensive pathway to fabricating thin and lightweight devices. Industrial pigments are of particular interest for use in organic solar cells due to their abundant nature, low cost, stability, and ease of synthesis and purification as compared to typical polymeric materials. Several research groups have demonstrated the use of industrial dyes, including quinacridone (QA), diketopyrrolopyrrole (DPP), squaraines, phthalocyanines, and borondipyrromethenes, in organic solar cells, light emitting diodes, and transistors. However, due to their low solubility, most of these devices have been fabricated via high vacuum vapor deposition. Herein, I report our recent efforts on the development of solution processable small molecules based on an industrial pigment, quinacridone (QA), for organic solar cells. A series of soluble quinacridone derivatives were synthesized by incorporating alkyl chains and thiophene units to improve the solubility, light absorption, and charge transporting properties. The effect of these changes in molecular structure on the physical properties and device performance were investigated. Bulk heterojunction organic solar cells containing these QA derivatives as electron donors were fabricated with a maximum efficiency of 2.22 %. In an effort to simplify the multi-step synthesis of soluble QAs, Tert-butoxycaronyl quinacridone (tBOC-QA), a soluble yellow precursor of industrial red pigment QA, was prepared by replacing the H atom of the NH group on QA with a t-BOC group. The tBOC-QA can be solution processed to form uniform thin films and converted from soluble to insoluble through thermal treatment to remove the solubilizing groups. This conversion allows for facile processing of multilayer p-i-n organic solar cells without the use of orthogonal solvents or conventional vapor deposition. The p-i-n devices show higher device performance than their bilayer and bulk heterojunction counterparts, exhibiting power conversion efficiencies as high as 0.83%.

We have investigated two electron donor co-polymers in organic thin-film solar cells. The co-polymers used in this study were based on alternating units of fluorene and thiophene, named as LaPPS 23 (Poly(9,9’-dihexilfluorene-2,7-dii-alt-,5-thiophene) and LaPPS 29 (Poly(9,9 -ndihexil-2,7-fluorenodiilvinylene-alt- 2,5thiophene). The structural difference between these materials is that LaPPS 29 presents a double bond connecting the fluorene and thiophene units whereas LaPPS 23 connects these units by a single bond. Different photo-physical characteristics can be attributed due to this double bond: LaPPS 23 presents maximum absorption peak at 400 nm while LaPPS 29 presents at 470 nm. Electrical transport properties were also affected. These copolymers were studied as active layer in bi-layer devices using vapor-deposited C60 as the electron acceptor. We report results of copolymers films prepared from solutions of chloroform and chlorobenzene. The solvent effect in the device performance can be related to the film morphology which was investigated by Atomic Force Microscopy and indicates interesting results. Bi-layer devices based on LaPPS 23 achieved a AM1.5 power conversion efficiency (PCE) of 1.1% using chloroform as solvent. For devices using chlorobenzene, a PCE of 1.5% was found. In the case of LaPPS 29 an inverted behavior was found: PCE of 1.7% was achieved using chloroform and 0.9% for chlorobenzene. Density functional theory was used to perform analysis of the geometric structure, ground state electronic structure and excited state for both co-polymers.

Conjugated organic materials have attracted great scientific interest due to their superb optical and electronic properties, which make them one of the excellent candidates for the active layer in organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs) and organic lasers (OLs). Since thiophene/phenylene co-oligomers were synthesized[1-3] and their good characterization has recently been confirmed[4, 5], these materials have been proposed to be an active material applicable to optoelectronic devices[6, 7]. It is therefore curious to see how the properties can change and satisfy various demands required in organic optoelectronics, when the thiophene unit is replaced by a similar analogue of furan. Here we report a comparative research of a thiophene/phenylene co-oligomer (2,5-bis(4-biphenylyl)bithiophene: BP2T) and newly synthesized its analog furan/phenylene co-oligomer (2,5-bis(4-biphenylyl)bifuran: BP2F). In laser-induced photoluminescence measurements, both BP2F and BP2T single crystals showed amplified spontaneous emission (ASE) phenomena, which can clearly be evidenced by gain -narrowing peaks at 519 nm and 556 nm, respectively. It should be noted that BP2F single crystal showed smaller threshold (216 μJ/cm2) than that of BP2T (312 μJ/cm2). Characteristics of field-effect transistors employing these single crystals of both materials as the active layer were also measured. BP2T showed ambipolar carrier transport behavior (μhole = 2×10-2 cm2 V-1 s-1; μelectron = 4×10-4 cm2 V-1 s-1), while BP2F only shows p-type unipolar behavior, whose carrier mobility (μhole = 12×10-2 cm2 V-1 s-1) is higher than that of BP2T. The differences between BP2T and BP2F can be interpreted by the higher HOMO and LOMO levels in BP2F as well as a larger gap between them. We will present the future scope on a basis of the present results. [1] H.Yanagi, T. Morikawa, S. Hotta, K. Yase, Adv. Mater., 2001, 13, 313. [2] S. Hotta, H. Kimura, S. A. Lee, T. Tamaki, J. Heterocycl. Chem., 2000, 37, 281. [3] S. Hotta, S. A. Lee, T. Tamaki, J. Heterocycl. Chem., 2000, 37, 25. [4] M. Ichikawa, R. Hibino, M. Inoue, T. Haritani, S. Hotta, T. Koyama and Y. Taniguchi, Adv. Mater., 2003, 15, 213. [5] Y. Yoshida, N. Tanigaki, K. Yase and S. Hotta, Adv. Mater., 2000, 12, 1587. [6] S. Z. Bisri, T. Takenobu, Y. Yomogida, H. Shimotani, T. Yamao, S. Hotta and Y. Iwasa, Adv. Funct. Mater., 2009, 19, 1728. [7] Y. Wang, R. Kumashiro, Z. Li, R. Nouchi and K. Tanigaki, Appl. Phys. Lett., 2009, 95, 103306.

Conventional bulk heterojunction solar cells are fabricated with a top cathode which requires a low work function metal for efficient electron extraction and such structures are not viable to large area processing due to the poor stability of the cathode materials. To circumvent this problem, an inverted cell geometry becomes one of the prototypical architectures for roll-to-roll printing processes with a bottom cathode prepared using a modified transparent electrode on a flexible substrate. In the inverted structure, the bottom cathode can be realized by forming a thin film of colloidal zinc oxide (ZnO) nanoparticles (NPs) on an indium tin oxide (ITO) substrate. However, colloidal ZnO NPs are known to have a large density of defects and it has been shown that up to 30% of the atomic bonds in a ZnO nanoparticle are dangling bonds. These defects in the ZnO electron transport layer give rise to loss in photocurrent in photovoltaic (PV) cells and typical inverted polymer PV cells exhibit low power conversion efficiencies (PCEs). It has been shown that a short exposure of devices to ultraviolet (UV) light results in enhanced device performance, which helps to temporarily fill the defect states. However, even this light soaking cannot completely prevent photocurrent lost from surface recombination due to the presence of a large density of defect states in the ZnO NPs film. We recently reported inverted solar cells1 with a power conversion efficiency of 7.3% using a new D-A polymer based on stille polycondensation of a distannyl-dithienogermole (DTG) derivative with 1,3-dibromo-N-octyl-thienopyrrolodione (TPD). Here, we present a simple processing method to enhance the inverted PDTG-TPD: PC71BM PV cells to 8.1%. The defects in ZnO films are passivated by UVO treatment, which is confirmed by the disappearance of defect emission in the photoluminescence spectrum and an increase in carrier lifetime determined from transient photo-current decay measurements. The work function of ZnO was studied by electroabsorption to prove that defects passivation does not change the surface energetic. Similar effects were observed on PV cells based on PDTS-TPD. The EQE of the optimum cell reaches 72%, indicating that inverted PV cells based on PDTG-TPD is promising for photovoltaic applications and defects passivation treatment on ZnO NPs films can significantly enhance the device performance. 1 C.M. Amb, S. Chen, K.R. Graham, J. Subbiah, C. E. Small, F. So. And J.R. Reynolds, JACS,133,10062 (2011)

Tensile drawing of blends comprising a conjugated polymer guest in a thermoplastic host is a versatile technique that can enable a high degree of uniaxial orientation of the guest polymer. Aligned conjugated polymer films produced in this way are of interest for many applications, most notably as emissive polarisers in the next generation of high brightness LCDs. Poly(9,9-dioctylfluorene) (PFO) is particularly suited for such studies due to its efficient pure-blue emission and excellent processability. Several reports on oriented blends of PFO and ultrahigh-molecular-weight polyethylene (UHMWPE) have already appeared in the literature [1,2]. However, the crucial issue of the PFO morphology within these blends has hardly been addressed, occasionally misinterpreted, and remains ambiguous. PFO can adopt several conformational phases; one of these, termed the β-phase, features chain segments with extended planar zigzag geometry. The resulting reduced exciton energy leads to efficient energy transfer to and emission from the extended chains. Due to increased chain linearity PFO has a higher natural anisotropy in the β-phase than in the glassy phase [3]. Previous studies, however, have reported only the glassy phase in drawn blend films. Here we present a study of dilute (≤1 wt%) PFO blends with two polyethylene hosts, namely UHMWPE and linear-low-density PE (LLDPE), that have contrasting molecular weights and degrees of branching. Tensile drawing is performed in two temperature regimes: above and below PFO’s glass transition temperature (T_g). Maximum draw ratios achieved are 100 and 12 for blends with UHMWPE and LLDPE respectively. The resulting films are studied by polarised optical and Raman spectroscopy. Pristine gel-processed UHMWPE/PFO blends contain a moderate β-phase fraction which is removed during drawing above PFO’s T_g but conserved for drawing below T_g. Melt-compounded LLDPE/PFO blends initially have negligible β-phase content; however, interaction with LLDPE branches during tensile drawing induces a small amount of β-phase. In both PE hosts, adoption of β-phase results in characteristically redshifted emission spectra and increased emission dichroism. These findings add to our understanding of PFO morphology in PE matrices and offer the prospect of improved performance photoluminescent polarisers. Ongoing work will focus on further optimising the optical properties of tensile-drawn films by controlling the PFO morphology in ternary UHMWPE/LLDPE/PFO blends. [1] He, B. et al. “Highly polarized blue luminescence from the oriented poly(9,9-dioctylfluorene)/polyethylene blending films” Macromol. 38, 6762 (2005) [2] Knaapila, M. et al. “Concentration effect on the oriented microstructure in tensile drawn polyfluorene-polyethylene blend” Macromol. 43, 299 (2010) [3] Grell, M. et al. “Chain geometry, solution aggregation and enhanced dichroism in the liquid-crystalline conjugated polymer poly(9,9-dioctylfluorene)” Acta Polym. 49, 439 (1998)

We studied the nanoscale structure in pure PCDTBT thin films and the photovoltaic blends with PCBM for bulk heterojunction solar cells. Using spectroscopic ellipsometry and grazing-incidence X-ray scattering, we show that in pure PCDTBT thin films there is a depth-dependent heterogeneity in molecular organization, i.e. molecular order in the form of π-π stacking that is stabilized near the buried substrate and conformational disorder that results from enhanced chain mobility at the film free-surface. We find that the conformational order of PCDTBT reduces upon thermal treatment, which is evidenced by the decreased π-π stacking coherence length between PCDTBT molecules via grazing incidence X-ray scattering and the reduced glass transition temperature of the PCDTBT:PC70BM blend via ellipsometry. We correlate the reduced conformational order with reduced hole-mobility in thermally annealed films, and suggest that this explains the failure of such annealing protocols to substantially improve device-efficiency. Our annealing studies demonstrate that the blend only undergoes coarse phase-separation when annealed at or above 155oC, suggesting a promising degree of morphological stability of PCDTBT:PC70BM blends.

Well-organized periodical stripes of conjugated polymers, including poly(3-hexylthiophene) (P3HT), poly(3-butylthiophene) (P3BT), and poly(3-butylthiophene)-b-poly(3-hexylthiophene) (P3BHT)), were formed by repeated “stick-slip” motion of the contact line in a “cylinder-on-Si” geometry. The interfacial interaction between the solute and the substrate effectively mediate the pattern formation. Subsequently, electrical conductivity was found to be significantly improved by chloroform vapor annealing. This facile, one-step deposition technique based on controlled evaporative self-assembly opens up a new avenue for organizing semicrystalline conjugated polymers into two-dimensional ordered patterns in a simple and controllable manner.

It has been known for more than a decade now that the field-effect mobilities (μFET) of charge carriers in π-stacked polymer organic semiconductor (OSC) films can vary over more than an order of magnitude for different surface modifications of a SiO2 dielectric. This is a surprisingly large effect that is often postulated to arise from a switch between edge-on (i.e., in-plane π-stacking) and face-on (out-of-plane π-stacking) lying-down orientations. Using regioregular poly(3-hexylthiophene) (rrP3HT) as model OSC on SiO2 gate dielectric surfaces terminated by well-defined self-assembled monolayers (SAMs), we show here evidence of a transition interfacial layer at the dielectric surface that is decisively controlled by van der Waals interaction with the surface. This ultrathin “wetting” layer can be face-on, which strongly degrades field-effect charge transport. Atomic force microscopy of ultrathin films reveals directly the occurrence of a significant population of face-on rrP3HT chains which decreases significantly across the series: trimethylsilyl > trifluromethyl > trimethyl ≈ hydroxyl-terminated SiO2, although the bulk and top surface organization of the chains in the thicker films are identical. Concomittantly, charge-modulation spectroscopy in both the infrared and optical spectral regions reveals that the face-on transition layer does not support delocalized field-induced polarons unlike the edge-on π-stacked layer. Finally detailed modeling of the μFET–temperature–hole density surface of the FETs reveals that face-on transition layer has a broaden transport density-of-states width, and Fermi level lies further away from the transport level. Together these provide firm evidence for the existence of an ultrathin transition layer that can have different orientation and packing than in the bulk of the OSC film, which resolves the long-standing mystery of how the interface affects μFET.

The development of Organic Photovoltaics (OPVs) has seen the use of a wide range of complimentary techniques for materials characterisation. Here we report and evaluate the use of helium ion microscopy (HeIM) for imaging the nanoscale structure in samples applicable for OPVs. For a poly(3-hexylthiophene)/[6,6]-phenyl C61-butric acid methyl ester (P3HT/PCBM) blend thin-film subject to a thermal anneal at 140°C, we identify a network structure that is not apparent at the film surface, with slightly elongated PCBM nanodomains. The absence of similar features in blend thin-films subject to different annealing treatments, or correlation between secondary electron yield and variations in surface topography suggest the HeIM is capable at imaging spatial variations in chemical structure with nanometer resolution. The calculated lateral spatial periodicity of the film (20±4nm) is consistent with other studies of this system, demonstrating HeIM as a promising technique for the characterization of organic semiconductor thin-films.

Organic photovoltaic (OPVs) are a promising technology for inexpensive solar applications. These devices, which use a donor and acceptor material to convert photons to electricity, are generally engineered to have bulk heterojunction (BHJ) morphologies. Controlling the morphology is crucial to the performance of the OPV device. One method for controlling the morphology is to use surfactants to promote self-assembled domains of the desired size. Because these surfactants, by their nature, are located at the interfaces between the electron donor (A) and electron acceptor (B) phases, they give us a unique opportunity to engineer the properties of the interface. The interface is important in OPVs because excitons, bound electron-hole pairs, must diffuse to an interface where they can dissociate in order for charges to be generated. If the exciton does not make it to the donor-acceptor interface within its lifetime, its energy will be lost when it relaxes to the ground state. Developing a way to funnel excitons to the interface should enhance device performance. Excitons can jump from one material to another by a Foerster resonance energy transfer (FRET) mechanism; therefore, by manipulating FRET, it is possible to control where excitons go. By placing a FRET energy acceptor (C) at the interface between the A and B phases, the excitons can be preferentially directed to the interface so that more excitons can be harvested within the exciton lifetime. Using a kinetic Monte-Carlo (KMC) simulation of FRET, we have examined several model systems to determine the effect of FRET on exciton harvesting. As a test case, we simulated lamellar A-B phases with and without a C-containing surfactant interfiacial layer for varying lamellar widths and different Foerster transfer radii (R0), which determine the strength of FRET between the energy donors and acceptors. We looked at how the exciton distributions changed, starting with only exciton diffusion, then adding FRET between A and B, and finally including the FRET from A to C and B to C. We evaluated additional morphologies based on domain size, purity, and interfacial width, with and without a FRET energy harvesting C-block at the interface. In order to assess realistic morphologies, we also coupled the KMC simulation with field theoretic simulations of morphologies for systems with surfactants. Using the KMC simulation we picked the best A, B, and C morphologies and R0 values for optimal exciton harvesting.

Despite improvements on the power conversion efficiencies (PCE) of organic photovoltaic (OPV) devices, very little is known about their thermomechanical properties and long term reliability. Our research has demonstrated quantitatively that the polymer/fullerene active layer in bulk heterojunction (BHJ) OPVs is cohesively the weakest for P3HT/PCBM device structures. Internal device cohesion is directly related to high manufacturing yields and improved long-term reliability. We postulate that by tuning the degree of polarity and interface interactions between the polymer and fullerene through intercalation and altering processing conditions, greater layer cohesion can be achieved. A range of different polymers and fullerenes as replacements for P3HT/PCBM were selected for this study. Data obtained from XPS of the debonded surfaces indicate that failure still occurs within the polymer/fullerene active layer for these new devices, but some possess much greater cohesion than P3HT/PCBM based devices. Additionally, the effects of composition and annealing conditions on active layer cohesion and device morphology are further examined. Ultimately, we are developing a metrology towards understanding optimal parameters, including molecular design and anneal temperature and time, in order to aid in the realization of OPV electronics with greater reliability and stability.

Donor–acceptor (D-A) type copolymers show great potential for the application as donor material in the active layer of organic solar cells. Such copolymers are designed to offer a more efficient harvesting of the solar spectrum, which is due to more complex repeat units. Their characteristic is the shape of the absorption spectrum exhibiting two prominent bands, but only little is known about the nature of latter as well as the elementary processes following photoexcitation. Solar cells comprised of such D-A copolymers and methanofullerenes yield improved efficiencies of up to 8% [1,2]. Here we present an in-depth study of the excitation dynamics in the donor-acceptor copolymer poly[N-9''-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2thienyl-2',1',3'-benzothiadiazole)] (PCDTBT). By carrying out comparative steady state absorption and photoluminescence (PL) measurements on PCDTBT we assign the two prominent absorption bands to two spatially separated co-monomer units on the polymer chain. An efficient energy transfer process prior to the radiative decay follows excitation of the higher energy band into the ground state resulting in a low energy monomer emission. The relaxation dynamics investigated by time-resolved PL spectroscopy are dominated by a redshift of the spectrum. This energetic relaxation indicates a low crystallinity of the neat copolymer phase. Adding PC70BM results in an efficient quenching of the PL. We find no evidence for a direct decay pathway from the higher energy band of the donor towards the fullerene acceptor. Instead, the internal energy transfer within PCDTBT precedes the charge transfer process, indicating that the energetic gap relevant for processes following this initial relaxation is the gap of the low energy band. Our findings indicate that effective coupling between copolymer building blocks leading to the energy transfer between D-A units and not the pre-separation of the photogenerated singlet excitons governs the photovoltaic performance of the blends. [1] Blouin, N.; Michaud, A.; Leclerc, M. Adv. Mater., 19, 2295-2300, 2007. Park, S. H., Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K. and Heeger, A. J. Nat Photon, 3(5):297–302, 2009 [2] Chen, H.-Y.; Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.; Yu, L.; Wu, Y. and Li, G. Nat Photon, 3(11):649–653, 2009.

Interfaces between different organic materials play a key role in determining organic semiconductor device characteristics. Here we present a physics-based one-dimensional model with the goal of exploring critical microscopic processes at organic/organic interfaces. Specifically, we envision a simple bilayer structure consisting of an electron transport layer (ETL) and a hole transport layer (HTL) with an interface between them. The key features of the model calculations focus on two aspects: (1) the microscopic physical processes at the interface, such as exciton formation, dissociation, and geminate recombination; (2) the treatment of the interface parameters and the discretization method. At the interface an electron on an ETL molecule may interact with a hole on an adjacent HTL molecule and form an exciton on either side. The exciton may subsequently diffuse into the relevant layer and recombine. This process may be important in light-emitting diodes because under forward bias the energy band offsets between the two materials lead to charge accumulation on both sides of the interface. By increasing the recombination rates, we show in the calculation that interface exciton formation increases the current density. Exciton dissociation is the reverse process. The strong effective field at the interface can cause excitons to dissociate into an electron in the ETL and a hole in the HTL. Geminate recombination occurs when the Coulomb interaction between the electron and the hole at the interface generated in the exciton dissociation causes the formation of an intermolecular excited state across the interface (exciplex), which then relaxes to the ground state. The exciton dissociation is the critical process in photovoltaic cells, and it is the source for the photocurrent. Geminate recombination reduces the current under photo-excitation and degrades the efficiency of a photovoltaic device. The relative impacts of the different processes are demonstrated in our calculation by varying the corresponding kinetic coefficients. As it is the aim of this work to explore effects associated with the organic/organic interface, its treatment in the numerical calculations is of critical importance. Instead of an abrupt change in the material parameters we model the interface as a continuous but rather sharp transition from the ETL to the HTL. The gradual change in chemical composition in the numerical code is implemented through a suitable non-uniform discretization grid. The model framework furthermore includes charge injection implemented as non-linear boundary conditions at the contacts, charge carrier transport, and bulk recombination/generation; exciton formation, exciton transport and relaxation, exciplex relaxation, etc. As an example, C60 and tetracene parameters are used for the ETL and HTL materials respectively.

To fabricate interpenetrating networks in bulk heterojunction morphology, P3HT nanowires are formed by solubility induced crystallization using marginal solvent. Bulk heterojunction photoactive layers are fabricated with poly(3-hexylthiophene) (P3HT) nanowires and small nanocrystals and their morphologies are demonstrated with three dimensional transmission electron microscopy (3D-TEM). The crystalline morphology of the photoactive layer containing P3HT nanowires is compared to the morphology only with small nanocrystals from good solvent. Photovoltaic performances are improved as increasing thicknesses of the photoactive layers including P3HT nanowires, resulting in the power conversion efficiency (PCE) up to 4.21 % with a short-circuit current (J sc) of 10.2 mA cm-2 and a fill factor (FF) of 68.9 %, while the active layer only with small nanocrystals present reduced PCE of 1.08 %. The influences of P3HT nanowires on the enhanced photocurrent generation are investigated with the view of photon absorption, exciton diffusion, charge transfer and charge transport, respectively.

We investigated the interface of poly(3-hexylthiophene) (P3HT) and C61-butyric acid methylester (PCBM) by using photoelectron spectroscopy (PES). These are the most widely used materials for bulk heterojunction (BHJ) organic solar cells due to their high efficiency. Study of the BHJ interfaces is difficult because the organic films are typically prepared by spin coating in ambient conditions. This is incompatible with the interface electronic structure probes such as photoelectron spectroscopy, which requires ultrahigh vacuum conditions. In addition, the study of interface requires gradual deposition of thin films that is also incompatible with the spin coating. In this work, we used electrospray vacuum deposition (EVD) technique to deposit P3HT and PCBM in high vacuum conditions, which allows us to form polymer thin films onto ITO substrate in a step-wise manner directly from solutions and enables us to use PES without exposing the sample to the ambient condition. When PCBM was deposited on the P3HT film, the measured HOMO(P3HT) – LUMO(PCBM) offset was about 0.71 eV, which is much close to the Voc of P3HT:PCBM solar cells.

Bulk heterojunction (BHJ) solar cells have been the primary focus of research in organic based devices due to their large junction area that improves excitons quenching and decreases recombination. However, the freed charge carriers in BHJ devices often suffer from poor charge transport to the electrodes. We have characterized the absorption, transport, internal quantum efficiency (IQE) and printability of the effects of planar-diffusive junctions (PDJ) in organic photovoltaic devices. We compared planar-diffusive junction devices with bilayer and BHJ solar cells based on SL05 (a pyrrolyl polymer)-PC70BM PDJ solar cells were fabricated by spin coating a mixture of orthogonal and non-orthogonal solvents, unlike the bilayer counterparts that only used orthogonal solvents. All solar cell architectures were fabricated with equivalent layer thicknesses, 30 nm to 85 nm, in order to characterize the effects PDJs on absorption, transport and IQE. At 30:70 chloroform:1,2 dichlorobenzene solvent ratios for PC70BM solutions, it was found that the integrated IQE increased by 34% compared to the optimized BHJ structure. Electron microscopy is used to analyze the composition of the solar cells as a function of thickness.

PCDTBT has been shown to be a promising polymer for organic photovoltaic devices. However, limited solubility of this material can make its deposition problematic. Here we report on the synthesis and device properties of a series of PCDTBT derivatives that have been designed for improved solubility. The materials synthesized include either additional alkoxy sidegroups or different moieties along the polymer backbone. In particular, we find that when PCDTBT is substituted with octyloxy substituents positioned on the benzothiadiazole unit, solutions (in chloroform) can be prepared at a concentration of up to 20mg/ml. Without such substituents, the solubility of the nominally equivalent PCDTBT is limited to 4mg/ml in chloroform. Cyclic voltammetry measurements of these new polymers indicates a deeper HOMO level that is consistent with measured open circuit voltage (Voc) values in excess of 0.9V; an improvement over PCDTBT where we find Voc limited to 0.82V. The improved solubility and higher Voc values identified in these new derivatives highlight their potential for photovoltaic applications.

Despite significant advances in the design and synthesis of conjugated polymers for use in organic photovoltaic (OPV) devices, they still suffer from a limited absorption range, preventing efficient photon harvesting from the full solar spectrum. One promising solution to this problem is to stack multiple photoactive layers with complementary absorption spectra. While record tandem devices have been reported for OPV devices, there is still tremendous opportunity for improvement. In addition to materials with complementary band gaps, a successful tandem architecture requires the incorporation of a transparent recombination layer between photoactive layers. This work describes the fabrication of tandem devices using novel low band gap donor materials such as c-PCPDTBT and Si-PCPDTBT in the photoactive layer, and TiOx, ZnO, and TBACa2Nb3O10 in the recombination layer. We also seek to increase device efficiency by using more reductive fullerene acceptors to increase the open circuit voltage of each sub-cell. We are exploring a fully solution-processable route to build efficient tandem OPV devices. This is not a trivial endeavor because careful consideration has to be taken on the choice of solvent (i.e. solvent used to deposit subsequent layers has to be innocuous to the underlying layer), on the surface energies of the various layers, and on tailoring the thickness of the various layers for adequate absorption of the solar spectrum and efficient charge collection. This presentation will give experimental insights for the fabrication of tandem OPV devices as these details are still absent in the literature.

Effective characterization of charge transport parameters within bulk heterojunction (BHJ) organic photovoltaic cells (OPV) such as charge carrier mobility, carrier concentration, and electric field profile are of vital importance for understanding how to best harvest photogenerated charges from OPV cells as well as to target ways for increasing the power conversion efficiency of these devices. Traditionally, researchers have employed vertical device structures as they are (or closely resemble) OPV devices. Unfortunately, this structure does not provide researchers with a means to spatially probe the device either electrically or optically along the direction of charge transport. We have developed a lateral organic photovoltaic device (LOPV) device structure which overcomes some of the limitations of the vertical device structure by allowing the measurement of electrical and optical properties along the transport direction of charge carriers. Two terminal photocurrent measurements have confirmed that LOPV devices exhibit space charge limited transport physics under appropriate electrical biasing conditions. Space charge limited currents have been thoroughly reported in vertical OPV devices but have recently been demonstrated in LOPV devices. Spatially resolved photocurrent measurements via local excitation of the active layer have been used determine charge transport parameters for LOPV cells made from P3HT:PCBM and PSBTBT:PCBM. These materials were chosen as a comparison to one another as well as due to the many reports that exist on these materials. From the spatially resolved photocurrent measurements, contrast between areas of high or low local photocurrent can determine the length of the space charge regions adjacent to each of the electrodes. The carrier population in the space charge region is dominated by electrons (holes) near the cathode (anode) with little carrier recombination. In between the two space charge regions, there is a region with symmetric carrier populations and where the recombination rate is approximately equal to the generation rate. Using the spatially resolved photocurrent information and the DC photocurrent measurements, the mobilities of both electrons and holes as well as the generation rate of charge carriers within the active layer can be determined using a previously developed one dimensional model which solves the current density, continuity, and Poisson’s equations as well as simulation results based on this model. This method of measuring both carrier mobilities is one of the few methods that can resolve the mobilities of both electrons and holes even if they are approximately equal.

TIPS-pentacene has attracted wide attention because of its good solubility and high carrier mobility. Morphology control is crucial for achieving reproducible device performance of TIPS-pentacene based organic electronics. In this study, monolayers of various organic molecules are prepared on the substrate by self assembly or the Langmuir-Blodgett technique, which is followed by the deposition of TIPS-pentacene films on top. Interplay between the morphological and electrical properties are investigated by current sensing atomic force microscopy. It is found that the substrate surface passivation has a significant impact on morphological and electrical properties of TIPS-pentacene films.

Storage of optical energy by photo-isomers of light absorbing molecules is a promising approach to capture and store solar radiation over an extended period of time. A good example of such a molecule is azobenzene which changes from the stable trans form to the meta-stable cis form upon absorption of near UV radiation (~3.5 eV). The amount of energy which can be stored, as given by the energy difference between the cis form and the trans form is 0.59 eV per molecule (MRS Bulletin, September 2011, pp 669-70), yielding a storage efficiency of 0.17. In this research, azobenzene molecules connected in a ring configuration were studied by first principle quantum mechanical calculations to investigate whether such configurations could be utilized to obtain more favorable storage properties. Such rings were synthesized by Dr. Norikane and co-workers at AIST and reported in a press release on December 2, 2010 (www.aist.go.jp). Atomic model of the ground state consists of 2 or 3 azobenzene molecules which are in the trans form connected as a ring. Upon optical excitation, one or more of the azobenzene molecules change from trans to cis form. Therefore excited state models include various combinations of trans and cis forms of the molecule, ranging from all trans to all cis. Calculations are carried out using the DFT method with B3LYP hybrid functional and Pople type basis sets augmented with polarization functions. Calculated energy difference between the trans form and the cis form of one azobenzene unit is 0.67 eV which agrees reasonably well with the above mentioned value. For the ring consisting of 2 azobenzene units, the first trans to cis transition increases the energy by 0.56 eV, and the second transition by 0.30 eV. Both of these values indicate a deterioration of the energy storage capability, possibly because two azobenzene units are not adequate to form a good ring. In the case of the ring consisting of 3 azobenzene units, the first trans to cis transition increases the energy by 0.63 eV, the second transition by 0.74 eV, and the third transition by 0.84 eV. Energy storage capability of the azobenzene molecules in the ring increases on average by ~10% as compared to independent molecules. The favorable result obtained with the ring of 3 azobenzenes provides an incentive to explore ring configurations which include more than 3 molecules.

Recently, there has been a renewed interest in the use of small molecules as the donor material in organic photovoltaic (OPV) devices. In the past 3-4 years efficiencies nearing 5% have been demonstrated using 2,4-bis[4-N,N-diisobutylamino)-2,6-dihydroxyphenyl]squarine (SQ) as the donor molecule with C₆₀ as the acceptor molecule. These molecules are particularly interesting systems to study from the perspective of x-ray diffraction as they generate highly crystalline and oriented thin film phases. This allows a much clearer picture of the molecular orientation within the device and therefore betters understanding of exactly why exciton splitting, charge transport and ultimately efficiency differ between donor molecules, as well as between different post-processing techniques for the same molecule. Furthermore, because squaraine based OPVs form efficient devices in the bilayer geometry rather than a bulk heterojunction geometry and because squaraine donor layers are typically very thin, ≈6nm, grazing incidence x-ray diffraction can be used to garner the precise molecular conformation in the bulk as well as at the donor:acceptor interface. We have used in-situ solvent and thermal annealing grazing incidence x-ray diffraction(GIXRD) experiments to understand how altering the squaraine packing, crystallite orientation and crystallite dimensions effect exciton splitting, carrier mobility and overall device efficiency.

The polymer-metal interface is crucial to the performance of organic photovoltaics because it is the site of charge injection and extraction. In this work, we use various phosphonic acid (PA) self-assembled monolayers (SAMs) to modify the transparent conducting oxide work function and observe changes in open-circuit voltage for bulk heterojunctions prepared on different PA SAMs—in some come cases with Voc’s exceeding that obtained with poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS). We explore the origin of improved work function for multiple material combinations, comparing the effects of the contact modification on the interfacial energy-level offsets and carrier recombination rates.

Dye sensitized solar cells typically employ molecular photon absorbers. The photon absorber can be very inexpensive and therefore may lead to new approaches to low cost solar energy conversion. Existing research on this topic shows only a few expensive-rare-earth based dyes with broad-spectrum absorption have the ability to transfer photo-excited electrons to other structures for collection. Most inexpensive dyes show strong absorption only in the near UV region. Few dyes exhibit absorption maxima near 700 nm, the spectral region where the sunlight energy maximum occurs. Consequently, there is a need to find inexpensive broad-spectrum dyes that can withstand prolonged exposure to light and the ambient environment. Good photovoltaic performance in a typical dye cell having a titanium-oxide (TiO2) electrode requires good electronic coupling between the lowest dye excited state, the unoccupied molecular orbital (LUMO), and the Ti 3d orbital in TiO2 (conduction band). Our focus was to examine the photo-absorption and electronic transfer of Saffron, a naturally occurring and relatively inexpensive dye, and contrast its performance with various groups of better-known sensitizers (e.g., anthocyanine) that have measurable near-IR absorption. Saffron shows surprisingly strong photovoltaic effects in the near IR spectrum and good overall performance relative to other naturally occurring dyes. The second part of our study examines the molecular structure-optical absorption relationship to guide the engineering of synthetic sensitizers that have absorption maxima near 700 nm and are capable of being incorporated into solar cells.

We used a wet grinding method to disperse pure metal oxide particles, having their intrinsic electronic and optical properties, prior to the formation of metal oxide films. During the grinding process, large clumps of the particle underwent de-aggregation to form a stable dispersed solution. The dispersed metal oxide solution was readily deposited onto various substrates and, thereafter, integrated into organic optoelectronics as buffer layers. The insertion of metal oxide interfacial layers significantly enhanced the long-term stability of P3HT:PCBM-based solar cells. We suspect that such a metal oxide suspension would also provide appropriate interconnection layers for organic tandem cells.

The materials research in organic semiconductors has been extended from amorphous to ordered materials including polycrystals and liquid crystals. We have modeled and investigated an electric hopping transport in theoretical manner for self-organized molecular systems in comparison with experimental results. At the same time, we discuss the contribution of permanent dipoles oriented to the energetic disorder in ordered molecular systems. The diagonal disorder is frequently discussed on the amorphous organic semiconductors. For the application of solar cells, the diagonal disorder provides one of the dominant contributions for recombination process of hole and electron charge pairs in charge transfer states. Thus our study will provide us the basic concept how we consider the charge transport in organic semiconductors having orientational- and translational- order parameters of molecules. In order to evaluate the contribution dominated by the orientational and translational order parameters, we derive the N-th moment of energy by averaging and by summing up all of Coulomb interaction due to dipole moments in those systems. These models provide us with the energetic disorder as a function of order parameters. Using our model, we predict that the angle between the dipole moment and molecular long axis is key factor for the order parameter dependence of energetic disorder. On the other hand, the molecular structure having negative orientational order parameter increases the diagonal disorder. The present model directly gives us an insight into the effect of ordered molecular alignment and molecular design for getting quality oriented organic semiconductors with high performance.

Bridged triphenylamines (N-Heterotriangulenes) gained much interest due to the increasing demand on p-type semiconductors in flat-panel or flexible display devices based on organic light-emitting diodes (OLEDs) as well as organic field-effect transistors (OFETs) with low power consumption and a high operation speed.[1-5] The herein reported N-heterotriangulene derivatives possess dimethylmethylene bridges in order to facilitate planarity as well as good solubility. Further advantages are the easy functionalization at the three para positions and the possibility of introducing substituents or functional groups at the bridges.[2-5] By controlling the stoichiometry of the halogenation agent, e.g., N-bromosuccinimide (NBS) or N-iodosuccinimide (NIS), the N-heterotriangulene can be selectively mono-, di- or tris-functionalized at the active positions, which opens several synthetic pathways. For instance, results the twofold bromination in a monomer which was successfully polymerized by Yamamoto coupling obtaining a molar mass up to 18,500 g/mol (PDI = 2.3). Application of spin-coated films of this "zig-zag" polymer as active layer in OFETs resulted in a field-effect mobility of μ = 3.5 × 10^-4 cm^2/Vs and an on/off-current ratio of 4 × 10^-3. By creating an A2B-type monomer bearing two bromines and one boronic ester, the synthesis of a hyperbranched polymer was possible with a molar mass up to 33,500 g/mol. The hole-transport application of this 2D-polymer within OLEDs is currently under investigation. Besides that, tris-brominated N-heterotriangulenes were successfully functionalized with either strong electron-donating or –accepting moieties, e.g., thiophene, pyrene, methoxy or cyano groups as potential building blocks for charge-transfer complexes. Selective bromination, iodination as well as borylation allowed furthermore the development of a strategy towards a hexameric macrocycle. The most critical step in the reported synthesis sequence concerned the dimerization of a bromine substituted N-heterotriangulene trimer, which was accomplished by Yamamoto-coupling. Besides standard characterizations, single crystal X-ray analysis as well as STM imaging of the macrocycle will be presented herein together with further functionalization possibilities. [1] S. Wang, M. Kivala, I. Lieberwirth, K. Kirchhoff, X. Feng, W. Pisula, K. Müllen, ChemPhysChem 2011, 12, 1648-1651. [2] Z. Fhang, T.-L. Teo, L. Cai, Y.-H. Lai, A. Samoc, M. Samoc, Org. Lett. 2009, 11, 1-4. [3] Z. Fang, X. Zhang, Y.H. Lai, B. Liu, Chem. Commun. 2009, 920-922. [4] Z. Jiang, T. Ye, C. Yang, D. Yang, M. Zhu, C. Zhong, J. Qin, D. Ma, Chem. Mater. 2011, 23, 771-777. [5] M. Bieri, S. Blankenburg, M. Kivala, C.A. Pignedoli, P. Ruffieux, K. Müllen, R. Fasel, Chem. Commun. 2011, 47, 10239-10241.

Exciton diffusion in organic semiconductor has crucial effects on the performance of organic solar cells. The exciton diffusion lengths of polymer semiconductors have not been extensively studied compared to small molecular semiconductors. Here, we accurately measured the exciton diffusion length of poly(3-hexylthiophene) (P3HT) as a function of crystalline order using spectrally resolved photoluminescence quenching. The extent of crystalline order of P3HT films was varied by using thermal treatments and measured by both X-ray scattering and atomic force microscopy. The extent of crystalline order was characterized by the mean crystal size and increased with the thermal treatments. The exciton diffusion length increases from 3nm to 8nm as the mean crystal size increase from 9nm to 31nm. This increase of exciton diffusion length results from the enhancement of Förster-mediated hopping with an increase in crystalline order. Our results clearly show that the exciton diffusion lengths of P3HT films monotonically increase with crystal size.

PCDTBT is one of the most promising p-type semiconducting materials with high power conversion efficiency up to 4.6% in organic solar cells. The high efficiency is caused by low HOMO level and high hole mobility of the polymer. However, the power conversion efficiency of PCDTBT can be further improved by optimization of the morphology of the photoactive layer. Here we controlled the morphology of the photoactive layer of PCDTBT/PCBM film by end group functionalization of PCDTBT and demonstrated the high efficiency as high as 6.0% in organic solar cells without any post treatments, additives and optical spacers. End group functionalized PCDTBT has clear fibrillar structures in the blend film with PCBM. These fibrillar structures are originated from the increased π-π stacking, which results in remarkable increment in hole mobility and light absorption. The end group modification of semiconducting polymers can be utilized to control the morphology of the photoactive layer for high performance organic solar cells

The polymer solar cells have great attraction on low-cost, flexible, and easy applications to large area devices. Recently, the power conversion efficiencies in the bulk hetero-junction polymer solar cells consisting of new polymer donor and fullerene acceptor blends have been reached close to 8%. But the large area polymer solar cell studies with new low band gap polymers are very limited and most of which is done with P3HT and PCBM blends. In this study, polymer solar cells are fabricated through blade method with donor-acceptor type low band gap polymer. Power conversion efficiency of 5.2% is achieved at 9 mm2 area cell. And each parameter which impact on the efficiency of the devices will be discussed.

Semi-conducting conjugated (co)polymers, exhibit numerous advantages. They are well soluble and can be processed easily in a large scale. Among this materials, rod-coil block copolymers, based on a conjugated rod and a coil block covalently linked, show very particular properties of self-assembly, and are very promising in organic electronic, as organic photovoltaic materials for example.[1] [2] Most of these copolymers can been obtained by using anionic or controlled/living radical polymerization for the synthesis of rod-coil block copolymers. [3] [4] In this contribution we present recent results about the controlled synthesis of end-functionalized poly(3-hexylthiophene) (P3HT) and their use as building blocks for original well-defined semiconducting copolymers. These copolymers are designed for the morphology control in optoelectronic devices, including organic photovoltaic. In this work, P3HT is a model semi-conducting block in order to develop a synthetic “toolbox” for semi-conducting copolymers. Several synthetic routes have been developed (using mainly nitroxyde mediated radical polymerization) in order to obtain pure and well-defined materials for optoelectronic applications. Various P3HT based “rod-coil like” copolymers (with various coil segments: polyacrylates, polyvinylpyridine …) have been synthesized and characterized. We present our recent results about block copolymers and more complex architectures such as grafted and “comb like” copolymers. [1] S. Barrau, T. Heiser, F. Richard, C. Brochon, C. Ngov, K. van de Wetering, G. Hadziioannou, D. V. Anokhin, D. A. Ivanov, Macromolecules 41 (2008), 2701. [2] F. Richard, C. Brochon, N. Leclerc, D. Eckhardt, T. Heiser, G. Hadziioannou, Macromolecular rapid communications, 29 (2008), 885. [3]. Sary, N., Richard, F., Brochon, C., Leclerc, N., Lévêque, P., Audinot, J., Berson, S., Heiser, T., Hadziioannou, G. and Mezzenga, R.,. Advanced Materials, 22(6) (2010) 763-768. [4] Brochon, C., Sary, N., Mezzenga, R., Ngov, C., Richard, F., May, M. and Hadziioannou, G., 2008. Journal of Applied Polymer Science, 110(6), 3664-3670.

We have developed mechanically robust, electrically conductive, free-standing and transparent hybrid nanomembranes made of densified carbon nanotube sheets (CNSs) that were coated with poly(3,4-ethylenedioxythiophene) (PEDOT) using vapor phase polymerization (VPP). We also show electrochemical capacitor performance of prepared nanomembranes needed for ultrathin and transparent supercapacitor applications. The hybrid nanomembranes with thickness of ~66 nm and low areal density of ~15 ug/cm2 consisted of highly aligned PEDOT-coated carbon nanotubes, and the membranes exhibited high mechanical strength and modulus of 135 MPa and 12.6 GPa, respectively. The hybrid nanomembranes recovered their original sheet shape in liquid after collapsed at liquid/air interface unlike previous carbon nanotube sheets. The electrochemical capacitance of the hybrid nanomembrane structure was 8 times higher than that of pristine 2-layer CNSs and reached values of ~62 F/g due to the effective integration of the PEDOT conducting polymer using VPP. The hybrid nanomembrane possessed high energy density of ~39 Wh/kg at 61 oC, and the nanomembrane attached on a glassy carbon showed rectangular shapes of cyclic voltammogram curves in high scan rates ranging from 100 to 1500 mV/s at room temperature. The hybrid nanomembranes will be applicable to sensors, actuators, optical devices, fuel cells as well as electrochemical capacitors.

Abstract Spin-coating a mixture solution of P3HT and PCBM on a cold substrate successfully generated a desirable bulk heterojunction (BHJ). This concept was based on the abrupt decrease in solubility of P3HT as solution temperature decreased. The selective precipitation of P3HT on the PEDOT:PSS-coated cold substrate facilitated a desirable vertical composition. The high crystallinity of P3HT suppressed movement of PCBM during thermal annealing, preventing aggregation of PCBM. The morphological excellence of the pristine film gave a comparable or better power conversion efficiency (PCE) with that made by the conventional spin-coating and thermal annealing. After thermal annealing, the device made via coating on a cold substrate showed ~40% increase in PCE from the BHJ solar cells made by the conventional method.

Poly(3-hexylthiophene) (P3HT)-Graphene nanocomposites were synthesized via in-situ oxidative polymerization, where graphene has been dispersed in a solution of 3-hexylthiophene monomer and chloroform prior to polymerization. The main thrust for this work was to investigate the optoelectronic properties of P3HT-Graphene nanocomposites under different graphene loading concentrations. Cyclic voltammetry was employed to evaluate the HOMO levels of the nanocomposites, while optical spectrophotometry (UV-Vis-NIR) was utilized to determine the optical bandgap of the composites. The information from the aforementioned techniques were used to estimate HOMO-LUMO energy level analysis. The results revealed a significant influence in optical bandgap of P3HT with increasing graphene content but little change in HOMO levels was observed. Furthermore, an extensive study aiming at the effect of graphene content on the optical constants of P3HT was performed using both spectrophotometric and ellipsometric methods. Photoluminescence spectra of the samples were also studied and the results showed no quenching effect of PL emission with increasing graphene content. Our studies indicate that the inclusion of graphene has an impact on the electronic and optical properties of P3HT, which can further benefit the organic device applications like, organic light emitting diodes, organic solar cells, organic field-effect transistors and polymer batteries.

Polyaniline (PANI) is a conductive polymer which has been studied with great interest in recent years due to its wide application range, as well as studying the formation of composites with other materials to obtain better properties. Recently PANI nanocomposites with graphite oxide (GO) and reduced graphite oxide (RGO) have been used for supercapacitor devices due to an increase in their electrochemical performance. In this work, we studied the effect of aqueous dispersion of GO on enzymatic polymerization of PANI and the characterization of resulting composites were studied. Enzymatic polymerization of aniline is an environmentally friendly method, whose polymerization is very different from chemical and electrochemical methods. On the other hand, GO colloidal properties are highly dependent on the average sheet size. Two dispersions with GO and GO nanocolloids (nGO) with lateral size of 12.50 μm and 247 nm respectively were used during the enzymatic oxidation of Aniline. The enzymatic polymerization of PANI was carried out in acidic medium using toluenesulfonic acid (TSA), horseradish peroxide (HRP) and hydrogen peroxide. The polymerization reaction was studied using 1, 2.5 and 5 wt % of GO and nGO dispersions. No changes were observed in enzyme activity in the polymerization reaction of Aniline in presence of GO and nGO dispersions. The morphology of resulting PANI-GO composites was studied by scanning electron microscopy (SEM). PANI colloids were deposited on GO and nGO colloidal particles. The PANI-GO colloids were characterized by ultraviolet–visible spectroscopy and Fourier transformed infrared spectroscopy, while colloidal stability was tested at different pHs. The UV-vis spectroscopy results revealed that the GO and nGO dispersions affect the electronic conjugation of PANI modifying its absorption spectrum. In addition the aggregates of PANI-GO colloids increased with the concentration of GO in the reaction media decreasing its colloidal stability. Finally results of transmission electron microscopy characterization, Raman spectroscopy and X-ray photoelectron spectroscopy analyses will be also discussed.

Despite their high cost, relatively poor absorption properties, and eventual devices that have low Voc, fullerene based materials have been the most commonly used electron acceptor materials for application in organic photovoltaics. In order to address these shortfalls, our group has done substantial work on designing, synthesizing, and testing a number of small molecule alternatives. Such alternatives are especially interesting due to the potential for selectively tuning electronic and physical characteristics such as energy levels that lead to higher Voc, band gaps, stronger absorption, and solubility. One recent success was a molecule we call PI-BT, that resulted in devices with 2.54% efficiency when used with the donor material poly-(3-hexylthiophene) (P3HT). The synthetic challenges will be detailed along with the progress towards new and potentially interesting analogues.

Dye sensitized solar cells (DSSC) have recently reached power conversion efficiencies (PCE) >12%, making them quite attractive for application in low cost solar energy technology.(1) Although ruthenium based sensitizing dyes are commonly used, metal-free sensitizing dyes are advantageous due to their higher molar absorption coefficients, ease of chemical modification, lower cost, lower environmental impact, and increased performance in solid-state DSSCs (ssDSSCs).(2,3) Typically, metal-free sensitizers consist of three moieties: an electron donor (D); an electron-rich conjugated bridging group (π); and an electron acceptor (A) which also serves to chemically bind the dye to the titania surface. These dyes are commonly referred to as D-π-A dyes and typically have a broad visible-light absorption spectrum and the ability to separate charges due to their photoinduced intramolecular charge transfer (PICT) properties.(2) To date, the world record sensitizing dye, Y123, has a maximum absorption at 532 nm and a PCE of 7.2%.(4,5) Moving forward, overall efficiency gains can be realized by lowering the bandgap of the sensitizing dye in order to red-shift the absorption and thereby harvest more solar photons. Using density functional theory, a variety of potential low bandgap sensitizing dyes were screened. Our group will present new sensitizing dyes with novel D, π, and A groups. Example D groups include alkyl amines that are typically more electron rich than the commonly used alkyl ethers. Introduction of a fully aromatic carbazole pi group has the potential to improve the dye stability compared to our previous work with a fluorene pi group.(6,7) A new dye architecture will also be explored using a pyran-based π-A group. Lastly the commonly used cyanoacrylic acid acceptor moiety will be replaced with nitroacrylic acid that molecular modeling has shown to provide lower band gap dyes. The synthesis and characterization of these new dyes along with preliminary DSSC device results will be presented. References 1. Cao, Y. M.; Bai, Y.; Yu, Q. J.; Cheng, Y. M.; Liu, S.; Shi, D.; Gao, F. F.; Wang, P. J. Phys. Chem. C 2009, 113 (15), 6290-6297. 2. Chen, R.; Yang, X.; Tian, H.; Wang, X.; Hagfeldt, A.; Sun, L. Chem. Mater. 2007, 19, 4007-4015. 3. Chen C.; Hsu, Y.; Chou, H.; Thomas, K.R.J.; Lin, J.T.; Hsu, C.P. Chem. Eur. J. 2010, 16, 3184-3193. 4. Tsao, H. N.; Yi, C.; Moehl, T.; Yum, J.-H., Zakeeruddin, S. M.; Nazeeruddin, M. K.; Grätzel, M. ChemSusChem. 2011, 4, 591-594. 5. Burschka, J.; Dualeh, A.; Kessler, F.; Baranoff, E.; Cevey-Ha, N.-L.; Yi, C.; Nazeeruddin, M. K.; Grätzel, M. J. Am. Chem. Soc. 2011, Oct. 5, Web Publication. 6. Nguyen, W. H. ; Bailie, C. ; McGehee, M. D. ; Sellinger, A. A Comparative Study of Different Acceptor Groups in D-π-A Dyes for Application in Dye Sensitized Solar Cells (DSSC). 2011, MRS Spring Meeting, poster presentation. 7. Blouin, N.; Michaud, A.; Leclerc, M. Adv. Mater. 2007, 19, 2295-2300.

Three new polythiophene derivatives with conjugated terthiophene-vinylene side chain, Poly{3-(5”-hexyl-[2,2’:5’,2”]terthiophenyl-5-vinyl)-thiophene}, P1, Poly{ 3-(5,5”-dihexyl-[2,2’:5’,2”]terthiophenyl-3’-vinyl)-thiophene}, P2 and Poly{3-(4,4”-dihexyl-[2,2’:5’,2”]terthiophene-3’-vinyl)-thiophene}, P3 were synthesized by stille coupling method and characterized via H1-NMR and GPC. The different conjugated side chains provide the ability to control the molecular organization and further impact the photo-physical and electrochemical properties. These Polythiophene films fabricated by spin-casting show a broader absorption range from 300 to 700 nm, significantly broader in comparison with the absorption of Poly(3-hexylthiophene); especially, the absorption spectrum of P3 displays a broad plateau and much stronger red shift in the wavelength range between 450 to 700 nm, which is attributed to the π-π* transition of the conjugated main chains. And the X-ray diffraction results reveal that, the ordered structure is enhanced by more organized conjugated side chains and driven by the alkyl chain attaching to them. Moreover, we have determined the HOMO level following the order, P1>P2>P3, through the cyclic voltammetry study; a result suggests that the donating property providing by the alkyl group will be changed in compliance with different substituted site, or relatively linked position between backbone and conjugated side chain. On the basis of our results, the broad absorption and lower HOMO level demonstrate P3 could be promising polymer photovoltaic material.

We have synthesized a new narrow bandgap alternating polyfluorene copolymer (PFTTDIONE) based on 2,7-dibromo-9,9-dioctylfluorene, 2,5-bis-(tributyl- stannyl)thiophene, and 2,6-bis-(5-bromo-3-hexyl-thiophen-2-yl)-anthraquinone (M1), via a Stille polymerization reaction. The optical properties, electrochemical properties, photovoltaic properties, hole mobility, and AFM morphology of the copolymers were investigated and discussed. The optical bandgap of PFTTDIONE is equal to the electrochemical bandgap (1.90 eV). PFTTDIONE exhibits an extended absorption band in the visible part of the spectrum with an absorption edge close to 650 nm. In order to investigate its photovoltaic properties, polymer solar cells (PSCs) devices based on PFTTDIONE were fabricated with a structure of ITO/PEDOT:PSS/ copolymer:PCBM/LiF/Al under the illumination of AM 1.5G, 100 mW/cm2. The bulk heterojuction (BHJ) polymer solar cells were fabricated with the conjugated polymer as the electron donor and 6,6-phenyl C61-butyric acid methyl ester (PCBM) as the electron acceptor. The power conversion efficiency (PCE) of the solar cells based on PFTTDIONE/PCBM (1:2) annealing at 110 oC for 20 min was 1.58 % with an open-circuit voltage (Voc) of 0.74 V, fill Factor of 35.7 %, and a short-circuit current (Jsc) of 5.99 mA/cm2.

Controlling thin film morphology is a key in optimizing the efficiency of polymer-based photovoltaic devices. Poly(3-hexylthiophene) and [6,6]-penyl-C61 butyric acid methyl ester (P3HT:PCBM) based solar cell performance is dictated by nanostructure of the active layer. To control the morphology of the active layer, we synthesized block copolymers (BCP) consisting of poly(3-hexylthiophene)-b-poly(3-(2-ethylhexythiophene). Then we applied solvent annealing to BCPs:PCBM active layers to control the morphology and improve the performance of solar cell. First of all, the performance of BCP:PCBM and P3HT:PCBM based solar cells were measured after thermal annealing, which is known as a conventional annealing method. P3HT:PCBM based solar cell showed higher power conversion efficiency than BCP:PCBM based solar cell after thermal annealing at optimum temperatures. On the other hand, BCP:PCBM system gave higher efficiency than not only that of P3HT:PCBM system after solvent annealing but also BCP:PCBM system after thermal annealing. In addition, we found strong relationship between the efficiency of BCP:PCBM based solar cell and the surface tensions of solvents for solvent annealing. To understand the solvent annealing effect, the active layers after solvent annealing were investigated by scanning transmission electron microscopy, scanning force microscopy, grazing incident angle X-ray diffraction (GIXD) and in situ GIXD as a function of time during solvent annealing. The domain sizes of BCP and PCBM in the active layers after solvent annealing were smaller and more continuity than those in the active layer after thermal annealing. These results indicate that BCPs induce optimum nanostructure of the active layer specifically by solvent annealing for high performance devices.

Organic photovoltaic cell (OPV) containing poly (3-hexylthiophene) (P3HT) as p-type polymer exhibits unique photovoltaic properties due to high charge carrier mobility of P3HT. In this study, we examined random copolymer and polymer blend methods to enhance P3HT-based OPV performance. In the random copolymer (RCP) method, we prepared novel RCPs consist of 3-hexylthiophene and 3-(6-X-hexyl)thiophene (X = Br, OH or SH) monomers to control crystallinity and affinity for PCBM. While Voc of RCPs containing Br or SH groups were slightly increased, power conversion efficiency (PCE) was decreased due to low Jsc. On the other hand, low band gap polymer PCPDTBT was blended with P3HT to expand light absorption region of P3HT. As expected, blend system exhibited slightly higher Jsc than P3HT at higher thickness region of active layer. For further improvement of blend system, we prepared block copolymer that contained P3HT combined with PCPDTBT through pai-conjugated bond. This block copolymer exhibited 15～30% higher performance than original homopolymers and blend. We concluded that block copolymer technology should have crucial advantage for high performance device.

The electronic structure at the donor/acceptor interface in an organic solar cell not only affects the photovoltage produced by the cell but also the efficiency with which absorbed photons are converted to directional flow of charges by means of photoexcited electron transfer. Rates of charge transfer from a donor to an acceptor molecule in thin films are believed to be sensitive functions of the relative molecular arrangements and thus macroscopic processing conditions. It is known from ultrafast pump-probe experiments that in common donor/acceptor material combinations the electron (or hole) transfer time is often sub-50 fs. However, the finite temporal width of the spectrally narrow laser pulse often precludes the precise determination of the transfer rate. In light of this, we have used the X-ray spectroscopy technique known as the core-hole-clock to study electron transfer dynamics on the 1 fs time scale. In order to probe the effect of the relative donor/acceptor orientation on the transfer rate, we have prepared very thin films with model small molecule interfaces using surface interactions to vary the relative molecular orientation. Our analysis of the electron transfer dynamics thereof will be presented.

Conventional polythiophene derivatives such as regioregular poly(3-hexyl thiophene) (rr-P3HT) have shown reasonably high power conversion efficiencies (PCE) up to 5% due to their relatively high short circuit current (J_sc) and fill factor (FF). Although rr-P3HT is a promising candidate as an electron donor for bulk heterojunction (BHJ) organic photovoltaic cells (OPVs), PCE of rr-P3HT has been limited by relatively low open circuit voltage (V_oc) and limited photon absorption at the long wavelength region.. Our approach to achieve better performance is to increase V_oc by introducing phenanthrene-based polymers as donor materials for OPVs. They have chance to achieve higher V_oc due to their deeper HOMO level than that of P3HT. However, many researchers have reported that phenanthrene homopolymers showed large energy bandgap (E_g) properties as high as 3 eV, resulting in reduced Jsc value. To solve this problem, donor-acceptor alternating copolymer (push-pull structure) concept was introduced. We used a diketopyrrolopyrrole(DPP) unit as an electron accepting moiety for donor-acceptor type. Also we synthesized copolymers with different solubilizing groups to examine the solubility effect of the polymer. The performance up to 2.1 % of a copolymer:PC₆₁BM device was achieved without any post-treatment. In this presentation, we will report the material synthesis, characterization, and device performance of these novel low band gap donor polymers.

We report the characteristics of a Ti-In-Sn-O (TITO) multicomponent electrode prepared by co-sputtering of ITO and anatase TiO2 targets for reducing high-cost indium content in the anode layer of organic solar cells (OSCs). The dependence of RF power on the electrical, optical, structural and surface properties of the TITO multicomponent electrodes was investigated in detail. The optimized TITO (80 W TiO2 co-sputtered ITO) electrode exhibited a low sheet resistance of 18.06 Ohm/square, a high average optical transmittance of 80.61 % and a root mean square roughness of 3.1 nm, which are acceptable electrodes for the fabrication of OSCs as an anode layer. The anatase TiO2 phase in the ITO matrix prevents the bixbyite structure from attaining the preferred orientation, producing a smooth surface of the TITO electrode. Moreover, the OSC with the optimized TITO anode showed an open circuit voltage (0.596 V), short circuit current (8.334 mA/cm2), fill factor (63.988 %) and power conversion efficiency (3.176 %) comparable to those of OSCs with a reference ITO electrode. This indicates that the TITO multicomponent electrode is a promising indium-saving multicomponent electrode that to reduces the indium content in ITO electrodes for low-cost OSCs.

The mechanical flexibility of transparent PEDOT:PSS films printed onto a flexible PET substrate using a gravure printing method was investigated using a lab-made bending test system. Gravure-printed PEDOT:PSS electrodes with a sheet resistance of 359 Ohm/square and a transparency of 88.92 % showed outstanding flexibility in several types of flexibility tests including outer/inner bending, twisting and stretching. Notable, the PEDOT:PSS electrode had a constant resistance change within an outer and inner bending radius of 10 mm. In addition, the stretched PEDOT:PSS electrode showed a fairly constant resistance change up to 4 %, which is more stable than the resistance change of conventional amorphous ITO electrode. The twisting test revealed that the resistance of the PEDOT:PSS electrode began to increase at an angle of 36 degree due to delamination of the film from the PET substrate. Despite the high sheet resistance of the PEDOT:PSS electrode, the flexible organic solar cells fabricated on the PEDOT:PSS electrode showed a power conversion efficiency of ~2% indicating the possibility of using gravure printed PEDOT:PSS as a flexible and transparent electrode for printing-based flexible organic solar cells.

Energy conversion into electrical power from ambient sources such as mechanical vibrations, acoustic energy, thermal gradients and electromagnetic waves, including solar energy, is vitally important for future wireless and portable electronics. Development of an integrated architecture that harvests multiple types of energies is desirable for effective exploitation of the energies available in nature. Hence, in this work, we report a hybrid architecture designed to harvest solar and mechanical energies, either separately or simultaneously. A patterned polyvinylidene fluoride trifluoroethylene (P(VDF-TrFE)) and conjugated polymer(P3HT) were used to fabricate the hybrid power generator. The presented hybrid power generator enables excellent piezoelectric harvesting performance with a high current density due to P3HT in an independent operation mode. Under room light illumination, the output performance of the hybrid cell is enhanced by separating the generated electrons and holes efficiently and prevents their recombination with internal electric field of P(VDF-TrFE). This work, clearly demonstrates the potential of hybrid approach for harvesting multi-type energies. Furthermore, this hybrid approach may also provide one of the candidates for photovoltaic power.

Recently, much attention has been focused on the organic semiconductors as most appropriate candidates for thin, large, and high resolution electronic devices. Both carrier conduction inside the organic semiconductors and carrier injection through metal/organic semiconductor interface perform a critical function in creating high performance organic field-effect transistors. In order to enhance carrier conduction which is related to the carrier mobility, molecules should have a strong interaction so that a charge-carrier transfer between neighbor molecules becomes more efficient. Moreover, it has recently become evident that the origin of high carrier conduction of some crystalline organic semiconductors, in which the molecular interactions are remarkably large, can be attributed to a band transport mechanism. The barrier height of carrier injection is determined by Fermi-level alignment at the interface between the organic semiconductor and the metal electrode. A general approach to lower the injection barrier height is to introduce some functional groups into the organic semiconductors in order to modify the energy levels of them. However, it often causes the different molecular interactions and the changes in the crystalline structure. Consequently, serious effects on the carrier transport mechanism and its characteristics are in most cases unavoidable. In the present study, we synthesized a new asymmetrically fluorine-substituted TIPS (FTIPS) pentacene derivative. The effects of fluorine substituent on the carrier conduction in the FTIPS pentacene are discussed on the basis of the single crystal X-ray structure analysis, theoretical calculations and organic field-effect transistor measurements.

Organic solar cells have attracted strong interests as an important source of renewable energy because of its inexpensive fabrication, portability and flexibility. Many research groups have studied to enhance the power conversion efficiency (PCE) of organic photovoltaic devices (OPVs). Electrostatic spray deposition (ESD) technique is a one potential fabrication method as an alternative to the conventional spin-coating. It makes it possible to fabricate polymer thin films directly into a high-vacuum system without significant solvent issue. In addition, morphology could be controlled by choosing adequate ESD parameters and, consequently, the device efficiency would be enhanced. To understand the correlation between efficiency and morphology as well as the morphology controllability of the ESD, we conducted comparative studies with series of devices fabricated with ESD and spin-coating techniques. The performances of OPVs based on the homogeneous blends of poly(3-Hexylthiophene) (P3HT) and 6,6-phenyl C61-butyric acid methyl ester (PCBM) films were investigated by exposing the AM 1.5 illumination. The morphologies of P3HT:PCBM films were measured using scanning electron microscope, atomic force microscope and line profiler. In order to understand electronic structures of OPVs with respect to the morphologies, in situ photoemission spectroscopy measurements were also carried out. We found the relation between morphology and PCE and the advantage of ESD methods for the morphology control. The PCE could be enhanced with various ESD parameters.

Dye-sensitized solar cells (DSSCs) have attracted tremendous attention due to their high power conversion efficiencies (PCE), low cost, and ease of fabrication. One strategy to improve the efficiency of DSSCs is to develop dyes that strongly absorb photons in the 400 - 900 nm range. However, it is extremely challenging to find a single sensitizing dye that can efficiently absorb in this range. Recently, the concept of energy relay dyes (ERD) to increase light absorption has opened up a new approach to broadly absorb light.¹ For example, highly photoluminescent ERDs dissolved in the liquid electrolyte of a DSSC undergoes Förster resonant energy transfer to the sensitizing dye that is attached to the titania. In this study, we report on the design and synthesis of novel soluble ERDs and sensitizing dyes that more efficiently absorb photons in the 400 - 900 nm range. The ERDs are based on modified DCM-based dyes and the sensitizing dyes on silicon-naphthalocyanines that shows (NIR) absorption. This report describes the synthesis and characterization of the ERD and sensitizing dyes with preliminary DSSC device results. 1. Nat. Photonics 2009, 3, 406–411, Nano Lett., 2010, 10, 3077–3083.

We present the synthesis and device characterization of new hole transport materials (HTMs) for application in solid-state dye sensitized solar cells (ssDSSCs). The new HTMs were designed to have similar functional groups and energy levels to the state-of-the-art HTM (spiro-OMeTAD) yet differ in solubility, molecular size and melting point. These new HTMs possess low glass transition and melting temperatures and may be melt infiltrated into the mesoporous titania network at low temperatures (<100 C). This could lead to enhanced pore filling in thick devices with resulting higher optical absorption and PCE values than the current commercial HTMs. We also synthesize different sized HTMs using 3-dimensional cores based on silsesquioxane platforms from which to attach moieties with hole conducting properties. Using standard device fabrication methods and Z907 as the sensitizing dye, we obtained power conversion efficiencies (PCE) of 2.94% in 2-μm-thick cells, rivaling the PCE obtained in control devices using spiro-OMeTAD. In 6-μm-thick cells, the device performance is higher than that obtained using spiro-OMeTAD, making these new HTMs promising for preparing high efficiency ssDSSCs.

Recently, as demand for lightweight portable devices increases, power sources for these applications must be lower in cost, thinner, lighter and mechanically flexible. In manufacturer of flexible solar cells by printing or roll-to-roll process, silver (Ag) is preferred to use as an anode metal because of their processability. However, because the Ag has the similar work-function value with that of ITO, symmetry breaking is not spontaneously built. Although symmetry breaking has been emphasized in inverted solar cells, the role of symmetry breaking is not often considered in conventional single solar cells because symmetry breaking was thought to be created spontaneously in conventional solar cells through the use of different work-function metals. In this work, we describe our investigation of the importance of symmetry breaking in BHJ solar cells with a conventional device structure. Titanium suboxide (TiOx) was used as the material that induces symmetry breaking and various cathode metals (Al, Ag, and Au) were used to explore the effects of the TiOx symmetry breaking layer. The inserted TiOx layer obviously extracted the same level of open circuit voltage (Voc) regardless of metal work function. Ultraviolet photoelectron spectroscopy (UPS) results indicated that the formation of the interface dipole between the TiOx symmetry breaking layer and metal electrode successfully modify the effective work functione of the cathode electrode, thereby leading to symmetry breaking in BHJ solar cells.

Studying organic photovoltaic cells (OPVCs) during the last decades led to novel insights and a certain understanding of the correlation between the structure of the active layers and performance/stability of the OPVC itself. Recent research also focuses on polymer-polymer solar cells, where the PCBM derivatives as acceptor component are substituted by acceptor polymers with suitable energy levels. The use of covalently bound diblock copolymers can lead to novel and more stable blend systems. While diblock copolymers containing both, coil-type and rigid, conjugated polymer blocks are known for some time. Rigid, all-conjugated diblock copolymers are a new, emerging class of functional polymer materials for OPVCs. A diblock copolymer consisting of poly(3-hexylthiophene) as donor and poly{[9,9-bis(2-octyldodecyl)fluorene-2,7-diyl]-alt-[4,7-di(thiophene-2-yl)-2,1,3-benzothiadiazole]-5′,5″-diyl} as acceptor block was synthesized and its behavior as a compatibilizer in ternary polymer blends for OPVC applications was studied. The synthetic procedure towards poly(3-hexylthiophene)-b-poly{[9,9-bis(2-octyldodecyl)fluorene-2,7-diyl]-alt-[4,7-di(thiophene-2-yl)-2,1,3-benzothiadiazole]-5′,5″-diyl} will be described as well as first OPVC experiments on ternary polymer-polymer-polymer blends with the diblock copolymer as one of the active components.^[1] [1] R. C. Mulherin, S. Jung, S. Huettner, K. Johnson, P. Kohn, M. Sommer, S. Allard, U. Scherf, N. C. Greenham, Nano Lett. 2011; published online DOI: 10.1021/nl202691n.

Anode buffer layers are essential to achieve higher efficiencies in organic solar cells. We studied the insertion of poly-bithiophene (PBT) electrochemically synthesized in aqueous medium as buffer layer in bulk heterojunction solar cells having poly[9,9-dioctyl-fluorene-co-bithiophene] and [6,6]-phenyl-C61-butyric acid methyl ester (F8T2:PCBM, 1:3) as active layer. PBT was electrochemically synthesized on poly(3,4-ethylenedioxithiophene):poly(styrene sulfonic acid) (PEDOT:PSS) deposited on indium tin oxide (ITO) substrate. We point out some advances related with the preparation of PBT films in aqueous medium and show that it influenced positively the open circuit voltage and increased about 1.5 times the short circuit current, increasing the power conversion efficiency from 1 % up to 2.9 % from devices without buffer layer or with 11 nm of PBT, respectively.

Organic solar cells based on blends of a conjugated polymer with a fullerene derivative offer promise as an inexpensive, printable, and flexible source of renewable energy. Many blends used in organic solar cells contain molecularly mixed polymer:fullerene bimolecular crystals, which form due to fullerene intercalation between the polymer side chains. We will demonstrate the ability to control intercalation by tuning the fullerene size and the linearity of the polymer side chains and will show the effect of fullerene intercalation on absorption, exciton splitting, and solar-cell performance. We will also present the determination of the detailed molecular structure of a polymer:fullerene bimolecular crystal using x-ray diffraction techniques, two-dimensional solid-state nuclear magnetic resonance (NMR) spectroscopy, and molecular mechanics simulations. The molecular structure contains interesting features such as twists in the polymer backbone that allow increased backbone-fullerene interactions and one-dimensional fullerene channels for electron transport. Moreover, we will show how molecular packing affects the solar-cell performance of polymer blends with indene-C60-bisadduct (ICBA) and explain why ICBA outperforms phenyl-c61-butyric acid methyl ester (PCBM) as the electron acceptor in some polymer:fullerene blends, whereas PCBM outperforms ICBA in many other blends.

The performance of organic photovoltaics (OPVs) based on conjugated polymers can be improved by choosing polymers whose light absorption has maximum overlap with the solar spectrum. Recently-synthesized low-bandgap polymers have been designed to absorb light in the near infrared (NIR) region to improve spectral coverage in combination with visible-light-absorbing materials. In many cases, excitation of the lowest energy feature in the absorbance of the NIR polymer does not result in significant charge generation despite strong absorption. Here, we investigate charge generation and extraction in novel low-bandgap copolymers composed of “push-pull” units including modified diketopyrrolopyrrole (DPP) that result in optically-induced electronic transitions across the NIR with high optical extinction coefficients. Bulk heterojunction photovoltaics were fabricated with these polymers and the electron-accepting fullerene, PC₇₁BM. A series of polymers with similar optical bandgaps, but differing HOMO energy levels were examined. Using current-voltage (J-V) curves under varying illumination and external quantum efficiency (EQE) measurements, we probed the photoinduced electron transfer from the polymer to the fullerene as a function of the photon energy. These measurements enable us to determine the critical energy required for charge generation by examining the tail of the EQE spectrum. In addition, we use quantum-chemical DFT calculations to elucidate the nature of the polymer’s electronic states to rationalize the observed results. These studies provide design rules for the electronic structure of efficient donors for fullerene-based acceptors and can be used to aid the molecular design of novel polymers with NIR absorbance for solar cells.

Covalent Organic Frameworks (COFs) are a novel class of organic crystalline frameworks linked by covalent boronate ester formation or by Schiff base formation.¹ Recently it has been reported that charge carriers can be transported along the Ï€-stacked framework and high charge carrier mobilities were observed.^2,3 Physical and chemical properties, such as thermal stability, absorbance spectrum and conductivity can be tailored by the choice of the appropriate building blocks. Due to the high and accessible internal surface area combined with well-defined crystalline structure these materials can be used as model systems to investigate ordered and interpenetrating networks of donor-acceptor sytems at the nanoscale.
Here we report for the first time a hole conducting thiophene-based Covalent Organic Framework, named TT-COF, which absorbs light over a wide spectral range and shows significant photoconductivity. Due to its high surface area and its 3 nm open pores the framework can adsorb large electroactive guest molecules, such as PCBM₆₀. Fluorescence quenching of the COF in presence of PCBM₆₀ indicates energy transfer from the electron donor TT-COF to the electron acceptor PCBM₆₀. Furthermore, TT-COF is thermally very stable and can be handled under ambient conditions. We will discuss the optoelectronic properties of host-guest systems based on this novel electroactive framework.

Acknowledgement:The authors acknowledge funding from the NIM cluster (LMU Munich)

Typical thermoelectric materials have been inorganic semiconductor materials in which there are trade-offs between the electrical conductivity and the Seebeck coefficient so that a lot of efforts are required to find the optimized concentration of carriers. And also those materials are so toxic and too expensive to be utilized for the mass production. Therefore, polymers have been tried as thermoelectric materials because of their low cost and high processability.[1,2] Considering the thermoelectric figure of merit, ZT = S2σT/k where S denotes the Seebeck coefficient, σ is the electrical conductivity, k is the thermal conductivity, and T is absolute temperature, polymers have a potential for good thermoelectric material due its low thermal conductivity of polymers. Especially PEDOT:PSS which has good electrical conductivity is proper material for thermoelectric applications. In this study, SWCNT are dispersed by a solution of sodium dodecyl sulfate (SDS) and mixed with PEDOT:PSS. Thermoelectric properties which consist of electrical conductivity, Seebeck coefficient, and thermal conductivity were measured as a function of SWCNT concentration at room temperature. Concentration of SWCNT was from 0wt% up to 100wt%. The electrical conductivity show maximum value ~ 1500 S/cm at SWCNT concentration of 40wt%, while the Seebeck coefficient and thermal conductivity remained insensitive to SWCNT concentration. The Seebeck coefficient is around 20 μV/K and thermal conductivity is 0.25~0.4 W/mK. We also fabricated thermoelectric device using PEDOT:PSS/SWCNT nanocomposite which shows about 200nW power output at 80K temperature difference.

The admixture of a few mol-percent of strong molecular acceptors with electron affinities comparable to the ionization energies (IEs) of organic semiconductors (OSCs) is typically used for molecular electrical p-type doping. There, integer electron transfer between the highest occupied molecular orbital (HOMO) of the OSC and the lowest unoccupied molecular orbital (LUMO) of the dopant is commonly regarded as the fundamental mechanism of molecular electrical doping. This charge transfer is believed to lead to a localized electron on the p-dopant and a mobile hole in the OSC matrix, which then accounts for the observed increased conductivity of the OSC. This process, however, entails a number of consequences that are in conflict with established concepts of organic-semiconductor physics, such as a charge-induced appearance of polaronic states in the fundamental gap of the OSC, which have not been observed to date. With the aim to observe such states in p-doped OSCs, which are expected to occur at, or close to the Fermi Energy (E_F) with reduced IE, we performed ultraviolet photoelectron spectroscopy (UPS) experiments on the prototypical OSC/dopant pair pentacene (PEN) and 2,3,5,6-tetrafluoro- 7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) using very high dopant concentrations of up to 100%. 10% doped PEN films exhibit slightly increased IE, the IE of 1:1 mixed (amorphous) films is even 0.85 eV higher than that of pure PEN and, most notably, the HOMO of p-doped PEN is always several 100 meV below E_F, which is in clear contrast to the expectation of PEN ionization by F4-TCNQ. Moreover, continuous wave electron paramagnetic resonance (cwEPR) results show that the concentration of charged molecules concurrently decreases from 10% to 100% dopant ratio. To remedy these inconsistencies, we suggest intermolecular orbital hybridization between the OSC HOMO and the dopant LUMO, leading to the formation of a doubly occupied bonding and an empty anti-bonding supramolecular hybrid orbital with a reduced fundamental gap, which is in fact found by optical absorption measurements and corroborated by density-functional theory (DFT) calculations on PEN/F4-TCNQ complexes. Based on similar results for various OSCs, we propose molecular electrical doping to be generally due to the presence of OSC/dopant hybrids within the OSC matrix exhibiting low-lying unoccupied states in the fundamental gap of the OSC. As available states are occupied following Fermi-Dirac statistics, only a small fraction of the hybrids is ionized at room temperature, which further explains the considerably high doping concentrations usually needed to achieve high conductivity. Controlling the degree of this hybridization thus emerges as strategy for the design of improved molecular dopants.

The commercialization of organic solar cells depends on many factors related to production costs, installation, power conversion efficiency and cell lifetime. While during the last years efficiencies have reached values of up to 8% in single junction cells, the outdoor lifetime of these devices remains well below those for commercial inorganic solar cells. Many experiments have been performed on improving device performance, however, the mechanism by which degradation influences the fundamental steps of exciton diffusion and charge separation has not been addressed. In this contribution we show how a combination of optical spectroscopy methods is capable of shining light on the most important loss processes in the active layer of regioregular poly[3-hexylthiophene] (P3HT) 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6) C61 (PCBM) solar cells. The aging is performed in ambient conditions upon illumination at AM1.5 conditions for different periods of time. By using steady state photoluminescence (PL) and photoinduced absorption (PIA) spectroscopy we pinpoint the fundamental excitations of these bulk-heterojuctions. Aging results in a drastic reduction in the yield of polarons and as well a loss in excitons. In order to quantify the time scales for the different processes we performed femtosecond PIA revealing that exciton and polaron trapping occurs on an ultrafast time scale, comparable to rate of photoinduced electron transfer. Deschler et al., Adv. Funct. Mater., 2011, accepted

Organic solar cells have attracted a great deal of attention in view of their potential for the fabrication of low-cost and flexible devices. Many different types of small molecules, polymers and colorants have been used in the fabrication of bulk-heterojunction (BHJ) solar cells. One type of molecular framework that has been explored in these type of devices are conjugated donor-acceptor or “push-pull” systems. Various type of donor and acceptor functionalities have been used in regards to small organic molecules and efficiencies up-to 5% have been reported so far. A common variety of donor-acceptor system consists of an oligothiophene with a triarylamine donor at one end and an acceptor at the other end. We report a cyanopyridone-containing donor-acceptor system, which includes cyanopyridone as an aromatizable acceptor, dithiophene as a short oligothiophene conjugated Π-bridge and triphenylamine as the donor functionality. The new materials based on this module were designed, synthesized, characterised and tested in BHJ solar cells. We have successfully scaled-up (10g) the chromatography-free synthesis of a key example and have demonstrated the improved performances in BHJ solar cells as compared with the conventional dicyanovinylidene analogues. Specifically, we report that cyanopyridone acceptor group in our model enhances the absorption profile of molecules and reduced the optical band-gap, when compared with standard dicyanovinylidene group. This in turn has led to increase in the performance of devices based on these new materials. Furthermore, we will present results showing power conversion efficiencies upto 3.02% for BHJ devices based on these materials when used as donor materials with PCBM-C60. The fabricated solar cells with these materials were completely solution processable, stable under inert and atmospheric conditions and processable at higher temperatures. A key compound is now being evaluated in larger scale device modules (10cm X 10cm).

Organic solar cells (OSCs) are actively being developed as a low-cost technology for solar-to-electric power conversion. Currently, major research efforts are focused on improving the cell efficiency through optimization of the bulk heterojunction (BHJ) architecture, a blend film consisting of a mixture of donor ((p-type) component and acceptor (n-type) component of various organic materials. High-efficiency OSCs are mostly based on BHJs with a solution-cast blend film of a conjugated polymer as the donor and a small molecule as the acceptor, such as P3HT:PCBM. For these polymeric BHJs, the ability to control the film morphology and crystallinity is essential, which is usually done through fine-tuning of a solution-cast process. For vacuum-deposited small molecules, which in principle offer many significant advantages and are well proven to be useful in the related organic light emitting diode (OLED) technology, however, the implementation of morphology and crystallinity control of BHJs is much more limited as the effect of solvent is absent. Recently, we have succeeded^*1 in producing high-quality and morphologically-oriented crystalline blend films based on small molecules by using a liquid as a non-sticking co-evaporant source during vacuum deposition of the blend film and observed striking improvements in OSC performance, particularly in the photocurrent generation with the use of a relatively thick (~400 nm) blend film for greater light absorption. Blend films based on H₂Pc and C₆₀ with much improved crystallinity have been produced by this method and confirmed by analysis using UV-Vis, XRD and FESEM. Used in organic solar cells, a variety of blend films produced by this method have achieved striking enhancement of short-circuit current density. *1: T. Kaji et al., Adv. Mater., 23, 3320-3325 (2011).

We examine the vacuum thermal deposition of polymeric semiconductors and their performance in photovoltaic devices with both planar and bulk heterojunction architectures. We investigate how chemical properties of different polythiophenes are modified on deposition, study the influence side groups have on the chemical, physical and electronic properties of these films, and demonstrate photovoltaic devices with efficiencies comparable to solution deposited equivalents. Vacuum thin film deposition onto polymer substrates is an inexpensive and commonly used method widely exploited in various fields of industry (e.g. packaging). Its prevailing advantages, such as complete absence of solvents, good control over the film homogeneousity and thickness, parallel and sequential deposition of complex multilayer structures, are all unmatched by solution processing. In organic photovoltaics, polymer semiconductors are often chosen over molecular materials due to their preferable response to morphological development during or after deposition. The vacuum thermal evaporation of poly(3-hexythiophene) (P3HT) and poly(thiophene) (PTh) semiconducting polymers with and without side groups is examined. Structural changes before and after evaporation determined using GPC, UV-Vis absorption spectroscopy, NMR, and FT-IR. NMR and FT-IR show that the polymers largely retain their chemical structure, however GPC and UV-Vis indicate that their conjugation length decreases. The role of side groups in relation to structural and electronic properties is investigated. Topography and grain structure of the polymer films are studied using MicroXAM, AFM and HRTEM. XRD and HRTEM data reveal enhanced molecular packing and crystallinity of PTh. This results in significantly improved charge transport properties with relatively high hole mobilities (10-4 cm2/Vs). Evaluation of PTh and P3HT electronic properties is performed using simple planar geometry solar cells with a C60 heterojunction. PTh/C60 devices exhibit almost a 70% increase in efficiency as compared to P3HT/C60 devices, demonstrating enhanced charge collection in PTh films through improved molecular order. Next, co-deposited bulk heterojunctions with different PTh:C60 volume ratios are fabricated and their morphology characterized by phase-contrast AFM and HRTEM. Post-production thermal annealing is shown to improve the interpenetrated polymer-fullerene network and enhance efficiency by as much as 100%. Performance of the devices with 40-60% of PTh is comparable to bulk heterojunctions of PTh and PCBM processed in solution. Currently we are developing co-deposition of polymer-polymer heterojunctions. We successfully demonstrate that vacuum thermal evaporation is suitable for the deposition of low solubility polymers. The deposition method has good potential for low-cost production of complex multilayer structures relevant to a plethora of electronic and optoelectronic applications.

Fullerene derivatives are frequently used as electron accepters in organic photovoltaic (OPV) devices due to their high electron affinities, high electron mobilities, and there favorable morphological interactions with many OPV polymers. These fullerene derivatives have substituted side groups to make them soluble in organic solvents used during OPV fabrication. The presence of these side groups also change the electronic properties, mobilities, recombination kinetics, and morphology, and hence the OPV characteristics of the devices. We have used cyclic voltometry (CV) to study the reduction states and their corresponding stabilities for various fullerene derivatives. We observed that indene substituted fullerenes have more stable one-electron reduction states than phenyl butyric acid methyl ester (PBM) substituted fullerenes. In addition, the twice reduced state of all fullerenes appears to be more thermally stable than either the once- or triply- reduced fullerene. We have also performed ultra-fast laser spectroscopy measurements to study the recombination kinetics within a polymer:fullerene system and found that the electrically inactive side groups serve as blockers that hinder charge recombination. Molecular dynamics (MD) studies have been used to determine the interactions between the fullerenes to gain insight into how the morphology can affect the charge recombination behavior. A series of OPV devices were fabricated and analyzed in an attempt to relate the PV parameters to the conclusions drawn from all the mentioned experiments. This work has given us better insight to the function of fullerene side groups and their affect on OPV operation. Having a better understanding of the affect of these side groups will help us further understand OPV device physics and will also assist in the design and development of new fullerene derivatives that will help further improve OPV device efficiencies.

The major factor driving the remarkable growth of OPV efficiency over the past few years is the development of low bandgap polymer absorbers and electron acceptors. This presentation discusses a particular class of the latter, namely non-fullerene, light absorbing electron acceptor materials. We have previously reported a full study on morphology and device optimization of OPV devices using a new organic electron accepting small molecule 2-[{7-(9,9-di-n-propyl-9H-fluoren-2-yl)benzo[c][1,2,5]thiadiazol-4-yl}methylene] malononitrile (K12) blended with P3HT.[1] Its absorption spectrum overlaps with that of P3HT but it is much stronger at the 400-500 nm range where P3HT absorbs weakly, allowing for greater overlap with the solar spectrum and potential enhanced device performance. Indeed we have shown that in the optimized P3HT:K12 bulk heterojunction devices, absorption by K12 also contributes to the photocurrent. Herein, we report on the photophysics of photo-induced free charge-carrier generation in films of P3HT:K12 blends, using steady-state and time-resolved photoluminescence (TRPL) and time-resolved microwave conductivity (TRMC). We find at very low K12 loadings of up to 5% in the blend, the conventional exciton dissociation at the donor/acceptor interface followed by electron transfer to the acceptor is taking place – much in the same way as a standard fullerene acceptor. This is rationalized by the observations of a nearly 90% steady-state PL quenching and faster TRPL decays compared to the pure polymer. Although charge-generation occurs the small increase of only ca. 4× in the TRMC photoconductance of blends compared to that of pure polymer implies that at such low acceptor concentration electrons are essentially trapped in isolated K12 molecules and do not contribute very much to the TRMC signal. At high K12 loadings, in which corresponding OPV devices produce the highest photocurrents, the magnitude of TRMC signal is enhanced by more than an order of magnitude, compared to that of the pure polymer, due to an increase in both the yield of free charge-carrier production and the electron mobility in the K12 phase as a result of the clustering and aggregation of K12 molecules. In this case, K12 also contributes to charge-generation by photo inducing hole transfer to P3HT. Our results have direct implications for the design and optimization of novel non-fullerene small molecule electron-acceptor for OPV applications and also open the possibility to tailor acceptors to harvest carriers via photo-excited hole transfer. Ref.: [1] Paul E. Schwenn, K. Gui, Alexandre M. Nardes, Karsten B. Krueger, Kwan H. Lee, Karyn Mutkins, Halina Rubinstein-Dunlop, Paul E. Shaw, Nikos Kopidakis, Paul L. Burn, Paul Meredith, Adv. Energy Mater. 2011, 1, 73-81.

The morphology of polymer/fullerene films is known to strongly affect the performance of bulk heterojunction organic photovoltaic (OPV) devices. Understanding factors that affect morphology is vital to optimizing device performance. For many if not most polymer/fullerene mixtures, high boiling temperature solvent additives have been used to “improve” the morphology, where “improvement” is measured by an increase in the power conversion efficiency (PCE) of completed devices. In previous work, our group showed that high boiling temperature solvent additives can have a strong effect of the diffusion rate of the fullerene in a BHJ mixture. Specifically we showed that additives that are good solvents increase the diffusion rate of fullerenes and poor solvents decrease the diffusion rate of fullerenes. Since mobile fullerenes aggregate and form large domains via Oswald ripening, suppression of fullerene diffusion is expected to be good for the longevity of OPV devices. The solvent additives do not evaporate out of the layers so studying the effect that the solvent additive has on morphology is very relevant to completed devices. In this presentation we will analyze the effect that high boiling temperature solvent additives have on the molecular structure of BHJ mixtures using differential scanning calorimetry (DSC) and solid state nuclear magnetic resonance (ss-NMR) relaxometry. DSC shows how the various solvent additives affect the glass transition temperatures (Tgs) and melting temperatures (Tms) of the mixture. However, we found that the DSC data in not unambiguous in that there are several changes in Tg and Tm of the three component mixture (polymer + fullerene + solvent additive). Temperature dependent ss-NMR relaxometry allows us to determine the temperature dependence of molecular motion and so will allow us to determine how the solvent additive either increases or decreases the ability of specific carbons on the polymer and fullerene to move. These measurements will be coupled to lifetime measurements of completed OPV devices.

A photoelectrochemical cell allows conversion of light to electricity under a semiconductor-electrolyte solution interface. Conjugated polymers have been studied for photoelectrochemical cell applications due to their high electron mobility, low cost and flexibility in fabrication of large cells. Many other polymers such as polyacetylenes, polyanilines (PANI), polyphenylene-vinylenes (PPV) and polythiophenes have also been of great interest for such applications. Although the photovoltaic systems with conjugated polymers found in hybrid structures with PbSe, CdS and CdSe have shown promising photo-conversion efficiencies, their applications are limited by the use of toxic precursors (e.g. Cd and Pb). In the past, we have studied the photoelectrochemical properties of nano-hybrid films fabricated by blending regioregular polyhexylthiophene (RRPHTh) conducting polymer with nanodiamond (ND) nanoparticles, zinc oxide (ZnO), and titanium oxide (TiO2) nanoparticles that are deposited on indium tin oxide (ITO) coated glass plates, n-type silicon, and gold coated glass substrates. The ND-RRPHTh films revealed better photoelectrochemical properties than RRPHTh, ZnO-RRPHTh and TiO2-RRPHTh nano-hybrid films. In the present study, we have studied the photoelectrochemical properties of poly (3-octylthiophene-2,5-diyl) (P3OT), blended with different ratios of ND. P3OT blended with ND was characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), UV-visible spectroscopy and cyclic voltammetry (CV). The properties of a photoelectrochemical cell under illumination were evaluated by applying a suitable voltage and measuring the resulting current density. It was found that 1:1 ratio of ND to P3OT illuminated in a 0.2 M LiClO4 electrolyte gives a promising photocurrent. This photocurrent was found to be 2- times higher than that given by simple P3OT polymer. The reported photoelectrochemical study has shown that the photo-induced electron transfer in ND-RRPHTh nano-hybrid films, in which the P3OT acts as the donor and ND acts as the acceptor, provides a molecular-level approach to high-efficiency photoelectrochemical conversion properties. The electrochemical response of ND-P3OT films in a different electrolyte (0.2 M tetra-butyl-ammonium-tetrafluoroborate (TBATFB), 0.2 M HCl) is being investigated as a next step of this study.

A very promising approach for solution processed organic solar cells is making use of small conjugated molecules. In these small molecules the effects of solubility, side chains, stacking and processing conditions are very pronounced in comparison to their polymeric counterparts. Enhanced stacking of the small molecules leads to higher charge mobility, but also makes obtaining the perfect layer morphology a challenge. Compared to the much more investigated polymer donor materials the small molecules are easier to synthesize and to purify and the molecules that are obtained do not have a distribution of sizes, batch to batch variation or end-group differences. These advantages make these small molecules a promising alternative. In this work we investigated the solar cell performance of a series of newly synthesized small molecules based on the diketopyrrolopyrrole (DPP) building block combined with a fullerene acceptor. A striking difference with published DPP copolymers is that the amount of fullerene can be largely reduced, which improves the efficiency of the solar cells since [60]PCBM does not contribute to current generation in the visible region of the solar spectrum. Up to now efficiencies of up to 3.2% have been reached using a star shaped molecule with DPP and thiophene rings. Also a series of linear DPP based small molecules was made varying the position of the solubilizing side chains. The electronic properties of the individual molecules are identical, but large differences are found in the stacking of the molecules. The absorption of in the film is largely influenced by the position of the side chain as is the morphology of a mixed film with fullerene obtained by spin casting. This ongoing research on DPP based small molecules will focus on the relation between molecular structure versus aggregation behavior and solar cell performance.

Understanding the charge transport in conjugated polymers is crucial to optimize the performance of organic optoelectronic devices such as light-emitting diodes, field-effect transistors and solar cells. The electrical current in polymeric hole-only diodes is space-charge limited, with a charge carrier mobility dominated by hopping between localized sites at the Fermi level in a Gaussian or density of states. The mobility has been investigated as a function of charge carrier density, electric field, and temperature. However, the interpretation is still under debate; numerous theoretical explanations for the observed functional dependences have been proposed, which all fit the reported data in the respective experimental ranges. In conventional diodes the electroluminescent material is sandwiched between two electrodes with an overlapping area in the order of square millimeters. The leakage current then prohibits the electrical characterization at low fields, a problem amplified at low temperatures. Similarly, application of high fields is hindered by breakdown under continuous DC bias at high voltages. Hence the available experimental range of data from which charge transport parameters can be extracted is limited. A prerequisite to distinguish between the various theoretical models is to determine the experimental current voltage characteristics over a much larger temperature and electric field range. To this end, we use a previously developed technology of molecular junctions[1], a versatile experimental testbed, in conjunction with low duty cycle pulse measurements to bypass the aforementioned issues and significantly expand the measurement window of poly(phenylene vinylene)-based hole-only diodes. The unprecedented wide temperature- and field range (0.01 MV/m to 350MV/m) has allowed us to test the validity of the reported theoretical models. The reliability of the extracted key transport parameters will be discussed. [1] H. B. Akkerman, P. W. M. Blom, D. M. de Leeuw, and B. de Boer, Nature 441, 69 (2006)

Recently the application of ionic liquids, alternatively room temperature molten salts, as electrolyte gates has been demonstrated to be a powerful tool in condensed matter and materials science research. Ionic liquids, used in place of traditional gate dielectric materials, allow for the accumulation of very high two- and three-dimensional charge densities (>10¹⁴ #/cm² and >10²¹ #/cm³ respectively) at low voltage (<5 V). Here we study the electrochemical gating of the benchmark semiconducting polymer poly(3-hexylthiophene) (P3HT) with the ionic liquid 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([EMI][FAP]). The extensive electrochemical stability window of [EMI][FAP] allowed for the robust and reproducible accumulation of 2 x 10²¹ hole/cm³, which corresponds to one hole (and stabilizing anion dopants) per every two thiophene rings. Displacement current measurements collect versus a calibrated reference electrode were converted to the vacuum scale allowing the mapping of the density of states (DOS) of the P3HT/[EMI][FAP] doped composite. The DOS was found to be highly structured and extremely broad, extending over 1.5 eV down to 6.7 eV below vacuum. Electrochemical transistor measurements up to the large attainable charge density revealed a finite potential and charge density window of high electrical conductivity in [EMI][FAP] gated P3HT, with a significant degree of charge accumulation preceding and succeeding measurable electrical conductivity. Electrical conductivity and hole mobility reached maximums of 85 S/cm and 0.86 cm²/V s at approximately 0.12 and 0.16 holes per thiophene ring respectively. The negligible vapor pressure of ionic liquids allowed the application of traditional vacuum cryogenic techniques in order to measure the simultaneous temperature and charge density dependence of hole transport throughout the entire window of finite conductivity. The hole transport was found to be thermally activated at all charge densities. The activation energy was non-monotonic, displaying a minimum of ~20 meV in the region of maximum conductivity and hole mobility. This complex charging and transport behavior was attributed to increasing disorder upon the incorporation of the ionic liquid into the P3HT. In order to verify the generality of this result, displacement current and conductivity measurements were extended to four other ionic liquids gating P3HT, and three other semiconducting polymers gated with [EMI][FAP].

Bulk-heterojunction (BHJ) polymer solar cells using poly(3-hexylthiophene) (P3HT) as an electron donor polymer and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor have attracted tremendous attention due to the simple processing steps and relatively high power conversion efficiency (PCE). Many studies related to the evolutions of surface morphology of active blend layer have indicated that thermal annealing treatment has significant influence on device characteristics. However, the important information of energy band alignments which might directly relate to the open-circuit voltage (V_oc) of devices has not been fully explored yet.
In this work, via ultraviolet photoemission spectroscopy (UPS), the energy levels of active layer surfaces with and without aluminum (Al) cathode coverage after thermal annealing have been monitored. With Al coverage on top of the active layer, the highest occupied molecular orbital (HOMO) of P3HT exhibits a great downward shift after annealing, but such an energy shift is not observed in the case without metal coverage. Besides, if Al is deposited on the pre-annealed P3HT/PCBM blend surface, the HOMO level of P3HT does not show obvious change. These results indicate that only annealing after Al deposition would induce the energy band shift of P3HT. On the other hand, the energy level of PCBM in the active layer does not change after annealing process with or without Al. The downward shift of P3HT’s HOMO induced by annealing with presence of Al would increase of energy difference between the HOMO of P3HT and the lowest unoccupied molecular orbital (LUMO) of PCBM, leading to the increase of the V_oc of solar cells. Furthermore, several studies show that there is a P3HT-rich region near the surface of the as-cast blended film caused by phase segregations, which would degrade the performance of the solar cells. Via UPS analysis, the apparent features belonged to PCBM are emerged and mixed with the spectra of P3HT near the Fermi level after annealing at 150 ^oC for 30 minutes. This phenomenon gives a direct evidence that annealing process applied on Al covered solar cell devices will induce the out-diffusion of PCBM toward cathode, which can provide more ideal hetero structures and better barrier-free pathways to maximize the electron extraction efficiency. To confirm the UPS measurements, devices with common structure ITO/PEDOT:PSS/P3HT:PCBM/Al under pre-anneal and post-anneal processes were compared. The device using post-annealing treatment shows a great improvement in Voc by 0.2 V, which can be explained from energy level offsets detected by the aforementioned UPS results. Moreover, a considerable increase in short-circuit current density by about 1 mA/cm² (12% increase) supports the UPS findings of evolutions in vertical distribution.

Solar energy conversion has been the promising research to meet the future energy crisis. There are various solar cells under development or in mass-production. Among them, flexible organic solar cell (OSC) has attracted significant attentions due to its low cost and flexibility. Consequently, a transparent conductive electrode such as Indium tin oxide (ITO) has its limitation for the use as an electrode due to its brittleness and high cost. Therefore, it needs to be replaced by the flexible high conductive electrodes in flexible OSC. Conductive Ï€-conjugated polymers have attracted considerable interests because of their possible applications as electrodes in a variety of organic devices. Among various conductive Ï€-conjugated polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) have received a particular attention due to its high conductivity and optical transparency in a visible wavelength. We have fabricated an ITO-free OSC by developing highly conductive and transparent tosylate-doped poly(3,4-ethylenedioxythiophene: p-toluene sulfonate) (PEDOT:PTS). The final OSC consisted of Glass/PEDOT:PTS/PEDOT:PSS/P3HT:PCBM/LiF/Al. The PEDOT:PTS was fabricated by vapor-phase oxidative polymerization of EDOT on the patterned UV-exposed octadecyltrichlorosilane (OTS) using Fe(PTS)₃ as an oxidant. The stability of the PEDOT:PTS-PEDOT:PSS interface was analyzed and successfully controlled. In addition, the optimization of the thickness in terms of transmission and conductivity power conversion efficiency of 1.4%. As a result, the ITO in OSC can be replaced with high conducting polymer such as PEDOT:PTS.

Naphthalene diimide (NDI) polymers have attracted a great deal of attention as an n-type organic material. Our recent work involves the diverse capability of the NDI monomer to make high performance n-type materials by using co-polymerization, ladderization, and cross-linking. Firstly, soluble naphthalene diimide-thiophene copolymers were used in organic field-effect transistors (OFETs). Subsequently, solution processible ladder polymers were created using an alkyl-substituted poly(benzoquinolinophenanthrolinedione) derivative, which were tested in OFETs. Finally, NDI-based polymers were used as an interfacial layer in OPVs. In this case, NDI-thiophene copolymers were cross-linked with bis(perfluorophenyl) azide (bis-PFPA) to form a robust solvent-resistant film, thereby preventing solvent-induced erosion during subsequent solution-based device processing. Chemical n-doping of this polymer film with (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI) formed a cross-linked electron transport layer. In organic solar cells utilizing the inverse OPV architecture, this system showed a substantial increase in power conversion efficiency, compared to systems using electron transport layers such as zinc oxide. We report on the OFET and OPV performance of these materials.

Organic semiconductors with controllable molecular packing and long range ordering are of great interests for the development of high-performance electronic devices. The charge carrier mobility, as well as directionality of charge transport and the associated anisotropy are the key parameter in describing organic semiconductors. Charge transport in the lateral direction favors high FET performance, while the vertical charge transport is desirable for diode-type devices such as OPVs. Molecules containing polycyclic aromatic cores have strong propensity of stacking into 1D columns as the preferred charge transporting pathway, thus emerge as a promising class of organic semiconductors with controllable transport anisotropy. A number of p-type disk-shaped molecules with mobilities comparable or even surpassing that of amorphous silicon have been developed over last decades. In contrast, n-type disk-shaped molecular systems are still much underdeveloped. Even more so, there lacks a detailed study of their electronic properties in the context of thin film devices. n-Type naphthalenetetracarboxylic diimide- and perylenetetracarboxylic diimide-based materials are increasingly attractive. Incorporating these electron deficient units into c₂-symmetric linear or c₃-symmetric disk-shaped molecular skeletons poses as an appealing approach towards achieving good optical, electronic and self-assembly properties. Herein, we report the investigation of (i) a series of novel n-type disk-shaped molecules that contain a triphenylene core fused with three naphthaleneimide or peryleneimide “arms”, as well as their corresponding monomeric pigments: naphthalene imidazole (NI) and perylene imidazole (PI), (ii) “sergeant” doping effect of linear perylene diimidazole (PDI) in “soldier” monomeric PI thin films. As a result of extended conjugation, the fusion has led to enhanced optical properties as well as well-aligned frontier orbital energies. Moreover, charge carrier mobilities of these compounds, measured both in the context of field effect transistors and by the space-charge limited-current (SCLC) model, show drastically different directional anisotropy. As revealed by X-ray scattering and atomic force microscopic (AFM) analyses, a strong correlation between the film morphology and the charge transport behavior has been established. We have also investigated the “sergeant-soldier” effect by doping small weight-percent of linear perylene diimidazole (PDI) into thin films of PI monomer. The doped FET device showed one order of magnitude higher electron mobility than pure PI device. It is postulated that the linear PDI with larger Ï€-surface nucleates the more ordered stacking between PI to form an efficient electron transport pathway, thus leading to the increase of FET electron mobilities. The present structure-property studies provide insightful information for achieving directional charge transport pathways in organic electronic devices.

One of factors determining the device properties of organic electronic devices such as organic field-effect transistors (OFETs) is an interface between metallic electrodes and organic semiconductors. Such an interface can be classified into two types: namely, Schottky and ohmic. An energy barrier for charge injection from the electrode into the organic semiconductor determines the contact type, and the barrier height can be effectively tuned by modifying the electrode surfaces with self-assembled monolayers (SAMs). However, if the SAMs are structurally unstable, anomalous hystereses are observed in device characteristics of OFETs [1], which is possibly due to the structural change of the SAMs. In this study, we exploit the instability to impart a switchable nature to the metal/organic-semiconductor interfaces. Helicenethiol SAMs were formed onto Au electrodes, and then a single crystal of rubrene was placed onto them. Current-voltage characteristics of the as-fabricated two-terminal device were a typical one for a double-Schottky device. The application of high voltages (±30 V) increased currents in the voltage region with the same polarity of the applied high voltage, which resulted in a switching to a single-Schottky device. The switching behavior is reversible so that we can switch it back by applying high voltage with the opposite polarity. This can be understood as reversible electric-field-induced Schottky-ohmic switching of the rubrene/Au interfaces where helicenethiol molecules are inserted in the form of SAMs. [1] R. Nouchi and Y. Kubozono, Org. Electron. 11, 1025 (2010).

We demonstrate the use of a vacuum-based, vapor phase technique for facile integration of a semiconducting polymer into polymer solar cells (PSCs). Proceeding from volatile monomeric units, polymer synthesis and film deposition occur simultaneously at modest vacuum without the need to consider the resultant polymer solubility. Hence, this technique extends the library of materials available for optimizing PSCs to insoluble and infusible polymers, which are typically considered difficult to process. Measured conductivities of the semiconducting polymer were below the detection limit of the equipment used (<10-4 S cm-1). Fourier Transform Infrared and X-ray photoelectron spectroscopy confirm the expected structure of the vapor deposited films. Cyclic voltammetry was used to determine the oxidation and reduction potentials and the electrochemical bandgap of the polymer. The optical bandgap was obtained from the measured UV-vis spectra. Bilayer heterojunction photovoltaic cells were fabricated on patterned ITO-coated glass substrates. The vapor-deposited semiconducting polymer was directly deposited onto the ITO to serve as the electron donor layer. The PV devices were completed by vacuum thermal evaporation of fullerene C60 as the electron acceptor layer, bathocuproine (BCP) as the exciton blocking layer, and silver (Ag) as the cathode. The resulting device structures were: ITO/polymer/C60/BCP (8 nm)/Ag (100 nm). Device performance was optimized by first varying the thickness of the C60 layer. The fill factor (FF) remained relatively constant with variation in C60 thickness, whereas the open circuit voltage (Voc), short-circuit current (Jsc), and power conversion efficiency (PCE) achieve a maximum for a thickness of ~ 30 nm of C60. The external quantum efficiency exhibit an edge that matches the edge of the absorption coefficient of the vapor-deposited polymer, confirming that the vapor-deposited polymer contributes to the photocurrent of the devices. Additional devices were fabricated using 30 nm of C60 and varying thickness of vapor-deposited polymer. The Jsc for the devices remained constant, but the FF decreased with increasing thickness of the polymeric donor layer, likely due to increased series resistance through the device. A maximum PCE of 0.8% under AM 1.5G (100 mW cm-2) is achieved using about 25 nm of polymer and 30 nm of C60. This is the highest efficiency achieved to date for the use of a vapor-phase deposition of the donor polymer for a PSC. This work demonstrates that the vapor phase technique used to deposit this polymer is a viable technique for the processing and design of polymer active layers for PSCs without solubility or substrate considerations. Additional polymers deposited by the same technique and their application to polymer solar cells will also be discussed. This work was supported by Eni SpA under the Eni-MIT Solar Frontiers Center.

Much attention has been paid to thin-film solar cells such as organic solar cells (OSCs) because they can be low-cost, lightweight, and flexible. The main drawback of thin-film solar cells, however, is relatively low light absorption due to their thin active layers. Therefore, it is necessary to develop proper light trapping schemes to increase the average path-length of incident ray. The path-length enhancement of 4n²/sin²(θ_a/2) is known as the theoretical limit of light trapping within ray-optical regime, where n is a refractive index and θ_a is an acceptance angle [1, 2]. However, as the active layer of thin-film solar cell is too thin to be textured for ray-optical light trapping, optical components should be set on a substrate, usually glass. But, rather small refractive index of glass (n=1.5) makes the enhancement much smaller than that of silicon solar cells (n=3.5). A reduced acceptance angle can compensate the small enhancement from the low refractive index, since the sun is always on its orbit during the day and the height of the sun varies annually only within ±23.5°. We found that with 47° of the acceptance angle, 17.5% and 31.7% of power conversion efficiency enhancements are expected for OSCs using PCPDTBT:PCBM and α-Si solar cells, respectively. The material of the electrodes is very important in the light trapping scheme, as the electrodes’ parasitic absorption limits the maximum absorption by the active layers. Our research shows that 27.6% of absorption enhancement can be achieved in the OSCs when aluminum cathode layer is replaced with silver, as silver shows higher reflectance than aluminum. We also found that light trapping is effective in small molecular OSCs. Because bilayer small molecular OSCs show strong trade-off between internal quantum efficiency (IQE) and absorption by the layer thicknesses, we can reduce the thickness of the active layers to increase IQE while maintaining high absorption through light trapping. The short-circuit current of our small molecular OSCs using CuPc/C60 as active materials is increased from 6.7mA/cm² up to 10 mA/cm². We will propose several configurations of thin-film solar cells including texturing, V-shape, and 1D concentrator array to implement the above-mentioned schemes. Also, we will present simulation results combining both ray-optics and transfer-matrix methods. [1] E. Yablonovitch, J. Opt. Soc. Am., A 72, 899 (1982). [2] P. Campbell and M. A. Green, IEEE Trans. Electron. Dev., 33, 234 (1986).

A vital parameter in the design of excitonic devices is the exciton diffusion length, which describes the characteristic distance an exciton travels in a material before recombining. Spectrally-resolved photoluminescence quenching (SR-PLQ) is an accurate and convenient method to measure this parameter[1]; however, it requires samples that are optically thick (i.e. the thickness is greater than the absorption length) at all wavelengths used in the measurement. This imposes a practical constraint, as many materials of interest cannot be fabricated in sufficiently thick films to meet this criterion. These include a variety of solution-processed materials, such as squaraine donors. We demonstrate a method to extend SR-PLQ to optically thin films by incorporating the effects of the optical field into a simulation of the exciton dynamics. This allows for the measurement of the diffusion length in films whose thickness is of the order of that used in devices. In this talk we will discuss measurements of the exciton diffusion lengths for several materials of interest in solar cells, including a family of functionalized squaraines, subphthalocyanine, PTCDA, etc., as well as measurements of the dependence of the diffusion length on the film thickness. [1] R. R. Lunt, N. C. Giebink, A. A. Belak, J. B. Benziger, and S. R. Forrest, “Exciton diffusion lengths of organic semiconductor thin films measured by spectrally resolved photoluminescence quenching”, J. Appl. Phys, 105, 053711 (2009)

The influence of the metal electrode on vertical segregation and doping in poly(3-hexylthiophene-2,5-diyl) (P3HT)/[6,6]- phenyl-C61 butyric acid methyl ester(PCBM) bulk-heterojunction (BHJ) organic photovoltaics has been investigated by neutron reflectometry and x-ray absorption spectroscopy. Here we present the first measurements of the BHJ vertical composition profile and doping with an intact cathode. Prior to heating, our neutron reflectivity results show a P3HT-rich skin at the cathode. Heating leads to PCBM enrichment at the cathode for all metals. Measurement of a sample heated without an electrode does not show PCBM enrichment at the BHJ-air interface indicating that the segregation is driven by the metal. X-ray absorption spectroscopy is used to measure the influence of heating at the cathode-BHJ interface. From the near edge x-ray absorption fine structure (NEXAFS) of the carbon K-edge we find that heating causes Ca and Al to dope PCBM. Ag does not dope PCBM because its electron work function is below the lowest occupied molecular orbital of PCBM. Ca L-edge NEXAFS shows that Ca on top of the BHJ is oxidized. However, after washing with water we find that Ca remains in the BHJ and that the trapped Ca is bonded to carbon and not oxygen. Finally, we fabricate photovoltaic devices to measure the effects of doping and vertical segregation. Heating improves the power conversion efficiencies of BHJ devices with Al and Ca electrodes as has been widely reported. The improvement is to some extent due to increased hole-blocking character of the cathode due to PCBM enrichment and doping. In contrast, the BHJ device with a Ag electrode is less efficient after heating. This is because a PCBM depleted layer forms BHJ layers with Ag electrodes upon heating, as seen with neutron reflectometry. In the depleted layer the concentration of PCBM is below its solubility limit in P3HT, which has been shown to result in only hole transport. In summary, we present a complete understanding for how vertical segregation affects the electronic function of organic solar cell layers. This understanding is based on measurement of the vertical concentration profile using neutron reflectometry and n-type doping by the metal measured using XAS.

The small molecule 5,5′-bis{7-(4-(5-hexylthiophen-2-yl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine}-3,3′-di-2-ethylhexylsilylene-2,2′-bithiophene (G9) has recently been reported,[1] and represents one material in a new class of narrow band gap molecular donors for OPV applications. The compound is built upon a D’A/D/A/D’ framework; D and A correspond to electron rich (donor segment) and electron deficient (acceptor segment) units. Central to this structure is the [1,2,5]thiadiazolo[3,4-c]pyridine (PT) acceptor unit, which as a. high electron affinity and leads to narrow band gaps when coupled with a strong donor. Recent reports have shown that the pyridal N-atom of the PT moiety is susceptible to coordination by Lewis acids, giving rise to a narrowing of optical band gaps via a decrease of both the HOMO and the LUMO energy levels, with the more substantial contribution from the LUMO.[2,3] We have investigated the interaction of thin films of G9 with two different hole-transport interlayers: PEDOT:PSS and solution processed NiOx (s-NiOx) using both XPS and UPS. As a control the interaction of a G9 film with the Lewis acid p-toluene sulfonic acid will also be discussed. For both the control and PEDOT:PSS experiments, a decrease in optical gap and shift in absorbance to the near-IR region were observed but with no change on s-NiOx. XPS results suggest an interaction between the pyridal N-atom and the Lewis acid (for PEDOT:PSS and control), but also reveal interactions with G9’s azole and thiophene units, manifested in changes in line shape in the S 2p and N 1s core levels. No changes in line shape were observed for thin films of G9 on s-NiOx. UPS results demonstrate an increase in the ionization energy in the near-surface region for thin films of G9 (relative to bulk values) on both PEDOT:PSS and s-NiOx interlayers. The work function of the interlayer was found to have some impact on device performance – in addition to Lewis acid/base interactions – and will be discussed. Bulk heterojunctions of G9 and PC70BM showed an increase in power conversion efficiency (PCE) from 2.2% to 4.7% when PEDOT:PSS was replaced with s-NiOx. The increased PCE was associated with increased open circuit voltage (from 540 to 730 mV) and short-circuit current (from 10.4 to 12.3 mA/cm2). No changes in the near-IR region of the measured external quantum efficiency were observed for either interlayer, indicating local absorbance changes are confined to the PEDOT:PSS/G9 interface. These results suggest that interfacial chemistries such as intermolecular interactions between interlayers and photoactive layers can significantly impact OPV device performance beyond energetic alignment. Such interfacial phenomena must be accounted for when choosing the appropriate interlayer for incorporation in bulk heterojunction architectures. [1] J. Mat. Chem. 2011, 121, 12700. [2] J. Am. Chem. Soc., 2009, 131, 10802.. [3] J. Am. Chem. Soc., 2011, 133 (12), 4632.

The open circuit voltage (V_OC) must be optimized to obtain the best efficiency in solar cells. In organic solar cells V_OC is directly proportional to the offset between the HOMO level of the electron donor material and the LUMO level of the electron acceptor material. Hence, engineering acceptor materials with a LUMO level that is as high as possible may increase the power conversion efficiency, provided that the driving force for charge transfer is preserved. We investigated the possibility of improving the performance of poly[9,9-didecanefluorene-alt-(bis-thienylene) benzothiadiazole] (PF10TBT) based solar cells. PF10TBT:PCBM optimized devices have an internal quantum efficiency (IQE) of 75%. Devices in which PF10TBT is blended with fullerene derivatives having higher LUMO energies than PCBM show the expected increase in V_OC but also a strong decrease in short circuit current (J_SC). By investigating the photophysics of the blends using near-IR luminescence and photoinduced absorption we found that the charge transfer state is only formed efficiently with PCBM. A clear correlation between the energy of the CT state and J_SC is found. Recombination of the CT state to the triplet state of the PF10TBT occurs in these blends, representing an additional contribution to the deviation of the IQE from unity.

The interplay between forward and backward electron transfer between donor and acceptor counterparts of polymer-fullerene bulk-heterojunctions is of crucial relevance for photovoltaic performance. Whereas the former is essential for an efficient charge generation, the latter can lead to an anew formation of neutral excitonic states with lower energy, e.g., triplet excitons. The presence of triplet excitons in the P3HT:LuN₃@PCBEH and their absence in P3HT:PC₆₀BM (P3HT - poly[3-hexylthiophene], LuN₃@PCBEH – endohedral fullerene derivative) [1] was found to be conclusive with the relative positions of the energy levels of triplet excitons located on the polymer and the fullerene LUMO. To generalize this assumption, we investigated the donor-acceptor blends with different donor polymers by using the optically detected magnetic resonance (ODMR). The technique is highly sensitive to the spin state of quasi-particles and allows distinguishing between charge transfer (CT) states (or polaron pairs) and triplet excitons. Further, the orientation of molecular units can be determined. The highly performing donor-acceptor copolymer benzodithiophene PTB7 [2] was found to be in a face-on configuration, while the P3HT monomers were perpendicular with respect to the substrate. Furthermore, we scrutinize the influence of fullerene acceptors on these systems. The PTB7:PC₇₀BM blend was found to exhibit both the CT and triplet states, whereas the P3HT:PC₇₀BM exhibited only CT states. Surprisingly, at higher fullerene loads in PTB7:PC₇₀BM (1:1.5), the formation of triplet excitons on PC₇₀BM was observed, too. We discuss these findings within the morphology and energy level driven electron back transfer picture. [1] M. Liedtke, A. Sperlich, H. Kraus, A. Baumann, C. Deibel, M. J. M. Wirix, J. Loos, C. M. Cardona, and V. Dyakonov, J. Am. Chem. Soc. 133, 9088–9094 (2011). [2] Y. Liang, Z. Xu, J. Xia, S.-T. Tsai, Y. Wu, G. Li, C. Ray, and L. Yu, Adv. Mater. 22, E135–E138 (2010).

A series of isoindigo-based low band gap polymers with cyclopentadithiophene (CPDT) as donor, decorated with different side chains such as 2-ethylhexyl, octyl and dodecyl, have been synthesized by Stille cross-coupling reaction. The influences of side chain on the optical, electrochemical and crystalline properties of polymers have been investigated. These polymers show broad and strong absorption covering entire visible range and their maxima absorption is shifted with different side chains. And quantitative analysis of the impact of side chain on the observed open circuit voltage (Voc) and short circuit current (Jsc) of the corresponding bulk heterojunction solar cell are demonstrated. The observed results account for the intermolecular interactions in the polymer/PCBM blends by correlation of the pre-exponential dark current term (Jso) with the difference in Voc and Jsc.

Donor-acceptor copolymers continue to attract attention as low band gap materials for organic photovoltaic (OPV) devices. Here, we report the synthesis and device performance of new copolymers incorporating isothianaphthene-based (ITN) monomeric units. With only four linear steps, we synthesized three different ITN monomers with electron-withdrawing substituents, and evaluated the relative electron withdrawing strength of the functionalities. We show that the substituents on ITN strongly influence the electronic nature of the copolymers and device performance. Ester and imide-based polymers behave as p-type materials and achieve device efficiencies as high as 3% in bulk heterojunction devices with phenyl C61-butyric acid methyl ester (PC61BM). In contrast, the nitrile-based polymer performs as an n-type material due to the strong electron withdrawing character of the cyano group. These new monomers, along with our corresponding studies on the effect of electron withdrawing substituents on OPV device properties, may help to guide the future design of acceptor monomers.

Mixtures of donor and acceptor organic materials are increasingly important candidates for optoelectronic devices like organic light emitting diodes (OLED’s) and organic photovoltaics (OPV’s). In the later class of device, there are two organic materials paradigms: small molecules and polymers. The most efficient OPV’s to date are solution-cast films of intimately-blended polymers and small molecules based on fullerenes. This device morphology is referred to as a bulk heterojunction (BHJ), so-called because it utilizes a 3D bicontinuous network of donor and acceptor materials with nanoscale domains that create interfaces throughout the film. Since interfaces are a necessity for exciton dissociation in OPV’s the high interfacial area in BHJ’s is an obvious advantage. Small molecules are not readily incorporated into such BHJ blends by solution casting since they have poor solubility. Organic molecular beam epitaxy of these materials is very well-developed and has led to good efficiencies in bilayer film devices where donor and acceptor materials are deposited sequentially to create one (essentially planar) interface for exciton dissociation [1]. This has led to an interest in the creation of mixed films on donor and acceptor small molecules that mimic the BHJ morphology. The best results have so far been obtained with 1:1 mixtures of copper phthalocyanine and fullerene-C60 [2]. However, it is somewhat surprising that mixtures do not result in dramatic efficiency improvements compared to the best bilayer OPV’s. This suggests important missing pieces in our understanding of the deep connection between films morphology, composition, and charge transport. Here, we pursue the question of the morphological and electrical properties of co-deposited and mixtures by considering donor-acceptor films created by vacuum codeposition of P3HT and C60. The recent demonstration of vacuum-deposition of P3HT [3] offers opportunities to explore new film morphologies for this canonical donor-acceptor couple. We have characterized these films with an atomic force microscopy (AFM), UV-Vis spectroscopy, and conducting AFM. Using AFM, we observe that the codeposition of C60 and P3HT results in smoother surfaces with domains of larger average size compared to the pure vacuum-deposited C60. In addition, conducting AFM current maps show that P3HT intermixed with C60 allows more efficient hole injection when compared with pure C60 films. [1]P. Peumans et al., J. Appl. Phys. 93, 3693 (2003). [2]S. Uchida et al., Appl. Phys. Lett. 84, 4218 (2004). [3]H. Wei et al., Appl. Surf. Sci. 255, 8593 (2009).