We report a comparison of charge carrier kinetics and recombination dynamics correlated with the device performances of PBDTTT-C and PBDTTT-CT in non-fullerene P(NDI2OD-T2) solar cells. The nanoscale morphological characteristics are found to be remarkably different for these two polymers in all-polymer bulk heterojunction (BHJ) blends. Important insights into the carrier dynamics and crystalline packing features of these different donor materials are provided in detail. The higher device performance of the PBDTTT-CT:P(NDI2OD-T2) device (2.78% compared to 1.56% for the PBDTTT-C:P(NDI2OD-T2) cell) is shown to result from efficient charge generation and faster carrier separation, reducing non-geminate recombination during charge collection, which is attributed to the perfect intermixing between the donor and acceptor polymer networks.

One-dimensional nanowires and microfibers from small molecules or π-conjugated polymers are important building blocks for the fabrication of nanoelectronic devices and provide model systems for charge transport in organic semiconductors under nanoscopic confinement. We investigated the relationship between molecular structure, supramolecular arrangement of the chromophores, and charge transport within self-assembled nanowires from perylene bisimides with oligopeptide-polymer side chains. The preparation of hierarchically structured microfibers of aligned nanofibrils allowed for a comprehensive structural characterization on all length scales with molecular level precision.⁽¹⁾ We prove that conformational flexibility between the hydrogen-bonded oligopeptides and the π-π stacked chromophores is a key requirement for synergistically enhanced π-π interactions and hydrogen bonding. The combination of beneficial π-π stacking, long-range order of the π-conjugated segments, and the directionality of the nanowires within thin films result in remarkably high conductivity after n-type doping. Moreover, we show that the capability to efficiently transport charge carriers through such nanowires ultimately depends on a delicate balance between π-π and other supramolecular interactions.⁽²⁾ Our results show that there is a complex interplay between the universal translation of molecular chirality into supramolecular helicity and the molecules&’ inherent propensity to form β-sheet-like aggregates.⁽³⁾ The result is a twofold odd-even effect in electronic circular dichroism spectra, depending on both the number of L-alanine units in the oligopeptides and the length of the alkylene spacer between the chromophores and oligopeptide substituents. We demonstrate that even derivatives with similar molecular structures and self-assembly behavior can have significantly different electron mobilities that appear to correlate with the nature of their electronic CD spectra. It is, hence, not generally sufficient to prepare one-dimensional aggregates of π-conjugated molecules in order to obtain self-assembled nanowires. Instead, the exact molecular arrangement within the aggregates and, consequently, even supposedly minor details of the molecular structure may play an important role.
(1) Marty, R.; Szilluweit, R.; Sanchez-Ferrer, A.; Bolisetti, S.; Adamcik, J.; Mezzenga, R.; Spitzner, E.-C.; Feifer, M.; Steinmann, S. N.; Corminboeuf, C.; Frauenrath, H. ASC Nano 2013, 7, 8498-8508.
(2) Marty, R.; Nigon, R.; Leite, D.; Frauenrath, H. J. Am. Chem. Soc. 2014, 136, 3919-3927.
(3) Marty, R.; Helbing, J.; Frauenrath, H. J. Phys. Chem. B. 2014, 118, 11152-11160.

Convective self-assembly (CSA) is a method generally used for depositing in a controlled manner nano- to micro-sized colloids onto a solid substrate under the action of solvent evaporation and capillary forces. By controlling some parameters such as substrate temperature, solution concentration, ambient humidity and temperature while translating the triple-contact line (solvent-air-substrate) a large variety of ordered structures have been obtained from different kinds of colloids (e.g. polymer, metallic)^1.2.3. Here, we adapt the CSA method to polymer solutions and show that by controlling the deposition speed, substrate temperature and oligomer/polymer concentration, CSA can lead to thin films comprised of molecules possessing highly oriented chain conformation, including crystals. For example, performing CSA on short chains of conjugated oligothiophenes (TH13) leads to highly ordered superstructures. Their optoelectronic properties are visibly changed in comparison to spin casted films of the same material that adopts rather disordered microstructure. Significantly different morphologies (porous films or random nanoparticles) with different optoelectronic properties can also be obtained when performing CSA on conjugated polyfluorene (PFO). The advantage of CSA method relies on its reproducibility and applicability over large surface area (i.e. squared centimetres) on any type of substrate that is compatible with the solvent used in solution preparation process.
References:
¹C. Leordean et al., J. Raman Spectrosc.45, 627 (2014).
²V. Saracut et al., ACS Applied Materials & Interfaces5, 1362 (2013).
³C. Farcau et al., Nanoscale4, 7870 (2012).

In spite of impressive improvement in device performance of the polymer-fullerene bulk-heterojunction photovoltaic cells over the past decade, the fact that their light absorption relies mostly on the polymer suggests inefficient use of the solar spectrum on single device level. Here, as a proof of concept, we demonstrate that blends of solution-processible titanyl phthalocyanines (TiOPc) nanoparticles [17-19] and polymer semiconductors [20, 21] can be fabricated into functioning bulk-heterojunction photovoltaic cells. Being a strong absorber in the red and near-IR region, TiOPc nanoparticles can contribute to significantly enhanced light absorption. The higher contrast in electron density between these TiOPc nanoparticles and polymer semiconductors also facilitates characterization of the nanoscale morphology by electron microscopy, making the establishment of a definitive structure-property relationship possible.

Photosynthetic organisms efficiently capture light and transport excitons to the reaction center by tuning the nature and magnitude of energetic disorder in their constituent antenna proteins. Due to their long-range order, large domain sizes and high oscillator strengths, one-dimensional tubular cyanine dye based J-aggregates have become a model system for understanding excitonic antennae and represent a route toward creating organic excitonic wires. We probe the temperature dependent disorder of matrix stabilized tubular J-aggregates, and observe signatures of exciton-exciton annihilation at extremely low photon fluxes, suggesting micron scale exciton diffusion lengths at room temperature, and band-like transport as we tune dynamic disorder in the system. In the past, photochemical instability has hindered the study of these materials and their implementation in practical devices. We stabilize these J-Aggregates against photobleaching and cryogenic damage in a sugar-based matrix enabling quantification of disorder, and robust measurements of exciton diffusion. The long diffusion lengths are among the largest ever measured for singlet excitons in a non covlalent organic supramolecular complex, and argue that quantum coherence can play a role in enhancing energy transfer in analogous highly ordered biological aggregates such as the chlorosome.

Perylenediimides (PDIs) are a promising alternative to fullerenes in organic photovoltaics (OPVs) but have historically performed poorly due to their tendency to form large crystalline domains, leading to excimer formation and trapping of charge carriers. These problems can be avoided by controlling the packing morphology of the PDI molecules so that excimer and trap formation is avoided, or by developing amorphous materials. Here we explore the properties of planar PDI monomers which are substituted at the 2,5,8,11-positions ("Headland Positions") and find that structures with substituents in these positions pack in slip-stacked columns which, despite a high degree of crystallinity, have greatly reduced rates of excimer formation when compared to unsubstituted PDI. Materials with substituents in the headland position undergo nearly quantitative charge transfer when mixed with a polymer donor and in the case of 2,5,8,11-tetraphenyl-PDI (Phenyl-PDI) geminate recombination is also minimized leading to efficient charge carrier generation and high OPV device efficiencies. The structure of the PDI can be further tuned by substituting the solubilizing alkyl chain at the imide position. When the solubilizing chain is sterically small (n-octyl) the molecules stack in a slip-stacked orientation and when it is large (1-ethylpropyl) they pack in a herringbone structure, but when the solubilizing chain is of intermediate size (3,7-dimethyloctyl) the packing structure can be controlled with solvent additives allowing for the fine tuning of active layer morphology. This degree of control allows for the fabrication of efficient OPV devices while maintaining the crystallinity of PDI.

In this talk the influence of backbone fluorination on the optical, electronic and morphological properties of various thiophene containing polymers will be discussed. We report that fluorination leads to an increase in polymer ionisation potential without a change in optical band gap. Increasing levels of backbone fluorination also result in an increase in the dielectric constant of the polymers. Fluorination is found to have a pronounced effect of the tendency of the polymers to aggregate, both in solution and the solid state, which leads to improved performance in field effect transistors in comparison to the non-fluorinated analogues.

Polymer solar cells attract considerable attention as a potential renewable energy technology. Typically, the light-harvesting active layer is composed of a fine blend of a polymeric electron-donor and an electron-acceptor such as a fullerene derivative. The precise nanostructure of these so-called bulk-heterojunctions is critical for achieving an optimal photovoltaic performance. However, polymer/fullerene blends are metastable materials and hence their nanostructure and photovoltaic performance tend to evolve with time. Thus, in particular elevated processing and operating temperatures pose a formidable challenge to the long-term stability of polymer solar cells. In the first part of my talk I will discuss the thermal stability of bulk-heterojunctions with respect to the glass transition temperature of the blend as well as the nucleation and growth kinetics of fullerene crystals. Then I will introduce two tools based on 1) nucleating agents and 2) fullerene mixtures, which permit to considerably enhance the thermal stability of this promising class of materials.

Many applications of conjugated polymers depend critically on excitation energy transport. Although bulk measurements regarding these processes have the inherent disadvantage of averaging over interesting areas, single-molecule spectroscopy is limited to single conjugated polymer chains, which may not reveal bulk-related mechanisms. Both experimental techniques have their unique advantages and have served well in obtaining a detailed understanding of organic materials. We use in situ solvent vapor annealing (SVA) to generate highly ordered aggregates composed of individual conjugated polymer chains to advance into the regime between a single chain and bulk film, that is, the mesoscopic size domain. The model system used shows a drastic change in photoluminescence characteristics in going from well-dissolved single chains to the bulk film, making it especially useful to follow the evolution from a single chain towards a mesoscopic object. By employing single-molecule and aggregate spectroscopy, we unravel the following non-intuitive behavior in the mesoscopic size regime: aggregation of multiple chains leads to coherent coupling between chromophores, resulting in a ten-fold increase in the excited state lifetime. This coupling between chromophores in aggregates is so efficient that strong photon antibunching evolves. Even particles containing dozens of individual chains still behave as single quantum emitters due to efficient excitation energy transfer, while the brightness is raised due to the increased absorption cross-section of the suprastructure. Excitation energy can delocalize between individual polymer chromophores in these aggregates by both coherent and incoherent coupling, which are differentiated by their distinct spectroscopic fingerprints. This mesoscale approach allows us to identify intermolecular interactions which do not exist in isolated chains and are inaccessible in bulk films where they are present but masked by disorder.

Organic conducting polymers are promising electrode materials for printable and flexible organic electronics. One of the most studied conducting polymers is PEDOT:PSS which is sufficiently conductive and transparent, but which shows some drawbacks, such as hygroscopy and acidity. In this communication we present a new approach to stabilize PEDOT in aqueous dispersions by replacing the PSS with a new family of polyanions based on a polystyrene backbone with (trifluoromethylsulfonyl)imide (TSFI) side groups. The PEDOT:PSTFSIK dispersions were obtained by oxidative polymerization of EDOT in aqueous PSTFSIK solution and were characterized regarding their composition, morphology, doping, rheological behavior and opto-electronic performance. These PEDOT:polyanion dispersions show excellent printability and good opto-electronic performance (238Ohm.sq^-1 at 91% transmittance) and were successfully integrated as flexible electrodes in OLED and OPV devices.

Organic photovoltaics (OPV) made from small molecule donors and fullerene derived acceptors have demonstrated prominent advantages. They generally exhibit intrinsically higher mobility and open circuit voltage. Besides, their defined molecular structures effectively reduced batch-to-batch variation for device performance and enable easily tuned absorption and energy levels with chemical structure modification. In order to meet the mass production requirements, printing, instead of spin-coating, of OPV solutions for device active layer fabrication must be used, and understanding the control of film morphology during this process is under urgent demand.
Using solution shearing printing technique, combined with in-situ x-ray scattering, we have investigated the drying dynamics and bulk-heterojunction (BHJ) morphology evolution of the OPV blend system based on p-DTS(FBTTh₂)₂ as donor and PC71BM as acceptor. We have investigated how this morphology changes as a function of the titrated acceptor amount, the types of solvent, and the additives used with the solvents. The BHJ films were shear-printed from solutions, with the solvent drying process monitored using grazing-incidence x-ray scattering. Information on donor crystallization and molecule packing as a function of time and processing conditions were derived. Ex-situ resonant soft x-ray scattering were used to extract phase separation size and relative domain purity. Our results reveals the phase separation and donor molecular packing process are largely affected by the amount of PC71BM acceptor added to the blend system and the morphological evolution during solvent drying is dictated by the temporal range of solvent evaporation process. Performance of devices made with active layers under these varied processing conditions is examined to reveal the morphology-performance relationship for this model system. This knowledge greatly advances the understanding of small molecule BHJ morphology formation, which is expected to provide valuable guidance over OPV material processing and molecular design.

Narrow bandgap semiconducting polymers have received much attention due to its unique properties (e.g. long-wavelength light absorption) and potential for applications in organic photovoltaics, infrared sensor, and infrared light photo-thermo-electric conversion device. In order to develop narrow bandgap polymers, extended π-conjugated systems (delocalized frontier molecular orbitals) are required. In the molecular design, we focus on the planar and rigid π-framework, which is effective for extending π-conjugation length and densely molecular packing. The thienoisoindigo (TII) and thiophene-flanked diketopyrrolopyrrole (TDPP) units are selected as high planar and rigid π-conjugated framework. Furthermore, selecting the side chains is also important. Long-branched-alkyl chains with branching site at the second carbon are the most commonly used for solubilizing groups. However, steric hindrance in this type of branched side chains usually prohibits the highly ordered interchain packing, and has a negative influence on charge-transport properties. In this work, in order to eliminate the steric impact, the hybrid siloxane-terminated side chain which is moved the branching point away from a π-conjugated backbone is incorporated into the π-conjugated backbone.
Six conjugated polymers based on TII and TDPP units bearing either branched 2-butyloctyl (BO) or siloxane-terminated undecyl (SiC11) groups have been designed and synthesized, including TII-based homopolymers PTII-SiC11 and PTII-BO, TII-TDPP-based copolymers P(TII-TDPP)-SiC11 and P(TII-TDPP)-BO, and TDPP-based homopolymers PTDPP-SiC11 and PTDPP-BO. We report the impact of backbone selection and side chain engineering on the optical, electrochemical, and carrier transport properties, as well as the solid state packing, for this set of polymers. TII homopolymers absorb near infrared wavelengths (~2000 nm) and have extremely narrow optical bandgaps below 0.6 eV with highly lying HOMO levels and low lying LUMO levels, originating from the flat and rigid TII framework. TII homopolymers exhibited p-channel mobilities of 10^-3-10^-2 cm²V^-1s^-1 whereas other four polymers exhibited a typical ambipolar transport with mobilities of 10^-4-10^-1 cm²V^-1s^-1 in FETs. Two-dimensional GIWAXS indicates that the TII unit prefers face-on rich lamellar orientations resulting in the bimodal face-on and the edge-on lamellar packing, and siloxane-terminated alkyl chain reinforces π-π interchain interaction in a crystalline film. We demonstrate that the incorporation of the TII unit in the polymer backbone is an effective way to develop ultra-narrow bandgap conductive polymers.

Development of organic printed electronics has been expanding to a variety of appications such as organic light-emitting diodes (OLEDs), organic photovoltaic cells (OPVs), electronic papers, wearable electronics and various sensors. It is expected that flexible, thin, and light-weight devices should bring an innovation to our life, which makes the R&D in this area competitive. Furthermore, the low cost and the ease of processing would be considered as important features. From this point of view, polymer based organic light emitting diodes (P-OLEDs) possesses the advantage over the small molecule based OLED as the former can be fabricated by wet process, on the other hand the latter requires the vacuum process.
The lifetime and efficiency of P-OLEDs have been reaching a certain level, but further improvement is desired. It is considered to be essential to understand the fundamental process governing the efficiency and the device degradation mechanism for the further improvement of P-OLED performance. In this study, we have focused on the formation of carrier traps during the device driving by the thermally stimulated current technique (TSC). We have developed a new technique which enable us to study the traps with deeper energy, which can not be analyzed the conventional TSC technique. Our study shows the formation of carrier traps whose energy is much deeper than the previously observed traps[1]. Furthermore, the formation of traps with deeper energy linearly increases as the photoluminescence (PL) intensity decreases. This suggests that the formation of deep trap is closely related to the formation of quenching sites, which results in the PL degradation, thus the device degradation.
[1] M. Nakahara et al., Jpn. J. Appl. Phys. 46, L636-L639 (2007)

A porphyrin array in the antenna pigments of natural light-harvesting complexes, including chlorophyll, which absorbs photons from light and then enables an energy transfer relay of photon-excited excitons to a reaction centre, is of great importance for light harvesting in nature. To mimic the natural system, self-assembled porphyrins structures are synthesized and their exciton migration characteristic are examined. However, their low stability to physical and chemical environments. Herein, a 2-dimensional porphyrin covalent array with J-J, J-H and H-H aggregated stacking configurations of transition dipoles was synthesised by Yamamoto coupling reaction and verified experimentally using X-ray diffraction and photophysical measurements combined with first-principles calculations of the structures. It is revealed by amplified fluorescence quenching experiment that the ordered array of light-sensitizing porphyrins with J-J and J-H configurations serves as a funnel to transport excitons via both Forster and Dexter energy transfer mechanisms, thereby enabling the rapid and long-range transfer of excitons to the exciton-accepting reaction centre upon panchromatic photon absorptions. This enhanced transfer boosts the collection of photons to the reaction centre by approximately 20-fold, far exceeding the absorbed light energy in the single porphyrin unit. Additionally, the covalent bonds between porphyrins, resulting in the formation of channels for excitons, exhibited chemical stability even in a strongly acidic solution, in addition to thermal stability up to a high temperature in air.

The origin of photo-generated charges in bulk organic semiconductors like P3HT is an important but still unclear question. However, there is evidence that the polaron pair separation and charge stability depend on the aggregation and microstructure.
Here, we present a model system of structurally well-defined organic nanowires from a broad variety of oligopeptide-substituted p- and n-type organic semiconductors. Synergistic π-π stacking of the cores and hydrogen bonding of the chiral oligopeptide substituents are the origin of strong electronic interactions of the chromophores along the nanowires. The supramolecular helicity of these systems results in strict one-dimensional nanoscopic confinement so that the nanowires comprise a single stack of co-facially stacked and helically arranged chromophores at their core.
These structural features are presumably the basis for a facile photo-generation of delocalized positive or negative polaron-like charge carriers with lifetimes of several days and charge densities of up to 4 mol% in the absence of any additional dopant. It is important to note that these lifetimes are orders of magnitude higher than what is typically observed for photo-generated charges in bulk organic semiconductors. We demonstrate that analogous non-chiral derivatives with comparable aggregation strength form two dimensional structures and show no generation of long lived photo-induced charge carriers, thus indicating the importance of the one-dimensional confinement Accordingly, only the helical nanowires show macroscopic electron or hole transport properties that correlate with their spectroscopic features.
The use of hydrogen-bonded groups as an additional structure-directing element could provide a general avenue for novel organic semiconductors with improved morphology and performance. Our system in particular provides a universal and structurally well-defined model for p- and n-type organic semiconductor nanostructures that may provide insights into the fundamental processes of charge generation and transport under nanoscopic confinement.

Molten organic crystalline materials generally show exothermic crystallization under cooling below their melting temperature (T_m), and the molecules revert to the energetically favorable crystalline phase. Even though molecules can be retained as a supercooled liquid by rapid cooling, they readily undergo a phase transformation to crystalline phase upon subsequent heat treatment. In contrast to this general observation, in this presentation, we discuss the molecular design of thermally stable supercooled liquid of diketopyrrolopyrrole (DPP) derivatives and their intriguing shear-triggered crystallization with dramatic optical property changes. Molten DPP8, one of the synthesized DPP derivatives, forms a stable supercooled liquid without crystallization even after slow cooling (0.2 ^oC/min) over a range of 100 ^oC below T_m and through multiple thermal cycles. More interestingly, under applied shear force, this supercooled liquid transforms to its crystal phase accompanied by a 25-fold increase in photoluminescence quantum efficiency and a color change.
By systematic investigation the correlation between the chemical structure of DPP series and supercooled liquid formation, we revealed that the origin of this thermally stable supercooled liquid is a subtle force balance between two different intermolecular interactions acting in opposite directions: aromatic interactions among the DPP core units and van der Waals interactions among the aliphatic side chains. By applying shear force to a supercooled liquid DPP8 film at different temperatures, we demonstrated direct writing of fluorescent patterns (at 25 ^oC) and propagating fluorescence amplification (at 120 ^oC), respectively. Shear-triggered crystallization of DPP8 is achieved even by living cell attachment on a supercooled liquid DPP8 film, demonstrating highly sensitive shear-triggered crystallization about million times more sensitive than typical mechanochromism observed in organic materials.
[1] Chung, K. et al. Shear-Triggered Crystallization and Light Emission of a Thermally Stable Organic Supercooled Liquid. ACS Cent. Sci.1, 94-102 (2015).

The discovery of electrical conductivity in polyacetylene, as well as photo- and electroluminescence in poly(p-phenylene vinylene)s has led to a growing interest in conjugated polymer structures for a vast array of applications, ranging from the development of sensors to Polymer Light Emitting Diodes (pLEDs), among others. The marked relationship between chemical structure and properties for this kind of materials has resulted in the development and study of novel polymer classes with different chemical structures or compositions in an attempt to improve optoelectronic properties, stability, costs or any combination of these.
In the current work, a new family of conjugated polymers obtained from readily available reactants by means of the Claisen-Schmidt reaction was modified by substituting one of the starting materials for different structural isomers; resulting in varying conjugation lengths and, consequently, optical properties. The polymers were prepared in alcoholic basic solutions from the different isomers of terephthalaldehyde and linear or cyclic alkyl ketones followed by a dehydration step in acidic media; resulting in yields of 54 to 94% and products ranging from insoluble powders to soluble resins depending on the nature of the reactants. Furthermore, a modified terephthalaldehyde (2,5-bis(octyloxyl)terephthalaldehyde) was employed in order to obtain soluble materials with the different ketones studied.
Structural characterization by Infrared Spectroscopy (FT-IR) confirmed the presence of the expected functional groups, as well as residual hydroxyl groups from an incomplete dehydration. Furthermore, ¹H Nuclear Magnetic Resonance (NMR-¹H) supported the predicted structure of a conjugated backbone with an open alkyl side chain or a closed ring, depending on the type of ketone used. Additionally, Gel Permeation Chromatography (GPC) of soluble samples determined that polymer chains consist of between 4 to 112 repetitive units, with polymer weight increasing as the dialdehyde isomer went from ortho- to para-.
Further characterization by UV-Vis Spectroscopy show optical band-gaps in the range of 2.80 to 3.23 eV for samples dissolved in THF. On the other hand, photoluminescent behavior was found for ortho- and para- polymers by means of PL spectroscopy, with no significant activity in meta- polymers.

The charge transport in organic semiconductors in field-effect transistor devices is confined mainly to the first monolayer of the molecules at the dielectric-semiconductor interface.^[1] Establishing perfectly ordered 2D-arrangement of simple p-systems is almost impossible due to different and complex growing modes, but can be achieved by chemical modification of respective molecules. By introducing a surface selective anchor group on a chained tail connected to the π-system, the molecules are able to assemble in a self-terminating process on dedicated surfaces. Self-assembled monolayers (SAMs) of different molecular structure and various functionalities are tried and tested in organic electronics.^[2,3]
Here we present an approach for the formation of densely packed, true 2D-confined π-systems by creating SAMs of semiconducting molecules. We employ the p-type organic semiconductor benzothieno[3,2-b][1]benzothiophene (BTBT) which is attached to alkylphosphonic acids of odd and even numbered chain lengths. The structure of the monolayers are investigated with grazing incidence X-ray diffraction and X-ray reflectivity, which has proven to be well-suited for vertical structure analysis.^[4] We can show that well-aligned monolayers with upright standing alkyl-chains are formed and that the active molecules are truly confined to a single 2D ππ-stack of about 0.7 nm thickness.
Self-assembled monolayer field effect transistors (SAMFETs) are built with the BTBT-alkyl-SAM as active monolayer. The good performance of the devices, even on channel areas of 20000 mu;m², reflect the effective hole transport in such confined transport channels. Implementing complementary logic is the key to low power, high noise margin electronic devices. BTBT-alkyl-SAMs are combined with perylene-diimide (n-type) derivatives to build CMOS-type inverters with excellent gain and low hysteresis, in which the BTBT-SAM serves as the p-channel and as dielectric for the n-channel in one FET.
[1] M. Mottaghi, G. Horowitz, Org. Electron.2006, 7, 528.
[2] H. Klauk, U. Zschieschang, J. Pflaum, M. Halik, Nature2007, 445, 745.
[3] M. Halik, A. Hirsch, Adv. Mater.2011, 23, 2689.
[4] A. Khassanov, H.-G. Steinrück, T. Schmaltz, A. Magerl, M. Halik, Acc. Chem. Res., [Online early access] DOI: 10.1021/acs.accounts.5b00022 , published Jun 15, 2015.

Polythiophenes have potential applications as semiconducting materials for organic electronic devices. However, the widespread usage of these polymers is limited by their low performance relative to inorganic semi-conductors. Polymers are soft materials with many conformational degrees of freedom. Limited understanding of how conformational disorder affects optical and electronic properties is a key source of difficulties in developing high performing materials. In this work, we develop a coarse-grained approach based on the tight binding approximation to model the electronic degrees of freedom of a single chain, taking into account conformational degrees of freedom. Particularly important is dihedral disorder, which disrupts extended electronic states. Our model is parameterized using density functional theory (DFT) calculations of the one-dimensional band structures for chains with imposed periodic variations in dihedral angles. The tight binding model predicts valence and conduction bands for these chain conformations that compare well to DFT results. As a first application of our model, we compute the optical absorption spectrum of polythiophene chains in solution. We find good overall agreement with the broadening of the absorption edge, which is a consequence of dihedral disorder of chains in solution as compared to all-trans chains.

The mechanisms behind the threshold-voltage shift in organic transistors due to functionalizing the gate dielectric with self-assembled monolayers (SAMs) are still under debate. We address the mechanisms by which SAMs determine the threshold voltage, by analyzing whether the threshold voltage depends on the gate-dielectric capacitance or not. We have investigated transistors based on five oxide thicknesses and two SAMs with rather diverse chemical properties, using the benchmark organic semiconductor dinaphtho[2,3-b:2&’,3&’-f]thieno[3,2-b]thiophene. Unlike several previous studies we have found that the dependence of the threshold voltage on the gate-dielectric capacitance is completely different for the two SAMs. In transistors with an alkyl SAM, the threshold voltage does not depend on the gate-dielectric capacitance and is determined mainly by the dipolar character of the SAM, whereas in transistors with a fluoroalkyl SAM, the threshold voltages exhibit linear dependence on the inverse of the gate-dielectric capacitance. Kelvin probe force microscopy measurements indicate this behavior is attributed to an electronic coupling between the fluoroalkyl SAM and the organic semiconductor.

Charge separation is one of the fundamental steps to carrier generation in photovoltaic devices. The binding energetics of excitons and the interfacial polaron pairs created from them directly limit device voltage and current. When considering commonly used device models (e.g., Giebink et al. and Renshaw et al.)^1,2, the dissociation kinetics of these polaron pairs are typically modeled with the Onsager-Braun relationship.³ We have explored the polaron pair binding energies derived from first principles calculations as the input to these models. We start with classical relationships for the electrostatics at interfaces and incorporate them into a semi-classical model. We then examine two example cases of polaron pair, one with Frenkel nature and one with a hybrid Frenkel/Wannier-Mott nature, using electronic structure calculation methods. The model predictions so obtained exhibit improved accuracy upon experimental verification.⁴
1N C Giebink, G P Wiederrecht, M R Wasielewski, and S R Forrest. 2010. “Ideal diode equation for organic heterojunctions. I. Derivation and application.” Phys. Rev. B. 82 (15).
²C. Kyle Renshaw and Stephen R Forrest. 2014. “Excited state and charge dynamics of hybrid organic/inorganic heterojunctions. I. Theory.” Phys. Rev. B. 90 (4).
³C. L. Braun. 1984. “Electric-Field Assisted Dissociation of Charge-Transfer States as a Mechanism of Photocarrier Production.” J. Chem. Phys. 80 (9):4157-4161.
⁴Morris S.E., Bilby D., Sykes M.E., Hashemi H., Waters M.J., Kieffer J., Kim J., and Shtein M. 2014. “Effect of axial halogen substitution on the performance of subphthalocyanine based organic photovoltaic cells.” Organic Electronics. 15 (12):3660 - 3665.

Characterizing organic semiconductors using conventional DC field-effect transistor measurements can often be difficult due to parasitic device effects which can mask the channel behavior. Disentangling parasitic effects and their temperature dependence from intrinsic channel properties is a prerequisite to concrete transport-microstructure correlations and the eventual development of predictive structure-function relationships. Therefore a detailed investigation of organic transistors, which includes complementary measurement techniques, is needed to discern the intrinsic properties of the channel. We report here on the use of impedance spectroscopy to disentangle the effects of contacts on device operation and extract the electrical properties of the channel which we correlate with conventional DC measurements. Using a transmission line model to fit the AC impedance of the transistor channel coupled with a parallel resistor-capacitor model of the contact impedance, we gain insight into charge transport within the channel while discriminating the effects due to contact impedance. We have previously shown that rapidly changing contact resistance can govern the current flow and result in ambiguity in calculations of the mobility (µ) and threshold voltage at low gate voltages in DC current-voltage measurements of vapor phase grown single crystal rubrene transistors with gold source and drain contacts. By replacing the gold with platinum contacts we observe the expected current-voltage characteristics indicative of the decreased influence of the contact impedance. We extend this study to regioregular poly(3-hexylthiophene-2,5-diyl) (rr-P3HT) transistors with distinct degree of order and defects which are attained by differentiating solidification kinetics either by the different solvents (i.e. chloroform and 1,2-dichlorobenzene) or spin speed. Impedance measurements show that contact resistance per channel width is two orders of magnitude higher in rr-P3HT with Au contacts (10⁹ Omega;/m) as compared to rubrene/Au (10⁷ Omega;/m) at high field (high |V_GS|), and lacks the strong gate dependence observed in rubrene transistors at low field. By changing the degree of ordering in rr-P3HT we observe the mobility in the channel increases by 10 x and the mobility shifts from exponential mu;(V_GS) behavior in rr-P3HT with low order and high paracrystallinity to constant mu;(V_GS) for highly ordered rr-P3HT with low paracrystallinity.

Conjugated polymers have an electron delocalization throughout their structure, this feature gives them special properties, such as radiation absorption, luminescence and semiconductivity. These materials have been extensively investigated for their potential application in the development of optoelectronic devices, because they present important characteristics such as light weight, high flexibility and solution processability, making these polymeric materials assets as light emitting diodes (PLEDS) and photovoltaics (PVs). However, since the conjugated structure possesses alternating single and double bonds, this results in a planar configuration and rigidity and solubility problems.
In this work the synthesis of a new family of photoluminiscent polymers by the aldol condensation reaction is presented. These polymers were prepared in a basic medium using linear alkyl ketones of different lengths in order to obtain alkyl side chains of varying length in the products and an aromatic dialdehyde; dehydration was performed on the resulting material in an acid medium resulting in yields of 81 to 94% and products ranging from insoluble to soluble powders with increasing ketone length.
Fourier Transformed Infrared Spectroscopy (FT-IR) was used for structural analysis of the products to corroborate the predicted functional groups; as well as to observe an increase in the signal corresponding to the alkyl side chains by increasing the length of the precursor ketone, this result is corroborated by ¹H Nuclear Magnetic Resonance (NMR-¹H) with which the backbone of the molecule was studied.
Optical characterization was performed by means of UV-Vis spectroscopy and Photoluminisence where red-shift in the emission maxima of the obtained materials with increasing side-chain length was obtained.
A trend of increasing solubility was found with increasing ketone size, with some products presenting high solubility; which allowed us to confirm the molecular weight of the polymers obtained by Gel Permeation Chromatography (GPC).

Organic thin-film transistors (TFTs) have great potential for various applications, such as flexible displays or sensors [1,2]. For some of these applications, the TFTs must be able to withstand temperatures in excess of 100 degrees Celsius, for example to permit the integration with devices or components that require high process temperatures, or to make it possible that the devices can be subjected to the standard sterilization protocols required for biomedical applications [3]. Kuribara et al. [3] have recently shown that the carrier mobility of low-voltage organic TFTs based on the small-molecule semiconductor dinaphtho[2,3-b:2&’,3&’-f]thieno[3,2-b]thiophene (DNTT) and encapsulated with a parylene/Au/parylene stack remains at about 70% of its initial value when the TFTs are subjected to a temperature of 130 degrees Celsius in a nitrogen-filled glove box. In this work, we have investigated how the thermal stability of low-voltage DNTT TFTs is affected by the encapsulation of the TFTs and by the ambient in which the thermal treatment is performed. We have fabricated DNTT TFTs with the same aluminum oxide/alkylphosphonic acid self-assembled monolayer gate dielectric employed by Kuribara et al. [3,4]. Some of the TFTs were encapsulated with a layer of vacuum-deposited Teflon [5], while others were left without encapsulation, and the thermal treatment was performed either in nitrogen or in air. We found that the encapsulation with Teflon has virtually no effect on the thermal stability of our TFTs. In contrast, the ambient in which the thermal treatment is conducted was found to have a measurable effect, but in an unexpected way: When the thermal treatment is carried out in nitrogen, the mobility drops to 70% of its initial value at a temperature of 160 degrees Celsius and to close to zero at 170 degrees Celsius, whereas when the treatment is performed in air, the mobility remains at 75% of its initial value up to a temperature of 160 degrees Celsius and at 60% up to 180 degrees Celsius. An explanation for the somewhat surprising observation that the thermal stability of the TFTs is better in air than in nitrogen was provided by scanning electron microscopy of the vacuum-deposited DNTT films before and after thermal treatment: While the DNTT films remain continuous and conducting when the heating is carried out in air, the semiconductor morphology undergoes a dramatic change, including the formation of large, thick crystals and a complete loss of percolation, when the heating is conducted in nitrogen.
[1] M. Noda et al., J. SID, vol. 19, p. 316 (2011)
[2] T. Someya et al., Adv. Mater., p. 3799 (2010)
[3] K. Kuribara et al., Nature Commun., p. 1721 (2012)
[4] S. Bisoyi et al., Org. Electronics, vol. 15, p. 3173 (2014)
[5] C. L. Fan et al., Org. Electronics, vol. 14, p. 2228 (2013)

Poly(3-hexylthiophene) (PHT) is one of the conducting polymers, and is widely used as donor polymers in organic thin-film solar cells. By alternating copolymerize donor molecules with acceptor molecules, the push-pull effect can lower LUMO energy levels and band gaps. As a result, it becomes possible to provide efficient light absorption and improvements in the photoelectric conversion efficiency. In this study, we evaluated the push-pull effect of alternate copolymers having hexylthiophene (HT) as a donor and benzothiadiazole (BT) as an acceptor molecules.
We calculated the quantum properties of three dimers (HT-BT, HT-HT, and BT-BT) using Gausian 09. LUMO orbitals of HT-BT molecules are localized at the acceptor molecule, suggesting that HT-BT have a push-pull effect. HT-HT and BT-BT did not show such phenomenon. HOMO and LUMO energies and band gaps of the dimers were calculated by TD-DFT (B3LYP / cc-pVDZ). The energy levels of the HOMO of the dimers are not significantly different. On the other hand, the LUMO energy level of HT-BT is lower than that of HT-HT and BT-BT due to the push-pull effect. Band gap energy of HT-BT is smaller than that of HT-HT. This fact suggested that the push-pull effect provide solar cells with excellent performances.
PHT-BT was synthesized by Suzuki-Miyaura coupling of 4,7-bis(4,4,5,5-tetramethyl- 1,3,2-dioxaborolan-2-yl)-2,1,3,-benzothiadiazole and 2,5-dibromo-3-hexylthiophene. The weight-average molecular weight (Mw) and polydispersity index of PHT-BT was 3,000 g mol-1 and 1.14. UV-vis absorption and cyclic voltammetry of those polymers were measured. In UV-vis absorption spectra of PHT-BT, maximum absorption peaks were observed at 325 nm and 561 nm. While the bandgap of PHT having Mw = 15, 800 was 2.16 eV determined from the band edge, the band gap of PHT-BT which is calculated from the band edge (710 nm) was 1.76 eV. This is consistent with the calculation data shown above. (1955 words)

Charge transfer (CT) excitons are Coulombically bound electron and hole pairs located in spatially separate regions across the donor/acceptor interface. CT excitons play an important role in both light emission of organic light emitting devices and the generation of photocurrent in organic photovoltaic. For some donor/acceptor blends the triplet CT states have lower electronic excitation energy than the singlet CT states, and in these materials the photoluminescence and photocurrent generation exhibit a non-trivial magnetic field dependence due to its effect on singlet-triplet intersystem crossing and reverse intersystem crossing rates. Recent nanoscale-imaging experiments have demonstrated that in these materials bound electron-hole pairs can move geminately over distance of 5-10nm, confirming the transport of CT excitons despite strong Coulombic attraction. These experiments can be understood in the context of a theoretical model combining kinetic Monte Carlo and the quantum master equation. The model contains a minimal set of physical elements, and yet is able to quantitatively reproduce the time- and magnetic field- dependent experimental results. More importantly, the model provides insights into properties that otherwise cannot be obtained from experiments. Here I present the details of this model along with the physical insight it has provided into this particular class of materials.

Boroxine rings are formed through the dehydration of boronic acids. While the dehydration of boronic acids is a reversible reaction, the stabilization of boroxine rings by the coordination of nitrogen atom in amine molecules to boroxines has been reported. Nowadays, star-shaped polymers having boroxine as a core and polymer as a side chain, are researched. Those are attractive synthetic materials because of its ability to form unique structures by covalent bonds or supramolecular interactions, and its reversible covalent chemistry and Lewis acidity derived from boron-containing cores. In this study, low molecular boroxine-amine complexes using phenylboronic acid were synthesized as model compounds. Boroxine-amine complexes are formed by mixing boronic acids with amine molecules in the molar ratio of 3 : 1. As model, complex with triphenylboroxine (TPB) and pyridine (Py) was prepared. From 1H NMR spectrum, TPBbull;Py complex was found to be more stable than non-coordinated boroxine, TPB, under water existence. Not only pyridine, but also pyrrolidine, methylpyrrolidine (MePyrr), pipelidine, methylpipelidine, and ethylamine coordinated to boroxine rings. The single crystal X-ray of TPBbull;MePyrr showed the crystal structure was similar to that of TPBbull;Py, while the N-B-C angle was slightly larger. This suggested that amines affected to the structure of the complexes. In addition, the synthesis of boroxine rings with π conjugated poly(3-hexylthiophene) (PHT) side chains were carried out. PHT was synthesized via catalyst transfer polycondensation. The number-average molecular weight of PHT was 2,800, and the molecular weight distribution was 1.25. Boronic acid terminated PHT (BP-PHT) was synthesized by inserting boronic ester group via Suzuki-Miyaura coupling and subsequent deprotection. Then, BP-PHT were mixed with pyridine in the mole ratio of 3 : 1. The formations of boroxine rings were confirmed by 1H NMR.

Small organic molecules have come to the forefront in past decade when discussing new trends in fabricating and understanding the physics of organic electronic devices, in particular due to the record high electronic mobilities and exciton diffusion lengths reported for these systems. These molecules belong to an intermediate regime where excitons are neither fully localized on individual molecules like the Frenkel excitons of most molecular crystals nor fully delocalized like the Wannier excitons of inorganic semiconductors.
Here we propose a radical new way of looking at excitonic behavior in organic crystalline semiconducting thin films, using substituted phthalocyanines derivatives as a case study. We focus on mixtures (alloys) of metal and metal-free Pc with octabutoxy side-chains (OBPc), fabricating films using a solution-processing, novel hollow pen-writing technique¹ that produce millimeter sized grains with long range macroscopic order. The solutions used in thin film deposition contained Co, Ni, Mn, or CuOBPc molecules mixed in ratios with the metal-free Pcs (H₂OBPc) ranging from 1:1 to 1000:1 (with 1000:1 containing roughly 1 metal Pc for every 1000 metal-free Pc). Excitons are investigated using temperature dependent absorption/transmission/linear dichroism and time and polarization- resolved photoluminescence (TRPL) spectroscopy. All optical characterization and grazing-incidence X-Ray scattering indicate a uniform mixing of the species is achieved in films without loss of the long range order previously observed in individual species.
The spectroscopy experiments produced two important results that offer great insight into the fundamental quantum mechanics of delocalized excitons in small molecule semiconductors. First, they indicate that the delocalization² of bandgap excitons extends over tens of lattice sites and second, the presence of the metal-Pc molecule inside the metal-free Pc film introduces a radiative transition that may involve localized electronic states of both species. Low temperature, high magnetic field Magnetic Circular Dichroism (MCD) reveal some of these systems also exhibit enhanced magnetic exchange interactions mediated by the same delocalized exciton states.
These studies also indicate that solution-processing deposition techniques in tandem with chemical synthesis design of small molecule soluble derivatives is a viable avenue for exploring organic analogues of alloyed systems such as SiGe where the role of conduction electrons is played by the delocalized π-electrons.
This material is based upon work supported by the National Science Foundation under Grant Nos. DMR-1062966, DMR-0821268, DMR-1056589, DMR-0722451
¹ I. Cour, Z. Pan, L. T. Lebruin, M. A. Case, M. Furis, R.L. Headrick, Organic Electronics13 419-424 (2012)
² N. Rawat, Z. Pan, L. W. Manning, C. J. Lamarche, I. Cour, R. L. Headrick, R. Waterman, A. R. Woll, M. Furis., J. Phys. Chem. Lett. 6, 1834-1840 (2015)

Semiconducting conjugated polymers have received considerable interest as gain media for lasers and amplifiers. In recent years, it has been realized that polymer technology is very well suited to develop datacom for providing low-cost ultra-high bandwidth services to the end-user due to its low cost easy fabricated through spin-coating and photochemical processing. A promising material for such applications is poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT). F8BT has a low stimulated emission (ASE) threshold and exhibits large net optical gain at 570 nm which is one of the polymethylmethacrylate (PMMA) low-loss transmission wavelengths. F8BT shows liquid crystallinity and can be readily aligned into a monodomain by use of an alignment layer. The refraction indexes of the materials are greatly affected by the orientation of the polymer chains.
Here we report our detailed study on F8BT plane waveguide structure which was obtained via patterning chain oriented area. The index exhibits significantly increased in the selected areas due to chain alignment but without thickness change of the films. Experimental investigation including microscopy images, polarized absorption and PL spectra and micro-Raman spectroscopy study of the patterned samples clearly shows the difference between the aligned area and isotropic area. The gain property study on chain oriented F8BT film shows that the net optical gain of oriented films are dependent on the excitation polarization, chain alignment axis and excitation geometry. High net gain coefficient and high thermal stability of ASE can be obtained by proposing specific optimised excitation configurations. A stronger waveguide confinement can also be achieved compared with an isotropic film due to the higher index in this direction. Furthermore, a series of binary blend films were made in different ratio of a red emission polymer (guest) in F8BT (host). The polymer chains were subsequently oriented to observe the relationship between the degree of chain orientation and the proportion of host and guest materials. Our demonstration suggest that patterning chain oriented area can be a promising approach to achieve plane waveguide device in optical fibre high-speed data communication systems.
We thank the National Natural Science Foundation of China (Grants 61376023 and 61136003), the National Key Basic Research Program of China (973 Program, 2015CB932203).

One of the target applications for organic field-effect transistors (OFETs) on flexible substrates is the backplane for active-matrix organic light emitting diode (AMOLED) displays. In contrast to an active-matrix liquid crystal displays (AMLCDs) pixel an OLED is not voltage but current driven. The current supplied by the driving circuit of the pixel determines its brightness. In literature, electrical bias stress measurements that investigate transistor performance under electrical stress do not keep the drain-source current constant, as it is nessary for AMOLED application. Instead - for the sake of a simple measurement setup, the gate-source bias is kept constant and changes of the drain-source current are detected. In most cases, the drain-source current decreases over time and, accordingly, the stress level decreases as well. Moreover, the extracted threshold shift value (#8710;V_th) that is a measure of the stress resistance of the OFET, is extracted from a transfer measurement that itself influences the value since during measurement the gate-source bias is inversed possibly leading to partial relaxation of the induced stress.
In this work we establish an alternative stress protocol that allows to maintain a constant stress level over the whole duration of the bias stress measurement. Moreover this method allows us to continuously measure #8710;V_th and avoid the need to interrupt the bias stress in order to record transfer characteristics. Therefore we implement a feedback control loop to our measurement software that continuously adjusts the gate-source voltage in order to keep the drain-source current at a constant level. Changes in the applied gate-source voltage can be directly correlated to #8710;V_th as long as other effects such as mobility degradation can be neglected.
We apply the two stress protocols to a p-type semiconducting polymer under various conditions. Differences between stress measurements using the conventional protocol and our feedback loop control method are presented.
The most prominent candidate to cause device degradation is water. Therefore we evaluate the influence of water on both, device performance and bias stress behavior and compare temperature dependent measurements in the range from room temperature to 100°C in vacuum and ambient condition.

Here, we developed 2,2&’-bipyridine-based materials end-capped with dipyridylphenyl groups, referred to as 4,4&’-BPy3 and 6,6&’-BPy3 for high-performance organic light-emitting devices (organic LEDs). These BPy derivatives possess elaborated pai-pai stacking arising from coplanarity of 2,2&’-bipyridine moiety with nitrogen atoms in trans position. In addition to the pai-pai interaction, peripheral pyridine rings can form intermolecular CHmiddot;middot;middot;N weak hydrogen bond network. These compounds have a deeper lowest unoccupied molecular orbital (LUMO) (Ea=3.0~3.1 eV) and a highest occupied molecular orbital (HOMO) (Ip=6.4~6.8 eV) energy level compared to conventional electron transport material, 2,2', 2"-(1,3,5-benzinetriyl)-tris (1-phenyl-1-H-benzimidazole) (TPBi) (Ea=2.8eV, Ip=6.3eV). These compounds show high thermal stability indicating morphorogical stability in thin solid film.
The X-ray structure of single crystal of BPy derivatives apparently reveals that coplanar conformation of the 2,2&’-bipyridine moiety with a dihedral angle of 0#730;. Further, weak intermolecular hydrogen-bond networks were also observed. We analyzed their molecular orientation in vacuum-deposited films using variable-angle spectroscopic ellipsometry (VASE). Consequently, these compounds formed a horizontally oriented film. Then, we employed BPy derivatives in fac-tris (2-phenylpyridine) iridium (III) [Ir(ppy)3] based organic LEDs as an electron transport layer (ETL). The device with 6,6&’-BPy3 exhibited low operating voltages of 2.8, and 3.2 V at luminances of 100 and 1000 cd m-2, respectively. At 1000 cd m-2, this device gave a power efficiency of 74 lm W-1, a current efficiency of 75 cd A-1, an external quantum efficiency of 21%.

Organic photovoltaics (OPVs) are a low-cost solar technology and single-junction polymer-OPVs have recently achieved power conversion efficiency (PCE) greater than 10%, a benchmark widely touted for enabling OPV commercialization. Therefore, considering the three key success parameters for any solar energy technology—cost, efficiency, and lifetime—improving device stability becomes the most imperative problem to solve in making OPV a market-competitive technology. So far, the steady improvement in the power conversion efficiency of polymer-based OPV has been driven by the development of new polymer absorbers without much consideration of their intrinsic stability to photo-degradation. In our group we are investigating the stability of active layers to identify components in their molecular structure that are susceptible to photo-oxidation. We have used a combinatorial materials approach to identify degradation pathways and design fundamentally stable polymer building blocks, which we present in a case study of high efficiency (>9%) materials. Furthermore, we have studied the effect of high boiling point solvent additives often used to optimize the active layer morphology during device fabrication. We find that residual amounts of the solvent additive remain in the film after deposition and that in some cases the presence of the additive is deleterious to the long-term photo-stability of the active layer.

Exciton generation, migration and dissociation are basic processes in the operation of organic solar cells, which need to be understood and tuned to utilise the full potential of these devices.¹ In a bulk heterojunction solar cell, the exciton formed in one component (donor) of the blend must diffuse to an interface with the acceptor, at which the offset of the energy levels leads to charge separation. Understanding organic solar cells therefore requires the understanding of exciton diffusion.² Materials with high exciton diffusion coefficient are desirable as solar cell materials, as this will ensure the excitons generated in the active layer reach the interface and generate charges. Conversely in organic light-emitting diodes and lasers small exciton coefficients are desirable to minimize the exciton-exciton annihilation.
Here we report a study of exciton diffusion using time-resolved fluorescence measurements in two solution processable small molecules, promising as organic photovoltaic materials. The materials studied are p-DTG(FBTTh₂)₂ and p-DTG(F2BTTh₂)₂ which differ in an additional fluorine substitution in the benzo[1,2,5]thiadiazole backbone. Exciton diffusion in these materials are investigated by surface quenching and exciton-exciton annihilation methods.³ In the surface quenching method, excitons generated in the bulk of the material diffuse to the quencher surface and undergo electron transfer, resulted in a faster decay of the fluorescence emission in the material. The diffusion coefficient and length can be derived by modelling these decays using one dimensional diffusion model. A threefold enhancement in the diffusion coefficient from 0.47 x 10^-3 cm² s^-1 to 1.4 x 10^-3 cm² s^-1 was observed on subtle changes in the substitution pattern in the conjugated backbone. Self-quenching of excitons (exciton-exciton annihilation) under high excitation fluence was also investigated in these materials to understand the changes in the diffusion coefficient with additive (Diiodooctane) and thermal annealing conditions. Substantial enhancement in diffusion coefficient was observed with small amount of additive and under thermal annealing. These could be directly attributed to the enhancement in the crystalline order as evident from the atomic force microscopie phase images. These studies give an insight into the variation in diffusion coefficient and lengths with subtle changes in the substituents in the conjugated backbone as well as the role of additive and annealing in these small molecule donors for organic solar cells
Reference:
1. Menke, S. M., Holmes, R. J. Energy Environ. Sci., 2014, 7, 499.
2. Ruseckas A., Shaw P. E., Samuel I. D.,W. Dalton Trans, 2009, 10040.
3. Masri, Z., Ruseckas, A., Emelianova, E. V., Wang, L., Bansal, A. K., Matheson, A., Lemke, H. T., Nielsen, M. M., Nguyen, H., Coulembier, O., Dubois, P., Beljonne, D. and Samuel, I. D. W. Adv. Energy Mater., 2013, 3, 1445.

PEDOT:PSS is still not the ideal hole-transport material for inverted planar perovskite solar cells, and that because of its inefficient electron blocking capability, in addition to its acidic behavior and hygroscopic nature that lead to its poor chemical stability. Here, we have a new organic hole transport molecules, 3,6-difluoro-octakis(4-methoxyphenyl)benzene-1,2,4,5-tetraamine, very easy to synthetize, has a good hole mobility, and relatively good stability. It has an excellent transparency in the visible light spectrum range and simple coating process. When used in inverted planar CH₃NH3PbI₃ solar cells, the efficiency is 12.5% with a Voc reaching than 1.07V, a Jsc of 17.5mA/cm² and a fill factor of 0.66.

Solution-processed OPVs utilizing small molecule donors (SMDs) have been the focus of intense international research in recent years, and have produced PCEs upwards of 10% in a relatively short period of time compared to devices containing their polymer donor counterparts. The allure of SMDs as candidates for OPV technologies is further enhanced based on the fact that they offer a solution the numerous and costly challenges associated with large-scale (and reproducible) production and purification of high performance polymers. Although SMDs have a higher and more reproducible tendency to crystalline in films, the precise control of morphology in the solar cell blend remains an important and ongoing effort. We have synthesized a series of SMDs, based on conjugated building blocks found in low-bandgap polymers, in order to understand how tuning the molecular structure changes the solution-processing parameters required to optimize charge generation, collection, and ultimately device performance. Data and trends derived from a combination of studies including x-ray diffraction, time-resolved microwave conductivity, ultrafast transient absorption spectroscopy, DFT calculations, and device fabrication and performance will be presented.

With a power conversion efficiency under one sun illumination 75% higher than the reference P3HT:PCBM solar cells [1], PBDT[2F]BT:fullerene blends enable performances so far unrivalled amoung solar cells based on wide-bandgap polymer. Using femto- to microsecond transient absorption, we investigate the origin of those outstanding properties. We find that geminate recombination losses of photogenerated charge carriers are nearly absent in PBDT[2F]BT:fullerene blends. Additional field-dependent measurement as well as morphological and energy levels characterization reveal optimal conditions for every parameter with efficient and field-independent charge generation, enabling excellent short-circuit current and fill factor. Strikingly, this outstanding generation is achieved in spite of a rather low offset between the polymer&’s excitons and the blend&’s charge transfer state energy levels, which allows for an open circuit voltage as high as 0.9 V. Interestingly, replacing the fluorine substituents by hydrogen in those polymers results in only moderate performances, thus highlighting the importance of molecular design. This is discussed in terms of energy levels and blends morphology.
[1] J. Wolf et al., accepted inChem. Mater., 2015

Intrinsic paramagnetic states have been studied extensively in cis- and trans-poly(acetylene) thin films. Unexpectedly, condensed-phase paramagnetic states were observed for selected small-molecule organic semiconductors, such as dibenzotetraphenyldiindenoperylene (DBP) and non-planar phthalocyanines (ClAlPc and Cl₂SiPc). These paramagnetic states were characterized using primarily X-band continuous-wave electron paramagnetic resonance (CW EPR) spectroscopy at room temperature. Paramagnetic states were only observed for selected molecular structures in the condensed phase, and extensive negative controls that lack an observed paramagnetic state were investigated. Various purification and chemical doping experiments were also conducted to confirm that the empirically observed paramagnetic states in DBP, ClAlPc and Cl₂SiPc do not arise due to external impurities, polarons (radical cations or anions) or simple chemical exchange. Further, the aforementioned paramagnetic states in DBP, ClAlPc and Cl₂SiPc were only observed to evolve upon molecular aggregation, with a detectable EPR signal appearing upon aggregation of approximately 4000 molecules. Coupled EPR and X-Ray diffraction studies reveal that the intensity of the EPR signal corresponding to the observed paramagnetic states is dependent on the crystal phases and domain wall density within the sample, similar to previous observations made for poly(acetylene). Theories for understanding the origin of these heretofore-uncharted observations will be discussed. Understanding the origin of these intrinsic paramagnetic states in the condensed phase will provide valuable insight into materials selection and design principles for organic electronic devices.

Organic self-assembly monolayers (SAM) are a popular system used in nanoscience as an easily characterized model of self-assembly and as an important tool for surface functionalization, stabilization, and templating. Predictable control of SAM organization on surfaces would be of great utility but is quite limited at this point. Experimentally observed TTF-TCNQ (TTF = tetrathiafulvalene; TCNQ = 7,7,8,8-tetracyanoquinodimethane) SAMs on noble metal surfaces show low dimensional organic charge-transfer complexes can result in highly organized structures even in lower coverage regimes. Until now there was no computational methodology to explain the self-assembly and predict ordering of such nanostructures and their variations with growth conditions. For bimolecular charge-transfer systems it was hypothesized that differential charging on the surface enables the electrostatic potential to be a significant interaction leading to long range order.
Starting from the TTF-TCNQ 2D structure we examine the interactions via Monte Carlo simulations of many possible (virtually innumerable) finite island structures for bimolecular charge-transfer complexes, using a scalable parallelized electrostatics code written in-house. The parameters for these simulations are derived from first principles density functional and hybrid functional calculations of the individual molecules and small clusters. We examine how modifying the differential charging in TTF-TCNQ affects the organization of the 2D islands on a surface, including stability, shape and mobility of molecular islands. We anticipate that charge-transfer complex SAMs based on a variety of organic molecules are possible and will also show highly organized phases. The code is compatible with materials by design methodologies to search for other promising 2D charge transfer molecules.
This research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility

Polarimeters are optical detectors that can measure the intensity and polarization state of light (i.e. the 2-dimensional Stokes parameters). Polarimeters are important tools for many applications, such as aerosol characterization, demultiplexing in telecommunications, and biological imaging. Currently, polarimeter imaging is commonly done with focal plane array (FPA) technology that relies on a super-pixel approach in which the FPA&’s pixels are segregated into 2x2 pixel unit cells. Each sub-pixel is composed of a polarizer followed by an inorganic photodetector. This architecture is robust and has high temporal resolution, however it requires complex fabrication and the super pixel design limits spatial resolution, and it is thus prone to imaging artifacts. Here, we present a polarimeter based on a vertical stack of semitransparent organic photovoltaic (OPV) detectors. The OPV device is designed with a biaxially oriented active polymer semiconductor layer that results in intrinsic polarized light sensitivity. A set of 3 semitransparent OPV devices are fabricated and stacked vertically with an optimized device design for high performance optical characterization. The vertical stack and semitransparent detecting capability improves upon the FPA design by measuring the light at a single spatial location while maintaining the high temporal resolution.
In this talk, we will discuss the device design and fabrication approach used to achieve the organic polarimeter including: transparent electrode design, orienting the active layer films, and stacking the individual OPV devices. We will also demonstrate the capabilities of the device to measure the Stokes parameters of incident light, and discuss the unique advantages of this approach for application implementation. Importantly, this work highlights the ability to exploit intrinsic organic electronic material properties to realize optoelectronic devices that are not feasible with inorganic semiconductors.

We report a method to characterize the density of gap states (DOS) in the wider energy range for organic thin film transistors (OTFTs) by a combination of gate-bias dependent field-effect mobility measurements [1] and current DLTS measurements with a bipolar rectangular weighting function in the unit of coulomb [2]. From gate-bias dependent field-effect mobility measurements at various temperatures in the range from 100 to 300 K, the DOS is obtained in the energy range near the band edge to 0.2 eV. Current DLTS at the same temperature range gives the DOS in the energy range from 0.15 to 0.55 eV. Moreover, from isothermal current DLTS at 300 K, it is possible to obtain the DOS with deeper energy levels depending on measurement time. Bottom gate top contact OTFTs were fabricated using alumina gate dielectric deposited by atomic layer deposition on thienothiophene derivatives (C10-DNTT) as the organic semiconductor. Cr and Au were evaporated as the gate and source/drain electrodes, respectively. The DOS was obtained in the energy range from 0.04 to 0.7 eV from the valence band edge by the combination of gate-bias dependent field-effect mobility and current DLTS measurements in the temperature range from 100 to 300 K. The area DOS was found to vary from 5.1x10¹⁴ cm^-2eV^-1 at 0.04 eV to 1.1x10¹² cm^-2eV^-1 at 0.70 eV. The DOS will be presented for other types of OTFTs.
The authors acknowledge Nippon Kayaku Co., Ltd for supplying the DNTT materials. This work was financially supported by NEDO.
[1] D. V. Lang et al., Phys. Rev. Lett. 93, 086802, 2004.
[2] K. Nakamura et al., Materials Research Society Fall meeting, M5.80, 2013.

We report the status of our photocrosslinker research program to develop sterically-hindered bis(fluorophenyl azide)s (sFPAs) for practical polymer organic semiconductor technologies. Previously we have reported that sFPA photocrosslinkers can be mixed into polymer organic semiconductors (OSCs) to effect general non-specific photocrosslinking when exposed to deep-ultraviolet light (254nm DUV) without degrading their essential semiconductor properties.[1] We have now elaborated and extended the initial set of sFPAs to produce an entire family of sFPAs each one suitable for specific applications in different polymer organic semiconductor, including for i-line lithography applications. We report that exciton and charge-carrier traps can be nearly fully eliminated even for the most demanding organic semiconductors. We report new device architectures, including photo-patterned transistor circuits that are now possible in conventional lithography tools, and discuss new insights into the structure-property relations of these photo-crosslinkers. This opens the possibility to elaborate advanced processing methodologies and develop sophisticated device architectures for polymer organic semiconductor devices that are no longer constraint by the limitations of solution processing.

Organic semiconductors have shown a wide array of applications including uses in photovoltaics, transistors, sensors and biocompatible electronics. Despite tremendous performance gains over the last decade, the field lacks clear design rules for a systematic advancement as well as a clear understanding as to the mechanisms of charge transport in these unique semiconductors.
We performed in-operando near edge X-ray absorption fine structure spectroscopy (NEXAFS) in transmission geometry on organic field effect transistors (oFET) in order to identify the polaron absorption spectrum and the dependence of the spectral features as a function of operating conditions, morphology and charge density. For our studies poly-3-hexylthiophene (P3HT) served as model system, due to the ease with which morphology can be tuned, and extensive literature on P3HT as an oFET active layer. Due to its p-type characteristics, in-situ operation of the transistor creates an accumulation layer of p-polarons, giving rise to a pre-carbon K-edge absorption peak.
Films of different morphology were obtained by varying the spin casting and annealing conditions. The absorption spectra were related to the morphological information obtained by grazing wide-angle X-ray diffraction (GIWAXD).
A careful understanding of the charge transport mechanism will enable to purposefully fine tune the chemistry, structure and morphology of organic semiconductors to enhance their performance.

Here, we report a highly crystalline and self-assembled 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-Pentacene) which is formed by a simple spin-coating method. Instead of using a blended formulation of a semiconducting small molecule and an insulating polymer matrix, we used a spherulite-crystalline domain of TIPS-pentacene which is self-assembled on partially crosslinked poly-4-vinylphenol:poly(melamine-co-formaldehyde) (pc-PVP:PMF) gate dielectric. The partial crosslinking of polymer gate-dielectrics allows TIPS-pentacene molecules in a solvent to permeate into it while maintaining its complex network even after the solvent evaporation. Experimentally, we found that the amount of crosslinking in pc-PVP:PMF affected the permeation and extraction of TIPS-Pentacene molecule in pc-PVP:PMF network. TIPS-pentacene molecule permeates into pc-PVP:PMF spontaneously during dwelling, and then the solvent evaporation during spinning boosts the extraction of TIPS-Pentacene solution onto pc-PVP:PMF surface. The residue of the solvent in pc-PVP:PMF bulk evaporates slowly, which encourages the self-assembly of TIPS-Pentacene molecules. For this reason, the size of crystalline TIPS-Pentacene grains increases with the amount of permeated TIPS-Pentacene solution, and finally grows to the millimeter size under the optimized condition (the surface energy of the pc-PVP:PMF layer is 54.46 mJ m-2 / the dwell time is 10 sec). As a result, a high-performance organic transistor with a maximum hole mobility of 3.40 cm²V^-1s^-1 was demonstrated. The mobility value is one of the highest mobility values for TIPS-pentancene field-effect transistor. Furthermore, The morphology and structural properties of TIPS-Pentacene/PVP:PMF films have been analyzed using a combination of polarized optical microscopy, atomic force microscopy, 2D-grazing incidence wide-angle X-ray scattering, and secondary ion mass spectrometry.

The molecular design of high-performance conjugated polymers is based on molecular-orbital energetics and crystal-engineering concepts that are capable of efficiently controlling the frontier orbital energy levels and the π-orbital overlap. The conventional notion that the energy levels and the band gaps are primarily governed by conjugated building blocks in the backbones has driven rapid progress in the molecular design of conjugated backbones. In recent years, however, studies have shown that side-chain engineering can significantly affect molecular packing and the π-planar distance, in addition to its conventional role as a solubilizer. Side-chain engineering via tuning of the alkyl-chain branching point has been proven to boost electrical performance in conjugated polymers. To date, however, despite the rapid progress in the conjugated backbone and the side-chain engineering, very little is known about an odd-even effect of alkyl side chains of conjugated polymers due to their relatively disordered or complex conformation compared with small molecules. Herein, a series of diketopyrrolopyrrole-based polymer semiconductors containing even or odd numbers of linear spacer groups are synthesized to investigate odd-even effect of alkyl side-chain spacer length on the molecular packing and charge transport characteristics. Furthermore, the optimal side-chain geometries for the most efficient charge transport are determined by systematic side-chain engineering. Investigation of structure-property relationship in this work can provide new insight into the molecular design of high-performance polymer semiconductors.

We report the results of an extensive investigation of metal-molecule-metal tunnel junctions based on oligophenylene dithiols (OPDs) bound to several types of electrodes (M₁minus;Sminus;(C₆H₄)nminus;Sminus;M₂, with and M_1,2 =Ag, Au, Pt) to examine the impact of molecular length (n) and metal work function (Phi;) on junction properties. Our investigation includes (1) measurements by scanning Kelvin probe microscopy (SKPM) of electrode work function changes caused by chemisorption of OPD self-assembled monolayers (SAMs), (2) measurements of junction current-voltage (I-V) characteristics by conducting probe atomic force microscopy (CP-AFM) in the linear and nonlinear bias ranges, and (3) direct quantitative analysis of the full I-V curves. Further, we employ transition voltage spectroscopy (TVS) to estimate the energetic alignment ε_h=E_F-E_HOMO of the dominant molecular orbital (HOMO) relative to the Fermi energy E_F of the junction. Where photoelectron spectroscopy data are available, the values agree very well with those determined by TVS. Using a single-level model, which we justify via quantum chemical calculations at post-DFT level and additional UV-visible absorption measurements, we are able to quantitatively reproduce the I-V measurements in the whole bias range investigated (~1.0 minus; 1.5 V) and to understand the behavior of ε_h and Γ (contact coupling strength) extracted from experiment. We find that Fermi level pinning induced by the strong dipole of the metal-S bond causes a significant shift of the HOMO energy of an adsorbed molecule, resulting in exhibiting a weak dependence with the work function Phi;. Both of these parameters play a key role in determining the tunneling attenuation factor (β) and molecule resistance (R). Correlation among Phi;, ΔPhi;, R, transition voltage V_t and ε_h and accurate simulation provide a remarkably complete picture of tunneling transport in these prototypical molecular junctions.

The electronic properties of organic semiconductor thin film are greatly influenced by the crystalline packing of molecules within each layer, which may depend on the processing methods and conditions. The crystallization mechanisms turn out to be both subtle and varied, with different behavior for different materials. Here we report an in-situ study of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C₈-BTBT) crystallization, which exhibits multiple transient phases during the deposition of the film. Films were prepared with controllable grain size and film morphology via hollow pen writing. Under very high writing speed (>20 mm/s) the films have an isotropic morphology, although the mobilities can be fairly high, up to 2.6 cm²/V.s. To understand the crystallization mechanisms in this very interesting new regime, we use real-time synchrotron X-ray scattering combined with optical video microscopy during deposition at different substrate temperatures. We observed a sequence of crystallization in which a layered liquid-crystalline (LC) type structure precedes inter-layer ordering. This indicates the formation of an intermediate state during the crystallization, which is analogous to the high temperature smectic LC phase of C₈-BTBT. For films deposited below 800C, preliminary results suggest that the films transform directly from the LC state to the final crystalline state. However, for films written at high speed and high temperature at or above 800C, a fast transition of several states is observed following the LC phase starting with a transient crystalline phase, which we denote as Cr1. Cr1 lasts for only ~0.6 seconds before the final crystalline structure (Cr2) is formed. We will discuss the implications of this discovery for control of metastable phases and properties of organic semiconductor thin films.

Long range ordering of semiconducting polymers is desirable for enhanced charge carrier mobility in organic electronics applications such as organic field-effect transistors (OFET) and organic photovoltaics (OPV). The time-dependent self-assembly of a model conjugated polymer, poly(3-hexylthiophene) (P3HT) has led to birefringent solutions with long range order consisting of aligned P3HT nanofibers. Herein it is shown that liquid crystalline behavior and the ability to preserve the alignment in the solid state depend not merely on the percentage of aggregates, but on the length of the fibers. Solutions aged for shorter times yielded isotropic fluids and thin films as characterized by polarized optical microscopy (POM), while solutions aged for a sufficient duration formed birefringent solutions with monodomain-like alignment on scales millimeters long in solution-filled capillaries and solid films. Thin films were deposited both by “drawing” using a controlled stage and micropipette, and by spincoating. Atomic force microscopy was used to observe nanofibers of increasing length and orientation with aging. As an alternative aggregation technique, ultrasonication of solutions resulted in similar proportions of aggregation to aging (as determined by UV-Vis), but produced shorter fibers that resulted in isotropic solutions and films. Atomic force microscopy was employed to observe changes in nanofiber length and orientation with time, x-ray diffraction was used to observe increases in crystallinity and Raman microspectroscopy further elucidated the changes in film anisotropy and ordering. An order of magnitude increase in charge carrier mobility was observed for devices fabricated from solutions aged for 4 days as compared to devices coated with fresh P3HT solution. However, the highest charge carrier mobility in this study was achieved by combining the two self-assembly methods of ultrasonication followed by aging, resulting in an excellent mobility of 0.15 cm²V^-1s^-1, which gives insight into the optimization of conjugated polymer self-assembly for high-performing organic electronics applications.

Data-driven approaches are particularly useful for computational materials discovery and design as they can be used for rapidly screening over a very large number of materials, thus suggesting lead candidates for further in-depth investigations. A central challenge of such approaches is to develop a numerical representation, often referred to as a fingerprint, of the materials. Inspired by recent developments in chem-informatics, we propose a class of hierarchical motif-based topological fingerprints for materials composed of elements such as C, O, H, N, F, etc., whose coordination preferences are well understood. A large benchmark set of over 50,000 organic molecules [1], and several hundred organic polymers [2] are considered to provide necessary data, which were computed using density functional theory calculations. We show that the proposed fingerprints, when representing either molecules or polymers, may be effectively mapped onto a variety of properties using a similarity-based learning model and hence can be used to predict relevant properties of a material, given that its fingerprint can be defined. Using this scheme, efficient and accurate data-driven prediction models are developed to rapidly estimate the band gap and dielectric constant of polymers falling within the same class of the systems used in the original training. Two procedures are introduced to demonstrate that the learning model can be inverted to identify the desired fingerprints and then, to reconstruct molecules which possess a set of targeted properties [3].
References
[1] R. Ramakrishnan et al., Sci. Data 1, 140022 (2014).
[2] V. Sharma et al., Nature Commun. 5, 4845 (2014)
[3] T. D. Huan, A. K. Mannodi-Kanakkithodi, and R. Ramprasad, arXiv:1503.07503

Printing techniques have been widely explored to electronic devices fabrication, due to low cost and facility of production. Among the different techniques, inkjet printing has been showed as a potential tool to fabricate organic field-effect transistors (OFETs) because it has several advantages, such as, easiness to print well-defined patterns with deposition accuracy and in small dimensions. Additionally, the printing designs can be changed easily and different materials can be deposited on the same substrate. This last advantage enables the fabrication of organic digital logic inverters, such as, organic CMOS (complementary metal oxide semiconductor). In this work, we fabricated OFETs with architecture top gate-bottom contact using the inkjet printing technique. As semiconductor was used the poly(3-hexylthiophene) (P3HT). The semiconductor was printed with a functional materials printer using different line deposition patterns, with the same or variable dispensing direction. Patterns of parallel and perpendicular lines with respect to direction of electronic conduction were printed. The proposal of this work is to evaluate the influence of deposition patterns in the OFETs performance. Other printing parameters were kept, as well as the polymeric dielectric layer, produced via spin-casting, and the evaporated metal electrodes. Microscopy techniques were used to analyze the morphology of the printed films. OFETs with high-performance were obtained for the case which parallel lines regarding direction of electronic conduction had the same dispensing direction. These devices reached on/off ratio up to 10⁴ and swing subthreshold of 3 V/dec. In OFETs with perpendicular lines regarding electronic conduction, the performance was similar to the two types of dispensing direction. This analysis showed that the best performance of OFETs was obtained for the printed parallel pattern in respect to direction of electronic conduction, due to its lower channel resistance.

In the area of solar energy conversion, organic photovoltaics (OPVs) represent an attractive alternative to their silicon-based counterparts because of their potential for low-cost roll-to-roll printing [1], and their intended application in light-weight mechanically conformable devices and in window-type semi-transparent PV modules [2]. Of all proposed OPV candidates, polymer donors are especially promising when used in conjunction with fullerene acceptors such as PCBM, achieving power conversion efficiencies (PCEs) of over 10% in bulk heterojunction (BHJ) solar cells [3]. With extrapolated lifetimes of over 15 years, polymer-fullerene BHJ solar cells are now one step closer to practical applications [4]. In OPV tandem devices of >10% PCE, [5] an efficient approach to harvesting sunlight across the full visible spectrum consists in combining a wide-bandgap polymer (1.8OC) of 0.6 V, fill-factor (FF) values of ca. 0.65, and PCEsof ca. 4% [8]. We developed poly(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b']dithiophene-3,4-difluoro-thiophene) (PBDT[2F]T) as an alternative to P3HT, and show that the wide-bandgap analogue (Egap 2.1 eV) can reach PCEs of ca. 7%, combining a large V_OC of ca. 0.9 V, high FFs of 0.72, and J_SC values consistent with those obtained with P3HT (ca. 10-11 mA/cm²) in the same standard device configuration [9]. To date, PBDT[2F]T is arguably the highest-performing polymer donor with a wide-bandgap of 2.1eV, and our transient absorption spectroscopy studies emphasize the minimal geminate recombination and a high energy charge transfer (CT) state that correlates with its high J_SC and V_OC. We detail the ordering and morphological patterns of PBDT[2F]T in a combination of GIXS and HR-TEM analyses, comparing the crystallinity of the polymer to that of P3HT.
References:
[1] R. Soslash;ndergaard, M. Hösel, D. Angmo, T. T. Larsen-Olsen, and F. C. Krebs, MaterialsToday, 2012, 15, number 1-2
[2] L. Wen, Q. Chen, F. Sun , S. Song , L. Jin & Y. Yu, Scientific Reports, 2014, 4 : 7036 | DOI: 10.1038/srep07036
[3] P.M. Beaujuge, A. Amassian et All., DOI: 10.1002/aenm.201500204
[4] W. R. Mateker, I. T. Sachs-Quintana, G. F. Burkhard, R. Cheacharoen, and M. D. McGehee, 2015, Chem.of Mater., DOI: 10.1021/cm504650a
[5] Chen, C.-C.; Chang, W.-H.; Yoshimura, K.; Ohya, K.; You, J.; Gao, J.; Hong, Z.; Yang, Y., Adv. Mater. 2014, 26, 5670-5677
[6] Dennler G., Appl Phys Lett., 2006, 89(7):073502
[7] Zhao, G.; He, Y.; Li, Y. Adv. Mater., 2010, 22, 4355minus;4358
[8] Dang, M. T.; Hirsch, L.; Wantz, G.; Wuest, J. D. Chem. Rev., 2013, 113, 3734minus;3765
[9] J. Wolf, F. Cruciani, A. El Labban, and P. M. Beaujuge, Chem. Mater., DOI: 10.1021/acs.chemmater.5b01520

Breath figure technique is a cost effective, water based self-assembly alternative to various patterning techniques available. Ordered microporous polymer structures produced by this technique has been shown to have potenital applications in tissue engineering, catalysis and superhydrophobic surfaces. In this study we have modified the breath fiugre patterning technique to obtain large area ordered patterns from polythiophene polymers, which are commercially available and highly sought for photovoltaic and sensor applications. Fluorescence Spectroscopy and SAXS/WAXS studies are used to charaterize the optical and morphological properties of these ordered microporous films. Compared of drop-cast thin films, these microporous thin films show increased crystallinity and stronger inter-chain interaction inside the honeycomb frame. Furthermore, these semi-transparent sub-cm large area patterns obtained by Breath Figure method are used for assessing the PV performance and exploring sensory applications.

In the past decades, there have been various efforts to achieve high performance organic solar cells, such as developing high performance active materials, adding processing additive in active layers, and inserting efficient interfacial layers. Among them, non-fullerene acceptors have been developed to overcome drawbacks of a conventional fullerene-based acceptor, [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), which has a weak visible absorption and a low-lying lowest unoccupied molecular orbitals (LUMO) energy. Naphthalimide-dicyanodistrylbenzene(DCS-NI) compounds are among the efficient non-fullerene acceptors because of their noticeable absorption coefficients in visible range and a destabilized LUMO compared to that of PCBM. In this work, we investigated the effect of alkoxy chain length in core phenyl unit of DCS-NI compounds, from methoxy to hexyloxy, and the changes of photovoltaic properties. The physical properties of different alkyl chains on DCS-NI compounds were quite different and these phenomena influenced their photovoltaic properties. When we fabricated bulk heterojunction solar cells with a high performance donor material, p-DTS(FBTTh₂)₂ , the solar cell exhibited systemically increased open circuit voltage, from 0.85V to 0.99V, and these changes were only resulted from the simple alkyl chain engineering. To further study the relationship between alkyl chain length and open circuit voltage, thin film morphologies and nano-scale structures were investigated. It was found that the changes of an open circuit voltage is correlated to inter-/intramolecular interactions and frontier orbital energy level changes. As a result, the bulk heterojunction solar cell, consisted of DCS-NI compounds with varying alkyl chain, exhibited relatively high power conversion efficiency ranged from 2.3% to 5.4%.

In comparison to polymeric counterparts, small molecule dyes/absorbers have many advantages, including monodispersity, well-defined structures, ease of synthesis and purification, more effective intermolecular π-π stacking, along with a better batch-to-batch reproducibility. Here we report our recent progress on computationally assisted smart molecular design and synthesis of very planar acceptor-donor-acceptor (A-D-A) dyes for organic photovoltaics (OPV). For conjugated molecules with A-D-A structure, many promising end-group acceptors have been reported. In this work, we synthesized conjugated small molecules utilizing strong and planar electron-withdrawing building blocks, such as 2-(benzo[d]thiazol-2-yl)acetonitrile or 1-octyl-3-methyl-1H-pyrazol-5(4H)-one, commonly found in photographic dyes. As a core donor block, we used either two-dimensional thienyl-substituted benzo[1,2-b:4,5-b&’]dithiophene (BDT-T) or a novel mono-dimensional 4,4&’,8,8'-tetrakis(alkoxy)-2,2'-bibenzo[1,2-b:4,5-b']dithiophene (bis-BDT) moiety to fine tune overlap of the dye absorption with the solar spectrum. Our design approach depends on theoretical calculations to fine tune optoelectronic properties of candidate materials prior to their synthesis, and contactless time-resolved microwave conductivity (TRMC) to measure the photoconductance of absorber/fullerene blends, prior to their optimization in devices. Our results demonstrate excellent correlation between measured photoconductance and OPV device characteristics. The evaluation of the selected absorbers includes also characterization of the optical, electronic, and structural properties of the absorber/fullerene films using UV-Vis absorbance, photoluminescence, cyclic voltammetry, and X-ray diffraction. Bulk heterojunction solar cells fabricated from blends of the investigated absorbers and fullerene exhibited FF>60%, Voc 800-920 eV and nonoptimized PCE 5%.

Polymeric semiconductors have attracted tremendous interest due to their solution processability and mechanical flexibility. Research on new polymeric semiconducting materials becomes more important to improve performance of organic thin film transistors (OTFT). One of the useful methods is introducing halogen atoms to molecular structure. This strategy can enhance electron mobility and ambient stability due to lower HOMO and LUMO energy levels caused from strong electron-withdrawing property of halogen atoms. Among others, chlorine atom can be easily applied by simple modification into polymer molecules compared to fluorine atom. Also, chlorinated polymers showed comparable device performance with fluorinated polymers. In addition, molecular ordering and thin film morphology of polymeric semiconductors affect the OTFT performance because strong intermolecular interaction is generated by ordered molecular structure and large domain. It facilitates intermolecular transport through hopping mechanism resulting in enhanced mobility. In this presentation, new alternating copolymers were synthesized by using chlorinated isoindigo unit and different donor moieties, bithiophene for PCIIBT and thienylenevinylene for PCIITV, via Stille polymerization. Interestingly, although TV unit is more planar structure caused from insertion of vinyl group between thiophene rings, PCIIBT containing bitiohphene exhibited much stronger intensity and highly extended lamellar diffraction peak up to (400) in out-of-plane X-ray diffraction (XRD) analysis after thermal annealing process. It indicated that PCIIBT had improved crystalline structure. Moreover, PCIIBT showed larger fibrillar structure than PCIITV in atomic force microscope (AFM) images. Therefore, PCIIBT exhibited higher mobility than PCIITV in OTFT due to enhanced intermolecular charge transport between polymer chains.

In this study we designed and synthesized novel soluble squaraine derivatives (named SQ-ER) with 2-ethylhexyl groups are developed as a donor material for solution-processible organic photovoltaic cells. SQ-ER derivatives are synthesized 3 steps from the corresponding 2-ethylhexylarylamine derivative, and can be easily prepared on a multigram scale of over 10g. The cost of SQ-ER is very inexpensive of below #65284;20/g. SQ-ER showed absorption maxima around 650 nm with a high extinction coefficient of over 3.0×10^-5M^-1cm^-1. SQ-EP showed high solubility into non-halogenated organic solvents, such as toluene. Also, these squaraine has deep HOMO level of 5.3 eV suggesting high open circuit voltage in bulk-heterojunction(BHJ) type OPV with a combination of fullerene derivatives. Using a combination of PC₇₁BM as acceptor material, these squaraines are used in BHJ photovoltaic cells with a structure of [ITO/M_OO₃ (6 nm)/SQ-ER:PC₇₁BM (70 nm)/BCP (10 nm)/Al (100 nm)] realizing power conversion efficiency of over 5.5% with a J_SC = 11.68 mA/cm², a V_OC = 1.00 V and a FF = 0.48, under AM1.5G 1 sun illumination at room temperature.

Crystalline and morphological evolution of organic semiconductors is strongly related with the surface condition of underlying substrates. Controlling this interface is important to improve the performance of organic devices. Herein we report an approach that utilizes an organic heterointerface to improve the crystallinity and control the morphology of an organic thin film.¹ Pentacene is used as an active layer above, and m-bis(triphenylsilyl)benzene is used as the bottom layer. Sequential evaporations of these materials result in extraordinary morphology with far fewer grain boundaries and myriad nanometre-sized pores. These peculiar structures were formed by difference in molecular interactions between the organic layers and the substrate surface. The pentacene film exhibits high mobility up to 6.3 cm² V^-1 s^-1, and the pore-rich structure improves the sensitivity of organic-transistor-based chemical sensors. Our approach opens a new way for the fabrication of nanostructured semiconducting layers towards high-performance organic electronics.
[1] B. Kang et al., Nat. Commun. 5:4752 doi: 10.1038/5752 (2014).

Organic field-effect transistors (OFETs) are one of the most reliable platforms to investigate the charge carrier injection and transport in organic semiconductors. It has been established that the mobility in most OFETs depends on the gate voltage. In particular, since an increase of gate voltage results in an increase of the charge carrier concentration, this dependence has been rationalized in terms of charge carrier concentration which concomitantly varies due to electronic structure (i.e. trap states) as an empirical power law: mu;prop;(V_GS-V_T-V_DS/2)^α. However, a self-consistent extraction method of the threshold voltage V_T and exponent of power law dependence α is still lacking in that the exponent α still remains as a fitting parameter with a strong sensitivity to threshold voltage V_T and that contact resistance is often neglected in the extraction of threshold voltage V_T. In this work, we propose a new method for the extraction of the threshold voltage and exponent of the power law dependant mobility simultaneously in a self-consistent manner. This method enables physically based extraction of both parameters in the linear regime transfer characteristics of OFETs measured with a standard laboratory data acquisition system, showing a good endurance to measurement noise. In addition, a rigorous analysis became possible on the effect of the electronic structure on the power law dependence of mobility characterized by the exponent α.

Several studies have been focused on the fabrication of organic complementary digital logic inverters (CMOS) and oscillators [1, 2]. These devices are the key for the development of digital integrated circuits and are considered necessary for organic electronic applications. The main challenge in the manufacture of the CMOS involves the connection of n-channel and p-channel OFETs that have comparable performances [3, 4]. It is known that the development of n-type organic semiconductors is lagged behind the development of p-type organic semiconductors. To supply this need, many efforts have been done on the development of new n-type organic semiconductors for application in OFETs. In the case of top-gate OFETs a perfect orthogonality between the dielectric solvent and the semiconductor layer is required, then the choice of the dielectric solvent is an important parameter to obtain high performance devices. In this context, we analyzed the performance of top-gate n-channel OFETs using different solvents for the dielectric layer. We used PNDI2OD-T2 as the semiconductor layer and the PMMA as dielectric layer. Some selective solvents, as methyl-ethyl-ketone (MEK), n-butylacetate (nBA) and dimethylsulfoxide (DMSO), were used to produce the dielectrics layers. All devices were characterized through output curves and transfer curves and their parameters were obtained from the saturation regime using the gradual channel approximation. All devices showed reasonably high values of field effect mobility (µ~10^-2 cm²/V s). However, the best performance, with high I_on/I_off ratio (~ 10³) and smaller values of subthreshold swing and threshold voltage were achieved for OFETs with a perfectly orthogonal solvent (MEK). The value of I_on/I_off ratio decreased one order of magnitude for the devices manufactured with a poor solvent orthogonality (nBA). Although, DMSO is a perfectly orthogonal solvent to PNDI2OD-T2, the resulting devices built did not show the expected performance because its leakage current was relatively high.
[1] Fabiano, S. et. al. Advanced Materials, 26, 7438 (2014).
[2] Mamada, M. et. al. Chemistry of Materials, 27, 141 (2015).
[3] Gili, E. et. al. Applied Physics Letters, 100, 123303-1 (2012).
[4] Baeg, K.-J. et al. Journal of Polymer Science: part B: Polymer Physics, 49, 62 (2011).

Conjugated polymers have received great attention as active semiconducting materials for various electronic devices such as organic photovoltaic cells and organic field-effect transistors. Molecular ordering and orientation of conjugated polymers are one of the crucial factors to determine the performance of organic electronics. In general, highly ordered and oriented molecular structures could enhance their charge transport properties, which leads to increase in device performance. In the chemical point of view, introduction of flat and planar molecules into the conjugated backbone is the most efficient approach for inducing well-ordered structure and high crystallinity. Among the diverse planar molecules such as ladder type, fused acenes, vinylene, etc., quinoidal molecules are promising moieties to increase rigidity and co-planarity due to their double bond linkage. Highly planar structure of quinoidal molecules promotes the formation of π-π stacking. In addition, quinoidal molecules commonly have extended π-electron delocalization compared with benzoid form; accordingly, they are beneficial to achieve strong intermolecular overlap of π-electron and well-ordered structure. However, incorporation of quinoidal structures into conjugated polymers is quite difficult because their termini are commonly capped with dicyanomethylene groups which are scarcely bonded to other aromatic molecules. In this research, we introduced quinoidal molecules, whose end-groups are substituted with benzene, into the conjugated backbone in order to increase structural planarity. We investigated the effect of quinoidal backbones on crystallinity in final polymers, and characterized their charge transport properties.

It is widely accepted that the presence of oxygen has important effects on conjugated polymer (CP) photophysics. The most drastic of these is the irreversible formation of high electron affinity sites that quench the polymer photoluminescence (PL). ^1,2 There is also some work suggesting the formation of reversible charge transfer states that quench luminescence. ^3,4 These oxygen charge transfer sites are thought to be responsible for the “blinking” behavior in individual polymer chain spectroscopy when those chains are sufficiently folded to allow efficient energy transfer to a reversibly quenched “funnel” location. As a consequence, the prospective PL from hundreds of chromophores can be suppressed.⁵ We have observed a very different and surprising effect that photooxidation can have on conjugated polymer chains, the modulation of energy transfer that triggers an increase in PL quantum efficiency. This change in the energy transfer topography creates extremely large spikes in the PL that we describe as “flaring”. The traditional held picture describing 2 types of chromophores in a CP chain does not explain our observations satisfactorily.^3,6 We conclude that the presence of a third very low energy weakly emissive chromophore is necessary to explain our findings and are able to document the properties of those chromophores as well. Our hypothesis can explain other phenomena seen in CPs like the reduction of PL quantum efficiency with increasing molecular weight.⁷ Selective photobleaching of the weakly emissive chromophore results in a large fold increase in PL from single polymer chains as well as a reduction in their PL fluctuations.
References:
1. Yan, M.; Rothberg, L.; Papadimitrakopoulos, F.; Galvin, M.; Miller, T.
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The continued study and development of polymer semiconductor devices relies on fundamental knowledge of structure-property relationships and harnessing direct control over properties that favor optimized morphologies for devices. We are investigating surface energy modification of known conjugated polymer systems. Surface energy is associated with the Flory-Huggins Interaction Parameter and is used to predict or characterize the miscibility of polymeric materials, yet it is often not reported in studies of conjugated polymer systems. Recently, we demonstrated that the surface energy of a simple conjugated polymer, P3HT, could be tuned via the incorporation of comonomers with heteroatom modified side chains. Resulting polymers could be reliably increased or decreased in surface energy without change in electronic and optical properties, due to physical decoupling of the heteroatoms from the backbone conjugation. Interestingly, polymers with oligo-ether side chains demonstrated high degrees of self-organization in as cast films, evidenced by vibronic features in UV-Vis and lamellar packing in GIXRD measurements. Additionally, semi-fluoro alkyl polymers featured higher melting and crystallization transitions in DSC studies, and larger spacing in lamellar packing compared to P3HT. Previously, we pointed to surface energy and miscibility as a factor in the formation of organic alloys between two polymer donors in ternary blend solar cells. We have demonstrated a method for tuning the surface energy of known polymers towards controlled miscibility of materials in active layers of organic photovoltaics.

Large-amplitude intermolecular vibrations in combination with complex shaped transfer integrals generate a thermally fluctuating energetic landscape. The resulting dynamic disorder and its intrinsic presence in organic semiconductors is one of the most fundamental differences to their inorganic counterparts. Dynamic disorder is believed to govern many of the unique electrical and optical properties of organic systems. However, the low energy nature of these vibrations makes it difficult to access them experimentally and because of this we still lack clear molecular design rules to control and reduce dynamic disorder.
Recently we reported a novel method based on electron diffraction that allows for direct probing of the most dominant lattice vibrational modes [1]. Since then we encountered strong thermal vibrations in every single organic material we studied (14 up to date), indicating that a large degree of dynamic disorder is a universal phenomenon in organic crystals.
In my talk I will present a molecular design strategy to avoid dynamc disorder. We found that small molecules that have their side chains attached to the long axis of their conjugated core are less likely to suffer from dynamic disorder effects [2]. In particular, we demonstrate that 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothio-phene (C8-BTBT) exhibits strongly reduced thermal vibrations in comparison to other molecules and relate its outstanding performance to its lower dynamic disorder. We rationalize the low degree of dynamic disorder in C8-BTBT with a better encapsulation of the conjugated cores in the crystal structure which helps reduce large amplitude thermal motions. Our work provides a general strategy for the design of new classes of very high mobility organic semiconductors with low dynamic disorder.
[1] S. Illig, A. S. Eggeman, A. Troisi, H. Sirringhaus and P. A. Midgley, Nat. Mater., 2013, 12, 1045-1049.
[2] S. Illig, A. S. Eggeman, A. Troisi, S. G. Yeates, J. E. Anthony and H. Sirringhaus, in review process.

The dynamic and thermodynamic properties of soft condensed systems undergo strong modifications when they are confined to small dimensions. The classical argument to rationalize this is that the size of the material comes into conflict with the characteristic lengthscale of a certain physical process. The physical process is forced then to develop in a different manner in order to adequate to the new spatial conditions, which leads to a material with a new physical behaviour and, eventually, to distinct properties. Physical processes leading to the development of the internal microstructure in molecular systems frequently have lengthscales in the nanometer range and thus will be sensitive to spatial restriction at the nanoscale. These include crystallization, melting, glass transition, phase separation and mesophase transitions, among others ¹.
Typical devices manufactured from π-conjugated molecular materials, such as FET and OPV cells, are based on architectures where the material is processed to have nanoscale dimensions. Therefore, it is to be expected that the material integrating such devices experiences some kind of finite size effect during the processing step. However, little attention has been paid so far to confinement effects on the structural features of π-conjugated systems even though it is well known that microstructure determines to a large extent the performance of these materials ². We have addressed this issue through an comprehensive study on the structure development in the model conjugated system p-DTS(FBTTh₂)_2. As confining medium, we have selected Anodic Aluminum Oxide (AAO) templates³, consisting of hexagonal arrays of cylindrical nanopores with diameters ranging from 25 to 400 nm.
Strong confinement effects have been found on p-DTS(FBTTh₂)₂. The analysis of the microstructure of the material as a function of the temperature and degree of confinement revealed a notably different phase behavior as compared to its bulk counterpart. Deep depressions of crystallization temperature, which denote the change of nucleation mechanism from a typical heterogeneous mechanism of bulk materials to a homogeneous mechanism in confinement. Moreover, we show that the temperatures of the phase transitions are dependent on the degree of confinement. More striking is the discovery of new thermotropic mesophase that is absent in the bulk material. We outline the complete microstructural behavior of p-DTS(FBTTh₂)₂ in a temperature-confinement phase diagram. Furthermore, the structure of each phase inside the pores was studied and we found that all the confined phases show high levels of texture. Thus, we show that p-DTS(FBTTh₂)₂ nanowires with uniaxial crystal orientation are obtained, which can be manipulated by different confinement and crystallization protocols.
1. Martín, J. et al.Phys. Rev. Lett.s 2010, 104, (19), 197801.
2. Treat, N. D.; Stingelin, N.; et al. Nat Mater 2013, 12, (7), 628-633.
3. Martín, J.et al. Nat. Commun. 2014, 5.

In order to achieve the next leap forward in organic photovoltaics (OPV), a fundamental understanding of how these devices function is required. From this fundamental understanding, design rules for new materials and processing can be derived. The physical processes that occur in the mixed phase of the bulk heterojunction are still poorly understood, and this study will try to address the morphological and electronic behavior of the materials. Here, we study the small molecule electron donor X2,^[1] which gained a lot of attention due to the crystalline nature and high efficiency (6.6%),^[1,2] which makes it a good model system. Interestingly, X2 has a wide range of blend ratios with the electron acceptor, [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM), that work well in a photovoltaic device, in contrast to most other OPV materials.^[1] In our study, the X2:PCBM blend ratio is varied between 1:0 and 0:1 in 0.05-0.1 increments, allowing systematic investigation of the X2:PCBM mixed phase. From Grazing-Incidence Wide Angle X-ray Scattering (GIWAXS) measurements, we determine the amount of PCBM that is intimately mixed with the X2, and how the X2 morphology and crystallinity evolves with blending.^[3] Preliminary results show that the orientation of X2 crystallites varies with blend ratio. Near-Edge X-Ray Absorption Fine Structure (NEXAFS) give insight in the how the X2 and PCBM molecular electronic configuration change with blending, while resonant-Soft X-ray Scattering (R-SoXS) is used to probe the size of the different domains.
The detailed information about the morphology and electronic state of both materials in the pure domains and the mixed phase from this study contributes to a better understanding on how the energy levels, which mainly determines the open circuit voltage in a photovoltaic device, of the materials change when blended with each other. The molecules and/or morphology can then be optimized to enhance the voltage and performance.
[1] Y. Huang, X. Liu, C. Wang, J. T. Rogers, G. M. Su, M. L. Chabinyc, E. J. Kramer, G. C. Bazan, Adv. Energy Mater.2014, 4, 1301886.
[2] X. Liu, Y. Sun, L. A. Perez, W. Wen, M. F. Toney, A. J. Heeger, G. C. Bazan, J. Am. Chem. Soc.2012, 134, 20609.
[3] J. A. Bartelt, Z. M. Beiley, E. T. Hoke, W. R. Mateker, J. D. Douglas, B. A. Collins, J. R. Tumbleston, K. R. Graham, A. Amassian, H. Ade, J. M. J. Fréchet, M. F. Toney, M. D. McGehee, Adv. Energy Mater.2013, 3, 364.

It has been demonstrated recently that singlet exciton fission can be exploited to enhance the efficiency of organic solar cells [1]. In these devices, optically generated singlet excitons are transformed into two triplets, each with energy up to half that of the singlet. Singlet fission therefore enables the device to collect as many as two electron-hole pairs per incident photon. It is well accepted that the efficient dissociation of triplets generated through singlet fission necessitates a specific energetic alignment between triplets in the donor/acceptor and the interfacial charge transfer (CT) state. However, the impact of film crystallinity on the energetics of singlet-fission-based systems has not been comprehensively studied. In particular, while disordered molecular aggregates may be preferred for their ability to undergo singlet fission, ordered, crystalline systems tend to be more efficient, likely due to the greater delocalization of excitons in ordered films.
Here, we have investigated two device systems, with either amorphous or crystalline rubrene (a singlet fission material) as the donor, and C₆₀ as the acceptor. We find that the donor crystallinity affects the CT state energy, such that in the amorphous system (CT=1.47 eV), charge transfer from a rubrene triplet (T₁=1.2 eV) is energetically unfavorable, but in the crystalline system (CT=1.2 eV), the same process is energetically allowed. The result is that, in the amorphous system, triplet exciton fusion (a process where two low energy triplets interact to form a higher energy singlet) presents an indirect pathway for charge generation, increasing rubrene&’s contribution to device performance, whereas in the crystalline device, triplet fusion is a loss pathway.
In our experiments, we demonstrate evidence of modified external quantum efficiency (EQE) in organic solar cells due to triplet fusion. By modulating the rate of triplet fusion via a background white light of varying intensity, we are able to observe changes in the EQE spectra of the amorphous and crystalline systems that are consistent with predictions based on measured device energetics. In the amorphous system, the improvement in EQE is quite significant (over 10%) even in a relatively low intensity background white light (~4 mW/cm²). In addition, we present EQE data in the presence of a background magnetic field, which is also known to modulate the rate of triplet fusion in organic films. The results of these studies underlie the development of high efficiency singlet-fission-based organic solar cells.
[1] D. Congreve et al., Science340, pp. 334-337 (2013).

The way organic conjugated molecules or polymer chains organize in the solid state affects the electronic and photophysical properties of the corresponding materials to a large extent. We have developed in our laboratory a multifaceted modelling scheme that encompasses classical molecular dynamics, quantum-chemistry and non-adiabatic quantum dynamics simulations and has been applied to assess multiple fundamental opto-electronic processes in conjugated materials used as active components in the field of organic electronics. Here, we will more specfically review work dealing with the modelling of charge transport in molecular crystals and conjugated poymers, singlet fission and exciton dissociation at organic interfaces, and the electronic structure of molecularly-doped conjugated materials. In all cases, we will highlight how these processes are sensitive to the relative arragement of the conjugated molecules or chromophores at the nanoscale.

Solution processing of small-molecule organic semiconductors (OSCs) offers a pathway towards facile, low-cost manufacturing of high performance electronic devices on large area, flexible substrates. Achieving a high carrier mobility in organic field effect transistors (OFET) requires the small-molecule OSC to form a highly crystalline film with domains connected through minimally disruptive grain and domain boundaries and with two-dimensional in-plane p-stacking. Yet, despite tremendous engineering progress the community has very little control over the nucleation and growth behavior of OSCs during solution printing. This stems in part from the lack of quantitative understanding of nucleation and crystallization mechanisms by which the OSC film forms. In this presentation, we describe the nucleation and growth mechanism of OSCs in drop-casting, spin-casting and blade-coating. In the first two, crystallization appears to proceed via a direct nucleation mechanism. However, small-molecule OSCs can crystallize via a two-step nucleation process similar to proteins, colloids and pharmaceutical molecules during blade-coating. This process is shown to be kinetically induced and is demonstrated to have tremendous benefit for thin film microstructure and morphology, resulting in excellent carrier transport both in pristine and blended OSCs. Two-step nucleation is shown to partly decouple the crystallization step from the solvent drying conditions, thus improving the manufacturability of several technologically important OSCs. We go on to show that further decoupling of OSC coating and crystallization can be utilized to pattern the crystallization of OSC films.

The influence of conjugated polymer and fullerene phase order on the respective electronic energy levels is currently widely discussed, but difficult to be measured directly, especially if ordered and amorphous phases occur within one and the same bulk heterojunction. Here we show that tuning domain order via ternary blending of amorphous and semi-crystalline polymer representatives induces transition energy changes as evidenced by activation of different luminescent recombination channels. These different specific charge-transfer transition energies could be unambiguously assigned to the nature of specific polymer:fullerene interfaces and their domain order at the interface.

Organic field effect transistors (OFETs) from single crystals require complex procedures for preparation of single crystals and then placing them at the exact right position. This is in contrast to the ease of fabrication of organic thin film transistors (OTFTs) through either spin coating, printing, or various evaporation methods. There have been a great deal of research in single crystal transistors as it offers high mobility and better device performance and stability due to less defects and grain boundaries than the thin films. Here we demonstrate a new type of transistor which can be prepared with similar ease as thin films but is based on single crystals. The fabrication is carried out with the aid of stencils mask pattern to yield a device structure we call Organic single crystallites Statistical Field Effect Transistor (OSFET). The concept of the statistical FET (SFET) structure is to first grow many crystallites on gate dielectric such that they form a discontinuous poly crystalline layer of sizes smaller than the channel length. Through one additive step that deposits conducting round disks, over the entire area, the crystallites become inter connected. In fact, the new device is now being composed of many transistors inter-connected in series and in parallel. The odds that a given crystallite is being contacted on both sides and the number of crystallites connecting two conducting circles are a statistical issue and hence the name of this structure: statistical field-effect transistor (SFET).
Both n and p-type solution processable small molecules which can form micro sized single crystals were prepared and used in the SFET structure. Our results suggest that unlike single crystal transistors, OSFETs do not require tedious fabrication process, can be easily prepared with the aid of a single additive step, and exhibit many advantages over both single crystal and thin film transistors.

A dramatic enhancement of optoelectronic properties for organic semiconductor was demonstrated by utilizing a graphene-organic heterointerface. A “quasi-freestanding” monolayer graphene was successfully transferred to various arbitrary substrates to serve as an atomically thin, transparent and highly conductive epitaxial template for the highly ordered organic crystal growth. Due to the “wetting-transparent” nature of the graphene templates, the growth mode, morphology and the crystallographic structure of the organic semiconductors exhibited a versatile tunability through the surface engineering of desired substrates. Furthermore, the anisotropic nature of optoelectronic properties of organic semiconductor molecules provided a significant enhancement in overall optoelectronic properties through the graphene-templated growth, including exciton diffusion length, optical absorption, charge carrier lifetime as well as the energetic landscape of the organic semiconductor surface. Especially, the exciton diffusion length of a model molecule, pentacene, increased up to nearly 100 nm, which was twice the value of conventionally assembled organic molecules. In application to the organic electronic devices, these peculiar optical behavior allowed the optical thickness of the organic thin films to be doubled, leading to yield 5 times higher power conversion efficiency in comparison to conventional planar heterojunction organic photovoltaic cells. Theoretical simulations as well as systematic studies on the film structure and optoelectrical properties were performed to corroborate our new findings.

Understanding the relationships between molecular/crystal structure and charge transport properties in organic semiconductors (OSCs) is crucial to facilitate the synthesis of high performance materials. A thorough multi-scale study of charge carrier mobility in OSCs, however, is a formidable task, especially for the purposes of screening a library of compounds with potential high performance. Predictive, structure-property modelling methods for materials design is an alternative avenue where complex relationships between structure and macroscopic material properties could be derived. Extending upon earlier work on the development of QSPR models for charge transfer rates in polycyclic aromatic hydrocarbons (Misra et al., JCTC, 2011, 7, 2549), here we report a QSPR study of a library of acenes and thienoacenes to predict reorganisation energy. We use molecular descriptors based on structural and electronic properties such as molecular signature, polarizibility and molecular orbital energy levels. Our training set involves about 150 experimentally known compounds that are used as active layers in organic field-effect transistors as p-type OSCs. Confirming earlier results, we find high correlation of reorganisation energy with the molecular energy difference between the neutral and cation species with a correlation coefficient of r²=0.98. A preliminary QSPR model that does not require full molecular and cation geometry optimizations however, show less promise (r²=0.56). This work is a preliminary step in our efforts to develop molecular and crystal structure-property relationships for in silico prediction of new high performing materials.

Organic light-emitting diodes (OLEDs) have become more prevalent commercially over the past decade. These electroluminescent devices utilize organic glass materials to transport charges, and because electrons have to be transferred between neighboring molecules to conduct, the relative orientation between molecules can affect the charge transfer. As a result molecular level details of the packing are critical to understanding material performance. OLED materials with different molecular designs were studied using angle dependent near edge X-ray absorption fine structure (NEXAFS) spectroscopy, and the results were compared with observations made by ellipsometry. In contrast to ellipsometry, which gives the orientation of the dipole of the whole molecules, NEXAFS has the ability to show the orientation of specific bonds of the molecules within the solid state film. The measurements show some molecules could be twisted more than the others depending on the specifics of the structure, and differences in the level of anisotropic packing were also seen between NEXAFS and ellipsometry on some molecules. These differences could be due to differences in molecular rigidity or in packing at the surface relative to the bulk.

Efficient charge transport is of paramount importance in all electronic devices. The inability of conjugated polymers to carry charges effectively remains a major limitation of their application in solar cells and transistors. Hence, much effort has been devoted to understanding how to design polymers that exhibit high carrier mobilities. While initially there was much emphasis on increasing long-range order and crystallinity, recent record-breaking materials exhibit poor crystallinity, as characterized by X-ray diffraction. These observations will be reconciled by making use of a theoretical charge transport model that includes the microstructure at the mesoscale to show that transport can be very effective in materials exhibiting only regions of local order rather than large crystallites. Furthermore, experimental results demonstrate that materials that have very weak diffraction but nonetheless have local order can have relatively high mobilities, in agreement with our model. Understanding these principles will help in designing high-mobility polymers that can find applications in optoelectronic devices.

Organic electronics is a promising technology for developing applications that require low-cost, light-weight, mechanically flexible electronic components. In order to form a solid foundation for this technology, more studies of operational and environmental stability, as well as mechanical durability, must be carried out. In this work, we investigate various popular solution-processed OFETs, including functionalized acenes and rylenes, for mechanical flexibility,^[1] as well as for several types of operational instabilities (e.g., the bias-stress effect and the gate-voltage sweep rate dependence of nominal mobility).^[2,3] In particular, we have fabricated n-type PDIF-CN₂ OFETs following simple solution based routes and demonstrated their operational stability, stability against thermal annealing to temperatures over 100 ^oC and resilience against photooxidation under intense illumination in oxygen atmosphere.^[4] The observed great operational and environmental stability of PDIF-CN₂ OFETs will be beneficial for integrating these n-type devices in practical organic electronics applications.
References:
[1] H. T. Yi , M. C. Payne , J. E. Anthony , and V. Pozdorov, Nat. Commun.3, 1259 (2012).
[2] Y. Chen, and V. Podzorov, Adv. Mater.24, 2679 (2012).
[3] Y. Chen, B. Lee, H. T. Yi, S. S. Lee, M. M. Payne, S. Pola, C.-H. Kuo, Y.-L. Loo, J. E. Anthony, Y. T. Tao, and V. Podzorov, Phys. Chem. Chem. Phys.14, 14142 (2012).
[4] Hee Taek Yi, Zhihua Chen, Antonio Facchetti, and Vitaly Podzorov, Adv. Funct. Mater. submitted 2015.

Air-stable solution-processable p-doped polymers with deep workfunctions up to 6.1 eV are demonstrated through thin-film contact doping and solution-state p-doping of high ionization potential (Ip) triphenylamine-fluorene copolymers using nitrosonium ion as an oxidant. The doping levels of the p-doped films are systematically varied to study changes in the workfunction, atomic core levels and valence band structure. New aspects of the physics of doped organic semiconductors are described here. As the polymer gets doped, the workfunction shifts quickly to the edge of the #61552;-band formed by the highest-occupied-molecular-orbital (HOMO) and levels off over a wide range of doping level below 1 hole per repeat unit. In particular, we have evidence that doping does not always lead to shifting of states into the gap with respect to vacuum level. The workfunction of heavily-doped triarylamine polymers can exceed the Ip of the neutral polymers and this is related to the energy of the molecular orbitals of the doped polymer. Furthermore we identified the role of the counter-balancing anion which provides stabilization or destabilization of the polaronic charge by the Madelung potential effect which can further tune the energy of the charge and thus affect the workfunction of the doped film. This experimental observations are supported by DFT quantum chemical calculations of the radical cation state of the polymers. The results here opens new opportunities to the design of ultrahigh workfunction materials and their application in organic electronic devices.

Organic photovoltaic (OPV) devices often undergo a period of ‘burn-in&’ during the early stages of operation; this stage describing the relatively rapid drop in power output before stabilising. Minimising the detrimental effects of burn-in is vitally important in order for OPVs to be considered a viable technology for renewable energy conversion. Here, we characterise the burn-in phase in-situ using transient photocurrent and photovoltage techniques, complementing standard current-voltage measurements and providing further insight into the evolution of a device than can otherwise be achieved through analysis of its primary solar cell metrics.
Our focus is on devices containing the polymer:fullerene blend PBDTTT-EFT:PC₇₁BM, which is among the current state-of-the-art semiconductor materials in the field. When measured under nitrogen, both inverted and normal architecture PBDTTT-EFT:PC₇₁BM bulk-heterojunction solar cells are found to lose upwards of 60% of their original efficiency after 24 hours continuous simulated solar illumination. For inverted PBDTTT-EFT:PC₇₁BM OPVs, a close correlation between the burn-in phase kinetics and an increase in charge build-up within the solar cell is determined, which acts to reduce the maximum attainable photocurrent. Under constant white-light bias the loss in solar cell photovoltage is correlated with an increase in charge carrier lifetime, tentatively attributed to a lower density of charge within the device. A range of experimental probes are subsequently employed ex-situ to characterise the cumulative effects of burn-in and separate bulk and interface-specific degradation mechanisms. Owing to the poor stability of the reference PBDTTT-EFT:PC₇₁BM OPV devices, the device processing protocol is modified to identify routes for stability enhancement in this initially promising solar cell blend.

The small-molecule semiconductors DNTT and its didecyl and diphenyl derivatives C₁₀-DNTT and DPh-DNTT have recently shown great potential for the realization of high-mobility, long-term-stable organic thin-film transistors (TFTs). In this work, low-voltage DNTT, C₁₀-DNTT and DPh-DNTT TFTs were fabricated on various substrates, and their static and dynamic performance was investigated. All TFTs were fabricated using a hybrid gate dielectric consisting of a thin, plasma-grown aluminum oxide (AlO_x) layer and an alkyl or fluoroalkylphosphonic acid self-assembled monolayer (SAM). Long-channel TFTs (L = 100 µm) fabricated on silicon substrates have effective carrier mobilities of 3.0 cm²/Vs (DNTT), 5.3 cm²/Vs (C₁₀-DNTT) and 5.9 cm²/Vs (DPh-DNTT), with on/off ratios of 10⁷. These results confirm the beneficial effect of the decyl and phenyl substituents on the carrier mobility of C₁₀-DNTT and DPh-DNTT TFTs in comparison to DNTT TFTs.
Using the transmission line method (TLM), we have also measured the contact resistances and the intrinsic carrier mobilities of TFTs based on the three semiconductors fabricated on four different types of substrates: silicon, glass, polyethylene naphthalate (PEN) foil and paper. It was found that both, the choice of the substrate and the choice of the SAM (alkyl or fluoroalkyl) have an influence not only on the intrinsic mobility, but also on the contact resistance of the TFTs. For example, the fibrous rough surface of the paper leads not only to smaller carrier mobilities, but also to larger contact resistances compared to TFTs fabricated on the smoother silicon, glass or PEN substrates. We also found that the extent to which the effective mobility decreases when the channel length is reduced from 100 µm to 1 µm is quite different for the three semiconductors, so that for short-channel TFTs (L = 1 µm) on PEN, C₁₀-DNTT is outperformed by DNTT, due to the larger contact resistance. Another interesting result is that the performance of DNTT TFTs is less affected by the choice of the substrate and the SAM than that of C₁₀-DNTT and DPh-DNTT TFTs. Of the three semiconductors, DPh-DNTT provides the smallest contact resistance and thus the best dynamic performance in short-channel TFTs: The signal delay measured in 11-stage ring oscillators based on DPh-DNTT TFTs with a channel length of 1 µm fabricated on PEN at a supply voltage of 4 V is 240 ns, which is the shortest delay reported for flexible organic TFTs at supply voltages below 10 V.

With outstanding electronic, optical, thermal and mechanical properties, graphene has drawn promising attraction for flexible electrodes in various organic optoelectronic devices. In these applications, ultrafast charge transfer rate from organic semiconductor to graphene can impact the overall device performance. Here, we propose a new mechanism in which the charge transfer rate can be controlled by varying the number of graphene layers and their stacking. The charge transfer rate from zinc phthalocyanine (ZnPc) crystals to graphene is measured by time-resolved photoemission spectroscopy. Surprisingly, the charge transfer to A-B stacked bilayer graphene is slower than that to both monolayer graphene and graphite and this anomalous behavior disappears when the two graphene layers are randomly-stacked. The observation is explained by a charge transfer model that accounts for the band structure difference in mono- and bilayer graphene, which predicts that the charge transfer rate depends non-intuitively on both the layer number and stacking of graphene. This work provides a new design strategy for optoelectronic devices using graphene as a conductor.

Organic-based solar cells are beginning to emerge with the potential to compete with other thin film photovoltaic (PV) technologies, with efficiencies of 12% recently demonstrated. Unique to the function of organic PVs are the creation of tightly bound excitons that can only be efficiently separated at a donor/acceptor (D/A) interface capable of providing the necessary energetic driving force for dissociation. The consequences of this are the need for long exciton diffusion lengths or a well controlled bulk heterojunction morphology, and the presence of charge transfer (CT) states, ground state complexes that exist at the D/A interface.
We have recently been exploring pinhole-free organic semiconductor-based thin films that feature crystalline grains of 100&’s of microns in extent. Because they are pinhole-free, we can apply them to vertical devices such as PV cells to explore various emergent phenomena. For example, we have found that CT states are more easily separated into free charge if they are delocalized; an aspect that becomes most feasible for highly ordered systems. I will discuss our recent efforts to understand, template, and control microcrystalline film morphology. These films show unprecedented thin film exciton diffusion lengths of 100&’s of nm. Also, we are able to directly measure photocurrent from multiple CT states and probe singlet fission-based photocurrent as well. These aspects have important consequences for achieving more efficient photocurrent generation.

A recent study has indicated that the energetic density of shallow trap states, specifically at the interface between the electron donor and the electron acceptor, could be responsible for the apparent high recombination order observed in organic photoactive blends [1]. In order to elucidate this issue, we carried out investigations on a material system that allows for well-controlled donor/acceptor interactions. Films of the small-molecule donor, H1, can be processed such that H1 molecules are either stacking with their pi-face perpendicular or parallel to the substrate. By evaporating a layer of the acceptor molecule C60 on the films, we study the effect of molecular orientation at the donor/acceptor interface on charge recombination using transient absorption spectroscopy. We compare the two bilayer systems to a bulk heterojunction also using H1, which is expected to have a mixture of face-on and edge-on donor/acceptor interactions as well as a much larger interface area.
[1 ] J. Gorenflot et al., J. Appl. Phys.115, 144502 (2014)

The time-dependence of space-charge built-up at the ITO / P3HT interface has been investigated for the first time using charge extraction by linearly increasing voltage (CELIV) in the dark. Thereby a double CELIV ramp with a repetition rate of 100 Hz is utilized to obtain a noise-reduced current signal. The two ramps of each double CELIV ramp are separated by a variable delay time of 2 to 2048µs in short-circuit. During the ramps space-charge is effectively extracted from the polymer establishing always the same non-equilibrium state. The temporal recovery of thermal equilibrium by injection of space charge is probed by the extracted charge from the second CELIV ramp utilizing various delay times. The investigated diode is ITO/P3HT/Ag where Ag forms a neutral contact with P3HT and therefore is not significantly contributing to the charge extraction. The measured build-up time constant for the space charge next to the ITO/P3HT interface amounts to 230 µs. The dynamics of such relatively slow charge transfer at the interface of the transparent conducting oxide and the organic semiconductor as well as time-limiting processes will be discussed by considering tunneling, thermal detrapping and diffusion of involved holes. It will be shown that thermal detrapping of electrons via vibronic states with subsequent tunneling to neighboring tail states of the DOS via Miller-Abrahams model, prior emptied by tunneling of electrons to empty states in ITO, is the time limiting process and responsible for the long build-up time. Changes occurring during oxygen doping will be introduced and the impact on the CELIV signals will be highlighted. It will be shown that the build-up time for the space charge shortens considerably and that the CELIV signals change in a characteristic way.

Enhancing the understanding of the relationship between processing conditions, morphology and power conversion efficiency (PCE) in bulk heterojunction (BHJ) organic photovoltaics (OPV) is very important.¹ The current understanding is that phase separation in the BHJ results in a morphology composed of electron donor and/or acceptor rich domains together with an intermixed phase at the interface between the two.² To date the ‘desirable&’ morphology that leads to improvement in PCE is achieved by trial-and-error processing such as thermal/solvent vapor annealing and chemical additives.^3,4 The lack of direct mechanistic understanding of BHJ morphologies is a result of the lack of ultra-sensitive, spatially resolving tools that are capable of characterizing the “buried” morphologies on the 10-20 nm scale as well as probing the chemical composition and extent of the mixed interface. We developed a new methodology capable of probing buried morphologies that combines Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS)⁵ and AFM. ToF-SIMS is a powerful technique with high chemical selectivity and resolution (few nm) in depth profiling, but poor lateral resolution, while AFM provides the desired lateral resolution. The buried morphology is revealed by the AFM studies after subsequent sputtering using ToF-SIMS. We show the applicability of this methodology to the classical poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM) BHJ. The P3HT signature is revealed in ToF-SIMS by the ³⁴S^- fragment; however, in order to have a specific signature for the acceptor, the fullerene was replaced by a deuterated-PCBM and the ²H^- fragment was used. The ToF-SIMS depth profile shows that the BHJ at the air interface is polymer-rich, while the bottom interface is PCBM-rich. The AFM images reveal that the first few nm of the film are covered by amorphous P3HT with crystalline P3HT residing below. The buried morphologies show the presence of ~10-20 nm domains which gradually increase in size to ~100 nm near the bottom of the film, indicative of PCBM-rich regions. We will also present the methodology that allows for the conversion of the ToF-SIMS sputtering time into depth and the calculation of the mixing length at the interfaces. We will discuss how different processing conditions affect the mixing length.
1. Liu, F.; Gu, Y.; Jung, J.; Jo, W.; Russell, T. J. Polym. Sci., Part B: Polym. Phys. 2012, 50, 1018.
2. Westacott, P.; Tumbleston, J.; Shoaee, S.; Fearn, S.; Bannock, J.; Gilchrist, J.; Heutz, S.; deMello, J.; Heeney, M.; Ade, H. et al. Energy Environ. Sci.2013, 6, 2756.
3. Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. Adv. Funct. Mater.2005, 15, 1617.
4. Peet, J.; Kim, J.; Coates, N.; Ma, W.; Moses, D.; Heeger, A. J.; Bazan, G. Nat. Mater. 2007, 6, 497.
5. Zimmerman, J. D.; Lassiter, B. E.; Xiao, X.; Sun, K.; Dolocan, A.; Gearba, R.; Vanden Bout, D. A.; Stevenson, K. J.; Wickramasinghe, P.; Thompson, M. E.; et al. ACS Nano2013, 7, 9268.

Here we report solution-processed n-channel OTFTs that exhibited very high field effect mobility of up to 9 cm²/Vs at room temperature and the observed negative temperature coefficient (dmu;/dT < 0). A new hybrid dielectric CDPA/AlO_x/SiO₂ was developed, where self-assembled-monolayer (SAM) of 12-cyclohexyldodecyl-phosphonic acid (CDPA) was attached on an ultra-thin layer of sol-gel deposited AlO_x on top of thermally-grown SiO₂ substrate. The resulting dielectric was found stable with varying temperature due to higher degree of metal-oxide structural integrity under optimized baking conditions. Enhanced electron mobility of 6,13-bis((triisopropylsilyl)ethynyl)-5,7,12,14-tetraazapentacene (TIPS-TAP) was achieved, offering a good platform to study temperature-dependence of mobility. Negative temperature coefficient (dmu;/dT < 0) was observed for the first time in n-channel thin film transistor based on small molecule semiconductor, suggesting band-like transport regime. By controlling thin film morphology with different fabrication techniques, we have found that grain boundaries play an important role in determining the charge transport behavior.

Solution-processed small-molecule (SPSM) bulk heterojunction (BHJ) solar cells have garnered significant attention due to the compatibility with large-area roll-to-roll manufacturing, besides the advantage of having higher crystallinity and lack of batch-to-batch variability as compared to their polymeric counterparts. The use of solvent additives has been adopted as a requirement to achieving high power efficiencies (PCE) in most of SPSM systems. However, recently some systems have been developed that achieve optimal performance without the need for additives, nor added annealing step. We explored a variety of small molecule donor materials in order to establish a framework that can help in defining the need (or not) of using additives, or annealing, for attaining optimal PCE from their blends with fullerene. We have investigated the ease of crystallization of three donor molecules, namely p-DTS(FBTTh₂)₂, p-SIDT(FBTTh₂)₂ and X2, using ultra-fast differential scanning calorimetry (Flash-DSC) and grazing incidence X-ray scattering. We have found that the resistance to crystallization of the molecular donors, by going through a transient mesophase (such as amorphous or liquid crystalline phase), is the main obstacle for the donor crystallization and donor/acceptor phase separation in the BHJ. Therefore, the additives, or alternatively thermal or solvent vapor annealing, are required under these conditions to provide the kinetics for donor crystallization and to facilitate phase separation from the acceptor. We go on and demonstrate a 7.6% efficient additive-free processed solar cell using a donor material that can easily crystallize and thus tend to phase separate from the fullerene without the need of an additive or annealing. Our findings should pave the way for the development of new materials that enable fabrication of high efficient organic solar cells using environmentally friendly large-area manufacturing in a single step.

Organic electronic materials have stirred a lot of excitement and research effort given their promise for low-cost deposition on flexible substrates, particularly for large-area electronics. But while, indeed, the potential for organic electronics is enticing and offers many advantages over conventional technologies, the number of successful adoptions of organic materials in consumer applications remains limited, mostly because of performance issues. The modest performance of organic devices, however, is often not intrinsic to their constituent materials, but it results from defects and impurities that give rise to trapping sites and significantly degrade device performance.
We quantitatively analyze the spectral density of trap states (DOS) in organic field-effect transistors based on several anthradithiophenes by using analytical methods, and we correlate it with the device mobility, subthreshold swing and threshold voltage. We evaluate devices fabricated by several solution-based methods to study the effect of the processing conditions in introducing or eliminating trapping states. The results are compared with the data obtained on vapor-grown single crystals. We show that the type of processing method greatly affects the DOS and performance of the resulting device. In addition, we find that the nature of the dielectric plays a key role in the resulting sub-gap DOS and we can lower the trap density two orders of magnitude by using the Cytop dielectric, for which we find a density of shallow traps of N_t = 10¹⁸ cm^-3 and deep traps of N_t = 6 x 10¹⁷ cm^-3.
We further analyzed the DOS in devices of different purity and composition. For diF TES ADT devices fabricated from a mixture of syn and anti isomers, which show a typical mobility of 2.7±0.7 cm²/Vs, we found a peak located around 0.4 eV, suggesting that a defect or impurity forms traps 0.4 eV from the valence band edge. The mobility was improved upon extensive purification of the organic semiconductor, and the DOS was reduced by about two orders of magnitude, but we were not able to completely remove that trap-state. We extinguished this peak by two different methods. The first one involved separating the syn and anti isomers present in the mixture by modifying the synthetic procedure. In the second method we vertically phase-separate the isomers by processing. Not only was the peak removed in this case, but the mobility became at least double, depending on the contact treatment and dielectric used.

Many organic device and energy-related semiconductors require facile and exclusive electron transport. While numerous low-bandgap polymers show high electron mobility, they are also typically ambipolar. We have shown that higher-bandgap molecules and polymers can combine usable electron mobility without appreciable hole transport. For example, the pyromellitic (benzenetetracarboxylic) diimide subunit promotes electron transport in molecular solids with field effect transistor mobility >0.1 cm²/Vs across strongly layered solids along with transparency and low processing temperatures. We determined the crystal structure of a dibromo derivative that illustrates the layerwise packing. Tetracarboxylic diimide copolymers with non-donor phenylene and ethynylene subunits also show exclusively n-type behavior. Besides use in solution-deposited transistors, these polymers are especially favorable for use as matrices in thermoelectric composites, where exclusive electron transport is especially desirable. Seebeck coefficients in the millivolts/kelvin range and power factors of 100 mu;W/mK² were obtained.

Photoconversion efficiency can be improved in solution-processable porphyrin heterojunction solar cells by increasing exciton diffusion lengths (L_D). Rapid and efficient exciton diffusion in porphyrin thin films permits excited states to migrate from the point of photogeneration to the donor-acceptor interface. Thus, it is important to understand the relationship between the structure/molecular orientation of the porphyrin material and how this affects the distance over which the exciton can migrate before it decays. We have studied the singlet exciton diffusion properties of solution-cast thin films of carboalkoxyphenylporphyrins for use as sensitizers for organic heterojunction solar cells. The photoluminescent singlet decay lifetime PL(t) of pristine porphyrin films and films lightly doped (0.02 - 1% wt.) with [6,6]-phenyl-C61-butryic acid methyl ester (PCBM) were used to obtain a relative quenching efficiency (Q). Q was used in a 3D exciton diffusion Monte Carlo simulation model to extract the exciton diffusion coefficient (D), L_D and the nanocomposition of the blend. This study shows an increase in diffusion lengths (L_D) from 15 to 40 nm by extending the length of peripheral carboalkoxy and alkoxyphenyl substituents on the porphyrin macrocycle. Extended PL decay times and exciton diffusion lengths occurred by increasing the alkyl chain length and branching of a meso-substituted carboalkoxyphenyl group, affecting porphyrin-porphyrin excitonic interactions and exciton hopping efficiencies. We present the effects of the peripheral porphyrin substituents, methods of processing the films, and molecular orientation on the exciton dynamics. We also discuss how extending singlet exciton diffusion lengths past the absorption depth may ultimately eliminate the requirement of nanostructured donor-acceptor interfaces in porphyrin-based solar cell devices.

Soluble organic semiconductors that can be processed using simple solution-based methods represent an emerging class of electronic materials for application in a wide range of optoelectronics. Due their generally modest performance characteristics, however, their use to date has been limited to relatively a few conventional applications. The development of advanced material systems and/or novel device concepts with enhanced performance characteristics is thus necessary if printed electronics are to become a reality. In this presentation I will discuss the development of various solution-processable organic blend semiconductors and their implementation in high-performance electronic devices and systems such as thin-film transistors, integrated circuits and energy harvesting devices. Particular emphasis will be placed on the apparent morphology-independent charge carrier transport characteristics observed in certain blend semiconductors and the associated advantages for large-area manufacturing.

Singlet fission provides a path towards more efficient solar cells by doubling the current from high-energy photons. Pentacene and tetracene are organic seminconductors typically used in singlet fission solar cells. However, they are not stable in ambient conditions and not solution processable. We show solar cells from TIPS-pentacene, as well as from terrylene derivatives. Those molecules have pracitcal advantages over the traditional polyacenes, and are hence a more realistic option for the application of singlet fission in solar cells.

Understanding the morphology of organic bulk heterojunction (BHJ) solar cells and how it can be influenced is essential for improving device performance. Using Transmission Electron Microscopy (TEM), we have previously found that phase separation and crystallinity might be expected in mixed small molecule BHJ layers [1]. Electron Spectroscopic Imaging (ESI) [2] and Electron Diffraction revealed that substrate and substrate temperature affect the morphology of the photoactive layer during co-evaporation of F₄ZnPc:C₆₀ BHJs. Depending on device architecture (inverted [3] or non-inverted solar cell), the underlying substrate layer may selectively enhance crystal growth of one of the materials, which thus provides a mechanism for phase separation during film formation. The different observed morphologies can in addition be correlated to device efficiency.
To investigate the BHJ layer morphology further and to compare it to solar cell device layers, we studied the 3D structure of such layers in more detail by electron tomography and FIBSEM nanotomography. For this study F₄ZnPC:C₆₀ was co-evaporated at different temperatures on layers of neat F₄ZnPc or C₆₀, respectively. Since so far ESI and electron diffraction did only provide information about material distribution in a two-dimensional projection of the sample, we now used tomographic methods to also gain insight into the three-dimensional distribution of the materials.
In this work, we present tomographic 3D reconstructions of co-evaporated F₄ZnPc:C₆₀ layers as they can be found in inverted and non-inverted solar cell devices and compare these to device-like layers as seen by FIBSEM nanotomography. From these reconstructions, we deduce that materials separate in the active layers in a distinctive pattern, which provides first ideas for a mechanistic model of the different device efficiencies of inverted and non-inverted solar cells. At present, we also explore the potential of reducing beam damage using low-dose and cryo-techniques to increase 3D resolution, enabling visualization of crystallinity.
Together these results contribute to the understanding of solar cell functionality; in particular they provide clear evidence that device performance can be manipulated by morphology in a predictable manner.
[1] D. Nanova et al., MRS Fall Meeting 2014
[2] M. Pfannmöller et al., Nano Letters 2011
[3] J. Meiss et al., Adv. Funct. Mat. 22, 405-414 (2012)
[4] This work was supported by the German Ministry of Research and Education, collaborative project #8222;LOTsE“ , grants 03EK3505M (I.W.), 03EK3505L (W.K.), and 03EK3505K (R.R.S)

The controlled crystallization of organic thin films holds great potential for enhancing the electrical and optical performance of organic electronic devices using existing active materials. To date however, quantitative understanding of the processes involved in device-relevant geometries has been limited. Here, the thin-film crystal growth of rubrene is characterized during annealing using polarized optical microscopy with a heated stage for in situ measurements, followed by atomic force microscopy and x-ray diffraction. Annealed films show temperature-dependent transitions from predominant growth of the triclinic polymorph to single-crystal orthorhombic grains, followed by a transition to spherulitic growth of the orthorhombic grains. The ability to track crystal morphology in situ with time permits determination of the crystal orientation, which is used in conjunction with crystal size measurements to extract the activation energies for crystallization of different crystal phases and planes. It is expected that this understanding of film evolution, orientation, and control over film composition can be applied to the optimization of thin-film transistors, as well as other organic electronic devices which use molecular template layers to realize epitaxial growth.

The efficiency of organic photovoltaic (OPV) cells has increased significantly over the past 14 years, in large part due to the introduction of the bulk heterojunction architecture based on the fullerene acceptor. This allowed for an optimization of the donor-acceptor interface, which enhances exciton dissociation while also maximizing light absorption due to increased device thickness. However, the necessity for a bulk heterojunction architecture is in part due to the limitations of the fullerene acceptor. Although fullerenes are excellent electron acceptors and have high electron mobility, they are relatively poor absorbers within the solar spectrum and their high electron affinity significantly limits the maximum open circuit voltage (V_oc). Recently, OPV efficiencies as high as 8.4% have been demonstrated using the non-fullerene acceptor alternatives SubPc and SubNc in a planar heterojunction device when paired with an α-6T donor [1]. This high efficiency is due to the complementary absorption spectra of each material as well as long range Förster exciton transfer from the larger bandgap material SubPc to the smaller bandgap material SubNc, allowing these excitons to then efficiently dissociate at the α-6T/SubNc interface. In order to better understand the high efficiency of the exciton dissociation and subsequent carrier extraction as well as the large V_oc achieved in these devices, it is important to understand the band alignment at each of these material interfaces. Ultraviolet, x-ray, and inverse photoemission spectroscopies (UPS, XPS, and IPES) were employed to measure the ionization energy (IE) and electron affinity (EA) of each material. In addition, UPS, XPS and IPES measurements were performed before and after depositing subsequent layers of SubPc and SubNc on α-6T as well as SubPc on SubNc in order to directly probe the energy level alignment at the interfaces of interest. These measurements indicate the existence of a small dipole at the α-6T and SubNc/SubPc interface, which increases the difference between the donor LUMO and acceptor HOMO and explains the large V_oc found in these devices. However, a 0.25 eV electron barrier was discovered between SubNc and SubPc, which may limit the efficiency of extracting electrons in the current device configuration. With this knowledge, future devices may be able to be improved by modifying SubNc to decrease its EA, modifying SubPc to increase its EA, or by introducing an interface dipole via a self-assembled monolayer or alternative method in order to reduce this barrier for electrons.
[1] K. Cnops, B. P. Rand, D. Cheyns, B. Verreet, M. A. Empl, and P. Heremans, “8.4% efficient fullerene-free organic solar cells exploiting long-range exciton energy transfer,” Nat. Commun., vol. 5, Mar. 2014

The heterojunction formed between the hydrogen and fluorine terminated copper phthalocyanines was investigated by ultraviolet photoelectron and X-ray absorption spectroscopy. Pinning at the Fermi level of the underlying electrode is observed for both materials—one p#8209;type and the other one n#8209;type. This results in an interfacial charge transfer accompanied by a sheet charge density at the organic/organic interface. An interlayer with co-facial intermolecular arrangement, which differs from the respective bulk structures, at the interface was found by both spectroscopy techniques; this interlayer, noteworthy, is unpinned. The overall electronic structure of this heterojunctions explains its behaviour as charge generation layer in planar heterojunction diodes

Improved materials, processing protocols, and device designs have brought organic electronic devices to the forefront of numerous emerging technologies. A critical bottleneck that remains, however, is a thorough understanding of the intimate relationships among chemical and molecular structure, processing, solid-state packing, and the underlying physical processes that determine device performance. Here we will discuss the development of molecular-based models that provide relevant materials-scale information through a combination of classical molecular dynamics approaches with quantum mechanics calculations. Our goal is to provide a molecular-scale description of these relationships in order to refine and offer novel design pathways for next generation organic semiconducting active layers.

Highly pi-extended acenes, e.g., pentacene[1], and thienoacenes[2] have been intensively studied as organic semiconductors applicable to the high-performance organic field-effect transistors (OFETs). Among recently developed organic semiconductors, dinaphtho[2,3-b;2',3'-f]thieno[3,2-b]thiophene (DNTT) is one of the best materials showing high mobility (> 1 cm²/V s) in the thin film transistor architecture and good ambient stability[3]. To explore further possibility of DNTT and its related materials, we have developed DNTT derivatives with linear alkyl[4], phenyl[5], or branched alkyl groups, and pi-extended analogues with eight aromatic rings fused in a linear manner[6,7]. For realizing these molecular modifications, new synthetic methods for effectively constructing the DNTT framework have been also developed[7-9]. These DNTT derivatives have shown promising characteristics as organic semiconductors; for example, higher mobility of up to 8 cm²/V s in thin film transistor (C10-DNTT), enhanced thermal stability up to 250 °C (DPh-DNTT), and solution processability (EH-DNTT). In this contribution, these recent advances on DNTT-based materials and the synthetic chemistry behind will be discussed.
Reference
[1] T. Jackson et al., IEEE Electron Device Lett. 18 (1997) 606. [2] K. Takimiya, et al. Adv. Mater. 23 (2011) 4347. [3] T. Yamamoto and K. Takimiya, J. Am. Chem. Soc. 129 (2007) 2224. [4] M. Kang, et al., Adv. Mater. 23 (2011) 1222.[5] M. Kang, et al., ACS Appl. Mater. Interfaces, 5 (2013) 2331. [6] K. Niimi, et al., J. Am. Chem. Soc. 133 (2011) 8732. [7] T. Mori, et al. J. Am. Chem. Soc. 135 (2013) 13900. [8] K. Niimi, et al., Org. Lett., 13 (2013) 3430. [9] M. Abe, et al. Chem. Mater., 27 (2015) 5049.

In recent years, the efficiency of solution-processable small molecule donors in bulk-heterojunction (BHJ) solar cells with fullerene acceptors has quickly improved from ca. 4%[1] to >9%[2]. At this point in time, small molecule and polymer donors are comparably efficient in BHJ solar cells, and yet the morphologies, phase composition and the texture of these two p-extended donor systems are uniquely different when intimately mixed with fullerenes. While fibrillar morphologies developing across the photoactive layer are often seen in efficient polymer-based BHJ devices,[3] BHJ thin films composed of small molecule donors and fullerenes tend to be of relatively fine-scale morphology,[4] with X-ray diffraction patterns usually showing pronounced local and mesoscale order.[2,5] Interestingly, compared to polymer donors,[6] the pattern of solubilizing substituents appended to the small molecule donor backbone can be even more critical, playing a determining role on (i) crystal packing in pure small molecule domains, (ii) correlated charge transport, (iii) the development of the BHJ morphology with fullerenes, and in turn (iv) BHJ solar cell efficiency.[7] Examining small molecule donors with analogous π-extended backbones, but distinct side chains, further points to the relative importance of functionalizing the middle unit with the appropriate substituents, while end group substituents may not be as critical.[7] As a general trend, we show that it is important to mitigate the propensity of the small molecule donors to crystalize in thin films via proper selection of the pattern of side chain substituents.[8] The highest-performing small molecule donor systems maintain pronounced local order in the BHJ thin films with fullerene acceptors.
[1] T.-Q. Nguyen et al. Adv. Funct. Mater. 2009, 19, 3063;
[2] D.J. Jones et al. Nat. Commun. 2015, 6, 6013; Y. Chen et al., JACS 2015, 137, 3886;
[3] R.A.J. Janssen et al. JACS, 2013, 135, 18942; P.M. Beaujuge et al. Adv. Mater. 2014, 26, 4357;
[4] G.C. Bazan, A.J. Heeger et al. Nat. Mater. 2012, 11, 44;
[5] T.-Q. Nguyen, G.C. Bazan, A.J. Heeger et al. JACS, 2014, 136, 3597;
[6] P.M. Beaujuge, J.M.J. Fréchet et al. JACS 2010, 132, 7595; J.M.J. Fréchet, M.D. McGehee, P.M. Beaujuge et al. JACS 2013, 135, 4656; P.M. Beaujuge et al. Chem. Mater. 2014, 26, 2829; J-L. Brédas, M.D. McGehee, P.M. Beaujuge et al. Chem. Mater. 2014, 26, 2299; K.R. Graham, A. Amassian, P.M. Beaujuge, M.D. McGehee et al. JACS 2014, 136, 9608; P.M. Beaujuge, F. Laquai et al. Adv. Energy Mater. 2015, DOI: 10.1002/aenm.201401778;
[7] P.M. Beaujuge et al. 2015, Submitted
[8] P.M. Beaujuge et al. 2015, Submitted

Elucidating the morphology at donor-acceptor interfaces is crucial to understand organic photovoltaic performance. One system that has undergone extensive experimental and computational investigation is the acene-fullerene interface. For example, in the pentacene/C₆₀ system, the intermolecular coupling is predicted to differ significantly for edge-on, face-on, and end-on interface configurations;[1] furthermore, modeling shows that the face-on configuration is morphologically unstable, resulting in mixing of the acene and fullerene.[2] Meanwhile, experimental work has struggled to understand the properties of these interfaces. We show that the tetracene/C₆₀ interfaces are chemically unstable and undergo a Diels-Alder reaction ([4+2] cycloaddition), creating adduct species at the donor-acceptor interface. While this reaction has been studied in solution—and in one case vibration milling of powders[3]—identification of the sub-monolayer of adduct formed at a bilayer interface is difficult using conventional techniques. In this work, we analyze the reaction using atom probe tomography, which provides not only molecular mass information, but also a three-dimensional volumetric image of the interface. We then use the presence of this adduct to explain changes in photovoltaic performance, demonstrating the value of atom probe tomography for studying small-molecule organic systems.
[1] Coropceanu, V. et al. Chem. Rev. 107 926 (2007)
[2] Yi, Y. et al. J. Am. Chem. Soc. 131 15777 (2009)
[3] Murata, Y. et al. J. Org. Chem. 64 3483 (1999)

In this presentation I will discuss some recent results focusing on the formation of hybrid photovoltaic (HPV) and light emitting diode (HLED) structures implementing vertically aligned ZnO nanorods as charge collecting and charge injection electrodes respectively.
In the first part of the presentation I will outline the methods we have developed to give controllable and reproducible nanorod growth in aqueous solution. I highlight the dependence of the nanorod structure on the processing conditions employed for growth and propose a relationship between the nature of the substrate and the resultant nanorod array (NRA) structure- highlighting the different structures that can be prepared by simple process modifications.
The core of the presentation details the infiltration of these NRAs with poly(3-hexylthiophene-2,5-diyl) (P3HT) and indene-C₆₀ bisadduct (IC₆₀BA) as photoactive materials for HPV applications and poly(9,9-dioctyluorene-alt-benzothiadiazole) (F8BT) for HLED devices. Using a combination of optical and electron microscopy with X-ray diffraction we probe the ordering and orientation of the polymers inside the NRAs as processing conditions are varied. Here the role of solvent, processing and post deposition anneal on the polymer morphology are highlighted and the optimum conditions identified for device preparation. In the final part of the presentation the influence of these processing parameters on measured performance are measured using both HPV and HLED devices.
References
1. J. M. Downing, M. P. Ryan, M. A. McLachlan, Thin Solid Films 2013, 539, 18.
2. J. C. D. Faria, A. C. Campbell, M. A. McLachlan, Adv. Funct. Mater. 2015, doi: 10.1002/adfm.201501411.

Drastic improvement in power conversion efficiencies (PCEs) of polymer-based solar cells (PSCs) has been made in the last decade, mainly by the devleopment of new donor-acceptor (D-A) semiconducting polymers. One of the largest issues in improving PCEs in PSCs is to realize a high short-circuit current (J_SC) and a high open-circuit voltage (V_OC) at the same time. This is interpreted as to create a semiconducting polymer with both a narrower band gap and deeper HOMO energy level. However, it is well-known that these two parameters are always trade-off. Whereas the narrow band gap can increase the light harvesting property and thus J_SC, it inevitably gives rise to a shallower HOMO energy level, resulting in a low V_OC, or to a deeper LUMO energy level, which diminishes the energy offset of LUMOs between the polymer and fullerene, in turn preventing the efficient charge separation at the polymer/fullerene interface. In this regard, synthetic chemist face a formidable challenge: to create new semiconducting polymers in order to manage the energetics between polymers and fullerenes and thus to minimize the trade-off.
Recently, we reported that a naphtho[1,2-c:5,6-cprime;]bis[1,2,5]thiadiazole (NTz)-based polymer, PNTz4T,[1] with a band gap of 1.56 eV showed PCEs as high as 10% in solar cells combined with PC₇₁BM.[2] Although the J_SC was high (19.4 mA/cm²) the V_OC was limited to 0.71-0.74 V because of the moderately deep HOMO energy level of -5.14 eV. Naphtho[1,2-c:5,6-cprime;]bis[1,2,5]oxadiazole (NOz), a new fused heterocycle as an oxygen analogue of NTz, can deepen both the HOMO and LUMO energy levels while maintaining the band gap as compared to NTz. This is likely as a result of stronger electron negativity of oxygen than sulfur. Thus, the incorporation of NOz into the semiconducting polymer is expected to realize both high J_SC and V_OC when it is used as the p-type material for PSCs. Here, we synthesized a copolymer of quaterthiophene and NOz, PNOz4T. PNOz4T was found to have a narrow band gap of 1.52 eV and deep HOMO energy levels of ~5.4 eV. PNOz4T exhibited a highly ordered π-π stacking structure with a distance of around 3.5 Å and a face-on backbone orientation, which are favorable for solar cells. The solar cells with an inverted structure using PNOz4T and PC₇₁BM demonstrated PCEs of ~8.9% with remarkably high V_OCs of ~1 V despite having such a narrow band gap. Notably, the photon energy loss, E_g-eV_OC was significantly reduced to 0.52-0.55 eV (PNOz4T) from 0.82-0.85 (PNTz4T).
[1] J. Am. Chem. Soc.2012, 134, 3498-3507. [2] Nat. Photon.2015, 9, 403.

A large diversity of π-systems exists, the vast majority of them transport electrical charges but only a few molecular structures qualify as best-performing organic semiconductors with mu; ge; 10 cm²/V.s. But charge carrier mobility is a materials and not a molecular property. One has, thus, to consider supramolecular order at all lengthscales. The best organic semiconductors self-organize into large plate-like monocrystals as evidenced in a recent review paper devoted to the question: “What Currently Limits Charge Carrier Mobility in Crystals of Molecular Semiconductors?” G. Schweicher, Y. Olivier, V. Lemaur, Y. H. Geerts, Isr. J. Chem.2014, 54, 595-620. Due to their electronic properties but also to their favorable crystalline morphology, BTBT derivatives exhibit record charge carrier mobility above 10 cm²/V.s. We will report our latest results on the molecular and supramolecular engineering of BTBT semiconductors, including: design by theory, synthesis, crystal engineering, calculation and observation of crystal morphology, and processing into single crystal thin films for transistor fabrication

Over the past decade, small molecule organic solar cells (SMOSCs) have been intensively investigated as a competitive technology of green energy with the ease of synthesis and reproducibility without batch-to-batch variation with well-defined structure and definite molecular weight. In particular, donor-acceptor push-pull structure is considered as a promising way to make low bandgap materials of SMOSCs. With this push-pull structure strategy, the sequence of donor and acceptor moieties significantly affects molecular secondary interaction, supramolecular structure, film morphology and photovoltaic properties.
To explore the efficient way of assembling donor and acceptor moieties on bulk heterojunction small molecule organic solar cells (SMOSCs), we successfully synthesized and characterized A-D-A type and D-A-D type triad molecules (ADA and DAD, respectively) using indolo[3,2-b]indole (IDID) as a donor moiety and diketopyrrolopyrrole (DPP) as an acceptor moiety. Comparing ADA and DAD with their optical, electrochemical properties, both compounds exhibited strong intramolecular charge transfer characteristics between donor and acceptor moieties, whereas ADA exhibited higher molar extinction coefficient, lower-lying HOMO level, and more stacked supramolecular structure than DAD. Furthermore, OSCs using blend of ADA as a donor and PC₆₁BM as an acceptor exhibited high PCE of 4.15% with high J_SC of 10.24mA/cm² and fill factor of 55.6%, whereas OSCs using DAD as a donor and PC₆₁BM as an acceptor exhibited PCE of 2.46% with J_SC of 6.62mA/cm² and fill factor of 57.2%. By comparison of optical, electrochemical properties and device performances, A-D-A type is considerably confirmed as suitable architecture using IDID and DPP moieties for donor of SMOSCs.

Conjugated polymers (CP) have garnered considerable attention due to their conductivity and luminescence properties, coupled with ease of processing and fine-tuning of their properties through chemical structure manipulation. Because of this, they are highly attractive for many optoelectronic applications ranging from LED&’s to sensors. Currently, manipulation of their chemical structure has mainly been carried out by the tailoring of monomers before polymerization, with most of the post-polymerization modifications aimed to improve material stability by crosslinking or elimination of labile side groups with little to no effect on optoelectronic properties.
The current work explores the chemical modification of the carbonyl moiety of poly(p-phenyl-4-octyl-pentanodione) -a photoluminescent conjugated polymer with ketone groups in its backbone obtained by means of the Claisen-Schmidt reaction between a lineal ketone and an aromatic dialdehyde- using different reactions in order to alter its optoelectronic properties without disrupting the conjugated π bond system. At the moment, modifications include the substitution of C=O bonds by C=C bonds using the Wittig reaction, by C=N bonds via reaction with amines -including the insertion of new alkyl or aryl groups-, and by oxime groups using hydroxylamine.
Infrared Spectroscopy (FT-IR) was used to follow the reaction, with C=O peak intensity decreasing as reaction time increased, which was quantified using a carbonyl index. Furthermore, ¹H Nuclear Magnetic Resonance (¹H-NMR) supported the predicted structure with newly inserted groups bonded directly to the main chain. Additionally, Gel Permeation Chromatography (GPC) determined that polymer chains experience a decrease of mean molecular weight and a narrowing polydispersity with increasing reaction times.
Further characterization by UV-Vis and PL Spectroscopy showed a decrease in optical band-gaps as well as a blue-shift in emission maxima when conjugation is increased with new side chains groups. On the other hand, the opposite effect is observed when the modifying groups do not increase conjugation length, as well as for the oxime modification.

PTB7:PC₇₀BM films are among the highest efficiency bulk heterojunction organic photovoltaic (OPV) devices, reaching record efficiencies of 9.2% [1]. This makes PTB7:PC₇₀BM an important model system for understanding the role of processing conditions in determining film morphology and for developing new processing conditions to improve performance. OPV film morphology determines the ease of exciton separation and charge transport through the film, and so control of the morphology will directly allow improvement in OPV devices.
This work concentrates on the effects of thermal annealing of spin-cast films of neat PTB7 and PTB7:PC₇₀BM. Annealing is a common processing technique for other OPV devices but its effects on PTB7 devices have not yet been studied. Typical annealing temperatures below 260°C increase the degree of crystallinity of the films, which is shown to slow photoluminescence decay and therefore decrease recombination in the film. Other studies on polymer:fullerene bulk heterojunctions have shown that thermal annealing increases crystallinity by allowing fullerenes to migrate out of polymer aggregates, encouraging phase segregation [2,3], and it is hypothesized that a similar effect is occurring in the case of PTB7.
Annealing at higher temperature (290°C) near the polymer melting point (303°C) is shown to lead to a significant decrease of the pi-pi stacking distance of PTB7, both in neat films and in PC₇₀BM blends, in addition to increasing crystallinity as for lower annealing temperatures. A smaller distance between polymer backbones is expected to increase wavefunction overlap and increase the carrier mobility in the pi-pi stacking direction, which will in turn improve the device efficiency. GIWAXS with in-situ annealing shows that the formation of this new structural motif is a gradual process, narrowing the possible mechanisms at play.
[1] Z. He, C. Zhong, S. Su, M. Xu, H. Wu, Y. Cao, Nat. Photon. 2012, 6, 591.
[2] J. Bartelt, Z. Beiley, E. Hoke, W. Mateker, J. Douglas, B. Collins, J. Tumbleston, K. Graham, A. Amassian, H. Ade, J. Fréchet, M. Toney, M. McGehee, Adv. Energy Mater. 2013, 3, 364.
[3] W-R. Wu, U-S. Jeng, C-J. Su, K-H. Wei, M-S. Su, M-Y. Chiu, C-Y. Chen, W-B. Su, C-H. Su, A-C. Su, ACS Nano 2011, 5, 6233

6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS-PEN)¹ is a prominent p-type semiconducting material for solution-processed organic field-effect transistors (OFETs). Several groups have reported OFETs based on TIPS-PEN by solution processes with high mobilities ranging from 0.1 cm² V^-1 s^-1 to 10 cm² V^-1 s^-1. Recently, we reported synthesis and single-crystal structure of 5,15-bis(triisopropylsilylethynyl)tetrabenzoporphyrin (TIPS-BP) ^2,3 which forms a 2D slip-stack packing. This packing mode is similar with that of TIPS-PEN. In order to estimate the mobility in this crystal structure, the charge transfer integrals between the highest occupied molecular orbitals (t_HOMO) were calculated using the obtained single-crystal structure, at GGA:BP/DZP level by the Amsterdam Density Functional (ADF) program. The calculated t_HOMOs are larger than those of TIPS-PEN indicating that TIPS-BP has a potential to show a higher mobility than TIPS-PEN. Therefore, we fabricated top-gate-bottom-contact OFETs based on TIPS-BP by thermal vacuum deposition, drop casting and dip coating with different conditions. We obtained average and highest hole mobilities of 0.92 and 1.42 cm² V^-1 s^-1, respectively. We will report the details of crystal structures and fabrication of OFETs based on TIPS-BP.
References: (1) Anthony, J. E. et al. J. Am. Chem. Soc. 2001, 123, 9482-9483. (2) Yamada, H. et al. J. Photopol. Sci. Tech.2013, 26, 213-216. (3) Yamada, H. et al. J. Porphyrins Phthalocyanines2015, 19, 465-478.

Low band gap (LBG) organic materials that absorb into the Near-InfraRed (NIR) are of great interest in the recent years for number of potential applications, in both military and civilian uses. Military applications include target acquisition, surveillance, night vision, homing, and tracking. Non military uses include thermal efficiency analysis, remote temperature sensing, short-ranged wireless communication, spectroscopy, but also energy. For example, the use of NIR-absorbing or NIR photovoltaic organic materials (small molecules or polymers) could extend the material&’s absorption into the NIR spectral region and even beyond 1000 nm wavelength, which in principle could enhance the current power conversion efficiency (PCE) of organic photovoltaics OPVs.
Even though semiconducting polymers with ultra LBGs have been synthesized before, the challenge in designing and synthesizing materials that have a good photoresponse beyond 900 nm and an appreciable PCE in polymer:fullerene solar cells is still open and lies (among others) in the precise energy level control that is required.
A new ultra low band gap (LBG) α,β-unsubstituted BODIPY-based conjugated polymer has been synthesized by conventional cross coupling polymerization techniques (Stille cross coupling) for the first time. The polymer exhibits a panchromatic absorption spectrum ranging from 300 nm to 1100 nm and an optical band gap (E_g^opt) of 1.15 eV, suitable for near infrared (NIR) organic photovoltaic applications as electron donor. Preliminary power conversion efficiency (PCE) of 1.1 % in polymer:[6,6]-phenyl-C71-butyric acid methyl ester (PC₇₀BM) 1:3 weight ratio bulk heterojunction (BHJ) solar cells has been achieved, demonstrating very interesting and promising photovoltaic characteristics, such as good fill factor (FF) and open circuit voltage (Voc). These results showing that by the proper chemical design, new α,β-unsubstituted BODIPY based NIR copolymers can be developed in the future with suitable energy levels matching those of PC₇₀BM towards more efficient NIR organic photovoltaics (OPVs).

Azulene, a nonalternant hydrocarbon composed of five- and seven-membered aromatic rings, has attracted much attention because of its unusual properties, including a large dipole moment and long-wavelength absorption. We focused on the connection of azulene units via the 2,6-position to form a linear and flat molecular shape. Here, we present the synthesis, structures, and FET characteristics of all four isomers of terazulene, namely 2,6prime;:2prime;,6Prime;-terazulene (TAz1), 2,2prime;:6prime;,2Prime;-terazulene (TAz2), 2,2prime;:6prime;,6Prime;-terazulene (TAz3), and 6,2prime;:6prime;,6Prime;-terazulene (TAz4).
All terazulene isomers showed well-defined herringbone packing in single crystals, and these structural features were accurately reflected in the thin film structures. All isomers showed n-type characteristics. In addition, p-type characteristics were observed in TAz2 and TAz3. The LUMOs of all isomers were distributed over the entire molecule and could overlap adjacent molecules. In contrast, in TAz1 and TAz4, the HOMOs were localized at one end of the azulene unit and could not overlap adjacent molecules. In TAz2 and TAz3, the HOMOs were slightly spread over the molecule and the bias was small. Thereby, TAz1 and TAz4 showed only n-type characteristics; TAz2 and TAz3 showed ambipolar operation in the OFET device. These results represent an unconventional concept, namely the achievement of polarity control of OFET by molecular orbital distribution control. This provides a new strategy for the molecular design of semiconductor materials.

Organic semiconductor nanowires have recently drawn attention due to their high degree of crystalline order and thus excellent electronic properties along the fiber axis direction. They have been incorporated as active materials into various kinds of electronic devices, such as thin film transistors, sensors and organic solar cells.^[1-3] For all of these applications, the internal order in the nanowires as well as their orientation are of crucial importance. Control over these parameters is desired to improve the device performance. For thin film transistors, sensors, or more complex logic devices such as inverters, an alignment of the nanowires parallel to the substrate but perpendicular to source and drain electrodes is highly beneficial for efficient charge transport across the device channel. Besides the alignment of the nanowires, a patterning of the active layer is highly advantageous, to avoid cross talk between individual devices.
Here, we report on a method providing for both, the patterning and the desired alignment of nanowires, by depositing them on pre-patterned substrates with designated surface areas. The substrates were fabricated with a standard lithographical process, using self-assembled monolayers (SAMs) containing different head group termination and thus offering differently functionalized surface areas, i.e. hydrophilic/oleophilic/fluorophilic.^[4] Organic semiconductor nanowires of the model compound dioctyl-perylenedicarboximide (C₈-PDI) were grown by a solution-based method as reported in literature.^[1,2] These nanowires were dispersed in a non-solvent and solution processed on the pre-patterned SAM structures. By the right choice of materials, the interactions between surface, solvent, and nanowires can be tuned such that the nanowires are deposited only on designated areas. Nanowires dispersed in dilute polymer solutions can increase the coverage of the semiconductor nanostructures on specific surface patterns, and the polymer can even be chosen to be electronically active to enhance the device performance. By employing adequate deposition processes or by adjusting the dimensions of the surface patterns to the size of the nanowires, the latter could also be aligned in a desired direction. Hence, our approach allows for the indirect patterning and alignment of semiconductor nanowires without affecting them by the actual lithography step. To validate the concept, aligned nanowire arrays were incorporated into thin film transistors and electrically characterized.
References:
^[1] Briseno, A. L.; Mannsfeld, S. C. B.; Reese, C., et al. Nano Letters 2007, 7, (9), 2847-2853.
^[2] Briseno, A. L.; Mannsfeld, S. C. B.; Jenekhe, S. A., et al. Materials Today 2008, 11, (4), 38-47.
^[3] Marty, R.; Szilluweit, R.; Sánchez-Ferrer, A., et al. ACS Nano 2013, 7, (10), 8498-508.
^[4] Wang, Z.; Mohammadzadeh, S.; Schmaltz, T., et al. ACS Nano 2013, 7, (12), 11427-34.

Self-assembled thin films are very versatile nanomaterials with great potential for various applications such as solar cells, electronic semiconductor components, protective coatings, membranes for fuel cells, among many others.
In this work were constructed self-assembled thin films based on the aromatic imide N, N '-(2-phosphonoethyl)-3,4,9,10-perylendiimide (PPDI) using the zirconium phosphonate method (ZP). The films were grown on silicon and ITO (Indium Tin Oxide) substrates. Prior to deposition of the first layer it was necessary to increase the density of hydroxyl groups on the surface of ITO substrate by an activation step in a solution of deionized water, ammonium hydroxide and hydrogen peroxide. In the case of the silicon substrates, priming with APTES (3-aminopropyltriethoxysilane, generating a NH₂-rich surface), followed by phosphorylation with phosphorus oxychloride (POCl₃), was necessary. The substrates were then immersed alternately in aqueous solutions of a zirconium salt and the imide PPDI, leading to multilayered films. Films with up to 20 layers were constructed.
The growth of the films was monitored by absorption spectroscopy in the UV-visible in the case of films on ITO and specular reflectance in the case of films on silicon. Two series of films were grown on ITO substrates to evaluate the effects of heat treatment.
The films characterization was made by atomic force microscopy (AFM) in contact mode and x-ray reflectivity (XRR), made at a synchrotron source. The topography obtained by AFM images show that the annealing provided a more complete coverage of the ITO substrate. The reflectivity curves of X-rays suggest that the silicon films are more organized than ITO films, demonstrating the influence of the substrate in the organization of self-assembled films grown on it.

Rational design of solubilizing sidechains in polymer semiconductors has recently attracted tremendous interest because of its capability to simultaneously induce high electrical performance and better mechanical softness of thin films. In this circumstance, exact understanding of thermally activated charge transport in terms of sidechains is absolutely required.
In this talk, we describe the first 3-dimensional (3D) charge conduction and molecular scale investigation in polymer semiconductors via comparative analysis using the isoindigo-based polymers with a siloxane-terminated side-chain (PII2T-Si) and a branched alkyl-terminated side-chain (PII2T-Ref). Interestingly, for PII2T-Si thin-film with a bimodal molecular orientation, unlike PII2T-Ref with a unimodal fashion, the distinct transition of the relevant crystallographic parameters including the π-stacking distance and the coherence length of the lateral crystallites was observed at different temperature regimes, thereby strongly mediating 3D charge conduction into the channel. Moreover, such structural property in PII2T-Si was turned out to be more effective transport path way. We believe that our findings will provide rational design rule to guide next generation polymer semiconductors for high-performance flexible organic electronics.

A major challenge for improving photonic devices comprising multiple material components is to distinguish between loss mechanisms occurring in the bulk layers and at material interfaces. Within the active layer, undesirable photophysics and poor transport properties can result in device efficiency losses.^1,2 At material interfaces, sub-optimal electrical contact and band-edge matching will also result in device deficiencies.³ These mechanisms must be identified and understood in order to enable mitigation via bottom-up device and materials design. We are establishing novel characterization tools which correlate the structural and opto-electronic properties of the active layer with device performance. We utilize optical experiments, particularly Raman spectroscopy^4,5, to screen electronic transitions and to probe molecular ordering in the organic active layer. We combine these with impedance spectroscopy, a powerful frequency resolved technique for studying the dielectric response and electrical transport phenomena of semiconductor-electrode systems.⁶ These complementary methods allow us to probe carrier energetics and dynamics and to relate these to electrically significant device properties. In this contribution we will present our results on studying carrier transport mechanisms in devices based on low band-gap polymers. We recently demonstrated the importance of correlating molecular structure and conformation with electrical phenomena in the donor-acceptor copolymer PCPDTBT (poly[2,6-(4,4-bis-2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b&’]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)].⁷ Here we apply Raman and impedance spectroscopies to diode and solar cell structures. We further relate these results to our previously developed model in order to correlate changes in the molecular ordering, due to film processing, with electrical losses in the active layer and at material interfaces. This approach enables us to identify significant loss mechanisms in complex device architectures, which instructs future device design and optimization.
(1) Proctor, C. M.; Kuik, M.; Nguyen, T.-Q. Prog. Polym. Sci.2013, 38, 1941.
(2) Ramanan, C.; Smeigh, A. L.; Anthony, J. E.; Marks, T. J.; Wasielewski, M. R. J. Am. Chem. Soc.2012, 134, 386.
(3) Nolasco, J. C.; Ramos-Ortiz, G.; Maldonado, J. L.; Barbosa-Garcia, O.; Ecker, B.; von Hauff, E. Appl. Phys. Lett.2014, 104, 043308.
(4) Martin, E.; Provencher, F.; Berube, N.; Cote, M.; Silva, C.; Doorn, S.; Grey, J. J. Mater. Chem. C2015.
(5) Tunc, A. V.; Giordano, A. N.; Ecker, B.; Da Como, E.; Lear, B. J.; von Hauff, E. J. Phys. Chem. C2013, 117, 22613.
(6) Ecker, B.; Posdorfer, J.; von Hauff, E. Sol. Energy Mater. Sol. Cells2013, 116, 176.
(7) Di Nuzzo, D.; Fontanesi, C.; Jones, R.; Allard, S.; Dumsch, I.; Scherf, U.; von Hauff, E.; Schumacher, S.; Da Como, E. Nat. Commun.2015, 6, 6460.

High-performance FETs based on organic active materials are of particular interest due to their compatibility with low-cost, high-throughput processing and mechanical compliance with soft tissues. Due to the relatively low charge carrier mobilities of organic materials, often high operational voltages have to be applied and result in severe limitations. To overcome these issues high gate capacitance materials are needed. One appealing approach is the fabrication of ultra-thin dielectric layers, which, however, are technologically very challenging to make and suffer often from high leakage problems.
Here, we discovered that a polar fluorinated PVDF-HFP dielectric, despite of a low ion concentration, is able to induce an electric double-layer charging effect under an applied gate voltage. This polymer dielectric is solution-processable with a high static capacitance of about 0.3 mu;F/cm² , even at a thickness of several micrometers. Furthermore it is highly compatible with solution processing of various organic semiconductors. Remarkably, the resulting devices showed both high current output and low bias stress in both ambient and aqueous conditions making it the ideal candidate for low-voltage and stable device operation.

We have recently developed ternary blend polymer solar cells based on poly(3-hexylthiophene) (P3HT), a fullerene derivative (PCBM), and a near-IR dye molecule such as silicon phthalocyanine bis(trihexylsilyl oxide) (SiPc). This approach can improve the light-harvesting efficiency in the near-IR region effectively. However, the improvement is still limited up to 10%. This is because dye loading concentrations are typically as small as 5 wt%. In order to improve furthermore, we should establish the interface engineering for selective dye loading into heterojunction in polymer/fullerene solar cells. Here, we designed various silicon phthalocyanine derivatives with different axial ligands to finely tune the surface energy. For example, BuSiPc6, which has four tert-butyl groups attached to the phthalocyanine core and two tri-n-hexylsilyl oxide groups in the axial ligand, has a surface energy smaller than SiPc. On the other hand, SiPcBz, which has two tribenzylsilyl oxide groups in the axial ligand, has a surface energy larger than SiPc. The location of these dye molecules in blend films of regionrandom P3HT (RRa-P3HT) and polystyrene (PS) was studied by AFM measurements. As a result, we found that BuSiPc6 is selectively loaded in RRa-P3HT domains, SiPcBz is selectively loaded in PS domains, and SiPc6 is selectively loaded at the RRa-P3HT/PS interface. These loading locations are in good agreement with that predicted from the surface energy. This finding shows that dye loading location can be controlled by careful materials design. On the basis of this molecular design strategy, we further develop more efficient near-IR dye molecules. We will demonstrate how such molecular design can improve the photovoltaic performance of ternary blend solar cells effectively.

The crystallization mechanisms of organic semiconductor thin films are of critical interest because of the possibility of stabilizing metastable polymorphs with superior electronic properties. In this study, the crystallization process and thin film structure of 6,13-bis(trisopropylsilylethynyl)(TIPS)-pentacene are monitored by optical video microscopy and time-resolved optical reflectometry. The results indicate that there are three regions that exist when writing the film: meniscus, supersaturated region and the solid film. Real-time reflectometry results show that absorption transition of the supersaturated region has a red sift compared to the meniscus due to formation of aggregates. After the crystallization is complete, the absorption peaks from the solid film are further shifted and broadened due to intermolecular interactions in the crystal. At deposition temperatures above 120 #8451;, a blue shift of absorption peaks of the solid film relative to the room temperature phase is attributed to the high temperature polymorph. As the sample is cooled to ambient temperature the films become strained, and we find that there is a strong correlation between cracking of the film, which releases the strain, and the observed shifting of the absorption peaks shift back to the room temperature positions. When the thickness is below a critical value where no cracking occurs, the high-temperature polymorph is stabilized at room temperature. We discuss these results in terms of a model where stabilization of the high temperature polymorph at room temparture is due to strain, rather than by confinement effects. Complementary grazing incidence wide angle X-ray scattering and carrier mobility measurements of these two polymorphs will also be reported.

In organic field-effect transistors (OFETs), series parasitic resistance between source and drain largely governs the overall device performance. Therefore, various characterization methods for the parasitic resistance and the real channel resistance in the OFETs have been proposed. Transfer (or transmission) line method (TLM) is the most popular and practical technique employed by many researchers. However, some research articles recently reported channel-length-dependence in the parasitic resistance of the OFETs with Schottky contacts, which causes the misevaluation of the series parasitic resistance. Thus, even the TLM procedure has not been versatile yet. In this study, a complementary way for analyzing the channel length dependence of the series parasitic resistance and the carrier mobility was proposed with the aid of device simulation considering Schottky barrier with a thermionic field emission (TFE) model. Actually, the gate-voltage-dependent parasitic resistance and hole mobility were evaluated for coplanar-type (bottom-gate, bottom-contact) pentacene thin-film transistors with gold contacts.
Device simulation was carried out with a Thin-film Organic Transistor Advanced Simulator (TOTAS), which was originally developed by our group. The TFE model was utilized as the boundary condition for the current continuity equations at the source electrode/semiconductor interface. Moreover, the normal thermionic emission model was chosen for the drain electrode/semiconductor interface.
The simulated current-voltage characteristics fit the experimental results in the linear regime for the pentacene transistors with the channel length ranging from 2 to 80 mu;m. Further numerical analysis clarified that the hole field-effect mobility in the transistor channel reveals smaller values with shorter channel length, even after getting rid of the influence of Schottky barrier. This result indicates that intermittent disordered layers with low carrier mobility possibly exist in the vicinity of the contact electrode. From a series of the experimental data and numerical investigation, we discussed the device operation of the coplanar-type OFETs in detail, by considering properly individual processes of carrier injection, carrier flow near the contact electrode, and actual carrier transport in the channel region, respectively.

Phthalocyanine is a stable ligand that hosts a metal ion in a planar conjugated ring, and some of the metal complexes are known to show unique magnetic properties in the assembly states. Thermodynamically stable β-form of (phthalocyaninate)cobalt(II) (Co(Pc)) is known to behave as a weak antiferromagnet. Porphyrine is considered as one of phthalocyanine analogs. We synthesized (tetramethylporphyrinato)cobalt(II) (Co(tmp)) and analyzed the crystal structure and measured the magnetic properties in the crystalline state. The crystal is composed of one-dimensional columns of face-to-face stacked planar Co(tmp). This crystal structure is rather similar to that of the β-form of Co(Pc). However, Co(tmp) was found to show weak ferromagnetic interactions. At the present stage, the difference is considered to originate from the slight difference in the stacking pattern, and we will discuss the details in our presentation.
Then, we prepared the non-templated films and the templated films on perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) using Organic Molecular Beam Deposition (OMBD) and investigated the properties of the films. We will present difference in the properties between bulk and films of Co(tmp) and the effect of PTCDA on Co(tmp) films.

Zinc oxide (ZnO) is n-type semiconducting material which owns the deep-lying valence band (~7.7 eV) with a large energy band-gap of 3.4 eV and high electron mobility. ZnO nanoparticles (NPs) can be easily prepared in the solution state through a lot of methods. Therefore, ZnO NPs have been developed to cathode interlayer for solution-processable optoelectronic devices However, ZnO NPs have a significant number of surface defects and a large energy barrier for electron injection to the lowest unoccupied molecular orbital (LUMO) of typical conjugated polymers. It still requires much effort to make full use of ZnO NPs without degrading the device performance. To solve the problem, we invent conjugated polyelectrolyte (CPE) passivated ZnO NPs (CPE:ZnO hybrid NPs) in the colloidal state. By introducing alkoxy side-chain tethered polyfluorene CPE, poly[(9,9-bis((8-(3-methyl-1-imidazolium)octyl)-2,7-fluorene)-alt-(9,9-bis(2-(2-methoxyethoxy)ethyl)-fluorene)] dibromide (F8imFO₄), into ZnO NPs, CPE:ZnO hybrid NPs are formulated and well dispersed in the solution state, unlike the case for non-alkoxy CPE which forms coagulation of NPs. XPS results reveal that the surface defects of ZnO NPs are effectively passivated as a result of the coordination bonds between the alkoxy side chains and bromide mobile ions of F8imFO₄ and ZnO NPs. This leads to suppressing a photoluminescence quenching at the interface between light-emitting polymer and CPE:ZnO hybrid NPs. We successfully demonstrated high-efficiency yellow-emitting poly(p-phenylene vinylene) (PPV) based polymer light-emitting diodes (PLEDs) using the CPE:ZnO hybrid NPs as a cathode interlayer. The device with the hybrid interlayer show the highest device efficiencies of 11.7 cd A^-1 at 5.2 V and 8.6 lm W^-1 at 3.8 V compared to both of ZnO only (4.8 cd A^-1 at 7 V and 2.2 lm W^-1 at 6.6 V) and CPE only (7.3 cd A^-1 at 5.2 V and 4.9 lm W^-1 at 4.2 V) devices

Singlet exciton fission (SF), the process by which a singlet exciton may be intermolecularly down-converted to two separate triplet excitons has been shown, much like multi-exciton generation (MEG) in inorganic materials, to offer an efficiency benefit in light-harvesting devices. In the case of a quantitative SF system, the thermodynamic limit of a single junction photovoltaic is found to be 45%, far above the 32% Shockley-Queisser limit. For this reason, the field attracted much recent attention, especially in the field of organic photovoltaics. Here we present the effect that synthetic manipulation of a canonical SF material, tetracene, has on its ability to undergo rapid and efficient singlet fission. We examine the structural, optical, and photophysical properties of a series of vapor deposited thin films of cyano-substituted diaryl tetracenes (TcCN) in order to understand the effect cyano-substitution has on the intermolecular SF process in these thin film samples. It is found that cyano-substitution results in a significantly more stable chromophore, more suitable for application in devices. Furthermore, it is found that these cyano groups lower the triplet energy of the chromophore, relative to tetracene, which results in an exoergic SF process (2E(T₁)-E(S₁) = 2 x 1.04 eV - 2.25 eV = -0.17 eV). For this reason, SF is found to be significantly more rapid in TcCN than the parent chromophore, tetracene. Singlet fission proceeds by two rates in the film samples, tau; = 0.8 ± 0.2 ps and tau; = 23 ± 3 ps, which is attributed to structural disorder within the film giving rise to two populations: one with a favorable interchromophore geometry, which undergoes rapid SF, and a second population in which the initially-formed singlet exciton must diffuse to a site at which this favorable geometry exists. A triplet yield analysis using the population dynamics of the transient absorption spectra indicates the formation of 1.6 ± 0.3 triplets per initial excited state.

The field of organic polymer-based light-emitting diodes (PLEDs) is growing. LEDs made entirely of inorganic compound semiconductor materials typically have large material embodied energies (due to high temperature and high energy-use fabrication processes) and are not compatible with the natural environment after degradation and being discarded (the majority of light sources are disposed of in landfills). Thus, PLEDs are investigated as alternatives in order to create devices with low embodied energies that are more biodegradable. Traditionally, PLEDs are made in a bottom-emitting configuration, where light is emitted through a thick glass substrate. This leads to a decrease in device power efficiency, as a good portion of the light is lost to total internal reflection. This issue can be mitigated to some extent using top-emitting devices which lose less light due to the thinner transparent layers through which the light travels. In addition, two of the constituent materials of bottom-emitting PLEDs—(poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)), PEDOT:PSS, the hole transport layer (HTL) and indium tin oxide (ITO), the anode—react negatively. PEDOT:PSS is slightly acidic and corrodes the ITO layer over time, leading to short device operational lifetimes. The lifetime can be improved using an inverted device configuration, which eliminates both the PEDOT:PSS HTL and expensive indium.
This particular work focuses on the use of a native metal oxide HTL of Ag₂O in top-emitting inverted PLED devices. Native oxide HTLs can be grown conformally on metal electrodes and have fewer processing steps compared to non-native HTLs such as PEDOT:PSS, NiO_x and MoO₃. A thin layer of native Ag₂O can be formed on the surface of an Ag electrode in only one step, making it an intriguing option to compare with non-native HTLs with the ongoing aim of improving device operational lifetime. Here, plasma exposure in an O₂/Ar gas mixture is employed as a means to grow Ag₂O on top of an Ag anode. Shorter plasma exposure times and lower power plasmas give smooth Ag₂O-coated Ag surfaces while longer exposure times form grainier surfaces. Energy dispersive X-ray spectroscopy shows increased oxygen content on the surface and reduction of Ag content, indicating formation of a native oxide. X-ray photoelectron spectroscopy further confirms the existence of Ag₂O by observation of low energy O1s and Ag3d electron binding energies. Preliminary results indicate Ag₂O thickness is less than 10 nm at short plasma exposure times.
Top-emitting inverted PLED devices containing poly(9,9-di-n-dodecylfluorenyl-2,7-diyl), PFO, active layers are fabricated with and without the Ag₂O HTL and their performance and operational lifetime will be compared conventional bottom-emitting and inverted bottom-emitting devices with non-native HTLs. Performance and operational lifetime testing involves current- and luminance-voltage as well as electroluminescence-time measurements.

Active layers of heterojunction organic field-effect transistors (OFETs) can be formed of binary blends of donor polymers, such as poly(3-hexylthiophene) (P3HT) and small-molecule acceptors (e.g. perylene perylenedicarboximide derivatives, PDI). The blends may reveal phase separated bicontinuous-like morphologies, which enable ambipolar transport of electrons and holes in the OFETs. As the ambipolar charge carrier transport is required for applications in complementary-like logic circuits, the research on the P3HT/PDI blends is technologically relevant and also scientifically interesting.
In this contribution we present a systematic study on the blends of P3HT with two PDI derivatives: N,Nprime;-dioctyl-3,4,9,10-perylenedicarboximide (PDI-C8) or N,Nprime;-dioctyl-1,7-dicyano- 3,4,9,10-perylenedicarboximide (PDI-C8/CN₂). The content of either PDI derivative was varied from 25 to 75 wt. % while all obtained results were compared with the properties of the pure components. In our research we used differential scanning calorimetry, X-ray diffraction (XRD) and microscopy to gain an insight into the formation of crystalline phases in the solvent-cast films. Our results indicated that the presence of the polar cyano-groups in the perylene aromatic bays modifies phase behavior of both PDI derivatives and also crystallization of P3HT in the blends. In the P3HT:PDI-C8 systems, increasing the content of PDI-C8 causes a drop in the maximum crystallinity of P3HT from 0.6 to 0.1. In contrast, PDI-C8/CN₂ exerts almost no influence on the max. crystallinity of P3HT, which stays at the level of approx. 0.6 irrespectively of PDI-C8/CN₂ content. Based on comparison to similar systems reported in literature and on our XRD data we attributed the differences in P3HT crystallinity to adsorption at the P3HT/PDI interface and epitaxial-like crystallization of the polymer in the blends with PDI-C8. In the case of P3HT:PDI-C8/CN₂ systems the interactions between blend components were found weaker. Further, the XRD results indicated that crystal structures of both PDI-C8 and PDI-C8/CN₂ were only insignificantly altered in the blends, while growth rates and sizes of the crystalline domains of both PDI derivatives were related to the blend composition. Further analysis revealed that the observed properties could be attributed to partial miscibility of the PDI derivatives with P3HT: while PDI-C8 revealed certain degree of solubility in the amorphous P3HT, PDI-C8/CN₂ was found rather immiscible with the polymer.

Electrostatic interactions in organic self-assembly monolayers (SAM) is ubiquitous but a long-range potential and considered weak in most of weakly- or un-charged molecules on surfaces in contrast to strong short-range potentials such as van der Waals and hydrogen bond interactions, which are critical interactions for forming SAM in most of uni-molecular systems at high coverage regime. An oppositely charged bi-molecular compound in charge transfer complexes is proposed to be one of perfect candidates for formation of SAM regardless of its coverage.
Following our previous report in which we studied of the nature of uni-molecular self-assemblies of TCNQ on noble metal surfaces,¹ here we investigate the low dimensional form of bi-molecular organic charge transfer complex TTF-TCNQ (TTF = tetrathiafulvalene; TCNQ = 7,7,8,8-tetracyanoquinodimethane) on Au(111) and Ag(111) using scanning tunneling microscopy/spectroscopy at 4.3 K. Among various stoichiometric phases, TTF-TCNQ SAM with a 1 to 1 stoichiometric ratio is observed at any coverage with less dependence on evaporation conditions. Combination of atomic resolution scanning tunneling micrographs, first-principles quantum mechanical computations, and classical electrostatic energy calculations allows us to understand the role of electrostatics in forming the SAM of the charge transfer complex.^2,3 The featured edge shapes of the SAM islands observed in the STM images are elucidated by the computational electrostatics analysis. Spatial distribution of zero-bias Kondo resonance peak in STS shows the charge transfer is uniform inside the island and up around the island edges. We anticipate that the creating charge-transfer complex SAM layers based on the long-range electrostatic force is a unique protocol for not only forming coverage-independence molecular ordered layers but also implementing novel electron correlation phenomena into the organic films.
References
¹C. Park et al. Weak competing interactions control assembly of strongly bonded TCNQ ionic acceptor molecules on silver surfaces. Phys. Rev. B90, 125432 (2014).
²S. Jeon et al. Confined metal surface electronic states in ordered charge transfer molecular lattices of TTF-TCNQ on Ag(111). in preparation.
³S. Jeon et al. Electrostatic control of donor-acceptor molecular structure on noble metal surfaces. in preparation.
Acknowledgements: This research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.

Charge carrier transport properties were investigated using computational methods for modified phthalocyanine-based organic semiconductors. To achieve efficient charge separation and transport in optoelectronic materials, small internal reorganization energies are desired. While many p-type organic semiconductors have been reported with low internal reorganization energies, few n-type materials with low reorganization energy are known. Metal phthalocyanines have long received extensive research attention in the field of organic device electronics due to their highly tunable electronic properties through modification of the molecular periphery. In this study, density functional theory (DFT) calculations are performed on a series of zinc-phthalocyanines (ZnPc) with various degrees of peripheral per-fluoroalkyl (-C₃F₇) modification. Introduction of the highly electron withdrawing groups on the periphery leads to a lowering in the energy of the molecular frontier orbitals as well as an increase in the electron affinity. Additionally, all molecules studies are found to be most stable in their anionic form, demonstrating their potential as n-type materials. However, the calculated internal reorganization energy slightly increases as a function of peripheral modification. By varying the degree of modification we develop a strategy for obtaining an optimal balance between low reorganization energy and high electron affinity for the development of novel n-type optoelectronic materials.

Fullerenes are an intriguing class of organic semiconductors that feature a high electron affinity and exceptional charge transport properties, desirable for a number of opto-electronic applications. Typically, a trade-off has to be made between the higher electron mobility of pristine fullerenes and the superior solubility of substituted (exohedral) fullerenes, which however require additional synthesis steps that result in a higher energy footprint and materials cost. We therefore revisit the use of pristine fullerenes and demonstrate that the increase in entropy associated with equimolar C₆₀:C₇₀ fullerene mixtures leads to significantly enhanced solubility in a variety of organic solvents. We utilize this ‘entropic solubilization&’ to prepare state-of-the-art opto-electronic devices with pristine fullerene mixtures: field-effect transistors with an electron mobility of 1 cm² V^-1 s^-1 and polymer solar cells with a highly reproducible power-conversion efficiency of 6 % as well as a thermally stable active layer.

Fully conjugated block copolymers, consisting of an electron donor and an electron acceptor block, can serve as the active layer in high-performance organic photovoltaic devices. We have recently demonstrated block copolymer photovoltaic devices with 3% power conversion efficiency using poly(3-hexylthiophene)minus;blockminus;poly-((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2&’,2"-diyl) (P3HT-b-PFTBT) block copolymers as the active material. Block copolymer device performance outperforms devices that contain P3HT/PFTBT blends by a factor of three due to the self-assembly of P3HT-b-PFTBT into alternating donor and acceptor domains. Furthermore, incorporating the donor-acceptor interface within the chemical structure enables model studies of charge and energy transfer. To this end, we have synthesized a novel series of block copolymers by substituting carbazole and phenyl moieties in place of fluorene in PFTBT, yielding poly(3-hexylthiophene)minus;blockminus;poly-((9-(9-heptadecanyl)-9H-carbazole)-1,4-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2&’,2"-diyl) (P3HT-b-PCDTBT) and poly(3-hexylthiophene)minus;blockminus;poly-((2,5-dihexylphenylene)-1,4-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2&’,2"-diyl) (P3HT-b-PBDTBT). These small perturbations to the chemical structure modify the highest occupied molecular orbital (HOMO) level of the electron acceptor by tenths of an eV. In PCDTBT the HOMO level is lower, closer to the HOMO of P3HT and in PBDTBT the HOMO is higher. We can further modify the structures by adding solubilizing hexyl side chains on the thienyl units of the acceptor block to fully disperse chains in solution and enable solution photoluminescence studies. We determine the relative absorbance and photoluminescence fractions of P3HT and the acceptor blocks in the block copolymers by deconvoluting the contributions of the individual blocks to the copolymer spectra. The photoluminescence data from block copolymers in solution suggest significant energy transfer from P3HT to all acceptors, suggesting that charge photogeneration in P3HT-b-PFTBT block copolymer films includes energy transfer from the donor to the acceptor followed by hole back transfer from the acceptor to the donor. Furthermore, in P3HT-b-PCT6BT, we observe no evidence of intrachain charge transfer, while in P3HT-b-PFT6BT and P3HT-b-PBT6BT we observe varying degrees of intrachain charge transfer. Thus, our results provide a measure of the critical driving force needed for charge transfer to occur.

Organic semiconductors have been comprehensively studied [1] over the past decades owing to their use in the next-generation electronics. In particular, one-dimensional materials, such as nanowires, nanofibres and nanotubes have enabled fundamental studies of anisotropic optoelectronic properties and helped pave the way for low-cost, large-area and flexible devices, e.g. field-effect transistors, solar cells and various types of sensors [2]. However, the synthesis and fabrication of nanostructured organic semiconductors has to date suffered from the lack of fine control over their physical dimensions, composition and alignment, which has produced materials that do not offer any real advantage over bulk materials.
Living crystallization-driven self-assembly (CDSA) is a process introduced by Wang et al. [3], which relies on diblock copolymer self-assembly in solution, yielding cylindrical micelles (nanocylinders) of low polydispersity (PDI < 1.2) and a well-defined core-corona structure. Through a careful choice of the diblock copolymer and its molecular weight, we can tune the size, structure and properties of the core and corona independently [4].
We show that diblock copolymer nanocylinders grown by CDSA are a promising new functional hierarchical system for use in organic electronics and, specifically, demonstrate for the first time, cylindrical micelle field-effect transistors. We show that the field-effect charge carrier mobility of nanocylinders with a regioregular poly(3-hexylthiophene) (P3HT) core is dependent on the length of the core-forming diblock, the length of the assembled nanocylinders and the composition of the corona. The saturation mobility we currently measure from thin films of randomly oriented 1µm-long micelles approaches 0.01 cm^-2 V^-1s^-1, with further improvements in these values envisaged for the near future.
References:
[1] H. Sirringhaus, 25th Anniversary Article: Organic Field-Effect Transistors: The Path Beyond Amorphous Silicon, Advanced Materials, 26, 1319-1335, 2014
[2] A. L. Briseno et al., Introducing organic nanowire transistors, Materials Today, 11 (4), 38-47, 2008
[3] X. Wang et al., Cylindrical Block Copolymer Micelles and Co-Micelles of Controlled Length and Architecture, Science, 317, 644-647, 2007
[4] J. Qian et al., Uniform,High Aspect Ratio Fiber-like Micelles and Block Co-micelles with a Crystalline π-Conjugated Polythiophene Core by Self-Seeding, J. Am. Chem. Soc., 136 (11), 4121-4124, 2014

Organic photovoltaic devices (OPVC) that can be fabricated via simple solution processing techniques are under intense investigation in academic and industrial laboratories because of their potential for mass production of flexible and cost-effective devices. Recent drastic improvements in the power conversion efficiency (PCE) of polymer solar cells are mainly attributed to the synthesis of ideally designed new photovoltaic materials. To achieve high PCE of OPVC, rational design of photo-active materials is required by considering high crystallinity. A series of photovoltaic polymers based on phenylene, thiopehene and difluorobenzothiadiazole moieties were designed and synthesized by modulating the intrachain noncovalent coulomb interactions, and their effects on the optical, morphological, electrical and photovoltaic characteristics were studied. Among them PPDT2FBT polymer formed well-distributed interpenetrating nano-fbrillar networked morphologies with PC₇₀BM, showing well-balanced hole and electron mobilities. Notably, PSCs based on these polymers exhibited PCEs of up to 9.39% in a 290 nm thick conventional single-cell device. The molecular weight effect of PPDT2FBT was also investigated systematically. Highly efficient all-PSCs showing greater than 5% PCE was demonstrated by blending PPDT2FBT_H as a donor with P(NDI2OD-T2) as an acceptor. Bulk heterojunction (BHJ) morphology was investigated by grazing incident X-ray scattering, resonant soft X-ray scattering, and other microscopy measurements.

For solution processes to fabricate crystalline thin films of organic semiconductors materials for printed electronics, a lot of dialkylated materials are proposed. We have reported the quality transistor material which is mono alkylated phenyl-BTBT derivative (Ph-BTBT-10) exhibiting high FET mobility over 10 cm²/Vs with the polycrystalline thin film fabricated by normal spin coting technique[1]. In the polycrystalline thin films of Ph-BTBT-10, thermal anneal at 120 ^oC for 15mins induced crystal structure change from mono-layer to bilayer structure and mobility increased from 2.1 to 14.7 cm²/Vs. We have also investigated mono-alkylated other Ph-BTBT derivatives having various alkyl chain length from C₆ to C₁₄, all Ph-BTBT derivatives show bilayer structure by thermal annealing. Especially, bilayer polycrystalline thin films in Ph-BTBT-14 show high FET mobility 18.9cm²/Vs by thermal annealing. We evaluated other mono alkyl materials also and they show bilayer structure and increasing of mobility by thermal annealing.
Thus, we conclude the idea of use of bilayer structure of mono alkyl organic transistor materials are good for realization of high mobility organic transistor materials.
[1] H. Iino, T. Usui, and J. Hanna, Nat. Commun., 6, 6828 (2015).

Synthesis and characterization of conjugated polymers and oligomers have become a major area of research owing to many potential applications. Both linear and branched oligo- and polythiophenes have been synthesized, fully characterized and used in applications such as field effect transistors and photovoltaics. Oligothiophenes are interesting owing to their easy synthesis, high electron density and potential applications in field effect transistors and solar cells. Here we report the synthesis and characterization of a series of soluble multidimensional oligothiophenes and investigation of optical and complexation properties. Optical and electrochemical studies suggest that the molecules adapt a nonplanar conformation. Electron deficient TCNQ and Hg(II) cations interact strongly with electron rich oligothiophenes to form charge transfer complexes, which led to significant quenching of photoluminescence. Such 3D conjugated materials could be used for the fabrication of photovoltaic devices or field effect transistors.
Choong PS, Valiyaveettil S, Synthesis and Characterization of Multi-arm Oligothiophenes, Manuscript in preparation.
Acknowledgement: The authors thank the National University of Singapore (NUS, Tier 1 Grant) for funding support and CPS thank NUS for a research scholarship.

Currently, many devices powered by solar cells require high voltage to operate and many polymer solar cells (PSCs) connected in series are required to power them. High V_oc solar cells are necessary to reduce the number of cells required to achieve the voltage requirements for these operations. We will describe a simple method to develop a high V_oc low band gap PSCs. In addition, two new AFM-based nanoscale characterization techniques are introduced to study the surface morphology and physical properties of the structured active layer. With the help of ternary solvent processing of the active layer and C₆₀ buffer layer, a bulk heterojuntion PSC with V_oc more than 0.9V and conversion efficiency 7.5% has been developed. To understand the role of ternary solvents on the morphology of the active layer, Pulsed-Force-Mode AFM (PFM-AFM) and Mode-Synthesizing AFM (MSAFM) are used for advanced analysis. Interestingly, MSAFM provides high sensitivity for direct visualization of the donor-acceptor

The power conversion efficiency (PCE) of organic solar cells has steadily improved in the last 3 decades with the binary bulk heterojunction (BHJ) emerging as the most popular structure. To continue this steady trend in efficiency, new approaches to improve the PCE are required. Recently, much success was achieved with ternary organic semiconductor systems. These systems generally consist of a polymer as the host electron donor, fullerene as the host electron acceptor and an infra-red sensitizer. The absorption spectra of these three types of molecules complement each other thus absorbing more light and potentially achieving higher PCE than the equivalent binary system. Controlling and understanding ternary systems on the nanometer-scale is challenging. Overlap between the fluorescence spectrum of one of the materials and the absorption spectrum of another is almost unavoidable in spectrally complimentary ternary systems, meaning that energy transfer is likely to occur. We experimentally determined the energy transfer properties of poly(3-hexylthiophene):[6,6]-phenyl- C61-butyric acid methylester: 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (P3HT:PCBM:DIBSq) ternary systems and utilised kinetic Monte Carlo models to investigate the impact of energy transfer on photovoltaic device performance in this popular ternary blend. It was found that the exciton dissociation efficiency was much improved due to long range energy transfer resulting in a larger optimum feature size compared with the P3HT:PCBM binary BHJ systems. Even though P3HT:PCBM:DIBSq forms an energetic cascade structure, exciton dissociation does not occur efficiently at the P3HT:DIBSq interface. However, this deficiency has no bearing on the device performance as energy transfer to the PCBM:DIBSq interface, where exciton dissociation is very efficient, occurs efficiently. Hence, energy transfer relaxes the energetic and morphological requirements of ternary blend solar cells allowing more organic semiconductor to be successfully matched in ternary organic solar cell architectures.

In spite of tremendous progress in molecular design, engineering and processing, only few small molecule organic semiconductors (OSCs) have reached field-effect mobilities higher than 10 cm² V^-1 s^-1, typically with single-crystal devices.
In a previous work, our group demonstrated the ability to modify charge transport properties of oligothiophenes via fine tuning of their crystal packing structures by the addition of bulky pendant groups. This gave rise to a rationally designed OSC, trimethyl-[2,2&’;5&’,2&’&’;5&’&’,2&’&’&’]quaterthiophen-5-yl-silane (4TTMS), which has an in-plane isotropic mobility of 0.1 cm² V^-1 s^-1 recorded with physical vapor transport (PVT) grown single-crystal field-effect transistors.
In this study, we have designed and synthesized a series of bulky end-capped BTBTs with the aim of tuning their crystalline packing. A combined theoretical and experimental study allowed us to identify 2,7-di-tert-butylBTBT as a new high-performance solution processable OSC with large and well-balanced transfer integrals as evidenced by quantum-chemical calculations. The relevant substitution by two tBu improves the orbital overlap of the molecules within their crystalline architecture. PVT grown single-crystal field-effect transistors show a remarkable average saturation mobility of 7.1 cm² V^minus;1 s^minus;1 (maximum mobility of 17 cm² V^minus;1 s^minus;1).

Conventional inorganic and hybrid (e.g., organohalide lead perovskite) semiconductors have low exciton binding energies due to their high static dielectric constants (ε_r > 10) allowing them to work in a predominantly ‘non-excitonic&’ regime at room temperature. However, organic semiconductors have low static dielectric constants (ε_r < 4) leading to exciton binding energies much greater than kT. As a result, homojunction organic diodes have very poor efficiency at room temperature, since neither thermal activation nor an external electric field is sufficient to overcome the exciton binding energy. Therefore, the bulk heterojunction active layer was designed where the energy offset between the electron affinities and/or ionization potentials of the electron donor and acceptor components can enhance the exciton dissociation yield.
Theoretical studies have proposed that increasing the dielectric constant of organic semiconductors could lead to enhanced performance by reducing the Onsager radius of the electron-hole pairs leading to weakly bound CT states and a reduction in both geminate and non-geminate recombination.[1] Some efforts have been made experimentally to synthesize ‘high&’ dielectric organic semiconductors. Recently, the introduction of glycol solubilizing groups has been shown to increase the static and low frequency dielectric constant of both polymeric and fullerene systems due to their ability to polarize in low frequency fields.[2]
This presentation will discuss the synthesis and characterization of a number of novel high static dielectric constant organic semiconductors incorporating glycol side-chains. Polymeric, non-polymeric (both crystalline and amorphous) and fullerene-based materials will be presented, some with static dielectric constants approaching 10. We will discuss how the introduction of glycol side-chains affects the morphology, charge transport and optoelectronic properties of the materials. Finally, the performance of these materials in OPV devices will be discussed. This presentation aims to provide insight into the significance of the dielectric constant in OPV devices, a topic that has often been overlooked in the field of organic optoelectronics.
[1] L. J. A. Koster, S. E. Shaheen and J. C. Hummelen, Adv. Energy Mater., 2012, 2, 1246-1253.
[2] S. Torabi, F. Jahani, I. Van Severen, C. Kanimozhi, S. Patil, R. W. A. Havenith, R. C. Chiechi, L. Lutsen, D. J. M. Vanderzande, T. J. Cleij, J. C. Hummelen and L. J. A. Koster, Adv. Funct. Mater., 2015, DOI: 10.1002/adfm.201402244, 150-157.

This lecture focuses on the challenging design, realization, characterization, understanding, and implementation of new materials for creating unconventional electronic circuitry. Fabrication methodologies to achieve these goals include high-throughput, large-area, high-resolution printing techniques. Materials design topics to be discussed include: 1. Rationally designed high-mobility p- and n-type organic semiconductors for printed organic CMOS, 2. Self-assembled and printable high-k nanodielectrics enabling ultra-large capacitance, low leakage, high breakdown fields, minimal trapped interfacial charge, and device radiation hardness, 3. Polycrystalline and amorphous oxide semiconductors for transparent and mechanically flexible electronics, 4. Combining these materials sets to fabricate a variety of high-performance thin-film transistor-based circuitries, 5. The relevance of these advances to unconventional photovoltaic materials. This presentation emphasizes the symbiosis between materials synthesis, computational modeling and simulation, materials characterization, and device fabrication and evaluation.

We examine the main factors that define charge transport in organic semiconductors. We consider both crystals based on a single molecule building block, such as oligoacenes, and bimolecular charge-transfer crystals in which one component acts as an electron donor and the other as an acceptor. We first discuss the state-of-the-art methodologies used in the derivation of the microscopic parameters (electron-vibration couplings, transfer integrals, band gaps, bandwidths, and effective masses) describing charge transport. In particular, we evaluate the impact that the amount of nonlocal Hartree-Fock exchange included in a hybrid density functional has on these parameters. In the case of disordered systems, we use a combination of electronic-structure calculations and molecular mechanics/molecular dynamics simulations complemented by ensemble and time average approaches to separate the static and dynamic disorder components. Finally, we examine the impact that the interplay between electronic interactions and electron-phonon interactions has on the charge-carrier mobility.

Conjugated polymers are widely applied as absorber materials in organic photovoltaics (OPV), where these materials offer attractive low-cost processing possibilities like printing, doctor blading or spray deposition. For solution-based OPV cells, the bulk-heterojunction (BHJ) concept is commonly used; i.e., the active layer is formed by an intimate mixture of the light-absorbing donor polymer and a fullerene derivative which acts as electron acceptor. An optimized BHJ layer requires specific phase segregation of the donor and acceptor components to allow optimum charge carrier photogeneration and charge carrier extraction towards the respective electrodes.
The long-term stability of OPV devices has been recognized as an important area of research, both in academia and industry. In fact, most BHJ systems show poor stability and often undergo macrophase segregation of the blend components, especially after prolonged exposure to heat (e.g., by sun exposure). Improvement of the morphological stability has been attempted by crosslinking of the polymer network using homopolymers and copolymers with crosslinkable units.
Here, we explore the concept of BJH layer crosslinking by bisazide derivatives. Such small-molecule crosslinkers are favorable because they can be added to almost any polymer system. Crosslinking is conducted by thermal treatment or exposure to UV light. It was shown previously that the addition of an aromatic bisazide derivative stabilizes the BHJ morphology without affecting the electronic properties of the blend [1]. PEG bisazides with different chain lengths (n = 13, 23, and 45) were synthesized by SPECIFIC POLYMERS and added to polymer-fullerene BHJ layers. For P3HT:PCBM:bisazide blend layers, it turned out that thermal treatment (at moderate 150°C) led to selective crosslinking of the PCBM phase. In a microscopy study, these layers showed excellent stability of the BJH morphology even after heating for several hours. Moreover, the crosslinked P3HT:PCBM layers showed a slightly higher OPV cell efficiency as compared to devices without crosslinker.
Cross-linking of high-performance PCE-10:PC71BM blend layers has been explored as well (PCE-10 is also known as PBDTTT-EFT or PTB7-Th). Here, thermal crosslinking at 150°C for 15 min led to a dramatic decrease of the solar cell efficiency (from 6.0 to 0.4 %). Alternative crosslinking by a 20-min exposure to UV light resulted in a moderate efficiency drop to ca. 3.5%. Optimization of the crosslinking procedure (either thermally or by UV exposure) is in progress. As a part of that, we also present a new class of bisazide-based crosslinkers with solvent additive properties for morphology adaption similar to using diiodooctane (DIO).
Literature
[1] L.Derue , O. Dautel , A. Tournebize , M. Drees , H. Pan , S. Berthumeyrie , B. Pavageau , E. Cloutet , S. Chambon , L.Hirsch , A. Rivaton , P. Hudhomme , A. Facchetti , and G. Wantz *, Adv. Mat. 2014,26, 5831 -5838

The ability to improve the performance of organic electronic devices, such as solar cells, is critically linked to the understanding and control of morphology, crystallinity of the active layer material within it. There are ranges of methods e.g., thermal or solvent annealing, addition of solvent additives commonly applied as processing tools to establish an optimum structure-property-processing nexus. Polyalkylthiophenes are an important class of model semiconducting polymer systems studied to explore the underlying structure-property relationships. Exciton diffusion is an important process to understand the full potential of solar cells, since in a bulk heterojunction, the exciton formed in the donor of the blend must diffuse to an interface with the acceptor finally leading to charge separation. Here we report a study of exciton diffusion using time-resolved fluorescence measurements by exciton-exciton annihilation. The annihilation process was studied by measuring time resolved fluorescence at different excitation intensities using a femto laser for excitation and a streak camera for detection.
Our work probes a random copolymer of polyalkylthiophenes, polyhexylthiophenes and polydodecylthiophenes, P3HT-co-P3DDT (hexyl: dodecyl = ca. 50:50) and introduces an extra effect of varying side chain length within it contributing to molecular packing, in comparison to homopolymers, e.g., P3HT or P3DDT. In comparison to P3HT, it has relatively lower melting temperature, well separated from degradation temperature, making it an interesting candidate to control the nucleation and growth of crystallites within it using thermal annealing and melt re-crystallization. Thermal annealing and melt re-crystallization of thin films showed up to 2-3 folds enhanced exciton diffusion coefficients in comparison to unprocessed neat polymers. Similar enhancement was also observed when those thermal protocols were followed with the addition of small quantities of either solvent additive (DIO, diiodooctane), or a classical polymer nucleant (DMDBS, Bis (3, 4-dimethylobenzylideno) sorbitol). Phase imaging by atomic force microscopy showed pronounced ordered/aligned lamellar periods for melt re-crystallized and thermally annealed samples. Absorption spectroscopy on those indicates an enhanced vibronic peak at ca. 600 nm wavelengths, mostly indicative of higher ordering from aggregation. Grazing incidence X-ray scattering (GIXS) studies were followed under synchrotron source, showing enhanced relative degree of crystallinity on thermally processed films. Separately, role of carbon di-sulphide (CS₂) solvent vapor annealing was also observed indicating enhanced ordering and exciton diffusion coefficient. Finally, our work convincingly supports enhancement of ordering and crystallinity in a polyalkylthiophene copolymer by different annealing conditions, leading to the enhancement of exciton diffusion coefficients.

Photoinduced charge transfer processes are key to the operation of optoelectronic devices, especially of organic photovoltaics (OPVs). Recently, evidence has come to light that electronic and vibrational coherence may play a role in efficient charge separation in donor acceptor systems, which are used in OPVs [1,2].
Here, we use an ultrafast three-pulse experiment to probe vibrational coherence during the photoinduced charge transfer process in a series of model polymer-fullerene systems. We directly measure the vibrational signatures of the excited states of the system, the singlet exciton and the hole polaron, and find that there is a transfer of vibrational coherence, for certain but not all vibrational modes, between the initially generated singlet exciton and the hole polaron. Furthermore, we find that the vibrational dynamics of the hole polaron formed via charge transfer significantly differ depending on the nature of the fullerene to which the electron is transferred. This allows us to measure the delocalization of the hole polaron wave function.
Our results elucidate the importance of understanding the ultrafast vibrational dynamics related to charge transfer process and how these vibrational modes couple the electronic states allowing for efficient charge generation in organic semiconductors.
References
[1] Y. Song et al., ‘Vibrational coherence probes the mechanism of ultrafast electron transfer in polymer-fullerene blends&’. Nature Commun. 5:4933, doi: 10.1038/ncomms5933, (2014).
[2] S. Gélinas et al., ‘Ultrafast Long-Range Charge Separation in Organic Semiconductor Photovoltaic Diodes&’. Science 343, 512-516, (2014).

Singlet fission is a multiple exciton generation process, in which one singlet exciton splits into two triplet excitons, and hence could potentially increase the maximum power conversion efficiency in organic photovoltaics. Here we study the dynamics of singlet excitons in a low-bandgap polymer film, poly[4,6-(dodecyl-thieno[3,4-b]thiophene-2-carboxylate)-alt-2,6-(4,8-dioctoxylbenzo[1,2-b:4,5-b]dithiophene)] (PTB1), by transient absorption spectroscopy. Upon the photoexcitation at an absorption edge, singlet excitons were observed immediately and decayed monotonically on a timescale of hundred picoseconds. Instead, a different transient species was observed 1 ns after the excitation. This long-lived species was assigned to triplet excitons generated through the intersystem crossing from singlet excitons. Upon the photoexcitation at a shorter wavelength, on the other hand, both singlet and triplet excitons were observed immediately after the excitation (<0.1 ps). We therefore ascribe such ultrafast triplet formation to the singlet fission into two triplets. We discuss the singlet fission mechanism on the basis of excitation wavelength and intensity dependence.

The molecular packing of conjugated polymers can substantially impact its optoelectronic properties. Poly(3-hexylthiphene) (P3HT) has been reported to be capable of self-assembling into nanowhiskers and nanofibers, which provide superior optoelectronic properties compared to its amorphous film. Here, the local morphology and electronic property studies beyond the simplest form of the P3HT nanowhiskers, monolayer nanowhisker, are carried out by atomic force microscopy (AFM) and its advanced modes. The P3HT nanowhiskers form layered nanowhisker structures with distinctive heights, indicating that the nanowhiskers aggregate in solution in a layer-by-layer fashion. The number of layers also increases over time, hinting that layered structures are more thermodynamically favored in solution. The polymer chain packing integrity of the monolayer nanowhiskers is probably compromised by structural fluctuations induced by its local environments, evidenced by our AFM measurements. The local contact potential difference values of overlapped, deformed, and triple-stacked nanowhiskers are substantially higher than that of straight section of nanowhiskers. The findings here provide insights on the effects of molecular packing and local adsorption environments on electronic properties of conjugated polymer aggregates that are difficult to resolve in bulk thin films.

We are combining near-field and wide-field optical spectroscopies with electrostatic force imaging to probe correlated optical and electronic (surface potential) properties in isolated crystalline nanowires of different organic semiconductors. We are particularly interested in the question of how different molecular packing motifs influence intra- and inter-chain exciton coupling in polymeric semiconductors (as signaled by particular spectroscopic signatures) and surface potential (as signaled by Kelvin Probe Force Imaging, KPFM). Recent theoretical work by Spano and coworkers have suggested interesting mixed H/J-type exciton coupling resulting from a combination of point-dipole coupling and charge-transfer interactions, the latter being strongly influenced by molecular chromophore alignment. In addition, this charge-transfer interaction also modulates the energy location of the Highest Occupied Molecular Orbital (HOMO) which can be measured directly via KPFM. Thus the combined optical and electronic spectroscopies on crystalline organic nanostructures make an interesting platform for quantitative tests of exciton coupling in these systems.

We would like to quantitatively understanding the structure-function relationship of organic photovoltaic (OPV) materials on the nanometer scale at a level allowing direct comparison with ab-initio modeling. Kelvin Probe Force Microscopy (KPFM) is a technique often used to study light-matter interactions on the nanoscale. To overcome the relatively slow timescale of the KPFM measurement, light intensity modulated KPFM was introduced in early 2008 to measure surface potential decay times in the range of µs [1]. Only recently however, this technique has been used in combination with background light illumination to measure the time dependency of surface photovoltages on samples such as polymer bulk heterojunction devices [2] and silicon nanocrystal solar cells [3]. The spatial and temporal resolution of KPFM needs to be improved in order to measure charge generation at a heterojunction on the molecular scale. We use alkali halide substrates cleaved in ultra high vacuum (UHV) to deposit materials used for molecular heterojunctions with sub monolayer coverage.
KPFM is a relative measurement between AFM tip and a sample. Light illumination often influences various parameters, such as the temperature of the cantilever and the tip-sample distance, which can adversely affect the KPFM signal. To reliably measure excited states of molecules, we therefore propose a differential KPFM measurement using an optical chopper, allowing us to directly examine the difference between dark and illuminated states, eliminating undesirable effects of the light illumination on the tip sample junction.
To characterize and understand this model system in detail, in particular the role of defects, we are complementing our UHV AFM/KPFM measurements with optical spectroscopy techniques on the same samples (under UHV conditions). In particular, we are performing THz spectroscopy measurements (in collaboration with the Cooke group at McGill University) to determine spatially averaged charge mobility at the nm scale and fs absorption spectroscopy (in collaboration with the Silva group at U. Montreal) to determine the energetics and time scales of charge separation.
References
[1] Takihara, M., Takahashi, T., & Ujihara, T. (2008). Minority carrier lifetime in polycrystalline silicon solar cells studied by photoassisted Kelvin probe force microscopy. Applied Physics Letters, 93(2), 021902. doi:10.1063/1.2957468
[2] Shao, G., Glaz, M. S., Ma, F., Ju, H., & Ginger, D. S. (2014). Intensity-Modulated Scanning Kelvin Probe Microscopy for Probing Recombination in Organic Photovoltaics. ACS Nano, 8(10), 10799-10807. doi:10.1021/nn5045867
[3] Borowik, L., Lepage, H., Chevalier, N., Mariolle, D., & Renault, O. (2014). Measuring the lifetime of silicon nanocrystal solar cell photo-carriers by using Kelvin probe force microscopy and x-ray photoelectron spectroscopy. Nanotechnology, 25(26), 265703. doi:10.1088/0957-4484/25/26/265703

The model conjugated polymer poly-(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) is known to form two distinct morphological phases in solution, known as a red phase, characterized by strong intramolecular excitonic coupling with spectral lineshapes akin to J aggregates, and a more disordered blue phase. The disorder-to-order transformation occurs by what appears to be a second-order phase transition (see Anna Ko#776;hler et al. ‘&’ An Orderminus;Disorder Transition in the Conjugated Polymer MEH-PPV &’&’, J.AM. Chem. Soc. 2012, 134, 11594-11601). Previous studies demonstrate that this phase transition in MEH-PPV dissolved in methyltetrahydrofuran (MeTHF) is dependent on the solvent polarity, solution concentration, and temperature. Here, we report formation of the red phase in a gel of MEH-PPV blended with ultra-high-molecular-weight (30,000 kDa) polystyrene dissolved in MeTHF (MEH-PPV:UHMWPS). The temperature-dependent absorption and photoluminescence spectra of MEH-PPV:UHMWPS (from 5.0 wt %, 5x10^-5 M solution) are similar to those measured in MEH-PPV solution at similar concentration. As in solution, the blue-to-red phase transition is observed at temperatures between 170 and 130 K during cooling of the gel, and the spectrum is almost entirely dominated by the lower energy, “red”, progression at 150 K. The transition is reversible and it also takes place during heating the gel. By means of two-dimensional photoluminescence spectroscopy (2DPL), we investigate the nature of intra- and interchain excitonic coupling in the blue and the red phases in the MEH-PPV:UHMWPS gel. 2DPL spectroscopy probes spectral correlations of photoexcitations via cross peaks in the 2D map of the fourth order photoluminescence excitation nonlinear coherent signal. This technique allows us, thus, to unravel with intricate detail the nature of band shifts and homogeneous broadening in the temporal evolution of the aggregated-phase signal of MEH-PPV, as well as to observe exciton dynamics.

Space-charge-limited current-voltage measurements (SCLC) on unipolar devices are commonly used to determine charge carrier mobilities in organic (Shivanna et al., 2014), inorganic and organic/inorganic hybrid (Shi et al., 2015) semiconducting thin films. SCLC measurements are appealing since they are simple to perform and can be applied to semiconductor films of similar thickness and properties to the active layers of optoelectronic devices, but are prone to misinterpretation. It is common to extract a charge carrier mobility by fitting an analytic form to the obtained current density-voltage (J-V) data for single-carrier devices, where the simplest and most commonly used equation is the Mott-Gurney square law (Mott & Gurney, 1940). This simple model does not allow for factors such as traps, doping and effects caused by injection barriers; in particular it is based on the assumption of a perfectly injecting contact. In practice, a built-in potential (V_bi) is likely to arise from using contacts of different work function. The effect is commonly corrected for by shifting the voltage axis using V_bi as a fitting parameter (Nicolai et al., 2012). However, the effect of V_bi on the J-V curves is hard to distinguish from the effect of trap states (Kirchartz, 2013) meaning that the V_bi and mobility extracted by this fitting can be unreliable. Here, we show using a combination of measurements on model systems and simulation how V_bi can be determined directly from the J-V curves even in the presence of traps states. We apply the method to determine mobility, V_bi, and trap distribution for single-carrier devices made from both small molecules and polymers.

To develop a model of polymer transport we start with detailed atomistic models of several polymers in bulk, study their electronic structure and the magnitude of electron-phonon coupling. We then use the detailed models to generate a mode general polymer model that can be used to relate the charge mobility with microscopic parameter. We discover that a number of microscopic parameters have no role in determining the mobility v.s. temperature curve which seems to depend on just two effective material parameters, only one of which substantially different from material to material. We use the model to look retrospectively at the best available polymers to explain their exceptional performances. The results are used to propose new chemical strategies for synthesizing polymer with increased performance.
Refs:
Fornari RP, Aragoacute; J, Troisi A, A very general rate expression for charge hopping in semiconducting polymers, J. Chem. Phys. 142, 184105, 2015
Fornari R, Troisi A, Narrower Bands with Better Charge Transport: the Counterintuitive Behaviour of Semiconducting Co-Polymers, Adv. Mater. 26, 7627-7631, 2014
Fornari RP, Troisi A, Theory of charge hopping along a disordered polymer chain, Phys. Chem. Chem. Phys. 16, 9997-10007, 2014

Intermolecular charge-transfer (CT) states formed at the interface between organic electron donor and acceptor materials are exploited in so called ‘exciplex&’-based OLEDs [1]. In contrast to conventional emitter based OLEDs, they allow simple color tunability, circumvent rare earth elements, and still exhibit a higher quantum yield than fluorescent emitters due to the low singlet-triplet-splitting. For organic solar cells, CT states have been considered as free carrier recombination centers, limiting their open-circuit voltage [2].
Furthermore, CT absorption at photon energies below the optical gap of both donor and acceptor material has been utilized to enable near-infrared photo-detection between 650nm and 950nm [3]. However, due to the low absorption coefficient in the spectral region of CT absorption, such devices comprise photo-active layers exceeding 10 micrometers in thickness. This requires high external voltages on the order of 100 V to extract the photo-generated carriers and implies very high dark currents limiting on-off ratios to below ten. Moreover, such thick devices are expected to exhibit slow response times.
In this work, we present a new photodetector device architecture of sub-micron thickness, based on organic molecular semi-crystalline materials, able to harvest long wavelength photons efficiently by intermolecular CT absorption. We demonstrate external quantum efficiencies (EQE) measured at short-circuit exceeding 20%, on-off ratios in the order of 10⁴ due to low dark currents and specific detectivities in the order of 10¹² Jones. The peak detection wavelength is 950nm, using a blend of C60 and an electron donor with an optical gap of 850nm. This new type of photo-detector competes in the near-infrared wavelength range with standard organic photo-detectors based on above gap absorption in terms of EQE and has the potential to extend their detection wavelength. The device is visible blind and can potentially be transparent with all advantages of organic opto-electronic devices, such as scalability, flexibility, light-weight and low-cost.
[1] K. Goushi, K. Yoshida, K. Sato and C. Adachi, Nat. Phot. 6(4), 253-258 (2012), DOI: 10.1038/nphoton.2012.31
[2] K. Vandewal, K. Tvingstedt, A. Gadisa, O. Inganäs, and J.V. Manca, Phys. Rev. B 81, 125204 (2010), DOI: 10.1103/PhysRevB.81.125204
[3] C. Yang, P. Tsai, S. Horng, K. Lee, S. Tzeng, H. Meng, J. Shy, and C. Shu, Appl. Phys. Lett. 92, 083504 (2008), DOI: 10.1063/1.2839397

Energy harvesting is the process extracting power from ambient sources (e.g., light, thermal gradients, and motion). Recently, this technology has attracted a great deal of interest as a potential inexhaustible source for low-power electronic devices (e.g., wireless sensor networks and wearable electronics). One-dimensional (1-D) nanostructures such as nanofibers are a promising material for energy-harvesting applications. To improve the performance of bulk heterojunction (BHJ) organic photovoltaic (OPV) devices, a 3-D nanofiber network composed of a semiconductor is a promising option for nanoscaled morphology control and molecular ordering in the active layer. In the present study, poly(3-hexylthiophene) (P3HT) nanofibers were prepared by self-assembly (the average width is about 20 nm; the average length > 500 nm). The OPV device containing the self-assembled P3HT nanofiber network and [6, 6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) were fabricated by solution process and showed a 20% better cell performance (the average power conversion efficiency was 3.1 % under the illumination of AM 1.5G, 100 mW/cm²) compared to the cell without the nanofiber network (conventional P3HT:PC₆₁BM BHJ OPV device); the open circuit voltage (V_oc) slightly decreased, but the short-circuit current (J_sc) and fill factor (FF) were increased. Our space-charge limited current measurements also demonstrated that the introduction of a nanofiber network into the active layer improved the hole transport mobility from 2.1 × 10^-6 cm² V^-1 s^-1 (for conventional P3HT:PC₆₁BM BHJ OPV device) to 8.6 × 10^-6 cm² V^-1 s^-1. These results indicated that the polymer semiconductor nanofiber network in the active layer functioned as a continuous carrier path and allowed the efficient extraction of charges, resulting in a better FF and increasing J_sc. Under a low-luminance environment, the thicker active layer is required to achieve efficient light absorption. For this purpose, the morphology control based on polymer semiconductor nanofiber networks is more effective for the optimization of the device performance. In addition, crystalline semiconductor nanofiber improved stability of OPV devices.

The structure and electronic properties of a series of donor-acceptor organic molecules are explored using ab initio calculations to understand the behavior of polaron pairs at the interface of the donor-acceptor junction in organic photovoltaics. Our results suggest that we may be able to control polaron pairs based on the asymmetry of the donor molecule in use. This leads to a better control of thermodynamic losses and open circuit voltage. Our calculations reveal the probability distribution of the formation of different types of polaron pairs, especially, depending on deposition order (i.e. donor on top of acceptor vs. acceptor on top of donor). The energetics of crystalline substrates with different surface terminations are mapped out using a single molecule of the partnering species. Accordingly, the interfacial structure and properties are different depending on whether the substrate is a donor or acceptor due to the incongruency between lattices and the disorder that develops in the contact layers of donor and acceptor, respectively.

The ability to adjust blend morphology is important for the development of organic photovoltaic solar cells. This can be done by controlling the structural order and crystallinity within the film. The degree of crystallinity of the polymer is also associated with many other properties of the film including the charge separation efficiency¹, charge transport^2-3 and degree of phase segregation between the components^4-6. As some of these effects are positive and some have a detrimental effect on the device performance it is often found that materials with intermediate crystallinities give the best power conversion efficiencies⁵. An elegant way of exploring the influence of degree of crystallinity on the physical properties is to use a blend of copolymers to vary the degree of order in the film.^4,5
Here, we investigated the important problem of how crystallinity affects exciton diffusion and fullerene distribution in organic solar cell materials. We controlled the degree of crystallinity of the film by blending amorphous and semi-crystalline copolymers. For this we studied two anthracene containing poly(p-phenylene-ethynylene)-alt-poly-(p-phenylene-vinylene) (PPE-PPV) copolymers (AnE-PV) that are used as donor materials for solar cell applications^5,7. The two polymers have the same conjugated backbone but differ in the number and position of branched side-chains, which leads them to be either semi-crystalline or amorphous⁵. The ability to modify the overall crystallinity of the film, by changing the blend ratio of amorphous to semi-crystalline polymer, makes blends of these polymers an ideal model system to investigate some important unanswered questions in OPV research. We studied exciton diffusion and fluorescence quenching by dispersing fullerene molecules in these blends of amorphous and semi-crystalline copolymers, and show that fullerene preferentially mixes into disordered regions of the polymer film. We also found that the diffusion length is more than two times higher in a semi-crystalline copolymer compared to an amorphous copolymer. Hence we show that polymer ordering in the film is very important for exciton diffusion and that a relatively small difference in molecular structure makes a large difference to the fullerene distribution in the blends.
1. Ohkita, H. et al., J. Am. Chem. Soc. 2008,130 (10), 3030-3042.
2. Jiang, X. M. et al., Advanced Functional Materials 2002,12 (9), 587-597.
3. Herrmann, D. et al., J. Am. Chem. Soc. 2011,133 (45), 18220-18233.
4. Campoy-Quiles, M.et. al., Organic Electronics 2009,10 (6), 1120-1132.
5. Kästner, C. et al., Journal of Materials Chemistry A 2013,1 (12), 3961-3969.
6. Jamieson, F. C. et. Al., Chemical Science 2012,3 (2), 485-492.
7. Egbe, D. A. M. et. Al., Macromolecules 2010,43 (3), 1261-1269.

Single crystals of organic semiconductors offer extraordinary optical properties arising from long range order of chromophores in the solid state. Superradiance, lasing and nonlinear optical processes are just few examples of the interesting collective and coherent phenomena that can be observed. However, the exploitation of organic single crystals in photonics has remained limited thus far, primarily because of the difficulties in integrating such material form with the most common photonic platforms. For example, it is challenging to prepare organic single crystals in the form of thin films for optical waveguides or planar photonic crystals and only a limited set of organic materials exhibits wire shaped single crystals for integration with optical fibers.
In this communication we present our most recent results in preparing hybrid single crystal photonic fibers and study their optical properties. Our structures are based on a hollow core glass optical fiber obtained by a standard fiber drawing technique from a glass preform. The hollow optical fibers can be prepared with an empty inner core with radius from 500 nm to 7 µm and a glass cladding having an outer diameter of 100 µm. A fiber with desired dimensions is then placed in contact with a melt of the organic semiconductor in a zone furnace kept above the melting temperature of the organic. The fiber core is filled by the organic semiconductor melt by capillary forces and subsequently cooled slowly below the crystal solidification temperature. We present polarized optical microscopy images demonstrating the homogeneous growth of different materials (anthracene, perylene, dibenzo-anthracene) as single crystals inside the hollow core fibers. Further experiments by X-ray diffraction confirm the homogeneous filling by a single crystal over several centimeters in length along the fiber. In addition, we discuss a series of optical experiments assessing the light guiding and attenuation characteristic of such fibres, both in the linear and nonlinear regime upon illumination with light from a lamp or femtosecond laser pulses. The results open the way towards the exploitation of organic single crystals in nanophotonics and offer a unique photonic structure to observe novel optical phenomena.

Exciton diffusion plays a critical role in the operation of many organic optoelectronic devices, most notably in photovoltaic cells (OPVs). Consequently, the exciton diffusion length (L_D) is an important parameter that helps to inform overall OPV design. Experimentally, exciton transport is probed by measuring end-of-life products, either photons (from radiative recombination) or charge carriers (from dissociation). Photoluminescence-based measurements have been widely used to extract L_D for a variety of materials, often in good agreement with an F#1255;rster model of exciton transport. While tracking luminescence provides an effective means of measuring L_D, non-luminescent (dark) materials, including many crystalline materials, remain inaccessible via such techniques. For dark materials, charge carriers are instead tracked, often as photocurrent from exciton dissociation in an OPV. Such measurements are complicated by the fact that photogenerated carriers are subject to recombination events prior to collection as current. Here, we present an alternate approach for measuring L_D that is equally applicable to luminescent and dark materials, that utilizes photovoltage instead of photocurrent to determine the number of excitons reaching the quenching interface in an OPV. The photovoltage-based measurement circumvents potential issues related to charge carrier recombination that complicate conventional OPV-based measurements of L_D. The photovoltage-based technique is used to measure L_D for dark metal phthalocyanine materials and to demonstrate competition between Förster and electron exchange energy transfer in the luminescent material boron subphthaloycanine chloride.

Since 1970s, conjugated polymers have received considerable attentions both from academia and industry due to their attractive advantages of light weight, low cost, easy fabrication and low energy consumption, etc., enabling them as potential promising materials in various fields, such as organic light-emitting diodes (OLEDs), organic solar cells (OSCs) and organic field-effect transistors (OFETs), especially on flexible substrates. To achieve such an attractive goal of plastic electronics, understanding the nature of charge transport in conjugated polymers is therefore of paramount importance for the development of high performance electronic devices as well as advancing their functional devices. However, it has been a big challenge for investigating charge transport in polymers due to their complex solid state structures. With this question as our research core, we have carried out systematical investigations at multi-levels from thin films to molecular scales in order to comprehensively reveal the intrinsic charge transport in conjugated polymers and realize the fabrication of high-performance polymer optoelectronic devices as well as novel functional devices.

Charge carrier mobility is an important characteristics of organic semiconductors that is affected by both intrinsic and extrinsic factors. The intrinsic factors relate to the molecular structure and packing of molecules in the crystals, as well as polaronic effects and thermal lattice fluctuations, while the extrinsic factors are charge trapping and scattering due to impurities and disorder. In this talk I will discuss the recent progress made in our group on: (a) synthesis of unique organic heterointerfaces, exhibiting trap healing effect, that can be used to measure charge transport and carrier mobility quite sensitively [1], and (b) development of novel measurement techniques that allow to dramatically improve the signal-to-noise ratio and sensitivity in Hall effect measurements even when applied to conventional OFET devices [2]. This two-fold approach, novel functional interfaces and new measurement techniques, allows us to gain significant further insights into the fundamental charge transport physics of organic semiconductors.
References.
[1]. B. Lee, Y. Chen, H. T. Yi, D. Fu, K. Czelen, H. Najafov and V. Podzorov, “Trap healing and ultra-low noise Hall effect at the surface of organic semiconductors”, Nature Mater.12, 1125-1129 (2013).
[2]. Y. Chen, H. T. Yi and V. Podzorov, "Sensitive Hall effect measurements in low-µ materials", in preparation (2015).

Over recent years there has been tremendous progress in developing low-temperature, solution-processible organic semiconductors that provide high charge carrier mobilities for both n-type and p-type device operation, good operational stability and other functionalities such as efficient electroluminescene, sensing or memory functions for a variety of applications. Here we are interested in understanding the charge transport physics of high mobility conjugated polymers and the relationship between molecular structure, polymer microstructure and charge transport. Many of the recently discovered high mobility polymers, in particular donor-acceptor copolymers, are characterised by a puzzling lack of pronounced crystalline order. In this presentation we will present our current understanding of the electronic structure and transport physics of these materials and of the reasons why these materials exhibit such high carrier mobilities.

This communication will display that single-crystals of organic semiconductors can be used in novel electronic devices. If field-effect transistors are easily and commonly produced with organic semiconductors, the Bipolar Transistor has not yet been successfully made with organic materials. Here, single-crystals of P and N type of various organic semiconductors are laminated to form a PNP stacks. The two PN junctions shows typical IV responses. Such a simple device shows a bipolar transistor behavior with current amplification. If performances of such Organic Heterojunction Bipolar Transistors (OHBT) are still below those of inorganic-based ones, the research presented here opens a new route for the development of a new generation of organic semiconductor based circuitry.

This talk will describe a series of scanning Kelvin probe microscopy (SKPM) experiments designed to examine the relationship between local work function, strain, and structural defects in crystalline organic semiconductors. One experiment involves recording the work function of single crystals of the benchmark semiconductor rubrene as a function of tensile and compressive strain. Another examines the work function changes associated with line dislocations in rubrene. Still another explores work function changes associated with microstrain in vapor-deposited thin films of pentacene. Collectively, the results demonstrate that work function variations in the range of 10-100 meV are very common in organic semiconductors due to strain and structural disorder, and thus they provide one mechanism for understanding the origin of shallow ‘tail states&’ in the electronic structure of these materials.