Symposium MD5 : Fundamentals of Organic Semiconductors---Synthesis, Morphology, Devices and Theory

In this presentation, we seek to provide a rationalization of the impact that intermolecular arrangements and interactions at the polymer/fullerene interfaces have on the performance of bulk-heterojunction solar cells. We discuss the results of combined electronic-structure calculations and molecular-dynamics simulations both for representative systems reported in the literature and new systems synthesized in our Center. In particular, we examine:

(i) the propensity of the fullerene molecules to dock preferentially on top of the electron-poor moiety or electron-rich moiety of the polymer, as a function of the nature and location of the polymer side chains; and

(ii) the impact that the packing arrangements have on the energetic distribution of the charge-transfer interfacial electronic states and their localization/delocalization characteristics.

This work is supported by King Abdullah University of Science and Technology, in the framework of its Solar & Photovoltaics Engineering Research Center (SPERC) and its Collaborative Research Grant Program (Award CRG3-62140391), and by ONR-Global (Award N62909-15-1-2003).

While improved materials, processing protocols, and device designs have ushered organic electronic devices onto the commercial landscape, there remains a need to establish a thorough understanding of the intimate relationships among chemical and molecular structure, processing, solid-state packing, and the underlying physical processes that determine material performance. Through the development and application of multiscale, theoretical materials chemistry approaches, we seek to develop the chemical insight behind these relationships that is necessary to refine and offer novel design pathways for next generation organic semiconducting active layers. In this presentation, we will focus on how such models reveal the influence of seemingly modest changes in chemical structure on the processing and solid-state packing of organic semiconducting active layers.

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. Results suggest that one may be able to control polaron pair behavior based on the asymmetry of the donor molecule. This allows for control of thermodynamic losses as well as open circuit voltage. These calculations also revealed the probability distribution of the formation of different types of polaron pairs, especially, as it relates to deposition order (i.e. donor on top of acceptor versus 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.

Extending photoresponse into the near-infrared (NIR) is one clear route to improving performance of both panchromatic and transparent organic photovoltaics (OPVs). However, current demonstrations of NIR active OPVs have been limited by low open-circuit voltages (V_OC) and low external quantum efficiencies (EQE). One reason for this is the challenge of optimizing energy level alignment within the tightened tolerance of a small bandgap. In this work, we show that V_OC and EQE can be simultaneously enhanced in organic salt-based OPVs via single-step anion exchanges. We demonstrate that V_OC gains are due to improved energy level alignment and that energy levels can be finely tuned by alloying various anions. These effects can be exploited to optimize energy level alignment for abitrary donor-acceptor pairings with novel low bandgap organic salts, and we extend this method to several new molecules with unprecedented V_OC (for their spectral range) and photoresponse from 950 nm to as far as 1500 nm. This work bypasses the V_OC bottleneck which previously hindered small molecule-based photovoltaics, and presents an exciting path forward for the design of efficient multi- and single-junction transparent photovoltaics.

We used Density Functional Theory (DFT) calculations to study the electronic structure of organic mixed-stack charge-transfer crystals. We investigate the impact that the amount of nonlocal Hartree-Fock exchange (HFE) included in a hybrid density functional has on the geometry, the normal vibrational modes, electronic coupling and electron-vibrational couplings in these systems. The crystal geometry and the frequencies of the phonons are found to be only modestly affected by the amount of HFE. In contrast, the electronic couplings and electron-vibration couplings show a strong dependence on this value. We compare the DFT results with results obtained within the G₀W₀ approximation as a way of benchmarking the optimal amount of HFE needed in a given functional. The electronic structures of a series of donor-acceptor crystals have been investigated in detail. The charge-transport, ferroelectric and non-linear optical properties of several systems that were investigated will be discussed.

The fullerene-based organic solar cells (OSC) with a very minute amount of polymer “donor” have recently attracted intensive research interest due to their simple structure and unique electrical performance especially large Voc. However, charge transport mechanism in such devices with “donor” concentration much lower than the percolation threshold is still unclear. Some studies propose the high Voc is the Schottky barrier height between the anode electrode and the fullerene LUMO. On the other hand, other authors argue that Voc is enhanced because CT states, the major pathways for bimolecular recombination in bulk-heterojunction OSC devices, are significantly reduced in these active layers. Studies based primarily on current density-voltage (J-V) measurements are unable to differentiate different mechanisms. Here we apply impedance spectroscopy (IS), low-energy external quantum efficiency (EQE) spectroscopy, and photoluminescence (PL) dynamics in addition to J-V to study PCBM-based organic solar cells with varying P3HT concentrations. The experimental results are compared to 1D drift-diffusion modeling using SCAPS software.[1] Strong frequency-dependent capacitance behaviors are observed in devices with very low donor concentrations, indicating significant charge accumulation due to space charge limited as well as trap limited transport process. In addition, the role of exciton-polaron annihilation due to poor exciton dissociation and polaron collection will be probed by PL dynamic studies. Finally, impacts of p-type doping of active layer in the fullerene-based OSCs on hole mobility and internal field improvements will be discussed.

Reference:
[1] M. Burgelman, P. Nollet, S. Degrave, Modelling polycrystalline semiconductor solar cells, Thin Solid Films. 361 (2000) 527–532.

This project is sponsored by National Science Foundation DMR-1305893

Limited lateral conductivities of the photo-active materials used in organic thin-film solar cells necessitate the use of semitransparent electrodes for current collection and lateral current transport. Due to the tradeoff between electrical conductivity and optical transmission, which should not underrun 80%, typical values for the sheet resistances of semitransparent electrodes deposited on glass amount to 10–20 Ω/sq. The power loss due to Joule heating and the accompanying voltage drop caused by this sheet resistance increases with cell length in current transport direction and thus defines an upper limit for practical solar cell lengths. In the several approaches existing for calculation of the power loss, the semitransparent electrode layer is either modeled as one lumped resistance in series to the solar cell or as distributed resistance across the whole length of the solar cell in current transport direction. We present a quantitative comparison between these two concepts to investigate the direct influence on the optimal solar cell geometry and to discuss the capabilities as well as limitations of each model. Furthermore the impact of the results on the serial interconnection of a monolithic thin film organic solar cell module in terms of optimal cell lengths and cell interconnection distance is evaluated. The computational study presented here is based on the material system PCDTBT:PC70BM for the photo-active layer material as an example and commonly used semitransparent conductive electrodes: ITO deposited on glass as well as on PET foil and highly doped PEDOT:PSS named PH1000 on PET foil.

Energy levels in the mixed polymer-fullerene phase of bulk heterojunction solar cells are significantly shifted from their values in the pure materials [1]. These shifts are important for solar cell performance: they create energy cascades between the mixed phase and pure donor and acceptor phases, which have been shown to improve geminate splitting and suppress bimolecular recombination. This work investigates the origin of these energy level shifts and explains their effect on the charge transfer (CT) state energy and open circuit voltage (Voc).

We use regiorandom P3HT:PCBM as our model system. Regiorandom P3HT is amorphous, and when blended with PCBM, forms only one amorphous mixed phase, allowing us to study the energetics of the mixed phase without effects of polymer crystallites. We measure the polymer ionization potential (IP) and fullerene electron affinity (EA) as a function of blend composition using cyclic voltammetry (CV). The polymer IP monotonically increases in magnitude by around 400 meV and the fullerene EA by around 200 meV as the fullerene content in the blend is increased from 29% to 71%. The effective band gap measured by CV increases by around 300 meV.

Computational modelling studies using molecular dynamics [2] and classical microelectrostatics [3] have suggested that electrostatic interactions give rise to dipoles at the molecular interface between polymer and fullerene. We hypothesize that the electrostatic potential created by these interfacial dipoles shifts the energy levels of individual molecules in the blend. To test this hypothesis, we design a simplified electrostatic model to compute the effects of interfacial dipoles on polymer and fullerene energy levels. We fit the energy level shifts predicted by this model to experimentally measured values, using the magnitude of the interfacial dipole as the only fitting parameter. With this model, we show that the energy level shifts can be quantitatively accounted for by interfacial dipoles of comparable magnitude to permanent dipole moments in the molecules.

Despite large changes in the effective band gap, the measured CT state energy and Voc shift by only small amounts and show no trend. We show that energetic disorder in the mixed phase results in broadening of all the densities of states. During normal solar cell operation, only the low-energy tail of the CT density of states is filled, and effectively sets Voc. While changes in blend composition produce large changes in the centres of the energy levels, their low-energy tails are only slightly affected, resulting in Voc not varying by much.

In conclusion, this work shows how energy levels in the mixed phase are shifted by dipoles at the polymer-fullerene interfaces. Tuning these energy level shifts is likely to be an important part of future strategies aimed at improving the efficiencies of organic solar cells.

[1] J. Am. Chem. Soc. 2014, 40, 14078
[2] Adv. Mater. 2013, 25, 878
[3] J. Phys. Chem. C 2013, 117, 12981

Rubrene is a promising material for organic electronics and optoelectronics; it forms crystalline films with high hole mobility and efficient electroluminescence. Recent studies have shown that the electronic properties of rubrene films can be tuned by substrate-induced strain, suggesting a new route towards the design of more efficient devices. Here, we present a first-principles analysis of the strain-induced changes to the mechanical, electronic, and optical properties of rubrene crystals. Density functional theory and many-body perturbation theory studies predict changes in hole motilities in excellent agreement with electrical conductivity measurements when a strain consistent with the experiment is applied. Furthermore, we predict that the optical absorption and nature of low-energy excitons within the crystal can be tuned by an applied strain as low as 1%. This work utilized resources at the Center for Nanoscale Materials, supported by the U.S. Department of Energy under Contract No. DE-AC02-06CH11357.

Organicoptoelectronics, such as organicphotovoltaics (OPVs) and organic light emitting diodes (OLEDs), offer the promise of low-cost flexible solar cells, displays, and light sources that have the potential to be manufactured on large-area plastic substrates. One of the key elements for improving efficiencies in organic optoelectronics is finding suitable cathode electrode materials to replace the reactive low work function metals, such as calcium or magnesium, that are typically used to either inject electrons into or extract electrons from the lowest unoccupied molecular orbital (LUMO) of a given organic semiconductor.Polyoxometalates (POMs) are a well-known large group of clusters with frameworks built from transition metal oxo anions linked by shared oxide ions, first reported by Jöns Jacob Berzelius in 1826.One of the most intriguing properties of POM clusters is their high ability to accept a large number of electrons with minimal structural modifications, a property that could enable them to play an important role as excellent electron conductors in electronic devices.
We report here on the preparation of efficient electron conducting layers consisting of lacunaryPOM clusters spin coated from a water/methanol solution between the organic semiconductor and the cathode electrode surfaces for application inorganic optoelectronics. The photoelectron spectroscopy study reveals that these POMsexhibit a high degree of reduction which introduces delocalized electrons that can hop over the metal centers creating thus near Fermi level states that overlap sufficiently with the Fermi level of the metal cathode acting as a highly conductive path for electron conduction through the cathode interface. It is verified also that our method is applicable to a wide range of different organic semiconducting materials and that can be used in state-of-the-art high-efficiency organic electronic devices, including OLEDs and bulk heterojunction (BHJ) OPVs.In particular, OLEDs based on the greenemitting poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,1’,3}-thiadiazole)] (F8BT) and a wide range of OPV devices based on photoactive layers composed of mixtures of a polymer donor, such as poly(3-hexylthiophene) (P3HT), poly[(9-(1-octylnonyl)-9H-carbazole-2,7-diyl)-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT) or poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2- b :4,5- b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4- b ] thiophenediyl}) (PTB7), and a fullerene acceptor, such as [6,6]-phenyl C71butyric acid methyl ester (PC₇₁BM), exhibited a large enhancement in their efficiencies when used the reduced POMs as electron conduction layers.

Despite the many advances, OPV devices still fall behind their inorganic counterparts in terms of power conversion efficiency and longevity. Up to now, disordered blends of donor and acceptor molecules yield the best solar cells. However, every device is a compromise between small and large phases, which favor charge carrier generation and charge carrier transport, respectively. Additionally, optimizing processing conditions is a non-trivial task, which is usually done empirically. Hence, new materials often lag behind expectations obtained from chemical intuition. Furthermore, synthetic chemists open up new routes to synthesize even more molecules applicable to OPV meaning that the sheer number of compounds available renders the experimental selection of suitable compounds difficult. Hence, in-silico pre-screening of such new compounds is desirable.
Although in-silico screening of new compounds is less laborious than the experimental analogue, only a limited number of compounds can be treated due to the manual effort involved. Hence, we employ automation routines in order to minimize manual intervention on runtime-restricted systems that allow for easy screening of a large number of compounds in addition to accelerating calculations by the use of an efficient implementation of electron core potentials [Song et al., J. Chem. Phys., 2015, 143(1)]. Material properties that need to be predicted correctly by the aforementioned screening procedures include ionization potentials, electron affinities and exciton binding energies and absorption spectra both, in vacuum and in solution. This means that correct predictions of absolute frontier orbital energies and optical gaps are necessary in these environments. We could already show the fast reproduction of absorption spectra in some of these cases [Gampe et al., Chem. Eur. J., 2015, 21(20)].
Frontier orbital energies are very important molecular properties that decide whether or not a materials system can produce a photocurrent under irradiation as the difference between these energies is the main driving force behind exciton separation at the interface. However, commonly employed DFT calculations grossly underestimate the fundamental gap. Recently, it was suggested to use the class of range-separated hybrid functionals to overcome this issue by tuning or optimizing the range-separation parameter to the system at hand without the need for empirical data. This is done by minimizing the misfit to Koopman’s theorem and an analogous, inverse variant concerning the LUMO.
We employ this technique to reproduce experimental molecular properties for a variety of dyes applicable to OPV, such as porphyrins. We also compare the results obtained for different functionals. The automation routines we employ render this a feasible approach for research in the field of OPV.
We acknowledge funding from the Bundesministerium für Bildung und Forschung (BMBF FKZ 03EK3507) and the Deutsche Bundesstiftung Umwelt (DBU).

Thin organic chromophore layers are the essential functional elements in devices for light to electric energy conversion and vice versa, i.e. in organic solar cells (OSCs) and organic light emitting diodes (OLEDs). In OLEDs the morphology of the active layer needs to be engineered for optimal directional characteristics of light emission while in OSCs the morphology of the active layer needs to be elaborately tuned for optimal power conversion efficiency. However, the morphology is generally challenging to target starting from molecular chromophore design. This is due to the fact that predictions of supra-molecular structures are still computationally very demanding at a high accuracy level and the formation of particular morphologies depends on various external physical, chemical and processing parameters. Therefore, various approaches have been developed to control the morphology. One promising approach to design defined supra-molecular structures that are thermodynamically stable is self-assembly. A prominent strategy to induce self-assembly is to introduce amphiphilicity to functional molecules. Beyond self-assembly, amphiphilicity additionally enables utilization of the Langmuir-Blodgett (LB) technique for superior control of molecular ordering and phase formation.
This presentation will focus on self-assembly, morphological and photonic properties of novel donor and novel acceptor molecules that were made amphiphilic. Using the LB-technique we are able to produce thin films of both donor and acceptor molecules with controlled morphology. The surface sensitive photothermal deflection spectroscopy[1-3] is not just applied to determine the UV-vis absorption spectra of even molecular monolayers, but also to monitor self-assembly on surfaces from solutions in situ. On the examples of C60-derivatives, but also merocyanines and novel thiazoles derivatives it is demonstrated how large the influence of the morphology on photonic properties might be and how to control morphologies by molecular assembling.[4]

1. Presselt, M., et al., Influence of Phonon Scattering on Exciton and Charge Diffusion in Polymer-Fullerene Solar Cells. Advanced Energy Materials, 2012. 2(8): p. 999-1003.
2. Presselt, M., et al., Sub-bandgap absorption in polymer-fullerene solar cells studied by temperature-dependent external quantum efficiency and absorption spectroscopy. Chemical Physics Letters, 2012. 542: p. 70-73.
3. Herrmann, F., et al., Influence of Interface Doping on Charge-Carrier Mobilities and Sub-Bandgap Absorption in Organic Solar Cells. The Journal of Physical Chemistry C, 2015. 119(17): p. 9036-9040.
4. Habenicht, S.H., et al., Tuning the Polarity and Surface Activity of Hydroxythiazoles – Extending Applicability of Highly Fluorescent Self-Assembling Chromophores to Supra-Molecular Photonic Structures. to be submitted, 2015.

The authors are grateful for financial support (BMBF FKZ 03EK3507).

Chirality plays a very important role in scientific and technological research fields with huge potential to be applied in key areas including chemical synthesis, pharmaceutics, catalysis, fundamental physics and biomedicals. However, the separation of chiral molecules is still a challenge for their practical use in chemical and pharmaceutical industries. The most widely used characterizations rely on the traditional methods of measuring the circular dichroism using circularly polarized light. Besides, chirality in nanostructures could potentially be very useful. However, the application of chiral nanostructures, particularly, organic chiral nanomaterials, is still in infancy. In this research, we have successfully synthesized chiral materials, i.e., CPDI-Ph derivatives, and self-assmbled them into nanowires. The prepared 1-D nanomaterials show excellent n-type organic field-effect transistor (OFET) performance and high photo-responsivity under monochromic light irradiations. More importantly, specific photo-responses were obtained from chiral semiconductors upon illuminating circularly polarized light on the active layer of the n-type OFETs, which demonstrates the first example for detecting circularly polarized light from n-type phototransistors based on photoactive 1-D nanomaterials.

The development of highly conductive conjugated polymers has been one of the major goals for organic electronics. Recently, conductivities of over 4000 S/cm was demonstrated for vapor phase polymerized poly(3,4-ethylenedioxythiophene):tosylate (PEDOT:Tos), rivaling ITO as the material of choice for transparent flexible electrodes. Substantial research has also been invested in the development of patterning techniques for such conductive polymers. These patterning techniques include laser cutting, ink jet printing, screen printing, transfer printing and photolithography. Each of these methods has its respective advantages and disadvantages. We report a method of patterning vapor phase polymerized PEDOT:Tos using UV-light and show how this new approach has several advantages over existing patterning techniques.
When patterning conductive polymers using photolithography, a mold of the pattern is first made using a photoresist. The polymer is then added to the mold after which the photoresist is dissolved in an organic solvent. This results in a step-like profile of the pattern edges. The procedure involves a number of steps and the chemicals used to strip the photoresist are often environmentally hazardous and can also damage the conductive polymer. Our approach is similar to that of photolithography in that the patterning is mediated by UV-light through a photomask. However, it has fewer processing steps and does not involve any photoresists or hazardous chemicals.
The patterning can either be done with a step-like profile of the pattern or having a continuous polymer film where the conductivity of the film is locally reduced. The reduction in conductivity is controlled by the radiation dose and can span over six orders of magnitude. The exposure also induces a change in the color of the polymer film. Line widths of 100 um can be done effortlessly and work is ongoing towards significantly smaller dimensions.
The simplicity and versatility of this novel patterning procedure show great promise for patterning of organic electronic circuits as well as electrochromic displays.

Donor–acceptor (D–A) semiconducting copolymers, PDPP-TVT-24 and PDPP-TVT-29, were synthesized by alternating diketopyrrolopyrrole (DPP) derivatives with difference alkyl side-chains (2-decylbutadecyl (24) and 7-decylnonadecyl (29)) and thiophene vinylene thiophene (TVT) for organic field-effect transistors (OFETs). PDPP-TVT-24 and PDPP-TVT-29 could form highly π-conjugated structures in as-spun films. In particular, the layer-like conjugated film morphologies could be developed via short-term thermal annealing above 150 °C for 10 min. The strong intermolecular interaction, originating from the fused DPP and D–A interaction, led to the spontaneous self-assembly of polymer chains within close proximity (with π-overlap distance of 3.55–3.70 Å) and formed unexpectedly long-range π-conjugation, which is favorable for both intra- and intermolecular charge transport. Unlike intergranular nanorods in the as-spun film, well-ordered layers in the 200 °C-annealed film could yield more efficient charge-transport pathways. The granular morphologies of the as-spun films produced a field-effect mobility (μ_FET) of 0.65 cm² V^–1 s^–1 (for PDPP-TVT-24) and 1.39 cm² V^–1 s^–1 (for PDPP-TVT-29) in OFETs based on a polymer-treated SiO₂ dielectrics, while the 200 °C-annealed films showed high μ_FET values of up to 3.4-3.7 cm² V^–1 s^–1. The difference of charge mobilities between PDPP-TVT-24 and PDPP-TVT-29 as-spun films might be originating from polymer chain flexibility with different alkyl side-chain length. The significantly high μ_FET observed for the thermally annealed copolymers films were mainly related to the extraordinarily layered π-conjugated crystals, owing to the strong intermolecular interactions and the spaced out side-chain arrangement that helped the polymer chains form well-interconnected crystal grains, thus providing efficient pathways for charge transport. The highly π-conjugated nature, excellent solution processability, electrical properties, and simple synthesis of PDPP-TVT-24 and PDPP-TVT-29 suggest that the copolymers can potentially be applied to high-throughput, roll-to-roll solution printing of low-cost OFET circuits and arrays.

Charge transport in organic semiconductors is typically described by hopping through localized electronic states in Gaussian density of states (GDOS). The efficiency of charge transport is determined by the position of Fermi level and the corresponding DOS. As the carriers fill the lower-lying states of the organic semiconductor, corresponding to a shift in the Fermi energy, additional charges will occupy states at relatively higher energies. Consequently, these additional charges on average require less activation energy to hop to neighbouring sites, resulting in a higher mobility with increasing carrier density. Carrier mobility in such devices can then be tuned by varying the pushing the Fermi level relative to the maximum of the DOS. However, there are limitations to how much the Fermi level can be shifted, primarily due to device breakdown considerations. As a result, the mobility in three terminal organic semiconductor based devices have been limited to low values.
We have examined the characteristics of Copper Phthalocyanine(CuPc) based organic field effect transistors in both the positive and negative drain-source voltage (V_DS) regime. Phthalocyanine based molecular materials are polycrystalline, thermally and chemically stable and are ideally suited for testing any carrier transport model. Moreover, they have the potential for application in organic solar cells, LEDs and field effect transistors. We find that the Fermi level can be moved closer to the maximum of the GDOS by applying a positive V_DS. In the positive V_DS regime, the I_DS-V_DS characteristics at zero gate voltage (V_G) are similar to the two terminal devices, but an external gate can still be used to modify the carrier density. We observe an increase of almost two orders of magnitude in the mobility in this regime, with mobility values comparable to those observed in single crystals of CuPc. The high mobility can be understood on the basis of a transition from hopping to band like transport, with an applied V_DS and V_G. Band mobilities calculated using first principle DFT calculations are found to be in good agreement with the observed experimental values.

Parallel-like ternary blends provide a possible solution to one of the inherent limitations of organic solar cells: the relatively narrow absorption width of the conjugated polymers used in the active layer. This study focuses on selecting donor polymer pairs for ternary blends which will exhibit the characteristic composition dependent voltage, while extending the absorption and increasing the current, thereby improving the power conversion efficiency. In order to investigate the effect of a common backbone moiety on performance, two donor moieties and two acceptor moieties were mixed and matched to synthesize four polymers: poly(benzodithiophene-dithienyl-benzothiadiazole), poly(benzodithiophene-dithienylbenzotriazole), poly(naphthodithiophene-dithienyl-benzothiadiazole), and poly(naphthodithiophene-dithienybenzotriazole). These polymers were then paired in photovoltaic devices (with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) as an electron acceptor) and their performance compared to binary devices made from each individual polymer. It was found that pairings in which the polymers shared a common donor moiety performed well relative to the binary blends, but pairings with either a common acceptor moiety or no common moiety performed poorly, as evidenced by a drop open-circuit voltage. Resonant soft X-ray scattering shows that each blend exhibits markedly different morphology, with P(NDT-DTBT) in particular dominating domain formation when included in a blend. In the most successful pairing, P(BnDT-DTBT) and P(BnDT-HTAZ), each polymer formed independent domains. For this set of polymers, in order to see parallel-like behavior it was essential that the ternary blend include a common donor moiety in the polymer backbone and exhibit independent, constructive domain formation. These insights will serve to guide future selection of polymer pairs for parallel-like ternary blends.

It is necessary to develop a method to control the electronic properties of single-walled carbon nanotubes (SWCNTs) for use in a wide range of areas such as electronics and photonics. For example, nitrogen doping into the SWCNTs is one of the methods for tuning their electronic properties, resulting in n-type SWCNTs in air. However, in previously-reported chemical vapor deposition using a nitrogen source, only nitrogen-doped (N-doped) defective SWCNTs can be synthesized and they do not show good conductivity. Here, we employed a defluorination-assisted nanotube substitution reaction, which was performed using ammonia gas, for the synthesis of N-doped crystalline SWCNTs that exhibit good conductivity and n-type conduction.
We synthesized highly crystalline SWCNTs (hc-SWCNTs) by the arc discharge method. The hc-SWCNTs were fluorinated at 250 °C using a gas mixture of F₂ (20%) and N₂ (80%) for 4 h. The fluorinated SWCNTs (F-SWCNTs) were characterized using X-ray photoelectron spectroscopy (XPS) and their C:F stoichiometry was determined to be CF_0.39. Next, T