β-Disubstituted Pentacene Derivatives: Thin Film Structural Properties and Four-Probe Field Effect Mobility

2,9- and 2,10-diphenylpentacene were synthesized by direct C-H borylation of ketal-protected pentacene, followed by halodeboronation, resolution of the dihalo isomers, Suzuki arylation, cleavage of the ketals and decarbonylation in the solid state. They were studied as main active components in organic field effect transistors (OFETs). Diphenyl substitution of pentacene affects the unit cell dimensions only slightly, preserving a face to edge molecular packing in the first layers of thin films evaporated on SiO2 substrates. Both isomers self-assemble into nanoribbons during the thin film growth upon vapor deposition. The similarity between the surface induced phases of the 2,9-isomer and unsubstituted pentacene leads to similar 4-probe hole mobilities, i. e. 0.13 cm2 V-1 s-1 for the former. Whereas 2,9-disubstitution thus does essentially preserve the thin film characteristics of unsubstituted pentacene, 2,10-disubstitution is detrimental to the molecular ordering in the thin films and therefore to the field effect mobility which is only 0.07 cm2 V-1 s-1. The known strong enhancement of field effect mobility observed upon diphenyl substitution of anthracene can thus not be emulated analogously with pentacene.

Aligned Conjugated Polymer Nanofiber Networks in an Elastomer Matrix for High-Performance Printed Stretchable Electronics

Conjugated polymer films are promising in wearable X-ray detection. However, achieving optimal film microstructure possessing good electrical and detection performance under large deformation via scalable printing remains challenging. Herein, we report bar-coated high-performance stretchable films based on a conjugated polymer P(TDPP-Se) and elastomer SEBS blend by optimizing the solution-processing conditions. The moderate preaggregation in solution and prolonged growth dynamics from a solvent mixture with limited dissolving capacity is critical to forming aligned P(TDPP-Se) chains/crystalline nanofibers in the SEBS phase with enhanced π-π stacking for charge transport and stress dissipation. The film shows a large elongation at break of >400% and high mobilities of 5.29 cm2 V-1 s-1 at 0% strain and 1.66 cm2 V-1 s-1 over 500 stretch-release cycles at 50% strain, enabling good X-ray imaging with a high sensitivity of 1501.52 μC Gyair-1 cm-2. Our work provides a morphology control strategy toward high-performance conjugated polymer film-based stretchable electronics.

Tuneable Anisotropic Plasmonics with Shape-Symmetric Conducting Polymer Nanoantennas

A wide range of nanophotonic applications rely on polarization-dependent plasmonic resonances, which usually requires metallic nanostructures that have anisotropic shape. This work demonstrates polarization-dependent plasmonic resonances instead by breaking symmetry via material permittivity. The study shows that molecular alignment of a conducting polymer can lead to a material with polarization-dependent plasma frequency and corresponding in-plane hyperbolic permittivity region. This result is not expected based only on anisotropic charge mobility but implies that also the effective mass of the charge carriers becomes anisotropic upon polymer alignment. This unique feature is used to demonstrate circularly symmetric nanoantennas that provide different plasmonic resonances parallel and perpendicular to the alignment direction. The nanoantennas are further tuneable via the redox state of the polymer. Importantly, polymer alignment could blueshift the plasma wavelength and resonances by several hundreds of nanometers, forming a novel approach toward reaching the ultimate goal of redox-tunable conducting polymer nanoantennas for visible light.

Theoretical Study of Charge Mobility in Crystal Porphine and a Computer Design of a Porphine-Based Semiconductive Discotic Liquid Mesophase

Organic semiconductors are the focus of numerous studies; they are used in electronic devices. Modern research involves the production of neuromorphic organic materials, including those based on liquid crystal materials. The purpose of this work involves the theoretical modeling of molecules (the "core with branches" type) to construct a discotic mesophase capable of performing the functions of a neuromorphic material. For this purpose, the conductivity of crystal porphine, which can act as the nucleus of a molecule of the "core with branches" type, was investigated. The Marcus theory charge mobility values for the hole and electron were 0.148 and 0.088 cm²/V·s, respectively (the MOO method for calculating transfer integrals), and 0.561 and 0.160 cm²/V·s (DIPRO method). Based on TD-HF (HF-3c level of theory) calculations, possible structures of molecules for the formation of a discotic mesophase are proposed.

Hydrogen bonding-induced high-performance stretchable organic semiconductors: a Review

Semiconductors are widely used in electron devices. With the development of wearable soft-electron devices, conventional inorganic semiconductors are unable to meet the demand because of their high rigidity and high cost. Thus, scientists construct organic semiconductors with high charge mobility, low cost, eco-friendly, stretchable, etc. Due to the excellent performance of stretchable organic semiconductors, they can be widely used as wearable soft-electron devices, such as stretchable organic field-effect transistors (OFETs), organic solar cells (OSCs), etc. Contains flexible display devices and flexible power sources, which are of great interest for applications of future electron devices. However, there are still some challenges that need to be solved. Commonly, enhancing the stretchability may cause the degradation of charge mobility, because of the destruction of the conjugated system. Currently, scientists find that hydrogen bonding can enhance the stretchability of organic semiconductors with high charge mobility. Thus in this review, based on the structure and design strategies of hydrogen bonding, various hydrogen bonding induced stretchable organic semiconductors are introduced. In addition, the applications of the hydrogen bonding induced stretchable organic semiconductors are reviewed. Finally, the stretchable organic semiconductors design concept and potential evolution trends are discussed. The final goal is to outline a theoretical scaffold for the design of high-performance wearable soft-electron devices, which can also further advance the development of stretchable organic semiconductors for applications.

Clean BN-Encapsulated 2D FETs with Lithography-Compatible Contacts

Device passivation through ultraclean hexagonal BN encapsulation has proven to be one of the most effective ways of constructing high-quality devices with atomically thin semiconductors that preserve the ultraclean interface quality and intrinsic charge transport behavior. However, it remains challenging to integrate lithography-compatible contact electrodes with flexible distributions and patterns. Here, we report the feasibility of a straightforward integration of lithography-defined contacts into BN-encapsulated two-dimensional field-effect transistors (2D FETs), giving rise to overall device quality comparable to the state-of-the-art results from the painstaking pure dry transfer processing. The electronic characterization of FETs consisting of WSe₂ and MoS₂ channels reveals an extremely low scanning hysteresis of ∼2 mV on average, a low density of interfacial charged impurities of ∼10¹¹ cm^-2, and generally high charge mobilities over 1000 cm² V^-1 s^-1 at low temperatures. The overall high device qualities verify the viability of directly integrating lithography-defined contacts into BN-encapsulated devices to exploit their intrinsic charge transport properties for advanced electronics.

Chlorination Enabling a Low-Cost Benzodithiophene-Based Wide-Bandgap Donor Polymer with an Efficiency of over 17

Three regioregular benzodithiophene-based donor-donor (D-D)-type polymers (PBDTT, PBDTT1Cl, and PBDTT2Cl) are designed, synthesized, and used as donor materials in organic solar cells (OSCs). Because of the weak intramolecular charge-transfer effect, these polymers exhibit large optical bandgaps (>2.0 eV). Among these three polymers, PBDTT1Cl exhibits more ordered and closer molecular stacking, and its devices demonstrate higher and more balanced charge mobilities and a longer charge-separated state lifetime. As a result of these comprehensive benefits, PBDTT1Cl-based OSCs give a very impressive power conversion efficiency (PCE) of 17.10% with a low nonradiative energy loss (0.19 eV). Moreover, PBDTT1Cl also possesses a low figure-of-merit value and good universality to match with different acceptors. This work provides a simply and efficient strategy to design low-cost high-performance polymer donor materials.

Accurate Analysis of Anisotropic Carrier Mobility and Structure-property Relationships in Organic BOXD Crystalline Materials

Charge mobility is an essential factor of organic crystalline materials. Although many investigators have made important progress, the exact relationship between the crystal structure and carrier mobility remains to be clarified. Fortunately, a series of bis-1,3,4-oxadiazole derivatives have been successfully prepared and reported. They have similar main molecular fragments but different crystal packing modes, which provide an ideal research objective for studying the effect of molecular packing on charge mobility in organic photoelectric conversion systems. In this work, the charge mobilities of these molecules are systematically evaluated from the perspective of first-principles calculation, and the effect of a molecular overlap on orbital overlap integral and final charge carrier mobility is fully discussed. It can be seen that the small intermolecular distance (less than 6 Å) is the decisive factor to achieve high electron mobility in π stacking, and better mobility can be obtained by increasing the hole migration distance appropriately. A larger dihedral angle of anisotropy is an important point limiting the charge mobility in the herringbone arrangement. It is hoped that the correlation results between the crystal structure and mobility can assist the experimental study and provide an effective way to improve the photoelectric conversion efficiency of the organic semiconductor devices and multiple basis for multiscale material system characterization and material information.

Quantitative Prediction of the Electro-Mechanical Response in Organic Crystals

Organic semiconductors' inherent flexibility makes them appealing for advanced applications such as wearable electronics, e-skins, or pressure sensors, and can even be used to enhance their intrinsic electronic properties. Unfortunately, these applications for organic materials are currently hindered by the lack of a quantitative understanding of the interplay between their electrical and mechanical properties. In this work, this gap is filled by presenting an accurate methodology able to predict quantitatively the effects of external deformation on the charge transport properties of any organic semiconductors. Three prototypical materials are investigated, showing that the experimental variation of charge carrier mobility with strain is fully reproduced, even in a wide range of deformations applied along different crystal axes. The results indicate that the intrinsic electro-mechanical response of the materials varies by orders of magnitude within the class of organic semiconductors, a difference rationalized observing that the mobility trend is primarily influenced by the transfer integrals' variation, rather than by a modification of the crystal phonons. In light of its robustness, accuracy, and low computational cost, this protocol represents an ideal tool to quantify the electro-mechanical response in new organic compounds, thus establishing a reliable route for a full exploitation of strain engineering in advanced technologies.

Charge Mobility in Discotic Liquid Crystals

Discotic (disk-shaped) molecules or molecular aggregates may form, within a certain temperature range, partially ordered phases, known as discotic liquid crystals, which have been extensively studied in the recent past. On the one hand, this interest was prompted by the fact that they represent models for testing energy and charge transport theories in organic materials. However, their long-range self-assembling properties, potential low cost, ease of processability with a variety of solvents and the relative ease of tailoring their properties via chemical synthesis, drove the attention of researchers also towards the exploitation of their semiconducting properties in organic electronic devices. This review covers recent research on the charge transport properties of discotic mesophases, starting with an introduction to their phase structure, followed by an overview of the models used to describe charge mobility in organic substances in general and in these systems in particular, and by the description of the techniques most commonly used to measure their charge mobility. The reader already familiar or not interested in such details can easily skip these sections and refer to the core section of this work, focusing on the most recent and significant results regarding charge mobility in discotic liquid crystals.

Exciton Transport in Molecular Semiconductor Crystals for Spin-Optoelectronics Paradigm

Organic semiconductors with long-range exciton diffusion length are highly desirable for optoelectronics but currently remain rare. Here, the estimated diffusion length of singlet excitons (L_D ) in 2,6-diphenyl anthracene (DPA) crystals grown by solvent evaporation was shown to be up to approximately 124 nm. These crystals showed a previously unseen parallelogram morphology with layer-by-layer edge-on molecular stacking, isotropic optical waveguiding, radiation rate and non-radiation rate constants of 0.15 and 0.26 ns^-1 respectively, as well as good field-effect transistor hole mobility and theoretically computed strong electronic couplings as high as 109 meV. Photoresponse experiments revealed that the photoconductivity of DPA crystals is surprisingly not related to the radiative pathway but associated with rapid exciton diffusion to the crystal surface for charge separation and carrier bimolecular recombination. Taken together, DPA was shown to be a promising semiconducting material for a new organic optoelectronics paradigm.

Azaisoindigo-Based Polymers with a Linear Hybrid Siloxane-Based Side Chain for High-Performance Semiconductors Processable with Nonchlorinated Solvents

Developing nonchlorinated solvent-processed polymeric semiconductors to avoid environmental concerns and health hazards caused by chlorinated solvents is especially urgent. Here, a molecular design strategy, composed of backbone fluorination and side chain optimization, is used for preparing high-solubility and high-performance azaisoindigo-based polymers. The effects of different backbones and side chains on the solubility, film crystallinity, molecular stacking, and charge transport properties are mainly investigated. A long linear hybrid siloxane-based chain (C₆-Si₇) is chosen to improve the solubility, while the incorporation of fluorine (F) is used to enhance the film crystallinity and charge mobility. By optimizing the backbone and side chain, both solubility and charge mobility of the azaisoindigo-based polymer are significantly improved. As a result, PAIIDBFT-Si films processed with toluene, tetrahydrofuran, ether, and alkanes, achieved charge mobilities of 4.14, 3.78, 2.14, and 2.34 cm² V^-1 s^-1, respectively. The current study provides an effective strategy for the design and synthesis of high-performance polymeric semiconductors processed with nonchlorinated solvents.

Tuning of Molecular Electrostatic Potential Enables Efficient Charge Transport in Crystalline Azaacenes: A Computational Study

The chemical versatility of organic semiconductors provides nearly unlimited opportunities for tuning their electronic properties. However, despite decades of research, the relationship between molecular structure, molecular packing and charge mobility in these materials remains poorly understood. This reduces the search for high-mobility organic semiconductors to the inefficient trial-and-error approach. For clarifying the abovementioned relationship, investigations of the effect of small changes in the chemical structure on organic semiconductor properties are particularly important. In this study, we computationally address the impact of the substitution of C-H atom pairs by nitrogen atoms (N-substitution) on the molecular properties, molecular packing and charge mobility of crystalline oligoacenes. We observe that besides decreasing frontier molecular orbital levels, N-substitution dramatically alters molecular electrostatic potential, yielding pronounced electron-rich and electron-deficient areas. These changes in the molecular electrostatic potential strengthen face-to-face and edge-to-edge interactions in the corresponding crystals and result in the crossover from the herringbone packing motif to π-stacking. When the electron-rich and electron-deficient areas are large, sharply defined and, probably, have a certain symmetry, calculated charge mobility increases up to 3-4 cm2V-1s-1. The results obtained highlight the potential of azaacenes for application in organic electronic devices and are expected to facilitate the rational design of organic semiconductors for the steady improvement of organic electronics.

Weakly Conjugated Phosphine Oxide Hosts for Efficient Blue Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes

The development of host materials with high first-triplet state (T₁) energy and high charge mobility is a key to achieve efficient true-blue organic light-emitting diodes (OLEDs), employing phosphorescence and thermally activated delayed fluorescence (TADF). An ether-bridged double triphenylphosphine oxide (TPPO) compound, bis(2-(diphenylphosphino)phenyl)ether oxide (DPEPO), was reported to have a very high T₁ energy of 3.3 eV but suffers from poor charge mobility. Here, five bridge-controlled multi-TPPO derivatives were studied through a combination of experiments and theory. We demonstrate that the push-pull electron capability of the bridge group governs the T₁ energy and electron mobility of these materials. Replacing the ether bridge by a bis(trifluoromethyl)methylene group can reduce the energy barrier for intramolecular electron exchange and consequently enhance the electron mobility by two orders of magnitude without lowering the T₁ energy. A blue TADF OLED employing this bis(trifluoromethyl)methylene-bridged compound achieves the same high external quantum efficiency but a much higher current density compared to the control device employing DPEPO. In contrast, a bridge group with strong electron-withdrawing capability, such as phosphine oxygen or sulfone, lowers the T₁ energy of the compound by enhancing the electronic coupling between TPPO subunits and inhibits intermolecular electron transfer by trapping the electron charge around the bridge.

High Vertical Carrier Mobilities of Organic Semiconductors Due to a Deposited Laid-Down Herringbone Structure Induced by a Reduced Graphene Oxide Template

High vertical carrier mobilities in organic semiconductor films are a challenging issue for fundamentally improving the performance of vertical devices. To achieve improvement in the vertical direction, a reduced graphene oxide (rGO) template is used with pentacene and DNTT having a herringbone structure enabling two-dimensional (2D) transport in comparison with CuPc having a slipped-stack structure. A thin-film structure and the optoelectrical properties of the oriented films are investigated with respect to molecular structures and packing modes. The rGO template induces a "laid-down" herringbone structure for pentacene and DNTT with a face-on orientation. Our results reveal that intermolecular dispersion energy is an additional important factor to form face-on states of molecules and influences face-on ratios in the films on rGO. Vertical charge mobilities of the films are significantly enhanced by the rGO template. Particularly, the DNTT film with a laid-down herringbone structure produces a vertical mobility as high as 0.27 cm² V^-1 s^-1, one of the highest values for ordinary thin films with several hundred nanometer thickness. These findings suggest that 2D transport is advantageous for vertical carrier transport also.

The Effects of Side Chains on the Charge Mobilities and Functionalities of Semiconducting Conjugated Polymers beyond Solubilities

Recent decades have witnessed the rapid development of semiconducting polymers in terms of high charge mobilities and applications in transistors. Significant efforts have been made to develop various conjugated frameworks and linkers. However, studies are increasingly demonstrating that the side chains of semiconducting polymers can significantly affect interchain packing, thin film crystallinity, and thus semiconducting performance. Ways to modify the side alkyl chains to improve the interchain packing order and charge mobilities for conjugated polymers are first discussed. It is shown that modifying the branching chains by moving the branching points away from the backbones can boost the charge mobilities, which can also be improved through partially replacing branching chains with linear ones. Second, the effects of side chains with heteroatoms and functional groups are discussed. The siloxane-terminated side chains are utilized to enhance the semiconducting properties. The fluorinated alkyl chains are beneficial for improving both charge mobility and air stability. Incorporating H bonding group side chains can improve thin film crystallinities and boost charge mobilities. Notably, incorporating functional groups (e.g., glycol, tetrathiafulvalene, and thymine) into side chains can improve the selectivity of field-effect transistor (FET)-based sensors, while photochromic group containing side chains in conjugated polymers result in photoresponsive semiconductors and optically tunable FETs.

Conductivity and Density of States of New Polyphenylquinoline

Electrical, photoelectrical, and optical properties of thin films of a new heat-resistant polyphenylquinoline synthesized using facile methods were investigated. An analysis of the obtained temperature dependences of the dark conductivity and photoconductivity indicates the hopping mechanism of conductivity over localized states arranging at the energy distance of 0.8 eV from the Fermi level located inside the band gap of the investigated material. The optical band gap of the studied material was estimated from an analysis of the spectral dependences of the photoconductivity and absorption coefficient before (1.8–1.9 eV) and after (2.0–2.2 eV) annealing at temperatures exceeding 100 °C. The Gaussian character of the distribution of the localized states of density inside the band gap near the edges of the bands was established. A mechanism of changes in the optical band gap of the investigating polymer under its annealing is proposed.

Board-like Fused-Thiophene Liquid Crystals and their Benzene Analogs: Facile Synthesis, Self-Assembly, p-Type Semiconductivity, and Photoluminescence

Fused-thiophene discotic liquid crystals were designed and easily synthesized by Suzuki coupling and FeCl₃ oxidized tandem cyclodehydrogenation reactions, including homo- and cross-coupling reactions. The resulting hexagonal and rectangular columnar mesomorphic supramolecular structures formed were characterized by polarizing optical microscopy, differential scanning calorimetry, and small-angle X-ray scattering. The charge carrier transport properties in the mesophases of two of the synthesized fused-thiophene discogens were measured by transient photocurrent time-of-flight (TOF) technique, revealing fast hole transport values in the range of 10^-3 to 10^-2  cm²  V^-1  s^-1 , thus demonstrating potential applications in electronic devices. The luminescent sanidic mesogens, with different extended π-conjugated systems, also emit blue, green, or red light, with absolute photoluminescent quantum yields as high as 18 %.

Insights into High Conductivity of the Two-Dimensional Iodine-Oxidized sp2-c-COF

A recent experiment [ Jin , E. ; Science 2017 , 357 , 673 - 676 ] shows that the conductivity of a two-dimensional (2D) sp²-carbon-hybridized π-conjugated covalent organic framework (sp²-c-COF) can be enhanced by as much as 12 orders of magnitude after iodine oxidation processing. To understand the physical mechanism underlying such a huge increase in the conductivity, we perform multiscale computations and find that the high conductivity of the iodine-oxidized 2D COF can be attributed to both hole transfer and ion transfer within the 2D COF. The computed dominant charge distribution corresponding to the valence band maximum (VBM) suggests that the delocalized π electrons occur mostly at the active reaction sites. The computed low ionization energy at the active reaction sites further supports that the 2D COF tends to lose electrons during iodine oxidation and to yield cationic COF and anionic triiodide I₃^-. Complementary classical molecular dynamics simulation shows a relatively high anion conductivity of 13.63 × 10^-2 S m^-1, consistent with the high conductivity measured from the experiment (7.1 × 10^-2 S m^-1). Meanwhile, we find that the cations in 2D COF can also induce a shift of the Fermi level to cross the valence band, thereby enhancing the hole mobility to 86.75 cm² V^-1 s^-1. For proposing a potential application of the highly conductive iodine-oxidized 2D sp²-c-COF, we design a prototypical model of the 2D spirally wound lithium-ion battery and find that it exhibits enhanced stability than a typical electrolyte material.

Inhibition Effect of Graphene on Space Charge Injection and Accumulation in Low-Density Polyethylene

Space charge injection and accumulation is attracting much attention in the field of dielectric insulation especially for electronic devices, power equipment and so on. This paper proposes using the inhibition effect of graphene for the injection and accumulation of space charge in low-density polyethylene (LDPE). Scanning electron microscope (SEM) and transmission electron microscopy (TEM) images were employed to observe the dispersion of graphene with a two-dimensional structure in LDPE. The time-dependent space charge dynamic behaviors of graphene/LDPE nanocomposites with the filler content of 0, 0.003, 0.005, 0.007 and 0.01 wt % were characterized by the pulsed electro-acoustic (PEA) test at 40, 60 and 80 °C, and the charge mobility was evaluated by its depolarization processes. The experimental results show that for the undoped LDPE film, large amounts of space charges were injected from the electrodes into samples, especially at 60 and 80 °C. The graphene/LDPE nanocomposites with a filler content of 0.005 wt % could markedly suppress the space charge injection and accumulation even at 80 °C, which is attributed to the large quantities of graphene-polymer in interface regions. These interface regions introduced numbers of deep trap sites within the forbidden band of nanocomposites, which can reduce the de-trapping rate of charges and suppress the space charge accumulation in the polymer bulks. The graphene/LDPE nanocomposites are suggested for dielectric applications, intending the inhibition of space charge injection and accumulation.

Null Exciton Splitting in Chromophoric Greek Cross (+) Aggregate

Exciton interactions in molecular aggregates play a crucial role in tailoring the optical behaviour of π-conjugated materials. Though vital for optoelectronic applications, ideal Greek cross-dipole (α=90°) stacking of chromophores remains elusive. We report a novel Greek cross (+) assembly of 1,7-dibromoperylene-3,4,9,10-tetracarboxylic tetrabutylester (PTE-Br₂ ) which exhibits null exciton coupling mediated monomer-like optical characteristics in the crystalline state. In contrast, nonzero exciton coupling in X-type (α=70.2°, PTE-Br₀ ) and J-type (α=0°, θ=48.4°, PTE-Br₄ ) assemblies have perturbed optical properties. Additionally, the semi-classical Marcus theory of charge-transfer rates predicts a selective hole transport phenomenon in the orthogonally stacked PTE-Br₂ . Precise rotation angle dependent optoelectronic properties in crystalline PTE-Br₂ can have consequences in the rational design of novel π-conjugated materials for photonic and molecular electronic applications.

Light-Tuned DC Conductance of Anatase TiO₂ Nanotubular Arrays: Features of Long-Range Charge Transport

Experimental results related to the photoactivated dc conductance of anatase TiO₂ nanotubular arrays (aTNTAs) under pulsed irradiation by a laser light inside and outside the fundamental absorption band are presented. It is found that the mobility and diffusion coefficients of charge carriers in the examined aTNTA are extremely low due to a strong charge-phonon coupling, abundance of shallow traps, and hopping conductivity between adjacent nanotubes. We consider that the confining electric field appeared within the array structure due to the difference in the local concentrations of excess electrons and holes at large values of the dc conductance suppresses the drift current. In this case, the dc conductance of such aTNTAs is mainly matured by the diffusion of mobile carriers. A recurrent kinetic model for evolution of the dc conductance of aTNTAs under laser irradiation has been proposed to interpret the experimental results.

A Facile Approach to Improve Interchain Packing Order and Charge Mobilities by Self-Assembly of Conjugated Polymers on Water

Development of facile and economic approaches for assembling organic semiconductors into more ordered structures toward high charge mobilities is highly demanding for the fabrication of organic circuits. Here a simple and facile approach is reported to prepare conjugated polymer thin films with improved crystallinities and charge mobilities by self-assembling semiconducting polymers on water. The formation of polymer thin films with more ordered structures is attributed to coffee ring effect induced by solvent-evaporation on water, and the hydrophobic nature of conjugated polymers that forces the polymer chains to pack densely and orderly on water surface. This approach is applicable to typical semiconducting polymers, and charge mobilities of their thin films are boosted remarkably. Finally, this new method can be utilized to easily fabricate the array of field-effect transistors with high charge mobilities in an economic way.

Fabrication of Completely Polymer-Based Solar Cells with p- and n-Type Semiconducting Block Copolymers with Electrically Inert Polystyrene

It is widely recognized that fullerene derivatives show several advantages as n-type materials in photovoltaic applications. However, conventional [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) exhibits weak absorption in the visible region, and poor morphological stability, due to the facile aggregation. For further improvement of the device performance and durability, utilization of n-type polymeric materials instead of PCBM is considered to be a good way to solve the problems. In this study, we fabricated completely polymer-based solar cells utilizing p- and n-type block copolymers consisting of poly(3-hexylthiophene) (P3HT) and poly{[N,N′-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} [P(NDI2OD-T2)], respectively, containing common polystyrene (PSt) inert blocks, which decreased the size of phase separated structures. Electron mobility in synthesized P(NDI2OD-T2)-b-PSt film enhanced by a factor of 8 compared with homopolymer. The root mean square roughness of the blend film of two block copolymers (12.2 nm) was decreased, compared with that of the simple homopolymers blend (18.8 nm). From the current density-voltage characteristics, it was confirmed that the introduction of PSt into both P3HT and P(NDI2OD-T2) improves short-circuit current density (1.16 to 1.73 mA cm⁻²) and power-conversion efficiency (0.24% to 0.32%). Better performance is probably due to the uniformity of the phase separation, and the enhancement of charge mobility.

Synergistic Effects of Selenophene and Extended Ladder-Type Donor Units for Efficient Polymer Solar Cells

Two pairs of polymer donor materials based on indacenodithiophene (IDT) and indacenodithieno[3,2-b]thiophene (IDTT) as the donor units are synthesized. Thiophene or selenophene is introduced as the π-bridge units and electron-deficient fluorine-substituted quinoxaline is used as acceptor unit. Selenophene-containing polymers PIDT-DFQ-Se and PIDTT-DFQ-Se show redshifted absorption and narrower bandgaps. Combined with IDTT donor unit, PIDTT-DFQ-Se shows the highest absorption coefficient. Both the IDTT unit and selenophene unit have positive effects on the hole mobilities, making PIDTT-DFQ-Se the highest one. The best power conversion efficiency of 7.4% is obtained from devices based on PIDTT-DFQ-Se:[6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₁ BM) with a J_sc of 12.6 mA cm^-2 , a V_oc of 0.89 V, and a fill factor (FF) of 0.66.

Rational In Silico Design of an Organic Semiconductor with Improved Electron Mobility

Organic semiconductors find a wide range of applications, such as in organic light emitting diodes, organic solar cells, and organic field effect transistors. One of their most striking disadvantages in comparison to crystalline inorganic semiconductors is their low charge-carrier mobility, which manifests itself in major device constraints such as limited photoactive layer thicknesses. Trial-and-error attempts to increase charge-carrier mobility are impeded by the complex interplay of the molecular and electronic structure of the material with its morphology. Here, the viability of a multiscale simulation approach to rationally design materials with improved electron mobility is demonstrated. Starting from one of the most widely used electron conducting materials (Alq₃ ), novel organic semiconductors with tailored electronic properties are designed for which an improvement of the electron mobility by three orders of magnitude is predicted and experimentally confirmed.

Exploiting Dispersion-Driven Aggregators as a Route to New One-Dimensional Organic Nanowires

The efficiency of charge carrier mobility in organic semiconductors is heavily dependent upon the long-range organization (i.e., morphology) and the local relative arrangement of the transporting molecules. Here, we exploit London dispersion forces as a design principle to construct compact one-dimensional (1-D) assemblies of quaterthiophene cores. We demonstrate that the substitution of quaterthiophene with dispersion-driven aggregators (e.g., [7]ladderanes, hydrogenated pyrenes, etc.) leads to the formation of highly stable and tightly packed 1-D supramolecular assemblies with electronic compactness superior to that of quaterthiophene crystals. Tunability and even tighter stacking arrangements can be achieved by inserting molecular linkers between the quaterthiophene fragments and the dispersion-driven components. The proposed 1-D nanowires represent an original route toward the rational design of efficient organic semiconductors.

Correlating Molecular Structures with Transport Dynamics in High-Efficiency Small-Molecule Organic Photovoltaics

Efficient charge transport is a key step toward high efficiency in small-molecule organic photovoltaics. Here we applied time-of-flight and organic field-effect transistor to complementarily study the influences of molecular structure, trap states, and molecular orientation on charge transport of small-molecule DRCN7T (D1) and its analogue DERHD7T (D2). It is revealed that, despite the subtle difference of the chemical structures, D1 exhibits higher charge mobility, the absence of shallow traps, and better photosensitivity than D2. Moreover, charge transport is favored in the out-of-plane structure within D1-based organic solar cells, while D2 prefers in-plane charge transport.

Efficient and balanced charge transport revealed in planar perovskite solar cells

Hybrid organic-inorganic perovskites have emerged as novel photovoltaic materials and hold great promise for realization of high-efficiency thin film solar modules. In this study, we unveil the ambipolar characteristics of perovskites by employing the transport measurement techniques of charge extraction by linearly increasing voltage (CELIV) and time-of-flight (TOF). These two complementary methods are combined to quantitatively determine the mobilities of hole and electron of CH3NH3PbI3 perovskite while revealing the recombination process and trap states. It is revealed that efficient and balanced transport is achieved in both CH3NH3PbI3 neat film and CH3NH3PbI3/PC61BM bilayer solar cells. Moreover, with the insertion of PC61BM, both hole and electron mobilities of CH3NH3PbI3 are doubled. This study offers a dynamic understanding of the operation of perovskite solar cells.

High efficiency air-processed dithienogermole-based polymer solar cells

The effect of air processing, with air exposure varying from minutes to hours prior to encapsulation, on photovoltaic device performance has been studied through a series of electrical characterizations and optical simulations for a donor/acceptor polymer-based organic solar cell based on poly(dithienogermole-alt-thienopyrrolodione) p(DTG-TPD)/PC71BM blends. A ∼10% degradation in power conversion efficiency was observed due to air processing with 10 min exposure time, with AM1.5 power conversion efficiencies (PCEs) decreasing from 8.5 ± 0.25% for devices processed in inert nitrogen atmosphere to 7.7 ± 0.18% for devices processed in ambient air. After 3 h air exposure, the PCE leveled off at 7.04 ± 0.1%. This decrease is attributed partially to interface issues caused by exposure of the electrode materials to oxygen and water and partially to a degradation of the hole transport in the active layer.

Theoretical Prediction of Isotope Effects on Charge Transport in Organic Semiconductors

We suggest that the nuclear tunneling effect is important in organic semiconductors, which we showed is absent in both the widely employed Marcus theory and the band-like transport as described by the deformation potential theory. Because the quantum nuclear tunneling tends to favor electron transfer while heavier nuclei decrease the quantum effect, there should occur an isotope effect for carrier mobility. For N,N'-n-bis(n-hexyl)-naphthalene diimide, electron mobility of all-deuteration on alkyls and all (13)C-substitution on the backbone decrease ∼18 and 7%, respectively. Similar isotope effects are found in the N,N'-n-bis(n-octyl)-perylene diimide. However, there is nearly no isotope effect for all-deuterated rubrene or tetracene. We have found that the isotopic effect only occurs when the substituted nuclei contribute actively to vibrations with appreciable charge reorganization energy and coupling with carrier motion. Thus, this prediction can shed light on the current dispute over the hopping versus band-like mechanisms in organic semiconductors.

Adjusting the Local Arrangement of π-Stacked Oligothiophenes through Hydrogen Bonds: A Viable Route to Promote Charge Transfer

We show that substituting quaterthiophene cores with strong H-bond aggregators, such as urea groups, provides an efficient way to adjust the mutual in-plane displacements of the semiconducting units and promote charge transfer. Our 2-D structure-property mapping reveals that the insertion of substituents induces up to 2.0 Å longitudinal and transversal displacements between the π-conjugated moieties. Some of these relative displacements lead to improved cofacial orbital overlaps that are otherwise inaccessible due to Pauli repulsion. Our results also emphasize that the fine-tuning of in-plane displacements is more effective than achieving "tighter" packing to promote charge-transfer properties.

Chain conformations dictate multiscale charge transport phenomena in disordered semiconducting polymers

Existing models for the electronic properties of conjugated polymers do not capture the spatial arrangement of the disordered macromolecular chains over which charge transport occurs. Here, we present an analytical and computational description in which the morphology of individual polymer chains is dictated by well-known statistical models and the electronic coupling between units is determined using Marcus theory. The multiscale transport of charges in these materials (high mobility at short length scales, low mobility at long length scales) is naturally described with our framework. Additionally, the dependence of mobility with electric field and temperature is explained in terms of conformational variability and spatial correlation. Our model offers a predictive approach to connecting processing conditions with transport behavior.