Molecular Semiconductors in Organic Photovoltaic Cells

Stereoregular Brush Polymers and Graft Copolymers by Chiral Zirconocene-Mediated Coordination Polymerization of P3HT Macromers

Two poly(3-hexylthiophene) (P3HT) macromers containing a donor polymer with a polymerizable methacrylate (MA) end group, P3HT-CH₂-MA and P3HT-(CH₂)₂-MA, have been synthesized, and P3HT-(CH₂)₂-MA has been successfully homopolymerized and copolymerized with methyl methacrylate (MMA) into stereoregular brush polymers and graft copolymers, respectively, using chiral ansa-zirconocene catalysts. Macromer P3HT-CH₂-MA is too sterically hindered to polymerize by the current Zr catalysts, but macromer P3HT-(CH₂)₂-MA is readily polymerizable via either homopolymerization or copolymerization with MMA in a stereospecific fashion with both C₂-ligated zirconocenium catalyst 1 and Cs-ligated zirconocenium catalyst 2. Thus, highly isotactic (with mm% ≥ 92%) and syndiotactic (with rr% ≥ 93%) brush polymers, it-PMA-g-P3HT and st-PMA-g-P3HT, as well as well-defined stereoregular graft copolymers with different grafted P3HT densities, it-P(M)MA-g-P3HT and st-P(M)MA-g-P3HT, have been synthesized using this controlled coordination-addition polymerization system under ambient conditions. These stereoregular brush polymers and graft copolymers exhibit both thermal (glass and melting) transitions with Tg and Tm values corresponding to transitions within the stereoregular P(M)MA and crystalline P3HT domains. Acceptor molecules such as C60 can be effectively encapsulated inside the helical cavity of st-P(M)MA-g-P3HT to form a unique supramolecular helical crystalline complex, thus offering a novel strategy to control the donor/acceptor solar cell domain morphology.

n-Type Doped Conjugated Polymer for Nonvolatile Memory

This study demonstrates a facile way to efficiently induce strong memory behavior from common p-type conjugated polymers by adding n-type dopant 2-(2-methoxyphenyl)-1,3-dimethyl-2,3-dihydro-1H-benzoimidazole. The n-type doped p-channel conjugated polymers not only enhance n-type charge transport characteristics of the polymers, but also facilitate to storage charges and cause reversible bistable (ON and OFF states) switching upon application of gate bias. The n-type doped memory shows a large memory window of up to 47 V with an on/off current ratio larger than 10 000. The charge retention time can maintain over 100 000 s. Similar memory behaviors are also observed in other common semiconducting polymers such as poly(3-hexyl thiophene) and poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene], and a high mobility donor-acceptor polymer, poly(isoindigo-bithiophene). In summary, these observations suggest that this approach is a general method to induce memory behavior in conjugated polymers. To the best of the knowledge, this is the first report for p-type polymer memory achieved using n-type charge-transfer doping.

Progress in Understanding Degradation Mechanisms and Improving Stability in Organic Photovoltaics

Understanding the degradation mechanisms of organic photovoltaics is particularly important, as they tend to degrade faster than their inorganic counterparts, such as silicon and cadmium telluride. An overview is provided here of the main degradation mechanisms that researchers have identified so far that cause extrinsic degradation from oxygen and water, intrinsic degradation in the dark, and photo-induced burn-in. In addition, it provides methods for researchers to identify these mechanisms in new materials and device structures to screen them more quickly for promising long-term performance. These general strategies will likely be helpful in other photovoltaic technologies that suffer from insufficient stability, such as perovskite solar cells. Finally, the most promising lifetime results are highlighted and recommendations to improve long-term performance are made. To prevent degradation from oxygen and water for sufficiently long time periods, OPVs will likely need to be encapsulated by barrier materials with lower permeation rates of oxygen and water than typical flexible substrate materials. To improve stability at operating temperatures, materials will likely require glass transition temperatures above 100 °C. Methods to prevent photo-induced burn-in are least understood, but recent research indicates that using pure materials with dense and ordered film morphologies can reduce the burn-in effect.

Enhancement of third-order nonlinear optical susceptibility of Alq3 in polar aprotic solvents

The influence of solvent polarity on nonlinear optical properties of tris-(8-hydroxyquinoline)-aluminum (Alq₃) was investigated by the degenerate four-wave mixing method at the 532 nm. It was obtained that the effective values of the third-order nonlinear optical susceptibility (χeff⟨3⟩) and the second-order hyperpolarizability (γ_eff) of Alq₃ depend on the solvent polarity. Additionally, it was found that Alq₃ dissolved in dimethyl sulfoxide has the highest values of χeff⟨3⟩ and γ_eff. Furthermore, two Stegeman's figures of merit were also calculated. The obtained results suggest that Alq₃ is also promising material for application in all-optical signal processing devices.

Indeno[1,2-b]fluorene-based novel donor–acceptor conjugated copolymers

A series of conjugated copolymers based on indeno[1,2-b]fluorene as donor unit with different acceptor units have been synthesized to explore the influences of molecular backbone planarization and acceptor electronegativity on charge transport and photovoltaic properties. Polymer incorporating 2,3-diphenylquinoxaline acceptor moiety shows poor light-harvesting capacity and inferior photovoltaic efficiency of 0.5% due to twisted geometry. By introducing the stronger acceptor thiadiazolo[3,4-c]pyridine in polymer, intramolecular charge transfer is enhanced, giving rise to improved absorption property and photovoltaic efficiency of 1.39%. However, the polymer backbone is still twisted. When thiophene-flanked diketopyrrolopyrrole (DPP) is incorporated as electron acceptor, the polymer exhibits a more planar molecular geometry, yielding a broader and red-shifted absorption spectrum as well as a significantly improved hole mobility of 1.46E-2 cm2 V−1 s−1. However, the photovoltaic device efficiency is only enhanced to be 1.69%. The low-lying lowest unoccupied molecular orbital of −3.95 eV as a result of the strong electron deficiency of the DPP unit may lead to the inefficient charge dissociation and increase the charge recombination, which may give rise to the limit photovoltaic performance.

Radical induced intermolecular linkage and energy level modifications of a porphyrin monolayer

A new method to directly modify the surface structure and energy levels of a porphyrin monolayer was examined in the molecular scale using scanning tunneling microscopy and spectroscopy (STM and STS) and presented in this communication. The exposure to atomic oxygen has induced highly ordered surface cross-linking and changed the occupied and unoccupied orbital levels of a cobalt(ii) octaethyl porphyrin (CoOEP) monolayer, and as a result, the HOMO-LUMO gap was reduced by ∼10%. Counterintuitively, the STM/STS data indicated that the reactive central Co atoms did not participate in the gas-surface reactions. Reflection-absorption infrared spectroscopy (RAIRS) measurements further indicated that the STM observed intermolecular linkages are stabilized via hydrogen bonding. This CoOEP + O˙ system also illustrates an example that the six-fold surface packing symmetry predominates the four-fold molecular symmetry in producing a three-fold symmetric surface cross-linking structure.

Thermoactivated Electrical Conductivity in Perylene Diimide Nanofiber Materials

Thermoactivated electrical conductivity has been studied on nanofibers fabricated from the derivatives of perylene tetracarboxylic diimide (PTCDI) both in the dark and under visible light illumination. The activation energy obtained for the nanofibers fabricated from donor-acceptor (D-A) PTCDIs are higher than that for symmetric n-dodecyl substituted PTCDI. Such difference originates from the strong dependence of thermoactivated charge hopping on material disorder, which herein is dominated by the D-A charge-transfer and dipole-dipole interactions between stacked molecules. When the nanofibers were heated above the first phase transition temperature (around 85 °C), the activation energy was significantly increased because of the thermally enhanced polaronic effect. Moreover, charge carrier density can be increased in the D-A nanofibers under visible light illumination. Consistent with the theoretical models in the literature, the increased charge carrier density did cause decrease in the activation energy due to the up-shifting of Fermi level closer to the conduction band edge.

Multipodal arrangement of push–pull chromophores: a fundamental parameter affecting their electronic and optical properties

Branching, along with the acceptor nature and π-system length, was revealed as a crucial factor affecting the fundamental optoelectronic properties of push–pull molecules.

Loss channels in triplet–triplet annihilation photon upconversion: importance of annihilator singlet and triplet surface shapes

Differences in triplet–triplet annihilation photon upconversion efficiencies between structurally similar annihilators can be understood in terms of singlet and triplet surface shapes.

Frontiers of solvent-free functional molecular liquids

The breakthrough of functional molecular liquids (FMLs) in cutting-edge research and their fundamental liquid features on the basis of molecular architectures are highlighted in this Feature Article.

Efficient small molecule photovoltaic donor based on 2,3-diphenyl-substituted quinoxaline core for solution-processed organic solar cells

An excellent PCE of 6.25% was achieved based on a novel OSM (TPACN)2Qx containing 2,3-diphenyl-substituted quinoxaline (Qx) as electrophilic core.

Self-assembled coronene nanofiber arrays: toward integrated organic bioelectronics for efficient isolation, detection, and recovery of cancer cells

Integrated coronene-based nanofiber array devices for circulating tumor cell isolation, detection, and recovery through electrical stimulation.

Supramolecular architectures self-assembled using long chain alkylated spin crossover cobalt(ii) compounds

Self-assembled hybrid supramolecular architectures formed between amphiphilic anion and long alkylated cationic cobalt(ii) complexes have been synthesized and characterized.

Design of Diketopyrrolopyrrole (DPP)-Based Small Molecules for Organic-Solar-Cell Applications

After the first report in 2008, diketopyrrolopyrrole (DPP)-based small-molecule photovoltaic materials have been intensively explored. The power conversion efficiencies (PCEs) for the DPP-based small-molecule donors have been improved up to 8%. Furthermore, through judicious structure modification, DPP-based small molecules can also be converted into electron-acceptor materials, and, recently, some exciting progress has been achieved. The development of DPP-based photovoltaic small molecules is summarized here, and the photovoltaic performance is discussed in relation to structural modifications, such as the variations of donor-acceptor building blocks, alkyl substitutions, and the type of conjugated bridges, as well as end-capped groups. It is expected that the discussion will provide a guideline in the exploration of novel and promising DPP-containing photovoltaic small molecules.

Not Only Columns: High Hole Mobility in a Discotic Nematic Mesophase Formed by Metal-Containing Porphyrin-Core Dendrimers

We report a new family of multifunctional liquid-crystalline porphyrin-core dendrimers that have coumarin functional groups around the porphyrin core. Porphyrin metalation strongly affects the photophysical properties, and therefore Zn^II and Cu^II derivatives have also been prepared. All the synthesized dendrimers form a nematic discotic mesophase. Their high tendency for homeotropic alignment makes these dendrimers excellent candidates for device applications, owing to their easy processability, spontaneous alignment between electrodes, and self-healing of defects because of their dynamic nature. The charge mobility values of these materials are the highest ever reported for a nematic discotic phase. Moreover, these values are similar to the highest values reported for ordered columnar mesophases, and this shows that a supramolecular organization in columns is not necessary to achieve high charge mobility.

Synthesis and spectroscopic properties of photochromic dithienylethene-functionalized subphthalocyanine conjugate

A subphthalocyanine-dithienylethene dyad has been synthesized and characterized by 1H-, [Formula: see text]C-NMR, HR-MS, UV-visible and emission spectroscopy. The results show that photoinduced isomerization of dithienylethene moiety from close-form to opened form can be achieved under visible light using subphthalocyanine as a light-harvesting unit and the fluorescence properties of subphthalocyanine could be modulated by the isomerization state of the dithienylethene moiety.

Organic Optoelectronic Materials: Mechanisms and Applications

Organic (opto)electronic materials have received considerable attention due to their applications in thin-film-transistors, light-emitting diodes, solar cells, sensors, photorefractive devices, and many others. The technological promises include low cost of these materials and the possibility of their room-temperature deposition from solution on large-area and/or flexible substrates. The article reviews the current understanding of the physical mechanisms that determine the (opto)electronic properties of high-performance organic materials. The focus of the review is on photoinduced processes and on electronic properties important for optoelectronic applications relying on charge carrier photogeneration. Additionally, it highlights the capabilities of various experimental techniques for characterization of these materials, summarizes top-of-the-line device performance, and outlines recent trends in the further development of the field. The properties of materials based both on small molecules and on conjugated polymers are considered, and their applications in organic solar cells, photodetectors, and photorefractive devices are discussed.

Sequence-defined oligo(ortho-arylene) foldamers derived from the benzannulation of ortho(arylene ethynylene)s

A Cu-catalyzed benzannulation reaction transforms ortho(arylene ethynylene) oligomers into ortho-arylenes.

A Cu-catalyzed benzannulation reaction transforms ortho(arylene ethynylene) oligomers into ortho-arylenes. This approach circumvents iterative Suzuki cross-coupling reactions previously used to assemble hindered ortho-arylene backbones. These derivatives form helical folded structures in the solid-state and in solution, as demonstrated by X-ray crystallography and solution-state NMR analysis. DFT calculations of misfolded conformations are correlated with variable-temperature ¹H and EXSY NMR to reveal that folding is cooperative and more favorable in halide-substituted naphthalenes. Helical ortho-arylene foldamers with specific aromatic sequences organize functional π-electron systems into arrangements ideal for ambipolar charge transport and show preliminary promise for the surface-mediated synthesis of structurally defined graphene nanoribbons.

Chemical Self-Doping of Organic Nanoribbons for High Conductivity and Potential Application as Chemiresistive Sensor

Intrinsically low electrical conductivity of organic semiconductors hinders their further development into practical electronic devices. Herein, we report on an efficient chemical self-doping to increase the conductivity through one-dimensional stacking arrangement of electron donor-acceptor (D-A) molecules. The D-A molecule employed was a 1-methylpiperidine-substituted perylene tetracarboxylic diimide (MP-PTCDI), of which the methylpiperidine moiety is a strong electron donor, and can form a charge transfer complex with PTCDI (acting as the acceptor), generating anionic radical of PTCDI as evidenced in molecular solutions. Upon self-assembling into nanoribbons through columnar π-π stacking, the intermolecular charge transfer interaction between methylpiperidine and PTCDI would be enhanced, and the electrons generated are delocalized along the π-π stacking of PTCDIs, leading to enhancement in conductivity. The conductive fiber materials thus produced can potentially be used as chemiresistive sensor for vapor detection of electron deficient chemicals such as hydrogen peroxide, taking advantage of the large surface area of nanofibers. As a major component of improvised explosives, hydrogen peroxide remains a critical signature chemical for public safety screening and monitoring.

Highly Efficient Inverted Perovskite Solar Cells With Sulfonated Lignin Doped PEDOT as Hole Extract Layer

Sulfonated-acetone-formaldehyde (SAF) was grafted with alkali lignin (AL) to prepare grafted sulfonated-acetone-formaldehyde lignin (GSL). Considering the rich phenolic hydroxyl groups in GSL, we detected a hole mobility of 2.27 × 10(-6) cm(2) V(-1) s(-1) with GSL as a hole transport material by space-charge-limited current model. Compared with nonconjugated poly(styrene sulfonic acid), GSL was applied as p-type semiconductive dopant for PEDOT to prepare water-dispersed

PEDOT

GSL.

PEDOT

GSL shows enhanced conductivity compared with that of

PEDOT

PSS. Simultaneously, the enhanced open-circuit voltage, short-circuit current density, and fill factor are achieved using

PEDOT

GSL as a hole extract layer (HEL) in sandwich-structure inverted perovskite solar cells. The power conversion efficiency is increased to 14.94% compared with 12.6% of

PEDOT

PSS-based devices. Our results show that amorphous GSL is a good candidate as dopant of PEDOT, and we provide a novel prospective for the design of HEL based on lignin, a renewable biomass and phenol derivatives.

Structural, electronic, and magnetic properties of tris(8-hydroxyquinoline)iron(III) molecules and their magnetic coupling with ferromagnetic surface: first-principles study

Using first-principles calculations, we have systematically investigated the structural, electronic, and magnetic properties of facial (fac-) and meridional (mer-) tris(8-hydroxyquinoline)iron(III) (Feq3) molecules and their interaction with ferromagnetic substrate. Our calculation results show that for the isolated Feq3, mer-Feq3 is more stable than the fac-Feq3; both Feq3 isomers have a high spin-state of 5 μB as the ground state when an on-site Hubbard-U term is included to treat the highly localized Fe 3d electrons; while the standard DFT calculations produce a low spin-state of 1 μB for mer-Feq3. These magnetic behaviors can be understood by the octahedral ligand field splitting theory. Furthermore, we found that fac-Feq3 has a stronger bonding to the Co surface than mer-Feq3 and an anti-ferromagnetic coupling was discovered between Fe and Co substrate, originating from the superexchange coupling between Fe and Co mediated by the interface oxygen and nitrogen atoms. These findings suggest that Feq3 molecular films may serve as a promising spin-filter material in spintronic devices.

Aligning self-assembled gelators by drying under shear

We show how drying under shear can be used to prepare aligned fibres and worm-like micelles from low molecular weight gelators. Shearing followed by drying leads to the dealignment before the water can be removed; continuous shear whilst drying is required to maintain the alignment. Combining a slow pH change with continuous shear allows alignment of the gelling fibres, which can then be dried.

Photon-absorbing charge-bridging states in organic bulk heterojunctions consisting of diketopyrrolopyrrole derivatives and PCBM

We have investigated the photo- and electrochemical properties of five diketopyrrolopyrrole (DPP) derivatives both experimentally and theoretically. In the experimental study, we found that a blend of a DPP derivative named D2 and phenyl-C61-butyric acid methyl ester (PCBM) exhibits the highest internal quantum efficiency (IQE) and power convergence efficiency (PCE) among the five derivatives investigated. In the theoretical study, we found that the open-circuit voltage can be estimated from the difference between the energy gap of frontier orbitals and the voltage loss and that the latter is suppressed when the IQE is large. Then, to investigate the factors that influence the IQE, investigations on charge recombination, hole transfer, and charge transfer induced by photoabsorption were conducted for the complexes of each DPP derivative and PCBM. It was found that D2/PCBM exhibits the largest charge-bridging upon photoabsorption, which leads to the highest IQE and PCE among the five DPP derivatives.

Syntheses, Charge Separation, and Inverted Bulk Heterojunction Solar Cell Application of Phenothiazine-Fullerene Dyads

A series of phenothiazine-fulleropyrrolidine (PTZ-C60) dyads having fullerene either at the C-3 aromatic ring position or at the N-position of phenothiazine macrocycle were newly synthesized and characterized. Photoinduced electron transfer leading to PTZ(•+)-C60(•-) charge-separated species was established from studies involving femtosecond transient absorption spectroscopy. Because of the close proximity of the donor and acceptor entities, the C-3 ring substituted PTZ-C60 dyads revealed faster charge separation and charge recombination processes than that observed in the dyad functionalized through the N-position. Next, inverted organic bulk heterojunction (BHJ) solar cells were constructed using the dyads in place of traditionally used [6,6]-phenyl-C61- butyric acid methyl ester (PCBM) and an additional electron donor material poly(3-hexylthiophene) (P3HT). The performance of the C-3 ring substituted PTZ-C60 dyad having a polyethylene glycol substituent produced a power conversion efficiency of 3.5% under inverted bulk heterojunction (BHJ) configuration. This was attributed to optimal BHJ morphology between the polymer and the dyad, which was further promoted by the efficient intramolecular charge separation and relatively slow charge recombination promoted by the dyad within the BHJ structure. The present finding demonstrate PTZ-C60 dyads as being good prospective materials for building organic photovoltaic devices.

Organic Light-Emitting Transistors: Materials, Device Configurations, and Operations

Organic light-emitting transistors (OLETs) represent an emerging class of organic optoelectronic devices, wherein the electrical switching capability of organic field-effect transistors (OFETs) and the light-generation capability of organic light-emitting diodes (OLEDs) are inherently incorporated in a single device. In contrast to conventional OFETs and OLEDs, the planar device geometry and the versatile multifunctional nature of OLETs not only endow them with numerous technological opportunities in the frontier fields of highly integrated organic electronics, but also render them ideal scientific scaffolds to address the fundamental physical events of organic semiconductors and devices. This review article summarizes the recent advancements on OLETs in light of materials, device configurations, operation conditions, etc. Diverse state-of-the-art protocols, including bulk heterojunction, layered heterojunction and laterally arranged heterojunction structures, as well as asymmetric source-drain electrodes, and innovative dielectric layers, which have been developed for the construction of qualified OLETs and for shedding new and deep light on the working principles of OLETs, are highlighted by addressing representative paradigms. This review intends to provide readers with a deeper understanding of the design of future OLETs.

Preparation, optical and electrochemical properties, and molecular orbital calculations of tetraazaporphyrinato ruthenium (II) bis(4-methylpyridine) fused with one to four diphenylthiophene units

2,5-Diphenyl-3,4-dicyanothiophene (1) and phthalonitrile (2) were mixed and treated with ruthenium (III) trichloride, 4-methylpyridine, and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in 2-ethoxyethanol at 135°C, to produce low-symmetrical tetraazaporphyrins (TAPs) (3), (4), (5), and (6) with one to three thiophene rings. Two thiophene-annelated tetraazaporphyrins were isolated as opposite and adjacent isomers 4 and 5. The structure of 3 was determined by X-ray crystallography, showing that the thiophene ring linked at the 3,4-positions on the tetraazaporphyrin scaffold deviates from the mean plane of the four central pyrrole nitrogen atoms (N1-N3-N5-N7). Optical and electrochemical properties of the products were examined by UV-vis and magnetic circular dichroism (MCD) spectroscopy, together with cyclic voltammetry. In the (1)H NMR spectra, the signals of 4-methylpyridine coordinating to the central ruthenium atom appeared at a higher magnetic field than those of uncoordinated 4-methylpyridine itself due to the shielding effect of the TAP ring. Increasing the number of fused thiophene rings resulted in 1) lower magnetic field shifts of the signals of axially coordinated 4-methylpyridine in the (1)H NMR spectra, 2) lower energy shifts of the Q band absorption in the UV-vis spectra, and 3) decreasing (cathodic shift) of the first oxidation potentials. The structures of simplified model compounds were optimized using the DFT method with the Gaussian 09 program at the B3LYP/LANL2DZ level for the Ru atom and the B3LYP/6-31G (d, p) level for the C, H, N, and S atoms. The optimized structures were utilized to calculate the NMR shielding constants, the HOMO and LUMO orbital energies, and the electronic absorption spectra.

Mechanoresponsive Luminescent Molecular Assemblies: An Emerging Class of Materials

The possibility to change the molecular assembled structures of organic and organometallic materials through mechanical stimulation is emerging as a general and powerful concept for the design of functional materials. In particular, the photophysical properties such as photoluminescence color, quantum yield, and emission lifetime of organic and organometallic fluorophores can significantly depend on the molecular packing, enabling the development of molecular materials with mechanoresponsive luminescence characteristics. Indeed, an increasing number of studies have shown in recent years that mechanical force can be utilized to change the molecular arrangement, and thereby the optical response, of luminescent molecular assemblies of π-conjugated organic or organometallic molecules. Here, the development of such mechanoresponsive luminescent (MRL) molecular assemblies consisting of organic or organometallic molecules is reviewed and emerging trends in this research field are summarized. After a brief introduction of mechanoresponsive luminescence observed in molecular assemblies, the concept of "luminescent molecular domino" is introduced, before molecular materials that show turn-on/off of photoluminescence in response to mechanical stimulation are reviewed. Mechanically stimulated multicolor changes and water-soluble MRL materials are also highlighted and approaches that combine the concept of MRL molecular assemblies with other materials types are presented in the last part of this progress report.

Charge transport dependent high open circuit voltage tandem organic photovoltaic cells with low temperature deposited HATCN-based charge recombination layers

Mechanisms of charge transport between the interconnector and its neighboring layers in tandem organic photovoltaic cells have been systematically investigated by studying electronic properties of the involving interfaces with photoelectron spectroscopies and performance of the corresponding devices. The results show that charge recombination occurs at HATCN and its neighboring hole transport layers which can be deposited at low temperature. The hole transport layer plays an equal role to the interconnector itself. These insights provide guidance for the identification of new materials and the device architecture for high performance devices.

Supramolecular Assembly of pH-Sensitive Triphenylene Derived π-Gelators and Their Application as Molecular Template for the Preparation of Silica Nanotubes

The gelation properties and mode of self-assembly of six asymmetrical hexaether triphenylene derivatives mono-functionalized with carboxylic and primary amine groups were investigated. The presence of a carboxylic and amine group attached to the triphenylene core generated stable, thermo- and pH-sensitive supramolecular π-organogels with a reversible response to both stimuli. In order to understand the gelation process, we studied the effect of the spacer length and found a different gelation scope for the acid and basic derivatives that accounts for a different supramolecular self-assembly. The presence of the basic group on the amino derivatives was used to guide and catalyze the templated in situ sol-gel polymerization of TEOS and allowed us, under controlled hydrolytic conditions, to prepare an entangled fibrillar network of silica nanotubes.

Strain effects on the work function of an organic semiconductor

Establishing fundamental relationships between strain and work function (WF) in organic semiconductors is important not only for understanding electrical properties of organic thin films, which are subject to both intrinsic and extrinsic strains, but also for developing flexible electronic devices. Here we investigate tensile and compressive strain effects on the WF of rubrene single crystals. Mechanical strain induced by thermal expansion mismatch between the substrate and rubrene is quantified by X-ray diffraction. The corresponding WF change is measured by scanning Kelvin probe microscopy. The WF of rubrene increases (decreases) significantly with in-plane tensile (compressive) strain, which agrees qualitatively with density functional theory calculations. An elastic-to-plastic transition, characterized by a steep rise of the WF, occurs at ∼0.05% tensile strain along the rubrene π-stacking direction. The results provide the first concrete link between mechanical strain and WF of an organic semiconductor and have important implications for understanding the connection between structural and electronic disorder in soft organic electronic materials.