Orthogonal Ambipolar Semiconductor Nanostructures for Complementary Logic Gates

We report orthogonal ambipolar semiconductors that exhibit hole and electron transport in perpendicular directions based on aligned films of nanocrystalline "shish-kebabs" containing poly(3-hexylthiophene) (P3HT) and N,N'-di-n-octyl-3,4,9,10-perylenetetracarboxylic diimide (PDI) as p- and n-type components, respectively. Polarized optical microscopy, scanning electron microscopy, and X-ray diffraction measurements reveal a high degree of in-plane alignment. Relying on the orientation of interdigitated electrodes to enable efficient charge transport from either the respective p- or n-channel materials, we demonstrate semiconductor films with high anisotropy in the sign of charge carriers. Films of these aligned crystalline semiconductors were used to fabricate complementary inverter devices, which exhibited good switching behavior and a high noise margin of 80% of 1/2 Vdd. Moreover, complementary "NAND" and "NOR" logic gates were fabricated and found to exhibit excellent voltage transfer characteristics and low static power consumption. The ability to optimize the performance of these devices, simply by adjusting the solution concentrations of P3HT and PDI, makes this a simple and versatile method for preparing ambipolar organic semiconductor devices and high-performance logic gates. Further, we demonstrate that this method can also be applied to mixtures of PDI with another conjugated polymer, poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene]) (PBTTT), with better hole transport characteristics than P3HT, opening the door to orthogonal ambipolar semiconductors with higher performance.

The Structural Origin of Electron Injection Enhancements with Fulleropyrrolidine Interlayers

The orientation of the substituent groups in a new class of work function modification layers, based on functionalized fulleropyrrolidines, is measured and found to directly account for the sign of the work function change.

Real-space visualization of conformation-independent oligothiophene electronic structure

We present scanning tunneling microscopy and spectroscopy (STM/STS) investigations of the electronic structures of different alkyl-substituted oligothiophenes on the Au(111) surface. STM imaging showed that on Au(111), oligothiophenes adopted distinct straight and bent conformations. By combining STS maps with STM images, we visualize, in real space, particle-in-a-box-like oligothiophene molecular orbitals. We demonstrate that different planar conformers with significant geometrical distortions of oligothiophene backbones surprisingly exhibit very similar electronic structures, indicating a low degree of conformation-induced electronic disorder. The agreement of these results with gas-phase density functional theory calculations implies that the oligothiophene interaction with the Au(111) surface is generally insensitive to molecular conformation.

Solvent washing with toluene enhances efficiency and increases reproducibility in perovskite solar cells

The fabrication process such as the amount of washing solvent, spin rates and time for reproducible preovskite solar cell has been optimized.

Controlled Directional Crystallization of Oligothiophenes Using Zone Annealing of Preseeded Thin Films

We demonstrate a simple route to directionally grow crystals of oligothiophenes, based on 2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene with degrees of polymerization of 2 (BTTT-2) and 4 (BTTT-4) via zone annealing (ZA) of preseeded films. ZA of spun-cast films of BTTT-2 does not yield highly aligned crystals. However, if the film is oven-annealed briefly prior to ZA, highly aligned crystals that are millimeters in length can be grown, whose length depends on the velocity of the ZA front. The precrystallized region provides existing nuclei that promote crystal growth and limit nucleation of new crystals in the melted region. Aligned crystals of BTTT-2 can be obtained even when the moving velocity for ZA is an order of magnitude greater than the crystal growth rate. The relative nucleation rate to the crystallization rate for BTTT-4 is greater than that for BTTT-2, which decreases the length over which BTTT-4 can be aligned to ∼500 μm for the conditions examined. The temperature gradient and moving velocity of ZA enable control of the length of the aligned crystalline structure at the macroscale.

Water Processable Polythiophene Nanowires by Photo-Cross-Linking and Click-Functionalization

Replacing or minimizing the use of halogenated organic solvents in the processing and manufacturing of conjugated polymer-based organic electronics has emerged as an important issue due to concerns regarding toxicity, environmental impact, and high cost. To date, however, the processing of well-ordered conjugated polymer nanostructures has been difficult to achieve using environmentally benign solvents. In this work, we report the development of water and alcohol processable nanowires (NWs) with well-defined crystalline nanostructure based on the solution assembly of azide functionalized poly(3-hexylthiophene) (P3HT-azide) and subsequent photo-cross-linking and functionalization of these NWs. The solution-assembled P3HT-azide NWs were successfully cross-linked by exposure to UV light, yielding good thermal and chemical stability. Residual azide units on the photo-cross-linked NWs were then functionalized with alkyne terminated polyethylene glycol (PEG-alkyne) using copper catalyzed azide-alkyne cycloaddition chemistry. PEG functionalization of the cross-linked P3HT-azide NWs allowed for stable dispersion in alcohols and water, while maintaining well-ordered NW structures with electronic properties suitable for the fabrication of organic field effect transistors (OFETs).

Adsorption-induced conformational isomerization of alkyl-substituted thiophene oligomers on Au(111): impact on the interfacial electronic structure

Alkyl-substituted quaterthiophenes on Au(111) form dimers linked by their alkyl substituents and, instead of adopting the trans conformation found in bulk oligothiophene crystals, assume cis conformations. Surprisingly, the impact of the conformation is not decisive in determining the lowest unoccupied molecular orbital energy. Scanning tunneling microscopy and spectroscopy of the adsorption geometries and electronic structures of alkyl-substituted quaterthiophenes show that the orbital energies vary substantially because of local variations in the Au(111) surface reactivity. These results demonstrate that interfacial oligothiophene conformations and electronic structures may differ substantially from those expected based on the band structures of bulk oligothiophene crystals.

Selective Nucleation of Poly(3-hexyl thiophene) Nanofibers on Multilayer Graphene Substrates

We demonstrate that graphene surfaces provide highly selective nucleation of poly(3-hexyl thiophene) (P3HT) nanofibers (NFs) from supersaturated solutions. Solvent conditions are identified that give rise to a wide hysteresis between crystallization and melting centered around room temperature, yielding metastable solutions that are stable against homogeneous nucleation for long periods of time but that allow for heterogeneous nucleation by graphene. Selective growth of P3HT crystals is found for multilayer graphene (MLG) supported on either Si or ITO substrates, with nucleation kinetics that are more rapid for MLG on Si but slower in both cases than for highly oriented pyrolytic graphite (HOPG). Although the NFs grow vertically from the substrate with face-on orientation of P3HT chains, we observe edge-on orientation in dried films, presumably due to capillary forces that cause collapse of the NFs onto the substrate during solvent evaporation.

Rubrene crystal field-effect mobility modulation via conducting channel wrinkling

With the impending surge of flexible organic electronic technologies, it has become essential to understand how mechanical deformation affects the electrical performance of organic thin-film devices. Organic single crystals are ideal for the systematic study of strain effects on electrical properties without being concerned about grain boundaries and other defects. Here we investigate how the deformation affects the field-effect mobility of single crystals of the benchmark semiconductor rubrene. The wrinkling instability is used to apply local strains of different magnitudes along the conducting channel in field-effect transistors. We discover that the mobility changes as dictated by the net strain at the dielectric/semiconductor interface. We propose a model based on the plate bending theory to quantify the net strain in wrinkled transistors and predict the change in mobility. These contributions represent a significant step forward in structure-function relationships in organic semiconductors, critical for the development of the next generation of flexible electronic devices.

Unconventional, chemically stable, and soluble two-dimensional angular polycyclic aromatic hydrocarbons: from molecular design to device applications

Polycyclic aromatic hydrocarbons (PAHs), consisting of laterally fused benzene rings, are among the most widely studied small-molecule organic semiconductors, with potential applications in organic field-effect transistors (OFETs) and organic photovoltaics (OPVs). Linear acenes, including tetracene, pentacene, and their derivatives, have received particular attention due to the synthetic flexibility in tuning their chemical structure and properties and to their high device performance. Unfortunately, longer acenes, which could exhibit even better performance, are susceptible to oxidation, photodegradation, and, in solar cells which contain fullerenes, Diels-Alder reactions. This Account highlights recent advances in the molecular design of two-dimensional (2-D) PAHs that combine device performance with environmental stability. New synthetic techniques have been developed to create stable PAHs that extend conjugation in two dimensions. The stability of these novel compounds is consistent with Clar's sextet rule as the 2-D PAHs have greater numbers of sextets in their ground-state configuration than their linear analogues. The ionization potentials (IPs) of nonlinear acenes decrease more slowly with annellation in comparison to their linear counterparts. As a result, 2-D bistetracene derivatives that are composed of eight fused benzene rings are measured to be about 200 times more stable in chlorinated organic solvents than pentacene derivatives with only five fused rings. Single crystals of the bistetracene derivatives have hole mobilities, measured in OFET configuration, up to 6.1 cm(2) V(-1) s(-1), with remarkable Ion/Ioff ratios of 10(7). The density functional theory (DFT) calculations can provide insight into the electronic structures at both molecular and material levels and to evaluate the main charge-transport parameters. The 2-D acenes with large aspect ratios and appropriate substituents have the potential to provide favorable interstack electronic interactions, and correspondingly high carrier mobilities. In stark contrast to the 1-D acenes that form mono- and bis-adducts with fullerenes, 2-D PAHs show less reactivity with fullerenes. The geometry of 2-D PAHs plays a crucial role in determining both the barrier and the adduct stability. The reactivity and stability of the 2-D PAHs with regard to Diels-Alder reactions at different reactive sites were explained via DFT calculations of the reaction kinetics and of thermodynamics of reactions and simple Hückel molecular orbital considerations. Also, because of their increased stability in the presence of fullerenes, these compounds have been successfully used in OPVs. The small-molecule semiconductors highlighted in this Account exhibit good charge-transport properties, comparable to those of traditional linear acenes, while being much more environmentally stable. These features have made these 2-D PAHs excellent molecules for fundamental research and device applications.

Controlled integration of oligo- and polythiophenes at the molecular scale

Hole mobilities greater than 0.1 cm² V⁻¹ s⁻¹ are attained in films that contain over 80% oligomer.

Molecular weight dependence of the morphology in P3HT:PCBM solar cells

In polymer-based photovoltaic devices, optimizing and controlling the active layer morphology is important to enhancing the device efficiency. Using poly(3-hexylthiophene) (P3HT) with well-defined molecular weights (MWs), synthesized by the Grignard metathesis (GRIM) method, we show that the morphology of the photovoltaic active layer and the absorption and crystal structure of P3HT are dependent on the MW. Differential scanning calorimetry showed that the crystallinity of P3HT reached a maximum for intermediate MWs. Grazing-incidence wide-angle X-ray diffraction showed that the spacing of the (100) planes of P3HT increased with increasing MW, while the crystal size decreased. Nonlinear crystal lattice expansions were found for both the (100) and (020) lattice planes, with an unusual π-π-stacking enhancement observed between 50 and 100 °C. The melting point depression for P3HT, when mixed with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), and, hence, the Flory-Huggins interaction parameter depended on the MW. PCBM was found to perturb the ordering of P3HT chains. In photovoltaic devices, P3HT with a MW of ∼20K showed the best device performance. The morphologies of these blends were studied by grazing-incidence small-angle X-ray scattering (GISAXS) and resonant soft X-ray scattering. In GISAXS, we observed that the low-molecular-weight P3HT more readily crystallizes, promoting a phase-separated morphology.

Intrinsic and extrinsic parameters for controlling the growth of organic single-crystalline nanopillars in photovoltaics

The most efficient architecture for achieving high donor/acceptor interfacial area in organic photovoltaics (OPVs) would employ arrays of vertically interdigitated p- and n- type semiconductor nanopillars (NPs). Such morphology could have an advantage in bulk heterojunction systems; however, precise control of the dimension morphology in a crystalline, interpenetrating architecture has not yet been realized. Here we present a simple, yet facile, crystallization technique for the growth of vertically oriented NPs utilizing a modified thermal evaporation technique that hinges on a fast deposition rate, short substrate-source distance, and ballistic mass transport. A broad range of organic semiconductor materials is beneficial from the technique to generate NP geometries. Moreover, this technique can also be generalized to various substrates, namely, graphene, PEDOT-PSS, ZnO, CuI, MoO3, and MoS2. The advantage of the NP architecture over the conventional thin film counterpart is demonstrated with an increase of power conversion efficiency of 32% in photovoltaics. This technique will advance the knowledge of organic semiconductor crystallization and create opportunities for the fabrication and processing of NPs for applications that include solar cells, charge storage devices, sensors, and vertical transistors.

Accurate atomic displacement parameters from time-of-flight neutron-diffraction data at TOPAZ

Accurate atomic displacement parameters (ADPs) are a good indication of high-quality diffraction data. Results from the newly commissioned time-of-flight Laue diffractometer TOPAZ at the SNS are presented. Excellent agreement is found between ADPs derived independently from the neutron and X-ray data emphasizing the high quality of the data from the time-of-flight Laue diffractometer.

Reversible, self cross-linking nanowires from thiol-functionalized polythiophene diblock copolymers

Poly(3-hexylthiophene)-block-poly(3-(3-thioacetylpropyl) oxymethylthiophene) (P3HT)-b-(P3TT) diblock copolymers were synthesized and manipulated by solvent-induced crystallization to afford reversibly cross-linked semiconductor nanowires. To cross-link the nanowires, we deprotected the thioacetate groups to thiols and they subsequently oxidized to disulfides. Cross-linked nanowires maintained their structural integrity in solvents that normally dissolve the polymers. These robust nanowires could be reduced to the fully solvated polymer, representing a novel, reversible cross-linking procedure for functional P3HT-based nanowire fibrils. Field-effect transistor measurements were carried out to determine the charge transport properties of these nanostructures.

Oligothiophene semiconductors: synthesis, characterization, and applications for organic devices

Oligothiophenes provide a highly controlled and adaptable platform to explore various synthetic, morphologic, and electronic relationships in organic semiconductor systems. These short-chain systems serve as models for establishing valuable structure-property relationships to their polymer analogs. In contrast to their polymer counterparts, oligothiophenes afford high-purity and well-defined materials that can be easily modified with a variety of functional groups. Recent work by a number of research groups has revealed functionalized oligothiophenes to be the up-and-coming generation of advanced materials for organic electronic devices. In this review, we discuss the synthesis and characterization of linear oligothiophenes with a focus on applications in organic field effect transistors and organic photovoltaics. We will highlight key structural parameters, such as crystal packing, intermolecular interactions, polymorphism, and energy levels, which in turn define the device performance.