Closing the loop between microstructure and charge transport in conjugated polymers by combining microscopy and simulation.

A grand challenge in materials science is to identify the impact of molecular composition and structure across a range of length scales on macroscopic properties. We demonstrate a unified experimental-theoretical framework that coordinates experimental measurements of mesoscale structure with molecular-level physical modeling to bridge multiple scales of physical behavior. Here we apply this framework to understand charge transport in a semiconducting polymer. Spatially-resolved nanodiffraction in a transmission electron microscope is combined with a self-consistent framework of the polymer chain statistics to yield a detailed picture of the polymer microstructure ranging from the molecular to device relevant scale. Using these data as inputs for charge transport calculations, the combined multiscale approach highlights the underrepresented role of defects in existing transport models. Short-range transport is shown to be more chaotic than is often pictured, with the drift velocity accounting for a small portion of overall charge motion. Local transport is sensitive to the alignment and geometry of polymer chains. At longer length scales, large domains and gradual grain boundaries funnel charges preferentially to certain regions, creating inhomogeneous charge distributions. While alignment generally improves mobility, these funneling effects negatively impact mobility. The microstructure is modified in silico to explore possible design rules, showing chain stiffness and alignment to be beneficial while local homogeneity has no positive effect. This combined approach creates a flexible and extensible pipeline for analyzing multiscale functional properties and a general strategy for extending the accesible length scales of experimental and theoretical probes by harnessing their combined strengths

Diffraction imaging of nanocrystalline structures in organic semiconductor molecular thin films

The properties of organic solids depend on their structure and morphology, yet direct imaging using conventional electron microscopy methods is hampered by the complex internal structure of these materials and their sensitivity to electron beams. Here, we manage to observe the nanocrystalline structure of two organic molecular thin-film systems using transmission electron microscopy by employing a scanning nanodiffraction method that allows for full access to reciprocal space over the size of a spatially localized probe (~2 nm). The morphologies revealed by this technique vary from grains with pronounced segmentation of the structure-characterized by sharp grain boundaries and overlapping domains-to liquid-crystal structures with crystalline orientations varying smoothly over all possible rotations that contain disclinations representing singularities in the director field. The results show how structure-property relationships can be visualized in organic systems using techniques previously only available for hard materials such as metals and ceramics

Hopping Transport and Rectifying Behavior in Long Donor–Acceptor Molecular Wires

We have developed a series of long donor (D)–acceptor (A) block molecular wires (D_<i>m</i>A_<i>n</i> or D_<i>m</i>CA_<i>n</i>: C, cyclohexane bridge; <i>m</i>, <i>n</i> = 1–4) attached to Au surfaces with lengths ranging from 3 to 10 nm in order to probe electrical rectification in the hopping regime. In each wire, the donor block was synthesized from the Au surface by stepwise imine condensation between 4,4′(5′)-diformyltetrathiafulvalene electron donors (D) and 1,4-diaminobenzene linkers, followed by the stepwise synthesis of the acceptor block using <i>N</i>,<i>N</i>′-di­(4-anilino)-1,2,4,5-benzenebis­(dicarboximide) electron acceptors (A) and terephthaldehyde linkers. Molecular junction measurements by conducting probe atomic force microscopy (CP-AFM) revealed that the D_<i>m</i>CA₁ (<i>m</i> = 1–4) wires exhibited electrical rectification with current rectification ratios as high as 30 at ±1.0 V when contacted with Au-coated tips and Au substrates; D_<i>m</i>A_<i>n</i> wires did not rectify, suggesting electronic decoupling of the D and A blocks is necessary for diode behavior. The forward bias condition for D_<i>m</i>CA₁ corresponded to negative potential on the acceptor block and positive potential on the donor block, as anticipated. Furthermore, the rectification ratio was a function of the wire architecture, length, and measurement temperature. Density functional theory (DFT) calculations of ground state neutral and ionized electronic structures and the experimental data for D_<i>m</i>CA₁ suggest that under forward bias the rate limiting transport step in these diodes is activated hole hopping from the HOMO level of the D block to the HOMO level of the A block; that is, hole-only transport pertains and it is sensitive to energy level alignment. Under reverse bias, the rate limiting transport step is relatively insensitive to temperature, which is consistent with a change in the rate limiting mechanism from hopping to tunneling. We propose a simple energy level model that rationalizes the change in transport mechanism and we suggest how these molecular diode structures might be further improved to achieve better rectification with simultaneous hole and electron transport in the D and A blocks, respectively

Charge Transport in 4 nm Molecular Wires with Interrupted Conjugation: Combined Experimental and Computational Evidence for Thermally Assisted Polaron Tunneling

We report the synthesis, transport measurements, and electronic structure of conjugation-broken oligophenyleneimine (CB-OPI 6) molecular wires with lengths of ∼4 nm. The wires were grown from Au surfaces using stepwise aryl imine condensation reactions between 1,4-diaminobenzene and terephthalaldehyde (1,4-benzenedicarbaldehyde). Saturated spacers (conjugation breakers) were introduced into the molecular backbone by replacing the aromatic diamine with <i>trans</i>-1,4-diaminocyclohexane at specific steps during the growth processes. FT-IR and ellipsometry were used to follow the imination reactions on Au surfaces. Surface coverages (∼4 molecules/nm²) and electronic structures of the wires were determined by cyclic voltammetry and UV–vis spectroscopy, respectively. The current–voltage (<i>I</i>–<i>V</i>) characteristics of the wires were acquired using conducting probe atomic force microscopy (CP-AFM) in which an Au-coated AFM probe was brought into contact with the wires to form metal-molecule-metal junctions with contact areas of ∼50 nm². The low bias resistance increased with the number of saturated spacers, but was not sensitive to the position of the spacer within the wire. Temperature dependent measurements of resistance were consistent with a localized charge (polaron) hopping mechanism in all of the wires. Activation energies were in the range of 0.18–0.26 eV (4.2–6.0 kcal/mol) with the highest belonging to the fully conjugated OPI 6 wire and the lowest to the CB3,5-OPI 6 wire (the wire with two saturated spacers). For the two other wires with a single conjugation breaker, CB3-OPI 6 and CB5-OPI 6, activation energies of 0.20 eV (4.6 kcal/mol) and 0.21 eV (4.8 kcal/mol) were found, respectively. Computational studies using density functional theory confirmed the polaronic nature of charge carriers but predicted that the semiclassical activation energy of hopping should be higher for CB-OPI molecular wires than for the OPI 6 wire. To reconcile the experimental and computational results, we propose that the transport mechanism is thermally assisted polaron tunneling in the case of CB-OPI wires, which is consistent with their increased resistance

Partial Fluorination as a Strategy for Crystal Engineering of Rubrene Derivatives

Through a close examination of the intermolecular interactions of rubrene (<b>1a</b>) and select derivatives (<b>1b</b>–<b>1p</b>), a clearer understanding of why certain fluorinated rubrene derivatives pack with planar tetracene backbones has been achieved. In this study we synthesized, crystallized, and determined the packing structure of new rubrene derivatives (<b>1h</b>–<b>p</b>). Previously, we proposed that introducing electron-withdrawing CF₃ substituents induced planarity by reducing intramolecular repulsion between the peripheral aryl groups (<b>1e</b>–<b>g</b>). However, we found that in most cases, further increasing the fluorine content of rubrene lead to twisted tetracene backbones in the solid state. To understand how rubrene (<b>1a</b>) and its derivatives (<b>1b</b>–<b>p</b>) pack in the solid state, we (re)­examined the crystal structures through a systematic study of the close contacts. We found that planar tetracene cores occur when close contacts organize to produce an <i>S</i> symmetry element about a given rubrene molecule. We report the first instance of rubrene derivatives (<b>1l</b> and <b>1n</b>) that pack in a two-dimensional brick motif. The prospects for new rubrene derivatives in semiconductors were estimated by calculating the reorganization energies of the monomers and transfer integrals of the dimers we observed. Our work allows for the rational design and improved crystal engineering of new rubrene derivatives