Solid-state Charge Transport Research Articles

Electronic Properties of [Ru(NH3)6][M(CN)6] Double-Complex Crystals and Their Thermal Decomposition to Prussian Blue Analogs

The complex ions, [Ru(NH3)6]2+/3+ and [Fe(CN)6]3-/4-, are well known redox couples exhibiting fast interfacial and homogeneous electron transfer rates in aqueous solution. Here we show that when mixed together, they form insoluble double-complex Ax[Ru(NH3)6][Fe(CN)6] crystals with rhombohedral symmetry, where A is a cationic species and 0≤x≤2. The analogous interactions of [Ru(NH3)6]2+/3+ with [Ru(CN)6]3-/4- produces Ax[Ru(NH3)6][Fe(CN)6] crystals. Electrical measurements of these single crystals exhibit ohmic current-voltage responses, with corresponding conductivities on the order of 0.1 mS/m. We propose a solid-state charge transport mechanism based on Marcus theory for homogeneous outer-sphere electron transfer.Thermal decomposition at mild temperatures removes ammine ligands and transforms these crystals into nanopolycrystalline Prussian blue analogs (PBAs) that retain the macroscopic geometry of the double-complex crystals but adopt the familiar highly defected porous crystal structure and intervalence charge transfer characteristics of PBAs.

Deciphering the Roles of Interfacial Amino Acids in Inter-Protein Charge Transport.

Elucidating charge transport (CT) through proteins is critical for gaining insights into ubiquitous CT chain reactions in biological systems and developing high-performance bioelectronic devices. While intra-protein CT has been extensively studied, crucial knowledge about inter-protein CT via interfacial amino acids is still absent due to the structural complexity. Herein, by loading cytochrome c (Cyt c) on well-defined peptide self-assembled monolayers to mimic the protein-protein interface, we provide a precisely controlled platform for identifying the roles of interfacial amino acids in solid-state CT via peptide-Cyt c junctions. The terminal amino acid of peptides serves as a fine-tuning factor for both the interfacial interaction between peptides and Cyt c and the immobilized Cyt c orientation, resulting in a nearly 10-fold difference in current through peptide-Cyt c junctions with varied asymmetry. This work provides a valuable platform for studying CT across proteins and contributes to the understanding of fundamental principles governing inter-protein CT.

Elucidating Design Rules toward Enhanced Solid-State Charge Transport in Oligoether-Functionalized Dioxythiophene-Based Alternating Copolymers.

This study investigates the solid-state charge transport properties of the oxidized forms of dioxythiophene-based alternating copolymers consisting of an oligoether-functionalized 3,4-propylenedioxythiophene (ProDOT) copolymerized with different aryl groups, dimethyl ProDOT (DMP), 3,4-ethylenedioxythiophene (EDOT), and 3,4-phenylenedioxythiophene (PheDOT), respectively, to yield copolymers P(OE3)-D, P(OE3)-E, and P(OE3)-Ph. At a dopant concentration of 5 mM FeTos3, the electrical conductivities of these copolymers vary significantly (ranging between 9 and 195 S cm-1) with the EDOT copolymer, P(OE3)-E, achieving the highest electrical conductivity. UV-vis-NIR and X-ray spectroscopies show differences in both susceptibility to oxidative doping and extent of oxidation for the P(OE3) series, with P(OE3)-E being the most doped. Wide-angle X-ray scattering measurements indicate that P(OE3)-E generally demonstrates the lowest paracrystallinity values in the series, as well as relatively small π-π stacking distances. The significant (i.e., order of magnitude) increase in electrical conductivity of doped P(OE3)-E films versus doped P(OE3)-D or P(OE3)-Ph films can therefore be attributed to P(OE3)-E exhibiting both the highest carrier ratios in the P(OE3) series, along with good π-π overlap and local ordering (low paracrystallinity values). Furthermore, these trends in the extent of doping and paracrystallinity are consistent with the reduced Fermi energy level and transport function prefactor parameters calculated using the semilocalized transport (SLoT) model. Observed differences in carrier ratios at the transport edge (ct) and reduced Fermi energies [η(c)] suggest a broader electronic band (better overlap and more delocalization) for the EDOT-incorporating P(OE3)-E polymer relative to P(OE3)-D and P(OE3)-Ph. Ultimately, we rationalize improvements in electrical conductivity due to microstructural and doping enhancements caused by EDOT incorporation, a structure-property relationship worth considering in the future design of highly electrically conductive systems.

Secondary Structure-Dependent Charge Transport in iLOV Protein Junctions.

The composition and structure of proteins are crucial for charge migration in the solid-state charge transport (CTp). Despite much progress, it is still challenging to explore the relationship between conformational change and CTp in the complex protein system. Herein, we design three improved light-oxygen-voltage (iLOV) domains, and efficiently regulate the CTp of the iLOV self-assembled monolayers (SAMs) by pH induced conformation variation. The current density can be controlled in the range of one order of magnitude. Interestingly, the CTp of iLOV displays negative linear relations with the β-sheet contents. Single-level Landauer fitting and transition voltage spectroscopy analysis suggest that β-sheet-dependent CTp would be related to the coupling between iLOV and electrodes. This work proposes a new strategy to explore the CTp in complex molecular system. Our findings deepen the understanding on protein structure-CTp relationship, and provide predictive mode of protein CTp responses for the design of functional bioelectronics.

Base Pair Stacking Modulates Solid-State Charge Transport in DNA Hairpins.

Particular interest has been focused on modulation of solid-state charge transport (CT) in DNA. Nevertheless, it remains challenging to do so in a sensitive and predictive manner due to the lack of a definite relationship between DNA base pair stacking and DNA CT. The challenges can be mainly attributed to the ill-defined systems, which may lead to ambiguous and even contradictory conclusions. Here, we use DNA hairpins to construct the well-defined self-assembled monolayers. We reveal nearly positive-linear correlations between DNA conformation and CT in the DNA hairpins regulated with metal ion chelation and DNA sequence. The correlations have been confirmed by the solid-state current-voltage characteristics and circular dichroism in solution. The enhanced CT via metal ion chelated DNA hairpins is mainly from the improved DNA energy coupling to electrodes, not the almost unchanged energy barrier when Hg ion-induced DNA conformational switches toward the canonical B-form.

Photocatalyzed electron exchange between organic chromophores and hematite nanoparticles and the role of solid-state charge transport

An increase in the fluorescence lifetime of rhodamine B dye, when in contact with hematite nanoparticles in methanol from mildly acidic to alkaline pH, is consistent with the transfer of electrons from excited dye to the hematite conduction band.

Impact of the Synthesis Method on the Solid-State Charge Transport of Radical Polymers.

There are conflicting reports in the literature about the presence of room temperature conductivity in poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), a redox active polymer with radical groups pendent to an insulating backbone. To understand the variability in the findings across the literature and synthetic methods, we prepared PTMA using three living methods - anionic, ATRP and RAFT polymerization. We find that all three synthetic methods produce PTMA with radical yields of 70 - 80%, controlled molecular weight, and low dispersity. Additionally, we used on-chip EPR to probe the robustness of radical content in solid films under ambient air and light, and found negligible change in the radical content over time. Electrically, we found that PTMA is highly insulating - conductivity in the range 10-11 S/cm - regardless of the synthetic method of preparation. These findings provide greater clarity for potential applications of PTMA in energy storage.

Correction: Impact of the synthesis method on the solid-state charge transport of radical polymers

Correction for ‘Impact of the synthesis method on the solid-state charge transport of radical polymers’ by Yiren Zhang et al., J. Mater. Chem. C, 2018, 6, 111–118.

Aqueous Processing for Printed Organic Electronics: Conjugated Polymers with Multistage Cleavable Side Chains.

The ability to process conjugated polymers via aqueous solution is highly advantageous for reducing the costs and environmental hazards of large scale roll-to-roll processing of organic electronics. However, maintaining competitive electronic properties while achieving aqueous solubility is difficult for several reasons: (1) Materials with polar functional groups that provide aqueous solubility can be difficult to purify and characterize, (2) many traditional coupling and polymerization reactions cannot be performed in aqueous solution, and (3) ionic groups, though useful for obtaining aqueous solubility, can lead to a loss of solid-state order, as well as a screening of any applied bias. As an alternative, we report a multistage cleavable side chain method that combines desirable aqueous processing attributes without sacrificing semiconducting capabilities. Through the attachment of cleavable side chains, conjugated polymers have for the first time been synthesized, characterized, and purified in organic solvents, converted to a water-soluble form for aqueous processing, and brought through a final treatment to cleave the polymer side chains and leave behind the desired electronic material as a solvent-resistant film. Specifically, we demonstrate an organic soluble polythiophene that is converted to an aqueous soluble polyelectrolyte via hydrolysis. After blade coating from an aqueous solution, UV irradiation is used to cleave the polymer’s side chains, resulting in a solvent-resistant, electroactive polymer thin film. In application, this process results in aqueous printed materials with utility for solid-state charge transport in organic field effect transistors (OFETs), along with red to colorless electrochromism in ionic media for color changing displays, demonstrating its potential as a universal method for aqueous printing in organic electronics.

(Invited) Solid-State Charge Transport in Redox-Active Radical Polymers

Elucidating the nanostructural and charge transport behavior of redox-active polymers has led to the implementation of these critical macromolecules in a number of energy conversion and energy storage applications. To date, however, the majority of the work associated with charge-conducting polymers has focused on systems where a large degree of π-conjugation exists along the macromolecular backbone. Here, we move away from this archetype through the design, characterization, and utilization of an emerging class of materials, radical polymers. In this context, we define radical polymers to be macromolecules that consist of non-conjugated backbones and contain stable radical sites on the pendant groups of each repeat unit. These stable radical sites are capable of undergoing oxidation-reduction reactions in a rather facile manner if there is a large population of the radical sites in relatively close (i.e., ~1 nm) proximity. In turn, this ability allows for charge to be transported in the solid state and in a straightforward manner despite the fact that these completely amorphous materials have very localized charge sites. Because the charge transfer sites are independent of the macromolecular architecture, this allows for myriad potential designs of radical polymers. Furthermore, because of this electronic decoupling between the pendant groups and the macromolecular backbone, the Singularly Occupied Molecular Orbital (SOMO) charge transport level of the radical group can be predicted easily. This information can be used in a straightforward manner to match energy and charge transfer levels of other electronically-active inorganic and organic materials. As such, radical polymers are beginning to emerge as useful materials in a number of hybrid and organic electronic applications. Here, we quantify the solid-state electrical conductivity and hole mobility of a model radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), for the first time. In particular, we establish that the solid-state conductivity (~10-3 S m-1) and the space charge-limited hole mobility (~10-4 cm2 V-1 s-1) are on par with common conjugated polymer semiconductors [e.g., poly(3-hexylthiophene) (P3HT)]. Furthermore, these charge transport properties are found to be highly dependent on the exact chemical nature of the stable radical pendant groups. This is confirmed using a variety of spectroscopic techniques that are coupled to electronic characterization measurements. Conversely, due to the localized nature of the charge transfer groups and the fact that these materials are well into the glassy state at room temperature, charge transport is found to be relatively temperature-independent, in contrast to many conjugated organic semiconductors. By developing these fundamental structure-property-performance relationships in radical polymers, we have been able to demonstrate their high performance ability in applications where transparent conducting polymer thin films are required [e.g., high efficiency organic photovoltaic (OPV) devices]. In particular, we demonstrate that insertion of the redox-active polymer at the organic semiconductor/metal electrode interface improves charge extraction and device lifetime of inverted polymer solar cells based on a blend of P3HT and a soluble fullerene derivative relative to devices that lack this redox-active polymer interlayer. Finally, we demonstrate that the readily-tuned synthesis of radical polymers affords the opportunity to generate well-defined block polymers in a straightforward manner. We demonstrate that these materials self-assemble easily on the ~30 nm scale, and that they form ordered thin film nanostructures. This opens the possibility of creating nanostructured electron conducting-ion conducting pathways using a single material. As such, radical polymer-based block polymers could be of immense promise in future solid-state flexible battery devices as well. Therefore, we will demonstrate the ability to use carefully-designed redox-active radical polymer chemistries and nanostructures to impact important energy conversion and energy storage devices in a positive manner.

Quantification of the solid-state charge mobility in a model radical polymer

We establish that an oft-used radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), has a solid-state hole mobility value on the order of 10−4 cm2 V−1 s−1 in a space charge-limited device geometry. Despite being completely amorphous and lacking any π-conjugation, these results demonstrate that the hole mobility of PTMA is comparable to many well-studied conjugated polymers [e.g., poly(3-hexylthiophene)]. Furthermore, we show that the space charge-limited charge carrier mobility of these macromolecules is only a weak function of temperature, in contrast to many thermally-activated models of charge transport in polymeric materials. This key result demonstrates that the charge transport in radical polymers is inherently different than that in semicrystalline, conjugated polymers. These results establish the mechanism of solid-state charge transport in radical polymers and provide macromolecular design principles for this emerging class of organic electronic materials.

Derivation of the Weber-Fechner law and the Loewenstein equation as the steady-state response of an Elovich solid state biological system

The Weber-Fechner law (response of an organism is a linear function of log of stimulus) has been widely used for description of physiological and psychological data for many years. It is shown here that the Weber-Fechner law is derivable in a simple way from the Elovich equation (−dx/dt=m exp(nx), which is observed experimentally in numerous physiological and biochemical systems, and which has a simple derivation from solid state charge transport across interfaces in these systems. It therefore seems reasonable to interpret data conforming to the Webner-Fechner law to imply that the observed phenomenon is rate-limited by interfacial charge transport in the cell. By a similar analysis, the Loewenstein equation, which may be considered an exact form of the Weber-Fechner law applicable to data over a wider range of values of the variables, is derived from a more exact form of the Elovich equation.