Alberto Salleo, Postdoctoral Faculty Sponsor
Charge Carrier Induced Structural Ordering And Disordering in Organic Mixed Ionic Electronic Conductors.
Advanced materials (Deerfield Beach, Fla.)
Operational stability underpins the successful application of organic mixed ionic-electronic conductors (OMIECs) in a wide range of fields, including biosensing, neuromorphic computing, and wearable electronics. In this work, we investigate both the operation and stability of a p-type OMIEC material of various molecular weights. Electrochemical transistor measurements reveal that device operation is very stable for at least 300 charging/discharging cycles independent of molecular weight, provided the charge density is kept below the threshold where strong charge-charge interactions become likely. When electrochemically charged to higher charge densities, we observe an increase in device hysteresis and a decrease in conductivity due to a drop in the hole mobility arising from long-range microstructural disruptions. By employing operando X-ray scattering techniques, we find two regimes of polaron-induced structural changes: 1) polaron-induced structural ordering at low carrier densities, and 2) irreversible structural disordering that disrupts charge transport at high carrier densities, where charge-charge interactions are significant. These operando measurements also reveal that the transfer curve hysteresis at high carrier densities is accompanied by an analogous structural hysteresis, providing a microstructural basis for such instabilities. This work provides a mechanistic understanding of the structural dynamics and material instabilities of OMIEC materials during device operation. This article is protected by copyright. All rights reserved.
View details for DOI 10.1002/adma.202310157
View details for PubMedID 38198654
- Tuning the Mobility of Indacenodithiophene-Based Conjugated Polymers via Coplanar Backbone Engineering CHEMISTRY OF MATERIALS 2023; 36 (1): 256-265
Polaron absorption in aligned conjugated polymer films: breakdown of adiabatic treatments and going beyond the conventional mid-gap state model.
This study provides the first experimental polarized intermolecular and intramolecular optical absorption components of field-induced polarons in regioregular poly(3-hexylthiophene-2,5-diyl), rr-P3HT, a polymer semiconductor. Highly aligned rr-P3HT thin films were prepared by a high temperature shear-alignment process that orients polymer backbones along the shearing direction. rr-P3HT in-plane molecular orientation was measured by electron diffraction, and out-of-plane orientation was measured through series of synchrotron X-ray scattering techniques. Then, with molecular orientation quantified, polarized charge modulation spectroscopy was used to probe mid-IR polaron absorption in the ℏω = 0.075 - 0.75 eV range and unambiguously assign intermolecular and intramolecular optical absorption components of hole polarons in rr-P3HT. This data represents the first experimental quantification of these polarized components and allowed long-standing theoretical predictions to be compared to experimental results. The experimental data is discrepant with predictions of polaron absorption based on an adiabatic framework that works under the Born-Oppenheimer approximation, but the data is entirely consistent with a more recent nonadiabatic treatment of absorption based on a modified Holstein Hamiltonian. This nonadiabatic treatment was used to show that both intermolecular and intramolecular polaron coherence break down at length scales significantly smaller than estimated structural coherence in either direction. This strongly suggests that polaron delocalization is fundamentally limited by energetic disorder in rr-P3HT.
View details for DOI 10.1039/d3mh01278f
View details for PubMedID 37982315
Controlling swelling in mixed transport polymers through alkyl side-chain physical cross-linking.
Proceedings of the National Academy of Sciences of the United States of America
2023; 120 (35): e2306272120
Semiconducting conjugated polymers bearing glycol side chains can simultaneously transport both electronic and ionic charges with high charge mobilities, making them ideal electrode materials for a range of bioelectronic devices. However, heavily glycolated conjugated polymer films have been observed to swell irreversibly when subjected to an electrochemical bias in an aqueous electrolyte. The excessive swelling can lead to the degradation of their microstructure, and subsequently reduced device performance. An effective strategy to control polymer film swelling is to copolymerize glycolated repeat units with a fraction of monomers bearing alkyl side chains, although the microscopic mechanism that constrains swelling is unknown. Here we investigate, experimentally and computationally, a series of archetypal mixed transporting copolymers with varying ratios of glycolated and alkylated repeat units. Experimentally we observe that exchanging 10% of the glycol side chains for alkyl leads to significantly reduced film swelling and an increase in electrochemical stability. Through molecular dynamics simulation of the amorphous phase of the materials, we observe the formation of polymer networks mediated by alkyl side-chain interactions. When in the presence of water, the network becomes increasingly connected, counteracting the volumetric expansion of the polymer film.
View details for DOI 10.1073/pnas.2306272120
View details for PubMedID 37603750
Role of aggregates and microstructure of mixed-ionic-electronic-conductors on charge transport in electrochemical transistors.
Synthetic efforts have delivered a library of organic mixed ionic-electronic conductors (OMIECs) with high performance in electrochemical transistors. The most promising materials are redox-active conjugated polymers with hydrophilic side chains that reach high transconductances in aqueous electrolytes due to volumetric electrochemical charging. Current approaches to improve transconductance and device stability focus mostly on materials chemistry including backbone and side chain design. However, other parameters such as the initial microstructure and microstructural rearrangements during electrochemical charging are equally important and are influenced by backbone and side chain chemistry. In this study, we employ a polymer system to investigate the fundamental electrochemical charging mechanisms of OMIECs. We couple in situ electronic charge transport measurements and spectroelectrochemistry with ex situ X-ray scattering electrochemical charging experiments and find that polymer chains planarize during electrochemical charging. Our work shows that the most effective conductivity modulation is related to electrochemical accessibility of well-ordered, interconnected aggregates that host high mobility electronic charge carriers. Electrochemical stress cycling induces microstructural changes, but we find that these aggregates can largely maintain order, providing insights on the structural stability and reversibility of electrochemical charging in these systems. This work shows the importance of material design for creating OMIECs that undergo structural rearrangements to accommodate ions and electronic charge carriers during which percolating networks are formed for efficient electronic charge transport.
View details for DOI 10.1039/d3mh00017f
View details for PubMedID 37089107
- Linking Phase Behavior to Performance Parameters in Non-Fullerene Acceptor Solar Cells ADVANCED ENERGY MATERIALS 2023
An ordered, self-assembled nanocomposite with efficient electronic and ionic transport.
Mixed conductors-materials that can efficiently conduct both ionic and electronic species-are an important class of functional solids. Here we demonstrate an organic nanocomposite that spontaneously forms when mixing an organic semiconductor with an ionic liquid and exhibits efficient room-temperature mixed conduction. We use a polymer known to form a semicrystalline microstructure to template ion intercalation into the side-chain domains of the crystallites, which leaves electronic transport pathways intact. Thus, the resulting material is ordered, exhibiting alternating layers of rigid semiconducting sheets and soft ion-conducting layers. This unique dual-network microstructure leads to a dynamic ionic/electronic nanocomposite with liquid-like ionic transport and highly mobile electronic charges. Using a combination of operando X-ray scattering and in situ spectroscopy, we confirm the ordered structure of the nanocomposite and uncover the mechanisms that give rise to efficient electron transport. These results provide fundamental insights into charge transport in organic semiconductors, as well as suggesting a pathway towards future improvements in these nanocomposites.
View details for DOI 10.1038/s41563-023-01476-6
View details for PubMedID 36797383
Influence of Side Chain Interdigitation on Strain and Charge Mobility of Planar Indacenodithiophene Copolymers.
ACS polymers Au
2023; 3 (1): 59-69
Indacenodithiophene (IDT) copolymers are a class of conjugated polymers that have limited long-range order and high hole mobilities, which makes them promising candidates for use in deformable electronic devices. Key to their high hole mobilities is the coplanar monomer repeat units within the backbone. Poly(indacenodithiophene-benzothiadiazole) (PIDTC16-BT) and poly(indacenodithiophene-thiapyrollodione) (PIDTC16-TPDC1) are two IDT copolymers with planar backbones, but they are brittle at low molecular weight and have unsuitably high elastic moduli. Substitution of the hexadecane (C16) side chains of the IDT monomer with isocane (C20) side chains was performed to generate a new BT-containing IDT copolymer: PIDTC20-BT. Substitution of the methyl (C1) side chain on the TPD monomer for an octyl (C8) and 6-ethylundecane (C13B) afford two new TPD-containing IDT copolymers named PIDTC16-TPDC8 and PIDTC16-TPDC13B, respectively. Both PIDTC16-TPDC8 and PIDTC16-TPDC13B are relatively well deformable, have a low yield strain, and display significantly reduced elastic moduli. These mechanical properties manifest themselves because the lengthened side chains extending from the TPD-monomer inhibit precise intermolecular ordering. In PIDTC16-BT, PIDTC20-BT and PIDTC16-TPDC1 side chain ordering can occur because the side chains are only present on the IDT subunit, but this results in brittle thin films. In contrast, PIDTC16-TPDC8 and PIDTC16-TPDC13B have disordered side chains, which seems to lead to low hole mobilities. These results suggest that disrupting the interdigitation in IDT copolymers through comonomer side chain extension leads to more ductile thin films with lower elastic moduli, but decreased hole mobility because of altered local order in the respective thin films. Our work, thus, highlights the trade-off between molecular packing structure for deformable electronic materials and provides guidance for designing new conjugated polymers for stretchable electronics.
View details for DOI 10.1021/acspolymersau.2c00034
View details for PubMedID 36785836
View details for PubMedCentralID PMC9912480
- Influence of Side Chain Interdigitation on Strain and Charge Mobility of Planar Indacenodithiophene Copolymers ACS POLYMERS AU 2022
Impact of Side Chain Hydrophilicity on Packing, Swelling and Ion Interactions in Oxy-bithiophene Semiconductors.
Advanced materials (Deerfield Beach, Fla.)
Exchanging hydrophobic alkyl-based side chains to hydrophilic glycol-based side chains is a widely adopted method for improving mixed-transport device performance, despite the impact on solid state packing and polymer-electrolyte interactions being poorly understood. Presented here is a Molecular Dynamics (MD) force field for modelling alkoxylated and glycolated polythiophenes. The force field is validated against known packing motifs for their monomer crystals. MD simulations, coupled with X-ray Diffraction (XRD), show that alkoxylated polythiophenes will pack with a 'tilted stack' and straight interdigitating side chains, whilst their glycolated counterpart will pack with a 'deflected stack' and an s-bend side chain configuration. MD simulations reveal water penetration pathways into the alkoxylated and glycolated crystals - through the pi-stack and through the lamellar stack respectively. Finally, the two distinct ways tri-ethylene glycol polymers can bind to cations are revealed, showing the formation of a meta-stable single bound state, or an energetically deep double bound state, both with a strong side chain length dependance. The minimum energy pathways for the formation of the chelates are identified, showing the physical process through which cations can bind to one or two side chains of a glycolated polythiophene, with consequences for ion transport in bithiophene semiconductors. This article is protected by copyright. All rights reserved.
View details for DOI 10.1002/adma.202204258
View details for PubMedID 35946142
Tuning Organic Electrochemical Transistor Threshold Voltage using Chemically Doped Polymer Gates.
Advanced materials (Deerfield Beach, Fla.)
Organic electrochemical transistors (OECTs) have shown promise as transducers and amplifiers of minute electronic potentials due to their large transconductances. Tuning OECT threshold voltage is important to achieve low-powered devices with amplification properties within the desired operational voltage range. However, traditional design approaches have struggled to decouple channel and materials properties from threshold voltage, thereby compromising on several other OECT performance metrics such as electrochemical stability, transconductance, and dynamic range. In this work, we utilize simple solution processing methods to chemically dope polymer gate electrodes, thereby controlling their work function, which in turn tunes the operation voltage range of OECTs without perturbing their channel properties. Chemical doping of initially air-sensitive polymer electrodes further improves their electrochemical stability in ambient conditions. Thus, we demonstrate, for the first time, OECTs which are simultaneously low-powered and electrochemically resistant to oxidative side reactions at ambient conditions. This approach shows that threshold voltage, which was once interwoven with other OECT properties, can in fact be an independent design parameter, expanding the design space of OECTs. This article is protected by copyright. All rights reserved.
View details for DOI 10.1002/adma.202202359
View details for PubMedID 35737653
- Critical analysis of self-doping and water-soluble n-type organic semiconductors: structures and mechanisms JOURNAL OF MATERIALS CHEMISTRY C 2022
Mixed Ionic-Electronic Conduction, a Multifunctional Property in Organic Conductors.
Advanced materials (Deerfield Beach, Fla.)
Organic mixed ionic-electronic conductors (OMIECs) have gained recent interest and rapid development due to their versatility in diverse applications ranging from sensing, actuation and computation to energy harvesting/storage, and information transfer. Their multifunctional properties arise from their ability to simultaneously participate in redox reactions as well as modulation of ionic and electronic charge density throughout the bulk of the material. Most importantly, the ability to access charge states with deep modulation through a large extent of its density of states and physical volume of the material enables OMIEC-based devices to display exciting new characteristics and opens up new degrees of freedom in device design. Leveraging the infinite possibilities of the organic synthetic toolbox, this perspective highlights several chemical and structural design approaches to modify OMIECs' properties important in device applications such as electronic and ionic conductivity, color, modulus, etc. Additionally, the ability for OMIECs to respond to external stimuli and transduce signals to myriad types of outputs has accelerated their development in smart systems. This perspective further illustrates how various stimuli such as electrical, chemical, and optical inputs fundamentally change OMIECs' properties dynamically and how these changes can be utilized in device applications.
View details for DOI 10.1002/adma.202110406
View details for PubMedID 35434865
- Redox-Active Polymers Designed for the Circular Economy of Energy Storage Devices ACS ENERGY LETTERS 2021; 6 (10): 3450-3457
- Ion Pair Uptake in Ion Gel Devices Based on Organic Mixed Ionic-Electronic Conductors ADVANCED FUNCTIONAL MATERIALS 2021
- A Stacked Hybrid Organic/Inorganic Electrochemical Random-Access Memory for Scalable Implementation ADVANCED ELECTRONIC MATERIALS 2021