Ruiheng Wu

All Publications

• Bridging length scales in organic mixed ionic-electronic conductors through internal strain and mesoscale dynamics. Nature materials Wu, R., Meli, D., Strzalka, J., Narayanan, S., Zhang, Q., Paulsen, B. D., Rivnay, J., Takacs, C. J. 2024

  Abstract

  Understanding the structural and dynamic properties of disordered systems at the mesoscale is crucial. This is particularly important in organic mixed ionic-electronic conductors (OMIECs), which undergo significant and complex structural changes when operated in an electrolyte. In this study, we investigate the mesoscale strain, reversibility and dynamics of a model OMIEC material under external electrochemical potential using operando X-ray photon correlation spectroscopy. Our results reveal that strain and structural hysteresis depend on the sample's cycling history, establishing a comprehensive kinetic sequence bridging the macroscopic and microscopic behaviours of OMIECs. Furthermore, we uncover the equilibrium and non-equilibrium dynamics of charge carriers and material-doping states, highlighting the unexpected coupling between charge carrier dynamics and mesoscale order. These findings advance our understanding of the structure-dynamics-function relationships in OMIECs, opening pathways for designing and engineering materials with improved performance and functionality in non-equilibrium states during device operation.

  View details for DOI 10.1038/s41563-024-01813-3

  View details for PubMedID 38409601

• Time-Resolved Structural Kinetics of an Organic Mixed Ionic-Electronic Conductor. Advanced materials (Deerfield Beach, Fla.) Paulsen, B. D., Wu, R., Takacs, C. J., Steinruck, H., Strzalka, J., Zhang, Q., Toney, M. F., Rivnay, J. 2020: e2003404

  Abstract

  The structure and packing of organic mixed ionic-electronic conductors have an especially significant effect on transport properties. In operating devices, this structure is not fixed but is responsive to changes in electrochemical potential, ion intercalation, and solvent swelling. Toward this end, the steady-state and transient structure of the model organic mixed conductor, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), is characterized using multimodal time-resolved operando techniques. Steady-state operando X-ray scattering reveals a doping-induced lamellar expansion of 1.6 A followed by 0.4 A relaxation at high doping levels. Time-resolved operando X-ray scattering reveals asymmetric rates of lamellar structural change during doping and dedoping that do not directly depend on potential or charging transients. Time-resolved spectroscopy establishes a link between structural transients and the complex kinetics of electronic charge carrier subpopulations, in particular the polaron-bipolaron equilibrium. These findings provide insight into the factors limiting the response time of organic mixed-conductor-based devices, and present the first real-time observation of the structural changes during doping and dedoping of a conjugated polymer system via X-ray scattering.

  View details for DOI 10.1002/adma.202003404

  View details for PubMedID 32864811

• Charge state-dependent ion condensation near conjugated polymer backbones MATERIALS HORIZONS Meli, D., Thomas, Q., Rolland, N., Freychet, G., Kousseff, C. J., Cavassin, P., Flagg, L. Q., Lemaur, V., Surendran, A., Hamid, Z., Griggs, S., Wu, R., Huerta, R. A., Duplessis, I. D., Paulsen, B. D., Marks, T. J., Lauhon, L. J., McCulloch, I., Richter, L. J., Beljonne, D., Rivnay, J. 2025

  Abstract

  Despite the technological appeal of polymeric organic mixed ionic/electronic conductors (OMIECs) for diverse applications, a deep understanding of the fundamentals of mixed charge transport in these materials, especially regarding the complex interplay between polymer, ion and solvent structure in determining transport, is lacking. Herein, extensive molecular dynamics (MD) simulations of a model OMIEC representing various electrochemically gated states are reported that reveal charge state-dependent counterion condensation. X-ray diffraction simulations based on the MD data predict a measurable change in the scattering intensity at the counterion absorption edge, indicative of counterion repositioning with charging. We leverage an operando resonant X-ray scattering technique to experimentally corroborate the simulated scattering and report excellent agreement between predicted and experimental data, confirming that counterions preferentially reside in the lamellar mid-plane of crystallites at low doping, and near the polymer backbone at higher doping. Driving forces for ion type-dependent spatial repositioning and implications thereof are discussed.

  View details for DOI 10.1039/d5mh01939g

  View details for Web of Science ID 001623316800001

  View details for PubMedID 41295917

• The Influence of Alkyl Spacers and Molecular Weight on the Charge Transport and Storage Properties of Oxy-Bithiophene-Based Conjugated Polymers. Angewandte Chemie (International ed. in English) Yu, H., Marks, A., Tuladhar, S. M., Siemons, N., Anderson, I., Bidinger, S., Keene, S. T., Quill, T. J., Wu, R., Gough, O., Wu, G., Eisner, F., Salleo, A., Rivnay, J., Malliaras, G. G., Barnes, P. R., McCulloch, I., Nelson, J. 2024: e202417897

  Abstract

  Conjugated polymers (CPs) with polar side chains can conduct electronic and ionic charges simultaneously, making them promising for bioelectronics, electrocatalysis and energy storage. Recent work showed that adding alkyl spacers between CP backbones and polar side chains improved electronic charge carrier mobility, reduced swelling and enhanced stability, without compromising ion transport. However, how alkyl spacers impact polymer backbone conformation and, subsequently, electronic properties remain unclear. In this work, we design two oxy-bithiophene-based CP series, each featuring progressively extended alkyl spacer lengths and two distinct molecular weight (MW) distributions. Using operando characterisations, we evaluate the (spectro)electrochemical and swelling properties of the polymer thin films, and their performance in organic field-effect transistors and organic electrochemical transistors. Surprisingly, alkyl spacers negatively impact the hole mobility of our polymers, with higher MW amplifying this effect. Using molecular dynamics simulations, we show that it is thermodynamically favourable for adjacent non-polar alkyl spacers to aggregate in polar electrolytes, leading to backbone twisting. Further spectroscopic measurements corroborate this prediction. Our findings demonstrate the active interactions between side chain structure, MW and electrolyte/solvent polarity in influencing polymer performance, underscoring the importance of considering solvation environment effects on polymer conformation when designing new mixed conducting CPs for electrochemical applications.

  View details for DOI 10.1002/anie.202417897

  View details for PubMedID 39714022

• Decoupling the Influence of Poly(3,4-Ethylenedioxythiophene)-Collagen Composite Characteristics on Cell Stemness ADVANCED SCIENCE Keate, R. L., Tropp, J., Wu, R., Petty II, A. J., Ameer, G. A., Rivnay, J. 2024; 11 (27): e2305562

  Abstract

  Conductive polymers (CPs) are widely studied for their ability to influence a myriad of tissue systems. While their mixed ionic/electronic conductivity is commonly considered the primary driver of these benefits, the mechanisms by which CPs influence cell fate remain unclear. In this study, CP-biomaterial interactions are investigated using collagen, due to its widespread prevalence throughout the body and in tissue engineering constructs. Collagen is functionalized with both electrostatically and covalently bound derivatives of the CP poly(3,4-ethylenedioxythiophene) (PEDOT) doped via backbone-tethered sulfonate groups, which enable high solubility and loading to the collagen biomatrix. Intrinsically doped scaffolds are compared to those incorporated with a commercially available PEDOT formulation, which is complexed with polyanionic polystyrene sulfonate (PSS). Low loadings of intrinsically doped PEDOT do not increase substrate conductivity compared to collagen alone, enabling separate investigation into CP loading and conductivity. Interestingly, higher PEDOT loading bolsters human mesenchymal stromal (hMSC) cell gene expression of Oct-4 and NANOG, which are key transcription factors regulating cell stemness. Conductive collagen composites with commercial PEDOT:PSS do not significantly affect the expression of these transcription factors in hMSCs. Furthermore, it is demonstrated that PEDOT regulates cellular fate independently from physical changes to the material but directly to the loading of the polymer.

  View details for DOI 10.1002/advs.202305562

  View details for Web of Science ID 001160742900001

  View details for PubMedID 38350724

  View details for PubMedCentralID PMC11251566

• Enhancement of Conjugated Polymer Microstructure and Mixed-Conducting Properties via Chalcogenophene Heteroatom Substitution CHEMISTRY OF MATERIALS Paulsen, B. D., Meli, D., Moser, M., Marks, A., Ponder Jr, J. F., Wu, R., Schafer, E. A., Strzalka, J., Zhang, Q., Mcculloch, I., Rivnay, J. 2024

  View details for DOI 10.1021/acs.chemmater.3c02309

  View details for Web of Science ID 001166498000001

• Synthetic Nuances to Maximize n-Type Organic Electrochemical Transistor and Thermoelectric Performance in Fused Lactam Polymers. Journal of the American Chemical Society Marks, A., Chen, X., Wu, R., Rashid, R. B., Jin, W., Paulsen, B. D., Moser, M., Ji, X., Griggs, S., Meli, D., Wu, X., Bristow, H., Strzalka, J., Gasparini, N., Costantini, G., Fabiano, S., Rivnay, J., McCulloch, I. 2022

  Abstract

  A series of fully fused n-type mixed conduction lactam polymers p(g7NCnN), systematically increasing the alkyl side chain content, are synthesized via an inexpensive, nontoxic, precious-metal-free aldol polycondensation. Employing these polymers as channel materials in organic electrochemical transistors (OECTs) affords state-of-the-art n-type performance with p(g7NC10N) recording an OECT electron mobility of 1.20 * 10-2 cm2 V-1 s-1 and a muC* figure of merit of 1.83 F cm-1 V-1 s-1. In parallel to high OECT performance, upon solution doping with (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI), the highest thermoelectric performance is observed for p(g7NC4N), with a maximum electrical conductivity of 7.67 S cm-1 and a power factor of 10.4 muW m-1 K-2. These results are among the highest reported for n-type polymers. Importantly, while this series of fused polylactam organic mixed ionic-electronic conductors (OMIECs) highlights that synthetic molecular design strategies to bolster OECT performance can be translated to also achieve high organic thermoelectric (OTE) performance, a nuanced synthetic approach must be used to optimize performance. Herein, we outline the performance metrics and provide new insights into the molecular design guidelines for the next generation of high-performance n-type materials for mixed conduction applications, presenting for the first time the results of a single polymer series within both OECT and OTE applications.

  View details for DOI 10.1021/jacs.2c00735

  View details for PubMedID 35257589

• Electrochemistry of Thin Films with In Situ/Operando Grazing Incidence X-Ray Scattering: Bypassing Electrolyte Scattering for High Fidelity Time Resolved Studies. Small (Weinheim an der Bergstrasse, Germany) Paulsen, B. D., Giovannitti, A., Wu, R., Strzalka, J., Zhang, Q., Rivnay, J., Takacs, C. J. 2021: e2103213

  Abstract

  Electroactive polymer thin films undergo repeated reversible structural change during operation in electrochemical applications. While synchrotron X-ray scattering is powerful for the characterization of stand-alone and ex situ organic thin films, in situ/operando structural characterization has been underutilized-in large part due to complications arising from supporting electrolyte scattering. This has greatly hampered the development of application relevant structure property relationships. Therefore, a new methodology for in situ/operando X-ray characterization that separates the incident and scattered X-ray beam path from the electrolyte is developed. As a proof of concept, the operando structural characterization of weakly-scattering, organic mixed conducting thin films in an aqueous electrolyte environment is demonstrated, accessing previously unexplored changes in the pi-pi peak and diffuse scatter, while capturing the solvent swollen thin film structure which is inaccessible in previous ex situ studies. These in situ/operando measurements improve the sensitivity to structural changes, capturing minute changes not possible ex situ, and have multimodal potential such as combined Raman measurements that also serve to validate the true in situ/operando conditions of the cell. Finally, new directions enabled by this in situ/operando cell design are examined and state of the art measurements are compared.

  View details for DOI 10.1002/smll.202103213

  View details for PubMedID 34549509

• The Effect of Alkyl Spacers on the Mixed Ionic-Electronic Conduction Properties of N-Type Polymers ADVANCED FUNCTIONAL MATERIALS Maria, I. P., Paulsen, B. D., Savva, A., Ohayon, D., Wu, R., Hallani, R., Basu, A., Du, W., Anthopoulos, T. D., Inal, S., Rivnay, J., McCulloch, I., Giovannitti, A. 2021

  View details for DOI 10.1002/adfm.202008718

  View details for Web of Science ID 000608924800001