Honors & Awards
REAXYS Chemistry PhD Prize Award, REAXYS (2017)
Outstanding PhD award 2017/2018, Imperial College London
Diplom, Universitat Karlsruhe (2012)
Master of Science, Imperial College of Science, Technology & Medicine (2014)
Doctor of Philosophy, Imperial College of Science, Technology & Medicine (2018)
Current Research and Scholarly Interests
I am interested in the development of redox-active polymeric organic semiconductors for energy storage and energy conversion devices. My research vision is to develop affordable, safe, and sustainable devices to pave the way for next-generation, low-carbon technologies.
- High-Gain Chemically Gated Organic Electrochemical Transistor ADVANCED FUNCTIONAL MATERIALS 2021
Reversible Electrochemical Charging of n-Type Conjugated Polymer Electrodes in Aqueous Electrolytes.
Journal of the American Chemical Society
Conjugated polymers achieve redox activity in electrochemical devices by combining redox-active, electronically conducting backbones with ion-transporting side chains that can be tuned for different electrolytes. In aqueous electrolytes, redox activity can be accomplished by attaching hydrophilic side chains to the polymer backbone, which enables ionic transport and allows volumetric charging of polymer electrodes. While this approach has been beneficial for achieving fast electrochemical charging in aqueous solutions, little is known about the relationship between water uptake by the polymers during electrochemical charging and the stability and redox potentials of the electrodes, particularly for electron-transporting conjugated polymers. We find that excessive water uptake during the electrochemical charging of polymer electrodes harms the reversibility of electrochemical processes and results in irreversible swelling of the polymer. We show that small changes of the side chain composition can significantly increase the reversibility of the redox behavior of the materials in aqueous electrolytes, improving the capacity of the polymer by more than one order of magnitude. Finally, we show that tuning the local environment of the redox-active polymer by attaching hydrophilic side chains can help to reach high fractions of the theoretical capacity for single-phase electrodes in aqueous electrolytes. Our work shows the importance of chemical design strategies for achieving high electrochemical stability for conjugated polymers in aqueous electrolytes.
View details for DOI 10.1021/jacs.1c06713
View details for PubMedID 34469688
Energetic Control of Redox-Active Polymers toward Safe Organic Bioelectronic Materials.
Advanced materials (Deerfield Beach, Fla.)
Avoiding faradaic side reactions during the operation of electrochemical devices is important to enhance the device stability, to achieve low power consumption, and to prevent the formation of reactive side-products. This is particularly important for bioelectronic devices, which are designed to operate in biological systems. While redox-active materials based on conducting and semiconducting polymers represent an exciting class of materials for bioelectronic devices, they are susceptible to electrochemical side-reactions with molecular oxygen during device operation. Here, electrochemical side reactions with molecular oxygen are shown to occur during organic electrochemical transistor (OECT) operation using high-performance, state-of-the-art OECT materials. Depending on the choice of the active material, such reactions yield hydrogen peroxide (H2 O2 ), a reactive side-product, which may be harmful to the local biological environment and may also accelerate device degradation. A design strategy is reported for the development of redox-active organic semiconductors based on donor-acceptor copolymers that prevents the formation of H2 O2 during device operation. This study elucidates the previously overlooked side-reactions between redox-active conjugated polymers and molecular oxygen in electrochemical devices for bioelectronics, which is critical for the operation of electrolyte-gated devices in application-relevant environments.
View details for DOI 10.1002/adma.201908047
View details for PubMedID 32125736
- Design and evaluation of conjugated polymers with polar side chains as electrode materials for electrochemical energy storage in aqueous electrolytes ENERGY & ENVIRONMENTAL SCIENCE 2019; 12 (4): 1349–57
The Role of the Side Chain on the Performance of N-type Conjugated Polymers in Aqueous Electrolytes
CHEMISTRY OF MATERIALS
2018; 39 (9): 2945–53
We report a design strategy that allows the preparation of solution processable n-type materials from low boiling point solvents for organic electrochemical transistors (OECTs). The polymer backbone is based on NDI-T2 copolymers where a branched alkyl side chain is gradually exchanged for a linear ethylene glycol-based side chain. A series of random copolymers was prepared with glycol side chain percentages of 0, 10, 25, 50, 75, 90, and 100 with respect to the alkyl side chains. These were characterized to study the influence of the polar side chains on interaction with aqueous electrolytes, their electrochemical redox reactions, and performance in OECTs when operated in aqueous electrolytes. We observed that glycol side chain percentages of >50% are required to achieve volumetric charging, while lower glycol chain percentages show a mixed operation with high required voltages to allow for bulk charging of the organic semiconductor. A strong dependence of the electron mobility on the fraction of glycol chains was found for copolymers based on NDI-T2, with a significant drop as alkyl side chains are replaced by glycol side chains.
View details for PubMedID 29780208
View details for PubMedCentralID PMC5953566
- Redox-Stability of Alkoxy-BDT Copolymers and their Use for Organic Bioelectronic Devices ADVANCED FUNCTIONAL MATERIALS 2018; 28 (17)
Controlling the mode of operation of organic transistors through side-chain engineering.
Proceedings of the National Academy of Sciences of the United States of America
2016; 113 (43): 12017-12022
Electrolyte-gated organic transistors offer low bias operation facilitated by direct contact of the transistor channel with an electrolyte. Their operation mode is generally defined by the dimensionality of charge transport, where a field-effect transistor allows for electrostatic charge accumulation at the electrolyte/semiconductor interface, whereas an organic electrochemical transistor (OECT) facilitates penetration of ions into the bulk of the channel, considered a slow process, leading to volumetric doping and electronic transport. Conducting polymer OECTs allow for fast switching and high currents through incorporation of excess, hygroscopic ionic phases, but operate in depletion mode. Here, we show that the use of glycolated side chains on a thiophene backbone can result in accumulation mode OECTs with high currents, transconductance, and sharp subthreshold switching, while maintaining fast switching speeds. Compared with alkylated analogs of the same backbone, the triethylene glycol side chains shift the mode of operation of aqueous electrolyte-gated transistors from interfacial to bulk doping/transport and show complete and reversible electrochromism and high volumetric capacitance at low operating biases. We propose that the glycol side chains facilitate hydration and ion penetration, without compromising electronic mobility, and suggest that this synthetic approach can be used to guide the design of organic mixed conductors.
View details for PubMedID 27790983
N-type organic electrochemical transistors with stability in water
Organic electrochemical transistors (OECTs) are receiving significant attention due to their ability to efficiently transduce biological signals. A major limitation of this technology is that only p-type materials have been reported, which precludes the development of complementary circuits, and limits sensor technologies. Here, we report the first ever n-type OECT, with relatively balanced ambipolar charge transport characteristics based on a polymer that supports both hole and electron transport along its backbone when doped through an aqueous electrolyte and in the presence of oxygen. This new semiconducting polymer is designed specifically to facilitate ion transport and promote electrochemical doping. Stability measurements in water show no degradation when tested for 2 h under continuous cycling. This demonstration opens the possibility to develop complementary circuits based on OECTs and to improve the sophistication of bioelectronic devices.
View details for DOI 10.1038/ncomms13066
View details for Web of Science ID 000385587100001
View details for PubMedID 27713414
View details for PubMedCentralID PMC5059848
Molecular Design of Semiconducting Polymers for High-Performance Organic Electrochemical Transistors.
Journal of the American Chemical Society
2016; 138 (32): 10252-10259
The organic electrochemical transistor (OECT), capable of transducing small ionic fluxes into electronic signals in an aqueous environment, is an ideal device to utilize in bioelectronic applications. Currently, most OECTs are fabricated with commercially available conducting poly(3,4-ethylenedioxythiophene) (PEDOT)-based suspensions and are therefore operated in depletion mode. Here, we present a series of semiconducting polymers designed to elucidate important structure-property guidelines required for accumulation mode OECT operation. We discuss key aspects relating to OECT performance such as ion and hole transport, electrochromic properties, operational voltage, and stability. The demonstration of our molecular design strategy is the fabrication of accumulation mode OECTs that clearly outperform state-of-the-art PEDOT-based devices, and show stability under aqueous operation without the need for formulation additives and cross-linkers.
View details for DOI 10.1021/jacs.6b05280
View details for PubMedID 27444189
View details for PubMedCentralID PMC4991841
- Operation mechanism of organic electrochemical transistors as redox chemical transducers JOURNAL OF MATERIALS CHEMISTRY C 2021
- The Effect of Alkyl Spacers on the Mixed Ionic-Electronic Conduction Properties of N-Type Polymers ADVANCED FUNCTIONAL MATERIALS 2021
Side Chain Redistribution as a Strategy to Boost Organic Electrochemical Transistor Performance and Stability.
Advanced materials (Deerfield Beach, Fla.)
A series of glycolated polythiophenes for use in organic electrochemical transistors (OECTs) is designed and synthesized, differing in the distribution of their ethylene glycol chains that are tethered to the conjugated backbone. While side chain redistribution does not have a significant impact on the optoelectronic properties of the polymers, this molecular engineering strategy strongly impacts the water uptake achieved in the polymers. By careful optimization of the water uptake in the polymer films, OECTs with unprecedented steady-state performances in terms of [muC* ] and current retentions up to 98% over 700 electrochemical switching cycles are developed.
View details for DOI 10.1002/adma.202002748
View details for PubMedID 32754923
- Temperature-resilient solid-state organic artificial synapses for neuromorphic computing SCIENCE ADVANCES 2020; 6 (27)
- Balancing Ionic and Electronic Conduction for High-Performance Organic Electrochemical Transistors ADVANCED FUNCTIONAL MATERIALS 2020
Polaron Delocalization in Donor-Acceptor Polymers and its Impact on Organic Electrochemical Transistor Performance.
Angewandte Chemie (International ed. in English)
Donor-acceptor (D-A) polymers are promising materials for organic electrochemical transistors (OECTs), as they minimize detrimental faradaic side-reactions during OECT operation, yet their steady-state OECT performance still lags far behind their all-donor counterparts. Here, we report three D-A polymers based on the diketopyrrolopyrrole unit that afford OECT performances similar to those of all-donor polymers, hence representing a significant improvement to the previously developed D-A copolymers. In addition to improved OECT performance, DFT simulations of the polymers and their respective hole polarons also revealed a positive correlation between hole polaron delocalization and steady-state OECT performance, providing new insights into the design of OECT materials. More importantly, we demonstrate how polaron delocalization can be tuned directly at the molecular level by selection of the building blocks comprising the polymers' conjugated backbone, thus paving the way for the development of even higher performing OECT polymers.
View details for DOI 10.1002/anie.202014078
View details for PubMedID 33259685
Reversible Electronic Solid-Gel Switching of a Conjugated Polymer
2020; 7 (2): 1901144
Conjugated polymers exhibit electrically driven volume changes when included in electrochemical devices via the exchange of ions and solvent. So far, this volumetric change is limited to 40% and 100% for reversible and irreversible systems, respectively, thus restricting potential applications of this technology. A conjugated polymer that reversibly expands by about 300% upon addressing, relative to its previous contracted state, while the first irreversible actuation can achieve values ranging from 1000-10 000%, depending on the voltage applied is reported. From experimental and theoretical studies, it is found that this large and reversible volumetric switching is due to reorganization of the polymer during swelling as it transforms between a solid-state phase and a gel, while maintaining percolation for conductivity. The polymer is utilized as an electroactive cladding to reduce the void sizes of a porous carbon filter electrode by 85%.
View details for DOI 10.1002/advs.201901144
View details for Web of Science ID 000492740700001
View details for PubMedID 31993279
View details for PubMedCentralID PMC6974956
- Highly selective chromoionophores for ratiometric Na+ sensing based on an oligoethyleneglycol bridged bithiophene detection unit JOURNAL OF MATERIALS CHEMISTRY C 2019; 7 (18): 5359–65
- Materials in Organic Electrochemical Transistors for Bioelectronic Applications: Past, Present, and Future ADVANCED FUNCTIONAL MATERIALS 2019; 29 (21)
- Nanoscale Ion-Doped Polymer Transistors NANO LETTERS 2019; 19 (3): 1712–18
Nanoscale Ion-Doped Polymer Transistors.
Organic transistors with submicron dimensions have been shown to deviate from the expected behavior due to a variety of so-called "short-channel" effects, resulting in nonlinear output characteristics and a lack of current saturation, considerably limiting their use. Using an electrochemically doped polymer in which ions are dynamically injected and removed from the bulk of the semiconductor, we show that devices with nanoscale channel lengths down to 50 nm exhibit output curves with well-defined linear and saturation regimes. Additionally, they show very large on-currents on par with their microscale counterparts, large on-to-off ratios of 108, and record-high width-normalized transconductances above 10 S m-1. We believe this work paves the way for the fabrication of high-gain, high-current polymer integrated circuits such as sensor arrays operating at voltages below |1 V| and prepared using simple solution-processing methods.
View details for PubMedID 30720280
- Influence of Water on the Performance of Organic Electrochemical Transistors CHEMISTRY OF MATERIALS 2019; 31 (3): 927–37
- Role of the Anion on the Transport and Structure of Organic Mixed Conductors ADVANCED FUNCTIONAL MATERIALS 2019; 29 (5)
Double doping of conjugated polymers with monomer molecular dopants
2019; 18 (2): 149-+
Molecular doping is a crucial tool for controlling the charge-carrier concentration in organic semiconductors. Each dopant molecule is commonly thought to give rise to only one polaron, leading to a maximum of one donor:acceptor charge-transfer complex and hence an ionization efficiency of 100%. However, this theoretical limit is rarely achieved because of incomplete charge transfer and the presence of unreacted dopant. Here, we establish that common p-dopants can in fact accept two electrons per molecule from conjugated polymers with a low ionization energy. Each dopant molecule participates in two charge-transfer events, leading to the formation of dopant dianions and an ionization efficiency of up to 200%. Furthermore, we show that the resulting integer charge-transfer complex can dissociate with an efficiency of up to 170%. The concept of double doping introduced here may allow the dopant fraction required to optimize charge conduction to be halved.
View details for DOI 10.1038/s41563-018-0263-6
View details for Web of Science ID 000456325600016
View details for PubMedID 30643236
Subthreshold Operation of Organic Electrochemical Transistors for Biosignal Amplification
2018; 5 (8): 1800453
With a host of new materials being investigated as active layers in organic electrochemical transistors (OECTs), several advantageous characteristics can be utilized to improve transduction and circuit level performance for biosensing applications. Here, the subthreshold region of operation of one recently reported high performing OECT material, poly(2-(3,3'-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)-[2,2'-bithiophen]-5-yl)thieno[3,2-b]thiophene), p(g2T-TT) is investigated. The material's high subthreshold slope (SS) is exploited for high voltage gain and low power consumption. An ≈5× improvement in voltage gain (AV) for devices engineered for equal output current and 370× lower power consumption in the subthreshold region, in comparison to operation in the higher transconductance (gm), superthreshold region usually reported in the literature, are reported. Electrophysiological sensing is demonstrated using the subthreshold regime of p(g2T-TT) devices and it is suggested that operation in this regime enables low power, enhanced sensing for a broad range of bioelectronic applications. Finally, the accessibility of the subthreshold regime of p(g2T-TT) is evaluated in comparison with the prototypical poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and the role of material design in achieving favorable properties for subthreshold operation is discussed.
View details for DOI 10.1002/advs.201800453
View details for Web of Science ID 000441869400040
View details for PubMedID 30128254
View details for PubMedCentralID PMC6097142
Direct metabolite detection with an n-type accumulation mode organic electrochemical transistor
2018; 4 (6): eaat0911
The inherent specificity and electrochemical reversibility of enzymes poise them as the biorecognition element of choice for a wide range of metabolites. To use enzymes efficiently in biosensors, the redox centers of the protein should have good electrical communication with the transducing electrode, which requires either the use of mediators or tedious biofunctionalization approaches. We report an all-polymer micrometer-scale transistor platform for the detection of lactate, a significant metabolite in cellular metabolic pathways associated with critical health care conditions. The device embodies a new concept in metabolite sensing where we take advantage of the ion-to-electron transducing qualities of an electron-transporting (n-type) organic semiconductor and the inherent amplification properties of an ion-to-electron converting device, the organic electrochemical transistor. The n-type polymer incorporates hydrophilic side chains to enhance ion transport/injection, as well as to facilitate enzyme conjugation. The material is capable of accepting electrons of the enzymatic reaction and acts as a series of redox centers capable of switching between the neutral and reduced state. The result is a fast, selective, and sensitive metabolite sensor. The advantage of this device compared to traditional amperometric sensors is the amplification of the input signal endowed by the electrochemical transistor circuit and the design simplicity obviating the need for a reference electrode. The combination of redox enzymes and electron-transporting polymers will open up an avenue not only for the field of biosensors but also for the development of enzyme-based electrocatalytic energy generation/storage devices.
View details for DOI 10.1126/sciadv.aat0911
View details for Web of Science ID 000443175500063
View details for PubMedID 29942860
View details for PubMedCentralID PMC6014717
- Lipid bilayer formation on organic electronic materials JOURNAL OF MATERIALS CHEMISTRY C 2018; 6 (19): 5218–27
Enhanced n-Doping Efficiency of a Naphthalenediimide-Based Copolymer through Polar Side Chains for Organic Thermoelectrics
ACS ENERGY LETTERS
2018; 3 (2): 278–85
N-doping of conjugated polymers either requires a high dopant fraction or yields a low electrical conductivity because of their poor compatibility with molecular dopants. We explore n-doping of the polar naphthalenediimide-bithiophene copolymer p(gNDI-gT2) that carries oligoethylene glycol-based side chains and show that the polymer displays superior miscibility with the benzimidazole-dimethylbenzenamine-based n-dopant N-DMBI. The good compatibility of p(gNDI-gT2) and N-DMBI results in a relatively high doping efficiency of 13% for n-dopants, which leads to a high electrical conductivity of more than 10-1 S cm-1 for a dopant concentration of only 10 mol % when measured in an inert atmosphere. We find that the doped polymer is able to maintain its electrical conductivity for about 20 min when exposed to air and recovers rapidly when returned to a nitrogen atmosphere. Overall, solution coprocessing of p(gNDI-gT2) and N-DMBI results in a larger thermoelectric power factor of up to 0.4 μW K-2 m-1 compared to other NDI-based polymers.
View details for PubMedID 29457139
Liquid-Solid Dual-Gate Organic Transistors with Tunable Threshold Voltage for Cell Sensing
ACS APPLIED MATERIALS & INTERFACES
2017; 9 (44): 38687–94
Liquid electrolyte-gated organic field effect transistors and organic electrochemical transistors have recently emerged as powerful technology platforms for sensing and simulation of living cells and organisms. For such applications, the transistors are operated at a gate voltage around or below 0.3 V because prolonged application of a higher voltage bias can lead to membrane rupturing and cell death. This constraint often prevents the operation of the transistors at their maximum transconductance or most sensitive regime. Here, we exploit a solid-liquid dual-gate organic transistor structure, where the threshold voltage of the liquid-gated conduction channel is controlled by an additional gate that is separated from the channel by a metal-oxide gate dielectric. With this design, the threshold voltage of the "sensing channel" can be linearly tuned in a voltage window exceeding 0.4 V. We have demonstrated that the dual-gate structure enables a much better sensor response to the detachment of human mesenchymal stem cells. In general, the capability of tuning the optimal sensing bias will not only improve the device performance but also broaden the material selection for cell-based organic bioelectronics.
View details for DOI 10.1021/acsami.7b09384
View details for Web of Science ID 000415140800057
View details for PubMedID 29039186
- Sodium and Potassium Ion Selective Conjugated Polymers for Optical Ion Detection in Solution and Solid State ADVANCED FUNCTIONAL MATERIALS 2016; 26 (4): 514–23
- Single and Multiple Additions of Dibenzoylmethane onto Buckminsterfullerene EUROPEAN JOURNAL OF ORGANIC CHEMISTRY 2013; 2013 (35): 7907–13