Hydration of a side-chain-free n-type semiconducting ladder polymer driven by electrochemical doping

We study the organic electrochemical transistors (OECTs) performance of the ladder polymer, poly(benzimidazobenzophenanthroline) (BBL) in an attempt to better understand how an apparently hydrophobic side-chain-free polymer is able to operate as an OECT with favorable redox kinetics in an aqueous environment. We examine two BBLs of different molecular masses from different sources. Both BBLs show significant film swelling during the initial reduction step. By combining electrochemical quartz crystal microbalance (eQCM) gravimetry, in-operando atomic force microscopy (AFM), and both ex-situ and in-operando grazing incidence wide-angle x-ray scattering (GIWAXS), we provide a detailed structural picture of the electrochemical charge injection process in BBL in the absence of any hydrophilic side-chains. Compared with ex-situ measurements, in-operando GIWAXS shows both more swelling upon electrochemical doping than has previously been recognized, and less contraction upon dedoping. The data show that BBL films undergo an irreversible hydration driven by the initial electrochemical doping cycle with significant water retention and lamellar expansion that persists across subsequent oxidation/reduction cycles. This swelling creates a hydrophilic environment that facilitates the subsequent fast hydrated ion transport in the absence of the hydrophilic side-chains used in many other polymer systems. Due to its rigid ladder backbone and absence of hydrophilic side-chains, the primary BBL water uptake does not significantly degrade the crystalline order, and the original dehydrated, unswelled state can be recovered after drying. The combination of doping induced hydrophilicity and robust crystalline order leads to efficient ionic transport and good stability.Comment: 24 pages, 5 figure

The Role of Side Chains and Hydration on Mixed Charge Transport in <i>n</i> ‐Type Polymer Films

Introducing ethylene glycol (EG) side chains to a conjugated polymer backbone is a well‐established synthetic strategy for designing organic mixed ion‐electron conductors (OMIECs). However, the impact that film swelling has on mixed conduction properties has yet to be scoped, particularly for electron‐transporting (n‐type) OMIECs. Here, the authors investigate the effect of the length of branched EG chains on mixed charge transport of n‐type OMIECs based on a naphthalene‐1,4,5,8‐tetracarboxylic‐diimide‐bithiophene backbone. Atomic force microscopy (AFM), grazing‐incidence wide‐angle X‐ray scattering (GIWAXS), and scanning tunneling microscopy (STM) are used to establish the similarities between the common‐backbone films in dry conditions. Electrochemical quartz crystal microbalance with dissipation monitoring (EQCM‐D) and in situ GIWAXS measurements reveal stark changes in film swelling properties and microstructure during electrochemical doping, depending on the side chain length. It is found that even in the loss of the crystallite content upon contact with the aqueous electrolyte, the films can effectively transport charges and that it is rather the high water content that harms the electronic interconnectivity within the OMIEC films. These results highlight the importance of controlling water uptake in the films to impede charge transport in n‐type electrochemical devices

Finishing the euchromatic sequence of the human genome

The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead

Anion-Dependent Doping and Charge Transport in Organic Electrochemical Transistors

We study the effects of different electrolyte anions on the mixed ionic/electronic transport properties of organic electrochemical transistors (OECTs) based on poly­(3-hexylthiophene-2,5-diyl). We show that the transport properties depend on the anion present in the electrolyte, with greater source-drain currents resulting from the use of molecular anions such as hexafluorophosphate and trifluoromethanesulfonylimide than from the use of smaller atomic anions such as fluoride or chloride. Using spectroelectrochemistry, we show the maximum doping level that can be achieved in an aqueous environment is also anion-dependent. Furthermore, we find that the average electronic carrier mobility at a given doping level depends on the chemistry of the compensating counterion. We further investigate this dependence by electrochemical quartz crystal microbalance measurements, showing the solvation of the dopant anions within the polymer is drastically different depending on the choice of the anion. Surprisingly, we find that the kinetics of the doping process in these OECTs is faster for bulkier anions. Finally, we use electrochemical strain microscopy to resolve ion-dependent differences in doping and local swelling at the nanoscale, providing further insight into the coupling between local structure and ion uptake. These measurements demonstrate that the identity of the compensating ion and its interaction with the polymer and solvent are important considerations for benchmarking and designing polymer materials for mixed ionic/electronic conduction applications

Cantilever Ringdown Dissipation Imaging for the Study of Loss Processes in Polymer/Fullerene Solar Cells

We use dissipation imaging to probe local changes in electronic properties of nanostructured semiconductor films due to local photochemistry. We make quantitative maps of electrostatic dissipation due to photogenerated carriers by measuring the ringdown time of an oscillating atomic force microscope cantilever. Using organic photovoltaic materials as a testbed, we study macroscopic device degradation as a function of photooxidation for three different film morphologies comprising the conjugated polymer poly­[[4,8-bis­[(2-ethylhexyl)­oxy]­benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene-2,6-diyl]­[3-fluoro-2-[(2-ethylhexyl)­carbonyl]­thieno­[3,4-<i>b</i>]­thiophenediyl]] (PTB7) and the fullerene derivative [6,6]-phenyl-C₇₁ butyric acid methyl ester (PC₇₁BM). We find that, judged by device performance, the stability of the macroscopic devices is sensitive to processing conditions, with films processed with the solvent additive 1,8-diiodooctane being the most stable. At the microscopic level, we compare the evolution of cantilever power dissipation as a function of photochemical degradation for three different polymer/fullerene blend morphologies and show that the changes in local power dissipation correlate with device stability. Using ringdown imaging to look at local dissipation in a highly phase-separated PTB7:PC₇₁BM film morphology, we show that cantilever power dissipation increases more rapidly over large fullerene aggregates than in well-mixed polymer/fullerene regions, suggesting that local photochemistry on the fullerene contributes strongly to the dissipation signal

Impact of varying side chain structure on organic electrochemical transistor performance: a series of oligoethylene glycol-substituted polythiophenes

The electrochemical doping/dedoping kinetics, and the organic electrochemical transistor (OECT) performance of a series of polythiophene homopolymers with ethylene glycol units in their side chains using both kosmotropic and chaotropic anion solutions were studied. We compare their performance to a reference polymer, the polythiophene derivative with diethylene glycol side chains, poly(3-{[2-(2-methoxyethoxy)ethoxy]methyl}thiophene-2,5-diyl) (P3MEEMT). We find larger OECT material figure of merit, μC*, where μ is the carrier mobility and C* is the volumetric capacitance, and faster doping kinetics with more oxygen atoms on the side chains, and if the oxygen atom is farther from the polythiophene backbone. Replacing the oxygen atom close to the polythiophene backbone with an alkyl unit increases the film π-stacking crystallinity (higher electronic conductivity in the undoped film) but sacrifices the available doping sites (lower volumetric capacitance C* in OECT). We show that this variation in C* is the dominant factor in changing the μC* product for this family of polymers. With more oxygen atoms on the side chain, or with the oxygen atom farther from the polymer backbone, we observe both more passive swelling and higher C*. In addition, we show that, compared to the doping speed, the dedoping speed, as measured via spectroelectrochemistry, is both generally faster and less dependent on ion species or side chain oxygen content. Last, through OECT, electrochemical impedance spectroscopy (EIS) and spectroelectrochemistry measurements, we show that the chaotropic anion PF6− facilitates higher doping levels, faster doping kinetics, and lower doping thresholds compared to the kosmotropic anion Cl−, although the exact differences depend on the polymer side chains. Our results highlight the importance of balancing μ and C* when designing molecular structures for OECT active layers