Efficient molecular doping of polymeric semiconductors driven by anion exchange

Modulation between capacitor and conductor for a redox-active 2D bis(terpyridine)cobalt(II) nanosheet via anion-exchange

Ionic polymers are intriguing materials whose functionality arises from the synergy between ionic polymer backbones and counterions. A key method for enhancing their functionality is the post-synthetic ion-exchange reaction, which is instrumental in improving the chemical and physical properties of polymer backbones and introducing of the functionalities of the counterions. Electronic interaction between host polymer backbone and guest ions plays pivotal roles in property modulation. The current study highlights the modulation of responses to external electric field in cationic bis(terpyridine)cobalt(II) polymer nanofilms through anion-exchange reactions. Initially, as-prepared chloride-containing polymers exhibited supercapacitor behaviour. Introducing anionic metalladithiolenes into the polymers altered the behaviour to either conductive or insulative, depending on the valence of the metalladithiolenes. This modulation was accomplished by fine tuning of charge-transfer interactions between the bis(terpyridine)cobalt(II) complex moieties and redox-active anions. Our findings open up new avenue for ionic polymers, showcasing their potential as versatile platform in materials science.

Ion-Exchange Doping of Semiconducting Single-Walled Carbon Nanotubes

Semiconducting single-walled carbon nanotubes (SWCNTs) are a promising thermoelectric material with high power factors after chemical p- or n-doping. Understanding the impact of dopant counterions on charge transport and thermoelectric properties of nanotube networks is essential to further optimize doping methods and to develop better dopants. This work utilizes ion-exchange doping to systematically vary the size of counterions in thin films of small and large diameter, polymer-sorted semiconducting SWCNTs with AuCl3 as the initial p-dopant and investigates the impact of ion size on conductivity, Seebeck coefficients, and power factors. Larger anions are found to correlate with higher electrical conductivities and improved doping stability, while no significant effect on the power factors is found. Importantly, the effect of counterion size on the thermoelectric properties of dense SWCNT networks is not obscured by morphological changes upon doping. The observed trends of carrier mobilities and Seebeck coefficients can be explained by a random resistor model for the nanotube network that accounts for overlapping Coulomb potentials leading to the formation of an impurity band whose depth depends on the carrier density and counterion size. These insights can be applied more broadly to understand the thermoelectric properties of doped percolating disordered systems, including semiconducting polymers.

Development of High Performance Thermoelectric Polymers via Doping or Dedoping Engineering

It is of great significance to develop high-performance thermoelectric (TE) materials, because they can be used to harvest waste heat into electricity and there is abundant waste heat on earth. The conventional TE materials are inorganic semimetals or semiconductors like Bi2Te3 and its derivatives. However, they have problems of high cost, scarce/toxic elements, high thermal conductivity, and poor mechanical flexibility. Organic TE materials emerged as the next-generation TE materials because of their merits including solution processability, low cost, abundant element, low intrinsic thermal conductivity, and high mechanical flexibility. Organic TE materials are mainly conducting polymers because of their high conductivity. Both the conductivity and Seebeck coefficient depend on the doping level, and they are interdependent. Hence, the TE properties of polymers can be improved through doping/dedoping engineering. There are three types of doping forms, oxidative (or reductive) doping, protonic acid doping, and charge transfer doping. Accordingly, they can be dedoped by different approaches. In this article, we review the methods to dope and dedope p-type and n-type TE polymers and the combination of doping and dedoping to optimize their TE properties. Secondary doping is also covered, since it can significantly enhance the conductivity of some TE polymers.

Universal Copolymerization of Crosslinked Polyether Electrolytes for All-Solid-State Lithium-Metal Batteries

Solid polymer electrolytes (SPEs) are pivotal in advancing the practical implementation of all-solid-state batteries. Poly(1,3-dioxane) (PDOL)-based electrolytes have attracted significant attention due to the pseudo-high conductivity achieved through sophisticated in situ polymerization methods; however, such PDOL-based electrolytes present challenges of crystallization over time and monomers residual during processing. In this study, integrating LiTFSI and LiDFOB as a universal copolymerization strategy for developing high-performance PDOL electrolytes with a wide range of epoxy crosslinkers is proposed. It is discovered that this approach leverages the protective effects of TFSI anions on the boron active center and catalyzes polymer chain growth via crosslinking. The homogenously crosslinked (benzene-centered) PDOL electrolyte exhibits remarkable thermo-mechanical stability (up to 100 °C), high ion migration number (tLi+ = 0.42), a wide electrochemical window (≈5.0 V vs Li+/Li), and high ionic conductivity (4.5×10-4 S cm-1). Notably, the crosslinked PDOL electrolyte is in the all-solid-state with minimal monomer/oligomer residual, exhibiting no crystallization during relaxation, delivering a robust performance in all-solid-state lithium metal batteries.

Near-infrared luminescent open-shell π-conjugated systems with a bright lowest-energy zwitterionic singlet excited state

Open-shell systems with extensive π-conjugation have fascinating properties due to their narrow bandgaps and spin interactions. In this work, we report neutral open-shell di- and polyradical conjugated materials exhibiting intriguing optical and magnetic properties. Our key design advance is the planarized geometry allowing for greater interaction between adjacent spins. This results in absorption and emission in the near infrared at 803 and 1050 nanometers, respectively, and we demonstrate a unique electronic structure where a bright zwitterionic excited state is the lowest-accessible electronic transition. Electron paramagnetic resonance spectroscopy and superconducting quantum interference device measurements reveal that our materials are open-shell singlets with different degrees of spin interactions, dynamics, and antiferromagnetic properties, which likely contributed to the formation of their emissive zwitterionic singlet excited state and near-infrared emission. In addition, our materials show reversible and stable electrochromic switching with more than 500 cycles, indicating their potential for optoelectronic and electrochemical energy storage applications.

Efficient molecular doping of polymeric semiconductors improved by coupled reaction

Exploring chemical doping method to improve the electrical conductivity of polymers is still very attractive for researchers. In this work, we report a developed method of doping a polymer semiconductor aided by the coupled reaction that commonly exists in biological systems where a non-spontaneous reaction is driven by a spontaneous reaction. During the doping process, the chemical reaction between the dopant and the polymer is promoted by introducing a thermodynamically favorable reaction via adding additives that are highly reactive to the reduction product of the dopant to form a coupled reaction, thus significantly improving the electrical conductivity of polymers by 3-7 orders. This coupled reaction doping process shows the potential of wide applications in exploring efficient doping systems to prepare functional conducting polymers, which could be a powerful tool for modern organic electronics.

Interface-Interactive Nanoarchitectonics: Solid and/or Liquid

The methodology of nanoarchitectonics is to construct functional materials using nanounits such as atoms, molecules, and nanoobjects, just like architecting buildings. Nanoarchitectonics pursues the ultimate concept of materials science through the integration of related fields. In this review paper, under the title of interface-interactive nanoarchitectonics, several examples of structure fabrication and function development at interfaces will be discussed, highlighting the importance of architecting materials with nanoscale considerations. Two sections provide some examples at the solid and liquid surfaces. In solid interfacial environments, molecular structures can be precisely observed and analyzed with theoretical calculations. Solid surfaces are a prime site for nanoarchitectonics at the molecular level. Nanoarchitectonics of solid surfaces has the potential to pave the way for cutting-edge functionality and science based on advanced observation and analysis. Liquid surfaces are more kinetic and dynamic than solid interfaces, and their high fluidity offers many possibilities for structure fabrications by nanoarchitectonics. The latter feature has advantages in terms of freedom of interaction and diversity of components, therefore, liquid surfaces may be more suitable environments for the development of functionalities. The final section then discusses what is needed for the future of material creation in nanoarchitectonics.

Liquid-Liquid and Liquid-Solid Interfacial Nanoarchitectonics

Nanoscale science is becoming increasingly important and prominent, and further development will necessitate integration with other material chemistries. In other words, it involves the construction of a methodology to build up materials based on nanoscale knowledge. This is also the beginning of the concept of post-nanotechnology. This role belongs to nanoarchitectonics, which has been rapidly developing in recent years. However, the scope of application of nanoarchitectonics is wide, and it is somewhat difficult to compile everything. Therefore, this review article will introduce the concepts of liquid and interface, which are the keywords for the organization of functional material systems in biological systems. The target interfaces are liquid-liquid interface, liquid-solid interface, and so on. Recent examples are summarized under the categories of molecular assembly, metal-organic framework and covalent organic framework, and living cell. In addition, the latest research on the liquid interfacial nanoarchitectonics of organic semiconductor film is also discussed. The final conclusive section summarizes these features and discusses the necessary components for the development of liquid interfacial nanoarchitectonics.

Sequential Cascade Doping of Conjugated-Polymer-Wrapped Carbon Nanotubes for Highly Electrically Conductive Platforms

To explore a highly conductive flexible platform, this study develops PIDF-BT@SWCNT by wrapping single-walled carbon nanotubes (SWCNTs) with a conjugated polymer, PIDF-BT, known for its effective doping properties. By evaluating the doping behaviors of various dopants on PIDF-BT, appropriate dopant combinations for cascade doping are selected to improve the doping efficiency of PIDF-BT@SWCNT. Specifically, using F4TCNQ or F6TCNNQ as the first dopant, followed by AuCl3 as the second dopant, demonstrates remarkable doping efficiency, surpassing that of the individual dopants and yielding an exceptional electrical conductivity exceeding 6000 S/cm. Characterization using X-ray photoelectron spectroscopy and Raman spectroscopy elucidates the doping mechanism, revealing an increase in the proportion of electron-donating atoms and the ratio of quinoid structures upon F4TCNQ/AuCl3 cascade doping. These findings offer insights into optimizing dopant combinations for cascade doping, showcasing its advantages in enhancing doping efficiency and resulting electrical conductivity compared with single dopant processes.

Enhancing the Conductivity and Thermoelectric Performance of Semicrystalline Conducting Polymers through Controlled Tie Chain Incorporation

Conjugated polymers are promising materials for thermoelectric applications, however, at present few effective and well-understood strategies exist to further advance their thermoelectric performance. Here a new model system is reported for a better understanding of the key factors governing their thermoelectric properties: aligned, ribbon-phase poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) doped by ion-exchange doping. Using a range of microstructural and spectroscopic methods, the effect of controlled incorporation of tie-chains between the crystalline domains is studied through blending of high and low molecular weight chains. The tie chains provide efficient transport pathways between crystalline domains and lead to significantly enhanced electrical conductivity of 4810 S cm-1, which is not accompanied by a reduction in Seebeck coefficient or a large increase in thermal conductivity. Respectable power factors of 173 µW m-1 K-2 are demonstrated in this model system. The approach is generally applicable to a wide range of semicrystalline conjugated polymers and could provide an effective pathway for further enhancing their thermoelectric properties and overcome traditional trade-offs in optimization of thermoelectric performance.

Counterion docking: a general approach to reducing energetic disorder in doped polymeric semiconductors

Molecular doping plays an important role in controlling the carrier concentration of organic semiconductors. However, the introduction of dopant counterions often results in increased energetic disorder and traps due to the molecular packing disruption and Coulomb potential wells. To date, no general strategy has been proposed to reduce the counterion-induced structural and energetic disorder. Here, we demonstrate the critical role of non-covalent interactions (NCIs) between counterions and polymers. Employing a computer-aided approach, we identified the optimal counterions and discovered that NCIs determine their docking positions, which significantly affect the counterion-induced energetic disorder. With the optimal counterions, we successfully reduced the energetic disorder to levels even lower than that of the undoped polymer. As a result, we achieved a high n-doped electrical conductivity of over 200 S cm-1 and an eight-fold increase in the thermoelectric power factor. We found that the NCIs have substantial effects on doping efficiency, polymer backbone planarity, and Coulomb potential landscape. Our work not only provides a general strategy for identifying the most suitable counterions but also deepens our understanding of the counterion effects on doped polymeric semiconductors.

Photocatalytic doping of organic semiconductors

Chemical doping is an important approach to manipulating charge-carrier concentration and transport in organic semiconductors (OSCs)1-3 and ultimately enhances device performance4-7. However, conventional doping strategies often rely on the use of highly reactive (strong) dopants8-10, which are consumed during the doping process. Achieving efficient doping with weak and/or widely accessible dopants under mild conditions remains a considerable challenge. Here, we report a previously undescribed concept for the photocatalytic doping of OSCs that uses air as a weak oxidant (p-dopant) and operates at room temperature. This is a general approach that can be applied to various OSCs and photocatalysts, yielding electrical conductivities that exceed 3,000 S cm-1. We also demonstrate the successful photocatalytic reduction (n-doping) and simultaneous p-doping and n-doping of OSCs in which the organic salt used to maintain charge neutrality is the only chemical consumed. Our photocatalytic doping method offers great potential for advancing OSC doping and developing next-generation organic electronic devices.

Enhancing the Thermoelectric Properties of Conjugated Polymers by Suppressing Dopant-Induced Disorder

Doping is a crucial strategy to enhance the performance of various organic electronic devices. However, in many cases, the random distribution of dopants in conjugated polymers leads to the disruption of the polymer microstructure, severely constraining the achievable performance of electronic devices. Here, it is shown that by ion-exchange doping polythiophene-based P[(3HT)1-x-stat-(T)x] (x = 0 (P1), 0.12 (P2), 0.24 (P3), and 0.36 (P4)), remarkably high electrical conductivity of >400 S cm-1 and power factor of >16 µW m-1 K-2 are achieved for the random copolymer P3, ranking it among highest ever reported for unaligned P3HT-based films, significantly higher than that of P1 (<40 S cm-1, <4 µW m-1 K-2). Although both polymers exhibit comparable field-effect transistor hole mobilities of ≈0.1 cm2 V-1 s-1 in the pristine state, after doping, Hall effect measurements indicate that P3 exhibits a large Hall mobility up to 1.2 cm2 V-1 s-1, significantly outperforming that of P1 (0.06 cm2 V-1 s-1). GIWAXS measurement determines that the in-plane π-π stacking distance of doped P3 is 3.44 Å, distinctly shorter than that of doped P1 (3.68 Å). These findings contribute to resolving the long-standing dopant-induced-disorder issues in P3HT and serve as an example for achieving fast charge transport in highly doped polymers for efficient electronics.

Transient-State Self-Bipolarized Organic Frameworks of Single Aromatic Units for Natural Sunlight-Driven Photosynthesis of H2O2

Constructing π-conjugated polymer structures through covalent bonds dominates the design of organic framework photocatalysts, which significantly depends on the selection of multiple donor-acceptor building blocks to narrow the optical gap and increase the lifetimes of charge carriers. In this work, self-bipolarized organic frameworks of single aromatic units are demonstrated as novel broad-spectrum-responsive photocatalysts for H2O2 production. The preparation of such photocatalysts is only to fix the aromatic units (such as 1,3,5-triphenylbenzene) with alkane linkers in 3D space. Self-bipolarized aromatic units can drive the H2O2 production from H2O and O2 under natural sunlight, wide pH ranges (3.0-10.0) and natural water sources. Moreover, it can be extended to catalyze the oxidative coupling of amines. Experimental and theoretical investigation demonstrate that such a strategy obeys the mechanism of through-space π-conjugation, where the closely face-to-face overlapped aromatic rings permit the electron and energy transfer through the large-area delocalization of the electron cloud under visible light irradiation. This work introduces a novel design concept for the development of organic photocatalysts, which will break the restriction of conventional through-band π-conjugation structure and will open a new way in the synthesis of organic photocatalysts.

A pattern recognition artificial olfactory system based on human olfactory receptors and organic synaptic devices

Neuromorphic sensors, designed to emulate natural sensory systems, hold the promise of revolutionizing data extraction by facilitating rapid and energy-efficient analysis of extensive datasets. However, a challenge lies in accurately distinguishing specific analytes within mixtures of chemically similar compounds using existing neuromorphic chemical sensors. In this study, we present an artificial olfactory system (AOS), developed through the integration of human olfactory receptors (hORs) and artificial synapses. This AOS is engineered by interfacing an hOR-functionalized extended gate with an organic synaptic device. The AOS generates distinct patterns for odorants and mixtures thereof, at the molecular chain length level, attributed to specific hOR-odorant binding affinities. This approach enables precise pattern recognition via training and inference simulations. These findings establish a foundation for the development of high-performance sensor platforms and artificial sensory systems, which are ideal for applications in wearable and implantable devices.

Dopant-induced Morphology of Organic Semiconductors Resulting in High Doping Performance

Doping plays a crucial role in modulating and enhancing the performance of organic semiconductor (OSC) devices. In this study, the critical role of dopants is underscored in shaping the morphology and structure of OSC films, which in turn profoundly influences their properties. Two dopants, trityl tetrakis(pentafluorophenyl) (TrTPFB) and N,N-dimethylanilinium tetrakis(pentafluorophenyl)borate (DMA-TPFB), are examined for their doping effects on poly(3-hexylthiophene) (P3HT) and PBBT-2T host OSCs. It is found that although TrTPFB exhibits higher doping efficiency, OSCs doped with DMA-TPFB achieve comparable or even enhanced electrical conductivity. Indeed, the electrical conductivity of DMA-TPFB-doped P3HT reaches over 67 S cm-1, which is a record-high value for mixed-solution-doped P3HT. This can be attributed to DMA-TPFB inducing a higher degree of crystallinity and reduced structural disorder. Moreover, the beneficial impact of DMA-TPFB on the OSC films' morphology and structure results in superior thermoelectric performance in the doped OSCs. These findings highlight the significance of dopant-induced morphological and structural considerations in enhancing the film characteristics of OSCs, opening up a new avenue for optimization of dopant performance.

2D Ionic Liquid-Like State of Charged Rare-Earth Clusters on a Metal Surface

Rare-earth complexes are vital for separation chemistry and useful in many advanced applications including emission and energy upconversion. Here, 2D rare-earth clusters having net charges are formed on a metal surface, enabling investigations of their structural and electronic properties on a one-cluster-at-a-time basis using scanning tunneling microscopy. While these ionic complexes are highly mobile on the surface at ≈100 K, their mobility is greatly reduced at 5 K and reveals stable and self-limiting clusters. In each cluster, a pair of charged rare-earth complexes formed by electrostatic and dispersive interactions act as a basic unit, and the clusters are chiral. Unlike other non-ionic molecular clusters formed on the surfaces, these rare-earth clusters show mechanical stability. Moreover, their high mobility on the surface suggests that they are in a 2D liquid-like state.

Chemical modification and doping of poly(p-phenylenes): A theoretical study

CONTEXT

Conjugated polymers (CPs) have been recognized as promising materials for the manufacture of electronic devices. However, further studies are still needed to enhance the electrical conductivity of these type of organic materials. The two main strategies for achieving this improvement are the doping process and chemical modification of the polymer chain. Therefore, in this article, we conduct a theoretical investigation, employing DFT calculations to evaluate the structural, energetic, and electronic properties of pristine and push-pull-derived poly(p-phenylene) oligomers (PPPs), as well as the analysis at the molecular level of the polymer doping process. As a primary conclusion, we determined that the PPP oligomer substituted with the push-pull group 4-EtN/CNPhNO2 exhibited the smallest HOMO-LUMO gap (Eg) among the studied oligomers. Moreover, we observed that the doping process, whether through electron removal or the introduction of the dopant anion ClO4-, led to a substantial reduction in the Eg of the PPP, indicating an enhancement in the polymer's electrical conductivity.

METHODS

DFT calculations were conducted using the PBE0 functional along with the Pople's split valence 6-31G(d,p) basis set, which includes polarization functions on all atoms (B97D/6-31G(d,p)).

High Thermoelectric Performance in Solution-Processed Semicrystalline PEDOT:PSS Films by Strong Acid-Base Treatment: Limitations and Potential

Thermoelectric (TE) generation with solution-processable conducting polymers offers substantial potential in low-temperature energy harvesting based on high tunability in materials, processes, and form-factors. However, manipulating the TE and charge transport properties accompanies structural and energetic disorders, restricting the enhancement of thermoelectric power factor (PF). Here, solution-based strong acid-base treatment techniques are introduced to modulate the doping level of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin films with preserving its molecular orientation, enabling to achieve a remarkably high PF of 534.5 µW m-1  K-2 . Interestingly, theoretical modeling suggested that further de-doping can increase the PF beyond the experimental value. However, it is impossible to reach this value experimentally, even without any degradation of PEDOT crystallinity. Uncovering the underlying reason for the limitation, an analysis of the relationship among the microstructure-thermoelectric performance-charge transport property revealed that inter-domain connectivity via tie-chains and the resultant percolation for transport are crucial factors in achieving high TE performance, as in charge transport. It is believed that the methods and fundamental understandings in this work would contribute to the exploitation of conducting polymer-based low-temperature energy harvesting.

Impact of doping on the mechanical properties of conjugated polymers

Conjugated polymers exhibit a unique portfolio of electrical and electrochemical behavior, which - paired with the mechanical properties that are typical for macromolecules - make them intriguing candidates for a wide range of application areas from wearable electronics to bioelectronics. However, the degree of oxidation or reduction of the polymer can strongly impact the mechanical response and thus must be considered when designing flexible or stretchable devices. This tutorial review first explores how the chain architecture, processing as well as the resulting nano- and microstructure impact the rheological and mechanical properties. In addition, different methods for the mechanical characterization of thin films and bulk materials such as fibers are summarized. Then, the review discusses how chemical and electrochemical doping alter the mechanical properties in terms of stiffness and ductility. Finally, the mechanical response of (doped) conjugated polymers is discussed in the context of (1) organic photovoltaics, representing thin-film devices with a relatively low charge-carrier density, (2) organic thermoelectrics, where chemical doping is used to realize thin films or bulk materials with a high doping level, and (3) organic electrochemical transistors, where electrochemical doping allows high charge-carrier densities to be reached, albeit accompanied by significant swelling. In the future, chemical and electrochemical doping may not only allow modulation and optimization of the electrical and electrochemical behavior of conjugated polymers, but also facilitate the design of materials with a tunable mechanical response.

Confined Space Nanoarchitectonics for Dynamic Functions and Molecular Machines

Nanotechnology has advanced the techniques for elucidating phenomena at the atomic, molecular, and nano-level. As a post nanotechnology concept, nanoarchitectonics has emerged to create functional materials from unit structures. Consider the material function when nanoarchitectonics enables the design of materials whose internal structure is controlled at the nanometer level. Material function is determined by two elements. These are the functional unit that forms the core of the function and the environment (matrix) that surrounds it. This review paper discusses the nanoarchitectonics of confined space, which is a field for controlling functional materials and molecular machines. The first few sections introduce some of the various dynamic functions in confined spaces, considering molecular space, materials space, and biospace. In the latter two sections, examples of research on the behavior of molecular machines, such as molecular motors, in confined spaces are discussed. In particular, surface space and internal nanospace are taken up as typical examples of confined space. What these examples show is that not only the central functional unit, but also the surrounding spatial configuration is necessary for higher functional expression. Nanoarchitectonics will play important roles in the architecture of such a total system.

2D Materials Nanoarchitectonics for 3D Structures/Functions

It has become clear that superior material functions are derived from precisely controlled nanostructures. This has been greatly accelerated by the development of nanotechnology. The next step is to assemble materials with knowledge of their nano-level structures. This task is assigned to the post-nanotechnology concept of nanoarchitectonics. However, nanoarchitectonics, which creates intricate three-dimensional functional structures, is not always easy. Two-dimensional nanoarchitectonics based on reactions and arrangements at the surface may be an easier target to tackle. A better methodology would be to define a two-dimensional structure and then develop it into a three-dimensional structure and function. According to these backgrounds, this review paper is organized as follows. The introduction is followed by a summary of the three issues; (i) 2D to 3D dynamic structure control: liquid crystal commanded by the surface, (ii) 2D to 3D rational construction: a metal-organic framework (MOF) and a covalent organic framework (COF); (iii) 2D to 3D functional amplification: cells regulated by the surface. In addition, this review summarizes the important aspects of the ultimate three-dimensional nanoarchitectonics as a perspective. The goal of this paper is to establish an integrated concept of functional material creation by reconsidering various reported cases from the viewpoint of nanoarchitectonics, where nanoarchitectonics can be regarded as a method for everything in materials science.

Theoretical Perspective of Enhancing Order in n-Doped Thermoelectric Polymers through Side Chain Engineering: The Interplay of Counterion-Backbone Interaction and Side Chain Steric Hindrance

Donor-acceptor (D-A) copolymers doped with n-type dopants are widely sought after for their potential in organic thermoelectric devices. However, the existing structural disorder significantly hampers their charge transport and thermoelectric performance. In this Letter, we propose a mechanism to mitigate this disorder through side chain engineering. Utilizing molecular dynamics simulations, we demonstrate that strong Coulomb interactions between counterions and charged polymer backbones induce a transition in the stacking arrangement of the polymer backbones from a slipped to a vertical configuration. However, the presence of side chain steric hindrance impedes the formation of closely packed and ordered vertical stacking arrangements, resulting in greater distances between adjacent backbones and a higher level of structural disorder in the doped films. Therefore, we propose minimizing side chain steric hindrance to enhance the structural order in doped films. Our findings provide essential insights for advancing high-performance thermoelectric polymers.

Charge transfer in superbase n-type doping of PCBM induced by deprotonation

N-type electronic doping of organic semiconductors (OSCs) by using superbase compounds shows high doping efficiency (H. Wei, Z. Cheng, T. Wu, Y. Liu, J. Guo, P.-A. Chen, J. Xia, H. Xie, X. Qiu, T. Liu, B. Zhang, J. Hui, Z. Zeng, Y. Bai and Y. Hu, Adv. Mater. 2023, 35, 2300084). While a deprotonation reaction is believed to trigger the doping process, the detailed mechanism therein is not yet fully understood. In the present work we theoretically study the electronic structure of the deprotonated Phenyl-C61-butyric acid methyl ester (PCBM) molecule, as well as the charge transfer (CT) between PCBM and its deprotonated species. We find that deprotonated PCBM without formation of a new bond between the deprotonated side chain and fullerene induces electronic structure with broken spin symmetry, where an in-gap state is singly occupied by an unpaired electron. A second scenario that we find to be possible is the formation of a new bond between the deprotonated side chain and a fullerene. This leads to a spin symmetric electronic structure with partially localized in-gap state, which is expected to contribute less to the effective doping. These results show that the deprotonated PCBM species without new bond formation predominantly accounts for the effective n-type doping of PCBM, an insight that will be useful for optimization of this recently discovered doping method.

Unraveling the crucial role of trace oxygen in organic semiconductors

Optoelectronic properties of semiconductors are significantly modified by impurities at trace level. Oxygen, a prevalent impurity in organic semiconductors (OSCs), has long been considered charge-carrier traps, leading to mobility degradation and stability problems. However, this understanding relies on the conventional deoxygenation methods, by which oxygen residues in OSCs are inevitable. It implies that the current understanding is questionable. Here, we develop a non-destructive deoxygenation method (i.e., de-doping) for OSCs by a soft plasma treatment, and thus reveal that trace oxygen significantly pre-empties the donor-like traps in OSCs, which is the origin of p-type characteristics exhibited by the majority of these materials. This insight is completely opposite to the previously reported carrier trapping and can clarify some previously unexplained organic electronics phenomena. Furthermore, the de-doping results in the disappearance of p-type behaviors and significant increase of n-type properties, while re-doping (under light irradiation in O2) can controllably reverse the process. Benefiting from this, the key electronic characteristics (e.g., polarity, conductivity, threshold voltage, and mobility) can be precisely modulated in a nondestructive way, expanding the explorable property space for all known OSC materials.

Materials Nanoarchitectonics at Dynamic Interfaces: Structure Formation and Functional Manipulation

The next step in nanotechnology is to establish a methodology to assemble new functional materials based on the knowledge of nanotechnology. This task is undertaken by nanoarchitectonics. In nanoarchitectonics, we architect functional material systems from nanounits such as atoms, molecules, and nanomaterials. In terms of the hierarchy of the structure and the harmonization of the function, the material created by nanoarchitectonics has similar characteristics to the organization of the functional structure in biosystems. Looking at actual biofunctional systems, dynamic properties and interfacial environments are key. In other words, nanoarchitectonics at dynamic interfaces is important for the production of bio-like highly functional materials systems. In this review paper, nanoarchitectonics at dynamic interfaces will be discussed, looking at recent typical examples. In particular, the basic topics of "molecular manipulation, arrangement, and assembly" and "material production" will be discussed in the first two sections. Then, in the following section, "fullerene assembly: from zero-dimensional unit to advanced materials", we will discuss how various functional structures can be created from the very basic nanounit, the fullerene. The above examples demonstrate the versatile possibilities of architectonics at dynamic interfaces. In the last section, these tendencies will be summarized, and future directions will be discussed.

Molecular Doping Modulation and Applications of Structure-Sorted Single-Walled Carbon Nanotubes: A Review

Single-walled carbon nanotubes (SWCNTs) that have a reproducible distribution of chiralities or single chirality are among the most competitive materials for realizing post-silicon electronics. Molecular doping, with its non-destructive and fine-tunable characteristics, is emerging as the primary doping approach for the structure-controlled SWCNTs, enabling their eventual use in various functional devices. This review provides an overview of important advances in the area of molecular doping of structure-controlled SWCNTs and their applications. The first part introduces the underlying physical process of molecular doping, followed by a comprehensive survey of the commonly used dopants for SWCNTs to date. Then, it highlights how the convergence of molecular doping and structure-sorting strategies leads to significantly improved functionality of SWCNT-based field-effect transistor arrays, transparent electrodes in optoelectronics, thermoelectrics, and many emerging devices. At last, several challenges and opportunities in this field are discussed, with the hope of shedding light on promoting the practical application of SWCNTs in future electronics.

Iodonium Initiators: Paving the Air-free Oxidation of Spiro-OMeTAD for Efficient and Stable Perovskite Solar Cells

To date, perovskite solar cells (pero-SCs) with doped 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-OMeTAD) hole transporting layers (HTLs) have shown the highest recorded power conversion efficiencies (PCEs). However, their commercialization is still impeded by poor device stability owing to the hygroscopic lithium bis(trifluoromethanesulfonyl)imide and volatile 4-tert-butylpyridine dopants as well as time-consuming oxidation in air. In this study, we explored a series of single-component iodonium initiators with strong oxidability and different electron delocalization properties to precisely manipulate the oxidation states of Spiro-OMeTAD without air assistance, and the oxidation mechanism was clearly understood. Iodine (III) in the diphenyliodonium cation (IP+ ) can accept a single electron from Spiro-OMeTAD and forms Spiro-OMeTAD⋅+ owing to its strong oxidability. Moreover, because of the coordination of the strongly delocalized TFSI- with Spiro-OMeTAD⋅+ in a stable radical complex, the resulting hole mobility was 30 times higher than that of pristine Spiro-OMeTAD. In addition, the IP-TFSI initiator facilitated the growth of a homogeneous and pinhole-free Spiro-OMeTAD film. The pero-SCs based on this oxidizing HTL showed excellent efficiencies of 25.16 % (certified: 24.85 % for 0.062-cm2 ) and 20.71 % for a 15.03-cm2 module as well as remarkable overall stability.

Significant increase of the photoresponse range and conductivity for a chalcogenide semiconductor by viologen coating through charge transfer

Widening the photoresponse range while enhancing the electrical properties of semiconductors could reduce the complexity and cost of photodetectors or increase the power conversion efficiency of solar cells. Surface doping through charge transfer with organic species is one of the most effective and widely used approaches to achieve this aim. It usually features easier preparation over other doping methods but is still limited by the low physicochemical stability and high cost of the used organic species or low improvement of electrical properties. This work shows unprecedented surface doping of semiconductors with highly stable, easily obtained, and strong electron-accepting viologen components, realizing the significant improvement of both the photoresponse range and conductivity. Coating the chalcogenide semiconductor KGaS2 with dimethyl viologen dichloride (MV) yields a charge-transfer complex (CTC) on the surface, which broadens the photoresponse range by nearly 300 nm and improves the conductivity by 5 orders of magnitude. The latter value surpasses all records obtained by surface doping through charge transfer with organic species.

Interplay between Side Chain Density and Polymer Alignment: Two Competing Strategies for Enhancing the Thermoelectric Performance of P3HT Analogues

A series of polythiophenes with varying side chain density was synthesized, and their electrical and thermoelectric properties were investigated. Aligned and non-aligned thin films of the polymers were characterized in the neutral and chemically doped states. Optical and diffraction measurements revealed an overall lower order in the thin films with lower side chain density, also confirmed using polarized optical experiments on aligned thin films. However, upon doping the non-aligned films, a sixfold increase in electrical conductivity was observed for the polythiophene with the lowest side chain density compared to poly(3-hexylthiophene) (P3HT). We found that the improvement in conductivity was not due to a larger charge carrier density but an increase in charge carrier mobility after doping with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). On the other hand, doped aligned films did not show the same trend; lower side chain density instead led to a lower conductivity and Seebeck coefficient compared to those for P3HT. This was attributed to the poorer alignment of the polymer thin films with lower side chain density. The study demonstrates that optimizing side chain density is a synthetically simple and effective way to improve electrical conductivity in polythiophene films relevant to thermoelectric applications.

Doping of molecular semiconductors through proton-coupled electron transfer

The chemical doping of molecular semiconductors is based on electron-transfer reactions between the semiconductor and dopant molecules; here, the redox potential of the dopant is key to control the Fermi level of the semiconductor1,2. The tunability and reproducibility of chemical doping are limited by the availability of dopant materials and the effects of impurities such as water. Here we focused on proton-coupled electron-transfer (PCET) reactions, which are widely used in biochemical processes3,4; their redox potentials depend on an easily handled parameter, that is, proton activity. We immersed p-type organic semiconductor thin films in aqueous solutions with PCET-based redox pairs and hydrophobic molecular ions. Synergistic reactions of PCET and ion intercalation resulted in efficient chemical doping of crystalline organic semiconductor thin films under ambient conditions. In accordance with the Nernst equation, the Fermi levels of the semiconductors were controlled reproducibly with a high degree of precision-a thermal energy of about 25 millielectronvolts at room temperature and over a few hundred millielectronvolts around the band edge. A reference-electrode-free, resistive pH sensor based on this method is also proposed. A connection between semiconductor doping and proton activity, a widely used parameter in chemical and biochemical processes, may help create a platform for ambient semiconductor processes and biomolecular electronics.

Extremely Electron-Withdrawing Lewis-Paired CN Groups for Organic p-Dopants

P-type chemical doping (p-doping) is a key technique to modulate the optical, electrical, and electronic properties of organic semiconductors. However, typical functional groups in organic p-dopants have insufficient electron-withdrawing strength, and the inevitable diffusion of dopants in host matrices degrades doping stabilities. Herein, we utilize extremely electron-withdrawing Lewis-paired CN groups as a new class of building blocks for designing unprecedentedly strong organic p-dopants with excellent doping stability. Various Lewis acids are paired with CN-functionalized conjugated molecules in the solution state, which strengthens the electron-withdrawing properties of CN groups almost twofold. The large dopants afford outstanding doping stability against continuous heating and long-term atmospheric exposure, which is promising for practical applications in devices. Given the broad applicability of this simple combinatorial approach, it may impact many fields of (opto)electronics.

New Chemical Dopant and Counterion Mechanism for Organic Electrochemical Transistors and Organic Mixed Ionic-Electronic Conductors

Organic mixed ionic-electronic conductors (OMIECs) have varied performance requirements across a diverse application space. Chemically doping the OMIEC can be a simple, low-cost approach for adapting performance metrics. However, complex challenges, such as identifying new dopant materials and elucidating design rules, inhibit its realization. Here, these challenges are approached by introducing a new n-dopant, tetrabutylammonium hydroxide (TBA-OH), and identifying a new design consideration underpinning its success. TBA-OH behaves as both a chemical n-dopant and morphology additive in donor acceptor co-polymer naphthodithiophene diimide-based polymer, which serves as an electron transporting material in organic electrochemical transistors (OECTs). The combined effects enhance OECT transconductance, charge carrier mobility, and volumetric capacitance, representative of the key metrics underpinning all OMIEC applications. Additionally, when the TBA+ counterion adopts an "edge-on" location relative to the polymer backbone, Coulombic interaction between the counterion and polaron is reduced, and polaron delocalization increases. This is the first time such mechanisms are identified in doped-OECTs and doped-OMIECs. The work herein therefore takes the first steps toward developing the design guidelines needed to realize chemical doping as a generic strategy for tailoring performance metrics in OECTs and OMIECs.

Remarkable Electrical Conductivity Increase and Pure Metallic Properties from Semiconducting Colloidal Nanocrystals by Cation Exchange for Solution-Processable Optoelectronic Applications

The authors report a strategic approach to achieve metallic properties from semiconducting CuFeS colloidal nanocrystal (NC) solids through cation exchange method. An unprecedentedly high electrical conductivity is realized by the efficient generation of charge carriers onto a semiconducting CuS NC template via minimal Fe exchange. An electrical conductivity exceeding 10 500 S cm^-1 (13 400 S cm^-1 at 2 K) and a sheet resistance of 17 Ω/sq at room temperature, which are among the highest values for solution-processable semiconducting NCs, are achieved successfully from bornite-phase CuFeS NC films possessing 10% Fe atom. The temperature dependence of the corresponding films exhibits pure metallic characteristics. Highly conducting NCs are demonstrated for a thermoelectric layer exhibiting a high power factor over 1.2 mW m^-1 K^-2 at room temperature, electrical wires for switching on light emitting diods (LEDs), and source-drain electrodes for p- and n-type organic field-effect transistors. Ambient stability, eco-friendly composition, and solution-processability further validate their sustainable and practical applicability. The present study provides a simple but very effective method for significantly increasing charge carrier concentrations in semiconducting colloidal NCs to achieve metallic properties, which is applicable to various optoelectronic devices.

Novel Organic Superbase Dopants for Ultraefficient N-Doping of Organic Semiconductors

Doping is a powerful technique for engineering the electrical properties of organic semiconductors (OSCs), yet efficient n-doping of OSCs remains a central challenge. Herein, the discovery of two organic superbase dopants, namely P2-t-Bu and P4-t-Bu as ultra-efficient n-dopants for OSCs is reported. Typical n-type semiconductors such as N2200 and PC₆₁ BM are shown to experience a significant increase of conductivity upon doping by the two dopants. In particular, the optimized electrical conductivity of P2-t-Bu-doped PC₆₁ BM reaches a record-high value of 2.64 S cm^-1 . The polaron generation efficiency of P2-t-Bu-doped in PC₆₁ BM is found to be over 35%, which is 2-3 times higher than that of benchmark n-dopant N-DMBI. In addition, a deprotonation-initiated, nucleophilic-attack-based n-doping mechanism is proposed for the organic superbases, which involves the deprotonation of OSC molecules, the nucleophilic attack of the resulting carbanions on the OSC's π-bonds, and the subsequent n-doping through single electron transfer process between the anionized and neutral OSCs. This work highlights organic superbases as promising n-dopants for OSCs and opens up opportunities to explore and develop highly efficient n-dopants.

Efficient and air-stable n-type doping in organic semiconductors

Chemical doping of organic semiconductors (OSCs) enables feasible tuning of carrier concentration, charge mobility, and energy levels, which is critical for the applications of OSCs in organic electronic devices. However, in comparison with p-type doping, n-type doping has lagged far behind. The achievement of efficient and air-stable n-type doping in OSCs would help to significantly improve electron transport and device performance, and endow new functionalities, which are, therefore, gaining increasing attention currently. In this review, the issue of doping efficiency and doping air stability in n-type doped OSCs was carefully addressed. We first clarified the main factors that influenced chemical doping efficiency in n-type OSCs and then explain the origin of instability in n-type doped films under ambient conditions. Doping microstructure, charge transfer, and dissociation efficiency were found to determine the overall doping efficiency, which could be precisely tuned by molecular design and post treatments. To further enhance the air stability of n-doped OSCs, design strategies such as tuning the lowest unoccupied molecular orbital (LUMO) energy level, charge delocalization, intermolecular stacking, in situ n-doping, and self-encapsulations are discussed. Moreover, the applications of n-type doping in advanced organic electronics, such as solar cells, light-emitting diodes, field-effect transistors, and thermoelectrics are being introduced. Finally, an outlook is provided on novel doping ways and material systems that are aimed at stable and efficient n-type doped OSCs.

Dithieno[3,2-f:2',3'-h]quinoxaline-Based Photovoltaic-Thermoelectric Dual-Functional Energy-Harvesting Wide-Bandgap Polymer and its Backbone Isomer

Both organic solar cells (OSCs) and organic thermoelectrics (OTEs) are promising energy-harvesting technologies for future renewable and sustainable energy sources. Among various material systems, organic conjugated polymers are an emerging material class for the active layers of both OSCs and OTEs. However, organic conjugated polymers showing both OSC and OTE properties are rarely reported because of the different requirements toward the OSCs and OTEs. In this study, the first simultaneous investigation of the OSC and OTE properties of a wide-bandgap polymer PBQx-TF and its backbone isomer iso-PBQx-TF are reported. All wide-bandgap polymers form face-on orientations in a thin-film state, but PBQx-TF has more of a crystalline character than iso-PBQx-TF, originating from the backbone isomeric structures of α,α '/β,β '-connection between two thiophene rings. Additionally, iso-PBQx-TF shows inactive OSC and poor OTE properties, probably because of the absorption mismatch and unfavorable molecular orientations. At the same time, PBQx-TF exhibits both decent OSC and OTE performances, indicating that it satisfies the requirements for both OSCs and OTEs. This study presents the OSC and OTE dual-functional energy-harvesting wide-bandgap polymer and the future research directions for hybrid energy-harvesting materials.

Molecular nanoarchitectonics: unification of nanotechnology and molecular/materials science

The development of nanotechnology has provided an opportunity to integrate a wide range of phenomena and disciplines from the atomic scale, the molecular scale, and the nanoscale into materials. Nanoarchitectonics as a post-nanotechnology concept is a methodology for developing functional material systems using units such as atoms, molecules, and nanomaterials. Especially, molecular nanoarchitectonics has been strongly promoted recently by incorporating nanotechnological methods into organic synthesis. Examples of research that have attracted attention include the direct observation of organic synthesis processes at the molecular level with high resolution, and the control of organic syntheses with probe microscope tips. These can also be considered as starting points for nanoarchitectonics. In this review, these examples of molecular nanoarchitectonics are introduced, and future prospects of nanoarchitectonics are discussed. The fusion of basic science and the application of practical functional materials will complete materials chemistry for everything.

Highly Conductive Ultrathin Layers of Conjugated Polymers for Metal-Free Coplanar Transistors with Single-Polymer Transport Layers

Although metal or oxide conductive films are widely used as electrodes of electronic devices, organic electrodes would be more favorable for next-generation organic electronics. Here, using some model conjugated polymers as examples, we report a class of highly conductive and optically transparent polymer ultrathin layers. Vertical phase separation of semiconductor/insulator blends leads to a highly ordered two-dimensional (2D) ultrathin layer of conjugated-polymer chains on the insulator. Afterwards, the thermally evaporated dopants on the ultrathin layer lead to a conductivity of up to 10³ S cm^-1 and a sheet resistance 10³ Ω/square for a model conjugated polymer poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophenes) (PBTTT). The high conductivity is due to the high hole mobility (∼ 20 cm² V^-1 s^-1), although doping-induced charge density is still in the moderate range of 10²⁰ cm^-3 with a 1 nm thick dopant. Metal-free monolithic coplanar field-effect transistors using the same conjugated-polymer ultrathin layer with alternatively doped regions as electrodes and a semiconductor layer are realized. The field-effect mobility of this monolithic transistor is over 2 cm² V^-1 s^-1 for PBTTT, one order higher than that of the conventional PBTTT transistor using metal electrodes. The optical transparency of the single conjugated-polymer transport layer is over 90%, demonstrating a bright future for all-organic transparent electronics.

Unlocking the Full Potential of Electron-Acceptor Molecules for Efficient and Stable Hole Injection

Understanding the hole-injection mechanism and improving the hole-injection property are of pivotal importance in the future development of organic optoelectronic devices. Electron-acceptor molecules with high electron affinity (EA) are widely used in electronic applications, such as hole injection and p-doping. Although p-doping has generally been studied in terms of matching the ionization energy (IE) of organic semiconductors with the EA of acceptor molecules, little is known about the effect of the EA of acceptor molecules on the hole-injection property. In this work, the hole-injection mechanism in devices is completely clarified, and a strategy to optimize the hole-injection property of the acceptor molecule is developed. Efficient and stable hole injection is found to be possible even into materials with IEs as high as 5.8 eV by controlling the charged state of an acceptor molecule with an EA of about 5.0 eV. This excellent hole-injection property enables direct hole injection into an emitting layer, realizing a pure blue organic light-emitting diode with an extraordinarily low turn-on voltage of 2.67 V, a power efficiency of 29 lm W^-1 , an external quantum efficiency of 28% and a Commission Internationale de l'Eclairage y coordinate of less than 0.10.

Extremely Suppressed Energetic Disorder in a Chemically Doped Conjugated Polymer

Chemical doping can be used to tune the optoelectronic properties of conjugated polymers (CPs), extending their applications as conducting materials. Unfortunately, chemically doped CP films containing excess dopants exhibit an increase in energetic disorder upon structural alteration, and Coulomb interactions between charge carriers and dopants also affect such disorder. The increase in energetic disorder leads to a broadening of the density of states, which consequently impedes efficient charge transport in chemically doped CPs. However, the molecular origins that are inherently resistant to such incidental increase of energetic disorder in chemically doped CPs have not been sufficiently explored. Here, it is discovered that energetic disorder in chemically doped CPs can be suppressed to a level close to the theoretical limit. Indacenodithiophene-co-benzothiadiazole (IDTBT) doped with triethyloxonium hexachloroantimonate (OA) exhibits disorder-free charge-transport characteristics and band-like transport behavior with astonishing carrier mobility as a result of reinforced 1D intramolecular transport. Molecular structure of IDTBT provides a capability to lower the energetic disorder that generally arises from the inclusion of heterogeneous dopants. The results suggest the possibilities of implementing disorder-free CPs that exhibit excellent charge transport characteristics in the chemically doped state and satisfy a prerequisite for their availability in the industry.

Molecular Machines and Microrobots: Nanoarchitectonics Developments and On-Water Performances

This review will focus on micromachines and microrobots, which are objects at the micro-level with similar machine functions, as well as nano-level objects such as molecular machines and nanomachines. The paper will initially review recent examples of molecular machines and microrobots that are not limited to interfaces, noting the diversity of their functions. Next, examples of molecular machines and micromachines/micro-robots functioning at the air-water interface will be discussed. The behaviors of molecular machines are influenced significantly by the specific characteristics of the air-water interface. By placing molecular machines at the air-water interface, the scientific horizon and depth of molecular machine research will increase dramatically. On the other hand, for microrobotics, more practical and advanced systems have been reported, such as the development of microrobots and microswimmers for environmental remediations and biomedical applications. The research currently being conducted on the surface of water may provide significant basic knowledge for future practical uses of molecular machines and microrobots.

Revealing the Electrophilic-Attack Doping Mechanism for Efficient and Universal p-Doping of Organic Semiconductors

Doping is of great importance to tailor the electrical properties of semiconductors. However, the present doping methodologies for organic semiconductors (OSCs) are either inefficient or can only apply to some OSCs conditionally, seriously limiting their general applications. Herein, a novel p-doping mechanism is revealed by investigating the interactions between the dopant trityl tetrakis(pentafluorophenyl) borate (TrTPFB) and poly(3-hexylthiophene) (P3HT). It is found that electrophilic attack of the trityl cations on thiophenes results in the formation of tritylated thiophenium ions, which subsequently induce electron transfer from neighboring P3HT chains to realize p-doping. This unique p-doping mechanism enables TrTPFB to p-dope various OSCs including those with high ionization energy (IE ≈ 5.8 eV). Moreover, this doping mechanism endows TrTPFB with strong doping capability, leading to doping efficiency of over 80% in P3HT. The discovery and elucidation of this novel doping mechanism not only points out that strong electrophiles are a class of efficient p-dopants for OSCs, but also provides new opportunities toward highly efficient doping of various OSCs.

Visualisation of individual dopants in a conjugated polymer: sub-nanometre 3D spatial distribution and correlation with electrical properties

While molecular doping is ubiquitous in all branches of organic electronics, little is known about the spatial distribution of dopants, especially at molecular length scales. Moreover, a homogeneous distribution is often assumed when simulating transport properties of these materials, even though the distribution is expected to be inhomogeneous. In this study, electron tomography is used to determine the position of individual molybdenum dithiolene complexes and their three-dimensional distribution in a semiconducting polymer at the sub-nanometre scale. A heterogeneous distribution is observed, the characteristics of which depend on the dopant concentration. At 5 mol% of the molybdenum dithiolene complex, the majority of the dopant species are present as isolated molecules or small clusters up to five molecules. At 20 mol% dopant concentration and higher, the dopant species form larger nanoclusters with elongated shapes. Even in case of these larger clusters, each individual dopant species is still in contact with the surrounding polymer. The electrical conductivity first strongly increases with dopant concentration and then slightly decreases for the most highly doped samples, even though no large aggregates can be observed. The decreased conductivity is instead attributed to the increased energetic disorder and lower probability of electron transfer that originates from the increased size and size variation in dopant clusters. This study highlights the importance of detailed information concerning the dopant spatial distribution at the sub-nanometre scale in three dimensions within the organic semiconductor host. The information acquired using electron tomography may facilitate more accurate simulations of charge transport in doped organic semiconductors.

Tuning Threshold Voltage of Electrolyte-Gated Transistors by Binary Ion Doping

Electrolyte-gated transistors (EGTs) operating at low voltages have attracted significant attention in widespread applications, including neuromorphic devices, nonvolatile memories, chemical/biosensors, and printed electronics. To increase the practicality of the EGTs in electronic circuits, systematic control of threshold voltage (Vth), which determines the power consumption and noise margin of the circuits, is essential. In this study, we present a simple strategy for systematically tuning Vth to almost half of the operating potential range of the EGT by controlling the electrochemical doping of electrolyte ions into organic p-type semiconductors. The type of anion in the ionogel determines Vth as well as other transistor characteristics, such as the subthreshold swing and mobility, because the positive hole carriers are the majority carriers. More importantly, Vth can be finely controlled by binary anion doping using ionogels with two anions with varying molar fractions at a fixed cation. In addition, the binary anion doping successfully controls the inversion characteristics of ion-gated inverters. As unlimited combinations of ion pairs are possible for ionogels, this study opens a route for controlling the device characteristics to expand the practicality and applicability of ionogel-based EGTs for next-generation ionic/electronic devices.

Strain-induced thermoelectricity in pentacene

The present work discusses a non-synthetic strategy to achieve a favorable thermoelectric response in pentacene via strain. It is found that a uni-axial strain is capable of inducing spatial anisotropy in the molecule. As a result, the transmission spectrum becomes highly asymmetric under a particular strained scenario, which is the primary requirement to get a favorable thermoelectric response. Different thermoelectric quantities are computed for the strain-induced pentacene using Green's function formalism following the Landauer-Büttiker prescription. Various scenarios are considered to make the present work more realistic, such as the effects of substrate, coupling strength between the molecule and electrodes, dangling bonds, etc. Such a scheme to enhance the thermoelectric performance in pentacene is technologically intriguing and completely new to the best of our knowledge.

Transporting holes stably under iodide invasion in efficient perovskite solar cells

Highly efficient halide perovskite solar cells generally rely on lithium-doped organic hole transporting layers that are thermally and chemically unstable, in part because of migration of iodide anions from the perovskite layer. We report a solution strategy to stabilize the hole transport in organic layers by ionic coupling positive polymer radicals and molecular anions through an ion-exchange process. The target layer exhibited a hole conductivity that was 80 times higher than that of the conventional lithium-doped layer. Moreover, after extreme iodide invasion caused by light-soaking at 85°C for 200 hours, the target layer maintained high hole conductivity and well-matched band alignment. This ion-exchange strategy enabled fabrication of perovskite solar cells with a certified power conversion efficiency of 23.9% that maintained 92% under standard illumination at 85°C after 1000 hours.