Drastic Improvement of Air Stability in an n-Type Doped Naphthalene-Diimide Polymer by Thionation

Pataki N, Rossi P, Erhardt A, Guchait S, Pallini F, McNeill C, Müller C, Caironi M, Sommer M. Solubilizing Benzodifuranone-Based Conjugated Copolymers with Single-Oxygen-Containing Branched Side Chains

Single-oxygen-containing branched side chains are designed and used to solubilize n-type copolymers consisting of BDF (benzodifuranone), isatin, and thiophene-based units. We present a simple synthetic approach to side chains with varying linker distances between the backbone and the branching point. The synthetic pathway is straightforward and modular and starts with commercially available reagents. The side chains give rise to excellent solubilities of BDF-thiophene copolymers of up to 90 mg/mL, while still being moderate in size (26-34 atoms large). The excellent solubility furthermore allows high molar mass materials. BDF-thiophene copolymers are characterized in terms of optoelectronic and thermoelectric properties. The electrical conductivity of chemically doped polymers is found to scale with molar mass, reaching ∼1 S/cm for the highest molar mass and longest backbone-branching point distance.

More Electropositive is More Electronegative: Atom Size Determines C=X Group Electronegativity

Opposite to what one might expect, we find that the C=X group can become effectively more, not less, electronegative when the Pauling electronegativity of atom X decreases down Groups 16, 15, and 14 of the Periodic Table. Our quantum-chemical analyses, show that, and why, this phenomenon is a direct consequence of the increasing size of atom X down a group. These findings can be applied to tuning and improving the hydrogen-bond donor strength of amides H2 NC(=X)R by increasingly withdrawing density from the NH2 group. A striking example is that H2 NC(=SiR2 )R is a stronger hydrogen-bond donor than H2 NC(=CR2 )R.

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.

Organic thermoelectric generators: working principles, materials, and fabrication techniques

Organic thermoelectricity is a blooming field of research that employs organic (semi)conductors to recycle waste heat through its partial conversion to electrical power. Such a conversion occurs by means of organic thermoelectric generator (OTEG) devices. The recent process on the synthesis of novel materials and on the understanding of doping mechanisms to increase conductivity has tremendously narrowed the gap between laboratory research and their application in actual applications. This Feature Article intends to highlight the impressive progress in materials and fabrication techniques for OTEGs made in recent years.

Why does thionating a carbonyl molecule make it a better electron acceptor?

The past decade has witnessed a surge of biomedical and materials applications of thiocarbonyl molecules (R₂CS), such as in photodynamic therapy, organic field-effect transistors, and rechargeable batteries. The success of these applications originates from thiocarbonyl's small optical gap in the visible region and the enhanced electron affinity compared to the carbonyl analogues (R₂CO). Although these observations seem to be contrary to the implication based on a simple electronegativity consideration (2.58 for sulfur and 3.44 for oxygen), a natural bond orbital (NBO) analysis gives a straightforward explanation for the LUMO-lowering effect of CO → CS substitution. In comparison to the valence (2p)_C/(2p)_O interactions in CO, the higher 3p orbital of sulfur and its weaker overlap with the 2p level of carbon result in a weaker antibonding interaction in NBO, a prominent contributor to the LUMO. Such an analysis also provides a semi-quantitative understanding of the electronic effect of substituents on or in π-conjugation with a (thio)carbonyl functionality. The intuitive concepts uncovered here offer a simple rule to predict the electronic properties of π-conjugated molecules that incorporate heavy heteroelements and would facilitate materials development.

Recent Developments of n-Type Organic Thermoelectric Materials: Influence of Structure Modification on Molecule Arrangement and Solution Processing

Organic semiconductor (OSCs) thermoelectric materials have been studied widely due to their low thermal conductivity and solution processing characteristics. Currently, the high conductivity (up to 1000 s cm^-1 ) has boosted the performance of p-type organic thermoelectric materials substantially. In contrast, the development of n-type organic thermoelectric materials is still limited by their low mobility, inferior air stability, and poor doping efficiency, which is relevant to the molecule structure and dopant dispersion. Herein, the recent development of n-type organic thermoelectric materials was reviewed with an emphasis on molecule structure modification and solution processing. Methods for optimizing conjugate structure were summarized from the effects of conjugated backbone modification and side chains diversification on molecular stacking. The primary n-type dopants were also summarized briefly. Especially, the role of solution aggregation controlling on film preparation and properties was given special attention. Additionally, the emergence of organic diradicals with low lowest unoccupied molecular orbital energy level and no doping was introduced, which shows great potential in n-type organic thermoelectric materials. All these endeavors have led to the development of n-type OSCs materials. This Review is aimed at illustrating the state-of-the-art progress and providing some guideline for the design of organic thermoelectric materials in the future.

Doping Approaches for Organic Semiconductors

Electronic doping in organic materials has remained an elusive concept for several decades. It drew considerable attention in the early days in the quest for organic materials with high electrical conductivity, paving the way for the pioneering work on pristine organic semiconductors (OSCs) and their eventual use in a plethora of applications. Despite this early trend, however, recent strides in the field of organic electronics have been made hand in hand with the development and use of dopants to the point that are now ubiquitous. Here, we give an overview of all important advances in the area of doping of organic semiconductors and their applications. We first review the relevant literature with particular focus on the physical processes involved, discussing established mechanisms but also newly proposed theories. We then continue with a comprehensive summary of the most widely studied dopants to date, placing particular emphasis on the chemical strategies toward the synthesis of molecules with improved functionality. The processing routes toward doped organic films and the important doping-processing-nanostructure relationships, are also discussed. We conclude the review by highlighting how doping can enhance the operating characteristics of various organic devices.

Unconventional Thermoelectric Materials for Energy Harvesting and Sensing Applications

Heat is an abundant but often wasted source of energy. Thus, harvesting just a portion of this tremendous amount of energy holds significant promise for a more sustainable society. While traditional solid-state inorganic semiconductors have dominated the research stage on thermal-to-electrical energy conversion, carbon-based semiconductors have recently attracted a great deal of attention as potential thermoelectric materials for low-temperature energy harvesting, primarily driven by the high abundance of their atomic elements, ease of processing/manufacturing, and intrinsically low thermal conductivity. This quest for new materials has resulted in the discovery of several new kinds of thermoelectric materials and concepts capable of converting a heat flux into an electrical current by means of various types of particles transporting the electric charge: (i) electrons, (ii) ions, and (iii) redox molecules. This has contributed to expanding the applications envisaged for thermoelectric materials far beyond simple conversion of heat into electricity. This is the motivation behind this review. This work is divided in three sections. In the first section, we present the basic principle of the thermoelectric effects when the particles transporting the electric charge are electrons, ions, and redox molecules and describe the conceptual differences between the three thermodiffusion phenomena. In the second section, we review the efforts made on developing devices exploiting these three effects and give a thorough understanding of what limits their performance. In the third section, we review the state-of-the-art thermoelectric materials investigated so far and provide a comprehensive understanding of what limits charge and energy transport in each of these classes of materials.

Naphthalene diimides: perspectives and promise

In this review, we describe the developments in the field of naphthalene diimides (NDIs) from 2016 to the presentday. NDIs are shown to be an increasingly interesting class of molecules due to their electronic properties, large electron deficient aromatic cores and tendency to self-assemble into functional structures. Almost all NDIs possess high electron affinity, good charge carrier mobility, and excellent thermal and oxidative stability, making them promising candidates for applications in organic electronics, photovoltaic devices, and flexible displays. NDIs have also been extensively studied due to their potential real-world uses across a wide variety of applications including supramolecular chemistry, sensing, host-guest complexes for molecular switching devices, such as catenanes and rotaxanes, ion-channels, catalysis, and medicine and as non-fullerene accepters in solar cells. In recent years, NDI research with respect to supramolecular assemblies and mechanoluminescent properties has also gained considerable traction. Thus, this review will assist a wide range of readers and researchers including chemists, physicists, biologists, medicinal chemists and materials scientists in understanding the scope for development and applicability of NDI dyes in their respective fields through a discussion of the main properties of NDI derivatives and of the status of emerging applications.

Persistent Conjugated Backbone and Disordered Lamellar Packing Impart Polymers with Efficient n-Doping and High Conductivities

Solution-processable highly conductive polymers are of great interest in emerging electronic applications. For p-doped polymers, conductivities as high a nearly 10⁵ S cm^-1 have been reported. In the case of n-doped polymers, they often fall well short of the high values noted above, which might be achievable, if much higher charge-carrier mobilities determined could be realized in combination with high charge-carrier densities. This is in part due to inefficient doping and dopant ions disturbing the ordering of polymers, limiting efficient charge transport and ultimately the achievable conductivities. Here, n-doped polymers that achieve a high conductivity of more than 90 S cm^-1 by a simple solution-based co-deposition method are reported. Two conjugated polymers with rigid planar backbones, but with disordered crystalline structures, exhibit surprising structural tolerance to, and excellent miscibility with, commonly used n-dopants. These properties allow both high concentrations and high mobility of the charge carriers to be realized simultaneously in n-doped polymers, resulting in excellent electrical conductivity and thermoelectric performance.

Sequential Doping of Ladder-Type Conjugated Polymers for Thermally Stable n-Type Organic Conductors

Doping of organic semiconductors is a powerful tool to optimize the performance of various organic (opto)electronic and bioelectronic devices. Despite recent advances, the low thermal stability of the electronic properties of doped polymers still represents a significant obstacle to implementing these materials into practical applications. Hence, the development of conducting doped polymers with excellent long-term stability at elevated temperatures is highly desirable. Here, we report on the sequential doping of the ladder-type polymer poly(benzimidazobenzophenanthroline) (BBL) with a benzimidazole-based dopant (i.e., N-DMBI). By combining electrical, UV-vis/infrared, X-ray diffraction, and electron paramagnetic resonance measurements, we quantitatively characterized the conductivity, Seebeck coefficient, spin density, and microstructure of the sequentially doped polymer films as a function of the thermal annealing temperature. Importantly, we observed that the electrical conductivity of N-DMBI-doped BBL remains unchanged even after 20 h of heating at 190 °C. This finding is remarkable and of particular interest for organic thermoelectrics.

Rigid Coplanar Polymers for Stable n-Type Polymer Thermoelectrics

Low n-doping efficiency and inferior stability restrict the thermoelectric performance of n-type conjugated polymers, making their performance lag far behind of their p-type counterparts. Reported here are two rigid coplanar poly(p-phenylene vinylene) (PPV) derivatives, LPPV-1 and LPPV-2, which show nearly torsion-free backbones. The fused electron-deficient rigid structures endow the derivatives with less conformational disorder and low-lying lowest unoccupied molecular orbital (LUMO) levels, down to -4.49 eV. After doping, two polymers exhibited high n-doping efficiency and significantly improved air stability. LPPV-1 exhibited a high conductivity of up to 1.1 S cm^-1 and a power factor as high as 1.96 μW m^-1  K^-2 . Importantly, the power factor of the doped LPPV-1 thick film degraded only 2 % after 7 day exposure to air. This work demonstrates a new strategy for designing conjugated polymers, with planar backbones and low LUMO levels, towards high-performance and potentially air-stable n-type polymer thermoelectrics.