Molecularly doped conjugated polymers with polar side chains are an emerging class of conducting materials exhibiting enhanced and thermally stable conductivity. Here, we study the electronic conductivity (σ) and the corresponding thermal stability of two polythiophene derivatives comprising oligoethylene glycol side chains: one having oxygen attached to the thiophene ring (poly­(3-(methoxyethoxyethoxy)­thiophene) (P3MEET)) and the other having a methylene spacer between the oxygen and the thiophene ring (poly­(3-(methoxyethoxyethoxymethyl)­thiophene) (P3MEEMT)). Thin films were vapor-doped with fluorinated derivatives of tetracyanoquinodimethane (F n TCNQ, n = 4, 2, 1) to determine the role of dopant strength (electron affinity) in maximum achievable σ. Specifically, when vapor doping with F4TCNQ, P3MEET achieved a substantially higher σ of 37.1 ± 10.1 S/cm compared to a σ of 0.82 ± 0.06 S/cm for P3MEEMT. Structural characterization using a combination of X-ray and optical spectroscopy reveals that the higher degree of conformational order of polymer chains in the amorphous domain upon doping with F4TCNQ in P3MEET is a major contributing factor for the higher σ of P3MEET. Additionally, vapor-doped P3MEET exhibited superior thermal stability compared to P3MEEMT, highlighting that the presence of polar side chains alone does not ensure higher thermal stability. Molecular dynamics simulations indicate that the dopant–side-chain nonbond energy is lower in the P3MEET:F4TCNQ mixture, suggesting more favorable dopant–side-chain interaction, which is a factor in improving the thermal stability of a polymer/dopant pair. Our results reveal that additional factors such as polymer ionization energy and side-chain–dopant interaction should be taken into account for the design of thermally stable, highly conductive polymers.

ParagraphConducting organic materials, such as doped organic polymers, 1 molecular conductors, 2, 3 and emerging coordination polymers, 4 underpin technologies ranging from displays to flexible electronics. 5 Realizing high electrical conductivity in traditionally insulating organic materials necessitates tuning their electronic structure through chemical doping. 6 Furthermore, even materials that are intrinsically conductive, such as single-component molecular conductors, 7,8 require crystallinity for metallic behavior. However, commercial conducting polymers are often purposefully amorphous to aid in durability and processability. 9,10 Using molecular design to engender high conductivity in undoped amorphous materials would enable tunable and robust conductivity in many applications, but there are no intrinsically conducting organic materials which maintain high conductivity when disordered. Here we show that the completely amorphous coordination polymer Ni tetrathiafulvalene tetrathiolate (NiTTFtt) displays intrinsic metallic conductivity. Despite its disordered structure, NiTTFtt exhibits remarkably high electronic conductivity (1280 S/cm) and intrinsically glassy metallic behavior. Analysis with advanced theory shows that these properties are enabled by strong molecular overlap and correlation that are robust to structural perturbations. This unusual set of structural and electronic features results in remarkably stable organic conductivity which is maintained in air for weeks and at temperatures up to 140 °C. Our results demonstrate that molecular design can enable metallic conductivity even in heavily disordered materials. This both raises fundamental questions about how band-like transport can exist in the absence of periodic structure as well as suggests exciting new applications for these materials.

With the ability to modulate electronic properties through molecular doping coupled with ease in processability, semiconducting polymers are at the forefront in enabling organic thermoelectric devices for thermal energy management. In contrast to uniform thermoelectric material properties, an alternative route focuses on functionally graded materials (FGMs) where one spatially controls and optimizes transport properties across the length of a thermoelectric material. While primarily studied in the context of inorganic materials, the concept of FGMs for organic thermoelectrics has not been explored. Herein, we introduce how molecular doping of semiconducting polymers enables spatial compositional control of thin-film FGMs. Specifically, we use sequential vapor doping of poly­[2,5-bis­(3-tetradecyl­thiophen-2-yl)­thieno­[3,2-b]­thiophene] (PBTTT) with the small molecule acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano­quinodimethane (F4TCNQ) to fabricate the simplest form of FGMsdouble-segmented thin films. The two thin-film segments are of equal length (7.5 mm) but each set to different doping levels. Our study focuses on understanding the thermoelectric properties (Seebeck coefficient, α, and electronic conductivity, σ) and structural properties (through X-ray scattering, UV–vis–NIR spectroscopy, and Raman spectroscopy) within and across the two segments. We observe the presence of a small diffuse interfacial region of 0.5–1 mm between the two segments where the doping level and transport properties vary continuously. Despite the diffuse interface, the measured α across the two segments is simply the average of α within each segment. Importantly, this experimental result is consistent with reported mathematical models describing the spatial average of α  in graded thermoelectric materials. Our results demonstrate the facile fabrication and characterization of functionally graded organic thermoelectric materials, providing guidelines for further development on more complex FGMs.

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Photothermoelectric (PTE) materials are promising candidates for solar energy harvesting and photodetection applications, especially for near-infrared (NIR) wavelengths. Although the processability and tunability of organic materials are highly advantageous, examples of organic PTE materials are comparatively rare and their PTE performance is typically limited by poor photothermal (PT) conversion. Here, we report the use of redox-active Sn complexes of tetrathiafulvalene-tetrathiolate (TTFtt) as transmetalating agents for the synthesis of presynthetically redox tuned NiTTFtt materials. Unlike the neutral material NiTTFtt, which exhibits n-type glassy-metallic conductivity, the reduced materials Li1.2Ni0.4[NiTTFtt] and [Li­(THF)1.5]1.2Ni0.4[NiTTFtt] (THF = tetrahydrofuran) display physical characteristics more consistent with p-type semiconductors. The broad spectral absorption and electrically conducting nature of these TTFtt-based materials enable highly efficient NIR-thermal conversion and good PTE performance. Furthermore, in contrast to conventional PTE composites, these NiTTFtt coordination polymers are notable as single-component PTE materials. The presynthetically tuned metal-to-insulator transition in these NiTTFtt systems directly modulates their PT and PTE properties.