High Thermoelectric Power Factor of Poly(3-hexylthiophene) through In-Plane Alignment and Doping with a Molybdenum Dithiolene Complex

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.

A Highly Conductive n-Type Polythiophene Derivative: Effect of Molecular Weight on n-Doping Behavior and Thermoelectric Performance

Here, we examine the impact of the molecular weight of an n-type conjugated polymer (n-PT2) on molecular doping and thermoelectric parameters. Two common dopants TDAE and N-DMBI with different doping mechanisms are used for molecular doping of n-PT2. It turns out that n-PT2 with a higher molecular weight is more miscible with the dopant, leading to more charge carriers. Moreover, the crystal structures and morphology of n-PT2 with a higher molecular weight are more tolerant against the intrusion of dopant molecules and charging. Finally, these factors work in synergy to endow the doped n-PT2 with the best conductivity and power factor (144 S cm-1/75.0 μW m-1 K-2 and 75.4 S cm-1/98.5 μW m-1 K-2 after doping by TDAE and N-DMBI, respectively). This study indicates that regulating the molecular weight allows for synergistic regulation of conductivity and Seebeck coefficient and is a feasible means to improve the performance for a given n-type organic thermoelectric material.

Controlling π-Conjugated Polymer-Acceptor Interactions by Designing Polymers with a Mixture of π-Face Strapped and Nonstrapped Monomers

Controlling π-conjugated polymer-acceptor complex interaction, including the interaction strength and location along the polymer backbone, is central to organic electronics and energy applications. Straps in the strapped π-conjugated polymers mask the π-face of the polymer backbone and hence are useful to control the interactions of the π-face of the polymer backbone with other polymer chains and small molecules compared to the conventional pendant solubilizing chains. Herein, we have synthesized a series of strapped π-conjugated copolymers containing a mixture of strapped and nonstrapped comonomers to control the polymer-acceptor interactions. Simulations confirmed that the acceptor is directed toward the nonstrapped repeat unit. More importantly, strapped copolymers overcome a major drawback of homopolymers and display higher photoinduced photoluminescence (PL) quenching, which is a measure of electron transfer from the polymer to acceptor, compared to that of both the strapped homopolymer and the conventional polymer with pendant solubilizing chains. We have also shown that this strategy applies not only to strapped polymers, but also to the conventional polymers with pendant solubilizing chains. The increase in PL quenching is attributed to the absence of a steric sheath around the comonomers and their random location along the polymer backbone, which enhances the probability of non-neighbor acceptor binding events along the polymer backbone. Thus, by mixing insulated and noninsulated monomers along the polymer backbone, the location of the acceptor along the polymer backbone, polymer-acceptor interaction strength, and the efficiency of photoinduced charge transfer are controllable compared to the homopolymers.

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.

Slippage- and load-induced changes in the crystalline orientation of oligo(3-methoxythiophene) powder to develop a gold-tone luster

The achievement of molecular orientation control by rubbing and pressing poly(3-alkylthiophene)s is a powerful technique to improve the performance of organic electronic devices. We report here that the rubbing and pressing of blackish-brown 3-methoxythiophene oligomer powders yield layer and tablet samples with gold tones, respectively. Specular reflectivity, colorimetric, and X-ray diffraction measurements reveal that this gold tone is caused by an increase in the ratio of edge-on lamellar crystallites to face-on ones, which is promoted by rubbing/pressing. In contrast to the 3-alkylthiophene polymer, which develops a dominant face-on lamellar structure, rubbing of the 3-methoxythiophene oligomer increases the relative amount of edge-on lamellar crystallites to face-on lamellar ones. Furthermore, gold tone development in the tablet samples is limited to the near-surface area, despite the fact that pressure is also applied to the tablet bulk. These specific chemical events are explained by considering the repulsive interactions between the 3-methoxythiophene backbone and the functional groups on the surface of the substrate employed during the rubbing/pressing processes. Despite the lower applied pressure, gold tone development by rubbing is accompanied by a higher reflective property than by pressing because of the formation of larger relative amounts and sizes of edge-on lamellar crystallites, which are responsible for the gold tone.

Rubbing and pressing of blackish-brown 3-methoxythiophene powder produced layers and tablets with gold tone luster, respectively, due to increased edge-on lamellar crystallites.

Double Doping of a Low-Ionization-Energy Polythiophene with a Molybdenum Dithiolene Complex

Doping of organic semiconductors is crucial for tuning the charge-carrier density of conjugated polymers. The exchange of more than one electron between a monomeric dopant and an organic semiconductor allows the polaron density to be increased relative to the number of counterions that are introduced into the host matrix. Here, a molybdenum dithiolene complex with a high electron affinity of 5.5 eV is shown to accept two electrons from a polythiophene that has a low ionization energy of 4.7 eV. Double p-doping is consistent with the ability of the monoanion salt of the molybdenum dithiolene complex to dope the polymer. The transfer of two electrons to the neutral dopant was also confirmed by electron paramagnetic resonance spectroscopy since the monoanion, but not the dianion, of the molybdenum dithiolene complex features an unpaired electron. Double doping allowed an ionization efficiency of 200% to be reached, which facilitates the design of strongly doped semiconductors while lessening any counterion-induced disruption of the nanostructure.

Alignment of organic conjugated molecules for high-performance device applications

High-performance organic semiconductor materials as the electroactive components of optoelectronic devices have attracted much attention and made them ideal candidates for solution-processable, large-area, and low-cost flexible electronics. Especially, organic field-effect transistors (OFETs) based on conjugated semiconductor materials have experienced stunning progress in device performance. To make these materials economically viable, comprehensive knowledge of charge transport mechanisms is required. The alignment of organic conjugated molecules in the active layer is vital to charge transport properties of devices. The present review highlights the recent progress of processing-structure-transport correlations that allow the precise and uniform alignment of organic conjugated molecules over large areas for multiple electronic applications, including OFETs, organic thermoelectric devices (OTEs), and organic phototransistors (OPTs). Different strategies for regulating crystallinity and macroscopic orientation of conjugated molecules are introduced to correlate the molecular packing, the device performance and charge transport anisotropy in multiple organic electronic devices. This article is protected by copyright. All rights reserved.

Polaron Delocalization Dependence of the Conductivity and the Seebeck Coefficient in Doped Conjugated Polymers

Conjugated polymers are promising materials for thermoelectrics as they offer good performances at near ambient temperatures. The current focus on polymer thermoelectric research mainly targets a higher power factor (PF; a product of the conductivity and square of the Seebeck coefficient) through improving the charge mobility. This is usually accomplished via structural modification in conjugated polymers using different processing techniques and doping. As a result, the structure-charge transport relationship in conjugated polymers is generally well-established. In contrast, the relationship between the structure and the Seebeck coefficient is poorly understood due to its complex nature. A theoretical framework by David Emin (Phys. Rev. B, 1999, 59, 6205-6210) suggests that the Seebeck coefficient can be enhanced via carrier-induced vibrational softening, whose magnitude is governed by the size of the polaron. In this work, we seek to unravel this relationship in conjugated polymers using a series of highly identical pro-quinoid polymers. These polymers are ideal to test Emin's framework experimentally as the quinoid character and polaron delocalization in these polymers can be well controlled even by small atomic differences (<10 at. % per repeating unit). By increasing the polaron delocalization, that is, the polaron size, we demonstrate that both the conductivity and the Seebeck coefficient (and hence PF) can be increased simultaneously, and the latter is due to the increase in the polaron's vibrational entropy. By using literature data, we also show that this phenomenon can be observed in two closely related diketopyrrolopyrrole-conjugated polymers as well as in p-doped P3HT and PANI systems with an increasing molecular order.

Understanding the evolution of the Raman spectra of molecularly p-doped poly(3-hexylthiophene-2,5-diyl): signatures of polarons and bipolarons

Molecular doping is a key process to increase the density of charge carriers in organic semiconductors. Doping-induced charges in polymer semiconductors result in the formation of polarons and/or bipolarons due to the strong electron-vibron coupling in conjugated organic materials. Identifying the nature of charge carriers in doped polymers is essential to optimize the doping process for applications. In this work, we use Raman spectroscopy to investigate the formation of charge carriers in molecularly doped poly(3-hexylthiophene-2,5-diyl) (P3HT) for increasing dopant concentration, with the organic salt dimesityl borinium tetrakis(penta-fluorophenyl)borate (Mes₂B⁺ [B(C₆F₅)₄]^-) and the Lewis acid tris(pentafluorophenyl)borane [B(C₆F₅)₃]. While the Raman signatures of neutral P3HT and singly charged P3HT segments (polarons) are known, the Raman spectra of doubly charged P3HT segments (bipolarons) are not yet sufficiently understood. Combining Raman spectroscopy measurements on doped P3HT thin films with first-principles calculations on oligomer models, we explain the evolution of the Raman spectra from neutral P3HT to increasingly doped P3HT featuring polarons and eventually bipolarons at high doping levels. We identify and explain the origin of the spectral features related to bipolarons by tracing the Raman signature of the symmetric collective vibrations along the polymer backbone, which - compared to neutral P3HT - redshifts for polarons and blueshifts for bipolarons. This is explained by a planarization of the singly charged P3HT segments with polarons and rather high order in thin films, while the doubly charged segments with bipolarons are located in comparably disordered regions of the P3HT film due to the high dopant concentration. Furthermore, we identify additional Raman peaks associated with vibrations in the quinoid doubly charged segments of the polymer. Our results offer the opportunity for readily identifying the nature of charge carriers in molecularly doped P3HT while taking advantage of the simplicity, versatility, and non-destructive nature of Raman spectroscopy.

Organic Thermoelectric Materials as the Waste Heat Remedy

The primary reason behind the search for novel organic materials for application in thermoelectric devices is the toxicity of inorganic substances and the difficulties associated with their processing for the production of thin, flexible layers. When Thomas Seebeck described a new phenomenon in Berlin in 1820, nobody could have predicted the future applications of the thermoelectric effect. Now, thermoelectric generators (TEGs) are used in watches, and thermoelectric coolers (TECs) are applied in cars, computers, and various laboratory equipment. Nevertheless, the future of thermoelectric materials lies in organic compounds. This paper discusses the developments made in thermoelectric materials, including small molecules, polymers, molecular junctions, and their applications as TEGs and/or TECs.

Achieving Efficient p-Type Organic Thermoelectrics by Modulation of Acceptor Unit in Photovoltaic π-Conjugated Copolymers

π-Conjugated donor (D)-acceptor (A) copolymers have been extensively studied as organic photovoltaic (OPV) donors yet remain largely unexplored in organic thermoelectrics (OTEs) despite their outstanding mechanical bendability, solution processability and flexible molecular design. Importantly, they feature high Seebeck coefficient (S) that are desirable in room-temperature wearable application scenarios under small temperature gradients. In this work, the authors have systematically investigated a series of D-A semiconducting copolymers possessing various electron-deficient A-units (e.g., BDD, TT, DPP) towards efficient OTEs. Upon p-type ferric chloride (FeCl3 ) doping, the relationship between the thermoelectric characteristics and the electron-withdrawing ability of A-unit is largely elucidated. It is revealed that a strong D-A nature tends to induce an energetic disorder along the π-backbone, leading to an enlarged separation of the transport and Fermi levels, and consequently an increase of S. Meanwhile, the highly electron-deficient A-unit would impair electron transfer from D-unit to p-type dopants, thus decreasing the doping efficiency and electrical conductivity (σ). Ultimately, the peak power factor (PF) at room-temperature is obtained as high as 105.5 µW m-1 K-2 with an outstanding S of 247 µV K-1 in a paradigm OPV donor PBDB-T, which holds great potential in wearable electronics driven by a small temperature gradient.

Molecular tetrominoes: selective masking of the donor π-face to control the configuration of donor-acceptor complexes

Understanding the doping mechanism in organic semiconductors and generating molecular design rules to control the doping process are crucial for improving the performance of organic electronics. Even though controlling the location and orientation of the dopant along the semiconductor backbone is an important step in the doping mechanism, studies in this direction are scarce as it is a challenging task. To address this, herein, we incorporated π-face masked (strapped) units in 1,4-bis(phenylethynylene)benzene (donor) to control the acceptor (dopant) location along the trimer, donor-acceptor binding strength, and acceptor ionization. Two strapped trimers, PCP and CPC, are synthesized with control over the location of the strapped repeat unit in the trimer. The trimers are complexed with the 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) acceptor in solution. DFT calculations show that DDQ residing on the non-strapped repeat unit (the percentage of this configuration is at least ca. 73%) has the highest binding energy for both PCP and CPC. The percentage of dopant ionization is higher in the case of strapped trimers (PCP and CPC) compared to that of linear control trimers (PLP and LPL) and the completely non-strapped (PPP) trimer. The percentage of dopant ionization increased by 15 and 59% in the case of PCP and CPC respectively compared to that of PPP.

Cross-Linking of Doped Organic Semiconductor Interlayers for Organic Solar Cells: Potential and Challenges

Solution-processable interlayers are important building blocks for the commercialization of organic electronic devices such as organic solar cells. Here, the potential of cross-linking to provide an insoluble, stable, and versatile charge transport layer based on soluble organic semiconductors is studied. For this purpose, a photoreactive tris-azide cross-linker is synthesized. The capability of the small molecular cross-linker is illustrated by applying it to a p-doped polymer used as a hole transport layer in organic solar cells. High cross-linking efficiency and excellent charge extraction properties of the cross-linked doped hole transport layer are demonstrated. However, at high doping levels in the interlayer, the solar cell efficiency is found to deteriorate. Based on charge extraction measurements and numerical device simulations, it is shown that this is due to diffusion of dopants into the active layer of the solar cell. Thus, in the development of future cross-linker materials, care must be taken to ensure that they immobilize not only the host but also the dopants.

Model of the Thermoelectric Properties of Anisotropic Organic Semiconductors

A model of charge hopping transport that accounts for anisotropy of localized states and Coulomb interaction between charges is proposed. For the anisotropic localized states, the degree of orientation relates exponentially to the ratio of conductivities in parallel and perpendicular directions, while the ratio of Seebeck coefficients stays nearly unaffected. However, the ratio of Seebeck coefficients increases if Coulomb interaction is screened stronger in a direction parallel to the predominant orientation of the localized states. This implies two different physical mechanisms responsible for the anisotropy of thermoelectric properties in the hopping regime: electronic state localization for conductivities and screening for Seebeck coefficients. This provides an explanation for the recent experimental findings on tensile drawn and rubbed polymer films.

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.

UV-to-IR Absorption of Molecularly p-Doped Polythiophenes with Alkyl and Oligoether Side Chains: Experiment and Interpretation Based on Density Functional Theory

The UV-to-IR transitions in p-doped poly(3-hexylthiophene) (P3HT) with alkyl side chains and polar polythiophene with tetraethylene glycol side chains are studied experimentally by means of the absorption spectroscopy and computationally using density functional theory (DFT) and tight-binding DFT. The evolution of electronic structure is calculated as the doping level is varied, while the roles of dopant ions, chain twisting, and π-π stacking are also considered, each of these having the effect of broadening the absorption peaks while not significantly changing their positions. The calculated spectra are found to be in good agreement with experimental spectra obtained for the polymers doped with a molybdenum dithiolene complex. As in other DFT studies of doped conjugated polymers, the electronic structure and assignment of optical transitions that emerge are qualitatively different from those obtained through earlier "traditional" approaches. In particular, the two prominent bands seen for the p-doped materials are present for both polarons and bipolarons/polaron pairs. The lowest energy of these transitions is due to excitation from the valence band to a spin-resolved orbitals located in the gap between the bands. The higher-energy band is a superposition of excitation from the valence band to a spin-resolved orbitals in the gap and an excitation between bands.