Polythiophene: From Fundamental Perspectives to Applications

Perspective Cite This: Chem. Mater. XXXX, XXX, XXX−XXX

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Polythiophene: From Fundamental Perspectives to Applications Thaneshwor P. Kaloni,*,† Patrick K. Giesbrecht,‡ Georg Schreckenbach,*,† and Michael S. Freund*,‡ †

Department of Chemistry, University of Manitoba, Winnipeg, MB R3T 2N2, Canada Department of Chemistry, Florida Institute of Technology, Melbourne, Florida 32901, United States

‡

ABSTRACT: The field of organic electronics has been heavily impacted by the discovery and development of π-conjugated conducting polymers. These polymers show great potential for integration into future optical and electronic devices due to their capacity to transition between semiconducting and conducting states as well as the ability to alter mechanical properties by controlled doping, chemical modification, and stacking or creating composites with other materials. Among π-conjugated polymers, polythiophene and its derivatives has been one of the most extensively studied and is widely investigated computationally and experimentally for use in electronic devices such as light-emitting diodes, water purification devices, hydrogen storage, and biosensors. Various theoretical modeling studies of polythiophene ranging from an oligothiophene approach to infinite chain lengths (periodic boundary conditions) have been undertaken to study a variety of electronic and structural properties of these polymers. In this review, we discuss the recent advances in the understanding of pristine polythiophene and its derivatives from fundamental perspectives to device applications. constituting the π-conjugated polymer can greatly alter the electronic and optical properties therein. Typical candidates utilized include acetylene, pyrrole, aniline, and thiophene.3−5 Moreover, it has been suggested that polyselenophene, polytellurophene, poly(1H-phosphole), and poly(1H-arsole) can also be developed based on binding energy calculations,81−87 thereby allowing a wide range of semiconducting organic materials to be developed. 1.2. Polythiophene and Its Derivatives. Polythiophene and its derivatives are of great interest in device applications88−94 due to the high stability of its (un)doped states, ease of structural modification, and solution processability.95,96 As such, polythiophene has been extensively studied using theoretical and experimental approaches.26,97−120 Several studies on the electronic structure of thiophenes using firstprinciples calculations were conducted to elucidate the role side-chain groups and dopants have on the overall electronic properties of the polymeric material, where the band gap can be altered from 3 to 1 eV based on dopant level or side-chain group employed.121−128 Furthermore, by altering the morphology and thus the geometry of the oligomer chains, the band gap and optical properties of polythiophene-based films can drastically be altered.127,129−140 In theoretical modeling, the approximation called the periodic boundary conditions seems to be a sufficient method with few limitations in its capacity to calculate the band gap in

1. INTRODUCTION 1.1. Conjugated Polymers. Since the discovery of polyacetylene by Heeger, MacDairmid, and Sherikawa in 1976,1 a new and emerging class of materials, π-conjugated conducting organic polymers, has intrigued scientists with their unique physical and chemical properties. Given the πconjugation arising from the alternating single and double carbon−carbon bonds within the polymer backbone, the πelectrons are effectively delocalized along the polymer chain, leading to one unpaired π-electron per carbon atom2−13 and allowing for charge transport along the polymer chain, and as a consequence, they display conducting or semiconducting behavior.14−28 As such, there is great interest toward their application in electronic and optical semiconducting devices1,29−39 where the electronic properties of such polymers can be engineered easily either through doping or chemical modification.13,40−44 The process of doping induces charge transfer to/from the conjugated backbone, leaving the system p/n-type doped32,45,46 and changing the polymer electrical properties and thus the electrochemical potential. Doping can be achieved chemically, electrochemically, or in the presence of an external electric field, as discussed in various two-dimensional (2D) materials.47−52 The electronic and optical properties of π-conjugated polymers can be tuned widely through doping, providing a transition from insulator to semiconductor to metal based on the dopant concentration.41,53−56 This paves the way for application of πconjugated polymer in transistors, photoresistors, light-emitting diodes, solid-state lasers, biosensors, and organic solar cells57−67 and sensors.67−80 As such, the selection of the monomer © XXXX American Chemical Society

Received: July 19, 2017 Revised: November 28, 2017

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DOI: 10.1021/acs.chemmater.7b03035 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials relation to experimental values.141,142 There have been only a few theoretical reports for doped oligothiophene where the dopants are not covalently bound to oligothiophene.102,103,125,143 Extensive first-principles calculations were performed to investigate the charge redistribution in Li-doped polyacetylene,144 where it was suggested that a large enhancement of the conductivity can be achieved by electron or hole doping.1,145−149 Density functional theory (DFT)-based calculations with and without a hybrid functional for Cl or Cl2 doped polypyrrole150−152 indicated that a hybrid functional is better at producing the experimental band gap, while the electronic structures of polythiophene-based devices are still not well-understood. For doped polythiophene, the main charge carriers are bipolarons or polaron pairs, where the pairs of the polarons are well-separated by radical cations.153 Moreover, the bipolarons are found to be localized over a certain portion of the polythiophene chain, while polaron pairs are found to be well-separated by a nearly undistorted portion of the polythiophene chain.153−155 Due to various experimental limitations, synthesis and structural properties of π-conjugated oligomers or polymers are difficult to understand.155,156 This is also the case for the theoretical calculations because of the various levels of theoretical aspects, including the choice of DFT functional (model chemistry) and, crucially, the choice of a simplified model for complex experimental situations.125,141,157 It was proposed that dopants can have a major influence on the electronic properties, in particular the formation of polarons or bipolarons in the π-conjugated polymers, which was demonstrated in experimental findings.149 Recently, the structural and electronic properties of doped polythiophene were extensively analyzed for various lengths of the unit cells such as for more than 10 thiophene rings.96 It has also been proposed that doped polythiophene can serve as a model for π-conjugated polymers in general, with its properties applied to other doped πconducting polymers.141 The effect of Li or Cl doping on the structural and electronic properties of polythiophene in various stacking geometries either in periodic or oligomer forms was studied recently in the framework of first-principles DFT-based model calculations. In particular, the structural and electronic properties of the periodic pristine monolayer polythiophene, bilayer polythiophene with normal stacking and flipped by 180°, and Li and Cl adsorption/intercalation were investigated.102 The obtained result demonstrates that the electronic structure can easily be tuned and controlled either by adsorption/intercalation or by creating multilayers in bulk or bilayer stacking sequences. 1.3. Scope and Goals of the Current Review. Due to their great capability to be integrated as light-absorbing materials in organic electronic devices, polythiophenes have been the subject of interest for many researchers over the last three and a half decades. One of the most exciting properties of these materials is the electrical conductivity resulting from the delocalization of electrons along the polymer backbone. However, the electrical conductivity is not the only interesting property resulting from electron delocalization, but also the optical responses of these materials with dramatic color shifts in response to changes in solvent, temperature, applied potential, and binding to other molecules. Many interesting and comprehensive review articles have been published on polythiophene-based materials, dating from 1981 to 2010.96,158−167 These reviews are an excellent guide to the highlights of the primary polythiophene literature from the last

three decades. Thus, in this review, we focused on the current developments in pristine polythiophene and its derivatives from fundamental perspectives to device applications.

2. GEOMETRY AND ELECTRONIC STRUCTURE OF POLYTHIOPHENE 2.1. Structural Analysis. The structural and electronic properties of pristine and doped polythiophene have been extensively studied.88,102,121,168,169 The structure for the monolayer polythiophene using DFT is depicted in Figure 1a, where the periodic supercell contains 16 C, 8 H, and 4 S

Figure 1. Optimized geometry (periodic calculations) of polythiophene for (a) pristine, (b) Li-doped, and (c) Cl-doped, where the green, yellow, cyan, brown, and pink spheres represent S, C, H, Li, and Cl atoms, respectively. Adapted with permission from ref 102. Copyright 2015 American Chemical Society.

atoms (i.e., four monomer units), while Figures 1b and c represent the corresponding Li- and Cl-doped polythiophene monolayer. The calculated structural parameters (bond length and angles) were found to be in good agreement with reported experimental and theoretical values.88,168,170 Upon doping, the Li atoms are located above the center of the pentagon, donating their fraction of the electrons to the aromatic backbone of the thiophene unit. In contrast, the Cl atom moves away from the polythiophene surface due to the electron density along the thiophene backbone.102 No deviation from the planar structure of the pristine monolayer of polythiophene was observed upon doping. The bulk and bilayer polythiophene have also been calculated (Figure 2), where the interlayer spacing amounts to 3.372 or 3.371 Å for pristine bulk (periodic stacking of the bilayer) or bilayer polythiophene, respectively. Only small deviations from the linear structure of its monolayer counterpart were observed in the pristine bilayer structure. Introduction of Li, intercalated into the bilayer (Figure 2b), resulted in a more planar structure, with the Li atoms residing in the center of the bilayer and with slightly larger interlayer spacing (3.653−3.881 Å). B


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Figure 2. Top and side views of the optimized structure of periodic bilayer polythiophene for (a) pristine, (b) Li intercalate, and (c) Cl intercalate. Adapted with permission from ref 102. Copyright 2015 American Chemical Society.

studies of the variation of the Li-doping (Li/C ratios of 0.25, 0.083, and 0.050; Figure 4) can vary the extent to which the polythiophene is n-doped.88 Similarly, p-doped states are observed when Cl is incorporated (Figure 3c), with the Cl atom gaining 0.54 electrons, the ortho-C atoms losing a charge of 0.05 electrons, and the meta-C atoms losing ca. 0.11 electrons. No electron density was lost from the S and H atoms in comparison to the undoped polythiophene monolayer. These results indicate that the band gap in polythiophene can thus be controlled through doping level of Li or Cl atoms, agreeing with experimental evidence. The binding energy was found to be −2.01 and −1.49 eV for Li- and Cl-doped polythiophene, respectively. This indicates that the formation of Cl-doped polythiophene is easier than that of Li-doped polythiophene. The band structures of pristine bilayer, pristine bulk, Li/Clintercalated bulk, and Li/Cl-intercalated bilayer polythiophene were calculated (Figure 5). A band gap of 4 meV was found for pristine bulk polythiophene, while a band gap of 0.81 eV was achieved for the bilayer (Figure 5b). Two well-separated bands close to the conduction regime and two bands close to the valence regime above/below the EF were found, similar to previous experimental and theoretical reports for π-stacked polymers.173 The gap between the two bands is about 0.41 eV, and due to the splitting of the bands, the band gap in the bilayer reduces as compared to the corresponding monolayer case. Li intercalation into the bulk polythiophene results in the film becoming n-doped, shifting the gap of the pristine case below the EF by 0.25 eV. Similarly, Li-intercalated bilayer polythiophene was found to be shifted by 0.52 eV, resulting in a gap of 0.51 eV, see Figure 5d. It is important to mention that there are two possible optical transitions (IR polaronic and optical polaronic) for light absorption; these can be described as π−π* transitions. This kind of behavior was observed in experiment for doped πconjugated polymers.32 Furthermore, the doping can be understood by two transitions with energies in the range of IR polaronic absorption for a small band gap and optical polaronic absorption for a larger band gap. Importantly, by doping, the conductivity can be increased, allowing such systems to be used in devices requiring high conductivity, i.e. photovoltaics, thermoelectrics, and spintronics.32,174,175 2.3. Pristine and Doped Polythiophene: Molecular Approach. The structure of polythiophene was also analyzed through molecular calculations, where four different oligomeric structures (2-ring, 4-ring, 6-ring, and 8-ring) were considered (HOMO/LUMO molecular orbitals shown in Figure 6). The 2-ring structure is a rather small model for the calculations, resulting in larger bond lengths and a larger HOMO−LUMO

Upon introduction of Cl dopants into the bilayer of polythiophene (Figure 2c), the polythiophene bilayer was found to also exhibit a planar structure with slightly larger interlayer spacing (3.783−3.801 Å). The Cl atoms now reside between the two layers away from the center of the thiophene rings, similar to the monolayer case. Computational studies involving para-toluenesulfonic acid (p-TSA) intercalated in polythiophene bilayers have also shown this movement of the anion away from the center of the bilayer, allowing for tighter packing of the thiophene units.171 Flipping of the second layer of polythiophene by 180° has also been considered for creating bulk or bilayer structures. Note that it is speculated a more favorable structure is formed based on donor−acceptor pair between the S atom and the carbon backbone, which has not been verified experimentally yet. The parallel bulk or bilayer structures, however, are energetically more favorable than their flipped counterparts. Total ground state energy difference between the parallel and flipped bulk or bilayer amounted to −0.69 and −1.12 eV, respectively. Similarly, the parallel bulk or bilayer polythiophene is stable by −1.09 or −0.88 eV and −1.03 or −0.36 eV after Li and Cl intercalation.103 2.2. Electronic Structure. The band structures of pristine, Li-doped, and Cl-doped monolayer polythiophene are depicted in Figure 3. A band gap of 1.08 eV is obtained for pristine

Figure 3. Electronic band structure for monolayer polythiophene in the periodic case for (a) pristine, (b) Li-doped, and (c) Cl-doped polymers. Adapted with permission from ref 102. Copyright 2015 American Chemical Society.

monolayer of polythiophene (Figure 3a), agreeing well with available reports.88,127,172 By analyzing the Löwdin populations, it is found that the C atoms ortho- to the S atom accept 0.09 electrons with the S atom donating 0.41 electrons to the carbon backbone. The meta-C atoms become positively charged through donating 0.17 electrons, with the H atoms becoming negatively charged by 0.13 electrons. Introduction of a Li atom results in n-doping of the polythiophene monolayer through electron donation from the Li atom, shifting the band gap 0.68 eV below the Fermi Level (EF; Figure 3b).88 Computational C


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Figure 4. Band structures of doped polythiophene. The three middle panels from left to right refer to different concentrations with Li/C ratios of 0.25, 0.083, and 0.050, while the panels on the far right and far left correspond to the undoped polymer. Adapted with permission from ref 88. Copyright 2009 AIP Publishing LLC.

be 2.94 eV for the 2-ring system and decreases to 1.87 to 1.54 eV to 1.39 eV for 4-ring to 6-ring to 8-ring, respectively. The HOMO−LUMO gap can easily be engineered by creating disorder180 or by doping181,182 in polythiophene. The calculated value of the band gap in the periodic case is similar to the HOMO−LUMO gap of the 6-ring or 8-ring geometry (1.2 eV). Formation of a bilayer of polythiophene results in a decrease in the band gap due to interlayer interactions and band splitting at EF. Similar to periodic calculations, the flipped state was also found to be slightly higher in energy (4−116 meV for 2−8 rings) with a larger HOMO−LUMO gap (1.338 eV for 8 rings) but still accessible under experimental conditions. Experimentally, it was demonstrated that the stacking of the π-conjugated thiophene is useful to obtain long-range charge and energy transport in novel nanoscale polymer-based devices such as optical, electronic switches, and thin film transistors.183−186 2.3.1. Effects of Chain Lengths and Functionals on the Electronic Properties. Recently, the effect of the chain length on the band gap was addressed. 132 The band gaps (fundamental, optical, and HOMO−LUMO) as a function of 1/N are plotted in Figure 7 for polythiophene derivatives poly(3-hexylthiophene) and the thieno[3,4-b]thiophene-altbenzodithiophene copolymer, where N is the number of C C bonds along the shortest path along the backbone of the chain used (N = 2−6). To determine the band gap of the polymers, an extrapolation of the band gap for planar oligomers was performed, given that the planar backbone can be assumed due to the delocalization of the π-electrons. 187 The fundamental and optical band gaps for an infinite planar chain of poly(3-hexylthiophene) and thieno[3,4-b]thiophenealt-benzodithiophene copolymer were found to be 1.87 and 1.65 eV, respectively, which are fairly close to experimental values of 1.84 and 1.68 eV, respectively.188 The band gap of planar poly(3-hexylthiophene) oligomers was found to be slightly higher than that of planar thieno[3,4-b]thiophene-altbenzodithiophene copolymer oligomers.189 In addition, the effects of the chain length on various energy gaps, orbital energies, ionization potential (IP), and electron affinity (EA) were addressed for a thiophene dimer (2T) and hexamer (6T) in comparison with the available experimental

Figure 5. Electronic band structure for periodic bulk and bilayer polythiophene: (a and b) pristine, (c and d) Li intercalation, and (e and f) Cl intercalation. Adapted with permission from ref 102. Copyright 2015 American Chemical Society.

Figure 6. Calculated HOMO and LUMO of polythiophene oligomers with an isovalue of ±0.05 electrons/Å3 for (a) 2-ring, (b) 4-ring, (c) 6ring, and (d) 8-ring. Adapted with permission from ref 102. Copyright 2015 American Chemical Society.

gap. By increasing the number of rings to 4, 6, and 8, the structural parameters and band gap become smaller and show convergence.124,176−179 The HOMO−LUMO gap is found to D


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lations of excitation energies, the HOMO−LUMO orbital energy gap (Δϵ) essentially serves as a rough estimate of the optical gap (ΔEO). The data presented in Table 1 reflect that for PBE, the TDDFT excitation energy is larger than the HOMO−LUMO orbital energy gap (Δϵ), while for the LC and HF functionals, the TDDFT excitation energy is smaller than the HOMO−LUMO orbital energy gap (Δϵ). For 6T and 12 T, a small HOMO−LUMO orbital energy gap (Δϵ) for PBE in conjunction with the small admixture of other orbital pairs renders the calculated optical gaps to be too low. In case of functionals with a large fraction of exact exchange, the mixing of the different occupied−unoccupied orbital pairs is in fact needed to explain the electronic transition. A nice overview of the HOMOs and LUMOs involved in the transition for different functionals is addressed in Figure 9, which indicates that the nature of the transition is totally dependent on the type of functional used in the calculations. The effects of electron donating/withdrawing groups on the electronic structure for oligomers composed of a basic unit consisting of a thiophene and a pyrrole ring connected by a −CHN− group were studied by employing DFT/TDDFT. Oligomers with various substituents in different positions were also considered.198 The HOMO−LUMO gaps, excitation energies, and the oscillator strengths were calculated using five functionals (B3LYP, B2PLYP, PBE0, PW6B95, and PWPB95). The obtained results indicate that substituents induce twisting in the conjugated backbone of the oligomers and thus increase the HOMO−LUMO gaps and the excitation energies compared to those of the pristine molecule. The HOMO, LUMO, and HOMO−LUMO gap (ΔH−L) of the basic monomer unit were calculated using various functionals and are shown in Figure 9. All of the other functionals provide larger ΔH−L gaps as compared to the B3LYP functional, while the ΔH−L obtained from B2PLYP and PWPB95 functionals are found to be largest among the functionals. Interestingly, all the functionals were found to have the same trend in the ΔH−L. 2.4. Effects of Substitution. The structural modification and electronic properties of substitutionally doped polythiophene were studied using periodic DFT calculations127 as well as realized in experiment via NMR studies.199 In particular, the methyl (CH3), amino (NH2), nitro (NO2) and Cl groups greatly modify the structural and electronic properties of thiophene monomers and dimers. In the case of pristine polythiophene monomers, dimers, and trimers, band gaps of 4.49, 2.93, and 2.21 eV were found computationally, which are close to the experimental band gaps of 5.37, 4.12, and 3.52 eV, respectively.200 The obtained values of the HOMO−LUMO gap for the monomer (dimer) are found to be 4.44 eV (2.97 eV), 4.22 eV (2.87 eV), 4.46 eV (2.75 eV), and 3.21 eV (2.22

Figure 7. Chain length versus band gap for polythiophene derivatives poly(3-hexylthiophene) (dotted lines) and thieno[3,4-b]thiophene-altbenzodithiophene copolymer (solid lines). Adapted with permission from ref 132. Copyright 2014 Elsevier Publishing Company.

data by employing Hartree−Fock (HF) theory and different hybrid and nonhybrid functionals.176 The orbital energy gap (Δϵ) was found to be 2.9 to 9.2 eV for 2T, while it becomes 1.6 to 7.0 eV for 6T, depending on the method used. The optical gaps obtained from a variety of different computational methods are shown in Table 1. The optical gaps (ΔEO) were obtained using time-dependent DFT (TDDFT) as a comparison.190,191 The optical gaps (ΔEO) obtained from HF and long-range corrected functionals (LC) are larger than the experimentally measured values; PBE and B3LYP underestimate the values, and the LC* approach produces optical gaps which are close to the experimental findings. It has been observed that the band gap and other energy parameters depend on the fraction of exchange correlation in the functional.192,193 In addition, in some cases, the band gap and other energy parameters obtained by using the B3LYP functional provide reasonable agreement with experimental reports for certain oligomers. The calculated values of the energy parameters for thiophene dimer (2T) and hexamer (6T) are discussed in detail in ref 176; the data are summarized in Table 1. The occupied−unoccupied orbital pair contribution to the transition density was calculated based on TDDFT in the framework of the LC-PBE0 and B3LYP functionals (6-31+G* basis), see Figure 8. The functionals with low fraction of exact exchange produce an almost pure HOMO−LUMO transition. For TDDFT calcu-

Table 1. Values of Negative HOMO Energy (−ϵH), LUMO Energy (−ϵL), IP, EA, Orbital Energy Gap (Δϵ), Fundamental Gap (ΔEF), and Optical Gap (ΔEO) of Thiophene Dimer/Hexamer (2T/6T) with Experimental Dataa

a

quantity

HF

PBE

B3LYP

HSE

LC

LC*

−ϵH IP −ϵL EA Δϵ ΔEF ΔEO

7.7/6.6 6.4/5.6 −1.5/-0.4 −0.8/0.3 9.2/7.03 7.2/5.4 4.2/2.9

5.1/4.5 7.4/5.9 2.2/2.9 −0.03/1.6 2.9/1.6 7.4/4.3 3.7/2.01

5.7/5.0 7.5/6.1 1.6/2.5 −0.05/1.45 4.1/2.6 7.5/4.6 3.9/2.3

5.6/4.9 7.5/6.1 1.8/2.7 −0.05/1.5 3.7/2.2 7.6/4.6 3.9/2.3

8.3/7.3 7.8/6.7 −0.3/0.9 −0.04/1.3 8.5/6.4 7.8/5.4 4.3/2.9

7.7/6.4 7.7/6.4 0.0/1.4 −0.01/1.3 7.7/5.0 7.7/5.0 4.1/2.6

exptl 7.7/6.4194,195 −0.05/1.2196

4.1/2.9197

LC* = LC−PBE0, and all the parameters are measured in eV. Adapted with permission from ref 176. Copyright 2013 American Chemical Society. E


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Figure 8. Contributions of occupied−unoccupied orbital pair to lowest-energy singlet transition obtained from TDDFT for 6T and 12T using the LC-PBE0 and B3LYP functionals (6-31+G** basis). Adapted with permission from ref 176. Copyright 2014 American Chemical Society.

Figure 9. HOMO−LUMO gaps (ΔH−L) calculated using five different functionals (B3LYP, B2PLYP, PW6B95, PWPB95, and PBE0). Adapted with permission from ref 198. Copyright 2013 American Chemical Society.

eV) for CH3, Cl, NH2, and NO2 substitutions, respectively. The nitro group induces the smallest HOMO−LUMO gap, which can be attributed to the fact that the nitro group has the largest electron-withdrawing capability. The calculated density of states (DOS) for pristine and substituted polythiophene is shown in Figure 10. It can be seen that there are few modifications in the electronic structure with a minor effect of the substituents on the HOMO−LUMO gap. A HOMO−LUMO gap of 1.19 eV was obtained for pristine polythiophene and becomes 1.19, 1.22, 1.14, and 1.27 eV for CH 3 , Cl, NH 2 , and NO 2 substitutions, respectively. As observed in the monomer and dimer calculations, the chloro and nitro groups increase the HOMO−LUMO gap due to their ability to withdraw electron density from the thiophene ring. Overall, the influence of the substituents on the electronic structure of polythiophene is greatly reduced by the transition from oligo- to polymer form.121 The structural parameters and electronic structure of polythiophene are also influenced by steric repulsion between substituents. The steric repulsion between the substituents can

Figure 10. DOS of polythiophene substituted by (a) CH3, (b) Cl, (c) NH 2 , and (d) NO2 , where dashed lines represent pristine polythiophene DOS. Adapted with permission from ref 127. Copyright 2012, Rittmeyer and Groβ; licensee Beilstein-Institut.

be reduced by separating the thiophene rings in the backbone of the polymer by a vinyl bridge (Figure 11a). In this scheme, the structure remains flat irrespective of the substituent. The inclusion of a vinyl bridge on polythiophene significantly reduces the HOMO−LUMO gap from 1.2 to 0.7 eV, see Figure 11b. The vinyl bridges reduce the aromaticity by modifying the structure toward a quinoid form, where the band gap of conjugated polymers depends on the degree of the quinoid versus the aromatic form of the backbone.126,201,202 The trend is found to be similar for all the substituents on polythiophene with the largest HOMO−LUMO gap observed for the nitrosubstituted polythiophene. The HOMO−LUMO gap of the vinyl-bridged polythiophene with an annulated phenyl ring is F


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(Figure 12d). Importantly, each of these doping species possesses distinct optical transitions.205 Due to the unpaired electrons in the half-filled orbitals, a polaron consists of a 1/2 spin, while a bipolaron has no spin, allowing differentiation of the two states by electron spin resonance (ESR) spectroscopy.206−208 The polaron/bipolaron levels are localized on a segment of the polymer chain for a polymer in an amorphous phase. The EF lies in the middle of the localized states of the polaron band for a disordered polaronic polymer (Figure 12b), while it lies between the valence band and the bipolaron band for a disordered bipolaronic polymer (Figure 12e).209,210 When a polymer exhibits crystalline domains, short interchain distances occur due to π-electronic density, which promotes the delocalization of the polaron wave function over several chains, becoming more metallic in nature.211 Similarly, a highly oxidized polymer can exhibit metallic behavior212−214 due to the creation of a polaron network in the half-filled polaron band (Figure 12c).215 The electronic structure of doped polythiophene was proposed to be equivalent to a Fermi glass, as depicted in Figure 12e, where doped polythiophene is described by a bipolaron network with an empty delocalized bipolaron band,216 which in fact merges into the delocalized valence band, see Figure 12f. Furthermore, polythiophene-based polymers exhibited higher Seebeck coefficients and lower thermal conductivities compared to those of metals while maintaining metallic electrical conductivity, forming a semimetallic material175 and making them useful for thermoelectric and spintronic applications.217−222 Analysis of the structure of polythiophene has found that the benzenoid form of the polythiophene, see Figure 13a, is

Figure 11. Vinyl-bridged polythiophene derivatives (a) structural drawing, and (b) HOMO−LUMO gap. Adapted with permission from ref 127. Copyright 2012, Rittmeyer and Groβ; licensee BeilsteinInstitut.

further reduced to 0.25 eV due to the increased conjugation in the backbone.

3. POLARONS Removal of the electrons from the top of the valence band of a polymer leads to the formation of charged defects such as polarons (radical cation) and bipolarons (dication), which are balanced by molecular/atomic counterions.175 The modifications in the bond lengths around the excess of charge provide the extent of the wave function of the polarons/bipolarons, which leads to two new in-gap states (i and i*), where a localized level is destabilized from the top of the valence band.175,203,204 For a polaron, the level i contains half-filled orbitals (Figure 12a), whereas for a bipolaron it is empty

Figure 13. Schematic representation of the neutral and charged polythiophene rings for (a) neutral aromatic benzenoid, (b) neutral quinonoid, (c) polaron, and (d) bipolaron. Adapted with permission from ref 88. Copyright 2009 AIP Publishing LLC. Figure 12. (a) Electronic structure of a polymer for a polaron, (b) the logarithm of the DOS lnN(E) for an amorphous polaronic polymer solid containing localized states around the EF, (c) a metallic network of polarons with the EF lying in a delocalized polaron band, (d) electronic structure of a polymer for a bipolaron, (e) lnN(E) for an amorphous bipolaronic polymer solid, and (f) a semimetallic network of bipolarons with the EF lying between the valence band and the empty bipolaron band, where i and i* represent the in-gap states induced by local structural distortions. Adapted with permission from ref 175. Copyright 2014 Nature Publishing Group.

energetically more favorable than its quinonoid form,88 see Figures 13a and b. The charge transfer due to donor or acceptor doping introduces polarons or bipolarons into the polymer, see Figures 13c and d, resulting in the formation of a localized quinonoid structure.223 Calculations employing B3LYP DFT using Li donor atoms found the polaron is energetically more stable than the bipolaron.102 These findings are in line with previous reports for Li-88 or Cl-doped141 G


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Figure 14. (a) Geometrical structure of a single bipolaron doped into a 100-ring polythiophene chain and (b) band gap as a function of the number of bipolarons in this chain. Adapted with permission from ref 226. Copyright 2010 American Chemical Society.

Figure 15. Schematic representation of polythiophene (a) undoped and (b) doped with corresponding energy diagrams for π−π* transitions. Adapted with permission from ref 227. Copyright 2014 American Chemical Society.

regime.227 However, a polaron, i.e., a positive charge and an unpaired electron are created on the polythiophene chain after an electron from the chain is removed, see Figure 15b. In this situation, two polaron states, namely P1 and P2, arise in the midgap region between the π−π* bands. The induction of the P1 and P2 states results in new optical transitions in the infrared region related to the two new states. This behavior leads to optical transparency in the visible spectral region,112,228,229 which can also be demonstrated by analyzing the absorption spectra of pristine and doped polythiophene. In addition, the photoluminescence of polarons in doped polythiophenes was investigated to estimate microscopic carrier mobility.227 The observed carrier mobility was found to be 2.0 cm2 V−1 s−1, a promising result for the use of polythiophenes in field-effect transistors constructed from two-dimensional materials.230−239

polythiophene. Instead of doping, polarons or bipolarons can also be created in polythiophene by suppressing and adding charge to it through application of an electric field.224 The increased presence of polarons and bipolarons is found to facilitate large increases in the electrical and thermal conductivity of conducting polymers from both theoretical and experimental studies.148,154,225 In addition, the polaronic and bipolaronic defect states in polythiophene are responsible for excitations as well as charge storage.226 It was found that the energy of distortion to form two polarons and one bipolaron are quite similar; however, the ionization energy decreases significantly for bipolarons compared to two polarons. As a result, a single bipolaron is more stable than two polarons.154 In ref 226, the structures of bipolaron-doped polythiophene containing 100 and 150 rings and considering a maximum of 12 bipolaronic defects were studied. The structural symmetry is found to be destroyed by increasing the concentration of bipolarons. If a single bipolaron is doped into a large, 100-ring polythiophene chain, the chain was found to contain four distinct quinonoid rings at the center, where ∼68% of the total charge is carried by the bipolaronic defect. Interestingly, the band gap was found to decrease upon increasing the concentration of bipolarons in polythiophene, see Figures 14a and b. Moreover, for 4−13 rings, a continuous transition to an aromatic structure is observed. The lowest optical absorption for pristine polythiophene results from π−π* transitions, as shown schematically in Figure 15a. Such a transition lies in the visible region, i.e. the optical

4. OPTICAL PROPERTIES The structural and optical properties of water-soluble polythiophene derivatives were addressed by Li and Shi70,240 with possible monomer structures shown in Figure 16. The first water-soluble polythiophene derivatives were synthesized in 1987, poly(3-thiophene-β-ethanesulfonate) and poly(3-(thiophene-δ-butanesulfonate)), see 1a and 1b in Figure 16. In addition, poly(3-[(S)-5-amino-5-carboxyl-3-oxapentyl]-2,5-thiophene hydrochloride) (2) was synthesized in 1991241 and applied for creating biosensors to monitor DNA hybridization, peptides, and proteins. 242−244 In 1996, a biotinylated H


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the experimental band gap in the case of semiconducting materials.236,257−264 In ref 256, various polymers such as polythiophene (PT), poly(dithiophene ethyne) (PDTE), poly(dithiophene vinylene) (PDTV), poly(dithiophene dicyanovinylene) (PDTDCNV), poly(dithiophene difluorovinylene) (PDTDFV), poly(dithiophene dichlorovinylene) (PDTDCV), and fullerene (C60) were studied, see Figure 17. Table 2 provides comparisons of various calculated energies, including band gap using DFT, G0W0, and BSE, agreeing with experimental values reported.

Figure 16. Structures of polythiophene derivatives (water soluble). Adapted with permission from ref 70. Copyright 2014 American Chemical Society. Figure 17. Structures of PT and various derivatives (PDTE, PDTV, PDTDCNV, PDTDFV, and PDTDCV, respectively) and fullerene (C60), where white, gray, blue, cyan, green, and yellow spheres represent H, C, N, F, Cl, and S atoms, respectively. Adapted with permission from ref 256. Copyright 2014 American Physical Society.

copolymer of water-soluble poly(3-alkoxy-4-methylthiophene) (3) was experimentally studied.245 A large series of cationic water-soluble poly(3-alkoxy-4-methylthiophenes) was also prepared later in 1996 (4−12).246−249 It was found that the above-mentioned polythiophene-based molecules exhibit conformational change/aggregation while interacting with DNA, protein, polysaccharide, and small analytes, which can be monitored with the help of absorption or emission spectroscopy. Furthermore, regioregular water-soluble polythiophenebased molecules were synthesized by cross-coupling reactions such as 2,5-poly(thiophene-3-propionic acid) (13) with a headto-tail regioregular chain250 and a cationic regioregular head-totail 2,5-poly(3-(6-N,N-dimethylhexylammonium thiophene)) (14).251 The optical spectra of water-soluble polythiophene-based molecules252 are strongly dependent on aggregation in solutions or solid state, regioregularity, and backbone conformations. Normally, optical properties are reflections of the chemical and electronic structures of the π-conjugated backbones; thus, polythiophene-based molecules with the same backbone structures and different side groups (either alkyl/ alkoxy or ionic groups) usually show similar optical behavior.253 The colorimetric response of polythiophene-based molecules (13) can be easily tuned by varying the size of their countercations.251 By addition of Bu4NOH to polythiophenebased molecules (13), a base with a cation larger than NH+4 , the absorption spectrum was found to be blue-shifted by 130 nm. Thus, a dramatic color change from purple to yellow was observed. In addition, it has been found that the absorption and emission spectra of water-soluble polythiophene can be dramatically changed in aqueous media containing surfactants due to the formation of ionic complexes through ionic selfassembly.250,254,255 Recently, the optical properties of polythiophene and various derivatives were studied theoretically by Samsonidze and coworkers256 using the GW approximation (quasi-particle selfenergy corrections method) and by solving the Bethe−Salpeter equation (BSE) to include electron−hole interaction. It was reported that the band gap obtained using the GW approximation is larger than that of GGA functionals, and the GW approximation was found to be a better approach to obtain

Moreover, the polymers listed in Table 2 show larger structural parameters compared to those of pristine polythiophene, which leads to folded bands. The obtained macroscopic absorption spectra (χ2) for these polythiophenebased molecules and fullerene are shown in Figure 18, where χ2 = Aωϵ2 and χ2 = Vωϵ2 for polythiophene-based molecules and C60, respectively; ϵ2, ω, A, and V are the imaginary part of the dielectric function, the photon energy, the cross-sectional area, and the volume of the supercell being used, respectively. The dashed blue curves in Figure 18 represent the joint density of quasiparticle states; the dashed green curves represent the optical transition matrix elements, and the red curves are for the excitonic effects. The optical excitation energies (Eoee) are indicated by the peaks observed in the red curves in Figure 18. The BSE exciton binding energies (Ebin = Egap − Eoee) are summarized in Table 2. For the C60 molecule, the obtained lowest peak of the dashed blue curve amounts to 4.56 eV, which has no matching peak in the dashed green curve, indicating that the calculated HOMO−LUMO optical transition for C60 molecule is quenched by symmetry.265 Thereby, the lowest peak in the red curve at 3.50 eV arises from optical transitions at higher energies than the HOMO−LUMO gap of 4.56 eV. A comparison of experimental and theoretical (using various computational approaches266) absorption energies of 49 different polythiophene-based molecules is addressed in ref 197, see Figure 19. All of the structures were optimized using the AM1 semiempirical method267 with the GAMESS package268 or DFT with the B3LYP hybrid functional269,270 in the 631G* basis in the Q-Chem package.271 The ZINDO semiempirical method was already tested at the CIS level with the ZINDO package.272 The 10 highest occupied and 10 lowest unoccupied orbitals were considered with the random phase approximation (RPA) method.273,274 First-principles calculations were performed at both the CIS and RPA levels, and TDDFT was used with the BLYP functional;270 in fact, it was I


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Table 2. DFT, G0W0, and BSE Results for an Isolated Polythiophene (PT), Poly(methyl thiophene) (PMeT), Poly(3hexylthiophene) (P3HT), Poly(dithiophene ethyne) (PDTE), Poly(dithiophene vinylene) (PDTV), Poly(3-hexyldithiophene vinylene) (P3HDTV), Poly(dithiophene dicyanovinylene) (PDTDCNV), Poly(dithiophene difluorovinylene) (PDTDFV), Poly(dithiophene dichlorovinylene) (PDTDCV), Fullerene (C60), [6,6]-Phenyl-C61 Butyric Acid Methyl Ester (PCBM), Polythiophene on Fullerene (PT/C60), and Poly(dithiophene vinylene) on Fullerene (PDTV/C60)a G0W0

DFT PT PMeT P3HT PDTE PDTV P3HDTV PDTDCNV PDTDFV PDTDCV C60 PCBM PT/C60 PDTV/C60

BSE

EVB

ECB

Egap

EVB

ECB

Egap

Eoee

Ebin

−4.26 −3.94 −3.79 −4.44 −4.19 −3.81 −5.26 −4.43 −4.53 −5.84 −5.43 −4.49 −4.37

−3.21 −2.87 −2.80 −3.36 −3.36 −3.12 −4.63 −3.46 −3.52 −4.19 −3.96 −4.06 −4.08

1.05 1.07 0.99 1.08 0.83 0.69 0.63 0.97 1.01 1.65 1.47 0.43 0.29

−5.35 −4.98

−2.25 −1.97

3.10 3.01

1.48 1.48

1.62 1.53

−5.52 −5.14

−2.39 −2.52

3.13 2.62

1.79 1.23

1.34 1.39

−6.04 −5.48 −5.58 7.31

−3.86 −2.54 −2.64 −2.75

2.18 2.94 2.94 4.56

1.00 1.45 1.46 3.50

1.18 1.49 1.47 1.06

a The EVB (eV), ECB (eV), Egap (eV), Eoee (eV), and Ebin (eV) represent the valence band maximum or HOMO, the conduction band minimum or LUMO, quasiparticle band gap, optical excitation energy, and binding energy of the lowest bright exciton, respectively. Adapted with permission from ref 256. Copyright 2014 American Physical Society.

Figure 18. Calculated optical absorption spectra of isolated polythiophene-based molecules and C60 employing the BSE functional. Red, dashed green, and dashed blue curves represent the macroscopic absorption spectra (χ2) with/without electron−hole interaction and the joint density of quasiparticle states, respectively. Adapted with permission from ref 256. Copyright 2014 American Physical Society.

shown that the TDDFT-BLYP method is the most accurate for aromatic hydrocarbons and polyenes.275,276 Six computational approaches, including ZINDO/CIS, ZINDO/RPA, HF/CIS, HF/RPA, TDDFT/TDA, and TDDFT, were used to calculate band gaps and absorption energies for various molecules and compare the data with experimentally observed values, see Table 3.277−286 The geometries predicted by the AM1 semiempirical method seem to be unreliable for performing single-point excited-state calculations, unlike DFT calculations. On the other hand, the semiempirical ZINDO/CIS method provides good agreement between experimental and computed optical absorption energies with small variation. However,

TDDFT methods (TDDFT/CIS and TDDFT/RPA) are quite accurate after systematic empirical correction. In addition, the HOMO and LUMO energy levels using different functionals for another set of polythiophene-based molecules are addressed in ref 287, see Figure 20, where SW refers to the square-wave voltammetry method. All of the molecules investigated exhibit very low-energy HOMO levels such as −5.94, −5.93, −5.87, and −5.78 eV,288 respectively, for the structures PBDTA-MIM, PMIM, PBDTO-MIM, and PTMIM (details about the structural arrangements can be found in ref 287). The HOMO level is slightly raised from PMIM to PT-MIM by providing a lower offset to the LUMO energy level of PCBM.289 Adding the BDT units results in deeper HOMO J


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Figure 19. Various polythiophene-based structures along with corresponding labeling (refer to Table 3). Adapted with permission from ref 197. Copyright 2002 American Chemical Society.

energy levels of −5.87 and −5.94 eV for PBDTO-MIM and PBDTA-MIM, respectively, as compared to PT-MIM (−5.78 eV) to PMIM (−5.93 eV). By replacing the alkoxy side chains with the less electron-donating alkyl chains (from PBDTOMIM to PBDTA-MIM), the HOMO energy level is found to be lower by 0.07 eV. Generally, the HOMO levels of donor− acceptor polymers are mainly estimated by the electron deficiency in the system.290−295 The HOMO levels are relatively less affected by the conjugation length of the polymers as compared to the LUMO levels, which indicates that the variation of the HOMO energy level mainly depends on the nature of the donor units attached. The estimated HOMO and LUMO energy levels and optical gap for different systems obtained by using various computational approaches are shown in Figure 20.

well-known Bi−Te system.302 These properties make polythiophene a promising material in the formation of hybrid organic/inorganic thermoelectric composites. One such composite involves the incorporation of Bi2Te3 nanoparticles into a polythiophene film, with the temperature dependence of the Seebeck coefficient, electrical conductivity, power factor, and thermal conductivity for the composite shown in (Figure 21).303,304 The Seebeck coefficients for this system were found to be negative for a large temperature range (see left top panel of Figure 21), indicating that the polythiophene/Bi2Te3 composite behaves as n-type thermoelectric material, where the thermoelectric transport is dominated by electron carriers. The absolute value of the Seebeck coefficient increases slightly over the temperature range from 300 to 473 K with a similar trend also observed for the electrical conductivity, see right top panel. The highest absolute value of the Seebeck coefficient was found to be 156 μV K−1 at 423 K, which is 10% larger than that of pristine Bi2Te3 (120−138 μV K−1) under the same conditions and much larger than the obtained value for a polyaniline/Bi2Te3 composite (10−30 μV K−1).305 Importantly, the electrical conductivity for the polythiophene/Bi2Te3 composite decreases dramatically with increasing polythiophene content (see left bottom panel of Figure 21), which can be attributed to the decrease in the carrier concentration and carrier mobility. The obtained values of the temperature versus thermal conductivity for the interface of polythiophene/Bi2Te3 are shown in the right bottom panel of Figure 21. Thermal conductivities of 0.33−0.55 μWm−1 K−2 were found for the temperature range

5. THERMOELECTRIC PERFORMANCE The thermoelectric properties such as Seebeck coefficient and electric conductivity of polythiophene and its derivatives have been studied extensively computationally (theoretically) and experimentally.296−300 The Seebeck coefficient for pristine polythiophene was found to decrease with increasing electric conductivity, while the thermoelectric power factor increased.296 The power factor was found to be 1.03 × 10−5 μWm−1 K−2 with a Seebeck coefficient of 23 μV K−1, comparable to those of other conducting polymers. The figure of merit (ZT = α 2σT/κ, where α, σ, κ, and T are the Seebeck coefficient, the electrical conductivity, the thermal conductivity, and the operating temperature)301 was found to be 1/3 of the K


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Table 3. Experimentally Observed Absorption Energy and Calculations Using Different Computational Approaches (CIS and RRA)a ZINDO 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

HF

exptl

AM1

DFT

AM1

5.10 4.11 3.50 3.18 2.98 2.87 5.96 4.49 3.91 3.38 3.25 5.93 4.40 3.78 3.43 4.90 3.76 3.19 2.96 3.81 3.23 2.99 2.83 5.58 4.96 4.58 4.44 4.35 4.34 4.32 4.82 3.87 3.10 4.38 3.83 5.58 5.93 5.90 3.45

0.15, 0.88 −0.17, −1.13 −0.26, −1.04 −0.33, −1.00 −0.37, −0.97 −0.41, −0.97 −0.28, −0.14 −0.27, −1.24 −0.34, −1.14 −0.33, −1.00 −0.42, −0.58 −0.53, 0.57 −0.33, −0.81 −0.42, −0.71 −0.25, −0.63 0.52, 0.62 0.12, −0.87 −0.06, −0.84 −0.14, −0.81 0.43, −0.66 −0.08, −0.89 −0.20, −0.89 −0.14, −0.77 −0.22, −0.35 −0.50, −1.50 −0.69, −1.54 −0.63, −1.46 −0.91, −1.65 −0.65, −1.46 −0.59, −1.41 0.50, 0.66 −0.11, −1.12 0.08, −0.84 0.95, 1.09 0.38, −0.39 0.16, −0.26 −0.20, −0.65 −0.15, −0.67 0.06, −0.80

0.16, 0.95 −0.06, −1.03 −0.14, −0.94 −0.21, −0.90 −0.25, −0.87 −0.30, −0.87 −0.08, −0.01 −0.04, −1.03 −0.11, −0.06 −0.12, −0.81 −0.04, −0.72 −0.39, 0.73 −0.15, −0.60 −0.23, −0.49 −0.25, −0.41 0.60, 0.75 0.05, −0.94 0.06, −0.74 −0.07, −0.04 −0.20, −1.22 −0.15, −0.95 −0.20, −0.89 −0.21, −0.83 −0.15, −0.13 −0.20, −1.23 −0.36, −1.26 −0.21, −1.12 −0.60, −1.39 −0.49, −1.31 −0.25, −1.13 0.47, 0.73 −0.05, −1.08 0.02, −0.79 1.06, 1.26 0.33, −0.45 0.27, −0.15 −0.04, −0.52 0.02, −0.49 0.05, −0.80

1.40, 1.05 0.59, 0.31 0.46, 0.21 0.40, 0.18 0.38, 0.17 0.34, 0.14 0.96, 0.56 0.60, 0.31 0.45, 0.20 0.16, 0.88 0.24, 0.06 0.73, 0.32 0.49, 0.78 0.31, 0.59 0.24, 0.52 1.61, 2.03 1.04, 0.79 0.65, 0.41 0.60, 0.39 1.68, 1.43 0.92, 0.68 0.68, 0.46 0.76, 0.56 1.25, 0.84 0.46, 0.17 0.20, −0.04 0.28, 0.06 −0.30, −0.10 0.22, 0.01 0.29, 0.09 1.63, 1.28 0.90, 0.62 0.95, 0.70 1.55, 1.25 1.41, 1.10 1.29, 0.89 0.93, 0.52 0.90, 0.50 0.87, 0.62

TDDFT DFT 1.93, 0.56, 0.44, 0.37, 0.34, 0.30, 1.21, 0.91, 0.76, 0.36, 0.74, 0.99, 0.18, 0.06, 0.02, 1.26, 0.66, 0.63, 0.57, 0.62, 0.58, 0.48, 0.45, 1.60, 0.86, 0.61, 0.79, 0.26, 0.37, 0.71, 1.91, 0.78, 0.81, 2.02, 1.22, 1.48, 1.17, 1.19, 0.73,

1.71 0.28 0.19 0.15 0.13 0.09 0.83 0.62 0.52 0.69 0.56 0.59 0.48 0.35 0.30 1.80 0.39 0.39 0.35 0.33 0.33 0.25 0.24 1.87 0.60 0.38 0.58 0.07 0.18 0.51 1.66 0.49 0.56 1.81 0.90 1.77 0.78 0.80 0.48

AM1

DFT

0.88, 0.77 0.10, −0.20 −0.15, −0.41 −0.35, −0.55 −0.51, −0.65 −0.65, −0.76 0.89, 0.44 0.16, −0.18 −0.23, −0.47 −0.64, −0.76 −0.95, −1.01 0.71, 0.23 0.16, −0.23 −0.20, −0.50 −0.44, −0.66 0.19, 0.16 −0.10, −0.26 −0.22, −0.43 −0.37, −0.52 −0.04, −0.09 −0.11, −0.27 −0.30, −0.44 −0.37, −0.47 −0.45, −0.46 −0.33, −0.57 −0.79, −0.95 −1.09, −1.15 −1.36, −1.45 −0.58, −0.78 −1.15, −1.18 0.27, 0.13 0.23, −0.08 0.17, −0.07 −0.16, −0.25 0.38, 0.13 −0.32, −0.32 −0.47, −0.56 −0.43, −0.50 0.09, −0.12

1.08, 0.70 0.02, −0.28 −0.22, −0.47 −0.41, −0.61 −0.56, −0.71 −0.71, −0.82 1.11, 0.68 −0.01, 0.05 −0.05, −0.26 −0.34, −0.43 −0.64, −0.69 0.94, 0.48 0.34, −0.02 −0.05, −0.32 −0.29, −0.49 0.33, 0.24 −0.27, −0.51 −0.31, −0.51 −0.48, −0.62 −0.15, −0.44 −0.23, −0.46 −0.42, −0.60 −0.55, −0.68 0.13, 0.03 −0.12, −0.31 −0.56, −0.69 −0.76, −0.81 −1.13, −0.71 −1.25, −1.28 −0.77, −0.80 0.29, 0.07 0.11, −0.23 0.05, −0.20 −0.15, −0.22 0.34, 0.05 0.03, 0.54 −0.35, −0.44 0.21, −0.34 0.09, −0.24

a

Adapted with permission from ref 197. Copyright 2002 American Chemical Society. The experimental data were adapted from refs 277−281 and 283−286.

300−473 K. While the figure of merit, ZT, is still lower than that of bulk Bi2Te3, doping the polythiophene film is expected to increase the electrical conductivity with minimal effects on its thermal conductivity, allowing for an optimization of the figure of merit of these polythiophene/inorganic hybrid thermoelectric composites.306−308 The thermoelectric properties for the interface of polythiophene/multiwalled carbon nanotube (MWCNT), another promising thermoelectric material, were also investigated, with the MWCNT content ranging from 30 to 50%.219 The figure of merit was found to increase by increasing the concentration of MWCNT, largely due to the large increase in the electrical conductivity. The highest value of the figure of merit was found to be 8.71 × 10−4 for the interface of polythiophene/MWCNT composite with 80% coverage of the MWCNT at 120 °C, while the Seebeck coefficient was found to be 22.7 μV K−1. These

values can be increased either through doping of the polythiophene film309 or introduction of a ternary thermoelectric material such as Bi2Te3.304,310 Polythiophene composites have also been studied for their thermoelectric properties, namely the superstructures of poly(3,4-ethylenedioxythiophene) with polythiophene, poly(3methylthiophene), and poly(3-hexylthiophene) in the presence of polystyrenesulfonate.221 The temperature dependence of the power factor for the three systems mentioned above is addressed in Figure 22. Power factors of 1.57, 4.43, and 5.79 μWm−1 K−2 were observed for the superstructures of poly(3,4ethylenedioxythiophene) with polythiophene, poly(3-methylthiophene), and poly(3-hexylthiophene) in the presence of polystyrenesulfonate at 300 K, respectively. Moreover, improvements in the electrical conductivity and Seebeck coefficient were obtained by tailoring the size of the nanostructures L


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Figure 22. Power factor as a function of operating temperature for the superstructures of poly(3,4-ethylenedioxythiophene) with (a) polythiophene, (b) poly(3-methylthiophene), and (c) poly(3-hexylthiophene). Adapted with permission from ref 221. Copyright 2013 American Chemical Society.

Figure 20. HOMO and LUMO energy levels using DFT (B3LYP/631G(d) level and SWV) and the optical band gaps. Adapted with permission from ref 287. Copyright 2013 American Chemical Society.

was found that by increasing the length of the polythiophene chain, the oscillation of the spin thermopower increases, enhancing the spin thermoelectric figure of merit due to the reduction of the HOMO−LUMO gap. By increasing the length of the polythiophene chain in the parallel configuration, the magnitude of the spin thermoelectric figure of merit is suppressed due to an enhancement in the oscillation at larger chain lengths, with the situation reversed for the antiparallel configuration. Moreover, the spin conductance increases and the thermal conductance decreases with increasing the length of the polythiophene chain. Thus, the magnitude of the spin figure

used.311,312 This study has been followed by several other research groups to obtain deep understanding and improve the thermoelectric properties of different varieties of polymers.313−322 Recently, the spin thermoelectric properties of polythiophene connected between three-dimensional ferromagnetic electrodes (see Figure 23 for the model being used) for both the parallel and antiparallel configurations were studied using a tight-binding model within the Green’s function approach.323 It

Figure 21. Observed value of the Seebeck coefficient (left top panel), electrical conductivity (right top panel), power factor (left bottom panel), and thermal conductivity (right bottom panel) as a function of temperature for the interface of polythiophene/Bi2Te3. Adapted with permission from ref 303. Copyright 2011 Springer. M


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Figure 23. Ferromagnetic/polythiophene/ferromagnetic junction for both the parallel and antiparallel configurations, where n represents the number of thiophene rings and 4j, 4j-1, 4j-2, and 4j-3 refer to the sites of carbon atoms in thiophene rings. Adapted with permission from ref 323. Copyright 2013 John Wiley and Sons.

polythiophene is illustrated in Figure 25b; it is still unknown whether chain alignment can be obtained without creating crystalline domains in the polythiophene.329 The morphology of the polythiophene was also altered to determine the effect on the resulting thermoelectric properties. Vertically aligned arrays of polythiophene nanofiber were synthesized on a metallic substrate (see Figure 25c), and the transport properties of isolated nanofibers from the array were studied. Polythiophene nanofibers with 200 nm diameter were synthesized in a tubular form with wall thicknesses of 40−80 nm, see Figure 25d. Some of the polythiophene nanofibers are found to be in the solid phase, and these were used to measure the thermal conductivity on single fibers. On the other hand, polythiophene nanofibers with diameters of 18−300 nm were found to be amorphous based on electron diffraction highresolution transmission electron microscopy. Interestingly, polythiophene nanotubes were synthesized for the first time, see Figure 25e. The thermal conductivity of individual nanofibers was measured using the suspended microbridge technique (see Figures 26a and b).330 The obtained values of the thermal conductivity of polythiophene nanofiber samples increase upon decreasing the diameter, see Figure 26a. The thermal conductivity of the polythiophene nanofibers with diameters of about 204 to 71 nm were found to increase from 100 to 350 K, see Figure 26b. However, the thermal conductivities of several other amorphous materials such as carbon black,331 SiO2,332 and hydrogenated silicon films333 were found to remain constant up to 300 K.332 The fibers exhibit a distinct behavior with respect to temperature versus the thermal conductivity as compared to fibers of polyethylene (c-PE) and polybenzobisoxazole (c-PBO), see Figure 26b.328 Indeed, the thermal conductivity of polythiophene nanofibers was found to be increased from 80 to 300 K, whereas c-PE and c-PBO were found to decrease over the same temperature window. At room temperature, the phonon scattering in the crystalline polythiophene fibers is anharmonic phonon−phonon scattering, whereas the polythiophene nanofibers (Figure 25) were found to be dominated by interchain scattering resulting from the increased disorder within the fibers.

of merit decreases for antiparallel and increases for parallel configurations. This can be attributed to the fact that the value of the spin conductance and spin thermopower decreases in the antiparallel configuration due to reduction of the carrier tunneling between the electrodes via the polythiophene chain. The insets in Figures 24a and b show the voltage dependence of the change of maximum value of the spin figure of merit for the parallel and antiparallel configurations, respectively.

Figure 24. Voltage dependency of the spin figure of merit for various lengths of thiophene rings for (a) parallel and (b) antiparallel configurations. The insets refer to the change of maximum value of the spin figure of merit with voltage for n = 5 thiophene rings. Adapted with permission from ref 323. Copyright 2013 John Wiley and Sons.

In a recent experiment, it was demonstrated that pristine polythiophene nanofibers can have a thermal conductivity of about 4.4 Wm−1 K−1, which is 4 times higher than that obtained for the bulk polymer so far.324−326 By increasing the crystallinity as well as aligning the crystallites of the polythiophene (see Figure 25a for the model being used), the thermal conductivity was increased.327 For instance, highmodulus commercial fibers have a thermal conductivity of up to 10 Wm−1 K−1, where the main contribution is coming from the anharmonic phonon−phonon scattering in the crystalline regions of the polymers. 328 The amorphous form of N


Perspective


Figure 25. (a) Schematic representation of the chain orientation of the semicrystalline form of polythiophene surrounded by amorphous form, (b) chain orientation of the amorphous form of polythiophene, (c) scanning electron microscopy image of vertical polythiophene nanofiber arrays on metallic substrate, (d) transmission electron microscopic image of a polythiophene nanofiber, where the inset shows a selected area of the electron diffraction patterns, and (e) high-resolution transmission electron microscopic image of a polythiophene nanotube wall in the amorphous form. Adapted with permission from ref 324. Copyright 2014 Nature Publication Group.

Figure 26. (a) Thermal conductivity of a single-fiber versus fiber diameter at room temperature, where the inset shows the scanning electron microscopy image of a polythiophene nanofiber in the suspended microbridge, and (b) the thermal conductivity of a single-fiber versus temperature, where c-PE, c-PBO, and a-PT refer to crystalline polyethylene, crystalline polybenzobisoxazole, and amorphous polythiophene, respectively. Adapted with permission from ref 324. Copyright 2014 Nature Publication Group.

6. APPLICATIONS In this section, we focus mostly on experimental perspectives, the reason being that this field is not nearly saturated despite having a long research history. Indeed, there are several tremendous new directions that have opened recently for both experimentalists and theoreticians, especially in the area of device applications of polythiophene-based materials due to their low cost, solution processability, highly variable conductivity, and the possible development of three-dimensional stacking devices. Examples include solar cells, field-effect transistors (FET), light-emitting diodes (LED), hydrogen storage, water purification, and DNA detection. Thus, there are indeed lots of possibilities for the study and practical use of polythiophene-based materials. 6.1. Resistive Memory Devices. Polythiophene and its derivatives such as tethered alkyl substituted polythiophenes and block copolymers such as poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PE-

DOT:PSS) and their composites are of great interest for polymer-based resistive memory storage.334,335 Dynamic doping of π-conjugated polymers can be obtained by creating a composite containing a sufficient level of immobilized anions to accommodate the polymer’s oxidized form as well as mobile cations to revert the polymer into its neutral state. By forming a heterojunction with a material that is able to accept these mobile cations and remain or become conductive, such as metal oxides, high and low conductive states necessary for a resistive memory element can be achieved.147,336 The electrodeposition of these heterojunctions is scalable down to the nanometer scale to create devices on existing crossbar structures.147,337 Further, recent work has demonstrated the ability to perform this memory storage along the polymer chain in donor− acceptor conjugated polymers, allowing access to even smaller dimensions.338 One such system employs a polythiophene film doped with both dodecyl sulfate and Li+ placed into a heterojunction with O


Perspective

Chemistry of Materials an electrodeposited WO3 film (Figure 27).147,339 Even in the absence of solvent and electrolyte, Li+ ions are mobile and can

In this experiment, the conductivity of the polythiophene layer was reversibly switched between the high and low conductance states with write and erase bias, respectively. The retention time of the ON state was found to be 1 × 104 s, and the write-readerase-read switching was found to be stable over 100 cycles. Moreover, a fast switching speed was observed by simultaneous monitoring of both write and read operations. It is expected that such devices are dimensionally stable. Therefore, they are promising candidates for next generation nonvolatile memory application. High hole mobility of poly(3-hexylthiophene) thin films has been demonstrated.342 Thus, the thin films of poly(3hexylthiophene) are expected to be potential candidates for studying charge transport phenomena.343,344 In addition, poly(3-hexylthiophene) has also been proposed as a potential material to construct junction diodes.59,66,345−347 The switching behavior of the Al/PEDOT:PSS/poly(methyl methacrylate)/indium−tin−oxide/polyethylene terephthalate memory device was addressed in ref 348. The writing, reading, and erasing voltage pulses for the current−time characteristics were considered as −4, +1, and +4 V, respectively, see Figure 28. The value of the applied voltage−time for the writing, erasing, and reading processes was 1 ms, while the erasing voltage pulse can be applied to change the device from the ON to the OFF state. The current at the reading voltage pulse is found to be about 2 × 10−11 A. The current at the reading voltage is found to be about 1 × 10−8 A after the writing voltage pulse was applied to turn the device from the OFF to ON state. A significant difference between ON and OFF currents demonstrates that the proposed device has good switching behavior. The I−V curves for the device indicate that the current bistability with an ON/OFF current ratio is found to be about 1× 103, which in fact is larger than that of the device without a poly(methyl methacrylate) buffer layer. The obtained results indicated that the device has strong potential for readonly memory (ROM) applications. Moreover, bipolar switching behavior of nonvolatile polymerbased memory devices made of PEDOT:PSS with glycerol was demonstrated.349 It was found that the OFF-current of the

Figure 27. (a) Current−voltage characteristics for sandwiched structures of polythiophene(dodecyl sulfate−Li+)/polypyrrole(dodecyl benzenesulfonate) on indium tin oxide. Adapted with permission from ref 339. Copyright 2012 Royal Society of Chemistry.

drift in response to an applied field.340 The Li+ ions move from the polythiophene(dodecyl sulfate−Li+ ) film to WO3 (see Figure 27b), resulting in an oxidized polythiophene and a reduced WO3 film and a higher conducting state with increasing current at positive biases, see Figure 27a. To prevent diffusion of the Li+ ions from the WO3 film into the polythiophene film and thereby retain the high conductive state, an oxidized polypyrrole(dodecyl benzenesulfonate) layer is deposited between the films. Any mechanism by which Li ions go through the polypyrrole(dodecyl benzenesulfonate) layer will result in an increase in the energy of the system, amounting to a barrier to ion transport. The slow ion motion observed between the films can be understood by analyzing the hysteresis curve in Figure 27a. It is expected that, by optimizing the barrier thicknesses and the applied voltage, these systems can be utilized as memory devices such as dynamic randomaccess memory (DRAM) or static random-access memory (SRAM).59,66 Very recently, a polyelectrolyte-gated memory device based on a poly(3-carboxypentylthiophene) was synthesized. It was demonstrated that the device can display nonvolatile bistable memory characteristics with low operating voltage (±3 V).341

Figure 28. (a) Structure of the proposed Al/PEDOT:PSS/poly(methyl methacrylate)/indium−tin−oxide/polyethylene terephthalate memory device. (b) Switching behavior of the input voltage (top) and output current (bottom) for write-read-erase cycles. Adapted with permission from ref 348. Copyright 2012 AIP Publishing LLC. P


Perspective


Figure 29. Measured I−V characteristics of the indium−tin−oxide/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) + functionalized multiwalled carbon nanotubes/Al devices for various concentrations of the functionalized multiwalled carbon nanotubes. Device schematic is shown on the right. Adapted with permission from ref 350. Copyright 2012 ScienceDirect.

memory devices increases without any effect on ON-current levels, which can be understood by achieving highly conductive current paths on the ON-state. The write-read-erase-read cycle test could be operated 105 times, and the ON-retention time is dependent on the amount of glycerol in the PEDOT:PSS film as well as the annealing temperature. Another type of memory device made of a small amount of up to 0.01 wt% of functionalized MWCNTs embedded in PEDOT:PSS with Al as the top electrodes on indium−tin− oxide substrates was synthesized.350,351 Even for the low resistance ON-state and the high resistance OFF-state, the information can be stored for hours. The system was found to be stable even after hundreds of write-read-erase-read cycles, which makes it a candidate for erasable and rewritable volatile memory devices. The threshold voltage for OFF to ON switching can be maintained just by adjustment through changing the concentration of functionalized MWCNTs within the polymer film. The device performance for various concentrations of functionalized MWCNTs for indium−tin−oxide/PEDOT:PSS + functionalized MWCNTs/Al is pictured in Figures 29a−f. The devices showed bistability even for very small concentration of the functionalized MWCNTs. By reducing the concentration of the functionalized MWCNTs, an enhancement on the threshold voltage was found, which in fact allows the tuning of memory retention. Devices made of only of PEDOT:PSS do not show the bistability, which indicates that the functionalized MWCNTs composite is an essential utilization of single polymer film memory devices. 6.2. Photovoltaics and Photoelectrochemistry. Solar energy conversion, including the use of organic solar cells, could be a potential route to tackle the energy crisis in the future.353,354 A schematic of an organic solar cell is shown in Figure 30, where a bulk heterojunction is formed between a ptype and n-type organic small molecules or polymers.352 One

Figure 30. Schematic for the formation of a polymer-based solar cell. In this design, a bulk heterojunction is formed between the p-type and n-type polymers employed. Adapted with permission from ref 352. Copyright 2015 American Chemical Society.

potential route to increase the efficiency of such organic solar cells has been suggested by many groups,163,355−358 which involves developing new organic materials with a high dielectric constant. This type of research focuses on lowering the Coulomb potential between the electrons and holes by increasing the concentration of free charges, which is useful for the solar cells. Given the promising electrical, optical, and physical properties of polythiophene films, their use in organic solar cells359−361 and dye-sensitized solar cells362−364 has significantly grown in the scientific community over recent years. The heterostructure of poly(3-hexylthiophene) and [6,6]phenyl-C61-butyric acid methyl ester was proposed as a potential candidate to construct organic solar cells with Q


Perspective


the placement of the valence and conduction bands relative to the reduction potential for hydrogen, the photoexcited electron upon illumination can be utilized to drive the reductive process in solution, provided no large energy barrier exists toward the reduction process. The band position of polythiophene films also allows their use in oxygen reduction to water390−397 for fuel cell applications. Given the large degree that these polymer films can be adjusted, the light absorption and extraction efficiency as well as the catalytic ability of the film can be optimized. However, due to the potential for degradation of the polymer film under the applied conditions, current avenues being explored involve the incorporation of protective layers, additional light absorbers, and introduction of solution-based electrocatalysts to help mitigate this issue and improve the cell’s efficiency. Alternatively, thiophene and its derivatives can also form heterostructures with other semiconducting materials for solar energy conversion. Experimentally, using the Heck reaction, coupling of olefins to the Si(111) surface functionalized by a mixed single layer consisting of methyl and thienyl groups was proposed.398 In this method, conjugated linkage was developed between the Si(111) surface and olefinic surface functionality, which indeed allows charge redistribution easily from the Si(111) surface to the mixed monolayer of methyl and thienyl groups. Such a synthetic method would be useful to construct surface derivatizable, air-stable silicon photoelectrodes.399 Alternatively, thiophene adsorption on a Si(111) surface can also tune the electronic band gap over a wide range, which could be a significant step toward applications in photoelectrochemistry.400 Furthermore, introduction of a PEDOT:PSS layer at an n-doped silicon surface results in the formation of a p-n diode,401−403 where the barrier to charge transport and the overall photovoltaic properties can be tuned based on the organic moiety present.404−408 Recently, the charge injection rates in nanosilicon-polythiophene bulk heterojunction solar cells were determined experimentally as well as theoretically.409−412 Characteristic times of the order of ∼10−100 fs were found for the electron injection from the photoexcited dodecathiophene or polythiophene to a hydrogen-passivated silicon clusters of ∼2.2 nm diameter if the thiophene chain lies approximately parallel to the silicon surface (physisorption). The chemisorption of the thiophene chain with an ethyl linker significantly improves the electron injection time as compared to the physisorption of the thiophene on the silicon surface. Moreover, the electron injection time was found to be much larger when the oligothiophene chain anchors perpendicularly to the silicon surface. This is attributed to the low weight of the oligothiophene LUMO on the chain end and the small overlap of this state with the unoccupied electron states of the nanocrystal, see Figure 32 for the model that was used. Therefore, chemisorption of the thiophene on the silicon surface has the advantage of offering more anchoring points per chain in oligothiophenes as compared to physisorption (see right panel of Figure 32), ensuring a smaller spacing between the oligothiophene chain and the silicon surface and improving the electronic coupling. Such a system has recently been realized experimentally by constructing hybrid photovoltaic thin films.413 6.3. Organic Light-Emitting Diodes. Polymer-based (organic) light-emitting diodes have attracted significant research interest because of their low power consumption, light weight, fast response, and comparative ease of integration

reasonable characteristic carrier lifetimes, and hence reasonable extraction efficiencies.365−372 Theoretically, it was proposed that incorporation of permanent dipole moments in the organic materials in fact lowers the Coulomb attraction/potential.372 One method of introducing this dipole into conjugated polymers is by linking the polymer with a fullerene derivative, resulting in large dipole moments in the polymer side chains.373−375 Fullerene-based polythiophene solar cells suffer from narrow absorption profiles, however, leading to the development of fullerene-free solar cells376−378 which show improved efficiencies (ca. 10%). Alternatively, introduction of a donor/acceptor pair within the polymer chain can also result in this necessary dipole moment,379 such as poly(thiophene-blockperylene diimide) copolymers.380 In addition, semiconducting conjugated polymers were fabricated through Stille polymerization, where the structures combined unsubstituted or (triisopropylsilyl)ethynyl substituted 2,6-bis(trimethylstannyl)benzo[1,2-b:4.5-b′]dithiophene that acts as a donor unit and benzotriazole with a symmetrically branched alkyl side chain that act as an acceptor unit.381 Power conversion efficiencies of 5.5 and 2.9% for (triisopropylsilyl)ethynyl substituted 2,6-bis(trimethylstannyl)benzo[1,2-b:4.5b′]dithiophene and benzotriazole-based devices, respectively, were obtained. While lower than state-of-the-art power conversion efficiencies (7−10%),382,383 the difference in power conversion efficiencies can be understood by the differences in the optimal morphology and carrier mobility, which in fact provides a potential route to improve the performance of the solar cells.378,384,385 Polythiophene films have also shown their use in photoelectrochemical cells. Preliminary work investigated the ability of polythiophene films such as poly(3-hexylthiophene) to photoelectrochemically or photocatalytically reduce water to hydrogen.387−390 Further work has shown the ability of a poly(3-hexylthiophene) film to reduce an organic substrate, anthraquinone-2,7-disulfonate, directly (Figure 31).386 Due to

Figure 31. Schematic for a photoelectrochemical system involving a polythiophene film for the reduction of anthraquinone-2,7-disulfonate (AQ27DS). Adapted with permission from ref 386. Copyright 2016 John Wiley and Sons. R


Perspective


Figure 32. (a) Physiosorption (left panel) and chemisorption (right panel) of a polythiophene chain on H-passivated silicon clusters. Adapted with permission from ref 409. Copyright 2013 American Chemical Society.

into electronic devices.66,414,415 The charge transport is an important factor to determine the performance of these devices. To achieve high performance of the organic light-emitting devices (OLEDs), the charge injection and transport from both anode and cathode should be balanced by excitons that were induced in the light emission layer.415 A light-emitting diode mainly consists of three layers embedded between two electrodes: (i) the hole injection/transport layer, (ii) electron-emitting layer, and (iii) electron-transporting layer, where each layer has to be optimized individually for charge injection, transport, and emission. A schematic of a typical OLED is shown in Figure 33. Due to the high conductivity,

Figure 34. (a) STM image of a polythiophene wire embedded on a Au(111) surface, (b) normalized conductance (G/G0) as a function of the distance between the tip and the sample for a polythiophene wire embedded in the junction for different voltages, (c) conductance (dI/ dV) spectrum, (d) light emission efficiency as a function of the voltage, and (e) atomistic structure of the polythiophene embedded between the gold electrode and linked on the top by the STM tip. Adapted with permission from ref 418. Copyright 2014 American Physical Society.

distance between tip and sample, and β determines the ability of the wire to transport current and becomes smaller and smaller upon increasing the applied voltage.421,422 The obtained dI/dV spectra for suspended polymers of various lengths are shown in Figure 34c. It is clear from this figure that the first resonance appears at V = −0.8 V and a second one occurs at V = 1.25 V, while another highly intense resonance was found at 2 V. For V = 1.4 eV, the quantum yield of the emission process was found to be 10−7 photon/electron. It increases up to 10−5 at higher voltage, see Figure 34d, which can be attributed to the larger emission bandwidth. An atomistic structure of the polythiophene embedded between the gold electrode and linked to the top by the STM tip is shown in Figure 34e. The authors of ref 418 suggest that the strong similarity of their experiment with OLEDs opens up the possibility to realize single-molecular optoelectronic devices made of polythiophene. 6.4. Field-Effect Transistors. The basic function of a transistor is to work as a switch and an amplifier, where the amount of current flowing between two terminals is determined by a voltage applied to a gate; thus, the transistor can function as ON/OFF or as a very sensitive transducer, providing memory elements similar to those discussed above in Section 6.1. This basic function makes transistors the fundamental building block of all logic circuits necessary for electronic devices. Thin film field-effect transistors (FETs) generally consist of a semiconducting layer separated from a gate electrode by a thin insulating gate dielectric.58,424,425 Experimentally, it was demonstrated that polythiophene and derivatives have played an important role in the development of printable electronics, organic electronics, and molecular electronic devices.426 In recent years, polythiophene films have been used in polymer-based FETs.423,427−431 In one study, the authors

Figure 33. Schematic for an OLED. Adapted with permission from ref 416. Copyright 2009 American Chemical Society.

reasonable ionization potential, wettability, and fair holeinjection ability, PEDOT:PSS is typically employed in OLEDs as a hole injection layer, similar to the ITO anode in Figure 33. However, the electrical properties of such a device can thereby become dependent on the presence of oxygen and water during fabrication. Alternative thiophene derivatives such as bithiophene vinylene have shown promise as light-emitting materials, where the color of the light-emitting can be tuned from red to purple to white simply by the monomer or substituent employed on the thiophene unit.416,417 Alternatively, a suspended single layer of pristine polythiophene molecule between the tip of a scanning tunneling microscope (STM) and a gold substrate was proposed as polythiophenebased light-emitting diode.418,419 Figure 34a shows the suspended polythiophene wire in the junction, where the STM tip is first located on the top of polythiophene deposited on a Au(111) surface, see Figure 34b.420 The total conductance can be defined as G(z) ∝ Gcexp−βz, where Gc is the conductance at contact, z is the S


Perspective


Figure 35. (a) I−V characteristics of a PBTTT/PMMA transistor, where the PBTTT molecular structure is shown in the upper right inset, and the transistor structure is shown in lower left inset. (b) CMS spectra for the accumulation region for isotropic nanoribbon PBTTT/PMMA transistors, where the inset shows the delocalized states, (c) CMS of a nanoribbon PBTTT film in accumulation and full depletion regimes, and (d) UV−visible absorption spectrum (red line) and first derivative of the absorption (black line) at 300 K. Adapted with permission from ref 423. Copyright 2013 American Chemical Society.

Figures 36a−c. The source electrode is usually grounded when voltage is applied through the gate electrode and drain

investigated the charge carriers induced in FETs based on a uniaxially aligned polythiophene derivative poly(2,5-bis(3alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) using electro-optical charge modulation spectroscopy (CMS). The I−V characteristics of a PBTTT/polymethymethalacrylate (PMMA) transistor are shown in Figure 35a, where linear and saturation mobilities of 0.10 and 0.32 cm2 V−1 s−1, respectively, were found. The CMS spectra for an isotropic nanoribbon of PBTTT transistors are addressed in Figure 35b; the obtained spectra agree well with a previous report.432 Neutral absorption is found to about 2.2 eV, which shows a positive peak in the ΔT/T plot, whereas charge induced, polaron-based absorption with two significant negative peaks at about 1.2−1.3 and 1.8−1.9 eV was observed. These two peaks might be originating from the same charged species because the shape of the spectra was independent of the magnitude of the applied gate voltage.222 The CMS spectra were measured for aligned nanoribbons of PBTTT films that were prepared by zone-casting and measured in accumulation regime of −30 V and full depletion regime of +10 V, where the modulating voltage was ±1 V, see Figure 35c. Neutral UV− visible absorption spectroscopy was carried out on PBTTT films at room temperature, see Figure 35d. Interestingly, the charge-induced absorption spectrum suggests that polymerbased field-effect transistors can be constructed and operated at room temperature.433 The working principles of polythiophene-based transistors are demonstrated in ref 434. A source and a drain electrode, also called channel width, are separated by a distance called the channel length, and are directly connected to the semiconducting layer. Commonly used structures of the transistors include (i) bottom contact/top gate, (ii) top contact/bottom gate, and (iii) bottom contact/bottom gate geometries, see

Figure 36. Molecular structure of self-doped polythiophene (rrP3HT) and different transistor prototypes: (a) bottom contact/top gate, (b) top contact/bottom gate, and (c) bottom contact/bottom gate. (d) Working principle of electrolyte-gated transistors. Adapted with permission from ref 434. Copyright 2014 Springer Berlin Heidelberg.

electrode. The source−drain voltage can be estimated just by measuring the potential difference between the source and drain, where the source acts as the charge-injecting electrode. A symbolic representation of the working principles of the polythiophene-based field-effect transistor is shown in Figure 36d. When a positive gate voltage is applied with respect to the source, electrons are injected, while holes are injected for T


Perspective

Chemistry of Materials negative gate voltage. The applied gate voltage determines the amount of accumulated charges in the channel and the current flow between the source and the drain electrode which, in principle, is the product of the charge carrier density, mobility, and the lateral electric fields. Normally, the charges accumulate only within a minute distance from the semiconductor− dielectric interface (within the first molecular layer). It is not necessary that all the induced charges are mobile and contribute to the current in the field-effect transistors. Such transistors have been realized experimentally, where SiO2 acts as a substrate, and a polythiophene-based polymer behaves as sample. It has been demonstrated that such transistors are highly stable in air.435−438 More recently, investigations have begun looking at organic FETs involving thiophene copolymers.440−446 In one such study, a high field-effect mobility of ∼3.6 cm2 V−1 s−1 was demonstrated experimentally for an organic transistor made of an indacenodithiophene−benzothiadiazole copolymer, creating a donor−acceptor pair within the polymer backbone. The molecular structure of the copolymer is shown in Figure 37.

Figure 37. Molecular structure of an indacenodithiophene− benzothiadiazole copolymer. Adapted with permission from ref 439. Copyright 2013 Nature Publishing Group.

Figure 38. Organic FET constructed from indacenodithiophene− benzothiadiazole copolymer: (a) top-gate, bottom-contact device, (b) transfer curves, and (c) mobility obtained based on the first derivative of the linear and second derivative of the saturation regime. Adapted with permission from ref 439. Copyright 2013 Nature Publishing Group.

The conjugated planes are found to exhibit a common as well as comprehensive orientation in both the noncrystalline regions and the ordered crystallites. The charge transport in highmobility semiconducting polymers is found to be quasi onedimensional, indicating only intermolecular hopping through the short π-stacking bridges.439 An organic FET constructed from indacenodithiophene−benzothiadiazole copolymer is addressed in Figure 38a, with its I−V characteristics shown in Figure 38b. The carrier mobility can be estimated by taking the first as well as second derivatives of the drain current for the linear and saturation regimes, see Figure 38c. The obtained values of the mobilities are 1.5 ± 0.2 cm2 V−1 s−1 for the linear and 2.0 ± 0.2 cm2 V−1 s−1 for the saturation regimes at room temperature with ON/OFF ratios of about 106, which depends on the lengths of the devices. A mobility 2−3 times higher was achieved in contrast to the earlier studies;447,448 the enhancement can be attributed to the larger molecular mass and smaller polydispersity.449 The mobilities could be further increased by employing fractionated materials with higher molecular masses and narrower polydispersity such that the quality and efficiency of the organic field-effect transistors can be improved. 6.5. Biosensors. Clean water with good microbiological quality is essential to keep humans healthy and free from diseases that are transferred by water.450−453 It has been reported that diarrheal diseases due to poor microbiological quality result in the deaths of about 4000−6000 children per day; a large percentage of these deaths are children.454 Water purification techniques are being developed and employed day

by day to improve hygiene and reduce the occurrence of waterborne diseases, but water distribution systems are increasingly contaminated by a wide range of microbes.455,456 Drinking water naturally contains viruses, bacteria, fungi, and parasites, and they can survive (i) even in the filtration and chlorination process, (ii) in the distribution system, such as household pipes or reservoirs, (iii) from retention or stagnation, and (iv) in the distribution system following a structural failure.454,457−459 To tackle the problem properly, a cationic water-soluble polythiophene was used for the rapid and direct assessment of the microbiological quality of water based on the heterotrophic plate count criteria applied in North America, especially in the United States and Canada.76,166,460 In the presence of polyanionic macromolecules such as DNA, RNA, and negatively charged particles, the cationic polythiophene shows a diminution of its initial fluorescence, see Figure 39 for the detailed steps involved. It also shows a wavelength shift in fluorescence and colorimetry when its conformation changes from a nonplanar to a planar form. The interactions between the cationic polythiophene biosensor and intact microbial particles decrease the magnitude of the fluorescence intensity without inducing a fluorescence wavelength shift, which indicates that this technique is useful for biosensing. U


Perspective


A schematic description of a polythiophene-based fluorescent DNA biosensor is presented in Figure 40. The fluorescent hybridization detection signal can be achieved upon conformational change of the cationic poly(3-alkoxy-4-methylthiophene) from a planar (duplex) form to a nonplanar (triplex) form.80,461 The polythiophene biosensor measured after 30 min of triplex formation shows that some DNA sequences emitted higher fluorescence signals than others. Moreover, the absorbance measurements showed that the difference in fluorescence signal is reflected by a faster switch from 530 nm (duplex) to 420 nm (triplex) absorbance peak for the high fluorescence signal sequences compared to the weak fluorescence signal, which indicates that the polythiophene conformational change is indeed affected by the sequence of the DNA components. The difference in fluorescence emission signal upon hybridization can be understood by the structure adopted in the duplex. The absence of intermolecular hydrogen bonding stabilizes the dsDNA structure, while ssDNA shows various structures directly mediated by its sequence; therefore, the variations might be responsible for the polythiophene-based biosensor structure in the duplex form.462 6.6. Batteries, Hydrogen Storage, and Supercapacitors. A polythiophene-based flow battery was realized experimentally with the goal of achieving a high cell potential.464 In ref 464, polythiophene microparticles were used for both anodic and cathodic redox couples. The polythiophene electrochemical redox potential was found to be −2.0 and +0.5 V for n-doping and p-doping, respectively. This indicates that the polythiophene-based flow battery can be operated at a cell potential of 2.5 V, which is the highest cell potential in a metal-free and all-organic redox flow battery. Such a battery also showed stable charge/discharge performance with a high energy efficiency of about 70%.

Figure 39. Operating principle of a polymer-based biosensor. The fluorescence is optimal when the cationic polythiophene biosensors is by itself; after addition of a microbial particle, a reduction in fluorescence without any wavelength shift is observed. Adapted with permission from ref 76. Copyright 2013 American Chemical Society.

Figure 40. Schematic representation of polymer-based hybridization detection system. The cationic poly(3-alkoxy-4-methylthiophene) biosensor is electrostatically attracted by DNA with different optical spectra for single-stranded (ss) DNA (duplex) and double-stranded (ds) DNA (triplex). The free cationic poly(3-alkoxy-4-methylthiophene) shows a nonplanar conformation with high fluorescence intensity (λmax = 400 nm). For the duplex form, it shows a planar conformation with very low fluorescence (λmax = 530 nm). Whereas, upon hybridization of the duplex with a dsDNA, the cationic poly(3-alkoxy-4-methylthiophene) conformation transfers from planar to nonplanar and thus regains much of its fluorescence emission properties (λmax = 420 nm). DNA detection is based on the fluorescence difference between duplex and triplex. Adapted with permission from ref 80. Copyright 2013 John Wiley and Sons. V


Perspective

Chemistry of Materials The synthesis of activated carbon derived from polythiophene is addressed in Figure 41. In this process, 2-

Figure 42. Schematic representation of 1D polythiophene nanofibers grown within a titanium dioxide nanotube array. Adapted with permission from ref 466. Copyright 2017 Royal Society of Chemistry.

Figure 41. Schematic representation of the synthesis process of polythiophene-based activated carbon with the following steps: (1) synthesis and (2) chemical activation of polythiophene. SEM images of (a) polythiophene and (b) activated carbon and (c) TEM image of the activated carbon. Adapted with permission from ref 463. Copyright 2011 ScienceDirect.

7. CONCLUSION AND OUTLOOK In this review, we covered the material properties and possible applications of pristine polythiophene and its derivatives in light of theoretical modeling and experimental realization with a view of future directions. Polythiophene-based polymers have been used in molecule-based electronic devices because of their capacity to transition from semiconductor to conductor and the fact that the material properties can be easily controlled by doping or chemical modification by creating superlattices with other materials or by stacking. Polythiophene-based polymers have already been widely used and extensively studied in electronic devices such as in field-effect transistors, memory devices, organic light-emitting diodes, solar cells, biosensors/ hydrogen storage, and batteries. As such, polythiophene-based molecules do have significant potential to be fully integrated into a wide range of next generation electronic devices. However, due to various experimental limitations, synthetic and structural properties of π-conjugated oligomers or polymers are still difficult to understand.155,156 As such, there has been a big push to predict the ability of polythiophene films and various derivatives for use in organic solar cells, FETs, and LEDs through computational approaches471−475 rather than experimental approaches, effectively creating a catalog of potential polythiophene-based materials. The challenge then lies in the effectiveness of the theoretical models used, which is limited by the various levels of approximations that have to be chosen and the difficulty of building physically meaningful yet tractable models of complex experimental situations.157,476 In the end, it should be noted that the B3LYP, HSE, LC, and LC* levels of computational approaches reproduced experimental parameters very well compared to those of HF and PBE; therefore, our suggestion is to employ B3LYP, HSE, LC, and LC* levels of computational approaches while dealing with πconducting polymers such as polythiophene and its derivatives.

thiophenemethanol is first polymerized with the help of FeCl3. The scanning electron microscopy (SEM) image is shown in Figure 41a, where the diameter of the polythiophene polymer was found to be about 0.3−1.3 μm. The polythiophene is then converted into activated carbon by chemical activation with KOH. The morphology of the activated carbon exhibits a graphene-like structure with highly vesiculated irregularly shaped particles of relatively large size of about 40 μm, see Figure 41b. The porosity of the activated carbon was found to be randomly distributed but uniformly sized, see Figure 41c. Highly porous carbon materials with tunable pore size, surface area, and pore volume of up to 3000 m2/g and 1.75 cm3/g, respectively, were synthesized for the first time with relatively high hydrogen storage capacity. The hydrogen storage capacity of the material was found to be about 5.71 wt%, which is indeed superior to the hydrogen uptake of other types of activated carbon having similar or even higher surface area, which is a great achievement in the area of hydrogen storage.465 A large enhancement in charge capacity was found for ultrathin polythiophene films, which preserves the high surface area and pore space of the nanostructures in which they were developed, ideal for application as pseudocapacitors.466,467 One such design is shown in Figure 42, where polythiophene nanofibers are electropolymerized within the pores of a titanium dioxide nanotube array.466 The nanostructures were found to exhibit excellent long-term cycling stability (>5000 cycles) with high Coloumbic efficiencies (99.76%) and high specific capacitance (1052 g−1), very important for application as pseudocapacitors. Furthermore, pseudocapacitors made of ultrathin polythiophene within activated carbon increase by up to 50 and 250% in specific and volumetric capacitance, respectively, as compared to the bare activated carbon, which indeed is a great step toward increasing the charge capacity for polythiophene-based materials.468−470

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: msfreund@fit.edu. ORCID

Patrick K. Giesbrecht: 0000-0003-2039-4791 W


Perspective


Conductivity in Doped Polyacetylene. Phys. Rev. Lett. 1977, 39, 1098− 1101. (2) Shirakawa, H.; Louis, E. J.; MacDiarmid, A. G.; Chiang, C. K.; Heeger, A. J. Synthesis of Electrically Conducting Organic Polymers: Halogen Derivatives of Polyacetylene (CH). J. Chem. Soc., Chem. Commun. 1977, 16, 578−580. (3) Kumar, M. R.; Freund, M. S. In Self-Doped Polymers; K. Mullen, T. M., Reynolds, J. R., Ed.; Royal Society of Chemistry: London, 2013. (4) Freund, M. S.; Deore, B. A. Self-Doped Conducting Polymers; Wiley and Sons: Hoboken, NJ, 2007. (5) Skotheim, T. A. In Handbook of Conducting Polymers; Dekker: New York, 1986; Vol. I and II. (6) McGehee, M. D.; Miller, E. K.; Moses, D.; Heeger, A. J. In Advances in Synthetic Metals: Twenty Years of Progress in Science and Technology; Bernier, P., Ed.; Elsevier: Amsterdam, 1999. (7) Plausinaitis, D.; Sinkevicius, L.; Mikoliunaite, L.; Plausinaitiene, V.; Ramanaviciene, A.; Ramanavicius, A. Electrochemical polypyrrole formation from pyrrole ’adlayer’. Phys. Chem. Chem. Phys. 2017, 19, 1029−1038. (8) Pham-Cong, D.; Park, J.; Kim, J.; Kim, J.; Braun, P.; Choi, J.; Kim, S.; Jeong, S.; Cho, C. Enhanced cycle stability of polypyrrole-derived nitrogen-doped carbon-coated tin oxide hollow nanofibers for lithium battery anodes. Carbon 2017, 111, 28−37. (9) Tan, Q.; Lu, S.; Lv, Y.; Xu, X.; Si, J.; Xiang, Y. Doping structure and degradation mechanism of polypyrrole-Nafion composite membrane for vanadium redox flow batteries. RSC Adv. 2016, 6, 103332−103336. (10) Kadac, K.; Nowaczyk, J. Polythiophene nanoparticles in aqueous media. J. Appl. Polym. Sci. 2016, 133, 1. (11) Raichman, D.; Ben-Shabat Binyamini, R.; Lellouche, J.-P. A new polythiophene-driven coating method on an inorganic INT/IF-WS2 nanomaterial surface. RSC Adv. 2016, 6, 4490−4504. (12) Brédas, J. L.; Street, G. B.; Thémans, B.; André, J. M. Organic Polymers Based on Aromatic Rings (Polyparaphenylene, Polypyrrole, Polythiophene): Evolution of the Electronic Properties as a Function of the Torsion Angle between Adjacent Rings. J. Chem. Phys. 1985, 83, 1323−1329. (13) Brédas, J. L.; Thémans, B.; Fripiat, J. G.; André, J. M.; Chance, R. R. Highly Conducting Polyparaphenylene, Polypyrrole, and Polythiophene Chains: An ab initio Study of the Geometry and Electronic-Structure Modifications upon Doping. Phys. Rev. B: Condens. Matter Mater. Phys. 1984, 29, 6761−6773. (14) Heeger, A. J. Semiconducting and Metallic Polymers: The Fourth Generation of Polymeric Materials (Nobel Lecture). Angew. Chem., Int. Ed. 2001, 40, 2591−2611. (15) MacDiarmid, A. G. Synthetic Metals”: A Novel Role for Organic Polymers (Nobel Lecture). Angew. Chem., Int. Ed. 2001, 40, 2581− 2590. (16) Shirakawa, H. The Discovery of Polyacetylene Film: The Dawning of an Era of Conducting Polymers (Nobel Lecture). Angew. Chem., Int. Ed. 2001, 40, 2574−2580. (17) Zhao, X.; Zhan, X. Electron Transporting Semiconducting Polymers in Organic Electronics. Chem. Soc. Rev. 2011, 40, 3728− 3743. (18) Hoofman, R. J.; de Haas, M. P.; Siebbeles, L. D.; Warman, J. M. Highly Mobile Electrons and Holes on Isolated Chains of the Semiconducting Polymer Poly(phenylene Vinylene). Nature 1998, 392, 54−56. (19) Noriega, R.; Salleo, A.; Spakowitz, A. J. Chain Conformations Dictate Multiscale Charge Transport Phenomena in Disordered Semiconducting Polymers. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 16315−16320. (20) Nicolai, H.; Kuik, M.; Wetzelaer, G.; de Boer, B.; Campbell, C.; Risko, C.; Brédas, J.; Blom, P. Unification of Trap-Limited Electron Transport in Semiconducting Polymers. Nat. Mater. 2012, 11, 882− 887. (21) Sotzing, G. A.; Lee, K. Poly(thieno[3,4-b]thiophene): A p- and n-Dopable Polythiophene Exhibiting High Optical Transparency in the Semiconducting State. Macromolecules 2002, 35, 7281−7286.

Georg Schreckenbach: 0000-0002-4614-0901 Michael S. Freund: 0000-0003-1104-2292 Notes

The authors declare no competing financial interest. Biographies Dr. Thaneshwor P. Kaloni obtained his M.Sc. from Tribhuvan University, Nepal in 2007. He received a Ph.D. in materials science and engineering from King Abdullah University of Science and Technology in 2013. He then moved to the University of Manitoba Department of Chemistry as a postdoctoral fellow in 2014 and worked on polythiophene-based conducting polymers for one and half years. In 2016, he moved to California State University Northridge as a research scientist. Currently, he is a postdoctoral fellow at the University of Arkansas, Fayetteville, where he is working on twodimensional structural phase transition and ferroelectricity in monochalcogenide monolayers. To date, he has published 37 papers. Patrick K. Giesbrecht received his B.Sc. and M.Sc. degrees from the University of Manitoba (Winnipeg, Canada) in 2015 and 2017, respectively. During his time there, he conducted research on the growth and surface functionalization of silicon microrods under the supervision of Professor Michael S. Freund as well as investigations into electrocatalysts for CO2 reduction for solar fuels applications under the supervision of Professor David E. Herbert. He is now a member of the professional staff at the Florida Institute of Technology in the Freund Group, where he is working on the development of membranes for solar fuel generation. Dr. Georg Schreckenbach is a Professor of Chemistry at the University of Manitoba, Canada. He received his Ph.D. in 1996 from the University of Calgary, Canada, under the supervision of the late Tom Ziegler. From 1997 to 2000, he was a Seaborg Institute Postdoctoral Fellow at Los Alamos National Laboratory, United States, working with Jeffrey Hay and Richard Martin. After holding positions at the CCLRC Daresbury Laboratory (UK) and Concordia University (Canada), he joined the University of Manitoba in 2003. Dr. Schreckenbach’s research interests span a wide range of computational and theoretical chemistry. They include quantum-chemical method development, chemistry of heavy elements (actinides and Hg), and materials (solar energy conversion, polymers, and 2D materials). Dr. Michael S. Freund has been Professor and Head of Chemistry at the Florida Institute of Technology since 2015. From 2002 to 2015, he was faculty at the University of Manitoba, where he attained the rank of tenured Professor of Chemistry and held the Canada Research Chair in Electronic Materials (Tier 1 and 2). He has published over 100 papers and has supervised approximately 50 students and postdoctoral associates. His research interests include electrochemistry, conducting polymers, and surface science.

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ACKNOWLEDGMENTS G.S. acknowledges funding from the Natural Sciences and Engineering Council of Canada (NSERC, Discovery Grant). M.S.F. acknowledges support by the Natural Sciences and Engineering Research Council (NSERC) of Canada, the Canada Research Chair program, Canada Foundation for Innovation (CFI), the Manitoba Research and Innovation Fund, the University of Manitoba, and the United States National Science Foundation (NSF) under the CCI Solar Fuels Program, Grant No. CHE-1305124.

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Polythiophene: From Fundamental Perspectives to Applications

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