Semiconducting Polymers Containing Tellurium - ACS Publications

Dec 24, 2014 - Group 16 atom (O, .... with the heavier group 16 elements selenium or tellurium.37,38 ...... (110) Luhman, W. A.; Holmes, R. J. Appl. P...
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Perspective pubs.acs.org/Macromolecules

Semiconducting Polymers Containing Tellurium: Perspectives Toward Obtaining High-Performance Materials Elisa I. Carrera and Dwight S. Seferos* Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada ABSTRACT: The development of high-carbon-content polymers for optoelectronics is an area of intense research; however, carbon-rich materials have certain limitations that arise from their composition. Some of these limitations can be overcome by the judicious incorporation of heavier elements which do not significantly change the carbon content to a point where it adversely affects cost and processability. Here we examine the use of tellurium as a heavy atom in the design of optoelectronic polymers. Group 16 atom (O, S, Se, Te) substitution is a promising strategy for the development of high performance materials for organic electronic applications. The use of tellurium in place of selenium or sulfur in conjugated polymers lends new properties to these materials such as red-shifted optical absorption, high polarizability, high dielectric constant, and strong intermolecular interactions. These properties are favorable for organic photovoltaics (OPVs) and organic field effect transistors (OFETs). In particular, extending the absorption range to the near-IR allows for more efficient solar harvesting since low-energy photons are most abundant. Additionally, strong Te−Te interactions lead to enhanced interchain electronic coupling, which is expected to facilitate charge transport. The use of polymers containing tellurophene, the tellurium analogue of the well-studied thiophene, has only recently begun to emerge in the literature. New synthetic routes have been developed, and there now exist a handful of tellurophene-containing polymers that have been synthesized and used to fabricate OPVs and OFETs. Their performance in OPVs has not surpassed that of their lighter chalcogen analogues; however, the use of tellurophene-containing materials is a young field, and continued efforts in the development of new materials and device optimization should lead to improved performance. In this Perspective we discuss the current status of tellurium-containing polymers in terms of their synthesis, properties, and performance. We highlight the challenges that have been overcome thus far and emphasize those that should be the focus of future work. This includes overcoming synthetic challenges and developing an understanding of the current limitations in device performance with tellurium-containing polymers through studies of materials properties and excited state dynamics. We also suggest new applications and directions for tellurium-containing materials beyond OPVs and OFETs.

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INTRODUCTION Decades after their introduction,1−11 poly(3-alkylthiophene)s (P3ATs) are still the most popular conjugated polymers as measured by the number of papers published and their citations. Extensive effort has been dedicated to perfecting their synthesis and to tuning their physical properties through structural modification. Facile monomer synthesis allows for the incorporation of various side chains and other functional groups into the final polymer structure. The monomers can be polymerized in a controlled manner by catalyst transfer polymerization (CTP),12−14 allowing access to high molecular weight, regioregular polymers with narrow dispersities and even more exotic polymer architectures including statistical,15−17 gradient,18−20 and block copolymers.21−23 Structure−property−function relationships and processing conditions have been carefully studied and optimized. To use organic photovoltaics (OPVs) as one example, poly(3-hexylthiophene) (P3HT) has a 3−4% average power conversion efficiency (PCE), and this efficiency has been continually validated by the many independent groups that publish P3HT data every year.24−32 Although not a record PCE, 3−4% is in fact quite good considering P3HT’s limitations of a high-lying highest © XXXX American Chemical Society

occupied molecular orbital (HOMO) level (leading to low open circuit voltage, Voc) and nonideal optical absorption of the solar spectrum (leading to low short circuit current, Jsc). Favorable morphology and phase separation with the electron acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are likely the reasons for these observations. This is quite remarkable given that P3HT was designed neither for OPV use nor for phase separation with PCBM. The research community has long identified P3HT’s limited absorption range as a limitation for optoelectronics including OPVs, electrochromic devices, and photodetectors. Two main approaches are used to tune the frontier molecular orbitals (FMOs) of polymers so that they absorb light further into the red or even near-infrared areas of the electromagnetic spectrum. The first is the copolymerization of electron-rich monomers such as thiophenes or thiophene derivatives with electron-deficient monomers in an alternating fashion to form donor−acceptor (D−A) polymers where the energy gap Received: November 14, 2014 Revised: December 10, 2014

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sized and used to achieve better performance than either P3AT or P3AS homopolymers. Indeed, there have now been several examples of seleniumfunctionalized conjugated polymers and small molecules that have improved properties over sulfur-based materials. Improvements in PCE are readily achieved when selenium substitution is combined with the D−A copolymer approach. For example, poly(indacenenodiselenophene-alt-difluorobenzothiadiazole) (PIDSe-DFBT) outperformed the analogous thiophene-based polymer with a 6.6% PCE (versus 5.9%) resulting from increased Jsc due to the narrower HOMO−LUMO gap.46 In another study, the use of selenophene spacers in the polymer poly(2,7-carbozole-alt-diselenienylbenzothiadiazole) (PCDSeBT) led to increased crystallinity and higher PCE than the analogous thiophene device (4.1% versus 3.6% PCE) due to Se−Se interactions.47 Recently, the use of selenophene spacers in poly(5,6-difluorobenzo-2,1,3-thiadiazole-4,7-diyl-alt-quaterchalcogenophene) with the quaterchalcogenophene composition Se2-Th2 was reported, with a slight improvement in device efficiency compared to the thiophene analogue, and this time in the inverted device architecture (7.34% versus 6.82% PCE).48 Selenophene-containing donor−acceptor polymers also exhibit high charge carrier mobilities in OFETs, and exceptionally high hole mobilities up to 12 cm2 V−1 s−1 are achieved for a device based on poly[2,5-bis(7-dodecyloctadecyl)pyrrolo[3,4-c]pyrrole-1,4-(2H,5H)-dione-(E)-{1,2-bis[5-(thiophen-2-yl)selenophen-2-yl]ethene}] (P-29-DPPDTSE).49 For more discussion on selenophene electronic materials readers are referred to recent review articles.37,44 In the examples noted above, the selenium-containing polymer acts as the hole transporter or electron donor; however, equally promising results have been obtained for selenophene-containing electron-transporting (n-type) polymers. The Jenekhe group reported the copolymerization of an alkylated naphthalene diimide with selenophene.50 The resultant copolymer has good crystallinity and high electron mobility. In a second study this same polymer was used as the electron acceptor in an OPV that reached 4.8% PCE, which is the current record efficiency for an all-polymer solar cell.51

between the HOMO and the lowest unoccupied molecular orbital (LUMO) is dramatically decreased compared to each individual monomer unit.33,34 The second approach, and the focus of this Perspective, is heavy atom substitution.35,36 Differences in size, electronegativity, polarizability, and increased strength of intermolecular interactions lead to favorable properties for organic electronic applications including decreased HOMO−LUMO gaps when heavy atoms are used in place of lighter ones. Additionally, the incorporation of heavy atoms does not significantly alter the carbon content to a level that compromises the low cost and processability of these materials. In the case of thiophene, sulfur may be replaced with the heavier group 16 elements selenium or tellurium.37,38 This approach leads to several new properties, opportunities, and challenges, each of which will be discussed below. A combination of the donor−acceptor and heavy atom substitution approaches may also be employed to further narrow the HOMO−LUMO gap and optimize FMO positions.39

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LESSONS LEARNED FROM SELENIUM When the first selenophene-containing conjugated polymers were introduced, their performance was not as good as analogous thiophene polymers. Although their synthesis is somewhat more involved compared to thiophenes, poly(3hexylselenophenes) (P3HSs) are regioregular polymers with low dispersities40 and have been widely commercially available for years. P3HS has a narrow HOMO−LUMO gap due to a slight destabilization of the HOMO energy and a more significant stabilization of the LUMO energy. This leads to a red-shifted absorption spectrum compared to P3HT. The increased size of selenium leads to larger carbon−heteroatom bond lengths. Selenophene oligomers have similar intra-ring C−C bond lengths but shorter inter-ring C−C bond lengths compared to thiophene oligomers, suggesting a greater contribution from the quinoid resonance form.37 As a result, selenophene oligomers have a higher degree of planarity which contributes to the narrow HOMO−LUMO gap through extension of the effective conjugation length. While the crystallinity and orientation of P3HS are similar to P3HT, the larger heteroatom leads to increased π-stacking distances. The lamellar spacing between polymer chains is decreased for poly(3-alkylselenophene)s (P3ASs) compared to P3ATs for a given side chain composition.41 Despite red-shifted optical absorption, OPV devices using P3HS as the donor material are less efficient than P3HT devices and have a 2.7% PCE in the typical bulk heterojunction (BHJ) architecture.42 While the Voc is comparable to P3HT, poor external quantum efficiency (EQE) and fill factor (FF) are observed, suggesting that charge recombination losses resulting from unfavorable mixing with the PCBM acceptor lead to lower than expected PCE. Attempts to improve morphology through block43 and statistical16 copolymerization with P3HT or through the use of various functionalized and nonfunctionalized selenophenes did not lead to improved device efficiencies over P3HT in typical BHJ devices; however, some of these exhibited good charge carrier mobilities in organic field effect transistor (OFET) devices.44 Recently, our group has reported P3AS nanowire solar cells with an improved efficiency of 2.9%, the highest to date for P3AS, likely due to improved phase purity of polymers and PCBM in the BHJ architecture.45 Moreover, statistical selenophene−thiophene nanowires can be synthe-

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PROPERTIES OF TELLURIUM The differences between tellurium and selenium are more pronounced compared to the differences between selenium and sulfur. A further red-shifted absorption is observed for tellurophene-based materials compared to sulfur and selenium analogues. Strong Te−Te interactions lead to strong interchain electronic coupling. From both a light-harvesting and chargetransport standpoint, tellurophene polymers are expected to give the best performance of the three chalcogenophenes. However, as was observed for the selenophene materials, heteroatom substitution leads to changes in physical and morphological properties that introduce new challenges in terms of achieving improved device performance. In the case of tellurophene, these challenges have yet to be overcome. Tellurium is a metalloid, and organotellurium compounds may be regarded as organometallic, leading to many fundamental differences compared to the lighter analogues. Similar to the results discussed above for selenophene oligomers, density functional theory calculations on tellurophene oligomers indicate that intra-ring C−C bond lengths are comparable to thiophene and selenophene oligomers; however, their inter-ring C−C bond distance is the shortest.52 This suggests that tellurophenes have the greatest quinoid B

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crystal packing, as in the case of 3,4-dimethoxytellurophene. Te−Te distances of 3.80−4.04 Å were determined by X-ray crystallography, which are significantly less than twice the van der Waals distance of Te (2.15 Å). Se−Se distances in the selenium analogue (3.78 Å) are roughly twice the van der Waals distance of Se (1.90 Å), demonstrating that Te−Te interactions are much stronger. These interactions play a significant role in the morphology of polytellurophenes as will be discussed further below.

character of the three chalcogenophene oligomers and polymers and should therefore have the narrowest HOMO− LUMO gap. Indeed, HOMO−LUMO gaps predicted from extrapolation of the HOMO−LUMO gap energy plotted against the reciprocal of the number of monomer units (1/ N) decrease in the order of thiophene (2.06 eV) to selenophene (1.85 eV) to tellurophene (1.48 eV) oligomers. The experimental HOMO−LUMO gaps of the polychalcogenophenes are similar in each case and follow the same trend: 2.0 eV,53 1.9 eV,53 and 1.5 eV52 for S, Se, and Te, respectively. The calculated geometry of tellurophene is highly planar and has the longest carbon−heteroatom bond length (2.100 Å) of the chalcogenophenes due to the large size of Te.54 This is comparable to the Te−C bond length obtained experimentally (2.055 Å) through elucidation of the microwave spectrum of tellurophene.55 Crystal structures of tellurophene derivatives have been reported including 2,2′-bitellurophene56 and 3,4dimethoxytellurophene (Figure 1),57 and the structures are very

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SYNTHESIS OF TELLUROPHENE-CONTAINING POLYMERS

The synthesis of tellurophene monomers and polymers presents a greater challenge than the lighter chalcogen analogues due to differences in reactivity and stability. This is likely the reason that polytellurophenes are quite rare. If tellurophene-based materials are to be used in organic electronic applications, it is imperative that low-cost and high-yielding synthetic routes are developed. The first tellurophene polymers were reported in the literature in the mid-1980s and mid-1990s.58 They were described as insoluble black powders or films that were synthesized by chemical oxidation. Electrochemical polymerization of tellurophene56 and higher tellurophene homologues such as 2,2′-bitellurophene56,59 and 2,2′:5,2″-tertellurophene59 were also described at that time. Copolymers of tellurophene and selenophene were synthesized by electrochemical polymerization of the appropriate hybrid terchalcogenophenes.59 These polymers were slightly conducting when doped (up to 10−6 S cm−1). However, due to their lack of solubility, full characterization of these polymers was not possible. Thus, the synthesis of substituted

Figure 1. Crystal structure of 3,4-dimethoxytellurophene aggregates. Reproduced with permission from ref 57.

similar to the computed geometries. Strong intermolecular interactions between tellurium atoms act as a driving force for

Chart 1. Structures of Chalcogen-Containing Conjugated Polymers

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Scheme 1. Various Condensation Polymerization Methods for the Synthesis of Tellurophene-Containing Copolymers

Scheme 2. Preparation of Pinacolborane-Substituted Chalcogenophene Monomers by Metallacycle Transfer from a Zirconacycle Intermediate

highlights the unique chemistry of tellurium compared to the lighter chalcogen analogues and demonstrates that postpolymerization tuning of the optoelectronic properties is possible by chemical oxidation. This type of behavior has also been studied by our group for conjugated tellurophene small molecules, where oxidative addition of halogens leads to red-shifted absorption properties, and these reactions are reversible using heat or light.63−65 More recently, Choi and co-workers have reported the copolymerization of bitellurophene with diketopyrrolopyrrole (DPP) via Suzuki condensation polymerization to give polymer P6.66 The effect of bromine coordination on optical properties as well as on performance in thin film transistors was studied and will be discussed further below. A few other D−A type copolymers containing tellurium have been reported using different synthetic methodologies. Our group reported the synthesis of a poly(cyclopentadithiophene)benzotelluradiazole (P9-Te) through postpolymerization modification of the selenium-containing analogue (P9-Se), which was synthesized by a Suzuki-type condensation polymerization.67 The postpolymerization modification involves reduction of P9-Se to the diamine, poly[4,4-bis(2-ethylhexyl)cyclopenta[2,1-b;3,4-b′]dithiophene-2,6-diyl-alt-2,3-diaminobenzo-1,4-diyl], with lithium aluminum hydride. Reoxidation with tellurium tetrachloride inserts tellurium into the diazole unit to produce P9-Te. Other types of palladium-catalyzed condensation polymerizations have also been employed using tellurophene as a comonomer. Unsubstituted tellurophene is conveniently prepared by a ring-closing reaction between sodium telluride and diacetylene gas following Sweat and Stephens’ procedure.68 Functionalization of tellurophene at the 2- and 5-positions with various reactive moieties yields monomers ready for polymerization by conventional or microwave-assisted Stille,69 Suzuki,70 and the newly reported ipso−arylative coupling (Scheme 1).70 Stille coupling polymerization has been used to synthesize copolymers including tellurophene−DPP copolymers (P7 and P8),70,71 chalcogeno-

tellurophenes to impart increased solubility in the polymers became necessary. The first soluble tellurophene-containing polymers were synthesized in the 1990s: one through Wittig condensation of tellurophene dialdehyde with a phenyldiphosphonium salt60 (P1, Chart 1) and another through the oxidative polymerization of a three-ring system comprising a central tellurophene and two flanking 3-butylthiophenes (P3).61 These copolymers were soluble in common organic solvents, allowing for characterization by 1H NMR spectroscopy, gel permeation chromatography, and solution absorption spectroscopy. The number-average molecular weights (Mn) of the polymers were quite low (Mn = 7.0 and 3.0 kg mol−1, respectively). It was not until 2009 that Bendikov and co-workers reported the first electrochemically polymerized substituted tellurophene, poly(3,4-dimethoxytellurophene) (P4).57 Compared to the thiophene and selenophene analogues, 3,4-dimethoxytellurophene has lower oxidation potentials (0.60 and 0.95 V) and a decreased HOMO−LUMO gap (1.51 eV, for the polymer). However, despite substitution on the tellurophene backbone, the polymer was insoluble and was unstable under doping conditions, a result that was different from the corresponding selenophene analogue. In 2010, our group reported the first palladium-catalyzed copolymerization of a halogenated bitellurophene monomer with a boronic ester-functionalized substituted fluorene unit to give low molecular weight copolymers (P5) (Mn = 3.1 kg mol−1).62 Evidence of molecular order was observed in the solid state optical absorption spectrum by the presence of a low-energy absorption shoulder indicative of πstacking. The organometallic nature of tellurium was exploited in this study. Specifically, molecular bromine adds to the tellurium center and changes the oxidation state, which leads to a dramatic red-shift in the optical properties and disrupts the molecular packing. Switching between the brominated and nonbrominated polymers was achieved through exposure of a polymer film to bromine vapor or heat, respectively. This D

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Scheme 3. (a) Synthesis of 3-Alkyltellurophene; (b) Synthesis of Poly(3-alkyltellurophene) by (i) Catalyst Transfer Polymerization and (ii) Suzuki-Coupling Condensation Polymerization

Figure 2. Optical absorption spectra of chalocogenophene polymers. Adapted with permission from (a) ref 75, (b) ref 67, and (c) ref 77.

phene−isoindigo copolymers (P10-S, P10-Se, and P10-Te),72 and cyclopentadithiophene−chalcogenophene copolymers P11-S, P11-Se, and P11-Te.73 P7 has also been synthesized by Suzuki coupling polymerization and microwave-assisted ipso−arylative polymerization.70 Another interesting synthetic route to alternating polymers containing chalcogenophenes has been developed by Rivard and co-workers in which Suzuki−Miyaura cross-coupling methodology is combined with zirconium-mediated metallacycle transfer chemistry (Scheme 2).74 This strategy involves ring-closing reactions of alkyl-spaced diyne precursors capped with pinacolborane groups with zirconium-dicyclopentadiene generated in situ. The zirconacycle can then be treated with S2Cl2, SeCl2, or bipy·TeCl2 to replace zirconium with sulfur, selenium, or tellurium, respectively. The chalcogenophene monomer can then be polymerized by Suzuki condensation with a desired dihalogenated monomer. The authors synthesized low molecular weight alternating thiophene− tetrahydrobenzochalcogenophene copolymers (P12-S, P12Se, and P12-Te) and a hybrid S−Se−Te-containing polymer (P13). As expected, the polymers exhibit red-shifted optical absorption in the order S to Se to Te. The advantage of such a synthesis is that a single precursor can be synthesized and the heteroatom can be substituted with any of the chalcogens in moderate to good yield. Our group developed a gram-scale route to 3-alkyltellurophene75 (Scheme 3a) through modifications of the 3hexylselenophene synthetic route.76 Beginning with 2-chloroN-methoxy-N-methylacetamide, the desired alkyl chain can be installed using the appropriate alkylmagnesium bromide reagent, followed by treatment with ethynylmagnesium bromide to afford the propargyl alcohol, which is the precursor for 3-alkyltellurophene. A ring-closing reaction occurs in the

presence of sodium telluride to give 3-hydroxy-3-alkyl-2hydrotellurophene, which is then dehydrated to give the desired 3-alkyltellurophene monomer. Iodination at the 2- and 5-positions yields a monomer that is ready for polymerization. Our group has used this methodology to synthesize a bis(trimethylsilylethynyl)-terminated tellurophene monomer, along with the thiophene and selenophene analogues, which were then treated with sodium methoxide, methanol, and bis(triethylphosphine)platinum(II) dichloride to afford platinum−acetylide chalcogenophene polymers P14-S, P14-Se, and P14-Te.77 These polymers exhibit red-shifted optical absorption showing that trends in group 16 substitution hold true for transition metal-containing polymers. In 2013, our group published the first synthesis of tellurophene homopolymers, poly(3-alkyltellurophene)s (P3ATe).75 One of the main advantages of P3AT and P3AS synthesis is the ability to polymerize the monomers in a controlled fashion through catalyst-transfer polymerization (CTP).12−14 This polymerization occurs by a chain-growth mechanism which allows for controllable molecular weights based on catalyst loading and leads to regioregular polymers with narrow dispersities. Our group employed CTP conditions to polymerize the hexyl-, ethylhexyl-, and dodecyl-substituted tellurophene monomers, yielding poly(3-hexyltellurophene) (P3HTe), poly(3-(2-ethylhexyl)tellurophene) (P3EHTe), and poly(3-dodecyltellurophene) (P3DDTe) (Scheme 3b).75 Regioregular polymers were obtained for P3DDTe and P3EHTe; however, poor solubility in the case of P3HTe made regiochemical determination difficult. The polymers had number-average molecular weights up to 11.3 kDa with dispersities around 2, a significant improvement from the previously reported tellurophene-containing polymers, yet still not as good as thiophene or selenophene polymers. E

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Macromolecules Regioregular P3DDTe was independently synthesized and studied by Kang and co-workers using a different synthetic approach.78 In that case, 3-dodecyltellurophene was asymmetrically functionalized with complementary iodine and boronic acid pinacol ester groups, followed by Suzuki coupling condensation polymerization (Scheme 3b) to give regioregular polymers with number-average molecular weights of 20 kDa and dispersities of 2.4. These are quite significant breakthroughs in the synthesis of polytellurophenes. The monomers could be synthesized on a large scale with relatively good yields, and these are the first examples of soluble homopolymers that could be fully characterized. Additionally, the degrees of polymerization were much higher than the previously reported copolymers.

Figure 3. X-ray diffraction patterns for P3HT,21 P3HS,41 and P3HTe75 with d-spacing indicated. Compiled with permission of the authors of the indicated references and with permission of The Royal Society of Chemistry.

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POLYMER PROPERTIES In all cases where the sulfur and selenium analogues are known, the tellurium-containing polymers exhibit red-shifted optical absorption properties (Figure 2), which is consistent with a decreased HOMO−LUMO gap. For example, the absorption maximum of P3DDTe is red-shifted by 90 nm (0.65 eV) and 45 nm (0.43 eV) compared to P3HT and P3HS, respectively, with an onset of absorption past 700 nm.75 The electrochemical HOMO−LUMO gap of P3HTe is 1.37 eV, which is significantly lower than P3HS (2.2 eV).40 Luminescence in these polymers is almost negligible. The effect of the side chain on the P3ATe backbone was studied by our group as described above. While the linear hexyl and dodecyl side chains give polymers with similar optical absorption spectra, the spectrum of the branched ethylhexyl side chain is blue-shifted. This is attributed to backbone twisting from the presence of the bulky side chain. Polymer solubility also varies with the heteroatom. Tellurophene polymers are the least soluble of the chalcogenophenes and tend to aggregate due to the strong Te−Te interactions. This is apparent in the optical absorption spectra of polymers P11-S, P11-Se, and P11-Te.73 The spectrum of each polymer contains a strong low-energy absorption attributed to aggregation of the polymer in solution. The intensity of this peak is related to the strength of the chalcogen−chalcogen interactions, with P11-Te having the most intense aggregation-induced absorption. Similar to P3HT, AFM images indicate that polytellurophenes form nanofibrils in the solid state. P3AT is known to crystallize in an “edge-on” fashion, with the π faces perpendicular to the substrate.79 The alkyl side chains slightly interdigitate, and the length and nature of the side chain affect the lamellar spacing, or d-spacing, between polymer backbones. For P3ATe, longer side chains or branching leads to larger dspacing. For example, the d-spacing determined by X-ray diffraction for P3HTe, P3EHTe, and P3DDTe is 12.4, 14.1, and 20.3 Å, respectively.75 Interestingly, with a hexyl side chain, the d-spacing decreases from P3HT (16.3 Å)21 to P3HS (15.5 Å)41 to P3HTe (12.4 Å)75 (Figure 3). The photophysical properties of tellurophenes and tellurophene polymers are relatively unknown. Large spin−orbit coupling constants from heavy atoms tend to promote intersystem crossing to triplet excited states. Thus, this phenomenon could be expected for tellurium. A recent publication reported the ultrafast photophysics of P3HT, P3HS, and P3HTe using transient absorption spectroscopy.80 P3HS and P3HTe show evidence of triplet exciton formation on the picosecond time scale (Figure 4). The evolution of these spectral features occurs an order of magnitude faster for

Figure 4. Temporal evolution of singlet emission/excited state absorption spectral region for P3HT, P3HS, and P3HTe as assayed at indicated probe energies on a picosecond time scale. The upper limit time constants of triplet formation for P3HS and P3HTe are indicated. Adapted with permission from ref 80.

P3HTe (time constant for the rate of formation, τ = 1.1 ps for P3HTe versus 18 ps for P3HS). This is consistent with an increased rate of intersystem crossing promoted by the heavier tellurium atom. Steady-state photoluminescence experiments on all three polymers shows that while P3HT exhibits fluorescence emission (fluorescence quantum yield, ΦF = 0.30), luminescence is significantly quenched for P3HS (ΦF = 4.2 × 10−3) and even more so for P3HTe (ΦF = 1.4 × 10−4), further supporting the increased rate of intersystem crossing for Te. A lack of phosphorescence from the triplet excited state in polytellurophene indicates that a nonradiative decay pathway is prominent.

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PUTTING TELLUROPHENE POLYMERS TO USE While tellurium-containing polymers offer the most promising properties of the group 16 elements, their use in organic electronics has not yet been widely studied. Only a handful of reports discuss tellurophene-based polymers in OPVs. Kang and co-workers reported the only example of an OPV device fabricated with a tellurophene homopolymer.78 A device efficiency up to 1.1% was achieved using regioregular P3DDTe as the donor material in a BHJ device with PC71BM as the acceptor material. A similar device efficiency (1.16%) was achieved for the donor−acceptor copolymer P10Te.72 In this study, the thiophene (P10-S) and selenophene (P10-Se) analogues were also prepared for comparison. Polymers with heavier chalcogens are expected to harvest a F

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Macromolecules greater portion of the solar spectrum, leading to higher Jsc. Consequently, P10-Se performed better in a device than P10-S (5.72% versus 3.98% PCE), resulting from increased photocurrent. The Te-containing analogue, however, did not follow this trend and had the poorest device performance (1.16%) due to a dramatic drop in the Jsc. A similar trend was observed for devices based on benzochalcogenophene−DPP small molecules, with device efficiencies of 4.2%, 5.8%, and 3.0% for S, Se, and Te, respectively.81 The improvement in Jsc and device efficiency going from sulfur to selenium confirms that heavyatom substitution is a viable approach to improve OPV performance; however, it is clear that the presence of tellurium poses new challenges that must be overcome in order to achieve better performance from these polymers. Recently, Grubbs and co-workers published a series of devices based on the tellurophene-containing donor−acceptor copolymer P7.70 Devices with different architectures were fabricated using both high and low molecular weight polymers. The maximum device efficiency of the best-performing device was 4.4%. In these devices, Jsc was dramatically improved compared to the previously discussed devices with onsets of photocurrent approaching 1000 nm (Figure 5). This is an optimal absorption edge for OPVs and is the main reason for the improved efficiency.82 The best performing tellurophenecontaining device had comparable efficiency to that of the thiophene analogue (4.7% PCE); however, the authors point out that the architecture of the champion device was quite

different from the thiophene-containing device and that direct comparisons should be made with caution. It is therefore important to point out that there are many factors that contribute to the overall efficiency of a device, and each component must be optimized to truly evaluate the OPV potential of a polymer, as evidenced by the many publications on polythiophene devices in the literature. As such, there is still a lot of opportunity and potential for tellurophene-containing polymers as OPV materials. Because of the narrow HOMO−LUMO gap of telluriumcontaining conjugated polymers and their ability to form strong Te−Te interactions, these materials should have high charge carrier mobilities in OFETs. While tellurium has been investigated in inorganic-based field effect transistors,83−85 studies of OFETs involving Te are quite rare, save for four publications in the past ten years. The first, published in 2004 by Otsubo and co-workers, was focused on 2,6-diphenylbenzo[1,2-b:4,5-b′]dichalcogenophene small molecules.86 The sulfur, selenium, and tellurium analogues were synthesized and studied. Herringbone-type crystal packing was observed for the selenium analogue and was thought to be the same for the S and Te analogues, which is promising considering pentacene and sexithiophene pack in a similar manner and have high OFET mobilities. All three analogues performed well as holeconducting (p-type) transistors. The authors were surprised to find that the Se analogue, rather than the Te analogue, performed best. The hole mobility, μh, of the isoindigotellurophene polymer P10-Te and its lighter chalcogen analogues was also determined by evaluating OFET device performance.72 The tellurium analogue performed best with a 0.072 cm2 V−1 s−1 hole mobility (versus 0.016 and 0.003 cm2 V−1 s−1 for Se and S, respectively). Choi and co-workers published a tellurophene-DPP polymer that is similar to P7 but with a different side chain (P8) and evaluated its performance in an OFET device, drawing comparisons to the analogous thiophene-DPP device.71 Both polymers exhibited p-type behavior. The tellurophene-containing polymer outperformed the thiophene counterpart, with hole mobilities of 0.48 and 1.47 cm2 V−1 s−1 for pristine and thermally annealed Te devices, respectively (versus 0.24 and 0.62 cm2 V−1 s−1 for pristine and thermally annealed S devices, respectively; Figure 6). The same group also studied OFET devices containing P6 (μh = 0.16 cm2 V−1 s−1 after annealing) and its bromine adduct.66 Interestingly, the devices were applied as sensors to detect bromine vapor due to increased drain current upon bromine exposure. These studies once again demonstrate that the heavy atom substitution approach may be a viable way to maximize device performance.

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FUTURE DIRECTIONS One reason for pursuing materials with heavy group 16 atoms for organic electronic applications is their red-shifted optical properties. All of the examples of tellurium-containing small molecules and polymers examined thus far have indeed shown these red-shifted optical properties. P3ATe homopolymers, for example, have HOMO−LUMO gaps around 1.4−1.6 eV,75 which is remarkably narrow for a polymer that does not contain a donor−acceptor electronic structure. These materials certainly have the potential for use in optoelectronics. However, from the few reports of devices containing tellurium-based polymers described above, it is obvious that these materials are not yet performing as well as they could. A key step to improving their performance is developing a better under-

Figure 5. (a) J−V curves and (b) IPCE measurements for solar cells prepared from P7. Device structure: indium tin oxide (ITO)/MoOx/ P7:PC71BM/LiF or TiOx/Al. Reproduced with permission from ref 70. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA. G

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molecular weight P7 and found that the high molecular weight polymers always performed better.70 Tellurium-containing polymers are less soluble than their lighter chalcogen analogues. Not only does this present a challenge in synthesizing high molecular weight polymers, but it also presents a major challenge for device fabrication since solution processing is commonly employed. Strong Te−Te interactions induce aggregation of the polymer chains which reduces solubility. Incorporation of longer linear or branched side chains is one way to improve polymer solubility. The hexyl side chain is the empirically optimized side chain for thiophene polymers because it balances solution processability with solidstate organization. However, this is not necessarily true for P3ATe or P3AS, for that matter. In fact, our group has shown that the heptyl side chain is better for P3AS processability and leads to a lamellar structure that more closely resembles P3HT.16 Recent work on poorly soluble donor−acceptor polymers provides further motivation for experimentally determining which side chains work best for tellurophenes.93 Poor active layer morphology is likely the major cause for the low Jsc and PCE in tellurium-containing OPVs. Films that contain blends of tellurophene−isoindigo copolymers (P10Te) and PCBM exhibit coarse phase separation, with large domains of pure materials.72 This type of morphology is detrimental to the Jsc because of short exciton diffusion lengths in OPVs. The low solubility of tellurophene polymers may be one reason why poor mixing of the donor and acceptor materials occurs. We also believe that larger domains occur for tellurium-containing polymers because strong Te−Te interactions facilitate aggregation. Thus, addressing the issues of solubility and processing described above might also lead to improved morphology of blended polymer−fullerene films. Yet the morphology of OPVs is quite complex. Even in the most well studied system (P3HT/PCBM), three phases (donor, acceptor, and mixed) are present in various amounts. The composition also varies from the top to the bottom of the active layer, and it is affected by processing conditions.94−96 Fullerenes may not be the ideal acceptor for tellurophenecontaining donors. The acceptor should suppress the overgrowth of tellurophene domains in the solid state, and new acceptor materials including n-type polymers and small molecules should be tested.97 Morphology and crystallinity also play important roles in charge carrier mobility. As discussed above, Te-containing polymers perform better in OFET devices than those containing S; however their performance is not always better than analogous Se-containing polymers. In the case of both P10-Te and P8 which outperformed their lighter chalcogen analogues, the improvements were attributed to more pronounced “edge-on” orientation and increased crystallinity in the Te-containing films.71,72 In the few reports of telluriumcontaining OFETs only the hole mobility is discussed. However, tellurophenes have low-lying LUMO energies which may facilitate electron injection and n-type transport.98 Moreover the combination of low-lying LUMO energies and high-lying HOMO levels in tellurophene-containing polymers may give these materials ambipolar character.99,100 The photophysics of polytellurophenes should also be an area of continued investigation. Despite evidence for intersystem crossing in tellurophene-containing polymers, phosphorescence is rarely observed. To our knowledge, poly{(E,E)-1,4-diethoxy-2,5-bis[2-(tellurophen-2-yl)ethynyl]benzene} (P2) is the only tellurophene-containing polymer to

Figure 6. Transfer curves of thin film transistor devices fabricated with (a) pristine and (b) thermally annealed films of P8. Reproduced with permission from ref 71. Copyright 2013 The Royal Society of Chemistry.

standing of their materials properties. Here we will address some issues and challenges that we believe should be the focus of research in the years to come. Several challenges have been overcome in the synthesis of tellurophene-containing polymers; however, future work should continue to focus on synthesis. One major challenge is to establish control over tellurophene polymerization using CTP. We can observe several shortcomings when applying CTP methods to tellurophene when compared to thiophene and selenophene. Lower than expected molecular weights (based on catalyst loading) and uncharacteristically high dispersities indicate that chain termination events occur before complete monomer consumption. This may be due to poor solubility, poor reaction kinetics, or poor catalyst association with the growing polymer chain. This should be addressed through mechanistic studies of tellurophene polymerization, side-chain modification to improve solubility, and improved catalyst design. Establishing greater control over this polymerization would provide several opportunities for future work. If fair comparisons are to be made between polytellurophenes and their lighter chalcogen analogues, the molecular weights and dispersities of all polymers should be roughly equal. The current methodology makes it difficult to target specific molecular weights and also leads to much higher dispersity than can be achieved for P3AT and P3AS. Additionally, an improved synthesis will make it possible to synthesize welldefined block or statistical copolymers with other monomers that undergo CTP.87,88 The development of new synthetic pathways to obtain high molecular weight polytellurophenes is also important since high molecular weight (>20 000 kDa) conjugated polymers typically perform better in devices.89−92 Grubbs and co-workers fabricated devices with low and high H

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Macromolecules exhibit photoluminescence;101 however, phosphorescent tellurophene small molecules are beginning to emerge in the literature. Aggregation-induced phosphorescence has been reported for tellurophene-containing pinacolborane monomer units, B-Te-6-B,102 as well as for pinacolborane-substituted tellurophenes and benzo[b]tellurophenes.103 This occurs in the solid state at room temperature (Figure 7). It would be

of excited state dynamics in P3HTe/PCBM blends is absolutely necessary in order to develop better performing systems. While triplet formation due to intersystem crossing is evident in pristine P3HTe, it remains to be determined whether intersystem crossing occurs in P3HTe/PCBM blends or whether charge transfer and separation occur more rapidly. Ultrafast spectroscopic techniques on blend films should provide insight as to which process is favored. CT in polymer/fullerene blends occurs on the subpicosecond time scale. 115,116 Based on previously determined rates of intersystem crossing in P3HTe, CT should occur more rapidly than intersystem crossing in P3HTe/PCBM blends. However, CT rates vary for different polymers and are also dependent on morphology.118,119 Thus, the rate should be determined for P3HTe/PCBM to verify this. If intersystem crossing does occur in P3HTe/PCBM blends, it is important to determine whether these triplets act as trap states, and if so, other acceptor materials with more appropriately matched energy levels should be investigated. Additionally, engineering tellurophene-containing polymers to have more appropriately matched energy levels with PCBM should be investigated. Another potential advantage of using tellurium in the molecular design of polymers for electronics is that they may have higher dielectric constants than lighter polymers. This is advantageous because it reduces the distance that an electron and hole must be separated to overcome their mutual Coulombic attraction. Recent work by Tsang, So, and coworkers has shed light on how low dielectric constants of BHJ systems lead to predictable losses in Voc.120 These studies propose that losses in Voc can be overcome and that an OPV will behave like an inorganic PV if the dielectric constant can be increased to ∼5. Heavy-atom polymers and small molecules should be investigated for this purpose. Composite materials should also be investigated because they offer several advantages. Blends of P3HT with insulating commodity plastics such as high-density polyethylene (HDPE) have been used to combine the favorable semiconducting properties of P3HT with the high mechanical strength of HDPE.121,122 OFET devices made with these blends retain their charge transport performance even with as little as 3% P3HT incorporation. This offers a major advantage in terms of cost since HDPE is inexpensive and the amount of the more costly semiconducting polymer is minimized. As researchers continue to develop more exotic semiconducting polymers (heavy group 16 and others), they should bear in mind that blending these materials with commodity plastics is a means to reduce materials cost. This is a unique advantage of semiconducting polymers relative to other emerging thin-film electronic materials. In summary, heavy group 16 atom substitution is a promising approach toward new polymer design for the purpose of maximizing performance of organic electronic devices. Incorporation of tellurium into conjugated polymers offers interesting materials properties that are not seen with the lighter chalcogen analogues. These include red-shifted optical absorption, strong heteroatom interactions, high polarizability, and high dielectric constant. Despite these favorable properties, polytellurophenes have not achieved record device performances. Continued efforts in the synthesis of new tellurophenebased materials, photophysical studies, and device optimization should lead to improvements in device performance. Although this Perspective focused mainly on tellurophenes, we would like to stress that selenophene-containing polymers still require a

Figure 7. Emission characteristics of B-Te-6-B in solution and in the solid state. Reproduced with permission from ref 102. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA.

worthwhile to understand what causes phosphorescence in these systems (and to determine ways of suppressing luminescence quenching in nonemissive tellurophenes) because phosphorescent tellurium compounds could be a cheaper alternative to some light-emitting metal phosphors used for OLEDs. Other tellurium-containing heterocyclic small molecules that exhibit luminescence include tellurorosamines,104 tellurorhodamines,105 and a tellurophene-DPP small molecule,106 which are fluorescent when oxidized to the telluroxide but are nonemissive in their reduced form. Similarly, our group reported a water-soluble tellurophene that was not emissive in the Te(II) or Te(IV) state but exhibited strong fluorescence when oxidized to the Te(VI) tellurone.107 The dependence of luminescence on the oxidation state of Te may offer utility as redox probes. An alternative optical application for telluriumcontaining compounds that deserves further investigation is as high refractive index materials due to the high polarizability of the tellurium atom. Because of their longer lifetime, triplet excitons have a greater diffusion length than singlet excitons.108−110 Thus, the use of a material that generates triplet excited states could be a good strategy for reducing recombination losses and improving photocurrent. Devices that use singlet fission donor materials function by harvesting triplet excitons, demonstrating that triplets can be converted into photocurrent.109,111−113 Despite their advantages, however, triplet excitons may also pose new challenges within a device. As with the charge transfer and separation of singlet excitons, favorable energetics between the donor and the acceptor are imperative. If the polymer triplet energy is lower than the energy of the charge transfer (CT) and charge-separated (CS) states, triplets can act as trap states leading to a loss of charge carriers.114−116 On the other hand, if the triplet energy is higher than the CT and CS energies, charge separation is energetically favorable, and triplets may be ionized and harvested. This has been observed for a platinum− acetylide−thiophene polymer/fullerene blend where intersystem crossing in the polymer is highly efficient and photocurrent is generated from triplet excitons.117 Based on the low device efficiencies achieved thus far for tellurophene-containing OPVs, it is clear that an understanding I

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Macromolecules lot of development and optimization, and many of the challenges associated with tellurophenes will be lessened or potentially alleviated if the community can form a better understanding of conjugated selenophene-based materials.

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postdoctoral fellows study the optical and electronic properties of semiconducting polymers and nanomaterials. He has authored or coauthored over 60 publications and has more than 10 patents and patent applications. Since beginning his independent career, he has been recognized with a DuPont Young Professor Grant (2011), Ontario Early Research Award (2011), Canada Research Chair (2012), and Alfred P. Sloan Research Fellowship (2013).

AUTHOR INFORMATION

Corresponding Author

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*E-mail [email protected] (D.W.S.). Notes

The authors declare no competing financial interest.

REFERENCES

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Biographies

Elisa I. Carrera is a Toronto native who received her B.Sc. in Chemistry from Ryerson University in 2012 after completing the cooperative education program. Through the co-op program, she has worked as a Research Assistant at the Xerox Research Centre of Canada on the synthesis of “green” resins as well as at Opalux Inc. where she participated in the research and development of tunable photonic color devices. After her undergraduate degree, she began graduate studies at the University of Toronto where she is currently pursuing her Ph.D. in Chemistry under the supervision of Professor Dwight Seferos. Her research is focused on synthesizing conjugated tellurophene small molecules and studying redox chemistry at the tellurium center with an emphasis on light-driven reactions.

Dwight Seferos is a Seattle, WA, native and attended Western Washington University, completing a B.S. degree in 2001. He entered the graduate school at the University of California, Santa Barbara, where he worked under the supervision of Guillermo Bazan on the synthesis and study of organic molecules with delocalized π-electron systems. After completing a Ph.D. in 2006, Seferos moved to Northwestern University where he was an American Cancer Society Postdoctoral Fellow with Chad Mirkin. In 2009, Seferos began his independent laboratory in the Chemistry Department at the University of Toronto where he and his highly talented team of students and J

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DOI: 10.1021/ma502307b Macromolecules XXXX, XXX, XXX−XXX