Thermoelectric Enhancement by Compositing ... - ACS Publications

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Thermoelectric Enhancement by Compositing Carbon Nanotubes into Iodine-Doped Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4phenylenevinylene] Murat Tonga,† Lang Wei,† Patrick S. Taylor,‡ Eugene Wilusz,§ Ljiljana Korugic-Karasz,‡ Frank E. Karasz,‡ and Paul M. Lahti*,† †

Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003, United States Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003, United States § Natick Soldier Research, Development and Engineering Center, Natick, Massachusetts 01760, United States ‡

S Supporting Information *

ABSTRACT: Free-standing iodine-doped composite samples of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) with carbon nanotubes (NTs) showed thermoelectric (TE) power factors (PFs) up to 33 μW·m−1· K−2 after optimizing multiple factors, including: (1) sample fabrication solvent, (2) doping time, (3) average MEH-PPV molecular weight, (4) NT fraction in the composite, and (5) use of single-wall versus multi-wall nanotubes (SWNT and MWNT, respectively). Composite fabrication from halogenated solvents gave the best TE performance after iodine doping times of 2−4 h; performance drops substantially in ∼20 h doped samples. TE performance dropped after at least 24 h of removal from iodine vapor but was fully restored upon re-exposure to the dopant. Longer-chain MEH-PPV gave not only mechanically stronger films but also higher PFs in doped SWNT composites. MWNT composites gave low PFs, attributed to poor NT dispersion. Scanning electron microscopy showed increasingly extensive network formation as NT fraction increased in the composites; this phase separation provides charge transport pathways that improve thermoelectric PFs. The results support a strategy of producing phase-separated materials having both electrical conduction enhanced regions and Seebeck thermopower retaining regions to maximize organic TE response. KEYWORDS: organic polymer thermoelectrics, MEH-PPV, single-wall carbon nanotubes, multi-wall carbon nanotubes, polymer−nanotube composites, doped polymer thermoelectrics, scanning electron microscopy

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INTRODUCTION

considered. The present submission follows this common practice in reporting PFs.

While thermoelectrics were primarily developed using inorganic materials, organic materials now also show much promise.1−6 Potential deficits in organic thermoelectric (TE) performance may be offset by their processability and reduced environmental impact, especially compared to inorganic TE materials containing toxic elements such as antimony, bismuth, and lead. Recent organic TE material studies have focused, for example, on poly(p-phenylenevinylene)s (PPVs),7,8 poly(3hexylthiophene) (P3HT),9−12 poly(2,7-carbazole),13 poly(2,7carbazolenevinylene),14 polyaniline,15−18 and poly(3,4-ethylenedioxythiophene) (PEDOT).19−23 TE depends on specific electrical conductivity, thermal conductivity, and thermoelectric power response to a temperature gradient (Seebeck coefficient): σ, κ, and S, respectively. Eq 1 defines the TE figure of merit, ZT, at temperature T. Thermal conductivity for organic polymers tends to fall roughly1,8,21 in the range 0.01−0.5 W·m−1·K−1. In the absence of thermal conductivity, the TE power factor, PF, is frequently reported1 using eq 2 instead of ZT, so neither thermal conductivity nor temperature dependencies are explicitly © XXXX American Chemical Society

ZT =

S2σT κ

PF = S2σ

(1) (2)

An important goal to improve organic TE response is to increase both σ and S simultaneously, a nontrivial task given their typically inversely related behavior. Because TE depends more strongly on the Seebeck coefficient than on electrical conductivity, maintaining higher Seebeck coefficients in conductivity-enhanced materials is important for organics, as described by Casian.24−26 In principle, this might be achieved in phase-separated materials, in which some domains enable sufficient charge conduction to allow electrical current flow, and other domains provide the needed Seebeck thermopower response to a temperature gradient. Received: November 16, 2016 Accepted: February 16, 2017

A

DOI: 10.1021/acsami.6b14695 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces The present contribution shows that TE performance improvement can be achieved by optimizing multiple preparation variables when compositing iodine (I2) doped poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) with single-wall carbon nanotubes (SWNT). MEH-PPV is a hole-transporting material when p-doped,27−29 while both SWNT and multi-wall carbon nanotubes (MWNT) in various forms can be excellent electrical conductors.30−33 Added carbon nanotubes (NTs) can increase thermal conductivity κ undesirably in eqs 1 and 2, but NT-induced increases in electrical conductivity could more than offset this problem while still retaining the p-doped MEH-PPV Seebeck response. Related strategies are described elsewhere.34−38 Phase separated polymer rich and NT rich regions in polymer/NT composites (Scheme 1) could be important to enable Scheme 1. Schematic for Making Thermoelectric Composites from Doped Blends of MEH-PPV with Carbon Nanotubes

Figure 1. Electrical conductivity of MEH-PPV-70k (low molecular weight) and MEH-PPV-150k (high molecular weight) films cast from DCE or DCB and then doped by exposure to iodine vapor for the times shown. Solid lines are eye guides.

PPV (Supporting Information, Figure S1), DCE or DCB were generally much superior for making well-dispersed solutions of MEH-PPV with high SWNT loads that yielded free-standing films after ambient evaporation. Therefore, most of the results described below used DCE or DCB for sample preparation. The MEH-PPV films increased in weight by 111−125% after 3 h of iodine vapor exposure and 138−142% after 20 h of exposure. Shorter doping times of 2−4 h gave significantly better electrical conductivity and higher Seebeck coefficients for all solvents tested with either MEH-PPV-70k or MEH-PPV150k. Table 1 shows the TE results obtained at room temperature for DCE and DCB fabricated samples at the time when electrical conductivity maximized and after 20 h. The resulting PFs were up to an order of magnitude larger at shorter doping times. From eq 1, assuming a quite conservative value of κ = 1 W·m−1·K−1 at room temperature, ZT ∼ 10−5 to

simultaneous increases in both electrical conductivity and Seebeck response. Achieving TE improvement in this way requires empirical tuning of MEH-PPV:NT ratios. The present results show that such a multivariable TE improvement strategy works by testing effects of blend composition, fabrication solvent, doping times, use of lower versus higher average molecular weight MEH-PPV, and choice of SWNTs versus MWNTs. Observable changes in film morphology demonstrate the phase-separation behavior that was originally targeted for improving TE performance.

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Table 1. Effect of Iodine Doping Exposure Time on MEHPPV Thermoelectric Behaviora

RESULTS AND DISCUSSION Effect of Fabrication Conditions on Thermoelectric Performance of Iodine-Doped MEH-PPV Samples. The effects of fabrication solvent, iodine doping time, and conjugated polymer molecular weight were tested using MEH-PPV without added carbon nanotubes. P-doped MEHPPV films form hole-transporting sites,27 but their maximum conductivity varied significantly for samples cast from different fabrication solvents. The p-doped electrical conductivities for both high and low molecular weight MEH-PPV films cast from various solvents were measured periodically at room temperature during exposure to iodine vapor for times up to 24 h (Supporting Information, Figure S1). Lower molecular weight polymer is designated as MEH-PPV-70k, and higher molecular weight polymer as MEH-PPV-150k. A range of solvent polarities was tested, from toluene to pyridine; likewise, a range of solvent boiling points was also tested, from dichloromethane (DCM) to 1,2-dichlorobenzene (DCB). Figure 1 exemplifies the electrical conductivities obtained over the first few hours of doping for samples fabricated from 1,2-dichloroethane (DCE) and DCB. Although a few other solvents gave higher conductivity for one or the other molecular weight of MEH-

doping time (h)

σ (S·cm−1)

S (μV·K−1)

PF (μW·m−1·K−2)

3

0.30

73

0.16

DCE

20

0.15

45

0.03

DCB

2.5

0.14

121

0.20

DCB

20

0.06

38

0.01

DCE

2.5

1.21

58

0.41

DCE

20

0.52

42

0.09

DCB

3

1.47

66

0.63

DCB

20

0.49

33

0.05

sample

solvent

MEHPPV-70k MEHPPV-70k MEHPPV-70k MEHPPV-70k MEHPPV150k MEHPPV150k MEHPPV150k MEHPPV150k

DCE

a

Electrical and thermoelectric characteristics were measured as soon as possible after sample removal from the iodine vapor doping chamber.

B


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ACS Applied Materials & Interfaces

Figure 2. Electrical conductivity (σ) and Seebeck coefficient (S) data for undoped MEH-PPV-70k (a) and MEH-PPV-150k (b) samples having added weight percent of SWnNT fabricated from DCE or DCB. Power factors for the same MEH-PPV-70k (c) and MEH-PPV-150k (d) blend samples.

10−4 for the doped polymers without NTs. Composite films between MEH-PPV and SWNT also gave much better PFs for shorter doping exposure times; therefore, the studies described below used these conditions unless otherwise stated. Optimizing MEH-PPV TE Blends with Carbon SWNTs. As mentioned earlier, solvent choices to fabricate MEHPPV:NT composites were limited by the desire to disperse the nanotubes effectively with maximum polymer dissolution. Chlorinated solvents such as DCE, DCB, and chlorobenzene (CB) were optimal for SWNT with MEH-PPV. The solvent mixtures were dried by passive evaporation in air in polytetrafluoroethylene troughs to give visually homogeneous, free-standing films for SWNT weight percentages up to 50% with MEH-PPV-70k and 60% with MEH-PPV-150k. Blending 1.2−1.5 nm carbon NTs (SWwNT, “w” for wider diameter) into MEH-PPV raised electrical conductivity up to ∼0.04 S·cm−1 at maximum NT load even before iodine doping (Supporting Information, Figure S2). Seebeck coefficients trended downward from S = 120−130 μV·K−1 in undoped samples of MEH-PPV-70k as the SWwNT load increased from 30%. A similar trend was seen for MEH-PPV-150k with SWwNT fabricated from DCB, but the corresponding samples fabricated from DCE showed variation only between 66 and 85 μV·K−1 (Figure S2). The undoped PF reached 0.02 μW·m−1· K−2 at maximum SWwNT:MEH-PPV ratios for samples

fabricated from DCE and reached 0.07−0.1 μW·m−1·K−2 at maximum SWwNT:MEH-PPV ratios from DCB. When narrower diameter 0.7−1.4 nm SWnNTs were used, sample conductivities even before iodine doping reached σ ∼ 20 S·cm−1 for MEH-PPV-70k and σ ∼ 70−90 S·cm−1 for MEH-PPV-150k at high NT contents (Figure 2). For samples cast from either DCE or DCB, the Seebeck thermopower at first dropped sharply as SWnNTs were added and then roughly stabilized for loads ≥20%. The highest PF = 3.2 μW·m−1·K−2 for 50% SWnNT in MEH-PPV-70k from DCE and 1.8 μW· m−1·K−2 for 50% SWnNT in MEH-PPV-150k from DCB (Figures 2c and d). Narrower carbon NT diameters have been associated with larger barriers to charge mobility;39,40 therefore, it seems unlikely that the better TE performance for SWnNT versus SWwNT is due to the diameter difference. Rather, it may arise from a higher weight fraction of nanotubes in the material. Use of SWnNTs gave relatively lower Seebeck coefficients compared to corresponding samples with SWwNTs: S = 30−50 μV·K−1 (Figure 2) instead of the above-mentioned S = 66−85 μV·K−1 for SWwNTs (Supporting Information, Figure S2). Decreased Seebeck coefficients often accompany increased electrical conductivity in TE materials, but even in these undoped samples, the greater conductivity enhancement outweighs a modest Seebeck coefficient decrease when using SWnNTs instead of SWwNTs; the result is roughly two orders of magnitude higher for undoped PFs from SWnNTs. The C


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Figure 3. Electrical conductivity (σ) and Seebeck coefficient (S) data for iodine-doped MEH-PPV-70k (a) and MEH-PPV-150k (b) samples added with weight percent of SWnNT fabricated from DCE or DCB. Power factors for the same MEH-PPV-70k (c) and MEH-PPV-150k (d) samples with SWnNT. All samples were doped for 2−3 h.

Table 2. De-Doping Thermoelectric Performance Parameters and Iodine Uptake/Retention for an MEH-PPV-150 K Blend with 50% w/w SWnNTs sample condition

conductivity (S·cm−1)

S (μV·K−1)

PF (μW·m−1·K−2)

weight (mg)

predoped after 2.5 h I2 doping 6 h ambient 15 h ambient 36 h ambient 60 h ambient after 2.5 h I2 redoping

34 430 177 132 104 93 415

39 23 30 32 33 36 29

5.3 24.4 16.5 13.8 11.4 11.8 32.8

5.9 11.2 7.2 6.7 6.7 6.5 10.5

a

TE measurements made as soon as possible after removal of sample from exposure to dopant. Sample fabrication from 1,2-dichloroethane. Sample progress from predoped to just-doped, then exposure to ambient conditions (open sample tube) for 60 h, and then 2.5 h of exposure to iodine vapor in a capped tube.

these conditions, either low or high molecular weight MEHPPV with 10−50% SWwNT gave doped-sample Seebeck coefficients in the range S ∼ 20−40 μV·K−1. Use of shorter, 2−3 h doping exposure times gave rather higher PFs for polymer/SWNT samples fabricated from either DCB or DCE relative to analogous samples doped for 20 h (Supporting Information, Figure S4). But, even the best PF = 0.5−0.6 μW· m−1·K2 for iodine doped polymer/SWwNT blends were inferior to PFs for similar samples using SWnNT, even before the latter were doped. Switching to polymer blends with SWnNT for iodine doped composites gave the best TE performance results. Doping for 2−3 h of MEH-PPV/SWnNT blends increased electrical conductivity by 10-fold or more relative to MEH-PPV alone,

results show the importance of testing different grades of carbon SWNTs when optimizing TE behavior such as that in the present study. Adding SWNTs to MEH-PPV clearly increased availability of effective conduction pathways in these composites even before doping. This supports a major motivation for use of polymer− nanotube blends to provide this better charge conduction relative to polymer alone. Doping the blends with iodine was expected to give even greater electrical conductivities due to production of hole-carrier sites in MEH-PPV. The electrical conductivity of MEH-PPV-70k films with 10−40% SWwNTs reached 4−5 S·cm−1 after prolonged iodine vapor doping; by the same procedure, MEH-PPV-150k with 40% wNT SWNTs reached 14 S·cm−1 (Supporting Information, Figure S3). Under D


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Figure 4. Conductivities and Seebeck coefficients (a) and power factors (b) for blends of MEH-PPV-70k with varying percentages (w/w) of MWNT all after iodine doping for 2.5 h; conductivities and Seebeck coefficients (c) and power factors (d) for MEH-PPV-150k with varying percentages (w/ w) of MWNT before and after iodine doping for 3 h. For 10−20% MWNT loads in MEH-PPV-70k and 10−30% MWNT loads in MEH-PPV-150k, samples were fabricated from 1,2-dichloroethane; for higher loads, samples were fabricated from chlorobenzene.

decrease essentially stops after ∼15 h with loss of ∼75% of initial postdoping weight gain. The weight loss is presumably due to iodine sublimation, because 2.5 h of redoping restored the sample weight to >90% of the first-time, just-doped weight. After 36 h of ambient dedoping, PF stabilized at ∼50% of the just-doped value (Table 2, Figure S5). Redoping in iodine vapor for 2.5 h then restored increased TE performance and gave the highest PF in our study: 33 μW·m−1·K−2 with σ = 415 S·cm−1 and S = 29 μV·K−1 (ZT ∼ 0.01, using the same assumption for κ described earlier). In other cases, doped composite samples showed readily measurable TE response after well over a year in an ambient environment. MEH-PPV Blends with Carbon MWNTs. Doped MEHPPV-70k blends with carbon MWNTs gave electrical conductivities lower than those of comparable samples with SWNTs. Solvent choices for fabricating MEH-PPV/MWNT blends were limited by poor nanotube dispersion: effective dispersion in DCE could only be achieved up to 20% MWNT and in CB up to 30−40% MWNT. As with SWNTs, different MWNT sources gave different results both before and after iodine doping. A blend of 30% MWNT with 3−20 nm diameters in MEH-PPV-70k fabricated from CB gave a maximum PF of only 0.04 μW·m−1·K−2 after doping; under the same conditions, 6−13 nm diameter MWNTs gave PF = 0.57 μW·m−1·K−2. In Figure 4 and subsequently in this article, results using the 6−13 nm outer diameter MWNTs are given due to their consistently higher PFs.

with conductivities considerably higher than those of the corresponding doped samples using SWwNT (compare Figure 3 to Supporting Information, Figure S3). MEH-PPV-70k/ SWnNT doped conductivities fabricated from either DCE or DCB reached σ ∼ 220−240 S·cm−1 at 50% SWnNT load; MEH-PPV-150k analogously reached σ ∼ 430−450 S·cm−1. From either DCE or DCB, both low and high molecular weight polymer blends had Seebeck coefficients S ∼ 20−30 μV·K−1 after doping for SWnNT loads of 30−50%. Notably, compared to the undoped sample values, doped sample Seebeck coefficients dropped only by a factor of two or less. The combination of steep conductivity increases and relatively limited Seebeck coefficient decreases yielded sharp PF increases as SWnNT load increased in doped composites. Representative samples of MEH-PPV-70k with 50% SWnNT gave similar PFs from DCB or DCE: for corresponding MEH-PPV-150k samples, the PFs from DCE are somewhat higher, up to 24 μW·m−1·K−2 at the highest SWnNT load in Figure 3d. Again conservatively estimating κ = 1 W·m−1·K−1 at room temperature, ZT ∼ 0.004−0.008 for the higher PF samples. Just-doped samples removed from the iodine-doping chamber undergo some dedoping after ambient exposure, increasing the Seebeck thermopower with decreased electrical conductivity and PF. However, redoping the dedoped samples restores electrical conductivity and PF performance to justdoped levels or even somewhat higher. Table 2 shows the results for a dedoping/redoping test of a MEH-PPV-150k plus 50% w/w SWnWT composite from DCE. Dedoping weight E


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S8−S9 and Figure 5. MEH-PPV-150k/SWNT samples have similar appearances, with DCE-cast samples looking somewhat

Interestingly, MEH-PPV/MWNT blends before iodine doping gave quite low Seebeck coefficients which, after doping, were comparable to those in composites with similar SWNT loads. This behavior was obtained for two different sources of similar MWNTs and was particularly notable with higher weight polymer. For 30% MWNT in MEH-PPV-150k cast from CB, S ∼ 2 μV·K−1; after 2.5 h of iodine doping, S = 25 μV·K−1 (Supporting Information, Figure S5). A 30% SWnNT blend in MEH-PPV-150k from DCB had S = 38 μV·K−1, which dropped to S = 21 μV·K−1 after 3 h of iodine doping. MEH-PPV-150k without any nanotubes doped for 20 h gave S = 30−40 μV·K−1. The contrast in Seebeck behavior of undoped MWNT versus SWNT composites was quite striking and suggests that adding MWNT is particularly disruptive to the Seebeck response of undoped MEH-PPV itself. Carbon MWNTs are more complex than SWNTs, and their behavior can vary substantially depending41 on nanotube diameter, number of shells, and surface oxidation. MWNTs may act here as charge trap sites that suppress Seebeck response in the blends; the trapping is alleviated when MEH-PPV chains are p-doped to provide new pathways for charge transport. Further experiments would be required to probe this further. After 2−3 h of iodine doping, both MEH-PPV-70k and MEH-PPV-150k with similar loads of MWNT gave somewhat higher conductivities at highest MWNT load for MEH-PPV150k (Figure 4). Seebeck coefficients were similar for both types of polymers at high MWNT loads (S ∼ 20 μV·K−1) and are similar to Seebeck coefficients in SWnNT-blend samples (Figure 3). The iodine doped PFs reached a maximum and then decreased as MWNT load increased with best TE performance from MEH-PPV-150k with 20% MWNTs fabricated from CB, giving a PF of only 0.95 μW·m−1·K−2. Carbon MWNTs presently are less costly than SWNTs, but the best TE results for MWNT blends with MEH-PPV were inferior to results using SWnNT blends. Therefore, MWNT blends were not further investigated save for morphological examination described below. Electron Microscopy of MEH-PPV/Nanotube Blends. Casian and co-workers recently described24 a theoretical model of how interactions between 1D chains of thermoelectric charge carriers in an organic crystal could be important to maximize TE response. Organic polymer morphology can be far different from organic crystalline order, but local chain contacts and phase boundaries in varying polymer morphologies could produce analogous contacts that influence electrical properties. Therefore, we used scanning electron microscopy (SEM) to probe morphological changes in the same polymer/SWNT samples used to measure TE properties. In addition to the SEM images given in the main article below, others are given in the Supporting Information (Figures S6−S10) as additional examples of surface morphology. SEM images of undoped samples of either MEH-PPV-70k or MEH-PPV-150k (no NTs) cast from DCE or DCB were typically smooth and continuous with some wrinkling and stippling, although a few DCE-cast samples appeared somewhat porous (Supporting Information, Figures S6). After 3 h of iodine doping, the sample appearances were not much changed, although it was often easier to see SEM contrast. Composites with SWNTs showed increased sample surface inhomogeneity, from speckling at low SWNT loads to widespread “marbling” at higher loads. MEH-PPV-70k/SWNT showed strong marbling when fabricated from either DCE or DCB at 30% NT load or higher, as exemplified in the Supporting Information, Figures

Figure 5. Scanning electron micrographs obtained at 1.0 kV with 30 μs dwell time. Top, MEH-PPV-70k/50% SWnNT from DCB; bottom, MEH-PPV-70k/50% SWnNT from DCE. Both samples were taken after iodine vapor doping.

more inhomogeneous. The SEM surface inhomogeneity presumably reflects phase separation of SWNT-rich regions as the NT load increases. The inhomogeneity is enhanced in the sample cast from lower-boiling DCE compared to that from DCB. Even film strength is affected because 40% SWnNT blends in MEH-PPV-70k cast from DCE would not give freestanding films but crumbled in multiple preparation attempts. On the basis of the apparent porosity seen in some SEM images (e.g., Supporting Information, Figure S6), it is tempting to attribute improvements in doped-sample conductivity and PF at high SWNT loads to iodine dopant vapor more readily perfusing the more porous morphologies. This is not necessarily the case, however. The formation of highly fibrillar networks at high NT loads (presumably NT rich, as mentioned earlier) could also provide more pathways for charge carrier transport to improve conductivity and TE performance. Although the most strikingly phase separated sample, highly fibrillar 50% SWnNT in MEH-PPV-150k cast from DCE in Figure 6, gave the highest PF, a smoother and less-obviously fibrillar sample with corresponding composition from DCB shown also in Figure 6 still gave a respectably high PF, as shown in Figure 3d. Overall, either increased porosity or F


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Figure 6. Scanning electron micrographs obtained at 1.0 kV with 30 μs dwell time. Top, MEH-PPV-150k/50% SWnNT from DCB; bottom, MEH-PPV-150k/50% SWnNT from DCE. Both samples were taken after iodine vapor doping.

Figure 7. Scanning electron micrographs obtained at 1.0 kV with 30 μs dwell time. Top, MEH-PPV-70k with 40% w/w MWNT from CB; bottom, MEH-PPV-150k with 40% w/w MWNT from CB. Both samples were taken after 3 h of doping with iodine vapor.

increased conduction pathways or both could contribute to better TE performance with increasing NT incorporation and would be consistent with the morphological variations observed. As mentioned earlier, higher loads of MWNTs could not be dispersed in most solvents for MEH-PPV solutions, so it is not surprising that polymer/MWNT samples showed high phase separation. Figure 7 (expanded view in Supporting Information, Figure S10) shows a SEM-imaged surface nearly covered by tangles of nanowires for the MEH-PPV-150k with the highest MWNT load (40%), fabrication of which required use of CB solvent. This image is similar to those reported by Prajongtat et al.42 for MEH-PPV/MWNT nanocomposites. The same MWNT load in MEH-PPV-70k (Figure 7, left) does not show such nanowires, but the SEM image is still quite heterogeneous in appearance. Although DCE gave reasonable NT dispersion up to 30% MWNT with MEH-PPV-70k, the higher NT loads again required use of CB. Given the inferior TE performance of the composites with MWNT compared to SWnNT, the MWNT blends were not further studied. Summary of TE Variable Influences. The results show how multiple variables influence TE performance in the conjugated polymer composites with carbon nanotubes. Longer chain lengths of MEH-PPV give higher electrical conductivity after iodine doping for both shorter and longer doping times.

Shorter doping exposure times of 2−3 h gave electrical conductivity and better TE performance 2−3-fold higher than that at 20 h both for polymer alone and the composites with SWNTs. There is a strong effect of fabrication solvent on the doped electrical conductivities for MEH-PPV-only samples and the composites. The solvent giving highest electrical conductivity for the polymer only (chloroform) did not give the best TE performance for the composites (DCE or DCB). This was attributed to differences in composite morphology from different fabrication solvents, as seen in the SEM images. There are similarly strong fabrication effects on polymer and large molecule morphology in organic solar cell electronic materials43−45 and in other polymer-based thermoelectric materials.4,6,46 Morphology is important to the composite materials’ TE performance, not just the individual component electronic properties. The composite morphological and TE changes are most dramatic for ≥30% weight fractions of carbon NTs. In the doped composites, the electrical conductivity tends to climb steeply at these compositions, while Seebeck thermopower tends drops slowly, with overall increase in PF due to the much greater increase in electrical conductivity. While increasing phase separation correlates with higher PFs for the SWNT composites, MWNT composites give variable, relatively low PF despite high phase separation seen by SEM. Overall, SWNTs G


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reported herein. Spectral grade CB, DCE, and DCB purchased from Fisher Scientific were used as received. Sample Preparation. MEH-PPV and MEH-PPV−nanotube blend solutions were prepared in spectral grade solvents at concentrations of 6−7 mg/mL for MEH-PPV blends with varying weight/weight percentages of SWNTs or MWNTs. The solutions were thoroughly mixed by sonication, stirred at room temperature for 18 h, and then added into casting wells incised into a polytetrafluoroethylene sheet. The film-casting sheet was then allowed to air-dry overnight. The resulting free-standing films were then removed and cut into rectangles of about 2.5 cm × 0.5 cm × (25−50) μm. A micrometer was used to measure sample thickness by averaging at several spots in the sample. Doping was carried out by placing pristine blend films into capped vials with a few crystals of iodine (I2) for a set time: both long (20 h) and short (2−3 h) durations were used. Electrical conductivity and thermoelectric measurements were then carried out as quickly as possible after sample removal from the doping chamber. Additional details are given in the Supporting Information. Sample Analyses. Sample specific conductivity was measured using a standard four-probe setup with platinum wire contacts. A Keithley 2440 source meter was used for conductivity measurements, and a Keithley 6182 nanovoltmeter was used for voltage measurements. Thermoelectric performance was evaluated using a custombuilt apparatus described previously47 and in the Supporting Information. A digital dual input thermometer was used to measure sample temperatures and confirm establishment of a stable thermal gradient using K-type thermocouples coated with polyvinyl chloride. Thermoelectric potentials were measured with thermal gradients ΔT = 0.1−10 °C. Seebeck coefficients were determined by plotting measured thermoelectric potentials vs applied ΔT. Only single solvent-cast composite strips (“tabs”) were used for measurementsno multistrip “laminated” stack samples were used. Further details are given in the Supporting Information. SEM was used to examine the same free-standing samples used to carry out thermoelectric testing. SEM samples were attached to a flat geometry sample holder using conductive double-faced adhesive tape. A model FEI Magellan 400 instrument was used to obtain SEM images at acceleration voltages of 1−5 kV.

give TE performance much better than that of MWNTs in these composites. Some commercial SWNTs give markedly better TE performance than others in the composites. The nonencapsulated iodine-doped composites lose about half of their PF by dedoping after ambient exposure, but redoping for typical iodine exposure time restores the PF. This indicates that partial dedoping is not destructive to the basic doped composite structure. This TE optimization suggests a procedure to do the same for analogous composites of carbon NTs with other conjugated organic polymers: (1) select 2−3 solvents for fabrication to get higher doped electrical conductivity for the polymer alone with the ability to disperse the NTs well; (2) select for highest TE using different grades of polymer varying in, e.g., chain length, regiospecificity, and dispersity; (3) blend increasing amounts of NTs for samples showing the best TE performance from steps 1−2; (4) during step 3, test NTs from different sources and of different types to establish the best TE; and (5) test for reversibility of both doping and dedoping if the dopant is volatile. Overall, in this study, 30−50% w/w SWnNT gave best PFs for both the lower and higher MW MEH-PPV in DCE after 2− 3 h of iodine doping (similar or slightly better PFs in DCB so long as 50% SWnNT). For a conservative assumption of κ = 1 W·m−1·K−1 at room temperature, ZT approaches 0.01 for the doped composites with the highest PFs.

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CONCLUSION This work showed that composite blends of MEH-PPV with carbon nanotubes provide significant thermoelectric power factors after iodine doping. Multiple optimization iterations to test effects of MEH-PPV chain length (molecular weight), amount and grade of SWNT added, sample fabrication solvent, and doping duration led to substantial optimization of thermoelectric power factors in iodine doped MEH-PPV/ SWNT blends, up to ∼33 μW·m−1·K−2. Multiday exposure of doped samples to ambient conditions results in loss of about half of the doped power factor, but redoping restores thermoelectric performance. The latter result indicates a need for encapsulation to retain exposure to high dopant levels and maintain best performance, at least for volatile dopants such as iodine. Of course, less volatile dopants, including organic acids or Lewis acids, can be tested in a manner similar to that used in this study with optimization of multiple variables for best TE performance. As a result, there is much promise for analogous tests of doped conducting polymer composites with carbon nanotubes to make thermoelectric materials that are effective enough to be useful and much less environmentally hazardous than many inorganic thermoelectric materials.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b14695. Electrical conductivity as a function of doping time for MEH-PPV samples cast from different solvents; thermoelectric performance values for undoped MEH-PPV samples blended with wide carbon SWNTs; thermoelectric performance values for MEH-PPV samples blended with wide carbon SWNTs iodine-doped for 20 h; thermoelectric performance values as a function of iodine doping time for MEH-PPV samples blended with wide carbon SWNTs; thermoelectric performance values for MEH-PPV samples blended with carbon MWNTs before and after 2.5−3 h of iodine doping; SEM images for MEH-PPV samples containing 0 and 30% SWNT; and expanded view SEM image for MEH-PPV-150k sample containing 30% MWNT (PDF)

EXPERIMENTAL SECTION

Materials. Iodine was purchased from Fisher Scientific and used as received. MEH-PPV samples were purchased from Sigma-Aldrich. In the various discussions, “low molecular weight” MEH-PPV with M̅ n = 70 000−100 000 Da and “high molecular weight” MEH-PPV with M̅ n = 150 000−250 000 Da are denoted MEH-PPV-70k and MEH-PPV150k, respectively. Two grades of SWNTs were purchased from Sigma-Aldrich: ≥50−70% carbon as SWNT with 1.2−1.5 nm diameters (SWwNT) and ≥80% carbon as SWNT with 0.7−1.4 nm diameters (SWnNT). MWNTs were purchased from Wako Pure Chemical Industries (3−20 nm diameters) and from Sigma-Aldrich (outer diameter 6−13 nm, inner diameter 2−6 nm, length 2.5−20 μm); the latter MWNTs were used for virtually all of the results

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Paul M. Lahti: 0000-0003-4255-4733 Notes

The authors declare no competing financial interest. H


Research Article


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ACKNOWLEDGMENTS This work was supported by the Defense Threat Reduction Agency via the Department of the Army through Contracts W911QY-09-1-0003 (P.S.T. and F.E.K.) and W911NF-14-20002 (M.T., L.W., and P.M.L.) from the United States Army Natick Soldier Research, Development and Engineering Center (NSRDEC). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSRDEC.

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