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Enhanced Molecular Packing of a Conjugated Polymer with High Organic Thermoelectric Power Factor Wei Ma, Ke Shi, Yang Wu, Zuoyu Lu, Hanyu Liu, Jie-Yu Wang, and Jian Pei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06899 • Publication Date (Web): 31 Aug 2016 Downloaded from http://pubs.acs.org on September 5, 2016

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ACS Applied Materials & Interfaces

Enhanced Molecular Packing of a Conjugated Polymer with High Organic Thermoelectric Power Factor Wei Ma‡*a, Ke Shi‡†b, Yang Wu a, Zuo-Yu Lu b, Han-Yu Liu b, Jie-Yu Wang b, Jian Pei b * a

State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, China

b

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Bioorganic Chemistry

and Molecular Engineering of Ministry of Education, Key Laboratory of Polymer Chemistry and Physics of the Ministry of Education, Center for Soft Matter Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China

KEYWORDS: organic thermoelectric materials,

n-type conjugated polymer, solution

processed, morphology, molecular packing

Abstract:

ACS Paragon Plus Environment

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The detailed relationship between film morphology and the performance of solution processed ntype organic thermoelectric (TE) devices is investigated. It is interesting to find that the better ordered molecular packing of n-type polymer can be achieved by adding a small fraction of dopant molecules, which is not observed before. The better ordered structure will be favorable for the charge carrier mobility. Meanwhile, dopant molecules improve free carrier concentration via doping reaction. As a result, a significantly enhanced electrical conductivity (12 S cm-1) and power factor (25.5 µW m-1 K-2) of TE devices is obtained. Furthermore, the phase separation of conjugated polymer/dopants is observed for the first time with resonant soft X-ray scattering. Our results indicate that the miscibility of conjugated polymers and dopants plays an important role on controlling the morphology and doping efficiency of TE devices.

Introduction Organic semiconductors are receiving increasing attention as promising thermoelectric (TE) materials by virtue of their low thermal conductivity, mechanical flexibility and ultralow-cost fabrication.1–4 Despite all the above advantages, further development of organic TE materials is impeded by their low electrical conductivity σ, which results in unsatisfying power factor S2σ (numerator of figure of merit ZT). As electrical conductivity is determined jointly by carrier charge q, free-carrier concentration n (cm-3) and carrier mobility µ (cm2 V-1 s-1) as σ = nqµ, chemical doping is a key strategy to maximize the electrical conductivity and thus power factor of organic semiconductors by modulating the free-carrier concentration and changing the carrier mobility.5–8 However, unlike conventional inorganic semiconductors, doping of organic materials usually requires introduction of a large molecular species that may influence the original ordered molecular packing9 and potentially leads to severe phase separation between dopants and organic semiconductors in the bulk. For instance, in most conjugated polymers such


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as polyacetylene, poly(phenylenevinylene) and poly(3-hexylthiophene), the highly ordered molecular packing is severely disrupted due to the intercalation of dopant molecules, leading to the decrease of carrier mobility.10–12 P(NDIOD-T2) (poly{N,N′-bis(2-octyl-dodecyl)-1,4,5,8napthalenedicarboximide-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)}) is one of rare examples that the doped film morphology and molecular packing remain unchanged compared with those of the undoped thin films.13 This is due to the highly ordered nature of P(NDIOD-T2), which causes the low miscibility of dopant molecules and P(NDIOD-T2), thus leading to the low doping efficiency (only ~1% dopant contributes free carrier to the film) and the poor electrical conductivity. After all, there has been no report showing that the introduction of dopants benefits for the crystallization of organic semiconductors and improves the molecular packing and film morphologies. As a consequence, the detailed investigation and optimization of the molecular interaction between dopants and conjugated polymers appear to be vital to obtain high performance TE materials. In our recent work, n-type conjugated polymer FBDPPV was developed with the highest electrical conductivities of 14 S cm-1 and power factors up to 28 µW m-1 K-2.14 The record performance for solution-processed organic TE materials provides the best research benchmark for us to elucidate the key factors to obtain high electrical conductivities and power factor of organic TE materials. The investigation of morphology evolution in doped films, i.e. the changing of the molecular packing and the phase separation, would have a dramatic impact on the free-carrier concentration and the carrier mobility. Therefore, not only electrical conductivity, even power factors are closely correlated to the film morphology considering its limited impact on the Seebeck coefficient. It obviously necessitates more effort to investigate the morphology dependence of doped films on electrical conductivities, and thus further understand the roles of


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dopants in structure-property relationship of organic TE materials and develop high performance TE materials. However, most recent efforts have been focused on the report of high performance organic TE materials, and few studies focus on structure evolution of dopants and conjugated polymers in doped films and its impact on TE performance is also not clear.

Figure 1. (a) Chemical structures of FBDPPV and N-DMBI; (b) schema for TE device and morphology evolution in thin films. In this work, we systematically studied the relationship between thin film morphology and the thermoelectric power factor of a solution processed n-type polymer FBDPPV doped by N-DMBI (chemical structures shown in Figure 1a). Herein, to understand the detailed doping process, especially from the view of the morphology, grazing incident wide-angle X-ray scattering (GIWAXS), atomic force microscopy (AFM) and resonant soft X-ray scattering (RSoXS) were utilized to figure out the evolution of film morphology with the increase of dopant ratio. In particular, the phase separation at small length-scale between dopants and FBDPPV is explored


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for the first time by resonant soft X-ray scattering, which is a novel technique and has been proved to be a powerful tool in probing phase separation of two or more components in block copolymers and bulk heterojunction solar cells.15–17 In this work, we observed the phase separation in doped films using this technique. Furthermore, the evolution of the molecular packing of FBDPPV as a function of the dopant amount is investigated by GIWAXS. Besides, X-ray photoelectron spectroscopy (XPS) and thermoelectric properties were also explored to further elucidate the relationship between film morphology and power factor. We find that the role of the dopant molecules is not limited to improve the free charge concentration through doping reaction. More importantly, better molecular packing of FBDPPV was achieved via the appropriate interaction with dopant molecules, implying that proper combination of dopant and conjugated polymer is beneficial to the crystallization of organic semiconductors and may improve the molecular packing and thus enhance the charge carrier mobility. By coupling of these two favorable contributions, the electrical conductivities and power factors are significantly improved.

Experimental Section

Grazing Incidence Wide-Angle X-ray Scattering (GIWAXS) Characterization: GIWAXS measurements were performed at beamline 7.3.318 at the Advanced Light Source. Samples were prepared on Si substrates using identical blend solutions as those used in devices. The 10 keV Xray beam was incident at a grazing angle of 0.12°-0.16°, selected to maximize the scattering intensity from the samples. The scattered X-rays were detected using a Dectris Pilatus 2M photon counting detector.


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Resonant Soft X-ray Scattering (R-SoXS): R-SoXS transmission measurements were performed at beamline 11.0.1.219 at the Advanced Light Source (ALS). Samples for R-SoXS measurements were prepared on a PSS modified Si substrate under the same conditions as those used for device fabrication, and then transferred by floating in water to a 1.5 mm × 1.5 mm, 100 nm thick Si3N4 membrane supported by a 5 mm × 5 mm, 200 µm thick Si frame (Norcada Inc.). 2-D scattering patterns were collected on an in-vacuum CCD camera (Princeton Instrument PI-MTE). The sample detector distance was calibrated from diffraction peaks of a triblock copolymer poly(isoprene-b-styrene-b-2-vinyl pyridine), which has a known spacing of 391 Å. The beam size at the sample is approximately 100 µm by 200 µm.

TE device fabrication and testing: Both FBDPPV and N-DMBI were dissolved in 1,2dichlorobenzene (o-DCB) with a concentration of 3 mg/ml. Aliquots of N-DMBI and polymer solutions were mixed at room temperature. All devices were fabricated using glass substrates. The gold electrodes were pre-patterned by photolithography on the surface with a channel length of 100 µm and a channel width of 500 µm for conductivity measurements and a channel length of 500 µm and a channel width of 2500 µm for Seebeck coefficient measurements. The substrates were subjected to cleaning using ultrasonication in acetone, cleaning agent, deionized water (twice), and isopropanol. Thin films were deposited on the treated substrates by spincoating at 1500 rpm for 60 s and annealed at 120 °C for 8 h. The doped thin films show good stability over 30 days under nitrogen atmosphere. The thickness of FBDPPV thin film is around 16 nm. The Seebeck coefficient measurements were done in vacuum. The Seebeck coefficient is calculated by S=Vtherm/∆T, where Vtherm is the thermal voltage obtained between the two ends of


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the device subject to a temperature gradient ∆T. The Vtherm was measured with Keithley 4200 SCS, and the temperature difference was introduced by Peliter elements and monitored by using an infrared camera FLIR A300 (thermal sensitivity < 50 mK). The accuracy of the measurements was verified by two resistive thermometers next to the electrodes. 4-Point conductivity measurements were conducted in an N2 glovebox with Keithley 4200 SCS.

XPS: X-ray photoelectron spectroscopy (XPS) was conducted on a Kratos AXIS Ultra-DLD Photoelectron Spectrometer under an ultrahigh vacuum of about 3×10-9 Torr with an unfiltered He I gas discharge lamp source (21.22 eV) and a monochromatic Al Kα source (1486.6 eV) as the excitation source, respectively. The instrumental energy resolution for XPS was 0.5 eV. Before measurements, all the samples were spin-coated on 1 cm × 1 cm native oxide silicon substrates in a N2 glove box and transferred through a transport system without air exposure in to the spectrometer analysis chamber.

Results and Discussions


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Figure 2. (a) 2D GIWAXS patterns; (b) scattering profiles in the in-plane and the out-of-plane directions for FBDPPV pure films and with 1 wt %, 3 wt %, 5 wt %, 7 wt %, 15 wt %, 20 wt % and 50 wt % dopant N-DMBI; (c) out-of-plane (010) coherence length (CL) of FBDPPV as a function of doping amount. The scattering intensity is offset for clarity.

Grazing incident wide-angle X-ray scattering (GIWAXS) is used to probe the molecular packing and orientation of pure FBDPPV and FBDPPV with different amount of dopant (N-DMBI). The 2D GIWAXS patterns and corresponding scattering profiles in the in-plane (IP) and the out-of-


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plane (OOP) directions are displayed in Figure 2a-b. Compared with the highly crystallized ntype conjugated polymer P(NDIOD-T2)13, FBDPPV pure film shows relatively weak diffraction peaks and thus lower molecular packing ordering. The diffraction peaks at q ≈ 0.17 and 0.34 Å-1 correspond to (100) and (200) lamellar packing, and the peak at q ≈ 1.75 Å-1 is originated from (010) π-π stacking. When 1 wt % dopant is added, the (h00) scattering peaks changed little in terms of the location and intensity but the out-of-plane scattering (010) peak becomes weaker, indicating that the “face-on” π-π packing of FBDPPV becomes less ordered. When 3 wt % dopant is added, the out-of-plane (h00) scattering peaks become weaker, but the out-of-plane (010) and in-plane (h00) scattering peaks are stronger compared to those with 1 wt % dopant. This suggests that “face-on” molecular packing is enhanced the thin films. When 5-7 wt % dopant is added, the out-of-plane (010) and the in-plane (h00) scattering peaks are further improved and the “face-on” molecular packing is still dominant. It should be noted that this is the first example showing that dopants improve molecular packing of the conjugated polymers (as demonstrated in Figure 1b). Although further addition of dopants (15 wt %-50 wt %) leads to an enhanced out-of-plane (h00), the decreased in-plane (h00) and out-of-plane (010) π-π stacking ordering demonstrate that the FBDPPV’s “face-on” molecular packing was destroyed at high dopant ration. To quantitatively analyze FBDPPV’s molecular packing, the coherence length of FBDPPV (010) peaks (the fitted results are displayed in Figure S1) is calculated as a function of the dopant ratio. The results are displayed in Figure 2c and Table 1. The out-of-plane (010) π-π stacking coherence length decreases at the very beginning and increases to the maximum when 5-7 wt % dopant is added, which is in agreement with the previous qualitative analysis. Thus, we can conclude that the small amount of dopant induces more ordered face-on molecular packing of FBDPPV, which would be advantageous for charge carrier transport.20–23


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The surface morphology is further investigated by AFM (Figure S2). It is observed that all the films with small dopant amount (less than 7 wt %) show smooth surface morphology as the un-doped film. When the dopant amount is higher than 15 wt %, the film surface becomes rough, which should result from the dopant aggregation. This indicates that the miscibility threshold for the dopant in the FBDPPV7 should be between 7-15 wt %. When the dopant amount exceeds 7 wt %, the dopant is unfavorable for the packing of FBDPPV molecules and extra dopant aggregates on the top surface of thin films.

Figure 3. R-SoXS proﬁles in log-scale for FBDPPV with 1 wt %, 3 wt %, 5 wt %, 7 wt % and 15 wt % dopant N-DMBI. The scattering induced by surface roughness or FBDPPV: N-DMBI is marked by the blue arrows. Resonant soft X-ray scattering (R-SoXS) is a novel technique emerged in recent years and, herein, is employed for the first time to explore the phase separation between dopants and conjugated polymers. A series of energies near carbon K-edge is used (see Figure S3 as an


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example). A resonant energy of 284.8 eV was selected due to the most significant features in the scattering profiles and thus corresponding to the highly enhanced materials contrast between different components.24–27 The median of the scattering distribution Smedian corresponds to the characteristic median length scale, ξ, of the corresponding log-normal distribution. The R-SoXS scattering profiles of FBDPPV doped with 1 wt %, 3 wt %, 5 wt %, 7 wt % and 15 wt % NDMBI are recorded and shown in Figure 3. There are multiple peaks in R-SoXS profiles. At the very low-q (q 600 nm) of the blended films, since similar scattering peaks at this range were observed when non-resonant energy of 270 eV is used (where the vacuum-materials optical contrast is high17,28) (shown in Figure S4). It is further noted that when the doping ratio is less than 7 wt %, the scattering at low-q is low, suggesting that macro-scale aggregation of N-DMBI on the thin film surface does not occur. The high scattering at this range for 3 wt % dopant is due to the parasitic scattering. When the doping ratio reaches 15 wt %, the scattering at low-q is significantly enhanced, demonstrating that the surface roughness is highly increased. This information is consistent with the surface roughness revealed by AFM height images. Furthermore, it is interesting to find that the phase separation between dopants and conjugated polymers is revealed by R-SoXS at high-q. These peaks are originated from dopants: polymers phase separation since the material contrast is highly enhanced at resonant energy while the peaks are not observed at non-resonant energy of 270 eV as shown in Figure S3. The detailed analysis revealed that all blends show almost identical double peaks at q ≈ 0.17 and 0.44 nm-1. These peaks correspond to the small phase separation with length scale ξ of 13 nm and 30 nm. Such small phase separation confirms that N-DMBI molecules tend to be miscible with FBDPPV. The identical scattering profiles indicate that similar phase separation is achieved


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within a large-scale variation of the doping amount. This small and similar phase separation suggests that the initially added dopants disperse into conjugated polymer phase and further added dopants do not severely aggregate in the bulk but form big aggregation on the surface of thin films.

Figure 4. (a) XPS spectra of FBDPPV with different dopant amount; (b) the ratio of peak area near 402 eV and 400 eV as a function of dopant amount.

To further analyze the doping level as a function of the dopant amount, the XPS measurement is carried out. During the doping reaction, the imidazole cation N-DMBI+ is produced with an absorption peak at 402 eV near the nitrogen K-edge at ~400 eV (the doping reaction is also confirmed by UV-Vis results as shown in Figure S5 and S6). Therefore, the doping level can be quantitatively described by monitoring the absorption peak at 402 eV, i.e. the greater the peak area at 402 eV is, the higher the doping level is. The XPS spectra near the nitrogen K-edge are


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shown in Figure 4(a). Two distinguished absorption peaks were observed, which correspond to the absorption of neutral N-DMBI and FBDPPV at 400 eV and that of newly generated cation NDMBI+ at 402 eV. It is found that the doping level (the ratio of peak areas at 402 and 400 eV) exponentially increases with dopant amount as shown in Figure 4(b). When the dopant amount is less than 7 wt %, the doping level increases very rapidly and then the doping level is saturated with dopant amount between 7-50 wt %, which implies no further doping reaction occurred. In other words, when doping ratio over 7 wt %, the excess N-DMBI does not act as dopant but aggregates on the top surface of the film.

Figure 5. Thermoelectric properties of the doped FBDPPV at different doping concentration. (a) Electrical conductivity, (b) Seebeck coefficient, (c) Power Factors.

To understand the relationship between the observed morphology and the thermoelectric property of FBDPPV, the electrical conductivity and Seebeck coefficient were measured. Electrical conductivity was measured via four-probe method and Seebeck coefficient was determined by imposing a temperature difference across the sample and measuring the thermovoltages (see experimental section for details). As shown in Figure 5, the electrical conductivity of films dramatically increases by adding the dopant and attains to maxima as the


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mass fraction of N-DMBI in solution is 7 wt %. The Seebeck coefficients are negative, confirming that n-type electrical transport is dominant. When the electrical conductivity increases, the Seebeck coefficient changes in opposite direction, which is in accord with their negative correlation with carrier concentrations.29 Further increasing the concentration of NDMBI leads to a rapid decrease of electrical conductivity and the Seebeck coefficient continues to decline but less significant. The combination of the electrical conductivity and Seebeck coefficient yields a power factor as high as 25.5 µW m-1 K-2 for the n-type polymers FBDPPV at room temperature.

Table 1. Summary of the thermoelectric parameters and structure parameters of FBDPPV doped by N-DMBI. Doping conditio n (wt%)

Conductivity (S/cm)

Seebeck Power Factor Coefficie (µW/mK2) nt (µK/V)

Ratio of CL of peaks at 402 Out-ofeV and 400 Plane (010) eV (Å)

1

0.0087±0.0018

1.14±0.06 1148 ± 2 8

16:84 37.95

3

1.56±0.25

-329±29

16.9±0.6

24:76

37.20

5

6.23±0.71

-210±20

25.5±2.0

32:68

39.54

7

12.02±1.86

-131±9

20.6±0.4

33:67

40.98

15

0.60±0.08

-52±11

0.10±0.06

33:67

35.34

20

0.06±0.01

-77±5

0.033±0.003

35:65

30.86

50

6.5 × 10-4 ± -75±17 6.4×10-4

3.7 × 4 ±0.6×10-4

10- 37:63

N/A


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On the basis of the morphology characterization and device performance results, we propose the following picture of structural evolution and discuss the impact of dopant on the molecular packing of FBDPPV and its thermoelectric performance. When small amount (1-3 wt %) of dopant N-DMBI is added into FBDPPV, doping reaction is occurred, which is the main contributor to improve electrical conductivities due to the considerably enhanced carrier concentration n. Although such small amount of dopant slightly disturbed the molecular ordering of FBDPPV and thus impacts carrier mobility µ, the overall electrical conductivities σ are still improved. It has been reported that the Seebeck coefficients shows the opposite correlation with carrier concentration n and electrical conductivities, i.e. Seebeck coefficient is proportional to n1/4 29

.

Due to the significantly improved carrier concentration n, the Seebeck coefficients are

negatively impacted, decreased from -1148 to -330 µK/V. In addition, due to the highly improved electrical conductivities σ, the PF is still improved. Further increasing the dopant amount to 5-7 wt %, the doping level is significantly enhanced as revealed by XPS, which leads to the highly increased carrier concentration n. Most importantly, the further added dopant induces the greater molecular π-π stacking of FBDPPV. Such ordered molecular packing will be favorable to the charge carrier transport and thus the electron mobility.20–23 These two simultaneously improved factors (charge carrier mobility and carrier concentration) further boost the electrical conductivities up to 12 S/cm. As the improved thin film morphology does not contribute to the Seebeck coefficients and higher carrier concentration is unfavorable for it, the Seebeck coefficients smoothly decreases as the dopant amount increases. Due to the significantly enhanced electrical conductivity, the power factor increases to as high as 25.5 µW m-1 K-2. We note here that the 5-7 wt % weight fraction corresponds to the ratio between the number of FBDPPV repeat unit and the number of N-DMBI molecule being 2:1 - 3:1 (see supporting


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information). This is well consistent with the report that the molecular number ratio of n-type conjugated polymer unit and N-DMBI molecule in doping reaction should be close to 2:1.30 When more dopants are added (> 10 wt %), the doping level becomes constant since the extra dopant aggregates on the surface of films and thus the effect on improving carrier concentration becomes limited. Meanwhile, the large amount dopants disturbed the π-π stacking of FBDPPV, which is deleterious for charge carrier mobility. As a result, the electrical conductivities decrease. As the variation of doping efficiency and carrier concentration is small, the Seebeck coefficients slightly decrease. Therefore, the power factors become very low.

Conclusions In conclusion, we highlighted for the first time the impact of dopants on the morphology evolution in governing the thermoelectric power factor of the n-type polymer and successfully obtained an efficient power factor of TE devices by using well mixed dopant in the low crystalized conjugated polymer. We find that the dopants not only improve free carrier concentration, but also enhance the molecular packing of the polymer. More fundamentally, our results demonstrate that rational selection of a pair of dopant and polymer with good miscibility and well defined molecular interaction is the key to enhance TE performance, which will stimulate the advances of n-type TE devices. Also, this work indicates that low crystallinity ntype conjugated polymers with high electron mobility are very good candidate to yield high efficient solution-processed TE device, which opens up a new frontier in understanding the critical structure-morphology-function relationship of n-type TE materials.

ASSOCIATED CONTENT


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Supporting Information. The fitted GIWAXS (010) profiles, AFM images, RSoXS results for many energies, UV-Vis absorption spectra are supplied as Supporting Information. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. ACKNOWLEDGMENT This work was supported by the Major State Basic Research Development Program (Nos. 2013CB933501, 2015CB856505) from the Ministry of Science and Technology, and National Natural Science Foundation of China (21504066, 21534003). X-ray data was acquired at beamlines 7.3.3 and 11.0.1.2 at the Advanced Light Source, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The authors thank Dr. Deng-Li Qiu from Bruker AXS GmbH Beijing Representative Office for AFM measurements. The authors thank Dazhen Huang, Dr. Chong-An Di and Dr. Ye Zou from Institute of Chemistry, Chinese Academy of Sciences for Seebeck coefficient and XPS measurements.


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REFERENCES (1)

Environ, E.; Poehler, T. O.; Katz, H. E. Environmental Science Prospects for Polymer-

Based Thermoelectrics : State of the Art and Theoretical. Energy Environ. Sci. 2012, 5 (5), 8110–8115. (2)

Bubnova, O.; Crispin, X. Environmental Science. Energy Environ. Sci. 2012, 5 (5),

9345–9362. (3)

Pentzer, E.; Mcgrail, B. T.; Sehirlioglu, A.; Pentzer, E. Polymer Composites for

Thermoelectric Applications. Angew. Chem. Int. Ed. 2015, 54 (54), 1710–1723. (4)

Zhang, Q.; Sun, Y.; Xu, W.; Zhu, D. Organic Thermoelectric Materials : Emerging Green

Energy Materials Converting Heat to Electricity Directly and Effi Ciently. Adv. Mater. 2014, 26 (26), 6829–6851. (5)

Mai, C.; Schlitz, R. a; Su, G. M.; Spitzer, D.; Wang, X.; Fronk, S. L.; Cahill, D. G.;

Chabinyc, M. L.; Bazan, G. C. Side-Chain Effects on the Conductivity, Morphology, and Thermoelectric Properties of Self-Doped Narrow-Band-Gap Conjugated Polyelectrolytes. J. Am. Chem. Soc. 2014, 136, 13478-13481. (6)

Zhang, Q.; Sun, Y.; Xu, W.; Zhu, D. Thermoelectric Energy from Flexible P3HT Films

Doped with a Ferric Salt of Triflimide Anions. Energy Environ. Sci. 2012, 5 (5), 9639–9644. (7)

Bubnova, O.; Khan, Z. U.; Malti, A.; Braun, S.; Fahlman, M.; Berggren, M.; Crispin, X.

Optimization of the Thermoelectric Figure of Merit in the Conducting Polymer Poly ( 3 , 4Ethylenedioxythiophene ). Nat. Mater. 2011, 10 (6), 429–433.


18

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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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(8)

Kim, G.; Shao, L.; Zhang, K.; Pipe, K. P. Engineered Doping of Organic Semiconductors

for Enhanced Thermoelectric Efficiency. Nat. Mater. 2013, 12 (8), 719–723. (9)

Cochran, J. E.; Junk, M. J. N.; Glaudell, a. M.; Miller, P. L.; Cowart, J. S.; Toney, M. F.;

Hawker, C. J.; Chmelka, B. F.; Chabinyc, M. L. Molecular Interactions and Ordering in Electrically Doped Polymers: Blends of PBTTT and F 4 TCNQ. Macromolecules 2014, 47, 6836–6846. (10) Masse, M. a; Schlenoff, J. B.; Karasz, F. E.; Thomas, E. L. Crystalline Phases of Electrically Conductive Poly(p-Phenylene-Vinylene). J. Polym. Sci. Part B Polym. Phys. 1989, 27 (27), 2045–2059. (11) Duong, D. T.; Wang, C.; Antono, E.; Toney, M. F.; Salleo, A. The Chemical and Structural Origin of Efficient P-Type Doping in P3HT. Org. Electron. 2013, 14 (5), 1330–1336. (12) Murthy, N. S.; Miller, G. G.; Baughman, R. H. Structure of Polyacetylene – Iodine Complexes. J. Chem. Phys. 2001, 89 (89), 2523. (13) Schlitz, R. A.; Brunetti, F. G.; Glaudell, A. M.; Miller, P. L.; Brady, M. A.; Takacs, C. J.; Hawker, C. J.; Chabinyc, M. L. Solubility-Limited Extrinsic N-Type Doping of a High Electron Mobility Polymer for Thermoelectric Applications. Adv. Mater. 2014, 26 (26), 2825–2830. (14) Shi, K.; Zhang, F.; Di, C.; Yan, T.; Zou, Y.; Zhou, X.; Zhu, D.; Wang, J.; Pei, J. Toward High Performance N ‑ Type Thermoelectric Materials by Rational Modi Fi Cation of BDPPV Backbones. J. Am. Chem. Soc. 2015, 137 (137), 6979–6982.


19


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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 22

(15) Yan, H.; Collins, B. A.; Gann, E.; Wang, C.; Ade, H. Correlating the Efficiency and Nanomorphology of Polymer Blend Solar Cells Utilizing Resonant Soft X-Ray Scattering. ACS Nano 2012, 6 (1), 677–688. (16) Wang, C.; Lee, D. H.; Hexemer, A.; Kim, M. I.; Zhao, W.; Hasegawa, H.; Ade, H.; Russell, T. P. Defining the Nanostructured Morphology of Triblock Copolymers Using Resonant Soft X-Ray Scattering. Nano Lett. 2011, 11(9), 3906-3911. (17) Collins, B. A.; Cochran, J. E.; Yan, H.; Gann, E.; Hub, C.; Fink, R.; Wang, C.; Schuettfort, T.; McNeill, C. R.; Chabinyc, M. L.; Ade, H. Polarized X-Ray Scattering Reveals Non-Crystalline Orientational Ordering in Organic Films. Nat. Mater. 2012, 11 (6), 536–543. (18) A. Hexemer, W. Bras, J. Glossinger, E. Schaible, E. Gann, R. Kirian, A. MacDowell, M. Church, B. Rude, H. Padmore. GIWAXS Beamline Paper. J. Phys. Conf. Ser. 2010, 247, 012007. (19) Gann, E.; Young, A. T.; Collins, B. A.; Yan, H.; Nasiatka, J.; Padmore, H. A.; Ade, H.; Hexemer, A.; Wang, C. Soft X-Ray Scattering Facility at the Advanced Light Source with RealTime Data Processing and Analysis. Rev. Sci. Instrum. 2012, 83 (4), 045110. (20) Albrecht, S.; Janietz, S.; Schindler, W.; Frisch, J.; Kurpiers, J.; Kniepert, J.; Inal, S.; Pingel, P.; Fostiropoulos, K.; Koch, N.; Neher, D. Fluorinated Copolymer PCPDTBT with Enhanced Open-Circuit Voltage and Reduced Recombination for Highly Efficient Polymer Solar Cells. J. Am. Chem. Soc. 2012, 134 (36), 14932–14944. (21) Liu, F.; Gu, Y.; Jung, J. W.; Jo, W. H.; Russell, T. P. On the Morphology of PolymerBased Photovoltaics. J. Polym. Sci. Part B Polym. Phys. 2012, 50 (15), 1018–1044.


20

Page 21 of 22

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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(22) Liu, Y.; Zhao, J.; Li, Z.; Mu, C.; Ma, W.; Hu, H.; Jiang, K.; Lin, H.; Ade, H.; Yan, H. Aggregation and Morphology Control Enables Multiple Cases of High-Efficiency Polymer Solar Cells. Nat. Commun. 2014, 5 (5), 5293. (23) Qian, D.; Ma, W.; Li, Z.; Guo, X.; Zhang, S.; Ye, L.; Ade, H.; Tan, Z.; Hou, J. Molecular Design toward Efficient Polymer Solar Cells with High Polymer Content. J. Am. Chem. Soc. 2013, 135 (135), 8464–8467. (24) Ma, W.; Tumbleston, J.; Wang, M.; Gann, E.; Huang, F.; Ade, H. Domain Purity, Miscibility, and Molecular Orientation at Donor/Acceptor Interfaces in High Performance Organic Solar Cells: Paths to Further Improvement. Adv. Energy Mater. 2013, 3 (3), 864–872. (25) Ma, W.; Ye, L.; Zhang, S.; Hou, J.; Ade, H. Competition between Morphological Attributes in the Thermal Annealing and Additive Processing of Polymer Solar Cells. J. Mater. Chem. C 2013, 1 (1), 5023–5030. (26) Ma, W.; Tumbleston, J. R.; Ye, L.; Wang, C.; Hou, J.; Ade, H. Quantification of Nanoand Mesoscale Phase Separation and Relation to Donor and Acceptor Quantum Efficiency, Jsc, and FF in polymer:Fullerene Solar Cells. Adv. Mater. 2014, 26 (26), 4234–4241. (27) Collins, B. A.; Li, Z.; Tumbleston, J. R.; Gann, E.; McNeill, C. R.; Ade, H. Absolute Measurement of Domain Composition and Nanoscale Size Distribution Explains Performance in PTB7:PC71BM Solar Cells. Adv. Energy Mater. 2013, 3 (3), 65–74. (28) Zhou, Y.; Kurosawa, T.; Ma, W.; Guo, Y.; Fang, L.; Vandewal, K.; Diao, Y.; Wang, C.; Yan, Q.; Reinspach, J.; Mei, J.; Appleton, A. L.; Koleilat, G. I.; Gao, Y.; Mannsfeld, S. C. B.;


21


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Page 22 of 22

Salleo, A.; Ade, H.; Zhao, D.; Bao, Z. High Performance All-Polymer Solar Cell via Polymer Side-Chain Engineering. Adv. Mater. 2014, 26, 3767–3772. (29) Glaudell, A. M.; Cochran, J. E.; Patel, S. N.; Chabinyc, M. L. Impact of the Doping Method on Conductivity and Thermopower in Semiconducting Polythiophenes. Adv. Energy Mater. 2014, 1401072. (30) Naab, B. D.; Guo, S.; Olthof, S.; Evans, E. G. B.; Wei, P.; Millhauser, G. L.; Kahn, A.; Barlow, S.; Marder, S. R.; Bao, Z. Benjamin D. Naab Song Guo Selina Olthof Eric G. B. Evans Peng Wei Glenn L. Millhauser Antoine Kahn Stephen Barlow Seth R. Marder Zhenan Bao. J. Am. Chem. Soc. 2013, 135 (135), 15018–15025.

The morphology features of n-type organic thermoelectric device are investigated. We find that dopants not only improve free carrier concentration, but also enhance the molecular packing of polymer and thus higher carrier mobility. More fundamentally, our results demonstrate that rational selection of a pair of dopant and polymer with good miscibility is the key to enhance TE performance, which will stimulate the advances of n-type TE device.

ToC figure


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Enhanced Molecular Packing of a Conjugated ... - ACS Publications

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