Significant Enhancement of Hole Transport Ability in Conjugated

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Significant Enhancement of Hole Transport Ability in Conjugated Polymer/Fullerene Bulk Heterojunction Microspheres Amandeep Jindal, Hiroaki Kotani, Soh Kushida, Akinori Saeki, Takahiko Kojima, and Yohei Yamamoto ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.8b00170 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Polymer Materials

Significant Enhancement of Hole Transport Ability in

Conjugated

Polymer/Fullerene

Bulk

Heterojunction Microspheres Amandeep Jindal,a Hiroaki Kotani,b Soh Kushida,a,c Akinori Saeki,d Takahiko Kojimab and Yohei Yamamoto*a,e a

Department of Materials Science, Faculty of Pure and Applied Sciences, University of Tsukuba,

1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan. b

Department of Chemistry, Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1

Tennodai, Tsukuba, Ibaraki 305-8571, Japan. c

Organisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld

270, Heidelberg 69120, Germany. d

Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1

Yamadaoka, Suita, Osaka 565-0871, Japan e

Tsukuba Research Centre for Energy Materials Science (TREMS), University of Tsukuba, 1-1-1

Tennodai, Tsukuba, Ibaraki 305-8573, Japan

ACS Paragon Plus Environment

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ACS Applied Polymer Materials 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

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KEYWORDS. microspheres, bulk heterojunction, polythiophene, fullerene, electrochemistry

ABSTRACT. Bulk heterojunction (BHJ) strategy requires morphology of wide area donoracceptor interfaces with high charge carrier mobilities through the bicontinuous charge transporting layers.

Here, we report formation of well-defined bulk heterojunction (BHJ)

microspheres from regiorandom poly(hexylthiophene) (rra-PHT)/phenyl-C61-butyric acid methyl ester (PCBM) mixture by a vapor diffusion method. By electrochemical oxidation, the BHJ microsphere exhibits enhanced generation of PHT cation species due to increased hole-transport property in comparison with a solution-cast film derived from rra-PHT/PCBM mixture without microsphere morphology. Photoconductivity and electrochemical stability of the microsphere are comparable or even higher than a cast film of irregular aggregates of regioregular P3HT.


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Conjugated polymer/fullerene donor-acceptor (DA) systems have been the focus of recent research attention with advantages of flexibility and wide-area processability at low cost.1– 3

Therein, optimization of the appropriate bulk heterojunction (BHJ) morphology is required for

obtaining

better

electronic,

optoelectronic

and

catalytic

properties.4–8

Poly(3-

hexylthiophene)/phenyl-C61-butyric acid methyl ester (P3HT/PCBM) BHJ is one of widely studied DA model systems because of their benefits such as high charge carrier mobility, charge separation efficiency, solution processability, and miscibility.3,9–11 Typical control factors that affect the BHJ are their morphology and the blending ratio of P3HT and PCBM.10,12–14 Therefore, it is necessary to achieve a morphology that gives both high charge carrier mobility and high charge separation efficiency, leading to generation of large number of P3HT cation species (holes) at the BHJ interfaces, making it valuable for numerous applications such as photovoltaics, electrocatalysis, and photocatalysis.14 For photocatalytic applications, in 2015, Haro et al. and Bourgeteau et al., reported on photoelectrochemical hydrogen evolution with P3HT/PCBM BHJ.15,16 The BHJ layer acts as photocathodes, where holes and electrons are efficiently generated through photoinduced charge separation at the D/A interfaces, and the generated electrons transport to the surface of the cathodes (TiOx or MoS3) and reduce protons to generate hydrogen gas. The polymer/fullerene BHJ photocatalytic layers are advantageous, because these layers are noble-metal free, solution processable, high absorptivity for visible light, and rather stable against photoirradiation. Also, there are several reports on conjugated polymer photocatalysts and electrocatalysts.17,18 Therein, microporous morphology has large surface area, which is important to efficiently access and reduce protons and generate hydrogen.


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In this paper, we fabricate BHJ microspheres from regiorandom poly(hexylthiophene) (rra-PHT, 1) and PCBM (2) (Figure 1a). The resultant microspheres display elongated photocarrier lifetime (), and the charge carrier mobility is nearly comparable with highcrystalline regioregular P3HT (rr-P3HT).19–21 Electrochemical measurements clearly reveal that the generation of PHT cation species in the BHJ microspheres is highly enhanced in comparison with the irregular aggregates of a drop-cast film from solution of rra-PHT/PCBM mixtures.22 Furthermore, the BHJ microspheres show better tolerance property against degradation by water as compared to microspheres made of rra-PHT only. The BHJ microspheres with a long photocarrier lifetime, high conductivity, and better tolerance are valuable for application to photo- and electrocatalysis. For fabrication of microspheres, rra-PHT 1 is utilized, because the low crystallinity by the regiorandom side chains is favorable for the formation of microsphere morphology.19–21 This is in sharp contrast to rr-P3HT with a rigid π-conjugated planes and high crystallinity, which hampers the formation of amorphous microspheres.20 Microspheres of 1/2 blend were prepared by a slow diffusion of MeOH vapor into a CHCl3 solution containing 1 and/or 2 in different weight ratios (1/2 = 10/0, 8/2, 5/5, 2/8, 10/0 w/w). For self-assembly, 5-mL vial containing 2 mL of CHCl3 solution of 1/2 (total concentration: 0.5 mg mL–1) was placed in a 50-mL vial containing 5 mL of MeOH (Figure 1b). After 3 days of vapor diffusion, precipitates were produced in the weight fraction of 2 ranging from 0 to 0.5 (1/2 = 10/0 to 5/5). The color of the precipitate changed from orange to brown as the fraction of 2 increased. Meanwhile, further increase of the fraction of 2 to 0.8 (1/2 = 2/8) and 1 (1/2 = 0/10) hardly gave precipitates. Scanning electron microscopy (SEM) showed that the precipitates with 1/2 = 10/0, 8/2, and 5/5 formed well-defined microspheres (Figure 1c–e, hereafter, these microspheres are


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abbreviated as MS10/0, MS8/2, and MS5/5, respectively). The resultant spherical structure preserved after removal of the solvent in the isolation procedure. Transmission electron micrograph (TEM) clearly displays that the resultant spheres are filled, not hollow (Figure S1 in the Supporting Information (SI)). On the other hand, the mixing ratio 1/2 of 2/8 and 0/10 hardly afforded microsphere but only resulted in the formation of irregular aggregates during the drying process (Figure 1f and g, these aggregates are abbreviated as IA2/8 and IA0/10, respectively). Judging from the absorbance of the dissolved precipitates in CHCl3, the actual weight ratios of 1/2 in the precipitates for the initial ratio of 8/2, 5/5, and 2/8 were determined as 6.7/3.3, 3.3/6.7, and 0.5/9.5, respectively (Figure S2 in SI). These results indicate that not all of 1 is included in the blend microsphere and some portion of 1 deposit at the inside of the vial as a red-colored layer. Also note that a mixture of rr-P3HT/PCBM hardly gives microspheres by the vapor diffusion method but just affords irregular aggregates (Figure S3 in SI).


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Figure 1. (a) Molecular structures of rra-PHT (1) and PCBM (2). (b) Schematic representation of the vapor diffusion method. (c–g) SEM micrographs of the resultant precipitates with the mixing ratio (1/2) of 10/0 (c), 8/2 (d), 5/5 (e), 2/8 (f), and 0/10 (g), and the schematic representations of the self-assembled precipitates with microsphere morphology (MS10/0, MS8/2, and MS5/5) and irregular aggregates (IA2/8 and IA0/10).

In order to investigate the redox properties of the microspheres, cyclic voltammetry (CV) experiments were conducted. As a control, solution-cast films of 1 (SF10/0) and 1/2 blend with the weight ratio 1/2 = 8/2 (SF8/2) were also prepared. As a working electrode, glassy carbon electrode (GCE,  = 1.6 mm) was used, which was coated with 10 µg of 1 (9.1, 11.1, 13.2, 16.7, 20, and 20 µL of MS10/0, MS8/2, MS5/5, IA2/8, SF10/0, and SF8/2, respectively), as determined by photoabsorption measurements. Figure 2a shows CV profiles of MS10/0 (red) and SF10/0 (black), where significant difference is observed in the oxidation waves at 0.6 V. For MS10/0, oxidation of 1 resulted in the generation of their cation species (1+) with the total charge (Q) of 88.7 µC in the potential range of 0.45–0.90 V. The baseline-subtracted peak current (Ip) value was 36.4 µA at 0.63 V, which corresponds to the ionization energy (Ei) of 4.63 eV.10 The Q value for MS10/0 was 20 times greater than that of SF10/0 (4.5 µC) with Ip of 0.42 µA at 0.87 V, corresponding to the Ei value of 4.87 eV (Figure 2a inset). These results indicate that the microsphere morphology can generate 20 times greater number of 1+ species with less ionization energy. The BHJ microspheres MS8/2 affords a much greater Q value of 125.1 µC with Ip of 56.8 µA at 0.65 V and Ei of 4.65 eV (Figure 2b). The sharper oxidation peak for MS8/2 as compared to MS10/0 indicates that PHT cation species form with less resistance.23 By blending PCBM into the


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microsphere, the electrochemical properties were preserved or even superior to that of the microsphere of 1. In contrast, SF8/2 showed a higher average Ei value of 4.75 eV with Q of only 5.4 µC with Ip of 0.2 µA at 0.9 V (Figure S4 in SI). On the other hand, MS5/5 and IA2/8 showed very small Q values (7.1 µC and 5.5 µC, respectively) with high Ei (4.80 and 4.70 eV, respectively, Figure 2b). The high Ei for MS5/5 should be derived from the larger amount of 2 in the microsphere, especially at the surface, which disturbs the electrochemical oxidation of 1. These results support that the well-dispersed microspherical morphology with smooth surface results in the efficient production of 1+ species. X-ray photoelectron spectroscopy (XPS) displays clear difference between microspheres and solution-cast films. Figure 2c shows S2p band of the sample SF10/0 and microspheres MS8/2 and MS10/0. The spectrum is fitted with the Gaussian doublet in the ratio of 2:1, with S2p3/2 and S2p1/2 peaks separated by 1.2 eV. The strong negative shift of 0.6 eV is observed for S2p doublet for the microspheres compared with the cast film, indicating that the HOMO level for the microsphere is closer to the Fermi level.24 It explains the high production of PHT+ cations in the microsphere morphology. The enhancement of the formation of 1+ in the microspheres is attributed to the increased hole transport property in the microsphere morphology, schematically shown in Figure 2d. Because the microspheres are produced as a thermodynamic product under slow precipitation process, the process allows the high degree of interchain packing, resulting in the increased hole hopping ability in comparison with the kinetically produced solution-cast film. Broadening of the S2p peak was observed for MS8/2, indicating that the S2p orbital in PHT is much delocalized.25 The possible reason of the delocalization in MS8/2 is attributed to the weak interactions of electron-donating PHT with closely placed electron-accepting ability of PCBM, leading to enhanced hole-generation for MS8/2 (Figure 2b). MS8/2 also shows reduction


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in the size of the microspheres (average diameter, dav = 2.1 µm) as compared to MS10/0 (dav = 2.4 µm, Figure S3).

Figure 2. (a) Cyclic voltammograms of MS10/0 (red) and SF10/0 (black) in 0.1 M TBAPF6 in acetonitrile. Inset shows a magnified CV profile of SF10/0. (b) Cyclic voltammograms of MS8/2 (red), MS5/5 (blue), and IA2/8 (green) in 0.1 M TBAPF6 in acetonitrile. Inset shows magnified CV profiles of MS5/5 (blue) and IA2/8 (green). (c) XPS spectra of the raw data (black dotted line), smoothed data (red line) and fit S2p doublet (green and purple lines) of MS8/2, MS10/0 and SF10/0.


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(d) Schematic representation of the cross-section of the microsphere in the electrochemical oxidation process, in which holes generate and delocalize subsequently.

In order to confirm the behavior of the charge carriers, flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements were carried out. TRMC is an electrodeless method, which gives information on the conductivity and lifetime of photogenerated charge carriers.26 Upon laser excitation at 355 nm, the microspheres show higher values of µmax (the product of photon-to-carrier conversion efficiency and the sum of charge carrier mobilities) than solution-cast films (Figure 3a–c and Table 1). Furthermore, photocarrier half-lifetimes (1/2) of the microspheres, defined as the time where the TRMC intensity decreases to half of its maximum value, were 700–4,700 times longer than those of the solution-cast films (Figure 3 and Table 1). These results coincide well with the CV results, where microspheres possess higher charge carrier mobility that benefits a long-range diffusion and reduced recombination, leading to the large values of Q by the electrochemical oxidation. Although the Q values of the microspheres in the redox cycle were almost identical (Qox = 88.7 µC and Qred = 86.2 µC for MS10/0, Qox = 125 µC and Qred = 121.7 µC for MS8/2), the shape of the oxidation and reduction waves was not symmetric (Figure 2a and b): the reduction peak was much broader with less Ip than the oxidation peak. This asymmetric redox behavior indicates that the oxidation of 1 to 1+ occurs much easier than the reduction of 1+ to 1. A possible reason is that the electrochemical oxidation of the microsphere generates delocalized holes in the πconjugated system, which flattens the neighboring π-conjugated plane and stabilizes the


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intermolecular π-stacking. As a result, reduction of 1+ needs larger energy than the oxidation of the microsphere.

Figure 3. (a–c) FP-TRMC profiles of cast films of microspheres (red) and solution cast films (black) with the mixing ratio (PHT/PCBM) of 10/0 (a), 8/2 (b), and 5/5 (c). (d) Bar chart of 1/2 for cast films of microspheres (red) and solution cast films (black). Table 1. Summary of µmax, 1/2, and Q of Microspheres and Solution-Cast Films of 1/2 Blends Obtained by FP-TRMC and CV Measurements Microspheres 1/2 ratio

Solution-Cast Films

µmax (10–4 cm2 V–1

µmax 1/2 (µs)

Q (µC)

(10–4 cm2 V–1

s–1) 10/0

2.7

s–1) 320

88.7

0.48

1/2 (µs) 0.34

Q (µC) 4.5


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8/2

3.0

1400

125

0.35

0.30

5.4

5/5

1.7

670

7.1

0.50

0.92

4.1

MS10/0 is electrochemically stable during 30 redox cycles (Figure 4a). On the other hand, MS8/2 was less stable, showing sharp decline of Ip during 30 redox cycles (Figure 4b). With regard to improving the stability, we found that microspheres produced by a fluorinated vial have higher sphericity with narrower size distribution as compared to those prepared by a normal vial, which enhances electrochemical stability. dav of the resultant microspheres was 2.1 µm with the standard deviation () of 0.4 µm. The  value is much smaller than the microspheres prepared using a normal vial without fluorination treatment ( = 0.8 for microspheres with dav = 2.1 µm, Figure S5 in SI). Microspheres with better monodispersity is attributed to the fluorophilic nature of the fluorinated vials with less interactive forces between the vial wall and polymers. The microspheres with narrow size distribution, prepared with the fluorinated vial, displayed much sharper oxidation waves with higher electrochemical stability by the redox cycles than those prepared with a normal vial (Figure 4c). For comparison, a solution-cast film of highly crystalline rr-P3HT showed higher Ip value due to the higher carrier mobility than the microspheres of 1 (Figure 4d). However, the electrochemical stability of the cast film of rr-P3HT was inferior to that of the microspheres of regiorandom 1, for which the Ip value decreased and the peak potential shifted to the higher value during redox cycles. These results further suggest that microsphere morphology is advantageous for the electrochemical stability against oxidative decomposition of PHT.


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Figure 4. Cyclic voltammograms of MS10/0 (a), MS8/2 (b), MS8/2 prepared with a fluorinated vial (c), and irregular aggregates of regioregular P3HT (d) with 1st (black), 20th (red) and 30th (blue) cycles in acetonitrile containing 0.1 M TBAPF6 as an electrolyte.

Finally, we investigated the tolerance properties of the polymer microspheres against degradation by water.27,28 The Ip value of MS10/0 was 36.4 µA (Qred = 89 µC) for dry acetonitrile, which was largely reduced by one third to 12.6 µA (Qred = 22 µC) for acetonitrile containing 3% of water (Figure 5a and c). In contrast, the BHJ microspheres MS8/2 shows higher tolerance against the degradation by water; the Ip value was 56.8 µA (Qred = 125 µC) for dry acetonitrile, while that still shows as high as 30.2 µA (Qred = 46 µC) for acetonitrile containing 3% of water (Figure 5b and c). Moreover, the oxidation peak is much sharper for the BHJ microspheres than only rra-PHT microsphere, further indicating that formation of PHT cation in MS8/2 offers less


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resistance as compared to MS10/0.23 A possible reason is referred to the PCBM on the surface of the microspheres, which protect PHT from an irreversible oxidation to form sulfoxide group in the thiophene moieties.27

Figure 5. (a, b) Cyclic voltammograms of MS8/2 (a) and MS10/0 (b) in acetonitrile containing 0.1 M TBAPF6 as an electrolyte with different water content: dry (black line), 1.25% (red line), 2.0% (purple line) and 3.0% v/v (pink line). (c) Plots of the amount of generated holes versus water content (vol%) of MS8/2 (black) and MS10/0 (red).

In summary, self-assembly of regiorandom PHT/PCBM blends affords BHJ microspheres that electrochemically produce PHT cation species with lesser energy as compared to the solution-cast film. PHT/PCBM ratio of 8/2 displays the highest degree of the total charge by the electrochemical oxidation. Transient microwave conductivity studies have confirmed that the photocarrier lifetime of the microspheres is 3 orders of magnitude longer than that of the solution-cast films with greater charge carrier mobilities. The BHJ microspheres with narrow size distribution display better electrochemical stability during multiple redox cycles. Moreover,


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BHJ microspheres show better tolerance against electrochemical degradation by water. Considering the high charge carrier mobility and redox stability, PHT/PCBM BHJ microspheres will be beneficial for constructing efficient and durable electro- and photocatalytic systems.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Materials and measurements, photoabsorption spectra, CV, SEM (PDF)

AUTHOR INFORMATION Corresponding Author *To whom correspondence should be addressed. [email protected] Funding Sources JSPS (JP17H05142, JP16H02081) University of Tsukuba Pre-strategic initiative TIA Kakehashi Asahi Glass Foundation


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ACKNOWLEDGMENT The authors thank Dr. Kentaro Tashiro in National Institute for Materials Science (NIMS), Japan, for his valuable discussion. This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas "π-System Figuration" (JP17H05142), and Scientific Research (A) (JP16H02081) from Japan Society for the Promotion of Science (JSPS), University of Tsukuba Pre-strategic initiative “Ensemble of light with matters and life”, TIA Kakehashi, and Asahi Glass Foundation.

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TOC Figure


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Significant Enhancement of Hole Transport Ability in Conjugated

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