Influence of molecular weight on silole-containing cyclopentadithiophene polymer and its impact on the electrochromic properties

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Inﬂuence of molecular weight on silole-containing cyclopentadithiophene

polymer and its impact on the electrochromic properties

Jen-Hsien Huang

a

, Annie Tzuyu Huang

a

, Chih-Yu Hsu

b

, Jiann-Tsuen Lin

b

, Chih-Wei Chu

a,c,n a

Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan b_{Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan}

c

Department of Photonics, National Chao-Tung University, Hsinchu 30010, Taiwan

a r t i c l e

i n f o

Article history: Received 7 July 2011 Received in revised form 2 November 2011

Accepted 11 November 2011 Available online 3 December 2011 Keywords: Electrochromic Polymer Molecular weight Electrochemistry Mobility

a b s t r a c t

In this article, we have systematically studied the effect of molecular weight on poly[(4,40 -bis(2-ethylhexyl)dithieno[3,2-b:20_,30_{-d]silole)-2,6-diyl-alt-(5,5}0_-thienyl-4,40_-dihexyl-2,20_{bithiazole)-2,6-diyl],} a novel cathodically coloring electrochromic polymer. The polymer with higher molecular weight can stack much better due to the strongerp–pn

interaction and was found to exhibit better charge transfer property and lower internal resistance. These results have shown to enhance the coloration efﬁciency of the polymer remarkably. For the molecular weight of 33.6 kg/mol, the coloration efﬁciency is 423 cm2_/C, much higher compared with the low molecular weight polymer (6.3 kg/mol, 226 cm2_/C).

1. Introduction

Conjugated polymers have received much attention through-out the course of the past two decades, stemming not only from their high conductivity in the doped state but also from a variety of optoelectronic and redox properties. One of these properties is electrochromism defined as a reversible optical absorbance/ transmittance change in response to an externally applied poten-tial. Compared with inorganic materials, conjugated polymers possess many superior properties over the counter part, including rapid response times, larger contrast, lower power requirements, multicolor, higher coloration efficiency (CE) and the capability to structurally modify the chemical structure to achieve specific color or function[1–6].

Among these properties, CE is the most important parameter for electrochromic materials deﬁned by the relationship between the injected/ejected charge, electrode area and the change in optical density (

D

OD) at a speciﬁc wavelength. An ideal or high CE electrochromic material should exhibit a large transmittance change (

D

%T) with a small injected/ejected charge. Recently, CE of conjugated polymers can be enhanced by blending carbon nanotubes or other conducting materials into the polymer matrix.

The resulting conductive network can lead to lower internal resistance and give rise to larger CE [7–10]. Alternatively, the improvement of CE can also be achieved through fabrication engineering, i.e. by controlling the growth rate of polymer films that can increase the degree of packing for polymer chains. Consequently, the unique stacking properties of conjugated poly-mers can result in higher charge mobility and reduced resistance thus leading to high CEs[11]. Besides these methods, another influenced factor comes from the regioregularity of the polymer chains. Jain et al. have reported that the regioregular water-soluble conjugated polymer exhibits better CE compared with the corresponding water-soluble regiorandom polymers[12]. Intrin-sically, the mechanism for the enhancement of CE is very similar either by controlling the film growth rate or the regioregularity of the polymer chains. These two methods both enhance the poly-mer chain packing and therefore better optoelectronic properties by forming stronger chain interactions. This indicates that the interaction between polymer chains is critical and it is important to explore the effect of molecular weight (MW) on the polymer based electrochromic properties.

Recent studies have shown that the variation of polymer MW can cause significant impact on the optoelectronic device perfor-mance. Kline et al. have concluded that poly(3-hexylthiophene) (P3HT) based field effect transistor with higher MW have higher mobility because the regions of crystalline order are less well defined[13]. The morphology and performance of bulk hetero-junction solar cells comprised of polythiophene and polyfluorene Contents lists available atSciVerse ScienceDirect

journal homepage:www.elsevier.com/locate/solmat

Solar Energy Materials & Solar Cells

n

Corresponding author at: Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan.

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based polymer have been investigated with different MWs. It was concluded that devices made of higher MW polymer exhibit higher performance because of the longer conjugation length of the polymer chain and better interconnections within the bicon-tinuous network[14–16]. Hosoi et al.[17]also fabricated poly-meric light-emitting diodes using poly(9,9-dioctylﬂuorene) (PDOF) with different MWs. They found that electroluminescence characteristics of devices using the PDOF with the high MW component were superior to those of the devices using PDOF with the low MW component [17]. However, the effect of MW on electrochromic properties is still not clear. It has been shown that, at similar thicknesses, the cyclopentadithiophene (CPDT)-based polymers have higher optical density change and CE compared to PEDOT and its derivatives[18,19]. It will be interesting to know the correlation between the MW and electrochromic properties. Here, we synthesized poly[(4,40

-bis(2-ethylhexyl)dithieno[3,2-b:20_,30_{-d]silole)-2,6-diyl-alt-(5,5}0_-thienyl-4,40_-dihexyl-2,20

-bithia-zole)-2,6-diyl] (Si-PCPDTTBT) with various MWs to investigate the impact of MW on its electrochromic properties.

2. Experimental 2.1. Synthesis

Monomers M1 and M2 were synthesized according to a reported procedure[20,21], and the characterization is described as follows. The synthetic route of copolymer Si-PCPDTTBT is shown inScheme 1, and the procedure is described as follows:

M1. Yield: 65%.1H NMR:

d

7.03 (d, 2H), 6.92 (d, 2H), 2.86 (t, 4H), 1.72 (m, 4H), 1.40–1.15 (m, 12H), 0.86 (t, 6H). EI-MS: m/z 656. M2. Yield: 92%.1_{H NMR:}

_d

_{7.06 (s, 2H), 1.68 (m, 2H), 1.4–}

1.13 (m, 16H), 0.90 (t, 6H), 0.83 (t, 6H), 0.74 (m, 4H), 0.32 (s, 18H). A solution of M1 (2.00 g, 2.68 mmol) and M2 (1.70 g, 2.68 mmol) in toluene (40 mL) was purged with Ar for 10 min and then Pd(PPh3)4 (0.150 g, 0.134 mmol) and

triphenylpho-sphine (0.21 g, 0.80 mmol) were added. After purging with Ar for 20 min, the mixture was heated under reﬂux for 48 h in an oil bath set at 110 1C under an Ar atmosphere. The mixture was cooled to room temperature, MeOH (100 mL) was added, and the precipitated polymer was ﬁltered out. After Soxhlet extraction with MeOH, hexane and CHCl3, the polymer was recovered from

the CHCl3 phase through reprecipitation with MeOH and then

dried under vacuum for 1 day (40%). The MW of polymer was controlled by changing the reaction time, which was monitored by GPC. In this study, Si-PCPDTTBT with four different MWs was synthesized. The reaction times were 6, 10, 24 and 48 h respec-tively. The detail information of the MWs for the as-prepared polymers are shown inTable 1.

2.2. Polymer thin-ﬁlm deposition

Prior to deposition, Si-PCPDTTBT was dissolved in 1,2,-dichlor-obenzene and then stirred for 12 h at 50 1C. Before polymer deposition, the ITO glasses (2 2 cm2_{) were ultrasonically}

cleaned in detergent, de-ionized water, acetone and isopropyl alcohol before the deposition. After routine solvent cleaning, the substrates were treated with UV ozone for 15 min. Consequently, the polymer ﬁlms were deposited through spin coating with a spin rate of 2000 rpm.

2.3. Characterization

Thermal gravimetric analysis (TGA) was conducted to character-ize the thermal stability of Si-PCPDTTBT powder using DuPont 951 from room temperature to 850 1C with a heating rate of 10 1C min1

in nitrogen atmosphere. Spectroelectrochemical data were recorded using a Shimadzu UV-1601PC spectrophotometer. Surface morphol-ogies of thin ﬁlms were obtained using atomic force microscopy (AFM, Digital instrument NS 3a controller with D3100 stage). X-ray diffraction (XRD) analyses were performed using a Philips X’Pert/ MPD instrument. For EIS analysis, the P3HT electrodes were

N S N S C6H13 C₆H₁₃ S S Br Br Si S S Sn Sn N S N S C6H13 C6H13 S S Si S S n Pd (0) Si-PCPDTTBT M1 M2

Scheme 1. Synthetic route of Si-PCPDTTBT copolymer. Table 1

Summary of the MWs, polydispersities (PDI) and yields of Si-PCPDTTBT samples in this study.

Sample of Si-PCPDTTBT Mna(kg/mol) Mwa(kg/mol) PDI Yield (%)

Low MW 6.3 12.2 1.93 61

Medium MW 12.7 15.7 1.23 55

22.9 27.3 1.19 57

High MW 33.6 38.9 1.15 56

a_{Molecular weights and polydispersity were measured by GPC, using THF as} an eluent, polystyrene as a standard. Mn—number average molecular weight. Mw—weight average molecular weight.

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characterized using a three-electrode system (the potentiostat mentioned above) equipped with an FRA2 module in the presence of 1.0 mM redox couples and 0.1 M LiClO4in aqueous solution. The

impedance spectra were recorded at the formal potential of the redox couple in the frequency ranging from 100 to 10,000 Hz. The impedance spectra were ﬁtted to an equivalent circuit model proposed by Sundfors et al.[22]and the model parameters were obtained using ZView software. Colorimetry measurements were performed using a Minolta CS-100 Chroma Meter; the sample was illuminated from behind using a D50 (5000 K) light source. A background measurement was taken from blank ITO in an electro-lyte solution held in a standard quartz cuvette.

3. Results and discussion

Table 1summarizes the MWs, polydispersities (PDI) and yield

of the Si-PCPDTTBT samples used in this study. It should be mentioned that the solubility of Si-PCPDTTBT becomes very poor with MW larger than 33.6 kg/mol (Mn). FromTable 1, it can be

found that increasing Si-PCPDTTBT MW also leads to lower PDI. In general, polymer with larger PDI would results in amorphous structure. This is due to the polymer chains disruption contrib-uted from the smaller polymer segment. However, for large MW polymer, the narrow PDI can enhance the crystallinity due to the better polymer chain packing [23]. TGA thermograms recorded under nitrogen atmosphere for the Si-PCPDTTBT with various MWs are shown inFig. 1. A drastic weight loss was observed in the temperature range of 400–500 1C for the Si-PCPDTTBT fol-lowed by a continuous gradual weight loss. In addition, the onset temperature of 10% weight loss for the polymer is found to increase with increasing MW. The decreasing thermal stability with the decreasing molecular weight is expected as it has been observed in many polymer systems.

The absorption spectra of the Si-PCPDTTBT ﬁlms with different MWs are shown inFig. 2. The absorption peaks show a trace of red shift in Si-PCPDTTBT thin ﬁlm. The corresponding images for the polymer solution with various MWs are shown in the inset of

Fig. 2. The conjugation length of the polymer increases with its

MW. Therefore, the extent of red shift in the absorption spectrum increases with its MW as well. Moreover, the vibronic peak appeared in the high MW Si-PCPDTTBT ﬁlm indicated the forma-tion of

p

–

p

stacking resulting in an ordered structure. The conformation of ordered structured facilitates the delocalization of

p

-electrons and an increase in conjugation length that results in a red shift of absorption spectrum.

Fig. 3 presents the optoelectrochemical spectral series

obtained from Si-PCPDTTBT ﬁlms with different MWs while changing from the reduced state to the fully oxidized state. In the neutral state, the polymer showed a

l

maxof 520 nm because

of the

p

–

p

n

transition. An optical band gap of 1.82 eV was calculated from the onset of the

p

–

p

n

transition. Upon stepwise oxidation, the reduction in absorption due to the decrease in

p

–

p

n

transition results in a very transmissive polymer ﬁlm which indicates that the Si-PCPDTTBT is a cathodically coloring material. The polymer shows a well-deﬁned isosbestic point at around 620 nm, which implies that only one kind of chromophoric species is involved. Moreover, we found that the Si-PCPDTTBT with higher MW exhibits larger optical contrast between dar-kened and bleached states. The

D

ODs of the Si-PCPDTTBT ﬁlms obtained between 0.0 and 1.0 V are 0.23, 0.33, 0.37 and 0.42 for the MW of 6.3, 12.7, 22.9 and 33.6 kg/mol, respectively.

The surface morphology of the Si-PCPDTTBT films was char-acterized by AFM as shown in Fig. 4. The morphologies of Si-PCPDTTBT films are distinctly different while varying the MWs. At low MW, the sample exhibits smooth surface topogra-phy with featureless morphology. With the increasing of the MWs, the surface morphologies become more and more uneven which may be originated from the enhancement of interchain stacking. The polymer films with 6.3 kg/mol, 12.7 kg/mol, 22.9 kg/mol and 33.6 kg/mol MW exhibit a root-mean-square sur-face roughness of 1.3 nm, 1.6 nm, 2.1 nm and 2.7 nm, respectively. The images suggest that the higher MW samples are more crystalline. 200 20 30 40 50 60 70 80 90 100 110 6.3 kg/mol 12.7 kg/mol 22.9 kg/mol 33.6 kg/mol Weight (%) Temperature (°C) 90 % Weight Loss 400 600 800

Fig. 1. Thermal stability of Si-PCPDTTBT with various MWs.

Low MW High 400 6.3 kg/mol 12.7 kg/mol 22.9 kg/mol 33.6 kg/mol Absorbance (a.u.) Wavelength (nm) 500 600 700 800

Fig. 2. Optical absorption of Si-PCPDTTBT with various MWs. The red-shift of the absorption indicates the larger orbital delocalization by virtue of longer conju-gated polymer chains (inset: the corresponding solution images). (For interpreta-tion of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

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To further investigate the crystallinity of the polymer ﬁlms, XRD was used to study the property of polymer ﬁlms. As shown in

Fig. 5a, the low MW Si-PCPDTTBT exhibits an amorphous pattern

indicating the disordered structure. Increased intensity is observed with increasing MW in the peak at 2

y

¼6.171, which corresponds to the layering distance, d100¼14.3 ˚A, between the

sheets of Si-PCPDTTBT chains associated with the plane perpen-dicular to their longitudinal axes. The increase of crystallinity can enhance charge transfer within the polymer ﬁlms because of the reduction of charge transfer barrier [15,24–28]. This can be conﬁrmed by the measurement of charge mobility as shown in

Fig. 4b. Here, we measure the hole mobility of Si-PCPDTTBT by

fabricating the hole only devices. The structure of hole only device is shown in the inset ofFig. 5b. To block the electron current, the polymer films were spin coated on the poly(ethylenedioxythio-phene):polystyrenesulfonate modified ITO substrate and capped with V2O5/Al. The curves were fitted with space-charge limited

current model to calculate charge mobility. The calculation used was J¼9

e0erm

V2_/8L3 _[29]_{, where}

_e0er

_{is the permittivity of the}

polymer,

m

is the carrier mobility and L is the device thickness. With the increasing of the MW, the calculated charge mobilities of Si-PCPDTTBT are 7.2 108_{, 8.3 10}8_{, 1.0 10}7 _{and 1.2}

107_m2_{/V s, respectively. The higher mobility of Si-PCPDTTBT is}

attributed from the stronger crystallinity and more favorable morphology.

To investigate the kinetics of the electron transfer at the polymer–solution interface, we characterized the polymer ﬁlms using electrochemical impedance spectroscopy (EIS). Fig. 6 dis-plays the impedance spectra of the polymer ﬁlms characterized under 1.0 mM K4Fe(CN)6, K3Fe(CN)6 and 0.1 M LiClO4 as the

supporting electrolyte. We ﬁtted the EIS data with the equivalent circuit [22] presented in the inset of Fig. 6. In the equivalent circuit, Rs is the solution resistance, Rct is the charge transfer

resistance due to electron transfer at the polymer–solution inter-face, Zwis the inﬁnite-length Warburg diffusion impedance due to

diffusion of the redox couple in the solution, Cdis the electronic

capacitance of the polymer ﬁlm and Zd is the ﬁnite-length

Warburg diffusion impedance due to diffusion of charge compen-sating counterions in the polymer film. The fitting results are presented inFig. 6, where the solid lines represent the modeling results obtained by fitting the dotted experimental data. The fitted results suggest that the values of Rctof the Si-PCPDTTBT are

1203, 957, 894 and 427

O

for the increasing of the MW from 6.3 to 33.6 kg/mol. The small value of Rctof the high MW ﬁlm was

resulted from its highly ordered structure, which provided a larger charge mobility, leading to a lower resistance at the

400 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.0 V 0.2 V 0.4 V 0.6 V 0.8 V 1.0 V Absorbance Wavelength (nm) 515 nm 400 0.0 0.1 0.2 0.3 0.4 0.5 _{0.0 V} 0.2 V 0.4 V 0.6 V 0.8 V 1.0 V Absorbance Wavelength (nm) 520 nm 400 0.0 0.1 0.2 0.3 0.4 0.5 _{0.0 V} 0.2 V 0.4 V 0.6 V 0.8 V 1.0 V Absorbance Wavelength (nm) 530 nm 400 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.0 V 0.2 V 0.4 V 0.6 V 0.8 V 1.0 V Absorbance Wavelength (nm) 560 nm 610 nm 500 600 700 800 500 600 700 800 500 600 700 800 500 600 700 800

Fig. 3. In situ UV–vis absorption spectra of ﬁlms of the Si-PCPDTTBT with various MWs at various potentials (0.0–1.0 V vs. Ag/Agþ

) in 0.1 M LiClO4/acetonitrile: (a) 6.3 kg/mol, (b) 12.7 kg/mol, (c) 22.9 kg/mol and (d) 33.6 kg/mol.

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polymer–electrolyte interface. We obtained the rate constant of the electron transfer (k0) using k0¼((RT)/(n2F2RctAC))[22], where

A is the area of the electrode, C is the bulk concentration of the redox couple and other symbols have their usual electrochemical meanings. Using the known values of Rct, we calculated the values

of k0 for the polymer ﬁlms to be 2.95 104, 3.71 104,

3.97 104 _{and 8.31 10}4_{cm/s while the MW is increased}

from 6.3 to 33.6 kg/mol. Thus, the Si-PCPDTTBT with higher MW results not only a larger mobility but also a decreased value of Rct, that leads to a larger value of k0.

Fig. 4. The AFM images of Si-PCPDTTBT ﬁlms with various MWs: (a) 6.3 kg/mol, (b) 12.7 kg/mol, (c) 22.9 kg/mol and (d) 33.6 kg/mol.

0.01 1E-3 0.01 0.1 1 10 100 6.3 kg/mol 12.7 kg/mol 22.9 kg/mol 33.6 kg/mol Curent density (A/m 2) Voltage (V) 4 33.6 kg/mol 22.9 kg/mol 12.7 kg/mol 6.3 kg/mol

Diffraction Intensity (a.u.)

2

5 6 7 8 9 10 _0.1 ₁

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The

D

OD as a function of charge ingress/egress are shown in

Fig. 7at a monochromatic wavelength of 550 nm. The relationship

was determined by switching voltages between a ﬁxed darkened

state (0.1 V) and different bleached states. By calculating the slope of

D

OD vs. charge plot, the CEs of Si-PCPDTTBT can be determined. The calculated CE of 33.6 kg/mol Si-PCPDTTBT is 423 cm2/C, which is much higher than that of 6.3 kg/mol one (226 cm2_/C).

As described above, the crystalline structure of the high MW Si-PCPDTTBT is responsible for its larger charge mobility and lower value of Rct at the polymer ﬁlm-electrolyte interface. These proper-ties can minimize the inject/eject charge. Moreover, the

D

OD also increases with increasing MWs as shown in Fig. 3. These factors both enhance the CE for higher MW Si-PCPDTTBT.

For electrochromic switching studies, polymer ﬁlms were coated on ITO-coated glass slides in the same manner as described above. The transmittance in response to the potential was repeatedly switched between 0 and 1.0 V (10 s for each). The 33.6 kg/mol Si-PCPDTTBT delivers a

D

%T of 48% at 580 nm. As depicted inFig. 8, the Si-PCPDTTBT thin ﬁlm reveals a switch-ing time of 2.2 s at 1.0 V for the bleached process and 2.4 s for the darkened process. The ratio of the bleached and the darkened switching time is 91.7%, indicating that charge injection/ejection was reversible during electrochemical reactions.

In addition to the optical spectra, we also performed colori-metry analysis, which provided a more precise way than optical spectra to deﬁne color. The CIE 1931 color space coordinates of the Si-PCPDTTBT with the applied voltage of 0 V and 1.0 V are determined to be (0.2521, 0.1121) and (0.2491, 0.2687), respec-tively. The polymer switched from deep purple neutral state at 0 V to pale-blue oxidized state at 1.0 V are shown in the inset of

Fig. 9. The luminance dependence on the applied potential for

Si-PCPDTTBT is also shown in the inset ofFig. 9. As expected for a cathodically coloring material, the luminance increases upon

0 0 200 400 600 800 1000 1200 1400 6.3 kg/mol 12.7 kg/mol 22.9 kg/mol 33.6 kg/mol -Z (Ohm) Z (Ohm) 200 400 600 800 1000 1200 1400

Fig. 6. EIS results of Si-PCPDTTBT ﬁlms with various MWs.

0.0 0.00 0.05 0.10 0.15 0.20 0.25 Δ OD Charge density (mC/cm2) 0.0 -0.05 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 Δ OD Charge density (mC/cm2) 0.0 -0.05 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 Δ OD Charge density (mC/cm2₎ 0.2 0.0 0.1 0.2 0.3 0.4 0.5 Δ OD Charge density (mC/cm2) 0.2 0.4 0.6 0.8 1.0 1.2 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.4 0.6 0.8 1.0 1.2 η = 322 cm2_/C _{η = 423cm}2_/C η = 281cm2_/C η = 226cm2_/C

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oxidation from 25.6% to 66.7%, as the intensity of the

p

–

p

n

transition diminishes.

4. Conclusion

We conclude that the MW of Si-PCPDTTBT has a substantial effect on the way that the polymer chains stacked on each other. This characteristic affects the charge mobility by three times as we increase the Si-PCPDTTBT MW from 6.3 to 33.6 kg/mole.

Moreover, the

D

OD of Si-PCPDTTBT also increases with increasing MW. Combining the two factors, the electrochromic CE can be enhanced from 226 to 423 cm2/C when the MW of polymers is increased from 6.3 to 33.6 kg/mol.

Acknowledgments

The authors are grateful to the National Science Council (NSC), Taiwan (NSC 98-2221-E-001–002 and 99-2120-M-009-008) and Academia Sinica for ﬁnancial support.

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Fig. 8. Optical transmittance changes for the 33.6 kg/mol Si-PCPDTTBT at 580 nm, stepped between 0.0 and 1.0 V (vs. Ag/Agþ

).

Fig. 9. CIE 1931 xy chromaticity diagram for the 33.6 kg/mol Si-PCPDTTBT and the relative luminance (%Y) at applied potentials ranging from 0 to 1.0 V (vs. Ag/Agþ

). The images are the Si-PCPDTTBT ﬁlm under darken and bleached states. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

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