The Dynamic Evolution from Chain Disorder to Order of PTB7

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Applications of Polymer, Composite, and Coating Materials

The Dynamic Evolution from Chain Disorder to Order of PTB7 Condensed State Structures under the External Fields Jiaxuan Ren, Xiaona Li, Tengning Ma, Bin Liu, Hao Zhang, Tao Li, and Dan Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08938 • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on July 30, 2018

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ACS Applied Materials & Interfaces

Abstract: In this research, the effect of external fields (solvent, temperature, solution concentration and external force) on dynamic evolution from chain disorder to order of Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro2-[(2ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]

(PTB7)

condensed

state

structures was explored by UV–vis Absorption Spectra, Atomic Force Microscope (AFM) and Transmission Electron Microscopy (TEM). It was found that PTB7 main chains presented amorphous conformations induced by the poor solvent 1, 2-dichloroethane (DCE). However, the local ordered aggregation appeared in amorphous conformations when the solubility of the poor solvent was again lowered by reducing temperature. It is worth noting that the size of ordered aggregation was further increased with the decrease of solution concentration or increase of external force. It was found that there were two main PTB7 absorption peaks in the UV–vis absorption spectra, we denoted A0−0 for the intensity of the lower energy absorption peak and A0−1 for the intensity of the higher energy absorption peak, respectively. The ratio R=A0-0/A0-1 was used to characterize the dynamic evolution from disorder to order of the PTB7 condensed state structures in absorption spectra. It increased from 0.94 for PTB7 amorphous state to 1.25 for PTB7 large size ordered aggregation. The dynamic evolution from chain disorder to order could also be distinctly observed by TEM. It was inferred that PTB7 condensed state structures (amorphous state, local ordered aggregation and large-scale ordered aggregation) might exist simultaneously because of the complexity of copolymer conformations. This research is meaningful to establish physical basis for the molecule design and the synthesis of materials to

1 Environment ACS Paragon Plus

ACS Applied Materials & Interfaces 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

enhance photoelectronic device efficiency based on condensed matter physics of conjugated polymer. Keywords: PTB7, condensed state structure, from disorder to order, external field, dynamic evolution

1. Introduction Conjugated polymers are widely applied for polymer solar cell (PSC), organic light emitting diode (OLED), field effect transistor (FET) and solid-state laser due to their low cost and processability.1-6 Low bandgap polymers used in PSCs are rapidly developing in recent years with power conversion efficiency (PCE) over 7%.7-10 Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[ (2ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) with structural formula shown in Figure 1 is a kind of representative low bandgap conjugated polymer, which is used for PSC with a certified efficiency of 9.2%.11 It is usually considered that OSC can be widely applied with PCE over 10%.12,13 Therefore, extensive work has focused on PTB7 material synthesis and device fabrication in order to further enhance photoelectronic device performance for the practical application.14-18 As we know, the photoelectronic device performance of conjugated polymers has shown strong dependence on photophysical properties.19-22 Besides chemical structure, the formation of complex condensed state structure and evolution were immediate causes of deciding the device performance. Hence, researching conjugated polymers chain conformation and aggregation was of great significance.23-30 As a kind of soft


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matter materials, in principle, condensed state structure of conjugated polymers can be controlled by external field. External field can induce the change of chain conformation

to

form

more

desirable

microstructure.31-33 Although

some

investigations have focused on the correlation between photophysical properties and aggregation structures induced by external fields,

such as homopolymer

poly(3-hexylthiophene) (P3HT) and poly(9,9-dioctylfluorene) (PFO),34-43 few researches devoted to dynamic evolution of copolymer condensed state structures like PTB7 under the external fields till now. Usually, the photoelectric devices are fabricated by spin-coating films from semidilute solution. In precursor solution, molecular chain conformation and chain aggregation can directly affect the condensed state structure of photoelectronic film.44 In a certain sense, effectively regulating and controlling the external fields in semidilute solutions, such as solvent field,45,46 temperature field, force field and time field (velocity field), etc. is crucial to decide conjugated polymer final condensed state structure and device performance. In this work, we investigated the effect of external fields (solvent, temperature, solution concentration, external force) on the dynamic evolution from chain disorder to order of the PTB7 condensed state structures. The dynamic evolution from chain disorder to order was explored by TEM. The formation conditions and mechanism of the above condensed state structure have been revealed. More details will be discussed below.



Figure 1. Molecular structural formula of PTB7.

2. Experimental Section

2.1. Solution Samples Preparation PTB7 was purchased from 1-Materials, Canada. The average molecular weight was 125,000 g/mol and the polydispersity index was 2.5. Three kinds of solvents, chlorobenzene (CB), 1, 2-dichloroethane (DCE) and chloroform (TCM) were produced from the Beijing Chemical Company, China, which were all chromatographically pure. PTB7 (CB) and PTB7 (DCE) solutions with concentration of 20 mg/mL, 15 mg/mL and 10 mg/mL, PTB7 (TCM) solution with concentration of 20 mg/mL were prepared respectively. And PTB7 (CB) dilute solution with concentration of 0.006 mg/mL has also been prepared in order to verify the experimental conclusion. All PTB7 solutions were stirred at 340 K in the dark for 12 h to dissolve PTB7 better. Quartz substrates (25 mm2) were dipped in a Piranha solution (H2SO4:H2O2=3:1) for 4 h and then cleaned in ultrasonic baths by using deionized


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water, ethanol, isopropanol, acetone, respectively and finally dried with nitrogen gas. All of the films were prepared by spin coating on quartz plate for 2 min at room temperature.

2.2. UV–Vis Measurement The UV–vis absorption spectra were obtained by the Shimadzu UV-3000 spectrophotometer, Japan. Film samples were characterized through transparent quartz plate and the 0.006 mg/mL diluted solution sample was measured through quartz cell with 1 cm path length. The tested spectral range was 300-800 nm. All test results were normalized at the top of the highest absorption peak.

2.3. Atomic Force Microscope (AFM) Measurement The morphology of PTB7 film was performed using the SII SPA-300 AFM measurement in tapping mode. The root mean square (RMS) surface roughness was measured in different solvents. The test range was 3×3 µm.

2.4. Transmission Electron Microscope (TEM) Measurement The images of high resolution transmission electron microscope (HR-TEM) were carried out through JEM-2100F, Japan. The magnified lattice fringes can illustrate the degree of polymer orderness.

3. Results and Discussion



3.1. The Effect of Solvent on PTB7 Condensed State Structure Solvent field was a kind of external field whose change can greatly affect polymer physical properties, it has been investigated in lots of researches.47-50 Polymer chains could be well dispersed in good solvent. However, in poor solvent, the attraction between chains was stronger than that between chain and solvent. It could result in the formation of molecular chain aggregation.51 Induced by poor solvent, PFO β-conformation appeared with ordered aggregation, which could enhance the charge carrier mobility.37-40,44 Formed P3HT nanorods or nanofibrils in poor solvent gave rise to higher charge carrier mobility and solar conversion efficiency.34 However, few investigations have been done on effect of poor solvent on PTB7 condensed state structure until now. 3.1.1 The exploration on good and poor solvents of PTB7 As we know, the roughness of polymer film derived from the formation of microstructures.52 The solubility of solvent could impact the polymer chain aggregation state, this state could be kept during the spin-coating process and sequentially influenced the roughness and morphology of polymer film.53,54 Therefore, solvent solubility of PTB7 could be inferred through the surface roughness and morphology of the film. In this research, surface roughness of PTB7 was measured by AFM. The films were spin-coated at 1000 rpm from 20 mg/mL CB, TCM and DCE solutions, respectively. These samples were denoted as PTB7 (CB), PTB7 (TCM) and PTB7 (DCE) to describe conveniently. AFM images were presented in Figure 2. It


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was obviously seen that the surface morphology of PTB7 (CB) and PTB7 (TCM) was smooth, the RMS surface roughness of PTB7 (CB) and PTB7 (TCM) was less than 1 nm (0.51 nm and 0.86 nm, respectively) as shown in Table 1. While the surface morphology of PTB7 (DCE) was obviously rough and its RMS surface roughness was obviously high (15.23 nm). Thus, it was considered that PTB7 was well dissolved in CB and TCM, while worse in DCE. It meant that CB and TCM were good solvents for PTB7, but DCE was poor solvent for PTB7. During the experimental process, there was another evidence to prove poor solvent DCE, which has been attached in Figure S1.

Figure 2. AFM images of PTB7 films prepared from different solvents, (a) CB; (b) TCM; (c) DCE.

Table 1. The RMS surface roughness and the ratio R of PTB7 films prepared from different solvents with the same concentration (20 mg/mL)

CB

TCM

DCE

good

good

poor

Solvents



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RMS/nm

0.51

0.86

15.2

R= A0-0/A0-1

1

0.97

0.94

3.1.2 The effect of poor and good solvents on PTB7 condensed state structure In order to explore the effect of good and poor solvent on PTB7 condensed state structure, the UV–vis absorption spectra of PTB7 (CB), PTB7 (TCM) and PTB7 (DEC) were performed. The results were shown in Figure 3 and the magnified image of main absorption peaks was presented in the inset. It was found that there were two main PTB7 absorption peaks between the wavelength range of 550 nm and 750 nm, we denoted A0−0 for the intensity of the lower energy absorption peak (longer wavelength, at 680nm) and A0−1 for the intensity of the higher energy absorption peak (shorter wavelength, at 620nm), respectively, in the UV–vis absorption spectra. It was clearly seen from poor solvent DCE to good solvent CB in Figure 3: First, the absorption peaks were red-shifted, the red-shift could compared with the main absorption peak for well-studied P3HT taking place mainly between 400 nm and 550 nm,55 which indicated the increase of main chain conjugation length;56,57 Second, absorption peak intensity A0−0 increased gradually, which meant π−π stacking was enhanced.58,59 In order to further illustrate the change, we used the parameter


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R=A0-0/A0-1 to quantify the ratio of absorption peak intensity, which was shown in Table 1. The lower energy (0-0) was dominant part from crystalline regions of the film and the higher energy (0-1) was the part from intrachain states. The ratio of A0−0 and A0−1 was related to the free exciton bandwidth of the aggregates (W) and the energy of the main intramolecular vibration (Ep) by the following expression: A0 − 0 1 − 0.24 W EP 2 ≈( ) A0 − 1 1 + 0.073W EP Here, W is the free exciton bandwidth of the aggregates and Ep is the energy of the main intramolecular vibration, which is coupled to the electronic transition. Assuming the C=C symmetric stretch (0.18 eV) dominates the coupling to the electronic transition, W could be estimated.60 W was related to the polymer chain conjugation length and intrachain orderness, a decrease of W could lead to an increase of

both

conjugation length and orderness of molecular chains. According to the formula, R=A0 0/A0 1 increased with the decrease of free exciton bandwidth W, which meant R −

−

increased with the conjugated length and orderness of PTB7 chains.52,61-63 When R was increased from 0.94 for PTB7 (DCE) to 1 for PTB7 (CB), it indicated that the size of chain ordered aggregation was gradually increased from poor solvent to good solvent. As demonstrated above, although CB and TCM were all good solvents for PTB7, it could be found from Figure 3 that the absorption peaks of PTB7(CB) was obviously red-shifted compared with PTB7 (TCM), and R increased from 0.97 for PTB7 (TCM) to 1 for PTB7 (CB) (shown in Table 1). It indicated that the ordered degree of PTB7 (CB) was higher than that of PTB7 (TCM). The reason will be discussed in section 3.3.



A0-1 A0-0

1.0 Normalized Absorption(a.u.)

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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DCE TCM CB

0.8 0.6 600

625

650

675

0.4 0.2 0.0 300

400

500

600

700

800

Wavelength(nm) Figure 3. UV–vis absorption spectra of PTB7 films prepared from 20 mg/mL DCE, TCM and CB solutions, respectively.

Effect of poor solvent on PTB7 was different from other conjugated polymers, such as PFO,35-40 P3HT,34,41-43 which could increase the chain orderness. The reason was mainly determined by the chemical structure of PTB7. PTB7 with many electronegative atoms and flexible side chains could form the aggregation even in good solvent CB (10 mg/mL).64 In poor solvent, the interaction between the PTB7 chain and solvent became weakened and the interaction between chains became strong, which made the chains occur crispation and form amorphous conformation. Hence, the ordered degree of PTB7 was reduced in poor solvent, just as R=0.94 for PTB7 (DCE) shown in Table 1.


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3.2. The Effect of Temperature on PTB7 Condensed State Structure The effect of solvent on PTB7 condensed state structure has shown the ordered degree of PTB7 was reduced in poor solvent. Jean-Jacques Simon et al.57 heated PTB7 solution from 298K to 353K, they found that ordered aggregation was broken with the increase of temperature and formed disordered structures. Based on this, we infer that lowering temperature may decrease the flexibility of PTB7 chains and induce PTB7 to form ordered aggregation and keep them. To testify the deduction, after placing in refrigerator (253 K) for 2 h, PTB7 CB and DCE solutions with concentration of 20 mg/mL were prepared and spin-coated with the speed of 1000 rpm at room temperature. These samples were denoted as PTB7 (low-CB) and PTB7 (low-DCE), respectively. The UV–vis absorption spectra were shown in Figure 4. It was found that the absorption peaks of PTB7 (low-CB) hadn’t obvious change after reducing the temperature of PTB7 (CB) solutions, which indicated that PTB7 condensed state structure wasn’t affected when the temperature decreased to 253 K. But absorption peaks of PTB7 (low-DCE) were slightly red-shifted and the width of the absorption peaks was widened as compared to PTB7 (DCE). At the same time, A0-0 peak of PTB7 (low-DCE) increased, which resulted in R increasing from 0.94 to 1. These results illustrated that the conjugation length of PTB7 main chain increased slightly in PTB7 (low-DCE), the π-π interaction between part of chain segments was enhanced and local ordered aggregation was formed as compared to PTB7 (DCE). It could be deduced that the interaction between the PTB7



chain and solvent was further weakened and the interchain interaction of PTB7 was further enhanced with the decreasing temperature of PTB7 (DCE) solution. Although low temperature led to higher molecular chain entanglement, the π-π interaction between part of chain segments was enhanced, which resulted in forming local ordered aggregation. After lowering the temperature of DCE solution, there was hysteresis of condensed state structure of PTB7 at room temperature since DCE was poor solvent for PTB7.34 Hence, the local ordered aggregation formed in low temperature could be kept. In addition, 10 mg/mL PTB7 (low-DCE) sample has also been prepared, the results was shown in Figure S2 and Table S1. It again proved that low temperature could increase the order degree of condensed state structure of PTB7 in poor solvent. According to the above results, we could conclude that R with 0.94 in 20 mg/mL PTB7 (DCE) (shown in Table 1) should be the minimum value of R for PTB7.

Figure 4. UV-vis absorption spectra of PTB7 films spin-coated at room temperature from (a) CB and (b) DCE solutions with different temperature.


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3.3. The Effect of Solution Concentration on PTB7 Condensed State Structure Polymer solutions with different concentrations could bring about different interchain distances, interactions between the chain segments, chain entanglement. The condensed state structure of polymer was decided by interchain or intrachain interaction or both.65 So, it was important to explore effect of solution concentration on PTB7 condensed state structure. However, few researches have been devoted to the effect of concentration on the condensed state structure of PTB7 in semidilute solution up to now. So, exploring this issue is very necessary to understand the formation mechanism of PTB7 condensed state structure in order to enhance device efficiency. To explore this issue, the films based on quartz plate were spin-coated at the speed of 1000 rpm from PTB7 (CB) 20 mg/mL, 15 mg/mL, 10 mg/mL solutions, respectively. The UV–vis absorption spectra were shown in Figure 5. The magnified image of two peaks was presented in the inset. It was clearly seen that there were mainly two kinds of variations about the absorption peaks with the decrease of PTB7 concentration: First, the absorption peaks were red-shifted, which indicated the increase of main chain conjugation length and the formation of more planar structure. Second, ratio R was increased, it indicated that intrachain orderness was enhanced and the size of chain ordered aggregation was also increased.65-67 From Table 2, it was found that R was gradually increased with the reduction of PTB7 concentration in CB and DCE solution, respectively. It illustrated that the size of chain ordered aggregation



was gradually increased with the decrease of PTB7 solution concentration both in good (CB) and poor (DCE) solvent.

1.4

Normalized Absorption

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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20mg/mL 15mg/mL 10mg/mL

CB 1.2

A0-1

1.0

A0-0

0.8 0.6

600 625 650 675 700

0.4 0.2 0.0 300

400

500

600

700

800

Wavelength(nm)

Figure 5. UV-vis absorption spectra of PTB7 films prepared from CB solutions with different concentrations (from 20 mg/mL to 10 mg/mL). The magnified image of two peaks of all samples was presented in the inset.

Table 2. The ratio R of PTB7 films prepared from different concentrations from 20 mg/mL to 10 mg/mL in good solvent CB and poor solvent DCE solutions, respectively, and the sample of diluted PTB7 CB solution with the concentration of 0.006 mg/mL.


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Concentration

20

15

10

（mg/mL）） (CB)

0.006 (solution)

1

1.08

1.14

1.25

0.94

0.95

0.98

---

R=A0-0/A0-1 (DCE) R=A0-0/A0-1

The driving force of forming the ordered aggregation with larger size in the lower concentration 10 mg/mL was explored. When the concentration of CB solution was 10 mg/mL, aggregation of PTB7 molecular chains have formed with radius 3.42 nm.64 With the decrease of concentration from 20 mg/mL to 10 mg/mL, the interchain interaction weakened, meanwhile, the entanglement of molecular chain reduced, which led to decrease disorder of PTB7 molecular chain. It indicated that interchain interaction was not the main reason to form the ordered aggregation. Inversely, the ordered aggregation with larger size in the lower concentration of 10 mg/mL was strongly derived from intrachain interaction. Furthermore, by comparing the UV–vis absorption spectrum of diluted PTB7 (CB) solution with the concentration of 0.006 mg/mL (shown in Figure 6), it was found that the ordered degree of PTB7 chain was higher (R≈1.25, shown in Table 2) with larger size ordered aggregation in diluted solution (0.006 mg/mL). It demonstrated again that PTB7 ordered aggregation with larger size was mainly derived from intrachain interaction rather than interchain



interaction. This conclusion was in accord with the report of Chen68 in which PTB7’s low-energy visible absorption was mainly due to self-aggregation-induced ordering. And, this larger size ordered aggregation in precursor solution could enhance π-π interaction of film. Besides, the absorption spectrum of PTB7 was scarcely dependent of the concentration (0.005-0.1 mg/mL).68 We inferred that R≈1.25 was the maximum value for PTB7 under lower concentration conditions. That is, the orderness of PTB7 condensed state structure was the highest with concentration of 0.006 mg/mL in CB solvent.

1.2


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10mg/mL 0.006mg/mL

CB

1.0 0.8 0.6 600 625 650 675 700 0.4 0.2 0.0 300

400

500

600

700

800

Wavelength (nm) Figure 6. UV-vis absorption spectra of diluted PTB7 (CB) solution with the concentration of 0.006 mg/mL and spin-coating film from 10 mg/mL CB solution. The magnified image of two peaks of two samples was presented in the inset.


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3.4. The Effect of Force Field on PTB7 Condensed State Structure As mentioned above, high molecular chain entanglement could reduce the ordered degree of PTB7 condensed state structure. In order to improve chain orderness, it was necessary to disentangle PTB7 chains before the film was formed. As we know, the process of film-forming by spin coating was a dynamical process combined with shear force and centrifugal force. With the increase of spin-coating speed, shear force and centrifugal force increased gradually. Polymer chains could be stretched out by the force field, which decreased the chain entanglement degree. In order to explore the effect of force field on PTB7 condensed state structure, the films were prepared from the concentration of 20 mg/mL CB solution and spin-coated at speeds from 1000 to 5000 rpm. Their UV–vis absorption spectra were shown in Figure 7. The magnified image of main absorption peaks was presented in the inset. From Figure 7, it was found that the absorption peaks were weakly red-shifted and parameter R was gradually increased with the increasing speed. It indicated that the size of chain ordered aggregation was gradually increased, which meant that the entanglement degree of PTB7 molecular chains was gradually decreased with the increase of shear force and centrifugal force. The result was like decreasing concentration shown in Figure 5. The ratio R of PTB7 films with different spin-coating speeds and concentrations in CB solutions was shown in Table 3. The results were all consistent with Figure 7. It was again proved that molecular chain entanglement was decreased by increasing spin-coating speed, which could promote the formation of ordered aggregation .



1.2


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CB A0-1 A 0-0

1.0

1k rpm 2k rpm 3k rpm 4k rpm 5k rpm

0.8 0.6

600

625

650

675

0.4 0.2 0.0 300

400

500

600

700

800

Wavelength (nm) Figure 7. UV-vis absorption spectra of PTB7 films prepared from 20 mg/mL CB solutions with different spin-coating speeds.

Table 3. The ratio R of PTB7 films prepared from different spin-coating speeds and concentrations of CB solutions Spin coating speed

1k rpm

2k rpm

3k rpm

4k rpm

5k rpm

20 mg/mL

1

1.01

1.04

1.06

1.08

15 mg/mL

1.06

1.12

1.13

1.15

---

10 mg/mL

1.14

1.16

1.17

1.18

---

As we know, the condensed state structure of film was directly affected by precursor solution as well as film-forming process. As mentioned above, induced by


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shear force and centrifugal force in the process of spin coating, PTB7 chain could be disentangled and the size of ordered aggregation was increased with conjugation length increase whatever in good solvent CB (Figure 7) or poor solvent DCE (Figure S3). However, the free volume and time of chain segment movement were necessary in the process of chain disentanglement. The free volume could be provided by solvent, but the time depends on the evaporation speed of solvent, which was related to the boiling point. The boiling points of CB, DCE, TCM were shown in Table 4. From Table 4, it was obviously seen that TCM solvent evaporated faster than CB solvent due to the low-boiling point at 334.2 K. Hence, shear force and centrifugal force had less time to drive PTB7 chains to stretch during spin-coating process. As a result, the ordered degree of PTB7 (TCM) was lower than that of PTB7 (CB) (shown in Figure 2) and the RMS surface roughness of PTB7 (TCM) was a little higher than PTB7 (CB) (shown Table 1). However, the boiling point of DCE solvent was 356.5 K, which was higher than that of TCM solvent (334.2 K). The R of PTB7 decreased from 0.97 for TCM to 0.94 for DCE shown in Table 1. It demonstrated that poor solvent (DCE) induced PTB7 to form more disordered aggregation, i.e., amorphous state. We considered that there were two reasons for the formation of amorphous state by spin coating from DCE solution: First, the boiling point of DCE was relatively low, which made force field lack enough time to stretch PTB7 chains; Second, it was also the main reason that DCE was a kind of poor solvent for PTB7, which led PTB7 molecular chains to entangle with each other. So, PTB7 amorphous state could be formed in DCE solvent.



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Table 4. The boiling point of different solvents with CB, DCE, TCM, respectively.

Solvents

CB

DCE

TCM

Boiling point/ K

405

356

334

I

3.5. PTB7 Condensed State Structures from Chain Disorder to Order and the Formation Mechanism

According to the above results, it was found that R was less than 1 (shown in Table 2) when PTB7 presented amorphous state, while R was equal to or bigger than 1 when PTB7 appeared local ordered aggregation or large-scale ordered aggregation. Above results demonstrated that parameter R is a crucial criterion for determining polymer molecular chains orderness. The same results could be confirmed with TEM measurement shown in Figure 8.


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Figure 8. TEM (left) and High resolution transmission electron microscope (HR-TEM) (right) images of PTB7 condensed structure from disorder to order: (a) PTB7 spin-coating film from 20 mg/mL DCE solution, (b) PTB7 spin-coating film from 20 mg/mL DCE solution in low temperature, (c) PTB7 spin-coating film from 10 mg/mL CB solution, (d) PTB7 diluted CB solution with the concentration of 0.006 mg/mL.

In Figure 8, TEM images showed the morphology of PTB7 under different conditions. In each image (right), high resolution transmission electron microscope (HR-TEM) could indicate the level of orderness of the PTB7 condensed state structure. In Figure 8a, there were not obvious lattice fringes in PTB7 spin-coating film from 20 mg/mL DCE solution, whose R was 0.94. It illustrated that PTB7 exhibited amorphous state due to R1. The above results were consistent with UV-vis spectra. Meanwhile, it indicated that the external field could induce PTB7 condensed state change from disordered to ordered structure. Through exploring the effect of external field (solvent, temperature, solution


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concentration and external force) on PTB7 condensed state structure, the dynamic evolution from chain disorder to order was found. It could be inferred that there were mainly three kinds of PTB7 condensed state structures as shown in Figure 9. Firstly, molecular chains were all tangled up to form amorphous state (Figure 9a). Secondly, when the entanglement density of molecular chain was high enough, the π-π interaction was enhanced between part of chain segments, which brought about local ordered aggregation (Figure 9b). Thirdly, driven by intrachain interaction, molecular chain self-assembly induced largely ordered aggregation (Figure 9c). As the complexity of polymer structures caused by molecular weight polydispersity and multilayered structures, three kinds of PTB7 condensed state structures may exist simultaneously. It has been reported that PTB7 film involves hierarchical nanomorphologies from several nanometers to tens of nanometers up to hundreds of nanometers in size.51,69 Changing one of them could change condensed state structure and photophysical properties of PTB7. Condensed state structure with ordered aggregation could enhance charge carrier mobility and photoelectric device efficiency.70 Therefore, through controlling external field to induce the ordered aggregation structure is of significance to enhance PTB7 carrier mobility and photoelectric device efficiency based on condensed matter physics of conjugated polymer.



Figure 9. Schematic diagram of three kinds of condensed state structures of PTB7. (a) amorphous state; (b) local ordered aggregation; (c) largely ordered aggregation by molecular chain self-aggregation.

4. Conclusions The dynamic evolution of PTB7 condensed state structure from chain disorder to order under the external fields (solvent, temperature, solution concentration and external force) was revealed. Induced by poor solvent DCE, PTB7 chains was entangled into amorphous state. When the solubility of poor solvent was further reduced by lowering temperature, local ordered aggregation was formed. With the decrease of solution concentration or increase of external force, PTB7 chain entanglement was lowered and the size of ordered aggregation was further increased. Parameter R change from 0.94 to 1.25 could demonstrate the dynamic evolution from chain disorder to order. TEM images have again proved the dynamic evolution of PTB7 condensed state structure. Due to the complexity of polymer structures, it was


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considered that three kinds of PTB7 condensed state structures might exist simultaneously. Firstly, molecular chains were all tangled up forming amorphous state. Secondly, driven by π-π interaction between fractional chain segments with high chain entanglement density, local ordered aggregation appeared. Thirdly, molecular chain self-assembly driven by intrachain interaction induced PTB7 chains to form largely ordered aggregation, which was mainly determined by the chemical structure of PTB7. Through controlling external field to increase the chain orderness is of importance to establish physical basis for the molecule design and synthesis of materials to enhance device efficiency based on condensed matter physics of conjugated polymer.

Associated Content Supporting Information To prove the conclusion again, the experimental phenomenon photo and UV–vis absorption spectra of PTB7 are presented in the Supporting Information.

Author Information Corresponding Author: Dan Lu E-mail: [email protected]. Phone: +86-136-2079-2963

ORCID iD: Dan Lu: 0000-0002-7537-3173



Notes The authors declare no competing financial interest.

Acknowledgements This work is supported by grants from the National Natural Science Foundation of China (91333103) and (21574053).

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Table of Contents Graphic


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The Dynamic Evolution from Chain Disorder to Order of PTB7

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