The Impact of Polymer Dynamics on Photoinduced Carrier Formation

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The Impact of Polymer Dynamics on Photo-induced Carrier Formation in Films of Semiconducting Polymers Yudai Ogata, Daisuke Kawaguchi, and Keiji Tanaka J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 17 Nov 2015 Downloaded from http://pubs.acs.org on November 18, 2015

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The Journal of Physical Chemistry Letters

The Impact of Polymer Dynamics on Photo-induced Carrier Formation in Films of Semiconducting Polymers Yudai Ogata†, Daisuke Kawaguchi‡*, and Keiji Tanaka†,§*

†

Department of Applied Chemistry, ‡Education Center for Global Leaders in Molecular Systems

for Devices and §International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka 819-0395, Japan

[email protected] and [email protected]

ACS Paragon Plus Environment

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ABSTRACT: A better understanding of the carrier formation process in photosemiconducting polymers is crucial to design and construct highly-functionalized thin film organic photodevices. Almost all studies published focus on the effect of structure on the photo-induced carrier formation process.

Here, we study the dynamics of polymer chain impacts on the carrier

formation process for a series of poly(3-alkylthiophene)s (P3ATs) with different alkyl side-chain lengths. The formation of polarons (P) from polaron pairs (PP) was accelerated at a temperature at which the twisting motion of thiophene rings occurs. Among all P3ATs employed, in P3AT with hexyl groups, or poly(3-hexylthiophene) (P3HT), it was easiest to twist the thiophene rings and generate P from PP. The activation energy for P formation was proportional to that of thiophene ring motion. This makes it clear that chain dynamics, in addition to the crystalline structure, is a controlling factor for the carrier formation process in photosemiconducting polymers.

TOC GRAPHICS


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Semiconducting polymers have attracted a great deal of attention as an essential material for thin film organic photodevices such as solar cells,1-3 field-effect transistors,4-5 and light-emitting diodes6-7 due to their high absorption coefficient, light weight, and excellent mechanical flexibility.

In the last two decades, efforts have been made to improve the photoelectric

characteristics of semiconducting polymers synthetically. Typical examples that emerged based on this strategy are polythiophenes and their analogues with various side chains. Since they possess excellent processability, based on a simple solution process, they have been widely studied from various viewpoints. For example, there have been many studies using regioregular poly(3-hexylthiophene) (P3HT).8 Although the performance of thin film devices using semiconducting polymers is controlled by many factors, there is no doubt that carrier mobility is one of them. Hence, it is essential to get a better understanding of carrier formation mechanisms. To this end, the dynamics of transient species in neat P3HT films has been theoretically and experimentally examined in recent years.912

Within a time range shorter than ca. 100 fs after photo-excitation, a hot-exciton dissociation

model was proposed to explain successive events at a high excitation energy.9 This model, which demonstrated the formation of free electron-hole pairs without Coulomb interaction, polarons (P), was experimentally verified by terahertz time domain spectroscopy.10 Femtosecond pump-probe transient absorption spectroscopy (TAS) is a powerful technique to address events happening over a few picoseconds. So far, comparing the decay time constant for singlet excitons (S) and the rise time constant for P, it was reported that P is formed from S.11 By contrast, we applied global fit analyses based on chemical kinetics, taking into account both the time constants of photoexcitons and the concentrations of various transient species, to TAS


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data. This led to a different conclusion that P was formed not from S but from polaron pairs (PP).12 It has been widely accepted that carrier mobility in films of photosemiconducting polymers is strongly dependent on the films’ aggregation states. For instance, carrier mobility in a P3HT film is enhanced by processing conditions, such as the selection of certain solvents or substrate surfaces which can lead to high crystallinity and appropriate orientation.13,14 In addition, carrier mobility is also affected by thermal molecular motion of photosemiconducting polymers. This has been hitherto observed only for P3HT12 and we are unsure if the relationship holds for all photosemiconducting polymers. If it does hold, then carrier mobility, in addition to crystalline structure, must be taken into account in the design and construction of thin film organic photodevices. This point is important because when these devices are used in real life, the surrounding temperature can change a lot, meaning that the device performance would be concurrently altered by the temperature-dependent polymer dynamics. The objective of this study is to investigate how polymer dynamics impact the photo-induced carrier formation process for poly(3-alkylthiophene)s (P3ATs) with different alkyl side-chain lengths, including P3HT, which is the most studied conjugated polymer. P3AT is a class of crystalline polymer composed of a rigid main chain of thiophene rings and flexible alkyl side chains. The side chain motion of P3ATs, which was polyethylene-like cooperative motion, was detected when the carbon number of the side chains (N) was larger than six.15-16 Also, based on experiments with some P3ATs, main chain motion based on the twisting of thiophene rings is evident.12,17-19 In the following discussion, we first confirm the carrier formation process for P3ATs in films by femtosecond pump-probe TAS measurements. Then, the relaxation behavior


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of P3ATs in the films is confirmed by dynamic mechanical analysis (DMA). Finally, the results obtained by both measurements are directly compared. As materials, regioregular poly(3-butylthiophene) (P3BT), P3HT, poly(3-octylthiophene) (P3OT), poly(3-decylthiophene) (P3DT) and poly(3-dodecylthiophene) (P3DDT) were purchased from Sigma-Aldrich, Inc. and used as received. Characteristics of P3ATs such as number-average molecular weight (Mn), polydispersity index (PDI), head-to-tail regioregularity, glass transition temperature (Tg), melting temperature (Tm) and degree of crystallinity (Xc) are summarized in Table 1. Here, Mn and PDI were determined by gel permeation chromatography with polystyrene standard. Regioregularity and Xc were determined by proton nuclear magnetic resonance (NMR) and Fourier-transform infrared spectroscopy, respectively. Tg and Tm were examined by differential scanning calorimetry. Details on the characteristics of the samples are described in the Supporting Information. Table 1. Characteristics of P3ATs used.

Polymer

Mn

PDI

Regioregularity/%

Tg / K

Tm / K

Xc / %

P3BT

22k

3.1

87

327

500

62.3

P3HT

26k

2.4

98

283

488

83.1

P3OT

58k

1.6

98

260

427

90.8

P3DT

89k

2.2

95

249

421

64.7

P3DDT

31k

1.8

98

240

332

71.9

P3AT films were prepared by spin-coating from chloroform solutions at 1000 rpm for 60 s onto commercially available 7.5 µm-thick polyimide (PI) and quartz substrates for DMA and TAS measurements, respectively. To unify the extent of the thermal relaxation among the samples, the P3AT films were dried under vacuum at a temperature being higher than the Tg by


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50 K for 12 h. The film thickness was confirmed to be ca. 300 nm by atomic force microscopy. Dynamic loss modulus of P3ATs in the films was examined as functions of frequency and temperature by DMA. Temperature dependence of femtosecond TAS was carried out using a pump and probe system. The analyses of transient absorption spectra for all P3ATs are described in the Supporting Information in detail. The data are shown in Figures S3 and S4. We followed our previous method for P3HT published elsewhere.12 First, we extracted the optical density (∆OD) for S, PP and P from transient absorption spectra. Second, we analyzed the data with coupled differential equations for the time dependence of concentration for S, PP and P, to take into account all the possible reactions among them. In the analyses, the rate constant for the transition process from i to j (kij) was the fitting parameter. Whether the kij value is finite, the corresponding transition process typically exits. We found that only four processes existed: S+SS0, PP+PPS0, PPP and PPP, where S0 is the ground state. This result was common for all P3ATs. Hence, it can be claimed that the photo-carrier generation process for P3ATs is independent of, or insensitive to, alkyl chain length. In general, the temperature dependence of kij provides the activation energy (∆E) of the corresponding transition. Figure 1(a) shows kPPP for P3ATs as a function of temperature. It should be noted that the scale of the ordinate is not the same among the panels. A larger kPPP value indicates a faster dissociation process from PP to P. Thus, it is clear that photo-carrier generation in P3HT is the fastest among the P3ATs employed at a given temperature. Also, as a general trend, the kPPP value remained constant in a lower temperature region, and then simply increased with temperature. The threshold temperature, defined as TPPP, was successfully determined for all P3ATs by the least square fitting technique using two linear functions.


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Figure 1(b) shows the N dependence of TPPP; it decreased with decreasing N. Since the dissociation of PP to P proceeded faster above TPPP, it is apparent that side-chains directly, or indirectly, contribute to the P formation dynamics from PP.

(a) 0.10 0.05 ps−1 0.05

P3BT

0 0.20

(b)

0.10 ps−1

400

P3HT

0 0.20 0.10 ps−1 0.10

350

TPP→P / K

0.10

kPP→P / ps−1

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


300

P3OT

0 0.10

250 0.05 ps−1

0.05

200

P3DT

0 0.10

2

4

6

8

10

12

14

N

0.05 ps−1 0.05

P3DDT

0 0

100

200

300

400

500

Temperature / K

Figure 1. (a) Temperature dependence of rate constants for P formation from PP (kPPP) in the P3AT films. (b) N dependence of TPPP. Figure 2(a) shows semi-logarithmic plots of rate constants from PP to P against inverse of temperature for the P3AT films. Assuming an Arrhenius type functional form, kPPP has a relation with absolute temperature (T) above TPPP as follows,

k PP→ P = A ⋅ exp(− ∆E PP→ P RT )

(1)

where A and R are frequency factor and the gas constant, respectively. Thus, the apparent activation energy for the PPP transition (∆EPPP) can be calculated from the slope of plots in Figure 2(a).

Panel (b) of Figure 2 shows how the ∆EPPP value depends on N.

The N


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dependence of pre-exponential factor (A) for the PPP transition is also provided in Supporting Information (Figure S5). The ∆EPPP value was minimized at N = 6, meaning that, of all the P3ATs, P3HT most easily formed P from PP.

(a)

(b) −2 −3

1. 2. 3. 4. 5.

2 3 1 4

10

P3BT P3HT P3OT P3DT P3DDT

∆EPP→P / kJ•mol−1

−1

ln(kPP→P / ps−1)

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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5

−4 −5 2.0

2.5

3.0

3.5

4.0

4.5

8 6 4 2 0

2

4

6

103•T−1 / K−1

8

10

12

14

N

Figure 2. (a) Arrhenius plots for kPPP in the P3AT films. (b) N dependence of ∆EPPP obtained from panel (a). DMA enables a direct understanding of polymer dynamics. Figure 3(a) shows the temperature dependence of the loss modulus (E") at a frequency (f) of 20 Hz for all P3ATs in the films. Three relaxation peaks were observed for all the P3ATs except for P3BT. The molecular motion of P3HT has been extensively analyzed by methods such as solid state NMR, impedance analysis and quasi-inelastic neutron scattering.17-20 Combining the DMA results and the information published, the peaks, from low to high temperature, can be associated with side chain motion, the twisting motion of the main chain and the deformation of the inter-lamellar crystalline region, and named β, α1 and α2 processes, respectively. In the case of P3BT, a single peak, assignable to the twisting motion of the thiophene rings

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was observed around 340 K. The relaxation

temperatures for the β, α1 and α2 processes, denoted as Tβ, Tα1 and Tα2, respectively, were determined by curve fitting with three Gaussian functions. Since the α1 relaxation process


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partially overlapped with the β one for P3DT and P3DDT, the determination error of Tα1 might be larger for P3DT and P3DDT than for other P3ATs. We had previously established that, using P3HT, the α1 relaxation process strongly impacts the P formation process.12 Hence, we focused on Tα1 here. Figure 3(b) shows Tα1 as a function of N. Tα1 decreased with increasing N. This can be simply explained in terms of the plasticizing effect of the alkyl side-chains.15 TPPP also decreased with increasing N and that relationship will be discussed later.

(a) 1. P3BT 4. P3DT f= 2. P3HT 5. P3DDT α1 3. P3OT

20 Hz

(b) 400

1

β α1

β

α2

2 β β

α2

α1

3

α1 α1

α2 α2

f = 20 Hz 350

4 5

T α1 / K

E″ / 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


300 250 200

2

4

6

8

10

12

14

N 100 150 200 250 300 350 400 450

Temperature / K

Figure 3. (a) Temperature dependence of dynamic loss modulus (E") for P3AT films at a frequency of 20 Hz. (b) N dependence of relaxation temperature for the α1 process (Tα1) for the P3AT films. To evaluate the difficulty in twisting the thiophene rings of the main chain, the apparent activation energy for the α1 process (∆H*α1) in P3ATs was evaluated. ∆H*α1 values were extracted based on Arrhenius plots of ln f and the reciprocal temperature for α1 relaxation. The raw data of the temperature dependence of E" as a function of frequency for the P3AT films are provided in Supporting Information (Figure S6(a)). Figure 4(a) shows Arrhenius plots for the α1 process in the P3AT films. The ∆H*α1 value can be extracted from the following equation (2):


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ln f ∝ − ∆H α*1 RT

(2)

Figure 4(b) shows the N dependence of ∆H*α1 for P3ATs. P3HT had the lowest ∆H*α1. To understand the peculiar N dependence of ∆H*α1, two competitive factors which may affect the twisting motion of thiophene rings in P3ATs should be taken into account. The first factor is the plasticizing effect. The acceleration of the molecular motion induced by the alkyl side chains becomes remarkable with increasing N,15,21 as seen in Figure 3(b). The other factor is the size scale of the molecular motion. For P3ATs, the alkyl side chains directly bind to thiophene rings. Thus, given that the twisting motion of the main chain thiophene rings is coupled with the alkyl side-chain motion, the size scale of the molecular motion becomes larger with increasing N. That is, the ∆H*α1 value increases with N. Results like this have been reported for the segmental motion of poly(n-alkyl methacrylate).22 Our results indicate that when N is greater than 6, the size scale effect becomes more important than the plasticizing effect.

(a)

(b) 7

300

1. P3BT 4. P3DT

5

∆H*α1 / kJ•mol−1

6 2. P3HT 5. P3DDT

ln(f / Hz)

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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3. P3OT

200

4 3

100

2 1 0 2.5

1

2

3 4 5

3.0

3.5

4.0

4.5

0 2

4

6

103•T−1 / K−1

8

10

12

14

N

Figure 4. (a) Relation between ln f and reciprocal absolute temperature for Tα1 in the P3AT films with various side-chain lengths. (b) N dependence of apparent activation energy (∆H*α1) for P3AT films.


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We finally come to the connection between carrier formation and polymer dynamics. Figure 5(a) shows the N dependences of TPPP and Tα1. For all the P3ATs, TPPP was closely related to Tα1, meaning that P formation from PP was strongly related to α1 relaxation. It is noteworthy that there exists a slight difference between TPPP and Tα1 for N = 12. This may be because the melting of side chains disturbs the precise determination of Tα1 for P3DDT. Figure 5(b) shows the relation between ∆H*α1 and ∆EPPP for P3ATs. ∆EPPP passes through the origin and is linearly proportional to ∆H*α1. Therefore, it can be concluded that P is more easily formed from PP as thiophene rings become more twistable. In that sense, P3HT is the best of the P3ATs in terms of photo-carrier generation due to the twistable nature of its thiophene rings.

(a)

(b) 10

400

TPP→P Tα1

∆EPP→P / kJ•mol−1

1. 2.

350

T/K

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


300 250 200 2

4

6

8

N

10

12

14

8

P3DDT

6

P3DT P3OT

4

P3HT

2 0

P3BT

0

50

100

150

200

250

∆H*α1 / kJ•mol−1

Figure 5. (a) N dependence of TPPP and Tα1. (b) ∆EPPP vs. ∆H*α1. In summary, we examined photo-carrier generation and molecular motion in a series of P3ATs with various alkyl side-chain lengths by temperature dependent TAS and DMA measurements. Our main finding is that P formation from PP is closely related to the twisting motion of thiophene rings, which is common to all P3ATs. Why P3HT is the best in terms of photo-carrier generation compared to the other P3ATs can be also understood on the basis of the chain


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dynamics. We believe that this fundamental knowledge will be used in the development of organic devices in the near future.

ASSOCIATED CONTENT

AUTHOR INFORMATION Corresponding Author [email protected] and [email protected] Author Contributions K. T. designed all the experiments and directed the work to completion.

D. K. provided

intellectual input for the analyses of the transient absorption spectra. Y. O. performed the experiments and analyses. All authors contributed to the writing of the manuscript. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This research was partly supported by JSPS KAKENHI Scientific Research on Innovative Area “New Polymeric Materials Based on Element-Blocks” (No. 15H00758) program, Grant-in-Aids for Scientific Research (A) (No. 15H02183) to K. T. and for Scientific Research (C) (No. 26410224) to D. K. from the Ministry of Education, Culture, Sports, Science and Technology,


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Japan. Y. O. acknowledges the financial support through JSPS Research Fellowships for Young Scientists (No. 27·3110). Supporting Information Available: Details of molecular characterization, sample preparation, and analyses of TAS and DMA. The supporting Information is available free of charge via the Internet at http://pubs.acs.org.

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Figure 1. (a) Temperature dependence of rate constants for P formation from PP (kPP→P) in the P3AT films. (b) N dependence of TPP→P. 392x333mm (120 x 120 DPI)


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Figure 2. (a) Arrhenius plots for kPP→P in the P3AT films. (b) N dependence of ∆EPP→P obtained from panel (a). 391x176mm (120 x 120 DPI)



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Figure 3. (a) Temperature dependence of dynamic loss modulus (E") for P3AT films at a frequency of 20 Hz. (b) N dependence of relaxation temperature for the α1 process (Tα1) for the P3AT films. 380x235mm (120 x 120 DPI)


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Figure 4. (a) Relation between ln f and reciprocal absolute temperature for Tα1 in the P3AT films with various side-chain lengths. (b) N dependence of apparent activation energy (∆H*α1) for P3AT films. 392x176mm (120 x 120 DPI)



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Figure 5. (a) N dependence of TPP→P and Tα1. (b) ∆EPP→P vs. ∆H*α1. 394x175mm (120 x 120 DPI)


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Table 1. Characteristics of P3ATs used. 206x68mm (120 x 120 DPI)



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TOC graphic 189x132mm (120 x 120 DPI)


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The Impact of Polymer Dynamics on Photoinduced Carrier Formation

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