Quantitative Study on β-Phase Heredity Based on Poly (9, 9

Nov 28, 2016 - ABSTRACT: In this work, the quantitative relationship in the heredity of ... The heredity based on PFO β-phase from the solution to th...
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Quantitative Study on β‑Phase Heredity Based on Poly(9,9dioctylfluorene) from Solutions to Films and the Effect on Hole Mobility Zeming Bai, Yang Liu, Tao Li, Xiaona Li, Bin Liu, Bo Liu, and Dan Lu* State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, 2699 Qianjin Avenue, Changchun 130012, China S Supporting Information *

ABSTRACT: In this work, the quantitative relationship in the heredity of β-phase from a solution to a thin film based on poly(9,9-dioctylfluorene) (PFO), the mechanism of β-phase formation, and the effects of β-phase contents on hole mobility were investigated. The heredity based on PFO β-phase from the solution to the thin film was characterized through UV−vis absorption. Results indicated that β-phase can be completely transferred from solutions to films during drying to form films. PFO β-phase was stable and could manage the dynamic changes from a liquid state to a thin-film state. The β-phase content was higher in the diluted solutions, and the reason was revealed through dynamic light scattering. Thus, a new structure model was constructed, and polymer chain aggregation was rendered unnecessary during PFO β-phase formation. The energy status of the β-phase was lower than that of the α-phase. Consequently, PFO chains were autonomously assembled to become orderly. The chemical environment of the low-concentration solution was more suitable than that of the high-concentration solution. The polymer chains in the former could more freely adapt to a flat geometry than those in the latter to facilitate interchain stacking. Chain aggregation was then observed through transmission electron microscopy. Photoinduced charge extraction with a linear increase in voltage was also performed to examine the charge density and hole mobility of PFO. Hole mobility could be enhanced by an order of magnitude when β-phase was increased from 0% to 5.4%. Thus, the presence of a small amount of ordered domains that can form interconnected channels could strongly enhance the carrier transport of materials in poorly ordered organic thin films, such as PFO. This condition is possibly beneficial for photoelectronic devices, and the adaptive nature of PFO chains in solutions to form a flat geometry is the main factor that promotes the order of the system.

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tion.12−14 Thus, the processing of functional condensed matter thin films, such as from single chain conformations to chain aggregation, should be controlled. In addition to thermal and solvent vapor annealing, chain conformations from a solution basis provide an enhanced control of the resulting thin film because the nonequilibrium condensed-state structure of most cast thin films cannot be easily changed. Chain aggregation can be controlled more efficiently by using prescribed solutions with known existing functional species that can be transferred to solid-state films. In practice, chain conformation in a

INTRODUCTION Conjugated polymers have been widely applied to optoelectronic devices, such as solar cells, field-effect transistors, polymer light-emitting diodes, and organic lasers,1−5 because of their excellent optical and electronical properties.6−10 In carbon-based platforms, the transport of charge carriers inside a soft film is a unique phenomenon11 but is considered a limiting factor in various platform applications. As such, enhanced electronically active systems have been developed. Manipulating the condensed-state structure of polymer thin films can be a more effective approach than chemical approaches to construct new materials. Macromolecular chain conformation can strongly influence carrier mobility because of the special nature of macromolecular structures and subsequent chain aggrega© XXXX American Chemical Society

Received: September 5, 2016 Revised: November 28, 2016 Published: November 28, 2016 A

DOI: 10.1021/acs.jpcc.6b08941 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 1. Schematic molecular diagram: (a) α-conformation and (b) β-conformation of PFO.

morphological structures during transfer. The dynamic evolution of the formed aggregated structure was examined through dynamic light scattering (DLS). Our results showed that β-phase formation was related to PFO solution concentration. Photoinduced charge extraction through a linear increase in voltage (photo-CELIV) was conducted to estimate hole mobility and charge density in PFO thin films under different processing procedures. Hole mobility could be enhanced by an order of magnitude when the β-phase contents increased from 0% to 5.4%. This research helps elucidate and control the molecular-chain-condensing process from solutions to films to fabricate photoelectric devices with high chargecarrier mobility, stability, and efficiency.

precursor solution should be regulated to form an ordered structure for optoelectronic films and devices. Precursor solutions with existing functional species not only introduce manipulating functional moieties in resulting films but also alter the dynamic path of chain condensed matter evolution. In integrated in situ film preparation and structural characterization experiments, a polymer film can be formed within the first 7−8 s during spin coating. More than 99% of the initial volume is lost within the first half of a second after the film is spun.15 Thus, the starting point of film preparation should be manipulated to provide another route that can counteract the rapid kinetics of chain condensed matter evolution. In this work, precursor processing should be applied to manipulate thin films. Poly(9,9-dioctylfluorene) (PFO) was used as a model system. This material is a suitable blue light material with interesting optical and electrical characteristics.12,16−26 PFO is also abundant in conformation. The material forms αand β-phases (Figure 1) depending on the torsional angle between repeating units. The β-phase shows a narrow emission band red-shifted by 100 meV from those observed in a glassy polymer.27−31 In general, the β-phase is a mesomorphic or weakly crystalline substance that can generate Bragg reflections.13,32 Compared with the α-phase, the β-phase displays an improved planarization of the main chain, which can increase the conjugation length and degree of order. Thus, the charge-carrier mobility, stability, and efficiency of PFO optoelectronic devices are enhanced when the β-phase is formed.23,25,33,34 PFO exhibits a unique feature in solutions. For example, Chen and Justino et al. observed the entanglement of PFO in toluene. PFO chains become isolated stiff polymers when their concentration is less than c* (>0.2 mg/mL).35,36 However, polymers overlap and form a network structure when the concentration exceeds c*. Monkman and Knaapila found PFO showing a thin and long rodlike conformation in a suitable solvent through small-angle neutron measurement. However, PFO can form large and thin sheetlike aggregation in poor solvents, where a fraction of chains or segments show β-phase conformation, and the resulting sheetlike aggregation produces a structural order.37 In general, β-phase in thin films can be obtained through thermal cycling, solvent vapor treatment, or dipping films in a mixed solvent/nonsolvent for various times.33,38 Although PFO has been investigated, its properties, including the relationship between chain aggregation and β-phase formation, have yet to be fully described. The quantitative heredity from solutions to films for PFO β-phase, its formed mechanism, and effect on hole mobility remain unclear. In this study, the quantitative relationship of PFO β-phase from solutions to films was revealed. The β-phase contents were quantitatively calculated during transfer from a solution to films. Our results demonstrated that β-phase can be completely transferred from solutions to films. Transmission electron microscopy (TEM) was applied to confirm the change in PFO

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. PFO was purchased from American Dye Source, Catalog No. ADS329BE. The weight-average molecular weight (Mw) was 75 000 g/mol, and the polydispersity index was 2.5. Toluene and ethanol were used as good and poor solvents, respectively. These reagents were purchased from Beijing Chemical Company, China and were all chromatographically pure. All the samples were dissolved in toluene by stirring at 75 °C first and then cooling down to room temperature for several minutes. Different percentage concentrations of ethanol were used as poor solvent to induce β-phase formation. To explore the relationship between the solution and films on the β-phase, the films were spin-cast from 10 mg/mL PFO solution. Next, the solution was diluted from the original concentration 10 mg/ mL to 0.05 mg/mL. By contrast, the low-concentration sample content was 0.05 mg/mL without dilution. The field-effect devices were fabricated with a structure of ITO/SiO2/PFO/Al. A 70 nm thick SiO2 insulator layer obtained by spin-coating a SiO2 sol−gel (homemade) on the ITO-coated glass was used. Additional details of the SiO2 insulator layer were reported in a previous work.40 The PFO layer was spin-coated on the SiO2 substrate in a glovebox, and the thickness of a PFO-active layer was 150 nm, which was characterized with a Veeco DEKTAD 150 surface profilometer. The Al (100 nm) cathode was deposited through a shadow-mask-defining active area of 0.05 cm2. Thermal annealing was carried out at 120 °C for 10 min under N2. 2.2. Calculation Method of β-Phase Contents and Charge-Carrier Mobility. The calculation methods of βphase contents in PFO solution or films and charge-carrier mobility have been reported in our previous research,40,41 and the calculation method on PFO β-phase content was also presented in the Supporting Information. 2.3. UV−Vis Measurement. UV−vis research was carried out by Shimadzu UV-3000 spectrophotometer. A quartz cuvette was used for measurement. 2.4. Light Scattering (LS) Measurements. An ALV/ CGS-3 LS spectrometer made in Germany equipped with an B

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Figure 2. (a). Normalized UV−vis absorption spectra of five PFO samples with ethanol percentage content variation from 0% to 40% in 0.05 mg/ mL PFO solution diluted from the original PFO solution of 10 mg/mL concentration. (b) UV−vis absorption spectra of five PFO films with ethanol percent variation from 0% to 40% spin-cast from the original PFO solution of 10 mg/mL concentration.

3. RESULTS AND DISCUSSION 3.1. Quantitative Relationship in PFO β-Phase Content between Solution and Film. In a previous research, we found that the proportion of the PFO β-phase can be obviously enhanced by the solvent field. Particularly, the percentage of poor solvent (ethanol) ranged from 10% to 90%.41 In this work, the 0%−40% ethanol solutions were used to induce βphase formation. Poor solvent solubility of PFO can induce the aggregation of molecular chains. The aggregation then alters the PFO’s ground-state electronic structure and can be detected by the change in absorption spectrum. The normalized UV−vis absorption spectrum was used to probe the β-phase proportion in solutions and films. Five samples dissolved by different ratios of toluene/ethanol are presented in Figure 2. All the curves were normalized at the isosbestic point (405 nm) identified previously in a relevant report to correct for precipitation effects.29,30 The main band around 391 nm corresponds to the α-phase, whereas the peak at 437 nm corresponds to the βphase. The normalized UV−vis absorption spectra of the PFO solution and film show a red shift in the main band with the appearance of a new peak at 402 nm. This change is ascribed to aggregation absorption and was proved by Monkman et al.21−25,33 In Figure 2, the increase in the β-phase is gradual from 0% to 30%. However, at 40% ethanol, the absorption peak of the β-phase in both solution and films concurrently rises and reaches the maximum value. The two kinds of solutions became cloudy when the ethanol concentration increased to 40%, and the phase separation, designated as macroscopic aggregation, occurred.41 During this process, network structure or sheetlike aggregation appeared as shown by TEM. Table 1 displays the change in β-phase contents with ethanol percentage content variation from 0% to 40% in both PFO solution and films at the PFO starting concentration of 10 mg/mL. In previous works, many researchers noticed that the conformational differences in solution were carried over to the spin-cast films.42−46 However, such studies only remained in the qualitative stage. Quantitative study on the conformational heredity from solution to films was first achieved through our experiments. The PFO β-phase contents were not only

ALV/LSE-7004 multiple-τ digital correlator was used in DLS. The JDS-Uniphase solid-state He−Ne laser with a output power of ca. 22 mW at the operating wavelength of 632.8 nm was used as light source. The LS cell was held in a thermostat refractive index matching vat filled with purified and dust-free toluene. In DLS, the intensity time correlation function g2(t) was measured, where g2(t) is the normalized scattering intensity autocorrelation function and t is the decay time. g2(t) can be related to the normalized first-order electric field time correlation function g1(t) via the Siegert relation as g2(t) = 1 + β|g1(t)|2·g1(t) is the dynamic structure factor and obtained from the linear fit model, whereas G(τ) is the van Hove normalized scattering intensity autocorrelation function, which can be calculated from the Laplace inversion of the measured g2(t). The details of the LS theory can be found elsewhere.37−39The viscosity of mixed solvent was measured using a Julabo 536 10 Ubbelohde viscometer. 2.5. TEM Measurements. TEM experiments were performed using a JEM-2100F with an accelerating voltage of 200 kV. The samples for TEM were floated away from the PFO solutions with ethanol proportions of 0%, 10%, 20%, 30%, and 40% and then picked up with a copper grid. The minimum range of electron diffraction is 500 nm; therefore, we selected the range of 500 nm for electron diffraction of the whole PFO films. 2.6. Atomic Force Microscopy (AFM) Measurements. AFM experiments were performed using a SII SPA-300. The samples for AFM were spin-cast from the PFO solutions with ethanol proportion of 0%, 10%, 20%, 30%, and 40%. The PFO concentration for all the solutions was 10 mg/mL. 2.7. Photo-CELIV Measurements. The CELIV consisted of a pulsed laser (Continuum Minilete TM Nd:YAG), a synthesized function generator (Stanford Research System DS345), a digital delay generator (Stanford Research System DG645), and an oscilloscope (Tektronix MSO 4054) for signal observation and recording. Photo-CELIV measurements were carried out under ambient conditions, and samples were irradiated through the ITO side by a single 10 ns, 355 nm laser flash with energy flux 20 μJ cm−2/pulse. C

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The Journal of Physical Chemistry C Table 1. β-Phase Contents with Different Percentages of Ethanol in PFO Solution and Films at the Starting PFO Concentration (10 mg/mL)

far and the interaction between the PFO chains was fairly weak. When the content of ethanol reached 10%, rodlike aggregation ensued (Figure 3b). This result suggests that a small PFO aggregation structure formed but Bragg reflection intensity was exceedingly weak and hardly observed. Hence, a β-phase was barely generated. The result agrees well with the data shown in Table 1 (β-phase content of 0% in solution and film). Nevertheless, the aggregation significantly changed as the ethanol content rose from 10% to 30% (Figure 3b−d). Not only was additional aggregation observed, but the intensity of Bragg reflection was obviously enhanced. Hence, increasing the poor solvent content not only generated increased aggregation but also formed an additional β-phase. Furthermore, PFO chains adopted a more rodlike aggregation, and the interaction between the rodlike aggregations strengthened. At 20% ethanol, the PFO chains began to overlap. When the ethanol content increased to 30%, the originally well-dissolved PFO chains begun to aggregate and form a network structure with more limited chain motion than that of formed rodlike aggregation (Figure 3d). Interchain interaction became sufficiently strong and generated overlapped packing. When the β-phase content reached the maximum value with the ethanol content of 40%, obvious sheetlike aggregation and strong intensity of Bragg reflection were observed (Figure 3e). This result indicated that considerable aggregation materialized in the PFO solution. As revealed by Chen et al.,46 the PFO-enriched phase was mesomorphic and consisted of sheetlike aggregation of nanometer lengths. Herein, the enhanced intensity of Bragg reflection indicated that the β-phase was microcrystalline (or weakly crystalline), which proved that transformation from disorder to order occurred with β-phase content increase. The driving force can be attributed to a kind of “forced proximity” caused by adding poor solvent to shorten the distances between the PFO chains. Furthermore, the side-chain interaction was enhanced, which supplied sufficient energy for the PFO backbone to overcome steric hindrance and attain backbone planarization.

ethanol (%) β-phase contents (in PFO solution) β-phase contents (in PFO films)

0%

10%

20%

30%

40%

0%

0%

5.5%

9.1%

36.1%

0%

0%

5.4%

9.0%

36.1%

enhanced with increasing ethanol content but also remained consistent throughout the solution and films (Table 1). The above-mentioned observations indicate that the β-phase was insufficiently stable and was not affected by the liquid− solid phase transition process. More importantly, the PFO βphase can be completely carried over from the solution to films. 3.2. Aggregation Structure from Solution to Films with Different PFO β-Phase Contents. In toluene solution, PFO was locally dissolved into single chains when the concentration was sufficiently low (≤10 mg/mL) at room temperature, in which chain shape remained as stiff rods.37 However, the absorption peak of the β-phase in our study was obviously reduced after a filtration experiment.49 This result demonstrated that β-phase aggregation was partially induced by the poor solvent (ethanol). In fact, the structural parameters of the aggregation of the PFO solution were calculated by Knaapila et al. and showed that PFO can form long, sheetlike particles with increased poor solvent content, but rodlike particles were dissolved to the molecular level.47,48 However, the aggregation shapes in PFO solution or films were obtained only by calculation method. To reveal the aggregation structure morphology, we conducted TEM. Five photographs with different poor solvent (ethanol) contents are presented in Figure 3. The inset is the Bragg reflection intensity with different PFO β-phase contents. When the ethanol content was 0% (Figure 3a), we found that neither a visible aggregation nor a Bragg reflection appeared. Hence, we inferred that the PFO dissolved to the molecular level in the toluene solution. At such instance, the distance between the PFO chains was sufficiently

Figure 3. TEM images with different ratios of toluene/ethanol: (a) 0%, (b) 10%, (c) 20%, (d) 30%, and (e) 40% ethanol. D

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The Journal of Physical Chemistry C 3.3. β-Phase with Different Starting Concentrations and Its Formation Mechanism. As stated above, the β-phase can be quantitatively transferred from solution to films, and the quantitative relationship exhibited a corresponding direct oneto-one relation from the solution to films at the high precursor solution concentration (10 mg/mL; Table 1). However, the low PFO starting concentration (e.g., 0.05 mg/mL) importantly revealed the intrinsic properties of the polymer chain. The high starting concentration (e.g.,10 mg/mL) was suitable for application in films and devices. Therefore, the two kinds of concentrations were representative concentrations for basic and application research. However, whether the β-phase contents were directly affected by the two kinds of typical concentrations was unknown. To explore this issue, we cast the films from PFO semidilute solution (10 mg/mL) to obtain the absorption spectrum. The absorption spectrum of the solution was obtained by diluting the semidilute solution (10 mg/mL) to 0.05 mg/mL. Interestingly, the absorption spectrum of the 0.5 mg/mL PFO solution displays an obvious higher absorption intensity at 437 nm (i.e., β-phase absorption peak; Figure 4)

calculated by the method in previous studies36,44). Second, the two samples with different starting concentrations (10 and 0.05 mg/mL) all showed the maximum β-phase content (around 36%) at 40% ethanol, which was consistent with our previous work.41,49 This result demonstrated that the β-phase can reach a maximum value and was fairly stable when the percent of the poor solvent ethanol exceeded 40%, which was of great significance to applications for photoelectricity devices. In a word, the above-mentioned two kinds of phenomena indicate that the PFO at low starting concentrations attained higher βphase contents than those at high starting concentrations. Furthermore, the two kinds of starting concentrations (0.05 and 10 mg/mL) achieved the maximum β-phase content of 36% when the percentage content of the poor solvent exceeded 40% and were sufficiently stable in the PFO solution as reported in our previous studies.41,49 To further explore the two interesting phenomena mentioned above, we used light scattering. For convenience, we chose to add 10% poor solvent ethanol for research. Chen and co-workers found that DLS is a more sensitive probe than small-angle neutron scattering for detecting PFO aggregation.41 Thus, the aggregation morphology in PFO solution at different starting concentrations (0.05 and 10 mg/mL) was characterized by DLS. Figure 5a shows the autocorrelation function of the two kinds of solutions, whereas Figure 5b presents their corresponding hydrodynamic radius distributions (R h). Although the two starting concentrations were different, the test concentration was controlled at 0.05 mg/mL. Compared with the low starting concentration, the high starting concentration presented a shift of the correlation function to a longer time along the relaxation time axis. Two kinds of molecular chain movement relaxation modes are shown in Figure 5b, i.e., the fast mode (for single-chain movement) and slow mode (for chain aggregation). For the low starting concentration (0.05 mg/mL) solution, the fast mode with a peak of 9 nm was attributed to the translational diffusion of PFO single chains with the motion of the flexible side chains, whereas the slow mode with a peak of 100 nm was ascribed to the aggregation considered to involve the β-phase. However, the Rh distribution at high starting concentration changed significantly, and the Rh peak of the slow mode shifted from 100 to 800 nm. Moreover, the fast mode disappeared, and a new relaxation mode with average Rh of 8 μm was observed instead. This mode was designated as the “slowest mode.” The appearance of these peaks indicated that by increasing the PFO starting concentration to 10 mg/mL, the well-dissolved PFO single chains aggregated to form a new aggregation structure, and the aggregation degree was sufficiently high, such that the chain movement was limited unlike in the PFO single chains (fast mode). We inferred this new aggregation structure to be a rodlike aggregation that can pack as a network structure, which was confirmed by TEM (Figure 3d). Usually, β-phase formation is considered to be caused by chain aggregation; greater aggregation results in higher β-phase contents.46,49 However, our results contradict this viewpoint. We considered that this issue must be understood in terms of the following three aspects. First, at low poor solvent (ethanol) content (e.g., < 10%) and high starting PFO concentration (e.g., 10 mg/mL), considerable aggregation was noted despite the dilution from 10 to 0.05 mg/mL. Hence, the large aggregation structure formed at the initial concentration was not affected by the dilution. This result was achieved because the aggregation increased the interchain interaction and

Figure 4. Normalized UV−vis absorption spectra of five PFO samples at different percentage contents of ethanol (0−40%) at low starting PFO concentration (0.05 mg/mL).

than that in Figure 2a with PFO high precursor solution concentration (10 mg/mL) in the ethanol content range of 10%−30%. Table 2 shows the β-phase contents under the low Table 2. β-Phase Contents with Different Percentage Contents of Ethanol at Low Starting Concentration (0.05 mg/mL) in PFO Solution ethanol percent (%) β-phase content

0%

10%

20%

30%

40%

0%

5.2%

11.2%

16.7%

36.2%

precursor solution concentration (0.05 mg/mL). Compared with Table 1 (10 mg/mL), Table 2 presents two kinds of notable phenomena. First, the β-phase content in Table 2 is higher than that of Table 1 at 10%−30% ethanol, especially at 10% ethanol content. Table 1 reveals that no β-phase was present in the films nor in solution, and no absorption peak at 437 nm was observed in Figure 2. However, Figure 4 shows a low peak at 437 nm with 5.2% β-phase content (Table 2, E

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Figure 5. (a) Solvent dependence of autocorrelation functions of the PFO solution. The result in black represents the sample mixed from 10 mg/mL and then diluted to 0.05 mg/mL. The result in red denotes the sample mixed directly at 0.05 mg/mL. (b) Corresponding hydrodynamic radius distribution (θ = 90°, T = 298 K). We chose 10% poor solvent ethanol for the experiment. The test concentration was controlled at 0.05 mg/mL.

Figure 6. AFM image under 20% and 30% ethanol contents.

increased to the maximum level, and the β-phase content in turn reached the maximum value. In our observations, chain aggregation was not a necessary condition in β-phase formation, but a formed β-phase must be accompanied by aggregation. Side chain interaction must be sufficiently strong to provide sufficient energy for the PFO main chain to overcome steric hindrance and planarize to form the β-phase. 3.4. Effect of Different β-Phase Contents on Charge Density and Mobility. Recent research has demonstrated that adding poor solvent can generate stiff chain segments within the polymer aggregation in casting solution. In this process, the poor solvent induces a short-range lamellar order within the polymer aggregation.39 In this work, we added ethanol as the poor solvent into well-dissolved PFO solution to induce largescale aggregation with regions of short-range order. This shortrange order (designated as β-phase) held a larger intrachain torsion angle than that of the α phase and exhibited a coplanar

reduced the flexibility of the side chain. The side chains of adjacent PFO chains then entangled and formed a stable structure that can be transferred from the precursor solution to the diluted solution. However, in this case, the side chain interaction was unable to provide sufficient energy for the PFO main chain to overcome the steric hindrance for planarity to form the β-phase. Second, when the poor solvent ethanol content was increased (e.g., 20% and 30%), two kinds of possibilities should be considered. On one hand, adding ethanol can increase the aggregation. Side chain interaction provides sufficient energy to induce PFO backbone planarization. On the other hand, the poor solvent can act as driving force, which enhances the repulsive force between the PFO chains and solvent molecules. Therefore, the aggregation is subjected to compressive force and results in PFO backbone planarization. Third, when the poor solvent (ethanol) content increased (e.g., 40%), both interchain and intrachain interactions of PFO F

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Figure 7. (a) Photo-CELIV transients at varying light intensity under holes with 0% β-phase content. (b) Photo-CELIV transients at varying light intensity under holes with 5.4% β-phase content. (c) Dependence of hole mobility on the density of light with different β-phase contents.

increased hole mobility but also enhanced charge density. By contrast, charge density variation did not influence hole mobility. Therefore, the ordered β-phase structure is the main factor that enhances hole mobility independent of charge density.

conformation that can enhance hole mobility. Figure 6 presents the AFM results of the morphological structure of the films at different ethanol contents. When the ethanol content exceeded 30%, the films displayed high roughness; thus, it was unsuitable as the active layer for fabricating photoelectric devices. Therefore, two kinds of different casting solutions with 0% (0% β-phase) and 20% ethanol (5.4% β-phase) were selected to explore the effect of different β-phase contents on hole mobility. The CELIV method was first used to study PFO devices. CELIV is a kind of method that involves the simultaneous determination of mobility and charge density. In our determination, SiO2 was selected as a MIS (metal− insulator−semiconductor) structure to obtain hole mobility. The mechanism involves hole and electron drifting upon application of linearly increasing voltage. Holes were extracted through the Al electrode, but electrons accumulated at the SiO2 interface. Photo-CELIV transients at varying light intensity for the cases of hole with different PFO β-phase contents are shown in Figures 7a and 7b. The peak was derived from charge density. This observation suggests that the charge density was enhanced with increasing light intensity (Figure 7b). By contrast, the devices with 5.4% β-phase showed a much higher charge density than those with 0% β-phase at different light intensities. Thus, the β-phase was beneficial to charge generation. Figure 7c reveals the dependence of the mobility on the density of light on the different β-phase contents. When the β-phase content was 0%, the average mobility reached 7.9 × 10−7 cm2/vs. However, the average mobility was increased to 4.0 × 10−6 cm2/vs when the β-phase content was 5.4%. Obviously, the hole mobility was increased by an order of magnitude, which proves the PFO β-phase can facilitate charge-carrier transport and generate an increased charge-carrier mobility. However, although the charge density was enhanced with increased light intensity, the mobility scarcely differed. This finding demonstrates that hole mobility was dominated by PFO β-phase change, but charge density did not affect the mobility.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b08941. Calculation method for the β-phase in the PFO solution and films and the fluorescence spectra of PFO with different β-phase contents (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].: +86 431 85167057. Fax: +86 431 85193421. ORCID

Dan Lu: 0000-0002-7537-3173 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by grants from the National Natural Science Foundation of China (21174049), (21574053), and (91333103).

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REFERENCES

(1) Thomas, A. K.; Brown, H. A.; Datko, B. D.; Garcia-Galvez, J. A.; GreyInterchain, J. K. Charge-Transfer States Mediate Triplet Formation in Purified Conjugated Polymer Aggregates. J. Phys. Chem. C 2016, 120, 23230−23238. (2) Zhang, W.; Pan, X.; Feng, X. F.; Wang, C. H.; Yang, Y. W.; Ju, H. X.; Zhu, J. F. Interface Properties between a Low Bandgap Conjugated Polymer and a Calcium Metal Electrode. Phys. Chem. Chem. Phys. 2016, 18, 9446−9452. (3) Inganas, O.; Zhang, F. L.; Andersson, M. R. Alternating Polyfluorenes Collect Solar Light in Polymer Photovoltaics. Acc. Chem. Res. 2009, 42, 1731−1739. (4) Schwartz, B. J. Conjugated Polymers as Molecules Materials: How Chain Conformation and Film Morphology Influence Energy Transfer and Interchain Interactions. Annu. Rev. Phys. Chem. 2003, 54, 141−172. (5) Wu, H. B.; Ying, L.; Yang, W.; Cao, Y. Progress and Perspective of Polymer White Light-Emitting Devices and Materials. Chem. Soc. Rev. 2009, 38, 3391−3400.

4. CONCLUSIONS In summary, solution precursor method could be applied as a new approach to manipulate the condensed-state structure of PFO thin films. PFO aggregations could be transferred from precursor solutions to thin films. The relationship between βphase content and precursor solution was also established. PFO concentration could affect the extension of chain aggregation and thus generate different β-phase contents. β-phase not only G

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The Journal of Physical Chemistry C (6) Zhang, D. Z.; Xu, R. L.; Sun, L.; Chen, C.; Gao, F. L.; Zhang, X. D.; Zhou, J. R.; Ruan, S. P. Mechanism of Polyfluorene Interlayer in Ultraviolet Photodetector: Barrier-Blocking Electron Transport and Light-Inducing Hole Injection. J. Phys. Chem. C 2016, 120, 26103− 26109. (7) Ullah, H.; Bibi, S.; Tahir, A. A.; Mallick, T. K. Density Functional Theory Study of Selenium-Substituted Low-Bandgap Donor−Acceptor−Donor Polymer. J. Phys. Chem. C 2016, DOI: 10.1021/ acs.jpcc.6b10441. (8) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Santos, D. A. D.; Bredas, J. L.; Logdlund, M.; Salaneck, W. R.; et al. Electroluminescence in Conjugated Polymers. Nature 1999, 397, 121−128. (9) Cheng, Y. J.; Yang, S. H.; Hsu, C. S. Synthesis of Conjugated Polymers for Organic Solar Cell Applications. Chem. Rev. 2009, 109, 5868−5923. (10) McGehee, M. D.; Heeger, A. J. Semiconducting (Conjugated) Polymers as Materials for Solid State Lasers. Adv. Mater. 2000, 12, 1655−1668. (11) Noriega, R.; Rivnay, J.; Vandewal, K.; Koch, F.; Stingelin, N.; Smith, P.; Toney, M. F.; Salleo, A. A General Relationship between Disorder, Aggregation, and Charge Transport in Conjugated Polymers. Nat. Mater. 2013, 12, 1038−1044. (12) Knaapila, M.; Winokur, M. J. Structure and Morphology of Polyfluorenes in Solutions and the Solid State. Adv. Polym. Sci. 2008, 212, 227−272. (13) Knaapila, M.; Monkman, A. P. Methods for Controlling Structure and Photophysical Properties in Polyfluorene Solutions and Gels. Adv. Mater. 2013, 25, 1090−1108. (14) Chen, M. C.; Hung, W. C.; Su, A. C.; Chen, S. H.; Chen, S. A. Nanoscale Ordered Structure Distribution in Thin Solid Film of Conjugated Polymers: Its Significance in Charge Transport across the Film and in Performance of Electroluminescent Device. J. Phys. Chem. B 2009, 113, 11124−11133. (15) Chou, K. W.; Yan, B. Y.; Li, R. P.; Li, E. Q.; Zhao, K.; Anjum, D. H.; Alvarez, S.; Gassaway, R.; Biocca, A.; Thoroddsen, S. T.; Hexemer, A.; Amassian, A. Spin-Cast Bulk Heterojunction Solar Cells: A Dynamical Investigation. Adv. Mater. 2013, 25, 1923−1929. (16) Miyamae, T.; Tsukagoshi, K.; Mizutani, W. Two-color Sum Frequency Generation Study of Poly(9,9-dioctylfluorene)/Electrode Interfaces. Phys. Chem. Chem. Phys. 2010, 12, 14666−14669. (17) Das, D.; Gopikrishna, P.; Singh, A.; Dey, A.; Iyer, P. K. Efficient Blue and White Polymer Light Emitting Diodes Based on a Well Charge Balanced, Core Modified Polyfluorene Derivative. Phys. Chem. Chem. Phys. 2016, 18, 7389−7394. (18) Choi, E. Y.; Mazur, L.; Mager, L.; Gwon, M.; Pitrat, D.; Mulatier, J. C.; Monnereau, C.; Fort, A.; Attias, A. J.; Dorkenoo, K.; et al. Photophysical Amplified Spontaneous Emission and Charge Transport Properties of Oligofluorene Derivatives in Thin Films. Phys. Chem. Chem. Phys. 2014, 16, 16941−16956. (19) Monkman, A. P.; Rothe, C.; King, S.; Dias, F. B. Polyfluorene Photophysics. Adv. Polym. Sci. 2008, 212, 187−225. (20) Scherf, U.; List, E. J. W. Semiconducting Polyfluorenes Towards Reliable Structure Property Relationships. Adv. Mater. 2002, 14, 477−487. (21) Chen, S. H.; Su, A. C.; Su, C. H.; Chen, S. A. Crystalline Forms and Emission Behavior of Poly(9,9-di-n-octyl-2,7-fluorene). Macromolecules 2005, 38, 379−385. (22) Chen, S. H.; Chou, H. L.; Su, A. C.; Chen, S. A. Molecular Packing in Crystalline PFO Poly(9,9-di-n-octyl-2,7-fluorene). Macromolecules 2004, 37, 6833−6838. (23) Chen, C. Y.; Chang, C. S.; Huang, S. W.; Chen, J. H.; Chen, H. L.; Su, C. I.; Chen, S. A. Phase-Separation-Induced Gelation of Poly(9,9-dioctylfluorene)/Methylcyclohexane Solution. Macromolecules 2010, 43, 4346−4354. (24) Bright, D. W.; Dias, F. B.; Galbrecht, F.; Scherf, U.; Monkman, A. P. The Influence of Alkyl-Chain Length on Beta-Phase Formation in Polyfluorenes. Adv. Funct. Mater. 2009, 19, 67−73.

(25) Knaapila, M.; Bright, D. W.; Stepanyan, R.; Torkkeli, M.; Almasy, L.; Schweins, R.; Vainio, U.; Preis, E.; Galbrecht, F.; Scherf, U.; Monkman, A. P. Network Structure of Polyfluorene Sheets as a Function of Alkyl Side Chain Length. Phys. Rev. E 2011, 83, 05180310−051803-11. (26) Mikhnenko, O. V.; Blom, P. W. M.; Loi, M. A. Sensitive Triplet Exciton Detection in Polyfluorene Using Pd-Coordinated Porphyrin. Phys. Chem. Chem. Phys. 2011, 13, 14453−14456. (27) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M.; et al. Two Dimensional Charge Transport in Self-Organized, High-Mobility Conjugated Polymers. Nature 1999, 40, 685−688. (28) Donley, C. L.; Zaumseil, J.; Andreasen, J. W.; Nielsen, M. M.; Sirringhaus, H.; Friend, R. H.; Kim, J. S. Effects of Packing Structure on the Optoelectronic and Charge Transport Properties in Poly(9,9di-n-octylfluorene-alt-benzothiadiazole). J. Am. Chem. Soc. 2005, 127, 12890−12899. (29) Kline, R. J.; McGehee, M. D.; Toney, M. F. Highly Oriented Crystals at the Buried Interface in Polythiophene Thin-Film Transistors. Nat. Mater. 2006, 5, 222−228. (30) Chunwaschirasiri, W.; Tanto, B.; Huber, D. L.; Winokur, M. J. Chain Conformations and Photoluminescence of PFO (Poly(di-noctylfluorene). Phys. Rev. Lett. 2005, 94, 107402-1−107402-4. (31) Arif, M.; Volz, C.; Guha, S. Chain Morphologies in Semicrystalline Polyfluorene: Evidence from Raman Scattering. Phys. Rev. Lett. 2006, 96, 025503-1−025503-4. (32) Chen, S. H.; Su, A. C.; Chen, S. A. Noncrystalline Phases in Poly(9,9-di-n-octyl-2,7-fluorene). J. Phys. Chem. B 2005, 109, 10067− 10072. (33) Peet, J.; Brocker, E.; Xu, Y. H.; Bazan, G. C. Controlled β Phase Formation in Poly(9,9-di-n-octylfluorene) by Processing with Alkyl Additives. Adv. Mater. 2008, 20, 1882−1885. (34) Lu, H. H.; Liu, C. Y.; Chang, C. H.; Chen, S. A. Self-Dopant Formation in Poly(9,9-di-n-octylfluorene) via a Dipping Method for Efficient and Stable Pure-Blue Electroluminescence. Adv. Mater. 2007, 19, 2574−2579. (35) Rahman, M. H.; Chen, C. Y.; Liao, S. C.; Chen, H. L.; Tsao, C. S.; Chen, J. H.; Liao, J. L.; Ivanov, V. A.; Chen, S. A. Segmental Alignment in the Aggregate Domains of Poly(9,9-dioctylfluorene) in Semidilute Solution. Macromolecules 2007, 40, 6572−6578. (36) Justino, L. L. G.; Ramos, M. L.; Knaapila, M.; Marques, A. T.; Kudla, C. J.; Scherf, U.; Almásy, L.; Schweins, R.; Burrows, H. D.; Monkman, A. P. Gel Formation and Interpolymer Alkyl Chain Interactions with Poly(9,9-dioctylfluorene-2,7-diyl) (PFO) in Toluene Solution: Results from NMR, SANS, DFT, and Semiempirical Calculations and Their Implications for PFO β-Phase Formation. Macromolecules 2011, 44, 334−343. (37) Knaapila, M.; Garamus, V. M.; Dias, F. B.; Almásy, L.; Galbrecht, F.; Charas, A.; Morgado, J.; Burrows, H. D.; Scherf, U.; Monkman, A. P. Influence of Solvent Quality on the Self-Organization of Archetypical Hairy Rods−Branched and Linear Side Chain Polyfluorenes: Rodlike Chains versus “Beta-Sheets” in Solution. Macromolecules 2006, 39, 6505−6512. (38) Grell, M.; Bradley, D. D. C.; Ungar, G.; Hill, J.; Whitehead, K. S. Interplay of Physical Structure and Photophysics for a Liquid Crystalline Polyfluorene. Macromolecules 1999, 32, 5810−5817. (39) Schmidt, K.; Tassone, C. J.; Niskala, J. R.; Yiu, A. T.; Lee, O. P.; Weiss, T. M.; Wang, C.; Fréchet, J. M. J.; Beaujuge, P. M.; Toney, M. F. A Mechanistic Understanding of Processing Additive-Induced Efficiency Enhancement in Bulk Heterojunction Organic Solar Cells. Adv. Mater. 2014, 26, 300−305. (40) Gao, Y. J.; Pivrikas, A.; Xu, B.; Liu, Y.; Xu, W. Q.; van Loosdrecht, P. H. M.; Tian, W. J. Measuring Electron and Hole Mobilities in Organic Systems: Charge Selective CELIV. Synth. Met. 2015, 203, 187−191. (41) Huang, L.; Huang, X. N.; Sun, G. N.; Gu, C.; Lu, D.; Ma, Y. G. Study of β Phase and Chains Aggregation Degrees in Poly(9,9H

DOI: 10.1021/acs.jpcc.6b08941 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C dioctylfluorene) (PFO) Solution. J. Phys. Chem. C 2012, 116, 7993− 7999. (42) Nguyen, T.; Doan, V.; Schwartz, B. J. Conjugated Polymer Aggregates in Solution: Control of Interchain Interactions. J. Chem. Phys. 1999, 110, 4068−4078. (43) Yang, C. Y.; Hide, F.; Diaz-Garcia, M. A.; Heeger, A. J.; Cao, Y. Microstructure of Thin Films of Photoluminescent Semiconducting Polymers. Polymer 1998, 39, 2299−2304. (44) Frank, C. W.; Rao, V.; Despotopoulou, M. M.; Pease, R. F.W.; Hinsberg, W. D.; Miller, R. D.; Rabolt, J. F. Structure in Thin and Ultrathin Spin-Cast Polymer Films. Science 1996, 273, 912−915. (45) Chen, S. H.; Su, A. C.; Chang, C. S.; Chen, H. L.; Ho, D. L.; Tsao, C. S.; Peng, K. Y.; Chen, S. A. Aging of Poly(2-methoxy-5-(2‘ethylhexyloxy)-1,4-phenylenevinylene)/Toluene Solutions and Subsequent Effects on Luminescence Behavior of Cast Films. Langmuir 2004, 20, 8909−8915. (46) Chen, J. H.; Chang, C. S.; Chang, Y. X.; Chen, C. Y.; Chen, H. L.; Chen, S. A. Gelation and Its Effect on the Photophysical Behavior of Poly(9,9-dioctylfluorene-2,7-diyl) in Toluene. Macromolecules 2009, 42, 1306−1314. (47) Knaapila, M.; Dias, F. B.; Garamus, V. M.; Almásy, L.; Torkkeli, M.; Leppanen, K.; Galbrecht, F.; Preis, E.; Burrows, H. D.; Scherf, U.; Monkman, A. P. Influence of Side Chain Length on the Self-Assembly of Hairy-Rod Poly(9,9-dialkylfluorene)s in the Poor Solvent Methylcyclohexane. Macromolecules 2007, 40, 9398−9405. (48) Surin, M.; Hennebicq, E.; Ego, C.; Marsitzky, D.; Grimsdale, A. C.; Müllen, K.; Brédas, J. L.; Lazzaroni, R.; Leclère, P. Correlation between the Microscopic Morphology and the Solid-State Photoluminescence Properties in Fluorene-Based Polymers and Copolymers. Chem. Mater. 2004, 16, 994−1001. (49) Huang, L.; Zhang, L. L.; Huang, X. N.; Li, T.; Liu, B.; Lu, D. Study of the α-Conformation of the Conjugated Polymer Poly(9,9dioctylfluorene) in Dilute Solution. J. Phys. Chem. B 2014, 118, 791− 799.

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DOI: 10.1021/acs.jpcc.6b08941 J. Phys. Chem. C XXXX, XXX, XXX−XXX