J. Phys. Chem. B 2010, 114, 7469–7473
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Morphology-Dependent Optoelectronic Properties of Blue Emitter Poly(p-phenylene) Synthesized with Chemical Vapor Deposition Polymerization Chang-Tao Hsiao and Shih-Yuan Lu* Department of Chemical Engineering, National Tsing-Hua UniVersity, HsinChu, Taiwan 30013, Republic of China ReceiVed: January 24, 2010; ReVised Manuscript ReceiVed: May 4, 2010
Poly(p-phenylene) (PPP), an important blue emitter, is difficult to be fabricated in thin film form with a wet process because of its insoluble and infusible characteristics resulting from its rigid backbone structure. Therefore, various indirect and multistep synthetic approaches have been proposed for fabrication of PPP films. In this article, a chemical vapor deposition polymerization process was developed to fabricate successfully PPP films in one-step directly without the often required postreaction thermal treatment. We further demonstrated that the polymer chain conformation of the film can be effectively adjusted through a simple surface treatment of the substrate, and the variations in film morphology induced by the substrate characteristics led to significant changes in the optoelectronic properties of the film. Introduction Conjugated polymers have been studied extensively and intensively in the past two decades because of their cost effectiveness, mechanical flexibility, good processability, and unique optoelectronic properties. These advantages make them potentially interesting for applications in organic light-emitting diodes (OLED),1 photovoltaics,2 and field effect transistors.3 Poly(p-phenylene) (PPP), an important blue emitter, has attracted much research attention because of its high thermal and oxidation resistances as well as simplicity in molecular structure. However, its rigid backbone, composed of aromatic rings of strong π-conjugations, is essentially insoluble and infusible, making it difficult to be fabricated in thin film form with a wet process. Therefore, various indirect, multistep synthetic approaches have been proposed for fabrication of PPP films such as the synthesis of soluble PPP precursors or derivatives to enable a spin-coating procedure4,5 and the vacuum deposition of PPP from its parent powders.6,7 Most of the preparation methods mentioned in the literature for PPP precursors, derivatives, and powders suffer a common disadvantage: the issues of metallic catalyst separation, product purification, and organic solvent disposal. In this study, PPP thin film synthesis with a dry, facile one-step chemical vapor deposition polymerization (CVDP) process based on a precursor monomer, bis-chloro-biphenylene, is reported. In recent years, various CVDP processes have been developed to synthesize successfully poly(p-phenylene vinylene) (PPV) and derivatives in the form of thin film or nanostructure, exhibiting excellent optoelectronic properties.8-12 In most CVDP processes, a dehydrohalogenation step at hightemperature pyrolysis is required to form the activated intermediate. After the condensation polymerization in a lowtemperature environment, a subsequent thermal conversion treatment under inert gas is necessary to form the wellconjugated structure. The present CVDP process, however, is free from the final thermal conversion treatment in producing the product film, a significant process simplification. Furthermore, the halide ions are removed during the high-temperature * Corresponding author.
pyrolysis, and thus there is no need for the troublesome postpolymerization separation and purification. We further studied the film morphology control through modulation of the surface characteristics of the collection substrate. The substrate was controlled to be hydrophilic or hydrophobic, and the resulting PPP film morphology changed accordingly. More importantly, the difference in film morphology led to significant variations in the optoelectronic properties of the film. The photoluminescence (PL) quantum yield of the film produced on hydrophobic substrates was found to be consistently higher than that on hydrophilic substrates, and the electric conductivity of the film deposited on hydrophobic substrates was significantly higher than that on hydrophilic substrates. This study demonstrated the feasibility of modulating the optoelectronic properties of conducting polymer films through morphology control achieved with surface characteristics adjustment of the collection substrate. Experimental Section Surface Pretreatment of Substrate. Bare (FEA microscope slides) and FTO glasses (Solaronix, Swiss) were used as the substrate. The substrate was cleaned with a mixture of sulfuric acid and hydrogen peroxide (H2SO4/H2O2 6:1 vol %) and deionized water under megasonic agitation. The hydrophilic substrate was obtained after rinsing with acetone and drying. To prepare hydrophobic substrates, the hydrophilic substrate was soaked in 6 wt % trimethyl-chloro-silane (TMCS) solution for ∼24 h to reach an adsorption equilibrium. Deposition of PPP Films. The CVDP setup is depicted in Figure 1. The temperatures of the precursor holder, furnace, and deposition zone are controlled at T1, T2, and T3, respectively. An amount of 0.25 g monomer precursor, 4,4′-bischloromethyl-1,1′-biphenyl, was loaded in the precursor holder (2) set at 110 °C (T1). The vapors of the monomer precursor were passed through the furnace (3) for pyrolysis at 800 °C (T2) and then deposited onto the collection substrate, hydrophilic or hydrophobic, in the deposition zone (4) maintained at 100 °C (T3). Samples produced at different deposition times were
10.1021/jp1006846 2010 American Chemical Society Published on Web 05/18/2010
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Figure 1. Setup for the CVDP process. (1) Carrier gas, N2; (2) precursor holder for 4,4′-bis-chloromethyl-1,1′-biphenyl with a temperature controller; (3) furnace with a quartz tube as the reaction chamber; (4) deposition zone with a collection substrate; (5) cold trap; and (6) vacuum pump.
collected to study the evolution of the morphology and optoelectronic properties of the film. The deposition rate was ∼6 nm/min. Characterizations. The film morphology was investigated with a scanning electron microscope (JSM-5600, JEOL). The surface roughness (Rms in units of nanometers) was measured with an atomic force microscope (NanoScope E, Digital Instruments, Inc.) under a tapping mode over an area of 100 µm2. The FT-IR spectroscopy was conducted with a spectrophotometer (Spectrum RX-I, Perkin-Elmer). The optical properties of the film were characterized with a UV-visible (U-3300, Hitachi) and PL (F-4500, Hitachi) spectrophotometers. The crystallographic structure of the film was studied with an X-ray diffractometer (MXP18, MAC Science, Cu KR). The current density-voltage (J-V) curves of the film, in a sandwich form of FTO/PPP/FTO, were recorded with a Keithley 236 measurement unit under the static mode. The sandwich structure of the test cells was constructed by clamping two FTO glasses together with one of the two FTO glasses deposited with the PPP film. Note that the deposition time used to create samples for the FTIR, XRD, and J-V measurements was 270 min, to ensure a full coverage of the substrate with the PPP deposit. Result and Discussion We first investigated the surface characteristics of the bare glass and fluorine-doped tin oxide (FTO)-coated glass substrates upon modification of TMCS. From Figure 2, it is evident that the hydrophobicity of both kinds of substrates was significantly enhanced with the TMCS treatment. Also, the substrate surface became hydrophobic upon deposition of the PPP film. The TMCS treatment did achieve a certain extent of adjustment on the surface characteristics of the substrate. We later showed that such variations were enough to induce drastic film morphology change and the consequent differences in optoelectronic properties of the films.
Figure 2. Static contact angle measurements for different surfaces.
deposition time (min)
30
90
270
C O Si Cl Cu
57.38 39.61 2.09 0.02 0.89
87.57 11.62
95.4 3.97
0.87
0.65
Next, we performed the SEM-EDS analyses on the films deposited at three increasing deposition times to investigate if any chlorine-containing side product remained in the film. As tabulated in Table 1, the chlorine signal disappeared after a deposition time of 90 min or longer. Even at a deposition time of 30 min, the chlorine content was very minor. This showed that the present CVDP process can avoid the troublesome postdeposition separation. Note here that the C signals came from the polymer film, Si signals from the glass substrate, O signals from both the glass substrate and the environment, and Cu signals from the copper tape. Also note that hydrogen cannot be detected with SEM-EDS. Table 1 also showed an increasing content of C as the deposition proceeded longer to generate thicker films. The Si signals coming from the substrate diminished as the film grew thicker to shield the substrate composition. To study the molecular structure of the product film, we conducted FT-IR spectroscopy. Figure 3a shows the resulting FT-IR spectrum of the deposited film. The two peaks located at 690 and 754 cm-1 came from the C-H out-of-plan deformation of the monosubstituted phenyl ring, the peak at 800 cm-1 was contributed by the C-H out-of-plane deformation of the 1,4-disubstituted benzene, the peak at 1033 cm-1 accounted for the in-plane deformation, and the peak at 1478 cm-1 came from the ring stretching (CdC) structure.13 The spectrum was in good agreement with that reported in the literature for PPP films.13 The X-ray diffraction patterns of the PPP films were also recorded and shown in Figure 3b for both hydrophilic and hydrophobic substrate cases. Both patterns exhibited three characteristic peaks in the 2θ ranges of 19.1-20.4, 21.0-22.8, and 26.3-28°. With the postulated orthorhombic unit cell, the following Miller indices can be assigned to these diffraction peaks: (110), (200), and (210) from left to right.14,15 Again, the XRD patterns agreed well with those reported in the literature for PPP films.14,15 Evidently, there was no appreciable difference between the two XRD patterns of Figure 3b. Nevertheless, there did exist a slight difference in the grain size of the two samples. The grain size, as estimated by the Scherrer equation based on
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Figure 3. (a) FT-IR spectrum of the PPP thin film. (b) XRD patterns of the PPP thin films deposited on hydrophilic and hydrophobic substrates. The grain sizes were estimated with the Scherrer equation. The asterisks denote the main peaks of each spectrum.
Figure 4. Schematic reaction mechanism of the CVDP process. The inset images show (A) monomer precursor and (B) PPP excited by a 365 nm UV light.
the (200) peak, of the film grown on the hydrophilic substrate (13 nm) was slightly larger than that of the film grown on the hydrophobic substrate (10 nm). A reasonable reaction mechanism is proposed and illustrated in Figure 4. The thermal energy supplied by the furnace caused the homolytic bond rupture of the precursor molecules to generate monoradicals and biradicals, which later underwent recombination reactions at the substrate surface to afford the PPP thin films.16 The inset images of Figure 4 show the appearance of two solutions containing the monomer precursor and PPP fragments excited by a 365 nm UV light. Evidently, there were no visible emissions from the monomer solution, whereas bright blue lights were emitted from the PPP sample. To understand the chain conformation of the deposited PPP films, we studied the morphological evolution of the deposits on hydrophilic and hydrophobic substrates. Figure 5 shows the morphological and surface roughness evolutions of the deposits on hydrophilic and hydrophobic substrates. The poor wetting between the growing PPP and the hydrophilic surface led to the vertical growth of the PPP grains in the early growth stage, as depicted in Figure 5a-1,a-2. On the contrary, the PPP would prefer to grow along the substrate surface if the surface is hydrophobic, forming smoother particle films. (See Figure 5b1-b-3.) At the deposition time of 30 min, the deposit amounts were not enough to cover the substrate, and the surface roughnesses were thus close to that of the bare substrate, 2.7 nm. At 90 min, the polymer deposits almost fully cover the substrate, but there were still openings to the substrate, particularly for the hydrophilic case, leading to the highest surface roughnesses. Further increase in deposition time enabled filling of the openings and smoothing of the surface, with the roughness decreasing from 52 to 37 to 17 nm for the hydrophilic
Figure 5. Top view SEM images for morphological evolution of the PPP deposits: (a-1-a-4) deposition times of 30, 90, 270, and 1,200 min on hydrophilic surfaces, respectively; (b-1-b-4) deposition times of 30, 90, 270, and 1200 min on hydrophobic surfaces, respectively. The inserted schematics (lower left) are the proposed film growth process to go with the SEM images. The images located at the upper right corners are local enlargements of the SEM images for easier visualization. The scale bar for upper right inset images is 350 nm. Also included are surface roughnesses determined with AFM data shown in lower left insets.
case and from 28 to 25 to 19 nm for the hydrophobic case. Note that the surface roughnesses of the hydrophilic case are larger than those of the corresponding hydrophobic case, mainly because of the different polymer growth habits induced by the substrate characteristics in the early growth stage. At 1200 min, the interactions between the growing PPP and the substrate surface were shielded by the already deposited PPP film and
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Figure 6. UV-visible spectra (left): (a) films deposited on hydrophilic surfaces, (c) films deposited on hydrophobic surfaces; and corresponding PL spectra (right): (b) films deposited on hydrophilic surfaces, (d) films deposited on hydrophobic surfaces.
thus became negligible, and the polymer growth habit for the hydrophilic case turned similar to that for the hydrophobic case (Figure 5a-4,b-4), giving close surface roughnesses. The schematic morphology evolutions are also inserted in Figure 5 to go with the top view SEM images and AFM surface roughness data. The optoelectronic properties of conjugated polymer films often depend on the conformation of the polymer chains and the way these chains pack in the films.17 We investigated how the morphological difference induced by the substrate characteristics affected the UV-visible absorption, PL emission, and electron conduction of the films. Figure 6 shows the UV-visible and PL spectra of the films prepared at deposition times of 30, 90, and 270 min on hydrophilic and hydrophobic substrates. All samples exhibited an on-set absorption at ∼405 nm and a main PL peak centering around 460 nm, typical for PPP films.1 Note that the on-set absorption wavelengths for samples prepared with deposition times of 30 and 90 min can be clearly determined by suitably magnifying the vertical axis. Both UV-visible and PL spectra agreed well with those reported in the literature.1 The intensities of the UV-visible absorptions and PL emissions increased with increasing deposition time, as they should. However, we did not observe evident differences between the corresponding spectra of the hydrophilic and hydrophobic cases. Deeper investigations were needed. An investigation of the PL quantum yield revealed the effect of the morphological difference. Here the PL quantum yields were determined using the same formula described in refs 11 and 12. In Figure 7, we plotted the PL quantum yield versus the deposition time for the six samples. Evidently, the PL quantum yields increased with increasing deposition time. This is probably because of the better developed conjugation structure of the polymer chains with longer deposition time. Second, the
Figure 7. PL quantum yield and PL quantum yield ratio versus the deposition time.
PL quantum yields of the hydrophobic case are consistently higher than the corresponding samples of the hydrophilic case. The quantum yield ratio, plotted in the same Figure, could reach as high as 2.5 for the short deposition time samples. A possible reason for this phenomenon is described as follows. The grain sizes were estimated with the Scherrer equation to be 13 and 10 nm for samples of the hydrophilic and hydrophobic cases, respectively. Smaller grains contain fewer polymer chains and thus fewer interchain interactions to promote electron-hole pair separations. Smaller grains also mean a larger number of grain boundaries. These grain boundaries retard charge transfer and thus are beneficial for confining charge carriers within the grains. Both effects are favorable for enhancing the electron-hole pair recombination, giving rise to higher quantum yields for the hydrophobic case. Figure 8 shows the J-V curves, measured at room temperature, of the PPP films grown from substrates of different characteristics. The J-V curves were collected via a Keithley
Properties of Blue Emitter Poly(p-phenylene)
J. Phys. Chem. B, Vol. 114, No. 22, 2010 7473 hopping between polymer grains is necessary for the overall charge transport.18 As previously discussed, there would be a larger number of grain boundaries existing in the film of the hydrophobic case because of the smaller grain size. Consequently, a higher charge carrier transport resistance results, further suppressing the current density of the hydrophobic case. In conclusion, a CVDP process was developed to fabricate successfully PPP films in one-step directly. With the present process, no postdeposition separation and thermal conversion are needed to afford the product films. The polymer chain conformation of the film can be effectively adjusted through a simple surface treatment of the substrate. The variations in film morphology induced by the substrate characteristics led to significant changes in the optoelectronic properties of the film.
Figure 8. Room-temperature J-V curves for PPP films grown from substrates of different characteristics.
Acknowledgment. We thank the National Science Council of the Republic of China (Taiwan) under grant NSC 98-2221E-007-036-MY3 and the Top program of the National TsingHua University for financial support. References and Notes
Figure 9. Schematic of charge carrier transport in PPP film.
236 measurement unit under the device format of FTO/PPP/ FTO. There can be observed a significant difference in current density, with the hydrophilic case achieving higher current densities. This significant difference in current density can be related to the morphological difference of the two cases. As illustrated in Figure 9, the orientation of the polymer grains played an important role here. For the hydrophilic case, the vertical growth habit induced by the nonwetting interaction between the growing PPP and the hydrophilic substrate made easy the charge carrier transport along the growth axis of the polymer grains, in line with the overall electron flow direction of the device. The situation for the hydrophobic case is just the opposite of that for the hydrophilic case. The orientation of the polymer grains is perpendicular to the overall electron flow direction, making it difficult to drive the electron flow within the neighboring polymer grains. Furthermore, the charge carrier
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