Effect of Aggregation on the Optical and Charge Transport Properties

Apr 10, 2008 - Andres Garcia and Thuc-Quyen Nguyen*. Center for Polymers and Organic Solids, Department of Chemistry and Biochemistry, UniVersity of ...
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J. Phys. Chem. C 2008, 112, 7054-7061

Effect of Aggregation on the Optical and Charge Transport Properties of an Anionic Conjugated Polyelectrolyte Andres Garcia and Thuc-Quyen Nguyen* Center for Polymers and Organic Solids, Department of Chemistry and Biochemistry, UniVersity of California, Santa Barbara, California 93106 ReceiVed: December 5, 2007; In Final Form: February 5, 2008

The steady-state optical and charge transport properties of the anionic polythiophene, poly[3-(potassium-4butanoate)thiophene-2,5-diyl], are reported. The degree of interchain aggregation in solution and in the film state can be controlled by solution processing conditions and the introduction of bulky organic counterions. Disruption of interchain contacts leads to significant changes in the absorption and emission properties. Scanning probe microscopy reveals various surface morphologies, depending on the solution history. Attempts to determine electronic charge carrier mobilities by using steady-state current-voltage (I-V) measurements using hole-only diodes gives rise to light-emitting electrochemical cell behavior, indicating ion motion and/ or redistribution within the films. The use of a pulsed I-V method operating at frequencies higher than the ion response times allows measurements of electronic carrier mobilities in hole-only diodes. Hole mobilities in films with different degrees of chain aggregation reveal an order of magnitude difference (∼10-8 to 10-9 cm2/Vs).

I. Introduction Conjugated polyelectrolytes (CPEs) are polymers having a conjugated backbone with pendent cationic or/and anionic groups. These molecular features provide materials with the optical and electronic properties of conjugated polymers in combination with the behavior of polyelectrolytes, which can be modulated by electrostatic forces. CPEs can be water soluble, a property which allows incorporation into biosensors that take advantage of their large optical cross sections.1-8 More recently, there has been an upsurge in using CPEs in optoelectronic devices with performances not attainable with conventional neutral conjugated polymers. Their solubility in polar solvents allows fabrication of multilayer polymer light-emitting diodes (PLEDs) with the CPE as the electron-transporting layer and minimum intermixing between the CPE/polymer interface.9-13 CPEs have also been used in single-component light-emitting electrochemical cells (LECs), in which a single layer of CPE sandwiched between two electrodes has both the electrochemical and emissive components.14-17 Organic solar cells with CPEs have also been examined.18-20 To further develop the applications of CPEs in optical and electronic devices there is a need to establish an understanding of their solution and solid-state properties. The structural attributes of CPEs provide additional parameters, relative to neutral counterparts, for fine-tuning desirable properties. For example, intra- and interchain interactions, and hence electronic properties, can be altered by parameters such ionic strength, pH, counterion(s) etc., as in traditional polyelectrolytes.21 Most studies have been performed in solution;22-30 however, there is a need to investigate solid-state properties, such as optical and electronic charge carrier transport, which are essential to their function in optoelectronic devices. Mobility values for CPEs are very scarce in the literature because electronic charge transport studies by conventional methods involved an applied * Corresponding author. E-mail: [email protected].

bias such as single charge carrier diodes, and field effect transistors do not work due to the interference of ion motion. In this contribution, we investigate the optical and charge transport properties of the anionic polythiophene, poly[3(potassium-4-butanoate)thiophene-2,5-diyl] (P3PBT in Figure 1). Steady-state spectroscopy shows that the degree of aggregation depends on the solution history and that it can be disrupted by the addition of cations or poly(ethylene glycol) (PEG). Solution aggregation can be transferred onto films similar to that observed with neutral conjugated polymers.31,32 Scanning probe microscopy reveals surface features that correspond to the differences in polymer association. The ability to control the degree of aggregation allows us to investigate its influence on electronic charge carrier transport. We show that pulsed bias methods can be used for electronic carrier mobility measurements without the complication of ion migration under the applied field. II. Experimental Methods P3PBT was purchased from Reike Metals. PEG (Mn ∼ 3000), tetrabutylammonium bromide (TBABr), and benzyltriethylammonium chloride (BnTEACl) were purchased from Aldrich; all materials were used as received. P3PBT stock solutions with concentrations of 5% (w/v) and 0.1% (w/v) were prepared with 18 MΩ deionized water and were stirred for 10 h at 80 °C before use. Solutions of P3PBT (5% w/v) with and without PEG or additional salts were used to spin-cast films. For solution spectroscopic measurements, 1 and 10 mm quartz cuvettes were used for high- and low-concentration solutions, respectively. Quartz substrates were used for film measurements. UV-vis absorption spectra were recorded on a Shimadzu UV-2401 PC diode array spectrometer. Fluorescence was measured by using a PTI Quantum Master fluorometer. Solution photoluminescence quantum yield measurements were carried out in water solutions using the optically dilute method.33 Solutions of fluorescein in water were used as a reference.

10.1021/jp711462p CCC: $40.75 © 2008 American Chemical Society Published on Web 04/10/2008

Optical and Charge Transport Properties of a CPE

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Figure 1. Chemical structures of the conjugated polyelectrolyte P3PBT, PEG, and ammonium salts.

Polymer films were spin-coated at 2000 rpm from solutions onto either quartz (spectroscopic measurements) or indium/tin oxide (ITO) (charged transport measurements) and were heated at 80 °C under a nitrogen atmosphere for 2 h. Substrates were cleaned by heating in 70:30 (v/v) H2SO4/H2O2 solution (quartz only), followed by successive rinsing and ultrasonic treatment in water, acetone, isopropyl alcohol, and then drying with N2 gas and several hours in an oven. The substrates were treated with UV/O3 prior to polymer deposition. Film thicknesses were measured with an atomic force microscope (AFM). Scanning probe measurements were preformed using a commercial scanning probe microscope (MultiMode with a Nanoscope Controller IIIa, Veeco Inc.). All scanning probe measurements were done under ambient conditions. Silicon probes with a spring constant of ∼5 N/m and a resonant frequency of ∼75 kHz (Budget Sensors) were used for tapping AFM measurements, whereas for conductive atomic force microcopy (C-AFM) measurements Pt-coated probes with a spring constant of ∼0.2 N/m and resonant frequency of ∼13 kHz were used. Bulk electronic charge transport measurements were performed under a nitrogen atmosphere through the use of holeonly diodes. These devices were fabricated by spin-casting a polymer film onto a patterned ITO substrate. After the filmdrying procedure described above, a 100 nm thick gold electrode (4.9 mm2 contact area) was thermally deposited at 10-7 Torr (Angstrom Engineering, Inc.) using a shadow mask. Currentvoltage (I-V) measurements were recorded with a Keithley 2602 or 4200 SCS. III. Results and Discussion III.A. Optical Properties of Solutions and Films. In neutral conjugated polymers, the degree of aggregation decreases at lower polymer concentrations and is independent of the stock solution concentration used to prepare low-concentration solutions because the polymer and solvent medium are, by and large, hydrophobic. In CPEs, there are several interactions to consider: hydrophobic-hydrophobic (conjugated backbone), hydrophilic-hydrophilic (charge group-solvent medium), and hydrophobic-hydrophilic (backbone-solvent medium). Thus, we anticipated that it is more difficult to disrupt the aggregation in CPEs by serial dilution, as compared to neutral conjugated polymers in good solvents, and the degree of aggregation in a dilute solution retains some of the characteristics from the original stock solution. In other words, the degree of aggregation is more dependent on the solution history. To investigate this

Figure 2. Solution absorption spectra at various concentrations diluted from 5% (w/v) (high concentration) (a) and 0.1% (w/v) (low concentration) (b). Spectra are normalized based on the 420 nm absorption peaks.

hypothesis, 0.1%, 0.01%, and 0.001% (w/v) P3PBT solutions were prepared from two stock solutions with different concentrations: 0.1% and 5% (w/v). The stock solutions were individually prepared by weighing the CPE powder, followed by addition of the solvent up to the appropriate volume. The absorption spectra of 0.1%, 0.01%, and 0.001% (w/v) solutions prepared from 5% and 0.1% (w/v) stock solutions are shown in Figure 2. The absorption spectra of 0.1%, 0.01%, and 0.001% (w/v) solutions prepared from 5% (w/v) stock solution are broad with multiple peaks (Figure 2a). An isosbestic point observed at 420 nm indicates the presence of two chromophoric states. No shift in the absorption maxima is observed upon dilution, but a slight decrease in the absorption maximum centered at 500 nm along with an increase in the band centered at 420 nm is observed. Increasing the polymer concentration leads to an increase in the intensity of the 500 nm peak. Therefore, we assign the absorption band centered at 500 nm to aggregates having strong interchain and/or intrachain π-π interactions with more electronic delocalization (lower energy).34 The absorption band centered at 420 nm is assigned to more isolated chains. Blue and red shifts of absorption bands can also be attributed to H- (blue shift) and J- (red shift) aggregates.35,36 The bluer shifted band centered at 420 nm is speculated to arise from isolated chains and not H-aggregates because, as will be shown later, stronger fluorescence is observed upon excitation of this band, whereas H-aggregates should exhibit weaker fluorescence than red-shifted J-aggregates due to the forbidden transition from the excited to the ground state. Dilute solutions prepared from a 0.1% (w/v) stock solution have a significantly lower fraction of aggregates, and no shift in the absorption maxima upon dilution is observed (Figure 2b). The larger fraction of aggregated chains observed in solutions prepared from higher concentration stock solutions indicate a concentration-dependent formation of aggregates. This has been observed in neutral conjugated polymers, in which increasing solution concentration increases interchain aggregation.37 A higher degree of aggregation is observed in dilute solutions prepared from 5% (w/v) stock solution than from 0.1% (w/v) stock solution. These observations suggest that the multiple interactions in CPEs lead to kinetically stable aggregates.

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Figure 3. Photoluminescence spectra excited at 420 (dashed line) and 500 nm (dotted line). Photoluminescence excitation spectrum (solid line) with emission collected at 600 nm.

Figure 3 shows the normalized photoluminescence (PL) spectra of a 0.001% (w/v) solution diluted from a 5% (w/v) stock solution upon excitation at 420 and 500 nm and the photoluminescence excitation (PLE) with the emission collected at 600 nm. The PL spectra at the two excitation wavelengths are broad with a maximum centered at ∼600 nm. The PL intensity at 600 nm decreases by ∼39%, and an emission shoulder at longer wavelength is observed upon excitation at 500 nm, as compared to 420 nm. No significant shift of the emission was observed with longer excitation wavelengths (500, 550, and 600 nm). PL quantum yields of 3.4% and 1.0% with respect to fluorescein were measured at 420 and 500 nm excitations, respectively. The higher PL quantum yield of the band centered at 420 nm than the red-shifted aggregated band leads us to assign this band as originating from more isolated chains rather than H-aggregates. The PLE spectrum collected at 600 nm as well shows very little contribution when the excitation wavelength is greater than 500 nm, consistent with the 420 nm absorption being due to single-chain species. Aggregates in general are known to quench fluorescence due to the longer radiative lifetimes of delocalized excited states,38,39 due to the forbidden transition from the excited to the ground state. The lower PL quantum yield of this band seems to indicate H-aggregates as the likely source, but the large relative red shift indicates an opposite assignment, J-aggregates. It is possible that although in J-aggregates the relaxation transition is allowed much more than in H-aggregates, this transition is allowed to a lower degree than isolated chains. Slightly higher PL intensities (∼8%) but similar PL behavior and quantum yields were observed with solutions diluted from a 0.1% (w/v) stock solution, consistent with decreased chain aggregation. Attempts to disrupt the aggregates by heating solutions up to 90 °C, by adding less polar solvents (THF, acetonitrile), or by adding the surfactant sodium dodecyl sulfate (SDS) did not yield observable results. The strength of the aggregate interactions is expected to be large because of the numerous π-π interactions between chains induced by the polar medium. Disruption of aggregation was achieved by adding large amounts (>50 monomer molar equiv) of TBABr and BnTEACl directly into the 0.001% (w/v) P3PBT solutions. Titration of the polymer solution with ammonium salt solutions had minor effects in reducing the fraction of aggregated species. However, adding the salts directly as a solid into the polymer solution causes an immediate change of the solution color from pink to yellow. Figure 4a shows the absorption and fluorescence spectra of 0.001% (w/v) P3PBT solutions diluted from a 5% (w/v) stock solution with the addition of solid ammonium salts. An increase in absorbance of the 420 nm peak and a decrease of the 500 nm band with the addition of BnTEACl and TBABr are observed. The bulkier TBABr salt effectively disrupts aggregated chains more than BnTEABr, as observed by narrower absorption and fluorescence bands and higher PL quantum yields

Figure 4. (a) Solution and absorption (solid lines) and PL (circles) spectra of P3PBT with addition of solid ammonium salts. (b) Absorption (solid lines) and PL (dashed) spectra of films spin-coated from pure P3PBT (black) and 95:10 (red) and 90:10 (blue) (w/w) P3PBT/ ammonium salt blend solutions.

(23% vs 4%, λexc ) 420 nm and 16% vs 1.5%, λexc ) 500 nm). The broader absorption and PL band in the presence of BnTEACl is indicative of aggregate formation with various sizes. The single-chain absorption maximum is slightly redshifted upon the addition of the ammonium salts, ∼3 nm with BnTEACl and ∼10 nm with TBABr. The red shifts can possibly be due to more open and elongated polymer coil conformations, leading to a slightly increased electronic delocalization. In neutral conjugated polymers, the interchain organization and aggregation in solution can survive the spin-coating process and be transferred into the film.31,32 Figure 4b shows the absorption and PL spectra of films spun cast from 5% (w/v) P3PBT solutions with and without ammonium salts (90:10 w/w). For the P3PBT film, the absorption (Figure 4b, solid curve) shows a stronger aggregate absorption band at 500 nm compared to the absorption spectra of solutions. Thus, there is higher degree of aggregation in film where most solvent is removed. The organic salts, as in solution, disrupt aggregated chains but are less effective in the disruption of aggregated chains in films than in solution. As with the observation in solution measurements, TBABr more effectively disrupts aggregation; only this salt yields films with observable PL (quantum yield of ∼1%). Films containing ammonium salts were found to be too rough for electronic measurements. Aggregate disruption in solution can be transferred to films by the addition of a small percentage of PEG. Films from 5% (w/v) P3PBT solutions with 90:10 (w/w) P3PBT/PEG show a substantial decrease of the aggregate absorption band, ∼73% lower than in pure P3PBT film. Unlike films of pure P3PBT, the addition of PEG gives rise to emission (Figure 5). PL quantum yields of ∼3% and 1% were measured for 95:5 and 90:10 (w/w) P3PBT/PEG films, respectively. The quality of the P3PBT/PEG blend films are much better than those obtained with ammonium salts. III.B. Film Morphology. Tapping mode AFM was used to examine the surface morphology of films prepared from pure P3PBT solution and from the P3PBT/PEG mixture. The tip in this measurement mode vibrates at its resonance frequency,

Optical and Charge Transport Properties of a CPE

Figure 5. Absorption (solid lines) and PL (circles) spectra of spun cast films from pure P3PBT (black) and 95:5 (red) and 90:10 (blue) (w/w) P3PBT/PEG solutions.

while scanning over the sample surface to give images of surface topography and phase simultaneously. Topographic images provide surface structure and roughness, whereas the phase image provides information on nanoscale variations in the surface “hardness” and “softness.” Phase contrast results when the tip and surface interaction results in a change in the oscillation of the cantilever as the energy of the vibrating tip is dampened by the sample; therefore, different materials (blend systems) or different molecular packing structure within a material (amorphous vs crystalline) exhibit different tip-sample interactions. By monitoring phase changes, localized “hard” (crystalline) and “soft” (amorphous) domains can be imaged. Topographic and phase images of pure, 95:5, and 90:10 (w/ w) P3PBT/PEG films are presented in Figure 6. The P3PBT film shows scattered tall features with a surface roughness of rms ) 1.7 nm (Figure 6a). A featureless and low-contrast phase image is expected for homogeneous amorphous films. However, the phase image of the P3PBT film reveals isolated domains, which are believed to be aggregate domains (Figure 6b). Contrast between amorphous and aggregated or crystalline domains of phase images in polymer films has been observed in several systems. The most recognizable is in poly(3hexylthiophene) (P3HT) films with crystalline fibers.40,41 The topographic image of the 95:5 P3PBT/PEG film (Figure 6c) shows phase segregation; however, the majority of the film is flat and homogeneous. Close inspection of the speculated P3PBT-rich domains surrounding the PEG domains (tall features) reveals no phase segregation between the aggregate and amorphous domains, as observed in pure P3PBT film (Figure 6a). The surface roughness decreases from rms ) 1.7 nm to rms ) 0.72 nm, consistent with the notion that PEG interferes with P3PBT aggregation. The taller domains in Figure 6c are assigned to PEG-rich domains because these domains increase with PEG concentration (Figure 6e); this assignment was further confirmed with C-AFM. C-AFM is a scanning probe technique, which examines surface topography and local current simultaneously42-45 and can provide information on the nanoscale charge transport properties of domains. In these measurements, a conducting tip acts as a nanoelectrode making contact with the sample and measuring current as a fixed voltage is applied to the sample while scanning. Since PEG-rich domains are nonconductive, there should be no or very low detectable current in these domains. From the current images (at -500 mV applied bias, see the Supporting Information), there is negligible current from the regions assigned as PEG-rich domains. Similar to the observations made via examination of topographic features, the phase images of the P3PBT/PEG films highlight the assigned isolated PEG-rich domains (Figure 6, parts d and f). The P3PBT aggregate domains are also not visible. Comparison of the phase images of P3PBT/PEG and P3PBT

J. Phys. Chem. C, Vol. 112, No. 17, 2008 7057 films thus shows further evidence that aggregated chains are disrupted by PEG. The homogenously distributed aggregate features observed in the P3PBT film phase image are not observed in the 95:5 (w/w) P3PBT/PEG blend film. It is possible, however, that the aggregate sizes are much smaller than in pure P3PBT film. Increasing the PEG concentration to 90:10 (w/w) in the P3PBT/PEG mixture leads to films in which the majority of the surface is believed to be covered with PEG (Figure 6, parts e and f). The surface roughness increases to rms ) 2.0 nm. The phase image shows a similar trend. Whether the PEG domains are located exclusively on the surface or penetrate through the entire film is not known, but we suspect that a disproportionate fraction of PEG will reach and reside at the surface after the spin-coating process because of its greater solubility in water and smaller molecular dimensions, relative to P3PBT. The overall set of observations shows that the introduction of PEG provides for a simple method to control interchain P3PBT interactions, and therefore electronic charge carrier transport, as will be described in section D. III.C. Light-Emitting Electrochemical Cells. CPEs provide interesting molecular systems by which the effects of ionic charge (and/or dipoles) on electronic charge transport can be investigated, a phenomenon that has not been fully investigated but has been speculated to influence charge transport.46,47 In this section, we focus on investigating the electronic charge transport properties and the effect of aggregation on the hole carrier mobility of P3PBT. The P3PBT/PEG blend system was chosen for the transport studies because it provides better quality films, when compared with the films attained upon introduction of ammonium salts. Additionally, the ionic content of the films is not altered significantly. The solubility of the P3PBT in water allows the formation of thick films (>80 nm) by spin-coating. Thus, it seems possible to fabricate hole-only diodes in which the P3PBT layer is sandwiched between two high work function electrodes with electron injection minimized in such a device structure. ITO and Au were chosen because they both have work functions relatively close to the highest occupied molecular orbital (HOMO) energy of alkyl substituted polythiophenes;48 thus, holes are expected to be the majority carrier injected. Often, the current density-voltage characteristics of a single electronic charge carrier can be fitted to the space charge limited current (SCLC) model, from which carrier mobility can be extracted.49-51 Attempts to determine carrier mobility in these CPE films by conventional hole-only diode I-V measurements were not successful. Pure P3PBT, 95:5, and 90:10 P3PBT/PEG films blend films were sandwiched between ITO substrate and Au electrodes. Electroluminescence is observed at both positive and negative bias scans for all devices. This behavior is typical of LECs, in which electrochemical doping of the conjugated polymer near the electrodes is accompanied by ion redistribution. Ion accumulation at the electrode interfaces reduces hole and electron injection barriers, allowing both carriers to be injected into the device with electron-hole recombination providing the emissive species.52,53 Observation of emission therefore indicates that both electrons and holes are injected. Under these conditions, it is not possible to measure the hole mobility across the film. The current density versus voltage (J-V), luminance versus voltage (L-V), luminance versus current density (L-J), and luminous efficiency versus current density (E-J) plots at positive bias are shown in Figure 7. From Figure 7a, one observes a decrease in the current density with increasing PEG content. This is likely a contribution from the increase in the

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Figure 6. AFM topographic (a, c, and e) and phase images (b, d, and f) of pure P3PBT film (a and b), 95:5 P3PBT/PEG (c and d), and 90:10 P3PBT/PEG (e and f) blends.

nonconducting PEG component and differences in film thicknesses: 80 nm for the pure P3PBT film and 92 and 100 nm for the 95:5 and 90:10 (w/w) P3PBT/PEG films, respectively. On the other hand, an increase in luminance, together with a decrease in the turn-on bias from 4.2 to 2.8 V, is observed in the 95:5 (w/w) P3PBT/PEG blend, as compared to the P3PBTonly device. The improved emission is believed to be due to the reduction of chain aggregation by PEG, which leads to the increase in the PL quantum yield. With the 90:10 (w/w) P3PBT/ PEG film one obtains the lowest luminance and highest turnon voltage. As shown in Figure 6e, the greater surface coverage of PEG possibly reduces charge injection from the electrode in these devices. Plots of L and E versus J are more appropriate to compare the performances of these devices (Figure 7, parts b and c). An identical trend is observed in both plots; upon increasing the PEG concentration in P3PBT films, there is an increase of L and E at a similar J. This increase in performance is largely believed to be due to an increase in the PL quantum yield of the PEG-containing films. Another factor that may be contributing to the changes in performance is the possible increase in ion conductivity in the blends facilitated by PEG. PEG is a shorter chain version of poly(ethylene oxide) (PEO), which is commonly used in LEC blends to facilitate ion motion. However, an increase in the time required to reach the steady state J, an indirect measure of ion motion, upon increasing PEG

was observed (∼27, ∼34, and ∼50 s at +5 V for pure P3PBT, 5:95, and 10:90 (w/w) P3PBT/EG films, respectively). Increased ion motion thus can be ruled out as a major contributor to the increase of E versus J with increasing PEG content. Ion redistribution and/or motion observed in P3PBT devices does not allow electronic charge carrier mobilities to be measured by conventional diode methods, but one can take advantage of the different response time of ions and electronic charge carriers to extract electronic carrier mobilities. III.D. Charge Transport Properties. The response times of ions and electronic charge carriers differ significantly, which makes it possible to investigate one with minor effects of the other. The response times of ions are known to be significantly longer than those of electronic charge carriers. Response times in milliseconds to minutes have been observed for ions, depending on the ease of the matrix to facilitate ion motion.52,54-56 Electronic charge carrier response times in microseconds are generally observed in conjugated polymers films.57,58 Hence, we envisioned performing I-V measurements at frequencies faster than the response time of ions to minimize the effect of ion redistribution/motion on the field distribution within the film. Identical devices to those discussed above were used for pulsed bias measurements. The bias was stepped-pulsed during scans with 500 ms off-times (0 V) and 5 ms on-time (0 V to 4 V to 0 V, with 0.2 V steps). The resulting J-V plots for the pure P3PBT film with and without pulsed bias scans are shown in

Optical and Charge Transport Properties of a CPE

Figure 7. (a) Plots of current density and luminance vs bias, (b) luminance vs current density, and (c) luminous efficiency vs current density of pure P3PBT (black), 90:10 (red), and 95:5 (blue) (w/w) P3PBT/PEG blend films sandwiched between ITO and Au electrodes.

Figure 8. Current density vs bias characteristics of a pure P3PBT diode with and without pulsed bias scans.

Figure 8. Most notable are the high current densities and large hysteresis observed in scans without pulsed bias. The large hysteresis between forward and reverse scans is due to slow ion redistribution and/or motion to and away from the polymer/ electrode interface. During forward scans double layer formation by accumulated ions and/or electrochemical doping near the polymer/metal interface reduces charge carrier injection barriers, and a sudden increase in current density is observed (∼2.5 V). During reverse scans the slow ion redistribution and/or motion away from the interface leads to current densities much larger than forward scans due to the accumulated ions already present near the interface. In scans with pulsed bias, orders of magnitude lower in current densities, no hysteresis and no light emission are observed. There is insufficient time for ion redistribution and/or motion at the interfaces in the pulsed experiments and

J. Phys. Chem. C, Vol. 112, No. 17, 2008 7059 hence insignificant modification of injection barriers and electrochemical doping. Lower current densities are observed due to the large electron injection barrier with either unmodified electrode, which leads to injection and transport of solely holes unlike scans without pulsed bias. Light emission is observed in scans with no pulsed bias as shown in Figure 7, whereas no light emission was observed in pulsed bias scans. Similar behavior was also observed at negative bias. The pulse effect on the J-V curves observed in P3PBT was not observed when the neutral conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) was used (Supporting Information). Care must be taken when choosing the pulse duration time and scan range when performing these measurements. Upon increasing the on-time up to 30 ms no changes in the plots was observed; above 30 ms a gradual increase in hysteresis and current density with increase pulse bias on-time takes place. The longer pulsed on-times allow more ions to reach the electrode/polymer interface and/or to redistribute during forward scans. A similar effect is observed upon changing the bias range because ions move more quickly at a larger bias. For example, with smaller bias (4 V, hysteresis is observed at 5 ms ontimes, which was not observed with smaller bias scan ranges. This observation is due to a combination of changes in the total on-time of the scan and the voltage-dependent response times of ions. The response time of ions decreases with increasing voltage, and hence bias pulses with shorter on-times are necessary. Pulsed bias scans were used to obtain hole-only diode behavior in devices fabricated from pure P3PBT and P3PBT/ PEG films. The blends exhibit similar behavior as the pure films (decrease current density, no hysteresis, and no light emission). The J-V characteristics for the P3PBT and P3PBT/PEG diodes are shown in Figure 9 at forward and reverse scans. Current density decreases upon increasing PEG concentration in films, possibly due to an increase in film thickness or decrease in carrier mobility or a combination of both. SCLC behavior where J ∝ V2 and J ∝ 1/L3 is observed in all three diodes allowing determination of carrier mobilities.48-51 Two regions are observed in the log J-log V plots in all three films, an ohmic region at low bias with J ∝ V due to intrinsic charge carriers and an SCLC region at higher bias with J ∝ V2 from which charge carrier mobilities can be determined using49,59-62

9 V2 J ) roµ 3 8 L

(1)

where o and r are the vacuum permittivity and polymer relative dielectric constant, respectively. L and µ are the thickness and hole mobility of the polymer films, respectively. The hole mobilities extracted from the fits using eq 1 are 5.1 × 10-8 cm2/Vs for P3PBT and 7.8 × 10-9 and 6.7 × 10-9 cm2/Vs for 95:5 and 90:10 (w/w) P3PBT/PEG. Close to an order of magnitude decrease in hole mobility of P3PBT/PEG film is believed to be a consequence of reducing the amount of aggregated species in the blend films by the addition of PEG. There is greater π-π overlap in aggregated species, which increases electronic coupling, leading to faster hopping rates and ultimately higher carrier mobilities across the film. This result agrees well with the optical characteristics and AFM data reported above. In comparison to pure P3PBT films, a larger decrease in carrier mobility (∼84% and ∼86% for 95:5 and

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Figure 9. (a) Semilog (log J-V) and (b) double log (log J-log V) plots of current density vs bias of P3PBT (black), 95:5 (blue), and 90:10 (red) (w/w) P3PBT/PEG blend films. Plot b also contains SCLC fit lines (J ∝ V2).

90:10 (w/w) P3PBT/PEG) than decrease in absorption of aggregate chains (∼60% and ∼75%) in the blends films is observed. This discrepancy is possibly due to hole transport being impeded by not only reduced chain aggregation but also by PEG domains, which cannot transport electronic charge carriers. The hole mobility for P3PBT is much lower than those measured for neutral polythiophene and its derivatives.61,63-65 It is unclear whether interactions between the intrinsic ionic charges of the CPE with the backbone or with electronic charge carriers affect the carrier transport. Dunlap et al.47 proposed a model taking into account the interactions between permanent dipoles and transport sites, and this model was used to explain the charge transport behavior of a molecular-doped polymer system. In this model energetic disorder is created by interactions between permanent dipoles and transport sites, and the energetic disorder (inversely related to carrier mobility) was found to be proportional to the dipole moment and the average density of dipoles. This can possibly explain the low mobilities obtained in our P3PBT system in which the large dipole moment originating from the ionic charge and counterion (∼15 D per monomer as estimated by DFT calculations) and the dipole density in the P3PBT films may lead to large disorder and hence low mobilities. Similar behavior has been observed experimentally in molecular-doped systems66-70 and polymer blends,71 but the proximity of the dipoles originating from the ionic units is much closer in CPEs than in molecular-doped polymers, which can possibly have a greater influence on carrier mobility. The effects of the ionic charge and counterion on electronic carrier mobility in CPEs are currently being investigated. IV. Conclusions Steady-state spectroscopic measurements, scanning probe microscopy, and charged transport measurements of the anionic polythiophene P3PBT have been examined in detail. The intrinsic ionic properties of P3PBT lead to high degree of polymer chain aggregation in water, as evidenced by spectroscopic measurements. Reducing aggregation in P3PBT by serial dilution is less effective than in neutral conjugated polymers

Garcia and Nguyen due to the presence of multiple interactions: hydrophobichydrophobic (conjugated backbone), hydrophilic-hydrophilic (charge group-solvent medium), and hydrophobic-hydrophilic (backbone-solvent medium). These factors allow one to finetune the interactions between conjugated polymer chains using techniques that have been applied to conventional polyelectrolytes. Aggregate formation can be controlled in P3PBT by solution processing conditions. Aggregates can also be disrupted by the addition by bulky ammonium salts or PEG, and the degree of aggregation observed in solutions can be transferred onto films and observed by AFM phase imaging. Phase contrast observed in AFM images of P3PBT film is assigned to aggregated domains which disappear in 95:5 (w/w) P3PBT/PEG films. The flexibility in controlling aggregation in P3PBT films allowed the investigation of its effects on electronic charge transport. Hole-only diodes fabricated from P3PBT films exhibit unanticipated LEC behavior as a result of ion redistribution/ motion reducing the electron injection barrier such that light emission is observed. The device efficiencies of the LECs improve with PEG concentration, and this effect is believed to be a consequence of the increased PL quantum yield. Ion motion and/or redistribution can be reduced with pulsed I-V measurements at frequencies faster than the response times of ions. Thus, it is possible to fabricate diode devices with high work function metals in which the majority of carriers injected are holes. Hole mobilities can thereby be calculated by using the SCLC model. The pulsed technique used here is likely to be of use for measuring the electronic charge transport properties of other CPEs or organic systems embedded with ions. Reduced aggregation in P3PBT/PEG blend leads to close to an order of magnitude reduction in hole mobility, compared to that of the pure P3PBT film. Changes in interchain contacts in solutions and films lead to changes in absorption, PL, and electronic charge transport behavior, important properties in optical and electronic devices such as PLEDs, LECs, solar cells, and field effect transistors (FETs). The ability to control polymer chain interactions in P3PBT is attractive since the electronic properties of conjugated polymers depend strongly on polymer chain interactions. This opens up the possibility to control polymer properties for specific applications. For LEDs and LECs, one would envision use films with low degree of aggregation and high PL quantum yield, whereas for applications that require high electronic charge carrier mobilities, such as FETs, films with high degree of aggregation would be more desirable. Acknowledgment. This work is supported by the NSF CAREER Award (DMR No. 0547639). Supporting Information Available: Conducting AFM data of 95:5 P3PBT/PEG film and the pulsed I-V measurements of an MEH-PPV film. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Liu, B.; Bazan, G. C. Chem. Mater. 2004, 16, 4467. (2) Chuyan, C.; Mikhailovsky, A.; Bazan, G. C. J. Am. Chem. Soc. 2007, 129, 11134. (3) Hong, J. W.; Hemme, W. L.; Keller, G. E.; Rinke, M. T.; Bazan, G. C. AdV. Mater. 2006, 18, 878. (4) Kim, I.; Dunkhorst, A.; Bunz, U. H. F. Langmuir 2005, 21, 7985. (5) Pinto, M. R.; Schanze, K. S. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 7505. (6) Chen, L.; McBranch, D. W.; Wang, H.; Helgeson, R.; Wudl, F.; Whitten, D. G. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 12287. (7) Kushon, S. A.; Ley, K. D.; Bradford, K.; Jones, R. M.; McBranch, D.; Whitten, D. Langmuir 2002, 18, 7245.

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