J. Phys. Chem. C 2010, 114, 22309–22315
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Temperature and Electric Field Dependent Electron Transport in Cationic and Anionic Conjugated Polyelectrolytes Andres Garcia,† Youngeup Jin,‡ Jacek Z. Brzezinski,† and Thuc-Quyen Nguyen*,† Department of Chemistry and Biochemistry, Center for Polymers and Organic Solids, UniVersity of California, Santa Barbara, California 93106, United States, and Department of Chemistry, Pusan National UniVersity, Busan 609-735, Korea ReceiVed: September 27, 2010; ReVised Manuscript ReceiVed: NoVember 4, 2010
The temperature and electric field dependence of electron mobilities in conjugated polyelectrolytes (CPEs) with structure variations of the ionic functionality and counterions were investigated to gain insight into the influence of these parameters on charge transport. Electron mobilities were determined by using electrononly diodes, and the temperature and electric field dependence were modeled with the Gaussian disorder model, the correlated Gaussian disorder model, and the polaronic correlated Gaussian disorder model for comparison. The transport characteristics were found to be dependent on the type of charge, as well as the choice of counterion, with the former baring a stronger influence on transport. Anionic CPEs are more susceptible to differences in the calculated energetic disorder than their cationic counterparts. Examination of cationic CPEs with different counterions show that, although the energetic disorder is similar, large differences in electron mobilities can be obtained. These observations provide useful structural handles to tailor and manipulate charge carrier mobilities. Introduction Charge transport in organic semiconducting materials plays a crucial role in the performance of devices such as field effect transistors (FETs), photovoltaics, light-emitting diodes (LEDs), and light-emitting FETs (LEFETs). Due to weak intra- and intermolecular π-π interactions in conjugated organic materials, charge transport can be easily influenced by a variety of factors, such as chemical structure, molecular geometry, chemical purity, and processing conditions. Much effort has been dedicated to the design of new materials with control of intra- and intermolecular interactions to enhance charge carrier mobility. Recently, a relatively new class of organic semiconducting materials, conjugated polyelectrolytes (CPEs), has been synthesized and used in various types of optoelectronic devices,1,2 such as biosensors,3 solar cells,4-6 light-emitting electrochemical cells (LECs),7 and electron injection/transport layers (EILs/ETLs) in polymer8-13 and small molecule14 based organic light-emitting diodes (PLEDs and OLEDs, respectively). CPEs are composed of a π-conjugated backbone with pendant groups bearing ionic functionalities. The ionic functionalities allow the use of polar media for processing leading to unique higher order structures.15 Additionally, CPEs provide routes for controlling intra- and intermolecular interactions not available in neutral conjugated polymers, such as changes in pH,16,17 ionic strength,16 and counterion.18,19 While CPEs have been widely and successfully employed in various applications,8-11 their charge transport behavior is not fully understood. In recent studies, changes in counterion,8,18,19 conjugated backbone,9 and the appended ionic functionality9 have been shown to influence charge carrier mobilities and PLED performance. Thus, there is the possibility to structurally modify counterion, appended charge, and chemi* Corresponding author. E-mail:
[email protected]. † University of California, Santa Barbara. ‡ Pusan National University.
Figure 1. Chemical structures of PFNF (a), PFNBIm4 (b), and PFCO2Na (c).
cal structure of the conjugated backbone to modulate intra-/ interchain interactions and thereby charge carrier transport. In this contribution, temperature and electric field dependent electron mobility measurements were performed by using electron-only diodes to gain greater insight into the influence of the ionic functionalities in CPEs on charge carrier transport. Several CPEs with identical conjugated backbones but with different counterions and appended charges were examined (Figure 1): poly[(9,9-bis(6′-(N,N,N-trimethylammonium)hexyl))fluorene-alt-co-1,4-phenylene] fluoride (PFNF), poly[(9,9-bis(6′(N,N,N-trimethylammonium)hexyl))fluorene-alt-co-1,4-phenylene] tetrakis(1-imidazoly)borate (PFNBIm4), and sodium poly[9,9bis(5′-pentanoate)fluorene-alt-co-1,4-phenylene] (PFCO2Na). Comparison of PFNF and PFNBIm4 provides insight into the influence of counteranion size (F- versus BIm4-), whereas comparison between PFNF and PFCO2Na allows one to examine the influence of charge reversal (cationic versus anionic charges). Experimental Methods Electron mobility measurements were performed using electrononly diodes as described in the literature9,20,21 which consist of a film sandwiched between two low work function electrodes
10.1021/jp109243x 2010 American Chemical Society Published on Web 11/23/2010
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Figure 2. Absorption (open symbols) and PL (solid symbols) spectra of PFNF (black circles), PFNBIm4 (red squares), and PFCO2Na (green triangles) films.
to ensure electrons are the majority injected carriers. PFNF, PFNBIm4, and PFCO2Na were synthesized according to previous reports8,22 and were dissolved either in methanol (PFNF and PFNBIm4) or in a 3:7 (v:v) water:methanol mixture (PFCO2Na) for spin coating. Electron-only diodes were fabricated by spin coating (900 rpm for 60 s) relatively thick CPE films (130, 180, and 110 nm, for PFNF, PFNBIm4, and PFCO2Na, respectively) onto thermally evaporated Al-coated glass substrates, followed by thermal evaporation of Ba (∼5 nm) and Al (∼100 nm) using a shadow mask. All device fabrication steps were done under an inert N2 atmosphere inside a glovebox. Temperature-dependent current density versus voltage (J-V) measurements from 250 to 300 K (10 K intervals) were measured under vacuum (10-4 Torr) with a Keithley 4200 SCS source meter and a vacuum variable-temperature probe station (Lake Shore Cryotronics, Inc.). All J-V measurements were performed with stepped-pulse voltage scans, as described in previous reports,19 to reduce ion motion, which can lead to modification of injection barriers in these mixed electronic/ionic charge conducting systems. Voltage measurements were performed with 500 ms off-times and 5 ms on-times for step-pulsed voltage scans. Quartz substrates were used for spectroscopic measurements, and the UV-vis absorption spectra were recorded with a Shimadzu UV-2401 PC diode array spectrometer, while fluorescence spectra were measured with a PTI Quantum Master fluorometer. Film morphology and film thicknesses were measured by atomic force microscopy (AFM) using a commercial scanning probe microscope (MultiMode with a Nanoscope Controller IIIa, Veeco Inc.). All scanning probe measurements were done under a dry N2 atmosphere. Silicon probes with a spring constant of ∼5 N/m and a resonant frequency of ∼75 kHz (Budget Sensors) were used for tapping mode measurements. Results and Discussions Differences in CPE charge carrier mobility with structural changes in counterion,18,19 conjugated backbone,9 and appended ionic components9 can arise from several other possibilities such as differences in electronic structure/properties of polymer chains, film morphology, and polymer chain interactions, etc. In this study CPEs with identical conjugated polymer backbones are investigated to discern the influence on electron mobility by differences in the counterion size and the appended ionic units, and not by changes in the electronic structure of the conjugated backbone. The film absorption and photoluminescence (PL) spectra of PFNF, PFNBIm4, and PFCO2Na are shown in Figure 2. All three CPEs exhibit similar absorption and PL spectra with similar maxima (∼375 nm and ∼425 nm for absorption and PL spectra), suggesting similar electronic
Figure 3. Temperature-dependent J-V plots from 250 to 300 K of PFNF (a), PFNBIm4 (b), and PFCO2Na (c) electron-only diodes.
structures and excited-state species. The optical band gap for the three materials is similar, 2.95 ( 0.004 eV. Indeed, similar CPEs have been shown to exhibit similar electronic properties such as oxidation and reduction potentials,23 in agreement with these spectroscopic measurements. These results are not surprising, since the ionic component is not in conjugation with the π system of the backbone and hence can only have weak effects on the highest occupied and lowest unoccupied molecular orbital (HOMO and LUMO, respectively) energies. Electron transport measurements were performed with electrononly diodes in which PFNF, PFNBIm4, and PFCO2Na films were sandwiched between bottom Al and top Ba electrodes. The relatively low work function of Ba and Al electrodes, 2.7 and 4.3 eV, respectively, form ohmic contacts for electron injection and collection (CPE LUMO ≈ 2.4 eV), allowing electrons to be the majority carriers. These devices were operated with high-frequency stepped-pulse voltage sweeps, at a frequency faster than the response of ions (milliseconds to seconds)13,24 to reduce ion migration and concomitant formation of ionic double layers. Ion accumulation at electrode interfaces can modify charge injection barriers25 and lower the injection barrier for both holes and electrons leading to LEC behavior as observed in other CPE systems.7,26-28 Operating devices with a stepped-pulse voltage sweep with a 5 ms on-time and a 500 ms off-time lead to devices that do not exhibit large hysteresis between forward and reverse scans, attributed to slow ion migration, along with no emission as shown in previous studies.19,29 Figure 3 shows the J-V plots for PFNF, PFNBIm4, and PFCO2Na devices collected between 250 and 300 K. All samples exhibit a steady increase in current density with increasing temperature. This trend implies that charge transport is temperature-activated due to charge localization as observed
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Figure 4. Temperature-dependent µo-E plots from 250 to 300 K of PFNF (a), PFNBIm4 (b), and PFCO2Na (c) with fitted lines.
in disordered organic films, as a result of relatively large energetic barriers (>kT) between transport sites. Excellent fits of J-V plots to space-charge-limited-current (SCLC) with a Poole-Frenkel electric field dependent mobility were obtained (eq 1).21,30
9 V2 J ) 8 εoεrµ 3 L
(1)
µ ) µo exp(γ√E) Here, εo is the vacuum permittivity, εr is the relative dielectric constant of the film, µ is the mobility at a specified electric field, V is the applied voltage, L is the thickness of the active layer, µo is the zero-field mobility, γ is a material-dependent factor determining the mobility electric field dependence, and E is the electric field defined as V/L in volts per centimeter. Thicknesses of films were determined with AFM, and the εr value used for all samples was 4, as reported for cationic and anionic CPE structures.31 This εr value is slightly larger than the commonly measured value of 3 used for mobility calculations in neutral conjugated polymers. As can be seen from the ln µ-E plots (Figure 4), all CPEs exhibit linear behavior at high electric field as commonly observed in neutral conjugated polymers and allows the determination of µo and γ values at all temperatures measured. At low field, there is a small deviation from the linear behavior. Similar observation has been reported for neutral amorphous conjugated polymers.32,33 The µo values of PFNF, PFNBIm4, and PFCO2Na determined using eq 1 are 5.3 × 10-4, 8.3 × 10-7, and 1.1 × 10-8 cm2/ (V · s), respectively. These values are much larger than that of
Figure 5. AFM images of PFNF (a), PFNBIm4 (b), and PFCO2Na (c).
the neutral counterpart (5.5 × 10-10 cm2/(V · s)). This observation suggests that the ionic components do not influence the charge transport. A drop by nearly 3 and 4 orders of magnitude in electron mobility is observed from PFNF to PFNBIm4 and from PFNF to PFCO2Na, respectively. The carrier mobilities follow trends similar to those previously reported as a function counterion size18 and pendant group charge.9 Film morphology can influence charge carrier transport in neutral conjugated polymers.34,35 Differences in crystallinity, grain boundaries, domain size, and phase separation between crystalline and amorphous domains have been shown to lead to orders of magnitude differences in carrier mobilities and hence have been investigated as a possible source for the large difference in µo values observed between CPEs. The film
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Figure 6. µo versus (1000/T)2 (a) and γ versus (1/kT)2 (b) plots of PFNF (black circles), PFNBIm4 (red squares), and PFCO2Na (green triangles) devices with fitted lines according to the GDM.
morphology of PFNF, PFNBIm4, and PFCO2Na devices measured by AFM are shown in Figure 5. All films are featureless, a signature of amorphous morphology with similar surface roughness (root mean square (rms) ) 1.6, 1.9, and 2.4 nm for PFNF, PFNBIm4, and PFCO2Na, respectively), as commonly observed in CPE films.18,14 The similarities in morphology between films imply that it is unlikely that such large differences in electron mobilities stem from differences in film morphology. To gain greater insight into the parameters influencing charge carrier transport, the temperature and electric field dependences of charge carrier mobilities were examined. Charge transport in disordered organic material is most commonly modeled by the Gaussian disorder model (GDM),36 which considers the transport of charge carriers via hopping between localized transport sites with energetic and positional disorder due to differences in local environments and distances between transport sites. The energies of the transport sites are random with a Gaussian shape distribution leading to the description of charge carrier mobilities according to the GDM by
2σ ) ] exp[C (( kTσ ) [ ( 3kT
µ ) µ∞ exp -
2
2
o
) ]
- Σ2 √E
(2)
where µ∞ is the mobility prefactor hypothetically related to the mobility as the temperature approaches infinity, σ is the rms width of the energetic disorder, k is Boltzmann’s constant, Co is a constant, and Σ is related to the positional disorder in transport sites. Experiments were carried out from 250 to 300 K. As seen from eq 2, the GDM successfully accommodates the 1/T2 and E dependence of µ, as commonly observed in organic films. From the µo versus (1000/T)2 plots (Figure 6a), σ and µ∞ values
can be determined using the electric field independent part of eq 2 from the slope and extrapolation of µo as T approaches infinity. From γ versus (1/kT)2 plots (Figure 6b) and the previously determined σ, Co and Σ values can be obtained from the slope and extrapolated γ values at (1/kT)2 ) 0, by using the field-dependent component of eq 2. A summary of the parameters extracted with the GDM are listed in Table 1. The interpretation of the obtained Co and Σ values is not discussed to a great depth since their interpretation in the literature remains inconclusive.37,38 However, we observed that the order of magnitude of these values is similar to what has been reported for polyfluorenes.37 The extrapolated intercepts of γ values in the (1/kT)2 plots consistently led to positive values for PFNBIm4, in contrast to negative values observed in PFNF and PFCO2Na. The change in sign of these extrapolated values and inability to obtained Σ values for all samples highlight the sensitivity of these values to fitting parameters and their difficult interpretation. Furthermore, we recognize that experimental limitations, as in other conjugated polymer systems,21,38-42 may not allow a sufficiently wide temperature range for greater precision. Comparison of σ and µ∞ values between CPEs may indicate that the decrease in electron mobility from PFNF to PFNBIm4 and from PFNF to PFCO2Na arises from different sources. The similar σ values determined in both PFNF (87 meV) and PFNBIm4 (83 meV) are somewhat unexpected given the orders of magnitude drop in µ from PFNF to PFNBIm4, which does not coincide with an increase in σ as commonly observed in neutral conjugated polymers. Carrier mobility in organic semiconductors is largely dictated by the energetic barriers between transport sites; lower carrier mobilities have been shown to have larger energetic barriers than higher mobility materials.43 However, the drop in electron mobility does coincide with a similar drop in µ∞ from 1.3 × 10-1 to 7.5 × 10-5 cm2/ (V · s) in PFNF to PFNBIm4. As T approaches infinity, the charge carrier mobility is expected to be limited by electronic or orbital overlap between polymer chains43 and therefore the drop in µ∞ from PFNF to PFNBIm4 may suggest a reduction in interchain contacts and/or aggregation as a result of differences in counteranion size. The larger BIm4 counterions are anticipated to minimize interchain contacts, leading to lower carrier mobilities. Additional evidence of reduced interchain contacts can be observed by spectroscopic measurements. An increase in the solid-state PL quantum yield from 20 to 43% is observed for PFNF and PFNBIm4, respectively. Generally, PL quenching is indicative of more pronounced aggregation.18 Several orders of magnitude drop in electron mobility from PFNF to PFCO2Na cannot be accounted for simply by differences in counterion size and possible decrease of interchain contacts since the ionic radii of sodium and fluoride are 1.02 and 1.33 Å, respectively. Both an increase in σ from 87 to 116 meV and a drop in µ∞ from 1.3 × 10-1 to 2.0 × 10-4 cm2/ (V · s) are observed from PFNF to PFCO2Na. It is difficult to disentangle their relative contributions since both effects can contribute to the decrease in carrier mobility. An interesting observation is that while both cationic CPEs exhibit similar σ values, PFCO2Na exhibits a larger σ value. The influence of dipole moments have been shown to affect charge carrier transport in organic materials with larger dipole moments
TABLE 1: Summary of Transport Parameters According to the GDM µ∞(cm2/(V · s)) PFNF PFNBIm4 PFCO2Na
-1
σ (meV) -2
1.3 × 10 ( 1.1 × 10 7.5 × 10-5 ( 2.3 × 10-5 2.0 × 10-4 ( 1.2 × 10-4
87 ( 4 83 ( 4 116 ( 6
C0 (cm1/2/(V1/2 · s1/2)) -4
Σ -4
4.6 × 10 ( 2.2 × 10 1.8 × 10-4 ( 2.1 × 10-5 2.3 × 10-4 ( 4.5 × 10-5
2.7 ( 0.2 NA 4.0 ( 1.7
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Figure 7. γ versus (1/kT)3/2 plots of PFNF (black circles), PFNBIm4 (red squares), and PFCO2Na (green triangles) devices with fitted lines according to the CGDM.
significantly reducing charge carrier transport.44-47 Hence, differences in dipole moments of the appended ionic substituents may be possibly influencing transport in these CPEs. Another commonly used charge carrier transport model is the correlated Gaussian disorder model (CGDM).48-50 This model highlights the importance of dipole moments in charge transport and may be useful in addressing the uncertainties raised in the preceding paragraph. While the GDM assumes transport sites with random energies and a Gaussian shape density of states, spatial correlation between the energy of transport sites is invoked in the CGDM by inclusion of long-range chargedipole interactions between injected carriers and randomly located and oriented dipoles. Charge-dipole interactions are also observed to lead to a Gaussian distribution in transport site energies and the CGDM leads to the ubiquitous µ ∝ exp(E) dependence within experimentally observed electric field ranges (
