Letter pubs.acs.org/JPCL
Hole Mobility of a Liquid Organic Semiconductor Brett A. Kamino,† Timothy P. Bender,†,‡,* and Richard A. Klenkler§ †
Department of Chemical Engineering and Applied Chemistry, The University of Toronto, 200 College Street, Toronto, Ontario, Canada, M5S 3E5 ‡ Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada, M5S 3H6 § Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ontario, Canada, L5K 2L1 S Supporting Information *
ABSTRACT: The first detailed study of charge transport through a liquid organic semiconductor (LOS) is reported with the goal of elucidating the effects of molecular motion on charge transport through molecular liquids. Using a liquid, silyl ether-substituted triarylamine, hole transport mobilities were obtained over a wide range of temperatures above the glass transition temperature of the material. Analysis of this data reveals that molecular motion(s) have a negligible effect on macroscopic charge transport through a molecular liquid. The results strongly resemble transport behavior found in conventional, disordered solids and suggest that silyl ether-substituted LOSs may be good candidates for integration into electronic devices, by those who are familiar with the application of traditional triarylamines, where their unique physical state can or could be exploited. SECTION: Plasmonics, Optical Materials, and Hard Matter
L
of a charge transport measurement through an LOS.11 In that report, Ribierre et al. show that the liquid semiconductor, N-(2ethylhexyl)carbazole, acts as a p-type semiconductor and has an order of magnitude higher hole mobility than its equivalent polymeric, solid-state analogue (poly(N-vinylcarbazole)). Additionally, the transport characteristics of several solid molecular semiconductors have been studied above their glass transition, but this type of study is limited by the possibility of crystallization of the material above the glass transition temperature (Tg) and the relatively high temperatures required to achieve a liquid state. In this work, we study the hole-transporting ability of a silyl ether-substituted triarylamine LOS: N,N-bis(4(triisopropylsilyloxy)phenyl)-3,4-dimethylaniline (2TIPS, Figure 1a), a material that is a liquid at room temperature (Tg = −28 °C). Time-of-flight photocurrent measurements12 were performed on samples of 2TIPS doped into a solid polymeric matrix and of 2TIPS as a neat liquid. The goals were to validate the charge transporting capability of a silyl ether-substituted triarylamine and to begin to understand whether charge transport in the liquid phase adheres to conventional theories explaining charge transport through amorphous molecular solids. In particular, we were interested to see whether the charge transport characteristics of an LOS can be described by the disorder formalism model using 2TIPS as a model system. Developed by Bässler and co-workers,13 this model has been used to successfully describe the transport behavior seen in
iquid organic semiconductors (LOSs) are an emerging class of materials for organic electronic devices. While conventional organic semiconducting materials are designed to form highly crystalline solids or morphologically stable glasses,1 LOSs are specifically intended to be free-flowing liquids at room temperature. Such materials present several unique processing advantages over their more typical counterparts, including solvent-free device processing and the presumed ability to easily achieve intimate contact with nanostructured or mesoporous surfaces. The utility of LOSs has been demonstrated in dye-sensitized solar cells,2 photorefractive devices,3 as host materials in liquid active layer light emitting diodes,4,5 and several other device types.6,7 However, this class of materials remains understudied. Until recently, only two LOSs were known in the literature: tris(4-methoxyethoxyphenyl)amine (TMEPA)2 and N-(2-ethylhexyl)carbazole.3−5 Our group has worked to expand the available set of LOSs by developing several general synthetic methods that, by incorporation of siloxane or silyl ether molecular fragments into conventional arylamine molecular structures (triarylamines and carbazoles), yield LOSs.8−10 We have shown that these silicon−arylamine hybrid materials retain their characteristic electrochemical properties while having vastly different physical properties from their parent materials, including presenting themselves as liquids and waxes. Despite the success of this strategy in producing a number of new LOSs, it remained unclear what effect the substitution of the siloxane or silyl ether groups would have on the hole-transporting properties of these materials, as charge transport through the liquid phase is poorly understood for organic semiconductors. We are aware of only a single example © 2012 American Chemical Society
Received: January 13, 2012 Accepted: March 22, 2012 Published: March 22, 2012 1002
dx.doi.org/10.1021/jz300058w | J. Phys. Chem. Lett. 2012, 3, 1002−1006
The Journal of Physical Chemistry Letters
Letter
applied bias) of charge was injected; this ensured that charge transport would not be space charge limited.12 Signal transients were collected at different field strengths, transport layer thicknesses, and temperatures. These measurements showed characteristics typical of triarylamines blended with an inert polymer (Figure 1a).16 In order to validate our technique, transients were measured as a function of transport layer thickness over a range of 5− 37 μm. Transit times (and hence mobility) were found to scale linearly with thickness (Figure S1), confirming that the transient signals are indeed due to transport across the entire thickness of the transport layer and that the calculated mobilities were not an artifact of the system. Field-dependent mobility over a temperature range of −30 to 65 °C was measured and is shown in Figure 2a for a 9 μm thick
Figure 1. Example of photogenerated transients through (a) 2TIPS in a polystyrene matrix (9 um) and (b) neat 2TIPS (50 um) with the calculated transit time shown (Note: the y-axis is linearly scaled while the x-axis is logarithmic). Device structures are illustrated next to their respective transients, and the chemical structure of 2TIPS is shown.
many conventional organic semiconducting materials, including those based on triarylamines. 2TIPS (Figure 1a) was chosen as a model compound, because we have previously shown that it has predictable electrochemical behavior8 and is a member of the well-studied triarylamine family of hole transport compounds.14 We began our study by investigating the basic transport properties of 2TIPS doped into an inert polymer matrix. Polymers commonly used include polystyrene and polycarbonate-A. Polystyrene was chosen as the inert matrix as it formed higher quality films when doped with 2TIPS than did polycarbonate-A. The time-of-flight cell (Figure 1a, inset) was assembled in a similar manner to what we have previously described.15 Specifically, a Mylar substrate metalized with an aluminum electrode was sequentially coated with a silane blocking layer, a charge generator layer (CGL, composed of a hydroxygallium phthalocyanine dispersion in a polymer binder) and a blade coated hole-transport layer consisting of 2TIPS and polystyrene at a 1:1 ratio (by weight). The cell was completed by pressure contact with a second metalized Mylar electrode. The time-of-flight measurements were performed as described by Melnyk and Pai,12 using a nitrogen pulse laser with a dye attachment (Laser Science VSL-337ND-S & DUO-220). The output laser wavelength was tuned to 650 nm, so as to be only absorbed by the CGL, and laser intensity was adjusted such that less than 1/10 CV (where, C is the device capacitance and V the
Figure 2. Field-dependent hole-mobility as a function of temperature for 2TIPS in a (a) polystyrene matrix and (b) as a neat liquid.
hole-transport layer. The transients were nondispersive as indicated by a plateau of current followed by a well-defined drop in current defining the transit time. Hole-transport mobilities were found to be weakly field dependent across the temperature range (Figure 1a). Comparing measurements at variable temperatures, we find that the mobility is strongly temperature activated until we reach the glass transition temperature of the polymer blend (measured to be 48 °C)17 where this dependence plateaus. This behavior at the glass transition temperature has been previously observed in doped polymers,18 molecular glasses,19 and polymeric semiconductors.20 In order to see whether 2TIPS behaved similarly to other triarylamines, the results were analyzed within the context of the disorder formalism: ⎫ ⎧ ⎡⎛ ⎞2 2⎤ ⎡ ⎛ ⎤ ⎪ ⎪ 2σ ⎞ ⎥ σ ⎟ ⎨C ⎢⎜ ⎟ − Σ2⎥E1/2⎬ exp⎪ μ(T , E) = μ0 exp⎢ − ⎜ ⎪ ⎝ ⎠ ⎥⎦ ⎢⎣ ⎝ 3kT ⎠ ⎥⎦ ⎩ ⎢⎣ kT ⎭ (1) 1003
dx.doi.org/10.1021/jz300058w | J. Phys. Chem. Lett. 2012, 3, 1002−1006
The Journal of Physical Chemistry Letters
Letter
most samples, increasing the field strength over 1 × 105 V/cm resulted in shorting the device. At very low temperatures, this effect was not observed, and higher field strengths were found to be necessary to achieve measurable signals. Much like the 2TIPS/polystyrene samples, results for the neat 2TIPS cells were interpreted within the context of the disorder formalism (eq 1). From Figure 2b, we can see that log μ scales with E1/2 showing a weak dependence on field strength like that which was observed for the solid sample. Plotting the temperaturedependent mobility at different field strengths again shows a linear relationship between log μ versus T−2 similar to the data observed for the solid 2TIPS/polystyrene system (Figure 3).
where μ is the mobility, μ0 is the mobility at infinite temperature and zero field, σ is the width of the density-ofstates in eV, k is the Boltzmann constant, T is the absolute temperature, Σ describes the positional disorder, C is an empirical constant, and E is the applied electric field. This equation describes the dependence of charge carrier mobility on temperature and electric field and predicts that log μ should scale linearly with the inverse square of temperature and the square of the electric field. Excluding the points at and above the glass transition temperature, the data collected for the 2TIPS/polystyrene films shows the expected dependence (Figure 2a). By extrapolating the mobilities to an electric field of zero, the values of μ0 and σ were found to be 1.92 × 10−3 cm2 V−1 S−1 and 0.104 eV, respectively. When compared to other triarylamines found in the literature, these values are in line with other well-known, non-liquid triarylamines, including TPD and TAPC.21,22 For the time-of-flight measurement of 2TIPS as a neat liquid, a different cell design was required. Empty cells were assembled by sandwiching two strips of Kapton (polyimide) film between the same two Mylar substrates described above and gently clipping them together (Figure 1b, inset). Once prepared, the empty cell was heated to 50 °C, and the neat 2TIPS was drawn into the cell by capillary action. Unfortunately, this setup produced weak photocurrents and highly dispersive transients, making accurate determination of the mobilities impossible. It was found that adding a thin (275 nm) layer of TPD on top of the hydroxy gallium phthalocyanine layer by physical vapor deposition increased photocurrent and improved the resolution of the transient so that a sharp and distinct transit time could be observed and defined. The cell design and an example transient are illustrated in Figure 1b. The additional TPD layer seems to improve charge injection between the phthalocyanine layer and 2TIPS, but the rising plateau in the transient is indicative of the slow release of a well of charge in the CGL. This manifests itself in a measured current that steadily increases over time until the transit time (Ttransit, Figure 1b) is reached, possibility indicating a charge injection barrier. If a charge injection barrier exists on a time scale similar to the transit time, the results of this experiment would certainly be invalidated.23 To investigate this possibility and validate the fidelity of the technique, transients were measured as a function of transport layer thickness over a range of 50−125 um (as defined by the thickness of the Kapton spacer). Again, transit times and mobility were found to scale linearly with thickness of the 2TIPS layer (Figure S2), thereby confirming that the mobility values being measured are independent of any charge injection issues. As a point of comparison, we measured the mobility of N-(2ethylhexyl)carbazole, an LOS with a known hole transport mobility.11 The compound was synthesized by the method outlined in the Supporting Information accompanying this Letter, and a time-of-flight cell with this material was prepared in the same manner as described above (Figure 1b), including the additional 275 nm TPD interlayer. Transients were obtained over a variety of fields, and the field-dependent mobilities agreed well with those previously reported for this material, albeit at a lower field strength (Figure S4). This confirmation of the known hole-transport mobility of N-(2-ethylhexyl)carbazole provides further validation of our experimental setup. Using a 2TIPS cell with a 50 um Kapton spacer and the TPD interlayer, field-dependent transients were collected between −40 and 60 °C (Figure 2b). However, it was only possible to collect transients through a narrow range of field strengths. For
Figure 3. Temperature dependence on hole mobility for 2TIPS doped in polystyrene (50 wt %) at 555 kV/cm and neat 2TIPS at 100 kV/cm. The glass transition temperature of the polymer blend is indicated as Tg at 48 °C.
Interestingly, the majority of the data points obtained on this line correspond to values above the Tg of 2TIPS (Tg = −28 °C).9 This relationship implies that charge transport through liquid 2TIPS is governed by temperature-activated hopping, as predicted by the disorder formalism and as is seen in most disordered solids. This relationship holds true for the entire range of values above the Tg of this compound, a span of approximately 90 °C. This represents the first investigation into the effect of temperature on charge mobility in an organic molecular liquid and invites discussion into the role of molecular motion on the charge transport process. Analysis of the extrapolated zero field mobilities gives μ0 and σ values of 8.89 × 10−2 cm2 V−1 s−1 and 0.115 eV, respectively. Comparing these parameters to those determined for the solid 2TIPS/ polystyrene system, we find that the prefactor mobility (μ0) scales with the increased hole-transport mobilities observed in this sample while the width of the density-of-states (σ) is somewhat larger than what was determined for the solid sample. This is contrary to the expected trend found in typical materials. In most solid-state examples, it is expected that the density of states decreases in energy when going from a dilute sample (2TIPS doped into polystyrene) to a more concentrated sample (neat 2TIPS).24 In this case, the observed increase in the density of states can be explained by the increased local motions of individual hole transport molecules when going from a solid to a liquid.25 Temperature-driven changes in the density-of-states due to molecular motion and solvation energy have been proposed to describe deviations in temperature-dependent mobility behavior above the glass transition temperature in solid systems.26 However, this is still a poorly understood phenomenon that produces very different behaviors for various material types. For most doped polymer systems (as observed for 2TIPS/ polystyrene in this letter), the glass transition is characterized 1004
dx.doi.org/10.1021/jz300058w | J. Phys. Chem. Lett. 2012, 3, 1002−1006
The Journal of Physical Chemistry Letters
Letter
Trapping in C60 Sensitized Photorefractive Polymers. J. Appl. Phys. 2007, 102, 033106. (4) Hirata, S.; Kubota, K.; Jung, H. H.; Hirata, O.; Goushi, K.; Yahiro, M.; Adachi, C. Improvement of Electroluminescence Performance of Organic Light Emitting Diodes with a Liquid Emitting Layer by Introduction of Electrolyte and a Hole-Blocking Layer. Adv. Mater. 2011, 23 (7), 889−893. (5) Xu, D.; Adachi, C. Organic Light-Emitting Diode with Liquid Emitting Layer. Appl. Phys. Lett. 2009, 95, 053304. (6) Hendricks, E.; Guenther, B. D.; Zhang, Y.; Wang, J. F.; Staub, K.; Zhang, Q.; Marder, S. R.; Kippelen, B. L.; Peyghambarian, N. Ellipsometric Determination of the Electric-Field-Induced Birefringence of Photorefractive Dyes in a Liquid Carbazole Derivative. Chem. Phys. 1999, 245, 407. (7) Ribierre, J. C.; Aoyama, T.; Muto, T.; André, P. Hybrid Organic− Inorganic Liquid Bistable Memory Devices. Org. Electron. 2011, 12 (11), 1800−1805. (8) Kamino, B. K.; Grande, J. B.; Brook, M. A.; Bender, T. P. Siloxane-Triarylamine Hybrids: Discrete Room Temperature Liquid Triarylamines via the Piers−Rubinsztajn Reaction. Org. Lett. 2011, 13, 154−157. (9) Kamino, B. K.; Castrucci, J.; Bender, T. P. Controlling the Physical and Electronic Properties of Arylamines through the Use of Simple Silyl Ethers: Liquid, Waxy and Glassy Arylamines. Silicon 2011, 3 (3), 125−137. (10) Kamino, B. K.; Mills, B. M.; Reali, C.; Bender, T. P. Liquid Triarylamines: The Scope and Limitations of Piers−Rubinsztajn Conditions for Obtaining Triarylamine−Siloxane Hybrid Materials. J. Org. Chem. 2011, 77 (4), 1663−1674. (11) Ribierre, J.-C.; Aoyama, T.; Muto, T.; Imase, Y.; Wada, T. Charge Transport Properties in Liquid Carbazole. Org. Electron. 2008, 9, 396−400. (12) Melnyk, A. R.; Pai, D. M. Physical Methods of Chemistry, 2nd ed.; Rossiter, B. W., Baetzold, R. C., Eds.; Wiley: New York, 1993; Vol. VIII, Chapter 5, pp 321−386. (13) Auweraer, M. V.; Schryver, F. C. D.; Borsenberger, P. M.; Bässler, H. Disorder in Charge Transport in Doped Polymers. Adv. Mater. 1994, 6 (3), 199−213. (14) Borsenberger, P. M.; Weiss, D. S. Organic Photoreceptors for Xerography; Marcel Dekker, Inc.: New York, 1998. (15) Klenkler, R. A.; Xu, G.; Graham, J. F.; Popovic, Z. D. Charge Transport Across Pressure-Laminated Thin Films of Molecularly Doped Polymers. App. Phys. Lett. 2006, 88, 102101. (16) Borsenberger, P. M. Hole Transport in Tri-p-tolylamine-Doped Bisphenol-A-polycarbonate. J. Appl. Phys. 1990, 68, 6263−6273. (17) The glass transition temperature of the 1:1 (weight by weight) blend/composite of 2TIPS and polystyrene was determined through differential scanning calorimetry. During this experiment, the blend/ composite was heated to 200 °C, and only the glass transition temperature was observed. No other thermal event(s) was(were) observed including the lack of any observable phase separation(s). (18) Abkowitz, M.; Stolka, M.; Morgan, M. Behavior of the Drift Mobility in the Glass Transition Region of Some Hole Transporting Amorphous Organic Films. J. Appl. Phys. 1981, 52 (5), 3453−3457. (19) Bässler, H.; Borsenberger, P. M. The Transition from Nondispersive to Dispersive Charge Transport in Vapor Deposited Films of 1 -Phenyl-3-p-diethylamino-styryl-5-pdiethylphenylpyrazoline (DEASP). Chem. Phys. 1993, 177, 763−771. (20) Abkowitz, M. A.; McGrane, K. M.; Knier, F. E.; Stolka, M. Electronic Transport in Si and Ge Backbone PolymersEffect of Thermal Transitions. Mol. Cryst. Liq. Cryst. 1990, 183, 157−169. (21) Borsenberger, P. M.; Gruenbaum, W. T.; Magin, E. H.; Sorriero, L. J. Hole Transport in Tri-p-tolylamine Doped Polymers: The Role of the Polymer Dipole Moment. Chem. Phys. 1995, 195, 435−442. (22) N,Ǹ -diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) and 1,1-bis(di-4-tolylaminophenyl)cyclohexane (TAPC). (23) Chen, I. Current Transients due to Delayed Charge Injection. Jpn. J. App. Phys. 1989, 28, 21.
by the onset of temperature-independent mobility. Other systems have shown an inversion in the mobility dependence20 or even a continuation of normal behavior well above the glass transition temperature.19 Our LOS was studied well above its glass transition and displays quite pedestrian behavior throughout this range. In fact, the only difference found was a slight increase in the density of states compared to the solid polymeric sample. If molecular motions were important to charge transport through a LOS, we would expect a deviation from normal temperature-dependent behavior due to an increase in physical diffusion at higher temperatures. The absence of such an effect strongly suggests that molecular motions have a negligible effect on the hole-transport mobility of our system. In summary, the charge transporting properties of a silyl ether-substituted liquid triarylamine, 2TIPS, was studied. As both a dopant in an inert polymer matrix and as a neat liquid, the charge transport molecule exhibited conventional holetransporting properties when studied using a time-of-flight technique. Most interestingly, the charge mobility of neat 2TIPS closely followed the temperature dependence predicted by the disorder formalism model for solid materials. This behavior is observed over a large range of temperatures above the Tg, and we conclude that molecular motion has a negligible effect on macroscopic charge transport in this material save for a small increase in the density of states. The conventional charge mobility behavior of these materials suggests that silyl ether-functionalized LOSs may be excellent targets for integration into novel, liquid electronic devices by those who are familiar with traditional solid triarylamines but who might want to exploit the novel liquid state of LOSs.
■
ASSOCIATED CONTENT
S Supporting Information *
Detailed information on data collection, time-of-flight cell construction, experimental set up, and additional figures, tables, and synthetic details are included in the electronic Supporting Information accompanying this letter. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We would like to thank the National Research Council of Canada (NSERC) for funding of this project through the Discovery Grant Program, the Bert Wasmund Fellowship and Hatch Scholarship for Sustainable Energy Research for additional funding, and Mr. Johann Junginger for much technical assistance.
■
REFERENCES
(1) Shirota, Y.; Kageyama, H. Charge Carrier Transporting Molecular Materials and Their Applications in Devices. Chem. Rev. 2007, 107, 953−1010. (2) Snaith, H. J.; Zakeeruddin, S. M.; Wang, Q.; Péechy, P.; Grätzel, M. Dye-Sensitized Solar Cells Incorporating a “Liquid” HoleTransporting Material. Nano Lett. 2006, 6, 2000. (3) Ribierre, J.; Aoyama, T.; Kobayashi, T.; Sassa, T.; Muto, T.; Wada, T. Influence of the Liquid Carbazole Concentration on Charge 1005
dx.doi.org/10.1021/jz300058w | J. Phys. Chem. Lett. 2012, 3, 1002−1006
The Journal of Physical Chemistry Letters
Letter
(24) Schein, L. B.; Borsenberger, P. M. Hole Mobilities in a Hydrazone-Doped Polycarbonate and Poly (styrene). Chem. Phys. 1993, 177, 773−781. (25) Borsenberger, P. M.; Pautmeier, L.; Bässler, H. Hole Transport in Bis(4-N,N-diethylamino-2-methylphenyl)-4-methylphenylmethane. J. Chem. Phys. 1991, 95 (2), 1258−1265. (26) Bässler, H. Charge Transport in Random Organic Photoconductors. Adv. Mater. 1993, 5 (9), 662−665.
1006
dx.doi.org/10.1021/jz300058w | J. Phys. Chem. Lett. 2012, 3, 1002−1006
