7908
J . Phys. Chem. 1991, 95, 7908-7914
Hole Diffusion In Triarytamine Polymer Films in a Contactlng Electrolyte. Initial Comparison with Hole Mobilities John S. Facci,* Martin Abkowitz, William Limburg, Fred Knier, John Yanus, and Dale Renfer Xerox Webster Research Center, 114-390, 800 Phillips Road, Webster, New York I4580 (Received: September 21, 1990; In Final Form: May 15, 1991)
The first comparison of hole diffusion and mobility in a discrete hopping charge-transport polymer is presented. Charge transport (hole diffusion) in a triarylamine-containing polymer in contact with a liquid electrolyte is measured electrochemically by steady-state voltammetry and characterized by a hole diffusion coefficient Dh. Dhrises with time spent in the oxidized state and is apparently due to electroanodic cross-linking of the polymer. That counterion transport is not rate limiting during the measurement of Dhin the un-cross-linked polymer is rigorously demonstrated. Comparison of activation data suggests that hole hopping in the un-cross-linked and cross-linked polymers proceeds by different mechanistic pathways. Time of flight (TOF) drift mobilities j~ are measured in the un-cross-linked polymer and compared with Dh via the Einstein equation over a range of temperatures. Predicted values of zero-field mobilities from Dhand the Einstein equation agree qualitatively with experimental mobilities. The predicted mobility activation energy, however, is somewhat low relative to the experimental results. This is attributed to the differences in the physical state of the polymer in the two measurements. Dh measurements are done in a liquid electrolyte while mobility measurements are done in the solid state. This leads to differences in the coupling of the electron-exchange step with microscopic polymer motions in the two techniques.
Introduction Charge transport among a network of fixed hopping sites in electrochemically reactive polymers is a ubiquitous phenomenon which is of both fundamental and applied importance, especially in areas such as electrophotographic imaging,' electrocatalysis,2J and molecular electronic devices4 Charge transport in electroactive polymers is a discrete hopping process in which the electron-hopping sites are identified with molecular redox sites. The hopping process involves a series of electron self-exchange reactions between neighboring reduced-oxidized molecular pairs as shown in eq I . In this equation, k,, is the apparent electron
self-exchange constant, Ox and Red are the oxidized (acceptor) and reduced (donor) forms of the electroactive center, and the star indicates a "tagged" molecule. The bonds are reminders that the electroactive species are associated with a polymer lattice. Their local microscopic motions, which are governed by the polymer chain, are therefore coupled to the electron-transfer step. The motion of charge-compensating counterions X-is also coupled to the electron-transfer reaction as indicated in the equation. Charge-transport rates, which are characterized by an electron (De) or hole (Dh) diffusion coefficient, can now be measured by an array of electroanalytical techniques. These include potential step chron~amperometry~ and more recently a family of steadystate voltammetric techniques which are based on the use of two terminal interdigitated arrays (IDA'S)and twin-electrode sandwich cells.b11 M ~ r r a y ~and . ' ~Faulkner" have pointed out that the (1) (a) Schaffert, R. M. Electrophotography; Wiley: New York, 1975.
(b) Pai, D. M . In Photoconductivity In Polymers; An Interdisciplinary Approach; Patsis; A . V., Seanor, D.A., Eds.; Technomic: Westport, CT, 1976; Chapter 2. (2) Murray, R. W . I n Elecrroanalytical Chemistry; Bard, A. J.,; Ed.; Marcel Dekker: New York, 1984; Vol. 13. Chapter 2, and references therein. (3) Andrieux, C. P.; Dumas-Bouchiat, J. M.; Saveant, J. M. J . Electroanal. Chem. 1982, 131, I . (4) Chisdey, C . E. D.; Murray, R. W. Science 1986, 231. 2 5 . ( 5 ) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundametnals and Applicafions; Wiley: New York, 1980, Chapter 5. (6) Jernigan, J . C.; Murray, R. W. J . A m . Chem. SOC.1987, 109, 1738. (7) Jernigan, J . C.; Murray, R. W. J . Phys. Chem. 1987, 91, 2030. (8) Jernigan, J . C.; Surridge, N . A.; Zvanut, M. E.; Silver, M.; Murray, R. W . 1. Phvs. Chem. 1989.89. 4620. (9) Chidsky, C. E. D.; Feldman, B.J.; Lundgren, C.; Murray, R . W. Anal. Chem. 1986, 58, 601. (IO) White, B. A.; Murray, R . W . J . A m . Chem. SOC.1987, 190, 2576.
coupling of counterion motion with charge diffusion can lead to very large errors in the transient measurement of D. Steady-state voltammetry circumvents this problem and is being widely applied to the measurement of electron transport rates in electroactive&I' and conducting In these experiments, bulk electrolysis of the film generates a mixed-dent polymer film and results in incorporation of charge-compensating counterions X-. In the present investigation the discussion will be limited to the steady-state voltammetry of redox polymer films on Au IDA'S contacted by a liquid electrolyte. Time-of-flight (TOF) techniques, which have been described largely in the solid-state physics literature,l are used to measure drift mobilities p in univalent electroactive films. These measurements are carried out on a capacitively charged metaltransport layer-metal sandwich device in a constant externally applied electric field. A small fraction of the applied charge is photoinjected at one of the metal-polymer interfaces and the time-resolved transit of charge to the other metal-polymer interface is measured. In the case of uncharged electroactive polymers, charge transport occurs in the absence of any counterions. This, in principle, allows the influence of counterion motion to be separated from other factors (such as polymer chain motions) which are coupled to electron hopping. Equation 1 provides a central theme for understanding the hopping process in electroactive polymers in diverse arenas such as hopping in the solid state (mixed-valent or univalent), and in polymers which are contacted by an electrolyte or plasticized by a bathing solvent vapor.6 An ultimate goal is the understanding of how molecular, electronic, and environmental factors influence hopping rates. Toward this end, we have undertaken a comparison of charge diffusion rates (obtained from an electroanalytical probe of transport) with a TOF determination of drift mobility. This comparison provides a test of the Einstein relation in the context of hopping transport (as opposed to ionic transport) and helps elucidate mechanistic characteristics of hopping in the solid state which are still not well understood. This has not, to the best of our knowledge, been done yet. In this and a following paper we are initiating a dicussion of these comparisons. ( I I ) Chen, X.; He, P.; Faulkner, L. R. J . Electroanal. Chem. 1987, 222, 223. ~~. (12) Kittlesen, G.P.; White, H . S.; Wrighton, M . S.J . Am. Chem. SOC. 1985, 107, 7373. ( I 3) Thackery, J. W.; White, H . S.;Wrighton, M. S.J . Phys. Chem. 1985,
89 - . , 5111 - .- - .
(14) Kittlesen, G.P.; White, H . S.;Wrighton, M . S. J . Am. Chem. Soc. 1984, 106, 7389.
( 1 5 ) Paul, E. W.; Ricco, A . J.; Wrighton. M. S.J . Phys. Chem. 1985,89, 1441.
0022-3654/91/2095-7908%02.50/00 I99 I American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 7909
Hole Diffusion in Triarylamine Polymer Films We have found that the condensation polymer PTPB, which incorporates into the polymer backbone reversibly oxidized tet-
PTPB
raphenylknzidine (TPB) moieties, is an ideal candidate for these comparisons. Steady-state voltammetric measurements of thin films of PTPB on interdigitated arrays immersed in LiC104/ CH$N are employed as the electroanalytical probe of Dh. In these measurements, high ionic conductivity, at least at a microscopic level, is required in order to prevent counterion transport from rate limiting the electron-transfer step. We establish in the studies below that counterion transport does not limit the electron-hopping rate. In addition, PTPB is ideal from a TOF standpoint as the drift mobility is characterized by trap-free hole transport.I6 Described below are studies of the temperature dependence of Dh in films of solvent cast PTPB, electrodeposited PTPB, and the model compound N,N’-( 3-hydroxyphenyl)-N,N’-diphenyL4,4’-diaminobiphenyI (VZ-(OH)~TPB, I). The
Figure 1. Schematic diagram of the Au microelectrode interdigitated array fabricated on Si wafers. The microelectrodes are 5 pm wide, 1 mm long, and 1000 A thick and the Si02gap is 5 pm wide. Au lead-in lines are 0.2 mm wide and terminated in 1 X 1 mm contact pads. A 2000-A polymer film is overcoated on the array.
p Transport Polymer I, m-tOH)zTPB
electrochemistry of I and PTPB are interesting in their own right and apropos to the comparison of D and p . PTPB is readily electroanodically cross-linked and, much to our surprise, Dh is greatly enhanced in cross-linked films. This fact is used to validate the comparison of electron-hopping diffusion and mobility.
Experimental Section Interdigitated Arrays. Microlithographically patterned Au IDA’S were designed and fabricated in-house. Three-inch Si wafers of ( 1 1 1 ) orientation (Polishing Corp. of America) were thermally oxidized forming a 500-nm Si02layer on Si. This served as an insulating base for the array. A 5.0-nm vapor-deposited Cr layer was then applied to the Si02layer as an aid in bonding a 100-nm electron beam deposited Au layer. Microelectrode arrays were patterned using a high-fidelity copy of the master mask (Perkin-Elmer, Dansbury, CT). Microlithography was performed by standard lift-off techniques. Each microelectrode is 5 pm wide, 1000 pm long, and 0.1 pm thick. Adjacent microelectrodes are electrically insulated from one another by a 5 pm wide 500 nm thick Si02 insulating gap. The 50 inner and 51 outer microelectrodes form the two terminals of the IDA, denoted W, and W,, respectively. W, and W, are employed as hole generator and collector electrodes, respectively. Patterned wafers were finally diced into 5 X 7 mm sections containing individual IDA devices. A pair of 1 mm X 1 mm contact pads and 0.2 mm wide Au lead-in lines on the wafer provided the electrical connection to the IDA device. Electrical contact to the pads was provided by homebuilt Au spring clips or Cu microalligator clips. The IDA is schematically shown in Figure 1. Some of the IDA’s were packaged in order to provide a better means of making electrical contact with the device. The diced wafer in this case was mounted with Epo-Tek H70E-2 epoxy on a 8 mm X 50 mm polyamide circuit board. Each circuit board was terminated with Au-plated Cu strips to which a standard card edge connector was coupled. Contacts from the device to the circuit board were made via a thermosonic Au ball wire bonder (Model 2460-11, Hughes Aircraft IPD). Wire bonds employed 0.025-mm Au wire which were encapsulated with a globtop encapsulant, Hysol ES2343 cross-linkable resin, in order to provide mechanical protection for the Au wires. (16) Facci, J.
S.;Stolka, M.Philos. Mag. B 1986, 54,
1.
Figure 2. Schematic diagram of the sandwich cell used in time-of-flight drift mobility measurements. Photogenerated holes transit the chargetransport polymer of thickness d in a constant electric field E .
Transport Measurements. Individual IDA’s were first cleaned in a mixture of concentrated nitric and sulfuric acids overnight in order to remove residual photoresist. IDA’s were then overcoated with thin films of solvent cast PTPB, electrodeposited FTPB, or electropolymerized I (poly-I) in order to measure charge (hole) transport rates electroanalytically in each of these systems. In each case, the polymer film is uniformly coated over the entire microelectrode assembly as well as the insulating SiO, gap between microelectrodes. Electroanalytical characterization of the polymer coated IDA was preceded by careful overcoating of all lead-in lines under a 30X stereomicroscope with a layer of solvent resistant silicone adhesive. An x-y translation stage was used to ensure accurate placement ( f 5 pm) of the adhesive. Films of PTPB were cast by evaporation of the solvent from 1-5 mg/mL solutions in chlorobenzene (HPLC grade, Aldrich). Electroanodic polymerization of I onto the Au IDA was done by sweeping through both the +1 and +2 oxidation waves of 2.0 mM I in 0.3 M LiC104/CH3CN. Electrodeposition initially occurs on the Au microelectrodes, growing laterally from the microelectrode edge into the insulating gap. Due to their proximity, adjacent microelectrodes become physically bridged with a thin film of poly-1. Electrodeposition was stopped when the gap was completely bridged (viewed optically at SOOX). The IDA’s were then rinsed to remove oligomeric or unreacted I. PTPB was electrodeposited in a manner similar to the electrodeposition of I. Figure 2 schematically shows the sandwich cell used in drift mobility measurements. A 10-1 5-pm polymer film is doctor blade cast onto a Au-coated Si wafer. The polymer is then overcoated with an evaporated AI film. A potential difference of 50-900 V ( 3 X IO4 to 6 X IOs V/cm) is applied to the sandwich cell with the AI electrode biased positive. A nanosecond laser pulse (A = 337 nm) is used to photooxidize a small number of TPB sites near the Al/polymer interface. Charge transport (flow of holes from the AI to the Au electrode) occurs in a constant external electric
7910 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991
Facci et al.
1
200 pA/cmZ
*
1
I
A, fyI
50 pA/cmZ
ic
B
E (V.) vs. SCE
Figure 3. Cyclic voltammograms of I at a vitreous carbon electrode ( A = 0.07 cm2)in 0.1 M TEAP/CH3CN. Panel A: 1.0 mM solution of I, 500 mV/s. Panel B: Film of electroanodically polymerized I, 50 mV/s.
field. The transit time of the packet of charge is measured from the current-time profile and related to the hopping mobility (vide infra). Instrumentation and Materials. A PAR Model 175 universal programmer and 173 potentiostat, Nicolet 4094C digital oscilloscope, and HP7074 digital pen plotter were used to perform, record, and digitally store voltammetric data. A Pine Instruments Model RDE-4X bipotentiostat was used in measurements requiring independent potential control of W, and W2. Standard degassable electrochemical cells with a Luggin probe were employed. Variable-temperature measurements were made using a nonisothermal cell arrangement in which the reference electrode was kept at room temperature while the cell temperature was varied. All potentials are referenced to a sodium chloride saturated calomel electrode (SSCE) of conventional design. Glassy carbon electrodes, 0.3 and 0.5 cm in diameter, were obtained from Bioanalytical Systems and Princeton Applied Research, respectively. Pt disk electrodes were Teflon shrouded and 0.29 cm2 in area. PTPB is formed by the condensation of M-(OH)~TPBwith diethylene glycol bis(chloroformate), [CIC(O)CH2CH2I20,in T H F in the prtsence of triethylamine. The details of the synthesis are presented elsewhere." Anhydrous LiC104 (G.F. Smith) was used as received but dried in a vacuum oven at 110 OC immediately prior to use. Tetra-n-butylammonium perchlorate (TBAP) was twice recrystallized from a mixture of ethyl acetate and petroleum ether. Tetraethylammonium perchlorate (TEAP) was recrystallized three times from water and dried in a vacuum oven at 110 OC. High-purity acetonitrile (B&J and EM Science), specified to contain
