J. Phys. Chem. 1984, 88, 4714-4717
4714
independent of the electric field for a wide range of concentrations. In this respect, TPD differs frqm all systems studied thus far. The field dependence at high fields can be best described by the exponential relationship proposed by the computer simulation of hopping transport.24 Some disagreement however still exists with the temperature dependence predicted by the model. The field
and temperature dependence of the mobility in the wide ranges in which the measurements were performed cannot all be explained in the framework of existing theories. Registry No. TPD, 65 18 1-78-4; bisphenol A polycarbonate (SRU), (bisphenol A)-(carbonicacid) (copolymer), 25037-45-0.
24936-68-3;
Trap-Controlled Hopping Transport D. M. Pai, J. F. Yanus, and M. Stolka* Xerox Webster Research Center, Webster, New York I4580 (Received: January 19, 1984) Transition from hopping to trap-controlled hopping transport has been observed in poly(N-vinylcarbazole) (PVK) doped l,l’-biphenyl]-4,4/-diamine(TPD). At very with another transporting material, N,N’-diphenyl-N,N’-bis(3-methylphenyl)-[ low concentrations of the additive, the transport is dominated by the slow release of charges from the additive sites acting as traps for transport via carbazole groups of the polymer. As the concentration of the additive is increased, the transport is dominated by hopping via the additive sites with the polymer acting as an inert binder. The large trapping effect of small concentrations of TPD is explained on the basis of the difference in ionization potentials of the two materials.
Introduction Charge transport in crystalline solids takes place in extended states, and the microscopic charge carrier mobility is determined by the nature of the band and the scattering the carrier experiences as a result of crystal lattice vibrations. In the presence of shallow traps, the free carrier motion is interrupted by the interaction with the traps. The number of trapping events during the course of the drift depends on the concentration of traps. As a result, at any given time, a fraction [nto/(nto no)] of carriers in transit reside in shallow traps, and the remaining carriers (h/(qo no)]] are “free”. The transit time in the presence of shallow traps is equal to the sum of microscopic transit time “while free” and the product of the number of trapping events a carrier experiences during transit and the trap release time. The drift mobility is then’ P = d n o / ( n t o + no)] where p,, is the microscopic mobility, no is the number of free carriers, and ntOis the number of carriers residing in the shallow traps. Trap-controlled drift mobilities have been reported in a number of inorganic and organic crystal^.^" On the other hand, charge transport in amorphous organic solids is a hopping It is dominated by conditions for charge exchange between neighboring hopping sites, which are discrete molecules or groups if the material is a polymer. It is tacitly assumed that the distribution of energy of hopping sites in “trapfree” hopping systems is narrow, and therefore approximately the same amount of thermal activation is needed to free charges from all the hopping sites. It is estimated’ that the energy distribution width is only about 0.1 eV. Trapping then should mean the presence of hopping sites that differ from the majority of sites in that they require substantially larger energy input to release the charge carriers back to the majority type of hopping sites. On the basis of these considerations it was proposed that hole transport via one molecule would be affected by trapping on
+
+
another molecule provided the latter has lower ionization potential? This was indeed demonstrated by results obtained with N-isopropylcarbazole (NIPC) and triphenylamine (TPA). Both materials display comparable charge transport characteristicss when dispersed individually in inert polymeric binders. Both have field-dependent carrier mobilities which are thermally activated. Since the gas-phase ionization potential of TPA is 0.48 eV lower than that of NIPC, TPA when present in small concentrations can become trapping molecule for transport via NIPC. In this paper we describe hole transport in two materials with markedly different transport characteristics, poly(N-vinylcarbazole) (PVK) and N,N’-diphenyl-N,N’-bis(3-methylphenyl)-[ l,l’-biphenyl]-4,4’-diamine(TPD).
TPD
PV K
Charge transport in PVK has been studied extensively.”’ The carrier mobility is strongly field dependent in the whole range of measured fields and typically low, from lo-* to IO” cm*/(V s), depending on the temperature and electric field. The activation energy of hole transport” at E = 5 X lo5 V/cm is -0.4 eV and also field dependent. The hole mobilities in TPD are independent of electric field up to los V/cm, particularly at high concentrations of TPD in the polymer matrix and in amorphous glass of TPD.12 Values of I.L exceeding cm*/(V s) have been measured even at electric fields well below lo4 V/cm. The apparent activation energy of hole transport in the amorphous glass of TPD below T8 (63 “C) is only 0.12 eV.
- -
-
terscience, New York, 1967. (2) W. E. Spear and J. Mort, Proc. Phys. Soc., London, 81, 130 (1963). (3) D. C. Hoesterey and G. M. Letson, J . Phys. Chem. Solids, 24, 1609
Experimental Section The synthesis and purification of N,N’-diphenyl-N,N’-bis( 3methylpheny1)-[ l,lr-biphenyl]-4,4/-diamine (TPD) has been described in the previous publication.12 PVK was purified by fivefold
(1963). (4) J. Mort, G. Pfister, and S . Grammatica, SolidState Commun., 18,693 (1976). ( 5 ) G. Pfister, Phys. Reu. B: Solid State, 16, 3676 (1977). ( 6 ) H. Scher and E. W. Montroll, Phys. Reu. B: Solid State, 12, 2455 (1975). (7) H. Bassler, G. Schonherr, M. Abkowitz, and D. M. Pai, Phys. Rev. E Condens. Matter, 26, 3105 (1982), and references therein.
(8) G. Pfister, S. Grammatica, and J. Mort, Phys. Reu. Lett., 37, 1360 (1976). (9) P. J. Regensburger, Photochem. Photobiol., 8, 429 (1968). (10) D. M. Pai, J . Chem. Phys., 52, 2285 (1970). (11) W. D. Gill, J . Appl. Phys., 43, 5052 (1972). (12) M. Stolka, J. F. Yanus, and D. M. Pai, preceding paper in this issue.
(1) A.
Rose, “Concepts in Photoconductivity and Allied Problems,” In-
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0 1984 American Chemical Societv 0022-3654/84/2088-4714$01.50/0
Trap-Controlled Hopping Transport precipitation from benzene solutions by methanol. Appropriate amounts of the polymer, PVK, and TPD were dissolved in methylene chloride. Films varying in thickness from 1 to 3 5 pm were coated by a drawbar technique on a thin (0.5 pm) layer of amorphous selenium which had been vapor deposited on an aluminum substrate. The TPD concentration in the further text is quoted as the ratio by weight of TPD to PVK. The drift mobilities were measured by the time of flight t e ~ h n i q u e . ’ ~The experimental arrangement has also been described in the previous paper. In this technique charge carriers are photogenerated in the selenium layer by a 5-ps light flash; the sheet of charges is then injected into the organic film. The transient current due to the drift of the thin sheet of charges is time resolved. The transit time is the time when the first wave of carriers reaches the substrate, and it appears as a demarcation between the flat portion and the large tail of the current vs. time curve. In this study, the current due to the carrier drift was displayed in double linear i - t rather than the log i - log t displays employed to study the very dispersive signals observed in some materials: Clear transit times due to the arrival at the substrate of a substantial fraction of the carrier was clearly visible in the i - t display when time of flight measurements were carried out on PVK, TPD, and TPD in PVK.
The Journal of Physical Chemistry, Vol. 88, No. 20, 1984 4715
, tVACUUM LEVEL
I
f
t
I
la 1
Ib )
------pVK
~
Id)
Figure 1. Schematics of hopping via high ionization potential compound in the presence of a low ionization potential compound: (a) hopping via high-I, material (no dopant); (b) hopping via high-I, material in the presence of a small amount of a low-I, compound (trapping at the sites of low I,); (c) hopping via both low-I, and high-I, material; (d) hopping via low-I, compound exclusively.
TABLE I: Ionization Potentials from Ea 1
Results and Discussion compd Y, eV eV I , , ~eV It has been postulated that charge carrier mobilities in disorTPD/TCNE 1.63 7.1 7.3 PVK~TCNE 2.06 1.6 1.8 dered organic systems such as solid solutions of active transport NIPC/TCNE 2.10 1.7 7.8 molecules in inert polymers are dominated by the rate of charge N-ETC/TCNE 2.02 1.6 1.7 exchange between neighboring localized state^.^,^ The “localized states” are isolated molecules uniformly dispersed in inert binder ‘ a = 0.72,b = 3.48;values from ref 16. b a = 0.92,b = 5.12;values resins or functional groups in certain polymers. The “holes” are from ref 15. positively charged molecules or groups, cation radicals; the ”electrons” are corresponding anion radi~a1s.I~The microscopic marizes values obtained for TPD, PVK, and model compounds mechanism of hole transport can therefore be interpreted as an for PVK, N-isopropylcarbazole (NIPC) and N-ethylcarbazole electric field driven chain of reversible redox reactions where (N-ETC), with tetracyanoethylene (TCNE) as an acceptor and neutral molecules or groups will repetitively transfer an electron methylene chloride as a solvent. Values of a and b are the emto their positively charged cation radical neighbors in the direction pirical constants for T C N E and a series of donors, obtained by of an electric field. Although the charge migration involves two a u t h o r ~ . ’ ~Values J ~ obtained by JohnsonZZare probably more well-defined ions (cation radicals in the case of hole transport), accurate since his data base includes a large number of donor it is not considered an ionic process as such since ions do not compounds including many aromatic amines. migrate and matter is not displaced, at least not beyond the range Even though the values of Zphave to be considered approximate, associated with vibrations, rotations, and diffusion of molecules the difference in ionization potentials is large. Since the Zpof TPD and ions in semirigid polymeric glasses. is lower by about 0.5-0.6 eV, TPD is expected to act as a trap The hole transfer or injection from a photoconductor can be for hole transport in PVK. described as a photooxidation step. One would therefore expect The effect of addition of TPD to PVK is shown in Figure 2. that the injection depends, among others, on the ionization poThe hole mobility in PVK at 22 OC and E = 5 X lo5 V/cm is tentials of the photoconductor and the hole transport material. 2.5 X 10” cm2/(V s) (point 1, Figure 2). Addition of as little Charge transfer among like species (hopping transport), on the as 0.05 wt. % of TPD (based on PVK) reduces the mobility, p, other hand, is dominated by factors that are included in the by about 2 orders of magnitude. The addition of more TPD apparent activation energy of charge transport. It could also be (region 2 on the curve in Figure 2) further reduces the hole expected that the addition of very small amounts of another hole achieved at the mobility to the minimum value of 1.5 X transport material with a lower oxidation potential than that of 0.02/1 weight ratio of TPD to PVK. From this point on, the the first material will result in the formation of traps. The cariers addition of more TPD to PVK (region 3, Figure 2) causes an in transit will stop at the site of molecules with lower ionization increase of p. The system reaches mobility of pure PVK at the potentials and will not be released until a neighboring neutral TPD/PVK ratio of 0.27/1. The mobility further increases with molecule gains statistically enough thermal energy to release an increasing TPD concentration (region 4), ultimately reaching electron and move it “uphill” to the site of the trap. (See Figure values in excess of cm2/(V s) for 100 wt.% TPD (in 1.) amorphous glass) with no PVK present. The ionization potentials of PVK and TPD were qualitatively At ratios of TPD to PVK larger than about 0.27/1, the mixture determined from the strength of charge-transfer complexes with has hole mobilities equal to those measured in TPD/polycarbonate an electron acceptor by using the following empirical f o r m ~ l a ~ ~ ~at’ ~the same TPD concentrations. This indicates that at these weight ratios of TPD/PVK, the transport proceeds exclusively v (eV) = a l p - b (1) via TPD sites with no participation of PVK. Under these conditions, PVK performs only the function of an inert binder resin where v is the energy of the lowest energy charge-transfer band, such as polycarbonate. At low TPD/PVK ratios, however, Z is the ionization potential of the donor, and a and b are constants transport proceeds primarily via PVK with TPD molecules acting Bependent upon the nature of the a c c e p t ~ r . ~ Table ~ - ’ ~ I sumas randomly distributed traps with slow release of carriers (region 2, Figure 2). Between those two regions, both materials participate (1 3) F. K.Dolezalek in “Photoconductivity and Related Phenomena”, J. in transport, TPD more so as its concentration is increased (the Mort and D. M. Pai, Eds., Elsevier, New York, 1976,pp 27-63. average intersite distance is decreased). Region 1 corresponds (14) J. Mort and G. Pfister, Polym.-Plast. Technol. Eng., 12,89 (1979). to the situation in Figure la. Region 2 corresponds to Figure lb; (15)E. M. Voigt and C . Reid, J . Am. Chem. Soc., 86, 3930 (1964). low concentration of TPD causes trapping. Region 3 corresponds (16)G.E.Johnson, unpublished data.
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4716 The Journal of Physical Chemistry, Vol. 88, No. 20, 1984
1
6
0
20
1
40
0
60
00
0
Pai et al.
I
PERCENT TPD
1
Figure 2. Hole drift mobility a t E = 5 X IOs V/cm (22 "C) in PVK doped with varying amounts of TPD (in weight percent TPD). The meaning of numbers 1-4 is explained in the text.
to Figure IC, where hopping proceeds mainly via TPD but also via PVK. The mobilities are low since the intersite distance between TPD units is still large. Region 4 corresponds to Figure Id. The distance between TPD sites is small, and the transfer of electron from carbazole in PVK is not energetically favored. Figure 3 shows the mobility as a function of TPD concentration in PVK expressed as the number of molecules per cm3. For comparison, the hole drift mobilities in TPD dispersed in polycarbonate are also presented. At low concentrations of TPD in PVK corresponding to region 2 of Figure 2, the drift mobility is approximately inversely proportional to the TPD concentrationas expected for an ideal system with a low concentration of traps. It is assumed that the introduction of traps has no effect on the mobility in the regions between the traps. For molecular concentrations larger than 4 X lozomolecules/cm3, the mobility is independent of the binder. In this region, the intermolecular distance between TPD molecules is small enough to dominate the transport regardless of the binder polymer. Hole mobility in PVK is highly field dependent over the whole measurable range of electric fields,9-'' while in the TPD/polycarbonate films the mobilities are independent of electric fields and have only shallow field dependence at high fieldsI2 (dashed curves in Figure 4). The difference in the field dependence is also apparent in the PVK/TPD systems. For example, in region 3 (Figure 2) the mobility is still highly field dependent, indicating that PVK is still involved in the transport. At TPD/PVK ratios greater than 0.2/1 the field dependence is shallow and approximately the same as that in TPD/polycarbonate (see Figure 4). The transition from the trap-controlled hopping transport to hopping transport is also evident from Figure 5 , which shows the activation energy, A, of hole transport as a function of concentration of TPD in PVK. The activation energy of transport for pure PVK at 5 X lo5 V/cm is approximately 0.4 eV. As very small amounts of TPD are added to PVK, the transport activation energy, A, increases up to -0.55 eV. This increase reflects the extra energy input needed for release of holes from TPD moiety back to PVK. The reduction of TPD cation radicals (trapped holes) by carbazole is an improbable process since the redox potential of alkylcarbazole is more positive than that of TPD. Based on the measured activation energy of PVK and TPDdoped PVK, the trap depth of TPD in PVK is approximately 0.15
I
I
k TPD CONCENTRATION (MOLECULES/CM3)
Figure 3. Hole drift mobility at E = 5 X lo5V/cm (22 "C) as a function of TPD concentration: solid curve, PVK/TPD; dashed curve, TPD/ polycarbonate.
16'7 , io3
/ I
0.02:1 1 l l l i l l l
104
I 1 1 l l 1 1 1 1
io5
1
, l l l l l L l
106
ELECTRIC FIELD (V/CM)
Figure 4. Hole drift mobility at 22 "C as a function of electric field for PVK/TPD (solid curves) and TPD/polycarbonate (dashed curves). The numbers indicate weight ratios of TPD to the host polymer.
eV, which is much smaller than the difference between the ionization potentials of PVK and TPD. Depending on the empirical
J . Phys. Chem. 1984, 88. 4717-4723 WEIGHT RATIO TPWMATRIX
lo-z
Id'
10
100%TPO
i
I
E= 5 x 105 V/CM 0.6-
-
VK MATRIX
4717
The trend, however, is in the right direction. The lower ionization potential material is a trap in the higher I, matrix. As the concentration of TPD is increased to such levels where hopping among TPD molecules becomes feasible, the activation energy begins to decrease. Eventually, it reaches values that are typical for TPD in polycarbonate (dashed line, Figure 5). This also indicates that in this region, hole transport proceeds via TPD with no PVK participation.
Conclusions Hole transport has been measured in poly(N-vinylcarbazole) (PVK) doped with various amounts of N,N'-diphenyl-N,N'-bis% (3-methylphenyl)-[l,l'-biphenyl]-4,4'-diamine (TPD). Charge ;j0.3\ \ carrier mobilities in PVK are drastically reduced (by up to 3 orders \ of magnitude) by the addition of small amounts of TPD which, POLYCARBONATE by itself at high concentration, is a more efficient transport ma0.2MATRIX A, terial than PVK. At higher concentrations of TPD, exceeding I -0.2/1 ratio to PVK, the charge transport proceeds exclusively 0.1 via the additive with no participation of carbazole units in PVK. This behavior is interpreted on the basis of differences in ionization potentials of the two materials. While the lower I, material (TPD) L L I I L l l L l llllIIJ is easily oxidized by the cation radicals of the carbazole groups, IOZ0 IOZ1 5x102' the transfer of charge back to PVK is energetically difficult. Since TPD CONCENTRATION ( MOLECULES/CM3) the distance between molecules of the additive at low concenFigure 5. Hole transport activation energy at B = 5 X lo5 V/cm as a trations is too large for charge transfer to occur, TPD becomes function of TPD concentration in PVK (solid line) and polycarbonate a charge trap. This feature demonstrates the versatility and utility (dashed line). The transport activation energy for PVK is denoted by of amorphous organic systems in studies of trapcontrolled hopping the arrow. and transitions from trap-dominated hopping to hopping transport which is not easily achieved in inorganic systems. The charge drift relation employed to calculate I,, this difference is 0.5-0.6 eV. velocities can be controlled over a wide range by changing the This simplistic picture of correlating the trap depth with the concentration of a low-I, transporting additive. difference in Zp assumes that the ionization potential of an isolated molecule of TPD is independent of the environment, which is Registry No. PVK, 25067-59-8;TPD, 65181-78-4;NIPC, 1484-09-9; N-ETC, 86-28-2. probably not the case. That could account for the discrepancy.
-
0.4
3
'
L
L_i
Long-Range Proton Hyperfine Coupling in Alicyclic Nitroxide Radicals by Resolution-Enhanced Electron Paramagnetic Resonance Magdi M. Mossoba, Keisuke Makino, Peter Riesz,* Radiation Oncology Branch, Clinical Oncology Program, Division of Cancer Treatment, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20205
and Ray C . Perkins, Jr. Varian Associates, Palo Alto, California 94303 (Received: March 8, 1984)
The free radicals 4-R-2,2,6,6-tetramethylpiperidine1-oxy1 (R = OH (Tempol), NH2(Tempamine), H2 (Tempo), and 0 (Tempone)) and 2-R-5,5-dimethylpyrrolidine1-oxy1 (R = COOH,CH,,CD3,OH,and OD)have been investigatedin aqueous solutions at room temperature by EPR using 90' out-of-phase detection. In contrast to in-phase EPR, the present approach provides a means for further identificationof radicals which is particularly useful for species that are otherwise indistinguishable. The superhyperfinecoupling constants of y- and 8-nuclei in piperidine derivatives, which were obtained by computer simulation, are in agreement with those calculated from literature 'HNMR data observed for the same compounds and indicate that Ternpol and Tempamine are stable in chair conformations, while Tempo and Tempone are rapidly interconverting between two identical chair and twist conformations, respectively. The data obtained for the pyrrolidine-1-oxy1 derivatives containing a carboxy or methyl group at C2 were consistent with slightly puckered rings while the derivative with a hydroxy substituent was found to favor a deformed ring with a pucker at C2.
protons that are three or four u bonds away. In the present study, long-range coupling could be obtained for substituted piperidine*Member of NIH ESR Center.
(1) Perkins. Jr.. R. C.. oaoer oresented at the 5th International EPR Symposium, Denver, CO, 19ki2: Clarkson, R. B. Varian Instrum. Appl. 1979, 13, 4.
This article not subject to U S . Copyright. Published 1984 by the American Chemical Society
