- Email: [email protected]
A study of hole mobility in diaryldiacetylenes
ELSEVIER
Synthetic Metals 78 (1996) 79-83
A study of hole mobility in diaryldiacetylenes A.V. Vannikov a,‘A.R. Tameev a, E.I. Mal’tsev a, G.H.W. Milburn b, A.J. Shand b, A.R. Weminck b, J. Wright b a Frumkin b Department
Iilsiitute of Electrochemistry, Russian Acade.my of Sciences, Leninski Prospect 31, Moscow I 17071, Russia of Applied Chemical and Physical Sciences, Napier University, 10 Colinton Road, Edinburgh EHIO .5DT, UK
Received 9 May 1995; revised 5 September 1995; accepted 13 October 1995
Abstract Thin homogeneous films exhibiting effectiveholetransportwith drift mobility valuesup to low4 cm2V-’ s-l areformedin the process of vacuumthermaldepositionof monomericdiaryldiacetylenes. Basictransportcharacteristics andtheir dependence on the electricfield, temperature andchemicalcompositionof the monomers aredetermined. Keywor&:
Diaryldiacetylene; Mobility
1. Introduction The extensive use of molecularly doped polymer layers as xerographic photoreceptors has led to a number of publications covering experimental and theoretical work on the subject of electron and hole transport in disordered molecular solids [ l-71. In thesesystems it is essential to distinguish whether conduction is controlled by the process of ionization of an individual transport molecule followed by inuamolecular structural relaxation (polaron formation) or by polymer/ dopant interactions. Polydiacetylene (PDA) single crystals are the only polymer single crystals that have a polyconjugated backbone structure and consequently exhibit uniquely high charge carrier mobilities for organic compounds (about 5 cm* V- ’ s- ‘) [ 8,9]. The structural and geometrical criteria that must be satisfiedfor the topochemical polymerization of diacetylene
single crystals [ 10,l l] are very restrictive, limiting the structure and number of polymer single crystals that can be obtained. Monomeric diaryldiacetylenes are low-molecular weight analogues of PDAs and are therefore of interest for fundamental studieson charge transport mobilities. Having aromatic side groups adjacent to the diacetylene unit createsa
molecule that is usually unable to undergo topochemical polymerization, but can often be thermally polymerized from the isotropic or liquid crystalline phases [ 12,131. Many diaryldiacetylenes have been prepared at Napier University and some have very good nonlinear optical properties (e.g. compound D(2) in this paper has a ,$‘) coeffi0379-6779/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved ccnrn170 L770,0~\~1C71 0
cient of 2.02+0.50 pm V-’ at 15% concentration in poly( methyl methacrylate) ) [ 14-161. In the present paper hole transport in diaryldiacetylenes of different structures is considered.
2. Experimental All the intermediates and diacetylenes were purified and satisfactorily characterized using standard analytical techniques. The diacetylenes were prepared using the CadiotChodkiewcz reaction between equimolar amounts of bromo and terminal acetylenes in the presence of catalytic amounts of copper(I) chloride. The bromoacetylenes were synthe,sized by chain extension of the corresponding aldehydes [ 171. The intermediate dibromoalkenes were converted into the bromoacetylenes by reflux with equimolar amounts of potassium tertiary-butoxide in toluene. Nitrophenylacetylene was prepared by reacting nitrobromobenzene with trimethylsilylacetylene in the presence of a palladium catalyst [ 181. Aminophenylacetylene wasproduced by reducing nitrophenylacetylene with zinc in aqueous ammonia and purified by steam distillation [ 191, Molecular structures of the investigated compounds are illustrated in Fig. 1. Samples were prepared by vacuum thermal sublimation from a resistance-heatedquartz crucible onto a substrate. The crucible-to-substrate distance was 5 cm. The crucible temperature was in the range 220-240 “C and was adjusted such that the deposition time ranged between 2.5 and 10 min. The diaryldiacetylene layer was vacuum-depos-
A.V. Vannikov
et al. /Synthetic
Metals
78 (1996)
79-83
c=c-c=c NOz D(2)
Fig. 1. Structures of the studied diaryldiacetylenes.
ited onto a Au-coated quartz substrate, which increased in temperature up to 70 “C during deposition. Then, 0.1-0.2 p,rn thick selenium and a semi-transparent Au top electrode were successively vacuum-deposited at 8 X 10m5 Torr onto the diaryldiacetylene layer. Charge carrier drift mobilities were measured by conventional time-of-flight techniques using a N2 laser pulse (h = 337 nm, pulse length 10 ns) [ 201. The maximum charge generated at the electrode surface was less than 0.1 CV, where C is the sample capacitance and V the applied voltage. The transient currents were measured with a digital C9-8 oscilloscope connected to a computer. Thicknesses of the diaryldiacetylene layers were 1.3-2.7 km, as determined by an MII-4 microinterferometer. Optical absorption spectra were recorded on a ‘Beckman DU-7’ spectrophotometer. 3. Results
The absorption spectra of diaryldiacetylene thin layers formed by vacuum deposition on quartz substrates and in chloroform solutions are presented in Fig. 2. The transient currents in the diaryldiacetylene layers caused by the migration of holes were analysed. Electron transport in the diaryldiacetylene systems was not detected. Fig. 3 (inset) shows a typical transient current signal obtained in a sandwich specimen having a plateau region followed by a short dispersion tail. This indicates that the injected charge carriers follow essentially the normal (nondispersive) mode of transport. The transit time ( tT) values were determined from the intersection of the asymptotes to the plateau and trailing edge of the signal. At a layer thickness of d and an average electric field strength F, an expression for the drift mobility ,u may be written in the form p = dl t,F. Figs. 3 and 4 show the dependence of hole mobility as a function of electric field for D( 1) layers in coordinates log pF and log P-F’.~, respectively. A comparison between the two curves indicates that in the latter case linear dependence provides a somewhat better description of the experimental data. A similar field dependence of p was observed for the other diaryldiacetylenes studied. Specific features of the dependence were noted: an increase of one order of magni-
tude in the field strength brought about the enhancement of p by 1.5 orders of magnitude in the layers of D ( 1) and D (2) (Figs. 4 and 5), while in the case of D(3) the mobility increased by the factor of 1.5-2 (Fig. 6). Typical temperature dependence curves of the drift mobility are presented in Fig. 7. It has been revealed that the temperature dependence of mobility has the form, p = exp( -En/ kT), where E, is the activation energy and k the Boltzmann constant.
4. Discussion
The absorption spectra, shown in Fig, 2, indicate that in the course of vacuum themlal deposition the diaryldiacetylenes undergo partial polymerization, This manifests itself by the broadening of the longwave monomer absorption band and/or a shift of the maximum position (I) towards low energies, indicative of an increased number of conjugated ITelectron bonds. The solid-state spectra of D(2) and D(3) show broadened absorption bands as compared to their respective spectra in solutions, indicating that a range of oligomerized diaryldiacetylenes, having different conjugation lengths, and unreactedmonomer are present in the deposited layer. The two absorption spectra of D( 1) differ with the solid-state absorption being shifted towards the longwave region by approximately 30 nm. This length of shift can be attributed to an effective doubling of the n-electron conjugation length [21-231. In D( 1) a greater degree of dimerization or partial oligomerization of the monomer molecules has occurred than observed in D( 2) and D( 3). During the thermal deposition of D( 1) the hydrogen-bonding interactions associated with the amine group created a favourable alignment for polymerization to occur. In view of the fact that polycrystalline layers are formed in the process of film formation, one would expect charge carrier mobility limitation. Since the ratio of the two mobility values in apolydiacetylenemonocrystal is pu /pL = lo3 [24] and that pII is equal to about 5 cm2 V- * s-’ [ 8,9], it may be that the value pw 10B3 cm* V- ’ s- ’ represents an upper limit to the mobility expectancy in polycrystalline films at room temperature. The measured room temperature p values
A.V. Vannikov
et al. /Synthetic
Metals
78 (1996)
81
79-83
electric field dependence of the mobility is expressed as follows: log p N F”.5
(2)
The intersection of the straight line dependence of Eq. (2) with the ordinate gives the mobility value at zero field (F = 0). The mobility activation energy at zero field can be
:t 05 Fig. 3. Field dependence of hole drift mobility in vacuum-deposited D( 1) at284K(X),292K(B)and327K(O)plottedaslog~,vs.F.Inset:a typical transient signal observed in vacuum-deposited films. Photocurrent measuredinD(1) (d=1.3mm,F=0.7X104Vcm-‘,T=284K).
K-06
m250ai350400450500 Wav.9klgth
0
4
1w
z
ml
Fig. 2. Optical absorption spectra of the diaryldiacetylenes in chloroform solutions of 2X lob3 M (dashed line) and in the solid state prepared by vacuum deposition on quartz substrate (solid line): (a) D( 1); (b) D(2);
8 200 8 (Field. V/crrp
3bo
0
Fig. 4. Field dependence of hole drift mobility in vacuum-deposited D( 1) at284K(X),292K(~)and327K(U)plottedaslog~~vs.Fo~s.
Cc) D(3).
of various diaryldiacetylene systems lie in a range from 10P4 to low6 cm2 V-’ s-‘. The charge carrier mobility values of polymers, e.g. undoped, polyconjugated some poly (phenylenevinylene) s [ 251, polyacetylenes [ 261 and phenyl-substituted polyacetylenes [ 271, also lie in the aforementioned range. It follows from Figs. 3-7 that the field and temperature dependences of the hole mobility (p) in diaryldiacetylenes may be described by the Gill empirical expression: ~=~oexp(-E,lk)X(1/T-1/To);E,=A-pF0.5
(1)
where A is the trap depth at zero field and /I is a constant. This expression is referred to as an Arrhenius type and the
0
loo
200
3Go
400
0
(Field,V/cr~$~ Fig. 5. Field dependence of pd in a vacuum-deposited film of D(2) at ( X.) 288 K, (4) 299 K and (0) 335 K.
82
A.V. Vannikov
et al. /Synthetic
Metals
78 (1996)
79-83
Table 1 Charge transport characteristics of the diaryldiacetylenes a Compound
A (eV)
p(F=O)
D(1)
0.20 0.47 0.17
2.1 x 2.2x 2.4x
D(2) D(3)
a Poole-Frenkel coefficient ,&=3.8
(cm’ V-r s-r) at T
10-6 10-6 lo-’
p (eV cm”2 V-r’*) 4.1 x 10-4 1O-4 lo-’
2.3 X 6.2x
T(K)
b (cm* V-r s-r)
To W)
292 299 298
8.6x
1O-4 9.0x 10-4 4.7 x 10-4
1125 447 533
X low4 eV cmr” V- I” for e=4. The Evalue was determined by capacitance measurements,
E-E-
-
ml
(Field V/cr~$~*~
Fig. 6. Field dependence of pd in a vacuum-deposited film of D( 3) at ( X ) 285 K, (I) 298 K and (0) 340 K.
-a
0.0018
at T
0.0024
0.0030 l/T,
0.C 16
l/K
Fig. 7. Temperature dependence of ,u~in vacuum-deposited films of D( 3) at (1) F= 1.1 X 104, (2) 7.2X lo4 and (3) 1.1 X 105cm2 V-r s-‘.
calculated by plotting E versus F and extrapolating the E= E( F) straight line dependence through the low field region to the intercept with the ordinate. The values of coefficient p are obtained from the log p versus F”.’ dependences. The data on charge transport in diaryldiacetylenes obtained in the present work are presented in Table 1. Expression ( 1) fits charge carrier behaviour in both polyconjugated macromolecular systems and saturated chain polymers doped with low-molecular transport molecules [ 28,291. There are several phenomenological approaches which consider the transport properties of polymers
[ 1,3,4,6,7], and they all lead to expression ( 1) to a greater or lesser extent. However, no model provides a general explanation for the scope of the characteristic transport variations observed in any of the polymer systems studied. In any system the p. value is a principal parameter which serves for the characterization of a system in the absence of barriers (traps) for charge carriers. In this study the experimental value of p was only comparable to the Poole-Frenkel constant in the case of D(l), showing that the Poole-Frenkel mechanism was not valid for all the diacetylenes. The data presented in Table 1 show that the mobility values at room temperature differ considerably, since they are specified by the hopping of charge carriers between transport sites which are the diacetylene monomers and different sized oligomers. The mobility is greatest for D(3) which appears to have the longest conjugated chains from interpreting the absorption profiles shown in Fig. 1 (lowest energy absorption). The h values (see Table 1) are broadly similar, which confirms that 110refers to crystalline structures which are free of the barriers to charge carrier transport. The activation energies of mobility values are similar for D( 1) and D(3) which both have electron-donor and electron-acceptor groups in their molecular structure. The activation energy of mobility is much higher for D (2) which has only electron-acceptor groups present in its molecular structure. For D( 1) in the high electric field region, the growth of hole mobility slows down with increasing applied voltage (Fig. 4). This seems likely to demonstrate the resemblance of charge carrier mobility behaviour to a recently observed effect in molecularly doped polymer systems where the ,u versus F function reaches a maximum, then decreases with increasing F [ 30,3 11. This effect is not detected in the case of monomer triarylamine transport molecules. It is evident in dimers, and more pronounced in tetramer transport molecules which play the role of active transport sites in inert polymer matrices. As follows from the above-mentioned results, it is in the predominantly dimerized diacetylene layers of D( 1) that the dependence p versus F falls off. The results of this investigation demonstrate that, in principle, the preparation by the vacuum deposition method of various diaryldiacetylenes can produce high quality thin layers, which may be suitable for use in various applications where effective hole transport is necessary, such as in the production of organicphotodiodes based on multi-layer, lowmolecular weight organic compositions.
A.V. Vannikov
et al. /Synthetic
Acknowledgements The research described in this publication was made possible in part by Grant No. 015 PP from the International Science and Technology Centre and the Russian fund for fundamental research (Project 93-03-09704a).
References [l] P.M. Borsenberger and D.S. Weiss, Organic Photoreceptors for Imaging Systems, Marcel Dekker, New York, 1993. [2] D.M. Pai and B.E. Springett, Rev. Mod. Phys., 65 (1993) 163. [3] L.B. Schein, Philos. Msg. B, 65 (1992) 795. [4] H. Bassler, Phys. StatusSolidi (b), 75 (1993) 15. [5] L.B. Schein and J.X. Mack, Chem. Phys. Lett., 149 (1988) 109. [6] R.A. Marcus, Annrt. Rev. Phys. Chem., 15 (1964) 155. [7] S.V. Novik0vandA.V. Vannikov, Chem. Phys. Lett., 182 (1991) 598. [8] B. Reimer and H. Bassler, Phys. Status Solidi (b), 85 (1978) 145. [9] D. Moses, M. Sinclair, S. Phillips and A.J. Heeger, Synth. Met., 28 (1989) D675. 1101 G. Wegner, J. Polym. Sci., Polym. Lett., 9 (1971) 133. : 111 G. Wegner, Pure Appl. Chem., 49 (1977) 443. [12] M.A. Schen, Proc. SPIE, 971 (1988) 178. [ 131 J. Tsibouklis, C.N. Campbell, A.R. Weminck, A.J. Shand and G.H.W. Milbum, Polym. Bull., 29 (1992) 661. 1141 C.N. Campbell, A.R. Weminck, A.J. Shand and G.H.W. Milbum, Liq. Cryst., 8 (1992) 5623.
Metals
78 (1996)
79-83
83
[15] J. Wright, A.R. Weminck, A.J. Shand, G.H.W. Milburn, H.J. Coles and D.A. McL. Smith, Mol. Cryst. Liq. Cryst., 258 (1995) 1. [16] J. Wright, A.R. Weminck, A.J. Shand and G.H.W. Milburn, Int. J. Nonlinear Opt. Phys., 3 (1994) N4-447. [17] B. Grant, R.J. Cox and N.J. Clecak, Mol. Cryst. Liq. Cryst., 51 (1979) 209. [I81 W.B. Austin,N. Bilow, W.J. KellaghanandK.S.Y.Lau,J. Org. Chem., 46 (1981) 2280. [ 191 J. Tsibouklis, Ph.D. Thesis, Napier University, 1988. 1201 P.M. Borsenberger, L. Pautmeir and H. Bassler, J. Chem. Phys., 94 (1991) 5447. 1211 D. Bloor, D.J. Ando, F.H. Preston and G.C. Stevens, Chem. Phys., 24 (1974) 407. [22] R.H. Baughman and R.R. Chance, J. Polym. Sci., 14 (1976) 2037. [23] G.N. Patel, R.R. Chance and J.D. Witt, J. Chem. Phys., 70 (1979) 4387. [24] K. Lochner, B. Reimer and H. Bassler, Chem. Phys. Lett., 41 (1976) 388. [25] A. Yu. Kryukov, A. Ch. Saidov and A.V. Vannikov, Thin Solid Films, 209 (1992) 84. [26] P.D. Townsend and R.H. Friend, Phys. Rev. B, 40 (1989) 3112. [27] Sh. Zhou, H. Hong, Y. He, D. Yang, T. Masuda and T. Higashimura, Polymer, 33 (1992) 2189. [28] M. Stolka and M.A. Abkowitz, Synth. Met., 41-43 (1991) 3385. [29] M. Van der Auweraer, F.C. De Schryver, P. Borsenberger and J.J. Fitzgerald, J. Phys. Chem., 97 (1993) 8808. [30] G. Verbeek, M. Van Der Auweraer, F.C. De Schryver, C. Geelen, D. Terre11 and S. De Meuter, Chem. Phys. Lett., 188 (1992) 85. [31] R.H. Young and N.G. Rule, Phys. Rev. Lett., 72 (1994) 388.
Synthetic Metals 78 (1996) 79-83
A study of hole mobility in diaryldiacetylenes A.V. Vannikov a,‘A.R. Tameev a, E.I. Mal’tsev a, G.H.W. Milburn b, A.J. Shand b, A.R. Weminck b, J. Wright b a Frumkin b Department
Iilsiitute of Electrochemistry, Russian Acade.my of Sciences, Leninski Prospect 31, Moscow I 17071, Russia of Applied Chemical and Physical Sciences, Napier University, 10 Colinton Road, Edinburgh EHIO .5DT, UK
Received 9 May 1995; revised 5 September 1995; accepted 13 October 1995
Abstract Thin homogeneous films exhibiting effectiveholetransportwith drift mobility valuesup to low4 cm2V-’ s-l areformedin the process of vacuumthermaldepositionof monomericdiaryldiacetylenes. Basictransportcharacteristics andtheir dependence on the electricfield, temperature andchemicalcompositionof the monomers aredetermined. Keywor&:
Diaryldiacetylene; Mobility
1. Introduction The extensive use of molecularly doped polymer layers as xerographic photoreceptors has led to a number of publications covering experimental and theoretical work on the subject of electron and hole transport in disordered molecular solids [ l-71. In thesesystems it is essential to distinguish whether conduction is controlled by the process of ionization of an individual transport molecule followed by inuamolecular structural relaxation (polaron formation) or by polymer/ dopant interactions. Polydiacetylene (PDA) single crystals are the only polymer single crystals that have a polyconjugated backbone structure and consequently exhibit uniquely high charge carrier mobilities for organic compounds (about 5 cm* V- ’ s- ‘) [ 8,9]. The structural and geometrical criteria that must be satisfiedfor the topochemical polymerization of diacetylene
single crystals [ 10,l l] are very restrictive, limiting the structure and number of polymer single crystals that can be obtained. Monomeric diaryldiacetylenes are low-molecular weight analogues of PDAs and are therefore of interest for fundamental studieson charge transport mobilities. Having aromatic side groups adjacent to the diacetylene unit createsa
molecule that is usually unable to undergo topochemical polymerization, but can often be thermally polymerized from the isotropic or liquid crystalline phases [ 12,131. Many diaryldiacetylenes have been prepared at Napier University and some have very good nonlinear optical properties (e.g. compound D(2) in this paper has a ,$‘) coeffi0379-6779/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved ccnrn170 L770,0~\~1C71 0
cient of 2.02+0.50 pm V-’ at 15% concentration in poly( methyl methacrylate) ) [ 14-161. In the present paper hole transport in diaryldiacetylenes of different structures is considered.
2. Experimental All the intermediates and diacetylenes were purified and satisfactorily characterized using standard analytical techniques. The diacetylenes were prepared using the CadiotChodkiewcz reaction between equimolar amounts of bromo and terminal acetylenes in the presence of catalytic amounts of copper(I) chloride. The bromoacetylenes were synthe,sized by chain extension of the corresponding aldehydes [ 171. The intermediate dibromoalkenes were converted into the bromoacetylenes by reflux with equimolar amounts of potassium tertiary-butoxide in toluene. Nitrophenylacetylene was prepared by reacting nitrobromobenzene with trimethylsilylacetylene in the presence of a palladium catalyst [ 181. Aminophenylacetylene wasproduced by reducing nitrophenylacetylene with zinc in aqueous ammonia and purified by steam distillation [ 191, Molecular structures of the investigated compounds are illustrated in Fig. 1. Samples were prepared by vacuum thermal sublimation from a resistance-heatedquartz crucible onto a substrate. The crucible-to-substrate distance was 5 cm. The crucible temperature was in the range 220-240 “C and was adjusted such that the deposition time ranged between 2.5 and 10 min. The diaryldiacetylene layer was vacuum-depos-
A.V. Vannikov
et al. /Synthetic
Metals
78 (1996)
79-83
c=c-c=c NOz D(2)
Fig. 1. Structures of the studied diaryldiacetylenes.
ited onto a Au-coated quartz substrate, which increased in temperature up to 70 “C during deposition. Then, 0.1-0.2 p,rn thick selenium and a semi-transparent Au top electrode were successively vacuum-deposited at 8 X 10m5 Torr onto the diaryldiacetylene layer. Charge carrier drift mobilities were measured by conventional time-of-flight techniques using a N2 laser pulse (h = 337 nm, pulse length 10 ns) [ 201. The maximum charge generated at the electrode surface was less than 0.1 CV, where C is the sample capacitance and V the applied voltage. The transient currents were measured with a digital C9-8 oscilloscope connected to a computer. Thicknesses of the diaryldiacetylene layers were 1.3-2.7 km, as determined by an MII-4 microinterferometer. Optical absorption spectra were recorded on a ‘Beckman DU-7’ spectrophotometer. 3. Results
The absorption spectra of diaryldiacetylene thin layers formed by vacuum deposition on quartz substrates and in chloroform solutions are presented in Fig. 2. The transient currents in the diaryldiacetylene layers caused by the migration of holes were analysed. Electron transport in the diaryldiacetylene systems was not detected. Fig. 3 (inset) shows a typical transient current signal obtained in a sandwich specimen having a plateau region followed by a short dispersion tail. This indicates that the injected charge carriers follow essentially the normal (nondispersive) mode of transport. The transit time ( tT) values were determined from the intersection of the asymptotes to the plateau and trailing edge of the signal. At a layer thickness of d and an average electric field strength F, an expression for the drift mobility ,u may be written in the form p = dl t,F. Figs. 3 and 4 show the dependence of hole mobility as a function of electric field for D( 1) layers in coordinates log pF and log P-F’.~, respectively. A comparison between the two curves indicates that in the latter case linear dependence provides a somewhat better description of the experimental data. A similar field dependence of p was observed for the other diaryldiacetylenes studied. Specific features of the dependence were noted: an increase of one order of magni-
tude in the field strength brought about the enhancement of p by 1.5 orders of magnitude in the layers of D ( 1) and D (2) (Figs. 4 and 5), while in the case of D(3) the mobility increased by the factor of 1.5-2 (Fig. 6). Typical temperature dependence curves of the drift mobility are presented in Fig. 7. It has been revealed that the temperature dependence of mobility has the form, p = exp( -En/ kT), where E, is the activation energy and k the Boltzmann constant.
4. Discussion
The absorption spectra, shown in Fig, 2, indicate that in the course of vacuum themlal deposition the diaryldiacetylenes undergo partial polymerization, This manifests itself by the broadening of the longwave monomer absorption band and/or a shift of the maximum position (I) towards low energies, indicative of an increased number of conjugated ITelectron bonds. The solid-state spectra of D(2) and D(3) show broadened absorption bands as compared to their respective spectra in solutions, indicating that a range of oligomerized diaryldiacetylenes, having different conjugation lengths, and unreactedmonomer are present in the deposited layer. The two absorption spectra of D( 1) differ with the solid-state absorption being shifted towards the longwave region by approximately 30 nm. This length of shift can be attributed to an effective doubling of the n-electron conjugation length [21-231. In D( 1) a greater degree of dimerization or partial oligomerization of the monomer molecules has occurred than observed in D( 2) and D( 3). During the thermal deposition of D( 1) the hydrogen-bonding interactions associated with the amine group created a favourable alignment for polymerization to occur. In view of the fact that polycrystalline layers are formed in the process of film formation, one would expect charge carrier mobility limitation. Since the ratio of the two mobility values in apolydiacetylenemonocrystal is pu /pL = lo3 [24] and that pII is equal to about 5 cm2 V- * s-’ [ 8,9], it may be that the value pw 10B3 cm* V- ’ s- ’ represents an upper limit to the mobility expectancy in polycrystalline films at room temperature. The measured room temperature p values
A.V. Vannikov
et al. /Synthetic
Metals
78 (1996)
81
79-83
electric field dependence of the mobility is expressed as follows: log p N F”.5
(2)
The intersection of the straight line dependence of Eq. (2) with the ordinate gives the mobility value at zero field (F = 0). The mobility activation energy at zero field can be
:t 05 Fig. 3. Field dependence of hole drift mobility in vacuum-deposited D( 1) at284K(X),292K(B)and327K(O)plottedaslog~,vs.F.Inset:a typical transient signal observed in vacuum-deposited films. Photocurrent measuredinD(1) (d=1.3mm,F=0.7X104Vcm-‘,T=284K).
K-06
m250ai350400450500 Wav.9klgth
0
4
1w
z
ml
Fig. 2. Optical absorption spectra of the diaryldiacetylenes in chloroform solutions of 2X lob3 M (dashed line) and in the solid state prepared by vacuum deposition on quartz substrate (solid line): (a) D( 1); (b) D(2);
8 200 8 (Field. V/crrp
3bo
0
Fig. 4. Field dependence of hole drift mobility in vacuum-deposited D( 1) at284K(X),292K(~)and327K(U)plottedaslog~~vs.Fo~s.
Cc) D(3).
of various diaryldiacetylene systems lie in a range from 10P4 to low6 cm2 V-’ s-‘. The charge carrier mobility values of polymers, e.g. undoped, polyconjugated some poly (phenylenevinylene) s [ 251, polyacetylenes [ 261 and phenyl-substituted polyacetylenes [ 271, also lie in the aforementioned range. It follows from Figs. 3-7 that the field and temperature dependences of the hole mobility (p) in diaryldiacetylenes may be described by the Gill empirical expression: ~=~oexp(-E,lk)X(1/T-1/To);E,=A-pF0.5
(1)
where A is the trap depth at zero field and /I is a constant. This expression is referred to as an Arrhenius type and the
0
loo
200
3Go
400
0
(Field,V/cr~$~ Fig. 5. Field dependence of pd in a vacuum-deposited film of D(2) at ( X.) 288 K, (4) 299 K and (0) 335 K.
82
A.V. Vannikov
et al. /Synthetic
Metals
78 (1996)
79-83
Table 1 Charge transport characteristics of the diaryldiacetylenes a Compound
A (eV)
p(F=O)
D(1)
0.20 0.47 0.17
2.1 x 2.2x 2.4x
D(2) D(3)
a Poole-Frenkel coefficient ,&=3.8
(cm’ V-r s-r) at T
10-6 10-6 lo-’
p (eV cm”2 V-r’*) 4.1 x 10-4 1O-4 lo-’
2.3 X 6.2x
T(K)
b (cm* V-r s-r)
To W)
292 299 298
8.6x
1O-4 9.0x 10-4 4.7 x 10-4
1125 447 533
X low4 eV cmr” V- I” for e=4. The Evalue was determined by capacitance measurements,
E-E-
-
ml
(Field V/cr~$~*~
Fig. 6. Field dependence of pd in a vacuum-deposited film of D( 3) at ( X ) 285 K, (I) 298 K and (0) 340 K.
-a
0.0018
at T
0.0024
0.0030 l/T,
0.C 16
l/K
Fig. 7. Temperature dependence of ,u~in vacuum-deposited films of D( 3) at (1) F= 1.1 X 104, (2) 7.2X lo4 and (3) 1.1 X 105cm2 V-r s-‘.
calculated by plotting E versus F and extrapolating the E= E( F) straight line dependence through the low field region to the intercept with the ordinate. The values of coefficient p are obtained from the log p versus F”.’ dependences. The data on charge transport in diaryldiacetylenes obtained in the present work are presented in Table 1. Expression ( 1) fits charge carrier behaviour in both polyconjugated macromolecular systems and saturated chain polymers doped with low-molecular transport molecules [ 28,291. There are several phenomenological approaches which consider the transport properties of polymers
[ 1,3,4,6,7], and they all lead to expression ( 1) to a greater or lesser extent. However, no model provides a general explanation for the scope of the characteristic transport variations observed in any of the polymer systems studied. In any system the p. value is a principal parameter which serves for the characterization of a system in the absence of barriers (traps) for charge carriers. In this study the experimental value of p was only comparable to the Poole-Frenkel constant in the case of D(l), showing that the Poole-Frenkel mechanism was not valid for all the diacetylenes. The data presented in Table 1 show that the mobility values at room temperature differ considerably, since they are specified by the hopping of charge carriers between transport sites which are the diacetylene monomers and different sized oligomers. The mobility is greatest for D(3) which appears to have the longest conjugated chains from interpreting the absorption profiles shown in Fig. 1 (lowest energy absorption). The h values (see Table 1) are broadly similar, which confirms that 110refers to crystalline structures which are free of the barriers to charge carrier transport. The activation energies of mobility values are similar for D( 1) and D(3) which both have electron-donor and electron-acceptor groups in their molecular structure. The activation energy of mobility is much higher for D (2) which has only electron-acceptor groups present in its molecular structure. For D( 1) in the high electric field region, the growth of hole mobility slows down with increasing applied voltage (Fig. 4). This seems likely to demonstrate the resemblance of charge carrier mobility behaviour to a recently observed effect in molecularly doped polymer systems where the ,u versus F function reaches a maximum, then decreases with increasing F [ 30,3 11. This effect is not detected in the case of monomer triarylamine transport molecules. It is evident in dimers, and more pronounced in tetramer transport molecules which play the role of active transport sites in inert polymer matrices. As follows from the above-mentioned results, it is in the predominantly dimerized diacetylene layers of D( 1) that the dependence p versus F falls off. The results of this investigation demonstrate that, in principle, the preparation by the vacuum deposition method of various diaryldiacetylenes can produce high quality thin layers, which may be suitable for use in various applications where effective hole transport is necessary, such as in the production of organicphotodiodes based on multi-layer, lowmolecular weight organic compositions.
A.V. Vannikov
et al. /Synthetic
Acknowledgements The research described in this publication was made possible in part by Grant No. 015 PP from the International Science and Technology Centre and the Russian fund for fundamental research (Project 93-03-09704a).
References [l] P.M. Borsenberger and D.S. Weiss, Organic Photoreceptors for Imaging Systems, Marcel Dekker, New York, 1993. [2] D.M. Pai and B.E. Springett, Rev. Mod. Phys., 65 (1993) 163. [3] L.B. Schein, Philos. Msg. B, 65 (1992) 795. [4] H. Bassler, Phys. StatusSolidi (b), 75 (1993) 15. [5] L.B. Schein and J.X. Mack, Chem. Phys. Lett., 149 (1988) 109. [6] R.A. Marcus, Annrt. Rev. Phys. Chem., 15 (1964) 155. [7] S.V. Novik0vandA.V. Vannikov, Chem. Phys. Lett., 182 (1991) 598. [8] B. Reimer and H. Bassler, Phys. Status Solidi (b), 85 (1978) 145. [9] D. Moses, M. Sinclair, S. Phillips and A.J. Heeger, Synth. Met., 28 (1989) D675. 1101 G. Wegner, J. Polym. Sci., Polym. Lett., 9 (1971) 133. : 111 G. Wegner, Pure Appl. Chem., 49 (1977) 443. [12] M.A. Schen, Proc. SPIE, 971 (1988) 178. [ 131 J. Tsibouklis, C.N. Campbell, A.R. Weminck, A.J. Shand and G.H.W. Milbum, Polym. Bull., 29 (1992) 661. 1141 C.N. Campbell, A.R. Weminck, A.J. Shand and G.H.W. Milbum, Liq. Cryst., 8 (1992) 5623.
Metals
78 (1996)
79-83
83
[15] J. Wright, A.R. Weminck, A.J. Shand, G.H.W. Milburn, H.J. Coles and D.A. McL. Smith, Mol. Cryst. Liq. Cryst., 258 (1995) 1. [16] J. Wright, A.R. Weminck, A.J. Shand and G.H.W. Milburn, Int. J. Nonlinear Opt. Phys., 3 (1994) N4-447. [17] B. Grant, R.J. Cox and N.J. Clecak, Mol. Cryst. Liq. Cryst., 51 (1979) 209. [I81 W.B. Austin,N. Bilow, W.J. KellaghanandK.S.Y.Lau,J. Org. Chem., 46 (1981) 2280. [ 191 J. Tsibouklis, Ph.D. Thesis, Napier University, 1988. 1201 P.M. Borsenberger, L. Pautmeir and H. Bassler, J. Chem. Phys., 94 (1991) 5447. 1211 D. Bloor, D.J. Ando, F.H. Preston and G.C. Stevens, Chem. Phys., 24 (1974) 407. [22] R.H. Baughman and R.R. Chance, J. Polym. Sci., 14 (1976) 2037. [23] G.N. Patel, R.R. Chance and J.D. Witt, J. Chem. Phys., 70 (1979) 4387. [24] K. Lochner, B. Reimer and H. Bassler, Chem. Phys. Lett., 41 (1976) 388. [25] A. Yu. Kryukov, A. Ch. Saidov and A.V. Vannikov, Thin Solid Films, 209 (1992) 84. [26] P.D. Townsend and R.H. Friend, Phys. Rev. B, 40 (1989) 3112. [27] Sh. Zhou, H. Hong, Y. He, D. Yang, T. Masuda and T. Higashimura, Polymer, 33 (1992) 2189. [28] M. Stolka and M.A. Abkowitz, Synth. Met., 41-43 (1991) 3385. [29] M. Van der Auweraer, F.C. De Schryver, P. Borsenberger and J.J. Fitzgerald, J. Phys. Chem., 97 (1993) 8808. [30] G. Verbeek, M. Van Der Auweraer, F.C. De Schryver, C. Geelen, D. Terre11 and S. De Meuter, Chem. Phys. Lett., 188 (1992) 85. [31] R.H. Young and N.G. Rule, Phys. Rev. Lett., 72 (1994) 388.