Dark and photoconductive properties of hydroxymethylferrocene

ELSEVIER

Synthetic Metals97(1998)63-68

Dark and photoconductive properties of hydroxymethylferrocene Aloke Kumar Chakraborty ‘, Raghu Nath Bera, Ashis Bhattacharjee 2, Biswanath Mallik * Deparmem

of Spectroscopy,

Indim

Association

for the Cultivation

of Science, Jadaqm-,

Calcutta

700 032, hdia

Received 29September 1997;received in revised form8June1998;accepted 1July1998

Abstract The dark electricalconductionpropertiesof hydroxymethyiferrocene havebeenstudiedasa functionof voltageat different sampIecell temperatures by usingsandwich-cell configuration.Theanalysisof theresultshasbeenbasedon thespacechargelimitedconduction(SCLC) theory.The distributionof trapsin this materialhasbeenfoundto be singlediscretelevel type.Photoconductivityin thismaterialhasbeen studiedasa functionof the intensityof exciting light source(mercurylamp; 125W) andsamplecell temperature.The resultshavebeen compared with theresultsof ferroceneand,in somecases, with otherferrocenederivatives.Anunusualtemperature-dependenceofphotocurrent hasbeenobserved.The possibleoriginof thisobservationhasbeendiscussed. 0 1998ElsevierScienceS.A. All rightsreserved. Keyvords:

Organometallic semiconductors; Photoconductivity; Darkconductivity; Trapdistribution; Intensitydependence

1. Introduction Ferrocene [ (CSHS)2Fe;abbreviatedasFcH] andits derivatives have drawn much attention in the contemporary organomerallic researchdue to their important physical [ 1] and electrochemical properties [ 21 aswe11as technological usefulness[31. From the recent studiesin our laboratory, an adsorption-inducedreversible structural phasetransition has beenpredicted in ferrocene andsomeof its derivatives [ 4,5]. The dark electrical transport properties, including the nature of trap distribution and the effects of adsorptionof vapors on the conductivity of ferrocene, have been found to be highly influenced by the substitution of different functional groups such as-( COOH), -(COOH)2, -( CHO), -( COCH,) and -( COC,H5) into the ferrocene ring [ 5-91. The nature of distribution of traps in these materialswas determined from the dark current-voltage (Z-V) characteristics. In FcH, Fc (COOH) and Fc(COOH) 2the natureof the distribution of traps was exponential type, whereas in the other ferrocene derivatives mentioned above the distribution of traps was singlediscretelevel type. Ferroceneis known to be a photoconductor [ 9, lo]. In the caseof materialshaving exponential distribution of traps the intensity dependenceof * Corresponding author.Tel.: + 91334734971;fax: +91 334732505; e-mail: spbm@,iacs.emet.in ’ Present address: Physics Department,

Eden Govt. GirIs’ CoUege, Dhaka,

Bangladesh. ” Present

address:

Department

of Physics,

St. Joseph’s

Point,Darjecling 734104, India. 0379~6779/98/$ - see from matter 8 P1150379-6779(98)00114-3

1998 Elsevier

College,

North

photocurrent provides valuable information on the nature of the distribution of traps and, hence, gives an opportunity to compare the results obtained from the dark conductivity measurements[ 91. Some results on such photoconductive studiesof FcH, Fc (COOH) and Fc(COOH) 2 have recently beenreported [ 91, where the sublinearlight intensity dependence of photocurrent in the caseof FcH has confirmed the existence of traps distributed exponentially in this material (indicated by dark Z-V measurement). In contrast, in the casesof Fc( COOH) and Fc( COOH),, the dependenceof photocurrent on light intensity hasbeenobservedto be superlinear, which implies that the substitutionof-( COOH) group in the FcH unit changes significantly some photoinduced electrical transport propertiesin the material incorporatedby the changesin the molecular structure. In all the ferrocene derivatives studied so far, the carbon atom present in the substitution groups was attached to the oxygen atom by a double bond. It was thought worthwhile to extend similar studies on conductivity (dark and photo) to a ferrocene derivative having no suchcarbon-oxygen bonding in the substitutiongroup. For this purposea -( CH,OH) substitutedferrocene derivative, hydroxymethylferrocene, (abbreviated as Fc( CH,OH) ) , wasselectedin order to explore how various structuredependenttransport parametersare affected by substitution of the -(CH,OH) group into the cyclopentadienyl ring of ferrocene. Some interesting and unusual results obtained from such experiments are described in the present paper. The earlier reported results [ 6,7,9] on dark and photocon-

Science S.A. All rights reserved.

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et al. /Synthetic

ductivity measurements of FcH, Fc(COOH) and Fc( COOH) 2 have been considered for comparison with the results obtained for Fc( CH,OH) .

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cell of the material were performed with forward as well as reverse bias. The values of current measured with both forward and reverse bias were almost equal which indicated that the junctions were ohmic.

2. Experimental 3. Results and discussion High purity microcrystalline hydroxymethylferrocene (in powder form) was obtained from Strem Chemical Co. Inc., Newburyport, MA, USA, and was used after further purification by recrystallization. The experiment was performed by the conventional sandwich cell technique [ 4,9,11]. The pure dry powdery material ( 10 mg) was placed between a conducting glass (iridium-tin oxide (ITO) coated) and a stainless-steel electrode, and kept inside a brass chamber, fashioned with Teflon and having a quartz window at the top for photoconductivity studies. Teflon spacers,2 mil(O.00508 cm) thick, maintained the separation between the electrodes. To maintain the sandwich cell, two spring clips were attached at a moderate pressure (about 0.035 MPa) at the two sides of the electrodes. The sample cells were prepared in air and in safe light illumination. All the measurements were made in controlled dry nitrogen atmosphere. Before the conductivity measurements, desorption of water vapor or any preadsorbed (if adsorbed during sample cell preparation) vapors from the sample cell was ensured by repeated heating and cooling treatments of the cell, initially in vacuum and finally in dry nitrogen gas atmosphere. A d.c. bias potential 27 V across the electrodes was applied from the voltage source of a programmable electrometer (model 6 17, Keithley Inst. Inc., Cleveland, OH, USA) in all the measurements, except the measurement on Z-V characteristics. For the measurement of I-Vcharacteristics, voltage up to 100 V was applied from the voltage source of the above-mentioned electrometer and, for higher voltages, a d.c. power supply (model PS 2500) of Hoefer Scientific Inst., San Francisco, USA, was used. The current was recorded by the above-mentioned electrometer. A proportional temperature controller (model RTE 110, Neslab Inst. Inc., NH, USA) was used to control the temperature of the sample cell. The temperature was measured by using a copper-constantan thermocouple attached at the top of the metal electrode and a microprocessor-controlled digital millivoltmeter. The details of the experimental set-up can be found elsewhere [ 4,9,11]. A mercury lamp ( 125 W) was used as the polychromatic light source for the photoconductivity measurements. The approximate integral photon flux reaching the sample cell, without using filters in the light path, was about 3.0 x lOI per cm2 and this intensity of light was considered as 100%. Various neutral density filters were interposed in the light beam path to control the light intensity in the sequence of 27,36,49 and 64%. The steady state values of dark as well as photocurrent were only noted. In this experiment the conductivity cell consisted of two junctions of stainless-steel electrode/metallocenes and metallocenes/ conducting glass. To check whether the junctions were rectifying or ohmic, the current measurements in a sandwich

3.1. Dark conductivity The steady-state dark current flowing in an insulatingmaterial in the ohmic region arises due to the drift of the thermal charge carriers present in the material. At sufficiently higher voltages, the dark conduction is dominated by the injected charge carriers from the electrodes through the ohmic contacts and gives rise to space charge limited current (SCLC) . We have measured the dark electrical current (1,) as a function of applied voltage (V) at different values of fixed temperature (r). As the pressure on the sandwich cell was constant (clipping pressure = 0.035 MPa) , the change in& was solely due to change in either the applied voltage or the temperature. The ohmic dark current, I,, is given by [9] Z,(ohmic)=n,qp(A/ti)V

(1) and for the simple case of a single discrete trapping level, space charge limited current (SCLC) can be written as r9,w31

Zd(SCL)=(9/8)r0s~L,A(V*/~‘)

(2)

where no is the thermally liberated free carrier density; 4 the electronic charge; A the area of the sample cell; d the interelectrode spacing; so the free space permittivity; E the dielectric constant; pu,=@ the effective drift mobility; p the microscopic mobility; f3the ratio of free to trapped charge carriers. The crossover from the ohmic to SCLC region takes place at a voltage V, given by [9,12,13] vt=(s/9>(qd2n&O&e)

(3)

To find out the nature of distribution of traps in Fc(CH,OH) , the Z-V characterisiics of this material were studied at different sample cell temperatures and the logarithmic plots of Z, against bias voltage (V) at different cell temperatures obtained are shown in Fig. 1I The currents at lower voltages are ohmic in nature, whereas at higher voltages the currents are proportional to the square (approximately) of the applied voltages at all the cell temperatures. This indicates that the trap distribution in Fc( CH,OH) is of the single discrete level type. The transition voltage (V,) from ohmic to the SCLC region has been found to decrease with increasing cell temperature as shown in Fig. 2. This decrease in V, may be due to the corresponding decrease in the (no/S) factor with the increase in cell temperature as suggested by Eq. (3). Comparing the results obtained from the dark Z-V measurements of Fc( CH,OH) with those of FcH, Fc(COOH) and

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65

It has been reported [ 121 that measurements of activation energies in the ohmic and SCLC regions provide an effective technique for locating localized levels in the case of materials having wide bandgaps, where the activation energy for electrical conduction involves some combination of energy required to raise carriers from their dominant levels to their corresponding transport band and of the energy required to create carriers in the dominant levels. When enough carriers are injected, the latter requirements may be suppressed. The dominant levels were evaluated using the following expressions [ 6,121 as given below: no=Nc(Nv/Nm)

exp[ -(EC-Em)

+ 1/2(E,-E,)]/kT

(4)

and 8=(N,/N,)

Fig. 1. Logarithmic plots of dark current against voltage for Fc(CH,OH) at different cell temperatures: (a), (b), (c), (d) and (e) represent 297, 303, 311, 316 and 323 K, respectively.

i

exp[-(E,-E,)]lkT

(5)

where EC-Em is the activation energy in the SCLC region ( Escr) and [ (EC-Em) + l/2( E, - E,) ] is that in the ohmic region (Eo) . We have evaluated the activation energy values in both the ohmic ( Eo) and SCLC regions (Esc-) for Fc( CH,OH) from the linear plots of log Id versus 1/T (Fig. 3) as 1.84 and 1.69 eV, respectively, which means that Fc (CH,OH) is a nonextrinsic material (i.e. E. and EsCL are not equal). The thermal activation energy value (E,) is related to the structure of the material. Generally, this value decreases with increasing number of T electrons or the degree of unsaturation associated with the molecular structure [ 141. But ferrocene and its derivatives studied do not show such a relation, The ferrocene derivatives having -( COOH) , -( COOH)2 and -(CH,OH) groups substituted in the ferrocene ring show activation energy values closer to that of FcH (E. values for FcH, Fc(COOH) and Fc(COOH), are 1.31, 1.45 and 1.62 eV, respectively; EscL for FcH, Fc (COOH) andFc( COOH) 2 are 1.31,1.26 and 1.62 eV, respectively [ 6,7,9] ). It has been

10: i.00 Fig. 2. Plot of logarithm

3.20 3.30 3.40 lOOO,'T(l/K) of V, vs. reciprocal of temperature

3550 for Fc(CH,OH)

Fc(COOH) Z [ 6,7,9], it is clear that the nature of trap distribution in ferrocene remains the same as the exponential distribution type after substitution of -( COOH) and -( COOH) 2 groups, whereas substitution of the -(CH,OH) group changes the trap distribution to the single discrete level type similar to the case of substitution of -( CHO) , -( COCH3) and -(COC,H,) groups. However, the transition voltage, V,, decreases with increasing temperature in the case of Fc( CH,OH) which is similar to that of FcH, Fc( COOH) and Fc (COOH) Z [ 6,7,9]. It is evident from Eq. (3) that V, has a thermal activation energy (E,,) equal to the difference of the activation energies in the ohmic (Eo) and SCLC (Es& regions. However, in the case of Fc (CH,OH) the calculated value of Evt ( = 0.58 eV) is higher than the value ofE, - EsCL ( =0.15 eV).

3.05

3.15

3.25 3.35 3.45 lOOO/T(l/K) Fig. 3. Plots of logarithm of dark current vs. reciprocal of temperature Fc(CH,OH) at two different bias voltages: (a) 27 V, (b) 500 V.

for

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reported [ 61 that the values of EO for Fc (CHO) , Fc( COCH,) and Fc(COC,H,) are 3.75, 3.7 and 2.85 eV, respectively, with the corresponding EscL for the materials being 2.36,3.2 and 2.55 eV. All the mono-group substituted FcH derivative studies are non-extrinsic in nature. From the results it is difficult to correlate the change in various transport parameters with the substitution groups in the ferrocene unit. In substituted ferrocene materials the effect of substitution of any group in one of the Cp rings is transmitted either to the central iron atom or to the other Cp ring by the induction effect [ 151. However, this substituent exerts greater effect upon the ring on which it is substituted. Thus, the observed behavior of the derived activation energy value in the studiedferrocenematerials may originate from the modifications in the n-electronic structure by the different substituent groups as a result of substitution in the cyclopentadienyl ring.

97 (1998) 63-68

6 5 i:

;' 10'

2

3

4

5

6

7

I 8 9 ,02

Intensity(%)

3.2. Photoconductivi~ 3.2.1. Dependence ofphotoconductivity exciting light source

on the intensity of

The photoconductivity is generally a function of the incident light intensity and measurement of the intensity dependence of photocurrent yields important information on the cooperation of traps and recombination centers. For the organic photoconductors as well as inorganic systems, the light intensity ZBand the photocurrent ZPhare generally related by the relation [ 16-181: IphaG (6) where the exponent y is the characteristic of the photoconducting system and it depends on the amount of recombination of the photogenerated charge carriers during their transport to the electrodes. A plot of log ZPhversus log 1, yields a straight line where the slope of this linear plot yields the value of 7. The intensity dependence of photocurrent of Fc( CH?OH) was studied at different sample cell temperatures and the results of such a study are shown in Fig. 4. The values of y estimated from the intensity dependence of ZPhat different cell temperatures are shown in Fig. 5. Similar studies were carried out for FcH [ 91 and the calculated y values at different cell temperatures for FcH are also shown in Fig. 5 for comparison. It is clear from this figure that, within the working range of temperature, the y values for FcH are smaller than unity, whereas for Fc( CH,OH) y values are around unity. The sublinear intensity dependence of photocurrent (0.5 < y < 1) as observed in the case of FcH is the result of cooperation of traps and recombination centers. The superlinear dependence of photocurrents on the light intensity with y> 1 as observed in the case of Fc( CH,OH) at certain temperatures can be due to the influence of several defect states with different electronic behaviors (hence, with different charge carrier recombination properties). These higher values of y may be caused by the presence of a set of traps, which are unoccupied in the dark but become increasingly filled with increasing light

Fig. 4. Logarithmic plots of photocurrent against intensity of exciting li$t source (mercury lamp; 125 W) for Fc(CH,OH) at different cell temperatures: 284 (A), 290 (B), 294 (C), 298 (D), 306 (E), 308 (F), 310 (G), 313 (H) and 317 K (I).

1.75

_EO

-

-1.25

- *-_

-l”“,

-i 0.75

1.25 t

Fig. 5. Plots of y vs. reciprocal (A) andFc(CH,OH) (B).

of temperature

for different

materials:

FcH

intensity. Fig. 5 indicates that, for FcH and Fc (CH,OH), tIie y values exhibit fluctuations with cell temperatures. The observed fluctuations in y values are interesting and quite unusual. 3.2.2. Temperaturedependenceof photoconducfivity

In the case of organic/organometallic materials the temperature dependence of the steady-state photocurrent (I,,) generally follows the expression [ 91: E

Zph=ZO’ exp - 2; i 1 where IO’ is the preexponential factor and Ephis the activation energy for the photoconduction. The slope of the linear plots

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et al. /Synthetic

of log Iph versus 1IT gives the value of Eph. For the photoexcitation at a particular light intensity, the values of Zph at different temperatures were taken from the experiment on the dependence of photocurrent upon the light intensity (Fig. 4) and the values of log Zphwere plotted against 1/T. The log Zph versus l/T plots at each intensity of exciting light for Fc(CH,OH) could not be fitted to a single straight line for the whole temperature region of study (Fig. 6) due to fluctuations of data points. Using a fresh sample cell, the experiment was performed, with a fixed intensity of light ( lOO%), for the measurement of photocurrents at different cell temperatures. In this case also the plot of log I,,,, versus 1/T did not show a single straight line in the entire temperature range studied and anomalous increase in current was observed for the value of cell temperature around 303 K (i.e. lO’/Tm3.3 K- ’ ) . It may be recalled that the plot of log Z, versus 1 /Tfor Fc(CH,OH) has shown a single linear plot (Fig. 3) in the same temperature region. The plot of log Id versus 1/T, considering the equilibrium value of Id after release of photoexcitation, was also observed to be linear (as Fig. 3) in the same temperature range, which indicated that the photoinduced changes are almost reversible. The values of Zphfor Fc ( CHzOH) have been observed to increase with increasing bias voltage. The evaluated slopes of the plots of log Zph versus log V (not shown here) for the material under study are well within the agreeable limits of the ohmic character of the I+, and, therefore, the injection of charge carriers from the electrodes does not seem to be present in the working range of voltage. The kind of log Zphversus l/T behavior observed in the present study indicates the temperaturerange-dependent photoconduction process and it seems that the photoconduction process in this material is highly influenced by some photoinduced effect. It has been reported that a phase transition occurs in FcH under the influence of external parameters such as temperature [ 201 and pressure [ 211. Some FcH derivatives exhibit various kinds of mesophase transitions associated with intermolecular interactions [22]. Again, FcH and some of its derivatives in the presence of adsorbed vapors have indicated [ 4,5] the possibility of adsorption-induced phase transition. Photoinduced phase transition in some quasi-one-dimensional molecular systems has been reported [ 23,241 in recent years. From these observations it appears that occurrence of a phase transition in FcH under suitable light illumination at a particular temperature is not unlikely. In fact, it has been reported that the anomalous photoconductivity observed in the FcH sample at a particular temperature range is the possible indication of reversible photoinduced phase transition in the material [ 191. Meier et al. [ 171 have reported reversible changes in photoconductivity and dark conductivity of polymeric IJ--cyanophthalocyaninato-cobalt( III) due to a phase transition that occurs by varying the temperature at about 285 K. It is understood from their studies [ 171 that the phase transition influences the charge-carrier trapping properties of the material in the phase transition region. It has been reported also by Meier et al. 1181 that the photo-

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1

,

I

I

3.2

3.3 3.4 3.5 3.6 lOOO/T(l/K) Fig. 6. Plots of logarithm of photocurrent vs. reciprocal of temperature for Fc( CH20H) at different exciting light intensities from a mercury lamp ( 125 W): (A), (B), (C), (D) and (E) represent 27, 36,49, 64 and 100% light intensity, respectively.

conductivity of phthalocyaninato+-thiagermanium( IV) ( (PcGeS),) decreases on going from low to high temperature at about 308 K without any significant change in dark conductivity. They have mentioned that the effect may be the result of a phase change of the crystal structure which can affect, for instance, the rate of generation of carriers. In amorphous silicon deposited from silane [ 251 maximum photoconductivity has been observed at some characteristic temperature T,,,. This T,,,, has been thought to make the transition between the two regimes of recombination of charge caniers. From the above discussion it is clear that the above-mentioned photoinduced phase transition in the material can affect the trapping/detrapping as well as the charge carrier recombination processes which ultimately modulates the rate of generation of charge carriers and causes the change in photoconductivity as observed in the present case. It appears that the fluctuations in y values with temperature is related to the temperature-range-dependent photoeffects on the charge carrier trapping/detrapping and charge carrier recombination processes. Similar processes have been thought to be responsible for the anomalous photoconductivity of ferrocene in a specific temperature range [ 191. Fluctuation in physical parameters near the phase transition region has been well recognized in various systems [ 26,271. In the present experiment fluctuation in photocurrent noted at various temperatures is possibly related to the phenomenon of photoinduced phase transition. Experimental techniques such as differential scanning calorimetry, scanning electron microscopy, optical absorption spectroscopy, etc., are powerful analytical techniques that

may contribute to characterizingthe materialsduring light

68

A.K. Chukraborty

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illumination. Unfortunately, such experiments under the same operating conditions as in the present study could not be performed due to lack of the necessary instrumental facilities.

4. Conclusions

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Acknowledgements A.K.C. thanks the ICCR, Government of India, New Delhi, for providing his research fellowship: the authorities ofIACS, Calcutta 700032, India, for providing laboratory and other facilities; and also the Government of Bangladesh for graYiting him study leave.

References [II [21

The study on dark conductivity of Fc (CH?OH) has shown that the distribution of traps in this material is of the single discrete level type and it is a non-extrinsic material. In the entire temperature range of study, a single straight line has been obtained from the plot of log Zdversus 1/T for both the ohmic and SCLC region (Fig. 3). The dark activation energy value of the different ferrocene derivatives possibly originates from the modifications in the n-electronic structure by the different groups substituted in the Cp ring. From the results it is difficult to correlate the change in various transport parameters with the substitution groups in the ferrocene unit. In contrast to the behavior of dark conductivity, the photoconductivity behavior of Fc(CH,OH) in the present study has indicated the temperature-range-dependent photoeffects. The equilibrium value of dark current (I,) obtained after release of photoexcitations has shown the linear plot of log Id versus 1 IT for the entire temperature range, indicating that the photoinduced changes are almost reversible. A superlinear dependence of photocurrent on the light intensity with y> 1 has been observed in certain temperatures in the case of Fc( CH,OH). Temperature-dependent fluctuations in y value have been observed in both FcH and Fc (CH,OH) . The evaluated slope of the plot of log ZPhversus log V for the material under study is well within the agreeable limits of the ohmic character of the ZPh and, therefore, the injection of charge carriers from the electrodes does not seem to be present in the working range of voltage. The observed temperature-dependent fluctuations in y values are interesting and quite unusual. Such fluctuations in yvalues with temperature are expected to be related to the temperature-dependent photoeffects on the charge carrier trapping/detrapping and charge carrier recombination processes, possibly originating from the photoinduced reversible phase transition in the material.

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N.J. Long, Angew. Chem., Int. Ed. Engl. 34 (1995) 21. A.P.F. Turner, I. Karube, G.S. Wilson (eds.), Biosensors, Oxford University Press, London, 1987. [31 C.E. Carraher, J.E. Sheets, CU. Pittman, Advances in Organometac and Inorganic Polymer Science, Marcel Dckker, New York, 1982. J. Phys. Chem. Solids 50 (1989) 1113. r41 B. Mall.&, 4. Bhattachajee, B. Mallik, Bull. Chem. Sot. Jpn. 64 ( 1991) 3129. [51 A. Bhattachajee, [61 A. Bhattachajee, B. Mallik, Indian J. Phys. 66A (1992) 369. B. Ma&k, Bull. Electrochem. 6 ( 1990) 780. 171 A. Bhattachatjee, [81 A. Bhattachajee, B. Mallik, J. Mater. Sci. 29 (1994) 4875. A. Bhattachajee, B. MaIlik, Bull, Chem. Sot. Jpn. [91 A.K. Chakraborty, 67 (1994) 607. (There was a computational error in the reported value (0.58) of y for Fc(COOH)~ at a cell temperature of 303.45 K, which has now been corrected to 1.05.) 1101 H. Meier, Organic Semiconductors, Verlag Chemie, GmbH, Weinheim, 1974, p. 165. 1111 B. Mallik, A. Ghosh, TN. Misra, Proc. Indian Acad. Sci. Part 1, 88A ( 1979) 25. [I21 G.G. Roberts. W. S&mid&n, Phys. Rev. 180 11969) 785. Cl31 K.C. Kao, H. Hwang, Electrical Transport in Solids, Pergamon Press, Oxford, 1981. H. Meier, Organic Semiconductors, Verlag Chemie, GmbH, Weinheim, 1974, p. 271. N.A. Nesmyanov, Fundamentals of Organic Chem[I51 A.N. Nesmyanov, istry, Vol. 4, Mir Publishers, Moscow, 1981, p. 142. 1161 H. Meier, Organic Semiconductors, Verlag Chemie, GmbH, Weinheim, 1974, p. 317. M. Hannck, J. Metz, Synth. Cl71 H. Meier, W. Albrecht, E. Zimmerhackl, Met. 11 ( 1985) 333. 1181 H. Meier, W. Albrecht, E. Zimmerhackl, M. Hanack, K. Fischer, J. Mol. Electron. 1 (1985) 47. B. Mall& Synth. Met. 73 ( 1995) 239. [W A.K. Chakraborty, 1201 G. Calvetin, G. Clecn, J.F. Berar, D. Andre, J. Phys. Chem. Solids 43 (1982) 785. [2X1 B. Karvaly, B. Mallik, G. Kemeny, J. Mater. Sci. Lett. 4 ( 1985) 982. [=I K. Iwai, M. Katada, I. Motoyama, H. &no, Bull. Chem. Sot. Jpn. 60 (1987) 1961. [=I S. Koshihara, Y. Segawa, Y. Tokura, T, Koda, K. Takeda, Synth. Met. 55-57 (1993) 103. 1241 Y. Tokura, S. Koshihara, Mol. Cryst. Liq. Cryst. 216 ( 1992) 3. Solids 15 ( 1974) 410. 1251 W.E. Spear, R.J. Loveland, J. Non-Cryst. [261 A.G. Naumovets, Contemp. Phys. 30 (1989) 187. in C. Domb, M.S. Green (eds.), Phase Transitionszd [271 J. Als-Nielsen, Critical Phenomena, Vol. 5A, Academic Press, London, 1976, pp. 143 and 145.