Dependence of Electrical and Optical Properties on the Morphology

Chapter 4

Dependence of Electrical and Optical Properties on the Morphology and Microstructure of Polyaniline 1

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S. S. Hardaker , K. Eaiprasertsak , J. Yon1,R.V. Gregory , G. X. Tessema , R. Ou , C. Cha , and R. J. Samuels 3

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Center for Advanced Engineering Fibers and Films and School of Textiles, Fiber, and Polymer Science, and Department of Physics and Astronomy, Clemson University, Clemson, SC 29634 School of Chemical Engineering, Georgia Institute of Technology, Atlanta, G A 30332

Downloaded by STANFORD UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: September 16, 1999 | doi: 10.1021/bk-1999-0735.ch004

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Since the discovery o f electroactive polymers i n the 1970's extensive work has been carried out to utilize these materials i n a variety o f applications normally not thought to be germane to organic polymers. Anticipated applications have ranged from "organic wires" made from fibers o f intrinsically conductive organic polymers to semiconductor devices, and light emitting diodes.[1] Use o f these polymers for battery electrodes and actuators is also finding applications i n a variety o f engineered high technology materials. Since the early work o f Shrikawa, MacDiarmid and Heeger, countless new polymers for electronic, and recently photonic, applications have been synthesized. [2] While many o f the descriptions o f charge transport i n these polymers is based on our knowledge o f the solid state physics and chemistry governing transport mechanisms i n metals and inorganic semiconductors, few models describing this phenomena i n polymers have been put forth. Most o f our understanding o f these materials is based upon the knowledge base already established for well-ordered and glassy metals, and on the crystalline state o f inorganic semiconductors. Organic polymers are, however, quite different than most o f their inorganic counter-parts. Organic polymers will have a variety o f differing structures depending not only on the chemical repeat unit o f the mers but also on the interactive forces between the polymer chains. These forces w i l l determine the final polymer morphology. In

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©1999 American Chemical Society

In Semiconducting Polymers; Hsieh, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.


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addition the processing o f these polymers w i l l have a substantial effect on the polymer's final morphology and structure, crystallinity, density, elastic properties, solubility, strength, bend modulus, stability, and a variety o f other properties. A n example o f a processing related effect on structure is the degradation o f molecular weight due to shearing i n a processing stage, exposure to high temperatures, or other conditions the polymer might be exposed to while being processed into an end item. Although the literature is rich with detailed accounts o f these effects on the mechanical properties o f organic polymers, such as the polyamides and polyesters used i n fiber production, there are very few papers devoted to these effects in electroactive polymers. Even fewer papers discuss the effect o f morphology development during processing on the electrical and optical properties o f these polymers. This contribution w i l l seek to demonstrate the effect o f various formation methodologies on the resultant electrical and optical properties. Polyaniline ( P A N I ) is a polymer o f particular interest due to its facile processing capability i n its non-conductive emeraldine base form. This chapter w i l l consider the effects o f polymer orientation on the refractive index i n all three dimensions o f polyaniline ( P A N I ) films formed by various casting techniques. These changes i n the refractive index can be related to the orientation o f the polymer molecular chains and therefore the molecular structure o f the formed polymer. Additionally we w i l l discuss the effect o f P A N I films prepared from solutions containing differing concentrations o f a reducing agent, w h i c h reduces the emeraldine base ( E B ) to the lecoemeraldine base ( L E B ) form o f the polymer resulting i n significantly different morphologies. Determination o f the thermopower o f films formed from P A N I i n different oxidation states w i l l be shown to be indicative o f the charge transport mechanisms at different temperatures. The effects o f differences i n polymer structure and morphology on final properties w i l l vary from polymer to polymer. Morphological effects on final electrical, optical, and mechanical properties o f the formed polymer w i l l i n general hold for all organic polymers whether or not they are electroactive. Although the observed changes reported here are specific to P A N I the effect o f these differences must be considered for all electroactive polymeric materials.

Experimental There are many different experimental methods described i n the literature regarding the chemical and electrochemical synthesis of polyaniline. [3] F o r the purposes of this writing we w i l l only discuss the effect o f morphological changes o f P A N I films formed from chemically synthesized polyaniline Synthesis conditions w i l l affect the properties o f the final polymer product due to changes i n molecular weight and interaction o f the growing polymer with either itself or neighboring polymer chains and possibly other molecular species present in the reaction mixture. Normally polyaniline is synthesized under acidic conditions leading to a doped electrically conductive powder. Subsequent dedoping o f polymer results i n the emeraldine base (EB) form that is soluble i n several different solvents. M u c h work i n the literature is reported on solvation o f the E B form i n N-methyl-2-pyrrolidinone ( N M P ) . We have


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found and reported i n previous work that di-methyl-propylene urea ( D M P U ) is i n fact a better solvent for the P A N I base forms than is ( N M P ) for processing purposes. [4] B y better solvent we mean that the polymer i n D M P U has a higher intrinsic viscosity and viscoelastic properties consistent with better solvation. The solvent from which P A N I is processed w i l l have a significant influence on the morphology o f the solid state polymer. I f for example a film is cast from a poor solvent, where the polymer/polymer interactions are strong resulting i n a tight ball chain configuration, the mechanical and electrical properties w i l l be considerably different than i f the chains were i n an expanded coil conformation by processing i n a much better solvent. Methods o f synthesis, dedoping, reduction o f E B to L E B , and solvation for the study described i n this chapter are outlined below.

Spin Coated and Drawn Films. Polyaniline was chemically synthesized by addition o f an ammonium persulfate solution (180g (NH )2S2C>8 i n 1.0M HC1) to a stirred aniline solution (73.6g i n 1.0M HC1). The oxidizing solution was added over two hours under constant stirring and the reaction mixture was held at about 0 ° C . After two additional hours, the reaction mixture was filtered and washed with 6.5 liters o f water, followed by deprotonation i n excess 3 wt% The deprotonated base polymer was filtered and washed with 6.0 liters o f water. The polymer was washed i n one liter o f methanol, filtered and dried. Molecular weights were estimated with gel permeation chromatography ( D M S O , 0 . 2 % L i B r , 7 5 ° C , polyvinyl pyridine standard) as Mn=35000, Mw=83000, Mw/Mn=2.4. N,N'-dimethyl propylene urea ( D M P U ) was chosen as the processing solvent for the preparation o f films v i a spin coating as this solvent inhibits the formation o f gels, allowing stable solutions o f P A N I to be prepared for solution processing.[5] A 7.5 wt % solution o f P A N I was spin-coated onto glass substrates with a spin time o f two minutes and a spin speed o f 750 rpm. The films were dried at 1 3 0 ° C for 90 seconds and allowed to cool to room temperature. The films were removed from the substrates by submersing i n water. The spin coated samples were hand drawn at 100°C to varying extension ratios to a maximum o f 1.63 at about 5 cm/min. 4

NH4OH.

O x i d a t i o n State Studies. For the oxidation state studies polyaniline was chemically synthesized b y addition o f an ammonium persulfate solution (in 1.0 M HC1) to a stirred aniline solution (in 1.0 M HC1) over a two and one half-hour period. The temperature o f the reaction mixture was controlled at -30 ° C . L i C l (6.0 M ) was present to prohibit freezing. For this reaction, the aniline:oxidant molar ratio was 2:1. The total reaction time was 24 hours after which the resultant emeraldine salt was washed with 6.0 L o f deionized water. The emeraldine salt was deprotonated by stirring i n 3 wt% N H 4 0 H for 25 hours and washed with 6.0 L deionized water. Oligomer was removed by stirring the emeraldine base i n methanol for 45 minutes followed by rinsing with 6.0 L methanol. G e l permeation chromatography (polystyrene standards, N-methyl-2-pyrrolidinone/0.05 M L i B r as eluent, 80 °C) was


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employed to determine the molecular weight o f the emeraldine base (Mn=32,000 and Mw=60,000). Solutions (9 wt%) o f the emeraldine base were prepared in N,N'-dimethyl propylene urea ( D M P U ) containing varying amounts o f phenyl hydrazine. The molar ratios o f phenyl hydrazine to emeraldine base (based on a four ring repeat unit) were R = 0.0, 0.36, 0.71, and 1.11. These solutions were spin coated onto silicon wafers at a spinning speed o f 500 rpm and a spinning time o f 30 seconds on a Headway Research Inc., P M 1 0 1 D T - R 7 9 0 spin coater. These films were then placed on a heating plate for 10 minutes at - 7 5 °C. The films were removed from the substrate by submersion i n deionized water and annealed at 200 °C for 20 minutes under a vacuum. I vapor was used to dope the films.


2

Differential scanning calorimetry ( D S C ) was carried out on a T A Instruments, M o d e l 2920 at a heating rate o f 20 °C per minute under nitrogen, with indium as a calibration standard. F o r the electrical measurements, samples were mounted across two copper blocks using carbon paint to make electrical contacts, with a silicon diode mounted near the sample to measure temperature. The resistivity (using a standard four probe technique) was measured as a dipstick containing the sample was slowly lowered into a liquid nitrogen dewar. After the temperature dependence o f the resistivity was measured, the sample was slowly withdrawn from the dewar and the thermoelectric power was measured. The two copper mounting blocks were controlled at two different temperatures and the gradient measured by a A u Fe(0.07%)-Cu thermocouple anchored near the ends o f the sample. Both the D C conductivity and thermoelectric power were measured i n the temperature range between 77 and 300 K . Although P A N I is easily synthesized, precaution must be taken to minimize exposure to aniline i n the polymerization process. Under certain conditions aniline w i l l react with itself to form the highly carcinogenic oligomer benzidine. This oligomer results from a head-to-head coupling o f the aniline monomer. Benzidine once formed and subsequently incorporated into the polymer chain as a defect i n the normal head to tail polymerization o f aniline is not considered to be dangerous. One must still be concerned with the possibility that unreacted aniline may well be present in the formed P A N I and an oligomerization reaction forming benzidine may occur i n the polymer. Recent work i n our laboratory clearly demonstrates that benzidine easily partitions into emeraldine base polyaniline. [6] A detailed discussion o f the incorporation o f benzidine i n formed P A N I and i n the starting reaction mixtures may be found i n reference 6. O p t i c a l Properties of P A N I F i l m s The three dimensional refractive indices o f the freestanding P A N I films were characterized with a modified prism-wave guide coupler (Metricon PC-2010) at 1550nm. The technique was previously developed and tested on a range o f different freestanding and spin coated polymer films[7,8] In this work N z is the refractive index along the optical symmetry axis, N y is the refractive index perpendicular to the symmetry axis i n the film plane, while N x is the refractive index normal to the film


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plane. The average refractive index, N a v = (Nz+Ny+Nx)/3, is proportional to the density and hence is a measure o f the crystallinity. The birefringence is defined as: N z y = N z - N y , N z x = N z - N x , and N y x = N y - N x and is a measure o f molecular orientation. In order to obtain accurate values o f the principal refractive indices i n the plane o f the sample, it is best to measure the refractive index as a function o f the angle, | , around the film. B y introducing a large number, 16 i n our case, o f angular measurements into the optical indicatrix equation, a precise identification is obtained o f the symmetry axis direction and o f both principal refractive indices i n the plane o f the film. The optical indicatrix equation i n the plane o f the film has the linear form: l/N+2 = 1/Ny2 + (1/Nz2 - l/Ny2)cos2

(1)

Experimentally the above equation is slightly modified to: 1/N2 = 1/Ny2+ (1/Nz2 - l/Ny2)cos2(-fl)

(2)

where B, is the angle between the assumed reference direction and the optical symmetry axis direction. To find the exact direction o f the optical symmetry axis, a linear least squares computer subroutine is used. [9] Figure 1 shows a linear indicatrix plot o f the in-plane refractive indices o f the P A N I film cast from D M P U and stretched to a draw ratio o f 1.63. This technique eliminates alignment errors i n the refractometer that can easily be missed by the more conventional technique o f simply measuring only along and perpendicular to the assumed symmetry axis direction. A further advantage is this measurement technique also yields 16 N x values to average, increasing the reliability o f that important parameter. A l l refractive index measurements made i n our laboratory utilized the angular indicatrix approach. Effect o f F i l m O r i e n t a t i o n . The effect on the three dimensional refractive indices o f fabricating film by spin coating is shown i n Figure 2. In spin coating a polymer droplet is deposited onto a spinning surface and is spread out uniformly from the center. Figure 2 shows that the resulting undrawn film orientation is random i n the film plane, N z = N y , but the principle refractive index i n the thickness direction, N x , is different. The spin coated film has a more planar-like structure. In contrast the D M P U cast undrawn film before drawing was isotropic. W i t h extension o f the freestanding spin coated film changes i n orientation occur i n all three film planes. N z increases with increasing draw ratio as the random molecules align increasingly in the draw direction. N y decreases with increasing draw, as does N x . The rate o f decrease i n N y is greater than that o f N x so that the two values approach each other at the highest draw ratio. Thus the spin coated film goes from a planar to almost a uniaxial structure with increasing extension. The average refractive index remains constant indicating there is no phase change with extension. [10]


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0.34

0.0

0.2

0.4 COS

0.6 2

0.8

1.0

(0)

Figure 1. Linear refractive index indicatrix plot for spin coated and drawn film with a draw ratio o f 1.63.



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The new near infrared wave guide coupling technique can be used for the determination o f the three dimensional refractive indices o f films opaque in the visible region. This technique is appropriate for other electroactive polymer systems providing a window exists i n the visible or near infrared spectrum where measurements can be made. Applying this technique to P A N I films has clearly shown that spin coated films show some planar character i n the thickness direction. Unidirectional stretching o f freestanding spin-coated P A N I film leads to increased orientation i n the deformation direction, and conversion from a planar to a more uniaxial structure. Similar studies have been carried out on P A N I films formed from NMPsolutions.[ll] Although these studies elucidate the effect o f morphological changes o f the optical properties o f P A N I films which have been cast and subsequently oriented, the technique can be applied to other electroactive polymers as well to determine the effect o f orientation and subsequent morphological changes on the formed polymer's optical properties. Effect of Structural Morphology on the Electrical Properties of PANI Films The chemical structure o f polyaniline i n various oxidation states is shown i n Figure 3. The fully reduced form is referred to as leucoemeraldine base ( L E B ) while the fully oxidized form is called pernigraniline ( P N B ) . Both L E B and P N B w i l l shift towards the E B form upon exposure to an oxidizing environment. [2] Although it is well known that the oxidation state o f polyaniline is an important characteristic o f this polymer, there are few reports o f its influence on the development o f morphology i n fibers and films. Previous work has shown that both films and fibers produced from solutions o f leucoemeraldine base i n N,N'-dimethyl propylene urea ( D M P U ) exhibit crystallinity and a melting transition. [12] Electrical transport properties o f chemically prepared polyaniline show a strong temperature dependence. In general, depending on the sample preparation and dopant species, the polymers may be metallic, insulating, or be i n the critical region for a metal-insulator transition. [13,14] It is also known that disorder plays a predominant role i n the transport process. The disorder can be homogeneous on a molecular scale, mesoscopic heterogeneity's with highly doped conducting clusters, or crystalline islands separated by insulating regions. The D C conductivity is thus influenced to a great deal b y the extent o f amorphous regions. However, because o f the absence o f a measuring current flow, the thermoelectric power is less dependent on these regions. Several models have been proposed for describing the temperature dependence and transport properties o f charge carriers, including quasi-one dimensional variable range hopping (Q-1D V R H ) , charging energy limited tunneling for granular metals, and three dimensional variable range hopping with a Coulomb gap.[15,16,17] In this section we w i l l elucidate the interconnection between structure, both molecular (i.e. oxidation state) and morphological, and electrical transport properties. The techniques employed for this study are differential scanning calorimetry for



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(a)

K > " - Q

(b) H

H

I J,

Figure 3. Three oxidation states o f polyaniline base: a) fully oxidized pernigraniline base ( P N B ) , b) partially oxidized emeraldine base ( E B ) , and c) fully reduced leucoemeraldine base ( L E B ) .



56 characterization o f thermal transitions, and temperature dependence o f conductivity and thermoelectric power. Films were spin coated from solutions containing molar ratios R = l . l l , 0.71, 0.36, and 0.00 o f phenyl hydrazine, a reducing agent, to emeraldine base (based on a four ring repeat unit). A s one might expect, the appearance o f the films depended strongly upon the level o f oxidation. The fully reduced films were a grey color with a poorly reflecting surface indicative o f the leucoemeraldine base. W i t h increasing oxidation, the films were more reflective and had a dark grey color, while the unreduced films were quite reflective and had the characteristic coppery color o f emeraldine base films. This optical behavior is attributed to the presence o f a crystalline phase (probably less than - 1 5 % ) i n the leucoemeraldine base form o f the films which causes a rough surface to form. The surfaces o f the films which were against the substrates are much more reflective. A s reported earlier, an endotherm was observed from differential scarining calorimetry consistent with melting o f the crystallites. [13] For the present study, D S C was chosen to investigate the effect that oxidation state has on the morphological development o f the prepared films. A discussion o f thermal transitions i n the emeraldine base form o f polyaniline may be found in the literature.[18,19] Figure 4 presents the D S C results on four annealed films. For the emeraldine base film (R = 0.00), as the temperature increases a broad exotherm centered at - 3 0 0 °C was observed, attributed to oxidation or crosslinking and characteristic o f emeraldine base. W i t h increasing R (i.e. more reduction) one can see the development o f an endothermic peak between - 3 0 0 and - 3 8 5 ° C , depending on oxidation state. A s a greater proportion o f the repeat units are reduced (as R increases), more o f the chain segments can crystallize, which is accompanied by an increase i n the magnitude o f the endotherm. Another effect is that the endotherm becomes sharper and shifts to higher temperatures with increasing R , believed to be due to increasing crystallinity i n conjunction with more perfect ordering i n the crystallites. When treated with I vapor, the reduced segments are oxidatively doped to the conducting state. Figure 5 presents the results o f the electrical conductivity temperature dependence as a function film oxidation level prior to doping. The room temperature conductivity o f the R = 0.00 film doped with 1.0 M HC1 was measured to be 0.22 S/cm. W i t h increasing R , the conductivity of the I doped films increases, and can be interpreted i n terms o f the quasi-ID variable range hopping mechanism (Equation 3):[16,14] 2

2

(3)

here y = Vi, and T = 485 K for the R = 0.36 film and T = 137 K for the R = 0.71 film. If fit to Equation 1, the R = 1.11 (most reduced) film exhibited a T o f 70 K . A decreasing T can be attributed to more intra- and interchain ordering resulting in 0

0

Q

Q


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R = 0.00

100

200

300

400

500

Temperature (°C)

Figure 4. Differential scanning calorimetry o f polyaniline spin coated films as a function o f chemical reduction: R = 0.00: -emeraldine base, R = 1.11: -leucoemeraldine base.


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greater charge derealization, consistent with the morphological picture obtained from the D S C results. However, for the R = 1.11 film, a slightly better fit is obtained when plotted against 1/T, indicative o f a semiconductor mechanism. This is i n fact born out by thermoelectric power measurements on this film i n which the thermoelectric power is proportional to 1/T. Figure 6 presents the thermoelectric power for the R = 0.36 film as a function o f 1/T. A prominent feature o f this figure is the transition at - 1 5 0 K , which is consistent with polaron scattering by phenyl ring vibrational modes as reported b y Pratt et.al.[20] This temperature corresponds to a P-transition observed by dynamic mechanical thermal analysis and temperature dependence o f the anisotropy i n a indicating a lack o f phenyl ring rotation below 150 K.[21] The change i n slope at the 150 K temperature is consistent with a change from a semiconductor transport mechanism to a variable range hopping mechanism. Ongoing work suggests that there may be another transition i n the thermoelectric power spectrum around 250 K . This transition has yet to be quantified. This work demonstrates that there is a clear connection between morphological structure as evidenced by the D S C results and electrical transport properties. W i t h increasing chemical reduction, there is less hindrance to crystallization and sharper, higher temperature endotherms are obtained from the D S C , characteristic o f larger, more perfect crystalline structures. This behavior is confirmed by the temperature dependence o f conductivity i n which T decreases with increasing chemical reduction. U n l i k e the other two samples (R = 0.36, 0.71), the R = 1.11 film exhibits the 1/T dependence o f thermoelectric power indicative o f a semiconductor transport mechanism. Although the L E B form o f P A N I slowly reoxidizes back toward the E B form the above study combined with the optical studies suggest that the structure formed when processed i n the L E B form is maintained after reoxidation and subsequent doping. This is consistent with the results obtained for fiber spinning from L E B where the formed fibers demonstrate superior mechanical properties when compared with fibers spun from E B systems.[5]


d c

0

Concluding Remarks This chapter reports on the effect o f morphological structure on the determined electrical and optical properties o f polyaniline when processed by different methodologies. Although the information reported is specific to P A N I the concept o f changes i n structural properties o f electroactive polymers due to processing or preprocessing steps is not unique to polyaniline but rather extends to all synthetic organic polymers. Such morphological changes affect not only the mechanical properties o f the formed polymer but also the electrical transport mechanisms as well as their optical properties. When these polymers are considered for device applications one must consider the route by which the materials are prepared and the resulting structure as this w i l l have demonstrative effects on the polymers, thermal and electrical transport, optical characteristics, failure mechanisms, and efficiency as hole or electron injectors.


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= l.n


= 0.71

l

1

1

1

1

50

60

70

80

90

1

•—

100

1000/T

i

1—

1 10

120

,/2

= 0.36

Figure 5. D C conductivity o f iodine doped polyaniline films as a function o f R, the molar ratio o f reducing agent to emeraldine base (based on a four ring repeat unit) o f the solution used to prepare films.

4.00e-6

2.00e-6 -I 2

1 3

1 4

1 5

1 6

1 7

1 8

1 9

1 10

. 11

1 12

r13

_1

1000/T ( K ) Figure 6. Thermoelectric power o f a polyaniline spin coated film. R = 0.36 is the molar ratio o f reducing agent to emeraldine base (based on a four ring repeat unit) o f the solution used to prepare film.


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References [1] [2]

[3]


[4] [5]

[6] [7]

[8] [9]

[10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

Proceedings of the "International conference on the Science oand Technology of Synthetic Metals" Snowbird Utah, US A July 1996 A . G. MacDiarmid, Conjugated Polymers and Related Materials, edited by W. R. Salanek, I. Lundstrom, and B. Ranby, Oxford University Press, New York, 1993, pp. 73 - 98. Genies, E . M . , Boyl, A . , Lapkowski, M . , Tsintavis, C.; Synthetic Metals, 1990 36, 139 R. Jain and R.V. Gregory, Synthetic Metals, 1995, 74, 263 A.P. Chacko, S. S. Hardaker, B. Huang, and R. V . Gregory, Mat. Res. Soc. Symp. Proc., 1995, 413, 503; S.S. Hardaker, A.P. Chacko, B. Huang, and R. V . Gregory, SPE-ANTEC '96, 1996, Vol. II, 1358. Watters, M . J. "Determination of Trace quantities of Benzidine in EmeraldineBase Polyaniline" M.S. Thesis, Clemson University, 1995 Hardaker, S., C.Y. Cha, S. Moghazy and R. J. Samuels, in "Advances in Polyimide Science and Technology," C. Feger, M . Khojasteh and M . Htoo, eds. Lancaster, Pa., Technomic Pub. Co., 1993, 571. Hardaker, S., Moghazy, S., Cha, C. and R.J. Samuels J. Polymer Sci., Part B: Polymer Physics, 1993, 31, 1951. R . E . Pepper and R.J. Samuels, Encyclopedia of Polymer Science and Engineering, H.F.Mark, N.M.Bikales, C.G.Overberger, G.Menges, and J.I. Kroschwitz, Eds., 1988, 14, 261. C . Cha, S. S. Hardaker, R .V. Gregory and R .J. Samuels, Synthetic Metals, 1997, 84, 743. R. Ou, T. Liu, H . Wang, S. S. Hardaker, L . Ding, B. Mattes. R. V . Gregory, and R. J. Samuels; SPE-ANTEC'98, 1998, Vol. II, 1351 A . P. Chacko, S. S. Hardaker, R. V . Gregory, and T. W. Hanks, Polymer, 1998, 39(14), 3289 H . K . Chaudhari and D. S. Kelkar, J. Appl. Poly. Sci., 1996, 62, 15 J. Joo, V . N . Prigodin, Y . G. Min, A . G. MacDiarmid, and A . J. Epstein, Phys. Rev. B, 1994, 50, 12226 E . P. Nakmedov, V . N . Prigodin, and A . N . Samukhin, Sov. Phys. Solid. Stat., 1989, 31, 368 B. Abeles, P. Sheng, M . D. Courts, and Y . Arie, Adv. Phys., 1975, 24, 407 B. I. Shklovskii and A . L . Efros, Electronic Properties of Doped Semiconductors, Springer-Verlag, New York, 1984. Y . Wei, G. W. Jang, K. F. Hsueh, E. M . Scherr, A . G. MacDiarmid, and A. J. Epstein, Polymer, 1992, 33, 314 A . J. Milton and A . P. Monkman, J. Phys. D: Appl. Phys., 1993, 26, 1468 F. L . Pratt, S. J. Blundell, W. Hayes, K . Nugamine, K . Ishida, and A . P. Monkman, Phys. Rev. Lett., 1997, 79, 2855 A . P. Monkman, P. N . Adams, P. J. Laughlin and E . R. Holland, Synthetic Metals, 1995, 69, 183


Dependence of Electrical and Optical Properties on the Morphology

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