UV photoemission spectroscopic studies on molecular aggregation of triphenylamines dispersed in polycarbonate thin films

Chetmcal Phyws 160 ( 1992) 289-297

North-Holland

UV photoemission spectroscopic studies on molecular aggregation of triphenylamines dispersed in polycarbonate thin films

Naoki Sato Department of Chemistry. College ofArts and Sxnces, Umrerslty of Tokyo. Komaba. Meguro. Tokyo 153. Japan

and

Masao Yoshikawa 311 Laboratory, Research and Development Center, RICOH Co. Ltd., Kohoku. Yokohama 223, Japan

Received 22 April 199 1

Ultraviolet photoemission spectra of triphenylamine (TPA) derlvatlves, N(phenyl),_,(p-tolyl), (n=O-3), were measured

not only m the gaseous and sohd states but also dispersed in poly(carbonate) (PC) thin films wth a weight ratlo 3/5 (for TPA/

PC). Correlations of the electromc structure of the composite films with experimental results on electromc transport are studied,

and also the degree of dispersion of the guest molecules m the films IS &cussed m terms of the polarization energy m the organic

molecular sohds.

1. Introduction

A system of electron-donating molecules dispersed in an inert polymer. i.e. a molecularly doped polymer, such as triphenylamine (TPA) dispersed in poly (carbonate) ( PC ), is widely used as material for the carrier (usually positive hole) transport layer (CTL) in a double-layered organic photoconductor (OPC). in which the CTL is separated in operational function from the carrier (photo-)generation layer

(CGL). for use in electrophotography. However, the molecular aggregation in such a composite system and its electronic structure have scarcely been studied, despite the great importance of understanding its electronic properties and also to determine the conditions most appropriate for its practical applica- tions, e.g. the structural and/or electronic require- ments for a guest molecule. These problems are also of fundamental interest, e.g. the correlation between the chemical structure of the molecules and the char- acteristics when they are dispersed in a polymer matrix. Such a study should not only provide fundamental information but should also lead to a more efficient system for use in reprography.

In this work, we measured the ionization energies of TPA derivatives, i.e. a series of phenyl(p-tolyl )amine compounds, N(phenyl),-,(p-tolyl), (n=O-3), both in the gaseous and solid states and also dispersed in PC thin films, using ultraviolet photoemission spectroscopy (UPS). The gas-phase results were examined to determine the electronic structure of the guest molecules with particular reference to experimental results on the electronic transport in their composite films. Comparison of the results in the solid and dispersed states showed the characteristic molecular aggregation of TPA in the polymer film.

2. Experimental

Triphenylamine (TPA) derivatives with para-

methyl substitution of the phenyl groups, N(phenyl),_,(p-tolyl). (n= l-3), were synthesized by condensation reactions of the appropriate diphen- ylamines with iodobenzenes, while TPA (n = 0) was purchased from Tokyo Kasei Kogyo Co., Ltd. They were purified by recrystallization followed by re-

0301-0104/92/$05.00 0 1992 Elsevier Science Pubhshers B.V. All rights reserved.

390 N Sate, M. Yoshtkawa / 1fPS studres on molecular aggregatlorz of TPA

peated vacuum sublimation, and the purity was

checked by differential scanning calorimetry (DSC) and thermogravimetric (TG ) analysis. The melting points, T,, and vaporization temperatures, T,, thus obtained are shown in fig. 1. The melting behaviors at 123,69, 108,and 117”Cofthesampleswithn=O to 3. respectively, were very sharp. These samples suffered no thermal decomposition after melting up to the vaporization temperatures around 192, 213, 2 19, and 228 o C for n = 0 to 3. respectively.

Bisphenol A poly( carbonate) (PC: poly [ oxy- carbonyloxy- 1,4-phenylene ( 1 -methylethylidene ) - 1,4- phenylene] ), commercially available as Panlite K- 1300 from Teijin Chemical Co., Ltd., was used with- out further purification.

He (I ) (hv = 2 1.22 eV ) photoelectron spectra of the

guest molecules in the gas phase were measured on a Perkin-Elmer PS- 18 photoelectron spectrometer with a heated probe. The heating temperatures were 93, 115. 135. and 127”CforN(phenyl),_,(p-tolyl),with II = 0 to 3, respectively. The photoelectron energy was analyzed with a 127’ cylindrical electrostatic deflec- tion analyzer with a resolution of ~40 meV. The spectra were calibrated with the atomic line spectra of argon, xenon and also helium. The helium line spectrum, excited by He( II) (hv=40.8 1 eV) radia- tion also produced at the light source, was useful to

250

200

Y :

2

100

50

‘.-‘.; TDSC

m

0 1 2 3 n

Fig. 1. Meltmg pomt T,,, and vaporization temperature T, of tn-

phenylamme (TPA) denvatlves, i.e. N(phenyl),_.(ptolyl), for

n = O-3. DSC and TG indicate differential scanning calorimetry

and thermogravimetrlc analysis. respectwely.

calibrate the spectra in the lower ionization energy

range, since it gave a peak at 4.99 eV on the ioniza-

tion energy scale, for He(I) photoelectron spectra

11 I. Solid-state photoemission spectra were measured

using thin films to eliminate surface charge effects.

The rather low T,,, and T, values for the four TPA

derivatives obtained above (see fig. 1) indicate that these materials cannot be maintained as thin films at

room temperature in vacua for a long time. Their neat

films were therefore vapor-deposited onto copper

substrates cooled at - 50’ C for in situ UPS measurements under a vacuum of 10-j Pa in the chamber.

Their thicknesses were about 20 nm. measured on a

quartz oscillator monitor. Neat PC films and com-

posite films of TPA derivatives and PC were cast from

the respective solutions using tetrahydrofuran (THF) as a common solvent: PC. or a TPA derivative and PC with weight ratio 3 : 5 was dissolved in THF with

a concentration of about 1 wt%. This weight ratio was

chosen to be close to that for similar systems applied

practically in electrophotography, and also so that

crystallization of the guest molecules did not occur.

The copper substrate was dipped into the above so-

lution and withdrawn at a well-controlled rate to ob-

tain a film of appropriate thickness, 9 to 20 nm, which

was checked using a surface roughness monitor and/

or by the optical absorbance at A,,,,, v 300 nm due to

the guest molecules. In addition, changes in the com-

position of the thicker films during the experiments

were monitored by optical absorbance. Decreases in

the absorbance were observed for the films of all the guest molecules above room temperature, particu-

larly in vacua. This was caused by selective sublima-

tion of the guest molecules from the PC matrix due

to their high vapor pressures.

The apparatus used for the measurements of the sample films was an improved version of that re-

ported in detail previously [ 21. The light source, a

water-cooled-capillary hydrogen discharge tube at- tached to a 0.5 m Seya-Namioka monochromator,

was used in the photon energy range from 7 to 10 eV.

The electron energy analysis was made with a spher- ical retarding-field analyzer with the ac modulation method. This enabled energy values to be determined with an experimental error of 5 0.1 5 eV. For

the measurements of the composite films the sub-

N. Sate. M. Yoshlkawa / UPSstudIes on molecular aggregation of TPA 291

strate was cooled down to about - 50°C to suppress the sublimation of the guest molecules.

3. Results and discussion

3. I. Gas phase photoelectron spectra of TP‘4 derlvatwes

The gas-phase He(I) photoelectron spectra for the four TPA derivatives are similar to one another, as shown in fig. 2. The first band in each spectrum, as- signed to a phenyl rt orbital [ 3 1, is separated from the higher-energy bands: this is a common characteristic of these compounds. Its peak position shifts to the lower-energy side with increasing number of methyl groups at the para positions of the phenyl groups, i.e. with n of N(phenyl),_,(p-tolyl), increasing from 0 to 3. Fig. 3 depicts the energy level correlation for the four compounds. Again only slight changes in the energy structure are observed.

The electrochemical oxidation potentials are compared with the adiabatic and vertical ionization energies in the gas phase, Zi and Ii, respectively, determined from the He (I) spectra for those compounds,

TPAs

16 14 12 10 8 I,/eV

Fig. 2. He(I) photoelectron spectra of trlphenylamine (TPA)

denvatives, i.e. N(phenyl),-,(p-tolyl). for n=O-3. m the gas

phase.

0 '1 2 3

6 52 _____ 6 L8_____

6 fJ2_____ 6 78__-_- 6 ?2- 667- 7 06- 6 cd.-

17 L21---

0 15--- a 77- a 71 - 865- a 6a=

a a5 9 27-

9 05- 9 35-

9 77- 100 -

103 - 10 35-

11 1 - (10 91 -

11 6 -

121 - 120 - 120 -

130 -

137 - 138 - 137 - 1LO - 1L.O -

ua - 150 - 151 - 152 -

163 -

168 - 166 -

175 -

Rg. 3. Energy level correlation for trlphenylamme (TPA) derw-

atwe molecules, 1.e. N(phenyl),_,(p-tolyl), for n=O-3. ob-

tamed with He(I) photoelectron spectroscopy. Dashed lmes m-

dicate adiabatic values.

as shown in fig. 4. The oxidation potentials were measured as half-wave potentials E, ,: by voltamme- try, carried out on the TPA derivatives in acetonitrile solution under argon at a concentration of ~0.8 mmol/dm3 containing 0.1 mol/dm3 tetrabutylam- monium perchlorate as electrolyte, using a platinum working electrode and a saturated calomel reference.

The cyclic voltammograms of the compounds with n=O and 1 for N(phenyl),_,,(p-tolyl). were slightly but definitely different from those of the compounds with n =2 and 3. The former are characterized by double peaks, particularly in the oxidation processes. This is in good agreement with the cyclic voltammogram of triphenylamine obtained under slightly different conditions [ 41. Electrochemical studies of substituted triphenylamines [ 4.5 ] have shown that such a voltammogram suffers from generation of N,N’-tetraphenylbenzidines via anodic oxidation

292 N. Sate. M. Yoshlkawa / UPS studies on molecular aggregation of TPA

7.0

65

3 2

I 1

0

0

0

0

0

0

0

0

4

I I 1 0

07 09 11

6/2/V

Fig. 4. Adiabatic (0 ) and vertical (0 ) lonizatlon energies of

trlphenylamme (TPA) derivatives. I.e. N (phenyl),_,(p-tolyl).

for n= O-3, m the gas phase, 1; and I;, respectively, versus elec-

trochemical oxidation potentials E,,?.

coupling of triphenylamines. Thus, the present results for the TPA compounds with n = 0 and 1 can be understood when taking the associated coupling reactions into account, while the compounds with n= 2 and 3 do not suffer such coupling because of the much higher stability of their radical cations generated in the electrochemical processes [ 4,5].

It can be seen from fig. 4 that neither the adiabatic (1: ) nor the vertical (1: ) ionization potential shows a linear relationship with a slope of 45” with the E, ,? value. However, such a relationship is only observed for pairs of compounds, i.e. for the compounds with n = 0 and 1 or for those with n = 2 and 3, but not for all the compounds. The deviation from a linear relationship has been attributed to the difference in solvation effects in the electrochemical processes. e.g. in the case of tetrathiafulvalene (TTF) derivatives [ 61. However, the discrepancy between the two linear regressions in the present case could be related to differences in oxidation coupling between the pairs of compounds, as mentioned above. These observations indicate the need for caution in the use of the oxidation potential, which is much easier to

obtain experimentally, as a measure of the ionization

energy of a particular compound.

3.2. Photoemssion spectra of neat TPA solids and the polarization energy

Thin films of the four TPAs show very similar photoemission spectra, at least in the photon energy range employed: the spectra, obtained with the same excitation source energy, hv=7.7.5 eV, are compared in fig. 5. The spectra exhibit sharp onsets on the high voltage side (or right-hand side) of the retarding voltage scale, which is proportional to the electron kinetic-energy scale (see, e.g. ref. [ 2 ] ), while the spectral features are only partially resolved. The clear onsets allow precise determination of the threshold ionization energies Is for the solid films, they are listed in table 1. The values again decrease with increasing number n of the methyl substitution in each phenyl ring: the difference in Zp for the change from n = 0 to 3 is notably smaller than that in 18.

As is widely known, molecular identity persists in most organic solids, so that there is a direct corre- spondence between the band features in the UPS

/ TPAs

kW=775e”

1

-2 0 2 L Retordlng voltage / V

Fig. 5. UV photoemlssion spectra of vapor-deposlted thm films

of trlphenylamine (TPA) derivatives, I.e. N (phenyl )9_n(p-to-

lyl), for n=O-3, obtained with the same excitation energy’ hv=7.75 eV.

Table 1

N Sate. M Yoshkawa / CrPS studies on molecular aggregation of TP.4 293

Iomzatton energtes of trtphenylamine denvattves in the gaseous and sohd states, and their differences, compared with the polanzation

energtes calculated using eq. (2) from the parameters also hsted. &I= I; -I:“. d: calculated from a Gfunctton potenttal model (refs.

[ 12.131). p: measured m this work.

Compound 1; (eV) 1:” (eV) U (cV) P+ (eV) oc (10m39Fmz) p (103kgmm’)

N(Ph), 6.82 5.64 1.1, 1.35 3.65 1.173

N(Ph)Z(Tol) 6.78 5.59 1.1, 1.32 3.87 1.164

N(Ph)(Tol)Z 6.52 5.52 I .oo 1.24 4.10 1.123

N(Tol), 6.48 5.42 I .o, 1.16 4.32 1.078

spectra from the gaseous and solid state, particularly for typical organic molecular solids such as poly- scenes. This makes the determination possible of the polarization energy P,, which is the electronic stabi- lization energy for an ionized molecule chiefly result- ing from electrostatic polarization induced on the surrounding molecules in the solid, from the difference in the ionization energies of the two phases [ 7,8]. Considering solid-state ionization in photoemission in more detail, the polarization energy as experimentally determined is defined as follows [ 8 ] :

p + =p-p 52 s (1)

Further, an approximate expression for P, in SI units has been derived from an evaluation of the interaction energy between an ion and the induced dipoles on the surrounding molecules. It generally leads to values in good agreement with those observed experimentally [ 8,9]. The relation indicates that

P, =6.99(e/4xeO)2cip”‘3 , (2)

where to, (Y. and p are the permittivity in vacuum, the mean molecular polarizability, and the molecular- packing density in the solid, respectively [ 81. This last term is defined as the number density of molecules in the unit crystal cell, and is also proportional to the mass density p, asp = (N,/M)p, where NA and A4 are the Avogadro constant and the molar mass, respectively.

The difference in the ionization energies of the gaseous and solid state for TPA derivatives

A,I= I; - Iih , (3)

as listed in table 1, will be discussed also on this basis. If the values of d and p (or p) are available, the polarization energies can be calculated using eq. (2).

Accordingly, we measured the p values of the four compounds and calculated their B values from a 6- function potential model [ 10, 1 1 ] using the data on the molecular geometries of triphenylamine [ 12 ] and when a methyl group is substituted in a benzene ring as well as the C-H bond lengths [ 13 1. The calculated value of P,, together with the p and d values obtained, are also listed in table 1.

Both A,I and P, exhibit very similar behavior with increasing n, the number of methyl substituents in each phenyl ring. However, one can notice that A,I is slightly smaller than P, for each compound, although the calculated values of P, should only be regarded as approximate. Besides, although the right- hand sides of eqs. ( 1) and (3) are the same, eq. ( 1) is valid only for the case where the molecular identity is completely unchanged in the solid. In other words, A,I can include any specific interaction other than van der Waals forces and/or any change in molecular ge- ometry (which can be positive or negative) in addition to the contribution from P,. These effects have been observed for iodine [ 141, organic iodine compounds [ 151. and N,N-diphenyl-N’-picrylhydrazine [ 16 1. Thus. the difference between A,Z and P, in table 1 can be ascribed chiefly to the change in molecular structure. i.e. that in the phenyl (orp-tolyl) twist angle. as pointed out previously [ 17 1: the change can lead to a decrease in the p value, while the crystallin- ity in a sample specimen film may also contribute to the value of A,Z (cf. refs. [ 18.191). The similar behavior of A,I and P, with increasing n may therefore have some significance.

3.3. Photoernission spectra of neat PC

Photoemission spectra for thin films of neat poly- ( carbonate ) (PC ), used as a binder or host material

N Sate, M Yoshrkawa / C’PS studres on molecular aggregation ofTP.4

I, /eV

Fig. 6. UV photoemlwon spectra for a thm film of neat

poly (carbonate) (PC) The abscissa IS the solid-state lomzatlon

energy.

m the present study, show gentle tails, which are in- creasingly noticeable in the spectra obtained with lower excitation energies, as demonstrated in fig. 6. These tails tend to obscure the onset of the spectrum, making it difficult to determine the ionization en-

ergy. The above phenomena are common in UV photoemission spectra of polymer films and can be ex- plained by impurities, such as unsaturated terminal groups, which can act as carrier traps inducing shal- low levels contributing to the tail. Bearing these fac- tors in mind, 1:” of PC has been determined to be 7.0 eV, which is still larger than Zi of TPA derivatives.

3.4. TP4/PC composite thin films

Judging from the thermal properties of TPA derivatives described earlier, there was doubt whether the composite thin films with PC, prepared as described in section 2, would be stable because of separative sublimation in vacua. To check this, the optical absorbance at the absorption maximum of the longest wavelength band in the electronic spectra of the composite films, which were thicker than those used for the UPS measurements, was measured as a function of time under various conditions. Fig. 7a shows the time dependence of the optical absorbance at /I mnx = 300 nm for each composite film in air and fig.

i \

0 100

t/h

1

200

Fig. 7. Time dependence of optical absorbance at I,,,,, - 300 nm

for TPA/PC composite films: (a) N(phenyl),_,(p-tolyl),/PC

m air. and (b) N(phenyl),(p-tolyl)/PC in various conditions:

at RT m au (0 ), m the vapor of the guest molecules (0 ), and

m vacua (0). and at 60°C m ax (B). Solid lmes are guides for

the eye.

7b shows the time dependence for a N(phenyl)2(p- tolyl)/PC composite film under the following conditions: at room temperature (RT) in air, at RT in the vapor of the guest molecules, at RT in vacua. and at 60°C in air. The third curve in fig. 7b measured at “RT in vacua” indicates that it is virtually impossi- ble to carry out UPS measurements on composite films. Sample specimen films of TPA/PC were therefore held on substrates cooled to ” - 50’ C during the measurement.

3.5. Photoemisslon and molecular dispersron in TPA/ PC composite films

Composite thin films of TPA derivatives with PC. with a weight ratio TPA: PC of 3: 5, show photoemission spectra with fairly clear features even for low- energy incident light, as depicted in fig. 8 where the abscissa is again the retarding voltage. In particular, the small spectral feature at highest electron kinetic energy for each spectrum is ascribed to photoemission from the TPA molecules embedded in PC, since keeping a specimen film in vacua for a long time eliminated the feature selectively, due to sublimation

N. Sato. M. Yoshkawa / UPS studies on molecular aggregation of TPA 295

-

-

-

/ rcu* -1

\

-WAS/PC hv=816eV

Retardmg voltage / V

Fig. 8. UV photoemission spectra for TPA/PC composite thin

films measured using hvz8.16 eV incident light, m comparison

with that of a PC thin film.

of guest molecules from the near-surface of the film (cf. the discussion concerned with fig. 7). Accord- ingly, the threshold ionization energies determined from the spectra of these composite films can be regarded as those of the TPA derivatives dispersed in PC. The threshold ionization energies of the dispersed systems, I::, are very similar to the corresponding I:“, particularly for the two TPA compounds with n= 0 and 1, as listed in table 2.

To discuss the polarization effects in these materials. the difference in the ionization energies of a TPA derivative in the gas phase and in the dispersed-in- polymer state,

AdI=I;-Iih. (4)

Table 2

must be examined in detail, since this value indicates the electrostatic environment of a TPA derivative molecule in PC. The values of A,J are compared with the corresponding ionization energy differences A,I in table 2. The two quantities, A,I and AJ, are much closer to each other for the two compounds with n= 0 and 1 in N(phenyl),_,(p-tolyl), than for those with n = 2 and 3, while all the values of AI are rather similar to one another. This similarity indicates that there is little difference in the polarizability for a charge (carrier) between TPA derivatives and PC in the condensed phases. However, assuming that the molecular structure of a TPA molecule is similar in both its neat solid and in the PC binder, the subtle difference between A,I and AJ for the four TPA derivatives, considered in pairs, further suggests that the aggregation of guest molecules in the composite films is more dense in the case of the compounds with n=O

and 1. This indicates that the molecular aggregation in the PC composite films of these two compounds is close to that in their neat solids, i.e. the guest molecules are not molecularly doped in PC in the strict sense of the term, whereas the molecular dispersion is more complete in the composite films of the other two compounds.

It appears to be difficult to characterize the dispersion of the guest molecules in such thzn composite films, and in particular to specify whether molecularly doping has occurred, directly from a structural analysis method, for example X-ray diffraction. However, X-ray diffraction patterns for composite films of TPA derivatives with PC which were prepared as much thicker (about 2 l-22 urn) films than those for the UPS measurements, were measured and compared with one another and also with the pattern of a neat PC film, as shown in fig. 9. The patterns for the four TPA/PC composite films are very similar to

Comparison of threshold iomzation energies of triphenylamine derwatwes between neat solids and composite poly(carbonate) films

(allvaluesmeV).A,I=I;-Z~h, A,I=I;-ILh

Compound

N(Ph), N(Ph),(Tol)

N(Ph)(Tol),

N(Tol), PC

I; I: I’h I ASI I:: AdI

6.82 7.06 5.6, 1.1, 5.6, 1.19

6.78 6.94 5.59 1.1, 5.60 1.1s

6.52 6.12 5.5, 1 .oo 5.58 0.94

6.48 6.67 5.4, 1 06 5.58 0.90 _ 7.0 _

296 N. Sato. M. Yoshikawa / UPS studies on molecular aggregation of TPrl

0 20 LO

29/”

Fig. 9. X-ray diffraction patterns ofTPA/PC composite films (of

thickness 21-22 pm) and a neat PC film (of thickness 13.5 km)

measured using a CuKoc source.

one another and are basically derived from that of the PC film, while a broad feature is observed to de- velop with increasing n at 20% 8 ‘. These results may reflect the large content of PC in the composite films and the variation in the degree of association of the guest molecules. The broad features at 28% 8”) corresponding to an intermolecular distance of about 1.1 nm, may be attributed to intermolecular spacings of TPA derivative molecules dispersed in the PC composite films (cf. the intermolecular spacings in a TPA neat crystal being estimated to be around 0.7 nm from the crystal data [ 201 ). These features indicate that there is an increase of the homogeneity in the molecular dispersion in the films with increasing n and sug- gest that there is heterogeneous or segregated dispersion of the compounds with small n and near molecular doping for those with large n in this case. However, it is difficult to characterize the molecular dispersion in the composite thin films solely from X- ray measurements.

Here again, it can be noted that a careful comparison between A,I and AdZ in terms of the polarization energy in organic molecular solids or in terms of the electrostatic polarization environments for a TPA derivative molecule, has enabled us to discuss the

guest molecular dispersion in the composite films. That is, the subtle difference between the A,Z and AdZ values observed for the two compounds with n = 0 and 1 in N(phenyl)3_n(P-tolyl)n indicates that the guest molecules are associated to some extent, or form clusters, in their PC composite films, while in the other two compounds, with n= 2 and 3, they are rather isolated in the films, as described above. This can therefore be regarded as an example of the characterization of the molecular aggregation in composite thin films through the observation of their electronic structures. These results support the speculations above from the X-ray diffraction patterns obtained for much thicker films.

Finally, results on electronic properties in the TPA/ PC composite systems have been shown to correlate with the above conclusions on the guest molecules. Since these systems are useful as materials for the positive hole transport layer in a double-layered organic photoconductor, their hole drift mobilities have been measured [ 211. Fig. 10 depicts the hole drift mobility ,B,, versus the square-root of the electric-field strength E observed at 25’ C for TPA/PC composite films of thickness of about 10 urn. Over the whole range of applied fields the ,LL,, values for the films of N(phenyl),_.(p-tolyl),,/PC increase with increasing value of n. Such behavior of pr, observed for the four TPA/PC systems corresponds well to the differences between AdZ and A,Z discussed above. This sug-

H 0 .

0 5 10 15

(E/10‘Vcm~‘)“2

Fig. 10. Electnc-field strength dependence of the hole drift mo-

bility measured at 20°C for TPA/PC composite films:

N(phenyl),_,(p-tolyl).forn=O (w). 1 (Oj.2 (O),and3 (0).

N. Sate. M. Yoshlkawa / UPS studies on molecular aggregatron ofTPA 297

gests that large values of ,u,, for the composite films are connected with a high degree of molecular aggregation in these films.

4. Conclusion

We have measured UV photoelectron spectra for triphenylamine (TPA ) derivatives, N (phenyl) 3_ n (p-tolyl), (n = O-3), in the gaseous and solid states and also when dispersed in poly (carbonate) (PC) thin films. to determine the threshold ionization energies for the respective states. The values obtained were discussed in terms of the polarization energy of organic solids, in particular to study the molecular aggregation in composite TPA/PC thin films. This enabled us to characterize the molecular dispersion of the TPA derivatives as the guest molecules in these systems. The evidence suggests that the TPA molecules with n = 0 and 1 are associated to some extent or form clusters in their PC composite films, while the molecules with n=2 and 3 are rather isolated and homogeneously dispersed in the films. Such a conclusion is consistent with results both from the X-ray diffraction patterns of much thicker films of these composite systems and from positive hole drift mobility measurements of the films. Further, the present study can be regarded as the characterization of the molecular dispersion in the composite films, which is difficult to determine by direct structural analysis alone, through the observation of their electronic structures.

Acknowledgement

The authors are grateful to Professor Kazuhiko Seki for useful discussion, to Professor Hiroo Inokuchi for

his interest and encouragement, and to the Nissha Aid for Academic Research for financial support to NS.

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Documents

UV photoemission spectroscopic studies on molecular aggregation of triphenylamines dispersed in polycarbonate thin films