Photogeneration in conducting polymers

Synthetic

ELSEVIER

Metals

69 (1995)

Photogeneration

613-619

in Conducting Polymers

E.M. Conwell and H.A. Mizes Wilson Center for Research &Technology, Webster, NY 14580, USA and Center for Photoinduced

Charge Transfer

University of Rochester, Rochester. NY 14627. USA Abstract A number of diverse phenomena, such as the small carrier yield per photon seen in picosecond photoconductivity, the different behavior of picosecond photoinduced absorption, PA, in dilute solutions and thin films, and the spectral distribution of PA in thin films, show that photogenerated positive and negative polarons in conducting polymers are for the most part bound in pairs on adjacent chains by their Coulomb attraction. The PA also gives additional evidencethatthe binding energy of the singlet exciton in poly (phenylene vinylene) is 0.4 eV, leading to a gap of 2.8 eV. 1. INTRODUCTION Much effort

After reviewing some of the experimental

has been devoted

to determining

the

nature of the excitations that are photogenerated in conducting polymers [l]. The most successful techniques have been picosecond photoinduced photoconductivity

absorption

PC. It was predicted

electric vector perpendicular

PA and

that light with

to the chains could create

electrons and holes on separate chains [2,3], which would then evolve into independent

or free polarons.

be shown in the next section, there PA and

PC that

As will

is a great deal of

evidence

from

generated

by only a small fraction of the photons.

free

polarons

are It has

for polaron

pairs, we summarize

evidence

our calculations

for

polaron pairs in poly (phenylene vinylene), PPV. Then we will show that polaron pairs can account for the ps PA data on PPV, transpolyacetylene, KH),, and polythiophene, PT. Because these materials are typical conducting polymers, we believe that photogeneration results principally in polaron pairs rather than free polarons in all of this group of conducting polymers. Finally we will suggest what the role of polaron pairs is in other phenomena poiymen.

that

have

been

studied

in these

been suggested that most of the polarons form pairs [4], or charge transfer complexes (51, bound by their

2. EVIDENCE AGAINST GENERATION OF FREE POLARONS

Coulomb attraction.

2.1 Photoconductivity

Photoinduced

charge transfer between

a pair of like

or unlike organic molecules to form a complex that is stable in the excited state, although not in the ground state, has been studied extensively by chemists. Such charge transfer complexes have been found between a

It has been established, by means of PA, that for electric vector of the light parallel to the chains, El, the excitations generated in (CH), are soliton pairs with a lifetime -1 ps (61. For E vector perpendicular to the chains,

El,

it was found

that

the

majority

of the

pair of molecules, e.g. phenyls, that happen to be parallel and at the right distance even though they are parts of different, not totally aligned, polymer chains. It

excitations are still soliton pairs with -1

should be noted that, even for photons with energy well

spectrum of these excitations was found to differ

beyond the absorption edg’e, a photogenerated electron and hole are much less likely to completely separate in a conducting polymer than in a three-dimensional semiconductor like Si or Ga As, for example. This is so because the conducting polymer is essentially an assembly of separate oligomers, the dielectric constant transverse to the chains is smaller by - a factor S and the carriers can more easily lose excess energy because they

that

are strongly modes.

carriers and the measured PC was a few cm*/Vs, in agreement with the ps mobility deduced from the decay

coupled

0379-6779/95/$09.50

0

to many,

1995 Elsevier

SSDI 0379-6779(94)02593-N

higher

Science

energy

optical

S.A. All rights reserved

-40%

ps lifetime,

but

of the excitations give rise to a sizable tail on the

PA which

persists for more than

of the

absorption

solitons in the

in having

red

[6].

a ns [6].

The

substantially

These

excitations

PA

from

greater were

assumed to be free polarons. However this identification can be shown to be inconsistent with the observed ps PC. For El the rapid decay of the photogenerated pairs led to the conclusion that only -1 Ohof the photons create free carriers [7]. This conclusion was reinforced by the fact that the mobility p calculated from this number of free

614

E.M. ConweU, HA. Mizes f Synthetic Metals 69 (1995) 613419

of the light-induced dichroism (8) and with the calculated mobility of drifting polarons and solitons [9]. For El, if 40°h of the excitations were free, long-lived polarons, the PC should be very much greater than that resulting from El. However, the PC for El was found to be only 1.7 times as large as that for El [lO,ll]. It is clear that the excitations associated with the long PA tail are not free polarons. For PPV, PT and other nondegenerate polymers the carriers of ps photocurrent are expected to be free polarons. Formation of bipolarons, which requires the meeting of two polarons of like sign, should take longer than picoseconds. Note also that in more recent, and presumably more ordered, PPV samples bipolarons, which were seen in earlier samples on a millisecond time frame, are no longer seen (121. For oriented

samples of PPV it was also found that

the ps photocurrent was almost independent of the direction of polarization of the light source relative to the chain direction [13], even though PA was larger by a factor 3 for light polarized perpendicular to the chains (41. With the peak photocurrent

comparable

to that in

2.2 Photoinduced

in

Absorption

Only 10 to ZOoAof the ps photogenerated PPV and its derivatives are excitons

excitations [21]. PA

experiments performed to identify the remaining 80 to 90% of the excitations showed, as in (CH),, a long-lived tail, lasting nanoseconds 14,211. In poly (2-methoxy-I,4 phenylene vinylene), M-PPV, over the range investigated the PA consists of two broad absorptions, peaked at -0.5 eV and -1.5 eV, the latter containing a weak secondary peak at 2.15 eV [41. For PPV the two main peaks are at about the same locations but the secondary peak is shifted to 2.3 eV [22]. The dynamics of the peaks are the same. As discussed at length in Refs. 4 and 23, the data eliminate the possibilities that the excitations giving rise to the long-lived PA are triplet excitons, singlet excitons, independent polarons, bipolarons or biexcitons. Hsu et al suggested that the excitations are polaron pairs on adjacent chains bound bytheir Coulomb attraction [4]. A similar long lived PA is seen in PT [24] and its derivative poly (octylthiophene), P30T. For P30T the PA peaks at 1.2 eV, with a secondary peak at 1.9 eV, just below the absorption edge at -2.0 eV 1251. No PA data were obtained in the infrared for PTor P3OT.

(CH), and the carrier drift mobility also expected to be comparable, -1%

it was concluded for this case also that only

of the photons absorbed create free carriers [ 131. A

similar

conclusion

can be reached

from

PC data

for

derivatives of PT [14].

More direct evidence that the excitations giving rise to the PA involve more than one chain is provided by the cases where soluble forms of the polymers are available. A soluble form hexyloxy)-p

Comparison that

of photoluminescence

of PPV oligomers

of PPV films to

led to the conclusion that

the

of PPV is poly (2-methoxy-5-(2’-ethyl-

phenylene

soluble form is P30T. long-lived

vinylene),

MEH-PPV.

For PT a

For both MEH-PPV and P3OT the

PA was found in thin films but not in dilute

singlet exciton in the films is confined to a single chain

solutions [25,261.

and -6 or 7 monomers, or -4OA. in length [15]. The observation that the threshold for ps PC in PPV coincides with the absorption edge led one group to conclude that photons with energy above the absorption edge produce

the PA dynamics match those of the photoluminescence,

free electrons and holes, i.e. the binding energy of the exciton is skT at 300 K [16]. This hardly seems a necessary conclusion in light of the fact that only -lo/b of the photons create free carriers.

It is not unreasonable

that a few % of excitons could be dissociated

by the

surface or by defects, in the same way that C60 doping causes exciton dissociation in PPV (171. Calculation of the exciton binding energy in PPV gives 0.4 eV iI81 and this value has also been obtained from measurement of the From optical photovoltaic response of PPV 1191. measurements Leng et al have deduced an exciton binding energy of 1.1 eV [20]. We will show later that the observed PA gives additional energy of 0.4 eV.

evidence for a binding

In the case of the MEH-PPV solution

indicating that the PA is due to singlet excitons (261. In MEH-PPV thin films, however, the photoluminescence shows different, over all more rapid, dynamics than the PA, which displays the long plateau-like tail seen in the other

PPV’s 1261.

Similar

behavior

was found

for

P3OT[25]. 3. CALCULATIONS FOR POLARON PAIRS TO calculate the properties of polaron pairs in KH), we used the SSH Hamiltonain (271, which has been used extensively to describe polarons in that material. For studying polaron pairs in PPV we first need a Hamiltonian suitable for describing single pOlarOnS in that material. We set up a tight-binding Hamiltonian in the spirit of the SSH Hamiltonian,

specifically (281

EM

E

Hz

m-1

+

Cl-( t

0



Conwell,

HA.

polyacetylene, -au

)c+

615

A&es / Synthetic Metals 69 (1995) 613619

cj I

portion,

z

the

lo%,

interchain

coupling

of the polaron

resulted

in a

being shifted to the

coupled chain 1321.

+au -c I2+ H.C.I.

(1)

To treat the case of a polaron pair on adjacent chains we added to Equation (1) the term giving the potential at

Here M is the number of monomers in the chain and indicates one of the pairs of nearest neighbors in the monomer. The sum is taken over all 9 pairs. te is the

v =i

c

eej lel

[(d,

)i

1: I:,)

+k, /cl )(dl

j

electronic coupling or transfer integral between neighboring n orbitals and cij> is the change in length of the bond, referred to an imagined initial state with all bonds equal in length. a is the ratio between electronic coupling change and bond length change and K is the effective spring constant, due to the o bonds. C is a stiffness constant adjusted to give the correct chain The length in a self-consistent calculation [ZSJ.

where the summation

parameters to, a and K were chosen so that calculations with Equation (1) gave values within 1% for 3 quantities:

i and j determined

(1) the difference between single bond length, 1.474 A, and double bond length, 1.355A, of the vinyl group as determined by MNDO; (2) the valence band width, 5.47

The resulting n and n* energy levels for the situation that the polarons are stationary and exactly opposite each other are shown schematically in Fig. 1. It is seen that the levels one the chain with P-are pulled down due

eV, obtained by Local Density Functional theory (301, (3) the energy gap, 2.8 eV 1181. The values that satisfy the criterion just given are to = 2.66 eV, a = 10.29 eVtA and K = 99.0 eV/Az. polyacetylene

Larger values than those familiar from

(271, were expected because the band gap

is twice as large, and the vinyl dimerization

40°h larger.

is over all the sites on the other

chain, ej is the charge on the jth site (determined

self-

consistently), (d$+l and (dl)‘ij the parallel and perpendicular components, respectively, of the distance between sites i and j, ~1 and EL the dielectric constants parallel and perpendicular to the chain direction, taken as 8 [321 and 3, respectively. Interchain coupling was also included, with the coupling (tl& between the atoms as described earlier.

to the attraction of P+. The polaron levels are pulled down more than the conduction or valence band levels because their wave functions are localized. distance

of the top

P- level from

the

The resulting band

edge

is

denoted by 0. The lower P level has moved into the valence band. On the chain with P+ levels are moved up

with the calculation From a self-consistent Hamiltonian (1) and the parameters just specified, the

due to the repulsion of P . The upper P+ level has moved into the conduction band, and the lower P+ level is

energy interval between

energy @ above the valence band.

the polaron level and the next

The introduction

of

to be 0.19 eV. It is larger, of course, for short chains [ZSJ.

interchain coupling was found to have little effect on the energy levels because tl is small [33]. The spacing

The half-width

between

level on a long chain of PPV (2 20 monomers) was found of the polaron is - 4 monomers or - 26A.

Significantly, a calculation of the polaron geometry using the unrestricted in good additional

Hartreee-Fock

agreement

with

PM3 method gave results This agreement is

ours.

the P- and

P+ levels in the gap is denoted

by

0.0 and 0, as well as E,, are dependent on chain length, the dependence of the latter two, shown in Ref. 33, being much larger,

evidence for the validity of our Hamiltonian.

Determining whether the polaron is stable in the presence of interchain interactions is more complicated than for (CH), because the interchain distance is different

Using Equations (1) and (2) we have also calculated the formation

energy for

a polaron pair in PPV. The total

energy of a chain is the sum of the lattice energy and the electronic energy. The formation energy was taken as

for every C atom in the monomer. To obtain an order of magnitude approximation, we took the total interchain

the difference

coupling for a monomer

consistent minimum energy of two coupled PPV chains in

from the interchain-coupling-

induced splitting of the bands at the band edge (301, and divided this up among the pairs of atoms according to their distance apart (281. The overall coupling is smaller than in polyacetylene and the polaron in PPV was found However, as in to be stable under this coupling.

between

two total

energies:

the self-

their ground state and the self-consistent minimum energy of two coupled PPV chains with a polaron pair. The resulting formation energy as a function of chain length is shown in Fig. 2. Shown for comparison are the S&O-O

transition energies measured for PPV oligomers,

616

which

E.M.

should

exciton.

represent

the

Even with allowance

Conwell, HA.

formation

energy

Mizes I Synthetic Metals 69 (1995) 613619

of an

for the possibility that the

calculations are uncertain by a few tenths of an eV it is clear that polaron pair formation formation,

4. COMPARISON WITH EXPERIMENTAL PA 4.1 PPV and PT

is favored over exciton

at least for chains less than 7 monomers long. a,@

The three possible photoinduced absorptions are and @ of Fig. 1. For PPV with chains 7 monomers

long, generally considered an average chain length for that case, we calculate @ = 0.45 eV, @ = 1.41 eV and @I = 2.35 eV. It is seen that these three values are within 0.1 eV of the three PA peaks found in PPV, as enumerated in section 2.2, thus in excellent agreement, perhaps better than could be expected for peak 0. Experimentally the PA is found to be strongly polarized along the polymer chain direction [4]. It is apparent that this condition will be satisfied for peaks @ and 0. The condition is also satisfied for peak @I because, by symmetry, the wavefunction must be a superposition of the situation shown in Fig. I, with P on the left chain,

Eg

P+ on the right, and the situation with P+ on the left chain, P- on the right. The net current corresponding to a transition must then vanish perpendicular to the chains.

ne ative ca ain

We emphasize that, as is apparent in Fig. 1, the sum of transitions @ and @ must equal E,. The fact that the experimental values for these transitions are 0.5 and 2.3

p;oszii;e

eV. resoectivelv. addina evidence Fig. 1 Energy levels and optical transitions for a polaron

UP to 2.8 eV, is independent

for a aao of 2.8 eV and an exciton bindinq

enerov of 0.4 eV.

pair (schematic)

For M-PPV the absorption

edge is at 2.2 eV rather

than 2.4 eV. If we take the shift in the absorption edge as the shift in the gap (i.e., assume the exciton binding energy is the same in M-PPV as in PPV) E, for M-PPV is 2.6 eV.

It is also reasonable

binding

to assume that the polaron

energy in the presence of the polaron of the

opposite sign, i.e., the energy 0,

is the same for M-PPV

as for PPV. We then find that transition

@ for M-PPV is

shifted down by 0.2 eV to 2.15 eV, where in fact it is seen experimentally

141. Note that there is not necessarily a

corresponding

shift downward

likely that the absorption edge

for transition

0.

is determined’by

than average chain lengths while the peak at 2

3

5

Number

10

20

30

60

of monomers

oligomers, measured by R. Mahrt, J.- P. Yang, A. Greiner, H. BIssler and D.D.D. C. Bradley., Makromol. Chem. Rapid formation

Note that the larger polaron

energy for the 60 monomer

not be real because the self-consistent case had not completely converged.

1.5 eV

should be determined by the most likely length of the charge transfer complex, which is probably shorter. In P30T the observed long-lived PA extends from 1 .l to 2.0 eV with a maximum at - 1.2 eV and a smaller

Fig. 2 Formation energy vs chain length for: 0 polaron pairs in PPV, calculated as described; A excitons on PPV

Commun. 11 (1990) 415.

It is longer

chain length may calculation for this

peak at 1.9 eV, just below the absorption edge at - 2 eV 1241. Somewhat similar data have been obtained for PT 1251. In principle, a Hamiltonian of the type (1) could be set up for PT, but unfortunately

calculations of the band

structure and the exciton binding energy available to guide the choice of parameters.

are not We can,

however, make reasonable estimates of where the peaks should fall from the following considerations. First, it is reasonable that the energy @ changes little when E,

E.M. Conwell. HA

617

Mizes I Synthetic Metals 69 (1995) 613-619

changes, provided the chain length is kept the same. In

relative

support of this we note that the distance of the polaron

independent

level from the nearest band edge calculated for long chains of PPV with the Hamiltonian (1) is 0.19 eV [28],

interchain distance and dielectric constants.

agreeing closely with 0.2 eV obtained for a long chain of (CH), [34], which has E, = 1.4 eV. For poly (3 hexyl thiophene), P3HT, the polaron-to-band energy has been measured as 0.4 eV by observing the optical absorption due to electrons and holes injected at the contacts j351. But it must be remembered that 0.4 eV does not apply to the polaron level in the long chain limit, but rather to

to

those of

on the P+ chain are essentially being determined primarily by

E,,

Measurements

of ps PA in (CH), are available only in

the range 0.3 to 0.5 eV [6].

At 0.3 eV PA is low and

decreasing with decreasing energy; it shows no evidence for a peak in the neighborhood of 0.1 eV. The PA is continuous

in the range 0.3 to 0.5 eV but, unlike PPV,

shows two local maxima in this range, at 0.35 eV and 0.45 eV (61, the latter being somewhat higher. Around these

that for the average conjugation length in the sample used, For PPV the polaron level is 0.4 eV from the band

maxima the PA is roughly symmetric over a few hundredths of an eV. Calculation of @ as a function of

edge

for a conjugation length between 6 and 7 monomers, thus - 40A (281. This should be in the neighborhood of the conjugation length for the PT sample used for the measurements. The quantity @ includes a small additional shift of the polaron level from

chain length in (CH), gives the result that it ranges from 0.36 to 0.38 eV for chains from 200 sites down to - 100, and then increases with decreasing chain length to 0.6 eV

the band edge due to the Coulomb effect of the oppositely charged polaron. We expect this shift in PT to be similar to that for PPV because it depends essentially

bimodal Gaussian distribution of conjugation lengths, centered at chains of 30 CH’s and 200 CH’s with standard deviations 50% of the peak value 1361. According to our

on the interchain distance and dielectric constants, which are quite similar for the different polymers in this group.

calculations, we can assign the local maximum at 0.35 eV to conjugation lengths from 200 down to - 100 sites. The maximum at 0.45 eV could be obtained with a Gaussian centered about 45 sites, larger than the 30 sites

if, on the strength of these arguments, we take the energy @ the same for P30T as for PPV, and the difference

in energy gaps as the difference

in absorption

at 30 sites. It is noteworthy that absorption and resonant Raman spectra in (CH), have been well fitted using a

edges of PPV and P3OT, 0.4 eV, the upper peak in P3OT

perhaps because the quality of (CH), samples had improved in the 6 years between the two experiments.

would be shifted down relative to that in PPV 0.4 eV, to

We can predict on the basis of our model that additional

1.95 eV. This is in excellent agreement with 1.9 eV at which the peak is found [24]. The agreement suggests

ps absorption

that the exciton binding

energy in PT is also -

should be found

corresponding to the transition

in (CH), around

1 eV,

0.

0.4 eV.

Making a similar shift downward for the - 1.5 eV peak in PPV we find the other peak in the visible for P3OT at 1.1

The above discussion has dealt mainly with the peaks in the PA, which arise from the polarons being exactly

eV, quite close to the observed value of .1.2 eV. A third peak should be found at - 0.5 eV when PA in this

opposite each other.

spectral region is explored.

polaron in its potential absorption

4.2 PA in (CH),

In an earlier paper, treating

PA in

PPV 1331, we showed that the zero point motion of each well causes a broadening

frequencies

comparable

spread in PA. Additional contributions of different

to the

of the

observed

broadening comes from the conjugation lengths, whose

Calculations of the energy levels due to a polaron pair in (CH), were carried out using the SSH Hamiltonian

also another

and the parameters ~1 = 11.0, Ed = 3.0, C = 1.266 o/K, with other parameters as in Ref. 27. For a chain length of

same chain but separated by a conjugation defect, e.g., an sp3. Due to the polaron width being - 4 monomers

100 CH’s we found the energy @ equal to 0.36 eV and @ equal to 0.1 eV. The small spacing between the P and P+ levels compared to PPV or PT is due to the smaller gap

the

of (Cl-i),. In fact @ is very close to the value that would be obtained by a rigid shift downward of 0 for PPV, 1.4 eV, by the difference in band gaps of PPV and PA, 2.8 1.4 eV. Thus the calculations for (CH), provide additional justification for our assumption that the energy 0 and

short parallel sections of the polymer chains for formation of the polaron pairs. For PPV we have found that even with a chain length of only 2 monomers the energy @I is within 0.3 eV of its value for 7 monomer chains and longer. Nevertheless single polarons can be

the downward

,generated,

shift of the energy levels on the P- chain

peaks are at different

energies.

It is necessary to consider

type of pair, where

Coulomb

effects

P+ and P- are on the

in this case are much

smaller,

allowing transitions much closer to those due to a single polaron. We note also that the theory requires only

for example

in situations where disorder or

618

EM.

defects result in there suitable to accommodate

Conwell, HA.

Mizes / Synthetic Metal

not being an adjacent chain the poiaron of opposite sign.

5. OTHER EFFECTSOF POLARON PAIRS Additional

consequences of the high quantum

of long-lived P+ P pairs can be expected. the

long tail

on the

yield

One of these is

photolimunescence,

which

lasts

69 (1995) 613419

absorption,

and the

small carrier

yield and

lack of

anisotropy in the photoconductivity, found in PPV, PT and (CH), can be ascribed to a photogenerated charge transfer state or polaron pairs. More generally, the state may be an excimer due to the admixture of a small amount of the exciton wavefunction which is quite close in energy 1391; it is not possible to determine this until

to the chain with the other polaron they may recombine non-radiatively. However there is also a chance for an

more accurate single polarons account for ps polaron pairs

exciton to be formed, with the subsequent probability of radiating within ps as is found for excitons directly photogenerated. That this process is not negligible is

predominant process in this group of conducting polymers. The creation of polaron pairs can account alS0 for other phenomena, such as the dc electric field and

demonstrated

magnetic field dependence of the steady state PC. Finally, we have shown that the experimental finding of

nanoseconds (261. When one polaron makes a transition

by the fact that PPV light emitting diodes

work, based as they are on a pair of polarons formed by injected carries coming together as an exciton and recombining radiatively.

calculations are done. Although some are undoubtedly directly generated to PC, we believe that photogeneration of rather than single polarons is the

PA peaks at 0.5 and 2.3 eV is independent evidence for a PPV gap of 2.8 eV and an exciton binding energy of 0.4 eV.

Another

effect is field-enhanced

PC. The large ps

transient PC observed in PPV [ 161 is field independent

up

to fields of 2x104 V/cm and is undoubtedly due to photocarriers directly generated by photons. This does not, however,

as suggested

in Ref. 16, eliminate

the

possibility of PC at longer times, or steady state PC, being greatly enhanced by a large electric field. This effect,

REFERENCES 1. A.J. Heeger et al, Rev. Mod. Phys. 60(1988)

781

2. D. Baeriswyl and K. Maki, Phys. Rev. 838 (1988) 135 3. P.L. Danielsen, Synth. Met.a(l987) 125 4. J.W.P. Hsu et al. Phys. Rev. B49(1994) 712 5. E.M. Conwell and H.A. Mizes, submitted publication

observed by Gailberger and Blssler [37], was attributed by them to excitons being dissociated by the field. However, exciton lifetime in PPV is short, typically - 20

6. L. Rothberg et al., Phys. Rev. Lett. 65 (1990) 100

ps, and the number created only 10 to 20% of the excitations (41. It is more likely that the increased PC is

7. M. Sinclair, D. Moses and A.J. Heeger, Commun. 59(1986) 343

due to the field dissociating P+ P- pairs, which are created in much greater abundance and live to ns. The pairs most easily dissociated by the field could be those created

on the same chain with

separating them.

a conjugation

defect

It is possible that such pairs account for energy of - 0.1 eV seen in the ps

the activation [ 161 and the steady state photocurrent photocurrent

[37]. A polaron that is one of a pair opposite each other on adjacent long chains is, according to the calculations described earlier, in a potential well 0.3 eV deep. Polaron

pairs may also account for the

magnetic

for

Solid State

8. Z. Vardeny et al., Phys. Rev. Lett. 49 (1982) 1657 9. 5. Jeyadev and E.M. Conwell, 5917

Phys. Rev. 8%

(1987)

10. S.D. Phillips and A.J. Heeger, Phys. Rev. 8% 6211

(1988)

11. 12. 13. 14.

H. Bleier et al., Europhys. Let-t. s(l987) 1397 D.A. Halliday et al., Adv. Mater. s (1993) 40 D.D.C. Bradelyet al., J.Phys. C2l(l986) L515 G.Yu et al., Phys. Rev. 642(1990) 3004

15. S.C. Graham et al., Synth Met.4l(l991) 1277 16. C.H. Lee et al., Phys Rev. B49(1994) 2396 17. C.H. Lee et al., Phys Rev. B48(1993) 15,245

field effect on PC observed in PPV 1381 and other polymers. jn fact, Frankevich et al suggested that the

18. P. Comes da Costa and E.M. Conwell., Phys. Rev. Bs (1993) 1993

effect is due to polaron pain, but they attributed the creation of the polaron pairs to dissociation of Frenkel

19. R.N. Marks et al., J.Phys.; Condens, Matter 5 (1994) 1379

excitons.

20. J.M. Leng et al., Phys. Rev. Lett. z1. (1994) 156 21. M. Yan etal., Phys Rev. Lett.72(1994) 1104 22. M.Yan., thesis (CUNY) 23. H.A. Mizes and E.M. Conwell., submitted publication

6. CONCLUSIONS We conclude that the long-lived

ps photoinduced

for

EM.

Conwell, HA

Mizes I Synthetic Metals 69 (1995) 613-619

24. B.Kraabel, Mol. Cryst. Liq. Cryst., in press 25. G.S. Kanner et al., Phys. Rev. Lett. 69(1992)

538

26.

AT&T

M. Yan. L.J. Rothberg and T.M.

Miller,

Bell

Labs, private communication 27. W.P. Su, R. Schrieffer and A.J. Heeger., Phys. Rev. 82 (1980) 2099 28.

H.A. Mizes and E.M. Conwell., Mol. Cryst. Liq. Cryst.,

in press 29. SStafstrOm

and K.A. Chao., Phys. Rev. 8%

(1984)

2098 30.

P.Gomes da Costa, R.Dandrea and E.M. Conwell.,

Phys. Rev. BQ (1993) 1800 31. H.A. Mizes and E.M. Conwell,

Phys. Rev. Lett. 70

(1993) 1505 32. J.Fink et al in “Electronic Properties of Conjugated Polymers.” eds. H.Kuzmany, M. Mehring and 5. Roth., Springer Series in Solid State Sciences, Vol. 76 (Berlin, 1987) p. 79 33. E.M. Conwell and H.A. Mizes., Mole. Cryst. Liq. Cryst., in press 34. D.K. Campbell and A.R. Bishop., Phys. Rev. B24(1981) 4859 35. K.E. Ziemelis et al., Phys. Rev. Let-t. 66(1991)

2231

36. G.P. Brivio and E.Mulazzi, Phys. Rev. B30(1984) 37. M. Gailberger and H. Bassler., Phys. Rev. 8% 8643 38. E.L. Frankevich et al., Phys. Rev, 8% l.Sokolik et al., Proc. SPIE m(1993) 171

876 (1991)

(1992) 9320;

39. See, for example, J.Michl and V. Bonacic-Koutecky, “Electronic Aspects of Organic Photochemistry” (Wiley, New York, 1990)

619