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Time resolution of ion pair formation in poly(N-vinylcarbazole)
Chemical Physics 178 ( 1993) 415-422 North-Holland
Time resolution of ion pair formation in poly
(N-vinylcarbazole
)
S. Neiplirek Institute of Macromolecular Chemistry Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic
Received 5 April 1993; in final form 9 August 1993
The laser-flash-induced transient optical absorption of poly(N-vinylcarbazole) thin films has been investigated. The rise time of the ion pair formation was estimated to be about 40 ns, their lifetime is longer than 10 us. The ion pair stability is probably due to self-trapping process and dimer radical cation formation.
1. Introduction
Photogeneration of charge carriers in polymers has been intensively studied for a long time, in poly (Nvinylcarbazole ) [ l-3 1, poly (diacetylene ) s [ 4,5 1, polyacetylene [ 6,7], poly(phenylacetylene) [ 8,9], and in other polymers (see the reviews [ lo- 12 ] ) , as well as in molecularly doped polymers [ 13,141. Whereas the charge carrier photogeneration in polyacetylene, poly ( phenylacetylene ) and poly ( diacetylene)s is a typical intrinsic process, the photogeneration in poly (N-vinylcarbazole ) and molecularly doped systems exhibits prevalently extrinsic behaviour. In the extrinsic case there are several possibilities of the formation of excited species (extimers and exciplexes), capable of participating in free carrier formation. Impurities or guest molecules, namely oxygen, may participate in the mechanism. Very often free charge carriers are generated via nonrelaxed exciplexes formed between singlet excitons and acceptor-like impurities or photooxidation products. The following mechanism was proposed [ 15 ] (see fig. 1) by Yokoyma and co-workers: D*+A+(D*...A)+(D’+...A’-)** 1 + D’+
.?.A’-:
D’+
+A’-
.
Here, (D*...A) represents an encounter complex, (D’+...A--)** a nonrelaxed exciplex state, De+.?. A’-)
an ion pair with an interionic distance r, and D’+ f A’- free charge carriers. Process 1 corresponds to thermalization and process 2 to dissociation. Thus, charge carrier photogeneration in poly (N-vinylcarbazole) (PVCA) can be depicted as a multistage process which involves: photoexcitation to a neutral molecular electronic state and the formation of a nonrelaxed exciplex state, ionization and thermalization of the excited state, leading to the formation of a Coulomb-field-bound geminate ion pair (chargetransfer state), and thermal dissociation of the pair into free carriers by Brownian motion subject to a combination of Coulombic and applied fields. Although this model represents a satisfactory description of the charge photogeneration in PVCA, some experimental data obtained recently need further discussion. Namely, while in molecular crystals or organic liquids the geminate recombination or a Coulombically bound geminate electron-hole pair (ion pair) is a diffusive process, completed within typically 10 to 100 ps [ 16,17 1, a delayed field collection experiment with pure PVCA film made by Mort et al. [ 18 ] revealed a ion pair lifetime of 100 s. Dynamic aspects of the ionic species such as exciplex, ion pair, and free ions (hole and electron) in polymer films are still beyond our knowledge. Recently an information was published concerning exciplex dynamics in PVCA films [ 193. Because fluorescence spectra showed the time-dependent shift, it was concluded that multiple exciplex and exterplex species, with various relative configurations, be-
0301-0104/93/% 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.
S. NeSptirek/Chemical Physics178 (1993) 415-422
416
D*+ A-
(D’...A)
-
ENCOUNTER CCMPLEX
THERMALIZATION
(Dt.A’)”
NONRELAXED EXCIPLEX STATE
t
-
( DxbT)
(1 )
ION PAIR
D?+A +e‘ DttA:
//ELD
p(F) Dt+
&m%l I
\
/
D*+A (D:,&)**__-__ -+ I I-
l-P(F)
(2) A7
CHARGE CARRIERS
(Dt AT)’ RELAXED FLUORESCENT EXCIPLEX STATE
1 (DT
o+A+
A:)_
hu,
* FLUCRkCENcE
D+A
j A
1 /
DA
Ll ___-_
Do rDA
r0 Separation
drstorce, r
Fig. 1. Schematic picture of the charge carrier photogeneration in poly(N-vinylcarbazole). Thermalization ( 1 ), dissociation (2). D: donor, A: acceptor, e-: free electron, hv: excitation energy, hur: energy of the fluorescent light, rDA:distance between donor and electron, r,,: thermalization distance of the ion pair.
tween donor carbazole and acceptor dopant were important. An unusual slow and nonexponential decay of exciplex fluorescence in the solid state, which is longer than 100 ns (typical exciplex lifetime in solution), with a long tail up to several microseconds and its dependence upon an applied electric field suggested that the geminate recombination of the ion pairs determined the exciplex fluorescence. From the optical measurements of PVCA doped with acceptor molecules it was concluded that the ionic absorption bands are mainly assigned to ion pairs or free ions (hole and electron) [ 201. This typical behaviour of solid PVCA films needs further detailed study. To obtain more information about long-lived species, laser-flash-induced transient optical absorption of thin films of PVCA was studied. If ionic species are formed during the excitation, one must expect photoinduced transient absorption with red shift caused by the interaction of excitons with electron-hole pairs, arising mainly from the increase of the molecular polarizability with electronic excitation. The results of this studies are presented in this work.
2. Experimental PVCA was obtained by radiation polymerization and purified by multiple precipitation from benzene solution into methanol. Films (thickness about 1 pm) were prepared from cyclohexanone-benzene solution by spin-coating on glass slides. The samples were irradiated with 15 ns flashes of 266 nm light produced by a Nd: YAG laser equipped with two frequency doublers. Optical absorption changes were measured in perpendicular direction using a 450 W (steady-state regime) xenon lamp, monochromator and a photomultiplier operated with five dynodes. During the measurement of the photoinduced transient absorption (0.5 ms) the power of the monitoring lamp was increased up to 40 kW to obtain higher light intensity of the monitoring light. The films were oriented at an angle of 45’ to both light beams.
3. Results Fig. 2a shows kinetic traces illustrating the forma-
S. Nefptirek/ ChemicalPhysics178 (1993) 415-422
I
417
I (Cl
(dl
Fig. 2. Kinetic traces of the transient absorption of the PVCA film recorded at 354 nm (A,=266 nm). Fresh sample; time scale 50 ns/ division (a), 200 ns/division (b), 1 ps/division (c). Sample kept during 3 months in air prior to irradiation; time scale 1 ps/division (d ) . The measure of the division is given in case (b ) .
tion of the transient optical absorption induced at IzCXC = 266 nm (excitation to the third singlet state) in a freshly prepared PVCA film measured (&= 354 nm) in vacua. Within the error limit, the half-life of the formation of the new absorption is x40 ns. Measuring on the 200 ns and 1 us/division time scale (figs. 2b and 2c), no decrease in the transient absorption was observed up to t= 10 l,tsfor the fresh sample. The dependence of the transient absorption at Iz&,s=354 nm recorded at 1.5 ~.tsafter excitation on the exposure dose (A,,,= 266 nm) is shown in fig. 3. The dependence is linear up to ~6 mJ cm-*. For higher power a sublinear dependence was observed. Fig. 4 shows the ground state absorption spectrum of a PVCA film (curve 2) [ 21 and the transient absorption spectrum (curve 1) taken at 1.5 us after excitation at A,,,= 266 nm. The absorbance of the transient absorption was about two orders of magnitude lower than that of the ground state, It is very interesting to compare the shapes of both curves. The induced spectrum has a shape very similar to that of the ground state spectrum of the PVCA film, but the positions of the maxima are shifted by about 255 cm-’ (average value) to the red region. The experi-
mental data are: Ground state absorption: 1= 344, 330 and 296 nm, transient absorption: 1~347, 332 and 299, red shift v= 25 1,183 and 333 cm-‘.
4. Discussion As mentioned above the charge carrier photogeneration in PVCA is an extrinsic process involving at least three steps: photoexcitation, thermalization and dissociation of the ion pairs. These pairs, in dissociation of which an external electric field is effective, are not formed from the relaxed fluorescent exciplex state (D’ +A’- ) *, but from the “nonrelaxed exciplex state” (D’+...A’-)**, which is produced from the “encounter complex” (D*...A) between a migrating singlet exciton D* and an electron acceptor A [ 15 1. The “nonrelaxed exciplex state” arises through a rapid electron transfer at a somewhat larger separation of the components in an encounter complex than in the relaxed fluorescent exciplex state. This process is possible since the carbazole chromophores are located around an acceptor in a high concentration, and D* is mobile from molecule to molecule. A thermal-
S. Nefptirek/ Chemical Physics 78 (1993) 415-422
3 t
-006
-0.06
2
-0.04 i VI 9 - 0.02
I
1t
oLo 200
16
0 Laser
power,
P
(mJ cr?l’1
Fig. 3. Dependence of the transient optical absorption at I= 354 nm, recorded at 1.5 ps after the flash, on the exposure dose. Temperature T= 296 K.
ization of the excess energy of the nonrelaxed exciplex state leads to the formation of the ion pairs with a separation distance r,,, the positive charge (hole) being located at the carbazole unit, and the negative charge (electron) probably at oxygen or impurities. It is known that the main impurities in films prepared from highly purified PVCA are dissolved oxygen, oxidation products and residual solvent molecules [ I]. The oxidation products probably contain carbonyl groups which, because of their high electron affinity [ 221, act as acceptors and may constitute efficient electron traps [ 231. These impurities might efficiently contribute to the photogeneration mechanism in forming strong charge transfer complexes with excited carbazole groups. Oxidation and photodegradation should proceed most rapidly in the surface region and result in an impurity concentration protile. In our samples one oxygen molecule was present per about ten to twenty carbazole units in the bulk of the sample and about one oxygen molecule per six carbazole units on the surface as determined by ESCA measurements. The dissociation of ion pairs into free carriers in
uo
280
320
Wavelength,
u d
360
400
A ( nm )
Fig. 4. Transient optical absorption spectrum of a PVCA film taken 1.5 us after the flash (curve 1). Temperature T=296 K, duration of flash about 15 ns. Curve 2: ground state absorption spectrum of PVCA taken from ref. [ 21 (the absorption coefficient scale was corrected according to ref. [ 2 1 ] ). The absorbance of the transient absorption was about two orders of magnitude lower than that of the ground state.
external electric field (as a competitive process to the geminate recombination) may be described in terms of the Onsager theory [ 24 ] as it follows from literature data [ l-3,25,26] and from fig. 5. According to this model the overall photogeneration efficiency v may be calculated as ~(8 T) =rto j%,
K T&(r) dr ,
(1)
where f(r, 1;; T) is the dissociation probability of bound ions separated by a distance r= 1r I= 1r, - r,, 1, r, and #j, being the position vectors of the negative and positive ion, respectively; g(r) represents the initial spatial distribution of bound pairs and dr is the volume element. The primary quantum effrciency vo, i.e. the number of photogenerated ion pairs per photon, is assumed to be independent of the applied field. Assuming that the distribution is spherically symmetrical and that charges in all pairs are separated by the same distance ro, we have g= (4~:) -‘6( r- ro), where 6 is the &function. For amorphous polymers the &function may be regarded only as a zero-order approximation. The diffusive motion of charges during the thermalization in dis-
S. NeSptirek/ Chemical Physics178 (1993) 415-422
I lo6
I
lo6
lo7 Electric
J 109
fleld,F(Vm-‘1
Fig. 5. Field dependence of the photogeneration effkiency u in PVCA film. The solid lines are calculated using Onsager dissociation theory for the constant distance of bound pairs rp2.9 nm (curve 1) and for distances distributed according to eq. (2) with the parameter cr= 1.6 nm (curve 2). Taken from ref. [26]. The measurements of q were carried out using the technique of emission-limited photoinduced discharge [ 27 1.The surface potential was measured with a rotary electrodynamic electrometer [ 281.
ordered structures leads to the distribution of radii of charge transfer states which can be assumed to be a Gaussian one [ 25,26,29]:
g(r) =
&
exp(-r2/a2) ,
where (31is the dispersion parameter. It may be demonstrated that in this case the initial separation distance of ion pairs, the dissociation of which is the most probable, is field-dependent: at high fields it tends to (Y,while at low fields it becomes approximately 2a 1301. Let us discuss the nature of the transient optical absorption spectrum. From fig. 4 it is seen that this spectrum is a copy of the ground state absorption of the PVCA film shifted into the red region. Thus, an effect suggestive of the Stark effect could be assumed. A possible explanation is a formation of coupled states
419
of molecular excitons with charge carriers [ 3 11. The attraction of excitons to charge carriers arises mainly from the increase in molecular polarizability on electronic excitation, so that the exciton energy decreases in the electric field of the charge or in the field of the ion pair. Under this assumption our optically induced absorption follows the formation of the ion pairs created by the laser flash excitation (the formation of free charge carriers by the dissociation of the ion pairs is weak at an electric field strength equal to zero). No direct spectral evidence concerning the cation radical of carbazole was found with our experimental arrangement probably because of low concentration of photogenerated ion pairs. From laser photolysis experiments with solutions of charge transfer complexes of PVCA [ 32,33 ] is known that absorption of carbazole radical cation is located at x 790 nm and carbazole dimer radical cation between 700 and 760 nm depending on its steric configuration. For the solid films of charge transfer complexes of PVCA, the peak of the transient optical absorption was found at x 750 nm using dynamic attenuated total reflection spectroscopy [ 201. This suggests that the carbazole dimer radical cation is formed in the solid films after the light excitation. Its lifetime was found to be x 3-5 us [ 201. Let us estimate the magnitude of the shift between the ground state and transient optical absorption spectra. In homogeneous condensed media the energy of molecular excitation can be expressed as E= AE+ B, where AE= El-E,,is the difference of two energy terms of an isolated molecule and B is the change in the interaction energy between the excited molecule and the surrounding ones. In molecular crystals the magnitude of B is of the order of 1000 cm-’ and it is just this quantity that is responsible for the red shift of crystal spectra in comparison to the vapour spectra. In the presence of an electric charge the value of B becomes a function of the distance r between the charge and the molecule which is electronicallyexcited,B=Bo+g(r),where&r) isthe change in the molecule-charge interaction energy arising from the excitation of the molecule. If F( r) is the electric field produced by the charge at the point r, then, considering only the dipole polarization of the molecule in the point-dipole approximation, one can express q(r) as
420
v(r) = - (ch-~0)F2(r)12,
S. NeSpiuek/ Chemical Physics178 (1993) 415-422
(3)
where (Y,, and cyo are static polarizabilities of the molecule in the excited and ground states. In most known organic solids ay,> CX~. Hence, the energy p(r) corresponds to attraction of the exciton to the charge. No direct information on the Acr value is available for the carbazole unit. According to ref. [ 2 11, for example, for the lowest singlet excited states of anthracene and tetracene ACY k: 1.5 x 10 -23 cm3. The electric field of the static charge in CGSE unit, is F(r) = t is the electric permittivity, so that e/cr2, -Aae2/2e2r4. Taking our average experip(r) = mental value p(r) = 255 cm-’ and Aa equal to the value for anthracene and tetracene, the distance between the charge (generated by the laser flash) and the molecule excited by the monitoring light can be obtained as 0.44 nm. This value is very close to the distance of two carbazole units at PVCA chains [ 2 11. Because of the strong dependence of q( r) on the distance ( re4) only carbazole units in the nearest neighbourhood of the ion pair are responsible for the discussed effect. It should be pointed out that the same distance (0.4 nm) was obtained from the thermoluminescence experiments [ 34,351. Starting the measurements at a temperature of z 160 K, the activation energy and the frequency factor of the relaxation process are practically constant, being 0.4 eV and lo-as-1, respectively (see fig. 6). The activation energy in the high temperature region is close to the activation energy of the hole drift mobility. Lower values of the frequency factor suggest that the recombination luminescence is controlled by the diffusive migration of the trapped charge carriers. The spectral distribution of the thermoluminescence consists of a luminescence band with a maximum near 2.4 eV [34,35] which coincides with the spectral characteristics of the phosphorescence of excimers. This fact suggests that the excitation process ends in the formation of a triplet state of an excimer and therefore the formation of dimer radical cations can be expected. This fact is in agreement with the result of transient optical absorption on thin solid PVCA films [ 201 mentioned above. Recently it has been found that the long-time geminate recombination behaviour in PVCA is controlled by traps that extend to an energy of about 0.6 eV below the distribution center of the intrinsic hop-
(b)
05
t
I
I
I
I
I
I
I
80 xwl 120 UO 160 180 200 220 ---T(K)
t..,
,I..
0
5 x) -Lg
I
15
Fig. 6. The thermoluminescence glow curve (a), the temperature dependence of mean activation energy (E) (b ) , and the energy distribution of the mean frequency factor (s) and of the trap concentration H(E) (c). Taken from ref. [ 341. The method of fractional thermostimulated luminescence was used [ 35,361.
ping states [ 371. Possible candidates for this are sandwich dimers active in excimer formation. Theoretical estimation of the depth of the traps using the self-consistent polarization field technique [ 38 ] makes 0.4 to 0.5 eV. The formation of dimers represent the reason for the long life-time of ion pairs (longer than 10 us from our experiments and FZ100 s from the collection electric field experiments [ 181 at room temperature). One can simply understand the behaviour in the framework of a self-trapping process. After the formation of the PVCA radical cation, the dimer radical cation is immediately formed due to the molecular dynamics of the side carbazole groups. This process is limited by the thermoactivated torsional vibration of the polymer pendant groups. According to the reference data, dielectric losses corresponding to the relaxation in PVCA occur at 2 10 K (measured at 1 kHz); an activation energy of ~~0.35 eV was observed [ 391. This indicates that the characteristic time of y-relaxations at room temperature is of the order of microseconds, which is the time associated with the self-localization of the excess charge. The observed life-time of the ion pairs (T> 10 ps in this work) is longer than the value presented for the solid charge-transfer complexes [ 20](3-5 ps for PVCA- 1,Cdicyanobenzene, PVCA-phthalic anhydride and PVCA-1,2,4,5-tetracyanobenzene). It
S. Neiptirek/ Chemical PhysicsI78 (1993) 415-422
could be pointed out that the shape of the kinetic traces of the transient absorption depended on the history of the sample. For samples stored 3 months in air and measured in air, a decrease in the transient absorption at longer times was observed (cf. fig. 2d). As follows from fig. 7, the decay is a first-order process (D= Doexp( - t/T), where D is the absorbance and t is time) with the lifetime 7= 4 p. The temperature dependence of the decay of the transient absorption is very weak. The activation energy EA is estimated to be less than 0.05 eV. The formation of the charge transfer complex with adsorbed oxygen which acts here as acceptor center thus makes the lifetime of ion pairs shorter but, on the other hand, the number of ion pairs (Q,) increases; thus, the acceptor doping increases quantum efficiency of the charge carrier photogeneration in PVCA. With regards to the facts mentioned above, the extrinsic mechanism of the charge carrier photogeneration in PVCA should be modified as follows D*+A+D+D’+...A’-)*D
orD’+...D’-+A
4D’+D?.A.-+D.++A.-+D, where D’+D is the dimer radical cation. The reality of the long lifetime of the ion pairs is reflected in the theoretical predictions of the timedependent Onsager model [ 401. The physical basis of this model is that ifan electric field can be applied on a time scale comparable to the geminate recombination time, then time-dependent photogeneration
421
would be observable. In this case the distribution function g(r, At) is time-dependent. The time evolution of g( r, At) in zero field can be obtained analytically from the classic Smoluchowski equation. Typical curves for the quantum efficiency at various field strengths, time delays, and thermalization distances r. are given in ref. [ 4 11. This model was used by Mort et al. [ 18 ] to fit the delayed-collection-field data. These authors obtained the values of the diffusion coefficient d- 10 -20 m2 s- ’ for T= 293 K and d-10-18m2s-1forT=373KThesesmaIlvaluesof the diffusion coefficient mean that in applied zero field the charge carriers are essentially immobile. The value is close to the “Anderson limit” [40], where the disorder in the system precludes any diffusion at all.
5. Conclusion The optically induced transient absorption between 280 and 380 nm in thin films of poly(N-vinylcarbazole) has been investigated. The absorbance of the transient absorption is about two orders of magnitude lower than the optical density of the ground state absorption of the film. The shape of the transient spectrum resembles very closely that of the ground state spectrum, but the positions of the maxima are shifted by about 255 cm-’ to the red region. A possible explanation is the formation of coupled states of molecular excitons with charge carriers. The formation time of bound ion pairs is about 40 ns, their lifetime is longer than 10 ps. The long lifetime is assumed to be due to dimer radical cation formation and self-trapping process.
Acknowledgement
0123456
7 Time,
0
9
The experiments were performed in Hahn-Meitner-Institut, Berlin. The authors wishes to acknowledge the financial support of the Kemforschungszentrum Karlsruhe and helpful discussions with Professor W. Schnabel.
t (ps)
Fig. 7. Dependence of the transient optical absorption on time for sample kept 3 months in air prior to irradiation. Replotted from fig. 2d.
References [ 1] G. Ptister and D.J. Williams, J. Chem. Phys. 6 ( 1974) 2416.
422
S. Nefpiuek / ChemicalPhysics178 (1993) 415-422
[2] P.M. Borsenberger andA.1. Ateya, J. Appl. Phys. 49 ( 1978) 4035. [ 31 P.J. Regensburger, Photochem. Photobiol. 8 (1968) 429. [ 41 V. Seiferheld, B. Ries and H. Biissler, J. Phys. C 16 ( 1983) 5189. [ 51K.J. Donovan and E.G. Wilson, J. Phys. C 19 (1986) L357. [ 6 ] J. Orenstein, Z. Varedeny, G.L. Baker, G. Eagle and S. Etemad, Phys. Rev. B 30 ( 1984) 786. [ 71 H. Kiess, R. Keller, D. Baeriswyl and G. Harbede, Solid State Commun. 44 (1982) 1443. [8] E.T. Kang, P. Ehrlich and W.A. Anderson, Mol. Cryst. Liq. Cryst. 106 (1984) 305. [9] J. PfIeger, S. Nesptirek and J. Vohlidal, Mol. Cryst. Liq. tryst. 166 (1989) 143. [lo] M. Stolka and D.M. Pai, Advan. Polym. Sci. 29 (1978) 1. [ 111 R.C. Pennwell, B.N. Ganguly and T.W. Smith, Macromol. Rev. 13 (1978) 63. M. Biswasand and T. Uryu, J. Macromol. Sci. Rev. Macromol. Chem. Phys. C 26 (1986) 249. P.M. Borsenberger, L.E. Contois and A.I. Ateya, J. Appl. Phys. 50 (1979) 914. J. Mort and G. Ptister, Polym. Plast. Technol. Eng. 12 (1979) 89. M. Yokoyma, Y. Endo and H. Mikawa, Bull. Chem. Sot. Japan49(1976)1538. [ 161 L.B. Schein, R.W. Anderson, R.C. Enck and A.R. McGhie, J. Chem. Phys. 71 (1979) 3189. [ 171 CL. Braun and T. Scott, J. Phys. Chem. 87 (1983) 4776. [ 18 ] J. Mort, M. Morgan, S. Grammatica, J. Noolandi and K.M. Hong, Phys. Rev. Letters 48 (1982) 1411. [ 191 H. Sakai, A. Itaya and H. Masuhara, J. Phys. Chem. 93 (1989) 5351. [ 201 A. Itaya, T. Yamada and H. Masuhara, Chem. Phys. Letters 174 (1990) 145. [ 211 M. Pope and C.E. Swenberg, Electronic processes in organic crystals (Clarendon Press, Oxford, 1982) p. 82 1. [22] H.H. Jaffe and M. Orchin, Theory and application of UV spectroscopy (Wiley, New York, 1965 ) p. 179.
[23] W. Klispffer, J. Chem. Phys. 50 (1969) 1689. [24] L. Onsager, Phys. Rev. 54 (1938) 554. [25] S. Ndptirek and V. Cimrova, Progr. Coll. Polym. Sci. 78 (1988) 78. [ 261 S. NeSpiuek, V. Cimrova and J. PfIeger, Colloid. Polym. Sci. 269 (1991) 556. [ 271 I. Chen, J. Mort and M.D. Tabak, Trans. IEEE Electron Devices 19 (1972) 413. [28] S. Neiptirek and K. Ulbert, Cs. Gas. Fyz, A 25 (1975) 144. [29] R.R. Chance and C.L. Braun, J. Chem. Phys. 64 (1976) 3573. [ 301 S. NeSpiirek and V. Cimrovl, in: 30th UIPAC International Symposium on Macromolecules, The Hague, 1985, Abstracts p. 45 1. [ 3 1 ] V.M. Agranovich and A. Zakhidov, Chem. Phys. Letters 68 (1979) 86. [32] M. Yamamoto, Y. Tsujii and A. Tsuchida, Chem. Phys. Letters 154 (1989) 559. [ 331 Y. Tsujii, K. Takami, A. Tsuchida, S. Ito, Y. Onogi and M. Yamamoto, Polym. J. 22 ( 1990) 3 19. [ 341 I. Tale, P. Butlers, S. NeSpiuek and J. PospISil, IN. Akad. Nauk Latv. SSR, Ser. Phys. ( 1987) 40. [ 351 P. Butlers, I. Tale, J. PospiSil and S. NeSptirek, Progr. Coll. Polym. Sci. 78 (1988) 93. [ 361 I. Tale, Phys. Status Solidi (a) 66 ( 1981) 65. [ 371 F. Stolxenburg, B. Ries and H. BLsler, Mater. Sci. 13 (1987) 259. [38] E.A. Silinsh, D.R. Balode, AI. Belkind, AK. Gailis, V.V. Grechov, A.J. Jurgis, L.F. Taure and S. NeSptirek, Proceedings of the 7th Molecules Crystals Symposium, Nikko, 1975, p. 145. [39] J.M. Pochan, D.F. Hinman and R. Nash, J. Appl. Phys. 10 (1975) 4115. [ 401 J. Noolandi and KM. Hong, J. Chem. Phys. 70 ( 1979) 3230. (411 J. Noolandi and KM. Hong, Chem. Phys. Letters 58 (1978) 575. [42] P.W. Anderson, Phys. Rev. 109 (1958) 1492.
Time resolution of ion pair formation in poly
(N-vinylcarbazole
)
S. Neiplirek Institute of Macromolecular Chemistry Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic
Received 5 April 1993; in final form 9 August 1993
The laser-flash-induced transient optical absorption of poly(N-vinylcarbazole) thin films has been investigated. The rise time of the ion pair formation was estimated to be about 40 ns, their lifetime is longer than 10 us. The ion pair stability is probably due to self-trapping process and dimer radical cation formation.
1. Introduction
Photogeneration of charge carriers in polymers has been intensively studied for a long time, in poly (Nvinylcarbazole ) [ l-3 1, poly (diacetylene ) s [ 4,5 1, polyacetylene [ 6,7], poly(phenylacetylene) [ 8,9], and in other polymers (see the reviews [ lo- 12 ] ) , as well as in molecularly doped polymers [ 13,141. Whereas the charge carrier photogeneration in polyacetylene, poly ( phenylacetylene ) and poly ( diacetylene)s is a typical intrinsic process, the photogeneration in poly (N-vinylcarbazole ) and molecularly doped systems exhibits prevalently extrinsic behaviour. In the extrinsic case there are several possibilities of the formation of excited species (extimers and exciplexes), capable of participating in free carrier formation. Impurities or guest molecules, namely oxygen, may participate in the mechanism. Very often free charge carriers are generated via nonrelaxed exciplexes formed between singlet excitons and acceptor-like impurities or photooxidation products. The following mechanism was proposed [ 15 ] (see fig. 1) by Yokoyma and co-workers: D*+A+(D*...A)+(D’+...A’-)** 1 + D’+
.?.A’-:
D’+
+A’-
.
Here, (D*...A) represents an encounter complex, (D’+...A--)** a nonrelaxed exciplex state, De+.?. A’-)
an ion pair with an interionic distance r, and D’+ f A’- free charge carriers. Process 1 corresponds to thermalization and process 2 to dissociation. Thus, charge carrier photogeneration in poly (N-vinylcarbazole) (PVCA) can be depicted as a multistage process which involves: photoexcitation to a neutral molecular electronic state and the formation of a nonrelaxed exciplex state, ionization and thermalization of the excited state, leading to the formation of a Coulomb-field-bound geminate ion pair (chargetransfer state), and thermal dissociation of the pair into free carriers by Brownian motion subject to a combination of Coulombic and applied fields. Although this model represents a satisfactory description of the charge photogeneration in PVCA, some experimental data obtained recently need further discussion. Namely, while in molecular crystals or organic liquids the geminate recombination or a Coulombically bound geminate electron-hole pair (ion pair) is a diffusive process, completed within typically 10 to 100 ps [ 16,17 1, a delayed field collection experiment with pure PVCA film made by Mort et al. [ 18 ] revealed a ion pair lifetime of 100 s. Dynamic aspects of the ionic species such as exciplex, ion pair, and free ions (hole and electron) in polymer films are still beyond our knowledge. Recently an information was published concerning exciplex dynamics in PVCA films [ 193. Because fluorescence spectra showed the time-dependent shift, it was concluded that multiple exciplex and exterplex species, with various relative configurations, be-
0301-0104/93/% 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.
S. NeSptirek/Chemical Physics178 (1993) 415-422
416
D*+ A-
(D’...A)
-
ENCOUNTER CCMPLEX
THERMALIZATION
(Dt.A’)”
NONRELAXED EXCIPLEX STATE
t
-
( DxbT)
(1 )
ION PAIR
D?+A +e‘ DttA:
//ELD
p(F) Dt+
&m%l I
\
/
D*+A (D:,&)**__-__ -+ I I-
l-P(F)
(2) A7
CHARGE CARRIERS
(Dt AT)’ RELAXED FLUORESCENT EXCIPLEX STATE
1 (DT
o+A+
A:)_
hu,
* FLUCRkCENcE
D+A
j A
1 /
DA
Ll ___-_
Do rDA
r0 Separation
drstorce, r
Fig. 1. Schematic picture of the charge carrier photogeneration in poly(N-vinylcarbazole). Thermalization ( 1 ), dissociation (2). D: donor, A: acceptor, e-: free electron, hv: excitation energy, hur: energy of the fluorescent light, rDA:distance between donor and electron, r,,: thermalization distance of the ion pair.
tween donor carbazole and acceptor dopant were important. An unusual slow and nonexponential decay of exciplex fluorescence in the solid state, which is longer than 100 ns (typical exciplex lifetime in solution), with a long tail up to several microseconds and its dependence upon an applied electric field suggested that the geminate recombination of the ion pairs determined the exciplex fluorescence. From the optical measurements of PVCA doped with acceptor molecules it was concluded that the ionic absorption bands are mainly assigned to ion pairs or free ions (hole and electron) [ 201. This typical behaviour of solid PVCA films needs further detailed study. To obtain more information about long-lived species, laser-flash-induced transient optical absorption of thin films of PVCA was studied. If ionic species are formed during the excitation, one must expect photoinduced transient absorption with red shift caused by the interaction of excitons with electron-hole pairs, arising mainly from the increase of the molecular polarizability with electronic excitation. The results of this studies are presented in this work.
2. Experimental PVCA was obtained by radiation polymerization and purified by multiple precipitation from benzene solution into methanol. Films (thickness about 1 pm) were prepared from cyclohexanone-benzene solution by spin-coating on glass slides. The samples were irradiated with 15 ns flashes of 266 nm light produced by a Nd: YAG laser equipped with two frequency doublers. Optical absorption changes were measured in perpendicular direction using a 450 W (steady-state regime) xenon lamp, monochromator and a photomultiplier operated with five dynodes. During the measurement of the photoinduced transient absorption (0.5 ms) the power of the monitoring lamp was increased up to 40 kW to obtain higher light intensity of the monitoring light. The films were oriented at an angle of 45’ to both light beams.
3. Results Fig. 2a shows kinetic traces illustrating the forma-
S. Nefptirek/ ChemicalPhysics178 (1993) 415-422
I
417
I (Cl
(dl
Fig. 2. Kinetic traces of the transient absorption of the PVCA film recorded at 354 nm (A,=266 nm). Fresh sample; time scale 50 ns/ division (a), 200 ns/division (b), 1 ps/division (c). Sample kept during 3 months in air prior to irradiation; time scale 1 ps/division (d ) . The measure of the division is given in case (b ) .
tion of the transient optical absorption induced at IzCXC = 266 nm (excitation to the third singlet state) in a freshly prepared PVCA film measured (&= 354 nm) in vacua. Within the error limit, the half-life of the formation of the new absorption is x40 ns. Measuring on the 200 ns and 1 us/division time scale (figs. 2b and 2c), no decrease in the transient absorption was observed up to t= 10 l,tsfor the fresh sample. The dependence of the transient absorption at Iz&,s=354 nm recorded at 1.5 ~.tsafter excitation on the exposure dose (A,,,= 266 nm) is shown in fig. 3. The dependence is linear up to ~6 mJ cm-*. For higher power a sublinear dependence was observed. Fig. 4 shows the ground state absorption spectrum of a PVCA film (curve 2) [ 21 and the transient absorption spectrum (curve 1) taken at 1.5 us after excitation at A,,,= 266 nm. The absorbance of the transient absorption was about two orders of magnitude lower than that of the ground state, It is very interesting to compare the shapes of both curves. The induced spectrum has a shape very similar to that of the ground state spectrum of the PVCA film, but the positions of the maxima are shifted by about 255 cm-’ (average value) to the red region. The experi-
mental data are: Ground state absorption: 1= 344, 330 and 296 nm, transient absorption: 1~347, 332 and 299, red shift v= 25 1,183 and 333 cm-‘.
4. Discussion As mentioned above the charge carrier photogeneration in PVCA is an extrinsic process involving at least three steps: photoexcitation, thermalization and dissociation of the ion pairs. These pairs, in dissociation of which an external electric field is effective, are not formed from the relaxed fluorescent exciplex state (D’ +A’- ) *, but from the “nonrelaxed exciplex state” (D’+...A’-)**, which is produced from the “encounter complex” (D*...A) between a migrating singlet exciton D* and an electron acceptor A [ 15 1. The “nonrelaxed exciplex state” arises through a rapid electron transfer at a somewhat larger separation of the components in an encounter complex than in the relaxed fluorescent exciplex state. This process is possible since the carbazole chromophores are located around an acceptor in a high concentration, and D* is mobile from molecule to molecule. A thermal-
S. Nefptirek/ Chemical Physics 78 (1993) 415-422
3 t
-006
-0.06
2
-0.04 i VI 9 - 0.02
I
1t
oLo 200
16
0 Laser
power,
P
(mJ cr?l’1
Fig. 3. Dependence of the transient optical absorption at I= 354 nm, recorded at 1.5 ps after the flash, on the exposure dose. Temperature T= 296 K.
ization of the excess energy of the nonrelaxed exciplex state leads to the formation of the ion pairs with a separation distance r,,, the positive charge (hole) being located at the carbazole unit, and the negative charge (electron) probably at oxygen or impurities. It is known that the main impurities in films prepared from highly purified PVCA are dissolved oxygen, oxidation products and residual solvent molecules [ I]. The oxidation products probably contain carbonyl groups which, because of their high electron affinity [ 221, act as acceptors and may constitute efficient electron traps [ 231. These impurities might efficiently contribute to the photogeneration mechanism in forming strong charge transfer complexes with excited carbazole groups. Oxidation and photodegradation should proceed most rapidly in the surface region and result in an impurity concentration protile. In our samples one oxygen molecule was present per about ten to twenty carbazole units in the bulk of the sample and about one oxygen molecule per six carbazole units on the surface as determined by ESCA measurements. The dissociation of ion pairs into free carriers in
uo
280
320
Wavelength,
u d
360
400
A ( nm )
Fig. 4. Transient optical absorption spectrum of a PVCA film taken 1.5 us after the flash (curve 1). Temperature T=296 K, duration of flash about 15 ns. Curve 2: ground state absorption spectrum of PVCA taken from ref. [ 21 (the absorption coefficient scale was corrected according to ref. [ 2 1 ] ). The absorbance of the transient absorption was about two orders of magnitude lower than that of the ground state.
external electric field (as a competitive process to the geminate recombination) may be described in terms of the Onsager theory [ 24 ] as it follows from literature data [ l-3,25,26] and from fig. 5. According to this model the overall photogeneration efficiency v may be calculated as ~(8 T) =rto j%,
K T&(r) dr ,
(1)
where f(r, 1;; T) is the dissociation probability of bound ions separated by a distance r= 1r I= 1r, - r,, 1, r, and #j, being the position vectors of the negative and positive ion, respectively; g(r) represents the initial spatial distribution of bound pairs and dr is the volume element. The primary quantum effrciency vo, i.e. the number of photogenerated ion pairs per photon, is assumed to be independent of the applied field. Assuming that the distribution is spherically symmetrical and that charges in all pairs are separated by the same distance ro, we have g= (4~:) -‘6( r- ro), where 6 is the &function. For amorphous polymers the &function may be regarded only as a zero-order approximation. The diffusive motion of charges during the thermalization in dis-
S. NeSptirek/ Chemical Physics178 (1993) 415-422
I lo6
I
lo6
lo7 Electric
J 109
fleld,F(Vm-‘1
Fig. 5. Field dependence of the photogeneration effkiency u in PVCA film. The solid lines are calculated using Onsager dissociation theory for the constant distance of bound pairs rp2.9 nm (curve 1) and for distances distributed according to eq. (2) with the parameter cr= 1.6 nm (curve 2). Taken from ref. [26]. The measurements of q were carried out using the technique of emission-limited photoinduced discharge [ 27 1.The surface potential was measured with a rotary electrodynamic electrometer [ 281.
ordered structures leads to the distribution of radii of charge transfer states which can be assumed to be a Gaussian one [ 25,26,29]:
g(r) =
&
exp(-r2/a2) ,
where (31is the dispersion parameter. It may be demonstrated that in this case the initial separation distance of ion pairs, the dissociation of which is the most probable, is field-dependent: at high fields it tends to (Y,while at low fields it becomes approximately 2a 1301. Let us discuss the nature of the transient optical absorption spectrum. From fig. 4 it is seen that this spectrum is a copy of the ground state absorption of the PVCA film shifted into the red region. Thus, an effect suggestive of the Stark effect could be assumed. A possible explanation is a formation of coupled states
419
of molecular excitons with charge carriers [ 3 11. The attraction of excitons to charge carriers arises mainly from the increase in molecular polarizability on electronic excitation, so that the exciton energy decreases in the electric field of the charge or in the field of the ion pair. Under this assumption our optically induced absorption follows the formation of the ion pairs created by the laser flash excitation (the formation of free charge carriers by the dissociation of the ion pairs is weak at an electric field strength equal to zero). No direct spectral evidence concerning the cation radical of carbazole was found with our experimental arrangement probably because of low concentration of photogenerated ion pairs. From laser photolysis experiments with solutions of charge transfer complexes of PVCA [ 32,33 ] is known that absorption of carbazole radical cation is located at x 790 nm and carbazole dimer radical cation between 700 and 760 nm depending on its steric configuration. For the solid films of charge transfer complexes of PVCA, the peak of the transient optical absorption was found at x 750 nm using dynamic attenuated total reflection spectroscopy [ 201. This suggests that the carbazole dimer radical cation is formed in the solid films after the light excitation. Its lifetime was found to be x 3-5 us [ 201. Let us estimate the magnitude of the shift between the ground state and transient optical absorption spectra. In homogeneous condensed media the energy of molecular excitation can be expressed as E= AE+ B, where AE= El-E,,is the difference of two energy terms of an isolated molecule and B is the change in the interaction energy between the excited molecule and the surrounding ones. In molecular crystals the magnitude of B is of the order of 1000 cm-’ and it is just this quantity that is responsible for the red shift of crystal spectra in comparison to the vapour spectra. In the presence of an electric charge the value of B becomes a function of the distance r between the charge and the molecule which is electronicallyexcited,B=Bo+g(r),where&r) isthe change in the molecule-charge interaction energy arising from the excitation of the molecule. If F( r) is the electric field produced by the charge at the point r, then, considering only the dipole polarization of the molecule in the point-dipole approximation, one can express q(r) as
420
v(r) = - (ch-~0)F2(r)12,
S. NeSpiuek/ Chemical Physics178 (1993) 415-422
(3)
where (Y,, and cyo are static polarizabilities of the molecule in the excited and ground states. In most known organic solids ay,> CX~. Hence, the energy p(r) corresponds to attraction of the exciton to the charge. No direct information on the Acr value is available for the carbazole unit. According to ref. [ 2 11, for example, for the lowest singlet excited states of anthracene and tetracene ACY k: 1.5 x 10 -23 cm3. The electric field of the static charge in CGSE unit, is F(r) = t is the electric permittivity, so that e/cr2, -Aae2/2e2r4. Taking our average experip(r) = mental value p(r) = 255 cm-’ and Aa equal to the value for anthracene and tetracene, the distance between the charge (generated by the laser flash) and the molecule excited by the monitoring light can be obtained as 0.44 nm. This value is very close to the distance of two carbazole units at PVCA chains [ 2 11. Because of the strong dependence of q( r) on the distance ( re4) only carbazole units in the nearest neighbourhood of the ion pair are responsible for the discussed effect. It should be pointed out that the same distance (0.4 nm) was obtained from the thermoluminescence experiments [ 34,351. Starting the measurements at a temperature of z 160 K, the activation energy and the frequency factor of the relaxation process are practically constant, being 0.4 eV and lo-as-1, respectively (see fig. 6). The activation energy in the high temperature region is close to the activation energy of the hole drift mobility. Lower values of the frequency factor suggest that the recombination luminescence is controlled by the diffusive migration of the trapped charge carriers. The spectral distribution of the thermoluminescence consists of a luminescence band with a maximum near 2.4 eV [34,35] which coincides with the spectral characteristics of the phosphorescence of excimers. This fact suggests that the excitation process ends in the formation of a triplet state of an excimer and therefore the formation of dimer radical cations can be expected. This fact is in agreement with the result of transient optical absorption on thin solid PVCA films [ 201 mentioned above. Recently it has been found that the long-time geminate recombination behaviour in PVCA is controlled by traps that extend to an energy of about 0.6 eV below the distribution center of the intrinsic hop-
(b)
05
t
I
I
I
I
I
I
I
80 xwl 120 UO 160 180 200 220 ---T(K)
t..,
,I..
0
5 x) -Lg
I
15
Fig. 6. The thermoluminescence glow curve (a), the temperature dependence of mean activation energy (E) (b ) , and the energy distribution of the mean frequency factor (s) and of the trap concentration H(E) (c). Taken from ref. [ 341. The method of fractional thermostimulated luminescence was used [ 35,361.
ping states [ 371. Possible candidates for this are sandwich dimers active in excimer formation. Theoretical estimation of the depth of the traps using the self-consistent polarization field technique [ 38 ] makes 0.4 to 0.5 eV. The formation of dimers represent the reason for the long life-time of ion pairs (longer than 10 us from our experiments and FZ100 s from the collection electric field experiments [ 181 at room temperature). One can simply understand the behaviour in the framework of a self-trapping process. After the formation of the PVCA radical cation, the dimer radical cation is immediately formed due to the molecular dynamics of the side carbazole groups. This process is limited by the thermoactivated torsional vibration of the polymer pendant groups. According to the reference data, dielectric losses corresponding to the relaxation in PVCA occur at 2 10 K (measured at 1 kHz); an activation energy of ~~0.35 eV was observed [ 391. This indicates that the characteristic time of y-relaxations at room temperature is of the order of microseconds, which is the time associated with the self-localization of the excess charge. The observed life-time of the ion pairs (T> 10 ps in this work) is longer than the value presented for the solid charge-transfer complexes [ 20](3-5 ps for PVCA- 1,Cdicyanobenzene, PVCA-phthalic anhydride and PVCA-1,2,4,5-tetracyanobenzene). It
S. Neiptirek/ Chemical PhysicsI78 (1993) 415-422
could be pointed out that the shape of the kinetic traces of the transient absorption depended on the history of the sample. For samples stored 3 months in air and measured in air, a decrease in the transient absorption at longer times was observed (cf. fig. 2d). As follows from fig. 7, the decay is a first-order process (D= Doexp( - t/T), where D is the absorbance and t is time) with the lifetime 7= 4 p. The temperature dependence of the decay of the transient absorption is very weak. The activation energy EA is estimated to be less than 0.05 eV. The formation of the charge transfer complex with adsorbed oxygen which acts here as acceptor center thus makes the lifetime of ion pairs shorter but, on the other hand, the number of ion pairs (Q,) increases; thus, the acceptor doping increases quantum efficiency of the charge carrier photogeneration in PVCA. With regards to the facts mentioned above, the extrinsic mechanism of the charge carrier photogeneration in PVCA should be modified as follows D*+A+D+D’+...A’-)*D
orD’+...D’-+A
4D’+D?.A.-+D.++A.-+D, where D’+D is the dimer radical cation. The reality of the long lifetime of the ion pairs is reflected in the theoretical predictions of the timedependent Onsager model [ 401. The physical basis of this model is that ifan electric field can be applied on a time scale comparable to the geminate recombination time, then time-dependent photogeneration
421
would be observable. In this case the distribution function g(r, At) is time-dependent. The time evolution of g( r, At) in zero field can be obtained analytically from the classic Smoluchowski equation. Typical curves for the quantum efficiency at various field strengths, time delays, and thermalization distances r. are given in ref. [ 4 11. This model was used by Mort et al. [ 18 ] to fit the delayed-collection-field data. These authors obtained the values of the diffusion coefficient d- 10 -20 m2 s- ’ for T= 293 K and d-10-18m2s-1forT=373KThesesmaIlvaluesof the diffusion coefficient mean that in applied zero field the charge carriers are essentially immobile. The value is close to the “Anderson limit” [40], where the disorder in the system precludes any diffusion at all.
5. Conclusion The optically induced transient absorption between 280 and 380 nm in thin films of poly(N-vinylcarbazole) has been investigated. The absorbance of the transient absorption is about two orders of magnitude lower than the optical density of the ground state absorption of the film. The shape of the transient spectrum resembles very closely that of the ground state spectrum, but the positions of the maxima are shifted by about 255 cm-’ to the red region. A possible explanation is the formation of coupled states of molecular excitons with charge carriers. The formation time of bound ion pairs is about 40 ns, their lifetime is longer than 10 ps. The long lifetime is assumed to be due to dimer radical cation formation and self-trapping process.
Acknowledgement
0123456
7 Time,
0
9
The experiments were performed in Hahn-Meitner-Institut, Berlin. The authors wishes to acknowledge the financial support of the Kemforschungszentrum Karlsruhe and helpful discussions with Professor W. Schnabel.
t (ps)
Fig. 7. Dependence of the transient optical absorption on time for sample kept 3 months in air prior to irradiation. Replotted from fig. 2d.
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