Electric field-induced fluorescence quenching and transient fluorescence studies in poly(p-terphenylene vinylene) related polymers

Chemical Physics ELSEVIER

Chemical Physics 227 (1998) 167-178

Electric field-induced fluorescence quenching and transient fluorescence studies in poly(p-terphenylene vinylene)related polymers N. Pfeffer a,1, D. Neher

a,* ,

M. Remmers

a,

C. Poga a,2, M. Hopmeier b, R. Mahrt b

a Max-Planck Institutf(tr Polymerforschung, Ackermannweg 10, 55128 Mainz Germany Fachbereich Physikalische Chemie und Zentrum Jfir Materialwissenschafien, Philipps-Unit,ersiti~t Marburg. Hans-Meerwein-Strasse, D-35032 Marburg, German),

Received 16 April 1997

Abstract Electric field induced fluorescence quenching has been investigated for a series of poly(p-terphenylene vinylene)s. The quenching efficiencies follow a strictly quadratic dependence on the applied field amplitude with maximum values of about 10% at 200 V/p,m. Quenching occurs predominately at higher emission energies, resulting in a distinct blue-shift between the electro-modulated signal and the photoluminescence spectra. These results provide evidence for the field assisted dissociation of neutral excitons within an inhomogeneously broadened density of states (DOS). Experiments are also performed on devices prepared by the Langmuir-Blodgett-technique in order to evaluate contributions by the Stark effect. These experiments prove the electric field-assisted separation of charges onto separate chains. Transient photoluminescence experiments show fluorescence decay times ranging between 100 ps and 200 ps. The increase in relaxation times for larger detection wavelengths gives evidence for spectral relaxation within the DOS. This leads to a consistent picture, where the balance between electric field assisted dissociation of excitons competes with the radiative decay as well as the non-radiative decay processes. Implications on the spectral properties of electroluminescent devices are further discussed. © 1998 Elsevier Science B.V.

1. Introduction Light emitting diode devices and photoconductors based on organic materials have been thoroughly investigated with respect to photonics application.

* Corresponding author. Fax: +49-613-379-100. i Present address: Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA Eindhoven,The Netherlands. 2 Present address: Allied Signal Inc., P.O. Box 1021, Morristown, NJ 07962, USA.

The function of these devices critically depends on the interplay of excitons, free carriers and photons such as the efficiency of formation and dissociation of excitons in presence of an electric field. In particular the various processes governing the spectral properties and efficiencies of LEDs are not fully understood. The experimental method of electric field modulated fluorescence (EMF) [1] is a direct way to study the dissociation of photo-generated excitons in the presence of an electric field. While photoconductivity experiments include various steps such as the

0301-0104/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PH S0301-0 104(97)001 96- 1

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N. Pfeffer et al. / Chemical Physics 227 (1998) 167-178

photo-generation of charge carriers, their transport through the material and the collection by the electrodes, EMF can be considered as a probe which basically measures the efficiency of electric-field assisted exciton dissociation into free charge carriers under the influence of an electric field [2,3]. EMF is thus rather insensitive to effects such as charge trapping or charge recombination effects. EMF has been studied in polymeric thin films based on poly(p-phenylene vinylene) [4-9]. Deussen [6] reported efficient quenching in an electroluminescent device made of blends of poly(p-phenylphenylene vinylene) (PPPV) and polycarbonate. Under reverse bias quenching efficiencies of up to 40% at a field of 2.7 × 10 s V / m were observed. The process was explained by the electric field-assisted dissociation of neutral excitons to give free electrons and holes located either on adjacent segments of the same chain or on neighbouring chains. Monte Carlo simulation of the field dependence of the fluorescence quenching efficiency gave an electron binding energy of 0.4 eV [6]. The drop of the quenching efficiency on dilution of the PPPV was explained by the decreased probability of finding nearby acceptor sites. A similar behaviour has been reported for molecular doped polymer films by the same authors [10]. Even stronger quenching efficiencies were observed when the sample was biased in forward direction, above the current onset. Presumable, the presence of charges gives an additional contribution to the quenching in agreement to observations by others [11,12]. Fluorescence quenching experiments on poly(p-phenylene vinylene) have shown quenching efficiencies of up to 34% at a field of 3 X l0 s V / m [7], similar to the values reported by Deussen et al. Time resolved field-induced luminescence quenching studies in poly(p-phenylphenylene vinylene) have been interpreted within the picture of an inhomogeneously broadened density of states (DOS), where initially photo-generated neutral excitons are subject to field-assisted dissociation into free charge carriers. Spectral relaxation of the exciton within the DOS reduces the probability of separation at longer time delays [4,5]. In this paper we report on a comparative study of the time-integrated electric field induced fluorescence quenching in three novel poly(p-terphenylene vinylene) related polymers [13]. Experiments have

also been performed on Langmuir-Blodgett films to eliminate contributions from the Stark-effect and to study the contribution of inter-chain charge carrier separation. The wavelength dependence of the electromodulated fluorescence indicates that quenching must occur at an early state of the exciton relaxation within the DOS. Time-resolved fluorescence experiments show a clear spectral relaxation of the excitons within the DOS.

2. Experimental 2.1. Sample preparation

The chemical structures of the three studied polymers [13] are shown on Fig. 1. P3V is a fully-conjugated strictly alternating polymer. In P3VE the conjugation is periodically interrupted by a ethenylene unit, which also effects the stiffness of the backbone. The copolymer P 3 / 5 V possesses a periodic modulation of the H O M O - L U M O separation along the backbone due to the inclusion of pentaphenylene-vinylene units, Both P3VE and P 3 / 5 V are statistical copolymers. Molecular weights M n of the used compounds were 168.000 (P3V), 109.000 (P35V) and 13.900 (P3VE) [13]. Quantum yields of fluorescence are up to 45% in the bulk [10]. While

o?- o?-o P3v

OH

°

I

Fig. 1. Chemical structure of poly(p-terphenylenevinylene)compounds.

N. Pfeffer et a l . / Chemical Physics 227 (1998) 167-178

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Wavelength [nm] Fig. 2. (a) Absorption and photoluminescence spectra of the polymers in the solid film. (b) PL (solid line) and EL (dotted line) spectra of an approximately 100 nm thick spincoated films of P35V. In the EL experiment the sample consists of the spincoated emission layer between an ITO and an aluminium charge injecting contact. The driving voltage was approximately 18 V.

169

the absorption and photoluminescence in solution of the three polymers are almost identical, the photoluminescence spectra in the solid film show some minor differences, which might be caused by packing effects or aggregation of the chains in the solid state (Fig. 2a). For comparison the electroluminescence (EL) (Fig. 2b) shows an extended tail towards longer wavelength. This behaviour will be discussed below. The sample structure for the EMF experiment is presented on Fig. 3. The polymers were spin-coated from toluene, after filtering the polymeric solution through 0.45 mm filters, on partially ITO covered fused silica substrates. 60 nm thick aluminium electrodes were evaporated on top. The active area was 4 mm 2. The thicknesses of the polymeric films (ranging between 90 and 150 nm) were measured by a Tencor alpha-step 200 profilometer. The Langmuir-Blodgett (LB) film sample had a slightly modified structure. Due to the high rigidity of the Langmuir films of the P3V derivatives a transfer onto a solid substrate was only possible if the polymer had been mixed with the analogous isopentoxy substituted poly(p-phenylene) (PPP) derivative (M, < 50.000) [14]. PPP has been shown to be processable with the LB technique yielding films in which the polymer chains are oriented parallel to the substrate plane and within this plane, possess a preferential orientation of the polymer backbone parallel to the dipping direction [15-17]. The LB multilayers were prepared using a Lauda Filmwaage FW-1 in a laminar flow box. Solutions with a concentration of 0.5 g/1 70:30 PPP:P3VE in

+ V (shot -1)

Aluminum I

Polymer ITO/fused Silica

V simot +Vo

i

[ f\

Fluorescence Excitation Light ~

•

Fig. 3. Sample geometry in the EMF and PL experiments.

V si~(ot

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N. Pfeffer et al. / Chemical Physics 227 (1998) 167-178

Chloroform (Merck Uvasol) were spread on the water surface (Milli-Q water, conductivity 10 -18 S/cm). The transfer was performed at 6°C at a surface pressure of 7.5 m N / m and a dipping speed of 1 cm/min. In order to improve the stability of the LB samples a 60 nm thick pure PPP layer was first coated onto the surface of the partially ITO covered fused silica substrate before LB deposition. The total film thickness was 110 nm.

2.2. Electric-field modulated fluorescence In the EMF experiments a function generator supplied a sinusoidal electric field across the ITO bottom and aluminium top electrode. Two modes were used: a bipolar mode with a sinusoidal voltage V = V0sin ~ot (with no DC offset) and a reverse bias mode with the sinusoidal voltage varying between 0 V and - V 0 (Fig. 3). In the second case, the voltage at the ITO electrode was always negative, that is the electroluminescent diode is driven only in reverse direction. In this case no electroluminescence was observed. The excitation light from a 450W Xenon arc lamp passed through a monochromator and was focused on the active area of the sample. The excitation light could be modulated by a chopper. The fluorescence was detected normal to the surface of the film, after being filtered by a second monochromator, by a Hamamatsu R928P photomultiplier in current mode. The PM signal was demodulated by a lock-in amplifier (EG&G 5210), working at either the fundamental or harmonic of the modulation frequency. Fluorescence spectra were measured by modulating the excitation beam with the chopper and no voltage applied across the sample. In the electromodulated fluorescence experiments a constant excitation flux was used.

2.3. Transient fluorescence experiments As excitation source in the time-resolved PL measurements we used 100 fs laser pulses at 3.1 eV, derived from a frequency-doubled mode-locked Ti: sapphire layer. To avoid sample degradation and to eliminate non-linear effects, the beam power was reduced to 1 mW at a repetition rate of 76 Mhz. This also ensured that we were in the linear excitation

regime. The samples were kept at room temperature. The luminescence from the sample was dispersed by a 0.64 m monochromator and detected by a Streakcamera to follow the PL decay with a time resolution of about 30 ps.

3. Results and discussion Fig. 4 shows the electromodulated fluorescence spectra measured in the reverse bias mode with an applied field amplitude o f - 1.5 X 108 V / m . Also shown are fluorescence spectra with no electric field applied for the three polymers. The modulated signals have been normalised to the maximum intensity of the corresponding PL spectra. In the reverse bias mode the lock-in amplifier was set to the fundamental frequency of the electric field. Similar results are obtained if the bipolar mode was used and the detection performed at the first harmonic (2 w). In this case, a smaller (approximately 1/3) signal was detected even at to. This effect can be linked to the asymmetric structure of the sample (see results by Deussen et al. [6]) or explained by build-in fields [18]. The normalised signal is independent of the frequency of the electric field tested between 30 Hz and 30 kHz and independent of the excitation light intensity varied between 50 n W / c m 2 and 0.5 m W / c m 2. The dependence of the quenching efficiency, taken at the wavelength of maximum signal, is proportional to the square of the electric field as shown on Fig. 5. Square-type electric field dependencies has recently reported by several authors [7,19]. It has been claimed that the strictly quadratic dependence of the quenching efficiency with the applied electric field allows one to identify the underlying charge generation mechanism [7]. The electric field induced dissociation of initially formed charge-transfer excitons (Onsager theory) [20] was excluded in favour of a two step process: the electric field separates the neutral singlet exciton leading to a charge transfer state which recombines non-radiative to the ground state or is further separated via diffusion into free carriers [21]. Kalinowski et al. [18] has further proposed a macrotrap model, where neutral excitons are trapped at the defected crystallite interfaces. The

N. Pfeffer et al./ Chemical Physics 227 (1998) 167-178 400

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171

ciation into free carriers. This model could fit the electric field dependence over a large range. Alternatively data have been explained by a modified Onsager-Noolandi-Hong-Popovic-approach, which also gives a quadratic dependence of the EMF signal on the electric field strength [22]. Work by Kalinowski et al. has further shown that these different models follow rather similar power laws at field strength typically used in our EMF experiments [ 18]. Even though an identification of the initial excitonic state from the functional dependence is not possible, the strictly quadratic increase identifies the observed quenching as an electric field induced modulation of the photo-induced charge generation process. As outlined above, several authors have proposed strong fluorescence quenching due to the presence of injected charges [6,11,12]. Based on recent EMF experiments on a-sexithiophene devices Fichou et al. concluded that one charge quenches the fluorescence of approximately 200 molecules. Charge carrier induced quenching should scale with the amount of charges injected into the film. Among the different models which have been used to describe the current-voltage behaviour of polymeric LEDs [23-25] only the space-charge limited current (SCLC) model gives a current density which depends quadratically on the applied voltage. In our experiments, with the devices driven under reverse bias, current densities are however too low to cause significant space charge effects.

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~.2

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6

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[nm]

Fig. 4. E M F spectra (solid line) recorded at 1.5 × 108 V / m for sinusoidal modulation. Q u e n c h i n g efficiencies are given relative to the PL emission intensity at the P L m a x i m u m (A = 455 nm). A l s o s h o w n are c o r r e s p o n d i n g c w - P L spectra (dashed line).

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*i' l'.s excess energy can in addition induce nearestneighbour charge separation forming a trapped CT state, which can further undergo field assisted disso-

•

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Electric Field [10 8 V/m] Fig. 5. Relative fluorescence q u e n c h i n g efficiencies as a function of the applied electric field.

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N. Pfeffer et al. / Chemical Physics 227 (1998) 167-178

Fig. 5 shows that the quenching efficiencies are quite different for the three polymers. Field-assisted separation of charge carriers might occur predominately on segments of the same polymer backbone (intra chain charge separation) or onto neighbouring molecules (inter chain charge carrier separation). Interestingly, the largest efficiencies are recorded for the polymer P3VE with a complete interruption of conjugation. In contrast the fully conjugated P3V homopolymers only shows weak quenching with efficiencies of up to 2%. As demonstrated below the excited state life times at zero bias are similar for all three compounds. The pronounced differences in the quenching efficiencies of the three P3V derivatives must be caused by a variation in the probability for field-assisted exciton breakage as a function of the backbone composition. Changes in the electronic backbone structure would mostly affect intra chain processes. Further, modifications of the chemical structure might also influence the stiffness of the backbone and thus the local packing of the chains. As a consequence, the probability of inter chain jumps might be altered. At this point we like to discuss the model of disoder-assisted dissociation of excitons in an inhomogeneously broadened density of states (DOS) as proposed by Gailberger and B~issler et al. [26]. Neutral excitons have binding energies of several tenth of eV. Field-assisted separation of these excitons into separated charge carriers thus requires strong electric fields. Dissociation of an exciton will however be favoured in case that the electron can jump onto a site of high electron affinity (or the hole onto a site with low ionisation energy). Energetic disorder in organic solids can be enhanced through a variation in the electronic structure of the organic components (e.g. the conjugation length). In P3VE the interruption of the 7r-conjugation is introduced by a copolymerisation scheme, yielding conjugated segments of different lengths. The high quenching efficiency observed in P3VE can be understood via the energetic stabilisation of the photogenerated charge carriers onto rather long poly(p-terphenylene vinylene) segments. A variation in the electronic structure along the backbone is absent in P3V but weakly present in P35V, the latter showing quenching efficiencies between that of P3V and of P3VE. We would further like to emphasise the difference in molecular weight.

While P3V and P35V have M, above 100.000, the P3VE molecules are rather short. Enhanced charge separation between face-to-face aligned polymers might also contribute to the high quenching efficiency of P3VE. The maximum quenching efficiency of 10% at a field of 2 × 108 V / m for P3VE is slightly lower than the values (15-18%) reported for P P P V / P C blends or for pure PPV devices at the same field [6,7]. Interestingly, the quenching efficiency in either LB films or spin-coated layers of the analogous iso-pentoxy substituted PPP is below the detection limit of approximately 0.1% at the same field strength, irrespective of molecular weight. For comparison, photoconductivity experiments on LED devices based on P3V showed a significant lower photocurrent than devices made from MEH-PPV [27]. Even lower photocurrents were found for devices made from PPP. There is an obvious correlation between the EMF quenching signal and the macroscopically detected photocurrent. This indicates that the probability of photogeneration of free charge carriers mainly governs the photoconductivity in this class of materials. In principle, the observed rise in quenching efficiency with increasing fraction of vinylene units in the main chain can be related to both intra chain or inter chain effects. Decreasing the fraction of phenylene units in the backbone (from PPP to PPV) clearly lowers the H O M O - L U M O separation. One might argue that this also effects the binding energy of either intra or inter chain singlet excitons. This explanation currently lacks any prove. Alternatively the packing of the polymer chains in the film will strongly influence the probability of field-assisted charge cartier separation on adjacent chains. It is well known that PPV forms a planar structure even in the ground state, thus allowing a face-to-face packing of the conjugated main chains in the solid state. In contrast P3V and PPP exhibit a cylindrical superstructure due to the torsion of adjacent phenylene units. However, the stilbenol units in P3V are almost planar [13,28]. This results in a different packing of the polymer chains in the solid state. PPP tends to form a hexagonal columnar packing in which the conjugated backbones are surrounded by flexible side chains. P3V forms a layered structure with layers of conjugated backbones separated by

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N. Pfeffer et al. / Chemical Physics 227 (1998) 167-178

surface. Further, polarised UV-Vis spectra of the studied LB-films showed the same optical anisotropy of the PPP and P3VE ~--rr *-transitions in the mixed layer. This indicates that both compounds experience the same aligning forces during transfer and, since PPP is known to be exclusively oriented parallel to the surface, suggests a parallel orientation also for P3VE. In this case the electric field applied to the sandwich devices is perfectly perpendicular to the chain direction and electric field exciton dissociation is only possible perpendicular to the conjugated polymer backbone. The exact analysis of the quenching efficiency in terms of the contribution of inter chain processes to the total signal of the spincoated samples is, however, not meaningful. Phase separation into domains of pure PPP and P3VE is most likely to occur in both the Langmuir layers and the spincoated film, respectively. Note, that the spincoated film shows a reduced quenching efficiency when compared to a film made from pure P3VE. This effect was already observed in blends of poly(p-phenylphenylene vinylene) in PC as men-

side chain layers. In these main chain layers interactions between adjacent chains are more likely to occur, compared to the hexagonal packing of PPP. This can explain the observed difference in quenching efficiency of P3V and PPP, presuming that inter chain processes play a significant role in the charge separation. Inter chain charge separation could be clearly proven by performing EMF experiments on the LB samples. Fig. 6 shows the fluorescence and EMF spectra for the LB sample and a spincoated blend layer of PPP with P3VE. Even though quenching efficiencies are lower for the LB samples a clear signal could be resolved. We have not explicitly analysed the orientation of the P3VE molecules in the mixed LB film but we presume that the polymer chains lie flat on the substrate surface for the following reasons: P3V related compounds do not transfer quantitatively onto a solid substrate, but the polymers form rather stable monolayers on the water surface with an area per molecule corresponding to a flat orientation of the polymer chains on the water

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Fig. 6. EMF spectra (solid line) of a blend layer of P3VE with iso-pentoxy substituted poly(p-phenylene) (PPP). Samples have been prepared by either (a) spincoating with PPP:P3VE= 60:40 or (b) by the LB technique with PPP:P3VE= 70:30. In the latter case, the polymer chains are oriented exclusively parallel to the substrate surface that is perpendicular to the direction of the applied electric field. In the EMF experiments the electric field amplitude was 1.5 :x: 108 V/m. Also shown are PL spectra (dashed line) with no field applied.

174

N. Pfeffer et al. / Chemical Physics 22 7 (1998) 1 6 7 - 1 7 8

tioned above. Clearly, the macroscopically observed quenching efficiency will depend on the size and dimension of the domains, which are different in the spin-coated film and the LB sample. Fig. 5 shows a distinct spectral difference between the electromodulated fluorescence and the PL spectra. The quenching efficiency is higher in all cases for shorter wavelength and decreases towards the tail of the PL. The shape of the spectra was independent of the angle of incidence of the light beams, thus excluding any interference effects on the emission spectrum [29,30]. We have been successful to describe these spectral features by taking into account a field induced red shift of the PL spectrum according to the quadratic Stark-effect: OIF

A I F O~

-AaFe--(cos-O c9t'

~

2 - "

sin O ) .

(1)

Here, F is the internal electric field strength and A c~ is the difference in the polarisabilities of the ground and the excited state. Eq. (2) is valid only if the light is detected in a direction parallel to the applied electric field and if there is one dominant component of the molecular polarizability, which is assumed to be parallel to the polymer backbone. # then describes the angle between the polymer backbone and the direction of the applied electric field. As shown in Fig. 7 the modulated signal can be convoluted into a component proportional to the original fluorescence spectrum (representing the fluorescence quenching) and a second contribution proportional to the first derivative of the fluorescence spectrum (attributed to the Stark effect). The analysis of the latter contribution yields values for A~ (given in 47re 0 ,~3) of 400, 600 and 1600 ,~3 for P3V, P 3 / 5 V and P3VE, respectively, which are smaller than values of PPV as deduced from electromodulated spectroscopy [31]. However, the relative tendency is quite astonishing since P3VE with a statistically interrupted conjugation yields the highest A o~ (compared to P3V and P3/5V). The Stark contribution seems to scale with the efficiency of fluorescence quenching. Stark effects can safely be excluded by comparison of the EMF signals of the LB-samples (Fig. 6). In the LB film, the chains are oriented strictly parallel to the substrate surface (O = 90 °) and the projection factor in Eq. (2) should vanish. The spectral signa-

t

i

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-5

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I 600

Wavelength [nm] Fig. 7. Convolution of the EMF spectra into a quenching component proportional to the cw-PL spectrum (dashed line) and a Stark component proportional to the first derivative of the spectrum (dotted line). The relative contributions have been chosen to give the best fit (solid line) to the experimental data shown by open squares.

ture of the signal and the absence of any modulation effect in case of pure PPP samples proved that the signal origins solely from the electric field-induced modulation of the P3VE fluorescence. Despite the difference in the molecular orientation the EOF spectrum of the LB sample compares well in position and shape with the spectrum of the corresponding spincoated films. A shift and a slight narrowing of the EOF spectrum is present in both cases. The Stark effect can therefore be ruled out as a possible explanation for the spectral shift. An alternative explanation arises from the above mentioned picture of an inhomogeneous broadening of the density of states (DOS). As already pointed out by Kersting et al. [4], quenching becomes rather difficult as soon as the excitons have relaxed to states deeper in the DOS. As illustrated in Fig. 8 the DOS of the exciton is lowered relative to the DOS of the free charge carrier by an amount equal to the exciton binding energy. This difference can be compensated by applying an external electric field. At moderate fields excitons in the upper part of the DOS can separate into free charge carriers but the dissociation of excitons at the bottom of the DOS is less likely to occur. Site selective fluorescence experiments have indeed shown, that states energetically deep in the DOS correspond to sites for which the

N. Pfeffer et aL / Chemical Physics 227 (1998) 167-178

175

in the solid film. All polymers show a fast non-exponential fluorescence decay. For longer times the decay slows down gradually and becomes quasi-exponential (Fig. 10). For comparison of the fast decay

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450

500

550

600

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Fig. 8. Scheme of the spectral relaxation and electric-field induced exciton dissociation processes in a inhomogeneously broadened density of states (DOS) for a) no field applied and b) with applied electric field [4]. Pt and P2 represent the state densities of the exciton and the charge carrier after transfer to a neighbouring site, respectively.

'--2. O3 .c_ tO¢

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probability of energy transfer is less than the probability of recombination [32]. Following an excitation, excitons are generated in the DOS manifold, which can either decay directly to the ground state (under emission of a photon), split into free carriers under the influence of an electric field or undergo spectral relaxation to the bottom of the DOS. Therefore, photoluminescence consists of photons emitted from excitons at different energies within the density-of-state distribution. At the same time reduced probability for exciton dissociation in the lower part of the DOS can explain the observed blue shift of the EMF spectra relative to the PL. Indeed the EMF spectra broaden if higher electric fields are applied (Fig. 9). In this picture the narrow and shifted EMF spectra at low electric fields resemble the shape of the fluorescence prior to relaxation to the bottom of the DOS. The quite pronounced difference in the shapes of the EMF and PL spectra indicates that considerable spectral relaxation must occur within the fluorescence lifetime. We have therefore performed transient fluorescence experiments on the three polymers

¢-

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W a v e l e n g t h [nm] Fig. 9. EMF spectra recorded at different modulation amplitudes of the sinusoidal electric field. Field amplitudes were: (---) 1.1 × 108 V / m ( . - - ) 1.9X108 V / m and ( - - ) 2.3×108

V/m.

176

N. Pfeffer et al. / Chemical Physics 227 (1998) 1 6 7 - 1 7 8

100-

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100

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Time [ps] Fig. 10. Time resolved luminescence traces at different detection wavelengths for the three polymers in the solid film at room temperature. For excitation 100 fs pulses at 400 nm were used.

in the different samples we define a decay time ~'D for the decrease to 1 / e of the maximum luminescence intensity. The observed decay times varies between 100 and 200 ps. Fig. 11 shows that there is a clear increase in relaxation time with decreasing detection energy. Having in mind the large quantum yields of fluorescence of up to 45%, the fast decay is quite surprising. However, the observed temporal behaviour of the luminescence can be explained by an interplay between radiative and non-radiative recombination channels in addition to a pronounced energy relaxation on the timescale of the fluorescence lifetime [33,34]. We would like to note that Tasch et al. [35] have observed field-induced changes in the photoluminescence of ladder-type poly(p-phenylene)s. The PL spectrum of this compound consists of two narrow peaks at 461 nm and 491 nm and a broader peak centered at 530 nm. In this case the quenching efficiency was the highest for the longest wavelength. This has been attributed to the field-response of two different photogenerated species.

Finally, we like to address briefly the spectral shape of the EL emission spectra. Enhanced EL emission in the tail of the PL spectrum have been frequently reported [14,36-38]. Pronounced effects have in particular been observed in LED-devices based on LB films of PPP or low molecular weight compounds [14,38]. The spectrum shown in Fig. 2b has been recorded at approximately 1.8 × 108 V / m . At this field excitons formed by the collision of electrons and holes might rediffuse into free carriers under the influence of the strong applied field. In this framework, red-shifted EL might result from the lower electric field-induced fluorescence quenching efficiency of excitons trapped on low-lying sites. The experimentally observed quenching efficiencies of up to 10% are, however, too small to account for the observed red shift of the EL solely by an enhanced quenching efficiencies at shorter emission wavelengths. The spectral shift thus indicates that different recombination centres dominate the PL and EL spectra, due to the difference in the excitation mechanism, which is the direct photo-induced singlet exciton formation in PL versus separate charge injecting in EL [14]. Most likely, charges injected into the film will predominately be trapped by states at the bottom at the DOS, resulting in a enhanced EL emission at longer wavelength [36]. Recent experiments on LEDs based on Langmuir-Blodgett emission layers have shown that the externally observed light origins from the recombination of electrons

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N. Pfeffer et al. / Chemical Physics 227 (1998) 167-178

which have travelled in average 2 0 - 4 0 n m into the film [39]. D e p e n d i n g on the mobility of the electrons the time needed to travel across this distance will be in the range of microseconds. This is orders of m a g n i t u d e larger than typical exciton life times of the studied conjugated polymers in the solid film.

4. Conclusion The experiments have proven a strong correlation b e t w e e n the molecular structure of the polymers and the efficiency of field-induced charge carrier separation. These effects are most likely governed by the electronic structure of the individual p o l y m e r chain itself as well as by the packing of the polymers in the solid film. The contribution of inter chain excitons could be p r o v e n by performing experiments on LB samples with the conjugated chains aligned perpendicular to the applied electric field. Differences in the spectral location and shape of the electric field modulated fluorescence signal and the PL spectra indicate the role of spectral relaxation. In this picture the narrow and shifted E M F spectra at low electric fields should closely resemble the shape of the fluorescence prior to relaxation to the bottom of the DOS. This leads to a p r o n o u n c e d blue shift of the E M F spectra. Spectral relaxation has b e e n p r o v e n by performing transient fluorescence experiments on thin films of the polymers. In contrast to this electroluminescence is e n h a n c e d at higher emission wavelengths which points on the role of states deep in the D O S as trapping centres for the injected charge carriers.

Acknowledgements W e like to thank Prof. G. W e g n e r ( M P I - P M a i n z ) and Prof. H. B~issler ( U n i Marburg) for support and fruitful discussions. This work was f i n a n c e d in part by the V o l k s w a g e n - S t i f t u n g and the EC u n d e r Brite-Euram B R E 2 - C T 9 3 - 0 5 9 2 .

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