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Electromodulation of fluorescence in hole-transporting materials (TPD, TAPC) for organic light-emitting diodes
Chemical Physics 256 (2000) 351±362
www.elsevier.nl/locate/chemphys
Electromodulation of ¯uorescence in hole-transporting materials (TPD, TAPC) for organic light-emitting diodes Waldemar Stampor * Department of Molecular Physics, Technical University of Gda nsk, Narutowicza 11=12, 80-952 Gda nsk, Poland Received 24 February 2000
Abstract Electric-®eld-modulated ¯uorescence (EMF) was measured in ®lms of N,N0 -diphenyl-N,N0 -bis(3-methylphenyl)-1,10 biphenyl-4,40 -diamine (TPD) and 1,10 -bis(di-4-tolylaminophenyl) cyclohexane (TAPC), the diamine compounds commonly used as hole-transporting materials in organic light-emitting diodes. External electric ®elds of 2 106 V/cm reduce the integral emission intensity from about 0.3% down to 10% dependent on the excitation photon energy and spectral emission range. The analysis of the experimental data provides evidence that the EMF in TPD is due to the ®eld imposed changes of the dissociation rate of localized molecular excited states. The observed dierence in the EMF for the short-wavelength (monomolecular) and long-wavelength (excimer) emission bands of TAPC can be ascribed to the dierent ®eld response of these two types of emitting species. All these results are consistent with a kinetic scheme that includes formation of molecular excited states, their localization and relaxation into charge pairs within extended trapping domains, and formation of excimer states at a ®eld-dependent rate mediated by the ®eld-assisted dissociation of the charge pairs. The 3D-Onsager theory of dissociation satisfactorily explains the electric ®eld and excitation wavelength dependence of EMF in TPD and that for the excimer emission of TAPC. The quadratic Stark eect on ¯uorescence quantum yield must have been invoked to understand the EMF characteristics of monomolecular emission of TAPC. Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: Electromodulation; Photogeneration; Onsager model; Stark eect; TPD; TAPC
1. Introduction Electric ®eld eect on properties of electronic excited states is of crucial importance in organic electroluminescent (EL) devices which, as a rule, operate under high electric ®elds. Since one of the important exciton decay pathways is its dissociation into a pair of charges, electromodulation of ¯uorescence (EMF) has been extensively used as a method to study charge carrier separation mech*
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anism in organic solids [1±18]. It is usually assumed that intrinsic photogeneration of charge in organic photoconductors proceeds via an intermediate step of a bound (geminate) electron±hole pair (a charge transfer (CT) state). The geminate pair can dissociate into separated carriers, relax by radiative or radiationless transition to the ground state, or due to geminate recombination regenerate an emitting molecular singlet state S1 . An external electric ®eld applied to the sample can change the rate constants of these processes, imposing a change of the ¯uorescence intensity in EMF experiments. Ecient ¯uorescence quenching is
0301-0104/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 ( 0 0 ) 0 0 1 2 3 - 3
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observed when dissociation of exciton states is strongly dependent on electric ®eld since then a direct competition between ¯uorescence and charge separation exists. The measured dependencies of the EMF signal on excitation wavelength and electric ®eld interpreted in the framework of various theoretical models allow the revelation of details of the processes involved in photogeneration mechanism. For example, electric®eld-induced luminescence quenching in doped poly(N-vinylcarbazole) [2,3], quinacridone [11] or a-sexithiophene [18] was successfully explained according to the three-dimensional Onsager model of geminate recombination [19]. A central assumption of the Onsager model, commonly used to analyse charge carrier photogeneration in organic semiconductors, is that the eciency for geminate e±h pair formation does not depend on the electric ®eld. On the other hand, in the case of metal-free pthalocyanine [4,5], thionaphthenoindole [7] and epindolidione [6,11] the EMF data were interpreted assuming an exponential function of the applied ®eld, kCT k0 exp bF , for a CT state formation rate constant kCT [20]. Recently, EMF experiments have been carried out in order to investigate charge photogeneration mechanism in a series of electroluminescent conjugated polymers: poly(phenyl-p-phenylenevinylene) (PPPV) [9], poly(p-phenylenevinylene) (PPV) [10], poly(pterphenylenevinylene) derivatives [14], ladder type poly(para-phenylene) (m-LPPP) [13] and in a typical low-molecular weight electroluminescent (EL) material tris(8-quinolinolato) aluminium (Alq3 ) [12]. In the case of similarity between photoluminescence (PL) and electroluminescence (EL) spectra [21,22], the emission is assumed to be underlain by the same excited states. Since organic LEDs operate under high electric ®elds (>106 V/cm), the EMF eect can strongly reduce EL quantum yield. For example, in electric ®elds as high as 3 106 V/cm, the quenching eect can reach about 60% in PPPV [9] and over 60% in Alq3 [12]. In PPPV, the EMF eect was modelled with Monte Carlo simulation, taking account of the electric ®eld-assisted dissociation of neutral excitons within inhomogeneously broadened density of states (DOS) [9]. Charge carrier separation in Alq3 is dominated by a ®eld-assisted hopping within a local environ-
mental potential (hopping separation model) rather than continuous diusion of a charge carrier in the Coulomb ®eld of its parent countercharge combined with the applied electric ®eld (Onsager model) [12]. The electric ®eld eect on photoluminescence in an organic superlattice structure consisting of alternating layers of Alq3 and aromatic diamine (TPD) has been investigated [8]; the photoluminescence intensity was reduced by applying reverse bias voltage to the multilayer structure with indium±tin oxide (ITO) and Mg:In electrodes. The decrease of photoluminescence from Alq3 layers was stronger when compared with that from TPD layers. However, the observed quenching eect, ascribed to the electric ®eld enhancement of the dissociation of photoexcited excitons, was not analysed quantitatively. In the present paper, we report EMF measurements on thin ®lms of two diamine derivatives: N,N0 -diphenyl-N,N0 -bis(3-methylphenyl)-1,10 -biphenyl-4,40 -diamine (TPD) and 1,10 -bis(di-4-tolylaminophenyl) cyclohexane (TAPC) being most commonly used as hole-transporting layers (HTL) in organic LEDs [21,22]. In addition, they can be employed to form an emissive layer in the blue light-emitting diodes [23±25]. Our aim is to follow the magnitude of the electric ®eld eect on PL as a function of the excitation wavelength and the applied electric ®eld, to reveal its mechanism in these two important HTL and EL materials. We show that the EMF eect observed in TPD and TAPC can be well understood on the grounds of the Onsager model of charge separation. However, the electric ®eld modulation of the short-wavelength part of the TAPC ¯uorescence spectrum can be at least in part due to the Stark eect.
2. Experimental details The measurements were performed on two diamine compounds: TPD and TAPC. The molecular structures of the compounds are shown in Fig. 1. The starting materials, commercially available TPD (Aldrich) and TAPC synthesized at Eastman Kodak laboratory by the procedure
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away from the excitation light path. The electrical signal derived from the photomultiplier consisted of a steady state and alternating components corresponding to the time-dependent ¯uorescencePsignal I(t) expanded in a Fourier series I t Inx t (n 0,1,2,. . .). The signal I0x was measured with a d.c. voltmeter, the second harmonic I2x (t) was processed by a lock-in ampli®er (Princeton Applied Research model 5210). The phase shift between the applied ®eld F(t) and the second harmonic I2x t was 90°. The quantity to be monitored as a function of electric ®eld and excitation wavelength is de®ned here as 2xEMF
Fig. 1. The molecular structure of TPD and TAPC.
described in Ref. [26], were carefully puri®ed by means of vacuum sublimation at 230±240°C. The EMF results were obtained with 50±600 nm thick TPD and TAPC ®lms in the sandwich cell arrangements Al/TPD/Al/quartz and Al/TAPC/ Al/quartz. Organic ®lms were evaporated onto thin semitransparent layers of oxidized aluminium on ultrasonically cleaned room-temperature quartz substrates at pressures of about 10ÿ3 Pa The second and deposited at a rate of about 1 A/s. semitransparent layer of Al was deposited on the top at a rate 2 nm/s. The thicknesses of the organic ®lms were measured using a Talystep-type digital thickness monitor ± Gimetr VIS model I. The active electrode area of the sample was 0.2 cm2 . All the samples had been stored in the dark for several days before measurements. The ¯uorescence of TPD or TAPC was excited by a light beam from a mercury lamp (Narva, HBO 200 W). The electromodulation of ¯uorescence (EMF) was induced by a sinusoidal ®eld (F t F0 sin xt with x/2p 175 Hz) applied to the sample. The global ¯uorescence or some spectral ranges of the ¯uorescence chosen by a set of appropriate glass ®lters (Schott or Corning) were detected by a photomultiplier tube (EMI 9863QB) placed behind the sample in a position
I2x F0 ; I0x F0
1
where I0x and I2x are the zeroth- and second-order Fourier components of the ¯uorescence intensity in the sinusoidal modulating ®eld with an amplitude F0 . Positive values of (2x)EMF signals mean ¯uorescence quenching in a sense that the increasing electric ®eld diminishes ¯uorescence intensity. All measured (2x)EMF signals were excellently reproducible. A more detailed description of the electromodulation measurements and EMF method is given elsewhere [7]. Electroabsorption (EA) signals were measured at the second harmonic (2x) of the applied ®eld frequency (x) by means of a conventional phasesensitive detection technique. Absorption and ¯uorescence spectra were recorded with a Zeiss Specord M10 spectrophotometer and a Spex Fluorolog 2 spectro¯uorometer, respectively.
3. Results The absorption (ABS) and ¯uorescence (FL) spectra of vacuum-evaporated ®lms are depicted in the upper part of Fig. 2 for TPD and in the upper part of Fig. 3 for TAPC. Vibronic structure of the TPD ¯uorescence spectrum is clearly seen (Fig. 2). In TAPC (Fig. 3), in addition to the ®rst emission band with a maximum at 372 nm, a broad band in the long-wavelength part of the FL spectrum (k > 400 nm) is observed. This broad band, appearing only in solid phase and decaying on
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Fig. 2. Top: absorption (Ð) and ¯uorescence ( ) spectra of a 100 nm thick TPD ®lm. Bottom: excitation spectrum of the second harmonic (2x) response of global ¯uorescence to a sinusoidal electric ®eld with an amplitude F0 8:5 105 V/cm.
the nanosecond scale of time, was assigned to singlet excimer emission [27]. In the spectral range of this band, the absorption spectrum is structureless, and the absorption coecient is negligibly small; so an assignment of this long-wavelength emission to aggregates can be ruled out. The ¯uorescence intensity is a linear function of excitation light intensity within the entire excitation range (1010 ± 1015 ) photons/cm2 s (Fig. 4) which excludes eects of exciton interactions on the ¯uorescence yield [28]. The lower parts of Figs. 2 and 3 show electric®eld-induced ¯uorescence quenching eciency ((2x)EMF signal) as a function of the excitation wavelength. In the case of TPD (Fig. 2), quenching of ¯uorescence within the main absorption band with maxima at 317 and 356 nm is approximately constant. Besides, this excitation spectrum reveals an abrupt decrease at the absorption edge and a
Fig. 3. Top: absorption (Ð) and ¯uorescence ( ) spectra of a 70 nm thick TAPC ®lm. Bottom: excitation spectrum of the second harmonic (2x) response of long-wavelength ¯uorescence (s) and short-wavelength ¯uorescence (h) to a sinusoidal electric ®eld with an amplitude F0 1:6 106 V/cm.
Fig. 4. Fluorescence intensity as a function of excitation light intensity (kex 313 nm) for a TAPC ®lm. The data for the longwavelength and short-wavelength emission bands are presented.
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sharp increase at the short-wavelength part where absorption connected with higher energy levels is of importance. In contrast to TPD, electric ®eld eect on ¯uorescence in TAPC (Fig. 3) depends on the spectral range of the detected ¯uorescence. Quenching of the long-wavelength broad emission band in TAPC (k > 400 nm) is several times higher than that of the short-wavelength band (360 < k < 400 nm). In both cases, the (2x)EMF signal is independent of the excitation wavelength within the ®rst low energy absorption band, and excitation spectra show nearly the same main features as in TPD. The EMF measurements were carried out on samples with dierent thicknesses, and no in¯uence of interference (microcavity effects) on the EMF data was observed. The EMF signals as a function of the applied electric ®eld are displayed in Figs. 5 and 6 for TPD and in Figs. 7 and 8 for TAPC. In the same ®gures, electroabsorption signals, (2x)EA, measured at the same wavelength of absorbed light as the excitation wavelength in EMF experiments, are also showed for comparison. The electroabsorption contribution to electromodulation of ¯uorescence (resulted from the electric-®eld-induced change of absorption coecient) was already subtracted
Fig. 5. The double logarithmic (2x)EMF signal±electric ®eld plot at the excitation wavelength kex 313 nm for a TPD ®lm. The squares stand for experimental data, the solid line represents the best ®t according to the 3D-Onsager model (e 2:8, The electroabsorption (EA) change of the g0 0:21, r0 19 A). transmitting light intensity with k 313 nm caused by the Stark eect is shown for comparison ( ).
355
Fig. 6. The double logarithmic (2x)EMF signal±electric ®eld plot at the excitation wavelength kex 260 nm for a TPD ®lm. The circles stand for experimental data, the solid line is a plot calculated according to the 3D-Onsager model (e 2:8, The ®eld dependence of the electroabg0 0:30, r0 21 A). sorption signal (EA) for k 313 nm light passing through the sample is shown by the dotted line.
Fig. 7. Electric ®eld eect on the long-wavelength (k > 400 nm) ¯uorescence band in TAPC. The circles and the squares stand for experimental data of (2x)EMF signal with excitation wavelength kex 313 for 70 nm and 58 nm thick ®lms, respectively. The lines are plots calculated according to dierent theoretical models as indicated in the ®gure. The Onsager The model ®t obtained with e 3:0, g0 0:90 and r0 16 A. NHP or HS model: b 0:022 lm/V and A 40; the Poole± Frenkel model: g0 0:8, bPF 1:9 10ÿ4 (cm/V)1=2 eV and APF 5:0 104 (for the meaning of the parameters see the text and Ref. [7]). The ®eld dependence of the EA signal for k 313 nm light passing through the sample is displayed with the dotted line.
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Fig. 8. Electric ®eld eect on the short-wavelength (360 < k < 400 nm) ¯uorescence band in TAPC. The solid line represents the linear ®t (in log±log scale) of EMF experimental data (squares) obtained with kex 313 nm. The dotted line represents the ®eld dependence of the EA signal for k 313 nm light passing through the sample. The values of the parameters corresponding to the Onsager model-based plot (- - -) are: e 3:0, g0 0:21 and r0 19 A.
from the all displayed EMF signals. The EA signals are at least several times smaller than the EMF signals, and they follow exactly a quadratic function of the applied ®eld re¯ecting the Stark shift of the excited energy levels. In all EMF measurements, ¯uorescence is quenched by the electric ®eld. Under excitation within the ®rst low energy absorption band, the maximum EMF signals at a ®eld amplitude 2 106 V/cm reach a value 3% for TPD (Fig. 5) and 10% for TAPC (Fig. 7). The ¯uorescence quenching in TPD (Fig. 5 for kex 313 nm and Fig. 6 for kex 260 nm) and the long-wavelength (k > 400 nm) ¯uorescence quenching in TAPC (Fig. 7) distinctively depart from a second-order function of the applied electric ®eld. The electric ®eld eect on shortwavelength ¯uorescence (360 < k < 400 nm) in TAPC (Fig. 8) is relatively small and increases exactly with the square of the ®eld strength.
4. Discussion The electromodulation of ¯uorescence in organic molecular solids can be interpreted in terms
of the following main possible mechanisms: (i) electric ®eld eect on electronic energy transfer, (ii) exciton±charge carrier interaction, (iii) the Stark eect on ¯uorescence spectra, and (iv) electric ®eld dependence of radiationless decay channels of excited states (dierent from those mentioned in (i) and (ii)), in particular, electric ®eld-enhanced dissociation of excited states into free and/or trapped charge carriers. A signi®cant electric ®eld eect on electronic energy transfer is expected to occur in systems of molecules with large permanent dipole moments [29]. In these systems, an electric ®eld can reduce or eliminate the possibility of energy transfer between pairs of neighbour molecules by making the energy mismatch comparable to/or larger than the excitation transfer interaction energy. Intermolecular radiationless energy transfer is in turn one of the processes determining the lifetime of excited states, and thus, any change in its rate will alter the population of excited states and, consequently, the intensity of luminescence. This eect cannot be of notable importance in the case of TPD and TAPC molecules with rather small dipole moments. Values of the ground state dipole moments, estimated by quantum chemical calculations, are only 0.3 D for TPD and 0.7 D for TAPC molecule [30]. Electromodulation of ¯uorescence is due to the exciton±charge carrier interaction if the excited states are eectively quenched by free or trapped charge carriers introduced into the molecular system in result of electrode injection and/or internal (bulk) photogeneration. The essential condition for this kind of EMF is the ecient electromodulation of the charge carrier concentration in the light absorption region of the sample. Therefore, good injecting (ohmic) contacts are required to ensure sucient charge injection or ejection through metal/organic solid interfaces. Indeed, the EMF signals as high as several percent were observed in anthracene and tetracene crystals with photoinjecting electrolytic or Au contacts [31,32]. This mechanism of EMF is irrelevant in explaining the observed strong ¯uorescence quenching in organic photoconductors supplied with rather poor injecting Al electrodes. We can safely disregard this as it comes from analyses of photoconductivity results [7,17,18].
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In the Stark eect on emission spectra, two cases should be considered: (a) the so-called Stark shift and (b) the change in quantum yield (integral intensity) of emission bands. The ®rst case is directly connected with the electric-®eld-induced shifts of appropriate energy levels due to the interaction of molecular dipoles (permanent and induced electric dipoles) with the applied electric ®eld. The EMF signal, measured at a speci®c wavelength, is then a linear combination of the ®rst and second energy derivatives of the ¯uorescence spectrum. Besides, in an isotropic medium, this signal increases strictly with the second power of the electric ®eld intensity [33]. The quantitative analysis of the EMF spectra, very similar to that extensively used in the well-known electroabsorption spectroscopy, allows one to evaluate the change in electric permanent dipole moment and/ or molecular polarizability upon photoexcitation [34±36]. At this point, we should note that the EMF results presented in Section 3 cannot be interpreted this way. The reason is that the EMF signals in TPD and TAPC originated from the full spectrum of ¯uorescence bands, so the Stark shift eect would not be detected at all. A further argument against attributing the EMF results to the Stark shift eect is provided by the clear deviation from a second power behaviour seen in Figs. 5 and 6 for TPD and in Fig. 7 for TAPC. The exception is the electromodulation of the short-wavelength component of the TAPC emission spectrum presented in Fig. 8. In this ®gure, the electric ®eld dependence of the EMF is exactly quadratic and the EMF signals are the same order of magnitude as those measured in electroabsorption. However, in the latter case, a more exact theory of the Stark eect ought to be taken into account, which leads to the change in quantum yield of ¯uorescence due to an electric ®eld-induced modi®cation of transition dipole moments [33]. In this kind of the Stark eect, the EMF signals follow, similarly as in the Stark shift eect, a quadratic function of the applied electric ®eld. Therefore, we suppose that the electric ®eld eect on short-wavelength ¯uorescence in TAPC can be at least in part due to the Stark change in quantum yield of ¯uorescence. This result will be discussed later on.
357
On the basis of the above reasoning, we infer that the ®eld-assisted thermal dissociation of excitons into free and/or trapped charge carriers is the most probable process for the quenching of ¯uorescence by an electric ®eld observed in TPD and TAPC ®lms. First, we will discuss in greater detail the EMF results in TPD. The ¯uorescence spectrum of a TPD ®lm displayed in the upper part of Fig. 2 is expected to be of molecular origin. We attribute the observed ¯uorescence quenching to a ®eld-dependent dissociation of coulombically bound electron±hole pairs (CT states) created in a ®eld-independent process from primary excited singlet Frenkel states. The dissociation process can be described in the framework of the threedimensional Onsager model [19]. According to this model, the escape probability of geminate recombination can be expressed as follows: XOns F 1 ÿ nÿ1
1 X
Pj rC =r0 Pj n;
2
j0
where Pj x is given by a recursion formula, Pj x Pjÿ1 x ÿ xj exp ÿx=j!;
3
P0 x 1 ÿ exp ÿx:
4
The ®eld dependence of the escape probability, XOns F , appears through the parameter n
er0 j F j : kT
5
In the above equations, rC represents the so-called ``Onsager distance'' at which the coulombic attraction energy is equal to the thermal energy, kT, rC
e2 4pe0 ekT
6
and r0 stands for a discrete characteristic thermalization length. The latter quantity corresponds to the initial electron±hole separation distance which, to be more realistic, should be modelled by an exponential or Gaussian distribution function [37,38]. Since the dissociation process of an electron±hole pair competes with geminate recombination into the ¯uorescent state, the ¯uorescence
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intensity is expected to be a function of the electric ®eld, I F
kf 1 ÿ g F I ; kf kn
7
where kf and kn are the rate constants of radiative and other (dierent from charge separation) nonradiative decay pathways for singlet emitting exciton states, respectively, and I is the rate at which excited states are produced. The primary quantum yield of geminate carrier pair formation, g0 , is usually assumed to be ®eld independent in the Onsager model of charge separation with the ®eld dependent quantum eciency of photogeneration expressed as g F g0 XOns F :
8
To compare this model with experiment, calculated values of I2x =I0x have been ®tted to the measured (2x)EMF signals as described earlier [7]. In the ®tting procedure, the primary quantum yield g0 and the initial e±h separation distance r0 were adjusted using the experimental value e 2:8 obtained from capacitance measurements for TPD ®lms [25]. The good agreement between the curves calculated based on the Onsager model and the experimental data is seen from Figs. 5 and 6. Under excitation kex 313 nm (within the ®rst low energy absorption band) the best ®t, with param is attained (the solid eters g0 0:21 and r0 19 A, line in Fig. 5). The independence of the EMF signals on excitation wavelength in this spectral range, shown in Fig. 2, indicates at a vibrationally relaxed singlet S1 state as a common precursor for formation of the geminate e±h pair. In this regard, the photogeneration mechanism in TPD seems to be very similar to that proposed by Noolandi and Hong [20], where the initial charge separation in organic photoconductors is considered to occur after the internal conversion and vibrational relaxation to the lowest excited singlet state of the molecules. The excitation of higher energy levels (above the ®rst absorption band) enlarges electromodulation of ¯uorescence because geminate pairs with greater r0 (so weaker bound) are then formed during the initial charge separation process. In fact, for excitation kex 260 nm, we get, in terms of the Onsager model, larger values of the
parameters g0 and r0 (the solid line with g0 0:30 in Fig. 6). The values of the primary and r0 21 A separation distance r0 , found in TPD, compare favourably with those obtained by the EMF method in other organic photoconductors [3,11,18]. Using the Gaussian distribution function of (e±h) pair radii, instead of a discrete value of the radius r0 , in the ®tting procedure would lead to shorter separation distances [37,39]. The rather high values of g0 suggest in turn the primary charge separation step in the TPD ®lm to be a very ecient process. This would imply that some more extended, intermolecular CT states exist close to the ¯uorescent localized Frenkel state and the photoexcited singlet states, fairly well delocalized, produce geminate pairs which undergo in turn an electric ®eld-dependent dissociation. To verify this hypothesis, the energy location of CT states in TPD should be established by an independent method. Actually, a preliminary analysis of the electroabsorption spectrum of the TPD ®lm revealed the polar states, most probably having partly intermolecular charge transfer character, placed in the spectral range of the ®rst singlet absorption band [30]. If S1 states and CT states are energetically degenerated, g0 in the Onsager theory can be considered as a CT fraction of the hybridized Frenkel/CT exciton. The electric ®eld could then change this fraction. The mixing of quasidegenerate Frenkel and CT excitons has been really observed in electroabsorption (EA) of quinacridone ®lms [40]. However, the eect was rather small and followed a quadratic function of the applied voltage. The stronger EMF signals could not be reconciled with this mechanism [11] like in the present case. Our values of the EMF signals presented in Fig. 5 agree well with those obtained for photoluminescence originating from TPD placed in multilayer structures TPD/Alq3 [8]. This comparison is reasonable because as shown in Fig. 5 of Ref. [8], the TPD photoluminescence quenching in multilayer structures rather does not depend on the TPD layer thickness. Considering the above-stated facts, the origin of the strong photoluminescence quenching observed in TPD monolayer in Ref. [8] (above 20% at the electric ®eld intensity of 106 V/cm) is not clear. We suppose that in a mirror-based cell arrangement
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studied in Ref. [8], electric-®eld-induced changes of optical properties of a cavity could be of importance, so the cavity eects could increase the EMF signals measured in monolayers. As can be seen in Figs. 7 and 8, a long-wavelength and short-wavelength part of TAPC emission spectrum are aected in a dierent manner by the external electric ®eld. This is not surprising if we take into account dierent natures of these two emission spectral components. According to Kalinowski et al. [27], the short-wavelength emission band originates from singlet excited states of tri(ptolyl)amine (TTA) subunits of TAPC molecule and the broad long-wavelength emission band in solid TAPC stems from singlet excimeric states formed by TTA moieties of neighbour TAPC molecules. We will ®rst discuss the mechanism of the ®eld-induced quenching of excimer-type ¯uorescence in TAPC. It is worthwhile to compare the present results of excimer ¯uorescence quenching with a similar quenching of exciplex ¯uorescence observed by Yokoyama and co-workers [2,3] in poly(N-vinylcarbazole) doped with a week electron acceptor. The authors of Refs. [2,3] considered that the ¯uorescent exciplex state is formed by a geminate recombination of the electron±hole pair state in competition with the ®eld-assisted dissociation into free charge carriers described in quantitative terms of the Onsager model. A very similar approach to the exciplex ¯uorescence quenching has been recently applied by Ohta and co-workers [15,16] for a mixture of ethyl carbazole and dimethyl terephthalate doped in poly(methyl methacrylate) (PMMA) polymer ®lms. We assume that the ®eld-induced excimer ¯uorescence quenching observed in TAPC could be of similar origin. The kinetic scheme that will be used to analyse the experimental data is shown in Fig. 9. According to this scheme, a relaxed singlet molecular exciton (M ) in a TAPC ®lm migrates until it reaches the so-called macrotrap [41,42] (spatially extended domain of crystal lattice perturbation). An encounter complex M Mt formed within the macrotrap is considered to have a favourable molecular conformation for the electron transfer. The key point of the charge separation mechanism presented in Fig. 9 is that electron±hole separation occurs at the expense of exciton localization en-
359
Fig. 9. The kinetic scheme used to describe the electric ®eld modulation of ¯uorescence (monomer excimer emission) in TAPC. The molecular exciton (M ) migrates forming an encounter complex, M Mt , at an extended lattice defect (macrotrap). The encounter complex M Mt can then relax to the excimer state (MM) with the probability 1 ÿ g0 or transform into the geminate (e±h) pair M Mÿ with the probability g0 . The geminate pairs undergo either ®eld-assisted charge separation M Mÿ at a rate de®ned by X(F), or form the excimer state with the probability 1 ÿ X F .
ergy resulting in a geminate pair M Mÿ formed with the ®eld independent probability g0 (see also Ref. [7]). The geminate pair at a separation distance r0 either dissociates thermally into free carriers with the electric ®eld-dependent probability, X(F), or recombines to give an excimer state (MM) . A direct decay path from the encounter complex state M Mt to the excimer state (MM) without involvement of the geminate pair state (with the probability of this process being (1 ÿ g0 )) is also possible. From this scheme, the electric ®eld dependence of excimer ¯uorescence intensity can be derived and expressed as follows: I F
kf0 1 ÿ g0 X F I : kf0 kn0
9
This expression is very similar to Eq. (7). In Eq. (9), kf0 and kn0 denote the appropriate (radiative and non-radiative, respectively) decay rate constants for excimer states, and I is the rate at which encounter complex states M Mt are formed. Inserting the Onsager expression (2) for X(F) into Eq. (9), and applying the same ®tting procedure, as previously used for TPD, the excimer ¯uorescence quenching in TAPC is successfully explained. As can be seen in Fig. 7, the experimental EMF data are reproduced excellently by the theoretical curve
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based on the Onsager theory with g0 0:9 0:1, and e 3:0 (the solid line in Fig. 7), r0 16 A which strongly supports the Onsager type (diusion-controlled) charge separation in TAPC ®lms. Furthermore, according to the scheme shown in Fig. 9, the very high value of g0 0:9 indicates that the formation of most excimer states in TAPC ®lms proceeds mainly through geminate (e±h) pair states M Mÿ . This gives good correspondence with exciplex formation mechanism in an electron donor and acceptor systems [2,3,15,16]. The recent comprehensive study of delayed ¯uorescence in methyl-substituted poly(para-phenylene) (MeLPPP) [43] also leads to a conclusion that excimer ¯uorescence is caused by the recombination of geminate pairs produced from singlet excitations. In principle, it is possible that geminate (e±h) pairs are formed directly from relaxed excimer states [39]. However, transient absorption and emission experiments [44,45], particularly an unusually slow and non-exponential decay of exciplex ¯uorescence in doped poly(N-vinylcarbazole), suggest the dynamics of exciplex formation to be determined by the recombination of primary (e±h) pairs. Therefore, we assume the excimer formation process via (e±h) pairs to be more probable also in our case of TAPC. We shall now compare the present interpretation of the EMF results based on the Onsager theory with other models of charge photogeneration, which are usually applied to elucidate electromodulation of ¯uorescence in organic photoconductors. In the Poole±Frenkel (PF) formalism [46], an electron±hole pair dissociation is treated as a one-step thermally activated overcoming the Coulomb barrier lowered by the external ®eld, a carrier diusion motion during charge separation process being completely disregarded. In the conventional formulations of the Onsager and PF models, a ®eld-independent yield of geminate pair formation, g0 , is usually assumed. On the contrary, an exponential ®eld dependence of the rate constant for electron±hole pair (CT) formation, kCT k0 exp bF ;
10
is inhered in the Noolandi±Hong±Popovic (NHP) model [5,20] or the hopping separation model (HS)
[7,12]. The quantum yield of charge separation can be expressed as follows: g F gPF F g0
exp bPF F 1=2 =kT APF exp bPF F 1=2 =kT
11
in the PF model, and g F gCT F
exp bF =kT A exp bF =kT
12
in the NHP or HS model. In Eqs. (11) and (12), APF and A are constants determining the branching ratio of recombination and generation channels at F 0 in the PF and NHP or HS models, respectively. To obtain the luminescence intensity I(F) in the relevant model, expression (11) or (12) should be substituted into Eq. (7) (for a further description of these models see Ref. [7] and references therein). However, in the present work, it has been found that the ®eld dependence of the EMF signals in TPD and TAPC can be consistently understood in the framework of the Onsager model. The PF model-based curves deviate distinctively from the experimental EMF data in the high ®eld range (see the dash-dotted line in Fig. 7). In addition, the most basic parameter bPF of this model is found to be over two times smaller than the theoretical value of (e3 =pee0 )1=2 predicted by the Poole±Frenkel expression. Similarly, the plots obtained from the models NHP or HS with an electric-®eld-dependent rate constant, kCT , expressed by Eq. (10), are in apparent disagreement with experimental data (see the best ®t shown in Fig. 7 by the dashed line). Finally, we should discuss the mechanism of short-wavelength ¯uorescence quenching in TAPC ®lms. The in¯uence of the electric ®eld on this type of ¯uorescence (Fig. 8) is signi®cantly weaker than that on excimer ¯uorescence (Fig. 7). This fact correlates well with the previously made assumption that the appropriate excimer states in TAPC ®lms are formed mainly via geminate pair states which can be eciently dissociated by the electric ®eld. According to the present results, the ®eldassisted dissociation of molecular Frenkel excitons, related to the short-wavelength (monomer) ¯uorescence quenching in TAPC, appears to be a less ecient process. The dierence arises pre-
W. Stampor / Chemical Physics 256 (2000) 351±362
sumably from dierent recombination sites available in a TAPC ®lm for excimer and Frenkel states, respectively. The exact quadratic ®eld dependence of monomer ¯uorescence quenching observed in TAPC (Fig. 8) cannot be easily rationalized on the basis of EMF models, involving ®eld-induced dissociation of excitons, applied in the present work. For example, as shown in Fig. 8, the slope of Onsager model-based curves, displayed in a double logarithmic plot, changes continuously with an increasing applied ®eld strength if only a ®eld range is not too small. Square-type electric ®eld dependencies of EMF quenching were observed also in other organic photoconductors [2,3,10,14,47]. To explain experimental data, the electric ®eld dependence of the initial charge separation distance r0 (Onsager theory) was postulated in Ref. [3]. It has been claimed that the quadratic ®eld dependence of ¯uorescence quenching in an isotropic medium is a direct consequence of symmetry [10]. When an external ®eld is applied to that medium, excited state dissociation rate should not depend on the direction of the applied electric ®eld. Consequently, the quenching eciency should be an even function of the electric ®eld, i.e., quadratic in the lowest order if a ®eld strength is not too high. In addition, as mentioned in the beginning of this section, the rather small quenching of short-wavelength (monomer) ¯uorescence in TAPC suggests that the quadratic Stark eect on quantum ¯uorescence yield can also contribute to electromodulation in TAPC. Unfortunately, the ®eld dependence of EMF signals, calculated according to dierent models, approximately follows the same power law in a narrow-®eld range which is typically accessible in experiment [7]. Therefore, at the present stage, it is not possible to make a conclusive statement concerning the origin of monomer ¯uorescence quenching in TAPC. It is noteworthy to mention here that the electric ®eld-induced quenching in MeLPPP [43] is also noticeably dierent for both types of observed ¯uorescence. However, in contrast with our results, the quenching eciency for intrinsic (monomer) ¯uorescence in MeLPPP is higher than that for excimer ¯uorescence. The distinctive electric ®eld behaviour of excimer ¯uorescence in MeLPPP has been explained by the
361
assumption that at the pumping intensities being employed in the experiment, the appropriate excimer sites were populated in the saturated regime. Since in our case the linear intensity dependence of both the ¯uorescence spectral components was observed (Fig. 4), the in¯uence of the non-linear processes on singlet and excimer population should be excluded.
5. Conclusions We have demonstrated that ¯uorescence of vacuum-evaporated ®lms of diamine derivatives (TPD and TAPC) is quenched by external electric ®eld. The dependence of ¯uorescence quenching on electric ®eld and excitation wavelength is well understood in terms of the ®eld-assisted dissociation of excitons into charge carriers, described by the three-dimensional Onsager model. The good agreement between the present EMF results and the Onsager theory indicates that the primary charge separation is rather a ®eld-independent process and the external electric ®eld in¯uences only the further, diusion-controlled step of charge separation. It has been found that the two bands of TAPC ¯uorescence spectrum, ascribed to singlet monomer and excimer emission, respectively, are dierently aected by the external electric ®eld. The very ecient quenching of excimer emission has been shown to be compatible with the assumption that formation of excimers proceeds via geminate pair intermediates, while the much weaker electromodulation of monomer emission seems to be a result of the quadratic Stark eect.
Acknowledgements The author would like to thank Prof. Jan Kalinowski for stimulating discussions and invaluable help. Dr. P.M. Borsenberger is gratefully acknowledged for the providing of TAPC material. The author also thanks Dr. M. Cocchi for measurement of ¯uorescence spectra of TPD and TAPC ®lms.
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[24] J. Kido, M. Kimura, K. Nagai, Science 267 (1995) 1332. [25] J. Kalinowski, P. Di Marco, V. Fattori, L. Giulietti, M. Cocchi, J. Appl. Phys. 83 (1998) 4242. [26] N.G. Rule, M.A. Berwick, L.E. Contois, US Patent 4,127,412, 28 November 1978. [27] J. Kalinowski, G. Giro, M. Cocchi, V. Fattori, P. Di Marco, Appl. Phys. Lett. 76 (2000) 2352. [28] V. Ern, J.L. Saint-Clair, M. Schott, G. Delacote, Chem. Phys. Lett. 10 (1971) 287. [29] D.M. Hanson, in: F.L. Carter (Ed.), Molecular Electronic Devices, Marcel Dekker, New York, 1982, p. 89. [30] J. Kalinowski, W. Stampor, G. Marconi, P. Di Marco, in preparation. [31] L. Peter, G. Vaubel, Phys. Stat. Sol. (b) 48 (1971) 587. [32] J. Kalinowski, J. Godlewski, R.R. Chance, J. Chem. Phys. 64 (1976) 2389. [33] W. Liptay, in: O. Sinanoglu (Ed.), Modern Quantum Chemistry, vol. 3, Academic Press, New York, 1965, p. 45. [34] R.M. Hochstrasser, L.J. Noe, Chem. Phys. Lett. 5 (1970) 489. [35] D.J. Lockhart, S.G. Boxer, Chem. Phys. Lett. 144 (1988) 243. [36] S. Umeuchi, Y. Nishimura, I. Yamazaki, H. Murakami, M. Yamashita, N. Ohta, Thin Solid Films 311 (1997) 239. [37] R.R. Chance, C.L. Brown, J. Chem. Phys. 64 (1976) 3573. [38] T.K. D aubler, V. Cimrov a, S. Pfeier, H.-H. H orhold, D. Neher, Advanced Materials 11 (1999) 1274. [39] V. Cimrov a, S. Nesp urek, Chem. Phys. 184 (1994) 283. [40] J. Kalinowski, W. Stampor, P. Di Marco, V. Fattori, Chem. Phys. 182 (1994) 341. [41] J. Kalinowski, J. Godlewski, P. Mondalski, Mol. Cryst. Liq. Cryst. 175 (1989) 67. [42] J. Kalinowski, J. Godlewski, P. Mondalskii, in: K. Sumino (Ed.), Defect Control in Semiconductors, vol. II, NorthHolland, Amsterdam, 1990, p. 1705. [43] Yu. Romanovskii, A. Gerhard, B. Schweitzer, R.I. Personov, H. B assler, Chem. Phys. 249 (1999) 29. [44] H. Sakai, A. Itaya, H. Masuhara, J. Phys. Chem. 93 (1989) 5351. [45] A. Itaya, T. Yamada, H. Masuhara, Chem. Phys. Lett. 174 (1990) 145. [46] J. Frenkel, Phys. Rev. 54 (1938) 647. [47] Y. Nishimura, N. Ohta, M. Yamamoto, I. Yamazaki, Mol. Cryst. Liq. Cryst. 315 (1998) 181.
www.elsevier.nl/locate/chemphys
Electromodulation of ¯uorescence in hole-transporting materials (TPD, TAPC) for organic light-emitting diodes Waldemar Stampor * Department of Molecular Physics, Technical University of Gda nsk, Narutowicza 11=12, 80-952 Gda nsk, Poland Received 24 February 2000
Abstract Electric-®eld-modulated ¯uorescence (EMF) was measured in ®lms of N,N0 -diphenyl-N,N0 -bis(3-methylphenyl)-1,10 biphenyl-4,40 -diamine (TPD) and 1,10 -bis(di-4-tolylaminophenyl) cyclohexane (TAPC), the diamine compounds commonly used as hole-transporting materials in organic light-emitting diodes. External electric ®elds of 2 106 V/cm reduce the integral emission intensity from about 0.3% down to 10% dependent on the excitation photon energy and spectral emission range. The analysis of the experimental data provides evidence that the EMF in TPD is due to the ®eld imposed changes of the dissociation rate of localized molecular excited states. The observed dierence in the EMF for the short-wavelength (monomolecular) and long-wavelength (excimer) emission bands of TAPC can be ascribed to the dierent ®eld response of these two types of emitting species. All these results are consistent with a kinetic scheme that includes formation of molecular excited states, their localization and relaxation into charge pairs within extended trapping domains, and formation of excimer states at a ®eld-dependent rate mediated by the ®eld-assisted dissociation of the charge pairs. The 3D-Onsager theory of dissociation satisfactorily explains the electric ®eld and excitation wavelength dependence of EMF in TPD and that for the excimer emission of TAPC. The quadratic Stark eect on ¯uorescence quantum yield must have been invoked to understand the EMF characteristics of monomolecular emission of TAPC. Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: Electromodulation; Photogeneration; Onsager model; Stark eect; TPD; TAPC
1. Introduction Electric ®eld eect on properties of electronic excited states is of crucial importance in organic electroluminescent (EL) devices which, as a rule, operate under high electric ®elds. Since one of the important exciton decay pathways is its dissociation into a pair of charges, electromodulation of ¯uorescence (EMF) has been extensively used as a method to study charge carrier separation mech*
Fax: +48-58-347-2821.
anism in organic solids [1±18]. It is usually assumed that intrinsic photogeneration of charge in organic photoconductors proceeds via an intermediate step of a bound (geminate) electron±hole pair (a charge transfer (CT) state). The geminate pair can dissociate into separated carriers, relax by radiative or radiationless transition to the ground state, or due to geminate recombination regenerate an emitting molecular singlet state S1 . An external electric ®eld applied to the sample can change the rate constants of these processes, imposing a change of the ¯uorescence intensity in EMF experiments. Ecient ¯uorescence quenching is
0301-0104/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 ( 0 0 ) 0 0 1 2 3 - 3
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observed when dissociation of exciton states is strongly dependent on electric ®eld since then a direct competition between ¯uorescence and charge separation exists. The measured dependencies of the EMF signal on excitation wavelength and electric ®eld interpreted in the framework of various theoretical models allow the revelation of details of the processes involved in photogeneration mechanism. For example, electric®eld-induced luminescence quenching in doped poly(N-vinylcarbazole) [2,3], quinacridone [11] or a-sexithiophene [18] was successfully explained according to the three-dimensional Onsager model of geminate recombination [19]. A central assumption of the Onsager model, commonly used to analyse charge carrier photogeneration in organic semiconductors, is that the eciency for geminate e±h pair formation does not depend on the electric ®eld. On the other hand, in the case of metal-free pthalocyanine [4,5], thionaphthenoindole [7] and epindolidione [6,11] the EMF data were interpreted assuming an exponential function of the applied ®eld, kCT k0 exp bF , for a CT state formation rate constant kCT [20]. Recently, EMF experiments have been carried out in order to investigate charge photogeneration mechanism in a series of electroluminescent conjugated polymers: poly(phenyl-p-phenylenevinylene) (PPPV) [9], poly(p-phenylenevinylene) (PPV) [10], poly(pterphenylenevinylene) derivatives [14], ladder type poly(para-phenylene) (m-LPPP) [13] and in a typical low-molecular weight electroluminescent (EL) material tris(8-quinolinolato) aluminium (Alq3 ) [12]. In the case of similarity between photoluminescence (PL) and electroluminescence (EL) spectra [21,22], the emission is assumed to be underlain by the same excited states. Since organic LEDs operate under high electric ®elds (>106 V/cm), the EMF eect can strongly reduce EL quantum yield. For example, in electric ®elds as high as 3 106 V/cm, the quenching eect can reach about 60% in PPPV [9] and over 60% in Alq3 [12]. In PPPV, the EMF eect was modelled with Monte Carlo simulation, taking account of the electric ®eld-assisted dissociation of neutral excitons within inhomogeneously broadened density of states (DOS) [9]. Charge carrier separation in Alq3 is dominated by a ®eld-assisted hopping within a local environ-
mental potential (hopping separation model) rather than continuous diusion of a charge carrier in the Coulomb ®eld of its parent countercharge combined with the applied electric ®eld (Onsager model) [12]. The electric ®eld eect on photoluminescence in an organic superlattice structure consisting of alternating layers of Alq3 and aromatic diamine (TPD) has been investigated [8]; the photoluminescence intensity was reduced by applying reverse bias voltage to the multilayer structure with indium±tin oxide (ITO) and Mg:In electrodes. The decrease of photoluminescence from Alq3 layers was stronger when compared with that from TPD layers. However, the observed quenching eect, ascribed to the electric ®eld enhancement of the dissociation of photoexcited excitons, was not analysed quantitatively. In the present paper, we report EMF measurements on thin ®lms of two diamine derivatives: N,N0 -diphenyl-N,N0 -bis(3-methylphenyl)-1,10 -biphenyl-4,40 -diamine (TPD) and 1,10 -bis(di-4-tolylaminophenyl) cyclohexane (TAPC) being most commonly used as hole-transporting layers (HTL) in organic LEDs [21,22]. In addition, they can be employed to form an emissive layer in the blue light-emitting diodes [23±25]. Our aim is to follow the magnitude of the electric ®eld eect on PL as a function of the excitation wavelength and the applied electric ®eld, to reveal its mechanism in these two important HTL and EL materials. We show that the EMF eect observed in TPD and TAPC can be well understood on the grounds of the Onsager model of charge separation. However, the electric ®eld modulation of the short-wavelength part of the TAPC ¯uorescence spectrum can be at least in part due to the Stark eect.
2. Experimental details The measurements were performed on two diamine compounds: TPD and TAPC. The molecular structures of the compounds are shown in Fig. 1. The starting materials, commercially available TPD (Aldrich) and TAPC synthesized at Eastman Kodak laboratory by the procedure
W. Stampor / Chemical Physics 256 (2000) 351±362
353
away from the excitation light path. The electrical signal derived from the photomultiplier consisted of a steady state and alternating components corresponding to the time-dependent ¯uorescencePsignal I(t) expanded in a Fourier series I t Inx t (n 0,1,2,. . .). The signal I0x was measured with a d.c. voltmeter, the second harmonic I2x (t) was processed by a lock-in ampli®er (Princeton Applied Research model 5210). The phase shift between the applied ®eld F(t) and the second harmonic I2x t was 90°. The quantity to be monitored as a function of electric ®eld and excitation wavelength is de®ned here as 2xEMF
Fig. 1. The molecular structure of TPD and TAPC.
described in Ref. [26], were carefully puri®ed by means of vacuum sublimation at 230±240°C. The EMF results were obtained with 50±600 nm thick TPD and TAPC ®lms in the sandwich cell arrangements Al/TPD/Al/quartz and Al/TAPC/ Al/quartz. Organic ®lms were evaporated onto thin semitransparent layers of oxidized aluminium on ultrasonically cleaned room-temperature quartz substrates at pressures of about 10ÿ3 Pa The second and deposited at a rate of about 1 A/s. semitransparent layer of Al was deposited on the top at a rate 2 nm/s. The thicknesses of the organic ®lms were measured using a Talystep-type digital thickness monitor ± Gimetr VIS model I. The active electrode area of the sample was 0.2 cm2 . All the samples had been stored in the dark for several days before measurements. The ¯uorescence of TPD or TAPC was excited by a light beam from a mercury lamp (Narva, HBO 200 W). The electromodulation of ¯uorescence (EMF) was induced by a sinusoidal ®eld (F t F0 sin xt with x/2p 175 Hz) applied to the sample. The global ¯uorescence or some spectral ranges of the ¯uorescence chosen by a set of appropriate glass ®lters (Schott or Corning) were detected by a photomultiplier tube (EMI 9863QB) placed behind the sample in a position
I2x F0 ; I0x F0
1
where I0x and I2x are the zeroth- and second-order Fourier components of the ¯uorescence intensity in the sinusoidal modulating ®eld with an amplitude F0 . Positive values of (2x)EMF signals mean ¯uorescence quenching in a sense that the increasing electric ®eld diminishes ¯uorescence intensity. All measured (2x)EMF signals were excellently reproducible. A more detailed description of the electromodulation measurements and EMF method is given elsewhere [7]. Electroabsorption (EA) signals were measured at the second harmonic (2x) of the applied ®eld frequency (x) by means of a conventional phasesensitive detection technique. Absorption and ¯uorescence spectra were recorded with a Zeiss Specord M10 spectrophotometer and a Spex Fluorolog 2 spectro¯uorometer, respectively.
3. Results The absorption (ABS) and ¯uorescence (FL) spectra of vacuum-evaporated ®lms are depicted in the upper part of Fig. 2 for TPD and in the upper part of Fig. 3 for TAPC. Vibronic structure of the TPD ¯uorescence spectrum is clearly seen (Fig. 2). In TAPC (Fig. 3), in addition to the ®rst emission band with a maximum at 372 nm, a broad band in the long-wavelength part of the FL spectrum (k > 400 nm) is observed. This broad band, appearing only in solid phase and decaying on
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Fig. 2. Top: absorption (Ð) and ¯uorescence ( ) spectra of a 100 nm thick TPD ®lm. Bottom: excitation spectrum of the second harmonic (2x) response of global ¯uorescence to a sinusoidal electric ®eld with an amplitude F0 8:5 105 V/cm.
the nanosecond scale of time, was assigned to singlet excimer emission [27]. In the spectral range of this band, the absorption spectrum is structureless, and the absorption coecient is negligibly small; so an assignment of this long-wavelength emission to aggregates can be ruled out. The ¯uorescence intensity is a linear function of excitation light intensity within the entire excitation range (1010 ± 1015 ) photons/cm2 s (Fig. 4) which excludes eects of exciton interactions on the ¯uorescence yield [28]. The lower parts of Figs. 2 and 3 show electric®eld-induced ¯uorescence quenching eciency ((2x)EMF signal) as a function of the excitation wavelength. In the case of TPD (Fig. 2), quenching of ¯uorescence within the main absorption band with maxima at 317 and 356 nm is approximately constant. Besides, this excitation spectrum reveals an abrupt decrease at the absorption edge and a
Fig. 3. Top: absorption (Ð) and ¯uorescence ( ) spectra of a 70 nm thick TAPC ®lm. Bottom: excitation spectrum of the second harmonic (2x) response of long-wavelength ¯uorescence (s) and short-wavelength ¯uorescence (h) to a sinusoidal electric ®eld with an amplitude F0 1:6 106 V/cm.
Fig. 4. Fluorescence intensity as a function of excitation light intensity (kex 313 nm) for a TAPC ®lm. The data for the longwavelength and short-wavelength emission bands are presented.
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sharp increase at the short-wavelength part where absorption connected with higher energy levels is of importance. In contrast to TPD, electric ®eld eect on ¯uorescence in TAPC (Fig. 3) depends on the spectral range of the detected ¯uorescence. Quenching of the long-wavelength broad emission band in TAPC (k > 400 nm) is several times higher than that of the short-wavelength band (360 < k < 400 nm). In both cases, the (2x)EMF signal is independent of the excitation wavelength within the ®rst low energy absorption band, and excitation spectra show nearly the same main features as in TPD. The EMF measurements were carried out on samples with dierent thicknesses, and no in¯uence of interference (microcavity effects) on the EMF data was observed. The EMF signals as a function of the applied electric ®eld are displayed in Figs. 5 and 6 for TPD and in Figs. 7 and 8 for TAPC. In the same ®gures, electroabsorption signals, (2x)EA, measured at the same wavelength of absorbed light as the excitation wavelength in EMF experiments, are also showed for comparison. The electroabsorption contribution to electromodulation of ¯uorescence (resulted from the electric-®eld-induced change of absorption coecient) was already subtracted
Fig. 5. The double logarithmic (2x)EMF signal±electric ®eld plot at the excitation wavelength kex 313 nm for a TPD ®lm. The squares stand for experimental data, the solid line represents the best ®t according to the 3D-Onsager model (e 2:8, The electroabsorption (EA) change of the g0 0:21, r0 19 A). transmitting light intensity with k 313 nm caused by the Stark eect is shown for comparison ( ).
355
Fig. 6. The double logarithmic (2x)EMF signal±electric ®eld plot at the excitation wavelength kex 260 nm for a TPD ®lm. The circles stand for experimental data, the solid line is a plot calculated according to the 3D-Onsager model (e 2:8, The ®eld dependence of the electroabg0 0:30, r0 21 A). sorption signal (EA) for k 313 nm light passing through the sample is shown by the dotted line.
Fig. 7. Electric ®eld eect on the long-wavelength (k > 400 nm) ¯uorescence band in TAPC. The circles and the squares stand for experimental data of (2x)EMF signal with excitation wavelength kex 313 for 70 nm and 58 nm thick ®lms, respectively. The lines are plots calculated according to dierent theoretical models as indicated in the ®gure. The Onsager The model ®t obtained with e 3:0, g0 0:90 and r0 16 A. NHP or HS model: b 0:022 lm/V and A 40; the Poole± Frenkel model: g0 0:8, bPF 1:9 10ÿ4 (cm/V)1=2 eV and APF 5:0 104 (for the meaning of the parameters see the text and Ref. [7]). The ®eld dependence of the EA signal for k 313 nm light passing through the sample is displayed with the dotted line.
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Fig. 8. Electric ®eld eect on the short-wavelength (360 < k < 400 nm) ¯uorescence band in TAPC. The solid line represents the linear ®t (in log±log scale) of EMF experimental data (squares) obtained with kex 313 nm. The dotted line represents the ®eld dependence of the EA signal for k 313 nm light passing through the sample. The values of the parameters corresponding to the Onsager model-based plot (- - -) are: e 3:0, g0 0:21 and r0 19 A.
from the all displayed EMF signals. The EA signals are at least several times smaller than the EMF signals, and they follow exactly a quadratic function of the applied ®eld re¯ecting the Stark shift of the excited energy levels. In all EMF measurements, ¯uorescence is quenched by the electric ®eld. Under excitation within the ®rst low energy absorption band, the maximum EMF signals at a ®eld amplitude 2 106 V/cm reach a value 3% for TPD (Fig. 5) and 10% for TAPC (Fig. 7). The ¯uorescence quenching in TPD (Fig. 5 for kex 313 nm and Fig. 6 for kex 260 nm) and the long-wavelength (k > 400 nm) ¯uorescence quenching in TAPC (Fig. 7) distinctively depart from a second-order function of the applied electric ®eld. The electric ®eld eect on shortwavelength ¯uorescence (360 < k < 400 nm) in TAPC (Fig. 8) is relatively small and increases exactly with the square of the ®eld strength.
4. Discussion The electromodulation of ¯uorescence in organic molecular solids can be interpreted in terms
of the following main possible mechanisms: (i) electric ®eld eect on electronic energy transfer, (ii) exciton±charge carrier interaction, (iii) the Stark eect on ¯uorescence spectra, and (iv) electric ®eld dependence of radiationless decay channels of excited states (dierent from those mentioned in (i) and (ii)), in particular, electric ®eld-enhanced dissociation of excited states into free and/or trapped charge carriers. A signi®cant electric ®eld eect on electronic energy transfer is expected to occur in systems of molecules with large permanent dipole moments [29]. In these systems, an electric ®eld can reduce or eliminate the possibility of energy transfer between pairs of neighbour molecules by making the energy mismatch comparable to/or larger than the excitation transfer interaction energy. Intermolecular radiationless energy transfer is in turn one of the processes determining the lifetime of excited states, and thus, any change in its rate will alter the population of excited states and, consequently, the intensity of luminescence. This eect cannot be of notable importance in the case of TPD and TAPC molecules with rather small dipole moments. Values of the ground state dipole moments, estimated by quantum chemical calculations, are only 0.3 D for TPD and 0.7 D for TAPC molecule [30]. Electromodulation of ¯uorescence is due to the exciton±charge carrier interaction if the excited states are eectively quenched by free or trapped charge carriers introduced into the molecular system in result of electrode injection and/or internal (bulk) photogeneration. The essential condition for this kind of EMF is the ecient electromodulation of the charge carrier concentration in the light absorption region of the sample. Therefore, good injecting (ohmic) contacts are required to ensure sucient charge injection or ejection through metal/organic solid interfaces. Indeed, the EMF signals as high as several percent were observed in anthracene and tetracene crystals with photoinjecting electrolytic or Au contacts [31,32]. This mechanism of EMF is irrelevant in explaining the observed strong ¯uorescence quenching in organic photoconductors supplied with rather poor injecting Al electrodes. We can safely disregard this as it comes from analyses of photoconductivity results [7,17,18].
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In the Stark eect on emission spectra, two cases should be considered: (a) the so-called Stark shift and (b) the change in quantum yield (integral intensity) of emission bands. The ®rst case is directly connected with the electric-®eld-induced shifts of appropriate energy levels due to the interaction of molecular dipoles (permanent and induced electric dipoles) with the applied electric ®eld. The EMF signal, measured at a speci®c wavelength, is then a linear combination of the ®rst and second energy derivatives of the ¯uorescence spectrum. Besides, in an isotropic medium, this signal increases strictly with the second power of the electric ®eld intensity [33]. The quantitative analysis of the EMF spectra, very similar to that extensively used in the well-known electroabsorption spectroscopy, allows one to evaluate the change in electric permanent dipole moment and/ or molecular polarizability upon photoexcitation [34±36]. At this point, we should note that the EMF results presented in Section 3 cannot be interpreted this way. The reason is that the EMF signals in TPD and TAPC originated from the full spectrum of ¯uorescence bands, so the Stark shift eect would not be detected at all. A further argument against attributing the EMF results to the Stark shift eect is provided by the clear deviation from a second power behaviour seen in Figs. 5 and 6 for TPD and in Fig. 7 for TAPC. The exception is the electromodulation of the short-wavelength component of the TAPC emission spectrum presented in Fig. 8. In this ®gure, the electric ®eld dependence of the EMF is exactly quadratic and the EMF signals are the same order of magnitude as those measured in electroabsorption. However, in the latter case, a more exact theory of the Stark eect ought to be taken into account, which leads to the change in quantum yield of ¯uorescence due to an electric ®eld-induced modi®cation of transition dipole moments [33]. In this kind of the Stark eect, the EMF signals follow, similarly as in the Stark shift eect, a quadratic function of the applied electric ®eld. Therefore, we suppose that the electric ®eld eect on short-wavelength ¯uorescence in TAPC can be at least in part due to the Stark change in quantum yield of ¯uorescence. This result will be discussed later on.
357
On the basis of the above reasoning, we infer that the ®eld-assisted thermal dissociation of excitons into free and/or trapped charge carriers is the most probable process for the quenching of ¯uorescence by an electric ®eld observed in TPD and TAPC ®lms. First, we will discuss in greater detail the EMF results in TPD. The ¯uorescence spectrum of a TPD ®lm displayed in the upper part of Fig. 2 is expected to be of molecular origin. We attribute the observed ¯uorescence quenching to a ®eld-dependent dissociation of coulombically bound electron±hole pairs (CT states) created in a ®eld-independent process from primary excited singlet Frenkel states. The dissociation process can be described in the framework of the threedimensional Onsager model [19]. According to this model, the escape probability of geminate recombination can be expressed as follows: XOns F 1 ÿ nÿ1
1 X
Pj rC =r0 Pj n;
2
j0
where Pj x is given by a recursion formula, Pj x Pjÿ1 x ÿ xj exp ÿx=j!;
3
P0 x 1 ÿ exp ÿx:
4
The ®eld dependence of the escape probability, XOns F , appears through the parameter n
er0 j F j : kT
5
In the above equations, rC represents the so-called ``Onsager distance'' at which the coulombic attraction energy is equal to the thermal energy, kT, rC
e2 4pe0 ekT
6
and r0 stands for a discrete characteristic thermalization length. The latter quantity corresponds to the initial electron±hole separation distance which, to be more realistic, should be modelled by an exponential or Gaussian distribution function [37,38]. Since the dissociation process of an electron±hole pair competes with geminate recombination into the ¯uorescent state, the ¯uorescence
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W. Stampor / Chemical Physics 256 (2000) 351±362
intensity is expected to be a function of the electric ®eld, I F
kf 1 ÿ g F I ; kf kn
7
where kf and kn are the rate constants of radiative and other (dierent from charge separation) nonradiative decay pathways for singlet emitting exciton states, respectively, and I is the rate at which excited states are produced. The primary quantum yield of geminate carrier pair formation, g0 , is usually assumed to be ®eld independent in the Onsager model of charge separation with the ®eld dependent quantum eciency of photogeneration expressed as g F g0 XOns F :
8
To compare this model with experiment, calculated values of I2x =I0x have been ®tted to the measured (2x)EMF signals as described earlier [7]. In the ®tting procedure, the primary quantum yield g0 and the initial e±h separation distance r0 were adjusted using the experimental value e 2:8 obtained from capacitance measurements for TPD ®lms [25]. The good agreement between the curves calculated based on the Onsager model and the experimental data is seen from Figs. 5 and 6. Under excitation kex 313 nm (within the ®rst low energy absorption band) the best ®t, with param is attained (the solid eters g0 0:21 and r0 19 A, line in Fig. 5). The independence of the EMF signals on excitation wavelength in this spectral range, shown in Fig. 2, indicates at a vibrationally relaxed singlet S1 state as a common precursor for formation of the geminate e±h pair. In this regard, the photogeneration mechanism in TPD seems to be very similar to that proposed by Noolandi and Hong [20], where the initial charge separation in organic photoconductors is considered to occur after the internal conversion and vibrational relaxation to the lowest excited singlet state of the molecules. The excitation of higher energy levels (above the ®rst absorption band) enlarges electromodulation of ¯uorescence because geminate pairs with greater r0 (so weaker bound) are then formed during the initial charge separation process. In fact, for excitation kex 260 nm, we get, in terms of the Onsager model, larger values of the
parameters g0 and r0 (the solid line with g0 0:30 in Fig. 6). The values of the primary and r0 21 A separation distance r0 , found in TPD, compare favourably with those obtained by the EMF method in other organic photoconductors [3,11,18]. Using the Gaussian distribution function of (e±h) pair radii, instead of a discrete value of the radius r0 , in the ®tting procedure would lead to shorter separation distances [37,39]. The rather high values of g0 suggest in turn the primary charge separation step in the TPD ®lm to be a very ecient process. This would imply that some more extended, intermolecular CT states exist close to the ¯uorescent localized Frenkel state and the photoexcited singlet states, fairly well delocalized, produce geminate pairs which undergo in turn an electric ®eld-dependent dissociation. To verify this hypothesis, the energy location of CT states in TPD should be established by an independent method. Actually, a preliminary analysis of the electroabsorption spectrum of the TPD ®lm revealed the polar states, most probably having partly intermolecular charge transfer character, placed in the spectral range of the ®rst singlet absorption band [30]. If S1 states and CT states are energetically degenerated, g0 in the Onsager theory can be considered as a CT fraction of the hybridized Frenkel/CT exciton. The electric ®eld could then change this fraction. The mixing of quasidegenerate Frenkel and CT excitons has been really observed in electroabsorption (EA) of quinacridone ®lms [40]. However, the eect was rather small and followed a quadratic function of the applied voltage. The stronger EMF signals could not be reconciled with this mechanism [11] like in the present case. Our values of the EMF signals presented in Fig. 5 agree well with those obtained for photoluminescence originating from TPD placed in multilayer structures TPD/Alq3 [8]. This comparison is reasonable because as shown in Fig. 5 of Ref. [8], the TPD photoluminescence quenching in multilayer structures rather does not depend on the TPD layer thickness. Considering the above-stated facts, the origin of the strong photoluminescence quenching observed in TPD monolayer in Ref. [8] (above 20% at the electric ®eld intensity of 106 V/cm) is not clear. We suppose that in a mirror-based cell arrangement
W. Stampor / Chemical Physics 256 (2000) 351±362
studied in Ref. [8], electric-®eld-induced changes of optical properties of a cavity could be of importance, so the cavity eects could increase the EMF signals measured in monolayers. As can be seen in Figs. 7 and 8, a long-wavelength and short-wavelength part of TAPC emission spectrum are aected in a dierent manner by the external electric ®eld. This is not surprising if we take into account dierent natures of these two emission spectral components. According to Kalinowski et al. [27], the short-wavelength emission band originates from singlet excited states of tri(ptolyl)amine (TTA) subunits of TAPC molecule and the broad long-wavelength emission band in solid TAPC stems from singlet excimeric states formed by TTA moieties of neighbour TAPC molecules. We will ®rst discuss the mechanism of the ®eld-induced quenching of excimer-type ¯uorescence in TAPC. It is worthwhile to compare the present results of excimer ¯uorescence quenching with a similar quenching of exciplex ¯uorescence observed by Yokoyama and co-workers [2,3] in poly(N-vinylcarbazole) doped with a week electron acceptor. The authors of Refs. [2,3] considered that the ¯uorescent exciplex state is formed by a geminate recombination of the electron±hole pair state in competition with the ®eld-assisted dissociation into free charge carriers described in quantitative terms of the Onsager model. A very similar approach to the exciplex ¯uorescence quenching has been recently applied by Ohta and co-workers [15,16] for a mixture of ethyl carbazole and dimethyl terephthalate doped in poly(methyl methacrylate) (PMMA) polymer ®lms. We assume that the ®eld-induced excimer ¯uorescence quenching observed in TAPC could be of similar origin. The kinetic scheme that will be used to analyse the experimental data is shown in Fig. 9. According to this scheme, a relaxed singlet molecular exciton (M ) in a TAPC ®lm migrates until it reaches the so-called macrotrap [41,42] (spatially extended domain of crystal lattice perturbation). An encounter complex M Mt formed within the macrotrap is considered to have a favourable molecular conformation for the electron transfer. The key point of the charge separation mechanism presented in Fig. 9 is that electron±hole separation occurs at the expense of exciton localization en-
359
Fig. 9. The kinetic scheme used to describe the electric ®eld modulation of ¯uorescence (monomer excimer emission) in TAPC. The molecular exciton (M ) migrates forming an encounter complex, M Mt , at an extended lattice defect (macrotrap). The encounter complex M Mt can then relax to the excimer state (MM) with the probability 1 ÿ g0 or transform into the geminate (e±h) pair M Mÿ with the probability g0 . The geminate pairs undergo either ®eld-assisted charge separation M Mÿ at a rate de®ned by X(F), or form the excimer state with the probability 1 ÿ X F .
ergy resulting in a geminate pair M Mÿ formed with the ®eld independent probability g0 (see also Ref. [7]). The geminate pair at a separation distance r0 either dissociates thermally into free carriers with the electric ®eld-dependent probability, X(F), or recombines to give an excimer state (MM) . A direct decay path from the encounter complex state M Mt to the excimer state (MM) without involvement of the geminate pair state (with the probability of this process being (1 ÿ g0 )) is also possible. From this scheme, the electric ®eld dependence of excimer ¯uorescence intensity can be derived and expressed as follows: I F
kf0 1 ÿ g0 X F I : kf0 kn0
9
This expression is very similar to Eq. (7). In Eq. (9), kf0 and kn0 denote the appropriate (radiative and non-radiative, respectively) decay rate constants for excimer states, and I is the rate at which encounter complex states M Mt are formed. Inserting the Onsager expression (2) for X(F) into Eq. (9), and applying the same ®tting procedure, as previously used for TPD, the excimer ¯uorescence quenching in TAPC is successfully explained. As can be seen in Fig. 7, the experimental EMF data are reproduced excellently by the theoretical curve
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W. Stampor / Chemical Physics 256 (2000) 351±362
based on the Onsager theory with g0 0:9 0:1, and e 3:0 (the solid line in Fig. 7), r0 16 A which strongly supports the Onsager type (diusion-controlled) charge separation in TAPC ®lms. Furthermore, according to the scheme shown in Fig. 9, the very high value of g0 0:9 indicates that the formation of most excimer states in TAPC ®lms proceeds mainly through geminate (e±h) pair states M Mÿ . This gives good correspondence with exciplex formation mechanism in an electron donor and acceptor systems [2,3,15,16]. The recent comprehensive study of delayed ¯uorescence in methyl-substituted poly(para-phenylene) (MeLPPP) [43] also leads to a conclusion that excimer ¯uorescence is caused by the recombination of geminate pairs produced from singlet excitations. In principle, it is possible that geminate (e±h) pairs are formed directly from relaxed excimer states [39]. However, transient absorption and emission experiments [44,45], particularly an unusually slow and non-exponential decay of exciplex ¯uorescence in doped poly(N-vinylcarbazole), suggest the dynamics of exciplex formation to be determined by the recombination of primary (e±h) pairs. Therefore, we assume the excimer formation process via (e±h) pairs to be more probable also in our case of TAPC. We shall now compare the present interpretation of the EMF results based on the Onsager theory with other models of charge photogeneration, which are usually applied to elucidate electromodulation of ¯uorescence in organic photoconductors. In the Poole±Frenkel (PF) formalism [46], an electron±hole pair dissociation is treated as a one-step thermally activated overcoming the Coulomb barrier lowered by the external ®eld, a carrier diusion motion during charge separation process being completely disregarded. In the conventional formulations of the Onsager and PF models, a ®eld-independent yield of geminate pair formation, g0 , is usually assumed. On the contrary, an exponential ®eld dependence of the rate constant for electron±hole pair (CT) formation, kCT k0 exp bF ;
10
is inhered in the Noolandi±Hong±Popovic (NHP) model [5,20] or the hopping separation model (HS)
[7,12]. The quantum yield of charge separation can be expressed as follows: g F gPF F g0
exp bPF F 1=2 =kT APF exp bPF F 1=2 =kT
11
in the PF model, and g F gCT F
exp bF =kT A exp bF =kT
12
in the NHP or HS model. In Eqs. (11) and (12), APF and A are constants determining the branching ratio of recombination and generation channels at F 0 in the PF and NHP or HS models, respectively. To obtain the luminescence intensity I(F) in the relevant model, expression (11) or (12) should be substituted into Eq. (7) (for a further description of these models see Ref. [7] and references therein). However, in the present work, it has been found that the ®eld dependence of the EMF signals in TPD and TAPC can be consistently understood in the framework of the Onsager model. The PF model-based curves deviate distinctively from the experimental EMF data in the high ®eld range (see the dash-dotted line in Fig. 7). In addition, the most basic parameter bPF of this model is found to be over two times smaller than the theoretical value of (e3 =pee0 )1=2 predicted by the Poole±Frenkel expression. Similarly, the plots obtained from the models NHP or HS with an electric-®eld-dependent rate constant, kCT , expressed by Eq. (10), are in apparent disagreement with experimental data (see the best ®t shown in Fig. 7 by the dashed line). Finally, we should discuss the mechanism of short-wavelength ¯uorescence quenching in TAPC ®lms. The in¯uence of the electric ®eld on this type of ¯uorescence (Fig. 8) is signi®cantly weaker than that on excimer ¯uorescence (Fig. 7). This fact correlates well with the previously made assumption that the appropriate excimer states in TAPC ®lms are formed mainly via geminate pair states which can be eciently dissociated by the electric ®eld. According to the present results, the ®eldassisted dissociation of molecular Frenkel excitons, related to the short-wavelength (monomer) ¯uorescence quenching in TAPC, appears to be a less ecient process. The dierence arises pre-
W. Stampor / Chemical Physics 256 (2000) 351±362
sumably from dierent recombination sites available in a TAPC ®lm for excimer and Frenkel states, respectively. The exact quadratic ®eld dependence of monomer ¯uorescence quenching observed in TAPC (Fig. 8) cannot be easily rationalized on the basis of EMF models, involving ®eld-induced dissociation of excitons, applied in the present work. For example, as shown in Fig. 8, the slope of Onsager model-based curves, displayed in a double logarithmic plot, changes continuously with an increasing applied ®eld strength if only a ®eld range is not too small. Square-type electric ®eld dependencies of EMF quenching were observed also in other organic photoconductors [2,3,10,14,47]. To explain experimental data, the electric ®eld dependence of the initial charge separation distance r0 (Onsager theory) was postulated in Ref. [3]. It has been claimed that the quadratic ®eld dependence of ¯uorescence quenching in an isotropic medium is a direct consequence of symmetry [10]. When an external ®eld is applied to that medium, excited state dissociation rate should not depend on the direction of the applied electric ®eld. Consequently, the quenching eciency should be an even function of the electric ®eld, i.e., quadratic in the lowest order if a ®eld strength is not too high. In addition, as mentioned in the beginning of this section, the rather small quenching of short-wavelength (monomer) ¯uorescence in TAPC suggests that the quadratic Stark eect on quantum ¯uorescence yield can also contribute to electromodulation in TAPC. Unfortunately, the ®eld dependence of EMF signals, calculated according to dierent models, approximately follows the same power law in a narrow-®eld range which is typically accessible in experiment [7]. Therefore, at the present stage, it is not possible to make a conclusive statement concerning the origin of monomer ¯uorescence quenching in TAPC. It is noteworthy to mention here that the electric ®eld-induced quenching in MeLPPP [43] is also noticeably dierent for both types of observed ¯uorescence. However, in contrast with our results, the quenching eciency for intrinsic (monomer) ¯uorescence in MeLPPP is higher than that for excimer ¯uorescence. The distinctive electric ®eld behaviour of excimer ¯uorescence in MeLPPP has been explained by the
361
assumption that at the pumping intensities being employed in the experiment, the appropriate excimer sites were populated in the saturated regime. Since in our case the linear intensity dependence of both the ¯uorescence spectral components was observed (Fig. 4), the in¯uence of the non-linear processes on singlet and excimer population should be excluded.
5. Conclusions We have demonstrated that ¯uorescence of vacuum-evaporated ®lms of diamine derivatives (TPD and TAPC) is quenched by external electric ®eld. The dependence of ¯uorescence quenching on electric ®eld and excitation wavelength is well understood in terms of the ®eld-assisted dissociation of excitons into charge carriers, described by the three-dimensional Onsager model. The good agreement between the present EMF results and the Onsager theory indicates that the primary charge separation is rather a ®eld-independent process and the external electric ®eld in¯uences only the further, diusion-controlled step of charge separation. It has been found that the two bands of TAPC ¯uorescence spectrum, ascribed to singlet monomer and excimer emission, respectively, are dierently aected by the external electric ®eld. The very ecient quenching of excimer emission has been shown to be compatible with the assumption that formation of excimers proceeds via geminate pair intermediates, while the much weaker electromodulation of monomer emission seems to be a result of the quadratic Stark eect.
Acknowledgements The author would like to thank Prof. Jan Kalinowski for stimulating discussions and invaluable help. Dr. P.M. Borsenberger is gratefully acknowledged for the providing of TAPC material. The author also thanks Dr. M. Cocchi for measurement of ¯uorescence spectra of TPD and TAPC ®lms.
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