Electromodulation of photoluminescence in vacuum-evaporated films of bathocuproine

Chemical Physics 410 (2013) 45–54

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Electromodulation of photoluminescence in vacuum-evaporated films of bathocuproine Maciej Mis´nik a, Karol Falkowski a, Wojciech Mróz a,b,c, Waldemar Stampor a,⇑ a

´ sk University of Technology, Narutowicza 11/12, 80-952 Gdan ´ sk, Poland Department of Electronic Phenomena, Gdan Istituto per lo Studio delle Macromolecole (ISMAC), Consiglio Nazionale delle Ricerche (CNR), Via Bassini 15, 20133 Milano, Italy c OPTOTEC S.p.A., Via G. Zenale 44, 20024 Garbagnate Milanese, Italy b

a r t i c l e

i n f o

Article history: Received 21 September 2012 In final form 24 October 2012 Available online 12 November 2012 Keywords: Electromodulation Photoluminescence Charge photogeneration Dimers Onsager model Bathocuproine OLEDs

a b s t r a c t Electric field-modulated photoluminescence (EML) was measured in vacuum-evaporated films of bathocuproine (BCP), electron-transporting material commonly used in organic light-emitting diodes (OLEDs). The external electric field of 106 V/cm strength decreases long-wavelength photoluminescence (PL) up to 10% but the same effect on short-wavelength PL is above one order of magnitude smaller. The distinctive difference between the EML characteristics for the short-wavelength (mono-molecular) and long-wavelength (associative species) emission of BCP films is a result of the different nature of relevant emissive states. Absorption, PL, EML and atomic force microscopy (AFM) measurements can be consistently explained assuming existence of dimer species in solid BCP with their population increasing during aging process of the films. Besides ground state absorption dimer states are assumed to be populated indirectly from molecular (Frenkel type) excitons diffusing to defected domains of the films where dissociate through an intermediate stage of geminate (e–h) pairs. The EML data are analyzed applying various models of (e–h) pair dissociation based on Poole–Frenkel, Braun, Onsager and Sano-Tachiya-Noolandi-Hong (STNH) theories. The Onsager theory explains satisfactorily the observed EML quenching effect for dimer-type PL. The Stark effect on fluorescence quantum yield should be possibly invoked to explain the EML characteristics of monomolecular emission of BCP. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Bathocuproine (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, in short BCP; see Fig. 1a), is the electron-transporting material with a relatively large band gap, commonly used as a hole-blocker/ exciton-confiner in organic light-emitting diodes (OLEDs) [1] and organic solar cells [2]. In addition, this material can be applied as a blue emitter in OLEDs [1,3]. The core of BCP molecule structure is the phenanthroline unit, which has an aromatic p-conjugated frame with two nitrogen N atoms placed in the 1 and 10 positions. The BCP molecule is non-planar with the dihedral angle of 47.9° between the phenanthroline plane and the substituted phenyl rings in the positions 4 and 7 [4]. The methyl groups substituted in the positions 2 and 9 has rather little effect on the molecular structure [4]. According to Refs. [4–6] the low energy singlet electronic transitions in a BCP molecule are highly localized in the phenanthroline moiety and contain contributions of p ? p⁄ and n ? p⁄ primary transitions due to the spatial overlap between respective molecular orbitals of n or p origin. The hole blocking property of BCP is attributed to the participation of N atoms in ⇑ Corresponding author. Tel.: +48 583472704; fax: +48 583472821. E-mail address: [email protected] (W. Stampor). 0301-0104/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.chemphys.2012.10.013

the frontier orbitals which do not accept the holes from the active layers of OLEDs. Electroabsorption in vacuum evaporated films of BCP has been recently studied in the framework of the Stark effect, and accordingly, the low energy singlet excited states in solid BCP were recognized as Frenkel excitons endowed with intramolecular charge-transfer (CT) character [7]. In OLEDs the emissive layer is subjected to electric fields of strength exceeding 106 V/cm. Under such a high electric field exciton dissociation into free charge carriers is significantly enhanced which results in reduction of population of emissive excited states. The exciton dissociation is considered as a one of quenching processes responsible for the observed high field roll-off in electroluminescence (EL) quantum efficiency of OLEDs which is currently under extensive debate [1]. The mechanism of exciton dissociation can be efficiently probed by electromodulation of photoluminescence experiments where the photoluminescence quenching effect is observed for a sample placed in an external electric field. This effect was studied intensively in organic EL emitters, among others, in poly(phenyl-pphenylenevinylene) (PPPV) [8], mer-tris(8-quinolato) aluminum (Alq3) [9,10] and fac-tris(2-phenylpyridine) iridium (Ir(ppy)3) [11,12].

M. Mis´nik et al. / Chemical Physics 410 (2013) 45–54

46

(a)

(b)

N

N

BCP Fig. 1. (a) The molecular structure of BCP. (b) Energy level diagram (neglecting interface dipoles) for BCP samples and Al electrodes as applied in EML measurements.

In the present paper we report on optical properties of thin vacuum evaporated films of BCP applying absorption, photoluminescence (PL) and electromodulation of photoluminescence (EML) spectroscopy. In comparison to BCP in dilute solution, the PL spectrum of BCP films exhibits an additional long-wavelength emission band in the green region of the spectrum. The distinctive difference in the EML characteristics for the short-wavelength (violet) and long-wavelength (green) PL bands of BCP was for the first time recognized. Our experiments clarify the origin of the green emission band which is attributed to the formation of dimers. The EML data are analyzed applying various models of (e–h) pair separation and we show that the Onsager theory explains satisfactorily the observed strong electric-field quenching effect for dimer-type PL. In contrast, the relatively weak quenching of monomolecular emission of BCP is probably underlain at least in part by the Stark effect on fluorescence quantum yield.

PL. Positive values of (2x)EML signals mean PL quenching. All measurements were carried out at room temperature. A more detailed description of the EML measurements is given elsewhere [12,15]. Ordinary absorption spectra were recorded either with a Perkin–Elmer Lambda 10 spectrophotometer or Perkin–Elmer Lambda 900 spectrometer. PL spectra were collected with a Perkin–Elmer LS 55B spectrofluorometer or with a setup consisting of Osram XBO 450 W xenon lamp followed by a Jobin Yvon Horiba Gemini 180 monochromator, used for excitation wavelength selection, and a Spex 270M monochromator combined with CCD, employed as a detector. Within the applied excitation light intensity range (1012-1015) ph/cm2 s the linear intensity dependence of the both PL bands (short-wavelength or long-wavelength) was observed, therefore the influence of the nonlinear processes on excited states population can be excluded. The topographic images of the BCP films were obtained using a scanning force microscope Nanosurf easy scan 2 (Schaefer) equipped with Si cantilever with normal spring constant of 0.05 N/m. The AFM experiments were done in the constant-force mode with an applied loading force of 20 nN and with the scanning frequency of 1 Hz. The Fourier components of PL intensity, Inx (n = 0, 2), required in formula (1), were calculated numerically according to various theoretical models which quantitate the PL quenching mechanism as described in Section 3.2. The details of numerical procedures with the relevant references can be found in our previous papers [12,17]. 3. Results and discussion 3.1. Absorption, photoluminescence and AFM measurements

ð2xÞEML ¼

I 2x I0x

ð1Þ

where I0x stands for (0x) – Fourier component and I2x – for the rms value of (2x) – Fourier component of I intensity of modulated

8

Abs 1.2

PL film

6

in CH2Cl2

0.8 4

0.4

0.0

violet emission

300

400 λ [ nm ]

green emission

500

ε [ 104 cm-1 M-1 ]

The starting material was BCP supplied by Aldrich (with a spectral purity of 99.95%) or Lumtec (with sublimation grade purity). The BCP fluorescence spectra of either supplier are very similar. The experiments were carried out on 100–200 nm thick films deposited by thermal evaporation in vacuum (103 Pa) on room temperature quartz substrates at a rate of about 0.1 nm/s. The samples in EML measurements were arranged in the sandwich cell configuration, Al/BCP/Al/quartz, supplied with two vacuum-evaporated semi-transparent aluminum electrodes. Aluminum with a rather poor injection ability of charge into organic materials was chosen to weaken PL quenching induced by exciton–charge carrier (polaron) interaction as argued in our previous papers [13–15] (see also energy level diagram for BCP samples and Al electrodes in Fig. 1b). The thickness of the BCP films was controlled with a crystal quartz microbalance during evaporation cycle and next verified with a Tencor Alpha Step 500 Profiler. The active electrode area of the samples was 0.2 cm2. The photoluminescence of BCP was excited by a light beam from a mercury lamp (Narva, HBO 200 W) followed by a SPM-2 Zeiss monochromator. The electromodulated photoluminescence was measured at the second harmonic (2x) of fundamental frequency (x) of applied electric field, F(t) = F0 sin(xt), (typically x/ 2p = 175 Hz), using phase-sensitive detection. The PL light was collected with a quartz lightguide followed by a set of appropriate cut-off glass filters (Schott and Corning) to form the beam with the proper spectral range detected by a photomultiplier tube (EMI 9863QB). The quantity to be recorded is EML signal defined as

Absorption (ABS) and typical photoluminescence (PL) spectra of a 120 nm-thick BCP film (solid lines) are compared with the ABS and PL spectra of BCP diluted in polar solvent CH2Cl2 (dashed lines) in Fig. 2. It is clearly seen that the ABS spectrum of the film displays all the features of the solution spectrum except for the long-wavelength tail (k > 370 nm) spreading below the solution absorption onset (see also Fig. 3). The enhanced long-wavelength tail may suggest some kind of defect sites with non-zero oscillator strength being distributed in energy to act as ground state absorption centers in the solid film. However, we are aware of additional complications arising from light scattering on crystallites which likely

Optical density D

2. Experimental and numerical details

2

0

Fig. 2. Absorption (ABS) and typical photoluminescence (PL) spectra of a 120 nmthick BCP film (solid lines) along with the ABS and PL spectra of BCP dissolved in CH2Cl2 (dashed lines). PL spectra excited at wavelength of 300 nm were measured shortly after deposition.

M. Mis´nik et al. / Chemical Physics 410 (2013) 45–54

(a)

BCP in CH2Cl2

x8

This work

-2

3x10 M -3 3x10 M -4 3x10 M -5 3x10 M

PL intensity [a.u.]

Abs [ a.u. ]

1.0

47

0.5

x 20

BCP solid film

1 month

3 days

1 day

x 200 1 hour

0.0 320

340

360

380

400

300

420

400

500 λ [ nm ]

600

λ [nm] Tang et al ref. [3] Chen et al ref. [20] Yang et al ref. [21]

PL intensity [a.u.]

contribute substantially to the tail of ABS spectrum in vacuum evaporated polycrystalline films (see for example [18]). The shape of the solution spectrum is independent of the BCP concentration (105–102 M in our experiments) and the optical density of the solution scales linearly within experimental error with solute concentration according to the Lambert–Beer law (Fig. 3). The rather small (red-type) matrix shift (<50 meV) of the ABS spectrum of the film with respect to that of solution indicates at non-resonance interaction between an excited molecule and its ground state solid environment to be not significant. This means the electronic states in BCP solid films being well spatially localized within a molecule can be assigned to Frenkel-type excitons as we argued previously in our electroabsorption study [7]. The PL spectrum of the BCP film recorded shortly after deposition (Fig. 2) in addition to the main band centered at 396 nm (violet emission) shows well pronounced features in the longwavelength region (green emission) which are absent in the dilute solution. The matrix (solvent) shift of the main band of the film in photoluminescence is similar to that in absorption. Though, shortwavelength PL (fluorescence) should have a monomer origin as that in the solution, the long-wavelength (green) fluorescence should be rather assigned to some kind of associative emitting species (dimers or excimers) with decay time of several ns remarkably longer than lifetime of 0.5 ns for singlet excited states of monomers in solid BCP films [19]. The peculiar feature of the PL spectrum of BCP films recognized in the present work is the significant increase in green PL intensity during aging process of films proceeding on day timescale (Fig. 4a). The dynamics of growth of the green PL band does not depend on ambient atmosphere since practically the same PL results were obtained for samples stored in air (Fig. 4a) as for ‘‘virgin’’ (as-deposited) samples encapsulated in glass tubes immediately after evaporation without breaking vacuum (not shown). One important issue of this study is to address the origin of the long-wavelength PL band. To explain the mechanism of long-wavelength emission we compared our PL spectra of BCP solid films with those available in literature (Fig. 4b). The distinct shoulder at 475 nm in the PL spectrum of solid BCP films observed by Tang and coworkers [3] (solid line in Fig. 4b) or by Chen and coworkers [20] (dashed line in Fig. 4b) was tentatively assigned to excimers. The both literature PL spectra correspond more or less to our PL spectra measured shortly after film deposition. In turn, in the PL spectrum recorded by Yang and coworkers [21] (dotted line in Fig. 4b) the intensity of longerwavelength emission is stronger than that of shorter-wavelength

(b)

300

400

500

600

λ [ nm ]

(c)

Yang at al ref. [23]

ITO / BCP / Al EL intensity [a.u.]

Fig. 3. A comparison of long-wavelength parts of absorption spectra of BCP in CH2Cl2 solutions for various molar concentrations as indicated in the figure (broken lines) and a 100 nm thick neat film (solid line).

300

400

500 λ [ nm ]

600

Fig. 4. Photoluminescence spectra of solid BCP films of different origin: (a) PL spectra measured in this work and (b) PL spectra taken from Refs. [3,20,21]. In part (a) of the figure the PL spectra for a 100 nm thick BCP film excited at the wavelength kexc = 300 nm were measured after aging periods in air as indicated in the figure. Very similar spectra were recorded during aging of the films encapsulated in glass tubes without breaking vacuum after deposition. For comparison, the electroluminescence spectrum measured in ITO/BCP/Al system, taken from Ref. [23], is displayed in part (c) of the figure.

emission, that is reversely than in Refs. [3,20]. The authors of the paper [21] ascribed this peculiarity to the lower purity of BCP starting material in their experiments in comparison to Refs. [3,20]. The long-wavelength emission of BCP was observed also under electrical excitation in electroluminescence spectra of multilayer diodes

M. Mis´nik et al. / Chemical Physics 410 (2013) 45–54

PL intensity [a.u.]

(a)

violet PL

λexc= 320nm -2

3x10 M -3 3x10 M -4 3x10 M -5 3x10 M

10

0 300

(b)

400 λ [ nm ]

500

600

green PL -2

PL intensity [a.u.]

with an indium-tin-oxide (ITO) anode and magnesium cathode: ITO/BCP/Alq3/Mg [3] or ITO/TPD/BCP/Alq3/Mg [22] in the range from 450 to 525 nm with a maximum at 490 nm ascribed to BCP excimers. The clear long-wavelength tail in EL spectrum of BCP was observed in a single-layer diode (Fig. 4c) [23]. Next we examine PL spectra of BCP in concentrated liquid solutions where associative species of molecules (excimers, dimers or other aggregates) should play the role. In case of dimers (strictly speaking physical dimers) associative interaction between neighboring molecules occurs both in ground and excited states (distinct PL and ABS dimer bands) but in excimer case association is limited only to excited state of the bimolecular complex and ground state of the complex is dissociative due to repulsive interactions of two ground-state molecules (distinct excimer PL band with no ABS band) [24,25]. Strongly red-shifted (0.8 eV from the monomer) PL band at 490 nm under UV excitation appeared in aqueous solutions of 1,10-phenanthroline (the core unit of BCP molecule) more concentrated than 103 M [26]. The excimer assignment of this PL band was based on the lack of distinctive features observed in ordinary (ground state) absorption spectra in comparison to dilute solutions. However, the concentration independence of the PL spectra of BCP solutions excited with UV light (Fig. 5a) are suggestive of no excimer formation in BCP case. As we can see in Fig. 5a, in strongly concentrated dichloromethane solutions of BCP (up to 3  10-2 M) where excimers are expected to be formed ‘‘spontaneously’’, only monomer emission is observed. Similarly as in Ref. [26], no new distinctive features in ABS spectra of concentrated BCP solutions can be discerned from monotonic long-wavelength background (Fig. 3). However, in the case of concentrated BCP solutions a new (non-monomer) longer-wavelength emission can be selectively excite with visible (violet) light with wavelength kexc > 380 nm (Fig. 5c). In particular, the green PL band of highly concentrated (3  10-2 M) BCP solution directly accessed optically under kexc = 400 nm excitation corresponds well with that spectrum for BCP polycrystalline powder (Fig. 5b). This means that in closely packed systems specific intermolecular configurations are forced to form a small minimum on a ground-state potential energy surface of interacted adjacent molecules. As a consequence, some non-zero ground-state absorption (though very weak and buried under long-wavelength absorption tail in our case) allows for selective direct optical excitation of low-energy emitting species. Based on the fact that excimers cannot be directly excited by light absorption we reject excimer formation in favor of dimer formation concept and we finally attribute the long-wavelength PL of the concentrated BCP solutions and films to the decay of excited states into weakly-bound ground states of dimers (or other molecular aggregates involving more than two molecules). In molecular organic solids composed of flat rigid molecules (like pyrene or phthalocyanines), van der Waals interactions in the ground state favor pair sandwich-type (face-to-face) structures in which molecular planes and transition dipole moments are parallel [24,25]. In anthracene and naphthalene crystals, which have the herringbone structure with neighboring molecules inclined to each other, slip along an appropriate crystallographic direction caused by dislocations may also result in the formation of some fraction of approximately parallel pairs [27,28]. According to the classical exciton theory, resonance exchange of excitation energy between pair molecules leads to splitting of excited state energy levels (so called exciton splitting) [29]. In case of an ideal sandwich of molecules with parallel transition dipole moments, a lower bonding state does not carry oscillator strength and its transition to the ground state should be forbidden due to symmetry restrictions imposed on exciton wavefunctions. Nevertheless, finite oscillator strength of the order of 0.01 arises from vibronic coupling [30] or charge-transfer contribution to the excited states in slightly distorted sandwich structures [31]. In addition, static disorder

3x10 M -3 3x10 M -4 3x10 M

λexc= 400 nm

2

1 powder

0 400

500 λ [ nm ]

600

700

(c) 1 PL intensity [a.u.]

48

1 λexc= 380 nm 2 λexc= 390 nm

2

3 λexc= 400 nm

2

4 λexc= 410 nm

3 solution -3 8x10 M

4 0 400

500

600

700

λ [ nm ] Fig. 5. Photoluminescence spectra of BCP in CH2Cl2 solutions excited with light of wavelengths: (a) kexc = 320 nm and (b) kexc = 400 nm for various molar concentrations as indicated in the figure. In part (c) of the figure PL spectra of a strongly concentrated solution (8  103 M) are displayed for various excitation wavelengths belonging to the spectral range of assumed dimeric absorption. In part (b) of the figure a PL spectrum of the powder (thick solid line) is added for comparison.

caused by local crystal irregularities can lead to oblique (transition dipoles inclined to their interconnected axis by a considerable tilt angle h) or even in-line (head-to-tail with h = 180°) alignments of pair molecules with quite emissive lower-energy exciton state [29]. Particularly, hindered rotations of phenyl side groups about single C–C bonds in a BCP molecular structure favor certainly an

M. Mis´nik et al. / Chemical Physics 410 (2013) 45–54

amorphous state with a wide distribution of intermolecular arrangements in solid BCP. In contrast to excimers, postulated in Refs. [3,20], the dimers proposed in this work are two-molecule aggregates stable in the ground state due to intermolecular interaction between conjugated p-electron systems of BCP enhanced most probably by strongly electronegative nitrogen atoms incorporated in pyridine rings [27,32] which implies subsequently the constituent molecules to be more closely spaced than for excimers (3.53 Å in pyrene-type excimers [25]) in a solid state of BCP. The dimer concept is strongly supported by crystal data [5,33] and quantum chemical calculations [4,5] where various dimer configurations have been recently identified in solid BCP. In fact, the dimer ‘‘antiparallel’’ configurations (nitrogen sides of the molecules opposite to each other) with the largest orbital overlap along c crystal axis are characterized by the perpendicular interplanar distance as short as 1.62 Å and the lateral displacements: 4.1 and 5.5 Å, along the molecular short and long axis, respectively [5]. Remarkably, this specific intermolecular arrangement reflects dramatically in the high electron mobility along the c crystal stack as large as 103 cm2/V s [4]. Despite the presence of dimers in BCP film samples, the main features of the film absorption spectrum are actually quite similar to those of the solution spectrum (Fig. 2), indicating the UV excitation of the BCP film creates prevailingly the monomer (Frenkel) excitons which provide by radiative decay the violet fluorescence. However, it is well known [24,25] that Frenkel excitons due to resonance (excitonic) interactions are mobile in crystal solids and during their lifetime migrate at room temperature usually by hopping (diffusion) to defect states (here possibly BCP dimers) that can collect the excitation energy and then decay radiatively giving rise the additional green fluorescence. We note here that due to a rather small overlap integral of BCP absorption and fluorescence spectra, the radiationless single-step (Förster-type) energy transfer induced by long-range electric dipole–dipole interactions is relatively inefficient process in concentrated BCP systems which is confirmed by the lack of green fluorescence under UV excitation even in the most concentrated solution with the average distance between BCP molecules of 57 Å (Fig. 5a). In turn, from clear observation of strong green fluorescence under UV excitation in BCP films we can infer that the energy transfer from monomer to dimer species in films is rather dominated by multi-step diffusive motion of Frenkel excitons. From Fig. 4a it is seen that the dimer-to-monomer PL ratio increases with aging of BCP films and as mentioned before this process proceeds independently of ambient atmosphere. Conceivably, increasing population of the dimer sites due to relaxation of intermolecular structure of BCP films during aging enables the relative dimer emission intensity to vary. It is well established that structural relaxation effects play an important role during film deposition, annealing and aging as reported for organic solid films [34]. Upon hot vapor deposition of the material on a room-temperature (or cold) substrate, the molecules condense initially in a random fashion, and next rearrange themselves to minimize the total free energy with timescale dependent on details of preparation procedure (substrate type and temperature, film thickness, evaporation speed and so on). Consequently, in BCP films which are initially amorphous after evaporation, during aging process intermolecular configurations can be formed which favor dimer emission. The structural relaxation proceeds on hour or day timescales, dependent on the film thickness and substrate material. As demonstrated in Fig. 4a for a 100 nm thick BCP film vacuum-evaporated on a room-temperature quartz substrate, the aging relaxation is nearly completed within a month providing an optical signature of fully developed green fluorescence. Polycrystallization and morphology of evaporated BCP films was investigated in detail by Mori and Masumoto [35,36] using

49

optical polarizing and atomic force microscopy (AFM). A rough surface and many small crystals were observed in SEM (scanning electron microscopy) images of BCP films vacuum-deposited on ITOcoated glass substrates [37]. The structural relaxation of BCP can be significantly enhanced by heating treatment of films after evaporation when thermal agitation enable to rearrange the positions and mutual orientations of molecules [38]. The poor thermal stability of a BCP film structure is regarded as an essential factor reducing the performance of organic multilayer electronic devices which apply frequently a BCP layer [36–39]. More importantly with this respect, the gradual ordering of films can be substantially reduced by so-called organic alloy method where two materials are evaporated simultaneously [35]. In particular, co-evaporation of BCP and Bphen (bathophenanthroline = BCP molecule without methyl groups) molecules allows for better structural stability of films since interaction between two different moieties creates energetically favorable BCP-Bphen hetero-dimer configurations, and, consequently, restricts substantially molecular motions. As observed by scanning AFM, the morphological changes of our BCP samples (Fig. 6) proceed in accord with the timescale relevant

Fig. 6. The scanning force micrographs of BCP films vacuum-evaporated on glass substrates recorded shortly after deposition (a) and 3 days after deposition (b). The dark-to-white color scale represents height differences of 10 and 50 nm in parts (a) and (b) of the figure, respectively. The horizontal streaks are artifacts of the horizontal scan direction.

M. Mis´nik et al. / Chemical Physics 410 (2013) 45–54

50

for changes of PL spectra of the same samples (Fig. 4a). It is clearly seen that the surface of the fresh BCP film (Fig. 6a) is smoother and more homogenous in comparison to that for the sample stored 3 days (Fig. 6b) which correlates well with the observed development of long-wavelength (green) emission (Fig. 4a) and connects convincingly the film relaxation with forming emissive dimer species. We also note here that the timescale of structural relaxation of our BCP samples (hours–days) is slower in comparison to that of BCP samples investigated in Ref. [35] (minutes–hours) but this is not surprising since the phenomenon to be critically dependent on substrate type and details of sample preparation procedure. In this context we add here that the highly condensed molecules in BCP films enable presumably the formation of a variety of dimer conformations revealing in the sample dependent relative intensities of PL spectral features (the maxima and shoulders in Fig. 4a). Further information on dimer and monomer excited states in BCP films are provided by electromodulated photoluminescence (EML) experiments which are especially suitable to differentiate these types of excited states.

The electric field characteristics of EML signals for a long-wavelength (green) and short-wavelength (violet) part of BCP emission spectrum are depicted in the Fig. 8a. The BCP films were excited with UV light having photon energy within the spectral range of the first absorption band. Most importantly, the green and violet emissions of BCP are affected in a totally different manner by the external electric field. The EML plots for green emission displayed in Fig. 8a distinctively depart from a second-order function of the applied electric field and tend to reach a ‘‘saturation’’ level in the high field range. The maximum quenching of green PL at an electric field exceeding 2  106 V/cm reaches about 11% in BCP films. In turn, the EML signals for violet PL increase approximately with the square of the applied electric field and are at least one order of magnitude smaller in comparison to those for the green PL. In the Fig. 8a electroabsorption signals (EA) measured at the same wavelength of absorbed light as the excitation wavelength in

(a)

0

green PL violet PL

10

4

-1

10

1 2 3

-2

10

r0/rC

η0

0.096 0.090 0.090

0.45 0.49 0.035

EML

Onsager model

-3

10

2

-4

10

EA 3

slope 2.0

1 5

6

10

10

Frms [ V/cm]

(b) Onsager model (a=0)

-1

10

(2ω)EML

The EML experiments were carried out on BCP films typically 2– 3 days after evaporation. We measured the relative change of PL intensity induced by an external electric field using modulation technique as determined by formula (1) in the experimental part of this work. The reproducibility of the (2x)EML signals was very good for different runs on the same sample and satisfactorily good for samples originated from different series. The circle points in Fig. 7 show (2x)EML signal for global (spectrally integrated) emission as a function of the wavelength of excitation light (kexc). For all wavelengths (kexc < 350 nm), PL intensity is reduced by an electric field. For kexc > 350 nm the photon intensity of PL was too small to measure EML signals under modulation regime. The PL quenching starts rather abruptly at about 350 nm and then increases more gradually with excitation photon energy within the first absorption band suggesting the opening of a nonradiative pathway for the decay of the Frenkel (monomer) excitons. Of various physical mechanisms, the field-assisted thermal dissociation of excitons into free and/or trapped charge carriers seems to be the most appropriate to interpret the observed PL quenching in organic thin films (see Refs. [13–17] and literature cited therein).

(2ω)EML, -(2ω)EA

3.2. Electromodulation of photoluminescence

STNH model a/rC = 0.042 κrC/D = 1

-3

10

κrC/D = 10

6

Frms = 10 V/cm

κrC/D = 100

(2ω)EML [ % ]

-5

10

5

10

6

10

Frms [ V/cm]

2

0 250

300

350 λ [ nm ]

400

Fig. 7. Excitation spectrum of the second harmonic (2x) response of global (wavelength integrated) photoluminescence to a sinusoidal electric field Frms = 106 V/cm (circles) in comparison to absorption spectrum (solid line) for a 100 nm thick BCP film.

Fig. 8. The double logarithmic plots displaying the dependence of (2x)EML signal on electric field at the excitation wavelength kexc = 313 nm for BCP films. The abscissa of the plots stands for the rms value of the external electric field strength. In part (a) of the figure the open symbols stand for experimental data showing the electric field effect on green PL (circles, squares and triangles for three different samples, respectively) and violet PL (stars for one sample). The solid lines represent the best fit according to the Onsager model assuming er = 3.0 and three different sets of parameters (g0 and r0/rc) as indicated in the figure. The electroabsorption (EA) change in the transmitting light intensity with k = 313 nm caused by the Stark effect is shown for comparison (full diamonds). The thin dotted lines in the figure have slopes 2.0 which correspond to the Stark effect. In part (b) of the figure the EML experimental data (circles taken from the part (a) of the figure) are compared with theoretical curves (broken lines) based on the STNH model assuming a/ rc = 0.042 and r0/rc = 0.096 for three different values of capture velocity j as indicated in the figure. For comparison the plot based on the Onsager model (solid line) is also shown.

M. Mis´nik et al. / Chemical Physics 410 (2013) 45–54

EML experiments are showed for comparison. The EA signals in BCP due to Stark effect follow exactly the second order function of electric field (see also Ref. [7]). Since the EA signals are at least one order of magnitude smaller than the EML signals, the ‘‘trivial’’ electromodulation of PL intensity caused by electric field-induced change in the number of photons absorbed in BCP samples can be safely neglected in interpretation of EML characteristics. The different responses of green and violet PL to an electric field are not surprising if we take into account the different origins of these two PL spectral components. According to reasoning presented in Section 3.1, the green PL is assigned to dimer excited states and violet PL – to monomer excitons. We will first discuss the mechanism of the field-induced quenching of dimer-type PL. As we argued in Section 3.1, under UV excitation dimer excited states in BCP films are mainly populated indirectly from monomer (Frenkel) excitons diffusing to sample regions favorable for dimer formation. However, it is not trivial to explain why dimer species carrying lower energy are quenched by an electric field more efficiently than higherenergy Frenkel excitons. To rationalize the EML quenching effect for dimers we adopt here the model previously applied for excimers in TAPC (1,10 -bis(di-4-tolylaminophenyl)cyclohexane) films [15]. The key assumption of this model is that formation of associative species (here dimers) proceeds with substantial contribution of electron–hole (e–h) pair intermediates which can be efficiently broken by an external electric field. According to this model, a relaxed Frenkel exciton (M⁄) in a BCP film migrates by diffusion until it reaches some special region (so-called macrotrap in Refs. [13,14]). An encounter complex (M⁄. . .M)t created within this region is considered to have a favorable molecular conformation for the electron transfer. The electron–hole separation (M+. . .M) occurs at the expense of exciton localization energy resulting in a geminate (e–h) pair formed at a separation distance r0 with the field independent probability g0. The geminate (e–h) pair is assumed to dissociate thermally into free carriers with the electric field-dependent escape probability (X), or recombine to a dimer state (MM)⁄. In addition, dimer states can be populated with the probability (1  g0) by exciton energy transfer when the encounter complex state (M⁄. . .M)t decays to the dimer state (MM)⁄ without involvement of the geminate (e–h) pair state. Based on this model, the PL intensity (I) is expected to be a function of the electric field (F),

IðFÞ ¼

51

kf ½1  g0 XðFÞI kf þ kn

ð2Þ

where kf and kn denote the rate constants of radiative and non-radiative decay pathways for emitting dimer states, respectively, and I⁄ stands for the production rate of encounter complex states (M⁄. . .M)t. Except for the escape probability, X(F), all parameters in formula (2) are assumed to be field independent which is typical for charge separation models based on the Onsager theory [40]. Many results confirming the applicability of Onsager theory to exciton dissociation in organic solids are well documented in literature [24,25]. The validity of the Onsager assumptions has been recently comprehensively analyzed by Arkhipov and Bässler [41]. In particular, if exciton dissociation starts from vibrationally relaxed exciton state in disordered solids the primary quantum yield g0 can be field dependent which is at variance with typical formulations of the Onsager theory (for further discussion and literature on this subject see also Ref. [17]). However, in our case of BCP films, exciton dissociation takes place in macrotraps (spatially extended domains produced by physical perturbation of crystal lattice), and, accordingly, energy excess released during exciton localization instead of being dissipated to phonons induces nearest neighbor charge separation, forming a trapped geminate (e–h) pair with a probability g0 independent of the applied electric field in full analogy to the Auger process on a lower energy scale as discussed in Ref. [13]. Under the assumption that geminate (e–h) pairs with a single ‘‘thermalization’’ distance of the ejected electron (r0) are formed, the relevant formulas (3)–(7) for escape probability X in the framework of various models are collected in Table 1. The escape probability X is defined here as the probability for the dissociation of (e–h) pair that is the escape of a charge carrier from its parent countercharge. In more sophisticated versions of the models, some distribution of r0 distances should be taken into account [24,25]. The calculated values of I2x/I0x have been fitted to the measured (2x)EMF signals as described in Section 2. The classical treatment of electron–hole separation process by Onsager [40] and Sano-Tachiya-Noolandi-Hong (STNH) [42,43], based on solving the Smoluchowski Equation, refers to the diffusion of an ion pair in the continuous three-dimensional medium in the presence of an applied electric field. In the STNH model the final geminate recombination step (carrier capture) proceeds on a sphere of finite radius (a) with a finite velocity (j). The commonly used Onsager theory based on his paper [40] (Onsager paper

Table 1 Formulas for escape probability of e–h pair dissociation in various models. Model

References

Onsager

[40]

Sano-Tachiya-Noolandi-Hong (STNH)

[42,43]

Poole–Frenkel (PF)

[50,51]

Braun (Br)

[52]

Hopping separation (HS)

[9,13]

Escape probability     k B T X1 rC er 0 F P m; P m; m¼1 er0 F kB T r0     r 1 X1 2r 0 2 2kB T XSTNH ðFÞ ¼ 1  C pffiffiffiffiffiffiffi al;0 bl Z2l Zl l¼0 eFr 0 r 0 2p rC

XOns ðEÞ ¼ 1 

Formula No.

Notes

(3)

P(m, x) is the incomplete gamma function of integral order m.

(4)

Z2l is the Z special function of the second kind of l order ! bPF F 1=2 f PF ðFÞ ¼ exp kB T  3 1=2 e bPF ¼

f PF ðFÞ APF þ f PF ðFÞ

(5)

f Br ðFÞ XBr ðFÞ ¼ ABr þ f Br ðFÞ

(6)

XPF ðFÞ ¼

pe0 er

XHS ðFÞ ¼

f HS ðFÞ AHS þ f HS ðFÞ

(7)

2I1 ðuÞ u b F 1=2 u ¼ PF kB T J1 is the Bessel function of the first order sinhðbFÞ f HS ðFÞ ¼ bF ed b¼ kB T d is the hopping distance

f Br ðFÞ ¼

M. Mis´nik et al. / Chemical Physics 410 (2013) 45–54

(a)

-1

(2ω)EML

PF model

0

10

10

-2

10

ε

AF

8.0 3.0 3.0

10 8 10 8 10

η0

6

1.0 1.0 0.5

-3

10

-4

10

5

6

10

10

Frms [ V/cm]

(b)

0

10

ε

ABr

8 4 3

4x10 6 10 6 10

-1

10

(2ω)EML

dated from 1938 year on geminate recombination) is a special case of a more general STNH treatment for a = 0. The Onsager model gives good frames to explain EML results for dimer-type PL in BCP films. In the fitting procedure, the primary quantum yield g0 and the ratio of the initial radius to the Onsager radius, r0/rc, were adjusted assuming the relative permittivity of the material er = 3 and temperature T = 298 K. The Onsager radius rc represents a distance, at which the Coulomb attraction energy of (e–h) pair is equal to the thermal energy, and here rc = e2/4pe0erkBT ffi 187 Å (e – elementary charge, kB – Boltzmann constant). The theoretical curves in Fig. 8a (the lines designated by 1 and 2) fitting experimental EML electric field characteristics (circles, squares and triangles for three different samples, respectively), are calculated according to the Onsager model formula (3) (see Table 1) with two sets of the model parameters: (r0/rc = 0.096, g0 = 0.45) for the curve 1, and (r0/rc = 0.090, g0 = 0.49) for the curve 2. The estimated values of radii, r0 = (16.8–18.0) Å, for (e–h) pairs contributing in formation of dimer species, correspond roughly to one intermolecular distance along a axis columnar stack (16.5 Å) or two average intermolecular distances (15.8 Å) in a crystal lattice of BCP [33]. This stays in accordance with the notion that a short range order of a crystal structure is preserved in organic films manufactured by vacuum-evaporation method which is well documented in literature (see for example [44]). Remarkably, the slightly smaller value of initial radius r0 = (16 ± 1) Å and nearly a factor of 2 larger the primary quantum yield g0 was obtained for vacuum-evaporated TAPC films [15] resulting in EML characteristics of excimer PL in TAPC very similar to those of dimer PL in BCP. We note that the PL quenching effect of 10% at high electric fields translates into a very low zero-field exciton dissociation efficiency: g(F = 0) = g0 exp(rc/r0) =(1–3)  105 which is rather typical for organic one-component photoconductors [24,25]. In Fig. 8b the experimental electric field EML characteristics are compared with those calculated according to the STNH model (see formula (4) in Table 1) for varying final recombination parameters jrc/D. The initial e–h separation distance r0 was taken from the fit based on the Onsager model (Fig. 8a) and the final recombination sphere radius a was assumed to be equal to the average intermolecular distance in the crystal lattice of BCP (7.9 Å [33]). A very good fit has been obtained with jrc/D = 100 (see the dashed line in Fig. 8b) and for jrc/D > 100 the STNH curves approach to the Onsager model-based curve, which gives the lower limit for the final recombination velocity j = 14 cm/s taking the diffusion coefficient of majority charge carriers (electrons in BCP) D = 2.6  107 cm2/s. The given above value of diffusion coefficient D was calculated using Einstein relation, D = lkBT/e with a typical value of charge carrier mobility l = 105 cm2/V s in electron-transporting films of Al quinolate complex (Alq3) for which the most reliable data are currently available [45]. The data for electron mobility in solid BCP (or Bphen and similar phenanthroline derivatives) range from 107 to 103 cm2/V s [4,46–48]. We should add here that various methods of mobility measurements lead usually to quite different results [45]. In addition, the electric field dependence of charge carrier mobility is frequently observed in disordered organic solids [25,49] which is not implemented in the STNH theory. The lower limit of final recombination velocity j in BCP films compares well with recent evaluations of this quantity in other organic thin films [17]. Remarkably, the estimated lower limit of final recombination rate constant j/a = 2  108 s1 in BCP films implies the upper bound of decay lifetime of dimer fluorescence s = 5  109 s which is consistent with the experimental data for transient PL measurements for this material [19]. Next, we check the possibility of interpretation of the present EML results with other models of exciton dissociation, which are usually applied to elucidate electromodulation of photoluminescence in organic photoconductors. A validity test of the Poole–

-2

10

-3

10

4

η0 = 1 Braun model

-4

10

5

6

10

10

Frms [ V/cm]

(c)

0

10

δ [ angstrom ]

AHS

7.7 7.7 5.1

100 200 20

-1

10

(2ω)EML

52

-2

10

-3

10

HS model -4

10

5

6

10

10

Frms [ V/cm] Fig. 9. The validation of various EML models for BCP films. The symbols stand for the experimental EML data for green PL taken from Fig. 8a. The lines are plots calculated according to various theoretical models with parameter values as indicated in the figure. For meaning of the parameters see Table 1 and references cited therein.

Frenkel (PF) [50,51], hopping-separation (HS) [9,13] and Braun (Br) [52] models is depicted in Fig. 9 which shows the field dependence of EML signals for BCP films and plots fitted according to

M. Mis´nik et al. / Chemical Physics 410 (2013) 45–54

various theoretical models. For short description of the models and the meaning of relevant parameters see Table 1. In contrast to the Onsager and STNH theory, in the PF and HS models (e–h) pair dissociation is treated as a one-step thermally activated jumping over the local potential energy barrier lowered by the external electric field, carrier diffusion with its multi-step random walk being completely disregarded during charge separation. Consequently, the electric field dependence of escape probability X(F) for (e–h) pairs can be derived according to the first order kinetics as

XðFÞ ¼

f ðFÞ A þ f ðFÞ

ð8Þ

where the field dependent function, f(F) = keh(F)/keh(0), is equal to the relative increase with field of (e–h) dissociation rate constant (keh) and A is a parameter determining the branching ratio of recombination and dissociation channels for (e–h) pairs at zero electric field (F = 0). The specific shape of the function f(F) depends on the model. In the framework of one-dimensional version of PF model which is usually applied in wide-band materials at high electric fields [50,51]: fPF(F) = exp(bPFF1/2/kBT) with a basic PF coefficient bPF = (e3/pe0er)1/2 dependent on the relative permittivity er. In terms of HS model [9,13]: fHS(F) = sinh(bF)/(bF) with b = ed/kBT where d denotes a hopping distance. Essentially, the same formula (8) is also applied in the Braun model, however, in this case, for the specific function f(F) should be inserted that derived by Onsager in his earlier paper [53] (Onsager paper dated from 1934 year on bulk recombination) for the relative increase of the dissociation constant in weak electrolytes,

fBr ðFÞ ¼

2I1 ðuÞ u

ð9Þ

where u = bPFF1/2/kBT and I1(u) is a modified Bessel function of the first kind of the first order. The formula (9) is frequently used to calculate the electric field-induced increase in dissociation probability of (e–h) pairs in organic systems, in particular, in OLEDs and organic solar cells (for a survey of relevant papers see Refs. [17,54]). Braun originally applied the formula (9) to describe the electric field dependence of dissociation of relaxed CT states in donor-acceptor organic solids [52]. Braun’s approach to dissociation of geminate (e–h) pairs is critically analyzed in Ref. [54] where the wrong assumptions of the Braun model are recognized. The most importantly, the formula (8) assuming the first order kinetics is invalid in description of multi-step character of geminate dissociation/ recombination of (e–h) pairs. In addition, the implicit assumption in applying the formula (9) to geminate recombination is the unrealistic requirement for infinitesimally small initial radius r0 ? 0 which results from the property of the Onsager solution (3):

lim

r 0 !0

XOns ðFÞ 2I1 ðuÞ ¼ f Br ðFÞ; ¼ u XOns ð0Þ

ð10Þ

as discussed in our previous paper [17]. The main message from this reasoning is that the proper and consistent description of geminate dissociation/recombination of (e–h) pairs in continuous medium can be accomplished only in the framework of the STNH model. As it follows from Fig. 9, the theoretical curves obtained on the basis of PF, Braun or HS models deviate remarkably from the experimental EML data. For the PF (Fig. 9a) and Braun (Fig. 9b) theorybased plots the moderately good agreement with experimental data has been achieved only in a low-electric field range and for a rather high value of dielectric constant er = 8 whereas in organic thin films typical values of er = 3–4. Even though, the plots obtained from the HS model (Fig. 9c) with AHS = 100–200 and a hopping distance d = 7.7 Å equal approximately to an average intermolecular separation in a BCP crystal satisfactorily reproduce

53

the low-field experimental data, the apparent disagreement between theory and experiment, similarly, as for the PF and Braun models, exists at high electric fields. In addition, we note the zero-field exciton dissociation efficiency: g(F = 0) = g0/(1 + A) in the HS model is at least 2 orders of magnitude larger than that calculated according to other models (PF, Braun, Onsager and STNH). We stress here once more that the PF, Braun and HS models rely on the formula (9) assuming in fact one-step charge separation process. This assumption is difficult to realize in organic solids with inherent narrow bands where the electronic wavefunctions are strongly localized in space and the potential energy barrier is extended within many lattice constants (rc > 100 Å). The apparent success of the Onsager–STNH theory in organic solids just originates from the fact that a carrier diffusion motion is consistently included during charge separation process. It can be concluded that the EML results for dimer-type PL of BCP films are well reproduced and reasonably understood using the Onsager theory. Finally, we should discuss the mechanism of short-wavelength (violet) PL quenching in BCP films. The influence of the electric field on this type of PL is significantly weaker than that on dimer-type PL (Fig. 8a). This fact correlates well with the previously made assumption that the dimer states in BCP films are populated mainly via geminate (e–h) pair states which can be efficiently separated by the electric field. According to the present results, the electric field-assisted dissociation of Frenkel excitons, related to the monomer-type PL quenching in BCP, appears to be a less efficient process. The difference arises presumably from different contribution of charge transfer resonance into dimer and monomer (Frenkel) states, respectively. This characteristic property of Frenkel excitons translates in terms of the Onsager model to a remarkably smaller value of the primary quantum yield of (e–h) pair formation g0 = 0.035 with the similar, as for dimers, initial (e–h) radius r0 = 16.8 Å (the line designated by number 3 in Fig. 8a). Even though, the fitting procedure leads to a reasonable set of the Onsager model parameters, the observed nearly quadratic field dependence together with the comparatively small values of EML signals in this relatively narrow applied electric field range suggest that the Stark effect on PL quantum yield possibly contributes to electromodulation of monomer PL, similarly, as in TAPC films [15].

4. Conclusions The present results prove the leading role of dimer species, presumably constituting a part of larger aggregates, in featuring the excited states responsible for the appearance of the long-wavelength photoluminescence (fluorescence) in vacuum-evaporated BCP films. Photoluminescence of BCP films is quenched by external electric field which is well understood in terms of the field-assisted dissociation of electronic excited states into charge carriers. Electromodulation of photoluminescence is successfully described by the three-dimensional Onsager model of geminate recombination which indicates that carrier diffusion underlies the charge separation mechanism. The observed efficient quenching of dimer-type photoluminescence is compatible with the assumption that formation of dimers proceeds via geminate (e–h) pair intermediates which can be more efficiently separated in comparison to more localized monomer (Frenkel-type) excitons. The relatively weak quenching of monomer emission of BCP is probably underlain at least in part by the Stark effect on fluorescence quantum yield. A comparison of the Sano-Tachiya-Noolandi-Hong results with the conventional Onsager model allowed estimating the lower limit of final recombination (capture) speed of (e–h) pairs in BCP films j = 14 cm/s which gives the back electron transfer rate constant kBET = j/a = 2  108 s1 between an electron and a hole separated at the average intermolecular distance in the crystal lattice. The

54

M. Mis´nik et al. / Chemical Physics 410 (2013) 45–54

main message from this work for BCP and our previous work [17] for other organic films is that final recombination between an electron and a hole occurs at a non-zero separation distance with a finite recombination speed. This should have impact on description of carrier recombination mechanism involved in organic devises such as solar cells and EL diodes and should be taken into account in optimization of their performance. Commonly applied Onsager theory of geminate recombination and Langevin theory of bulk recombination assume the final recombination step proceeds at zero separation between an electron and a hole. As discussed recently in Ref. [55] the slow back electron transfer can be the main reason that the Langevin theory of bulk recombination fails in organic solar cells. Acknowledgments This work was financially supported by Polish National Science Centre under Grant DEC-2011/03/B/ST7/01888. References [1] J. Kalinowski, Organic Light Emitting Diodes: Principles, Characteristics, and Processes, Marcel Dekker, New York, 2005. [2] P. Peumans, A. Yakimov, S.R. Forrest, J. Appl. Phys. 93 (2003) 3693. [3] H.Q. Tang, H.X. Liao, L.H. Zhu, Chem. Phys. Lett. 381 (2003) 605. [4] H.Z. Gao, C.S. Qin, H.Y. Zhang, S.X. Wu, Z.M. Su, Y. Wang, J. Phys. Chem. A 112 (2008) 9097. [5] H. Li, J.-L. Bredas, C. Lennartz, J. Chem. Phys. 126 (2007) 164704. [6] J.H. Seo, C.Y. Kim, S.J. Kang, K.-H. Joo, C.N. Whang, A. Moewes, G.S. Chang, J. Chem. Phys. 126 (2007) 064706. [7] A. Tykocki-Piłat, W. Stampor, Photonics Lett. Poland 3 (2011) 64. [8] M. Deussen, M. Scheidler, H. Bässler, Synth. Met. 73 (1995) 123. [9] W. Stampor, J. Kalinowski, P. Di Marco, V. Fattori, Appl. Phys. Lett. 70 (1997) 1935. [10] J. Szmytkowski, W. Stampor, J. Kalinowski, Z.H. Kafafi, Appl. Phys. Lett. 80 (2002) 1465. _ [11] J. Kalinowski, W. Stampor, J. Me˛zyk, M. Cocchi, D. Virgili, V. Fattori, P. Di Marco, Phys. Rev. B 66 (2002) 235321. _ [12] W. Stampor, J. Me˛zyk, Chem. Phys. 337 (2007) 151. [13] J. Kalinowski, W. Stampor, P. Di Marco, J. Chem. Phys. 96 (1992) 4136. [14] J. Kalinowski, W. Stampor, P. Di Marco, J. Electrochem. Soc. 143 (1996) 315. [15] W. Stampor, Chem. Phys. 256 (2000) 351. [16] W. Stampor, Chem. Phys. 315 (2005) 259. [17] K. Falkowski, W. Stampor, P. Grygiel, W. Tomaszewicz, Chem. Phys. 392 (2012) 122. [18] W. Stampor, J. Kalinowski, P. Di Marco, Chem. Phys. 134 (1989) 385. [19] Mróz, W., Meinardi, F. Private communication.

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