Electromodulation of photoluminescence in vacuum-evaporated films of fac-tris(2-phenylpyridine)iridium(III)

Chemical Physics 337 (2007) 151–160 www.elsevier.com/locate/chemphys

Electromodulation of photoluminescence in vacuum-evaporated films of fac-tris(2-phenylpyridine)iridium(III) Waldemar Stampor *, Jakub Me˛z_ yk Department of Molecular Physics, Gdan´sk University of Technology, Narutowicza 11/12, 80-952 Gdan´sk, Poland Received 16 May 2007; accepted 10 July 2007 Available online 20 July 2007

Abstract Electromodulation of photoluminescence (EML) has been investigated in vacuum-evaporated films of Ir(ppy)3 (tris(2-phenylpyridine)iridium) commonly used as a phosphorescent emitter in organic light-emitting diodes. The quenching of photoluminescence up to 30% is observed in the external electric field applied to sandwich cells Al/Ir(ppy)3/Al/glass. The electric field characteristics, the emission and excitation spectra of EML signals are interpreted in terms of field-induced dissociation of excited states into free and/or trapped charge carriers according to the Onsager theory. In addition, the existence of internal electric fields of (1–3) · 105 V/cm in Ir(ppy)3 films is inferred from the (1x)EML signals measured at the first harmonic of the applied electric field frequency, in accordance with previous electroabsorption results.  2007 Elsevier B.V. All rights reserved. Keywords: Electromodulation; Charge photogeneration; Onsager model; Iridium complex; OLEDs

1. Introduction Cyclometalated iridium complexes have been recently used in organic light-emitting diodes (OLEDs) to demonstrate efficient electrophosphorescence with the near unity internal quantum efficiency (QE) [1,2]. Such high quantum efficiencies result from efficient intersystem crossing because the iridium atom has a strong coupling between its orbital angular momentum and its spin, which mixes the singlet and triplet states of organic complex and thus permits to harvest light from the all excitons – both singlets and triplets – populated after an electron and hole combination in operating OLEDs. Both fluorescent and phosphorescent OLEDs typically experience a pronounced decrease in the QE at large current densities flowing through devices subject to the high electric field [3]. There are three main exciton quenching

*

Corresponding author. Tel.: +48 583471568; fax: +48 583472821. E-mail address: [email protected] (W. Stampor).

0301-0104/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2007.07.016

processes responsible for the high field drop in QE of OLEDs: (i) exciton–exciton interaction, (ii) exciton–charge carrier interaction, and (iii) exciton dissociation. The quenching mechanisms operating in iridium-based phosphor OLEDs are currently under intense debate [4–11]. The roll-off in QE observed in such devices was originally attributed to triplet–triplet annihilation [4,5]. However, recent steady state [7] and transient [8,9] photoluminescence (PL) measurements on such materials at varying intensities of the exciting light showed that exciton–exciton interaction is too weak to explain strong electrophosphorescence quenching (as high as 90% [4,9]) observed in Irbased phosphor devices. Instead, combinations of (i)–(iii) mechanisms have been employed to describe the quenching effect. Thus, annihilation of excitons on charge carriers and exciton dissociation were proposed to coexist in operating OLEDs, with the former mechanism prevailing within the lower-field range [9,10]. Accordingly, the high field (>106 V/cm) quenching effects in electrophosphorescence were attributed to the electric field-enhanced dissociation of electron–hole pairs prior to exciton formation [6,9,10].

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In OLEDs with Ir(flz)3-doped emitting layer (flz = fluorenyl-pyrazolato derivative) [9] the decrease in QE as a function of applied electric field strength was fitted satisfactorily using the Poole–Frenkel model of charge separation [12]. In turn, in OLEDs with Ir(ppy)3 (ppy = phenylpyridine) embedded in TPD (diamine derivative) matrix, the Onsager model of charge pair dissociation [13] appeared to be more appropriate [6,10]. On the contrary, fieldinduced exciton dissociation was excluded as a relevant process in OLEDs comprising Ir(ppy)3:TCTA (TCTA = carbazolyltriphenylamine) or Ir(piq)3:NPB (piq = phenylisoquinoline, NPB = diamine derivative) systems because the PL transients recorded on reverse-biased OLED cells were only slightly influenced by high external electric field [11]. Consequently, an unified model taking into account both triplet–triplet and triplet–charge quenching mechanisms [11] was applied to reproduce the QE decrease in such OLEDs, with the former mechanism dominating in the high current density range. The magnitude of the high field (or high current density) imposed QE reduction observed in various OLEDs [1–11] changes from as small as about 20% to as high as above 90%. On the basis of this short survey we can safely conclude that contributions from particular quenching mechanisms (i)–(iii) in different OLEDs depend on individual characteristics of iridium phosphor emitters (ligand type, neat or doped film, dopant concentration, matrix type). In the present paper, we report on electromodulation of photoluminescence (EML) in thin neat films of Ir(ppy)3. Applying low excitation intensities (<1015 photons/cm2 s) and weakly injecting electrodes allowed us to ignore exciton–exciton and exciton–charge carrier interactions, and thus to examine the electric field-induced PL quenching due to exciton dissociation alone. The EML is a well established method which enables to investigate in detail dissociation of excitons and charge separation mechanism in organic solids (for the recent surveys of literature on this subject, see Refs. [14–16]). The preliminary EML results for Ir(ppy)3 films have already been presented [6]. However, in our previous study only the EML measurements in steady state (dc) electric field were carried out which restricted our data to a high electric field range. In the present paper, we study the quenching effect applying a more sensitive ac electromodulation technique. In the (ac)EML experiments we could extend electric field range and record the emission and excitation spectra of EML signals. Comparing the (2x)EML emission spectrum for a Ir(ppy)3 film with the recently reported PL spectrum for a single crystal of Ir(ppy)3 [17] we discuss the possible involvement of crystal defect states in exciton dissociation in Ir(ppy)3 films. The all observed EML results are satisfactorily rationalized on the grounds of the Onsager model of charge separation. Additionally, the analysis of the (1x)EML signals measured at the first harmonic of the applied electric field frequency (x) enabled us to evaluate the internal electric fields built into Ir(ppy)3 films.

2. Experimental and numerical details 2.1. Material and samples The fac-Ir(ppy)3 material (for molecular structure see Fig. 1a) was prepared and purified as described in Ref. [18]. Here fac stands for facial arrangement (with C3 symmetry) of phenylpyridine ligands where the three pyridine N atoms are located opposite the three phenyl C atoms in a pseudo-octahedral complex. The fac stereoisomer being more stable energetically [19] than a less symmetrical meridional form prevails in vacuum-evaporated layers of Ir(ppy)3 [20,21]. The neat films of Ir(ppy)3 were deposited by thermal evaporation (215–232 C) in vacuum (103 Pa) at a rate of 10 nm/min onto room temperature quartz substrates. The quartz plates were cleaned in solvents and ultrasonically in an isopropanol bath before loading into a vacuum deposition chamber. The thickness of the organic films was measured with a Tencor Alpha Step 500 Profiler. The EML results were obtained with 100–400 nm thick films in the sandwich cells supplied with two vacuum-evaporated semi-transparent aluminium electrodes, Al/Ir(ppy)3 film/Al/quartz substrate (Fig. 1b). Aluminium electrodes with rather poor charge injection ability were chosen to

N

Ir N

N

fac-Ir(ppy)3

Fig. 1. (a) The molecular structure of fac-Ir(ppy)3. (b) A schematic diagram of the sandwich cell arrangement used in EML measurements.

W. Stampor, J. Me˛z_ yk / Chemical Physics 337 (2007) 151–160

eliminate electromodulation of photoluminescence due to exciton–charge carrier interaction [6,10,14]. 2.2. Electromodulation measurements The photoluminescence was excited by a light beam from a mercury (Narva, HBO 200W) or xenon (Osram, XBO 450W) lamp followed by a SPM-2 Zeiss monochromator. In the (ac)EML experiments the modulation of photoluminescence was induced by a sinusoidal voltage, U0 sin(xt), applied to the sample (typically x/2p = 175 Hz). The photoluminescence light was collected with a quartz light guide followed by a monochromator (SDMC1 Optometrics) and a set of appropriate cut-off glass filters (Schott and Corning) and then detected by a photomultiplier tube (EMI 9863QB). The ac PL signal consisted of a steady-state (I0x) and alternating components corresponding to the time-dependent PL intensity, I(t), expanded in a Fourier series IðtÞ ¼ I 0x þ I 1x sinðxt þ u1 Þ þ I 2x sinð2xt þ u2 Þ þ   

ð1Þ

The steady state signal, I0x, was measured with a dc digital meter. The alternating signals, I1x(t) and I2x(t), were measured simultaneously by two lock-in amplifiers (Princeton Applied Research model 5210) referenced to the first (1x) and second (2x) harmonic of the applied external voltage frequency, respectively. The quantities to be monitored are defined here as I 1x ; I 0x I 2x ð2xÞEML ¼ ; I 0x

ð1xÞEML ¼

ð2Þ ð3Þ

where I1x and I2x are the first and second order Fourier components (amplitudes) of the photoluminescence intensity. The EML signals were stored, averaged and processed by a computer on-line. The phase shift between the applied voltage and the second harmonic response, I2x(t), was found to be u2 = 90 ± 10, which means photoluminescence quenching in a sense that the increasing electric field diminishes PL intensity. The emission spectra of I2x signals were corrected for photomultiplier and monochromator sensitivity. The phase shift (u1) between the modulating voltage and the first harmonic response, I1x(t), was equal, within ±10 error, to 0–180, dependent on the sample position. When the sandwich cell was reversed to the opposite position with respect to the direction of applied modulating field direction, the phase of (1x)EML signal changed by 180. The phase shifts of the EML signals were independent of the amplitude of external voltage applied to the sample. The EML signals (amplitudes and phases) did not depend on the applied field frequency within the accessible in experiment frequency range (50–5000 Hz). The sensitivity of the (ac)EML experimental setup was better than 104. In the dc EML experiments the photoluminescence quenching efficiency,

ðdcÞEML ¼ 

DI IðU ¼ 0Þ  IðU Þ ; ¼ I0 IðU ¼ 0Þ

153

ð4Þ

was calculated from PL signals measured without (I0) and with a dc voltage (U) applied to the sample (I). To reach the reproducibility better than 10%, all PL intensities were averaged during 60 s. In such experiments the sensitivity of the setup was 102. Electric field-induced change in the number of photons absorbed in Ir(ppy)3 films (due to electroabsorption) is at least two orders of magnitude smaller than the EML signals for all applied excitation light wavelengths and electric field strengths (confer Ref. [20]). All measurements were carried out at room temperature. Further details of the experimental setup and electromodulation method are described elsewhere [14,22]. 2.3. Numerical procedures Applying sinusoidal electromodulation we measure the PL intensity at the first and second harmonics of the applied electric field frequency. The recorded (1x)EML and (2x)EML signals (defined by Eqs. (2) and (3)) are compared with those calculated on the basis of various theoretical models as considered in Sections 3.2 and 3.3. The Inx Fourier components: Z T x IðF ðtÞÞ dt; ð5aÞ I 0x ¼ 2p 0 Z T x I 1x ¼ IðF ðtÞÞ sinðxtÞ dt; ð5bÞ p 0 Z T x I 2x ¼ IðF ðtÞÞ cosð2xtÞ dt; ð5cÞ p 0 are calculated numerically using a FORIF SSP procedure. We note that for sinusoidal modulating field, F(t) = F0 sin(xt), according to Eq. (5b) the I1x Fourier component disappears [14]. Therefore, any non-zero (1x)EML signals detected at the first harmonic mean the presence of an internal electric field (Fi) in the sample. Then the effective modulating field can be expressed as F ðtÞ ¼ F i þ F 0 sinðxtÞ: ð6Þ The value of Fi can be evaluated from the magnitude of (1x)EML signals as described in Section 3.3. It is also worth to note that the (2x)EML signals are independent of Fi for Fi  F0. The relation Fi  F0 is well fulfilled in typical high-field EML experiments on sandwich Al/organic film/Al cells [22]. The analysis of (2x)EML effect, free of internal fields Fi, allows to test the various theoretical EML models and thus to quantify the PL quenching mechanism as discussed in Section 3.2. 3. Results and discussion 3.1. Absorption and photoluminescence spectra The absorption spectrum of a neat Ir(ppy)3 film (thick solid line in Fig. 2) is nearly the same as that for Ir(ppy)3

154

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ABS

PL film

Intensity [ a. u. ]

crystal EML

I2ω

EML I2ω/I0ω 300

400

500

600

λ [ nm ] Fig. 2. The excitation (squares) and emission (circles) spectra of the EML signals for a 310 nm thick vacuum-evaporated film of Ir(ppy)3. The rms value of external electric field Frms = 1.0 · 106 V/cm. The EML excitation spectrum was recorded with emission light wavelength kem = 525 nm. The EML emission spectrum was measured with excitation light wavelength kexc = 365 nm. The absorption and photoluminescence spectra of vacuumevaporated films are shown with solid lines. For comparison, the photoluminescence spectrum of a Ir(ppy)3 single crystal taken from Ref. [17] is displayed with the dotted line.

in solution as reported previously [20,21,23]. An intense ultraviolet band in the range 250–320 nm is assigned typically to the singlet–singlet ligand-centred, p–p* transition (S0 ! 1LC). In turn, a weaker absorption band extending into the visible region (320–510 nm) is attributed to metal-to-ligand-charge-transfer transitions (MLCT) [8,23– 28]. More intense features with a main maximum at 385 nm are conventionally associated with singlet–singlet, d–p* (S0 ! 1MLCT) transitions, and weakly manifesting shoulders in the longer-wavelength range (450–510 nm) – with singlet–triplet, d–p* (S0 ! 3MLCT) transitions and their vibrational satellites. The transitions to 3MLCT states being formally spin-forbidden acquire intensities by an effective mixing with higher energy spin-allowed transitions due to strong spin–orbit coupling of iridium atom [29]. The TD DFT (time-dependent density functional theory) calculations by Hay [19], neglecting spin–orbit coupling, revealed in the MLCT absorption region several ‘‘pure’’ singlet and triplet transitions involving ‘‘metal’’ orbitals (5d-Ir) significantly mixed with p ligand orbitals. The two lowest energy MLCT states are located at 492 nm (2.52 eV) and 453 nm (2.74 eV) on the basis of electroabsorption measurements [20]. The first state at 2.52 eV is certainly the lowest triplet state, T1, and the energy of the second state (2.74 eV) corresponds well with the energy of the singlet state, S1, calculated by the TD DFT method [19,30]. Thus, the singlet–triplet splitting energy in Ir(ppy)3, DE(S1–T1), can be evaluated as low as 0.22 eV (1800 cm1) which is in good accordance not only with TD DFT results (0.21 eV [19] and 0.25 eV [30]) and the evaluation based on absorption data (0.19 eV [26]) but also with an ordering scheme for organo-transition-metal compounds proposed

recently by Yersin [31]. For comparison, the magnitude of DE(S1–T1) for organic molecules involving only p–p* transitions is typically greater than 1 eV. A significant reduction of S1–T1 splitting in organo-transition-metal complexes is due to a large metal orbital contribution into the relevant electronic wavefunctions [31]. An iridium core of organic complex acts as an efficient electronic coupler which extends the excitation over three ligands and induces a rather large (on average) spatial separation between interacting electrons. As a result strongly delocalized MLCT excited states, containing charge transfer (dp*) and neutral (pp*) configurations, with considerably reduced electron– electron exchange interaction are created. It is worth to note here that due to a high degree of spin–orbit interaction, a spin labelling for MLCT states (singlet or triplet) is often no longer adequate in such complexes [32]. This is fully confirmed by the TD DFT calculations [33] where including spin–orbit coupling of 5d electrons as a perturbation a strong mixing between 1MLCT and 3MLCT configurations in Ir(ppy)3 has been demonstrated. The photoluminescence spectrum of a 50 nm thick evaporated Ir(ppy)3 film (thin solid line in Fig. 2) consists of a broad band with its intensity maximum position (at 523 nm), red-shifted by several nm in comparison to that measured in various solvents [18,24–26]. The observed PL emission is commonly identified as phosphorescence from the 3MLCT state. This assignment originally based on PL lifetime measurements and solvent dependence of PL spectra [18] was later confirmed by quantum chemical calculations [19,30,33] and extensive experimental works [8,24– 26,34–36] including: (i) the temperature dependence of the emission band structure [34], (ii) the rigidochromic (blue) band shift of PL spectra on going from a room temperature fluid solution to a low temperature glass [24,26], (iii) the Stokes shift and band structure analysis for Ir(ppy)3 placed in Shpol’skii crystals [35], (iv) the high magnetic field effects on low temperature PL spectra [34] and (v) the cyclic voltammetry measurements of reduction/oxidation potentials [25,36]. Additionally, at low temperatures a weak higher-energy PL band was observed (at 455 nm for Ir(ppy)3 dispersed in PMMA [24] or at 396 nm for neat films [28]) which was attributed to emission from a 3LC [24] or 1MLCT state [28]. All experimental data confirm that intersystem-crossing in Ir(ppy)3 proceeds fast with near unity efficiency [28,38] which is consistent with the recent femtosecond time-resolved absorption measurements [37] and quantum mechanical calculations showing very high density of spin-mixed states (about 70 states within the lowest 1 eV) [33]. Maybe the most complete picture of the Ir(ppy)3 emissive state has been emerged from a comprehensive study by Finkenzeller and Yersin [34] where PL spectra and decay times of Ir(ppy)3 dissolved in THF (tetrahydrofuran) were registered in a wide temperature range and high magnetic fields. From the data analysis three PL active components of the lowest 3MLCT state were identified revealing the large value of the zero-field-splitting (83 cm1) which

W. Stampor, J. Me˛z_ yk / Chemical Physics 337 (2007) 151–160

is a consequence of strong spin–orbit interaction introduced by the Ir-5d electrons into the corresponding triplet wavefunctions. The transition from the lowest triplet sublevel to the ground state being strictly forbidden gains the oscillator strength by coupling with Hertzberg–Teller active vibrations. In turn, the spin-allowed transitions from the two higher energy substates show a usual Franck–Condon activity involving totally symmetric vibrations of the ground state. At ambient temperature, the individual properties of triplet substates are smeared out and one finds only an averaged behavior in a broad and rather featureless PL band (Fig. 2). The solid state effect on photoluminescence of neat Ir(ppy)3 films prepared by evaporation appears mainly in considerable reduction of the PL quantum efficiency and additional inhomogeneous broadening of the PL spectrum [8,21]. In comparison to measurements in dilute solutions where nearly a single decay time of 1–2 ls is observed [18,21], the PL decay in neat films is about one order of magnitude faster and non-monoexponential [21,39,40], which indicates at additional non-radiative decay channels in the solid state. It is regarded that intermolecular interactions dictated by molecular packing in the solid state are responsible for the observed quenching effects. The existence of weakly emissive luminescence quenchers such as excimers, ground state aggregates and other energy traps in Ir(ppy)3 films was proposed [7,8,21,40]. The solid-state self-quenching processes controlled by dipole–dipole interactions similar to those occurring in the Fo¨rster energy transfer model were also considered in this context [41]. Due to difficulty of growing quality Ir(ppy)3 crystals of sufficient size, only recently the full details of fac-Ir(ppy)3 crystal structure have been reported by Breu and coworkers [17]. For our further reasoning it is useful to summarize here the main conclusions of that work. The space group of a fac-Ir(ppy)3 crystal turns out to be trigonal centrosymmetric, P  3. Despite a very corrugated molecular shape imposed by a propeller-like structure of octahedron complex, Ir(ppy)3 molecules interpenetrate effectively in the crystal lattice to attain a close packing controlled by intermolecular dispersive (London) and electrostatic (coulombic) forces. The six molecules of the unit cell are stacked rigorously in three columns running along c with the C3 molecular axis collinear with the c crystal axis. In the two homochiral columns at 1/3, 2/3, z and 2/3, 1/3, z (Wyckhoff positions, ‘‘2b’’ and ‘‘2c’’, respectively) molecules with a fixed chirality, K and D, respectively, have permanent dipole moments (parallel to a C3 molecular axis [19,20]) arranged in a head-to-tail position. In the third racemic column at 0, 0, z (Wyckhoff position ‘‘2a’’) molecules alternate their chirality successively and realize a tail-to-tail arrangement of dipole moments. The Ir–Ir distance within ˚ . From an alternative point of view, each column is 8.402 A the molecules ‘‘2b’’ and ‘‘2c’’ being at the same ‘‘height’’ along c form two honeycomb-like hexagonal layers in the ab plane at z = 0 and z = 1/2, respectively. The Ir–Ir distance between adjacent molecules in the hexagonal layers

155

˚ ) and the resulting cavity in the is relatively large (9.709 A center of the honeycomb is filled closely by two molecules, at z = 1/4 and z = 3/4, respectively, belonging to the third (racemic) column. The planar ppy ligands of adjacent molecules within the racemic columns are engaged in T-shaped (edge-to-face) motifs. Although the described above molecular arrangements minimize the total interaction energy of the ideal lattice, the real crystals are liable to systematic twinning (by merohedry) which interchanges the symmetry related molecules in the racemic column and thus disrupts the rigorous alternation of molecules along c. Consequently, at twin boundaries a tail-to-tail alignment of dipoles is converted into a more favorable head-to-tail one and adjacent ppy ligands are no longer engaged in a T-shaped but rather in a parallel displaced geometry (offset face-to-face) with a shifted p-stack involving half of a ligand. Such rearrangements of molecular packing and the existence of many distorted regions at boundaries of thin lamellar domains stacked along c profoundly affects the photophysical properties of Ir(ppy)3. According to Breu and coworkers [17], this inherent intrinsic disorder is not only a reason for a nonlinear second order activity of nominally centrosymmetric Ir(ppy)3 crystals, but also has a pronounced effect on photoluminescence spectra. The PL spectrum of a fac-Ir(ppy)3 single crystal (dotted line in Fig. 2) is characterized by the appearance of a distinct shoulder (at 507 nm) at the shorter-wavelength side of the main band (at 545 nm) which is not present in the spectrum of the dissolved compound. While the main band was assigned to the emission from regular domains, the shorter-wavelength shoulder with its intensity varying strongly from crystal to crystal was ascribed to the twinned domains boundaries. The specific intermolecular interactions occurring within the bulk and interface regions are also displayed in distinctively different pressure-induced shifts of either emission bands under high pressure applied to crystal Ir(ppy)3 samples [17]. Recent AFM studies [21] revealed in vacuum-evaporated Ir(ppy)3 films many microdomains of rod-like shape resembling the natural shape of Ir(ppy)3 single crystals [17]. This fact strongly suggests that Ir(ppy)3 films exhibit the polycrystalline character. It seems to be therefore reasonable to compare PL spectra of Ir(ppy)3 films with those for single crystals. As can be seen in Fig. 2, the PL spectrum of a thin Ir(ppy)3 film exhibits a single inhomogeneously broadened asymmetric band without any well-resolved features. The PL intensity maximum is observed at the wavelength which falls between the positions of the main peak and the shoulder in the PL crystal spectrum. A further insight into the nature of emissive electronic states in solid Ir(ppy)3 can be provided by investigations of the high electric field effect on PL spectra of thin Ir(ppy)3 films. 3.2. (2x)EML results The plot of the PL intensity detected at the second harmonic of the electromodulating field (I2x) as a function of

W. Stampor, J. Me˛z_ yk / Chemical Physics 337 (2007) 151–160

100

Ir(ppy)3

10-1

λem=525 nm

ΔΙ / Ι0 0ω

emission wavelength is shown by circles in Fig. 2. This emission EML spectrum for a Ir(ppy)3 film was obtained at kexc = 365 nm excitation with the rms electric field strength of 1.0 · 106 V/cm. For all wavelengths the PL intensity is reduced by the applied electric field. The EML spectrum has its maximum red-shifted by about 10 nm and is noticeably sharper relative to the ordinary PL spectrum of a film. The peak position of the EML spectrum coincides well with the position of a shoulder in the PL spectrum of a crystal. In addition, from the EML data we can deduce that the PL quenching efficiency (DI/I) within the shorter-wavelength spectral region being approximately constant (5% at the electric field of 106 V/cm) is several times larger in comparison to that measured at the longer-wavelength part. This means that some higher-energy emitting species, although buried under the inhomogeneously broadened ordinary PL band, are strongly affected by the electric field and give then the relatively large EML signals. We propose here that the observed EML effect should be in large attributed to the molecular aggregates similar to those responsible for the higher-energy shoulder detected in the PL spectrum of a single crystal (see Section 3.1). Further arguments for this assignment are given by a quantitative analysis of the electric field dependence of EML signals as will be discussed below. The square points in Fig. 2 show electric field-induced photoluminescence quenching efficiency (I2x/I0x) as a function of the excitation wavelength (kexc). The PL quenching gradually increases with the excitation photon energy within the first lower-energy (MLCT) absorption band in Ir(ppy)3. A sharper increase appears in the spectral range of the second absorption maximum connected with the higher-energy (1LC) exciton states. The electric field dependence of PL quenching efficiency (calculated according to Eqs. (3) and (4)) measured at kem = 525 nm and under excitation within the MLCT absorption band is depicted in Fig. 3. The EML data obtained by the dc and ac methods are shown for samples with two different thickness (d = 150 nm and d = 310 nm). Due to a poor sensitivity, the dc method could be applied for electric field intensities higher than 106 V/cm. As can be seen, the values of the (2x)EML and (dc)EML signals are in good agreement with each other and independent of the sample thickness. The electroabsorption signals (2x)EA measured at the same wavelength of absorbed light as the excitation wavelength used in the EML experiments are shown for comparison. Since the EA signals are about two orders of magnitude smaller than the EML signals, the electroabsorption contribution to the electromodulation of photoluminescence can be safely neglected. The EML plot in Fig. 3 distinctively departs from a second-order function of the applied electric field and a trend to saturation is observed in the high field range. The maximum quenching of photoluminescence at an electric field of 3 · 106 V/cm reaches 30%. The features of EML in Ir(ppy)3 cannot be rationalized in the framework of the Stark effect where energy levels

10-2

EML

Frenkel model EA

2ω

156

10-3

Onsager model slope 2.0

10-4 105

106

F [ V/cm ] Fig. 3. The relative PL intensity change (calculated according to Eqs. (3) and (4)) as a function of the electric field strength for Ir(ppy)3 films displayed in the log–log scale. The (2x)EML data (squares) and (dc)EML data (circles) for films with thickness d = 150 nm (open symbols) and d = 310 nm (filled symbols) are shown. For (2x)EML data the abscissa F stands for the rms value of the electric field. The excitation and emission light wavelengths are kexc = 405 nm or 436 nm, and kem = 525 nm. The solid line is a plot representing the best fit based on the 3D-Onsager model ˚ and g0 = 0.9. The dashed line is obtained according to the with r0 = 15.5 A Poole–Frenkel model with er = 3.5 and APF = 107 (for definition of APF, see Ref. [22]). The field dependence of the electroabsorption signal (EA) for k = 436 nm light passing through the sample (d = 310 nm) is shown by the dotted line. The EA signal increases exactly with the square of the applied electric field, in accordance with the Stark effect.

shift or redistribution of oscillator strength between different states induces the exact square-type field dependence of EML signals. The pronounced Zeeman interaction between the substates of the lowest Ir(ppy)3 triplet state leading to changes in PL intensities has been recently observed under application of high magnetic fields at 1.5 K [34]. In principle, the Stark analogue of the mechanism proposed in Ref. [34] is conceivable to operate in Ir(ppy)3 at low temperatures and high electric fields. However, at room temperature a fast thermalization within the triplet manifold occurs [34] and the photoluminescence is then governed by the average behaviour of the equilibrated substates. Besides, positive and negative signals should be typically observed in the Stark-type EML spectra with intensities scaled with F2 (see for example Ref. [42]) which is not the case. The triplet–charge carrier interaction can be also safely ruled out as the process responsible for the strong PL quenching observed in Al/Ir(ppy)3/Al system. As we argued previously [6,10], the Al electrodes have only weakly charge-injecting properties due to a relatively large energy mismatch (above 1 eV, see Ref. [6] for details) between the Al Fermi level and the relevant energy levels of Ir(ppy)3 frontier orbitals. Consequently, the field-assisted dissociation of excitons into free and/or trapped charge carriers is the most proba-

W. Stampor, J. Me˛z_ yk / Chemical Physics 337 (2007) 151–160

ble process responsible for the photoluminescence quenching effect observed in the solid Ir(ppy)3. To describe the exciton dissociation, the three-dimensional Onsager model of geminate recombination [13] has been applied. In this model, dissociation of excitons is a two-step process. After excitation charge carriers are first separated to a certain distance r0 creating a geminate e–h pair (CT state). In a second step, the charge carriers start a Brownian random walk (in an isotropic continuous medium) under the combined action of Coulomb and external electric fields. Assuming that geminate pairs with a single thermalization distance r0 are formed, the escape probability can be expressed as follows [13,43]: 1 X P m ðrC =r0 ÞP m ðnÞ; ð7Þ XOns ðF Þ ¼ 1  n1 m¼1

where Pm(x) is the incomplete gamma function of integral order m and n¼

er0 jF j : kT

ð8Þ

In formula (7), rC represents the coulombic capture radius (Onsager radius) at which the Coulomb attraction energy is equal numerically to the thermal energy, kT. In more sophisticated approaches, some distribution of r0 distances should be taken into account [44]. Since an electron–hole pair dissociation competes with its geminate recombination into the emissive state, the PL intensity is expected to be a function of the electric field, IðF Þ ¼

kf ½1  g0 XOns ðF ÞI  ; kf þ kn

ð9Þ

where kf and kn are the rate constants for radiative and non-radiative decay pathways of emitting states, respectively, g0 is the yield of geminate pair formation, and I* is the production rate of ‘‘hot’’ excitons. Except for the escape probability (X) all parameters in formula (9) are assumed to be electric field independent. This also includes the primary quantum yield g0, however, in some different from Onsager approaches the field dependence of this quantity is also taken into account (see for example Refs. [16,22]). The field independence of g0 is in accordance with the conventional formulations of the Onsager theory in organic molecular solids [44]. It is assumed there that an e–h pair is formed indirectly by an autoionization of a Franck–Condon molecular state or directly via an intermolecular charge-transfer (CT) transition where the both processes do not require any electric field assistance. This is essentially a case of the present work because the EML results for Ir(ppy)3 were obtained with the excitation light having photon energy at least 0.5 eV above the energy of the lowest excitonic transition in this material. However, since the fast intersystem crossing (S* ! T*) is considered to take place in Ir(ppy)3 after photoexcitation [37], the vibrationally ‘‘hot’’ triplet (T*) rather than singlet (S*) states should be regarded as precursors of geminate e–h pairs on the grounds of the Onsager model of exciton dissocia-

157

tion. On the other hand, this distinction may be less important for MLCT states in iridium complexes where due to strong mixing between 1MLCT and 3MLCT states the term ‘‘intersystem crossing’’ has no well-defined meaning for such states [32]. The Onsager model provides a good framework for a quantitative description of the EML effect in Ir(ppy)3 films. The calculated values of I2x/I0x have been fitted to the measured (2x)EML signals using the numerical procedures as described in Section 2.3 and Ref. [45]. In the fitting procedure, the primary quantum yield g0 and initial separation distance r0 were adjusted assuming the relative permittivity er = 3. For the EML results displayed in Fig. 3 the good agreement between theory (solid line in Fig. 3) and experiment was attained with g0 = 0.9 ± 0.1 and r0 = ˚ . The obtained value of r0 is close to the twice 15.5 ± 0.5 A intermolecular distance in the columnar stack along c axis ˚ [17]) indicating that in fac-Ir(ppy)3 crystal structure (16.8 A the primary acts of exciton dissociation in Ir(ppy)3 take place within the c columns rather than the ab planes. The rationale for this fact is not only the smaller distance between the adjacent molecules in the c columns in comparison to that in the ab planes in regular crystal domains but also the face-to-face (laterally displaced) arrangement of ppy ligands at twin boundaries which may favour the charge transfer along the c direction (confer Section 3.1). Furthermore, according to Ref. [17] the intermolecular distances at the twin boundaries should be slightly shorter ˚ observed in the bulk of the lamellar domains than the 8.4 A which goes well with our estimated values of r0 smaller ˚ corresponding to the two nearest neighbour than 16.8 A separation distances along the c axis. This fact taken together with the previous assignment of the emission EML spectrum (Fig. 2) suggests that the molecular aggregates similar to those occurring at crystal twin boundaries can be also involved in the exciton dissociation in Ir(ppy)3 films. Therefore, our present results are in accordance with the idea that a short range order of a crystal structure is preserved in Ir(ppy)3 films which is not unique but rather typical for vacuum-evaporated organic films (see for example [46]). The large value of g0 found in solid Ir(ppy)3 reflects a high degree of charge and energy delocalization of the relevant excited states. A similar observation has been recently made in star-burst amines [15] where highly delocalized excited states of those dendrimer systems exhibit also large dissociation abilities. In the same way as in Ref. [15] we speculate here that the efficient primary charge separation step in the solid state is determined not only by favourable electron exchange integrals between neighbour molecules but also by efficient energy/charge delocalization within the area of molecule itself. While the first factor entails naturally the molecular arrangements within the c columns in Ir(ppy)3, the latter one should be ascribed to strong spin–orbit coupling introduced by heavy atoms in such organo-metallic complexes [31]. A continuous increase of the EML effect with excitation photon energy

W. Stampor, J. Me˛z_ yk / Chemical Physics 337 (2007) 151–160

(squares in Fig. 2) shows that in the solid state the primary dissociation of ‘‘hot’’ excitons is a process competing against their conversion and/or vibrational deactivation to the lowest emissive state. In turn, a more abrupt (steplike) increase of the EML effect occurring within the higher photon energy range (where the LC states are excited) suggests that dissociation of the ‘‘hot’’ LC excitons contributes also to the total EML effect due to the favourable arrangement of adjacent ligands as argued above. The importance of specific intermolecular interactions in neat Ir(ppy)3 films can be easily recognized if we compare the present EML results with those for Ir(ppy)3 dispersed in solid matrices [10,11,47]. In the latter case the quenching of Ir(ppy)3 emission under application of high electric fields is significantly weaker. For example, in a TPD:Ir(ppy)3 system the Ir(ppy)3 emission intensity is only reduced by 2% at an electric field of 2 · 106 V/cm [10]. In this context we recall also the recent transient EML experiments where PL decays affected by external electric fields are measured in TCTA:Ir(ppy)3 [11] and PVK:Ir(ppy)3 [47] systems. In contrast to stationary measurements, the transient EML experiments allow to distinct amplitude-type quenching (when population of excited states is decreased) from rate-type quenching (when the lifetime of emissive state is reduced) [48,49]. Interestingly, both in TCTA:Ir(ppy)3 and PVK:Ir(ppy)3 systems the decay time of Ir(ppy)3 emission was independent of electric field and the amplitudetype PL quenching was observed. In particular, an amplitude decrease of 5% has been detected at a field strength of 2.5 · 106 V/cm in the former system [11]. That result is in full accordance with the Onsager model of exciton dissociation where the population of emissive states is established by ‘‘hot’’ exciton dissociation events and then the decay time of relaxed emissive states should not depend on electric field. It would be instructive to make similar experiments for neat Ir(ppy)3 films and we address this issue for our future research. Finally, we note that the Poole–Frenkel (PF) model [12] is inappropriate to describe the charge separation in Ir(ppy)3 films. In the PF formalism a geminate e–h pair is assumed to dissociate by a single thermally-activated jump over the coulombic barrier lowered by the external field in the down-field direction. At high electric fields the onedimensional version of the PF model is usually applied [44]. The dissociation process is then described in terms of the first order kinetic rate constant, pffiffiffiffi bPF F ¼ k 0 exp ; kT

3.3. (1x)EML results When electromodulation is induced by the sinusoidal voltage without any external steady state component (UDC = 0 V), as takes place in the present measurements, (1x)EML effect measured at the first harmonic of the applied electric field frequency should not appear in fully symmetric and isotropic systems due to symmetry reasons. Therefore, the nonzero (1x)EML signals (Fig. 4) indicate a real asymmetry of Al/Ir(ppy)3/Al/glass cells prepared by vacuum-evaporation. It is well established that the electronic structure of the sandwich devices with nominally symmetric configuration is not in fact symmetric. Electro-

4

Ir(ppy)3 λ em =525 nm

5

Fi [10 V/cm]

0 1.0 2.0

-4



k eh

dependence of EML signals in comparison to the Onsager model. Failure to describe charge separation mechanism according to the PF model is frequently reported in organic solids (see for example Refs. [14,50]). The PF formalism rests on premise that a charge carrier escapes Coulomb attraction of its counterpart jumping over the potential barrier in a one step process. This assumption is difficult to realize in organic solids where the Onsager radius is ˚ for equal to many intermolecular distances (rC = 170 A typical value of er = 3) and the electronic wavefunctions are strongly localized in space. The possible explanation of the apparent success of the Onsager model originates just from the fact that it takes into account the multi-step character of charge separation process in terms of carrier diffusion. How valid is the Onsager formalism of carrier continuous diffusion in organic solids was widely discussed in the context of geminate recombination on a discrete lattice using the Monte Carlo simulation method [50] and the master equation approach [51].

I1ω / I0ω [ 10-2]

158

3.0

ð10Þ 0

where k0 is the zero-field rate constant and the basic parameter of the model, bPF depends on the relative permittivity er of the material (for detailed description of the PF model and its application to EML see Refs. [14,22]). The theoretical curve obtained on the basis of the PF model (broken line in Fig. 3) deviates distinctively from the experimental data and gives the stronger electric field

1

2

Frms [106 V/cm] Fig. 4. The (1x)EML signal (calculated according to Eq. (2)) as a function of the rms value of modulating electric field for a 310 nm thick Ir(ppy)3 film. The circles stand for experimental data and the solid lines are theoretical curves calculated according to the Onsager model for different values of internal electric fields as indicated in the figure. The values of the Onsager model parameters are the same as in Fig. 3.

W. Stampor, J. Me˛z_ yk / Chemical Physics 337 (2007) 151–160

modulation of absorption (EA) or luminescence (EML) spectroscopy offers an excellent opportunity to investigate and quantify the effect by probing the average internal electric fields (Fi) built into the samples (for detailed background information, see Refs. [45,52]). The combined analysis of the (1x)EA and (2x)EA response has been recently applied to a wide range of molecular organic materials which made it possible to evaluate values of Fi between 4 · 104 and 3 · 105 V/cm for sandwich arranged cells, Al/organic film/Al/glass substrate [52]. The observed features of Fi were explained by the asymmetrical distribution of space charge in the near electrode regions associated with different defect (of structural and/or impurity origin) characteristics on the substrate and non-substrate side of the samples. The similar approach can be also applied to the present (1x)EML results for Ir(ppy)3. In Fig. 4 the (1x)EML signal versus the rms value of modulating electric field is displayed for a Al/Ir(ppy)3/Al/ glass system. Theoretical plots are calculated according to the Onsager model, inserting F(t) = Fi + F0 sin(xt) into Eq. (9) with different values of Fi as indicated in the figure. The results show that the internal electric field Fi is not constant and increases with the amplitude of the applied field from Fi = 1 · 105 V/cm under the low field regime up to above 2 · 105 V/cm at the higher fields. From EA measurements on Ir(ppy)3 films [52] we obtained Fi = (2 ± 1) · 105 V/cm approximately independent of the applied field. This apparent inconsistency can be explained by the different time scales of phenomena involved in EA and EML. While electroabsorption proceeds during the very short interaction time of light with molecule (1016–1015 s), the EML effect is averaged during the exciton lifetime which is orders of magnitude longer. Therefore, the internal electric fields monitored in EA and EML experiments can be of different characters as observed also for other organic systems [45]. It is worth to note here, however, that electric fields Fi of (1–3) · 105 V/cm are too weak to affect noticeably the (2x)EML effect described in Section 3.2. 4. Conclusions Electromodulation of photoluminescence in vacuumevaporated films of Ir(ppy)3 has been investigated in sandwich arranged cells Al/Ir(ppy)3/Al/glass. The electric field characteristics, the emission and excitation spectra of EML signals are rationalized in terms of the field-assisted dissociation of excitons into free and/or trapped charge carriers. The charge separation mechanism is described well by the three-dimensional Onsager model which indicates that charge carriers overcome the potential barrier by the diffusive motion rather than one-step jumps. An analysis of (1x)EML signals provides information on internal electric fields of (1–3) · 105 V/cm occurring in Ir(ppy)3 films, in accordance with previous electroabsorption measurements for this material.

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