Electromodulation of monomer and excimer phosphorescence in vacuum-evaporated films of platinum (II) complexes of 1,3-di(2-pyridyl)benzenes

Organic Electronics 14 (2013) 2880–2888

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Organic Electronics journal homepage: www.elsevier.com/locate/orgel

Electromodulation of monomer and excimer phosphorescence in vacuum-evaporated films of platinum (II) complexes of 1,3-di(2-pyridyl)benzenes Wojciech Mróz a,b, Karol Falkowski a, Maciej Mis´nik a,c, Ester Rossi d, Marcella Balordi d, Waldemar Stampor a,⇑ a

´ sk University of Technology, Narutowicza 11/12, 80-233 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, Italyc Tele and Radio Research Institute, Vacuum Measurement Laboratory, Ratuszowa 11, 03-450 Warszawa, Poland d Dipartimento di Chimica Inorganica, Metallorganica e Analitica ‘‘Lamberto Malatesta’’ dell’Università degli Studi di Milano, UdR INSTM di Milano, 20133 Milano, Italy b c

a r t i c l e

i n f o

Article history: Received 5 July 2013 Accepted 9 August 2013 Available online 29 August 2013 Keywords: Electromodulation Photoluminescence Phosphorescence Excimers Exciton dissociation Onsager model

a b s t r a c t Electric field-modulated photoluminescence (EML) measurements are presented for vacuum-evaporated films of cyclometallated Pt (II) complexes of 1,3-di(2-pyridyl) benzenes used as triplet emitters in organic light-emitting diodes (OLEDs). The excimer phosphorescence is quenched by the external electric field of 2.5 MV/cm up to 25% but the same effect on monomer phosphorescence is one order of magnitude smaller. The higher quenching effect for triplet excimers than triplet monomers in solid films of Pt complexes is rationalized assuming excimers to be populated within excimer-active domains of the films through an intermediate stage of geminate (e–h) pairs derived from dissociated monomer excitons. The EML data for excimers are successfully described in the framework of Sano– Tachiya–Noolandi–Hong (STNH) theory of geminate (e–h) pair recombination where the final recombination step (e–h capture) proceeds on a sphere of finite radius (a) with a finite speed. The conventional Onsager theory (a = 0) is sufficient to explain the EML quenching effect for monomers. The results are important for explaining the decrease of electroluminescence quantum efficiency observed in OLEDs working under high electric fields. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Platinum (II) complexes containing N^C^N – coordinating tridentate ligands in the form of 1,3-di(2-pyridyl) benzene derivatives (for molecular structures of some examples see Fig. 1) have been recently demonstrated as efficient phosphorescent emitters in organic light-emitting diodes (OLEDs) [1–6]. The strong spin–orbit coupling introduced by the Pt atom allows for efficient intersystem crossing between singlet and triplet excited states enabling emission from triplet states (phosphorescence). In turn, ⇑ Corresponding author. Tel.: +48 583472704; fax: +48 583472821. E-mail address: [email protected] (W. Stampor). 1566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2013.08.012

the rigidity of their molecular skeleton involving a tridentate ligand favors radiative over non-radiative decay pathways which leads to a high quantum yield of phosphorescence. In addition, their square-planar structures enable them to interact facially with each other, thereby facilitating formation of bi-molecular states, either in the ground state (dimers), or in the excited state (excimers). In fact, this type Pt complex – based emitters show usually two phosphorescence bands located in the bluish-green and red regions of the visible spectrum which are assigned to monomolecular and bi-molecular excited states, respectively [7,8]. The relative contribution of both spectral components to the total emission output can be controlled by changing the Pt complex concentration in a host matrix of the OLED

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F

N

N

N

N Pt

Pt

N

N

NCS

Pt

Cl

MePtNCS

FPtCl Cl

MePtCl Fig. 1. The molecular structures of Pt complexes investigated.

emissive layer which can be applied in electroluminescent diodes emitting white light (WOLEDs) [1,3,5–7]. The external electroluminescence (EL) quantum efficiency uEL (calculated as a number of collected photons per an injected charge carrier) of OLED usually exhibits non-monotonous behavior with applied electric field strength (or electric current density). A typical example of voltage dependence of uEL for a FPtCl-based EL diode is displayed in Fig. 2b and others for Pt dipyridylbenzene complexes with essentially similar uEL behavior can be found in literature [2,4]. This non-monotonous electric-field

EL [a.u.]

(a) 1.0

FPtCl

0.5

0.0

500

600

700

800

λ [nm]

ϕEL [%]

(b)

0.3

0.2

0.1

0.0

8

12

16

20

U [V] Fig. 2. Electroluminescence (EL) spectrum (a) and external EL quantum efficiency (uEL) vs voltage applied to a 165 nm – thick EL diode: ITO/ PEDOT:PSS/PVK/65% PVK:30% PBD:5% FPtCl/Ba/Al. The EL spectrum in the part (a) was recorded at the applied voltage of 16 V.

behavior of uEL is frequently observed for EL emitters, both in electrophosphorescent and electrofluorescent diodes, with single-layer or multi-layer structures [9,10]. In general, the initial field increase in EL quantum efficiency is followed by the roll-off preceded by a more or less broad maximum dependent on the individual features of an OLED. According to the present knowledge, the field evolution of uEL can be ascribed to various mechanisms. Firstly, when an OLED does not operate under space charge limited conditions, the uEL initially increases with applied voltage due to the increasing efficiency of electrode charge injection [9]. Secondly, when defect states with lower luminescence efficiency are involved their filling up with increasing voltage can also lead to increase in uEL [11]. Next, when recombination zone of charge carriers in an OLED structure moves under the influence of electric field, the voltage dependence of light output coupling factor is expected to occur due to concomitant change in side waveguiding modes responsible for light losses in the normal photon flux of the planar EL cell [2,12]. In addition, the effect of high electric fields on the decay of emissive excited states should be considered since various quenching mechanisms like exciton–exciton and exciton–charge carrier (polaron) interactions, or exciton dissociation set in, leading to a reduction in uEL [9,13–16]. In OLEDs the emissive layer is subjected to electric fields of strength exceeding 106 V/cm – under such strong electric field exciton dissociation into free charge carriers must not be neglected. In the present paper we investigate mechanism of exciton dissociation in Pt (II) complexes of dipyridylbenzenes (Fig. 1) applying electromodulation of photoluminescence (EML) spectroscopy where the photoluminescence (PL) quenching effect is measured for a sample placed in an external electric field. In comparison to dilute solutions, the PL spectra of Pt complexes in solid films are dominated by an additional broad long-wavelength band attributed to triplet excimers. The distinctive difference in the EML characteristics for the triplet mono-molecular excitons and triplet bi-molecular excited states (excimers) of Pt complexes has been recognized. The EML data for excimers are successfully described assuming excimers to be formed through an intermediate stage of geminate (e–h) pairs. Geminate (e–h) pair dissociation process is rationalized in the framework of Sano–Tachiya–Noolandi–Hong (STNH)

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theory [17,18] where the final recombination step (e–h capture reaction) is assumed to proceed with a finite speed on a sphere of finite radius (a). We show that the conventional Onsager theory [19] assuming, a = 0, is adequate only to dissociation process of more localized monomolecular (Frenkel type) excitons. 2. Experimental and numerical details

3. Results and discussion

The Pt complexes (Fig. 1) were kindly supplied by authors of Ref. [6] where details of synthesis procedure are given. The 100–200 nm thick neat films of Pt complexes were deposited by thermal evaporation in vacuum (103 Pa) on room temperature quartz substrates at a rate of (0.10.2) nm/s. The EML measurements were carried out in the sandwich cell configuration, Al/Pt complex/Al/quartz, supplied with two vacuum-deposited semi-transparent aluminum electrodes. Al electrodes with rather poor charge injection ability were chosen to reduce PL quenching induced by exciton–charge carrier (polaron) interaction as argued in our previous papers [20–22]. The thickness of Pt complex films was controlled with a crystal quartz microbalance during evaporation cycle and next verified with a KLA-Tencor Alpha Step 500 Profiler. The active electrode area of samples was 0.2 cm2. The photoluminescence of Pt complexes was excited by a light beam from a mercury lamp (Narva, HBO 200W) followed by a SPM-2 Zeiss monochromator. The electromodulated photoluminescence was measured at the second harmonic (2x) of applied sinusoidal electric field, F(t) = F0 sin(xt), (typically x/2p = 175 Hz), using a lock-in amplifier (EG&G Princeton Applied Research, model 5210). The PL light was collected with a quartz lightguide followed by a set of appropriate cut-off glass filters (Schott and Corning) and then detected by a photomultiplier tube (EMI 9863QB). The recorded (2x)EML signal is defined as

ð2xÞEML ¼

I 2x I0x

The Fourier components of PL intensity, Inx (n = 0, 2), were calculated numerically according to various theoretical models which quantify 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 [23,24].

ð1Þ

where I0x stands for (0x)-Fourier component and I2x – for the rms value of (2x)-Fourier component of I intensity of electromodulated PL. Positive values of (2x)EML signals mean PL quenching. A more detailed description of the EML measurements is given elsewhere [22,23]. Ordinary absorption spectra were recorded either with a Perkin–Elmer Lambda 10 spectrophotometer or a Perkin–Elmer Lambda 900 spectrometer. PL spectra were collected with a Perkin–Elmer LS 55B spectrofluorometer or with a setup consisting of an 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 a chargecoupled device (CCD) photon detector. Device fabrication details of OLEDs (Fig. 2) are given in Ref. [6]. The emitting layer comprising poly(N-vinylcarbazole) (PVK), 2-(4-biphenyl)-5(4-tert-butylphenyl)-1,3,4oxadiazole (PBD), and Pt complex was spin coated from a CH2Cl2 solution (8 mg mL1) consisted of 65% PVK, 30% PBD and 5% Pt complex by mass.

3.1. Absorption and photoluminescence spectra The photophysical properties of FPtCl, MePtCl and MePtNCS complexes in solution are essentially very similar [6]. The optical absorption (ABS) and photoluminescence (PL) spectra of Pt complexes diluted in a polar solvent CH2Cl2 are displayed in Fig. 3. An intense UV absorption band at k < 320 nm is attributed to ligand-centered (LC) electronic p–p* transitions, while weaker absorption bands extending into the visible region (340–450 nm) correspond to singlet–singlet electronic transitions and their vibronic satellites of mixed MLCT/LC character with a significant admixture of d–p*, metal-to-ligand – charge-transfer (MLCT) [25,26]. A weakly manifesting band at around 495 nm associated with a singlet–triplet (S0 ? T1) transition of MLCT/LC character being formally spin-forbidden acquires intensity by an effective mixing with higher energy spin-allowed transitions due to strong spin–orbit coupling of platinum atom [25]. This picture of electronic transitions in Pt dipyridylbenzene complexes has been firmly supported by quantum–mechanical calculations of frontier molecular orbitals (MO) for MePtCl and related molecules [27]. In particular, TD DFT (time-dependent density functional theory) calculations, neglecting spin–orbit coupling, revealed several higher-energy occupied MOs involving p ligand orbitals significantly mixed with 5d-Pt orbitals and lower-energy empty MOs almost completely of p* character localized on the ligand. Accordingly, the MLCT character of S0 ? T1 and S0 ? S1 transitions was evaluated for 25% and 32%, respectively [27]. All three complexes are highly luminescent with 0.5– 0.7 quantum yields, relatively long emission decay times of several microseconds in degassed CH2Cl2 and PL quenching by molecular oxygen [6] which is typical for the phosphorescence of this class Pt complexes [25,28,29]. The PL spectra of complexes (Fig. 3) display vibrationally structured profiles ascribed to T1 ? S0 electronic transition with a main vibronic progression of about 1300 cm1 characteristic of aromatic ring stretching modes. The highest PL peak of vibrational 0–0 origin at 501–504 nm, mirror-symmetry related with an absorption peak at 495 nm, exhibits Stokes shift of 45 meV which is much smaller than that for Pt complexes with their electronic transitions of pure LC character (for example, 0.41 eV in PtOEP – platinum octaethyl porphyrin [30]) While the shape of the ABS spectra does not change, in the PL spectra of all three Pt complexes in CH2Cl2 a broad structureless band at longer wavelength (k > 640 nm) emerges progressively with increasing concentration (Fig. 3). This band appearing not only in fluid but also in solid solutions (e.g. in polyvinylcarbazole) and neat

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(a)

(a)

-3

2.1 x 10-3 M

2.1 x 10 M

-5

10 M in CH2Cl2

4.2 x 10 M 1.5

4.2 x 10 M

-4

1.0

x 10

ABS

2

PL 0.5

0

300

400

500

600

700

0.0

800

6

2

1.7 x 10-5 M

4.2 x 10 M

300

0.4

PL

600

700

film 100 %

6.2 x 10 -5 M 0.8

MePtNCS

ABS

PL

0.4

2

0

300

400

500

600

700

800

Normalized PL [a.u.]

Normalized ABS [a.u.]

1 x 10-3 M 2.5 x 10 -4 M

x 10

1.5

1.0

MePtNCS PL ABS

0.5

0.0

300

400

500

600

700

800

Fig. 4. A comparison of ABS and PL spectra of FPtCl (a) and MePtNCS (b) for various molecular systems: neat vacuum evaporated films (thickness d = 100 nm), solid solutions in PVK prepared by spin-coating and liquid solutions in CH2Cl2 with concentrations of Pt complexes as indicated in the figure. Excitation spectra of the (2x)EML signal for global (wavelength integrated) photoluminescence of vacuum evaporated FPtCl films (Frms = 106 V/cm) are displayed in part (a) of the figure (circles).

2 x 10-3 M

6.2 x 10-5 M

4

film 100 % 40 % in PVK

800

λ [nm]

2 x 10-3 M

800

λ [nm]

0.0 500

ABS, PL [a.u.]

Normalized PL [a.u.]

Normalized ABS [a.u.]

ABS 2

700

-5

0.8

MePtCl

6

600

6x10 M in CH2 Cl2

x 30

(c)

500

-4

4

400

400

6x10-5 M in CH2Cl2

1.7 x 10 M

300

-5

0

-5

0

film 100 % 40 % in PVK

EML

1

(b) 2.1 x 10 M

ABS

10 M in CH2Cl2

-3

2.1 x 10-3 M

PL

FPtCl

3

λ [nm]

λ [nm]

(b)

EML [%], ABS, PL [a.u.]

-5

4.2 x 10 M

Normalized PL [a.u.]

Normalized ABS [a.u.]

2.1 x 10 M

FPtCl

4

film 100%

-4

-5

0.0

λ [nm] Fig. 3. Photoluminescence (PL) and absorption (ABS) spectra of FPtCl (a), MePtCl (b) and MePtNCS(c) complexes in CH2Cl2 solutions for various molar concentrations as indicated in the figure. The photoluminescence was excited with light of wavelengths kexc = 350 nm.

evaporated films (Fig. 4) can be assigned to excimers due to tendency of square-planar structures of Pt complexes to form face-to-face sandwich pairs of molecules in close-packed systems [7,28]. In case of excimers associative interaction between neighboring molecules is limited exclusively to the excited state of the aggregate and the ground state of the bi-molecular entity is dissociative

which leads to a distinct excimer PL band with no any ABS fingerprints [31,32]. In fact, the ABS spectra of neat films and concentrated solutions display all the features of the dilute solution spectra where only monomer species exist. In vacuum evaporated films of Pt complexes their PL is overwhelmingly dominated by longer-wavelength excimer band (Fig. 4) and in case of MePtNCS only a small bump can be observed in the spectral range of shorterwavelength monomer emission. It is worth to mention here that a closely related to FPtCl, a bromo-substituted BrPtCl complex belonging to the same family of Pt complexes concerned (Fig. 1) crystallizes in two different forms which exhibit completely different PL spectra rationalized in terms of the spatial arrangements of adjacent molecules [8]. While a less tightly packed needle-like crystals with nearest-neighbor molecules staggered with one another display green structured phosphorescence of mono-molecular origin, a cuboid crystals favoring larger overlapping of facially oriented and closely spaced molecules give the broad-band red emission attributed to excimers. Vacuum evaporated films of such compounds, which probably contain a mixture of these two, or probably even more polymorphic forms, can thus exhibit various

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phosphorescence components. In addition, domains of the sample with only one monomer-type lattice may exhibit similar behavior due to the presence of defects at which adjacent pairs of molecules are suitably located to be within the range of excimer interaction. The appearance of excimer emission is then a result of competition in rate between relaxation of the respective molecular excitons on the one hand and the trapping of molecular excitons at sites favorable for excimer formation on the other. Further details on excimer and monomer excited states in Pt complexes are provided by electromodulated photoluminescence (EML) experiments. 3.2. Electromodulation of photoluminescence The EML experiments were carried out on Pt complexes in the form of solid films supplied with Al electrodes. The measured signal is the relative change of PL intensity induced by an external electric field as determined by formula (1) in the experimental part of this paper. The circles in Fig. 4a show (2x)EML signal for global (spectrally integrated) emission as a function of the wavelength of excitation light (kexc). The EML data are shown for a 100 nm – thick vacuum evaporated neat film of FPtCl complex. For all wavelengths (kexc < 450 nm) PL intensity is reduced by an applied electric field. The PL quenching effect is approximately constant for excitation with light having photon energy within the spectral range of the first absorption band (with maxima at 390 nm and 430 nm) where Frenkel-type excitons of LC/MLCT origin are involved. In addition, this EML excitation spectrum exhibits a sharp increase at the shorter-wavelength part of the spectrum where photon absorption associated with higher energy (LC) excited states becomes of importance. The observed PL quenching effect as much as several percent at electric field of 106 V/cm strength (Fig. 5) indicates opening of a non-radiative decay channel of the excited states. Of various physical mechanisms, the field-assisted thermal dissociation of excitons into free and/or trapped charge carriers seems to be the most appropriate mechanism to rationalize the observed PL quenching in organic photoconductive films (see Refs. [20–24] and literature cited therein). The electric field characteristics of EML

global PL 10-1

(2ω)EML

λexc = 313 nm 10-2

IðFÞ ¼

10-3 10-4

105

signals for solid films of all three Pt complexes are essentially very similar (Fig. 5). In case of MePtCl complex blended with polyvinylcarbazole (PVK) under UV excitation both singlet PVK and singlet Pt complex excitons are generated. However, due to highly efficient energy transfer (Förster type) from PVK to Pt complex molecules, the PL quenching effect is underlain by prevailing dissociation of Pt complex excitons as observed previously in hole transporting host matrix – iridium complex guest systems [15]. The characteristic feature of the present results is different response to the electric field of the short-wavelength (green) and long-wavelength (red) portion of the PL spectrum which is depicted for vacuum-evaporated FPtCl films excited with light, kexc = 405 nm, within the spectral range of the LC/MLCT absorption band (Fig. 6), or kexc = 313 nm, relevant to higher energy LC absorption (Fig. 7). Whereas the red phosphorescence originated from triplet excimers is quenched by the external electric field of 2.5  106 V/ cm up to 25%, the same effect on green phosphorescence of monomer origin is one order of magnitude smaller. The EML experimental plots displayed in Figs. 6 and 7 distinctively depart from a second-order function of the applied electric field, in contrast to electroabsorption (EA) plots, measured at the same wavelength as the PL excitation wavelength, which are underlain by the Stark effect. Since the EA signals are at least one order of magnitude smaller than the EML signals measured for the same samples, the electric field-induced change in the number of photons absorbed in samples can be safely disregarded in interpretation of EML effect. Upon photoexcitation of a ‘‘hot’’ molecular exciton state its dissociation is usually assumed to proceed in two successive steps [31,32]. In a first step electron is ejected from a molecule, and next separated to a certain thermalization distance r0, creating a geminate (e–h pair) (charge-transfer, or CT state) with independent of electric field probability g0. In a second step, charge carriers start a Brownian random walk under the action of the combined Coulomb and applied electric field, until finally they either recombine regenerating the emissive state, or escape the Coulomb attraction contributing to a carrier current. According to this approach, the overall electric fielddependence of exciton dissociation is governed by the probability of dissociation of (e–h) pair (so called escape probability) X. Since the dissociation process of an (e–h) pair competes with geminate recombination into an emissive state, the PL intensity is expected to be a function of the electric field (F),

100% FPtCl 100% MePtNCS 40% MePtCl :60% PVK

106

F [V/cm] Fig. 5. Electric field effect on global PL for various Pt complexes in solid films (d = 100 nm). The (2x)EML signal as a function of rms value of applied electric field is displayed. The photoluminescence was excited with light of wavelengths kexc = 313 nm.

kf ½1  g0 XðFÞI ; kf þ kn

ð2Þ

where kf and kn are the rate constants of radiative and other non-radiative (different from charge separation) decay pathways for emitting states, respectively, and I* is the production rate of ‘‘hot’’ excitons. Except for the escape probability X(F), all parameters in formula (2) are assumed to be field independent. Some points should be considered in the context of the EML models based on the formula (2). It is assumed there that an (e–h) pair is formed by an autoionization of a

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FPtCl

(a)

red PL

-1

10

EML

(2ω)EML , -(2ω)EA

green PL λexc = 405 nm

-2

10

Onsager model

-3

10

EA

-4

10

2

1

(2ω)EML , (2ω)EA

(a)

slope 2.0

5

FPtCl 10

-2

10

10

-3

10

2 -4

5

6

10

10

F [V/cm]

FPtCl

(b)

-1

10

red PL 10

-3

10

(e-h) pair distribution

red PL

STNH model

10-3

(e-h) pair distribution Dirac

Dirac

-4

10

Gauss

10

FPtCl 10-1 10-2

(2ω)EML

(2ω)EML

-2

STNH model

EA 1 slope 2.0

10

F [V/cm]

(b)

EML

Onsager model

6

10

red PL green PL λexc = 313 nm

-1

5

10

10-4

6

F [V/cm] Fig. 6. The double logarithmic plots displaying the dependence of (2x)EML signal on the rms value of the applied electric field at the excitation wavelength kexc = 405 nm for 100 nm-thick FPtCl vacuum evaporated films. In part (a) of the figure the electric field effect on red PL (circles) and green PL (triangles) is shown. The lines represent the best fit according to the Onsager model (final recombination radius a = 0). In part (b) of the figure the EML experimental data for red PL (circles taken from the part (a) of the figure) are compared with theoretical curves based on the STNH model assuming the final recombination sphere radius a/rC = 0.038 and the initial (e–h) pair distributions according to a Dirac delta function (solid lines) and a Gauss function (dashed line). The final recombination speed parameter is j rC/D = 1 (thick solid line), or j rC/ D = 0.1 (thin solid line) for a Dirac delta distribution, and j rC/D = 1 for a Gaussian distribution. The r0 radius of both distributions is r0/rC = 0.075 and the width of a Gaussian distribution b/rC = 0.01. The primary efficiencies of (e–h) pair production are g0 = 0.7 for red PL and g0 = 0.12 for green PL. In all calculations relative dielectric permittivity er = 3.0 is assumed. In part (a) of the figure the electroabsorption (EA) change in the transmitting light intensity with k = 405 nm caused by the Stark effect is shown for comparison (diamonds). The EA signals follow the linear fit with a slope 2.0 (thin dotted line) which corresponds to the Stark effect. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Franck–Condon molecular state which do not require any electric field assistance. This is essentially a case of the present work because the EML results for Pt complexes were obtained with the excitation photon energy at least 0.5 eV above the energy of absorption threshold. However, if exciton dissociation starts from vibrationally relaxed exciton states, the primary quantum yield g0 of (e–h) pair formation in disordered solids can be field dependent which is well documented in literature (for further discussion on this subject see Refs. [24,33]).

10

Gauss

5

10

6

F [V/cm] Fig. 7. The double logarithmic plots displaying the dependence of (2x)EML signal on the rms value of the applied electric field at the excitation wavelength kexc = 313 nm. All symbols and designations are the same as in Fig. 6. The lines represent the best fit according to the Onsager model or the STNH model with final recombination parameters: a/rC = 0.044 and j rC/D = 5. The initial radius is r0/rC = 0.088 and the width of Gaussian distribution b/rC = 0.02. The primary efficiencies of (e–h) pair production (g0) for red and green PL are the same as for kexc = 405 nm excitation (Fig. 6). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Next, since the fast (at a rate higher than 1012 s1 [34]) intersystem crossing (S* ? T*) immediately after photoexcitation is considered to take place in Pt complexes, similarly as in Ir(ppy)3 [35], the vibrationally ‘‘hot’’ molecular triplet (T*) rather than singlet (S*) states should be regarded as precursors of geminate (e–h) pairs in exciton dissociation process as discussed in Ref. [23] for Ir(ppy)3 films. Further, to describe the efficient EML quenching effect for triplet excimers in Pt complex films, excimer formation process is assumed to proceed mainly by charge-transfer process with substantial contribution of (e–h) pair intermediate states in analogy to the EML model proposed previously for singlet excimers in 1,10 -bis(di-4-tolylaminophenyl) cyclohexane (TAPC) [22] and singlet excited dimers in bathocuproine (BCP) [36]. Accordingly, dissociation of ‘‘hot’’ triplet molecular excitons (M*) takes place within some special ‘‘excimer-active’’ domains of a film where molecules adopt a favorable molecular conformation for the electron transfer. The electron–hole separation occurs here at the expense of molecular exciton

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localization energy resulting in a geminate triplet (e–h) pair formed with probability g0. The created geminate (e–h) pair dissociates then into free carriers with escape probability (X), or recombines to a bi-molecular triplet excimer state (MM)* with probability (1  X). Alternatively, triplet excimers can be populated by exciton energy transfer process with probability (1  g0) when the encounter complex between molecules – one excited and another one at some separation unexcited (M*  M), decays to the excimer state (MM)* without involvement of the intermediate (e–h) pair state. Notably, the excimer formation process with involvement of (e–h) pairs is supported by recent time-dependent PL measurements for solid films of similar dipyridylbenzene Pt complexes at room temperature [8]. The rise time of excimer phosphorescence sr = 10 ns for neat films is largely shorter than monomolecular phosphorescence decay time (s = 3.4 ls) indicating that only selected configurations with appropriate orientations and sufficient orbital overlaps of neighbor molecules can serve as the excimer forming sites – the molecular excitons need 10 ns to transfer their electronic excitation energy to these sites. This PL rise time (sr), as suggested by authors of Ref. [8], can be attributed to the (e–h) pair lifetime if excimer formation process follows charge-transfer step. Otherwise, the excimer formation time for any configuration of closely-spaced molecules in neat Pt complex films would be falling in the time range below 1 ns as in the case of dimer-type crystal lattice of pyrene with all molecules arranged in parallel pairs [37]. The escape probability X was calculated in this paper according to the Onsager [19] and Sano–Tachiya–Noolandi–Hong (STNH) [17,18] theory, solving the Smoluchowski equation for 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 reaction, surface quenching) proceeds on a sphere of finite radius (a) with a finite velocity (j). The general solution for X(r, h), with r being an initial intrapair separation and h being a polar angle, derived by Sano and Tachiya [17] and independently by Noolandi and Hong [18] involves many time-consuming mathematical manipulations, and therefore a special solution for a = 0 given by Onsager [19] is often used to describe (e–h) pair dissociation in organic solids [31,32]. The exact formulas for X(r, h) can be found in original papers [17–19]. If g(r, h) represents the initial distribution of pair separations, the averaged escape probability can be obtained by integration,

X¼

Z

Xðr; hÞgðr; hÞds;

ð3Þ

where ds is the volume element. In the present paper we have used isotropic distributions described by Dirac delta or Gauss functions:

gðr; hÞ ¼

1 dðr  r0 Þ; 4pr 2

gðr; hÞ ¼ g 0 exp 

ð4Þ

ðr  r 0 Þ2 2

b

! ;

ð5Þ

where r0 is the mean intrapair distance, b is the width of the distribution and g0 is the normalization factor. The analytical expressions for averaged escape probability X(F) are available for models assuming an initial pair distribution in the form of the Dirac delta function. The relevant formulas and numerical procedures used in the present paper together with original references are given and described in detail in our previous publication [24]. The calculated values of I2x/I0x have been fitted to the measured (2x)EML signals as described in Section 2. The EML data for monomer-type PL (green phosphorescence) are adequately fit to the Onsager model with a Dirac delta distribution of (e–h) pair initial radii (the model designated here as Ons-Dirac) which is shown in Figs. 6a and 7a for excitation wavelengths of 405 nm and 313 nm, respectively. 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 (e–h) Coulomb attraction energy is equal to the thermal energy, and, here rc = e2/4pe0erkBT ffi 187 Å (e – elementary charge, e0 – permittivity of free space, kB – Boltzmann constant). The Ons-Dirac model-based curves in Figs. 6a and 7a (solid lines designated by 1) fitting experimental EML results for green phosphorescence (triangles) are obtained with approximately the same g0 = 0.12, and slightly different radii: r0/rc = 0.075 (r0 = 14.0 Å) and r0/rc = 0.088 (r0 = 16.5 Å), respectively. The estimated values of (e–h) pair radii for FPtCl correlate well with r0 = (13.0–15.1) Å for monomer phosphorescence of Pt – octaethyl porphyrin (PtOEP) films [30]. Using Onsager model-based formula gOns(F = 0) = g0exp(rC/r0), we estimate the zero-field charge photogeneration efficiency as 2  107  1  106 which is comparable to that for PtOEP films [30]. Though, crystallographic parameters for FPtCl are unknown at the moment, we recall here the generally accepted notion that a short range order of a crystal structure is preserved in vacuum-evaporated films. Consequently, there exists a strong correlation between crystal distances and values of r0 derived from EML experiments for solid films manufactured by vacuum evaporation. In fact, as observed for vacuum evaporated films of various small-molecule materials, under excitation within the first low-energy absorption band r0 radii are approximately equal to one or two specific, or two average – crystal lattice distances (see Table 2 in our recent paper [24] and references therein). With this respect two types of molecular packing in a crystal lattice are especially favorable for primary exciton dissociation. In the first case, with sexithiophene (a-6T) and quinacridone (c-QAC) being typical examples, crystal structures show layered arrangement with closely spaced molecular layers and the value of r0 correspond then to a distance between adjacent layers. In the second case, the direction of ‘‘easy’’ dissociation in a crystal lattice is established by strong charge-transfer coupling within a molecular stack as in aluminum and iridium complexes (mer-Alq3, or fac-Ir(ppy)3) where the values of r0 correspond to the twice intermolecular distances along the columnar stack.

W. Mróz et al. / Organic Electronics 14 (2013) 2880–2888

The Ons-Dirac model is rather not appropriate to describe the EML results for excimer-type PL (red phosphorescence) in Pt complex films which is depicted in Figs. 6a and 7a for two kinds of excitation regime, respectively. The theoretical curves (solid lines designated by number 2) calculated with g0 = 0.7 and the same values of r0/rc as for monomer-type PL clearly depart from the experimental EML data for red phosphorescence (circles) both in the low electric field and high electric field ranges. The low-field disagreement can be largely suppressed, if instead of Dirac, we apply a Gaussian distribution of (e–h) pairs regarding this way some positional disorder of molecular film structure (Ons-Gauss model). However, the best Ons-Gauss model based curves (broken lines) obtained with the distribution width, b/rC = 0.01–0.02, noticeably deviate in the high-field region and this feature cannot be removed in the framework of this model. For excimers, which are bi-molecular excited states delocalized in space by resonance exchange of electronic excitation energy and charge, a more proper description of EML effect should be based on the STNH theory regarding a finite radius (a) of a final (e–h) pair recombination sphere. In Figs. 6b and 7b the experimental electric field EML characteristics are compared with those calculated according to the STNH theory assuming Dirac or Gaussian distributions of initial (e–h) pairs (STNH-Dirac or STNHGauss models, respectively). The initial (e–h) separation distances r0 were taken from the fit based on the Onsager model (Fig. 6a or Fig. 7a) and the final recombination sphere radii were assumed to be equal to a/rC = 0.038 (Fig. 6b), or a/rC = 0.044 (Fig. 7b) which are close to the average intermolecular distances in small-molecule organic crystals (7–9 Å). The STNH-Dirac model-based curves (thick solid lines in Figs. 6b and 7b), obtained with a final recombination speed parameter j rC/D = 1 (Fig. 6b), or j rC/D = 5 (Fig. 7b), fit well the experimental data (circles), especially in the medium – and high-electric field regions. For smaller values of j rC/D, the STNH-Dirac model-based curves deviate distinctly upwards in the high-field range which is exemplified for a recombination speed 10 times smaller (j rC/D = 0.1) by the thin solid line in Fig. 6b. In turn, for larger values of j rC/D, or smaller values of final (e–h) radius (a/rC), the STNH model-based curves bend downwards in the high-field region and come close to the Onsager model-based curves (not shown). The rather poor STNH-Dirac model-based fit in the low-field range can be improved noticeably, in a similar way, as in the case of the Onsager model-based plots, if we allow for a Gaussian distribution of r0 radii (STNH-Gauss model) remaining the other parameters unchanged (broken lines in Figs. 6b and 7b). We note that the estimated above values of j rC/D = 1–5 translate to the final carrier recombination velocity: j = (0.14–0.70) cm/s, or j = (1.4–7.0) cm/s, taking a diffusion coefficient of majority charge carriers (holes): D = 2.6  107 cm2/s, or D = 2.6  106 cm2/s, respectively. The given above values of D were calculated using Einstein relation, D = lkBT/e, with typical in organic films charge carrier mobilities: l = 105 cm2/V s, or l = 104 cm2/V s, respectively. The data for hole mobilities in solid organic Pt complexes, or related compounds, range from

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107 cm2/V s to 103 cm2/V s [38–40]. Apart from p–p stacking interactions, both the film quality and morphological features of films dependent on post-treatment conditions were found to affect the carrier mobility. According to the most reliable data analysis available for electrontransporting films of Al quinolate complex Alq3, various methods of mobility measurements lead usually to results different by 2–3 orders of magnitude [41,42]. In addition, the electric field dependence of charge carrier mobility of Poole–Frenkel type, is usually observed in disordered organic solids [32,43], which is not currently implemented in the formalism of the STNH theory. The present evaluation of j in FPtCl films compares well with those made recently in other organic materials – for example, j = 2 cm/s was predicted in hole transporting films of Ir phenylpyridine complex Ir(ppy)3 [24]. For a classical ion pair of finite lifetime (s), the recombination velocity may be evaluated [44] as, j = a/s = 8 cm/s, with a = 8 Å (mean intermolecular distance), and s = sr = 10 ns taken from transient PL experiments [8], assuming the rise time of excimer phosphorescence (sr) to be limited by the lifetime of a final (e–h) pair. This rough estimation of final recombination velocity is consistent with that derived from a thorough analysis of the EML results in the framework of the STNH theory. It can be then concluded that the STNH theory gives better description of EML results for excimer-type PL of Pt complex films in comparison to the Onsager theory. According to the present results, the estimated values of r0 radii for (e–h) pairs contributing in formation of excimer species correspond to those for (e–h) pairs taking part in dissociation of monomer excitons. The larger electric field effect on excimer than monomer PL arises presumably from different contribution of charge transfer resonance into bimolecular (excimer) and mono-molecular (Frenkel) excited states, respectively, as observed recently for dimer and monomer singlet states in vacuum evaporated films of bathocuproine (BCP) [36]. This characteristic property of bi-molecular excited states, in comparison to monomer states, can be quantified in terms of the STNH theory in larger values of the primary quantum yield of (e–h) pair formation (g0) and finite size of final recombination sphere (a).

4. Summary and conclusions Electromodulation of photoluminescence (EML) in Pt (II) dipyridylbenzene complex films is due to the field-assisted dissociation of electronic excited states into charge carriers. The EML quenching effect has been successfully described by the Sano–Tachiya–Noolandi–Hong (STNH) and Onsager theory of geminate recombination using Dirac delta, or Gaussian distributions of (e–h) pairs which indicates that carrier diffusion motion controls the charge separation mechanism. The efficient quenching of excimer phosphorescence is explained assuming excimers to be populated mainly through an intermediate stage of (e–h) pairs which can be broken by the electric field. Dissociation of Frenkel excitons, related to the monomer-type PL quenching, appears to be a less efficient process which results presumably from different contribution of charge transfer resonance into bi-molecular (excimer) and

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mono-molecular (Frenkel) excited states, respectively. Applying Sano–Tachiya–Noolandi–Hong theory the final recombination (capture) speed of (e–h) pairs in vacuum evaporated films has been estimated, j = 0.1–7 cm/s, which gives the back electron transfer rate constant kBET = j/a = 107–108 s1 between an electron and a hole separated at the average intermolecular distance. This is in contrast to the commonly applied Onsager theory of geminate recombination and Langevin theory of bulk recombination which assume the final recombination process proceeds at zero separation between an electron and a hole (a = 0). This should have been taken into account in description of organic devises such as solar cells and EL diodes where understanding of (e–h) recombination mechanism is essential for their optimal performance design. In particular, according to the present EML results, the exciton dissociation effect as high as 25% should contribute to the high electric field roll-off of EL quantum efficiency observed in OLEDs based on Pt dipyridylbenzene complex emitters. Acknowledgment This work was financially supported by Polish National Science Centre under Grant DEC-2011/03/B/ST7/01888. References [1] J. Kalinowski, M. Cocchi, D. Virgili, V. Fattori, J.A.G. Williams, Adv. Mater. 19 (2007) 4000. [2] M. Cocchi, D. Virgili, V. Fattori, J.A.G. Williams, J. Kalinowski, Appl. Phys. Lett. 90 (2007) 023506. [3] M. Cocchi, J. Kalinowski, D. Virgili, V. Fattori, S. Develay, J.A.G. Williams, Appl. Phys. Lett. 90 (2007) 163508. [4] X.H. Yang, Z.X. Wang, S. Madakuni, J. Li, G.E. Jabbour, Adv. Mater. 20 (2008) 2405. [5] M. Cocchi, J. Kalinowski, L. Murphy, J.A.G. Williams, V. Fattori, Org. Electronics 11 (2010) 388. [6] W. Mróz, C. Botta, U. Giovanella, E. Rossi, A. Colombo, C. Dragonetti, D. Roberto, R. Ugo, A. Valore, J.A.G. Williams, J. Mater. Chem. 21 (2011) 8653. [7] B. D’Andrade, S.R. Forrest, Chem. Phys. 286 (2003) 321. [8] J. Kalinowski, M. Cocchi, L. Murphy, J.A.G. Williams, V. Fattori, Chem. Phys. 378 (2010) 47. [9] J. Kalinowski, Organic Light Emitting Diodes: Principles, Characteristics, and Processes, Marcel Dekker, New York, 2005. [10] J. Kalinowski, V. Fattori, M. Cocchi, J.A.G. Williams, Coord. Chem. Rev. 255 (2011) 2401. [11] M. Segal, M.A. Baldo, R.J. Holmes, S.R. Forrest, Z.G. Soos, Phys. Rev. B 68 (2003) 075211.

[12] V. Bulovic, V.B. Khalfin, G. Gu, P.E. Burrows, D.Z. Garbuzov, S.R. Forrest, Phys. Rev. B 58 (1998) 3730. [13] M.A. Baldo, C. Adachi, S.R. Forrest, Phys. Rev. B 62 (2000) 10967. _ [14] J. Kalinowski, W. Stampor, J. Me˛zyk, M. Cocchi, D. Virgili, V. Fattori, P. Di Marco, Phys. Rev. B 66 (2002) 235321. [15] J. Kalinowski, W. Stampor, J. Szmytkowski, D. Virgili, M. Cocchi, V. Fattori, C. Sabatini, Phys. Rev. B 74 (2006) 085316. [16] S. Reineke, K. Walzer, K. Leo, Phys. Rev. B 75 (2007) 125328. [17] H. Sano, M. Tachiya, J. Chem. Phys. 71 (1979) 1276. [18] J. Noolandi, K.M. Hong, J. Chem. Phys. 70 (1979) 3230. [19] L. Onsager, Phys. Rev. 54 (1938) 554. [20] J. Kalinowski, W. Stampor, P. Di Marco, J. Chem. Phys. 96 (1992) 4136. [21] J. Kalinowski, W. Stampor, P. Di Marco, J. Electrochem. Soc. 143 (1996) 315. [22] W. Stampor, Chem. Phys. 256 (2000) 351. _ [23] W. Stampor, J. Me˛zyk, Chem. Phys. 337 (2007) 151. [24] K. Falkowski, W. Stampor, P. Grygiel, W. Tomaszewicz, Chem. Phys. 392 (2012) 122. [25] J.A.G. Williams, A. Beeby, E.S. Davies, J.A. Weinstein, C. Wilson, Inorg. Chem. 42 (2003) 8609. [26] J.A.G. Williams, S. Develay, D.L. Rochester, L. Murphy, Coord. Chem. Rev. 252 (2008) 2596. [27] W. Sotoyama, T. Satoh, H. Sato, A. Matsuura, N. Sawatari, J. Phys. Chem. A 109 (2005) 9760. [28] S.J. Farley, D.L. Rochester, A.L. Thompson, J.A.K. Howard, J.A.G. Williams, Inorg. Chem. 44 (2005) 9690. [29] A.F. Rausch, L. Murphy, J.A.G. Williams, H. Yersin, Inorg. Chem. 51 (2012) 312. [30] J. Kalinowski, W. Stampor, J. Szmytkowski, M. Cocchi, D. Virgili, V. Fattori, P. Di Marco, J. Chem. Phys. 122 (2005) 154710. [31] M. Pope, E.C. Swenberg, Electronic Processes in Organic Crystals and Polymers, Oxford University Press, New York, 1999. [32] M. Schwoerer, H.C. Wolf, Organic Molecular Solids, Wiley VCH, Weinheim, 2007. [33] V.I. Arkhipov, H. Bässler, in: W. Brütting (Ed.), Physics of Organic Semiconductors, Wiley-VCH, Weinheim, 2005, pp. 183–269. [34] E.O. Danilov, I.E. Pomestchenko, S. Kinayyigit, P.L. Gentili, M. Hissler, R. Ziessel, F.N. Castellano, J. Phys. Chem. A 109 (2005) 2465. [35] K.C. Tang, K.L. Liu, I.C. Chen, Chem. Phys. Lett. 386 (2004) 437. [36] M. Mis´nik, K. Falkowski, W. Mróz, W. Stampor, Chem. Phys. 410 (2013) 45. [37] J. Birks, Photophysics of Aromatic Molecules, Wiley, London, 1970. [38] Y.Y. Noh, J.J. Kim, Y. Yoshida, K. Yase, Adv. Mater. 15 (2003) 699. [39] C.M. Che, H.F. Xiang, S.S.Y. Chui, Z.X. Xu, V.A.L. Roy, J.J. Yan, W.F. Fu, P.T. Lai, I.D. Williams, Chem. Asian J. 3 (2008) 1092. [40] X.Y. Zhao, C. Piliego, B.S. Kim, D.A. Poulsen, B. Ma, D.A. Unruh, J.M.J. Frechet, Chem. Mater. 22 (2010) 2325. [41] S. Barth, P. Muller, H. Riel, P.F. Seidler, W. Riess, H. Vestweber, H. Bässler, J. Appl. Phys. 89 (2001) 3711. [42] H. Mu, I. Reddy, J. Hunz, P. Severs, S. Patil, J. Phys. D 43 (2010) 195103. [43] P.M. Borsenberger, D.S. Weiss, Organic Photoreceptors for Xerography, Marcel Dekker, New York, 1998. [44] C. Braun, J. Chem. Phys. 80 (1984) 4157.