Influence of doping on spin-dependent exciton formation in Alq3 based OLEDs

Organic Electronics 8 (2007) 249–255 www.elsevier.com/locate/orgel

Influence of doping on spin-dependent exciton formation in Alq3 based OLEDs Fernando A. Castro a,b,*, George B. Silva a, Frank Nu¨esch b, L. Zuppiroli c, Carlos F.O. Graeff a b

a Departamento de Fı´sica e Matema´tica, FFCLRP – USP, Av. Bandeirantes 3900, 14040-901 Ribeira˜o Preto-SP, Brazil ¨ berlandstr. 129, Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Functional Polymers, U CH-8600 Du¨bendorf, Switzerland c Laboratoire d’optoe´lectronique deˆs mate´riaux mole´culaire, IMX-STI-EPFL, CH-1015 Lausanne, Switzerland

Available online 20 July 2006

Abstract We have used electrically detected magnetic resonance (EDMR) to investigate the effect of doping and trapped charges on spin-dependent exciton formation in aluminum(III) 8-hydroxyquinoline (Alq3) based OLEDs. As dopant we have used 4-(dicyanomethylene)-2-methyl-6-{2-[(4-diphenylamino)-phenyl]ethyl}-4H-pyran (DCM-TPA). We observed significant differences (in g-factor, DHpp and amplitude) between EDMR signals from DCM-TPA doped and undoped devices at room temperature. The signal from the DCM-TPA doped OLED was found to be strongly dependent on temperature, with signal intensity increasing by two orders of magnitude below 200 K. It was attributed to spin-dependent exciton formation, which our results indicate that takes place between a trapped electron in the dopant and a hole from Alq3, in the current/bias range investigated. The observed differences between doped and undoped devices are discussed in terms of a change in exciton precursor reaction rates and/or decrease in spin coherence time. Preliminary results from rubrene doped OLEDs indicate a similar process.  2006 Elsevier B.V. All rights reserved. PACS: 71.35.y; 73.40.c; 76.90.+d; 85.60.Jb Keywords: Doping; Exciton; OLED; Electrically detected magnetic resonance

1. Introduction Organic electronics has been the subject of intensive research during the last decade. Since the * Corresponding author. Address: Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for ¨ berlandstr. 129, CH-8600 Du¨bendorf, Functional Polymers, U Switzerland. Tel.: +41 44 823 4584; fax: +41 44 823 4012. E-mail address: [email protected] (F.A. Castro).

discovery of the first efficient electroluminescent material aluminum(III) 8-hydroxyquinoline (Alq3) [1], several organic molecules emitting light from blue to red have been produced. However, due to the intrinsic optical properties of organic solids, these materials often show broad emission spectra with unsatisfactory color purity. For several applications, such as color displays or electrically pumped lasers, not only color tunability is important but also color purity or color saturation is an important

1566-1199/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2006.06.007

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factor. Doping a host matrix with highly fluorescent dyes has proven to be an efficient strategy to achieve narrow emission spectra [2]. Although doping of the electroactive host may bring other advantages, such as increased device lifetime and efficiency [3], the mechanism of light generation is still not fully understood. In particular the process of exciton formation on the doping molecule is subject of debate. While many studies suggest energy-transfer mechanisms [4], others show evidence of charge transfer mechanisms [5]. As a matter of fact, the process of exciton formation itself after charge injection in organic devices is still subject to controversy [6–8], even for undoped materials. One of the main discussions is centered on the relative percentage of singlet and triplet exciton formation. Due to simple spin statistics, one would expect 25% singlets and 75% triplets to be formed, what would limit the efficiency of organic light emitting diodes (OLEDs). However, many experimental and theoretical works [9,10], in particular on conjugated polymers, claimed efficiencies higher than 25%; while other groups claimed singlet generation fraction below this value [11]. We have recently reported the observation of singlet exciton formation probability smaller than 25% for Alq3-based OLEDs using electrically detected magnetic resonance (EDMR) [8]. Typically, in an EDMR experiment spin level transitions induced by magnetic resonance are measured through changes in device current. The key to EDMR is that many of the processes that lead to charge transport and recombination in semiconductors are strongly dependent on spin selection rules, or the spin states of interacting electrons. One important characteristic of this technique is that it allows the investigation of devices in real working conditions at the same time as it is able to probe the wave functions of unpaired spin sites and its surrounding with higher sensitivity than conventional electron spin resonance (ESR). Although for several years EDMR has been used to characterize inorganic semiconductors [12], only recently Alq3-based devices have been studied using this and other magnetic resonance related techniques [13,14]. In this work, we used EDMR to investigate the effect of doping and trapped charges on spin-dependent exciton formation. State-of-the-art undoped and DCM-TPA doped Alq3 based OLEDs were investigated. At room temperature, significant differences were found between EDMR signatures from doped and undoped devices. The signal from

DCM-TPA doped OLEDs was found to be strongly dependent on temperature, with signal intensity increasing by two orders of magnitude below 200 K. Our results indicate that exciton formation in these doped devices occurs between an electron trapped in the dopant and a hole in the Alq3. We discuss these differences in terms of a decrease in spin coherence time, or a change in exciton precursor reaction rates. Preliminary results from rubrene doped OLEDs are also discussed. 2. Experimental All organic light emitting diodes used in this work were prepared by thermal evaporation in high vacuum (<5 · 107 mbar). Prior to deposition, indium–tin–oxide (ITO) coated glass substrates were cleaned in a sequence of ultrasonic baths using ethanol, acetone, detergent and Milli-Q water, respectively. Then the organic layers were deposited in sequence. For the undoped OLED, used as a reference, the sequence comprises a 10 nm thick copper phthalocyanine (CuPc) layer, used to improve hole injection, followed by a 40 nm thick N,N 0 -diphenylN,N 0 -bis(1-naphthyl)-1,1 0 biphenyl-4,400 diamine (aNPD) hole transporting layer and a 60 nm thick Alq3 electron transporting and emissive layer. Subsequently, a 0.8 nm thick LiF layer was deposited followed by a 100 nm thick Al layer. The doped devices investigated in this work were prepared in the same way, except that instead of the 60 nm Alq3 layer, a 20 nm thick Alq3 doped layer (co-evaporation of Alq3 with 1 wt.% of doping molecules) was deposited followed by a 40 nm thick undoped Alq3 layer. The doped device structure can be seen in Fig. 1 together with the molecular structures of Alq3 and the doping molecules used in this work: 4-(dicyanomethylene)-2-methyl-6-{2-[(4-diphenylamino)-phenyl]ethyl}-4H-pyran (DCM-TPA) and rubrene. To avoid air-induced degradation, the devices were directly transferred to an inert gas glove box without being exposed to the atmosphere. Samples with lateral dimensions no greater than 3.0 mm were then contacted and sealed inside an ESR quartz tube. EDMR measurements were done using a specially designed computer interfaced K-band (24 GHz) spectrometer, in the temperature range from 100 to 300 K. The spin-dependent conductivity changes were measured by modulating the static magnetic field H0 and using lock-in detection of the current

F.A. Castro et al. / Organic Electronics 8 (2007) 249–255

Fig. 1. On the left side: molecular structure of tris-(8-hydroxy quinoline) (Alq3), 4-(dicyanomethylene)-2-methyl-6-{2-[(4-diphenylamino)-phenyl]ethyl}-4H-pyran (DCM-TPA), and rubrene. On the right side: structure of the doped devices used in this work. The undoped device contains a 60 nm thick Alq3 layer instead of the 20 nm doped layer plus 40 nm Alq3 layer.

changes. The magnetic-field modulation was typically 133 Hz. The EDMR signal (Dr/r) is calculated normalizing the current changes: (I  I0)/I0, where I0 is the current out of resonance. Electroluminescence spectra of DCM-TPA doped and Alq3 undoped OLEDs operating at 0.75 mA/cm2 were measured using an integrating sphere. 3. Results

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the same bias current. The bias current/voltage was chosen to make sure that the device was always electroluminescent. The general behaviour of Dr/r with temperature is different for Alq3 doped or undoped devices. For the doped OLED a drastic signal decrease is observed for temperatures above 200 K, while the EDMR signal from undoped samples is weakly dependent on T. Since EDMR is measured using a lock-in amplifier, not only the amplitude is measured but also the phase difference with respect to the reference signal. So, if two components have different relaxation times, one expects different phase shifts with respect to the lock-in reference signal and this can be used to separate the EDMR signal components. Detailed information about this procedure can be found on the works by Graeff et al. [8] and Dersch et al. [15]. In Fig. 3 we compare an EDMR signal for the doped device at 110 K and at 250 K showing the signal in both channels 1 and 2 of the lock-in. For T = 300 K the signal was too weak. The signal at 250 K was scaled to match the intensity of the one at 110 K for better visualization. Making use of a rotation matrix we changed the phase between channels one and two for each spectrum in order to eliminate the signal from channel 2. In the inset it is shown that this is not possible at 110 K. As a matter

Fig. 2 shows the temperature (T) dependence of the EDMR signal amplitude from a DCM-TPA doped OLED (full circles) and an Alq3 undoped OLED (open squares). For each sample all measurements were performed operating the device at

4 3

Δσ/σ (x10-4)

2

DCM-TPA doped OLED 24.086 GHz Channel 1 T = 110 K T = 250 K

Channel 2 T = 110 K T = 250 K

1 0 Channel 2

-1

1x10-4

110 K

-2 250 K

Δσ/σ

-3 -4

1x10-5

850

855

855

858

861

860

865

864

Magnetic Field (mT)

10-6

DCM-TPA doped OLED Undoped Alq3 OLED

90

120

150

180

210

240

270

300

Temperature (K)

Fig. 2. Amplitude of EDMR signal as a function of temperature for an Alq3 OLED doped with 1 wt.% DCM-TPA (full circles) and an undoped Alq3 OLED (open squares).

Fig. 3. EDMR spectra of a DCM-TPA doped OLED at 110 K (solid and dashed lines) and at 250 K (open square and dotted line) in both channels 1 and 2 of the lock-in amplifier. The spectrum taken at 250 K was scaled to match the 110 K amplitude for better visualization. The inset shows a magnification of the signals in channel 2, dislocated vertically. For each temperature, the phase between the signals from both channels was changed in order to eliminate the signal in channel 2. Note that a residual signal cannot be eliminated from the spectrum at 110 K.

F.A. Castro et al. / Organic Electronics 8 (2007) 249–255 1 Undoped OLED DCM-TPA doped 250K

Δσ/σ

of fact it was only possible for spectra taken at temperatures above 200 K. This is an indication of a change in spin coherence time with temperature around 200 K, the same ‘‘threshold’’ temperature where a strong decrease in signal amplitude was observed. Making use of the procedure mentioned above, we noticed that the signal at 110 K can be decomposed into two components one with g-factor of 2.0042 and DHpp = 1.82 ± 0.01 mT (bias independent) and the other with a g-factor of 2.0039 and DHpp between 3.00 ± 0.05 and 4.20 ± 0.05 mT (increasing with increasing bias). This signal is similar to the one seen for the undoped OLED, where the signal attributed to holes [8] has gh = 2.0042 and DHpph = 1.5 mT and the signal attributed to electrons has ge = 2.0032 and DHppe also dependent on bias, between 2.0 and 3.4 mT. As we recently reported [8], the signal from these Alq3 undoped OLEDs comes from spin-dependent exciton formation due to different cross sections for anti-parallel and parallel spin pairs prior to exciton formation. In that work, unipolar Alq3 devices were also investigated, what lead to the conclusion that holes coming from a-NPD were injected in the Alq3 layer and excitons were formed by a hole and an electron in the Alq3. In both cases, hole- and electron-only devices, the EDMR signals were at least 25 times less intense than the signal from the OLED. Although the separation of different contributions to the EDMR signal at 250 K due to different spin-dependent relaxation times was not possible using phase analysis, we fitted the spectra using a sum of Gaussian functions. We did not use the signal of DCM-TPA doped OLED at room temperature, since the signal to noise was not good enough to render a reasonable fit. To decrease the free parameters, we used the g factors obtained from phase analysis of the spectra at low temperatures as ‘‘almost’’ fixed parameters. ‘‘Almost’’ meaning that these values were allowed to fluctuate in a given range around the central value obtained at low temperatures. The signal from the DCM-TPA doped OLED at 250 K could, in this way, be decomposed into two components. One with g-factor of 2.0042 and DHpp = 1.5 ± 0.1 mT and the other with g-factor of 2.0038 and DHpp = 2.5 ± 0.1 mT. In Fig. 4 typical EDMR spectra are presented for both undoped OLED at room temperature and DCM-TPA doped OLEDs at 250 K. The spectra were scaled to the right-side peak in order to better visualize the spectral differences. We can clearly see

0

24.086 GHz

-1 855

858

861

Magnetic Field (mT)

Fig. 4. Typical EDMR spectra of an undoped Alq3 OLED (full line) at room temperature and of a DCM-TPA doped OLED (dashed line) at 250 K. Signals were scaled to the same value for the right-side peak.

that both g factor and peak-to-peak linewidth (DHpp) are different, as well as the signal amplitude (shown in Fig. 2). By comparing the EDMR spectral characteristics of the signals components, the main difference between doped and undoped OLEDs stems from the anion-related signal. Fig. 5 shows the normalized electroluminescence (EL) spectrum of a DCM-TPA doped OLED and the relative EL spectrum of an undoped OLED, both operating at the same current density at room temperature. The red shift is a clear indication that

Electroluminescence Intensity (a.u.)

252

Undoped OLED DCM-TPA doped OLED

1.0

0.8

0.6

0.4

0.2

0.0 400

500

600

700

800

Wavelength (nm)

Fig. 5. Normalized electroluminescence spectrum of a DCMTPA doped OLED (dashed line) and the relative EL spectrum of an undoped Alq3 OLED at room temperature. Spectra were taken for devices operating at 0.75 mA/cm2.

F.A. Castro et al. / Organic Electronics 8 (2007) 249–255

light emission is coming from DCM-TPA molecules. Note that the higher EL intensity of the doped sample reflects its higher efficiency [16]. 4. Discussion The results presented above provide two main findings. First, the EDMR signal on DCM-TPA doped samples are strongly dependent on temperature, especially in the transition from room temperature to 200 K, where a two order of magnitude change in signal amplitude is observed. The other important finding is that the spectroscopic characteristics of the signal, g factor and DHpp, are distinct from undoped samples. Our results indicate that the EDMR signal both in undoped and doped samples, are coming from spin-dependent exciton formation. In the following we will discuss possible mechanisms for the strong temperature dependence on doped diodes. One possibility would be a quench in exciton formation. This is clearly not the case. In fact, the external quantum efficiency of DCM-TPA doped diodes is higher than those found in undoped samples [16]. However, as seen in Fig. 5, the emission color is shifted from green to red. In other words, doping with DCM-TPA is responsible for a strong decrease in the characteristic green emission of Alq3. One explanation for this effect is that excitons formed in the Alq3 are energy transferred to the dopant molecule by a Fo¨rster mechanism [4]. Another possibility is that electrons are trapped in the dopant which then reacts with a hole in Alq3, directly forming an exciton [5]. EDMR is not probing the exciton state, but the state just before exciton formation, or in other words its precursor pair [8]. This precursor pair, formed by a cation (hole) and anion (electron) can have different spin states. These states determine the rate at which excitons are formed, and are assumed to be different for spin parallel pairs with respect to spin anti-parallel pair states, as discussed in more detail in [8,7]. Due to the spin conservation rules obeyed in a Fo¨rster energy-transfer mechanism, one would expect no change in the EDMR signal, neither in amplitude nor in g-factor and lineshape. In other words, the signal of the doped or undoped samples would be indistinguishable. By contrast, our results show that, under all experimental conditions used, a clear difference in the spectroscopic characteristics (g-factor and DHpp) between doped and undoped samples is observed

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(see Fig. 4). Additionally, a two orders of magnitude change in Dr/r occur between 200 K and room temperature. Thus, we have no indication from EDMR that excitons are being formed in Alq3, in the experimental conditions used. Therefore our results strongly favour electron trapping as the main mechanism for the change in emission color with doping. The fact that the lowest unoccupied molecular orbital (LUMO) of these DCM-TPA molecules is located around 0.48 eV [16] below the LUMO of Alq3 suggests that the doping molecules are negatively charged prior to exciton formation due to hole transfer from Alq3. This is also confirmed by numerical modelling, where an important space charge density due to trapped electrons in the doped layers is observed [17]. Why would thus the electron trapping in DCMTPA quench the EDMR signal at room temperature? At least two mechanisms could be mentioned; both related to a change in the reaction rates of the spin precursor pairs. The signal observed by EDMR related to exciton formation relies on the fact that the reaction rate of anti-parallel spins (RAP) is different from that of parallel (RP) spins. If they are the same, one has the commonly assumed 75% chance of forming triplet states against 25% of singlet exciton states. The other consequence of this assumption, not so commonly discussed, is that in this case Magnetic Resonance is not useful to probe these states. EDMR, as well as optically detected magnetic resonance, ODMR, can probe systems where RAP and RP are different. With differences in RAP and RP there is an increased formation of either triplets or singlet states. In other words, one could have, for example, a 22% probability of singlet exciton formation. Bringing the system into resonance is equivalent to bringing the system to the condition of RAP = RP. In summary, if one assumes that in the DCM-TPA doped Alq3 at room temperature RAP = RP, bringing the precursor pair spin systems into magnetic resonance condition will have no effect on the triplet or singlet exciton formation rate, in other words, the EDMR or ODMR signal is quenched. At this point one cannot discard this explanation; however one would have to assume that RAP has different temperature dependence than RP to explain the temperature dependence of Dr/r. The second possible explanation for the signal quenching comes from the dynamics of the process. The EDMR as well as the ESR signal amplitude are dependent on the microwave power used (Pl). For the Alq3 OLED the dependence is shown in

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10

broadened envelope line far below saturation. The quantity s is the saturation factor defined as

-3

Δσ/σ

s¼

10-4

T = 300 K Δσ/σ from Undoped OLED Δσ/σ α Pw Δσ/σ α (Pw)1/2

10-5 1

10

100

Microwave power (mW)

Fig. 6. Amplitude of EDMR signal of an undoped Alq3 OLED as a function of microwave pffiffiffiffiffiffi power intensity (Pw). Dependence change from aPw to a P w at around 30 mW.

Fig. 6. In this figure one can observe a transition from Dr/r / Pl to / (Pl)1/2 at approximately 30 mW. This behaviour has already been discussed in the literature [18] related to a transition from unsaturated to weakly saturated condition. Kawachi et al., have described in detail the EDMR dependence on power for a-Si:H based transistor. In our previous study on undoped Alq3 OLEDs [8], we have compared X-band and K-band EDMR lineshape analysis combined with theoretical works on HOMO and LUMO molecular orbital and shown that the lineshape of the cationic and anionic states are dominated by proton hyperfine interaction. In what concerns the broadening mechanisms found in magnetic resonance, in our OLEDs one finds inhomogeneous broadening, as found in a-Si:H. For an inhomogeneously broadened line [18], the dependence of the EDMR signal intensity Dr/r at the resonance field H0 and DHpp on the microwave power can be expressed as follows: pffiffi ð1Þ Dr=r / v0 H 0 cT 002 H 21 s;   2 1 DH pp ¼ pffiffiffi ð2Þ 0 ; 3 cT 2 where c is the gyromagnetic ratio, H1 is the microwave magnetic field, v0 is the static susceptibility and T 02 is defined as 1 1 1 ¼ pffiffi þ  ; T 02 sT 2 T 2

ð3Þ

where T 2 is the apparent spin–spin relaxation time which reflects the linewidth of the inhomogeneously

1 1þ

c2 H 21 T 1 T 2

;

ð4Þ

where T1 and T2 are the spin–lattice and spin–spin relaxation times. So in the case of DCM-TPA, the signal quenching could result from a strong decrease in T1,2 of the anion spin system, or an increase in the exciton precursor reaction rate. By numerical simulation using Eq. (1), it is trivial to calculate that a two order of magnitude change in Dr/r, implies at least a two order of magnitude change in T1,2. In both situations, from what has been discussed, it would be ‘‘equivalent’’ to virtually decrease the applied microwave power. To support this finding one could in fact have made a measurement of Dr/r as a function of Pl. However, this was not possible in a reasonable range due to the weakness of the EDMR signal in the doped sample at room temperature. From simple arguments it is hard to argue why T1 should decrease so strongly in DCM-TPA. On the other hand, the higher external quantum efficiency observed in DCM-TPA doped OLEDs compared to undoped, could come from an increase in exciton precursor reaction rates, or in other words in a decrease in the spin coherence time. Note that in undoped OLEDs, as in the doped ones below 200 K (Fig. 3), it is not possible to put the signal completely in phase with the reference signal from the lock-in. However, as we have shown, this becomes possible for DCMTPA doped devices above 200 K. These results are evidence that indeed there is a change in spin coherence time with temperature in the dopant. A detailed explanation of the temperature dependence would involve a detailed understanding of the EDMR spin dynamics. A few words on the lineshape observed in the doped samples. Through the decomposition of the EDMR signal, we observed that the g-factor for the electron related signal is the most affected, while the hole related signal shows nearly the same g-factor as for the undoped OLED. This observation reinforces the interpretation of DCM-TPA electron trapping. Unfortunately in the case of DCM-TPA there are no theoretical works on cationic and anionic states of this molecule, more importantly inside an Alq3 matrix. Notice, however, that a broader line should be expected as a response to the more localized nature of the electron trapped in the LUMO state of this molecule. The small g-shift to higher

F.A. Castro et al. / Organic Electronics 8 (2007) 249–255

values with respect to the anionic state in Alq3 could also be explained by this higher localization. The fact that both components behave differently with temperature is evidence that indeed we are able to probe both spins participating in the spin-dependent exciton formation. Finally, we have also made some preliminary EDMR studies on rubrene doped Alq3 OLEDs. The signals show small temperature dependence and very small amplitudes, in the order of 106– 107. Although more studies are necessary to clarify which spin-dependent mechanism is responsible for the signal, we speculate that similarly to what was discussed earlier a decrease in spin coherence time or a change in exciton precursor reaction rates could be responsible for the small signal. 5. Conclusion We have investigated the effect of doping and trapped charges on spin-dependent exciton formation in Alq3 based OLEDs using electrically detected magnetic resonance. Strong differences (in g-factor, DHpp and amplitude) were found between EDMR signals from DCM-TPA doped and undoped devices at room temperature. The signal from DCM-TPA doped OLED was found to be strongly dependent on temperature, with signal intensity increasing by two orders of magnitude below 200 K. Our results indicate that a direct reaction between a trapped electron in the dopant with a hole from Alq3 is the main mechanism for exciton formation in the DCM-TPA doped OLEDs, in the current/bias range investigated. The differences, probably due to these trapped states, are discussed in terms of a decrease in spin coherence time or a change in exciton precur-

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sor reaction rates. Preliminary results from rubrene doped OLEDs indicate a similar process. References [1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [2] A.A. Shoustikov, Y. You, M.E. Thompson, IEEE J. Sel. Top. Quantum Electron. 4 (1998) 3. [3] M.A. Baldo, M.E. Thompson, S.R. Forrest, Nature 403 (2000) 750. [4] R.N. Bera, Y. Sakakibara, M. Tokumoto, K. Yase, Jpn. J. Appl. Phys. Part 1 42 (12) (2003) 7379. [5] X. Gong, J.C. Ostrowski, D. Moses, G.C. Bazan, A.J. Heeger, Adv. Funct. Mater. 13 (6) (2003) 439. [6] M. Reufer, M.J. Walter, P.G. Lagoudakis, A.B. Hummel, J.S. Kolb, H.G. Roskos, U. Scherf, J.M. Lupton, Nature Mater. 4 (4) (2005) 340. [7] M. Wohlgenannt, K. Tandon, S. Mazumdar, S. Ramasesha, Z.V. Vardeny, Nature 409 (6819) (2001) 494. [8] C.F.O. Graeff, G.B. Silva, F. Nu¨esch, L. Zuppiroli, Eur. Phys. J. E 18 (2005) 21. [9] J.S. Wilson, A.S. Dhoot, A.J.A.B. Seeley, M.S. Khan, A. Ko¨hler, R.H. Friend, Nature 413 (2001) 828. [10] D. Beljonne, A.J. Ye, Z. Shuai, J.L. Bredas, Adv. Funct. Mater. 14 (7) (2004) 684. [11] M. Segal, M.A. Baldo, R.J. Holmes, S.R. Forrest, Z.G. Soos, Phys. Rev. B 68 (7) (2003) 075211. [12] C.F.O. Graeff, in: H.S. Nalwa (Ed.), Encyclopedia of Nanoscience and Nanotechnology, vol. 2, American Scientific Publishers, Stevenson Ranch, CA, 2004, pp. 1–745. [13] F.A. Castro, G.B. Silva, L.F. Santos, R.M. Faria, F. Nuesch, L. Zuppiroli, C.F.O. Graeff, J. Non-Cryst. Solids 338–340 (2004) 622. [14] G. Li, C.H. Kim, P.A. Lane, J. Shinar, Phys. Rev. B 69 (2004) 165311. [15] H. Dersch, L. Schweitzer, J. Stuke, Phys. Rev. B 28 (1983) 4678. [16] F. Nu¨esch, L. Zuppiroli, D. Berner, C. Ma, X. Wang, Y. Cao, B. Zhang, Res. Chem. Intermed. 30 (4–5) (2004) 495. [17] D. Berner, F. Nuesch, E. Tutis, C. Ma, X. Wang, B. Zhang, L. Zuppiroli, J. Appl. Phys. 95 (7) (2004) 3749. [18] G. Kawachi, C.F.O. Graeff, M.S. Brandt, M. Stutzmann, Jpn. J. Appl. Phys. 36 (1A) (1997) 121.