Improving the efficiency of light-emitting diode based on a thiophene polymer containing a cyano group

Available online at www.sciencedirect.com

Organic Electronics 8 (2007) 641–647 www.elsevier.com/locate/orgel

Improving the efficiency of light-emitting diode based on a thiophene polymer containing a cyano group S. Cheylan

a,*

, H.J. Bolink b, A. Fraleoni-Morgera c, J. Puigdollers d, C. Voz d, I. Mencarelli c, L. Setti c, R. Alcubilla d, G. Badenes a

ICFO – Institut de Ciencies Fotoniques, Mediterranean Technology Park, Av. del Canal Olı´mpic s/n, 08860 Castelldefels (Barcelona), Spain b Instituto de Ciencia Molecular, Universidad de Valencia, P.O. Box 22085, 46071 Valencia, Spain Departamenti de Quimica Industriale e Materiali, Universite di Bologna – V. Risorgimento 4, 40136 Bologna, Italy d Departamento de Ingenierı´a Electro´nica, Universidad Polite´cnica de Catalun˜a – UPC, Campus Nord Edifici C4, c/Jordi Girona 1-3, 08034 Barcelona, Spain a

c

Received 12 February 2007; received in revised form 23 April 2007; accepted 26 April 2007 Available online 5 May 2007

Abstract We report on the overall improvement of a single layer organic light-emitting diode device based on poly{[3-hethylthiophene]-co-3-[2-(p-cyano-phenoxy)ethyl]thiophene} or namely PTOPhCN. This polymer was recently developed by adding a cyano group as a side-chain substituent of the thiophenic backbone onto the main polymer chain and showed promising results for light-emitting diode devices. Using an improved device layout, bright red electroluminescence was obtained at 4 V and showed a luminance of about 400 cd/m2 at 8 V with current densities in the order of 6000 A/m2.  2007 Elsevier B.V. All rights reserved. PACS: 42.70.a; 82.35.Cd; 85.60.Jb Keywords: Organic light-emitting diodes; Polymer; Thiophene; Cyano group; Luminance efficiency

1. Introduction Organic light-emitting diodes (OLEDs) have been investigated intensively for the past decade or so. A variety of approaches are being investigated to improve their quantum efficiency and lifetime. Extensive work has been done on the surface prop* Corresponding author. Tel.: +34 93 553 4033; fax: +34 93 553 4000. E-mail address: [email protected] (S. Cheylan).

erties of the most commonly used anode, a layer of indium tin oxide (ITO), modified by chemical and physical treatments. Such treatments have been shown to affect the electrode work function, the surface energy, the morphology, the sheet resistance and therefore the OLEDs efficiency and lifetime [1–4]. Another approach consists in modifying the metal used for the cathode, such as using a mixture of metals such as Ba/Al or Mg/Ag [5] in order to improve the electron injection, or increasing its transparency to improve the output efficiency of

1566-1199/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2007.04.009

S. Cheylan et al. / Organic Electronics 8 (2007) 641–647

the device. At present, devices exhibiting charge transport layers (hole or electron), or buffer layers such as SiO2 or LiF for the electron injection improvement [6–8], are commonly reported for their high efficiency and lifetime. Finally, substantial performance enhancement may be obtained directly modifying the properties of the active organic material by way of rational design of the chemical structure of the photoactive material. For example, adding a functional group to the main polymer chain, or using polymer blends, have resulted in highly efficient devices [9–12]. Following this line of research, we have synthesized a novel thiophene-based polymer by adding a functional cyano group to its side-chain and investigated its potential use in OLEDs [13]. Among all the conjugated polymers developed in the past years for device application, polythiophene was selected due to its high chemical stability and structural tailorability, which can be usefully exploited to design proper structures for the targeted aims (color emission, physical properties such as glass transition temperatures, etc.) [14]. The cyano group (CN) has been widely reported to provide high electron affinity [10], a characteristic which is highly desirable for OLED applications because it allows the use of more stable metal electrodes (e.g. aluminium) for electron injection. In our earlier work [11], we demonstrated that this novel thiophene-based polymer bearing a cyano group in its side-chain, namely PTOPhCN, showed promise for OLED applications. Orange-red photoluminescence (PL) was observed for either spin coated films or chloroform-based solution of the polymer. In addition, red electroluminescence (EL) was obtained for a single layer device, although the EL threshold voltage was high, i.e. about 28 V. This work presents the progress obtained on the use of PTOPhCN as the active layer for OLED. Inserting charge injection layers at the electrodeorganic interface, we can now report electroluminescence at 4 V with a high current density of the order of 6000 A/m2, and a luminance of about 400 cd/m2. The insertion of a PEDOT-PSS layer between the anode and the organic layer improved the hole injection and transport, while a more appropriate cathode was used for the electron injection. Compared to linear polythiophenes reported in the literature and used in our reference device, OLEDs using the PTOPhCN polymer show a significant improvement in luminance and hence in current efficiency of 0.08 cd/A which is, to our knowledge, the

best result reported so far for an ‘‘all-thiophene’’ electroluminescent polymer. 2. Experimental Poly{[3-hethylthiophene]-co-3-[2-(p-cyano-phenoxy)ethyl]thiophene} abbreviated as PTOPhCN was prepared by oxidative polymerization. Such synthesis enables to obtain in a cheap and easy way gram quantities of the polymer, which chemical structure can be seen in Fig. 1a. For extensive details on the chemical synthesis, refers to [13,15]. PTOPhCN was dissolved in chloroform at 5 mg/mL and thoroughly purged with N2(g). Before spin coating the solutions were filtered over a 0.45 lm PTFE filter. Thin films were prepared from the chloroform based-solution via spin coating on patterned ITO coated glass plates. The ITO glass plates were extensively cleaned, using a sequence of soap, water and isopropanol ultrasonic washing steps and just before the deposition of the organic film were exposed to UV-Ozone for 20 min. A 100 nm thick PEDOT-PSS (HCStarck) hole injection layer was first deposited followed by an 80 nm thick layer of the light-emitting polymer. Ba (5 nm) and Al (80 nm) were used as cathode for the devices, which were evaporated under vacuum (<1 · 106 mbar). Devices were characterised in an inert atmosphere (<0.1 ppm H2O and O2). Film thickness was deter-

a

O

S

0.53

S

C N

0.47

b 200

Intensity (a.u.)

642

100

0 300

400

500

600

700

800

Wavelength (nm) Fig. 1. (a) Chemical structure of PTOPhCN. (b) Optical absorption ( ) and emission (s) spectra of a 220 nm thick spin coated layer of PTOPhCN.

S. Cheylan et al. / Organic Electronics 8 (2007) 641–647

3. Results and discussion

1.5 1.0

Current (mA)

mined using an Ambios XP1 profilometer. Current density and luminance versus voltage were measured using a Keithley 2400 source meter, and a photodiode coupled to a Keithley 6485 picoampmeter using a Minolta LS100 to calibrate the photocurrent. An Avantes luminance spectrometer was used to measure the EL spectrum. The quantum yield measurements were performed in N2 ambient, using the quantum yield measurement system model C9920-01 from HAMAMATSU, for a thin layer of polymer spin coated on quartz substrates. The system is made up of an excitation light source that uses a xenon lamp and a monochromator, an integration sphere and a multi-channel spectrometer.

643

0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.5 -2.0 -1.5 -1.0 -0.5

0.0

0.5

1.0

1.5

Potential (V) vs SCE Fig. 2. Cyclovoltammetric response of a thin film of PTOPhCN (about 150 nm) deposited on a Pt slide, with a voltage sweep of 50 mV/s. HOMO level at 5.1 eV and LUMO level at 2.6 eV were calculated. The electrochemical bandgap is of 2.5 eV.

3.1. Photophysical characteristics Details about the chemical synthesis and the structural properties of the polymer can be found in previous works [13–16]. Fig. 1b shows the absorption and photoluminescence spectra of PTOPhCN for a spin coated layer about 220 nm thick. From the absorption edge, an optical band gap of 2.0 eV may be estimated, which corresponds to orange emission. The PL spectrum shows evident structuration and differs markedly from the absorption one, which is broad and misses any trace of sharp peaks and/or shoulders. In particular, two sharp peaks at 609 nm and 636 nm, with the presence of a bump around 665 nm, are visible, which origin is still under investigation. An important parameter for emitting organic materials is their luminous efficiency, i.e. the ratio between the number of photons of light radiated from the material and the number of photons of light absorbed by the material. The luminescence quantum yield of a thin layer of PTOPhCN was measured to be about 0.08 (8%). Such a low result is consistent with the well known relatively poor photoluminescence efficiency of these compounds [17,18]. As a final observation on the absorption and emission properties of PTOPhCN, from Fig. 1b a large Stokes’ shift of 0.75 eV between the PL and the Absorption is evident, with a low self-absorption, which makes this material also a promising candidate for laser applications. To investigate the structure of the deposited layer, X-ray diffraction (XRD) measurements were conducted on a thick 300 nm film of PTOPhCN spin-coated on a glass substrate (not shown here),

measured as-deposited. No sharp peaks due to the diffraction by a crystalline structure of the X-rays were present, leading to conclude that the structure of the PTOPhCN layer as deposited is amorphous. In order to have an indication on the energy levels of PTOPhCN, electrochemical measurements have also been performed on a spin-coated thin film (about 150 nm) of the polymer. As is visible from Fig. 2, the polymer showed clear oxidation and reduction onsets at 0.73 and 1.75 V, respectively. Based on the most recent literature considerations on the subject [16], the expressions: ELUMO ¼ eðEred onset þ 4:4Þ EHOMO ¼ eðEox onset þ 4:4Þ have been used to calculate the absolute HOMO and LUMO levels. This resulted in HOMO and LUMO levels of PTOPhCN of 5.1 and 2.6 eV, respectively. From these values an electrochemical bandgap of 2.5 eV was found. 3.2. Current density– voltage – luminance characteristics A single layer light-emitting device was fabricated using PTOPhCN as the active material, with a 220 nm thick layer and Al electrodes [13]. For this device EL was reported for a voltage around 28 V, with current densities of the order of 2500 A/m2 which resulted in low power efficiency. In order to improve the efficiency, various changes in the device layout were examined. Fig. 3 shows the current density (J) versus the applied voltage (Vapplied) curves of

644

S. Cheylan et al. / Organic Electronics 8 (2007) 641–647

a 2

Current density (A/m )

500 400 300 200 100 0 0

5

10

15

20

25

30

Vapplied (V) 600 500

2

Current density (A/m )

b

400 300 200 100 0 0.0

0.5

1.0

1.5

(Vappl - Vbi)/L (MV/cm) Fig. 3. Curves of current density J as a function of applied voltage Vapplied (a) and (Vapplied Vbi)/L (b) of the thiophenebased diode structures ITO/PTOPhCN/Al with a layer of PTOPhCN of around 220 nm (n) and 80 nm (*). Also shown is the measurement for a device with an 80 nm thick layer of PTOPhCN, with Ba/Ag as material used for the electrode (h). Vbi was calculated using Ref. [29] with values of the work function of metals from the literature and values of the energy level of PTOPhCn from electrochemistry.

the basic device previously mentioned as well as of a device with a thin active layer of about 80 nm. As can be seen, the use of a thinner layer leads to a dramatic lowering of the operating voltage of the device (threshold voltage going from 12 V down to 5 V). Furthermore, the EL was observed at around 15 V for the thinner device instead of 28 V. As the data in Fig. 3b clearly demonstrates, this decrease in voltage threshold is purely associated with the layer thickness as the electric field strength is constant for both devices. Indeed, the curves of current density plotted as a function of (Vapplied Vbi)/L fall nearly on top of each other, indicating that the device is contact limited instead of space charge limited as there is no change in E over the distance due

to the enhanced charge injection at higher fields [19]. Various reports have dealt with the improvement of device performance with decreasing thickness of the active layer [20,21], thus consolidating this interpretation. The current is controlled almost exclusively by holes injected at the ITO contact, due to the high energy barrier of about 1.8 eV present at the PTOPhCN/Al interface for the electron injection. The electron injecting contact plays no real part in determining the J/V characteristics of the device, but is the limiting factor for the device efficiency in terms of luminance. In order to remediate this unbalanced charge distribution in the device, to improve the electron injection and to increase the luminance, a third device was fabricated using Ba/Ag electrode instead of Al electrode. As mentioned previously, although Al is widely used as the cathode material due to its stability and processability, its injection efficiencies are generally inferior to those of Ba/ Ag, Mg/Ag or Ca cathodes. In that case, the use of Ba/Ag as a cathode should increase significantly the electron injection in the device, as Ba has a work function of about 2.7 eV, while Al has a work function of 4.4 eV. Therefore, the metal electrode was changed from Al to Ba/Ag (Ag is used to protect the oxidation-sensitive Ba contact) for a diode structure with a thin 80 nm of PTOPhCN whose J/V characteristic can be seen in Fig. 3 as well. When Ba/Ag is used as the electrode instead of Al, current densities of around 330 A/m2 and luminance of 20 cd/m2 are reached at 4 V. Such an increase in current density and luminance concurs with an increased injection of electrons due to the reduced energy barrier at the polymer/cathode interface, leading to a better charge balance and therefore better radiative recombination rate. Fig. 3b exhibits a change in the current density curves as a function of (Vapplied  Vbi)/L when Ba is used as an electrode. This result corroborates that a PTOPhCNbased device with ITO and Al as electrodes is injection limited, while now, using a Ba electrode instead of Al, one contact of the device is no longer limited. Furthermore, it has been reported that adding a thin Ba layer, the distance from the excitons in the PTOPhCN layer to the metallic electrode is increased, thus resulting in a reduced energy transfer from the excitons to the metal electrode, improving the light output efficiency of the device [22]. Finally, a standard step in OLED fabrication in order to improve the overall efficiency of the devices is to use a thin intermediate layer of PEDOT-PSS

S. Cheylan et al. / Organic Electronics 8 (2007) 641–647

between the ITO and the active organic layers. Extensive studies have been carried out and report that this thin uniform layer has several beneficial effects. It can planarise the otherwise rough ITO surface, modify its wetting properties for subsequent organic layer deposition and increase the anode work function thus facilitating the hole injection [7,20,22]. Fig. 4 displays the J/V and L/V curves for the following diodes structures: ITO/ PTOPhCN/Ba/Ag (device A) and ITO/PEDOTPSS/PTOPhCN/Ba/Ag (device B), with a PTOPhCN layer of around 80 nm for both devices. The data obtained with a diode based on a linear standard polythiophene (P3HT) of similar thickness is also added in order to compare the efficiency. Indeed, in this work, the addition of a PEDOTPSS layer also resulted in higher current densities, around 6000 A/m2 at 8 V, as well as an equally strong increase in luminance, from 20 cd/m2 for a single layer device up to 400 cd/m2 for the device

a

with the PEDOT-PSS hole injection layer. The higher current densities obtained show that the PEDOT-PSS layer improved the hole injection from the anode as expected. The concurrent increase in luminance shows that sufficient electrons are present in the device to recombine with the added holes, resulting in a higher recombination rate. A better charge balance has hence been reached. Fig. 5 shows the normalized photoluminescence and electroluminescence spectra taken at room temperature of a thin film of the thiophene-based polymer. The EL spectrum was obtained for an applied bias of 5 V, for the device structure ITO/PEDOTPSS/PTOPhCN/Ba/Ag, with a PTOPhCN layer of around 80 nm, while the PL spectra were obtained for a layer of about 220 nm. Both EL and PL spectra exhibit similar characteristics, with a good overlap, peaking around 600 nm. The EL spectrum actually contains a small artefact at 650 nm from the experimental set-up. At 5 V, the luminance from

10000

10000

1000

1000

2

Current Density (A/m )

645

100

100

10

10

1

1

0.1

0.1

0.01

0.01

1E-3

1E-3

1E-4

1E-4 -2

0

2

4

6

8

Vapplied (V)

b

1000

1000

100

Luminance (cd/m2)

100

10

10

1

1

0.1

0.1 0.01

0.01

1E-3

1E-3 -2

0

2

4

6

8

Vapplied (V) Fig. 4. Current density (a) and luminance (b) as a function of Vapplied for the following diode structure: ITO/PTOPhCN/Ba/Ag (h) as well as the diode structure ITO/PEDOTPSS/PTOPhCN/Ba/Ag (s), with a PTOPhCN layer of around 80 nm. The data obtained with a similar structure diode based on P3HT (m) are also added for comparison.

646

S. Cheylan et al. / Organic Electronics 8 (2007) 641–647

12000

PL EL

Intensity (a.u)

10000 8000 6000 4000 2000 0 400

500

600

700

800

900

Wavelength (nm) Fig. 5. Room temperature photoluminescence and electroluminescence of the thiophene-based diode structure: ITO/PEDOTPSS/PTOPhCN/Ba/Ag, with a PTOPhCN layer of around 80 nm. The electroluminescence spectrum was obtained for an applied bias of 5 V.

the EL signal is about 30 cd/m2 and the maximum luminance obtained at 8 V is 400 cd/m2. Efficiency of the order of 0.08 cd/A for current densities of about 1000 A/m2 were obtained for our PTOPhCN-based devices, which represents very promising results for electroluminescent devices based on polythiophenes. Indeed so far, luminous efficiencies of about 0.08–0.12 cd/A have been reported for such devices [23,24]. For example, Barta et al. reported efficiencies of up to 0.28% (0.12 cd/A) for regioregular poly(alkylthiophenes) but this was achieved at high applied voltages of around 38 V [23]. Other regiorandom polythiophene-based devices showed luminous efficiencies ranging from 0.0004 cd/A (for LB layers of P3HT) [24] to 0.08 cd/A (for a pristine uretane-substituted polythiophene, PURET) [25], up to values of 0.66 cd/ A for dye-doped PURET [25]. Furthermore, a similar device based on synthesized P3HT (regioregular, 94% head to tail) was also fabricated and its luminuous efficiency obtained is about 0.002 cd/A, confirming that better device performance is obtained while using PTOPhCN as the active layer of the device. The I/V and L/V curves of this device are shown in Fig. 4. In any case PTOPhCN being a regiorandom polythiophene, it is expected that its charge transport ability would be limited by its intrinsialy disordered structure (and indeed, the polymer is amorphous in its solid state [13]). From a practical point of view, an EL device should possess not only a high brightness but also a low operation current density. Therefore, although PTOPhCN-based devices show apprecia-

ble performances, there is still room for improvements in terms of optimizing their luminance efficiency. Nevertheless, this new polythiophene shows a significant increase in efficiency compared to previously reported polythiophenes, indicative of the gain that can be obtained by further optimizing the chemical structure and device optimization. Now, looking at the effect of the CN group on the polymer properties, it could seem at first that its role is to increase the electron affinity of the polymer. Indeed, in many cyano-substituted polymers, such as for example CN-MEH-PPV, this is the case [26,27]. However, in PTOPhCN the cyano group is not linked to the backbone via conjugated bonds, hence a direct conjugative effect is not expected to take place. On the other hand, for other thiophene-based polymers substituted in side-chain with a polar moiety through an ethylenic spacer, NMR and reactivity evidences show that some kind of electronic communication between the backbone and the polar moiety is present, in the solvated state [28]. In view of this, the found enhancement of EL given by the presence of the OPhCN group may be ascribed to (i) a possible electronic effect transmission through the ethylenic spacer, (ii) a spatial vicinity of the polar moiety to the backbone, or to a combination of the two factors. At this stage of our investigations it is not possible to find out if these or other factors are the ones influencing the device behaviour and more investigation is underway to clarify this point. However, it did not escape from our notice that the mentioned possible mechanism of EL enhancement could have interesting consequences for clarifying the basic behaviour of organic electronics-based systems and devices. 4. Conclusion A new orange-red light-emitting thiophene-based polymer (PTOPhCN) containing a cyano group in the side-chain has been used in OLED devices. A successful electroluminescent diode based on a single organic layer was fabricated, showing bright EL around 4 V for a luminance of 20 cd/m2. Introducing a PEDOT-PSS layer resulted in an overall improvement of the device, with luminance of about 400 cd/m2 and current densities of about 6000 A/m2 at 8 V, giving a maximum luminous efficiency of 0.08 cd/A. Although such device shows good efficiency in terms of luminance, it still exhibits current densities too high for being used in practical devices and further optimization of the device is underway.

S. Cheylan et al. / Organic Electronics 8 (2007) 641–647

To our knowledge, due to the easy synthesis, good filmability, redox potentials and device performance, this is the best ‘all-thiophene’ electroluminescent polymer described so far. The presence of the cyano group as a side-chain substituent of the polymer plays without any doubt a role in determining its interesting performances, but wether this is due to a direct conjugative effect exerted on the backbone, or to other factors such as spatial vicinities of polar groups, still remains to be completely clarified, and work is underway in this sense. Acknowledgements S.C. and H.J.B. acknowledge support from the Spanish Government, from the Ramon y Cajal program. This work was carried out with the financial support of the Spanish Ministry of Education and Science through grants TEC2005-02716/MIC, TEC2006-10665/MIC and the European Commission through the European Network of Excellence PHOREMOST (FP6-511616). References [1] J.-S. Kim, F. Cacialli, R.H. Friend, Thin Solid Films 445 (2003) 358. [2] J.-S. Lim, P.-K. Shin, Appl. Surf. Sci. 253 (2003) 3828. [3] S. Jung, N.G. Park, M.Y. Kwak, B.O. Kim, K.H. Choi, Y.J. Cho, Y.K. Kim, Y.S. Kim, Opt. Mater. 21 (2002) 235. [4] T.P. Nguyen, P. Le Rendu, N.N. Dinh, M. Fourmigue´, C. Me´zie`re, Synth. Met. 138 (2003) 229. [5] M.Y. Chan, S.L. Lai, F.L. Wong, O. Lengyel, C.S. Lee, S.T. Lee, Chem. Phys. Lett. 371 (2003) 700. [6] S.T. Zhang, Z.J. Wang, J.M. Zhao, Y.Q. Zhan, Y. Wu, Y.C. Zhou, X.M. Ding, X.Y. Hou, Appl. Phys. Lett. 84 (2004) 2916. [7] D. Xu, Z. Deng, X. Li, Z. Chen, C. Liang, Appl. Surf. Sci. 253 (2007) 3378.

647

[8] H. Kanno, N.C. Giebink, Y. Sun, S.R. Forrest, Appl. Phys. Lett. 89 (2006) 023503. [9] G. Gigli, M. Anni, M. Theander, R. Cingolani, G. Barbarella, L. Favaretto, O. Ingana¨s, Synth. Met. 119 (2001) 581. [10] M.-T. Lee, H.-H. Chen, C.-H. Liao, C.-H. Tsai, C.H. Chen, Appl. Phys. Lett. 85 (2004) 3301. [11] J.M. Xu, S.C. Nq, H.S.O. Chan, Macromolecules 34 (2001) 4314. [12] T.-Q. Nguyen, I.B. Martini, J. Liu, B.J. Schwartz, J. Phys. Chem. B 104 (2000) 237. [13] S. Cheylan, A. Fraleoni-Morgera, J. Puigdollers, C. Voz, L. Setti, R. Alcubilla, G. Badenes, Thin Solid Films 497 (2006) 16. [14] J.C. Gustafsson-Carlberg, O. Ingana¨s, M.R. Andersson, C. Booth, A. Azens, C.G. Granqvist, Electrochim. Acta 40 (1995) 2233. [15] A. Fraleoni-Morgera, C. Della-Casa, M. Lanzi, P. CostaBizzarri, Macromolecules 36 (2003) 8617. [16] W.-C. Wu, C.-L. Liu, W.-C. Chen, Polymer 47 (2006) 527, and references therein. [17] F. Cacialli, Synth. Met. 76 (1996) 145. [18] N.C. Greenham, I.D.W. Samuel, G.R. Hayes, R.T. Philips, Y.A.R.R. Kessener, S.C. Moratti, A.B. Holmes, R.H. Friend, Chem. Phys. Lett. 241 (1995) 89. [19] B.K. Crone, P.S. Davids, I.H. Campbell, D.L. Smith, J. Appl. Phys. 84 (1998) 833. [20] P.W.M. Blom, C. Tanase, D.M. de Leeuw, R. Coehoom, Appl. Phys. Lett. 86 (2005) 092105. [21] I.D. Parker, J. Appl. Phys. 75 (1994) 1656. [22] F. Li, Appl. Phys. Lett. 70 (1997) 1233. [23] P. Barta, F. Cacialli, R. Friend, M. Zago´rska, J. Appl. Phys. 84 (1998) 6279. [24] A.J. Pal, T.P. Ostergard, R.M. Osterbacka, J. Paloheimo, H. Stubb, IEEE J. Sel. Top. Quant. Electron. 4 (1998) 137. [25] A. Kaur, M.J. Cazeca, S.K. Sengupta, J. Kumar, S.K. Tripathy, Synth. Met. 126 (2002) 283. [26] M. Cheng, Y. Xiao, W.-L. Yu, Z.-K. Chen, Y.-H. Lai, W. Huang, Thin Solid Films 363 (2000) 110. [27] M.M.D. Ramos, H.M.G. Correia, R. Mendes Ribeiro, A.M. Stoneham, Synth. Met. 147 (2004) 281. [28] A. Fraleoni-Morgera, C. Della-Casa, P. Costa-Bizzarri, M. Lanzi, A. Missiroli, Macromolecules 38 (2005) 3170. [29] W. Bru¨tting (Ed.), Physics of Organic Semiconductors, WILEY-VCH Verlag GmbH & Co KGaA, 2005.