π-Stacked polybenzofulvene derivatives as hosts for yellow and red emitting OLEDs

π-Stacked polybenzofulvene derivatives as hosts for yellow and red emitting OLEDs

Materials Letters 142 (2015) 197–200 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet π...

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Materials Letters 142 (2015) 197–200

Contents lists available at ScienceDirect

Materials Letters journal homepage: www.elsevier.com/locate/matlet

π-Stacked polybenzofulvene derivatives as hosts for yellow and red emitting OLEDs Wojciech Mróz a, Francesca Villafiorita-Monteleone a, Mariacecilia Pasini a, Giorgio Grisci a,b, Marco Paolino b, Vincenzo Razzano b, Andrea Cappelli b,1, Chiara Botta a,n a

Istituto per lo Studio delle Macromolecole (CNR), Via E. Bassini 15, 20133 Milano, Italy Dipartimento di Biotecnologie, Chimica e Farmacia and European Research Centre for Drug Discovery and Development, Università degli Studi di Siena, Via Aldo Moro 2, 53100 Siena, Italy

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art ic l e i nf o

a b s t r a c t

Article history: Received 14 October 2014 Accepted 1 December 2014 Available online 10 December 2014

A new class of π-stacked polymers possessing interesting charge transport properties is represented by polydibenzofulvene derivatives. We study the electroluminescence properties of three π-stacked polybenzofulvene derivatives and compare them to those of the standard π-stacked poly(N-vinylcarbazole). Solution processable devices using one of these polymers as host with benzothiadiazole based dyes display performances overcoming those of the standard poly(N-vinylcarbazole) host. From a photophysical study, the superior performances of the polybenzofulvene containing a fluorene unit at the position 6 of the indene nucleus are explained to arise from its ability to self-assemble with the dyes in supramolecular organizations that reduce their microaggregation. & 2014 Elsevier B.V. All rights reserved.

keywords: Optoelectronics Organic light emitting diodes π-Stacked polymers Dyes Polybenzofulvene

1. Introduction Solution-processed organic optoelectronic devices such as lightemitting diodes (OLEDs) offer many advantages for large-area manufacture and show increasing levels of performance in both solid state lighting and display applications [1]. Devices based on blends of soluble polymeric hosts with small dyes are of particular interest since they are obtained by an extremely simple technology. The main drawback of this class of devices is the low compatibility among different chemical components of the polymer and the dyes that often causes inhomogeneities in the thin films properties and timeevolution of phase separation during device operation. The search of new classes of transparent host polymers with charge transport properties, able to blend dyes with negligible phase segregation, is therefore intensively investigated. The most used polymer host in OLEDs, for both fluorescent and phosphorescent dyes, is poly(N-vinylcarbazole) (PVK, Fig. 1) [2]. This commercially available polymer possesses good electrical properties and film forming capability that, together with its optical transparency, makes it an optimal host for most emitting dyes. Its fluorescence originates from π-stacked carbazole moieties possessing mainly two kinds of conformation, one partially overlapping and another fully overlapping [3]. When used as host in blends, its

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Corresponding author. Tel.: þ 39 02 23699734; fax: þ39 02 70636400. E-mail addresses: [email protected] (A. Cappelli), [email protected] (C. Botta). 1 Tel.: þ39 0577 234320.

http://dx.doi.org/10.1016/j.matlet.2014.12.002 0167-577X/& 2014 Elsevier B.V. All rights reserved.

blue electroluminescence is generally suppressed by virtue of both energy transfer to the dye and charge trapping processes at the dye sites [4]. However, weakly red emissive species often observed in PVK based OLEDs, discussed in terms of different mechanisms, including aggregation, electroplex, or electromers, limit its use as host for red emitting dyes [5]. Another class of π-stacked polymers possessing interesting charge transport properties is represented by polydibenzofulvene (PDBF) derivatives, investigated by Nakano and co-workers since 2001 [6]. This class of polymers shows a close similarity with PVK even though their use in OLEDs has never been reported so far. Their stacked structure is not based on complete face-to-face stacking of the fluorene units but rather to a slightly twisted one, producing a helical conformation of the polymeric chain that can induce peculiar supramolecular structures [7]. Polybenzofulvene derivatives (i.e. P0) are π-stacked polymers showing promising optical and electrical properties, which suggested a potential use in optoelectronic applications [8]. Intriguing photophysical properties were observed when a fluorene chromophore was introduced in the phenylindene scaffold. In particular, these polymers showed self-assembly abilities when blended with a small conjugated benzothiadiazoles dye (i.e. 4,7-di(2-thienyl)benzo-2,1,3-thiadiazole, DBT) [9], reducing unwanted dye microaggregation phenomena at dye concentrations up to 10%. This peculiar ability of polybenzofulvene derivatives P1 and P2 to self-assemble with dyes in supramolecular organizations able to reduce their microaggregation suggests their use as host in polymeric OLEDs. In the present work, we evaluate the potential of this new class of polymers in optoelectronics by using them as hosts in OLEDs

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O

O

N O n

n

PVK

PDBF

O

n

P0

O O

O O

N

S

S

n

P2

n

P1

N

N S

DBT

S

S

N

S

S

S

10 mg/mL and a rotation of 700 rpm were used. OLED composed of 100% PVK was prepared from a solution in chlorobenzene with concentration 15 mg/mL and rotation 2000 rpm. All other types of devices were prepared from tetrahydrofuran solutions having concentration of 10 mg/mL and rotations in the range of 400– 800 rpm were applied. After the deposition of emissive layer cathode consisting of 10 nm of calcium and 80 nm of aluminum was thermally evaporated at pressure 10  6 mbar. A cross-sectional SEM micrograph of a device is reported in Supplementary material. Electrical characterization of devices was performed with Keithley 2602 combined with calibrated photodiode under nitrogen atmosphere. External quantum efficiency (EQE) was obtained using Lambertian source assumption, by placing an OLED on photodiode surface and collecting the light emitted in half sphere solid angle as reported elsewhere [13]. Thickness of organic layers was about 130– 200 nm, as measured with a Bruker Dektak XT profilometer. Electroluminescence (EL) and photoluminescence spectra were recorded using a monochromator (Spex 270 M) equipped with liquid-nitrogen cooled CCD and corrected for the instrumental response. PL spectra are excited with a monochromated Xe lamp and PL excitation profiles (PLE) are obtained by using Rhodamine B as standard. PL QYs are obtained with a home-made integrating sphere [14].

DBTT

Fig. 1. Structures of some polymeric hosts and small molecule guests DBT and DBTT.

fabricated by a fully solution processable procedure and comparing them with PVK as a standard. Devices with the simple structure ITO/PEDOT:PSS/polymer/Ca/Al have been prepared with both the pure polymers and their blends with two benzothiadiazole based dyes (i. e. DBT and 4,7-bis(5’-hexyl-2,2’-bithienyl-5-yl)-2,1,3-benzothiadiazole, DBTT) emitting in the yellow and red spectral region. The photophysical properties of benzothiadiazoles based dyes are very interesting, due both to their high photoluminescence (PL) quantum yields (QY) and to the red-shifted emission induced by the charge transfer character of their electronic transitions [10]. However, the incorporation of benzothiadiazole based red emitting molecules in solution processed OLEDs needs proper functionalization of the dyes in order to avoid aggregation quenching phenomena [11]. We show that, within the class of polybenzofulvene derivatives, those containing the fluorene unit can be used as active layers in OLEDs fabrication. Moreover, one of these polymers, when used as host with benzothiadiazole based dyes, displays optoelectronic properties superior to those of the standard PVK thanks to its ability to reduce microaggregration phenomena.

2. Materials and methods PVK (Mw ¼25000-50000) was purchased from Aldrich, whereas polybenzofulvene derivatives P0, P1 and P2 were synthesized as reported elsewhere [8,9] and DBT and DBTT were synthesized according to the literature [10,12]. Glass substrates with ITO pattern were cleaned ultrasonically in distilled water, acetone and isopropanol. Subsequently a 50 nm film of poly(3,4ethylene-dioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) (H. C. Starck Clevios PVP.AI 4083) was spincoated from water solution through nylon filter (pore size 0.45 μm) and annealed at 100 1C for 10 min. On such prepared substrates, appropriate active layers were spincoated in a nitrogen filled glovebox. In the case of diodes consisting of pure (100%) polybenzofulvene derivatives P0-P2, degassed toluene solutions showing a concentration of about

3. Results and discussion Emission efficiency and PL peak positions of films of the polybenzofulvene derivatives P0, P1 and P2 slightly depend on the deposition techniques due to different supramolecular organizations induced during solvent evaporation. Therefore, while the photophysical properties of the polymers have already been reported for solutions and solution cast films [8,9], in Table 1 we show the PL properties of spincoated films obtained with the polymers in the same conditions employed for device fabrication. The parent polymer P0 does not show any electroluminescence; however, by adding the fluorene unit to the phenylindene group, the polymers become electroluminescent. Performances of simple single layer OLED structures ITO/ PEDOT:PSS/EML/Ca/Al are compared for the different emitting layers (EML). Devices fabricated with P1 are quite unstable (see Supplementary material), showing a weak red-shifted EL probably related to the formation of fluorenone defects [15]. On the other hand, the devices based on the P2 polymer are stable and show EL spectra corresponding to the PL (see Fig. 2). We observed that fluorene introduction at the position 6 of the indene nucleus increases the polymer conjugation and enhances the PL efficiency with respect to both P0 and P1 [9]. The best electroluminescent Table 1 PL properties of spincoated films and device characteristics for different emitting layer (EML) for the ITO/PEDOT:PSS/EML/Ca/Al device structure. EML

λPL a[nm]

PLQY [%]

λELa[nm]

EQEmax [%]

P0 P1 P2 PVK-DBT (95:5) P2-DBT (95:5) PVK-DBTT (95:5) PVK-DBTT (90:10) PVK-DBTT (80:20) P2-DBTT (95:5) P2-DBTT (90:10) P2-DBTT (80:20)

465 468 503 588 552 655 646 651 640 631 638

7 10 15 50 85 50 42 38 63 46 41

— 533 505 566 562 655 646 700 644 632 652

— 0.001 0.027 0.042 0.065 0.02 0.06 0.0006 0.075 0.075 0.03

a

Wavelength at the emission peak.

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Electroluminescence

Fig. 2. PL (solid lines) and EL (dashed lines) spectra of PVK (thin line), P1 (gray) and P2 (black) polymers (left panel). Device structures are ITO/PEDOT:PSS/Polymer/Ca/Al, operating voltages 7 V, 8 V and 18 V, respectively. Energy levels of materials used in devices with a scheme of the device cross-section (right panel).

600

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800

900

1000

Wavelength (nm) Fig. 3. EL spectra of devices ITO/PEDOT/blend/Ca/Al with different concentrations of DBTT (5%, 10% and 20%w from top to bottom) in the polymers P2 (solid lines) and PVK (dashed lines). In the inset: PLE spectra of spincoated films of DBTT (dotted line) and 20%w blends of DBTT in PVK, P1 and P2 (from bottom to top) measured at the emission peak maxima. Spectra are shifted vertically for clarity.

performances of P2, among the class of polybenzofulvene derivatives, appears therefore in agreement with its photophysical properties. Motivated by the peculiar ability of these polybenzofulvene derivatives, in particular P2, to form highly emissive self-assembled films with the yellow emitting dye 4,7-di(2-thienyl)-benzo-2,1,3-thiadiazole (DBT) [9], we evaluated P2 as a host in devices for yellow emission in comparison with standard PVK. As shown in Table 1, OLEDs based on P2 blend containing 5% (w/w) of DBT as the active layer display EL External Quantum Efficiency (EQE) higher than those of the corresponding blend in PVK. In order to obtain red emitting OLED devices, we blend P2 with a red emissive BT based dye, DBTT (see Fig.1), possessing high emission efficiency when aggregation is suppressed [16]. In order to test the ability of P2 to envelope this dye, we compared the dye PL excitation profile with those of its blends in the different polymers. As shown in the inset of Fig. 3, P2 is by far the most effective in excitation energy transfer towards the red emitting dye, showing a strong enhancement at wavelengths below 400 nm, where the polymer absorbs (see Supplementary material). Since the Foerster radius is similar for the three polymers (see Supplementary

material) this difference can be explained with a reduced dye aggregation in the P2 blend, with respect to the other two polymer blends, similarly to what observed for the smaller dye DBT [9]. In Table 1 the OLED performances of DBTT blends in P2 and PVK are compared for different dye concentrations. P2 blends display the best EQE values at all the dye concentrations. The thermal stability of the blends has been verified to be up to about 100 1C (see Supplementary material) The comparison of the EL spectra of the DBTT blends (see Fig. 3) shows that in PVK, by increasing the dye concentration, a progressive red-shift of the EL band occurs. This red-shift, accompanied by a drastic reduction of the device EQE, is associated to the EL contribution from dye microaggregates [16]. The comparison of the film morphology of PVK and P2 blends with DBTT clearly shows the presence of dye aggregates in the PVK blend, while the P2 blend, obtained in the same conditions, does not display any evidence of aggregation (see Supplementary material). We therefore attribute the better performance of the P2 host to its higher embedding capability, able to reduce dye microaggregation even at concentrations as high as 20% (w/w).

4. Conclusion We have studied the electroluminescence properties of a new class of π-stacked polybenzofulvene derivatives in comparison with those of the standard π-stacked poly(N-vinylcarbazole). Among the three polybenzofulvene derivatives P0, P1 and P2, the polymer bearing the fluorene moiety at the position 6 of the indene nucleus (i. e. P2) displays the best electroluminescence features. The ability of this polymer to embed benzothiadiazole based dyes emitting in the yellow and in the red region allows fabrication of polymer blends with reduced dye microaggregation. We show that the use of these blends as active layers in fully solution processable OLEDs produces yellow and red emitting devices whose performances overcome those obtained with the standard poly(N-vinylcarbazole).

Acknowledgments The authors are grateful to Dr Eugenio Paccagnini (Università di Siena) for the SEM experiments. This work has been partially supported by Cariplo Project EDONHIST 2012-0844 and Regione Lombardia Project “Tecnologie e materiali per l’utilizzo efficiente dell’energia solare” (decreto 3667/2013).

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Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2014.12.002. References [1] Sasabe H, Kido J. J Mater Chem C 2013;1:1699–707. [2] Brunner K, van Dijken A, Boerner H, JJAM Bastiaansen, Kiggen NMM, Langeveld BMW. J Am Chem Soc 2004;126:6035–42. [3] Ye T, Chen J, Ma D. Phys Chem Chem Phys 2010;12:15410–3. [4] Shen F, Xia H, Zhang C, Lin D, He L, Ma Y. J Phys Chem B 2004;108:1014–9; Giovanella U, Botta C, Papagni A, Tubino R, Miozzo L. Appl Phys Lett 2005;87:171910–3. [5] Araujo KAS, Guimarães PSS, Cury LA, Akcelrud L, Sanvitto D, De Giorgi M, et al. Org Electron 2012;13:2843–9. [6] Nakano T, Takewaki K, Yade T,, Okamoto Y. J Am Chem Soc 2001;123:9182–3. [7] Nakano T. Polym J 2010;42:103–23.

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