Efficient p-phenylene based OLEDs with mixed interfacial exciplex emission

Efficient p-phenylene based OLEDs with mixed interfacial exciplex emission

Electrochimica Acta 182 (2015) 524–528 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/elect...

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Electrochimica Acta 182 (2015) 524–528

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Efficient p-phenylene based OLEDs with mixed interfacial exciplex emission P. Dataa,b,c,* ,1, R. Motykab , M. Lapkowskib,c , J. Suwinskic , S. Jursenasd, G. Kreizad , A. Miasojedovasd , A.P. Monkmana a

Physics Department, Durham University, South Road, Durham DH1 3LE, United Kingdom Faculty of Chemistry, Silesian University of Technology, M. Strzody 9, 44-100 Gliwice, Poland Center of Polymer and Carbon Materials, Polish Academy of Science, M. Curie-Sklodowskiej 34, 41-819 Zabrze, Poland d Institute of Applied Research, Vilnius University, Sauletekio 9-III, LT-10222 Vilnius, Lithuania b c

A R T I C L E I N F O

A B S T R A C T

Article history: Received 5 August 2015 Received in revised form 19 September 2015 Accepted 19 September 2015 Available online 25 September 2015

Organic electronics, mainly due to the advancement of OLED (Organic Light Emitting Diode) technology, is a fast developing research area, and has already revolutionized the displays market. This direction presents the use of exciplex emitters and thermally activated delayed fluorescence (TADF) in OLEDs. This is shown through electrochemical characterisation of six p-phenylene derivatives for application in optoelectronic devices and presents the possibility the compounds’ use as OLED emitters. In these OLED devices, it is established that selenophene based compounds with a “heavy-atom effect” can be used as potential emitters when exciplex phenomena are involved. ã 2015 Elsevier Ltd. All rights reserved.

Keywords: exciplex OLED TADF selenophene thiophene

1. Introduction Some of the first polymeric and small molecules developed as OLED emitters were synthesized from of p-phenylenevinylenes [1–4] and thiophenes [5–8] as they are both electrochemically stable and highly fluorescent materials. The most difficult to obtain are blue emission OLED devices, usually their efficiency is couple times lower than for green and red based emitters. Blue (high energy) emitters are necessary as they are part of every display in a form of small micrometer pixel. One method used to decrease the emission wavelength is to insert molecules containing heavy atoms (Se, Te, Ir etc.) into the molecular structure. This however, unfortunately also leads to a decrease in photoluminescence quantum yield due to “heavy-atom” effects [9–11]. Iridium compounds have previously been used as OLED emitters due to their efficiently intermixed singlet and triplet states, producing effective heavy atom spin orbit coupling, yielding nearly 100% emission output via phosphorescence [12–15]. The quantum efficiency of a normal fluorescent emitting device is limited to 25%, upon electrical excitation, meaning that 75% of triplet

* Corresponding author. Tel.: +48606782634. E-mail address: [email protected] (P. Data). ISE member.

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http://dx.doi.org/10.1016/j.electacta.2015.09.110 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.

excitons are be wasted [16,17]. In order to increase the efficiency of devices, it is possible to link the processes of fluorescence and phosphorescence and get 100% efficiency, employing a TADF process [18,19]. In this process the excited singlet and triplet state must have very similar energies so the molecule at triplet excited state may move back to the singlet excited state with thermal activation. The Molecule at excited singlet state then relaxes by emitting a photon. The maximum efficiency of this process is 100% [18,19]. The recent idea of devices generating photons such as in light emitting diodes, is used in exciplex - bimolecular systems (BMs), which consist of two different low molecular weight compounds, one of which is the electron donor (D) and the second is the electron-acceptor (A) [20]. The application of the BMs in optoelectronic systems allows for much greater freedom of the choice of the active compounds. The length of the emitted wave in a BM system is independent of the energy gap in a single compound but is determined by the energy gap for the exciplex, where the HOMO (Highest Occupied Molecular Orbital) energy level is the same as the electron-donor compound and the LUMO (Lowest Unoccupied Molecular Orbital) energy level is the same as for electron-acceptor compound. In this case the emitted wavelength is adjusted by appropriate selection of the D and A compounds, which allows for the use of soluble, small molecular compounds [21–23].

P. Data et al. / Electrochimica Acta 182 (2015) 524–528

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Fig. 1. Investigated compounds, containing either oxygen, sulphur or selenium heteroatoms.

2. Experimental Section 2.1. Materials Cyclic voltammetric measurements were made in 1.0 mM concentrations of all monomers. These electrochemical studies were conducted in 0.1 M solutions of Bu4NBF4, 99% (Sigma Aldrich) in dichloromethane (DCM) solvent, CHROMASOLV1, 99.9% (Sigma Aldrich) at room temperature. UV–vis spectroelectrochemical measurements were performed on an ITO (Indium Tin Oxide) quartz glass working electrode coated with polymers. All organic evaporated compounds were purified by Creaphys organic sublimation system, BCP - bathocuproine (Sigma Aldrich), NPB - N, N0 -Di-1-naphthyl-N,N0 -diphenylbenzidine (TCI-Europe), TAPC 4,40 -Cyclohexylidenebis[N,N-bis(4-methylphenyl) benzenamine] (Sigma Aldrich), TPBi - 2,20 ,200 -(1,3,5-Benzinetriyl)-tris(1-phenyl1-H-benzimidazole) (LUMTEC), LiF (99.995%, Sigma Aldrich), Aluminium wire (99.9995%, Alfa Aesar). Investigated p-phenylene compounds (EE)-1,4-di(iso-propoxy)2,5-bis[2-(furan-2-yl) ethenyl]benzene (1a), (EE)-1,4-di(iso-propoxy)-2,5-bis[2-(tiophen-2-yl) ethenyl]benzene (1b), (EE)-1,4-di (iso-propoxy)-2,5-bis[2-(selenophen-2-yl) ethenyl]benzene (1c), 1,4-di(iso-propoxy)-2,5-bis(furan-2-yl) benzene (2a), 1,4-di(isopropoxy)-2,5-bis(thiophen-2-yl) benzene (2b), 1,4-di(iso-propoxy)-2,5-bis(selenophen-2-yl) benzene (2c) and their derivatives were previously described [24,25] and synthesized according to our previously published procedures [26,27].

Fig. 2. DPV curves of investigated compounds. Measurement conditions: scan rate 50 mV/s, Ag/AgCl - quasireference electrode, calibrated against a ferrocene/ ferrocenium redox couple.

investigated compounds were recorded using a UV-Vis-NIR spectrophotometer Lambda 950 (Perkin Elmer). Fluorescence measurements of dilute solutions, compounds embedded in a polystyrene (PS) matrix and in neat films, were conducted by exciting at the absorption band maximum using an 150 W xenon arc lamp light source passed through a monochromator and measured using a back-thinned CCD (Charge Coupled Device)

2.2. Measurements Electrochemical investigations were carried out using an Eco Chemie Company AUTOLAB potentiostat PGSTAT20. The electrochemical cell was comprised of a platinum electrode with an 1 mm diameter disc as the working electrode for electrochemical measurements and ITO glass as the working electrode for spectroelectrochemical measurements. An Ag/AgCl electrode was used as the reference electrode and a platinum coil as the auxiliary electrode. OLED devices were fabricated using precleaned ITO (Indium-Tin Oxide) coated glass substrates purchased from Ossila with a sheet resistance of 20 V/cm2 and an ITO thickness of 100 nm. The formed OLED devices had a pixel size of 2 mm by 1.5 mm. The small molecule and cathode layers were thermally evaporated using the Kurt J. Lesker Spectros II deposition apparatus at 106 mbar. All organic materials and aluminum were deposited at a rate of 1 Å s1 and the LiF layer was deposited at 0.1 Å s1. Absorption spectra of the dilute solutions of the

Table 1 Absorption, fluorescence and phosphorescence data of the investigated compounds in dilute (106) toluene solutions.

e

lABSmax lFLmax lPHmax PLQY, tFL

1

3

[nm]b

[nm]c

[nm]d

[%]e

[ns]f

[eV]g

[eV]h

38425 43100 40360 35725 20850 31300

398 402 408 354 359 373

452 462 471 381 402 410

553 544 528 531 513 506

53 60 9.6 42 18 1.3

2.01 2.02 0.73 1.52 0.63 0.16

2.95 2.87 2.82 3.37 3.25 3.13

2.22 2.44 2.61 2.41 2.55 2.67

[Lmol1 cm1]a 1a 1b 1c 2a 2b 2c a b c d e f g h

pp

Molar absorptivity coefficient at the absorption band maximum. Absorption band maximum. Fluorescence band maximum. Phosphorescence band maximum. Photoluminescence quantum yield. Fluorescence lifetime at fluorescence band maximum. Energy of singlet state. Energy of triplet state.

pp

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Fig. 3. Characteristics of working OLED devices based on investigated compounds as emitters. a) Electroluminescence spectra, b) Current density vs. bias, c) Luminance vs. bias, d) EQE vs. current efficiency, e) Luminous power efficiency vs. current density, f) Device efficiency vs. the luminance. Insets are the device structures.

spectrophotometer, PMA-11 (Hamamatsu). For these measurements, the dilute solutions of the investigated compounds were prepared by dissolution in a spectral grade solvent at a concentration of 106 M. The PS films containing the dispersed compounds at a concentration between 0.06–1 wt% were prepared by mixing the dissolved compounds and PS in toluene and casting the solutions on quartz substrates in an ambient air. Drop-casting from toluene solutions (1 103 M) was also employed in order to prepare neat films of the compounds. Fluorescence quantum yields (QY) of the solutions were estimated by using an integrated sphere method. An integrating sphere (Sphere Optics) coupled to a CCD spectrometer via an optical fibre was also employed to measure the h of the neat films. Fluorescence transients of the samples were measured at the fluorescence band maximum using a timecorrelated single photon counting system PicoHarp 300 (PicoQuant) utilizing a semiconductor diode laser (repetition rate

1 MHz, pulse duration 70 ps, emission wavelength 371 nm) as an excitation source. The fluorescence and phosphorescence spectra were also obtained using an EKSPLA SL312 Nd:YAG laser (150 ps), a home built single pass dye laser connected to a Stanford computer optics 4Picos gated intensified CCD camera (200 ps) and a Jobin Yvon Triax 190 spectrograph. These measurements were completed in a Cryophysics displex helium cryostat (10 K). IV characteristic of OLED devices were conducted in a 10 inch integrating sphere (Labsphere) connected to a Source Meter Unit. 3. Results and Discussion Six p-phenylene derivatives were investigated as materials for OLED emitters (Fig. 1). The group was chosen mainly to describe the influence of heteroatoms upon emission output and device efficiency.

P. Data et al. / Electrochimica Acta 182 (2015) 524–528 Table 2 OLED characteristics at 100 cd/m2 and 1000 cd/m2.

1a 1b 1c 2a 2b 2c 1a 1b 1c 2a 2b 2c

Brightness [cd/m2]

EQE [%]

Luminous Power Efficiency [Lm/W]

Current Efficiency [cd/A]

100 100 100 100 100 100 1000 1000 1000 1000 1000 1000

1.63 2.62 0.58 2.87 4.04 1.15 0.87 1.43 0.21 2.44 2.63 0.10

1.05 4.79 0.87 0.84 2.44 1.20 0.46 0.91 0.26 0.48 0.24 0.08

1.95 7.19 1.65 1.43 4.29 1.05 1.23 2.65 0.79 1.11 0.94 0.15

As a first step of analysis, the IP (Ionization Potential) and EA (Electron Affinity) were estimated [28–31] from DPV (Differential Pulse Voltammetry) analysis giving visible peaks of oxidation and reduction processes (Fig. 2). The IP and EA values gained from these electrochemical measurements (Table S1) assist in designing the OLED structures in further studies. The full electrochemical investigation was described in previous investigation [24]. Optical properties of synthesized heteroarene derivatives were assessed in various environments, including in dilute solutions of toluene and ethanol, in a rigid polystyrene matrix and the wet-cast films. Absorbance and photoluminescence (PL) spectra and fluorescence decay transients are presented in Figs. S1-3. Optical parameters are summarized in Table 1 and Table S1, S2. The PL spectra systematically shift to the longer wavelengths for compounds 2a < 2b < 2c (impact of O, S and Se substituents). A systematic increase in photoluminescence quantum yield (PLQY) from 1.3% to 42%. for compounds 2c < 2b < 2a is also evident, which is mainly due to reduced rates of non-radiative recombination (from 0.16 in 2c to 2.6 ns in 2a). The main origin of nonradiative recombination is most likely intersystem crossing. There is also evidence of ring twisting of the 5-member rings, seen for compound 2b (highlighted when comparing its lifetime in toluene against its lifetime in a rigid polymer matrix). Owing to their flat p-conjugated electron system, compounds 1a, 1b and 1c show redshifted fluorescence, enhanced emission QY (up to 60%) and a

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reduced rate of non-radiative recombination, as compared to series 2 compounds. Due to the 1 series flat structure, emission from wet cast films feature self-trapped exciton bands (Fig. S1). In spite of low the fluorescence efficiency of some of the compounds, we fabricated OLEDs containing the following structure: ITO/NPB (60 nm)/1a (40 nm)/TPBi (20 nm)/BCP (20 nm)/LiF (1 nm)/Aluminium (70), ITO/NPB (40 nm)/TAPC (20 nm)/1b or 1c (40 nm)/TPBi (20 nm)/BCP (20 nm)/LiF (1 nm)/ Aluminium (70), ITO/NPB (40 nm)/2a or 2b (20 nm)/TPBi (20 nm)/ BCP (20 nm)/LiF (1 nm)/Aluminium (70), ITO/NPB (40 nm)/2c (30 nm)/TPBi (20 nm)/BCP (20 nm)/LiF (1 nm)/Aluminium (70). The electroluminescence spectra of these OLEDs are similar to the emission spectra of pure compounds (Fig. S1 and Fig. 3a) meaning that emission mainly originates from the investigated compounds. The lowest turn-on voltage was found for 1a based devices (2.7 V) and highest for 2c based devices (4.5 V, Fig. 3b). OLEDs with thiophene derivatives demonstrated best characteristics in both of series of compounds, with an external quantum efficiency (EQE) of 2.63% (2b) and 1.43% (1b) at 1000 cd/m2 and an EQE of 4.04% (2b) and 2.62% (1b) at 100 cd/m2 (Table 2, Fig. 3). The highest luminance were observed for 2a and 2b, ca. 2200 cd/m2 (Fig. 3c) as well as with EQE. The thiophene derivatives had greatest the luminous power efficiency and current efficiency at low luminance ea.100 cd/m2 (4.79 lm/W and 7.19 cd/A respectively for 1b) and at high luminance ea.1000 cd/m2 (0.91 lm/W and 2.65 cd/A respectively for 1b) in their series (Fig. 3e,f). Overall it can be concluded that devices made from the series 2 are better due to the small electrochemical stability of the vinyl bond found in the 1 series. Another observation is that the roll-off is much lower in devices based on series 2 compounds (Fig. 3d). In series 1, the energy levels of the transporting layers were too high, resulting in the problem of balanced charge carriers condition. Higher EQEs (<100 cd/m2 luminance) were observed for 2b (4.04%), 2a (3.08%), 1b (2.60%), 2c (2.48%) and 1a (2.18%). When electroluminescence spectra were compared with spectra of the pure compounds, differences in compounds 2b and 2c were observed. There was no possibility of the emission originating from other supporting layers so mixed emission was investigated. One type of the emission, which has recently been investigated and could explain our behaviour is TADF emission. Unfortunately we didn't observe a charge transfer state (Fig. S2) and the triplet energy is much lower than singlet (Table 1, Fig. 4a, Fig. S4a) which

Fig. 4. a) Structures and singlet and triplet levels of investigated molecules in the emitter layer. Triplet and singlet levels of the excitonic state have been taken at the onsets from the emission spectra. b) HOMOdon-LUMOacc levels and the energy difference of the formed exciplex. c) Emission of the device with 2c compound compared to pure 2c in a solid layer and 50% 2c in NPB in a solid layer.

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means we cannot observe TADF process. The second choice for these observed spectral differences could be exciplex emission and for that we compare the IP and EA energies of pure compounds obtained from electrochemical measurements. When the energy levels were compared, only NPB could form blue emitting exciplex but to be sure we prepared mixed layers of 2b and 2c with NPB and TPBi. When emission spectra were compared with pure compounds only differences in emission were observed for mixed layers with NPB. The energies calculated from the exciplex emission onset were similar to the difference between the HOMO of the donor and LUMO of the acceptor, providing strong evidence of exiplex emission (Fig. 4, Fig. S4). The electroluminescence spectra of 2c based device is similar to the emission of pure materials. To explain this we compared the electroluminescence emission with the emission from pure materials and exciplexes (Fig. 4, Fig. S4) To be sure that we are forming exciplex moieties we investigated each emissive species using time resolved emission spectroscopy. All of the decays have two ‘cascade’ like features and in both mixtures a clear distinction between PF (prompt fluorescence) and DF (delayed fluorescence) regimes is observed. The PF lifetime of 2b and 2c in NPB are 18.1 ns and 13.0 ns respectively. The DF lifetimes are much higher, around 5.8 ms for 2b in NPB and 23.9 ms for 2c in NPB (Fig. S5b). Upon further inspection, we found out that the emission of those devices is not from the pure compounds but is attributed to in a huge part to the exciplex emission formed by the 2b or 2c and NPB which explains such high efficiency [19]. Moreover, it is showed that we were able to increase the efficiency of materials that had quite low efficiency to form quite usable OLED devices. 4. Conclusions In summary, using an electrochemical and spectroscopic investigation it was possible to set-up OLED multilayer models. Data gained from electrochemical studies showed a difference in HOMO-LUMO levels of NPB and selenophene or thiophene small molecule derivatives and proved the possibility of exciplex formation. Characteristics of OLED devices highlighted the possibility such compounds to be used in further applications, as fluorescence emitters as well as exciplex based emitters. Mixed emission from selenophene emitters and its exciplex form showed the possibility of using of such “heavy-atom” emitters in future OLED design. Acknowledgements Research was funded by the Polish National Science Centre grant no. 2011/03/N/ST5/04362 and the Polish Ministry of Science and Higher Education Mobility Plus Project no. 932/MOB/2012. The European Social Fund funded research at Vilnius University under the Global Grant measure. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2015.09.110. References [1] C.L. Miaoyin, N.G. Pschirer, M. Baumgarten, K. Mullen, Polyphenylene-Based Materials for Organic Photovoltaics, Chem. Rev. 110 (2010) 6817. [2] J. Heinze, B.A. Frontana-Uribe, S. Ludwig, Electrochemistry of Conducting Polymers—Persistent Models and New Concepts, Chem. Rev. 110 (2010) 4724.

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