Efficient Phosphorescence by Reducing Intrachain Chromophore

ARTICLE pubs.acs.org/JPCC

Efficient Phosphorescence by Reducing Intrachain Chromophore Interactions in Dendrimer-Containing Polymers Jack W. Levell,† Wen-Yong Lai,‡,§ Robert J. Borthwick,‡ Shih-Chun Lo,‡ Paul L. Burn,*,‡ and Ifor D. W. Samuel*,† †

Organic Semiconductor Centre, Scottish Universities Physics Alliance (SUPA), School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9SS, United Kingdom ‡ Centre for Organic Photonics & Electronics (COPE), School of Chemistry and Molecular Biosciences, The University of Queensland, Chemistry Building, Queensland 4072, Australia ABSTRACT:

Poly(dendrimers) comprised of a poly(styrene) backbone with dendrimer side chains containing an iridium(III) complex core, firstgeneration biphenyl dendrons, and (2-ethylhexyl)oxy surface groups show increased viscosity compared to their individual dendrimer components, which is important for inkjet printing processes. However, intrachain interchromophore interactions lead to lower photoluminescence quantum yields even in solution relative to the simple isolated dendrimers. We demonstrate that the phosphorescence efficiency of a polymer can be enhanced by incorporating the dendrimer monomer unit(s) into a copolymer with poly(styrene) spacer units. The poly(styrene) spacer units remove the intrachain interchromophore interactions between the chromophores in solution. The copolymer gives a >50% increase in solution photoluminescence quantum yields (to 94%) and an improved organic light-emitting diode performance with an external quantum efficiency of 6.7% at 100 cd/m2 at 11 V when compared to the homopolymer with the same dendrimer side chain.

1. INTRODUCTION Organic light-emitting diodes (OLEDs) are attractive devices for displays and solid-state lighting applications where they have achieved high power efficiencies.1 These high efficiencies have been attained by the development of phosphorescent emitters, such as fac-tris(2-phenylpyridyl)iridium(III) [Ir(ppy)3],2,3 in which the heavy atom leads to spin
orbit coupling, thus allowing harvesting of the singlet and triplet excitons that are generated during device operation. As a consequence, the use of phosphorescent emitters allows OLEDs with 100% internal quantum efficiencies to be achieved.4 These high-efficiency small molecule [e.g., Ir(ppy)3] based devices generally have complex multilayer architectures that are produced by vacuum deposition methods that are difficult to scale for large-area devices. In contrast, conjugated polymers and dendrimers can be dissolved in a range of solvents and solution-processed, allowing for low-cost processing. In particular, inkjet printing offers a way to make arrays of organic devices for displays.5 Light-emitting r 2011 American Chemical Society

dendrimers consist of a core, conjugated dendrons, and surface groups. By suitable choice of the core, they can be either fluorescent or phosphorescent, with the latter being far more attractive for OLEDs because of the higher efficiency it provides. Efficient phosphorescent OLEDs comprised of a solution-processed light-emitting dendrimer layer and an electron transport/ hole blocking layer have been produced.6
11 One of the keys of their success is that the dendrimer structure can be engineered at the molecular level to control the intermolecular interactions. Thus, dendrimers can be designed so that intermolecular interactions that lead to the quenching of the luminescence can be avoided while still allowing charge transport to occur through a film. Although dendrimer solutions are suitable for spin-coating,

Received: May 10, 2011 Revised: November 2, 2011 Published: November 03, 2011 25464

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Figure 1. Structures of copolymer 1, homopolymer 2, model dendrimer 3, and monomer 4.

unlike conjugated polymers, the viscosities tend to be low,12,13 and below those necessary for inkjet printing.14,15 Conjugated polymers are the largest class of solution-processed light-emitting materials, and the vast majority of work reported has been on fluorescent materials. However, phosphorescent polymers have been made by attaching phosphorescent pendant groups to a polymer backbone.16
23 This is preferable to simply including small molecules in a polymer blend24
26 as it avoids issues of phase separation. In principle, either conjugated or nonconjugated polymer backbones could be used. However, the use of conjugated polymers such as poly(fluorene) is problematic for phosphorescent emitters as they generally have low triplet energies, enabling backtransfer from the phosphorescent emitter to the polymer, leading to a reduction in the photoluminescence quantum yield (PLQY).17,22 As a result, we have taken the approach of using a nonconjugated polymer backbone to create phosphorescent poly(dendrimers) to provide solutions with increased viscosity for inkjet printing applications.12,13,27 The use of nonconjugated polymer backbones with small molecule complex side chains has been reported,16,18
20 although the solubility of the homopolymers has tended to be lower than that of their dendronized equivalents.28 The homopolymer poly(dendrimers) we have developed have had good PLQYs in spite of there being a dendrimer on every monomer unit, although not as high as the structurally similar individual dendrimers. The reduction in the PLQY for the polymeric materials was ascribed to the presence of intrachain interchromophore interactions, which was confirmed by time-resolved photoluminescence (PL) measurements that showed there was more than one lifetime component in PL decay in dilute solution—a direct indication of the presence of multiple emissive environments.22,27 When blended with a host

material, the solution-processable poly(dendrimers) can be used in OLEDs, giving modest performance.13,27,29 While the use of homopolymer poly(dendrimers) is very promising, the fact that the dendrimer side chains do not fully control the intrachain interchromophore interactions that lead to the quenching of the PL means that further materials development is required. In the present work, we explore a copolymer19,28 strategy for alleviating the intrachain interchromophore interactions that lead to the reduction in PLQY. We have prepared the copolymer poly(dendrimer)-co-(styrene) 1 and compare its optoelectronic properties to the corresponding homopolymer poly(dendrimer) 227 and a model dendrimer, 3, to determine the effects of the poly(styrene) “spacer groups”. The structures of the materials used in this study are shown in Figure 1.

2. EXPERIMENTAL METHODS 2.1. Synthesis and Measurements. All commercial reagents were used as received unless otherwise stated. The 1H NMR spectrum was recorded in deuterated chloroform using a 400 MHz Bruker spectrometer; EH = 2-ethylhexyl, Pr = n-propyl, L H = ligand phenyl/pyridyl H, SP H = surface phenyl H, PB H = polymer backbone H, and PBP H = polymer backbone phenyl H. Gel permeation chromatography (GPC) was carried out on a Waters system comprising a Waters 1515 isocratic HPLC pump, Waters 2414 refractive index detector, Waters 2489 UV/vis detector, Waters 717 plus autosampler, and Waters HPLC grade Styragel HT3 THF 7.8  300 mm columns (two in series) with the columns held at 40 °C with a flow rate of 1 mL/min. The results were analyzed using the Empower program. A solution of 4 (10.0 mg, 0.006 mmol), styrene (58.2 mg, 0.56 mmol), and 1,1-azobis(cyclohexanecarbonitrile) (4.4 mg, 25465

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The Journal of Physical Chemistry C 0.02 mmol) in N-methyl-2-pyrrolidinone (0.4 mL) was placed in a Schlenk tube. The solution was deoxygenated by cooling it in a dry ice bath, placing it under vacuum, and then backfilling with argon. This was repeated three times before the mixture was heated at 80 °C for 24 h. After being cooled to room temperature, the mixture was poured into methanol (10 mL) that was being stirred. The precipitate was collected by filtration, redissolved in dichloromethane (1 mL), and then the polymer was precipitated by pouring the solution into methanol (10 mL) that was being stirred. The precipitate was collected at the filter and dried to give a bright green-yellow powder of 1 (34 mg, 50%): TGA5% 380 °C; λmax(CH2Cl2)/nm 269, 289sh, 326sh, 387sh, 450sh; δH (400 MHz, CDCl3) 0.61 (br m, Pr CH3), 0.89
1.00 (br m, EH), 1.25
3.00 (br m, EH, PB H, Pr H), 3.90
3.97 (br m, EH), 4.20
4.37 (br m, NCH3), 6.31
7.30 (br m, SP H, LH, PBP H), ̄ w = 1.3  7.60
7.81 (br m, L H), 7.91
8.12 (br m, L H); GPC M ̄ n = 7.3  103, polydispersity 1.8. 104, M Copolymer 1 was prepared under conditions similar to those used for the formation of homopolymer poly(dendrimer) 2.27 Copolymer 1 was synthesized by copolymerization of dendrimer monomer 427 (Figure 1) with styrene with a feed ratio of 1:99 (by mole) using 1,1-azobis(cyclohexanecarbonitrile) as the initiator and N-methyl-2-pyrrolidinone as the solvent. After the reaction was heated at 80 °C for 24 h under nitrogen, 1 was obtained in 50% yield. The signals of the 1H NMR spectrum were broader than those of the monomer units, and analysis of the integration indicated that the ratio of the “dendrimer” to “styrene” monomer units in 1 was approximately 1:75, which is a little less than the feed ratio. However, it should be noted that estimation of the ratio was difficult due to the overlap of the signals of each of the “monomer” units and the different relaxation rates of the protons on the different components in the copolymer. Thermogravimetric analysis (TGA) under nitrogen showed that 1 was thermally stable with a 5% weight loss observed by 380 °C. Gel permeation chromatography against poly(styrene) standards showed that 1 had an Mw of 1.3  104 and a polydispersity index of 1.8. Given the relative molecular weights of the dendrimer and styrene monomers and the ratio of the two components from the 1H NMR, this indicates that there would be one or two dendrimer monomer units in each copolymer chain. However, it should be noted that for the homopolymer poly(dendrimers) gel permeation chromatography was found to underestimate the molecular weights by at least a factor of 3,29 and hence, the copolymer may well be larger and contain more dendrimer units. A further issue that needs to be considered in preparing random copolymers is the possibility that a copolymer is in fact not formed. That is, the individual monomer types polymerize independently and/or one of the monomers is merely a guest in the polymer of the second monomer. Although the broadness of the 1H NMR signals of both components suggested that this was not the case, we carried out gel permeation analysis at two different wavelengths, one at which the poly(styrene) and dendrimer units would absorb (254 nm) and the second at which only the dendrimer units would absorb (360 nm). The Mp and general shape of the two gel permeation chromatography traces were essentially the same (Figure 2), showing that a true copolymer was formed. 2.2. Photophysical and Device Measurements. Solution absorption and photoluminescence measurements were carried out with the samples dissolved in dichloromethane. The solutions were frozen
thawed
degassed for three cycles prior to the PL measurements. The solution PLQYs were measured using an excitation wavelength of 360 nm. Both sample and reference solutions were at

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Figure 2. GPC of copolymer 1 showing the detector response to changes in absorption at 254 nm (thick red line) and 360 nm (thin black line).

a concentration that gave an optical density of ∼0.1. A solution of quinine sulfate in 0.05 M sulfuric acid, which is known to have a quantum yield of 0.55, was used as the standard.30 The accuracy of these measurements is estimated to be (10% of the stated value. Film PLQY measurements were performed using an integrating sphere31 as part of a Hamamatsu C9920-02 measurement system32 with an excitation wavelength of 325 nm. Films of a 20 wt % concentration of both materials in 4,40 -(N,N0 -dicarbazolyl)biphenyl (CBP) were spin-coated from dichloromethane solutions at a concentration of 20 mg/mL at a spin speed of 2000 rpm onto fused silica substrates. All photoluminescence spectra were recorded in a Jobin Yvon Horiba Fluoromax 2 fluorimeter, and absorption spectra were recorded using a Varian Cary 300 spectrophotometer. The time-resolved PL measurements were performed using the time-correlated single photon counting technique. A pulsed 393 nm Picoquant GaN laser diode was used as the excitation source, and a cooled Hamamatsu R3809U-64 microchannel plate was used as the detector. The emission wavelength was selected using a monochromator centered at 530 nm. The instrument response function of the system was measured to be 200 ps full width at half-maximum. For OLED fabrication, indium tin oxide (ITO) coated glass substrates were etched with 37% hydrochloric acid and zinc powder. The substrates were cleaned by sonication in acetone and propan-2-ol followed by oxygen plasma ashing. The emissive layer was spin-coated from a dichloromethane solution of 20 wt % polymer in CBP at a concentration of 10 mg/mL and spin speed of 2000 rpm. This resulted in ∼100 nm thick films. Sequential layers of 1,3,5-tris[1-phenyl-1H-benzimidazol-2-yl]benzene (TPBI) (60 nm), LiF (0.7 nm), and aluminum (>100 nm) were deposited by thermal evaporation at a pressure of 2  10
6 mbar to complete the device. A shadow mask was used to give a final active area of 6 mm2. A cooled Andor DV420-BV CCD spectrometer was used to collect the electroluminescence spectra. The current and voltage were recorded using a Keithley source-measure unit, and the light emission was measured using a calibrated photodiode. The OLED efficiency was calculated assuming Lambertian emission.33

3. RESULTS AND DISCUSSION 3.1. Photophysical Properties in Solution. The absorption and photoluminescence spectra of dilute solutions of the materials in dichloromethane are shown in Figure 3. The absorption at 25466

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Figure 3. Absorption and PL spectra of copolymer 1, homopolymer 2, and model dendrimer 3 in dichloromethane. Absorption spectra have been normalized to an optical density of 0.1 at 360 nm. PL was excited at 360 nm and normalized at the peak of emission.

Figure 4. Time-resolved photoluminescence of polymers 1 and 2 in degassed dichloromethane solution. Excitation was at 393 nm and detection at 530 nm. The data for homopolymer 2 are shown with red plus signs with a black fit line; the data for copolymer 1 are shown with black open squares and a gray fit line.

Table 1. Photophysical Properties of 1, 2, and 3 in Solution

interchromophore interactions; the concentrations of solutions used for the PLQY measurements are low so that interchain interactions can be ignored. The intrachain interactions arise from having one dendrimer on every monomer unit in homopolymer 2, and hence, the pendant dendrimer units are physically constrained to be close to one another. What is perhaps remarkable is given the closeness of the chromophores in 2 the PLQY is still as high as 61%. The aim of preparing the copolymer was to have a polymer in which intrachain interchromophore interactions of the dendrimer side groups could be negated. This has been very successful, and copolymer 1 has a PLQY of 94%. The fact that copolymer 1 and model dendrimer 3 have the same high PLQYs suggests that the dendrimer side chains in 1 are acting as individual chromophores, which is consistent with the ratio of dendrimer to styrene “monomer” units in the polymer. To probe the photophysical properties further, we undertook time-resolved PL measurements. The PL lifetimes of the solutions are shown in Figure 4, with the parameters used for fitting the decays summarized in Table 1. The model dendrimer 3 has a monoexponential lifetime as expected for a compound with a single emissive chromophore. However, for homopolymer 2 the decay is multiexponential, showing that there are multiple emissive environments that correspond to intrachain interchromophore interactions. In contrast, copolymer 1 shows a single-exponential decay with a lifetime close to that of the model dendrimer 3, confirming that there are no intrachain interchromophore interactions. 3.2. Blended Film and Device Properties. For device studies we blended the materials 1 and 2 with CBP because of its favorable charge transport properties for OLEDs. The PLQYs of the 20 wt % blended films of 1 and 2 were 67% and 42%, respectively. These are both lower than the solution PLQYs of the materials and suggest that some intermolecular chromophore interactions occur in the blend with CBP. Nevertheless, the PLQY of 1 blended with CBP is very high, and considerably higher than that of the homopolymer 2 at the same weight percent blend ratio. However, it should be noted that the relative concentration of dendrimer chromophores is significantly less for copolymer 1 at the same weight percent. The structure of the OLEDs was ITO/polymer:CBP/TPBI/LiF/Al. The TPBI was introduced as an electron transport/hole blocking layer as it has been successfully used in other dendrimer-based devices to improve performance.9,10 The device performance at a brightness

material

PLQY

lifetime (μs) (pre-exponential factor)

model dendrimer 3

0.92

1.60

homopolymer 2 copolymer 1

0.61 0.94

1.03 (0.58); 1.95 (0.42) 1.50

wavelengths shorter than 350 nm is generally associated with singlet transitions and in particular the π
π* transitions of the dendrons and ligands. The weaker absorptions at longer wavelength are normally ascribed to the so-called metal-to-ligand charge transfer states of the iridium(III) complexes.34 However, for the polymers in addition to the ligand and dendron π
π* transitions there is also a component due to the polystyrene backbone that has an absorption peak around 290 nm.35 The emission spectra of the model dendrimer 3, homopolymer 2, and copolymer 1 are very similar. Indeed the photoluminescence spectra of 1 and 2 are so close that they are hard to distinguish in Figure 3. The overlap of the spectra can be understood from the fact that the emissive chromophores are all similar. That is, the complex of each of the materials is comprised of a phenyltriazole ligand and two 2-phenylpyridyl ligands. Then for each material first-generation biphenyl-based dendrons with (2-ethylhexyl)oxy surface groups are attached to the two 2-phenylpyridyl ligands. The PL spectra have peaks at 510 nm for 3 and 517 nm for 1 and 2, which corresponds to green emission in spite of the phenyltriazole ligand. Homoleptic iridium(III) complexes with phenyltriazole ligands emit blue light,36 whereas Ir(ppy)3 provides green phosphorescence. Hence, these complexes are a further example of where emission is only seen from transitions associated with the ligand with the longest conjugation length.37 The fact that the phenyltriazole ligand does not substantially perturb the emission relative to that of the homoleptic dendrimer comprised of the same first-generation dendrons and 2-phenylpyridyl ligands can be seen from the Commission Internationale de l0 Eclairage (CIE) coordinates of the photoluminescence, which are (0.32, 0.62) for 1 and (0.36, 0.61) for the homoleptic dendrimer.8 The solution photophysical properties of the materials are summarized in Table 1. The model dendrimer 3 has a very high PLQY of 92%, while the PLQY of homopolymer 2 is lower at 61%.27 The reduction in the PLQY of the homopolymer poly(dendrimer) 2 relative to the model dendrimer 3 is due to intrachain

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Table 2. OLED Properties of the Homoploymer 1 and Copolymer 2 [ITO/Polymer:CBP (20 wt %)/TPBI/LiF/Al] blended

EQE (%) at

voltage (V) at

material

film PLQY

100 cd/m2

100 cd/m2

homopolymer 227 copolymer 1

0.42 0.67

6.2 6.7

13.2 11.0

results largely from its increased PLQY. Finally, the emission spectrum of the OLEDs is shown in Figure 5b. The electroluminescence (EL) and PL spectra are essentially the same, meaning that there is the same emissive chromophore in both cases. Given that in a device the charge carriers are most likely to recombine on the lowest energy sites, the fact that EL is not red-shifted relative to the PL is a further indication that the dendrimer chromophores on the copolymer backbone do not suffer from strong aggregation effects.

4. CONCLUSIONS We have demonstrated a strategy for improving the PL and EL efficiency of poly(dendrimer)s by ensuring that the emissive dendrimer units on the polymer backbone are not subject to intrachain interchromophore interactions in solution. The reduction in interchromophore interactions also leads to higher solid-state PLQY and OLED efficiencies. ’ AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected] (P.L.B.); [email protected] (I.D.W.S.). Present Addresses §

Key Laboratory for Organic Electronics & Information Displays (KLOEID) and Institute of Advanced Materials (IAM), Nanjing University of Posts & Telecommunications (NUPT), 9 Wenyuan Road, Nanjing 210046, China.

’ ACKNOWLEDGMENT P.L.B. is a recipient of an Australian Research Council Federation Fellowship (Project FF0668728). J.W.L. and I.D.W.S. are grateful to the Engineering and Physical Sciences Research Council of the United Kingdom for financial support. Figure 5. (a) External quantum efficiency and luminance versus applied voltage for OLEDs made with copolymer 1 (20 wt % blended with CBP). (b) EL (thin black line) and solution PL (thick red line) spectra for copolymer 1.

of 100 cd/m2 is shown in Table 2, and the brightness and external quantum efficiency (EQE) curves as a function of voltage for copolymer 1 are shown in Figure 5a. The theoretical upper limit on the EQE of a device is set by the PLQY of the film and the outcoupling of light (normally taken to be 20% given the refractive index of typical organic materials).38 We note that as the blends are 80% CBP the refractive indices of the blended films of 1 and 2 will be very similar, and hence, the fraction of light outcoupled will also be very similar. For homopolymer 2 the EQE is 6.2% at a brightness of 100 cd/m2, which is close to the theoretical maximum efficiency of 8.4% based on the film PLQY of 42% and indicates that good charge balance has been achieved in the device.27 In contrast, the devices containing copolymer 1 had a maximum efficiency of 6.7% compared to its theoretical maximum of 13% based on the film PLQY. This implies that the charge balance in these devices is not perfect. As the TPBI hole blocking layer should ensure that the holes do not pass through the device, the lower EQE is ascribed to reduced hole transport through the emissive layer because of the insulating nature of the poly(styrene) units. Nevertheless, copolymer 1 is still capable of higher efficiency and lower drive voltages than homopolymer 2, which

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