Alcohol-Soluble

ARTICLE pubs.acs.org/JPCC

Optical and Charge Transport Properties of Water/Alcohol-Soluble Quinacridone Derivatives for Application in Polymer Light Emitting Diodes P. Zalar, T. V. Pho, A. Garcia, B. Walker, W. Walker, F. Wudl,* and T.-Q. Nguyen* Center for Polymers and Organic Solids and Department of Chemistry & Biochemistry, University of California, Santa Barbara, California 93106, United States

bS Supporting Information ABSTRACT: Sodium N,N0 -bis(3-sulfonylpropyl)quinacridone (Na+QPSO3), tetraphenylphosphonium N,N0 -bis(3-sulfonylpropyl)quinacridone (Ph4P+QPSO3), sodium N,N0 -bis(6-sulfonylhexyl)quinacridone (Na +QHSO3 ), and tetraphenylphosphonium N,N0 -bis(6-sulfonylhexyl)quinacridone (Ph4P+QHSO3) are employed as solution-processable electron injection layers (EILs) in multilayer polymer light emitting diodes utilizing poly(2-methoxy-5-(20 -ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) as the emissive layer. The electron mobilities of these oligoelectrolytes are comparable with conjugated polyelectrolytes, but the synthesis is much simpler. The choice of counterion can be used to tune electron mobilities and optical properties. Increased aggregation in solution positively influences device performance, yielding devices that outperform those that use a barium cathode with no barrier for electron injection. These oligoelectrolyte EILs were also tested in devices utilizing a blue emissive small molecule. The ease of synthesis of these quinacridone salts makes them very attractive candidates for commercial applications.

I. INTRODUCTION Conjugated polyelectrolytes (CPEs) are a class of polymer whose repeat units are made up of cationic or anionic functional groups. This type of functionality yields polymers that are soluble in polar solvents. CPEs also allow for the modification of solution and solid-state properties by the choice of counterion or anionic/ cationic backbones. Due to their solubility, CPEs have found applications where polar media are preferred such as in biosensors1 and in chemical sensors.2 Another advantage of their solubility in polar solvents is that multilayer device structures can be solution processed, whereby the solvents used for the deposition of each layer are of orthogonal polarity leading to minimal intermixing of the layers during film deposition.3 CPEs have been used in single layer light emitting electrochemical cells (LECs),4 solar cells,5 and more recently have been used as electron injection layers (EILs) for polymer light emitting diodes3b,6 (PLEDs) and for n-type organic field-effect transistors.7 Using CPEs as EILs, high performance can be achieved in these devices using high work function and stable metal cathodes such as aluminum, gold, or silver. While CPEs have excellent film-forming properties, they suffer from problems such as batch-to-batch variation in terms of molecular weight and polydispersity, difficulty in purification, low electron mobilities, and poor solubility. Additionally, PLEDs utilizing CPEs as EILs show a slow turn-on time.8 Recently, conjugated oligoelectrolytes have been used in place of CPEs due to their ease of synthesis and purification.9 The turn-on time for devices utilizing oligoelectrolytes as EILs is much faster than r 2011 American Chemical Society

devices that used CPE EILs. However, previous work attempting to employ oligoelectrolytes in PLEDs yielded nonuniform films with extremely rough surfaces.9 Nevertheless, improved electron injection was still observed relative to the devices fabricated using an aluminum electrode without an oligoelectrolyte layer. In search of conjugated frameworks for new electron injection materials with better film formation and simpler synthesis procedures, we found quinacridone. Quinacridone backbones have seen extensive use as a pigment in high quality industrial coatings.10 Quinacridone-based pigments are known to have exceptional photochemical, chemical, and thermal stability.11 Quinacridone materials have previously been used to form nanowires via self-assembly,12 as a dopant in Alq3 based organic light emitting diodes,13 and even as an electron donor material in organic photovoltaics.14 Recently, we reported the synthesis of charged quinacridones and showed that it is possible to use these materials as EILs in PLEDs.15 In this contribution, the optical and charge transport properties of sodium N,N0 -bis(3-sulfonylpropyl)quinacridone (Na+QPSO3), tetraphenylphosphonium N,N0 -bis(3-sulfonylpropyl)quinacridone (Ph4P+QPSO3), sodium N,N0 -bis(6-sulfonylhexyl)quinacridone (Na+QHSO3), and tetraphenylphosphonium N,N0 -bis(6-sulfonylhexyl)quinacridone (Ph4P+QHSO3) are presented (Figure 1). These materials were synthesized in one step and in a large quantity. In addition, these electrolytes were tested as Received: March 8, 2011 Revised: August 2, 2011 Published: August 03, 2011 17533

dx.doi.org/10.1021/jp202214g | J. Phys. Chem. C 2011, 115, 17533–17539

The Journal of Physical Chemistry C

Figure 1. Chemical structure of Fa3B (a). Chemical structure of X+QRSO3. R = 3 (P), 6 (H) and X+ = Na+, PPh4+ (b), and the LED device structure (c).

EILs in PLEDs using poly[2-methoxy-5-(20 -ethyl-hexyloxy)1,4-phenylenevinylene] (MEH-PPV) as the emissive layer. Application of the EIL to blue emitting material, tris(7,10diphenylfluoranthen-8-yl)benzene (Fa3B), was also investigated. The choice of counterions and alkyl chain length were found to affect optical properties, film morphology, electron transport, and device performance.

II. EXPERIMENTAL METHODS Toluene and anhydrous methanol were purchased from Sigma Aldrich and used as received. All solutions containing water were prepared using 18 MΩ water to ensure that no other ions were present. Fa3B was synthesized according to the previous procedure.16 N,N0 -Bis(6-bromohexyl)quinacridone was synthesized using a previously reported procedure.15,17 A solution of Na2SO3 (1.18 g, 9.6 mmol) in H2O (20 mL) was added to a mixture of N,N0 bis(6-bromohexyl)quinacridone (1.0 g, 1.6 mmol) in EtOH (20 mL). The mixture was refluxed for 2 days. Additional Na2SO3 (0.59 g, 4.8 mmol) and H2O (20 mL) were added, and the mixture was refluxed for another 2 days. Ph4PCl (2.40 g, 6.4 mmol) and H2O (50 mL) were added, and the mixture was extracted with DCM (3). The combined organic extracts were dried over MgSO4 and concentrated in vacuo. The residue was recrystallized from hot acetone to give Ph4P+QHSO3 (1.5 g, 1.1 mmol, 73%) as red crystals. 1H NMR (CD2Cl2): δ 8.65 (s, 2H), 8.42 (d, J = 7.8 Hz, 2H), 7.89 (t, J = 7.4 Hz, 8H), 7.73 (m, 18H), 7.61 (t, J = 8.1 Hz, 16H), 7.53 (d, J = 8.7 Hz, 2H), 7.19 (t, J = 7.4 Hz, 2H), 4.45 (t, J = 7.4 Hz, 4H), 2.71 (t, J = 7.7 Hz, 4H), 1.96 (br, 4H), 1.81 (p, J = 6.7 Hz, 4H), 1.61 (p, J = 6.7 Hz, 4H), 1.55 (p, J = 6.7 Hz, 4H). 13C NMR (CD2Cl2): δ 178.17, 142.86, 136.31, 135.17, 135.03, 131.19, 128.20, 126.84, 121.62, 121.15, 118.48, 117.77, 115.53, 113.82, 52.31, 46.89, 29.35, 27.50, 27.37, 26.07. MS (ESI) m/z: 319 (M 2Ph4P)2, 977 (M Ph4P). A solution of NaI (0.3 g, 2.0 mmol) in acetone (75 mL) was added to a suspension of Ph4P+QHSO3 (1.05 g, 0.8 mmol) in acetone (300 mL). The mixture was stirred for 45 min at room temperature, resulting in a red precipitate that was filtered and washed with acetone. Yield: 0.55 g, 99%. 1H NMR (DMSO-d6): δ 8.65 (s, 2H), 8.38 (d, J = 7.8 Hz, 2H), 7.877.85 (m, 4H), 7.32 (t, J = 7.0 Hz, 2H), 4.54 (t, J = 6.8 Hz, 4H), 2.44 (t, J = 7.1 Hz, 4H), 1.88 (br, 4H), 1.63 (p, J = 7.0 Hz, 4H), 1.56 (p, J = 6.2 Hz, 4H), 1.48 (p, J = 6.6 Hz, 4H). 13C NMR (DMSO-d6): δ 176.68, 141.95, 135.20, 135.03, 127.04, 125.70, 120.94, 120.32, 115.68,

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113.11, 51.43, 45.56, 28.26, 26.56, 26.16, 25.27. MS (ESI) m/z: 319 (M 2Na)2, 661 (M Na). Solid-state absorption, photoluminescence (PL), and quantum yield measurements were performed on quartz substrates that were cleaned by heating in 70:30 (v:v) H2SO4:H2O2 solution followed by sonication in water twice for 10 min each, in acetone for 30 min, and in 2-propanol for 1 h. The substrates were dried under nitrogen gas and subsequently in an oven at 120 °C overnight. Substrates were treated in UV/O3 (UVO Cleaner 42, Jelight Co. Inc.) for 1 h prior to film deposition. Films were deposited from 2% w/v solutions of Na+QPSO3 and Na+QHSO3 in 1:1 water:methanol and 3% w/v solutions of Ph4P+QPSO3 and Ph4+PQHSO3 in methanol. These solutions were stirred overnight at 40 °C. Solutions used for optical studies were serially diluted to 0.001% w/v concentrations for solution-state absorption, photoluminescence, and quantum yield measurements. All absorption data were obtained using a Shimadzu UV2401PC spectrophotometer. Fluorescence spectra were measured on a Photon Technology International Quantum Master fluorometer. Solution-state quantum yield measurements were performed using the optically dilute method with fluoroscein in water as a reference. Solid-state quantum yield measurements were performed using an integrating sphere and a Ti:Al2O3 laser (λex = 488 nm). Surface morphology images of EIL films were performed using a MultiMode atomic force microscope (AFM) with Nanoscope Controller IIIa (Veeco Inc.). All AFM measurements were performed in an inert atmosphere of nitrogen to prevent surface alteration by moisture in air.18 Silicon probes with a spring constant of ∼5 N/m and resonant frequencies of 75 kHz (Budget Sensors) were used for tapping mode measurements. Film thicknesses were determined by AFM. Corning 1737 glass substrates patterned with 140 nm of indiumtin oxide (ITO) were cleaned using the same procedure as for glass substrates discussed above. A 70 nm thick layer of poly(3,4-ethylenedioxythiophene):polystyrene sulfonic acid, PEDOT:PSS, (Baytron P 4083, H.C. Starck Inc.) is then spin coated on top of the ITO/glass substrate in air and annealed at 140 °C for 45 min. For the emissive layer, a 55 nm thick film of MEH-PPV (Canton OLED King Optoelectric Materials Co., Ltd.) was spin coated at 1500 rpm for 60 s from a 0.5% w/v solution in toluene atop of the PEDOT:PSS layer in a nitrogen atmosphere and left to dry for 20 min before deposition of the next layer. For blue emissive devices, Fa3B was spin coated at 2000 rpm for 60 s from a 1% w/v solution in toluene in a nitrogen atmosphere. Subsequently, X+QRSO3 solutions were spin coated atop of the MEH-PPV or Fa3B layer at 1500 rpm for 60 s in a nitrogen atmosphere. Na+QPSO3 and Na+QHSO3 films were spin coated from a 0.05% w/v solution in 50:50 and 30:70 water:methanol, yielding ∼20 and ∼15 nm thick films, respectively. Ph4P+QPSO3 and Ph4P+QHSO3 films were spin coated from a 0.5% w/v solution in 100% methanol, giving ∼18 and ∼20 nm thick films, respectively. After the devices were left to dry in a 104 Torr vacuum overnight, the 100 nm thick aluminum cathode was deposited by thermal evaporation at a base pressure of 106 Torr. Reference devices were made by thermal evaporation of 5 nm of barium capped with 100 nm of aluminum or with 100 nm of aluminum only. All devices were tested in a nitrogen atmosphere using a Keithley 2602 and multimeter coupled with a photodiode. Electron-only diodes were prepared by using an (aminopropyl)trimethoxysilane self-assembled monolayer (SAM) 17534

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modified-ITO surface. SAM-ITO substrates were prepared using a procedure previously reported by Sfez et al.19 This procedure reduces the work function of ITO from 4.7 to 4.3 eV.20 Films were spin coated in a nitrogen atmosphere from 2% w/v 1:1 water:methanol for Na+QPSO3 and Na+QHSO3 and from 3% w/v in methanol for Ph4P+QPSO3 and Ph4P+QHSO3 giving film thicknesses of ∼100, ∼85, ∼120, and ∼100 nm, respectively. The films were dried in a 104 Torr vacuum overnight. Subsequently, a 5 nm layer of barium was thermally evaporated and capped with 100 nm of aluminum. Current densityvoltage (JV) curves for electron only devices were obtained in a nitrogen atmosphere using a Keithley 4200 Semiconductor Characterization System.

the vibronic transitions of 02 (∼475 nm), 01 (∼510 nm), and 00 (∼545 nm). In the PL spectra, one observes a vibronic shoulder between 600 and 615 nm. The Na+QPSO3 and Ph4P+QPSO3 spectra do not exhibit any spectral shifts relative to one another, indicating that choice of counterions has little or no impact on the solution absorption and PL. However, the solution PL quantum yield (Table 1) for a larger counterion, Ph4P+QPSO3, is much higher (30%) than that of the Na+QPSO3 (21%). It is possible that the larger counterion is more effective in separating the molecules in a polar medium, decreasing strong aggregation similar to what has been observed in CPEs.21 The Na+QHSO3 and Ph4P+QHSO3 absorption and PL spectra show a red shift of ∼8 nm relative to their propyl analogues. We believe that the longer alkyl chain could also be aiding the aggregation of the Na+QHSO3 and Ph4P+QHSO3 compounds leading to the observed red shift. Changes in the optical band gap as a result of the longer alkyl chain could also be responsible for this shift. The solution PL quantum yield drops slightly from 21% for Na+QPSO3 to 18% for the longer alkyl chain compound, Na+QHSO3, in water (Table 1). A similar drop is also observed for Ph4P+QHSO3. However, in methanol, longer alkyl chain compounds have slightly higher PL quantum yields (Table 1). Figure 2b compares absorption and PL spectra for 0.001% Na+QHSO3 in water and methanol and for a Na+QHSO3 film spun from 1:1 v/v watermethanol. The absorption and PL spectra in water are red-shifted 17 nm relative to the methanol spectra, indicating a higher degree of aggregation in water solutions. The solution PL quantum yield measurements support this conclusion (Table 1). The PL quantum yield of Na+QHSO3 is drastically reduced from 80% in methanol to 18% in water. Similar shifts in the

III. RESULTS AND DISCUSSION 1. Optical Properties. Figure 2a shows the normalized absorption and PL spectra of 0.001% w/v Na+QPSO3, Na+QHSO3, Ph4P+QPSO3, and Ph4P+QHSO3 solution in water. All compounds show absorption peaks corresponding to

Figure 2. (a) Absorption and PL spectra of X+QHSO3 and X+QPSO3 in water and (b) of Na+QHSO3 in methanol (black solid line), water (dotted-dashed red curve), and film (dotted curve).

Figure 3. JV characteristics for electron-only diodes with the device structure ITO/APTMS/X+QRSO3/Ba/Al. The solid line is the fit to the space charge limited current model.

Table 1. Photophysical Properties of X+QRSO3 Solutions and Filmsa H2O UV

PL

UV

λPL (nm)

ΦPL (%)

541

566

21%

541

565

30%

549

570

547

568

λabs (nm) Na QPSO3 Ph4P+QPSO3 Na+QHSO3 Ph4P+QHSO3 +

film

CH3OH PL

λabs (nm)

UV

λPL (nm)

ΦPL (%)

528

557

78

526

552

74

18%

525

554

25%

527

550

λabs (nm)

PL λPL (nm)

ΦPL (%)

535

615

2.7

536

617

1.6

80

536

617

1.2

78

534

597

4.1

Na+QPSO3 and Na+QHSO3 films were deposited from 1:1 water:methanol whereas Ph4P+QPSO3 and Ph4+PQHSO3 films were deposited from methanol. a

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Figure 4. Surface morphology of (a) Na+QPSO3, (b) Na+QHSO3, (c) Ph4P+QPSO3, and (d) Ph4P+QHSO3 obtained in nitrogen using tapping mode AFM. The images were collected between the aluminum electrodes of the electron-only diodes. The image sizes are 5 μm 5 μm.

absorption and PL spectra are also observed for other compounds (Table 1). A red shift in the solution absorption and a large drop in solution PL quantum yield of neutral quinacridones upon aggregation have been reported by M€ullen et al.22 The PL spectrum of the Na+QHSO3 film cast from 50:50 water:methanol is broader and red-shifted 65 nm compared to the solution PL spectrum. No vibronic structure is observed in the film PL spectrum. This observation is a testament to the degree of inhomogeneity that is present in the solid-state film. The low PL quantum yield of Na+QHSO3 (1.2%) and the other quinacridone (1.64.1%) salts indicates that a high degree of selfquenching takes place within the films. The choice of counterion does not make a significant difference on the solid-state quantum yields as was observed for the CPEs studied by Yang et al.21 2. Electron Transport Properties. Electron-only diodes were fabricated to understand the electron transporting properties of the four compounds and to test their viability as electron injecting/transporting layers in PLEDs. SAM-modified ITO was used as an electrode instead of Al to avoid possible oxidation of the aluminum surface upon the deposition of X+QRSO3 from watermethanol solvent, leading to the formation of an insulator at the Al/active layer interface. SAM-modified ITO has a work function of 4.3 eV, similar to that of aluminum.20 Figure 3 shows the JV characteristics of the four electrononly devices. The materials containing the sodium counterion have much higher current densities at lower biases. These JV curves can be fitted to the MottGurney law for space charge limited current (SCLC)23,24 from which the charge carrier mobility can be extracted; J ¼

9 V2 ε0 εr μ 3 8 L

where εr is the dielectric constant of the medium, ε0 is the vacuum permittivity, μ is the charge carrier mobility, L is the film thickness, and V is the voltage applied.

Using the SCLC model to fit the JV curves, the electron mobilities are 3.9 105 cm2/(V 3 s) for Na+QPSO3, 3.8 105 cm2/(V 3 s) for Na+QHSO3, 2.9 107 cm2/(V 3 s) for Ph4P+QPSO3, and 1.6 107 cm2/(V 3 s) for Ph4P+QHSO3. The materials with the sodium counterion have essentially the same electron mobility, while the phosphonium materials have mobilities that are 2 orders of magnitude smaller than their sodium analogues. The alkyl chain length does not seem to affect the electron mobility, while the choice of counterion has a significant impact on the mobilities. These SCLC electron mobilities are the same order of magnitude as those of CPE electron injection materials reported previously in the literature.25,26 Next, we examined the impact of different counterions on the film morphology. The different electron mobility can possibly be explained by differences in the surface morphology of the electron-only devices displayed in Figure 4. Tapping mode AFM images were collected from regions between the aluminum electrodes and under a dry nitrogen environment. The surfaces of the Na+QPSO3 and Na+QHSO3 films are rough (7.1 and 21.2 nm) and composed of nanostructures (Figure 4a,b). Longer alkyl chains result in short fiber-like structures. In the case of the propyl and hexyl phosphonium materials, the film surface is much smoother (0.28 and 0.53 nm) and featureless, similar to that of amorphous materials (Figure 4c,d). This observation also confirmed that the bulky nature of the Ph4P+ counterion may interfere with molecular packing and hence intermolecular charge transport as discussed above. Bulky counterions have been previously shown to disrupt solid-state molecular packing.8 Figure 5 shows the JV, luminance versus voltage (LV), and luminescence efficiency versus current density (LEJ) data for the ITO/PEDOT:PSS/MEH-PPV/EIL(quinacridone salt)/Al devices and the two reference devices (ITO/PEDOT:PSS/ MEH-PPV/Al and ITO/PEDOT:PSS/MEH-PPV/Ba/Al). All devices using quinacridone EILs have a low turn-on bias, ∼1.9 V, comparable to that of the device using Ba as a cathode. Among the devices with EIL/Al cathodes, the device with Na+QHSO3 EIL 17536



Figure 5. JV (a), luminancevoltage (b), and luminance efficiency current density (c) characteristics of PLEDs utilizing different EILs: Na+QPSO3 (red circles), Na+QHSO3 (green squares), Ph4P+QPSO3 (blue triangles), and Ph4P+QHSO3 (purple crosses). Reference devices with Ba/Al (solid black diamond) and Al (open black diamond) cathodes are also included for comparison.

has the best performance and even better than that of the barium reference device that has no electron injection barrier. The barium device has a maximum luminescence of 12 800 cd/m2 whereas the Na+QHSO3 device has a maximum of 13 400 cd/m2. Both of these devices turn on at ∼1.9 V. The shorter alkyl chain Na+QPSO3 device has a similar turn-on bias but much lower luminance (8460 cd/m2), ∼40% less than the Na+QHSO3 device. Efficiencies of the Na+QPSO3 and Na+QHSO3 devices (1.65 and 1.22 cd/A at a current density of 300 mA/cm2, respectively) are higher than that of the barium device (0.85 cd/A) and by far surpass that of the MEH-PPV/Al device (0.0065 cd/A). The Ph4P+QPSO3 and Ph4P+QHSO3 devices have lower performances than their sodium counterparts. Both devices have a maximum luminescence of approximately 560 cd/m2 and efficiency of 0.11 cd/A, 1 order of magnitude lower than those of the Na+QPSO3 and Na+QHSO3 devices. The difference in performance between Na+QPSO3, Ph4P+QPSO3, Na+QHSO3, and Ph4P+QHSO3 devices follow the trend of the electron mobility study discussed above: higher electron mobility, better device performance. The electron mobilities of the sodium materials are 2 orders of magnitude higher (105 versus 107 cm2/(V 3 s)) than those of the phosphonium materials, and the device performance is also better by a factor of 15. This observation highlights that both efficient electron injection and high electron mobility are crucial to obtaining high efficiency in multilayer PLEDs utilizing a thick EIL. When the EIL/Al is compared to the Al cathode, the

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Figure 6. JV (a), LV (b), and LEJ (c) characteristics of devices utilizing a thin EIL cast from dilute solutions of Na+QHSO3 (green squares) and Ph4P+QHSO3 (purple crosses). Reference devices with Ba/Al (solid black diamond) and Al (open black diamond) cathodes are also included for comparison.

improvement in the device performance is due to much more efficient electron injection whereas for the case of the Ba devices, the improvement is due to less quenching of the excitons by the metal cathode. With the EIL, the recombination zone is moved much further away from the Ba cathode. It is well-known that in PLEDs, due to much lower electron mobility in emitting polymers,27 the charge recombination happens near the metal cathode and, as a result, some emissions are quenched by the metal implantation.28 The efficiencies of the devices with sodium counterions exceed those previously reported for CPEs as EILs, demonstrating that π-bonded small molecular electrolytes are a promising class of materials for EILs in LEDs. Next, we investigated the influence of the counterion choice on devices utilizing a thin EIL. It has been shown that for a thick EIL (>5 nm), ion motion plays an important role in efficient electron injection whereas for a thin EIL, the formation of an interfacial dipole is more important. Thus, for a thin EIL, molecular packing and electron transport of these materials should not play an important role in their function as an EIL. Na+QHSO3 and Ph4P+QHSO3 layers were spin-coated from a dilute solution (0.005% w/v), yielding ∼3 nm thick films. Figure 6 shows the JV, LV, and LEJ characteristics for two reference devices and the ITO/PEDOT:PSS/MEH-PPV/ Na+QHSO3 or Ph4P+QHSO3/Al devices. The turn-on bias of the devices with thin EILs is ∼2.7 V. Both devices have a similar performance and are slightly higher than the aluminum control device. The maximum luminescence of both devices with EIL/Al cathodes is ∼60 cd/m2 at ∼6.0 V. The luminescence 17537



Figure 7. JV (a), LV (b), and LEJ (c) characteristics of Fa3B based OLEDs with (red filled circles) and without (black filled diamonds) Na+QHSO3 EIL.

efficiency of these devices are similar, but slightly higher (0.06 cd/A) than that of the aluminum reference device (0.04 cd/A). This observation is quite different from the case of CPEs, where the devices with thin EILs still perform as well as the Ba control device.29 To understand this difference in device performance, we examined the surface morphology of devices with thin EILs using tapping mode AFM (Supporting Information). Unlike the case of CPEs, Na+QHSO3 and Ph4P+QHSO3 spun from a dilute solution do not form a continuous film atop of the MEHPPV layer. Circular features are observed possibly due to dewetting of a thin hydrophilic layer atop a hydrophobic surface. This noncontinuous Ph4P+QHSO3 or Na+QHSO3 film does not work efficiently as an EIL in PLEDs. Organic LEDs (OLEDs) utilizing a blue emissive layer, Fa3B (see Figure 1 for chemical structure), also exhibit better performance upon the addition of a quinacridone salt EIL. Figure 7 shows the JV, LV, and LEJ characteristics for the ITO/PEDOT:PSS/Fa3B/Na+QHSO3/Al devices and the Al reference device. The device turn-on bias is reduced from 9.8 to 4.8 V when the Na+QHSO3 EIL is inserted between the emissive layer and the Al cathode. The luminance also increases by 264 times from 21.5 cd/m2 to 5800 cd/m2. The device efficiency reaches 1.8 cd/A, over 3 orders of magnitude improvement as compared to that of the Al control device (4.29 103 cd/A).

IV. CONCLUSION In summary, the optical and charge transport properties of four quinacridone salts (Na+QPSO3, Na+QHSO3, Ph4P+QPSO3, Ph4P+QHSO3) have been characterized. The performance of these materials as EILs in OLEDs was also

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investigated. Increased alkyl chain length leads to a higher degree of aggregation in solution and greater order in the solid state. A higher degree of aggregation is observed for X+QRSO3 in water than in methanol as evidenced in the red shift of the absorption and PL and a factor of 3 drop in the PL quantum yield. A large red shift in the PL (up to 60 nm) and a large drop in the PL quantum yield (up to a factor of 66) is observed for X+QRSO3 films. The bulky counterion, tetraphenylphosphonium, is capable of interfering with molecular packing as evidenced by absorption, photoluminescence, PL quantum yield, and surface morphology. As a consequence, the electron mobilities of the sodium salts are 2 orders of magnitude higher than their tetraphenylphosphonium analogues due to the greater degree of intermolecular charge transport afforded by the smaller size. The charge-transport characteristics of these materials affect the performance of the tetraphenylphosponium materials as EILs in the MEH-PPV LEDs. All four quinacridone salts function effectively as EILs in PLEDs. The turn-on bias is ∼1.9 V, similar to that of the barium cathode device, where there is no barrier for electron injection. Among the four EILs tested, the Na+QHSO3 compound is shown to improve the efficiency of the MEH-PPV devices even better than the commonly used barium cathode in the control device (1.65 cd/A versus 0.85 cd/A). We also demonstrate the versatility of Na+QHSO3 as electron injection materials by testing its function with a blue emissive small molecule. Quinacridone salts are a promising class of electrolytes that could easily find applications in places where CPEs have previously been successful. However, for this class of material, the synthesis is much simpler, they can be easily purified and produced in large quantities.

’ ASSOCIATED CONTENT

bS

Supporting Information. Surface morphology of PLEDs with thick and thin EILs and absorption and PL spectra of Fa3B. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mails: T.-Q.N., [email protected]; F.W., wudl@chem. ucsb.edu.

’ ACKNOWLEDGMENT We thank Dr. Alexander Mikhailovsky for assistance with the measurement of solution and solid-state quantum yields. P.Z. is supported by the DOE-BES (DE-SC0002368), and T.V.P. is supported by the ConvEne IGERT Program (NSF-DGE 0801627). A. G. is supported by the NSF-CAREER award. T.Q. N. thanks the Camille Dreyfus Teacher Scholar Program. ’ REFERENCES (1) (a) Chen, L.; McBranch, D. W.; Wang, H.-L.; Helgeson, R.; Wudl, F.; Whitten, D. C. Proc. Natl. Acad. Sci. U. S. A. 1999, 96, 12287–1229. (b) Wang, D. L.; Gong, X.; Heeger, P. S.; Rininsland, F.; Bazan, G. C.; Heeger, A. J. Proc. Natl. Acad. Sci. U. S. A. 2002, 99, 49–53. (c) Duan, X. .; Liu, L. B.; Feng, F. D.; Wang, S. Acc. Chem. Res. 2010, 43, 260–270. (d) Jin, Y.; Yang, R.; Suh, H.; Woo, H. Y. Macromol. Rapid Commun. 2008, 29, 1398–1402. (e) Liu, B.; Bazan, G. C. Chem. Mater. 2004, 16, 4467–4476. 17538


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