Ambipolar Charge Transport in Polymer Light-Emitting Diodes

Apr 3, 2009 - California 93106;, and Organic Vision Incorporated, 2485 Guenette, Saint Laurent,. Quebec, Canada, H4R 2E9. ReceiVed: December 24, 2008 ...
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7398

J. Phys. Chem. C 2009, 113, 7398–7404

Ambipolar Charge Transport in Polymer Light-Emitting Diodes Xiong Gong,*,† Yali Yang,‡ and Steven Xiao‡ Center for Polymers and Organic Solids, UniVersity of California at Santa Barbara, Santa Barbara, California 93106;, and Organic Vision Incorporated, 2485 Guenette, Saint Laurent, Quebec, Canada, H4R 2E9 ReceiVed: December 24, 2008; ReVised Manuscript ReceiVed: February 04, 2009

Ambipolar charge-transport semiconducting polymer containing both a hole-transporting moiety, triphenylaniline (TPD), and an electron-transporting moiety, oxadiazole (OXD), in the main chain has been synthesized and investigated. OXD units allow an improvement in luminous efficiency (LE) of the polymer light-emitting diodes (PLEDs). One hundred times enhanced LE was observed from PLEDs made by the polymer with OXD units as compared with that PLEDs made by the polymer without OXD units. These results indicate that enhanced LE was attributed to an improvement in the charge balance of electrons and holes inside this ambipolar polymer. 1. Introduction

SCHEME 1: Synthesis Procedure of P-TPD

Polymeric light-emitting diodes (PLEDs) continue to be extensively investigated due to their potential applications in large-area and flexible displays by wet processes, such as screen printing or inkjet printing.1-3 Moreover, it is of great importance for the development of PLEDs used for solid-state lighting application.4,5 However, semiconducting polymers generally show either high hole-transport mobility or high electrontransport mobility. Moreover, semiconducting polymers generally are intrinsically p-type conductors, that is, the hole mobility is consistently much larger than the electron mobility, resulting in the imbalance recombination region to near the metal electrode and therefore in lower device efficiency and stability due to luminescent quenching and metal diffusion into emissive polymers from the metal electrode.6,7 Two approaches have been applied to enhance the luminous efficiency of PLEDs. One is to apply the hole-injection/ transporting layer and the electron-injection/transporting layer to balance the charge carriers. However, the fabrication of multilayer PLEDs by the solution process is usually difficult.5,8 Another approach is to blend small molecules into polymers to facilitate hole/electron-transporting properties. However, phase separation during storage and operation often occur in the blending system due to immiscibility of the different molecules, resulting in a reduction of device operation lifetime.9,10 In order to solve the above problems and achieve high-performance PLEDs, the polymer containing both a hole-transporting moiety (HTM) and an electron-transporting moiety (ETM) is desirable. In the past few years, lots of ambipolar polymers have been studied and reported.11-18 For example, Shu et al. reported a highly efficient blue-light-emitting copolymer with HTM triphenylamine (TPA) and ETM oxadiazole (OXD) at the C-9 position of fluorene (PF-TPA-OXD).11 They also reported another fluorene-based copolymer, PFA-OXD, which possesses bipolar charge-transporting functionalities, through the incorporation of HTM triphenylamine (TPA) into the polymer backbone and the ETM oxadiazole (OXD) group onto the C-9 * To whom correspondence should be addressed. E-mail: xgong@ physics.ucsb.edu. † University of California at Santa Barbara. ‡ Organic Vision Incorporated.

position of fluorene.12 In 2007, instead of triphenylamine (TPA), carbazoles (CAZ) were incorporated to fluorene as HTM and oxadiazole (OXD) as ETM to prepare another blue-lightemitting polyfluorene derivative (PF-CBZ-OXD) by Shu et al.13 Zhang et al. reported a luminescent PPV-type polymer containing triphenylamine (TPA) and oxadiazole (OXD) in the main chain, not incorporated with polyfluorene.14 The small-molecule hybrids with HTM triarylamine (TPA) or carbazole (CAZ)

10.1021/jp811396j CCC: $40.75  2009 American Chemical Society Published on Web 04/03/2009

Ambipolar Charge Transport in PLEDs

J. Phys. Chem. C, Vol. 113, No. 17, 2009 7399

SCHEME 2: Synthesis Procedure of OXD-TPD-PPV and OXD-(TPD)2-PPV

incorporated with ETM oxadiazole (OXD) were synthesized by Kamtekar et al. in 2006.15 Paik et al. published silicon-based PPV-type copolymers containing HTM carbazole (CAZ) and ETM oxadiazole (OXD) by the Heck coupling reaction.15 More ambipolar polymers with different structures were reported by Peng et al. and Xun et al.17,18 We reported high-performance electrophosphorescent PLEDs by end-capped electron-transporting moieties with poly(9,9-dioctylfluorene) (PFO) blended with organometallic compounds.5,19 In addition, ambipolar charge transport in organic/polymeric field-effect transistors (FETs) is great of importance for development complementary technology similar to Si metal oxide semiconductor (SMOS) technology, which has been widely used for various electronics. In the growing field of organic/polymeric FETs and/or organic/polymeric light-emitting FETs, the need to develop materials supports both p-type and n-type charge carriers is evident.20,21 In this study, we present the synthesis and investigation on the charge-injection/transporting properties of novel polymers containing both HTM (TPD) and ETM (OXD) and, furthermore, report the performance of PLEDs made with these novel polymers.

2. Results and Discussion 2.1. Synthesis of Materials. The polymers under investigation are poly(4-butyl-N,N-diphenylaniline) (P-TPD), poly[1,3,4oxadiazole-2,5-diyl-1,4-phenylene-1,2-ethenediyl-1,4-phenylene[(4butylphenyl)imino]-1,4-phenylene-1,2-ethenediyl-1,4phenylene] (OXD-TPD-PPV), and poly[1,3,4-oxadiazole-2,5diyl-1,4-phenylene-1,2-ethenediyl-1,4-phenylene[(4butylphenyl)imino][1,1′-biphenyl]-4,4′-diyl[(4-butylphenyl)imino]1,4-phenylene-1,2-ethenediyl-1,4-phenylene] (OXD-(TPD)2PPV). Their chemical structures and synthesis route are shown in Schemes 1 and 2, and all these polymers are now commercially available from Organic Vision Inc. The conventional Wittig and Wittig-Horner methods were adapted for the synthesis of these polymers. Triethylphosphonium salts instead of tributylphosphonium salts were selected for the monomer.14 The details of the synthesis of these polymers are described in the Experimental Section. 2.2. Absorption and Photoluminescence Spectra. Figure 1a shows the absorption spectra from the thin films cast from toluene solutions. One absorption band was observed from the P-TPD, which is attributed to the π-π* transition of TPD. Two absorption bands were observed from both OXD-TPD-PPV and

7400 J. Phys. Chem. C, Vol. 113, No. 17, 2009

Figure 1. (a) Absorption spectra and (b) PL spectra of thin films of P-TPD, OXD-TPD-PPV, and OXD-(TPD)2-PPV.

Figure 2. Cyclic voltammetry of ferrocence, P-TPD, OXD-TPD-PPV, and OXD-(TPD)2-PPV in acetonitrile solutions of TBAPF6 (0.1 M) at a scan rate of 40 mV/s.

OXD-(TPD)2-PPV. One is attributed to the π-π* transition of TPD; another is attributed to the π-π* transition of PPV.14 The photoluminescence (PL) spectra of the thin films of P-TPD under the optical excitation of 340 nm and PL spectra of the thin films of OXD-TPD-PPV and OXD-(TPD)2-PPV under the optical excitation of 430 nm are shown in Figure 1b. The maximum peak appearing around 515 nm for P-TPD is attributed to the π*-π transition of TPD; the maximum peaks

Gong et al.

Figure 3. Current density versus electric field (J vs E) for (a) “electrononly” devices, Yb/polymers/Ba/Al, and (b) “hole-only” devices, ITO/ polymers/Au.

appeared around 540 and 530 nm for OXD-TPD-PPV and OXD(TPD)2-PPV, respectively. Both peaks are attributed to the π*-π transition of PPV. 2.3. Electrochemistry. The typical cyclic voltammetry (CV) curves are shown in Figure 2. The energy levels of P-TPD, OXD-TPD-PPV, and OXD-(TPD)2-PPV were calculated using Ag/Ag+ as the reference electrode, -4.65 eV, which is calibrated by the ferrocence (FOC) redox system with 4.80 eV. The oxidation potentials were derived from the onset of electrochemical p-doping; the highest occupied molecular orbital (HOMO) energy levels were calculated according to the empirical formula EHOMO ) -([Eonset]ox + 4.6) (eV).22 The onset of oxidation potentials for P-TPD, OXD-TPD-PPV, and OXD(TPD)2-PPV are 0.85, 0.86, and 0.82 V, respectively. Therefore, The HOMO energy levels of P-TPD, OXD-TPD-PPV, and OXD-(TPD)2-PPV are -5.45, -5.46, and -5.42 eV, respectively. The band gaps obtained from the absorption spectra (Figure 1) of P-TPD, OXD-TPD-PPV, and OXD-(TPD)2-PPV were 2.85, 2.53, and 2.48 eV. The lowest unoccupied molecular orbital (LUMO) was -2.60 eV for P-TPD, -2.93 eV for OXDTPD-PPV, and -2.94 eV for OXD-(TPD)2-PPV. 2.4. Charge-Transporting Property. In order to investigate the charge injection into and transport in P-TPD, OXD-TPDPPV, and OXD-(TPD)2-PPV, electron-only and hole-only devices were fabricated with the following structures: Yb/ polymers/Ba/Al and ITO/polymers/Au. Because of the low work functions of the metals used for the electrodes in the electron-

Ambipolar Charge Transport in PLEDs

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Figure 5. Luminous efficiency (LE, cd/A) as a function of current density (J, mA/cm2) from the devices made with P-TPD, OXD-TPDPPV, and OXD-(TPD)2-PPV.

linearity at a voltage of approximately 4.5 V, which is the turnon voltage for the device made by polymers. The deviation from linear at lower voltage is likely originated from a thermionic emission contribution to the current. 24 If the injected charge is tunneling through a triangular barrier at one of the polymers interfaces, the constant k in eq 1 is given by

8πφ /2√2m* 3qp 3

Figure 4. Fowler-Nordheim plot for 100 nm thickness of polymers for (a) “electron-only” devices, Yb/polymers/Ba/Al, and (b) “hole-only” devices, ITO/polymers/Au.

only devices (Yb and Ba/Al) and the high work functions of the metals used for the electrodes in the hole-only devices (ITO and Au), holes are not injected into electron-only devices and electrons are not injected into hole-only devices. 23 Figure 3 shows the current density (J) versus electric field (E) for electron-only and hole-only devices. For the electrononly devices (Figure 3a), the current density increased in the following order: OXD-TPD-PPV > OXD-(TPD)2-PPV > P-TPD. Hence, cooperation of electron-transporting moieties, OXD onto PPV, results in better injection of electrons from low work function metals. On the contrary, for the hole-only devices (Figure 3b), the observed current density increased in the opposite order: P-TPD > OXD-TPD-PPV > OXD-(TPD)2-PPV. Hence, P-TPD has better injection/transportation of holes from high work function metals. Thus, cooperation of electrontransporting molecule, OXD onto PPV, provides a method for fine tuning the charge injection/transport, without significantly altering the bulk electronic properties of the semiconducting polymer. Figure 3 demonstrates that the current densities for both “holeonly” and “electron-only” devices are dependent on the electric field. Such field-dependent behavior is usually associated with tunneling processes. Fowler-Nordheim tunneling theory predicts24

-κ I ∝ F exp F 2

( )

κ)

(2)

Here φ is the barrier height and m* is the effective mass of the holes in polymers. Assuming the effective mass equals the free electron mass and the electric field is constant across the polymers layer, the calculated barrier heights for hole tunneling into “hole-only” and electron tunneling into “electron-only” devices are 0.4-0.6 and 0.2-0.4 eV, respectively. The data displayed in Figure 4a and 4b clearly demonstrate that the holeinjection barrier from ITO into P-TPD is lowered compared with that of OXD-TPD-PPV and OXD-(TPD)2-PPV. Similarly, the electron-injection barrier from metal electrodes into OXDTPD-PPV and OXD-(TPD)2-PPV is lowered compared with that of P-TPD. 2.5. Polymer Light-Emitting Diodes. To exploit the better balancing of charge injection/transport into P-TPD, OXD-TPDPPV, and OXD-(TPD)2-PPV, single-layer PLEDs were fabricated by employing P-TPD, OXD-TPD-PPV, and OXD-(TPD)2PPV as the emissive layer. The device structure is ITO/PEDOT: PSS/polymers/Al. Figure 5 shows the luminous efficiency (cd/ A) as the function of current density. Devices fabricated with OXD-TPD-PPV and OXD-(TPD)2-PPV showed a significant increase in LE over identical devices made with P-TPD. Therefore, increasing electron injection/transport to approach balanced electron and hole currents by using OXD-TPD-PPV and OXD-(TPD)2-PPV as the emissive layer results in improved LE. 3. Conclusions

(1)

where I is the current, F is the electric field strength, and k is a parameter that depends on the barrier shape. Figure 4 shows a plot of ln(I/F2) vs 1/F for both types of devices with a 100 nm thick polymer layer. It can be seen that the plot approaches

An ambipolar charge-transport semiconducting polymer containing both a hole-transporting moiety, triphenylaniline (TPD), and an electron-transporting moiety, oxadiazole (OXD), in the main chain has been synthesized. The optical properties of, electronic properties of, charge injection into, and transport into P-TPD, OXD-TPD-PPV, and OXD-(TPD)2-PPV have been investigated. The results demonstrated that the polymer contain-

7402 J. Phys. Chem. C, Vol. 113, No. 17, 2009 ing both TPD and OXD has ambipolar charge-transport ability. One hundred times enhanced luminous efficiency was observed from PLEDs made by the polymer with OXD units as compared with that PLEDs made by the polymer without OXD unit. These results indicate that enhanced luminous efficiency can be attributed to an improvement in the charge balance of electrons and holes in this ambipolar polymer. 4. Experimental Section 4.1. Synthesis Details. N,N′-Bis(4-butylphenyl)benzidine [INT-448]. A clean 2 L 3-neck round-bottom flask was dried with a hot gun for 5 min and then rinsed with toluene (regular) two times. Then, it was equipped with a condenser on the middle neck and a nitrogen inlet and glass stopper on the side necks. A magnetic stirring bar was placed into the flask, and the whole set up was place on a magnetic stirrer and swept with nitrogen for 30 min. Into the flask, 600 mL of toluene (regular) was poured and degassed with nitrogen bubbles for another 30 min. A 1.80 g amount of palladium acetate (8 mmol, 1/25 equiv) and 8.62 g of (oxidi-2,1-phenylene)-bis-diphenylphosphine (16 mmol, 2/25 equiv) were dissolved in toluene, and the resulting light yellow solution was stirred at room temperature until it turned into a thick and light yellow suspension (about 30 min). To this suspension, 65.67 g of 4-butylaniline (0.44 mol, 2.2 equiv) and 62.40 g of dibromobiphenyl (0.20 mol, 1.0 equiv) were added. A 49.38 g amount of sodium tert-butoxide (0.44 mol, 2.2 equiv) was added, which immediately caused a redbrown mixture. Then, the mixture was heated to reflux. A 1 mL TLC sample of the reaction mixture had to be taken out each hour to monitor the reaction progress. TLC condition: toluene/hexanes (50/50) as developing solvents, primary amine Rf 0.10, secondary amine Rf 0.45, and tertiary amine Rf 0.78. After about 5 h, when TLC showed that the spot of the tertiary amine was bigger and bigger and the spot of the primary amine was smaller and smaller, the reaction was quenched with the pouring of 600 mL of water. The reaction mixture was transferred into a 2 L of separatory funnel, and the aqueous layer was separated and extracted with 3 × 200 mL of toluene. The organic layers were combined, washed with 2 × 200 mL of water, poured into a 4 L of beaker, and acidified with concentrated hydrochloric acid (37%) until pH < 3. The resulting brown mixture was washed with 2 × 200 mL of toluene to remove the byproduct of tertiary amine and then neutralized with 20% NaOH aqueous solution until pH > 10. The resulting brown-purple mixture was extracted with 3 × 200 mL of toluene, and the combined toluene extracts were washed with 2 × 100 mL of water and dried over anhydrous sodium sulfate overnight. The drying agent was removed by filtration, and the solvent filtrate was removed by rotary evaporation. The resulting brown solid residue was poured with 500 mL of ethanol, and the precipitates were filtered and washed with 2 × 100 mL of ethanol. A white flake (66.78 g, 74.4%) was obtained after recrystallization from ethanol. Mp: 172-174 °C. MS (M+): 448. Anal. Calcd for C32H36N2: C, 85.67; H, 8.09; N, 6.25 Found: C, 85.57; H, 8.06; N, 6.37 N,N′-Bis(4-butylphenyl)-N,N′-bis[4-(diethoxymethyl)phenyl)benzidine [INT-805]. A clean and dry 1 L 3-neck round-bottom flask, equipped with an electronic thermometer, was rinsed with anhydrous toluene twice and swept with nitrogen for 30 min. Then, 600 mL of anhydrous toluene was poured and degassed with nitrogen bubbles for another 30 min. A 0.22 g amount of palladium acetate (1.0 mmol, 1/20 equiv) and 0.20 g of tri-tertbutylphosphine (4.0 mmol, 4/20 equiv) were dissolved in toluene, and the resulting yellow solution was stirred at room

Gong et al. temperature until it turned into a thick and light yellow suspension (about 30 min). To this suspension, 8.97 g of N,N′bis(4-butylphenyl)benzidine (INT-448) (20 mmol, 1.0 equiv) and 11.40 g of 4-bromobenzaldehyde diethyl acetal (44 mmol, 2.2 equiv) were added, and the resulting light yellow mixture was heated until it turned into a yellow solution. The solution was added 4.23 g of sodium tert-butoxide (44 mmol, 2.2 equiv), which immediately caused a red solution. The red solution was stirred at 80 °C for 24 h. Then the reaction mixture was allowed to cool down, and the reaction was quenched with 200 mL of brine. The aqueous layer was separated and extracted with 2 × 100 mL of toluene. The toluene layers were combined, washed with 100 mL of brine and 2 × 100 mL of water, and dried over sodium sulfate. The drying agent was removed by filtration, and the solvent was removed by rotary evaporation. The brown tar residue was purified with a silica gel column with petroleum ether/ethyl acetate (4/1) as an eluent. A 15.90 g amount of a yellow tar was collected directly for the next step. TLC condition for the desired product is petroleum ether/ethyl acetate (4/1) as a mixed developing solvent, Rf 0.87, with a green fluorescence. N,N′-Bis(4-butylphenyl)-N,N′-bis(4-formalphenyl)benzidine [INT-656]. In a clean 250 mL one-neck round-bottom flask, 15.90 g of INT-805 (19.75 mmol) was refluxed with 100 mL of 3.0 N HCl overnight. A black cake was observed when the mixture was allowed to cool down to room temperature, and the reaction mixture was extracted with 100 mL of ethyl acetate and 2 × 100 mL of ether. The organic layers were combined, washed with 2.0 N sodium carbonate aqueous solution until no bubbles came out and further with 2 × 100 mL of water, and dried over sodium sulfate overnight. The drying agent was removed by filtration, and the solvent was removed by rotary evaporation. The brown tar residue was purified by a silica gel column with petroleum ether/ethyl acetate (4/1) as an eluent. A 11.05 g amount of yellow oil was collected with a yield of 85.2%. TLC condition for the desired product is petroleum ether/ ethyl acetate (4/1) as a mixed developing solvent, Rf 0.58, with a green fluorescence. 1H NMR (acetone-d6, δ ppm): 9.85 (s, 2H, OdCH-), 7.75 (m, 8H, Ar-H), 7.30-6.90 (m, 16H, Ar-H), 2.65 (t, 4H, J ) 7.73 Hz, -CH2CH2CH2CH3), 1.80-1.30 (m, 8H, -CH2CH2CH2CH3), 0.97 (t, 6H, J ) 7.35 Hz, -CH2CH2CH2CH3). N-(4-Butylphenyl)-N,N-bis(4-formalphenyl)aniline [INT357]. A 1 L 3-neck flask, equipped with a condenser and an electronic thermometer, was swept with nitrogen for 30 min and placed in an ice bath. A 50 mL amount of anhydrous DMF (47.20 g, 640 mmol, 10 equiv) was introduced via a syringe and cooled to 0 °C, and 60 mL of phosphrous oxychloride (100.00 g, 640 mmol, 10 equiv) was dropwise added via a syringe to keep the internal temperature not up to 10 °C. The resulting orange cake was allowed to warm to room temperature and heated to 35 °C until it turned an orange-yellow solution. A light yellow solution of 19.5 g of (4-butylphenyl)diphenylamine (INT-301) (64 mmol, 1.0 equiv) in 600 mL of 1,2dichloroethane was then added, and the resulting red solution was heated to reflux for 48 h (TLC monitoring condition is petroleum ether/ethyl acetate 4/1, Rf 0.71; fluorescence is green). The reaction mixture was slowly poured into a well-stirred ice water (750 mL), and the resulting mixture was stirred for 2 h and extracted with chloroform to make an organic layer of about 900 mL. The organic layer was washed with water until it turned into a clear solution, which was dried over sodium sulfate. The drying agent was removed by filtration, and the solvent was removed by rotary evaporation. The black liquid residue was purified by a silica gel column with petroleum ether/ethyl acetate

Ambipolar Charge Transport in PLEDs (4/1) as an eluent. A 13.91 g amount of yellow oil was obtained after removal of solvents. N,N′-Bis(p-toluoyl)hydrazine [INT268]. A 34.01 g amount of p-toluoyl chloride (220 mmol, 2.2 equiv) was added dropwise to a solution of 10.50 g of hydrazine dihydrochloride (100 mmol, 1.0 equiv) in 44.52 g of triethylamine (440 mmol, 4.4 equiv) and 500 mL of chloroform at room temperature in a 1 L of one-neck round-bottom flask. The resulting mixture was stirred at 50 °C for 10 h and allowed to cool to room temperature and form a white solid precipitate. The white solid was washed with 2 × 200 mL of water and 2 × 200 mL of methanol and collected 44.00 g of the title product with a yield of 82.0%. Mp: 257-259 °C. 1HNMR (DMSO-d6, δ ppm): 10.4 (s, 2H, -NH), 7.8 (d, 4H, J ) 8.12 Hz, Ar-H), 7.3 (d, 4H, J ) 8.08 Hz, Ar-H), 2.3 (s, 6H, -CH3). 2,5-Bis(p-toluoyl)-1,3,4-oxadiazole [INT-250].14 In a 1 L 3-neck round-bottom flask and under nitrogen, a mixture of 8.05 g of N,N′-bis(p-toluoyl) hydrazine (INT-268) (30 mmol) and 250 mL of POCl3 was refluxed for 12 h. The excess POCl3 was then distilled out, and the residue was carefully and slowly poured into water. Filtration and then recrystallization from ethanol of the crude solid product gave 5.94 g of the title compound (79.1%) as a needle-like white crystal. Mp: 164-169 °C. 1H NMR (CDCl3, δ ppm): 8.02 (d, 4H, J ) 8.03 Hz, Ar-H), 7.33 (d, 4H, J ) 8.05 Hz, Ar-H), 2.44 (s, 6H, -CH3). 2,5-Bis(4-bromomethylphenyl)-1,3,4-oxadiazole [INT-408].14 A 1 L 3-neck round-bottom flask was swept with nitrogen for 30 min. A 9.63 g amount of 2,5-bis(p-toluoyl)-1,3,4-oxadiazole (INT-250) (38.5 mmol, 1.0 equiv) and 20.56 g of N-bromosuccinimide (NBS, 115.5 mmol, 3.0 equiv) were well distributed in 350 mL of CCl4, which made a beige suspension. A 1.86 g (7.7 mmol, 1/5 equiv) amount of benzoyl peroxide was added to the suspension as an initiator, and the resulting reaction mixture was heated to reflux for 1 h. The resulting light yellow solution was continued to reflux overnight and finally became an off-white suspension. The suspension was allowed to cool to room temperature. The precipitates were filtered, washed with hot water (2 × 25 mL), dried under suction, and recrystallized from THF. A 4.08 g amount of a beige solid was collected with a yield of 26.0% (TLC conditions, 100% hexane, Rf ) 0.53). 2,5-Bis[(4-(diethylphosphateno)methylphenyl]-1,3,4-oxadiazole [INT-522]. A 250 mL 3-neck round-bottom flask, equipped with an electronic thermometer, was swept with nitrogen for 30 min. A 4.08 g amount of 2,5-bis(4-bromomethylphenyl)1,3,4-oxadiazole (INT-408) (10 mmol) and 50 mL of triethyl phosphite were charged into the flask, and the resulting whitebeige suspension was stirred at 130 °C overnight and became a light yellow solution. The solution was allowed to cool to room temperature and stay in a freezer overnight. The precipitates were filtered, washed with cool hexane (2 × 50 mL), and dried under suction and in the air overnight. A 2.20 g amount of a beige powder was collected with a yield of 42.7% (TLC conditions, 100% THF, Rf ) 0.35). 1H NMR (DMSO-d6, δ ppm): 8.14 (d, 4H, J ) 8.00 Hz, Ar-H), 7.60 (d, 4H, J ) 7.03 Hz, Ar-H), 4.03 (m, 8H, -OCH2CH3), 3.27 (d, 4H, J ) 21.36 Hz, -CH2P), 1.22 (t, 12H, J ) 6.96 Hz, -OCH2CH3). Poly(4-butyl-N,N-diphenylaniline) [Poly-TPD]. A 1 L 3-neck round-bottom flask was dried with a torch under a flow of N2 and then allowed to cool to room temperature. A 600 mL amount of toluene was poured into the flask and degassed with N2 bubbles for another 30 min. A 0.45 g amount of Pd(OAc)2 (2.0 mmol, 2/40 equiv) and 1.62 g of P(t-Bu)3 (8.0 mmol, 8/40 equiv) were added, and the resulting light yellow solution was stirred for 30 min to activate the catalyst. A 12.48 g amount of

J. Phys. Chem. C, Vol. 113, No. 17, 2009 7403 4,4′-dibromobiphenyl (40 mmol, 1.0 equiv), 17.92 g of N,N′bis(4-butylphenyl)benzidine (INT-448) (40 mmol, 2.0 equiv), and 9.22 g of NaOtBu (96 mmol, 2.4 equiv) were successively added, and the resulting mixture, under N2 atmosphere, was heated for 12 h at 85 °C. Then the reaction was allowed to cool to room temperature and quenched with 100 mL of H2O. The resulting mixture was stirred for a while and then totally transferred into a 1 L separatory funnel. The aqueous layer was separated and extracted with 100 mL of toluene. The organic layers were combined, washed with 50 mL of water, and dried over anhydrous MgSO4 overnight. The drying agent was removed by filtration, and the solvent was removed by rotary evaporation. The solid residue was poured into 500 mL of methanol under stirring. The crude polymer was collected by filtration and further purified by Organic Vision Inc. patented technology (U.S. patent 6,894,145) to yield 11.2 g of fibrous solid. The molecular weight of this polymer was analyzed by standard gel permeation chromatography (GPC) techniques, and the weight-average molar mass is around 89 000 with a polydispersity (PDI) of 2.3 against polystyrene standards. Poly[1,3,4-oxadiazole-2,5-diyl-1,4-phenylene-1,2-ethenediyl1,4-phenylene[(4-butylphenyl)imino]-1,4-phenylene-1,2-ethenediyl-1,4-phenylene] [OXD-TPD-PPV]. In a dried 1 L 3-neck flask, 13.05 g of 2,5-bis[(4-(diethylphosphato)methylphenyl)1,3,4-oxadiazole (INT-522) (25.0 mmol, 1.0 equiv) and 8.9 g of N-(4-butylphenyl)-N,N′-bis(4-formalphenyl)aniline (INT-357) (25.0 mmol, 1.0 equiv) were dissolved in 400 mL of THF (regular) under continuous flowing of nitrogen. Into the resulting black solution, 11.2 g of potassium tert-butoxide (50.0 mmol, 2.0 equiv) was added, which immediately caused a yellow suspension. The suspension was stirred overnight at room temperature. The reaction mixture was poured into 2 L of methanol, and the resulting yellow precipitates were stirred briefly and centrifuged. The yellow fine solids were washed with 3 × 100 mL of methanol, 3 × 100 mL of water, and 3 × 100 mL of methanol and dried under suction and then in a vacuum oven overnight at 65 °C to collect the crude polymer of 6.4 g. The collected crude polymer was further purified by Organic Vision Inc. patented technology (U.S. patent 6,894,145) to yield 5.2 g of fibrous solid. The molecular weight of this polymer was analyzed by standard gel permeation chromatography (GPC) techniques, and the weight-average molar mass is around 54 000 with a polydispersity (PDI) of 3.4 against polystyrene standards. Poly[1,3,4-oxadiazole-2,5-diyl-1,4-phenylene-1,2-ethenediyl1,4-phenylene[(4-butylphenyl)imino][1,1′-biphenyl]-4,4′-diyl[(4butylphenyl)imino]-1,4-phenylene-1,2-ethenediyl-1,4-phenylene] [OXD-(TPD)2-PPV]. In a dried 1 L 3-neck flask, 13.05 g of 2,5-bis[(4-(diethylphosphato)methylphenyl)-1,3,4-oxadiazole (INT-522) (25.0 mmol, 1.0 equiv) and 16.4 g of N,N′bis(4-butylphenyl)-N,N′-bis(4-formalphenyl)benzidine (INT656) (25.0 mmol, 1.0 equiv) were dissolved in 500 mL of THF (regular) under continuous flowing of nitrogen. Into the resulting black solution, 11.2 g of potassium tert-butoxide (50.0 mmole, 2.0 equiv) was added, which immediately caused a yellow suspension. The suspension was stirred overnight at room temperature. The reaction mixture was poured into 2 L of methanol, and the resulting yellow precipitates were stirred briefly and centrifuged. The yellow fine solids were washed with 3 × 100 mL of methanol, 3 × 100 mL of water, and 3 × 100 mL of methanol and dried under suction and then in a vacuum oven overnight at 65 °C to collect the crude polymer of 9.2 g. The collected crude polymer was further purified by Organic Vision Inc. patented technology (U.S. patent 6,894,145) to yield 7.1 g of fibrous solid. The molecular weight of this polymer

7404 J. Phys. Chem. C, Vol. 113, No. 17, 2009 was analyzed by standard gel permeation chromatography (GPC) techniques, and the weight-average molar mass is around 57 000 with a polydispersity (PDI) of 2.7 against polystyrene standards. 4.2. Electrochemistry. Electrochemical measurements were performed using cyclic voltammetry (CV) at room temperature in a conventional three-electrode cell with a polymer thin film spin coated onto indium tin oxide (ITO) glass as the working electrode. Pt gauze was used as the counter electrode, and Ag/ Ag+ was used as the reference electrode, with 0.1 M tetra-nbutylammonium hexafluorophosphate (TBAPF6) in acetonitrile as the electrolyte. Scan rates were varied between 50 and 400 mV/s. Ferrocene was used as an internal standard, and its halfwave potential was taken as +0.45 V against Ag/AgCl. 4.3. Spectroscopy Measurement. For optical measurements, thin films of polymers were prepared by spin casting on quartz substrates from toluene (20 mg/mL). All films had approximately the same thickness. The absorption spectra of films were measured on a Shimadzu UV-2401PC UV-vis recording spectrophotometer. PL spectra of films were measured on a Spex Fluoromax-2 spectrometer. 4.4. Device Fabrication and Measurement. For investigation of the charge injection into and transport in polymers, “electron-only” and “hole-only” devices were fabricated with the following structures: Yb/polymers/Ba/Al and ITO/polymers/ Au. Thin films of polymers were prepared by spin casting from toluene (20 mg/mL). The metal electrode, Yb, Ba/Al, and Au were deposited by thermal evaporation at 4 × 10-7 Torr. Single-active-layer configurations PLEDs with poly(3,4ethylene dioxythiophene):poly(styrene sulfonic acid) (PEDOT: PSS) on indium tin oxide (ITO) as the hole-injecting bilayer electrode. The devices configurations are ITO/PEDOT:PSS/ polymers/Al. PEDOT:PSS was first spun cast onto an ITO surface; then the emitting layer was spun cast to a film with a thickness of approximately 100 nm. The Al cathode was deposited through a shadow mask by thermal evaporation at 4 × 10-7 Torr. Device performance was measured inside the drybox. The current-voltage--luminance characteristics were measured using a Keithley 236 source measurement unit. The EL spectra were measured using an Oriel Ocean CCD spectrograph. The luminance was obtained in a forward viewing direction (detector subtended 0.01 sr solid) angle using a calibrated silicon

Gong et al. photodiode.25 The luminous efficiency was converted from measured luminance. References and Notes (1) Burroughes, J. H.; Bradley, D.D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature (London) 1990, 347, 539. (2) Gustafsson, G.; Cao, Y.; Treacy, G. M.; Klavetter, F.; Colaneri, N.; Heeger, A. J. Nature (London) 1992, 357, 477. (3) Heeger, A. J. Angew. Chem., Int. Ed. 2001, 40, 2591. (4) Heeger, A. J. Solid State Commun. 1998, 107, 673. Heeger, A. J. ReV. Modern Phys. 2001, 73, 681. (5) Gong, X.; Wang, S.; Moses, D.; Bazan, G. C.; Heeger, A. J. AdV. Mater. 2005, 17, 2053. (6) Tessler, N.; Harrison, N. T.; Friend, R. H. Appl. Phys. Lett. 1998, 73, 732. (7) Scott, J. C.; Kaufman, J. H.; Brock, P. J.; DiPietro, R.; Salem, J.; Goitia, J. A. J. Appl. Phys. 1996, 79, 2745. (8) Meerholz, K. Nature (London) 2005, 437, 327. (9) Zhan, X.; Liu, Y.; Wu, X.; Wang, S.; Zhu, D. Macromolecules 2002, 35, 2529. (10) Campbell, A. J.; Bradley, D. D. C.; Antoniadis, H. J. Appl. Phys. 2001, 89, 3343. (11) Shu, C.-F.; Dodda, R.; Wu, F.-I.; Liu, M. S.; Jen, A. K.-Y. Macromolecules 2003, 36, 6698. (12) Wu, F.-I.; Shih, P.-I.; Shu, C.-F; Tung, Y.-L.; Chi, Y. Macromolecules 2005, 38, 9028. (13) Shu, C.-F. J. Phym. Sci., Part A: Polym. Chem. 2007, 45, 2925. (14) Zhang, Y.; Hu, Y.; Li, H.; Wang, L.; Jing, X.; Wang, F.; Ma, D. J. Mater. Chem. 2003, 13, 773. (15) Kamtekar, K. T.; Wang, C.; Bettington, S.; Batsanov, A. S.; Perepichka, I. F.; Bryce, M. R.; Ahn, J. H.; Rabinal, M.; Petty, M. C. J. Mater. Chem. 2006, 16, 3823. (16) Paik, K. L.; Baek, N. S.; Kim, H. K.; Lee, J.-H.; Lee, Y. Macromolecules 2002, 35, 6782. (17) Peng, Z.; Ba, Z.; Galvin, M. E. Chem. Mater. 1998, 10, 2086. (18) Xun, S.; Zhou, Q.; Li, H.; Ma, D.; Wang, L.; Jing, X.; Wang, F. J. Phys. Sci., Part A: Polym. Chem. 2008, 46, 1566. (19) Gong, X.; Ma, W.; Ostrowski, J. C.; Bechgaard, K.; Bazan, G. C.; Moses, D.; Heeger, A. J. AdV. Funct. Mater. 2004, 14, 393. (20) Zaumseil, J.; Donley, C.; Kim, L.; Friend, R. H.; Sirringhaus, H. AdV. Mater. 2006, 18, 2708. (21) Swensen, J.; Soci, C.; Heeger, A. J. Appl. Phys. Lett. 2005, 87, 253511. (22) Li, Y. F.; Cao, Y.; Gao, J.; Wang, D.; Yu, G.; Heeger, A. J. Synth. Met. 1999, 99, 243. (23) Parker, I. D. J. Appl. Phys. 1994, 75, 1658. (24) Fowler, R. H.; Nordheim, L. Proc. R. Soc. London Ser. 1928, A 119, 173. (25) Mu¨llen, K., Ed.; Electroluminescence-from Synthesis to DeVices; Wiley-VCH: New York, 2006; p 151.

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