Novel Electroactive and Photoactive Molecular Materials Based on

10786

J. Phys. Chem. B 2005, 109, 10786-10792

Novel Electroactive and Photoactive Molecular Materials Based on Conjugated Donor-Acceptor Structures for Optoelectronic Device Applications Xiaobo Sun, Yunqi Liu,* Xinjun Xu, Chunhe Yang, Gui Yu, Shiyan Chen, Zhehui Zhao, Wenfeng Qiu, Yongfang Li, and Daoben Zhu* Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, P.R. China ReceiVed: February 24, 2005; In Final Form: April 14, 2005

Four donor-acceptor functionalized molecular materials with symmetrical structures have been synthesized and investigated for their use in optoelectronic applications. These π-conjugated molecules consist of one electron-donating moiety, for instance, carbazole, triphenylamine, or phenothiazine at the center, and two acceptors at each side. Introduction of different donor moieties decreases the band gaps allowing a finetuning of the optical and electrical properties. These materials exhibit multifunctional properties, such as a red light-emitting behavior and a large photovoltaic effect. Red organic light-emitting diodes were fabricated in a facile nondoping configuration based on these materials. Saturated red-emission is observed with a CIE of x ) 0.64 and y ) 0.33, and an external quantum efficiency of 0.19%. In addition, our first observation of photovoltaic response in the π-conjugated molecule with donor-acceptor-donor structure is reported. The organic single-component photovoltaic cells were fabricated and characterized. Their open-circuit voltage and short-circuit current density are 1.1 V and 0.07 mA cm-2, respectively. The photovoltaic effect corresponds to the absorption characteristics of the compound and depends on the nature of the electron-donating group.

Introduction Extensive studies have shown that conjugated organic materials exhibit a variety of interesting optical, electrical, photoelectric, and magnetic properties in the solid state.1 The development of organic electroactive and photoactive materials has been greatly progressed due to their potential applications in optoelectronic devices, such as electroluminescence (EL) devices, photovoltaic devices, thin film transistors, and solid-state lasers.2-5 The advantages of organic functional materials over inorganic ones are their ease of processing and the tunability of their properties through a simple chemical modification. Furthermore, the use of functional groups has endowed the molecular materials with unique and interesting optoelectronic properties.6-8 An intramolecular charge-transfer compound is one of the most important molecular materials, which is functionalized by electron-donating (D) and electron-accepting (A) groups through a π-conjugated linker. In the initially excited states, charge transfer and charge separation of these compounds endow them unique optical and electric properties. The molecules with D-π-A structures have attracted much academic and technological research interest during the past few years because of their great potential applications as electroactive and photoactive materials in molecular electronics, such as biochemical fluorescent technology,9 nonlinear optics,10,11 electrogenerated chemiluminescence,12 organic light emitting diodes (OLEDs),13,14 and photovoltaic cells.6,15 The advantage of the donor-acceptor materials is that their physical properties could be easily tuned over a wide range by appropriate chemical modification to the structures of donors or acceptors. Since the first red OLED was fabricated by using the red laser dye, 4-(dicyanomethylene)-2methyl-6-(p-dimethyl-aminostyryl)-4H-pyran (DCM),13 many of * Address correspondence to these authors. E-mail: [email protected].

its analogues have been designed for red emisssion.14 The emission color, fluorescence quantum yield, energy levels, and thermal stability of the red emitters have been varied obviously with changes of donors or acceptor groups. However, these red emitters almost have an asymmetrical structure (D-π-A), and few reports are paid to molecules with symmetrical structures (D-π-A-π-D or A-π-D-π-A).16,17 But some of symmetrical molecule based devices have exhibited very excellent performance.17 Furthermore, it has been found that symmetrical molecules could show large two-photon absorption effects.11,18 In addition, new polymers have been synthesized with covalently linking donor and acceptor in the polymer chain for singlecomponent photovoltaic cells. The covalent bond enables a predefined control over the distance between the donor and acceptor to lessen phase separation.19 The performance of this polymer photovoltaic cell also can be improved by changing the donor or acceptor groups.20 Those results afford us a motivation to design and synthesize new donor-acceptor materials and to probe the structure-functional property relationships. Here, we describe the synthesis and investigation of novel compounds based on symmetrical A-π-D-π-A structures. Carbazole, triphenylamine, and phenothiazine groups were chosen as donors, which have showed excellent thermal and electrochemical stability, electron donating ability, and electrooptical properties in optoelectronic devices.21-23 The strong electronwithdrawing cyano group used as acceptor has been proven to increase electron affinity and facilitate electron injection.24 The electroactive and photoactive properties of the materials were observed and investigated. Pure red nondoping electroluminescence devices were obtained by a facile vacuum vapor deposition with these amorphous red emitters. A single-component photovoltaic cell with the conjugated material as the active layer was fabricated and characterized. To our knowledge, there is

10.1021/jp0509515 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/10/2005

Novel Electroactive and Photoactive Molecular Materials no report on the applications of these acceptor-donor-acceptor functional compounds in the optoelectronic fields, which can exhibit both light-emitting behavior and a photovoltaic effect. Experimental Section Materials. Isophorone, malononitrile, triphenylamine, and 2-ethylhexyl bromide were purchased from Acros and used as received. Carbazole and phenothiazine were obtained from the Beijing Chemical Plant with further purification prior to use. All reactions were performed under a nitrogen atmosphere. Solvents for photophysical measurements were all distilled after dehydration according to conventional methods. Aldehyde-aryl substitutes (compound 1-4) were prepared according to the methods described in the literature.25 Instruments and Measurements. 1H NMR spectra were recorded on a Bruker DMX 300/400 NMR spectrometer. Chemical shift data for each signal were reported in ppm units with tetramethylsilane (TMS) as internal reference, where δ (TMS) ) 0. Mass spectra were obtained on the AEI-MS50MS spectrometer (EI-MS) and Bruker BIFLEXØ spectrometer for matrix assisted laser desorption ionization with time-of-flight mode (MALDI-TOF-MS). Elemental analyses were performed on a Carlo-Erba-1106 instrument. Thermogravimetric analysis (TGA) was carried out with a Perkin-Elmer thermogravimeter (Model TGA7) under a dry nitrogen gas flow at a heating rate of 20 °C min-1. The UV-vis absorption spectra were measured with a Hitachi Model U-3010 spectrometer. The photoluminescence (PL) spectra were recorded on a Hitachi F-4500 fluorescence spectrophotometer. Electrochemical measurements were recorded on a computer-controlled EG&G Potentiostat/Galvanostat model 283 at room temperature. Cyclic voltammetry (CV) measurements were obtained in dried freshly distilled CH2Cl2 solutions containing Bu4NPF6 (0.1 M) as supporting electrolyte with a platinum button working electrodes, a platinum wire counter electrode, and an Ag/AgCl reference electrode. The concentration of the compounds used in the experiments was 1 × 10-3 M. The scan rates were 50 mV s-1. The reference electrode was calibrated to be 0.456 V versus the standard hydrogen electrode with a ferrocene standard solution. The highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) levels were calculated by assuming the energy level of ferrocene/ferrocenium as being -4.8 eV.26 3-Dicyanomethylidene-1,5,5-trimethylcyclohex-1-ene (5). A solution of isophorone (16.5 mL, 110 mmol), malononitrile (6.6 g, 100 mmol), piperidine (1.8 mL, 18.2 mmol), glacial acetic acid (0.40 mL, 7.0 mmol), and acetic anhydride (0.2 g, 2.0 mmol) in N,N-dimethylformamide (55 mL) was stirred at room temperature for 6 h and then refluxed at 120 °C for 4 h under nitrogen atmosphere. After being cooled to room temperature, the reaction mixture was poured into water. The crude solid was collected and washed with brine, then treated with boiling water and filtered. Recrystallization from 2-propanol/ water (8:3, v/v) gave 5 as fine brown needles: 15.2 g, yield 85%; MS (EI+) m/z 186 (M+); 1H NMR (300 MHz, CDCl3) δ [ppm] 1.01 (s, 6H), 2.02 (t, 3H), 2.18 (s, 2H), 2.51 (s, 2H), 6.61 (t, 1H). Elemental analysis calculated [%] for C12H14N2: C, 77.38; H, 7.58; N, 15.04. Found: C, 77.30; H, 7.60; N, 15.32. N-Propyl-3,6-bis[2-(3-dicyanomethylene-5,5-dimethylcyclohex-1-enyl)vinyl]carbazole (PDHC). N-Propyl-3,6-diformylcarbazole (1) (0.265 g, 1.0 mmol) and 5 (0.558 g, 3 mmol) were dissolved in acetonitrile solution (20 mL). Piperidine (20 mol % of the aldehyde) was added as a catalyst and the mixture was refluxed for 10 h. The reaction mixture was cooled and poured into water. The product was extracted into CH2Cl2 and

J. Phys. Chem. B, Vol. 109, No. 21, 2005 10787 the combined extract was dried over anhydrous MgSO4. After the solvent was evaporated under reduced pressure, the residue was purified by column chromatography (silica gel, CH2Cl2/ petroleum ether 3:1, v/v) to afford PDHC as a crimson solid with metallic luster (462 mg, 77%). MS (MALDI-TOF) m/z 601.6 (M+); 1H NMR (400 MHz, CDCl3) δ [ppm] 1.01 (t, 3H), 1.11 (s, 12H), 1.93-1.96 (m, 2H), 2.53 (s, 4H), 2.61 (s, 4H), 4.31 (t, 2H), 6.88 (s, 2H), 7.08 (d, 2H), 7.28 (d, 2H), 7.44 (d, 2H), 7.69 (d, 2H), 8.28(s, 2H); FT-IR (KBr, cm-1) υ 3037, 2960, 2874, 2217 (s, -CN), 1598, 1554, 1483, 1385, 1335, 1215, 1154, 959, 877, 801, 735. Elemental analysis calculated [%] for C41H39N5: C, 81.83; H, 6.53; N, 11.64. Found: C, 81.59; H, 6.80; N, 11.19. N-(2-Ethylhexyl)-3,6-bis[2-(3-dicyanomethylene-5,5-dimethylcyclohex-1-enyl)vinyl]carbazole (EDHC). The compound was prepared in a similar procedure as that used for PDHC from N-(2-ethylhexyl)-3,6-diformylcarbazole (2) and 5 in 72% yield. Column chromatography (silica gel, CH2Cl2/ petroleum ether 2:1, v/v) gave EDHC as a crimson solid with metallic luster. MS (MALDI-TOF) m/z 671.1 (M+); 1H NMR (400 MHz, CDCl3) δ [ppm] 0.87 (t, 3H), 0.95 (t, 3H), 1.13 (s, 12H), 1.28-1.33 (m, 4H), 1.37-1.42 (m, 4H), 2.06-2.09 (m, 1H), 2.55 (s, 4H), 2.61 (s, 4H), 4.19 (t, 2H), 6.89 (s, 2H), 7.08 (d, 2H), 7.28 (d, 2H), 7.41 (d, 2H), 7.69 (d, 2H), 8.28 (s, 2H); FT-IR (KBr, cm-1) υ 3037, 2958, 2870, 2217 (s, -CN), 1719, 1598, 1554, 1483, 1386, 1335, 1192, 1154, 960, 878, 801, 735. Elemental analysis calculated [%] for C46H49N5: C, 82.23; H, 7.35; N, 10.42. Found: C, 81.75; H, 7.33; N, 10.13. 4,4′-Bis[2-(3-dicyanomethylene-5,5-dimethylcyclohex-1enyl)vinyl]triphenylamime (BDHT). The compound was prepared in a similar procedure as that used for PDHC from N,N-bis(p-formylphenyl)phenylamine (3) and 5 in 69% yield. Column chromatography (silica gel, CH2Cl2/petroleum ether 2:1, v/v) gave BDHT as a dark red solid with metallic luster. MS (TOF-EI+) m/z 637 (M+); 1H NMR (400 MHz, CDCl3) δ [ppm] 1.08 (s, 12H), 2.46 (s, 4H), 2.60 (s, 4H), 6.81 (s, 2H), 6.876.91 (d, 2H), 6.99-7.03 (d, 2H), 7.07-7.09 (d, 4H), 7.17 (t, 3H), 7.35 (d, 2H), 7.39-7.42 (d, 4H); FT-IR (KBr, cm-1) υ 3035, 2958, 2863, 2217 (s, -CN), 1591, 1555, 1501, 1393, 1322, 1175, 964, 850, 732. Elemental analysis calculated [%] for C44H39N5: C, 82.86; H, 6.16; N, 10.98. Found: C, 82.96; H, 6.42; N, 10.91. N-Propyl-3,7-bis[2-(3-dicyanomethylene-5,5-dimethylcyclohex-1-enyl)vinyl]phenothiazine (PDHP). The compound was prepared in a similar procedure as that used for PDHC from N-propyl-3,7-diformyl-phenothiazin (4) and 5 in 63% yield. MS (MALDI-TOF) m/z 633.1 (M+); 1H NMR (400 MHz, CDCl3) δ [ppm] 1.03 (t, 3H), 1.07 (s, 12H), 1.84-1.89 (m, 2H), 2.44 (s, 4H), 2.59 (s, 4H), 3.84 (t, 2H), 6.81 (s, 2H), 6.84-6.90 (br, 6H), 7.24-7.29 (m, 4H); FT-IR (KBr, cm-1) υ 3035, 2959, 2870, 2217 (s, -CN), 1605, 1553, 1471, 1394, 1336, 1264, 1196, 1154, 961, 877, 803, 732. Elemental analysis calculated [%] for C41H39N5S: C, 77.69; H, 6.20; N, 11.05. Found: C, 77.49; H, 6.48; N, 10.85. Fabrication and Testing of the OLED. Per-cleaned ITOcoated glass (sheet resistance 30 Ω/0) was used as substrate and an anode. N,N′-Diphenyl-N,N′-bis(1-naphenyl)-1,1′-biphenyl-4,4′-diamine (NPB), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), and tris(8-hydroxyquinoline)aluminum (Alq3) were purchased from Aldrich Chemical Co. and purified by a train sublimation method. The LEDs were fabricated on ITO using a conventional vacuum vapor deposition in a vacuum of 2 × 10-4 Pa. A quartz crystal oscillator placed near the substrate was used to measure the thickness of the thin films, which were

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Sun et al.

SCHEME 1: Synthetic Route to the Main Intermediates and Aim Products

calibrated ex situ with an Ambios Technology XP-2 surface profilometer. The emitting area of the EL devices was about 4 mm2. Brightness and chromaticity coordinates were measured with a Spectra scan PR 650 photometer. Current-voltage (IV) characteristics were measured with a Hewlett-Packard 4140B semiconductor parameter analyzer. All the measurements were performed under ambient atmosphere at room temperature. Fabrication and Testing of the Photovoltaic Cells. The organic layer was prepared simply by means of spin-coating on the per-cleaned ITO glass covered with poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS) by spin coating. PEDOT was purchased and used without further purification. The sandwiched type photovoltaic cells were fabricated after the Al layer was covered on the organic layer through a mask via vacuum deposition. The effective area of the cell was about 12 mm2. The I-V measurements were conducted on a computer-controlled Keithley 236 source measurement system. A xenon lamp with AM1.5 filter was used as the white light source. All the measurements were performed under ambient atmosphere at room temperature. Results and Discussion Synthesis and Characterization of Materials. Scheme 1 describes the synthetic procedures and structures of the main intermediates and the symmetric products. These acceptordonor-acceptor functional materials consist of one of the electron-donating moieties, including carbazole, triphenylamine, or phenothiazine, at the center and two identical electronacceptor moieties, 3-dicyanomethylidene-1,5,5-trimethylcyclohex-1-ene (5), at each end. The electron-acceptor 5 has only one π-electron conjugation path and consequently only one methyl is active for participation in the next condensation reaction with a donor-containing aldehyde,14b,d so the symmetrical target products could be synthesized by a facile process with high purity and yield. The key intermediates, aryl dialdehyde monomers (1-4), were directly obtained by the Vilsmeier reaction under a large excess of POCl3/DMF (∼10 equiv) in good yield.25 The target compounds were synthesized by

Knoevenagel condensation reaction of 1 mol of desired aryl aldehyde with 3 equiv of 5 with good yield and high purity. Quantities in the range of several grams of four compounds have been obtained, and were fully characterized by 1H NMR spectra, MS, and element analysis. The results were consistent with proposed structures. They could dissolve in normal organic solvents, such as toluene, chloroform, and THF, etc. The synthesis and purification procedures are simple and most of the reagents are cheap. This is a significant advantage for commercial applications, where minimizing production cost is important. Furthermore, symmetrical structure will benefit to increase thermal stabilities of the materials.27 The thermal properties of these new materials were characterized by TGA measurements. Their thermal decomposition temperatures (Td) are all around 390 °C (see Table 1), indicating that the symmetrical donor-acceptor materials possess excellent thermal stability. This is a necessary feature for functional materials for applications in thin film molecular devices. Optical Properties. The solution-phase UV-vis absorption and PL spectra of the four compounds were recorded at room temperature. Figure 1 shows the absorption and PL spectra of the compounds in chloroform. We find that photophysical properties of these compounds could be easily tuned by changing the donor groups with different donating abilities. The optical data are listed in Table 1. These new compounds all show multiple absorption bands. Both of the compounds PDHC and EDHC have three bands centered at about 303, 355-374, and 469-487 nm, which may be deduced to carbazole moiety, π-π* transition, and charge-transfer transition, respectively. The compounds BDHT and PDHP have two bands centered at about 354-375 and 501-523 nm, which correspond to π-π* transition and charge-transfer transition. Consistent with the predicted trend, the absorption maxima of the compounds exhibit large substantial red shift with different donor groups, which could be explained by the fact that the donor groups have different electron-donating abilities, i.e., carbazole < triphenylamine < phenothiazine. This suggests a significant intramolecular charge-transfer behavior between the donors and accep-

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TABLE 1: Thermal and Optical Properties of the Compounds compd

Td [°C]

toluene

λmaxabs [nm] CH2Cl2

PDHC

399

EDHC

391

BDHT

389

PDHP

383

303 355 469 304 375 473 354 505 371 501

303 356 478 304 364 482 357 520 373 510

CHCl3 303 375 486 303 374 487 357 525 375 523

toluene

λmaxem [nm] CH2Cl2

3.1 × 104

550

580

590

3.0 × 104

547

581

591

4.7 × 104

583

636

644

4.2 × 104

654

702

720

 (M-1 cm-1) CHCl3

CHCl3

TABLE 2: Electrochemical Potentials, Energy Levels, and Band Gap of the Compounds in CH2Cl2 Solution

tors. In the fluorescence spectra, the emission peaks are red shifted in the same order PDHC e EDHC < BDHT < PDHP. Furthermore, the red shift of emission peaks is even greater than that of the absorption maxima, which shows that the intramolecular charge-transfer effect enhances the electron-donating ability of the aromatic donor group. Correspondingly, the band gap of the dye with the push-pull system becomes small.28 The relatively large Stoke shifts of these compounds indicate that the intramolecular charge-transfer molecules undergo structural reorientation. In different solvents, the absorption and emission spectra of four compounds all exhibit positive solvatochromism with polarity increasing (see Table 1). For instance, the absorption maxima of PDHC are shifted from 469 to 486 nm, and the emission peaks of PDHC are dramatically shifted from 550 to 590 nm with the solvents changed from toluene to CHCl3. The red shift in polar solvents indicates that the compound exhibits charge-transfer character both in the ground state and in the excited state. Any solvent stabilization of the ground state is revealed by a shift in the absorption spectrum. Similarly, the solvent stabilization of the excited state is revealed by a shift in the emission spectrum. A larger solvent-induced shift in emission spectra as compared to the shift in absorption spectra was observed. This positive solvatochromism indicates that the stabilization of the excited state is more than that of the ground state by polar solvents, due to stronger interaction between molecules in the excited state and surrounding solvents.29 Electrochemical Properties. Cyclic voltammetry was employed to investigate redox behavior. The four compounds are electrochemically stable. Figure 2 shows the cyclic voltammograms in CHCl3. There are a couple of reversible oxidation peaks and a single irreversible reduction peak, observed for all the compounds under CV conditions. The reversible oxidation process is probably attributed to the donor group and the irreversible reduction to the acceptor group. This fact indicates

that these compounds may exhibit a p-type semiconducting property.30 The electrochemical data and band gaps are listed in Table 2. With the different donors, the HOMO levels of four compounds show a more positive reduction (from -5.43 to -5.09 eV), but their LUMO levels are almost the same. The band gaps also become small with the increase of electron donating ability. These results are consistent with their PL spectra. All the HOMO levels of these compounds are higher than that of NPB, one of the most widely used hole-transport materials.31 The oxidation process in an electrochemical cell is closely related to hole-injection from the HOMO level of material. So, these materials may provide for more facile holeinjection and hole-transport when they are used as active materials in LED. Electroluminescence Properties. The new donor-acceptor compounds can be deposited as pristine thin film. The red light emitting devices (ITO/NPB/PDHC, EDHC, or BDHT/BCP/Alq3/ Al) were fabricated by a sequential thermal deposition. To achieve high performance in organic EL devices, it is necessary to attain hole-electron transport balance and effective recombination of holes and electrons in the emitting layer. So this type of configure was used to study the properties of EL devices,

Figure 1. Normalized UV-vis absorption and photoluminescence spectra of the compounds in dilute solution at room temperature.

Figure 2. Cyclic voltammograms of the four compounds in CH2Cl2 solution.

compd

Eoxa

PDHC EDHC BDHT PDHP

1.08 1.07 0.91 0.74

Ered

HOMO [eV]

LUMO [eV]

Egel [eV]

Egopt [eV]

-0.88 -0.89 -0.86 -0.85

-5.42 -5.41 -5.26 -5.09

-3.46 -3.45 -3.49 -3.50

1.97 1.96 1.77 1.59

1.85 1.84 1.73 1.65

a

a E and E ox red data are in units of mV with reference to the ferrocene standard. Egel Electrochemical band gap is derived from the difference between the LOMU and HOMO levels. Egopt optical band gap is derived from the onset of the absorption peaks (Eg ) 1240/λonset).

10790 J. Phys. Chem. B, Vol. 109, No. 21, 2005

Figure 3. Proposed energy level diagram of a red LED with the PDHC as the emitting layer (the HOMO and LUMO levels of NPB, BCP, and Alq3 are rooted in ref 31).

Sun et al.

Figure 5. EL spectra of the double layer device ITO/NPB/PDHC/ BCP/Alq3/Al.

TABLE 3: EL Data of the Devices ITO/NPB/PDHC (or EDHC, BDHT)/BCP/Alq3/Al λmax of EL spectra [nm] max ext quant effic [%] CIE [x, y]

Figure 4. Current density-voltage-brightness characteristics of the devices ITO/NPB/PDHC (or EDHC, BDHT)/BCP/Alq3/Al.

in which the red emitters, NPB, BCP, and Alq3 were used as the emitting layer, electron-blocking layer, hole-blocking layer, and electron-transporting layer, respectively. The proposed energy level diagram of a red LED provides further insight into the roles of four functional materials playing in the device (Figure 3). In this configuration, hole and electron are all restricted in the emission layer, because of the large difference of HOMO levels between the emitters and BCP, and the large difference of LUMO levels between the emitters and NPB. The current density-voltage-brightness characteristics of devices are shown in Figure 4. Compared to PDHC and EDHC as the emitting layer, the device with BDHT has lower turn-on voltage, which is due to its higher HOMO level. Though PDHC and EDHC have a similar donor group, the performance of the device based on EDHC is better than that of the device based on PDHC. The steric hindrance of EDHC is enhanced by the 2-ethylhexyl-substituted carbazole group, so the property in the solid state has been improved. For all the devices, the EL spectra resemble the corresponding solid-state PL spectra. This suggests that the emission originates from the single excited states of new emitters formed by recombination of injected electrons and holes. With PDHC as the emitting layer, the device showed a narrow-peak emission at 629 nm with a full width at halfmaximum of 73 nm (Figure 5), which was narrower than those of doping14c,16a,32 or nondoping16b devices from donor-acceptor analogues reported. When the radiometric EL spectrum was converted into chromaticity coordinates on the CIE 1931 diagram, a most saturated red emission from the nondoping devices is obtained (x ) 0.64, y ) 0.33), which is almost the same as that (x ) 0.64, y ) 0.33) of National Television System Committee (NTSC) standard for red color. The nonoptimized performance characteristics of the devices for the three com-

PDHC

EDHC

BDHT

629 0.16 0.64, 0.33

620 0.12 0.64, 0.35

672 0.19 0.62, 0.33

pounds are all listed in Table 3. Although the device has relatively lower external quantum efficiency (0.16%) with the CIE (x ) 0.64, y ) 0.33), the results are still better than or comparable with those of some red doping devices or nondoping devices at the same CIE reported,16 which were fabricated by using symmetrical DCM analogues as emitting layers. So these A-π-D-π-A compounds would be a kind of potential material for achieving pure red OLEDs. Characteristics of a Photovoltaic Device. In contrast, the device based on BDHT as an emissive layer exhibited moderate efficiency, but showed lower luminescence. However, the BDHT-based device exhibited a clear increase in reverse current upon an increase in illumination, indicating an occurrence of photovoltaic effect. To study this abnormal phenomenon, novel single-component photovoltaic cells were fabricated. The devices with the four donor-acceptor compounds as active layers were fabricated with a simple structure (ITO/PEDOT:PSS/active layer/Al). It was very surprising that the large photovoltaic effect was only observed from the device based on BDHT. But no such effect (or negligibly small) was found in the devices based on the other three compounds. Comparing the structures of the four compounds, the donor group is the only difference. Thus the large photovoltaic effect observed is relative to the donor group in this donor-acceptor system. An important issue for improving the efficiency of photovoltaic devices is the selection and development of new molecules having good overlap of their absorption spectrum with the terrestrial solar emission spectrum and high absorption coefficients.33 In view of the absorption spectrum of BDHT in the solid state, there is a good overlap of its broad optical absorption with visible solar emission (Figure 6). Moreover, the extinction coefficients of BDHT are higher than others. Therefore, BDHT may be a good material for singlecomponent photovoltaic cells. The donor and acceptor part are linked by covalent bonds, so the problem with phase separation is avoided in this device. The I-V characteristics of the BDHT-based devices were measured in the dark and under simulated AM1.5 conditions (Figure 7). Under simulated solar illumination (80 mW cm-2) through the transparent ITO anode, an open circuit voltage (VOC) of 1.14 V and a short circuit current (ISC) of 0.07 mA cm-2 have been acquired. From the I-V curve, a fill factor (FF )

Novel Electroactive and Photoactive Molecular Materials

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Figure 6. UV-vis absorption spectrum of BDHT in thin film.

each side. They possess good solubility, thermal stability, and electrochemical stability. These materials could exhibit multifunctional properties, a red light-emitting property, and a large photovoltaic effect. Saturated red-emission with the CIE (0.64, 0.33) is observed from the nondoping devices. Our first observation on photovoltaic response in the π-conjugated donor-acceptor molecular series is reported with the singlecomponent photovoltaic cells. Its open-circuit voltage is 1.1 V, and the short-circuit current density is 0.07 mA cm-2. The clear photovoltaic effect corresponds to the absorption characteristics of the compounds and depends on the nature of the electrondonating groups. Intensive studies on electroluminescence and photovoltaic properties by optimizing the configurations of the devices and modifications of the structures of these donoracceptor materials are in progress. Acknowledgment. This research is financially supported by the National Natural Science Foundation of China (NSFC), the Major State Basic Research Development Program, and the Chinese Academy of Sciences. References and Notes

Figure 7. Linear and semilogarithmic (inset) representations of the current density-voltage characteristics of the cell ITO/PFDOT/BDHT/ Al. The continuous line represents data obtained under 70 mW cm-2 white light illumination, while the dashed line plots data measured in the dark.

24%) and an energy conversion efficiency (η ) 0.03%) were calculated. These results are better than or comparable with those of single-component photovoltaic cells reported with conjugated polymers (ISC ) 0.008-0.012 mA cm-2, VOC ) 0.97-1.20 V, and FF ) 25-26%)19 and phthalocyanine-fullerene dyad (VOC ) 0.32 V, ISC ) 0.2 mA cm-2, FF ) 26%, and η ) 0.02%).15 The high VOC may be contributed to the level of band-bending.34 The low ISC is possibly relative to poor transport characteristics, because the initial efficient charge generation may be inferred from fluorescence quenching in the films.19 Though the mechanism of the photovoltaic effect is not fully understood yet, the electronic property of the electron-donating group is believed to play a crucial role in exhibiting a photovoltaic effect. The strong electron-acceptor coupled with the donor group enhances and stabilizes the charge separation process, then the dissociation and migration of charges to the opposite electrodes lead to the photocurrent.6 The photovoltaic characteristics can be further improved by using a lower work function cathode and changing the layer thickness of the organic layer, etc. The modifications of structures of these donor-acceptor materials and further study of the structure-property relationship are under investigation for photoelectric applications. Conclusions This study reports the synthesis and characterization of electroactive and photoactive molecular materials bearing one of the electron-donating moieties including carbazole, triphenylamine or phenothiazine at the center and two acceptors at

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