Solution-Processable Multiarmed Organic Molecules Containing

Jan 21, 2009 - Solution-Processable Multiarmed Organic Molecules Containing Triphenylamine and DCM. Moieties: Synthesis and Photovoltaic Properties...
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J. Phys. Chem. C 2009, 113, 2636–2642

Solution-Processable Multiarmed Organic Molecules Containing Triphenylamine and DCM Moieties: Synthesis and Photovoltaic Properties Guangjin Zhao,†,§ Guanglong Wu,† Chang He,† Fu-Quan Bai,‡ Hongxia Xi,†,§ Hong-Xing Zhang,‡ and Yongfang Li*,† Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China, State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin UniVersity, Changchun 130023, China, and Graduate UniVersity of Chinese Academy of Sciences, Beijing 100039, China ReceiVed: NoVember 6, 2008; ReVised Manuscript ReceiVed: December 13, 2008

Two new solution-processable multiarmed organic molecules, biarmed B(TPA-DCM-TPA) and triarmed T(TPA-DCM-TPA), were synthesized by a Heck reaction and characterized by UV-vis absorption spectroscopy and electrochemical cyclic voltammetry. The two molecules possess a D-π-A-π-D structure with triphenylamine (TPA) as donor (D) unit and 2-{2,6-bis-[2-(4-styryl)-vinyl]-pyran-4-ylidene}malononitrile (DCM) as acceptor (A) unit. The geometry and electronic properties of the molecules in ground-state as well as the absorption spectroscopic properties on the basis of the optimized ground-state structures were investigated by theoretical calculations. B(TPA-DCM-TPA) and T(TPA-DCM-TPA) films show broad and strong absorption band in the wavelength range of 300∼709 and 300∼755 nm with the lower band gap of 1.75 and 1.65 eV, respectively. B(TPA-DCM-TPA) was used as electron donor to fabricate organic solar cells (OSCs) with methanofullerene [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as electron acceptor. The OSC with a structure of ITO/PEDOT:PSS/B(TPA-DCM-TPA):PCBM (1:2)/Ca/Al delivered a power conversion efficiency of 0.73% under the illumination of AM 1.5, 100 mW/cm2. 1. Introduction There has been a great deal of research on bulk-heterojunction (BHJ) polymer solar cells (PSCs) composed of polymer donors and soluble fullerene acceptors, for its advantages of easy fabrication, low cost, and capability to fabricate flexible devices.1-4 By the efforts on polymer photovoltaic materials and device physics, the power conversion efficiency (PCE) of the PSCs has reached the values of 4-6%.5-9 And the PCEs of the PSCs are predicted to have an efficiency of 10% based on theoretical models.10 So there is still a long way to go before approaching the theoretical efficiency. In a parallel effort, organic solar cells (OSCs) based on small molecules with vacuum-evaporating interpenetrating multilayer structure have also reached PCEs as high as 5-6%.11-13 Most importantly, these types of small molecules have well-defined molecular structures, definite molecular weights without any distribution, and strictly controlled purity by column chromatography or sublimation. These are the advantages of the conjugated small organic molecules over the conjugated polymers which always suffer from impurity, broad molecular weight distributions, and batch to batch variations. However, the fabrication of OSCs associated with vacuum evaporation is much more expensive and difficult than the fabrication of PSCs by a solution-processable method. Therefore, organic small molecules with good solution-processibility could be promising photovoltaic materials for solution-processable OSCs. * To whom correspondence should be addressed. E-mail: [email protected]. Tel: 86-10-62536989. Fax: 86- 10-62559373. † Chinese Academy of Sciences. ‡ Jilin University. § Graduate University of Chinese Academy of Sciences.

For past few years, solution-processable OSCs based on dendrimers,14-16 oligomers,17-22 molecules with D-A structure,23-30 liquid crystalline molecules,31,32 and other organic molecules33-37 have attracted impressive interests. And the PCEs of the OSCs have approached 1-2%.14,20a,21,26,34,35 These efficiencies are still low when compared to either vacuum evaporated OSCs or solution-processable PSCs. So there is large room for further design and synthesis of high efficiency solutionprocessable organic photovoltaic materials. Organic glasses derived from triphenylamine (TPA) derivatives have been widely investigated and developed as electroluminescent and hole-transporting materials.38,39 TPA-based compounds can be viewed as three-dimensional (3D) systems, which can lead to amorphous materials with isotropic optical and charge-transport properties. So organic small molecules based on a 3D system are viewed as one of the promising photovoltaic materials40 for OSCs. Due to these advantages for 3D TPAbased compounds, many research groups have designed and synthesized such materials for photovoltaic applications.24-26,28,29 Here we report two novel solution-processable TPA-containing organic molecules B(TPA-DCM-TPA) and T(TPA-DCMTPA) (Scheme 1) for OSCs. The compounds include two and three D-π-A-π-D arms, respectively, with TPA as donor units (D), 2-{2,6-bis-[2-(4-styryl)-vinyl]-pyran-4-ylidene}malononitrile (DCM) as acceptor units (A). The consideration of the molecular design is that (1) the spatially nonplanar TPA group is used as the electron-rich moiety and DCM containing 2-pyran-4-ylidenemalononitrile (PM) derivatives is used as the electron-deficient moiety for getting broad absorption and suitable electronic energy levels for appropriate donor materials. (2) The combination of a 3D TPA group with linear π-conjugated systems DCM could avoid the anisotropic electronic properties and molecular orientation to the substrate, which are

10.1021/jp809795p CCC: $40.75  2009 American Chemical Society Published on Web 01/21/2009

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SCHEME 1: Synthetic Routes of B(TPA-DCM-TPA) and T(TPA-DCM-TPA)

detrimental for solar cells because they strongly reduce the absorption cross section for the incident light as well as the efficiency of charge transport through the cell thickness.41,42 B(TPA-DCM-TPA) and T(TPA-DCM-TPA) films show broad absorption bands covering from 350 to 709 and 350 to 755 nm, respectively. The OSCs based on B(TPA-DCM-TPA) were fabricated by spin-coating the blend solution of B(TPA-DCMTPA) as donor and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as acceptor. (We did not report the photovoltaic property of T(TPA-DCM-TPA) here, due to the poor filmforming property of the molecule blended with PCBM.) The PCE of the OSC reached 0.73% under the illumination of AM 1.5, 100 mW/cm2, which is one of the higher values reported so far for the solution-processable OSCs.

2. Experimental Section 2.1. Computational Methodology. The density functional theory (DFT) with Becke’s three-parameter functional and the Lee-Yang-Parr functional (B3LYP)43-46 and 6-31G* basis set was employed to investigate the geometry and electronic properties of B(TPA-DCM-TPA) and T(TPA-DCM-TPA) in ground state. No symmetry constraints were imposed during the optimization process. On the basis of the optimized structures in the ground state, the spectroscopic properties related to the absorption for the molecules were calculated by the semiempirical ZINDO method (Zerner’s spectroscopic parametrization of the intermediate neglect of differential overlap Hamiltonian).47 All of the calculations were accomplished by using the Gaussian 0348 program package.

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Zhao et al.

Figure 1. Calculated HOMO and LUMO energy levels and surface plots for B(TPA-DCM-TPA) and T(TPA-DCM-TPA) at B3LYP/6-31G* level.

2.2. Materials. All chemicals were purchased from Aldrich or Acros Chemical Co. and were used without further purification, except that DMF was freshly distilled prior to use. PCBM was purchased from American Dye Source (ADS) Inc. (purity >99.0%). 2-{2,6-bis-[2-(4-bromo-phenyl)-vinyl]-pyran-4-ylidene}-malononitrile (1),49 biphenyl-(4-vinylphenyl)-amine (2), and N,N,N-tri(4-ethenylphenyl)-aniline (4)50 were synthesized according to the procedures described in the literatures. The synthesis routes of B(TPA-DCM-TPA) and T(TPA-DCM-TPA) are shown in Scheme 1. 2.3. Synthesis of Compound 3. Under argon flow, 4.0 g (8.2 mmol) of 1, 1.5 g (5.5 mmol) of 2, 20.0 mg of Pd (OAc)2, 422 mg (1.3 mmol) of n-Bu4NBr, and 6.7 g (81.7 mmol) of NaOAc were added to a two-necked flask with condenser, balloon, and septum, and then 100 mL of degassed DMF was injected by syringe. The above mixture solution was heated to 100 °C and kept at the temperature for 24 h. Then the solution was cooled to room temperature and poured into distilled water. The precipitate was filtered, washed with water, and dissolved in dichloromethane dried over MgSO4. After column separation (SiO2, petroleum ether/methane chloride 1:1), 2.6 g of 3 was obtained with a yield of 48.1%. 1H NMR (400 MHz, CDCl3, δ (ppm)): 7.59 (2H, d), 7.54 (4H, d), 7.48 (2H, d), 7.45 (2H, d), 7.40 (2H, d), 7.29 (2H, d), 7.12 (4H, d), 7.06 (4H, m), 6.94 (2H, d), 6.76 (2H, d), 6.71 (2H, s). MALDI-TOF MS: 695.2. 2.4. Synthesis of B(TPA-DCM-TPA) and T(TPA-DCMTPA). Under Argon flow, 517 g (0.7 mmol) of 3, 72 mg (0.2 mmol) of 4, 15.0 mg of Pd (OAc)2, 34 mg (0.1 mmol) of n-Bu4NBr, and 541 mg (6.7 mmol) of NaOAc were added to a two-necked flask with condenser, balloon, and septum, and then 20 mL of degassed DMF was injected by syringe. The above mixture solution was heated to 100 °C and kept at the temperature for 24 h. Then the solution was cooled to room temperature and poured into distilled water. The precipitate was filtered, washed with water, and dissolved in dichloro-

Figure 2. Simulated absorption spectra with Gaussian curves based on the data calculated under the semiempirical ZINDO method for B(TPA-DCM-TPA) and T(TPA-DCM-TPA).

methane,dried over MgSO4. After column separation (SiO2, petroleum ether/methane chloride 1:1), 103 mg of B(TPA-DCMTPA) and 90 mg of T(TPA- DCM-TPA) were obtained with yields of 31.6% and 18.6%, respectively. B(TPA- DCM-TPA): 1 H NMR (400 MHz, CDCl3, δ (ppm)): 7.58 (12H, d), 7.53 (4H, d), 7.44 (4H, d), 7.40 (4H, d), 7.35 (2H, d), 7.28 (6H, d), 7.17 (4H, m), 7.12 (12H, m), 7.06 (6H, d), 7.03 (4H, d), 6.95 (1H, d), 6.77 (4H, d), 6.71 (4H, s), 5.70 (1H, d), 5.22 (1H, d) MALDI-TOF MS: 1554.7. T(TPA-DCM-TPA): 1H NMR (400 MHz, CDCl3, δ (ppm)): 7.55-7.58 (48H, d), 7.40 (18H, d), 7.30 (2H, d), 7.17 (6H, d), 7.12 (18H, d), 7.06 (18H, m), 6.94 (3H, d), 6.76 (6H, d), 6.71 (6H, s), 5.75 (3H, d), 5.48 (3H, d), 5.15 (6H, m). MALDI-TOF MS: 2170.5. 2.5. Instruments and Measurements. 1H NMR spectra were measured on a Bruker DMX-400 spectrometer. Absorption spectra were taken on a Hitachi U-3010 UV-vis spectrophotometer. The electrochemical cyclic voltammetry was conducted

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Figure 4. Cyclic voltammograms of B(TPA-DCM-TPA) and T(TPADCM-TPA) films on a platinum electrode in 0.1 mol/L Bu4NPF6.

TABLE 2: Electrochemical Potentials and Energy Levels of B(TPA-DCM-TPA) and T(TPA-DCM-TPA)

B(TPA-DCM-TPA) T(TPA-DCM-TPA)

Figure 3. UV-vis absorption spectra of (a) B(TPA-DCM-TPA) and T(TPA-DCM-TPA) in chloroform solution and (b) B(TPA-DCM-TPA) and T(TPA-DCM-TPA) films.

TABLE 1: UV-vis Absorption Data for B(TPA-DCM-TPA) and T(TPA-DCM-TPA) UV-vis absorption spectra solution

ox Eonset (V)

red Eonset (V)

EHOMO (eV)

ELUMO (eV)

Eec g (eV)

0.57 0.62

-1.24 -1.09

-5.28 -5.33

-3.47 -3.62

1.81 1.71

weight ratio of 1:2 was filtered through a 0.45 µm filter and spin coated on top of the PEDOT:PSS layer. The thickness of the active layer was about 80 nm as measured by Ambios Technology XP-2 profilometer. The negative electrode consisted of Ca (∼10 nm) capped with Al (∼100 nm). The thermal evaporation was done under a shadow mask in a base pressure of ca. 10-4 Pa. The active area of a device was 4-6 mm2. The current-voltage (I-V) measurement of the devices was conducted on a computer-controlled Keithley 236 Source Measure Unit. A xenon lamp coupled with AM 1.5 solar spectrum filters was used as light source, and the optical power at the sample was around 100 mW/cm2. The input photon to converted current efficiency (IPCE) was measured by Stanford Research Systems model SR830 DSP lock-in amplifier coupled with WDG3

film

compound

λmax (nm)

λmax (nm)

B(TPA-DCM-TPA) T(TPA-DCM-TPA)

308,406,476 310,407,481

317,420,485 312,423,494

λonset (nm) Eopt g (eV) 709 755

1.75 1.64

on a Zahner IM6e electrochemical workstation, in 0.1 mol/L tetrabutylammonium hexafluorophosphate (Bu4NPF6) acetonitrile solution, with a platinum disk coated with the sample film as the working electrode, a Pt wire as counter electrode and Ag/Ag+ (0.1 M) as the reference electrode. 2.6. Fabrication and Characterization of OSC Devices. The OSCs were fabricated in the configuration of the traditional sandwich structure with an indium tin oxide (ITO) glass positive electrode and a metal negative electrode. Patterned ITO glass with a sheet resistance of 30 Ω/0 was purchased from CSG HOLDING Co., Ltd. (China). The ITO glass was cleaned by sequential ultrasonic treatment in detergent, deionized water, acetone, and isopropanol and then treated in an ultraviolet-ozone chamber for 0.5 h (Ultraviolet Ozone Cleaner, Jelight Company, USA). Then PEDOT:PSS (poly (3,4-ethylene dioxythiophene): poly(styrene sulfonate)) (Baytron P VP AI 4083 H. C. Stark Germany) was filtered through a 0.45 µm filter and spin coated at 1000 rpm for 60 s on top of the ITO. Subsequently, the PEDOT:PSS film was baked at 150 °C for 30 min in the air. The thickness of the film was around 60 nm. The B(TPA-DCMTPA) and PCBM blend solution in 1,2-dichlorobenzene with a

Figure 5. I-V curves of the OSC based on B(TPA-DCM-TPA):PCBM (1:2 w/w) under the illumination of AM1.5, 100 mW/cm2.

TABLE 3: Photovoltaic Performance of the OSCs Based on the Blend of B(TPA-DCM-TPA) and PCBM (1:2 w/w) under the Illumination of AM1.5, 100 mW/cm2 negative electrode Al Ca/Al

Voc (V)

Isc (mA/cm2)

FF (%)

PCE (%)

0.64 0.77

2.49 2.37

41 40

0.65 0.73

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Figure 6. IPCE and absorption spectra of the OSCs based on the blend of B(TPA-DCM-TPA) and PCBM film.

monochromator and 500 W xenon lamp. The light intensity at each wavelength was calibrated with a standard single-crystal Si photovoltaic cell. The atomic force microscopy (AFM) measurement of the surface morphology of samples was conducted on a Nanoscope III (DI, USA) in contacting mode with 5 µm scanners. 3. Results and Discussion 3.1. Theoretical Calculations. Predicting the behavior of both the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy levels for new compounds is crucial to make a rational design of organic photovoltaic materials. Recently, several research groups have successfully predicted the HOMO and LUMO energy levels for some kinds of photovoltaic materials by theoretical calculations.21,26,28a,51 In the present work, we estimated the HOMO and LUMO energy levels for B(TPA-DCM-TPA) and T(TPADCM-TPA) by using the DFT-B3LYP method and the 6-31G* basis set. DFT-B3LYP/6-31G* has been found to be an accurate formalism for calculating the structural and optical properties of many molecular systems.52,53 Figure 1 gives the geometry and the HOMO and LUMO surface plots of the optimized ground-state structures for B(TPADCM-TPA) and T(TPA-DCM-TPA). Core and terminal TPA groups of the molecules show 3D structure, and DCM linear structure between the two TPA groups places the coplanar arm. The HOMO of the compounds contains terminal TPA character, as well as the following 2,4-bis-ethenyl-phenyl group on one of the arms, whereas the LUMO is located predominantly on the DCM group on two of the arms. The calculated HOMO and LUMO energies for B(TPA-DCM-TPA) are -5.04 and -2.70 eV, respectively, and the corresponding results of T(TPADCM-TPA) are -5.04 and -2.77 eV, respectively. The HOMO and LUMO energy levels for B(TPA-DCM-TPA) and T(TPADCM-TPA) are almost same, which can be attributed to the same characteristics and contribution of the HOMO and LUMO for B(TPA-DCM-TPA) and T(TPA-DCM-TPA), respectively. The calculated band gaps of the two compounds are 2.33 and 2.27 eV, respectively. These values are in agreement with the values obtained from both the electrochemical and optical measurements for the molecules (vide infra), Egopt ) 1.75 eV, Egec ) 1.81 eV for B(TPA-DCM-TPA), and Egopt ) 1.64 eV, Egec ) 1.71 eV for T(TPA-DCM-TPA), respectively, with some margin of error. Compared with the HOMO and LUMO energy levels for PCBM, the two compounds are suitable to be used as the photovoltaic donors.

Zhao et al. How the absorption of a new material matches with the solar spectrum is an important factor for the application as a photovoltaic material, and a good photovoltaic material should have broad and strong visible absorption characteristics. On the basis of the optimized ground-state structures mentioned above, the electronic structures related to absorptions for B(TPA-DCMTPA) and T(TPA-DCM-TPA) were calculated by the semiempirical ZINDO method. The simulated Gaussian type absorption spectra (the half-wave width is 20 nm) are shown in Figure 2. B(TPA-DCM-TPA) and T(TPA-DCM-TPA) have very similar absorption spectra ranging from 240 to 508 nm. B(TPA-DCMTPA) has three absorption peaks at 298, 358, and 445 nm, respectively, and those of T(TPA-DCM-TPA) are at 303, 362, and 445 nm, respectively. The experimental absorption spectra for both B(TPA-DCM-TPA) and T(TPA-DCM-TPA) in chloroform also have three peaks, respectively (vide infra). We can see intuitively that there are some differences between computational and experimental absorption data for the two compounds. The reasons are that (1) the computational absorptions of B(TPA-DCM-TPA) and T(TPA-DCM-TPA) were just the behavior of a single gas molecule without consideration of intermolecular interaction and solvent effect which could exist in dilute solution and (2) the semiempirical ZINDO method, which can induce some error, was employed to study the absorptions of the two materials for saving the computational resources. 3.2. Synthesis. The synthetic routes of B(TPA-DCM-TPA) and T(TPA-DCM-TPA) are shown in Scheme 1. During the synthesis of 3, the weight ratio of 1 and 2 should be kept at 1.5:1. Other weight ratios, such as 1:1 or 2:1, resulted in byproduct of 2-[2,6-bis-2-1-vinyl]-pyran-4-ylidene]-malononitrile (TPA-DCM-TPA) or unreacted 1. The target compounds of B(TPA-DCM-TPA) and T(TPA-DCM-TPA) were synthesized from 3 and 4 by a Heck reaction. It should be noted that the yields of B(TPA-DCM-TPA) (31.6%) and T(TPA-DCMTPA) (18.6%) are low. The reason may be that the large amount of the electron-deficient group -CN in the compounds results in the large polarity of molecules, which makes the target compounds have some loss during the purification. B(TPADCM-TPA) is well soluble in common organic solvents, such as dichloromethane, chloroform, and THF. However the solubility of T(TPA-DCM-TPA) is poorer than that of B(TPA-DCMTPA) due probably to the large size of the molecule. 3.3. Optical Properties. The absorption spectra of B(TPADCM-TPA) and T(TPA-DCM-TPA) solutions in chloroform show a broad absorption band covering the wavelength range from 300 to 615 nm, as shown in Figure 3a. For B(TPA-DCMTPA) dilute solution, there are three absorption peaks at 308, 406, and 476 nm, and the former two absorption peaks correspond to the π-π absorption of the molecule, whereas the visible absorption band with a peak at 476 nm could be assigned to the intramolecular charge transfer (ITC) transition between the TPA moiety and PM unit. The absorption spectrum of T(TPA-DCM-TPA) solution is quite similar to that of the B(TPA-DCM-TPA) solution, with three peaks at 310, 407, and 481 nm, respectively. Figure 3b shows the absorption spectra of B(TPA-DCM-TPA) and T(TPA-DCM-TPA) films. In comparison with their dilute solutions, the absorption of the films is red-shifted slightly. The B(TPA-DCM-TPA) and T(TPADCM-TPA) thin films exhibit absorption onsets at 709 and 755 nm, corresponding to the optical band gap of 1.75 and 1.64 eV, respectively. The data on the absorption spectra are listed in Table 1 for a clear comparison.

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Figure 7. AFM (5 µm × 5 µm) topography (a) and phase images (b) of B(TPA-DCM-TPA):PCBM (1:2 w/w) blend film.

3.4. Electrochemical Properties. Cyclic voltammetry was performed to investigate the electrochemical properties and to estimate the HOMO and LUMO energy levels of B(TPA-DCMTPA) and T(TPA-DCM-TPA).54 As shown in Figure 4, both B(TPA-DCM-TPA) and T(TPA-DCM-TPA) exhibit reversible p-doping/dedoping (oxidation and rereduction) processes. The onset oxidation potentials are 0.57 and 0.62 V vs. Ag/Ag+ for B(TPA-DCM-TPA) and T(TPA-DCM-TPA), respectively, and their onset reduction potentials are -1.24 and -1.09 V vs. Ag/ Ag+, respectively. The HOMO and LUMO energy levels of the compounds were calculated from the onset potentials according to the equations55

HOMO ) -e(Eox onset + 4.71)(eV) LUMO ) -e(Ered onset + 4.71)(eV) ox red Eec g ) e(Eonset-Eonset)(eV)

The electrochemical data of the materials are summarized in Table 2. In comparison with the HOMO level of -6.1 eV and LUMO level of -4.1 eV for PCBM, the energy levels of B(TPA-DCM-TPA) and T(TPA-DCM-TPA) are suitable for being used as donor materials blended with PCBM as acceptor in OSCs. 3.5. Photovoltaic Properties. The photovoltaic property of B(TPA-DCM-TPA) was investigated by fabricating the OSC devices with ITO as positive electrode, the blend of B(TPADCM-TPA) (donor) and PCBM (acceptor) (1:2 w/w) as active layer, Al or Ca(10 nm)/Al as negative electrode. The photovoltaic property of T(TPA-DCM-TPA) was not reported in this paper, due to the poor film-forming property of the blend of T(TPA-DCM-TPA)andPCBM.Figure5showsthecurrent-voltage (I-V) curves of the OSCs under the illumination of AM 1.5, 100 mW/cm2, and Table 3 compares the photovoltaic performances of the OSCs with different negative electrodes. The OSC with a structure of ITO/PEDOT:PSS/B(TPA-DCM-TPA): PCBM(1:2 w/w)/Al delivered an open-circuit voltage (Voc) of 0.64 V, a short-circuit current (Isc) of 2.49 mA/cm2, a calculated fill factor (FF) of 0.41, and a PCE of 0.65%. For further improving the PCE of the OSCs, we deposited a thin film of Ca (∼10 nm) between active layer and Al to form a Ca/Al negative electrode. Then the photovoltaic performance of the OSC was improved to a Voc of 0.77 V, and Isc of 2.37 mA/cm2, a calculated FF of 0.40, and a PCE of 0.73%. Figure 6 shows the IPCE curves of OSCs based on B(TPADCM-TPA). The shape of the curves is similar to the corresponding absorption spectrum of B(TPA-DCM-TPA):PCBM blend film (also shown in Figure 6), which indicates that the visible absorptions of both components contributed to the photovoltaic conversion, and the maximum quantum efficiency

of the device with the Ca/Al negative electrode reached 19.0% at 525 nm. The AFM images of the blend film of B(TPA-DCMTPA) with PCBM, as shown in Figure 7, present a small domains and a smooth surface (low roughness value). The good photovoltaic behavior obtained with B(TPA-DCM-TPA) as donor should be ascribed to the synergistic effects of its enhanced absorption of incident light and high quality uniform spin-cast film. 4. Conclusion We designed and synthesized two solution processable multiarmed TPA-based organic molecules, biarmed B(TPADCM-TPA) and triarmed T(TPA-DCM-TPA), for the application in OSCs. The electronic distribution and absorption spectra of the molecules were analyzed by theoretical calculations. The two compounds were characterized by UV-vis absorption spectroscopy and electrochemical cyclic voltammetry. B(TPADCM-TPA) and T(TPA-DCM-TPA) films show broad absorption bands covering the wavelength range of 300∼709 and 300∼755 nm, respectively. Both B(TPA-DCM-TPA) and T(TPA-DCM-TPA) have appropriate HOMO (-5.28 and -5.33 eV) and LUMO (-3.47 and -3.62 eV) energy levels for being used as donor materials with PCBM as acceptor in OSCs. The OSCs based on the blend of B(TPA-DCM-TPA) and PCBM (1:2, w/w) reached 0.73% under the illumination of AM 1.5, 100 mW/cm2. Acknowledgment. This work was supported by NSFC (Nos. 50803071, 20821120293, 50633050, and 20721061). References and Notes (1) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. (2) Gu¨nes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. ReV. 2007, 107, 1324. (3) Thompson, B. C.; Fre´chet, J. M. J. Angew. Chem., Int. Ed. 2008, 47, 58. (4) (a) Li, Y. F.; Zou, Y. P. AdV. Mater. 2008, 20, 2952. (b) Hou, J. H.; Tan, Z. A.; Yan, Y.; He, Y. J.; Yang, C. H.; Li, Y. F. J. Am. Chem. Soc. 2006, 128, 4911. (5) (a) Ma, W. L.; Yang, C. Y.; Gong, X.; Lee, K.; Heeger, A. J. AdV. Funct. Mater. 2005, 15, 1617. (b) Kim, Y. Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; Mcculloch, I.; Ha, C.-S.; Ree, M. Nat. Mater. 2006, 5, 197. (6) Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T.-Q.; Dante, M.; Heeger, A. J. Science 2007, 317, 222. (7) Li, G.; Shrotriya, V.; Huang, J. S.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat. Mater. 2005, 4, 864. (8) Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A. J.; Bazan, G. C. Nat. Mater. 2007, 6, 497. (9) Wong, W.-Y.; Wang, X.-Z.; He, Z.; Djurisˇic´, A. B.; Yip, C.-T.; Cheung, K.-Y.; Wang, H.; Mak, C. S. K.; Chan, W.-K. Nat. Mater. 2007, 6, 521.

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