Solution-Processable Organic Molecule Photovoltaic Materials with

May 9, 2013 - Two solution-processable acceptor–donor–acceptor (A-D-A) structured organic molecules with bithienyl-substituted benzodithiophene (B...
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Solution-Processable Organic Molecule Photovoltaic Materials with Bithienyl-benzodithiophene Central Unit and Indenedione End Groups Suling Shen,†,‡ Pei Jiang,† Chang He,*,† Jing Zhang,† Ping Shen,† Yi Zhang,† Yuanping Yi,*,† Zhanjun Zhang,‡ Zhibo Li,† and Yongfang Li*,† †

Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100049, China

‡

S Supporting Information *

ABSTRACT: Two solution-processable acceptor−donor−acceptor (A-D-A) structured organic molecules with bithienyl-substituted benzodithiophene (BDTT) as central and donor unit, indenedione (ID) as acceptor unit and end groups, and thiophene (T) or bithiophene (bT) as πbridges, D1 and D2, are designed and synthesized for the application as donor materials in organic solar cells (OSCs). Two corresponding molecules with alkoxy side chains on BDT, DO1, and DO2 are also synthesized for comparison. The four compounds possess broad absorption covering the wavelength range 450−740 nm and relatively lower HOMO energy levels from −5.16 to about −5.19 eV. D2 and DO2 with bithiophene π-bridges demonstrate stronger absorbance and higher hole mobilities than the compounds with thiophene π-bridges. The power conversion efficiency (PCE) values of the OSCs based on the organic compounds/PC70BM (1.5:1, w/w) are 6.75% for D2, 5.67% for D1, 5.11% for DO2, and 4.15% for DO1. The results indicate that the molecules with thienyl conjugated side chains and bithiophene π-bridges show better photovoltaic performance. The PCE of the D2-based OSC are among the highest values in the OSCs based on the solution-processed organic small molecules. KEYWORDS: organic solar cells, D-A-D structured molecules with conjugated side chains, solution-processable organic photovoltaic materials properties with PCE reaching 6−7% recently,7d,9,10b which is benefitted from their stronger intermolecular interactions resulting in higher hole mobility and higher FF of the OSCs. Two-dimensional-conjugated (2D-conjugated) polymers have been successfully applied as donor materials in polymer solar cells (PSCs).11−14 Especially, the 2D-conjugated polymers based on benzo[1,2-b:4,5-b′]dithiophene (BDT) unit with bithienyl conjugated side chains 4,8-bis(5-hexylthiophen-2yl)benzo[1,2-b:4,5-b′]dithiophene (BDTT) showed broad absorption, lower HOMO energy level, higher hole mobility, and good photovoltaic performance,12−14 which are benefitted from the planar molecular structure of BDT unit15a,b and the effect of 2D-conjugation.11b Obviously, the BDTT unit with bithienyl conjugated side chains is an excellent donor building block in constructing the copolymer donor materials in PSCs. Here, based on the above consideration, we introduced the BDTT unit into solution-processable organic small molecules and synthesized two solution-processable A-D-A structured organic molecules with BDTT as central building block and donor unit, indenedione (ID) as acceptor unit and end groups,15c and thiophene (T) or bithiophene (bT) as π-bridges, D1 and D2. We also synthesized two control molecules with

1. INTRODUCTION Bulk-heterojunction organic solar cells (OSCs) based on solution-processable organic molecules as donor have attracted great attention recently, due to their advantages of low cost fabrication by solution-processing and good reproducibility of their photovoltaic performance benefitted from the high purity and definite molecular weight of the organic small molecules.1 For improving the power conversion efficiency (PCE) of the bulk-heterojunction OSCs, the most important is the design and synthesis of new solution-proccessable organic molecule donor materials. There are three types of solution-proccessable organic molecule photovoltaic donor materials reported in the literature up to now. The first is the triphenylamine (TPA)-based molecules,2−4 the second is the hyperbranched molecules,5,6 and the third is the planar linear A-D-A structured molecules based on thiophene oligomers7 or other donor and acceptor units.8−10 The TPA-based molecules and hyperbranched molecules possess good solubility, but their photovoltaic properties are limited by their weak intermolecular interaction when blended with the acceptor [6,6]-phenyl C-butyric acid methyl ester (PCBM), which results in lower hole mobility and poorer fill factor (FF) (usually only ca. 0.4) of the OSCs based on the molecules as donor. The maximum PCE of the OSCs based on these types of molecules is only 4.3%.3d In comparison, the planar molecules show promising photovoltaic © XXXX American Chemical Society

Received: March 9, 2013 Revised: May 7, 2013

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2,3c 5,15 and 615 were synthesized according to the literature. The compounds are soluble in common organic solvents, such as CHCl3, THF, and toluene. The thermal stability of the compounds was investigated by thermogravimetric analysis (TGA) under nitrogen atmosphere at a heating rate of 10 °C/min, as shown in Figure 1. The

alkoxy side chains on BDT unit, DO1 and DO2. The molecular structures of the four molecules are shown in Scheme 1. The Scheme 1. Synthesis Routes of D1, DO1, D2, DO2: (a) CH3CH2CN, Piperidine, N2, Reflux at 80 °C for 24 h; (b, c, d, e) Pd(PPh3)4, Toluene, N2, Reflux at 115 °C for 24 h

Figure 1. TGA plots of D1, DO1, D2, and DO2 with a heating rate of 10 °C/min under N2 atmosphere.

temperatures with 5% weight loss for D1, DO1, D2, and DO2 are 372, 345, 365, and 336 °C, respectively. The thermal stability of the compounds is good enough for the application in optoelectronic devices. In addition, D1 and D2 with thiophene conjugated side chains show even better thermal stability than the corresponding alkoxy-substituted molecules DO1 and DO2, which agrees with the better thermal stability of the 2D-conjugated polymers than the alkoxy-substituted polymers based on the BDT unit.12a 2.2. Absorption Spectra. Figure 2 shows the absorption spectra of the compounds solutions and films. Benefited from their D−A molecular structures with thiophene or bithiophene π-bridges, the absorption spectra of the compounds in chloroform solutions exhibit a strong and broad absorption band in the wavelength range from 450 to 650 nm (see Figure 2a). Interestingly, the absorption peaks of DO2 and D2 with bithiophene π-bridges are enhanced significantly and redshifted a little than that of DO1 and D1 with thiophene πbridges. The peak absorption coefficient of D1, DO1, D2, and DO2 is listed in Table 1 for a clear comparison. The results indicate that extending the π-bridges from thiophene to bithiophene is beneficial to enhancing the absorption coefficient of the compounds. Absorption spectra of the compounds films (see Figure 2b) are significantly broadened and red-shifted by ca. 90 nm in comparison with their solutions, and they show a new vibronic peak at the long wavelength direction (for D2, the new vibronic peak is at ca. 662 nm) close to absorption edge, indicating that some ordered structure and strong π−π stacking interaction exist between the molecular backbones in the solid films. Similar to the solution absorptions, the absorbance of DO2 and D2 films with bithiophene π-bridges is much stronger than that of DO1 and D1 films. The peak absorbance of D2 film reaches ca. 1 × 105 cm−1. The absorption edges of D1, DO1, D2, and DO2 films are at ca. 770, 780, 775, and 773 nm, respectively, corresponding to the optical band gaps (Egopt) of 1.61, 1.59, 1.60, and 1.60 eV, respectively, as listed in Table 1. 2.3. X-ray Diffraction Patterns. The X-ray diffraction (XRD) patterns of the compounds films were shown in Figure 3. All of the four compounds show a diffraction peak at 2θ around 5°, indicating that some ordered structure does exist in the compounds films. The XRD peaks of DO1 and DO2 are

compounds show good solubility in common organic solvents, high thermal stability for the bithienyl-substituted molecules even better than the alkoxy-substituted molecules, and broad absorption in the visible region with absorption edges reached ca. 750 nm. The solution absorbance of DO2 and D2 with bithiophene π-bridges is significantly enhanced in comparison with that of DO1 and D1 with thiophene π-bridges. The best photovoltaic performance of the OSCs based on D2 as donor and PC70BM as acceptor demonstrated a power conversion efficiency (PCE) of 6.75% with a high open circuit voltage (Voc) of 0.92 V, a short circuit current density (Jsc) of 11.05 mA/cm2, and a fill factor (FF) of 66.4% under the illumination of AM 1.5G, 100 mW cm−2. The PCE of 6.75% and FF of 66.4% are among the highest values for the solution-processable organic small molecules photovoltaic materials. The results indicate that the solution-proceessible organic molecules with BDTT central unit and indenedione end groups are promising donor materials for the bulk-heterojunction OSCs.

2. RESULTS AND DISCUSSION 2.1. Synthesis and Thermal Stability. The four compounds were synthesized through Stille coupling reaction (see Scheme 1) with good yields of over 50%. Monomers 1,16 B

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corresponding compounds with alkoxy side chains, due to the larger size of the hexyl substituted thiophene side chains than the alkoxy side chains. Interestingly, the d-spacing of the molecules with bithiophene π-bridges is smaller than that of the corresponding molecules with thiophene π-bridges. 2.3. Electronic Energy Levels. The HOMO (the highest occupied molecular orbital) and LUMO (the lowest unoccupied molecular orbital) energy levels of conjugated polymers and conjugated organic compounds semiconductors can be estimated by electrochemical cyclic voltammetry.17 Figure 4 shows the cyclic voltammograms of the four

Figure 4. Cyclic voltammograms of the compounds films on Pt electrode in an acetonitrile solution of 0.1 mol/L Bu4NPF6 (Bu = butyl) with a scan rate of 100 mV/s.

Figure 2. UV−vis absorption spectra of D1, DO1, D2 and DO2 in chloroform solutions (a) and in solid films on the quartz plate (b).

compounds films. The onset oxidation potentials (Eox) and onset reduction potentials (Ered) are 0.85 V and −0.78 V vs Ag/ AgCl for D1, 0.84 V and −0.78 V vs Ag/AgCl for DO1, 0.82 V and −0.80 V vs Ag/AgCl for D2, and 0.82 V and −0.82 V vs Ag/AgCl for DO2. The redox potential of ferrocene (FeCp2) is 0.46 V vs Ag/AgCl in our measurement system (see Figure 4), and we take the energy level of Fc/Fc+ as −4.8 eV. Then, the HOMO and LUMO energy levels of the compounds films were estimated from the Eox and Ered values according to the following equations: HOMO = −e(Eox + 4.34) (eV), and LUMO = −e(Ered + 4.34) (eV), where the potential values of Eox and Ered are versus Ag/AgCl. The HOMO and LUMO energy levels of the four compounds estimated from the electrochemical measurement are also listed in Table 1. The length of the π-bridges and the conjugated side chains influence the HOMO and LUMO energy levels only a little. In comparison with the LUMO of −3.91 eV and HOMO of −5.93 eV for PC60BM and the LUMO of −3.91 eV and HOMO of −5.87 eV for PC70BM,18 the energy levels of the compounds are very well suited for its application as donor material in the OSCs with PC60BM or PC70BM as acceptor. Also, the relatively lower HOMO energy level could be beneficial to higher open circuit voltage of the OSCs based on the compounds as donor.19 2.4. Hole Mobility of the Compounds Films. Hole mobilities of the four compounds films were measured by the space-charge-limited current (SCLC) method20 with a holeonly device structure of ITO/PEDOT:PSS/sample film/Au. We measured the data of current density (J) versus voltage (V) for the devices based on the four compounds films. We first drew the plots of log(J) vs log(V) and confirmed that at the higher voltage the currents are indeed the space-charge-limitedcurrent (the slope of log (J) vs log(V) is 2, see Figure S1 in the

Table 1. Energy Bandgaps, Electronic Energy Levels, and Solution Absorption Coefficient of the Compounds compds

Eopt g (eV)

HOMO (eV)

LUMO (eV)

D1 DO1 D2 DO2

1.61 1.59 1.60 1.60

−5.19 −5.18 −5.16 −5.16

−3.56 −3.56 −3.54 −3.52

ε solution in chloroform (M−1cm−1) 0.737 0.776 1.002 0.945

× × × ×

105 105 105 105

Figure 3. XRD patterns of the compounds films.

stronger than those of D1 and D2, due probably to better layer by layer packing of the molecules with the smaller alkoxy side chains in DO1 and DO2 films. The XRD peak positions of D1, DO1, D2, and DO2 are at 2θ = ca. 4.4°, 4.8°, 4.7°, and 5.6°, corresponding to the d-spacing between the molecular layers is 2.02, 1.85, 1.89, and 1.58 nm, respectively. The d-spacing of the compounds with thiophene side chains is larger than that of the C

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Figure 5. Plot of ln(JL3/V2) versus (V/L)0.5 of the device ITO/PEDOT:PSS/sample film/Au for the measurement of hole mobility.

Supporting Information (SI)). Then, we drew a straight line based on the data at the higher voltages (space-charge-limitedcurrent region) in the plots of ln(JL3/V2) vs (V/L)0.5, as shown in Figure 5, for the calculation of hole mobility. The hole mobilities calculated from the straight lines are 2.04 × 10−4 cm2 V−1 s−1 for D1, 2.82 × 10−2 cm2 V−1 s−1 for D2, 1.71 × 10−4 cm2 V−1 s−1 for DO1, and 2.63 × 10−2 cm2 V−1 s−1 for DO2. The hole mobilities of D2 and DO2 with bithiophene π-bridges are two orders higher than those of D1 and DO1 with thiophene π-bridges. Additionally, those of D1 and D2 with thienyl conjugated side chains are a little higher than those of corresponding DO1 and DO2 with alkoxy side chains. 2.5. Theoretical Calculations. In order to deeply understand the structure and physicochemical properties of the compounds, we carried out theoretical calculations on the molecular geometries, electronic wave functions of frontier orbitals, and absorption spectra in chloroform for all four molecules. The detailed computational methodologies were described in the SI. The optimized geometries are shown in Figure S2 in the SI. For all these molecules, the molecular backbones present similar geometries. The terminal indenedione electron-withdrawing groups are nearly coplanar with the linked thiophene or bithiophene groups, while there is a twist angle of ca. 30° between the central BDT unit and the πbridge thiophene groups. The central BDT unit also displays a twist angle of ca. 54° with the side conjugated thienyl group for D1 and D2 and is nearly perpendicular to the plane which the main chain of the alkoxy substituent lies in for DO1 and DO2. Figure 6 shows the calculated electronic wave functions of HOMO and LUMO for D2 (the results for the other studied molecules are similar to D2 and can be found in Figure S3 in the SI). The electronic wave functions of the frontier orbitals are delocalized on the whole main chain; however, relatively larger densities are found on the central BDT group for the HOMO but on the terminal indenedione groups for the LUMO. The calculated HOMO and LUMO energies, and

Figure 6. DFT-wB97X/6-31G** calculated electronic wave functions of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) in chloroform for D2.

excitation energies and oscillator strengths of the lowest singlet excited states (S1) for the four compounds, are shown in Table S1 in the SI. The calculated optical absorption spectra for all the studied molecules are shown in Figure 7. The overall trends (in both absorption positions and strengths) compare well with the experimental results. The main peak is featured by the

Figure 7. TDDFT-wB97X/6-31G** calculated optical absorption spectra in chloroform based on the optimized geometries. D

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molecule donor materials is mixed into the fullerene acceptor phase while the fullerene molecules could not be mixed into the donor material phase, so that a higher concentration of the donor materials is needed to form the donor/acceptor interpenetrating networks in the active layers for efficient hole transport. However, in the devices with higher fullerene acceptor concentration at the optimized condition, some fullerene molecules are mixed into the donor material phase, so that a higher concentration of the fullerene acceptor is needed to form the donor/acceptor interpenetrating networks for efficient electron transport. Figure 8a shows the J−V curves of the OSCs based on the four organic molecules as donor and PC70BM as acceptor with

transition from the ground state (S0) to the lowest singlet excited state (S1), which is dominated by the configuration from HOMO to LUMO. Due to a certain amount of charge transfer from HOMO to LUMO and less steric hindrance around the terminal electron-withdrawing indenedione groups, this will benefit the electron transfer of the excitons from the donor molecules to the fullerene acceptors in the bulkheterojunction OSCs. 2.6. Photovoltaic Performance. For investigating the photovoltaic properties of the compounds, bulk-heterojunction OSCs were fabricated with the organic molecules as donor (D) and PC60BM or PC70BM as acceptor (A) in the device structure of ITO/PEDOT:PSS/photoactive layer/Ca/Al. We first performed the optimization of photovoltaic performance of the OSCs based on the bithienyl-substituted molecule D1 as donor and PC60BM as acceptor by changing D/A weight ratios from 1:1.5 to 2.5:1. The photovoltaic performance data of the OSCs were listed in Table S2 in the SI. The device with the D/ A weight ratio of 1.5:1 showed the best photovoltaic performance with a high open circuit voltage (Voc) of 1.03 V, a short-circuit current density (Jsc) of 9.78 mA/cm2, and a fill factor (FF) of 56%, leading to a higher power conversion efficiency (PCE) of 5.64%. By using PC70BM instead of PC60BM as acceptor at the D/A weight ratio of 1.5:1, the PCE of the OSC was almost the same (5.67%), with the same Voc of 1.03 V (see Table S2 in the SI). The high Voc of 1.03 V for the OSCs based on D1 should be benefitted from the lower HOMO level (−5.19 eV) of D1. Then, we optimized the photovoltaic performance of another bithienyl-substituted molecule D2 with PC70BM as acceptor by changing the D/A weight ratios from 2:1 to 1:2 in the active layer of the devices. The current density−voltage (J−V) curves of the OSCs with different D2/PC70BM weight ratios under the illumination of AM 1.5G, 100 mW/cm2 are shown in Figure S4 in the SI, and the photovoltaic parameters of the OSCs are listed in Table 2 for a clear comparison. The optimized weight Table 2. Device Performance Parameters of the BulkHeterojunction OSCs Based on the Organic Molecules As Donor (D) and PC70BM As Acceptor (A) with Different D/ A Weight Ratios, Under the Illumination of AM1.5G, 100 mW/cm2 donor molecule D/A ratio D2

D1 DO1 DO2

2:1 1.5:1 1:1 1:1.5 1:2 1.5:1 1.5:1 1.5:1

Voc (V) Jsc (mA cm−2) 0.87 0.92 0.95 0.96 0.83 1.03 0.91 0.92

7.61 11.05 10.13 8.39 6.43 10.07 9.47 8.58

FF (%)

PCE (%)

54.1 66.4 56.8 59.4 41.0 54.7 48.2 64.8

3.58 6.75 5.47 4.79 2.19 5.67 4.15 5.11

Figure 8. (a) Current density−voltage curves of the OSCs based on the organic molecules/PC70BM (1.5:1 w/w) under the illumination of AM 1.5G, 100 mW/cm2; (b) EQE spectra of the OSCs based on the organic molecules/PC70BM (1.5:1, w/w).

the donor/acceptor weight ratio of 1.5:1 under the illumination of AM 1.5G, 100 mW/cm2. The photovoltaic performance parameters of the OSCs were also compared in Table 2. Obviously, D2 and D1 with conjugated thienyl side chains show better photovoltaic performance than that of the corresponding molecules DO2 and DO1 with alkoxy side chains on BDT unit. The OSC based on the bithienylsubstituted molecule D2 with bithiophene π-bridge demonstrated the highest power conversion efficiency (PCE) of 6.75% with a high fill factor (FF) of 66.4%. Also, the bithienylsubstituted molecule D1 with thiophene π-bridges showed a PCE of 5.67% with a high Voc of 1.03 V. The PCE of 6.75% and FF of 66.4% for the OSC based on D2, and the Voc of 1.03 V for the OSC based on D1 are among the highest values for the solution-processed OSCs. The good photovoltaic performance should be benefitted from the broad and strong absorption,

ratio of D2/PC70BM is also 1.5:1, and under the optimized conditions, PCE of the OSC based on D2 reached 6.75% with a Voc of 0.92 V, a Jsc of 11.05 mA/cm2, and a FF of 66.4%. The relatively higher concentration of D1 and D2 donors at the optimized condition agrees with those of the OSCs based on the planar D−A structured molecules,8−10 but it is much higher than those of the optimized donor/acceptor weight ratios in PSCs (from 1:121 to 1:422) and in the OSCs based on the TPA-containing and hyperbranched molecules (from 1:23,4 to 1:45). Probably, some of the planar D−A structured E

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molecules with alkoxy side chains on BDT, DO1 and DO2. The compounds are soluble in common organic solvents, possess broad absorption covering the wavelength range 450− 740 nm, and have relatively lower HOMO energy levels of −5.16 ∼ −5.19 eV. Among the four compounds, D1 and D2 with bithienyl conjugated side chains show higher thermal stability, and D2 and DO2 with bithiophene π-bridges possess higher hole mobility and stronger absorption. The PCE values of the OSCs based on the organic molecules/PC70BM (1.5:1, w/w) under the illumination of AM 1.5G, 100 mW cm−2 are 6.75% for D2, 5.67% for D1, 5.11% for DO2, and 4.15% for DO1. The results indicate that the photovoltaic performance of the molecules with bithienyl conjugated side chains is better than that of the corresponding molecules with alkoxy side chains on BDT unit, and the molecules with bithiophene πbridges show better photovoltaic performance than that of the corresponding molecules with thiophene π-bridges.

higher hole mobility, and relatively lower HOMO energy level of the molecules with bithienyl conjugated side chains. For the two molecules with alkoxy side chains on BDT unit, the PCE is 5.11% with Voc of 0.92 V, Jsc of 8.58 mA/cm2, and FF of 64.8% for DO2 and 4.15% with Voc of 0.91 V, Jsc of 9.47 mA/cm2, and FF of 48.2% for DO1. The relatively lower PCE of the OSC based on DO1 could be due to the lower hole mobility of DO1, which results in lower fill factor of the device. The external quantum efficiency (EQE) curves of the OSCs based on the organic molecules/PC70BM (1.5:1, w/w) were shown in Figure 8b. The EQE curves cover a broad wavelength range from 320 to 740 nm, consistent with the absorption of the active blend layers of the organic molecules donor and PC70BM acceptor. D2 showed the highest EQE value with the maximum reaching 60% at 581 nm. The current integrated from the EQE curve with the AM 1.5G spectrum is 10.5 mA/ cm2 for the OSC based on D2/PC70BM, which agrees well with the Jsc value of 11.05 mA/cm2 of the device within 5% difference. The integration current from the EQE curves for other OSCs is also in good agreement with the corresponding Jsc values of the devices. 2.7. Morphologies of the Active Layers. Surface morphology of the blend films of the organic molecules and PC70BM (1.5:1, w/w) spin-coated from chloroform solution were studied by tapping-mode atomic force microscopy (AFM). Figure 9 shows the AFM height and phase images of

4. EXPERIMENTAL SECTION Materials. All reagents were obtained from Aldrich, Acros, and TCI Chemical Co, and used as received. Tetrahydrofuran (THF) was dried over Na/benzophenoneketyl and freshly distilled prior to use. CH3CH2CN and piperidine were dried over molecular sieve. Measurements and Instruments. Nuclear magnetic resonance (NMR) spectra were taken on a Bruker DMX-400 spectrometer. MALDI-TOF spectra were recorded on a Bruker BIFLEXβ. Absorption spectra were taken on a Hitachi U-3010 UV−vis spectrophotometer. The film on quartz used for UV measurements was prepared by spin-coating with 1% chloroform solution. TGA measurement was performed on a Perkin-Elmer TGA-7 apparatus. The electrochemical cyclic voltammogram was obtained using a Zahner IM6e electrochemical workstation in a 0.1 mol/L tetrabutylammonium hexafluorophosphate (Bu4NPF6) acetonitrile solution. A Pt electrode coated with the sample film was used as the working electrode; a Pt wire and Ag/AgCl were used as the counter and reference electrodes, respectively. XRD measurement was performed on a Philips X’Pert using a Cu Kα line. Fabrication and Characterization of Organic Solar Cells. Organic solar cells (OSCs) were fabricated in the configuration of the traditional sandwich structure with an ITO positive electrode and a metal negative electrode. Patterned ITO glass with a sheet resistance of 30 Ω−1 was purchased from CSG Holding Co., Ltd. (China). The ITO glass was cleaned in an ultrasonic bath of acetone and isopropanol and treated by UVO (ultraviolet ozone cleaner, Jelight Company, U.S.A.). Then, a thin layer (30 nm) of PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)) (Baytron PVP A1 4083, Germany) was spin-coated on the ITO glass. Subsequently, the photosensitive layer was prepared by spin-coating the blend solution of the organic compounds donor (D) and PC70BM acceptor (A) with different D/A ratios on the top of the PEDOT:PSS layer. The concentration of the solution was 12 mg/mL in chloroform. The thickness of the photoactive layer was measured using an Ambios Technology XP-2 profilometer. Finally, the Ca(30 nm)/Al(ca. 70 nm) electrode was vacuum evaporated on the photoactive layer under a shadow mask in the vacuum of ca. 10−4 Pa. The active area of the device is 4 mm2. The current density−voltage (J−V) measurement of the device 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 100 mW/cm2. Synthesis. The synthesis routes of the compounds are shown in Scheme 1. The detailed synthetic processes are as follows. 3: 1H-indene-1,3(2H)-dione (3.03 g, 20.74 mmol), monomer 1 (6.25 g, 22.81 mmol), piperidine (2 mL), and freshly distilled acetonitrile (22.8 mL) were mixed under argon and refluxed at 80 °C under argon for 24 h. The reaction mixture was cooled to room temperature, dissolved in dichloromethane, washed with brine, and dried by anhydrous magnesium sulfate. After evaporation of the

Figure 9. AFM height images (2 μm × 2 μm) (top) and phase images (bottom) of the blend films of D1/PC70BM, DO1/PC70BM, D2/ PC70BM, and DO2/PC70BM with D/A weight ratio of 1.5:1.

the blend films of the four organic molecules and PC70BM. The average roughness (Rav) of the D1/PC70BM, DO1/PC70BM, D2/PC70BM, and DO2/PC70BM films are 0.783 nm, 1.397 nm, 0.890 nm, and 1.73 nm, respectively. The result implies that the donor materials of D1 and D2 with bithienyl conjugated side chains have better miscibility with PC70BM molecules in the blend film. The phase images in Figure 9 demonstrate that the donor/acceptor interpenetrating network of the blend film of D2 is better than those of the blend films of D1/PC70BM, DO1/PC70BM, and DO2/PC60BM. The good photovoltaic performance of the D2/PC70BM active layer should be also related to the suitable morphology of the active layer.

3. CONCLUSIONS We designed and synthesized a series of solution-processable planar and conjugated A-D-A structured organic molecules with benzodithiophene (BDT) as central building block and donor unit, indenedione (ID) as acceptor unit and end groups, and thiophene (T) or bithiophene (bT) as π-bridges. The molecules include two molecules with bithienyl conjugated side chains on BDT unit, D1 and D2, and two corresponding F

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7.40(s, 2H), 7.19(s, 2H), 4.23(s, 4H), 2.97−2.93(s, 8H), 1.88−1.85(s, 2H), 1.76−1.63(m, 12H), 1.54−1.35(m, 36H), 1.08−1.05(t, 6H), 0.99−0.97(m, 18H). 13C NMR (CDCl3, 300 MHz) δ (ppm): 202.20, 201.32, 142.05, 140.42, 134.86, 134.66, 129.79, 122.88, 122.67, 116.80, 69.36, 104.51, 41.88, 40.76, 40.06, 39.43, 31.75, 31.65, 30.55, 29.43, 29.33, 29.12, 23.22, 22.69, 22.61, 14.26, 14.14, 14.12. Elemental analysis calcd. for C86H102O6S6: C, 72.53; H, 7.22. Found: C, 71.89; H, 7.33

solvent, the residue was purified by column chromatography (petroleum/dichloromethane, 5: 1) to produce 5 g monomer 3 as a yellow solid with a yield of about 60%. GC/MS: 402(M+). 1H NMR (400 MHz, CDCl3) δ (ppm): 7.96−7.95 (t,2H), 7.835 (s, 1H), 7.81− 7.77 (m,2H), 7.599(s, 1H), 2.64−2.60(t,2H), 1.66−1.58(m,3H),1.38− 1.25(m, 6H), 0.91−0.88(t, 2H). 13C NMR (CDCl3, 600 MHz) δ (ppm):190.02, 189.58, 144.05, 142.15, 141.89, 140.55, 136.91, 135.17, 125.45, 124.47, 123.09, 31.57, 29.50, 29.18, 28.86, 22.59, 14.09. Elemental analysis calcd. for C20H19BrO2S: C, 59.56; H, 4.75. Found: C, 59.70; H, 5.05. 4: The synthesis and purification processes of 4 are similar with those of 3, with 2 used instead of 1. MALDI-TOF: MS, 569.1; calcd. for C30H33BrO2S2, 569.62. 1H NMR (400 MHz, CDCl3) δ (ppm): 8.09−8.07 (s, 1H), 7.97−7.94 (t, 2H), 7.78−7.75 (t, 2H), 7.21 (s, 1H), 7.07 (s, 1H), 2.94−2.90 (t, 2H), 2.62−2.57 (t, 2H), 1.71−1.60 (m, 4H), 1.42−1.25 (s, 12H), 0.93−0.88 (s, 6H). 13C NMR (CDCl3, 600 MHz) δ (ppm): 190.61, 189.99, 158.83, 147.19, 143.94, 142.02, 140.39, 135.88, 134.88, 134.70, 133.08, 130.72, 127.19, 126.18, 122.89, 122.69, 122.38, 111.53, 31.61, 31.48, 29.60, 29.55, 29.05, 28.92, 22.59, 22.57, 14.10, 14.07. Elemental analysis calcd. for C30H33BrO2S2: C, 62.26; H, 5.84. Found: C,63.58; H, 5.87. D1: Under argon atmosphere, monomer 5 (0.5 g) and monomer 3 (0.523 g) are dissolved in 80 mL of anhydrous toluene. Then, 30 mg Pd(PPh3)4 is added, and the mixture is refluxed under inert atmosphere for 24 h. After cooling, the solvent is evaporated and the residue dissolved in methylenechloride. The organic phase is washed with water and dried with MgSO4. After evaporation of solvent, the residue is purified by chromatography on silica gel using a chloroform/petroleum ether (1/1, v/v) mixture as eluent to get 0.575 g D1 as a black solid in ca. 80% yield. MALDI-TOF: m/z 1166.6; calcd. for C70H70O4S6, 1166.36. 1H NMR (400 MHz, CDCl3) δ (ppm): 7.97−7.95(t, 4H), 7.87−7.83(t, 6H), 7.77−7.75(t, 4H), 7.38− 7.37(d, J = 4.0 Hz, 2H), 6.98−6.97(d, J = 4.0 Hz, 2H), 2.96−2.95(t, 4H), 2.90−2.86(t, 4H), 1.87−1.80(m, 4H), 1.75−1.70(m, 4H), 1.57− 1.50(d, J = 8.4 Hz, 6H), 1.47−1.25(m, 20H), 0.95−0.91(m, 10H). 13C NMR (CDCl3, 400 MHz) δ (ppm): 190.23, 189.65, 147.81, 145.11, 144.59, 142.46, 142.09, 140.56, 139.71, 137.38, 136.82, 136.11, 136.03, 135.53, 135.08, 134.85, 128.30, 124.68, 124.43, 124.17, 123.70, 123.05, 122.85, 31.69, 31.67, 31.64, 30.41, 30.29, 29.61, 29.36, 29.12, 22.71, 14.18, 14.17. Elemental analysis calcd. for C70H70O4S6: C, 72.00; H, 6.04. Found: C, 72.72; H, 6.21. DO1: The synthesis and purification processes are similar with D1, but monomer 6 was used instead of 5. MALDI-TOF: m/z 1091.7; calcd. for C66H74O6S4, 1091.55. 1H NMR (400 MHz, CDCl3) δ (ppm): 7.98−7.97 (m, 4H), 7.93−7.91 (t, 4H), 7.81−7.78 (m, 4H), 7.73 (s, 2H), 4.28−4.26 (d, J = 8.0 Hz, 4H), 2.98−2.94 (t, 4H), 1.92− 1.88 (m, 2H), 1.86−1.72 (m, 4H), 1.71−1.61(m, 4H), 1.44−1.21(m, 26H), 1.08−1.06(m, 4H), 1.00−0.92(m, 4H), 0.91−0.83(m, 8H). 13C NMR (CDCl3, 300 MHz) δ (ppm): 190.37, 189.69, 144.88, 144.40, 142.60, 142.28, 140.72, 136.18, 135.57, 135.20, 135.00, 130.18, 124.82, 123.15, 120.76, 78.91, 40.86, 31.77, 30.59, 29.83, 29.40, 24.01, 23.25, 22.76, 14.19, 11.45. Elemental analysis calcd. for C66H74O6S4: C, 72.62; H, 6.83. Found: C, 72.45; H, 6.65. D2: The synthesis and purification processes are similar to those of D1, but monomer 4 was used instead of 3. The obtained D2 is a black solid with ca. 51% yield. MALDI-TOF: m/z 1498.8; calcd. for C90H98O4S8, 1498.52. 1H NMR (400 MHz, CDCl3) δ (ppm): 8.04 (s, 2H), 7.95−7.91 (t, 4H), 7.75−7.73 (m, 4H), 7.73 (s, 2H), 7.37−7.36 (s, 2H), 7.31 (s, 2H), 7.12 (s, 2H), 6.97−6.96 (d, J = 4.0 Hz, 2H), 2.99−2.95 (t, 4H), 2.92−2.88 (t, 4H), 2.84−2.80 (t, 4H), 1.85−1.82 (m, 4H), 1.71−1.68 (m, 8H), 1.37−1.33 (m, 36H), 0.96−0.91 (m, 18H). 13C NMR (CDCl3, 300 MHz) δ (ppm): 192.48, 158.97, 147.74, 142.19, 140.54, 137.14, 136.81, 134.89, 133.49, 133.03, 129.94, 128.14, 126.98, 126.50, 124.59, 122.93, 122.28, 31.92, 30.61, 29.68, 29.49, 29.15, 22.77, 14.22, 11.86. Elemental analysis calcd. for C90H98O4S8: C, 72.05; H, 6.58. Found: C, 71.57; H, 6.81. DO2: The synthesis and purificatino process for DO2 is similar to that of DO1, but monomer 4 was used instead of 3. MALDI-TOF: m/ z 1423.0; calcd. for C86H102O6S6, 1422.6. 1H NMR (400 MHz, CDCl3) δ (ppm): 8.09(s, 2H), 7.96(s, 4H), 7.77(s, 4H), 7.52(s, 2H),

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ASSOCIATED CONTENT

S Supporting Information *

Computational methodologies, DFT-B3LYP/6-31G** optimized geometries in chloroform, DFT-wB97X/6-31G** calculated electronic wave functions of the HOMO and the LUMO in chloroform based on the optimized geometries, J−V curves of the OSCs based on D2/PC60BM with different weight radios under the illumination of AM 1.5G, 100 mW/cm2, etc. This information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y. Li), [email protected] (C. He), [email protected] (Y. Yi). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by The Ministry of Science and Technology of China (No. 2010DFB63530, 2011AA050523), NSFC (Nos. 50933003, 21021091) and the Chinese Academy of Sciences (No. KGCX2-YW-399+9-1, GJHZ1124).

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REFERENCES

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