Solution-Processable Organic Molecule with Triphenylamine Core

Feb 10, 2010 - E-mail: [email protected] (Y.L.) or [email protected] (Y.Z.)., †. Chinese ... Small Molecule Solution-Processed Bulk Heterojunction Sol...
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J. Phys. Chem. C 2010, 114, 3701–3706

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Solution-Processable Organic Molecule with Triphenylamine Core and Two Benzothiadiazole-Thiophene Arms for Photovoltaic Application Yi Yang,† Jing Zhang,† Yi Zhou,*,‡ Guangjin Zhao,† Chang He,† Yongfang Li,*,† Mattias Andersson,‡ Olle Ingana¨s,‡ and Fengling Zhang‡ Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, People’s republic of China, and Biomolecular and Organic Electronics, Department of Physics, Chemistry and Biology, Linko¨ping UniVersity, S-581 83, Linko¨ping, Sweden ReceiVed: NoVember 14, 2009; ReVised Manuscript ReceiVed: January 14, 2010

A new solution-processable biarmed organic molecule with triphenylamine (TPA) core and benzothiadiazolehexylthiophene (BT-HT) arms, B(TPA-BT-HT), has been synthesized by a Heck reaction, and characterized by UV-vis absorption, cyclic voltammetry, and theoretical calculation. Photovoltaic properties of B(TPABT-HT) as light-harvesting and electron-donating material in organic solar cells (OSCs), with [6,6]-phenylC61-butyric acid methyl ester (PC60BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) as acceptors, were systematically investigated. The performance of the OSCs varied significantly with B(TPA-BT-HT)/ fullerene weight ratio, active layer thickness, and solvents used for spin-coating the active layer. The optimized device with the B(TPA-BT-HT)/PC70BM weight ratio of 1:2 and a thickness of 55 nm with the active layer spin-coated from DCB solution shows a power conversion efficiency of 1.96% with a short-circuit current density of 5.50 mA/cm2 and an open-circuit voltage of 0.96 V under the illumination of AM1.5, 100 mW/cm2. Introduction Solution processable organic photovoltaic materials and organic solar cells (OSCs) have drawn great attention in recent years,1-20 due to their advantages of easy fabrication, low cost, light weight, and flexibility like polymer solar cells (PSCs), as well as their high purity and definite molecular weight, which may give the photovoltaic properties of the organic molecules good reproducibility. Recently, the power conversion efficiency (PCE) of 1.7-4% has been reached for solution processed OSCs,2-4,20 which indicates that the organic small molecules including dendrimers are also promising for use as high efficiency photovoltaic materials in the near future. Triphenylamine (TPA)-containing molecules have attracted special research interests for the solution-processable organic optoelectronic molecules, because of their good solutionprocessability benefiting from the three-dimensional propeller structure of TPA. Shirota et al have developed a series of TPAbased compounds for the application in organic light-emitting diodes (OLEDs) as hole-transporting or electroluminescent materials.6,21,22 Meanwhile, Roncali’s group has done some pioneer work for using TPA derivatives as electron donor materials for photovoltaic conversion.7-10 Our group also synthesized a series of D-A structured molecules with TPA as the donor unit and benzothiadiazole or DCM as the acceptor unit for the application in OSCs.15-20 The TPA-containing D-A structured molecules were designed to take advantages of (1) the higher hole-transporting mobility of TPA,21 (2) the extended absorption spectrum of the D-A structure toward longer wavelength by an intramolecular charge transfer (ICT),23,24 and * To whom correspondence should be addressed. E-mail: [email protected] (Y.L.) or [email protected] (Y.Z.). † Chinese Academy of Sciences. ‡ Linko¨ping University.

(3) the higher oxidation potential of the D-A molecules for a higher open circuit voltage of the OSCs with the molecules as donor.25 Among the molecules, a star-shaped molecule with TPA as core and three benzothiadiazole-hexylthiophene (BT-HT) arms, S(TPA-BT-HT), displayed high photovoltaic performance: the PCE of the OSC based on S(TPA-BT-HT)/PC70BM reached 2.39% under the illumination of AM.1.5, 100 mW/cm2.20 For investigating the relationship between the molecular structure and the photovoltaic properties of the TPA-BT-HTcontaining molecules, herein, we report a new solutionprocessable D-A structured molecule, B(TPA-BT-HT), with the propeller-shaped TPA as core and two BT-HT arms (see Scheme 1). B(TPA-BT-HT) also shows good solubility in common organic solvents, and broad absorption in the wavelength range from 300 to 600 nm. The OSC device based on B(TPA-BT-HT) as donor and PC70BM as acceptor displayed a power conversion efficiency of 1.96% with a short-circuit current density of 5.50 mA/cm2 and an open-circuit voltage of 0.96 V under the illumination of AM1.5, 100 mW/cm2. The photovoltaic performance of two-armed B(TPA-BT-HT) is a little lower than that of the star-shaped three-armed S(TPA-BT-HT), but it is still among the best photovoltaic performance for solution processable small molecule photovoltaic donor materials. Experimental Section Chemicals. 3-Bromothiophene, 1-bromohexane, iodine, nbutyllithium (2.88 mol/L in hexane), pyridine, tri-n-butyltin chloride, thionyl chloride, bromine, bromine hydride, phosphorus oxychloride, Pd(PPh3)2Cl2, Ph3MePBr, tetrabutylammonium bromide, sodium acetate, palladium acetate, and DMF were obtained from Acros Organics. Tetrahydrofuran (THF) and diethyl ether were dried over Na/benzophenoneketyl and freshly distilled prior to use. PC60BM and PC70BM were purchased from

10.1021/jp910836t  2010 American Chemical Society Published on Web 02/10/2010

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SCHEME 1: Synthesis Route for B(TPA-BT-HT)

American Dye Source (ADS) Inc. and used as received without further purification. Measurements. 1H NMR and 13C NMR spectra were taken on a Bruker DMX-400 spectrometer. MALDI-TOF spectra were recorded on a Bruker BIFLEXIII. Elemental analyses were carried out on a flash EA 1112 elemental analyzer. 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% methylene chloride solution. The cyclic voltammetry was performed with use of a Zahner IM6e electrochemical workstation with a Pt disk, Pt plate, and Ag/Ag+ electrode as the working electrode, counter electrode, and reference electrode, respectively, in a 0.1 mol/L tetrabutylammonium hexafluorophosphate (Bu4NPF6) acetonitrile solution. The thin film on the Pt disk, formed by drop-casting the molecule solution in THF (analytical reagent, 1 mg/mL), was used as the working electrode. Device Fabrication and Characterization. OSC devices were fabricated with the configuration of ITO/PEDOT:PSS (40 nm)/active layer (40-90 nm)/Al (100 nm). The ITO glass was cleaned and a thin layer of PEDOT:PSS (poly-(3,4-ethylenedioxythiophene):poly(styrene sulfonate)) was spin-cast on it from a PEDOT:PSS aqueous solution (Baytron AI 4083 from H. C. Starck) at 2000 rpm and dried subsequently at 150 °C for 30 min in air, then it was transferred to a glovebox. The active layer of the blend of B(TPA-BT-HT) and PC60BM or PC70BM was spin-coated onto the PEDOT:PSS layer. Al as the negative electrode was deposited in vacuum onto the active layer at a pressure of ca. 5 × 10-5 Pa. The size of the device was defined by the mask when depositing Al and was approximately 6 mm2. The thickness of the active layer was determined by an Ambios Tech. XP-2 profilometer. The current-voltage (I-V) characteristics were measured on a computer-controlled Keithley 236 Source-Measure Unit. A xenon lamp coupled with AM 1.5 solar spectrum filters was used as the light source, and the optical power at the sample was around 100 mW/cm2. All the measurements were performed under ambient atmosphere at room temperature.

Synthesis. 4,4′-(Phenylazanediyl)dibenzaldehyde. Under a nitrogen atmosphere, phosphorus oxychloride (47.5 mL) was added into DMF (36.3 mL) dropwise at 0 °C. After the addition, the mixture was stirred vigorously for 1 h at 0 °C, then triphenylamine was added into the solution. The solution was heated to 95 °C and kept at that temperature for 4 h. Then the reaction mixture was cooled to room temperature, poured into 500 mL of ice water, and neutralized with NaOH solution (2 mol/L, ca. 750 mL). The organic was extracted with methylene chloride (5 × 100 mL). The combined organic was washed with brine (2 × 100 mL) and dried over anhydrous sodium sulfate. After evaporation of the solvent, the residue was purified by column chromatography on silica gel, using a petroleum ether/ethyl acetate mixture (4:1), and produced 4.2 g of yellow solid pure product with a yield of 68.4%. GC/MS: 301 (M+). 1H NMR (CDCl3, 400 MHz): δ (ppm) 9.93 (s, 2H), 7.45 (t, 2H), 7.32 (t, 1H), 7.23 (m, 6H). N,N-(4-Vinylphenyl)benzenamine. To a solution of CH3PPh3Br (5.22 g, 14.6 mmol) in THF at -78 °C was added 9.1 mL (14.6 mmol) of n-butyllithium (1.6 M in hexane) by syringe. The mixture was warmed to room temperature slowly. 4,4′-(Phenylazanediyl)dibenzaldehyde (2 g, 6.6 mmol) in THF (20 mL) was added dropwise to the mixture, and the resulting mixture was kept at room temperature for 12 h. The mixture was poured into water and extracted with methylene chloride. The organic layer was washed with brine and dried over anhydrous sodium sulfate. After evaporation of the solvent, the residue was purified by column chromatography on silica gel column with hexane, and produced 1.72 g of white solid N,N(4-vinylphenyl)benzenamine with a yield of 87.2%. 1H NMR (CDCl3, 400 MHz): δ (ppm) 7.35 (m, 7H), 7.15 (d, 2H), 7.08 (d, 4H), 6.80 (m, 2H), 5.78 (d, 2H), 5.28 (d, 2H). 4-Bromo-7-(4-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole. Under a nitrogen atmosphere, a mixture of 4,7-dibromobenzo[c][1,2,5]thiadiazole (27.78 g, 94.49 mmol), (4-hexylthiophene2-yl)tributylstannane (35.9 g, 78.74 mmol), and Pd(PPh3)2Cl2 (552 mg, 0.79 mmol) was dissolved in degassed anhydrous tetrahydrofuran (250 mL). The solution was kept at 80 °C for 24 h. The mixture was poured into water and extracted with

A New Solution-Processable Biarmed Organic Molecule

Figure 1. UV-vis absorption and PL spectra of B(TPA-BT-HT) in chloroform solution and in the film state.

methylene chloride. The organic layer was washed with brine and dried over anhydrous sodium sulfate. After evaporation of the solvent, the residue was purified by column chromatography on silica gel, using a petroleum ether/methylene chloride mixture (10:1), and produced 10 g of red solid pure product with a yield of 33.3%. GC/MS: 382 (M+). 1H NMR (CDCl3, 400 MHz): δ (ppm) 7.96 (s, 1H), 7.84 (d, 1H), 7.70 (d, 1H), 7.07 (s, 1H), 2.71 (t, 2H), 1.73 (m, 2H), 1.41 (m, 2H), 1.34 (m, 4H), 0.92 (t, 3H). 13C NMR (CDCl3, 400 MHz): δ (ppm) 153.75, 151.73, 144.46, 138.06, 132.28, 129.60, 127.29, 125.45, 122.03, 112.02, 31.69, 30.60, 30.44, 29.03, 22.63, 14.10. N,N-(4-(2-(7-(4-Hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazol4-yl)Winyl)phenyl)benzenamine (B(TPA-BT-HT)). A mixture of 4-bromo-7-(4-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (1 g, 2.62 mmol), N,N-(4-vinylphenyl)benzenamine (354 mg, 1.20 mmol), Pd(OAc)2 (5 mg), NaOAc (1.96 g, 23.9 mmol), and n-Bu4NBr (123 mg, 0.38 mmol) was dissolved in degassed N,N-dimethylformamide (50 mL). The solution was kept under a nitrogen atmosphere at 100 °C for 24 h. The mixture was poured into water. The precipitate was filtered, washed with water, dissolved in methylene chloride, and dried over anhydrous sodium sulfate. After evaporation of the solvent, the residue was purified by column chromatography on silica gel, using a petroleum ether/methylene chloride mixture (5:1), and produced 400 mg of red solid B(TPA-BT-HT) with a yield of 40%. MALDI-TOF MS: 897.6, calcd for C54H51N5S4 897. 1H NMR (CDCl3, 400 MHz): δ (ppm) 7.97(s, 2H), 7.93 (d, 2H), 7.80 (d, 2H), 7.62 (d, 2H), 7.55 (d, 2H), 7.51 (s, 1H), 7.33 (t, 4H), 7.20 (d, 2H), 7.15 (d, 2H), 7.10 (m, 4H), 2.71 (t, 4H), 1.74 (m, 4H), 1.40 (m, 12H), 0.92 (t, 6H). 13C NMR (CDCl3, 400 MHz): δ (ppm) 153.85, 152.71, 147.41, 147.09, 144.35, 139.26, 132.54, 132.06, 129.48, 129.30, 128.92, 127.92, 126.33, 125.86, 125.64, 125.16, 123.87, 123.76, 122.89, 121.41, 31.74, 30.70, 30.49, 29.09, 22.67, 14.15. Results and Discussion Photophysical Properties. Figure 1 shows the normalized absorption spectra and photoluminescent (PL) spectra of B(TPABT-HT) solution in CHCl3 and its as-cast thin film. There are two groups of absorption bands between 300 and 600 nm for the solution. The peaks at ca. 332-346 nm could be assigned to π-π* transitions and the absorption maximum at 502 nm should result from an intramolecular charge transfer (ICT) between the donor part (TPA) and the acceptor end groups (BT-

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Figure 2. Cyclic voltammogram of B(TPA-BT-HT) film on Pt electrode in an acetonitrile solution of 0.1 mol/L Bu4NPF6 (Bu ) butyl) with a scan rate of 100 mV/s.

HT) as had been designed. The molar extinction coefficient of B(TPA-BT-HT) reached 56 000 M-1 · cm-1 at 502 nm. The same feature was observed in the thin film absorption spectrum with a red-shift about 25 nm, which could be explained by stronger intermolecular interaction in the condensed solid state. From the onset (642 nm) of the film absorption spectrum, the energy band gap of B(TPA-BT-HT) in the thin film was calculated to be 1.93 eV. The absorption spectrum and the energy bandgap of the two-armed B(TPA-BT-HT) is very similar with that of the star-shaped S(TPA-BT-HT).20 PL spectra of B(TPA-BT-HT) in chloroform solution and in thin film, as also shown in Figure 1, display a red emission peak with maxima at ca. 646 and 636 nm, respectively. The results indicate that B(TPA-BT-HT) could be used as a redemitting electroluminescent organic material in organic lightemitting diodes. Electrochemical Properties. Electrochemical properties of the compound were investigated by cyclic voltammetry. Figure 2 shows the cyclic voltammogram of B(TPA-BT-HT) film on the Pt working electrode in an acetonitrile solution of 0.1 mol/L Bu4NPF6 (Bu ) butyl) with a scan rate of 100 mV/s. The compound exhibited reversible reduction/reoxidation (n-doping/ dedoping) processes, indicating the stability of its n-doping state onset ) and reduction before n-dedoping. The onset oxidation (Eox onset ) of B(TPA-BT-HT) were 0.56 and -1.61 V potentials (Ered vs. Ag/Ag+, respectively, which fell into a similar potential range to other BT-based small molecules.26,27 The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of B(TPA-BT-HT) were estimated according to the following equations:28

HOMO ) -e(Eonset + 4.71) (eV) ox

(1)

LUMO ) -e(Eonset red + 4.71) (eV)

(2)

onset ECV - Eonset g ) e(Eox red ) (eV)

(3)

Where the unit of the potentials is V vs. Ag/Ag+. The calculated HOMO and LUMO energy levels of B(TPA-BT-HT) are -5.27 and -3.10 eV, respectively, which is similar to that of S(TPABT-HT).20 The LUMO level of B(TPA-BT-HT) is ca. 0.8 eV higher than that (-3.91 eV)29 of PCBM, which guarantees the efficient exciton charge separation at the interface of B(TPA-

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Figure 3. Molecular orbital surfaces of the HOMO and LUMO of B(TPA-BT-HT), obtained at the semiempirical pm3 level.

TABLE 1: Photovoltaic Properties of the OSCs Based on the Blend of B(TPA-BT-HT):PCBMs (CB solvent) with Different Weight Ratios and Active Layer Thickness (different spin-coating speed) PC60BM

PC70BM

B:PCBM (w/w)

thickness (nm)

Jsc (mA/cm )

Voc (V)

FF

PCE (%)

thickness (nm)

Jsc (mA/cm2)

Voc(V)

FF

PCE (%)

1:2

81 70 58 85 70 60 89 73 62

1.84 2.42 2.16 2.10 2.55 2.64 2.18 2.19 2.27

0.93 0.92 0.89 0.89 0.90 0.89 0.87 0.88 0.86

0.30 0.32 0.34 0.33 0.38 0.41 0.30 0.32 0.33

0.50 0.72 0.66 0.61 0.87 0.96 0.57 0.61 0.65

80 73 54 82 73 50 82 75 55

4.33 4.81 5.30 3.67 4.32 4.67 4.00 4.01 4.15

0.90 0.90 0.91 0.92 0.92 0.92 0.82 0.84 0.87

0.28 0.31 0.32 0.27 0.28 0.31 0.31 0.32 0.32

1.10 1.32 1.54 0.91 1.12 1.30 1.01 1.08 1.13

1:3 1:4

2

TABLE 2: Photovoltaic Properties of OSCs Based on the Blend Layer of B(TPA-BT-HT):PC70BM (1:2 w/w) from Different Solvent Solutions with Different Spin-Coating Speeds with the solvent of CB thickness Jsc Voc (nm) (mA/cm2) (V) 73 63 54 47

4.81 5.38 5.30 5.00

0.90 0.90 0.91 0.89

with the solvent of DCB

PCE thickness Jsc Voc FF (%) (nm) (mA/cm2) (V) 0.31 0.32 0.32 0.33

1.32 1.55 1.54 1.47

70 63 55 41

4.78 5.12 5.50 3.42

0.92 0.92 0.96 0.88

FF

PCE (%)

0.37 0.37 0.37 0.36

1.62 1.76 1.96 1.09

BT-HT) and PCBMs in the OSCs with B(TPA-BT-HT) as donor and PCBM as acceptor.30 The HOMO level of B(TPA-BT-HT) is 0.51 eV lower-lying than that (-4.76 eV)31 of P3HT, indicating higher stability against oxidation and thus an advantage for solar cell application. In addition, the deeper HOMO level of B(TPA-BT-HT) is beneficial for higher open circuit voltage (Voc) of the OSCs with B(TPA-BT-HT) as donor

Figure 4. Electron and hole mobilities of the blend film of B(TPABT-HT):fullerene from FET measurements. (n stands for electron mobility and p stands for hole mobility.)

material, because the Voc is usually proportional to the difference between the LUMO level of the acceptor and the HOMO level of the donor, and the deeper the HOMO of the donor, the larger the difference will be. Theoretical Calculation. The geometry and the HOMO and LUMO energy levels of B(TPA-BT-HT) have been investigated by means of theoretical calculation with the Gaussian 03 program package at the semiempirical pm3 level. Methyl groups were used in place of the hexyl groups to reduce computation time. As illustrated in Figure 3, the core TPA group shows 3D structure, and BT-HT groups act as the linear coplanar arms. This geometry could result in good solution processability. The HOMO distributes in the whole molecule with higher density on the TPA unit, whereas the LUMO is located predominantly on the BT group on one arm of the molecule. The broad distribution of the HOMO should be good for the hole transport through the molecules when it is used as donor material in the OSCs.

Figure 5. J-V curves of the OSCs based on B(TPA-BT-HT): PC60BM and B(TPA-BT-HT):PC70BM with different weight ratios, under the illumination of AM1.5, 100 mW/cm2. (The inset shows the IPCE spectrum of the OSC based on B(TPA-BT-HT):PC70BM (1:2, DCB).)

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Figure 6. AFM images (height) of B(TPA-BT-HT):PCBM mixtures.

Photovoltaic Properties. Bulk heterojunction OSCs with use of B(TPA-BT-HT) as donor and fullerene derivative PC60BM or PC70BM as acceptor were fabricated with the structure of ITO/PEDOT:PSS/B(TPA-BT-HT):fullerene/Al. To further study the influence of donor/acceptor ratios and active layer thickness, devices with different weight ratios (1:2, 1:3, and 1:4) and active layer thickness (spin speeds range from 2000, to 3000, to 4000 rpm) were fabricated and characterized. The performance parameters of 18 solar cells, with chlorobenzene as solvent in spin-coating the active layer, are summarized in Table 1. From the data listed in Table 1, it can be seen that the main difference between the photovoltaic performances of the devices at different device conditions appeared in the short-circuit currents (Jsc), while the open-circuit voltage (Voc) and the fill factor (FF) remained relatively constant. Higher Jsc values were observed for the devices with PC70BM as the electron acceptor than those with PC60BM as the acceptor. The reason could be the higher absorption coefficient of PC70BM in the visible region than that of PC60BM.32,33 Furthermore, the donor/acceptor weight ratio had an influence on the performances as well. For the devices with PC70BM as acceptor, the optimized B(TPA-BTHT):PC70BM weight ratio is 1:2 with the device PCE of 1.54%, while for the devices with PC60BM as acceptor, the optimized B(TPA-BT-HT):PC60BM weight ratio is 1:3 with the PCE of 0.96%. The effect of the B(TPA-BT-HT):PCBM weight ratio on the photovoltaic performance can be explained as follows. There is a balance between the absorbance and the charge transporting network of the active layer in the PSC devices. Too low content of PCBM will limit the electron transporting ability, while too high content of PCBM will decrease the absorbance and hole transporting ability of the active layer. The different optimized weight ratios for PC60BM and PC70BM could be due to their different electron transport and visible absorption properties. We also used different solvent (CB and DCB) in the preparation of the active layer for optimizing the device performance of the OSCs based on B(TPA-BT-HT): PC70BM with the weight ratio of 1:2. As listed in Table 2, when using DCB as the solvent, the FF (0.36-0.37) of the devices was improved compared to that (0.31-0.33) of the devices with the solvent of CB. The improved FF values could benefit from better morphology, which will be discussed later (see Figure 6), of the active layer in the device fabricated with the solvent of DCB. However, even the improved FF of 0.37 is low, and indicative of poor charge mobility balance. It may suffer from the lower hole mobility of the organic molecule. In our FET determination of electron and hole mobility, we consistently find that there is a considerable inbalance between them (Figure 4). This would cause a low fill factor, as observed. The best device with the solvent of DCB and with the active layer thickness of 55 nm at

the spin-speed of 3500 rpm showed a PCE of 1.96% with a Voc of 0.96 V, a Jsc of 5.50 mA/cm2, and a FF of 0.37. The photovoltaic performance of the two-armed B(TPA-BT-HT) is a little lower than that of the star-shaped S(TPA-BT-HT), but it is still among the best photovoltaic performance for solution processable small molecule photovoltaic donor materials. The J-V curves of three typical OSCs are shown in Figure 5. For the devices with PC70BM as acceptor, the Jsc is greater than 5 mA/cm2. However, the current density increased to 7-8 mA/cm2 at -1 V, which indicates that a larger number of excitons could be produced and dissociated at the interface of B(TPA-BT-HT)/PC70BM and lower Jsc should result from poorer charge transport in the active layer. The influence of the fullerene derivative acceptors and the solvent on the morphology of the active layer was studied by AFM measurement. Figure 6 shows the AFM height images of the blend films of B(TPA-BT-HT)/PCBM formed from CB or DCB solutions. The surface of B(TPA-BT-HT)/PC60BM film formed from CB solution was quite uniform with a rms roughness of ∼0.5 nm (see Figure 6a), and that of the B(TPABT-HT)/PC70BM film formed from DCB solution was even smoother with a roughness of 0.377 nm (see Figure 6c), which is reasonable when using a high-boiling solvent. However, the film of B(TPA-BT-HT)/PC70BM formed from CB solution is inhomogeneous (see Figure 6b). The poorer morphology could be the reason of the lower FF value with the solvent of CB as mentioned above. Conclusion A new solution-processable organic molecule B(TPA-BTHT) with TPA core and two BT-HT arms has been synthesized. OSCs have been fabricated by solution spin-coating with B(TPA-BT-HT) as donor and PC60BM or PC70BM as acceptor. The influence of the weight ratio of B(TPA-BT-HT):fullerene, active layer thickness, and solvent used in the device fabrication on the photovoltaic performance have been investigated. The Jsc values of the devices with PC70BM as acceptor are obviously higher than those of the devices with PC60BM as acceptor, which results from the stronger visible absorption of PC70BM than PC60BM. For the OSCs based on B(TPA-BT-HT):PC70BM, DCB is a better solvent than CB in preparing the active layer. The optimized OSC with the B(TPA-BT-HT):PC70BM weight ratio of 1:2, an active layer thickness of 55 nm (at a spin-coating speed of 3500 rpm), and with the solvent of DCB displayed a power conversion efficiency of 1.96% with a Voc of 0.96 V and a Jsc of 5.50 mA/cm2, under the illumination of AM1.5, 100 mW/cm2. The results indicate that the solution-processable multiarmed molecules with TPA core are promising for the application in OSCs.

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