Novel solution processable small molecule containing new electron-withdrawing group and oligothiophene for photovoltaic applications

Solar Energy Materials & Solar Cells 98 (2012) 343–350

Contents lists available at SciVerse ScienceDirect

Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat

Novel solution processable small molecule containing new electron-withdrawing group and oligothiophene for photovoltaic applications Zaifang Li, Qingfeng Dong, Bin Xu, Weidong Cheng, Shiyu Yao, Xiaoyu Zhang, Shanpeng Wen, Hui Li, Yujie Dong, Wenjing Tian n State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, PR China

a r t i c l e i n f o

abstract

Article history: Received 6 July 2011 Received in revised form 26 October 2011 Accepted 29 October 2011 Available online 6 December 2011

A novel electron-withdrawing group 5-(2,6-dimethyl-4H-pyran-4-ylidene)-1,3-diethyl-2-thioxo-dihydropyrimidine-4,6(1H,5H)-dione (PD) was designed and synthesized according to Knoevenagel condensation. On this basis, a small molecule 8TPDC8 with oligothiophene as electron-donating unit and PD as electron-accepting unit was designed and synthesized. UV–vis absorption spectrum displayed that 8TPDC8 possesses a relatively broad absorption range (from 300 to 900 nm). The cyclic voltammetry investigation displayed that the highest occupied molecular orbital (HOMO) energy level of 8TPDC8 was  5.28 eV, which promised good air stability and high open circuit voltage (Voc) for photovoltaic application. The bulk heterojunction (BHJ) photovoltaic devices were fabricated with 8TPDC8 as the donor material and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor material. A power conversion efficiency of 1.28% has been obtained under the illumination of AM 1.5, 100 mW/cm2. & 2011 Elsevier B.V. All rights reserved.

Keywords: Novel electron-withdrawing group Small molecule Broad absorption range Photovoltaic devices

1. Introduction Photovoltaic devices based on organic polymers and small molecules are evolving into a promising cost-effective alternative to silicon-based solar cells due to their low-cost fabrication through solution processing, light weight, as well as excellent compatibility with flexible substrates [1]. In contrast with polymers [2], solution processable small molecules possess many advantages, such as high purity, well-defined molecular structures, and definite molecular weights. Until now, profound progress has been achieved in the synthesis of new solution processable small molecules and their photovoltaic applications [3–8]. In order to enlarge the absorption spectra of organic conjugated molecules, donor–acceptor (D–A) structure molecules were designed and synthesized [3,4,10–12]. As usual, in D–A small molecules, sulfonyldibenzene [9], 2-(2,6-dimethyl-4H-pyran-4ylidene) malononitrile (PM) [8,13], malononitrile [7], benzothiadiazole [10], diketopyrrolopyrrole [11], and squaraine [5], etc. were used as the electron-withdrawing groups. However, few small molecules exhibited the extremely desirable absorption

n

Corresponding author. Tel.: þ86 431 85166368; fax: þ 86 431 85193421. E-mail address: [email protected] (W. Tian).

0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2011.10.035

spectra matching well with the solar spectrum because of the relatively weak [7,9,10] or strong [5,11] electron-withdrawing abilities of the electron-withdrawing groups in small molecules. For example, the D–A small molecules with sulfonyldibenzene as the electron-accepting unit only covered the absorption range from 300 to 500 nm [9] Molecules based on malononitrile or PM showed the main absorption range from 300 to 650 nm [7,13]. Molecules containing benzothiadiazole also displayed the main absorption range from 300 to 650 nm [10]. Meanwhile the small molecules based on diketopyrrolopyrrole and oligothiophene exhibited its absorption range mainly from 500 to 800 nm [11], And the small molecules based on squaraine demonstrated its absorption range of 550–900 nm [5]. As for the absorption spectra of D–A molecules, the wavelength of the absorption peaks of the electron-donating unit, electron-accepting unit and p–pn interactions between molecules is usually less than ca. 400 nm, while that of the absorption peak of ICT generally is longer than ca. 480 nm. Therefore, the narrow absorption spectra (300–650 nm) could mainly be caused by the relatively weak ICT, because of the weak electron-withdrawing ability of electron-accepting groups. In the molecules containing diketopyrrolopyrrole or squaraine group, the ICT is so strong that there is an absorption valley (400–500 nm) appearing between the absorption of p–pn interaction and the ICT in the absorption spectrum. So, electron-withdrawing groups with too weak or strong electron-withdrawing

344

Z. Li et al. / Solar Energy Materials & Solar Cells 98 (2012) 343–350

Fig. 1. The chemical structure of PD and 8TPDC8.

ability are not very suitable for achieving D–A molecules with a desirable absorption spectra. In this article, we designed and synthesized a new electronwithdrawing group PD, incorporating 1,3-diethyl-2-thioxo-dihydro-pyrimidine-4,6-dione with 2,6-dimethyl-4H-pyran-4-one by a knoevenagel reaction (Fig. 1). We also synthesized 8TPDC8 by using PD as electron-accepting group and oligothiophenes as electron-donating group (Fig. 1). We chose oligothiophene as the electron-donating unit due to its excellent coplanarity, high carrier mobility, good air stability, and strong electron-donating ability [8,11]. The symmetrical combination of a PD unit with alkylated thiophenevinyl assured longer conjugated length and better coplanarity, which could enhance the intermolecular interactions and further reduce the band gap of the molecules. UV–vis absorption spectrum displayed that the 8TPDC8 thin film covered a rather broad absorption range from 300 to 900 nm, which is an extremely desirable absorption spectrum for photovoltaic applications. The objective of this project is aiming at the development of new series of PD derivatives as donors for organic solar cells, we report here preliminary results on BHJ solar cell based on 8TPDC8 as donor and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) as acceptor. A relatively high power conversion efficiency (PCE) of 1.28% was achieved for the photovoltaic devices based on 8TPDC8:PCBM (1:2) under simulated air mass 1.5 global irradiation (100 mW/cm2).

2. Experimental 2.1. Materials All reagents and chemicals were purchased from commercial sources (Aldrich, Across, Fluka) and used without further purification unless stated otherwise. All solvents were distilled over appropriate drying agent(s) prior to use and were purged with nitrogen. Compound 6 was synthesized according to a previous reference [14]. The synthesis route was shown in Scheme 1. 2.2. Synthesis of compounds 3,4-Dioctyllthiophene (1): Mg (4.80 g, 0.20 mol) was placed in 60 mL of dry ether, a grain of I2 was added as catalyst. The solution was heated to 40 1C for stimulating the reaction. 1-Bromooctane (40 mL, 0.25 mol) was added dropwise over 2 h, and the reaction mixture was stirred for another hour. This solution was then transferred slowly via a cannula to a mixture of 3,4dibromothiophene (9.09 mL, 0.08 mol) and 200 mg of Ni(dppp)Cl2

in 50 mL of dry ether while cooling on ice. The reaction mixture was stirred for 16 h at room temperature and subsequently poured out in 300 mL of ice/water containing 10 mL of concentrated HCl. The product was extracted with ether and the combined organic layers were washed with plenty of water and brine, successively. The organic extracts were dried over anhydrous MgSO4, evaporated and purified with column chromatography on silica gel with petroleumether as the eluant to give a slight yellowish oil. 18.26 g (0.059 mol, yeild 74.3%). 1H NMR (300 MHz): d(ppm) 6.884 (s, 2H, –Ph), 2.498 (t, 4H, –CH2), 1.614 (m, 4H, –CH2), 1.297 (m, 20H, –CH2), 0.886 (t, 6H, –CH3). Elem. Anal. Calcd. for C20H36S: C, 77.85; H, 11.76. Found: C, 77.82; H, 11.77. 2,5-Dibromo-3,4-dioctylthiophene (2): Compound 1 (13.44 g) was dissolved in 80 mL of THF at room temperature, then 14.4 g N-bromosuccinimide was added over a period of 5 min. The solution was stirred at room temperature for 8 h. The solvent was then removed in vacuum and hexane (500 mL) was then added (to precipitate all the succinimide). The mixture was filtered through a silica plug and the solvent was removed in vacuum. Distillation under vacuum gave 15.23 g gas a colorless oil. 1H NMR (300 MHz): d(ppm) 2.511 (t, 4H, –CH2), 1.471(m, 4H, –CH2), 1.283 (m, 20H, –CH2), 0.890 (t, 6H, –CH3). Elem. Anal. Calcd. for C20H34Br2S: C, 51.51; H, 7.35. Found: C, 51.52; H, 7.32. 5-Bromo-3,4-dioctylthiophene-2-carbaldehyde (3): n-Buli (7.5 mL of 2.5 M solution in hexane) was added dropwise to a solution of compound 2 (8.0 g) in THF (80 mL) at  78 1C under argon. The solution was stirred at  78 1C for 2 h, then dried DMF (2.2 mL) was added quickly and it was kept at room temperature and stirred for 24 h before being poured into water. The product was extracted with ether. The organic layer was subsequently washed with water and brine and dried over MgSO4, and the solvent was removed by rotary evaporation. The crude product was purified by column chromatography with CH2Cl2:petroleumether (1:2) to give a red brown oil 4.06 g (9.80 mmol, yeild 57.1%). 1H NMR (300 MHz, CDCl3, TMS): d(ppm) 9.909 (s, 1H, –CHO), 2.871 (t, 2H, –CH2), 2.541 (t, 2H, –CH2), 1.536 (m, 4H, –CH2), 1.283 (m, 20H, –CH2), 0.961 (m, 6H, –CH3). Elem. Anal. Calcd. for C21H35BrOS: C, 60.71; H, 8.49. Found: C, 60.73; H, 8.50. 5-(2,6-Dimethyl-pyran-4-ylidene)-1,3-diethyl-2-thioxo-dihydro-pyrimidine-4,6-dione (4): 2,6-Dimethyl-pyran-4-one (2.48 g, 20 mmol) and 1,3-diethyl-2-thioxo-dihydro–pyrimidine-4,6-dione (4 g, 20 mmol) were dissolved in a mixture of acetic anhydride (50 mL), The solution refluxed with vigorous stirring for 24 h. The reaction mixture was cooled to room temperature and then poured into water. After filtration, the residue was recrystallized from ethanol and the yellow solid 3.86 g (12.6 mmol, 63%) was obtained. 1H NMR (300 MHz, CDCl3, TMS): d(ppm) 8.829 (s, 2H, –PD), 4.580 (m, 4H, –CH2), 2.471(s, 6H, –CH3), 1.313 (t, 6H, –CH3). 13 C NMR (75 MHz, CDCl3, TMS): d(ppm) 177.688, 165.579, 161.703, 158.182, 111.592, 96.546, 43.210, 20.628, 12.419. Elem. Anal. Calcd. For C15H18N2O3S: C, 58.80; H, 5.92. Found: C, 58.79; H, 5.95. 5-(2,6-bis((E)-2-(5-bromo-3,4-dioctylthiophen-2-yl)vinyl)-4Hpyran-4-ylidene)-1,3-diethyl-2-thioxo-dihydropyrimidine-4,6(1H, 5H)-dione (5): A mixture of 5-bromo-3,4-dioctylthiophene-2carbaldehyde(3.55 g), 5-(2,6-dimethyl-pyran-4-ylidene)-1,3diethyl-2-thioxo-dihydro-pyrimidine-4,6-dione (1.19 g), piperidine (10 drops), and n-propyl alcohol were refluxed under N2 for 24 h. The reaction mixture was cooled to room temperature and poured into water and extracted with dichloromethane. The combined organic extractions were washed three times with water, dried over anhydrous MgSO4, evaporated under vacuum, and purified with column chromatography on silica gel with dichloromethane:petroleumether (2:1) as the eluate to a red solid 2.57 g was obtained (2.33 mmol, 60.0%). 1H NMR (300 MHz,

Z. Li et al. / Solar Energy Materials & Solar Cells 98 (2012) 343–350

345

Scheme 1. Synthesis routes of 8TPDC8.

CDCl3, TMS): d(ppm) 8.894 (s, 2H, –PD), 7.581 (d, 2H, J¼15.6 Hz, –vinylic), 6.588 (d, 2H, J¼15.6 Hz, -vinylic), 4.606 (m, 4H, –CH2), 2.693 (t, 4H, –CH2), 2.532 (t, 4H, –CH2), 1.518 (m, 8H, –CH2), 1.292 (m, 46H, –CH2, –CH3), 0.892 (t, 6H, –CH3), 0.826 (t, 6H, –CH3). 13C NMR (75 MHz, CDCl3, TMS): 177.527, 161.736, 160.019, 156.212, 144.974, 143.360, 135.159, 127.623, 118.090, 113.848, 112.925, 97.534, 43.358, 31.841, 31.787, 31.537, 29.625, 29.454, 29.390, 29.302, 29.252, 29.205, 28.395, 28.037, 22.638, 22.609, 14.066, 14.004, 12.526. Elem. Anal. Calcd. for C57H84Br2N2O3S3: C, 67.74; H, 6.88. found: C, 67.72; H, 6.90.

Compound (7): n-Butyllithium (1.7 mL of a 2.5 M solution in hexane) was added dropwise into a solution of compound 9 (0.992 g, 4 mmol) in tetrahydrofuran (THF, 100 mL) at  78 1C under an atmosphere of dry argon. After the solution was stirred at 78 1C for 2 h, 0.968 g (4.80 mmol) of trimethylstannyl chloride was added to the solution, and the new solution was warmed to room temperature. The solvent was evaporated by rotary evaporation after the solution was stirred for 12 h. The residue was dissolved in dichloromethane and filtered. The filtrate was dried in vacuum to yield 1.46 g of a yellow solid. The compound

346

Z. Li et al. / Solar Energy Materials & Solar Cells 98 (2012) 343–350

was used for the next reaction as obtained, with no further purifications. 1H NMR (300 MHz, CDCl3, TMS): d(ppm) 7.278 (d, 1H, –Th), 7.199 (d, 1H, –Th), 7.162 (d, 1H, –Th), 7.094 (d, 2H, –Th), 7.012 (t, 1H, –Th), 0.389 (m, 9H, –CH3). Elem. Anal. Calcd. for C15H16S3Sn: C, 43.81; H, 3.92. found: C, 43.85; H, 3.90. 5-{2.6-Bis-[2-(3,4-dioctyl-[2,20 ;500 ,200 ;500 ,2000 ]quaterthiophen-5-yl)vinyl]-pyran-4-ylidene}-1,3-diethyl-2-thioxo-dihydro-pyrimidine4,6-dione (8TPDC8): Compound 10 (752 mg, 1.4 mmol), Compound 11 (562 mg, 0.51 mmol), and 20 mg (PPh3)4Pd(0) (2 mol% with respect to the compound 5) were dissolved in a mixture of toluene (8 mL) and DMF (2 mL, 4/1 volume ratio). The solution was stirred under the Ar atmosphere and refluxed with vigorous stirring at 120 1C for 24 h. The resulting solution was then poured into water and was extracted with chloroform. The extract was washed with brine and dried over anhydrous magnesium sulfate. After evaporation of the solvent, the residue was purified with column chromatography on silica gel with dichloromethane/ petroleumether (2:1) as the eluant was a dark green solid 546 mg (0.38 mmol, 74.5%). 1H NMR (300 MHz, CDCl3, TMS): d(ppm) 8.903 (s, 2H, –PD), 7.680 (d, 2H, J¼15.3 Hz, –vinylic), 7.242 (t, 2H, –Th), 7.210 (m, 2H, –Th), 7.142 (m, 8H, –Th), 7.045 (m, 2H, –Th), 6.671 (d, 2H, J¼15.6 Hz, –vinylic), 4.624 (m, 4H, –CH2), 2.738 (m, 8H, –CH2), 1.605 (m, 8H, –CH2), 1.451 (m, 8H, –CH2), 1.325 (m, 38H, –CH2, –CH3), 0.861 (m, 12H, –CH3). 13C NMR (75 MHz, CDCl3, TMS): 177.610, 161.837, 160.400, 156.271, 147.188, 140.411, 137.984, 136.985, 136.882, 135.532, 134.613, 134.107, 133.756, 128.021, 127.953, 127.418, 124.714, 124.627, 124.468, 124.107, 123.925, 118.108, 113.116, 43.407, 31.914, 31.870, 31.771, 30.431, 29.908, 29.858, 29.717, 29.496, 29.349, 29.302, 28.047, 27.781, 22.713, 22.681, 14.124, 14.064, 12.612. MALDI-TOF MS: calcd for C81H98N2O3S9 1436.2; found 1436.7. Elem. Anal. Calcd. for C81H98N2O3S9: C, 67.74; H, 6.88. Found: C, 67.76; H, 6.85.

photovoltaic area as defined by the geometrical overlap between the bottom ITO electrode and the top cathode was 5 mm2. The J–V characteristics were recorded using Keithley 2400 Source Meter in the dark and under simulated AM 1.5 illumination (100 mW/cm2) by Solar Simulators (SCIENCETECH SS-0.5 K). The spectral response was recorded by SR830 lock-in amplifier under short circuit condition when devices were illuminated with a monochromatic light from a Xeon lamp. Films thickness was measured by Veeco DEKTAK 150 surface profilometer (100 nm for 1600 rpm, 70 nm for 2500 rpm, and 50 nm for 3500 rpm). AFM images were measured by using a Nanoscope IIIa Dimension 3100. All fabrication and characterizations were performed in an ambient environment.

2.3. Instruments and measurements

3.2. Thermal properties

1

H NMR and 13C NMR spectra were measured using a Bruker AVANCE-500 NMR spectrometer and a Varian Mercury-300 NMR, respectively. The elemental analysis was carried out with a Thermoquest CHNS-Ovelemental analyzer. The time-of-flight mass spectra were recorded with a Kratos MALDI-TOF mass system. Differential scanning calorimetry (DSC) were performed under nitrogen flushing at a heating rate of 20 1C/min with a NETZSCH (DSC-204) instrument. Thermogravimetric analysis were performed on a Perkine Elmer Pyris 1 analyzer under nitrogen atmosphere (100 mL/min) at a heating rate of 10 1C/min. UV–vis absorption spectra were measured using a Shimadzu UV-3600 spectrophotometer. Electrochemical measurements of these derivatives were performed with a Bioanalytical Systems BAS 100 B/W electrochemical workstation. Atomic force microscopy (AFM) images of blend films were carried out using a Nanoscope IIIa Dimension 3100.

3. Results and discussion 3.1. Material synthesis and structural characterization The general synthetic routes toward all compounds were outlined in Scheme 1. The compounds 4 and 5 were prepared through Knoevenagel condensation. The structures of these compounds were confirmed by 1H NMR and 13C NMR spectra. The data were included in the Experimental Section. In 1H NMR spectroscopy of compound 5, the coupling constant (J 15.5 Hz) of olefinic protons indicates that Knoevenagel reaction afforded pure all-trans isomers. The final products (shown in Scheme 1) were prepared by the well-known palladium-catalyzed still coupling reactions. Molecular weight of 8TPDC8 was determined by Kratos MALDI-TOF. These results are consistent with the proposed structure and molecule weight. Furthermore, 8TPDC8 exhibited excellent solubility in common organic solvents such as chloroform, tetrahydrofuran, and chlorobenzene.

Differential scanning calorimetry (DSC) was performed to investigate the thermal properties of 8TPDC8. Fig. 2 shows the DSC curve of the small molecule 8TPDC8 after they are purified through recrystallization. When 8TPDC8 was heated, the endothermic peak due to melting temperature was observed at ca. 178 1C. Cooling scans exhibited oppositely exothermic peak due to crystallization at 119 1C, which indicates that 8TPDC8 is crystalline. To investigate the stability of 8TPDC8, we also measured its thermogravimetric analysis (Fig. 3). 8TPDC8 exhibited good thermal stability with 5% weight-loss temperatures (Td) higher than 394 1C.

178°C 2.4. Fabrication and characterization of photovoltaic devices

enhance

For device fabrication, the ITO glass was precleaned and modified by a thin layer of PEDOT:PSS, which was spin-cast from a PEDOT:PSS (Bayer PVP Al 4083) aqueous solution (H.C. Starck) on the ITO substrate, and the thickness of the PEDOT:PSS layer is about 50 nm. The active layer contained a blend of small molecules as electron donor and PCBM as electron acceptor, which was prepared from a chloroform solution with different weight ratios (Small molecules:PCBM¼1:2). After spin-coating the blend from solution at 1600, 2500, and 3500 rpm, respectively, devices were completed by evaporating a 0.6 nm LiF layer and then protected by 100 nm of Al at a base pressure of 5  10  4 Pa. The effective

8TPDC8

119°C 50

75

100 125 150 Temperature (°C)

175

200

Fig. 2. Differential scanning calorimetry (DSC) measurement of 8TPDC8, scan rate 10 1C min  1.

Z. Li et al. / Solar Energy Materials & Solar Cells 98 (2012) 343–350

3.3. Optical properties The normalized UV–vis absorption spectrum of 8TPDC8 in dilute chloroform solution (concentration 10  5 M) is shown in Fig. 4, and the main optical properties are listed in Table 1. 8TPDC8 shows two absorption peaks at 412 and 555 nm, which can be assigned to the intrinsic absorption of p–pn transition of the molecule backbone and ICT transition between

1.0

394°C 8TPDC8

weight ratio

0.8

0.6

0.4

0.2

0.0 0

200

400 Temperature (°C)

600

800

Fig. 3. Thermogravimetric analysis (TGA) measurement of the small molecule 8TPDC8.

1.2 8TPDC8-S

1.0 Absorption (a.u.)

8TPDC8-F 0.8

8TPDC8:PCBM

0.6 0.4 0.2 0.0 300

400

500

600 700 800 Wavelength (nm)

900

1000

Fig. 4. Normalized absorption spectrum of 8TPDC8 in chloroform solution (S), in a film (F) and 8TPDC8:PCBM (1:2) film.

347

oligothiophene and PD group. Moreover, the relatively high absorption coefficient (62,370) was calculated from the Beer’s law equation. The UV–vis spectrum of 8TPDC8 in thin film reveals a pronounced broadening of the absorption band and a red shift of the absorption peak 619 nm. The absorption band-edge 775 nm leads to the estimated band gap (Eg) of 1.60 eV for 8TPDC8, which could attribute to the stronger intermolecular interactions in the solid state [8,11]. It is so excited that the absorption of 8TPDC8 thin film covered the broad ranges from 300 to 900 nm, which could be caused by the following two reasons. First, there exists strong ICT inside the molecule because of the strong electronwithdrawing ability of PD and the relatively strong electrondonating ability of oligothiophene. Second, the excellent coplanarity of 8TPDC8 could also enhance the p–pn stacking effect and thus enlarging the absorption of the molecule. We also measured the UV–vis absorption spectrum (Fig. 4) of the blend film (8TPDC8:PCBM¼1:2). It can be seen that the blend film displayed a narrow absorption spectrum compared that in the pure solid thin film, which could be caused by the existing of excess PCBM in the solid film. The excess PCBM can destroy immensely the intermolecular p–p stacking in the solid state and thus the red shift of the absorption spectra. Therefore, the blend film displayed more narrow absorption range (300–750 nm) than that of pure film (300–850 nm). 3.4. Electrochemical properties Fig. 5 shows the cyclic voltammetry (CV) diagrams of PD and 8TPDC8 using TBAPF6 as a supporting electrolyte in methylene dichloride solution with platinum button working electrodes, a platinum wire counter electrode and an Ag/AgNO3 reference electrode under the N2 atmosphere. Ferrocene was used as the internal standard. The redox potential of Fc/Fc þ which has an absolute energy level of  4.8 eV relative to the vacuum level for calibration is located at 0.11 V in 0.1 M TBAPF6/methylene dichloride solution [15]. The electrochemical measurement results of PD were 0.74 V (oxidation potential) and 1.18 V (reduction potential), respectively. Therefore, the calculated energy levels of PD were  5.43 eV (HOMO) and  3.51 eV (LUMO), which are both very ideal energy levels for constructing donor materials. Table 1 shows the calculated HOMO (  5.28 eV) and LUMO energy level (  3.60 eV) from the redox potential of 8TPDC8. The HOMO value of  5.28 eV indicates that 8TPDC8 may possess good air stability [16] and relatively high Voc, because Voc is related to the difference between the LUMO energy level of the acceptor and the HOMO energy level of the donor within the active layer. The relatively low LUMO value of 3.60 eV also promises effective charge transfer from the donor (8TPDC8) to acceptor (PCBM) [15]. Furthermore, the electrochemical band gap Eg,ec (1.68 eV, list in Table 1) is well in agreement with the optical band gap Eg,opt (1.60 eV).

Table 1 Optical and electrochemical data of 8TPDC8. Molecule

In solutiona

labs max (nm) (emax (M 8TPDC8 a b c

1

cm

In filmb

1

))

412, 555(62370)

ledge

labs max (nm)

ledge

(nm)

(nm)

(nm)

696

436, 619

775

1  10  5 M in anhydrous chloroform. 8TPDC8 was spin-coated from a 10 mg/mL chloroform solution. The optical band gap (Eg,opt) was obtained from film absorption edge.

c

Eonset Ox

Eonset Red

Electrochem.

Optical

(V)/HOMO (eV)

(V)/LUMO (eV)

Eg,ec (eV)

Eg,opt (eV)c

0.58/  5.28

 1.10/  3.60

1.68

1.60

348

Z. Li et al. / Solar Energy Materials & Solar Cells 98 (2012) 343–350

3.5. Theoretical calculation The geometry and electronic properties of 8TPDC8 have been investigated by means of theoretical calculation with the Gaussian 03 program package at a hybrid density functional theory (DFT) level (Fig. 6). H atoms were used in place of the n-octyl groups to limit computation time. The calculation results demonstrate that the electron density of the HOMO energy level distributes mainly on the oligothiphene unit, while that of the LUMO energy level mainly delocalizes on the PD unit, indicating a charge-transfer nature of HOMO-LUMO from the electron-donating unit to the PD-acceptor unit. Furthermore, the calculated HOMO energy level ( 5.14 eV) is nearly consistent with the electrochemical measurement while the LUMO energy level (2.88 eV) exhibits some bias.

energy level of the donor within the active layer [17–19]. Furthermore, it can be seen that the solvents and spin speeds have almost no effect on the Voc. To gain further insight into what might affect the PCE of the photovoltaic device, atomic force microscopy (AFM) was employed to probe the blended film morphological properties with a ratio of 8TPDC8/PCBM (1:2 w/w, 2500 rpm). Fig. 9 shows the AFM topographical height and phase images of the blended film. It is clearly shown by the height image (Fig. 9a) that this blended film exhibits a relatively smooth surface with a rootmean-square (rms) roughness of 0.68 nm, which demonstrates that 8TPDC8 possesses good compatibility with PCBM. As we know that the optimal scale of the phase separation is that of the exciton diffusion length, and the separated phases must be

3.6. Photovoltaic performance

Fig. 6. Molecular orbital surfaces of the HOMO and LUMO of 8TPDC8 obtained at B3 LYP/6-31G* level.

1

Current Density (mA cm-2)

To demonstrate the application potential of 8TPDC8 as an electron donor in organic solar cells, devices with 8TPDC8 as donor and PCBM as acceptor were fabricated with the structure of ITO/PEDOT:PSS/8TPDC8:PCBM/LiF/Al. The current–voltage (J–V curves are presented in Figs. 7 and 8, and the corresponding data are listed in Table 2. In order to study the influence of solvents, photovoltaic devices with different solvents (chlorobenzene and chloroform) were fabricated at the same spin speed of 2500 rpm. From the data listed in Table 2, it can be seen that the main difference between the photovoltaic performance of the two devices (2500 rpm) appeared in the short-circuit currents (Jsc), while the open-circuit voltage (Voc) and the fill factor (FF) remained relatively constant. The higher Jsc (4.81 mA/cm2) and PCE (1.28%) obtained from chloroform solution than those obtained from chlorobenzene solution (Jsc ¼3.17 mA/cm2, PCE 0.88%) could be due to the excellent solubility of the former. To further investigate the influence of the active layer thickness, devices with different spin speeds (range from 1600, to 2500, to 3500 rpm) were fabricated and characterized. The current–voltage (J–V curves are presented in Fig. 8, and the corresponding data are also listed in Table 2. It can be seen that the device with the active layer thickness of 70 nm exhibited the highest PCE of 1.28%, which could be caused by the appropriate thickness for both absorbance and carrier transport. Too thick active layer will limit the carrier transport ability, while too thin active layer will decrease the absorbance. As for Voc, the relatively high Voc of the device based on 8TPDC8/PCBM could be explained by the low HOMO energy level of 8TPD ( 5.28 eV), because Voc is related to the difference between the LUMO energy level of the acceptor and the HOMO

8TPDC8:PCBM---Chlorobenzene 8TPDC8:PCBM---Chloroform

0 -1 -2 -3 -4 -5 0.0

0.2

0.4 0.6 Voltage (V)

0.8

1.0

Fig. 7. Current–voltage curves of photovoltaic cell based on 8TPDC8:PCBM (1:2) with chlorobenzene and chloroform as solvents respectively, under illumination of AM 1.5, 100 mW/cm2.

Fig. 5. Cyclic volatammetry curves of PD (a) and 8TPDC8 (b) solutions on platinum electrode in 0.1 mol/L n-Bu4NPF6 in CH2Cl2 solution, at a scan rate of 100 mV/s.

1

40

0

35

-1

8TPDC8:PCBM

25

-3 -4

-6

20 15

8TPDC8:PCBM (1600rmp) 8TPDC8:PCBM (2500rmp) 8TPDC8:PCBM (3500rmp)

-5

-7 -0.2

349

30

-2 EQE (%)

Current density (mA cm-2)

Z. Li et al. / Solar Energy Materials & Solar Cells 98 (2012) 343–350

10 5

0.0

0.2

0.4

0.6

0.8

1.0

Voltage (V)

0 400

Fig. 8. Current–voltage curves of photovoltaic cell based on 8TPDC8:PCBM(1:2) under illumination of AM 1.5, 100 mW/cm2 with different spin speeds (1600, 2500, and 3500 rpm).

Table 2 Characteristic current–voltage data from device with the weight ration of 8TPMC8:PCBM¼ 1:2, testing at standard AM 1.5 G conditions. Solvents

Rotate speed (rp m)/ thickness (nm)

Voc (v)

Jsc (mA/ cm2)

FF

PCE (%)

Chlorobenzene Chloroform Chloroform Chloroform

2500 1600/100 2500/70 3500/50

0.88 0.89 0.92 0.90

3.17 3.25 4.81 3.80

0.32 0.25 0.29 0.29

0.88 0.82 1.28 1.01

500

600

700

800

900

Wavelength (nm) Fig. 10. External quantum efficiency (EQE) curve for device using 1:2 blend of 8TPDC8:PCBM.

contiguous to allow for low-resistance charge transport pathways from the active layer to the electrodes [22,23]. Unfortunately, the devices fabricated using 8TPDC8:PCBM did not exhibit the ideal phase separation and thus the bad the series resistance (9.3 O cm2), the parallel resistance (385 O cm2) and the exceptionally low fill factors (FF ¼0.29) [24]. To further confirm the accuracy of the measurements, the external quantum efficiency (EQE) of the device based on 8TPDC8 and PCBM was performed. It can be seen that the EQE mainly covered the absorption range from 350 to 750 nm, which is very similar to the UV–vis absorption spectrum of the same blend film (Fig. 4).

4. Conclusions

Fig. 9. Tapping mode AFM (1  1 mm2) topographical (a) and phase (b) images of 8TPDC8/PCBM (1:2 w/w) film spin-coated from chloroform.

contiguous to allow for low-resistance charge transport pathways from the active layer to the electrodes [20,21]. Unfortunately, the devices fabricated using 8TPDC8:PCBM did not exhibit the ideal phase separation (Fig. 9) and thus increased the probability of exciton recombination. Therefore, the short-circuit current of the device based on 8TPDC8:PCBM is only 4.81 mA/cm2 (Fig. 10). Furthermore, we also measured the series resistance (9.3 O cm2) and the parallel resistance (385 O cm2). The bad resistance mainly caused by the relatively poor film-forming ability of these small molecules, which results the non-desired phase separation in the active layer. As we know that the optimal scale of the phase separation between these constituents is that of the exciton diffusion length, and the separated phases must be

To summarize, a new solution processable Donor–Acceptor (D–A small molecule (8TPDC8) was designed and synthesized with a new electron-withdrawing group (PD) as the electronwithdrawing group and oligothiophene as the electron-donating group. The UV–vis absorption spectrum showed that 8TPDC8 thin film covered a rather broad absorption spectrum from 300 to 900 nm, which is an extremely desirable absorption range for photovoltaic applications. The Cyclic voltammetry investigation displayed that the HOMO energy levels of 8TPDC8 were relatively low, which promised good air stability and high open circuit voltage (Voc) for organic solar cell application. Preliminary results on solar cells exhibit a relatively high Voc (0.92 V) and power conversion efficiency of 1.28%.

Acknowledgment This work was supported by the State Key Development Program for Basic Research of China (Grant no. 2009CB623605), the National Natural Science Foundation of China (Grant no. 20874035), and the Project of Jilin Province (20080305). References [1] (a) S. Gunes, H. Neugebauer, N.S. Sariciftci, Conjugated polymer-based organic solar cells, Chemical Reviews 107 (2007) 1324–1338; (b) E. Bungaard, F.C. Krebs, Low bandgap polymers for organic photovoltaics, Solar Energy Materials and Solar Cells 91 (2007) 954–985;

350

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11]

[12]

Z. Li et al. / Solar Energy Materials & Solar Cells 98 (2012) 343–350

(c) F.C. Krebs, T. Tromholt, M. Jørgensen, Upscaling of polymer solar cell fabrication using full roll-to-roll processing, Nanoscale 2 (2010) 873–886. (a) F.C. Krebs, R.B. Nyberg, M. Jørgensen, Influence of residual catalyst on the properties of PPVs, Chemistry of Materials 16 (2004) 1313–1318; (b) K.T. Nielsen, K. Bechgaard, F.C. Krebs, Removal of palladium nanoparticles from polymer materials, Macromolecules 38 (2005) 658–659. N.M. Kroneneberg, M. Deppish, F. Wurthner, H.W.A. Ledemann, K. Deing, K. Meerholz, Bulk heterojunction organic solar cells based on merocyanine colorants, Chemical Communications 48 (2008) 6489–6491. T. Rousseau, A. Cravino, T. Bura, G. Ulrich, R. Ziessel, J. Roncali, BODIPY derivatives as donor materials for bulk heterojunction solar cells, Chemical Communications (2009) 1673–1675. F. Silvestri, M.D. Irwin, L. Beverina, A. Facchetti, G.A. Pagani, T.J. Marks, Solution processable bulk-heterojunction solar cells using a small molecule acceptor, Journal of the American Chemical Society 130 (2008) 17640–17641. B. Walker, A.B. Tamayo, X. Dung Dang, P. Zalar, J.H. Seo, A. Garcia, M. Tantiwiwat, T.Q. Nguyen, Nanoscale phase separation and high photovoltaic efficiency in solution-processed, small-molecule bulkheterojunction solar cells, Advanced Functional Materials 19 (2009) 3063–3069. W.F. Zhang, S.C. Tse, J.P. Lu, Y. Tao, M.S. Wong, Solution process able donor–acceptor oligothiophenes for bulk-heterojunction solar cells, Journal of Materials Chemistry 20 (2010) 2182–2189. Z.F. Li, J.N. Pei, Y.W. Li, B. Xu, M. Deng, Z.Y. Liu, H. Li, H.G. Lu, Q. Li, W.J. Tian, Synthesis and photovoltaic properties of solution processable small molecules containing 2-pyran-4-ylidenemalononitrile and oligothiophene moieties, Journal of Physical Chemistry C 114 (2010) 18270–18278. K.P. Li, J.L. Qu, B. Xu, Y.H. Zhou, L.J. Liu, P. Peng, W.J. Tian, Synthesis and photovoltaic properties of novel solution-processable triphenylamine-based dendrimers with sulfonyldibenzene cores, New Journal of Chemistry 33 (2009) 2120–2127. J. Zhang, Y. Yang, C. He, Y.J. He, G.J. Zhao, Y.F. Li, Solution-processable starshaped photovoltaic organic molecule with triphenylamine core and benzothiadiazole-thiophene arms, Macromolecules 42 (2009) 7619–7622. A.B. Tamayo, B. Walker, T.Q. Nguyen, A low band gap, solution processable oligothiophene with a diketopyrrolopyrrole core for use in organic solar cells, Journal of Physical Chemistry C 112 (2008) 11545–11551. (a) Z.F. Li, Q.F. Dong, Y.W. Li, B. Xu, M. Deng, J.N. Pei, J.B. Zhang, F.P. Chen, S.P. Wen, Y.J. Gao, W.J. Tian, Design and synthesis of solution processable small molecules towards high photovoltaic performance, Journal of Materials Chemistry 21 (2011) 2159–2168;

[13]

[14]

[15]

[16]

[17]

[18]

[19]

[20]

[21] [22] [23]

[24]

(b) Z.F. Li, Q.F. Dong, B. Xu, H. Li, S.P. Wen, J.N. Pei, S.Y. Yao, H.G. Lu, P.F. Li, W.J. Tian, Solar Energy Materials and Solar Cells 98 (2011) 2272–2280. L.L. Xue, J.T. He, X. Gu, Z.F. Yang, B. Xu, W.J. Tian, Efficient bulk-heterojunction solar cells based on a symmetrical D-u-A-u-D organic dye molecule, Journal of Physical Chemistry C 113 (2009) 12911–12917. M. Melucci, G. Barbarella, M. Zambianchi, P.D. Pietro, A. Bongini, Solutionphase microwave-assisted synthesis of unsubstituted and modified a-quinque- and sexithiophenes, Journal of Organic Chemistry 69 (2004) 4821–4828. B.C. Thompson, Y.G. Kim, J.R. Reynolds, Spectral broadening in MEH-PPV: PCBM-based photovoltaic devices via blending with a narrow bandgap cyanovinylene–dioxy thiophene polymer, Macromolecules 38 (2005) 5359–5362. D.M.D. Leeuw, M.M.J. Simenon, A.R. Brown, R.E.F. Einerhand, Stability of n-type doped conducting polymers and consequences for polymeric microelectronic devices, Synthetic Metals 87 (1997) 53–59. C.J. Brabec, A. Cravino, D. Meissner, N.S. Sariciftci, T. Fromherz, M.T. Rispens, L. Sanchez, J.C. Hummelen, Origin of the open circuit voltage of plastic solar cells, Advanced Functional Materials 11 (2001) 374–380. M.C. Scharber, D. Mhuhlbacher, M. Koppe, P. Denk, C. Waldauf, A.J. Heeger, C.J. Brabec, Design rules for donors in bulk-heterojunction solar cells— towards 10% energy-conversion efficiency, Advanced Materials 18 (2006) 789–794. G. Dennler, M.C. Scharber, T. Ameri, P. Denk, K. Forberich, C. Waldauf, C.J. Brabec, Design rules for donors in bulk-heterojunction tandem solar cells towards 15% energy-conversion efficiency, Advanced Materials 20 (2008) 579–583. P. Peumans, S. Uchida, S.R. Forrest, Efficient bulk heterojunction photovoltaic cells using small-molecular-weight organic thin films, Nature 425 (2003) 158–162. H. Hoppe, N.S. Sariciftci, Morphology of polymer/fullerene bulk heterojunction solar cells, Journal of Materials Chemistrys 16 (2006) 45–61. C.V. Hoven, X.D. Dang, R.C. Coffin, J. Peet, T.Q. Nguyen, G.C. Bazan, Advanced Materials 22 (2010) 63–66. M. Campoy-Quilers, T. Ferenczi, T. Agostinelli, P.G. Etchegoin, Y. Kim, T.D. Anthopoulos, P.N. Stavrinou, D.D.C. Bradley, J. Nelson, Morphology evolution via self-organization and lateral and vertical diffusion in polymer:fullerene solar cell blends, Nature Materials 7 (2008) 158–164. G.D. Wei, S.Y. Wang, K. Renshaw, M.E. Thompson, S.R. Forrest, Solutionprocessed squaraine bulk heterojunction photovoltaic cells, American Chemistry Society Nano 4 (2010) 1927–1934.