A planar copolymer for high efficiency polymer solar. cells

Transcription

1 A planar copolymer for high efficiency polymer solar cells Ruiping Qin,, Weiwei Li,, Cuihong Li, Chun Du, Clemens Veit, Hans-Frieder chleiermacher, Mattias Andersson, Zhishan Bo,,, * Zhengping Liu,, * Olle Inganäs, Uli Wuerfel, Fengling Zhang,, * Laboratory of Polymer Physics and Chemistry, Institute of Chemistry CA, Beijing , China, Biomolecular and Organic Electronics, Department of Physics, Chemistry and Biology, Linköping University, E-58183, Linköping, weden, College of Chemistry, Beijing Normal University, Beijing , China, Fraunhofer Institute for olar Energy ystems IE, Germany Institute of Chemistry Linköping University Beijing Normal University Fraunhofer Institute for olar Energy ystems IE 1

2 Experimental section Materials and instruments: Unless otherwise noted, all chemicals were purchased from Aldrich or Acros and used without further purification. olvents were dried using standard procedures. The catalyst precursor Pd(PPh 3 ) 4 was prepared according to the literature 1 and stored in a chlenk tube under nitrogen. 4,7-Dibromo-5,6- bis(octyloxy)benzo-2,1,3-thiadiazole 2 and 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9-octyl- 9H-carbazole 3 were prepared according to literature procedures. All reaction were performed under an atmosphere of nitrogen and monitored by TLC with silica gel 60 F254 (Merck, 0.2 mm). Column chromatography was carried out on silica gel ( mesh). 1 H and 13 C NMR spectra were recorded on a Bruker DM 300 or AV 400 spectrometer in CDCl 3. The gel permeation chromatography (GPC) measurements were performed at 150 o C on a PL-220 (Polymer Laboratories) chromatography connected to a differential refractometer with 1,2,4-trichlorobenzene as an eluent. Elemental analyses were performed on a Flash EA 1112 analyzer. The powder X-ray diffraction (XRD) patterns were collected using a PANalytical X' Pert PRO MPD diffractometer with Cu KR radiation. The electrochemical behavior of HX-1 was investigated by using cyclic voltammetry (CHI 630A Electrochemical Analyzer) with a standard three-electrode electrochemical cell in a 0.1 M tetrabutylammonium tetrafluoroborate solution in CH 3 CN at room temperature at atmosphere with a scanning rate of 0.1 V/s -1. A glassy carbon working electrode, a Pt wire counter electrode, and an Ag/AgNO 3 (0.01M in CH 3 CN) reference electrode were used. The experiments were calibrated with the standard ferrocene/ferrocenium (Fc) redox system and assumption that the energy level of Fc is 4.8 ev below vacuum. 4 Electronic absorption spectra were obtained on a HIMADZU UV-visible spectrometer model UV-1601PC. Fluorescence spectra were recorded on a Varian FLR025. TGA was carried out on a Pyris 1 TGA instrument under a nitrogen atmosphere at a heating rate of 10 C/min to record the thermal gravimetric analysis (TGA) trace. ynthesis: C 8 H 17 O OC 8 H C 17 8 H 17 O OC 8 H 17 C 8 H 17 O OC 8 H 17 Br O [Pd] NB Br Br + B O N N N N N N Br 5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)benzo-[c][1,2,5]-thiadiazole (2) 2

3 A mixture of 4,7-dibromo-5,6-bis(octyloxy)benzo-[c][1,2,5]-thiadiazole 1 (10.0 g, mmol) and 4,4,5,5-tetramethyl-2-(thiophen-2-yl)-1,3,2-dioxaborolane (10.0 g, 47.6 mmol), Na 2 CO 3 (10.0 g, mmol), toluene (150 ml), and water (50 ml) was carefully degassed before and after Pd(PPh 3 ) 4 (1.0 g, mmol) was added. The reaction mixture was stirred at 90 o C under nitrogen atmosphere for 10 days. The organic layer was separated; the aqueous one was extracted with CH 2 Cl 2 (3 100 ml); the combined organic layers were dried over anhydrous Na 2 O 4 and evaporated to dryness. The residue was chromatographically purified on silica gel column eluting with CH 2 Cl 2 /hexane (1:10, v:v) to afford 2 as a yellow oil (4.96 g, 49%). 1 H NMR (400 MHz, CDCl 3 ): δ (d, 2H), (d, 2H), (t, 2H), (t, 4H), (m, 4H), (m, 4H), (m, 16H), (t, 6H). 13 C NMR (100 MHz, CDCl 3 ) δ , , , , , , , 72.41, 29.92, 28.43, 27.61, 27.37, 24.05, 20.76, Anal. Calcd. for C 30 H 40 N 2 3 : C, 64.71; H, 7.24; N, Found: C, 62.84; H, 7.16; N, ,7-Bis(5-bromothiophen-2-yl)-5,6-bis(octyloxy)benzo-[c][1,2,5]-thiadiazole (3) A mixture of 2 (4.9 g, 8.8 mmol), N-bromosuccimide (NB) (3.2 g, mmol), and chloroform (50 ml) was stirred at room temperature in dark for 24 hours. The solvent was removed under reduced pressure, the residue was chromatographically purified on silica gel column eluting with CH 2 Cl 2 /hexane (1:10, v:v) to afford 3 as an orange crystal (4.96 g, 90%). 1 H NMR (400 MHz, CDCl 3 ): δ (d, 2H), (d, 2H), (t, 4H), (m, 4H), (m, 4H), (16, H), (t, 6H). 13 C NMR (100 MHz, CDCl 3 ): δ , , , , , , , 74.54, 31.80, 30.25, 29.44, 29.37, 25.90, 22.66, Anal. Calcd. for C 30 H 38 Br 2 N 2 O 2 3 : C, 50.42; H, 5.36; N, Found: C, 50.84; H, 5.50; N, Poly(2-(5-(5,6-bis(octyloxy)-4-(thiophen-2-yl)benzo[c][1,2,5]thiadiazol-7-yl)thiophen-2-yl)-9-octyl- 9H-carbazole) (HX-1). A mixture of 3 (1.345 g, mmol), 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9-octyl-9H- 3

4 carbazole (1.000 g, mmol), NaHCO 3 (6.0 g, 71.4 mmol), H 2 O (30.0 ml), and THF (150.0 ml) was carefully degassed before and after Pd(PPh 3 ) 4 (22 mg, 19 μmol) was added. The mixture was stirred and refluxed for 2 days under nitrogen. The mixture was poured into methanol (200 ml) and the resulted precipitate was collected by filtration. The crude polymer was dissolved in hot chloroform (300 ml) and filtered. The filtration was concentrated to about 100 ml and precipitated into acetone (400 ml). The precipitated was collected by filtration and dried under high vacuum to afford polymer HX-1 as a black solid (1.16 g, 74%). A number average molecule weight of 16.6 kg/mol, a weight average molecular weight of 51.4 kg/mol, and a PDI of 3.1 were determined by gel permeation chromatography (GPC) at 150 o C with 1,2,4-trichlorobenzene as an eluent vs polystyrene standards. 1 H NMR (400 MHz, 1,2-dichlorobenzene-d 4 ) δ 8.92 (m, 2H), 8.15 (m, 2H), 7.80 (m, 6H), 4.35 (m, 6H), 2.20 (m, 4 H), 1.98 (m, 2H), (m, 30 H), 0.94 (m, 9H). 13 C NMR (100 MHz, solid state): δ , , , , , , , 73.72, 42.61, 30.82, 23.53, Anal. Calcd. for [C 50 H 61 N 3 O 2 3 ] n : C, 72.16; H, 7.39; N, Found: C, 71.75; H, 7.54; N, Reference: (1) Tolman, C. A.; eidel, W. C.; Gerlach, D. H. J. Am. Chem. oc. 1972, 94, (2) Bouffard, J.; wager, T. M. Macromolecules 2008, 41, (3) Fu, Y.; Bo, Z. Macromol. Rapid Commun. 2005, 26, (4) Pommerehne, J.; Vestweber, H.; Guss, W.; Mahrt, R. F.; Bässler, H.; Porsch, M.; Daub, J. Adv. Mater.1995, 7,

5 HX-1 Intensity Figure 1. X-ray diffraction pattern of powdery HX-1 sample. (A) Absorbance (a.u.) 0,4 0,3 0,2 0,1 0,0 a b c Wavelength (nm) (B) PL Intensity (a.u) 6x10 6 5x10 6 4x10 6 3x10 6 2x10 6 1x10 6 a b c Wavelength (nm) PL Intensity (a.u.) 2x10 4 1x10 4 Wavelength (nm) b c (C) Absorption coefficient (M -1 cm -1 ) Wavelength (nm) P3HT HX-1 Figure 2. UV-vis absorption (A) and PL spectra (B) of film samples. a: HX-1; b: HX-1/PC 71 BM spin-coated from DCB solution; c: HX-1/PC 71 BM spin-coated from DCB:diiodooctance (2.5%). (C) UV absorption spectra of HX-1 and P3HT in chloroform solution (concentration: mg/ml). 5

6 11, , ,0 740 J C [ma/cm 2 ] 9,5 9,0 8,5 8,0 7,5 I II III V OC [ma/cm 2 ] I II III 0, , Fill factor 0,60 0,55 0,50 0,45 0,40 I II III Efficiency I II III Figure 3. The statistical graphs of the performances of three group solar cells, group I and II were spin-coated from DCB:DIO solutions with polymer concentration of 10.0 g/l and 5.0 g/l and group III are control cells from pure DCB solution with polymer concentration of 5.0 g/l, where squares represent mean values and filled circles are values of the cell with the highest PCE. Current density [ma/cm 2 ] Voltage [mv] Figure 4. I-V characteristics of five solar cells on one substrate. 6

7 Table 1. Parameters of five solar cells on the same substrate. ubstrate Pixel Jsc (ma/cm2) 9,57 9,48 9,49 9,76 9,62 Voc (V) 0,83 0,82 0,81 0,81 0,8 FF 0,69 0,69 0,69 0,69 0,68 PCE (%) 5,5 5,4 5,3 5,4 5,3 Figure 5. AFM images of HX-1/PC71BM film prepared by spin-coating from 1,2-dichlorobenzene solution (top) without 1,8-diiodooctane and (bottom) with 1,8-diiodooctane (2.5%). 7

8 Figure 6. Differential scanning calorimetry of HX-1 at a heating rate of 20 C/min (second round). Polymer olar Cell Fabrication Polymer solar cells (PCs) were fabricated with the device configuration of ITO/PEDOT:P/Polymer: PC 71 BM (1:2.5)/LiF/Al. The conductivity of ITO was 20 Ω/ and PEDOT:P is Baytron AI 4083 from H.C. tarck. HX-1 is only soluble in DCB when heated solution to 90 o C, so it is necessary to spin coat active layers from DCB at elevated temperature. The ITO glass was cleaned and a thin layer of PEDOT: P (Baytron AI 4083) was spin coated at 3000 rpm and dried subsequently at 120 C for 10 min in an oven. The substrates were then transferred to a glove box where all following preparation steps and the characterization took place. The active layers containing HX-1 and PC 71 BM (1:2.5, w/w) were spin-coated on the top of ITO/PEDOT:P from DCB solution with or without DIO in the glove 8

9 box. The DCB solutions were heated at 90 o C for an hour before spin-coating. The prepared films were kept in a vacuum chamber overnight before thermally evaporated 0.3 nm LiF followed by 100 nm Al at a pressure of mbar at room temperature. ix cells were fabricated on one substrate with an effective area of cm 2. Device characterization The measurement of devices was conducted in the glove box. The temperature while measuring the IV curves was approximately 30 C. Detail information on the setup for device characterization can be found in the following reference (Prog. Photovolt: Res. Appl. 2008; 16: ). 9