научная статья по теме SYNTHESIS AND PHOTOVOLTAIC PROPERTIES OF ALTERNATING CONJUGATED POLYMERS DERIVED FROM INDOLO[3,2-B]CARBAZOLE AND THIOPHENE/THIENO[3,2-B]THIOPHENE-CORED BENZOSELENADIAZOLE Физика

ВЫСОКОМОЛЕКУЛЯРНЫЕ СОЕДИНЕНИЯ, Серия А, 2011, том 53, № 6, с. 864-874

ФУНКЦИОНАЛЬНЫЕ ПОЛИМЕРЫ

SYNTHESIS AND PHOTOVOLTAIC PROPERTIES OF ALTERNATING CONJUGATED POLYMERS DERIVED FROM INDOLO[3,2-b]CARBAZOLE AND TfflOPHENE/THIENO[3,2-b]TfflOPHENE-CORED BENZOSELENADIAZOLE1

© 2011 n Bin Li", b, Yangjun Xiab, Junfeng Tongb, Caixia Yang", and Duowang Fanb

a College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, China, 730070 b Key Laboratory of Optoelectronic Technology and Intelligent Control of Ministry Education, Lanzhou Jiaotong University,

Lanzhou, China, 730070 e-mail:yjxia73@126.com (Yangjun Xia), yangcx@nwnu.edu.cn (Caixia Yang) Received October 29, 2010 Revised Manuscript Received January 21, 2011

Abstract—Two alternating narrow band gap (NBG) copolymers derived from 5,11-di(N-9-heptadeca-nyl)-indolo[3,2-b]carbazole (ICZ) and 4,7-di(thien-2-yl)-2',1',3'-benzoselenadiazole (DSeBT) or 4,7-di(thieno[3,2-b]thien-2-yl)-2',1',3'-benzoselenadiazole (DTSeBT), were synthesized and named PICZ-DSeBT and PICZ-DTSeBT, respectively. The PICZ-DSeBT shows good solubility in common organic solvent, and the PICZ-DTSeBT is soluble in hot o-dichlorobenzene (ODCB) and not good soluble in chloroform, toluene etc. The chemical structure, molecular weight and fundamental physical properties of the copolymers were characterized by *H NMR, gel permeation chromatography (GPC), cyclic voltam-metry (CV), thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) etc. Potential application of the copolymers to be employed as electron donor material and PC61BM ([6,6]-phenyl-C61 butyric acid methyl ester) for photovoltaic solar cells (PSCs), were investigated. PSCs based on the blends of PICZ-DSeBT/PC61BM (w : w; 1 : 2) or PICZ-DTSeBT/PC61BM (w : w; 1 : 2) with devices configuration as ITO/PEDOT : PSS/blend/Al, show the power conversion efficiencies (PCEs) of 1.06% and 1.52%, with the open circuit voltage (Foc) of 0.75 V and 0.70 V, short circuit current densities (Jsc) of 3.45 mA/cm2 and 5.30 mA/cm2 under an AM1.5 simulator (100 mW/cm2) and the photocurrent response on-set wavelength extending up to 760 nm and 800 nm, respectively. It indicates that the NBG copolymers are viable electron donor materials for PSCs.

INTRODUCTION

In recent years, conjugated polymers (CPs) based on bulk heterojunction solar cells have gained increasing attention as a renewable and clean energy source due to the advantages such as: low cost, light weight, easy fabrication by solution processing, flexibility while comparing with the silicon-based solar cells [1, 2], and many CPs such as: poly(MDMO-PPV) [3], poly(MEH-PPV) [4], poly(3-hexylthiophene) (P3HT) [5], polythiophene derivatives [6], polymers based on thiophene derivatives [7, 8] and narrow band gap (NBG) CPs based on fluorene [9, 10] or carbazole [11, 12], have been reported. Among them, the NBG co-polymers based on carbazole have been paid much attention to achieving the high performance photovoltaic cells for the NBG polymers based on carbazole should combined good light harvesting properties of the NBG unit, good chemical stability and transport properties of the carbazole in recently. For example, Leclerc et al. reported an alternating NBG conjugated polymer: poly[^-9'-heptadecanyl-2,7-carbazole-aft-5,5'-(4',7'-di-2-thien-2-yl-2',1',3'-benzothiadiaz-

1 Статья печатается в представленном авторами виде.

ole)] (PCDTBT), the PCDTBT/PC61BM-based photovoltaic solar cells (PSCs) with power conversion efficiency (PCE) of 3.6% was achieved [12]. More recently, the researcher from Alan Heeger group increased efficiency of the solar cell to 6.1% by morphology control [13], they also demonstrated that the PCDTBT showed good stability in nitrogen and air [14]. Qin et al. reported an alternating NBG CPs based on carbazole named as HXS-1, an PCE of 5.4% for the HXS-1/PC61 BM-based PSCs, an PCE of 5.4% was achieved [15].

As good optical and chemical analogue to polycar-bazole (PCZ), poly-indolo[3,2-6]carbazole (PICZ) and PICZ derivatives have a more extended coplanar fused structure, which enables a more enhanced n-conjugation, n-n intermolecular interactions and the improved charge transporting mobility as compared to PCZ. The NBG CPs based on indolo[3,2-6]carbazole were not only expected to have similar chemical stability and light harvesting properties, but also show better ^-transporting properties than that for NBG CPs based on carbazole. Recently, Lu et al. reported a series of alternating NBG CPs derived from indolo[3,2-¿]carbazole and thiophene-cored 2',1',3'-benzothia-

diazole, the PSCs based on the polymers and PC61BM with PCE of 3.6% have been demonstrated [16]. Zhou et al. reported two alternating NBG CPs, derived from indolo[3,2-b]carbazole and 4,7-di(2-thienyl)-2',1',3'-benzothiadiazole and 4,7-di(2,2'-bithien-5-yl)-2',1',3'-benzothiadiazole, respectively, the PSCs based on the polymers and PC61BM with PCEs between 1.29% to 2.07% were demonstrated [17]. More recently, we reported an alternating NBG CPs derived from indolo [3,2-b]carbazole and 4,7-di(thieno[3,2-b]thien-2-yl)-2' ,1' ,3' -benzothiadiazole (PICZ-DT-BT), the photovoltaic cells based on the polymers and PC61BM with PCE of 2.4% were achieved [18]. However, the band gaps of these polymers of 1.8—2.0 eV were needed to decreased to matching the solar irradiation spectra [16—18]. On the other hand, as compared to the well documented 2',1',3'-benzothiadiaz-ole (BT), an electron deficient unit, which was widely used as the backbone in NBG CPs [9, 10, 12, 13], its analogue 2',1',3'-benzoselenadiazole (BSe) was more effective in extending the absorption spectrum towards the infrared region and viable electron deficient unit to achievement of NBG CPs. Such as, Hou et al. demonstrated that as a result of incorporating the BSe unit in benzo[1,2-b;4,5-b]dithiophene (BDT) based on polymers, the band gaps can be lowered from 1.70 eV (BDT-BT) to 1.52 eV (BDT-BSeT) [19]. Zhao et al. reported an alternating NBG polymer derived from 2,7-carbazole and 4,7-di(thien-2yl)-2',1',3'-benzosel-enadiazole, named as PCZDBSe. The PCZDBSe showed the mobility of 3.9 x 10-4 cm2/V.S, highest occupied molecular orbital (HOMO) energy level of - 5.28 eV and band gap of 1.73 eV. The PCE of 2.6% has been demonstrated in the PSCs based on PCZDBSe [20]. It indicated BSe was more effective in extending the absorption spectrum of copolymers towards the infrared region and a promising candidate for the well documented BT unit for photovoltaic CPs [12, 20].

In this paper, two alternating NGB CPs with band gap between 1.59—1.66 eV, which were derived from 5,11-di(^-9-heptadecanyl)indolo[3,2-b]carbazole (ICZ) and thiophene or thieno[3,2-b]thiophene-cored benzoselenadiazole, named as PICZ-DSeBT and PICZ-DTSeBT, respectively, were synthesized by Suzuki coupling reaction. The chemical structure, molecular weight and fundamental physical properties of the copolymers were characterized by 1H NMR, gel permeation chromatography (GPC), cyclic voltam-metry (CV), thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) etc. Potential applications of the copolymers to be employed as electron donor material in PSCs were investigated.

EXPERIMENTAL

General Methods

1H NMR spectra were recorded on a Bruker DRX 300 spectrometer operating at 300 MHz and were referred to tetramethylsilane (TMS). GC-MS and FAB-

MS were obtained on TRANCE2000, Fiunigan.Co., and VG ZAB-HS, respectively. Thermal gravimetric analysis (TGA) was conducted on a TGA 2050 thermal analysis system (TA instruments) under a heating rate of 30°C/min and a nitrogen flow rate of 20 mL/min. DSC was run on a DSC 204 F1 (NETZSCH) thermal analysis system. The sample was heated from 30°C to 200°C at a rate of 20°C/min under nitrogen flow of 20 mL/min. Analytical GPC was obtained using a Waters GPC 2410 in tetrahydrofuran (THF) via a calibration curve of polystyrene standards. Elemental analysis were performed on a Vario EL Elemental Analysis Instrument (Elementar Co.). UV-visible absorption spectra were measured on a UV-2550 spectrophotometer (Shimadzu Co.). PL spectra were taken by Spec-tro fluorophotometer 970CRT with 400 nm excitation. Atomic Force Microscope (AFM) images were obtained using a Nanoscope IIIa-01 Scanning Probe Microscope System with tapping mode. The microwave assisted polymerization reactions were carried on a mo-nomicrowave reactor system (NOVA, Shanghai Preekem Co.). Cyclic voltammetry (CV) was measured on a CHI electrochemical workstation (Shanghai Hua-chen Co.) at a scan rate of 50 mV/s with a nitrogen-saturated solution of 0.1 M tetrabutylammonium hexafluo-rophosphate (Bu4NPF6) in acetonitrile (CH3CN) with platinum and saturated calomel electrodes (SCE) as the working and reference electrodes, respectively.

Fabrication and Characterization of the Photovoltaic Cells

PSCs were fabricated using a sandwich structure of ITO/PEDOT : PSS/blend/Al on pre-patterned indium-tin oxide (ITO) with a sheet resistance 10— 20 fi/D. The substrate was ultrasonically cleaned with acetone, detergent, deionized water and 2-propanol subsequently. Oxygen plasma treatment was made for 10 min as the final step of substrate cleaning to improve the contact angle just before film coating. Onto the ITO glass, a layer of polyethylenedioxythiophene-polystyrene sulfonic acid (PEDOT:PSS) film with a thickness of 50 nm was spin-coated from its aqueous dispersion (Baytron P 4083, Bayer AG), aiming at improving the hole injection and avoiding the possibility of leakage. PEDOT:PSS film was dried at 80° C for 2 h in the vacuum oven. The solution of the PICZ-DSeBT and PC61BM with different weight ratios in toluene or PICZ-DTSeBT and PC61BM with different weight ratios in hot o-dichlorobenzene, were prepared in a nitrogen-filled dry box and spin-coated on the top of the ITO/PEDOT:PSS surface. The typical thickness of the blend layers were 70—80 nm. Then 200 nm thick alum

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