Full donor-type conjugated polymers consisting of alkoxy- or alkylselenophene-substituted benzodithiophene and thiophene units for organic photovoltaic devices

Synthetic Metals 168 (2013) 23–30

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Full donor-type conjugated polymers consisting of alkoxy- or alkylselenophene-substituted benzodithiophene and thiophene units for organic photovoltaic devices Yun-Sun Byun a , Ji-Hoon Kim a , Jong Baek Park a , In-Nam Kang b , Sung-Ho Jin c , Do-Hoon Hwang a,∗ a

Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 609-735, Republic of Korea Department of Chemistry, The Catholic University of Korea, Gyeonggi-do, Republic of Korea c Department of Chemistry Education and Interdisciplinary Program of Advanced Information and Display Materials, Pusan National University, Busan 609-735, Republic of Korea b

a r t i c l e

i n f o

Article history: Received 24 December 2012 Received in revised form 7 February 2013 Accepted 8 February 2013 Available online 15 March 2013 Keywords: Alkylselenophene-substituted benzodithiophene Conjugated polymers Organic solar cell

a b s t r a c t Full donor-type conjugated polymers containing benzodithiophene and thiophene derivative units were synthesized as electron donors for organic photovoltaic devices. The alkoxy-substituted benzo[1,2b:4,5-b ]dithiophene (BDT) monomer, 2,6-bis(trimethyltin)-4,8-di(2-ethylhexyloxyl)benzo[1,2-b:4,5b ]dithiophene, was polymerized with 2,5-dibromothiophene through a Pd(0)-catalyzed Stille coupling reaction. To enhance the interchain interactions between polymers chains, an alkylselenophenesubstituted BDT derivative was newly synthesized, and copolymerized with the same counter monomer parts. The two newly synthesized polymers were characterized for use in organic photovoltaic devices as electron donors. Measured optical band gap energies of the polymers were 2.10 and 1.96 eV, depending on polymer structure. Field-effect transistors were fabricated using the polymers to measure their hole mobilities, which ranged from 10−3 to 10−5 cm2 V−1 s−1 depending on the polymer structure. Bulk heterojunction organic photovoltaic cells were fabricated using conjugated polymers as electron donors and a [6,6]-phenyl C71 -butyric acid methyl ester (PC71 BM) as an electron acceptor. One fabricated device showed a power conversion efficiency of 2.73%, an open-circuit voltage of 0.72 V, a short-circuit current of 7.73 mA cm−2 , and a fill factor of 0.46, under air mass (AM) 1.5 global (1.5 G) illumination conditions (100 mW cm−2 ). © 2013 Elsevier B.V. All rights reserved.

1. Introduction Organic photovoltaic cells (OPVs) have attracted considerable attention because of their advantages such as easy manufacturing with low costs, simple fabrication, light weight, and capability to assemble flexible large-area devices. One of main challenges with OPVs is the design and synthesis of high-performance donor and acceptor materials [1–6]. Short-circuit current density (Jsc ), opencircuit voltage (Voc ), and fill factors (FF) are important parameters in OPV devices, because the power conversion efficiencies (PCEs) of the device is proportional to these parameter values. The value of Voc is proportional to the difference between the energy levels of the highest occupied molecular orbital (HOMO) of a donor and that of the lowest unoccupied molecular orbital (LUMO) of an acceptor [7]. The value of Jsc depends primarily on photo-absorption properties and charge carrier mobility. In order to obtain high

∗ Corresponding author at: Department of Chemistry, Pusan National University, Busan 609-735, Republic of Korea. Tel.: +82 51 510 3893; fax: +82 51 516 7421. E-mail address: [email protected] (D.-H. Hwang). 0379-6779/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.synthmet.2013.02.013

PCEs, the donor polymers must have optical band-gap energies between 1.2 and 1.8 eV, HOMO energy levels between −5.2 and −5.8 eV, and LUMO energy levels between −3.7 and −4.0 eV [8]. In particular, in band-gap control, the modulation of molecular energy levels (HOMO and LUMO) of conjugated polymers is of great importance for photovoltaic material design; electron donating and withdrawing functional groups have been used in molecular design for this purpose. For example, a new alkylthienyl-substituted benzo[1,2-b:4,5-b ]dithiophene (BDT) with two-dimensional (2-D) conjugated structure [9], which exhibits very interesting properties and a promising photovoltaic performance, has been designed and synthesized. Generally, broad and strong absorption bands may be achieved in 2-D conjugated polymers owing to an extended side chain. Intensive studies on 2-D conjugated polythiophenes have been characteristically performed [10,11], and these polythiophenes were found to exhibit two main absorption peaks: one that originates from the conjugated main chain and the other from the conjugated side chain. Consequently, broad and strong absorption bands can be achieved in 2-D conjugated polythiophenes. On the other hand, since 2-D conjugated polythiophenes have bigger conjugated planes than their one-dimensional conjugated

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counterparts do, a better interchain – overlapping may be formed, and higher charge mobility may be achieved. Many recent studies have designed and synthesized a new BDT derivative with a 2-D conjugated structure, by introducing alkylthienyl groups to 4 and 8 positions of the BDT unit [12,13]. Initially, a polymer backbone (PBDTTT) was selected to investigate photovoltaic properties of the 2-D conjugated BDT-polymer. conjugated polymers containing aromatic Recently selenophene building blocks have been reported as donor polymers for OPVs, because the selenophene unit could enhance the inter-chain interactions between polymer chains due to the strong Se–Se interactions [14]. Furthermore, the inclusion of selenophene building block could lower the HOMO energy level of the resulting polymers comparing with the corresponding thiophene building block [15]. Therefore synthesis of alkylselenophenyl-substituted BDT would be very interesting as a new building block for donor polymers for OPVs. In this study, we synthesized two new conjugated polymers consisting of alkoxy- or alkylselenophenesubstituted benzodithiophene as monomer units, and thiophene as a counter-monomer unit. 2,6-Bis(trimethyltin)-4,8-di(2ethylhexyloxyl)benzo[1,2-b:4,5-b ]dithiophene (ABDT) and 2,6bis(trimethyltin)-4,8-bis(5-dodecylselenophen-2-yl)benzo[1,2-b; 4,5-b ]dithiophene (SeBDT) were synthesized and copolymerized with 2,5-dibromothiophene (T) through Stille coupling polymerization to produce fully conjugated semiconducting polymers, poly[4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b ]dithiophene-altthiophene] (PABDT-T) and poly[4,8-bis(5-dodecylselenophen2-yl)benzo[1,2-b:4,5-b ]dithiophene-alt-thiophene] (PSeBDT-T); the thermal, optical, electrochemical, and photovoltaic properties of the polymers were investigated. Synthetic routes and chemical structures of the monomers and polymers are shown in Schemes 1 and 2.

tris(dibenzylideneacetone)dipalladium(0) were purchased from Strem Chemicals. [6,6]-Phenyl C71 -butyric acid methyl ester (PC71 BM) was purchased from EM Index. 4,8-Dehydrobenzo[1,2b:4,5-b ]dithiophene-4,8-dione (1), 4,8-di(2-ethylhexyloxyl)benzo [1,2-b:4,5-b ]dithiophene (2), and 2,6-bis(trimethyltin)-4,8-di(2ethylhexyloxyl)benzo[1,2-b:4,5-b ]dithiophene (ABDT) were synthesized based on methods previously described in literature [16].

2. Experimental

Composite solutions (1:2 wt%) of the polymers and [6,6]-phenyl C71 -butyric acid methyl ester (PC71 BM) were prepared using 1,2dichlorobenzene as the solvent. Polymer photovoltaic devices were fabricated with a typical sandwich structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/polymer:PC71 BM/LiF/Al. ITO-coated glass substrates were cleaned using a successive cleaning procedure, which included sonication in detergent, distilled water, acetone, and 2-propanol. A 40-nm-thick layer of PEDOT:PSS (Clevios P) was spin coated on a cleaned ITO substrate exposed to ozone for 20 min. The PEDOT:PSS layer was baked on a hot plate at 150 ◦ C for 15 min. The active layer was spin coated from a pre-dissolved composite

2.1. Materials Oxalyl chloride, N,N-diethylamine, 1-bromooctane, 1-bromododecane, 2-ethylhexylbromide, selenophene, 2-(tributylstannyl) thiophene, tert-butyllithium, n-butyllithium, zinc powder, trimethyltin chloride, and 2,5-dibromo thiophene, tin(II) chloride dehydrate were purchased from Sigma–Aldrich. Thiophene-3carboxylic acid, thiophene, 3-bromothiophene, and N-bromosuccinimide were purchased from Alfa Aesar. 1,3-bis(diphenylphosphino)propane nickel(II) chloride, tri-o-tolylphosphine,

2.2. Measurements 1 H and 13 C NMR spectra were recorded on a Varian Mercury Plus 300 MHz spectrometer, and chemical shifts were recorded in units of ppm, with chloroform as the internal standard. Absorption spectra were measured using a JASCO JP/V-570 model. Molecular weights of the polymers were determined via gel permeation chromatography (GPC) analysis relative to a polystyrene standard using a Waters high-pressure GPC assembly (model M590). For the absorption coefficient of the polymer films, the measured absorption coefficient values from the UV-visible spectrophotometer were divided by their thicknesses in the cm unit. Thermal analyses were carried out on a Mettler Toledo TGA/SDTA 851e under an N2 atmosphere, with a heating and cooling rate of 10 ◦ C min−1 . Elemental analysis was performed using a Vario Micro Cube in the Korea Basic Science Institute (Busan, Korea), and cyclic voltammetry (CV) was performed on a CH Instruments Electrochemical Analyzer. CV measurements were carried out in acetonitrile solutions containing 0.1 M tetrabutylammonium tetrafluoroborate (TBABF4 ) as the supporting electrolyte, using Ag/AgNO3 as the reference electrode, a platinum wire as the counter electrode, and a platinum working electrode.

2.3. Fabrication of photovoltaic devices

Scheme 1. Synthetic routes and chemical structures of BDT-based units.

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Scheme 2. Synthetic routes for preparation of the polymers.

solution after filtering the solution through a 0.1 ␮m syringe filter. The device structure was completed via deposition of both a LiF (∼0.5 nm) and an Al cathode (120 nm) as top electrodes onto the polymer active layer, under vacuum (3 × 10−6 Torr) in a thermal evaporator. The thickness of the active layer was measured using a KLA Tencor Alpha-step IQ surface profilometer, with an accuracy of ±1 nm. The current density–voltage (J–V) characteristics of all polymer photovoltaic cells were determined by illuminating the cells with simulated solar light (AM 1.5 G) with an intensity of 100 mW cm−2 , using a McScience Lab50 1000 W solar simulator. Electronic data were recorded using a McScience K101 Lab20 source-measure unit, and all characterizations were performed in ambient environment. The illumination intensity used was calibrated by employing a standard Si photodiode detector from PV measurements Inc., which was calibrated at the National Renewable Energy Laboratory (NREL). The measurement was carried out after masking the entire area of the fabricated device, except the active cell area. External quantum efficiency (EQE) was measured as a function of wavelength in the range of 360–800 nm, using a halogen lamp as the light source; calibration was performed using a silicon reference photodiode. All characterization steps were carried out in ambient laboratory atmosphere. The active area of the solar cells was 0.09 cm2 . 2.4. Synthesis of monomers and polymers 2.4.1. Synthesis of 4,8-di(2-ethylhexyloxyl)benzo[1,2-b:4,5b ]dithiophene (2) [16] 1 H NMR (300 MHz, CDCl , ı): 7.48 (d, 2H), 7.36 (d, 2H), 4.27 (d, 3 4H), 2.01 (m, 2H), 1.56 (m, 4H), 1.27 (q, 12H), 0.88 (t, 12H). 13 C NMR (75 MHz, CDCl3 , ı): 144.97, 132.04, 130.58, 126.36, 120.74, 74.36, 40.31, 30.98, 30.06, 29.89, 29.80, 26.51, 14.54. Anal. calcd. for C26 H38 O2 S22 : C, 69.91; H, 8.57; O, 7.13; S, 14.36. Found: C, 69.89; O, 7.09; H, 8.54; S, 14.38. 2.4.2. Synthesis of 2,6-bis(trimethyltin)-4,8-di(2-ethylhexyloxyl) benzo[1,2-b:4,5-b ]dithiophene (ABDT) [16] 1 H NMR (300 MHz, CDCl , ı): 7.51 (s, 2H), 4.27 (d, 4H), 2.01 (m, 3 2H), 1.56 (m, 4H), 1.27 (q, 12H), 0.88 (t, 12H), 0.45 (s, 18 H). 13 C NMR (75 MHz, CDCl3 , ı): 143.11, 140.45, 134.02, 132.98, 128.02, 73.59, 31.93, 30.56, 29.71, 29.67, 29.37, 26.13, 22.69, 14.11. Anal. calcd. for C32 H54 O2 S2 Sn2 : C, 49.76; H, 7.05; O, 4.14; S, 8.30. Found: C, 49.71; H, 6.98; O, 4.10; S, 8.33.

2.4.3. Synthesis of 2-dodecylselenophene (3) Selenophene (5.0 g, 38 mmol), and 250 mL of THF were added to a flask under an inert atmosphere. The solution was cooled to −78 ◦ C and 29.2 mL of tert-butyllithium (49 mmol, 1.7 M in pentane) was added. The solution was subsequently stirred at −78 ◦ C for 1 h, and 13.7 mL of 1-boromododecane (57 mmol) was added in one portion. The reaction mixture was stirred for 2 h, and the cooling bath was removed afterwards. The reaction mixture was heated to room temperature and stirred for 12 h. The resulting solution was poured into 200 mL of methanol and extracted by ethyl acetate thrice. The organic layer was separated and dried with anhydrous MgSO4 . After the solvent was removed under vacuum, a yellowish liquid was obtained (7.5 g, 66% yield). 1 H NMR (300 MHz, CDCl3 , ı): 7.80 (d, 1H), 7.14 (m, 1H), 6.94 (d, 1H), 2.87 (t, 2H), 1.87 (m, 2H), 1.69–1.25 (m, 28H), 0.87 (t, 3H). 13 C NMR (75 MHz, CDCl3 , ı): 153.65, 129.26, 128.12, 126.36, 33.14, 32.99, 32.84, 32.21, 30.00, 29.93, 29.84, 29.74, 29.64, 29.40, 22.97, 14.37. Anal. calcd. for C16 H28 Se: C, 64.20; H, 9.43. Found: C, 63.84; H, 9.39. 2.4.4. Synthesis of 4,8-bis(5-dodecylselenophen-2-yl)benzo[1,2b;4,5-b ]dithiophene (4) Compound 3 (4.0 g, 20 mmol) and 250 mL of THF were added into a flask under an inert atmosphere. The solution was cooled down by an ice-water bath, and 22.7 mL of n-butyllithium (45 mmol, 2.0 M in pentane) was added to the flask dropwise. The reactant was warmed up to 50 ◦ C and stirred for 2 h. Subsequently, 4,8-dehydrobenzo[1,2-b:4,5-b ]dithiophene-4,8-dione (1.1 g, 5 mmol) in THF (20 mL) was added, and the mixture was stirred for 1 h at 50 ◦ C. After cooling down to ambient temperature, SnCl2 ·2H2 O (8.2 g, 36 mmol) in 10% HCl (16 mL) was added, and the mixture was stirred for an additional 2 h. The resulting solution was poured into 200 mL of methanol and extracted by ethyl acetate thrice. The organic layer was separated and dried with anhydrous MgSO4 . The obtained crude product was purified through silica gel column chromatography using methylene chloride/hexane as an eluent. A yellow solid was obtained after removing solvents (1.2 g, 24% yield). 1 H NMR (300 MHz, CDCl3 , ı): 7.65 (d, 2H), 7.46 (d, 2H), 7.29 (d, 2H), 6.91 (d, 2H), 2.91 (t, 4H), 1.77 (m, 4H), 1.44–1.28 (m, 20H), 0.89 (t, 6H). 13 C NMR (75 MHz, CDCl3 , ı): 155.49, 142.68, 139.03, 136.47, 130.33, 127.60, 126.59, 123.73, 33.18, 32.73, 32.15, 29.88, 29.79, 29.58, 29.47, 22.91, 14.32. Anal. calcd. for C42 H58 S2 Se2 : C, 64.26; H, 7.45; S, 8.17. Found: C, 64.11; H, 6.97; S, 7.98.

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2.4.5. Synthesis of 2,6-bis(trimethyltin)-4,8-bis(5-dodecylselenophen-2-yl)benzo[1,2-b;4,5-b ]dithiophene (SeBDT) Compound 4 (1.0 g, 1.2 mmol), TMEDA (N,N,N ,N -tetramethylethylenediamine) (0.4 g, 3.2 mmol) and 100 mL of THF were added into a flask under an inert atmosphere. The solution was cooled to −78 ◦ C, and 1.8 mL of n-butyllithium (3.6 mmol, 2.0 M in pentane) was added. After the solution was stirred at −78 ◦ C for 1 h, 3.6 mL of trimethyltin chloride (3.6 mmol, 1.0 M in THF) was added in one portion. The reaction mixture was stirred for 2 h, and the cooling bath was removed afterwards. The reaction mixture was heated to room temperature and stirred for 12 h. The resulting solution was poured into 200 mL of cold water and extracted by ethyl acetate thrice. The organic layer was separated and dried with anhydrous MgSO4 . After the solvent was removed under vacuum, the residue was crystallized in methanol. A yellow solid was obtained (1.1 g, 75% yield). 1 H NMR (300 MHz, CDCl3 , ı): 7.70 (s, 2H), 7.43 (d, 2H), 7.07 (d, 2H), 2.97 (t, 4H), 1.76 (m, 4H), 1.46–1.26 (m, 36H), 0.87 (t, 6H), 0.38 (s, 18H). 13 C NMR (75 MHz, CDCl3 , ı): 155.10, 143.57, 143.31, 142.48, 137.29, 130.17, 126.56, 125.66, 124.84, 33.26, 33.14, 32.83, 32.71, 32.19, 29.93, 29.71, 29.63, 29.39, 22.96, 14.39, -8.05. Anal. calcd. for C48 H74 S2 Se2 Sn2 : C, 51.91; H, 6.72; S, 5.77. Found: C, 49.89; H, 6.69; S, 5.08. 2.5. General polymerization procedure The two copolymers were prepared through the same synthetic procedure, using a Stille coupling reaction of diarylbromide and bis(tributylstannyl) compounds. [17] A reaction mixture of tris(dibenzylideneacetone)dipalladium (0.01 g) and trio-tolylphosphine (0.01 g) in chlorobenzene (15 mL) was stirred at 150 ◦ C for two days, and an excess amount of 2-bromothiophene and 2-(tributylstannyl)thiophene (the end-capper) were subsequently added; stirring was continued for 2 h. The reaction mixture was cooled to 50 ◦ C and added into (vigorously stirred) 200 mL of methanol. A fibrous polymer was collected via filtration, which was further purified by extracting for two days using a Soxhlet apparatus with acetone as the solvent to remove low molecular weight oligomers, and catalyst residues. The obtained polymer was dissolved in chloroform and re-precipitated in methanol, twice. 2.5.1. Synthesis of poly[4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4, 5-b ]dithiophene-alt-thiophene] (PABDT-T) 2,6-Bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)-benzo[1,2-b:4, 5-b ]dithiophene (300 mg, 0.78 mmol) was used with 2,5dibromothiophene (94 mg, 1.0 equiv.), tris(dibenzylideneacetone) dipalladium (7.4 mg, 2.6 ␮mol), tri-o-tolyphosphine (9.9 mg,

10.4 ␮mol), and anhydrous chlorobenzene (15 mL) for the polymerization. A red solid was obtained after successive purification processes (315 mg, 59% yield). 1 H NMR (300 MHz, CDCl3 , ı): 7.25–7.08 (br, 2H), 7.08–6.99 (br, 2H), 4.19 (br, 4H), 1.95–1.28 (br, 18H), 0.90–0.85 (br, 12H). 2.5.2. Synthesis of poly[4,8-bis(5-dodecylselenophen-2yl)benzo[1,2-b:4,5-b ]dithiophene-alt-thiophene] (PSeBDT-T) 2,6-Bis(trimethyltin)-4,8-bis(5-dodecylselenophen-2-yl)-benzo[1,2-b:4,5-b ]dithiophene (300 mg, 0.27 mmol) was used with 2,5-dibromothiophene (65 mg, 1.0 equiv.), tris(dibenzylideneacetone)dipalladium (7.4 mg, 2.6 ␮mol), tri-o-tolyphosphine (9.9 mg, 10.4 ␮mol), and anhydrous chlorobenzene (15 mL) for the polymerization. A dark red solid was obtained after successive purification processes (186 mg, 36% yield). 1 H NMR (300 MHz, CDCl3 , ␦): 7.66∼7.43 (br, 4H), 7.30∼7.04 (br, 2H), 7.02∼6.99 (br, 2H), 2.97∼2.93 (br, 4H), 1.80∼1.18 (br, 40H), 0.90–0.85 (br, 6H). 3. Result and discussion 3.1. Synthesis and characterization of monomers and polymers 1 H and 13 C NMR spectra of the newly synthesized 2,6-bis(trimethyltin)-4,8-bis(5-dodecylselenophen-2-yl)benzo[1,2-b;4,5b ]dithiophene (SeBDT) are shown in Fig. 1(a) and (b), respectively. Characteristic proton peaks in the trimethyltin moiety (Hh ), terminal methyl group (Ho ), and ␣-methylene protons of selenophene (Hd ) showed a singlet at 0.38, a triplet at 0.87, and a triplet at 2.97 ppm, respectively. Characteristic aromatic proton peaks at the thiophene ring (Ha ), as well as the selenophene ring (Hb and Hc ) showed a singlet at 7.70, a doublet at 7.43, and a doublet at 7.07 ppm, respectively. The other proton peaks in the spectrum were consistent with the SeBDT monomer structure. Characteristic carbon peaks in the trimethyltin moiety (Cv ), termimal methyl group (Cu ), and ␣-methylene of selenophene (Cj ) showed at −8.09, 14.32, and 22.90 ppm, respectively. Ten other aliphatic carbons (Hk –Ht ) were superimposed in the peaks at 29.58, 29.88, 32.14, 32.66, and 33.22 ppm. The nine aromatic carbons are clearly showed in the down field between 120 and 160 ppm. All the polymers were synthesized via polycondensation of bis(arylbromide)s and bis(aryltrimethyltin)s through a Pd(0)-catalyzed Stille coupling reaction. The synthesized polymers showed good solubility in common organic solvents, such as chloroform, toluene, chlorobenzene, and 1,2-dichlorobenzene. The molecular weights and polydispersity index (PDI) of the synthesized polymers were determined using gel permeation

Fig. 1. (a) 1 H and (b) 13 C NMR spectra of 2,6-bis(trimethyltin)-4,8-bis(5-dodecylselenophen-2-yl)benzo[1,2-b;4,5-b ]dithiophene (SeBDT) in CDCl3 .

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Fig. 2. TGA plots of polymers with a heating rate of 10 ◦ C min−1 under an inert atmosphere.

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650 nm, with peak absorption at approximately 500 nm. UV–visible absorption maxima of PABDT-T and PSeBDT-T were found at 538 and 518 nm in solution, respectively, and at 500 and 527 nm, respectively, in the film state. PSeBDT-T showed higher absorption intensities compared to PABDT-T, in the short wavelength region of the spectrum from 350 to 450 nm, owing to absorption of the selenophene units. UV–visible absorption spectra of the polymer thin films showed a similar absorption trend as the polymer solutions, but the absorption maxima and edges of the polymer films marginally moved to the longer wavelength region of the spectrum compared to corresponding polymer solutions, probably owing to enhanced interchain – interactions between the polymer chains, as shown in Fig. 4. Moreover, there were distinct red-shifts in PSeBDT-T compared to PABDT-T, in both solution and thin films; an indication of dependence of the polymer structure unit on selenophene side chains. This may be attributed to the enhanced interchain interactions between polymer chains, induced by strong opt Se–Se interactions [14]. Optical band-gap energies (Eg ) of the polymer thin films were estimated by measuring their UV–visible absorption onsets and were determined for PABDT-T and PSeBDT-T, thin films as 2.10 and 1.96 eV, respectively. 3.3. Electrochemical properties

Fig. 3. Normalized absorption spectra of polymers in chloroform solution.

chromatography (GPC) with a polystyrene standard in a chloroform eluent. The measured weight-averaged molecular weights (Mw ) of PABDT-T and PSeBDT-T were 8700 (PDI = 2.4) and 13,000, respectively. Thermal stability of the synthesized polymers was investigated using a thermogravimetric analysis (TGA) as shown in Fig. 2. The synthesized polymers exhibited good thermal stability, losing less than 5% of their weight on heating from approximately 320 to 420 ◦ C. PSeBDT-T showed lower decomposition temperatures (Td ) than PABDT-T. The measured decomposition temperature of PSeBDT-T and PABDT-T were 365 and 321 ◦ C, respectively. The molecular weights and thermal properties of the polymers are summarized in Table 1.

Cyclic voltammetry (CV) was conducted to investigate electrochemical redox behaviors, as well as to determine HOMO and LUMO energy levels of the polymers [18]. CV was performed in a solution of Bu4 NBF4 (0.10 M) in acetonitrile at a scan rate of 100 mV s−1 , at room temperature under an argon atmosphere. A platinum plate was used as the working electrode; a platinum wire, as the counter electrode; and a Ag/AgNO3 electrode, as the reference electrode. The HOMO energy level was measured from oxidation onsets of the polymers. Electrochemical onsets were determined at the position where the current starts to differ from the baseline. To get an accurate redox potential, the reference electrode was calibrated using a ferrocene/ferrocenium (Fc/Fc+ ) couple, the redox potential of which was assumed to have an absolute energy level of −4.80 eV relative to the vacuum level [19,20]. The formal potential of Fc/Fc+ was measured as 0.08 V against Ag/Ag+ . Therefore, the HOMO energy values were calculated using the equation: EHOMO = −(Eonset,ox + 4.72) eV where Eonset, ox is the onset oxidation potential versus Ag/Ag+ . The HOMO energy levels of PABDT-T and PSeBDT-T were found to be −5.04 and −5.14 eV, respectively; these levels are relatively higher compared to those of other reported D–A type polymers [21–23]. The LUMO energy levels were determined by combining the HOMO energy levels with optical band gap energies of the polymers and were estimated for PABDT-T and PSeBDT-T as −2.92 and

3.2. Optical properties UV–visible absorption spectra of the polymers in a chloroform solution and thin films are shown in Figs. 3 and 4. UV–visible absorption spectra of the synthesized polymer solution in chloroform showed a broad absorption band in the range of 350 to Table 1 Molecular weight and thermal stability data of the polymers. Polymers

Mn a (g mol−1 )

Mw a (g mol−1 )

PDIa

Td (◦ C)b

PABDT-T PSeBDT-T

8700 13,000

20,000 27,000

2.4 2.1

321 365

a Mn , Mw , and PDI of the polymers were determined by gel permeation chromatography using polystyrene standards in CHCl3 . b Temperature at 5% weight loss with a heating rate of 10 ◦ C min−1 under nitrogen.

Fig. 4. The absorption coefficients of the polymer films.

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shows the transfer curves of the OTFT devices fabricated using the polymers as the active layer. The OTFTs fabricated using the polymers as the active layer exhibited typical p-channel transistor characteristics. The field-effect mobility was calculated in the saturation regime using the following equation: Ids =

WCi (Vgs − Vth )2 2L

where Ids is the drain-source current in the saturated region, W and L are the channel width and length, respectively,  is the fieldeffect mobility, Ci is the capacitance per unit area of the insulating layer (SiO2 , 300 nm), and Vgs and Vth are the gate and threshold voltages, respectively [24,25]. The measured hole mobilities of the fabricated OTFTs using PABDT-T and PSeBDT-T were 3.6 × 10−4 and 2.0 × 10−3 cm2 V−1 s−1 , respectively. The characteristics of the fabricated OTFTs are summarized in Table 3. Fig. 5. Energy diagram of the HOMO–LUMO energy levels of the polymers and PC71 BM.

−3.11 eV, respectively. PSeBDT-T showed somewhat deeper HOMO energy levels compared to PABDT-T, suggesting that the electrondonating ability of the alkylselenophene side group, conjugated to the main chain is slightly weaker than that of the alkoxy side group. The HOMO/LUMO energy levels of the polymers are illustrated in Fig. 5, and the UV–visible absorption properties, optical band gaps, and HOMO/LUMO energy levels of the polymers are summarized in Table 2. 3.4. TFT characteristics of polymer thin films To measure the field-effect hole mobility of the polymers, organic thin film transistors (OTFTs) were fabricated on a silicon wafer using a bottom contact geometry (channel length L = 12 ␮m, and width, W = 120 ␮m) under a nitrogen atmosphere. Fig. 6

3.5. Photovoltaic properties Photovoltaic devices were fabricated using the synthesized polymers as p-type electron donors and [6,6]-phenyl C71 -butyric acid methyl ester (PC71 BM) as n-type acceptors. Photovoltaic cells were constructed with a configuration of ITO/PEDOT:PSS/polymer:PC71 BM/LiF/Al. The layers in the device consisted of PEDOT:PSS (60 nm), the active layer (80 nm), LiF (0.50 nm), and Al (100 nm). The fabricated devices showed the highest short-circuit currents (Jsc ), open-circuit voltage (Voc ) and power-conversion efficiencies (PCE) at a composition ratio of 1:2(device fabrication conditions are described in greater detail in Section 2). The performances of the fabricated photovoltaic cells are summarized in Table 3, and their current–voltage (J–V) curves are shown in Fig. 7. The highest PCEs of devices fabricated using PABDT-T and PSeBDT-T were measured to be 2.03, 2.73%, respectively. The measured open-circuit voltages of the photovoltaic cells fabricated using PABDT-T and PSeBDT-T were 0.68 and 0.72 V,

Table 2 Optical and electrochemical properties of the polymers. Polymers

PABDT-T PSeBDT-T a b c

max, abs (nm)

max (nm)

edge (nm)

Solutiona

Filmb

Filmb

538 518

543 527

584 610

Optical opt Eg (eV)c

HOMO (eV)

LUMO (eV)

2.10 1.96

−5.04 −5.14

−2.94 −3.18

1 × 10−5 M in anhydrous chloroform. Polymer film on a quartz plate by spin-casting from a solution in chloroform at 900 rpm for 10 s. Calculated from the absorption band edge of the copolymer films, Eg = 1240/edge .

Fig. 6. Transfer characteristics of OTFTs fabricated using the polymers as active layer at a constant source-drain voltage of −80 V. (a) PABDT-T and (b) PSeBDT-T.

Y.-S. Byun et al. / Synthetic Metals 168 (2013) 23–30

29

Table 3 Summary of characteristics of photovoltaic devices with ITO/PEDOT:PSS/Polymers:PC71 BM (1:2)/LiF/Al configurations. Polymers

Voc a (V)

Jsc a (mA cm−2 )

FFa

PCEa (%)

 (cm2 /Vs)b

Ion /Ioff

PABDT-T PSeBDT-T

0.68 0.72

7.24 7.73

0.39 0.46

2.03 2.73

3.6 × 10−4 2.0 × 10−3

102 102

a b

b

Photovoltaic properties of copolymer/PC71 BM-based devices spin-coated from a 1,2-dichlorobenzene solution for polymers. The field-effect carrier mobilities of the polymers were measured by fabricating thin film transistors (TFTs) with bottom contacted geometry using Au electrodes.

respectively. In the CV results, PSeBDT-T showed deeper HOMO energy levels compared to PABDT-T; therefore, it may be inferred that photovoltaic cells fabricated using the former would show higher Voc than the latter would [26]. Photovoltaic device fabricated using PSeBDT-T (Voc = 0.72 V) showed a higher Voc than PABDT-T did (Voc = 0.68 V), as expected. Measured short-circuit currents of devices fabricated using PABDT-T and PSeBDT-T were 7.24 and 7.73 mA/cm2 , respectively. Measured Jsc values of the fabricated devices were consistent with UV–visible absorption spectra, and measured mobility data of the polymers. PSeBDT-T showed the wider absorption spectrum and higher hole mobility than PABDT-T. The best OPV performance was observed in the device fabricated using a PSeBDT-T/PC71 BM active layer, which reached a PCE of 2.73%, with a Jsc of 7.73 mA cm−2 , a Voc of 0.72 V, and a FF of 0.46, under AM 1.5 G irradiation (100 mW cm−2 ). Fig. 8 shows an atomic force microscopy (AFM) image (2 ␮m × 2 ␮m scan area) of the blend film (polymer:PC71 BM = 1:2),

Fig. 7. J–V characteristics of the photovoltaic devices with ITO/PEDOT:PSS/ polymers:PC71 BM (1:2)/LiF/Al configurations.

Fig. 8. Tapping-mode AFM height images of thin films cast from (a) PABDT-T:PC71 BM (1:2) (b) PSeBDT-T:PC71 BM (1:2).

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The photovoltaic cell fabricated using PSeBDT-T showed the higher Voc , Jsc , FF and PCE values than those of the device fabricated using PABDT-T. Acknowledgments This research was supported financially by the Ministry of Knowledge Economy (MKE) under the New & Renewable Energy Program through a KETEP grant (No. 20103020010050) and a National Science Foundation (NRF) grant funded by the Korean government (MEST) (No. 2011-0011122) through GCRC SOP (Grant No. 2011-0030668). References Fig. 9. External quantum efficiency (EQE) curves of the PSeBDT-T:PC71 BM solar cell device (1:2 ratio).

in order to better understand device characteristics. The root mean square (RMS) roughness of the active layers was 3.0 nm for PABDT-T, and 1.8 nm for PSeBDT-T. The morphology of thin films is very important in the performance of bulk hetero junction OPVs [27–29]. An active layer composed of PSeBDT-T and PC71 BM (1:2) showed homogeneous and well-distributed donor and acceptor domains, indicating that the layer was able to form a good p–n junction, for efficient charge generation and transport. The accuracy of the photovoltaic measurements was confirmed using the external quantum efficiency (EQE) of the devices. Fig. 9 shows the EQE curves of the OPVs fabricated under the same optimized conditions as those used in J–V measurements. In order to confirm PCE, the EQE of the device based on PBDTSe-T:PC71 BM, illuminated by monochromatic light was determined and was found to show efficient photo-response in a broad range from 350 to 850 nm. The highest EQE of around 50% was observed in a broad range from 450 to 600 nm. In order to evaluate the accuracy of photovoltaic measurement results, Jsc values were calculated by integrating EQE data with the AM 1.5 G reference spectrum and were found to be very close to J–V measurements. The onset wavelength of the photon-to-current conversion was 720 nm for the devices. 4. Conclusion A series of -conjugated polymers consisting of a full donor system were synthesized and characterized for use in p-type donor materials in organic photovoltaic cells. 2,6-Bis(trimethyltin)4,8-di(2-ethylhexyloxyl)benzo[1,2-b:4,5-b ]dithiophene (ABDT) or 2,6-bis(trimethyltin)-4,8-bis(5-dodecylselenophen-2-yl)benzo [1, 2-b;4,5-b ]dithiophene (SeBDT) were copolymerized with 2,5dibromothiophene (T) through a Pd(0)-catalyzed Stille coupling reaction. PSeBDT-T showed a wider absorption spectrum, smaller band gap energy, and deeper HOMO energy level compared with those of PABDT-T. PSeBDT-T also showed higher field-effect hole mobility than PABDT-T due to the enhanced interchain interaction between polymer chains, induced by strong Se–Se interactions.

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