Synthesis and Characterization of Novel Low-Bandgap
Triphenylamine-Based Conjugated Polymers with Main-Chain Donors and Pendent
Acceptors for Organic Photovoltaics
DURYODHAN SAHU,1HARIHARA PADHY,1DHANANJAYA PATRA,1JEN-HSIEN HUANG,2CHIH-WEI CHU,2,3HONG-CHEU LIN1 1Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan, Republic of China 2Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan, Republic of China
3Department of Photonics, National Chiao Tung University, Hsinchu, Taiwan, Republic of China
Received 17 August 2010; accepted 13 September 2010 DOI: 10.1002/pola.24389
Published online 22 October 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT:A series of novel low-bandgap triphenylamine-based conjugated polymers (PCAZCN, PPTZCN, and PDTPCN) consisting of different electron-rich donor main chains ( N-alkyl-2,7-carbazole, phenothiazine, and cyclopentadithinopyrol, respectively) as well as cyano- and dicyano-vinyl electron-acceptor pendants were synthesized and developed for poly-mer solar cell applications. The polypoly-mers covered broad absorption spectra of 400–800 nm with narrow optical band-gaps ranging 1.66–1.72 eV. The highest occupied molecular or-bital and lowest unoccupied molecular oror-bital levels of the polymers measured by cyclic voltammetry were found in the range of5.12 to 5.32 V and 3.45 to 3.55 eV, respectively. Under 100 mW/cm2 of AM 1.5 white-light illumination, bulk heterojunction photovoltaic devices composing of an active
layer of electron-donor polymers (PCAZCN, PPTZCN, and PDTPCN) blended with electron-acceptor [6,6]-phenyl-C61 -bu-tyric acid methyl ester or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) in different weight ratios were investigated. The photovoltaic device containing donor PCAZCN and acceptor PC71BM in 1:2 weight ratio showed the highest power conver-sion efficiency of 1.28%, withVoc¼ 0.81 V, Jsc¼ 4.93 mA/cm2, and fill factor¼ 32.1%.VC 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 5812–5823, 2010
KEYWORDS:bulk heterojunction; conducting polymers; conju-gated polymers; donor–acceptor; heteroatom-containing poly-mers; low bandgap; polymer solar cells; synthesis; triphenylamine derivatives
INTRODUCTION Solar cells based on polymers have attracted intense research interests in recent years because of their unique advantages of low cost, light weight, and compatibil-ity for making flexible and large area devices.1To date, solu-tion-processed bulk heterojunction (BHJ) polymer solar cell (PSC) devices composed of an active layer of electron-donor polymers blended with electron-acceptor fullerene deriva-tives, such as [6,6]-phenyl-C61-butyric acid methyl ester
(PC61BM) or [6,6]-phenyl-C71-butyric acid methyl ester
(PC71BM), have been developed and reached power
conver-sion efficiency (PCE) values up to 7.7%.2–4 Moreover, the
PSC devices with appropriate industrial processing technique of traditional silicon-based solar cells have also been extended.5 Nevertheless, unlike silicon-based solar cells, these PCEs of PSC devices are not sufficient for commerciali-zation, though there are lot of advantages of polymers. To seek higher efficiencies, there have been various research efforts focused on the design and synthesis of polymers with new electron-rich and electron-deficient units along their backbones.6–20 In general, to acquire higher PCE values in BHJ solar cells, the developments of new materials were
focused on: (i) fused aromatic rings in the conjugated poly-mer backbones that endow with strong absorption spectra, which can harvest more sunlight in the whole solar spec-trum,6–8to increase the Jscvalues; (ii) tunable intramolecular
charge transfer (ICT) from donor to electron-acceptor units that lower the bandgaps;9–11 (iii) the mini-mum difference (should be 0.3 eV) between the highest occupied molecular orbital (HOMO) level of the polymer and the lowest unoccupied molecular orbital (LUMO) level of the PCBM acceptor in the active layer, which provides an effi-cient driving force for the charge separation to get a high open circuit voltage (Voc);
6(d,e),12,14
and (iv) the excited elec-tron-hole pairs need to be dissociated into free charge car-riers with high yields.3(a)
During the last decade, many articles have been reported based on conjugated polymers with electron-donating and electron-accepting moieties combined through p-conjugated linkers in the polymer backbones, which reduce the optical bandgaps and allow the absorptions of the polymers to match well with the solar spectrum.16–19In donor–acceptor
Additional Supporting Information may be found in the online version of this article. Correspondence to: H.-C. Lin (E-mail: linhc@mail.nctu.edu.tw) Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 5812–5823 (2010)VC2010 Wiley Periodicals, Inc.
(D-A) polymers, the electron-donor units provide deeper HOMO levels, and the electron-acceptor units are responsible for the electronic bandgaps of the polymers.20 Furthermore,
to get high PCE values, the strategy to design conjugated polymers, where the electron-acceptors were placed at the pendants connected with electron-rich main chains, was found to be evenly effective and showed promising perform-ances with PCE values up to 4.5%.21 These architectures composing of electron-acceptor pendants and electron-donor main chains have the unique advantages of allowing charge separation through sequential transfers of electrons from the main chains to the side chains and then to PCBM, though there are limited reports explored in PSC applications.21 Numerous conjugated units, such as 2,7-carbazole,6 cyclo-pentadithiophene,7 cyclopentadithinopyrol,8(a–c)
phenothia-zine,9(c–e)and triphenylamine10,21(a–c) containing electron-rich heteroatoms (i.e., N, S, and Si), have been extensively used in the polymer designs for the applications of PSCs. As a donor moiety, 2,7-carbazole derivatives showed both smaller bandgaps and deeper HOMO energy levels, which enabled higher light harvestings and Vocvalues (up to 0.9 V),
respec-tively.6 For instance, Heeger et al. reported that a polymer (PCDTBT) containing 2,7-carbazole as a donor moiety showed a PCE value of 6.1% along with the best ever inter-nal quantum efficiency of 100%.6(c) Moreover, the introduc-tion of alkyl chains in donor moieties of polymers provides good balance between solubility and crystallinity in polymer films and also has impacts on PCE values.22 Furthermore, the introduction of electron-withdrawing cyano- or dicyano-vinyl groups to polymer backbones lowers their LUMO lev-els,23 tunes their electrooptical properties,11,24 and also
enhances their electrochemical stabilities.10(b),25
In this study, we report the synthesis and characterization of three novel low-bandgap D-A triphenylamine-based conju-gated polymers (PCAZCN, PPTZCN, and PDTPCN), where a triphenylamine derivative (M1) bearing an electron-with-drawing cyano- and dicyano-vinyl pendant was copolymer-ized with three different electron-rich donor (M2–M4) blocks, those are, N-alkyl-2,7-carbazole, -3,7-phenothiazine, and -2,6-dithinopyrol, respectively, via Suzuki and Stille reac-tions (see Fig. 1 and Schemes 1 and 2). These highly elec-tron-rich donor groups, including triphenylamine derivative (M1)21(c),26 and electron-donor blocksM2–M4, in the back-bones of the main-chain copolymers endowed with strong and broad absorption spectra to obtain superior harvesting of sunlight and also suitable molecular energy levels to ac-quire good charge separations and transportations as well as high values of Voc. Solution-processed BHJ PSC devices
com-posed of an active layer of electron-donor polymers blended with electron-acceptor PC61BM or PC71BM were developed,
and their photovoltaic properties were investigated as well. EXPERIMENTAL
Materials
4-(Tridecan-7-yl)-4H-dithieno[3,2-b:20,30-d]pyrrole was pre-pared according to the published procedure.8(c) All other chemicals were purchased from Aldrich, ACROS, Fluka, or
TCI. Toluene, tetrahydrofuran (THF), and diethyl ether were distilled over sodium/benzophenone. Chloroform (CHCl3)
was purified by refluxing with calcium hydride and then dis-tilled. If not otherwise specified, the other solvents were degassed by nitrogen 1 h before use.
Measurements and Characterization
1
H and13C NMR spectra were measured using Varian Unity 300 MHz spectrometer. Elemental analyses were performed on a HERAEUS CHN-OS RAPID elemental analyzer. Thermo-gravimetric analyses (TGA) were conducted with a TA Instru-ments Q500 at a heating rate of 10 C/min under nitrogen. The molecular weights of polymers were measured by gel permeation chromatography (GPC) using Waters 1515 sepa-ration module (concentsepa-ration: 1 mg/1 mL in THF; flow rate: 1 mL/1 min), and polystyrene was used as a standard with THF as an eluent. UV–visible absorption spectra were recorded in dilute THF solutions (106M) on a HP G1103A. Solid films of UV–vis spectra measurements were spin coated on a glass substrate from THF solutions with a concentration of 10 mg/mL. Cyclic voltammetry (CV) measurements were performed using a BAS 100 electrochemical analyzer with a standard three-electrode electrochemical cell in a 0.1 M tet-rabutylammonium hexafluorophosphate solution (in acetoni-trile) at room temperature with a scanning rate of 100 mV/ s. During the CV measurements, the solutions were purged with nitrogen for 30 s. In each case, a carbon working elec-trode coated with a thin layer of polymers, a platinum wire as the counter electrode, and a silver wire as the quasi-refer-ence electrode were used, and Ag/AgCl (3 M KCl) electrode was served as a reference electrode for all potentials quoted herein. The redox couple of ferrocene/ferrocenium ion (Fc/ Fcþ) was used as an external standard. The corresponding HOMO and LUMO levels were calculated using Eox/onset and
Ered/onset for experiments in solid films of polymers, which
were performed by drop-casting films with the similar thick-ness from THF solutions (ca. 5 mg/mL). The onset potentials were determined from the intersections of two tangents FIGURE 1Molecular structures of polymers (PCAZCN, PPTZCN, and PDTPCN).
drawn at the rising currents and background currents of the CV measurements.
Fabrication of PSCs
The PSCs in this study were composed of an active layer of blended polymers (polymer:PCBM) in solid films, which were sandwiched between a transparent indium tin oxide (ITO) anode and a metal cathode. Before the device
fabrica-tion, ITO-coated glass substrates (1.5 1.5 cm2) were
ultra-sonically cleaned in detergent, deionized water, acetone, and isopropyl alcohol. After routine solvent cleaning, the sub-strates were treated with UV ozone for 15 min. Then, a modified ITO surface was obtained by spin coating a layer of poly(ethylene dioxythiophene):polystyrenesulfonate (PEDOT: PSS) (30 nm). After baking at 130 C for 1 h, the
sub-strates were transferred to a nitrogen-filled glove box. Then, SCHEME 1Synthetic routes of monomers (M1–M4).
on the top of PEDOT:PSS layer, the active layer was prepared by spin coating from blended solutions of polymer blends polymer:PC61BM (with 1:1 w/w) andPCAZCN:PC71BM (with
1:1, 1:2, 1:3, and 1:4 w/w) subsequently with a spin rate about 1000 rpm for 60 s, and the thickness of the active layer was typically about 80 nm. Initially, the blended solu-tions were prepared by dissolving both polymers and PCBM in 1,2 dichlorobenzene (20 mg/1 mL), followed by continu-ous stirring for 12 h at 50C. In the slow-growth approach, blended polymers in solid films were kept in the liquid phase after spin coating by using the solvent with a high boiling point. Finally, a calcium layer (30 nm) and a subse-quent aluminum layer (100 nm) were thermally evaporated through a shadow mask at a pressure below 6 106Torr. The active area of the device was 0.12 cm2. All PSC devices were prepared and measured under ambient conditions. The solar cell measurements were done inside a glove box under simulated AM 1.5G irradiation (100 mW/cm2) using a Xenon lamp-based solar simulator (Thermal Oriel 1000 W). The external quantum efficiency (EQE) spectra were obtained at short-circuit conditions. The light source was a 450 W Xe lamp (Oriel Instrument, model 6266) equipped with a water-based IR filter (Oriel Instrument, model 6123NS). The light output from the monochromator (Oriel Instrument, model 74100) was focused on the photovoltaic cell under test.
Fabrication of Hole- and Electron-Only Devices
The hole- and electron-only devices in this study contain polymer blends polymer:PC61BM (1:1) blend films
sand-wiched between transparent ITO anode and cathode. The devices have been prepared following the same procedure as the fabrication of BHJ devices, except that in the hole-only devices, Ca was replaced with MoO3 (with work function U
¼ 5.3 eV) and for the electron-only devices, the PEDOT:PSS layer was replaced with Cs2CO3(with work functionU ¼ 2.9
eV). In hole-only devices, MoO3 was thermally evaporated
with a thickness of 20 nm and then capped with 50 nm of Al on the top of the active layer. On the other hand, Cs2CO3
was thermally evaporated in the electron-only devices with a thickness of 2 nm on the top of transparent ITO. For both devices, annealing of the active layer was performed at 130
C for 20 min.
Synthesis of Monomers (M1–M4) and Polymers (PCAZCN, PPTZCN, and PDTPCN)
The synthetic procedures of the monomers (M1–M4) and polymers (PCAZCN, PPTZCN, and PDTPCN) are given below, and the other intermediates are described in the Supporting Information.
3-(4-(Bis(4-bromophenyl)amino)phenyl)-2-(5-formylthiophen-2-yl)acrylonitrile (M1)
Phosphorousoxychloride (0.45 mL, 4.8 mmol) was added to a solution of 3-(4-(bis(4-bromophenyl)amino)phenyl)-2-(thi-ophen-2-yl)acrylonitrile (1) (2 g, 3.7 mmol) and DMF (0.5 mL, 6.5 mmol) in 1,2-dichloroethane (50 mL) cooled in an ice bath. The solution was then warmed up to room temper-ature and heated to reflux overnight. The reaction was cooled to room temperature and poured into a saturated aqueous sodium acetate solution (100 mL) and stirred for several hours to complete the hydrolysis. The reaction mix-ture was extracted with dichloromethane and washed with water, then the organic layer was dried over MgSO4, and concentrated in rotary evaporator. The crude product was purified by column chromatography (silica) using a mixture of hexane:ethyl acetate (10:1) as an eluent to yield a yellow-ish crystal (1.34 g, 64%). 1 H NMR (300 MHz, CDCl3),d (ppm): 9.90 (s, 1H), 8.02 (d, J ¼ 4.2 Hz, 1H), 7.97 (s, 1H), 7.89 (d, J ¼ 8.7 Hz, 2H), 7.57– 7.54 (m, 5H), 7.08 (d, J ¼ 9 Hz, 4H), 7.02 (d, J ¼ 8.7 Hz, 2H). 13C NMR (300 MHz, DMSO-d6), d (ppm): 182.7, 150.2, 149.1, 145.2, 142.7, 142.5, 137.2, 133.1, 131.6, 127.4, 127.3, 127.0, 126.3, 121.4, 118.1, 116.9, 102.2. MS (FAB): m/z [Mþ] SCHEME 2Synthetic routes of polymers (PCAZCN, PPTZCN, and PDTPCN).
563; calcd m/z [Mþ] 561.94. Anal. Calcd for C26H16Br2N2OS:
C, 55.34; H, 2.86; N, 4.96. Found: C, 55.41; H, 3.36; N, 4.92. 2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9-(tridecan-7-yl)-9H-carbazole (M2)
A solution of compound 6 (3 g, 5.91 mmol) in anhydrous THF (80 mL) was cooled to 78 C under nitrogen and stirred at this temperature for 5 min in the flame-dried two-necked round-bottom flask. n-Butyllithium (5.4 mL of 2.5 M solution in hexane, 13.6 mmol) was added dropwise, using a syringe, and the mixture was stirred at78C for 1 h, then warmed up to 0C for 15 min, and cooled again to 78C. 2-Isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3.62 mL, 17.73 mmol) was added rapidly to the solution, and the resulting mixture was warmed to room temperature and stirred overnight. The reaction mixture was poured into water and extracted dichloromethane with a little brine wash. The organic extracts were dried over magnesium sul-fate, and the solvent was removed by rotary evaporator. The obtained product was further purified by recrystallization from acetone to yield the title product as a white solid (2.56 g, 72.2%). 1 H NMR (300 MHz, CDCl3), d (ppm): 8.12 (br, 2H), 8.03 (br, 1H), 7.89 (br, 1H), 7.66 (d, J ¼ 7.8 Hz, 2H), 4.70 (m, 1H), 2.35–2.31 (m, 2H), 1.98–1.92 (m, 2H), 1.39 (s, 24H), 1.05– 0.95 (m, 16H), 0.78 (t, J ¼ 6.6 Hz, 6H).13C NMR(300 MHz, CDCl3), d (ppm): 142.15, 138.91, 126.26, 124.81, 120.25, 118.31, 115.66, 83.91, 56.45, 34.03, 31.78, 29.33, 26.90, 25.15, 22.76, 14.22: m/z [Mþ] 602; calcd m/z [Mþ] 601.45. Anal. Calcd for C37H57B2NO2: C, 73.88; H, 9.55; N, 2.33.
Found: C, 73.68; H, 9.68; N, 2.58.
3,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-10-(tridecan-7-yl)-10H-phenothiazine (M3)
The monomerM3 was synthesized by a similar procedure to that ofM2, but the obtained product was purified by recrys-tallization from methanol to yield a white solid (yield: 74%).
1 H NMR (300 MHz, CDCl3),d (ppm): 7.53–7.50 (m, 4H), 6.86 (d, J ¼ 8.1 Hz, 2H), 3.68 (m, 1H), 2.12–2.00 (m, 2H), 1.85– 1.77 (m, 2H), 1.52–1.25 (m, 40H), 0.85 (t, J ¼ 6.3 Hz, 6H). 13 C NMR (300 MHz, CDCl3), d (ppm): 147.95, 133.94, 133.88, 125.92, 116.85, 83.87, 64.44, 34.57, 31.90, 29.37, 27.61, 25.06, 22.75, 14.30: m/z [Mþ] 634; calcd m/z [Mþ] 633.42. Anal. Calcd for C37H57B2NO4S: C, 70.14; H, 9.07; N,
2.21. Found: C, 70.09; H, 9.03; N, 2.09. 2,6-Bis(tributylstannyl)-4-(tridecan-7-yl)-4H-dithieno[3,2-b:20,30-d]pyrrole (M4)
A solution of compound 9 (2 g, 5.53 mmol) in anhydrous THF (50 mL) was cooled to 78 C under nitrogen and stirred at this temperature for 5 min in the flame-dried two-necked round-bottom flask. n-Butyllithium (4.6 mL of 2.5 M solution in hexane, 11.6 mmol) was added dropwise, using a syringe, and the mixture was stirred at78C for 1 h, then warmed up to 0C for 15 min, and cooled again to 78C. Tri-n-butyltin chloride (3.30 mL, 12.17 mmol) was added rapidly to the solution, and the resulting mixture was warmed to room temperature and stirred overnight. The
reaction mixture was poured into water and extracted with diethyl ether. The combined organic layers were washed with a little brine and dried over MgSO4. After, the solvent
had been removed under reduced pressure to afford M4 (4.26 g, 82%). 1 H NMR (300 MHz, CDCl3),d (ppm): 6.97 (s, 2H), 4.26–4.22 (m, 1H), 2.03 (m, 2H), 1.87 (m, 2H), 1.64–1.59 (m, 12H), 1.41–1.12 (m, 40H), 0.96–0.86 (m, 24H).13C NMR (300 MHz, CDCl3), d (ppm): 143.90, 134.25, 122.07, 119.09, 59.83, 35.38, 31.78, 29.36, 28.05, 27.46, 27.34, 26.73, 22.75, 15.83, 14.20, 13.89, 11.13. m/z [Mþ] 942; calcd m/z [Mþ] 941.40. Anal. Calcd for C45H83NS2Sn2: C, 57.52; H, 8.90; N, 1.49.
Found: C, 57.02; H, 9.20; N, 1.49.
Polymerization Procedures for Polymer Precursors PCAZCHO and PPTZCHO
Polymerization steps for PCAZCHO and PPTZCHO were car-ried out through the palladium(0)-catalyzed Suzuki coupling reactions. In a 50-mL flame-dried two-necked round-bottom flask, 1 equiv ofM1 and 1 equiv of M2 or M3 and Pd(PPh3)4
(1.5 mol %) were dissolved in a mixture of toluene (mono-mer ¼ 0.5 M) and 2 M Na2CO3(3:1). The solution was first
put under a nitrogen atmosphere and vigorously stirred at 90–95 C for 3 days. After reaction completion, an excess of bromobenzene was added to the reaction, and then 1 h later, excess of phenylboronic acid was added and the reaction refluxed overnight to complete the end-capping reaction. The polymer was purified by precipitation in methanol/water (10:1), filtered through 0.45-lm nylon filter, and washed on Soxhlet apparatus using hexane, acetone, and CHCl3. The
CHCl3 fraction was reduced to 40–50 mL under reduced
pressure, precipitated in methanol/water (10:1, 500 mL), fil-tered through 0.45-lm nylon filter, and finally air-dried overnight.
Poly[2-(5-formylthiophen-2-yl)-3-(4-(phenyl(4-(9-(tridecan-7-yl)-9H-carbazol-2-yl)phenyl)amino) phenyl)acrylonitrile] (PCAZCHO)
Dark black solid (yield: 76%).1H NMR (ppm, CDCl3):d 9.88
(s, 1H), 8.17 (m, 2H), 7.88–7.85 (m, 2H), 7.78–7.71 (m, 6H), 7.61 (s, 1H), 7.49 (m, 4H), 7.42–7.36 (m, 4H), 7.24–7.21 (m, 2H), 4.69 (m, 1H), 2.38 (m, 2H), 2.17 (m, 2H), 1.26–1.16 (m, 16H), 0.78 (t, J ¼ 6.00 Hz, 6H). GPC (THF, polystyrene stand-ard): Mn¼ 13,266 g/mol, Mw¼ 21,140 g/mol, PDI ¼ 1.59.
Poly[2-(5-formylthiophen-2-yl)-3-(4-(phenyl(4-(10-(tridecan-7-yl)-10H-phenothiazin-3-yl)phenyl)amino) phenyl)acrylonitrile] (PPTZCHO)
Dark black solid (yield: 75%). 1H NMR (300 MHz, CDCl3),d
(ppm): 9.85 (s, 1H), 7.81–7.79 (m, 2H), 7.68 (s, 1H), 7.52– 7.44 (m, 6H), 7.34 (m, 4H), 7.24–7.21 (m, 4H), 7.10–7.00 (m, 2H), 7.00–6.98 (m, 2H), 3.73 (m, 1H), 2.11 (m, 2H), 1.87 (m, 2H), 1.58 (m, 4H), 1.27 (m, 12H), 0.85 (m, 6H). GPC (THF, polystyrene standard): Mn¼ 10,121 g/mol, Mw¼ 18,825 g/
mol, PDI¼ 1.86.
Synthetic Procedure for Polymer Precursor PDTPCHO Polymerization steps for PDTPCHO were carried out through the Stille reactions. In a 50-mL flame-dried two-necked
round-bottom flask, equimolar of M1 (0.25 g, 0.44 mmol) and M4 (0.42 g, 0.44 mmol) followed by Pd(PPh3)4 (1 mol
%) were dissolved in 8 mL of dry toluene and deoxygenated with nitrogen for 30 min. The reaction mixture was stirred at 110C for 3 days, and then an excess amount of 2-bromo-thiophene was added to end cap the trimethylstannyl groups for 4 h. The reaction mixture was cooled to 40C and added slowly into a vigorously stirred mixture of methanol/acetone (3:1). The polymers were collected by filtration and repreci-pitation from methanol. The crude polymers were further purified by washing with acetone and EA for 2 days in a Soxhlet apparatus to remove oligomers and catalyst residues. Poly[2-(5-formylthiophen-2-yl)-3-(4-(phenyl(4-(4-(tridecan-7-yl)-4H-dithieno[3,2-b:20,30-d]pyrrol-2-yl) phenyl)amino)phenyl)acrylonitrile] (PDTPCHO)
Dark black solid (yield: 70%). 1H NMR (300 MHz, CDCl3),d
(ppm): 9.88 (s, 1H), 7.84–7.81 (m, 2H), 7.71–7.61 (m, 4H), 7.46 (s, 1H), 7.40–7.41 (m, 2H), 7.23–7.14 (m, 8H), 4.22 (m, 1H), 2.04 (m, 2H), 1.87 (m, 2H), 1.25–1.16 (m, 16H), 0.81 (m, 6H). GPC (THF, polystyrene standard): Mn¼ 5037 g/mol,
Mw¼ 6760 g/mol, PDI¼ 1.34.
Synthetic Procedures for Polymers PCAZCN, PPTZCN, and PDTPCN
Poly[2-((5-(1-cyano-2-(4-(phenyl(4-(9-(tridecan-7-yl)- 9H-carbazol-2-yl)phenyl)amino)phenyl)vinyl)thiophen-2-yl)methylene)malononitrile] (PCAZCN)
To a solution of PCAZCHO (200 mg, 0.26 mmol) and malo-nonitrile (172 mg, 5.2 mmol) in 15 mL of CHCl3, 0.2 mL of
pyridine was added. The mixture solution was stirred over-night at room temperature, then the resulting mixture was poured into methanol, and the precipitate was filtered off and washed with water. The resulted black color polymer was purified by repeated precipitation from its THF solution to methanol (175 mg, 82%). 1H NMR (300 MHz, CDCl 3),d (ppm): 8.18 (m, 2H), 7.91–7.88 (m, 2H), 7.78–7.73 (m, 6H), 7.63 (s, 1H), 7.57 (s, 1H), 7.50– 7.48 (m, 3H), 7.42–7.37 (m, 5H), 7.24–7.20 (m, 2H), 4.70 (m, 1H), 2.38 (m, 2H), 2.15 (m, 2H), 1.26–1.15 (m, 16H), 0.78 (t, J ¼ 6.00 Hz, 6H). GPC (THF, polystyrene standard): Mn ¼
13,781 g/mol, Mw¼ 21,103 g/mol, PDI ¼ 1.53. Anal. Calcd
C, 80.86%; H, 6.41%; N, 8.73%. Found: C, 79.73%; H, 6.18%; N, 8.40%.
Poly(2-((5-(1-cyano-2-(4-(phenyl(4-(10-(tridecan-7-yl)-10H-phenothiazin-3yl)phenyl)amino)phenyl)vinyl) thiophen-2-yl)methylene)malononitrile) (PPTZCN)
The synthesis procedure forPPTZCN was followed using the same procedure as that of PCAZCN. After purification afforded a black solid (81%).
1
H NMR (300 MHz, CDCl3),d (ppm): 7.85–7.82 (m, 2H), 7.77
(s, 1H), 7.74 (s, 1H), 7.60–7.46 (m, 6H), 7.34 (m, 4H), 7.25– 7.21 (m, 4H), 7.12–6.98 (m, 4H), 3.73 (m, 1H), 2.17 (m, 2H), 1.87 (m, 2H), 1.58 (m, 4H), 1.27 (m, 12H), 0.85 (m, 6H). GPC (THF, polystyrene standard): Mn ¼ 10,503 g/mol, Mw ¼
19,010 g/mol, PDI¼1.81. Anal. Calcd C, 77.75%; H, 6.16%; N, 8.40%. Found: C, 76.95%; H, 6.22%; N, 8.12%.
Poly(2-((5-(1-cyano-2-(4-(phenyl(4-(4-(tridecan-7-yl)-4H-dithieno[3,2-b:20,30-d]pyrrol-2-yl)phenyl)amino)phenyl) vinyl)thiophen-2-yl)methylene)malononitrile) (PDTPCN) The synthesis procedure forPDTPCN was followed using the same procedure as PCAZCN. After purification afforded a black solid (76%). 1 H NMR (300 MHz, CDCl3), d (ppm): 7.88–7.85 (m, 2H), 7.79–7.76 (m, 2H), 7.66–7.60 (m, 4H), 7.55 (s, 1H), 7.48 (s, 1H), 7.24–7.08 (m, 8H), 4.23 (m, 1H), 2.04 (m, 2H), 1.88 (m, 2H), 1.25–1.11 (m, 16H), 0.81 (m, 6H). GPC (THF, polysty-rene standard): Mn ¼ 5237g/mol, Mw ¼ 6820 g/mol, PDI¼
1.30. Anal. Calcd C, 73.76%; H, 5.82%; N, 8.60%. Found: C, 72.29%; H, 5.82%; N, 8.28%.
RESULTS AND DISCUSSION
Synthesis and Structural Characterization
The general synthetic routes of monomers M1–M4 and poly-mers (PCAZCN, PPTZCN, and PDTPCN) are shown in Schemes 1 and 2, respectively. Synthesis of 4-(tridecan-7-yl)-4H-dithieno[3,2-b:20,30-d]pyrrole (9)8(c) was reported by known literature procedures. Monomers (M1–M4) were sat-isfactorily characterized by1H NMR, 13C NMR, MS spectros-copy, and elemental analyses. As shown in Scheme 2, poly-mer precursors PCAZCHO and PPTZCHO were prepared by the well-known Pd(0)-catalyzed Suzuki polymerization between triphenylamine dibromide monomerM1 and dibor-onic esters of 2,7-carbazole (M2) and phenothiazine (M3), respectively, and polymer precursor PDTPCHO was synthe-sized by Pd(0)-catalyzed Stille polymerization between tri-phenylamine dibromide monomers M1 and M4. Further-more, to increase the stability of polymers, end-caping reactions were carried out on all polymers. The obtained polymers were further purified by washing on Soxhlet appa-ratus using hexane and acetone and later on extracted by CHCl3. The CHCl3 fraction was concentrated to 40–50 mL
under reduced pressure, precipitated in methanol, filtered through 0.45-lm nylon filters, and finally dried under reduced pressure at room temperature. After purification and drying, all polymers were obtained as black solids in overall good yields (70–76%).
Condensations of aldehydes in polymer precursors PCAZ-CHO, PPTZPCAZ-CHO, and PDTPCHO with excess of malononitrile in CHCl3 solutions in the presence of pyridine afforded the
corresponding polymers PCAZCN, PPTZCN, and PDTPCN in excellent yields (76–82%; see Fig. 1).11(b)All final polymers (PCAZCN, PPTZCN, and PDTPCN) exhibited good solubilities in common organic solvents, such as THF, CHCl3, toluene,
and chlorobenzene at room temperature.
The molecular weights of polymers PCAZCN, PPTZCN, and PDTPCN were determined by GPC against monodisperse polystyrene standards in THF and are summarized in Table 1. The number-average molecular weights (Mn) and the
weight-average molecular weights (Mw) obtained in all
poly-mers were in the range of 5237–13,781 and 6820–21,103, respectively, with polydispersity indices (PDI ¼ Mw/Mn)
determined by TGA are shown in Figure S1 of the Support-ing Information and summarized in Table 1. The TGA ther-mograms of polymers PCAZCN, PPTZCN, and PDTPCN revealed the decomposition temperatures (Td with 5%
weight loss) at 393, 385, and 377C, respectively.
Optical Properties
The optical properties of these D-A polymers (PCAZCN, PPTZCN, and PDTPCN) were investigated by UV–vis spec-troscopy in THF solutions as well as in solid films. Figure 2(a,b) shows the normalized UV–vis absorption spectra of polymers in THF solutions (106M) and solid films, respec-tively, and the data are illustrated in Table 2. Polymer PCAZCN in solution showed two absorption peaks [see Fig. 2(a)], where one peak at 374 nm is attributed to the p–p* transitions of their conjugated polymer backbones and another peak at 536 nm is correspondent to the ICT interac-tions between their polymer main chains and pendent acceptor groups.21 In polymer PPTZCN, the high-energy p– p* transition bands situated at 377and 459 nm are consist-ent with the reported phenothiazine-based homopolymers or copolymers.27 The low-energy peak appeared at 525 nm is due to the ICT transitions occurring between their polymer main chains and pendent acceptor groups similar to that of PCAZCN. However, in case of polymer PDTPCN, both p–p* transition and ICT transition bands overlapped and showed a single absorption peak at 437 nm and the ICT transition band tailed up to 700 nm. In contrast to those solution spec-tra of all D-A polymers (PCAZCN, PPTZCN, and PDTPCN) shown in Figure 2(b) and Table 2, the absorption spectra in solid films were red shifted (17–61 nm) and broadened to longer wavelength region tailed up to 750 nm. These batho-chromic shifts in solid films are attributed to the
intermolec-ular interactions in the solid state, which indicate the effi-cient packing of the polymer backbones in solid films.13The
optical bandgaps (Eopt
g ) of polymers PCAZCN, PPTZCN, and
PDTPCN calculated from the absorption onsets in the solid films (konset, film) were in the range of 1.66–1.72 eV. These
low bandgaps of polymers could be attributed to the addi-tional ICT transitions from electron-rich main-chain donors to the pendent electron-acceptor cyano- and dicyano-vinyl TABLE 1Molecular Weights and Thermal Properties of
Polymers Polymer Mna Mwa PDIa Td(C)b PCAZCN 13,781 21,103 1.53 393 PPTZCN 10,503 19,010 1.81 385 PDTPCN 5,237 6,820 1.30 377 a
Molecular weights and polydispersity were measured by GPC, using THF as an eluent, polystyrene as a standard.Mn, number-average
mo-lecular weight;Mw, weight-average molecular weight. b
Temperature (C) at 5% weight loss measured by TGA at a heating rate of 20C/min under nitrogen.
FIGURE 2Normalized UV–vis spectra of polymers (PCAZCN, PPTZCN, and PDTPCN) in (a) dilute THF solutions (1 106M) and (b) solid films.
TABLE 2Optical Properties of Polymers
Polymer kabs, sol(nm) a
kabs, film(nm) b
konset, film(nm) Eoptg ,film(eV) c PCAZCN 374, 536 391, 591 748 1.66 PPTZCN 377, 459, 525 393, 485, 586 729 1.70 PDTPCN 437 454 719 1.72 a In THF dilute solutions. b
Spin coated from THF solutions.
c
The optical bandgaps were obtained from the equationEopt
acceptors.9–11,21 Among these polymers (PCAZCN, PPTZCN, and PDTPCN), PCAZCN has the broadest absorption spec-trum toward a higher absorption wavelength attributed to a better packing of the polymer backbones, which leads to a more extensive delocalization of the electrons in the solid state.13
Electrochemical Properties
To understand the electronic structures of D-A polymers (PCAZCN, PPTZCN, and PDTPCN), the HOMO and LUMO lev-els were investigated by the CV measurements in solid films with Ag/AgCl as a reference electrode, calibrated by ferro-cene (E1/2(ferrocene)¼ 0.45 mV vs. Ag/AgCl). The HOMO and
LUMO levels were estimated by the oxidation and reduction potentials from the reference energy level of ferrocene (4.8 eV below the vacuum level) according to the following equa-tion:28 EHOMO/LUMO ¼ [(Eonset 0.45)4.8] eV, and the
results are summarized in Table 3. As shown in Figure S2 of the Supporting Information, all polymers PCAZCN, PPTZCN, and PDTPCN exhibited two quasi-reversible oxidations or p-doping/dedoping (oxidation/rereduction) processes with onset oxidation potentials (Eox/onset) 0.97, 0.85, and 0.77 V,
respectively. Nevertheless, the n-doping/dedoping (reduc-tion/reoxidation) processes showed irreversible reduction potentials with onset reduction potentials (Ered/onset) of
0.84 V (PCAZCN), 0.90 V (PPTZCN), and 0.80 V (PDTPCN), respectively. On the basis of their corresponding Eox/onset and Ered/onset values, the HOMO/LUMO levels of
PCAZCN, PPTZCN, and PDTPCN were 5.32/3.51, 5.20/ 3.45, and 5.12/3.55 eV, respectively. The obtained LUMO levels of the polymers were in the desirable range to afford efficient driving force for charge separation.12The data show that the different donor segments have almost no effects on the LUMO energy levels20(b)but affect the HOMO energy lev-els that are responsible for the Vocvalues, which were reflected
in the photovoltaic properties correspondingly.12 In addition, the electrochemical bandgaps (Ecv
g) ofPCAZCN, PPTZCN, and
PDTPCN calculated from Ecv
g ¼ (Eox/onset Ered/onset) were
1.81, 1.75, 1.67 eV, respectively, which lay within the acceptable range of errors to those obtained from their absorption spectra (Eopt
g ).
Photovoltaic Properties
The potential applications of D-A polymers (PCAZCN, PPTZCN, and PDTPCN) in PSCs were explored by fabricating
BHJ photovoltaic devices with a configuration of ITO/ PEDOT:PSS (30 nm)/polymer:PC61BM blend (80 nm)/
Ca(30 nm)/Al(100 nm). The polymer solutions for the active layer were prepared by blending polymers (PCAZCN, PPTZCN, and PDTPCN) and PC61BM in a weight ratio of 1:1
initially, and later the active-layer compositions were modi-fied with various weight ratios of the previous optimum polymer blended with PC71BM (owing to a broader
absorp-tion and a higher absorpabsorp-tion coefficient of PC71BM than
PC61BM). The J–V characteristics of the PSC devices based on
polymers (PCAZCN, PPTZCN, and PDTPCN) and PC61BM
(1:1 w/w) are shown in Figure 3, and the photovoltaic prop-erties, that is, open circuit voltage (Voc), short circuit current
density (Jsc), fill factor (FF), and PCE values, are illustrated
in Table 4. The Vocvalues of polymers, which are dependent
on the energy levels of the active-layer components,18 corre-late well with the cyclovoltometric results, that is, polymers with lower HOMO levels have higher Voc values and vice
versa. As a result, polymerPCAZCN has the highest Vocvalue
(0.79 V), and polymer PDTPCN has the lowest Voc value
(0.63 V). As shown in Figure 4, the absorbance spectra of polymer:PC61BM blend (1:1 w/w) in solid films measured
from the PSC devices by using an ITO/PEDOT substrate as a reference were broadened toward longer wavelength region. Besides, the carrier transporting properties, that is, hole mo-bility (lh) and electron mobility (le), of blended polymers in
the active layer are also important parameters, which could affect the PCE values of PSC devices. To investigate the car-rier transporting properties, which are accountable for the variation in FF and Jsc value in PSC devices,
29,30
the space charge limited current method31 was used to evaluate
lh
and le mobilities of polymer blends in polymer films of
PCAZCN, PPTZCN, and PDTPCN:PC61BM (1:1 w/w), and the
data are listed in Table 4. It is perceptible that, compared with PPTZCN (le ¼ 2.7 108 cm2/V s and lh ¼ 3.6
109cm2/V s) andPDTPCN (le¼ 2.2 108cm2/V s and
lh¼ 2.7 109cm2/V s), the hole and electron mobilities
TABLE 3Electrochemical Properties of Polymers
Polymer Eox,onset (V)a Ered,onset (V)a HOMO (eV)b LUMO (eV)b Ecv g (eV) PCAZCN 0.97 0.84 5.32 3.51 1.81 PPTZCN 0.85 0.90 5.20 3.45 1.75 PDTPCN 0.77 0.80 5.12 3.55 1.67 a
Onset oxidation and reduction potentials measured by cyclic voltam-metry in solid films.
b
HOMO/LUMO¼ [(Eonset 0.45) 4.8] eV, where 0.45 V is the value
for ferrocene versus Ag/Agþand 4.8 eV is the energy level of ferrocene below the vacuum.
FIGURE 3 Current–voltage (J–V) curves of polymer solar cells containing polymer blends polymer:PC61BM ¼ 1:1 (w/w) under the illumination of AM 1.5G, 100 mW/cm2.
ofPCAZCN (le¼ 5.8 108cm 2
/V s andlh¼ 7.4 109
cm2/V s) were much higher to induce larger Jsc (3.99 mA/
cm2) and FF (32.0%) values. The poor FF values of all poly-mers might be attributed to the possibility of recombination of holes and electrons in these polymers.20(d) The stronger and broader absorption spectrum at 550–650 nm of PCAZCN:PC61BM in the active layer (Fig. 4) increased its
photocurrent response, and thus to show a broader and higher EQE value of11% (see Fig. S3 of the Supporting In-formation). Therefore, the device containingPCAZCN:PC61BM
showed the highest Jsc value (3.99 mA/cm2), which was
larger than those of the devices containingPPTZCN:PC61BM
(1.91 mA/cm2) and PDTPCN:PC61BM (1.78 mA/cm 2
).21(b) According to the atomic force microscopy (AFM) images of Supporting Information Figure S4, it was observed that a highest Jsc value (3.99 mA/cm2) was induced in the device
ofPCAZCN:PC61BM (1:1 w/w) having the smoothest surface
with a root-mean-square roughness (Rrms) of 0.26 nm, and Jscvalues were reduced to 1.91 and 1.78 mA/cm2by
increas-ing the roughness of PPTZCN:PC61BM (Rrms ¼ 0.28 nm)
and PDTPCN:PC61BM (Rrms ¼ 0.36 nm), respectively. This
is due to the large-scaled phase separation, which decreased the diffusional escape probabilities for mobile charge car-riers, and hence to increase the charge recombination, which
is fully consistent with the Jsc and PCE values obtained by
the copolymers.9(e,f) On the basis of the above reasonable facts, the PSC device containing polymerPCAZCN as an elec-tron donor and PC61BM as an electron acceptor (with 1:1
weight ratio) showed the best performance of a highest PCE ¼ 1.01% with Voc ¼ 0.79 V, Jsc ¼ 3.99 mA/cm
2
, and FF ¼ 32.0%. However, because of the poor dissociation of excitons at the polymer/acceptor interface and transport of free charge carriers toward the collecting electrodes,30 the PCE values ofPPTZCN (0.38%) and PDTPCN (0.34%) decreased significantly. The best performance of the PSC devices con-taining polymer PCAZCN was optimized by fabricating BHJ PSC devices using PCAZCN as a donor and PC71BM as an
acceptor in different weight ratios of 1:1, 1:2, 1:3, and 1:4. The J–V curves and EQE curves of the PSC devices based on PCAZCN:PC71BM in four different blended ratios (1:1, 1:2,
1:3, and 1:4) are shown in Figures 5 and 6, respectively, and data are illustrated in Table 5. The PSC device based on PCAZCN:PC71BM in 1:2 weight ratio obtained the best PCE
value of 1.28% with Voc ¼ 0.81 V, Jsc ¼ 4.93 mA/cm2, and
FF ¼ 32.1%. Indeed, the weight ratios between polymer donors and PCBM acceptor played a key role in all PCE val-ues.6(a),32As shown in Table 5, because of the poor interfa-cial contacts between the donor polymers and PC71BM
TABLE 4Photovoltaic Properties of Polymer Solar Cell Devices with a Configuration of ITO/ PEDOT:PSS/Polymer:PC61BM/Ca/Al a Polymer/ PC61BM (1:1) le (cm2/V s) lh (cm2/V s) Voc (V) Jsc (mA/cm2) FF (%) PCE (%) PCAZCN 5.8 108 7.4 109 0.79 3.99 32.0 1.01 PPTZCN 2.7 108 3.6 109 0.65 1.91 30.6 0.38 PDTPCN 2.2 108 2.7 109 0.63 1.78 30.3 0.34 a
Measured under AM 1.5 irradiation, 100 mW/cm2
.
FIGURE 4Absorption spectra of polymer blends of poly-mer:PC61BM ¼ 1:1 (w/w) measured from solar cell devices by using the substrate of ITO/PEDOT as a reference.
FIGURE 5 Current–voltage (J–V) curves of polymer solar cells containing polymer blends PCAZCN:PC71BM in different weight ratios under the illumination of AM 1.5G, 100 mW/cm2.
acceptor, the other polymer blends of PCAZCN:PC71BM ¼
1:3 and 1:4 (w/w) with higher PCBM contents offered much lower PCE values (see Table 5), which led to the suppression
of charge separation/transportation within the active layer.33 Figure 6 shows EQE curves of PSC devices containing poly-mer PCAZCN in different weight ratios of PCAZCN:PC71BM
¼ 1:1, 1:2, 1:3, and 1:4. The photocurrent responses dis-played a maximum EQE value of 17% at 550 nm (at PCAZCN:PC71BM ¼ 1:1 and 1:2 w/w), which are
corre-spondent to that of the absorbance spectra of polymer blends in solid films measured from the PSC devices (see Fig. 4). The similarities between the absorption spectrum and EQE of polymer blend containing PCAZCN indicate that the photogeneration of charges originates predominantly FIGURE 6External quantum efficiency (EQE) of polymer solar
cells containing polymer blends PCAZCN:PC71BM in different weight ratios.
TABLE 5 Photovoltaic Properties of Polymer Solar Cell Devices with a Configuration of ITO/PEDOT:PSS/PCAZCN:PC71BM/Ca/Al a
PCAZCN:PC71BM Voc(V) Jsc(mA/cm2) FF (%) PCE (%)
1:1 0.83 4.23 30.5 1.07
1:2 0.81 4.93 32.1 1.28
1:3 0.78 3.57 29.4 0.82
1:4 0.78 3.25 29.6 0.75
a
Measured under AM 1.5 irradiation, 100 mW/cm2
.
FIGURE 7AFM images of polymer blends PCAZCN: PC71BM in different weight ratios. (a) 1:1 (w/w), (b) 1:2 (w/w), (c) 1:3 (w/w), and (d) 1:4 (w/w).
following the absorption of light by polymer.6(a),32It is worth noticing that the EQE values remain almost same for PSC devices containing PCAZCN:PC71BM ¼ 1:1 and 1:2 (w/w).
However, the EQE values decreased significantly by increas-ing the PC71BM proportion, which is likely due to the
increase in imbalance of charge carrier mobilities at higher PC71BM concentrations.14 Therefore, as shown in Table 5,
the PCE values as well as FF and Jsc values of PSC devices
containingPCAZCN:PC71BM in 1:3 and 1:4 (w/w) decreased
correspondingly compared with that in 1:1 and 1:2 (w/w). In addition, the behavior of the photovoltaic properties can be well analyzed from the surface topography of the active layer. The AFM images of polymer blends PCAZCN:PC71BM
as cast films in different weight ratios are given in Figure 7. The average Rrms of PCAZCN:PC71BM ¼ 1:1, 1:2, 1:3, and
1:4 (w/w) were 0.17, 0.16, 0.18, and 0.21 nm, respectively, which indicates that the smoother surface has a higher PCE value ascribed to the better solubility and interfacial contacts of the polymer blend.
CONCLUSIONS
A series of novel low-bandgap conjugated triphenylamine-based polymers (PCAZCN, PPTZCN, and PDTPCN) consisting of different electron-rich donors (including 2,7-carbazole, phenothiazine, and dithianopyrol) in the main chains along with cyano- and dicyano-vinyl acceptors in the side chains were designed and copolymerized successfully by Pd(0)-cata-lyzed Suzuki or Stille coupling reaction. The electron-rich do-nor groups including triphenylamine in the polymer main chains endowed with strong and broad absorptions to get superior harvesting of sunlight and tunable HOMO levels and the electron-withdrawing cyano- and dicyano-vinyl in the side chains effectively reduced the bandgaps. The BHJ PSC device containing electron-donor polymer PCAZCN (bearing 2,7-carbazole units) and electron-acceptor PC71BM in 1:2
weight ratio afforded the highest PCE value of 1.28% with Voc¼ 0.81 V, Jsc¼ 4.93 mA/cm2, and FF¼ 32.1%. However,
because of poor carrier transporting characteristics of PPTZCN and PDTPCN, their PCE values were reduced significantly.
The authors are grateful to the National Center for High-Performance Computing for computer time and facilities. The financial supports of this project provided by the National Sci-ence Council of Taiwan (ROC) through NSC 97-2113-M-009-006-MY2, National Chiao Tung University through 97W807, and Energy and Environmental Laboratories (charged by Chang-Chung Yang) in Industrial Technology Research Institute (ITRI) are acknowledged.
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