Containing Phenothiazine Donor and Various Benzodiazole
Acceptors for Polymer Solar Cells
HARIHARA PADHY,1JEN-HSIEN HUANG,2DURYODHAN SAHU,1DHANANJAYA PATRA,1 DHANANJAY KEKUDA,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 6 June 2010; accepted 15 July 2010 DOI: 10.1002/pola.24273
Published online 20 September 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT:A series of soluble donor-acceptor conjugated poly-mers comprising of phenothiazine donor and various benzodia-zole acceptors (i.e., benzothiadiabenzodia-zole, benzoselenodiabenzodia-zole, and benzoxadiazole) sandwiched between hexyl-thiophene linkers were designed, synthesized, and used for the fabrication of polymer solar cells (PSC). The effects of the benzodiazole acceptors on the thermal, optical, electrochemical, and photo-voltaic properties of these low-bandgap (LBG) polymers were investigated. These LBG polymers possessed large molecular weight (Mn) in the range of 3.855.13 10
4
with high thermal decomposition temperatures, which demonstrated broad absorption in the region of 300750 nm with optical bandgaps of 1.801.93 eV. Both the HOMO energy level (5.38 to 5.47 eV) and LUMO energy level (3.47 to 3.60 eV) of the LBG polymers were within the desirable range of ideal energy level. Under 100 mW/cm2 of AM 1.5 white-light illumination, bulk
heterojunction PSC devices containing an active layer of elec-tron donor polymers mixed with elecelec-tron acceptor [6,6]-phe-nyl-C61-butyric acid methyl ester (PC61BM) or [6,6]-phenyl-C71 -butyric acid methyl ester (PC71BM) in different weight ratios were investigated. The best performance of the PSC device was obtained by using polymer PP6DHTBT as an electron do-nor and PC71BM as an acceptor in the weight ratio of 1:4, and a power conversion efficiency value of 1.20%, an open-circuit voltage (Voc) value of 0.75 V, a short-circuit current (Jsc) value of 4.60 mA/cm2, and a fill factor (FF) value of 35.0% were achieved. VC 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4823–4834, 2010
KEYWORDS:conjugated polymers; copolymerization; donor-acceptor; heteroatom-containing polymers; phenothiazine derivatives; solar cells
INTRODUCTION Despite the poor long-term stability, poly-mer solar cell (PSC) devices based on conjugated polypoly-mers as electron donors and fullerene derivatives as electron acceptors are of broad interests because of the advantages of low cost, light-weight flexible devices, tunable electronic properties, and ease of processing for the conversion of solar energy to electricity.1–7 Although poly(3-hexylthiophene)
(P3HT) is proven to be one of the most efficient donor mate-rials ever tested in PSCs for giving the power conversion effi-ciency (PCE) up to 5%,2 further enhanced PCE values are limited because of both lower photocurrent generation and intrinsic absorption properties. To conquer these problems, low-bandgap (LBG) polymers composed of electron-rich (do-nor) and electron-deficient (acceptor) units have been uti-lized recently in PSCs with fullerene derivatives, such as [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or [6,
6]-phenyl-C71-butyric acid methyl ester (PC71BM), yielding a
PCE value up to 7.7%.3 PSCs consisting of such
donor-acceptor (D-A) LBG polymers have attracted more attention owing to their tunable optical, electrochemical, electronic, and photovoltaic properties.6 Incorporation of wide ranges
of donors and acceptors into LBG polymers can manipulate the electronic structures, that is, the highest occupied molec-ular orbital (HOMO) and lowest unoccupied molecmolec-ular or-bital (LUMO) levels through the partial intramolecular charge transfer (ICT) in the D-A systems.8 By optimizing materials and device structures, photovoltaic parameters, such as the short-circuit current (Jsc) and open-circuit voltage (Voc), can
be further improved to obtain higher PCE values in the PSCs. In solar cell devices, Jsc is determined by the creation and
subsequent dissociation of excitons at the polymer/acceptor interface followed by transport of free charge carriers towards the collecting electrodes,9 V
oc is primarily
deter-mined by the effective band gap of the bulk hetero-junction (BHJ) film.7(a) For this purpose, the electron donor polymer should exhibit a band gap between 1.2 and 1.9 eV, which Correspondence to: H.-C. Lin (E-mail: linhc@cc.nctu.edu.tw)
corresponds to a HOMO energy level between 5.8 and 5.2 eV and a LUMO energy level between 4.0 and 3.8 eV.1(c)Again, if the energy difference between the LUMO
lev-els of polymer and acceptor is less than 0.3 eV,10the driving force for charge separation will be reduced, and Voc can be
reduced by raising the HOMO level. Consequently, it is of great importance to match the energy levels of the polymer and acceptor carefully to develop BHJ solar cells with high efficiencies.
Among all heterocyclic compounds, phenothiazine contains both electron-rich sulfur and nitrogen heteroatoms. The elec-tron-rich nature of phenothiazine contributes for the efficient electron donor and hole transporting materials in polymers and organic molecules for photo-induced charge separation and it has been also proven as a superior electron donor for reductive quenching.11 Because of their unique electro-opti-cal properties, these materials are potential candidates for diverse applications for light-emitting diodes,12 solar cells, chemiluminescence devices,13and organic field effect transis-tors.12(b),14Phenothiazine ring hampers stacking aggregation and intermolecular excimer formation in the main chain of the polymer due to its nonplanar structure.15 However, till now only a limited number of phenothiazine-based polymers for photovoltaic devices have been explored.16
Addition of electron-withdrawing imine nitrogen to a conju-gated polymer backbone generally enhances its electron-accepting properties and makes it susceptible to n-doping (reduction). Benzodiazole units are, in that sense, typical examples of such units containing imine nitrogen.6(d) 2,1,3-Benzothiadiazole is a widely used electron acceptor for the synthesis of D-A polymers. For example, copolymers of ben-zothiadiazole with fluorene,17 silafluorene,18 carbazole,19 dithienosilole,20 dithienocyclopentadiene,21 and dithieno[3,2-b:20,30-d]pyrroles22 were synthesized and applied to PSCs, yielding PCE values in the range of 0.185.4%. Recently, many photovoltaic papers have reported LBG copolymers made of electron donors and acceptors sandwiched between two thiophene units.17–23 Incorporation of acceptor units in the midst of two thiophene units, alleviate the severe steric hindrance between the electron donors and acceptors, resulting in more planar structures to facilitate interchain associations and improve the hole mobilities of the LBG poly-mers. Despite these advantages, addition of thiophene units could induce solubility problems and yield low molecular weights in polymers.17(a) To utilize the aforementioned mer-its of thiophene unmer-its, structural modifications, such as incor-poration of alkyl or alkoxy chains on the 3- and/or 4-posi-tion of thienyl units17(c) or addition of supplementary alkylated thiophene units,17(d)have been outfitted to acquire higher molecular weights and better solubilities than the original polymers without any soluble side-chains.
To have better photophysical, electrochemical, and photovol-taic properties in the resulting LBG polymers, the incorpora-tion of phenothiazine donor units with various acceptor units are very intriguing and thus to motivate this study. Herein, we report the design, synthesis, properties, and de-vice applications of phenothiazine-based alternating
conju-gated D-A polymers, in which the acceptor benzodiazole units include benzothiadiazole, benzoselenodiazole, and ben-zoxadiazole sandwiched between two hexyl thiophene units. These polymers were synthesized by palladium (0)-catalyzed Suzuki coupling reactions. The effects of D-A strengths on the electronic and optoelectronic properties of the LBG poly-mers were also investigated. In addition, the PSC devices fab-ricated by polymer/PC61BM or polymer/PC71BM blends
sandwiched between a transparent anode (ITO/PEDOT:PSS) and a cathode (Ca) were explored.
EXPERIMENTAL Materials
All chemicals and solvents were reagent grades and pur-chased from Aldrich, ACROS, Fluka, TCI, TEDIA, and Lancas-ter Chemical Co. Toluene, tetrahydrofuran, and diethyl ether were distilled over sodium/benzophenone to keep anhy-drous before use. Chloroform (CHCl3) was purified by
reflux-ing with calcium hydride and then distilled. If not otherwise specified, the other solvents were degassed by nitrogen 1 h prior to use.
Measurements and Characterization
1
H-NMR and 13C-NMR spectra were recorded on a Varian Unity 300 MHz spectrometer using CDCl3solvent. Elemental
analyses were performed on a HERAEUS CHN-OS RAPID ele-mental analyzer. Thermogravimetric analyses (TGA) were conducted with a TA Instruments Q500 at a heating rate of 10 C/min under nitrogen. The molecular weights of poly-mers were measured by gel permeation chromatography (GPC) using Waters 1515 separation module (concentration ¼ 1 mg/mL in THF; flow rate ¼ 1 mL/min), and polystyrene was used as a standard with THF as an eluant. UVvisible absorption spectra were recorded in dilute chlorobenzene solutions (105 M) and on solid films (spin-coated with a spin rate 1000 rpm for 60 s on glass substrates from chlorobenzene solutions with a concentration of 10 mg/mL) on a HP G1103A. Cyclic voltammetry (CV) measurements were performed using a BAS 100 electrochemical analyzer with a standard three-electrode electrochemical cell in a 0.1-M solution of tetrabutylammonium hexafluorophosphate ((TBA)PF6) in acetonitrile at room temperature with a
scan-ning rate of 100 mV/s. During the CV measurements, the sol-utions were purged with nitrogen for 30 s. In each case, a carbon working electrode coated with a thin layer of poly-mers, a platinum wire as the counter electrode, and a silver wire as the quasi-reference 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 ferro-cene/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-cast-ing films with the similar thickness from THF solutions (5 mg/mL). The onset potentials were determined from the intersections of two tangents drawn at the rising currents and background currents of the cyclic voltammetry (CV) measurements. Surface morphology images of thin films (on
glass substrates) were obtained using atomic force micros-copy (AFM, Digital instrument NS 3a controller with D3100 stage).
Device Fabrication and Characterization of PSCs
The PSCs in this study were composed of an active layer of blended polymers (Polymer: PCBM) in solid films, which was sandwiched between a transparent indium tin oxide (ITO) anode and a metal cathode (Ca). Prior to 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 130C for 1 h, the substrates were transferred to a nitrogen-filled glovebox. Then, on the top of PEDOT:PSS layer, an active layer was prepared by spin coating from blended solutions of poly-mers:PC61BM (with 1:1 w/w) and PP6DHTBT:PC71BM (with
1:1, 1:3, and 1:4 w/w) subsequently with a spin rate1500 rpm for 60 s, and the thickness of the active layer was typi-cally80 nm. Initially, the blended solutions were prepared by dissolving both polymers and PCBM in 1,2-dichloroben-zene (20 mg/mL), followed by continuous 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 cal-cium layer (30 nm) and a subsequent 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 testing was done inside a glove box under simulated AM 1.5G irradiation (100 mW/cm2) using a Xenon lamp based solar simulator (Ther-mal Oriel 1000W). The light intensity was calibrated by a mono-silicon photodiode with KG-5 color filter (Hamamatsu). The external quantum efficiency (EQE) action spectrum was obtained at short-circuit condition. 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 onto the photovoltaic cell under test.
Synthesis of Monomers and Polymers General Synthetic Procedures for 4a–4c
In a 100-mL flame-dried two-neck flask fitted with a con-denser, 1.00 equiv of dibromoarene (3a–3c), 2.2 equiv of 2-(4-hexylthiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2), and 0.03 equiv of tetrakis (triphenylphosphine) palla-dium was added. The mixture was degassed and purged nitrogen. Afterward, 40 mL of anhydrous toluene and 2 M aqueous potassium carbonate solution (8 mL) was added. The reaction mixture was heated to 90C with vigorous stir-ring until reaction completion by TLC analyses (24 h). The mixture was poured into water (100 mL) and extracted with methylene chloride. The organic layer was washed thrice with water, once with brine and dried over magnesium
sul-fate. The solvent was evaporated and the residue was puri-fied by column chromatography on silica gel with hexane/ ethyl acetate ¼ 20/1 to give the products. Their chemical characterization analyses are shown as follows:
4,7-Bis(4-hexylthiophen-2-yl)-2,1,3-benzothiadiazole (4a) Orange needles (yield: 88%); mp 75–77 C. 1H-NMR (ppm, CDCl3): d 7.97 (dd, 2H), 7.82 (d, J ¼ 1.8 Hz 2H), 7.04 (dd, 2H), 2.66 (t, J ¼ 7.5 Hz, 4H), 1.70 (m, 4H), 1.251.53 (m, 12H), 0.90 (t, J ¼ 6.7 Hz, 6H). 13C-NMR (ppm, CDCl3): d 153.02, 139.75, 128.42, 127.90, 127.21, 126.38,126.15,. 31.68, 29.70, 29.65, 29.03, 22.67, 14.14. 4,7-Bis(4-hexylthiophen-2-yl)-2,1,3-benzoselenadiazole (4b) Purple solid (yield: 87%); mp 8283 C. 1H-NMR (ppm, CDCl3):d 7.87 (d, J ¼ 1.2 Hz, 2H), 7.71 (s, 2H), 7.04 (d, J ¼ 1.5 Hz, 2H), 2.68 (t,J ¼ 7.8 Hz, 4H), 1.68 (m, 4H), 1.201.43 (m, 12H), 0.90 (t, J ¼ 6.9 Hz, 6H).13C-NMR (ppm, CDCl3):d 158.19, 143.98, 139.29, 128.87, 127.42, 125.75,121.83,. 31.68, 30.56, 30.45, 29.04, 22.62, 14.10. 4,7-Bis(4-hexylthiophen-2-yl)-2,1,3-benzoxadiazole (4c) Yellow solid (yield: 92%); mp 7879 C. 1
H-NMR (ppm, CDCl3):d 7.95 (d, J ¼ 1.2 Hz, 2H), 7.57 (s, 2H), 7.02 (d, J ¼ 1.2 Hz, 2H), 2.67 (t,J ¼ 7.6 Hz, 4H), 1.70 (m, 4H), 1.201.43 (m, 12H), 0.89 (t, J ¼ 7.2 Hz, 6H).13C-NMR (ppm, CDCl3):d 148.081, 145.28, 137.77, 30.40, 126.32, 122.31, 121.88, 31.90, 30.83, 30.65, 29.24, 22.85, 14.34.
General Bromination Procedures for 5a–5c
In a 100-mL flask, 1.00 equiv of 4,7-di(4-hexyl-2-thienyl)-arene (4a4c) was added into THF under nitrogen flow. Af-ter solids were dissolved completely, 2.10 equiv N-bromosuc-cinimide (NBS) was added in portion wise. The reaction mix-tures were stirred at a room temperature for 5 h. Subsequently, hexane was added into the mixture, and the white precipitate formed was filtered off. In addition, the fil-trate was extracted with ethyl acetate, and the organic layer was washed with brine followed by being dried over anhy-drous sodium sulfate. After that, the residue was purified by column chromatography on silica gel with hexane/methylene chloride¼ 1/2 to give the products. Their chemical charac-terization analyses are shown as follows:
4,7-Bis(5-bromo-4-hexyl-2-thienyl)-2,1,3-benzothiadiazole (5a) Red solid (yield: 94%); mp 101103 C. 1H-NMR (ppm, CDCl3):d 7.75 (s, 2H), 7.71 (s, 2H), 2.63 (t, J ¼ 7.2 Hz, 4H),
1.67 (m, 4H), 1.331.40 (m, 12H), 0.89 (t, J ¼ 7.1 Hz, 6H).
13
C-NMR (ppm, CDCl3): d 152.19, 143.01, 138.46, 128.03,
125.25, 124.80, 111.59, 31.62, 29.74, 29.67, 28.96, 22.64, 14.11. Element Anal. Calcd for C26H30Br2N2S3: C, 49.84%; H,
4.83%; N, 4.47%; Found: C, 49.62%; H, 5.02%; N, 4.62%. EIMS (m/z): calcd for C26H30Br2N2S3, 626.53; found, 626.
4,7-Bis(5-bromo-4-hexyl-2-thienyl)-2,1,3-benzoselenadiazole (5b)
Purple solid (yield: 96%); mp 9294 C. 1H-NMR (ppm, CDCl3):d 7.65 (s, 2H), 7.64 (s, 2H), 2.62 (t, J ¼ 7.2 Hz, 4H),
1.67 (m, 4H), 1.331.42 (m, 12H), 0.90 (t, J ¼ 6.9 Hz, 6H). (ppm, CDCl3): d 157.71, 142.62, 138.73, 127.63, 126.75,
Element Anal. Calcd for C26H30Br2N2S2Se: C, 46.37%; H,
4.49%; N, 4.16. Found: C, 46.78%; H, 5.14%; N, 4.33%. EIMS (m/z): calcd for C26H30Br2N2S3, 673.43; found, 674.
4,7-Bis(5-bromo-4-hexyl-2-thienyl)-2,1,3- benzoxadiazole (5c) Orange solid (yield: 93%); mp 108110 C. 1H-NMR (ppm,
CDCl3):d 7.77 (s, 2H), 7.38 (s, 2H), 2.60 (t, J ¼ 7.2 Hz, 4H),
1.62 (m, 4H), 1.331.42 (m, 12H), 0.90 (t, J ¼ 6.9 Hz, 6H).
13
C-NMR (ppm, CDCl3): d 147.42, 143.88, 137.08, 129.71,
125.68, 121.35, 111.56, 31.59, 29.66, 29.64, 28.94, 22.60, 14.10. Element Anal. Calcd for C26H30Br2N2OS2: C, 51.15%;
H, 4.95%; N, 4.59%. Found: C, 51.30%; H, 5.52%; N, 4.27%. EIMS (m/z): calcd for C26H30Br2N2S3, 610.47; found, 610.
10-Hexyl-3,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-10H-phenothiazine (7)
A solution of 3,7-dibromo-10-hexyl-10H-phenothiazine (4.41 g, 10 mmol) in anhydrous tetrahydrofuran (100 mL) was cooled to78C under nitrogen and stirred at this tempera-ture for 5 min in the flame-dried two-neck round-bottom flask.n-Butyl lithium (8.4 mL of 2.5 M solution in hexane, 21 mmol) was added dropwise, using a syringe, and the mixture was stirred at 78 C, warmed to 0 C for 15 min, and cooled again at78C for 15 min. 2-Isopropoxy-4,4,5,5-tet-ramethyl-1,3,2-dioxaborolane (6.13 mL, 30 mmol) was added rapidly to the solution, and the resulting mixture was warmed to room temperature and stirred overnight. The mixture was poured into water and extracted with ether. The organic extracts were washed with brine and dried over magnesium sulfate. The solvent was removed by rotary evap-oration, and the residue was recrystalized from acetone to obtain 3.96 g (74%) of the title product as a slight yellow solid ; mp 212214 C. 1H-NMR (ppm, CDCl 3): d 7.54 (m, 4H), 6.80 (d,J ¼ 7.8 Hz, 2H), 3.84 (t, J ¼ 6.9 Hz, 2H), 1.78 (m, 2H), 1.4 (m, 2H), 1.32 (s, 24H), 1.25 (m, 4H), 0.86 (t,J ¼ 7.2 Hz, 3H). 13C-NMR (ppm, CDCl3): d 147.47, 134.23, 133.97, 124.15, 114.90, 83.91, 47.71, 31.62, 26.89, 26.72, 25.06, 22.78, 14.21. ELEM. ANAL. Calcd for C30H43B2NO4S: C,
67.31%; H, 8.10%, N, 2.62%. Found: C, 66.47%; H, 7.73% N, 2.93%. EIMS (m/z): calcd for C30H43B2NO4S, 535.35; found,
536.
General Polymerization Procedure
All polymerization steps were carried out through the palla-dium(0)-catalyzed Suzuki coupling reactions. In a 50-mL flame dried two-neck flask, 1 equiv of 10-hexyl-3,7-bis (4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-10H-phenothiazine (7), 1 equiv of bis(bromo-4-hexyl-2-thienyl) arene (5a5c), and Pd(PPh3)4(1.5 mol %) were dissolved in a mixture of
toluene ([monomer] ¼ 0.5 M) and aqueous 2 M Na2CO3
(2:3). The solution was first put under a nitrogen atmos-phere and vigorously stirred at 9095C for 45 days. Af-ter reaction completion, an excess of bromobenzene was added to the reaction then 1 h later, excess of phenylboronic acid was added and the reaction refluxed overnight to com-plete 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 hex-ane, acetone, and chloroform. The chloroform fraction was
reduced to 4050 mL under reduced pressure, precipitated in methanol/water (10:1, 500 mL), filtered through 0.45lm nylon filter and finally air-dried overnight.
Poly[(10-hexyl-10H-phenothiazine-3,7-ylene)-alt-(4,7-bis(4-hexylthien-2-yl)-2,1,3-benzothiadiazole) 20,200-diyl] (PP6DHTBT)
Dark orange solid (yield: 71%). 1H-NMR (ppm, CDCl3): d
8.02 (br, 2H), 7.84 (br, 2H), 7.32 (br, 4H), 6.93 (br, 2H), 3.92 (br, 2H), 2.74 (br, 4H), 1.88 (br, 2H), 1.72 (br, 4H), 1.50 (br, 4H), 1.18-1.40 (br, 14H), 0.810.90 (br, 9H). Anal. Calcd C, 70.45%; H, 6.85%, N, 5.60%. Found: C, 69.78%; H, 6.97% N, 5.42%. Poly[(10-hexyl-10H-phenothiazine-3,7-ylene)-alt-(4,7-bis(4-hexylthien-2-yl)-2,1,3- benzoselenadiazole) 20,200-diyl] (PP6DHTBSe)
Dark black solid (yield: 76%). 1H-NMR (ppm, CDCl
3):d 7.92 (br, 2H), 7.77 (br, 2H), 7.32 (br, 4H), 6.94 (br, 2H), 3.92 (br, 2H), 2.74 (br, 4H), 1.88 (br, 2H), 1.71 (br, 4H), 1.50 (br, 4H), 1.18-1.40 (br, 14H), 0.810.90 (br, 9H). Anal. Calcd C, 66.30%; H, 6.45%, N, 5.27%. Found: C, 64.49%; H, 6.38% N, 4.94%. Poly[(10-hexyl-10H-phenothiazine-3,7-ylene)-alt-(4,7-bis(4-hexylthien-2-yl)-2,1,3- benzoxadiazole) 20,200-diyl] (PP6DHTBX)
Dark solid (yield: 69%). 1H-NMR (ppm, CDCl3): d 8.00 (br,
2H), 7.53 (br, 2H), 7.30 (br, 4H), 6.91 (br, 2H), 3.89 (br, 2H), 2.69 (br, 4H), 1.88 (br, 2H), 1.69 (br, 4H), 1.48 (br, 4H), 1.18-1.40 (br, 14H), 0.810.90 (br, 9H). Anal. Calcd C, 71.99%; H, 7.00%, N, 5.72%. Found: C, 72.00%; H, 6.84% N, 5.75%. RESULTS AND DISCUSSION
Synthesis and Structural Characterization
The general synthetic routes of monomers 5a–5c and 7 are shown in Scheme 1. Synthesis of 2-(4-hexylthiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2),24(a) 4,7-dibromo-2,1,3-benzothiadiazole (3a),24(b)
4,7-dibromo-2,1,3-benzosele-nadiazole (3b),23(c) and 4,7-dibromo-2,1,3-benzoxadiazole (3c)6(d) were prepared by following the literature proce-dures. Hexyl-thiophene units were added to both sides of each acceptor units through the Suzuki coupling reaction between 2-(4-hexylthiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2) and dibromo arenes (3a3c) in presence of a catalyst Pd(PPh3)4. Next, these compounds were
bromi-nated with NBS to produce monomers5a5c. The diboronic ester monomer (7) was prepared according to the literature method,16(d) that is, alkylation of phenothiazine with 1-bro-mohexane, followed by bromination with molecular bromine, then lithiation of dibromo compound with n-Buli and quenching with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxa-borolane produced monomer 7. Monomers (5a5c and 7) were satisfactorily characterized by 1H NMR, 13C NMR, MS spectroscopy, and elemental analyses. As shown in Scheme 2, three alternating polymers PP6DHTBT, PP6DHTBSe, and PP6DHTBX were prepared with the well-known Suzuki poly-merization between the diboronic ester of phenothiazine (7)
and the dibromide monomers (5a5c). The obtained poly-mers were further purified by washing on Soxhlet apparatus using hexane, acetone, and chloroform. The chloroform frac-tion was reduced to 4050 mL under reduced pressure, pre-cipitated in methanol, filtered through 0.45lm nylon filters and finally dried under reduced pressure at room tempera-ture. After purification and drying, all polymers were obtained as fibrous solids in overall good yields (6976%).
The chemical structures of the polymers were confirmed with1H-NMR and elemental analysis. The1H-NMR spectra of polymers are demonstrated in Figure 1, where the broaden-ing signals of1H-NMR spectra in both aromatic and aliphatic regions were observed as a result of polymerization. The polymers exhibited good solubilities in common organic sol-vents, such as THF, chloroform, toluene, and chlorobenzene at room temperature.
SCHEME 1Synthetic routes of monomers (5a5c and 7).
The molecular weights of the polymers were determined by gel permeation chromatography (GPC) against monodisperse polystyrene standards in THF are summarized in Table 1. These results show that reasonable molecular weights were obtained in these polymers, which had number-average mo-lecular weights (Mn) ranging 38,50051,300 and
weight-av-erage molecular weights (Mw) ranging 64,500101,700,
respectively, with polydispersity indices (PDI ¼ Mw/Mn)
ranging 1.671.98. The thermal properties of the polymers determined by thermogravimetric analysis (TGA) are shown in Figure 2 and summarized in Table 1. The TGA thermo-grams of the polymers revealed (5% weight loss) decomposi-tion temperatures (Td) in the range of 401434 C,
indica-tive of excellent thermal stabilities. Optical Properties
The normalized UVvis absorption of the synthesized poly-mers in dilute chlorobenzene solutions (concentration 105 M) and solid films are shown in Figure 3, and the main opti-cal properties are listed in Table 2. The absorption spectra of polymers, that is, PP6DHTBT, PP6DHTBSe, and PP6DHTBX, exhibited two distinct broad absorption peaks. The short-wavelength absorption peaks have been attributed
to a delocalized p-p* transition in the polymer chains and long-wavelength absorption peaks attributed to a localized transition between the donor-acceptor (D-A) charge transfer states in polymer segments. The high energy transition bands situated at 300–400 nm are consistent with the reported phenothiazine homopolymers12(b)or phenothiazine containing copolymers.12(c) The low energy peaks appeared at 500600 nm, with tailing the absorption around 700 nm are due to the ICT happening inside these phenothiazine based D-A conjugated polymers. The maximum absorption wavelengths (kmax,abs) for PP6DHTBT, PP6DHTBSe, and
PP6DHTBX in solutions were located at 515, 552, and 522 nm, respectively, while those in solid films at 552, 582, and 553 nm, respectively. As illustrated in Table 2, the optical band gaps (Eg
opt
) of PP6DHTBT, PP6DHTBSe, and PP6DHTBX in solid films, which were estimated from the absorption edges of UV-vis spectra, were 1.93, 1.80, and 1.90 eV, respec-tively. Compared with UVvis absorption spectra in solu-tions, all polymers in solid films had a red shift (3037 nm), FIGURE 11H-NMR spectra of polymers in CDCl
3. Labels of x and y are CDCl3and H2O, respectively.
TABLE 1Molecular Weights and Thermal Properties of Polymers Polymer Yield (%) Mna (104) Mwa (104) PDI (Mw/Mn) Tdb (C) PP6DHTBT 71 4.07 7.54 1.85 434 PP6DHTBSe 76 5.13 10.17 1.98 401 PP6DHTBX 79 3.85 6.45 1.67 417 a
Molecular weights and polydispersity index (PDI) values were meas-ured by GPC, using THF as an eluent, polystyrene as a standard.Mn,
number average molecular weight. Mw, weight average molecular
weight.
b
Temperature (C) at 5% weight loss measured by TGA at a heating
this could be attributed to the interchain associations and aggregations in solids. The maximum absorption wavelength (kmax,abs ¼ 552 nm) of PP6DHTBT in solid film was
red-shifted compared with that (540 nm) of its analogue F8TBT (phenothiazine units replaced with fluorene units).25(a) Although PP6DHTBSe bearing alkyl chains at 4-position of thiophene units revealed kmax,abs ¼ 582 nm in solid film,
which also red shifted to its fluorene-based polymer ana-logue bearing alkyl-chain free thiophenes, PFO-DBTSe (570
nm).25(b)Since the side-chain functionalization usually cause steric hindrance to affect the coplanarity of the conjugated backbone, side-chain functionalized polymer has a blue shift in the absorption spectra compared with its side-chain free polymer analog.26 It suggests that the phenothiazine unit possesses stronger electron-donating capability (stronger degree of delocalization and the stronger ICT) than the fluo-rene unit, thus to improve the effective conjugation length along the phenothiazine-based polymer backbone.16(d) Because of the presence of Selenium (Se) atom, which has larger size and is more electron rich than both S and O atoms, PP6DHTBSe had a more red-shifted absorption wave-length (kmax,abs) compared with the other two polymers
(PP6DHTBT and PP6DHTBX). Similar results were reported for the polymers containing benzoselenadiazole units,27
where the presence of imine nitrogens in benzodiazole units can stabilize the quinoid resonance structures by the most electron rich Se atom.27(d)
Electrochemical Properties
The energy band structures, that is, HOMO and LUMO levels, of the polymers were investigated by cyclic voltammetry (CV) measurements to understand the charge injection proc-esses of these polymers in their PSC devices. The cyclic vol-tammograms of the polymers in solid films are displayed in Figure 4 and the related CV data (formal potentials, onset potentials, HOMO and LUMO levels, and band gaps) are sum-marized in Table 3. Ag/AgCl was served as a reference elec-trode and it was calibrated by ferrocene (E1/2(FC/FCþ)¼ 0.45
eV vs. Ag/AgCl). The HOMO and LUMO energy levels were estimated by the oxidation and reduction potentials from the reference energy level of ferrocene (4.8 eV below the vac-uum level) according to the following equation28: EHOMO/
ELUMO¼ [(Eonset Eonset(FC/FCþvs. Ag/Agþ)) 4.8] eV, where
4.8 eV is the energy level of ferrocene below the vacuum level and Eonset(FC/FCþ vs. Ag/Agþ) ¼ 0.45 eV. All polymers
exhibited one quasi-reversible p-doping/dedoping (oxida-tion/rereduction) process at positive potentials and one quasi-reversible or reversiblen-doping/dedoping (reduction/ reoxidation) process at negative potentials, which are good signs of high structural stability in the charged state. The HOMO levels were in the range of 5.38 to 5.47 eV, which were estimated from the onset oxidation potentials (Eox/onset) of polymers (1.031.12 V). The LUMO levels were
FIGURE 3Normalized UV-vis spectra of polymers in (a) dilute chlorobenzene solutions and (b) solid films, respectively.
TABLE 2Optical Properties of Polymers
Polymer
Solutiona Solid Filmb
kmax,abs (nm) kedge (nm) Egopt (eV)c kmax,abs (nm) kedge (nm) Egopt (eV)c PP6DHTBT 337,515 607 2.04 347,552 642 1.93 PP6DHTBSe 349,552 656 1.89 358,582 689 1.80 PP6DHTBX 330,522 610 2.03 341,553 652 1.90 a
In chlorobenzene dilute solution.
b
Spin coated from chlorobenzene solution.
c
in the range of 3.47 to 3.60 eV, which were estimated from the onset reduction potentials (Ered/onset) of polymers
(0.75 to 0.88 V). As all HOMO levels were below the air oxidation threshold (ca. 5.27 eV or 0.57 V vs. SCE),29 the polymers should show good air stabilities. More importantly, the introduction of phenothiazine unit in the PP6DHTBT polymer backbone decreases the energy band gap in contrast to its fluorene analog F8TBT,25(a) which may be due to the presence of electron-rich sulfur and nitrogen heteroatoms of the phenothiazine unit that renders the resulting conjugated backbone more electron-rich.16(d) On the other hand, the LUMO energy levels are clearly affected by the electron-defi-cient centers of the benzodiazole comonomers, stronger elec-tron-deficient units resulting in lower LUMO energy levels. It has been found that the band gaps of these polymers were affected due to stronger ICT interactions between the donor phenothiazine unit and acceptor benzodiazole units. It is worth also noting that the band-gap values directly meas-ured by CV (Eg
ec
between 1.83 and 1.95 eV) and the optical band-gap values estimated from UV–vis spectra (Eg
opt
between 1.80 and 1.93 eV) are relatively in good agreement.
All these electrochemical characteristics are within the desir-able range for the ideal polymers to be utilized in the or-ganic photovoltaic applications.
Photovoltaic Properties
To investigate the potential use of polymers PP6DHTBT, PP6DHTBSe, and PP6DHTBX in PSCs, the bulk heterojunction (BHJ) solar cell devices comprising of these polymers as electron donors and fullerene derivatives (PC61BM or
PC71BM) as an electron acceptor in their active layer were
fabricated with a structure of ITO/PEDOT:PSS (30 nm)/poly-mer:PCBM blend (80 nm)/Ca (30 nm)/Al (100 nm). The blended solutions were prepared with polymers and PC61BM
in a weight ratio of 1:1 (w/w) initially, and later the active layer compositions were modified with various weight ratios for the previous optimum polymer with PC71BM. The current
density (J) versus voltage (V) curves of the PSCs are shown in Figure 5; the open circuit voltage (Voc), short circuit
cur-rent density (Jsc), fill factor (FF), and the PCE values of the
devices are summarized in Table 4. In BHJ solar cell devices, Voc is determined by the difference between HOMO level of
the electron donor polymer and LUMO level of the electron acceptor material (PCBM).7(a) Because of negligible differen-ces in HOMO levels of all polymers ((5.38) (5.47) eV), there were minor variations in Voc values (0.690.65 V).
With the similar Voc values and fill factor (29.132.1%) in
the devices containing the polymers blended with PC61BM in
a weight ratio of 1:1 (w/w), it was evident that due to the major variations of the Jsc values (1.92, 1.43, and 1.24 mA/
cm2) in polymers PP6DHTBT, PP6DHTBSe, and PP6DHTBX, they are crucially affected to have the PCE values of 0.41, 0.28, and 0.25, respectively. Among these PSC devices con-taining polymers, the best performance was the PSC device containing PP6DHTBT:PC61BM (1:1 w/w) with a highest PCE
value of 0.41%, Voc ¼ 0.67 V, Jsc¼ 1.92 mA/cm2, and FF¼
32.1%.
Since the best performance of PSC device was observed in the previous optimum polymer blend PP6DHTBT:PC61BM
(1:1 wt %) as an active layer, the PSC devices as a function of polymer blends PP6DHTBT:PC71BM in various weight
compositions (1:1, 1:3, and 1:4 w/w) were fabricated owing FIGURE 4Cyclic voltammograms of polymers.
TABLE 3Electrochemical Properties of Polymersa
Polymer
Oxidation Potential (V vs. Ag/Agþ)
Reduction Potential
(V vs. Ag/Agþ) Energy Levelb(eV) Band Gap eV)
Eox/onsetc Eox/od Ered/onsetc Ered/od HOMO LUMO Egec Egopt
PP6DHTBT 1.07 1.36 0.88 0.97 5.42 3.47 1.95 1.93
PP6DHTBSe 1.03 1.31 0.80 1.01 5.38 3.55 1.83 1.80
PP6DHTBX 1.12 1.40 0.75 0.99 5.47 3.60 1.87 1.90
a
Reduction and oxidation potentials measured by cyclic voltammetry in solid films.
b
EHOMO/ELUMO¼ [(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.
c
Onset oxidation and reduction potentials.
d
to a broader absorption and a higher absorption coefficient of PC71BM than PC61BM.
1(d)
The absorption spectra of the polymer blends PP6DHTBT:PC71BM (1:1, 1:3, and 1:4 w/w)
prepared under the same conditions as the process of device fabrication are demonstrated in Figure 6(a). The current– voltage characteristics of these devices are also shown in Figure 5, and their related photovoltaic properties are illus-trated in Table 4. The optimum photovoltaic performance with the maximum PCE value of 1.20% (Voc ¼ 0.75 V, Jsc¼
4.60 mA/cm2, and FF¼ 35.0%) was obtained in the PSC de-vice having a weight ratio of PP6DHTBT:PC71BM ¼ 1:4.
Using lower weight ratios of PCBM in blended polymer PP6DHTBT:PC71BM (1:1 and 1:3 w/w) led to reductions in
the Jsc values due to the inefficient charge separation and
electron transporting properties, resulting in the lower PCE results.30
The Voc values observed in PP6DHTBT:PC71BM solar cells
were fairly stable (0.730.75 V) in all polymer blend compo-sitions (1:11:4 w/w) with PC71BM (Table 4), which are
comparable to that of some of donor acceptor polymer:fuller-ene BHJ solar cells.17–22The EQE curves of the PSC devices are also plotted in Figure 6(b) to compare with the absorp-tion spectra of the polymer blends PP6DHTBT:PC71BM
shown in Figure 6(a). It is apparent that the PSC devices exhibited a very broad response range covering from 400 to 700 nm, where the EQEs were within 30%. The main reason for the low EQE values of the PSC devices are due to the lim-ited absorbances of the active layer as shown in Figure 6(a). In BHJ solar cell devices, the absorptions of the long wave-length region are contributed by the polymers, and the absorptions in the short wavelength region are mainly from PC71BM. However, the peak values of the absorbances in the
long wavelength region are only 0.170.30, so it means that
FIGURE 5Currentvoltage curves of PSCs using poly-mer:PCBM blends under the illumination of AM 1.5G, 100 mW/ cm2.
TABLE 4Photovoltaic Properties of Polymer Solar Cell Devices with the Configuration of ITO/PEDOT: PSS/Polymer:PCBM/Ca/Ala Polymer Polymer/ PCBM (w/w) Voc (V) Jsc (mA/cm2) FF (%) PCE (%) PP6DHTBT 1:1 (C61) 0.67 1.92 32.1 0.41 PP6DHTBSe 1:1 (C61) 0.65 1.43 30.5 0.28 PP6DHTBX 1:1 (C61) 0.69 1.24 29.1 0.25 PP6DHTBT 1:1 (C71) 0.73 2.95 34.0 0.74 PP6DHTBT 1:3 (C71) 0.73 3.80 33.1 0.88 PP6DHTBT 1:4 (C71) 0.75 4.60 35.0 1.20 a
Measured under AM 1.5 irradiation, 100 mW/cm2
.
FIGURE 6(a) Absorbance spectra of PP6DHTBT:PC71BM thin films measured from the solar cell devices by using an ITO/ PEDOT substrate as a reference. (b) EQE of PP6DHTBT:PC71BM solar cells.
only some portions of light were absorbed in the PSC de-vices, which might be due to the small thickness of the active layer (80 nm).
Carrier transport properties, including hole and electron mobilities of PP6DHTBT:PC71BM (1:4 wt %) were evaluated
by fabricating the hole- and electron-only devices. The
de-vices were prepared following the same procedure as the fabrication of BHJ devices, except that Ca was replaced with MoO3 (U ¼ 5.3 eV) in the hole-only devices, and the
PEDOT:PSS layer was replaced with Cs2CO3(U ¼ 2.9 eV) for
the electron-only devices. The electron and hole mobilities were determined precisely by fitting the plots of the dark current versus voltage (J–V) curves for single carrier devices to the space charge limited current (SCLC) model. The dark current is given by J ¼ 9e0erlV2/8L3, where e0er is the
per-mittivity of the polymer, l is the carrier mobility, and L is the device thickness. The hole and electron mobilities of PP6DHTBT:PC71BM (1:4 wt %) are 3.68 109cm2/Vs and
1.76 108cm2/Vs, respectively. The electron mobility was much higher (i.e., ca. 1 order of magnitude) than the hole mobility, resulting in an imbalance in the hole and electron transport in the blended polymer film. Because of the poor hole mobility and the imbalance of the hole and electron transport in the blended polymer film, the device was lim-ited to have a low FF value, which could be another reason for the lower PCE value.31 From the AFM images of
PP6DHTBT:PC71BM with various weight ratios (Fig. 7), we
observed that the roughness is increased from 0.67 nm (with PCE ¼ 0.74%) to 1.48 nm (with PCE ¼ 0.88%) and 2.60 nm (with PCE ¼ 1.20%) as the weight ratio of PP6DHTBT:PC71BM changed from 1:1 to 1:3 and 1:4 w/w
ratio, respectively. Therefore, we can summarize that the increased PCE value caused by a higher content of PC71BM
in the polymer blend of PP6DHTBT:PC71BM ¼ 1:4 w/w was
induced by the larger roughness in the polymer blend. In addition, the optimum PSC device (without annealing, PCE¼ 1.20%) containing polymer blend of PP6DHTBT:PC71BM ¼
1:4 (w/w) was further investigated for the thermal anneal-ing effects. As shown in Table 5, the PCE values of 1.14, 1.05, and 0.86% were obtained at the thermal annealing (20 min) of 50, 100, and 150C, respectively. Finally, the thermal annealing effects were proven to have no substantial increase on the solar cell device performance.
CONCLUSIONS
In summary, a series of new LBG polymers containing the phenothiazine unit as an electron donor conjugated with var-ious benzodiazole acceptors via hexyl-thiophene linkers were synthesized and characterized. These polymers show strong absorptions in the range of 300–700 nm and have ideal ranges of HOMO and LUMO levels (with optical bandgaps of 1.80–1.93 eV). Bulk heterojunction PSCs were fabricated from the polymer blends consisting of these LBG polymers FIGURE 7AFM images of PP6DHTBT: PC71BM blend films.
(a) 1:1 (w/w), (b) 1:3 (w/w), and (c) 1:4 (w/w) ratios.
TABLE 5 Annealing Effects on Polymer Solar Cell Device Containing PP6DHTBT:PC71BM (1:4 wt%) Annealing Temperature (C) Voc (V) Jsc(mA/cm2) FF (%) PCE (%) 50 0.73 4.99 31.3 1.14 100 0.74 4.37 32.6 1.05 150 0.67 4.09 31.2 0.86
(PP6DHTBT, PP6DHTBSe, and PP6DHTBX) as an electron do-nor and PC61BM/PC71BM as an electron acceptor. With the
similar Voc values and fill factor in the PSC devices
contain-ing the polymers blended with PC61BM in a weight ratio of
1:1 (w/w), it was found that due to the major variations of the Jsc values (1.92, 1.43, and 1.24 mA/cm
2
) in polymers PP6DHTBT, PP6DHTBSe, and PP6DHTBX, they are crucially affected to have the PCE values of 0.41, 0.28, and 0.25, respectively. The PSC device containing a polymer blend of PP6DHTBT:PC71BM (1:4 wt %) exhibited the best device
performance with a PCE value of 1.20%, an open-circuit volt-age (Voc) of 0.75 V, a short-circuit current (Jsc) of 4.60 mA/
cm2, and a fill factor (FF) of 0.35. The optimization of photo-voltaic properties in the PSC devices containing polymer blends PP6DHTBT:PC71BM can be adjusted by the
morphol-ogy variations with different weight ratios of PC71BM, which
were observed to have higher roughnesses with larger PC71BM contents, and thus to substantially increase the PCE
values of the PSC devices. Finally, this study revealed that these new phenothiazine-based LBG polymers will have potential applications for the flexible electronic devices. We 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 Science Coun-cil of Taiwan (ROC) through NSC 97-2113M-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.
REFERENCES AND NOTES
1 (a) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Adv Funct Mater 2001, 11, 15–26; (b) Gu¨nes, S.; Neugebauer, H.; Sariciftci, N. S. Chem Rev 2007, 107, 1324–1338; (c) Dennler, G.; Scharber, M.; Brabec, C. J. Adv Mater 2009, 21, 1323–1338; (d) Thompson, B. C.; Fre´chet, J. M. J. Angew Chem Int Ed Engl 2008, 47, 58–77; (e) Hoppe, H.; Sariciftci, N. S. J Mater Chem 2006, 16, 45–61; (f) Bundgaard, E.; Krebs, F. C. Sol Energy Mater Sol Cells 2007, 91, 954–985; (g) Kroon, R.; Lenes, M.; Hummelen, J. C. Polym Rev 2008, 48, 531–582.
2 (a) Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Em-ery, K.; Yang, Y. Nat Mater 2005, 4, 864–868; (b) Ma, W. L.; Yang, C. Y.; Gong, X.; Lee, K.; Heeger, A. J Adv Funct Mater 2005, 15, 1617–1622; (c) Li, G.; Shrotriya, V.; Yao, Y.; Yang, Y. J Appl Phys 2005, 98, 043704.
3 (a) Liang, Y. Y.; Wu, Y.; Feng, D. Q.; Tsai, S. T.; Son, H. J.; Li, G.; Yu, L. P. J Am Chem Soc 2009, 131, 56–57; (b) Liang, Y. Y.; Feng, D. Q.; Wu, Y.; Tsai, S. T.; Li, G.; Ray, C.; Yu, L. P. J Am Chem Soc 2009, 131, 7792; (c) Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat Photonics 2009, 3, 297–303; (d) Hou, J. H.; Chen, H. Y.; Zhang, S. Q.; Chen, R. I.; Yang, Y.; Wu, Y.; Li, G. J Am Chem Soc 2009, 131, 15586–15587; (e) Chen, H.-Y.; Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.; Yu, L.; Wu, L.; Li, G. Nat Photonics 2009, 3, 649–653.
4 (a) Li, G.; Yao, Y.; Yang, H.; Shrotriya, V.; Yang, G.; Yang, Y. Adv Funct Mater 2007, 17, 1636–1644; (b) Li, G.; Shrotriya, V.; Yao, Y.; Huang, J. S.; Yang, Y. J Mater Chem 2007, 17, 3126–3140; (c) Nguyen, L H.; Hoppe, H.; Erb, T.; Gunes, S.; Gobsch, G.; Sariciftci, N. S. Adv Funct Mater 2007, 17, 1071–1078; (d) Guo, X. G.; Kim, F. S.; Jenekhe, S. A.; Watson, M. D. W. J Am Chem Soc 2009, 131, 7206–7207; (e) Hsu, S. L. C.; Lin, Y. C.; Lee, R. F.; Sivakumar, C.; Chen, J. S.; Chou, W. Y. J Polym Sci Part A: Polym Chem 2009, 47, 5336–5343; (f) Zhang, S. M.; Fan, H. J.; Liu, Y.; Zhao, G. J.; Li, Q. K.; Li, Y. F.; Zhan, X. W. J Polym Sci Part A: Polym Chem 2009, 47, 2843–2852.
5 (a) Xin, H.; Kim, F. S.; Jenekhe, S. A. J Am Chem Soc 2008, 130, 5424–5425; (b) Chang, Y. T.; Hsu, S. L.; Chen, G. Y.; Su, M. H.; Singh, T. A.; Diau, E. W. G.; Wei, K. H. Adv Funct Mater 2008, 18, 2356–2365; (c) Xin, H.; Ren, G.; Kim, F. S.; Jenekhe, S. A. Chem Mater 2008, 20, 6199–6207; (d) Yao, Y.; Hou, J. H.; Xu, Z. X.; Li, G.; Yang, Y. Adv Funct Mater 2008, 18, 1783–1789; (e) Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A. J.; Bazan, G. C. Nature Mater 2007, 6, 497–500.
6 (a) Zhu, Z. G.; Waller, D.; Gaudiana, R.; Morana, M.; Muhl-bacher, D.; Scharber, M.; Brabec, C. Macromolecules 2007, 40, 1981–1986; (b) Zhang, F. L.; Bijleveld, J.; Perzon, E.; Tvingstedt, K.; Barrau, S.; Inganas, O.; Andersson, M. R. J Mater Chem 2008, 18, 5468–5474; (c) Li, Y. F.; Zou, Y. P. Adv Mater 2008, 20, 2952–2958; (d) Blouin, N.; Michaud, A.; Gendron, D.; Wakim, S.; Blair, E.; Plesu, R. N.; Bellette, M.; Durocher, G.; Tao, Y.; Leclerc, M. J Am Chem Soc 2008, 130, 732–742; (e) Soci, C.; Hwang, I.-W.; Moses, D.; Zhu, Z.; Waller, D.; Gaudiana, R.; Bra-bec, C. J.; Heeger, A. J Adv Funct Mater 2007, 17, 632–636; (f) Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nyuyen, T.-Q.; Heeger, A. J. Science 2007, 317, 222–225; (g) Bundgaard, E.; Krebs, F. C. Sol Energy Mater Sol Cells 2007, 91, 1019–1025; (h) Chen, C. P.; Chan, S. H.; Chao, T. C.; Ting, C.; Ko, B. T. J Am Chem Soc 2008, 130, 12828–12833.
7 (a) Scharber, M. C.; Muehlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger A. J.; Brabec, C. J. Adv Mater 2006, 18, 789–793; (b) Zhang, F. L.; Mammo, W. L.; Anderson, M.; Admassie, S. M.; Andersson, R.; Inganas, O. Adv Mater 2006, 18, 2169–2173; (c) Wienk, M. M.; Turbiez, W.; Gilot, G.; Janssen, R. A. Adv Mater 2008, 20, 2556–2560; (d) Lee, S. K.; Cho, N. S.; Cho, S.; Moon, S. J.; Lee, J. K., Bazan, G. C. J Polym Sci Part A: Polym Chem 2009, 47, 6873–6882; (e) Wan, M. X.; Wu, W. P.; Sang, G. Y.; Zou, Y. P.; Liu, Y. Q.; Li, Y. F. J Polym Sci Part A: Polym Chem 2009, 47, 4028–4036.
8 (a) Bre’das, J. L. J Chem Phys 1985, 82, 3808–381; (b) Ron-cali, J. Chem Rev 1997, 97, 173–206.
9 Blom, P. W. M.; Mihailetchi, V. D.; Koster, L. J. A.; Markov, D. E. Adv Mater 2007, 19, 1551–1566.
10 Bredas, J.-L.; Beljonne, D.; Coropceanu, V.; Cornil, J. Chem Rev 2004, 104, 4971–5003.
11 (a) Jenekhe, S. A.; Lu, L.; Alam, M. M. Macromolecules 2001, 34, 7315–7324; (b) Bates, W. D.; Chen, P. Y.; Dattelbaum, D. M.; Jones, W. E.; Meyer, T. J. J Phys Chem A 1999, 103, 5227–5231; (c) Pfennig, B. W.; Chen, P. Y.; Meyer, T. J. Inorg Chem 1996, 35, 2898–2901.
12 (a) Cho, N. S.; Park, J. H.; Lee, S. K.; Lee, J.-H.; Shim, H.-K.; Park, M.-J.; Hwang, D.-H.; Jung, B.-J. Macromolecules 2006,
39, 177–183; (b) Hwang, D.-H.; Kim, S.-K.; Park, M.-J.; Lee, J.-H.; Koo, B.-W.; Kang, I.-N.; Kim, S.-H.; Zyung, T. Chem Mater 2004, 16, 1298–1303; (c) Park, M. J.; Lee, J.; Jung, I. H.; Park, J. H.; Hwang, D. H.; Shim, H. K. Macromolecules 2008, 41, 9643–9649.
13 (a) Lai, R. Y.; Kong, X.; Jenekhe, S. A.; Bard, A. J. J Am Chem Soc 2003, 125, 12631–12639; (b) Fungo, F.; Jenekhe, S. A.; Bard, A. J Chem Mater 2003, 15, 1264–1272; (c) Sun, D.; Roso-kha, S. V.; Koich, J. K. J Am Chem Soc 2004, 126, 1388–1401. 14 Zou, Y.; Wu, W.; Sang, G.; Yang, Y.; Liu, Y.; Li, Y. Macromo-lecules 2007, 40, 7231–7237.
15 Kong, X.; Kulkarni, A. P.; Jenekhe, S. A. Macromolecules 2003, 36, 8992–8999.
16 (a) Tang, W. H.; Kietzke, T.; Vemulamada, P.; Chen, Z. K. J Polym Sci Part A: Polym Chem 2007, 45, 5266–5276; (b) Sang, G. Y.; Zou, Y. P.; Li, Y. F. J Phys Chem C 2008, 112, 12058–12064; (c) Wong, W. Y.; Chow, W. C.; Cheung, K. Y.; Fung, M. K.; Djurisic, A. B.; Chan, W. K. J Organometallic Chem 2009, 694, 2717–2726; (d) Li, Y. W.; Xue, L. L.; Li, H.; Li, Z. F.; Xu, B.; Wen, S. P.; Tian, W. J. Macromolecules 2009, 42, 4491–4499; (e) Li, K. C.; Hsu, Y. C.; Lin, J. T.; Yang, C. C.; Wei, K. H.; Lin, H. C. J Polym Sci Part A: Polym Chem 2008, 46, 4285–4304; (f) Huang, J. H.; Li, K. C.; Wei, H. Y.; Chen, P. Y.; Lin, L. Y.; Kekuda, D.; Lin, H. C.; Ho, K. C.; Chu, C. W. Org Elec-tronics 2009, 10, 1109–1115.
17 (a) Svensson, M.; Zhang, F. L.; Veenstra, S. C.; Verhees, W. J. H.; Hummelen, J. C.; Kroon, J. M.; Inganas, O.; Andersson, M. R. Adv Mater 2003, 15, 988–991; (b) Slooff, L. H.; Veenstra, S. C.; Kroon, J. M.; Moet, D. J. D.; Sweelssen, J.; Koetse, M. M. Appl Phys Lett 2007, 90, 143506; (c) Shi, C.; Yao, Y.; Yang, Y.; Pei, Q. J Am Chem Soc 2006, 128, 8980–8986; (d) Wang, E. G.; Wang, M.; Wang, L.; Duan, C. H.; Zhang, J.; Cai, W. Z.; He, C.; Wu, H. B.; Cao, Y. Macromolecules 2009, 42, 4410–4415. 18 (a) Boudreault, P. T.; Michaud, A.; Leclerc, M. Macromol Rapid Commun 2007, 28, 2176–2179; (b) Wang, E. G.; Wang, L.; Lan, L. F.; Luo, C.; Zhuang, W. L.; Peng, J. B.; Cao, Y. Appl Phys Lett 2008, 92, 033307.
19 Blouin, N.; Michaud, A.; Leclerc, M. Adv Mater 2007, 19, 2295–2300.
20 (a) Hou, J. H.; Chen, H. Y.; Zhang, S. Q.; Li, G.; Yang, Y. J. Am Chem Soc 2008, 130, 16144–16145; (b) Liao, L.; Dai, L. M.; Smith, A.; Durstock, M.; Lu, J. P.; Ding, J. F.; Tao, Y. Macromo-lecules 2007, 40, 9406–9412.
21 (a) Muhlbacher, D.; Scharber, M.; Morana, M.; Zhu, Z.; Wal-ler, D.; Gaudiana, R.; Brabec, C. Adv Mater 2006, 18, 2884–2889; (b) Moule, A. J.; Tsami, A.; Bunnagel, T. W.;
For-ster, M.; Kronenberg, N. M.; Scharber, M.; Koppe, M.; Morana, M.; Brabec, C. J.; Meerholz, K.; Scherf, U. Chem Mater 2008, 20, 4045–4050.
22 Yue, W.; Zhao, Y.; Shao, S.; Tian, H.; Xie, Z.; Geng, Y.; Wang, F. J Mater Chem 2009, 19, 2199–2206.
23 (a) Zhang, F.; Perzon, E.; Wang, X.; Mammo, W.; Andersson, M. R.; Inganas, O. Adv Funct Mater 2005, 15, 745–750; (b) Ash-raf, R. S.; Hoppe, H.; Shahid, M.; Gobsch, G.; Sensfuss, S.; Klemm, E. J Polym Sci Part A: Polym Chem 2006, 44, 6952–6961; (c) Wang, F.; Luo, J.; Yang, K.; Chen, J.; Huang, F.; Cao, Y. Macromolecules 2005, 38, 2253–2260.
24 (a) Gautrot, J. E.; Hodge, P.; Cupertio, D.; Helliwell, M.; New J Chem 2007, 31, 1585–1593; (b) Tsami, A.; Bunnagel, T. W.; Farrell, T.; Scharber, M.; Choulis, S. A.; Brabec, C. J.; Scherf, U. J Mater Chem 2007, 17, 1353–1355.
25 (a) McNeill, C. R.; Halls, J. J. M.; Wilson, R.; Whiting, G. L.; Berkebile, S.; Ramsey, M. G.; Friend, R. H.; Greenham, N. C. Adv Funct Mater 2008, 18, 2309–2321; (b) Iuo, J.; Hou, Q.; Chen, J.; Cao, Y. Synth Metals 2006, 156, 470–475.
26 (a) Cho, S.; Seo, J. H.; Kim, S. H.; Song, S.; Jin, Y.; Lee, K.; Suh, H.; Heeger, A. J. Appl Phys Lett 2008, 93, 263301; (b) Zhou, H. X.; Yang, L. Q.; Xiao, S. Q.; Liu, S. B.; You, W. Macro-molecules 2010, 43, 811–820; (c) Zhang, S. M.; Guo, Y. L.; Fan, H. J.; Liu, Y.; Chen, H. Y.; Yang, G. W.; Zhan, X. W.; Liu, Y. Q.; Li, Y. F.; Yang, Y. J Polym Sci Part A: Polym Chem 2009, 47, 5498–5508; (d) Song, S.; Jin, Y.; Kim, S. H.; Moon, J.; Kim, K.; Kim, J. Y.; Park, S. H.; Lee, K.; Suh, H. Macromolecules 2008, 41, 7296–7305.
27 (a) Yang, R.; Tian, R.; Hou, Q.; Yang, W.; Cao, Y. Macromo-lecules 2003, 36, 7453–7460; (b) Jung, I. H.; Kim, H.; Park, M. J.; Kim, B.; Park, J. H.; Jeong, E.; Woo, H. Y.; Yoo, S.; Shim, H. K. J Polym Sci Part A: Polym Chem 2010, 48, 1423–1432; (c) Huang, F.; Hou, L.; Shen, H.; Yang, R.; Hou, Q.; Cao, Y. J Polym Sci Part A: Polym Chem 2006, 44, 2521–2532; (d) Ono, K.; Tanaka, S.; Yamashita, Y. Angew Chem Int Ed 1994, 33, 1977–1980.
28 (a) Chen, Y. Y.; Tao, Y. T.; Lin, H. C. Macromolecules 2006, 39, 8559–8566; (b) Li, K. C.; Hsu, Y. C.; Lin, J. T.; Yang, C.; Wei, K. H.; Lin, H. C. J Polym Sci Part A: Polym Chem 2009, 47, 2073–2092.
29 de Leeuw, D. M.; Simenon, M. M. J.; Brown, A. R.; Einhard, R. E. F. Synth Met 1997, 87, 53–59.
30 Baek, N. S.; Hau, S. K.; Yip, H. L.; Acton, O.; Chen, K. S.; Jen, A. K. Y. Chem Mater 2008, 20, 5734–5736.
31 Hou, J.; Chen, T. L.; Zhang, S.; Chen, H.-Y.; Yang, Y. J Phys Chem C 2009, 113, 1601–1605.
