Main-Chain Copolymers Containing Electron Deficient Bithiazole
Units for Organic Solar Cells
DHANANJAYA PATRA,1DURYODHAN SAHU,1HARIHARA PADHY,1DHANANJAY 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 11 July 2010; accepted 21 August 2010 DOI: 10.1002/pola.24356
Published online 5 October 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT:A series of low-band-gap (LBG) donor–accepor con-jugated main-chain copolymers (P1–P4) containing planar 2,7-carbazole as electron donors and bithiazole units (4,40 -dihexyl-2,20-bithiazole and 4,40-dihexyl-5,50-di(thiophen-2-yl)-2,20 -bithia-zole) as electron acceptors were synthesized and studied for the applications in bulk heterojunction (BHJ) solar cells. The effects of electron deficient bithiazole units on the thermal, optical, electrochemical, and photovoltaic (PV) properties of these LBG copolymers were investigated. Absorption spectra revealed that polymers P1–P4 exhibited broad absorption bands in UV and visible regions from 300 to 600 nm with opti-cal band gaps in the range of 1.93–1.99 eV, which overlapped with the major region of the solar emission spectrum. More-over, carbazole-based polymers P1–P4 showed low values of the highest occupied molecular orbital (HOMO) levels, which
provided good air stability and high open circuit voltages (Voc) in the PV applications. The BHJ PV devices were fabricated using polymers P1–P4 as electron donors and (6,6)-phenyl-C61 -butyric acid methyl ester (PC61BM) or (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) as electron acceptors in different weight ratios. The PV device bearing an active layer of polymer blend P4:PC71BM (1:1.5 w/w) showed the best power conver-sion efficiency value of 1.01% with a short circuit current den-sity (Jsc) of 4.83 mA/cm2, a fill factor (FF) of 35%, and Voc ¼ 0.60 V under 100 mW/cm2 of AM 1.5 white-light illumination. VC 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 5479–5489, 2010
KEYWORDS:bithiazole units; bulk heterojunction; copolymers; donor–acceptor
INTRODUCTION In the 21st century, to reduce carbon emis-sions and green house effects, solar energy is one of the ‘‘green’’ and ‘‘sustainable energy’’ sources to create better environment. Recently, organic semiconducting materials, includingp-conjugated polymers1and small molecules,2have
been used in various optical and electronic devices because of their unique advantages, such as light weight, low-cost production, and large area device fabrication by solution pro-cess.3The highly efficient organic solar cell devices belong to
the bulk heterojunction (BHJ) solar cells, in which p-conju-gated polymers are used as electron donors and the fuller-ene derivatives, such as [6,6]-phenyl-C61-butyric acid methyl
ester (PC61BM) or [6,6]-phenyl-C71-butyric acid methyl ester
(PC71BM), as electron acceptors. After an extensive
investiga-tion on polymer solar cells (PSCs), the BHJ devices based on polymer blends (with various weight ratios and thicknesses) of poly(3-hexylthiophene) (P3HT) and PC61BM were taken as
standard devices. However, the enhancements of power con-version efficiency (PCE) values in these devices are quite dif-ficult because of low open circuit voltage (Voc) values (0.6
V) and large band gaps, which limit their net light harvesting capabilities. Hence, the utilization of newly developed low-band-gap (LBG) conjugated polymers likely to be the promis-ing alternatives of P3HT for PSCs. Recently, PCE values up to 6.0–7.7% were obtained by using LBG conjugated polymers in the BHJ solar cells as electron donors.4Nevertheless, these PCEs are not sufficient for commercialization of PSCs. There-fore, promising efforts are required to develop new donor– acceptor (D–A) polymer structures with higher molecular crystallinity which can result in better p–p stacking, extended absorption, higher mobility, and balanced charge transport to get higher PCE values in PSCs.1(e)
Later, there were several reports on D–A PSCs,5–14 which harvest maximum solar spectrum ranging from visible to near infrared absorptions which appealed high short circuit current density (Jsc) values. It has been verified that Voc is
directly proportional to the difference between the highest occupied molecular orbital (HOMO) levels of donor poly-mers and the lowest unoccupied molecular orbital (LUMO)
Correspondence to: H.-C. Lin (E-mail: [email protected])
levels of acceptor PCBM derivatives (i.e., PC61BM and
PC71BM). 1(c,d),3
In BHJ solar cells, where PCBM is used as an acceptor, the ideal band gap (in order to achieve a high Voc
value) of donor polymer should be in the range of 1.2–1.9 eV which corresponds to a HOMO energy level between5.8 and 5.2 eV and a LUMO energy level between 4.0 and 3.8 eV.7,1(c,d)
Furthermore, to facilitate efficient electron transfer from donor to acceptor, the minimum energy differ-ence between LUMO levels of electron donor and acceptor should be ca. 0.3 eV.8,10(b,c) Consequently, to obtain the desired molecular energy levels of the conjugated LBG poly-mers, electron-donating groups or electron-withdrawing groups can be substituted alternatively in the polymer back-bones either to raise the HOMO energy level or to reduce the LUMO energy level.1(c),3
Conjugated polymers having D–A architectures have been extensively studied by using fused heterocyclic electron rich segments, such as carbazole,10 dibenzosilole,11 cyclopentadi-thiophene,12 dithienopyrrole,13 dithienosilole,14 fluorene,15 and phenothiazine16 as an electron donating building block for PSCs as well as organic field effect transistors. Owing to the easy modulations of physical properties, it has been pro-ven that 2,7-carbazole derivatives are one of the excellent potential donor candidates for BHJ solar cells.10(b)Using 2,7-carbazole-based alternating copolymer (PCDTBT) as an elec-tron donor, Leclerc et al. achieved a PCE value of 3.6%.10(a) Hence, with improving absorption characteristics and charge-carrier mobilities, Heeger et al. reported an ever high PSC device containing PCDTBT with a PCE value of 6.1% and an internal quantum efficiency approaching ca. 100%.4(c)
The five-membered heterocyclic electron deficient moiety, that is, thiazole, induces larger p–p stacking and higher coplanarity17,18 in D–A polymers so as to have a stronger tendency to self-assemble into stacked solid structures, which not only minimize steric hindrances but also provide extended conjugation lengths. Introduction of thiazole units with electron-withdrawing imine nitrogen (AC¼¼N) generally enhances the electron-accepting (n-doping) properties of the D–A polymers. Moreover, thiazole-based polymers exhibit high oxidative stabilities which favor the polymers to lower its HOMO energy level and thus to increase their open circuit voltages.20Though Shim et al. firstly reported that the
poly-mer containing bithiazole and fluorine units achieved a low PCE value of 0.52%,20(b) we reached a much higher PCE value of 3.04% using a copolymer containing bithiazole and cyclopentadithiophene units recently.21(a) As a consequence, the copolymers containing the planar electron-withdrawing bithiazole units as acceptors and 2,7-carbazole units as donors to produce D–A polymers will be very interesting LBG polymers for the applications of PSCs. In addition, Li et al. have newly reported one D–A copolymer containing 2,7-carbazole and bithiazole moieties as electron-donor and electron-acceptor segments, respectively, but only possessed a maximum PCE value of 0.30%.10(e)
In this article, we synthesized and characterized a series of copolymers consisting of a planar 2,7-carbazole moiety with
conducting thiophene (thiophene or bithiophene) as tron-donating segments and bithiazole derivatives as elec-tron-accepting segments. The copolymers were synthesized by Pd(0)-catalyzed Stille coupling polymerization with 1:1 (molar ratio) donor–acceptor ratio. The resulting polymers P1–P4 exhibited broad absorption bands located in the UV– visible regions from 300 to 600 nm with optical band gaps of 1.99–1.93 eV. From the preliminary investigation, the pho-tovoltaic (PV) performance of the PSC device containing P4 (as an electron donor) blended with PC71BM (as an acceptor)
showed the best PCE value of 1.01% with a Voc¼ 0.60 V, a
Jsc¼ 4.83 mAcm2, and a fill factor (FF) of 35.0% measured
under 100 mW/cm2of AM 1.5 white-light illumination.
EXPERIMENTAL
Materials
All chemicals and solvents were reagent grades and pur-chased from Aldrich, ACROS, Fluka, TCI, TEDIA, and Lancas-ter Chemical Co. Toluene and tetrahydrofuran (THF) were distilled from sodium-benzophenone under nitrogen before use. Unless otherwise specified, the other solvents were degassed by nitrogen 1 h before use. All the other chemicals were used as received.
Measurements and Characterization
1
H and 13C NMR spectra were recorded on a Varian Unity 300 MHz spectrometer using CDCl3 solvent and chemical
shifts were reported asd values (ppm) relative to an internal tetramethylsilane standard. Elemental analyses were per-formed on a HERAEUS CHN-OS RAPID elemental analyzer. Thermogravimetric analyses (TGA) were conducted with a TA Instruments Q500 at a heating rate of 10 C/min under nitrogen. Gel permeation chromatography (GPC) analyses were conducted on a Waters 1515 separation module using polystyrene as a standard and THF as an eluent. UV–Visible (UV–vis) absorption spectra were recorded in dilute THF sol-utions (106 M) on a HP G1103A spectrophotometer. Thin films for UV–vis measurements were spin-coated on a glass substrate from THF solutions with a concentration of 5 mg/ mL. Cyclic voltammetry (CV) measurements were performed using a BAS 100 electrochemical analyzer with a standard three-electrode electrochemical cell in a 0.1M tetrabutylam-monium hexafluorophosphate (TBAPF6) 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 work-ing electrode coated with a thin layer of copolymers, a plati-num 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 poten-tials quoted herein. The redox couple of ferrocene/ferroce-nium ion (Fc/Fcþ) was used as an external standard. The corresponding HOMO and LUMO levels were calculated using Eox/onsetandEred/onsetfor experiments in solid films of
poly-mers, which were performed by drop-casting films with the similar thickness from THF solutions (ca. 5 mg/mL). The LUMO levels of PC61BM or PC71BM employed were in accordance
from the intersections of two tangents drawn at the rising currents and background currents of the CV measurements. Device Fabrication and PV Measurements of PSCs The polymer PV cells in this study were composed of an active layer of blended polymers (P1–P4:PCBM) in solid films, which were sandwiched between a transparent indium tin oxide (ITO) anode and a metal cathode. Before the device fabrication, ITO-coated glass substrates (1.5 1.5 cm2) were ultrasonically cleaned in detergent, deionized water, acetone, and isopropyl alcohol sequentially. After routine solvent cleaning, the substrates were treated with UV ozone for 15 min. Then, a modified ITO surface was obtained by spin-coating a layer of poly(ethylene dioxythiophene):polystyrene-sulfonate (PEDOT:PSS) (30 nm). After baking at 130 C for
1 h, the substrates were transferred to a nitrogen-filled glove box. The PSC devices were fabricated by spin-coating solu-tions of blended polymers (P1–P4):PCBM (with various weight ratios of a copolymer and one of PCBMs, that is, PC61BM or PC71BM) onto the PEDOT:PSS modified substrates
at 1500 rpm for 60 s (ca. 80 nm), and placed in a covered glass Petri dish. Initially, the blended polymer solutions were prepared by dissolving both copolymers and PC61BM (with a
1:1 weight ratio) initially and then with various weight ratios for the optimum copolymer with PC71BM in 1,2
dichloroben-zene (DCB) (20 mg/mL), followed by continuous stirring for 12 h at 50 C. In the slow-growth approach, blended poly-mers 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 subsequent aluminum layer (100 nm) were thermally evaporated through a shadow mask at a pressure below 6 106 Torr. All PSC devices were prepared and measured under ambient conditions, where the active area of the devices was 0.12 cm2. 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 (Thermal Oriel 1000W). The external quan-tum 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 PV cell under test.
Synthesis of Monomers and Polymers 4,40-Dibromo-2-nitrobiphenyl (1)
4,40-Dibromobiphenyl (20 g, 64 mmol) in 300 mL of glacial acetic acid was heated (100C) to dissolve completely. Then, 90 mL of fuming nitric acid was added dropwise for a period of 30 min. The resulting mixture was further stirred vigo-rously for 1 h at 100C to get a reddish brown precipitate. The reaction mixture was cooled to room temperature and poured into ice cold water. The precipitate was filtered and washed with excess of water, then the obtained product was further purified by recrystalization from ethanol to get a yel-low solid (20.30 g, 88.72%). 1H NMR (300 MHz, CDCl 3), d (ppm): 8.03 (d,J ¼ 3.0 Hz, 1H), 7.75 (dd, J ¼ 9.0 Hz, J ¼ 3.0 Hz, 1H), 7.55 (d, J ¼ 9.0 Hz, 2H), 7.30 (d, J ¼ 9.0 Hz, 1H), 7.15 (d,J ¼ 6.0 Hz, 2H). 2,7-Dibromocarbazole (2)
Mixture of compound 2 (20 g, 56.02 mmol) and triphenyl-phosphine (36.73 g, 140.05 mmol) were dissolved in 220 mL of DCB and the reaction mixture was refluxed for 12 h. The excess DCB was removed by high vacuum distillation and the residue was purified by column chromatography (Silica gel) using a mixture of hexane:ethyl acetate (7:3) to get a white solid (12.90 g, 70.87%). 1H NMR (300 MHz, CDCl3),d (ppm): 8.08 (br, 1H), 7.87 (d, J ¼ 8.7 Hz, 2H), 7.56
(d,J ¼ 1.5 Hz, 2H), 7.35 (dd, J ¼ 1.8 Hz, J ¼ 8.4 Hz, 2H). 1-Hexylheptanol (3)
In a 500 mL flame-dried two-neck round bottom flask, ethyl formate (10 mL,123.78 mmol) was dissolved in 100 mL of anhydrous THF and cooled to78C under N2atmosphere.
A freshly prepared hexylmagnesium bromide, which was obtained by adding 1-bromohexane (48.90 mL, 346.59 mmol) to a suspension of magnesium turning (10.40 g, 433.24 mmol) in dry THF (150 mL), was added dropwise into the previous solution, and then the reaction mixture was stirred overnight at room temperature. The reaction was quenched by the addition of MeOH, and then followed by adding saturated aqueous NH4Cl. The crude compound was
extracted three times with ethyl acetate. The combined or-ganic fractions were washed with brine, dried over MgSO4,
and concentrated by rotary evaporation. After vacuum distil-lation, the final compound was isolated as a white solid (21.87 g, 87.94%).1H NMR (CDCl3, 300 MHz):d (ppm): 3.58
(m, 1H), 1.46–1.25 (m, 21H), 0.87 (t,J ¼ 6.4 Hz, 6H). Tridecan-7-yl 4-Methylbenzenesulfonate (4)
In a 250 mL flame-dried two neck round bottom flask, 1-hexylheptanol (10.0 g, 49.91 mmol), Et3N (17.40 mL, 124.77
mmol), and Me3NHC1 (4.77 g, 49.91 mmol) were mixed in
40 mL of CH2Cl2and then cooled to 0–5C. A solution of
p-toluenesulfonyl chloride (11.90 g, 62.38 mmol) in CH2Cl2(39
mL) was added dropwise over 90 min and kept the reaction at room temperature. After 2 h, water was added and the crude compound was extracted with CH2Cl2. The organic
fraction was washed with water and brine, and dried over MgSO4, and concentrated by rotary evaporation.
Subse-quently, the crude product was purified by column chroma-tography (Silica gel, hexane/ethylacetate 9:1) to yield a vis-cous colorless liquid (15.15 g, 85.60%). 1H NMR (300 MHz, CDCl3):d (ppm): 7.79 (d, J ¼ 8.2 Hz, 2H);7.32 (d, J ¼ 8.1 Hz,
2H); 4.53 (m, J ¼ 6.0 Hz, 1H); 2.43 (s, 3H); 1.52 (m, 4H); 1.22 (m, 20H); 0.88 (t,J ¼ 6.9 Hz, 6H).
2,7-Dibromo-9-(Tridecan-7-yl)-9H-Carbazole (5)
2,7-Dibromocarbazole (4.0 g, 12.30 mmol) and potassium hy-droxide powder (3.45 g, 61.50 mmol) were dissolved in 50 mL of dimethylsulfoxide (DMSO) at 60 C. Then, a solution of tridecan-7-yl 4-methylbenzenesulfonate (6.25 g, 18.45 mmol) with 30 mL of DMSO was added dropwise through a dropping funnel over 1.5–2 h and stirred overnight. The reaction mixture was cooled to room temperature and poured into 500 mL of water. The crude compound was extracted with ethylacetate and washed with brine. The com-bined organic layer was dried over MgSO4and concentrated
in rotary evaporator. The crude compound was purified by column chromatography (silica gel) using hexane as an elu-ent to give a white solid (4.62 g, 74.03%). 1H NMR (300 MHz, CDCl3): d (ppm): 7.90 (br, 2H); 7.70 (s, 1H); 7.54 (s,
1H); 7.33 (d, J ¼ 6.0 Hz, 2H); 4.42 (m, 1H); 2.19 (m, 2H); 1.91 (m, 2H); 1.15 (m, 16H); 0.83 (t, J ¼ 6.3 Hz, 6H). 13C NMR (75 MHz, CDCl3): d (ppm): 130.61; 130.15; 122.58;
121.71; 121.48; 114.75; 112.39; 57.22; 33.76; 31.79; 29.25;
26.98; 22.76; 14.23; EIMS (m/z): Anal. Calcd for C25H33Br2N:
C, 59.18; H, 6.56; N, 2.76. Found: C, 59.58; H, 6.12; N, 2.77. MS (FAB): m/z [Mþ] 505.0; calcd m/z [Mþ] 505.10.
General Synthetic Procedures of Polymers P1-P4
The synthetic route of copolymers is shown in Scheme 1. Into a 25 mL two-necked flask, 2,7-dibromo-9-(tridecan-7-yl)-9H-carbazole, 2,5-bis(tributylstannyl)thiophene (or 5,50
bis(tributylstannyl)-2,20-bithiophene), and 5,50-dibromo-4,40 -dihexyl-2,20-bithiazole (or 5,50 -bis(5-bromothiophen-2-yl)-4,40-dihexyl-2,20-bithiazole were added. The mixture was deoxygenated with nitrogen for 30 min, after which dry tolu-ene (15 mL) and Pd(PPh3)4(1 mol %), was transferred into
the mixture in a dry environment. The reaction mixture was stirred at 110C for 3 days, and then an excess amount of 2-bromothiophene was added to end-cap the trimethyl-stannyl groups for 4 h. The reaction mixture was cooled to 40 C and added slowly into a vigorously stirred mixture of methanol/acetone (3:1). The polymers were collected by fil-tration and reprecipitation from methanol. The crude poly-mers were further purified by washing with acetone and EA for 2 days in a Soxhlet apparatus to remove oligomers and catalytic residues.
P1
Following the general polymerization procedure, compound 5 (0.5 equiv), M1 (0.5 equiv), and compound 6 (1.0 equiv) were used in this polymerization to acquire a red powder. Yield: 72%. GPC: Mw: 41,900; polydispersity index (PDI):
1.62; 1H NMR (300 MHz, CDCl3): d (ppm) 8.10 (broad),
7.79–7.39 (broad), 7.12 (s), 4.62 (broad), 2.84 (broad), 1.83 (broad), 1.75–0.80 (broad), 0.77–0.61(broad). Anal. Calcd for (C51H65N3S4)n: C, 72.21; H, 7.72; N, 4.95. Found: C, 71.81; H,
7.59; N, 4.87.
P2
Following the general polymerization procedure, compound 5 (0.5 equiv), M2 (0.5 equiv), and compound 7 (1.0 equiv) were used in this polymerization to acquire a deep red pow-der. Yield: 69%. GPC: Mw: 25,100; PDI: 1.36; 1H NMR (300
MHz, CDCl3):d (ppm) 8.10 (broad), 7.89–7.22 (broad), 7.12
(broad), 4.62 (broad), 2.97 (broad), 1.83 (broad), 1.75–0.80 (broad), 0.77–0.61(broad). Anal. Calcd for (C59H69N3S6)n: C,
69.98; H, 6.87; N, 4.15. Found: C, 69.18; H, 6.79; N, 4.25.
P3
Following the general polymerization procedure, compound 5 (0.5 equiv), M1 (0.5 equiv), and compound 6 (1.0 equiv) were used in this polymerization to acquire a black powder. Yield: 66%. GPC: Mw: 8,880; PDI: 1.20;
1
H NMR (300 MHz, CDCl3): d (ppm) 8.10 (broad), 7.79–7.19 (broad), 7.15
(broad), 4.62 (broad), 2.99 (broad), 1.85 (broad), 1.75–0.80 (broad), 0.77–0.61(broad). Anal. Calcd for (C59H69N3S6)n: C,
69.98; H, 6.87; N, 4.15. Found: C, 69.30; H, 6.98; N, 4.22.
P4
Following the general polymerization procedure, compound 5 (0.5 equiv), M2 (0.5 equiv), and compound 7 (1.0 equiv) were used in this polymerization to acquire a black powder. Yield: 64%. GPC: Mw: 8600; PDI: 1.19;
1
H NMR (300 MHz, CDCl3): d (ppm) 8.10 (broad), 7.79–7.39 (broad), 7.11
(broad), 4.59 (broad), 2.97 (broad), 1.81 (broad), 1.75–0.80 (broad), 0.77–0.61(broad). Anal. Calcd for (C67H73N3S8)n: C,
68.38; H, 6.25; N, 3.57. Found: C, 67.82; H, 6.38; N, 3.64.
RESULTS AND DISCUSSION
Syntheses and Characterization
The synthetic routes of 2,7-carbazole-based donor monomer (compound 5) and polymers P1–P4 are outlined in Scheme 1. Compound 5 was adequately characterized by 1H NMR,
13
C NMR, MS spectroscopies, and elemental analyses. The thiophene- and bithiophene-based donor monomers (com-pounds 6 and 7, respectively) were prepared according to the methods described elsewhere.22 In addition, the syn-thetic procedures of bithiazole-based acceptor monomers M1 and M2 were also reported earlier by our group.21(a)In this study, polymers P1–P4 consisting of 2,7-carbazole and thiophene (or 2,20-bithiophene) as electron-donating moeities and bithiazole as electron-accepting moieties were synthe-sized by Pd(0)-catalyzed Stille coupling polymerization in toluene at 110C with a feed-in molar ratio of m:n¼ 1:1. All these copolymers are readily soluble in common organic solvents such as chloroform, THF, and chlorobenzene at room temperature and completely soluble in high boiling point solvents (e.g., chlorobenzene) at high temperature. The molecular weights of polymers P1–P4 determined by GPC against polystyrene standards in THF are summarized in Ta-ble 1. These results show that consideraTa-ble molecular weights with high yields (64–72% after Soxhlet extractions) were obtained in these copolymers, where the average mo-lecular weights (Mw) were in the range of 41,900–8600 with
PDI (PDI ¼ Mw/Mn) values of 1.62–1.19. The thermal
stabil-ities of conjugated polymers play an important role for opto-electronic applications. As shown in Figure 1, the thermal stabilities of polymersP1–P4 were investigated by TGA, and their corresponding results are summarized in Table 1. All polymers showed good thermal stabilities and exhibited Td
values (temperatures at 5% weight loss by a heating rate of 10 C/min under nitrogen) between 361 and 452C, where the Td value was reduced as the molecular weight
decreased.23
Optical Properties
The photophysical features of the copolymers were investi-gated by UV–vis absorption spectroscopy in dilute THF solu-tions and spin-coated films on glass substrates, which are
TABLE 1 Molecular Weights and Thermal Properties of Polymers P1–P4 Polymer Mwa Mna PDIa (Mw/Mn) Yield (%) Tdb (C) P1 41,900 25,900 1.62 72 452 P2 25,100 18,500 1.36 69 423 P3 8,900 7,400 1.20 66 418 P4 8,600 7,200 1.19 64 361 a
Molecular weights (MnandMw) and polydispersity index (PDI) values
were measured by GPC, using THF as an eluent, polystyrene as a stand-ard.Mn, number average molecular weight;Mw, weight average
molec-ular weight.
b
Temperature (C) at 5% weight loss measured by TGA at a heating rate of 10C/min under nitrogen.
presented in Figures 2(a) and 2(b), respectively. The normal-ized absorption spectra of polymers P1–P4 and their optical data, including the absorption wavelengths (kmax,abs) and the
optical band gaps (Eopt
g ), and absorpation coefficients (amax),
are summarized in Table 2. All polymers (P1–P4) shows rel-atively high absorption coefficients (amax, calculated from
Beer’s law) with the range of 4.2–5.7 and 2.0–4.2 104M1 cm1 in dilute solutions and solid films, respectively, which assures the copolymers to harvest enough photons. The absorption wavelengths (kmax,abs) of polymers P1, P2, P3,
and P4 in dilute solutions were located at 455, 459, 464, and 466 nm, respectively, which can be attributed to p–p* transition of the conjugated copolymer backbones and the p–p interaction between the electron donor (carbazole) and acceptor (bithiazole) units. It is obvious that, by tuning the numbers of thiophene units in the polymer-conjugated heter-ocyclic main-chains, the absorption spectra of carbazole-based copolymers will be effectively influenced (in both solu-tions and solid films). In contrast to solusolu-tions (see Table 2), the absorption wavelengths (kmax,abs) of polymersP1–P4 in
solid films were found red-shifted to the range of 463–503
nm obviously. These red-shifted wavelengths in solid films are ascribed to the interchain associations andp–p stackings of these copolymers as well as the highly rigid and planar
FIGURE 1TGA measurements of polymers P1–P4 with a heat-ing rate of 10C/min.
FIGURE 2Normalized absorption spectra of P1–P4 in dilute chloroform solutions.
TABLE 2Optical and Electrochemical Properties of Polymers P1–P4
Solutiona Solid Filmb amax(10 4
M1cm1) Energy Levels Band Gapsg
Polymer kmax,abs (nm) kmax,abs (nm) konset,abs (nm) Solutiond Solid Filme Eox onset (V)/HOMOf(eV) Ered onset (V)/LUMOf(eV) Egec (eV) Eopt g (eV) P1 (314)c455 314, 463 623 4.2 2.0 1.07/5.42 0.75/3.60 1.82 1.99 P2 (364)c459 464 632 4.5 2.1 1.05/ 5.40 0.76/3.59 1.81 1.96 P3 464 490 626 5.2 3.4 1.03/5.38 0.77/3.58 1.80 1.98 P4 466 290, 504 642 5.7 4.2 0.99/5.34 0.80/3.55 1.79 1.93 a In THF dilute solution. b
Spin coated from THF solution on glass surface.
c
Shoulder peak.
d
Absorption coefficient determined at kmaxin THF. e
Absorption coefficient of the solid film at kmax.
f
EHOMO/ELUMO ¼ [(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/FCþvs.Ag/Agþ)¼ 0.45 eV.
g
Band gaps, electrochemical band gapEec
g ¼ Eox/Onset–Ered/Onsetand
optical band gapEopt
segments of polymer backbones.24,6(b) Moreover, the red shifts of absorption wavelengths from solutions to solid films are 8, 5, 26, 38 nm for polymersP1–P4, respectively. Hence, the larger numbers of thiophene units in P3 and P4 with longer conjugation lengths induced strongerp–p stackings in solid films, and thus to have larger red-shifted absorption wavelengths.25
As shown in Table 2, the optical band gaps (Eopt
g ) of
poly-mers P1–P4 in solid films were found in the range of 1.93– 1.99 eV, which were determined from the cutoffs of the absorption wavelengths. The optical band gaps of the copoly-mers were reduced fromP1 to P2 and from P3 to P4 owing to the enhancement of electron donating capabilities, be-cause more thiophene units and longer conjugation lengths were introduced in the polymer backbones.6(b),23 These results imply that the light harvesting capabilities along with optical band gaps can be tuned by electron D–A segments in the polymer backbone, which is one of the efficient ways to design the organic PV materials.
Electrochemical Properties
To investigate the redox behavior of the random copolymers and determine their electronic states (i.e., HOMO/LUMO lev-els), the electrochemical properties of polymersP1–P4 were investigated by CV. The oxidation and reduction cyclic vol-tammograms of the copolymers are shown in the Figure 3. The electrochemical properties, such as onset potentials of oxidation and reduction, that is, the estimated positions of the upper edges of the valence band (HOMO) and the lower edges of the conduction band (LUMO), respectively, and elec-trochemical band gaps are summarized in Table 2. The CV measurements were carried out in a 0.1M TBAPF6 solution
(in acetonitrile) at a scan rate of 100 mV/s under nitrogen. A carbon electrode, which was coated with the polymer film by dip coating, was used as a working electrode and Ag/ AgCl was served as a reference electrode, and it was cali-brated by ferrocene (E1=2ferrocene ¼ 0.45 mV versus 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 vacuum level) according to the following equation21(a),26:EHOMO/ELUMO¼ [(Eonset–
Eonset(FC/FCþ vs. Ag/Agþ)) 4.8] eV and band gap ¼ Eonset/ox–
Eonset/red (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). It can be seen that polymers P1–P4 possess quasi-reversible p-doping/dedoping (oxidation/rereduction) proc-esses at positive potentials and reversible n-doping/dedop-ing (reduction/reoxidation) processes at negative potentials. The onset oxidation and reduction potentials of polymers P1–P4 were in the ranges of 1.07–0.99 V and (0.75)– (0.82) V, respectively, from which the estimated HOMO and LUMO levels were found in the range of (5.34)–(5.42) eV and (3.55)–(3.60) eV, respectively. The lower HOMO energy levels of the polymers were desirable for high open circuit voltages of PSCs, as the polymers were taken as do-nor materials.8The noticeably higher oxidation potentials of P1–P4 can be explained by that the resulting conjugated copolymers were more electron deficient because of the nitrogen atoms in their planarp-conjugated systems.18(c),19(b)
On the other hand, the LUMO energy level of the electron donor (polymer) has to be positioned above the LUMO energy level of the electron acceptor (PCBM) at least 0.3 eV, so the exciton binding energy of polymer could be overcome and result in efficient electron transfer from donor to acceptor.3 The high reduction potentials of polymers P1–P4 represent high electron affinities to make these copolymers suitable donors to inject and transport electrons to PCBM acceptor in PSC devices.3(a),20(b)The differences between the band gap values directly measured by CV (Eec
g between 1.79
and 1.82 eV) and the optical band gap values obtained from UV–vis spectra (Eopt
g between 1.93 and 1.99 eV) lied within
an acceptable range of errors.
PV Properties
To investigate the potential applications of copolymers in PSC devices, BHJ solar cells were fabricated by using poly-mers P1–P4 as electron donors and fullerene [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as an electron
acceptor with a device configuration of ITO/PEDOT:PSS(30 nm)/(P1–P4):PCBM(1:1 w/w)(80 nm)/Ca(30 nm)/Al(100 nm). This weight ratio of polymer blends with PCBM (P1– P4:PCBM ¼ 1:1 w/w) was found to have the optimum PCE value. Figure 4 shows the J-V curves of all PSCs containing P1–P4 under the condition of AM 1.5 at 100 mW/cm2, and the open circuit voltage (Voc), short circuit current density
(Jsc), FF, and PCE values of the devices are summarized in
Table 3. To have the great performance in PSC devices, DCB was chosen as the solvent to obtain the blended polymer active layers with good film qualities. The obtained PCE val-ues of polymers P1–P4 were in the range of 0.36–0.57%, whereP3 and P4 possessed the highest PCE value (0.57%). However, the similar alternating copolymer reported by Li et al., which comprised of a planar carbazole unit as an elec-tron donor and a bithiazole unit as an elecelec-tron acceptor
FIGURE 3Cyclic voltammograms of P1–P4 in solid films at a scan rate of 100 mV/s.
sandwiched between thiophene units, only achieved a lower PCE value of 0.30%.10(e)
Although both PSC devices containing P3 and P4 possessed the highest PCE value (PCE¼ 0.57%), P4 generated a higher Jscvalue, a higher absorption coefficient, and an efficient red
shift in UV–vis spectrum compared with those ofP3. There-fore, the PSC device containing P4 was chosen to be opti-mized in further PV studies. To acquire the advantage of a higher absorption coefficient of PC71BM27 than PC61BM, the
BHJ PSC devices with different weight ratios of P4 (as an electron donor) and PC71BM (as an electron acceptor) were
fabricated, and their J–V characteristics and PV properties are illustrated in Figure 5(a) and Table 4, respectively. The optimum PCE value of 1.01% was obtained in the PSC device having a weight ratio ofP4:PC71BM¼ 1:1.5 (with Voc¼ 0.60
V,Jsc¼ 4.83 mA/cm2, and FF¼ 35%). Using a lower weight
ratio of PCBM in blended polymerP4:PC71BM (1:1 w/w) led
to a reduction in theJsc value, which could be attributed to
the inefficient charge separation and electron transporting properties, resulting in the lower PCE value.28 However, loading larger weight ratios of PCBM in blended copolymers
TABLE 3Photovoltaic Properties of Polymer Solar Cell (PSC) Devices with a Configuration of ITO/PEDOT:PSS/P1–P4:PC61BM(1:1 w/w)/Ca/Ala Active Layerb (Polymer:PC61BM¼1:1) Voc (V) Jsc (mA/cm2) FF (%) PCE (%) P1 0.53 1.93 35 0.36 P2 0.56 2.31 36 0.46 P3 0.62 2.52 37 0.57 P4 0.58 2.87 34 0.57 a
Measured under AM 1.5 irradiation, 100 mW/cm2
.
b
Active layer of blended polymers with the weight ratio of P1– P4:PC61BM¼1:1.
FIGURE 5(a)J–V characteristics of ITO/PEDOT:PSS/P4:PC71BM/ Ca/Al under illumination of AM 1.5 at 100 mW/cm2. (b) EQE curves of PSC devices based on polymer blends P4/PC71BM in various weight ratios.
TABLE 4 Photovoltaic Propertiesaof Bulk-Heterojunction PSC Devices Containing Different Weight Ratios of Blended Polymers P4:PC71BM and Blend Film Roughness by AFM Measurements Weight Ratios of Blended P4:PC71BM Voc (V) Jsc (mA/cm2) FF (%) Rrms (nm)b PCE (%) 1:1 0.60 3.30 39 0.20 0.77 1:1.5 0.60 4.83 35 0.17 1.01 1:2 0.58 3.42 28 0.22 0.55 a
Measured under AM 1.5 irradiation, 100 mW/cm2
.
bR
rms: root-mean-square values of roughnesses measured from AFM
images. FIGURE 4J–V characteristics of ITO/PEDOT:PSS/P1-P4:PC61BM
P4:PC71BM (1:2 w/w) also reduced theJsc and PCE values,
which could be probably attributed to the increased aggrega-tion of PCBM so as to influence the separaaggrega-tion of charges. Hence, bothJsc and PCE values decreased with larger PCBM
molar ratios of 1:2 (w/w) because of the reasons described here.15(e) To investigate the different efficiencies of the PSC
devices, the EQEs for polymer P4 blended with PC71BM in
various weight ratios were further investigated in Figure 5(b), where the PSC devices exhibited a very broad response range covering from 400 to 700 nm with the maximum EQE values of 27%, 34%, and 22% for P4:PC71BM ¼ 1:1, 1:1.5,
and 1:2 (w/w), respectively. Therefore, the photocurrent generation in the PSC device withP4:PC71BM¼ 1:1.5 (w/w)
is higher and leading to the highest PCE value because of more light harvest in the visible region.
Surface morphology of the active layer is also the key param-eter for device performance in PSC devices.29The AFM topo-graphic images of the polymer blends of P4:PC71BM in
vari-ous weight ratios (1:1, 1:1.5, and 1:2) are presented at Figure 6(a–c) and their root-mean-square values of rough-ness (Rrms) are presented in Table 4. It is clearly seen that
all the phase images possessed almost similar coarse surfa-ces, which were attributed to the domains of highly stacked polymer chains inP4.21(a)The most coarse surface ofRrms¼
0.22 nm in P4:PC71BM ¼ 1:2 (w/w) indicated a large scale
phase separation, which might decrease the diffusional escape probability for mobile charge carriers and thus to increase charge recombination.30,16(d) However, the decrease of PC71BM content in P4:PC71BM ¼ 1:1 (w/w) reduced the
Rrms value to 0.20 nm, which led to a similar Jscvalue with
that of P4:PC71BM ¼ 1:2 (w/w). Compared with the other
blending ratios ofP4:PC71BM, a smoothest surface withRrms
¼ 0.17 nm was obtained in P4:PC71BM ¼ 1:1.5 (w/w),
which enhanced the Jsc value and yielded the highest
effi-ciency (PCE¼ 1.01%) in PSCs.30
CONCLUSIONS
In conclusion, a series of conjugated main-chain copolymers consisting of 2,7-carbazole electron-donating unit and bithia-zole electron-accepting unit were synthesized by Pd(0)-cata-lyzed Stille coupling polymerization. Carbazole-based poly-mers exhibited broad absorption bands located in the UV and visible regions from 300 to 600 nm with optical band gaps of 1.93–1.99 eV. The HOMO and LUMO energy levels of the polymers can be finely tuned via the molecular engineer-ing of donor/acceptor moieties and conjugated linkers inside the copolymers, which possessed relatively lower HOMO lev-els for PSC applications. The BHJ PV devices using polymers P1–P4 as electron donors and PC61BM as electron acceptors
were fabricated, and the optimization of PSC devices with P4:PC71BM in different weight ratios were investigated.
Finally, the PV device bearing an active layer of polymer blendP4:PC71BM (1:1.5 w/w) showed the best PCE value of
1.01%, with a short circuit current density (Jsc) of 4.83 mA/
cm2, a FF of 35%, and Voc¼ 0.60 V under 100 mW/cm2of
AM 1.5 white-light illumination. AFM images reveled that there were a better mixing between polymers and PC71BM
to generate a less scale phase separation. Although the PCE values of all PSC devices were not sufficiently high, the tuna-ble optoelectronic properties could be achieved by the struc-tural modifications of electron donor and acceptor units. The authors thank to the National Center for High-performance Computing for computer time and facilities. The financial
FIGURE 6AFM images of blended polymer P4:PC71BM spin coated from DCB in the ratios of (a) 1:1 (w/w), (b) 1:1.5 (w/w), and (c) 1:2 (w/w) with a size of 1 1 lm2.
supports of this project provided by the National Science Coun-cil of Taiwan (ROC) through NSC 97-2113-M-009-006-MY2, National Chiao Tung University through 97W807, and Energy and Environmental Laboratories (charged by Dr. Chang-Chung Yang) in Industrial Technology Research Institute (ITRI) are acknowledged.
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