Quinoxaline-Based Donor-Acceptor Conjugated Polymers
and Their Applications on Thin Film Transistors
and Solar Cells
MEI-HSIU LAI,1CHU-CHEN CHUEH,1WEN-CHANG CHEN,1,2JYH-LIH WU,3FANG-CHUNG CHEN3 1Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan
2Institute of Polymer Science and Engineering, National Taiwan University, Taipei 106, Taiwan 3Department of Photonics and Institute of Electro-optical Engineering, National Chiao Tung University, Hsinchu 300, Taiwan
Received 16 October 2008; accepted 20 November 2008 DOI: 10.1002/pola.23219
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Synthesis, properties, and optoelectronic device applications of four new bis-[4-(2-ethyl-hexyloxy)-phenyl]quinoxaline(Qx(EHP))-based donor-acceptor conju-gated copolymers are reported, in which the donors are thiophene(T), dithio-phene(DT), dioctylfluorene(FO), and didecyloxyphenylene(OC10). The optical band gaps (Eg) of PThQx(EHP), PDTQ(EHP), POC10DTQ(EHP), and PFODTQ(EHP) estimated from the onset absorption are 1.57, 1.65, 1.77, and 1.92 eV, respectively. The smallest Egof PThQx(EHP) among the four copolymers is attributed to the bal-anced donor/acceptor ratio and backbone coplanarity, leading to a strong intramolec-ular charge transfer. The hole mobilities obtained from the thin film transistor (TFT) devices of PThQx(EHP), PDTQ(EHP), POC10DTQ(EHP), and PFODTQ(EHP) are 2.52 104, 4.50 103, 4.72 105, and 9.31 104cm2V1s1, respectively, with the on-off ratios of 2.00 104, 1.89 103, 4.07 103, and 2.30 104. Polymer solar cell based on the polymer blends of PFODTQ(EHP), PThQx(EHP), POC10DTQ(EHP), and PDTQ(EHP) with [6, 6]-phenyl C61-butyric acid methyl ester (PCBM) under illumination of AM1.5 (100 mW cm2) solar simulator exhibit power conversion efficiencies of 1.75, 0.92, 0.79, and 0.43%, respectively. The donor/ acceptor strength, molecular weight, miscibility, and energy level lead to the differ-ence on the TFT or solar cell characteristics. The present study suggests that the prepared bis[4-(2-ethyl-hexyloxy)-phenyl]quinoxaline donor-acceptor conjugated copolymers would have promising applications on electronic device applications.
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VC2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 973–985, 2009
Keywords: charge transfer; conjugated polymers; copolymerization; donor-acceptor; polycondensation; quinoxaline; solar cells; thin film transistor
INTRODUCTION
Donor-acceptor (D-A) copolymers have attracted significant scientific interest recently as their electronic and optoelectronic properties can be
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 47, 973–985 (2009)
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VC2008 Wiley Periodicals, Inc.
Additional Supporting Information may be found in the online version of this article.
Correspondence to: W.-C. Chen (E-mail: [email protected]. tw)
manipulated through intramolecular charge transfer (ICT).1–24 Such polymers may have potential applications in various organic elec-tronic devices, such as light-emitting diodes,3–8 field effect transistors,9–16 and photovoltaic cells.17–24
Among D-A conjugated copolymers, the elec-tron-donating moieties of fluorene,3,5,14,15 thio-phene,13,25–29 dialkoxylphenylene,30,31 and carba-zole32 have been reported. On the other hand, thieno[3,4-b]pyrazine (TP), 2,1,3-benzothiadiazole (BT), quinoxaline (Q), and pyridine (Py) are generally employed as the electron-accepting moieties. We are particularly interested in the quinoxaline-based copolymers due to their good charge-transfer characteristics and stability for device applications.33–35We found that poly(thio-phene-alt-alkylquinoxaline)-based thin film tran-sistor exhibited a medium high hole mobility with a relatively high on/off ratio13and ambipolar car-rier transport by blending with poly(benzobisimi-dazophenanthroline).16 A recent report by Inga¨-nas’s group demonstrated that fluorene-quinoxa-line alternating copolymer exhibited a high power conversion photovoltaic efficiency due to its bal-anced electron and hole mobilities.36 Another study by Scherf ’s group discovered the
quinoxa-line/ologothiophene copolymers having the unex-pected independence of photophysical properties with the thiophene segment length.26 We believe that the intramolecular charge transfer and back-bone planarity played important roles on the aforementioned results.
In this article, we report the synthesis, proper-ties, and device applications of bis-[4-(2-ethyl-hex-yloxy)-phenyl]quinoxaline-based donor-acceptor conjugated copolymers, in which the donors include dioctylfluorene, didecyloxyphenylene, and thiophene. These polymers were synthesized by palladium(0)-catalyzed Suzuki or Stille coupling reaction, as shown in Scheme 1. The long bis[4-(2-ethyl-hexyloxy)-phenyl] side groups on the qui-noxaline moiety significantly improve the polymer solubility for device applications. The effects of do-nor-acceptor strength and backbone planarity on the electronic and optoelectronic properties of the studied copolymers were investigated. Further-more, field-effect carrier mobility obtained from the bottom gate thin film transistor was corre-lated with the polymer structure. Polymer solar cell devices fabricated by polymer/PCBM blends sandwiched between a transparent anode (ITO/ PEDOT:PSS) and a cathode (Ca) were also explored. The present study revealed that the
Scheme 1. Synthesis of DTQ(EHP), PThQx(EHP), PDTQ(EHP) and PFODTQ (EHP), and POC10DTQ(EHP).
new polymers had potential applications for flexi-ble electronic devices.
EXPERIMENTAL
Materials
N-bromosuccinimide (NBS), tributyl(thien-2-yl)stannane, bis(triphenylphosphine)-dichloro palladium(II) [(PPh3)2Cl2Pd(II)],
9,9-dioctylfluor-ene-2,7-diboronic acid bis(1,3-propanediol)ester, tetrakis (triphenylphosphine)-palladium(0) [(PPh3)4
Pd(0)], trioctylmethyl ammonium chloride
(aliquatVR
336), phenyl boronic acid, bromoben-zene, and [6, 6]-phenyl C61-butyric acid methyl ester (PCBM) were purchased from Aldrich (Mis-souri, USA) or Acros (Geel, Belgium) and used without further purification. Ultra-anhydrous solvents and common organic solvents, such as THF and toluene, were purchased from Tedia (Ohio, USA).
The following monomers were prepared accord-ing to literature procedures: 4,40-dihydroxybenzil,37 4,40-bis(2-ethylhexyloxyl)benzyl,38 5,8-dibromo-2,3-bis-[4-(2-ethyl-hexyloxy)-phenyl]-quinoxaline (Qx(EHP), 1),39 2,5-bis(trimethylstannyl) thio-phene,40 and 2,5-didecyloxyphenylene-1,4-bis-(4,4,5,5-tetramethyl-1,3,2-dioxaborolate) (DP).31
2,3-Bis(4-(2-ethylhexyloxy)phenyl)-5,8-dithien-2-yl-quinoxalines (2)
To a solution of Qx(EHP) (1.0 g, 1.43 mmol) and tributyl(thien-2-yl)stannane (1.59 g, 4.29 mmol) in toluene (10 mL), (PPh3)2Cl2Pd(II) (20 mg) was
added. The mixture was refluxed for 24 h. After cooling to room temperature, the mixture was poured into methanol and the precipitate was fil-tered off. The crude product was dried under vac-uum at 50C to obtained a brown-yellow powder (904 mg, yield: 90%).1H NMR (500 MHz, CD2Cl2)
d (ppm): 0.90–0.96 (12H, s, ACH3), 1.34–1.54
(16H, br, ACH2A), 1.74–1.75 (2H, s, ACHA),
3.91–3.92 (4H, s, AOCH2A), 6.92–6.93 (4H, s, ArAH), 7.19–7.20 (2H, s, ArAH), 7.53–7.54 (2H, s, ArAH), 7.70–7.72 (2H, s, ArAH), 7.88 (2H, s, ArAH), 8.14 (2H, s, ArAH). 2,3-Bis(4-(2-ethylhexyloxy)phenyl)-5,8-bis[50 -bromo-dithien-2-yl-quinoxalines] (DTQ(EHP)), (3) 2,3-bis(4-(2-ethylhexyloxy)phenyl)-5,8-dithien-2-yl-quinoxalines (2) (1.35 g, 1.92 mmol) was dis-solved in THF, and NBS (N-bromosuccinimide)
(752 mg, 4.22 mmol) was immediately added. After the mixture was stirred at room tempera-ture for 3 h, it was poured into methanol and the precipitate was filtered off. The crude product was dried under vacuum at 50C to obtain an or-ange powder (1.49 g, yield: 90%). 1H NMR (500 MHz, CD2Cl2) d (ppm): 0.91–0.98 (12H, s, ACH3),
1.34–1.54 (16H, br,ACH2A), 1.76 (2H, s, ACHA),
3.92 (4H, s,AOCH2A), 6.94–6.96 (4H, s, ArAH),
7.15 (2H, s, ArAH), 7.60 (2H, s, ArAH), 7.66 (2H, s, ArAH), 8.10 (2H, s, ArAH).
General Procedure of Polymerization
As shown in the Scheme 1, Stille coupling reac-tion was used to synthesize two thiophene/qui-noxaline-based copolymers, PThQx(EHP) and PDTQ(EHP). Monomers and (PPh3)4Pd(0) were
dissolved in toluene, and then the solution was stirred under nitrogen atmosphere and refluxed with vigorous stirring for 72 h. The end groups were capped by refluxing for another 12 h each with phenyl boronic acid and bromobenzene (both 1.0 equivalent with respect to the dibromo mono-mer). After cooling to room temperature, the resulting solution was dropped into a mixture of methanol and D.I. water. The precipitated solid was filtered and collected. The crude product was extracted in a Soxhlet apparatus with acetone for 24 h to remove the oligomer and catalyst residues. It was then dried under vacuum at 60C for 24 h to obtain the polymer product.
Suzuki cross-coupling reaction was used to syn-thesize PDTQ(EHP), POC10DTQ(EHP), and PFODTQ(EHP), as shown in Scheme 1. Donor monomers (DP and fluorene), acceptor monomers DTQ(EHP), and (PPh3)4Pd(0) were dissolved in a
mixture of toluene (15 mL) and aqueous 2 M K2CO3(ca. 10 mL) with several drops of aliquatV
R 336. The following handling steps are all the same as described earlier in the Stille coupling reaction.
Poly[thiophene-2,5-diyl-alt-2,3-bis(4-(2-ethylhexy-loxy)phenyl)-5,8-quinoxaline] (PThQx(EHP)) 557 mg (0.8 mmol) of Qx(EHP) and 328 mg (0.8 mmol) of 2,5-bis(trimethylstannyl) thiophene were used to afford dark solid (270 mg, yield: 52%).1H NMR (500 MHz, CD2Cl2) d (ppm): 0.88–
0.92 (12H, s,ACH3), 1.31–1.52 (16H, br,ACH2A),
1.70–1.83 (2H, s, ACHA), 3.66–3.89 (4H, s, AOCH2A), 6.77–6.89 (4H, s, ArAH), 7.43–7.98
(%): C, 77.6; H, 7.5; N, 4.5; S, 5.2; O, 3.9. Found: C, 74.5; H, 7.1; N, 4.3; S, 6.3. Weight average mo-lecular weight distribution (Mw) from
gel-permea-tion chromatography (GPC) was 5040, with a polydispersity index (PDI) of 1.26.
Poly[2,3-bis(4-(2-ethylhexyloxy)phenyl)-5,8-dithien-2-yl-quinoxaline] (PDTQ(EHP))
688 mg (0.8 mmol) of DTQ(EHP) and 464 mg (0.8 mmol) of bis(tributyltin) were used to afford dark solid (400 mg, yield: 71%). 1H NMR (500 MHz, CD2Cl2) d (ppm): 0.69 (12H, s, ACH3), 1.01–1.82
(18H, br,ACH2A and ACHA), 3.69–3.89 (4H, br, AOCH2A), 5.72–8.04 (14H, br, ArAH). Anal.
Calcd. for C44H48N2O2S2 (%): C, 75.4; H, 6.9; N,
4.0; S, 9.2; O, 4.6. Found: C, 71.9; H, 7.0; N, 3.5; S, 8.3. Mwand PDI from GPC were 11,560 and 1.68,
respectively.
Poly[2,5-didecyloxy-1,4-phenylene-alt-2,3-bis[4-(2-ethylhexyloxy)phenyl)-5,8-dithien-2-yl-quinoxaline] (POC10DTQ(EHP))
688 mg (0.8 mmol) of DTQ(EHP) and 514 mg (0.8 mmol) of monomer DP were used to afford dark-purple solid (560 mg, yield : 63%). 1H NMR (500 MHz, CD2Cl2) d (ppm): 0.83–0.98 (18H, d, ACH3),
1.22–1.88 (44H, br, ACH2A), 1.93–1.65 (6H, d,
AOCACH2A and ACHA), 3.90–4.18 (8H, q,
AOCH2A), 6.84–6.92 (4H, s, ArAH), 7.20–8.16
(12H, br, ArAH). Anal. Calcd. for
C70H92N2O4S2(%): C, 77.2; H, 8.5; N, 2.6; S, 5.9.
Found: C, 77.0; H, 9.1; N, 2.4; S, 5.5. Mwand PDI
from GPC were 14,350 and 1.67, respectively.
Poly[2,5-didecyloxy-1,4-phenylene-alt-2,3-bis(4-(2-ethylhexyloxy)phenyl)-5,8-dithien-2-yl-quinoxaline] (PFODTQ(EHP))
688 mg (0.8 mmol) of DTQ(EHP) and 447 mg (0.8 mmol) of 9,9-dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester were used to afford red solid (520 mg, yield: 58%). 1H NMR (500 MHz, CD2Cl2) d (ppm): 0.74 (6H, s, ACH3), 0.95–1.09(d,
18H,ACH3and 24H,ACH2A), 1.38–1.52 (16H, d,
ACH2A), 1.70–1.80 (2H, s, ACHA), 2.18 (4H, s,
ACH2A), 3.98 (4H, s, AOCH2A), 6.80–7.01 (4H, s,
ArAH), 7.54–8.20 (16H, br, ArAH). Anal. Calcd. for C73H88N2O2S2 (%): C, 80.5; H, 8.1; N, 2.6; S,
5.9; O, 2.9. Found: C, 80.8; H, 8.6; N, 2.4; S, 5.6. Mw and PDI from GPC were 53,070 and 2.38,
respectively.
Characterization
1
H NMR spectra were recorded by Bruker Avance DRX 500 MHz spectrometer in dichloromethane-d2. Gel permeation chromatographic (GPC)
analy-sis was performed on a Lab Alliance RI2000 instrument (one column, MIXED-D from Polymer Laboratories) connected with one refractive index detector from Schambeck SFD Gmbh. All GPC analyses were performed on polymer/THF solu-tion at a flow rate of 1 mL/min at 40C and cali-brated with polystyrene standards. Elemental analyses were performed with a Heraeus varioIII-NCSH instrument.
Thermal decomposition temperatures (Td) of
the prepared polymers were characterized by a TA instrument TGA 951 thermogravimetric ana-lyzer (TGA) at a heating rate of 20C/min under a nitrogen atmosphere from room temperature to 800 C, and differential scanning calorimetry (DSC) measurements were performed under a nitrogen atmosphere at a heating rate of 10 C/ min from 0 to 300C using a TA instrument DSC-910S.
The absorption spectra and photoluminescence (PL) spectra were recorded at room temperature with a Jasco model UV/VIS/NIR V-570 spectrome-ter and Fluorolog-3 spectrofluoromespectrome-ter (Jobin Yvon), respectively. For the solution spectra, poly-mers were dissolved in THF (ca. 1 mg/mL) and then put in a quartz cell for measurement. For the thin film spectra, polymers were first dis-solved in CHCl3(ca. 10 mg/mL); then the solution
spin-coated at a speed rate of 500 rpm for 20 s onto quartz substrate.
The electrochemical properties of the polymer films were investigated by a Bioanalytical System Model CV-27 potentiostat and a BAS X-Y recorder with a 0.1 M acetonitrile or DMF solution contain-ing Tetrabutylammonium Perchlorate (TBAP) as the electrolyte. TBAP was obtained from commer-cial sales and recrystallized twice from ethyl ace-tate and then dried in vacuum prior to use. Plati-num wire and ITO glass (polymer films area about 0.5 0.7 cm2) were used as auxiliary and working electrodes, respectively, and the solution of the polymer in THF (ca. 1 mg/mL) was used to prepare the polymer film on ITO glass by drop coating. All cell potentials were taken with the use of an Ag/AgCl, KCl (sat.) reference electrode. Then, the cyclic voltammetry in films were exam-ined by the three-electrode cell at a voltage scan rate of 0.1 V s. Furthermore, the reference elec-trode was calibrated through running the cyclic
voltammetry of ferrocene without any polymer film coated on ITO glass. The obtained potential values were then converted to versus ferrocene. The energy parameters EA and IP were esti-mated from the measured redox potentials on the basis of the prior works on conjugated poly-mers which have shown that: IP¼ (Eox
onsetþ 4.8) and EA¼ (Ered
onset þ 4.8), where the onset poten-tials are in volts (versus Fcþ/Fc) and IP and EA
are in electron volts.41,42 The electronic struc-ture parameters, HOMO and LUMO, were investigated with the relations of HOMO¼ IP
and LUMO ¼ EA by assuming no
configura-tion interacconfigura-tions.
Polymer Thin Film Transistors
Polymer thin film transistor was fabricated with a bottom-contact and bottom-gate configuration on the heavily n-doped silicon wafers. The source/ drain regions were defined by a 130 nm Au layer through a regular shadow mask, and the channel length (L) and width (W) were 25 and 500/ 1000 lm, respectively. A thermally grown 200 nm SiO2used as the gate dielectric with a capacitance
of 68 nF cm2. The aluminum was used to create a common bottom-gate electrode. Afterward, the substrate was modified with octyltrichlorosilane (OTS) coupling agent to promote the molecular chain ordering of the polymer semiconductor at the gate dielectric/semiconductor interface. 0.5 wt % polymer solution in chlorobenzene was filtered through 0.50-lm pore size PTFE membrane sy-ringe filters, spin-coated at a speed rate of 1000 rpm for 60 s onto the silanized SiO2/Si substrate
and annealed at 100C overnight under vacuum. Output and transfer characteristics of the OTFT devices were measured using a Keithley 4200 semiconductor parametric analyzer. All the pro-cedures and electronic measurements were per-formed in ambient air. The structures of polymer thin films on SiO2/Si substrates were
character-ized by means of tapping mode atomic force mi-croscopy (AFM) with a Nanoscope 3D controller (Digital Instruments, Santa Barbara, CA) at room temperature. Commercial silicon cantile-vers (Nanosensors, Germany) with typical spring constants of 21–78 Nm1 were used, and images were taken continuously with the scan rate of 0.8 Hz. The processing and annealing conditions of thin film of polymer samples are the same as the device fabrication to simulate the polymer tran-sistor structures.
Polymer Solar Cells
Polymer solar cell devices were fabricated by spin-coating a blend of polymer/PCBM
sand-wiched between a transparent anode (ITO/
PEDOT:PSS) and a cathode (Ca). First,
PEDOT:PSS was spin-coated on the patterned ITO glass and then an active layer spin-coated (800 rpm) from the dichlorobenzene solution ( 0.8 wt %), followed by postannealing treatment at 110 C. Then, Ca (30 nm) /Al (100 nm) was vapor deposited to serve as cathode. Keithley 2400 was used to measure the curves of current density versus voltage (J-V curves), and the effi-ciency was calculated under the 100 mW cm2 simulated sunlight (AM 1.5G).
RESULTS AND DISCUSSION
Polymer Structure Characterization
Figure 1 shows the1H NMR spectra of monomer (DTQ(EHP)) and the corresponding polymer (PFODTQ(EHP)) in d-dichloromethane. For the spectrum of DTQ(EHP), the signals in the ranges of 0.91–1.76 and 3.93 ppm are assigned to the pro-tons of alkoxy group on the quinoxaline. The sig-nals in 6.94–8.10 ppm are attributed to the pro-tons on the quinoxaline and thiophene moieties. The1H-NMR spectrum of PFODTQ(EHP) shows
Figure 1. 1H NMR spectra of DTQ(EHP) and PFODTQ(EHP) in CD2Cl2. Labels of x and y are CD2Cl2and H2O, respectively.
similar proton signal positions as those of DTQ(EHP) except the signals in 0.74–2.18 ppm due to the n-alkyl group on the fluorene moiety. The numbers of protons estimated from peak inte-gration are in a fair agreement with the corre-sponding chemical structure of the copolymers. The1H spectra of other polymers (Supp. Info. Fig. S1) also agree well with the proposed polymer structures.
All the polymers are readily soluble in common organic solvents, such as THF, chloroform, DMF, and dichlorobenzene. The weight-averaged molec-ular weights (Mw) of the polymers determined
by GPC are in the range of 5040–53,070 with PDI of 1.26–2.38. The low molecular weight of PThQx(EHP) is probably due to the low mono-mer reactivity by the Stille coupling reaction or the steric hindrance of the long 4-(2-ethylhexyl-oxy)phenylene group on the quinoxaline. The ele-mental analyses of the four copolymers are in a reasonable agreement with the theoretical esti-mation. The slightly larger deviation on the car-bon and sulfur contents of PThQx(EHP) and PDTQ(EHP) could be due to the end group effect, since the two copolymers have low molecular weights.
Figure 2 shows TGA and DSC curves of the studied copolymers. The thermal decomposition temperatures (Td, 95 wt % residue) of the four
copolymers are in the range of 400–441 C, indi-cating their good thermal stability. Among the four copolymers, only PFODTQ(EHP) shows a glass transition temperature (Tg) at 142 C. The
rigid backbone of the other three copolymers
prob-ably limits the chain motion and thus Tg is not
observed.
Optical Absorption and Photoluminescence Properties
The UV–visible absorption spectra of the synthe-sized polymers in dilute THF solutions and thin films are shown in Figure 3, and their correspond-ing absorption maxima (kabs
max) are summarized in Table 1. The kabs
max of PThQx(EHP), PDTQ
(EHP), POC10DTQ(EHP), and PFODTQ
(EHP) solutions in the visible region are observed at 566, 579, 536, and 524 nm, respectively, while those of polymer films at 612, 586, 576, and 538 nm. In comparison, the kabs
max are much more red-shifted than those of parent poly(quinoxaline),1 polythiophene (nonregioregular),1 polyfluorene5 or poly(dioctylphenylene).30It suggests the signif-icance of the intramolecular charge transfer between the donor and bis[4-(2-ethyl-hexyloxy)-phenyl]quinoxaline. The optical band gaps (Eg) of
PThQx(EHP), PDTQ(EHP), POC10DTQ
Figure 2. TGA curves of the four copolymers at a heating rate of 20C/min under nitrogen atmosphere. The insert shows the DSC curves of PFODTQ(EHP) at a heating rate of 10C/min under nitrogen atmos-phere.
Figure 3. Normalized UV–visible spectra of the four copolymers in (a) dilute THF solutions and (b) solid films, respectively.
(EHP), and PFODTQ(EHP) estimated by the onset absorption are 1.57, 1.65, 1.77, and 1.92 eV, respectively. PThQx(EHP) has a smaller Egthan
PDTQ(EHP) since it has the more balanced donor-to-acceptor ratio (1:1) and leads to a higher intramolecular charge transfer. Such result is similar to that reported previously on the thio-phene-thienopyrazine copolymers.15Furthermore,
the unsubstituted thiophene moieties of
PDTQ(EHP) are less bulky than the moieties of dioctylfluorene and didecyloxyphenylene in POC10DTQ(EHP) and PFODTQ(EHP), respec-tively, which enhance the coplanarityof the poly-mer backbone and enlarge the polypoly-mer conjuga-tion length. In summary, the lower Eg of
PDTQ(EHP) than those of POC10DTQ(EHP) or PFODTQ(EHP) is probably resulted from the higher backbone coplanarity of the five-member thiophene moiety in comparison with the six-member phenylene ring.
The absorption spectra of PThQx(EHP) and PDTQ(EHP) show blue shifts compared with the n-octylbiphenyl substituted thiophene-quinoxa-line copolymers26 but red-shift in comparison with those of the alkyl-substituted copolymers.1It suggests that the significance of the side chain on the backbone planarity of thiophene-quinoxaline copolymers, which leads to the difference on the photophysical properties. The kabs
max of PFODT-Q(EHP) film is similar to that of APFO-15 reported in the literature,36 in which the APFO-15 has a meta-position substitution of dialkoxy group on the phenylene ring.
The normalized photoluminescence (PL)
absorption spectra of the four copolymers in dilute THF solution and thin film are shown in Figure 4.
The PL measurements are excited at the wave-length of 500–580 nm based on their optical absorption peaks. The kPL
max of PThQx(EHP),
Table 1. Optical and Electrochemical Properties of PThQx(EHP), PDTQ(EHP), POC10DTQ(EHP), and PFODTQ(EHP) kabs max (soln) (nm)a k abs max (film) (nm) Eopt g (eV)b Oxidation (vs Fcþ/Fc) Reduction (vs Fcþ/Fc) Eelec g (eV) Eonset (V) HOMO (eV) Eonset (V) LUMO (eV) PThQx(EHP) 350, 566 364, 612 1.57 0.22 5.02 1.70 3.10 1.92 PDTQ(EHP) 366, 579 386, 586 1.65 0.23 5.03 1.70 3.10 1.93 POC10DTQ(EHP) 356, 388c, 536 368c, 400, 576 1.77 0.18 4.98 1.84 2.96 2.02 PFODTQ(EHP) 390, 524 396, 538 1.92 0.58 5.38 1.50 3.30 2.08 aIn THF dilute solution.
bEstimated from the absorption edge of the film. cEmission shoulder.
Figure 4. Normalized PL spectra of the four copoly-mers in (a) dilute THF solutions and (b) solid films, respectively.
PDTQ(EHP), POC10DTQ(EHP), and PFODT-Q(EHP) in THF are 636, 678, 630, and 612 nm, respectively, while those in solid films are 740, 742, 702, and (636, 658) nm. The large red shifts of the kPLmax in the solid films compared with their solutions are probably resulted from the enhanced interchain interaction and thus promote energy transfer in the aforementioned polymer films.
Electrochemical Characteristics
The electrochemical characteristics of the four copolymers were investigated by cyclic
voltamme-try and summarized in Table 1. Figure 5 shows the cyclic voltammograms of the four polymer films. All of the prepared copolymers exhibit quasi-reversible reductive reaction but only PFODTQ(EHP) has a reversible oxidation. The LUMO levels of PThQx(EHP) and PDTQ (EHP) estimated from the onset reduction are 3.10 eV, with the HOMO potentials estimated from the onset oxidation in the range of5.02 to 5.03 eV. It suggests that the HOMO/LUMO lev-els of the two polymers do not change signifi-cantly even though the thiophene content varies. Furthermore, the electrochemical band gaps of PThQx(EHP) and PDTQ(EHP) estimated from cyclic voltammetry (1.92 versus 1.93 eV) are very close. The LUMO levels of POC10DTQ(EHP)
and PFODTQ(EHP) are 2.96 and 3.30 eV,
respectively, with their HOMO levels are 4.98 and 5.38 eV. In comparison with the HOMO and LUMO levels of poly(9,9-octylfluorene) (PFO) at 5.6 and 2.0 eV,31 the energy levels of PFODTQ(EHP) have been significantly var-ied due to the modulated ICT strength. The elec-trochemical band gaps are 0.2–0.4 eV larger than the corresponding optical ones, which is probably due to the exciton binding energy of conjugated polymers.31 However, the trend between the polymer structure and the obtained band gap is similar.
Thin Film Transistor Characteristics
The charge transporting characteristics of the four polymers were explored by their bottom con-tact thin film transistor (TFT) devices and sum-marized in Table 2. Figure 6 shows the TFT char-acteristic curves of all four copolymers on the OTS-modified SiO2. These polymers showed
typi-cal p-type I-V characteristics (drain current Id
versus drain voltage Vdat different gate voltages
Vg) when operated in the accumulation mode
Figure 5. Cyclic voltammograms of the four poly-mer films on ITO glass at a scanning rate of 0.1 V s1 in DMF/acetonitrile containing 0.1 M TBAP.
Table 2. TFT Characteristics of the Four Copolymers, PThQx(EHP), PDTQ(EHP), POC10DTQ(EHP), and PFODTQ(EHP)
Polymer Mobility (Cm2V1s1) On/Off RMS (nm)
PThQx(EHP) 2.52 104 2.00 104 1.68
PDTQ(EHP) 4.50 103 1.89 103 1.96
POC10DTQ(EHP) 4.72 105 4.07 103 0.61
operation. In the saturation region (Vd[ Vg Vt), Idcan be described by eq 1: Id¼WColh 2L ðVg VtÞ 2 (1) Here, W and L are the channel width and length, respectively. Cois the capacitance of the gate
insu-lator per unit area (SiO2, 200 nm, Co ¼ 17 nF
cm2). The saturation region mobility of the stud-ied polymers was calculated from the transfer characteristics involving plotting (Id)1/2versus Vg.
The estimated hole mobilities of PThQx(EHP) and PDTQ(EHP) are 2.52 104 and 4.50 103 cm2 V1 s1, respectively, with the corre-sponding on-off ratios of 2.00 104 and 1.89 103 in the ambient conditions. Both of them pos-sess similar threshold voltage (VT) around17 V,
but PDTQ(EHP) shows a significantly higher p-channel mobility as compared to PThQx(EHP). It may be attributed to the larger molecular
weight of PDTQ(EHP) than that of
PThQx(EHP). In addition, the estimated hole mobility of POC10DTQ(EHP) and PFODTQ (EHP) are 4.72 105and 9.31 104, respec-tively, with the corresponding on-off ratios of 4.07 103
and 2.30 104. Processing solvent plays an important role on the resulted surface structured of semiconducting polymers and their TFT mobili-ties.43However, the surface morphologies of these two polymers are smooth and without significant aggregation, as shown in Figure 7. It indicates that the polymer structure leads to the difference on the carrier mobility. Even though the molecu-lar weights of POC10DTQ(EHP) and PFODTQ (EHP) are much higher than PDTQ(EHP), the
hole mobilities of POC10DTQ(EHP) and
PFODTQ(EHP) are still lower than that of PDTQ(EHP). It indicates the importance of intramolecular charge transfer on the TFT mobil-ity. By annealing above the Tg of PFODTQ
(EHP), its mobility is slightly improved to 1.30 103cm2V1s1, which could be realized through the insignificant variation of polymer surface mor-phology, as shown in Supporting Information (Supp. Info. Fig. S3).
Characterization of Polymer Solar Cells
Photovoltaic devices were fabricated from poly-mer/PCBM blends of a sandwiched structure com-posed of a transparent anode (ITO/PEDOT:PSS) and a cathode (Ca). The photovoltaic properties of the four copolymers are summarized in Table 3. Figure 8 shows the current-voltage (J-V) charac-teristics of the aforementioned device measured under illumination of AM1.5 (100 mW cm2) solar simulator. Although different blend ratios (1:1, 1:2, and 1:4) of copolymer to PCBM are used, only the power conversion efficiency (PCE) of PFODTQ (EHP) increases at a higher PCBM content. For
Figure 6. (a) Transfer characteristics of the polymer TFT devices with an OTS-modified surface and annealed at 100 C, where Vds ¼ 100 V. (b) and (c) are the output characteristics of PDTQ(EHP) and PDOC10DTQ(EHP) based TFT devices.
example, power conversion efficiency (PCE) of PThQx(EHP)/ PCBM decreases from 0.92% of 1:1 ratio to 0.65% of 1:2 ratio, and 0.51% of 1:4 ratio. In comparison, the PCE of the PFODTQ (EHP)/PCBM with the ratios of 1:1, 1:2, and 1:4 are 1.16, 1.24, and 1.75%, respectively. Such vari-ation of the copolymer/PCBM ratio with PCE is attributed to the formation of charge transfer exciton, which leads to the higher hole mobility of the fluorine-based copolymers at a larger PCBM content.36,44–46
The PCE of the PThQx(EHP), PDTQ(EHP), and POC10DTQ(EHP) with the 1:1 blend ratio to PCBM are 0.92, 0.43, and 0.79%, respectively. The Isc of POC10DTQ(EHP) is relatively small,
possibly due to its lowest hole mobility among the four copolymers. Comparing PDTQ(EHP), PThQx(EHP) shows a relatively higher PCE than PDTQ(EHP) despite its low molecular
weight. It may be explained by their surface structures shown in Figure 9. As shown in the fig-ure, the surface of PThQx(EHP) is homogeneous with a smaller root mean square (RMS) rough-ness of 2.54 nm. However, PDTQ(EHP) has a large RMS roughness of 10.67 nm, probably from the PCBM aggregation (white part). The low mis-cibility between the PDTQ(EHP) and PCBM explains its poor solar cell characteristics. As shown in Table 3, the open-circuit voltage (Voc) of
PFODTQ(EHP) reached as large as 0.79 V, which may be resulted from the relatively low
HOMO energy level (5.14 eV) of PFODTQ
(EHP) compared with the other copolymers (ca. 4.8 eV). Voc is an approximate measure of the
difference between the oxidation potentials of the donor polymer and the reduction potential of PCBM. Furthermore, the good film quality (due to high Mw) and medium high hole mobility are
Figure 7. AFM tapping mode topographical images of (a) POC10DTQ(EHP) and (b) PFODTQ(EHP) thin films on the SiO2/Si substrate.
Table 3. Polymer Solar Cell Characteristics of the Four Copolymers, PThQx(EHP), PDTQ(EHP), POC10DTQ(EHP), and PFODTQ(EHP)
Polymer Polymer:PCBM ratio Voc(V) Isc(mA cm2) FF PCE (%)
PThQx(EHP) 1:1 0.69 2.91 0.46 0.92 PDTQ(EHP) 1:1 0.39 2.24 0.49 0.43 POC10DTQ(EHP) 1:1 0.67 3.02 0.39 0.79 PFODTQ(EHP) 1:1 0.73 4.48 0.36 1.16 1:2 0.75 4.29 0.39 1.24 1:4 0.79 6.05 0.37 1.75
probably contributed to good short-circuit current
(Isc) of PFODTQ(EHP). The photocurrent
increased at a higher PCBM loading provides the formation of adequate electron and hole percola-tion paths and leads to efficient charge collecpercola-tion to the electrodes.36 The PCE of PFODTQ(EHP) device reaches 1.75% based on the
aforemen-tioned optimization. Although the PCE of
PFODTQ(EHP) is lower than that of similar
polymer with 3.5%,36 it could be improved through device optimization.
CONCLUSIONS
In this study, we have successfully synthesized four quinoxaline-based donor-acceptor copolymers for thin film transistor (TFT) and solar cell appli-cations. PThQx(EHP) has the smallest band gap of 1.57 eV among the four copolymers due to a strong intramolecular charge transfer. The hole mobilities obtained from the TFT devices of the four copolymers are in the range of 4.72 105– 4.50 103 cm2 V1s1with the on-off ratios of 1.89 103to 2.30 104. Polymer solar cells based on the blend of copolymer with PCBM blend ex-hibit power conversion efficiencies of 0.43–1.75%. The donor/acceptor strength, molecular weights, miscibility, and energy level lead to the difference on the TFT or solar cell characteristics. The elec-tronic and optoelecelec-tronic properties suggest that the prepared bis[4-(2-ethyl-hexyloxy)-phenyl] quinoxaline-containing copolymers would have potential applications for electronic devices.
The financial supports of National Science Council (NSC96-2221-E-002-021), Excellent Research Projects of National Taiwan University, and Ministry of Figure 8. J-V characteristics of polymer solar cells
using polymer/PCBM blends under the illumination of AM 1.5G, 100 mW cm2. The ratio of polymer/ PCBM is 1:1 except PFODTQ(EHP) with both the ratios of 1:1 and 1:4.
Figure 9. AFM tapping mode topographical images of (a) PThQx(EHP)/PCBM and (b) PDTQ(EHP)/PCBM blends on ITO glass coating with PEDOT:PSS. [Color figure can be viewed in the online issue, which is available at www.interscience. wiley.com.]
Economic Affairs of Taiwan (96-Ec-17-A-08-S1-015) are highly appreciated.
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