Synthesis and Characterization of Pyrido[3,4-b]pyrazine-Based Low-Bandgap Copolymers for Bulk Heterojunction Solar Cells

pubs.acs.org/Macromolecules Published on Web 07/12/2010 r 2010 American Chemical Society DOI: 10.1021/ma100522a

Synthesis and Characterization of Pyrido[3,4-b]pyrazine-Based

Low-Bandgap Copolymers for Bulk Heterojunction Solar Cells

Department of Materials Science and Engineering, National Chiao Tung University, 300 Hsinchu, Taiwan, ROC

Received March 8, 2010; Revised Manuscript Received June 16, 2010

ABSTRACT: We have used Stille polycondensation to prepare a series of low-bandgap copolymers, P1-P4, by conjugating the electron-accepting pyrido[3,4-b]pyrazine (PP) moieties with the electron-rich benzo[1,2-b:3,4-b0]dithiophene (BDT) or cyclopentadithiophene (CPDT) units. P1 and P3 are based on PP and BDT units while P2 and P4 are based on PP and CPDT units. All of these polymers exhibited excellent thermal stability and sufficient energy offsets for efficient charge transfer and dissociation, as determined through thermogravimetric analyses and cyclic voltammetry, respectively. The bandgaps of the polymers could be tuned in the range 1.46-1.60 eV by using the two different donors, which have different electron-donating abilities. The three-component copolymers, P3 and P4, incorporating the thiophene and bithio-phene segments, respectively, absorbed broadly, covering the solar spectrum from 350 to 800 nm. The morphologies of the blends of P3 and P4 with [6,6]-phenyl-C70-butyric acid methyl ester (PC70BM) were more

homogeneous than those of P1 and P2; in addition, devices incorporating the P3 and P4 blends exhibited superior performance. The best device performance resulted from an active layer containing the P4:PC70BM

blend; the short-circuit current was 10.85 mA cm-2and the power conversion efficiency was 3.15%.

Introduction

In recent years, conjugated polymers possessing extended delocalizedπ-electron systems have been studied extensively for their application in bulk heterojunction (BHJ) solar cells, which generally feature a polymeric donor and a fullerene-based accep-tor.1-4Regioregular polythiophene derivatives have been widely investigated for BHJs because of their highly crystallizable state, leading to good light harvesting in the visible spectrum and excellent carrier mobility. Power conversion efficiencies (PCEs) of up to 5% have been achieved when using poly(3-hexylthio-phene)/[6,6]-phenyl-C61-butyric acid methyl ester (P3HT/PCBM)

composites as the photoactive layer.5-8Nevertheless, the effici-ency of this system is difficult to improve because of the limited absorption range of P3HT, which absorbs photons only at wave-lengths less than 650 nm and merely ca. 22% of the absorbed photons from sunlight.9In attempts to harvest more photons and further improve the efficiencies of polymer solar cells (PSCs), several conjugated polymers featuring electron donor-acceptor (D-A) units in main chain-conjugated configurations10-14_and

side chain-attached architectures15-19have been developed because of their tunable electronic properties, ambipolar charge transport abilities, and enlarged spectral absorption ranges. Low-bandgap D-A polymers have attracted considerable attention because their absorption ranges are broader than that of P3HT; for example, the presence of intramolecular charge transfer (ICT) bands arising from push/pull interactions between the D and A units can extend the absorption into the near-infrared (NIR) region of the solar spectrum.20-26 The properties of D-A polymers can be tuned by varying the structures of the D and A units.

Several D-A conjugated polymers based on electron-accepting pyrido[3,4-b]pyrazine (PP) moieties have been described previously.

These PP-containing polymers have displayed higher electron-accepting abilities relative to that of quinoxaline-containing polymers because the former feature more electron-withdrawing nitrogen atoms in the fused ring.27-29The poly(pyridopyrazine-phenylene) polymer has been used in NIR light-emitting diodes;30

another polymer, in which the PP moieties were conjugated with thiophene/fluorene units, has been used in field-effect transistors, with hole mobilities of up to 4.4 10-3cm2V-1s-1.31To date, only a few PSCs prepared from PP-containing polymers have been investigated;32-35they have exhibited moderate PCEs (up to 1.1%)34 because of their low molecular weights and/or poor solubility.

In this study, we used Stille polycondensation to prepare four PP-containing low-bandgap polymers of high molecular weight and exhibiting good solubility. The presence of octyloxyphenyl groups attached to the PP moieties increased the solubility and, thereby, extended the conjugation length of the synthesized polymers. Furthermore, because several low-bandgap polymers incorporating benzo[1,2-b:3,4-b0]dithiophene (BDT)21,36,37 and cyclopentadithiophene (CPDT)38-41 units in their backbones exhibit high electron-donating ability and good performance when applied in PSCs, we incorporated these two electron-rich donors into our polymer backbones by conjugating them with electron-withdrawing PP moieties. Using this approach allowed us to fine-tune the absorption spectra and energy levels because of the modulated ICT strength between the PP moieties and the BDT/CPDT units, thereby leading to PSCs exhibiting varied photovoltaic performance. Because our synthesized polymers, P1 and P2 absorb too little in the visible range, where the radiated intensity of photons in the solar spectrum was quite high, due to their low-bandgap characteristics; therefore, we synthesized the three-component random copolymers P3 and P4, through further incorporation of thiophene/bithiophene segments, to extend the light-harvesting range. P3 and P4 displayed very broad absorption ranges, covering the solar spectrum from the

visible to a significant portion of the NIR region. Thus, we expected the performance of the devices prepared from P3 and P4 to be significantly improved.

Experimental Section

Materials. 1-Bromo-3-octyloxybenzene (1),42 1,2-bis(3-octy-loxyphenyl)ethane-1,2-dione (2),42 3,4-diamino-2,5-dibromo-pyridine (3),27 benzo[1,2-b:4,5-b0]dithiophene-4,8-dione (4),21 1,5-bis(trimethylstannyl)-4,8-dioctylbenzo[1,2-b:4,5-b0 ]dithio-phene (M2),21 4,4-bis(2-ethylhexyl)-2,6-bis(trimethylstannyl)-4H-cyclopenta[2,1-b:3,4-b0]-dithiophene (M3),38 and 5,50 -di-bromo[2,20]-bithiophene (M5)44 were prepared according to reported procedures. [6,6]-Phenyl-C70-butyric acid methyl ester

(PC70BM) was purchased from Nano-C.

2,5-Dibromothio-phene (M4) and all other reagents were used as received without further purification, unless stated otherwise.

Measurements and Characterization.1H and13_{C NMR}

spec-tra were recorded using a Varian UNITY 300 MHz spectro-meter. Mass spectra were obtained using a JEOL JMS-HX 110 spectrometer. Differential scanning calorimetry (DSC) was performed using a Perkin-Elmer Pyris 1 unit operated at heat-ing and coolheat-ing rates of 20 and 40°C min-1, respectively; the glass transition temperatures (Tg) were determined from the

second heating scan. Thermogravimetric analysis (TGA) was undertaken using a TA Instruments Q500; the thermal stabilities of the samples were determined under a N2 atmosphere by

measuring their weight losses while heating at a rate of 20°C min-1. Size exclusion chromatography (SEC) was performed using a Waters chromatography unit interfaced with a Waters 1515 differential refractometer; polystyrene was the standard and THF was the eluant. UV-Vis spectra of dilute chloroform solutions (1 10-5M) were measured using a Hitachi U-4100 spectrophotometer. Solid films for UV-Vis analysis were ob-tained by spin-coating dichlorobenzene (DCB) solutions of the copolymers (10 mg mL-1) onto quartz substrates. Cyclic vol-tammetry (CV) was performed using a BAS 100 electrochemical analyzer operated at a scan rate of 50 mV s-1; the solvent was anhydrous acetonitrile containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte.

The potentials were measured against a Ag/Agþ(0.01 M AgNO3)

reference electrode; ferrocene/ferrocenium ion (Fc/Fcþ) was used as the internal standard. The onset potentials were deter-mined from the intersection of two tangents drawn at the rising and background currents of the cyclic voltammogram. Topo-graphic images of the copolymer:PC70BM films (surface area:

5 5 μm2) were obtained through atomic force microscopy (AFM) in the tapping mode under ambient conditions using a Digital Nanoscope IIIa instrument. Transmission electron microscopy (TEM) images of the copolymer:PC70BM films

were recorded using a FEI T12 TEM operating at 120 KeV. The thickness of the active layer was measured using a Veeco Dektak 150 surface profiler. Hole-only mobility measurements were performed using the device structure of ITO/PEDOT:PSS/ P1-P4:PC70BM (1:4, w/w)/Au. The hole mobilities of the

blends were determined by fitting the dark J-V curve into the space-charge-limited current (SCLC) model,17,43based on the equation

J¼ 9 8ε0εrμh

V2 L3

Here ε0 is the permittivity of free space, εr is the dielectric

constant of the material,μhis the hole mobility, V is the

vol-tage drop across the device, and L is the thickness of the active layer.

Fabrication and Characterization of Photovoltaic Devices. Indium tin oxide (ITO) coated glass substrates were cleaned stepwise in detergent, water, acetone, and isopropyl alcohol (ultrasonication; 20 min each) and then dried in an oven for 1 h; subsequently, the substrates were treated with UV ozone for

10 min prior to use. A thin layer (ca. 20 nm) of poly(ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, Baytron P VP AI 4083) was spin-coated at 5000 rpm onto the ITO sub-strates. After baking at 140°C for 20 min in air, the substrates were transferred into a N2-filled glovebox. The copolymers

P1-P4 were codissolved with PC70BM in DCB (weight ratio,

1:4; total concentration, 25 mg mL-1) and stirred continuously for 12 h at 50°C. The photoactive layer was obtained by spin-coating the blend solution onto the ITO/PEDOT:PSS surface at 800 rpm for 60 s without further special treatment. The thick-nesses of the photoactive layers were ca. 75-95 nm. Finally, an Al layer (100 nm) was thermally evaporated through a shadow mask under a vacuum of less than 1 10-6Torr. The effective layer area of one cell was 0.04 cm2. The current density-voltage (J-V) characteristics were measured using a Keithley 236 source-meter. The photocurrent was measured under simulated AM 1.5 G irradiation at 100 mW cm-2using a Xe lamp-based Newport 66902 150W solar simulator. The spectrum of the solar simulator was calibrated using a PV-measurement (PVM-154) monosilicon solar cell (NREL calibrated) to minimize spectral mismatch; a silicon photodiode (Hamamatsu S1133) was em-ployed to check the uniformity of the exposed area. External quantum efficiency (EQE) was measured using a system estab-lished by Optosolar, Inc. Monochromatic light was created from a 500-W Xe lamp source passing through a monochromator. The photocurrent of the device was detected using a lock-in amplifier under short-circuit conditions by illuminating the monochromatic incident beam. A calibrated mono silicon diode exhibiting a response at 350-1000 nm was used as a reference.

5,8-Dibromo-2,3-bis(3-octyloxyphenyl)pyrido[3,4-b]pyrazine (M1). A mixture of 1 (0.91 g, 1.95 mmol), 2 (0.52 g, 1.95 mmol), and acetic acid (20 mL) was heated at 50°C for 4 h under N2. The

mixture was poured into water (100 mL) and the resulting precipitate collected by filtration. The crude product was puri-fied through column chromatography (EtOAc/hexane, 1:10) and then recrystallized twice from ethanol to afford M1 (0.91 g, 67%) as a yellow solid. 1H NMR (300 MHz, CDCl3): δ 0.87-0.91 (m, 6H), 1.30-1.44 (m, 20H), 1.69-1.78 (m, 4H), 3.83-3.89 (m, 4H), 6.95-6.99 (m, 2H), 7.15-7.28 (m, 6H), 8.76 (s, 1H).13C NMR (75 MHz, CDCl3):δ 14.1, 22.7, 26.0, 29.0, 29.2, 29.3, 31.8, 68.1, 115.6, 115.7, 117.0, 117.3, 120.1, 122.4, 122.5, 129.5, 135.8, 138.3, 138.4, 142.4, 146.2, 147.2, 156.1, 158.2, 159.1, 159.2. MS (m/z): [M]þcalcd for C35H43Br2N3O2,

697.1; found, 697. Anal. Calcd for C35H43Br2N3O2: C, 60.27; H,

6.21; N, 6.02. Found: C, 60.32; H, 6.61; N, 6.05.

Alternating Copolymer P1. A mixture of M1 (100 mg, 0.143 mmol), M2 (111 mg, 0.143 mmol), and tri(o-tolyl)phosphine (3.6 mg, 8.0 mol %) was dissolved in dry chlorobenzene (CB) (3 mL) and degassed for 15 min. Tris(dibenzylideneacetone)-dipalladium (2.6 mg, 2.0 mol %) was added under N2and then

the reaction mixture was heated at 130°C for 36 h. After cooling to room temperature, the solution was added dropwise into methanol (100 mL). The crude polymer was collected, dissolved in chlorobenzene, and reprecipitated in methanol. The solid was washed with methanol, acetone, and chloroform in a Soxhlet apparatus. The chloroform solution was concentrated and then added dropwise into methanol. Finally, the polymer was col-lected and dried under vacuum to give P1 (113 mg, 80%).1H NMR (300 MHz, CDCl3):δ 0.76-2.10 (m, 60H), 3.67-4.41 (m,

8H), 6.84-7.80 (m, 10H), 8.84 (br, 1H). Anal. Calcd: C, 74.42; H, 8.29; N, 4.27. Found: C, 73.56; H, 8.02; N, 4.23.

Alternating Copolymer P2. Using a polymerization procedure similar to that described above for P1, a mixture of M1 (120 mg, 172 mmol) and M3 (125 mg, 172 mmol) in dry CB (3 mL) was copolymerized to give P2 (110 mg, 68%).1_{H NMR (300 MHz,} CDCl3):δ 0.62-0.67 (m, 12H), 0.75-1.01 (m, 22H), 1.15-1.45 (m, 22H), 1.66-2.08 (m, 8H), 3.97 (br, 4H), 7.07 (br, 2H), 7.36-7.48 (m, 6H), 7.88 (s, 1H), 8.67 (s, 1H), 9.18 (s, 1H). Anal. Calcd: C, 76.63; H, 8.68; N, 4.47. Found: C, 72.26; H, 8.08; N, 4.11.

Alternating Copolymer P3. Using a polymerization procedure similar to that described above for P1, a mixture of M1 (60.0 mg, 0.086 mmol), M2 (133 mg, 0.172 mmol), and M4 (20.8 mg, 0.086 mmol) in dry CB (2.8 mL) was copolymerized to give P3 (106 mg, 82%). 1H NMR (300 MHz, CDCl3): δ 0.77-0.95

(m, 46H), 1.18-1.48 (m, 146H), 3.94-4.32 (m, 12H), 7.05-7.47 (m, 14H), 8.98 (br, 1H). Anal. Calcd: C, 72.32; H, 7.94; N, 2.78. Found: C, 72.00; H, 7.88; N, 2.87.

Alternating Copolymer P4. Using a polymerization procedure similar to that described above for P1, a mixture of M1 (72.1 mg, 0.103 mmol), M3 (151 mg, 0.206 mmol), and M5 (33.5 mg, 0.103 mmol) in dry CB (2.8 mL) was copolymerized to give P4 (108 mg, 70%). 1_{H NMR (300 MHz, CDCl} 3): δ 0.66-1.15 (m, 68H), 1.32-1.53 (m, 18H), 1.75-1.98 (m, 12H), 3.97-4.10 (m, 4H), 6.95-7.17 (m, 8H), 7.24-7.55 (m, 6H), 7.82 (br, 1H), 8.62 (s, 1H), 9.16 (s, 1H). Anal. Calcd: C, 74.20; H, 8.10; N, 2.79. Found: C, 73.57; H, 7.97; N, 2.81.

Results and Discussion

Synthesis and Characterization of the Copolymers. As indicated in Scheme 1, we prepared the symmetric 1,2-dike-tone 2 presenting two 3-octyloxyphenyl groups through a Grignard reaction between 3-octyloxyphenylmagnesium bromide (1) and oxalyl chloride in the presence of CuBr and LiBr. Condensation of 2 with the diamine 3 afforded the monomer M1; the distannyl monomer M2 was prepared through dilithiation of compound 4 with n-BuLi, followed by quenching with trimethylstannyl chloride. With the mono-mers M1-M5 in hand, we prepared the PP-based alternating/ random copolymers P1-P4 through Stille polycondensa-tions using various feed molar ratios of the corresponding monomers (Scheme 2). P1 and P3, which featured electron-donating BDT units, had weight-average molecular weights (Mw) of 398 and 563 kg mol-1, respectively, and

polydisper-sities of 2.65 and 2.54, respectively, as determined through gel permeation chromatography (GPC) using polystyrene standards. The donor moieties of the polymers P2 and P4 were highly coplanar fused CPDT units; their values of Mw

were 101 and 89 kg mol-1, respectively, with polydispersities of 1.65 and 1.79, respectively. The relatively high molecu-lar weights of P1 and P3, relative to those of P2 and P4, presumably resulted from the high reactivity of the BDT moieties in their polymerization reactions. The solubility of the copolymers in various solvents was determined at a concentration of 5 mg/mL. We found that P1 and P3 were completely soluble in THF, CHCl3, CB, or DCB at room

temperature and became soluble in toluene at 60 °C; P2 and P4 were completely soluble in THF, CHCl3, CB, DCB,

or toluene at room temperature. We also synthesized

nonsubstituted polymers through reacting the PP unit with-out any alkyl chain with the BDT or CPDT units, and both of them have insoluble precipitation in toluene, THF, CB or DCB. We characterized the synthesized monomers and polymers using1H and13C NMR spectroscopy, mass spec-trometry, and elemental analysis. We estimated the copoly-mers’ composition using the ratio of the integrated value of the proton peak on the PP unit (-N-CH-) to that of the proton peak on the thiophene/bithiophene units (thiophene-H) from their NMR spectra; the output composition (m:n) for P3 and P4 was 0.97:1 and 1:0.98, respectively, and they are very close to the feed ratios.

Scheme 1. Synthetic Routes of the Monomers

Scheme 2. Synthetic Routes of the Copolymers.a

a

We investigated their thermal behavior through DSC analysis, which revealed no obvious thermal transitions for P1 and P3 in the temperature range from 40 to 300°C; in contrast, P2 and P4 exhibited distinct glass transition tem-peratures (Tg) at 68 and 77°C, respectively (Figure 1). Thus,

P1-P4 displayed amorphous properties. We attribute the slightly higher value of Tgobtained for P4 to the presence in

its backbone of the more rigid bithiophene segment, which presumably enhanced the chain rigidity45while reducing the number of the alkyl groups in the polymer chain. Figure 2 displays the TGA curves of these polymers. The 5% weight loss temperature (Td) of each polymer was greater than

330°C; such good thermal stability is an important prere-quisite for a polymer’s application to PSCs. Table 1 sum-marizes the molecular weights, polydispersities, and thermal properties of the polymers.

Photophysical Properties. Figure 3 presents absorption spectra of the copolymers P1-P4, recorded for both dilute (1 10-5 M) CHCl3 solutions and solid films; Table 2

summarizes the spectral data. In solution, P1 and P2 dis-played absorption peaks close to 400 nm that we assign to their localizedπ-π* transition bands and at 668 and 776 nm, respectively, that we attribute to ICT interactions between

the PP acceptor moieties and the BDT/CPDT donor units. The absorption maximum of P2 was red-shifted by ca. 108 nm, relative to that of P1, because of the stronger ICT effect in P2 than that in P1, indicating that the fused CPDT unit has stronger electron-donating ability than does the BDT unit. The three-component copolymer P3 exhibited two combined, but distinct, absorption bands of almost equal intensity, with absorption maxima at 468 and 616 nm, respectively. We attribute the higher-energy band to the absorption of the BDT-thiophene segment, consistent with reports of related polymers in the literature,36and the lower-energy band to ICT of the BDT-PP segment. The spectrum of P3 exhibited an absorption maximum having a hypso-chromic shift of ca. 50 nm relative to that of P1 because the incorporation of the higher-energy BDT-thiophene segment shifted the absorption to the shorter wavelength region. P4 exhibited similar optical properties as those of P3, with absorption maxima at 505 and 676 nm that we attribute to the CPDT-bithiophene and CPDT-PP segments, respec-tively. The absorption spectra of P1-P4 in the solid state were similar to their corresponding solution spectra, with slight red shifts of ca. 6-18 nm of their absorption maxima, indicating there some intermolecular interactions occurred in their solid states. Figure 3 reveals that P3 and P4 absorbed broadly in the region 350-800 nm, extending from the visible to a portion of the NIR region of the solar spectrum; such an absorption is a desirable property for generating higher photocurrents in solar cells.

The optical bandgaps (Egopt) of P1-P4, estimated from the

onsets of absorption in their solid films, were 1.58, 1.46, 1.60, and 1.49 eV, respectively. Generally, the incorporation of electron donating groups reduces the optical bandgaps by raising the energy levels of the highest occupied molecular orbital (HOMO).46We suspect that the optical bandgaps of P2 and P4 were lower than those of P1 and P3 because the incorporation of the more strongly electron-donating and highly coplanar CPDT units resulted in stronger ICT inter-actions when conjugated with the PP acceptors.12,47

Figure 1. DSC traces of the copolymers P2 and P4, recorded at a heating rate of 20°C min-1under a N2atmosphere.

Figure 2. TGA thermograms of the copolymers P1-P4, recorded at a heating rate of 20°C min-1under a N2atmosphere.

Table 1. Polymerization Data and Thermal Properties of the Copolymers feed ratio copolymer M1 M2 M3 M4 M5 Mw (104) Mn (104) PDI Tg (°C) Td (°C) P1 0.5 0.5 39.8 15.0 2.65 n.d.a 332 P2 0.5 0.5 10.1 6.1 1.65 68 412 P3 0.25 0.5 0.25 56.3 22.1 2.54 n.d.a 330 P4 0.25 0.5 0.25 8.9 5.0 1.79 77 410

a_{Glass transition temperature was not detectable.}

Figure 3. UV-vis absorption spectra of the copolymers P1-P4 (a) in dilute chloroform solutions (1 10-5M) (b) as solid films.

Electrochemical Properties. We used CV to investigate the redox behavior of the copolymers and obtain their HOMO and lowest occupied molecular orbital (LUMO) energy levels. Figure 4 displays the electrochemical behavior of the copolymers as solid films; Table 2 summarizes the relevant data. All synthesized polymers exhibited partially reversible oxidations, except for P3, which underwent an irreversible oxidation. The onset potentials of P1 and P3 were 0.42 and 0.40 V, respectively, arising essentially from the oxidation of the electron-donating BDT units,36although the cyclic vol-tammograms revealed indistinct oxidation peaks of the BDT units intermixed in the hybrid oxidation region. P2 and P4 exhibited oxidation onsets of 0.38 and 0.33 V, respectively, resulting from the oxidation of the CPDT units.40In their re-duction traces, P1-P4 exhibited reversible rere-duction peaks, with similar onset potentials between-1.48 and -1.52 V, which we assign to reductions of the electron-accepting PP moieties; these values are comparable with those reported previously for other PP-containing copolymers.27 _{On the}

basis of these onset potentials, we estimated the HOMO and LUMO energy levels according to the energy level of the ferrocene reference (4.8 eV below vacuum level).23,48,55 Accordingly, the HOMO energy levels of P1-P4 were 5.22, 5.18, 5.20, and 5.13 eV, respectively, implying that they varied with respect to the modulated ICT strengths resulting from the presence of the electron donors exhibiting various electron-donating abilities.12,20,47The LUMO energy levels of P1-P4 were all located within a reasonable range (3.28-3.32 eV) and were significantly greater than that of PC70BM

(ca. 4.2 eV);13,20 thus, we would expect efficient charge transfer/dissociation to occur in their devices.13,49-51 In addition, the electrochemical bandgaps (Egec) of P1-P4,

estimated from the difference between the onset potentials for oxidation and reduction, were in the range 1.83-1.92 eV; i.e., they were slightly larger than their corresponding optical bandgaps. This discrepancy between the electrochemical and optical bandgaps presumably resulted from the exciton binding energies of the polymers31 and/or the interface barrier for charge injection.52

Hole Mobility. Figure 5 displays the hole mobilities of the devices incorporating the P1-P4:PC70BM blends with a

blend ratio of 1:4 (w/w). The hole mobilities of P2 and P4 blends were 1.67 10-4and 2.36 10-4cm2V-1s-1, respectively while that of P1 and P3 blends are 7.81 10-5 and 1.37 10-4cm2V-1s-1, respectively. The fact that the hole mobility of the P4:PC70BM blend being higher than that

of P1-P3 blends is probably due to the more homogeneous morphology of the P4 blend.

Photovoltaic Properties. Next, we investigated the photo-voltaic properties of the polymers in bulk heterojunction solar cells having the sandwich structure ITO/PEDOT:PSS/ polymer:PC70BM (1:4, w/w)/Al, where the photoactive

layers had been spin-coated from dichlorobenzene solutions. Figure 6 presents the J-V curves of these PSCs; Table 3 summarizes the data. The devices prepared from P1-P4: PC70BM blends exhibited open circuit voltages (Voc) of 0.74,

0.66, 0.71, and 0.63 V, respectively; each value is related to the difference between the HOMO energy level of the poly-mer and the LUMO energy level of PC70BM.51We suspect

that the P4 blend provided the lowest value of Vocbecause of

its relatively higher-lying HOMO energy level. The short-circuit current densities (Jsc) of the devices incorporating the

P3 and P4 blends (6.40 and 10.85 mA cm-2, respectively) were significantly greater than those of P1 (4.69 mA cm-2)

Table 2. Optical and Redox Properties of the Copolymers absorption,λmax(nm) solution film Eg opt (eV)a Eonset ox (V) Eonset red

(V) HOMO (eV)b LUMO (eV)b Eg ec (eV)c P1 668 680 1.58 0.42 1.48 5.22 3.32 1.90 P2 776 779 1.46 0.38 1.49 5.18 3.31 1.87 P3 468, 616 494, 634 1.60 0.40 1.52 5.20 3.28 1.92 P4 505, 676 529, 690 1.49 0.33 1.50 5.13 3.30 1.83

a_{Estimated from the onset wavelength absorptions of the solid films.}b_{Calculated from the corresponding onset potentials.}c_{Calculated form the}

difference between the onset potentials for oxidation and reduction.

Figure 4. Cyclic voltammograms of the copolymers P1-P4 as solid films.

Figure 5. Dark J-V curves for the hole-dominated carrier devices incorporating the P1-P4:PC70BM blends, each prepared at a blend

ratio of 1:4 (w/w).

Figure 6. J-V characteristics of PSCs incorporating P1-P4:PC70BM

and P2 (6.26 mA cm-2). Because the light-harvesting cov-erages of the former pair were significantly broader than those of the latter pair, more available photos from the solar radiation could be absorbed by P3 and P4, thereby leading to higher photocurrents. The device incorporating the P3 blend exhibited a value of Vocof 0.71 V, a value of Jscof 6.40 mA

cm-2, a fill factor (FF) of 0.517, and a PCE of 2.35%; the device featuring the P4 blend displayed even greater perfor-mance, with values of Voc, Jsc, FF, and PCE of 0.63 V, 10.85

mA cm-2, 0.461, and 3.15%, respectively. Figure 7 displays the external quantum efficiency (EQE) curves of the devices incorporating the P1-P4:PC70BM blends at weight ratio

of 1:4. These devices exhibited a significantly broad EQE responses extending from the visible to the NIR, with a maximum intensity ranging between 460 and 480 nm. We attribute their higher EQE responses in the visible region to the corresponding higher absorbance of the blend, resulted from both the intrinsic absorption of the polymer and the presence of a high content of PC70BM, which also absorbs

significantly at 400-500 nm. In contrast, these devices dis-played relatively lower EQE responses at wavelengths above 700 nm because of the moderate absorbance of the polymer blend. The device of P4 blend exhibited a higher EQE response than that of other polymer blends, with a maximum of 54% at 480 nm, corresponding to its higher photocurrent. The theoretical short-circuit current density obtained from integrating the EQE curve of the P4 blend is 9.90 mA/cm2, that is reasonably agreed to the measured Jscof 10.85 mA/

cm2 _{AM 1.5G. For the P1-P3 blends, the discrepancy}

between the theoretical and the measured Jsc is around

10%. To optimize the device performance of the most efficient polymer P4, devices of various compositions incor-porating the P4:PC70BM blend were investigated. Figure 8

presents the J-V curves of the PSCs; Table 4 summarizes the data. The power conversion efficiencies of P4 devices in-creased with the weight ratio of PC70BM owing to the

enhanced photocurrent and fill factor. A high percentage (80%) of PC70BM was required in the P4 blend to obtain

such high efficiency. Similar phenomena have been observed in other amorphous polymer:PC70BM systems.2,13,42

Moreover, when exploring the decisive factors affecting the efficiencies of PSCs we must consider not only the

absorption and energy levels of polymers but also the surface morphologies of the polymer blends.53,54_{Figure 9 displays}

the surface morphologies determined through AFM mea-surements. Samples of the P1-P4:PC70BM blends were

prepared using procedures identical to those used to fabri-cate the active layers of the devices. In each case, we observed coarse phase separation in the images of the polymer blends, except for the P4 blend, which displayed a moderately homogeneous morphology. The P1 and P2 blends feature somewhat larger domains than those of the P3 and P4 blends; the root-mean-square roughness of the former pair were 4.06 and 8.05 nm, respectively, significantly larger than those of the latter pair (3.50 and 1.53 nm, respectively). The greater phase segregation and rougher surfaces of the P1 and P2 blends presumably arose because of poor miscibility with PCp70BM;a result of the greater numbers of alkyl groups

on the repeat units in these polymer chains.55,56This phe-nomenon was further confirmed by their respective TEM images. Figure 10 shows the TEM images of P1-P4: PC70BM blends, where the aggregated PC70BM size in the

P2 blend is in the range of 200-500 nm and is significantly larger than that in the P4 blend (ca. 70 nm), indicating the relatively better miscibility between P4 and PC70BM because

of the smaller number of alkyl chains. A similar phenomenon was also observed for comparing P1 with P3 blends; the aggregated PC70BM size in P1 was larger than that in P3.

Accordingly, we suspect that the miscibility of the P3 and P4 blends increased after the incorporation of their correspond-ing thiophene and bithiophene segments because of the reduced number of alkyl groups in the backbone. The AFM and TEM images clearly reveal that the P4 blend was more homogeneous and smoother than the other three; therefore, its device provided the highest photocurrent. We suggested that the lower power conversion efficiencies of P1 and P2 based heterojunction devices as compared to that of P3 and P4 based devices result from not only their narrower absorption region but more likely from the phase separation within the blends, as demonstrated by relatively larger PC70BM aggregations in the P1 and P2 blends. Even though

a few BDT37or CPDT39based polymers have demonstrated higher devices’ power conversion efficiencies, we have used a

Table 3. Photovoltaic Properties of Polymer Solar Cells Incorporating P1-P4:PC70BM Blends Prepared at 1:4 Weight Ratios polymer thickness (nm) Voc(V) Jsc(mA cm-2) FF PCE (%)

P1 95 0.74 4.69 34.5 1.20

P2 80 0.66 6.26 44.1 1.82

P3 92 0.71 6.41 51.7 2.35

P4 75 0.63 10.85 46.1 3.15

Figure 7. EQE curves of PSCs incorporating P1-P4:PC70BM blends,

each prepared at a blend ratio of 1:4 (w/w).

Figure 8. J-V characteristics of PSCs incorporating P4:PC70BM

blends prepared at various weight ratios (w/w).

Table 4. Photovoltaic Properties of Polymer Solar Cells Incorporating P4:PC70BM Blends Prepared at Various Weight Ratios P4:PC70BM (w/w) thickness (nm) Voc(V) Jsc(mA cm-2) FF PCE (%) 1:1 84 0.63 7.01 33.3 1.47 1:2 80 0.63 9.16 42.1 2.43 1:3 78 0.62 9.92 43.6 2.68 1:4 75 0.63 10.85 46.1 3.15

different acceptor, PP, for our synthesized polymers; the differences in the intrinsic physical/electronic properties and the blend morphology between our PP based polymers and the BDT or CPDT based polymers result in different device’s power conversion efficiencies.

Conclusion

We have prepared a series of new pyrido[3,4-b]pyrazine (PP)-based low-bandgap copolymers conjugated with electron-donat-ing benzo[1,2-b:3,4-b0]dithiophene (BDT) and/or cyclopenta-dithiophene (CPDT) units. The absorption spectra, bandgaps, and energy levels of the polymers can be tuned by using the two different donors, which exhibit different electron-donating abili-ties. Because the CPDT unit exhibited higher electron-donating ability relative to that of the BDT unit, its presence resulted in stronger ICT interactions when conjugated with PP moieties. Therefore, the absorptions of the CPDT-containing polymers were significantly red-shifted relative to those of the BDT-containing

polymers. Moreover, P3 and P4 absorbed broadly, covering the solar spectrum from the visible to the NIR, because of the presence of the thiophene/bithiophene segments in their polymer backbones; accordingly, their corresponding devices functioned with improved photocurrents. Furthermore, among our four tested polymer blends, the P4 blend possessed the most homo-geneous surface, presumably contributing to its more efficient carrier transport and dissociation. As a result, the best device performance was obtained when using the P4:PC70BM blend,

characterized by a high short-circuit current of 10.85 mA cm-2 and a resulting PCE of 3.15%.

Acknowledgment. We thank the National Science Council for financial support through project NSC 98-2120-M-009-006.

Supporting Information Available: Figures showing 1_H

NMR spectra of the synthesized polymers. This material is available free of charge via the Internet at http://pubs.acs.org. Figure 9. Topographic AFM images of copolymer:PC70BM (1:4, w/w) blends incorporating (a) P1, (b) P2, (c) P3, and (d) P4.

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