All-Polymer Solar Cells Based on Fully Conjugated Block Copolymers

Dec 10, 2012 - All-polymer solar cells using the P3HT:P3HT-PNBI-P3HT blend films ..... Jeong , Sooyong Lee , Jooyeok Seo , Hyemi Han , Youngkyoo Kim...
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All-Polymer Solar Cells Based on Fully Conjugated Block Copolymers Composed of Poly(3-hexylthiophene) and Poly(naphthalene bisimide) Segments Kazuhiro Nakabayashi* and Hideharu Mori Department of Polymer Science and Engineering, Graduate School of Science and Engineering, Yamagata University, 4-3-16, Jonan, Yonezawa 992-8510, Japan S Supporting Information *

ABSTRACT: Fully conjugated donor−acceptor block copolymers composed of poly(3-hexylthiophene) and poly(naphthalene bisimide) segments, P3HT-PNBI-P3HTs, were synthesized using quasi-living Grignard metathesis polymerization and the Yamamoto coupling reaction. Broad absorption in a range of 350−850 nm, which corresponded to the optical band gap of 1.46 eV, was observed for the P3HT-PNBI-P3HT thin films. In addition, the optical band gap decreased to 1.38 eV (the light absorption band extended to 893 nm) by thermal annealing, which was much smaller than those of previously reported donor− acceptor block copolymers (1.6−1.8 eV). The annealed P3HT:P3HT-PNBI-P3HT blend film (1:1 by weight) also exhibited broad absorption in the range of 350−950 nm. Cyclic voltammetry demonstrated that the P3HT-PNBI-P3HT thin films exhibited the oxidation and reduction properties derived from the P3HT and PNBI segments. The HOMO and LUMO levels were in the range of 5.57−5.60 and 4.22−4.27 eV, respectively. All-polymer solar cells using the P3HT:P3HT-PNBI-P3HT blend films achieved a power conversion efficiency of 1.28% with open-circuit voltage of 0.56 V, short-current of 4.57 mA/cm2, and fill factor of 0.50.

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INTRODUCTION Organic photovoltaics (OPVs) based on conjugated polymeric materials have received considerable attention in recent years because of their advantages (e.g., low cost, light weight, flexibility, and facile large-scale fabrication) compared to silicon-based solar cells.1−6 To date, polymer/fullerene (fullerene-based) OPVs, in which the active layers are composed of hole-transporting (i.e., donor; D) polymeric materials and electron-transporting (i.e., acceptor; A) fullerene derivatives, have achieved power-conversion efficiencies (PCEs) of over 7%.7,8 The four fundamental steps of the energy conversion process are as follows: (i) absorption of light and generation of excitons in the donor domains, (ii) diffusion of the excitons to the D−A interfaces (the exciton diffusion lengths before recombination are ca. 10−20 nm), (iii) dissociation of the excitons and generation of charge, and (iv) charge transport and charge collection. The development of organic solar cells requires donor and acceptor materials that can efficiently facilitate these four fundamental steps. A large variety of donor materials, such as low-band-gap polymers1−10 and poly(3© 2012 American Chemical Society

hexylthiophene) (P3HT)-based rod−coil block copolymers,11−14 have in fact been developed, thereby facilitating significant improvement of the PCEs. On the other hand, acceptor materials are, compared to donor materials, far less developed, and the fullerene derivative [6,6]-phenyl C61 butyric acid methyl ester (PCBM) is the only conventional acceptor material for organic solar cells.15 One of the major reasons for using PCBM as an acceptor material in OPVs is its high electron mobility (ca. 0.002 cm2/V·s).16 However, there are also considerable drawbacks for use in OPVs: (i) negligible light absorption in the visible-near IR regions, (ii) relatively poor photochemical and chemical stability, (iii) less compatibility with donor polymeric materials, and (iv) high cost for synthesis and purification. The investigation of non-fullerene acceptor polymeric materials that can potentially replace PCBM in OPVs is, thus, increasingly necessary, which gives rise to the Received: October 17, 2012 Revised: November 28, 2012 Published: December 10, 2012 9618

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atmosphere. After the removal of solvents, the crude product was purified by column chromatography (dichloromethane:hexane = 1:1.5) to yield a red solid (1.42 g, 93%). 1H NMR (CDCl3, δ, ppm): 8.74 (s, 2H), 7.14 (s, 4H), 4.07 (d, 4H), 2.69 (t, 4H), 1.95 (m, 2H), 1.70 (m, 4H), 1.23 (m, 98H), 0.88 (m, 18H). N,N′-Bis(2-decyl-1-tetradecyl)-2,6-bis(5′-bromo-4′hexylthiophenyl)naphthalene-1,4,5,8-tetracarboxylic Acid Bisimide (2). To the dry THF solution (60 mL) of 1 (1.42 g, 1.1 mmol) was added N-bromosuccinimide (NBS) (0.8 g, 4.4 mmol) under a nitrogen atmosphere. The resulting solution was refluxed for 16 h. After cooling to room temperature, the solution was washed with brine, dried over MgSO4, and evaporated. The residue was purified by column chromatography (dichloromethane:hexane = 1:2) to yield a red solid (1.49 g, 93%). 1H NMR (CDCl3, δ, ppm): 8.71 (s, 2H), 7.05 (s, 2H), 4.08 (d, 4H), 2.64 (t, 4H), 2.57 (s, 6H), 1.94 (m, 2H), 1.63 (m, 4H), 1.22 (m, 98H), 0.88 (m, 18H). D−A Block Copolymer. By way of example, synthesis of P3HT1PNBI-P3HT1 is shown as follows: The dry THF solution (2 mL) of Ni(COD)2 (0.062 g, 0.224 mmol), 2,2′-bipyridyl (bpy) (0.035 g, 0.224 mmol), and cyclooctadiene (COD) (0.027 mL, 0.224 mmol) was stirred at 80 °C for 30 min prior to the inject of the dry THF solution (8 mL) of 2 (0.1 g, 0.07 mmol) and P3HT1 (0.029 g, 0.175 mmol based on the repeating unit) under a nitrogen atmosphere, and the reaction was continued at 80 °C for 24 h. After cooling to room temperature, the reaction mixture was poured into methanol/ hydrochloric acid. The precipitated solid was purified by Soxhlet extraction with acetone and chloroform for 24 h each. The chloroform fraction was filtered with Celite, concentrated, and then poured into methanol to give red powders (94 mg, 85%). 1H NMR (CDCl3, δ, ppm): 8.85 (s), 6.98 (s), 4.12 (brs), 2.80 (m), 2.00 (brs), 2.80 (m), 2.00 (brs), 1.71 (m), 1.15−1.60 (m), 0.86 (m). m/n = 56/8 (calculated from 1H NMR spectra). Mn = 21 800, Mw/Mn = 1.28 (determined by SEC). Device Fabrication and Measurements. The typical procedure of ITO/PEDOT:PSS/P3HT:P3HT-PNBI-P3HT/Ca/Al architecture is as follows: Commercially available prepatterned 15 Ω/□ sheet resistance indium tin oxide (ITO) substrates were cleaned and plasmaetched piror to coating with a 30 nm layer of poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), spin-coated at 4000 rpm for 40 s, and subsequently annealed under flowing nitrogen at 120 °C for 10 min. Substrates were allowed to cool under a nitrogen atmosphere and then transferred to a glovebox for active layer deposition. P3HT (Rieke Metals, Mw = 50 000−70 000):P3HT-PNBI-P3HT blend−chlorobenzene solution was spincoated at 700 rpm for 90 s, and the active layer was annealed for 15 min. The blend solution that 5 mg of each polymer dissolved in 1 mL of chlorobenzene (1:1 by weight, concentration = 10 mg/mL) was prepared in a glovebox. Then the top electrode consisted of Ca interlayer (20 nm) and Al electrode (80 nm) was vacuum-deposited. The J−V characteristics of the devices were measured by using a direct-current voltage and a current source/monitor (Bunko-Keiki, BSO-X500L) in a nitrogen atmosphere under AM1.5G simulated solar light at 100 mW cm−2. The light intensity was corrected with a calibrated silicon photodiode reference cell (Bunko-Keiki, BS-520). Characterization. The 1H (400 MHz) NMR spectrum was recorded with a JEOL JNM-ECX400. The UV−vis spectra were recorded on a JASCO V-630BIO UV−vis spectrophotometer. The number-average molecular weight (Mn) and molecular weight distribution (Mw/Mn) were estimated by size exclusion chromatography (SEC) using a HLC-8220 system. The column set was as follows: a guard column (TSK guard column HXL-L) and three consecutive columns (G4000HXL, G3000HXL, and G2000HXL) eluted with THF at a flow rate of 1.0 mL/min. Polystyrene standards were employed for calibration. Cyclic voltammetry experiments for the polymer thin films were performed on a BAS electrochemical analyzer (model 660C). A three-electrode cell was used with platinum electrodes as both the counter and working electrodes. Thermal analysis was performed on a Seiko EXSTAR 6000 DSC 6200 at a heating rate of 10 °C/min for differential scanning calorimetry (DSC) under a nitrogen atomsphere. Silver/silver ion (Ag in 0.1 M AgNO3

possibility of fabricating all-polymer (polymer/polymer) solar cells. All-polymer solar cells offer potential advantages over conventional fullerene-based OPVs, such as more efficient light absorption due to acceptor polymer and relatively high opencircuit voltages. The PCEs of all-polymer solar cells are, however, mostly lower than 2%, which is far less than those of fullerene-based OPVs.17−21 Thus, the further development of non-fullerene acceptor materials themselves and the establishment to fabricate all-polymers solar cells are a currently active pursuit. Block copolymers that contain both donor and acceptor segments (D−A block copolymers) are promising candidates for use as non-fullerene acceptor materials. The acceptor (electron-transporting) properties are provided by the acceptor segments, and the D−A architecture leads to broad light absorption to the visible-near IR region. To date, several rod− coil D−A block copolymers22−24 have been reported; however, despite their interesting acceptor properties derived from the acceptor segments, the performance in OPVs was limited because of the presence of the flexible and aliphatic coil segments, which act as insulators. Based on these considerations, fully conjugated D−A block copolymers (rod−rod D−A block copolymers) are expected to be the ideal block copolymer structure for use as non-fullerene acceptor materials in OPVs. Nevertheless, few reports on fully conjugated D−A block copolymers have been established.25−28 This may result from the fact that, to date, the development of fully conjugated D−A block copolymers, including the synthetic methodology, remains a challenging area. Herein, we designed novel fully conjugated D−A block copolymers, poly(3-hexylthiophene)-block-poly(naphthalene bisimide)-block-poly(3-hexylthiophene) P3HT-PNBI-P3HTs, which consist of regioregular P3HT-based donor and PNBIbased acceptor segments. Heteroaromatic moieties containing nitrogen atoms (e.g., pyridine,29 quinoline,30 perylene bisimide,31−33 naphthalene bisimide,32,34−36 and diketopyrrolopyrrole37,38) are well-known to exhibit electron-transporting properties. Among them, the high electron mobilities of PNBI-based semiconducting materials (ca. 0.06 cm2/V·s)32 is a noteworthy issue, discussing alternatives of PCBM with high electron mobility. P3HT-PNBI-P3HTs were synthesized from regioregular P3HT and N,N′-bis(2-decyl-1-tetradecyl)-2,6-bis(5′-bromo-4′-hexylthiophenyl)naphthalene-1,4,5,8-tetracarboxylic acid bisimide in the presence of a nickel catalyst under Yamamoto coupling conditions to yield the desired block copolymers in high yield (>85%). As expected, broad light absorption in a range of 350−850 nm was observed in the P3HT-PNBI-P3HTs thin films. The electrochemical properties and the performances of all-polymer solar cells using P3HTPNBI-P3HTs were investigated in detail.

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EXPERIMENTAL SECTION

Materials. All reagents and solvents were used as received unless otherwise stated. Tetrahydrofran (THF) was refluxed over sodium benzophenone under a nitrogen atmosphere for 2 h and then distilled just before use. Poly(3-hexylthiophene)s (P3HTs),39 N,N′-bis(2decyl-1-tetradecyl)-2,6-dibromonaphthalene-1,4,5,8-tetracarboxylic acid bisimide (NBI),34 and 5-tributylstannyl-3-hexylthiophene40 were synthesized according to previous literatures. N,N′-Bis(2-decyl-1-tetradecyl)-2,6-bis(4′-hexylthiophenyl)naphthalene-1,4,5,8-tetracarboxylic Acid Bisimide (1). The dry toluene solution (60 mL) of NBI (1.32 g, 1.2 mmol), 5tributylstannyl-3-hexylthiophene (1.65 g, 3.6 mmol), and Pd(PPh3)4 (69 mg, 0.06 mmol) was refluxed for 16 h under a nitrogen 9619

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Scheme 1. Synthesis of P3HT-PNBI-P3HTs

solution) was used as the reference electrode. Ferrocene/ferrocenium (Fc/Fc+) was used as an internal standard. The potential values obtained in reference to Ag/Ag+ were converted to the values relative to the saturated calomel electrode (SCE). Tapping mode AFM observation was performed with an Agilent AFM 5500, using microfabricated cantilevers with a force constant of ∼34 N/m.

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RESULTS AND DISCUSSION Synthesis and Characterization. N,N′-Bis(2-decyl-1tetradecyl)-2,6-dibromonaphthalene-1,4,5,8-tetracarboxylic acid bisimide (NBI) was synthesized according to previous literature.34 Then N,N′-bis(2-decyl-1-tetradecyl)-2,6-bis(5′bromo-4′-hexylthiophenyl)naphthalene-1,4,5,8-tetracarboxylic acid bisimide (2) was synthesized using the Stille coupling reaction between NBI and 5-tributylstannyl-3-hexylthiophene, followed by bromination. As can be seen in Scheme 1, the synthetic route to poly(3-hexylthiophene)-block-poly(naphthalene bisimide)-block-poly(3-hexylthiophene) (P3HTPNBI-P3HTs) consists of a two-step reaction. In the first step, two monobromo-terminated regioregular poly(3hexylthiophene)s with different molecular weights (P3HT1 and P3HT2) were synthesized from 2-bromo-5-iodo-3hexylthiophene using the quasi-living Grignard metathesis method, which allows the precise tuning of the molecular weight by adjusting the ratios of the initiator and monomer.39,41 Their precise polymerization degrees (PDs) were estimated to be 28 and 18 by the integration of the two small triplets at 2.53 and 2.61 ppm (both 2H) compared to the integration of the peaks of the methylene protons of the repeating units at 2.80 ppm (2H) in the 1H NMR spectra (Figure 1).41 These PDs corresponded to molecular weights of 4600 and 3000, respectively (Table 1). In the second step, D−A block copolymers (P3HT1-PNBIP3HT1 and P3HT2-PNBI-P3HT2) were synthesized from P3HT and 2 by Yamamoto coupling reaction (Scheme 1). After the reaction, the obtained products were purified by Soxhlet extraction with acetone and chloroform to remove low molecular weight products and metal catalysts. The chloroform fraction was then concentrated and poured into methanol to

Figure 1. 1H NMR spectra (CDCl3) of P3HT2 (top) and P3HT2PNBI-P3HT2 (bottom).

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Table 1. Molecular Weights of P3HTs and P3HT-PNBIP3HTs calcd from 1H NMR P3HT1 P3HT2 P3HT1-PNBI-P3HT1 P3HT2-PNBI-P3HT2 a

mol wt

PD

4600 3000 18700 16700

28/0 18/0 56/8 36/9

determined by SEC Mn (Mw/Mn)a 9400 6300 21800 26000

(1.06) (1.07) (1.28) (1.60)

Determined by SEC using polystyrene standards in THF.

yield red powders. In the 1H NMR spectrum of P3HT2-PNBIP3HT2, two peaks corresponding to the aromatic rings of the P3HTs and PNBI segments are observed at 6.97 and 8.85 ppm (see Figure S1 in Supporting Information), and the peak at 2.53 ppm (a″) disappears. These results indicate that the formation of a linkage between the P3HT and PNBI segments was successfully carried out to yield the desired block copolymers. Note that the obtained products in this system are technically a mixture of desired block copolymers (main products) and byproducts (e.g., coupled-P3HT, P3HT-PNBI diblock copolymers), and yet, the possible reaction to form byproducts could be controlled by optimizing the reaction conditions. The sharp and unimodal profile of the obtained block copolymer in the size exclusion chromatography (SEC) analysis, which is clearly shifted to shorter elution times as compared to P3HT, also supports the suppression to form byproducts (Figure 2).

Figure 3. DSC profiles of (i) PNBI, (ii) P3HT, and (iii) P3HT1PNBI-P3HT1.

ized in Table 2. In the solution state, broad absorption spectra to ca. 700 nm are observed for P3HT1-PNBI-P3HT1 and Table 2. Optical Properties of Polymer Thin Filmsa λmax (nm) P3HT1-PNBI-P3HT1b P3HT1-PNBI-P3HT1 (50 °C)c P3HT1-PNBI-P3HT1 (100 °C)c P3HT1-PNBI-P3HT1 (200 °C)c P3HT2-PNBI-P3HT2b P3HT2-PNBI-P3HT2 (200 °C)c

379, 379, 378, 378, 383, 391,

557, 603 557, 603 554 524, 547 546 521

λedge (nm)

Egopt (eV)d

750 750 775 838 850 893

1.65 1.65 1.60 1.48 1.46 1.39

Thin films were prepared from the chloroform solutions. bWithout thermal annealing. cAnnealed for 30 min under a nitrogen atmosphere. d Calculated from Egopt = 1240/λedge (eV). a

P3HT2-PNBI-P3HT2, which are likely to be superpositions of the homopolymer spectra (see Figure S2 in Supporting Information). In the thin film state, each polymer exhibits broadened and red-shifted absorption spectra compared to those found for the solution state (Figure 4a). Peaks at ca. 610 nm due to the π−π stacking of P3HT segments are clearly observed for the P3HT1-PNBI-P3HT1 and P3HT2-PNBIP3HT2 thin films, and their optical band gaps (Egopt) were estimated to be 1.65 and 1.46 eV, respectively, from their λedge values (750 and 850 nm). The absorption bands of P3HT2PNBI-P3HT2 thin film in the ranges of 350−500 and 650−800 nm, which are derived from PNBI segments, are stronger than those of the P3HT1-PNBI-P3HT1 thin film because of the higher content of PNBI segments in P3HT2-PNBI-P3HT2. These results demonstrate that the absorption behavior strongly depends on the composition of each segment (m/n) in P3HT-PNBI-P3HTs. The effects of thermal annealing on the absorption spectra of the P3HT-PNBI-P3HT thin films are presented in Figure 4b. The intensity of the two absorption bands derived from the PNBI segments of the P3HT1-PNBIP3HT1 thin film is gradually enhanced with an increase in the annealing temperature, implying that thermal annealing induced reorientation of the polymer chains (especially, the PNBI segments). The P3HT2-PNBI-P3HT2 thin film also exhibited similar absorption behavior as a result of thermal annealing. Figure 4c shows a comparison of the absorption spectra between the annealed P3HT1-PNBI-P3HT1 and P3HT2-PNBI-P3HT2 thin films (at 200 °C). The higher PNBI content in P3HT2-PNBI-P3HT2 results in stronger

Figure 2. SEC profiles of P3HT1 and P3HT1-PNBI-P3HT1.

The PDs (m/n) of P3HT1-PNBI-P3HT1 and P3HT2-PNBIP3HT2 were respectively calculated to be 56/8 and 36/9 from the integration of the aromatic peaks of the P3HT and PNBI segments (i.e., 6.97 and 8.85 ppm), corresponding to total molecular weights of 18 700 and 16 700 for P3HT1-PNBIP3HT1 and P3HT2-PNBI-P3HT2. The thermal transition analysis of polymers was measured by differential scanning calorimetry (DSC) under a nitrogen atmosphere. In the DSC heating scans (Figure 3), the melting points of P3HT (Mn = 9400) and PNBI (Mn = 9000) homopolymers are observed in 109 and 204 °C, respectively. On the other hand, P3HT1PNBI-P3HT1 exhibits two melting transitions at 115 and 199 °C, which is consistent with those of PNBI and P3HT. The similar thermal behavior was also observed in the fully conjugated D−A block copolymer reported previously.28 Optical Properties. The optical absorption spectra of D−A block copolymers in dilute chloroform solution and as thin films were investigated. Their optical properties are summar9621

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Figure 4. Absorption spectra of (a) as-spun polymer thin films, (b) annealed P3HT1-PNBI-P3HT1 thin film, (c) annealed D−A block copolymer thin films (at 200 °C), and (d) P3HT:P3HT1-PNBI-P3HT1 blend film.

Figure 5. CV profiles of P3HT1-PNBI-P3HT1 (top) and P3HT2-PNBI-P3HT2 (bottom) thin films.

and P3HT2-PNBI-P3HT2 thin films decreased to 1.48 and 1.39 eV by the annealing at 200 °C, respectively. The Egopt

absorptions at 350−500 and 650−800 nm as it did in the asspun thin films. The Egopt values of the P3HT1-PNBI-P3HT1 9622

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Table 3. Electrochemical Properties of Polymer Thin Films P3HT1-PNBI-P3HT1 P3HT2-PNBI-P3HT2 a

Eredonset (V)a

Eredmax (V)a

Eoxonset (V)a

Eoxmax (V)a

HOMO (eV)b

LUMO (eV)c

Egel (eV)d

−0.13 −0.18

−0.25, −0.70 −0.37, −0.67

1.17 1.20

2.02 2.06

5.57 5.60

4.27 4.22

1.30 1.38

Versus SCE. bHOMO was calculated from HOMO = Eoxonset + 4.4 (eV). cLUMO was calculated from LUMO = Eredonset + 4.4 (eV). Electrochemical band gap (Egel) was calculated from Egel = HOMO − LUMO (eV).

d

All-Polymer Solar Cells. The potential application of P3HT-PNBI-P3HTs as acceptor materials in all-polymer solar cells was investigated by using P3HT as a donor material. The device architecture is as follows: ITO/PEDOT:PSS/ P3HT:P3HT-PNBI-P3HTs/Ca/Al. The current density voltage (J−V) curves of the fabricated all-polymer solar cells under 100 mW/cm2 AM1.5G solar illumination are shown in Figure 6. The Voc, short current (Jsc), fill factor (FF), and PCEs are

values of P3HT-PNBI-P3THs are much smaller than those of other reported D−A block copolymers (1.6−1.8 eV),25−27 giving them better potential for light harvesting and exciton generation. The absorption behavior of the P3HT:P3HT1-PNBI-P3HT1 blend film (1:1 by weight) was investigated as shown in Figure 4d. The as-spun blend film exhibits broad absorption in the range of 350−850 nm. The two absorption bands (at 350−500 and over 650 nm) are gradually enhanced with increasing annealing temperature, and the λedge value extended to ca. 950 nm. As well as the block copolymer films, the P3HT:P3HT2PNBI-P3HT2 blend film showed similar light absorption behaviors as P3HT:P3HT1-PNBI-P3HT1 blend film (see Figure S3 in Supporting Information). The broad absorption behavior of the P3HT:P3HT-PNBI-P3HT system indicates massive potential for efficient light harvesting and exciton generation. Electrochemical Properties. The electronic state (HOMO/LUMO levels) of the polymer thin films was investigated by cyclic voltammetry (CV). To determine the HOMO/LUMO levels, each measurement was calibrated with the saturated calomel electrode (SCE). The cyclic voltammograms are shown in Figure 5, and the electrochemical data are summarized in Table 3. Figure 5 illustrates that the P3HT1PNBI-P3HT1 and P3HT2-PNBI-P3HT2 thin films have one oxidation peak due to the P3HT segments and two reduction peaks due to the PNBI segments. The Eoxmax values of the P3HT1-PNBI-P3HT1 and P3HT2-PNBI-P3HT2 thin films are observed to be 2.02 and 2.06 V (vs SCE), which are comparable to that of P3HT. The P3HT1-PNBI-P3HT1 and P3HT2-PNBI-P3HT2 thin films, however, had higher Eoxonset values of 1.17 and 1.20 V compared to P3HT (0.93 V) as a result of the incorporation of the PNBI segments with strong electron-withdrawing properties. From these Eoxonset values, the HOMO levels (HOMO = Eoxonset + 4.4 eV42) were respectively estimated to be 5.57 and 5.60 eV for the P3HT1-PNBI-P3HT1 and P3HT2-PNBI-P3HT2 thin films. In the reduction waves, P3HT1-PNBI-P3HT1 and P3HT2-PNBI-P3HT2 exhibit Eredmax values comparable to those of PNBI; the observed Eredonset values are −0.13 and −0.18 V, and the LUMO levels (LUMO = Eredonset + 4.4 eV42) were then estimated to be 4.27 and 4.22 eV, respectively. In contrast to the HOMO levels, the LUMO levels were comparable to that of PNBI (4.30 eV) regardless of the incorporation of P3HT segments. Considering that the LUMO level of PCBM is ca. 4.20 eV,2 P3HT-PNBIP3HTs are expected to function as electron-transporting materials. In addition, P3HT-PNBI-P3HTs can also transport holes because of their relatively low HOMO levels (ca. 5.60 eV). From a standpoint of donor materials, low HOMO levels are required to achieve high open-circuit voltage (Voc) which is directly related to the HOMO/LUMO offset of the donor and acceptor materials.6−8 In this respect, the potential application of P3HT-PNBI-P3HTs as donor materials with low HOMO levels in fullerene-based OPVs is also interesting.

Figure 6. J−V characteristics of (a) P3HT:P3HT1-PNBI-P3HT1 and (b) P3HT:P3HT2-PNBI-P3HT2 solar cells under AM1.5G (100 mW/cm2) illumination.

summarized in Table 4. The as-spun P3HT:P3HT1-PNBIP3HT1 system gave the low PCE of 0.50% with Voc of 0.51 V, Jsc of 2.12 mA/cm2, and FF of 0.47. Thermal annealing resulted in a drastic improvement of the Jsc, and finally, the PCE of 1.20% with Voc of 0.59 V, Jsc of 3.28 mA/cm2, and FF of 0.62, was achieved in the P3HT:P3HT1-PNBI-P3HT1 system (annealed at 200 °C). The P3HT:P3HT2-PNBI-P3HT2 system exhibited a similar photovoltaic trend; the PCEs were improved from 0.37% (with Voc of 0.49 V, Jsc of 1.60 mA/cm2, and FF of 0.47) to 1.28% (with Voc of 0.56 V, Jsc of 4.57 mA/ cm2, and FF of 0.50). In addition to the thermal annealing conditions, other factors relating to device fabrication (e.g., solvents, additives, thickness of layers, and the ratio of donor and acceptor materials) also affect the photovoltaic performance,17−21 and thus, we believe that the further improvement of 9623

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by thermal annealing should contribute to the trends observed for the PCEs.

Table 4. Performance of All-Polymer Solar Cells Using the P3HT:P3HT-PNBI-P3HT System composition P3HT:P3HT1-PNBIP3HT1a

P3HT:P3HT2-PNBIP3HT2a

Voc (V)

Jsc (mA/cm2)

FF

PCE (%)

as-spun

0.51

2.12

0.47

0.50

annealed at 100 °Cb annealed at 200 °Cb as-spun

0.54

3.03

0.48

0.79

0.59

3.28

0.62

1.20

0.49

1.60

0.47

0.37

0.53

3.53

0.39

0.74

0.56

4.57

0.50

1.28

conditions

annealed at 100 °Cb annealed at 200 °Cb

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CONCLUSIONS The first example of the synthesis of fully conjugated donor− acceptor block copolymers incorporating PNBI segments (P3HT-PNBI-P3HTs) and the fabrication of all-polymer solar cells using fully conjugated D−A block copolymers as acceptor materials was presented herein. P3HT-PNBI-P3HTs were synthesized using quasi-living Grignard metathesis polymerization and the Yamamoto coupling reaction. The 1H NMR, SEC, and DSC analyses suggested that the efficient synthesis of P3HT-PNBI-P3HTs was achieved. Broad light absorption bands in the range of 350−850 nm were observed in the P3HT-PNBI-P3HT thin films. The optical band gap decreased from 1.46 to 1.38 eV by thermal annealing, which was much smaller than those of previously reported D−A block copolymers (1.6−1.8 eV). CV measurement demonstrated that the P3HT-PNBI-P3HT thin films exhibited the oxidation and reduction properties derived from the P3HT and PNBI segments. The HOMO and LUMO levels were in the ranges of 5.57−5.60 and 4.22−4.27 eV, respectively. All-polymer solar cells fabricated using the P3HT:P3HT-PNBI-P3HT blend film gave a best performance PCE of 1.28% with Voc of 0.56 V, Jsc of 4.57 mA/cm2, and FF of 0.50. These results demonstrated that fully conjugated D−A block copolymers incorporating PNBI segments are potentially applicable as no-nfullerene acceptor materials, and more efficient photovoltaic performance can be achieved by optimization of chemical structure and the fabrication conditions of these devices.

a

The active layers were prepared from the P3HT:P3HT-PNBI-P3HT (1:1 by weight) chlorobenzene solution. bAnnealed for 15 min under a nitrogen atmosphere.

the PCEs of the P3HT:P3HT-PNBI-P3HT system can be achieved by optimizing these factors. The effect of surficial morphology on the photovoltaic performance of the P3HT:P3HT-PNBI-P3HT blend films was investigated by atomic force microscopy (AFM) measurement. Figure 7 shows the AFM height images of the P3HT:P3HT2PNBI-P3HT2 blend film before and after thermal annealing. The surface of the annealed blend film is rather smoother than that of the as-spun blend film (see the scale bar). The morphological change and enhanced light absorption induced

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ASSOCIATED CONTENT

* Supporting Information S

Synthetic procedure, 1H NMR spectrum of PNBI, and absorption behaviors of P3HT-PNBI-P3HT. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Ziruo Hong for the discussions and advice to fabricate devices. This work is supported by the Japan Society for the Promotion of Science (JSPS) for providing the main support for this work by Grant-in-Aid for Research Activity Start-up (23850004).

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REFERENCES

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