A Benzoselenadiazole-Based Low Band Gap Polymer: Synthesis and

Feb 4, 2013 - *E-mail: [email protected] (K.T.); hashimoto@light. .... and Selenophene Incorporation for Bulk Heterojunction Solar Cells ..... L...
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A Benzoselenadiazole-Based Low Band Gap Polymer: Synthesis and Photovoltaic Application Erjun Zhou,† Junzi Cong,† Kazuhito Hashimoto,*,† and Keisuke Tajima*,‡,§ †

Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ‡ Japan Science and Technology Agency (JST), Precursory Research for Embryonic Science and Technology (PRESTO), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ABSTRACT: A novel alternating copolymer with a low band gap (Eg = 1.55 eV), PBDT−DTBSe, based on benzodithiophene (BDT) and benzoselenadiazole (BSe) units with thiophene as a π-conjugated bridge, was synthesized and characterized. When 1,8-diiodooctane was used as a solvent additive to optimize the mixing morphology, the maximum power conversion efficiency reached by a polymer solar cell based on PBDT−DTBSe/PC70BM was 5.18%, which was slightly higher than that of the benzothiadiazole (BT)-based analogue (5.01%). These results demonstrated the promising effectiveness of benzoselenadiazole as an electron-deficient unit for the design of the donor−acceptor photovoltaic polymers.

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INTRODUCTION For the past two decades, solution-processed semiconducting polymers, as important functional materials, have been applied to low-cost printed electronic devices such as polymer lightemitting diodes (PLEDs), field-effect transistors (FETs), sensors, photodetectors, and polymer solar cells (PSCs).1 The polymers can be molecularly engineered to possess suitable optical and electronic properties through synthesis and assembly in the films for the requirements in each type of device. For PSCs application,2 further improvement on the power conversion efficiency (PCE) is expected to follow from the development of novel conjugated polymers with higher carrier mobility and broader absorption of the solar spectrum, especially in the red and near-infrared regions.3 It has been well demonstrated that the utilization of electron-donating (D) and electron-accepting (A) building blocks in the polymer backbone is one of the most promising and attractive strategies for designing organic semiconducting polymers. The absorption spectra and energy levels of D−A type polymers can be modulated by adjusting the contribution of intramolecular charge transfer (ICT).4 Because many kinds of electron-rich building blocks contain thiophene unit, a simple method to tune the properties of the polymers is replaced the sulfur (S) atom in thiophene with a selenium (Se) atom. Se atom is much larger in size and less electronegativity compared with the S atom, and thus Secontaining polymers are expected to be more effective in extending the absorption spectrum toward the infrared region. In addition, the Se-containing unit is more polarizable than the S analogue; thus, the interchain Se···Se interactions could improve charge mobility of the polymers. The red-shifted © 2013 American Chemical Society

absorption spectrum and higher charge carrier mobility should contribute to higher photovoltaic performance. In fact, some Se-containing electron-rich building blocks, such selenophene,5 benzodiselenophene,6 and selenolo[3,2-b]thiophene,7 have been used in the design of semiconductor polymers and show higher FET mobility and promising photovoltaic performance. On the other hand, for electron-deficient building blocks, the effect of the substitution of S with Se has been investigated less extensively. 4,7-Dithien-2-yl-2,1,3-benzothiadiazole (DTBT) unit has been used as electron-deficient unit and copolymerized with many types of donor segments.8 When applied to PSCs, polymers based on DTBT and carbazole units show a maximum PCE of 7.1%.9 However, when 4,7-dithien-2-yl2,1,3-benzoselenadiazole (DTBSe)-based polymers, where S is changed to Se in DTBT, were used in PSCs, the photovoltaic performance was disappointing,10 and the maximum PCE reached only 2.5%.10d Thus, it is still necessary to develop novel BSe-based polymers and compare the properties of DTBT and DTBSe-based D−A type copolymers in detail. In this study, we chose benzodithiophene (BDT)11 as the electron-donating building block and synthesized PBDT− DTBSe, a new copolymer of the D−A type. To make a clear comparison and elucidate the relationship between properties and structure when a S atom is replaced with a Se atom, the corresponding polymer based on DTBT and BDT was also synthesized and characterized in parallel (Scheme 1). The basic Received: December 18, 2012 Revised: January 21, 2013 Published: February 4, 2013 763

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Scheme 1. Chemical Structures and Synthetic Routes to PBDT−DTBSe and PBDT−DTBT

Figure 1. UV−vis absorption spectra of PBDT−DTBSe and PBDT−DTBT (a) in CHCl3 solution and (b) in film on quartz plates spin-coated from CHCl3 solution.

Using the same coupling reaction, PBDT−DTBT was synthesized in parallel with Mn of 22.0 kg mol−1 and PDI of 2.1. Optical and Electrochemical Properties. The absorption spectra of PBDT−DTBSe in CHCl3 and in film, together with those of PBDT−DTBT, are shown in Figure 1. In CHCl3 solution, PBDT−DTBSe showed two main absorption bands with maxima at 446 and 673 nm, while PBDT−DTBT exhibited two peaks at 425 and 630 nm. The peak at the longer wavelength can be attributed to the ICT transition, and the other was possibly the result of higher energy transitions. Both polymers exhibit the similar absorption coefficients (∼3.8 × 104 L mol−1 cm−1) in the CHCl3 solutions at their absorption maxima. In films, the peaks of maximum ICT absorption were at 680 nm for PBDT−DTBSe and 646 nm for PBDT−DTBT, which were red-shifted by 7 and 16 nm, respectively, compared with those in CHCl3 solution. The optical band gaps of the PBDT− DTBSe and PBDT−DTBT film were estimated as 1.55 and 1.72 eV from the absorption onset (800 and 720 nm). These results indicate that replacing a sulfur atom with a selenium

properties of PBDT−DTBT and PBDT−DTBSe, such as their respective absorbance, energy levels, crystalline structures, and charge mobility in the films, were compared, and the potential of DTBSe as a building block for photovoltaic polymers was investigated.

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RESULTS AND DISCUSSION

Material Synthesis. PBDT−DTBSe was synthesized via Pd(PPh3)4-catalyzed Stille coupling of 2,6-bis(trimethyltin)-4,8di(2-octyldodecyloxy)benzo[1,2-b;3,4-b′]dithiophene and 4,7bis(5-bromothien-2-yl)-2,1,3-benzoselenadiazole. To increase the solubility of the target polymer, two bulky 2-octyldodecyloxy side chains were introduced to the BDT building block. The polymer exhibited good solubility in common organic solvents such as chloroform (CF), chlorobenzene (CB), and odichlorobenzene (DCB). The number-average molecular weight (Mn) of the polymer was 20.1 kg mol−1 with a polydispersity index (PDI) of 2.1, which was measured by gel permeation chromatography (GPC) using CHCl3 as an eluent. 764

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the out-of-plane measurement, the XRD pattern of PBDTDTBSe revealed a large diffraction peak at 4.3°, with a small peak at 8.5°, which can be assigned to the (100) and (200) diffractions of a layered structure. However, the in-plane measurement revealed only (010) diffraction due to intermolecular π−π stacking. The interlayer and interchain stacking distances of PBDT−DTBSe were calculated as 2.05 and 0.372 nm, respectively, while PBDT−DTBT showed an XRD pattern similar to that of PBDT−DTBSe, but with a shorter interchain stacking distance of 0.363 nm. The above results indicate a high degree of crystallinity for PBDT−DTBSe and PBDT−DTBT in the film, with the bulky side chains oriented vertically with respect to the substrate, while intermolecular π−π stacking is parallel to the substrate, as schematically shown in Figure 3b (edge-on orientation). Replacing a S atom with a Se atom seems to have minimal impact on the molecular orientations of the polymers, only slightly increasing the intermolecular π−π stacking distance. The high degree of crystallinity of PBDT−DTBSe and PBDT−DTBT in the films suggests a high hole mobility, which is an important requirement for effective photovoltaic polymers. The mobility was measured by using field-effect transistors (FETs) with a bottom-gate, top-contact device configuration built on n-doped silicon wafers. Figure 4 shows the typical transfer curves of both polymers. PBDT−DTBSe shows a hole mobility of 5.4 × 10−3 cm2 V−1 s−1 with on/off ratio of 5× 103, while PBDT−DTBT gives a lower hole mobility of 2.6 × 10−3 cm2 V−1 s−1 and on/off ratio of 1× 103. The threshold voltages for both polymers are −12 and −22 V, respectively. This increase of hole mobility by replaced S atom with Se atom was also found in other D−A type polymers.5−7 Although the FET mobility might not be directly correlated to the photovoltaic performance since the situation of the charge transport is quite different from each other, the 2 times higher hole mobility of PBDT−DTBSe than that of PBDT−DTBT could suggest more effective charge transport to the electrodes in the bulk-heterojunction solar cells. Photovoltaic Properties. The photovoltaic properties of PBDT−DTBSe and PBDT−DTBT were investigated in BHJ solar cells with [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) as the acceptor. PSCs were fabricated with a typical sandwich structure of glass/ITO/PEDOT:PSS/active layer/ Ca/Al. The ratio of the polymer to PC70BM was optimized as 2:1, 1:1, and 1:2, and the devices with a ratio of 1:1 showed the

atom is a simple way to red-shift the absorption spectra of BTcontaining D−A type polymers to the long-wavelength region.12 Cyclic voltammetry (CV) was performed to determine the HOMO and LUMO energy levels of PBDT−DTBSe and PBDT−DTBT. The CV curve was recorded by using a Ag/Ag+ electrode as a reference, which was calibrated against the redox potential of ferrocene/ferrocenium (Fc/Fc+), whose ionization energy was assumed to be 4.8 eV below the vacuum level. The CV curves are shown in Figure 2. Based on the onset values of

Figure 2. Cyclic voltammogram of PBDT−DTBSe and PBDT− DTBT films on a platinum plate in acetonitrile solution of [Bu4N]PF6 (0.1 mol L−1, Bu = butyl) at a scan rate of 50 mV s−1.

oxidation potentials (0.38 and 0.46 V) of the polymers, the HOMO energy levels of PBDT−DTBSe and PBDT−DTBT were calculated to be −5.18 and −5.26 eV, respectively. The LUMO energy levels were calculated from the onset values of the reduction potentials, namely −3.48 eV for PBDT−DTBSe and −3.50 eV for PBDT−DTBT. The higher-lying HOMO of PBDT−DTBSe compared with that of PBDT−DTBT resulted in decreasing the VOC of PSC, since VOC of the devices shows a primary correlation with the energy difference between the HOMO of the donor and the LUMO of the fullerene acceptor. Crystalline Structures and Hole Mobility. X-ray diffraction (XRD) measurements were carried out to understand the crystalline structures and molecular orientations of the polymers in solid state. Thin films were spin-coated on Si/ SiO2 substrates from a CB solution. As shown in Figure 3a, in

Figure 3. (a) XRD patterns along the out-of-plane and in-plane axes of PBDT−DTBSe and PBDT−DTBT thin solid films. (b) A schematic representation of the possible packing of PBDT−DTBSe in the thin solid film. 765

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Table 1. Device Characteristics of PSCs Fabricated from PBDT−DTBSe/PC70BM and PBDT−DTBT/PC70BM active layer VOC (V)

JSC (mA cm−2)

FF

PCE (%)

CB

0.62

8.84

0.59

3.23

CB/DIO (97/3 v/v) DCB

0.60

13.58

0.64

5.18a

0.72

11.16

0.62

5.01b

0.70

10.43

0.65

4.78

polymer PBDT− DTBSe

PBDT− DTBT

solvent

CB/DIO (99/1 v/v)

a The average value of five devices was 5.11% bThe average value of five devices was 4.90%.

ing nanoscale morphology, thus improving the PCE of PSCs for polymer/fullerene systems.13 Therefore, we added DIO to the PBDT−DTBSe/PC70BM system, in which the active layer spin-coated from a CB/DIO blend presented a more favorable morphology, as expected (Figure 5). The observed DIO effects on morphology are attributed to the selective solubility of DIO to fullerene. As a result, with the introduction of DIO, 3 vol %, in CB, the VOC decreased slightly, but JSC and FF improved substantially, resulting in a higher PCE of 5.18%. It is noteworthy that both the PCE and JSC (13.58 mA cm−2) of the PBDT−DTBSe/ PC70BM system are the highest for any BSe-containing polymer reported thus far. For PSCs based on PBDT−DTBT/PC70BM, a PCE of 5.01% was achieved simply by using pure DCB as a processing solvent, and AFM images revealed that a smooth film has been achieved (Figure 6). As expected, using DIO as an additive did not entail any further improvement in performance for PBDT−DTBT/PC70BM systems. Figure 7 shows plots of the external quantum efficiency (EQE) of the photovoltaic devices under monochromatic light illumination. For PBDT−DTBSe/PC70BM, the shapes of the EQE curves of the devices spin-coated from pure CB and CB/ DIO blend (97/3 v/v) solutions are similar, but the latter exhibits higher values. The highest EQE values of the two plots are 0.47 and 0.68, respectively. For PBDT−DTBT/PC70BM, the maximum response of EQE is similar to that of PBDT− DTBSe (0.65 at 400 nm), but the response extends only to 720 nm, which is shorter than that of the PBDT−DTBSe/PC70BM system by ca. 80 nm. This should be the cause of the lower JSC under irradiation with simulated solar light. The JSC calculated from the integral of EQE curves and AM 1.5 spectrum are 13.28 and 10.90 mA cm−2 for PBDT−DTBSe and PBDT− DTBT, respectively, which are consistent with the experimental JSC values under the simulated solar light (match within 2%).

Figure 4. Transfer curves of field-effect transistors based on PBDT− DTBSe and PBDT−DTBT.

highest performance. Figure 5 shows the J−V curves of the devices with the highest photovoltaic performance under

Figure 5. J−V curves under an AM 1.5 illumination (100 mW cm−2) for PSCs based on PBDT−DTBSe/PC70BM and PBDT−DTBT/ PC70BM spin-coated from different solutions.

AM1.5 illumination (100 mW cm−2). The corresponding VOC, short-circuit current (JSC), fill factor (FF), and PCE of the devices are also summarized in Table 1. For the PBDT−DTBSe/PC70BM system, using pristine CB as a processing solvent, only moderate PCE of 3.23% was achieved, mainly due to the moderate JSC of 8.84 mA cm−2. Since JSC generally depends on the mixing morphology of the films, the film surface morphology was observed by atomic force microscopy (AFM). As shown in Figure 6, large domains can be seen in the blend film processed from pristine CB, which indicates that the nonoptimal nanoscale structure and the large phase separation lowers the charge separation and transport. It has been shown in many studies that a small amount of 1,8diiodooctane (DIO) can drastically enhance the interpenetrat-

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CONCLUSION We synthesized a novel BSe-based copolymer with a low band gap. The slightly higher photovoltaic performance of PBDT− DTBSe/PC70BM compared with PBDT−DTBT/PC70BM suggests that BSe is a promising building block for constructing photovoltaic polymers. Furthermore, both the PCE (5.18%) and JSC (13.58 mA cm−2) of the PBDT-DTBSe/PC70BM system are the highest for any BSe-containing polymer reported thus far. Since various BT-based photovoltaic polymers with high performance have been reported, it might be possible to achieve higher conversion efficiency through modifying existing 766

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Figure 6. AFM height images of PBDT−DTBSe/PC70BM and PBDT−DTBT/PC70BM composite films spin-coated from different solutions. by evaporation and precipitated into methanol, and the polymer was collected as a dark blue solid. Yield: 370 mg (82%). 1H NMR (CDCl3, 400 MHz): δ (ppm) 8.5 (br, 2H), 7.9−7.3 (br, 6H), 4.2 (br, 4H), 1.9−0.8 (br, 78H). Mn = 20.1 kg mol−1; polydispersity = 2.1. Poly[4,8-di(2-octyldodecyloxy)benzo[1,2-b;3,4-b]-dithiophene-2,6-diyl-alt-4,7-dithien-2-yl-2,1,3- benzoselenadiazole5′,5″-diyl] (PBDT−DTBSe). A procedure similar to the synthesis of PBDT−DTBSe was adopted by using monomer 1 (554.5 mg, 0.5 mmol), monomer 3 (229.1 mg, 0.5 mmol), dry toluene (12 mL), and Pd(PPh3)4 (3%, 17 mg). The polymer was collected as a dark red solid. Yield: 380 mg (70%). 1H NMR (CDCl3, 400 MHz): δ (ppm) 8.5 (br, 2H), 8.0−7.2 (br, 6H), 4.2 (br, 4H), 1.8−0.7 (br, 78H). Mn = 22.0 kg mol−1; polydispersity = 2.1. Characterization. 1H NMR (400 MHz) spectra were measured using a JEOL Alpha FT-NMR spectrometer equipped with an Oxford superconducting magnet system. Absorption spectra were measured using a JASCO V-660 spectrometer. The concentration of the polymers in CHCl3 solutions was 5 × 10−3 g L−1, and the films were spin-coated from CHCl3 solutions (∼5 g L−1). Cyclic voltammograms (CVs) were recorded on an HSV-100 (Hokuto Denkou) potentiostat. A Pt plate coated with a thin polymer film was used as the working electrode. A Pt wire and an Ag/Ag+ (0.01 M of AgNO3 in acetonitrile) electrode were used as the counter and reference electrodes (calibrated against Fc/Fc+), respectively. Atomic force microscopy (AFM) was conducted in tapping mode with a NanoNavi probe station and an Simage unit (SII NanoTechnology Inc., Japan). XRD patterns were recorded on a Rigaku SmartLab X-ray diffractometer. The films were prepared by spin-coating chlorobenzene solutions of the polymers (10 g L−1) on Si substrates and thermally annealed at 80 °C for 30 min. Fabrication and Characterization of Field-Effect Transistors. Transistors were built on highly doped n-type (100) Si substrates (