Influence of Terminal Imide Units on Properties and Photovoltaic

Journal of Photopolymer Science and Technology

Volume 30, Number 5 (2017) 557－560 Ⓒ 2017SPST Communication

Influence of Terminal Imide Units on Properties and Photovoltaic Characteristics for Benzothiadiazolebased Nonfullerene Acceptors Shreyam Chatterjee, Yutaka Ie*, and Yoshio Aso* The Institute of Scientific and Industrial Research (ISIR), Osaka University, 8-1, Mihogaoka, Ibaraki, Osaka 567-0047, Japan *[email protected]; [email protected] Development of new semiconducting materials has become an important subject to develop organic photovoltaics. In this letter, new electron-accepting -conjugated molecules, which are composed of thiophene-linked benzothiadiazole (T-BTz) as a central unit and phthalimide (PI) or naphthalimide (NI) as a terminal unit, were designed and synthesized to investigate the influence of the terminal units on the properties and photovoltaic characteristics. The utilization of NI led to red-shifted absorption and increased electronaccepting characteristics, compared to those of PI. Furthermore, the photovoltaic device based on T-BTz-NI under the combination with poly(3-hexylthiophene) as a donor showed improved power conversion efficiency of 1.16%. These results indicate that NI become a good candidate for a terminal unit of non-fullerene acceptors. Keywords: Organic photovoltaics, Acceptor, Semiconducting materials, Structureproperty relationship

1. Introduction Lightness, flexibility, low cost, and ease of process make the bulk heterojunction (BHJ) organic photovoltaics (OPVs) as a promising sustainable energy source [1,2]. The active layer of OPVs consists of donor and acceptor semiconductors, and fullerene-based materials have remained as the yardstick for acceptors in OPVs [3-6]. However, fullerene derivatives have faced on the drawbacks such as weak absorption over the range of intense regions of the solar spectrum and the difficulty of manipulating energy levels and miscibility with donor materials, which generates a motivation to investigate non-fullerene acceptor materials for further advancement in OPVs [7]. Recently, we reported that an electron-accepting π-conjugated system consisting of benzothiadiazole (BTz) as a central unit and phthalimide (PI) as a terminal unit linked with acetylene bonds showed good OPV characteristics with a power conversion efficiency (PCE) of 1.58% under the combination with poly(3hexylthiophene) (P3HT) as a donor [8]. We also reported that naphthalimide (NI) functions as a Received April Accepted May

3, 2017 12, 2017

terminal unit of nonfullerene acceptors [9]. Based on these results, to directly compare the potential of PI and NI as terminal units, we fixed thiophenelinked BTz (T-BTz) as a central unit and designed new electron-accepting -conjugated compounds TBTz-PI and T-BTz-NI (Fig. 1). In this work, we investigated the synthesis, properties, and photovoltaic characteristics of these compounds.

Fig. 1. Chemical structures of T-BTz-PI (blue) and TBTz-NI (red).

2. Experimental Column chromatography was performed on silica 557

J. Photopolym. Sci. Technol., Vol. 30, No. 5, 2017

gel, KANTO Chemical silica gel 60N (40–50 μm). Preparative GPC was performed on Japan Analytical Industry LC-918 equipped with JAIGEL 1H/2H. TGA was performed under nitrogen with Shimadzu TGA-50. UV–vis spectra were recorded on a Shimadzu UV-3600 spectrophotometer. The surface morphology of organic films was observed by atomic force microscopy (Shimadzu, SPM9600). Organic photovoltaic devices were prepared with a structure of indium tin oxide (ITO)/poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/active layer/Ca/Al. ITO-coated glass substrates were first cleaned by ultrasonication in toluene, acetone, H2O, and 2-propanol for 15 min, respectively, followed by O2 plasma treatment for 20 min. The ITO-coated glass substrates were then activated by ozone treatment for 1.5 h. PEDOT:PSS was spin-coated on the ITO surface at 3000 rpm for 1 min and dried at 135 °C for 10 min. Under this condition, the thickness of PEDOT:PSS is ca. 30 nm. The active layers were then prepared by spincoating on the ITO/PEDOT:PSS electrode at 1000 rpm for 2 min in a glove box. Then, the active layer was annealed at 140 °C for 15 min. Ca and Al electrodes were evaporated on the top of active layer through a shadow mask to define the active area of the devices (0.09 cm2) under a vacuum of 10−5 Pa to thicknesses of 30 and 70 nm, respectively, determined by a quartz crystal monitor. After sealing the device from the air, the photovoltaic characteristics were measured under simulated AM 1.5G solar irradiation (100 mW cm−2) (SAN-EI ELECTRIC, XES-301S). The J–V characteristics of photovoltaic devices were measured by using a KEITHLEY 2400 source meter. The EQE spectra were measured by using Soma Optics Ltd. S-9240. The thickness of active layer was determined by KLA Tencor Alpha-step IQ. 3. Results and discussion The synthetic route of target compounds T-BTzPI and T-BTz-NI is shown in Scheme 1. Since we selected acetylene as a linker, palladium-catalyzed Sonogashira–Hagihara coupling was adopted to construct the -conjugated framework. The presence of dialkylfluorenes on the imide terminal unit ensured adequate solubility in common organic solvent such as chloroform, chlorobenzene, and odichlorobenzene (o-DCB). Both the acceptors were fully characterized by NMR, mass spectrometry, and elemental analysis. The thermal gravimetrical analysis (TGA) 558

indicates that T-BTz-PI and T-BTz-NI possess good thermal stability with 5%-weight-loss temperature of 404 and 343 C, respectively (Fig. 2).

Scheme 1. Synthesis of T-BTz-PI and T-BTz-NI.

Fig. 2. TGA curves of T-BTz-PI (blue) and T-BTz-NI (red).

The photophysical properties of the T-BTz-PI and T-BTz-NI in a chloroform solution and thin film were measured by UV–vis absorption spectrometry (Fig. 3). The main absorption bands of T-BTz-PI and T-BTz-NI appeared at about 490 and 503 nm, respectively. A red-shifted absorption of T-BTz-NI compared to that of T-BTz-PI is derived from the extension of -conjugation from PI to NI. The absorption spectra of the films were red-shifted compared to those in solution. The optical energy gaps (∆Egopt) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of T-BTz-PI and T-BTz-NI were extracted from the absorption onset to be 2.21 and 2.13 eV, respectively.


Fig. 3. UV-vis absorption spectra in chloroform solution (solid line) and as film (dashed line) for T-BTz-PI (blue) and T-BTz-NI (red).

The electrochemical behavior of T-BTz-PI and TBTz-NI was investigated by cyclic voltammetry (CV) measurements in o-DCB/acetonitrile (CH3CN) (10:1) solution (Fig. 4). The potential was calibrated using a ferrocene/ferrocinium (Fc/Fc+) redox couple. Both the compounds showed multiple reduction waves [9], and the first reduction potential of T-BTz-NI is positively shifted compared to that of T-BTz-PI, indicating that the NI unit has a stronger electron-accepting characteristic than PI. From the first reduction potentials, the LUMO levels of T-BTz-PI and T-BTz-NI were estimated to be −3.24 and −3.43 eV, respectively. Based on ELUMO and ∆Egopt, EHOMO values are calculated to be −5.45 and −5.56 eV for T-BTz-PI and T-BTz-NI, respectively. These energy levels are suitable for acceptors in OPVs. The space-charge-limited current (SCLC) measurements of T-BTz-PI and TBTz-NI pristine films showed that the electron mobility of 5.4 × 10−9 cm2 V−1 s−1 and 3.4 × 10−8 cm2 V−1 s−1, respectively.

Fig. 4. Cyclic voltammograms of T-BTz-PI (blue) and TBTz-NI (red) in o-DCB/CH3CN containing 0.1 M TBAPF6.

BHJ solar cells with a conventional device structure of glass/ITO/PEDOT:PSS/active layer/Ca/Al were utilized for this investigation. The active layers were composed of a blend of P3HT and the newly synthesized acceptors (T-BTz-PI or TBTz-NI). The fabrication conditions of the active BHJ layer were optimized as the blend composition of 1:1 ratio, a concentration of 14 mg mL–1 in chloroform for spin-coating, and thermal annealing at 140 C for 15 min. The best-performance current density−voltage (J−V) characteristics and external quantum efficiency (EQE) spectra are shown in Fig. 5, and their key photovoltaic parameters are summarized in Table. 1. As we expected, both compounds functioned as acceptors. These devices exhibited typical OPV J−V curves with higher opencircuit voltage (VOC) when compared with typical P3HT/PC61BM-based devices [10], reflecting the high-lying LUMO energy levels of these acceptors relative to that of PC61BM (−3.81 eV). Compared to the device composed of P3HT/T-BTz-PI, which shows a PCE of 0.35%, the T-BTz-NI-based device showed a superior PCE of 1.05%. The PCE improvement is attributed to the increased shortcircuit current density (JSC). The EQE spectra of the devices exhibited photoresponse between 300 nm and 700 nm with a maximum of 12% and 34% for P3HT/T-BTz-PI and P3HT/T-BTz-NI blends, respectively.

Fig. 5. (a) J−V curves of based on P3HT/T-BTz-PI (blue) and P3HT/T-BTz-NI (red) under illumination (solid line) and dark (dashed line) and (b) EQE spectra of these devices. 559


Table 1. Key photovoltaic parameters of OPV devices Blend Jsc FF PCE Voc (V)

(mA cm−2)

P3HT/T-BTz-PI

0.78

1.16

0.38

0.35

P3HT/T-BTz-NI

0.75

3.92

0.36

1.16

(%)

To investigate the morphology of the blend films, we performed atomic force microscopy (AFM) measurements. As shown in Fig. 6, AFM images of the blend films showed significant different morphologies with the average roughness (Ra) of 2.8 nm for the P3HT/T-BTz-PI film and 1.7 nm for the P3HT/T-BTz-NI films. The relatively large Ra value of the P3HT/T-BTz-PI film indicates a poor intermixing between the donor and acceptor with large domains, causing poor exciton dissociation leading to the lower JSC [9].

Fig. 6. AFM images of the (a) P3HT/T-BTz-PI and (b) P3HT/T-BTz-NI.

The charge-transporting characteristics of T-BTzPI and T-BTz-NI were investigated by the SCLC method (Fig. 7). The electron-only devices of the blend films showed that P3HT/T-BTz-NI has one order higher electron mobility (2.4 × 10−6 cm2 V−1 s−1) than that of P3HT/T-BTz-PI (3.2 × 10−7 cm2 V−1 s−1). This behavior is in good agreement with the mobility of pristine films and may be one reason to get the higher Jsc from the P3HT/T-BTz-NI-based OPV device.

Fig. 7. J−V characteristics of electron-only device for P3HT/T-BTz-PI (blue) and P3HT/T-BTz-NI (red).

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4. Conclusion In summary, to investigate the potential of PI and NI as a terminal unit of non-fullerene acceptors, we have designed and synthesized T-BTz-PI and TBTz-NI. Photophysical and electrochemical measurements indicated that the replacement of PI with NI led to red-shifted absorption and increased electron-accepting characteristics. Conventional OPV devices fabricated from T-BTz-NI as an acceptor with P3HT as a donor showed superior photovoltaic performance of up to 1.16% compared to the corresponding T-BTz-PI-based devices. This result indicates that naphthalimide (NI) unit is superior to phthalimide (PI) terminal unit under the combination with benzothiadiazole-based central unit. We are now currently investigating the potential of NI unit to acceptor-donor-acceptor systems as nonfullerene acceptors for OPVs. Acknowledgements This work was supported by ACT-C program from the Japan Science and Technology Agency and a Grant-in-Aid for Scientific Research (B) (16H04191) and Innovative Areas (JP25110004) and "Dynamic Alliance for Open Innovation Bridging Human, Environmental and Materials" from The Ministry of Education, Culture, Sports, Science and Technology, Japan. References 1. G. Dennler, M. C. Scharber, and C. J. Brabec, Adv. Mater., 21 (2009) 1323. 2. M. A. Green, K. Emery, Y. Hishikawa, W. Warta, and E. D. Dunlop, Prog. Photovoltaics, 23 (2015) 805. 3. N. S. Sariciftci, L. Smilowitz, A. J. Heeger, and F. Wuld, Science, 258 (1992) 1474. 4. T. M. Clarke and J. R. Durrant, Chem. Rev., 110 (2010) 6736. 5. G. Li, R. Zhu, and Y. Yang, Nat. Photonics, 6 (2012) 153. 6. Y. Li, Chem. Asian J., 8 (2013) 2316. 7. A. Anctil, C. W. Babbitt, R. P. Raffaelle, and B. J. Landi, Environ. Sci. Technol., 45 (2011) 2353. 8. Y. Ie, S. Jinnai, M. Nitani, and Y. Aso, J. Mater. Chem. C, 1 (2013) 5373. 9. S. Chatterjee, Y. Ie, M. Karakawa, and Y. Aso, Adv. Funct. Mater., 26 (2016) 1161. 10. M. T. Dang, L. Hirsch, G. Wantz, and J. D. Wuest, Chem. Rev., 113 (2013) 3734.