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A novel D–π–A small molecule with N-heteroacene as acceptor moiety for photovoltaic application
Dyes and Pigments 122 (2015) 231e237
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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
A novel DepeA small molecule with N-heteroacene as acceptor moiety for photovoltaic application Chengyuan Wang a, 1, Takuya Okabe b, 1, Guankui Long a, Daiki Kuzuhara b, Yang Zhao a, Naoki Aratani b, ***, Hiroko Yamada b, c, **, Qichun Zhang a, d, * a
School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma 630-0192, Japan CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan d Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 April 2015 Received in revised form 25 June 2015 Accepted 27 June 2015 Available online 8 July 2015
Organic p-conjugated small molecules have attracted much attention for developing high performance organic photovoltaics (OPVs) due to their well-defined molecular structure, easily controlled energy levels and absorption, and more accurate simulation-experiment match. A novel small molecule combing a N-heteroacene (acceptor moiety) and benzo[1,2-b:4,5-b0 ]dithiophene (donor unit) together through a p-conjugated bridge (3,300 -dioctyl-2,20 :50 ,200 -terthiophene), has been synthesized and characterized. The blended films of this small molecule with different acceptors have a wide absorption in visible region, which makes it possible for application in OPVs. The as-fabricated devices with PEDOT:PSS as an anode buffer layer show the power conversion efficiency at around 1%. By replacing PEDOT:PSS with MoO3, the power conversion efficiency is almost doubled up to 1.97%. AFM images and XRD patterns are employed to investigate the morphologies of the active layer. Consistent with the JeV curves and EQE spectra, the higher power conversion efficiency probably comes from the good alignment of the HOMO energy level of the small molecule with the work function of anode buffer layer and reduced chemical interactions between active layer and anode buffer layer. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Small molecule-based photovoltaics DepeA structure N-heteroacenes Molybdenum trioxide Synthesis Devices' fabrication
1. Introduction Organic photovoltaics (OPVs) have been demonstrated to be a potentially new generation of power source to address the increasing and serious energy crisis. Compared with the conventional inorganic photovoltaics, solution-processed OPVs have lower fabrication cost, light-weight, and high flexibility, which make them more attractive in practical applications [1e7]. Currently, bulk heterojunction (BHJ) structure devices have become the standard OPVs' architecture [8e10]. Organic p-conjugated semiconducting
* Corresponding author. School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore. ** Corresponding author. Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma 630-0192, Japan. *** Corresponding author. E-mail addresses: [email protected] (N. Aratani), [email protected] (H. Yamada), [email protected] (Q. Zhang). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.dyepig.2015.06.029 0143-7208/© 2015 Elsevier Ltd. All rights reserved.
materials, including polymers and small molecules, are commonly used as donors and acceptors in OPVs. Among all acceptors, fullerene and its derivatives are widely employed due to their good stability and low LUMO energy levels [11e13]. To our knowledge, donor materials play a more important role in the generation and transportation of charge carrier because the possibility of broader absorbance in the visible and near-infrared region can contribute to better harvest of sunlight [14,15]. Organic conjugated small molecules have drawn a lot of researchers’ attention because (1) organic small molecules with well-defined molecular structures are able to keep good batch-to-batch consistency, and (2) their more accurate simulation-experiment match is in favour of demonstrating property-structure relationship for deeper understanding of intrinsic charge carrier generation and transportation mechanism and providing rational molecular designing strategies [16e23]. To date, much effort has been devoted to develop diverse small molecule materials, and the power conversion efficiencies (PCE) have been optimized up to 9% for single junction devices, however, these results are still far away from the theoretical efficiency
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(20e24%) [24,25]. Clearly, there is still a long way to develop ultimate donor materials with higher performance. Recently, large N-heteroacenes have drawn much attention for their applications in organic electronics such as field-effect transistors, phototransistors, light-emitting diodes, photoelectrochemical cells, and photovoltaics [26e32]. In our previous study, 4,11-bis((triisopropylsilyl)thynyl)-1H-imidazo[4,5-b]phenazin (BIP) has been illustrated to be a useful acceptor moiety with moderate electron withdrawing ability [33,34]. The existence of electron-deficient pyridine or pyrazine units in BIP enables it to induce strong intramolecular charge transfer and wide absorbance in visible region, which benefits the generation of charge carriers [35e38]. The large conjugated backbone of the BIP moiety probably contributes to enhance the conjugation of the whole molecule, and also facilitate the migration of charge carriers. In this work, we would like to connect BIP with a typical benzo[1,2-b:4,5-b0 ] dithiophene (BDT) unit by a p-conjugated bridge (3,300 -dioctyl2,20 :50 ,200 -terthiophene (3DTT)), which also could increase the conjugation, to build up a DepeA molecule BDT-3DTT-BIP and investigate its OPV performance [39e44]. 2. Experimental section 2.1. Materials and characterization methods The chemical reagents and solvents were used directly as received from commercial companies without further purification. High-resolution mass spectra were collected on a JEOL spiral TOF JMS-S3000 spectrometer. 1H NMR and 13C NMR spectra were collected on a JEOL JNM-AL300 and JEOL JNM-ECX400 spectrometers. UVevis absorption spectra were measured on a JASCO UV/ VIS/NIR Spectro-photometer V-670. Ionization potential was evaluated with atmospheric photoelectron spectroscopy (Riken Keiki, AC-3). Currentevoltage (JeV) curves were recorded from a Keithley 2611B SYSTEM Source Mater unit under AM 1.5G illumination at intensity of 100 mW cm2 using a solar simulator (Bunko-keiki, CEP-2000RP). The external quantum efficiency (EQE) was measured under illumination of monochromatic light using the same system at intensity of 1.25 mW cm2. The surface morphology of organic films was investigated by SPA400, SPI3800N AFM (Seiko instruments Inc.) in a tapping mode using silicon probes with a resonant frequency of ~138 kHz and a force constant of 16 N m1. The out-of-plane X-ray diffraction (XRD) was carried out on a RINTTTRⅢ/NM diffractometer equipped with a rotating anode (Cu Ka radiation, l ¼ 1.5418 Å). All calculations were carried out using Gaussian 09.
2.3. Molecule synthesis Scheme 1 shows the synthetic route to BDT-3DTT-BIP. The single trimethylstannyl substituted BDT 2 was synthesized by a modified procedure based on the literature [45]. The recrystallized product was used directly for next step reaction, although there was still some double trimethylstannyl substituted BDT byproduct. Intermediate 3 was prepared through a reported procedure [46,47]. Compound 4 was obtained by Stille coupling reaction in 42.3% yield. The existence of double trimethylstannyl substituted BDT by-product probably lowered the yield. The target compound BDT-3DTT-BIP was synthesized as a dark red powder with Na2S2O5 as a catalyst in 20.3% yield, and was characterised with highresolution mass, 1H NMR and 13C NMR spectra (Supporting Information (SI)). 2.3.1. Synthesis of compound 2 In a dried flask, benzo[1,2-b;4,5-b0 ]dithiophene (207 mg, 1.08 mmol) was dissolved in anhydrous THF (15 mL) under argon atmosphere. The mixture was cooled down to 78 C, and nbutyllithium (0.75 mL, 1.2 mmol, 1.6 M in hexane) was added. The system was kept at 78 C for 2 h and trimethyltin chloride (427 mg, 2.14 mmol) was added in one portion. The solution was slowly warmed up to room temperature and stirred overnight. The reaction was quenched by adding diethyl ether (50 mL), which was washed with sodium bicarbonate solution and water. The organic layer was separated and solvent was evaporated. The solid residue was recrystallized from acetonitrile to obtain 2 as a white powder 252 mg in yield 65.9%. 1H NMR d H (CDCl3, 300 MHz): 8.10e8.43 (2H, d, AreH), 7.37e7.51 (2H, m, AreH), 7.37-7.28 (1H, d, AreH), 0.26e0.51 (9H, s, CH3 3). 2.3.2. Synthesis of compound 4 In a dried flask, compound 2 (167 mg, 0.47 mmol) and compound 3 (351 mg, 0.59 mmol) were dissolved in anhydrous toluene (20 mL) under argon atmosphere. Tetrakis(triphenylphosphine) palladium(0) (59 mg, 0.05 mmol) was added and the resulting solution was heated to reflux overnight. After cooling to room temperature, the solvent was evaporated to afford the crude product, which was further purified by flash column chromatography over silica gel with methylene chloride (DCM) and hexane (DCM: hexane ¼ 1: 4) to afford the pure product 137 mg as an
2.2. OPV device fabrication Indiumetineoxide (ITO)-patterned glass substrates (20.0 20.0 mm, 15 U per square) were cleaned stepwise in water Semico clean 56, water, and isopropyl alcohol under ultrasonization for 10 min each. After the UV/O3 treatment for 20 min, (3,4ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS, Clevios P VP AI4083, 200 mL) was spin-coated at 3000 rpm for 30 s in air followed by a thermal annealing treatment at 130 C for 10 min in air. The thickness of the resulting PEDOT: PSS layer was about 30 nm. The substrates were transferred to a N2-filled glove box (<10 ppm O2 and H2O). BDT-3DTT-BIP and [6,6]-phenyl-C61butyric acid methyl ester (PC61BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) (10 mg mL1, 100 mL) in CHCl3 were spincoated at 1500 rpm for 30 s in a glove box. After preparation of the organic layers, calcium (10 nm) and aluminum (70 nm) were vapor deposited at high vacuum (1.0 104 Pa) through a shadow mask that defined an active area to 4.0 mm2.
Scheme 1. Synthetic route to BDT-3DTT-BIP.
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orange colour powder in yield of 42.3%. Bp, 79 C (760 mmHg), IR (KBr), cm1: 3435, 2922, 2851, 1662, 1429, 1245, 1156, 862, 799, 748, 649. 1H NMR d H (CDCl3, 400 MHz) 9.80e9.88 (1H, s, Ar-CHO), 8.17e8.25 (2H, s, AreH), 7.56e7.65 (1H, s, AreH), 7.45e7.50 (1H, d, AreH), 7.41e7.45 (1H, s, AreH), 7.32e7.38 (1H, d, AreH) 7.26e7.30 (1H, d, AreH), 7.13e7.19 (2H, m, AreH), 2.74e2.89 (4H, m, CH2 2), 1.64e1.77 (4H, m, CH2 2), 1.28e1.48 (20H, m, CH2 10), 0.82e0.94 (6H, m, CH3 2); d C (CDCl3, 101 MHz) 182.56, 141.16, 140.92, 140.42, 140.28, 139.08, 138.10, 137.85, 137.68, 137.60, 137.03, 136.40, 135.72, 134.78, 130.14, 128.30, 127.89, 127.19, 126.37, 123.01, 118.83, 116.68, 116.49, 31.89, 31.87, 30.52, 30.33, 29.62, 29.51, 29.45, 29.42, 29.28, 29.26, 22.68, 14.12. HR-MS (MOLDI-TOF) m/z Found: [M]þ 688.1990; molecular formula C39H44OS5 requires [M]þ 688.1996. 2.3.3. Synthesis of BDT-3DTT-BIP In a round bottom flask, 1,4-bis((triisopropylsilyl)thynyl)-2,3diaminalphenazine (159 mg, 0.28 mmol), compound 3 (165 mg, 0.24 mmol) were dissolved in DMF (25 mL). Sodium metabisulfite (61 mg, 0.32 mmol) was added and the resulting solution was heated to 120 C for 12 h. After cooling to room temperature, the mixture was poured into ice water (100 mL) and filtered to get crude product, which was further purified by flash column chromatography over silica gel with DCM and hexane (DCM: hexane ¼ 1:3) to get pure product 61 mg, yield 20.4%. Bp, 253 C (760 mmHg), IR (ATR), cm1: 2925, 2863, 2359, 2341, 1584, 1557, 1529, 1514, 1493, 1463, 1434, 1412, 1390, 1340, 1308, 1210, 1065, 1014, 882, 751, 669. 1H NMR d H (CD2Cl2, 400 MHz) 9.74e10.04 (1H, s, AreNH), 7.94e8.22 (4H, m, AreH), 7.60e7.72 (2H, d, AreH), 7.50e7.58 (1H, s, AreH), 7.31e7.36 (1H, m, AreH), 7.27e7.31 (1H, s, AreH), 7.17e7.22 (1H, d, AreH), 7.12e7.17 (1H, m, AreH), 7.03e7.10 (2H, m, AreH), 2.69e2.83 (4H, m, CH2 2), 1.67e1.78 (4H, m, CH2 2), 1.23e1.46 (62H, m, CH 6, CH2 10, CH3 12), 0.84e0.93 (6H, m, CH3 2); d C (CD2Cl2, 101 MHz) 153.41, 142.86, 141.44, 141.26, 138.56, 138.16, 138.07, 137.89, 137.62, 137.47, 136.75, 135.66, 135.17, 133.11, 131.08, 130.45, 129.96, 128.91, 128.45, 127.92, 127.72, 126.64, 123.47, 119.16, 117.12, 116.87, 32.51, 31.03, 30.81, 30.28, 30.25, 30.21, 30.16, 30.10, 30.03, 29.92, 23.29, 23.25, 19.32, 14.48, 14.45, 12.23. HR-MS (MOLDI-TOF) m/z Found: [Mþ1]þ 1239.5380; molecular formula C73H90N4Si2S5 requires [Mþ1]þ 1239.5308. 3. Results & discussion 3.1. Optical properties of BDT-3DTT-BIP The optical properties of the BDT-3DTT-BIP film and its blended films with PC61BM or PC71BM were investigated by UVevis absorption spectra. The normalized absorption spectra are shown in Fig. 1. BDT-3DTT-BIP film has two maximum absorbance peaks (lmax ¼ 458 nm, 547 nm) in the visible region. The onset of absorption (lonset) is 665 nm, which determined the optical band gap of 1.86 eV. When blended with PC61BM in a 1: 1 ratio, the lmax at 547 nm blue-shifted to 508 nm, meanwhile, the absorption of BDT3DTT-BIP/PC71BM blended film in a 1: 1 ratio also has a blue-shift (lmas ¼ 531 nm), but the maximum absorption peak is still 23 nm red-shifted compared to the BDT-3DTT-BIP/PC61BM blended film. Thus, the BDT-3DTT-BIP/PC71BM has a wider absorption range and modest miscibility with PC71BM, which could facilitate the charge separation at the donor/acceptor interfaces. The blue-shift of BDT3DTT-BIP/PC61BM and BDT-3DTT-BIP/PC71BM blended films might come from the partially disorder stacking of local BDT-3DTTBIP molecules compared to its neat film. Each of the blended films covers a broad range of absorbance in visible region from 400 nm to 642 nm, suggesting they have the potential to obtain a high shortcircuit current density (Jsc).
Fig. 1. Normalized UVevis absorption spectra of BDT-3DTT-BIP film (magenta) and BDT-3DTT-BIP/PC61BM blended film in 1: 1 ratio (navy) and BDT-3DTT-BIP/PC71BM blended film in 1: 1 ratio (violet). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.2. Theoretical calculation Theoretical calculation was carried out to simulate the intrinsic molecular configuration and electron density distribution. The geometry structure of BDT-3DTT-BIP was optimized by using DFT calculations (B3LYP/6-31G*) [48,49] and the frequency analysis was followed to assure that the optimized structures were stable states. The optimized structure of BDT-3DTT-BIP shows a broad “V” shape conformation, and the two octyl chains are in the same side. The calculated HOMO and LUMO energy levels are 5.05 eV and 2.72 eV, respectively. The band gap is calculated to be 2.33 eV, which is 0.47 eV larger than experimental results. Ionization energy of BDT-3DTT-BIP was measured to be 5.35 eV, which established the HOMO energy level (SI), and LUMO energy level was determined to be 3.49 eV from the difference between the HOMO energy level and optical band gap. As shown in Fig. 2, the HOMO energy level is distributed on the BDT and 3DTTT units, while the LUMO energy level is mainly located on the BIP moiety, which indicates that the lowest transition (from HOMO to LUMO) shows intramolecular charge transfer character. The calculated molecular ground state dipole moment is 4.90 D, which is also consistent with the electron density distribution of the HOMO and LUMO energy levels. The stronger intramolecular charge transfer could facilitate the charge separation at the donor/acceptor interfaces, thus improving the device performance.
3.3. OPV devices performance of BDT-3DTT-BIP The OPV devices were fabricated with a typical BHJ structure of ITO/anode buffer layer/BDT-3DTT-BIP:acceptor/Ca/Al through a solution-processing method. Poly(3,4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS) or molybdenum oxide (MoO3) was adopted as anode buffer layer, and PC61BM or PC71BM was used as acceptor, respectively, and the optimized weight ratio of BDT-3DTT-BIP with PC61BM or PC71BM was 1: 1.2 (SI). The thickness of the active layers is ~90 nm. The detailed parameters of the optimal device performance under AM 1.5G illumination (100 mW cm2) are summarized in Table 1. 1,8-diiodooctane (DIO) was added to the mixed donor: acceptor solution and might enhance the crystallization of BDT-3DTT-BIP [50]. The PCE of ITO/ PEDOT:PSS/BDT-3DTT-BIP:PC61BM/Ca/Al device with 0.5% DIO addition by volume was double enhanced from 0.45% to 0.9%
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decrease the generation and transportation of charge carriers. The OPV devices were further optimized by using PC71BM as acceptor and MoO3 as anode buffer layer. The ITO/MoO3/BDT-3DTTBIP:PC71BM/Ca/Al device displayed the ultimate performance with PCE up to 1.97%. Fig. 3(a) shows the JeV curves of three types of optimized devices with different anode buffer layer or acceptor. The ITO/ PEDOT:PSS/BDT-3DTT-BIP:PC61BM/Ca/Al device with 0.5% DIO addition by volume had an optimized PCE of 0.90%. The opencircuit voltage (Voc), Jsc and fill factor (FF) was 0.57 V, 4.42 mA cm2 and 36%, respectively. In the same optimized conditions, when the acceptor was replaced by PC71BM, the Voc, Jsc and PCE had a minor increase, probably resulting from the improved absorption of PC71BM in visible region, while the FF had little change. The device was further optimized by changing anode buffer layer from PEDOT:PSS to MoO3, which showed a PCE of 1.97%, increased by 89.4%. Although the Jsc had a minor decrease, the Voc was increased by 26%, which was probably from the greater alignment of the work function of MoO3 (HOMO energy level in the range of 5.3 to 5.7 eV) with the HOMO energy level of BDT3DTT-BIP [51]. The FF also had a large increase by 60%. The enhancement was probably coming from the reduced chemical interaction between BDT-3DTT-BIP and anode buffer layer, because the N atoms in BIP could react with the acidic PEDOT:PSS, which thus decreased the OPV performance [51e53].
Fig. 2. Electron density distribution of BDT-3DTT-BIP.
compared to the device without DIO addition. Thermal annealing has already been proved to be effective for controlling film morphology to facilitate charge carrier separation and transportation, while it did not work well to improve the performance of our devices [1,16]. The thermal decomposition temperature of BDT3DTT-BIP was 372 C, suggesting it was stable enough for annealing treatment. When the ITO/PEDOT:PSS/BDT-3DTT-BIP:PC61BM/Ca/Al device was thermal treated at 100 C, the PCE was significantly decreased from 0.45% to 0.18%. Increased domain size in annealed film was observed in AFM images (Fig. S4) and this result might Table 1 Parameters of optimal device performance. Devices a
PC61BM as acceptor PC61BM as acceptorb PC61BM as acceptorc PC71BM as acceptord MoO3 as buffer layere
Voc (V)
Jsc (mA cm2)
FF (%)
PCE (%)
0.61 0.65 0.57 0.65 0.82
2.63 1.32 4.42 4.62 4.29
28 21 36 35 56
0.45 0.18 0.90 1.04 1.97
a ITO/PEDOT:PSS/BDT-3DTT-BIP:PC61BM/Ca/Al thermal annealing. b ITO/PEDOT:PSS/BDT-3DTT-BIP:PC61BM/Ca/Al annealed at 100 C. c ITO/PEDOT:PSS/BDT-3DTT-BIP:PC61BM/Ca/Al and without thermal annealing. d ITO/PEDOT:PSS/BDT-3DTT-BIP:PC71BM/Ca/Al and without thermal annealing. e ITO/PEDOT:PSS/BDT-3DTT-BIP:PC71BM/Ca/Al and without thermal annealing.
device without DIO addition and device without DIO addition and device with 0.5% DIO addition device with 0.5% DIO addition device with 0.5% DIO addition
Fig. 3. (a) JV curves and (b) EQE plots of the ITO/PEDOT:PSS/BDT-3DTT-BIP:PC61BM/ Ca/Al device (square), ITO/PEDOT:PSS/BDT-3DTT-BIP:PC71BM/Ca/Al device (triangle), and ITO/MoO3/BDT-3DTT-BIP:PC71BM/Ca/Al device (sphere).
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The external quantum efficiency (EQE) of the as-fabricated devices was investigated under illumination of monochromatic light 1.25 mW cm2, the corresponded plots are shown in Fig. 3(b). ITO/ PEDOT:PSS/BDT-3DTT-BIP:PC61BM/Ca/Al device had high conversion of absorption (EQE > 10%) from 309 nm to 611 nm, with two maximum EQE value at 396 nm (37%) and 521 nm (31%), respectively. As PC71BM was used as acceptor, the range corresponded to high photon conversion (EQE > 10%) was expanded from 311 nm to 644 nm, which was 31 nm broader than the device with PC61BM as acceptor. The larger photo conversion range was corresponded to the stronger absorption of BDT-3DTT-BIP/PC71BM blended film and the higher Jsc in JeV curve of ITO/PEDOT:PSS/BDT-3DTTBIP:PC71BM/Ca/Al device. The maximum EQE values were increased to 42% at 399 nm and 43% at 508 nm, respectively. Keeping PC71BM as the acceptor, when the anode buffer layer was replaced with MoO3, the EQE curve showed a similar tendency but with decreased EQE at 35% (384 nm) and 34% (522 nm), respectively. The higher EQE efficiency of ITO/PEDOT:PSS/BDT-3DTTBIP:PC71BM/Ca/Al device probably resulted from the existence of leakage current in the dark. 3.4. Morphology analysis The morphology of the fabricated devices was investigated by tapping-mode atomic force microscopy (AFM). As shown in Fig. 4(a), when BDT-3DTT-BIP/PC61BM blended film spin-coated on ITO/PEDOT:PSS substrate without addition of DIO, the film was smooth and well continuous with a small roughness (RMS ¼ 0.48 nm). As DIO was added, the film showed a large increase of RMS from 0.48 nm to 15 nm (Fig. 4(b)). The increased crystallinity of the blended film were observed and could possibly enhance the transportation of charge carriers, thus contribute to the improved Jsc (from 2.63 to 4.42 mA cm2) and FF (from 28% to 36%). Meanwhile, the BDT-3DTT-BIP/PC71BM blended films on ITO/ PEDOT:PSS or ITO/MoO3 substrate (Figs. 4(c), (d)) showed similar morphology with close roughness after adding DIO (RMS ¼ 18.2 nm and RMS ¼ 20.2 nm, respectively). However, the rough surface of
Fig. 4. AFM images of corresponding films (a) BDT-3DTT-BIP/PC61BM blended film without DIO, (b) BDT-3DTT-BIP/PC61BM blended film with 0.5% DIO, (c) BDT-3DTTBIP/PC71BM blended film with 0.5% DIO on PEDOT:PSS, (d) BDT-3DTT-BIP:PC71BM blended film with 0.5% DIO on MoO3.
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the active layer could induce bad contact situation between the electrodes and active layer, which did not favor for the transportation of charge carriers. This might explain why all the devices with the addition of DIO had relatively low Jsc. The performance enhancement of ITO/MoO3/BDT-3DTT-BIP:PC71BM/Ca/Al device might have little relevance to the morphology. To further confirm the enhanced crystallinity of the blended films after adding DIO, the X-ray diffraction (XRD) patterns of the corresponding blend film with or without DIO were investigated. As shown in Fig. 5, the BDT-3DTT-BIP/PC61BM blended film showed no diffraction peak. After adding DIO, a diffraction peak at 2q ¼ ca. 4.6 was observed, which corresponded to the d100 spacing value of 19.3 Å. The d100 spacing value is the distance between the planes of the major conjugated backbone separated by the longer alkyl chain (C8H17) [54]. The obvious XRD peak suggested that highly organized assembly of BDT-3DTT-BIP molecules after adding the DIO to the active layer. The BDT-3DTT-BIP/PC71BM blended film showed similar diffraction peak position when DIO was added, but the peak was much sharper than that of the BDT-3DTT-BIP/ PC61BM blended film, suggesting it had higher crystallinity. When the anode buffer layer (PEDOT: PSS) was changed to MoO3, a peak in same position was observed with a minor decreased sharpness. In a previous study, people had already demonstrated that the donor molecules have stronger nucleation and crystallinity on PEDOT: PSS surface because of certain interactions, which explained the lower crystallinity of BDT-3DTT-BIP/PC71BM blended film on MoO3 surface [55]. The enhanced PCE probably mainly come from the more aligned energy level and reduced chemical reaction between active layer and PEDOT: PSS.
4. Conclusions In conclusion, a novel DepeA small molecule BDT-3DTT-BIP with a large N-heteroacene as the acceptor moiety has been synthesized and characterised. The standard BHJ structure OPV devices were investigated through a solution-processing method with different acceptor materials and different anode buffer layers. When PEDOT:PSS was used as buffer layer, the devices showed PCE
Fig. 5. XRD patterns of corresponding films (magenta: BDT-3DTT-BIP/PC61BM blended film without DIO, navy: BDT-3DTT-BIP/PC61BM blended film with 0.5% DIO, orange: BDT-3DTT-BIP/PC71BM blended film with 0.5% DIO on PEDOT: PSS, violet: BDT-3DTTBIP/PC71BM blended film with 0.5% DIO on MoO3). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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around 1% with PC61BM or PC71BM as acceptors. The ultimate performance was achieved by using MoO3 as the anode buffer layer, the PCE was almost doubled. The AFM images and XRD patterns indicates the anode buffer layer had no obvious influence on film morphology, and combining with the JeV curves and EQE plots, the more aligned work function of the anode buffer layer and reduced chemical interactions between the buffer layer and active layer played an important role to enhance the PCE of the devices. Although there is a gap of efficiency between BDT-3DTT-BIP and the highest record, its special property such as annealing-free in processing suggests that large N-heteroacenes have the potential to be applied as good acceptor moieties, and further investigation to synthesize novel suitable materials and improve device performance is under progress.
Acknowledgements Q.Z. acknowledges the financial support AcRF Tier 1 (RG133/14) and Tier 2 (ARC 20/12 and ARC 2/13) from MOE, CREATE program (Nanomaterials for Energy and Water Management) from NRF, and New Initiative Fund from NTU, Singapore. This work was partly supported by Grants-in-Aid for Scientific Research (KAKENHI) Nos. 25288092 (H.Y.), 26105004 (H.Y.) and 25107519 (N.A.) from the Japan Society for the Promotion of Science (JSPS).
Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2015.06.029.
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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
A novel DepeA small molecule with N-heteroacene as acceptor moiety for photovoltaic application Chengyuan Wang a, 1, Takuya Okabe b, 1, Guankui Long a, Daiki Kuzuhara b, Yang Zhao a, Naoki Aratani b, ***, Hiroko Yamada b, c, **, Qichun Zhang a, d, * a
School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma 630-0192, Japan CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan d Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 April 2015 Received in revised form 25 June 2015 Accepted 27 June 2015 Available online 8 July 2015
Organic p-conjugated small molecules have attracted much attention for developing high performance organic photovoltaics (OPVs) due to their well-defined molecular structure, easily controlled energy levels and absorption, and more accurate simulation-experiment match. A novel small molecule combing a N-heteroacene (acceptor moiety) and benzo[1,2-b:4,5-b0 ]dithiophene (donor unit) together through a p-conjugated bridge (3,300 -dioctyl-2,20 :50 ,200 -terthiophene), has been synthesized and characterized. The blended films of this small molecule with different acceptors have a wide absorption in visible region, which makes it possible for application in OPVs. The as-fabricated devices with PEDOT:PSS as an anode buffer layer show the power conversion efficiency at around 1%. By replacing PEDOT:PSS with MoO3, the power conversion efficiency is almost doubled up to 1.97%. AFM images and XRD patterns are employed to investigate the morphologies of the active layer. Consistent with the JeV curves and EQE spectra, the higher power conversion efficiency probably comes from the good alignment of the HOMO energy level of the small molecule with the work function of anode buffer layer and reduced chemical interactions between active layer and anode buffer layer. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Small molecule-based photovoltaics DepeA structure N-heteroacenes Molybdenum trioxide Synthesis Devices' fabrication
1. Introduction Organic photovoltaics (OPVs) have been demonstrated to be a potentially new generation of power source to address the increasing and serious energy crisis. Compared with the conventional inorganic photovoltaics, solution-processed OPVs have lower fabrication cost, light-weight, and high flexibility, which make them more attractive in practical applications [1e7]. Currently, bulk heterojunction (BHJ) structure devices have become the standard OPVs' architecture [8e10]. Organic p-conjugated semiconducting
* Corresponding author. School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore. ** Corresponding author. Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma 630-0192, Japan. *** Corresponding author. E-mail addresses: [email protected] (N. Aratani), [email protected] (H. Yamada), [email protected] (Q. Zhang). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.dyepig.2015.06.029 0143-7208/© 2015 Elsevier Ltd. All rights reserved.
materials, including polymers and small molecules, are commonly used as donors and acceptors in OPVs. Among all acceptors, fullerene and its derivatives are widely employed due to their good stability and low LUMO energy levels [11e13]. To our knowledge, donor materials play a more important role in the generation and transportation of charge carrier because the possibility of broader absorbance in the visible and near-infrared region can contribute to better harvest of sunlight [14,15]. Organic conjugated small molecules have drawn a lot of researchers’ attention because (1) organic small molecules with well-defined molecular structures are able to keep good batch-to-batch consistency, and (2) their more accurate simulation-experiment match is in favour of demonstrating property-structure relationship for deeper understanding of intrinsic charge carrier generation and transportation mechanism and providing rational molecular designing strategies [16e23]. To date, much effort has been devoted to develop diverse small molecule materials, and the power conversion efficiencies (PCE) have been optimized up to 9% for single junction devices, however, these results are still far away from the theoretical efficiency
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(20e24%) [24,25]. Clearly, there is still a long way to develop ultimate donor materials with higher performance. Recently, large N-heteroacenes have drawn much attention for their applications in organic electronics such as field-effect transistors, phototransistors, light-emitting diodes, photoelectrochemical cells, and photovoltaics [26e32]. In our previous study, 4,11-bis((triisopropylsilyl)thynyl)-1H-imidazo[4,5-b]phenazin (BIP) has been illustrated to be a useful acceptor moiety with moderate electron withdrawing ability [33,34]. The existence of electron-deficient pyridine or pyrazine units in BIP enables it to induce strong intramolecular charge transfer and wide absorbance in visible region, which benefits the generation of charge carriers [35e38]. The large conjugated backbone of the BIP moiety probably contributes to enhance the conjugation of the whole molecule, and also facilitate the migration of charge carriers. In this work, we would like to connect BIP with a typical benzo[1,2-b:4,5-b0 ] dithiophene (BDT) unit by a p-conjugated bridge (3,300 -dioctyl2,20 :50 ,200 -terthiophene (3DTT)), which also could increase the conjugation, to build up a DepeA molecule BDT-3DTT-BIP and investigate its OPV performance [39e44]. 2. Experimental section 2.1. Materials and characterization methods The chemical reagents and solvents were used directly as received from commercial companies without further purification. High-resolution mass spectra were collected on a JEOL spiral TOF JMS-S3000 spectrometer. 1H NMR and 13C NMR spectra were collected on a JEOL JNM-AL300 and JEOL JNM-ECX400 spectrometers. UVevis absorption spectra were measured on a JASCO UV/ VIS/NIR Spectro-photometer V-670. Ionization potential was evaluated with atmospheric photoelectron spectroscopy (Riken Keiki, AC-3). Currentevoltage (JeV) curves were recorded from a Keithley 2611B SYSTEM Source Mater unit under AM 1.5G illumination at intensity of 100 mW cm2 using a solar simulator (Bunko-keiki, CEP-2000RP). The external quantum efficiency (EQE) was measured under illumination of monochromatic light using the same system at intensity of 1.25 mW cm2. The surface morphology of organic films was investigated by SPA400, SPI3800N AFM (Seiko instruments Inc.) in a tapping mode using silicon probes with a resonant frequency of ~138 kHz and a force constant of 16 N m1. The out-of-plane X-ray diffraction (XRD) was carried out on a RINTTTRⅢ/NM diffractometer equipped with a rotating anode (Cu Ka radiation, l ¼ 1.5418 Å). All calculations were carried out using Gaussian 09.
2.3. Molecule synthesis Scheme 1 shows the synthetic route to BDT-3DTT-BIP. The single trimethylstannyl substituted BDT 2 was synthesized by a modified procedure based on the literature [45]. The recrystallized product was used directly for next step reaction, although there was still some double trimethylstannyl substituted BDT byproduct. Intermediate 3 was prepared through a reported procedure [46,47]. Compound 4 was obtained by Stille coupling reaction in 42.3% yield. The existence of double trimethylstannyl substituted BDT by-product probably lowered the yield. The target compound BDT-3DTT-BIP was synthesized as a dark red powder with Na2S2O5 as a catalyst in 20.3% yield, and was characterised with highresolution mass, 1H NMR and 13C NMR spectra (Supporting Information (SI)). 2.3.1. Synthesis of compound 2 In a dried flask, benzo[1,2-b;4,5-b0 ]dithiophene (207 mg, 1.08 mmol) was dissolved in anhydrous THF (15 mL) under argon atmosphere. The mixture was cooled down to 78 C, and nbutyllithium (0.75 mL, 1.2 mmol, 1.6 M in hexane) was added. The system was kept at 78 C for 2 h and trimethyltin chloride (427 mg, 2.14 mmol) was added in one portion. The solution was slowly warmed up to room temperature and stirred overnight. The reaction was quenched by adding diethyl ether (50 mL), which was washed with sodium bicarbonate solution and water. The organic layer was separated and solvent was evaporated. The solid residue was recrystallized from acetonitrile to obtain 2 as a white powder 252 mg in yield 65.9%. 1H NMR d H (CDCl3, 300 MHz): 8.10e8.43 (2H, d, AreH), 7.37e7.51 (2H, m, AreH), 7.37-7.28 (1H, d, AreH), 0.26e0.51 (9H, s, CH3 3). 2.3.2. Synthesis of compound 4 In a dried flask, compound 2 (167 mg, 0.47 mmol) and compound 3 (351 mg, 0.59 mmol) were dissolved in anhydrous toluene (20 mL) under argon atmosphere. Tetrakis(triphenylphosphine) palladium(0) (59 mg, 0.05 mmol) was added and the resulting solution was heated to reflux overnight. After cooling to room temperature, the solvent was evaporated to afford the crude product, which was further purified by flash column chromatography over silica gel with methylene chloride (DCM) and hexane (DCM: hexane ¼ 1: 4) to afford the pure product 137 mg as an
2.2. OPV device fabrication Indiumetineoxide (ITO)-patterned glass substrates (20.0 20.0 mm, 15 U per square) were cleaned stepwise in water Semico clean 56, water, and isopropyl alcohol under ultrasonization for 10 min each. After the UV/O3 treatment for 20 min, (3,4ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS, Clevios P VP AI4083, 200 mL) was spin-coated at 3000 rpm for 30 s in air followed by a thermal annealing treatment at 130 C for 10 min in air. The thickness of the resulting PEDOT: PSS layer was about 30 nm. The substrates were transferred to a N2-filled glove box (<10 ppm O2 and H2O). BDT-3DTT-BIP and [6,6]-phenyl-C61butyric acid methyl ester (PC61BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) (10 mg mL1, 100 mL) in CHCl3 were spincoated at 1500 rpm for 30 s in a glove box. After preparation of the organic layers, calcium (10 nm) and aluminum (70 nm) were vapor deposited at high vacuum (1.0 104 Pa) through a shadow mask that defined an active area to 4.0 mm2.
Scheme 1. Synthetic route to BDT-3DTT-BIP.
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orange colour powder in yield of 42.3%. Bp, 79 C (760 mmHg), IR (KBr), cm1: 3435, 2922, 2851, 1662, 1429, 1245, 1156, 862, 799, 748, 649. 1H NMR d H (CDCl3, 400 MHz) 9.80e9.88 (1H, s, Ar-CHO), 8.17e8.25 (2H, s, AreH), 7.56e7.65 (1H, s, AreH), 7.45e7.50 (1H, d, AreH), 7.41e7.45 (1H, s, AreH), 7.32e7.38 (1H, d, AreH) 7.26e7.30 (1H, d, AreH), 7.13e7.19 (2H, m, AreH), 2.74e2.89 (4H, m, CH2 2), 1.64e1.77 (4H, m, CH2 2), 1.28e1.48 (20H, m, CH2 10), 0.82e0.94 (6H, m, CH3 2); d C (CDCl3, 101 MHz) 182.56, 141.16, 140.92, 140.42, 140.28, 139.08, 138.10, 137.85, 137.68, 137.60, 137.03, 136.40, 135.72, 134.78, 130.14, 128.30, 127.89, 127.19, 126.37, 123.01, 118.83, 116.68, 116.49, 31.89, 31.87, 30.52, 30.33, 29.62, 29.51, 29.45, 29.42, 29.28, 29.26, 22.68, 14.12. HR-MS (MOLDI-TOF) m/z Found: [M]þ 688.1990; molecular formula C39H44OS5 requires [M]þ 688.1996. 2.3.3. Synthesis of BDT-3DTT-BIP In a round bottom flask, 1,4-bis((triisopropylsilyl)thynyl)-2,3diaminalphenazine (159 mg, 0.28 mmol), compound 3 (165 mg, 0.24 mmol) were dissolved in DMF (25 mL). Sodium metabisulfite (61 mg, 0.32 mmol) was added and the resulting solution was heated to 120 C for 12 h. After cooling to room temperature, the mixture was poured into ice water (100 mL) and filtered to get crude product, which was further purified by flash column chromatography over silica gel with DCM and hexane (DCM: hexane ¼ 1:3) to get pure product 61 mg, yield 20.4%. Bp, 253 C (760 mmHg), IR (ATR), cm1: 2925, 2863, 2359, 2341, 1584, 1557, 1529, 1514, 1493, 1463, 1434, 1412, 1390, 1340, 1308, 1210, 1065, 1014, 882, 751, 669. 1H NMR d H (CD2Cl2, 400 MHz) 9.74e10.04 (1H, s, AreNH), 7.94e8.22 (4H, m, AreH), 7.60e7.72 (2H, d, AreH), 7.50e7.58 (1H, s, AreH), 7.31e7.36 (1H, m, AreH), 7.27e7.31 (1H, s, AreH), 7.17e7.22 (1H, d, AreH), 7.12e7.17 (1H, m, AreH), 7.03e7.10 (2H, m, AreH), 2.69e2.83 (4H, m, CH2 2), 1.67e1.78 (4H, m, CH2 2), 1.23e1.46 (62H, m, CH 6, CH2 10, CH3 12), 0.84e0.93 (6H, m, CH3 2); d C (CD2Cl2, 101 MHz) 153.41, 142.86, 141.44, 141.26, 138.56, 138.16, 138.07, 137.89, 137.62, 137.47, 136.75, 135.66, 135.17, 133.11, 131.08, 130.45, 129.96, 128.91, 128.45, 127.92, 127.72, 126.64, 123.47, 119.16, 117.12, 116.87, 32.51, 31.03, 30.81, 30.28, 30.25, 30.21, 30.16, 30.10, 30.03, 29.92, 23.29, 23.25, 19.32, 14.48, 14.45, 12.23. HR-MS (MOLDI-TOF) m/z Found: [Mþ1]þ 1239.5380; molecular formula C73H90N4Si2S5 requires [Mþ1]þ 1239.5308. 3. Results & discussion 3.1. Optical properties of BDT-3DTT-BIP The optical properties of the BDT-3DTT-BIP film and its blended films with PC61BM or PC71BM were investigated by UVevis absorption spectra. The normalized absorption spectra are shown in Fig. 1. BDT-3DTT-BIP film has two maximum absorbance peaks (lmax ¼ 458 nm, 547 nm) in the visible region. The onset of absorption (lonset) is 665 nm, which determined the optical band gap of 1.86 eV. When blended with PC61BM in a 1: 1 ratio, the lmax at 547 nm blue-shifted to 508 nm, meanwhile, the absorption of BDT3DTT-BIP/PC71BM blended film in a 1: 1 ratio also has a blue-shift (lmas ¼ 531 nm), but the maximum absorption peak is still 23 nm red-shifted compared to the BDT-3DTT-BIP/PC61BM blended film. Thus, the BDT-3DTT-BIP/PC71BM has a wider absorption range and modest miscibility with PC71BM, which could facilitate the charge separation at the donor/acceptor interfaces. The blue-shift of BDT3DTT-BIP/PC61BM and BDT-3DTT-BIP/PC71BM blended films might come from the partially disorder stacking of local BDT-3DTTBIP molecules compared to its neat film. Each of the blended films covers a broad range of absorbance in visible region from 400 nm to 642 nm, suggesting they have the potential to obtain a high shortcircuit current density (Jsc).
Fig. 1. Normalized UVevis absorption spectra of BDT-3DTT-BIP film (magenta) and BDT-3DTT-BIP/PC61BM blended film in 1: 1 ratio (navy) and BDT-3DTT-BIP/PC71BM blended film in 1: 1 ratio (violet). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.2. Theoretical calculation Theoretical calculation was carried out to simulate the intrinsic molecular configuration and electron density distribution. The geometry structure of BDT-3DTT-BIP was optimized by using DFT calculations (B3LYP/6-31G*) [48,49] and the frequency analysis was followed to assure that the optimized structures were stable states. The optimized structure of BDT-3DTT-BIP shows a broad “V” shape conformation, and the two octyl chains are in the same side. The calculated HOMO and LUMO energy levels are 5.05 eV and 2.72 eV, respectively. The band gap is calculated to be 2.33 eV, which is 0.47 eV larger than experimental results. Ionization energy of BDT-3DTT-BIP was measured to be 5.35 eV, which established the HOMO energy level (SI), and LUMO energy level was determined to be 3.49 eV from the difference between the HOMO energy level and optical band gap. As shown in Fig. 2, the HOMO energy level is distributed on the BDT and 3DTTT units, while the LUMO energy level is mainly located on the BIP moiety, which indicates that the lowest transition (from HOMO to LUMO) shows intramolecular charge transfer character. The calculated molecular ground state dipole moment is 4.90 D, which is also consistent with the electron density distribution of the HOMO and LUMO energy levels. The stronger intramolecular charge transfer could facilitate the charge separation at the donor/acceptor interfaces, thus improving the device performance.
3.3. OPV devices performance of BDT-3DTT-BIP The OPV devices were fabricated with a typical BHJ structure of ITO/anode buffer layer/BDT-3DTT-BIP:acceptor/Ca/Al through a solution-processing method. Poly(3,4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS) or molybdenum oxide (MoO3) was adopted as anode buffer layer, and PC61BM or PC71BM was used as acceptor, respectively, and the optimized weight ratio of BDT-3DTT-BIP with PC61BM or PC71BM was 1: 1.2 (SI). The thickness of the active layers is ~90 nm. The detailed parameters of the optimal device performance under AM 1.5G illumination (100 mW cm2) are summarized in Table 1. 1,8-diiodooctane (DIO) was added to the mixed donor: acceptor solution and might enhance the crystallization of BDT-3DTT-BIP [50]. The PCE of ITO/ PEDOT:PSS/BDT-3DTT-BIP:PC61BM/Ca/Al device with 0.5% DIO addition by volume was double enhanced from 0.45% to 0.9%
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decrease the generation and transportation of charge carriers. The OPV devices were further optimized by using PC71BM as acceptor and MoO3 as anode buffer layer. The ITO/MoO3/BDT-3DTTBIP:PC71BM/Ca/Al device displayed the ultimate performance with PCE up to 1.97%. Fig. 3(a) shows the JeV curves of three types of optimized devices with different anode buffer layer or acceptor. The ITO/ PEDOT:PSS/BDT-3DTT-BIP:PC61BM/Ca/Al device with 0.5% DIO addition by volume had an optimized PCE of 0.90%. The opencircuit voltage (Voc), Jsc and fill factor (FF) was 0.57 V, 4.42 mA cm2 and 36%, respectively. In the same optimized conditions, when the acceptor was replaced by PC71BM, the Voc, Jsc and PCE had a minor increase, probably resulting from the improved absorption of PC71BM in visible region, while the FF had little change. The device was further optimized by changing anode buffer layer from PEDOT:PSS to MoO3, which showed a PCE of 1.97%, increased by 89.4%. Although the Jsc had a minor decrease, the Voc was increased by 26%, which was probably from the greater alignment of the work function of MoO3 (HOMO energy level in the range of 5.3 to 5.7 eV) with the HOMO energy level of BDT3DTT-BIP [51]. The FF also had a large increase by 60%. The enhancement was probably coming from the reduced chemical interaction between BDT-3DTT-BIP and anode buffer layer, because the N atoms in BIP could react with the acidic PEDOT:PSS, which thus decreased the OPV performance [51e53].
Fig. 2. Electron density distribution of BDT-3DTT-BIP.
compared to the device without DIO addition. Thermal annealing has already been proved to be effective for controlling film morphology to facilitate charge carrier separation and transportation, while it did not work well to improve the performance of our devices [1,16]. The thermal decomposition temperature of BDT3DTT-BIP was 372 C, suggesting it was stable enough for annealing treatment. When the ITO/PEDOT:PSS/BDT-3DTT-BIP:PC61BM/Ca/Al device was thermal treated at 100 C, the PCE was significantly decreased from 0.45% to 0.18%. Increased domain size in annealed film was observed in AFM images (Fig. S4) and this result might Table 1 Parameters of optimal device performance. Devices a
PC61BM as acceptor PC61BM as acceptorb PC61BM as acceptorc PC71BM as acceptord MoO3 as buffer layere
Voc (V)
Jsc (mA cm2)
FF (%)
PCE (%)
0.61 0.65 0.57 0.65 0.82
2.63 1.32 4.42 4.62 4.29
28 21 36 35 56
0.45 0.18 0.90 1.04 1.97
a ITO/PEDOT:PSS/BDT-3DTT-BIP:PC61BM/Ca/Al thermal annealing. b ITO/PEDOT:PSS/BDT-3DTT-BIP:PC61BM/Ca/Al annealed at 100 C. c ITO/PEDOT:PSS/BDT-3DTT-BIP:PC61BM/Ca/Al and without thermal annealing. d ITO/PEDOT:PSS/BDT-3DTT-BIP:PC71BM/Ca/Al and without thermal annealing. e ITO/PEDOT:PSS/BDT-3DTT-BIP:PC71BM/Ca/Al and without thermal annealing.
device without DIO addition and device without DIO addition and device with 0.5% DIO addition device with 0.5% DIO addition device with 0.5% DIO addition
Fig. 3. (a) JV curves and (b) EQE plots of the ITO/PEDOT:PSS/BDT-3DTT-BIP:PC61BM/ Ca/Al device (square), ITO/PEDOT:PSS/BDT-3DTT-BIP:PC71BM/Ca/Al device (triangle), and ITO/MoO3/BDT-3DTT-BIP:PC71BM/Ca/Al device (sphere).
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The external quantum efficiency (EQE) of the as-fabricated devices was investigated under illumination of monochromatic light 1.25 mW cm2, the corresponded plots are shown in Fig. 3(b). ITO/ PEDOT:PSS/BDT-3DTT-BIP:PC61BM/Ca/Al device had high conversion of absorption (EQE > 10%) from 309 nm to 611 nm, with two maximum EQE value at 396 nm (37%) and 521 nm (31%), respectively. As PC71BM was used as acceptor, the range corresponded to high photon conversion (EQE > 10%) was expanded from 311 nm to 644 nm, which was 31 nm broader than the device with PC61BM as acceptor. The larger photo conversion range was corresponded to the stronger absorption of BDT-3DTT-BIP/PC71BM blended film and the higher Jsc in JeV curve of ITO/PEDOT:PSS/BDT-3DTTBIP:PC71BM/Ca/Al device. The maximum EQE values were increased to 42% at 399 nm and 43% at 508 nm, respectively. Keeping PC71BM as the acceptor, when the anode buffer layer was replaced with MoO3, the EQE curve showed a similar tendency but with decreased EQE at 35% (384 nm) and 34% (522 nm), respectively. The higher EQE efficiency of ITO/PEDOT:PSS/BDT-3DTTBIP:PC71BM/Ca/Al device probably resulted from the existence of leakage current in the dark. 3.4. Morphology analysis The morphology of the fabricated devices was investigated by tapping-mode atomic force microscopy (AFM). As shown in Fig. 4(a), when BDT-3DTT-BIP/PC61BM blended film spin-coated on ITO/PEDOT:PSS substrate without addition of DIO, the film was smooth and well continuous with a small roughness (RMS ¼ 0.48 nm). As DIO was added, the film showed a large increase of RMS from 0.48 nm to 15 nm (Fig. 4(b)). The increased crystallinity of the blended film were observed and could possibly enhance the transportation of charge carriers, thus contribute to the improved Jsc (from 2.63 to 4.42 mA cm2) and FF (from 28% to 36%). Meanwhile, the BDT-3DTT-BIP/PC71BM blended films on ITO/ PEDOT:PSS or ITO/MoO3 substrate (Figs. 4(c), (d)) showed similar morphology with close roughness after adding DIO (RMS ¼ 18.2 nm and RMS ¼ 20.2 nm, respectively). However, the rough surface of
Fig. 4. AFM images of corresponding films (a) BDT-3DTT-BIP/PC61BM blended film without DIO, (b) BDT-3DTT-BIP/PC61BM blended film with 0.5% DIO, (c) BDT-3DTTBIP/PC71BM blended film with 0.5% DIO on PEDOT:PSS, (d) BDT-3DTT-BIP:PC71BM blended film with 0.5% DIO on MoO3.
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the active layer could induce bad contact situation between the electrodes and active layer, which did not favor for the transportation of charge carriers. This might explain why all the devices with the addition of DIO had relatively low Jsc. The performance enhancement of ITO/MoO3/BDT-3DTT-BIP:PC71BM/Ca/Al device might have little relevance to the morphology. To further confirm the enhanced crystallinity of the blended films after adding DIO, the X-ray diffraction (XRD) patterns of the corresponding blend film with or without DIO were investigated. As shown in Fig. 5, the BDT-3DTT-BIP/PC61BM blended film showed no diffraction peak. After adding DIO, a diffraction peak at 2q ¼ ca. 4.6 was observed, which corresponded to the d100 spacing value of 19.3 Å. The d100 spacing value is the distance between the planes of the major conjugated backbone separated by the longer alkyl chain (C8H17) [54]. The obvious XRD peak suggested that highly organized assembly of BDT-3DTT-BIP molecules after adding the DIO to the active layer. The BDT-3DTT-BIP/PC71BM blended film showed similar diffraction peak position when DIO was added, but the peak was much sharper than that of the BDT-3DTT-BIP/ PC61BM blended film, suggesting it had higher crystallinity. When the anode buffer layer (PEDOT: PSS) was changed to MoO3, a peak in same position was observed with a minor decreased sharpness. In a previous study, people had already demonstrated that the donor molecules have stronger nucleation and crystallinity on PEDOT: PSS surface because of certain interactions, which explained the lower crystallinity of BDT-3DTT-BIP/PC71BM blended film on MoO3 surface [55]. The enhanced PCE probably mainly come from the more aligned energy level and reduced chemical reaction between active layer and PEDOT: PSS.
4. Conclusions In conclusion, a novel DepeA small molecule BDT-3DTT-BIP with a large N-heteroacene as the acceptor moiety has been synthesized and characterised. The standard BHJ structure OPV devices were investigated through a solution-processing method with different acceptor materials and different anode buffer layers. When PEDOT:PSS was used as buffer layer, the devices showed PCE
Fig. 5. XRD patterns of corresponding films (magenta: BDT-3DTT-BIP/PC61BM blended film without DIO, navy: BDT-3DTT-BIP/PC61BM blended film with 0.5% DIO, orange: BDT-3DTT-BIP/PC71BM blended film with 0.5% DIO on PEDOT: PSS, violet: BDT-3DTTBIP/PC71BM blended film with 0.5% DIO on MoO3). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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around 1% with PC61BM or PC71BM as acceptors. The ultimate performance was achieved by using MoO3 as the anode buffer layer, the PCE was almost doubled. The AFM images and XRD patterns indicates the anode buffer layer had no obvious influence on film morphology, and combining with the JeV curves and EQE plots, the more aligned work function of the anode buffer layer and reduced chemical interactions between the buffer layer and active layer played an important role to enhance the PCE of the devices. Although there is a gap of efficiency between BDT-3DTT-BIP and the highest record, its special property such as annealing-free in processing suggests that large N-heteroacenes have the potential to be applied as good acceptor moieties, and further investigation to synthesize novel suitable materials and improve device performance is under progress.
Acknowledgements Q.Z. acknowledges the financial support AcRF Tier 1 (RG133/14) and Tier 2 (ARC 20/12 and ARC 2/13) from MOE, CREATE program (Nanomaterials for Energy and Water Management) from NRF, and New Initiative Fund from NTU, Singapore. This work was partly supported by Grants-in-Aid for Scientific Research (KAKENHI) Nos. 25288092 (H.Y.), 26105004 (H.Y.) and 25107519 (N.A.) from the Japan Society for the Promotion of Science (JSPS).
Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2015.06.029.
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