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Solution-processed small molecules based on indacenodithiophene for high performance thin-film transistors and organic solar cells
Organic Electronics 15 (2014) 1155–1165
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Solution-processed small molecules based on indacenodithiophene for high performance thin-film transistors and organic solar cells Wenli Tang a,b, Dazhen Huang b, Chang He b,⇑, Yuanping Yi b, Jing Zhang b, Chongan Di b,⇑, Zhanjun Zhang a,⇑, Yongfang Li b,⇑ a b
University of Chinese Academy of Sciences, Beijing 100049, China Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e
i n f o
Article history: Received 18 January 2014 Received in revised form 10 March 2014 Accepted 11 March 2014 Available online 22 March 2014 Keywords: Solution-processable organic molecules Organic field-effect transistors Organic solar cells Indacenodithiophene Hole mobility
a b s t r a c t Solution-processed indacenodithiophene (IDT)-based small molecules with 1,3-indanedione (ID) as terminal acceptor units and 3,30 -hexyl-terthiophene (IDT-3Th-ID(I)) or 4,40 -hexyl-terthiophene (IDT-3Th-ID(II)) as p-bridges, have been designed and synthesized for the application in organic field-effect transistors (OFETs) and organic solar cells (OSCs). These molecules exhibited excellent solubility in common organic solvents, good filmforming ability, reasonable thermal stability, and low HOMO energy levels. For the OFETs devices, high hole motilities of 0.52 cm2 V1 s1 for IDT-3Th-ID(I) and 0.61 cm2 V1 s1 for IDT-3Th-ID(II) were achieved, with corresponding high ION/IOFF of ca. 107 and 109 respectively. The OSCs based on IDT-3Th-ID(I)/PC70BM (2:1, w/w) and IDT-3Th-ID(II)/PC70BM (2:1, w/w) without using any treatment of solvent additive or thermal annealing, showed power conversion efficiencies (PCEs) of 3.07% for IDT-3Th-ID(I) and 2.83% for IDT-3Th-ID(II), under the illumination of AM 1.5G, 100 mW/cm2. The results demonstrate that the small molecules constructed with the highly p-conjugated IDT as donor unit, 3Th as p-bridges and ID as acceptor units, could be promising organic semiconductors for highperformance OFETs and OSCs applications. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Organic optoelectronic devices based on solution-processing method compatible with roll-to-roll processing, have received tremendous scientific and industrial interest in recent years due to their cost-effective and large-scale reproductive advantages [1–7]. Compared to polymers, solution-processed small-molecules have the advantages
⇑ Corresponding authors. Fax: +86 10 62559373. E-mail addresses: [email protected] (W. Tang), huangdz@ iccas.ac.cn (D. Huang), [email protected] (C. He), [email protected] (Y. Yi), [email protected] (J. Zhang), [email protected] (C. Di), zhangzj@ucas. ac.cn (Z. Zhang), [email protected] (Y. Li). http://dx.doi.org/10.1016/j.orgel.2014.03.005 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.
of well-defined molecular structure, definite molecular weight, simple synthesis, and high purity without batch-to-batch variations, thus gaining increasing attention recently [6,8–10]. At the early time, solution-proccessable organic molecules containing triphenylamine (TPA) moiety have been investigated as organic optoelectronic materials [11–20]. These molecules possess good filmforming ability, wide and strong absorption and adjustable energy levels. Although intensive investigations have been paid for structure design of the TPA-based molecules, it is difficult to form ordered intermolecular packing caused by the propeller-like geometry of TPA moiety, which results in inferior hole mobility. Further improving the performance of these materials presents serious challenges
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in chemistry. One strategy is to introduce planar moiety to replace TPA moiety for enhancing their molecular packing interaction in film. Recently, the planar and linear molecules show promising photovoltaic properties with PCE surpassing 6% [21–23], which is benefitted from their stronger intermolecular interactions resulting in higher hole mobility and higher FF of the OSCs. The indacenodithiophene (IDT) unit is an attractive donor building block for designing polymers in organic filed-effect transistors (OFETs) and OSCs because of its rigid and coplanar structure of the IDT unit [24–31]. These polymers feature low HOMO energy thereby large VOC values (ca. 0.9 V) in OSCs and strong p-p stacking in solid thus high hole motilities (up to 102 103 cm2 V1 s1) [27,30]. Their interesting properties inspired several groups to incorporate IDT as a building block to construct new small molecules. Thus IDT-based small-molecules flanked by different acceptors of benzothiadiazole [32] or diketopyrrolopyrrole [33] have been investigated. High efficiency up to 4.72% has been realized, indicative of promising future of these IDT-small-molecules. The integration of IDT with different acceptor units can offer the opportunity to further tune the bandgaps and energy levels of the organic molecules. In this paper, we report two new IDT-based small molecules IDT-3Th-ID(I) and IDT-3ThID(II) comprising the same A-p-D-p-A type framework, that is, IDT unit as the donor unit and central building block, 1,3-indanedione (ID) as the terminal acceptor units (Scheme 1) and terthiophene group as the p-bridge. The choice of 1,3-indanedione (ID) as acceptor units is under the consideration that ID has high electron affinities with strong optical absorptions and possesses a good planar structure [9,34]. As expected, the two materials indeed show broad and efficient sunlight harvesting and good stacking in the film. In the two molecules, the defined molecular structure with the same framework but different side chain positions provide the opportunities for systematic studies on the effects of the side chain on the OFET
and OSC performance. Through the research on the optoelectric performance, IDT-3Th-ID(I) exhibited a high hole mobility of 0.52 cm2 V1 s1 with a high on/off current ratio of 107 in OFETs, and a power conversion efficiency (PCE) of 3.07% with a high open-circuit voltage (VOC) of 0.82 V in OSCs; IDT-3Th-ID(II) exhibited a high hole mobility of 0.61 cm2 V1 s1 with a high on/off current ratio of 109 in OFETs, and a PCE of 2.83% with a VOC of 0.81 V in OSCs. These results demonstrate that the small molecules constructed by the highly p-conjugated IDT as donor units and ID as acceptor unit could be promising organic semiconductors for high-performance OFETs and OSCs applications. 2. Experimental section 2.1. Measurements and instruments MALDI-TOF spectra were recorded on a Bruker BIFLEXIII. Nuclear magnetic resonance (NMR) spectra were taken on a Bruker DMX-400 spectrometer. Absorption spectra were taken on a Hitachi U-3010 UV–vis spectrophotometer. The film on quartz used for UV measurements was prepared by spin-coating with 1% chloroform solution. The TGA measurement performed on a Perkin–Elmer TGA-7 apparatus. The electrochemical cyclic voltammograms were obtained using a Zahner IM6e electrochemical workstation in a 0.1 mol/L tetrabutylammonium hexa-fluorophosphate (Bu4NPF6) acetonitrile solution. Pt electrode coated with the sample film was used as the working electrode; Pt wire and Ag/AgCl were used as the counter and reference electrodes respectively. 2.2. Synthesis of compounds Unless otherwise specified, all chemicals were purchased from J&K and Alfa without further purification. Several important intermediates indacenodithiophene (IDT)
Scheme 1. General chemical structure of IDT-3Th-ID (I) and IDT-3Th-ID (II).
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Scheme 2. Synthetic route of IDT-3Th-ID (I) and IDT-3Th-ID (II); (I) n-BuLi, THF, at 78 °C under Ar; (II) n-BuLi, DMF, POCl3, saturated sodium acetate; (III) NBS, DMF, chloroform (IV, VI) Pd (PPh3)4, toluene, at 120 °C reflux for 48 h under Ar; (V, VII) CHCl3, Pyridine, stirred for 12 h at 60 °C under Ar.
(monomer 2 in Scheme 2) and 5-bromo-3,30 -dihexyl-2,20 ,5, 2-terthiophene-2-carbaldehyde (Br3TCHO) (monomer 1 in Scheme 2) were prepared as reported. Toluene and THF was distilled from sodium. 2.2.1. Synthesis of 3Th (2) To a solution of 3-hexylthiophene (20 g, 119 mmol) in anhydrous THF (200 ml) at 78 °C, n-BuLi (52.4 ml, 2.5 M in hexane, 131 mmol) was added dropwise, and the mixture was stirred at this temperature under Ar for 2 h. Then tributylchlorostannane (37 ml, 325.5 mmol) was added, and the mixture was stirred at 78 °C for 30 min. Then, 2,5-dibromothiophene (9.68 g, 40 mmol) in 20 ml anhydrous THF, and tetrakis (triphenylphosphine) palladium (0) (1 g, 0.8 mmol) were added. After stirring the mixture reflux for 24 h, it was allowed to cool at room temperature. Then the mixture was poured into a lot of dilute hydrochloric acid and extracted with dichloromethane. The combined organic layer was washed with water and brine, dried over anhydrous Na2SO4. After removing the solvent under vacuum, the obtained crude product was purified by silica gel column chromatography (hexane) to obtain
compound 2 (14.4 g, 86%) as a yellow liquid. GC/MS: 416 (M+). 1H NMR (400 MHz, CDCl3) d(ppm): 7.22–7.19 (s, 2H), 7.02–7.04 (s, 2H), 6.90–6.88 (s, 2H), 2.90–2.94 (q, 2H), 2.52–2.56 (q, 2H), 1.55–1.61(m, 2H), 1.63– 1.71(m, 2H), 1.32–1.38 (m, 12H) 0.91–0.96 (q, 6H).
2.2.2. Synthesis of 3TCHO (3) About 0.5 ml DMF (0.5 ml, 6.49 mmol) was added to a cold solution of 3T (2.7 g, 6.49 mmol) in 1,2-dichloroethane (30 ml) and stirred at 0 °C for 30 min, then POCl3 (0.6 ml, 6.49 mmol) was added and stirred at 0 °C for 2 h. After being stirred at 40 °C for 12 h, 100 ml saturated sodium acetate solution was poured into the mixture and stirred for 1 h, and then extracted with dichloromethane. The combined organic layer was washed with water and brine, dried over Na2SO4. After removal of solvent, it was purified by silica gel column chromatography using a mixture of acetic ether and petroleum ether (20:1) as eluant, and obtained yellow liquid 3TCHO (2.6 g, 76%). GC/MS: 574(M+) 1H NMR (400 MHz, CDCl3) d(ppm): 10.01 (s,1H) 7.22 (s, 1H), 7.02–7.04 (s, 2H), 6.89–6.87 (s, 2H),
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2.90–2.94 (q, 2H), 2.52–2.55 (q, 2H), 1.55–1.61 (m, 2H), 1.63–1.71 (m, 2H), 1.32–1.37 (m, 12H) 0.91–0.97(q, 6H). 2.2.3. Synthesis of Br3TCHO (4) N-Bromosuccinimide (0.9 g, 5 mmol) in 20 ml DMF was added slowly into a solution of 3TCHO (2.6 g, 5 mmol) in 30 ml chloroform at 0 °C. After being stirred for 12 h at room temperature, the reaction mixture was poured into ice water (100 ml) and extracted with CH2Cl2. The organic layer was thoroughly washed with water, aqueous sodium sulfite and brine and again with water, and then dried over Na2SO4. After removal of solvent, it was purified by silica gel column chromatography using a mixture of acetic ether and petroleum ether (20:1) as eluent, and obtained yellow oil Br3TCHO (2.8 g, 93%). GC/MS: 524(M+). 1H NMR (400 MHz, CDCl3) d(ppm): 10.00 (s, 1H) 7.22 (s, 1H), 7.02–7.04 (t, 2H), 6.90 (s, 1H), 2.90–2.94 (q, 2H), 2.52– 2.55 (q, 2H), 1.55–1.61(m, 2H), 1.63–1.71 (m, 2H), 1.32– 1.37 (m, 12H) 0.92–0.97 (q, 6H). 13C NMR (400 MHz, CDCl3) d(ppm):181.51, 154.01, 143.42, 138.31, 136.07, 135.07, 126.99, 126.81, 126.56, 125.21, 124.63, 108.96, 31.80, 31.75, 31.49, 29.79, 29.77, 29.19, 29.10, 28.75, 22.79, 22.75, 14.29, 14.25. 2.2.4. Synthesis of IDT-3Th-CHO (II) A solution of 4 (1.2 g, 1.78 mmol) and compound 6 (1.1 g, 0.89 mmol) in dry toluene (40 mL) were degassed triple times with argon followed by the addition of Pd(PPh3)4 (103 mg, 0.089 mmol). After being stirred at 120 °C for 48 h under argon, the reaction mixture was poured into water (50 mL) and extracted with CHCl3. The organic layer was washed with water, and then dried over Na2SO4. After removal of solvent, the crude product was purified by column chromatography on silica gel using a mixture of dichloromethane and petroleum ether (2:1) as eluent to afford compound IDT-3Th-CHO(II) (0.82 g, 58%) as a red solid. MALDI-TOF: m/z 1793.2 calculated for C114H134O2S8 1792.8. 1H NMR (400 MHz, CDCl3) d(ppm): 9.98 (s, 2H), 7.42 (s, 2H), 7.18–7.23 (m, 8H), 7.02–7.09 (m, 18H), 2.90–2.94 (q, 4H), 2.72–2.76 (q, 4H), 2.54–2.58 (q, 8H), 1.57–1.71 (m, 16H),1.30–1.39 (m, 48H), 0.87– 0.89 (m, 24H). 13C NMR (400 MHz, CDCl3)d(ppm): 181.47, 156.26, 153.98, 153.36, 145.75, 141.82, 141.68, 141.53,140.44,138.81, 138.12, 135.85, 135.27, 134.71 133.91, 131.80, 128.48, 127.99, 127.46, 126.91, 12.37, 124.44, 121.94, 117.49, 63.17, 35.68, 31.82, 31.72, 31.65, 31.43, 30.50, 29.73, 29.39, 29.24, 29.07,28.88, 22.70, 22.64, 14.19. 2.2.5. Synthesis of IDT-3Th-ID (II) Under argon atmosphere, IDT-3Th-CHO(II) (300 mg, 0.16 mmol) was dissolved in a solution of dry CHCl3 (20 ml) and two drops of pyridine and then 1,3-indanedione (93 mg, 0.64 mmol) were added and the resulting solution was stirred for 12 h, under argon, at 60 °C. The reaction mixture was then extracted with CHCl3, washed with water and dried over Na2SO4. After removal of solvent, it was chromatographed on silica gel using a mixture of dichloromethane and petroleum ether (2:1) as eluent to afford IDT-3Th-ID (II) as a black solid (223 mg, 68% yield). MALDI-TOF: m/z 2049.2 calculated for C132H142O4S8
2049.06. 1H NMR (400 MHz, CDCl3) d(ppm): 8.18 (s, 2H), 8.04–8.06 (m, 4H), 7.83–7.88 (m, 4H), 7.48–7.54 (m, 4H), 7.48–7,51 (m, 4H), 7.22–7.36 (m, 4H), 7.10–7.21 (m, 16H), 3.02–3.61 (m, 4H), 2.84–2.88 (m, 4H), 2.66–2.70 (m, 8H), 1.66–1.85 (m, 16H), 1.41–1.53 (m, 48H), 0.96– 1.36 (m, 24H). 13C NMR (400 MHz, CDCl3)d(ppm): 187.0, 153.24, 151.13, 150.02,141.72, 141.68, 141.53, 141.80, 140.226, 139.21, 139.04, 138.42, 135.45, 133.61, 133.15, 128.57, 128.05, 127.61, 126.96, 119.30, 118.43, 63.20, 35.73, 35.63, 31.87, 31.79, 31.51, 30.33, 30.14, 29.74, 29.41, 29.30, 22.75, 22.60,14.25. Elemental analysis: Calculated for C132H142O4S8, C, 77.30; H, 6.93; found C, 77.08; H, 7.01. 2.2.6. Synthesis of IDT-3Th-CHO (I) The synthesis and purification processes were similar with IDT-3Th-CHO (II) but monomer 1 was used instead of compound 4. We obtained a red solid. (1.03 g, yield 65%) MALDI-TOF: m/z 1793.1 calculated for C114H134O2S8 1792.8. 1H NMR (400 MHz, CDCl3) d(ppm): 9.83(s, 2H), 7.59(s, 2H),7.38 (m, 2H), 7.34–7.36 (m, 6H), 7.17–7.19 (m, 8H), 7.08–7.09 (m, 8H), 7.00 (s, 2H), 2.80–2.84 (m, 4H), 2.73–2.77 (m, 4H), 2.55–2.58 (m, 8H), 1.55–1.67 (m, 16H), 1.30–1.39 (m, 48H), 0.79–0.98 (m, 24H). 13C NMR (400 MHz, CDCl3) d(ppm): 182.51, 156.77, 153.51, 141.73, 141.16, 140.50, 140.38, 139.49, 139.09, 138.26,136.69, 135.23, 134.50, 128.52, 126.32, 126.05, 119.83, 117.35, 63.18, 35.70, 31.83, 31.44, 30.69, 30.55, 30.38, 29.74, 29.62, 29.37,29.27,19.31,14.20,13.83. 2.2.7. Synthesis of IDT-3Th-ID (I) The synthesis and purification processes were similar with IDT-3Th-ID (II) but IDT-3Th-CHO (I) was used instead of IDT-3Th-CHO (II). We obtained a black solid. (235 mg, yield 67%) MALDI-TOF: m/z 2049.4 calculated for C132H142O4S8 2049.08. 1H NMR (400 MHz, CDCl3) d(ppm): 7.94–7.97 (m, 4H), 7.79–7.89 (s, 2H), 7.76–7.78 (m, 4H), 7.38–7.39 (m, 4H), 7.22–7.27 (m, 4H), 7.15–7.20 (m, 8H), 7.08–7.13 (m, 8H), 7.01 (s, 2H), 2.84–2.88 (m, 4H), 2.76– 2.80 (m, 4H), 2.55–2.59 (m, 8H), 1.54–1.68 (m, 16H), 1.25–1.42 (m, 48H), 0.85–0.91(m, 24H). 13C NMR (400 MHz, CDCl3) d(ppm):187.79, 156.65, 153.34, 141.91, 141.32, 140.26, 140.13, 139.49, 139.09, 138.26,136.69, 135.23, 133.61,133.15, 128.57, 128.05, 127.61, 126.96 126.05, 119.83, 117.35, 63.17, 35.74, 31.88, 31.80, 31.71, 31.50, 30.53, 30.29, 29.85, 29.41, 29.31, 22.78,22.75,14.25. Elemental analysis: Calculated for C132H142O4S8: C, 77.30; H, 6.93; found. C, 77.16, H, 7.02. 3. Results and discussion 3.1. Thermal stability and electrochemical properties The compounds are soluble in common organic solvents, such as CHCl3, THF, chlorobenzene and toluene. The thermal stability of the compounds was investigated by thermogravimetric analysis (TGA), under nitrogen atmosphere at a heating rate of 10 °C/min, as shown in Fig. S1 in Supporting Information. The temperatures with 5% weight loss are at 390 °C for IDT-3Th-ID(I) and 364 °C
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for IDT-3Th-ID(II), respectively, which indicates that the thermal stability of the materials is good enough for the application in optoelectronic devices. Electrochemical cyclic voltammetry was conducted to estimate the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for IDT-3Th-ID(I) and IDT-3Th-ID(II) (Fig. 1 and Table 1). The HOMO, LUMO and the electrochemical band gaps ðEcv g Þ were estimated from the empirical equation of EHOMO/LUMO = e (Eonset(ox/red) + 4.4) (eV). The HOMO and LUMO were respectively estimated to be 5.27 and 3.52 eV for IDT-3Th-ID(I), and 5.25 and 3.55 eV for IDT-3Th-ID(II). Detailed results of the electrochemical measurement of the compounds were listed in Table 1. The energy levels were suitable for the application as donors in OSC with PC70BM as acceptor. 3.2. Theoretical calculations In order to deeply understand the structure and physicochemical properties of the small molecules, we carried out theoretical calculations on the molecular geometries and potential energy at the DFT-B3LYP/6-31G, in which all alkyl chains were replaced by ethyl groups for quicker calculations. From the molecular geometries and potential energy curves shown in Fig. 2, we can find both of the two molecules have good planar structures from the side view, which will benefit the p–p stacking. Following this, we also checked the torsion angles in the planar structure shown in Fig. S2 in Supporting Information. From IDT core to ID end unit, there were three torsion angles for both molecules, which are 15.995°, 16.135° and 9.475° for IDT-3Th-ID(I) and 27.740°, 11.824° and 9.627° for IDT-3Th-ID(II), respectively. Because of the main influence of the first torsion angle on the steric hindrance, the smaller one indicated a better molecular planar structure for IDT-3Th-ID(I). Afterwards, we examined the changing tendency of potential energy varying with rotation angle (shown in Fig. 2b) and related the configuration change with temperature. For IDT-3Th-ID(I) (the red line), the energy gap between optimized configuration (room temperature at ca. 26°) and the
Fig. 1. Cyclic voltammograms of IDT-3Th-ID(I) and IDT-3Th-ID(II) films on Pt electrode in an acetonitrile solution of 0.1 mol/L Bu4NPF6 (Bu = butyl) with a scan rate of 100 mV/s.
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planar configuration (the one at 0°) is negligible. Therefore, most of the molecules tend to keep the planar configuration. With gentle increasing temperature, more molecules would change to this state. But if the temperature is too high, this configuration could not keep this planar state and would rotate to adjust this higher energy state, thus, changing to the rotated configuration. This would confirm the material mobility changing tendency, which would be discussed in the following OFET part. For IDT-3Th-ID(II) (green line), we could find that there were three minimal potential energy configurations (around 0°, 50° and 150°). And IDT-3Th-ID(II) stayed in the second minimal potential energy configuration (around 50°) at room temperature. As the temperature increased, the molecules tend to rotate to the third minimal state (around 150°) because of their relatively low energy gap compared with rotating to the first minimal state (the one at 0°). However, the rotation angle gap between every two closed minimal configurations is so large that the molecule must experience the non-minimal configuration before its reaching. As known, the steric hindrance of this kind non-minimal configuration was much larger than the minimal one, which would result in poor charge transportation. Therefore, comparing the initial state, the charge transportation ability will decrease during this process. However, as long as the rotation reached another minimal configuration, the steric hindrance would dramatically reduce and the transporting ability improves much. So in this part we conclude that IDT-3Th-ID(I) had better molecular planar structure than IDT-3Th-ID(II). But, their molecular configuration and charge transportation ability varied with their energy state (or external temperature) in different way. 3.3. Optical absorption properties To establish the effect of thermal annealing on the optical properties, the UV–vis absorption spectra of the two small molecules were measured (Fig. 3). Generally the solid films showed a red shift of ca. 20 nm for the two molecules comparing with those of their solutions. Thermal annealing influences the absorption spectra of the films significantly. The pristine IDT-3Th-ID(I) film at room temperature (Fig. 3a and c) displayed the absorption peak at 576 nm. After thermal annealing at 100 °C, the absorption spectra blue-shifted a little, further increasing annealing temperature to 120 °C and 140 °C leads to red-shift of the absorption, and the absorption of the film after annealed at 140 °C shows the largest red-shift. The phenomenon could be related to the change of configuration and torsion angle of the molecule at different annealing temperatures as mentioned above from DFT calculation. According to the DFT calculation, molecule in a stable configuration at room temperature would rotate to another configuration with increasing temperature. The change of molecular configuration could lead to increasing molecular steric hindrance which results in blue-shift of its absorption or decreasing molecular steric hindrance which lead to redshift of its absorption. The thermal annealing at 100 °C for IDT-3Th-ID(I) film may result in the increased molecular steric hindrance and the treatment at 140 °C may decrease the molecular steric hindrance and improve p–p
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Table 1 Optical and electrochemical data of compounds IDT-3Th-ID(I) and IDT-3Th-ID(II). Compound
kmax
IDT-3Th-ID(I) IDT-3Th-ID(II)
558 570
solution
(nm)
kabs
edge
(nm)
719 734
kmax
film
576 582
(nm)
ðEopt g Þ 1.72 1.69
film
(eV)
Ecv g (eV)
HOMO (eV)
LUMO (eV)
1.75 1.70
5.27 5.25
3.52 3.55
Fig. 2. The optimized molecular geometries (a) and potentials (b) of IDT-3Th-ID(I) and IDT-3Th-ID(II) by DFT calculation.
stacking of the molecule. It should be noticed here that an absorption shoulder emerged for the films annealed at 120 °C and higher temperature, which further confirms the reorganization and enhanced p–p stacking of the molecules in the annealed film. Similarly, the absorption spectrum of IDT-3Th-ID(II) film is also blue shifted after thermal annealing at the temperature till to 120 °C in comparison with that at room temperature. Then the absorption spectrum is red-shifted and enhanced after thermal annealing at the higher temperature from 120 to 140 °C, and achieved the largest red-shift and emerged a shoulder peak for the film annealed at 140 °C (Fig. 3b and d). The higher annealing temperature of IDT-3Th-ID(II) than IDT3Th-ID(I) for the transit from blue-shift to red-shift of absorption spectra and appearance of shoulder peak indicates that IDT-3Th-ID(I) could form p–p stacking more easily than IDT-3Th-ID(II), as evidenced by the XRD and AFM test which will be discussed later. 3.4. Characteristics of organic field-effect transistors To investigate the organic field-effect transistor (OFET) properties of the compounds, we fabricated bottom-gate/
top-contact (BG/TC) type OFETs on Si/SiO2 substrates, with the solution concentration of 4 mg/mL in chloroform. We observed p-type OFET characteristics for both molecules and the results were shown in Table 2. For IDT-3Th-ID(I), we got the optimized hole mobility of 0.52 cm2 V1 s1, threshold voltage of 0.38 V and on–off ratio of 107. And the corresponding output and transfer curve were shown in Fig. 4a and b. Similarly, for IDT-3Th-ID(II), the best hole mobility reached 0.61 cm2 V1 s1 with threshold voltage of 0.87 V and the on–off ratio of 109. The related output and transfer curves are presented in Fig. 4c and d. We also investigated the effect of annealing temperature on the mobility, which was summarized in Fig. 4e. For the OFETs thermal annealed at 100 °C, 120 °C, 140 °C and 160 °C, the mobility of IDT-3Th-ID(I) experienced continuous increase from 4.3 103 cm2 V1 s1 for the pristine device without annealing to 7.6 102 cm2 V1 s1 for the device annealed at 100 °C, 3.22 101 cm2 V1 s1 for the device annealed at 120 °C, and 3.3 101 cm2 V1 s1 for the device annealed at 140 °C, but it turned to decrease to 1.95 101 cm2 V1 s1 for the OFET annealed at 160 °C. However, the mobility of IDT-3Th-ID(II) decreased from 9.8 103 cm2 V1 s1 for the pristine
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Fig. 3. UV–vis absorption spectra of IDT-3Th-ID (I) (a and c) and IDT-3Th-ID (II) (b and d) annealed at various temperatures. Panels c and d show enlarged views of panel a and b, respectively.
Table 2 Optimized device performance of the OFETs based on IDT-3Th-ID(I) and IDT-3Th-ID(II) annealed at 140 °C.
a b c
Compound
l (cm2 V1 s1) ava(maxb)
Vthreshold (V) ava(minc)
Ion/Ioff ava(maxb)
IDT-3Th-ID(I) IDT-3TH-ID(II)
0.33(0.52) 0.42(0.61)
5.30(0.38) 4.10(0.87)
107(107) 108(109)
Average values. Maximum values. Minimum values.
device without thermal annealing to 4.34 103 cm2 V1 s1 for the device annealed at 100 °C and 4.96 104 cm2 V1 s1 for that annealed at 120 °C, then it increased to 4.2 101 cm2 V1 s1 for the device annealed at 140 °C and 3.8 101 cm2 V1 s1 for that annealed at 160 °C. Interestingly, the annealed OFET devices of the two molecules reached the optimized performance at 140 °C, while the hole mobilities of the two molecules varied with annealing temperature in different way. The phenomena could be caused by different side chain positions of the p-bridges. In comparison with the side chains on 3-position of the terthiophene p-bridge in IDT-3Th-ID(I), the side chains on 4-position of the p-bridge in IDT-3Th-ID(II) are closer to the IDT core, which results in larger steric hindrance. Different position of the side chains in the two molecules could result in different configuration change tendency of the molecules with increasing annealing temperature, so that the mobilities of the two molecules changed with annealing temperature in different ways. 3.5. Thin-film morphology: X-ray diffraction and atomic force microscopy results For further understanding the effect of thermal annealing on the hole mobilities of the two molecules, we
measured the X-ray diffraction (XRD) patterns and atomic force microscopy (AFM) images of the films. Fig. 5 shows the XRD patterns of the two compounds processed at different temperatures. For IDT-3Th-ID(I) after annealing at 120 °C, a small crystal peak emerged at 2h = 5.49° with dspacing of 1.61 nm between the molecular layers (calculated from the Bragg equation k = 2dsin h) (see Fig. 5a). But for those annealed at 140 °C and 160 °C, typical crystal peaks emerged at 2h = 5.36° and 2h = 5.21°, with the dspacing of 1.65 nm and 1.69 nm respectively. Obviously, the material experienced gradual crystal formation process by the thermal annealing from 100 °C to 160 °C, which could benefit the charge transportation. Therefore, the mobility of the compound increased with increasing the thermal-annealing temperature from 100 °C to 140 °C. But the thermal annealing at 160 °C increased d-spacing by 0.04 nm, indicating enlarged distance between two planar molecules, so that resulted in the mobility decrease for further increasing thermal annealing temperature from 140 °C to 160 °C. However for IDT-3Th-ID(II) in Fig. 5b, there was no diffraction peak even at 120 °C. The XDR result indicates that the material was still in amorphous state after thermal annealing at the temperature up to 120 °C, so that its mobility was low even after the thermal annealing. Nevertheless,
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Fig. 4. Transfer (a, c) and output (b, d) characteristics of the OFETs based on IDT-3Th-ID(I) and IDT-3Th-ID(II) annealed at 140 °C. The hole mobility of IDT3Th-ID (I) and IDT-3Th-ID (II) annealed at various temperatures (e).
Fig. 5. XRD patterns of IDT-3Th-ID(I) (a) and IDT-3Th-ID(II) (b) films annealed at different temperature.
after thermal annealing at higher temperature of 140 °C and 160 °C, the XRD peaks appeared at 2h = 5.41° and 5.38°, corresponding to the d-spacing of 1.63 and 1.64 nm, respectively. The appearance of the crystalline structure agrees with the great increase of hole mobility of IDT-3Th-ID(II) after thermal annealing at the higher temperature. In order to further investigate the microscope structure of the films in the OFETs, AFM images of the films were measured, as shown in Fig. 6. The surface roughness of
the films increased significantly with increasing annealing temperature from 0.128 to 2.53 nm for IDT-3Th-ID(I) (from Fig. 6a–d) and from 0.851 to 7.53 nm IDT-3Th-ID(II) (from Fig. 6e–h). The widths and domain sizes of clustered nanofibers were also increased with increasing the annealing temperature, which would be beneficial to improve charge transport. The AFM results are in agreement with the effect of the annealing temperature on the hole mobility of the compounds mentioned above.
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Fig. 6. AFM images (3 lm 3 lm scans) of the thin films annealed at different temperatures for IDT-3Th-ID(I) (a, b, c and d) and IDT-3Th-ID(II) (e, f, g and h), Images on the left and right show phase and topographic mode images, respectively.
Fig. 7. (a) Current density–voltage characteristics of the OSCs based on the blends of compounds/PC70BM (2:1, w/w) under the illumination of AM 1.5, 100 mW/cm2; (b) IPCE spectra of the OSCs devices.
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Table 3 Photovoltaic performance of the OPVs with the device structure of ITO/PEDOT: PSS/organic molecules: PC70BM (2:1, w/w)/Ca/Al, under the illumination of AM 1.5G, 100 mW/cm2.
a b
Active layer
VOC (V)
JSC (mA/cm2)a
JSC (mA/cm2)b
FF (%)
PCE (%)
IDT-3Th-ID(I): PC70BM IDT-3Th-ID(II): PC70BM
0.82 0.81
8.15 7.99
7.95 7.50
46 44
3.07 2.83
Measured from J–V curves. Calculated from IPCE.
3.6. Photovoltaic properties The photovoltaic properties of the compounds were investigated by fabricating the OSC devices with the two compounds as donor in the device structure of ITO/PEDOT: PSS/photoactive layer (60 nm)/Ca/Al. The photoactive layer was formed by spin-coating the blend solution of IDT-3ThID(I)(or IDT-3Th-ID(II))/PC70BM (2:1, w/w) in chloroform. Under standard measurement conditions, the optimized J–V curves were shown in Fig. 7a, and related parameters were summarized in Table 3. We got the optimized performance with a high VOC of 0.82 V, and achieved a reasonable power conversion efficiency (PCE) of 3.07% and 2.83% for IDT-3Th-ID(I) and IDT-3Th-ID(II), respectively. The higher VOC should benefit from the lower HOMO energy level of the two compounds donor. In addition, we also measured the incident photon-to-converted current efficiency (IPCE) spectra, as shown in Fig. 7b. The integrated photocurrents from the IPCE spectra are consistent with the JSC values quite well (see Table 3). The relatively higher efficiency of IDT-3Th-ID(I) should be from its good p–p stacking ability and its nanofiber morphology in the active layer, which would benefit the formation of suitable interpenetrating donor/acceptor networks, thus, results in higher efficiency.
4. Conclusions Two linear A-p-D-p-A structured molecules with ID as terminal acceptor unit, IDT as core and donor unit, and 3Th with different alkyl chain positions as p-bridges, IDT3Th-ID(I) and IDT-3Th-ID(II), were designed and synthesized for the application in OFETs and OSCs. According to the electrochemical measurement, optimized structure modeling, theoretical calculation and spectrum characterization, we found that the two compounds showed different intermolecular interactions, self-aggregation tendency, film morphology, and different configuration and charge transportation changing tendency with annealing temperature. For the OFET application, IDT-3Th-ID(I) demonstrated the optimized hole mobility of 0.52 cm2 V1 s1, threshold voltage of 0.38 V and on–off ratio of 107, and IDT-3Th-ID(II) showed an even higher hole mobility of 0.61 cm2 V1 s1, threshold of 0.87 V and the on–off ratio of 109. For the application in OSCs, the two molecules demonstrated higher VOC of 0.82 V, benefited from the lower HOMO energy level of the molecules. The OSCs based on IDT-3Th-ID(I)/PCBM70 (2:1, w/w) and IDT-3Th-ID(II)/ PCBM70 (2:1, w/w) showed power conversion efficiencies (PCEs) of 3.07% and 2.83% respectively. These results demonstrate that the small molecules with the highly p-conjugated IDT as donor unit, 3Th as p-bridges and ID as
acceptor units, could be promising organic semiconductors for the application in OFETs and OSCs. Acknowledgments This work was supported by The Ministry of Science and Technology of China (2014CB643501) and NSFC (Nos. 51373181 and 91333204). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.orgel.2014.03.005. References [1] A.C. Arias, J.D. MacKenzie, I. McCulloch, J. Rivnay, A. Salleo, Chem. Rev. 110 (2010) 3. [2] (a) J. Chen, Y. Cao, Acc. Chem. Res. 42 (2009) 1709; (b) Y.F. Li, Acc. Chem. Res. 45 (2012) 723. [3] Y.-J. Cheng, S.-H. Yang, C.-S. Hsu, Chem. Rev. 109 (2009) 5868. [4] Y. Li, P. Sonar, L. Murphy, W. Hong, Energy Environ. Sci. 6 (2013) 1684. [5] Y. Wen, Y. Liu, Y. Guo, G. Yu, W. Hu, Chem. Rev. 111 (2011) 3358. [6] T. Bura, N. Leclerc, S. Fall, P. Lévêque, T. Heiser, P. Retailleau, S. Rihn, A. Mirloup, R. Ziessel, J. Am. Chem. Soc. 134 (2012) 17404. [7] Y. Shirota, H. Kageyama, Chem. Rev. 107 (2007) 953. [8] Y. Lin, Y. Li, X. Zhan, Chem. Soc. Rev. 41 (2012) 4245. [9] S.L. Shen, P. Jiang, C. He, J. Zhang, P. Shen, Y. Zhang, Y.P. Yi, Z.G. Zhang, Z.B. Li, Y.F. Li, Chem. Mater. 25 (2013) 2274. [10] Y. Chen, X. Wan, G. Long, Acc. Chem. Res. 46 (2013) 2645. [11] S. Roquet, A. Cravino, P. Leriche, O. Alévêque, P. Frère, J. Roncali, J. Am. Chem. Soc. 128 (2006) 3459. [12] (a) C. He, Q.G. He, Y.P. Yi, G.L. Wu, F.L. Bai, Z.G. Shuai, Y.F. Li, J. Mater. Chem. 18 (2008) 4085; (b) J. Zhang, Y. Yang, C. He, Y.F. Li, Sci. China Chem. 54 (2011) 695. [13] J. Zhang, Y. Yang, C. He, Y.J. He, G.J. Zhao, Y.F. Li, Macromolecules 42 (2009) 7619. [14] J. Zhang, D. Deng, C. He, Y.J. He, M.J. Zhang, Z.-G. Zhang, Z.J. Zhang, Y.F. Li, Chem. Mater. 23 (2010) 817. [15] H.X. Shang, H.J. Fan, Y. Liu, W.P. Hu, Y.F. Li, X.W. Zhan, Adv. Mater. 23 (2011) 1554. [16] Z. Li, Q. Dong, Y. Li, B. Xu, M. Deng, J. Pei, J. Zhang, F. Chen, S. Wen, Y. Gao, W. Tian, J. Mater. Chem. 21 (2011) 2159. [17] L. Piot, F. Silly, L. Tortech, Y. Nicolas, P. Blanchard, J. Roncali, D. Fichou, J. Am. Chem. Soc. 131 (2009) 12864. [18] L. Gao, J. Zhang, C. He, S.L. Shen, Y. Zhang, H.T. Liu, Q.J. Sun, Y.F. Li, Sci. China Chem. 56 (2013) 997. [19] A. Cravino, S. Roquet, O. Alévêque, P. Leriche, P. Frère, J. Roncali, Chem. Mater. 18 (2006) 2584. [20] J. Roncali, Acc. Chem. Res. 42 (2009) 1719. [21] J. Zhou, X. Wan, Y. Liu, Y. Zuo, Z. Li, G. He, G. Long, W. Ni, C. Li, X. Su, Y. Chen, J. Am. Chem. Soc. 134 (2012) 16345. [22] Z. Li, G. He, X. Wan, Y. Liu, J. Zhou, G. Long, Y. Zuo, M. Zhang, Y. Chen, Adv. Energy Mater. 2 (2012) 74. [23] Z.B. Henson, G.C. Welch, T. van der Poll, G.C. Bazan, J. Am. Chem. Soc. 134 (2012) 3766. [24] C.-P. Chen, S.-H. Chan, T.-C. Chao, C. Ting, B.-T. Ko, J. Am. Chem. Soc. 130 (2008) 12828. [25] Y.-C. Chen, C.-Y. Yu, Y.-L. Fan, L.-I. Hung, C.-P. Chen, C. Ting, Chem. Commun. 46 (2010) 6503.
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Solution-processed small molecules based on indacenodithiophene for high performance thin-film transistors and organic solar cells Wenli Tang a,b, Dazhen Huang b, Chang He b,⇑, Yuanping Yi b, Jing Zhang b, Chongan Di b,⇑, Zhanjun Zhang a,⇑, Yongfang Li b,⇑ a b
University of Chinese Academy of Sciences, Beijing 100049, China Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e
i n f o
Article history: Received 18 January 2014 Received in revised form 10 March 2014 Accepted 11 March 2014 Available online 22 March 2014 Keywords: Solution-processable organic molecules Organic field-effect transistors Organic solar cells Indacenodithiophene Hole mobility
a b s t r a c t Solution-processed indacenodithiophene (IDT)-based small molecules with 1,3-indanedione (ID) as terminal acceptor units and 3,30 -hexyl-terthiophene (IDT-3Th-ID(I)) or 4,40 -hexyl-terthiophene (IDT-3Th-ID(II)) as p-bridges, have been designed and synthesized for the application in organic field-effect transistors (OFETs) and organic solar cells (OSCs). These molecules exhibited excellent solubility in common organic solvents, good filmforming ability, reasonable thermal stability, and low HOMO energy levels. For the OFETs devices, high hole motilities of 0.52 cm2 V1 s1 for IDT-3Th-ID(I) and 0.61 cm2 V1 s1 for IDT-3Th-ID(II) were achieved, with corresponding high ION/IOFF of ca. 107 and 109 respectively. The OSCs based on IDT-3Th-ID(I)/PC70BM (2:1, w/w) and IDT-3Th-ID(II)/PC70BM (2:1, w/w) without using any treatment of solvent additive or thermal annealing, showed power conversion efficiencies (PCEs) of 3.07% for IDT-3Th-ID(I) and 2.83% for IDT-3Th-ID(II), under the illumination of AM 1.5G, 100 mW/cm2. The results demonstrate that the small molecules constructed with the highly p-conjugated IDT as donor unit, 3Th as p-bridges and ID as acceptor units, could be promising organic semiconductors for highperformance OFETs and OSCs applications. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Organic optoelectronic devices based on solution-processing method compatible with roll-to-roll processing, have received tremendous scientific and industrial interest in recent years due to their cost-effective and large-scale reproductive advantages [1–7]. Compared to polymers, solution-processed small-molecules have the advantages
⇑ Corresponding authors. Fax: +86 10 62559373. E-mail addresses: [email protected] (W. Tang), huangdz@ iccas.ac.cn (D. Huang), [email protected] (C. He), [email protected] (Y. Yi), [email protected] (J. Zhang), [email protected] (C. Di), zhangzj@ucas. ac.cn (Z. Zhang), [email protected] (Y. Li). http://dx.doi.org/10.1016/j.orgel.2014.03.005 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.
of well-defined molecular structure, definite molecular weight, simple synthesis, and high purity without batch-to-batch variations, thus gaining increasing attention recently [6,8–10]. At the early time, solution-proccessable organic molecules containing triphenylamine (TPA) moiety have been investigated as organic optoelectronic materials [11–20]. These molecules possess good filmforming ability, wide and strong absorption and adjustable energy levels. Although intensive investigations have been paid for structure design of the TPA-based molecules, it is difficult to form ordered intermolecular packing caused by the propeller-like geometry of TPA moiety, which results in inferior hole mobility. Further improving the performance of these materials presents serious challenges
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in chemistry. One strategy is to introduce planar moiety to replace TPA moiety for enhancing their molecular packing interaction in film. Recently, the planar and linear molecules show promising photovoltaic properties with PCE surpassing 6% [21–23], which is benefitted from their stronger intermolecular interactions resulting in higher hole mobility and higher FF of the OSCs. The indacenodithiophene (IDT) unit is an attractive donor building block for designing polymers in organic filed-effect transistors (OFETs) and OSCs because of its rigid and coplanar structure of the IDT unit [24–31]. These polymers feature low HOMO energy thereby large VOC values (ca. 0.9 V) in OSCs and strong p-p stacking in solid thus high hole motilities (up to 102 103 cm2 V1 s1) [27,30]. Their interesting properties inspired several groups to incorporate IDT as a building block to construct new small molecules. Thus IDT-based small-molecules flanked by different acceptors of benzothiadiazole [32] or diketopyrrolopyrrole [33] have been investigated. High efficiency up to 4.72% has been realized, indicative of promising future of these IDT-small-molecules. The integration of IDT with different acceptor units can offer the opportunity to further tune the bandgaps and energy levels of the organic molecules. In this paper, we report two new IDT-based small molecules IDT-3Th-ID(I) and IDT-3ThID(II) comprising the same A-p-D-p-A type framework, that is, IDT unit as the donor unit and central building block, 1,3-indanedione (ID) as the terminal acceptor units (Scheme 1) and terthiophene group as the p-bridge. The choice of 1,3-indanedione (ID) as acceptor units is under the consideration that ID has high electron affinities with strong optical absorptions and possesses a good planar structure [9,34]. As expected, the two materials indeed show broad and efficient sunlight harvesting and good stacking in the film. In the two molecules, the defined molecular structure with the same framework but different side chain positions provide the opportunities for systematic studies on the effects of the side chain on the OFET
and OSC performance. Through the research on the optoelectric performance, IDT-3Th-ID(I) exhibited a high hole mobility of 0.52 cm2 V1 s1 with a high on/off current ratio of 107 in OFETs, and a power conversion efficiency (PCE) of 3.07% with a high open-circuit voltage (VOC) of 0.82 V in OSCs; IDT-3Th-ID(II) exhibited a high hole mobility of 0.61 cm2 V1 s1 with a high on/off current ratio of 109 in OFETs, and a PCE of 2.83% with a VOC of 0.81 V in OSCs. These results demonstrate that the small molecules constructed by the highly p-conjugated IDT as donor units and ID as acceptor unit could be promising organic semiconductors for high-performance OFETs and OSCs applications. 2. Experimental section 2.1. Measurements and instruments MALDI-TOF spectra were recorded on a Bruker BIFLEXIII. Nuclear magnetic resonance (NMR) spectra were taken on a Bruker DMX-400 spectrometer. Absorption spectra were taken on a Hitachi U-3010 UV–vis spectrophotometer. The film on quartz used for UV measurements was prepared by spin-coating with 1% chloroform solution. The TGA measurement performed on a Perkin–Elmer TGA-7 apparatus. The electrochemical cyclic voltammograms were obtained using a Zahner IM6e electrochemical workstation in a 0.1 mol/L tetrabutylammonium hexa-fluorophosphate (Bu4NPF6) acetonitrile solution. Pt electrode coated with the sample film was used as the working electrode; Pt wire and Ag/AgCl were used as the counter and reference electrodes respectively. 2.2. Synthesis of compounds Unless otherwise specified, all chemicals were purchased from J&K and Alfa without further purification. Several important intermediates indacenodithiophene (IDT)
Scheme 1. General chemical structure of IDT-3Th-ID (I) and IDT-3Th-ID (II).
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Scheme 2. Synthetic route of IDT-3Th-ID (I) and IDT-3Th-ID (II); (I) n-BuLi, THF, at 78 °C under Ar; (II) n-BuLi, DMF, POCl3, saturated sodium acetate; (III) NBS, DMF, chloroform (IV, VI) Pd (PPh3)4, toluene, at 120 °C reflux for 48 h under Ar; (V, VII) CHCl3, Pyridine, stirred for 12 h at 60 °C under Ar.
(monomer 2 in Scheme 2) and 5-bromo-3,30 -dihexyl-2,20 ,5, 2-terthiophene-2-carbaldehyde (Br3TCHO) (monomer 1 in Scheme 2) were prepared as reported. Toluene and THF was distilled from sodium. 2.2.1. Synthesis of 3Th (2) To a solution of 3-hexylthiophene (20 g, 119 mmol) in anhydrous THF (200 ml) at 78 °C, n-BuLi (52.4 ml, 2.5 M in hexane, 131 mmol) was added dropwise, and the mixture was stirred at this temperature under Ar for 2 h. Then tributylchlorostannane (37 ml, 325.5 mmol) was added, and the mixture was stirred at 78 °C for 30 min. Then, 2,5-dibromothiophene (9.68 g, 40 mmol) in 20 ml anhydrous THF, and tetrakis (triphenylphosphine) palladium (0) (1 g, 0.8 mmol) were added. After stirring the mixture reflux for 24 h, it was allowed to cool at room temperature. Then the mixture was poured into a lot of dilute hydrochloric acid and extracted with dichloromethane. The combined organic layer was washed with water and brine, dried over anhydrous Na2SO4. After removing the solvent under vacuum, the obtained crude product was purified by silica gel column chromatography (hexane) to obtain
compound 2 (14.4 g, 86%) as a yellow liquid. GC/MS: 416 (M+). 1H NMR (400 MHz, CDCl3) d(ppm): 7.22–7.19 (s, 2H), 7.02–7.04 (s, 2H), 6.90–6.88 (s, 2H), 2.90–2.94 (q, 2H), 2.52–2.56 (q, 2H), 1.55–1.61(m, 2H), 1.63– 1.71(m, 2H), 1.32–1.38 (m, 12H) 0.91–0.96 (q, 6H).
2.2.2. Synthesis of 3TCHO (3) About 0.5 ml DMF (0.5 ml, 6.49 mmol) was added to a cold solution of 3T (2.7 g, 6.49 mmol) in 1,2-dichloroethane (30 ml) and stirred at 0 °C for 30 min, then POCl3 (0.6 ml, 6.49 mmol) was added and stirred at 0 °C for 2 h. After being stirred at 40 °C for 12 h, 100 ml saturated sodium acetate solution was poured into the mixture and stirred for 1 h, and then extracted with dichloromethane. The combined organic layer was washed with water and brine, dried over Na2SO4. After removal of solvent, it was purified by silica gel column chromatography using a mixture of acetic ether and petroleum ether (20:1) as eluant, and obtained yellow liquid 3TCHO (2.6 g, 76%). GC/MS: 574(M+) 1H NMR (400 MHz, CDCl3) d(ppm): 10.01 (s,1H) 7.22 (s, 1H), 7.02–7.04 (s, 2H), 6.89–6.87 (s, 2H),
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2.90–2.94 (q, 2H), 2.52–2.55 (q, 2H), 1.55–1.61 (m, 2H), 1.63–1.71 (m, 2H), 1.32–1.37 (m, 12H) 0.91–0.97(q, 6H). 2.2.3. Synthesis of Br3TCHO (4) N-Bromosuccinimide (0.9 g, 5 mmol) in 20 ml DMF was added slowly into a solution of 3TCHO (2.6 g, 5 mmol) in 30 ml chloroform at 0 °C. After being stirred for 12 h at room temperature, the reaction mixture was poured into ice water (100 ml) and extracted with CH2Cl2. The organic layer was thoroughly washed with water, aqueous sodium sulfite and brine and again with water, and then dried over Na2SO4. After removal of solvent, it was purified by silica gel column chromatography using a mixture of acetic ether and petroleum ether (20:1) as eluent, and obtained yellow oil Br3TCHO (2.8 g, 93%). GC/MS: 524(M+). 1H NMR (400 MHz, CDCl3) d(ppm): 10.00 (s, 1H) 7.22 (s, 1H), 7.02–7.04 (t, 2H), 6.90 (s, 1H), 2.90–2.94 (q, 2H), 2.52– 2.55 (q, 2H), 1.55–1.61(m, 2H), 1.63–1.71 (m, 2H), 1.32– 1.37 (m, 12H) 0.92–0.97 (q, 6H). 13C NMR (400 MHz, CDCl3) d(ppm):181.51, 154.01, 143.42, 138.31, 136.07, 135.07, 126.99, 126.81, 126.56, 125.21, 124.63, 108.96, 31.80, 31.75, 31.49, 29.79, 29.77, 29.19, 29.10, 28.75, 22.79, 22.75, 14.29, 14.25. 2.2.4. Synthesis of IDT-3Th-CHO (II) A solution of 4 (1.2 g, 1.78 mmol) and compound 6 (1.1 g, 0.89 mmol) in dry toluene (40 mL) were degassed triple times with argon followed by the addition of Pd(PPh3)4 (103 mg, 0.089 mmol). After being stirred at 120 °C for 48 h under argon, the reaction mixture was poured into water (50 mL) and extracted with CHCl3. The organic layer was washed with water, and then dried over Na2SO4. After removal of solvent, the crude product was purified by column chromatography on silica gel using a mixture of dichloromethane and petroleum ether (2:1) as eluent to afford compound IDT-3Th-CHO(II) (0.82 g, 58%) as a red solid. MALDI-TOF: m/z 1793.2 calculated for C114H134O2S8 1792.8. 1H NMR (400 MHz, CDCl3) d(ppm): 9.98 (s, 2H), 7.42 (s, 2H), 7.18–7.23 (m, 8H), 7.02–7.09 (m, 18H), 2.90–2.94 (q, 4H), 2.72–2.76 (q, 4H), 2.54–2.58 (q, 8H), 1.57–1.71 (m, 16H),1.30–1.39 (m, 48H), 0.87– 0.89 (m, 24H). 13C NMR (400 MHz, CDCl3)d(ppm): 181.47, 156.26, 153.98, 153.36, 145.75, 141.82, 141.68, 141.53,140.44,138.81, 138.12, 135.85, 135.27, 134.71 133.91, 131.80, 128.48, 127.99, 127.46, 126.91, 12.37, 124.44, 121.94, 117.49, 63.17, 35.68, 31.82, 31.72, 31.65, 31.43, 30.50, 29.73, 29.39, 29.24, 29.07,28.88, 22.70, 22.64, 14.19. 2.2.5. Synthesis of IDT-3Th-ID (II) Under argon atmosphere, IDT-3Th-CHO(II) (300 mg, 0.16 mmol) was dissolved in a solution of dry CHCl3 (20 ml) and two drops of pyridine and then 1,3-indanedione (93 mg, 0.64 mmol) were added and the resulting solution was stirred for 12 h, under argon, at 60 °C. The reaction mixture was then extracted with CHCl3, washed with water and dried over Na2SO4. After removal of solvent, it was chromatographed on silica gel using a mixture of dichloromethane and petroleum ether (2:1) as eluent to afford IDT-3Th-ID (II) as a black solid (223 mg, 68% yield). MALDI-TOF: m/z 2049.2 calculated for C132H142O4S8
2049.06. 1H NMR (400 MHz, CDCl3) d(ppm): 8.18 (s, 2H), 8.04–8.06 (m, 4H), 7.83–7.88 (m, 4H), 7.48–7.54 (m, 4H), 7.48–7,51 (m, 4H), 7.22–7.36 (m, 4H), 7.10–7.21 (m, 16H), 3.02–3.61 (m, 4H), 2.84–2.88 (m, 4H), 2.66–2.70 (m, 8H), 1.66–1.85 (m, 16H), 1.41–1.53 (m, 48H), 0.96– 1.36 (m, 24H). 13C NMR (400 MHz, CDCl3)d(ppm): 187.0, 153.24, 151.13, 150.02,141.72, 141.68, 141.53, 141.80, 140.226, 139.21, 139.04, 138.42, 135.45, 133.61, 133.15, 128.57, 128.05, 127.61, 126.96, 119.30, 118.43, 63.20, 35.73, 35.63, 31.87, 31.79, 31.51, 30.33, 30.14, 29.74, 29.41, 29.30, 22.75, 22.60,14.25. Elemental analysis: Calculated for C132H142O4S8, C, 77.30; H, 6.93; found C, 77.08; H, 7.01. 2.2.6. Synthesis of IDT-3Th-CHO (I) The synthesis and purification processes were similar with IDT-3Th-CHO (II) but monomer 1 was used instead of compound 4. We obtained a red solid. (1.03 g, yield 65%) MALDI-TOF: m/z 1793.1 calculated for C114H134O2S8 1792.8. 1H NMR (400 MHz, CDCl3) d(ppm): 9.83(s, 2H), 7.59(s, 2H),7.38 (m, 2H), 7.34–7.36 (m, 6H), 7.17–7.19 (m, 8H), 7.08–7.09 (m, 8H), 7.00 (s, 2H), 2.80–2.84 (m, 4H), 2.73–2.77 (m, 4H), 2.55–2.58 (m, 8H), 1.55–1.67 (m, 16H), 1.30–1.39 (m, 48H), 0.79–0.98 (m, 24H). 13C NMR (400 MHz, CDCl3) d(ppm): 182.51, 156.77, 153.51, 141.73, 141.16, 140.50, 140.38, 139.49, 139.09, 138.26,136.69, 135.23, 134.50, 128.52, 126.32, 126.05, 119.83, 117.35, 63.18, 35.70, 31.83, 31.44, 30.69, 30.55, 30.38, 29.74, 29.62, 29.37,29.27,19.31,14.20,13.83. 2.2.7. Synthesis of IDT-3Th-ID (I) The synthesis and purification processes were similar with IDT-3Th-ID (II) but IDT-3Th-CHO (I) was used instead of IDT-3Th-CHO (II). We obtained a black solid. (235 mg, yield 67%) MALDI-TOF: m/z 2049.4 calculated for C132H142O4S8 2049.08. 1H NMR (400 MHz, CDCl3) d(ppm): 7.94–7.97 (m, 4H), 7.79–7.89 (s, 2H), 7.76–7.78 (m, 4H), 7.38–7.39 (m, 4H), 7.22–7.27 (m, 4H), 7.15–7.20 (m, 8H), 7.08–7.13 (m, 8H), 7.01 (s, 2H), 2.84–2.88 (m, 4H), 2.76– 2.80 (m, 4H), 2.55–2.59 (m, 8H), 1.54–1.68 (m, 16H), 1.25–1.42 (m, 48H), 0.85–0.91(m, 24H). 13C NMR (400 MHz, CDCl3) d(ppm):187.79, 156.65, 153.34, 141.91, 141.32, 140.26, 140.13, 139.49, 139.09, 138.26,136.69, 135.23, 133.61,133.15, 128.57, 128.05, 127.61, 126.96 126.05, 119.83, 117.35, 63.17, 35.74, 31.88, 31.80, 31.71, 31.50, 30.53, 30.29, 29.85, 29.41, 29.31, 22.78,22.75,14.25. Elemental analysis: Calculated for C132H142O4S8: C, 77.30; H, 6.93; found. C, 77.16, H, 7.02. 3. Results and discussion 3.1. Thermal stability and electrochemical properties The compounds are soluble in common organic solvents, such as CHCl3, THF, chlorobenzene and toluene. The thermal stability of the compounds was investigated by thermogravimetric analysis (TGA), under nitrogen atmosphere at a heating rate of 10 °C/min, as shown in Fig. S1 in Supporting Information. The temperatures with 5% weight loss are at 390 °C for IDT-3Th-ID(I) and 364 °C
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for IDT-3Th-ID(II), respectively, which indicates that the thermal stability of the materials is good enough for the application in optoelectronic devices. Electrochemical cyclic voltammetry was conducted to estimate the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for IDT-3Th-ID(I) and IDT-3Th-ID(II) (Fig. 1 and Table 1). The HOMO, LUMO and the electrochemical band gaps ðEcv g Þ were estimated from the empirical equation of EHOMO/LUMO = e (Eonset(ox/red) + 4.4) (eV). The HOMO and LUMO were respectively estimated to be 5.27 and 3.52 eV for IDT-3Th-ID(I), and 5.25 and 3.55 eV for IDT-3Th-ID(II). Detailed results of the electrochemical measurement of the compounds were listed in Table 1. The energy levels were suitable for the application as donors in OSC with PC70BM as acceptor. 3.2. Theoretical calculations In order to deeply understand the structure and physicochemical properties of the small molecules, we carried out theoretical calculations on the molecular geometries and potential energy at the DFT-B3LYP/6-31G, in which all alkyl chains were replaced by ethyl groups for quicker calculations. From the molecular geometries and potential energy curves shown in Fig. 2, we can find both of the two molecules have good planar structures from the side view, which will benefit the p–p stacking. Following this, we also checked the torsion angles in the planar structure shown in Fig. S2 in Supporting Information. From IDT core to ID end unit, there were three torsion angles for both molecules, which are 15.995°, 16.135° and 9.475° for IDT-3Th-ID(I) and 27.740°, 11.824° and 9.627° for IDT-3Th-ID(II), respectively. Because of the main influence of the first torsion angle on the steric hindrance, the smaller one indicated a better molecular planar structure for IDT-3Th-ID(I). Afterwards, we examined the changing tendency of potential energy varying with rotation angle (shown in Fig. 2b) and related the configuration change with temperature. For IDT-3Th-ID(I) (the red line), the energy gap between optimized configuration (room temperature at ca. 26°) and the
Fig. 1. Cyclic voltammograms of IDT-3Th-ID(I) and IDT-3Th-ID(II) films on Pt electrode in an acetonitrile solution of 0.1 mol/L Bu4NPF6 (Bu = butyl) with a scan rate of 100 mV/s.
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planar configuration (the one at 0°) is negligible. Therefore, most of the molecules tend to keep the planar configuration. With gentle increasing temperature, more molecules would change to this state. But if the temperature is too high, this configuration could not keep this planar state and would rotate to adjust this higher energy state, thus, changing to the rotated configuration. This would confirm the material mobility changing tendency, which would be discussed in the following OFET part. For IDT-3Th-ID(II) (green line), we could find that there were three minimal potential energy configurations (around 0°, 50° and 150°). And IDT-3Th-ID(II) stayed in the second minimal potential energy configuration (around 50°) at room temperature. As the temperature increased, the molecules tend to rotate to the third minimal state (around 150°) because of their relatively low energy gap compared with rotating to the first minimal state (the one at 0°). However, the rotation angle gap between every two closed minimal configurations is so large that the molecule must experience the non-minimal configuration before its reaching. As known, the steric hindrance of this kind non-minimal configuration was much larger than the minimal one, which would result in poor charge transportation. Therefore, comparing the initial state, the charge transportation ability will decrease during this process. However, as long as the rotation reached another minimal configuration, the steric hindrance would dramatically reduce and the transporting ability improves much. So in this part we conclude that IDT-3Th-ID(I) had better molecular planar structure than IDT-3Th-ID(II). But, their molecular configuration and charge transportation ability varied with their energy state (or external temperature) in different way. 3.3. Optical absorption properties To establish the effect of thermal annealing on the optical properties, the UV–vis absorption spectra of the two small molecules were measured (Fig. 3). Generally the solid films showed a red shift of ca. 20 nm for the two molecules comparing with those of their solutions. Thermal annealing influences the absorption spectra of the films significantly. The pristine IDT-3Th-ID(I) film at room temperature (Fig. 3a and c) displayed the absorption peak at 576 nm. After thermal annealing at 100 °C, the absorption spectra blue-shifted a little, further increasing annealing temperature to 120 °C and 140 °C leads to red-shift of the absorption, and the absorption of the film after annealed at 140 °C shows the largest red-shift. The phenomenon could be related to the change of configuration and torsion angle of the molecule at different annealing temperatures as mentioned above from DFT calculation. According to the DFT calculation, molecule in a stable configuration at room temperature would rotate to another configuration with increasing temperature. The change of molecular configuration could lead to increasing molecular steric hindrance which results in blue-shift of its absorption or decreasing molecular steric hindrance which lead to redshift of its absorption. The thermal annealing at 100 °C for IDT-3Th-ID(I) film may result in the increased molecular steric hindrance and the treatment at 140 °C may decrease the molecular steric hindrance and improve p–p
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Table 1 Optical and electrochemical data of compounds IDT-3Th-ID(I) and IDT-3Th-ID(II). Compound
kmax
IDT-3Th-ID(I) IDT-3Th-ID(II)
558 570
solution
(nm)
kabs
edge
(nm)
719 734
kmax
film
576 582
(nm)
ðEopt g Þ 1.72 1.69
film
(eV)
Ecv g (eV)
HOMO (eV)
LUMO (eV)
1.75 1.70
5.27 5.25
3.52 3.55
Fig. 2. The optimized molecular geometries (a) and potentials (b) of IDT-3Th-ID(I) and IDT-3Th-ID(II) by DFT calculation.
stacking of the molecule. It should be noticed here that an absorption shoulder emerged for the films annealed at 120 °C and higher temperature, which further confirms the reorganization and enhanced p–p stacking of the molecules in the annealed film. Similarly, the absorption spectrum of IDT-3Th-ID(II) film is also blue shifted after thermal annealing at the temperature till to 120 °C in comparison with that at room temperature. Then the absorption spectrum is red-shifted and enhanced after thermal annealing at the higher temperature from 120 to 140 °C, and achieved the largest red-shift and emerged a shoulder peak for the film annealed at 140 °C (Fig. 3b and d). The higher annealing temperature of IDT-3Th-ID(II) than IDT3Th-ID(I) for the transit from blue-shift to red-shift of absorption spectra and appearance of shoulder peak indicates that IDT-3Th-ID(I) could form p–p stacking more easily than IDT-3Th-ID(II), as evidenced by the XRD and AFM test which will be discussed later. 3.4. Characteristics of organic field-effect transistors To investigate the organic field-effect transistor (OFET) properties of the compounds, we fabricated bottom-gate/
top-contact (BG/TC) type OFETs on Si/SiO2 substrates, with the solution concentration of 4 mg/mL in chloroform. We observed p-type OFET characteristics for both molecules and the results were shown in Table 2. For IDT-3Th-ID(I), we got the optimized hole mobility of 0.52 cm2 V1 s1, threshold voltage of 0.38 V and on–off ratio of 107. And the corresponding output and transfer curve were shown in Fig. 4a and b. Similarly, for IDT-3Th-ID(II), the best hole mobility reached 0.61 cm2 V1 s1 with threshold voltage of 0.87 V and the on–off ratio of 109. The related output and transfer curves are presented in Fig. 4c and d. We also investigated the effect of annealing temperature on the mobility, which was summarized in Fig. 4e. For the OFETs thermal annealed at 100 °C, 120 °C, 140 °C and 160 °C, the mobility of IDT-3Th-ID(I) experienced continuous increase from 4.3 103 cm2 V1 s1 for the pristine device without annealing to 7.6 102 cm2 V1 s1 for the device annealed at 100 °C, 3.22 101 cm2 V1 s1 for the device annealed at 120 °C, and 3.3 101 cm2 V1 s1 for the device annealed at 140 °C, but it turned to decrease to 1.95 101 cm2 V1 s1 for the OFET annealed at 160 °C. However, the mobility of IDT-3Th-ID(II) decreased from 9.8 103 cm2 V1 s1 for the pristine
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Fig. 3. UV–vis absorption spectra of IDT-3Th-ID (I) (a and c) and IDT-3Th-ID (II) (b and d) annealed at various temperatures. Panels c and d show enlarged views of panel a and b, respectively.
Table 2 Optimized device performance of the OFETs based on IDT-3Th-ID(I) and IDT-3Th-ID(II) annealed at 140 °C.
a b c
Compound
l (cm2 V1 s1) ava(maxb)
Vthreshold (V) ava(minc)
Ion/Ioff ava(maxb)
IDT-3Th-ID(I) IDT-3TH-ID(II)
0.33(0.52) 0.42(0.61)
5.30(0.38) 4.10(0.87)
107(107) 108(109)
Average values. Maximum values. Minimum values.
device without thermal annealing to 4.34 103 cm2 V1 s1 for the device annealed at 100 °C and 4.96 104 cm2 V1 s1 for that annealed at 120 °C, then it increased to 4.2 101 cm2 V1 s1 for the device annealed at 140 °C and 3.8 101 cm2 V1 s1 for that annealed at 160 °C. Interestingly, the annealed OFET devices of the two molecules reached the optimized performance at 140 °C, while the hole mobilities of the two molecules varied with annealing temperature in different way. The phenomena could be caused by different side chain positions of the p-bridges. In comparison with the side chains on 3-position of the terthiophene p-bridge in IDT-3Th-ID(I), the side chains on 4-position of the p-bridge in IDT-3Th-ID(II) are closer to the IDT core, which results in larger steric hindrance. Different position of the side chains in the two molecules could result in different configuration change tendency of the molecules with increasing annealing temperature, so that the mobilities of the two molecules changed with annealing temperature in different ways. 3.5. Thin-film morphology: X-ray diffraction and atomic force microscopy results For further understanding the effect of thermal annealing on the hole mobilities of the two molecules, we
measured the X-ray diffraction (XRD) patterns and atomic force microscopy (AFM) images of the films. Fig. 5 shows the XRD patterns of the two compounds processed at different temperatures. For IDT-3Th-ID(I) after annealing at 120 °C, a small crystal peak emerged at 2h = 5.49° with dspacing of 1.61 nm between the molecular layers (calculated from the Bragg equation k = 2dsin h) (see Fig. 5a). But for those annealed at 140 °C and 160 °C, typical crystal peaks emerged at 2h = 5.36° and 2h = 5.21°, with the dspacing of 1.65 nm and 1.69 nm respectively. Obviously, the material experienced gradual crystal formation process by the thermal annealing from 100 °C to 160 °C, which could benefit the charge transportation. Therefore, the mobility of the compound increased with increasing the thermal-annealing temperature from 100 °C to 140 °C. But the thermal annealing at 160 °C increased d-spacing by 0.04 nm, indicating enlarged distance between two planar molecules, so that resulted in the mobility decrease for further increasing thermal annealing temperature from 140 °C to 160 °C. However for IDT-3Th-ID(II) in Fig. 5b, there was no diffraction peak even at 120 °C. The XDR result indicates that the material was still in amorphous state after thermal annealing at the temperature up to 120 °C, so that its mobility was low even after the thermal annealing. Nevertheless,
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Fig. 4. Transfer (a, c) and output (b, d) characteristics of the OFETs based on IDT-3Th-ID(I) and IDT-3Th-ID(II) annealed at 140 °C. The hole mobility of IDT3Th-ID (I) and IDT-3Th-ID (II) annealed at various temperatures (e).
Fig. 5. XRD patterns of IDT-3Th-ID(I) (a) and IDT-3Th-ID(II) (b) films annealed at different temperature.
after thermal annealing at higher temperature of 140 °C and 160 °C, the XRD peaks appeared at 2h = 5.41° and 5.38°, corresponding to the d-spacing of 1.63 and 1.64 nm, respectively. The appearance of the crystalline structure agrees with the great increase of hole mobility of IDT-3Th-ID(II) after thermal annealing at the higher temperature. In order to further investigate the microscope structure of the films in the OFETs, AFM images of the films were measured, as shown in Fig. 6. The surface roughness of
the films increased significantly with increasing annealing temperature from 0.128 to 2.53 nm for IDT-3Th-ID(I) (from Fig. 6a–d) and from 0.851 to 7.53 nm IDT-3Th-ID(II) (from Fig. 6e–h). The widths and domain sizes of clustered nanofibers were also increased with increasing the annealing temperature, which would be beneficial to improve charge transport. The AFM results are in agreement with the effect of the annealing temperature on the hole mobility of the compounds mentioned above.
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Fig. 6. AFM images (3 lm 3 lm scans) of the thin films annealed at different temperatures for IDT-3Th-ID(I) (a, b, c and d) and IDT-3Th-ID(II) (e, f, g and h), Images on the left and right show phase and topographic mode images, respectively.
Fig. 7. (a) Current density–voltage characteristics of the OSCs based on the blends of compounds/PC70BM (2:1, w/w) under the illumination of AM 1.5, 100 mW/cm2; (b) IPCE spectra of the OSCs devices.
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Table 3 Photovoltaic performance of the OPVs with the device structure of ITO/PEDOT: PSS/organic molecules: PC70BM (2:1, w/w)/Ca/Al, under the illumination of AM 1.5G, 100 mW/cm2.
a b
Active layer
VOC (V)
JSC (mA/cm2)a
JSC (mA/cm2)b
FF (%)
PCE (%)
IDT-3Th-ID(I): PC70BM IDT-3Th-ID(II): PC70BM
0.82 0.81
8.15 7.99
7.95 7.50
46 44
3.07 2.83
Measured from J–V curves. Calculated from IPCE.
3.6. Photovoltaic properties The photovoltaic properties of the compounds were investigated by fabricating the OSC devices with the two compounds as donor in the device structure of ITO/PEDOT: PSS/photoactive layer (60 nm)/Ca/Al. The photoactive layer was formed by spin-coating the blend solution of IDT-3ThID(I)(or IDT-3Th-ID(II))/PC70BM (2:1, w/w) in chloroform. Under standard measurement conditions, the optimized J–V curves were shown in Fig. 7a, and related parameters were summarized in Table 3. We got the optimized performance with a high VOC of 0.82 V, and achieved a reasonable power conversion efficiency (PCE) of 3.07% and 2.83% for IDT-3Th-ID(I) and IDT-3Th-ID(II), respectively. The higher VOC should benefit from the lower HOMO energy level of the two compounds donor. In addition, we also measured the incident photon-to-converted current efficiency (IPCE) spectra, as shown in Fig. 7b. The integrated photocurrents from the IPCE spectra are consistent with the JSC values quite well (see Table 3). The relatively higher efficiency of IDT-3Th-ID(I) should be from its good p–p stacking ability and its nanofiber morphology in the active layer, which would benefit the formation of suitable interpenetrating donor/acceptor networks, thus, results in higher efficiency.
4. Conclusions Two linear A-p-D-p-A structured molecules with ID as terminal acceptor unit, IDT as core and donor unit, and 3Th with different alkyl chain positions as p-bridges, IDT3Th-ID(I) and IDT-3Th-ID(II), were designed and synthesized for the application in OFETs and OSCs. According to the electrochemical measurement, optimized structure modeling, theoretical calculation and spectrum characterization, we found that the two compounds showed different intermolecular interactions, self-aggregation tendency, film morphology, and different configuration and charge transportation changing tendency with annealing temperature. For the OFET application, IDT-3Th-ID(I) demonstrated the optimized hole mobility of 0.52 cm2 V1 s1, threshold voltage of 0.38 V and on–off ratio of 107, and IDT-3Th-ID(II) showed an even higher hole mobility of 0.61 cm2 V1 s1, threshold of 0.87 V and the on–off ratio of 109. For the application in OSCs, the two molecules demonstrated higher VOC of 0.82 V, benefited from the lower HOMO energy level of the molecules. The OSCs based on IDT-3Th-ID(I)/PCBM70 (2:1, w/w) and IDT-3Th-ID(II)/ PCBM70 (2:1, w/w) showed power conversion efficiencies (PCEs) of 3.07% and 2.83% respectively. These results demonstrate that the small molecules with the highly p-conjugated IDT as donor unit, 3Th as p-bridges and ID as
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