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Diketopyrrolopyrrole-based conjugated small molecules bearing two different acceptor moieties for organic solar cells
Synthetic Metals 221 (2016) 39–47
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Diketopyrrolopyrrole-based conjugated small molecules bearing two different acceptor moieties for organic solar cells Eun Yi Ko, Suna Choi, Gi Eun Park, Dae Hee Lee, Min Ju Cho, Dong Hoon Choi* Department of Chemistry, Research Institute for Natural Sciences, Korea University, 5 Anam-dong, Sungbuk-gu, Seoul 136-701, South Korea
A R T I C L E I N F O
Article history: Received 10 September 2016 Received in revised form 2 October 2016 Accepted 4 October 2016 Available online xxx Keywords: Organic semiconductor Conjugated small molecule Diketopyrrolopyrrole Solar cell Power conversion efficiency
A B S T R A C T
Three A1-D-A2-D-A1-type conjugated molecules (A: acceptor, D: donor), containing diketopyrrolopyrrole (DPP) acceptors, another weaker acceptor, and thiophenes as common donors, were synthesized. The molecules exhibited strong absorption from 500 nm to the near-IR wavelength region, and different molecular energy levels. The three molecules synthesized were applied in solar cells made with phenylC71-butyric acid methyl ester, and thienopyrroledione (TPD) (DPP)2 exhibited the highest power conversion efficiency of 3.37% owing to its appropriate internal morphology and relatively high open circuit voltage, which is conferred by its relatively low-lying highest occupied molecular orbital level. ã 2016 Elsevier B.V. All rights reserved.
1. Introduction Donor-acceptor (D-A) conjugated polymers and small molecules have received considerable attention as components of semiconducting devices, thin-film transistors (TFTs), and organic solar cells (OSCs) [1,2]. In these molecules, the absorption band and energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) may be tuned by choosing appropriate electron donor and acceptor units. Conjugated small molecules have attracted particular interest owing to their easier synthesis and purification and higher crystallinity than polymers [3–7]. Furthermore, batch-to-batch variations in structure and properties are lessened in the preparation of small molecules. This higher molecular precision results in more reproducible film fabrication and better understanding of molecular structure-characteristics relationships [8]. Recently, high power conversion efficiencies (PCEs) of over 9% have been reported for OSCs fabricated with conjugated D-A type small molecules [9,10]. The combination of D and A units in an alternating sequence is an effective strategy for broadening the light-absorption wavelength band of a material, and for tuning the HOMO and LUMO energy levels to provide a low band gap. In addition, these small molecules display strong intra- and intermolecular interactions and efficient charge transport [11].
* Corresponding author. E-mail address: [email protected] (D.H. Choi). http://dx.doi.org/10.1016/j.synthmet.2016.10.005 0379-6779/ã 2016 Elsevier B.V. All rights reserved.
Diketopyrrolopyrrole (DPP) is a well-known acceptor moiety in conjugated D-A small molecules [5,12–15]. DPP has a high extinction coefficient in the visible-wavelength region, strong electron-withdrawing properties, and high coplanarity. In addition, the synthesis of DPP is widely known, and it is easy to introduce various alkyl chains onto the N-position of the DPP backbone, which improves solubility in common organic solvents and promotes lamella-forming crystallization [16]. Owing to these advantages, various kinds of DPP-containing small molecules with A-D-A or D-A-D structural motifs have been frequently applied in electronic and optoelectronic applications. However, these A-D-A or D-A-D type conjugated small molecules have only a single electron-donating or -accepting unit; therefore, the HOMO or LUMO energy levels of the molecule depend on the kind of donor or acceptor. However, the introduction of another donor or acceptor moiety can provide various D and A combinations, and thus more effective tunability of electrochemical properties [17]. The development of p-extended alternating D-A structures may be an effective strategy to obtain high charge mobility [18]. Although p-extended molecular structures have many advantages, there are surprisingly few reports of studies investigating high-performing semiconducting small molecules. The limited number of reported p-extended molecules with A1-D-A2-D-A1 or D1-A-D2-A-D1 structures exhibit relatively low PCEs in bulk heterojunction (BHJ)-type SCs. These small molecules have tunable absorption bands, energy levels, and morphologies, achieved by changing the chemical structure of the component moieties. For instance, A1-D-A2-D-A1-type small molecules containing DPP and
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isoindigo (IID) moieties Exhibit 0.42–2.36% PCEs in OSCs, but they show significantly different absorption spectra and film morphologies depending on the structure of the D or A1 unit [19]. Another study on D1-A-D2-A-D1 structures containing DPP and different donor core and end group units has been performed to investigate the structure-related device performance in terms of energy levels and morphologies [18]. Investigations into p-extended small molecules by incorporating two different electron acceptors are valuable for understanding the structure-property relationships of these materials. In this study, we synthesized three new A1-D-A2-D-A1 type conjugated small molecules containing the weak acceptors thiophene carboxylate (3EhT), thienopyrroledione (TPD), and fluorinated thienothiophene (TT). The structure of 3EhT(DPP)2, TPD(DPP)2 and TT(DPP)2 are shown in Fig. 1. The thiophene-based acceptors are located in the middle of the small molecules. These units are already known to exhibit good performance in D-A systems [20–22]. DPP, which is a stronger acceptor, is attached to either side of the central unit. We investigated the variation of molecular properties for these three small molecules depending on the core acceptor units. They all show broad absorption bands extending from the visible to near-IR region, and unique molecular arrangements resulting in different device performances. Conventional BHJ-type SCs were fabricated using a blended solution of phenyl-C71-butyric acid methyl ester (PC71BM) and each small molecule. The SC fabricated using TPD(DPP)2 and PC71BM provided a PCE of over 3.0% by virtue of the finer solid-state morphology of TPD(DPP)2, and its higher open circuit voltage (Voc) of 0.80 V, which is conferred by its relatively low-lying HOMO energy levels. 2. Materials and methods All the starting reagents were purchased from TCI, SigmaAldrich, Acros Organics, and Alfa Aesar, and were used without any further purification. Poly(3,4-ethylenedioxylthiophene)-poly(styrenesulfonate) (PEDOT:PSS; Heraeus Clevios P VP Al4083) was purchased from Semiphion Company, and PC71BM was purchased
from Nano-C. Compounds 1, 3, 4 and 5 were synthesized according to literature methods [13,18,19,23,24]. 2.1. Syntheses 2.1.1. Synthesis of 2,5-bis(2-ethylhexyl)-3-(5-phenylthiophen-2-yl)-6(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiophen-2-yl) pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (2) 2,5-Bis(2-ethylhexyl)-3-(5-phenylthiophen-2-yl)-6-(thiophen2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (1) (1.0 g, 1.7 mmol) was dissolved in 25 mL tetrahydrofuran (THF), and 2-isopropoxy4,4,5,5-tetramethyl-1,3,2-dioxaborolane (0.8 g, 4.2 mmol) was added. The mixture was cooled to 30 C and lithium diisopropylamide (LDA) (2.0 mL, 2 M) was added dropwise. After sufficient reaction time, 1 M HCl solution was added to quench the reaction, and the organic layer was extracted with methylene chloride. The extract was concentrated in vacuo and added to methanol to precipitate the product as a purple solid. Yield: 72% (0.86 g). 1H NMR (CDCl3, 500 MHz): d (ppm) 9.00 (d, J = 5.0 Hz, 1H), 8.89 (d, J = 5.0 Hz, 1H), 7.72 (d, J = 5.0 Hz, 1H), 7.68 (d, J = 10.0 Hz, 2H), 7.48 (d, J = 5.0 Hz, 1H), 7.44 (t, J = 10.0 Hz, 2H) 7.37 (d, J = 5.0 Hz, 1H), 4.07 (m, 4H), 1.90 (m, 2H), 1.26–1.40 (m, 30H), 0.86–0.92 (m, 12H). Elemental Anal. Calcd for C42H55BN2O4S2: C, 69.40; H, 7.63; B, 1.49; N, 3.85; O, 8.80; S, 8.82; Found: C, 70.15; H, 7.68; N, 3.68; O, 8.42; S, 8.72 2.1.2. Synthesis of 2-ethylhexyl 5,5Z-bis(2,5-bis(2-ethylhexyl)-3,6dioxo-4-(5-phenylthiophen-2-yl)-2,3,5,6-tetrahydropyrrolo[3,4-c] pyrrol-1-yl)-[2,20 :50 :200 :500 ,2000 :5000 ,2Z-quinquethiophene]-300 carboxylate (3EhT(DPP)2) (6) 2,5-Bis(2-ethylhexyl)-3-(5-phenylthiophen-2-yl)-6-(5(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (2) (0.20 g, 0.28 mmol), 2ethylhexyl 5,500 -dibromo-[2,20 :50 ,200 -terthiophene]-30 -carboxylate (3) (0.062 g, 0.11 mmol), Pd2(dba)3, and P(o-tol)3 were dissolved in 15 mL toluene. Then, 2 M aqueous potassium carbonate and methanol were added (toluene:methanol:water = 3:1:1 v/v). The temperature was raised to 90 C for 12 h, and the resulting dark
Fig. 1. Structures of the small molecules 3EhT(DPP)2, TPD(DPP)2, and TT (DPP)2.
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blue mixture was purified with silica-gel column chromatography (eluent = chloroform/hexane, 9:1 v/v). The purified solution was added to methanol to obtain precipitates. Yield: 81% (143 mg). 1H NMR (CDCl3, 500 MHz): d (ppm) 8.98 (m, 4H), 7.66 (d, J = 10.0 Hz, 4H), 7.53 (s, 1H), 7.41–7.48 (m, 7H), 7.34–7.37 (m, 3H), 7.31 (d, J = 5.0 Hz, 1H) 7.21–7.25 (m, 2H), 7.13 (d, J = 5.0 Hz, 1H), 4.24 (d, J = 5.0 Hz, 2H), 4.07 (m, 8H), 1.93 (m, 5H), 1.28–1.44 (m, 38H) 0.86– 0.96 (m, 32H). Elemental Anal. Calcd for C93H108N4O6S7: C, 69.71; H, 6.79; N, 3.50; O, 5.99; S, 14.01; Found: C, 69.56; H, 7.01; N, 3.53; O, 5.62; S, 14.28; MS (MALDI-TOF, m/z): [M]+ Calcd for C93H108N4O6S7: 1600.63. Found: 1599.37.
Grazing-incidence X-ray diffraction (GI-XRD) measurements were conducted at the 9A (U-SAXS) beamline (energy = 11.07 keV, pixel size = 79.6 mm, l = 1.120 Å, 2u = 0 –20 ) at the Pohang Accelerator Laboratory (PAL). The film samples were fabricated in the same way as films for TFTs, i.e., by spin-coating the small molecule solutions on n-octyltrichlorosilane (OTS)-treated Si/SiO2 wafers. The films were thermally annealed at 100 or 120 C for 10 min. Atomic force microscopy (AFM) images were obtained with an advanced scanning probe microscope (XE-100, PSIA).
2.1.3. Synthesis of 6,60 -(5',5000 -(5-(2-ethylhexyl)-4,6-dioxo-5,6dihydro-4H-thieno[3,4-c]pyrrole-1,3-diyl)bis([2,20 -bithiophene]-50 ,5diyl))bis(2,5-bis(2-ethylhexyl)-3-(5-phenylthiophen-2-yl)pyrrolo [3,4-c]pyrrole-1,4(2H,5H)-dione) (TPD(DPP)2) (7) 2,5-Bis(2-ethylhexyl)-3-(5-phenylthiophen-2-yl)-6-(5(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (2) (0.20 g, 0.28 mmol), 1,3-bis (5-bromothiophen-2-yl)-5-(2-ethylhexyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (4) (0.065 g, 0.11 mmol), Pd2(dba)3, and P(otol)3 were used for this synthesis. The procedure was the same as that used in the synthesis of 3EhT(DPP)2. Yield: 85% (152 mg). 1H NMR (CDCl3, 500 MHz): d (ppm) 8.98 (m, 4H), 7.94 (d, J = 5.0 Hz, 4H), 7.63 (m, 2H), 7.33-7.47 (m, 10H), 7.18 (m, 2H), 4.03 (m, 8H) 3.61 (d, J = 5.0 Hz, 2H), 1.92 (m, 5H), 1.30–1.42 (m, 38H) 0.84–0.98 (m, 32H). Elemental Anal. Calcd for C94H107N5O6S7: C, 69.38; H, 6.63; N, 4.30; O, 5.90; S, 13.79; Found: C, 69.59; H, 6.53; N, 4.19; O, 6.01; S, 13.68; MS (MALDI-TOF, m/z): [M]+ Calcd for C94H107N5O6S7: 1625.63. Found: 1628.91.
For the fabrication of bottom-gate top-contact (BGTC) TFT devices, n-doped Si/SiO2 wafers were cleaned using the literature method [25]. After UV/ozone treatment for 20 min, OTS was used to treat the gate dielectric, forming a self-assembled OTS monolayer. Then, a 10 mg mL1 chloroform solution of the semiconductor was spin-coated at 3000 rpm for 40 s to generate a thin film on the OTS-layer. The thin films were then annealed on a hot plate in air at a fixed temperature for 10 min. Source and drain electrodes (thickness = 70 nm) were thermally evaporated under high vacuum, and the performances of the devices were determined in air using a Keithley 4200 SCS analyzer. The fieldeffect saturated carrier mobility (m) was calculated from the equation in the literature [25].
2.1.4. Synthesis of 2-ethylhexyl 4,6-bis(50 -(2,5-bis(2-ethylhexyl)-3,6dioxo-4-(5-phenylthiophen-2-yl)-2,3,5,6-tetrahydropyrrolo[3,4-c] pyrrol-1-yl)-[2,20 -bithiophen]-5-yl)-3-fluorothieno[3,4-b]thiophene2-carboxylate (TT(DPP)2) (8) 2,5-Bis(2-ethylhexyl)-3-(5-phenylthiophen-2-yl)-6-(5(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (2) (0.20 g, 0.28 mmol), 2ethylhexyl 4,6-bis(5-bromothiophen-2-yl)-3-fluorothieno[3,4-b] thiophene-2-carboxylate (5) (0.070 g, 0.11 mmol), Pd2(dba)3, and P(o-tol)3 were used for this synthesis. The procedure was the same as that used in the synthesis of 3EhT(DPP)2. Yield: 74% (136 mg). 1H NMR (CDCl3, 500 MHz): d (ppm) 8.97 (m, 4H), 7.66 (d, J = 5.0 Hz, 1H), 7.57 (d, J = 5.0 Hz, 3H), 7.40–7.45 (m, 2H), 7.29–7.37 (m, 8H), 7.17 (m, 4H) 4.33 (d, J = 5.0 Hz, 2H), 4.04 (m, 8H), 1.91 (m, 5H), 1.29– 1.46 (m, 38H), 0.87–0.98 (m, 32H). Elemental Anal. Calcd for C95H107FN4O6S8: C, 68.06; H, 6.43; F, 1.13; N, 3.34; O, 5.73; S, 15.30; Found: C, 67.98; H, 6.50; N, 3.42; O, 6.92; S, 15.18; MS (MALDI-TOF, m/z): [M]+ Calcd for C95H107FN4O6S8: 1674.59. Found: 1674.38. 2.2. Instruments and measurements 1
H NMR spectra were recorded on a Bruker NMR 500 MHz spectrometer in deuterated chloroform. An elemental analyzer (Flash 2000 Thermo Fisher Scientific) was used for measuring the C, H, N, O and S contents. The molecular mass of each compound was determined by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry (LRF20, a Bruker Daltonics). The absorption spectra of the small molecules as both films and chloroform solutions were recorded using a UV–vis absorption spectrometer (Agilent 8453, photodiode array, l = 190– 1100 nm). Cyclic voltammetry (CV) was performed at a 20 mV s1 scan rate in distilled acetonitrile solution bearing 0.10 M tetrabutylammonium hexafluorophosphate using an eDAQ EA161 potentiostat.
2.3. Fabrication of TFTs
2.4. Fabrication of OSCs Conventional OSCs were fabricated with indium-tin-oxide (ITO)-coated glass, LiF and Al as electrodes, and a photosensitive layer. ITO glasses were washed using acetone, water, and isopropyl alcohol; then, UV/ozone treatment was performed for 20 min. PEDOT:PSS was spin-coated at 4000 rpm for 30 s onto the washed ITO glasses, and then the substrates were heated at 155 C for 15 min. Solutions containing one of the three small molecules and PC71BM (1.5 wt%, 1:1 wt ratio) were prepared in chloroform containing 1,8-diiodooctane (DIO) 1%. These solutions were spin-coated at 3000 rpm for 40 s to form a photoactive layer. Then, LiF (thickness of 1 nm) and Al (thickness of 100 nm) were evaporated under high-vacuum conditions. The current densityvoltage (J-V) characteristics were measured with a Keithley 2400 source meter under AM 1.5G illumination. 3. Results and discussion 3.1. Synthesis and properties of the small molecules DPP-based p-type small molecules were designed and synthesized using compounds 3, 4 and 5 as weak acceptors, and DPP (compound 2) anchored onto the edge moieties. The phenyl group was selected as an end group for blocking the side reactions. In addition, to achieve good solubility in organic solvents, branched alkyl side chain, 2-ethylhexyl groups were introduced to the accepting units. The core units such as thiophene carboxylate (3EhT), thienopyrroledione (TPD), fluorinated thienothiophene (TT) were used for the synthesis, which were employed in previous literatures [20–22]. The overall synthetic procedure is displayed in Scheme 1. The small molecules were named from the core units, i.e., 3EhT(DPP)2, TPD(DPP)2, and TT(DPP)2. Compound 2 was synthesized with compound 1 and 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane. The core acceptor units 3, 4, and 5, were prepared by methods cited in literature [18,19,21,22]. The small molecules were synthesized via Suzuki couplings between compound 2 and compound 3, 4, or 5 using Pd2(dba)3 and P(o-tol)3 as catalysts. The structures of the synthesized molecules were characterized by NMR and MALDI-TOF mass spectroscopy.
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Scheme 1. Synthetic procedure for the three conjugated small molecules. i. 2-Isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, LDA, THF, 30 C. ii. Suzuki reaction: Pd2(dba)3, P(o-tol)3, K2CO3, toluene, H2O, MeOH, 90 C.
1.0 0.8
TPD(DPP)2 TT(DPP)2
0.4 0.2 400
UV–vis absorption spectra of the three small molecules were measured in chloroform solution and as films (Fig. 2). In solution, the maximum absorption peaks of 3EhT(DPP)2, TPD(DPP)2, and TT (DPP)2, which can be assigned to strong intramolecular charge transfer bands, are observed at 629, 637, and 645 nm, respectively. The film sample was prepared by spin coating a 1 wt% chloroform solution of the small molecule onto the glass. The absorption bands of the films are broadened and red shifted relative to those of the solutions owing to strong intermolecular interactions. 3EhT(DPP)2 and TPD(DPP)2 show similar absorption spectra as films, but with different peak intensities for the 0–0 and 0–1 transitions. The stronger 0–1 transition peak of TPD(DPP)2 indicates that it exhibits stronger aggregation behavior owing to its symmetric and planar molecular structure [29]. The optical band gap energies (Egopt) of 3EhT(DPP)2, TPD(DPP)2, and TT(DPP)2 obtained from the absorption edges are 1.59, 1.58, and 1.46 eV, respectively, and were used to calculate the LUMO energy levels.
(b)
3EhT(DPP)2
0.6
0.0
3.2. Optical and electrochemical properties
600 Wavelength (nm)
800
Normalized Absorption (a.u.)
(a)
Normalized Absorption (a.u.)
To investigate the thermal properties of each molecule, differential scanning calorimetry (DSC) was used, and the results are given in Fig. S1. The three small molecules exhibit clear melting behavior during the heating cycle. 3EhT(DPP)2 exhibits lower thermal transition temperature (185 C) than TPD(DPP)2 (223 C) and TT(DPP)2 (214 C). TPD(DPP)2 has the highest melting temperature owing to its more symmetric and planar structure. In the cooling cycle, only 3EhT(DPP)2 exhibits a clear crystallization temperature (125 C), indicating that it has a lower degree of crystallinity than the other two molecules. Theoretical calculations were also performed using density functional theory (DFT) at the B3LYP/6-31G(D) level of theory using the Spartan’10 program to understand the electron distribution in the small molecules, as shown in Fig. S2. The small molecules show similar planar structures, owing to possible S–O and S–F interactions with functional groups, such as carboxylate and fluorine atom. Thus, TPD(DPP)2 and TT(DPP)2 are thought to display greater backbone planarity among the three molecules [26–28].
1.0 0.8
3EhT(DPP)2 TPD(DPP)2 TT(DPP)2
0.6 0.4 0.2 0.0
400
600
800
Wavelength (nm)
Fig. 2. UV–vis absorption spectra of the three small molecules (a) in chloroform solution and (b) as thin films.
E.Y. Ko et al. / Synthetic Metals 221 (2016) 39–47
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(b)
(a)
Fig. 3. (a) Cyclic voltammograms of thin films of the small molecules, and (b) energy diagrams of the small molecules.
Dilute chloroform solutions and films were employed to investigate the molar extinction coefficients (e) and absorption coefficients (a) of the materials (Fig. S3 and Table S1). 3EhT(DPP)2 has the highest e (5.68 105 M1 cm1) and a (1.19 105 cm1). This suggests that 3EhT(DPP)2 will exhibit better light-harvesting ability in the wavelength range of solar emission than the other molecules. The electrochemical properties of the small molecules were measured by conducting CV analysis of films on Pt electrodes (Fig. 3). Suitable HOMO, LUMO and band gap energies are crucial for photoinduced exciton generation in the prepared BHJ structures. The estimated oxidation onset potentials were determined using the absolute energy level of ferrocene (4.8 eV) as reference. 3EhT(DPP)2, TPD(DPP)2, and TT(DPP)2 show HOMO energy levels of 5.21, 5.29, and 5.21 eV determined from the oxidation potentials of 0.77, 0.85, and 0.77 V, respectively. The LUMO energy levels were determined using the relationship LUMO (eV) = HOMO (eV) Egopt (eV) [30], and are 3.62, 3.71 and 3.75 eV for 3EhT(DPP)2, TPD(DPP)2, and TT(DPP)2, respectively. The measured optical and electrochemical properties are tabulated in Table 1. 3.3. TFT performances The effect of the acceptor moiety in the small molecules on their charge transport properties was investigated by fabricating TFTs. The transfer curves of the TFTs are shown in Fig. 4 and the measured data are listed in Table 2. The highest charge carrier mobilities of the thermally annealed films are 5.84 103, 5.85 102, and 5.78 102 cm2 V1 s1 for 3EhT(DPP)2, TPD (DPP)2, and TT(DPP)2, respectively. The TFT made of a thermally annealed 3EhT(DPP)2 film shows a similar charge carrier mobility to that of the TFT prepared with a pristine 3EhT(DPP)2 film. However, TPD(DPP)2 and TT(DPP)2 show superior mobilities after thermal annealing. According to the
GI-XRD patterns (Fig. 5), TPD(DPP)2, which has planar structure shows many scattered patterns even in pristine film indicating possibly high degree of molecular packing. As shown in Fig. 5 and Table 3, 3EhT(DPP)2, TPD(DPP)2, and TT(DPP)2 show increased crystallinity after thermal annealing, as revealed by the fact that the crystallite grain size, as determined using the Scherrer equation [31], increases. In particular, TPD(DPP)2 and TT(DPP)2 revealed larger crystallites than 3EhT(DPP)2, which results from the distorted structure of 3EhT(DPP)2. The full width at half maximum (FWHM) of (100) diffraction peaks became smaller, indicating the improved crystallinity. Both TPD(DPP)2 and TT (DPP)2 show a significant improvement in carrier mobility after thermal annealing. The XRD pattern of 3EhT(DPP)2 indicates that the p–p stacking distance increases upon thermal annealing, although the crystallinity is increased (Table 3). Thus, no significant annealing effect was observed in the performance of the 3EhT(DPP)2-based TFT. The slightly distorted molecular structure of 3EhT(DPP)2 may interrupt charge transport compared to TPD(DPP)2 and TT(DPP)2. The surface morphologies of the thermally annealed active layers in the TFT devices were measured by AFM, as shown in Fig. 6. All three active layers show an increase in surface roughness after thermal annealing. The images obtained of pristine TPD(DPP)2 and TT(DPP)2 films exhibit significant differences to those obtained from the corresponding annealed films, which may indicate an increase in crystallinity upon thermal treatment. In brief, the improved crystallinity and compact surface coverage is thought to contribute to the increase in carrier mobility for TFTs upon thermal annealing. 3.4. OPV performance Conventional BHJ SCs were fabricated with the configuration ITO/PEDOT:PSS/photoactive layer/LiF/Al. Blended chloroform solutions of the small molecules and PC71BM were prepared to form
Table 1 Optical and electrochemical properties of the small molecules. Compound
3EhT(DPP)2 TPD(DPP)2 TT(DPP)2 a b c
lonseta (nm)
Absorption (nm) Solution
Film
629 637 645
657, 707 657, 708 693, 754
Film. Values were obtained from the cyclic voltammograms. HOMO (eV) Egopt (eV).
782 787 847
Egopt
1.59 1.58 1.46
a
(eV)
Eoxb (V)
0.77 0.85 0.77
Energy level (eV) HOMOb
LUMOc
5.21 5.29 5.21
3.62 3.71 3.75
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Fig. 4. Transfer curves of (a) 3EhT(DPP)2, (b) TPD(DPP)2, and (c) TT(DPP)2. VDS = 80 V.
Table 2 Performances of the three small molecule-based TFTs.
3EhT(DPP)2 TPD(DPP)2 TT(DPP)2
T ( C)
m (cm2 V1 s1)
Ion/Ioff
Vth (V)
pristine 100 pristine 120 pristine 100
4.28 103 5.84 103 2.26 103 5.85 102 2.51 103 5.78 102
105 105 105 107 104 107
5.68 0.98 4.90 0.78 1.87 1.72
the photoactive layer. The measured J-V characteristics and the external quantum efficiencies (EQEs) of the small molecule-based SCs are displayed in Fig. 7. The device performances were measured at 100 mW cm2 supplied by a solar simulator (Oriel, 1000 W). The measured parameters are listed in Table 4. The addition of DIO enhances Jsc, which contributes to an increase in the PCE of the small molecule-based SCs. The SCs containing 3EhT(DPP)2 and TPD(DPP)2 show similar PCEs, although they exhibit a problematic trade-off between Jsc and Voc. The SC containing 3EhT(DPP)2, having the highest e and a, exhibits a PCE of 3.29% and a higher Jsc of 8.31 mA cm2. Since Voc is proportional to the difference between the HOMO level of the small molecule and the LUMO level of the PC71BM, TPD(DPP)2 exhibits the highest Voc (=0.80 V) of the three molecules owing to its lower lying HOMO level (5.29 eV).
In order to investigate the relationship between the surface morphologies and performances of the SCs, AFM images of the photoactive layers were recorded (Fig. 8). Without addition of DIO, the films show large aggregated domains, grain boundaries, and high roughness, which indicates poor miscibility, and results in the interruption of exciton diffusion and inhibited migration of separated charges in the films. Furthermore, it induces a relatively low Jsc in SC devices derived from pristine blend films. However, the three small molecules display much smaller domain sizes and smoother surfaces by introducing DIO. These finer morphologies are thought to contribute to enhanced charge transport properties,
Table 3 2D GI-XRD peak assignments and corresponding parameters for small molecules before and after thermal annealing. Ta ( C)
3EhT (DPP)2 TPD(DPP)2 TT(DPP)2 a
pristine 100 pristine 120 pristine 100
p p Stacking
Lamellar Spacing qz (Å1) d (Å)
FWHM (Å1) Da (Å) qz (Å1) d (Å)
0.4697 0.4322 0.3946 0.3946 0.4243 0.4183
0.06863 0.05136 0.05363 0.04439 0.07917 0.03636
13.37 14.53 15.91 15.91 14.80 15.01
82.47 110.2 105.5 127.5 71.48 155.6
1.709 1.672 1.691 1.680 1.690 1.692
3.633 3.715 3.672 3.698 3.675 3.670
The grain size was calculated using the Scherrer equation, D = (Kl/bcosu).
Fig. 5. 2D GI-XRD patterns of 3EhT(DPP)2 (a, d), TPD(DPP)2 (b, e), and TT(DPP)2 (c, f). Pristine films (a–c) and films annealed at 100 C (d, f) and 120 C (e).
E.Y. Ko et al. / Synthetic Metals 221 (2016) 39–47
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Fig. 6. AFM height images (5 mm 5 mm) of the small molecule films. Topography images (left) and phase images (right) of 3EhT(DPP)2 (a, d), TPD(DPP)2 (b, e), and TT(DPP)2 (c, f). Pristine films (a–c) and films annealed at 100 C (d, f) and 120 C (e).
Fig. 7. (a) J-V characteristics of BHJ SCs and (b) EQE spectra of the films made from the small molecules and PC71BM.
resulting in higher Jsc values. The performance of the TPD(DPP)2bearing SC exhibits a drastic increase in PCE from 0.67 to 3.37% upon addition of DIO. The addition of DIO may improve the nanophase segregation in the blend matrix, increasing the Jsc value in all the devices. In particular, the lowest-lying HOMO level of TPD
(DPP)2 also contributes to an increase in Voc compared to that of the other two SCs. Therefore, the TPD acceptor unit, which has symmetric and planar molecular structure, is considered to be more effective for increasing the PCE of BHJ SCs with PC71BM. 4. Conclusion
Table 4 Photovoltaic characteristics of SCs made from the small molecules and PC71BM.
3EhT(DPP)2 TPD(DPP)2 TT(DPP)2
DIO
Jsc (mA cm2)
Voc (V)
FF
h (%)
– 1% – 1% – 1%
2.11 8.31 1.60 7.86 1.68 6.48
0.72 0.70 0.80 0.80 0.68 0.70
0.57 0.56 0.52 0.54 0.55 0.59
0.86 3.29 0.67 3.37 0.63 2.69
Average power conversion efficiencies are in the parenthesis.
(0.77) (3.18) (0.62) (3.20) (0.56) (2.61)
Newly synthesized A1-D-A2-D-A1 type conjugated molecules containing a relatively weak A2 accepting unit and an A1 diketopyrrolopyrrole moiety were prepared. By selecting different central weak accepting units, the absorption behavior and molecular energy levels of the small molecules were varied. Conventional SCs were fabricated with each molecule and PC71BM by adding a small amount of DIO. The TPD(DPP)2-based SC exhibited a higher Voc value owing to the position of its lowest-
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Fig. 8. AFM height images (5 mm 5 mm) of photoactive layers. Topography images (left) and phase images (right) of 3EhT(DPP)2 (a, d), TPD(DPP)2 (b, e), and TT(DPP)2 (c, f). Blended films (a–c) and blended films with DIO 1% (d–f).
lying HOMO level. The TPD(DPP)2-bearing SC exhibited a promising power conversion efficiency of 3.37%. Compared to the other two weak acceptors, the TPD unit exhibited improved performance in BHJ SC devices. Acknowledgements This research was supported by Science and Technology (NRF20100020209) and by the National Research Foundation of Korea (NRF2012R1A2A1A01008797), and the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education. Grazing incidence X-ray diffraction (GI-XRD) measurements were performed using Beam line 9A at the Pohang Accelerator Laboratory (PAL, Pohang, Korea). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. synthmet.2016.10.005. References [1] S.-Y. Liu, W.-Q. Liu, C.-X. Yuan, A.-G. Zhong, D. Han, B. Wang, M.N. Shah, M.-M. Shi, H. Chen, Dyes Pigm. 134 (2016) 139–147. [2] H. Zheng, J. Wang, W. Chen, C. Gu, J. Ren, M. Qiu, R. Yang, M. Sun, J. Mater. Chem. C 4 (2016) 6280–6286. [3] Y. Sun, J. Seifter, L. Huo, Y. Yang, B.B.Y. Hsu, H. Zhou, X. Sun, S. Xiao, L. Jiang, A.J. Heeger, Adv. Energy Mater. 5 (2015) 1400987. [4] K. Do, N. Cho, S.A. Siddiqui, S.P. Singh, G.D. Sharma, J. Ko, Dyes Pigm. 120 (2015) 126–135.
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Diketopyrrolopyrrole-based conjugated small molecules bearing two different acceptor moieties for organic solar cells Eun Yi Ko, Suna Choi, Gi Eun Park, Dae Hee Lee, Min Ju Cho, Dong Hoon Choi* Department of Chemistry, Research Institute for Natural Sciences, Korea University, 5 Anam-dong, Sungbuk-gu, Seoul 136-701, South Korea
A R T I C L E I N F O
Article history: Received 10 September 2016 Received in revised form 2 October 2016 Accepted 4 October 2016 Available online xxx Keywords: Organic semiconductor Conjugated small molecule Diketopyrrolopyrrole Solar cell Power conversion efficiency
A B S T R A C T
Three A1-D-A2-D-A1-type conjugated molecules (A: acceptor, D: donor), containing diketopyrrolopyrrole (DPP) acceptors, another weaker acceptor, and thiophenes as common donors, were synthesized. The molecules exhibited strong absorption from 500 nm to the near-IR wavelength region, and different molecular energy levels. The three molecules synthesized were applied in solar cells made with phenylC71-butyric acid methyl ester, and thienopyrroledione (TPD) (DPP)2 exhibited the highest power conversion efficiency of 3.37% owing to its appropriate internal morphology and relatively high open circuit voltage, which is conferred by its relatively low-lying highest occupied molecular orbital level. ã 2016 Elsevier B.V. All rights reserved.
1. Introduction Donor-acceptor (D-A) conjugated polymers and small molecules have received considerable attention as components of semiconducting devices, thin-film transistors (TFTs), and organic solar cells (OSCs) [1,2]. In these molecules, the absorption band and energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) may be tuned by choosing appropriate electron donor and acceptor units. Conjugated small molecules have attracted particular interest owing to their easier synthesis and purification and higher crystallinity than polymers [3–7]. Furthermore, batch-to-batch variations in structure and properties are lessened in the preparation of small molecules. This higher molecular precision results in more reproducible film fabrication and better understanding of molecular structure-characteristics relationships [8]. Recently, high power conversion efficiencies (PCEs) of over 9% have been reported for OSCs fabricated with conjugated D-A type small molecules [9,10]. The combination of D and A units in an alternating sequence is an effective strategy for broadening the light-absorption wavelength band of a material, and for tuning the HOMO and LUMO energy levels to provide a low band gap. In addition, these small molecules display strong intra- and intermolecular interactions and efficient charge transport [11].
* Corresponding author. E-mail address: [email protected] (D.H. Choi). http://dx.doi.org/10.1016/j.synthmet.2016.10.005 0379-6779/ã 2016 Elsevier B.V. All rights reserved.
Diketopyrrolopyrrole (DPP) is a well-known acceptor moiety in conjugated D-A small molecules [5,12–15]. DPP has a high extinction coefficient in the visible-wavelength region, strong electron-withdrawing properties, and high coplanarity. In addition, the synthesis of DPP is widely known, and it is easy to introduce various alkyl chains onto the N-position of the DPP backbone, which improves solubility in common organic solvents and promotes lamella-forming crystallization [16]. Owing to these advantages, various kinds of DPP-containing small molecules with A-D-A or D-A-D structural motifs have been frequently applied in electronic and optoelectronic applications. However, these A-D-A or D-A-D type conjugated small molecules have only a single electron-donating or -accepting unit; therefore, the HOMO or LUMO energy levels of the molecule depend on the kind of donor or acceptor. However, the introduction of another donor or acceptor moiety can provide various D and A combinations, and thus more effective tunability of electrochemical properties [17]. The development of p-extended alternating D-A structures may be an effective strategy to obtain high charge mobility [18]. Although p-extended molecular structures have many advantages, there are surprisingly few reports of studies investigating high-performing semiconducting small molecules. The limited number of reported p-extended molecules with A1-D-A2-D-A1 or D1-A-D2-A-D1 structures exhibit relatively low PCEs in bulk heterojunction (BHJ)-type SCs. These small molecules have tunable absorption bands, energy levels, and morphologies, achieved by changing the chemical structure of the component moieties. For instance, A1-D-A2-D-A1-type small molecules containing DPP and
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isoindigo (IID) moieties Exhibit 0.42–2.36% PCEs in OSCs, but they show significantly different absorption spectra and film morphologies depending on the structure of the D or A1 unit [19]. Another study on D1-A-D2-A-D1 structures containing DPP and different donor core and end group units has been performed to investigate the structure-related device performance in terms of energy levels and morphologies [18]. Investigations into p-extended small molecules by incorporating two different electron acceptors are valuable for understanding the structure-property relationships of these materials. In this study, we synthesized three new A1-D-A2-D-A1 type conjugated small molecules containing the weak acceptors thiophene carboxylate (3EhT), thienopyrroledione (TPD), and fluorinated thienothiophene (TT). The structure of 3EhT(DPP)2, TPD(DPP)2 and TT(DPP)2 are shown in Fig. 1. The thiophene-based acceptors are located in the middle of the small molecules. These units are already known to exhibit good performance in D-A systems [20–22]. DPP, which is a stronger acceptor, is attached to either side of the central unit. We investigated the variation of molecular properties for these three small molecules depending on the core acceptor units. They all show broad absorption bands extending from the visible to near-IR region, and unique molecular arrangements resulting in different device performances. Conventional BHJ-type SCs were fabricated using a blended solution of phenyl-C71-butyric acid methyl ester (PC71BM) and each small molecule. The SC fabricated using TPD(DPP)2 and PC71BM provided a PCE of over 3.0% by virtue of the finer solid-state morphology of TPD(DPP)2, and its higher open circuit voltage (Voc) of 0.80 V, which is conferred by its relatively low-lying HOMO energy levels. 2. Materials and methods All the starting reagents were purchased from TCI, SigmaAldrich, Acros Organics, and Alfa Aesar, and were used without any further purification. Poly(3,4-ethylenedioxylthiophene)-poly(styrenesulfonate) (PEDOT:PSS; Heraeus Clevios P VP Al4083) was purchased from Semiphion Company, and PC71BM was purchased
from Nano-C. Compounds 1, 3, 4 and 5 were synthesized according to literature methods [13,18,19,23,24]. 2.1. Syntheses 2.1.1. Synthesis of 2,5-bis(2-ethylhexyl)-3-(5-phenylthiophen-2-yl)-6(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiophen-2-yl) pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (2) 2,5-Bis(2-ethylhexyl)-3-(5-phenylthiophen-2-yl)-6-(thiophen2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (1) (1.0 g, 1.7 mmol) was dissolved in 25 mL tetrahydrofuran (THF), and 2-isopropoxy4,4,5,5-tetramethyl-1,3,2-dioxaborolane (0.8 g, 4.2 mmol) was added. The mixture was cooled to 30 C and lithium diisopropylamide (LDA) (2.0 mL, 2 M) was added dropwise. After sufficient reaction time, 1 M HCl solution was added to quench the reaction, and the organic layer was extracted with methylene chloride. The extract was concentrated in vacuo and added to methanol to precipitate the product as a purple solid. Yield: 72% (0.86 g). 1H NMR (CDCl3, 500 MHz): d (ppm) 9.00 (d, J = 5.0 Hz, 1H), 8.89 (d, J = 5.0 Hz, 1H), 7.72 (d, J = 5.0 Hz, 1H), 7.68 (d, J = 10.0 Hz, 2H), 7.48 (d, J = 5.0 Hz, 1H), 7.44 (t, J = 10.0 Hz, 2H) 7.37 (d, J = 5.0 Hz, 1H), 4.07 (m, 4H), 1.90 (m, 2H), 1.26–1.40 (m, 30H), 0.86–0.92 (m, 12H). Elemental Anal. Calcd for C42H55BN2O4S2: C, 69.40; H, 7.63; B, 1.49; N, 3.85; O, 8.80; S, 8.82; Found: C, 70.15; H, 7.68; N, 3.68; O, 8.42; S, 8.72 2.1.2. Synthesis of 2-ethylhexyl 5,5Z-bis(2,5-bis(2-ethylhexyl)-3,6dioxo-4-(5-phenylthiophen-2-yl)-2,3,5,6-tetrahydropyrrolo[3,4-c] pyrrol-1-yl)-[2,20 :50 :200 :500 ,2000 :5000 ,2Z-quinquethiophene]-300 carboxylate (3EhT(DPP)2) (6) 2,5-Bis(2-ethylhexyl)-3-(5-phenylthiophen-2-yl)-6-(5(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (2) (0.20 g, 0.28 mmol), 2ethylhexyl 5,500 -dibromo-[2,20 :50 ,200 -terthiophene]-30 -carboxylate (3) (0.062 g, 0.11 mmol), Pd2(dba)3, and P(o-tol)3 were dissolved in 15 mL toluene. Then, 2 M aqueous potassium carbonate and methanol were added (toluene:methanol:water = 3:1:1 v/v). The temperature was raised to 90 C for 12 h, and the resulting dark
Fig. 1. Structures of the small molecules 3EhT(DPP)2, TPD(DPP)2, and TT (DPP)2.
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blue mixture was purified with silica-gel column chromatography (eluent = chloroform/hexane, 9:1 v/v). The purified solution was added to methanol to obtain precipitates. Yield: 81% (143 mg). 1H NMR (CDCl3, 500 MHz): d (ppm) 8.98 (m, 4H), 7.66 (d, J = 10.0 Hz, 4H), 7.53 (s, 1H), 7.41–7.48 (m, 7H), 7.34–7.37 (m, 3H), 7.31 (d, J = 5.0 Hz, 1H) 7.21–7.25 (m, 2H), 7.13 (d, J = 5.0 Hz, 1H), 4.24 (d, J = 5.0 Hz, 2H), 4.07 (m, 8H), 1.93 (m, 5H), 1.28–1.44 (m, 38H) 0.86– 0.96 (m, 32H). Elemental Anal. Calcd for C93H108N4O6S7: C, 69.71; H, 6.79; N, 3.50; O, 5.99; S, 14.01; Found: C, 69.56; H, 7.01; N, 3.53; O, 5.62; S, 14.28; MS (MALDI-TOF, m/z): [M]+ Calcd for C93H108N4O6S7: 1600.63. Found: 1599.37.
Grazing-incidence X-ray diffraction (GI-XRD) measurements were conducted at the 9A (U-SAXS) beamline (energy = 11.07 keV, pixel size = 79.6 mm, l = 1.120 Å, 2u = 0 –20 ) at the Pohang Accelerator Laboratory (PAL). The film samples were fabricated in the same way as films for TFTs, i.e., by spin-coating the small molecule solutions on n-octyltrichlorosilane (OTS)-treated Si/SiO2 wafers. The films were thermally annealed at 100 or 120 C for 10 min. Atomic force microscopy (AFM) images were obtained with an advanced scanning probe microscope (XE-100, PSIA).
2.1.3. Synthesis of 6,60 -(5',5000 -(5-(2-ethylhexyl)-4,6-dioxo-5,6dihydro-4H-thieno[3,4-c]pyrrole-1,3-diyl)bis([2,20 -bithiophene]-50 ,5diyl))bis(2,5-bis(2-ethylhexyl)-3-(5-phenylthiophen-2-yl)pyrrolo [3,4-c]pyrrole-1,4(2H,5H)-dione) (TPD(DPP)2) (7) 2,5-Bis(2-ethylhexyl)-3-(5-phenylthiophen-2-yl)-6-(5(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (2) (0.20 g, 0.28 mmol), 1,3-bis (5-bromothiophen-2-yl)-5-(2-ethylhexyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (4) (0.065 g, 0.11 mmol), Pd2(dba)3, and P(otol)3 were used for this synthesis. The procedure was the same as that used in the synthesis of 3EhT(DPP)2. Yield: 85% (152 mg). 1H NMR (CDCl3, 500 MHz): d (ppm) 8.98 (m, 4H), 7.94 (d, J = 5.0 Hz, 4H), 7.63 (m, 2H), 7.33-7.47 (m, 10H), 7.18 (m, 2H), 4.03 (m, 8H) 3.61 (d, J = 5.0 Hz, 2H), 1.92 (m, 5H), 1.30–1.42 (m, 38H) 0.84–0.98 (m, 32H). Elemental Anal. Calcd for C94H107N5O6S7: C, 69.38; H, 6.63; N, 4.30; O, 5.90; S, 13.79; Found: C, 69.59; H, 6.53; N, 4.19; O, 6.01; S, 13.68; MS (MALDI-TOF, m/z): [M]+ Calcd for C94H107N5O6S7: 1625.63. Found: 1628.91.
For the fabrication of bottom-gate top-contact (BGTC) TFT devices, n-doped Si/SiO2 wafers were cleaned using the literature method [25]. After UV/ozone treatment for 20 min, OTS was used to treat the gate dielectric, forming a self-assembled OTS monolayer. Then, a 10 mg mL1 chloroform solution of the semiconductor was spin-coated at 3000 rpm for 40 s to generate a thin film on the OTS-layer. The thin films were then annealed on a hot plate in air at a fixed temperature for 10 min. Source and drain electrodes (thickness = 70 nm) were thermally evaporated under high vacuum, and the performances of the devices were determined in air using a Keithley 4200 SCS analyzer. The fieldeffect saturated carrier mobility (m) was calculated from the equation in the literature [25].
2.1.4. Synthesis of 2-ethylhexyl 4,6-bis(50 -(2,5-bis(2-ethylhexyl)-3,6dioxo-4-(5-phenylthiophen-2-yl)-2,3,5,6-tetrahydropyrrolo[3,4-c] pyrrol-1-yl)-[2,20 -bithiophen]-5-yl)-3-fluorothieno[3,4-b]thiophene2-carboxylate (TT(DPP)2) (8) 2,5-Bis(2-ethylhexyl)-3-(5-phenylthiophen-2-yl)-6-(5(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (2) (0.20 g, 0.28 mmol), 2ethylhexyl 4,6-bis(5-bromothiophen-2-yl)-3-fluorothieno[3,4-b] thiophene-2-carboxylate (5) (0.070 g, 0.11 mmol), Pd2(dba)3, and P(o-tol)3 were used for this synthesis. The procedure was the same as that used in the synthesis of 3EhT(DPP)2. Yield: 74% (136 mg). 1H NMR (CDCl3, 500 MHz): d (ppm) 8.97 (m, 4H), 7.66 (d, J = 5.0 Hz, 1H), 7.57 (d, J = 5.0 Hz, 3H), 7.40–7.45 (m, 2H), 7.29–7.37 (m, 8H), 7.17 (m, 4H) 4.33 (d, J = 5.0 Hz, 2H), 4.04 (m, 8H), 1.91 (m, 5H), 1.29– 1.46 (m, 38H), 0.87–0.98 (m, 32H). Elemental Anal. Calcd for C95H107FN4O6S8: C, 68.06; H, 6.43; F, 1.13; N, 3.34; O, 5.73; S, 15.30; Found: C, 67.98; H, 6.50; N, 3.42; O, 6.92; S, 15.18; MS (MALDI-TOF, m/z): [M]+ Calcd for C95H107FN4O6S8: 1674.59. Found: 1674.38. 2.2. Instruments and measurements 1
H NMR spectra were recorded on a Bruker NMR 500 MHz spectrometer in deuterated chloroform. An elemental analyzer (Flash 2000 Thermo Fisher Scientific) was used for measuring the C, H, N, O and S contents. The molecular mass of each compound was determined by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry (LRF20, a Bruker Daltonics). The absorption spectra of the small molecules as both films and chloroform solutions were recorded using a UV–vis absorption spectrometer (Agilent 8453, photodiode array, l = 190– 1100 nm). Cyclic voltammetry (CV) was performed at a 20 mV s1 scan rate in distilled acetonitrile solution bearing 0.10 M tetrabutylammonium hexafluorophosphate using an eDAQ EA161 potentiostat.
2.3. Fabrication of TFTs
2.4. Fabrication of OSCs Conventional OSCs were fabricated with indium-tin-oxide (ITO)-coated glass, LiF and Al as electrodes, and a photosensitive layer. ITO glasses were washed using acetone, water, and isopropyl alcohol; then, UV/ozone treatment was performed for 20 min. PEDOT:PSS was spin-coated at 4000 rpm for 30 s onto the washed ITO glasses, and then the substrates were heated at 155 C for 15 min. Solutions containing one of the three small molecules and PC71BM (1.5 wt%, 1:1 wt ratio) were prepared in chloroform containing 1,8-diiodooctane (DIO) 1%. These solutions were spin-coated at 3000 rpm for 40 s to form a photoactive layer. Then, LiF (thickness of 1 nm) and Al (thickness of 100 nm) were evaporated under high-vacuum conditions. The current densityvoltage (J-V) characteristics were measured with a Keithley 2400 source meter under AM 1.5G illumination. 3. Results and discussion 3.1. Synthesis and properties of the small molecules DPP-based p-type small molecules were designed and synthesized using compounds 3, 4 and 5 as weak acceptors, and DPP (compound 2) anchored onto the edge moieties. The phenyl group was selected as an end group for blocking the side reactions. In addition, to achieve good solubility in organic solvents, branched alkyl side chain, 2-ethylhexyl groups were introduced to the accepting units. The core units such as thiophene carboxylate (3EhT), thienopyrroledione (TPD), fluorinated thienothiophene (TT) were used for the synthesis, which were employed in previous literatures [20–22]. The overall synthetic procedure is displayed in Scheme 1. The small molecules were named from the core units, i.e., 3EhT(DPP)2, TPD(DPP)2, and TT(DPP)2. Compound 2 was synthesized with compound 1 and 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane. The core acceptor units 3, 4, and 5, were prepared by methods cited in literature [18,19,21,22]. The small molecules were synthesized via Suzuki couplings between compound 2 and compound 3, 4, or 5 using Pd2(dba)3 and P(o-tol)3 as catalysts. The structures of the synthesized molecules were characterized by NMR and MALDI-TOF mass spectroscopy.
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E.Y. Ko et al. / Synthetic Metals 221 (2016) 39–47
Scheme 1. Synthetic procedure for the three conjugated small molecules. i. 2-Isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, LDA, THF, 30 C. ii. Suzuki reaction: Pd2(dba)3, P(o-tol)3, K2CO3, toluene, H2O, MeOH, 90 C.
1.0 0.8
TPD(DPP)2 TT(DPP)2
0.4 0.2 400
UV–vis absorption spectra of the three small molecules were measured in chloroform solution and as films (Fig. 2). In solution, the maximum absorption peaks of 3EhT(DPP)2, TPD(DPP)2, and TT (DPP)2, which can be assigned to strong intramolecular charge transfer bands, are observed at 629, 637, and 645 nm, respectively. The film sample was prepared by spin coating a 1 wt% chloroform solution of the small molecule onto the glass. The absorption bands of the films are broadened and red shifted relative to those of the solutions owing to strong intermolecular interactions. 3EhT(DPP)2 and TPD(DPP)2 show similar absorption spectra as films, but with different peak intensities for the 0–0 and 0–1 transitions. The stronger 0–1 transition peak of TPD(DPP)2 indicates that it exhibits stronger aggregation behavior owing to its symmetric and planar molecular structure [29]. The optical band gap energies (Egopt) of 3EhT(DPP)2, TPD(DPP)2, and TT(DPP)2 obtained from the absorption edges are 1.59, 1.58, and 1.46 eV, respectively, and were used to calculate the LUMO energy levels.
(b)
3EhT(DPP)2
0.6
0.0
3.2. Optical and electrochemical properties
600 Wavelength (nm)
800
Normalized Absorption (a.u.)
(a)
Normalized Absorption (a.u.)
To investigate the thermal properties of each molecule, differential scanning calorimetry (DSC) was used, and the results are given in Fig. S1. The three small molecules exhibit clear melting behavior during the heating cycle. 3EhT(DPP)2 exhibits lower thermal transition temperature (185 C) than TPD(DPP)2 (223 C) and TT(DPP)2 (214 C). TPD(DPP)2 has the highest melting temperature owing to its more symmetric and planar structure. In the cooling cycle, only 3EhT(DPP)2 exhibits a clear crystallization temperature (125 C), indicating that it has a lower degree of crystallinity than the other two molecules. Theoretical calculations were also performed using density functional theory (DFT) at the B3LYP/6-31G(D) level of theory using the Spartan’10 program to understand the electron distribution in the small molecules, as shown in Fig. S2. The small molecules show similar planar structures, owing to possible S–O and S–F interactions with functional groups, such as carboxylate and fluorine atom. Thus, TPD(DPP)2 and TT(DPP)2 are thought to display greater backbone planarity among the three molecules [26–28].
1.0 0.8
3EhT(DPP)2 TPD(DPP)2 TT(DPP)2
0.6 0.4 0.2 0.0
400
600
800
Wavelength (nm)
Fig. 2. UV–vis absorption spectra of the three small molecules (a) in chloroform solution and (b) as thin films.
E.Y. Ko et al. / Synthetic Metals 221 (2016) 39–47
43
(b)
(a)
Fig. 3. (a) Cyclic voltammograms of thin films of the small molecules, and (b) energy diagrams of the small molecules.
Dilute chloroform solutions and films were employed to investigate the molar extinction coefficients (e) and absorption coefficients (a) of the materials (Fig. S3 and Table S1). 3EhT(DPP)2 has the highest e (5.68 105 M1 cm1) and a (1.19 105 cm1). This suggests that 3EhT(DPP)2 will exhibit better light-harvesting ability in the wavelength range of solar emission than the other molecules. The electrochemical properties of the small molecules were measured by conducting CV analysis of films on Pt electrodes (Fig. 3). Suitable HOMO, LUMO and band gap energies are crucial for photoinduced exciton generation in the prepared BHJ structures. The estimated oxidation onset potentials were determined using the absolute energy level of ferrocene (4.8 eV) as reference. 3EhT(DPP)2, TPD(DPP)2, and TT(DPP)2 show HOMO energy levels of 5.21, 5.29, and 5.21 eV determined from the oxidation potentials of 0.77, 0.85, and 0.77 V, respectively. The LUMO energy levels were determined using the relationship LUMO (eV) = HOMO (eV) Egopt (eV) [30], and are 3.62, 3.71 and 3.75 eV for 3EhT(DPP)2, TPD(DPP)2, and TT(DPP)2, respectively. The measured optical and electrochemical properties are tabulated in Table 1. 3.3. TFT performances The effect of the acceptor moiety in the small molecules on their charge transport properties was investigated by fabricating TFTs. The transfer curves of the TFTs are shown in Fig. 4 and the measured data are listed in Table 2. The highest charge carrier mobilities of the thermally annealed films are 5.84 103, 5.85 102, and 5.78 102 cm2 V1 s1 for 3EhT(DPP)2, TPD (DPP)2, and TT(DPP)2, respectively. The TFT made of a thermally annealed 3EhT(DPP)2 film shows a similar charge carrier mobility to that of the TFT prepared with a pristine 3EhT(DPP)2 film. However, TPD(DPP)2 and TT(DPP)2 show superior mobilities after thermal annealing. According to the
GI-XRD patterns (Fig. 5), TPD(DPP)2, which has planar structure shows many scattered patterns even in pristine film indicating possibly high degree of molecular packing. As shown in Fig. 5 and Table 3, 3EhT(DPP)2, TPD(DPP)2, and TT(DPP)2 show increased crystallinity after thermal annealing, as revealed by the fact that the crystallite grain size, as determined using the Scherrer equation [31], increases. In particular, TPD(DPP)2 and TT(DPP)2 revealed larger crystallites than 3EhT(DPP)2, which results from the distorted structure of 3EhT(DPP)2. The full width at half maximum (FWHM) of (100) diffraction peaks became smaller, indicating the improved crystallinity. Both TPD(DPP)2 and TT (DPP)2 show a significant improvement in carrier mobility after thermal annealing. The XRD pattern of 3EhT(DPP)2 indicates that the p–p stacking distance increases upon thermal annealing, although the crystallinity is increased (Table 3). Thus, no significant annealing effect was observed in the performance of the 3EhT(DPP)2-based TFT. The slightly distorted molecular structure of 3EhT(DPP)2 may interrupt charge transport compared to TPD(DPP)2 and TT(DPP)2. The surface morphologies of the thermally annealed active layers in the TFT devices were measured by AFM, as shown in Fig. 6. All three active layers show an increase in surface roughness after thermal annealing. The images obtained of pristine TPD(DPP)2 and TT(DPP)2 films exhibit significant differences to those obtained from the corresponding annealed films, which may indicate an increase in crystallinity upon thermal treatment. In brief, the improved crystallinity and compact surface coverage is thought to contribute to the increase in carrier mobility for TFTs upon thermal annealing. 3.4. OPV performance Conventional BHJ SCs were fabricated with the configuration ITO/PEDOT:PSS/photoactive layer/LiF/Al. Blended chloroform solutions of the small molecules and PC71BM were prepared to form
Table 1 Optical and electrochemical properties of the small molecules. Compound
3EhT(DPP)2 TPD(DPP)2 TT(DPP)2 a b c
lonseta (nm)
Absorption (nm) Solution
Film
629 637 645
657, 707 657, 708 693, 754
Film. Values were obtained from the cyclic voltammograms. HOMO (eV) Egopt (eV).
782 787 847
Egopt
1.59 1.58 1.46
a
(eV)
Eoxb (V)
0.77 0.85 0.77
Energy level (eV) HOMOb
LUMOc
5.21 5.29 5.21
3.62 3.71 3.75
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E.Y. Ko et al. / Synthetic Metals 221 (2016) 39–47
Fig. 4. Transfer curves of (a) 3EhT(DPP)2, (b) TPD(DPP)2, and (c) TT(DPP)2. VDS = 80 V.
Table 2 Performances of the three small molecule-based TFTs.
3EhT(DPP)2 TPD(DPP)2 TT(DPP)2
T ( C)
m (cm2 V1 s1)
Ion/Ioff
Vth (V)
pristine 100 pristine 120 pristine 100
4.28 103 5.84 103 2.26 103 5.85 102 2.51 103 5.78 102
105 105 105 107 104 107
5.68 0.98 4.90 0.78 1.87 1.72
the photoactive layer. The measured J-V characteristics and the external quantum efficiencies (EQEs) of the small molecule-based SCs are displayed in Fig. 7. The device performances were measured at 100 mW cm2 supplied by a solar simulator (Oriel, 1000 W). The measured parameters are listed in Table 4. The addition of DIO enhances Jsc, which contributes to an increase in the PCE of the small molecule-based SCs. The SCs containing 3EhT(DPP)2 and TPD(DPP)2 show similar PCEs, although they exhibit a problematic trade-off between Jsc and Voc. The SC containing 3EhT(DPP)2, having the highest e and a, exhibits a PCE of 3.29% and a higher Jsc of 8.31 mA cm2. Since Voc is proportional to the difference between the HOMO level of the small molecule and the LUMO level of the PC71BM, TPD(DPP)2 exhibits the highest Voc (=0.80 V) of the three molecules owing to its lower lying HOMO level (5.29 eV).
In order to investigate the relationship between the surface morphologies and performances of the SCs, AFM images of the photoactive layers were recorded (Fig. 8). Without addition of DIO, the films show large aggregated domains, grain boundaries, and high roughness, which indicates poor miscibility, and results in the interruption of exciton diffusion and inhibited migration of separated charges in the films. Furthermore, it induces a relatively low Jsc in SC devices derived from pristine blend films. However, the three small molecules display much smaller domain sizes and smoother surfaces by introducing DIO. These finer morphologies are thought to contribute to enhanced charge transport properties,
Table 3 2D GI-XRD peak assignments and corresponding parameters for small molecules before and after thermal annealing. Ta ( C)
3EhT (DPP)2 TPD(DPP)2 TT(DPP)2 a
pristine 100 pristine 120 pristine 100
p p Stacking
Lamellar Spacing qz (Å1) d (Å)
FWHM (Å1) Da (Å) qz (Å1) d (Å)
0.4697 0.4322 0.3946 0.3946 0.4243 0.4183
0.06863 0.05136 0.05363 0.04439 0.07917 0.03636
13.37 14.53 15.91 15.91 14.80 15.01
82.47 110.2 105.5 127.5 71.48 155.6
1.709 1.672 1.691 1.680 1.690 1.692
3.633 3.715 3.672 3.698 3.675 3.670
The grain size was calculated using the Scherrer equation, D = (Kl/bcosu).
Fig. 5. 2D GI-XRD patterns of 3EhT(DPP)2 (a, d), TPD(DPP)2 (b, e), and TT(DPP)2 (c, f). Pristine films (a–c) and films annealed at 100 C (d, f) and 120 C (e).
E.Y. Ko et al. / Synthetic Metals 221 (2016) 39–47
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Fig. 6. AFM height images (5 mm 5 mm) of the small molecule films. Topography images (left) and phase images (right) of 3EhT(DPP)2 (a, d), TPD(DPP)2 (b, e), and TT(DPP)2 (c, f). Pristine films (a–c) and films annealed at 100 C (d, f) and 120 C (e).
Fig. 7. (a) J-V characteristics of BHJ SCs and (b) EQE spectra of the films made from the small molecules and PC71BM.
resulting in higher Jsc values. The performance of the TPD(DPP)2bearing SC exhibits a drastic increase in PCE from 0.67 to 3.37% upon addition of DIO. The addition of DIO may improve the nanophase segregation in the blend matrix, increasing the Jsc value in all the devices. In particular, the lowest-lying HOMO level of TPD
(DPP)2 also contributes to an increase in Voc compared to that of the other two SCs. Therefore, the TPD acceptor unit, which has symmetric and planar molecular structure, is considered to be more effective for increasing the PCE of BHJ SCs with PC71BM. 4. Conclusion
Table 4 Photovoltaic characteristics of SCs made from the small molecules and PC71BM.
3EhT(DPP)2 TPD(DPP)2 TT(DPP)2
DIO
Jsc (mA cm2)
Voc (V)
FF
h (%)
– 1% – 1% – 1%
2.11 8.31 1.60 7.86 1.68 6.48
0.72 0.70 0.80 0.80 0.68 0.70
0.57 0.56 0.52 0.54 0.55 0.59
0.86 3.29 0.67 3.37 0.63 2.69
Average power conversion efficiencies are in the parenthesis.
(0.77) (3.18) (0.62) (3.20) (0.56) (2.61)
Newly synthesized A1-D-A2-D-A1 type conjugated molecules containing a relatively weak A2 accepting unit and an A1 diketopyrrolopyrrole moiety were prepared. By selecting different central weak accepting units, the absorption behavior and molecular energy levels of the small molecules were varied. Conventional SCs were fabricated with each molecule and PC71BM by adding a small amount of DIO. The TPD(DPP)2-based SC exhibited a higher Voc value owing to the position of its lowest-
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Fig. 8. AFM height images (5 mm 5 mm) of photoactive layers. Topography images (left) and phase images (right) of 3EhT(DPP)2 (a, d), TPD(DPP)2 (b, e), and TT(DPP)2 (c, f). Blended films (a–c) and blended films with DIO 1% (d–f).
lying HOMO level. The TPD(DPP)2-bearing SC exhibited a promising power conversion efficiency of 3.37%. Compared to the other two weak acceptors, the TPD unit exhibited improved performance in BHJ SC devices. Acknowledgements This research was supported by Science and Technology (NRF20100020209) and by the National Research Foundation of Korea (NRF2012R1A2A1A01008797), and the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education. Grazing incidence X-ray diffraction (GI-XRD) measurements were performed using Beam line 9A at the Pohang Accelerator Laboratory (PAL, Pohang, Korea). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. synthmet.2016.10.005. References [1] S.-Y. Liu, W.-Q. Liu, C.-X. Yuan, A.-G. Zhong, D. Han, B. Wang, M.N. Shah, M.-M. Shi, H. Chen, Dyes Pigm. 134 (2016) 139–147. [2] H. Zheng, J. Wang, W. Chen, C. Gu, J. Ren, M. Qiu, R. Yang, M. Sun, J. Mater. Chem. C 4 (2016) 6280–6286. [3] Y. Sun, J. Seifter, L. Huo, Y. Yang, B.B.Y. Hsu, H. Zhou, X. Sun, S. Xiao, L. Jiang, A.J. Heeger, Adv. Energy Mater. 5 (2015) 1400987. [4] K. Do, N. Cho, S.A. Siddiqui, S.P. Singh, G.D. Sharma, J. Ko, Dyes Pigm. 120 (2015) 126–135.
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