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Acetylene-bridged D–A–D type small molecule comprising pyrene and diketopyrrolopyrrole for high efficiency organic solar cells
Organic Electronics 14 (2013) 2341–2347
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Acetylene-bridged D–A–D type small molecule comprising pyrene and diketopyrrolopyrrole for high efficiency organic solar cells Jeong-Wook Mun a, Illhun Cho a, Donggu Lee b, Won Sik Yoon a, Oh Kyu Kwon a, Changhee Lee b, Soo Young Park a,⇑ a Creative Research Initiative Center for Supramolecular Optoelectronic Materials and WCU Hybrid Materials Program, Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, Republic of Korea b School of Electrical Engineering and Computer Science, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-747, Republic of Korea
a r t i c l e
i n f o
Article history: Received 4 February 2013 Received in revised form 22 May 2013 Accepted 23 May 2013 Available online 7 June 2013 Keywords: Organic solar cells (OSCs) D–A–D type Acetylene bridge Diketopyrrolopyrrole (DPP) Pyrene
a b s t r a c t To explore effects of acetylene-incorporation, acetylene-bridged D–A–D type small molecules ((HD/OD)-DPP-A-PY) using pyrene as a donor and diketopyrrolopyrrole as an acceptor were successfully synthesized and characterized. (HD/OD)-DPP-A-PY exhibited planar back-bone, conjugation extension, enhanced light absorption, and low HOMO energy level. Combined with the advanced properties, solution-processed OSCs based on a blend of HDDPP-A-PY as a donor and [6,6]-phenyl-C71-butyric-acid-methyl-ester (PC70BM) as an acceptor exhibited PCEs as high as 3.15%. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Developing small molecule organic solar cells (SMOSCs) has emerged as one of the most promising research field due to their advantages such as versatility in molecular design, well defined chemical structure, easy purification, and reproducibility without batch to batch variation [1]. Based on these advantages, the tremendous effort has been devoted to develop appropriate donor materials for SMOSCs [2]. Among them, donor–acceptor– donor (D–A–D) type triad molecule incorporating intra-molecular charge transfer (ICT) characteristics via covalent bonding of electron donor and acceptor has emerged as a promising molecular architecture for high performance SMOSCs [3]. Recently, various kinds of small molecules which incorporate 1,4-diketo-3,6-dithienylpyr-
⇑ Corresponding author. Tel.: +82 2 880 8327; fax: +82 2 886 8331. E-mail address: [email protected] (S.Y. Park). 1566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2013.05.035
rolo[3,4-c]pyrrole (DPP) as an electron acceptor of an ICT system, has been reported. Holmes et al. reported a series of liquid crystalline organic donor molecules consisting of hexa-peri-hexabenzocoronene (FHBC) and DPP unit with 1.59% of power conversion efficiencies (PCEs) [3d], and Nguyen et al. also reported a donor molecule comprising the benzofuran-substituted DPP with 4.4% of PCEs [3a]. Recently, pyrene-DPP-pyrene triad with thienylene bridging to either 1- or 2-position of pyrene was developed by Frechet et al. to show high PCEs: over 4% PCEs was obtained for 2-pyrene triad while only 0.7% PCE was obtained for 1-pyrene triad [4]. However, it should be noted that the 2-pyrene derivatives require much more demanding synthesis than that of 1-pyrene derivatives due to the different reactivity of 1- and 2-positions of pyrene, i.e. 1-position is preferentially activated to the electrophilic aromatic substitution [5]. To further increase the PCEs of pyrene-DPP-pyrene triad in this work, we decided to insert acetylene linkage
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between pyrene and DPP. The acetylene-incorporation was intended to benefit reduced highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) band gap through extended conjugation, and also the increased ionization potential of the triad molecule based on the relatively larger electron-withdrawing characteristics of sp-hybridization over sp2-hybridization [6]. It is well known that the former is effective in increasing short-circuit current (Jsc), while the latter in increasing open-circuit voltage (Voc) [6]. To explore such effects of acetylene insertion, we designed and synthesized synthetically more feasible 1-pyrene triads with and without acetylene linkers (HD/OD)-DPP-A-PY and OD-DPP-PY, respectively. Through comprehensive evaluation of theoretical, photophysical, thermal and electrochemical properties, we could confirm that the acetylene incorporated material shows smaller band gap, stabilized HOMO level, and higher thermal stability. The solution-processed OSCs based on HD-DPP-A-PY and PC70BM blend shows PCE of 3.15%.
2. Results and discussion 2.1. Synthesis (HD/OD)-DPP-A-PY was synthesized according to the pathway shown in Scheme 1, while OD-DPP-PY was synthesized according to the literature procedure [4]. Compound 1 was synthesized through a known procedure [7]. The final product, (HD/OD)-DPP-A-PY, was synthesized through Sonogashira reaction of 1(HD/OD) and 2, while OD-DPP-PY was synthesized according to the reported procedure [4]. Both (HD/OD)-DPP-A-PY and OD-DPP-PY were highly soluble in common organic solvents such as CHCl3, THF and chlorobenzene, which enabled the solution processing of organic solar cell (OSC) fabrication.
2.2. Theoretical calculations Theoretical molecular orbital calculation was carried out using Gaussian 09 at B3LYP/6-31G level to characterize optimized ground state geometry and electron density of HOMO and LUMO states. In optimized ground state geometry, DPP-A-PY showed planar non-twisted conformation due to small torsion angle between pyrene and thiophene moiety than that of DPP-PY (5.7–5.9° for DPP-A-PY and 38.0–50.1° for DPP-PY) (Fig. 1) [8]. The coplanarity facilitates the internal charge transfer between electrondonating pyrene and electron-withdrawing DPP group. Overall conjugation length of DPP-A-PY is relatively increased in comparison with that of DPP-PY (see Fig. S1 in Supplementary material) and thereby the band gap is reduced (see Table 1). As the DPP moiety has a strong tendency for p–p stacking [7], this structural planarity would synergetically reinforce the intermolecular interaction between neighboring molecules to give enhance transport property of DPP-A-PY compared to that of DPP-PY.
2.3. Photophysical, electrochemical, and thermal properties Fig. 2 shows (a) the UV–Vis absorption spectra of HDDPP-A-PY, OD-DPP-A-PY and OD-DPP-PY in CHCl3 solution, and (b) those of spin-coated film from CHCl3 solution; Table 1 lists the measured data. It is clearly noted that acetylene incorporated materials ((HD/OD)-DPP-A-PY) exhibit the red shifted and broadened absorption spectrum compared to that of OD-DPP-PY in solution. Furthermore, HDand OD-DPP-A-PY show higher molar extinction coefficients of 59,400 M 1 cm 1 at 632 nm and 57,300 M 1 cm 1 at 631 nm, respectively (cf. 42,300 M 1 cm 1 at 590 nm for OD-DPP-PY). The film spectra of compounds show redshifted absorption relative to the solution spectra, which is attributed to the structural reorganization and intermolecular stacking in the solid state [9]. The films of (HD/OD)DPP-A-PY show vibrationally resolved absorption spectrum which extends into the near IR region (750 nm) with absorption band maximum at 685 nm. Very distinctive and featured absorption at 685 nm in (HD/OD)-DPPA-PY indicate that incorporation of acetylene p-spacer results in more planar structure and strong intermolecular interactions [9]. In fact, the optical band gap estimated from the absorption edge of (HD/OD)-DPP-A-PY (1.69 eV) in thin film is smaller than that of OD-DPP-PY (1.78 eV). Bathochromically shifted absorption onset and reduced optical band gap of (HD/OD)-DPP-A-PY in the film state are consistent with the DFT calculation (vide supra) and attests the role of acetylene linker in extending the conjugation and planarization. The electrochemical properties of compounds in thin film were investigated using cyclic voltammetry (LUMO energy level was calculated from optical band gap and the electrochemically measured HOMO energy level, see Fig. S2). The HOMO and LUMO energy levels of HD-DPP-A-PY, OD-DPP-A-PY and OD-DPP-PY were 5.72/ 4.03 eV, 5.72/ 4.03 eV, and 5.51/ 3.73 eV, respectively. Consistent with our expectation and also with the DFT calculation results, the introduction of acetylene moiety between pyrene and DPP clearly lowered the HOMO energy level, which is beneficial for high Voc (The data are summarized in Table 1) [6]. The thermal property of as-synthesized compounds was investigated by differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA) under an N2 atmosphere (see Figs. S3 and S4 and Table 1). HD- and OD-DPP-A-PY exhibited better thermal stability with melting temperature of about 178 °C and 180 °C and decomposition temperature (5% weight loss) of about 407 °C and 381 °C compared to those of OD-DPP-PY (124 °C and 360 °C), again indicating the positive role of acetylene incorporation. 2.4. Photovoltaic properties To demonstrate the potential application of a series of materials as a donor in solution-processed OSCs, devices were fabricated with configuration of ITO/PEDOT:PSS (30–40 nm)/(HD/OD)-DPP-A-PY:PC70BM blend (ca. 100 nm)/Al (100 nm). The same devices with OD-DPP-PY:PC70BM as an active layer were also fabricated as a control according to the previously reported procedure [4]. Optimal (HD/OD)-DPP-A-PY:PC70BM devices were
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Scheme 1. Synthetic routes of (HD/OD)-DPP-A-PY and OD-DPP-PY.
Fig. 1. The calculated optimized ground state geometry of (a) DPP-A-PY and (b) DPP-PY. Top view and front view obtained using Gaussian 09 at the B3LYP/ 6-31G level.
obtained when using CHCl3 solutions at a concentration of 20 mg/mL with (HD/OD)-DPP-A-PY:PC70BM blending ratio of 7:3 by weight, and annealed after cathode deposition at 100 °C for 5 min. Fig. 3 shows the current density versus voltage (J–V) and incident photon conversion efficiency (IPCE) characteristics. The control device comprising ODDPP-PY/PC70BM (1:4 w/w) blend showed Voc of 0.79 V, Jsc
of 2.38 mA cm 2, and fill factor (FF) of 27.2%, resulting in the PCE of 0.51%. This control device result is consistent with the earlier report [4]. On the other hand, the devices of OD-DPP-A-PY/PC70BM (7:3 w/w) blend showed Voc of 0.87 V, Jsc of 6.20 mA cm 2, and FF of 41.9%, resulting in the PCE of 2.25%. As the optimized molecular structure for the highest performance, the HD-DPP-A-PY:PC70BM
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Table 1 Optical, electrochemical and thermal properties of (HD/OD)-DPP-A-PY and OD-DPP-PY.
a b c d e f g
Compound
kabs,sol (nm)a
kabs,
HD-DPP-A-PY OD-DPP-A-PY OD-DPP-PY
352, 423, 595, 632 352, 423, 597, 630 338, 397, 590
364, 428, 618, 682 364, 428, 614, 682 343, 410, 590
film
(nm)b
konsets,
film
(nm)a
Bgc (eV)a
733 732 695
1.69 1.69 1.78
EHOMOd (eV)
ELOMOe (eV)
5.72 5.72 5.51
4.03 4.03 3.73
Tmf (°C)
Tdg (°C)
178 180 124
408 381 360
Measured from chloroform solution (concentration of 10 5 M). Spin-coated from 0.5 wt% chloroform solution. Optical band gap was obtained from film absorption edge. Measured using drop-casted film from 3 wt% chloroform solution. Measured from optical band-gap and HOMO energy level. Measured from DSC in N2 (rate 5 °C/min). 5% Weight loss temperature – measured from TGA in N2 (rate 5 °C/min).
Fig. 2. UV–Vis absorption spectra of compound (HD/OD)-DPP-A-PY and OD-DPP-PY in (a) CHCl3 solution and (b) spin coated film from CHCl3 solution.
(7:3 w/w) blend showed Voc of 0.85 V, Jsc of 8.89 mA cm 2, and FF of 41.7%, resulting in the PCE of 3.15%. This device exhibited a broad IPCE plateau between 300 nm and 750 nm with a maximum of 46% at 577 nm (The data are summarized in Table 2). It is first noted that the Voc for (HD/OD)-DPP-A-PY is distinctively higher than that of OD-DPP-PY by 0.1 eV. This increased Voc is consistent with the lowered HOMO energy level of acetylene incorporated OD-DPP-A-PY (5.72 eV) compared to that of OD-DPPPY (5.51 eV). Most evidently, the Jsc for (HD/OD)-DPP-A-PY (8.89/6.20 mA cm 2) have significantly increased from that of OD-DPP-PY (2.38 mA cm 2); the current density for
Fig. 3. Characteristic (a) J–V curves and (b) IPCE spectrum of solar cells fabricated from (HD/OD)-DPP-A-PY and OD-DPP-PY under simulated AM 1.5 solar irradiation of 100 mW cm 2.
Table 2 Photovoltaic properties of (HD/OD)-DPP-A-PY and OD-DPP-PY blended with PC70BM.
a b c d
Compound (blend ratio)
Jsca
Vocb
FFc
PCEsd
Max. PCEsd
OD-DPP-A-PY (7:3) HD-DPP-A-PY (7:3) OD-DPP-PY (1:4)
6.20 8.89 2.38
0.87 0.85 0.79
41.9 41.7 27.2
1.96 2.95 0.46
2.25 3.15 0.51
Short circuit current (mA cm 2). Open circuit voltage (V). Fill factor. Power conversion efficiency (%).
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Fig. 4. Tapping mode AFM height images (5 lm 5 lm) of OD-DPP-PY/PC70BM blend film (a), OD-DPP-A-PY/PC70BM blend film (b), and HD-DPP-A-PY/ PC70BM blend film (c) from CHCl3 (after thermal treatment).
(HD/OD)-DPP-A-PY is about 3-/2-times higher than that of OD-DPP-PY. This enhanced property is unambiguously attributed to acetylene-induced planar back-bone facilitating the intermolecular interaction and enhanced absorption at longer wavelength. 2.5. Film morphology To investigate the acetylene-incorporation effect on the supramolecular structure in bulk heterojunction type active layer, we examined film surface topography using tapping mode atomic force microscope (AFM). The OD-DPPPY/PC70BM blend film (a) exhibits relatively homogeneous and flat surface (Fig. 4a) with a root-mean-squared (RMS) roughness of 0.5 nm. On the other hands, (HD/OD)-DPPA-PY/PC70BM blend films show clearly phase-separated, highly entangled, and ordered networks (Fig. 4b and c) with a RMS roughness of 0.8 nm for OD-DPP-A-PY/PC70BM blend film (b) and of 1 nm for HD-DPP-A-PY/PC70BM blend film (c), respectively. This is most likely originating from the enhanced intermolecular interaction between acetylene incorporated molecules as was alternatively evidenced by the DFT calculation and also by their superior optical and thermal properties. Through this better organized bulk heterojunction network structures which are the key requirement of efficient charge separation and transport, highly improved short circuit current and fill factor can clearly be explained.
3. Conclusion To explore the effects of acetylene-incorporation, acetylene-bridged D–A–D type small molecules ((HD/OD)-DPPA-PY) using pyrene as a donor and diketopyrrolopyrrole as an acceptor were synthesized and their photophysical, electrochemical, thermal, and photovoltaic device properties were investigated. Consistent with the expectation and DFT calculation result (HD/OD)-DPP-A-PY exhibited planar back-bone, conjugation extension, enhanced light absorption, and low HOMO energy level. Combined with the advanced properties, solution-processed OSCs based on HD-DPP-A-PY exhibited PCEs of 3.15%. All together, we clearly demonstrated beneficial effect of acetylene bridging in D–A–D type SMOSC.
4. Experimental section 4.1. Synthesis 4.1.1. Synthesis of 1(OD) Compound 1(OD) was synthesized through a literature procedure (Scheme S1) [7]. 1H NMR (300 MHz, CDCl3, d): 8.62 (d, J = 4.17, 2H), 7.22 (d, J = 4.17, 2H), 3.93 (d, J = 7.74, 4H), 1.88 (m, 2H), 1.22 (m, 64H), 0.89 (m, 12H). 13 C NMR (500 MHz, CDCl3, d): 161.38, 139.39, 135.28, 131.41, 131.18, 118.93, 108.02, 46.35, 37.75, 31.92, 31.88, 31.18, 29.97, 29.63, 29.55, 29.48, 29.35, 29.28, 26.18, 22.66, 14.10. High-resolution mass spectrometry (HRMS) mass-to-charge ratio (m/z): [M]+ calculated for C54H86Br2N2O2S2, 1016.4497; found 1016.4513. 4.1.2. Synthesis of 1(HD) Compound 1(HD) was synthesized through a literature procedure (Scheme S1) [7]. 1H NMR (300 MHz, CDCl3, d): 8.62 (d, J = 4.23, 2H), 7.22 (d, J = 4.20, 2H), 3.93 (d, J = 7.71, 4H), 1.87 (m, 2H), 1.29 (m, 48H), 0.88 (m, 12H). 13 C NMR (500 MHz, CDCl3, d): 161.39, 139.39, 135.27, 131.41, 131.17, 118.93, 108.03, 46.33, 37.75, 31.86, 31.74, 31.17, 29.96, 29.62, 29.48, 29.28, 26.17, 26.14, 22.66, 22.61, 14.09, 14.07. High-resolution mass spectrometry (HRMS) mass-to-charge ratio (m/z):[M]+ calculated for C46H70Br2N2O2S2, 904.3245; found 904.3233. 4.1.3. Synthesis of 2 The mixture of 1-bromopyrene (1.10 g, 3.91 mmol), PdCl2(PPh3)2 (0.55 g, 0.78 mmol), and CuI (14.9 g, 0.78 mmol) in diisopropylamine(30 mL) and toluene (15 mL) were stirred at 0 °C under Ar atmosphere for 30 min. Then trimethylsilylacetylene (1.65 mL, 11.73 mmol) was added to the mixture and stirred at 80 °C for 24 h. After evaporation of the solvent, the residue was purified by column chromatography on a silica gel (n-Hex) to afford a yellow solid (1.0 g, 82%). 1H NMR (300 MHz, CDCl3, d): 8.58 (d, J = 9.12, 1H), 8.23 (m, 8H), 0.39 (s, 9H). 13C NMR (500 MHz, CDCl3, d): 132.25,131.37, 131.20, 131.03, 129.91, 128.38, 128.20, 127.21, 126.18, 125.63, 125.54, 124.35, 124.25, 117.62, 104.10, 100.21, 29.70. High-resolution mass spectrometry (HRMS) mass-to-charge ratio (m/z): [M]+ calculated for C21H18Si, 298.1178; found 298.1182.
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4.1.4. Synthesis of OD-DPP-A-PY 1(OD) (250 mg, 0.24 mmol), CuI (3.5 mg, 0.018 mmol), and Pd(PPh3)4 (21.3 mg, 0.018 mmol) in diisopropylamine (2.3 mL) and toluene (5.1 mL) were stirred under Ar atmosphere at 0 °C for 30 min. Compound 2 (183 mg, 0.61 mmol) and tetrabutylammonium fluoride (160 mg, 0.61 mmol) then added, and the mixture was heated to 60 °C for 24 h. The mixture was poured into water and extracted with CHCl3. The organic layer was then dried over anhydrous MgSO4. After evaporation of the solvent, the residue was purified by column chromatography on a silica gel (CHCl3/n-Hex = 2: 8 ? CHCl3) and recrystallization using EA to afford a dark violet solid (210 mg, 65%). 1H NMR (300 MHz, CDCl3, d): 9.00 (d, J = 4.14, 2H), 8.62 (d, J = 9.39, 2H) 8.27 (m, 16 H), 7.56 (d, J = 4.11, 2H), 4.11 (d, J = 7.44, 4H), 2.03 (m, 2H), 1.39 (m, 64H), 0.82 (m, 12H). 13 C NMR (500 MHz, CDCl3, d): 161.46, 139.37, 135.70, 132.90, 131.88, 131.78, 131.20, 130.99, 130.72, 129.47, 128.84, 128.70, 128.58, 127.15, 126.33, 125.89, 125.85, 125.18, 124.53, 124.40, 124.19, 116.62, 108.90, 97.47, 88.09, 46.50, 37.90, 31.90, 31.34, 30.11, 29.72, 29.66, 29.58, 29.36, 26.34, 22.66, 14.07. High-resolution mass spectrometry (HRMS) mass-to-charge ratio (m/z):[M + H]+ calculated for C90H105N2O2S2, 1309.7617 ; found 1309.7634. Elemental Analysis calculated for C90H104N2O2S2: C 82.52, H 8.00, N 2.14, O 2.44, S 4.90; found: C 82.45, H 8.06, N 2.15, O 2.41, S 5.05. 4.1.5. Synthesis of HD-DPP-A-PY The reaction conditions and workup were same as for DPP-A-PY(OD), except 1(HD) (250 mg, 0.24 mmol) was used. After the reaction, the residue was purified by column chromatography on a silica gel (CHCl3/n-Hex = 2:8 ? CHCl3) and recrystallization using EA to afford a dark violet solid (230 mg, 69%). 1H NMR (300 MHz, CDCl3, d): 8.99 (d, J = 3.81, 2 H), 8.61 (d, J = 9.06, 2H) 8.27 (m, 16H), 7.56 (d, J = 4.14, 2H), 4.11 (d, J = 7.50, 4H), 2.04 (m, 2H), 1.39 (m, 48H), 0.84 (m, 12H). 13C NMR (500 MHz, CDCl3, d): 161.46, 139.38, 135.69, 132.90, 131.87, 131.78, 131.19, 130.99, 130.71, 129.47, 128.84, 128.70, 128.58, 127.15, 126.33, 125.89, 125.85, 125.18, 124.53, 124.39, 124.18, 116.61, 108.90, 97.46, 88.09, 46.49, 37.89, 31.90, 31.82, 31.32, 30.09, 29.75, 29.57, 29.34, 26.32, 26.28, 22.67, 14.08. High-resolution mass spectrometry (HRMS) mass-to-charge ratio (m/z): [M + H]+ calculated for C82H89N2O2S2, 1197.6365; found 1197.6344. Elemental Analysis calculated for C82H88N2O2S2: C 82.23, H 7.41, N 2.34, O 2.67, S 5.35; found: C 82.27, H 7.46, N 2.35, O 2.60, S 5.39. 4.1.6. Synthesis of OD-DPP-PY 1(OD) (200 mg, 0.20 mmol), pyren-1-yl-boronic acid (106 mg, 0.43 mmol), Pd(PPh3)4 (23.7 mg, 0.01 mmol), THF (20 mL), and 2 N K2CO3 (10 mL) were added to a two-necked round bottom flask under Ar atmosphere. The solution was stirred at 75 °C for 24 h. The mixture was poured into water and extracted with CHCl3. The organic layer was then dried over anhydrous MgSO4. After evaporation of the solvent, the residue was purified by column chromatography on a silica gel (CHCl3/n-Hex = 2:8 ? CHCl3) and being precipitated into MeOH to afford
a dark violet solid (136 mg, 55%). 1H NMR (300 MHz, CDCl3, d): 9.16 (d, J = 3.84 Hz, 2H), 8.58 (d, J = 9.36 Hz, 2H) 8.26 (m, 16H), 7.60 (d, J = 3.78, 2H), 4.17 (d, J = 6.45, 4H), 2.13 (m, 2H), 1.40 (m, 64H), 0.87 (m, 12H). 13C NMR (500 MHz, CDCl3, d): 161.91, 148.43, 140.12, 136.26, 131.70, 131.47, 130.91, 130.46, 129.38, 129.04, 128.57, 128.36, 128.24, 128.16, 127.28, 126.37, 125.77, 125.43, 125.16, 124.71, 124.50, 108.2, 46.52, 38.09, 31.89, 31.42, 30.13, 29.68, 29.64, 29.58, 29.33, 26.43, 22.64, 14.07. High-resolution mass spectrometry (HRMS) mass-to-charge ratio (m/z): [M + H]+ calculated for C86H105N2O2S2, 1261.7617; found 1261.7620. Elemental Analysis calculated for C86H104N2O2S2: C 81.86, H 8.31, N 2.22, O 2.54, S 5.08; found: C 81.56, H 8.33, N 2.18, O 2.88, S 4.99. 4.2. Characterization Chemical structures were fully identified by 1H NMR (Bruker, Avance-300), 13C NMR (Bruker, Avance-500), GCMass (JEOL, JMS-700), and elemental analysis (EA1110, CE Instrument). The thermal properties of the compounds were obtained using DSC and TGA under an N2 atmosphere, with a DSC-Q1000 model and a TA instruments Q50 model, respectively. UV–Vis spectra were recorded on a SHIMADZU UV-1650PC. HOMO level was obtained from the cyclic voltammetry measurements. Cyclic voltammetric measurements were performed using a 273A (Princeton Applied Research) with a one-compartment electrolysis cell consisting of a platinum working electrode, a platinum wire counter-electrode, and a quasi Ag+/Ag electrode as reference. Measurements were performed in a 0.5 mM acetonitrile solution with tetrabutylammonium tetrafluoroborate as the supporting electrolyte, at a scan rate of 50 mV/s. Each oxidation potential was calibrated using ferrocene as a reference. LUMO level was calculated from the HOMO level and the optical band gap, which was obtained from the edge of the absorption spectra. The surface morphologies were measured using atomic force microscopy in the tapping mode (AFM, Multimode with a Nano Scope V Controller, Bruker). 4.3. Fabrication and characterization of organic solar cells (OSCs) The organic solar cells in this study were fabricated by following method. Patterned ITO glass substrates (10 X/ square) were cleaned in an ultrasonic bath with trichloroethylene, acetone, and isopropyl alcohol for 10 min, respectively and then blow dried with a N2 stream. A 30– 40 nm of PEDOT:PSS (Batron P VP AI 4083) was then spin-coated onto the substrate (5000 rpm/30 s). The films were dried at 150 °C for 20 min. Subsequently, the (HD/ OD)-DPP-A-PY:PC70BM (>99.0%, ADS) solutions were deposited through spin casting at 1000 rpm/40 s. The thickness of (HD/OD)-DPP-A-PY:PC70BM films were around 130 nm. Al electrodes were deposited via thermal evaporation with thickness of 100 nm. The active area of these solar cells was 0.09 cm2. The current density–voltage (J–V) characteristics of the solar cells were measured with a Keithley 4200 source measurement unit. The solar cell performances were
J.-W. Mun et al. / Organic Electronics 14 (2013) 2341–2347
characterized under AM1.5G condition with an illumination intensity of 100 mW cm 2 generated by an Oriel Sol 3A solar simulator. J–V characteristics of the cells with illumination were measured using a metal mask of 0.09 cm2. The intensity dependent measurements have been performed with various neutral density filters. Incident photon to current conversion efficiency (IPCE) was measured using Oriel QE/IPCE Measurement Kit which composed of 300W Xenon Lamp, monochromator (74125), the order sorting filter wheel, the Merlin lock-in amplifier (70104) and the chopper.
[3]
Acknowledgements This research was supported by Basic Science Research Program (CRI; RIAMI-AM0209(0417-20090011)) and WCU (World Class University) Project (R31-2008-000-10075-0) through National Research Foundation of Korea funded by the Ministry of Education, Science and Technology.
[4]
[5]
[6]
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.2013.05.035. [7]
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Acetylene-bridged D–A–D type small molecule comprising pyrene and diketopyrrolopyrrole for high efficiency organic solar cells Jeong-Wook Mun a, Illhun Cho a, Donggu Lee b, Won Sik Yoon a, Oh Kyu Kwon a, Changhee Lee b, Soo Young Park a,⇑ a Creative Research Initiative Center for Supramolecular Optoelectronic Materials and WCU Hybrid Materials Program, Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, Republic of Korea b School of Electrical Engineering and Computer Science, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-747, Republic of Korea
a r t i c l e
i n f o
Article history: Received 4 February 2013 Received in revised form 22 May 2013 Accepted 23 May 2013 Available online 7 June 2013 Keywords: Organic solar cells (OSCs) D–A–D type Acetylene bridge Diketopyrrolopyrrole (DPP) Pyrene
a b s t r a c t To explore effects of acetylene-incorporation, acetylene-bridged D–A–D type small molecules ((HD/OD)-DPP-A-PY) using pyrene as a donor and diketopyrrolopyrrole as an acceptor were successfully synthesized and characterized. (HD/OD)-DPP-A-PY exhibited planar back-bone, conjugation extension, enhanced light absorption, and low HOMO energy level. Combined with the advanced properties, solution-processed OSCs based on a blend of HDDPP-A-PY as a donor and [6,6]-phenyl-C71-butyric-acid-methyl-ester (PC70BM) as an acceptor exhibited PCEs as high as 3.15%. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Developing small molecule organic solar cells (SMOSCs) has emerged as one of the most promising research field due to their advantages such as versatility in molecular design, well defined chemical structure, easy purification, and reproducibility without batch to batch variation [1]. Based on these advantages, the tremendous effort has been devoted to develop appropriate donor materials for SMOSCs [2]. Among them, donor–acceptor– donor (D–A–D) type triad molecule incorporating intra-molecular charge transfer (ICT) characteristics via covalent bonding of electron donor and acceptor has emerged as a promising molecular architecture for high performance SMOSCs [3]. Recently, various kinds of small molecules which incorporate 1,4-diketo-3,6-dithienylpyr-
⇑ Corresponding author. Tel.: +82 2 880 8327; fax: +82 2 886 8331. E-mail address: [email protected] (S.Y. Park). 1566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2013.05.035
rolo[3,4-c]pyrrole (DPP) as an electron acceptor of an ICT system, has been reported. Holmes et al. reported a series of liquid crystalline organic donor molecules consisting of hexa-peri-hexabenzocoronene (FHBC) and DPP unit with 1.59% of power conversion efficiencies (PCEs) [3d], and Nguyen et al. also reported a donor molecule comprising the benzofuran-substituted DPP with 4.4% of PCEs [3a]. Recently, pyrene-DPP-pyrene triad with thienylene bridging to either 1- or 2-position of pyrene was developed by Frechet et al. to show high PCEs: over 4% PCEs was obtained for 2-pyrene triad while only 0.7% PCE was obtained for 1-pyrene triad [4]. However, it should be noted that the 2-pyrene derivatives require much more demanding synthesis than that of 1-pyrene derivatives due to the different reactivity of 1- and 2-positions of pyrene, i.e. 1-position is preferentially activated to the electrophilic aromatic substitution [5]. To further increase the PCEs of pyrene-DPP-pyrene triad in this work, we decided to insert acetylene linkage
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between pyrene and DPP. The acetylene-incorporation was intended to benefit reduced highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) band gap through extended conjugation, and also the increased ionization potential of the triad molecule based on the relatively larger electron-withdrawing characteristics of sp-hybridization over sp2-hybridization [6]. It is well known that the former is effective in increasing short-circuit current (Jsc), while the latter in increasing open-circuit voltage (Voc) [6]. To explore such effects of acetylene insertion, we designed and synthesized synthetically more feasible 1-pyrene triads with and without acetylene linkers (HD/OD)-DPP-A-PY and OD-DPP-PY, respectively. Through comprehensive evaluation of theoretical, photophysical, thermal and electrochemical properties, we could confirm that the acetylene incorporated material shows smaller band gap, stabilized HOMO level, and higher thermal stability. The solution-processed OSCs based on HD-DPP-A-PY and PC70BM blend shows PCE of 3.15%.
2. Results and discussion 2.1. Synthesis (HD/OD)-DPP-A-PY was synthesized according to the pathway shown in Scheme 1, while OD-DPP-PY was synthesized according to the literature procedure [4]. Compound 1 was synthesized through a known procedure [7]. The final product, (HD/OD)-DPP-A-PY, was synthesized through Sonogashira reaction of 1(HD/OD) and 2, while OD-DPP-PY was synthesized according to the reported procedure [4]. Both (HD/OD)-DPP-A-PY and OD-DPP-PY were highly soluble in common organic solvents such as CHCl3, THF and chlorobenzene, which enabled the solution processing of organic solar cell (OSC) fabrication.
2.2. Theoretical calculations Theoretical molecular orbital calculation was carried out using Gaussian 09 at B3LYP/6-31G level to characterize optimized ground state geometry and electron density of HOMO and LUMO states. In optimized ground state geometry, DPP-A-PY showed planar non-twisted conformation due to small torsion angle between pyrene and thiophene moiety than that of DPP-PY (5.7–5.9° for DPP-A-PY and 38.0–50.1° for DPP-PY) (Fig. 1) [8]. The coplanarity facilitates the internal charge transfer between electrondonating pyrene and electron-withdrawing DPP group. Overall conjugation length of DPP-A-PY is relatively increased in comparison with that of DPP-PY (see Fig. S1 in Supplementary material) and thereby the band gap is reduced (see Table 1). As the DPP moiety has a strong tendency for p–p stacking [7], this structural planarity would synergetically reinforce the intermolecular interaction between neighboring molecules to give enhance transport property of DPP-A-PY compared to that of DPP-PY.
2.3. Photophysical, electrochemical, and thermal properties Fig. 2 shows (a) the UV–Vis absorption spectra of HDDPP-A-PY, OD-DPP-A-PY and OD-DPP-PY in CHCl3 solution, and (b) those of spin-coated film from CHCl3 solution; Table 1 lists the measured data. It is clearly noted that acetylene incorporated materials ((HD/OD)-DPP-A-PY) exhibit the red shifted and broadened absorption spectrum compared to that of OD-DPP-PY in solution. Furthermore, HDand OD-DPP-A-PY show higher molar extinction coefficients of 59,400 M 1 cm 1 at 632 nm and 57,300 M 1 cm 1 at 631 nm, respectively (cf. 42,300 M 1 cm 1 at 590 nm for OD-DPP-PY). The film spectra of compounds show redshifted absorption relative to the solution spectra, which is attributed to the structural reorganization and intermolecular stacking in the solid state [9]. The films of (HD/OD)DPP-A-PY show vibrationally resolved absorption spectrum which extends into the near IR region (750 nm) with absorption band maximum at 685 nm. Very distinctive and featured absorption at 685 nm in (HD/OD)-DPPA-PY indicate that incorporation of acetylene p-spacer results in more planar structure and strong intermolecular interactions [9]. In fact, the optical band gap estimated from the absorption edge of (HD/OD)-DPP-A-PY (1.69 eV) in thin film is smaller than that of OD-DPP-PY (1.78 eV). Bathochromically shifted absorption onset and reduced optical band gap of (HD/OD)-DPP-A-PY in the film state are consistent with the DFT calculation (vide supra) and attests the role of acetylene linker in extending the conjugation and planarization. The electrochemical properties of compounds in thin film were investigated using cyclic voltammetry (LUMO energy level was calculated from optical band gap and the electrochemically measured HOMO energy level, see Fig. S2). The HOMO and LUMO energy levels of HD-DPP-A-PY, OD-DPP-A-PY and OD-DPP-PY were 5.72/ 4.03 eV, 5.72/ 4.03 eV, and 5.51/ 3.73 eV, respectively. Consistent with our expectation and also with the DFT calculation results, the introduction of acetylene moiety between pyrene and DPP clearly lowered the HOMO energy level, which is beneficial for high Voc (The data are summarized in Table 1) [6]. The thermal property of as-synthesized compounds was investigated by differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA) under an N2 atmosphere (see Figs. S3 and S4 and Table 1). HD- and OD-DPP-A-PY exhibited better thermal stability with melting temperature of about 178 °C and 180 °C and decomposition temperature (5% weight loss) of about 407 °C and 381 °C compared to those of OD-DPP-PY (124 °C and 360 °C), again indicating the positive role of acetylene incorporation. 2.4. Photovoltaic properties To demonstrate the potential application of a series of materials as a donor in solution-processed OSCs, devices were fabricated with configuration of ITO/PEDOT:PSS (30–40 nm)/(HD/OD)-DPP-A-PY:PC70BM blend (ca. 100 nm)/Al (100 nm). The same devices with OD-DPP-PY:PC70BM as an active layer were also fabricated as a control according to the previously reported procedure [4]. Optimal (HD/OD)-DPP-A-PY:PC70BM devices were
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Scheme 1. Synthetic routes of (HD/OD)-DPP-A-PY and OD-DPP-PY.
Fig. 1. The calculated optimized ground state geometry of (a) DPP-A-PY and (b) DPP-PY. Top view and front view obtained using Gaussian 09 at the B3LYP/ 6-31G level.
obtained when using CHCl3 solutions at a concentration of 20 mg/mL with (HD/OD)-DPP-A-PY:PC70BM blending ratio of 7:3 by weight, and annealed after cathode deposition at 100 °C for 5 min. Fig. 3 shows the current density versus voltage (J–V) and incident photon conversion efficiency (IPCE) characteristics. The control device comprising ODDPP-PY/PC70BM (1:4 w/w) blend showed Voc of 0.79 V, Jsc
of 2.38 mA cm 2, and fill factor (FF) of 27.2%, resulting in the PCE of 0.51%. This control device result is consistent with the earlier report [4]. On the other hand, the devices of OD-DPP-A-PY/PC70BM (7:3 w/w) blend showed Voc of 0.87 V, Jsc of 6.20 mA cm 2, and FF of 41.9%, resulting in the PCE of 2.25%. As the optimized molecular structure for the highest performance, the HD-DPP-A-PY:PC70BM
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Table 1 Optical, electrochemical and thermal properties of (HD/OD)-DPP-A-PY and OD-DPP-PY.
a b c d e f g
Compound
kabs,sol (nm)a
kabs,
HD-DPP-A-PY OD-DPP-A-PY OD-DPP-PY
352, 423, 595, 632 352, 423, 597, 630 338, 397, 590
364, 428, 618, 682 364, 428, 614, 682 343, 410, 590
film
(nm)b
konsets,
film
(nm)a
Bgc (eV)a
733 732 695
1.69 1.69 1.78
EHOMOd (eV)
ELOMOe (eV)
5.72 5.72 5.51
4.03 4.03 3.73
Tmf (°C)
Tdg (°C)
178 180 124
408 381 360
Measured from chloroform solution (concentration of 10 5 M). Spin-coated from 0.5 wt% chloroform solution. Optical band gap was obtained from film absorption edge. Measured using drop-casted film from 3 wt% chloroform solution. Measured from optical band-gap and HOMO energy level. Measured from DSC in N2 (rate 5 °C/min). 5% Weight loss temperature – measured from TGA in N2 (rate 5 °C/min).
Fig. 2. UV–Vis absorption spectra of compound (HD/OD)-DPP-A-PY and OD-DPP-PY in (a) CHCl3 solution and (b) spin coated film from CHCl3 solution.
(7:3 w/w) blend showed Voc of 0.85 V, Jsc of 8.89 mA cm 2, and FF of 41.7%, resulting in the PCE of 3.15%. This device exhibited a broad IPCE plateau between 300 nm and 750 nm with a maximum of 46% at 577 nm (The data are summarized in Table 2). It is first noted that the Voc for (HD/OD)-DPP-A-PY is distinctively higher than that of OD-DPP-PY by 0.1 eV. This increased Voc is consistent with the lowered HOMO energy level of acetylene incorporated OD-DPP-A-PY (5.72 eV) compared to that of OD-DPPPY (5.51 eV). Most evidently, the Jsc for (HD/OD)-DPP-A-PY (8.89/6.20 mA cm 2) have significantly increased from that of OD-DPP-PY (2.38 mA cm 2); the current density for
Fig. 3. Characteristic (a) J–V curves and (b) IPCE spectrum of solar cells fabricated from (HD/OD)-DPP-A-PY and OD-DPP-PY under simulated AM 1.5 solar irradiation of 100 mW cm 2.
Table 2 Photovoltaic properties of (HD/OD)-DPP-A-PY and OD-DPP-PY blended with PC70BM.
a b c d
Compound (blend ratio)
Jsca
Vocb
FFc
PCEsd
Max. PCEsd
OD-DPP-A-PY (7:3) HD-DPP-A-PY (7:3) OD-DPP-PY (1:4)
6.20 8.89 2.38
0.87 0.85 0.79
41.9 41.7 27.2
1.96 2.95 0.46
2.25 3.15 0.51
Short circuit current (mA cm 2). Open circuit voltage (V). Fill factor. Power conversion efficiency (%).
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Fig. 4. Tapping mode AFM height images (5 lm 5 lm) of OD-DPP-PY/PC70BM blend film (a), OD-DPP-A-PY/PC70BM blend film (b), and HD-DPP-A-PY/ PC70BM blend film (c) from CHCl3 (after thermal treatment).
(HD/OD)-DPP-A-PY is about 3-/2-times higher than that of OD-DPP-PY. This enhanced property is unambiguously attributed to acetylene-induced planar back-bone facilitating the intermolecular interaction and enhanced absorption at longer wavelength. 2.5. Film morphology To investigate the acetylene-incorporation effect on the supramolecular structure in bulk heterojunction type active layer, we examined film surface topography using tapping mode atomic force microscope (AFM). The OD-DPPPY/PC70BM blend film (a) exhibits relatively homogeneous and flat surface (Fig. 4a) with a root-mean-squared (RMS) roughness of 0.5 nm. On the other hands, (HD/OD)-DPPA-PY/PC70BM blend films show clearly phase-separated, highly entangled, and ordered networks (Fig. 4b and c) with a RMS roughness of 0.8 nm for OD-DPP-A-PY/PC70BM blend film (b) and of 1 nm for HD-DPP-A-PY/PC70BM blend film (c), respectively. This is most likely originating from the enhanced intermolecular interaction between acetylene incorporated molecules as was alternatively evidenced by the DFT calculation and also by their superior optical and thermal properties. Through this better organized bulk heterojunction network structures which are the key requirement of efficient charge separation and transport, highly improved short circuit current and fill factor can clearly be explained.
3. Conclusion To explore the effects of acetylene-incorporation, acetylene-bridged D–A–D type small molecules ((HD/OD)-DPPA-PY) using pyrene as a donor and diketopyrrolopyrrole as an acceptor were synthesized and their photophysical, electrochemical, thermal, and photovoltaic device properties were investigated. Consistent with the expectation and DFT calculation result (HD/OD)-DPP-A-PY exhibited planar back-bone, conjugation extension, enhanced light absorption, and low HOMO energy level. Combined with the advanced properties, solution-processed OSCs based on HD-DPP-A-PY exhibited PCEs of 3.15%. All together, we clearly demonstrated beneficial effect of acetylene bridging in D–A–D type SMOSC.
4. Experimental section 4.1. Synthesis 4.1.1. Synthesis of 1(OD) Compound 1(OD) was synthesized through a literature procedure (Scheme S1) [7]. 1H NMR (300 MHz, CDCl3, d): 8.62 (d, J = 4.17, 2H), 7.22 (d, J = 4.17, 2H), 3.93 (d, J = 7.74, 4H), 1.88 (m, 2H), 1.22 (m, 64H), 0.89 (m, 12H). 13 C NMR (500 MHz, CDCl3, d): 161.38, 139.39, 135.28, 131.41, 131.18, 118.93, 108.02, 46.35, 37.75, 31.92, 31.88, 31.18, 29.97, 29.63, 29.55, 29.48, 29.35, 29.28, 26.18, 22.66, 14.10. High-resolution mass spectrometry (HRMS) mass-to-charge ratio (m/z): [M]+ calculated for C54H86Br2N2O2S2, 1016.4497; found 1016.4513. 4.1.2. Synthesis of 1(HD) Compound 1(HD) was synthesized through a literature procedure (Scheme S1) [7]. 1H NMR (300 MHz, CDCl3, d): 8.62 (d, J = 4.23, 2H), 7.22 (d, J = 4.20, 2H), 3.93 (d, J = 7.71, 4H), 1.87 (m, 2H), 1.29 (m, 48H), 0.88 (m, 12H). 13 C NMR (500 MHz, CDCl3, d): 161.39, 139.39, 135.27, 131.41, 131.17, 118.93, 108.03, 46.33, 37.75, 31.86, 31.74, 31.17, 29.96, 29.62, 29.48, 29.28, 26.17, 26.14, 22.66, 22.61, 14.09, 14.07. High-resolution mass spectrometry (HRMS) mass-to-charge ratio (m/z):[M]+ calculated for C46H70Br2N2O2S2, 904.3245; found 904.3233. 4.1.3. Synthesis of 2 The mixture of 1-bromopyrene (1.10 g, 3.91 mmol), PdCl2(PPh3)2 (0.55 g, 0.78 mmol), and CuI (14.9 g, 0.78 mmol) in diisopropylamine(30 mL) and toluene (15 mL) were stirred at 0 °C under Ar atmosphere for 30 min. Then trimethylsilylacetylene (1.65 mL, 11.73 mmol) was added to the mixture and stirred at 80 °C for 24 h. After evaporation of the solvent, the residue was purified by column chromatography on a silica gel (n-Hex) to afford a yellow solid (1.0 g, 82%). 1H NMR (300 MHz, CDCl3, d): 8.58 (d, J = 9.12, 1H), 8.23 (m, 8H), 0.39 (s, 9H). 13C NMR (500 MHz, CDCl3, d): 132.25,131.37, 131.20, 131.03, 129.91, 128.38, 128.20, 127.21, 126.18, 125.63, 125.54, 124.35, 124.25, 117.62, 104.10, 100.21, 29.70. High-resolution mass spectrometry (HRMS) mass-to-charge ratio (m/z): [M]+ calculated for C21H18Si, 298.1178; found 298.1182.
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4.1.4. Synthesis of OD-DPP-A-PY 1(OD) (250 mg, 0.24 mmol), CuI (3.5 mg, 0.018 mmol), and Pd(PPh3)4 (21.3 mg, 0.018 mmol) in diisopropylamine (2.3 mL) and toluene (5.1 mL) were stirred under Ar atmosphere at 0 °C for 30 min. Compound 2 (183 mg, 0.61 mmol) and tetrabutylammonium fluoride (160 mg, 0.61 mmol) then added, and the mixture was heated to 60 °C for 24 h. The mixture was poured into water and extracted with CHCl3. The organic layer was then dried over anhydrous MgSO4. After evaporation of the solvent, the residue was purified by column chromatography on a silica gel (CHCl3/n-Hex = 2: 8 ? CHCl3) and recrystallization using EA to afford a dark violet solid (210 mg, 65%). 1H NMR (300 MHz, CDCl3, d): 9.00 (d, J = 4.14, 2H), 8.62 (d, J = 9.39, 2H) 8.27 (m, 16 H), 7.56 (d, J = 4.11, 2H), 4.11 (d, J = 7.44, 4H), 2.03 (m, 2H), 1.39 (m, 64H), 0.82 (m, 12H). 13 C NMR (500 MHz, CDCl3, d): 161.46, 139.37, 135.70, 132.90, 131.88, 131.78, 131.20, 130.99, 130.72, 129.47, 128.84, 128.70, 128.58, 127.15, 126.33, 125.89, 125.85, 125.18, 124.53, 124.40, 124.19, 116.62, 108.90, 97.47, 88.09, 46.50, 37.90, 31.90, 31.34, 30.11, 29.72, 29.66, 29.58, 29.36, 26.34, 22.66, 14.07. High-resolution mass spectrometry (HRMS) mass-to-charge ratio (m/z):[M + H]+ calculated for C90H105N2O2S2, 1309.7617 ; found 1309.7634. Elemental Analysis calculated for C90H104N2O2S2: C 82.52, H 8.00, N 2.14, O 2.44, S 4.90; found: C 82.45, H 8.06, N 2.15, O 2.41, S 5.05. 4.1.5. Synthesis of HD-DPP-A-PY The reaction conditions and workup were same as for DPP-A-PY(OD), except 1(HD) (250 mg, 0.24 mmol) was used. After the reaction, the residue was purified by column chromatography on a silica gel (CHCl3/n-Hex = 2:8 ? CHCl3) and recrystallization using EA to afford a dark violet solid (230 mg, 69%). 1H NMR (300 MHz, CDCl3, d): 8.99 (d, J = 3.81, 2 H), 8.61 (d, J = 9.06, 2H) 8.27 (m, 16H), 7.56 (d, J = 4.14, 2H), 4.11 (d, J = 7.50, 4H), 2.04 (m, 2H), 1.39 (m, 48H), 0.84 (m, 12H). 13C NMR (500 MHz, CDCl3, d): 161.46, 139.38, 135.69, 132.90, 131.87, 131.78, 131.19, 130.99, 130.71, 129.47, 128.84, 128.70, 128.58, 127.15, 126.33, 125.89, 125.85, 125.18, 124.53, 124.39, 124.18, 116.61, 108.90, 97.46, 88.09, 46.49, 37.89, 31.90, 31.82, 31.32, 30.09, 29.75, 29.57, 29.34, 26.32, 26.28, 22.67, 14.08. High-resolution mass spectrometry (HRMS) mass-to-charge ratio (m/z): [M + H]+ calculated for C82H89N2O2S2, 1197.6365; found 1197.6344. Elemental Analysis calculated for C82H88N2O2S2: C 82.23, H 7.41, N 2.34, O 2.67, S 5.35; found: C 82.27, H 7.46, N 2.35, O 2.60, S 5.39. 4.1.6. Synthesis of OD-DPP-PY 1(OD) (200 mg, 0.20 mmol), pyren-1-yl-boronic acid (106 mg, 0.43 mmol), Pd(PPh3)4 (23.7 mg, 0.01 mmol), THF (20 mL), and 2 N K2CO3 (10 mL) were added to a two-necked round bottom flask under Ar atmosphere. The solution was stirred at 75 °C for 24 h. The mixture was poured into water and extracted with CHCl3. The organic layer was then dried over anhydrous MgSO4. After evaporation of the solvent, the residue was purified by column chromatography on a silica gel (CHCl3/n-Hex = 2:8 ? CHCl3) and being precipitated into MeOH to afford
a dark violet solid (136 mg, 55%). 1H NMR (300 MHz, CDCl3, d): 9.16 (d, J = 3.84 Hz, 2H), 8.58 (d, J = 9.36 Hz, 2H) 8.26 (m, 16H), 7.60 (d, J = 3.78, 2H), 4.17 (d, J = 6.45, 4H), 2.13 (m, 2H), 1.40 (m, 64H), 0.87 (m, 12H). 13C NMR (500 MHz, CDCl3, d): 161.91, 148.43, 140.12, 136.26, 131.70, 131.47, 130.91, 130.46, 129.38, 129.04, 128.57, 128.36, 128.24, 128.16, 127.28, 126.37, 125.77, 125.43, 125.16, 124.71, 124.50, 108.2, 46.52, 38.09, 31.89, 31.42, 30.13, 29.68, 29.64, 29.58, 29.33, 26.43, 22.64, 14.07. High-resolution mass spectrometry (HRMS) mass-to-charge ratio (m/z): [M + H]+ calculated for C86H105N2O2S2, 1261.7617; found 1261.7620. Elemental Analysis calculated for C86H104N2O2S2: C 81.86, H 8.31, N 2.22, O 2.54, S 5.08; found: C 81.56, H 8.33, N 2.18, O 2.88, S 4.99. 4.2. Characterization Chemical structures were fully identified by 1H NMR (Bruker, Avance-300), 13C NMR (Bruker, Avance-500), GCMass (JEOL, JMS-700), and elemental analysis (EA1110, CE Instrument). The thermal properties of the compounds were obtained using DSC and TGA under an N2 atmosphere, with a DSC-Q1000 model and a TA instruments Q50 model, respectively. UV–Vis spectra were recorded on a SHIMADZU UV-1650PC. HOMO level was obtained from the cyclic voltammetry measurements. Cyclic voltammetric measurements were performed using a 273A (Princeton Applied Research) with a one-compartment electrolysis cell consisting of a platinum working electrode, a platinum wire counter-electrode, and a quasi Ag+/Ag electrode as reference. Measurements were performed in a 0.5 mM acetonitrile solution with tetrabutylammonium tetrafluoroborate as the supporting electrolyte, at a scan rate of 50 mV/s. Each oxidation potential was calibrated using ferrocene as a reference. LUMO level was calculated from the HOMO level and the optical band gap, which was obtained from the edge of the absorption spectra. The surface morphologies were measured using atomic force microscopy in the tapping mode (AFM, Multimode with a Nano Scope V Controller, Bruker). 4.3. Fabrication and characterization of organic solar cells (OSCs) The organic solar cells in this study were fabricated by following method. Patterned ITO glass substrates (10 X/ square) were cleaned in an ultrasonic bath with trichloroethylene, acetone, and isopropyl alcohol for 10 min, respectively and then blow dried with a N2 stream. A 30– 40 nm of PEDOT:PSS (Batron P VP AI 4083) was then spin-coated onto the substrate (5000 rpm/30 s). The films were dried at 150 °C for 20 min. Subsequently, the (HD/ OD)-DPP-A-PY:PC70BM (>99.0%, ADS) solutions were deposited through spin casting at 1000 rpm/40 s. The thickness of (HD/OD)-DPP-A-PY:PC70BM films were around 130 nm. Al electrodes were deposited via thermal evaporation with thickness of 100 nm. The active area of these solar cells was 0.09 cm2. The current density–voltage (J–V) characteristics of the solar cells were measured with a Keithley 4200 source measurement unit. The solar cell performances were
J.-W. Mun et al. / Organic Electronics 14 (2013) 2341–2347
characterized under AM1.5G condition with an illumination intensity of 100 mW cm 2 generated by an Oriel Sol 3A solar simulator. J–V characteristics of the cells with illumination were measured using a metal mask of 0.09 cm2. The intensity dependent measurements have been performed with various neutral density filters. Incident photon to current conversion efficiency (IPCE) was measured using Oriel QE/IPCE Measurement Kit which composed of 300W Xenon Lamp, monochromator (74125), the order sorting filter wheel, the Merlin lock-in amplifier (70104) and the chopper.
[3]
Acknowledgements This research was supported by Basic Science Research Program (CRI; RIAMI-AM0209(0417-20090011)) and WCU (World Class University) Project (R31-2008-000-10075-0) through National Research Foundation of Korea funded by the Ministry of Education, Science and Technology.
[4]
[5]
[6]
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.2013.05.035. [7]
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