Efficient Organic Solar Cells with Star-Shaped Small Molecules

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Efficient Organic Solar Cells with Star-Shaped Small Molecules Comprising of Planar Donating Core and Accepting Edges Sanghyun Paek, Hyeju Choi, Jangkeun Sim, Kihyoung Song, Jae Kwan Lee, and Jaejung Ko J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5071709 • Publication Date (Web): 04 Nov 2014 Downloaded from http://pubs.acs.org on November 11, 2014

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The Journal of Physical Chemistry

Efficient Organic Solar Cells with Star-Shaped Small Molecules Comprising of Planar Donating Core and Accepting Edges Sanghyun Paek, † Hyeju Choi, † Jangkeun Sim, † Kihyoung Song, § Jae Kwan Lee, ‡,* and Jaejung Ko†,* †

Department of New Material Chemistry, Korea University, Chungnam, 330-700, Republic of

Korea, §Department of Chemistry, Korea National University of Education, Chungbuk 363-791, Republic of Korea, ‡Department of Chemistry Education, Chosun University, Gwangju, 501759, Republic of Korea.

*Corresponding Authors. Tel.: +82-62-230-7319 (J. K. Lee), +82-44-860-1337 (J. Ko); Fax: +82-62-232-8122 (J. K. Lee), +82-44-860-5369 (J. Ko); e-mail: [email protected] (J. K. Lee), [email protected] (J. Ko)

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ABSTRACT. High efficiency small molecules were synthesized and characterized in solutionprocessed organic solar cells. These were star-shaped comprising of fused triphenylamine donating core, hexyl cyano acetate or methylene malononitrile accepting edges, and alkylsubstituted therthiophene bridge unit, exhibiting a noteworthy power conversion efficiency of 4.18% in solar cell device from bulk-heterojunction films with PC71BM.

INTRODUCTION Solution-processed organic solar cells (OSCs) have been of great interests in scientific research owing to facile fabrication via various printing technologies such as roll-to-roll, inkjet, and doctor blade methods, and low-cost and efficient mass production.1-5 Over the past few decades, there has been considerable efforts to obtain high power conversion efficiencies (PCE) above 10% of OSCs from developments of high performance semiconducting materials such as lowband gap π-conjugated polymer donors and fullerene derivative acceptors, effective functions with surface plasmon resonancing, charge transporting, optical spacing, and buffering in device structures, and morphological engineering of photosensitive films by post heat treatment, drying condition of casting solutions, or processing additives.6-15 Recently, He et al. reported the most promising PCEs of up to 9.2% in OSCs with bulk hetetrojunction (BHJ) films from π-conjugated low-bandgap

polymers

containing

benzodithiophene

(poly(thieno[3,4-b]thiophene-alt-

benzodithiophene) (PTB) skeleton and [6,6]-phenyl-C(61 or 71)-butyric acid methyl ester (PC(61 or 71)BM).

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Although OSCs have revealed outstanding performances, these low-bandgap polymer

are still expensive suffering from multi-steps synthesis and reproducibility and purification for


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uniform molecular weight and high dispersity. In this regard, small molecule-based organic semiconductor may be more fascinated. During the past few years, enormous research progresses have been focused on development of highly efficient semiconducting small-molecules for high performing solution-processed small-molecule OSCs (SMOSCs) as alternative to polymer solar cells (PSCs).17-23 Recently, Zhou et al. reported the excellent benzo[1,2-b:4,5-b’]dithiophene (BDT)-based small molecules in SMOSCs with high PCEs above 8%.24 The notable performing small molecules for SMOSCs were often comprised of electrondonating groups and electron-accepting groups bridged with via π-conjugation.25 These structures have increased intramolecular charge transfer (ICT), resulting in enhanced molar absorptivity as well as a narrow bandgap. We have also developed the effective organic semiconductors from various structure-performance relationships and have reported various triarylamine-based electron donating units in conjunction with versatile accepting groups for solution-processed SMOSCs.26-35 Especially, triarylamine motifs could induce effective hole transport through stabilizing holes generated by the exciton dissociation. However, despite the good hole stabilization that triarylamine donors afford, its nonplanar structure might be a critical barrier in achieving efficient intermolecular stacking that is necessary for effective charge transport in a BHJ system. Thusly, we recently reported a new and unique planar star-shaped small molecule with fused triphenly (TPA) donor moieties and symmetric three branches, which formed the donor-acceptor-donor (D-A-D) skeleton, and composed of the dithieno(3,2-b;2',3'd)silole (DTS) donor, benzothiadiazole (BT) acceptor, and hexylterthiophene donor units.36 The planar structure with a fused TPA core showed an enhanced hole mobility than that with nonplanar TPA core. However, its branched building block seems to be conducted to the complicated D-(D-A-D)3 backbone structure. Moreover, the hidden acceptor unit at the edge position in this star-shaped structure might be inefficient at transferring charge to PCBM. In light


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of this, we have attempted to develop simple and efficient star-shaped planar organic semiconductors with a D3A backbone structure composed of a planar TPA donor core and acceptor edges. Hereby, we synthesized novel star-shaped planar organic semiconductors: 2,6,10-tris-(3,3'',3''',4'tetraoctyl[2,2':5',2'':5'',2'''-quaterthiophen]-5'''-methylenemalononitrile-5-yl)-4,4,8,8,12,12Hexamethyl-4H,8H,12H-benzo[1,9]quinolizino[3,4,5,6,7,-defg]acridine

(DMM-TPA-

[T3MMN]3,1) and 2,6,10-tris-(3,3'',3''',4'-tetraoctyl[2,2':5',2'':5'',2'''-quaterthiophen]-5'''-[hexyl-2cyanoacrylate]-5-yl)-4,4,8,8,12,12-Hexamethyl-4H,8H,12H-benzo[1,9]quinolizino[3,4,5,6,7,defg]acridine (DMM-TPA-[T3HCA]3, 2), consisting of a TPA core fused with a dimethylmethylene (DMM) bridge and methylene malononitrile (MMN) and hexyl cyanoacetate (HCA), respectively, bridged via an alkyl-substituted terthiophene π-conjugated bridge and investigated their photovoltaic characteristics in this work. These star-shaped D3A skeletons seemed to be simple and facilitate charge transfer from the acceptor units exposed at the edge position to PCBM. The BHJ films from these new materials and PC71BM were employed in OSCs fabrication, where displayed an enhanced photovoltaic performance compared with those with PC61BM causing by a superior spectral response in visible-light absorption region, and the noteworthy photovoltages of 0.86 V compared to the previously reported material. The most efficient SMOSCs possessing DMM-TPA-[T3MMN]3 (1) exhibited a high PCE of 4.18%, and was fabricated using a PC71BM BHJ film and TiOx functional layer inserted between BHJ layer and metal electrode. (Scheme 1)

EXPERIMENTAL SECTION


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General Methods. Most reactions were performed under inert atmosphere. Solvents were purified by distillation prior to use in reaction. All reagents were obtained from commercial suppliers such as Sigma-Aldrich, Alfa Aesar, and TCI. Synthesis of TiOx. TiOx material was synthesized according to methods reported previously. Titanium (IV) isopropoxide (10 mL) was drop-wised to 2-methoxyethanol (50 ml) solution with ethanolamine (5 ml). This solution underwent two times with heating to 80 °C for 2 h and to 120 °C for 1 h, then was evaporated to make dense TiOx solution above 120°C. The typical TiOx precursor solution was used after diluting with isopropyl alcohol. The TiOx film was prepared from spin-casting on the photoactive BHJ layer followed by thermal drying to 80 °C for 10 min in air (thicknesses of 10–20 nm). Measurements and Instruments. 1H NMR,

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C NMR spectra were measured by using a

Varian Mercury 300 spectrometer. Elemental analyses and Mass analysis were carried out with a Carlo Elba Instruments (CHNS-O EA 1108) and a JEOL JMS-SX102A instrument, respectively. The absorption spectra were measured from a Perkin-Elmer Lambda 2S UV-visible spectrometer. Cyclic voltammetry was measured from a BAS 100B (Bioanalytical Systems, Inc.) with a three electrode system comprising of a reference electrode with 0.1 M Ag/Ag+ acetonitrile solution (MF-2062, Bioanalytical System, Inc.), platinum counter electrode wire with diam. 1.0 mm, and platinum working electrode (MF-2013, Bioanalytical System, Inc.). The redox potentials were obtained in 0.1 M (n-C4H9)4N-PF6 in CHCl3 with 100 mV s-1 scan rate (vs. external Fc/Fc+ reference). Current-Voltage (J-V) curves were measured under 100 mW/cm2 AM 1.5G irradiation (Oriel 91193), which intensity of light was adjusted with National Renewable Energy Laboratory (NREL)-calibrated Si solar cell (PV measurement Inc.), and recorded using a


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Keithley model 2400 digital source meter. The external quantum efficiency were obtained from incident photon-to-current conversion efficiency (IPCE) measuring system (PV measurements). Fabrication of OSCs. BHJ films were prepared and optimized according to previously reported methods.21 The PEDOT:PSS (Heraeus, Clevios P VP.AI 4083) was spin-cast on the indium tin oxide (ITO)-coated glass substrates, which were cleaned from sonication in detergent, acetone, and isopropyl alcohol, with a thickness of ~40 nm followed by drying for 10 min at 140 °C in air. The synthesized material, (1 or 2) was mixed with PC71BM to give BHJ composite in a ratio of 1:1~1:4 w/w in chlorobenzene. This mixed solutions was spin-cast on PEDOT:PSS film followed by drying for 10 min at 80 °C in air. Subsequently, the Al electrode was deposited with thickness of ~100 nm on the BJH film under high vacuum system of 10–7 Torr. Synthesis of Semiconducting Small Molecules. 2,6,10-tris-(3,3'',3''',4'-tetraoctyl[2,2':5',2'':5'',2'''-quaterthiophen]-5-yl)-4,4,8,8,12,12Hexamethyl-4H,8H,12H-benzo[1,9]quinolizino[3,4,5,6,7,-defg]acridine (iv). 2,6,10-tribromo4,4,8,8,12,12-Hexamethyl-4H,8H,12H-benzo[1,9]quinolizino[3,4,5,6,7,-defg]acridine (iii) (0.2 g, 0.33

mmol)

and

1,3,2-Dioxaborolane,

4,4,5,5-tetramethyl-2-(3,3'',3''',4'-

tetraoctyl[2,2':5',2'':5'',2'''- quaterthiophen]-5-yl) (ii) (0.72 g, 1.32 mmol) were dissolved in degassed THF (2 mL). Pd(PPh3)4 (2 mg, 0.016 mmol) and anhydrous K2CO3 (0.22 g, 1.65 mmol) in THF/H2O mixed solution were added in this solution under a nitrogen atmosphere, then was carried out refluxing for 36 hrs. After finishing the reaction, the separated organic layer was dried over anhydrous magnesium sulfate and filtered. The column chromatography was performed to purify the final compound. Yield: 40 %. MS: m/z 1608 [M+]. 1H NMR (300 MHz, CDCl3): δ 7.60 (s, 6H), 7.19 (d, J = 4.8 Hz, 3H), 7.15 (s, 3H), 7.12 (d, J = 3.6 Hz, 3H), 7.08 (d, J = 3.9 Hz, 3H), 6.95 (d, J = 5.1 Hz, 3H), 2.81 (m, 12H), 1.67 (m, 12H), 1.35 – 1.25 (m, 36H),


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0.88 (m, 18H).

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C NMR (75 MHz, CDCl3): δ 142.15, 140.72, 139.75, 136.28, 135.85, 131.01,

130.49, 130.20, 129.22, 129.10, 126.21, 125.73, 125.26, 123.83, 121.20, 35.90, 33.45, 31.88, 30.92, 29.89, 29.55, 29.45, 22.87, 14.33. Anal. Calc. for C99H117NS9: C, 73.87; H, 7.33; N, 0.87. Found: C, 73.88; H, 7.24; N, 0.85. 2,6,10-tris-(3,3'',3''',4'-tetraoctyl[2,2':5',2'':5'',2'''-quaterthiophen]-5'''-carbaldehy-de-5-yl)4,4,8,8,12,12-Hexamethyl-4H,8H,12H-benzo[1,9]quinolizino[3,4,5,6,7,-defg]acri-dine (v). To a stirred

solution

of

2,6,10-tris-(3,3'',3''',4'-tetraoctyl[2,2':5',2'':5'',2'''-quaterthiophen]-5-yl)-

4,4,8,8,12,12-Hexamethyl-4H,8H,12H-benzo[1,9]quinolizino[3,4,5,6,7,-defg]acridine (iv) (0.1 g, 0.06 mmol) in CHCl3 was added phosphorus oxychloride (38 mg, 0.24 mmol) and DMF at 0 oC. The solution was refluxed for 3 h, then was neutralized using sodium carbonate and extracted with CH2Cl2. After finishing the reaction, the separated organic layer was dried over anhydrous magnesium sulfate and filtered. The column chromatography was performed to purify the final compound. Yield: 95 %. MS: m/z 1692 [M+]. 1H NMR (300 MHz, CDCl3): δ 9.84 (s, 3H), 7.61 (s, 9H), 7.28 (d, J = 3.9 Hz, 3H), 7.17 (s, 3H), 7.16 (d, J = 3.6 Hz, 3H), 2.85 (m, 12H), 1.69 (m, 12H), 1.46 – 1.34 (m, 36H), 0.89 (m, 18H).

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C NMR (75 MHz, CDCl3): δ 182.58, 142.91,

141.49, 141.29, 140.34, 140.26, 139.19, 138.73, 134.30, 131.17, 130.60, 129.39, 128.60, 127.99, 121.32, 35.95, 33.45, 31.91, 31.83, 30.87, 30.49, 30.00, 29.69, 29.38, 22.87, 22.81, 14.31. Anal. Calc. for C102H117NO3S9: C, 72.34; H, 6.96; N, 0.83. Found: C, 72.31; H, 6.91; N, 0.82. 2,6,10-tris-(3,3'',3''',4'-tetraoctyl[2,2':5',2'':5'',2'''-quaterthiophen]-5'''methylenemalononitrile-5-yl)-4,4,8,8,12,12-Hexamethyl-4H,8H,12Hbenzo[1,9]quinolizino[3,4,5,6,7,-defg]acridine (compound 1). Compound v (0.15 g, 0.088 mmol) and malononitrile (58 mg, 0.88 mmol) dissolved in dry CHCl3 were stirred for 1 h with a few drops of triethylamine at room temperature. After finishing the reaction, the organic layer extracted with CH2Cl2 was dried over anhydrous magnesium sulfate and filtered. The column


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chromatography was performed to purify the final compound. Yield: 80 %. Mp: 83 oC. MS: m/z 1836 [M+]. 1H NMR (300 MHz, CDCl3): δ 7.70 (s, 3H), 7.61 (s, 6H), 7.55 (s, 3H), 7.37 (d, J = 3.6 Hz, 3H), 7.20 (d, J = 3.9 Hz, 3H), 7.18 (s, 3H), 2.86 (m, 12H), 1.69 (m, 12H), 1.43 – 1.25 (m, 36H), 0.91 (m, 18H).

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C NMR (75 MHz, CDCl3): δ 149.94, 144.08, 143.29, 141.92, 140.69,

140.17, 133.23, 132.18, 131.22, 130.62, 128.95, 128.37, 121.28, 114.56, 113.62, 35.94, 33.42, 31.77, 30.77, 30.26, 30.07, 29.55, 22.80, 14.30. Anal. Calc. for C111H117N7S9: C, 72.54; H, 6.42; N, 5.34. Found: C, 72.44; H, 6.45; N, 5.29. 2,6,10-tris-(3,3'',3''',4'-tetraoctyl[2,2':5',2'':5'',2'''-quaterthiophen]-5'''-[hexyl-2cyanoacrylate]-5-yl)-4,4,8,8,12,12-Hexamethyl-4H,8H,12H-benzo[1,9]quinolizino[3,4,5,6,7,defg]acridine (compound 2). Compound v (0.15 g, 0.088 mmol) and hexylcyanoacetate (0.15 g, 0.88 mmol) dissolved in dry CHCl3 were stirred for 6 h with a few drops of triethylamine at room temperature. After finishing the reaction, the organic layer extracted with CH2Cl2 was dried over anhydrous magnesium sulfate and filtered. The column chromatography was performed to purify the final compound. Yield: 40 %. Mp: 80 oC. MS: m/z 2146 [M+]. 1H NMR (300 MHz, CDCl3): δ 8.21 (s, 3H), 7.62 (s, 6H), 7.61 (s, 3H), 7.33 (d, J = 3.9 Hz, 3H), 7.18 (d, J = 4.2 Hz, 3H), 7.17 (s, 3H), 4.29 (t, 6H), 2.86 (m, 12H), 1.70 (m, 18H), 1.43 – 1.33 (m, 54H), 0.91 (m, 27H).

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C NMR (75 MHz, CDCl3): δ 163.25, 146.08, 142.96, 141.99, 141.57, 140.98,

140.45, 139.21, 133.84, 132.92, 131.15, 130.58, 128.97, 128.57, 128.38, 126.00, 125.47, 121.26, 116.17, 97.65, 66.69, 35.93, 33.45, 31.90, 31.80, 31.57, 31.14, 30.81, 30.38, 30.02, 29.89, 29.56, 29.39, 28.70, 25.67, 22.86, 22.81, 22.70, 14.33, 14.31, 14.21. Anal. Calc. for C129H156N4O6S9: C, 72.16; H, 7.32; N, 2.61. Found: C, 72.11; H, 7.35; N, 2.60.

RESULTS AND DISCUSSION


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Scheme 2 shows the synthetic approaches of designed small molecules. All reactions were performed under inert atmosphere. The i, ii, and iii were prepared via methods reported previously.37,38 The iv was readily prepared by Suzuki coupling of ii and iii with Pd(PPh3)4 in THF/H2O. Carbaldehyde v was synthesized via the Vilsmeier-Haack reaction with compound iv. Then, DMM-TPA-[T3MMN]3 (1) and DMM-TPA-[T3HCA]3 (2) were readily prepared through Knoevenagel condensation of carbaldehyde v with malononitrile and hexyl cyanoacetate, respectively. The structures of synthesized products were confirmed from 1H and

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C NMR

spectroscopy, elemental analysis, and mass spectrometry. These materials were well soluble in polar aprotic solvent such as dichloromethane and chloroform and aromatic solvent such as chlorobenzene and toluene. Figure 1 shows the (a) UV-visible absorption spectra of 1 and 2 in both solution and solid-state form and the (b) cyclic voltammograms of 1 and 2 in dichloromethane. The results were summarized in Table 1. The absorption spectra of 1 (red) and 2 (black) in solution (solid line) presented two typical characteristic bands in 300-700 nm. These bands were investigated by calculation from time-dependent density-functional theory (TD-DFT) using the B3LYP/6-31G* model. Figure S1 and Table S1 show the simulated energy levels of these materials in vacuum state (see Supporting Information). Absorption bands in longer wavelengths could be induced by ICT from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO). In particular, energy level of LUMO+1 was slightly higher than that of LUMO. Thus, the rather broad absorption spectra of these materials at longer wavelength were caused by overlapping ICT bands as a result of HOMO→LUMO and HOMO→LUMO+1 excitations. The absorption bands at 350–500 nm showed strong oscillator strengths of f = 0.63-1.63 and are could be determined as π-π* transitions originated from HOMO-1→LUMO, HOMO-1→LUMO, and HOMO-1→LUMO+2 excitations. The 1 and 2 in solution exhibited high molar absorptivity


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of 99,000 M-1·cm-1 at 530 nm and 83,500 M-1·cm-1 at 498 nm, respectively, resulting in the ~30 nm red-shifted ICT band and higher molar absorptivity of 1 compared to those of 2 (Figure 1a and Table 1). These results indicated that the ICT from the planar TPA core to the MMN acceptor edge was more effective owing to the stronger electron-accepting strength of the MMN acceptor compared to that of the HCA acceptor. The absorption bands of both 1 and 2 in the solide-state (dashed line) broadened and red-shifted compared to those in solution. These note that the planar structures of 1 and 2 lead to more effective molecular networks via intermolecular π-π interactions. The energy bandgaps of 1 and 2 were determined using cyclic voltammograms in solution-state because these materials in solid-state thin films could not be measured owing to the stripping of the film from the electrode. The calculated energy levels of 1 and 2 for HOMO/LUMO were 5.031/3.669 eV and 4.977/3.671 eV, respectively. These HOMO levels could be comparable to those of star-shaped materials reported previously. Based on these results, we expect BHJ OSCs based on 1 and 2 to exhibit superior Voc values. Figure 2 shows the structures of TD-DFT calculated 1 and 2 using the B3LYP/6-31G* model. The orbital density for HOMO and LUMO of 1 and 2 were preponderantly distributed on the DMM-TPA donating core and the MMN (or HCA) accepting edge, respectively, exhibiting the orbital location of push-pull type small molecules. Interestingly, the LUMO and LUMO+1 were located on the one and the others of symmetrical three accepting units, respectively, and the HOMO-1 of these materials was present on the alkyl-substituted terthiophene linker. These were consistent with the results shown in Figure 1, and were assigned from the calculated in vacuo energy levels shown in Figure S1 and Table S1. Also, these present that ICT from the DMMTPA donating core to the MMN or HCA accepting edges in 1 and 2 effectively facilitate to transfer an excited electron to PCBM. In particular, DMM-TPA core stabilizes hole generated by exciton dissociation, resulting in effective transport of charge carrier.


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UV-visible absorption spectra of 1/PC71BM (black) and 2/PC71BM (red) (1:3 w/w) films were shown in Figure 3a. These films were fabricated with the optimum ratio determined by the best device performance. Normalized absorption bands of 2/PC71BM BHJ films showed similar spectral characteristics with that of its pristine solid-state film without PC71BM, but that observed for 1/PC71BM BHJ films exhibited a rather blue-shift of absorption peak than that of its pristine solid-state film. These could be interpreted as an interruption of the π-π packing interaction caused by the PC71BM BHJ film. The stronger ICT band intensity of 1/PC71BM in 550-750 nm range should mainly be due to its stronger molar absorptivity compared to that of 2/PC71BM, because they have similar film thicknesses and have with same ratio of PC71BM. The hole mobilities of the 1 and 2 were investigated by the space-charge-limited current (SCLC) J-V characteristics determined in hole-only devices under the dark condition. Figure 3b shows darkcurrent curves in ITO/PEDOT:PSS/donor:PC71BM(1:3)/Au, which was bias function in built-in voltage obtained from the work function difference between PEDOT:PSS-coated ITO and Au and abide by Ohm’s law at low voltages due to the presence of thermal free carriers. A trapfilled-limit (TFL) region in the presence of carrier traps was observed in between the Ohmic and trap-free SCLC domains. The SCLC is determined by the Mott–Gurney law (1):39 J = (9/8)ε·µ(V2/L3)

(1)

Where µ and ε are the carrier mobility and static dielectric constant of the medium, respectively. The evaluated hole mobilities of materials using the Mott-Gurney law (ε = 3ε0) were 4.84 × 10-5 and 3.53 × 10-5 cm2/V·s, respectively. Although the hole mobility of 1 was slightly higher than that of 2, it appeared to have a negligible influence on the photocurrents of these materials. Next, molecules 1 and 2 were investigated the photovoltaic performances via PC71BM BHJ films. A study of more than 150 solar cells gave an optimal ratio of 1:3 in most efficient BHJ


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photovoltaic devices with 1 or 2 with PC71BM, which thickness were around 100 nm. Figure 4 displays the J-V curves under AM 1.5 irradiation (100 mW/cm2), and the IPCE spectra of the 1 (or 2)/PC71BM based solar cells, which were also fabricated under optimized processing conditions with and without TiOx to give additional functions such as an optical spacer and buffer layer11,12 between the BHJ film and metal electrode. The related values were in Table 2. The IPCE spectra as shown in Figure 4b were well-matched in their optical absorptions, indicating a close relationship with photocurrents observed in the J-V curves. Conventionally fabricated devices incorporating 1/PC71BM and 2/PC71BM presented the following photovoltaic performances (Figure 4a and Table 2): PCE (maximum/average) of 3.74/3.60% with a shortcircuit current density (Jsc) of 9.34 mA·cm-2, a fill factor (F.F) of 0.46, and open-circuit voltage (Voc) of 0.86 V and PCE of 2.89/2.71% with Jsc = 7.66 mA·cm-2, F.F = 0.44, and Voc = 0.86 V, respectively, resulting in a noteworthy photovoltage value of 0.86 V, which was higher than that (0.74 V) of our previously reported star-shaped materials. The higher Jsc of 1/PC71BM could be mainly attributed to the highly intense spectral response resulting from a more efficient ICT compared to that of 2/PC71BM. Meanwhile, all devices fabricated using these materials:PC71BM BHJ with a TiOx functional layer exhibited better photovoltaic performances than those without TiOx. The best PCEs (maximum/average) of 4.18/4.01% with a Jsc of 10.23 mA·cm-2, a F.F of 0.48, and Voc of 0.86 V and PCE of 3.55/3.44% with Jsc = 8.92 mA·cm-2, F.F = 0.47, and Voc = 0.85 V were obtained for devices fabricated using 1/PC71BM and 2/PC71BM films with a TiOx functional layer, respectively, resulting in 11~23% increments of PCE with remarkably enhanced Jsc values compared with those of their counterparts without TiOx. These results might be due to an optical spacing effect as well as a reinforced contact between BHJ film and Al electrode insertion of TiOx layer, affecting light absorptivity and charge collection of the device. Conclusion


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We demonstrated the synthesis of new star-shaped planar small molecules: DMM-TPA[T3MMN]3 (1) and DMM-TPA-[T3HCA]3 (2), consisting of a TPA core fused with a dimethylmethylene (DMM) bridge and methylene malononitrile (MMN) and hexyl cyanoacetate (HCA), respectively, which were bridged by an alkyl-substituted terthiophene π-conjugated bridge and their photovoltaic performances in solution-processed OSCs. These star-shaped D3A skeletons could facilitate charge transfer from the acceptor units exposed at the edge position to PCBM. We believe that the findings reported in this study may herald the guaid of a new direction in the development of highly efficiency materials for use in solution-processed SMOSCs.

Acknowledgment. This research was supported by the Converging Research Center Program through the Ministry of Science, ICT and Future Planning, Korea (2013K000203), the International Science and Business Belt Program through the Ministry of Education, Science and Technology (no. 2012K001573), and the ERC (the Korean government (MEST)) program (no. 2013004800). Supporting Information Available: Calculation of energy level, AFM image of BHJ films. This material is available free of charge via the Internet at http://pubs.acs.org.

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23. Li, W.; Du, C.; Li, F.; Zhou, Y.; Fahlman, M.; Bo, Z.; Zhang, F. Benzothiadiazole-Based Linear and Star Molecules: Design, Synthesis, and Their Application in Bulk Heterojunction Organic Solar Cells. Chem. Mater. 2009, 21, 5327-5334 24. Zhou, J. et al. Solution-Processed and High-Performance Organic Solar Cells Using Small Molecules with a Benzodithiophene Unit. J. Am. Chem. Soc. 2013, 135, 8484-8487 25. Jeong, B. S.; Choi, H.; Cho, N.; Ko, H. M.; Lim, W.; Song, K.; Lee, J. K.; Ko, J. Molecular Engineering of Diketopyrrolopyrrole-Based Photosensitizer for Solution Processed Small Molecule Bulk Heterojunction Solar Cells. Sol. Energy Mater. Sol. Cells 2011, 95, 1731-1740 26. So, S.; Choi, H.; Kim, C.; Cho, N.; Ko, H. M.; Lee, J. K.; Ko, J. Novel Symmetric Squaraine Chromophore Containing Triphenylamine for Solution Processed Small Molecule Bulk Heterojunction Solar Cells. Sol. Energy Mater. Sol. Cells 2011, 95, 34333441 27. So, S.; Choi, H.; Ko, H. M.; Kim, C.; Paek, S.; Cho, N.; Song, K.; Lee, J. K.; Ko, J. Novel Unsymmetrical Push–Pull Squaraine Chromophores for Solution Processed Small Molecule Bulk Heterojunction Solar Cells. Sol. Energy Mater. Sol. Cells 2012, 98, 224232 28. Ko, H. M.; Choi, H.; Paek, S.; Kim, K.; Song, K.; Lee, J. K.; Ko, J. Molecular Engineering of Push-Pull Chromophore for Efficient Bulk Heterojunction Morphology in Solution Processed Small Molecule Organic Photovoltaics. J. Mater. Chem. 2011, 21, 7248-7253


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29. Kim, J.; Ko, H. M.; Cho, N.; Peak, S.; Lee, J. K.; Ko, J. Efficient Small Molecule Organic Semiconductor Sontaining Sisdimethylfluorenyl Amino Benzo[b]thiophene for High Open Circuit Voltage in High Efficiency Solution Processed Organic Solar Cell. RSC Adv. 2012, 2, 2692-2965 30. Cho, N.; Kim, J.; Lee, J. K.; Ko, J. Synthesis and Characterization of Push-Pull Organic Semiconductors with Various Acceptors for Solution-Processed Small Molecule Organic Solar Cells. Tetrahedron 2012, 68, 4029-4036 31. Kim, J.; Cho, N.; Ko, H. M.; Kim, C.; Lee, J. K.; Ko, J. Push-Pull Organic Semiconductors Comprising of Bis-dimethylfluorenyl Amino Benzo[b]thiophene Donor and Various Acceptors for Solution Processed Small Molecule Organic Solar Cells. Sol. Energy Mater. Sol. Cells 2012, 102, 159-166 32. Choi, H.; Ko, H. M.; Cho, N.; Song, K.; Lee, J. K.; Ko, J. Electron-Rich Anthracene Semiconductors Containing Triarylamine for Solution-Processed Small-Molecule Organic Solar CellsChemSusChem 2012, 5, 2045-2052 33. Cho, N.; Song, K.; Lee, J. K.; Ko, J. Facile Synthesis of Fluorine-Substituted Benzothiadiazole-Based Organic Semiconductors and Their Use in Solution-Processed Small-Molecule Organic Solar Cells. Chem. Euro. J. 2012, 18, 11433-11439 34. Paek, S.; Cho, N.; Song, K.; Jun, M. J.; Lee, J. K.; Ko, J. Efficient Organic Semiconductors

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35. Lee, J. K.; Jeong, B. S.; Kim, J.; Kim, C.; Ko, J. Synthesis and Photochemical Characterization of Fumaronitrile-Based Organic Semiconductor and Its Use in SolutionProcessed Small Molecule Organic Solar Cells. J. Photochem. Photobiol. A: Chem. 2013, 251, 25-32 36. Paek, S.; Cho, N.; Cho, S.; Lee, J. K.; Ko, J. Planar Star-Shaped Organic Semiconductor with Fused Triphenylamine Core for Solution-Processed Small-Molecule Organic Solar Cells. Org. Lett. 2012, 14, 6326–6329 37. Do, K.; Kim, D.; Cho, N.; Paek, S.; Song, K.; Ko, J. New Type of Organic Sensitizers with a Planar Amine Unit for Efficient Dye-Sensitized Solar Cells. Org. Lett. 2012, 14, 222-225 38. Lim, N.; Cho, N.; Paek, S.; Kim, C.; Lee, J. K.; Ko, J. High-Performance Organic Solar Cells with Efficient Semiconducting Small Molecules Containing an Electron-Rich Benzodithiophene Derivative. Chem. Mater., 2014, 26, 2283–2288 39. Mihalietchi, V. D.; Xie, H.; de Boer, B.; Koster, L. J. A.; Blom, P. W. M. Charge Transport and Photocurrent Generation in Poly(3-hexylthiophene):Methanofullerene Bulk-Heterojunction Solar Cells. Charge Transport and Photocurrent Generation in Poly(3-hexylthiophene):Methanofullerene Bulk-Heterojunction Solar Cells. Adv. Funct. Mater. 2006, 16, 699-708


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Table 1. Optical and electrochemical properties of the materials 1 and 2.

Mat.

1

2

λabs[a]/nm (ε/M-1cm-1) 420 (103,000), 530 (99,000) 419 (98,000), 498 (83,500)

λPL[a]/ m

732

723

Eonset, ox (V)/

Eonset, red (V) /

Eopt

E0-0

HOMO (eV)[b]

LUMO (eV)[b]

(eV)[c]

(eV)[d]

0.231

-1.131

/ -5.031

/ -3.669

1.362

1.97

0.177

-1.129

/ -4.977

/ -3.671

1.306

2.05

[a] Absorption and photoluminescence spectra were measured in chlorobezene. [b] Redox potential of the compounds were measured in CH2Cl2 with 0.1M (n-C4H9)4NPF6 with a scan rate of 50 mVs-1 (vs. Fc/Fc+). The HOMO and LUMO were deduced from the oxidation and reduction onsets under the assumption that the energy level of the standard Fc/Fc+ system was 4.8 eV below the level in vacuo. [c] Eopt was calculated from the absorption thresholds from absorption spectra in chlorobezene. [d] E0-0 was calculated from the absorption and emission cross peak in chlorobezene.

Table 2. Photovoltaic performances of the BHJ solar cells composed 1, 2 /PC71BM from chlorobenzene[a] Mat.

TiOx

Jsc (mAcm-2)

Voc (V)

FF

η max/ave (%)

1

X

9.38

0.86

0.46

3.74 / 3.60

1

O

10.22

0.86

0.47

4.18 / 4.01

2

X

7.65

0.86

0.44

2.89 / 2.71

2

O

8.91

0.85

0.47

3.55 / 3.44

[a] The optimum devices were fabricated with fluorinated benzothiadiazole series/PC71BM films which were spincast (3000rpm, 60s). The performances are determined under simulated 100 mW/cm2 AM 1.5G illumination. The light intensity using calibrated standard silicon solar cells with a proactive window made from KG5 filter glass traced to the National Renewable Energy Laboratory (NREL). The active area of device is 4 mm2.


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Scheme Captions Scheme 1. Molecular structures of the planar star-shaped DMM-TPA-[T3MMN]3 (1) and DMM-TPA-[T3HCA]3 (2)

and device architecture of solution processed small molecule

organic solar cell. Scheme 2. Schematic diagram for the synthesis of DMM-TPA-[T3MMN]3 (1) and DMMTPA-[T3HCA]3 (2). Figure Captions Figure 1. (a) UV-visible absorption spectra in both chlorobenzene (solid line) and thin-film form (dashed line) and (b) cyclic voltammograms of the 1 (red) and 2 (black) in dichloromethane/TBAHFP (0.1 M), scan speed 100 mV/s, potentials vs. external Fc/Fc+ Figure 2. Isodensity surface plots of the 1 and 2, calculated by the time dependent-density functional theory (TD-DFT) using the B3LYP functional/6-31G* basis set Figure 3. (a) UV-vis absorption spectra and (b) space charge limitation of current J-V characteristics of the 1 (red) (or 2 (black))/PC71BM BHJ (weight ratio of 1:3) films, which holeonly devices (ITO/PEDOT:PSS/Donor:PC71BM/Au). Figure 4. (a) Current (J)-voltage (V) curves under AM 1.5 conditions (100 mW/cm2) and (b) IPCE spectra of the 1 (red) (or 2 (black))/PC71BM BHJ solar cells fabricated under optimized processing condition with (solid line)/without (dashed line) insertion of TiOx layer.


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Scheme 1. Paek et. al.


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Scheme 2. Paek et. al.


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Figure 1. Paek et. al.


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[TOC]


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Efficient Organic Solar Cells with Star-Shaped Small Molecules

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