Article pubs.acs.org/JPCC
Highly Efficient Vacuum-Processed Organic Solar Cells Containing Thieno[3,2‑b]thiophene-thiazole Jihun Kim,†,§ Hyun-Sub Shim,‡,§ Horim Lee,† Min-Soo Choi,‡ Jang-Joo Kim,*,‡ and Yongsok Seo*,† †
Intellectual Textile System Research Center (ITRC) and RIAM, Department of Materials Science and Engineering, College of Engineering, Seoul National University, Kwanakro 1, Kwanakgu, Seoul 151-744, Republic of Korea ‡ Department of Materials Science and Engineering and the Center for Organic Light Emitting Diode, Seoul National University, Kwanakro 1, Kwanakgu, Seoul 151-744, Republic of Korea S Supporting Information *
ABSTRACT: Two novel electron-donor molecules based on donor−π-conjugated linker−acceptor structure with compact packing and intramolecular charge-transfer characteristics were synthesized for the preparation of efficient organic solar cells. The donor molecules featuring an electron-rich triphenyl amine as the electron-donor unit, dicyanovinylene as the acceptor unit, and π-conjugated linkers of thienothiophene, thiophene, and thiazole units were synthesized. The π-conjugated linkers were carefully designed to have a planar structure, an efficient conjugation length, and appropriate energy levels for a compact packing in the solid state. The vacuum-processed solar cells fabricated using the donor molecules of DTTh and DTTz exhibited average power-conversion efficiencies (PCEs) of 5.4 and 6.2% (the highest PCE obtained was 6.37%) under AM 1.5G illumination with an intensity of 100 mW cm−2.
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INTRODUCTION Organic solar cells (OSCs) have attracted significant attention because they may potentially be prepared using low-cost processes and may be applied in a variety of applications.1,2 Recent reports described the preparation of solution-processed OSCs, based on conjugated polymers or small molecules, that displayed excellent power-conversion efficiencies (PCEs) of up to ∼10.6% or ∼9%, respectively.3,4 By contrast, vacuumdeposited OSCs offer certain advantages (the device performances are reproducible and the purity of the materials may be readily controlled via sublimation) but display relatively lower efficiencies.5−9 Among the vacuum-deposited OSCs reported thus far, metal−phthalocyanine (M-Pc) compounds have been widely used as donor materials because of their high absorption coefficient in the visible and near-infrared regions and their good thermal stability.10−13 M-Pc compounds, however, provide relatively low open circuit voltages VOC because of the small difference between the highest occupied molecular orbital (HOMO) level of the M-Pc compounds and the lowest unoccupied molecular orbital (LUMO) level of the fullerene derivatives. The M-Pcs also provide a low fill factor FF due to the difficulties associated with forming charge-transport pathways in the codeposited layers.10,11 New donor materials have been developed in an effort to overcome the drawbacks of MPc compounds. A series of 2,2′-dicyanovinylene (DCV) acceptor−oligothiophene donor (A-D-A) compounds have been found to provide good transport properties and a high VOC with a PCE of ∼6.9%.9,14 Recently, a series of donor− acceptor (D-A) structures that incorporate arylamines as the © 2014 American Chemical Society
electron-donating component and a variety of electronaccepting moieties, such as 2,1,3-benzothiadiazole, pyrimidine, and dicyanovinylene, have been proposed.6,15,16 This combination provided high photovoltaic performances through the D-A molecular structure with a highly polar character and coplanar conformation. The PCE of this structure could potentially reach 6.6%. The properties of the donor materials are critical for the device performance, and the efficiency of an OSC prepared using small molecules can be improved through the appropriate design of donor materials. A popular strategy for preparing highly efficient organic solar cells includes incorporating organic compounds having a push− pull structure. This approach can produce high-performance donor materials with strong and broad absorption, suitable HOMO and LUMO energy levels, and strong intra- and intermolecular interactions. The triphenylamine (TPA) electron-donating moiety has been widely used in the metal-free organic sensitizers used in dye-sensitized solar cells17 and semiconducting small molecules used in OSCs2,19 because TPA-containing molecules offer excellent stabilities, good electron-donating properties, high hole-transporting properties, and good solubility, and they tend to stabilize the holes after exciton separation.18−21 The thiazole unit has been used as an electron-accepting compound based on the electron-withdrawing nitrogen of imine and its structural similarity to thiophene. Organic compounds containing a variety of thiazole Received: February 19, 2014 Revised: May 10, 2014 Published: May 10, 2014 11559
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derivatives, such as bithiazole, thiazolothiazole, and benzobisthiazole, have been introduced into organic light-emitting diodes (OLEDs), organic filed-effect transistors (OFETs), and OSCs.21 In OSCs, thiazole-based D-A alternative copolymer donors have shown promising performances, whereas thiazolebased small molecules have received less attention for use in OSCs;21 however, simple thiazole units may potentially be used in high-performance OSCs. This paper reports the synthesis of novel electron-donor molecules based on a D-π-conjugated linker-A structure that provides compact packing and intramolecular charge-transfer characteristics for the preparation of efficient organic solar cells. The donor molecules having a D-π-A structure and featuring an electron-rich triphenyl amine as the electron-donor unit, dicyanovinylene as the acceptor unit, and π-conjugated linkers composed of thienothiophene, thiophene, and thiazole units were synthesized. The π-conjugated linkers were carefully designed to provide a planar structure, an efficient conjugation length, and appropriate energy levels and to induce compact packing in the solid state. The novel thiazole-based compound could provide a highly efficient organic solar cell with a PCE of 6.2% (the highest PCE obtained was 6.37%) under AM 1.5G illumination with the intensity of 100 mW cm−2.
phenyl)thieno[3,2-b]thiophen-2-yl)thiazol-5-yl)methylene)malononitrile (denoted as DTTh and DTTz, respectively). Triphenylamine was used as the electron-donor group.20,21 The π-conjugated linker in DTTh was composed of the planar thieno[3,2-b]thiophene and thiophene rings, which increased the conjugated length, broadened the absorption spectrum, and induced compact crystal packing. A thiazole unit was introduced into DTTz to lower the HOMO level of the molecule and increase the VOC. Thieno[3,2-b]thiophene (1) and 2-bromothieno[3,2-b]thiophene (2) were synthesized according to the methods reported previously.27,28 4-(Diphenylamino)phenylboronic acid (3), 5-bromothiophene-2carbaldehyde (6), and bathocuproine (BCP) were purchased from Aldrich. 2-Bromothiazole-5-carbaldehyde (9) was purchased from Alfa Aesar. The synthetic route to DTTh and DTTz are outlined in Scheme 2 and are described in detail below. N,N-Diphenyl-4-(thieno[3,2-b]thiophene-2-yl)aniline (4). To a stirred solution of compound 2 (757.826 mg, 3.4585 mmol) and 4-(diphenylamino)phenylboronic acid (1 g, 3.4585 mmol) in anhydrous THF (60 mL) were added 2 N potassium carbonate aqueous solution (30 mL) and Pd(PPh3)4 (399.65 mg, 0.34585 mmol) under an argon atmosphere. The solution was stirred for 24 h at 79 °C in a round flask. The reaction mixture was added to water and stirred for 1 h. The mixture was then extracted with dichloromethane, washed with brine and water, and dried over anhydrous MgSO4. The residue was purified by column chromatography (hexane/dichloromethane = 4:1). Yield, 71%. 1H NMR (CDCl3, 400 MHz): δ 7.48 (d, 1H), 7.46 (d, 1H), 7.38 (s, 1H), 7.27 (m, 6H), 7.14 (d, 2H), 7.11 (d, 2H), 7.06 (m, 4H). 13C NMR (CDCl3, 300 MHz): δ 147.4, 130.5, 130.4, 128.4, 127.6, 125.5, 124.3, 123.6, 122.5, 120.7, 118.5, 115.5, 113.2. MS m/z = 383. 2-((5-Bromothiophene-2-yl)methylene)malononitrile (7). Malononitrile (1.037 g, 15.702 mmol) and β-alanine (28.1 mg, 0.3155 mmol) were added to a solution of compound 6 (1 g, 5.234 mmol) in dichloroethane (25 mL) and ethanol (25 mL) under argon atmosphere. The solution was stirred for 16 h at 85 °C in a round flask. After being cooled to room temperature, the reaction solution was separated by decantation, and the solvent was removed under reduced pressure. The crude product was purified by recrystallization from ethanol to give brown crystals (7). Yield, 72%. 1H NMR (CDCl3, 400 MHz): δ 7.72 (s, 1H), 7.47 (d, 1H), 7.21 (d, 1H). 13 C NMR (CDCl3, 300 MHz): δ 139.9, 133.1, 130.7, 126.6, 113.6, 113.0, 78.6. MS m/z = 240. 2-((2-Bromothiazol-6-yl)methylene)malononitrile (10). Malononitrile (1.03 g, 15.6 mmol) and β-alanine (28 mg, 0.3144 mmol) were added to a solution of compound 9 (1 g, 5.2 mmol) in dichloroethane (25 mL) and ethanol (25 mL) under an argon atmosphere. The solution was stirred for 16 h at 85 °C in a round flask. After being cooled to room temperature, the reaction solution was separated by decantation and the solvent was removed under reduced pressure. The crude product was purified by recrystallization from ethanol to give brown crystals (10). Yield, 68%. 1H NMR (CDCl3, 300 MHz): δ 8.08 (s, 1H), 7.91 (s, 1H). 13C NMR (CDCl3, 300 MHz): δ 152.5, 148.0, 146.7, 134.4, 112.6, 82.9. MS m/z = 241. 2-((5-(5-(4-(Diphenylamino)phenyl)thieno[3,2-b]thiophen-2-yl)thiophen-2-yl)methylene)malononitrile (8). Under an argon atmosphere, n-butyllithium (0.492 mL, 1.23 mmol) was added dropwise to the solution of compound 4 (393 mg, 1.025 mmol) in THF (30 mL) at −78 °C. After the
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