www.advmat.de www.MaterialsViews.com COMMUNICATION © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 1554 Adv. Mater. 2011, 23, 1554–1557 Huixia Shang, Haijun Fan, Yao Liu, Wenping Hu,* Yongfang Li,* and Xiaowei Zhan* This paper is dedicated to Prof. Daoben Zhu for his 70th birthday A Solution-Processable Star-Shaped Molecule for High-Performance Organic Solar Cells Organic solar cells (OSCs) exhibit tremendous potential in the world’s energy strategy due to their predominant advan- tages such as low cost, light weight, and large-area fabrication on flexible substrates. [1–4] Small molecule semiconductors for bulk heterojunction (BHJ) OSCs are attractive because of their advantages over their polymer counterparts, which include well- defined molecular structure, definite molecular weight, and high purity without batch to batch variations. [5–7] In recent years, a great amount of effort has been dedicated to develop this class of materials, including dendritic oligothiophenes, [8–10] star- or X-shaped molecules, [11–15] linear analogs with donor–acceptor– donor (D–A–D) structures, [16–21] fused polycyclic arene, [22] and other organic dyes. [23–25] However, the highest power conver- sion efficiencies (PCEs) reported for small molecule/fullerene derivative BHJ OSCs were only ≈3–4.4%, [5–7] lower than that ( ≈6–8%) of polymer/fullerene derivative BHJ solar cells. [1–4] The lower efficiency of small molecul BHJ OSCs became the big- gest hindrance to their application. Triphenylamine (TPA) has been regarded as a promising unit for efficient photovoltaic materials due to its good electron- donating and high hole-transporting capabilities. [26] Benefiting from its special propeller starburst molecular structure, amor- phous materials with isotropic optical and charge-transporting properties could be expected when combining TPA with linear π-conjugated systems. Solution-processable TPA-based small molecules have been widely investigated for application in OSCs with PCEs below 3%. [12–15,27–29] On the other hand, solution- processed BHJ OSCs based on oligothiophenes exhibited PCEs as high as 3.7%. [8–11,30] Here, we report the synthesis and characterization of a new 3D, star-shaped, D–A–D organic small molecule with TPA as the core and donor unit, benzothiadiazole as the bridge and acceptor unit, and oligothiophene as the arm and donor unit (S(TPA-BT-HTT), where S is star, BT is benzo- thiadiazole, and HTT is hexylterthiophene; Scheme 1 ). Solution- processed OSCs based on a blend of S(TPA-BT-HTT) and [6,6]-phenyl-C 71 -butyric-acid-methyl-ester (PC 71 BM) afforded a PCE as high as 4.3% without any post-treatments, e.g., thermal annealing, solvent annealing, or additive addition. The synthetic route for compound S(TPA-BT-HTT) is shown in Scheme 1. The intermediate compound 4-bromo-7-[5’’- n- hexyl-(2,2 ′;5 ′,2 ′′-terthiophene)-5-yl]-benzo[c][1,2,5]thiadiazole ( 3) was synthesized by a Stille coupling reaction of compound 4,7-dibromobenzo[c][1,2,5]thiadiazole ( 1) with 5-( n-hexyl)-5 ′′- (tributylstannyl)-2,2 ′;5 ′,2 ′′-terthiophene ( 2). A Stille coupling reaction of 3 with tris(4-(tributylstannyl)phenyl)amine ( 4) afforded S(TPA-BT-HTT). The terthiophene unit was used to extend the conjugated length in order to broaden the absorption spectrum. The electron-withdrawing building block, benzothia- diazole, was introduced to lower the highest occupied molecular orbital (HOMO) level (and in the end increase the open circuit voltage of the OSC) and to extend absorption via intramolecular D–A charge transfer. S(TPA-BT-HTT) was fully characterized using spectroscopic methods and elemental analysis. S(TPA- BT-HTT) is soluble in common organic solvents such as chlo- robenzene, chloroform, and THF due to the solubilizing three n-hexyl substituents. The thermal properties of S(TPA-BT-HTT) were investigated by thermogravimetric analysis (TGA) and dif- ferential scanning calorimetry (DSC). This star-shaped molecule exhibits excellent thermal stability with decomposition tempera- ture (5% weight loss) at ca. 420 °C (Figure S1, Supporting Infor- mation). The DSC traces for this compound do not show any peaks indicative of the phase transitions from room temperature to 250 °C, suggesting that it could be amorphous. Figure 1a shows the normalized UV-vis absorption spectra of S(TPA-BT-HTT) in dilute CHCl 3 solution and in a thin film. S(TPA-BT-HTT) in solution exhibits strong absorption, with three peaks at 328, 380, and 512 nm; the extinction coefficient is as large as 8.4 × 10 4 L mol -1 cm -1 (at 512 nm). The absorption maximum of 512 nm is red-shifted by 83 nm relative to that of tris[4-(5 ′′-hexyl-5-terthienyl)phenyl]amine, S(TPA-HTT), which has a similar skeleton structure without the benzothiadiazole acceptor unit. [31] The significant red-shift of the absorption is due to intramolecular D–A charge transfer in S(TPA-BT-HTT). A thin film of S(TPA-BT-HTT) shows significant absorption throughout the visible region (300–700 nm), which is red- shifted 26 nm relative to that in solution. The red-shift indicates that self-organization behavior exists somewhat in the film. The optical band gap estimated from the absorption edge of the thin film is 1.9 eV, similar to that of the widely used electron donor regioregular poly(3-hexylthiophene) (P3HT). DOI: 10.1002/adma.201004445 Dr. H. Shang, H. Fan, Y. Liu, Prof. W. Hu, Prof. Y. Li, Prof. X. Zhan Beijing National Laboratory for Molecular Sciences Key Laboratory of Organic Solids Institute of Chemistry Chinese Academy of Sciences Beijing 100190, P. R. China E-mail: xwzhan@iccas.ac.cn; liyf@iccas.ac.cn; huwp@iccas.ac.cn Dr. H. Shang, H. Fan, Y. Liu Graduate University of Chinese Academy of Sciences Beijing 100049, P. R. China Dr. H. Shang Department of Chemistry Handan College Handan 056005, P. R. China