A solution-processable D–A–D small molecule based on isoindigo for organic solar cells Miao Yang • Xuewen Chen • Yingping Zou • Chunyue Pan • Bo Liu • Hong Zhong Received: 28 June 2012 / Accepted: 22 August 2012 Ó Springer Science+Business Media, LLC 2012 Abstract A new linear D–A–D organic small molecule (M1), with triphenylamine as electron donor (D) unit and isoindigo (ID) as electron acceptor (A) unit, was synthe- sized by Stille coupling reactions. It exhibits broad and strong absorption (300–700 nm), a relatively low HOMO energy level (-5.30 eV), low band gap (1.69 eV), and moderate hole mobility (2.49910 -4 cm 2 /Vs). Solution- processed small molecule bulk-heterojunction solar cells based on M1: PC 61 BM (1:3, w/w) blend film exhibits a power conversion efficiency of 0.84 % with an open-circuit voltage (V oc ) of 0.78 V, under the illumination of AM1.5, 100 mW/cm 2 . Introduction Bulk heterojunction (BHJ) organic solar cells (OSCs) have generated great interest because of their unique features such as easy fabrication, low cost, lightweight, and flexi- bility [1–4]. In the past few years, great progress in BHJ polymer solar cells (PSCs) has been witnessed [5–8]; power conversion efficiencies (PCEs) of higher than 8 % have recently been achieved from PSCs based on blends of polymer donors and fullerene acceptors [9, 10]. Recently, solution-processed small molecule OSCs have also attrac- ted great attention because of small molecule donor materials having advantages over polymers, such as well- defined molecular structure, definite molecular weight, and high purity without batch-to-batch variations [11–13]. To date, the highest published PCE of solution-processed BHJ OSCs based on small molecular donor materials are over 6%[14]. Triphenylamine (TPA) has been regarded as a promising candidate for organic photovoltaic materials because of its good electron-donating and high hole-transporting capa- bilities [15]. TPA-based small molecules, including linear D–A–D small molecules with TPA as the terminal group and star-shaped molecules with TPA as core have been widely investigated for application in OSCs [16–24]. Li and Zhan et al. [18, 21–24] have reported a series of solution-processable TPA-based small molecules for high performance OSCs. Recently, they have reported a TPA- based linear D–A–D small molecule with thiazolothiazole (TT) as acceptor unit giving PCE up to 3.73 % [24], which is among the highest reported for solution-processed BHJ OSCs based on small molecules. Isoindigo (ID) has symmetrical lactam structure with strong electron-withdrawing character. Recently, isoindi- go-based conjugated polymers have demonstrated high mobilities for organic field effect transistors [25]; PSCs from isoindigo-based conjugated polymers have shown high photovoltaic performance [26–29]; and a PCE up to 6.3 % has been reported [28]. These results indicate the great potential of ID as electron-accepting candidate for high performance OSCs. However, to our best knowledge, there has been only onearticle on ID-based small molecules as electron donors in small molecule BHJ OSCs. Reynolds et al. [30] first reported two ID-based oligothiophenes for BHJ OSCs, and a PCE up to 1.76 % has been achieved. In this study, we designed and synthesized a new ID-based D–A–D small molecule with TPA as donor (D) unit and isoindigo as acceptor (A) unit (M1, M. Yang  X. Chen  Y. Zou (&)  C. Pan  H. Zhong College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, People’s Republic of China e-mail: yingpingzou@csu.edu.cn B. Liu State Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083, People’s Republic of China 123 J Mater Sci DOI 10.1007/s10853-012-6831-2