Research paper Efficient spirobifluorene-core electron-donor material for application in solution-processed organic solar cells M. Nazim a , Sadia Ameen a , M. Shaheer Akhtar b , Hyung Shik Shin a,⇑ a Energy Materials & Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Chonbuk National University, Jeonju 561-756, Republic of Korea b New & Renewable Energy Material Development Center (NewREC), Chonbuk National University, Jeonbuk, Republic of Korea article info Article history: Received 24 June 2016 In final form 25 September 2016 Available online 28 September 2016 Keywords: 3,5-Bis (trifluoromethyl) benzene Electrochemical properties Spirobifluorene Small molecule Organic solar cells abstract Efficient spirobifluorene-based organic small molecule (RTh-Sp-CF 3 ) was synthesized in a simple manner via Suzuki coupling reaction containing an alkyl bithiophene as donor and 3,5-bis (trifluoromethyl) ben- zene as acceptor unit. The spirobifluorene-based small molecule was utilized as an electron-donor mate- rials with well-known electron-acceptor material, phenyl-C 61 -butyric acid methyl ester (PC 61 BM) in the solution-processed small molecule organic solar cells (SMOSCs). The incorporation of 3,5-bis (trifluo- romethyl) benzene unit as electron-acceptor has significantly tuned the energy levels of small molecule and obtained the HOMO and LUMO energy levels of 5.35 eV and 3.92 eV, respectively. SMOSCs fabri- cated with RTh-Sp-CF 3 accomplished an overall power conversion efficiency (PCE) of 2.12% with short circuit current (J SC ) of 8.42 mA/cm 2 and the open-circuit voltage (V OC ) of 0.66 V. The reasonable J SC and V OC of devices might be attributed to the presence of strong electron-withdrawing fluorine units in RTh- Sp-CF 3 , which resulted from the improved absorption and electrochemical properties. Ó 2016 Elsevier B.V. All rights reserved. 1. Introduction Organic solar cells especially, bulk-heterojunction organic solar cells have owned considerable interest, both academically and industrially due to their versatile merits of low cost, large-scale fabrication, flexibility and diversity of electron-donors [1–4]. Apart from other organic solar cells, the small molecule organic solar cells (SMOSCs) have been growing as an alternative to polymer counterparts in solution-processed bulk-heterojunction (BHJ) owing to their simple molecular structures, easy synthesis and purification techniques [5–7]. Recently, SMOSCs have encountered as highly promising materials for solar cell application, after reach- ing the highest power conversion efficiencies (PCEs) over 9% [8,9]. For the further improvement in PCEs, it is necessary to design and synthesize new effective small organic molecules for SMOSCs. In this regards, the incorporation of the electron-donor (D) and electron-acceptor (A) units in organic small molecule plays a cru- cial role for the exciton-formation and its diffusion toward D-A interface which affects charge-transport properties [10–13]. In general, the fabrication of OSCs is carried out by utilizing fullerene derivatives especially, [6,6]-phenyl-C 61 butyric acid methyl ester (PC 61 BM) or [6,6]-phenyl-C 71 butyric acid methyl ester (PC 71 BM) as an electron-acceptor materials because of their good electron accepting and electron-transporting ability. PCBM molecules are easily blended with electron-donor materials and produce the excellent film morphologies for excellent charge dissociation and transport. In spites of these good properties, it is difficult to tune the optical or absorption properties and energy level of PCBM molecules by simple chemical modification. Recently, Zhan and Yao described the utilization of non-fullerene organic acceptors in place of PCBM derivatives for the development of high efficiency OSCs [14]. Even though, non-fullerene derivatives show excellent optoelectronic properties, but they do not blend properly with donor molecules and create aggregates of up to hundreds of nanometer over photoactive film which is larger in size to the exciton-diffusion length. As a result, the excitons might be quenched before they reach at the donor-acceptor (D-A) interface and fails to dissociate the excitons [15,16]. On the other hand, PC 61 BM or PC 71 BM is easily blended with donor materials, and forms nanoscale aggregates which are similar in size to the exciton-diffusion length [17]. However, the performances of SMOSCs depend on various fabrication parameters such as thick- ness of films, the composition ratio of donor/acceptor, and anneal- ing time as well as annealing temperature of devices [18–20]. Oligothiophenes have recently been employed as organic electron-donors (D) unit owing to well-defined and planar struc- ture, good solubility, and high electron-density for the designing http://dx.doi.org/10.1016/j.cplett.2016.09.065 0009-2614/Ó 2016 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: hsshin@jbnu.ac.kr (H.S. Shin). Chemical Physics Letters 663 (2016) 137–144 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett