Spirobifluorene-core electron-donor material for bulk-heterojunction solar cells

An efficient spirobifluorene-based organic small molecule (RTh-Sp-CF3) is synthesized in a simple manner via Suzuki coupling reaction containing an alkyl bithiophene as donor and 3,5-bis (trifluoromethyl) benzene as acceptor unit. The spirobifluorene-based small molecule is utilized as an electron-donor materials with wellknown electron-acceptor material, phenyl-C61-butyric acid methyl ester (PC61BM) in the solution-processed small molecule organic solar cells (SMOSCs). The incorporation of 3,5-bis (trifluoromethyl) 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 fabricated with RTh-Sp-CF₃ accomplishes an overall power conversion efficiency (PCE) of ~2.12% with short circuit current (JSC) of ~8.42 mA/cm² and the open-circuit voltage (VOC) of ~0.66 V. The reasonable JSC and VOC of devices might be attributed to the presence of strong electron-withdrawing fluorine units in RTh-Sp-CF₃, which results from the improved absorption and electrochemical properties.