Efficient spirobifluorene-core electron-donor material for application in solution-processed organic solar cells

Efficient spirobifluorene-based organic small molecule (RTh-Sp-CF₃) was 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 was utilized as an electron-donor materials with well-known 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 accomplished an overall power conversion efficiency (PCE) of ∼2.12% with short circuit current (J_SC) of ∼8.42 mA/cm² 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-CF₃, which resulted from the improved absorption and electrochemical properties.