A molecular breakwater-like tetrapod for organic solar cells

Abstract

We report the synthesis and characterization of a tetrapodal breakwater-like small molecule, SO, containing a tetraphenylsilane core and four cyanoester functionalized terthiophene arms. SO possesses a deep lying HOMO energy level of −5.2 eV and a narrow bandgap of 1.9 eV. Absorption, X-ray scattering and differential scanning calorimetry (DSC) measurements indicate crystalline nature of this compound but very slow crystallization kinetics. Solar cells employing SO and phenyl-C₆₁-butyric acid methyl ester (PCBM) were fabricated and evaluated. Relatively low performance was obtained mainly due to the lack of optimal phase separation under various processing conditions including thermal annealing, slow-cooling and solvent annealing. Addition of poly(thienylene vinylene) (PTV), poly(3-hexylthiophene) (P3HT) and a platinum-containing low bandgap conjugated polymer Pt–BODIPY, into the SO/PCBM blend was found to induce device favorable phase separation and the polymers were found to act as the primary hole conductor. Such ternary blend devices showed cooperatively improved performances over binary devices employing either SO or the individual conjugated polymer alone.