A-D-A-Type Oligothiophenes Containing Benzothiadiazole Terminal Units for Small Molecule Organic Solar Cells

Abstract

AbstractThe electron-deficient, fused-heterocyclebenzo[c][1,2,5]thiadiazole (BTDA) is investigated as acceptor group in A-D-A-type oligothiophenes in order to correlate their relative acceptor strength with opto-electronic and photovoltaic properties. In this respect, two novel BDTA-capped oligothiopheneswere synthesized and characterized by optical and electrochemical measurements. They showed broad absorptions in the visible spectrum and HOMO-LUMO energies well suited for organic solar cells. The attachment of terminal BTDA acceptor units to the conjugated oligothiophene backbone resulted in a hypsochromic shift in UV-Vis absorption and larger band gap in comparison to previously reported analogous dicyanovinylene (DCV)-substituted oligothiophenes indicating that BDTA is a weaker acceptor than DCV. Vacuumprocessed m-i-p (metal-intrinsic-p-doped)-type bilayer solar cells using these co-oligomers as donor and C60 as acceptor gave moderate power conversion efficiencies of around 1.0%. Bulk-heterojunction (BHJ) solar cells prepared by solution-processing using fullerene PC61BM as acceptor generated slightly lower efficiencies of 0.9%, whichwere increased to 1.5% by using the higher fullerene PC71BM. It was found that the cell efficiencies were mostly limited by the low photocurrent densities due to moderate light absorption in the bilayer devices and low fill factors coming from inefficient charge transport in the solutionprocessed BHJ devices.