EN
The 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.