Understanding the Photovoltaic Behavior of A–D–A Molecular Semiconductors through a Permutation of End Groups

The facile synthesis
of a series of benzodithiophene (BDT)- and
indacenodithiophene (IDT)-based A–D–A oligomers with
different end groups is reported, and their properties are studied
by optical spectroscopy, electrochemistry, and density functional
theory calculations. The permutation of central and terminal units
tunes the optoelectronic properties and photovoltaic device characteristics
in a predictable way, aiding in the rational design of small molecule
semiconducting materials. Among the three rhodanine-derived terminal
groups, N-alkylthiazolonethione revealed the strongest
electron-withdrawing character, resulting in the lowest band gap,
the highest stability, and the best photovoltaic device performance.
The crystallographic analysis of two IDT derivatives yielded a highly
unusual three-dimensional packing of the conjugated backbone, which
is likely responsible for the remarkable photovoltaic performance
of such A–D–A semiconductors.