“Roller-Wheel”-Type Pt-Containing Small Molecules and the Impact of “Rollers” on Material Crystallinity, Electronic Properties, and Solar Cell Performance

We
report the synthesis, characterization, and detailed comparison
of a series of novel Pt-bisacetylide containing conjugated small molecules
possessing an unconventional “roller-wheel” shaped structure
that is distinctly different from the “dumbbell” designs
in traditional Pt-bisacetylide containing conjugated polymers and
small molecules. The relationships between the chemical nature and
length of the “rollers” and the electronic and physical
properties of the materials are carefully studied by steady-state
spectroscopy, cyclic voltammetry, differential scanning calorimetry,
single-crystal X-ray diffraction, transient absorption spectroscopy,
theoretical calculation, and device application. It was revealed that
if the roller are long enough, these molecules can “slip-stack”
in the solid state, leading to high crystallinity and charge mobility.
Organic solar cells were fabricated and showed power conversion efficiencies
up to 5.9%, out-performing all existing Pt-containing materials. The
device performance was also found to be sensitive to optimization
conditions and blend morphologies, which are a result of the intricate
interplay among materials crystallinity, phase separation, and the
relative positions of the lowest singlet and triplet excited states.