Engineering Thin Films of a Tetrabenzoporphyrin toward Efficient Charge-Carrier Transport: Selective Formation of a Brickwork Motif

Tetrabenzoporphyrin
(BP) is a p-type organic semiconductor characterized by the large,
rigid π-framework, excellent stability, and good photoabsorption
capability. These characteristics make BP and its derivatives prominent
active-layer components in organic electronic and optoelectronic devices.
However, the control of the solid-state arrangement of BP frameworks,
especially in solution-processed thin films, has not been intensively
explored, and charge-carrier mobilities observed in BP-based materials
have stayed relatively low as compared to those in the best organic
molecular semiconductors. This work concentrates on engineering the
solid-state packing of a BP derivative, 5,15-bis­(triisopropylsilyl)­ethynyltetrabenzoporphyrin
(TIPS-BP), toward achieving efficient charge-carrier transport in
its solution-processed thin films. The effort leads to the selective
formation of a brickwork packing that has two dimensionally extended
π-staking.
The maximum field-effect hole mobility in the resulting films reaches
1.1 cm2 V–1 s–1, which
is approximately 14 times higher than the record value for pristine
free-base BP (0.070 cm2 V–1 s–1). This achievement is enabled mainly through the optimization of
three factors; namely, deposition process, cast solvent, and self-assembled
monolayer that constitutes the dielectric surface. On the other hand,
polarized-light microscopy and grazing-incident wide-angle X-ray diffraction
analyses show that there remains some room for improvement in the
in-plane homogeneity of molecular alignment, suggesting even higher
charge-carrier mobilities can be obtained upon further optimization.
These results will provide a useful basis for the polymorph engineering
and morphology optimization in solution-processed organic molecular
semiconductors.