Rational Control of Packing Arrangements in Organic Semiconducting Materials toward High-Performance Optoelectronics

Organic semiconducting materials have sparked a great deal of interest because of their structural versatility, lightweight, mechanical flexibility, as well as low temperature and large area fabrication, opening up possibilities for the development of next-generation electronic devices. Packing arrangements of organic semiconducting materials influence significantly the optoelectronic performance by alteration of electronic couplings, band structures, and exciton behaviors. The packing structures of small-molecule organic semiconductors can be typically classified into herringbone, slipped, and brickwork motifs. The preferred packing arrangement depends on the steric hindrance driven by the molecular structure and the weight of contribution of each interaction term, which are closely associated with the unpredictable and uncontrollable process of crystal nucleation and growth, involving lots of multiple variables such as the weak and subtle intramolecular or intermolecular interactions in organic materials. Therefore, it remains a long-standing challenge to tailor precisely the packing arrangements for high-performance or multifunctional organic semiconducting materials. In addition, the in-depth relationship between packing arrangements and optoelectronic properties is far from clear, preventing the development of high-performance organic optoelectronic materials.

Herein, we summarize our recent progress on the control of packing arrangements of organic semiconducting materials toward high-performance optoelectronics, shedding light on the structure–property relationship. First, we discuss the functionalization at the conjugated backbone of molecular materials to enhance carbon/hydrogen (C/H) ratios, constructing more dense herringbone or slipped packing structures with superior carrier mobilities. Next, we present the regulation of packing arrangements of organic semiconductors based on the same molecular structures, namely, control of the crystal polymorph. There is a very small energy gap between the highest occupied molecular orbital (HOMO) and HOMO–1 for C6-DBTDT; thus, the electronic couplings between (HOMO–1)s or along different directions have significant impacts on the charge transport behaviors. Finally, we demonstrate the role of the second component in the packing arrangements of organic optoelectronic materials. By nonstoichiometric ratio molecular doping, we have tailored the packing modes from traditional herringbone packing to face-to-face columnar stack with sufficient delocalization of radicals, showing acid-responsive high conductivity for one-dimensional (1D) organic nanomaterials. By stoichiometric ratio cocrystal engineering, we have achieved halogen-bonded or hydrogen-bonded cocrystal materials with different packing motifs or modification proportions. Short intermolecular contacts in a segregated-stack material give rise to larger radiative decay selectivity, accounting for an enhanced amplified spontaneous emission property. A cocrystal material with 2:1 modification not only exhibits stronger electronic couplings but also shows an extended distance between molecules, possessing an improved carrier mobility by 4 orders of magnitude relative to the single-component material. We believe that the rational control of packing arrangements of organic materials will open up possibilities for the development of high-performance optoelectronics.