Organic Semiconducting Alloys with Tunable Energy Levels

Continuous
band structure tuning, e.g., doping with different atoms,
is one of the most important features of inorganic semiconductors.
However, this can hardly be realized in organic semicondutors. Here,
we report the first example of fine-tuning organic semiconductor band
structures by alloying structurally similar derivatives into one single
phase. By incorporating halogen atoms on different positions of the
backbone, BDOPV derivatives with complementary intramolecular or intermolecular
charge distributions were obtained. To maximize the Coloumbic attractive
interactions and minimize repulsive interactions, they form antiparallel
cofacial stacking in monocomponent or in alloy single crystals, resulting
in efficient π orbital overlap. Benefiting from self-assembly
induced solid state “olefin metathesis” reaction, it
was observed, for the first time, that three BDOPV derivatives cocrystallized
in one single crystal. Molecules with different energy levels serve
like the dopants in inorganic semiconductors. Consequently, as the
total number of halogen atoms increased, highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels
of the alloy single crystals decreased monotonously in the range from
−5.94 to −6.96 eV and −4.19 to −4.48 eV,
respectively.