Halogenation of a nonplanar molecular semiconductor to tune energy levels and bandgaps for electron transport

Though peripheral halogen substitution is a known strategy to lower the lowest unoccupied (LUMO) and highest occupied (HOMO) molecular orbital energy levels of planar molecular semiconductors, this strategy has not been explored in conformationally contorted systems. We demonstrate that substitution of peripheral hydrogens with fluorine and chlorine can effectively lower the energy levels of contorted hexabenzocoronene (cHBC) despite its nonplanar conformation. The HOMO energy level lowers comparably with fluorine and chlorine substitution. Due to chlorines ability to accommodate more electron density than fluorine, chlorination lowers the LUMO energy level more effectively compared to fluorination (31-60 meV/F versus 53-83 meV/Cl), resulting in a narrowing of the optical bandgap. We find the preference for electron transport to increase with increasing halogenation of cHBC. As an example, thin-film transistors fabricated with 8F-8Cl-cHBC demonstrated electron mobilities as high as 10^-2 cm²/(V s) and solar cells with 8F-8Cl-cHBC and poly(3-hexylthiophene), P3HT, showed power-conversion efficiencies as high as 1.2%.