Doping of an organic molecular semiconductor by substitutional cocrystallization with a molecular n-dopant

Dopants for organic molecular semiconductors that yield immobile dopant ions are necessary for the creation of stable molecular semiconductor p–n junctions, the basis for almost all traditional inorganic semiconductor devices. We present evidence for the substitutional cocrystallization of tris(4-nitrophenyl)methyl radical (1) with small amounts of tris4-(dimethylamino)phenylmethyl radical (2), resulting in n-doped 1 with immobile 2+ counterions. Cyclic voltammetry indicates that electron transfer from 2 to 1 is favored by 0.51 eV. The powder X-ray diffraction patterns of pure 1 and 1 doped with 2 are very similar, indicating substitutional cocrystallization. The electrical conductivity of doped 1 increases with increasing concentration of 2, and the conductivity is constant over time. Variable-temperature conductivity measurements of 1 doped with 2% and 5% 2 indicate that the activation energy of conduction is 0.32 eV at both dopant concentrations.