Intrinsic intermolecular photoinduced charge separation in organic radical semiconductors.

Organic radicals based on tris(2,4,6-trichlorophenyl)methyl (TTM) radicals show efficient photoluminescence from excitons in the spin-doublet manifold, but their potential in charge photogeneration remains unexplored. Here we report that when TTMs are in contact, photoexcitation generates TTM anion-TTM cation pairs. These can decay radiatively or be fully separated under an electric field bias. We use a triphenyl-substituted TTM (P3TTM) in which the phenyl end groups enhance intermolecular interactions. In dilute (5 wt%) films in a wide-energy-gap organic semiconductor host, we observe prompt photoluminescence from the excited radical at 645 nm, and a delayed component, beyond 1 μs, at 800 nm due to recombination of P3TTM anion-cation pairs. Measurements of photocurrent made with diode structures with 100% P3TTM showed close-to-unity charge collection efficiency in reverse bias. We have found 'homojunction' intermolecular charge separation, made possible when the extra energy for double occupancy of the non-bonding radical level on the anion is lower than the energy of the doublet exciton. This opens possibilities for light harvesting using single-material molecular semiconductors.