Charge-separation energy in films of π-conjugated organic molecules

We use inverse photoelectron spectroscopy (IPES) and ultraviolet photoelectron spectroscopy (UPS) to investigate unoccupied and occupied electronic states of five organic semiconductor materials: CuPc (copper phthalocyanine), PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride), α-6T (α-sexithiophene), α-NPD (N,N′-diphenyl-N,N′-bis(l-naphthyl)-l,l′ biphenyl-4,4′′ diamine), and Alq3 (tris(8-hydroxy-quinoline)aluminum). The transport gap, Et, is the difference between the highest occupied and lowest unoccupied molecular orbitals, measured via UPS and IPES. The charge separation energy, or exciton binding energy, is the difference between Et and the optical gap, Eopt, measured via absorption. Et-Eopt in these correlated materials ranges from 0.4 to1.4 eV.