Energy level alignment at molecular semiconductor/GaAs(100) interfaces: Where is the LUMO?

The energy position of the lowest unoccupied molecular orbital (LUMO) in 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) and dimethyl-3,4,9,10-perylenetetracarboxilic diimide (DiMePTCDI) was determined by investigating the energy level alignment at molecular semiconductor/GaAs(100) interfaces. Interface dipoles are found to form at the interfaces of both organic materials and consequently, the vacuum level alignment rule does not hold. The interface dipoles vary as a function of the difference in electron affinities of substrate and organic layer and the HOMO-LUMO transport gap of PTCDA is determined to 2.44 ~ 2.55 eV. Taking into account the difference in the optical band gaps of PTCDA and DiMe-PTCDI the transport gap of DiMe-PTCDI is determined to 2.36 ~ 2.47 eV.