Electronic structures of unoccupied states in lithium phthalocyanine thin films of different polymorphs studied by IPES

Electronic structures of the unoccupied states in lithium phthalocyanine (LiPc) thin films of the x- and α-polymorphs were directly observed with inverse photoemission spectroscopy (IPES). The energy position of the lowest energy band and the lineshape of the second lowest energy band were different between the obtained IPE spectra for the two forms of the films. By comparing with energy levels in a LiPc molecule and its dimers calculated using the density functional method, we interpreted such differences as follows: the first band placed in lower energy for the α-form is due to more stabilized singly occupied molecular orbital (SOMO) and the observed splitting of the second feature for the x-form is lead from the removed degeneracy of 6eg levels caused by a large intermolecular interaction along the molecular stacking axis.