Electronic structure of the organic semiconductor copper phthalocyanine and K-CuPc studied using photoemission spectroscopy

The changes of the electronic structure of copper phthalocyanine (CuPc) caused by the intercalation with potassium are studied using core-level and ultraviolet photoemission spectroscopy. The analysis of the valence-band spectra allows the estimation of the energy gap relevant for transport, which is substantially larger than the energy gap obtained using optical methods, showing that solid CuPc has to be regarded as a correlated material. Furthermore, our experiments indicate that there is structural (polaronic) relaxation of the CuPc molecules upon charging and that the lowest unoccupied molecular orbital of CuPc is more concentrated in the central part of the phthalocyanine molecules.