Direct and inverse photoemission spectroscopy studies of potassium intercalated films of two organic semiconductors

A combined direct and inverse photoemission spectroscopy study of the occupied and unoccupied states of the organic semiconductors ZnPc and α-[N,N^′-diphenyl-N,N^′-bis(1-naphthyl)-1, 1^′-biphenyl-4,4^″ diamine] in the pristine and reduced state is presented. The splitting of the lowest unoccupied molecular orbital observed upon potassium intercalation leads to an evaluation of the size of correlation effects in both molecular systems. As expected, the Fermi level is found to shift towards the vacuum level upon intercalation. However, the results clearly demonstrate that the Fermi level in potassium intercalated organic semiconductors cannot a priori be assumed to be pinned at the onset of the lowest unoccupied molecular orbital in all cases.