Organic-​organic interfaces and unoccupied electronic states of thin films of perylene and naphthalene derivatives

Thin films of N,N′-Bis(benzyl)-3,4,9,10-perylenetetracarboxylic diimide (BPTCDI, Fig. 1b) and N,N′-Bis(benzyl)-1,4,5,8-naphthalenetetracarboxylic diimide (BNTCDI, Fig. 1d) were thermally deposited in UHV on 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA, Fig. 1a) and 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA, Fig. 1c) film surfaces, respectively, in order to form organic–organic interfaces so that molecules constituting the interfacing layers differ by the substituent group. The surface potential and the density of unoccupied electron states (DOUS) located 5–25 eV above the Fermi level (EF) were measured during the film deposition using an incident beam of low-energy electrons according to the total current electron spectroscopy (TCS) method. Analysis of the TCS data allowed us to assign the π( band located 5–7.5 eV above EF for all the four films under study and the higher located σ*1 and σ*2 bands and the splitting within them. In order to perform the analysis the molecules were hypothetically divided into benzene-like, conjugated and non-conjugated fragments that may individually contribute to the peaks in the DOUS bands. It was shown that a non-conjugated fragment would serve for decreasing of the energy corresponding to the σ*1 and σ*2 bands and the sub-bands within them while an addition of a benzene-like fragment would do the opposite. The BPTCDI/PTCDA and BNTCDI/NTCDA interfaces were found non-reacted and a 4.1±0.1 eV work function value for both BPTCDI and BNTCDI films was determined, which is about 0.25 eV lower than the work functions of the PTCDA and the NTCDA films.