Evolution of the electronic structure of alkali metal-doped copper-phthalocyanine (CuPc) on different metal substrates

The evolutions of the electronic structure in Cs-doped copper-phthalocyanine (CuPc) on different metal substrates have been investigated with photoemission spectroscopy. As the Cs doping ratio increases, the Fermi level of the system starts to shift towards the lowest unoccupied molecular orbital (LUMO) of CuPc, and then becomes saturated, as it aligns with the LUMO. The shift of all the energy levels has a linear dependence on the logarithm of the Cs doping ratio, with a slope of about 0.13 eV. By comparing the results of the samples deposited on different metal substrates, we found out that this slope indicates the broadening of the LUMO induced by alkali metal doping and is independent of the workfunction of the metal substrate.