The transport gap of organic semiconductors studied using the combination of direct and inverse photoemission

Influence of film thickness and air exposure on the transport gap of manganese phthalocyanine

“…This value is close to that reported for CuPc, i.e. (0.5 ± 0.2) eV, 12 or (0.6 ± 0.4) eV. 11 We attribute the optical absorption peaks at 1.38 eV and 1.74 eV in Fig.…”

“…11,12 The optical band gap can thus be smaller by up to 1.4 eV than the transport band gap. 11 The occupied electronic states of MnPc were intensively studied by ultraviolet photoelectron spectroscopy (UPS).…”

“…These values should represent MnPc not affected by the substrate any more. For comparison, the values reported for one of the most investigate Pc molecule, CuPc, for which the 3d states of the central metal ion are well separate from the π states of the organic ligand, 12 are also given in the table. The much smaller transport gap of MnPc (1.2 ± 0.3) eV compared to CuPc (2.2 ± 0.3) eV is the result of additional electronic levels close to the Fermi level, 7 both in the HOMO and the LUMO region.…”

“…It is broad due to the convolution of the original LUMO with the Gaussian type experimental resolution of approximately 0.5 eV and was thus deconvoluted in order to reduce the influence of the instrumental broadening. 12 The inset of Fig. 1(b) exemplifies the onset determination for the spectra of a 20 nm thick film with backgrounds subtracted.…”

“…The difference between the HOMO and LUMO onsets determines the transport gap (E t ). 11,12 In the case of very thin films of 1 nm and 2 nm an extra Gaussian was used in the fit to compensate for the Co substrate signal at the Fermi level. Due to image potential contributions at the metal-organic contact in the nanometer range a reduction of the barrier height is expected.…”