The Role of the Ionization Potential in Vacuum‐Level Alignment at Organic Semiconductor Interfaces

Low-density band-gap states in pentacene thin films probed with ultrahigh-sensitivity ultraviolet photoelectron spectroscopy

“…ClAlPc͑1 ML͒/ HOPG gives s = 4.92 eV, which is larger than threshold ionization potentials of Pn films ͑4.71-4.75 eV͒ with a standing molecular orientation. 6 As observed in a previous study, it was suggested that ϳ0.01 electrons per Pn molecule are transferred from the DOGS in the Pn overlayer to the HOPG and thereby the E F is located close to the highest occupied molecular orbital ͑HOMO͒ of Pn. 6 The UPS experiments were performed by using a new high-precision UPS apparatus with an electron energy analyzer ͑MBS A-1͒ and a monochromatic He I ␣ radiation ͑h = 21.218 eV͒ source ͑MBS L-1 and M-1͒.…”

“…Many studies using ultraviolet photoelectron spectroscopy ͑UPS͒ have been performed to clarify the ELA mechanism on weakly interacting organic interfaces, and have shown that the vacuum level ͑VL͒ misalignment and drastic variation of the Fermi level ͑E F ͒ in the organic band gap appear in various organic interface systems. [1][2][3][4][5][6][7][8] Because thermal equilibrium concept strictly requires that the Fermi levels of the organic film and the electrode coincide at the interface after contact, our understanding of the ELA mechanism still lacks consistency with this requirement. 5,6 The mystery on the ELA mechanism must be related to the density of gap states ͑DOGS͒ to control the E F .…”

“…The substrate spectrum, ClAlPc͑0.8 ML͒/HOPG, has a sharp ClAlPc HOMO peak with satellites due to hole-vibration coupling, indicating that the 0.8-ML ClAlPc consists of large well-oriented 1 ML domains with Cl atoms protruding outside. 6 Upon 0.8-ML Pn deposition, a broad HOMO of Pn appears on the low-E B side of the ClAlPc HOMO and the ClAlPc HOMO stays at the same E B . These results confirm that ͑i͒ the broad Pn HOMO originates from the HOMO of standing Pn 6,11 and ͑ii͒ the standing Pn molecules interact very weakly with the underlying ClAlPc.…”

“…As a sample system in which gap states are expected, we selected a highly oriented pyrolytic graphite ͑HOPG͒ surface modified with a monolayer ͑ML͒ of chloroaluminum phthalocyanine ͑ClAlPc͒, as a high-work function ͑ s ͒ substrate, 5,6 and Pn as an organic overlayer. ClAlPc͑1 ML͒/ HOPG gives s = 4.92 eV, which is larger than threshold ionization potentials of Pn films ͑4.71-4.75 eV͒ with a standing molecular orientation.…”

Charge equilibration and potential steps in organic semiconductor multilayers

“…Consequently, a detailed understanding of interface properties, energy level alignment in particular, is critical for guiding the improvement of device performance. The energy level alignment at all-organic and hybrid organic-inorganic interfaces has been the subject of extensive research efforts in recent years [3][4][5][6][7][8][9][10][11][12].…”