A. Kahn

Role of the deep-lying electronic states of MoO3 in the enhancement of hole-injection in organic thin films

The electronic structures of vacuum-deposited molybdenum trioxide (MoO3) and of a typical MoO3/hole transport material (HTM) interface are determined via ultraviolet and inverse photoelectron spectroscopy. Electron affinity and ionization energy of MoO3 are found to be 6.7 and 9.68 eV, more than 4 eV larger than generally assumed, leading to a revised interpretation of the role of MoO3 in hole injection in organic devices. The MoO3 films are strongly n-type. The electronic structure of the oxide/HTM interface shows that hole injection proceeds via electron extraction from the HTM highest occupied molecular orbital through the low-lying conduction band of MoO3.

Conjugated organic molecules on metal versus polymer electrodes: Demonstration of a key energy level alignment mechanism

Ultraviolet photoemission spectroscopy is used to determine the energy level alignment at interfaces between three electroactive conjugated organic molecular materials, i.e., N,N′-bis-(1-naphthyl)-N,N′-diphenyl1-1,1-biphenyl1-4,4′-diamine; para-sexiphenyl; pentacene, and two high work function electrode materials, i.e., gold and poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate). Although both electrode surfaces have a similar work function (∼5 eV), the hole injection barrier and the interfacial dipole barrier are found to be significantly smaller for all the interfaces formed on the polymer as compared to the metal. This important and very general result is linked to one of the basic mechanisms that control molecular level alignment at interfaces with metals, i.e., the reduction of the electronic surface dipole contribution to the metal work function by adsorbed molecules.

Organic molecular films on gold versus conducting polymer: Influence of injection barrier height and morphology on current–voltage characteristics

The current–voltage characteristics I(V) of model organic devices are studied under ultra-high-vacuum conditions. Active materials are N,N′-bis-(1-naphthyl)-N,N′-diphen-yl1-1,1-biphenyl1-4,4′-diamine (α-NPD) and pentacene, electrode materials are polycrystalline Au and the conductive polymer poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS). Despite a similar work function of electrode material surfaces (∼5 eV), hole injection from PEDOT/PSS is significantly more efficient than from Au, due to a smaller hole injection barrier. Hole injection characteristics from Au electrodes for devices made from α-NPD are independent of deposition sequence and substrate used. Pentacene devices exhibit serious asymmetries in that respect. These are caused by a strong dependence of morphology and preferred molecular orientation on the substrate for the crystalline material.

Electronic states at aluminum nitride (0001)-1×1 surfaces

We investigate the electronic structure of aluminum nitride (0001)-1×1 surfaces via direct and inverse photoemission spectroscopy. Bulk and surface sensitive measurements on clean surfaces and surfaces exposed to oxygen or cesium demonstrate the existence of filled and empty surface states which extend more than 1 eV beyond the valence- and conduction-band edges. The filled states are tentatively associated with Al dangling or back bonds. The measurement of the top of the valence band upon removal of the filled states leads to a determination of an electron affinity equal to 1.9±0.2 eV. The empty surface states are presumed to play a role in the pinning of the Fermi level in the upper part of the gap and are consistent with the anticipated metallicity of the surface.

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.

Measurement of interface potential change and space charge region across metal/organic/metal structures using Kelvin probe force microscopy

We report on high-resolution potential measurements across complete metal/organic molecular semiconductor/metal structures using Kelvin probe force microscopy in inert atmosphere. It is found that the potential distribution at the metal/organic interfaces is in agreement with an interfacial abrupt potential changes and the work function of the different metals. The potential distribution across the organic layer strongly depends on its purification. In pure Alq3 the potential profile is flat, while in nonpurified layers there is substantial potential bending probably due to the presence of deep traps. The effect of the measuring tip is calculated and discussed.

INVESTIGATION OF THE EARLY STAGES OF ZNSE EPITAXY ON GAAS(001) VIA SCANNING TUNNELING MICROSCOPY

We present a scanning tunneling microscopy (STM) study of the initial stages of ZnSe deposition on the GaAs(001)-(2×4) surface. The deposition of elemental Se and of ZnSe on the bare GaAs surface induces considerable atomic disorder attributed to the Se–As exchange reaction. The deposition of elemental Zn weakens the 2× periodicity of the surface but induces no apparent changes in the STM images of the As dimers. Comparison of STM images of submonolayers of ZnSe on GaAs with and without a Zn pretreatment suggests that Zn reduces the interaction of Se with the GaAs surface.