Molecular level alignment at organic semiconductor-metal interfaces

Hill, Ian G.; Rajagopal, A.; Kahn, Antoine; Hu, Ying

doi:10.1063/1.121940

Cited by 447 publications

(297 citation statements)

References 15 publications

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“…show no dependence on dipoles at the metalorganic interface, which have opposite signs for PTCDA on Ag and Au [8]. The dipole at the PTCDA/Ag interface corresponds to electron transfer from the metal to interface molecules, which gives rise to the filled gap states at −0.6 and −1.8 eV on the 4Å UPS spectrum.…”

Section: Transport Gapmentioning

confidence: 99%

Electronic polarization at surfaces and thin films of organic molecular crystals: PTCDA

Tsiper

Soos

Gao

et al. 2002

Chemical Physics Letters

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270

166

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The electronic polarization energies, P = P+ + P−, of a PTCDA (perylenetetracarboxylic acid dianhydride) cation and anion in a crystalline thin film on a metallic substrate are computed and compared with measurements of the PTCDA transport gap on gold and silver. Both experiments and theory show that P is 500 meV larger in a PTCDA monolayer than in 50Å films. Electronic polarization in systems with surfaces and interfaces are obtained self-consistently in terms of charge redistribution within molecules. I. TRANSPORT GAPThe electronic structure of organic molecular crystals is strikingly different from the conventional inorganic semiconductors, such as Si, in that the electronic polarization of the dielectric medium by charge carriers constitutes a major effect, with energy scale greater than transfer integrals or temperature [1,2]. The transport gap E t for creating a separated electron-hole pair has a substantial (1-2 eV) polarization energy contribution [3] and exceeds the optical gap by ∼ 1 eV. Limited overlap rationalizes the modest mobilities of organic molecular solids. Devices such as light-emitting diodes, thin film transistors, or photovoltaic cells require charge transport and are consequently based on thin films, quite often deposited on metallic substrates [4,5]. Organic electronics relies heavily on controlling films with monolayer precision, on forming structures with several thin films, and on characterizing the interfaces. The positions of transport states and mechanisms for charge injection are among the outstanding issues for exploiting organic devices. We focus here on the electronic polarization of crystalline thin films near surfaces and interfaces. We find that electronic polarization, and hence E t , in a prototypical organic crystal is significantly different at a free surface, near a metal-organic interface, in thin organic layers, and in the bulk.Weak intermolecular forces characterize organic molecular crystals, whose electronic and vibrational spectra are readily related to gas-phase transitions [1,2]. Due to small transfer integrals, charge carriers are molecular ions embedded in the lattice of neutral molecules. The transport gap E t in the crystal is derived from the charge gap for electron transfer in the gas phase, I(g) − A(g), which is the difference between the ionization potential and the electron affinity. But crystals have electrostatic interactions even in the limit of no overlap, and charge carriers are surrounded by self-consistent polarization clouds. In contrast to polaronic effects, electronic polarization is instantaneous and directly affects the positions of energy levels. Formation of polarization clouds is associated with stabilization energy P + for cations (the "holes") and P − for anions (the "electrons"). The combination P = P + + P − occurs in E t = I(g) − A(g) − P . Since Coulomb interactions are long-ranged, polarization clouds extend over many lattice constants and P depends on the proximity to surfaces and interfaces.

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Section: Transport Gapmentioning

confidence: 99%

Electronic polarization at surfaces and thin films of organic molecular crystals: PTCDA

Tsiper

Soos

Gao

et al. 2002

Chemical Physics Letters

Self Cite

270

166

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“…1,2 These interface level alignments have been widely investigated in the last decade: since the SchottkyMott limit (where use of the vacuum level alignment is made) was disproved 3,4 many different mechanisms have been proposed to explain the barrier formation at MO interfaces: chemical reactions and the formation of gap states in the organic gap; 5-8 orientation of molecular dipoles; 9,10 or compression of the metal electron tails at the MO interface due to the Pauli repulsion. 7,[11][12][13] It has also been suggested that the tendency of the charge neutrality level (CNL) of the organic material to align with the interface Fermi level 14,15 plays also an important role; this mechanism is associated with the induced density of interface states (IDIS) and the charge transfer between the organic and the metal.…”

Section: Introductionmentioning

confidence: 99%

C6H6/Au(111): Interface dipoles, band alignment, charging energy, and van der Waals interaction

Abad

Dappe

Martínez

et al. 2011

The Journal of Chemical Physics

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We analyze the benzene/Au(111) interface taking into account charging energy effects to properly describe the electronic structure of the interface and van der Waals interactions to obtain the adsorption energy and geometry. We also analyze the interface dipoles and discuss the barrier formation as a function of the metal work-function. We interpret our DFT calculations within the induced density of interface states (IDIS) model. Our results compare well with experimental and other theoretical results, showing that the dipole formation of these interfaces is due to the charge transfer between the metal and benzene, as described in the IDIS model.

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“…Since the Schottky-Mott model was disproved [11,12] by the observation of interface dipoles, several mechanisms have been proposed to explain the energy level alignment at organic interfaces: chemical reaction and the formation of gap states in the organic material [13]; orientation of molecular dipoles [14]; or compression of the metal electron tails at the MO interface due to Pauli repulsion [15,16]. Recently, some of us suggested [17] that an additional important mechanism is the tendency of the charge neutrality level (CNL) of the organic material to align with the metal Fermi level (at MO interfaces) or the CNL of the other organic material (at heterojunctions).…”

mentioning

confidence: 99%

Barrier Formation at Organic Interfaces in a Cu(100)-benzenethiolate-pentacene Heterostructure

et al. 2008

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Molecular level alignment at organic semiconductor-metal interfaces

Cited by 447 publications

References 15 publications

Electronic polarization at surfaces and thin films of organic molecular crystals: PTCDA

Electronic polarization at surfaces and thin films of organic molecular crystals: PTCDA

C6H6/Au(111): Interface dipoles, band alignment, charging energy, and van der Waals interaction

Barrier Formation at Organic Interfaces in a Cu(100)-benzenethiolate-pentacene Heterostructure

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