Charge-separation energy in films of π-conjugated organic molecules

Hill, Ian G.; Kahn, Antoine; Soos, Z. G.; Pascal, Robert A.

doi:10.1016/s0009-2614(00)00882-4

Cited by 748 publications

(645 citation statements)

References 26 publications

Supporting

Mentioning

576

Contrasting

Unclassified

Order By: Relevance

“…P S is 0.41 eV less than the bulk value P = 1.82 eV [16]. The difference is consistent with experimental estimates [10] and corresponds to the surface correction c = 1 − P S /P = 0.23, where the value inferred [3] from UPS and IPES spectra was c ∼ 0.25. Figure 3 gives results for PTCDA layers of finite thickness.…”

Section: Resultssupporting

confidence: 87%

“…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.…”

Section: Transport Gapmentioning

confidence: 96%

“…We use PTCDA on Ag rather than Au because the Au(5d) levels interfere with UPS of a monolayer. E S t includes a 0.2 eV intramolecular vibrational contribution [3] that reduces the UPS/IPES gap. This correction is the same for all PTCDA films.…”

Section: Transport Gapmentioning

confidence: 99%

See 2 more Smart Citations

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

View full text Add to dashboard Cite

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.

show abstract

Section: Resultssupporting

confidence: 87%

Section: Transport Gapmentioning

confidence: 96%

See 1 more Smart Citation

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

View full text Add to dashboard Cite

show abstract

“…In contrast, measuring the energetic location of the LUMO is more challenging. The most common methods to determine the LUMO of a monolayer or thin-film are inverse photoemission (IPES) [11], two-photon photoemission [12,13], scanning tunneling microscopy [14], or measurement of the standard reduction potential in an electrochemical cell [15].…”

mentioning

confidence: 99%

“…Figure 2 illustrates three different ways often used to designate the HOMO-LUMO separation for Alq 3 . In the panel on the left, the LUMO and HOMO energies measured by IPES and UPS are indicated [11]. This is commonly referred to as the ''transport gap'' [16].…”

mentioning

confidence: 99%

The role of interfaces in organic spin valves revealed through spectroscopic and transport measurements

Drew

Szulczewski

Nuccio

et al. 2011

Physica Status Solidi (b)

View full text Add to dashboard Cite

Interfaces between dissimilar materials are critical to the performance of many electronic devices. In electrical devices where the active transport is through an organic semiconductor (OSC) layer there are metallic electrodes to supply and remove the charge carriers. At metal/molecule and molecule/metal interfaces the electrical contact is usually not Ohmic. As a result, in most metal-organic semiconductors-metal devices charge injection is mainly governed by tunneling across the barriers. When the metal electrodes are ferromagnetic the density of occupied and unoccupied electronic states is spindependent, which presents further subtleties to the electron transfer. In this article we summarize some of the important experimental results that have advanced our understanding of spin-polarized electron transfer across ferromagnetic metal (FM)/OSC interfaces. In particular, we highlight key spectroscopic studies that have revealed insights into the nature of the electronic structure between OSCs and FMs, in particular between Alq 3 and Co and Fe surfaces. We discuss the relationship between energy level diagrams for these systems and transport measurements made on spin-valve devices.

show abstract

Untitled

Jiang¹,

Österbacka²,

Korovyanko³

et al. 2002

Adv. Funct. Mater.

321

308

View full text Add to dashboard Cite

Using a variety of optical probe techniques we studied the steady state and transient dynamics of charged and neutral photoexcitations in thin films of poly‐3‐alkyl thiophene with regioregular order, which forms self‐assembled lamellae structures with increased interchain interaction, as well as regiorandom order that keeps a chain‐like morphology. In regiorandom polythiophene films we found that intrachain excitons with correlated photoinduced absorption and stimulated emission bands are the primary photoexcitations; they give rise to a moderately strong photoluminescence band, and long‐lived triplet excitons and intrachain charged polarons. In regioregular polythiophene films, on the contrary we found that the primary photoexcitations are excitons with much larger interchain component; this results in lack of stimulated emission, vanishing intersystem crossing, and a very weak photoluminescence band. The long‐lived photoexcitations in regioregular polythiophene films are interchain excitons and delocalized polarons (DP) within the lamellae, with very small relaxation energy. The characteristic properties of the DP species are thoroughly investigated as a function of the alkyl side group of the polymer backbone, film deposition conditions and solvents used, as well as at high hydrostatic pressure. The quantum interference between the low energy absorption band of the DP species and a series of photoinduced infrared active vibrations, which give rise to antiresonances that are superimposed on the electronic absorption band is studied and explained by a Fano‐type interference mechanism, using the amplitude mode model.

show abstract

Charge-separation energy in films of π-conjugated organic molecules

Cited by 748 publications

References 26 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

The role of interfaces in organic spin valves revealed through spectroscopic and transport measurements

Untitled

Contact Info

Product

Resources

About