Formation of the Interfacial Dipole at Organic‐Organic Interfaces: C<sub>60</sub>/Polymer Interfaces

Osikowicz, Wojciech; Jong, M. P. de; Salaneck, W. R.

doi:10.1002/adma.200700622

Cited by 159 publications

(160 citation statements)

References 17 publications

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“…For most organic materials, the dipoles at both interfaces are very similar, and this is expected because of the similar surface chemistry of Al at both interfaces, where the Al surface is spontaneously oxidized. 39 Some organic materials, such as PTB7 and PEO, exhibit small differences in the surface WF that can be attributed to the different contact morphologies at the organic/metal interface. For pre-deposited Al, the interface is distinct without intermixing of organic and metal molecules.…”

Section: Resultsmentioning

confidence: 99%

Dipole formation at organic/metal interfaces with pre-deposited and post-deposited metal

Zhong

Zhang

et al. 2017

NPG Asia Mater

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In organic electronic devices, the interfacial dipole at organic/metal interfaces is critical in determining the carrier injection or extraction that limits the performance of the device. A novel technique to enable the direct measurement of underburied dipoles is developed and demonstrated. By tilting the shadow mask by a small angle, metal atoms diffuse into the opening slit to form an ultrathin metal layer during the evaporation process. As the ultrathin metal layer cannot screen out the dipole-induced surface work function change, the dipole strength and direction at the organic/metal interface can be revealed. It was found that the polarity of the organic material, the Fermi-level pinning and the interface morphology all play important roles in dipole formation. By comparing the energy level shifts at the organic/pre-deposited metal and organic/post-deposited metal interfaces, the dipole formed by molecular interactions could be distinguished from the dipole formed by Fermi-level pinning. NPG Asia Materials (2017) 9, e379; doi:10.1038/am.2017.56; published online 19 May 2017 INTRODUCTIONIn organic electronic devices, such as organic solar cells and organic light-emitting diodes, energy-level alignment at the organic/metal interface is critical for device performance because it determines the carrier injection barrier and device built-in voltage. 1,2 A small energylevel offset between the organic material's charge transport level and the metal's Fermi level is always favored to form an ohmic contact at the interface. Therefore, metals with low work function (WF), such as Ba, Ca, Li, Cs and Al, are popular choices for cathodes to facilitate electron injection or extraction. 3-5 However, low WF metals are intrinsically unstable. As a consequence, much effort has been made to develop electrode modification materials to enable the use of stable but high WF metals as cathodes. 6 Both inorganic materials, such as alkali-metal salts and metal oxides, and organic materials, such as organic surfactants, have been successfully applied in devices. [7][8][9][10][11][12] Through the formation of a dipole layer at the organic/metal interface, induced by the interfacial interaction, the interfacial energy-level alignment is modified, leading to a reduction in the charge injection barrier. 1,13 Various molecular structures are being examined to find the best modification materials for different devices.To select the desired interfacial material, the most common characterization is measuring the WF change on substrates of the cathode metal before and after depositing the interfacial materials on top 14,15 (Figure 1a). A reduced WF indicates that the electron injection barrier is lowered and hence can be used at the cathode. In contrast, an increased WF implies that the material can be used to enhance hole injection at the anode. The deduction is straightforward for inverted devices for which the layer deposition sequence, that is, cathode

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Section: Resultsmentioning

confidence: 99%

Dipole formation at organic/metal interfaces with pre-deposited and post-deposited metal

Zhong

Zhang

et al. 2017

NPG Asia Mater

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“…[19] layers of CuPc and C60 are deposited onto a Mg substrate and onto a ITO substrate. CuPc and C60 layers have experimental pinning levels W + CuPc = 4.4 eV and W − C60 = 4.5 eV [29,30]. Mg and ITO have work functions W Mg = 3.7 eV, and W ITO = 5.0 eV, respectively.…”

Section: Discussionmentioning

confidence: 99%

Charge equilibration and potential steps in organic semiconductor multilayers

Brocks

Çakır

Bokdam

et al. 2012

Organic Electronics

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Substantial potential steps ∼ 0.5 eV are frequently observed in organic multilayers of donor and acceptor molecules. Often such potential steps depend on the order in which the individual layers are deposited, or on which substrate they are deposited. In this paper we outline a model for these potential steps, based upon integer charge transfer between donors and acceptors, charge equilibration across the multilayer, and simple electrostatics. Each donor, acceptor, or substrate material is characterized by a pinning level, and the potential profile can be deduced from the sequential order of the layers, and the differences between their pinning levels. For particular orderings we predict that intrinsic potential differencess lead to electric fields across individual layers, which may falsely be interpreted as band bending.

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“…Arkhipov et al 12 were the first who suggested a model for dissociation of EHPs at a DA interface based on the assumption that a dipolar layer can be formed at the interface. Recently, the existence of dipoles has been confirmed experimentally for the interfaces between fullerene and various polymers 35 and theoretically for the interface between fullerene and pentacene. 36 These results strongly support the model for dissociation of EHPs at the DA organic interfaces based on the existence of the dipolar layer.…”

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confidence: 99%

Calculating the Efficiency of Exciton Dissociation at the Interface between a Conjugated Polymer and an Electron Acceptor

Baranovskiǐ

Wiemer

Nenashev

et al. 2012

J. Phys. Chem. Lett.

101

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The decisive feature of any material designed for photovoltaic applications is the dissociation efficiency of photogenerated excitons. This efficiency is essentially governed by the Coulomb attraction between electrons and holes. Because the dielectric constant in organic materials is rather low (ε r ≈ 3), the exciton binding energy is much larger than the thermal energy at room temperature, so that thermally governed dissociation seems improbable. Experiments show that while the dissociation probability for electron−hole pairs is indeed very low in the bulk samples, it becomes close to unity at intrinsic interfaces between two organic materials, an electron donor (usually a conjugated polymer) and an electron acceptor (usually a fullerene derivative). The driving force for this dissociation is still a matter of controversy. This Perspective provides a theoretical analysis of possible mechanisms for the efficient dissociation of electron−hole pairs at internal organic interfaces despite the strong Coulomb attraction between the charges. I t is one of the main challenges in the research on organic materials to reveal the mechanism responsible for the efficient dissociation of electron−hole pairs (EHPs) created by light. The interest of researchers in the dissociation problem is caused mainly by perspectives of photovoltaic applications of organic semiconductors. Such materials usually exhibit very high light absorption coefficients, which, combined with lowcost processing, makes them promising for applications in solar cells. 1,2 However, it is not the efficient light absorption itself but rather the combination of the efficient absorption with the efficient photocurrent generation that makes a material favorable for photovoltaic applications. The decisive step in photocurrent generation is the dissociation of EHPs created by light. In the dissociation process, the electron and hole must overcome their mutual Coulomb attraction, determined by the energywhere e is the elementary charge, ε r is the relative dielectric constant of the surrounding media, ε 0 is the permittivity of vacuum, and r is the electron−hole separation. While in inorganic semiconductors with ε r > 10 the binding energy of excitons is on the order of 10 meV, in organic semiconductors, in which ε r is typically between 2 and 4, 3 the binding energy of excitons is on the order of 1 eV. 4 The puzzling question arises then regarding the mechanism that could provide an efficient dissociation of EHPs with such a huge binding energy as compared to the thermal energy at room temperature, kT ≃ 0.025 eV.Numerous experimental and theoretical studies have been dedicated to the dissociation problem of EHPs in organic semiconductors. There is no chance to review all of them in the current report. Interested readers can find a comprehensive description of the research field in recent review articles. 1,3 Here, we only briefly describe the state of the research field.The first organic solar cells tested in the 1970s used a single material, sandwiched...

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Formation of the Interfacial Dipole at Organic‐Organic Interfaces: C₆₀/Polymer Interfaces

Cited by 159 publications

References 17 publications

Dipole formation at organic/metal interfaces with pre-deposited and post-deposited metal

Dipole formation at organic/metal interfaces with pre-deposited and post-deposited metal

Charge equilibration and potential steps in organic semiconductor multilayers

Calculating the Efficiency of Exciton Dissociation at the Interface between a Conjugated Polymer and an Electron Acceptor

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Formation of the Interfacial Dipole at Organic‐Organic Interfaces: C60/Polymer Interfaces

Cited by 159 publications

References 17 publications

Dipole formation at organic/metal interfaces with pre-deposited and post-deposited metal

Dipole formation at organic/metal interfaces with pre-deposited and post-deposited metal

Charge equilibration and potential steps in organic semiconductor multilayers

Calculating the Efficiency of Exciton Dissociation at the Interface between a Conjugated Polymer and an Electron Acceptor

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Product

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Formation of the Interfacial Dipole at Organic‐Organic Interfaces: C₆₀/Polymer Interfaces