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In applications, surface states and bulk doping concentration are important parameters of inorganic semiconductors, as they determine the bulk properties and substantially influence the properties of interfaces in devices, foremost the electron energy level alignment. In this work, we provide a qualitative model to describe the influence of surface state density and bulk donor concentration on the work function increase upon deposition of strong organic molecular acceptors onto the surface of n-doped inorganic semiconductors. This work function increase due to electron transfer to the molecular layer has two contributions: the formation of an interface dipole and a change of the near-surface space charge region inside the inorganic semiconductor, referred to as surface band bending. By using different surface preparation methods, we show how the surface state density limits the surface band bending change and enhances the interface dipole, both measured independently by photoelectron spectroscopy. In addition, we show that bulk donor concentration variation of the inorganic semiconductor has minor influence on the ratio of the two contributions to the work function change, at least for low to moderate donor concentrations up to 1019 cm−3. [ABSTRACT FROM AUTHOR]

Usability of inorganic/organic semiconductor hybrid structures for light-emitting applications can be intrinsically limited by an unfavorable interfacial energy level alignment causing charge separation and nonradiative deactivation. Introducing cascaded energy transfer funneling away the excitation energy from the interface by transfer to a secondary acceptor molecule enables us to overcome this issue. We demonstrate a substantial recovery of the light output along with high inorganic-to-organic exciton conversion rates up to room temperature. [ABSTRACT FROM AUTHOR]

Abstract: Ultraviolet photoemission spectroscopy was used to determine the work function φ and the hole injection barriers Δh of interfaces formed between pentacene and two fundamentally different electrode materials: (i) conductive polymers comprising poly(3,4-ethylenedioxythiophene) and sulfonate moieties, and (ii) Au with chemisorbed Cl. φ of these substrates before pentacene deposition covers a wide range of 4.25–5.15eV for the polymers, and extends to 5.8eV for Au–Cl. Despite this huge variation in φ, Δh after pentacene deposition is found to be almost identical for all cases (≉0.35eV). The mechanism responsible for Δh being independent of initial surface φ is proposed to be an interfacial charge transfer reaction leading to cationic pentacene species. These findings are compared to interfaces between pentacene and clean metal surfaces, where Δh was found to decrease with increasing substrate φ. [Copyright &y& Elsevier]

The formation of small gold clusters at the initial stage of the metal growth on a thin film of the conjugated organic semiconductor diindenoperylene (DIP) leads to charging during ultraviolet photoemission spectroscopy (UPS) investigations. Additional irradiation of the sample with visible light was successfully applied to re-establish charge neutrality on the sample surface. By varying the light energy, we demonstrate that the optical formation of excitons and/or polarons results in efficient electron transfer from DIP to the positively charged Au clusters and thus the elimination of charging in UPS. This allows the collection of reliable information on the interface electronic structure. [Copyright &y& Elsevier]

Ultraviolet photoelectron spectroscopy was used to investigate neutral methyl viologen (1,1′-dimethyl-1H,1′H-[4,4′]bipyridinylidene, MV0) deposited on Au(111). As a result of molecule-to-metal electron transfer, the work function of Au(111) was decreased from 5.50 to 3.30 eV. The energy levels of electron transport layers deposited on top of modified Au surfaces were shifted to higher binding energies compared to layers on pristine Au, and the electron injection barrier was reduced by 0.80 eV for tris(8-hydroxyquinoline)aluminum (Alq3) and by 0.65 eV for C60. The air-stable donor MV0 can thus be used to facilitate electron injection into organic semiconductors even from high work function metals. [ABSTRACT FROM AUTHOR]

Interface formation between the organic semiconductor α-sexithiophene (6T) and polar as well as nonpolar ZnO surfaces is investigated. The growth mode of the organic layer is strongly influenced by the orientation of the ZnO surface. No indication for chemisorption of 6T on ZnO is found by photoelectron spectroscopy. The energy level alignment at the 6T/ZnO interface is of type-II facilitating electron transfer from the organic to the inorganic part and hole transfer in the other direction, rendering this heterostructure interesting for photovoltaic applications. [ABSTRACT FROM AUTHOR]

The electronic properties of interfaces play a decisive role in determining the electrical characteristics of organic photovoltaic cells. In this paper, it is proposed that the mechanism responsible for the energy level alignment observed upon interface formation involves charge transfer between the electrode and the organic semiconductor. The amount of charges transferred is determined by occupying the density of states of the semiconductor according to Fermi–Dirac statistics and the observed energy level shifts are then governed by electrostatics. Within the proposed framework, we also discuss and numerically model the impact of the coupling strength between the molecular and the metal states. Photoelectron spectroscopy studies on two types of technologically relevant model interfaces, namely a metal/insulator/organic semiconductor heterojunction and a donor/acceptor heterojunction supported on a metal electrode, are then presented. The experimental observations, ranging from band bending to interface dipole formation, are shown to emerge naturally from the same theoretical framework. These results unambiguously demonstrate the essential role of the metal electrode in governing the energy level alignment, even significantly away from the metal surface. [ABSTRACT FROM AUTHOR]

Abstract: We report that solution-based treatment of Cu electrodes with strong electron acceptor molecules significantly decreases the contact resistance towards organic semiconductors, which is advantageous for applications such as organic field-effect transistors (OFETs). Spin-coating solutions of tetracyanoquinodimethane (TCNQ) or tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) onto Cu electrodes results in strongly chemisorbed acceptor (sub-)monolayers, which increase the electrode work function from 4.5eV (bare Cu) up to 5.2eV (F4-TCNQ-treated) even in air, as evidenced by X-ray photoelectron spectroscopy and photoelectron yield spectroscopy. The use of such modified electrodes in flexible OFETs with poly(3-hexylthiophene)-dithienyltetrafluorobenzene (P3HT-TFT) as semiconductor lead to a twofold increase of the on-current in the saturation regime and a decrease of the threshold voltage from −20V (bare Cu) to −7V (F4-TCNQ-treated). These results confirm that this simple solution-based process is viable for lowering organic/metal contact resistances in organic electronic devices. [Copyright &y& Elsevier]

Abstract: Ultraviolet photoelectron spectroscopy (UPS) was used to determine the energy level alignment at organic–organic conductor–semiconductor and semiconductor–semiconductor hetero-interfaces that are relevant for organic optoelectronic devices. Such interfaces were formed by in situ vacuum sublimation of small molecular materials [C60 and pentacene (PEN)] and ex situ spin-coating of poly(3-hexylthiophene) (P3HT), all on the common substrate poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS). We found that the deposition sequence had a significant impact on the interface energetics. The hole injection barrier (HIB) of C60 on PEDOT:PSS could be changed from 1.0eV (moderate hole injection) to 1.7eV (good electron injection) by introducing a layer of P3HT. The HIB of P3HT/PEDOT:PSS was increased by 0.35eV due to an interfacial PEN layer. However, PEN deposited on PEDOT:PSS and P3HT/PEDOT:PSS exhibited the same value. These observations are explained by material-dependent dipoles at the interfaces towards PEDOT:PSS and substrate dependent inter-molecular conformation. [Copyright &y& Elsevier]

Interfaces between an organic material and another organic material or a metal play an essential role in organic light-emitting diodes (OLEDs). Photoelectron spectroscopy in the vacuum-ultraviolet range has been used to characterise the alignment of energy levels and to discover possible chemical interaction at such interfaces. The systems investigated here are the interfaces of active conjugated organic molecules (pentacene, para-sexiphenyl, and N,N′-bis-(1-naphthyl)-N,N′-diphenyl1-1,1biphenyl1-4,4′-diamine) with materials used for carrier injection. Low work function metals such as samarium or alkali are used for electron injection. Materials with high work function such as gold or poly(3,4-ethylenedioxythiophene)/polystyrenosulfonate are used for hole injection. In this paper, we will discuss the differences in metal-organic interaction (physisorption versus stronger bonding) and energy level alignment at organic–metal and organic–organic interfaces. [Copyright &y& Elsevier]