Large π-conjugated molecules, when in contact with a metal surface, usually retain a finite electronic gap and, in this sense, stay semiconducting. In some cases, however, the metallic character of the underlying substrate is seen to extend onto the first molecular layer. Here, we develop a chemical rationale for this intriguing phenomenon. In many reported instances, we find that the conjugation length of the organic semiconductors increases significantly through the bonding of specific substituents to the metal surface and through the concomitant rehybridization of the entire backbone structure. The molecules at the interface are thus converted into different chemical species with a strongly reduced electronic gap. This mechanism of surface-induced aromatic stabilization helps molecules to overcome competing phenomena that tend to keep the metal Fermi level between their frontier orbitals. Our findings aid in the design of stable precursors for metallic molecular monolayers, and thus enable new routes for the chemical engineering of metal surfaces.

The fundamental limits of inorganic semiconductors for light emitting applications, such as holographic displays, biomedical imaging and ultrafast data processing and communication, might be overcome by hybridization with their organic counterparts, which feature enhanced frequency response and colour range. Innovative hybrid inorganic/organic structures exploit efficient electrical injection and high excitation density of inorganic semiconductors and subsequent energy transfer to the organic semiconductor, provided that the radiative emission yield is high. An inherent obstacle to that end is the unfavourable energy level offset at hybrid inorganic/organic structures, which rather facilitates charge transfer that quenches light emission. Here, we introduce a technologically relevant method to optimize the hybrid structure's energy levels, here comprising ZnO and a tailored ladder-type oligophenylene. The ZnO work function is substantially lowered with an organometallic donor monolayer, aligning the frontier levels of the inorganic and organic semiconductors. This increases the hybrid structure's radiative emission yield sevenfold, validating the relevance of our approach.

We discuss density functional theory calculations of hybrid inorganic-organic systems that explicitly include the global effects of doping (i.e., position of the Fermi level) and the formation of a space-charge layer. For the example of tetrafluoro-tetracyanoquinodimethane on the ZnO(0001[over ¯]) surface we show that the adsorption energy and electron transfer depend strongly on the ZnO doping. The associated work function changes are large, for which the formation of space-charge layers is the main driving force. The prominent doping effects are expected to be quite general for charge-transfer interfaces in hybrid inorganic-organic systems and important for device design.

Substantial variations in the electronic structure and thus possibly conflicting energetics at interfaces between hybrid perovskites and charge transport layers in solar cells have been reported by the research community. In an attempt to unravel the origin of these variations and enable reliable device design, we demonstrate that donor-like surface states stemming from reduced lead (Pb) directly impact the energy level alignment at perovskite (CHNHPbICl) and molecular electron acceptor layer interfaces using photoelectron spectroscopy. When forming the interfaces, it is found that electron transfer from surface states to acceptor molecules occurs, leading to a strong decrease in the density of ionized surface states. As a consequence, for perovskite samples with low surface state density, the initial band bending at the pristine perovskite surface can be flattened upon interface formation. In contrast, for perovskites with a high surface state density, the Fermi level is strongly pinned at the conduction band edge, and only minor changes in surface band bending are observed upon acceptor deposition. Consequently, depending on the initial perovskite surface state density, very different interface energy level alignment situations (variations over 0.5 eV) are demonstrated and rationalized. Our findings help explain the rather dissimilar reported energy levels at interfaces with perovskites, refining our understanding of the operating principles in devices comprising this material.

We have used ultraviolet and inverse photoemission spectroscopy to determine the transport gaps (E t ) of C 60 and diindenoperylene (DIP), and the photovoltaic gap (E PVG ) of five prototypical donor/ acceptor interfaces used in organic photovoltaic cells (OPVCs). The transport gap of C 60 (2.5 6 0.1) eV and DIP (2.55 6 0.1) eV at the interface is the same as in pristine films. We find nearly the same energy loss of ca 0.5 eV for all material pairs when comparing the open circuit voltage measured for corresponding OPVCs and E PVG . V C 2012 American Institute of Physics.The energy level alignment at the donor/acceptor (D/A) heterojunction of an organic photovoltaic cell (OPVC) is decisive for its performance. In particular, the energy offset between the lowest unoccupied molecular orbital level of the acceptor [LUMO(A)] and the highest occupied molecular orbital level of the donor [HOMO(D)] sets an upper limit for the open circuit voltage V oc . 1-6 This has been expressed as e Á V oc ¼ HOMO(D) À LUMO(A) À D, where e is the elementary charge and D a loss term, which has been suggested to be related to the exciton binding energy 2 or radiative and nonradiative temperature dependent losses. 1,3,5 The HOMO(D)/ LUMO(A) offset is denoted in various ways in the literature, such as charge transfer gap, intermolecular gap, or donor/ acceptor gap and is often estimated by optical spectroscopy, 5,6 or electrical characterization, e.g., cyclic voltammetry, 6 reverse saturation current analysis, 2,7 or temperature dependent measurements of the open circuit voltage. 4,5 To avoid ambiguity, we use the term photovoltaic gap (E PVG ) (cf Fig. 1). To minimize energy losses during the photon harvesting process, it is desirable to maximize E PVG within the constraint of keeping the LUMO-LUMO (DE L ) and HOMO-HOMO level offsets (DE H ) sufficiently large to drive charge separation across the D/A junction. To quantify D for unraveling its physical origin, it is mandatory to have reliable E PVG values for comparison with corresponding V oc values. Unfortunately, simple models for estimating the energy levels at organicorganic interfaces are often invalid (e.g., vacuum level alignment 8,9 ), and more involved models have been brought forward. 10,11 For the time being, experimental determination of interface energetics is indispensable to understand the processes inside an OPVC based on reliable values of E PVG , but only few pertinent studies have been conducted to date. [12][13][14][15] The present study focuses on E PVG values at prototypical organic D/A pairs formed between four organic semiconductors [sexithiophene (6T), fullerene (C 60 ), diindenoperylene (DIP, chemical structure shown in Fig. 1), and poly(3-hexylthiophene) (P3HT)] determined using the combination of ultraviolet and inverse photoemission spectroscopy (UPS and IPES). The experiment for DIP/C 60 demonstrates that E PVG can be reliably inferred from measuring the offset between the D/A HOMO levels, once the acceptor's transport gap (E t ) is known and no changes of E...

We show that the work function (Φ) of ZnO can be increased by up to 2.8 eV by depositing the molecular electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). On metals, already much smaller Φ increases involve significant charge transfer to F4TCNQ. No indication of negatively charged F4TCNQ on ZnO is found by photoemission spectroscopy. This fundamental difference is explained by a simple electrostatic model that identifies the bulk doping and band bending in ZnO as key parameters. Varying Φ of the inorganic semiconductor enables tuning the energy-level alignment at ZnO/organic semiconductor interfaces

Recently, Niederhausen et al. [Phys. Rev. B 86, 081411(R) (2012)] have reported on the energy level alignment of C 60 adsorbed on a bilayer α-sexithiophene (6T) film on Ag(111). The possibility of charge transfer from the metal to the C 60 through the bilayer 6T as discussed by the authors may have a strong impact on understanding the energy level alignment (ELA) at organic-organic (O-O) heterojunctions grown on electrodes. In this paper, we aim at a comprehensive picture of the ELA at O-O interfaces on a metal. We carry out a detailed investigation of the same pair of materials on Ag(111) as employed previously, however, with varying 6T interlayer thickness. The results allow unambiguous identification of integer charge transfer towards a fraction of the C 60 molecules as the mechanism leading to the formation of interface dipoles. Varying the 6T interlayer thickness also reveals the dependence of the observed features on the C 60 -metal distance. This dependence is quantitatively addressed by electrostatic considerations involving a metal-to-overlayer charge transfer. From this, we demonstrate the important role of dipole-dipole interaction potentials in the molecular layer and electric fields resulting from interface dipole formation for the energy level alignment. These findings provide a deeper understanding of the fundamental mechanisms that establish electronic equilibrium at molecular heterojunctions and will aid the prediction of an accurate energy level alignment at device relevant heterojunctions, e.g. in organic opto-electronic devices.

We investigate hybrid charge transfer states (HCTS) at the planar interface between α-NPD and ZnO by spectrally resolved electroluminescence (EL) and external quantum efficiency (EQE) measurements. Radiative decay of HCTSs is proven by distinct emission peaks in the EL spectra of such bilayer devices in the NIR at energies well below the bulk α-NPD or ZnO emission. The EQE spectra display low energy contributions clearly red-shifted with respect to the α-NPD photocurrent and partially overlapping with the EL emission. Tuning of the energy gap between the ZnO conduction band and α-NPD HOMO level (Eint) was achieved by modifying the ZnO surface with self-assembled monolayers based on phosphonic acids. We find a linear dependence of the peak position of the NIR EL on Eint, which unambiguously attributes the origin of this emission to radiative recombination between an electron on the ZnO and a hole on α-NPD. In accordance with this interpretation, we find a strictly linear relation between the open-circuit voltage and the energy of the charge state for such hybrid organic-inorganic interfaces.