Metal-organic Interface Research Articles

Metal–organic coordinated multilayer film formation: Quantitative analysis of composition and structure

Metal–organic coordinated multilayers are self-assembled thin films fabricated by alternating solution–phase deposition of bifunctional organic molecules and metal ions. The multilayer film composed of α,ω-mercaptoalkanoic acid and Cu (II) has been the focus of fundamental and applied research with its robust reproducibility and seemingly simple hierarchical architecture. However, internal structure and composition have not been unambiguously established. The composition of films up to thirty layers thick was investigated using Rutherford backscattering spectrometry and particle induced X-ray emission. Findings show these films are copper enriched, elucidating a 2:1 ratio for the ion to molecule complexation at the metal–organic interface. Results also reveal that these films have an average layer density similar to literature values established for a self-assembled monolayer, indicating a robust and stable structure. The surface structures of multilayer films have been characterized by contact angle goniometry, ellipsometry, and scanning probe microscopy. A morphological transition is observed as film thickness increases from the first few foundational layers to films containing five or more layers. Surface roughness analysis quantifies this evolution as the film initially increases in roughness before obtaining a lower roughness comparable to the underlying gold substrate. Quantitative analysis of topographical structure and internal composition for metal–organic coordinated multilayers as a function of number of deposited layers has implications for their incorporation in the fields of photonics and nanolithography.

Enhancement of electrical properties of vanadyl phthalocyanine derivative by PCBM

Purpose – The purpose of this paper is to investigate the effect of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) on improvement of physical and electrical properties of vanadyl phthalocyanine derivative. The correlation between the physical characteristics of the active layers, comprising vanadyl 2,9,16, 23-tetraphenoxy-29H,31H-phthalocyanine (VOPcPhO) and PCBM, and the electrical properties of metal/organic/metal devices have been studied. The use of soluble vanadyl phthalocyanine derivative makes it very attractive for a variety of applications due to its tunable properties and high solubility. Design/methodology/approach – The sandwich type structures Al/VOPcPhO/Al and Al/VOPcPhO:PCBM/Al were fabricated by spin casting the active organic layers between the top and bottom (aluminum) electrodes. The stand-alone (VOPcPhO) and composite (VOPcPhO:PCBM) thin films were characterized by X-ray diffraction, atomic force microscopy, UV/Vis and Raman spectroscopy. The electronic properties of the metal/organic/metal devices were studied using current-voltage (I-V) characteristics in dark at room temperature. Findings – The values of barrier height for Al/VOPcPhO/Al and Al/VOPcPhO:PCBM/Al devices were obtained from the forward bias I-V curves and were found to be 0.7 eV and 0.62 eV, respectively. The present study indicates that the device employing VOPcPhO:PCBM composite film as the active layer, with better structural and morphological characteristics, results in reduced barrier height at the metal-organic film interface as compared to the one fabricated with the stand-alone film. Research limitations/implications – It is shown that doping VOPcPhO with PCBM improves the crystallinity, morphology and junction properties. Practical implications – The spin coating technique provides a simple, less expensive and effective approach for preparing thin films. The soluble VOPcPhO is conveniently dissolved in a number of organic solvents. Originality/value – The physical properties of the VOPcPhO:PCBM composite thin film and the electrical properties of the composite thin-film-based metal/organic/metal devices have not been reported in the literature, as far as our knowledge is concerned.

Local electronic structure at organic–metal interface studied by UPS, MAES, and first-principles calculation

Understanding and controlling local electronic structures at organic–metal interfaces are crucial for fabricating novel organic-based electronics, as in the case of heterojunctions in semiconductor devices. Here, we report recent studies of valence electronic states at organic–metal interfaces (especially those near the Fermi level of a metal substrate) by the combined analysis of ultraviolet photoemission spectroscopy (UPS), metastable atom electron spectroscopy (MAES), and first-principles calculations. New electronic states in the HOMO (highest occupied molecular orbital)–LUMO (lowest unoccupied molecular orbital) gap formed at an organic–metal interface are classified as a chemisorption-induced gap state (CIGS) and a complex-based gap state (CBGS). The CIGS is further characterized by an asymptotic feature of the metal wave function in the chemisorbed species. The CIGSs in alkanethiolates on Pt(111) and C60 on Pt(111) can be regarded as damping and propagating types, respectively. The CBGSs in K-doped dibenzopentacene (DBP) are composed of DBP-derived MOs and K sp states and distributed over the complex film. No metallic structures were found in the K1DBP and K3DBP phases, suggesting that they are Mott–Hubbard insulators due to strong electron correlation. The local electronic structures of a pentacene film bridged by Au electrodes under bias voltages were examined by an FET-like specimen. The pentacene-derived bands were steeply shifted at the positively biased electrode, reflecting the p-type character of the film.

(Invited) Role of the Metal-Organic Interfaces in the Dark Current-Voltage Characteristics of Organic Solar Cells

. We present a model for the current-voltage curves of organic solar cells (OSCs), operating at dark conditions. The model relates the free charge density at the contact interface with the current density in OSCs. The strength and novelty of the model lie in the use of appropriate and physically-based boundary conditions for the charge and potential at the metal-organic interfaces. These boundary conditions contain information for the doping close to interfaces and for built-in voltage and band bending due to asymmetric contacts. We show that the values for the boundary conditions obtained from symmetric single-carrier diodes can be successfully used also for asymmetric double-carrier devices, such as OSCs, irrespectively of the dominant limit (space-charge, injection or diffusion) in the charge transport. Motivation. Organic solar cells (OSCs) are a promising alternative for powering portable devices directly from sunlight [1]. To support these applications, analyses and physical modeling of OSCs are required. In particular, the electrical conduction of the OSCs is described by the drift-diffusion differential equations. However, the solutions of these equations depend on the boundary conditions (BCs) at the contacts. Model and BCs. OSCs are asymmetric, with contacts to materials with different work functions, which result in built-in voltage Vbi and generate a voltage V between the contacts under illumination. Previously in [2-4], by relating to the charge density at interfaces, we have proposed a model for the current density – voltage (J-V) curves of single-carrier organic diodes, considering high bias and conduction limited by space-charge (SCLC) and injection (ILC). However, the OSCs are double-carrier devices, and their photovoltaic operation is at low bias, 0<V<Vbi , for which the diffusion dominates in the charge transport. We are now validating the model from [2-4] also for the diffusion regime at low bias of double-carrier diodes. In our model, the boundary conditions for charge concentrations at their respective injecting contacts are Eqs. (1), where p and n are the hole and electron concentrations, respectively, and the model parameters K­ 1p , K­ 1n , mp, mn, K2p and K2n depend on the organic materials, energy barriers at interfaces and doping [5]. These parameters can be extracted from experimental J-V curves (symbols in Fig. 1) for electron-only Yb/PPV/Ca and hole-only ITO/Ag/PPV/Ag devices [6], by fitting the experimental J-V curves with the curves obtained from the numerical solutions of the transport equations, using (1) as boundary conditions at the injection interfaces. The carrier densities at their respective extracting electrodes are assumed constant (2), since p(L) and n(0) are small, owing to the large Φ 4 /kT>>1 and Φ 1 /kT>>1, where, Φ 1 to Φ 4 are the barriers seen at the interfaces, (see inset of Figure 1) and NC is the density of sites. Then, the charge boundary conditions extracted from single-carrier devices are introduced in the model of the double-carrier device. Inspecting equations and results from numerical simulations, the potential distribution in the semiconductor V(x) can be approximated by a linear function (3), where b, b=kT/q[ln(q 2 NcL 2/2kTε)-2], is a band-bending related parameter [7]. This suggests that the hole Jp and electron Jn current densities are (4) and (5). With the above boundary conditions, our J−V model (solid lines in Fig. 1) is in an excellent agreement with published experimental results at dark conditions, both for the aforementioned single-carrier devices and for the double-carrier Ag/PPV/Ca diode. Conclusions. In this work, we present a model for J-V curves of OSCs that highlights the importance of the carrier density at interfaces. The model is valid for single- and double-carrier organic devices at dark conditions. Acknowledgments. This work was supported by the Project mP_TIC_5 from Campus de Excelencia Internacional BioTic Granada. [1] H. Hoppe and N.S. Sariciftci, Journal of Materials Research, 19, 1924-1945, 2004. [2] P. López Varo, J. A. Jiménez Tejada, J. A. López Villanueva and M. J. Deen, Org. Electron., 15, 2526 (2014). [3] P. Lara Bullejos, J. A. Jiménez Tejada, S. Rodríguez-Bolivar, M. J. Deen, O. Marinov, J. App. Phys. 105, 084516 (2009). [4] M. J. Deen, O. Marinov, U. Zschieschang, H. Klauk, IEEE Trans. El, Dev., 56, 2962 (2009). [5] P. López Varo, J. A. Jiménez Tejada, J. A. López Villanueva and M. J. Deen, Org. Electron., 15, 2536 (2014). [6] T. van Woudenbergh, P. W. M. Blom, and J. N. Huiberts, Appl. Phys. Lett. 82, 985 (2003). [7] P de Bruyn, A.H.P. van Rest, G.A.H. Wetzelaer, D.M. de Leeuw, and P.W.M. Blom Phys. Rev. Lett. 111, 186801 (2013). Figure 1

Interface Dipole and Growth Mode of Partially and Fully Fluorinated Rubrene on Au(111) and Ag(111)

Thin films of fully and partially fluorinated rubrene deposited on Au(111) and Ag(111) were investigated using ultraviolet and X-ray photoelectron spectroscopy. We demonstrate that fluorination of the molecules is an efficient way for tuning the metal–organic interface dipole and the hole injection barrier. Moreover, the results indicate that the pronounced electrostatic dipole moment of partially fluorinated rubrene (F14-RUB) has a strong impact on the growth mode of these molecules. Most notably, we infer that the first layer of F14-RUB on Au(111) and Ag(111) is formed by molecules with alternating orientation of their dipole moments whereas the second layer shows a nearly uniform orientation.

Improved Performance of Organic LEDs with Modified Metal-Organic Interface

Incorporation of thin layers of strong acceptor tetrafluro-tetracyanoquinodi methane (F4TCNQ) over conducting substrate Indium-Tin-Oxide (ITO) modified its work function. The enhancement depended on the thickness of F4TCNQ film. These modified ITO substrates were used as hole injection contacts in OLEDs which show enhanced device current, lower operating voltages and enhanced current efficiency.

Engineering the metal–organic interface by transferring a high-quality single layer graphene on top of organic materials

We present a new method for transferring chemical vapor deposition (CVD)-grown graphene onto the surfaces of organic materials directly. Raman and near edge X-ray absorption fine structure measurements prove that high-quality and single layer graphene/organic thin films can be obtained with minimized impurity introduction. In-situ synchrotron radiation photoemission spectroscopy combined with ultraviolet photoelectron spectroscopy experiments demonstrate that the inserted graphene can not only act as a buffer layer to reduce the interfacial chemical reactions between the deposited Al and organic materials, but also tune the metal/organic interface electronic structure significantly. This new graphene transfer technique may have a great potential in the application of engineering the metal–organic interface properties, which is one of the key technologies for the optimal design and fabrication of organic electronic and optoelectronic devices.

Anomalous coarsening driven by reversible charge transfer at metal-organic interfaces.

The unique electronic properties and functional tunability of polycyclic aromatic hydrocarbons have recently fostered high hopes for their use in flexible, green, portable, and cheap technologies. Most applications require the deposition of thin molecular films onto conductive electrodes. The growth of the first few molecular layers represents a crucial step in the device fabrication since it determines the structure of the molecular film and the energy level alignment of the metal–organic interface. Here, we explore the formation of this interface by analyzing the interplay between reversible molecule–substrate charge transfer, yielding intermolecular repulsion, and van der Waals attractions in driving the molecular assembly. Using a series of ad hoc designed molecules to balance the two effects, we combine scanning tunnelling microscopy with atomistic simulations to study the self-assembly behavior. Our systematic analysis identifies a growth mode characterized by anomalous coarsening that we anticipate to occur in a wide class of metal–organic interfaces and which should thus be considered as integral part of the self-assembly process when depositing a molecule on a conducting surface.

Spectroscopic fingerprints of work-function-controlled phthalocyanine charging on metal surfaces.

The electronic character of a π-conjugated molecular overlayer on a metal surface can change from semiconducting to metallic, depending on how molecular orbitals arrange with respect to the electrode's Fermi level. Molecular level alignment is thus a key property that strongly influences the performance of organic-based devices. In this work, we report how the electronic level alignment of copper phthalocyanines on metal surfaces can be tailored by controlling the substrate work function. We even show the way to finely tune it for one fixed phthalocyanine-metal combination without the need to intercalate substrate-functionalizing buffer layers. Instead, the work function is trimmed by appropriate design of the phthalocyanine's supramolecular environment, such that charge transfer into empty molecular levels can be triggered across the metal-organic interface. These intriguing observations are the outcome of a powerful combination of surface-sensitive electron spectroscopies, which further reveal a number of characteristic spectroscopic fingerprints of a lifted LUMO degeneracy associated with the partial phthalocyanine charging.

Effect of Structure and Thickness on the Electrical Characteristics of QLED Device

For the interface heterostructure between metal and organic layer of Quantum dot light-emitting diodes (QLED), tunneling injection model Fowler-Nordheim (F-N) was used to analyze the electrical characteristics of QLED. Through analyzing different thickness and the potential barrier of metal organic layer interface of QLED structure, simulation by Matlab obtained the J-V characteristic curves of the injection limit current density and the driving voltage. The simulation results showed that reducing the whole device thickness and the barrier height of metal organic layer interface of QLED structure, the energy consumption will be decreased. That is to say, carriers injection properties can be improved with the thinner device and lower barrier height, operating at the same voltage.

Optoelectronic simulation and thickness optimization of energetically disordered organic solar cells

In this work a complete semi-classical model of an organic solar cell is presented. The different aspects of conversion of light to electricity are taken into account. Correct models for density of state and organic-metal interface are considered in order to include the effect of energetically disorder material properties. Most of the parameters for the model are taken from literature while some were fixed by fitting with several experimental current-voltage characteristics. The comparison between modeling results and experimental data shows consistency and are in good agreement. Finally the model is used to investigate the optimization of hole transport (PEDOT) and active (P3HT:PCBM) layer thicknesses in order to maximize the cell efficiency. The simulation of the efficiency of the cell with varying thickness shows a fine tuning between the exciton generation and the charge recombination, giving clear indications on the optimization of cell performance.

Electrical characterization of controlled and unintentional modified metal–organic contacts

A method to characterize metal–organic contacts subjected to controlled technological treatments or unintentional degradation processes is proposed. The procedure is useful to characterize different fundamental aspects of a metal–organic structure such as the height of the interface energy barrier, the presence of impurities or trapping effects and the carrier mobility. Current–voltage curves in organic diodes are analyzed and the value of the free carrier density at the metal–organic interface is extracted and discussed. The charge carrier density is chosen for this analysis as this is one of the key physical parameters in the understanding of the physical processes involved in the device operation. The extracted charge, when described as a function of the current density, gives information about the doping and impurity concentration, the effective barrier seen by the carriers at the metal–organic interface or effective barriers seen by the carriers in the hopping processes across the organic material. An important advantage of the proposed procedure is the low computational time. Also, the procedure aims to provide a quick analysis for researchers on how the physical properties of the devices are evolving when they are technologically altered or degraded.

Space-charge and injection limited current in organic diodes: A unified model

Space-charge and injection limited currents in organic diodes have been analyzed using a unified model. Both currents have been modeled using the transport equations combined with a proper value of the free charge density at the metal–organic interface. The method has been applied to diodes with different organic materials, metal contacts and lengths of the organic materials. This unified model accurately reproduces published current–voltage curves for a variety of diode structures operating at different temperatures and voltages. The results of the method, in the injection regime, have been compared with the results of pure injection models developed by other authors. The reduction of the computational time and the number of parameters in our model are important advantages of our procedure. Moreover, it is an alternative where the injection models start to fail: at low applied voltages, close to the Ohmic regime, and for low heights of the energy barrier at the interface. The treatment is complemented with a compact model that relates the current density j with the free charge-carrier density at the interface pf(0): pf(0)=K1jm+K2, where the parameter K1 depends on the barrier height at the interface, m depends on the organic material and K2 controls the flat zone at low currents to include the dependence with thermal carriers and impurities.

Erratum: Interfacial properties at the organic-metal interface probed using quantum well states [Phys. Rev. B86, 155453 (2012)

Erratum: Interfacial properties at the organic-metal interface probed using quantum well states [Phys. Rev. B<b>86</b>, 155453 (2012)

Coordination Reactions and Layer Exchange Processes at a Buried Metal–Organic Interface

The reactive interface between phthalocyanine (2HPc) and a Cu(111) surface was investigated with X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption/reaction (TPD/TPR). 2HPc reacts with Cu(111) to form copper(II) phthalocyanine (CuPc). For 2HPc submonolayers, the reaction starts already below 240 K. Thin 2HPc multilayers (4 monolayers) are completely converted into CuPc at elevated temperatures (500 K). To understand how molecules can react even though they are initially not in contact with the Cu surface, TPD/TPR studies with a NiPc interlayer between Cu and 2HPc were performed. These studies reveal the operation of an exchange mechanism, by which CuPc or NiPc molecules from the first layer are replaced by 2HPc molecules from higher layers. Once a 2HPc molecule comes into contact with the Cu surface, the molecule can react with the surface, forming CuPc, which can then be replaced by another unreacted 2HPc molecule. These findings have important implications for metal–organic int...

Formation of metal–organic interface states studied with 2PPE

Experimental and theoretical investigations on the formation mechanism of interface-specific electronic states at metal–organic interfaces are reviewed. Model systems discussed are thin films of 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) and 1,4,5,8-naphthalene-tetracarboxylic acid dianhydride (NTCDA) on the silver single crystal surfaces Ag(111) and Ag(100). Time and angle-resolved two-photon photoemission (2PPE) reveals the occurrence of unoccupied electronic states that have no counterpart in the bulk metal or the organic overlayer. The interface states resemble Shockley-surface states of clean metals. They are strongly dispersive and show an ultra-short lifetime in the range of tens to hundred femtoseconds. The energy position of the interface states is to a large extent determined by the adsorption height of the molecules and their local surface density while the wave function experiences a strong influence from the adsorption geometry of the molecules.

Direct observation of adsorption geometry for the van der Waals adsorption of a single π-conjugated hydrocarbon molecule on Au(111)

Weak van der Waals adsorption of π-conjugated hydrocarbon molecules onto the gold surface, Au(111), is one of the essential processes in constructing organic-metal interfaces in organic electronics. Here we provide a first direct observation of adsorption geometry of a single π-conjugated hydrocarbon molecule on Au(111) using an atomically resolved scanning tunneling microscopy study combined with van der Waals density functional methodology. For the purpose, we utilized a highly symmetric π-conjugated hydrocarbon molecule, dehydrobenzo[12]annulene (DBA), which has a definite three-fold symmetry, the same as the Au(111) surface. Interestingly, our observations on an atomically resolved scale clearly indicate that the DBA molecule has only one adsorption configuration on Au(111) in spite of the weak van der Waals adsorption system. Based on the precisely determined adsorption geometry of DBA/Au(111), our calculation results imply that even a very small contribution of the interfacial orbital interaction at the organic-metal interface can play a decisive role in constraining the adsorption geometry even in the van der Waals adsorption system of a π-conjugated hydrocarbon molecule on the noblest Au(111) surface. Our observations provide not only deeper insight into the weak adsorption process, but also new perspectives to organic electronics using π-conjugated hydrocarbon molecules on the Au surface.

Electron injection at the PTCDA/metal interface detected by electron paramagnetic resonance

Electron paramagnetic resonance absorption was observed for samples prepared by vacuum vapor deposition of thin films of 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) on metal films of magnesium (Mg), aluminum (Al) and silver (Ag) but not on gold (Au) or PTCDA on quartz. The resonance absorption is seen as a Lorentzian line with g-value of 2.0030 and a linewidth of 0.19mT. The signal is attributed to PTCDA anions at the metal organic interface. The relative values of the metal work function and the PTCDA electron affinity appear to determine the occurrence of the EPR absorption.

Tuning gap states at organic-metal interfaces via quantum size effects

Organic-metal interfaces are key elements in organic-based electronics. The energy-level alignment between the metal Fermi level and the molecular orbital levels determines the injection barriers for the charge carriers at the interfaces, which are crucial for the performance of organic electronic devices. Dipole formation at the interfaces has been regarded as the main factor that affects the energy-level alignment. Several models have been proposed for the mechanism of dipole formation in the context of the interface between organic molecules and a bulk metal crystal surface, at which surface states were mostly used to probe the interfacial properties. Here we report that when the bulk metal crystal is replaced by a uniform metal thin film, the resulting two-dimensional quantum-well states will be able to not only probe but also modify the interfacial electronic structures, such as gap states, that have no counterpart at the organic-bulk crystal interface.

Computational comparison of conventional and inverted organic photovoltaic performance parameters with varying metal electrode surface workfunction

Inverted polymer-based bulk-heterojunction organic photovoltaic (BHJ-OPV) device designs have enabled a breakthrough in operational lifetime through use of stable electrode materials. To date, there have not been systematic performance parameter comparisons between conventional and inverted devices that consider a range of different metal electrodes and presence of native metal oxides at the organic-metal interface. Here, we systematically compute optical and electronic performance parameters for both conventional and inverted BHJ-OPV devices for 15 different electrode types covering a range of workfunctions. We quantitatively demonstrate that (1) high-workfunction metal electrodes (Au, Pd, Ni) are ideal for high-efficiency inverted device performance; and (2) native metal oxide on metal electrodes (e.g., CuO/Cu, Ag2O/Ag, NiO/Ni), which dramatically reduce conventional device efficiencies, can result in highly efficient inverted BHJ-OPV devices (efficiency of up to 6.7% for the P3HT:PCBM system). This work is an important advance over prior studies as it predicts the electrode materials and configurations that can lead to both high efficiency and high stability BHJ-OPV devices.