Electrode Work Function Engineering with Phosphonic Acid Monolayers and Molecular Acceptors: Charge Redistribution Mechanisms

The uses of self‐assembled monolayers (SAMs) of dipolar molecules or of adsorbed molecular acceptors on electrode materials are common strategies to increase their work function, thereby facilitating hole injection into an organic semiconductor deposited on top. Here it is shown that a combination of both approaches can surpass the performance of the individual ones. By combined experimental and theoretical methods it is revealed that in a three‐component system, consisting of an indium‐tin‐oxide (ITO) electrode, a carbazole‐based phosphonic acid SAM, and a molecular acceptor layer on top of the SAM, charge transfer occurs from the ITO through the SAM to the acceptor layer, resulting in an electrostatic field drop over the charge‐neutral SAM. This result is in contrast to common expectations of either p‐doping the carbazole of the SAM or charge transfer complex formation between the carbazole and the acceptor molecules. A high work function of 5.7 eV is achieved with this combined system; even higher values may be accessible by exploiting the fundamental charge redistribution mechanisms identified here with other material combinations.     

Calculating the Universal Energy‐Level Alignment of Organic Molecules on Metal Oxides

Recently, Greiner et al. [Nat. Mater. 2012, 11, 76 ] published a survey of the level alignment of about 40 metal oxide/organic molecule interfaces. They observed a striking regularity in the electronic level alignment of the highest occupied molecular orbital (HOMO) and the Fermi level that depends solely on the difference between the substrate work function and the ionization energy of the molecule independent of the details of the electronic structure of the oxide. The authors could reproduce their data under the assumption of thermodynamic equilibrium occupation of the HOMO using four adjustable parameters. A model that quantifies well‐established concepts in heterojunction physics and achieves the same result without any adjustable parameters is presented here. This approach explains why the level alignment is rather independent of the experimental details, such as the electronic structure of the oxide, defects in the oxide, and the thickness of oxide and overlayer. The model can also be extended to organic molecules or polymers on metals without any intermediate oxide as long as certain conditions are met. It also sheds new light on the large polaronic binding energy required to interpret the electronic level alignment of metal‐polymer interfaces.     

Depolarization Effects in Self‐Assembled Monolayers: A Quantum‐Chemical Insight

Many recent experimental studies have demonstrated that the deposition of a self‐assembled monolayer (SAM) made of polar molecules on a metal surface can significantly modulate its work function and hence the barrier for hole and electron injection in optoelectronic devices. The permanent dipole moment associated with the backbone of the molecules plays a key role in defining the amplitude and direction of the work‐function shift. We illustrate here via quantum‐chemical calculations performed on model systems that the dipole moment of molecules is significantly reduced going from the isolated state to the SAM. Such depolarization effects that are most often neglected thus reduce the work‐function shift and have to be taken in account to control and understand charge‐injection barriers in devices at a quantitative level.     

Using Self‐Assembling Dipole Molecules to Improve Charge Collection in Molecular Solar Cells

Surface modification of indium tin oxide (ITO)‐coated substrates through the use of self‐assembled monolayers (SAMs) of molecules with permanent dipole moments has been used to control the anode work function and device performance in molecular solar cells based on a CuPc:C₆₀ (CuPc: copper phthalocyanine) heterojunction. Use of SAMs increases both the short‐circuit current density (J_sc) and fill factor, increasing the power‐conversion efficiency by up to an order of magnitude. This improvement is attributed primarily to an enhanced interfacial charge transfer rate at the anode, due to both a decrease in the interfacial energy step between the anode work function and the highest occupied molecular orbital (HOMO) level of the organic layer, and a better compatibility of the SAM‐modified electrodes with the initial CuPc layers, which leads to a higher density of active sites for charge transfer. An additional factor may be the influence of increasing electric field at the heterojunction on the exciton‐dissociation efficiency. This is supported by calculations of the electric potential distribution for the structures. Work‐function modification has virtually no effect on the open‐circuit voltage (V_oc), in accordance with the idea that V_oc is controlled primarily by the energy levels of the donor and acceptor materials.     

Surface‐Transfer Doping of Organic Semiconductors Using Functionalized Self‐Assembled Monolayers

Controlling charge doping in organic semiconductors represents one of the key challenges in organic electronics that needs to be solved in order to optimize charge transport in organic devices. Charge transfer or charge separation at the molecule/substrate interface can be used to dope the semiconductor (substrate) surface or the active molecular layers close to the interface, and this process is referred to as surface‐transfer doping. By modifying the Au(111) substrate with self‐assembled monolayers (SAMs) of aromatic thiols with strong electron‐withdrawing trifluoromethyl (CF₃) functional groups, significant electron transfer from the active organic layers (copper(II) phthalocyanine; CuPc) to the underlying CF₃‐SAM near the interface is clearly observed by synchrotron photoemission spectroscopy. The electron transfer at the CuPc/CF₃‐SAM interface leads to an electron accumulation layer in CF₃‐SAM and a depletion layer in CuPc, thereby achieving p‐type doping of the CuPc layers close to the interface. In contrast, methyl (CH₃)‐terminated SAMs do not display significant electron transfer behavior at the CuPc/CH₃‐SAM interface, suggesting that these effects can be generalized to other organic‐SAM interfaces. Angular‐dependent near‐edge X‐ray absorption fine structure (NEXAFS) measurements reveal that CuPc molecules adopt a standing‐up configuration on both SAMs, suggesting that interface charge transfer has a negligible effect on the molecular orientation of CuPc on various SAMs.     

Molecular Heterojunctions of Oligo(phenylene ethynylene)s with Linear to Cruciform Framework

Electrical transport properties of molecular junctions are fundamentally affected by the energy alignment between molecular frontier orbitals (highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO)) and Fermi level (or work function) of electrode metals. Dithiafulvene (DTF) is used as substituent group to the oligo(phenylene ethynylene) (OPE) molecular wires and different molecular structures based on OPE3 backbone (with linear to cruciform framework) are achieved, with viable molecular orbitals and HOMO–LUMO energy gaps. OPE3, OPE3–DTF, and OPE3–tetrathiafulvalene (TTF) can form good self‐assembled monolayers (SAMs) on Au substrates. Molecular heterojunctions based on these SAMs are investigated using conducting probe–atomic force microscopy with different tips (Ag, Au, and Pt) and Fermi levels. The calibrated conductance values follow the sequence OPE3–TTF > OPE3–DTF > OPE3 irrespective of the tip metal. Rectification properties (or diode behavior) are observed in case of the Ag tip for which the work function is furthest from the HOMO levels of the OPE3s. Quantum chemical calculations of the transmission qualitatively agree with the experimental data and reproduce the substituent effect of DTF. Zero‐bias conductance, and symmetric or asymmetric couplings to the electrodes are investigated. The results indicate that improved fidelity of molecular transport measurements may be achieved by systematic studies of homologues series of molecular wires applying several different metal electrodes.     

The Effects of Embedded Dipoles in Aromatic Self‐Assembled Monolayers

Using a representative model system, here electronic and structural properties of aromatic self‐assembled monolayers (SAMs) are described that contain an embedded, dipolar group. As polar unit, pyrimidine is used, with its orientation in the molecular backbone and, consequently, the direction of the embedded dipole moment being varied. The electronic and structural properties of these embedded‐dipole SAMs are thoroughly analyzed using a number of complementary characterization techniques combined with quantum‐mechanical modeling. It is shown that such mid‐chain‐substituted monolayers are highly interesting from both fundamental and application viewpoints, as the dipolar groups are found to induce a potential discontinuity inside the monolayer, electrostatically shifting the core‐level energies in the regions above and below the dipoles relative to one another. These SAMs also allow for tuning the substrate work function in a controlled manner independent of the docking chemistry and, most importantly, without modifying the SAM‐ambient interface.     

Energy‐Level Alignment at the Organic/Electrode Interface in Organic Optoelectronic Devices

It is commonly believed that the work‐function reduction effect of the cathode interfacial material in organic electronic devices leads to better energy‐level alignment at the organic/electrode interface, which enhances the device performance. However, there is no agreement on the exact dipole direction in the literature. In this study, a peel‐off method to reveal the buried organic/metal interface to examine the energy‐level alignment is developed. By splitting the device at different interfaces, it is discovered that oppositely oriented dipoles are formed at different surfaces of the interfacial layer. Moreover, the function of the electrode interface differs in different device types. In organic light‐emitting diodes, the vacuum‐level alignment generally occurs at the organic/cathode interface, while in organic photovoltaic devices, the Fermi‐level pinning commonly happens. Both are determined by the integer charge‐transfer levels of the organic materials and the work‐function of the electrode. As a result, the performance enhancement by the cathode interfacial material in organic photovoltaic devices cannot be solely explained by the energy‐level alignment. The clarification of the energy‐level alignment not only helps understand the device operation but also sets up a guideline to design the devices with better performance.     

Controlling Electron and Hole Charge Injection in Ambipolar Organic Field‐Effect Transistors by Self‐Assembled Monolayers

Controlling contact resistance in organic field‐effect transistors (OFETs) is one of the major hurdles to achieve transistor scaling and dimensional reduction. In particular in the context of ambipolar and/or light‐emitting OFETs it is a difficult challenge to obtain efficient injection of both electrons and holes from one injecting electrode such as gold since organic semiconductors have intrinsically large band gaps resulting in significant injection barrier heights for at least one type of carrier. Here, systematic control of electron and hole contact resistance in poly(9,9‐di‐n‐octylfluorene‐alt‐benzothiadiazole) ambipolar OFETs using thiol‐based self‐assembled monolayers (SAMs) is demonstrated. In contrast to common believe, it is found that for a certain SAM the injection of both electrons and holes can be improved. This simultaneous enhancement of electron and hole injection cannot be explained by SAM‐induced work‐function modifications because the surface dipole induced by the SAM on the metal surface lowers the injection barrier only for one type of carrier, but increases it for the other. These investigations reveal that other key factors also affect contact resistance, including i) interfacial tunneling through the SAM, ii) SAM‐induced modifications of interface morphology, and iii) the interface electronic structure. Of particular importance for top‐gate OFET geometry is iv) the active polymer layer thickness that dominates the electrode/polymer contact resistance. Therefore, a consistent explanation of how SAM electrode modification is able to improve both electron and hole injection in ambipolar OFETs requires considering all mentioned factors.     

Using Self‐Assembling Dipole Molecules to Improve Hole Injection in Conjugated Polymers

Surface modification of indium‐tin‐oxide (ITO)‐coated substrates through the use of self‐assembled monolayers (SAMs) of molecules with permanent dipole moments has been used to control the ITO work function and device performance in polymer light‐emitting diodes based on a polyfluorene hole transporting copolymer. Measured current–voltage characteristics of the devices reveal greatly increased hole injection currents from the SAM‐altered electrodes with higher work function, in agreement with an expected reduction in the barrier for hole injection. In particular, it is shown that the SAM‐modified electrode with the highest work function provides an ohmic contact for hole injection into the studied polymer. Injection from the widely used poly(2,3‐ethylenedioxythiophene)/polystyrenesulphonic acid (PEDOT:PSS)‐coated ITO anode system, is less efficient compared with some of the studied SAM‐coated ITO anodes despite the significantly higher work function measured by a Kelvin probe. This apparently anomalous situation is attributed to the inhomogenities in the injection processes that occur over the area of the device when the PEDOT:PSS‐coated ITO electrode is used.     

Redox‐Induced Asymmetric Electrical Characteristics of Ferrocene‐Alkanethiolate Molecular Devices on Rigid and Flexible Substrates

The electrical properties of ferrocene‐alkanethiolate self‐assembled monolayers (SAMs) on a high yield solid‐state device structure are investigated. The devices are fabricated using a conductive polymer interlayer between the top electrode and the SAM on both silicon‐based rigid substrates and plastic‐based flexible substrates. Asymmetric electrical transport characteristics that originate from the ferrocene moieties are observed. In particular, a distinctive temperature dependence of the current (i.e., a decrease in current density as temperature increases) at a large reverse bias, which is associated with the redox reaction of ferrocene groups in the molecular junction, is found. It is further demonstrated that the molecular devices can function on flexible substrates under various mechanical stress configurations with consistent electrical characteristics. This study enhances the understanding of asymmetric molecules and may lead to the development of functional molecular electronic devices on both rigid and flexible substrates.     

Dynamic Photoswitching of Electron Energy Levels at Hybrid ZnO/Organic Photochromic Molecule Junctions

The functionality of interfaces in hybrid inorganic/organic (opto)electronic devices is determined by the alignment of the respective frontier energy levels at both sides of the heterojunctions. Controlling the interface electronic landscape is a key element for achieving favourable level alignment for energy and charge transfer processes. Here, it is shown that the electronic properties of polar ZnO surfaces can be reversibly modified using organic photochromic switches. By employing a range of surface characterization techniques combined with density functional theory calculations, it is demonstrated that self‐assembled monolayers (SAMs) of photochromic phosphonic acid diarylethenes (PA‐DAEs) can be employed to reversibly change the electronic properties of polar ZnO/SAM structures by light stimuli. The highest occupied molecular orbital level of PA‐DAE is raised by 0.7 eV and the lowest unoccupied one lowered by 0.9 eV, respectively, upon illumination by ultraviolet light and the levels shift back to their original position upon illumination by green light. The results thus provide a pathway to tailor hybrid interface electronic properties in a dynamic manner upon simple light illumination, which can be exploited to reversibly tune the electrical properties of photoswitchable (opto)electronic devices.     

The Swiss‐Army‐Knife Self‐Assembled Monolayer: Improving Electron Injection,Stability, and Wettability of Metal Electrodes with a One‐Minute Process

A novel Self‐assembled Monolayer (SAM) forming molecule bisjulolidyldisulfide (9,9'‐disulfanediylbis(2,3,6,7‐tetrahydro‐1H,5H‐pyrido[3,2,1‐ij]quinoline)) is demonstrated which lowers the work function of metal surfaces by ≈1.2 eV and can be deposited in a 1 min process. Bisjulolidyldisulfide exists in a stable disulfide configuration prior to surface exposure and can therefore be stored, handled, and processed in ambient conditions. SAM from bisjulolidyldisulfide are deposited on metal surfaces (Au and Ag), including inkjet printed Ag on polyethylene terephthalate substrates, investigated by photoelectron and infrared spectroscopy, and used as electrodes in n‐type organic field effect transistor (OFET). Treatment of electrodes in OFET devices with with bisjulolidyldisulfide‐SAMs reduces the contact resistance by two orders of magnitude and improves shelf life with respect to pristine metal electrodes. The presented treatment also increases the surfaces wettability and thereby facilitates solution processing of a subsequent layer. These beneficial properties for device performance, processing, and stability, combined with ease of preparation and handling, render this SAM‐forming molecule an excellent candidate for the high‐throughput production of flexible electronic devices.     

Cover Picture: Formation of Metal Nano‐ and Micropatterns on Self‐Assembled Monolayers by Pulsed Laser Deposition Through Nanostencils and Electroless Deposition (Adv. Funct. Mater. 10/2006)

The cover illustrates two‐step fabrication of metal micro‐ and nanostructures on self‐assembled monolayers (SAMs) by pulsed laser deposition and electroless deposition. Metal–SAM–metal junctions are a key component of molecular electronic devices. Pt was deposited in a micropattern by pulsed laser deposition through a stencil. XPS maps show how the Pt pattern is developed into a Cu pattern using electroless deposition as reported by Ravoo, Brugger, Reinhoudt, Blank, and co‐workers on p. 1337. The Cu pattern can also be observed by optical microscopy (background). Patterns of noble‐metal structures on top of self‐assembled monolayers (SAMs) on Au and SiO₂ substrates have been prepared following two approaches. The first approach consists of pulsed laser deposition (PLD) of Pt, Pd, Au, or Cu through nano‐ and microstencils. In the second approach, noble‐metal cluster patterns deposited through nano‐ and microstencils are used as catalysts for selective electroless deposition (ELD) of Cu. Cu structures are grown on SAMs on both Au and SiO₂ substrates and are subsequently analyzed using X‐ray photoelectron spectroscopy element mapping, atomic force microscopy, and optical microscopy. The combination of PLD through stencils on SAMs followed by ELD is a new method for the creation of (sub)‐micrometer‐sized metal structures on top of SAMs. This method minimizes the gas‐phase deposition step, which is often responsible for damage to, or electrical shorts through, the SAM.     

Charge Transport through Oligoarylene Self‐assembled Monolayers: Interplay of Molecular Organization,Metal–Molecule Interactions,and Electronic Structure

The electrical properties of two molecular wires?a novel aryl moiety, 6‐(5‐pyridin‐2‐ylpyrazin‐2‐yl)pyridine‐3‐thiol (PPPT), and the well studied 1,1';4',1''‐terphenyl‐4‐thiol (TPT)?organized in self‐assembled monolayers (SAMs) are measured using metal–molecule–metal (MMM) mercury‐drop junctions. Current measured at the same bias voltage through PPPT is found to be more than one order of magnitude lower than through TPT. To interpret and understand these results, characterization of the structure, organization of the SAMs, and theoretical analyses of the molecular systems are discussed. X‐ray photoelectron spectroscopy (XPS) and near‐edge X‐ray absorption fine structure spectroscopy (NEXAFS) indicate that although PPPT forms high‐quality SAMs on both Au and Ag substrates, it exhibits a lower packing density (by 20 %) and less orientational order than TPT. In addition, electronic structure calculations with density functional theory (DFT) reveal that the electron‐withdrawing nitrogen atoms in the PPPT aryl backbone stabilize the valence molecular electronic structure and pull negative charge from the thiol sulfur. This behavior can influence both charge‐injection barriers and metal–molecule binding interactions in the MMM junctions. The current–voltage data are interpreted on the basis of a hole‐tunneling, through‐bond mechanism. Conductance analysis through a model for off‐resonant tunneling transport suggests that a comparatively small difference in the charge‐injection barrier can explain the factor of ten difference in observed conduction.     

Electroactive Self‐Assembled Monolayers for Enhanced Efficiency and Stability of Electropolymerized Luminescent Films and Devices

In order to further improve the efficiency and stability of luminescent electrochemical polymerization (EP) films and devices, electroactive self‐assembled monolayers (SAMs) of carbazolyl alkanethiol are successfully designed and applied to modify Au electrode and covalently graft the deposited EP films. The analysis of the formation and coverage of the SAMs by atomic force microscopy (AFM), cyclic voltammetry (CV), and the theoretical calculation provide consistent results indicating the SAM molecules are densely packed and standing upright (liquid‐like) on the Au surface. In addition, ultraviolet photoelectron spectroscopy (UPS), CV, UV, AFM, and sonication treatment reveal that the close‐packed electroactive SAMs are effective at enhancing the work function of electrode, increasing the deposition rate of EP precursor as well as elevating the cross‐linking efficiency and the adhesive property of subsequent EP films. This is a simple and very efficient method for improving the performance of EP device, which has potential applications in display devices.     

Single Component Self‐Assembled Monolayers of Aromatic Azo‐Biphenyl: Influence of the Packing Tightness on the SAM Structure and Light‐Induced Molecular Movements

Aiming at modulating the packing density within functional self‐assembled monolayers (SAMs), two azo‐biphenyl derivatives AZO1 and AZO2 comprising a terminal sulfur anchor group have been designed and synthesized. While AZO1 allows for a coplanar arrangement of both biphenyl subunits, additional steric repulsion due to two methyl side groups attached to the footing biphenyl of AZO2 results in an increased intermolecular distance within the SAM, providing additional free volume. SAMs of both derivatives on gold and platinum substrates have been formed and thoroughly investigated by photoelectron (XPS) and near‐edge absorption fine structure (NEXAFS) spectroscopy as well as cyclic voltammetry and scanning tunneling microscopy. These measurements confirmed the formation of tightly packed SAMs for AZO1 , while AZO2 formed SAMs consisting of less organized and more loosely packed molecules. Optical investigations of both azo derivatives in solution as well as their SAMs displayed efficient photoisomerization in solution and in SAMs. Comparable maximal cis/trans ratios of ca. 0.9 have been observed in all cases upon irradiation at λ = 370 and 360 nm for AZO1 and AZO2 , respectively. The thermally induced cis → trans back reaction on AZO1 was found to be slower by a factor of 3 in SAMs as compared to solution, while AZO2 displayed comparable rates of the back reaction in both environments. This behavior can be explained by the different nature of molecular isomerization in the two SAM systems: whereas the isomerization in AZO1 SAMs takes place in a highly coordinated, collective way and involves many adjacent molecules, AZO2 species behave rather individually even packed in SAMs, such that their isomerization process is similar in SAMs and in solutions.     

A Solvent‐Free Solution: Vacuum‐Deposited Organic Monolayers Modify Work Functions of Noble Metal Electrodes

The energy level alignment between organic semiconductors (OSCs) and the respective (metal) electrodes in organic electronic devices is of key importance for efficient charge carrier injection. For many years, researchers have attempted to control this energy level alignment by means of functional self‐assembled monolayers or the insertion of thin injection layers (made, e.g., of doped OCSs or pure dopants). The present work demonstrates an alternative to these approaches, namely the use of phthalocyanine monolayers as contact primers, which are deposited onto noble metal electrodes by means of vacuum deposition. It is shown that polar as well as non‐polar phthalocyanines modify the work functions of clean Au(111) and Ag(111) surfaces as a function of their coverage and thus enable quantitative control of the metal work functions. This behavior is successfully replicated for the respective polycrystalline metal surfaces and it is found that full monolayers can even withstand air exposure when protected by sacrificial multilayers, which are afterward removed by thermal desorption.     

Fermi Level Pinning and Orbital Polarization Effects in Molecular Junctions: The Role of Metal Induced Gap States

Understanding the alignment of molecular orbitals and corresponding transmission peaks with respect to the Fermi level of the electrodes is a major challenge in the field of molecular electronics. In order to design functional devices, it is of utmost importance to assess whether controlled changes in the electronic structure of isolated compounds are preserved once they are inserted in the molecular junctions. Here, light is shed on this central issue by performing density functional theory calculations on junctions including diarylethene‐based molecules. It is demonstrated that the chemical potential equalization principle allows to rationalize the existence or not of a Fermi level pinning (i.e., same alignment in spite of a varying ionization potential in the isolated compounds), pointing to the essential role played by metal induced gap states (MIGS). It is further evidenced that the degree of level pinning is intimately linked to the degree of orbital polarization when a bias is applied between the two electrodes.