Organic and printed electronics technologies require conductors with a work function that is sufficiently low to facilitate the transport of electrons in and out of various optoelectronic devices. We show that surface modifiers based on polymers containing simple aliphatic amine groups substantially reduce the work function of conductors including metals, transparent conductive metal oxides, conducting polymers, and graphene. The reduction arises from physisorption of the neutral polymer, which turns the modified conductors into efficient electron-selective electrodes in organic optoelectronic devices. These polymer surface modifiers are processed in air from solution, providing an appealing alternative to chemically reactive low–work function metals. Their use can pave the way to simplified manufacturing of low-cost and large-area organic electronic technologies.

http://science.sciencemag.org/content/sci/336/6079/327.full.pdf

A Universal Method to Produce Low―Work Function Electrodes for Organic Electronics

In this Review, we summarize recent work on modeling of organic/metal and organic/organic interfaces. Some of the models discussed have a semiempirical approach, that is, experimentally derived values are used in combination with theory, and others rely completely of calculations. The models are categorized according to the types of interfaces they apply to, and the strength of the interaction at the interface has been used as the main factor. We explain the basics of the models, their use, and give examples on how the models correlate with experimental results. We stress that given the complexity of organic/metal and organic/organic interface formation, it is crucial to know the exact way in which the interface was formed before choosing the model that is applicable, as none of the models presented covers the whole range of interface interaction strengths (weak physisorption to strong chemisorption).

Energy‐Level Alignment at Organic/Metal and Organic/Organic Interfaces

This article provides an overview on the recent development of solution processed organic, inorganic, and hybrid interfacial materials for bulk-heterojunction polymer solar cells. The introduction of proper interfacial materials to optimize the electronic and electrical properties between the interfaces of the light-harvesting active layer and the charge-collecting electrode has become an important criterion to improve the performance of polymer solar cells. The electronic processes at these interfaces play a critical role in determining the efficiency for photon-to-electricity conversion. An ideal interface requires the formation of Ohmic contact with minimum resistance and high charge selectivity to prevent charge carriers from reaching the opposite electrodes. For long-term stability of polymer solar cells, interfaces with matched surface energy are required to prevent interfacial dewetting and delamination. Several classes of interfacial materials including inorganic metal oxides, crosslinkable charge-transporting materials, conjugated polymer electrolytes, self-assembled functional molecules, and graphene-based materials are highlighted and the integration of these interfacial materials with new low bandgap polymers and fullerene derivatives as active materials in different device architectures is also discussed.

Recent advances in solution-processed interfacial materials for efficient and stable polymer solar cells

Transition-metal oxides improve power conversion efficiencies in organic photovoltaics and are used as low-resistance contacts in organic light-emitting diodes and organic thin-film transistors. What makes metal oxides useful in these technologies is the fact that their chemical and electronic properties can be tuned to enable charge exchange with a wide variety of organic molecules. Although it is known that charge exchange relies on the alignment of donor and acceptor energy levels, the mechanism for level alignment remains under debate. Here, we conclusively establish the principle of energy alignment between oxides and molecules. We observe a universal energy-alignment trend for a set of transition-metal oxides--representing a broad diversity in electronic properties--with several organic semiconductors. The trend demonstrates that, despite the variance in their electronic properties, oxide energy alignment is governed by one driving force: electron-chemical-potential equilibration. Using a combination of simple thermodynamics, electrostatics and Fermi statistics we derive a mathematical relation that describes the alignment.

Universal energy-level alignment of molecules on metal oxides

The rapid development of novel organic technologies has led to significant applications of the organic electronic devices such as light-emitting diodes, solar cells, and field-effect transistors. There is a great need for conducting polymers with high conductivity and transparency to act as the charge transport layer or electrical interconnect in organic devices. Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonic acid) (PEDOT:PSS), well-known as the most remarkable conducting polymer, has this role owing to its good film-forming properties, high transparency, tunable conductivity, and excellent thermal stability. In this Review, various of interesting physical and chemical approaches that can effectively improve the electrical conductivity of PEDOT:PSS are summarized, focusing especially on the mechanism of the conductivity enhancement as well as applications of PEDOT:PSS films. Prospects for future research efforts are also provided. It is expected that PEDOT:PSS films with high conductivity and transparency could be the focus of future organic electronic materials breakthroughs.

Effective Approaches to Improve the Electrical Conductivity of PEDOT:PSS: A Review

Photoelectron spectroscopy has been used to map out energy level alignment of conjugated polymers at various organic-organic and hybrid interfaces. Specifically, we have investigated the hole-injection interface between the substrate and light-emitting polymer. Two different alignment regimes have been observed: (i) Vacuum-level alignment, which corresponds to the lack of vacuum-level offsets (Schottky–Mott limit) and (ii) Fermi-level pinning, where the substrate Fermi level and the positive polaronic level of the polymer align. The observation is rationalized in terms of spontaneous charge transfer whenever the substrate Fermi level exceeds the positive polaron/bipolaron formation energy per particle. The charge transfer leads to the formation of an interfacial dipole, as large as 2.1 eV.

Fermi-level pinning at conjugated polymer interfaces

Since the Nobel Prize awarded discovery that some polymers or “plastics” can be made electronically conducting, the scientific field of organic electronics has arisen The use of conducting polymers in electronic devices is appealing, because the materials can be processed from a liquid phase, much like ordinary non-conducting plastics This gives the opportunity to utilize printing technologies and manufacture electronics roll-to-roll on flexible substrates, ultimately at very low costs Even more intriguing are the possibilities to achieve completely novel functionalities in combination with the inherent compatibility of these materials with biological species Therefore, organic electronics can be merged with biology and medicine to create organic bioelectronics, ie organic electronic devices that interact with biological samples directly or are used for biological applications This thesis aims at giving a background to the field of organic bioelectronics and focuses on how electrochemical devices may be utilized An organic electronic wettability switch that can be used for lab-on-a-chip applications and control of cell growth as well as an electrochemical ion pump for cell communication and drug delivery are introduced Furthermore, the underlying electrochemical structures that are the basis for the above mentioned devices, electrochemical display pixels etc are discussed in detail In summary, the work contributes to the understanding of electrochemical polymer electronics and, by presenting new bioelectronic inventions, builds a foundation for future projects and discoveries

A Solid-State Organic Electronic Wettability Switch

The fundamentals of the energy level alignment at anode and cathode electrodes in organic electronics are described. We focus on two different models that treat weakly interacting organic/metal (and organic/organic) interfaces: the induced density of interfacial states model and the so-called integer charge transfer model. The two models are compared and evaluated, mainly using photoelectron spectroscopy data of the energy level alignment of conjugated polymers and molecules at various organic/metal and organic/organic interfaces. We show that two different alignment regimes are generally observed: (i) vacuum level alignment, which corresponds to the lack of vacuum level offsets (Schottky-Mott limit) and hence the lack of charge transfer across the interface, and (ii) Fermi level pinning where the resulting work function of an organic/metal and organic/organic bilayer is independent of the substrate work function and an interface dipole is formed due to charge transfer across the interface. We argue that the experimental results are best described by the integer charge transfer model which predicts the vacuum level alignment when the substrate work function is above the positive charge transfer level and below the negative charge transfer level of the conjugated material. The model further predicts Fermi level pinning to the positive (negative) charge transfer level when the substrate work function is below (above) the positive (negative) charge transfer level. The nature of the integer charge transfer levels depend on the materials system: for conjugated large molecules and polymers, the integer charge transfer states are polarons or bipolarons; for small molecules' highest occupied and lowest unoccupied molecular orbitals and for crystalline systems, the relevant levels are the valence and conduction band edges. Finally, limits and further improvements to the integer charge transfer model are discussed as well as the impact on device design.

https://iopscience.iop.org/article/10.1088/0953-8984/19/18/183202/pdf

Electronic structure of hybrid interfaces for polymer-based electronics

Photoelectron spectroscopy was employed to examine the energetics, and therefore charge injection barriers, at top and bottom contact configurations of gold and conjugated polymers, i.e., polymer spin coated on gold and vapor-deposited gold on polymer interfaces. Very similar results are obtained for both ex situ (contaminated) and in situ (clean) prepared interfaces: a 0.7–0.8eV decrease in the vacuum energy levels is consistently observed as compared to bare polycrystalline gold. These observations are explained by changes of the metal work function upon contacting either polymers or contaminants, associated with the reduction of the electron density tail that extends outside the metal surface.

Energetics at Au top and bottom contacts on conjugated polymers

Thin films of V(TCNE)x were deposited in ultrahigh vacuum using a film growth technique based on in situ chemical vapor deposition of tetracyanoethylene, TCNE, and bis-benzene vanadium, V(C6H6)2 The in situ preparation method enabled, for the first time, experimental analysis of oxygen-free films X-ray magnetic circular dichroism measurements recorded at the V L2,3 edge confirmed room temperature magnetic ordering A combination of conventional photoelectron spectroscopy (PES) and resonant photoemission (RPE) measured at the V L3 edge shows that the highest occupied electronic state is V(3d) derived The rearrangements of the TCNE- related valence electronic states observed in PES and the evidence of V(3d) and TCNE- p(p*) orbital overlap contained in RPE spectra, indicate that strong, covalent type bonding occurs between the vanadium and the TCNE molecules © 2006 The American Physical Society

Carl Tengstedt

Papers

Fermi-level pinning at conjugated polymer interfaces

A Solid-State Organic Electronic Wettability Switch

Electronic structure of hybrid interfaces for polymer-based electronics

Energetics at Au top and bottom contacts on conjugated polymers

X-ray magnetic circular dichroism and resonant photomission of V(TCNE)x hybrid magnets.