We explore the different mechanisms through which self-assembled monolayers can tailor energy level alignment at metal-organic semiconductor interfaces. We show that the large work function variation that can be induced by the self-assembled monolayer on gold has limited ability to tailor the interface energy level alignment of a subsequent organic semiconductor overlayer. (C) 2015 Elsevier B.V. All rights reserved.

We report a method for inkjet-printing an organic semiconductor layer on top of the electrolyte insulator layer in polyelectrolyte-gated OFETs by using a surface modification treatment to overcome the underlying wettability problem at this interface. The method includes depositing an amphiphilic diblock copolymer (P3HT-b-PDMAEMA). This material is designed to have one set of blocks that mimics the hydrophobic properties of the semiconductor (poly(3-hexylthiophene) or P3HT), while the other set of blocks include polar components that improve adhesion to the polyelectrolyte insulator. Contact angle measurements, atomic force microscopy, and X-ray photoelectron spectroscopy confirm formation of the desired surface modification film. Successful inkjet printing of a smooth semiconductor layer allows us to manufacture complete transistor structures that exhibit low-voltage operation in the range of 1 V.

The energy level alignment in metal-organic and organic-organic junctions of the widely used materials tris-(8-hydroxyquinoline)aluminum (Alq₃) and 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA) is investigated. The measured alignment schemes for single and bilayer films of Alq₃ and NTCDA are interpreted with the integer charge transfer (ICT) model. Single layer films of Alq₃ feature a constant vacuum level shift of ∼0.2–0.4 eV in the absence of charge transfer across the interface. This finding is attributed to the intrinsic dipole of the Alq₃ molecule and (partial) ordering of the molecules at the interfaces. The vacuum level shift changes the onset of Fermi level pinning, as it changes the energy needed for equilibrium charge transfer across the interface.

Device function in organic electronics is critically governed by the transport of charge across interfaces of dissimilar materials. Accurate measurements of energy level positions in organic electronic devices are therefore necessary for assessing the viability of new materials and optimizing device performance. In contrast to established methods that are used in solution or vacuum environments, here we combine Kelvin probe measurements performed in ambient environments to obtain work function values with photoelectron spectroscopy in air to obtain ionization potential, so that a complete energy level diagram for organic semiconductors can be determined. We apply this new approach to study commonly used electron donor and acceptor materials in organic photovoltaics (OPV), including poly(3-hexylthiophene) (P3HT), [6,6]-phenyl C61 butyric acid methyl ester (PCBM), and ZnO, as well as examine new materials. Band alignments across the entire OPV devices are constructed and compared with actual device performance. The ability to determine interfacial electronic properties in the devices enables us to answer the outstanding question: why previous attempts to make OPV devices using 6,13-bis(triisopropylsilylethynyl) (TIPS)-pentacene as the electron donor were not successful.

Organic electronics is a field covering all applications and devices where one or several of the active components are made of organic material, such as organic light emitting diodes, organic solar cells, organic thin film transistors, organic magnets for spintronics etc. In all of the applications mentioned above, transport of charges across both inorganic/organic and organic/organic interfaces play a key role for device performance. In order to achieve high efficiencies and longer life-times, proper matching of the electronic energy levels of the different materials is needed.

The aim of the research presented in this thesis has been to explore different routes to optimize interface energetics and gain deeper knowledge of the mechanisms that govern charge transport over the interface. Photoelectron spectroscopy (PES) is a method well suited to study both interactions between different materials taking place at surfaces as well as interface energetics.

One way to achieve proper matching of interfaces energy levels is by adding a dipole layer. In the three first papers presented in the thesis, the method of adding a monolayer of small organic molecules to change the work function of the surface is investigated. We start with a model system consisting of a nickel surface and PPDA molecules where we have strong interaction and mixing of orbitals between the molecule and the metal surface. The second system consists of a gold surface and TDAE molecules with weaker interaction with integer electron transfer and finally in the third paper an organic surface VPP-PEDOT-Tos is modified, with TDAE, to create a transparent low work function organic electrode. In the fourth paper, we focus on gaining deeper understanding of the Integer Charge Transfer (ICT) model and the mechanisms governing the alignment of energy levels at organic/(in)organic interfaces and in the fifth paper we continue to challenge this model by using it to predict the behavior of a bilayer device, in terms of energy level alignment.

Ultraviolet photoelectron spectroscopy measurements in combination with the integer charge transfer (ICT) model is used to obtain the energy-level alignment diagrams for two common types of bulk-heterojunction solar cell devices based on poly(3-hexylthiophene) or poly(2-methoxy-5-(3,7 -dimethyloctyloxy)- 1,4-phenylene vinylene) as the donor polymer and (6,6)phenyl- C61-butric-acid as the acceptor molecule. A ground-state interface dipole at the donor/acceptor heterojunction is present for both systems, but the origin of the interface dipole differs, quadrupole-induced in the case of poly(2-methoxy-5-(3,7-dimethyl-octyloxy)-1,4-phenylene vinylene), and ICT state based for poly(3-hexylthiophene). The presence of bound electron-hole charge carriers (CT states) and/or interface dipoles are expected to enhance exciton dissociation into free charge carriers, thus reducing the probability that charges become trapped by Coulomb forces at the interface followed by recombination.

In organic-based electronics, interfacial properties have a profound impact on device performance. The lineup of energy levels is usually dependent on interface dipoles, which may arise from charge transfer reactions. In many applications, metal-organic junctions are prepared under ambient conditions, where direct overlap of the organic system from the metal bands is prevented due to presence of oxides and/or hydrocarbons. We present direct experimental and theoretical evidence showing that the interface energetic for such systems is governed by exchange of an integer amount of electrons.

All-plastic light emitting diodes require the design and fabrication of low work function plastic electrodes. Here, we show that the work function of poly(3,4-ethylenedioxythiophene)-polystyrenesulfonic acid (PEDOT-PSS) can be decreased from 4.8 eV to 3.9 eV by surface reaction with the strong electron-donor tetrakis(dimethylamino)ethylene (TDAE). The surface modification was characterized by photoelectron spectroscopy and optical spectroscopy. The low work function plastic electrode was used in a first prototype for all-plastic light emitting diodes.

Novel applications for flexible electronics, e.g., displays and solar cells, require fully flexible, transparent, stable, and low-work-function electrodes that can be manufactured via a low-cost process. Here, we demonstrate that surface chemistry constitutes a route to producing transparent low-work-function plastic electrodes. The work function of the conducting polymer poly(3,4-ethylenedioxythiophene)-tosylate, or PEDOT-Tos, is decreased by submonolayer surface redox reaction with a strong electron donor, tetrakis-(dimethylamino)ethylene (TDAE), allowing it to reach a work function of 3.8 eV. The interface formed between TDAE and PEDOT is investigated in a joint experimental and theoretical study using photoelectron spectroscopy and quantum chemical calculations. © 2006 American Chemical Society.