Charge Injection Layers in Inorganic /Organic Interfaces

Research in the field of organic electronics has surged in recent years, not least due to its promising impact on the development of new, affordable, lightweight, mechanically flexible and environmentally friendly electronic products. In these devices, charge injection layers (CILs) are commonly added between the inorganic electrode and the active organic material to optimize charge injection (respectively extraction) barriers and exciton lifetimes in organic light emitting devices (OLEDs) or photovoltaic cells (OPVs). Via charge-transfer induced or intrinsic molecular dipoles, these layers alter the electron potential landscape at the interface, thereby shifting the relative position of substrate and adsorbate levels. Most studies in this field have focused on the effect of the CIL on the effective work function on the substrate. The question beyond, how CILs affect the morphology of subsequently deposited organic material, and in particular what the effect on eventual charge transfer processes at the interface is, remains open. The importance of the question is reinforced by the observation that contaminations like H or OH groups, which are commonly frequent on inorganic substrates such as ZnO, can be viewed as a kind of CIL as well. Therefore, a density-functional theory (DFT) study based on advanced exchange-correlation functionals (including hybrid and non-local functionals), as well as many-body perturbation theory, such as the GW approach and the random-phase- approximation (RPA), is proposed. These functionals mix in a fraction of exact exchange, thereby reducing the self-interaction error which may lead to spurious charge transfer and level alignment in conventional DFT approaches. In this project, the influence of various CILs on the morphology, electronic levels and quasi-particle energies of the active organic material is analyzed for the example of different combinations of CILs and prototypical organic materials (such as pentacene) adsorbed on zinc oxide substrates. In collaboration with experimental partners, the mechanisms of bonding and interface dipole formation at potentially technologically relevant interfaces will be investigated. A particular aspect will also be the thermodynamic stability of the CIL in the ternary system, monitoring adsorbate-induced phase changes or displacement of the layer. Computationally exploring various different hypothetical geometries will furthermore allow devising structure-to-property relationships. The achieved general insight into these interfaces will help developing new surface modifications to improve the efficiency of present-day devices.

Many products containing organic electronics can now be found in everyday life. These include, e.g., displays in mobile phones or OLED-TVs. To a large degree, the performance of these devices is determined by the interface between inorganic electrodes and the active organic material. To improve the performance, so-called charge-injection layers (CILs) can be inserted between the two components. The purpose of this project was to understand the impact of these CILs in more detail.One of the main findings of this project was that upon use of these CILs, the distribution of charge directly at the interfaces changes drastically. When organic materials are deposited directly onto a metal electrode, any charge that is transferred to the organic material is distributed evenly among all molecules in the first layer. This is depicted in the figure to the left. In contrast, when a CIL is used say, e.g., a doublelayer of NaCl the charge flows only to some molecules, while other molecules remain electrically neutral. This situation is schematically shown in the figure to the right. The difference in charge distribution affects barriers for charge injection as well as charge- transport properties across the interface, which are crucial for device operation. It may thus be the main reason for the observed impact of CILs.Before these insights could be obtained, however, a deeper understanding of the methods commonly employed to theoretically describe these interfaces had to be developed. The most popular computational method to study interfaces is a method called density functional theory. Although this theory is in principle exact, several approximations have to be applied to make it solvable for real systems. Naturally, these introduce errors. Most prominent among them is the so-called self-interaction error, i.e., the interaction of an electron with itself. In this project, we were able to show that for strongly interacting systems, this error is mostly irrelevant. However, when CILs are used, the interaction between electrode and organic material becomes much weaker. In this situation, the self- interaction error must be properly corrected. This can be done, e.g., by using an advanced class of functionals (called hybrid functionals) or by using many-body perturbation theory.