Opportunities for energy level tuning at inorganic/organic semiconductor interfaces

Abstract

The aim of this Perspective is to provide an overview of approaches that can be employed to tune the energy level alignment at interfaces between inorganic and organic semiconductors for use in electronic and optoelectronic devices. The approaches include tailoring intramolecular dipolar bond distribution, controlling molecular orientation at interfaces, and the insertion of a molecularly thin interlayer that abruptly shifts the electrostatic potential between the two semiconductors and, thus, affords level tuning. With these state of the art methods, the frontier energy levels at an inorganic/organic heterojunction can be varied up to ca. 3 eV, i.e., covering the energy gap of most semiconductors. By combining two or more of these approaches or by employing interfacial molecular switches, it is envisioned that unconventional and dynamically switchable interfacial energy level scenarios can be created, enabling expanded or superior device functionality.

Energetic disorder impacts energy-level alignment of alpha-sexithiophene on hydrogen-terminated silicon and silicon oxide

“…On Si the shift of VL (0.09 eV) by 6 T deposition is almost parallel to that of Si 2p (0.06 eV). Downwards band bending of similar magnitude upon contact formation of 6 T was observed on other Si surfaces [37] and the associated electron doping into the conduction band of Si is in accordance with the donor character of 6 T. Within the 6 T layer no energy-level bending can be observed, which can be expected based on general considerations film [3,9,10]: The transport gap of 6 T thin films is 2.8 eV [27] and the HOMO-onset in the multilayer film on Si can be found at 1.13 eV below E F , which is a rather mid-gap position. Furthermore, no tailing states at the low BE side of the HOMO can be observed (figure 1(c)) and, consequently, the position of E F is mainly determined by VL of the substrate, i.e., effective VL-alignment takes place.…”

“…The energy-level alignment (ELA) at the ubiquitous interfaces of (opto-)electronic applications is decisive for device performance [1][2][3]. Actually, hybrid organic-inorganic interfaces can add functionality to devices, e.g., they can serve as charge-carrier selective contacts for photovoltaic cells [4,5] and thin layers of organic semiconductors (OSCs) can tune ELA at such interfaces [6].…”

“…The ELA between inorganic semiconductors like Si and OSCs can be rather complex, e.g., for rodlike organic semiconductors like pentacene or tetracene the orientation of the long molecular axis with respect to the surface ('standing' or 'lying' molecules) is decisive for ELA [7,8]. In the absence of chemical interaction, ELA is mainly determined by the substrate work function (WF) and the ionization energy (IE) and electron affinity (EA) of the OSC thin film [3,5]. In addition to these quantities also the width of the density of states (DOS) of the frontier molecular orbitals (highest occupied molecular orbital, HOMO and lowest unoccupied molecular orbital, LUMO) of the OSC thin film plays an important role for ELA, in particular, if Fermi-level pinning is involved [9,10].…”

Advanced characterization of organic–metal and organic–organic interfaces: from photoelectron spectroscopy data to energy-level diagrams

“…The functionality of virtually all organic and hybrid (opto-)electronic devices depends on interface energetics, and a thorough understanding of energy-level alignment mechanisms at organic-metal and organic-organic interfaces is indispensable for further efficiency improvements [1][2][3][4][5]. For example, the energy-level offset at the donor-acceptor interface in organic photovoltaic devices is crucial for exciton dissociation [6][7][8]; chemisorbed molecular monolayers on metals can tune the substrate work functions by an interfacial charge transfer [9,10] and allow, consequently, to lower charge injection barriers into electrodes [11,12].…”

Conductivity in Thin Films of Transition Metal Coordination Complexes

“…Modifications of the electronic and physical properties are inherently coupled to these industrial processes and prototyping of new devices is slow, especially when compared to alternative semiconducting materials – such as organic and metal–organic based semiconductors. As such, simple tunability of the physical and electrical properties is a desirable trait for any material that will be employed in electronic devices; tunability that is accessible through simple, well established organic synthesis methods even more so. , These materials offer large synthetic design-spaces and simpler, often low-energy, fabrication methods, − including the exciting prospect of self-assembly methods. − Our study is aimed at investigating how the physical, chemical, and electronic properties of semiconductors based on metal coordination complexes can be modified through the substitution of the metal centers in two coordination materials.…”