Organic heterojunctions: Contact-induced molecular reorientation, interface states, and charge re-distribution

Distributions of Potential and Contact-Induced Charges in Conventional Organic Photovoltaics

The interfaces of dissimilar materials play central roles in photophysical events in organic photovoltaics (OPVs). Depth profiles of electrostatic potential and contact-induced charges determine the energy-level lineup of the frontier orbitals at electrode/organic and organic heterointerfaces. They are critical for the elementary processes in an OPV cell, such as generation and diffusion of free carriers. A simple electrostatic model describes the energetics in organic heterojunctions supported by an electrode, and experiments via photoelectron spectroscopy and the Kelvin probe method validate the potential distribution in the stacking direction of the device. A comparative study has clarified the significance of Fermi-level pinning and resulting electrostatic fields in determining the energy-level alignment. In this review, we discuss how parameters of device constituents affect the distributions of potential and the dark charges in conventional OPVs comprising metallophthalocyanine and C60 as donor and acceptor, respectively. The results of previous studies, together with additional numerical simulations, suggest that a number of the factors influence the depth profiles of the dark charge and potential, such as the work function of bottom materials, layer thickness, structural inhomogeneity at interfaces, top electrode, and stacking sequence.

Rectification ratio and direction controlled by temperature in copper phthalocyanine ensemble molecular diodes

Organic diodes and molecular rectifiers are fundamental electronic devices that share one common feature: current rectification ability. Since both present distinct spatial dimensions and working principles, the rectification of organic diodes is usually achieved by interface engineering, while changes in molecular structures commonly control the molecular rectifiers' features. Here, we report on the first observation of temperature-driven inversion of the rectification direction (IRD) in ensemble molecular diodes (EMDs) prepared in a vertical stack configuration. The EMDs are composed of 20 nm thick molecular ensembles of copper phthalocyanine in close contact with one of its fluorinated derivatives. The material interface was found to be responsible for modifying the junction's conduction mechanisms from nearly activationless transport to Poole-Frenkel emission and phonon-assisted tunneling. In this context, the current rectification was found to be dependent on the interplay of such distinct charge transport mechanisms. The temperature has played a crucial role in each charge transport transition, which we have investigated via electrical measurements and band diagram analysis, thus providing the fundamentals on the IRD occurrence. Our findings represent an important step towards simple and rational control of rectification in carbon-based electronic nanodevices.

From flat to tilted: gradual interfaces in organic thin film growth

We investigate domain formation and local morphology of thin films of α-sexithiophene (α-6T) on Au(100) beyond monolayer coverage by combining high resolution scanning tunneling microscopy (STM) experiments with electronic structure theory calculations and computational structure search. We report a layerwise growth of highly-ordered enantiopure domains. For the second and third layer, we show that the molecular orbitals of individual α-6T molecules can be well resolved by STM, providing access to detailed information on the molecular orientation. We find that already in the second layer the molecules abandon the flat adsorption structure of the monolayer and adopt a tilted conformation. Although the observed tilted arrangement resembles the orientation of α-6T in the bulk, the observed morphology does not yet correspond to a well-defined surface of the α-6T bulk structure. A similar behavior is found for the third layer indicating a growth mechanism where the bulk structure is gradually adopted over several layers.

Tailoring the growth and electronic structures of organic molecular thin films

In the rapidly developing electronics industry, it has become increasingly necessary to explore materials that are cheap, flexible and versatile which have led to significant research efforts towards organic molecular thin films. Organic molecules are unique compared to their inorganic atomic counterparts as their properties can be tuned drastically through chemical functionalization, offering versatility, though their extended shape and weak intermolecular interactions bring significant challenges to the control of both the growth and the electronic structures of molecular thin films. In this paper, we will review the self-assembly process and how to establish long-range ordered organic molecular thin films. We will also discuss how the electronic structures of thin films are impacted by the molecule's local electrostatic environment and its interaction with the substrate, within the context of controlling interfacial energy level alignment between organic semiconductors and electrodes in electronic devices.

Acoustic extraordinary transmission manipulation based on proximity effects of heterojunctions

Heterojunctions between two crystalline semiconductor layers or regions can always lead to engineering the electronic energy bands in various devices, including transistors, solar cells, lasers, and organic electronic devices. The performance of these heterojunction devices depends crucially on the band alignments and their bending at the interfaces, which have been investigated for years according to Anderson's rule, Schottky-Mott rule, Lindhard theory, quantum capacitance, and so on. Here, we demonstrate that by engineering two different acoustic waveguides with forbidden bands, one can achieve an acoustic heterojunction with an extraordinary transmission peak arising in the middle of the former gaps. We experimentally reveal that such a transmission is spatially dependent and disappears for a special junction structure. The junction proximity effect has been realized by manipulating the acoustic impedance ratios, which have been proven to be related to the geometrical (Zak) phases of the bulk bands. Acoustic heterojunctions bring the concepts of quantum physics into the classical waves and the macroscopic scale, opening up the investigations of phononic, photonic, and microwave innovation devices.

Phthalocyanine arrangements on Ag(100): From pure overlayers of CoPc and F16CuPc to bimolecular heterostructure

We have utilized scanning tunneling microscopy (STM) and low energy electron diffraction to determine the structural properties of two types of metal-phthalocyanines (MPcs), i.e., cobalt-phthalocyanine (CoPc) and hexadecafluorinated copper-phthalocyanine (F₁₆CuPc) on the Ag(100) surface. For coverage close to one monolayer, both systems form long-range ordered structures with square unit cells. The size and rotation of the unit cell with respect to the silver lattice depend on the chemical composition of MPc. Both types of molecules prefer adsorption with around a 30° angle between the molecular axis and the [011] silver direction. The CoPcs mainly arrange in a (5 × 5)R0 phase; however, two additional local arrangements, a 26 × 26 R 1 1 ○ and a (7 × 7)R0, were detected by STM. The F₁₆CuPcs form a 29 × 29 R 2 2 ○ structure. The co-adsorption of CoPc and F₁₆CuPc on the Ag(100) surface in a 1:1 ratio leads to the formation of a compositionally ordered chessboard-like 5 2 × 5 2 R 4 5 ○ structure. During filled states imaging, the different appearance of the central part of each MPc allows us to distinguish CoPcs from F₁₆CuPcs. Regardless of the applied voltage polarity, the ligands of F₁₆CuPcs appear brighter than the ligands of CoPcs.

Dynamics of Additive Migration to Form Cathodic Interlayers in Organic Solar Cells

Migration of additives to organic/metal interfaces can be used to self-generate interlayers in organic electronic devices. To generalize this approach for various additives, metals, and organic electronic devices it is first necessary to study the dynamics of additive migration from the bulk to the top organic/metal interface. In this study, we focus on a known cathode interlayer material, polyethylene glycol (PEG), as additive in P3HT:PC₇₁BM blends and study its migration to the blend/Al interface during metal deposition and its effect on organic solar cell (OSC) performance. Using dynamic secondary ion mass spectroscopy (DSIMS) depth profiles and X-ray photoelectron spectroscopy surface analysis (XPS), we quantitatively correlate the initial concentration of PEG in the blend and sequence of thermal annealing/metal deposition processes with the organic/Al interfacial composition. We find that PEG is initially distributed within the film according to the kinetics of the spin coating process, i.e., the majority of PEG accumulates at the bottom substrate, while the minority resides in the film. During electrode evaporation, PEG molecules kinetically "trapped" near the film surface migrate to the organic/Al interface to reduce the interfacial energy. This diffusion-limited process is enhanced with the initial concentration of PEG in the solution and with thermal annealing after metal deposition. In contrast, annealing the film before metal deposition stalls PEG migration. This mechanism is supported by corresponding OSC devices showing that V_oc increases with PEG content at the interface, up to a saturation value associated with the formation of a continuous PEG interlayer. Presence of a continuous interlayer excludes the driving force for further migration of PEG to the interface. Revealing this mechanism provides practical insight for judicious selection of additives and processing conditions for interfacial engineering of spontaneously generated interlayers.

Mapping Energy Levels for Organic Heterojunctions

An organic semiconductor thin film is a solid-state matter comprising one or more molecules. For applications in electronics and photonics, several distinct functional organic thin films are stacked together to create a variety of devices such as organic light-emitting diodes and organic solar cells. The energy levels at these thin-film junctions dictate various electronic processes such as the charge transport across these junctions, the exciton dissociation rates at donor-acceptor molecular interfaces, and the charge trapping during exciton formation in a host-dopant system. These electronic processes are vital to a device's performance and functionality. To uncover a general scientific principle in governing the interface energy levels, highest occupied molecular orbitals, and vacuum level dipoles, herein a comprehensive experimental research is conducted on several dozens of organic-organic heterojunctions representative of various device applications. It is found that the experimental data map on interface energy levels, after correcting variables such as molecular orientation-dependent ionization energies, consists of three distinct regions depending on interface fundamental physical parameters such as Fermi energy, work function, highest occupied molecular orbitals, and lowest unoccupied molecular orbitals. This general energy map provides a master guide in selection of new materials for fabricating future generations of organic semiconductor devices.

Energy-level alignment at interfaces between manganese phthalocyanine and C60

We have used photoelectron spectroscopy to determine the energy-level alignment at organic heterojunctions made of manganese phthalocyanine (MnPc) and the fullerene C₆₀. We show that this energy-level alignment depends upon the preparation sequence, which is explained by different molecular orientations. Moreover, our results demonstrate that MnPc/C₆₀ interfaces are hardly suited for application in organic photovoltaic devices, since the energy difference of the two lowest unoccupied molecular orbitals (LUMOs) is rather small.

Energy level alignment at planar organic heterojunctions: influence of contact doping and molecular orientation

Planar organic heterojunctions are widely used in photovoltaic cells, light-emitting diodes, and bilayer field-effect transistors. The energy level alignment in the devices plays an important role in obtaining the aspired gap arrangement. Additionally, the π-orbital overlap between the involved molecules defines e.g. the charge-separation efficiency in solar cells due to charge-transfer effects. To account for both aspects, direct/inverse photoemission spectroscopy and near edge x-ray absorption fine structure spectroscopy were used to determine the energy level landscape and the molecular orientation at prototypical planar organic heterojunctions. The combined experimental approach results in a comprehensive model for the electronic and morphological characteristics of the interface between the two investigated molecular semiconductors. Following an introduction on heterojunctions used in devices and on energy levels of organic materials, the energy level alignment of planar organic heterojunctions will be discussed. The observed energy landscape is always determined by the individual arrangement between the energy levels of the molecules and the work function of the electrode. This might result in contact doping due to Fermi level pinning at the electrode for donor/acceptor heterojunctions, which also improves the solar cell efficiency. This pinning behaviour can be observed across an unpinned interlayer and results in charge accumulation at the donor/acceptor interface, depending on the transport levels of the respective organic semiconductors. Moreover, molecular orientation will affect the energy levels because of the anisotropy in ionisation energy and electron affinity and is influenced by the structural compatibility of the involved molecules at the heterojunction. High structural compatibility leads to π-orbital stacking between different molecules at a heterojunction, which is of additional interest for photovoltaic active interfaces and for ground-state charge-transfer.