The ionization energy of α-sexithiophene and p-sexiphenyl in 2D and 3D thin films grown on silicon oxide surfaces

The ionization energy (IE) of molecular thin films is sensitive to the molecular environments surrounding ionized molecules. In this study, we investigate the evolution of the IE on dimension-controlled 2D and 3D thin films of rod-like molecules, namely α-sexithiophene (α-6T) and p-sexiphenyl (p-6P), grown on SiO2 surfaces via photoelectron yield spectroscopy. In 2D thin films, the IE of α-6T and p-6P showed nearly constant values of 4.93 eV and 5.50 eV, respectively. In 3D thin films, however, with increasing coverage, the IE of the α-6T thin films gradually decreased to 4.84 eV at 15 ML (ML: monolayer) and the IE of the p-6P thin films gradually increased to 5.65 eV at 7 ML. These experimental findings underline the less significant impact of the domain size on the IE in 2D thin films and the significant impact of the neighboring layers on the IE in 3D thin films.

Roughness evolution in strongly interacting donor:acceptor mixtures of molecular semiconductors. An in situ, real-time growth study using x-ray reflectivity

The evolution of surface roughness in binary mixtures of the two molecular organic semiconductors (OSCs) diindenoperylene (DIP) as electron-donor and 1, 3, 4, 5, 7, 8-hexafluoro-tetracyano naphthoquinodimethane (F6TCNNQ) as electron-acceptor is studied. We co-deposit DIP and F6TCNNQ in vacuum with varying relative molar content while keeping a molar excess of DIP in order to produce phase-heterogeneous mixtures. The excess DIP phase segregates in pristine crystallites, whereas the remaining mixed phase is constituted by DIP:F6TCNNQ co-crystallites. We calculate the surface roughness as function of film thickness by modelling x-ray reflectivity data acquired in situ and in real-time during film growth. To model the experimental data, two distinct approaches, namely the kinematic approximation and the Parratt formalism, are applied. A comparative study of surface roughness evolution as function of DIP:F6TCNNQ mixing ratio is carried out implementing the Trofimov growth model within the kinematic approximation. Depending on the thickness regime, mixing ratio-specific trends are identified and discussed. To explain them, a growth mechanism for binary heterogeneous mixtures of strongly interacting OSCs is proposed.

Energy level alignment at organic/inorganic semiconductor heterojunctions: Fermi level pinning at the molecular interlayer with a reduced energy gap

A lying (L) molecular interlayer between ZnO and standing (S) sexiphenyl molecules leads to “concealed” Fermi level pinning.

Charge separation at an organic/inorganic nano-hybrid interface: atomistic simulations of a para-sexiphenyl ZnO system

A prototypical organic/inorganic interface is considered which is formed by vertical stacking of 20 para-sexiphenyl molecules physisorbed on a ZnO nano-cluster of 3903 atoms. Charge separation kinetics at the interface are investigated for their dependence on ultrafast optical excitation. In order to analyze the spatio-temporal evolution of the Frenkel exciton in the organic part and the formation of charge separated states a first principles parameterized Hamiltonian is introduced and the related time-dependent Schroedinger equation is solved. By determining the interface absorption spectrum the optically addressable states can be uncovered. The work continues our previous studies of J. Phys. Chem. Lett., 2018, 9, 209, but with a changed type of surface passivation. This prevents trapping of electrons close to the surface. Charge separated states are formed by direct optical excitation and also by exciton decay at the interface. Electron migration away from the interface into bulk regions becomes possible. The hole stays close to the interface for all excitation scenarios. Finally, it is demonstrated that energetic disorder is of minor influence.

Ultrafast Crystallization Dynamics at an Organic-Inorganic Interface Revealed in Real Time by Grazing Incidence Fast Atom Diffraction

The poor structural properties of organic-inorganic interfaces and their variability represent the main cause of device under-performance. Understanding and controlling the development of these properties in real time has been a difficult experimental challenge. Using a recent technique based on grazing incidence fast atom diffraction (GIFAD), we were able to directly observe during deposition structural transitions in a perylene monolayer on Ag(110). Crystallization from the liquid phase occurs into two distinct structures with drastically different dynamics. Transition to the most compact packing occurs by self-organization only after a second layer has started to build up; subsequent incorporation of molecules from second to first layer triggers an ultrafast crystallization on a macroscopic sale. The final compact crystalline structure shows a long-range order and superior stability, which opens good perspectives for producing in a controlled manner highly ordered hybrid interfaces for photovoltaics and molecular electronics.

Atomistic Simulations of Charge Separation at a Nanohybrid Interface: Relevance of Photoinduced Initial State Preparation

Charge separation kinetics at a nanohybrid interface are investigated in their dependence on ultrafast optical excitation. A prototypical organic/inorganic interface is considered. It is formed by a vertical stacking of 20 para-sexiphenyl molecules physisorbed on a ZnO nanocluster of 3783 atoms. A first principle parametrized Hamiltonian is employed, and the photoinduced subpicosecond evolution of Frenkel-excitons in the organic part is analyzed besides the formation of charge separated states across the interface. The interface absorption spectrum is calculated. Together, the data indicate that the charge separation is based on the direct excitation of the charge separated states but also on the migration of created Frenkel excitons to the interface with subsequent decay. Further, the photoinduced interface dynamics are compared with data resulting from direct set-ups of an initially excited state. Mostly such set-ups lead to substantially different charge separation processes.

Charge and energy migration in molecular clusters: A stochastic Schrödinger equation approach

The performance of stochastic Schrödinger equations for simulating dynamic phenomena in large scale open quantum systems is studied. Going beyond small system sizes, commonly used master equation approaches become inadequate. In this regime, wave function based methods profit from their inherent scaling benefit and present a promising tool to study, for example, exciton and charge carrier dynamics in huge and complex molecular structures. In the first part of this work, a strict analytic derivation is presented. It starts with the finite temperature reduced density operator expanded in coherent reservoir states and ends up with two linear stochastic Schrödinger equations. Both equations are valid in the weak and intermediate coupling limit and can be properly related to two existing approaches in literature. In the second part, we focus on the numerical solution of these equations. The main issue is the missing norm conservation of the wave function propagation which may lead to numerical discrepancies. To illustrate this, we simulate the exciton dynamics in the Fenna-Matthews-Olson complex in direct comparison with the data from literature. Subsequently a strategy for the proper computational handling of the linear stochastic Schrödinger equation is exposed particularly with regard to large systems. Here, we study charge carrier transfer kinetics in realistic hybrid organic/inorganic para-sexiphenyl/ZnO systems of different extension.

Thin film growth studies using time-resolved x-ray scattering

Thin-film growth is important for novel functional materials and new generations of devices. The non-equilibrium growth physics involved is very challenging, because the energy landscape for atomic scale processes is determined by many parameters, such as the diffusion and Ehrlich-Schwoebel barriers. We review the in situ real-time techniques of x-ray diffraction (XRD), x-ray growth oscillations and diffuse x-ray scattering (GISAXS) for the determination of structure and morphology on length scales from Å to µm. We give examples of time resolved growth experiments mainly from molecular thin film growth, but also highlight growth of inorganic materials using molecular beam epitaxy (MBE) and electrochemical deposition from liquids. We discuss how scaling parameters of rate equation models and fundamental energy barriers in kinetic Monte Carlo methods can be determined from fits of the real-time x-ray data.

Limits of size scalability of diffusion and growth: Atoms versus molecules versus colloids

Understanding fundamental growth processes is key to the control of nonequilibrium structure formation for a wide range of materials on all length scales, from atomic to molecular and even colloidal systems. While atomic systems are relatively well studied, molecular and colloidal growth are currently moving more into the focus. This poses the question to what extent growth laws are size scalable between different material systems. We study this question by analyzing the potential energy landscape and performing kinetic Monte Carlo simulations for three representative systems. While submonolayer (island) growth is found to be essentially scalable, we find marked differences when moving into the third (vertical) dimension.

Thermally driven smoothening of molecular thin films: Structural transitions in n-alkane layers studied in real-time

We use thermal annealing to improve smoothness and to increase the lateral size of crystalline islands of n-tetratetracontane (TTC, C44H90) films. With in situ x-ray diffraction, we find an optimum temperature range leading to improved texture and crystallinity while avoiding an irreversible phase transition that reduces crystallinity again. We employ real-time optical phase contrast microscopy with sub-nm height resolution to track the diffusion of TTC across monomolecular step edges which causes the unusual smoothing of a molecular thin film during annealing. We show that the lateral island sizes increase by more than one order of magnitude from 0.5 μm to 10 μm. This desirable behavior of 2d-Ostwald ripening and smoothing is in contrast to many other organic molecular films where annealing leads to dewetting, roughening, and a pronounced 3d morphology. We rationalize the smoothing behavior with the highly anisotropic attachment energies and low surface energies for TTC. The results are technically relevant for the use of TTC as passivation layer and as gate dielectric in organic field effect transistors.