On the Nature of Charge-Injecting Contacts in Organic Field-Effect Transistors

Organic field-effect transistors (OFETs) are key enabling devices for plastic electronics technology, which has a potentially disruptive impact on a variety of application fields, such as health, safety, and communication. Despite the tremendous advancements in understanding the OFET working mechanisms and device performance, further insights into the complex correlation between the nature of the charge-injecting contacts and the electrical characteristics of devices are still necessary. Here, an in-depth study of the metal-organic interfaces that provides a direct correlation to the performance of OFET devices is reported. The combination of synchrotron X-ray spectroscopy, atomic force microscopy, electron microscopy, and theoretical simulations on two selected electron transport organic semiconductors with tailored chemical structures allows us to gain insights into the nature of the injecting contacts. This multiple analysis repeated at the different stages of contact formation provides a clear picture on the synergy between organic/metal interactions, interfacial morphology, and structural organization of the electrode. The simultaneous synchrotron X-ray experiments and electrical measurements of OFETs in operando uncovers how the nature of the charge-injecting contacts has a direct impact on the injection potential of OFETs.

Efficient evaluation of Coulomb interactions in kinetic Monte Carlo simulations of charge transport

The application of predictive and reliable modeling techniques for the simulation of charge transport in functional materials is an essential step for the development of advanced platforms for electronics, optoelectronics, and photovoltaics. In this context, kinetic Monte Carlo (KMC) methods have emerged as a valuable tool, especially for the simulation of systems where charge transport can be described by the hopping of charge carriers across localized quantum states, as, for example, in organic semiconductor materials. The accuracy, computational efficiency, and reliability of KMC simulations of charge transport, however, crucially depend on the methods and approximations used to evaluate electrostatic interactions arising from the distribution of charges in the system. The long-range nature of Coulomb interactions and the need to simulate large model systems to capture the details of charge transport phenomena in complex devices lead, typically, to a computational bottleneck, which hampers the application of KMC methods. Here, we propose and assess computational schemes for the evaluation of electrostatic interactions in KMC simulations of charge transport based on the locality of the charge redistribution in the hopping regime. The methods outlined in this work provide an overall accuracy that outperforms typical approaches for the evaluation of electrostatic interactions in KMC simulations at a fraction of the computational cost. In addition, the computational schemes proposed allow a spatial decomposition of the evaluation of Coulomb interactions, leading to an essentially linear scaling of the computational load with the size of the system.

Noncovalent passivation of supported phosphorene for device applications: from morphology to electronic properties

Simulations suggest efficient routes for the non-covalent passivation of supported phosphorene with alkanes, highlighting strategies to prevent surface degradation phenomena.

Nanoscale morphology and electronic coupling at the interface between indium tin oxide and organic molecular materials

The correlation between nanoscale morphology and charge injection rates at the interface between an organic semiconductor layer and a transparent metal oxide electrode was investigated by integrating molecular dynamics simulations with electronic structure calculations. The simulation approach proposed has been applied to the analysis of the hole injection mechanism at the interface between an amorphous layer of tris[(3-phenyl-1H-benzimidazol-1-yl-2(3H)-ylidene)-1,2-phenylene]Ir (DPBIC), a hole transport and emitter molecule, and the surface of indium tin oxide (ITO), a material commonly used as anode in OLEDs. The link between interface morphology and charge injection was investigated by implementing a two-step, top-down simulation approach. Namely, nanoscale molecular aggregation phenomena at the organic/electrode interface were first assessed by molecular dynamics simulations, mimicking different processing conditions, and followed by density functional theory calculations of the electronic coupling between molecular levels and the manifold of electrode states involved in the charge injection process. The correlation between structural parameters and electronic coupling suggests a significant role of specific molecule/electrode configurations on charge transport processes at the interface, resulting in a broad distribution of charge injection rates, and highlights the link between molecular structure, nanoscale aggregation and processing in the realization of heterointerfaces for efficient charge injection in organic electronic devices.

Spatial and orientational dependence of electron transfer parameters in aggregates of iridium-containing host materials for OLEDs: coupling constrained density functional theory with molecular dynamics

The integration between molecular dynamics and constrained density functional theory allows to evaluate charge transport parameters in bulk organic semiconductors.

A self-assembled lysinated perylene diimide film as a multifunctional material for neural interfacing

We report the design, synthesis and structure-property investigation of a new perylene diimide material (PDI-Lys) bearing lysine end substituents. Water processed films of PDI-Lys were prepared and their self-assembly, morphology and electrical properties in both inert and air environments were theoretically and experimentally investigated. With the aim of evaluating the potential of PDI-Lys as a biocompatible and functional neural interface for organic bioelectronic applications, its electrochemical impedance as well as the adhesion and viability properties of primary neurons on the PDI-Lys films were studied. By combining theoretical calculations and electrical measurements we show that due to conversion between neutral and zwitterionic anions, the PDI-Lys film conductivity increased significantly upon passing from air to an inert atmosphere, reaching a maximum value of 6.3 S m^-1. We also show that the PDI-Lys film allows neural cell adhesion and neuron differentiation and decreases up to 5 times the electrode/solution impedance in comparison to a naked gold electrode. The present study introduces an innovative, water processable conductive film usable in organic electronics and as a putative neural interface.

Theoretical insights on morphology and charge transport properties of two-dimensional N,N′-ditridecylperylene-3,4,9,10-tetra carboxylic diimide aggregates

An integrated computational approach, based on molecular dynamics and density functional theory, reveals an interplay between morphology, processing and charge transport properties in layered aggregates of PTCDI-C13.

An ultra-low bandgap diketopyrrolopyrrole (DPP)-based polymer with balanced ambipolar charge transport for organic field-effect transistors

Molecular design on new A₁–A₂ type polymer pDTDPP–TTF exhibiting ultra-low bandgap and balanced ambipolar charge transport was conducted for solution-processed OFETs.

Facile synthesis of arylthiophenyl-functionalized diketopyrrolopyrrole derivatives via direct C–H arylation: characterization and utilization in organic electronic devices

We report the synthesis and characterization of three arylthiophenyl-functionalized diketopyrrolopyrrole derivatives with different end-capping groups via direct C–H arylation.