Metallic nanowire networks have huge potential in devices requiring transparent electrodes. This article describes how the electrical resistance of metal nanowire networks evolve under thermal annealing. Understanding the behavior of such films is crucial for the optimization of transparent electrodes which find many applications. An in-depth investigation of silver nanowire networks under different annealing conditions provides a case study demonstrating that several mechanisms, namely local sintering and desorption of organic residues, are responsible for the reduction of the systems electrical resistance. Optimization of the annealing led to specimens with transmittance of 90% (at 550 nm) and sheet resistance of 9.5 Ω sq⁻¹. Quantized steps in resistance were observed and a model is proposed which provides good agreement with the experimental results. In terms of thermal behavior, we demonstrate that there is a maximum thermal budget that these electrodes can tolerate due to spheroidization of the nanowires. This budget is determined by two main factors: the thermal loading and the wire diameter. This result enables the fabrication and optimization of transparent metal nanowire electrodes for solar cells, organic electronics and flexible displays.

The past few years have seen a considerable amount of research devoted to nanostructured transparent conducting materials (TCM), which play a pivotal role in many modern devices such as solar cells, flexible light-emitting devices, touch screens, electromagnetic devices, and flexible transparent thin film heaters. Currently, the most commonly used TCM for such applications (ITO: Indium Tin oxide) suffers from two major drawbacks: brittleness and indium scarcity. Among emerging transparent electrodes, silver nanowire (AgNW) networks appear to be a promising substitute to ITO since such electrically percolating networks exhibit excellent properties with sheet resistance lower than 10 Ω/sq and optical transparency of 90%, fulfilling the requirements of most applications. In addition, AgNW networks also exhibit very good mechanical flexibility. The fabrication of these electrodes involves low-temperature processing steps and scalable methods, thus making them appropriate for future use as low-cost transparent electrodes in flexible electronic devices. This contribution aims to briefly present the main properties of AgNW based transparent electrodes as well as some considerations relating to their efficient integration in devices. The influence of network density, nanowire sizes, and post treatments on the properties of AgNW networks will also be evaluated. In addition to a general overview of AgNW networks, we focus on two important aspects: (i) network instabilities as well as an efficient Atomic Layer Deposition (ALD) coating which clearly enhances AgNW network stability and (ii) modelling to better understand the physical properties of these networks.

Hole mobility in $N,N^{\prime }$-diphenyl-$N,N^{\prime }$-bis(1-naphtylphenyl)-$1,1^{\prime }$-biphenyl-$4,4^{\prime }$-diamine ($\alpha$-NPD) is evaluated by electrical characterization in the ac regime. The frequency-dependent complex admittance and impedance of the structure consisting of the organic layer, grown by thermal evaporation, sandwiched by indium tin oxide and aluminum electrodes, are measured as functions of the applied dc voltage. The capacitance response shows negative values for frequencies below a characteristic value depending on the bias and ranging from $0.1\mathrm{Hz}$ up to $20\mathrm{Hz}$. It increases with the modulation frequency and reaches a peak, the magnitude and position of which are functions of the applied voltage. For higher frequencies, a minimum can be observed before the capacitance increases again up to a constant value. A final decreasing occurs at frequency of $4\times {10}^6\mathrm{Hz}$. The analysis of the experimental data is performed by a detailed theoretical study of the steady-state and small-signal electrical characteristics of the device. Numerical calculations are based on the solution of the basic semiconductor equations for the system consisting of two electrodes connected by the semiconducting channel formed by the organic layer. The description explicitly includes a continuous distribution of trap density of states and a field-dependent carrier mobility. The spatially dependent charge carrier and occupied trap concentrations, as well as the various components to the total current density, are obtained for the dc and ac regimes and are analyzed for given bias and frequency. Based on a formalism used in the study of inorganic semiconductors, the results of the simulation show that the inductive contribution to the capacitance response originates from the modulation of the hole concentration in the organic material, leading to the corresponding carrier transit time. Moreover, the low-frequency behavior of the capacitance curves could be explained by the presence of a band of defect states which modifies the charge distribution within the organic layer and the injection of electrons from the cathode. We show that the latter contribution is also responsible for the negative values of the capacitance measured below $10\mathrm{Hz}$. Good agreement is observed between the experimental and theoretical electrical characteristics, in particular for the differential susceptance results and the subsequent hole mobility values. Our approach can be a useful contribution for the methodology of obtaining mobilities from admittance measurements as it allows one to clarify the physical origin of the measured frequency-dependent capacitance and to check for the experimental procedure. This work finally leads to the formulation of the conditions under which small-signal ac measurements can be used to determine carrier mobility in organic devices.