Conductive oxide-based dispersions from non-critical raw materials for functional nanoinks

Project Information

CONDINKS

Grant agreement ID: 101069295

DOI

10.3030/101069295

Project closed

EC signature date 30 March 2022

Start date 1 September 2022

End date 31 August 2024

Funded under

• European Research Council (ERC)

Total cost

No data

EU contribution

€ 150 000,00

Coordinated by

FONDAZIONE ISTITUTO ITALIANO DI TECNOLOGIA
Italy

Periodic Reporting for period 1 - CONDINKS (Conductive oxide-based dispersions from non-critical raw materials for functional nanoinks)

Reporting period: 2022-09-01 to 2024-08-31

Summary of the context and overall objectives of the project

Transparent conductive oxides (TCOs) combine high transparency in the visible with high electrical conductivity, features that make them optimal candidates in a variety of optoelectronic applications. Recent efforts focus on the implementation of inks of transparent conductors, which brings along the opportunity of solution processing and the use of flexible substrates. In fact, conductive inks are used in applications ranging from flat-panel displays, flexible displays, touch-screens, thin-film or organic photovoltaics. In particular, Sn:In2O3 (ITO) is a very promising material because of its well-established properties. However, recently alternatives to ITO have goverened he market such as Ag nanowires and Cu nanoparticles, Indium has been removed from the critical raw materials list recently. Hence, it is time to reinvest TCOs as well-established alternatives to Ag and Cu, reaching similar properties at a similar or lower cost. In CONDINKS, we have identified some of the major issues, limiting the performance of TCO nanocrystals. The two major issues are related to 1) their electronic structure at the nanoscale that results in the formation of depletion regions near the nanocrystal surface and dopant segregation. Segregated on the surface of the nanocrystal, dopants might remain inactive and, hinder charge transport, hence, do not contribute to the conductivity. Segregated only to the core of the nanocrystal, might result into large depletion layers, which have a same effect. Both effects are well known also in the bulk. However, these effects have extensions in the nanometer regime and hence become significant in particular on the nanoscale. In ITO nanocrystals, it has been shown that depletion regions largely limit the conductivity of nanocrystal-based films. Precise dopant placement within the volume of the nanocrystal instead allows enhancing the conductivity by 10-fold for equal dopant concentration. As dopants add significantly to the cost of the system, it is imperative to understand how their performance can benefit from designing their location in the crystal through nanoscale dopant engineering. Hence, in this project we will synthesize well defined core-shell nanostructures that allow extracting precise dopant placement. This synthesis will be upscaled and thin film measurements performed.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The development of advanced materials with tunable electrical and electronic properties, particularly transparent conducting oxides (TCOs), has been a primary focus in scientific and technological research, as oxides provide superior stability with respect to many other materials. In particular, when selecting materials containing elements that are non-critical. The conductivity of TCO based nanocrystal films remains on the lower side of commercially available inks. In fact in our comparison study we reveal that the resistivity of silver nanowire films usually falls between 10 to 100 µΩ·cm, depending on factors like the density of the nanowires, film thickness, and processing conditions. Copper (Cu) nanoparticle printed films are an emerging alternative to silver and other conductive materials, especially in flexible electronics. he resistivity of copper nanoparticle films typically ranges from 30 to 100 µΩ·cm. Copper films can be more susceptible to oxidation compared to silver, which can increase resistivity over time if not properly protected. While copper nanoparticle films can achieve respectable conductivity, their resistivity is generally higher than that of silver nanoparticle films but can still be competitive. In our study we focussed our attention on indium tin oxide (ITO) nanocrystals (NCs) as a valid alternative to Cu and silver. ITO nanoparticle printed films offer typically low to moderate resistivity, which can be optimized through careful control of composition, thickness, and post-processing, making them highly suitable for transparent conductive applications. Core-shell nanostructures have emerged as a promising platform due to their versatility in modifying intrinsic properties and creating novel functionalities. In that case, a base metal oxide nanocrystal (MO NCs) is produced, serving as the core, and its corresponding core-shell nanostructure is fabricated. In our case, we selected the base material, ITO nanocrystals, with well-defined optical and electronic properties, including a tunable localized surface plasmon resonance (LSPR) in the near-infrared region, a bandgap in the ultraviolet range, and high conductivity. By encapsulating these ITO nanocrystals (core) with an external shell of Indium Oxide (In3O2) ITO/In2O3 core/shell NCs are produced, which significantly modify their intrinsic properties. The core-shell interface introduces novel physical and chemical phenomena that we target to optimize. We tested the hypothesis that shell formation has an influence on the conductivity and upon precise dopant placement the conductivity can be optimized whilst keeping lower the doping density.
Towards this aim, we first targeted the large-scale synthesis of both materials, optimizing synthesis processes to ensure reproducibility and product quality. Subsequently, thin films were fabricated using spin coating, exploring various deposition conditions to obtain films with optimal properties. The films were characterized using electrical conductivity techniques to evaluate the impact of the core-shell structure on device performance, as well as post processing treatments. The device fabricated in this study is a thin-film electrode. The combination of high conductivity and structural stability achieved through the optimized synthesis and fabrication processes makes these thin films suitable for applications in optoelectronic devices. The core-shell structure, particularly the ITO/In2O₃ films, is critical for improving the charge transport and distribution within the films. The results obtained in this study provide deeper insights into the relationship between nanoscale structure and macroscopic properties of these materials, as well as identifying potential applications in flexible electronics, optoelectronics, and energy-related fields. Taken together, the conductivity measurements provided valuable insights into the electrical properties of ITO and ITO/In2O3 thin films, indicating that both the quality of the films and their conductivity improve more significantly with higher annealing temperatures and the use of octane compared to hexane. The superior performance of octane may be attributed to its ability to produce better film quality, potentially due to the differences in solvent interactions and evaporation dynamics, which lead to improved nanoparticle dispersion and a more uniform film morphology. Additionally, the research underscores the advantages of core-shell materials in enhancing conductivity. The interface in core-shell structures plays a critical role in optimizing transport properties, while the core materials exhibit more uniform electrical behavior. These findings highlight the importance of understanding charge transport at interfaces to design materials with improved electrical characteristics.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

We are currently discussing with printing companies that are experts in the nanoscale printing of thin conductive lines. Target of this discussing is to identify potential future application of our key result on the upscaled core-shell materials.