Residual polymer stabiliser causes anisotropic electrical conductivity during inkjet printing of metal nanoparticles

Digitally printed electronics are gaining traction due to their design flexibility and faster time-to-market. Inkjet printing using colloidal materials like metal nanoparticles, conductive polymers, and magnetic nanoparticles has shown promise in various applications, from flexible electronics to solar cells. However, the performance of printed components often lags behind traditional manufacturing methods, primarily due to challenges in manufacturing techniques and material choices. Metal nanoparticles are frequently used for their conductivity, typically requiring a two-step process: solvent evaporation (pinning) upon printing followed by low-temperature sintering (120–200 °C). More recently, 3D inkjet printing has enabled the co-deposition of multiple materials, such as dielectric and conductive materials, to create complex multi-functional objects. Despite the potential, the lower and anisotropic conductivity (both intra-layer and inter-layer) of printed metal nanoparticle layers compared to bulk metals hinders widespread adoption. Previous studies have linked conductivity to thermal treatment and possible organic residues, but the precise mechanism remains unclear. Organic molecules, used as stabilizers in inks to maintain nanoparticle dispersion, may hinder performance even in small amounts. Advanced surface-sensitive techniques like TOF-SIMS and XPS, combined with GCIBs, are ideal for characterizing the chemical composition and distribution of organic materials within these complex structures, providing nanometer-scale resolution.

Extensive research has been dedicated to inkjet printing of various materials for electronics. Kamyshny and Magdassi (2019) reviewed conductive nanomaterials for 2D and 3D printed flexible electronics. Ko et al. (2007) demonstrated all-inkjet-printed flexible electronics using laser sintering of metal nanoparticles. Lewis and Ahn (2015) explored 3D printed electronics. Saleh et al. (2017) presented 3D inkjet printing of electronics using UV conversion. Previous work has highlighted the impact of sintering temperature on the conductivity of printed metal nanoparticle layers (Vaithilingam et al., 2017; Sowade et al., 2015; Magdassi et al., 2010; Vaithilingam et al., 2018). However, the specific role of organic residues from stabilizing agents like PVP and their influence on the anisotropic conductivity remained poorly understood. Studies using TOF-SIMS and XPS have been employed to analyze complex materials, including perovskite solar cells (Bailey et al., 2015; Tiddia et al., 2019), but a detailed investigation focusing on the impact of PVP residues in inkjet-printed silver nanoparticles was lacking.

This study used commercially available silver nanoparticle (AgNP) ink containing polyvinylpyrrolidone (PVP) as a stabilizer. The ink was inkjet-printed onto silicon/silicon dioxide (Si/SiO2) substrates using a Fujifilm Dimatix Materials Printer. Printing was conducted on a heated stage (90°C) for solvent evaporation (pinning). Samples were then subjected to various sintering temperatures in an argon-filled oven (up to 500°C). Electrical resistivity was measured using a four-probe method (Keithley 2400 sourcemeter and Keithley 7500 multimeter). Planar (intra-layer) and vertical (inter-layer) resistivity were determined. Morphological analysis was performed using scanning electron microscopy (SEM, JEOL 7100 F FEG-SEM). Detailed chemical analysis was conducted using several techniques: Time-of-flight secondary ion mass spectrometry (TOF-SIMS, IONTOF GmbH ToF-SIMS IV and 3D OrbiSIMS) for 3D chemical mapping and depth profiling; X-ray photoelectron spectroscopy (XPS, Kratos AXIS ULTRA) to analyze the chemical state of elements. High-resolution XPS spectra were obtained with a monochromatized Al Kα X-ray source, and peak fitting was performed using Casa XPS software to quantify the different forms of PVP. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were also performed to characterize the ink's thermal behavior. For the multi-material 3D printed sample, a conductive silver pattern encapsulated in tri(propylene glycol) diacrylate (TPGDA) was created using a PixDro LP50 printer with simultaneous UV curing/sintering, and analyzed using TOF-SIMS and transmission electron microscopy (TEM, JEOL 2000 FX TEM) to examine the interface between the dielectric and conductive layers.

TOF-SIMS and XPS analyses confirmed the presence of PVP on the surface of the printed AgNP layers. A "coffee ring" effect was observed, with PVP concentrating at the edges of printed droplets. In multilayer samples, ToF-SIMS depth profiling revealed that PVP accumulated at the interfaces between successive AgNP layers, with an average period of 282.8 nm matching the layer thickness. A higher resolution ToF-SIMS depth profile, using a lower energy etching beam, resolved an oscillating pattern within a single layer (period ~80.7nm), related to AgNP/PVP core-shell structures. XPS analysis distinguished between interacting (C–N–Ag) and non-interacting (C–N) PVP, showing that non-interacting PVP tends to migrate to the surface during sintering. Planar resistivity decreased significantly after sintering at 100°C, but increased at temperatures above 300°C due to void formation. Vertical resistivity remained three orders of magnitude higher than planar resistivity, demonstrating anisotropic conductivity. The presence of PVP was detected even after sintering at temperatures between 100 and 230°C where resistivity was highly anisotropic. In a 3D printed encapsulated conductive pattern, PVP accumulated at the dielectric/conductive interface, and TEM revealed AgNP interpenetration into the dielectric at the top interface, which could affect the conductive path. TGA revealed significant weight loss between 150 and 250 °C due to solvent evaporation.

The findings demonstrate that the anisotropic conductivity observed in inkjet-printed AgNP structures is a direct consequence of the distribution of residual PVP. The accumulation of PVP at interfaces between layers creates insulating barriers, hindering electron transport in the vertical direction. The coffee ring effect during pinning and the subsequent layering of AgNPs trap PVP at interfaces, leading to a significant difference in planar and vertical resistivity. This highlights the importance of controlling the distribution of organic residues in inkjet-printed electronic devices, which can be crucial for charge transfer processes and the overall device functionality, as recently shown for inkjet-printed graphene transistors. The observed increase in resistivity at higher sintering temperatures (above 300°C) suggests that optimizing sintering conditions is critical to balance the removal of PVP with the prevention of void formation that can significantly decrease conductivity.

This study provides a detailed understanding of how residual polymer stabilizers, specifically PVP, influence the electrical conductivity of inkjet-printed metal nanoparticles. The concentration of PVP at interfaces leads to anisotropic conductivity, significantly limiting device performance. This understanding suggests strategies for improving nanomaterial inks, such as using alternative stabilizers that are removable or breakable at lower temperatures (UV or IR). Optimizing sintering methods, and controlling the ink composition are crucial for overcoming the limitations of currently available nanoparticle inks and realizing the full potential of 3D printed electronics. Future work could focus on developing novel ink formulations with readily removable or photo-degradable stabilizers to enhance conductivity and reduce anisotropy.

This study focused primarily on AgNPs with PVP as a stabilizer. The findings may not be directly generalizable to other metal nanoparticles or different stabilizer molecules. The specific sintering parameters used in this study might also need to be adjusted depending on the substrate material and the desired device structure. Future studies could extend this work to other nanoparticle systems and investigate the effect of different processing techniques for optimizing the electrical properties.