Correlation between electrical and mechanical properties of silver nanowire electrodes studied by complementary in situ microscopy techniques
Organic solar cells are cost- and resource efficient alternative devices for power generation. Their application in flexible geometries is hindered, if the used electrodes are made from brittle transparent conductive oxides (TCOs). By replacing TCOs with percolation-based networks made of five-fold twinned silver nanowires (AgNWs), device flexibility can be achieved. However, the complex interplay between network structure and topology with the electro-mechanical behavior under load is not yet fully understood. In this work, we show a correlative and scale bridging approach to combine electrical and mechanical measurements with in situ microscopy techniques to gain further insight into the behavior of AgNW electrodes coated on flexible substrates. Going from light microscopy (LM) via scanning electron microscopy (SEM) to transmission electron microscopy (TEM) enables a systematic understanding of failure behavior and the underlying mechanisms at different length scales. Tensile testing of AgNW networks with concurrent electrical measurements were conducted in LM enabling the analysis of strain uniformity at a large scale as well as the evolution of electrical resistance. Transferring the tensile tests to SEM, allows for a direct correlation of the conductive behavior to the micro-mechanical stress state of the nanowire network. The tests reveal a behavior which strongly depends on the relationship between straining and coating direction. Straining parallel to the wire alignment leads to a failure of wires in tensile mode, which is reflected in a steep increase in ΔR/R0. Straining perpendicular unveiled various non-catastrophic deformation mechanisms, reflected in only a minor increase in ΔR/R0. To reveal further details about the fracturing and deformation of wires, in situ TEM tensile tests were conducted. Especially, wire kinking, as one of the more prominent deformation mechanisms, was further investigated. Kinks are formed to accommodate compressive strain resulting from the Poisson effect and are characteristic for the microstructure and size of the AgNWs. Therefore, different silver nanowire sizes were tested to determine, if a correlation between nanowire diameter and kinking angle is prevalent. With the network texture influencing the electrical properties and with a better understanding of the wire deformation, the fabrication of AgNW electrodes can be tailored to improve mechanical flexibility.
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|Extended Abstract||Figure to illustrate the different used techniques||a) Light microscopy image with overlayed strain map created by Bezier surface fitting. b) Scanning electron microscopy image showing nanowire kinking (white) and fracturing (turquois). c) Transmission electron microscopy image showing buckled silver nanowires (orange). d) Plot of averaged ΔR/R0 at different amounts of strain for nanowire networks strained parallel and perpendicular to the coating direction.||2 MB||Download|