Despite significant progress in reducing demand for the precious metal, silver continues to account for a significant chunk of the cost in PV cell manufacturing. And as demand for solar continues to rise, the overall availability of silver may become a serious issue in the coming decades if reductions in its use do not continue at pace, with some models predicting silver demand from the PV industry could exceed supply as soon as 2030.
Over the past decade, the PV industry and research community have made much progress in reducing the amount of silver required per cell, introducing various methods to reduce the amount of silver content in metallization paste without losing conductivity, and printing ever thinner lines onto cells.
On the printing side, Germany’s Fraunhofer Institute for Solar Energy Systems (ISE) estimates that over the past two decades, the width of metallization ‘fingers’ printed onto cells has reduced by 7 microns per year on average. But the difficulty of pushing any amount of paste through such tiny screen structures means that further reductions, beyond the currently achievable limit of around 19 microns, represent a major challenge.
A few approaches to silver-free metallization have caught the industry’s eye in recent years, and substituting either aluminum or a mix of copper and nickel are promising approaches. Manufacturers, however, seem likely to cling on to silver paste as the most efficient metallization material, and strategies to continue to reduce the amounts required will be of value to the industry for years to come.
Screen utility index
With this in mind, ISE has developed a system to model the effectiveness of any screen architecture in reference to the properties of a given silver paste. The model allows researchers to calculate a value it calls the ‘screen utility index’, to quantify the printability of different screen architectures in relation to available types of paste. The theoretical backing of this value, along with simulations and experiments to validate, are found in the paper A model for screen utility to predict the future of printed solar cell metallization, published in Scientific Reports.
“This data will allow the industry to improve their decision-making process for novel screen configurations without requiring complex mathematical modeling of the process mechanics,” the ISE scientists state. “The goal of the presented approach is to further improve the metallization of Si-solar cells in mass production in terms of increased cell efficiency and reduced production cost. Especially, the reduction of silver consumption per cell by an improved fine-line screen printing process is crucial when facing the predicted silver production crisis.”
One of the study’s key findings is that for manufacturers to keep reducing the thickness of screen printed lines, new materials with higher tensile strength will need to be investigated, as the pressure required to push paste through a gap so small will be too much for the stainless steel or tungsten commonly used today.
Suggestions for new materials here include fiber glass and carbon fiber materials, and note that carbon nanotubes would represent the finest possible wire with the highest possible tensile strength, and while they have not been used in any application comparable to a screen printing mesh, recent developments with the materials could place an industrial process within reach in the next few years. “The ultimate tensile strength of one of these potential wire or fiber materials needs to be higher than the minimum requirement for the desired screen utility index,” the scientists explain. “…New technologies for the mass production of very thin and strong wires or fibers have to be developed in order to prevent an upcoming dead-end of ultra-fine line metallization of Si-solar cells.”
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