The processes currently used to manufacture photovoltaic devices are suitable to achieve a minimum sustainable price, but low profit margins and high capital expenditures on equipment severely limit growth capacity, while also preventing introduction of newer innovative technologies into the market.
Approximately 2 Mkg of silver ink and pastes are consumed each year. Although the vast majority of this silver is consumed as silver paste to metalize photovoltaic panels, there is substantial interest in drop-on-demand (DoD) printable silver inks for low-cost electronics fabrication and rapid prototyping of electronic circuits. Most commercially available DoD silver inks are particle-based, consisting of a colloidal suspension of silver particles or nanoparticles dispersed in a fluid containing solvents, humectants, and other chemicals to aid in stabilization, dispersion, and sintering. These inks work reasonably well, with newer nanoparticle inks showing good electrical conductivity with low, sub-100 °C sintering temperatures. However, adoption of particle-based inks is still restricted by their high costs, limited commercial availability, and difficulties with nozzle clogging. Organometallic inks have been widely researched as a method to bypass some of the challenges with particle-based inks by printing chemical precursors that, once printed, react to form solid metals. These inks, also called metal-organic complex inks, or reactive inks, consist of dissolved metal salts, chelating agents, and reducing agents along with solvents that adjust viscosity, evaporation rate, and surface tension for DoD droplet stabilization.
Reactive inks can be easier to synthesize than nanoparticle inks (as exampled by the silver diamine ink demonstrated by Walker et al.) and bypass the oxidation issues that often degrade metallic particle-based inks (as exampled by the copper reactive ink demonstrated by Rosen et al.). Despite these advantages, reactive inks have seen very little adoption beyond the research community because, until recently, most reactive inks required high temperatures to initiate the metal complex reduction reaction. These higher temperatures negated the benefits of reactive inks and limited the types of substrates they could be printed on.
Researchers at Arizona State University have developed a system for the morphological control of reactive inks for high-precision DoD printing, with a particular application in photovoltaic manufacturing. This innovation centers around a controllable approach for patterning dopants/seed layers and etching grooves at different scales in a cost-efficient manner. The system comprises a substrate and a printer jet head having a nozzle (to dispense a reactive metal ink and a solvent onto the substrate), wherein the solvent and substrate temperature are controlled during deposition of the reactive metal ink onto the substrate to produce a dense film in the absence of sintering.
In another embodiment, the invention provides a system of optimizing morphology and electrical properties of silver printed on a substrate. The temperature of the substrate is maintained between ~60 °C and ~80 °C during deposition of the reactive metal ink onto the substrate. A contact angle of a dispensed droplet relative to the substrate is less than 15° and a concentration of the solvent to reactive metal ink ranges between 1:1 and 10:1, such that the dense film provides a media resistivity of less than 2 μΩ⋅cm and an optimized morphology of low porosity.
This innovation is covered by U.S. Pat. No. 11,077,664.
• Reactive inks for
o Photovoltaic cells
o Printed electronics