Flexible, custom-designed microfluidic printing technology
CoExtrusion (or CoEx) is a PARC technology for creating structured fluid coatings. At the heart of this technology is a custom-designed microfluidic stack that takes two or more input fluids and interdigitates them to create a patterned film.
PARC has demonstrated two main applications for CoEx so far. The first is printing high-aspect-ratio conductive traces, and the second is creating interdigitated or “striped” battery electrode films for improved rate performance.
CoEx is a highly flexible technology and can be adapted to meet various application needs. We have demonstrated high-speed deposition (40+ cm/s), and our custom microfluidic stack and dispensing system enables the printing of fluids of a wide range of viscosities and particle loadings. Furthermore, the nature of the printhead allows us to extrude fine scale features without the need for nanoparticle inks. Because we can take multiple fluid inputs and create an interdigitated (i.e. striped) pattern on the substrate, we can create films with enhanced internal surface or interface area. CoEx is a non-contact printing method compatible with roll-to-roll processing and maskless printing.
- Printed interconnects
- Fuel cells
- Electrochemical batteries
How the Technology Works
CoEx technology is best explored in its use cases. These two applications demonstrate the technology.
Application 1: High Aspect Ratio Metallization for Silicon Photovoltaic Cells
Photovoltaic cells receive incident solar energy and convert it to electrical energy. In order to carry that photogenerated current off of the cell, solar cells typically have metal traces (called gridlines) that conduct electrical current to the rest of the system. The conventional way of producing those gridlines is by screen printing (like as is done with textiles) a silver-containing paste onto the solar cell.
We have used CoEx to print high-aspect-ratio silver lines onto solar cells that improve performance. In this case, we print two fluids (a silver-containing fluid and a viscous sacrificial fluid), and the sacrificial fluid is used to shape and focus the functional silver ink (see Figure 1 below). Figure 2 compares silver lines made with the conventional process (left) and the CoEx process (right). Compared to the conventionally screen-printed gridline, the CoEx gridline is narrower, reducing shadowing on the solar cell and thus increasing the power generated. More importantly, the high aspect ratio of the CoEx gridline results in lower resistance. It is very difficult to create narrow and tall lines like this in a single step through conventional printing methods. After printing, the cell is fired at a high temperature and the sacrificial fluid is burned off, leaving behind a conductive silver gridline. Because of this reduced shadowing of the cell, switching from screen printing to CoEx results in a 5% relative performance increase. For example, CoEx makes a 20% efficient solar cell 21% efficient, with immediate improvements to performance.
Application 2: Battery Electrode Manufacturing
Background: A conventional battery electrode (such as those in EV’s or cell phones) consists of multiple repeating stacks of anodes, cathodes, and intervening separators. Anodes and cathodes contain the “active” components that contribute to the energy capacity of a battery. During the course of charging and discharging, ions travel between the cathode and the anode through the separator. One thing that limits how fast a battery can be charged or discharged is how long it takes for ions to get from one side of the battery to the other. If fast charging or discharging is desired, battery manufacturers address this by creating very thin anodes and cathodes – making sure that ions never have very far to go. But it also means that the battery is full of “passive” components like current collectors and separator films that don’t contribute to energy capacity. Higher energy density can be achieved by creating very thick anodes and cathodes – but because the electrode layers are so thick, it takes ions a long time to travel from one side to the other and power density is reduced.
The CoEx process for batteries creates a structure similar to the one illustrated in Figure 3, where we interdigitate regions of high energy density with regions of high ionic transport. In doing so, we create lots of thin regions where ions can travel very quickly from one side of the cell to the other, creating thick electrodes with the performance of thin electrodes. In doing so, we can improve the capacity of the battery pack by more than 10% depending on the charging and discharging rate. CoEx can serve as a drop-in replacement for slot dies in conventional battery manufacturing, increasing performance with minimum changes to the manufacturing line.
Contact PARC today to discuss how our CoEx technology can be used for your application.
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