The search for more powerful solar cells and miniaturized electronics is the subject of much research around the world. Scientists at the University of Ottawa have come up with new technology they think is pretty exciting. Mathieu de Lafontaine, a postdoctoral researcher at the University of Ottawa and a part-time physics professor, and Karin Hinzer, vice-dean, research, and University Research Chair in Photonic Devices for Energy at the Faculty of Engineering, say their discovery paves the way for a new era of miniaturization in the field of electronic devices.
The team has achieved a milestone by manufacturing the first back-contact micrometric photovoltaic cells. The cells, with a size twice the thickness of a strand of hair, have significant advantages over conventional solar technologies, reducing electrode-induced shadowing by 95% and potentially lowering energy production costs by up to three times. The breakthrough is the result of a research partnership between the University of Ottawa, the Université de Sherbrooke in Quebec and the Laboratoire des Technologies de la Microélectronique in Grenoble, France.
“These micrometrtic photovoltaic cells have remarkable characteristics, including an extremely small size and significantly reduced shadowing. Those properties lend themselves to various applications, from densification of electronic devices to more efficient solar cells, lightweight nuclear batteries for space exploration, and miniaturization of devices for telecommunications and the internet of things,” Hinzer says.
Miniature Solar Cells Explained
Here is the summary of a research paper published December 20, 2023 in the journal Cell Reports Physical Science:
Three dimensional (3D) interconnects increase chip power density and enable miniaturization. Photonic chips require new processes to enable transitioning to 3D interconnects. We fabricate 3D interconnects on a multijunction solar cell, leveraging processes such as III-V heterostructure plasma etching, gold electrodeposition, and chemical-mechanical polishing to integrate through substrate vias to the heterostructure.
Wafer bonding is used to handle 20-μm-thin III-V films. The strategy enables us to demonstrate photonic power devices having areas 3 orders of magnitude smaller compared to standard chips. The design also yields a small shading factor below 3%. Compared to miniaturized photonic power devices with two-dimensional connections, 3D interconnects achieve a 6-fold increase in wafer area use. These improvements will enhance the power yield per wafer while unlocking high-density and miniaturized devices for applications such as power over fiber, the internet of things, and microconcentrator photovoltaics.
de Lafontaine added, “This technological breakthrough promises significant benefits for society. Less expensive, more powerful solar cells will help accelerate the energy shift. Lightweight nuclear batteries will facilitate space exploration, and miniaturization of devices will contribute to the growth of the internet of things and lead to more powerful computers and smartphones. The development of these first back-contact micrometric photovoltaic cells is a crucial step in the miniaturization of electronic devices.”
The researchers focus on what they call “shading” of silicon based electronic devices. For those that are measured in centimeters, that “shading” caused by what they call metalized surfaces (typically copper) is around 6 percent. But as the silicon device gets reduced in size, the size of the connectors remains the same. That means as miniaturization increases, the “shading” from the electrical connections can approach 70 percent. Using the new 3-D connection process, the connections for miniaturized silicon based electronic components can be kept to less than 3 percent.
The Long Road From The Lab To Commercial Production
The researchers are aware of the challenges that are common when seeking to bring new technology out of the lab and into production. They discuss some of the hurdles in their paper.
“The enhanced level of complexity from a 3D architecture has three main challenges:
- increased failure risks
- increased manufacturing costs
- requirement for specialized tools
The increased failure risks can be explained by the number of technological steps required as it is more than 10-fold to that required for standard contacts. Furthermore, adding vias creates an enhanced shunting risk. Several technological steps such as plasma etching and atomic layer deposition (ALD) are already well known and used in the CMOS industry. Therefore, the knowledge of this mature industry can also mitigate the risk associated to device failure.
The increased manufacturing cost from the additional technological steps is unavoidable. It is currently difficult to assess the manufacturing cost of X-TSV μcells because no industrial transfer has occurred yet, but it is still possible to do some preliminary projections.
The microfabrication steps required on standard multi-junction solar cells represent approximately 10% over the total cost of the device. Assuming that the manufacturing cost increases linearly with the number of steps, TSV contact microfabrication would represent 100% of the current standard contact multijunction solar cell, since the number of fabrication steps is 10-fold higher. From this estimation, X-TSV cells would be 1.9 times the cost of a current standard contact multijunction solar cell.
However, there are several mitigating factors to consider. The manufacturing cost increase must be considered in the context that it also comes with an active wafer use for power generation increase. For (miniaturized) cells, it has been shown that the active area can be increased 6-fold compared with miniaturized standard contact solar cells. This large improvement comes from the standard front metallization not being suited for miniaturized device, whereas 3D interconnects are independent of the device area. Assuming an identical conversion efficiency can be realized with further development, these projections indicate that the cost per watt associated to this technology could be reduced 3-fold by using (miniaturized) μcells instead of standard contact μcells.
Therefore, considering the projected pros and cons, using 3D interconnects would be a pathway toward reducing the cost per energy yield of miniaturized cells. This is a positive projection considering the fact that the current challenge is to reduce the cost of III-V based solar cells. Note that using 3D interconnects could also reduce the cost of cell assembly, as surface mounting technology assembly processes could be used instead of more expensive wirebonding interconnection processes.
Specialized Tooling For Miniature Solar Cells Needed
When the topic is batteries, researchers are careful to match new technologies to existing manufacturing techniques. Asking manufacturers to rip out existing equipment and replace it with new tooling can be a deal breaker, whatever the benefits to society as large might be. The paper deals with that subject specifically:
Specialized tools that are not common in the multijunction photovoltaic industry will be required to manufacture TSV contacts. To name a few, plasma etching, ALD and chemical-mechanical planarization (CMP) are not currently used in multijunction solar cell manufacturing, which suggests that a paradigm shift will be required for X-TSV manufacturing. However, those three techniques have been well known for several decades in the CMOS industry. This aspect gives additional weight to the relevance of the CMOS industry in the scope of manufacturing these devices. Learning from the CMOS industry could therefore mitigate the risks associated to this last challenge. This aspect is of significant importance considering the multiple challenges associated to TSV reliability.
I am not a scientist nor have I ever played on on TV, but if I tried to explain this to my granddaughter, I might say that while it is possible to make electronic stuff smaller, everything has to be connected to everything else in order for it to work. The components may get smaller but the connections do not. By devising a new way to make those connections in 3 dimensions, the researchers have a found our how to miniaturize the connections along with the components. de Lafontaine and Hinzer would probably laugh at my simplistic explanation but that is my sense of their discovery.
Some of us wonder in an age when artificial intelligence and robots are converging at a dizzying pace whether smaller, more powerful electronics are really necessary but such concerns seldom figure into basic research projects, where the objective is discovering the limits of what is possible. Someone else is in charge of determining whether some new technology is commercially viable or can improve the human condition.
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