David Savastano, Editor05.27.09
iTi Solar workstations at NREL. |
In order to bring the costs of silicon-based and thin film PV to parity with hydrocarbons, advancements in production and materials must be made. Presently, solar cells are manufactured by a wide variety of technologies, most notably screen printing, as well as vacuum deposition, spin coating, heat treatments, laser-based concepts, wet chemical etching and wet chemical deposition.
Promising work is being conducted by the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) for using inkjet to create flexible thin-film and rigid silicon-based solar cells. NREL recently installed inkjet and spray workstations supplied by iTi Solar in its advanced Atmospheric Processing Platform, which is located in the Process Development and Integration Laboratory at the National Center for Photovoltaics.
A new division of imaging Technology international (iTi) Corporation, iTi Solar is dedicated to advancing the economics of clean, renewable energy through the use of industrial inkjet technologies.
Bruce Morgan, iTi’s CEO, said that iTi started working on printed electronics (PE) applications in 2002, but that adoption has been slower than expected. Still, he sees tremendous possibilities for inkjet in the PV market.
“When we started eight years ago, we thought there would be really hot demand and a killer application,” Morgan said. “It looked like there would be movement on the printed circuit board (PCB) side, as inkjet is an additive process that can be digitally controlled and it is very cost effective. The recession from hell hit and has helped put inkjet back in the forefront again. The potential for inkjet in PE applications is huge.”
It is looking as though PV may indeed be the “killer application” that Morgan anticipated. Even as the economy slumps, capital investment in PV continues to grow, as countries worldwide see the importance of reducing and even eliminating the need for hydrocarbon-based products.
“The issue is timing,” Morgan said. “It is imperative to drive down the cost of solar energy to grid parity with hydrocarbon processes. We see solar as an area that will take off, as it is not being starved for capital. There is capital investment coming from everywhere, from Washington and Germany to Japan, for economic, environmental and national security reasons. We need to eliminate our dependence on oil, but that can only happen when the cost of solar approximates other sources of energy for electrical generation.”
iTi Solar's VJet. |
Dr. Maikel van Hest, senior scientist, NREL, said that the agency’s goal is to make solar cells less expensive, and added that inkjet may play a role in that process.
“Inkjet is a very viable process, especially in cases where you need to place materials in exact locations, for example contacts,” Dr. van Hest said. “You can also use inkjet to produce uniform film thicknesses. With the new equipment from iTi Solar, we can now produce full scale wafers, and if you can produce 6.2 inch square wafers, you can certainly scale it up.”
“The PV market’s interest in inkjet is still relatively high, but commercialization has been slow,” Morgan said. “NREL’s initiative proves that inkjet is an important tool for solar cell fabrication.”
As an additive process, inkjet has the potential to greatly simplify the fabrication of solar panels by reducing the number of manufacturing steps, making more efficient use of expensive materials, and eliminating much of the vacuum equipment that often requires a clean room environment. It can also be used on a greater number of substrates, and drop placement is digitally controlled.
“Inkjet is an additive process, a huge plus as you put down only what you need, and with silver inks costing more than $1,000 a liter, the less you waste, the better,” Morgan said. “Inkjet is a highly controllable digital process which allows you to put down less fluid to achieve the same goal. Inkjet can also be used to put down resists for wet chemical etching and metallization which holds the promise of greatly reducing the cost of manufacturing the metal connector grid for silicon wafers.”
Inkjet could also help reduce costs through the choice of substrates.
“Silicon wafers represent half or more of the cost of silicon solar cells,” Morgan said. “With a non-contact process like inkjet, you can use thinner substrates and therefore less silicon. In thin film, substrates are relatively inexpensive but the materials are costly.Inkjet can minimize the use of these expensive fluids. Non-contact processes also create fewer problems with substrates, which also allows greater choice of substrates.”
“Right now, silicon PV are primarily printed using screen, and silicon wafers 200 microns thick will sometimes break during the process,” Dr. van Hest added. “With inkjet, which is a non-contact process, we can cut the silicon to 100 microns thick, which would be a dramatic cost savings.”
On the flexible PV side, iTi Solar recently announced that it was selected by Solarcoating Machinery GmbH (ScM) to provide the first digital inkjet system for ScM’s integrated roll-to-roll solar cell manufacturing production line. iTi Solar’s industrial single-pass inkjet printing technology will be integrated into ScM’s flexible, roll-to-roll manufacturing line process.
Still, Morgan noted that inkjet has some challenges to overcome in the PV space, although he added
“There are two huge challenges in the solar market,” Morgan said. “First, the early attempts to use inkjet were not brilliantly successful, as it turns out that inkjet is deceptively simple yet not trivial to implement properly. iTi was founded in 1992 and has been in PE since 2002, so we understand the complexity of the implementation issues. The second is jetting reliability. If you get an air bubble in the ink system, it will clog a nozzle.In many applications, loss of a nozzle can lead to shorts or other defects that can impact the performance of the cell.
“The challenge is consistent printhead reliability.Ink formulation is one challenge as inkjettable fluids must have rigorous quality controls during manufacturing to provide good jetting reliability,” he added. “Ink supply systems must be carefully constructed to avoid air intake and the resultant potential for nozzle failure. You have to be aware of these limitations, but the good news is that new generation printheads coming from Dimatix, Samsung and Xaar offer the promise of better jetting reliability.
“Many companies are working on the fluid side, including MacDermid, Sun Chemical, Dow, Xerox, Kodak, Hewlett-Packard and Cabot,” said Morgan. “Every big player in industrial chemicals is working to ensure the availability of high performance jettable fluids for PV fabrication, and there will be many materials choices available to solar cell manufacturers.”
Inkjet has made major inroads in printing, notably in wide format and labels, and Morgan expects the same will hold true in PV.
“Inkjet is an exciting new technology that is actually 35 years old, and at the end of the day, inkjet takes over every field it goes in,” Morgan concluded. “You can’t compete with its control, cost and productivity. Inkjet is a technology whose time is really coming in PV.”