A team of researchers from the Massachusetts Institute of Technology has found a way to passivate silicon at room temperature, which could be a significant boon to solar-cell production and other silicon-based technologies as it makes manufacturing process significantly less expensive.
Silicon, the material of high-tech devices from computer chips to solar cells, requires a surface coating before use in these applications. The coating “passivates” the material, tying up loose atomic bonds to prevent oxidation that would ruin its electrical properties. Passivation is essential: without it, silicon’s surface is oxidized as soon as it’s exposed to air, impeding its performance as a solar cell.
Typically, silicon surfaces are passivated with a coating of silicon nitride, which requires heating a device to 400°C, making it costly and limiting the kinds of materials that can be added to the devices. Conventional silicon-nitride passivation is one of the more expensive parts, and one of the more finicky parts, in the processing of silicon for solar cells and other uses, so replacing part of silicon nitride’s functionality with a simplified, robust organic layer has the potential to be a big win.
The team of researchers from MIT developed a process that decomposes organic vapors over wires heated to 300°C, but the silicon itself never goes above 20°C, room temperature. Heating those wires requires much less power than illuminating an ordinary light bulb, so the energy costs of the process are quite low. The MIT team has tested silicon chips with the new polymer coating in place for more than 200 hours, observing no degradation at all in performance.
“The electrical properties did not change,” said Karen Gleason an engineering professor at MIT.
The low temperature of the silicon chip in this process means that it could be combined with other materials, such as organic compounds or polymers, which would be destroyed by the higher temperature of the conventional coating process. This could enable new applications of silicon chips, for example, as biosensors following bonding with compounds that react with specific biological molecules.
The energy used in manufacturing silicon solar cells is a critical concern because every bit of cost savings helps to make them more competitive with other sources of electricity. The lower temperatures could significantly reduce manufacturing costs, the MIT researchers say. The new process also has an added benefit, providing an anti-reflective coating that improves a solar cell’s overall efficiency.
Both the conventional process and the new process take place in a vacuum chamber. Liquid reactants evaporate, then adsorb and react on the surface. The process can easily be scaled to the size of conventional solar cells, according to the researchers. Additionally, the materials involved are all commercially available, so implementing the new method for commercial production could be a relatively quick process. In fact, the next step for the team is to scale up the process from laboratory-scale to production levels that could lead to commercialization
“Lowering the cost of manufacturing equipment, including that used to apply the passivating and antireflection coating, is one of the three steps that’s needed to drive down the price of solar modules to widespread grid competitiveness,” said Tonio Buonassisi, an engineering professor at MIT.