Multi-crystalised silicon is a cheap conductor used in almost all modern solar cells. However its low cost manufacture introduces impurities that reduce efficiency.
“We call it ‘dirty silicon’,” explains Professor Bruce Hamilton from the Department of Electrical and Electronic Engineering. “As it is ‘cooked’, transition metals leach out of the steel furnace and into the silicon crystals. These metals cause defects which remove the energy that sunlight gives to electrons.”
Scientists are developing new manufacturing methods to reduce the level of impurities in the silicon and boost the efficiency of photovoltaic cells, without raising production costs and with lower carbon emission.
Professor Hamilton leads a project called ‘Efficiency Enhancement of Silicon Photovoltaic Solar Cells by Passivation’. His research team is using unique spectroscopic tools to understand the fundamental physics of silicon defects.
They have already enjoyed some early success: their spectroscopic analyses have revealed some highly disruptive defects. “Initial results show that titanium forms clusters within silicon crystals. These nano scale tennis ball-like structures are very efficient at capturing electrons, but they are hard to detect,” explains Professor Hamilton, who is now carrying out further work to calculate the rate of electron capture by titanium.
So is there a way to manufacture titanium-free multi-crystalised silicon?
“The obvious solution would be to ‘getter’ titanium defects – that is, to move the metal clusters to the edge of the silicon sheet,” says Professor Hamilton. “But experiments show that these clusters are difficult to diffuse: gettering is not an option.”
An alternative solution under investigation involves a process known as passivation: binding atoms to metal impurities to alter their electrical properties and wipe out their ability to steal energy from electrons flowing through the silicon. Current passivation methods use hydrogen atoms, although this is not without problems.
At hot temperatures the hydrogen atoms dissociate from the silicon defects – hardly ideal in a technology designed for sunny days. Larger, heavier atoms may be more effective, according to Professor Hamilton. “We are investigating the efficacy of fluorine in passivation,” he comments. “If it works, manufacturers will be able to produce cheap ‘dirty silicon’, then use our technologies to overcome impurities and boost its efficiency.
“Current solar cells made with expensive silicon are around 20% efficient. What if we can make cheap silicon even more efficient than this?”
Professor Hamilton suggests that alternatives to hydrogen passivation could become common within a few years if his experiments prove effective. An efficiency boost of just 2% would be worth $20 billion globally and represent a significant increase in solar generation and saving in carbon emissions.
Name: Efficiency Enhancement of Silicon Photovoltaic Solar Cells by Passivation
Lead researcher: Professor Bruce Hamilton
Research group: Microelctronics and Nanostructures
Research institute: Photon Science Institute
Industry partners: Elkem ASA; Fraunhofer; MEMC Electronic Materials SpA; National Renewable Energy Laboratory; University of Aveiro; University of Oxford.
Dates: September 2012 to August 2015
If it works, manufacturers will be able to produce cheap ‘dirty silicon’, then use our technologies to overcome impurities and boost its efficiency.