Hydrofluoric acid eliminated from silicon salvage process

South Korean scientists have developed a sustainable process to reclaim silicon wafers from old solar panels and used the salvaged silicon to build new solar cells.

Harnessing the sun’s energy using photovoltaic technology reduces society’s reliance on fossil fuels. Solar cell waste will increase dramatically in coming decades because of the millions of solar panels now being installed annually – each with a finite lifetime of about 25 years. In light of this, the waste electrical and electronic equipment (WEEE) directive now imposes recycling requirements on solar panel manufacturers. Fabricating new panels using recycled components not only maintains the solar industry’s green credibility, but also drives down the cost of solar energy significantly.

Many current technologies to recycle silicon solar cells involve breaking the silicon wafers upon removal from the panel. Once removed, the wafers are typically stripped of their impurities using hydrofluoric acid. As well as being harmful to the environment, hydrofluoric acid can penetrate deep into human tissue causing severe burns and even death after contact with the skin.

Nochang Park and his colleagues at the Korea Electronics Technology Institute and the Korea Interfacial Science and Engineering Institute have developed a new method to recycle end-of-life silicon solar panels. Park tells Chemistry World that this is the first process of its kind that does not use highly toxic chemicals such as hydrofluoric acid. The panels are first heated to 480°C in a furnace, which vaporises the glue that holds the silicon wafers inside. Surprisingly, if the temperature is ramped up by exactly 15°C per minute, no wafers break during the heating process. Once an unbroken wafer has been removed from the panel, its silver electrode is stripped from the top surface using nitric acid. The anti-reflective coating, emitter and the p–n junction layers are then pulverised in a grinding machine. Finally, potassium hydroxide etches away the aluminium electrode from the rear side of the wafer. New solar cells built using the reclaimed wafers perform virtually the same as those containing brand new silicon.

Steven Girard, who works on the sustainable synthesis of silicon nanostructures at the University of Wisconsin–Whitewater, US, describes the process as remarkable. ‘What really sets this method apart from the others is that it’s simple, scalable, inexpensive and much less toxic … I’m actually pretty blown away that they can show 100% recovery of unbroken silicon wafers.’ Efraín Ochoa, a silicon solar cell specialist at the University of Malaga in Spain is just as impressed. ‘This work presents a step forward for recycling silicon wafers, an option with the potential to have a great impact for the renewable energy industry.’