Anthony King
A dab of chilli spice boosts the amount of energy solar cells can harvest from sunlight. The compound capsaicin, which puts the heat into chilli peppers, allowed the surface of a perovskite solar cell to reach an efficiency of 21.88% (Joule, doi: 10.1016/j.joule.2020.12.009).
Perovskite solar cells are promising for next-generation photovoltaic technology. They work by sandwiching together two types of semiconductors, an n-type with an excess of electrons, and a p-type with a deficiency of electrons. When a photon of light energy hits the solar cell, it causes extra electrons to flow as current in one direction.
Currently, the best solar cells convert ca 25% of sunlight into electricity. ‘Adding dopants can give an n-type semiconductor more electrons or can make a p-type even more deficient in electrons,’ says Jon Major, a physicist at the University of Liverpool, UK, who was not involved in the research. This should make the cells even more efficient.
The researchers in China sought additives to modify the electronic structure of their perovskite solar cells and chose capsaicin because of its electrical, chemical and optical properties, explains senior author Qinye Bao at East China Normal University in Shanghai.
Their best device doped with capsaicin has the highest values for a polycrystalline MAPbI3 (methylammonium lead iodide) based p-i-n perovskite solar cell reported to date, notes Bao. ‘The bare device without capsaicin only shows an efficiency of 19.1% with less stability,’ he adds.
Part of the improvement is due to the additive allowing the surface energetics of the active layer to transform from p- to n-type character, Bao comments. ‘The resulting n-type perovskite surface will energetically enhance electron extraction at the upper contact with an electron transport layer.’
This isn’t a fundamental redesign of the solar cell, notes Major, but rather alters the p-type structure in a way that boosts performance. ‘The surface region became more p-type, so that reduced the number of electrons present in that region, and that was beneficial for how the device performed,’ he says. He notes, however, that perovskite solar cells are still a work in progress, especially since their stability is often problematic, due to degradation in the presence of moisture and in sunlight.
In December 2020, a UK company reported that it had set a world record for converting sun’s energy into electricity in a perovskite solar cell. Oxford PV announced that it had converted 29.52% of solar energy into electricity in a technology that involves coating ordinary silicon solar cells with a thin film of perovskite to make an affordable cell.
The company plans to sell these solar cells to the public in 2022 and claims they should generate 20% more power than current silicon solar cells.
‘Silicon [solar cells] require thick, highly crystallised silicon wafers, which take a long time to make and are expensive,’ says Major, as well as being heavy. ‘They are only cost effective because of the [scale of the] microelectronics industry in the Far East.’ He predicts silicon wafers will eventually be replaced by superior technology, just like LEDs replaced cathode ray TVs.
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