University researchers at opposite ends of the globe are in a race to perfect innovative applications to improve the output of photovoltaic cells, offering the potential to revolutionise the uptake of solar power generation.
Researchers at Pittsburgh University, Pennsylvania, in the United States, have developed a glass that, although appearing opaque, allows high-level light penetration – a property that could help improve solar cell performance.
Created by etching the glass to create tiny grass-like structures on its surface, the nanoscale engineering produces minute, perpendicular, grass-like blades of glass (nanograss), the versatility of which surprised researchers further when it was discovered that, simply by applying water, the glass changed from hazy to clear, a useful property in creating opacity-controlled smart windows.
(a) Cross-section SEM images of nanostructured grass-like glass with (i) 2.5, (ii) 4.5, (iii) 6, and (iv) 8.5 μm height, and (b) (i) 15° tilted and (ii) overhead view of 6 μm height hazy glass. Source – viewmedia
The method could provide a cheaper alternative for the production of switchable glass, which currently uses electricity to create a haze in glass to control privacy, glare and the penetration of sunlight.
According to a paper by the Pittsburgh research team published in the American Optical Society journal, Optica, in December last year,optoelectronic applications including solar cells and light-emitting diodes (LEDs) could benefit from better power conversion and extraction efficiencies via the glass substrate’s high transparency and high haze properties.
The nanograss is produced by a reactive ion etching process in fused silica with the height controlled by etch time.
Tests showed shorter nanograss (up to 2.5 μm high) improved the anti-reflection properties of the glass while longer grass tended to increase haze. Ultra-high haze, over 99 per cent, was achieved with longer nanograss (up to 6 μm high), though the transmission decreased slightly to less than 92 per cent.
Contour plots of (a) total transmission (%) and (b) haze (%) as a function of wavelength and nanograss height. Source – viewmedia
Researchers experimented with glass etched with nanograss structures from 0.8 to 8.5 microns in height with “blades” measuring a few hundred nanometers in diameter.
Various fluids with a similar index of refraction as the glass were applied to switch the haze of the glass substrate.
“Switchable glass available today is quite expensive because it uses transparent conducting layers to apply a voltage across the entire glass,” said Swanson School of Engineering lead researcher Paul Leu.
Optical images of smooth glass and glass with 2.5 and 6 μm height nanograss when placed (a) directly on paper with text and (b) about 1 cm above. Source – viewmedia
“Our glass would be potentially less expensive to make because its opacity can be switched in a matter of seconds by simply applying or removing liquid.”
The unique characteristic of the nanograss glass to switch opacity was discovered by accident when water was used to clean the glass.
When the water passed between the hydrophilic nanostructures, the nanograss glass acted as a flat substrate. Because the water had a similar index of refraction to the glass, the light did not pass through it, but when the water is removed the light is scattered when it hits the nanostructures, making the glass hazy.
Scattering ability of (a) flat fused silica and (b) 6 μm height nanograss glass. The scattering ability is demonstrated by shining a laser through a sample onto a target. The rings on the target are spaced 5 cm apart. The distance between the sample and target is 30 cm. Source – viewmedia
The nanograss glass is being developed to improve the ability of solar cells to capture light and turn it into power, designed to prevent light from being reflected off the surface of solar cells while helping to scatter the light that enters the glass, enabling more light to reach the semiconductors, where it is converted into electricity.
Meanwhile, on the other side of the world, researchers at the School of Photovoltaic and Renewable Energy Engineering at the University of New South Wales (UNSW), in Sydney, Australia, have dramatically improved the performance of spray-on perovskite solar cells.
Perovskites have been used in the production of solar cells for less than a decade and the ramping up of its efficiency over the past 12 months has set new world records.
The UNSW team achieved a 12.1 per cent efficiency rating for a 16 square centimetre perovskite solar cell.
Senior Research Fellow at the Australian Centre for Advanced Photovoltaics (ACAP), Anita Ho-Baillie, first announced the success at the Asia-Pacific Solar Research Conference in late 2016, with the new cell at least 10 times more efficient than certified high-efficiency perovskite solar cells previously on record, confirmed by Newport Corp international testing centre in Bozeman, Montana, in the United States.
Ho-Baillie, who came to Australia from Hong Kong as a teenager, said perovskite solar cell development was a hot area of research.
Anita Ho-Baillie holds a perovskite solar cell. Source – UNSW
“Perovskites came out of nowhere in 2009, with an efficiency rating of 3.8%, and have since grown in leaps and bounds,” she said.
“These results place UNSW amongst the best in the world producing state-of-the-art high performance perovskite solar cells.”
Perovskite, a structured crystalline compound in which a hybrid inorganic-organic lead or tin halide-based material acts as a light-harvesting active layer, is cheap to produce, simple to manufacture and can be printed or sprayed onto surfaces.
The material’s high absorption coefficient enables ultrathin films of around 500 nm to absorb the visible solar spectrum.
“The versatility of solution deposition of perovskite makes it possible to spray-coat, print or paint on solar cells,” Ho-Baillie said. “We hope one day to be able to spray it on any building fabric, device or even cars.”
Currently, most commercial solar cells are made from a highly purified silicon crystal which needs to be baked above 800˚C.
Perovskites, made at low temperatures and 200 times thinner than silicon cells, are something of a Holy Grail in the quest for cost-effective solar energy, as those currently produced are prone to fluctuating temperatures and moisture, functioning for only a few months without protection.
Research teams worldwide are in a race to extend the durability of perovskite solar cells, which would make them a highly-commercially competitive alternative to silicon solar cells.
In a clear indication of the scientific world’s determination to win that race, in July, 2017, researchers at South Korea’s Ulsan National Institute of Science and Technology (UNIST) set a new world record of 22.1 per cent for perovskite solar cell efficiency.
In August last year, the IMEC Research Hub in Belgium reported on the development of a new perovskite solar cell with a conversion efficiency of 23.9 per cent.
The race continues.
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First published in Solar Magazine
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