Coloured glass

Solar windows � crazy glazing?

Windows could soon be employed as powerful solar panels thanks to recent materials breakthroughs by UK-based Oxford Voltaics.

If you thought power-generating windows were a gimmick, think again. US energy analyst NanoMarkets forecasts the building-integrated photovoltaics market will top $16bn by 2017 and solar cell glazing – replacing window glass with a semi-transparent photovoltaic  panel – is a fast-growing industry sector. One company poised to cash in is UK-based Oxford Photovoltaics. Researchers from the University of Oxford spin-off have developed thin-film, dye-sensitised solar cells that can be printed onto glass and other surfaces. The glazing panels are semi-transparent, made in a variety of colours and tints, and chief executive Kevin Arthur reckons the manufacturing cost of the product is half that of today’s lowest-cost thin-film technology.

“We’re using the simplest screen printing processes to make a product from readily available, low-cost ‘ingredients’, that can be scaled up quickly on a substrate [glass] that people prefer to use,” says Arthur. “Adding the photovoltaic layer to a window would represent a fairly trivial additional cost and in return you would get electrical energy and lower the building’s carbon footprint.”

Thin-film solar cells – fabricated by depositing one or more thin layers of photovoltaic material on a substrate – were originally developed to avoid the costly silicon processing of conventional solar cells. Dye-sensitised versions, however, are a relatively new type of photovoltaic first developed by Michael Grätzel in 1991.

The cells typically comprise a semiconducting layer of transparent titanium dioxide covered with a dye that absorbs sunlight. The titanium dioxide and dye are immersed in an electrolyte and sandwiched and sealed between two plates; a transparent and metal electrode (see ‘How the cells work’). Oxford Photovoltaics has replaced the volatile liquid electrolyte with a porous titanium oxide scaffold to which the ink ‘sticks’, enabling entire solar modules to be screen-printed onto glass or other surfaces. And while other thin-film photovoltaics contain scarce elements and toxic rare earth metals, the researchers claim their modules are fabricated using only readily available and environmentally-benign materials.

“We also don’t have to worry about extensive sealing and encapsulation, whichis an issue for the electrolyte dye solar cell,” explains Dr Henry Snaith, developer of the technology and company founder. “We can process [the solar cells] over large areas very easily. You can go up to 4m by 5m for a single solar cell module; compare this to a 12in silicon wafer and you’ve got a completely different selling proposition.”

Snaith and Arthur intend to have their pilot line up and running within a year and will initially manufacture 10cm by 10cm panels, with a view to rapidly scaling up 1.1m by 1.3m versions. But it’s not all about size, and Snaith and his research team are also looking to boost the solar conversion efficiency of these panels.

Dye-sensitised cells manufactured by some competing businesses have conversion efficiencies of around 10 per cent but Snaith has only achieved 5 per cent in his laboratory. “We’re looking at new routes to increase the light absorption in the thin film. This will take two to three years but will increase efficiency to at least 10 per cent,” he says.

Another stumbling block for the company is the lifetime of the solar cells. The building industry will demand power-generating windows that can still deliver 80 per cent of their starting power after 25 years but dye-sensitised photovoltaics degrade when exposed to ultraviolet light. Research is already underway worldwide to tackle this problem; for example a barrier layer with ultraviolet stabilisers to protect the solar cell could be integrated into the cell.

“We’re also increasing research in this area and have resolved early teething issues with the stability of these cells,” explains Snaith. “I don’t want to say a number [for how long the PVs remain stable], but it’s on the way up. It’s now a case of optimising a system that can reach qualification.”

Crystal clear

But as companies focus on boosting the conversion efficiencies and lifetimes of their technologies, what about the actual transparency? Most solar cell glazing panels showcased so far are, at best, translucent.

A recent materials breakthrough from US researchers could bring fully-transparent power-generating windows closer to our homes. Rather than pursuing dye-sensitised photovoltaic panels, chemists from the US Department of Energy’s Brookhaven National Laboratory and Los Alamos National Laboratory have instead added carbon-based fullerenes to a semiconducting polymer, to fabricate transparent thin films. The films have a distinct honeycomb structure that absorbs light and produces electric charge.

“This is the first report of a material that blends semiconductors and fullerenes to absorb light and efficiently generate charge and charge separation,” explains Dr Mircea Cotlet from Broohkhaven’s Center for Functional Nanomaterials. “The films are only a couple of nanometres thick so should be highly transparent... and could be sprayed onto glass.”

Using basic laboratory equipment, Cotlet and colleagues fabricate the thin-film by first mixing a so-called conjugated semi-conducting polymer with spherical carbon ‘buckyballs’, known as fullerenes, in an organic solvent. They then spray micrometer-sized water droplets across a thin layer of the polymer solution. These droplets self-assemble into an array of hexagonal cells within the solution and the solvent evaporates to leave a transparent thin film.

Adding fullerenes to a conjugated polymer is a common strategy when developing organic photovoltaics but the hexagonal honeycomb structure makes this material different. According to the Cotlet, most of the polymer gathers at the edges of the hexagons, producing a honeycomb ‘scaffold’ that efficiently absorbs light. The rest of the polymer spreads very thinly across the hexagon centres, leaving these regions transparent.

“The final effect is sort of like a mosquito net,” he adds. “You still have plenty enough transparency to see through.”

A photovoltaic future?

Importantly, Cotlet and colleagues have demonstrated efficient charge transfer across the film and are now keen to implement the structure into devices including sensors, organic solar cells and of course, photovoltaic windows. And while the thin-films only currently measure 1mm by 1mm, Cotlet asserts: “This is big enough to start developing solar cells in the laboratory, and scaling up should be straight-forward as we can easily multiply the size of our experimental station. Looking ahead, the technology to apply these films is already there, look at how anti-reflecting coatings are put on the wind-shields of cars.”

The team reckon we could see an operational device in about five years, but there’s a clear trade-off. “Because of the very high transparency, I would still not expect the photovoltaic to have a conversion efficiency of much more than 1 per cent,” Cotlet admits. “But it [would be] cheap to make and if you can get some electricity out of it, then why not?”

Oxford Photovoltaic’s Snaith agrees, confirming his research team is also working on the development of fully-transparent solar panels. “There is a big market for fully-transparent windows, it is feasible and it will happen,” he says. “You could very well have a solar cell that is 10 per cent efficient... and be completely transparent in the visible, but it’s more likely this technology would get commercialised as soon as efficiency reached 5 per cent.” *

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