Solar reaching for the sky

C&I Issue 21, 2011

The Willis Tower in Chicago, US, previously known as the Sears Tower, stands 442m high. When it was completed in 1973, it was the tallest building in the world until the Petronas Towers in Kuala Lumpur, Malaysia, surpassed it in 1998. However, the building’s great height may also be valuable from a more practical standpoint. The owners of the skyscraper are currently installing photovoltaic glass units on the south-facing 56th floor windows, using a prism to reflect the light onto a horizontal PV cell without completely covering the window. Harnessing all of the tower’s windows for solar power generation could potentially provide as much electricity as a 10 acre solar power plant.

A more efficient use of space might place the photovoltaic cells directly on the window, and other skyscrapers have done exactly that on their unoccupied floors. But harnessing the light hitting windows for solar energy has the drawback of blocking light from entering the building. Instead, researchers are seeking to develop transparent photovoltaic cells that can do both: generate power and provide a view.

A golden solution?

Photovoltaic organic cells, comprising a layer of organic electronic materials between two electrodes, are of current interest in the search for transparency as they function well in low light conditions. ‘Crucially, organic solar cells offer the advantages of being extremely lightweight, compatible with flexible substrates, potentially very low cost and – being primarily carbon based – environmentally friendly,’ says Ross Hatton from the University of Warwick, UK. Hatton and colleague Tim Jones have developed a method to make a gold film that could be used as a transparent conductor for photovoltaic cells.

The electronics industry typically uses indium-tin oxide as a transparent electrode, but indium is relatively rare and expensive. Indium-tin oxide is also unstable and has high surface roughness, cracking easily when applied to flexible substrates. To address these problems, Hatton and Jones have developed an ultra-thin gold electrode – just eight billionths of a metre thick. ‘This ultra-low thickness means that even at the current high gold price the cost of the gold needed to fabricate one square metre of this electrode is only around £4.50,’ says Hatton. Gold is already used in the electronics industry due to its stability. Ultra-thin films are also resistant to solvents and mechanical abrasion, says Hatton. They are smooth and chemically well-defined, which makes them useful as organic solar cell components.

Hatton and Jones have increased the transparency of the electrode by using polystyrene balls to make a random array of small holes through the gold film. Reducing the thickness even further or making more holes may also increase the transparency, but this would lower its electrical conductivity.

The gold electrode could be mass-produced fairly easily, Hatton says, as ‘roll-to-roll evaporation of metal films is also a well-established process in the packaging industry’. Hatton added that mass-producing the electrodes could significantly increase the amount of gold needed, but may not have as much of an impact on worldwide gold usage. Technology needs currently make up about 11% of the worldwide demand for gold, compared with jewellery’s 54% and investment’s 35%.

However, other experimental transparent devices that use the visible spectrum to generate power tend to be limited, regardless of which electrodes they use, with either low efficiencies or low light transmission. About 10–35% of the visible light impinging on such devices passes directly through them, whereas conventional window glass transmits 55–90% of the visible light that hits it, depending on any added tints.

Efforts to improve the cell’s efficiency reduce transmission and vice versa, because both efficiency and transmission are competing for the same areas of the spectrum.

Other spectral areas

Researchers Richard Lunt and Vladimir Bulovic at the Massachusetts Institute of Technology in the US have focused instead on improving the efficiency of PV cells using other wavelengths of light such as near-infrared radiation. They began by changing the materials responsible for energy generation. ‘Our chemical formulation utilises excitonic organic semiconductors that allow for structured absorption with transparency in the visible solar spectrum and absorption in the near-infrared where power is directly generated from near-infrared light,’ says Lunt.

Currently their cell has an efficiency of 1.7%. By combining the cell with a near-infrared mirror, Lunt and Bulovic increased its efficiency and transparency. They aim to produce a transparent photovoltaic cell with an efficiency of between 8 and 12%, which would be among the highest reported efficiencies to date for organic solar cells.

They plan to achieve this in two ways. The first method would be to use more of the infrared spectrum by stacking different cells together that use different radiation wavelengths, as they would not compete for the same area of the spectrum.

The second is by optimising the cell design. ‘Organic solar cells typically are limited by the ability to separate excitons into free charges,’ Lunt says. When the organic layer of the photovoltaic cell absorbs light, electrons move from the highest occupied orbital to the lowest unoccupied orbital, forming the excitons. Exciton pairs then dissociate to form free charge carriers that move to the electrodes, resulting in electrical current. Changing the structure to a ‘bulk-hetero junction’ architecture would increase the available area to make more excitons, thus potentially increasing the efficiency by up to two or three-fold.

However, organic photovoltaic cells often have a relatively short lifetime. As yet, the total lifespan of the cell is unknown. Work to extend the life of organic light-emitting diodes, a technology with properties similar to Lunt and Bulovic’s organic cell, suggests that increasing its longevity will be fairly straightforward. For example, Lunt says that coating the interior surface of a double pane window would protect the cells from any weather or window washing destruction. The cell is suitable for coating directly on to the window glass during manufacture or being coated onto a flexible film that may be rolled onto existing windows.

It is still too early to estimate costs for installing this type of solar energy harvesting technology. Nevertheless, adding the technology while constructing buildings or replacing windows would incur a proportionally low cost, as the cost of the glass, frames and installation would be the same with or without the solar component. ‘These costs, for the glass, infrastructural frame, installation and labour, contribute roughly one-quarter to one-half of the thin film solar installation costs. So there is definitely significant potential to reduce the effective cost of solar installation,’ says Lunt.

Manipulating infrared

Making use of novel reflective coatings could also increase the transparent cells’ efficiency. Many windows are already coated to reduce glare and light transmission, but those properties cannot be manipulated. Sarbajit Banerjee at the University of Buffalo in the US has shown that the synthetic compound vanadium oxide can be manipulated to be transparent to infrared light at lower temperatures, but switches to reflect infrared light as it heats up.

Vanadium oxide belongs to a group of chemical compounds that exhibit this switchable reflective tendency, but forcing the change was not possible before except at high temperatures. Banerjee and his team prepared the vanadium oxide as a nanomaterial, which lowered the compound’s ‘trigger point’ at which it changes from reflective or transparent from 670°C to 320°C.

Combining the vanadium oxide with tungsten or molybdenum wires lowered the temperature even further, to -140°C. The researchers found that a small presence of the ‘doped’ atoms significantly reduced the trigger temperature, which will improve the visible light transparency of any coating made with vanadium oxide.

A window coated with vanadium oxide could lower heating and cooling costs by reflecting excess radiation in the summer and letting more radiation in during the winter. ‘I think as humble windowpanes start to “smarten up,” consumers will see big benefits reflected in decreased heating and air conditioning bills,’ says Banerjee.

Beyond energy conservation, such switchable window coatings could also play a bigger role in security. ‘There are a couple of applications one could envisage – for instance, for defence-related applications wherein windows are required that can provide switchable electromagnetic interference shielding or switchable heating/cooling capabilities,’ he adds.

Forcing the phase transition with electricity instead of heat is key. ‘On a broader scale, the ability to switch vanadium oxides using current or voltage makes these materials possible candidates for integration within electronic devices. To achieve switchability within an electronic device such as an integrated circuit, it is far easier to be able to induce phase transformations by varying voltage instead of by changing temperature,’ he says.

Once the vanadium oxide nanomaterial has been heated up once, its usefulness after cooling down is also a concern. ‘A particularly notable advantage to using nanostructures of vanadium oxide instead of bulk powders is that the material can be switched back and forth thousands of times without any degradation in performance, whereas large crystals tend to shatter with repeated cycling,’ says Banerjee. Other window coatings do not fare well in the long run because they cannot handle the switching back and forth.

The next steps are to make larger quantities of vanadium oxide nanomaterial, and test the coatings in windows. ‘For instance, we might want to have slightly different compositions and sizes of vanadium oxide nanomaterials for windows in frigid Buffalo as compared with balmy Miami,’ says Banerjee. Banerjee also noted the importance of combining vanadium oxide coatings with other emerging window technologies, such as self-cleaning coatings and double-paned windows filled with insulating gases like argon. ‘The vanadium oxide technology is a promising platform that is additive to other energy-saving window technologies,’ he says.

Smart windows already on the market may not see these additions for a few years, but combining advances in organic solar cells’ transparency and efficiency with novel reflection technologies will make truly smart windows that can provide renewable energy without obstructing the view.

Erin Boland worked as an intern on C&I during summer 2011 and is now based in Indiana, US.

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