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IN DEPTH: Rethinking the solar cell

A mineral discovered in Russia’s Ural Mountains almost 200 years ago is being touted as the key to unlocking future PV technologies with efficiencies of up to 50% — and at a price that could undercut conventional crystalline silicon (CSi) inside a decade.

Perovskites, salt-like crystals that dissolve rapidly in water or even high humidity, have been used in experimental solar cells for some time. The breakthrough came five years ago with a cell that converted light into electricity with an efficiency of 3.8%.

But this year, several research groups ramped up development of the technology, successfully designing perovskite PV cells with reported efficiencies of up to 19.3% — suddenly competitive with market-dominating CSi, and cadmium-telluride and copper-indium-gallium-selenide thin-film, which have efficiencies of 17-23% — as well as a potentially game-changing inexpensive, spray-on cell with 11% efficiency.

And perovksites — which made their debut as a dye replacement in dye-sensitised solar cells, as they did a better job of absorbing both visible and infrared light, and so harvest sunlight very efficiently — can be made on the cheap, the main ingredients being bulk chemicals such as ammonia, lead and iodine.

In commercial production they will stack up well against CSi and thin-film, which owe their high performance to expensive materials and energy-intensive crystal growth and vapour-deposition methods.

Best yet, unlike conventional PV produced at high temperatures, perovskite cells can be manufactured from solution at much lower temperatures, with researchers pursuing the idea of fabricating perovskite PV using low-cost, low-energy techniques such as 3D printing.

Perovskites’ rapid rise is exceptional in the solar R&D world, where efficiency improvements have historically been a game of decimal-point advances achieved over many years.

Several companies, including Oxford University spin-out Oxford Photovoltaics, expect to have cells ready for market inside four years.

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That the technology has suddenly leapt from relative obscurity into the commercial limelight is in large part due to researchers’ new-found ability to make consistent, high-quality batches of the cells.

To this point, there has been a wide variation in how effectively individual perovksite cells can convert light into electricity, a quandary caused by the variable size of the crystals in different cells — larger crystals mean electrons move more freely and so produce more electricity.

The two-step process used to make perovskite cells involves coating an electrode with lead iodide solution and allowing it to dry before a solution of methylammonium iodide is applied. As the two layers come together, perovskite crystals are formed. A top electrode completes the cell.

By tightly controlling the concentrations in the solution recipes as well as the processing conditions, larger crystals can be made, creating a more efficient cell.

Fine-tuning fabrication of the technology is only part of the challenge, however.

Another key hurdle being overcome by researchers is dealing with perovskites’ solubility. Because even humidity can cause the crystalline materials to break down and leak methylammonium, ways of sealing the cells are being explored, as are new combinations of the core ingredients.

And then there is the lead. Investment in the technology has been slowed somewhat by concerns over the environmental impact of the lead-based cells. Although infinitesimal (by one calculation, a year’s lead emissions from a coal-fired power plant are ten times the amount emitted in building a one-terawatt perovskite PV array) these are seen as a potential show-stopper. ​

Perovskite cells that substitute tin for lead have already been developed in at least two labs. The perovskites with tin — a metal that sits just above lead in the periodic table and shares a similar electronic structure — are much less stable in air than their lead counterparts and currently convert light into electricity with an efficiency of only 6%.

But the first tin perovskite prototypes are seen as a good start in fashioning an environmentally more benign version of the cell and could move towards commercial viability as rapidly as the leaded sort.

Cheap, safe, durable and capable of being manufactured in large panels: beyond the utility-scale applications of perovskites-powered solar lies the building-integrated PV (BIPV) market.

Work is under way between several perovskites technology developers and glass manufacturers to create PV glazing. Windows lacquered with perovskite — which could have a tint of any colour imaginable, given that the crystals could be “tuned” — are expected to be able to generate electricity with a more than commercially adequate 6-8% efficiency for BIPV, costing only 10% more than normal panes.

This flexibility might just win the day for perovskites. Researchers are looking into myriad crossover applications, from use as the light-absorbing heart of high-tech quantum-dot-based solar designs to marrying perovskites PV technology with conventional CSi by processing it as additional layers on top of the silicon. It could be coloured to capture more of the solar spectrum, thus boosting the efficiency of current state-of-the-art panels.

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