Scientists at Northwestern University and the U.S. Department of Energy have found that perovskite cells, one of the most promising solar technologies of recent years, can repay their energy cost over 10 times faster than traditional silicon-based solar cells. The finding confirms that, once issues related to cell longevity are ironed out, perovskite cells could soon bring us solar energy on the cheap, and do so with less impact on the environment over their lifetime.

Solar panel installations are doubtlessly having a positive impact on the environment, but quantifying their carbon footprint with some degree of precision – which is useful for comparing them to other means of energy production, including other renewables – is not a straightforward process. To get a more complete picture, it's important to consider not only the carbon emissions saved during the panel's operating life, but also the amount of energy that goes into materials processing, manufacture, repair, maintenance and, once it is no longer useful, disposal of the panel.

According to this metric, called the cradle-to-grave life cycle assessment, a typical solar panel takes a fairly long time, between two and three years, to offset the energy costs that went into producing it. This is because silicon-based solar panels must be manufactured inside a clean room using high-purity crystalline silicon wafers that can only form inside specialized high-temperature furnaces.

Scientists at Northwestern University have now calculated that, by contrast, perovskite-based solar cells have an energy payback time (EPBT) of only two to three months. According to the researchers, this is not only much faster than a silicon-based cell, but also significantly better than any other type of commonly available solar cell.

Energy payback time, or EPBT, for some of the best-known types of solar cells(Credit: Northwestern University)

Perovskite cells are the fastest-growing technology in the solar arena. Although they aren't quite as efficient at converting sunlight into electricity as silicon-based cells, they are catching up very quickly. More importantly, they are much cheaper to produce than normal panels, meaning that their commercialization could lead to a drastic drop in the cost of clean electricity.

Unlike traditional silicon-based cells, perovskites can be manufactured at a very low energy cost, without the need for sophisticated equipment, and in very few steps. A solution containing the electrode materials is coated onto a substrate and, once it evaporates, this solution produces dense layers of crystallized perovskite at a fraction of the cost and energy expenditure of other common solar panels.

According to the study, which analyzed the detailed energy expenditure for two different types of perovskite cells, raw materials contribute about 80 percent of the primary energy consumption for making the panels, suggesting that a better choice of materials could reduce the energy costs even further.

There are indeed plenty of issues with the current choice of materials for perovskite cells, which often make use of potentially toxic lead to absorb sunlight and improve conversion efficiency. The researchers also found that the use of gold, another common raw material, was even more problematic, since the process of mining this precious metal is extremely damaging to the environment.

But perhaps the biggest issue that perovskite cells are currently facing is that they are unable to brave the environment, since they are partly made from organic molecules that degrade quickly when exposed to the elements. Most perovskite cell designs currently lack a protective layer that could lengthen their lifetime, as this would reduce conversion efficiency.

Because of their very short lifetime, the researchers found that the overall CO2 impact of perovskite cells is still significantly higher than that of traditional silicon-based cells, which are much more durable with a reported average lifetime of approximately 20 years.

But if these issues are solved (which the researchers say could happen in as little as two years), perovskite cells could indeed rise to take the lion's share of the solar landscape in the near future, providing clean energy while having an even lower impact on the environment than the solar cells of today.

A paper describing the study appears in the latest issue of the journal Energy & Environmental Science.

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