35th EU PVSEC, 24 - 28 September 2018
SQUARE - Brussels Meeting Centre, Brussels, Belgium

19 December 2016

Perovskite Solar Cells – Impressive & Fast Efficiency Developments, Now It’s About Research On Stability

The development speed of perovskite solar cell efficiencies is breathtaking. When the technology was reported first in 2009, efficiencies were mere 3.8%. But in only 7 years, single-junction perovskite cell efficiencies skyrocketed beyond 20%, getting increasingly closer to the level of incumbent crystalline silicon technology.

Perovskite is a structured compound made of a hybrid organic-inorganic lead or tin halide-based material, which is used as the active PV layer. The fundamental advantages of this new material class are twofold - perovskites crystals can be manufactured at low temperatures of about 200 °C, and they can be produced at low cost. Perovskite solar cells are also several folds thinner than typical crystalline wafer-based cells. They enable simple application methods, such as spraying and printing, provide flexibility in choosing different substrates, such as glass or plastic, which in the end offers sheer endless product designs.

Perovskite + Silicon = Good fit

Perovskites can be even deposited on crystalline silicon solar cells to realize tandem cell structures to maximize utilization of incident sunlight. In fact, this is one research focus in the field of perovskites today as their wide bandgap makes them a perfect fit for a top cell in silicon tandem cell structures. At the last EU PVSEC in Munich in June, a research team from EPFL Neuchatel led by Christophe Baliff attained a 25.2% efficiency on a tandem cell structure of perovskites and silicon heterojunction cells. The Swiss research group has developed a low-temperature near infrared (NIR)-transparent perovskite cell of 0.25 cm2 aperture area with 16.5% steady-state efficiency. This means the top perovskite cell absorbs mostly that part of the sunlight, to which the underlying silicon cell is blind. Employing a larger 1 cm2 perovskite top cell, which had a lower efficiency of 14.5%, still resulted in a 23% final efficiency for the stack.

Going solo also possible

It is also possible to design a solar cell purely with perovskites, which is another strong branch of research. In April 2016, the Korean Research Institute of Chemical Technology and Ulsan National Institute of Science and Technology have demonstrated the highest efficiency for a perovskite single junction cell, reaching 22.1% on of 0.04 cm2 aperture area.

Since the material offers the flexibility to tune the band gap, layers sensitive to different bands of sunlight can be stacked to optimize absorption of incident light. In October 2016, a joint research team from Stanford and Oxford led by Michael McGehee, and Henry Snaith demonstrated 20.3% efficiency in laboratory testing for a dual-layer perovskite solar cell. The scientists developed an infrared absorbing 1.2 EV bandgap perovskite cell reaching 14.8% efficiency, which was stacked with a 1.6 eV bandgap substance in p-i-n configuration and ITO top contact attaining a power conversion ability of 15.8%. While the 20.3% tandem cell efficiency was lower than the record cell, its size was 0.2 cm2, which is about twice as big as the Korean record cell.

This is nowhere close to today’s commercial 6-inch crystalline silicon cell sizes (225 cm2), but researchers from the Australian Centre for Advanced Photovoltaics (ACAP) have been working in this direction and reported a 12.1% efficient 16 cm2 perovskite cell this December. The team has also attained a 11.5% efficiency for a four-cell mini module of the same area and a 18% power conversion for a 1.2 cm2 cell.  

Preparing commercialization in Europe

Although very young and the cells still being very small, perovskites are not just a technology that is only being worked on in laboratories of universities or research institutes. UK-based Oxford Photovoltaics Ltd. (Oxford PV) is in the process of commercializing perovskite cell technology. In November, the spin-off from Oxford University, co-founded by Snaith, acquired the CIS thin-film production line in Germany that was formerly operated by Bosch Solar CISTech GmbH. Oxford PV plans to newly equip the factory to “provide modern, pilot-scale capacity to scale-up its world-leading perovskite technology to industry-standard wafer size and to perfect the manufacturing processes necessary for commercial deployment.” Moreover, Oxford PV signed a joint development agreement with an unidentified “global manufacturer of solar cells and modules”

Several investors are obviously convinced about Oxford PV’s business plans based on perovskite technology. The company has been recently very successful in attracting investments – it announced this December an equity investment of £8.1 million, after raising £8.7m in October.

Some challenges ahead– and the biggest is stability

But no technology comes without limitations -  and in case of perovskites it’s their stability. The cells degrade when exposed to common aging elements - light, air and moisture. Depending on material and exposure conditions perovskite solar cells often see performance degradations only after hours or days. There is progress. For example, McGehee and Snaith’s research group have evaluated the thermal stability of bare perovskite cells by subjecting them to heating for 4 days at 100 °C under nitrogen and observed no changes in absorption spectra and thus the stability. Then in an article – ‘Low-cost solar cells poised for commercial breakthrough’ - published in Science magazine this December, Standford’s McGehee also reported that changing an organic component from the perovskites structure and encapsulating it has shown no degradation for 6 weeks following the standard durability test regime of 85 °C temperature and 85% relative humidity.

However, perovskites will have to compete with incumbent silicon where manufacturers offer performance warranties of 25-years or more. That’s why Diane Wang from UNSW in Sydney concluded in an article on ‘Stability of Perovskite Solar Cells’ published in April in Solar Energy Materials and Solar Cells Journal, “For perovskite solar cells to achieve the required stability, future research must focus on improving the intrinsic stability of the perovskite absorber layer, carefully designing the device geometry, and finding durable encapsulant materials, which seal the device from moisture.”

At the upcoming EU PVSEC in Amsterdam, under Topic 3 Thin Film Technology, we will again devote a significant portion to perovskite solar cells. We are looking forward to receiving your abstracts by 10 February 2017.


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Impressive: In less than a decade, researchers were able to improve efficiencies for perovskite-based solar cells from nearly nil to over 20%. In combination with an HJT cell, perovskite efficiencies even come close to the record of crystalline silicon based technology, though at a much smaller device size.