The “Spin” Around Perovskite Solar Cells Has Only Just Begun

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The buzz around perovskite solar cells has been building to a fever pitch ever since the technology first emerged in 2009, opening up the potential to send the cost of solar power plunging down without sacrificing efficiency. Apparently we ain’t seen nothing yet. Researchers are finally beginning to notice that the electron spin factor can be harnessed to boost performance even more.

A Long Journey For Perovskite Solar Cells

For all the research effort extended over the past 15 years very little has shown up in the marketplace. However, a perovskite-silicon combination has begun to appear. Perovskite solar cell fans also point out that the R&D timeline for perovskite technology is progressing with remarkable speed compared to other photovoltaic formulas.

The initial starting point for perovskite solar cell conversion efficiency was just 3.8% in 2009, and today it’s climbing towards the 30% conversion efficiency mark. Researchers are also resolving an issue with durability. A team at the US Department of Energy’s Brookhaven National Laboratory, for example, calculates a 30-year lifespan for their new perovskite device with a reasonably impressive solar conversion efficiency of 17.4%.

In another sign of approaching commerce-readiness, Toyota recently spearheaded a new ¥5.5 billion round of funding for the perovskite specialist EneCoat, through its Woven Capital investment branch. EneCoat has come up with a hybrid perovskite solar cell in a thin film format, which could be applied to cars among other objects (see lots more perovskite background here).

The Electron Spin Angle

In terms of future progress, perovskite solar cell researchers have spotted a new opportunity in the field of electron spin control. “Most current-day optoelectronic devices rely on the interconversion between charge and light. However, spin is another property of electrons, and control over the spin could enable a wide plethora of new effects and functionality,” explains the Energy Department’s National Renewable Energy Laboratory.

An NREL research team has been looking into the potential for deploying different types of perovskites to control electron spin, with the aim of improving energy efficiency in optoelectronic devices including solar cells.

In a conventional semiconductor, spin control typically requires the application of a magnetic field at temperatures below zero degrees Celsius. Building on previous research, earlier this year the  DOE team was able to achieve the same effect by deploying two different kinds of perovskites.

“Using chiral perovskites, the researchers were able to transform an LED to one that emits polarized light at room temperature and without a magnetic field,” DOE explains.

Full details are available in the study titled, “Room-temperature spin injection across a chiral perovskite/III–V interface” published in the journal Nature in June.

For those of you on the go, the first author of the study, Matthew Hautzinger, offers up a brief summary.

“You can buy an LED analogous to what we used for 14 cents, but with the chiral perovskite incorporated, we’ve transformed an already robust (and well understood) technology into a futuristic spin-control device,” Hautzinger says.

Chirality The Key To A More Durable Perovskite Solar Cell

If you caught that thing about chiral perovskites, so have other researchers. A team from the School of Engineering at the Hong Kong University of Science and Technology, for example, has been leveraging the power of chiral structures to vault a new, highly durable perovskite solar cell into commercial development.

The HKUST team was inspired by the mechanical strength of spring-like chiral structures, similar to a helix. Chirality refers to objects that are not identical to their mirror image.  Chiral structures are ubiquitous throughout nature and in physical objects including coils, screws, and the alpha helix of DNA.

“The team inserted chiral-structured interlayers based on R-/S-methylbenzyl-ammonium between the perovskite absorber and electron transport layer to create a strong, elastic heterointerface,” HKUST explains.

“The encapsulated solar cells retained 92% of their initial power conversion efficiencies after 200 cycles between −40°C and 85°C for 1,200 hours, tested under the International Electrotechnical Commission (IEC) 61215 solar cell standards,” they add.

“It is really the dawn for the commercialization of perovskite solar cells,” enthuses HKUST Associate Professor Zhou Yuanyuan, referring to the resolution of the durability obstacle.

Meet The Printable, 20-Year Perovskite Solar Cell Of Tomorrow, Soon

Meanwhile, scientists another Hong Kong institution, the University of Hong Kong, have apparently seen enough. With a funding assist from the Hong Kong government, the team is already laying plans to build a 20-megawatt capacity pilot production line to fabricate perovskite solar cells with a 20-year lifespan. They also report a conversion efficiency of 26% for their device.

If you’re wondering how that compares to conventional silicon technology, efficiency and longevity are only part of the perovskite formula. Cost is also a key element, and in that area perovskites beat silicon by a wide margin. Unlike silicon, perovskite materials can be disposed as an ink or coating on a thin, lightweight, flexible film. That saves money on the manufacturing side, and it also enables perovskite devices to be deployed in situations where conventional silicon is too stiff and heavy.

The UHK team also reports a manufacturing temperature as low as 100 degrees Centigrade, which saves considerably more energy than the high-temperatures needed for silicon processing. “The final comparable cost of energy for perovskite solar cells can be just half that of silicon polar cells,” the team emphasizes.

If all goes according to plan, solar cells will come off the pilot line in about 18 months, to be deployed on a trial basis by the electronics manufacturer Abes Technology Group and Towngas Energy Academy, the R&D branch of Towngas Smart Energy.

Onwards And Upwards For Spin Control

As for those spin control and chiral angles, keep an eye out for much more activity in that area. Here in the US, the Energy Department has tasked a new research collaboration with understanding and controlling “emergent properties in hybrid perovskites for new energy technologies.” The intent is to move beyond the prototypical perovskite-enabled hybrid organic-inorganic semiconductor structure to explore atypical structures as well as those without perovskites.

Called CHOISE for the “Center for Hybrid Organic-Inorganic Semiconductors for Energy,” the collaboration aims at achieving “unprecedented control” over a shortlist of emerging properties including spin, charge, and light-matter interactions.

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Photo: Spin control and chiral structures are front and center in the next generation of lower-costing, higher-performing perovskite solar cells (courtesy of US DOE).