Sunny skies ahead

Perovskites make up at least 38% of the Earth’s volume; yet, until 1987, this crystal structure template was largely ignored by scientists. Today, thanks to their ability to ‘sense’ external stimulation — such as that from pressure, heat, or light — and relay this in the form of an electrical current, perovskites have potential in applications ranging from electronic people-counters to cutting-edge solar cells.

Compared to current photovoltaic cells largely based on silicon, solar cells using perovskite are cheap and are made with simple production techniques and equipment requirements. Their downfall, however, is stability. The solar cells have a limited lifespan because of the perovskite’s vulnerability to moisture, weather, and light.

Making perovskite cells stable, and taking them from the lab to real-world application is a major venture for King Abdulaziz City for Science and Technology (KACST). The Saudi governmental science institution has teamed up with world-renowned energy researcher Michael Graetzel and his team from Switzerland’s École Polytechnique Fédérale de Lausanne (EPFL).

At the core of every solar cell is a semiconductor susceptible to having its electrons ‘knocked loose’ by light particles called photons, leaving a free electron and a hole where it used to be. Solar cells take advantage of this property by capturing free electrons and creating an electrical current, which is then utilized, for instance to power equipment, before the electrons are returned to the semiconductor, replenishing the holes from which they came.

“In solar cells, we have a ‘golden triangle’ of efficiency, cost, and stability,” says KACST nanotechnologist Abdulrahman Al Badri. “Any solar cell needs to meet these requirements before it can be commercialized.” In KACST’s solar village, silicon solar panels installed 35 years ago are still functional today. “However, people have been searching for an alternative.”

Silicon has long been the go-to semiconductor used in solar cells due to its abundance and status as the material of choice for microelectronics. But silicon isn’t a fantastic absorber of sunlight and its processing requires expensive equipment. Alternatives have been suggested, but these have often shown poor efficiencies or have toxic components.

“Perovskite has advantages over silicon in terms of the way we produce it. It is a very easy process. We don’t need a clean room, high temperatures, or electronic equipment, all of which are costly,” says Al Badri. In terms of environmental/safety impact, perovskite does contain small amounts of lead; however, this is something that KACST is working to eliminate.

Perovskite, an ionic crystal structure chemically known as calcium titanate (CaTiO₃), was discovered in 1839 and named after the Russian mineralogist Lev Aleksevich von Perovski. The term perovskite now covers a whole family of crystalline materials that form a lattice. These materials have the general atomic ratio ABX₃, where A and B are positively charged ions (cations), and X is a negatively charged ion (anion). In recent years, lead-halide perovskites (containing lead and a halogen ion) have been investigated heavily as light absorbers in solar cells.

To improve the utility of perovskite, scientists first investigated changing the composition of lead-halide perovskites by introducing multiple A cations, using combinations of the chemicals formadinium and methylammonium, and drawing together the beneficial effects of both. In recently published research, EPFL discusses how this combination allows for greater light harvesting near the infrared range and improved material stability.

Following this, KACST and EPFL focused their attention on understanding how other elements can benefit perovskite function, and have published their 2017 research results in the journals Science, the Royal Society of Chemistry, and the American Chemical Society. “We’re now adding some inorganic material such as caesium and rubidium. When you add small percentages of caesium to the mix, it keeps the perovskite stable,” explains Al Badri.

Alongside caesium, the researchers have also found that incorporating rubidium into the mixture can improve the photovoltaic performance of the perovskite solar cell by reducing the amount of ‘recombination.’

Recombination happens when electrons re-associate with holes. In photovoltaic cells, recombination means fewer electrons are available to produce electricity, and the cell is less efficient. In their 2017 Journal of Physical Chemistry paper, Al Badri and his colleagues show that the addition of rubidium boosts electron transport within the perovskite cells’ electrodes, and lowers recombination processes. In that particular paper, rubidium increased the power conversion efficiency from 18.64% to 19.53%.

At the February 2017 Stanford Energy Seminar in California, Graetzel described the current generation of four-cation cells as being able to provide “very good” voltage and showing excellent stability under a “double whammy” of stress conditions: drawing the maximum amount of power under maximum sunlight.

One of the major stabilizing benefits of adding mixed cations is that the right combination causes the mixture to automatically assume the desired ‘black’ crystal structure, which is able to efficiently absorb a greater band of light wavelengths, and more effectively produce power. At room temperature, some single-cation perovskites, such as ones containing just formadinium or caesium as their A cation, form a non-cubic arrangement known as the ‘yellow phase’, with inferior optical properties. When caesium and formadinium are combined, however, they automatically assume the desired black configuration. “You can even mechanically just grind those two materials [to get black-phase perovskite],” says Graetzel during his seminar. “So, black magic.”

With the refinement of perovskite composition yielding ever-increasing solar cell efficiency, Al Badri says the future lies in taking these achievements from proof-of-concept studies, and translating them into viable solar solutions: “Our focus is now to scale it up. We want to have an entire solar module made of perovskite cells so we can prove that perovskite can be competitive with other solar cells.”

In order to do this, one of the challenges will be maintaining the photovoltaic efficiency of the solar cells when the process is made mass-producible: “We’ve achieved over 20% efficiency in the lab, but our commercial focus will be 15%. If we can achieve that in large-scale modules, we’re in very good shape,” says Al Badri.

Al Badri is confident that the technology could have a high impact on the field of renewable energy and is contemplating what forms that could take. One potential application is the electrolysis of seawater, in which cheap and efficient perovskite cells could herald a new age in splitting water to form hydrogen for fuel cells.

This collaborative project between KACST and EPFL started in 2015 and will run until 2020 with the aim of making clean energy accessible to all.