Researchers in alternative energy have been excited about perovskite solar technology for years now. They have also struggled to overcome serious challenges

A perovskite is a mineral crystal comprising calcium, titanium, and oxygen. Other minerals, technically perovskite structures, have the same structure, but different elements.

The distinction between perovskite and perovskite structure matters only in geology. The solar industry uses the terms interchangeably.

Certain perovskites have several properties that make them useful in solar applications, including superconductivity and catalytic properties.

They are incorporated into organic photovoltaic thin films, an aspect of nanotechnology.

A perovskite solar cell comprises a layer of perovskites sandwiched between two other layers to receive and conduct electrons. Electrodes connect the outer layers, and one must allow light to shine through it. Many materials useful for these layers absorb moisture or react with oxygen, causing the perovskites to degrade.

They show great promise in laboratory conditions, but research has not yet progressed to the point of making practical perovskite solar cells.

The promise of perovskite solar technology

Current (first generation) solar technology uses wafers to make solar cells. About 90% of the wafers use silicon.

Most of the ones on the market are about 15% efficient at converting sunlight to electricity. The less common gallium arsenide cells are more efficient.

The second generation uses thin film nanotechnology and has about a 5% market share. It boasts 20% efficiency of its cells and 17.5 % efficiency of its modules.

Using perovskite structures with thin film is part of the third generation of solar technology. It uses a hybrid of organic and inorganic material, usually with either lead or tin halide. Among other intrinsic properties, perovskites can absorb a broader portion of the light spectrum than traditional cells.

They are cheap, relatively simple to manufacture and lightweight. Therefore. they promise both greater efficiency and lower cost. It takes only something like an ink-jet printer to apply them to the thin film.

Problems with perovskite solar technology

Perovskite solar technology still has hurdles to pass before it can become commercially viable. Perovskite solar cells only last a matter of months outdoors, where conventional silicon panels usually come with a 25-year guarantee.

Even if someone develops perovskite solar cells that last two and a half years, they won’t be competitive with silicon panels.

That’s about a tenth the life of conventional panels. Unless such a perovskite panel costs less than a tenth as much as silicon, it will therefore be more expensive in the long run.

Several conditions that cause them to degrade. Moisture is the worst. It changes the crystalline structure and destroys its ability to absorb visible light. Heat and ultraviolet light also degrade perovskites.

Film quality can likewise affect the stability of perovskite crystals. Some common components of the films easily absorb moisture.

It has already taken a lot of experimentation to produce crystals that last for months instead of minutes. Perovskite crystals combine an organic or inorganic component, a metal, and a halide in a specific arrangement of atoms.

The elements themselves matter less than the structure, which for a perovskite, resembles salt. Scientists must find a stable crystalline structure that efficiently converts light to electricity.

In addition, most perovskites used in solar technology contain lead. There is no safe level of exposure to lead. Decayed or damaged perovskite cells pour this toxic substance into the environment.

The search for solutions to problems with perovskites

Other metals, like tin or bismuth, can substitute for lead, but at the cost of reduced efficiency.

Lead perovskite solar cells can top 20% efficiency. Tin perovskite cells so far have only 6% efficiency. Bismuth likewise absorbs light less well, so it must be made thicker.

Scientists have as much trouble finding suitable substitutes for components of the thin film.

Encapsulation of devices in an inert environment can maintain perovskite stability by minimizing their exposure to heat and humidity and converting ultraviolet light to visible light. But the whole structure gains weight as a result.

A team of researchers at Stanford University notes that the lab has never studied such a fragile material as perovskites. But they think they have discovered a way to make them more durable by mimicking the compound eye of insects. A fly’s eye, for example has hundreds of light-sensitive segments, separated by a structure comparable to a honeycomb.

An insect eye has built in redundancy. If one segment gets damaged, the others continue to operate. The scaffolding that surrounds the segments shields them from damage. When the Stanford team surrounded perovskites with it, it had minimal effect on the cells’ ability to generate electricity.

Next, they exposed cells to harsh conditions for six weeks: temperatures of 185 degrees Fahrenheit at 85% relative humidity. They continued to operate efficiently during that time. It’s still only six weeks under laboratory conditions, not decades of operation in production conditions.

Perovskite solar technology won’t be suitable commercially for years to come. But researchers keep finding ways to improve their durability and decrease toxicity. It’s hard to predict which new technologies will prove viable.

Given the improvements to perovskite solar cells in the fairly short time researchers have been investigating them, they appear to be on their way to eventual success. But they won’t challenge first generation solar technology any time soon.

Sources:
Make perovskite solar cells stable / Yang Yang and Jingbi You, Nature. April 11, 2017
Perovskites and perovskite solar cells: an introduction / Ossila. April 2017
Perovskite solar / Perovskite-info. [2017]
A promising new solar technology with a troubling old toxicity problem / Jeff McMahon, Forbes. December 7, 2016
Stanford researchers mimic structure of insects eyes to make perovskite solar cells more durable / Steve Hanley, Clean Technica. September 3, 2017

Photo credits:
Perovskite crystal. Public domain from Wikimedia Commons
Thin film solar installation. Public domain from Wikimedia Commons
Dew on thistle. Public domain
Fly eyes. Public domain from Wikimedia Commons