Andrew BurgerWe humans have looked around us and received inspiration from the natural world throughout the course of our evolution. Many such observations have led to the invention of tools, technology and theories that have changed the way we work and live, augmenting what in many instances are our comparatively meager sensory and physical attributes.
One of the latest examples of what has come to be called “biomimicry” originates at Stanford University, where solar energy researchers have drawn inspiration from the compound eyes of a fly to create a honeycomb scaffolding made from low-cost epoxy resin commonly used in electronics manufacturing to enhance the stability of perovskite solar photovoltaic (PV) cells without sacrificing anything in the way of power conversion efficiency.
Commercial prospects are global and mass market in scope and scale, with the potential to produce perovskite PV energy cells and products in ribbons, adhesive strips and flexible forms that can conform to almost any size or shaped surface regardless of what the underlying material is made from.
The Stanford University research team has already filed a provisional patent for their perovskite PV lattice structure, and they are moving forward with the aim of further improving their durability and power/energy conversion efficiency. Coincidentally, they are focused on developing new form factors, such as flexible perovskite PV sheets and ribbons, as well as designs for industrial tools and processes that can scale production up to commercial levels, Reinhold Dauskardt, Stanford University professor of materials science and engineering and senior author of the research study published in Energy & Environmental Science, told Solar Magazine.
My What Beautiful Eyes…
The original perovskite is a mineral – calcium titanium oxide (CaTiO3) – discovered in the Ural Mountains of Russia by Gustav Rose in 1839 and named after Russian mineralogist Lev Perovski (1792–1856). Mineralogists have since expanded that to encompass a growing class of minerals of common structure – that forms crystals akin to table salt (sodium chloride, NaCl). They are commonly found in nature as layered mineral formations, but new types of synthetic PV perovskites are being created in materials science research labs around the world.
“Perovskites are promising, low-cost materials that convert sunlight to electricity as efficiently as conventional solar cells made of silicon,” Dauskardt explained in a Stanford University news report.
The problem is that perovskites are extremely unstable and mechanically fragile. They would barely survive the manufacturing process, let alone be durable long term in the environment.
Researchers have been experimenting with various combinations of chemical element, such as manganese, which serve as core elements of perovskite PV microcell structures in their search for one or more that exhibit optimum combinations of power/energy conversion efficiency, durability, ease of manufacturing and other attributes, such as color and form factor.
Significant gains in perovskite PV power/energy conversion efficiency have come fast in recent years, rising to surpass 18 percent in lab conditions as of June 2016. It’s the fastest such rise in industry history and has now reached levels comparable to that for today’s commercial crystalline silicon PV cells, which account for more than 90 percent of solar PV installations worldwide.
With a grant from the Stanford Precourt Institute for Energy and additional support from the National Science Foundation, Dauskardt and colleagues set about addressing the issue of perovskite PV cell durability and found inspiration in the compound eyes of a fly. As he explained:
We were inspired by the compound eye of the fly, which consists of hundreds of tiny segmented eyes. It has a beautiful honeycomb shape with built-in redundancy: If you lose one segment, hundreds of others will operate. Each segment is very fragile, but it’s shielded by a scaffold wall around it.
Custom-Designed Photovoltaic Perovskites
As solar energy researcher Bert Conings explains in a TEDxUHasselt presentation, researchers are aiming to boost perovskite PV power/energy conversion efficiency further by stacking cells that can produce electrical charge from the entire frequency spectrum of sunlight.
Also key to the intense research interest, custom-designed PV perovskites are inexpensive and straightforward in terms of fabrication. “We make the perovskite materials using precursor solutions, this is one of their main benefits, inexpensive solution processing,” Daushardt explained to Solar Magazine.
Another important advantage perovskites offer is that a variety of chemical elements can serve as core building blocks in constructing PV microcells. That gives researchers much more flexibility to mix and match PV perovskites of different chemical composition to max out their power/energy conversion efficiency, or match the specific requirements of particular applications and use cases.
All told, perovskite PV cells and products could substantially reduce the cost of producing electricity from solar energy and greatly expand the types of surfaces on which they can be applied. That encompasses anything from clothing, wearable accessories, furniture or just about any type of device or piece of equipment to windows and pretty much any building surface, whether curved or flat.
Perovskite PV cells could substantially reduce the cost of producing electricity & greatly expand the types of surfaces on which they can be applied.
The perovskite PV honeycomb cells the Stanford research team created were rectangular, but they could be made into any shape or form, Dauskardt said in response to an email. “We are certainly interested in developing cells on curved surfaces, since there would be many applications for these in designs including solar cells,” he added.
Enhancing Perovskite PV Durability...and Much More
Using the compound eye as a model, the researchers created a compound solar PV energy cell made up of honeycomb of perovskite PV microcells using standard lithographic techniques that can be found in microelectronics manufacturing facilities, Dauskardt explained. Each one is encapsulated in a hexagon-shaped scaffold that measures only 0.02 inches (500 microns) in width.
“We make hundreds of individual cells in each device, but this is simply determined by the lab scale processing we conduct. With scale-up tens or hundreds of thousands of cells could be made for each device,” he told Solar Magazine.
Of critical significance, the perovskite PV microcells continued to produce electricity at the same levels – around 12 percent – when embedded in the honeycomb scaffolding. “We got nearly the same power-conversion efficiencies out of each little perovskite cell that we would get from a planar solar cell,” Dauskardt was quoted in the Stanford U. news report. “So we achieved a huge increase in fracture resistance with no penalty for efficiency.”
To test their durability, the research team exposed their perovskite PV honeycomb cells to temperatures of 185 F (85 C) and 85 percent relative humidity for six weeks. They continued to generate electricity at relatively high rates of efficiency even under these extremely harsh conditions, according to the report.
Furthermore, the manufacturing process would be readily transferable to a large scale manufacturing facility, according to Dauskardt. “That is one of the ideas of the approach: we can make the structures using a range of methods that are amenable to large-scale manufacturing, including roll-to-roll processing,” he elaborated.
“Long term reliability is a central focus of our work – we really need to demonstrate the ability to be as reliable as silicon modules to have a large impact on residential solar power, although for other applications, for example in transportation systems, the lighter weight and improved form factors would be beneficial even with 10 years of reliable life.”
Looking ahead, the research team is already busy undertaking related projects aimed at improving the honeycomb scaffold device architectures, addressing improved processing methods, and increasing the reliability of their perovskite PV innovation. They are already producing pervoskite PV microcells and macro cells that are much more efficient in terms of power/energy conversion, which is determined by the composition of the perovskite and the materials used in their charge transport layers, Dauskardt said. comment↓
