In a new study, researchers from Stanford University have put together an insect-inspired design to protect fragile perovksite photovoltaic devices from deteriorating when exposed to heat, moisture or mechanical stress. The results of this research have been published in the journal Energy & Environmental Science (E&ES).
According to the researchers most solar devices, like rooftop panels, use a flat, or planar, design. However, that approach doesn't work well with perovskite solar cells.
Reinhold Dauskardt - Professor of materials science and engineering and senior author of the study - said that perovskites are promising, low-cost materials that convert sunlight to electricity as efficiently as conventional solar cells made of silicon.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.
Nicholas Rolston, a co-lead author of the E&ES study, said that perovskites are the most fragile materials ever tested in the history of their lab. This fragility is related to the brittle, salt-like crystal structure of perovskite, which has mechanical properties similar to table salt.
To address the durability challenge, the Stanford team turned to nature. The researchers 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.
Figure: Stanford University - A compound perovskite solar cell illuminated from below
Using the compound eye as a model, the researchers created a compound solar cell consisting of a vast honeycomb of perovskite microcells, each encapsulated in a hexagon-shaped scaffold just 0.02 inches (500 microns) wide.
The scaffold is made of an inexpensive epoxy resin widely used in the microelectronics industry. It's resilient to mechanical stresses and thus far more resistant to fracture.
Figure: Stanford University - Device architecture
Tests conducted during the study revealed that the scaffolding had little effect on the perovskite's ability to convert light into electricity.
What was interesting was the researchers got nearly the same power-conversion efficiencies out of each little perovskite cell that they would get from a planar solar cell - achieving a huge increase in fracture resistance with no penalty for efficiency.
One question the researchers wanted to answer was - "could the new device withstand the kind of heat and humidity that conventional rooftop solar panels endure?"
To find out, the researchers exposed encapsulated perovskite cells to temperatures of 185 degrees Fahrenheit (85 degrees Celsius) and 85 percent relative humidity for six weeks. Despite these extreme conditions, the cells continued to generate electricity at relatively high rates of efficiency.
Dauskardt and his colleagues have filed a provisional patent for the new technology. To improve efficiency, they are studying new ways to scatter light from the scaffold into the perovskite core of each cell.
"We are very excited about these results. It's a new way of thinking about designing solar cells. These scaffold cells also look really cool, so there are some interesting aesthetic possibilities for real-world applications."
Reinhold Dauskardt - Professor of materials science and engineering and senior author of the study
Scaffold-reinforced perovskite compound solar cells
Brian L. Watson | Nicholas Rolston | Adam D. Printz | Reinhold H. Dauskardt
Abstract
The relative insensitivity of the optoelectronic properties of organometal trihalide perovskites to crystallographic defects and impurities has enabled fabrication of highly-efficient perovskite solar cells by scalable solution-state deposition techniques well suited to low-cost manufacturing. Fracture analyses of state-of-the-art devices, however, have revealed that both the perovskite active layer and adjacent carrier selective contacts are mechanically fragile—a major obstacle to technological maturity that stands to significantly compromise their thermomechanical reliability and operational lifetimes. We report a new concept in solar cell design, the compound solar cell (CSC), which addresses the intrinsic fragility of these materials with mechanically reinforcing internal scaffolds. The internal scaffold effectively partitions a conventional monolithic planar solar cell into an array of dimensionally scalable and mechanically shielded individual perovskite cells that are laterally encapsulated by the surrounding scaffold and connected in parallel via the front and back electrodes. The CSCs exhibited a significantly increased fracture energy of ∼13 J m−2—a 30-fold increase over previously reported planar perovskite (∼0.4 J m−2)—while maintaining efficiencies comparable to planar devices. Notably, the efficiency of the microcells formed within the scaffold is comparable to planar devices on an area-adjusted basis. This development is a significant step in demonstrating robust perovskite solar cells to achieve increased reliability and service lifetimes comparable to c-Si, CIGS, and CdTe solar cells.
