An ultra-thin protective coating is proving to be sufficient to protect a perovskite solar cell from the damaging effects of outer space and harden it against environmental elements on Earth, according to a newly released study from the US Department of Energy’s National Renewable Energy Laboratory (NREL).
NREL research was funded by the US Department of Defense’s Operational Energy Capability Improvement Fund (OECIF) and conducted for the Air Force Research Laboratory (AFRL) to develop cost-effective innovative energy sources to power armed forces worldwide.
The research is the latest attempt to determine the effectiveness of perovskites for use in space applications where they would be exposed to protons, alpha particles, atomic oxygen and other stressors. The possibility of using perovskites to generate electricity in space is enticing as they offer a cheaper and lighter weight option for other technologies with the potential to achieve efficiencies similar to current space PV technologies.
As on Earth, perovskite solar cells must have a corresponding durability. However, the environment in space is significantly different. While the biggest challenges on Earth are related to the weather, in space perovskites must solve the problems posed by radiation bombardment and extreme temperature changes. Perovskites are showing signs of better radiation tolerance than many other solar cells, but much testing remains to be done.
Researchers ran simulations last year to demonstrate how exposure to radiation in space would affect perovskites. They noted that next-generation technology would work in space, but pointed to the need to encapsulate the cell in some way to provide additional protection.
In the follow-up research Ahmad Kirmani, lead author of the latest energy of nature paper, simulations said that a micron-thick layer of silicon oxide would maintain the efficiency and extend the lifetime of perovskite solar cells in space. For comparison: the micrometer-thick layer is about 100 times thinner than a typical human hair.
Kirmani said the silicon oxide layer can reduce the weight of conventional radiation barriers used for other solar cells by more than 99% and serves as a first step in developing lightweight and low-cost packaging for perovskites.
High-energy protons travel through perovskite solar cells without doing much damage. However, low-energy protons are more common in space and wreak more havoc on perovskite cells, knocking atoms out of position and causing efficiency to steadily decrease. The low-energy protons interact with matter much more easily, and adding the silicon oxide layer protected the perovskite from damage, even from the low-energy protons.
“We thought it was impossible for the silica to provide protection from long-range fully penetrating particles like the high-energy protons and alpha particles,” Kirmani said. “However, the oxide layer also proved to be a surprisingly good barrier.”
The results are detailed in the article “Metal oxide barrier layers for terrestrial and space-based perovskite photovoltaics”. Co-authors are NREL’s David Ostrowski, Kaitlyn VanSant, Rosemary Bramante, Karen Heinselman, Jinhui Tong, Bart Stevens, William Nemeth, Kai Zhu and Joseph Luther; and several key collaborators working with the University of North Texas and University of Oklahoma team. VanSant has the unique position of a postdoctoral fellow at NASA conducting research at NREL.
The researchers found that exposure to a flux of low-energy protons caused unprotected perovskite solar cells to lose only about 15% of their original efficiency. A larger concentration of particles destroyed the cells, while the protected perovskites showed what the scientists called “remarkable resilience”. With the simple barrier, the cells showed no damage.
In addition to making the cells more resilient in space, the researchers tested how the barrier could offer benefits in more conventional applications. They then exposed the perovskite solar cells to an uncontrolled humidity and temperature environment for several days to mimic storage conditions. The protected cells retained their initial efficiency of 19%, while the unprotected cells showed a significant deterioration from 19.4% to 10.8%. The oxide layer also offered protection when other perovskite compositions, which were typically more moisture sensitive, were exposed to water.
Also, the perovskite solar cells were exposed to a test chamber where they were bombarded with ultraviolet photons, similar to the environment in low Earth orbit. The photons interacted with oxygen to create atomic oxygen. The unprotected cells were destroyed after eight minutes. The protected cells maintained their initial performance at 20 minutes and had only a slight decline at 30 minutes.
The simulations and experiments showed that by reducing radiation damage, the lifespan of protected solar cells deployed in Earth orbit and in space would be extended from months to years.
“Power conversion efficiency and operational stability of perovskite solar cells have been the two main areas of focus for the community so far,” he said. “We’ve made great strides and I think we’re well into the point where we could get pretty close to the targets required for industrialization. However, to really enable this market entry, packaging is the next target.”
Since perovskite solar cells can be deposited on a flexible substrate, the emerging technology combined with the protective layer of silicon oxide enables their use for various terrestrial applications such as drone propulsion.
NREL is the US Department of Energy’s primary national laboratory for research and development in renewable energy and energy efficiency. NREL is operated for DOE by the Alliance for Sustainable Energy LLC.