Bridging the Gap Between Field and Lab for Perovskite Solar Cells
As solar technology hits commercial markets, the need for reliable research on solar cell performance grows. A research team at the U.S. Department of Energy's National Renewable Energy Laboratory focuses on simultaneous stressors to improve data on perovskite solar cells.
Researchers face a multitude of challenges when testing new technologies. One of the primary obstacles is finding efficient and cost-effective means to mimic real-world conditions. The problem with much of lab testing is finding a way to test variables using a simultaneous methodology that can replicate field conditions.
Laboratory testing of solar cells. Image used courtesy of NREL
The control of the lab is rarely thought of as a liability, but it can compromise the integrity of results that will not reflect outcomes once technology is operational in commercial markets. Engineers have already found significant gaps between lab testing and field performance with various green technologies.
For example, lithium-ion battery (Li-ion) research has created lab setups to replicate performance stressors related to electric vehicle use, and it has become clear that lab conditions are not adequately mimicking driver behavior. One Stanford study focused on mapping these differences between driver performance and lab testing, highlighting the growing awareness of this gap as it pertains to emerging clean technologies.
Perovskite Solar Cells and Simultaneous Testing
Metal halide perovskite solar cells (PSCs) are gaining significant attention in the research community because they are not only cost effective but are capable of exceptional power conversion efficiencies (PCEs).
PSC research milestones since 2008. Image used courtesy of ResearchGate
When researching this kind of solar technology, scientists often test how temperature and light exposure impact performance, but they do not necessarily test them at the same time.
The artificial separation of such variables can be detrimental to the reliability of results. It is the compounding effects of heat, light, humidity, and other environmental stressors that cause faster battery degradation when they happen simultaneously. Thus, separating them is less than ideal.
In addition to accelerating solar cell degradation, the combination of environmental stressors can also create performance issues that are never present in a lab setting.
Kai Zhu, a senior scientist in the Chemistry and Nanoscience Center at the U.S. Department of Energy's National Renewable Energy Laboratory (NREL), is designing research methods and developing testing protocols that can show how PSCs will perform after six months in a natural field environment.
Zhu has noted the practical necessity of such testing protocols. Solar cell technology is rapidly evolving, and waiting six months or a year for field testing results hampers the feasibility of executing improvements.
Promising Results After Thermal Cycling
In order to more closely mimic field conditions, Zhu and his team subjected the PSCs to aggressive thermal cycling that would adversely impact the cells more than typical natural environments. They exposed the cells to temperatures that ranged from -40°C to 85°C, and the results were promising. After completing 1,000 cycles, the PSCs averaged a 5% degradation.
Their findings suggest that it is most important to combine light and high temperatures in particular when testing PSCs to ensure that results reasonably match outdoor performance. he PSC cells Zhu tested under these conditions performed well overall. The cells maintained 93% of their maximum efficiency after approximately 5,030 hours of continuous operation.
Other researchers are working on not just improving testing protocols but enhancing PSC stability which has been a major obstacle to commercialization. Inverted PSCs with a p-i-n architecture demonstrate improved long-term stability while maintaining the high PCEs that make this particular solar cell advantageous for application.
Structure of PSCs with p-i-n architecture. Image used courtesy of Liu, et al
The development of Zhu’s testing protocols and the efforts to improve the efficiency of inverted PSCs together constitute an important step toward commercialization. Closing the gap between lab conditions and field environments is always challenging, but PSC researchers are well on their way to accomplishing this goal.