At present, society relies largely on burning fossil fuels for energy needs, but this isn't viable in the future because of global climate change and increasing energy demands. Thus, a transition is building toward renewable sources of energy, the most prominent being sunlight. Commercial solar panels have primarily used silicon for decades, but now a new material, perovskite, is revolutionizing the solar energy field.
The solar cells using this material could fulfill the promise of affordable solar energy in the near future. The sunlight-to-energy conversion efficiencies of perovskite solar cells have jumped from 3.8 percent to an impressive 20.1 percent in past years (commercial silicon panels are ~17 percent efficient). The use of the toxic element lead and the long-term stability of perovskites remain major challenges. At the Argonne-Northwestern Solar Energy Research (ANSER) center, a team led by Mercouri Kanatzidis and Robert Chang has achieved major milestones to solve this.
Currently, perovskite materials that give the most efficient solar cells consist of lead, iodine, and a small organic compound. Lead carries a health risk for researchers, manufacturers, and consumers, increasing safety requirements and cost. The team demonstrated ~5 percent efficiency based on tin-perovskites, paving the way for a new generation of earth-abundant and environmentally friendly solar cells. The team expects incremental increase in efficiencies to reach 20 percent using a combination of fundamental and engineering improvements. Good as their word, they replaced the iodine with a mixture of iodine and bromine, increasing light absorption and boosting the efficiencies to ~6 percent.
To optimize the assembly process, the researchers replaced the traditional method of sequential deposition of materials on the cell from top to bottom with a novel approach. They prepared the top and bottom parts separately, then sandwiched them together. This method minimizes the surface defects, cracks, and holes inherent in layering films. Chang compares this to the holes in Swiss cheese. "If you sandwich two pieces together, the holes are covered up," he said.
The tin-based materials can even lend their advantages to the predecessor dye-sensitized solar cells (DSSC). The dye cells use small organic molecules for light absorption on an interface of a liquid and solid medium.The liquid electrolyte was the bottleneck for durability of the device due to corrosion and leakage issues. It was replaced by a tin-perovskite, made of cesium, tin, and iodine, to create all solid-state solar cells with comparable efficiencies.
The practicality of perovskites has led to a wide adoption of perovskite solar cells in laboratories both for research and education. "Perovskite studies can be done on the benchtop," said Kanatzidis. "This has led to rapid progress in the field." The preparation is so easy that undergraduates in chemistry labs at Northwestern routinely make perovskite solar cells and solar panels. "Students get excited when they light LEDs with their solar panels," said Duyen Cao, one of the creators of this experiment.
Following the experiment in spring 2015, two students even started independent research projects on the subject. Some students told Cao that they plan to pursue a career in sustainable energy. Cao and her fellow researchers published a paper and a short how-to video to facilitate adoption of this lab at other universities in hopes of encouraging more students to go into solar energy fields.
This humble crystal structure is igniting a revolution in solar energy research. At a recent symposium on perovskites, ANSER researchers emphasized the attainable future progress for this technology. Director Michael Wasielewski and Director of Operations and Outreach Dick Co are delighted to see how the Energy Frontier Research Center's approach to interdisciplinary research has led to new, surprising discoveries. "The all solid-state DSSC work that brought global attention to the use of perovskites in solar cells was a union between inorganic chemistry and device physics that came about after an EFRC center meeting," said Co. Studies at the ANSER center have demonstrated the versatility of perovskites and are poised to continue advancements in solar energy.
