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Increasing the Efficiency and Stability Understanding of Tandem Solar Cells

NUANCE researchers were part of a study that reported perovskite-silicon tandem solar cells showed an efficiency of 26.7 percent and long-term stability

Northwestern Engineering researchers and international collaborators have demonstrated the highest-ever efficiency reported for perovskite-silicon tandem solar cells. The solar cells showed an efficiency of 26.7 percent as well as exhibited long-term stability of 80 percent efficiency preserved up to 1,000 hours of continuous operation.

Northwestern’s Vinayak Dravid and Hee Joon Jung, using state-of-the-art instrumentation at the Northwestern Atomic and Nanoscale Characterization Experimental (NUANCE) Center, provided critical insight, down to the atomic and molecular scale, as to how and why the complex architecture of the tandem solar cell improved performance so significantly. This atomic and molecular-scale understanding will aid future developments in this rapidly moving field.

Vinayak Dravid

The solar cell industry has been built dominantly on a silicon-based solar cell, but its efficiency has stalled in recent years, with no practical progress for some time. The new tandem solar cells promise to deliver improved efficiency and stability for about the similar cost as current commercially available solar cells, potentially within the next decade.

Along with the NUANCE researchers, collaborators on the study include the Korea Advanced Institute of Science and Technology (KAIST), Seoul National University, National Renewable Energy Laboratory (NREL), University of Colorado, and Sejong University.

The study, “Efficient, Stable Silicon Tandem Cells Enabled by Anion-Engineered Wide-Bandgap Perovskites” was published recently by the journal Science.

The KAIST and NREL groups developed the tandem stack concept and developed processing to make the solar cells and measure the efficiency, which was validated by measurements at NREL.

The atomic and nanoscale explanation of the improved performance was provided by the aberration-corrected S/TEM analysis at NUANCE by Jung, a co-first author of the study and a postdoctoral fellow in materials science and engineering at the McCormick School of Engineering. He is advised by Dravid, the Abraham Harris Professor of Materials Science and Engineering and founding director of NUANCE.

When Jung zoomed in at the atomic scale using an aberration-corrected transmission electron microscope at NUANCE, it looked like 2D halide perovskites at first glance. However, something was quite different in the structure — namely, lots of defects. 

Hee Joon Jung

“It is always a fascinating moment as a microscopist to observe the unexpected atomic arrangement against the prediction,” Jung said. “It was thrilling to conclude that this phase is the elusive 2D PbI2 which was not initially predicted.”

Jung’s images showed the interface phase to be 2D lead iodide (PbI2), potentially the key for long-term stability and high efficiency, but with many planar defects and interlayer dopants due to volatile anions. This interface phase provided mitigating conditions for electrical transport as well as chemical passivation to reduce carrier-recombination, thus improving efficiency.

The groups at KAIST and NREL used a chemical-additive solution for 2D halide-perovskites to fabricate the 2D halide-perovskites within 3D halide-perovskites, and they also conducted efficiency measurements. After Jung’s atomic structure analysis, the efficiency was better understood.

“These are exceedingly difficult measurements to make because these materials are susceptible to damage during preparation and analysis,” Dravid said.

The tandem cell establishes perovskites-halide with commercial silicon solar cells, allowing the solar cell industry to continue production with the addition of the perovskites-halides.

The stability stems from engineering the mixing ratio of the molecules building the two-dimensional layer, working as a passivation layer but with unusually conductive qualities. This tandem solar cell shows that solar quantum efficiency could soon reach 30 percent with long stability.