The quiet experiment that changed solar power

Long before perovskite solar cells began smashing efficiency records and transforming the future of clean energy, their modern design first emerged from a quiet experiment at Northwestern University.

More than a decade ago, Northwestern chemists and materials scientists reported in Nature the first solid-state solar cell based on a halide perovskite semiconductor — an advance that ultimately helped launch one of the fastest-growing revolutions in solar energy.

To build the new type of solar cell, the scientists replaced a liquid electrolyte with a solid, thin-film inorganic perovskite semiconductor. Perovskites carried multiple advantages, the team found. Unlike liquid electrolytes, perovskites did not leak, evaporate or degrade. The material also could both absorb sunlight and carry an electrical charge. So, the solid layer moved electricity through the device while helping generate power — all in one material.

The new design reached about 10% efficiency — a major milestone at the time.

“At the time, our work solved a longstanding problem in dye sensitized cells: the instability and leakage associated with liquid electrolytes,” said Northwestern’s Mercouri Kanatzidis, who led the 2012 study. “Our seemingly simple idea — replacing the liquid electrolyte with a solid perovskite semiconductor — foreshadowed the basic architecture that would soon define perovskite photovoltaics.”

In the years that followed, scientists around the world shifted to even more powerful lead-based perovskites, which soon crushed efficiency records and drew massive global attention.

“Many of the core concepts that underpin modern perovskite solar cells were present in that early Northwestern study,” said Kanatzidis, the Charles E. and Emma H. Morrison Professor of Chemistry at Northwestern’s Weinberg College of Arts and Sciences. “Our paper showed that perovskites can be processed from solution into continuous films and that they can deliver double-digit efficiencies in thin architectures.”

Today, perovskite solar cells are among the most intensely studied energy technologies globally. Their efficiencies now rival traditional silicon solar panels. And stacked “tandem” versions already have surpassed 30% efficiency in laboratory tests.

Perovskite concepts also have expanded far beyond solar cells. They are now used in LED displays, X-ray and gamma-ray detectors, lasers and many other high-tech devices. Although later milestones in this fast-moving and highly competitive landscape often overshadow that original Northwestern study, technical reviews still recognize the 2012 work as the first solid-state solar cell that used a perovskite absorber and hole conductor.

“We showed that a simple solid perovskite layer could replace unstable liquid components while also functioning as an effective solar absorbing semiconductor that generated the electrical power driving the device,” Kanatzidis said. “This new idea helped launch an entire generation of solar materials and technologies that now transform how the world captures energy from the sun.”