ASU, Stanford researchers achieve record-breaking efficiency with tandem solar cell

Researchers at Arizona State University and Stanford University are exploring new methods to improve the performance of solar-cells - such as stacking two different solar cell technologies in a tandem cell - in this case silicon and perovskite solar cell technologies.

The results of their work - published in the February 17 issue of Nature Energy - outline the use of perovskite and silicon to create a tandem solar cell capable of converting sunlight to energy with an efficiency of 23.6 percent, just shy of the all-time silicon efficiency record.

The high-performance tandem cell's layers are each specially tuned to capture different wavelengths of light. The top layer, composed of a perovskite compound, was designed to excel at absorbing visible light. The cell’s silicon base is tuned to capture infrared light.

The perovskite used in the tandem cell came courtesy of Stanford researchers who fabricated the compound and tested the materials. The research team at ASU provided the silicon base and modeling to determine other material candidates for use in the tandem cell’s supporting layers.

The researchers noted that low-cost and highly efficient, perovskites have been limited by poor stability, degrading at a much faster rate than silicon in hot and humid environments. In addition, perovskite solar cells have suffered from parasitic absorption, in which light is absorbed by supporting layers in the cell that don’t generate electricity.

McGehee said that his compound achieves record stability, perovskites remain delicate materials, and so making it difficult to employ in tandem solar technology.

However, McGehee was able to apply a tin oxide layer with help from chemical engineering professor Stacey Bent and doctoral student Axel Palmstrom of Stanford. The pair developed a thin layer that protects the delicate perovskite from the deposition of the final conductive layer without contributing to parasitic absorption, further boosting the cell’s efficiency.

This top conductive layer is applied using a process called sputtering deposition, which historically has led to damaged perovskite cells. The deposition of the final conductive layer wasn’t the only engineering challenge posed by integrating perovskites and silicon.

According to the researcers the silicon wafers are placed in a potassium hydroxide solution during fabrication, which creates a rough, jagged surface. This texture, ideal for trapping light and generating more energy, works well for silicon, but perovskite prefers a smooth — and unfortunately reflective — surface for deposition.

Additionally, the perovskite layer of the tandem cell is less than a micron thick, opposed to the 250-micron-thick silicon layer. This means when the thin perovskite layer was deposited, it was applied unevenly, pooling in the rough silicon’s low points and failing to adhere to its peaks.

The researchers developed a method to create a planar surface only on the front of the silicon solar cell using a removable, protective layer. This resulted in a smooth surface on one side of the cell, ideal for applying the perovskite, while leaving the backside rough, to trap the weakly absorbed near-infrared light in the silicon.

The success of the tandem cell is built on existing achievements from both teams of researchers. In October 2016, McGehee and post-doctoral scholar Tomas Leijtens fabricated an all-perovskite cell capable of 20.3 percent efficiency. The high-performance cell was achieved in part by creating a perovskite with record stability, marking McGehee’s group as one of the first teams to devote research efforts to fabricating stable perovskite compounds.

Zachary Holman, an assistant professor of electrical engineering at the Ira A. Fulton Schools of Engineering, said, "The best silicon solar cell alone has achieved 26.3 percent efficiency." Zachary, added, "Now we’re gunning for 30 percent with these tandem cells, and I think we could be there within two years."

Zachary, continued, "In many solar cells, we put a layer on top that is both transparent and conductive - so light can go through and conductive so we can take electrical charges off it."

Zachary concluded, "We have eight projects with different universities and organizations, looking at different types of top cells that go on top of silicon so far out of all our projects, our perovskite/silicon tandem cell with Stanford is the leader."

Michael McGehee, a materials science and engineering professor at Stanford’s College of Engineering, said, "We have improved the stability of the perovskite solar cells in two ways." Michael, added, "First, we replaced an organic cation with cesium. Second, we protected the perovskite with an impermeable indium tin oxide layer that also functions as an electrode."

Zhengshan (Jason) Yu, an electrical engineering doctoral student at ASU, said, "It was difficult to apply the perovskite itself without compromising the performance of the silicon cell. " Jason, added, "With the incorporation of a silicon nanoparticle rear reflector, this infrared-tuned silicon cell becomes an excellent bottom cell for tandems."