Next-generation solar materials are cheaper and more environmentally friendly to manufacture than traditional silicon solar cells, but hurdles remain to make devices strong enough to withstand real-world conditions. A new technique developed by a group of international scientists could facilitate the development of efficient and stable perovskite solar cells, named for their unique crystal structure that is excellent at absorbing visible light.

Scientists including Penn State professor Nelson Dzadeh reported in the journal Nature Energy about their new method of creating longer-lasting perovskite solar cells that still achieve a high efficiency of 21.59% in converting sunlight into electricity.

Perovskites are a promising solar technology because the cells can be made at room temperature using less energy than traditional silicon materials, making them more affordable and more sustainable to manufacture, according to Dzadeh, the John and Willie Leone Associate Professor of Energy and Mining Engineering. Family Faculty, Department of Energy and Mining Engineering and co-author of the study.

But scientists say the leading candidates used to make these devices—hybrid organic-inorganic metal halides—contain organic components that are sensitive to moisture, oxygen, and heat, and that exposure to real-world conditions can rapidly degrade performance.

One solution involves the use of completely inorganic perovskite materials such as lead cesium iodide, which has good electrical properties and excellent environmental tolerance. However, this material is polymorphic, meaning it has many phases with different crystal structures. Two photoactive phases are useful for solar cells, but can easily transform into an undesirable non-photoactive phase at room temperature, causing defects and reducing the efficiency of solar cells, scientists say.

Scientists say they combined two photoactive polymorphs of lead cesium iodide to create a phase heterojunction that can suppress conversion to an undesirable phase. Heterojunctions are formed by stacking different semiconductor materials, such as layers in a solar cell, with different optoelectronic properties. These connections in solar devices can be adapted to help absorb more energy from the sun and convert it into electricity more efficiently.

“The best part of this work is that it shows that using two polymorphs of the same material is the right way to make phase heterojunction solar cells,” Dzadeh said. “This increases the stability of the material and prevents conversion between the two phases. The formation of a coherent interface between the two phases allows electrons to flow easily through the device, resulting in increased energy conversion efficiency. This is exactly what we have shown in this study.”

The researchers produced a device that achieved a power conversion efficiency of 21.59% and excellent stability, one of the highest rates for this type of approach. According to Dzadeh, the devices retained more than 90% of their initial efficiency after 200 hours of storage in ambient conditions.

“When scaled from the laboratory to an actual solar module, our design demonstrated a power conversion efficiency of 18.43% for a solar cell area larger than 7 square inches (18.08 square centimeters),” Dzadeh said. “These initial results highlight the potential of our approach to design ultralarge perovskite solar modules and reliably assess their stability.”

Dzade modeled the structure and electronic properties of the heterojunction at the atomic scale and found that combining two photoactive phases creates a stable and coherent interface structure that supports efficient charge distribution and transport—desirable properties for high-efficiency solar devices.

Zadeh’s colleagues at Chonnam University in South Korea developed a unique double deposition method to fabricate the device; deposited one phase using hot air technology and the other using thermal evaporation from three sources. Sawantha S. Mali, a research professor at Chonnam University in South Korea and lead author of the paper, said adding small amounts of molecular and organic additives during the deposition process further improved the electrical properties, efficiency and stability of the device.

“We believe that the dual deposition technique we developed in this work will have important implications for the production of highly efficient and stable perovskite solar cells,” said Nelson Dzadeh, associate professor of energy and mining engineering in the John & Willie Department of Energy. Leon. and mining engineering and co-author of the study.

The double deposition technique could pave the way for the development of additional solar cells based on all inorganic perovskites or other halide perovskite compounds, the researchers said. In addition to expanding the technique to different compositions, the researchers say future work will also include making current-phase heterojunction cells more robust under real-world conditions and scaling them up to the size of conventional solar panels.

“We believe that thanks to this approach it will be possible to increase the efficiency of this material by more than 25% in the near future,” Dzadeh said. “And when we do that, commercialization is just around the corner.”