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Simply bringing 2D and 3D materials in contact changed the optical properties of the 3D layer, even when heat and pressure were not applied.
A team of scientists from Korea University, the University of Toledo, and Seoul National University has built a three-dimensional perovskite solar cell with an efficiency of over 26 percent and an operational lifetime of over 24,000 hours under laboratory test conditions. The researchers also used halide perovskites in their work, which have been difficult to fabricate in the past. 
As silicon-based solar cells reach their maximum energy-conversion potential, scientists have turned their attention to perovskites, which promise not just higher-efficiency solar cells but also more economical ones. While multiple attempts have been made to commercialize pervoskite-based solar cells, their stability has raised questions about long-term deployment. 
Jun Hong Noh, a professor at Korea University, has been working on a concept for solar cells in which charge-transport layers are placed on both sides of the absorber to passivate surfaces and interfaces. This approach has been used in silicon heterojunction (HIT) solar cells, but Noh’s idea uses halide perovskites, which are difficult to fabricate in this fashion. 
To overcome architectural challenges, Noh and colleagues turned to two-dimensional (2D) halide perovskites with a wide bandgap. These materials can absorb higher-energy light, such as blue or ultraviolet, but not lower-energy light, such as red or infrared. In their previous work, the researchers developed a method that requires no chemicals to form a 2D/3D junction. 
By applying heat and pressure to a 2D film brought into contact with a 3D film, the researchers grew a crystalline 2D layer on the 3D surface. The research team sought to understand how these parameters, including the carbon chain length of the organic cation in 2D halide perovskites, influence film growth, but found something they weren’t expecting. 
The team found that simply bringing 2D and 3D materials into contact altered the optical properties of the 3D layer, including its photoluminescence, even without heat or pressure. 
“Interestingly, these changes were reversible and strongly dependent on the organic cation,” said Noh in a press release. “When we further found that this contact interaction significantly influences phase transitions in the 3D perovskite, and that it originates from interactions between the organic cations of the 2D and 3D layers, we were genuinely excited.”
Adding thermal treatment to the two films in contact that are already interacting possibly leads to structural evolution in the 3D layer, the researchers hypothesized. To prove this, the team applied it in FAPbI₃ perovskite films, which typically see imperfect crystallization. Their hypothesis proved correct, when the films reached lattice parameters very close to the theoretical values, the team had computed. 
Even powders of the FAPbI₃ films made by the research team maintained a more stable phase than FAPbI₃ made through conventional methods. 
“Efficiency losses originate from trap states at surfaces and within the bulk, which are directly linked to defects. Likewise, phase transitions are known to initiate at defects. Therefore, achieving a near-perfect crystal structure is one of the most critical challenges in this field,” added Noh in the press release. 
The researchers integrated their perovskite films into conventional solar cells and found efficiency improved to 26.25 percent. While perovskite-based solar cells typically face durability challenges, these cells demonstrated an operational lifetime of 24,000 hours under accelerated testing. 
The 2D/3D film contact process is highly scalable and can be used to manufacture larger films with fewer defects. The team is now working on applying this approach to all perovskite tandem solar cells in which low-bandgap perovskites need to be deposited on top of wide-bandgap layers at low temperatures. 
The research findings were published in Nature Energy. 
Ameya is a science writer based in Hyderabad, India. A Molecular Biologist at heart, he traded the micropipette to write about science during the pandemic and does not want to go back. He likes to write about genetics, microbes, technology, and public policy.
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