Contact between 2D and 3D perovskites reshapes crystal order, lifting efficiency to 26.25%

 

Perovskites, a class of material with a characteristic crystal structure that can convert light into electricity, have proved to be promising for the development of more affordable, flexible, and efficient solar cells than the silicon cells on the market today.

Despite their potential, perovskite solar cells have not yet reached their full potential and still exhibit some limitations, particularly in terms of operational stability and durability over time. The ability of these cells to efficiently produce electricity from sunlight, without rapidly deteriorating over time, is known to rely on the quality of underlying perovskite films.

Researchers at Korea University, University of Toledo, and Seoul National University recently devised a new approach to attain high-quality perovskite layers with fewer imperfections and a regular atomic arrangement. Halide perovskite films created with strategy, introduced in a paper published in Nature Energy, were found to boost both the efficiency and stability of solar cells.

"For more than a decade, I have believed that placing charge-transport layers composed of materials similar to the light-absorbing halide perovskite on both sides of the absorber would be highly beneficial for passivating its surfaces and interfaces," Jun Hong Noh, senior author of the paper, told Tech Xplore.

"This concept is analogous to structures used in well-established technologies such as GaAs and silicon heterojunction (HIT) solar cells.

However, unlike those systems, halide perovskites cannot be easily fabricated as conventional n- and p-type semiconductors for such architectures."

To address the challenges associated with the development of solar cells based on halide perovskites, Noh and his colleagues turned to two-dimensional (2D) halide perovskites with a wide-bandgap.

A wider bandgap means that the energy gap between the valence band (i.e., where electrons typically sit in materials) and the conduction band (i.e., where electrons can move freely and conduct electricity) is larger. Wide-bandgap materials can absorb higher-energy light (e.g., blue or ultraviolet) but not lower energy light (e.g., red or infrared).

"We discussed this concept in detail in our recent review paper published in Nature Photonics," said Noh. "As part of this effort, in 2021 we reported a solid-phase in-plane growth method in Nature Energy that enables the formation of a 2D/3D junction without chemical reactions between the two layers.

"By bringing a 2D film into contact with a 3D perovskite film and applying heat and pressure, we were able to grow a highly crystalline 2D layer on top of the 3D surface."

An effect that could enhance perovskite solar cells

Building on their earlier works, Noh and his colleagues set out to better understand how parameters such as temperature and the carbon-chain length of the organic cation in 2D halide perovskites influenced the growth of the films. Their efforts led to an unexpected discovery.

They found that simply bringing 2D and 3D materials into contact significantly changed the optical properties of the 3D layer, particularly its photoluminescence (PL), or how much light it emitted after absorbing light. This occurred even without applying heat or pressure to the materials.

"Interestingly, these changes were reversible and strongly dependent on the organic cation," said Noh. "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."

The contact-induced cationic interactions uncovered by the researchers are reversible. As part of the recent study, the team tried to ensure that the 3D perovskite film remained in an improved and more ordered state even after the interaction had taken place, as this state would ultimately be driving improvements in performance.

"To achieve this, we introduced a thermal treatment after bringing the 2D and 3D layers into contact," explained Noh. "Since the two materials are already interacting at the interface, applying heat provides additional energy during this interaction, which we hypothesized would lead to a fundamentally different structural evolution in the 3D layer compared to conventional processing."

The researchers specifically applied their contact-induced improvement strategy to pristine FAPbI₃ perovskite films, which generally suffer from imperfect crystallization. They found that with their approach, additive-free FAPbI₃ films could be structurally refined, reaching lattice parameters very close to their theoretical ideal values.

"The efficiency and stability of perovskite solar cells are strongly governed by how crystallographically perfect the perovskite film is," said Noh.

"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."

The FAPbI₃ films prepared by the researchers were found to exhibit a suppressed phase transition to the non-perovskite phase. Even powders obtained by scraping the resulting films appeared to maintain a more stable phase than regular FAPbI₃ films.

Boosting solar cell efficiency and long-term stability

To further assess the potential of their approach, Noh and his colleagues integrated the perovskite films they created into real solar cells. The resulting solar cells reached high efficiencies of 26.25%, exhibiting operational lifetimes of around 24,000 hours under accelerated testing.

"FAPbI₃ perovskite is one of the most important compositions for high-efficiency perovskite solar cells," said Noh. "However, it is inherently difficult to fabricate highly crystalline FAPbI₃ films. To address this, many previous studies have relied on various additives to improve crystallinity.

"That said, the long-term effects of such additives on device stability are still not fully understood, and their absence of negative impact cannot be guaranteed. We believe our work is significant because it demonstrates a method to produce highly crystalline, near-ideal FAPbI₃ films without the use of additives."

A further advantage of the team's 2D/3D film contact process is that it could easily be scaled and used to reliably produce larger films with fewer defects. In the future, other researchers could try to adapt the approach and apply it to other halide perovskites.

"Through this work, we have uncovered a new type of interaction between 2D and 3D perovskites," said Noh. "Moving forward, we are particularly interested in exploring whether this effect can be further enhanced by using even more structurally perfect 2D and 3D films. We expect that improving the quality of both contacting layers could amplify and clarify the phenomena we have observed."

As part of their next studies, Noh and his colleagues would also like to apply the approach they developed to systems that need to be processed at lower temperatures. These include, for instance, all-perovskite tandem solar cells, in which low-bandgap perovskite films need to be deposited on top of wide-bandgap layers at low temperatures.

"In such cases, it is difficult to supply sufficient heat and time for proper crystallization," added Noh. "We believe that our contact-induced crystallization strategy could enable high-quality crystal formation even under these constrained conditions, potentially leading to significant performance improvements in tandem devices."