The fundamental scientific challenge addressed in FREENERGY is the oxidative stability of Sn2+ in halide perovskites under solar cell operation. State-of-the-art approaches aim to identify the best antioxidants to preserve the perovskite from external sources of degradation, such as oxygen and water, or reduce the interaction with them. While these approaches enabled longer material lifetimes, they did not directly face the intrinsic oxidative instability of Sn2+, resulting in the photovoltaic efficiency still being far from the real potential of this material.
The principal investigator pursued a strategy that tackles the challenge of tin oxidation from different angles, enabling the following main results:
1. Tuning the lattice parameters by engineering the perovskite composition. The oxidative stability of Sn2+ within the bulk of the perovskite crystals hinges on the formation energy of lattice defects. Enhancing the formation energy of the lattice defects enabled the reduction of defect concentration and thus the stabilization of the perovskite in solar cells' working condition. FREENERGY demonstrated that tin-based perovskite is intrinsically stable in solar cell working conditions.
2. Controlling the perovskite film microstructure by using small organic molecules as additives into the processing from solution. The defect concentrates at the surface of the crystalline grains forming the film of the perovskite into solar cells. Making a more uniform grain distribution reduces the amount of surfaces and, consequently, the concentration of defects, including the oxidized tin. This result and the resulting improved performance of tin-based perovskite solar cells were achieved by using small molecules to control the crystallization of the perovskite film.
3. The solar cells are made by interfacing the perovskite film with the other material components within the device. The chemical interaction of the perovskite with the other material comprising the device is critical for the functioning of the solar cells. The project made a critical contribution to the field, demonstrating a strategy to passivate and engineer the interface using specific supramolecular chemical interactions.
The main results described above have been reported in more than 20 peer-reviewed scientific publications addressing specific aspects of interest of FREENERGY. The principal investigator wrote a perspective peer-reviewed paper that provides an overall understanding of the results produced ("Stable Tin-Based Perovskite Solar Cells; ACS Energy Lett. 2023, 8, 4, 1896–1899"). In parallel to publications, the main advances of the project have been systematically presented at international conferences, disseminating the most advanced results and collecting the feedback of colleagues.
