“An understanding of the physical properties is as important as the efficiency of the photovoltaic devices. We have learned how to treat the material, how to make it clean, and this is an important starting point to access the intrinsic properties of these systems.”Maria Loi
Researchers from the University of Groningen have used x-rays to show in situ crystal growth of lead-free perovskite films that hold promise as an alternative to lead-based versions, which are toxic and not as stable. With the material being able to efficiently turn light into electricity, it could help in the development of the next generation of solar panels.
Hybrid perovskites for photovoltaic cells have a distinctive structure with anions forming an octahedron about a central cation in an idealized cubic unit cell, with the corners taken by other, larger cations, and different ions then being able to make different perovskites. Although for solar cells the most effective perovskites use lead as the central cation, tin-based alternatives, such as formamidinium tin iodide, have been developed to avoid toxicity.
As reported in Advanced Functional Materials [Dong et al. Adv. Funct. Mater. (2020) DOI: 10.1002/adfm.202001294], the study involved mixing 3D formamidinium tin iodide crystals with layered materials containing the organic cation phenylethylammonium (PEA), as adding small amounts of PEA brings greater stability and efficiency. This understanding led the team to develop an X-ray diffraction technique that showed the rapid formation of thin films in real-time during spin-coating from solution.
In investigating the film formation to assess if the oriented crystals grew from the substrate surface upwards, they found the opposite was true, that they grew at the air/solution interface. With the addition of different amounts of the 2D PEA, pure 3D perovskite crystals started to form at the surface but also in the bulk of the solution, while adding a small amount of the 2D material suppressed bulk crystallization and ensured the crystals only occurred at the air/solvent interface. This produced films composed of aligned 3D-like perovskite crystals and a small amount of 2D-like perovskite at the bottom of the film, producing a stable and efficient photovoltaic material.
As researcher Maria Loi told Materials Today, “An understanding of the physical properties is as important as the efficiency of the photovoltaic devices. We have learned how to treat the material, how to make it clean, and this is an important starting point to access the intrinsic properties of these systems.”
If tin-based perovskites can achieve similar efficiency as for lead, it will a useful breakthrough in photovoltaics The team now hope to realize the perovskite such as through the use of solvents, temperature or specific perovskite/substrate interactions to break up the formation of the insulating layer. Other applications could be in light emitting diodes and lasers since tin-based perovskites are very good light emitters.