Periodic Reporting for period 5 - MIX2FIX (Hybrid, organic-inorganic chalcogenide optoelectronics)
Berichtszeitraum: 2023-08-01 bis 2024-04-30
A new problem that our world confronts is that the commercialization of emergent devices with uncertain health and environmental compatibility raises serious concerns and hampers societal acceptance. The problem applies also to Optoelectronics market which is a key area with applications in our everyday life such as in solar panels, TV displays, smartphones, street lights etc. To address the risk of low environmental impact, researchers typically adopt sophisticated recycling procedures which however add cost in technology. This presents a grand challenge that MIX2FIX will address directly. Such devices should be timely replaced by equally smart but simultaneously sustainable alternatives. An emerging technology, called lead halide organic-inorganic perovskites, is a characteristic example of a device being at the stage of pre-commercialization. Solar cell and LEDs have shown boundary-pushing efficiencies thus far but at the expense of environmental compatibility; perovskites can decompose in humid air and release water soluble, toxic lead into the environment through the biogeochemical cycle. Therefore it is very challenging to unleash perovskite technology by developing the next generation of optoelectronic devices with the exclusive use of eco-friendly materials, by preserving similarly high performance and low cost. To tackle this challenge, MIX2FIX proposes to develop a new class of solution-processable optoelectronic devices based on air-stable, non-toxic metal chalcogenides endowed with an organic part, which will facilitate solution-processing and potentially enrich the compounds with the spectacular properties of halide perovskites. To achieve this, the CoG project has set the following overall objectives: (i) developing optoelectronically-active, organic-inorganic chalcogenide thin films films that have never been explored before, by mimicking strategies from established perovskite technology, (ii) devising means to improve their optoelectronic quality so as to be comparable with the best single-crystal semiconductors and (iii) implementing optimized materials into boundary-pushing PV and LED devices. Addressing these objectives, this project will therefore permit the transition for emerging optoelectronic materials from toxic lead halide perovskites to green hybrid chalcogenides.
In the first half of the project we have worked on the development of new hybrid materials based on various chalcogenides. After an extensive review of literature, we decided to limit our focus on nanoribbon-structured compounds which give photovoltaic efficiencies over 10% (Sb2S(Se)3) and three-dimensional cubic (or cubic-like) materials which can ensure isotropic charge transport (such as AgBiS2, Ag8SnS6, chalcogenide perovskites). Initially, we have fabricated Sb2S3 films of good optoelectronic quality and either modified their surface with organic cations (methylammonium bromide) or incorporated organic matrix (methylammonium acetate) within the Sb3+-precursors. In the first case, the methylammonium cations most probably altered the surface of the material but the optical properties were not improved. In the second case, the Sb2S3 lattice could afford up to 100% uptake of methylammonium cations, however the peaks of the parent compound (Sb2S3) seemed to dominate the crystalline compound. It is likely that the methylammonium cations leave the lattice upon evaporation at 300 0C; a low-temperature strategy should be adopted in the future in order to preserve the organic part within the inorganic lattice. On the other hand, we have performed a lot of work on the nanoparticle-synthesis. We have synthesized nanocrystals of various sulfides such as AgBiS2, Ag8SnS6 and TiSx with controllable size and ideal optical properties (with a bandgap inside the visible). This approach has two key benefits: a bottom-up and top-down synthesis can both be adopted, thus avoiding the high-temperature synthesis of other standard methodologies. Moreover, we can have access to phases that are not accessible with other conventional approaches (e.g. solid-state synthesis). Preliminary studies on ion exchange have already started in order to replace silver cations with formamidinium cations. Beyond the above studies, we have fabricated a large range of solar cells with chalcogenide materials to serve as a platform for the hybrid compounds which will be synthesized in the second phase of the project.
The incorporation of organic matric within chalcogenide compounds goes beyond state of the art since nobody has ever tried to synthesize hybrid chalcogenides for optoelectronics. Unfortunately, we did not have positive results thus far concerning the properties of these hybrids. However, until the end of the project, we expect to extend our methodology to various chalcogenides (those which have been synthesized so far) adopting bottom-up and top-down approaches. We expect to have easier results in the case of chalcogenide perovskites (of the type of CaSnS3) where the corner-shared octahedra are already built and the void between them should be simply filled by an organic dication (diammonium). These results will enable the development of novel functional hybrids at the boundaries of perovskite and chalcogenide thin films. With this, optoelectronics with efficiency and stability, comparable or higher than those of lead halide perovskite or chalcopyrite devices, will be demonstrated.