The negative impacts of climate change become increasingly visible. The major reason for climate change is our non-sustainable way of life, in particular regarding burning fossil fuels. Despite the dramatic situation, humankind seems not willing to drastically change their energy-demanding habits. Thus, a fast technological solution is the only way to mitigate the most severe consequences. Here, photovoltaics comes into play, which hardly causes CO2 emission during operation. However, the fabrication of conventional silicon modules releases CO2 and requires a lot of energy during refining the silicon and producing wafers. This drawback could be overcome by thin-film technologies, which require much less material and energy during manufacturing. Most promising are metal-halide perovskites here due to their excellent optoelectronic properties despite being processed from solution and with precursors of much lower purity than silicon.

Power conversion efficiencies of perovskite solar cells reached more than 25% and, employed in tandems cells, even more than 30%. However, achieving high long-term stability remains challenging. Beyond external factors such as humidity, further culprits are mobile ions in the perovskite, phase-instabilities, and reactions with other materials in the device stack. All these properties are related to interfaces in the solar cells. In fact, for such a thin film (perovskites are, interfaces dominate the overall behaviour of the solar cell. Thus, a lot of effort has already been dedicated to interfaces. However, the conventional interface control has been mainly based on a trial-and-error approach without considering distinctive characteristics of various surface termination originating from organic and
inorganic composites in halide perovskite crystals. 

More importantly, a lack of collective understanding regarding the electronic properties at heterojunction interfaces hinders perovskite solar cells (PSCs) from reaching the theoretical performance. Beyond the interface itself, the bottom layer additionally determines the bulk properties by influencing the perovskite growth. Thus, understanding and tailoring the functionality of interfaces is key for highly efficient and stable PSCs and thus subject of this research project.

The goal is to achieve stable perovskite solar cells by a holistic strategy on tailoring and understanding both top and bottom  interface. This approach requires a highly interdisciplinary team of chemists, materials scientists, and device physicists, which is
provided by the Korean-Swiss consortium. In terms of materials, covered by the Korean side, we will go beyond the state of the art of conventional interface passivation strategies and work on surface reconstruction methods on the top interface to make the “interface region” more resilient during stress occurring under operation.

For the bottom interface we will focus on strain control by  heteroepitaxy since strain has been ecently identified as a reason for enhanced recombination and reduced long-term stability due to undesired phase tran sitions. Evaluating the effect of such combined interface + close-by-regions engineering on the optoelectronic properties requires subtle and accurate optoelectronic characterization. Those will be undertaken by the Swiss partner, who will extend met ods that are established for homogeneous bulk properties towards providing spatial resolution. We will achieve this goal by combining advanced photoluminescence and transient electrical measurements with numerical device modelling. They will be complemented by characterization on the nanoscale using colocalized optical and atomic force microscopy on cross sections of operational devices.