Perovskite-silicon tandem solar cells achieve significantly higher efficiencies than silicon solar cells on their own. One particularly attractive method is co-evaporation of the perovskite precursor molecules on top of the silicon subcell. Scientists at HZB have analysed film growth on the nanoscale at BESSY II and found a new way to improve the quality of the perovskite layer: adding a thin seed layer of caesium chloride between the two sub-cells promotes uniform perovskite growth and suppresses the formation of undesired lead iodide at the interface.
Monolithic perovskite-silicon tandem solar cells are characterised by a thin perovskite layer, which can be produced via the so-called co-evaporation of precursor compounds, a well-established industrial process. As a hole transport layer for the perovskite, so called SAM are used, organic molecules that self-assemble to a monolayer. However, both the SAM and the perovskite layer must form as uniform as possible on the silicon surface. But as the SAM is not a perfect monolayer and in addition, the silicon surface has a nanostructure to improve light absorption, this is very tricky. At HZB, a team has now thoroughly investigated this problem using various methods, all available at HZB, and developed a solution that is also of interest for industrial production.
BESSY II reveals how layers form
Using a near-field scanning probe microscope (IR-s-SNOM) in the infrared range at BESSY II, they identified areas of varying thickness within the SAM layer, ranging from monolayers to several layers and even agglomerations of these molecules. They were also able to analyse in greater detail how SAM layers form on planar and industrially relevant textured silicon substrates. This revealed that SAM molecules accumulate in the texture-valleys, resulting in a layer of uneven thickness.
With X-ray photoemission electron microscopy (XPEEM) at BESSY II the scientists could investigate the growth of the co-evaporated perovskite on different areas of the SAM-coated silicon substrate. Their results show that the perovskite layer was unable to compensate for the underlying inhomogeneities, and that unwanted lead iodide had formed at the interface towards the silicon.
A seed layer helps
However, the team implemented and analysed an advanced solution: a layer of caesium chloride (CsCl) at the interface, which acts as a seed layer. 'This enables the uniform growth of co-evaporated perovskite layers on textured silicon and prevents the formation of interface defects caused by uneven coverage of the hole transport layer,' says Dr Viktor Škorjanc, first author of the paper. As a consequence, the layer quality is significantly improved. This boosts the efficiency of the perovskite-silicon tandem solar cells prepared by Stefanie Severin to 30.3 percent which is an outstanding value for evaporation-based perovskites.
The new vacuum process does not require solvents and might therefore improve the stability of the tandem cells, which is up to now a major hurdle on the way of commercialisation 'This represents a real step forward in translating record efficiencies from the laboratory into reliable, industrially manufacturable tandem solar technologies,' says Dr Marcel Roß, team leader of evaporated perovskite solar cells at HZB.
