For the future industrialisation of perovskite solar cells, the usage of green (non-toxic) solvents is necessary to reduce the negative impact on humans and the environment and hence to reduce costs for the enclosure of deposition machines in production lines and expensive health and safety measures.

Therefore, we deposit all layers in the cell stack (except for the vacuum-deposited electrodes) with slot die-coating and using solely green solvents. For the perovskite layer, we examined and optimised a scalable two-step deposition method. To obtain a stoichiometric perovskite layer, the precursor ratio must be carefully controlled to minimize residual lead iodide as well as excess organic cations such as formamidinium iodide. Both species are known to reduce the device performance. For instance, surplus formamidinium iodide can cause a pronounced reduction in short‑circuit current, while PbI₂ has in addition been shown to compromise long‑term operational stability of perovskite solar cells [1]. Achieving high efficiency and durability therefore requires precise compositional tuning, as deviations toward either lead‑rich or cation‑rich conditions significantly degrade device performance.

In this work, we focus on the homogeneity of solar cell efficiencies on a substrate size of 10×10 cm², with each solar cell having an active area of 0.5 cm², as this is an important prerequisite for reaching good module efficiencies. On each 10×10 cm²-substrate, we can measure up to 40 solar cells to evaluate the homogeneity over the whole area. In the next step, we prepared modules on 5×5 cm²-substrates with an area of 13 cm² containing nine monolithically interconnected cell stripes.

We investigate the influence of different parameters on solar cell characteristics and homogeneity, e.g. the used solvent for the second step, influences of additives and finally annealing temperature after second step layer. Analytical methods like x-ray diffraction (XRD), external quantum efficiency (EQE) and scanning electron microscopy (SEM) were used to further characterise the layers and evaluate their quality (e.g. complete conversion to perovskite), and to detect possible optimisation pathways.

With all slot die-coated perovskite solar cells we reached power conversion efficiencies (PCE) above 16 % on an active area of 0.5 cm² with usage of solely green solvents for all layers. For 13 cm²-modules we were able to reach an average VOC over all nine sub-cells, which is comparable to that of cells from the same experiment.