The worldwide push for electrification as a means of reducing CO2 emissions and mitigating climate change, as well as geopolitical factors causing instability on the prices of natural gas, have brought the importance of renewable energy generation to an all time high. Solar power is a key element to meet this increasing demand for clean energy, and research on photovoltaics is critical for the effort of making the technology available and fully diffused on the market. Scientists point out the usage of multijunction, or tandem, devices as a way to surpass the efficiency limits of solar cells, while keeping the cost of installation and external electronics constant. Moreover, thin film technologies allow for more responsible material usage, with reduced carbon footprint, and shorter energy pay-back time due to reduced production costs.

Perovskite-on-chalcogenides tandems have the advantage of combining an emerging technology with rapidly-increasing efficiency of perovskite solar cells with another thin film material, which already has space on the PV market. The two absorbing layers can have complimenting band gaps, due to the possibility of fine tuning by compositional variance for current matching. With this, the perovskite and chalcogenide technologies have the potential to generate highly efficient tandem devices. 

The most important challenge for the realisation of monolithic deposition of the perovskite top cell is the high surface roughness of the CIGS bottom cell [1]. This can cause a non-uniform distribution of the perovskite layer when using traditional deposition methods, such as spin coating, which relies on a smooth surface for the radiosymmetric flow of precursor solution. Spray coating is a technique well suited for coating of rough surfaces [2]. Moreover, Ultrasonic spray coating can generate smaller and more evenly distributed droplets, causing a better penetration of the deposited material on pores and crevices, which can reduce the roughness of the fabricated layers [3]. 

In this work we build on the previously reported ultrasonic spray coating of perovskite absorbing layers [4] and use the technique for the coating of rough CIGS surfaces to obtain 2-terminal monolithic tandem devices. The top surface roughness of the perovskite layer is compared to the original CIGS surface roughness by scanning electron microscopy of device cross sections. An interlayer of Me-4PACz is conformally deposited, also by ultrasonic spray coating, as a self assembled monolayer (SAM) for enhanced charge transport between the two absorbing layers. The usage of ultrasonic spray coating for the deposition of multiple layers of the stack represents an important step in the road to commercialization of the technology, as it introduces a scalable method for the fabrication of devices, which can be used for large area deposition with a high throughput, resulting in cheaper manufacturing.