Currently, the most efficient multijunction solar cells under AM1.5g spectrum are triple junction solar cells with a power conversion efficiency of 39.5%, higher than the previous six junction solar cell record [1]. These high-efficiency multijunction solar cells are made of epitaxially grown III-V materials. Over the recent years metal halide perovskites have proven to be an excellent alternative material class with potentially low process costs for multijunction solar cells. The record efficiency of 32.5 % for a dual junction perovskite-silicon tandem solar cells shows the extraordinary optoelectronic material properties [2]. Research on perovskite-perovskite-silicon triple junction solar cells is however still in its early stage and requires further development. In our work, we address some of the main challenges to realize monolithic two-terminal perovskite-perovskite-silicon triple junction solar cells.

A monolithically integrated triple junction solar cell consists of multiple layers processed sequentially on top of each other. Therefore, process compatibility with underlying layers is of great importance. Especially the solvent involved in the processing of top perovskite can be harmful to the middle perovskite since they share similar solvent systems. Here, we have adapted the deposition of perovskite solar cell that allows fast coating of the top perovskite layer without adversely affecting its morphology. We replaced static spin coating by dynamic spin coating which prevents dissolution of the underlying middle perovskite layer. Moreover, instead of using antisolvent, a nitrogen flow is used to accelerate the removal of solvent from the precursor during the spin coating similar to the approach developed in [3].

In addition, in monolithic triple-junction solar cells, the sub cells are connected in series through recombination layers. The recombination layer has shown to be one of the key factors influencing the performance of all-perovskite tandem solar cells. In this work, the performance of triple junction solar cells employing ultra-thin metal layers as well as thin transparent conductive oxide (TCO) between the two perovskite sub cells are compared.

Finally, precise characterization of solar cells is highly important in research and development. Measurement procedure is more complex in triple-junction solar cells compared to single junction devices as it is known from III-V solar cells [4,5]. Especially in two-terminal structures where access to the individual sub cells is not possible. One important aspect is correct IV measurement, which requires adjustment of sun simulator spectrum [4]. However, so far, all the reported perovskite based triple junction solar cells are only measured with non-adjusted spectrum which can lead to large uncertainties. In this work the IV measurement is done with a multi-light source solar simulator with adjusted spectrum.

Overall, this work addresses some key challenges for the development of monolithic two-terminal perovskite-perovskite-silicon triple junction solar cells. A systematic investigation is carried out to realize a damage-free processing method for the deposition of the perovskite top cell and optimization of the recombination layer between the two perovskite sub cells. We realized a first device with encouraging values for Voc of 2.4 V and for FF of 79 %. The high FF proves the good material quality and transport properties (i.e., working recombination layers and high shunt resistance). This is an indication that no pinholes or other shunting path is created which can happen due to process incompatibilities. The efficiency is limited by the low Jsc of 7 mA/cm² which will be improved by adapted materials and reducing parasitic absorption. We further implement the precise global solar cell characterization for a reliable and correct measurement of our triple junction solar cell.