Solar photovoltaics (PV) is expected to be a critical contributor to mitigating the effects of climate change by helping to satisfy net zero emissions. Since crystalline silicon-based solar cells are close to their practical efficiency limit, further reducing the balance of system (BoS) costs is only possible by increasing the cell efficiencies. The most promising candidate is perovskite/silicon (Si) tandem solar cell technology, which allows efficient solar spectrum harvesting. This relatively new technology attracts attention due to its potential to dominate the PV market; however, it also brings challenges that must be overcome, like stability and scalability concerns. This thesis project focuses on optimizing and characterizing recombination junctions (RJs) for monolithic perovskite/Si tandem solar cells aimed at improved performance and stability. Tandem solar cell PV parameter measurements, encapsulated stability measurements, and thin film characterizations are performed for RJ developments. The optimizations are performed for tandem solar cells with solution-processing and hybrid methods. Self-assembled monolayer (SAM) molecules and transparent conductive oxide (TCO) recombination layer (RL) combinations are optimized to obtain tandems with hybrid technique. In addition, the influence of the thickness of TCO RL on the tandem devices’ performance is also investigated, particularly solution-processed tandems. The improvements are observed by thinning down the thickness of TCOs regardless of the material type. 3 Characterizations revealed that ultra-thin ( 5 nm) amorphous indium zinc oxide (IZO) RL allows more workfunction shift, homogeneous surface potential distribution with SAM deposition, and better carrier recombination suppression at the perovskite/hole transport layer (HTL) interface. Ultra-thin RL idea is combined with some optical improvements in the device architecture, and stable high-efficient perovskite/Si tandem solar cells with 32.5% power conversion efficiency (PCE) and 80% fill factor (FF) values are realized. In addition, the preliminary examples of tandem devices with a larger active area (4 cm2 ) are presented. Finally, the remaining challenges and alternative concepts are also discussed.