Novel Organic Interlayers for Perovskite Solar Cells: From Synthesis to Device Integration and Performance Characterisation

Abstract

Perovskite solar cells have gained significant attention in recent years as an alternative photovoltaic technology due to their rapidly increasing efficiency, rising from 3.8% in 2009 to a staggering 28% in 2026. Combined with their low projected manufacturing cost, solution processability, and potential to be fabricated as both transparent and flexible devices, they represent highly promising materials for large-scale commercialization. Their appeal is further enhanced by their compatibility with silicon semiconductors in tandem solar cell architectures, enabling efficiencies that significantly exceed those of single-junction solar cells and surpass the Shockley–Queisser limit. A substantial portion of the efficiency improvements achieved in recent years can be attributed to optimizations at the interface between the hole-transporting layer (HTL) and the active perovskite layer, which had previously represented a major performance bottleneck. The incorporation of organic ammonium salts has been shown to passivate interfacial defects, resulting in improved efficiency and stability. Likewise, the introduction of phosphonic acid self-assembled monolayers (SAMs) as HTL materials has simplified device architectures and eliminated the need for conventionally used HTLs such as PTAA and NiOx, which often exhibit suboptimal energy-level alignment with the perovskite absorber and consequently contribute to significant energy losses. Despite these advances, the exact mechanisms underlying the beneficial effects of these interlayer materials remain poorly understood, and only a small fraction of the potentially suitable organic building blocks has been explored. For ammonium-salt interlayers, most reported materials are not capable of forming 2D perovskites. Since 2D perovskites are known to enhance stability and passivate defects, it remains unclear whether ammonium salts specifically designed to form a thin 2D perovskite layer at the interface could provide superior device performance and stability. Similarly, phosphonic acid SAMs are overwhelmingly based on carbazole-derived structures. While carbazole-based SAMs have proven highly successful, it is not yet understood whether the carbazole core possesses unique advantages or whether alternative aromatic chromophores could deliver comparable or even improved performance. More broadly, the molecular characteristics that govern the effectiveness of SAM-based interlayers remain insufficiently understood. Therefore, the aim of this thesis is to investigate new organic ammonium salts and phosphonic acid SAMs with diverse aromatic cores for incorporation as a bottom interlayer/HTL in p-i-n devices. Through this, the work seeks to elucidate the key molecular parameters that influence their performance as interfacial Introduction layers, thereby contributing to the establishment of structure-property relationships that may help guide the rational design of next-generation interlayers for efficient and stable perovskite solar cells. In this way expanding the roster of available materials and improving the solar cell performance above the current state of the art, focusing on stability, efficiency, and scalability.