Self-assembled monolayers as a universal tool for tuning interfacial charge selectivity and passivation in perovskite solar cells

Metal halide perovskite solar cells (PSCs) combine outstanding optoelectronic properties with solution processability, yet their long-term stability and reproducibility remain limited. Among the limiting factors, interfaces and charge transport in charge-selective materials play a significant role. This work focuses on optimizing the buried interface of PSCs, employing self-assembled monolayers (SAMs) as a precise strategy to address challenges in interfacial energetics and chemistry. SAMs are monomolecular layers of multifunctional molecules that anchor to indium tin oxide (ITO), the transparent electrode, while simultaneously tailoring the electronic properties of the interface to create a charge-selective contact. Their functionality arises from the interplay of two independent aspects: electronic selectivity, which defines the interfacial energetics for selective charge extraction, and interfacial passivation, which improves perovskite film quality and performance and stability in PSCs. SAMs are already widely applied as hole-selective contacts in lead-based PSCs, but their potential remains less explored in other contexts. This thesis aims to demonstrate the universality of SAMs as interface modifiers and to disentangle the contributions of selectivity and passivation across different perovskite systems. Two radically different case studies are investigated: hole-selective SAMs in tin-based (Sn) PSCs and electron-selective SAMs in lead-based (Pb) PSCs. We combined molecular design with advanced spectroscopic and device-level characterizations, including transient surface photovoltage, time-resolved photoluminescence, and stability testing under operational conditions. The findings highlight both the opportunities and the limitations of SAMs in these underexplored systems. In tin-based PSCs, new SAM molecules were designed to overcome the intrinsic shortcomings of commonly used materials, achieving improved perovskite film quality and device performance. In lead-based PSCs, systematic screening of novel electron-selective SAMs revealed that while electronic selectivity is readily achieved with electron acceptors, stability and interfacial quality remain major bottlenecks. Taken together, the results provide proof of concept for the universality of SAMs as interfacial modifiers and offer guiding principles for balancing charge selectivity with interfacial passivation. By shedding light on two distinct and underexplored fields, this work aims at extending the scope of SAMs as a versatile strategy for interfacial engineering in perovskite photovoltaics.