Abstract

Metal halide perovskites have emerged as promising photoactive materials for solar cell applications due to their high absorption coefficient, long charge carrier diffusion length, sharp absorption band edge, and tunable bandgap, along with their compatibility with scalable fabrication techniques. While single-junction perovskite solar cells have achieved efficiencies of up to 27%, surpassing the Shockley-Queisser limit requires the development of multi-junction tandem architectures. This thesis focuses on interfacial engineering strategies for perovskite tandem solar cells, with a particular emphasis on self-assembled monolayers to enhance device performance and stability.

The first part of this work will be directed toward developing a multi-functional passivation technique for nickel oxide/perovskite interface in order to fabricate perovskite/silicon tandem solar cells. Sputtered NiOx is particularly interesting for perovskite/Si solar cells due to low temperature processing of polycrystalline NiOx films, which result in multitude of surface defects. By introducing a dye molecule N719 at the NiOx/perovskite interface we aim to achieve concurrent passivation of both NiOx and perovskite, while also improving energetic alignment and charge extraction at the interface. Resulting passivation technique is transferable to the textured surface of the Si bottom cell, which allows for the fabrication of high-efficiency double-junction perovskite/Si tandem solar cells.

The second part of this research focuses on developing 1 μm thick Pb-Sn narrow-bandgap perovskite films for all-perovskite tandem solar cells. With a bandgap of 1.2 eV, Pb-Sn perovskite is a crucial material for the back sub-cell in double-junction configurations, requiring sufficient near-infrared absorption to complement the wide-bandgap front sub-cell. However, fabricating thick Pb-Sn perovskite films presents challenges due to rapid crystallization dynamics and the tendency of Sn²⁺ to oxidize into Sn⁴⁺. To address these issues, this work employs dopant-assisted crystallization control and interfacial engineering of both the top and buried surfaces. Through these strategies, the fabrication of high-efficiency all-perovskite tandem solar cells is achieved.

The third part of this study focuses on replacing the commonly used polymeric PEDOT:PSS hole-transporting layer with a self-assembled monolayer molecule. PEDOT:PSS is widely utilized in Pb-Sn perovskite solar cells due to its shallow valence band maximum, which aligns well with Pb-Sn perovskites. However, the sulfonic acid groups in PSS facilitate oxidation reactions with iodide and Sn²⁺ in the perovskite, compromising the stability of Pb-Sn perovskite films. To mitigate these stability issues, a carbazole-based self-assembly monolayer molecule with an ionic head group is introduced at the ITO/perovskite interface, resulting in high-efficiency Pb-Sn perovskite solar cells with enhanced open-circuit voltage.